First-principles calculations using density functional theory and two methods in comparison, Quantum ESPRESSO and Siesta, are done on large supercells which describe different placements of two identical adsorbed alkali metal atoms (of either Na, or K species) on the monolayer of boron carbide BC3. The energy of single-atom adsorption over the center of C6 ring, of the C4B2 hexagon and over a boron atom have been preliminarily estimated, the effect of applying the Grimme D2 correction on the adsorption characteristics evaluated, and the comparison of these results with available data discussed. The interaction of two identical Na or K atoms adsorbed at close enough distances (less than 10 Angstrom) is negligible if the adsorption occurs at the opposite sides of the BC3 layer, but creates a steep repulsive potential at distances less than 8 Angstrom if both atoms are adsorbed on the same side of the monolayer. Relaxation patterns resulting from the two K atoms being trapped at adjacent adsorption sites in the lattice are explained. The results suggest that the density of adsorbed K atoms on BC3 can be interestingly high.
We overview recent results on intrinsic frictional properties of adsorbed monolayers, composed of mobile hard-core particles undergoing continuous exchanges with a vapor phase. In terms of a dynamical master equation approach we determine the velocity of a biased impure molecule - the tracer particle (TP), constrained to move inside the adsorbed monolayer probing its frictional properties, define the frictional forces exerted by the monolayer on the TP, as well as the particles density distribution in the monolayer.
In quartz crystal microbalance (QCM) studies of the friction between an adsorbed monolayer film and a metallic substrate, the films are observed to slide relative to the substrate under inertial forces of order $10^{-14}dyn$ per film atom, a force much smaller than all theoretical estimates of the force that surface defects are capable of exerting. In this letter we propose, in order to resolve this issue, that if the defect potentials have a range of greater than an atomic spacing, the net force on a relatively stiff film due to the defects is likely to be extremely small. Line defects (e.g.,…
We study the response of an adsorbed monolayer under a driving force as a model of sliding friction phenomena between two crystalline surfaces with a boundary lubrication layer. Using Langevin-dynamics simulation, we determine the nonlinear response in the direction transverse to a high symmetry direction along which the layer is already sliding. We find that below a finite transition temperature, there exist a critical depinning force and hysteresis effects in the transverse response in the dynamical state when the adlayer is sliding smoothly along the longitudinal direction.
We comment and discuss the findings and conclusions of a recent theoretical study of the diffraction of He atoms from a monolayer of Xe atoms adsorbed on the graphite (0001) surface [Khokonov et al., Surf. Sci. 496(2002)L13]. By revisiting the problem we demonstrate that all main conclusions of Khokonov et al. that pertain to the studied system are at variance with the available experimental and theoretical evidence and the results of multiple scattering calculations presented in this comment.
Alkali metal atoms (Li, Na, K, Rb, Cs), dimers and (2$\times$2) monolayers on a graphite (0001) surface have been studied using density functional theory, pseudopotentials, and a periodic substrate. The adatoms bind at the hollow site (graphite hexagon), with Li lying closest to (1.84 Å) and Cs farthest (3.75 Å) from the surface. The adsorption energies range between $0.55-1.21$ eV, and the energy ordering of the alkali adatoms is Li$>$Cs$\ge$Rb$\ge$K$>$Na. The small diffusion barriers (0.02-0.21 eV for the C-C bridge) decrease as the atom size increases, indicating a flat potential energy sur…
Using the first-principles and Monte Carlo methods, here we systematically study magnetic properties of monolayer octagonal-tetragonal phosphorus with 3d transition-metal (TM) adatoms. Different from the puckered hexagonal black phosphorus monolayer (phosphorene or $α$-P), the octagonal-tetragonal phase of 2D phosphorus (named as ot-P or $ε$-P in this article) is buckled with octagon-tetragon structure. Our calculations show that all TMs, except the closed-shell Zn atom, are able to strongly bind onto monolayer $ot$-P with significant binding energies. Local magnetic moments (up to 6 $μ$B) on …
Simulation evidence of an incipient triple point is reported for xenon submonolayers adsorbed on the (111) surface of platinum. This is in stark contrast to the "normal" triple point found in simulations and experiments for xenon on the basal plane surface of graphite. The motions of the atoms in the surface plane are treated with standard 2D "NVE" molecular dynamics simulations using modern interactions. The simulation evidence strongly suggests an incipient triple point in the 120-150 K range for adsorption on the Pt (111) surface while the adsorption on graphite shows a normal triple point …
Sc is the lightest transition metal that could help to achieve the goal of high theoretical capacity. Hence, we here explored the performance of ScS_{2} monolayer as a cathode material for alkali-ion batteries (Li, Na, K) and other multi-valent metal-ion batteries (Mg, Al). Previous studies on ScS_{2} have focused only on the fundamental electronic and magnetic properties of the ScS_{2} monolayer, but not on its possible applications. Our first-principles calculations show that 2D ScS_{2} is able to deliver a large theoretical capacity of 491.36 mAh g^{-1} for alkali-ions and 324.29 mAh g^{-1}…
Topological semimetals in ferromagnetic materials have attracted enormous attention due to the potential applications in spintronics. Using the first-principles density functional theory together with an effective lattice model, here we present a new family of topological semimetals with a fully spin-polarized nodal loop in alkaline-metal monochalcogenide $MX$ ($M$ = Li, Na, K, Rb, Cs; $X$ = S, Se, Te) monolayers. The half-metallic ferromagnetism can be established in $MX$ monolayers, in which one nodal loop formed by two crossing bands with the same spin components is found at the Fermi energ…
We present microscopic investigations on the two-dimensional arrangement of cobalt phthalocyanine molecules on a graphite (HOPG) substrate in the low coverage regime. The initial growth and ordering of molecular layers is revealed in high resolution scanning tunneling microscopy (STM). On low coverages single molecules orient mostly along one of the substrate lattice directions, while they form chains at slightly higher coverage. Structures with two different unit cells can be found from the first monolayer on. A theoretical model based on potential energy calculations is presented, which rela…
We investigated the possibility of superconductivity in monolayer hexagonal boron nitride (h-BN) doped using each group-1 (Li, Na, K) and group-2 (Be, Mg, Ca, Sr, Ba) atom via ab-initio calculations. Consequently, we reveal that Sr- and Ba-doped monolayer h-BN and Ca-doped monolayer h-BN with 3.5% tensile strain are energetically stable and become superconductors with Tc values of 5.83 K, 1.53 K, and 12.8 K, respectively, which are considerably higher than those of Ca-, Sr-, and Ba-doped graphene. In addition, the momentum-resolved electron-phonon coupling (EPC) constant shows that the scatter…
We have investigated the influence of the substrate on the fluorescence of adsorbed organic molecules. Monolayer films of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI), a supramolecular network formed from PTCDI and melamine, and perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) have been deposited on hexagonal boron nitride (hBN). The principal peaks in the fluorescence spectra of these films were red-shifted by up to 0.37 eV relative to published measurements for molecules in helium droplets. Smaller shifts (~0.03 eV) arising from interactions between neighbouring m…
We present the first large-scale molecular dynamics simulations of hexane on graphite that completely reproduces all experimental features of the melting transition. The canonical ensemble simulations required and used the most realistic model of the system: (i) fully atomistic representation of hexane; (ii) explicit site-by-site interaction with carbon atoms in graphite; (iii) CHARMM force field with carefully chosen adjustable parameters of non-bonded interaction; (iv) numerous $\ge$ 100 ns runs, requiring a total computation time of ca. 10 CPU-years. This has allowed us to determine correct…
Molecular dynamics simulations are used to study contact between a rigid, nonadhesive, spherical tip with radius of order 30nm and a flat elastic substrate covered with a fluid monolayer of adsorbed chain molecules. Previous studies of bare surfaces showed that the atomic scale deviations from a sphere that are present on any tip constructed from discrete atoms lead to significant deviations from continuum theory and dramatic variability in friction forces. Introducing an adsorbed monolayer leads to larger deviations from continuum theory, but decreases the variations between tips with differe…
The Self-Assembly of Nano-Objects (SANO) code we implemented demonstrates the ability to predict the molecular self-assembly of different structural motifs by tuning the molecular building blocks as well as the metallic substrate. It consists in a two-dimensional Grand Canonical Monte-Carlo (GCMC) approach developed to perform atomistic simulations of thousands of large organic molecules self-assembling on metal surfaces. Computing adsorption isotherms at room temperature and spanning over the characteristic sub-micrometric scales, we confront the robustness of the approach with three differen…
Ab initio calculations within the density-functional theory formalism are performed to investigate the ground state, electric charge doping, and electronic properties of titanium and titanium dioxide monolayers adsorbed on a graphene surface. A new ground state structrure of Ti monolayer adsorbed on graphene is reported which is shown to be stable up to T = 500 K. Effects due to lower and higher Ti adatoms coverage are studied. We find that the adsorbed Ti provides a strong n-type doping which supports recent experimental observations. On the other hand, TiO_2 can induce both p- and n-type dop…
The interplay between the torsional potential energy and the scaling of the 1-4 van der Waals and Coulomb interactions determines the stiffness of flexible molecules. In molecular simulations often ad-hoc values for the scaling factor (SF) are adopted without adequate justification. In this letter we demonstrate for the first time that the precise value of the SF has direct consequences on the critical properties and mechanisms of systems undergoing a phase transition. By analyzing the melting of n-alkanes (hexane C6, dodecane C12, tetracosane C24) on graphite, we show that the SF is not a uni…
Covalent substrates can give rise to a variety of magnetic interaction mechanisms among adsorbed transition metal atoms building atomic nanostructures. We show this by calculating the ground state magnetic configuration of monoatomic 3d chains deposited on a monolayer of Cu$_2$N grown on Cu(001) as a function of $d$ filling and of adsorption sites of the one dimensional nanostructures.
Density functional theory is used to systematically study the electronic and magnetic properties of doped MoS$_2$ monolayers, where the dopants are incorporated both via S/Mo substitution or as adsorbates. Among the possible substitutional dopants at the Mo site, Nb is identified as suitable p-type dopant, while Re is the donor with the lowest activation energy. When dopants are simply adsorbed on a monolayer we find that alkali metals shift the Fermi energy into the MoS$_2$ conduction band, making the system n-type. Finally, the adsorption of charged molecules is considered, mimicking an ioni…
We discuss Molecular Dynamics (MD) computer simulations of tetracosane (C24H50) monolayer physisorbed onto a graphite surface. The alkane molecules are simulated with explicit hydrogens, and the graphite substrate is represented as an all-atom struc-ture having six graphene layers. The tetracosane dynamics modeled in the fully ato-mistic manner agree well with experiment. The low-temperature ordered solid orga-nizes in rectangular centered structure, incommensurate with underlying graphite. Above T = 200 K, as the molecules start to lose their translational and orientational order via gauche d…
Path-integral Monte Carlo calculations have been performed to study the $^4$He adsorption on both sides of a single $α$-graphyne sheet. For investigation of the interlayer correlation between the upper and the lower monolayer of $^4$He adatoms, the $^4$He-substrate interaction is described by the sum of the $^4$He-C interatomic pair potentials, for which we use both Lennard-Jones and Yukawa-6 anisotropic potentials. When the lower $^4$He layer is a C$_{4/3}$ commensurate solid, the upper-layer $^4$He atoms are found to form a Kagomé lattice structure at a Mott insulating density of 0.0706 Å$^{…
Through the positive role of alkali halides in realizing large area growth of transition metal-di-chalcogenide layers has been validated, the film-growth kinematics has not yet been fully established. This work presents a systematic analysis of the MoS$_2$ morphology for films grown under various pre-treatment conditions of the substrate with sodium chloride (NaCl). At an optimum NaCl concentration, the domain size of the monolayer increased by almost two orders of magnitude compared to alkali-free growth of MoS$_2$. The results show an inverse relationship between fractal dimension and areal …
The possibility of modifying the work function of electrodes is important for optimizing the energy barriers for charge-injection (extraction) at the interface to an organic material. In this study, we perform density-functional-theory calculations to investigate the impact of dithiol-terminated polyethylene glycol (PEG(thiol)) based self-assembled monolayers (SAMs) with different numbers of PEG repeat units on the work function of the Au(111) surface. We find that a monolayer of PEG(thiol) decreases the work function of the Au(111) surface, where the magnitude of this reduction strongly depen…
Using first-principles calculations we show that the work function of noble metals can be decreased or increased by up to 2 eV upon the adsorption of self-assembled monolayers of organic molecules. We identify the contributions to these changes for several (fluorinated) thiolate molecules adsorbed on Ag(111), Au(111) and Pt(111) surfaces. The work function of the clean metal surfaces increases in this order, but adsorption of the monolayers reverses the order completely. Bonds between the thiolate molecules and the metal surfaces generate an interface dipole, whose size is a function of the me…
Possible high thermoelectric power factor in alkali-metal-intercalated BC$_3$: anisotropic multiple valleys originating from the van Hove singularity of graphene
We study the response of an adsorbed monolayer on a disordered substrate under a driving force using Brownian molecular-dynamics simulation. We find that the sharp longitudinal and transverse depinning transitions with hysteresis still persist in the presence of weak disorder. However, the transitions are smeared out in the strong disorder limit. The theoretical results here provide a natural explanation for the recent data for the depinning transition of Kr films on gold substrate.
Alkali-metal adsorption on the surface of materials is widely used for in situ surface electron doping, particularly for observing unoccupied band structures by angle-resolved photoemission spectroscopy (ARPES). However, the effects of alkali-metal atoms on the resulting band structures have yet to be fully investigated, owing to difficulties in both experiments and calculations. Here, we combine ARPES measurements on cesium-adsorbed ultrathin bismuth films with first-principles calculations of the electronic charge densities and demonstrate a simple method to evaluate alkali-metal induced ban…
The study of the adsorption phenomenon of helium began many decades ago with the discovery of graphite as a homogeneous substrate for investigation of physically adsorbed monolayer films. In particular, helium monoatomic layers on graphite were found to exhibit a very rich phase diagram. In the present work we have investigated the adsorption phenomenon of helium atoms on graphene and silicene substrates by means of density functional theory with Born-Oppenheimer approximation. Helium-substrate and helium-helium interactions were considered from first principles. Vibrational properties of adso…
The adsorption characteristics of alkali, alkaline earth and transition metal adatoms on silicene, a graphene-like monolayer structure of silicon, are analyzed by means of first-principles calculations. In contrast to graphene, interaction between the metal atoms and the silicene surface is quite strong due to its highly reactive buckled hexagonal structure. In addition to structural properties, we also calculate the electronic band dispersion, net magnetic moment, charge transfer, workfunction and dipole moment of the metal adsorbed silicene sheets. Alkali metals, Li, Na and K, adsorb to holl…
Using electron diffraction, we show that free electron metals such as sodium and potassium form a highly regular quasiperiodic monolayer on the fivefold surface of icosahedral Al-Pd-Mn and that the quasiperiodicity propagates up to the second layer in sodium. Our photoelectron spectroscopy results show that the quasicrystalline alkali metal adlayer does not exhibit a pseudogap near the Fermi level, thought to be charactersitic for the electronic structure of quasicrystalline materials. Calculations based on density functional theory provide a model structure for the quasicrystalline alkali met…
A free-electron like band has recently been observed in a monolayer of PTCDA (3,4,9,10-perylene tetracarboxylic dianhydride) molecules on Ag(111) by two-photon photoemission [Schwalb et al., Phys. Rev. Lett. 101, 146801 (2008)] and scanning tunneling spectroscopy [Temirov et al., Nature 444, 350 (2006)]. Using density functional theory calculations, we find that the observed free-electron like band originates from the Shockley surface state band being dramatically shifted up in energy by the interaction with the adsorbed molecules while it acquires also a substantial admixture with a molecular…
Based on the fact that the underpotential electrochemical deposition of Ag atoms on the $Au(100)$ surface exhibits sharp first-order phase transitions at well-defined values of the (coexistence) chemical potential ($μ_{coex}$), we performed extensive simulations aimed at investigating the hysteretic dynamic behavior of the system close to coexistence upon the application of a periodic signal of the form $μ(t) = μ_{coex} + μ_{o}sin(2Πt/τ) $, where $μ_{o}$ and $τ$ are the amplitude and the period of the sweep, respectively. For relatively short periods and small enough amplitudes the system beco…
We report scanning tunneling microscopy observations of Ge deposited on the Si(111)-7x7 surface for a sequence of sub-monolayer coverages. We demonstrate that Ge atoms replace so-called Si adatoms. Initially, the replacements are random, but distinct patterns emerge and evolve with increasing coverage, till small islands begin to form. Corner adatom sites in the faulted half unit cells are preferred. First-principles density functional calculations find that adatom substitution competes energetically with a high-coordination bridge site, but atoms occupying the latter sites are highly mobile. …
LEED experiments show that Li adsorbed at Cu(100) surfaces at room temperature induces a (2x1) missing row substrate reconstruction while adsorption at lower temperatures, T=180 K, results in an unreconstructed Cu(100)+c(2x2)--Li overlayer structure. Substrate reconstruction has not been observed for Na nor for K adsorption. In order to study the specific reconstruction behavior of the Li adsorbate ab initio DFT calculations have been performed on Cu(100)+Ad, Ad = Li, Na, K systems at coverages Theta_Ad=0.25-0.5 with and without reconstruction. The calculations show that the (2x1) MR reconstru…
In this letter we study the valley degeneracy splitting of the transition-metal dichalcogenides monolayer by first-principles calculations. The local magnetic moments are introduced into the system when the transition-metal atoms are adsorbed to the monolayer surface. Zeeman effect arising from the local magnetic moment at transition-metal atom sites lifts the valley degeneracy. Anomalous charge, spin and valley Hall Effects can be accessed due to valley splitting when we can only excite carriers of one valley. The valley splitting depends on the direction of magnetization and thus can be tune…
We have performed a density functional theory study of the possible layered geometries occurring after dehydrogenation of a self-assembled monolayer (SAM) of biphenyl-thiol molecules (BPTs) adsorbed on a Au(111), as it has been experimentally observed for low energy electron irradiated SAMs of 4'-nitro-1,1'-biphenyl-thiol adsorbed on a Au(111) surface. [Eck, W. et al., Advanced Materials 2000, 12, 805] Cross-link formation between the BPT molecules has been analyzed using different models with different degrees of complexity. We start by analyzing the bonding between biphenyl (BP) molecules in…
Using density functional theory, we investigate the electronic and magnetic properties of $3d$ transition-metal adatoms adsorbed on a monolayer of Mn on W(110). Mn/W(110) has a noncollinear cycloidal spin-spiral ground state with an angle of 173$^\circ$ between magnetic moments of adjacent Mn rows. It allows to rotate the spin orientation of an adsorbed magnetic adatom quasi-continuously. Therefore, this surface is ideally suited for manipulating the spin direction of individual atoms and exploring their magnetic properties using scanning tunneling microscopy (STM). The adsorbed V and Cr trans…
We carried out first-principles calculations within Density Functional Theory to investigate the structural, electronic and magnetic properties of boron-nitride (BN) honeycomb structure functionalized by adatom adsorption, as well as by the substitution of foreign atoms for B and N atoms. For periodic high density coverage, most of $3d$ transition metal atoms and some of group 3A, 4A, and 6A elements are adsorbed with significant binding energy and modify the electronic structure of bare BN monolayer. While bare BN monolayer is nonmagnetic, wide band gap semiconductor, at high coverage of spec…
The first theoretical study of a dimer's adsorption process at homogenous surface is presented. By using the RSA algorithm, we show example monolayers, discuss estimations of random jamming coverage and measure the surface blocking function, which could be used for calculating real systems kinetics. We also found the correlation function for generated coverages and analyzed orientational ordering inside the adsorbed monolayer. Results were compared with theoretical and experimental data. Keywords: adsorption, RSA, monolayer, colloid particles
Transition metal dichalcogenide (TMDC) monolayers present a remarkable multifunctional material with potential to enable the development of a wide range of novel devices. However, the functionalities observed often fall short of the expectation, which hinders the device development. Here we demonstrate that the optical, catalytic, and thermal functionalities of TMDC monolayers can all be substantially enhanced by up to orders of magnitude with the intercalation of water molecules or small cations (H+ and Li+) between the monolayers and underlying substrates. In contrast, the same molecules or …
In order to search for states specific to insulator/metal interfaces, we have studied epitaxially grown interfaces with element-selective near edge X-ray absorption fine structure (NEXAFS). An extra peak is observed below the bulk edge onset for LiCl films on Cu and Ag substrates. The nature of chemical bonds as probed by X-ray photoemission spectroscopy and Auger electron spectroscopy remains unchanged, so we regard this as evidence for metal-induced gap states(MIGS) formed by the proximity to a metal, rather than local bonds at the interface. The dependence on the film thickness shows that t…
Materials exhibiting multiple stable phases can be used as functional components in electronic and optical applications if the phase transition is controllable. Group-VI transition metal dichalcogenides (TMDs, MX$_2$, where M=Mo, W and X = S, Se) are known to undergo charge induced transitions from semi-conducting H phases to metallic T phases. This occurs, for example, when bulk TMDs are exfoliated with the aid of alkali ion intercalants. However, it is difficult to experimentally decouple the effect of composition-dependent phase transition barriers from indirect effects related to the exfol…
We investigate the adsorption sites of $3d$ transition metal (TM) adatoms by means of low-temperature scanning tunneling microscopy and spectroscopy. Co and Ni adatoms were adsorbed on two types of graphene on SiC(0001), i.e. pristine epitaxial monolayer graphene (MLG) and quasi-free-standing monolayer graphene (QFMLG). In the case of QFMLG, two stable adsorption sites are identified, while in the case of MLG, only one adsorption site is observed. Our experimental results reveal the decoupling efficiency as a crucial parameter for determining the adsorption site as well as the electronic prope…
Light alkali metals have usually been considered as simple metals due to their monovalency and high conductivity. In these metals ionic pseudopotentials are weak and the nearly free electron model (NFE) becomes quite accurate at normal conditions. However, very recent experiments have shown that at high pressures their electronic properties deviate radically from the NFE model and even become unexpected good superconductors. In this work we present ab initio calculations to analyze the deviation from simplicity in a lithium monolayer (ML) when pressure is applied. We have seen that as a result…
Interfaces formed between monolayer transition metal dichalcogenides (TMDs) and (metallo)phthalocyanine molecules are promising in energy applications and provide a platform for studying mixed-dimensional molecule-semiconductor heterostructures in general. An accurate characterization of the frontier energy level alignment at these interfaces is key in the fundamental understanding of the charge transfer dynamics between the two photon absorbers. Here, we employ the first-principles substrate screening $GW$ approach to quantitatively characterize the quasiparticle electronic structure of a ser…
The nanofriction of Xe monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapor deposition and transferred to the QCM electrodes with a polymer stamp. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below …
