Formation of (n x 1)-O/Ag(110) overlayers and the role of step-edge atoms

Spatially Extended Charge Density Wave Switching by Nanoscale Local Manipulation in a VTe2 Monolayer

Monolayer transition metal dichalcogenide VTe2 exhibits multiple charge density wave (CDW) phases, mainly (4 × 4) and (4 × 1). Here we report facile dynamic and tens-of-nanometer scale switching between these CDW phases with gentle bias pulses in scanning tunneling microscopy. Bias pulses purposely stimulate a reversible random CDW symmetry change between the isotropic (4 × 4) and anisotropic (4 × 1) CDWs, as well as CDW phase slips and rotation. The switching threshold of ∼1.0 V is independent of bias polarity, and the switching rate varies linearly with the tunneling current. Density functional theory calculations indicate that a coherent CDW phase switching incurs an energy barrier of ∼2.0-3.0 eV per (4 × 4) unit cell. While there is a challenge in understanding the observed large-area CDW random fluttering, we provide some possible explanations. The ability to manipulate electronic CDW phases sheds new light on tailoring CDW properties on demand.

Step-Edge Directed Metal Oxidation

Metal surface oxidation is governed by surface mass transport processes. Realistic surfaces have many defects such as step edges, which often dictate the oxide growth dynamics and result in novel oxide nanostructures. Here we present a comprehensive and systematic study of the oxidation of stepped (100), (110) and (111) Cu surfaces using a multiscale approach employing density functional theory (DFT) and reactive force field molecular dynamics (MD) simulations. We show that the early stages of oxidation of these stepped surfaces can be qualitatively understood from the potential energy surface of single oxygen adatoms, namely, adsorption energies and Ehrlich-Schwöbel barriers. These DFT predictions are then validated using classical MD simulations with a newly optimized ReaxFF force field. In turn, we show that the DFT results can be explained using a simple bond-counting argument that makes our results general and transferable to other metal surfaces.

Thermodynamic and spectroscopic properties of oxygen on silver under an oxygen atmosphere

Comparing experimental and theoretical XPS and XANES suggest that unreconstructed atomic oxygen is not present on the silver surface at oxygen chemical potentials relevant for epoxidation.

O(2) dissociation before the onset of added row nucleation on Ag(110): an atomistic scanning tunnelling microscopy view

We present here a scanning tunnelling microscopy (STM) study on O(2) adsorption at Ag(110) at T = 175 K, i.e. in the temperature range between the onset of O(2) dissociation and the formation of the added row reconstruction. In agreement with previous studies at lower surface coverage, we observe several structures forming upon O(2) dissociation, including Ag-O complexes randomly distributed on the surface. We suggest that the presence of the latter structures, characterized by a large cross section for low energy electrons, can account for the marked decrease of the surface electron reflectivity and for the corresponding increase of the diffuse elastic intensity previously reported in a HREELS investigation of the same system.

Structure and thermal fluctuation of one-dimensional AgO chains on Ag(110) surfaces studied with density functional theory and Monte Carlo simulations

The structures of continuous and truncated AgO chains on Ag(110) surfaces are studied by using density functional theory (DFT) calculations and the thermal fluctuations of truncated chains are simulated by using the Monte Carlo method. Although it is known that oxygen elimination by CO from one-dimensional AgO chains takes place exclusively at chain ends when the chains keep a linear structure at low temperatures, the structure of chain ends has been unexplored. The DFT calculations reveal that oxygen-terminated chains are more stable than silver-terminated ones and have an enhanced density of states near the Fermi level at the terminal oxygen, which is consistent with scanning tunneling microscope (STM) observations. The Monte Carlo simulations with pairwise interactions between AgO units reproduce characteristic features observed in STM studies, including the existence of an onset temperature for the chain fluctuations and the oxygen-coverage dependence of average chain length. The onset temperature, on one hand, is largely controlled by attractive interactions in the direction parallel to chain growth. On the other hand, the spatial distribution of fragmented AgO chains depends strongly on repulsive interactions in the direction perpendicular to chains. In particular, the repulsive interactions ranging ten units of the lattice constant in the direction perpendicular to the AgO chains are essential to mimic STM observations, where fragmented chains almost keep the mutual distance inherent to the (nx1)-O phase even under thermal fluctuations.

Xenon as a probe for minority sites on solid surfaces

Atomic-level microscopies have proved useful to map solid-surface sites directly, but, because of their lack of chemical specificity, they are less adept at identifying unique chemical activity on those sites. Here we present a dual-titration approach developed to probe minority sites on solid surfaces with unique chemical properties of potential relevance to heterogeneous catalysis. Our methodology involves the initial dosing of a chemical probe such as carbon monoxide or ammonia to drive its selective adsorption onto specific sites with particular chemical activity, and the subsequent adsorption of xenon to help identify the nature of those sites. A combination of photoelectron and temperature-programmed desorption spectroscopies are used to characterize the Xe adsorption. The chemistry of oxygen-modified Ni(110) single crystals was probed to test this technique. It was observed that whereas CO does not discriminate among the various sites present on those surfaces, ammonia binds preferentially to the end of -Ni-O rows and modifies their local electrostatic potential. In addition, it was determined that adsorbed CO aids in a reversible surface reconstruction involving the coalescence of fragmented surface -Ni-O rows at high (>350 K) temperatures.

In Situ Observation of CO Oxidation on Ag(110)(2×1)-O by Scanning Tunneling Microscopy: Structural Fluctuation and Catalytic Activity

On the added-row reconstructed Ag(110)(nx1)-O surfaces where one-dimensional -Ag-O-Ag-O- chains arrange periodically, the clean-off reaction of O adatoms by CO was investigated using variable temperature scanning tunneling microscopy (VT-STM). Based on the in situ STM observations of the surface structure variation in the course of the reaction at various temperatures, we found that the reaction kinetics are significantly affected by the structural transition of AgO chains from a solid straight line configuration to dynamically fluctuating configurations. Below 230 K where the chains are straight, the reaction takes place only at the end of the chains, so that the reaction progresses in the zero-order kinetics with the reaction front propagating along the chain. The temperature dependence of the reaction rates yields the activation barrier of 41 kJ/mol and the preexponential factor of 1.7 x 10(3) cm(-2) s(-1). At room temperature, the reaction rate is drastically accelerated when almost half of the O adatoms are eliminated and the chains start fluctuating. The dynamic formation of active sites equivalent to the end of chains upon the chain fluctuation results in the nonlinear increase of the reaction rate.

Role of structural fluctuation in a surface reaction studied by scanning tunneling microscopy: the CO+O-->CO2 clean-off reaction on Ag(110)-(2x1)-O

The kinetics of the clean-off reaction of O adatoms by CO on Ag(110)-(2x1)-O is investigated by scanning tunneling microscopy. The reaction is accelerated in the lower O coverage range where AgO chains with (nx1) (n> or =4) configurations show significant structural fluctuation. Simulations based on the Ising model are used to provide a quantitative understanding of the acceleration, which originates from the dynamical formation of active O adatoms by fluctuation of AgO chains.