Investigation with low-energy electron diffraction of the adsorbate-induced metal relaxations in the Cu(100)-(2 x 2)-S surface structure

Modulating the Electronic Properties of Graphene by Self-Organized Sulfur Identical Nanoclusters and Atomic Superlattices Confined at an Interface

Ordered atomic-scale superlattices on a surface hold great interest both for basic science and for potential applications in advanced technology. However, controlled fabrication of superlattices down to the atomic scale has proven exceptionally challenging. Here we develop a segregation method to realize self-organization of S superlattices at the interface of graphene and S-rich Cu substrates. Via scanning tunneling microscope measurements, we directly image well-ordered identical nanocluster superlattices and atomic superlattices under the cover of graphene. Scanning tunneling spectra show that the superlattices in turn could modulate the electronic structure of top-layer graphene. Importantly, a special-ordered S monatomic superlattice commensurate with a graphene lattice is found to drive semimetal graphene into a symmetry-broken phase-the electronic Kekulé distortion phase-which opens a bandgap of ∼245 meV.

Sulfur Atoms Adsorbed on Cu(100) at Low Coverage: Characterization and Stability against Complexation

Using scanning tunneling microscopy, we characterize the size and bias-dependent shape of sulfur atoms on Cu(100) at low coverage (below 0.1 monolayers) and low temperature (quenched from 300 to 5 K). Sulfur atoms populate the Cu(100) terraces more heavily than steps at low coverage, but as coverage approaches 0.1 monolayers, close-packed step edges become fully populated, with sulfur atoms occupying sites on top of the step. Density functional theory (DFT) corroborates the preferential population of terraces at low coverage as well as the step adsorption site. In experiment, small regions with p(2 × 2)-like atomic arrangements emerge on the terraces as sulfur coverage approaches 0.1 monolayer. Using DFT, a lattice gas model has been developed, and Monte Carlo simulations based on this model have been compared with the observed terrace configurations. A model containing eight pairwise interaction energies, all repulsive, gives qualitative agreement. Experiment shows that atomic adsorbed sulfur is the only species on Cu(100) up to a coverage of 0.09 monolayers. There are no Cu-S complexes. In contrast, prior work has shown that a Cu₂S₃ complex forms on Cu(111) under comparable conditions. On the basis of DFT, this difference can be attributed mainly to stronger adsorption of sulfur on Cu(100) as compared with Cu(111).

The LVV Auger line shape of sulfur on copper studied by Auger photoelectron coincidence spectroscopy

We have studied the line shapes of Cu(0 0 1)-p (2 × 2)S L2VV and L3VV Auger decay by means of Auger photoelectron coincidence spectroscopy. Measuring the LVV Auger spectrum in coincidence with S 2p1/2 and 2p3/2 photoelectrons respectively, we have been able to separate the two overlapping Auger spectra and determine their intrinsic line shapes. The two Auger transitions, though shifted in energy, display an identical line shape whose main features can be qualitatively understood considering a single particle approximation but are better described within a Cini-Sawatzky (CS) approach. Comparison between the experimental and the CS calculated spectra confirms that a substantial part of the Auger lines (∼20%) can be ascribed to decay events accompanied by the excitation of one additional electron-hole pair in the valence band. For the first time, the locality of the Auger process combined with the surface sensitivity of the APECS technique and its ability to separate overlapping structures are used to study Auger transitions taking place at the the surface states of a S/noble-metal interface.

In situ video-STM studies of adsorbate dynamics at electrochemical interfaces

The dynamic behavior of individual adsorbates at electrochemical interfaces was studied directly by in situ high-speed scanning tunneling microscopy, using sulfur adsorbed on Cu(100) electrodes in 0.01 M HCl solution as an example. By dosing from diluted Na(2)S solutions S(ad) coverages of a few percent can be prepared, with the sulfur adsorbates occupying positions within the c(2x2) lattice of coadsorbed chloride. S(ad) tracer diffusion occurs via hopping between neighboring c(2x2) lattice sites at considerably higher rates than those of sulfur on Cu(100) under UHV conditions, indicating a pronounced influence of the electrochemical environment on the adsorbate surface dynamics. The diffusion barrier linearly increases by 0.5 eV per V with potential and is strongly affected by neighboring S(ad) and surface defects. The S(ad)-S(ad) interactions extend over approximately 7 A. They are repulsive between nearest-neighbor and attractive between next-nearest-neighbor sites, respectively, and result in significantly reduced diffusion barriers. S(ad) on the upper terrace side of steps are transiently trapped and exhibit lower diffusion rates, leading to the formation of small metastable p(2x2) domains. Attractive interactions between S(ad) and domain boundaries in the c(2x2) adlayer result in boundary pinning as well as transient trapping and enhanced diffusion of S(ad) along the boundary.

Quantitative measurements of adsorbate-adsorbate interactions at solid-liquid interfaces

The interactions between adsorbates at a solid-liquid interface were studied by video-rate STM for the case of sulfur on Cu(100) electrode surfaces in HCl solution. Quantitative data were obtained by analyzing the S(ad) dimer dynamics within the surrounding c(2 x 2)-Cl adlattice as well as the adsorbate configurations. The interactions are repulsive for S(ad) separated by one or two lattice spacings and attractive at a separation of square root of 2 with energies comparable to adsorbates at the solid-vacuum interface. The S(ad) diffusion barriers are significantly reduced in the vicinity of a neighboring adsorbate.

Pathways for SO(2) dissociation on Cu(100): density functional theory

The dissociation of SO(2) on Cu(100) and the diffusion of the co-adsorbed decomposition products S and O were investigated using density functional theory-based calculations. Two dissociation pathways were considered: (P1) [Formula: see text] and (P2) [Formula: see text], the difference being in the formation of the intermediate product SO. It is found that P1 is favored kinetically with a total effective dissociation barrier of 0.78 eV compared to P2 which has 1.58 eV. The transition state leading to the formation of O+SO is found to be a result of the weakened interaction between the O of SO and the surface while the transition state for breaking SO is seen to be that of the repulsive nature of co-adsorbed S and O. The co-adsorbed S has a lower diffusion barrier of 0.41 eV compared to O which has a barrier ranging from 0.49 to 0.95 eV.