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MnSb nanocrystallites and thin films have been grown on highly oriented pyrolytic graphite (HOPG) by codeposition of Mn and Sb in ultrahigh vacuum. The surface morphologies of these MnSb structures were investigated using in situ scanning tunneling microscopy (STM). With 5 nm nominal deposition, MnSb nanocrystallite chains with a typical width of 50 nm are formed along the HOPG step edges. A continuous MnSb film is formed after Mn and Sb codeposition on a HOPG precovered with an Sb wetting layer. The MnSb thin film surface exhibits mostly hexagonal-shaped MnSb(0001) plateaus of a few atomic layers in height. Atomic-resolution STM images revealed 2×2 and (23×23)R30° reconstructions on the MnSb(0001) surface and a 2×1 superstructure on MnSb(1011). X-ray photoelectron spectroscopy study showed that the Mn 2p peaks shift 2.5 eV with respect to that of elemental Mn, indicating the formation of MnSb compound. Vibrating sample magnetometry measurement showed that the MnSb film is ferromagnetic at room temperature, and is composed of mostly (0001)-oriented grains as well as some with other orientations. [ABSTRACT FROM AUTHOR]

A structural model for GaAs(100)'s sulfur passivating layer contains dimeric S-Ga in a (2 x 1) superstructure on a complete Ga layer. The model's basis is experimental data and low energy electron diffraction studies. The structure has inherently stable electrostatic characteristics that induce good surface passivation.

High resolution low-energy electron diffraction, scanning tunneling microscopy (STM), and transmission electron microscopy (TEM) have been used to study the temperature dependence of a vicinal (stepped) Si(111) surface with a polar angle of 6° from (111) along an azimuth rotated about 10° away from the high symmetry [112] direction. At the (1×1) to (7×7) reconstructive transition, the kinks at the step edges precipitate into a phase with high step density and increased rotation from the [112] direction, leaving behind a [112] oriented phase with triple-layer height steps and (7×7) reconstruction. The inclination and azimuth of the orientation of the kinked phase change continuously with temperature. When the surface is cooled to room temperature, STM images confirm a coexistence between the [112] oriented step-tripling phase and the kinked phase. In the step-tripling phase, the steps are very straight (kinks are rare) and oriented in the [112] direction. There is a small fraction of single-layer height steps between the predominant triple-layer height steps. All the terraces in this phase are (7×7) reconstructed. In the kinked phase, steps with a high density of kinks are bunched together and rotated to a direction about 45° away from [112]. TEM images of the surface topography also confirm the phase coexistence on a macroscopic scale. This phase separation (azimuthal faceting) is reversible, and can be understood thermodynamically by analogy with phase separation in a two-component fluid. [ABSTRACT FROM AUTHOR]

Germanium was deposited onto highly oriented pyrolytic graphite (HOPG) with and without antimony in ultra-high vacuum. The surface morphology was analyzed using in situ scanning tunneling microscopy (STM) at room temperature (RT). The film grows exclusively in 3D island mode and was affected significantly by substrate defects. At initial stage, nucleation of cluster occurred at step edges and defect sites. Later, we found various types of Ge nanostructures on HOPG in different deposition conditions and stages, including cluster chains, cluster islands, nanowires, and double layer ramified islands at RT. Compact Ge islands were observed when depositing at a substrate temperature of 450 K or after an annealing at 600 K following RT deposition. In addition, the pre-deposited Sb on graphite enhances the sticking probability and suppresses the surface diffusion of Ge atoms, resulting in a significant increase in Ge cluster island density on HOPG terraces. [ABSTRACT FROM AUTHOR]

The growth of Si and Ge on silicon nitride surfaces has been investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectrometer (AES). In the early stages, Si or Ge nanoclusters appeared irrespective of the different substrates. When annealed, the Si clusters were more stable against coalescence than those of Ge. As these clusters continued to grow, crystalline facets started to form. Both Si and Ge islands grew predominantly with (111)-oriented top facets on the crystalline Si3N4(0001)/Si(111). By contrast, they both grew in random orientation on the amorphous Si3N4 surface. Low-index facets such as (111) and (001) coexisted with high-index facets such as (113). [ABSTRACT FROM AUTHOR]

In solar energy harvesting using solar cells and photocatalysts, the photoexcitation of electrons and holes in semiconductors is the first major step in the solar energy conversion. The lifetime of carriers, a key factor determining the energy conversion and photocatalysis efficiency, is shortened mainly by the recombination of photoexcited carriers. We prepared and tested a series of ZnO/TiO2-based heterostructures in search of designs which can extend the carrier lifetime. Time-resolved photoluminescence tests revealed that, in ZnO/TiO2 core–shell structure the carrier lifetime is extended by over 20 times comparing with the pure ZnO nanorods. The performance improved further when Ag nanoparticles were integrated at the ZnO/TiO2 interface to construct a Z-scheme structure. We utilized these samples as photoanodes in a photoelectrochemical (PEC) cell and analyzed their solar water splitting performances. Our data showed that these modifications significantly enhanced the PEC performance. Especially, under visible light, the Z-scheme structure generated a photocurrent density 100 times higher than from the original ZnO samples. These results reveal the potential of ZnO-Ag-TiO2 nanorod arrays as a long-carrier-lifetime structure for future solar energy harvesting applications. [ABSTRACT FROM AUTHOR]

We present a low-temperature scanning tunneling microscopy investigation revealing the formation of a long range ordered α-sexithiophene (6T) monolayer nanostripe array on highly ordered pyrolytic graphite, with thiophene ring units oriented parallel to the surface. The 6T monolayer nanostripe is an effective molecular nanotemplate for the assembly of an ordered “zigzag” C60 chain array, accompanied by a slight structural modification of the underlying 6T nanostripes due to donor-acceptor intermolecular interactions between 6T and C60 molecules. [ABSTRACT FROM AUTHOR]

Ultra-fine Au nanoparticles (NPs) show great application potential in catalysis. Size-tunable Au NPs have been fabricated on MoS2 covered with monolayer 3,4,5,10-perylene tetracarboxylic dianhydride (PTCDA), and the morphological evolution as a function of Au deposition amount was investigated using scanning tunneling microscopy (STM). The PTCDA molecules act as a surfactant to stabilize ultra-fine Au NPs. Molecular scale STM images show that on MoS2 the Au NPs with PTCDA molecules on top can be formed with height and lateral size down to 1.3 nm and 3.5 nm, respectively. By controlling the deposition amount and annealing temperature, the size of Au NPs can be tuned. After annealing at 270 °C to remove PTCDA, Au NPs with a linear size ≤5 nm can be obtained on MoS2(0001), facilitating the characterization of their intrinsic physical and chemical properties using various analytical techniques. In addition, photoemission spectroscopy data reveal charge transfer from Au NPs to PTCDA, indicating that the NPs possess more reactive chemical properties than bulk Au. [ABSTRACT FROM AUTHOR]