Lorenz Romaner

The interface energetics of self-assembled monolayers on metals.

Self-assembled monolayers (SAMs) of organic molecules generally modify the surface properties when covalently linked to substrates. In organic electronics, SAMs are used to fine-tune the work functions of inorganic electrodes, thereby minimizing the energy barriers for injection or extraction of charge carriers into or out of an active organic layer; a detailed understanding of the interface energetics on an atomistic scale is required to design improved interfaces. In the field of molecular electronics, the SAM itself (or, in some cases, one or a few molecules) carries the entire device functionality; the interface then essentially becomes the device and the alignment of the molecular energy levels with those of the electrodes defines the overall charge-transport characteristics. This Account provides a review of recent theoretical studies of the interface energetics for SAMs of π-conjugated molecules covalently linked to noble metal surfaces. After a brief description of the electrostatics of dipole layers at metal/molecule interfaces, the results of density functional theory calculations are discussed for SAMs of representative conjugated thiols on Au(111). Particular emphasis is placed on the modification of the work function of the clean metal surface upon SAM formation, the alignment of the energy levels within the SAM with the metal Fermi level, and the connection between these two quantities. To simplify the discussion, we partition the description of the metal/SAM system into two parts by considering first an isolated free-standing layer of molecules and then the system obtained after molecule-metal bond formation. From an electrostatic standpoint, both the isolated monolayer and the metal-molecule bonds can be cast in the form of dipole layers, which lead to steps in the electrostatic potential energy at the interface. While the step due to the isolated molecular layer impacts only the work function of the SAM-covered surface, the step arising from the bond formation influences both the work function and the alignment of the electronic levels in the SAM with respect to the metal Fermi energy. Interestingly, headgroup substitutions at the far ends of the molecules forming the SAM are electrostatically decoupled from the metal-thiol interface in densely packed SAMs; as a result, the nature of these substituents and the binding chemistry between the metal and the molecules are two largely unrelated handles with which to independently tune the work function and the level alignment. The establishment of a comprehensive atomistic picture regarding the impact of the individual components of a SAM on the interface energetics at metal/organic junctions paves the way for clear guidelines to design improved functional interfaces in organic and molecular electronics.

Theoretical study of PTCDA adsorbed on the coinage metal surfaces, Ag(111), Au(111) and Cu(111)

A thorough understanding of the adsorption of molecules on metallic surfaces is a crucial prerequisite for the development and improvement of functionalized materials. A prominent representative within the class of π-conjugated molecules is 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) which, adsorbed on the Ag(111), Au(111) or Cu(111) surfaces, shows characteristic trends for work-function modification, alignment of molecular levels with the substrate Fermi energy and binding distances. We carried out density functional theory (DFT) calculations to investigate to what extent these trends can be rationalized on a theoretical basis. We used different density functionals (DF) including a fully non-local van der Waals (vdW) DF capable of describing dispersion interactions. We show that, rather independent of the DF, the calculations yield level alignments and work-function modifications consistent with ultra-violet photoelectron spectroscopy when the monolayer is placed onto the surfaces at the experimental distances (as determined from x-ray standing wave experiments). The lowest unoccupied molecular orbital is occupied on the Ag and Cu surfaces, whereas it remains unoccupied on the Au surface. Simultaneously, the work function increases for Ag but decreases for Cu and Au. Adsorption distances and energies, on the other hand, depend very sensitively on the choice of the DF. While calculations in the local density approximation bind the monolayer consistently with the experimental trends, the generalized gradient approximation in several flavors fails to reproduce realistic distances and energies. Calculations employing the vdW-DF reveal that substantial bonding contributions arise from dispersive interactions. They yield reasonable binding energies but larger binding distances than the experiments.

Epitaxy of Rodlike Organic Molecules on Sheet Silicates—A Growth Model Based on Experiments and Simulations

During the last years, self-assembled organic nanostructures have been recognized as a proper fundament for several electrical and optical applications. In particular, phenylenes deposited on muscovite mica have turned out to be an outstanding material combination. They tend to align parallel to each other forming needlelike structures. In that way, they provide the key for macroscopic highly polarized emission, waveguiding, and lasing. The resulting anisotropy has been interpreted so far by an induced dipole originating from the muscovite mica substrate. Based on a combined experimental and theoretical approach, we present an alternative growth model being able to explain molecular adsorption on sheet silicates in terms of molecule−surface interactions only. By a comprehensive comparison between experiments and simulations, we demonstrate that geometrical changes in the substrate surface or molecule lead to different molecular adsorption geometries and needle directions which can be predicted by our growth model.

The electronic structure of mixed self-assembled monolayers.

The electronic structure of mixed self-assembled monolayers (SAMs) on Au(111) surfaces is modeled using slab-type density-functional theory calculations. The studied molecules have a dipolar character induced by polar and electron donating or accepting tail-group substituents. The resulting electronic structure of mixed layers is found to differ qualitatively from a simple superposition of those of the respective pure layers. Specifically, the positions of the frontier electronic states are shifted relative to the metal Fermi level, with the sign and magnitude of that shift depending on the dipole moment of the molecules and the mixing ratio in the film. This appears counterintuitive considering previous investigations, in which it has been shown that, for densely packed layers, tail-group substituents have no impact on the interfacial energy-level alignment. The seeming contradiction can be lifted by considering the local electrostatic interactions within the films in both mixed and homogeneous monolayers. Beyond that, we show that mixed SAMs provide an efficient tool for continuously tuning substrate work functions over a range that far exceeds that accessible by merely changing the coverage of homogeneous layers, with the net effect depending linearly on the mixing ratio in agreement with recent experimental findings.

Doping Molecular Wires

The concept of doping inorganic semiconductors enabled their successful application in electronic devices. Furthermore, the discovery of metal-like conduction in doped polymers started the entire field of organic electronics. In the present theoretical study, we extend the concept of doping to monomolecular wires suspended between two metal electrodes. Upon doping, the conductivity of representative model systems is found to increase by 2 orders of magnitude. More importantly, by providing a thorough understanding of the underlying mechanisms, our results pave the way for the development of novel molecular components envisioned as functional units in nanoscale devices.

Comparison of thermal and electrical degradation effects in polyfluorenes

Abstract The origin of the low energy photoluminescence (PL) and electroluminescence (EL) peaks emerging upon degradation of highly blue emissive polyfluorenes (PFs) has been intensively debated in literature during the last years and has recently been identified as the emission from an exciton and/or charge trapping on-chain keto defect. In this work we compare several polyfluorenes with respect to their stability against degradation, namely a poly(9,9 dialkylfluorene) with two hexahydrofarnesyl sidebranches (PF 111/12), a slightly branched PF 2/6 with tetrabromospiro(fluorene-9,9′-fluorene) as branch (PF A193) and the same polyfluorene with triphenylamine endcappers (PF A207). The degradation of these polymers upon storage at an elevated temperature in vacuum and in air is compared using PL and infrared (IR) spectroscopy. We find that the stability of the spiro-type PFs is significantly improved compared to the regular PF. Essentially the same trend is observed in operated devices.

Ab initio calculations of grain boundaries in bcc metals

In this study, we compute grain boundary (GB) properties for a large set of GBs in bcc transition metals with a special focus on W, Mo and Fe using ab initio density functional theory (DFT) and semi-empirical second nearest neighbour modified embedded atom method (2NN-MEAM) potentials. The GB properties include GB energies, surface energies, GB excess volume and work of separation, which we analyse and then compare to experimental data. We find that the used 2NN-MEAM potentials can predict general trends of GB properties, but do not always reproduce the GB ground state structure and energy found with DFT. In particular, our results explain the experimental finding that W and Mo prefer intergranular fracture, while other bcc metals prefer transgranular cleavage.

The influence of keto defects on photoexcitation dynamics in polyfluorene

Abstract The optical properties of two differently substituted types of polyfluorenes, 9-monoalkylated PF (mono-PF) and 9,9-dialkylated PF (bi-PF) where studied by means of photo-induced absorption (PIA) and ultrafast pump and probe measurements. The photo-induced absorption was complemented by measurements on a fluorene–fluorenone copolymer, which can be seen as a model substance for the polyfluorenes containing keto-defect sites. By differential transmission measurements we show that for the 9-monoalkylated PF measurements the singlet and triplet signal is strongly reduced compared to the 9,9-dialkylated PF. Instead, the polaron signal becomes the dominant feature.

The roles of Eu during the growth of eutectic Si in Al-Si alloys

Controlling the growth of eutectic Si and thereby modifying the eutectic Si from flake-like to fibrous is a key factor in improving the properties of Al-Si alloys. To date, it is generally accepted that the impurity-induced twinning (IIT) mechanism and the twin plane re-entrant edge (TPRE) mechanism as well as poisoning of the TPRE mechanism are valid under certain conditions. However, IIT, TPRE or poisoning of the TPRE mechanism cannot be used to interpret all observations. Here, we report an atomic-scale experimental and theoretical investigation on the roles of Eu during the growth of eutectic Si in Al-Si alloys. Both experimental and theoretical investigations reveal three different roles: (i) the adsorption at the intersection of Si facets, inducing IIT mechanism, (ii) the adsorption at the twin plane re-entrant edge, inducing TPRE mechanism or poisoning of the TPRE mechanism, and (iii) the segregation ahead of the growing Si twins, inducing a solute entrainment within eutectic Si. This investigation not only demonstrates a direct experimental support to the well-accepted poisoning of the TPRE and IIT mechanisms, but also provides a full picture about the roles of Eu atoms during the growth of eutectic Si, including the solute entrainment within eutectic Si.

Ab initio search for cohesion-enhancing impurity elements at grain boundaries in molybdenum and tungsten

We report high throughput density functional theory (DFT) calculations to simulate segregation of s- and p-elements in Mo and W. First, the preference of solutes for interstitial or substitutional positions in the bulk is evaluated and then the segregation energies for the solutes to interstitial and different substitutional sites at a grain boundary (GB) and a free surface (FS) are computed. We show that several solutes change their site preference from substitutional to interstitial position upon segregation to the GB. With the segregation energies to GB and FS, the changes in cohesion can be calculated and GB cohesion enhancing solutes can be identified. The results show striking similarity for both W and Mo. In addition, we collected the available literature data from experimental and theoretical side, which we consequently compare to our results. From our results and the comparison to literature, we identify B, C and Be as potential alloying additions for an increased GB cohesion in Mo and W.