No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Weak competing interactions control assembly of strongly bonded TCNQ ionic acceptor molecules on silver surfaces
Abstract
The energy scales of interactions that control molecular adsorption and assembly on surfaces can vary by several orders of magnitude, yet the importance of each contributing interaction is not apparent a priori. Tetracyanoquinodimethane (TCNQ) is an archetypal electron acceptor molecule and it is a key component of organic metals. On metal surfaces, this molecule also acts as an electron acceptor, producing negatively charged adsorbates. It is therefore rather intriguing to observe attractive molecular interactions in this system that were reported previously for copper and silver surfaces. Our experiments compared TCNQ adsorption on noble metal surfaces of Ag(100) and Ag(111). In both cases we found net attractive interactions down to the lowest coverage. However, the morphology of the assemblies was strikingly different, with two-dimensional islands on Ag(100) and one-dimensional chains on Ag(111) surfaces. This observation suggests that the registry effect governed by the molecular interaction with the underlying lattice potential is critical in determining the dimensionality of the molecular assembly. Using first-principles density functional calculations with a van der Waals correction scheme, we revealed that the strengths of major interactions (i.e., lattice potential corrugation, intermolecular attraction, and charge-transfer-induced repulsion) are all similar in energy. The van der Waals interactions, in particular, almost double the strength of attractive interactions, making the intermolecular potential comparable in strength to the diffusion potential and promoting self-assembly. However, it is the anisotropy of local intermolecular interactions that is primarily responsible for the difference in the topology of the molecular islands on Ag(100) and Ag(111) surfaces. We anticipate that the intermolecular potential will become more attractive and dominant over the diffusion potential with increasing molecular size, providing new design strategies for the structure and charge transfer within molecular layers.
... This structure is stabilized by dipole-dipole interactions between the cyano endgroups and the quinone center of neighboring molecules. This assembly is very similar to typical self-assembled TCNQ islands on weakly interacting substrates [5,23,[47][48][49]. When measured at a lower bias voltage (e.g., at V = 0.2 V in Figure 4a), the molecules appear with featureless elliptical shape, reflecting only the topographic extent of the molecules. ...
... This is significantly smaller than reso- nances typically observed on metal surfaces, where strong hybridization effects lead to widths of the order of approx. 500 meV [5,48]. The narrow width thus reflects that MoS 2 acts as a decoupling layer from the metal substrate. ...
The electronic structure of molecules on metal surfaces is largely determined by hybridization and screening by the substrate electrons. As a result, the energy levels are significantly broadened and molecular properties, such as vibrations are hidden within the spectral line shapes. Insertion of thin decoupling layers reduces the line widths and may give access to the resolution of electronic and vibronic states of an almost isolated molecule. Here, we use scanning tunneling microscopy and spectroscopy to show that a single layer of MoS 2 on Ag(111) exhibits a semiconducting bandgap, which may prevent molecular states from strong interactions with the metal substrate. We show that the lowest unoccupied molecular orbital (LUMO) of tetracyanoquinodimethane (TCNQ) molecules is significantly narrower than on the bare substrate and that it is accompanied by a characteristic satellite structure. Employing simple calculations within the Franck–Condon model, we reveal their vibronic origin and identify the modes with strong electron–phonon coupling.
... This structure is stabilized by dipole-dipole interactions between the cyano endgroups and the quinone center of neighboring molecules. This assembly is very similar to typical self-assembled TCNQ islands on weakly interacting substrates [5,23,[47][48][49]. When measured at lower bias voltage (e.g., at V = 0.2 V in Figure 4a), the molecules appear with featureless elliptical shape, reflecting only the topographic extent of the molecules. ...
... In turn, we do not observe any Importantly, the 470-meV resonance has a rather narrow width of ∼ 100 meV. This is significantly smaller than typically observed on metal surfaces, where strong hybridization effects lead to widths of the order of ∼ 500 meV [5,48]. The narrow width thus reflects that MoS 2 acts as a decoupling layer from the metal substrate. ...
The electronic structure of molecules on metal surfaces is largely determined by hybridization and screening by the substrate electrons. As a result, the energy levels are significantly broadened and molecular properties, such as vibrations are hidden within the spectral lineshapes. Insertion of thin decoupling layers reduces the linewidths and may give access to the resolution of electronic and vibronic states of an almost isolated molecule. Here, we use scanning tunneling microscopy and spectroscopy to show that a single layer of MoS$_{2}$ on Ag(111) provides a semiconducting band gap that may prevent molecular states from strong interactions with the metal substrate. We show that the lowest unoccupied molecular orbital (LUMO) of tetra-cyano-quino-dimethane (TCNQ) molecules is significantly narrower than on the bare substrate and that it is accompanied by a characteristic satellite structure. Employing simple calculations within the Franck-Condon model, we reveal their vibronic origin and identify the modes with strong electron-phonon coupling.
... The electrostatic repulsion of anionic groups on the surface of PAA leads to its dispersibility rather than the expansion after water absorption, which also explains the high viscosity of the PAA solutions. Its dispersing property is about 15-20 times that of CMC and SA (Rustamov et al., 2001;Aoki et al., 2012;Suzuki et al., 2016). Heating, neutral salts, and organic acids have little effect on the dispersibility of PAA, which exhibits increased viscosity under alkaline conditions (Suzuki et al., 2016). ...
... when the Cl − concentration increased from 0 to 10 g/L. It can be seen from the results that the efficiency of PAA-n-FeS removal for Cr(VI) was enhanced when anion was added Studies had shown that with the increasing ionic strength, the diffusion layer of the colloidal particles became thinner and the repulsive potential between the particles reduced, resulting in easier agglomeration between the particles Park et al., 2014). Due to the negative charge on the surface of PAA-n-FeS (shown in Fig. 1), when Ca 2+ and Mg 2+ were added to the solution, the surface charge characteristics of the particles were changed, and the possibility of agglomeration between the particles was significantly increased. ...
Nano ferrous sulfide (n-FeS) colloids show an excellent performance in the application of remediation in situ soil and groundwater. However, due to the interfacial effect and high reactivity of the nano sized FeS, n-FeS easy to agglomerate, which reduces their remediation efficiency. In this study, a novel composite colloid was synthesized using polyacrylic acid salt (PAA) and n-FeS. The PAA-n-FeS colloid was used to remove Cr(VI) in water remediation, and its removal mechanism and efficiency were explored. The results showed that the hydrodynamic diameter of PAA-n-FeS ranged from 65.04 to 90.09 nm and the zeta potential was from -27 to -54 mV at pH varying from 4.5 to 9.0. PAA was coated on the surface of n-FeS, which improved the dispersibility and stability of n-FeS by increasing the steric hindrance and electrostatic repulsion between n-FeS particles. Moreover, the Cr(VI) maximum removal amount PAA-n-FeS was 432.79 mg/g, which was significantly higher than that of n-FeS (218.29 mg/g) and PAA (12.32 mg/g). The mechanism of PAA-n-FeS removal of Cr(VI) was mainly derived from its own reducibility. The reaction products were mainly Cr(OH)3, Cr(III)-Fe(III), Cr2O3, and Cr2S3. This research not only finds a new stabilizer for preventing n-FeS agglomeration, but also provides a novel n-FeS composited colloid for promoting the practical application to Cr(VI) removal from water.
... The adsorption of the two molecules is well studied on metal surfaces [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]. The electron-acceptor nature makes the work-function of the substrate metal surface increased. ...
We examined the mechanism of the large difference of the doping effect of two molecules of tetracyanoquinodimethane(TCNQ) and 2,3,5,6-Tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ) on the field effect transistor (FET) property, which is composed of the atomically thin MoS2 channel. The electron-acceptor behaviors of the two molecules are detected by examining the threshold voltage shift (ΔVth) of the drain current vs gate voltage curve plot. It shows only a mild shift of ∼4 V for the TCNQ but it shows a large value of ∼33 V for the F4-TCNQ molecule. The similar rapid change with small amount of F4-TCNQ deposition was detected as the Fermi level shift in the ultra violet photoemission spectroscopy (UPS). The DFT calculation shows the flat lying configuration for both molecules after the structural optimization, and the difference of the charge transfer to the substrate is not large enough to explain the difference of theΔVth. Instead, we want to propose a model in which the bonding of the F atom of the F4-TCNQ molecule at the S-defect contribute to the initial rapid variation of theΔVth.
... The reasons mentioned above led to an increased trend of mobility of As(V). The diffusion layer became thinner with the increasing ionic strength, as the repulsive potential energy between colloids was reduced and the coagulation of colloids and As(V) was promoted (Park et al., 2014). The change of zeta potential of the HA-Fe colloid under different ionic strengths can also verify this phenomenon (Fig. 1). ...
... Due to highly ordering and epitaxial growth of F 4 TCNQ on Au(111) the charge is spatially distributed and a space charge region extending up to a coverage of 10 ML into the organic film can be found [266]. Both molecules have been intensively studied on single crystal surfaces [267][268][269][270][271][272] as well as in devices like photovoltaic cells [273] or transistors [274,275]. STM investigations (see Fig. 5.5 (b)) revealed a brick-like adsorption geometry with stabilizing hydrogen bonds between neighboring molecules for TCNQ on Au(111) [268]. ...
Metal/organic interfaces as they appear between electrodes and organic semiconductors in organic electronics decisively determine device properties of transistors, light emitting diodes, or photovoltaic cells. The interactions within the organic semiconductor and between organic adsorbate and metallic substrate lead to characteristic properties of the particular interface. These properties, namely the binding strength, the adsorption geometry, and the electronic structure, have been studied with comprehensive surface sensitive experimental methods like high-resolution electron energy-loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). The use of single crystal metal surfaces as substrates and self-assembling small organic molecules as adsorbates lead to insights into structure-property relationships that will contribute to the further development of materials and devices. The first part of this work investigates the bonding strength between metal substrates and organic adsorbates. With the quantification of binding energies of simple aromatic molecules on coinage metal surfaces by means of TPD, this part enters questions of basic surface science. Besides the delivery of benchmarks of unrivalled accuracy for the further development of computational methods to model binding properties of adsorbate-covered surfaces the focus of this part also lays on the first investigation of the extraordinary coverage dependency of the binding energy of such systems. The second part is about the self-assembly of small-molecule organic semiconductors on metal surfaces, and how this arrangement is influenced by the molecular structure. This part covers the elucidation of adsorption geometries of N-heteropolycyclic aromatic molecules on the Au(111) surface by means of vibrational HREELS. Moreover, electronic HREELS enabled us to get insight into the electronic structure of these interfaces. To maximize the interaction between metal bands and the pi-system of the adsorbate the planar molecules prefer a planar adsorption geometry. This presetting of a flat geometry works subsequently as a template for further layers which leads to a growth mechanism and therefore film structure significantly different from that of the bulk crystal. The last part of this work studies the influence of organic adsorbate films on collective electronic properties of the metal surface with angle-resolved HREELS. Characteristic collective excitations of a two-dimensional electron gas present on the pristine gold surface are strongly influenced in their properties by adsorbate layers, e.g., they show a strongly enhanced intensity and a varied dispersion relation.
... Such a quantitative picture should be generic enough to test by applying the same ideas to the numerous disordered porous molecular assemblies known to date. 4,36 Another important test would be to follow the evolution of the pore-size distribution; the results would clarify the scaling of the pore size distribution but might be difficult to obtain in practice if the coverage window where disordered pores exist is not large. Finally, we note that kinetic Monte Carlo simulations of pore networks would be very valuable if reasonable estimates for relevant vacancy attachment rates could be made. ...
A scanning tunneling microscopy study of anthraquinone (AQ) on the Au(111) surface shows that the molecules self-assemble into several structures depending on the local surface coverage. At high coverages, a close-packed saturated monolayer is observed, while at low coverages, mobile surface molecules coexist with stable chiral hexamer clusters. At intermediate coverages, a disordered 2D porous network interlinking close-packed islands is observed in contrast to the giant honeycomb networks observed for the same molecule on Cu(111). This difference verifies the predicted extreme sensitivity [J. Wyrick et al., Nano Lett. 11, 2944 (2011)] of the pore network to small changes in the surface electronic structure. Quantitative analysis of the 2D pore network reveals that the areas of the vacancy islands are distributed log-normally. Log-normal distributions are typically associated with the product of random variables (multiplicative noise), and we propose that the distribution of pore sizes for AQ on Au(111) originates from random linear rate constants for molecules to either desorb from the surface or detach from the region of a nucleated pore.
In this study, we report on the self-assembly of the organic electron donor 2,3,6,7,10,11-hexamethoxytriphenylene (HAT) on graphene grown epitaxially on Ir(111). Using scanning tunneling microscopy and low-energy electron diffraction, we find that a monolayer of HAT assembles in a commensurate close-packed hexagonal network on graphene/Ir(111). X-ray and ultraviolet photoelectron spectroscopy measurements indicate that no charge transfer between the HAT molecules and the graphene/Ir(111) substrate takes place, while the work function decreases slightly. This demonstrates that the HAT/graphene interface is weakly interacting. The fact that the molecules nonetheless form a commensurate network deviates from what is established for adsorption of organic molecules on metallic substrates where commensurate overlayers are mainly observed for strongly interacting systems.
We have studied the self-assembly of 7,7,8,8-Tetracyanoquinodimethane molecules on the (3√2×√2)R45° reconstruction of the SnCu(001) surface alloy by means of X-ray photoemission spectroscopy, scanning tunneling microscopy, near-edge X-ray absorption fine structure spectroscopy, and density-functional theory calculations. Our results show that surface alloying strongly attenuates the chemical interaction of the molecule with the surface, but it does not inhibit the charge transfer from the substrate to the molecules. The assembly mechanism of the molecules is completely modified with respect to the bare Cu(001) surface. We show that, on the SnCu(100) surface alloy, the strong CN-Cu interaction drives the formation of different coordination structures with native Cu adatoms. We found that the flexible coordination chemistry of Cu allows the formation of 3 different stable phases, each one with the Cu ions in a different coordination geometry (coordinations 4, 3, and 2). Moreover, we show that both the formation of lateral H-bonds between adjacent molecules and the interaction of the Cu ion with the substrate play determinant roles in the stabilization of the structures.
The adsorption of tetracyanoquinodimethane and of the closely related derivative tetrafluorotetracyanoquinodimethane on the (111) surfaces of the coinage metals, namely, copper, silver, and (unreconstructed) gold, has been studied by dispersion-corrected ab initio density functional theory calculations. In order to separate the molecule–substrate interaction from the effects of molecule–molecule interaction, only the isolated molecules are considered. The results show that, in this case, the strength of the interaction of both molecules with the surfaces decreases in the expected order Cu > Ag > Au. The total amount of charge transfer, however, behaves in a different way, being larger for Ag and smaller for Cu and Au. This trend can be explained by a combination of the differences in the work functions of the three metals and the amount of backdonation between the molecule and the metal.
Optically-induced processes in organic materials are essential for light harvesting, switching, and sensor technologies. Here we studied the electronic properties of the tetracyanoquinodimethane(TCNQ)/Au(111) interface by using two-photon photoemission spectroscopy. We found a significant increase of the sample work function due to UV light illumination, while the electronic structure of the TCNQ-derived states remain unaffected. Thereby the work function of this interface can be tuned over a wide range via the photon dose. We assigned this to a photo-induced metal to molecule electron transfer creating negative ions. The electrons are bound by a small potential barrier. Thus, thermal-activation reverses the process resulting in original work function value. The presented photo-induced charge transfer at the TCNQ/Au(111) interface can be used for continuous work function tuning across the substrate's work function which can be applied in device-adapted hole-injection layers or organic UV-light sensors.
Bulk molecular ionic solids exhibit fascinating electronic properties, including electron correlations, phase transitions and superconducting ground states. In contrast, few of these phenomena have so far been observed in low-dimensional molecular structures, including thin films, nanoparticles and molecular blends, not in the least because most of such structures have so far been composed of nearly closed-shell molecules. It is therefore desirable to develop low-dimensional molecular structures of ionic molecules toward fundamental studies and potential applications. Here we present detailed analysis of monolayer-thick structures of the canonical TTF-TCNQ (tetrathiafulvalene 7,7,8,8-tetracyanoquinodimethane) system grown on low-index gold and silver surfaces. The most distinctive property of the epitaxial growth is the wide abundance of stable TTF/TCNQ ratios, in sharp contrast to the predominance of 1:1 ratio in the bulk. We propose the existence of the surface phase-diagram that controls the structures of TTF-TCNQ on the surfaces, and demonstrate phase-transitions that occur upon progressively increasing the density of TCNQ while keeping the surface coverage of TTF fixed. Based on direct observations, we propose the binding motif behind the stable phases and infer the dominant interactions that enable the existence of the rich spectrum of surface structures. Finally, we also show that the surface phase diagram will control the epitaxy beyond monolayer coverage. Multiplicity of stable surface structures, the corollary rich phase diagram and the corresponding phase-transitions present an interesting opportunity for low-dimensional molecular systems, particularly if some of the electronic properties of the bulk can be preserved or modified in the surface phases.
Collective magnetic properties are usually associated with the d or f
electrons that carry the individual magnetic moments. A fully
spin-polarized ground state based on π electrons has been predicted
in half-filled flat-band organic materials, but has remained
experimentally challenging to realize. Here we show that isolated
tetracyano- p-quinodimethane molecules deposited on graphene epitaxially
grown on Ru(0001) acquire charge from the substrate and develop a
magnetic moment of 0.4μB per molecule. The magnetic moment
survives even when the molecules form into a dimer or a monolayer, with
a value of 0.18μB per molecule for the monolayer. The
self-assembled molecular monolayer develops spatially extended
spin-split electronic bands, and we visualized the ground-state spin
alignment using spin-polarized scanning tunnelling microscopy. The
observation of long-range magnetic order in an organic layer adsorbed on
graphene paves the way for incorporating magnetic functionalities into
graphene.
Using low-temperature scanning tunneling microscopy (STM) distance-voltage characteristics at constant current are measured on Cu(111), Cu(110), Au(111), and Au(100) surfaces. Resonant tunneling features are observed that arise from image potential states distorted by the strong electric field of the STM tip. Distance-voltage characteristics are measured over a wide range of current setpoints and the energetic position and width of the resonant features show a significant dependence on this parameter. The lowest energy feature is found to be nearly independent of STM tip condition, while higher energy resonances show substantial tip dependence. The results are discussed and compared with recent calculations.
The authors study numerically the transition by breaking of analyticity which occurs in the incommensurate ground state of the Frenkel-Kontorova model (1938) when the amplitude lambda of its periodic-potential V(u) is increased beyond a critical value lambda c. A brief review of the properties of this transition and its connection with the standard map is given. They consider four quantities which are critical when lambda goes to lambda c from upper values: the gap in the phonon spectrum, the coherence length of the ground state, the Peierls-Nabarro barrier and the depinning force. The numerical method is discussed and the authors show in particular that the mapping method is unpracticable for lambda > lambda c. They observe the transition by breaking of analyticity and show that in the stochastic region ( lambda > lambda c) the ground state is never chaotic. Nevertheless in this region metastable chaotic states can be obtained. The numerical calculations, performed with a ratio of the atomic mean distance to the period of the potential V(u), l/2a=(3- square root 5)/2 equivalent to the golden mean, show that critical exponents can be defined for the four critical quantities studied. Their values reveal two scaling laws which are empirically explained.
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
The electronic properties and the function of hybrid inorganic-organic systems (HIOS) are intimately linked to their interface geometry. Here we show that the inclusion of the many-body collective response of the substrate electrons inside the inorganic bulk enables us to reliably predict the HIOS geometries and energies. This is achieved by the combination of dispersion-corrected density-functional theory (the DFT+ van der Waals approach) [Phys. Rev. Lett. 102, 073005 (2009)], with the Lifshitz-Zaremba-Kohn theory for the nonlocal Coulomb screening within the bulk. Our method yields geometries in remarkable agreement (≈0.1 Å) with normal incidence x-ray standing wave measurements for the 3, 4, 9, 10-perylene-tetracarboxylic acid dianhydride (C(24)O(6)H(8), PTCDA) molecule on Cu(111), Ag(111), and Au(111) surfaces. Similarly accurate results are obtained for xenon and benzene adsorbed on metal surfaces.
The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blöchl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
An electronic band with quasi-one-dimensional dispersion is found at the interface between a monolayer of a charge-transfer complex (TTF-TCNQ) and a Au(111) surface. Combined local spectroscopy and numerical calculations show that the band results from a complex mixing of metal and molecular states. The molecular layer folds the underlying metal states and mixes with them selectively, through the TTF component, giving rise to anisotropic hybrid bands. Our results suggest that, by tuning the components of such molecular layers, the dimensionality and dispersion of organic-metal interface states can be engineered.
Organic/metal interfaces control the performance of many optoelectronic organic devices, including organic light-emitting diodes or field-effect transistors. Using scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations, we show that electron transfer at the interface between a metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. These structural modifications mediate new intermolecular interactions through the creation of stress fields that could not have been predicted on the basis of gas-phase neutral tetracyano-p-quinodimethane conformation.
We present a parameter-free method for an accurate determination of long-range van der Waals interactions from mean-field electronic structure calculations. Our method relies on the summation of interatomic C6 coefficients, derived from the electron density of a molecule or solid and accurate reference data for the free atoms. The mean absolute error in the C6 coefficients is 5.5% when compared to accurate experimental values for 1225 intermolecular pairs, irrespective of the employed exchange-correlation functional. We show that the effective atomic C6 coefficients depend strongly on the bonding environment of an atom in a molecule. Finally, we analyze the van der Waals radii and the damping function in the C6R(-6) correction method for density-functional theory calculations.
The capability to control the type and amount of charge carriers in a material and, in the extreme case, the transition from metal to insulator, is one of the key challenges of modern electronics. By employing angle-resolved photoemission spectroscopy we find that a reversible metal to insulator transition and a fine-tuning of the charge carriers from electrons to holes can be achieved in epitaxial bilayer and single layer graphene by molecular doping. The effects of electron screening and disorder are also discussed. These results demonstrate that epitaxial graphene is suitable for electronics applications, as well as provide new opportunities for studying the hole doping regime of the Dirac cone in graphene.
We demonstrate tuning of Schottky energy barriers in organic electronic devices by utilizing chemically tailored electrodes. The Schottky energy barrier of Ag on poly[2-methoxy], 5-(2{prime}-ethyl-hexyloxy)- 1,4-phenylene was tuned over a range of more than 1 eV by using self-assembled monolayers (SAM{close_quote}s) to attach oriented dipole layers to the Ag prior to device fabrication. Kelvin probe measurements were used to determine the effect of the SAM{close_quote}s on the Ag surface potential. {ital Ab} {ital initio} Hartree-Fock calculations of the molecular dipole moments successfully describe the surface potential changes. The chemically tailored electrodes were then incorporated in organic diode structures and changes in the metal/organic Schottky energy barriers were measured using an electroabsorption technique. These results demonstrate the use of self-assembled monolayers to control metal/organic interfacial electronic properties. They establish a physical principle for manipulating the relative energy levels between two materials and demonstrate an approach to improve metal/organic contacts in organic electronic devices. {copyright} {ital 1996 The American Physical Society.}
The energy-level alignment at interfaces between three electroactive conjugated organic materials and Au was systematically varied by adjusting the precoverage of the metal substrate with the electron acceptor tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Photoelectron spectroscopy revealed that electron transfer from Au to adsorbed F4-TCNQ was responsible for lowering the hole-injection barrier by as much as 1.2 eV. This novel interface modification scheme is independent of the charge transfer complex formation ability of the organic materials with the electron acceptor.
The electronic properties of interfaces between two different solids can differ strikingly from those of the constituent materials. For instance, metallic conductivity-and even superconductivity-have recently been discovered at interfaces formed by insulating transition-metal oxides. Here, we investigate interfaces between crystals of conjugated organic molecules, which are large-gap undoped semiconductors, that is, essentially insulators. We find that highly conducting interfaces can be realized with resistivity ranging from 1 to 30 kohms per square, and that, for the best samples, the temperature dependence of the conductivity is metallic. The observed electrical conduction originates from a large transfer of charge between the two crystals that takes place at the interface, on a molecular scale. As the interface assembly process is simple and can be applied to crystals of virtually any conjugated molecule, the conducting interfaces described here represent the first examples of a new class of electronic systems.
A density functional calculation is performed to analyze the tetracyanoquinodimethane (TCNQ)/Au(111) interface, including the effect of the molecular charging energy on the transport gap. We find that the adsorbed TCNQ molecules are bent so that the edge N atoms are closer to the Au(111) surface. Theoretical
scanning tunneling microscopy (STM) imaging is carried out and compared with the experimental STM for the TCNQ/Au(111) self-assembled structure, finding good agreement, and validating the interface geometry obtained in our calculations. We show that the alignment between the metal and the organic levels is controlled by the charge transfer between the two materials and the dipole created in the molecule because of its deformation when adsorbed in Au(111). The calculated transport gap is 3.1 eV.
Epitaxial growth of organic charge-transfer salts composed of molecular cations and anions will potentially allow the synthesis of thin films of molecular conductors with strong electron correlations and electron–phonon interactions, highly distinct properties compared to thin organic semiconductors and traditional self-assembled monolayers. Here, we report on ionic decomposition of the charge transfer salt β″-(BEDT-TTF)2SF5CH2CF2SO3 into two surface-supported phases on Ag(111), each of which has a cation/anion ratio different from the 2:1 stoichiometry of the bulk. The films were grown by thermal evaporation of the bulk crystal. Subsequent reassembly of the constituent bis(ethylenedithio)tetrathiafulvalene molecules (BEDT-TTF) and the SF5CH2CF2SO3 anions into long-range ordered structures on the Ag(111) surface produced well-ordered molecular islands with either 1:1 or 3:1 stoichiometric ratios of BEDT-TTF:SF5CH2CF2SO3. Tunneling spectroscopy revealed that both surface structures could be considered insulating. However, the charge-transfer interaction between cations and anions still persists, producing electronic states that are distinct from those of pure BEDT-TTF molecules on a silver surface. Density functional theory calculations of adsorbed molecules show that they remain ionic, with adsorption and intermolecular binding energies comparable to that of bulk cohesive energies. The diversity of surface-supported multicomponent molecular structures derived from charge-transfer salts and the perseverance of cation and anion molecules despite thermal decomposition of the bulk all provide rich opportunity toward achieving correlated electron properties in ultrathin molecular films.
To explore the properties of electrons at interfaces, image-potential states on metal surfaces with physisorbed overlayers are being examined. Using angle-resolved two-photon photoemission we have measured changes in the binding energy and dispersion of image states for a Xe monolayer on the Ag(111) surface. The binding energy of the n = 1 state shifts from its clean surface value of −0.76 ± 0.02 eV to a value of −0.67 ± 0.02 eV at monolayer coverage. Dispersion measurements show that the effective mass (m∗) of the image potential state decreases from (1.35 ± 0.10)me to the free-electron value of (1.00 ± 0.05)me. A model i the only effect of the adsorbate is to change the work function of the metal approximates the binding energy shift, but fails to account for the effective mass. Finally, we suggest a model in which the unique electronic structure of the first adsorbate layer shifts the electron probability density outward from the surface. The dielectric screening expected within the layer thus becomes less significant. The coupling of the electron to the metal substrate is lessened and consequently the effective mass approaches that of a free electron.
The organic molecule tetracyanoquinodimethane (TCNQ) is a strong charge acceptor with a large tendency to form metal-organic charge transfer compounds. Using a low temperature scanning tunneling microscope we characterize the self-organization of TCNQ in ultra-thin films on a Au(1 1 1) surface and its electronic structure. We find that the molecules are weakly adsorbed on the metal, forming extended networks stabilized by hydrogen bonds. Despite its strong acceptor character, we find a negligible charge transfer from the metal surface.
Applying k⃗-resolved inverse photoemission spectroscopy to the Ag(100) surface, we have found two different types of unoccupied electronic surface states: (i) Near-normal-incidence, image-potential surface states produce a steplike spectral feature about 0.5 eV below the vacuum level; (ii) near the X― point of the surface Brillouin zone, an s-like Shockley-type surface state appears at EF+3.8 eV, in agreement with electroreflectance measurements by Kolb et al. Both surface states are quenched by the adsorption of water at 110 K.
We report a combined experimental and theoretical study of the unoccupied electronic states of the neutral molecular organic materials TTF (tetrathiafulvalene) and TCNQ (7,7,8,8-tetracyano-p-quinodimethane) and of the one-dimensional metallic charge transfer salt TTF-TCNQ. The experimental density of states (DOS) is obtained by x-ray absorption near edge spectroscopy (XANES) with synchrotron light and the predicted DOS by means of first-principles density functional theory calculations. Most of the experimentally derived element-specific XANES features can be associated to molecular orbitals of defined symmetry. Because of the planar geometry of the TTF and TCNQ molecules and the polarization of the synchrotron light, the energy dependent σ or π character of the orbitals can be inferred from angular dependent XANES measurements. The present work represents the state of the art analysis of the XANES spectra of this type of materials and points out the need for additional work in order to elucidate the governing selection rules in the excitation process.
Bound and excited negative ion states of gaseous 7,7,8,8‐tetracyanoquinodimethan (TCNQ) are studied from experiments involving collisions of electrons and fast cesium beams with TCNQ. The electron affinity of TCNQ is measured to be 2.8+0.05−0.3 eV by the collisional ionization technique. TCNQ attaches electrons with energies of ∼0, 0.7, and 1.3 eV (values represent peaks in cross section) to form long‐lived compound negative ions which are metastable with respect to autodetachment. The lifetime for the decay of TCNQ−∗ decreases from ∼2×10−3 sec at ∼0 eV to ∼10−4 sec at ∼4 eV. Evidence for a third compound negative ion state is seen as a peak in the TCNQ− signal at ∼3.2 eV. This state is metastable with respect to autodetachment and dissociation into the products C11N3H−3+HCN0.
It is shown that the classical image potential plays an important role
in the correct interpretation of vacuum-tunneling experiments. The
logarithmic derivative of the tunnel current with respect to the
electrode separation is nearly constant and approximately equal to the
square root of the average work function. From the experimental
tunnel-voltage dependence with the electrode separation we obtain the
barrier height as a function of distance.
A density functional calculation is performed to analyze the tetracyanoquinodimethane (TCNQ)/Au(111) interface, includ-ing the effect of the molecular charging energy on the transport gap. We find that the adsorbed TCNQ molecules are bent so that the edge N atoms are closer to the Au(111) surface. Theoretical scanning tunneling microscopy (STM) imaging is carried out and compared with the experimental STM for the TCNQ/Au(111) self-assembled structure, finding good agreement, and validating the interface geometry obtained in our calculations. We show that the alignment between the metal and the organic levels is controlled by the charge transfer between the two materials and the dipole created in the molecule because of its deformation when adsorbed in Au(111). The calculated trans-port gap is 3.1 eV.
We report a combined experimental and theoretical study of the modulation of surface charge transfer on the tetrathiafulvalene (TTF)/Au(111) interface as a function of coverage in the submonolayer regime by combining low-temperature scanning tunneling microscopy, high-resolution photoemission spectroscopy using synchrotron radiation, and density functional theory (DFT) calculations. The modulation is induced by the competition between long-range repulsive Coulombic interactions and short-range attractive hydrogen-bonding interactions. The system shows the characteristic pattern evolution, from monomeric stripes at low coverages to two-dimensional islands, with the formation of labyrinths in the crossover.
The recent emergence of molecular films as candidates for functional electronic materials has prompted numerous investigations of the underlying mechanisms responsible for their structure and formation. This review describes the role of epitaxy in molecular organization on crystalline substrates. A much-needed grammar of epitaxy is presented that classifies the various modes of epitaxy according to transformation matrices that relate the overlayer lattice to the substrate lattice. The different modes of epitaxy can be organized hierarchically to reflect the balance of overlayer–substrate and molecule–molecule energies. In the case of molecular overlayers, the mismatch of overlayer and substrate symmetries commonly leads to coincident epitaxy in which some of the overlayer lattice points do not reside on substrate lattice points. Analyses of numerous reported epitaxial molecular films reveal that coincidence is quite common even though, based on overlayer–substrate interface energies alone, not as energetically favorable as commensurism. The prevalence of coincidence can be attributed to overlayer elastic constants, associated with molecule–molecule interactions within the overlayer, that are larger than the elastic constants of the overlayer–substrate interface. This condition facilitates prediction of the epitaxial configuration and overlayer structure through simple and comparatively efficient geometric modeling that does not require the input of potential energies, while revealing the role of phase coherence between the overlayer and substrate lattices.
Self-assembly represents a promising strategy for surface functionalisation as well as creating nanostructures with well-controlled, tailor-made properties and functionality. Molecular self-assembly at solid surfaces is governed by the subtle interplay between molecule–molecule and molecule–substrate interactions that can be tuned by varying molecular building blocks, surface chemistry and structure as well as substrate temperature.In this review, basic principles behind molecular self-assembly of organic molecules on metal surfaces will be discussed. Controlling these formation principles allows for creating a wide variety of different molecular surface structures ranging from well-defined clusters, quasi one-dimensional rows to ordered, two-dimensional overlayers. An impressive number of studies exist, demonstrating the ability of molecular self-assembly to create these different structural motifs in a predictable manner by tuning the molecular building blocks as well as the metallic substrate.Here, the multitude of different surface structures of the natural amino acid cysteine on two different gold surfaces observed with scanning tunnelling microscopy will be reviewed. Cysteine on Au(110)-(1×2) represents a model system illustrating the formation of all the above mentioned structural motifs without changing the molecular building blocks or the substrate surface. The only parameters in this system are substrate temperature and molecular coverage, controlling both the molecular adsorption state (physisorption versus chemisorption) and molecular surface mobility. By tuning the adsorption state and the molecular mobility, distinctly different molecular structures are formed, exemplifying the variety of structural motifs that can be achieved by molecular self-assembly.
An overview is given of the progress of ideas and the very large body of experimental work which has marked the quest for organic superconductivity and the present search for new materials with the highest possible Tc. Focus is on TTF-TCNQ, on which a lot of precursor work has been accomplished, as well as on the (TM)2X series of organic salts where organic superconductivity has been discovered and further studies on the electron localization due to correlations (Mott localization) have been carried on. Throughout, the importance of hydrostatic pressure in tuning the properties of molecular conductors is emphasized.
Deposition of monolayers of strong electron donors or acceptors on metal surfaces in many cases results in a metal-independent work function as a consequence of Fermi-level pinning. This raises the question whether in such a situation molecular dipoles, which are also frequently used to tune the interface energetics, still can have any impact. We use density functional theory to show that the spatial position of the dipoles is the determining factor and that only dipoles outside the immediate metal-molecule interface allow work-function changes beyond the pinning limit.
The binding energies of image-potential states are studied in a systematic way for metals [Cu(111), Cu(100), Cu(110), Ag(111), Au(111), and Au(100)], a semimetal [Sb(100)], and a layered compound (1T-${\mathrm{TiS}}_{2}$). Data for Au(111) show that image-potential states exist even in the presence of bulk states. The coupling with bulk states is weak as indicated by the narrow ($\le${}0.1 eV) width of the image states. The binding energies of the lowest state cluster around 0.7 eV for a variety of metal and semimetal surfaces indicating a universal phenomenon. This value is close to the hydrogenic binding energy of (1/16) Ry $\sim${}0.85 eV for the n=1 state, which indicates that the simple Coulombic image potential dominates the energy balance.
We have measured the binding energy of the image-potential states on Cu(100) and Ag(100) surfaces with two-photon photoemission spectroscopy. We find EB=0.57 (0.18)+/-0.02 eV for Cu(100) and 0.53 (0.16)+/-0.02 eV for Ag(100) for the n=1 (n=2) states, respectively. These values are compared with the nearly hydrogenic binding energies of 0.77-0.83 eV obtained for the (111) surfaces of Cu, Ag, and Ni using the same method. The comparison shows that the binding energy does not depend on the material as long as the surface structure remains constant but changes markedly with the crystal orientation.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
Stark-shifted image-potential states were measured with an STM tip for benzene adsorbed on a Cu(111) surface. A single benzene molecule locally shifts the position of the first image state toward the Fermi level by 0.2 eV relative to its position on the clean surface. The energetic position of this molecule-modified state shifts to lower energy with increasing coverage of benzene on the surface. This is attributed to local surface potential changes that are correlated with the lowering of the crystal work function due to adsorption of benzene.
Epitaxial graphene thermally grown on 6H-SiC(0001) can be p-type doped via a novel surface transfer doping scheme by modifying the surface with the electron acceptor, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Synchrotron-based high-resolution photoemission spectroscopy reveals that electron transfer from graphene to adsorbed F4-TCNQ is responsible for the p-type doping of graphene. This novel surface transfer doping scheme by surface modification with appropriate molecular acceptors represents a simple and effective method to nondestructively dope epitaxial graphene for future nanoelectronics applications.