Exploring covalently bonded diamondoid particles with valence photoelectron spectroscopy
Tobias Zimmermann, Robert Richter, Andre Knecht, Andrey A. Fokin, Tetyana V. Koso, Lesya V. Chernish, Pavel A. Gunchenko, Peter R. Schreiner, Thomas Möller, Torbjörn Rander
aa r X i v : . [ phy s i c s . a t m - c l u s ] A ug Exploring covalently bonded diamondoid particles with valence photoelectronspectroscopy
Tobias Zimmermann, a) Robert Richter, Andre Knecht, Andrey A. Fokin,
2, 3
Tetyana V. Koso, Lesya V.Chernish, Pavel A. Gunchenko, Peter R. Schreiner, Thomas M¨oller, and Torbj¨orn Rander Institut f¨ur Optik und Atomare Physik, Technische Universit¨at Berlin, EW 3-1, Hardenbergstr. 36, 10623 Berlin,Germany Department of Organic Chemistry, Kiev Polytechnic Institute, pr. Pobedy 37, 03056 Kiev,Ukraine Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, 35392 Giessen,Germany (Dated: 18 July 2018)
We investigated the valence electronic structure of diamondoid particles in the gas phase, utilizing valencephotoelectron spectroscopy. The samples were singly or doubly covalently bonded dimers or trimers of thelower diamondoids. Both the bond type and the combination of bonding partners are shown to affect theoverall electronic structure. For singly bonded particles, we observe a small impact of the bond on theelectronic structure, whereas for doubly bonded particles, the connecting bond determines the electronicstructure of the highest occupied orbitals. In the singly bonded particles a superposition of the bondingpartner orbitals determines the overall electronic structure. The experimental findings are supported bydensity functional theory computations at the M06-2X/cc-pVDZ level of theory.PACS numbers: 33.60.+q, 36.40.Cg, 73.22.-f, 79.60.Jv, 81.07.-b, 82.80.PvKeywords: diamondoids, electronic structure, ionization potential, nanodiamonds, quantum confinement,size-dependence, valence photoelectron spectroscopy
I. INTRODUCTION
The electronic structure of nanoparticles define theirelectrical, chemical and optical properties. Hence, inves-tigating these forms the basis for developing new applica-tions in nanotechnology. A comprehensive understandingof the various possibilities to modify the electronic struc-ture of a particle is required when aiming to tailor com-pounds for specific applications. Research in this areacan advance, for instance, the development of electronphotoemitters or nano-electronics. Diamondoids are perfectly size- and shape-selectable, hydrogen passivated, sp -hybridized carbonnanostructures. As such, they are an ideal class ofparticles for the study of effects induced by manipu-lation of geometry and chemical composition on theelectronic structure in nanoscale systems. In addition,functionalization of these particles presents anotherpossibility to tune their electronic structure.
Apartfrom the exploration of their existence in crude oil, continuous improvements in the field of synthesis ofdiamondoids have led to a rise in their popularityduring the last ten years. Diamondoids are of interestfor the oil industry and environmental protection, and are also used for medicinal applications, amongothers. Of particular interest in nanoparticles is thesize-dependence of their electronic structure, oftenreferred to in recent literature as quantum confine-ment (QC) effects. As they are perfectly size-selectable, a) [email protected] diamondoids enjoy great popularity in the investigationof such effects. Recently, Schreiner et al. synthesized diamondoidparticles with extraordinary long CC-bonds.
Ex-cept for a study on the smallest singly bonded par-ticle, their electronic structure have not been investi-gated. To our knowledge this is the case also for suchparticles connected with CC-double-bonds, for whichresults for only one compound have been reported inthe literature. The present work deals with the va-lence electronic structure of such singly and doublybonded diamondoid particles. The idea to combine lowerdiamondoids is comparable to a modular design prin-ciple. It is motivated by the question whether such acombination of lower diamondoids can imitate the elec-tronic properties of larger, pristine diamondoids. II. EXPERIMENTAL
We utilized photoelectron spectroscopy (PES) for allmeasurements in this work. A Scienta SES-2002 hemi-spherical electron analyzer was used. The samples in thespectrometer focus region were ionized at 21 .
22 eV with aHe-lamp (SPECS UVS 10/35). A resistively heated ovenwas used to bring the samples into the gas phase, seeTable I for an overview of the various samples and theexperimental parameters. While the ambient chamberpressure was in the mid 10 − mbar range, chamber pres-sures during measurements were always kept constant inthe mid 10 − mbar range. To calibrate the spectrome-ter energy scale, and to test the resolution, Xe gas (Air No. Sample VaporizationTemperature ( ◦ C) adamantane 25 diamantane 60 − triamantane 70 − − − − − adamantylidene-diamantane 70 − diamantylidene-diamantane 110 − (di-adamantylidene)-diamantane 140 − di-pentacycloundecane 80 − − mbar range. Liquide, 99.995% purity) was used. The experimentalresolution was 50 meV for all samples.
III. COMPUTATIONS
Computations using density functional theory(DFT) were performed for all substances to assistthe interpretation of the recorded spectra. We utilizedGaussian09 with the M06-2X functional togetherwith a cc-pVDZ (correlation-consistent) basis set, as thismethod was previously successfully used to describe theoptimized geometries of the singly bonded particles. Second derivatives were computed analytically to con-firm that all structures are minima (NIMAG = 0). Allorbital energies and isosurfaces were generated withthis method. The stick spectra were convolved with aGaussian function to take the vibronic broadening andthe spectrometer resolution into account. This methodshows good agreement with the measured photoelectronspectra for both the pristine diamondoids and thediamondoid dimers, without a rigorous but expensiveFranck-Condon analysis for each sample. The adiabaticionization potentials (IP) were computed by subtractingthe total energy of the particle ion from the total energyof the particle ground state. We also observe that thepristine diamondoid IPs are more accurately computedby the M06-2X functional than the B3LYP functional;the latter has been used extensively previously fordiamondoid systems.
IV. RESULTS
Measurements were performed on singly and doublybonded diamondoid particles, see Figure 1. Henceforth,we will refer to them by the notation in Table I. For the singly bonded particles we investigated homo-dimers,where both diamondoids are of the same type ( , ), and hetero-dimers, with bonding partners of differenttypes ( , ). For doubly bonded particles a hetero-dimer ( ), a homo-dimer ( ) and a hetero-trimer( ) were investigated. For reference purposes, acompound with a CC-double-bond ( ) and the first threepristine diamondoids ( - ) were also analyzed. The re-sults will be split in two sections according to the typeof bonding. A. Singly bonded particles
The valence photoelectron spectra of , , and are shown in Figure 2, together with the computedspectrum of each sample. Comparing the spectra ofthe dimers with the respective monomers reveals thatthe overall structures of the photoelectron bands showsimilarities in both cases, e.g., the number of individualbands, although the dimers have broader bands shiftedto lower binding energies and lack the distinct ionizationonset present in the monomers.All IPs presented in this work are adiabatic ionizationpotentials and were determined by the method describedby Lenzke et al. . For pristine diamondoids the changein IP with the diamondoid size is well known and hasbeen ascribed to QC effects. The diamondoid particlesshow a more complex geometry compared to the pristinediamondoids. However, the diamondoid particle IPs alsoshow a decrease with increasing size, as can be seen inFigure 2. Hence, diamondoid dimers seem to be sub-jected to QC effects as well.Figure 3 shows the computed highest occupied molec-ular orbitals (HOMOs) of the singly bonded diamondoidhomo-dimers together with their associated monomers.The orbitals are delocalized and symmetrically dis-tributed over the entire cage structures, i.e., there is nodistinction between individual CC-bonds in the system,for both monomers and dimers. Calculations show thatthis is also true for the deeper lying valence orbitals inthe case of equal bonding partners. The dimerization oftwo identical pristine diamondoids leads to an increase ofthe orbital volume in comparison to the situation of themonomer by a factor of roughly two. This resembles thesituation in pristine diamondoids where the HOMO vol-ume also increases with size. This volume increase leadsto a lowering of the IP, and can be ascribed to QC. Optimized geometries for the singly bonded dimer ionsshow a considerable elongation of the central bond ofup to 1 ˚A, indicating dissociation upon ionization. Thisexplains the flat and featureless ionization onsets of thehomo-dimers in comparison to the monomers.Studying the change of IPs for diamondoid hetero-dimers (Figure 4), we observe a distinct behavior fordimers containing different bonding partners. Both stud-ied hetero-dimers include triamantane ( ) as one bondingpartner. In combination with adamantane ( ) the dimer = = = = FIG. 1. A graphical overview of the samples investigated in this work, as denoted in Table I. The lower pristine diamondoids( - ) are followed by the singly bonded particles ( - ) and the doubly bonded particles ( - ). Another doublybonded compound is used for reference purposes ( ). PSfrag replacements 12 11 10 9 8Binding energy (eV) I n t e n s i t y ( a r b . un i t s ) hν = 21 .
22 eV
FIG. 2. The valence spectra of the two singly bonded homo-dimers (top) and their corresponding pristine monomers (bot-tom). The ionization potentials, denoted by black arrows inthe figure, change to lower binding energies for the diamon-doid particles towards the single molecules. ( ) IP is significantly lower than for dimer con-taining diamantane ( ). However in both cases the dimerIPs differ only slightly from the IP of pristine triaman-tane. The computed HOMOs for the two hetero-dimers(Figure 5) still show delocalization over the whole dimer,but with a tendency towards localization on the triaman-tane moiety. For , the HOMO-1 is nearly completelylocalized to the triamantane part of the dimer. In thecase of this localization is less pronounced and theorbital extends also to the smaller bonding partner side.PSfrag replacements HOMO HOMO-1
FIG. 3. The HOMOs (left) of the two singly bonded homo-dimers (bottom) and their corresponding pristine monomers(top) in comparison to their HOMO-1 (right). The M06-2Xfunctional together with a cc-pVDZ basis set was utilized forthe computations. An isovalue of 0 .
02 is used for all isosur-faces presented in this work.
A comparison of the IPs for the bonding partners in theirpristine form (Table II) reveals that the HOMO of dif-fers by about 0 .
75 eV from while the difference is only0 . and . This difference in bindingenergies of the monomer highest occupied orbitals leadsto a higher localization in the triamantane moiety than inthe adamantyl or diamantyl moieties of the dimers (Fig-ure 5). The confinement of the orbital is higher for PSfrag replacements 12 11 10 9 8Binding energy (eV) I n t e n s i t y ( a r b . un i t s ) hν = 21 .
22 eV
FIG. 4. The valence spectra of the two singly bonded hetero-dimers (top) and their corresponding pristine monomers (bot-tom). In contrast to the homo-dimers the ionization poten-tials, denoted by black arrows in the figure, do not strictlyshift to lower energies but depend on the bonding partnersinvolved. than for and therefore the IP of is lower. Thecomputations thus suggest that the hetero-dimer elec-tronic structures are a superposition of the monomer or-bitals involved. With exception of , the agreementbetween the measured data and computations is good.This indicates that in most cases an approach of com-bined orbitals is applicable to singly bonded diamondoiddimers, regardless of whether the bonding partners areof the same type or not.Table II summarizes the measured IPs together withthe computed IPs of the singly bonded particles pre-sented in this work. The agreement between the experi-mental and the computed values is almost within rangeof the experimental error for the pristine diamondoids.Even though for the dimers, the experimental and com-puted values differ up to roughly 1 eV, the computationsreflect the decrease of IP with increasing bonding part-ner size accurately. Further study using more elaboratecomputations would be needed to gain better absoluteagreement with experimental values.
B. Doubly bonded particles
The photoelectron spectra of the doubly bonded par-ticles, shown in Figure 6, differ mainly in the ionizationonset region from the singly bonded particles; here, anisolated π -band with vibrational fine structure can beseen. This vibrational progression can be ascribed tothe C=C-stretch mode, well known from the photoelec-PSfrag replacements HOMO HOMO-1
FIG. 5. The HOMOs (left) of the two singly bonded hetero-dimers and in comparison to their HOMO-1 (right).The M06-2X functional together with a cc-pVDZ basis setwas utilized for the computations. tron spectra of other alkenes. The fact that the rela-tive intensity of the first vibrational band is higher inthe spectrum of than for the other compoundssupports the assignment of this feature to the doublebond as there are two of them in that case. Studieson the photoluminescence of hydrogenated amorphouscarbon show that the π -states of sp -sites in an overall sp -matrix form the valence-band edge. The dom-inant ionization onset of the sp -feature in the doublybonded diamondoid particles indicate similar behaviorin a few-atom size regime, far away from the bulk. Theenergetic position from this band changes only slightlywith particle size, and therefore, the IPs of the doublybonded structures lie within a small energy region around7 . pσ -electrons for 2,3-dimethylbutene (”tetramethylethylene”), whose struc-ture is comparable to the central CC-double-bond andthe next four neighbor atoms.Computations of the HOMOs (Figure 7) show a stronglocalization to the CC-double-bonds. The probabilitydensity at the diamondoid cages is low for the HOMOwhich explains the weak dependence on the overall size ofthe particles. QC effects are reduced due to the fact thatthe π -electrons are nearly unaffected by the sizes of thesurrounding diamondoid cages. In contrast, the HOMO-1 is distributed over the σ -bonds of the diamondoid cagesand therefore a size dependence of this orbital energy canbe expected. The small differences of the measured IPsfor the particles under consideration can be explainedwith the screening of the π -electrons by the surrounding σ -electrons. This results in a relatively constant HOMOenergy in comparison to the shift of the HOMO-1 energy.Hence, the energetic distance of the HOMO and HOMO-1 decreases with increasing size of the bonding partners.From studies on pristine diamondoids a decrease of QC No. Compound Experimental IP (eV) Computed IP (eV) adamantane 9 . . diamantane 8 . . triamantane 8 . . . . . . . . . . PSfrag replacements 12 10 8 6Binding energy (eV) I n t e n s i t y ( a r b . un i t s ) hν = 21 .
22 eV
FIG. 6. The valence spectra of the doubly bonded substances.The spectrum of was smoothed for presentation. TheIPs, denoted by black arrows, vary only slightly for the doublybonded particles presented in this work. effects with increasing diamondoid size is known. Thus,for larger bonding partners, a saturation of the decreasingenergetic distance of HOMO and HOMO-1 is expected atsome point. Further study may show if this saturationleads to a change of energetic ordering of the uppermostoccupied orbitals.The analysis of indicates that these characteristicsapply not only for doubly bonded diamondoid parti-cles but also for sp -hybridized molecules joined by CC-double-bonds in general. With 22 carbon atoms beingthe smallest doubly bonded substance of the study, com-pound shows the highest IP of the analyzed samples.PSfrag replacements HOMO HOMO-1
FIG. 7. The HOMOs (left) of the doubly bonded diamon-doid particles ( - ) and a doubly bonded referencecompound ( ) and the HOMO-1 (right). The M06-2X func-tional together with a cc-pVDZ basis set was utilized for thecomputations. V. CONCLUSIONS
We have studied the valence electronic structure of dia-mondoid particles. The influence of the bonding partnersand the types of connecting bond have been investigated.For singly bonded particles, the central CC-bond has onlylittle impact on the energy levels of the dimers under con-sideration. Moreover, we observe that a combination ofthe bonding partner orbitals describes the overall elec-tronic structure well. A consequence of this combinationprocess can be seen through an analysis of the particleIPs. While for the homo-dimers we measure IPs well be-low the corresponding monomers, the change of IPs for
No. Compound Experimental IP (eV) Computed IP (eV) adamantylidene-diamantane 7 . . diamantylidene-diamantane 7 . . (di-adamantylidene)-diamantane 7 . . di-pentacycloundecane 7 . . hetero-dimers strongly depends on the particle compo-sition. QC effects can be seen for the homo-dimer IPsbut seem to be nearly absent for IPs of hetero-dimers.DFT computations show symmetrically distributed va-lence orbitals for dimers with equal bonding partners. Asin pristine diamondoids, the HOMOs are delocalized overthe entire molecule. In hetero-dimers, the constituent or-bitals are asymmetrically distributed. The HOMO tendsto be localized to the larger bonding partner due to itslower IP. Hence the overall IP resembles that of the largerbonding partner, and QC effects are less pronounced.For doubly bonded particles, the CC-double-bond hasa clear and identifiable impact on the electronic struc-ture. It appears as an isolated electronic feature withvibrational fine-structure at the ionization onset in thephotoelectron spectra. The IPs vary only slightly withthe size of the constituents. Computations show a stronglocalization of the HOMO to the CC-double-bond andthe influence on the HOMO of the surrounding carbonatoms is restricted to screening. The HOMO-1 is delocal-ized over the σ -bonds of the diamondoid cage structuresand can be identified as a distinct shoulder in the photo-electron spectra.To further understand the influence of the combina-tion of single diamondoids to larger particles on the elec-tronic structure of the resulting system, more studieson this class of compounds are highly desirable. Us-ing singly bonded particles enables researchers to im-itate the electronic properties of higher diamondoids,thereby circumventing the problem of extremely low syn-thesis/isolation yields for higher diamondoids. Further-more, the study of singly bonded dimers is an interestingroute to gain insight into the role of dispersive forceson the long central CC-bonds. The further investigationof doubly bonded particles assists the comprehension ofcarbon nanostructures with both sp - and sp -hybridizedmoieties. Besides the already explored parameters size,shape, and functionalization, the combination of pristinediamondoids into particles with different bonding situa-tions opens a completely new degree of freedom to ex-plore with regards to electronic structure and band gaptuning. ACKNOWLEDGMENTS
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