Carl Ventrice

Novel electronic and magnetic properties of ultrathin chromium oxide films grown on Pt(111)

The growth of epitaxial metal–oxide films on lattice-mismatched metal substrates often results in the formation of unique overlayer structures. In particular, epitaxial chromium oxide films grown on Pt(111) exhibit a p(2×2) symmetry through the first two monolayers of growth which is followed by a (√3×√3)R30° phase that is attributed to the growth of a Cr2O3(0001) overlayer. Ultraviolet photoelectron spectroscopy measurements have been performed on the CrOx/Pt(111) system. The electronic structures of CrO2, Cr2O3, and Cr3O4 were calculated using the linear muffin-tin orbital method in the atomic sphere approximation. Comparison of the photoemission valence band spectra with the calculated density of states indicates that the CrOx initially grows in a cubic spinel Cr3O4 structure. Beyond ∼0.2 monolayers, the metallic behavior of the CrOx overlayer begins a transformation to an insulating state. The measured valence emission for the p(2×2) phase beyond ∼0.2 monolayers is more consistent with either a γ-Cr2O...

Influence of Chemisorbed Oxygen on the Growth of Graphene on Cu(100) by Chemical Vapor Deposition

Understanding the influence that copper substrate surface symmetry and oxygen impurities have on the growth of graphene by chemical vapor deposition is important for developing techniques for producing high-quality graphene. Therefore, we have studied the growth of graphene by catalytic decomposition of ethylene in an ultrahigh-vacuum chamber on both a clean Cu(100) surface and a Cu(100) surface predosed with a layer of chemisorbed oxygen. The crystal structure of the graphene films was characterized with in situ low energy electron diffraction. By heating the clean Cu(100) substrate from room temperature to the growth temperature in ethylene, epitaxial graphene films were formed. The crystal quality was found to depend strongly on the growth temperature. At 900 °C, well-ordered two-domain graphene films were formed. Predosing the Cu(100) surface with a chemisorbed layer of oxygen before graphene growth was found to adversely affect the crystal quality of the graphene overlayer by inducing a much higher d...

GMR multilayers on a new embossed surface

It has been shown that the deposition of magnetoresistive multilayers on stepped, corrugated or V-grooved surfaces can increase the magnitude of giant magnetoresistance (GMR). The primary reason for this enhancement of GMR is that the in-the-substrate-plane current crosses multiple magnetic layers which results in the mixed current-in-plane and current perpendicular to plane modes called current at an angle to the plane mode. In our studies, we use a novel substrate consisting of nano-hemispheres organized in a regular hexagonal array. The substrate was produced by anodization of Al and subsequent etching of alumina membrane. Scanning electron microscopy was used to investigate larger areas and cross-sectional images of the embossed surface, whereas detailed analysis of the surface structure was made by high resolution atomic force microscopy. We deposited uncoupled Co/Cu multilayers on the alumina substrate with an 8-nm-thick Fe buffer using magnetron sputtering. Our preliminary studies of the magnetotransport using a physical property measurement system (quantum design) demonstrated that the samples on the new substrate have an enhanced GMR effect compared to the samples with similar composition deposited on smooth (100) Si wafers. Because of the inexpensive method of fabrication of the embossed substrate, the GMR structures deposited on this substrate have a potential for use in magnetic sensors.

Surface morphology and electronic structure of Ni/Ag(100)

The growth morphology and electronic structure of Ni on Ag(100) has been studied with scanning tunneling microscopy (STM) and synchrotron based angle resolved photoemission spectroscopy. At deposition temperatures at or below 300 K, STM reveals Ni cluster growth on the surface along with some subsurface growth. Upon annealing to 420 K, virtually all Ni segregates into the subsurface region forming embedded nanoclusters. The electronic structure of Ni d bands in the unannealed surface shows dispersion only perpendicular to the surface whereas the annealed surface has Ni d bands that exhibit a three-dimensional-like structure. This is a result of the increased Ni d–Ag sp hybridization bonding and increased coordination of the embedded Ni nanoclusters.

Growth Morphology and Electronic Structure of Ultra-Thin TaO x Films on Ag(100)

A study of the growth morphology and electronic structure of TaO x films on the Ag(100) substrate has been performed to determine the properties of ultra-thin TaO x films without the influence of a mixed interfacial oxide (i.e., a disordered SiO 2 /TaO x interface for growth on Si). The TaO, films were grown by thermal evaporation of Ta in an oxygen atmosphere of 1 × 10 1−6 Torr. Growth on a Ag(100) surface held at room temperature results in an amorphous TaO x overlayer, as determined by low energy electron diffraction. The onset of ordering of these films occurs for a post-anneal at ∼500°C. A diffraction pattern that corresponds to a multi-domain overlayer structure is observed for anneals at ∼550°C. Deposition of Ta without oxygen results in the formation of Ta islands. These results indicate that there is a very weak adsorbatesubstrate interaction. Photoemission measurements of the TaO x films show the formation of a band gap with the valence band maximum residing at 3.5 eV below the Fermi level. Core level shifts of ∼3.5 eV are observed for the Ta with no indication of metallic Ta at the surface.