Brandon Fisher

Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs

Borophene: Boron in two dimensions Although bulk allotropes of carbon and boron differ greatly, small clusters of these elements show remarkable similarities. Boron analogs of two-dimensional carbon allotropes such as graphene have been predicted. Now Mannix et al. report the formation of two-dimensional boron by depositing the elemental boron onto a silver surface under ultrahigh-vacuum conditions (see the Perspective by Sachdev). The graphene-like structure was buckled, weakly bonded to the substrate, and metallic. Science, this issue p. 1513; see also p. 1468 A two-dimensional boron allotrope forms after depositing its elemental vapor on a silver surface in vacuum. [Also see Perspective by Sachdev] At the atomic-cluster scale, pure boron is markedly similar to carbon, forming simple planar molecules and cage-like fullerenes. Theoretical studies predict that two-dimensional (2D) boron sheets will adopt an atomic configuration similar to that of boron atomic clusters. We synthesized atomically thin, crystalline 2D boron sheets (i.e., borophene) on silver surfaces under ultrahigh-vacuum conditions. Atomic-scale characterization, supported by theoretical calculations, revealed structures reminiscent of fused boron clusters with multiple scales of anisotropic, out-of-plane buckling. Unlike bulk boron allotropes, borophene shows metallic characteristics that are consistent with predictions of a highly anisotropic, 2D metal.

Atomic-Scale Investigation of Graphene Grown on Cu Foil and the Effects of Thermal Annealing

We have investigated the effects of thermal annealing on ex-situ chemically vapor deposited submonolayer graphene islands on polycrystalline Cu foil at the atomic-scale using ultrahigh vacuum scanning tunneling microscopy. Low-temperature annealed graphene islands on Cu foil (at ∼430 °C) exhibit predominantly striped Moiré patterns, indicating a relatively weak interaction between graphene and the underlying polycrystalline Cu foil. Rapid high-temperature annealing of the sample (at 700-800 °C) gives rise to the removal of Cu oxide and the recovery of crystallographic features of the copper that surrounds the intact graphene. These experimental observations of continuous crystalline features between the underlying copper (beneath the graphene islands) and the surrounding exposed copper areas revealed by high-temperature annealing demonstrates the impenetrable nature of graphene and its potential application as a protective layer against corrosion.

Silicon Growth at the Two-Dimensional Limit on Ag(111)

Having fueled the microelectronics industry for over 50 years, silicon is arguably the most studied and influential semiconductor. With the recent emergence of two-dimensional (2D) materials (e.g., graphene, MoS2, phosphorene, etc.), it is natural to contemplate the behavior of Si in the 2D limit. Guided by atomic-scale studies utilizing ultrahigh vacuum (UHV), scanning tunneling microscopy (STM), and spectroscopy (STS), we have investigated the 2D limits of Si growth on Ag(111). In contrast to previous reports of a distinct sp(2)-bonded silicene allotrope, we observe the evolution of apparent surface alloys (ordered 2D silicon-Ag surface phases), which culminate in the precipitation of crystalline, sp(3)-bonded Si(111) nanosheets. These nanosheets are capped with a √3 honeycomb phase that is isostructural to a √3 honeycomb-chained-trimer (HCT) reconstruction of Ag on Si(111). Further investigations reveal evidence for silicon intermixing with the Ag(111) substrate followed by surface precipitation of crystalline, sp(3)-bonded silicon nanosheets. These conclusions are corroborated by ex situ atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Even at the 2D limit, scanning tunneling spectroscopy shows that the sp(3)-bonded silicon nanosheets exhibit semiconducting electronic properties.

Inclined-substrate deposition of biaxially textured magnesium oxide thin films for YBCO coated conductors

Highly textured MgO films were grown by the inclined-substrate deposition (ISD) technique at a high deposition rate. A columnar grain with a roofing-tile-shaped surface was observed in these MgO films. X-ray pole figure, and /and x-scan were used to characterize in-plane and out-of-plane textures. MgO films deposited when the incline angle a was 55 and 30 exhibited the best in-plane and out-of-plane texture, respectively. High-quality YBCO films were epitaxially grown on ISD-MgO-buffered Hastelloy C substrates by pulsed laser deposition. Tc ¼ 88 K, with sharp transition, and jc values of � 2 � 10 5 A/cm 2 at 77 K in zero field were observed on films 5 mm wide and 1 cm long. This work has demonstrated that biaxially textured ISD MgO buffer layers deposited on metal substrates are excellent candidates for fabrication of high-quality YBCO coated conductors. 2002 Elsevier Science B.V. All rights reserved.

Development of coated conductors by inclined substrate deposition

Abstract Inclined substrate deposition (ISD) offers the potential for rapid production of high-quality biaxially textured buffer layers suitable for YBa 2 Cu 3 O 7− δ (YBCO)-coated conductors. We have grown biaxially textured magnesium oxide (MgO) films on Hastelloy C276 (HC) substrates by ISD at deposition rates of 20–100 A/s. Scanning electron microscopy of the ISD MgO films showed columnar grain structures with a roof-tile-shaped surface. X-ray pole figure analysis revealed that the c -axis of the ISD MgO films is titled at an angle ≈32° from the substrate normal. A small full-width at half maximum of ≈9° was observed for the φ -scan of MgO films. YBCO films were grown on ISD MgO buffered HC substrates by pulsed laser deposition and were determined to be biaxially aligned with the c -axis parallel to the substrate normal. The orientation relationship between the ISD template and the top YBCO film was investigated by X-ray pole figure analysis and transmission electron microscopy. A transport critical current density of J c =5.5×10 5 A/cm 2 at 77 K in self-field was measured on a YBCO film that was 0.46-μm thick, 4-mm wide, 10-mm long.

Pulsed laser deposition of YBCO films on ISD MgO buffered metal tapes

Biaxially textured magnesium oxide (MgO) films deposited by inclined-substrate deposition (ISD) are desirable for rapid production of high-quality template layers for YBCO-coated conductors. High-quality YBCO films were grown on ISD MgO buffered metallic substrates by pulsed laser deposition (PLD). Columnar grains with a roof-tile surface structure were observed in the ISD MgO films. X-ray pole figure analysis revealed that the (002) planes of the ISD MgO films are tilted at an angle from the substrate normal. A small full-width at half maximum (FWHM) of ≈9° was observed in the -scan for ISD MgO films deposited at an inclination angle of 55°. In-plane texture in the ISD MgO films developed in the first ≈0.5 μm from the substrate surface, and then stabilized with further increases in film thickness. Yttria-stabilized zirconia and ceria buffer layers were deposited on the ISD MgO grown on metallic substrates prior to the deposition of YBCO by PLD. YBCO films with the c-axis parallel to the substrate normal have a unique orientation relationship with the ISD MgO films. An orientation relationship of YBCO100∥MgO111 and YBCO010∥MgO110 was measured by x-ray pole figure analyses and confirmed by transmission electron microscopy. A Tc of 91 K with a sharp transition and transport Jc of 5.5 × 105 A cm−2 at 77 K in self-field were measured on a YBCO film that was 0.46 μm thick, 4 mm wide and 10 mm long.

Growth and properties of YBCO-coated conductors fabricated by inclined-substrate deposition

YBCO-coated conductors with high current-carrying capability are desirable for electric power transmission applications. Inclined-substrate deposition (ISD) is capable of producing high-quality biaxially textured template films, which are important for fabrication of YBCO-coated conductors. We have grown biaxially textured ISD-MgO template films on flexible metallic substrates at deposition rates of 2-10 nm/sec. Columnar grains with a roof-tile-shaped surface structure were observed on the ISD-MgO films. X-ray pole figure analysis revealed that the ISD-MgO film is biaxially textured and its c-axis is titled at an angle from the substrate normal. Strontium ruthenium oxide (SRO) buffer films were epitaxially grown on ISD-MgO by pulsed laser deposition prior to the deposition of YBCO. Low /spl phi/-scan full-width at half maximum (FWHM) values of 6/spl deg/ and 7/spl deg/ were observed for YBCO and SRO, respectively. T/sub c/ of 91 K with a sharp transition and transport J/sub c/ over 1.4 MA/cm/sup 2/ at 77 K in self-field were measured on YBCO coated conductors grown with ISD MgO architectures using a SRO buffer.

Comparison between bulk and nanoscale copper-silicide: Experimental studies on the crystallography, chemical, and oxidation of copper-silicide nanowires on Si(001)

Self-assembled copper-silicide (Cu-Si) nanowires were prepared by the evaporation of Cu onto Si(001) under high vacuum conditions. The Cu-Si nanowires were studied in situ by scanning electron microscopy. Crystallographic, structural, and chemical properties of the nanowires were investigated by transmission electron microscopy at cross-sections of these nanowires fabricated by focused ion beam. In addition, chemical of nanowires were also studied by scanning transmission electron microscopy. The morphology of Cu-Si nanowires due to oxidation at ambient conditions was investigated as well. Experimental results obtained from the Cu-Si nanowires were compared to bulk data reported previously by other authors.

Growth and properties of YBCO-coated conductors on biaxially textured MgO films prepared by inclined substrate deposition

YBa2Cu3O7?? (YBCO) films were fabricated on SrRuO3 (SRO)-buffered MgO templates grown on Hastelloy C276 metallic substrates, on which the MgO layers had been deposited by inclined substrate deposition (ISD) using electron beam evaporation. YBCO and SRO films were deposited by pulsed laser deposition (PLD). ISD-MgO substrates fabricated with two different substrate inclination angles (? = 35? and 55?) were used to grow YBCO films. High transport critical current density, Jc = 1.4 ? 106?A?cm?2, has been measured at 77?K in self-field for YBCO film grown on ISD-MgO with ? = 35?, whereas YBCO film grown on ISD-MgO with ? = 55? had a lower Jc = 0.5 ? 106?A?cm?2. X-ray pole figure patterns revealed a cube-on-cube orientation relationship among YBCO, SRO, and ISD-MgO films, with the c-axis of the YBCO film being tilted. X-ray ? and -scans revealed good in-plane and out-of-plane textures of YBCO film grown on ISD-MgO substrate. The YBCO film grown on ISD-MgO with substrate inclination angle ? = 35? had a YBCO(005) -scan full width at half maximum (FWHM) = 5.8? and a YBCO(007) ?-scan FWHM = 2.8?, while YBCO film with ? = 55? had YBCO(005) -scan FWHM = 5.4? and YBCO(007) ?-scan FWHM = 2.6?.

Texture development of MgO buffer layers grown by inclined substrate deposition

Biaxially textured magnesium oxide (MgO) films used as template layers for YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO)-coated conductors have been grown efficiently and consistently by inclined substrate deposition (ISD). Further improvement in texture and a decrease in surface roughness were obtained by depositing a homoepitaxial MgO layer on the ISD MgO layer at an elevated temperature and flat angle. The texture of the ISD layer was studied as a function of thickness by X-ray diffraction and scanning and transmission electron microscopy. Surface roughness of the ISD and homoepitaxial layers was investigated by atomic force microscopy. Based on the results, the optimal thickness of the ISD layer was determined.