21 Search Results

Alloy oxidation is complex and involves several critical processes that lack understanding on the atomic level. Here, we report an atomistic picture of the initial-stage oxidation of stepped Cu₃Au(100) using a combination of surface science tools and modeling to illuminate the microscopic processes underlying oxygen-adsorption-induced structural and compositional changes. Pristine Cu₃Au(100) consists of wide CuAu-terminated terraces and narrow Cu-terminated terraces separated by monatomic steps. Counterintuitive to the common expectations of the adsorbate-induced surface segregation of the more reactive alloy component, our observations demonstrate that the oxygen adsorption leads to the exfoliation of the outermost CuAu layer, thereby exposing the innermore » Cu plane to O attack. This occurs via the oxygen-assisted abstraction of Au and Cu atoms from step edges and CuAu terraces, which generates many Cu adatoms aggregating into Cu clusters and Au adatoms dissolving into the bulk. The oxygen adsorption onto fourfold hollow sites of the exposed Cu plane results in nucleation and growth of the c(2×2)-O superstructure, which can be fit well by the Johnson-Mehl-Avrami-Kolmogorov theory with a site-saturated nucleation mechanism.« less

Recent developments of the scattering-type scanning near-field optical microscope at cryogenic temperatures (cryogenic s-SNOM or cryo-SNOM) have led to many breakthroughs in the studies of low energy excitations in quantum materials. However, the simultaneous demands on vibration isolation, low base temperature, precise nano-positioning, and optical access make the construction of a cryo-SNOM a daunting task. Adding to the overhead space required for a cryo-SNOM is the atomic force microscopy control, which predominantly utilizes a laser-based detection scheme for determining the cantilever tapping motion. In this work, we provide an alternative and straightforward route to performing s-SNOM using metal-coated Akiyama probes,more » where the cantilever tapping motion is detected through a piezoelectric signal. We show that the Akiyama-based cryo-SNOM attains high spatial resolution, good near-field contrast, and is able to perform imaging with a significantly more compact system compared to other cryo-SNOM implementations. Our results firmly establish the potential of s-SNOM based on self-sensing piezo-probes, which can easily accommodate far-infrared wavelengths and high magnetic fields in the future.« less

Surface segregation is a phenomenon that depends on the delicate interplay between thermodynamic driving forces and kinetic obstacles, for which elevated temperature is often needed to enhance the atom mobility and reach equilibrium. Using the classic system of ${\mathrm{Cu}}_3\mathrm{Au}\left(100\right)$ under nonisothermal conditions, in this study, we show an adatom process underlying transient surface segregation dynamics through the temperature-change-driven creation and annihilation of thermal vacancies in the bulk and the resulting bulk-surface mass exchanges. This is demonstrated by monitoring the surface composition evolution of ${\mathrm{Cu}}_{more »}$ 3 Au ( 100 ) with temperature changes between 250 and $500{}^{°}\mathrm{C}$, showing that the increase in temperature decreases monotonically the surface Au concentration as a result of the transfer of more Cu than Au from the bulk to the surface to form Cu-rich clusters of adatoms. Such a bulk thermal defect effect is expected to be universal in inducing the disparity in the bulk-surface mass exchanges of dissimilar atoms in multicomponent materials because of the inherent differences in the vacancy formation energies of the constituent atoms.« less

Crystal structure and composition are inter-dependent and decoupling their effects on surface reactivity is challenging. Using low-energy electron microscopy to spatially and temporally resolve the oxide film growth during the oxidation of NiAl(100), we differentiate such coupled effects by monitoring oxide growth while simultaneously fine-tuning the surface structure and composition during oxidation. We demonstrate that the oxidation of chemically ordered surfaces results in compact oxide island growth whereas non-compact oxide growth during the surface phase transition. By incorporating the surface phase transition induced chemical disordering into kinetic Monte Carlo simulations, we illustrate that the non-compact oxide growth is induced bymore » the composition effect on the surface diffusion of oxygen, which can be described by the concept of “ant in the labyrinth”.« less

Abstract Surface and subsurface are commonly considered as separate entities because of the difference in the bonding environment and are often investigated separately due to the experimental challenges in differentiating the surface and subsurface effects. Using in-situ atomic-scale transmission electron microscopy to resolve the surface and subsurface at the same time, we show that the hydrogen–CuO surface reaction results in structural oscillations in deeper atomic layers via the cycles of ordering and disordering of oxygen vacancies in the subsurface. Together with atomistic calculations, we show that the structural oscillations in the subsurface are induced by the hydrogen oxidation-induced cyclic lossmore » of oxygen from the oxide surface. These results demonstrate the propagation of the surface reaction dynamics into the deeper layers in inducing nonstoichiometry in the subsurface and have significant implications in modulating various chemical processes involving surface–subsurface mass transport such as heterogeneous catalysis, oxidation, corrosion and carburization.« less

Charge transfer between dissimilar atoms is an essential step for many chemical processes such as corrosion and heterogeneous catalysis, but directly probing the charge transfer has been a challenge. Using the oxygen-copper system as an example, we show that synchrotronbased ambient-pressure X-ray photoelectron spectroscopy can be employed to monitor the charge transfer between adsorbates and metal surfaces. Here, it is shown that oxygen chemisorption on Cu surfaces results in an Auger process that differs from the photo-excitation-induced Coster-Kroning transition and can be used to derive the degree of charge transfer in combination with ab initio calculations. The identified chemisorption-induced Augermore » process may have broader implications for its use as a fingerprint to monitor bond formation and charge transfer between dissimilar atoms.« less

Using ambient-pressure X-ray photoelectron spectroscopy, here we report the real-time monitoring of dynamic surface composition evolution of Cu₃Au(100) in response to the imposed environmental stimuli. Segregation of Au to the pristine surface under ultrahigh vacuum annealing leads to the phase separation with pure Au at the surface and alloyed Au in the subsurface. Upon switching to an oxidizing atmosphere, oxygen adsorption drives the surface segregation of Cu along with inward migration of pure Au to the subsurface. Switching to a H₂ atmosphere results in oxygen loss from the oxygenated surface, thereby promoting Au surface segregation and reverting the surface tomore » the pristine state with the Au termination. Finally, these measurements demonstrated the tunability of the surface composition of the binary alloy by utilizing the interplay between the tendency of segregating a more noble constituent to the surface and the tendency to segregate the more reactive one with the chemical stimuli.« less

Here, we have employed ambient-pressure X-ray photoelectron spectroscopy to monitor the full pathway of copper oxidation from the initial stages of oxygen chemisorption on the clean surface to oxide nucleation and growth and then to growing continuous oxide layer. By increasing oxygen pressure in a stepwise way from pO₂ = 10^–7 to 1 Torr at T = 350 °C, we show that the oxidation of Cu(100) takes place by dissociative chemisorption of oxygen to first form the p(2 × 2) phase at pO₂ = 1 × 10^–7 Torr, which then transforms to the missing-row reconstruction with a higher oxygen coveragemore » and upon increasing pO₂ from 1 × 10^–6 to 1 × 10^–3 Torr. After reaching pO₂ = 1 × 10^–2 Torr, nucleation and growth of Cu₂O islands occurs by consuming the regions. Increasing pO₂ to 0.1 Torr results in the continued Cu₂O growth with increased surface roughness. The surface is quickly covered with a CuO layer upon increasing pO₂ to 1 Torr, at which the surface oxidation occurs via the CuO/Cu₂O bilayer growth. The measurements delineate the range of pO₂ required for the crossover between the different stages of the surface oxidation and may have practical importance by tuning the oxidation conditions that allow for controlling the oxide phase and atomic structure of the surface.« less

Using synchrotron-based ambient-pressure X-ray photoelectron spectroscopy, we report the tuning of the deoxygenation process of bulk dissolved oxygen in copper via a combination of H₂ gas flow and elevated temperature. Here, we show that a critical temperature of ~580 °C exists for driving segregation of bulk dissolved oxygen to form chemisorbed oxygen on the Cu surface, which subsequently reacts with hydrogen to form OH species and then H₂O molecules that desorb from the surface. This deoxygenation process is tunable by a progressive stepwise increase of temperature that results in surface segregation of oxygen from deeper regions of bulk Cu. Usingmore » atomistic simulations, we show that the bulk-dissolved oxygen occupies octahedral sites of the Cu lattice and the deoxygenation process involves oxygen migration between octahedral and tetrahedral sites with a diffusion barrier of ~0.5 eV.« less