A comparison is made of the sensitivities for detecting hydrogen with Pt-coated single ZnO nanorods and thin films of various thicknesses (20–350 nm). The Pt-coated single nanorods show a current response of approximately a factor of 3 larger at room temperature upon exposure to $500\mathrm{ppm}{\mathrm{H}}_2$ in ${\mathrm{N}}_2$ than the thin films of ZnO. The power consumption with both types of sensors can be very small (in the nW range) when using discontinuous coatings of Pt. Once the Pt coating becomes continuous, the current required to operate the sensors increases to the $\mu \mathrm{W}$ range. The optimum ZnO thin film thickness under our conditions was between 40–170 nm, with the hydrogen sensitivity falling off outside this range. The nanorod sensors show a slower recovery in air after hydrogen exposure than the thin films, but exhibit a faster response to hydrogen, consistent with the notion that the former adsorb relatively more hydrogen on their surface. Both ZnO thin and nanorods cannot detect oxygen.

Single $\mathrm{ZnO}$ nanowire metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated using nanowires grown by site selective molecular-beam epitaxy. When measured in the dark at $25°\mathrm{C}$ , he depletion-mode transistors exhibit good saturation behavior, a threshold voltage of $\sim -3\mathrm{V}$ , and a maximum transconductance of order $0.3\mathrm{mS∕mm}$ . Under ultraviolet $\left(366\mathrm{nm}\right)$ illumination, the drain–source current increase by approximately a factor of 5 and the maximum transconductance is $\sim 5\mathrm{mS}∕\mathrm{mm}$ . The channel mobility is estimated to be $\sim 3{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ , which is comparable to that reported for thin film $\mathrm{ZnO}$ enhancement mode MOSFETs, and the on∕off ratio was $\sim 25$ in the dark and $\sim 125$ under UV illumination.

Single ZnO nanorods with diameters of $\sim 130\mathrm{nm}$ were grown on Au-coated ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrates by catalyst-driven molecular beam epitaxy. Individual nanorods were removed from the substrate and placed between Ohmic contact pads and the current–voltage characteristics measured as a function of temperature and gas ambient. In the temperature range from $25\text{to}150°\mathrm{C}$ , the resistivity of nanorods treated in ${\mathrm{H}}_2$ at $400°\mathrm{C}$ prior to measurement showed an activation energy of $0.089\pm 0.02\mathrm{eV}$ and was insensitive to the ambient used (${\mathrm{C}}_2{\mathrm{H}}_4,{\mathrm{N}}_2\mathrm{O},{\mathrm{O}}_2$ or 10% ${\mathrm{H}}_2$ in ${\mathrm{N}}_2$ ). By sharp contrast, the conductivity of nanorods not treated in ${\mathrm{H}}_2$ was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors.

Single ZnO nanorods with Ohmic contacts at either end exhibit large changes in current upon exposing the surface region to polar liquids introduced through an integrated microchannel. The polar nature of the electrolyte introduced led to a change of surface charges on the nanorod, producing a change in surface potential at the semiconductor∕liquid interface. The nanorods exhibit a linear change in conductance between $p\mathrm{H}$ 2 and 12 of $8.5\mathrm{nS}∕p\mathrm{H}$ in the dark and $20\mathrm{nS}∕p\mathrm{H}$ when illuminated with ultraviolet $\left(365\mathrm{nm}\right)$ light. The nanorods show stable operation with a resolution of $\sim 0.1p\mathrm{H}$ over the entire $p\mathrm{H}$ range. The results indicate that ZnO nanorods may have applications in integrated chemical, gas, and fluid monitoring sensors.