Haiqian Wang

Negative differential-resistance device involving two C60 molecules

Negative differential-resistance (NDR) molecular device is realized involving two C60 molecules, one is adsorbed on the tip of a scanning tunneling microscope and the other is on the surface of the hexanethiol self-assembled monolayer. The narrow local density of states features near the Fermi energy of the C60 molecules lead to the obvious NDR effect. Such controllable tunneling structure and the associated known electronic states ensure the stability and reproducibility of the NDR device.

Concept of a high-resolution miniature spectrometer using an integrated filter array

A high-resolution miniature spectrometer has been demonstrated by utilizing a 128-channel integrated filter array, fabricated by using the combinatorial deposition technique, as a dispersive component whose passbands range from 722.0 to 880.0 nm with a bandwidth (or spectral resolution) from 1.7 to 3.8 nm and an average channel interval of 1.2 nm. The miniature spectrometer is smaller than 1 cm3 without any moving parts. This kind of miniature spectrometer has the advantages of very low payload, high resolution, and high reliability simultaneously, which are especially urgently needed for space applications.

Transparent and conductive oxide films with the perovskite structure: La- and Sb-doped BaSnO3

We report that (La0.07Ba0.93)SnO3 films grown epitaxially on SrTiO3(001) substrates by laser ablation at 760°C show a cubic perovskite structure of lattice constant 4.121A, excellent optical transmittance in the visible range, and a weak metallic behavior with low resistivity of about 4mΩcm within 10–300K. The transparent perovskite oxide films are n-type conductors, with carrier concentration and mobility at room temperature of about 2×1021cm−3 and 0.69cm2∕Vs, respectively, and a direct allowed band gap of 4.02eV. The effect of deposition temperature on structural, optical, and electric properties of the Sb-doped Ba(Sn0.93Sb0.07)O3 films was also demonstrated.

What can a scanning tunneling microscope image do for the insulating alkanethiol molecules on Au(111) substrates

A low temperature scanning tunneling microscope (STM) has been employed to investigate the insulating alkanethiol self-assembled monolayers chemisorbed on Au(111) substrates. The STM images show clear intramolecular patterns, which are voltage- and site-dependent. Theoretical simulations, using the density functional theory, reproduce the experimental STM images. Our results show that due to the chemisorption, there are new states appeared in the energy gap of the alkanethiol, and they are mainly composed of Au and S orbitals, mixed with a small amount of orbitals at the alkyl part. The STM only images the states localized at the tail carbon–hydrogen groups since the Au and S atoms are located farther from the STM tip, and the images can reflect the surface topography of such standing molecular layers.

Self-assembly of one-dimensional molecular and atomic chains using striped alkanethiol structures as templates

Well-ordered striped structures are developed from alkanethiol self-assembled monolayers on an Au(111) surface following well-controlled annealing processes. We demonstrate here that such regular concave–convex molecular structures can be used as the templates for growing one-dimensional molecular and atomic chains. By depositing C60 molecules onto the striped surface, C60 bimolecular chains are self-assembled. Due to the breaking of C–S bonds under certain conditions, residual S atoms can form a S monoatomic chain between two adjacent stripe pairs of thoroughly lying-down molecules. Possible growth mechanisms are discussed.Well-ordered striped structures are developed from alkanethiol self-assembled monolayers on an Au(111) surface following well-controlled annealing processes. We demonstrate here that such regular concave–convex molecular structures can be used as the templates for growing one-dimensional molecular and atomic chains. By depositing C60 molecules onto the striped surface, C60 bimolecular chains are self-assembled. Due to the breaking of C–S bonds under certain conditions, residual S atoms can form a S monoatomic chain between two adjacent stripe pairs of thoroughly lying-down molecules. Possible growth mechanisms are discussed.

Single-crystalline transparent and conductive oxide films with the perovskite structure: Sb-doped SrSnO3

Perovskite-type transparent and conductive oxide films, Sb-doped SrSnO3 (SSSO), were grown on SrTiO3(001) substrates by the pulsed laser deposition method. It is revealed that the films can be grown at relatively lower temperatures and show high single crystallinity with a cubic perovskite structure with lattice constant of 4.036A. These films show good optical transmittance in the visible range, with the direct allowed band gap of 4.53eV, and behave as an n-type semiconductor with resistivity of 23mΩcm at room temperature. The effect of growth temperature on structural, electrical, and optical properties of the SSSO films was probed, and a diode based on n-SSSO∕p-La0.67Ca0.33MnO3 structure was also demonstrated.

Fractal independently tunable multichannel filters

A simple fractal rule has been presented in this letter for the construction of one-dimensional photonic crystals with orthogonal defect states, which can be used as independently tunable multichannel filters. Each channel of the filters can be tuned independently. It overcomes the difficulty and provides an easy way for the design of multichannel filter with specific channels. In order to prove the validity of such a fractal rule, a series of two-channel filters have been fabricated by using a low frequency magnetron sputtering system. The experimental results agree with theoretical ones very well.

Effects of discrete energy levels on single-electron tunneling in coupled metal particles

We report the measurements of single-electron tunneling (SET) spectra for individual and coupled ultrasmall palladium nanoparticles using scanning tunneling microscopy. Extra fine structures, in addition to the Coulomb blockade and staircases in current–voltage curves, was observed for individual Pd particles of about 2 nm in diameter, which is attributed to the interplay of the SET effect and the effects of discrete energy levels due to significant quantum confinement effects in nanoparticles. The origination of the negative differential resistance effect in series-coupled Pd particles is also attributed to the discreteness of energy levels of both the coupled Pd particles.

Influence of surface null potential on nonvolatile bistable resistive switching memory behavior of dilutely aluminum doped ZnO thin film

We report a correlation between surface null potential and bistable resistive switching effect in dilutely Al-doped ZnO nearly transparent thin film. The nearly symmetrical bistable resistive switching was observed at low operating potential (±1 V) with good repeatability and stability, driven by surface null potential. We report that above null potential, oxygen vacancies in the proximity of aluminum provide systematic development of conducting paths. While, the switching effect was also observed to be dopant driven in the proximity to ±1 V. The phenomenon was explained using migration of Al3+ in ZnO matrix, which dominates over oxygen vacancies.

Synthesis and Photocatalytic Activity of One-dimensional ZnO-Zn2SnO4 Mixed Oxide Nanowires

Mixed oxide photocatalysts, ZnO-Zn2SnO4 (ZnO-ZTO) nanowires with different sizes were prepared by a simple thermal evaporation method. The ZnO-ZTO nanowires were characterized with a scanning electron microscope, X-ray diffraction, high-resolution transmission electron microscopy, energy-dispersive spectrometer, and X-ray photoelectron spectra. The photocatalytic activity of the ZnO-ZTO mixed nanowires were studied by observing the photodegradation behaviors of methyl orange aqueous solution. The results suggest that the ZnO-ZTO mixed oxide nanowires have a higher photocatalytic activity than pure ZnO and Zn2SnO4 nanowires. The photocatalyst concentration in the solution distinctly affects the degradation rate, and our results show that higher photodegradation efficiency can be achieved with a smaller amount of ZnO-ZTO nanowire catalyst, as compared to the pure ZnO and ZTO nanowires. Moreover, the photocatalytic activity can also be enhanced by reducing the average diameter of the nanowires. The activity of pure ZnO and ZTO nanowires are also enhanced by physically mixing them. These results can be explained by the synergism between the two semiconductors.