We have developed the capability to elucidate interfacial reaction dynamics using an arguably unique combination of supersonic molecular beams combined with in situ STM visualization. These capabilities have been implemented in order to reveal the complex spatiotemporal correlations that govern the oxidation of graphitic systems spanning atomic-, nano-, and meso-length scales. In this study, the 3 nm periodic moir & eacute; pattern of monolayer and bilayer graphene on Ru(0001) provides a diverse palette of potential scattering and binding sites at the interface for ground state atomic oxygen. We resolve the site-specificity of atomic oxygen placement on the moir & eacute; lattice for both monolayer and bilayer graphene on Ru(0001) with atomic resolution. Angle- and energy-controlled scattering of O(3P) on these interfaces reveals an incisive side-by-side comparison of preferential reactivity of the monolayer surface compared to a more free-standing bilayer graphene ruthenium interface. Morphologically dependent reactivity of many layered graphene (HOPG) and monolayer graphene on Ru(0001) reveal anisotropic on-surface reactivity dependent on the presence of proximal reacted sites or local regions. The kinetics of on-surface oxidation are additionally shown to influence the morphology of surface products by varying the temperature of the interface and flux of reactant species. Such correlations are important in chemisorption, catalysis, materials oxidation and erosion, and film processing-and tunable moir & eacute; templated adsorption is a route to well-ordered self-assembled 2D materials for use in next-generation platforms for quantum devices and catalysis. Taken together, these results highlight a new direction in the examination of interfacial reaction dynamics where incident beam kinetic energy and angle of incidence can be used as reaction control parameters, with outcomes such as site-specific reactivity, changes for overall time-evolving mechanisms, and the relative importance of non-adiabatic channels in adsorption all linked to the on-surface fate of chemisorbed species. Surface scattering paired with scanning-probe microscopy provides unique insights to on-surface chemical dynamics and reveals morphologically dependent interfacial reactivity.

This paper examines the reactive surface dynamics of energy- and angle-selected N2 dissociation on a clean Ru(0001) surface. Presented herein are the first STM images of highly energetic N2 dissociation on terrace sites utilizing a novel UHV instrument that combines a supersonic molecular beam with an in situ STM that is in-line with the molecular beam. Atomically resolved visualization of individual N2 dissociation events elucidates the fundamental reactive dynamics of the N2/Ru(0001) system by providing a detailed understanding of the on-surface dissociation dynamics: the distance and angle between nitrogen atoms from the same dissociated N2 molecule, site specificity and coordination of binding on terrace sites, and the local evolution of surrounding nanoscopic areas. These properties are precisely measured over a range of impinging N2 kinetic energies and angles, revealing previously unattainable information about the energy dissipation channels that govern the reactivity of the system. The experimental results presented in this paper provide insight into the fundamental N2 dissociation mechanism that, in conjunction with ongoing theoretical modeling, will help determine the role of dynamical processes such as energy transfer to surface phonons and nonadiabatic excitation of electron-hole pairs (ehps). These results will not only help uncover the underlying chemistry and physics that give rise to the unique behavior of this activated dissociative chemisorption system but also represent an exciting approach to studying reaction dynamics by pairing the angstrom-level spatiotemporal resolution of an in situ STM with nonequilibrium fluxes of reactive gases generated in a supersonic molecular beam to access highly activated chemical dynamics and observe the results of individual reaction events.

This study examines the reactive surface dynamics of GaAs(110) oxidation with molecular oxygen at room temperature over a range of impinging kinetic energies. Visualization of the surface by scanning tunneling microscopy (STM) after exposures to O-2 with kinetic energies of 0.4-1.2 eV provides morphological and kinetic data that were obtained utilizing a novel instrument that combines a supersonic molecular beam with an in-line, in-situ STM. Oxidation was found to proceed by two morphologically distinct, competing mechanisms: a spatially homogeneous process with randomly distributed chemisorbed oxygen atoms leading to layer-by-layer oxide growth, and a spatially heterogeneous process with oxides nucleating on structural surface defects and growing vertically and laterally with continued exposure. Both oxidation mechanisms exhibit enhanced reactivity with increasing kinetic energy. Only trace oxidation was observed with O-2 kinetic energies below 0.7 eV; a rapid increase in the rate of oxidation from 1.0 to 1.2 eV was found with homogeneous and heterogeneous oxidation proceeding simultaneously until full surface coverage was reached. In addition, the relative rates of the two mechanisms appear to change with O-2 kinetic energy: spatially homogeneous oxidation is expected to dominate at lower kinetic energies (< 0.7 eV) while the heterogenous growth of oxide islands increasingly dominates with higher kinetic energies (>= 1.0 eV). The results obtained in this study conclusively demonstrate that a heterogenous oxidation mechanism is activated on GaAs(110) at high O-2 kinetic energies, and reveal that thin oxide layers can be achieved with higher efficiency at room temperature using molecular beams of oxygen. These results provide vital information about the morphological evolution of the surface in conjunction with the overall kinetics, and identify a controlled method of enhanced oxidation at moderate temperatures that could potentially improve abruptness at oxide interfaces and be used in the fabrication of GaAs semiconductor devices.

The detailed mechanism and kinetics for the oxidative erosion and ablation of highly oriented pyrolytic graphite (HOPG) with molecular oxygen has been examined by monitoring the spatiotemporal evolution of the reacting interface. This has been accomplished using a new, unique gas-surface scattering instrument that combines a supersonic molecular beam with a scanning tunneling microscope (STM) in ultrahigh vacuum. Using this new instrument, we are able to tightly control the energy, angle, and flux of impinging oxygen along with the surface temperature and examine the reacted surface spanning atomic, nano, and mesocopic length-scales. We observe that different oxidation conditions produce morphologically distinct etching features: anisotropic channels, circular pits, and hexagonal pits faceted along crystallographic directions. These outcomes depend upon independent effects of oxygen energy, incident angle, and surface temperature. Reaction probability increased with beam energy and demonstrated non-Arrhenius behavior with respect to surface temperature, peaking at around 1375 K. At the incident collision energies used, it was found that beam impingement angle had only minor effects on the reaction probability and etch pit Morphology. Comparison of the relative reactivity of higher grade versus lower grade HOPG indicates that the formation of etched channels largely depends on the presence of grain boundaries. We have also observed the transition to multilayer etching. The influence of structural inhomogeneities such as defects and grain boundaries can now be assessed by real-time visualization of reacting interfaces. For example, the insertion of intentionally created point defects via ion sputtering leads to marked enhancement in interfacial reactivity. The approach used herein has allowed us to correlate time-evolving surface morphology with atomic-level interfacial kinetics and dynamics, providing new insight into the reactivity of materials in aggressive, energetic environments.

Direct chemical dynamics simulations at high temperatures of reaction between O-3(2) and graphene containing varied number of defects were performed using the VENUS-MOPAC code. Graphene was modeled using (5a,6z)-periacene, a poly aromatic hydrocarbon with 5 and 6 benzene rings in the armchair and zigzag directions, respectively. Up to six defects were introduced by removing carbon atoms from the basal plane. Usage of the PM7/unrestricted Hartree-Fock (UHF) method, for the simulations, was validated by benchmarking singlet-triplet gaps of n-acenes and (5a,nz) periacenes with high-level theoretical calculations. PM7/UHF calculations showed that graphene with different number of vacancies has different ground electronic states. Dynamics simulations were performed for two O-3(2) collision energies E-i of 0.4 and 0.7 eV, with the incident angle normal to the graphene plane at 1375 K. Collisions on graphene with one, two, three, and four vacancies (1C-, 2C-, 3C-, and 4C-vacant graphene) showed no reactive trajectories, mainly due to the nonavailability of reactive sites resulting from nascent site deactivation, a dynamical phenomenon. On the other hand, O-3(2) dissociative chemisorption was observed for collisions on four- (with a different morphology), five- and six-vacant graphene (4C-2-, 5C- and 6C-vacant graphene). A strong morphology dependence was observed for the reaction conditions. On all reactive surfaces, larger reaction probabilities were observed for collisions at E-i = 0.7 eV. This is in agreement with the nucleation time measured by supersonic molecular beam experiments wherein about 2.5 times longer nucleation time for O-2 impinging at 0.4 eV compared with 0.7 eV was observed. Reactivity at both collision energies, viz., 0.4 and 0.7 eV, showed the following trend: 5C- < 6C- < 4C-vacant graphene. Formation of carboxyl/semiquinone (C=O)- and ether (-C-O-C-)-type dissociation products was observed on all reactive surfaces, whereas a higher probability of formation of the ether (-C-O-C-) group was found on 4C-vacant graphene on which dangling carbon atoms are present in close proximity. However, no gaseous CO/CO2 formation was observed on any of the graphene vacancies even for simulations that were run up to 10 ps. This is apparently the result of the absence of excess oxygen atoms that can aid the formation of larger groups, the precursors for CO/CO2 formation. Although the results of this study do not provide a conclusive understanding of the mechanism of graphene/graphite oxidation, this work serves as an initial study attempting to understand the O-3(2) dissociative chemisorption dynamical mechanism on defective-graphene/graphite surfaces at high temperatures.