Commensurate Monolayer Research Articles

Formation and structural characterization of two-dimensional wetting water layer on graphite (0001).

Understanding the structure and wettability of monolayer water is essential for revealing the mechanisms of nucleation, growth, and chemical reactivity at interfaces. We have investigated the wetting layer formation of water (ice) on the graphite (0001) surface using a combination of low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). At around monolayer coverages, the LEED pattern showed a (2 × 2) periodicity and STM revealed a hydrogen-bonded hexagonal network. The lattice constant was about 9% larger than that for ice Ih/Ic crystals, and the packing density was 0.096 Å-2. These results indicate that an extended ice network is formed on graphite, different from that on metal surfaces. Graphite is hydrophobic under ambient conditions due to the airborne contaminant but is considered inherently hydrophilic for a clean surface. In this study, the hydrophilic nature of the clean surface has been investigated from a molecular viewpoint. The formation of a well-ordered commensurate monolayer supports that the interaction of water with graphite is not negligible so that a commensurate wetting layer is formed at the weak substrate-molecule interaction limit.

Direct-space-self-consistent-phonon treatment of monolayer structures and dynamics.

Computations, which would have been intractable just a few years ago, are now possible on desktop workstations. Such is the case for the application of the Self-Consistent-Phonon (SCP) approximation to large monolayer clusters on structured surfaces, combining a SCP approach to the system dynamics with a random walk approach to finding the optimum positions of the adsorbed atoms. This combination of techniques enables the investigation of the stability, structure, and dynamics of incommensurate adsorbed monolayers at low temperatures. We refer to this approach as the Direct-Space-Self-Consistent-Phonon framework. We present the application of this framework to the study of rare-gas and molecular hydrogen adsorbates on the graphite basal-plane surface and (for xenon) the Pt(111) surface. The largest cluster size consists of 4096 particles, a system that is large enough to examine incommensurate phases without significant adverse boundary effects. The existence of "pseudo-gaps" in the phonon spectrum of nearly commensurate monolayers is demonstrated, and the implication of such "pseudo-gaps" for the determination of the location of any commensurate ↔ incommensurate phase transition is explored. The stability of striped incommensurate structures vs hexagonal incommensurate structures is examined. The inherent difficulties of using this approach for the highly quantum monolayer solids is shown to generate some particular problems. Nevertheless, we demonstrate that this approach to the stability, structure, and dynamics of quantum monolayer solids is a very useful tool in the theorist's arsenal. By implication, this approach should also be useful in the study of adsorption on graphene and carbon nanotubes at low temperatures.

A simulation study of the low temperature phase diagram of the methane monolayer on graphite: a test of potential energy functions.

We have used molecular simulation with two intermolecular potential models, TraPPE-UA and TraPPE-EH, the latter of which accounts for the tetrahedral shape, to study the effects of shape on methane adsorption on graphite. Both models give good descriptions of the vapour-liquid equilibria in the bulk phase, but adsorption on graphite is better described by the TraPPE-EH model. Molecular configurations in the monolayer, show the variation with temperature of the registry sites for the carbon and hydrogen atoms of the methane molecules. At temperatures below 70 K, the centre of mass (COM) of the molecules is in registry with the centre of the carbon hexagons. For temperatures above 70 K, a commensurate monolayer is initially formed as at low temperatures, then as the loading is increased the first layer remains in registry, but the COM of the methane molecules in the first layer shifts to the top of the graphite carbon atoms with the C-H bond pointing to carbon atoms in the second shell of a C-hexagon. At temperatures above 93 K, the first adsorbate layer goes through these two commensurate states and then undergoes a transition to an incommensurate solid. Finally, for temperatures greater than 110 K methane behaves like a pseudo spherical molecule.

SAMPLE: Surface structure search enabled by coarse graining and statistical learning

In this publication we introduce SAMPLE, a structure search approach for commensurate organic monolayers on inorganic substrates. Such monolayers often show rich polymorphism with diverse molecular arrangements in differently shaped unit cells. Determining the different commensurate polymorphs from first principles poses a major challenge due to the large number of possible molecular arrangements. To meet this challenge, SAMPLE employs coarse-grained modeling in combination with Bayesian linear regression to efficiently map the minima of the potential energy surface. In addition, it uses ab initio thermodynamics to generate phase diagrams. Using the example of naphthalene on Cu(111), we comprehensively explain the SAMPLE approach and demonstrate its capabilities by comparing the predicted with the experimentally observed polymorphs.

Self-Assembly of Di(pyrazol-1-yl)pyridine-benzoic Acid on Underpotentially Deposited Ag from Solution.

Monolayers of 4-(2,6-di(1 H-pyrazol-1-yl)pyridine-4-yl)benzoic acid (DPP-BA) on Au substrates modified by an underpotential-deposited bilayer of Ag were studied by X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy, and scanning tunneling microscopy. Highly crystalline layers are formed with molecules coordinatively bonding to the surface through the carboxylate moiety in a bidentate configuration. The molecules assemble to rows characterized by densely packed upright-orientated DPP units occupying an area of 41 Å2. The DPP units adopt a trans-trans conformation as inferred from the N 1s XPS spectra taken at different photon energies. Their alignment alternates between adjacent rows, giving rise to a herringbone pattern. The pronouncedly different structure of the weakly commensurate self-assembled monolayer (SAM) of DPP-BA on Ag compared to the commensurate SAM of a DPP thiol on Au is a manifestation of the shift from substrate-directed assembly in the latter case to assembly dominated by intermolecular interactions in the former case.

Effect of solid-like nitrogen contact layers on graphite: anisotropy of tangential pressure and orientational order

Molecular simulation has provided a significant progress in understanding behavior of two-dimensional molecular layers of various gases deposited on a solid surface. Substantial efforts have been made to describe the state of noble gases, nitrogen, methane, carbon dioxide etc. on the graphite surface. As a result, mechanism of solidification and melting of molecular layers, formation of structures commensurate with substrate, orientational ordering of non-spherical molecules and other distinctive features of 2D layers are mainly clear for at least 2 decades. Some quantitative refinements in modeling of such systems and adjusting parameters basing on experimental data are still continuing, but the activity in fundamental study in this area has been weakened. In part, this is due to difficulties in reliable evaluating of thermodynamic functions of 2D systems, especially in the crystalline form, which hinders exact evaluation of the 2D liquid–solid transition parameters. Recently new possibilities were demonstrated in the framework of a methodology based on kinetic Monte Carlo and explicit accounting for the 2D gas–solid and gas–liquid interface with an adjustable external potential imposed on the gas phase (Ustinov and Do in J Colloid Interface Sci 366:216–223, 2012; Ustinov in J Chem Phys 140: 074706, 2014a, in J Chem Phys 141:134706, 2014b, in J Chem Phys 142: 074701, 2015). This study aims at an extension of the approach to adsorption of nitrogen on the graphite surface. A special attention is paid to effect of commensurate nitrogen monolayer on graphite, manifesting itself as the appearance of a significant tangential pressure directly in the graphite layer. This pressure has to be accounted for in all thermodynamic equations such as the Gibbs–Duhem equation and involved into simulation schemes realizing the NPT ensemble. In the case of nitrogen at low temperatures the herringbone structure of the crystalline layer has proved to result in a significant anisotropy of that pressure. The developed approach can be extended to 3D systems and various fluids confined in nanoporous materials.

Frictional transition from superlubric islands to pinned monolayers.

The inertial sliding of physisorbed submonolayer islands on crystal surfaces contains unexpected information on the exceptionally smooth sliding state associated with incommensurate superlubricity and on the mechanisms of its disappearance. Here, in a joint quartz crystal microbalance and molecular dynamics simulation case study of Xe on Cu(111), we show how superlubricity emerges in the large size limit of naturally incommensurate Xe islands. As coverage approaches a full monolayer, theory also predicts an abrupt adhesion-driven two-dimensional density compression on the order of several per cent, implying a hysteretic jump from superlubric free islands to a pressurized commensurate immobile monolayer. This scenario is fully supported by the quartz crystal microbalance data, which show remarkably large slip times with increasing submonolayer coverage, signalling superlubricity, followed by a dramatic drop to zero for the dense commensurate monolayer. Careful analysis of this variety of island sliding phenomena will be essential in future applications of friction at crystal/adsorbate interfaces.

Origin of the moiré pattern in thin Bi films deposited on HOPG

Thin Bi(110) films deposited on highly oriented pyrolytic graphite (HOPG) exhibit a pronounced moir\'e pattern; here the origin of the moir\'e pattern is investigated using scanning tunneling microscopy (STM) and spectroscopy (STS), high-resolution transmission electron microscopy (HR-TEM), and density functional theory (DFT). It is shown that the moir\'e pattern forms only on islands which are misoriented by $\ensuremath{\sim}{30}^{\ensuremath{\circ}}$ with respect to the usual substrate symmetry direction. Two models of the moir\'e pattern are presented: (i) a commensurate monolayer construction (CMC) for rectangular overlayer symmetry on hexagonal substrates and (ii) a qualitative model based on simple superposition of a Bi overlayer on graphene. The CMC model has previously been applied only to systems with pure hexagonal symmetry. Both models generate moir\'e patterns with key parameters (period, angles of the pattern measured with respect to the main HOPG and Bi crystal directions) that are consistent with the experimental results, but development of a fully predictive/quantitative model remains an outstanding challenge. The electronic structure of the moir\'e pattern is investigated using STS and DFT, and it is found that the local density of states (LDOS) is modulated by the moir\'e pattern. These results are consistent with a picture in which a small distortion of Bi atomic positions at the film-substrate interface results in periodic modulation of the LDOS, hence allowing observation of the moir\'e pattern in STM images.

Spin structure of Rashba-split electronic states of Bi overlayers on Cu(1 1 1)

We investigate the unoccupied Rashba-type spin-orbit split band structure of the commensurate and incommensurate Bi monolayer on Cu(111) with spin- and angle-resolved two-photon-photoemission spectroscopy. Because of the unique geometrical structure of these Bi monolayers on Cu(111), it can be expected that both in-plane and out-of-plane potential gradients play an important role for the Rashba-type spin-structure in these systems. Our spin-resolved data of spin-split states in Bi/Cu(111) confirm the expected Rashba behavior of the in-plane spin-components that is caused by the out-of-plane potential gradient. But in addition, we indeed find out-of-plane spin components with different magnitudes in both monolayer Bi/Cu(111) systems, which we therefore attribute to the structurally induced in-plane potential gradients.

Characterization of early growth stages of Pb/Ge(001)

Early stages of thin Pb film growth on Ge(001) substrate, exhibiting quantum size effects (QSE), are characterized by means of Photoelectron Diffraction and Helium Atom Scattering. Pb is found to form a commensurate first monolayer, while an ordered layer-by-layer growth only sets in after deposition of 4 monolayers. In the intermediate coverage range no long range order of the overlayer is established and we find that uncorrelated islands of preferred four-layer thickness are formed. Continuous Pb film with long range order emerges through islands coalescence close to a coverage of 4 monolayers, upon which a more regular layer-by-layer growth mode sets in.

Reverse Self-Assembly: (111)-Oriented Gold Crystallization at Alkylthiol Monolayer Templates

It has long been known that thiol-terminated molecules self-assemble as commensurate monolayers on Au(111) surfaces. By spreading floating octadecanethiol monolayers on aqueous solutions of chloroauric acid (HAuCl4) and using x rays to reduce the gold ions as well as to probe the structure, we have observed the nucleation of (111)-oriented Au nanoparticles at thiol surfaces. This process may be similar to the formation of biogenic gold by bacteria. The thiol monolayer acts as a "soft template," changing its structure as Au crystals form so that there is a sqrt[3]×sqrt[3] commensurate relationship.

Properties of a Suggested Commensurate Monolayer Solid of Kr/NaCl(001)

It is shown that a commensurate square monolayer solid of Kr/NaCl(001) can be stabilized with a model incorporating a rather large energy corrugation amplitude. Then a square bilayer is formed under a compression and preempts the formation of an incommensurate triangular monolayer lattice. The lattice dynamics of the commensurate monolayer may be complex, because the modeling admits the possibility that it has a (2 × 2) unit cell with four Kr atoms.

Two commensurate hydrogen-bonded monolayer structures of melamine on Ag(111)

Melamine (1,3,5-triazine-2,4,6-triamine) was deposited on the Ag(111) surface under ultrahigh vacuum conditions. It forms two different monolayer structures, which were investigated by low energy electron diffraction and scanning tunneling microscopy. The α-phase is a honeycomb structure containing two molecules per unit-cell. The molecular orientation within the unit-cell is determined by six hydrogen bonds. The α-phase is kinetically preferred upon deposition at room-temperature and can be transferred to the thermodynamically more stable β-phase by annealing at 333 K. The β-phase has an oblique unit-cell containing four molecules and shows a higher surface density with additional hydrogen bonds between adjacent amino groups. Both structures are commensurate. While the structural motif of the α-phase has been observed before on Au(111) and Ag–Si(111) surfaces, the structure of the β-phase has been so far only theoretically predicted.

Growth and structure of pentacene films on graphite: Weak adhesion as a key for epitaxial film growth

The microstructure of pentacene films grown on the basal plane of graphite has been investigated. By combining various complementary techniques including scanning tunneling microscopy, atomic force microscopy, x-ray diffraction, thermal desorption spectroscopy, and x-ray absorption spectroscopy the molecular orientation, crystalline structure, and morphology of the films as well as their thermal stability have been characterized in detail as a function of the film thickness. Initial film growth leads to the formation of a commensurate monolayer consisting of flat-lying molecules while upon subsequent deposition epitaxially ordered (022)-oriented pentacene films are formed which adopt the Siegrist phase. The detailed analysis shows that this epitaxial growth of films with an essentially recumbent molecular orientation is brought about by a slight rotation of the molecules in the first layer around their long molecular axis upon deposition of overlying molecular layers. Such a structural modification is unusual and becomes possible by the rather weak adsorption energy on graphite. In contrast, a very different film structure including an upright orientation of molecules even in the first layer is found on nonperfect but rough graphite surfaces leading to the formation of (001)-oriented films which initially reveal the thin-film phase and continue to grow in the Campbell phase of pentacene.

Structural transformations of nitrogen adsorbed on graphite: Monte Carlo studies of spatial heterogeneity in multilayer system

We present numerical Monte Carlo studies of nitrogen multilayers adsorbed on the basal plane of graphite. The analysis is focused on the system spatial heterogeneity and its influence on structures and phase transitions. The simulations have been carried out for surface coverage from monolayer to four layers, in canonical ensemble, in the temperature range from 5 to 100 K. An intricate phase situation is observed due to the competition between intermolecular and N(2)-graphite interactions. The commensurate monolayer is stabilized by the graphite corrugation. The multilayer commensurate structure is only metastable at low temperatures. Its stable structure is triangular, 1.08 times denser than the commensurate one stabilized by the N(2)-N(2) interactions. The multilayer structure is strongly spatially nonuniform, the individual layer structure changes from herringbone in the first layer to pinwheel arrangement in the fourth one. Two structural phase transitions, orientational order-disorder, and melting, are observed in each layer. Their mechanism and transition temperatures show strong variations depending on the position of the layer and the number of layers in the system.

Stability analysis of the commensurate monolayer solid of xenon/graphite

A stability analysis based on model calculations of the grand potential finds that the transition from hexagonal incommensurate to commensurate monolayer solid of xenon/graphite is continuous with increasing pressure, in agreement with experimental observations. An atomic-scale interaction model gives an internally consistent account of the thermal expansion of the solid at the two-dimensional sublimation curve and of the chemical potential increase for isothermal compression from monolayer condensation to the commensurate solid. An estimate is given for the corrugation energy of xenon/graphite.

Electrocrystallization of Bi on Au(111) in an acidic chloroaluminate ionic liquid

Abstract The electrodeposition of Bi on Au(1 1 1) in an acidic chloroaluminate ionic liquid has been investigated by cyclic voltammetry and in situ STM. Two different UPD processes of Bi have been observed at 1.2 and 0.9 V versus Al/Al(III). Multiple domain structures are found for the first UPD, whereas a uniaxially commensurate monolayer is formed for the second one. More interestingly, high-resolution STM images reveal some meta-stable intermediate superstructure reported here for the first time. The co-adsorption of AlCl 4 − and Bi adatoms is clearly demonstrated. In the OPD region, different morphologies of Bi crystals have been obtained: a needle-like structure when the potential is swept continuously to 0.6 V, and ultrathin films of Bi when the potential is jumped from 1.3 to 0.4 V.

Commensurate monolayers on surfaces: Geometry and ground states

We present a procedure for the generation of all rigid commensurate monolayer-surface structures of a given symmetry, up to a certain number of adsorbate particles, ${N}_{\mathit{ads}}$, in the unit cell. It is shown that the minimum energy structures in each unit cell are related to a well-defined sequence of Fourier terms of the single-particle--surface potential. This fact allows the prediction of stable commensurate monolayer-surface structures with the only knowledge of Fourier coefficients of the atom-surface potential. The impact of the presented theory for theoretical and experimental determination of atomic and molecular monolayer ground states, as well as its extension to higher dimensions (i.e., for intercalated crystals), is discussed.

Geometric and electronic structure of commensurate4Ar∕Ag(111)−(7×7)R19.1°by density functional theory

We report the optimization of a $4\mathrm{Ar}∕\mathrm{Ag}(111)\text{\ensuremath{-}}(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.1\ifmmode^\circ\else\textdegree\fi{}$ monolayer using first-principles density functional theory. The adsorption energies resulting from our ground-state minimizations are similar for all three anchor sites. A commensurate adsorbate cell is comprised of four argon atoms which settle into three different configurations. We refer to these sites as top, threefold hcp hollow, and threefold fcc hollow sites. Each of these structures has one Ar atom located at one of the base sites of the Ag(111) surface. The remaining three Ar atoms are found near the bridge sites of the silver surface. In both of the hollow site structures, the bridge locations are measurably off of the exact geometric bridge positions while maintaining the $\sqrt{7}R19.1\ifmmode^\circ\else\textdegree\fi{}$ unit cell. Our results show that both the hcp and fcc hollow sites have slightly lower adsorption energies than the top site, by only 1.5 and $4.5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, respectively (per cell of four Ar atoms). The vertical harmonic vibration frequencies were computed for the top, hcp, and fcc hollow sites. They measured 5.22, 5.01, and $5.21\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, respectively. We studied the possible cause for this high-order commensurate monolayer. We have concluded that neither steric repulsion nor hybridization of the electrons of the Ar atoms is the reason for the commensurability in our model. We propose that the causal mechanism is the Novaco-McTague [Phys. Rev. Lett. 38, 1286 (1977)] theory for orientational epitaxy.

Energetics and dynamics of unoccupied electronic states at theh−BN∕Ni(111)interface

Adsorbed on the nickel 111 surface, hexagonal boron nitride h-BN forms a commensurate monolayer, which retains much of the band structure of layered bulk h-BN, specifically the wide band gap. This article explores its unoccupied band structure with femtosecond time-resolved two-photon photoelectron spectroscopy in a two-color setup, and presents spectra, dispersion, and cross-correlation measurements. Two distinct, dispersive intermediate states are observed at the origin of the surface Brillouin zone : a -related interface state at 1.51 eV above the Fermi level, which forms the conduction band, and an image-potential state at 0.65 eV below the vacuum level. With 260 fs for the image state and 110 fs for the interface state the lifetimes of the two unoccupied states are remarkably high due to the very small overlap of the corresponding wave functions with the nickel bulk states.