Atomic assembly of metal surfaces and interfaces

Inert gas ion impacts can be used to manipulate atomic assembly processes during the growth of metallic superlattices but the detailed mechanisms are not well understood. Molecular dynamics simulations are used to investigate the effects of ion incident angle and fluence upon the reassembly and structure of a copper surface partially covered with cobalt asperities. In the low ion energy regime, increasing the ion fluence decreases the cobalt layer surface roughness while gradually leading to an increase in the degree of interfacial mixing. The flattening of asperities occurs by direct (athermal) ion activation of an Ehrlich-Schwoebel mechanism of atom jumping. Intermixing of cobalt surface atoms in an underlying copper layer is found to occur by a knock-on process and the lowest energy barriers for this occur in low-index 〈1 1 0〉 and 〈1 1 2〉 crystal (channeling) directions. The mechanistic insights gained from the study are used to simulate the ion assisted growth of a Cu/Co/Cu multilayer system. Using ion parameters chosen to selectively activate atomic assembly mechanisms that promote flat, unmixed interfacial structures, it is shown that Cu/Co/Cu multilayer structures with high quality, smooth and chemically sharp interfaces can be obtained by using oblique, low energy, moderate fluence ion assistance with an ion mass that is similar to the atomic mass of the metals.     

Frontiers in infrared spectroscopy at surfaces and interfaces

An overview of advances in infrared (IR) spectroscopy is presented. Recent results in several areas of topical interest, including examples in biocompatibility, dielectric films, in situ chemical reactions, and electron relaxation in nanoparticles, are highlighted. Major advances in IR experimental methods include the development of accelerator-based sources of IR light and the application of novel techniques to examine complex systems. These advances, and their role in elucidating crucial insights about surfaces and interfaces, are illustrated by recent work in the literature. After reviewing the current state of the art, promising future directions are discussed. In particular, superb opportunities are expected to develop in a broad range of scientific disciplines, e.g., biology, device engineering, chemistry, and physics.     

Soft X-ray photoelectron spectroscopy of compound semiconductor surfaces and interfaces

Photoelectron spectroscopy is one of the leading techniques in the study of solid surfaces. In particular, X-ray photoelectron spectroscopy (XPS) — also known as ESCA (electron spectroscopy for chemical analysis) — is extensively used in materials science. This paper discusses the advantages of coupling the soft X-ray continuum of synchrotron radiation with this technique (SXPS). The primary advantage of high surface sensitivity is illustrated with case studies of clean semiconductor surfaces and of the initial stages of formation of metal/semiconductor and semiconductor/semiconductor interfaces. The prospects for soft X-ray photoelectron microscopy are briefly discussed.     

Electron energy-loss spectroscopy from polar ZnO surfaces

Reflection electron energy-loss spectra (ELS) in the 2–30-eV range were obtained from ZnO ($000\overline{1}$)O and (0001)Zn faces. Measurements were performed on atomically clean surfaces and on surfaces subjected to different treatments, including exposures to atomic hydrogen and molecular oxygen. The spectra were compared with ELS, UPS, XPS and UV reflection results obtained by other investigators. An identification of the observed electronic transitions is attempted.     

Real-time reflectivity and topography imagery of depth-resolved microscopic surfaces

We have constructed an interference microscope that produces, in real time, reflectivity and topography images of surfaces with depth discrimination better than 1>mu;m . Intensity and phase images are obtained at the rate of 50 per second by use of a multiplexed lock-in detection and MMX assembler-optimized calculation routines. With a wavelength of 0.84microm , depth discrimination of 0.7microm and lateral resolution of 0.3microm were demonstrated, in good agreement with theory. Two-dimensional cross-sectional reflectivity and topography images taken at different depths in an integrated circuit are presented.     

Photo- and cathodoluminescence investigation of ZnO films

Photo- and cathodoluminescence spectra of ZnO films obtained by magnetron sputtering and annealed at different temperatures have been studied. The effect of irradiation in a scanning electron microscope on the cathodoluminescence spectra of these films is investigated. It is shown that even relatively small irradiation durations result in a noticeable change in signal intensity. Both signal enhancement and reduction can occur depending on the annealing temperature. Data obtained are explained by defect formation and rearrangement stimulated by electron irradiation.     

Hydrogen desorption effect on cathodoluminescence of ZnO

The effects of the low-energy electron beam on ZnO single crystals were investigated by cathodoluminescence (CL), Time-of-Flight Electron Stimulated Desorption (TOF-ESD) and Temperature Programmed Desorption (TPD). Under e-beam irradiation, the ultraviolet (UV) emission decreases exponentially for O-face, while it increases and then decreases for Zn-face. Meanwhile, a large desorption of H⁺ is observed by TOF-ESD for both O- and Zn-faces. On the other hand, an increase of H in the irradiated volume is observed after stopping the e-beam irradiation. Similar H₂ desorption is found by TPD for both faces. These data suggest that the decrease of the UV intensity is related to H desorption.     

ZnO filled opal arrays: Photo and cathodoluminescence studies

The photo and cathodoluminescence of zinc oxide (ZnO) filled opal arrays, as well as ZnO nanocrystals prepared by a chemical deposition method were investigated. The photoluminescence (PL) from the arrays was studied using angular resolution. The PL spectra of the filled opal exhibited a dip corresponding to the array’s photonic band gap. ZnO nanocrystals embedded in the opal matrix demonstrated quenched excitonic emission while the opal matrix showed enhanced emission. This effect is explained by an energy transfer from ZnO nanocrystals to the opal matrix.     

Nanoscale metal pillar arrays on elastomeric substrates for surface-enhanced Raman spectroscopy platform

The nanostructures found in nature sometimes have elaborate, three-dimensional structures that consist of soft and flexible constituents, and which exhibit diverse mechanical and optical functions. Here, we introduce a facile, low-cost and scalable nanofabrication approach based on a hot embossing process that can replicate sub-micron to nanoscale features on elastomeric substrates. We have further developed this technique to achieve polymer/metal heterostructure nanopillar arrays via conformal coating of Au films on polymeric templates. Each nanopillar displays a smooth surface and a constant diameter along the vertical direction. Raman spectroscopy studies revealed that the metallic nanostructures decorated with methylene blue exhibited a dominant Raman peak at 1624 cm⁻¹ that was enhanced more than 3000 times and seven times relative to bare planar Si and Au-coated planar polystyrene substrates, respectively. These results indicate that our nanopillar array can be exploited as a flexible, large area platform for surface-enhanced Raman spectroscopy.     

Electron spectroscopy studies of adsorption and oxidation processes at metal surfaces

A review of the problems involved and the success achieved in extracting analytical and chemical information from adsorption and oxidation studies at c     

Growth,structure, and cathodoluminescence of Dy-doped ZnO nanowires

Dy-doped ZnO nanowires have been prepared using high-temperature and high-pressure pulsed-laser deposition. The morphology, structure, and composition of the as-prepared nanostructures are characterized by field emission scanning electron microscopy, X-ray diffraction, Raman scattering spectrometry, X-ray photoelectron spectrometry, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The alloying droplets are located at the top of the as-prepared Dy-doped ZnO nanowires, which means that the growth of the Dy-doped ZnO nanowires is a typical vapor-liquid-solid process. The luminescence properties of Dy-doped ZnO nanowires are characterized by cathodoluminescence spectra and photoluminescence spectra at low temperature (8 K). Two peaks at 481 and 583 nm, respectively, are identified to be from the doped Dy³⁺ ions in the CL spectra of Dy-doped ZnO nanowires.     

Magnetism at surfaces and interfaces

The current state-of-the-art of ab-initio calculations of the magnetic structures of surfaces and interfaces is highlighted by presenting results obtained with the recently developed full-potential linearized augmented plane wave method for thin films. In particular, spin density maps, (induced) magnetic moments and hyperfine-fields are presented for the clean metal surfaces Fe(001), Ni(001) and Pt(001). The magnetic moments on an interface are discussed for the prototypical case Ni/Cu.     

Photo- and cathodoluminescence of the cetylpyridinium chloride: ZnO nanocomposites

The cathodoluminescence and photoluminescence (PL) spectra of ZnO nanoparticles (NPs) embedded into the cetylpyridinium chloride (CPCL) matrix were studied. The composites were obtained by drying an aqueous suspension of CPCL and ZnO NPs, with NaCl and with NaOH additives. We observed that only NaOH addition lead to a significant increase in the PL intensity which we attribute to the surface chemistry of the ZnO NPs. We propose that thin ZnOH₂ and Na₂ZnO₂ layers form on the surface of the NPs; these layers present an increased number of oxygen vacancies, which act as emitting centres.     

Phase composition diagnostics of surfaces,thin films,and interfaces by Auger electron spectroscopy

The methods of phase composition diagnostics of materials by Auger spectra, developed in IMT RAS over a number of years, are presented. Examples of their use are generally in the determination of phase depth profiles. However, the considered methods can be also used for the composition diagnostics of small size objects (nanostructures) under conditions of significant noisiness of the Auger spectra and when the studied objects probably leave an analyzed region.     

Analysis of semiconductor surfaces and interfaces

Recent development in the experimental and theoretical analysis of semiconductor surfaces is described. Special attention is given to the Secondary Ion Mass Spectroscopy technique and to its use in the ultrasensitive elemental analysis of semiconductors. Applications to III–V compounds are described.