The sensitivity for detecting hydrogen with multiple ZnO nanorods is found to be greatly enhanced by sputter-depositing clusters of Pd on the surface. The resulting structures show a change in room- temperature resistance upon exposure to hydrogen concentrations in N2 of 10–500ppm of approximately a factor of 5 larger than without Pd. Pd-coated ZnO nanorods detected hydrogen down to <10ppm, with relative responses of >2.6% at 10ppm and >4.2% at 500ppm H2 in N2 after a 10min exposure. There was no response at room temperature to O2. Approximately 95% of the initial ZnO conductance after exposure to hydrogen was recovered within 20s by exposing the nanorods to either air or pure O2. This rapid and easy recoverability make the Pd-coated nanorods suitable for practical applications in hydrogen-selective sensing at ppm levels at room temperature with <0.4mW power consumption.

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High-level theoretical methods (UB3LYP/6-311++g(2df,p), RMP2/6-311++g(d,p), CBS-4M, CBS-Q, and G3) were used to study the structures and bond dissociation energies (BDE) of the X-Y-NO molecules. The data were used to evaluate the previous experimental and theoretical results. It was found that the syn conformation is favored by CH 3 -Y-NO, C 2 H 5 -Y-NO, and CH 3 O-Y-NO (Y ) C, N, O, S), whereas the anti conformation is favored by CH 3 CO-Y-NO and Ph-Y-NO (Y ) C, N, O). For Ph-S-NO, the syn conformation is preferred because of the long S-N bond. When X is an alkyl substituent, the Y-NO BDEs increase in the order X-S-NO (∼30 kcal/mol) < X-CH 2 -NO (∼40 kcal/mol) < X-O-NO (∼43 kcal/ mol) < X-NH-NO (48 kcal/mol). When X is an aromatic substituent, the Y-NO BDEs increase in the order X-O-NO (∼21 kcal/mol) < X-S-NO (26 kcal/mol) < X-CH 2 -NO (∼30 kcal/mol) < X-NH-NO (∼35 kcal/mol). The solvent effects of acetonitrile on the free energy change of C-NO and N-NO homolysis are significant, which are about 3-5 kcal/mol. The solvent effects of acetonitrile on the free energy change of O-NO and S-NO homolysis are relatively small, which are about 1-2 kcal/mol. Finally, we found that the remote substituent effects on C-NO, N-NO, O-NO, and S-NO BDEs have F + values of -0.4∼-0.9, 1.7-1.8, 3.2-3.9, and 1.2-1.7 kcal/mol. These values are significantly different from those on the C-H (0.4-0.6 kcal/mol), N-H (3.4-4.6 kcal/mol), O-H (4.1-5.7 kcal/mol), and S-H (2.0-3.8 kcal/ mol) BDEs. Therefore, the ground effects are important for the net substituent effects on BDEs.

There is renewed emphasis on development of robust solid-state sensors capable of uncooled operation in harsh environments. The sensors should be capable of detecting chemical, gas, biological or radiation releases as well as sending signals to central monitoring locations. We discuss the advances in use of GaN-based solid-state sensors for these applications. AlGaN/GaN high electron mobility transistors (HEMTs) show a strong dependence of source/drain current on the piezoelectric polarization-induced two-dimensional electron gas (2DEG). Furthermore, spontaneous and piezoelectric polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors to detect gases, polar liquids and mechanical pressure. AlGaN/GaN HEMT structures have been demonstrated to exhibit large changes in source–drain current upon exposing the gate region to various block co-polymer solutions. Pt-gated GaN Schottky diodes and Sc2O3/AlGaN/GaN metal-oxide semiconductor diodes also show large change in forward currents upon exposure to H2. Of particular interest is detection of ethylene (C2H4), which has strong double bonds and hence is difficult to dissociate at modest temperatures. Apart from combustion gas sensing, the AlGaN/GaN heterostructure devices can be used as sensitive detectors of pressure changes. In addition, large changes in source–drain current of the AlGaN/GaN HEMT sensors can be detected upon adsorption of biological species on the semiconductor surface. Finally, the nitrides provide an ideal platform for fabrication of surface acoustic wave (SAW) devices. The GaN-based devices thus appear promising for a wide range of chemical, biological, combustion gas, polar liquid, strain and high temperature pressure-sensing applications. In addition, the sensors are compatible with high bit-rate wireless communication systems that facilitate their use in remote arrays.