Showing papers by "Hua Zhang published in 2002"

Thermal Desorption Behavior and Binding Properties of DNA Bases and Nucleosides on Gold

Abstract: A comparison of the binding of DNA bases (adenine, cytosine, guanine, and thymine) and nucleosides (2‘deoxyadenosine, 2‘deoxycytidine, 2‘deoxyguanosine, and thymidine) to gold thin films is presented. Desorption of monolayer/submonolayer and multilayer films of the adsorbates on gold studied via temperature-programmed desorption (TPD) and reflection−absorption infrared (RAIR) spectroscopy reveals that there are major differences in the binding affinities of the different bases to gold, for example, thymine ΔHdes = 111 ± 2 kJ/mol compared to guanine ΔHdes = 146 ± 2 kJ/mol. The differences can be rationalized by molecular structures of the bases and their binding modes to gold surfaces deduced from IR data. Similar trends in desorption energies, shifted to lower desorption energy by more than 10 kJ/mol, are observed for deoxynucleoside layers on gold thin films.

Comparison of ultraviolet light-emitting diodes with peak emission at 340 nm grown on GaN substrate and sapphire

Abstract: Based on AlInGaN/AlInGaN multiquantum wells, we compare properties of ultraviolet light-emitting diodes (LED) with peak emission at 340 nm grown on free-standing hydride vapor phase epitaxially grown GaN substrate and on sapphire. For the LED grown on GaN substrate, a differential resistance as low as 13 Ω and an output power of more than one order of magnitude higher than that of the same structure grown on sapphire are achieved. Due to higher thermal conductivity of GaN, output power of the LEDs saturates at higher injection currents compared to the devices grown on sapphire.

Self-assembly of polyphenylene dendrimers into micrometer long nanofibers: An atomic force microscopy study

Abstract: Individual polyphenylene dendrimer 1 and their self-assembled nanostructures, prepared by spin-coating and solvent casting on various substrates such as mica, silanized mica, and highly oriented pyrolytic graphite (HOPG), have been investigated by noncontact atomic force microscopy. Besides globular clusters and monolayers, polyphenylene dendrimer 1 self-organizes into micrometer long nanofibers on a HOPG surface. Fibrillar nanostructures have also been visualized on a silanized mica surface, while on a mica surface the dendrimers only aggregate into globular clusters. Two possibilities for the development of dendrimer nanofibers are proposed.

Probing carboxylic acid groups in replaced and mixed self-assembled monolayers by individual ionized dendrimer molecules: An atomic force microscopy study

Abstract: Polyphenylene dendrimer molecules, G2Td(COOH)16, were ionized (G2 ions) and then adsorbed on a carboxylic acid (COOH) group terminated self-assembled monolayer (SAM) on a Au substrate through the linkage with Cu2+ ions. The individual G2 ions were observed by noncontact atomic force microscopy (NCAFM). The strong interaction between the G2 and Cu2+ ions, the latter of which are preadsorbed on the COOH SAM, led to a compression or deformation of the SAM and resulted in a decreased height of the G2 ions measured by NCAFM as compared to a theoretical model of the dendrimer molecules. The present method offers an approach to bind dendrimer molecules to a solid substrate and to study the nanoscale behavior of two kinds of thiol molecules in replaced and mixed SAMs.

Effect of Na doping on flux pinning of YBa1.9Na0.1Cu3O7-d

Abstract: We have prepared Na-doped YBa2Cu3Oy (YBa1.9Na0.1Cu3Oy +40mol%Y211) (YBNCO) and Na-free YBa2Cu3Oy (YBCO) samples by the Melt-Textured Growth (MTG) method to study the effect of doped Na ion on flux pinning. The ac susceptibility curves (acs) as well as the hysteresis loops were measured for the samples. Then the effective pinning energy (U(T,Hdc,J)), irreversibility line (Hirr(T)) and critical current density (jc(Hdc)) were determined, where T, Hdc and J are temperature, dc magnetic field and current density, respectively. We found that, with Na doping, the Hirr(T) line shifted to lower temperature while the Jc(Hdc) and U(T,Hdc,J) became smaller. It indicates that the Na ions play a negative role in the flux pinning of YBCO. The appearance of the second peak in the Jc(Hdc) curves and the enhancement of anisotropy in YBNCO further support this finding.