Zhuwei Zhang

Path‐Guided Wrinkling of Nanoscale Metal Films

The experimental part of this work was supported by the funds from NSFC (10974037), NBRPC (2010CB934102), International S&T Cooperation Program (2010DFA51970) and Eu-FP7 (No. 247644). The modeling and simulations were supported by the US National Science Foundation (Grants No. 0926851). We thank Dr. Zhang Jianming for helpful discussions.

Growth of large-domain YBa2Cu3Ox with new seeding crystals of CaNdAlO4 and SrLaGaO4

Abstract Single crystals of CaNdAlO 4 and SrLaGaO 4 were used as seeds to grow large domains of YBa 2 Cu 3 O x for levitation applications. These crystals have high melting temperatures (> 1500°C) and similar lattice structures to that of YBa 2 Cu 3 O x . In a seeded melt-texturing method developed previously, the single crystals of CaNdAlO 4 , SrLaGaO 4 , and NdBa 2 Cu 3 O x were used as seeds for comparison. After melt processing, scanning electron microscopy analysis did not reveal any major differences in all these seeded melt-textured samples. However, the levitation forces in the samples seeded with single crystals of CaNdAlO 4 and SrLaGaO 4 increased considerably compared to that of the sample seeded with NdBa 2 Cu 3 O x . A model is proposed to describe the domain growth mechanism during seeded melt processing.

The hrem observation of cross-sectional structure of carbon nanotubes

Abstract Cross-sectional structural features of carbon nanotubes have been investigated by high resolution electron microscopy (HREM). Carbon nanotube-bundles with 0.2–0.6 mm in diameter and 5–10 mm in length were produced by means of an arc-discharge method, which is suitable for preparing TEM cross-sectional samples of carbon nanotubes. Our HREM observations have shown that most of carbon nanotubes have a polyhedral and elliptical shape in a cross section, and there are many defects in the microstructure of carbon nanotubes, including edge-type dislocations and variable spacing between adjacent tube sheets, revealing that the seamless cylindric and scroll-shaped graphene sheets co-exist within the same nanotube. The abnormal structure features are closely related to the non-equilibrium growth conditions of carbon nanotubes in the arc-discharge process, and also to the accommodations of various strains taking place simultaneously in tube sheets.

Diode-pumped self-Q-switched single-frequency 946-nm Nd 3+ ,Cr 4+ :YAG microchip laser

A stable low-threshold self-Q-switched diode-pumped 946-nm Nd(3+), Cr(4+):YAG microchip laser operating in single frequency and fundamental transverse mode is reported. The output characteristics of the microchip laser were investigated under cw and pulsed pumping. By combination of the infrared microchip and an external frequency doubler, 473-nm pulses with a conversion efficiency of 18% were achieved.

Laser direct writing of nanoreliefs in Sn nanofilms.

High-resolution (approximately 200 nm) nanoreliefs, which possess a controllable height change (Deltah, up to film thickness) and transmittance or reflectance, have been successfully fabricated in 12-nm-thick Sn films by using 532 nm pulsed laser direct writing. Different from current micro/nanofabrication techniques, the height change of the nanoreliefs is generated by a laser-induced-thickening process. The majority of the height change comes from a balling and coarsening effect rather than oxidation of grains. Because both optical density and Deltah of the nanoreliefs are almost linear to laser power, the optical images can highly resemble the topographic images. This technique is useful for fabricating complicated nanorelief structures and fine images.

Efficient blue cw Nd:YAG microchip laser with two intracavity frequency doublers

A microchip cavity including a half-monolithic Nd:YAG- KNbO(3) chip and a second KNbO(3) crystal is presented. An output power of up to 30 mW at 473 nm pumped by a single stripe 1-W at 808-nm diode is achieved. The laser can be operated in either single-longitudinal or multilongitudinal mode by rotation of the second frequency-doubling crystal to 0 degrees or 90 degrees from the first one. The cw output noise is less than 2% for both cases.

Investigation of dislocations and traps in MBE grown p-InGaAs/GaAs heterostructures

Dislocations and traps in MBE grown p-InGaAs/GaAs lattice-mismatched heterostructures are investigated by Cross-section Transmission Electron Microscopy (XTEM), Deep Level Transient Spectroscopy (DLTS) and Photo-luminescence (PL). The misfit dislocations and the threading dislocations observed by XTEM in different samples with different In mole fractions and different InGaAs layer thickness generally satisfy the Dodson-Tsaos plastic flow critical layer thickness curve. The XTEM, DLTS and PL results are consistent with each other. The threading dislocations in bulk layers introduce three hole trap levels H1, H2 and H5 with DLTS activation energies of 0.32 eV, 0.40 eV, 0.88 eV, respectively, and one electron trap E1 with DLTS activation energy of 0.54 eV. The misfit dislocations in relaxed InGaAs/GaAs interface induce a hole trap level H4 with DLTS activation energy between the range of 0.67-0.73 eV. All dislocation induced traps are non-radiative recombination centers which greatly degrade the optical property of the InGaAs/GaAs layers

Study on readout durability of super-RENS disk

Characteristics essential for the readout durability of a superresolution near-field structure (super-RENS) disk are studied experimentally by using a home-built optical measuring setup and atomic force microscope, based on a simplified PtOx super-RENS disk. The experimental results show that for a super-RENS disk with constant structure and materials, readout signals including transmittance and reflectance vary with changes in bubble shape and size, indicating that the readout durability of the disk has a strong dependence on bubble stability, which is closely related to the thickness of the cover layer, the recording power and readout power, and the mechanical properties of the dielectric layer. Based on our experimental results, the main direction for improving readout durability is also proposed.

Cross-sectional transmission electron microscopic study on carbon nanotubules

Structural features of carbon nanotubules produced by arc-discharge have been investigated by high resolution electron microscopy (HREM) along the cross-sectional direction of tubules. Frequently, nested hollow tubules formed by successive cylindrical graphite sheets are found to be polyhedral or elliptical in cross-section which are perpendicular to the tube-axis. Varied spacing between adjacent tube sheets are observed and edge-type dislocations can be distinguished in some tubules. These abnormal structural features are related to accommodations of various strains taking place simultaneously in tube sheets.

Transparency conversion mechanism and laser induced fast response of bimetallic Bi/In thin film

Transparency conversion mechanism and laser induced fast response velocity of bimetallic Bi/In thin film is studied. Heat-treatment and laser exposure with different pulse width induced transparency is investigated by using ultraviolet-visible (UV-VIS) spectrometer, X-ray diffraction (XRD), Auger Electron Scan (AES), microscope and field emission scanning electron microscopy (FESEM). Research results show that oxidation is regarded as the reason for heat treated and long-pulsed laser exposure induced transparency conversion. Laser ablation is demonstrated to be the main reason for short-pulsed (~7ns) laser induced transparency conversion. For Bi/In thin film covered with a protection layer of (ZnS)0.85(SiO2)0.15 thin film, it exhibit fast response as fast as less than 100ns. The conclusions contribute to understanding and development of materials for thermal resist, photomask, optical storage medium and transparent conductive oxide with better performance.