Metal Adatoms Research Articles

3d oxide molecules to tailor large magnetic anisotropy energies on MgO films

Designing systems with large magnetic anisotropy energy (MAE) is desirable and critical for nanoscale magnetic devices. A recent breakthrough achieved the theoretical limit of the MAE for 3d transition metal atoms by placing a single Co atom on a MgO(100) surface, a result not replicated by standard first-principles simulations. Our paper, incorporating Hubbard-U correction and spin-orbit coupling, successfully reproduces and explains the high MAE of a Co adatom on a MgO (001) surface. We go further by exploring ways to enhance MAE in 3d transition metal adatoms through different structural geometries of 3d−O molecules on MgO. One promising structure, with molecules perpendicular to the surface, enhances MAE while reducing substrate interaction, minimizing spin fluctuations, and boosting magnetic stability. Additionally, we demonstrate significant control over MAE by precisely placing 3d−O molecules on the substrate at the atomic level. Published by the American Physical Society 2024

In Situ Closing the van der Waals Gap of Two-Dimensional Materials.

Self-intercalation in two-dimensional (2D) materials, converting 2D materials into ultrathin covalently bonded materials, presents great possibilities for studying a new family of quantum-confined materials with the potential to realize multifunctional behavior. However, understanding the mechanisms and associated in situ kinetics of synthesizing self-intercalated 2D (ic-2D) materials, particularly at the atomic scale, remains elusive, greatly hindering the practical applications of ic-2D crystals. Here, we successfully in situ synthesized ic-2D thin films via thermal annealing of their parental TMDCs inside an electron microscope. We atomically visualized the evolution from TaS2 and NbS2 into the corresponding ic-2D Ta1+xS2 and ic-2D Nb1+xS2, respectively, by in situ scanning transmission electron microscopy (STEM). The self-intercalation process in TaS2 is atomically realized by metal adatom edge adsorption and subsequent diffusion in an atom-by-atom manner. On the other hand, MoS2 and MoSe2 tend to coalesce into metal crystals under the same annealing conditions, suggesting that the self-intercalation process is predominantly controlled by thermodynamic factors as further verified by density functional theory (DFT). By varying the ramping rate and annealing temperature, the coverage and spatial arrangement of the filling sites can be precisely tuned, ranging from 2a × a, a × a, or Ta trimers, as predominantly gauged by kinetic factors. Our work sheds light on the thermodynamics and growth kinetics involved in ic-2D formation and paves the way for growing highly crystalline ic-2D materials with intercalation concentration and topology-dependent properties.

Structure Determination of F4TCNQ on Ag(111): A Systematic Trend in Metal Adatom Incorporation.

A structure determination of the commensurate phase formed by 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (F4TCNQ) absorbed on Ag(111) is reported. Initial characterization was performed using low-energy electron diffraction and synchrotron radiation photoelectron spectroscopy, with quantitative structural data being provided by normal incident X-ray standing waves (NIXSW) and surface X-ray diffraction (SXRD). NIXSW data show the F4TCNQ molecule to adopt a "twisted" conformation on the surface, previously found to be associated with metal adatom incorporation into a 2d-metal-organic framework for F4TCNQ on Au(111), Ag(100), and Cu(111). SXRD results provide direct evidence of the presence of Ag adatoms that are found to occupy near-bridge or fcc hollow sites with respect to the underlying surface, at an adsorption height of 2.69 ± 0.10 Å. The results show a consistent pattern of behavior for F4TCNQ adsorption on the (111) surfaces of Cu, Ag, and Au.

Stable, while Still Active? A DFT Study of Cu, Ag, and Au Single Atoms at the C3N4/TiO2 Interface.

Hybrid DFT calculations are employed to compare the adsorption and stabilization of Cu, Ag, and Au atoms on graphitic C3N4 and on the heterojunction formed by g- C3N4 and TiO2. While Cu and Ag can be strongly chemisorbed in form of cations on g- C3N4, Au is only weakly physisorbed. On g- C3N4/TiO2, all coinage metal adatoms can be strongly chemisorbed, but, while Cu and Ag forms cations, Au form an Au- species. Ab Initio Molecular Dynamics simulations confirm that the metal adatoms on g-C3N4 are highly mobile at room temperature, while they remain confined in the interfacial spacing between C3N4 and TiO2 on the heterojunction, being both stably bound and reachable for the reactants in a catalytic cycle. Doping g- C3N4/TiO2 with metal single atoms permits thus to generate catalytic systems with tunable charge and chemical properties and improved stability with respect to bare C3N4. Moreover, the changes in the electronic structure of g- C3N4/TiO2 induced by the presence of the metal single atoms are beneficial also for photocatalytic applications.

Element-specific cluster growth on the two-dimensional metal–organic network

A porous two-dimensional metal–organic network (2D-MON) on a substrate captures deposited metal atoms and metal clusters growing in the pores of the 2D-MON. We found that the growth mechanisms of Ag, In, and Pd clusters in the 2D-MON synthesized from 1,3,5-tris(4-bromophenyl)benzene molecules on Ag(111) are different from each other, and the difference derives from the interaction of an adatom with the 2D-MON. Ag and Pd clusters grow from the 2D-MON since the interaction of Ag and Pd adatoms with the 2D-MON is attractive. In clusters grow inside of the pores of the 2D-MON since the interaction between an In adatom and the 2D-MON is repulsive. The growth process of metal clusters is determined by the element-specific behavior of metal adatoms in the pores, taking into account interactions with the 2D-MON.

Nature of point defects in monolayer MoS2 and the MoS2/Au(111) heterojunction

Deposition of MoS2 on Au(111) alters the electronic properties of MoS2. In this study, we investigate the free-standing MoS2 monolayer and the MoS2/Au(111) heterostructure, with and without strain, as well as defects of interest in memristive and neuromorphic applications. We focus on the so-called atomristor devices based on monolayer materials that achieve resistive switching characteristics with the adsorption and desorption of metal adatoms. Our study confirms that the formation of midgap states is the primary mechanism behind the resistive switching. Our results show that strain lowers the adsorption/desorption energies of Au+defect structures of interest, leading to more favorable switching energies, but simultaneously reduces the switching ratio between states of differing conductivities. The presence of the Au(111) substrate additionally introduces non-uniform amounts of strain and charge transfer to the MoS2 monolayer. We propose that the induced strain contributes to the experimentally observed n- to p-type transition and Ohmic to Schottky transition in the MoS2 monolayer. The charge transfer leads to a permanent polarization at the interface, which can be tuned by strain. Our study has important implications on the role of the electrode as being a source of the observed variability in memristive devices and as an additional degree of freedom for tuning the switching characteristics of the memristor device.

First principles study of hydrogen storage on B-doped SiC monolayers through light transition metal atoms

In this first-principles study, based on Density Functional Theory, we assess the capacity of metal-decorated, boron-doped, graphene-like monolayers of silicon carbide (SiC) to adsorb hydrogen molecules. To enhance the binding of metal adatoms on SiC monolayers, these were substitutionally doped with boron atoms. Alkaline, alkaline-earth, and transition metal adatoms were considered and their hydrogen storage capabilities were compared. The results show that alkaline-earth metal adatoms are not suitable for hydrogen storage. On the other hand, sodium- and potassium-decorated B-doped SiC monolayers adsorb the largest number of H2 molecules per adatom, but their adsorption energies are insufficient for an adequate hydrogen storage. Titanium and scandium adatoms are the most suitable for hydrogen storage since they exhibit good adsorption energies and up to four and five H2 molecules per adatom, respectively. Moreover, the estimated potential barriers for diffusion of these two adatoms on the B-doped SiC monolayers indicate that the probability of clustering is very low. Moreover, within the ideal-gas approximation, it is estimated that hydrogen can be stored in the Ti- and Sc-decorated monolayers at room temperature and atmospheric pressure. Furthermore, if SiC monolayers were doped with boron atoms in concentrations similar to those reported for graphene, it is estimated that the gravimetric capacities could reach 5.1 wt% and 6.3 wt% for Ti-decorated and Sc-decorated monolayers, respectively, which are close to the target hydrogen-storage capacities envisioned for the near future.

Diffusion and magnetization of metal adatoms on single-layer molybdenum disulfide at elevated temperatures

The present work models temperature-dependent (500−1300K) diffusion dynamics of Ag, Au, and Cu adatoms on MoS2 as well as electronic and magnetic properties of adatom (Ag, Au, and Cu)/MoS2 systems. Modeling is done by means of ab initio molecular dynamics (AIMD) simulations that account for van der Waals corrections and electronic spin degrees of freedom in the framework of density functional theory. It is found that Ag and Au adatoms exhibit super-diffusive motion on MoS2 at all temperatures, while Cu adatoms follow a random walk pattern of uncorrelated surface jumps. The observed behavior is consistent with AIMD-calculated effective migration barriers Ea (EaAg=190±50meV, EaAu=67±7meV, and EaCu=300±100meV) and can be understood on the basis of the considerably flatter potential energy landscapes encountered by Ag and Au adatoms on the MoS2 surface (corrugation of the order of tens of meV), as compared to Cu adatoms (corrugation >100meV). Moreover, evaluation of the electronic and magnetic properties of AIMD configurations suggest that Ag, Au, and Cu monomer adsorption induces semimetallic features in at least one spin channel of the adatom/MoS2 electronic structure at elevated temperatures. The overall results presented herein may provide insights into fabricating 2D-material-based heterostructure devices beyond graphene.

Theoretical insights into the interplay between metal-organic and covalent bonding in single-layer molecular networks formed by halogen dissociation.

Synthesis via dehalogenative coupling due to thermal annealing is one of the most common routes of growing metal-organic and covalent polymer networks on catalytic metal surfaces. We present a computational approach taking into account both metal-coordinated and covalent C-C bonding interactions, which drive the self-assembly of tetrabrominated polyarene molecules into single-layer ordered and disordered nanostructures. The proposed coarse-grained lattice model is simulated using the Monte Carlo method. We investigate the annealing effect in ensembles of nearly and fully dehalogenated molecules, accordingly decreasing the concentration of dissociated (chemisorbed) halogen atoms, to account for the desorption process. The results suggest that dissociated halogens may be at least partially responsible for fragmentation of metal-organic networks on the Cu and Au surfaces. The simulations also show that fragmented covalent networks are mostly disordered or characterized by short-range glass-like order, but larger domains of these phases can be obtained after removing the split off Br atoms. We additionally examine the potential formation of fragments with a hybrid structure consisting of oligomer chains linked side-to-side by metal adatoms.

Adsorption and solar light activity of noble metal adatoms (Au and Zn) on Fe(111) surface: a first-principles study.

Noble metals such as gold (Au), zinc (Zn), and iron (Fe) are highly significant in both fundamental and technological contexts owing to their applications in optoelectronics, optical coatings, transparent coatings, photodetectors, light-emitting devices, photovoltaics, nanotechnology, batteries, and thermal barrier coatings. This study presents a comprehensive investigation of the optoelectronic properties of Fe(111) and Au, Zn/Fe(111) materials using density functional theory (DFT) first-principles method with a focus on both materials' spin orientations. The optoelectronic properties were obtained employing the generalized gradient approximation (GGA) and the full-potential linearized augmented plane wave (FP-LAPW) approach, integrating the exchange-correlation function with the Hubbard potential U for improved accuracy. The arrangement of Fe(111) and Au, Zn/Fe(111) materials was found to lack an energy gap, indicating a metallic behavior in both the spin-up state and the spin-down state. The optical properties of Fe(111) and Au, Zn/Fe(111) materials, including their absorption coefficient, reflectivity, energy-loss function, refractive index, extinction coefficient, and optical conductivity, were thoroughly examined for both spin channels in the spectral region from 0.0 eV to 14 eV. The calculations revealed significant spin-dependent effects in the optical properties of the materials. Furthermore, this study explored the properties of the electronic bonding between several species in Fe(111) and Au, Zn/Fe(111) materials by examining the density distribution mapping of charge within the crystal symmetries.

Roughness Factors of Electrodeposited Nanostructured Copper Foams

Copper-based electrocatalytic materials play a critical role in various electrocatalytic processes, including the electroreduction of carbon dioxide and nitrate. Three-dimensional nanostructured electrodes are particularly advantageous for electrocatalytic applications due to their large surface area, which facilitates charge transfer and mass transport. However, the real surface area (RSA) of electrocatalysts is a crucial parameter that is often overlooked in experimental studies of high-surface-area copper electrodes. In this study, we investigate the roughness factors of electrodeposited copper foams with varying thicknesses and morphologies, obtained using the hydrogen bubble dynamic template technique. Underpotential deposition (UPD) of metal adatoms is one of the most reliable methods for estimating the RSA of highly dispersed catalysts. We aim to illustrate the applicability of UPD of lead for the determination of the RSA of copper deposits with hierarchical porosity. To find the appropriate experimental conditions that allow for efficient minimization of the limitations related to the slow diffusion of lead ions in the pores of the material and background currents of the reduction of traces of oxygen, we explore the effect of lead ion concentration, stirring rate, scan rate, monolayer deposition time and solution pH on the accuracy of RSA estimates. Under the optimized measurement conditions, Pb UPD allowed to estimate roughness factors as high as 400 for 100 µm thick foams, which translates into a specific surface area of ~6 m2·g−1. The proposed measurement protocol may be further applied to estimate the RSA of copper deposits with similar or higher roughness.

Applying recurrent neural networks to predict the work function of metal adatom adsorption on graphene

The barrier height has a large impact on the performance of transistors, lasers, and solar cells. To tune such a barrier height, a bidirectional long short-term memory network (LSTM) model is proposed to predict the work function of adsorbed metal atoms on graphene. The largest relative errors predicted by the proposed model compared to those obtained from first-principles calculations are less than 10 %. However, more than one order of magnitude difference in field-emission currents can be caused by such errors according to calculations. It implies that the proposed model can be used to quickly screen surface structures with its work function being around a target work function before performing first-principles calculations. Thus, it can save a lot of computing costs and time costs. The selected adsorbed metal atoms at different sites on graphene can be implemented to improve the performance of graphene-based devices by using the proposed method.PACS numbers85.30.De, 85.30.Tv, 73.40.Kp.

Stability of Iridium Single Atoms on Fe3O4(001) in the mbar Pressure Range.

Stable single metal adatoms on oxide surfaces are of great interest for future applications in the field of catalysis. We studied iridium single atoms (Ir1) supported on a Fe3O4(001) single crystal, a model system previously only studied in ultra-high vacuum, to explore their behavior upon exposure to several gases in the millibar range (up to 20 mbar) utilizing ambient-pressure X-ray photoelectron spectroscopy. The Ir1 single adatoms appear stable upon exposure to a variety of common gases at room temperature, including oxygen (O2), hydrogen (H2), nitrogen (N2), carbon monoxide (CO), argon (Ar), and water vapor. Changes in the Ir 4f binding energy suggest that Ir1 interacts not only with adsorbed and dissociated molecules but also with water/OH groups and adventitious carbon species deposited inevitably under these pressure conditions. At higher temperatures (473 K), iridium adatom encapsulation takes place in an oxidizing environment (a partial O2 pressure of 0.1 mbar). We attribute this phenomenon to magnetite growth caused by the enhanced diffusion of iron cations near the surface. These findings provide an initial understanding of the behavior of single atoms on metal oxides outside the UHV regime.

Single-atom catalysis for carbon dioxide dissociation using greigite-supported M1/Fe3S4(111) (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) under electrostatic fields

Single transition metal adatoms (M) supported on a solid surface – here the reactive greigite Fe3S4(111) surface – combine the best properties of homogeneous and heterogeneous catalytic systems and are highly selective materials capable of altering the pathway of difficult reactions. We have employed state-of-the-art first-principles simulations to investigate the process of carbon dioxide (CO2) dissociation on the single-atom catalyst (SAC) M1/Fe3S4(111). We discuss how reconstructions of the symmetrical stacking sequence of charged atomic planes determines the possible stable non-polar terminations of the Fe3S4(111) surface. The thermodynamic stability and the work function are the main descriptors that are affected by external electrostatic fields, both for the most stable termination of the pristine Fe3S4(111) surface as well as the M1/Fe3S4(111) catalysts. We present the electron density plots for the M1/Fe3S4(111) surfaces, which show that the M adatom forms covalent metal-support interactions (CMSI). In general, positive external electrostatic fields enhance the adsorption properties of the M1/Fe3S4(111) catalysts towards the CO2 molecule, which activates chemically upon interaction. Our energy profiles for the dissociation of the CO2 molecule show that carefully selected transition metal adatoms, such as V, Cr and Co, and positive external electrostatic fields can be used to tune the catalytic properties of SACs.

Highly Sensitive and Selective DNA Sequencing Device Using Metal Adatom Adsorption on 2D Phosphorene.

Two-dimensional (2D) material revolutionarily extends the technique capability of traditional nanopore/nanogap-based DNA sequencing devices. However, challenges associated with DNA sequencing on nanopores still remained in improving the sensitivity and specificity. Herein, by first-principles calculation, we theoretically studied the potential of transition-metal elements (Cr, Fe, Co, Ni, and Au) anchored on monolayer black phosphorene (BP) to act as all-electronic DNA sequencing devices. The spin-polarized band structures appeared in Cr-, Fe-, Co-, and Au-doped BP. Remarkably, the adsorption energy of nucleobases can be significantly enhanced on BP with Co, Fe, and Cr doping, which contribute to the enlarged current signal and lower noise levels. Furthermore, the order of nucleobases in terms of their adsorption energies onto the Cr@BP is C > A > G > T, which exhibits more distinct adsorption energies than Fe@BP or Co@BP. Therefore, Cr-doped BP is more effective to avoid ambiguity in recognizing various bases. We thus envisaged a possibility of a highly sensitive and selective DNA sequencing device based on phosphorene.

Hydrogen storage on tin carbide monolayers with transition metal adatoms

In this work, we employ Density Functional Theory to study the effects of decoration with transition metal (TM) atoms—Ag, Au, Cu, Sc, Ti and Pd—on the H2 adsorption properties of tin carbide monolayers (SnC-ML), as a prospective material for hydrogen storage. The results indicate that TM adatoms are strongly bonded to the SnC-ML and that electronic charge is transferred from the adatoms to the SnC-ML. In particular, it is found that Sc and Ti are chemisorbed on SnC-ML with strong binding energies. The most stable adsorption site for these metal atoms is above Sn atoms of the SnC-ML. Also, these TM atoms exhibit the higher hydrogen-storage capacities with up to four hydrogen molecules per adatom. In contrast, the other studied metals have at most 2 hydrogen molecules adsorbed. Approximate temperature- and pressure-dependent curves suggest that, to storage hydrogen, Sc- and Ti-decorated SnC-ML should be cooled under freezing temperatures, or kept at 1 MPa and 2.5 MPa, respectively, which are much lower pressures than those currently used in vehicular tanks, which attain pressures of 35 MPa. These results indicate that Sc and Ti decorated SnC-ML can be useful as hydrogen-storage solid-state devices.

Superconducting and transport properties of biphenylene network monolayer with alkaline metal adatoms

To explore novel two-dimensional superconductors, the recently prepared biphenylene network (BPN) monolayer with alkaline metal adatoms was investigated theoretically. Our study suggests that the superconducting critical temperature (Tc) of BPN monolayer could be increased from 0.03 to 11.22 K by adsorption of Li atoms, which induces increased density of states at the Fermi Level, softened vibrations and electron pockets at the Fermi surface. The diffusion barrier of Li/Na adatoms is very low (<0.25 eV), and it tends to decrease for high coverage. Average conductance was proposed to quantitatively evaluate the intrinsic conductivity and electronic transport anisotropy of two-dimensional materials. The adsorption of alkaline metal atoms enhances the transport anisotropy more than twice, and the direction with better conductance derives from the connected delocalized multi-center π bonds. Our study suggests that adsorption of metal atoms is an effective way to functionalize BPN monolayers, which endows them potential applications in future superconducting and electronic devices. It also implies that carbon monolayer allotropes like BPN monolayer may provide good platforms for both fundamental research and device applications, which stimulates more explorations in various fields.

Interaction of individual Ag atoms with graphene on Rh(111): Adsorption, migration, and cluster formation

The interaction of Ag atoms with graphene moiré on Rh(111) was studied by scanning tunneling microscopy. Upon adsorption at 15 K, the Ag atoms existed mainly as monomers at specific sites in the moiré unit cell because thermal diffusion was suppressed. The Ag atoms exhibited a dI/dV peak at ∼0.5 V above the Fermi level, which was assigned to Ag(5s)-C(2pz) anti-bonding state. At elevated temperature, the Ag adatoms were thermally activated to diffuse and form clusters, wherein the activation barrier depended on the moiré periodicity. Furthermore, migration was induced by injecting tunneling electrons into the anti-bonding state. This work provides atomic-scale insight into the aggregation process of metal adatoms on metal-supported graphene.

Assessing factors that determine adatom migration and clustering on a thin film oxide; Pt1 and Rh1 on the “29” CuxO/Cu(1 1 1) surface

The dynamical nature of single-site catalysts under non-equilibrium conditions poses significant challenges in the full characterization of their active sites. The “29” oxide is a thin CuxO film grown on Cu(111) which provides a well-defined surface on which to study the structure and chemistry of atomically dispersed precious metals. A combination of experimental and first-principles approaches is used to investigate the factors that influence the mobility of metal adatoms and their clustering tendencies. Our work elucidates how a homogenous coordination environment of neutral single-atoms can be obtained when its tendency to break oxide bonds within its vicinity is low. We demonstrate that this in turn affects the chemical activity of atomically dispersed species on thin-film oxides, as defects on the oxide would in fact allow such species to be more accessible to CO. We thus highlight the importance of understanding the relationship between defects and atomically dispersed active sites upon designing single-site catalysts.

Mechanisms of adsorbing hydrogen gas on metal decorated graphene

Hydrogen is a key player in global strategies to reduce greenhouse gas emissions. In order to make hydrogen a widely used fuel, we require more efficient methods of storing it than the current standard of pressurized cylinders. An alternative method is to adsorb ${\mathrm{H}}_{2}$ in a material and avoid the use of high pressures. Among many potential materials, layered materials such as graphene present a practical advantage as they are lightweight. However, graphene and other 2D materials typically bind ${\mathrm{H}}_{2}$ too weakly to store it at the typical operating conditions of a hydrogen fuel cell, meaning that high pressure would still be required. Modifying the material, for example by decorating graphene with adatoms, can strengthen the adsorption energy of ${\mathrm{H}}_{2}$ molecules, but the underlying mechanisms are still not well understood. In this work, we systematically screen alkali and alkaline-earth metal decorated graphene sheets for the static thermodynamic adsorption of hydrogen gas from first principles and focus on the mechanisms of binding. We show that there are three mechanisms of adsorption on metal decorated graphene and each leads to distinctly different hydrogen adsorption structures. The three mechanisms can be described as weak van der Waals physisorption, metal adatom facilitated polarization, and Kubas adsorption. Among these mechanisms, we find that Kubas adsorption is easily perturbed by an external electric field, providing a way to tune ${\mathrm{H}}_{2}$ adsorption. This work is foundational and builds our understanding of ${\mathrm{H}}_{2}$ adsorption under idealized conditions.