Qinlin Guo

Quintuple-layer epitaxy of thin films of topological insulator Bi2Se3

We report the growth of atomically smooth, single crystalline Bi2Se3 thin films on Si(111) by using molecular beam epitaxy. Scanning tunneling microscopy, low-energy electron diffraction, X-ray photoelectron emission spectroscopy and Raman spectroscopy were used to characterize the stoichiometry and crystallinity of the film. The film grows in a self-organized quintuple-layer by quintuple-layer mode, and atomically smooth film can be obtained with the thickness down to one quintuple-layer (~1nm).Atomically smooth, single crystalline Bi2Se3 thin films were prepared on Si(111) by molecular beam epitaxy. Scanning tunneling microscopy, low-energy electron diffraction, x-ray photoelectron emission spectroscopy, and Raman spectroscopy were used to characterize the stoichiometry and crystallinity of the film. The films grow in a self-organized quintuple layer by quintuple-layer mode, and atomically smooth films can be obtained, with controllable thickness down to one quintuple layer (∼1 nm).Atomically smooth, single crystalline Bi2Se3 thin films were prepared on Si(111) by molecular beam epitaxy. Scanning tunneling microscopy, low-energy electron diffraction, x-ray photoelectron emission spectroscopy, and Raman spectroscopy were used to characterize the stoichiometry and crystallinity of the film. The films grow in a self-organized quintuple layer by quintuple-layer mode, and atomically smooth films can be obtained, with controllable thickness down to one quintuple layer (∼1 nm).

Surface Structural Evolution in Iron Oxide Thin Films

Ordered iron oxide ultrathin films were fabricated on a single-crystal Mo(110) substrate under ultrahigh vacuum conditions by either depositing Fe in ambient oxygen or oxidizing preprepared Fe(110) films. The surface structure and electronic structure of the iron oxide films were investigated by various surface analytical techniques. The results indicate surface structural transformations from metastable FeO(111) and O-terminated Fe(2)O(3)(0001) to Fe(3)O(4)(111) films, respectively. The former depends strongly on the oxygen pressure and substrate temperature, and the latter relies mostly upon the annealing temperature. Our experimental observations are helpful in understanding the mechanisms of surface structural evolution in iron oxides. The model surfaces of Fe-oxide films, particularly O-terminated surfaces, can be used for further investigation in chemical reactions (e.g., in catalysis).

Layer-by-layer growth of polar MgO(111) ultrathin films

By alternate deposition of Mg and exposure of O2, layer-by-layer growth, polar MgO(111) ultrathin films with Mg-terminated or O-terminated surfaces have been successfully fabricated on Mo(110) substrate. The surface geometric structure and electronic structures of the polar MgO(111) films were investigated using surface analysis techniques including low-energy electron diffraction and photoelectron emission and electron energy loss spectroscopies. The results indicate that the O-terminated surface is of an insulating character, while for Mg-terminated surface, a prominent new surface state at 2-3 eV and appreciable density of states near Fermi level have been observed. The polar oxide films provide ideal model surfaces for further investigation of support-particle system.

On the thermal stability of copper deposits on a (0001) sapphire surface

Abstract The thermal stability of copper deposited on a sapphire, α-Al 2 O 3 (0001)-1 × 1 substrate in UHV has been studied by Auger electron spectroscopy and electron energy-loss spectroscopy. The results indicate formation of a bond between the initially deposited copper and oxygen sites, and a Cu(I) state. The ultrathin Cu films, deposited upon room-temperature substrates, are stable on the substrate at temperatures below 430°C. For comparison, an ultraviolet photoelectron spectroscopy study was made on Cu deposited on Al 2 O 3 /A1(111).

Thickness-dependent metal–insulator transition in V2O3 ultrathin films

In this study, V2O3 ultrathin films about 5–20 nm thick were prepared on Al2O3 (0001) substrates through a reactive evaporation process. Auger electron spectroscopy and x-ray photoelectron spectroscopy have been used in situ to characterize their compositions and chemical states. Electric resistance measurements show that V2O3 films transform from metallic to semiconducting with the decrease of film thickness, which results from the a1g level rising because the lattice mismatch between the substrate and the film expands the c/a parameter ratio. No temperature-induced metal–insulator transition (like that in bulk V2O3) was observed in V2O3 thin films at low temperature. We conclude that stress plays a major role in suppressing the temperature-induced metal–insulator transition.

HREELS and LEED studies on Cu deposited on 1 × 1 reconstructed α-Fe2O3(0001) surfaces

High-resolution electron energy-loss spectroscopy (HREELS), low-energy electron diffraction, and X-ray photoelectron spectroscopy have been used to study clean 825 K-preannealed α-Fe2O3-1 × 1 (haematite) surfaces, an α-Fe2O3-(0001)-1 × 1 surface reconstructed with Fe3O4(111)-1 × 1 and to study Cu deposited on room-temperature surfaces of those. Three pronounced losses, at 47.5, 55.5 and 78.0 meV, of the surface phonons for the clean α-Fe2O3(0001) were observed. By deposition of copper, CuO vibrational features observed by HREELS indicate formation of a Cu(I) state for the very low coverages. Increased submonoloayer amounts of Cu result in clustering of the copper, leading for both the α-Fe2O3(0001)-1 × 1 and the reconstructed composite substrate surfaces to Cu(111) epitaxial growth.

Initial Oxidation and Interfacial Diffusion of Zn on Faceted MgO(111) Films

The interaction of zinc and faceted MgO(111) thin films prepared on a Mo(110) substrate was investigated in situ by using various surface analysis techniques, including X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, high-resolution electron energy loss spectroscopy, and low-energy electron diffraction. The results revealed that three-dimensional Zn islands exist on the faceted MgO(111) films and that no chemical interaction takes place at the interface at room temperature. Initially, deposited Zn is stable at temperatures below 400 K and diffuses into MgO at temperatures above 425 K. A portion of Zn is oxidized at approximately 10 (-6) mbar O 2 at room temperature. An interfacial phase of Zn x Mg 1- x O was formed after Zn was exposed to approximately 10 (-6) mbar O 2 at temperatures >or=500 K. The faceted structure on the MgO(111) surface is of a disadvantage for the epitaxial growth of ZnO films.

Insulating phase at low temperature in ultrathin La0.8Sr0.2MnO3 films.

Metal-insulator transition is observed in the La0.8Sr0.2MnO3 thin films with thickness larger than 5 unit cells. Insulating phase at lower temperature appeared in the ultrathin films with thickness ranging from 6 unit cells to 10 unit cells and it is found that the Mott variable range hopping conduction dominates in this insulating phase at low temperature with a decrease of localization length in thinner films. A deficiency of oxygen content and a resulting decrease of the Mn valence have been observed in the ultrathin films with thickness smaller than or equal to 10 unit cells by studying the aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy of the films. These results suggest that the existence of the oxygen vacancies in thinner films suppresses the double-exchange mechanism and contributes to the enhancement of disorder, leading to a decrease of the Curie temperature and the low temperature insulating phase in the ultrathin films. In addition, the suppression of the magnetic properties in thinner films indicates stronger disorder of magnetic moments, which is considered to be the reason for this decrease of the localization length.

Growth and electronic structure of Cu on Cr2O3(0001)

The deposition of Cu at room temperature on a Cr2O3(0001) substrate is studied by x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and low-energy-electron diffraction. The results indicate that at RT Cu is highly dispersed on the substrate at initial deposition. X-ray induced Auger spectra, Auger parameter and ultraviolet photoelectron spectroscopy show that at the initial coverage the deposited Cu is in the Cu(I) state due to the interaction of Cu with the Cr2O3 substrate; Cu becomes metallic at Cu coverages of }4

Atomic structure of Sr-induced reconstructions on the Si(100) surface

>> 4 monolayer equivalent. The formation of Cu two-dimensional or quasi-2D patches is followed by the formation of Cu three-dimensional clusters. Cu grows epitaxially on the Cr2O3(0001) films as Cu(111)R30° as observed by low-energy-electron diffraction.