X.S. Chen

Electronic Structures of S-Doped Capped C-SWNT from First Principles Study

The semiconducting single-walled carbon nanotube (C-SWNT) has been synthesized by S-doping, and they have extensive potential application in electronic devices. We investigated the electronic structures of S-doped capped (5, 5) C-SWNT with different doping position using first principles calculations. It is found that the electronic structures influence strongly on the workfunction without and with external electric field. It is considered that the extended wave functions at the sidewall of the tube favor for the emission properties. With the S-doping into the C-SWNT, the HOMO and LUMO charges distribution is mainly more localized at the sidewall of the tube and the presence of the unsaturated dangling bond, which are believed to enhance workfunction. When external electric field is applied, the coupled states with mixture of localized and extended states are presented at the cap, which provide the lower workfunction. In addition, the wave functions close to the cap have flowed to the cap as coupled states and to the sidewall of the tube mainly as extended states, which results in the larger workfunction. It is concluded that the S-doped C-SWNT is not incentive to be applied in field emitter fabrication. The results are also helpful to understand and interpret the application in other electronic devices.

Non-universal critical Casimir force in confined 4He near the superfluid transition

We present the results of a one-loop calculation of the effect of a van der Waals type interaction potential similar tox(-d-sigma) on the critical Casimir force and specific heat of confined He-4 near the superfluid transition. We consider a He-4 film of thickness L. In the region Lgreater than or similar toxi (correlation length) we find that the van der Waals interaction causes a leading non-universal non-scaling contribution of O(xi(2) L-d-sigma) to the critical temperature dependence of the Casimir force above T-lambda that dominates the universal scaling contribution similar toe(-L/xi) predicted by earlier theories. For the specific heat we find subleading non-scaling contributions of O(L-1) and O(L-d-sigma)

Optical and local current studies on InAs/GaAs quantum dots

By optimizing the growth technique of molecular beam epitaxy (MBE), weve prepared high quality InAs quantum dot samples, with high density (up to 1.2 x 1011 cm-2) and highly homogeneous size. Strong room temperature photoluminescence (PL) have been observed. The PL study reveals four confined-states QD transitions, which shows filling effect of the excited states. Dependence of these recombinations on excitation intensity and temperature has also been investigated. Conductive atomic force microscopy (C-AFM) has been used to probe the local current of InAs surface quantum dots (SQDs) on doped GaAs layer. It is found that current about picoampere can be drawn from individual QD while bias of a few hundred mV is applied between tip and the sample. Further I-V studies discover that surface quantum dots usually has a threshold bias near 200 mV for current transmission, whereas bias of 400 mV is needed when the conductive tip is located on wetting layer.