Binding and Electronic Level Alignment of π -Conjugated Systems on Metals
Antoni Franco-Cañellas, Steffen Duhm, Alexander Gerlach, Frank Schreiber
BBinding and Electronic Level Alignment of π -Conjugated Systems on Metals Antoni Franco-Ca˜nellas ID , Steffen Duhm ID , Alexander Gerlach ID , and Frank Schreiber ID ∗ Institut f¨ur Angewandte Physik, Universit¨at T¨ubingen,Auf der Morgenstelle 10, 72076 T¨ubingen, Germany Institute of Functional Nano & Soft Materials (FUNSOM),Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices and JointInternational Research Laboratory of Carbon-Based Functional Materials and Devices,Soochow University, Suzhou 215123, People’s Republic of China (Dated: May 1, 2020)We review the binding and energy level alignment of π -conjugated systems on metals, a fieldwhich during the last two decades has seen tremendous progress both in terms of experimentalcharacterization as well as in the depth of theoretical understanding. Precise measurements ofvertical adsorption distances and the electronic structure together with ab-initio calculations haveshown that most of the molecular systems have to be considered as intermediate cases between weakphysisorption and strong chemisorption. In this regime, the subtle interplay of different effects suchas covalent bonding, charge transfer, electrostatic and van der Waals interactions yields a complexsituation with different adsorption mechanisms. In order to establish a better understanding ofthe binding and the electronic level alignment of π -conjugated molecules on metals, we provide anup-to-date overview of the literature, explain the fundamental concepts as well as the experimentaltechniques and discuss typical case studies. Thereby, we relate the geometric with the electronicstructure in a consistent picture and cover the entire range from weak to strong coupling. Keywords: π -conjugated molecules on metals; vertical adsorption distances; energy-level alignment; X-raystanding waves, photoelectron spectroscopy CONTENTS
I. Introduction 1II. General considerations and fundamentals 3A. Interface energetics 3B. Role of the substrate 5C. Role of the molecule 7D. Role of in-plane interactions 7III. Experimental methods 8A. The X-ray standing wave technique 81. Concept of XSW measurements 82. Experimental considerations 9B. Photoelectron spectroscopy 101. Ultraviolet photoelectron spectroscopy 102. X-ray photoelectron spectroscopy 12C. Complementary techniques 12IV. Case studies 17A. Overview and compilation of adsorptiondistances 17B. Weakly interacting systems 17C. Strongly interacting systems 18D. Intermediate cases 211. Fluorination 212. Core substitutions of phthalocyanines 223. Functional groups 244. Surface modification and decoupling 25E. Chemical reactions at interfaces 251. On-surface formation of porous systems 262. Self-metalation reactions of porphyrins 263. The dissociation reaction of azobenzene 27 4. Surface-mediated trans-effects of MePc 28F. Heterostructures 28V. Summary and conclusions 29Acknowledgments 31References 32
I. INTRODUCTION
The interface between π -conjugated organic semicon-ductor molecules and metals is at the heart of a num-ber of important scientific questions, both from a fun-damental as well as from an applied perspective. It is akey issue for the different energy-level alignment (ELA)schemes as well as for charge carrier injection/extractionefficiencies and related issues in organic (opto)electronicsdevices . At the same time, already the question of theinteraction and binding is non-trivial, in particular, forsystems which are between the limiting cases of (clearlyweak) physisorption and (clearly strong) chemisorption.Importantly, there is a subtle interplay between geomet-ric and electronic structure, with a frequently substantial(but not necessarily dominating) contribution of disper-sion interactions, which makes predictions of the metal-organic interface rather challenging, if only “simple rules”are employed. Rather advanced theoretical methods,developed in the last decade, have enabled substantialprogress . In parallel with that, a satisfactory under-standing of these systems requires the experimental de-termination of both the exact adsorption (i.e., binding) a r X i v : . [ c ond - m a t . m t r l - s c i ] A p r geometry as well as the resulting electronic structure in-cluding possible charge transfer, interface dipoles, andshifts of the electronic levels. Fortunately, the last yearshave also seen tremendous progress in experimental re-sults, so that we are now looking at a reasonably largeand representative set of experimental data on a numberof systems, which allow a more comprehensive discussion.This is the main goal of the present review.We shall first emphasize the importance of the struc-tural properties. In line with the motivation above, it hasbecome clear that the precise knowledge of the moleculararrangement on the surface is necessary to assess and in-terpret the electronic properties and eventually the ELA.The nature of the interaction of (aromatic) π -systemswith metals is less obvious than, say, CO on Ni(111) orother chemisorbing systems , which can be safely as-sumed to exhibit a well-established chemical bond on theone hand, and, say, noble gases, which are obviously phy-sisorption, i.e. dispersion-interaction dominated on theother hand . The interaction and interface for the in-termediate case has been subject to intense research withtwo largely complementary approaches:I. Experimental high-precision determination of ad-sorption distances, mostly using the X-ray standingwave (XSW) technique. Remarkably, while XSWhad been developed in the 1960s for the localizationof interstitial dopants in the bulk and thereafterused also for simple adsorbates on surfaces , thefirst investigations of larger aromatic compoundswere published only in 2005 . Since then, nu-merous studies using the XSW technique have re-vealed that π -conjugated molecules on metals showa surprisingly rich phenomenology, e.g., with sig-nificant distortions of the molecules on noble metalsurfaces. Of course, also other techniques such asphotoelectron diffraction (PhD) , rod-scans in X-ray diffraction or LEED I-V , which are usedfor structural investigations on surfaces, have theirmerits but do not exhibit the same precision and/orelement specificity as the XSW technique.II. Quantum theoretical calculations that managed toinclude long-range dispersion forces in density func-tional theory (DFT) codes , which became morepopular than previous attempts involving, e.g.,Hartree-Fock self-consistent field wave functions or Møller-Plesset perturbation theory (MP2) . Dif-ferent schemes going beyond standard DFT weredeveloped to tackle the fundamental problem, howto treat exchange-correlation effects. While thoseapproximations with dispersion corrections involveincreased computational costs, they have becomemore and more accurate for calculating the adsorp-tion geometry of organic molecules on metals. Inthis context, the reader seeking more informationis referred to reviews of vdW-corrected DFT and to Ref. 9 for its application in the context ofmetal-organic interfaces. FIG. 1. Sketch of the fundamental quantities and phenom-ena central to this review: φ is the substrate work function,∆ p − b the change of surface dipole due to the push-back ef-fect, CT the charge transfer effects between adsorbate andsubstrate, vdW are van der Waals forces, ∆ mol the intramolec-ular dipoles intrinsic as well as adsorption induced, d adsorption the average adsorption distance of the molecule and “bond”refers to the possible formation of chemical bonds betweenthe molecule and the substrate. The magnitude of the energyshifts (dashed arrows) is intimately related to the adsorptiongeometry (solid arrows) as will be discussed in the main text. Regarding the electronic properties of such systems, ithas already been recognized in the 1970s that the elec-tronic structure of molecular solids is considerably dif-ferent to that in the gas phase . However, it tooktwenty more years until “energy-level alignment” and“interface dipoles” for π -systems at interfaces came intothe focus of research and the seminal review by Ishiiet al. has been published . In the last decade a sys-tematic understanding and phenomenology has been es-tablished . In particular, ELA is now relativelywell studied for multilayers on inert substrates, i.e., ifsubstrate-adsorbate interactions can be neglected .However, this is frequently not the case on metal sub-strates, and it is clear that the complete binding scenarioincluding possible distortions of the adsorbate is requiredfor a thorough understanding of ELA .As mentioned above, it is by now accepted thatELA at organic-metal interfaces is of utmost impor-tance for the performance of organic (opto)electronic de-vices . Moreover, energy-levels and thus chargeinjection barriers can be tuned by engineering the sub-strate work function . This can be done by pre-covering a metal electrode with a monolayer of an elec-tron acceptor (donor) for increasing (decreasing) the ef-fective substrate work function and thus lowering thehole (electron) injection barrier into subsequently de-posited organic layers . The contact formation atsuch strongly coupled interfaces goes usually along with acomplex electronic scenario involving donation and back-donation of charges (see Figure 1) . Furthermore, theadsorption distances including a possible intramoleculardistortion impacting the molecular dipole are essential.This is why in-depth discussion of the electronic struc-ture usually requires a precise determination of the geo-metric structure (Figure 1), and why XSW results have akey role in this context. Several original research articles(e.g. Refs. 66, 71, 73–81) and, more recently, some re-view articles and book contributions (Refs. 9, 51, 82–86) FIG. 2. Chemical structure of the main molecules reviewedhere (green for carbon, blue for oxygen, red for nitrogen, pinkfor fluorine and white for hydrogen). (a) Perylene. (b) Diin-denoperylene (DIP). (c) Perylene-3,4,9,10-tetracarboxylic di-anhydride (PTCDA). (d) Perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI). (e) Pentacene (PEN) if the pe-ripheral atoms are hydrogen or perfluoropentacene (PFP)if they are fluorine. (f) 6,13-pentacenequinone (P2O). (g)5,7,12,14-pentacenetetrone (P4O). (h) (Metal) phthalocya-nines (MePc) with or without perfluorination. (i) 2,3,5,6-tetrafluoro-7,7,8,8 tetracyanoquinodimethane (F TCNQ). (j)(Metal) tetraphenylporphyrin (MeTPP). have demonstrated that correlating electronic structureand vertical adsorption heights gives new insights.This review is organized as follows: Initially, we ex-plain the basic concepts of organic-metal contact forma-tion (Sec. II), followed by some experimental consider-ations related to XSW, photoelectron spectroscopy andcomplementary techniques (Sec. III). After providing acomprehensive list of XSW data obtained for conjugatedorganic molecules (COMs, representative chemical struc-tures are shown in Figure 2) on metals, which may serveas reference and general overview, we discuss several typ-ical adsorbate systems (Sec. IV). In each case, we explorethe geometric and electronic structure of these systemsas well as how these properties are related for differentadsorption scenarios. Finally, we shall summarize theimportant findings (Sec. V).
II. GENERAL CONSIDERATIONS ANDFUNDAMENTALS
First, we shall introduce the basic quantities, con-cepts and phenomena that describe and govern the metal-organic interface, in particular with respect to the differ-ent effects influencing the ELA in the monolayer regime.
A. Interface energetics
The most relevant energy-levels at an organic-metal in-terface in the limiting case of physisorption are shown inFigure 3. A metal has electrons occupying energy-levelsup to the Fermi level E F . The energy to bring themto the vacuum level (VL) corresponds to the metal workfunction φ . In the COM the most important energy-levels are those of the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital(LUMO), which are also referred to as the frontier molec-ular orbitals. The energy difference between the HOMOand the LUMO defines the transport gap E trans . Be-cause typical exciton binding energies of COM thin filmsare in the range of several hundred meV , i.e. muchhigher than for most inorganic semiconductors, the op-tical gap E opt is considerably smaller than E trans90–92 ,with the latter being the relevant parameter for ELAand the charge-transport characteristics of the thin film.We note that the ionization energy (IE), which is definedas the energy necessary to move an electron from theHOMO to the vacuum, and the electron affinity (EA),which is the energy necessary to bring an electron fromthe vacuum to the LUMO, cannot be considered as mate-rials parameters: The collective impact of intramoleculardipole moments, which depend on the molecular orienta-tion within the thin film, influences the IE as well as theEA . Therefore, one has to determine these valuesfor each specific thin film structure.For the IE and EA often the onsets of experimen-tally determined HOMO and LUMO levels are used ,( cf. Figure 3) because the onsets govern the transportproperties . However, the onset of a peak measured by(inverse) photoemission depends, naturally, on the ex-perimental resolution. Furthermore, the signal-to-noise-ratio can also play a significant role for the onset, espe-cially if the peak shape is not simply Gaussian and/orgap states are involved . Whether the use of onsetsor peak maxima is more beneficial depends on the spe-cific adsorbate/substrate system and the scientific ques-tion. Unfortunately, no general convention has been es-tablished yet and, consequently, great care has to betaken when comparing values from different publicationsor when comparing experiment and theory.For COM thin films polarization leads to a rearrange-ment of energy-levels in the solid state compared to thegas phase . The polarizability of metals is, ingeneral, much higher than that of organic thin films. Theimage-charge effect (often called screening) leads thus toa further narrowing of the transport gap in monolay-ers on a metal substrate compared to multilayers as shown in Figure 3. Moreover, even for physisorptionthe vicinity of a metal leads to broadening of the energy-levels through electronic, quantum-mechanical interac-tion of the localized molecular states with the continuumof metal states .Upon contact formation of a COM and a metal, vac-uum level alignment is rather the exception than the FIG. 3. Schematic energy-level diagram of a weakly interact-ing organic-metal interface. The metal is characterized by itswork function φ , which is the energetic difference between thevacuum level (VL) and the Fermi level E F . In the shown lim-iting case of physisorption, the vacuum level shift ∆VL is dueto the push-back effect. For the organic adsorbate the den-sity of states of the frontier molecular orbitals (HOMO andLUMO) are approximated as Gaussian peaks and are shownfrom monolayer to multilayer coverage. The energetic differ-ence between E F and the onset of the HOMO level definesthe hole injection barrier (HIB). The ionization energy (IE),the electron affinity (EA), the transport gap E trans and theoptical gap E opt are usually taken from multilayer measure-ments. rule . There are various reasons for vacuumlevel shifts ∆VL upon contact formation, which are notrestricted to metal substrates, but may also take placewhen the molecules are adsorbed on inorganic semicon-ductors and insulators . The magnitude of in-terface dipoles is often related to vertical adsorption dis-tances and the most relevant possible contributions asthey are illustrated in Figure 4 are:I. Push-back effect ∆ p − b caused by the Pauli repulsionbetween the electrons of the adsorbate and the metalII. Charge transfer between adsorbate and substrateIII.
Chemical bond formation between adsorbate andsubstrateIV.
Molecular dipole moment ∆ mol , which can be in-trinsic (polar COMs) or due to adsorption induceddistortionsFor the physisorbed system shown in Figure 3 only thepush-back effect is considered. For systems with strongerinteractions the impact of the interfacial coupling on FIG. 4. Dipoles at organic-metal interfaces. (a) The electrondensity spilling out of a clean metal surface gives rise to asurface dipole. (b) This surface dipole is weakened by theadsorption of COMs (push-back effect due to Pauli repulsion).(c), (d) Charge transfer yields an additional interface dipole.The charge transfer can be integer (ICT) or fractional (FCT).(e) Adsorption induced distortions or the adsorption of polarCOMs can lead to additional dipoles. ∆VL has to be taken into account. In general, whether anadsorbate is physisorbed or chemisorbed on a substrateis clearly defined by adsorption energies and can beaccessed theoretically . However, the adsorptiontype is not directly accessible by standard experimen-tal techniques. To overcome this issue we use a simpledefinition based on peak shifts between mono- and multi-layer in photoemission data : For rigid shifts of valenceelectron features (typically the HOMO-derived peak) andcore-levels we assume physisorption and chemisorption inall other cases. Within this definition it becomes appar-ent that the pentacene oxo-derivative P2O is physisorbedon Ag(111) and P4O is chemisorbed on the same sub-strate. Thus, we use schematic energy-level diagramsbased on photoemission data of P2O and P4O onAg(111) (Figure 5) to illustrate the impact of organic-metal coupling strength on interface dipoles and ELA.The push-back effect, which leads to ∆ p − b , is relatedto the electron density spilling out into vacuum at cleanmetal surfaces . Push-back takes place at virtuallyall organic-metal interfaces as the surface dipole part ofthe metal work function will be decreased by the merepresence of the molecular monolayer . There isa clear correlation between ∆ p − b and adsorption dis-tances . For physisorbed systems the push-backeffect is often the main contribution to ∆VL and canbe held responsible for most of the 0.60 eV shift at theP2O/Ag(111) interface.For the discussion of interfacial charge transfer it ishelpful to distinguish between integer and fractional charge transfer (Figure 4c and d) . While thelatter is usually related to chemical bond formation, theformer can also occur for weakly interacting systemsand is then a result of Fermi level pinning . Thishappens for high (or low) substrate work functions forwhich a vacuum level controlled ELA would lead to asituation with the HOMO (LUMO) being above (be-low) the Fermi level. In such cases thermodynamic equi-librium is maintained by an interfacial charge transfer.
FIG. 5. Schematic energy-level diagrams for P2O and P4Oon Ag(111). On the left the Ag substrate with its work func-tion φ and Fermi level E F is displayed. The middle pan-els correspond to a P2O monolayer, with the position of thevacuum level (VL), the position of the (former) LUMO, theHOMO position and the energetic position of C 1s and O 1score-levels. In the right panels the corresponding values formultilayer coverage are displayed. All binding energy valuesare given in eV, energy axes are not to scale. The molecularstructures on the bottom show possible resonance structuresin the monolayer. The energy-level diagrams are drawn usingUPS and XPS data published in Refs. 78 and 118. Thus, notably, also in the absence of any specific in-terfacial interaction charge transfer across an organic-inorganic interface can take place. Interestingly, theHOMO- (LUMO-) levels are typically pinned several hun-dred meV below (above) E F . This is dueto a certain degree of disorder in molecular thin filmsleading to a broadening of HOMO and LUMO density-of-states (DOS) . The relationship between DOSshape and ELA has been addressed in several publica-tions and is beyond the scope of this re-view. Likewise, for ELA at organic-organic interfaces,the reader is referred to Refs. 38, 41, 106, 133–136.The above mentioned screening effect leads to rigidenergy-level shifts (typically several hundred meV) ofvalence and core-levels to higher binding energies be-tween monolayer and multilayer coverage of organic thinfilms . This is the case for physisorbed P2O onAg(111) (Figure 5). For chemisorbed systems, the ex-pected shifts due to screening can be overcompensatedby the strong chemical coupling at the organic-metal in-terface. This becomes apparent for P4O on Ag(111);in this particular case, chemisorption goes along with afilling of the former LUMO. The charge transfer counter- FIG. 6. Vertical adsorption heights of P2O and P4O onAg(111). Bold numbers refer to experimental and italic num-bers to theoretical results, black numbers to carbon and rednumbers to oxygen. For better visibility, the molecular dis-tortions are not drawn to scale. Taken from Ref. 78. acts the VL decrease by push back leading to a constantVL upon contact formation. The apparent vacuum levelalignment is, however, most likely coincidental. For re-lated systems also a pronounced increase in the effectivemetal work function has been observed . Suchsystems will be discussed in more detail in Sec. IV C. Therelatively strong chemisorption of P4O on Ag(111) leadsto a rehybridization of the molecules in the monolayer(a possible resonance structure is shown in the bottomof Figure 5). This is in line with the experimentally de-termined vertical adsorption distances (Figure 6), whichshow a pronounced distortion of P4O upon adsorptionon Ag(111) .Overall, the PxO/Ag(111) systems (Figure 5) demon-strate some potential pitfalls in interpreting energy-leveldiagrams: For P2O/Ag(111) an apparent interface dipolemimics strong interaction, whereas the charge transferat the P4O/Ag(111) interface leads to apparent vacuumlevel alignment. Thus, additional information is neces-sary to fully understand and describe organic-metal in-terface energetics. In particular, a precise knowledge ofthe vertical adsorption distance is necessary for a properdescription of the adsorption behavior. B. Role of the substrate
As discussed above, one can distinguish two limitingcases within the domain of metal-organic interactions,namely, physisorption and chemisorption. The adsorp-tion distances are therefore expected to range betweenthe sum of the van der Waals radii (cid:80) r vdW for purephysisorptive bonding and the sum of the covalent radii (cid:80) r cov for pure chemisorptive bonding. The correspond-ing values for carbon atoms interacting with the threenoble metals are given in Tab. I. Obviously, the differ-ences between van der Waals and covalent bonding fora given substrate material (being 1 . − . reactivity of those materials is a key factor. For thatpurpose, the electronic properties of the substrates haveto be discussed in some detail .In metals, narrow d - and broad sp -bands form thevalence-band states, where the latter are more likely tointeract with a given adsorbate. At a certain distance,the molecular orbitals will start to overlap with thoseof the surface atoms. Initially, the adsorbate orbitalswill broaden and shift in energy (see Figure 3) as a con-sequence of the interactions with the rather delocalized sp -electrons and only if the d -orbitals are involved willthe adsorbate levels split into bonding and antibondingstates, generally one being below and the other abovethe metal band. In this context, one can relate the inter-action strength and the degree of chemisorption to thedifferent orbitals involved. For instance, the term weakchemisorption is used for the case where only sp -orbitalsare involved. When d -orbitals are also at play, the fillingof the bonding and antibonding states influences the in-teraction strength as well. Thus, a strong bond is associ-ated to the filling of only bonding states. Conversely, thepartial or total filling of antibonding states induces a re-pulsive interaction that counteracts the attractive forcesexerted by the sp -electrons. The degree of filling is re-lated to the relative position of the d -states with respectto the Fermi level. Also, the broadening of these statesis responsible for the degree of repulsion with the adsor-bate states. Indeed, a broader state increases the overlapwith the adsorbate orbitals and subsequently the costof orthogonalizing the wave functions to avoid Pauli re-pulsion. In light of this, moving from left to right inthe periodic table, i.e. from transition to coinage metals,the outmost d -states shift down in energy away from theFermi level , thus explaining the decreasing reactivitywithin this series. The broadening of the band, on theother hand, increases when moving down the column orfrom right to left in the periodic table, which explainswhy Cu is said to be more reactive than Au. This trendis also reflected in the averaged vertical adsorption dis-tances d H of the carbon atoms in the molecular back-bone of adsorbates on such surfaces. Figure 7 shows that d H decreases for each perylene derivative on the (111)-surfaces of noble metals in the order Au–Ag–Cu. Thisfinding can be considered as a qualitative trend for mostCOMs on these surfaces, but precise quantitative predic-tions can only be done if the nature of the adsorbate istaken into account.For the interaction with a given adsorbate, not onlythe chemical composition of the bulk crystal is impor-tant, but also its surface structure and termination. Boththe transfer of charge across the interface and the for-mation of bonds often need some energy barriers to beovercome. For all metal substrates the work function φ decreases with increasing “openness” of the surface be-ing considered. Thus, closed-packed surface structures,i.e. fcc(111), bcc(110), and hcp(001), show the highest FIG. 7. Experimentally determined vertical adsorption dis-tances of perylene (derivatives) on the (111)-surfaces of thenoble metals. Adapted from Ref. 80 with permission. Thedata is taken from these references: Perylene and PTCDIfrom Ref. 80, PTCDA on Au(111) from Ref. 148, PTCDA onAg(111) from Ref. 149, PTCDA on Cu(111) from Ref. 150,DIP from Ref. 151. φ and the lowest reactivity. Likewise, defects, step edgesand kinks act as interaction centers for adsorbates, whichin some cases migrate across the flat terraces until theyfind a suitable location. In all these cases, the electronicand/or chemical interaction, with the extreme case of ad-sorbate dissociation, is favored by the lower energy bar-riers caused by elements that disrupt the surface poten-tial landscape due to dangling bonds or excess/defect ofcharges, which may be recovered by the interaction withthe adsorbate.Of particular importance for CT effects is the pres-ence of surface states, which form as a consequence ofthe reduced coordination of the topmost atoms comparedto those in the bulk . Due to the termination ofthe crystal and the change of the electronic band struc-ture new states confined to a region very close to thesurface may exist. While these states appear even onperfect surfaces, the presence of defects, impurities oreven adsorbates may create new interface states localizedaround them. Similar to the doping in semiconductors, TABLE I. Selected substrate parameters: Atomic number Z ; sum of van der Waals radii (cid:80) r vdW for carbon and noble metalatoms ; sum of covalent radii (cid:80) r cov for carbon and noble metal atoms ; lattice plane spacing d for the (111) Braggreflection; corresponding photon energy E Bragg = hc/ d in back-reflection ( θ Bragg ≈ ◦ ); work function φ subs of the baresubstrates . Note that the small surface relaxations of Cu(111) and Ag(111) are often neglected for the determinationof the adsorption distances, whereas the reconstruction of Au(111) should be taken into account. Z (cid:80) r vdw (˚A) (cid:80) r cov (˚A) d (˚A) E Bragg (keV) φ subs (eV) (111) surfaceCu 29 3.17 2.08 2.086 2.972 ∼ Ag 47 3.49 2.21 2.357 2.630 ∼ Au 79 3.43 2.12 2.353 2.634 ∼ × √
3) herringbone reconstruction surface/interface states may act as a center for charge ex-change or reaction when a certain adsorbate is present.In the context of this review, a large fraction of thestudies in the literature have focused on the (111)-surfaces of Au, Ag and Cu. These are relatively inert andless prone to reacting with aromatic adsorbates. Also, forthe noble metals they are the ones with the lowest en-ergy, meaning that they are preferred in evaporation pro-cesses, giving them a slightly higher practical relevancethan, e.g., (110) and (100). Recently, also other orien-tations of the noble metals , alloys as wellas ZnO have been investigated, see the list inSec. IV. For more details on the substrate surface withoutadsorbates, we refer to Ref. 160
C. Role of the molecule
The description of organic molecules is largely basedon the concept of localized bonds . On metal surfaces,however, this approach might be questioned and is scru-tinized, e.g., by specific chemical modifications of the π -conjugated systems being investigated. It is well knownand understood how functional groups impact gas phaseproperties of COMs . The particular nature ofthose functional groups may stabilize the COM or mod-ify the HOMO-LUMO gap and other energy-levels. Forinstance, electronegative side-groups like fluorine gener-ally increase the EA and render the COM thus more n-type .For molecules in contact with the metal substrate func-tionalization can lead to additional effects like fosteringor hindering intermolecular interactions and thereby in-creasing or decreasing the interaction strength. Thatway, e.g. perfluorination of pentacene reinforces the re-pulsion with metal substrates and can change the inter-action from chemisorption to physisorption . Moreover,in the contact layer the desired functionalization effectcan even be nullified as shown in the bottom of Figure 5for P4O: The possible resonance structure of weakly in-teracting P2O molecules in the contact layer to Ag(111)are identical to the gas phase structure. Importantly, theconjugation does not extend over the pentacene backbonebut is broken by the keto-groups. For chemisorbed P4Oon Ag(111), however, by re-hybridization on the surface the conjugation can extend over the entire backbone ofthe molecule and thereby resemble PEN molecules . Forperylene derivatives, substitution can lead to significantdifferences of the adsorption distances and adsorptioninduced distortions , which are especially pronouncedon the relatively reactive Ag(111) and Cu(111) surfaces(Figure 7).Notably, all site-specific interactions affect alsothe electronic structure and can therefore be usedto tailor interface energetics . In general,a competition of adsorbate-substrate interaction be-tween the π -system of the COM and the functionalgroups can take place. For example, a submono-layer of the acceptor molecule HATCN (1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile, C N ) is lyingflat on Ag(111) to maximize the interaction of the π -system and the substrate. Increasing the coverage to afull monolayer, however, induces a re-orientation of theHATCN to an edge-on geometry due to the efficient inter-action of the cyano-groups with the substrate . More-over, the flexibility of the COM plays also an impor-tant role. While peripheral substitution of the moleculesoften leads to large adsorption induced molecular dis-tortions , functional groups belonging to a centralpart of the conjugated molecular backbone induce no oronly negligible distortions – even at strongly coupledorganic-metal interfaces. D. Role of in-plane interactions
While the focus of this review is on the vertical inter-actions, we may briefly comment on the impact of lat-eral forces. Obviously, the influence of the surroundingmolecules dominates the purely organic environment ofthe multilayer regime, most prominently through the π – π interactions of adjacent molecules . For a mono-layer on a metal, though, the molecule-molecule (i.e. in-plane) interactions are usually much weaker than thosebetween the molecules and the substrate. Thus, lateralinteractions are often only a small correction, and thesubstrate largely controls the interface properties and theELA. There are two notable exceptions, though. One isfor molecules with a large intrinsic molecular dipole. Forthese, the electrostatic interaction of nearby molecules,which can be experimentally tuned via the molecular cov-erage, influences the alignment of the molecular dipolesand is directly responsible for the overall interface dipole,which in turn induces work-function changes of the sub-strate . The other is for heteromolecular monolayersadsorbed on metal substrates . In this case, com-bining pairs of donor-acceptor molecules has been provento be an effective method to tune the metal work func-tion .From a more fundamental perspective, it is knownthat an increased intermolecular interaction can weakenthe molecule-substrate coupling and vice versa , asevidenced by changes in the adsorption distance andthe frontier orbitals of the molecule. In this regard,for homomolecular systems, the balance favoring oneor the other may be tuned by changing the tempera-ture , the coverage as well as the nature ofthe substrate . A nice example of this is found inRef. 177 with STM and XSW measurements of PTCDAtaken at RT, which show the well-known herringbonestructure, and at LT, where the first layer becomes dis-ordered. The decrease of the intermolecular interactionsat LT lowers the average adsorption distance of PTCDAand increases the bending of the oxygen atoms towardsthe surface and goes along with an increased filling of theformer LUMO level , all pointing towards an enhancedcoupling with the substrate. For a detailed discussion ofthe in-plane arrangement of molecules and their epitaxywith the substrate, the reader is referred to Refs. 183–185. III. EXPERIMENTAL METHODS
Pivotal to this review are studies performed with theXSW technique and photoelectron spectroscopy. In thissection we will give a general overview of the fundamen-tals and the experimental challenges of these, mainlywithin the context of organic-metal interfaces. Someother complementary techniques in this context will alsobe mentioned without going into much detail or claimingto be exhaustive.
A. The X-ray standing wave technique
The XSW technique is an interferometric method thatexploits the standing wave field I XSW created by Braggdiffraction of the incoming X-ray beam. By measuringcharacteristic photoemission signals, that are related tothe local field strength at the position of the excitedatomic species, one can derive high-precision and chemi-cally sensitive adsorption distances of molecules on singlecrystals (see Figure 8 for a schematic).
1. Concept of XSW measurements
In 1964 Boris W. Batterman first demonstrated thatthe fluorescence intensity emitted from a single crystal,illuminated with X-rays, changed characteristically whenrocking the crystal around the Bragg condition due to thepresence of an X-ray standing wave field . By match-ing the maxima and minima of the fluorescence signalwith the reflected X-ray intensity around the Bragg an-gle, he correlated them with the relative position of theatomic planes of the crystal. A few years later, he usedthis concept to locate, within a silicon single crystal, theposition of arsenic atoms, used as dopants, relative to thesilicon atomic planes . Soon thereafter, it was exploitedthat the XSW field extends outside the crystal surface ,opening up the door to not only the mapping of dopantswithin a crystal structure and the study of buriedinterfaces , but also the location of adsorbates onthe crystal surface . For the latter, initially, adsorbedatoms were studied , Langmuir-Blodgett films andatomic layers followed . Some smaller molecules de-posited on different surfaces started to be studied in the1990s , while, the first measurements of conjugatedorganic molecules on metal surfaces came a few yearslater .Without entering into the exact mathematical deriva-tion of the XSW field, which is based on dynamicaldiffraction theory , one can explain the basic princi-ple of the XSW technique using the fundamental equa-tion Y p ( hν ) = 1 + R + 2 √ R f H cos( ν − πP H ) (1)which relates the normalized photo yield Y p from a givenchemical species, the intrinsic reflectivity R of the crys-tal and the relative phase ν between the incoming andthe reflected wave with the two structural parameters f H and P H . The so-called coherent position P H , which takesvalues between 0 and 1 (being both geometrically equiv-alent), is directly related to the (mean) position of thespecies being considered via d H = ( n + P H ) d n = 0 , , , . . . (2)where n introduces an ambiguity that stems from theperiodicity of the XSW field of period d ( cf. Table I).In most cases, this ambiguity can be removed with com-mon sense and the physical constraints of the system.The index H in Eq. (1) and (2) refers to the reciprocallattice vector of the Bragg reflection employed in the ex-periment. The coherent fraction f H is related to the ver-tical ordering of the species contributing to a given d H .It assumes values between 0 and 1, with 0 as the out-come of randomly distributed emitters around d H and 1the case where all are adsorbing at d H . Different effectscontribute to the decrement of f H , for instance, thermalvibrations and static disorder. We note that, besides theobvious reason that the adsorption distance only makessense within the first adsorbed layer of molecules, the sig-nificance of the experimental P H values is limited by f H . FIG. 8. Schematics of the XSW-field formation. (a) An incoming X-ray plane wave with wave vector k interferes with theBragg-reflected wave with the corresponding wave vector k H and creates an interferences field inside as well as above the crystalsurface. Here, the periodicity between maxima (or minima) of intensity d XSW equals the lattice plane spacing d along thediffraction direction H . (b) For a given scattering geometry the interference field is stationary, but if one scans the incidentangle or photon energy around the Bragg condition, the intensity profile I XSW can be shifted by d /
2. Since the absorption ofX-rays around the Bragg conditions, which depends on the relative position of the atoms ( z/d ) within the field, determinesthe number of emitted photoelectrons, the photo yield variation Y p reveals the corresponding adsorption distance. Importantly,different elements within the adsorbed molecule, as symbolized by the green and red color of the atoms, can be distiguishedusing their specific core-level signal. In the inset, two simulated XSW scans are shown, with the normalized reflectivity (blueline) and the photoelectron yields as a function of the beam energy relative to the Bragg condition (green and red line). Thecoherent positions P H of the two species, as introduced in Eq. (1), can be easily converted to the adsorption distance. Thestrong dependence of the photoelectron yield with respect to the position of the emitter within the XSW field, which is theorigin of the precision of this technique, can be readily seen by the characteristic modulation of these curves. In other words, for a highly disordered layer ( f H ≈ f H can be used as con-fidence parameter for the obtained adsorption distanceand secondly, coverages below or equal to a full mono-layer are desirable to avoid artificially decreasing f H .We note that for practical purposes Eq.(1) has to berefined to account, e.g., for the broadening of the reflec-tivity curve due to the monochromator and the imperfec-tions of the crystals. Also, non-dipole effects in the pho-toemission process, which affect the angular distributionof the photoelectrons , have to be considered in thedata analysis.
2. Experimental considerations
Generally, datasets for two experimental quantities arerequired to model the XSW field and subsequently ex-tract the position of a given species relative to the latticeplanes of the crystal, i.e. the reflectivity R = R ( E ) andphoto yield Y p ( E ) when scanning around the Bragg con-dition E = E Bragg . The reflectivity can be measuredwith a camera directed at a fluorescence screen conve-niently located in the chamber and the photo yield is ex-tracted from fluorescence, Auger or photoelectron spec-troscopy data from the species of interest: Here, we re- strict our discussion to photoelectrons, which are mea-sured through XPS scans performed with different pho-ton energies around E Bragg ( cf. Table I).The use of the XSW technique is constrained by ratherdemanding experimental requirements. Certainly, thefirst major challenge is the indispensable crystal qual-ity of the substrate, both at the surface as well as inthe bulk, which is responsible for the coherence of thestanding wave field. In addition to the high photon fluxrequired for these experiments the need to tune the X-rayenergy limits the usage of the technique to synchrotronfacilities . Here, beamlines with insertion devices, crys-tal monochromators and complex X-ray optics can pro-vide a stable and highly brilliant X-ray beam, that can be(de-)focused to avoid beam damage on the samples. Thereader is referred to Refs. 208–210 for a more detailedexplanation of the beamline requirements.The experimental geometry, namely, the relative direc-tion of the incoming beam with respect to the sample andthe electron analyzer is essential. It can be shown thatwhen creating the interference field in back-reflection(see Figure 8), i.e. the incoming and the Bragg-reflectedbeam being almost perpendicular to the diffracting crys-tal planes (diffraction angles θ Bragg close to 90 ◦ ), the in-trinsic angular width of the reflectivity curve is largest.Thereby, the need for nearly perfect crystallinity ofthe substrates is relaxed . XSW experiments per-formed under these conditions are referred to as normal-0incidence (NI)XSW and have become standard for mea-suring adsorption distances of larger molecules on metals.Recently, it has been demonstrated that dedicatedbeamlines such as I09 at Diamond Light Source (UK) ,which is operational since 2013, can implement significantimprovements in performance and usability comparedto previous installations. Due to the optimized exper-imental setup and data-acquisition methods the signal-to-background and signal-to-noise ratio of the photoelec-tron spectra could be improved without risking extensivebeam damage even for molecular systems. If the electronanalyzer is positioned at an angle of 90 ◦ with respect tothe incident X-ray beam (as realized at I09), the sub-strate background in the spectra is strongly suppressedand also the non-dipole contributions to the photoelec-tron yield are minimized. Overall, the challenges asso-ciated with XSW measurements have to some degreeshifted away from the technical side and more towardsthe sample preparation and data analysis. Indeed, byusing a proper core-level model to account for the dif-ferent contributions to the photoelectron yield one canextract adsorption distances for the chemically inequiv-alent species within a molecule. Hence, the systematiccombination of XSW experiments with high-resolutionXPS allows to resolve intramolecular distortions thatwere not accessible before and thereby extend the sig-nificance of XSW results beyond average adsorption dis-tances . For that matter, accurate and prefer-ably theory-backed core-level models are necessary.Over the years, different software packages have beenused for handling (NI)XSW data. For fitting the core-level spectra the commercial CasaXPS has becomevery popular. For the analysis of the resulting photo-electron yield data, on the other hand, there are var-ious specialized tools available. Recently, Bocquet etal. discussed the general formalism and contributed anew open source program with graphical user interface(
Torricelli ) that facilitates the fitting of XSW data.
As also pointed out in Ref. 206, the analysis can be non-trivial, if the large angular aperture of the analyzer andthe finite tilt of the sample are considered. As a con-cluding remark, we also note that via off-normal XSWmeasurements, i.e. using a Bragg reflection with a finitein-plane component of H , it is in principle possible totriangulate the position of adsorption sites . Since forlarge adsorbate molecules this can be difficult , our fo-cus is on the vertical structure along the surface normal. B. Photoelectron spectroscopy
Photoelectron spectroscopy (PES) is a well-knownand established technique to determine the electronicstructure of solids and is described in detail in variousbooks and review articles . In this section we will,thus, deal with issues specific to PES on organic thinfilms including the main pitfalls and obstacles.
FIG. 9. Schematic valence energy-levels and UP spectrum ofa COM monolayer on a metal substrate. The sample is illu-minated by photons with energy hν . Photoelectrons from themetal Fermi level E F have the highest kinetic energy ( E F kin ).Usually, the Fermi level is used as energy reference to definethe binding energy E B of valence electron features. Inelasti-cally scattered secondary electrons lost the information abouttheir initial state and the position of the secondary electroncutoff E SECOkin allows to determine the vacuum level (VL) ofthe sample. The sketch is strongly simplified, in particularthe measurement process itself, i.e., the impact of the spec-trometer on measured kinetic energies, is neglected. Moredetailed sketches can be found, e.g., in Refs. 1 and 219.
1. Ultraviolet photoelectron spectroscopy
First, we will focus on ultraviolet photoelectron spec-troscopy (UPS). Figure 9 displays on the left side twoenergy-levels (HOMO and HOMO-1) of an organic thinfilm on a metal substrate, for which the continuous oc-cupied DOS is shown. The sample is irradiated withmonochromatic UV-light with photon energy hν and theresulting photoemission intensity is shown on the rightside of Figure 9. An electron analyzer measures the ki-netic energy E kin and intensity of photoelectrons. Theresulting spectra are usually plotted as function of bind-ing energy E B with the Fermi level serving as energyreference ( E F B = 0 eV). The information depth is limitedby the inelastic mean free path of photoelectrons. Theso-called “universal curve” gives a value of ∼ . Consequently,for (sub)monolayer coverages of a flat lying COM film ona metal, molecular features and substrate features appearconcomitantly.In addition to valence electron features, also secondaryelectrons (gray in Figure 9) contribute to the spectrum.These electrons have been inelastically scattered in thesample and thus lost the information about their initialstate. However, they can be used to determine the VLof the sample, since at a certain kinetic energy ( E SECOkin )the energy of secondary electrons is not sufficient to over-come the surface potential of the sample. At this energytheir intensity is dropping to zero, which is often calledsecondary-electron cutoff (SECO). The VL of the sample(w.r.t. E F ) is given by the difference of the photon energyand the whole width of the spectrum, i.e.:VL = hν − ( E F kin − E SECOkin ) . (3)This rather simplified description is sufficient to deter-mine the VL of samples with a homogenous surface po-1 FIG. 10. UP spectra of pentacene deposited on the (111)-surfaces of noble metals measured with the He I excitation line. χ denotes the nominal pentacene thickness. For PEN on Au(111) the SECO region (a), a valence survey spectrum (b), and azoom into the region close to the Fermi level (c) are shown. For PEN on Ag(111) (d) and on Cu(111) (e) only the zooms areshown. All valence electron spectra are measured with an emission angle of 45 ◦ . The lines are guides for the eye and mark theevolution of HOMO and FLUMO (former LUMO) features of PEN on Au(111) and Cu(111), respectively. The energy scale ofthe plots of the secondary electron spectra is corrected by the analyzer work function and the applied bias voltage. Thus, theSECO position corresponds to the VL position above the Fermi level. Adapted from Ref. 182. tential. However, as mentioned above, the adsorption ofCOMs usually modifies the work function of clean metalsurfaces. For submonolayer coverages, or in the case ofisland growth, the sample features local surface poten-tials . Depending on the lateral dimensionsof these inhomogeneities, either two separate SECOs canbe observed (for large island sizes) or the SECO positionis determined by the area-weighted mean of the local sur-face potentials. A detailed description and guidelines onhow to analyze SECOs are given in Ref. 245. This pub-lication describes, furthermore, how to determine IEs oforganic thin films, which are defined by the SECO andthe onset of the HOMO-derived peak .In general, for discussing interfacial interactions of-ten “monolayer” and “multilayer” energy-levels are com-pared ( cf. Figure 5). The thickness of vacuum-sublimedthin films in organic molecular beam deposition (OMBD)is usually measured by a quartz-crystal micro balanceand corresponds, thus, to a nominal mass thickness. Inthat process, layer-by-layer growth is rather the excep-tion than the rule and island or Stranski-Krastanov (is-land on wetting layer) growth dominates . Thus,the first step in interpreting photoemission data is toidentify the spectrum which is most dominated by mono-layer contributions. A first hint gives the evolution of theSECO as adsorption induced charge rearrangements of-ten saturate upon monolayer formation. For PEN, whichmay be regarded as the “fruit fly” of organic surface sci-ence , thickness-dependent UP spec-tra on Au(111), Ag(111) and Cu(111) are shown inFigure 10 as a typical example of UPS at organic-metalinterfaces. Indeed, for PEN on Au(111) the VL (as de- duced from the SECO position in Figure 10a) decreasesrapidly up to a nominal PEN thickness of 4 ˚A. However,this does not mean that a nominal thickness of 4 ˚A cor-responds to a closed monolayer. It simply tells that fromthis thickness on, subsequently deposited molecules growpredominantly in multilayers.The suppression of substrate features, e.g., the Au d -bands in a BE range from 2 to 8 eV in Figure 10b or theFermi-edge in Figure 10c, with increasing coverage can beused to estimate the growth mode of the adsorbate. How-ever, the applicability of the universal curve to organicthin films has been questioned and only qualitativestatements are straightforward. For PEN/Au(111) thesubstrate Fermi-edge is still visible for a nominal cover-age of 96 ˚A, which corresponded to more than twenty lay-ers of flat lying PEN. This clearly shows that the growthmode is not layer-by-layer. For the spectra with a nom-inal thickness of 96 ˚A on Ag(111) and Cu(111), on theother hand, the Fermi-edge is (almost) invisible, point-ing to less pronounced island growth.The shape of HOMO-derived UPS peaks is oftennot simply Gaussian. For well ordered monolayersand sufficient experimental resolution, hole-phonon cou-pling becomes evident in UP spectra as can beseen by the high-BE shoulder of the HOMO-derived peakin the spectra for a nominal coverage of 2 ˚A on Au(111)and Ag(111) (Figures 10c and d). Furthermore, factorslike the measurement geometry and the photon energyimpact photoemission intensities. For example, the emis-sion from the HOMO of flat-lying π -conjugated moleculeshas typically a maximum for an emission angle of 45 ◦ anda minimum for normal emission .2The spectra in Figure 10 are measured with an hemi-spherical analyzer and angle-integrated over ± ◦ along k x , which is a typical measurement geometry. Alsosuch angle-integrated spectra can reflect energy dispers-ing features, in particular if rotational domains related tothe substrate symmetry are involved. This explains theHOMO-shape of PEN in multilayers on Ag(111), in whichPEN adopts a herringbone arrangement and exhibitsa band dispersion . Notably, also former LUMO de-rived energy-levels of organic monolayers on metals canshow intermolecular energy dispersion . For PENon Au(111) the multilayer growth mode is still underdebate and, hence, the multilayer HOMO featureshave not been unambiguously assigned . Overall, greatcare has to be taken when comparing measurements ob-tained in different experimental setups. For example,in an early publication of PEN on Cu(111) the formerLUMO-derived peak just below the Fermi level (Fig-ure 10e) has been overlooked . Moreover, small differ-ences in, e.g., temperature, substrate cleanness, evapora-tion rate or impurities, can have a significant impact onorganic thin film growth and, consequently, the electronicstructure .
2. X-ray photoelectron spectroscopy
For XPS the electronic structure of the sample isprobed with X-rays, whose higher photon energy makecore-levels accessible . Core-levels provideinformation about the local chemical environment of theatoms, which gives rise to so-called chemical shifts in XPspectra . Figure 11a shows the C 1s spectrum of aPTCDI multilayer on Au(111) illustrating the strongchemical shift between carbon atoms in the functionalgroups (C=O) and in the perylene backbone. The chem-ically inequivalent carbon atoms within the molecule canbe precisely resolved if the shift in energy is large enough,which is often the case for carbon bound to electronega-tive atoms (e.g. O and F). The binding-energy (or core-level) shifts associated with the chemical structure canbe further modified by the molecular environment, forinstance if there are strong intermolecular interactions,and/or by the proximity of the substrate. Intrinsic orextrinsic peak broadening is another complication whendescribing core-level signals. Its origin is manifold anda proper description often requires electronic structurecalculations.The fine structure of the core-levels can only be re-solved using a sufficiently high energy resolution, whichis feasible when measuring at synchrotron radiation fa-cilities or with monochromatized lab-sources. Generally,monolayer spectra may include fingerprints of (chemical)interactions with the substrate . For PTCDI onAu(111) the interaction is weak and therefore the mul-tilayer (Figure 11a) and monolayer (Figure 11c) spectraare – except for a rigid shift due to screening – almostidentical. PTCDI monolayers on Ag(111) and Cu(111), FIG. 11. Influence of the environment on the core-level sig-nals. Multilayer spectrum (a) of the C 1s signal of PTCDI (b)compared to (sub)-monolayer coverages of the same moleculeadsorbed on the noble metals (c). Figure adapted fromRef. 80. however, are chemisorbed , which is reflected in non-rigid shifts of the components attributed to differentchemical environments of the multilayer versus the mono-layer spectra; in particular, the energetic spacing betweenthe C=O-derived peak and the main peak decreases forthe PTCDI monolayer (Figure 11c). In addition to theinsight in the chemistry of adsorbates, chemical shifts inXPS facilitate the experimental determination of indi-vidual vertical binding distances for each carbon speciesusing the XSW technique (Figure 7). C. Complementary techniques
The XSW technique is mostly applied to the measure-ment of adsorption distances and molecular distortionsperpendicular to the surface. Alternatively, photoelec-tron diffraction (PhD) provides a full, local 3D po-sitioning of a given species with respect to the surfaceatoms . However, while rather successful for cer-tain (preferably simple) systems, the data analysis andinterpretation are not straightforward since inequivalentpositions may be difficult to decouple, thus challeng-ing its application to larger adsorbates . Similarly,LEED I-V may provide a 3D picture of an adsorbate ,3but the analysis of the data is computationally expensiveand requires some initial guess of the adsorbate position.Both, PhD and LEED I-V require a certain degree of reg-istry/commensurability between the adsorbate and thesubstrate, which limits their use to systems with in-planeorder. Surface X-ray diffraction , and specificallythe so-called rod scans (along q z ) for the vertical struc-ture, are slightly less demanding to model (thanks to theapplicability of the kinematic, i.e. single-scattering, ap-proximation), but the sensitivity to light elements is lim-ited because of low X-rays scattering cross-sections. Im-portantly, it is very difficult to obtain element-specific po-sitions needed to determine possible distortions/bendingof the adsorbates. In contrast, relative positions, suchas tilt angles, can be inferred with NEXAFS by exploit-ing the geometry-dependent absorption of X-rays with-out the need of long-range order . We note thatother popular techniques in the study of inorganic sur-faces and interfaces such as reflection high-energy elec-tron diffraction (RHEED), scanning-electron microscopy(SEM) or ion scattering-based techniques are not verycommon in our context because of the probable beamdamage induced by the high energy of incoming parti-cles. Some notable exceptions can be found though .Although not the focus of this review, a full char-acterization of the interface geometry involves the in-plane structure of the adsorbed layers and their registrywith the surface atoms. Scanning-tunneling microscopy(STM) and low-energy electron diffraction(LEED) are the most popular techniques in thiscontext in addition to grazing-incidence X-ray diffrac-tion , which provides the highest resolution. STMoffers real-space images with atomic resolution, whichcan be combined with local spectroscopy measurements.However, any quantitative determination of vertical ad-sorption structures with STM is still challenging . No-tably, with atomic force microscopy (AFM) under UHVconditions one can estimate vertical bonding distances,although input from DFT-modeling is necessary to ex-tract absolute numbers . LEED offers reciprocal-spaceinformation that is averaged over a large sample area.With more elaborate versions such as LEED I-V, men-tioned above, and spot-profile analysis LEED (SPA-LEED) a precise description of the adsorbate unit cellcan be achieved . Finally, low-energy electron mi-croscopy (LEEM) provides in particular real-time infor-mation of the in-plane arrangement and morphology dur-ing growth .In Sec. III B photoemission spectroscopy was intro-duced as a tool to study the energy-level alignmentand interfacial coupling by using the energy infor-mation of photoelectrons. Beyond that, i.e. by ex-ploiting also the momentum information of the elec-trons , new possibilities arise,which – although not within the scope of this article –shall be briefly summarized here. By angle-resolvedand photon-energy dependent UPS measurement pos-sible in-plane and out-of-plane band dispersions of organic thin films can be accessed. More-over, even for largely angle-integrated measurementsthe photoelectron angular distribution (PAD) providesinsight into, e.g., the orientation of COMs on thesurface . The vibrational fine structure ofHOMO-peaks allows to assess charge reorganization en-ergies and thus to estimate hopping mobilities by a ‘first-principle’ experiment . Moreover, the develop-ment of instrumentation over the last decades has made itpossible to measure photoelectron reciprocal-space maps,often termed “orbital tomography”, which can be usedto reconstruct molecular orbitals in real space and/or toprecisely assign photoemission intensities to a particularmolecular orbital .Two-photon photoemission (2PPE) spectroscopy pro-vides insight into electron dynamics of interfacestates . In conjunction with real-space informa-tion, e.g., by LEED or STM, detailed insight in organic-metal coupling is possible . Accessing the unoccu-pied density of states by inverse photoemission (IPES)is demanding, as cross-sections and overall energy reso-lution are notoriously low and, most importantly, beamdamage can be a problem for organic thin films .Some of these issues can be overcome by low-energy in-verse photoemission spectroscopy (LEIPS) . Scan-ning tunneling spectroscopy (STS), as a local probe, ac-cesses unoccupied states as well as occupied states closeto E F and can furthermore identify site-specific inter-actions at organic-metal interfaces , also mea-surable with high-resolution electron energy-loss spec-troscopy (HREELS) . With in situ optical differ-ential reflectance spectroscopy (DRS) optical propertiescan be measured during deposition .Temperature-programmed desorption (TPD), with aproper modelling of the data, is used to measure theadsorption energy of a particular adsorbate on a givensubstrate . This parameter is important fora precise and quantitative distinction between adsorp-tion regimes and has become, together with the adsorp-tion distance, a benchmark parameter for state-of-the-art DFT calculations . Other uses include the study ofthermally-activated on-surface reactions , the as-sessment of the thin-film growth, desorption kinetics andthe thermal stability of a given system . Finally, in-situ IR spectroscopy provides insight into the vibra-tional modes and changes thereof upon adsorption. It isalso useful for identifying unknown sample compositionsand may give information on changes in the adsorbatecharge . Of course, there are also many other spec-troscopic techniques including, e.g., Raman and photo-luminescence, that can be applied to study some of theisssues discussed here, but rather indirectly and outsidethe scope of this review.
TABLE II. (On the following pages) – List of experimen-tal and element-resolved adsorption distances d H determinedwith the XSW technique for COMs on (111) noble metal sur-faces. Molecule Comment Signal d H (˚A)Pentacene (PEN) derivatives on Cu(111) F PEN
C 1 s – total 2 . s – PEN backbone 2 . s – C(1,2) 2 . s – C(3,4) 2 . s – C–F 3 . s . C 1 s .
34O 1 s . C 1 s .
25O 1 s . C 1 s . C 1 s . s . Perylene derivatives on Cu(111)
DIP s . Mobile phase C 1 s . s – NH . s – NH 2 . s . s – N–Cu 2 . RT C 1 s . s . C 1 s .
61O 1 s – carb. 2 .
73O 1 s – anh. 2 . C 1 s – perylene core 2 . s – C=O 2 . s – C–H 2 . s – C–C+C–C–O 2 . s . s . C 1 s . s . Phthalocyanine (Pc) derivatives on Cu(111)
CuPc s . s . s . s . s . s . s . s . s . s . s . s . CuPc C 1 s .
61N 1 s .
70F 1 s . C 1 s .
68F 1 s . Pc 0.7 ML C 1 s . s . s . s . s . . Molecule Comment Signal d H (˚A) VOPc s N 1 s V 2 p O 1 s ZnPc s . s . p / . ZnPc
C 1 s . s . s . p / . Porphyrin derivatives on Cu(111) s . s – aminic 2 . s – iminic 2 . s . s – aminic 2 . s – iminic 1 . s . s . s . s . C 1 s – C–C 2 . s – C–N 2 . s . p . Other compounds on Cu(111)
Azobenzene subML, 60 K C 1 s – C–C 2 . s – C–N 2 . s . s − COHON s . s . TCNQ C 1 s − N 1 s . s .
60 K C 1 s . s . s . s . ML, 150 K C 1 s . ML. 150 K C 1 s . Pentacene (PEN) derivatives on Ag(111)
P2O C 1 s .
32O 1 s . C 1 s .
69O 1 s . s . s . s . s . C 1 s . s . Molecule Comment Signal d H (˚A)Perylene derivatives on Ag(111) DIP s . relaxed ML O 1 s . . compressed ML O 1 s . . C 1 s . s – total/O KLL 2 . s – carb. 2 . s – anh. 3 . C 1 s .
86O 1 s – carb. 2 .
68O 1 s – anh. 2 . LT C 1 s . s – carb. 2 . s – anh. 2 .
300 K C 1 s . s – total 2 . s – carb. 2 . s – anh. 2 .
100 K C 1 s . s – total 2 . s – carb. 2 . s – anh. 2 . Surf. Pb Ag alloy C 1 s site A 3 . s site B 3 . s – carb. site A 3 . s – carb. site B 3 . s – anh. site A 3 . s – anh. site B 3 . f – bare 0 . f – PTCDA 0 . K-doped ML(K PTCDA) C 1 s perylene core 3 . s C=O 3 . s – carb. 3 . s – anh. 3 . p –PTCDA 3 . p –Ag 3 . C 1 s – perylene core 2 . s – C=O 2 . s – C–H 2 . s – C–C+C–C–O 2 . s . s . C 1 s . s . Phthalocyanine (Pc) derivatives on Ag(111)
CuPc s . s . p / . s . s . p / . s . s . p / . s . s . p / . s . s . p / . Molecule Comment Signal d H (˚A) s . s . p / . s . s . p / . SurfacePb Ag alloy C 1 s . s . p / . f – bare 0 . f – CuPc 0 . CuPc C 1 s .
25F 1 s . No dosing C 1 s – C–C 2 . s – C–N 2 . s . p / . dosing C 1 s – C–C 2 . s – C–N 2 . s . p / . O dosing C 1 s – C–C 2 . s – C–N 2 . s . p / . Pc s . s . s . s . s . s . s . s . s . s . s . s . s . s . d . s . s . d . / . s . s . s . p / . Other compounds on Ag(111)
Azobenzene
C 1 s − N 1 s .
210 K C 1 s − N 1 s .
210 K C 1 s . s .
80 K C 1 s . S 1 s . PYT s . s – carb. 2 . s – nitr. 2 . s . s . N 1 s . Molecule Comment Signal d H (˚A)Other compounds on Ag(111) (cont.) TCNQ
C 1 s – C–H 2 . s – C–C 2 . s – C–N 2 . s . K-doped ML(K-TCNQ) C 1 s – C–H 2 . s – C–C 2 . s – C–N 2 . s . p – bare 2 . p – TCNQ 3 . Pentacene (PEN) derivatives on Au(111)
P4O s . Perylene derivatives on Au(111)
DIP s . C 1 s . C 1 s . C 1 s . s . C 1 s . s . Phthalocyanine (Pc) derivatives on Au(111)
CuPc s . s . p / . s . s . p / . s . s . p / . s . s . p / . CuPc
C 1 s .
25F 1 s . Various COMs on other surfaces
MnPcon Cu(001) (002) reflection C 1 s total 2 . s – C–C 2 . s – C–N 2 . s . p / . (200) reflection C 1 s – perylene core 2 . s – C=O 2 . s – carb. 2 . s – anh. 2 . s – perylene core − C 1 s – C=O − O 1 s – carb. − O 1 s – anh. − CoTPPon Ag(100) (200) reflection C 1 s – core 3 .
10C 1 s – phenyl rings 2 .
45N 1 s . p / . Molecule Comment Signal d H (˚A) PTCDAon Ag(100) s – total 2 . s – perylene core 2 . s – C=O 2 . s – total 2 . s – carb. 2 . s – anh. 2 . s – total − C 1 s – perylene core − C 1 s – C=O − O 1 s – total − NTCDAon Ag(100) s – total 2 . s – carb. 2 . s – anh. 2 . s – total 2 . s – perylene core 2 . s – C=O 2 . s – total 2 . s – carb. 2 . s – anh. 2 . s total 2 . s – perylene core 2 . s – C=O 2 . s – total 2 . s – carb. 2 . s – anh. 2 . K-doped(K-PTCDA),(220) reflection C 1 s – total 2 . s – perylene core 2 . s – C=O 2 . s – total 2 . s – carb. 2 . s – anh. 2 . s . s − s − s − s − Grapheneon SiC(0001)
UHV-grown0.5 ML C 1 s – second layer − C 1 s – first layer (1) 2 . s – first layer (2) 2 . s – second layer − C 1 s – first layer (1) 2 . s – first layer (2) 2 . s – second layer − C 1 s – first layer (1) 2 . s – first layer (2) 2 . C 1 s – graphene 4 . Gr buffer layer(BL) bare C 1 s – BL bare 2 . s – BL bare doped 2 . s – BL 2 . s – BL doped 2 . s – graphene 5 . s – graphene doped 5 . s – graphene doped 5 . s – graphene 4 . s – graphene doped 4 . s – graphene doped 4 . s – interstitial 2 . N 1 s – total − N 1 s – strongly bound 2 . s – weakly bound 3 . s – total − B 1 s – strongly bound 2 . s – weakly bound 3 . IV. CASE STUDIESA. Overview and compilation of adsorptiondistances
We organize the case studies according to the strengthof the interaction with the substrate. In the followingsections, we present some well-studied systems that illus-trate the current understanding of the different adsorp-tion regimes, i.e. (vdW-dominated) physisorption, clearchemisorption and different cases in between. Most of thesystems have been studied with both XSW and UPS.For a general overview, we also refer to Table II whichprovides a comprehensive list with adsorption distancesof COMs on metals obtained by XSW measurements. Forreasons of space only the molecule, the substrate and theadsorption distance for the different elements (if applica-ble) are reported.
B. Weakly interacting systems
Weakly interacting systems, which are dominated bydispersion forces and lack stronger covalent interactions,represent the limiting case of physisorptive bonding. Be-cause they may be considered as test ground for DFTcalculations with van der Waals (vdW) corrections, pre-cise XSW results have become particularly important forevaluating the accuracy of these methods. A prototypicalsystem falling in this category would be simply benzeneon Ag(111), which was also pursued in Refs. 8 and 374.With regard to possible applications in organic electron-ics, though, larger acenes or similar systems have greaterpractical relevance due to their more suitable energy-levels and smaller HOMO-LUMO gap, as well as greaterthermal stability, since benzene desorbs already at 300 K.As model system we may consider diindenoperylene(DIP, see Figure 2), a π -conjugated organic semicon-ductor with excellent optoelectronic device performance,which has been studied over the last decade both inthin films and in monolayers on noble metal sur-faces . With respect to its chemical structure,DIP is a relatively simple, planar hydrocarbon with-out heteroatoms. In contrast to the intensely studiedPTCDA , i.e. a molecule with the same perylenecore but with four carbonyl groups, the specific DIP–substrate interaction is not complicated by polar sidegroups – see Figure 7 which illustrates the significant in-fluence of funtional groups on the bonding distances forthese molecules. Moreover, the influence of intermolec-ular (lateral) interactions is expected to be smaller thanfor PTCDA.Generally, the reliable prediction of the equilibriumstructure and energetics of hybrid inorganic/organic sys-tems from first principles represents a significant chal-lenge for theoretical methods due to the interplay of,generally, covalent interactions, electron transfer pro-cesses, Pauli repulsion, and vdW interactions. Recent FIG. 12. Comparison of DFT calculations performed for DIPadsorbed on the three (111)-surfaces of Cu, Ag and Au usingthe Perdew, Burke und Ernzerhof (PBE) exchange-correlationfunctional with and without vdW corrections. Note that insome cases, i.e. if no vdW interactions are included, the out-come would be no binding at all. Taken from Ref. 151 years have seen substantial efforts to incorporate vdWinteractions into density functional theory (DFT) cal-culations in order to determine the structure and sta-bility of π -conjugated organic molecules on metal sur-faces . This is particularly important for sys-tems with significant vdW contributions to the overallbonding (i.e. in the absence of covalent interactions, etc.)such as most π -conjugated molecules on weakly interact-ing substrates.The effect of dispersion forces is nicely demonstratedin Figure 12 by comparing DFT results obtained withthe PBE exchange-correlation functional with and with-out including vdW interactions. It was found that dis-persion corrected DFT calculations applied to DIP onthree different noble metal surfaces yield vertical bond-ing distances that agree very well with the experimen-tal data. The XSW results averaged over all carbonspecies of DIP, i.e. d H = 2 . ± .
03 ˚A for Cu(111),3 . ± .
04 ˚A for Ag(111) and, taking the reconstructionof the gold surface into account, 3 . ± .
03 ˚A for Au(111),differ less than 0.12 ˚A from the minima of the calculatedadsorption energies E ads ( z ) as they are marked by ar-rows in Fig. 12. As expected, those energies follow thetrend | E ads (Cu) | > | E ads (Ag) | > | E ads (Au) | (i.e., withthe strongest interaction for the most reactive substrate,which matches the discussion in Sec. II B). Importantly,the rather shallow and broad minima of E ads ( d ), whichcorrespond to the equilibrium distances, form only if thevdW corrections are included, otherwise there is no sta-ble adsorption at all. Moreover, Figure 12 shows that onCu(111) the Pauli repulsion sets in rather weakly (a lesssteep E ads ( d ) for small distances) compared to Ag(111)and Au(111), which is due to significant interaction be-8tween DIP and Cu(111), i.e. contributions beyond thevdW attraction.In order to understand the contribution of the vdWinteractions in more detail, first one has to consider theimpact of the specific symmetry on the vdW interac-tion (which for individual atoms goes as (distance) − ):Integrating the vdW energy of a single atom over thesemi-infinite substrate yields the atom-surface vdW en-ergy as C ( z − z ) − , where C determines the interactionstrength between atom and surface , z corresponds tothe distance of the atom to the uppermost surface layer,and z indicates the position of the surface image plane.Naively, one might attempt to determine the C coef-ficients for the different surfaces from all the two-bodyatom-atom vdW energies and thereby neglect any inter-actions of the substrate atoms with each other. However,it can be shown that the dielectric function, i.e. the col-lective electronic response of the underlying solid, has astrong influence on the interaction strength .Using the Lifshitz-Zaremba-Kohn (LZK) expression forcalculating the C coefficients, one obtains (in units ofHartree · Bohr ) 0.35 for Cu, 0.35 for Ag and 0.33 forAu, which leads to essentially the same interaction en-ergy at large distances for DIP on Cu(111), Ag(111),and Au(111) (Figure 12). However, at shorter molecule-surface distances, which include the equilibrium distance,the adsorption energy is determined by an interplay be-tween the vdW attraction and the Pauli repulsion witha possible covalent component. The Pauli repulsion fol-lows roughly the trend of decreasing vdW radii, with afaster onset in terms of the molecule-surface distance forAu (the largest vdW radius), and then decreases for Agand Cu. Therefore, for Au the balance between vdW at-traction and the Pauli repulsion is obtained further awayfrom the substrate (i.e. at larger adsorption distances)than for Cu, which in turn makes the adsorption ener-gies lower for Au than for Cu, in contrast to the possiblenaive expectation of Au with its higher electron densityand polarizability leading to stronger interactions thanCu.Generally, we note that semi-local DFT calculations,i.e. the different versions of the generalized gradient ap-proximation (GGA), might not provide very accurateenergy-levels . More advanced methods, however, arecomputationally prohibitively expensive because of thelarge number of atoms within the unit cell of largermolecules on surfaces . For weakly interacting systems,one may not expect major changes of the electronic struc-ture. Nevertheless, even for purely vdW-driven systemsthere will be at least variations of the molecular energy-levels and the vacuum level. As discussed in Sec. II A, thepush-back effect decreases the interfacedipole of the clean metal surface. However, recently itwas shown that vdW interactions can also cause sig-nificant charge rearrangements in the vicinity of the ad-sorbed COM, as shown for DIP on Ag(111) in Figure 13.Using DIP on noble metals as a model system, we be-lieve that the limiting case of weakly interacting systems FIG. 13. Left panel: vdW effect on the electron density dis-tribution ∆ n ( r ) vdW upon adsorption of DIP on Ag(111), ac-cumulation is in blue, depletion is in red. Right panel: Theintegral of ∆ n ( r ) vdW plotted as a function of z , the axis per-pendicular to the surface. A dipole-like density redistributionemerges at the interface. Taken from Ref. 125. – although indeed not as simple as at first assumed –is essentially understood. The key for this are state-of-the-art vdW-corrected DFT calculations in combinationwith very precise experimental data from XSW and othertechniques, which agree within less than ∼ . They found that the adsorp-tion distance of d Ads = 3 . ± .
02 ˚A and the adsorptionenergy of 0 . ± .
05 eV, which were measured for thisclearly physisorptive system, are in excellent agreementwith their DFT calculations.Again, we note that there is a gradual transition fromtruly weakly interacting systems such as DIP or ben-zene on Au(111) to those on Ag(111) or Cu(111), whichare more reactive due to the increased orbital overlap ofmolecular states at smaller adsorption distances (see alsoSec.II B).
C. Strongly interacting systems
In the other limiting case, the coupling and the inter-action between molecule and substrate is so strong thatthere is, inter alia , significant charge donation and/orback donation, significant shifts of energy-levels andpresumably an associated significant distortion of themolecule (but, notably, not yet a chemical reaction).In Sec. II we used P4O on Ag(111) as example for astrongly coupled system. The schematic energy-level di-agram (Figure 5) shows the charge transfer from the sub-9
FIG. 14. (a) UP spectra of the clean Au(111) surface (grayshadows) and monolayer (F )TCNQ films on Au(111) (redcurves). For F TCNQ two charge transfer states (CT andCT ) close to the substrate Fermi level E F are apparent. Theshift of the secondary-electron cutoff (left panel) to lowerbinding energy upon F TCNQ deposition evidences a workfunction increase. (b) N 1s core-level: No charge transfertakes place into the TCNQ monolayer and all molecules areneutral (N ). In contrast, in the F TCNQ monolayer almostall molecules are charged (N − ). Taken from Ref. 330. strate into the former LUMO of P4O in the monolayer.This CT goes along with strong chemical shifts of thecore-levels and a bending of the P4O oxygen atoms be-low the plane of the carbon backbone (Figure 6) . Over-all, P4O re-hybridizes in the contact layer to Ag(111)(possible resonance structures shown in the bottom ofFigure 5). P4O exhibits a similar chemisorptive behav-ior on Cu(111), but physisorbs on Au(111) . In con-trast, F TCNQ chemisorbs on virtually all clean metalsurfaces showing a qualitatively similar be-havior. These systems shall hence be discussed as modelsystems for strongly coupled organic-metal interfaces.The EA of F TCNQ in multilayer thin films on Auis 5.08 eV to 5.25 eV and thus larger than the workfunctions of most clean noble metals . Therefore, onecan expect a charge transfer into the LUMO of F TCNQin monolayers on such substrates to increase the effectivework function and maintain thermodynamic equilibrium.In fact, monolayers of F TCNQ increase the work func-tions of Au(111) , Ag(111) and Cu(111) . Asan example Figure 14a shows UP spectra of F TCNQon Au(111) and compares them with spectra of theunfluorinated parent molecule, TCNQ, which interactsonly weakly with Au(111) . The increase in the workfunction (evidenced by the SECO shift) is accompaniedby two charge transfer peaks (CT and CT ) close to E F . Likewise, the N 1s core-level shows strong chemicalshifts from mono- to multilayer F TCNQ coverage (Fig-ure 14b), which are caused by the charge transfer into
FIG. 15. Calculated occupation (in percent) of the lowest 60molecular orbitals of F TCNQ in a monolayer on Cu(111).The full (open) circles and solid (dashed) lines correspond tothe orbitals which are occupied (unoccupied) in the isolatedmolecule. Taken from Ref. 66. F TCNQ.The work function increase on Au(111) and all othermetal surfaces is smaller than 1 eV . How-ever, a complete filling of the LUMO of the moleculesin the monolayer would lead approximately to a workfunction increase of around 5 eV . The calculated oc-cupation of the lowest 60 molecular orbitals (MOs) fora F TCNQ monolayer on Cu(111) as displayed in Fig-ure 15 indicates that the charge donation to the LUMOis, in fact, accompanied by a back-donation to deeper ly-ing MOs. Especially, the HOMO-9 to HOMO-12 levelsare involved and each is only 80% to 90% occupied af-ter adsorption. They correspond to σ -orbitals localizedon the four nitrile groups of the molecule, which partici-pate most prominently in the chemical bonding with thesubstrate. Summing over all MOs gives a net negativecharge of ∼ e per F TCNQ molecule, which is signif-icantly less than 2 e corresponding to a complete fillingof the F TCNQ LUMO. In addition, as discussed belowin more detail, adsorption induced conformation changes– shown in Figure 16 for F TCNQ on Ag(111) – lead toadditional interface dipole moments.The significant molecular charging causes aromatiza-tion of the central quinone ring and makes the moleculestructurally flexible. This allows the molecule to bendand hybridize with the substrate through the lone elec-tron pairs of nitrogen . On Cu(111), the fluorine atomsare found ∼ . This is a consequence of the carbon atomscarrying the nitrile groups re-hybridizing from sp toward sp upon contact formation . Additionally, the strongmolecule-metal interaction leads to marked changes inbond lengths within F TCNQ. In the gas phase themolecule adopts a fully planar, quinoid-like geometry .Adsorption on the Cu(111) surface results in a nearly aro-matic ring . In the context of strong chemisorption andhybridization with metal surfaces also surface adatomshave been discussed, this applies especially to F TCNQ0on Au(111) and for TCNQ on Ag(111) .For strongly coupled systems, fractional charge trans-fer including donation and back-donation is usually ob-served. The impact of such charge rearrangements onthe VL shall be discussed with the example of F TCNQon Ag(111) . For a better understanding of vacuumlevel shifts caused by organic/inorganic contact forma-tion, the calculated total change of the VL, i.e., the in-terface dipole ∆VL, is often decomposed into two con-tributions : i ) the molecular contribution∆VL mol related to the surface-normal component of themolecular dipole (permanent or adsorption induced) and ii ) the contribution due to the interfacial charge re-arrangement (including charge transfer from or to themetal), the so-called bond dipole ∆VL bond , i.e.:∆VL = ∆VL mol + ∆VL bond . (4)An (infinitely) extended dipole layer results in a shiftof the vacuum level by∆VL = 1 ε A µ ⊥ , (5)where µ ⊥ refers to the surface-normal component of thedipole moment per molecule in the monolayer (includingdepolarization effects ), ε to the vacuum permit-tivity and A to the area per molecule. In the gas phaseF TCNQ is planar, thus, all contributions to ∆VL mol are due to adsorption-induced conformation changes. Inthe distorted conformation of the monolayer (Figure 16),each individual F TCNQ molecule possesses a dipole mo-ment of − .
69 D. In effect, it points away from the metalsurface and would result in a work function decrease by − .
85 eV. In addition to the distortion, however, also∆VL bond due to adsorption-induced charge rearrange-ments, ∆ ρ bond , has to be taken into account. ∆ ρ bond is calculated as the difference of the total electron den-sity of the combined metal-organic interface ρ sys and thenon-interacting densities of metal ρ metal and monolayer ρ monolayer :∆ ρ bond = ρ sys − ( ρ metal + ρ monolayer ) . (6)From ∆ ρ bond , ∆VL bond is then obtained by solvingthe Poisson equation. For F TCNQ, ∆VL bond amountsto +1.70 eV. The net effect, i.e., the sum of ∆VL bond and ∆VL mol , is a work function increase by +0.85 eV,which fits very well with the experimental value of0.65 eV . For molecules that undergo charge-transferreactions with the surface, ∆VL mol and ∆VL bond are not independent. Rather, any error in the description of thebending is made up for by a change in charge transfer,making ∆VL, which is the experimental observable, avery robust quantity .Figure 16 shows the x - y -plane integrated charge den-sity rearrangements ∆ ρ bond ( z ) of a monolayer F TCNQon Ag(111). The pronounced electron depletion directlyabove the top metal layer is attributed to push-back.The largest electron accumulation can be found in the
FIG. 16. Top: charge-density rearrangement, ∆ ρ bond , uponadsorption of a densely packed F TCNQ monolayer on aAg(111) surface integrated over the x - y plane within the unitcell; Bottom: resulting total charge transferred, Q bond . Thevertical line denotes the maximum value of Q bond . The struc-ture of the combined system is shown in the background as aguide to the eye. e corresponds to the (positive) elementarycharge and positive ∆ ρ bond values correspond thus to elec-tron accumulation and negative values to electron depletion.Taken from Ref. 441. π -electron region of F TCNQ. The dip in ∆ ρ bond ( z ) inthe region of the CN groups is consistent with the de-creased σ -electron density in that part of the molecule( cf. Figure 15) .In general, such strongly coupled systems can beused for work function engineering and consequently forenergy-level tuning of subsequently deposited organiclayers. This has been first demonstrated for the molec-ular acceptor TCAQ, which lowers the hole injectionbarrier into 6T layers on Au and Ag . Other exam-ples of strongly coupled electron accepting moleculeson metal surfaces include PEN , DIP ,PTCDA , PTCDI ,TAT , FAQ , HATCN , Pcs ,and TCNQ . Most of the above mentionedCOMs can serve different purposes in addition to beinga suitable ELA modifier . For example,F TCNQ is also a popular molecular dopant .Furthermore, work function modification is not restrictedto metal surfaces and electron donating COMscan also lower effective work functions bythe reversed process as electron accepting molecules,i.e., by an electron transfer from the adsorbate to thesubstrate. Strongly interacting organic-metal systemshave thus a significant relevance for applications.1
D. Intermediate cases
Between the two extreme scenarios discussed above,there exist plenty of systems whose phenomenology canneither be described by only considering “chemical” in-teractions nor vdW attraction alone. Most interestingly,these cases may tend towards one or the other side de-pending on the particular characteristics of the system.The following consists mostly of prototypical systems ofthe kind “molecule A on substrate B”, which are takenas a reference to discuss the interfacial changes that oc-cur when A or B are (slightly) modified. In addition,particular cases that exemplify the possibilities of sur-face/interface tuning are also outlined. We note thatsome systems may fit in two or more subsections.
1. Fluorination
Among the many options to functionalize a COMthrough specific chemical modifications , fluorination,i.e. the substitution of peripheral hydrogen atoms by fluo-rine, is one of the most widely employed. In the gas phaseand thin films, it increases the resistance to oxidation,changes the electron affinity, modifies the intrinsic molec-ular dipole as well as the optical properties . At theinterface, fluorination modifies the ELA and the natureof the substrate-molecule and molecule-molecule interac-tions. One of the most illustrative examples when dis-cussing the effects of fluorination is the case of PEN andits perfluorinated derivative PFP adsorbed on coppersurfaces . Even on the moderately reac-tive (111) noble metal surfaces, PEN molecules can ex-perience strong interactions.The hybridization of the molecular states with the sur-face atoms renders a completely filled LUMOon Cu(111) well below the Fermi level and a remark-ably short adsorption distance (Figure 17) . In con-trast, PFP shows no LUMO filling, no sign of hybridiza-tion is seen in XPS and the average adsorption dis-tance of carbon is ∼ ∼ . DFT calculations with vdW-corrections yielded adsorption geometries with average adsorptiondistances in perfect agreement with experiments and gavea more precise description of the actual arrangement:PEN would adsorb forming a small canoe-like shape withthe short molecular edges slightly above the average car-bon distance, whereas PFP would adsorb in a strong V-shape with the central carbon atoms, being the most re-active in acenes, very close to the surface and the shortedges ∼ PEN , with fluorine atoms only at the short molecular edges, adsorbed on Cu(111)revealed that the selective fluorination of PEN only yieldsa local conformational change. Despite the increased ad-sorption distance of the fluorine and carbon atoms nearby(see Figure 17) the structural, electronic and chemicalproperties of the PEN backbone remain unaffected be-cause the strong interaction of the core with the copperatoms prevails .On the less interacting silver surface, PEN has beenshown to have different growth phases that depend on thetemperature as well as on the coverage . Such be-havior has been defined as “soft chemisorption” sincefrom TPD a remarkable thermal stability is found andNEXAFS shows a significant modification of the PEN or-bitals in the monolayer , but there is no trace of LUMOfilling and the molecules form a disordered liquid-likephase at RT. Indeed, different studies have reported thedisorder present in the first PEN layer in contact withAg(111) with the interesting and controver-sial fact that an ordered second layer may grow ontop . Upon cooling, ordered areas are found inSTM but no clear diffraction pattern is observablein LEED . Notably, cooling as well as increasing thecoverage modify the adsorption distances, with the re-markable displacement of +0 .
14 ˚A in adsorption distanceupon coverage increase from 0.5 to 0.75 ML at RT as aconsequence of the shifting balance between intermolec-ular and substrate-molecule interactions (see Sec. II D).In this situation, fluorination of PEN has a similar effectas on copper, namely, the molecule-substrate interactiondecreases. As reported by G¨otzen et al. the TPD spec-trum of PFP, compared to that of PEN, does not showa monolayer feature, which indicates that the bondingstrength for the latter is higher. This appears in linewith the increased adsorption distance of PFP comparedto PEN for a similar coverage (2.98 ˚A vs. 3.16 ˚A )and the absence of CT to the LUMO . Similar toPEN on Ag(111), temperature, coverage and even thepreparation method seem to impact the supramoleculararrangement of PFP: monolayers prepared via desorptionof a multilayer appear as ordered patches that leave sub-strate regions uncovered at T <
130 K, then becomedisordered and homogeneously distributed all over thesurface at
T >
160 K. On the contrary, (sub)monolayersprepared via direct evaporation of the desired coverageadopt ordered arrangements at LT (dislocation network)and RT (Moir´e pattern) .On gold, both PEN and PFP, show a clear physi-sorptive behavior with no evidence of LUMO filling, norhybridization of the molecular orbitals . Quiteinterestingly, despite the a priori higher ionization en-ergy of PFP, an almost identical HIB was measured forboth molecules on gold, which comes along with a larger(by 0.45 eV) VL shift for PEN . The authors arguedthat the weaker pushback effect and the unexpected ELAshould be explained by a much larger adsorption distanceof PFP compared to PEN . Recently, direct XSW mea-surements have confirmed this . Another interesting2
FIG. 17. Adsorption geometry of PEN, F PEN and PFP on Cu(111) that combines experimental data obtained with XSW (inbold) and state-of-the-art DFT calculations with vdW corrections (in italics). The calculations are obtained from Ref. 481, themeasured adsorption distances for PEN and PFP from Ref. 74 and F PEN from Ref. 170. Note that only values for differentcarbon positions are included. In addition, the average adsorption distance of the fluorine atoms in F PEN is 3.40 ˚A and inPFP 3.08 ˚A . Figure adapted from Ref. 170 with permission. finding, which indicates a considerable interaction evenwithin the physisorptive regime, was reported by Lo etal. : Using STM it was found that PEN may change thesurface reconstruction of Au(111) and thereby suggestinga stronger interaction with the substrate than PFP. Yet,the PEN molecules appear to be mobile while PFP formsassemblies that are stabilized by intermolecular interac-tions .For the sake of completeness, we shall mention thatthe influence of fluorination on the metal-organic inter-face has been studied also for phthalocyanines ,rubrene and thiophenes . Of course not only (111)surfaces, but also several others have been studied, e.g.,PFP on Ag(110) or F PEN on Au(100) .As concluding remark, it is worth noting that withinthe monolayer regime the combination of fluorinated andnon-fluorinated compounds has been shown to be an ef-fective way to tune the work function of a metal sub-strate and, in the thin-film regime, the ionization en-ergy as well . In both cases, this method pro-vides a suitable pathway to systematically modify theinterface properties and adapt them to the particular de-vice requirements.
2. Core substitutions of phthalocyanines
All COMs discussed so far are intrinsically non-polarand therefore do not offer the possibility to tune theELA by a permanent molecular dipole moment, whosemagnitude and orientation may influence the vacuumlevel and thus the ELA . Animportant class of polar COMs are particular por-phyrins and phthalocyanines, as they offer numerouspossibilities of functionalization through substitutionof the central metal atom and insertion of furtherheteroatoms . Sinceonly a few adsorption distances have been measured forporphyrins by XSW , we focus on Pcs. We shall dis-cuss SnPc as example for a Pc for whichthe central atom is simply too big to fit into the aro-matic macrocycle and which is thus polar in the gasphase. Thus, it can adsorb in two different flat-lying ge-
FIG. 18. Side views of a molecule in the Sn-up (upper partof the figure) and Sn-down position (lower part) on Ag(111).For a coverage of 0.8 monolayers (left side) Sn-up and Sn-down oriented molecules are present on the surface and thevertical bonding distances (as measured by XSW) are given.For a monolayer coverage (right side) only Sn-down orientedmolecules are present on the surface. Taken from Ref. 76 withpermission. ometries (Figure 18), i.e. with the Sn atom either below(Sn-down) or above the molecular plane (Sn-up). Fora submonolayer coverage on Ag(111) both orientationswere found and the adsorption distances have been mea-sured by XSW . For a full monolayer coveragesubstrate mediated intermolecular interactions lead to areorientation of the Sn-up molecules and only Sn-downcan be found on the surface . In this case the tin atomplays a crucial role in the coupling with the substrateleading to pronounced charge rearrangements, which arenegligible for the Sn-up orientation . Overall, forsuch systems the orientation has a significant impact onthe intermolecular interaction. However, the moleculardipole moments are rather weak and hence the impacton the vacuum level marginal.For “umbrella-shaped” Pc molecules with an addi-tional heteroatom attached to the central metal ion thesituation is different. For example, the dipole momentof ClAlPc in the gas phase is 1.87 D and, according3 FIG. 19. Energy-level diagrams for monolayers (ML) of (a) ClAlPc/Au(111), (b) CuPc/Au(111), and (c) ClAlPc/HOPG. Foreach system, UPS measurements of the as-grown (AG) film at room temperature (RT) and of the annealed (AN) film at RTand low temperature (LT) are shown. Upon annealing and cooling, vacuum level (VL) shifts and changes of HOMO states areobserved. Here, ⊥ corresponds to the Cl-up and the reversed symbol to the Cl-down orientation. Taken from Ref. 506 withpermission. to equation (5), the collective impact of these dipoleson the vacuum level (∆VL mol ) for an aligned monolayershould yield a ∆VL value of several hundred meV. For as-deposited ClAlPc on Au(111) a mixed Cl-up/down ori-entation has been observed and the resulting ∆VL = − .
45 eV (Figure 19a) has been mainly ascribed to thepush-back effect . Strikingly, aligning the moleculesto a Cl-up orientation by annealing leads to a furtherVL decrease (∆VL = − .
89 eV) and the total ∆VL islarger than that of planar CuPc on the same substrate(Figure 19b). Apparently, the permanent dipole momentof ClAlPc is decreasing the vacuum level – whereas infact a Cl-up orientation should lead to an VL increase(by +0 .
47 eV), which was indeed observed for ClAlPc onHOPG (Figure 19c). On inert HOPG the dipole mo-ment of the adsorbate is thus not changed in the con-tact layer to the substrate. On metal substrates, how-ever, adsorption induced bond-length changes, which canlead to a partial depolarization of the COM on the sur-face, are frequently observed . In addition, inter-facial charge rearrangements due to strong interactionscan further impact the vacuum level, which has beensuggested to be the reason for the unexpected ELA ofClAlPc on Au(111) . A similar behavior has beenobserved on Ag(111): Also on this substrate annealingchanges a mixed orientation of a ClAlPc monolayer to apredominant Cl-up arrangement and concomitantly de-creases VL . Interestingly, for very low deposition ratesof ClAlPc on Ag(111) ( ∼ d ambiguity of the XSWtechnique (Eq. (2)) can hinder a straightforward assign-ment of adsorption distances and even the orientation (X-down or X-up). For example, the DFT-modeled adsorp- tion distances of GaClPc on Cu(111) in the Cl-up andCl-down orientation do not match the experimentalvalues . It turns out that in the most likely adsorptiongeometry the Cl atom is dissociated . Moreover, asmentioned above, the “up” and “down” orientation canalso coexist, as also observed for VOPc on Cu(111) .In these cases, having a very well characterized systemmay help to address this issue .Another challenge for XSW measurements are theabove mentioned pronounced distortions of the π -systemleading to significantly different adsorption distanceof the carbon atoms. For example, for ClAlPc onCu(111) in the Cl-down orientation the DFT-modeledadsorption distances of individual carbon atoms differ byup to 1.11 ˚A (Figure 20). The strong distortion is a conse-quence of a charge transfer from the Cu(111) into ClAlPc,which is mainly localized on two of the four ClAlPc lobesas shown by STM . This involves a symmetry reduc-tion of ClAlPc from 4-fold in the gas phase [and the Cl-uporientation on Cu(111)] to 2-fold in the Cl-down orienta-tion on Cu(111). Similar symmetry reductions have beenobserved also for other Pcs on different substrates, e.g.,for CuPc on Cu(111) and on Ag(100) , for FePc onCu(111) and for PtPc as well as PdPc on Ag(111) .In general, both orientations (Cl-up or Cl-down) havebeen observed for vacuum-sublimed ClAlPc on the (111)-surfaces of noble metals . Moreover, the ori-entation can be changed by, e.g., the deposition rate ,post-deposition annealing or by pulsing using an STMtip and can thus act as molecular switches .4 FIG. 20. DFT-based adsorption geometry of ClAlPc onCu(111). Taken from Ref. 524 with permission.
3. Functional groups
Obviously, also COMs other than phthalocyanines canbe functionalized. We already discussed the impact ofoxygen substitution on the coupling of pentacene withmetal substrates (Figures 5 and 6). In that case, theoxygen atoms break the conjugation of the pentacenebackbone and the impact on the gas phase properties(namely, it increases IE and EA) as well as on the cou-pling with metal substrates is severe . Also nitrogensubstitution is frequently used to increase the EAs ofCOMs . One example is the nitrogen-substituted ter-rylene analogue TAT . However, in this case,the nitrogen atoms are a central part of the TAT π -system and, although a nitrogen-specific interaction inTAT monolayers on Ag(111) takes place, the vertical ad-sorption distances are not substantially affected . Wealso briefly discussed the impact of functionalization ofperylene (Figure 7). Intriguingly, already the substitu-tion with indeno-groups, i.e., the change from perylene toDIP, changes the adsorption behavior significantly .In contrast, the functionalization of perylene with oxygen(PTCDA) or with oxygen and nitrogen (PTCDI) does not change the averaged adsorption distances of the car-bon atoms on the (111)-surfaces of noble metals (Fig-ure 7) . However, the adsorption distances ofthe atoms in the functional groups differ notably andthe ELA is considerably different: The energy-levels ofPTCDA are Fermi level pinned on the (111)-surfaces ofnoble metals and virtually all substrates , whereas forPTCDI the ELA is vacuum-level controlled . The cou-pling of PTCDA (and to minor extent also of PTCDI)to metals has been subject to extensive research (seeRefs. 18, 75, 80, 148, 150, 408, 431, 434, 536–544 as wellas the review papers 83, 85, and 433).In the following, we will focus on molecular functional-izations that change the orientation of the COM on metalsurfaces. It is well known that the organic-inorganic ELAdepends on the orientation of the COMs in the molecu-lar thin film . For all cases discussed so far,the molecules have a lying-down orientation in the con-tact layer to the metal, as such a face-on orientationmaximizes the wave function overlap between adsorbate and substrate. Monolayers of tilted or standing COMson metal surfaces are rather exceptional and in mostcases the result of a transition from flat-lying in a looselypacked monolayer to a standing or tilted orientation ina closely packed monolayer . In some otherrare cases the molecular surface unit cell includes twomolecules with one of them lying flat and the other onebeing tilted . On other surfaces, e.g., on metal ox-ides, standing orientations of (polar) COMs in monolay-ers are successfully used for ELA engineering .The question arises how this can be achieved for organic-metal interfaces, i.e., what are the driving forces for amolecular semiconductor to adopt a tilted or standingorientation on a clean metal surface?One of the first experimental demonstration of a COMwith a standing orientation on a clean metal surface hasbeen made for the electron accepting COM HATCN onAg(111) . In a combined UPS, TPD, RAIRS, DFTand Kelvin probe study Br¨oker et al. showed that, upto a threshold coverage, HATCN adopts a lying-downorientation on Ag(111). Increasing the coverage leadsto an orientational transition to standing molecules, i.e.,HATCN forms a transient monolayer on Ag(111). Inthe standing monolayer ∆VL is almost 1 eV and thusconsiderably larger than for monolayers of lying-downelectron accepting molecules on the same surface suchas F TCNQ , PTCDA or FAQ . For HATCNspecific interactions of the peripheral molecular cyanogroups with the metal are believed to be one of the driv-ing forces for an orientational transition, since for theedge-on conformation the CT becomes more localized onthe C ≡ N docking groups. In contrast, for the face-onconformation the whole molecule is involved in the in-teraction with the substrate, including the σ -electrons ofthe C ≡ N groups as well as the entire π -system .While HATCN has been an early example showing alarge ∆VL for a monolayer of edge-on COMs, a morerecent example is dinitropyrene-tetraone (NO -PyT) onAg(111), which also exhibits a transient monolayer struc-ture and where ∆VL for the edge-on orientation evenamounts to ∼ . The unsubstitutedparent molecule pyrene-tetraone (PyT), which is flat-lying for all coverages, only increases the work functionof Ag(111) by ∼ -PyT and PyT onAg(111) as measured by UPS is nearly identical: Theformer LUMO is partially filled due to a CT from thesubstrate and the work function increases by ∼ -PyT . This wasattributed to the bulky NO groups “pushing away” thecarbon skeleton from the substrate. In fact, the oxy-gen atoms in these groups have adsorption distances of2.75 ˚A. The valence electron structure and the measuredadsorption distances could be reproduced quite well by5 FIG. 21. (a) Experimentally determined vacuum level changes (∆VL) upon stepwise deposition of pyrene-tetraone (PyT) andits derivatives on Ag(111). (b) Calculated cumulative charge transfer. The averaged position of the carbon atoms (for lyingmolecules) and the topmost Ag plane are indicated by vertical dashed lines. (c) Adsorption induced charge rearrangementsderived by DFT calculations for the adsorption of an upright standing NO -PyT monolayer, averaged in the direction perpen-dicular to the paper plane. Br-PyT, which also shows a rather large increase of the substrate work function, is included for thesake of completeness. It could not be unambiguously shown whether the molecules adsorb intact or whether Br atoms detachduring the adsorption process. Adapted from Ref. 404 with permission. DFT modelling with vdW corrections. .While these calculations were confirming the ∆VLdata for PyT, the results for NO -PyT were at vari-ance. This can be attributed to an orientational tran-sition of NO -PyT to a standing monolayer. As XSWis intrinsically limited to lying (sub)monolayers, one hasto rely on DFT for the adsorption geometry. Indeed,DFT modelling of a standing monolayer of NO -PyT onAg(111) yields almost the same ∆VL as the measure-ments . Figure 21b compares the charge rearrange-ments upon adsorption. For PyT and lying NO -PyTthey are qualitatively similar and, moreover, they alsofit with the charge rearrangements of F TCNQ uponadsorption on the same substrate (Figure 16). In allcases, the minimum of charge density rearrangements(i.e., the maximum in electron density accumulation) canbe found between the metal surface and the molecular π -system. For standing NO -PyT the minimum is lo-cated at the NO docking groups (Figure 21c). More-over, the electron accumulation extends more than 10 ˚Aabove the surface and thus much further than for ly-ing NO -PyT. Notably, the averaged charge transfer permolecule is smaller for standing (0.32 e ) than for lying(0.71 e ) NO -PyT. The dipole moment, however, is in-creased due the larger charge separation, which causesin turn a pronounced work function increase. Finally,one finds that the electron affinity measured for standingNO -PyT molecules is significantly increased because ofelectrostatic effects.
4. Surface modification and decoupling
Another area, in which the connection between elec-tronic and geometric structure becomes evident, are efforts towards decoupling adsorbates from the metalsubstrate, which are often related to effects of chargetransfer or exciton lifetimes. In the spirit of pio-neering studies, such as the decoupling of Xe fromPd(001) by the adsorption of Kr monolayers , salt lay-ers may be used to decouple COMs from metal sur-faces . Oxidation of Cu(100) via O -dosingdecouples deposited PTCDA molecules from the surfaceand hinders organic-metal charge transfer: The averagedadsorption distance of the PTCDA carbon atoms on theoxygen-reconstructed ( √ × √ R ◦ Cu(100) surface is3.27 ˚A and thus much larger than that on pristineCu(100) (2.46 ˚A) . The PTCDA/Ag(111) model sys-tem has also been studied with respect to doping byK . The experimental and theoretical results pointtowards a reduced electronic coupling between the adsor-bate and the substrate, which goes hand in hand with anincreasing adsorption distance of the PTCDA moleculescaused by a bending of their carboxylic oxygen away fromthe substrate and towards the potassium atoms . Inprinciple, the organic-metal interaction strength can alsobe decreased by molecular functionalization with bulkyside-groups . However, only for one system adsorp-tion distances have been measured by XSW and func-tionalizing azobenzene by alkyl groups only increases theaveraged adsorption distance of the carbon atoms onAg(111) by 0.14 ˚A compared to the unsubstituted par-ent molecule . E. Chemical reactions at interfaces
Chemical reactions at surfaces involving COMmolecules, a very important topic in the context of catal-ysis and surface functionalization, have been addressed6in recent publications (see for instance Refs. 49, 51, 437,559–564). Therefore, we shall only highlight some casesthat involved a precise determination of the geometricstructure by XSW: i ) on-surface formation of porous sys-tems , ii ) self-metalation reactions of porphyrins , iii ) the dissociation reaction of azobenzene (AB) and iv ) surface-mediated trans -effects involving phthalo-cyanines.
1. On-surface formation of porous systems
The perylene derivative 4,9-diaminoperylene-quinone-3,10-diimine (DPDI) has been shown to dehydrogenateand become 3deh-DPDI after annealing of a submono-layer adsorbed on Cu(111) . After loosing the hydro-gen atoms, the two nitrogen atoms coordinate to copperadatoms and form a highly ordered nanoporous network(Figure 22a). Matena and coworkers studied the chemicaland structural changes induced by the formation of thenetwork . Initially, the core-level signature of nitrogenis composed of two peaks separated by 1.8 eV that belongto the amine (NH ) and imide (NH) groups, the latterbeing ∼ ∼ ∼ groups (Figure 22b). This was interpretedin terms of the interplay between molecule-substrate vs.intermolecular interactions, which is clearly balanced to-wards the latter upon network formation . More pre-cisely, the obtained adsorption distances with respect tothe surface correspond to a physisorptive scenario, imply-ing that the molecule is decoupled from the surface andthe bonding occurs only through the copper adatoms.Interestingly, DFT calculations of the network with andwithout the surface indicate that a planar geometry, withthe copper adatoms at the same plane as the molecules,is disrupted by the presence of the surface that pulls thecopper adatoms closer and thus bend the molecule. Con-sequently, the adatoms mediate the intermolecular inter-actions acting as coordination centers but also influencethe bonding between the molecules and the substrate .
2. Self-metalation reactions of porphyrins
As introduced in Sec. IV D 2, porphyrins as well as ph-thalocyanines can host a metal ion within the molecu-lar core (Figure 2). For metal-free molecules, these canalso be incorporated through various metalation reac-tions , whereby a H -molecule is released and FIG. 22. On-surface formation of a 2D porous network andthe related chemical and structural molecular changes. (a)Schematics of the reaction leading to the network forma-tion. The annealing at 200 ◦ C of a submonolayer coverageof DPDI induces the complete de-hydrogenation of the amineand imine groups, which are stabilized by the mediation ofthe Cu adatoms thus acting as the coordination centers forthe network formation. Figure adapted from Ref. 385 withpermission. the ion becomes coordinated to the central nitrogenatoms. Similar to what has been discussed in the pre-vious paragraph, the metalation reaction can be followedby monitoring the change in the N 1s core-level signal (seeFigure 23a), i.e. from two clearly distinguishable aminic(or pyrrolic, –NH–) and iminic (–N=) nitrogen speciesfor the free-base molecule towards one single species forthe equally-coordinated nitrogen atoms .A particular case of metalation is realized throughthe direct incorporation of surface atoms, the so-called self-metalation reaction . In this con-text, the thermally induced self-metalation of 2 H -tetraphenylporphyrin (2HTPP, Figure 2j) adsorbed onCu(111) and the subsequent formation of copper(II)-tetraphenylporphyrin (CuTPP) is one of the most thor-oughly studied reaction . For instance,in a temperature-dependent STM and XPS study ofthis reaction it was found that along with the self-metalation (starting at 400 K) the molecule undergoesa gradual hydrogen loss until a total de-hydrogenationoccurs at 500 K. As imaged with STM (Figure 23a),2HTPP molecules appear rather planar, with the phenylgroups parallel to the surface but the increasing lossof hydrogen reduces the steric repulsion between phenylrings and enables their rotation . Interestingly, the fullde-hydrogenation again renders a flat molecule. Conse-quently, the adsorption geometry has possible contribu-tions from the metalation, which relaxes the strong in-teraction of the nitrogen atoms with the substrate, andalso from the rotation of the functional groups. In orderto study the influence of the metalation on the verticaladsorption distance, B¨urker et al. followed the changesin the adsorption upon self-metalation at 500 K to avoidstrong contributions of the rotating phenyl groups to theconformational properties. Thus, for the free-base por-phyrin the two inequivalent nitrogen atoms have two dis-tinct adsorption distances (see Figure 23b), i.e. the iminic7 FIG. 23. (a) Evolution of the nitrogen N 1s core-level signal of2HTPP as a function of the temperature and the correspond-ing STM images. Image taken from Ref. 569 with permis-sion. (b) Adsorption distances of the average carbon atomsand the nitrogen species (aminic in dark and iminic in lightblue) before and after annealing at 500 K. Image adapted fromRef. 389 with permission. ones closer to the surface as a result of the stronger in-teraction with the copper atoms . Because both nitro-gen species are located below the average carbon adsorp-tion distance, the macrocycle takes a saddle-like shapeon the surface. Upon metalation at 500 K, the incor-poration of the copper atoms lifts this distortion, sincethe preferential interaction of the iminic nitrogens withthe substrate is switched off (Figure 23b). Interestingly,the average carbon adsorption distance remains virtuallyunchanged, although the vertical order is increased (asdeduced from the higher coherent fractions). The au-thors therefore conclude that the metalation of 2HTPPrelaxes the molecular core without impacting the overalladsorption distance of the molecule and rather possible
FIG. 24. XSW measurements performed for FePc adsorbedon Ag(111) in the case of no-, H O- and NH -dosing. The trans -effect increases in the order H O < NH < Ag(111). (a)To-scale schematics of the XSW data (note that the graphicrepresentation of the n ambiguity of the d hkl = d ( n + P hkl ).(b) Detail of the change of the Fe adsorption distance forthe different cases, which clearly describes the structural im-plications of the surface trans -effect. Figure reprinted fromRef. 397 with permission. rotations and/or bending of the phenyl groups determinethe overall adsorption geometry . Notably, recent DFTcalculations of 2HTPP adsorbed on Cu(111) have shownthat an inverted macrocycle reproduces the experimentaldata better than a saddle-shape geometry .
3. The dissociation reaction of azobenzene
As prototypical molecular switches azobenzene (AB)and its derivative tetrabutyl-AB (TBA) have been stud-ied with XSW on Cu(111) and on Ag(111) .Willenbockel et al. reported a coverage-dependent dis-sociation of AB on Cu(111), which itself is not ob-served on the Ag(111) surface . The authors at-tribute the difference to the balance between molecule-molecule and substrate-molecule interactions. More pre-cisely, the stronger bond between the nitrogen atoms ofthe (–N=N–) azo-bridge and the copper substrate forcesAB to decompose into two phenyl nitrene molecules toaccommodate the increasing molecular packing. In con-trast, the N–Ag bond is weaker and allows the moleculeto deform upon coverage increase. Through a sophisti-cated vector analysis of the XSW data the authorsobtained tilt and rotational angles for the phenyl ringson both substrates. The derived rotation of those groupson silver is larger than on copper, which is considered asindication for the increased flexibility of the N–Ag bond.Interestingly, in another study it was found that theisomerization reaction of AB, which is essential for theswitching mechanism, can be quickly reversed by CTfrom the substrate to the molecule, thus preventing theswitching effect to be measured . This would explain8why the switching is observed on Au(111) but not onAg(111) .
4. Surface-mediated trans-effects of MePc
In inorganic chemistry, it is known that adding a newligand to a metal ion influences the bond between theion and the other previously present ligands. One candistinguish two cases: the ligands are opposed (trans)to each other or the ligands are at the same side (cis).The new coordination affects the ground-state properties,the length as well as the thermodynamic and vibrationalproperties of the other bonds. In an analogous situa-tion, it was seen that one can reproduce the trans -effectwith metal-ion-containing molecules adsorbed on a metalsurface, where the substrate acts as one of the ligands.This is known as surface trans -effect. For the particu-lar case of of metal phthalocyanines (MePc) adsorbed onAg(111), a study with complementary PES, STM andDFT determined that Co and Fe ions are forming bondswith the substrate, but not Zn . Interestingly, upondosing of nitric oxide (NO), the changes in the electroniccharacteristics indicate that the ion-to-substrate bond isweakened or entirely suppressed and DFT calculationsshow that the Ag–Me bond length is increased .In this context, it was recently found that the coordina-tion of ligands with different reactive character to the Feion of FePc adsorbed on Ag(111) indeed changes the Ag-to-Fe adsorption distance . More precisely, H O andammonia (NH ) were dosed, which renders an increasing trans -effect in the order H O < NH < Ag(111), thus oneexpects that the Fe atom should show a larger adsorptiondistance when the Ag(111) surface is trans to the ammo-nia than to water. As shown in Figure 24, the XSWresults confirmed this scheme as the Fe atom is rathershifted away from the surface when NH is dosed com-pared to H O. This behavior was also reproduced withvdW-corrected DFT.
F. Heterostructures
Organic heterostructures in the monolayer or bilayerregime on clean metals may be consideredas model systems for organic-organic interfaces, i.e. theessential component for most electronic devices appli-cations. Most studies in the area are dealing with bi-molecular mixed layers , whereas only a fewstudies focus on bilayers . For both typesof heterostructures CuPc and PTCDA on Ag(111) havebecome popular . Figure 25ashows the experimentally determined adsorption geome-tries of PTCDA and CuPc in their respective monolayerson Ag(111) and in the bimolecular mixed layer . Strik-ingly, PTCDA is lifted up in the bimolecular system,whereas CuPc is pushed down. Naively, this could leadto the notion that the coupling of PTCDA with Ag(111) FIG. 25. Bimolecular mixed layer of PTCDA and CuPc onAg(111). (a) Vertical adsorption geometry as revealed byXSW. The adsorption heights of all involved atomic speciesare illustrated for the bimolecular monolayer (colored spheres)and the homomolecular monolayer (grey spheres). (b) Chargedensity difference plot showing depletion (blue) and accumu-lation (red) of electronic charge in a plane parallel to the sur-face (in a height of maximum DOS of the LUMO orbitals).(c) Projected DOS of the π -orbitals of PTCDA and CuPc inthe homomolecular PTCDA/Ag(111) (left), the homomolecu-lar CuPc/Ag(111) (middle) and the bimolecular layer (right).Energies are aligned with the vacuum level, the Fermi ener-gies are indicated by black lines revealing the work functions.Taken from Ref. 71 with permission. decreases and that of CuPc with Ag(111) increases. How-ever, the situation is more complex and Stadtm¨uller etal. showed by means of STM, STS, orbital tomographyand DFT modelling that the adsorption height changesare driven by intermolecular interactions, which are in-creased by the equalization of adsorption heights. Asillustrated in Figure 25b, which highlights the charge re-arrangement between PTCDA and CuPc, the acceptorcharacter of PTCDA and the donor character of CuPcare increased in the bimolecular system. Consequently,the LUMOs of PTCDA and CuPc move away from thecommon Fermi level in opposite directions (Figure 25c).Overall, this example shows how observables such as ver-tical adsorption distances, frontier orbital binding ener-gies and charge transfer are linked and influence eachother.For bilayer systems, the most fundamental question iswhether the deposition sequence reflects the actual ar-rangement in the heterostructure. At room temperature,this is the case for CuPc deposited on a closed mono-layer of PTCDA on Ag(111) . However, for the reversedeposition sequence, i.e., PTCDA on CuPc, molecularexchange occurs and PTCDA replaces CuPc in the con-tact layer to Ag(111) . One could expect that this is9 FIG. 26. (a) Bilayer formation (top) vs. molecular exchange(bottom). In both cases, CuPc (blue) has been vacuum-sublimed on a closed monolayer of P4O (red) or P2O (green)on Ag(111). (b) Vacuum level shift (∆VL) between cleanAg(111) and a monolayer of the respective COM. Binding-energy shift between monolayer and multilayer of the HOMO-maximum (∆HOMO) and the C 1s peak of the molecularbackbone (∆C π ) and the functional group (∆C funct ) of therespective COM on Ag(111). Averaged bonding distance (d H )of the carbon atoms in the molecular core in sub-monolayerson Ag(111). Taken from Ref. 118 with permission. related to the different interaction strength of the adsor-bates with the substrate, which is weaker for CuPc thanfor PTCDA. This assumption has been tested by usingP2O and P4O monolayers on Ag(111), which have beenintroduced as reference systems for physisorption andchemisorption (Figures 5 and 6). Indeed, subsequentlydeposited CuPc molecules can replace P2O in the con-tact layer to Ag(111), while they remain on top of P4Oon Ag(111) (Figure 26a) . The different behavior ofthe CuPc/PxO/Ag(111) bilayer systems allows, thus, toconclude that the interaction of CuPc with Ag(111) is be-yond physisorption (although still relatively “weak” ).As mentioned in Sec. II A, rigid shifts of valence andcore-levels observed for monolayer and multilayer cover-age can serve as indicator for organic-metal interactionstrength. These shifts are shown in Figure 26b for P2O,P4O, CuPc and PTCDA on Ag(111). With the excep-tion of the shift between HOMO position in the mono-layer and the multilayer (∆HOMO) all indicators showthat the interaction strength with Ag(111) increases inthe order P2O–CuPc–PTCDA–P4O. The largest shiftshave been found for the core-levels of carbon atoms in functional groups (∆C funct ), which might be the bestindicator for the interaction strength. Notably, all thedata are taken from measurements of monomolecular sys-tems , but still allow to predict the se-quential arrangement in heterostructures. However, weare aware that also other factors such as the particularmolecular weight or shape also impact possible molecularexchange processes .Figure 26b also includes vacuum level shifts betweenthe clean Ag(111) surface and the respective mono-layer and the vertical adsorption heights. As discussedthroughout this review, several often competing factorsimpact dipoles at organic-metal interfaces. The “correct”order of the ∆VLs might thus be merely coincidental.Vertical adsorption distances are a better indicator (fora detailed discussion see Sec. V). However, they havethe disadvantage of requiring measurements at highlyspecialized beamlines at synchrotron radiation facilities,whereas the photoelectron spectroscopy based indicatorscan be accessed with standard lab equipment. V. SUMMARY AND CONCLUSIONS
As discussed in this review, the contact formation ofspecific adsorbate-substrate systems is by now reasonablywell understood. At the same time, numerous studiesaddressing the relation of structural and electronic prop-erties at organic-metal interfaces havedemonstrated that there are actually no “simple rules”and that a prediction of the energy-level alignment re-quires significant computational efforts.Nevertheless, we can identify a few general trends thatconnect the adsorption geometry and the energy-levelalignment. For example, a clear correlation was found forthe shift of the Shockley surface state ∆ E IS on clean met-als due to adsorption of a molecular monolayer ( cf. Fig-ure 27). Apparently, this shift is related to the organic-metal coupling strength and can be explained using arelatively simple one-dimensional model potential . Acloser inspection of Figure 27, however, reveals that mostof the data points refer to Ag(111) surfaces and that thetwo outliers on the left of the calculated model curve cor-respond to energy shifts on Cu(111) surfaces. This indi-cates that the situation is more complicated and that insome cases effects beyond LUMO filling play a role forthe surface state shift.Elaborating on this issue, Figure 28 shows adsorptiondistances d H of carbon atoms in an aromatic environmentof seven COMs, for which they have been determinedon all three (111)-surfaces of the noble metals. Whilethe plot is certainly simplistic (possible coverage and/ortemperature effects are neglected) and the selection ofmolecules is to some degree arbitrary, it highlights someimportant findings for organic-metal interfaces. Obvi-ously, for all COMs the adsorption distances decrease inthe order Au–Ag–Cu (see also Sec. II B). Moreover, onAu(111) and Cu(111) the bonding distances exhibit a0 FIG. 27. Energy shift ∆ E IS of the interface state with re-spect to the Shockley surface state on the pristine metal as afunction of the carbon-metal distance d C . The solid red lineshows the calculated results for a carbon layer on Ag(111).Taken from Ref. 587 with permission. rather narrow distribution of only ∼ ∼ FIG. 28. Compilation of vertical adsorption distances d H for carbon atoms within the molecular backbone of selectedCOMs. The plot includes those systems for which XSWresults on all (111)-surfaces of the noble metals Au, Ag,and Cu are available. The data shown here are taken fromRefs. 17, 18, 77, 78, 80, 148, 150, 151, 171, 179, 386, and 396. shifts (Figures 11 and 26) on this surface. We notethat having virtually the same vertical adsorption dis-tances, does not imply that the energy-level alignmentis identical: PTCDA is Fermi-level pinned on all thethree surfaces (and virtually all other substrates ),whereas the ELA of PTCDI is vacuum level controlled onthe (111)-surfaces of the noble metals . The influenceof site-specific interactions is even more pronounced forP4O on these surfaces (black symbol in Figure 28), show-ing adsorption distances which differ by more than 1 ˚Adue to the re-hybridization of P4O on Ag(111) (Figure 5)and Cu(111). In fact, the vertical adsorption distancesof PEN and P4O on Cu(111) are rather similar (2.34 ˚Aand 2.25 ˚A, respectively) . While P4O is Fermi-levelpinned, the ELA of P2O and PEN are vacuum level con-trolled on these three surfaces .A common approach to reduce the organic-metalinteraction strength is (per)fluorination of the adsor-bate . Comparing CuPc and F CuPc (green sym-bols in Figure 26) shows that on Ag(111) this methodis indeed working and the repulsive interaction of thefluorine atoms prevent coupling beyond physisorption –as observed for CuPc/Ag(111). On Cu(111), however,where the adsorption distances of both phthalocyaninemolecules are rather similar, the attractive interactionbetween the substrate and the Pc core is already toostrong and the fluorine atoms cannot “repel” the entiremolecule. Consequently, for F CuPc/Cu(111) (andother perfluorinated Pcs such as F ZnPc/Cu(111) ) asignificant molecular distortion with the fluorine atomsabove the carbon backbone is found.Overall, these results demonstrate that the interplaybetween adsorption geometry and electronic structure iscomplex and measuring the element-specific adsorptiondistances of π -conjugated molecules on metals is essen-tial for understanding the interface dipoles and thus the1energy-level alignment. Because of the different drivingforces for charge rearrangements upon contact forma-tion, such as push-back effect or chemical-bond forma-tion, the bonding behavior cannot be characterized usingfew parameters like the metal work function, the ioniza-tion energy and electron affinity of the organic thin film.While XSW has become a well-established high-precisiontechnique in the field of metal-organic interfaces, it hasnot yet been used extensively to study all relevant sys-tems. However, we are confident that the results reviewedhere and, most importantly, new state-of-the-art facili-ties, such as beamline I09 at the Diamond Light Sourcewill encourage further systematic studies of such a vividand interesting area of surface science. ACKNOWLEDGMENTS
The authors thank the European Synchrotron Radi-ation Facility (ESRF) and the Diamond Light Source(DLS) for making their facilities available to us. Wethank the different local contacts and beamline scientistthat helped us during the numerous XSW experimentsat ID32 (ESRF, until 2011) and I09 (DLS, since 2013),in particular J¨org Zegenhagen and Tien-Lin Lee.It is a pleasure to acknowledge interactions with a largenumber of colleagues in the field, including in alphabeti-cal order J. Banerjee, C. B¨urker, B. Detlefs, D. A. Dun-can, G. Heimel, O. T. Hofmann, T. Hosokai, S. Kera,N. Koch, C. Kumpf, J. Niederhausen, I. Salzmann, A.Sch¨oll, M. Sokolowski, P. K. Thakur, F. S. Tautz, A.Tkatchenko, N. Ueno, A. Vollmer, Q. Wang, E. Zojerand others, too many to name them all.Financial support from the Deutsche Forschungsge-meinschaft (DFG), the Soochow University-Western Uni-versity Center for Synchrotron Radiation Research, the111 Project of the Chinese State Administration of For-eign Experts Affairs and the Collaborative InnovationCenter of Suzhou Nano Science & Technology (NANO-CIC) is gratefully acknowledged.2 ∗ [email protected] Ishii H, Sugiyama K, Ito E and Seki K 1999 Energylevel alignment and interfacial electronic structures at or-ganic/metal and organic/organic interfaces
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