Adsorption Height Research Articles

Hydrogen diffusion on Fe surface and into subsurface from first principles

The chemisorption of atomic hydrogen on metal surface and into subsurface is of great importance to understand the fundamental diffusion mechanism in heterogenous catalysis and hydrogen-induced embrittlement. Using spin-polarized density functional theory, we show that hydrogen prefers the quasi four-fold hollow site near typical Hollow site on Fe(110) and that four-fold hollow site on Fe(100). A very weak surface reconstruction effect induced by hydrogen coverage is reported. Two three-dimensional (3D) potential energy surfaces (PESs) are constructed for modelling hydrogen diffusion on Fe surface and into subsurface by interpolating ab initio energy points (∼1200 for each surface). We appraise the accuracy of PES and plot some contours of potential energies at different adsorption heights including the important subsurface regions. Furthermore, possible minimum energy pathways for hydrogen diffusion on Fe surface and into subsurface are searched out based on these 3D PESs using a mesh method. These pathways are in good agreement with those obtained from the nudged elastic band method. Some trapping regions into subsurface for hydrogen chemisorption are shown and the diffusion coefficient is estimated by classical transition state theory.

Organometallic Bonding in an Ullmann-Type On-Surface Chemical Reaction Studied by High-Resolution Atomic Force Microscopy.

The on-surface Ullmann-type chemical reaction synthesizes polymers by linking carbons of adjacent molecules on solid surfaces. Although an organometallic compound is recently identified as the reaction intermediate, little is known about the detailed structure of the bonded organometallic species and its influence on the molecule and the reaction. Herein atomic force microscopy at low temperature is used to study the reaction with 3,9-diiododinaphtho[2,3-b:2',3'-d]thiophene (I-DNT-VW), which is polymerized on Ag(111) in vacuum. Thermally sublimated I-DNT-VW picks up a Ag surface atom, forming a CAg bond at one end after removing an iodine. The CAg bond is usually short-lived, and a CAgC organometallic bond immediately forms with an adjacent molecule. The existence of the bonded Ag atoms strongly affects the bending angle and adsorption height of the molecular unit. Density functional theory calculations reveal the bending mechanism, which reveals that charge from the terminus of the molecule is transferred via the Ag atom into the organometallic bond and strengths the local adsorption to the substrate. Such deformations vanish when the Ag atoms are removed by annealing and CC bonds are established.

Adsorption Behavior of Nonplanar Phthalocyanines: Competition of Different Adsorption Conformations.

Using density functional theory augmented with state-of-the-art van der Waals corrections, we studied the geometric and electronic properties of nonplanar chlorogallium-phthalocyanine GaClPc molecules adsorbed on Cu(111). Comparing these results with published experimental data for adsorption heights, we found indications for breaking of the metal–halogen bond when the molecule is heated during or after the deposition process. Interestingly, the work-function change induced by this dissociated geometry is the same as that computed for an intact adsorbate layer in the “Cl-down” configuration, with both agreeing well with the experimental photoemission data. This is unexpected, as the chemical natures of the adsorbates and the adsorption distances are markedly different in the two cases. The observation is explained as a consequence of Fermi-level pinning due to fractional charge transfer at the interface. Our results show that rationalizing the adsorption configurations on the basis of electronic interface properties alone can be ambiguous and that additional insight from dispersion-corrected DFT simulations is desirable.

Identical Binding Energies and Work Functions for Distinct Adsorption Structures: Olympicenes on the Cu(111) Surface.

Reliability is one of the major concerns and challenges in designing organic/inorganic interfaces for (opto)electronic applications. Even small structural differences for molecules on substrates can result in a significant variation in the interface functionality, due to the strong correlation between geometry, stability, and electronic structure. Here, we employed state-of-the-art first-principles calculations with van der Waals interactions, in combination with atomic force microscopy experiments, to explore the interaction mechanism for three structurally related olympicene molecules adsorbed on the Cu(111) surface. The substitution of a single atom in the olympicene molecule switches the nature of adsorption from predominantly physisorptive character [olympicene on Cu(111)], to an intermediate state [olympicene-derived ketone on Cu(111)], then to chemisorptive character [olympicene radical on Cu(111)]. Despite the remarkable difference in adsorption structures (by up to 0.9 Å in adsorption height) and different nature of bonding, the olympicene, its ketone, and its radical derivatives have essentially identical binding energies and work functions upon interaction with the metal substrate. Our findings suggest that the stability and work functions of molecular adsorbates could be rendered insensitive to their adsorption structures, which could be a useful property for (opto)electronic applications.

On adatomic-configuration-mediated correlation between electrotransport and electrochemical properties of graphene

The electron-transport properties of adatom–graphene system are investigated for different spatial configurations of adsorbed atoms: when they are randomly-, correlatively-, or orderly-distributed over different types of high symmetry sites with various adsorption heights. The results are obtained numerically using the quantum-mechanical Kubo–Greenwood formalism. A band gap may be opened only if ordered adatoms act as substitutional atoms, while there is no band gap opening for adatoms acting as interstitial atoms. The type of adsorption sites strongly affect the conductivity for random and correlated adatoms, but practically does not change the conductivity when they form ordered superstructures with equal periods. Depending on electron density and type of adsorption sites, the conductivity for correlated and ordered adatoms is found to be enhanced in dozens of times as compared to the cases of their random positions. The correlation and ordering effects manifest weaker or stronger depending on whether adatoms act as substitutional or interstitial atoms. The conductivity approximately linearly scales with adsorption height of random or correlated adatoms, but remains practically unchanged with adequate varying of elevation of ordered adatoms. Correlations between electron transport properties and heterogeneous electron transfer kinetics through potassium-doped graphene and electrolyte interface are investigated as well.

In Russian

The ethylene complexes with the copper atom, copper cluster Cu 2 , and the Cu(100) surface have been studied using density functional theory. It has been found that ethylene possesses three main type of bonding, namely, mono- π , mono- σ and di- σ . Ethylene does form the mono -π -bonded complex with the copper surface. Thus, the adsorption height (h ads ) and energy (E ads ) equal to 2.234 A and 0.47 eV, respectively. When reacting with the copper atom, ethylene forms the 2 A' complex corresponding to the singlet state ethylene. For the triplet state ethylene complex geometry optimization, a computational procedure has been developed, which includes the frontier orbitals mixing. The obtained stable 2 B 2 complex is lying higher than the 2 A' state complex by 0.47 eV. The energy difference has been ascribed to the singlet-triplet excitation of ethylene interacting with the copper species, while the 2 B 2 complex can be achievable upon the thermal activation.

Switching Molecular Kondo Effect via Supramolecular Interaction.

We apply supramolecular assembly to control the adsorption configuration of Co-porphyrin molecules on Au(111) and Cu(111) surfaces. By means of cryogenic scanning tunneling microscopy, we reveal that the Kondo effect associated with the Co center is absent or present in different supramolecular systems. We perform first-principles calculations to obtain spin-polarized electronic structures and compute the Kondo temperatures using the Anderson impurity model. The switching behavior is traced to varied molecular adsorption heights in different supramolecular structures. These findings unravel that a competition between intermolecular interactions and molecule-substrate interactions subtly regulates the molecular Kondo effect in supramolecular systems.

Quantitative Prediction of Molecular Adsorption: Structure and Binding of Benzene on Coinage Metals.

Interfaces between organic molecules and solid surfaces play a prominent role in heterogeneous catalysis, molecular sensors and switches, light-emitting diodes, and photovoltaics. The properties and the ensuing function of such hybrid interfaces often depend exponentially on molecular adsorption heights and binding strengths, calling for well-established benchmarks of these two quantities. Here we present systematic measurements that enable us to quantify the interaction of benzene with the Ag(111) coinage metal substrate with unprecedented accuracy (0.02Å in the vertical adsorption height and 0.05eV in the binding strength) by means of normal-incidence x-ray standing waves and temperature-programed desorption techniques. Based on these accurate experimental benchmarks for a prototypical molecule-solid interface, we demonstrate that recently developed first-principles calculations that explicitly account for the nonlocality of electronic exchange and correlation effects are able to determine the structure and stability of benzene on the Ag(111) surface within experimental error bars. Remarkably, such precise experiments and calculations demonstrate that despite different electronic properties of copper, silver, and gold, the binding strength of benzene is equal on the (111) surface of these three coinage metals. Our results suggest the existence of universal binding energy trends for aromatic molecules on surfaces.

Approaching truly freestanding graphene: the structure of hydrogen-intercalated graphene on 6H-SiC(0001).

We measure the adsorption height of hydrogen-intercalated quasifreestanding monolayer graphene on the (0001) face of 6H silicon carbide by the normal incidence x-ray standing wave technique. A density functional calculation for the full (6√3×6√3)-R30° unit cell, based on a van der Waals corrected exchange correlation functional, finds a purely physisorptive adsorption height in excellent agreement with experiments, a very low buckling of the graphene layer, a very homogeneous electron density at the interface, and the lowest known adsorption energy per atom for graphene on any substrate. A structural comparison to other graphenes suggests that hydrogen-intercalated graphene on 6H-SiC(0001) approaches ideal graphene.

Nanoscale Fluid Structure of Liquid-solid-vapour Contact Lines for a Wide Range of Contact Angles

We study the nanoscale behaviour of the density of a simple fluid in the vicinity of an equilibrium contact line for a wide range of Young contact angles θY ∈ [ 40°,135° ]. Cuts of the density profile at various positions along the contact line are presented, unravelling the apparent step-wise increase of the film height profile observed in contour plots of the density. The density profile is employed to compute the normal pressure acting on the substrate along the contact line. We observe that for the full range of contact angles, the maximal normal pressure cannot solely be predicted by the curvature of the adsorption film height, but is instead softened – likely by the width of the liquid-vapour interface. Somewhat surprisingly however, the adsorption film height profile can be predicted to a very good accuracy by the Derjaguin-Frumkin disjoining pressure obtained from planar computations, as was first shown in [Nold et al., Phys. Fluids, 26, 072001, 2014] for contact angles θY< 90°, a result which here we show to be valid for the full range of contact angles. This suggests that while two-dimensional effects cannot be neglected for the computation of the normal pressure distribution along the substrate, one-dimensional planar computations of the Derjaguin-Frumkin disjoining pressure are sufficient to accurately predict the adsorption height profile.

Molecular Exchange in a Heteromolecular PTCDA/CuPc Bilayer Film on Ag(111)

The future success of organic semiconductors in electronic or spintronic devices depends crucially on the ability to control the properties of molecular thin films. Metal contacts as well as interfaces formed by different organic materials are of equal importance in this context. A model system contributing to the improvement of the fundamental understanding of such interfaces is the heteromolecular bilayer film formed by 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) grown on a well ordered CuPc monolayer on Ag(111). Using complementary experimental techniques, we are able to reveal a molecular exchange across this heteromolecular interface. At the initial stage of the PTCDA deposition, some of these molecules diffuse into the CuPc layer and displace CuPc molecules to the second layer. This inhibits the formation of a smooth interface between both species and results in a structurally disordered heteromolecular CuPc-PTCDA film in the first and randomly arranged CuPc molecules as well as ordered PTCDA islands in the second layer. While the second organic layer is electronically decoupled from the underlying layer, the first layer, although disordered, shows a charge reorganization and an adsorption height alignment of CuPc and PTCDA as it is known for highly ordered heteromolecular monolayer structures on Ag(111). The molecular exchange, which we consistently find in all our experimental data, is the result of a lower adsorption energy gain of PTCDA on Ag(111) compared to CuPc on Ag(111).

Adsorption of large hydrocarbons on coinage metals: a van der Waals density functional study.

The adsorption of organic molecules onto the close-packed facets of coinage metals is studied, and how accurately adsorption heights can be described by using recent advances of the van der Waals density functional (vdWDF), with optPBE/vdWDF, optB86b/vdWDF, vdWDF2, and rev/vdWDF2 functionals is illustrated. The adsorption of two prototypical aromatic hydrocarbons is investigated, and the calculated adsorption heights are compared to experimental literature values from normal incident X-ray standing wave absorption and a state-of-the-art semi-empirical method. It is shown that both the optB86b/vdWDF and rev/vdWDF2 functionals describe adsorption heights with an accuracy of 0.1 Å, compared to experimental values, and are concluded as reliable methods of choice for related systems.

Modeling adsorption and reactions of organic molecules at metal surfaces.

ConspectusThe understanding of adsorption and reactions of (large) organic molecules at metal surfaces plays an increasingly important role in modern surface science and technology. Such hybrid inorganic/organic systems (HIOS) are relevant for many applications in catalysis, light-emitting diodes, single-molecule junctions, molecular sensors and switches, and photovoltaics. Obviously, the predictive modeling and understanding of the structure and stability of such hybrid systems is an essential prerequisite for tuning their electronic properties and functions. At present, density-functional theory (DFT) is the most promising approach to study the structure, stability, and electronic properties of complex systems, because it can be applied to both molecules and solids comprising thousands of atoms. However, state-of-the-art approximations to DFT do not provide a consistent and reliable description for HIOS, which is largely due to two issues: (i) the self-interaction of the electrons with themselves arising from the Hartree term of the total energy that is not fully compensated in approximate exchange-correlation functionals, and (ii) the lack of long-range part of the ubiquitous van der Waals (vdW) interactions. The self-interaction errors sometimes lead to incorrect description of charge transfer and electronic level alignment in HIOS, although for molecules adsorbed on metals these effects will often cancel out in total energy differences. Regarding vdW interactions, several promising vdW-inclusive DFT-based methods have been recently demonstrated to yield remarkable accuracy for intermolecular interactions in the gas phase. However, the majority of these approaches neglect the nonlocal collective electron response in the vdW energy tail, an effect that is particularly strong in condensed phases and at interfaces between different materials.Here we show that the recently developed DFT+vdWsurf method that accurately accounts for the collective electronic response effects enables reliable modeling of structure and stability for a broad class of organic molecules adsorbed on metal surfaces. This method was demonstrated to achieve quantitative accuracy for aromatic hydrocarbons (benzene, naphthalene, anthracene, and diindenoperylene), C60, and sulfur/oxygen-containing molecules (thiophene, NTCDA, and PTCDA) on close-packed and stepped metal surfaces, leading to an overall accuracy of 0.1 Å in adsorption heights and 0.1 eV in binding energies with respect to state-of-the-art experiments. An unexpected finding is that vdW interactions contribute more to the binding of strongly bound molecules on transition-metal surfaces than for molecules physisorbed on coinage metals. The accurate inclusion of vdW interactions also significantly improves tilting angles and adsorption heights for all the studied molecules, and can qualitatively change the potential-energy surface for adsorbed molecules with flexible functional groups. Activation barriers for molecular switches and reaction precursors are modified as well.

Adsorption height alignment at heteromolecular hybrid interfaces

The formation of metalorganic hybrid interfaces is determined by the fine balance between molecule-substrate and molecule-molecule interactions at the interface. Here, we report on a systematic investigation of interfaces between a metal surface and organic monolayer films that consist of two different molecular species, one donor and one acceptor of electronic charge. Our x-ray standing wave data show that in heteromolecular structures, the molecules tend to align themselves to an adsorption height between those observed in the respective homomolecular structures. We attribute this alignment effect to a substrate-mediated charge transfer between the molecules, which causes a mutual enhancement of their respective donor and acceptor characters. We argue that this effect is of general validity for $\ensuremath{\pi}$-conjugated molecules adsorbing on noble metal surfaces.

Publisher’s Note: Self-Assembly of Bicomponent Molecular Monolayers: Adsorption Height Changes and Their Consequences [Phys. Rev. Lett. 112, 117602 (2014)

Received 1 April 2014DOI:https://doi.org/10.1103/PhysRevLett.112.149901© 2014 American Physical Society

Adsorption Characteristics of Thiophene on Cu and Ni(100): Role of Van Der Waals

We apply density functional theory, with and without the inclusion of self-consistent van der Waals (vdWs) interactions (optB86, optB88, optPBE, revPBE, rPW86), to study the adsorption of thiophene (C4H4S) on Cu(100) and Ni(100). Our calculations reveal that the C4H4S molecule adsorbs, on either substrate, with its molecular plane parallel to the surface with the sulfur close to the bridge site.The inclusion of vdWs interactions results in a significant increase in the binding energy of thiophene on Cu(100) (from 0.12 eV to up to 0.77 eV), while the adsorption height is also modified from 3.2 A down to, at most, 2.38 A, depending on the functional used. The Ni(100) case presents a similar behavior for the binding energy (enhancement from 1.56 eV to up to 2.34 eV), but the adsorption heights increase from 2.12 Å up to 2.32 Å.In addition to adsorption geometry and energetics, we present the results and analysis of the electronic properties (charge transfer, changes in the d-band of the substrate, and change in the work function) of these two systems to complement our understanding of the molecule-substrate bonding.Our results suggest that the adsorption characteristics are dependent on the type of functional used; opt-type functionals (optB86, optB88, optPBE) are found to produce stronger bonding as compared to PBE, revPBE and rPW86.This work is funded by the U.S. Department of Energy Basic Energy Science under Contract No DE-FG02-11ER16243

An Extensive Computational Study of the Adsorption of Thiophene on Transition Metal Surfaces: Role of Van Der Waals

Van der Waals (vdWs) interactions play a significant role in the determination of the adsorption characteristics at the interface between a molecule and a substrate. In this study, self-consistent inclusion of vdW interactions in density functional theory provides a good perspective to understand the interaction between organic adsorbates and inorganic interfaces. We present the results of adsorption of thiophene (C4H4S) on various transition metal surfaces with the goal of comparing the performance of five different vdW functionals (optB86, optB88, optPBE, revPBE, rPW86). Seven metallic substrates (100) are used for our study; three coinage metals (Au, Ag, Cu) and four reactive metals (Pt, Pd, Rh, Ni). The results show that vdWs inclusion enhances the interaction for Ag ( 0.08 to 0.73 eV), Au (0.14 to 0.86 eV), Cu (0.12 eV to 0.77 eV), Ni(1.56 to 2.34 eV), Pt (1.6 to 2.51 eV), Pd (1.67 to 2.54), Rh (1.74 to 2.96 eV). In addition, we performed calculations for adsorption heights along with analysis of the electronic changes (charge transfer, changes in the d-band of the substrate, and change in the work function) to complement our understanding of these systems. This work is funded by the U.S. Department of Energy Basic Energy Science under Contract No DE-FG02-11ER16243

Self-Assembly of Bicomponent Molecular Monolayers: Adsorption Height Changes and Their Consequences

Codeposition of two molecular species [copper phtalocyanine (CuPc, donor) and perfluoropentacene (PFP, acceptor)] on noble metal (111) surfaces leads to the self-assembly of an ordered mixed layer with a maximized donor-acceptor contact area. The main driving force behind this arrangement is assumed to be the intermolecular C-H ⋯ F hydrogen-bond interactions. Such interactions would be maximized for a coplanar molecular arrangement. However, precise measurement of molecule-substrate distances in the molecular mixture reveals significantly larger adsorption heights for PFP than for CuPc. Most surprisingly, instead of leveling to increase hydrogen-bond interactions, the height difference is enhanced in the blends as compared to the heights found in single-component CuPc and PFP layers. The increased height of PFP in mixed layers points to an overall reduced interaction with the underlying substrate, and its influence on electronic properties like the interface dipole is investigated through work function measurements.

Formation of metal–organic interface states studied with 2PPE

Experimental and theoretical investigations on the formation mechanism of interface-specific electronic states at metal–organic interfaces are reviewed. Model systems discussed are thin films of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) on the silver single crystal surfaces Ag(111) and Ag(100). Time and angle-resolved two-photon photoemission (2PPE) reveals the occurrence of unoccupied electronic states that have no counterpart in the bulk metal or the organic overlayer. The interface states resemble Shockley-surface states of clean metals. They are strongly dispersive and show an ultra-short lifetime in the range of tens to hundred femtoseconds. The energy position of the interface states is to a large extent determined by the adsorption height of the molecules and their local surface density while the wave function experiences a strong influence from the adsorption geometry of the molecules.

Electrostatic Interaction and Commensurate Registry at the Heteromolecular F16CuPc–CuPc Interface

Tailoring the properties of molecular thin films and interfaces will have decisive influence on the success of future organic electronic devices. This is equally true for metal–organic and hetero–organic contacts as they occur, for example, in donor–acceptor systems. Here, we report on the structure formation and interaction across such a heteromolecular interface. It is formed by monolayers of F16CuPc and CuPc stacked on a Ag(111) surface. We investigated the lateral and vertical structure using spot-profile analysis low energy electron diffraction and normal incidence X-ray standing waves, and performed pair potential calculations to understand the driving forces for the structure formation. Most surprisingly, for one phase we found a commensurate registry between the two organic layers, usually a sign for a strong (chemisorptive) interaction often involving metallic states of the surface. However, because the organic bilayer is not commensurate with the underlying Ag substrate in our case, the dominating factor must be the intermolecular interaction. Pair potential calculations suggest a site-specific adsorption that leads to a commensurate registry at the heteromolecular interface. The adsorbate system was further characterized by measuring adsorption heights, indicating flat-lying molecules and a CuPc–F16CuPc layer spacing of 3.06 Å.