We have combined scanning tunneling microscopy with inelastic electron tunneling spectroscopy (IETS) and density functional theory (DFT) to study a tetracyanoethylene monolayer on Ag(100). Images show that the molecules arrange in locally ordered patterns with three non-equivalent, but undeterminable, adsorption sites. While scanning tunneling spectroscopy only shows subtle variations of the local electronic structure at the three different positions, we find that vibrational modes are very sensitive to the local atomic environment. IETS detects sizeable mode frequency shifts of the molecules located at the three topographically detected sites, which permits us to determine the molecular adsorption sites through identification with DFT calculations.
We report our results on the adsorption of noble gases such as argon, krypton and xenon on a graphene sheet, using Grand Canonical Monte Carlo (GCMC) simulations. We calculated the two-dimensional gas-liquid critical temperature for each adsorbate, resulting in fair agreement with theoretical predictions and experimental values of gases on graphite. We determined the different phases of the monolayers and constructed the phase diagrams. We found two-dimensional incommensurate solid phases for krypton, argon and xenon, and a two-dimensional commensurate solid phase for krypton.
wo-dimensional (2D) materials unique properties and their promising applications in post-silicon microelectronics have attracted significant attention in the past decade. Recently, ferromagnetic order with out-of-plane easy axis in a monolayer of CrI3 has been observed and reported, with a Curie temperature of 45 K. Here we study, using density functional theory (DFT) calculations, how hydrogen and oxygen adsorption affects the structural, electronic, and magnetic properties of a CrI3 monolayer. Our results show that while the structure remains almost unchanged by the adsorption of hydrogen, a…
Two dimensional transition metal dichalcogenides appear as good candidates for gas sensing and catalysis. Here, by means of density functional theory, we characterize the adsorption and dissociation of selected diatomic molecules (CO, H$_2$, O$_2$, and NO) on the MoSe$_2$ monolayer. We consider that these processes occur on the pristine $1H$-MoSe$_2$ monolayer and in the vicinity of an isolated Se vacancy. The presence of Se vacancies both enhances the molecular adsorption and reduces the energy needed for dissociation, making it energetically favorable for the case of O$_2$ and NO molecules. …
We perform first-principles calculations of electronic and dielectric properties of bismuthene functionalized with small ligands using first-principle calculations. We show that all functionalized structures have topological insulating (TI) behavior with a sizeable gap by calculating the Z$_2$ topological invariant. Furthermore the adsorption of all groups induce a quasi-planar structure to the initially pristine bismuthene structure. Finally we show that the dielectric properties show a large anisotropy with two main in plane absorption peaks.
Gas sensing mechanism of H2S, NH3, NO2 and NO toxic gases on transition metal dichalcogenides based Janus MoSSe monolayers are investigated using the density functional theory. The pristine and defect included MoSSe layers are considered as a host material for adsorption study. Three types of defects (i) molybdenum vacancy, (ii) selenium vacancy, and (iii) sulfur/selenium vacancy are studied to understand their impact on electronic properties and sensing of these gas molecules. The formation energy is computed to predict the stability of these defects and noticed that selenium vacancy is the m…
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…
B$_5$Se this work we investigate the Li adsorption properties of a hybrid 2D material, namely monolayer B5Se with first principles calculations. 2 dimensional B$_5$Se was found to have a distorted hexagonal structure with five B atoms and one Se atom at the vertices of each hexagon. The density functional theory (DFT) calculations are performed with the generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhoff (PBE) exchange correlation functional. The results were inclusive of van der Waals corrections with Grimmes DFT-D2 scheme. Electrode performance metrics, as the most preferred…
Layered transition metal trichalcogenides MPX$_3$ (M: transition metal; X: S, Se) demonstrate a wide spectrum of properties and are widely proposed as effective materials for the water splitting reactions. Among these materials, NiPX$_3$ are the most promising ones due to the match their electronic structures for the oxygen and hydrogen evolution reactions. Here, we present first steps of a detailed theoretical description on the adsorption of water molecules on pristine and defected (chalcogen vacancies) surfaces of NiPX$_3$ and show that in all cases a physisorption takes the place with adso…
Nanomaterials are prone to influence by chemical adsorption because of their large surface to volume ratios. This enables sensitive detection of adsorbed chemical species which, in turn, can tune the property of the host material. Recent studies discovered that single and multilayer molybdenum disulfide (MoS_2) films are ultra sensitive to several important environmental molecules. Here we report new findings from ab initio calculations that reveal substantially enhanced adsorption of NO and NH3 on strained monolyaer MoS2 with significant impact on the properties of the adsorbates and MoS2 lay…
The effect of vacancy and water adsorption on the electronic structure of semiconducting 2D trichalcogenide material CrPX$_3$ (X: S, Se) is studied using state-of-the-art density functional theory (DFT) approach. It is found that chalcogen vacancies play a minor role on the electronic structure of CrPX$_3$ in the vicinity of the Fermi level leading to the slightly reduced band gap for these materials, however, inducing strongly localised defect states which are placed in the energy gap formed by the valence band states. Our DFT calculations show that the interaction of water molecules with CrP…
Fuel cells in vehicles are the leading cause of carbon monoxide emissions. CO is one of the most dangerous gases in the atmosphere, as it binds to the hemoglobin in blood cells 200 times easier than O2. As the amount of CO in the blood stream increases, the level of oxygen decreases, which can lead to many neurological problems. To reduce the amount of CO in the atmosphere, scientists have focused on the adsorption of oxygen. The best substrates used today are platinum and palladium monolayers, which are very expensive. Because of this, researchers have searched for cheap materials, such as Mo…
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 geometric and electronic properties of Bi-adsorbed monolayer graphene, enriched by the strong effect of substrate, are investigated by first-principles calculations. The six-layered substrate, corrugated buffer layer, and slightly deformed monolayer graphene are all simulated. Adatom arrangements are thoroughly studied by analyzing the ground-state energies, bismuth adsorption energies, and Bi-Bi interaction energies of different adatom heights, inter-adatom distance, adsorption sites, and hexagonal positions. A hexagonal array of Bi atoms is dominated by the interactions between the buffe…
We investigate the variation of the oscillation frequency of the Mg$^{2+}$ and O$^{2-}$ ions in the magnesium oxide lattice due to the interactions of the surface with water monolayers by means of Low Energy Electron Diffraction. Our key result is a new technique to determine the adsorbate vibrations produced by the water monolayers on the surface lattice as a consequence of their change in the surface Debye temperature and its chemical shift. The latter was systematically investigated for different annealing times and for a constant external thermal perturbation in the range of 110-300 K in o…
We demonstrate the tunability of the photoluminescence (PL) properties of monolayer (1L)-MoS2 via chemical doping. The PL intensity of 1L-MoS2 was drastically enhanced by the adsorption of p-type dopants with high electron affinity, but reduced by the adsorption of n-type dopants. This PL modulation results from switching between exciton PL and trion PL depending on carrier density in 1L-MoS2. Achievement of the extraction and injection of carriers in 1L-MoS2 by this solution-based chemical doping method enables convenient control of optical and electrical properties of atomically thin MoS2.
Two-dimensional (2D) materials have been studied extensively as monolayers 1-5, vertical or lateral heterostructures 6-8. To achieve functionalization, monolayers are often patterned using soft lithography and selectively decorated with molecules 9,10. Here we demonstrate the growth of a family of 2D materials that are intrinsically patterned. We demonstrate that a monolayer of PtSe2 can be grown on a Pt substrate in the form of a triangular pattern of alternating 1T and 1H phases. Moreover, we show that, in a monolayer of CuSe grown on a Cu substrate, strain relaxation leads to periodic patte…
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…
We study the effect of surface adsorption of 27 different adatoms on the electronic and magnetic properties of monolayer black phosphorus using density functional theory. Choosing a few representative elements from each group, ranging from alkali metals (group I) to halogens (group VII), we calculate the band structure, density of states, magnetic moment and effective mass for the energetically most stable location of the adatom on monolayer phosphorene. We predict that group I metals (Li, Na, K), and group III adatoms (Al, Ga, In) are effective in enhancing the n-type mobile carrier density, …
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…
Isolated and full monolayer adsorption of various carboranethiol (C$_2$B$_{10}$H$_{12}$S) isomers on gold (111) surface have been investigated using both the standard and van der Waals density functional theoretical calculations. The effect of differing molecular dipole moment orientations on the low energy adlayer geometries, the binding characteristics and the electronic properties of the self-assembled monolayers of these isomers have been studied. Specifically, the binding energy and work function changes associated with different molecules show a correlation with their dipole moments. The…
Two-dimensional materials composed of transition metal carbides and nitrides (MXenes) are poised to revolutionize energy conversion and storage. In this work, we used density functional theory (DFT) to investigate adsorption of Mg and Na adatoms on five M$_{2}$CS$_{2}$ monolayers (where M= Mo, Nb, Ti, V, Zr) for battery applications. We assessed the stability of the adatom (i.e. Na and Mg)-monolayer systems by calculating adsorption and formation energies, as well as voltages as a function of surface coverage. For instance, we found that Mo$_{2}$CS$_{2}$ cannot support a full layer of Na nor e…
The evolution of titanyl-phthalocyanine (TiOPc) thin films on Ag(111) has been investigated using IRAS, SPA-LEED and STM. In the (sub)monolayer regime various phases are observed that can be assigned to a 2D gas, a commensurate and a point-on-line phase. In all three phases the non-planar TiOPc molecule is adsorbed on Ag(111) in an oxygen-up configuration with the molecular pi-conjugated backbone oriented parallel to the surface. The commensurate phase reveals a high packing density, containing two molecules at inequivalent adsorption sites within the unit cell. Both molecules assume different…
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…
We report a first-principles calculation that models the effect of iron (Fe) atoms on the adsorption of a tungsten (W) atom on W(100) surfaces. The adsorption of a W atom on a clean W(100) surface is compared with that of a W atom on a W(100) surface covered with a monolayer of Fe atoms. The total energy of the system is computed as the function of the height of the W adatom. Our result shows that the W atom first adsorbs on top of the Fe monolayer. Then the W atom can replace one of the Fe atoms through a path with a moderate energy barrier and reduce its energy further. This intermediate sit…
First principles density functional calculations are used to study the early oxidation stages of the Mg(0001) surface for oxygen coverages 1/16 <= Theta <= 3 monolayers. It is found that at very low coverages O is incorporated below the topmost Mg layer in tetrahedral sites. At higher oxygen-load the binding in on-surface sites is increased but at one monolayer coverage the on-surface binding is still about 60 meV weaker than for subsurface sites. The subsurface octahedral sites are found to be unfavorable compared to subsurface tetrahedral sites and to on-surface sites. At higher coverages ox…
First-principles calculations are performed to investigate the effects of the adsorption of gas molecules (CO, NO, NO2, H2S, N2, H2O, O2, NH3 and H2) on the electronic properties of atomically thin indium selenium (InSe). Our study shows that the lone-pair states of Se are located at the top of the valence band of InSe and close to the Fermi energy level, implying its high sensitivity to external adsorbates. Among these gas molecules, H2 and H2S are strong donors, NO, NO2, H2O and NH3 are effective acceptors, while CO and N2 exhibit negligible charge transfer. The O2 molecule has very limited …
Two-dimensional ferroelectrics are important quantum materials which have found novel application in nonvolatile memories, however, the effects of reversible polarization on chemical reactions and interaction with environments are rarely studied despite of its importance. Here, based on the first-principles calculations, we found distinct gas adsorption behaviors on the surfaces of ferroelectric In2Se3 layer and the reversible gas caption and release controlled by ferroelectric switch. We rationalize the novel phenomena to the synergistic effect of the different electrostatic potential and ele…
Atomic oxygen adsorption on the Mo(112) surface has been investigated by means of first-principles total energy calculations. Among the variety of possible adsorption sites it was found that the bridge sites between two Mo atoms of the topmost row are favored for O adsorption at low and medium coverages. At about one monolayer coverage oxygen atoms prefer to adsorb in a quasi-threefold hollow sites coordinated by two first-layer Mo atoms and one second layer atom. The stability of a structural model for an oxygen-induced $p(2\times 3)$ reconstruction of the missing-row type is examined.
Nickel and Rhodium (001) surfaces display a similar - as from STM images - clock reconstruction when half a monolayer of C/Ni, N/Ni or O/Rh is adsorbed; no reconstruction is observed instead for O/Ni. Adsorbate atoms sit at the center of the black squares of a chess-board, $c(2\times 2)$, pattern and two different reconstructions are actually compatible with the observed STM images - showing a $(2\times 2)p4g$ pattern - according to whether a rotation of the black or white squares occurs. We report on a first - principles study of the structure of X/Ni(001) and X/Rh(001) surfaces (X=C,N,O) at …
In this work we study the multilayer adsorption of polyatomic species on homogeneous and heterogeneous bivariate surfaces. A new approximate analytic isotherm is obtained and validated by comparing with Monte Carlo simulation. Then, we use the well-known Brunauer-Emmet-Teller's (BET) approach to analyze these isotherms and to estimate the monolayer volume, $v_\mathrm{m}$. The results show that the value of the $v_\mathrm{m}$ obtained in this way depends strongly on adsorbate size and surface topography. In all cases, we find that the use of the BET equation leads to an underestimate of the tru…
Hydrogen adsorption on graphene can be increased by functionalization with Ti. This requires dispersing Ti islands on graphene as small and dense as possible, in order to increase the number of hydrogen adsorption sites per Ti atom. In this report, we investigate the morphology of Ti on nanocrystalline graphene and its hydrogen adsorption by scanning tunneling microscopy and thermal desorption spectroscopy, and compare the results with equivalent measurements on single-crystalline graphene. Nanocrystalline graphene consists of extremely small crystal grains of < 5 nm size. Ti atoms preferentia…
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
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…
In this paper we propose a mechanism for the induction of energy gaps in the spectrum of graphene and its bilayer, when both these materials are covered with water and ammonia molecules. The energy gaps obtained are within the range 20-30 meV, values compatible to those found in experimental studies of graphene bilayer. We further show that the binding energies are large enough for the adsorption of the molecules to be maintained even at room temperature.
A quantitative structural investigation is reported, aimed at resolving the issue of whether substrate adatoms are incorporated into the monolayers formed by strong molecular electron acceptors deposited onto metallic electrodes. A combination of normal-incidence X-ray standing waves, low energy electron diffraction, scanning tunnelling microscopy and X-ray photoelectron spectroscopy measurements demonstrate that the systems TCNQ and F4TCNQ on Ag(100) lie at the boundary between these two possibilities and thus represent ideal model systems with which to study this effect. A room-temperature c…
Halogenated aromatic molecules assemble on surfaces forming both hydrogen and halogen bonds. Even though these systems have been intensively studied on flat metal surfaces, high-index vicinal surfaces remain challenging, as they may induce complex adsorbate structures. The adsorption of 2,6-dibromoanthraquinone (2,6-DBAQ) on flat and stepped gold surfaces is studied by means of van der Waals corrected density functional theory. Equilibrium geometries and corresponding adsorption energies are systematically investigated for various different adsorption configurations.~It is shown that bridge si…
We use Ca doping during growth of one and two monolayer thick MgO films on Ag(100) to identify the adsorption sites of individual adatoms with scanning tunneling microscopy. For this we combine atomic resolution images of the bare MgO layer with images of the adsorbates and the substitutional Ca atoms taken at larger tip-sample distance. For Ho atoms, the adsorption sites depend on MgO thickness. On the monolayer, they are distributed on the O and bridge sites according to the abundance of those sites, 1/3 and 2/3 respectively. On the MgO bilayer, Ho atoms populate almost exclusively the O sit…
