Recent Scanning Tunneling Microscopy Studies Research Articles

Post-dissociation Dynamics of N2 on Ru(0001): How Far Can the “Hot” N Atoms Travel?

Due to the high barrier and large exoergicity, the dissociation of N2 impinging on Ru(0001) produces ballistic N atoms that can travel significant distances from the impact site, as shown by a recent scanning tunneling microscopy study [Wagner, J. J. Phys. Chem. C 2022, 126, 18333−18342]. In this work, the “hot” nitrogen atom dynamics following N2 dissociation is investigated theoretically on a high-dimensional potential energy surface based on a neural network representation of density functional theory data. Quasi-classical trajectory simulations for N2 dissociation with several initial conditions revealed that typically only one N atom undergoes significant migration, while the other is often trapped near the impact site. Regardless of the initial condition, the average final separation between the two N atoms is typically less than 10 Å, about 1 order of magnitude less than the experimental report (66 ± 28 Å). The relatively short migration distance of the hot N atom found in our simulations is attributed to a high diffusion barrier and fast energy dissipation to surface phonons. The theory–experiment discrepancy presents a challenge to the quantitative understanding of hot atom dynamics on metal surfaces.

√3 × 2 and √3 × √7 Charge Density Wave Driven by Lattice Distortion in Monolayer VSe2

A charge density wave (CDW), an ordered modulation of electron distribution and lattice distortion, is one of the intriguing phenomena observed in transition metal dichalcogenides. Recent STM studies reported a new CDW phase with √3 × 2 and √3 × √7 periodicities in monolayer (ML) VSe2 grown on graphene, which is totally different from the 4 x 4 x 3 CDW periodicity in bulk. Although the emergence of new CDW phase is of great research interest, the origin of the new modulation in ML VSe2 has not been clearly investigated. In this report, we conduct a systematic study to understand the nature of the √3 × 2 and √3 × √7 CDW in ML VSe2 using scanning tunneling microscopy (STM). Bias dependent topography and differential conductance (dI/dV) mapping indicate that the √3 × 2 and √3 × √7 modulations are mostly driven by strong lattice distortions of the Se atoms rather than by charge orderings. In addition, STM topography reveals that the v3 × 2 modulation corresponds to a gap feature, and the v3 × v7 modulation corresponds to an isolated Se atom in the distorted lattice structures. Our work provides prerequisite information to understand the emergence of √3, × 2 and √3, × √7 CDW in ML VSe2.

Coinage Metal–Sulfur Complexes: Stability on Metal(111) Surfaces and in the Gas Phase

We provide a comprehensive theoretical assessment at the level of density functional theory (DFT) of the stability of various coinage metal–sulfur complexes, both in the gas phase and also for the complexes adsorbed on the (111) surface of the same coinage metal. Our primary interest lies in the latter where earlier scanning tunneling microscopy (STM) experiments were interpreted to suggest the existence of adsorbed S-decorated metal trimers, sometimes as a component of more complex adlayer structures. Recent STM studies at 5 K directly observed other isolated adsorbed metal–sulfur complexes. For these adsorbed species, we calculate various aspects of their energetics including a natural measure of stability corresponding to their formation energy from sulfur adsorbed on terraces and from metal atoms that are in thermal equilibrium with the substrate. From this perspective, our DFT analysis shows that all of Ag2S3, Ag3S3, and many larger complexes on Ag(111) are strongly stable, Cu2S3 is stable, and some larger complexes are marginally stable on Cu(111), but only Au4S4 on Au(111) is stable. Results are consistent with STM observations for Cu(111) and Ag(111) surfaces but appear to deviate slightly for Au(111). A systematic analysis relating stability in the gas phase with that of adsorbed species is achieved within the framework of Hess’s law. This analysis also unambiguously elucidates various energetic contributions to stability.

Repulsion-Induced Surface-Migration by Ballistics and Bounce.

The motion of adsorbate molecules across surfaces is fundamental to self-assembly, material growth, and heterogeneous catalysis. Recent Scanning Tunneling Microscopy studies have demonstrated the electron-induced long-range surface-migration of ethylene, benzene, and related molecules, moving tens of Angstroms across Si(100). We present a model of the previously unexplained long-range recoil of chemisorbed ethylene across the surface of silicon. The molecular dynamics reveal two key elements for directed long-range migration: first 'ballistic' motion that causes the molecule to leave the ab initio slab of the surface traveling 3-8 Å above it out of range of its roughness, and thereafter skipping-stone 'bounces' that transport it further to the observed long distances. Using a previously tested Impulsive Two-State model, we predict comparable long-range recoil of atomic chlorine following electron-induced dissociation of chlorophenyl chemisorbed at Cu(110).

Magnetic adatoms as memory bits: A quantum master equation analysis

Due to underlying symmetries the ground states of magnetic adatoms may be highly stable, which opens perspectives for application as single-atom memory. A specific example is a single holmium atom (with $J=8$) on a platinum (111) surface for which exceptionally long lifetimes were observed in recent scanning tunneling microscopy studies. For control and read-out the atom must be coupled to electronic contacts. Hence the spin dynamics of the system is governed by a quantum master equation. Our analysis shows that in general it cannot be reduced to a classical master equation in the basis of the unperturbed crystal-field Hamiltonian. Rather, depending on parameters and control fields, "environment induced superselection" principles choose the appropriate set of basis states, which in turn determines the specific relaxation channels and lifetimes. Our simulations suggest that in ideal situations the lifetimes should be even longer than observed in the experiment. We, therefore, investigate the influence of various perturbations. We also study the initialization process of the state of the Ho atom by applied voltage pulses and conclude that fast, high fidelity preparation, on a $100\,\text{ns}$ timescale, should be possible.

Density functional theory study of the initial oxidation of the Pt(111) surface

We used density functional theory calculations to examine the initial stages of oxidation of the Pt(111) surface. Consistent with prior studies, our calculations predict that oxygen atoms adsorb on fcc sites and form $p(2\ifmmode\times\else\texttimes\fi{}2)$ and $p(2\ifmmode\times\else\texttimes\fi{}1)$ structures at coverages of 0.25 and 0.50 ML, respectively. In addition to various surface configurations of oxygen on fcc sites, we examined subsurface oxygen and clustering of oxygen atoms on the surface. We find that subsurface oxygen is not the precursor to the oxidation of the Pt(111) surface. Instead, we predict a strong preference for the formation and growth of one-dimensional Pt oxide chains within the $p(2\ifmmode\times\else\texttimes\fi{}1)$ structure. In particular, at coverages above 0.50 ML, additional oxygen atoms prefer to aggregate between the close-packed oxygen rows formed by the $p(2\ifmmode\times\else\texttimes\fi{}1)$ structure and induce large buckling $(\ensuremath{\sim}1.8\text{ }\text{\AA{}})$ and modification of the charge of the surface Pt atoms. The result is an oxide compound with threefold and fourfold Pt-O coordination that grows as a one-dimensional chain running parallel to the oxygen rows of the $p(2\ifmmode\times\else\texttimes\fi{}1)$ structure. Furthermore, half of the oxygen atoms in the Pt oxide chains reside near hcp sites, contrary to some reports that oxygen atoms reside only on the fcc sites on Pt(111). Our results agree well with a recent scanning tunneling microscopy study and suggest a precursor mechanism to the oxidation of metal surfaces involving Pt oxide chain formation and growth on terraces at moderate oxygen coverages. Our results should have important implications to current models of NO and CO oxidation on Pt(111) and potentially on studies of the initial oxidation of other transition-metal and bimetallic surfaces.

Recent STM, DFT and HAADF-STEM studies of sulfide-based hydrotreating catalysts: Insight into mechanistic, structural and particle size effects

The present article will highlight some recent experimental and theoretical studies of both unpromoted MoS 2 and promoted Co–Mo–S and Ni–Mo–S nanostructures. Particular emphasis will be given to discussion of our scanning tunnelling microscopy (STM), density functional theory (DFT), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) studies which have provided insight into the detailed atomic structure. In accordance with earlier theoretical studies, the experimental studies show that the Ni–Mo–S structures may in some instances differ from the Co–Mo–S analogues. In fact, the Co–Mo–S and Ni–Mo–S structures may be even more complex than previously anticipated, since completely new high index terminated structures have also been observed. New insight into the HDS mechanism has also been obtained and complete hydrogenation and hydrogenolysis pathways for thiophene hydrodesulfurization (HDS) have been calculated on the type of structures that prevail under reaction conditions. It is seen that important reaction steps may not involve vacancies, and special brim sites are seen to play an important role. Such studies have also provided insight into inhibition and support effects which play an important role in practical HDS. Recent STM studies have shown that MoS 2 clusters below 2–3 nm may exhibit new structural and electronic properties, and a large variety of size-dependent structures have been identified. In view of the large structure sensitivity of hydrotreating reactions this is expected to give rise to large effects on the catalysis.

Hydrogen Dissociation Dynamics on Precovered Pd Surfaces: Langmuir is Still Right

A recent scanning tunneling microscopy study by Mitsui et al. [Nature (London) 422, 705 (2003)] challenged the well-accepted picture based on early studies of Langmuir that an ensemble of at least two empty, catalytically active sites is required for the dissociative adsorption; instead, aggregates of three or more vacancies should be necessary. We have performed the first ab initio molecular dynamics study of the adsorption dynamics on a precovered surface providing detailed insights into the coverage dependence of the adsorption probability. The simulations show that there is no need to refine the Langmuirian picture: A dimer vacancy is still sufficient to dissociate hydrogen provided the kinetic energy of the molecules is large enough to overcome the relatively small adsorption barrier. In addition, we elucidate further aspects of the dissociation dynamics at precovered surfaces.

Negative Differential Resistance in Transport through Organic Molecules on Silicon

Recent scanning tunneling microscopy studies of individual organic molecules on Si(001) reported negative differential resistance (NDR) above a critical applied field, observations explained by a resonant tunneling model proposed prior to the experiments. Here we use both density functional theory and a many-electron GW self-energy approach to quantitatively assess the viability of this mechanism in hybrid junctions with organic molecules on Si. For cyclopentene on p-type Si(001), the frontier energy levels are calculated to be independent of applied electric fields, ruling out the proposed mechanism for NDR. Guidelines for achieving NDR are developed and illustrated with two related molecules, aminocyclopentene and pyrroline.

From atom-resolved scanning tunneling microscopy (STM) studies to the design of new catalysts

The scanning tunneling microscope (STM) is today established as a unique tool for resolving the atomic-scale structure of surfaces. STM studies of model systems relevant to heterogeneous catalysis have made it possible to address and resolve many important, fundamental questions related to catalytic processes by imaging, in direct space, the atomic-scale structure of catalytically relevant model systems, e.g. adsorbate-covered single crystal surfaces or nanoclusters supported either on metals or oxide surfaces. These studies are normally carried out under well-controlled vacuum or pressure conditions. Here we discuss three recent STM studies of model catalyst systems, all illustrating how the insight gained from fundamental studies of idealized model systems has been successfully linked to studies on real catalyst systems operating under realistic conditions, and how this interplay has facilitated the development of new and superior high surface area catalysts.

Collective modes and quasiparticle interference on the local density of states of cuprate superconductors

The energy, momentum and temperature dependence of the quasiparticle local density of states (LDOS) of a two-dimensional $d_{x^2-y^2}$-wave superconductor with random disorder is investigated using the first-order T-matrix approximation. The results suggest that collective modes such as spin/charge density waves are relevant low-energy excitations of the cuprates that contribute to the observed LDOS modulations in recent scanning tunneling microscopy studies of $\rm Bi_2Sr_2CaCu_2O_x$.

Fundamental Electronic Properties and Applications of Single‐Walled Carbon Nanotubes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structures and thermodynamic phase transitions for oxygen and silver oxide phases on Ag{1 1 1}

With density functional theory, we have examined oxygen adsorption at surface and subsurface sites of Ag{1 1 1}. The microscopic structure of Ag oxide epitaxed to Ag{1 1 1} has also been determined. In agreement with a recent scanning tunneling microscopy study, non-stoichiometric oxide growth is favoured over the previously assumed stoichiometric growth. An ab initio phase diagram for O on Ag{1 1 1} has been constructed from the adsorption free energy of the various O and Ag oxide phases. The key finding is that under real conditions for ethylene epoxidation the active catalyst is likely to be non-stoichiometric Ag oxide.

Fundamental electronic properties and applications of single-walled carbon nanotubes.

Recent scanning tunneling microscopy studies of the intrinsic electronic properties of single-walled carbon nanotubes (SWNTs) are overviewed in this Account. A brief theoretical treatment of the electronic properties of SWNTs is developed, and then the effects of finite curvature and broken symmetry on electronic properties, the unique one-dimensional energy dispersion in nanotubes, the interaction between local spins and carriers in metallic nanotubes systems, and the atomic structure and electronic properties of intramolecular junctions are described. The implications of these studies for understanding fundamental one-dimensional physics and future nanotube device applications are also discussed.

Comment on "Structure of the Mn-induced Cu(100) c(2 x 2) surface"

We have studied the atomic arrangement of Mn upon deposition on Cu(100) at room temperature using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Scanning tunneling microscopy reveals that Mn is already incorporated at very low coverages (\ensuremath{\sim}0.005 ML Mn). Both LEED and STM show that the surface alloy is characterized by a considerable corrugation where Mn atoms are displaced outside by 0.30 \AA{}. Our findings are at variance with a recent STM study [Phys. Rev. B 52, 2735 (1994)] which reports a corrugation of approximately 0.12 \AA{} and claims Mn incorporation only above a critical Mn coverage which should be larger than 0.06 ML. \textcopyright{} 1996 The American Physical Society.

Surface Damage During Kev Ion Irradiation: Results of Computer Simulations

AbstractMD simulations have been employed to investigate damage processes near surfaces during keV bombardment of metal targets. For self-ion implantation of au, Cu, and Pt in the range of 5-20 keV, we have found that the proximity of the surface leads to significantly more damage and atomic mixing in comparison to recoil events occurring in the crystal interior. IN some cases, large craters are formed in a micro-explosive event, while in others a convective flow of atoms to the surface creates adatoms and leaves dislocations behind. Both the amount damage created in the surface and its morphology depend sensitively on the details of the energy deposition along individual ion trajectories. the results of these simulations will be summarized and compared to recent scanning tunneling microscopy studies of individual ion impacts in Pt and Ge.

FULLERENES ON METAL AND SEMICONDUCTOR SURFACES: SCANNING TUNNELING MICROSCOPY STUDIES

In this paper, we review our recent scanning tunnelling microscopy studies of fullerenes C 60 and C 70. We focus our work on adsorption and reaction of these fullerenes on Si(111)7×7 and Si (100)2×1 semiconductor surfaces, as well as on Cu (111)1×1 and Ag (111)1×1 noble metal surfaces. By using STM, the adsorption geometry and reconstructions of fullerenes are directly observed on these surfaces, and the intramolecular structures are revealed in high resolution STM images. It is found that fullerenes are strongly bonded to the Si(100) and Si(111) surfaces so that the free rotation of C 60 and C 70 in their bulk phases is stopped. Local orderings with c(3×4) and c(4×4) reconstructions are observed for the first layer of C60 on the Si(100) surface. Well-ordered crystalline multilayer films can be formed on the Si(100) surface. The intramolecular structures observed for C 60 on the Si(100) surface are interpreted based on the local charge distribution on the molecule, in good agreement with a recent theoretical calculation. The Cu (111)1×1 and Ag (111)1×1 surfaces are found to be good substrates for ordered C60 and C 70 film growth. While the interaction of fullerenes with Ag(111) surface is weaker than that on the Cu(111) surface, rotation of C 60 in bulk phase is stopped on both surfaces. Well-ordered commensurate reconstructions are observed for C 60 and C 70 on both surfaces. Bias-voltage-dependent intramolecular structures are observed for the first layer of both C60 and C 70, which reveal the local charge density distribution on these molecules. The results are in good agreement with a recent theoretical calculation. The interesting behavior of the C 60/ C 70 mixture on Cu(111) surface is also reported.

Scanning tunnelling microscopy of the interaction of hydrogen with silicon surfaces

Abstract This article focuses on recent scanning tunnelling microscopy studies that have led to an improved understanding of the interaction of hydrogen with silicon surfaces. The structure and bonding of the Si(111)-7 × 7 and Si(100)-2 × 1 surfaces are described together with the adsorption and desorption of hydrogen from these surfaces. The role of hydrogen in the passivation of silicon surfaces and silicon chemical vapour deposition are also discussed.

Photoemission study of the Si(111)- sqrt 3 x sqrt 3 -Pb mosaic phase: Observation of a large charge transfer.

The Pb-induced \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 mosaic phase of the Si(111) surface has been studied with Si 2p core-level and angle-resolved photoelectron spectroscopy. The results from these measurements provide evidence for a model with a 1:1 mixture of Pb and Si adatoms for the \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 mosaic phase, which was proposed in a recent scanning tunneling microscopy study. We observe quite large surface core-level shifts which are interpreted as due to a charge transfer between the different adatom dangling-bond orbitals.

Scanning tunnelling microscopy of Z-DNA.

Scanning tunnelling microscopy (STM) has been used to map the surface topography of inorganic materials at the atomic level, and is potentially one of the most powerful techniques for probing biomolecular structure. Recent STM studies of calf thymus DNA and poly(rA).poly(rU) have shown that the helical pitch and periodic alternation of major and minor grooves can be visualized and reliably measured. Here we present the first STM images of poly(dG-me5dC).poly(dG-me5dC) in the Z-form. Both the general appearance of the fibres and measurements of helical parameters are in good agreement with models derived from X-ray diffraction.