Faqiang Xu

A sulfur passivation for GaAs surface by an organic molecular, CH3CSNH2 treatment

A sulfur passivation method for GaAs, CH3CSNH2 treatment has been developed. It is quite effective for removing the surface oxide layer and forming the sulfide passivation layer on GaAs surface. The enhancements of the PL intensity reveal the reduction of the surface recombination velocity and the reduction of density of defect states by this treatment. The synchrotron radiation photoemission spectroscopy measurements show that sulfur atoms bond both Ga and As atoms. After being annealed, a stable sulfur passivation layer is terminated on the surface due to the As2S3 component react with GaAs into the GaS component.

Charge transfer dynamics of 3,4,9,10-perylene-tetracarboxylic-dianhydride molecules on Au(111) probed by resonant photoemission spectroscopy

Charge transfer dynamics across the lying-down 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) organic semiconductor molecules on Au(111) interface has been investigated using the core-hole clock implementation of resonant photoemission spectroscopy. It is found that the charge transfer time scale at the PTCDA∕Au(111) interface is much larger than the C 1s core-hole lifetime of 6 fs, indicating weak electronic coupling between PTCDA and the gold substrate due to the absence of chemical reaction and∕or bonding.

Correlation between electronic structure and magnetic properties of Fe-doped ZnO films

Fe-doped ZnO films with different Fe concentrations that display ferromagnetism at room temperature have been prepared by plasma assisted molecular beamepitaxy (p-MBE) techniques. Synchrotron-based measurements of photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), resonant photoemission spectroscopy (RPES), and superconducting quantum interference device (SQUID) were performed to investigate the electronic structure and magnetic properties of the films. It was found by Fe 2p PES and XAS that the dominant valence state of Fe ions is Fe3+ and that the configuration of Fe ions varies from tetrahedral sites to octahedral sites as the Fe concentration increases. Results of RPES indicate that the electronic states related to Fe2+ also exist near the Fermi level and that the distribution of Fe 3d electronic states in the valence band varies with different Fe concentrations. Correlations of the magnetic properties with the electronic structure of Fe-ZnO films have established that the electronic states related to Fe2+ and localized defects like Zn vacancies play an important role for ferromagnetism of Fe-ZnO films, while Fe3+ ions at octahedral sites destabilize the ferromagnetic interactions

First-principles studies of the electronic and optical properties of 6H–SiC

Abstract We study the electronic and optical properties of hexagonal 6H–SiC crystal, using ab initio full potential augmented plane wave method. The density of states (DOS) and band structure are presented based on local density function theory. From the electronic structure calculation, the imaginary part of the dielectric function has been obtained directly using the joint DOS and the optical matrix elements. With band gap correction, the real part of dielectric function can be derived from the imaginary part by the Kramers–Kronig relationship. The reflectivity for normal incidence is also calculated. The resulting spectrum is in good agreement with available experimental data in a wide energy range.

Photophysical and electrical properties of organic waveguide nanorods of perylene-3,4,9,10-tetracarboxylic dianhydride

The single crystalline nanostructure of organic semiconductors provides a very promising class of materials for applications in modern optoelectronic devices. However, morphology control and optoelectronic property modulation of high quality single crystalline samples remain a challenge. Here, we report the morphology-controlled growth of single crystalline nanorod arrays of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). We demonstrate that, unlike PTCDA film, PTCDA nanorods exhibits optical waveguide features, enhanced absorption, and Frenkel excitation emission in the visible region. Additionally, we measured the electrical properties of PTCDA nanorods, including the conductivity along the growth direction of the nanorod, which is roughly 0.61 S·m–1 (much higher than that of pure crystalline PTCDA films).

Normal-state electronic structure in the heavily overdoped regime of Bi1.74 Pb0.38 Sr1.88 Cu O6+δ single-layer cuprate superconductors : An angle-resolved photoemission study

We explore the electronic structure in the heavily overdoped regime of the single layer cuprate superconductor Bi1.74Pb0.38Sr1.88CuO6+delta. We found that the nodal quasiparticle behavior is dominated mostly by phonons, while the antinodal quasiparticle lineshape is dominated by spin fluctuations. Moreover, while long range spin fluctuations diminish at very high doping, the local magnetic fluctuations still dominate the quasiparticle dispersion, and the system exhibits a strange metal behavior in the entire overdoped regime.

Size Effects on the Luminescence Properties of Polymer Nanowires

Poly(3-(2-methoxyphenyl)thiophene) (PMP-Th) nanowires were fabricated using porous anodic alumina (PAA) as template through electrochemical polymerization by cyclic voltammetry. The control on the size of nanowires was confirmed by electron microscopy. The results indicated that the luminescence spectra of PMP-Th nanowires in PAA nanochannels were blue-shifted and emission intensity was enhanced compared to the emission of the PMP-Th film. Moreover, the luminescent spectra of PMP-Th nanowires were size dependent, which may result from the change in the degree of confinement of nanowires in PAA. Förster energy transfer from PAA to PMP-Th molecules is considered to be responsible for the enhancement of luminescence from PMP-Th nanowires in PAA. The results show that the emission properties of polymers with nanostructures can be tuned by controlling their size.

THE Fe ULTRATHIN FILM ON ZnO(000-1) SURFACE: GROWTH, STRUCTURE, AND INTERACTION

We report the electronic structure of the interface of the ultrafilm Fe film with the O-terminated ZnO(000-1) polar surface studied by synchrotron radiation photoemission. It was found that Fe grows in a Stranski–Krastanov mode. The turning point of the growth mode was at the Fe coverage of ~2 A. At low coverage, about 1 ML (monolayer) of iron (ca. 1.5 A) was oxidized by the terminative oxygen atoms on the ZnO(000-1) surface. Analysis of the valence band and the Fe3p photoemission spectra indicates that below 2 A coverage, only Fe2+ species could be detected at the Fe/ZnO interface, after that the formation of a metallic iron overlayer was observed. A sharp Fermi edge appeared at 5.1 A coverage, indicating the formation of the bulk metallic Fe film. Furthermore, work function of the sample decreased to a minimum of 4.5 eV at 0.2 A Fe, and then to 4.9 eV with the formation of a dipole layer at the interface.

PHOTOEMISSION STUDY OF THE GADOLINIUM/GaAs(100) INTERFACE WITH SYNCHROTRON RADIATION

Soft-X-ray photoemission spectroscopy was used to characterize the Gd/GaAs(100)-interface formation at room temperature. At low Gd coverage (<1 A), the interface is near-abrupt, because no evidence of reaction is observed. With increasing Gd coverage, photoemission signals from chemically reacted product at the interface are observed, causing some intermixing between the overlayer and the substrate. For As atoms, they remain near the interface and have little diffusion. Ga atoms, however, are not kept near the interface, and they can diffuse into the Gd overlayer and segregate onto the surface instead. From the observed variations with metal coverage of binding energies and relative intensities of photoemission signals from the reacted layer, a profile of the interface structure is proposed, and some parameters (decaying length, segregation density and solution density, etc.) have been obtained. The results show that the deposition of Gd onto the GaAs(100) surface induces limited substrate disruption except for some diffusion and segregation of Ga atoms into the metal overlayer. This paper demonstrates that the disruption and epitaxial growth are not mutually exclusive in the Gd/GaAs(100) system.

Modulation of exciton transition in crystalline nanostructures of an organic semiconductor

In organic semiconductors, photophysical properties are closely related to the exciton transition (ET). Therefore, modulating ET has great significance for the photophysical properties enhancement of organic semiconductors. Researchers can modulate the ET in organic semiconductor crystals and films by applying high pressure and annealing processes which respectively affects the molecular distance in crystals and the molecular orientation in films. Yet, how to simplify the way of modulating ET in organic semiconductors remains a great challenge. Herein, we prepared 3,4,9,10-perylenetetracarboxylic dianhydride crystalline films and nanorods, and successfully modified the type of the governing ET in PTCDA crystallites so that realized the switch of emission wavelength. Particularly, we exploring ET modulation mechanism. This will be helpful for the photophysics increment of organic semiconductors.