J. F. Wendelken

Island morphology and adatom energy barriers during homoepitaxy on Cu(001)

Abstract Island morphologies and island separations were determined from the two-dimensional diffraction patterns measured with high-resolution low-energy electron diffraction during sub-monolayer homoepitaxy on the Cu(001) surface. In the temperature range considered, islands were found to have square shapes with edges along the [110] direction indicating a low energy-barrier for diffusion along close packed step edges. By measuring the temperature dependence of the average island separation in the diffusion limited steady-state growth regime, the energy barrier for adatom diffusion and the binding energy for small islands were determined.

A cryogenic Quadraprobe scanning tunneling microscope system with fabrication capability for nanotransport research.

We describe the development and the capabilities of an advanced system for nanoscale electrical transport studies. This system consists of a low temperature four-probe scanning tunneling microscope (STM) and a high-resolution scanning electron microscope coupled to a molecular-beam epitaxy sample preparation chamber. The four STM probes can be manipulated independently with subnanometer precision, enabling atomic resolution STM imaging and four-point electrical transport study of surface electronic systems and nanostructured materials at temperatures down to 10 K. Additionally, an integrated energy analyzer allows for scanning Auger microscopy to probe chemical species of nanostructures. Some testing results are presented.

LEED observations of oxygen ordering on Cu(110)

LEED observations were used to study oxygen adsorbed on Cu(110) and to follow changes in surface structure produced by 5 min anneals at up to 880 K. Factors determining the surface structure include the surface temperature Ts at exposure, the amount of exposure, and the maximum Ts after exposure. For surfaces exposed at Ts = 83 ± 1 K to between 0.3 and 100 langmuir (L) of room-temperature molecular oxygen, the LEED patterns upon exposure and after anneals are described for the first time in a detailed and systematic way. Upon exposure the surface exhibits a (1 × 1) pattern, although diffuse third-order reflections corresponding to an increase in the [110] dimension of the unit mesh are visible for coverages θ ≳ 0.50. A (3 × 1) pattern, which may be due to a molecular configuration, is seen after annealing at 180 K. After annealing at 330 K or higher, a simple relationship exists between oxygen coverage and surface structure, as inferred from LEED patterns; if θ ≲ 0.50, a (2 × 1) pattern is seen; if 0.50 ≲ θ ≲ 0.67, a superposition of a (2 × 1) and c(6 × 2) pattern is seen; if θ ≅ 0.67, a c(6 × 2) pattern is seen; and if θ ≳ 0.67, a c(6 × 2) pattern with additional reflections is seen. For surfaces exposed at 300 ≲ Ts ≲ 320 K to up to 105 L oxygen, the LEED patterns are similar to those reported by others; here too, however, previously unreported behavior is presented. The observations are interpreted in light of UPS and recent XPS data, which give information regarding the chemical state (atomic or molecular) and amount of the adsorbate.

The adsorption and desorption of ferrocene on Ag(100)

The molecular adsorption and desorption of ferrocene, Fe(C5H5)2, on Ag(100) have been studied by both photoemission and thermal desorption. Photoemission results indicate that the initially adsorbed surface species closely resemble that of molecular ferrocene. The molecule is adsorbed with the cyclopentadienyl (C5H5) ring ligands parallel to the surface, as determined by electron energy-loss spectroscopy. The shift in photoemission binding energies relative to the gas phase is largely independent of the molecular orbital. The ultraviolet light does lead to partial fragmentation of the ferrocene, but fragmentation occurs only in the presence of incident radiation. The energetics of molecular desorption are influenced by lateral interactions within the molecularly adsorbed film.

An XPS study of oxygen adsorption on Cu(110)

Abstract XPS was used to study oxygen adsorption on Cu(110) and to follow changes in oxygen coverage θ resulting from isochronal anneals at up to 880 K. Clean surfaces or surfaces preexposed to oxygen and then annealed to effect adsorbate ordering were exposed at a surface temperature of 83 ± 1 K (denoted as T l or between 300 and 320 K (denoted as T h ) to up to 10 5 L of room-temperature molecular oxygen. Spectra for clean surfaces exposed at T l or T h exhibited peaks near 530.15 ± 0.2 eV BE having FWHMs of about 1.9 or 1.6 eV, respectively. These peaks are attributed to photoemission from 1s levels of chemisorbed oxygen. Normalized areas under these peaks were used to obtain curves of θ versus exposure and sticking probability versus θ. Adsorption at both T l and T h is describable in terms of a mobile-precursor kinetics, with initial sticking probabilities of about 0.77 and 0.23, respectively. Chemisorption at T l was greatly inhibited by the presence of a preadsorbed layer of ordered atomic oxygen. The broadest O 1s XPS spectra remained broad even after anneals to 180 K but began to sharpen during anneals at 230 K. Anneals at up to 480 K produced no measurable change in θ. Higher-temperature anneals reduced θ to about 0.50 if θ ≳ 0.50 initially; no change in θ was observed if θ ≲ 0.50 initially.

Magnetic nanostructures fabricated by scanning tunneling microscope-assisted chemical vapor deposition

We have successfully used scanning tunneling microscope-assisted chemical vapor deposition to fabricate magnetic nanostructures as fine as 5 nm wide and <2 nm high using ferrocene [Fe(C5H5)2] as the metal-organic source gas. The physical properties of these nanostructures were qualitatively characterized and ex situ magnetic force microscopy measurements indicate these features are strongly magnetic.

Imaging and manipulation of the competing electronic phases near the Mott metal-insulator transition

The complex interplay between the electron and lattice degrees of freedom produces multiple nearly degenerate electronic states in correlated electron materials. The competition between these degenerate electronic states largely determines the functionalities of the system, but the invoked mechanism remains in debate. By imaging phase domains with electron microscopy and interrogating individual domains in situ via electron transport spectroscopy in double-layered Sr3(Ru1-xMnx)2O7 (x = 0 and 0.2), we show in real-space that the microscopic phase competition and the Mott-type metal-insulator transition are extremely sensitive to applied mechanical stress. The revealed dynamic phase evolution with applied stress provides the first direct evidence for the important role of strain effect in both phase separation and Mott metal-insulator transition due to strong electron-lattice coupling in correlated systems.

High coverages of oxygen on Cu(110) investigated with XPS, LEED, and HREELS

Oxygen sites and coverages involved in the oxidation of Cu(110) have been investigated using HREELS, XPS and LEED. High coverages of atomic oxygen were formed using low exposures of O2 by adsorbing at 15–20 K, irradiating with X-rays, and subsequently warming the crystal to 300 K. Coverages were determined from O 1s areas in XPS. HREELS measurements disclose three surface sites and one subsurface site during the initial oxidation process. Long-bridge and tilted 3- or 4-coordinated sites are occupied before oxygen moves below the surface to form Cu2O. Oxide growth proceeds in islands. After the oxide appears, a third surface site is occupied when O atoms are insufficiently mobile to enter bulk sites. Only p(2 × 1) and c(6 × 2) long-range structures are observed with LEED.

Resonance electron scattering by physisorbed and chemisorbed O2 on Pt(111)

Abstract We have investigated resonance electron scattering by both physisorbed and chemisorbed molecular O 2 on Pt(111). In each case, the molecular σ*-shape resonance is manifest in the vibrational excitation cross section but the resonance energy is lowered in comparison with the free molecule. We attribute the shift in energy (2.25 eV) in the physisorbed molecule primarily to an image potential effect. We identify the increased bond length of the chemisorbed molecule as an important factor in the larger shift (5.75 eV) observed in this case. This conclusion is supported by a consideration of data for adsorbed molecules containing the CO group.

Vicinal and on-axis surfaces of 6H-SiC(0001) thin films observed by scanning tunneling microscopy

Abstract Surfaces of 6H-SiC(0001) homoepitaxial layers deposited on vicinal (∼3.5° off (0001) towards [1120]) and on-axis 6HSiC wafers by chemical vapour deposition have been investigated using ultra-high vacuum scanning tunneling microscopy. Undulating step configurations were observed on both the on-axis and the vicinal surfaces. The former surface possessed wider terraces than the latter. Step heights on both surfaces were ∼0.25 nm corresponding to single bilayers containing one Si and one C layer. After annealing at T >1100°C for 3–5 min in UHV, selected terraces contained honeycomb-like regions caused by the transformation to a graphitic surface as a result of Si sublimation. A model of the observed step configuration has been proposed based on the observation of the [2110] or [1210] orientations of the steps and energetic considerations. Additional deposition of very thin (∼2 nm) SiC films on the above samples by gas source molecular beam epitaxy was performed to observe the evolution of the surface structure. Step bunching and growth of 6HSiC layers and formation of 3CSiC islands were observed on the vicinal and the on-axis surfaces, respectively, and controlled by the diffusion lengths of the adatoms.