L. C. Tien

Hydrogen-selective sensing at room temperature with ZnO nanorods

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 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.

Hydrogen sensing at room temperature with Pt-coated ZnO thin films and nanorods

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 500ppmH2 in N2 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 μW 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.

Depletion-mode ZnO nanowire field-effect transistor

Single 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°C, he depletion-mode transistors exhibit good saturation behavior, a threshold voltage of ∼−3V, and a maximum transconductance of order 0.3mS∕mm. Under ultraviolet (366nm) illumination, the drain–source current increase by approximately a factor of 5 and the maximum transconductance is ∼5mS∕mm. The channel mobility is estimated to be ∼3cm2∕Vs, which is comparable to that reported for thin film ZnO enhancement mode MOSFETs, and the on∕off ratio was ∼25 in the dark and ∼125 under UV illumination.

Electrical transport properties of single ZnO nanorods

Single ZnO nanorods with diameters of ∼130nm were grown on Au-coated Al2O3 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 25to150°C, the resistivity of nanorods treated in H2 at 400°C prior to measurement showed an activation energy of 0.089±0.02eV and was insensitive to the ambient used (C2H4,N2O,O2 or 10% H2 in N2). By sharp contrast, the conductivity of nanorods not treated in H2 was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors.

pH measurements with single ZnO nanorods integrated with a microchannel

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 pH 2 and 12 of 8.5nS∕pH in the dark and 20nS∕pH when illuminated with ultraviolet (365nm) light. The nanorods show stable operation with a resolution of ∼0.1pH over the entire pH range. The results indicate that ZnO nanorods may have applications in integrated chemical, gas, and fluid monitoring sensors.

Pt/ZnO nanowire Schottky diodes

Pt Schottky diodes were formed on single ZnO nanowires grown by site-selective molecular-beam epitaxy and then transferred to SiO2-coated Si substrates. The diodes exhibit excellent ideality factors of 1.1 at 25 °C and very low (1.5×10−10A, equivalent to 2.35Acm−2, at −10V) reverse currents. The nanowire diodes show a strong photoresponse, with the current–voltage characteristics becoming ohmic under ultraviolet illumination (366 nm light). The on-off current ratio of the diodes at 0.15∕−5V was ∼6. These results show the ability to manipulate the electron transport in nanoscale ZnO devices.

Anisotropic x-ray absorption effects in the optical luminescence yield of ZnO nanostructures

The authors have found that the directionality of the orbital populated following core-level x-ray absorption of a hexagonal nanostructure has a strong influence on the resulting optical luminescence yield spectra. For ZnO, there is an enhancement of the band gap exciton luminescence following O 1s to 2pz relative to 2px,y excitation. The defect luminescence O 1s excitation spectrum also shows sensitivity to the nature of the defect (surface or bulk).

Modeling and Fabrication of ZnO Nanowire Transistors

ZnO is attracting attention for application in transparent nanowire (NW) transistors because of the ease of synthesis of ZnO nanostructures, their good transport properties, the availability of heterostructures, and the possibility for optoelectronic integration. A variety of both top and bottom gate n-type ZnO NW transistors have been reported, showing generally high on/off ratios (104 - 107), subthreshold voltage swings of 130-300 mV/dec, and excellent drain-current saturation. Much higher electron mobilities and improved device stability are found when surface passivation is employed, pointing to the importance of controlling surface charge density. Simulations show that defects such as grain boundaries lead to a decrease of drain current. While the dc characteristics of such devices are generally reasonable, there have been no reports of the RF or high-speed switching performance. Additional work is needed on optimized gate dielectrics, reliability, and functionality of ZnO NW transistors.

ZnO-BASED NANOWIRES

ZnO has properties well-suited to UV/visible light emitters, transparent thin film electronics and a variety of gas and chemical sensor applications. There has been extensive interest in recent times in synthesis of ZnO nanowires by a number of methods using both catalyst and catalyst-free approaches. In this paper we review our recent results on developing functional nanowires in the ZnMgO systems and show some applications in hydrogen gas sensing, pH sensing, transparent transistors and UV detectors. In terms of sensors, the main selling points are large surface-to-volume ratio for improved detection sensitivity and also low power requirements.

Cubic (Mg,Zn)O nanowire growth using catalyst-driven molecular beam epitaxy

We report on the growth of Mg-rich cubic (Mg,Zn)O nanowires using a catalysis-driven molecular-beam-epitaxy method. Nanowires were grown on both Si and Al 2 O 3 substrates coated with a nominally 2-nm-thick layer of Ag. The (Mg,Zn)O nanowires were grown with a Zn and Mg cation flux, with an O 2 /O 3 mixture serving as the oxidizing species. The growth temperature was 400 °C. Under these conditions, nanowires were observed to grow on the Ag sites. The nanowire diameter was on the order of 90 nm. (Mg,Zn)O nanowires as long as 2 μm were realized. High-resolution transmission electron microscope imagery shows the nanowires had single-phase cubic rock salt structure (Mg,Zn)O with a growth direction along the [100]. The presence and compositional distribution of Mg and Zn in the single nanowire were confirmed using a compositional line-scan, profiled across the nanowire, by energy dispersive spectrometry with scanning transmission electron microscopy.