Guangcheng Xiong

Amorphous silica nanowires: Intensive blue light emitters

We report large-scale synthesis of silica nanowires (SiONWs) using an excimer laser ablation method. Silica was produced in the form of amorphous nanowires at a diameter of ∼15 nm and a length up to hundreds micrometers. The SiONWs emit stable and high brightness blue light at energies of 2.65 and 3.0 eV. The intensity of the emission is two orders of magnitude higher than that of porous silicon. The SiONWs may have potential applications in high-resolution optical heads of scanning near-field optical microscope or nanointerconnections in future integrated optical devices.

Nanoscale silicon wires synthesized using simple physical evaporation

We report the large-scale synthesis of silicon nanowires (SiNWs) using a simple but effective approach. High purity SiNWs of uniform diameters around 15 nm were obtained by sublimating a hot-pressed silicon powder target at 1200 °C in a flowing carrier gas environment. The SiNWs emit stable blue light which seems unrelated to quantum confinement, but related to an amorphous overcoating layer of silicon oxide. Our approach can be used, in principle, as a general method for synthesis of other one-dimensional semiconducting, or conducting nanowires.

Resistive Switching in

Al/CeOx/Pt devices with nonstoichiometric CeOx (1.5<x<2) films were fabricated. The unique resistive switching (RS) behaviors for resistive random access memory applications, including stable and sharp bipolar RS processes and a multilevel and self-stop set process without current compliance and without excessive requirement on a high-voltage electroforming process, were demonstrated. A multifilament switching model based on the distribution characteristics of oxygen vacancies in CeOx films is proposed to explain the observed RS behaviors.

\hbox{CeO}_{x}

Perfect epitaxial growth of superconducting YBa2Cu3O7−x thin films have been achieved on (100) SrTiO3, (110) SrTiO3, and (100) ZrO2 substrates using a modified planar dc magnetron sputtering system. The films exhibit zero resistances at 87–90 K with transition widths of about 2 K. The critical current density of 1.4×106 A/cm2 at 77 K so far has been measured on the (100) SrTiO3 substrate. The epitaxial orientation of the thin films was influenced by the substrate orientation, the substrate temperature, and the oxygen partial pressure. The quality of growth and the epitaxial orientation of the films were examined by electron channeling, x‐ray diffraction, and reflection high‐energy electron diffraction techniques. The preferential epitaxial orientation is discussed.

Films for Nonvolatile Memory Application

Carrier injection performed in doped-manganite junction exhibits stable resistance switching effect and variety of interface barrier adjusted by electric fields, which demonstrates roles of the carrier movement across interface and suggests the importance of injected carriers’ staying. All observations suggest that the staying of injected carriers in the functional oxides leads to a carrier self-trapping mechanism to understand resistance switching phenomenon. More results in other oxide junctions and epitaxial doped manganite films support carrier self-trapping model and exhibit the importance of heterointerface on stabilizing the self-trapping carriers. Superconductivity achieved and adjusted by carrier injection can be distinct evidence that the self-trapping carriers as minority played important roles in operating of oxides’ intrinsic property. This phenomenon could relate to a novel area of researches and applications in oxide electronics.

Epitaxial growth of superconducting YBa2Cu3O7−x thin films by reactive magnetron sputtering

Abstract Epitaxial growth of La 0.5 Sr 0.5 CoO 3 (LSCO) film has been achieved by pulsed laser deposition. Widely used substrates, MgO, SrTiO 3 and LaAlO 3 were chosen to examine the effect of the substrates on the growth of LSCO films. The influence of the substrate temperatures ( T s ) on the characteristics of the LSCO films was studied in a temperature range of 500–850°C. The LSCO films deposited at substrate temperatures of T s =800–850°C show good epitaxial features, better than that of the films deposited at lower T s . The experimental results reveal that the lattice mismatch and the difference of the expansion coefficients between the substrates and the LSCO films are important in the growth process of the LSCO films.

Resistance and superconductivity switching caused by carrier injection: Evidences of self-trapping carriers in oxide electronics

Abstract Cerium oxide (CeO 2 ) is the potential dielectric candidate for the Si-based devices due to its stable chemical properties, high dielectric constant and good lattice match with silicon. In this paper, the impacts of process conditions on the structural and electrical characteristics of CeO 2 thin films deposited on Si(100) substrates have been studied. Epitaxial CeO 2 thin films with different crystal orientations on Si(100) substrates have been grown by pulsed laser deposition (PLD). The structural and electrical characteristics of CeO 2 thin films have been characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), and high frequency capacitance–voltage measurements (C–V).

Influences of substrates and substrate temperatures on characteristics of epitaxial La0.5Sr0.5CoO3 thin films

The growth mechanism and surface topography of YBa2Cu3O7 − δ (YBCO) and BiSrCaCuO-2201 (BSCCO) epitaxially grown superconducting films have been studdied by scanning tunneling microscopy. We report here two growth mechanisms: screw dislocation growth and layered growth, and also the surface characterization and surface modification of thin films under a controlled manner. High-quality epitaxial thin films were grown in situ by excimer laser ablation. YBCO thin films were epitaxially grown with the c-axis perpendicular to the SrTiO3 (100) substrate surface. On the flat substrate, the YBCO films were nucleated and grown in the screw dislocation manner. On the titled substrate (α<1 deg.), the films show layered growth. The etching caused by field-induced evaporation has been utilized to observe the initial stage of growth. Epitaxially grown BSCCO-2201 films were firstly synthesized in our laboratory with the c-axis perpendicular to the ZrO2 substrates. Layered growth has been observed by STM. No screw dislocations were identified in our samples. Larger atomic flat areas were present, which are more stable than YBCO films. As a parent phase for the BSCCO 2212, 2223 series, the study of the growth mechanism of 2201 films is very important to the synthesis of superconducting 2212, 2223 or other artificial layered thin films.

Crystal-orientation controlled epitaxial CeO2 dielectric thin films on Si(100) substrates using pulsed laser deposition

Epitaxial double-perovskite Sr2FeMoO6 films have been grown by pulsed laser deposition (PLD) on SrTiO3 substrates. High-quality metallic Sr2FeMoO6 thin films with low resistivity were obtained by deposited in a mixture gas of hydrogen and argon. Resistivity versus temperature measurements, magnetic measurement, and x-ray diffraction indicate that using a mixture forming gas of hydrogen and argon in PLD is a flexible method for preparing high-quality epitaxial Sr2FeMoO6 films. In (011) cross-section high-resolution transmission electron microscopy images of epitaxial Sr2FeMoO6 films, the clear superlattice structure of ordered cations of Fe and Mo is shown.

The growth mechanism and topography of superconducting YBa2Cu3O7-δ and BiSrCaCuO-2201 films studied by scanning tunneling microscopy

Abstract Superconducting HgBa 2 CaCu 2 O y (Hg-1212) films have been fabricated on (110) NdGaO 3 , (100) SrTiO 3 , (100) LaAlO 3 and (100) Y–ZrO 2 substrates by a two-step process. Good reproducible superconducting properties, zero resistance temperatures ( T c )>115 K and critical current density ( J c )>4000 MA/m 2 at 77 K in zero field, are obtained. X-ray diffraction patterns indicate that the films have an epitaxial structure with the c -axis perpendicular to the surface. Scanning electron microscopy measurements show a layered growth structure with square and octagonal grains on the Hg-1212 film surfaces. (110) NdGaO 3 is demonstrated to be a good substrate for growing the Hg-cuprate thin films. A lower superconducting transition temperature is observed in films deposited on Y–ZrO 2 substrates, which can be attributed to variations of the microstructure in the films.