Single Adatom Research Articles

Characterizing the surface compositions of supported bimetallic PtSn clusters: Effects of cluster-support interactions and surface adsorbates

PtSn bimetallic clusters on TiO2(110) and highly oriented pyrolytic graphite (HOPG) surfaces have been characterized by scanning tunneling microscopy, low energy ion scattering (LEIS), X-ray photoelectron spectroscopy, and temperature programmed desorption (TPD); density functional theory (DFT) calculations have also been performed to better understand adsorption of CO and D2 on the PtSn surfaces. On TiO2 at coverages of 2 ML of Pt and 2 ML of Sn, exclusively bimetallic clusters are formed for both orders of deposition because clusters of the first metal completely cover the surface such that all atoms of the second metal are incorporated into the existing clusters. In contrast, on HOPG, the high mobility and weak cluster-support interactions on HOPG result in much larger 2 ML monometallic clusters (∼30 Å high) that do not completely cover the surface, and deposition of the second metal produces larger clusters as well as smaller ones. Despite the difference in cluster morphologies for the different orders of deposition and supports, the LEIS experiments demonstrate that in all cases, the PtSn clusters are rich in Sn at the surface, as expected based on the lower surface free energy for Sn compared to Pt. Furthermore, the +0.2 eV shift in the Sn(3d5/2) binding energy observed on all surfaces in the presence of Pt is consistent with PtSn alloy formation. Deposition of 2 ML of Sn on TiO2 produces two-dimensional clusters with oxidation of Sn and reduction of titania at the cluster-support interface, but addition of Pt to the Sn clusters causes Sn to diffuse away from this interface, leaving Sn in the metallic state. TPD experiments on 2 ML Pt/TiO2 with increasing coverages of Sn show that the number of adsorption sites for D2 sharply decreases to nearly zero at 0.5 ML, while CO adsorption decreases to zero only at much higher Sn coverages of 2 ML. DFT studies for Sn modified Pt surfaces and bulk structures demonstrate that for CO adsorption at low Sn coverages (≤0.25 ML), the strong Pt-CO interactions induce diffusion of Pt to the cluster surface and the formation of a bulk Pt3Sn alloy, whereas D2 adsorption does not lead to interactions with the Pt surface that are strong enough to induce alloy formation. A single Sn adatom prevents D2 adsorption on four neighboring Pt atoms via site-blocking and the donation of electron density to Pt.

Surface thermomigration of 2D voids

In a thermal gradient, surface nanostructures have been experimentally observed to move due to thermomigration. However, analytical models that describe the thermomigration force acting on surfaces are still controversial. In this work, we start from a thermodynamic approach based on the Massieu function, which is used to describe thermomigration of single adatoms, to develop an expression for the velocity of thermomigrating 2D holes. The model can be simplified in two limiting cases: (i) When the hole motion is limited by adspecies diffusion, the velocity is independent from the hole size (as in our experiments). (ii) If the hole motion is limited by the attachment or detachment of species to or from steps, then the velocity is proportional to the hole width. We have studied the thermomigration of 2D monatomic deep holes on Si(100) using low energy electron microscopy. From the velocity measurements taken at different temperatures, we find, using our model, that the sum of the migration energy and the adatom creation energy is 1.95 ± 0.16 eV. This value is consistent with those found by other authors, reinforcing the validity of our thermomigration model.

Blue phosphorene on Au(111): theoretical, spectroscopic and diffraction analysis reveal the role of single Au adatoms.

In investigating the monoatomic layers of P, several stable two-dimensional (2D) allotropes have been theoretically predicted. Among them, single-layer blue phosphorus (BlueP) appears to deliver promising properties. After initial success, where the structure of BlueP triangular patches on Au(111) was conceived on the basis of scanning tunneling microscopy (STM) and density functional theory (DFT), the surface structure model was revisited multiple times with increasing accuracy and insight of theoretical calculations and experimental datasets. Interestingly, the quest for a reliable atomic structure model of BlueP on Au(111) turned out to be very contentious and challenging, particularly considering the possible incorporation of Au atoms in the 2D sheet of P. This article proposes an extended report on theoretical findings that can be extracted from DFT calculations of the orbital projected band structure and employed for an efficient comparison protocol between the calculations and experimental datasets obtained from angle-resolved photoemission spectroscopy (ARPES). The findings, together with experimental and simulated data from STM imaging and surface X-ray diffraction (SXRD), show a clear way to verify the presence and characterize the stabilizing effect of foreign atoms in 2D materials.

Can P3S and C3S monolayers be used as anode materials in metal-ion batteries? An answer from first-principles study.

With the urgent need for efficient energy storage devices, significant attention has been directed to researching and developing promising anode materials for metal-ion batteries. Through density functional study, we successfully predicted the electrochemical performance of P3S and C3S monolayers for the first time, which could be used in alkali metal (Li, Na, and K)-ion batteries. Our study examines the energetic, dynamic, and thermal stabilities of pristine monolayers. The electronic structures of the pristine nanosheets are wide-gap semiconductors. After single metalation on the monolayers, the composite systems become metallic. Charge-density difference (CDD) analysis indicates that charge transfer occurs from alkali metal atoms to the P3S and C3S monolayers, and Bader charge analysis quantifies the amount of charge transfer. We analyzed how readily a single adatom diffuses within the 2D structures. One example is the diffusion of K on C3S, which has a low barrier value of 0.06 eV and seems practically barrierless. Our predicted composite systems report considerable theoretical storage capacity (C); for example, hexalayer K-adsorbed C3S shows a storage capacity of 1182.79 mA h g-1. The estimated open-circuit voltage (OCV) values suggest that the C3S monolayer is a promising anode material for Li-, Na-, and K-ion batteries, whereas the P3S monolayer is suitable as a cathode material for Li-, Na-, and K-ion batteries.

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.

On-surface synthesis of ballbot-type N-heterocyclic carbene polymers.

N-Heterocyclic carbenes (NHCs) are established ligands for metal complexes and surfaces. Here we go beyond monomeric NHCs and report on the synthesis of NHC polymers on gold surfaces, consisting of ballbot-type repeating units bound to single Au adatoms. We designed, synthesized and deposited precursors containing different halogens on gold surfaces under ultrahigh vacuum. Conformational, electronic and charge transport properties were assessed by combining low-temperature scanning tunneling microscopy, non-contact atomic force microscopy, X-ray photoelectron spectroscopy, first-principles calculations and reactive force field simulations. The confirmed ballbot-type nature of the NHCs explains the high surface mobility of the incommensurate NHC polymers, which is prerequisite for their desired spatial alignment. The delicate balance between mobility and polymerization rate allows essential parameters for controlling polymer directionality to be derived. These polymers open up new opportunities in the fields of nanoelectronics, surface functionalization and catalysis.

Stability of Single Gold Atoms on Defective and Doped Diamond Surfaces.

Polycrystalline boron-doped diamond (BDD) is widely used as a working electrode material in electrochemistry, and its properties, such as its stability, make it an appealing support material for nanostructures in electrocatalytic applications. Recent experiments have shown that electrodeposition can lead to the creation of stable small nanoclusters and even single gold adatoms on the BDD surfaces. We investigate the adsorption energy and kinetic stability of single gold atoms adsorbed onto an atomistic model of BDD surfaces by using density functional theory. The surface model is constructed using hybrid quantum mechanics/molecular mechanics embedding techniques and is based on an oxygen-terminated diamond (110) surface. We use the hybrid quantum mechanics/molecular mechanics method to assess the ability of different density functional approximations to predict the adsorption structure, energy, and barrier for diffusion on pristine and defective surfaces. We find that surface defects (vacancies and surface dopants) strongly anchor adatoms on vacancy sites. We further investigated the thermal stability of gold adatoms, which reveals high barriers associated with lateral diffusion away from the vacancy site. The result provides an explanation for the high stability of experimentally imaged single gold adatoms on BDD and a starting point to investigate the early stages of nucleation during metal surface deposition.

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.

Inelastic Light Scattering in the Vicinity of a Single-Atom Quantum Point Contact in a Plasmonic Picocavity.

Electromagnetic fields can be confined in the presence of metal nanostructures. Recently, subnanometer scale confinement has been demonstrated to occur at atomic protrusions on plasmonic nanostructures. Such an extreme field may dominate atomic-scale light-matter interactions in "picocavities". However, it remains to be elucidated how atomic-level structures and electron transport affect plasmonic properties of a picocavity. Here, using low-temperature optical scanning tunneling microscopy (STM), we investigate inelastic light scattering (ILS) in the vicinity of a single-atom quantum point contact (QPC). A vibration mode localized at the single Ag adatom on the Ag(111) surface is resolved in the ILS spectrum, resulting from tip-enhanced Raman scattering (TERS) by the atomically confined plasmonic field in the STM junction. Furthermore, we trace how TERS from the single adatom evolves as a function of the gap distance. The exceptional stability of the low-temperature STM allows to examine distinctly different electron transport regimes of the picocavity, namely, in the tunneling and QPC regimes. This measurement shows that the vibration mode localized at the adatom and its TERS intensity exhibits a sharp change upon the QPC formation, indicating that the atomic-level structure has a crucial impact on the plasmonic properties. To gain microscopic insights into picocavity optomechanics, we scrutinize the structure and plasmonic field in the STM junction using time-dependent density functional theory. The simulations reveal that atomic-scale structural relaxation at the single-atom QPC results in a discrete change of the plasmonic field strength, volume, and distribution as well as the vibration mode localized at the single atom. These findings give a qualitative explanation for the experimental observations. Furthermore, we demonstrate that strong ILS is a characteristic feature of QPC by continuously forming, breaking, and reforming the atomic contact and how the plasmonic resonance evolves throughout the nontunneling, tunneling, and QPC regimes.

Tuning a Two-Impurity Kondo System by a Moiré Superstructure.

Two-impurity Kondo models are paradigmatic for correlated spin-fermion systems. Working with Mn atoms on Au(111) covered by a monolayer of MoS_{2}, we tune the interadatom exchange via the adatom distance and the adatom-substrate exchange via the location relative to a moiré structure of the substrate. Differential-conductance measurements on isolated adatoms exhibit Kondo peaks with heights depending on the adatom location relative to the moiré structure. Mn dimers spaced by a few atomic lattice sites exhibit split Kondo resonances. In contrast, adatoms in closely spaced dimers couple antiferromagnetically, resulting in a molecular-singlet ground state. Exciting the singlet-triplet transition by tunneling electrons, we find that the singlet-triplet splitting is surprisingly sensitive to the moiré structure. We interpret our results theoretically by relating the variations in the singlet-triplet splitting to the heights of the Kondo peaks of single adatoms, finding evidence for coupling of the adatom spin to multiple conduction electron channels.

Microscopic theory of spin relaxation of a single Fe adatom coupled to substrate vibrations

Understanding the spin-relaxation mechanism of single adatoms is an essential step towards creating atomic magnetic memory bits or even qubits. Here we present an essentially parameter-free theory by combining \textit{ab-initio} electronic and vibrational properties with the many-body nature of atomic states. Our calculations account for the millisecond spin lifetime measured recently on Fe adatoms on MgO/Ag(100) and reproduce the dependence on the number of decoupling layers and the external magnetic field. We show how the atomic interaction with the environment should be tuned in order to enhance the magnetic stability, and propose a clear fingerprint for experimentally detecting a localized spin-phonon excitation.

Effects of strong spin-orbit coupling on Shiba states from magnetic adatoms using first-principles theory

Magnetic impurities at surfaces of superconductors can induce bound states referred to as Yu–Shiba–Rusinov states (i.e. Shiba states) within superconducting (SC) gaps. For superconductors with strong spin–orbit coupling (SOC), Shiba states arising from even single magnetic adatoms are too complex to be fully understood using effective models alone because SOC cannot be treated perturbatively and multiple orbitals are strongly mixed with spin projections. Here we investigate Shiba states of single magnetic adatoms at the surface of strongly spin-orbit coupled SC Pb, by solving the fully relativistic Dirac–Bogoliubov–de Gennes equations using multiple scattering Green’s function methods. For Fe and Co adatoms on Pb(110), we show that the Shiba states are better characterized by total angular momentum, J, and its projections on the z axis, m J . As a hallmark of the SOC effect, the Shiba states show a strong dependence of the orientation of the adatom moment. As the orientation of the Fe/Co moment changes, the deepest Shiba states merge at zero energy. This zero-energy state disappears with an additional non-magnetic adatom next to the magnetic adatom, although the other Shiba states unchange. For a Mn adatom on Pb, our Shiba states overall agree with experiments. The characteristics of our Shiba states are also observed with the similar energies and characters in the experiments. The deepest Shiba states that we compute, however, do not appear as close to the Fermi level as the experimental data. It would be interesting to compute the Shiba states with continuously varying vertical distances of the Mn adatom from the surface or with varying the charge state of the adatom, and to calculate the spatial dependence of the spectral density. Our findings will be also useful for understanding of Shiba states for dimers and longer spin chains on the Pb surface considering noncollinear magnetic structures in them.

Systematic study of Mn atoms, artificial dimers, and chains on superconducting Ta(110)

Magnetic adatoms coupled to an $s$-wave superconductor give rise to local bound states, so-called Yu-Shiba-Rusinov states. Focusing on the ultimate goal of tailoring chains of such adatoms into a topologically superconducting phase, we investigate basic building blocks - single Fe and Mn adatoms and Mn dimers on clean superconducting Ta(110) - using scanning tunneling microscopy and spectroscopy. We perform a systematic study of the hybridizations and splittings in dimers, and their dependence on the crystallographic directions and interatomic spacings, in order to identify potentially interesting chain geometries for this novel sample type. Subsequently, we study the spin structure as well as the length dependent Shiba band structure in Mn chains of those geometries using spin-resolved scanning tunneling spectroscopy. All results are compared to the according properties of structurally identical dimers and chains on the previously studied Nb(110), which has almost identical surface structure and electronic properties, but an about three times smaller spin-orbit interaction.

Molecular dynamics simulations of defective carbon nanotubes on the aging and friction properties of nitrile butadiene rubber composites

AbstractThe thermal‐oxidative aging (TOA) and tribological properties of rubber had attracted attention in the petrochemical field. In this study, the mechanical and tribological properties of defect‐free, Stone–Wales (SW), and single adatom (SA) defective carbon nanotubes (CNTs)/nitrile butadiene rubber (NBR) composites were investigated using molecular dynamics simulations. Poly (1,2‐dihydro‐2,2,4‐trimethyl‐quinoline) (antioxidant RD) was added to the composites to analyze their TOA mechanisms. Antioxidant RD provided the best protective effect in SW‐CNTs/RD/NBR composites, and their TOA properties were enhanced. In addition, the Young's modulus, bulk modulus, and shear modulus, friction coefficient, and wear rate of the SW‐CNTs/RD/NBR composites were improved. The effects of SW‐CNTs on the RD/NBR composites were elucidated from a molecular perspective. The surface structures of three groups of CNTs were analyzed, and the SW‐CNTs exhibited a stronger adsorption ability and binding effect. It was determined that the antioxidant RD and NBR could be more stably dispersed between the molecular chains in the SW‐CNTs/RD/NBR composites.

Non-Majorana modes in diluted spin chains proximitized to a superconductor

Spin chains proximitized with superconducting condensates have emerged as one of the most promising platforms for the realization of Majorana modes. Here, we craft diluted spin chains atom by atom following a seminal theoretical proposal suggesting indirect coupling mechanisms as a viable route to trigger topological superconductivity. Starting from single adatoms hosting deep Shiba states, we use the highly anisotropic Fermi surface of the substrate to create spin chains characterized by different magnetic configurations along distinct crystallographic directions. By scrutinizing a large set of parameters we reveal the ubiquitous emergence of boundary modes. Although mimicking signatures of Majorana modes, the end modes are identified as topologically trivial Shiba states. Our work demonstrates that zero-energy modes in spin chains proximitized to superconductors are not necessarily a link to Majorana modes while simultaneously identifying other experimental platforms, driving mechanisms, and test protocols for the determination of topologically nontrivial superconducting phases.

Strength and fracture behaviors of ultralong carbon nanotubes with defects

The presence of structural defects can result in large discrepancies between theoretical and experimental values of the strength of CNTs. To reveal the relationship between the strength and defects of CNTs, experiments accompanied with an experiment-based molecular dynamics simulation were conducted in this paper, where defects were introduced using oxygen plasma irradiation and the strength of the defective CNTs was measured by uniaxial tensile experiments. It was found that the strength of CNTs was controlled by the feature length of defects and the introduction of the single adatom, C–C bond breaking, and vacancy defect could decrease the strength by 8.2%, 23.2%, and 18.5%, respectively. Especially, when these defects synergized, the strength of CNTs ranged from 0 to 76.8% of the theoretical strength, which was confirmed by the experiments. Moreover, by defining cracks based on the atomic stress distributions around defects, the fracture of CNTs obeyed the Griffith's brittle fracture criterion and the atomic stress distributions along the crack plane obeyed the classical fracture mechanics theory except for some deviation near the crack tip. This work gives a perspective on reasons for the theory-experiment discrepancies on the strength of CNTs over the years, obtains the quantitative relationship between the size of defects and the strength of CNTs and provides a research paradigm for verifying the validity of fracture mechanics theories at nanoscale. • The adatom, C–C bond breaking, and vacancy defects are introduced to CNT by oxygen plasma irradiation. • The feature length of defects controls the strength of CNTs when these defects synergize. • The fracture of CNTs obeys the Griffith's brittle fracture criterion by regarding defects as cracks. • Atomic stress distributions obeys the classical fracture mechanics theory except for some deviation near the crack tip. • The characteristic length in GES is to separate two atoms from the maximum interaction to the interaction can be ignored.

Tracking single adatoms in liquid in a transmission electron microscope.

Single atoms or ions on surfaces affect processes from nucleation1 to electrochemical reactions2 and heterogeneous catalysis3. Transmission electron microscopy is a leading approach for visualizing single atoms on a variety of substrates4,5. It conventionally requires high vacuum conditions, but has been developed for in situ imaging in liquid and gaseous environments6,7 with a combined spatial and temporal resolution that is unmatched by any other method-notwithstanding concerns about electron-beam effects on samples. When imaging in liquid using commercial technologies, electron scattering in the windows enclosing the sample and in the liquid generally limits the achievable resolution to a few nanometres6,8,9. Graphene liquid cells, on the other hand, have enabled atomic-resolution imaging of metal nanoparticles in liquids10. Here we show that a double graphene liquid cell, consisting of a central molybdenum disulfide monolayer separated by hexagonal boron nitride spacers from the two enclosing graphene windows, makes it possible to monitor, with atomic resolution, the dynamics of platinum adatoms on the monolayer in an aqueous salt solution. By imaging more than 70,000 single adatom adsorption sites, we compare the site preference and dynamic motion of the adatoms in both a fully hydrated and a vacuum state. We find a modified adsorption site distribution and higher diffusivities for the adatoms in the liquid phase compared with those in vacuum. This approach paves the way for in situ liquid-phase imaging of chemical processes with single-atom precision.

Interplay of magnetic states and hyperfine fields of iron dimers on MgO(001)

Individual nuclear spin states can have very long lifetimes and could be useful as qubits. Progress in this direction was achieved on MgO/Ag(001) via detection of the hyperfine interaction (HFI) of Fe, Ti and Cu adatoms using scanning tunneling microscopy. Previously, we systematically quantified from first-principles the HFI for the whole series of 3d transition adatoms (Sc-Cu) deposited on various ultra-thin insulators, establishing the trends of the computed HFI with respect to the filling of the magnetic s- and d-orbitals of the adatoms and on the bonding with the substrate. Here we explore the case of dimers by investigating the correlation between the HFI and the magnetic state of free standing Fe dimers, single Fe adatoms and dimers deposited on a bilayer of MgO(001). We find that the magnitude of the HFI can be controlled by switching the magnetic state of the dimers. For short Fe-Fe distances, the antiferromagnetic state enhances the HFI with respect to that of the ferromagnetic state. By increasing the distance between the magnetic atoms, a transition toward the opposite behavior is observed. Furthermore, we demonstrate the ability to substantially modify the HFI by atomic control of the location of the adatoms on the substrate. Our results establish the limits of applicability of the usual hyperfine hamiltonian and we propose an extension based on multiple scattering processes.

Selective decorating of BC3 and C3N nanosheets with single metal atom for hydrogen storage

Employing first-principles calculations, we have studied the structure, stability and hydrogen storage efficiency of pristine and defective BC3 and C3N monolayer functionalized by a variety of single metal adatoms. It is found that single Sc adatom, acting as an optimal dopant on perfect BC3 monolayer, is able to adsorb up to nine H2 molecules as strongly as around 0.24 eV/H2, which allows for a hydrogen storage capacity of 7.19 wt% for Sc atoms stably adsorbing on double sides of BC3 monolayer with eighteen H2 molecules (18H2@2Sc/BC3). Moreover, the desorption temperature and thermodynamical stability of multiple H2 adsorbed Sc-decorated BC3 sheet have been addressed and the saturate configuration of 18H2@2Sc/BC3 is predicted to be stable at mild temperatures and pressures, i.e. less than 250 K at 1 bar, or larger than 24 bar at room temperature. This study indicates that the Sc-decorated BC3 monolayer could be a potential H2 storage candidate, and provides an instructive guidance for designing metal-functionalized carbon-based sheets in hydrogen storage.

Yu-Shiba-Rusinov States in a Superconductor with Topological Z_{2} Bands.

A Yu-Shiba-Rusinov (YSR) state is a localized in-gap state induced by a magnetic impurity in a superconductor. Recent experiments used an STM tip to manipulate the exchange coupling between an Fe adatom and the FeTe_{0.55}Se_{0.45} superconductor possessing a Z_{2} nontrivial band structure with topological surface states. As the tip moves close to the single Fe adatom, the energy of the in-gap state modulates and exhibits a zero-energy crossing followed by an unusual return to zero energy, which cannot be understood by coupling the magnetic impurity to the superconducting topological surface Dirac cone. Here, we numerically and analytically study the YSR states in superconductors with nontrivial Z_{2} bands and show the emergence of the two zero-energy crossings as a function of the exchange coupling between the magnetic impurity and the bulk states. We analyze the role of the topological surface states and compare in-gap states to systems with trivial Z_{2} bands. The spin polarization of the YSR states is further studied for future experimental measurement.