Chemical Engineering at the University of Florida

ZnO Nanowires for Sensing And Device Applications

Micro-patterned growth of ZnO nanorod arrays on silicon substrates can be achieved with using a low temperature aqueous method is demonstrated. ZnO nanocrystals were used as seeds for producing well-aligned, wurtzite ZnO nanorods. The shape of the ZnO nanorods was sensitive to the orientation of substrate as well as the molar composition of the chemical precursors. ZnO nanorods generally have wurtzite structure and higher tendency to grow along the <001> direction. The effect of The obtained ZnO nanorod arrays grown on (100) Si are typically hexagonal-shaped, and are shorter than those grown on (111) Si wafer. Needle shaped ZnO nanorods grow preferably in the <001> direction. As expected, substantially higher intensity of the XRD pattern is obtained for the [002] diffraction peak from nanorod arrays grown on (100) Si substrate and comparably lower [002] peak resulting from relatively small portion of (001) plane in the vertical orientation of nanorods grown on (111) Si substrate.

Figure Caption: Field emission SEM image of three dimensional arrays of ZnO nanorods grown on a) (100) Si substrate, and b) (111) Si substrate, X-ray diffraction pattern (Cu Ka) of ZnO nanorods grown on c) (100) Si and d) (111) Si. The insets show enlarged view of single ZnO nanrods (b, c) and crystal plane of hexagonal structure (d).

Depending on the molar composition of zinc acetate hexahydrate in the nutrient solution, different forms of mature ZnO rods can be obtained. The spherulitic prism shape was also found in this experiment when the molar ratio of zinc acetate hexahydrate was more than 0.625, the diameter of ZnO rod decreases from 1 micron to several hundred nanometers as the molar composition increases.

Figure Caption: Field emission SEM images of ZnO nanorod arrays. (a) rice (b) needle (c) spherulitic prism, (d) rod shape. (ratio of Zn(NO3)2·6H2O to C6H12N4, from (a) to (d), 1:1, 1:0.8, 1:0.7, 1:0.6).

UV sensors

A Schottky diode based single nanowire device was used as the UV sensor. Selected area diffraction patterns showed the nanowires to be single-crystal. The nanowires were illuminated with above bandgap light at 254 or 366 nm from a Hg arc lamp with power density of 0.1Wcm^-2 and current-voltage characteristics of the nanowire at the fixed 0.25 V pulse voltage was measured. The photoresponse is much faster than that reported for ZnO nanowires grown by thermal evaporation from ball-milled ZnO powders and likely is due to the reduced influence of the surface states seen in that material. The generally quoted mechanism for the photoconduction is creation of holes by the illumination that discharge the negatively charged oxygen ions on the nanowire surface, with detrapping of electrons and transit to the electrodes. The recombination times in high quality ZnO measured from time-resolved hotoluminescence are short, on the order of tens of ps, while the photoresponse measures the electron trapping time. In our nanowires, the electron trapping times are on the order of tens of seconds and these trapping effects are only a small fraction of the total 4 photoresponse recovery characteristic. Note also the fairly constant peak photocurrent as the lamp is switched on, showing that that any traps present have discharged in the time frame of the measurement.

Figure Caption: A single ZnO nanowirwe across two metal contacts

MOSFETs

Single ZnO nanowire metal-oxide semiconductor field effect transistors (MOSFETs) were fabricated using nanowires grown by site selective MBE. E-beam lithography was used to pattern sputtered Al/Pt/Au electrodes contacting both ends of a single nanowire. The separation of the electrodes was ~7 µm. Au wires were bonded to the contact pad for current –voltage (I-V) measurements. (Ce,Tb)Mg₁₁Al₁₉O with thickness 50nm was selected as the gate dielectric as it exhibits a large band gap sufficient to yield a positive band offset with respect to ZnO. The top gate electrode was e-beam deposited Al/Pt/Au.

Figure Caption: (Left) A cross section schematic of a single ZnO nanowire field effect transistor. (Right) An SEM of the ZnO nano MOSFET.

When measured in the dark at 25°C, the depletion-mode transistors exhibit good saturation behavior, a threshold voltage of ~-3V and a maximum transconductance of order 0.3 mS/mm. Under UV (366nm) illumination, the drain-source current increase by approximately a factor of 5 and the maximum transconductance is ~ 5 mS/mm. The channel mobility is estimated to be ~3 cm²/V.s, 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. While the dc characteristics of such devices are generally reasonable, there have been no reports of the rf or high speed switching performance. This is important because it will establish the effect of parasitic capacitances on the high speed performance of nanowire transistors.

Figure Caption: Drain IV characteristics of a single ZnO nanowire field effect transistor

Hybrid pn Junction LEDs

Zinc oxide (ZnO) is an attractive candidate for UV light emission since it is an environmentally friendly material which be grown at low temperatures on cheap transparent substrates and has both a direct wide band gap of 3.3 eV and a very large exciton binding energy of 60meV,important for robust light emission. These properties make ZnO light emitting diodes (LEDs) potentially useful in efficient solid state lighting where white light can be achieved by pumping of an appropriate polymer overlayer, as is used in GaN white LEDs. In addition, it has been suggested that semiconducting nanowires may offer additional advantages for light emission due to the increased junction area, reduced temperature sensitivity, enhanced polarization dependence of reflectivity and improved carrier confinement in 1-D nanostructures.

Figure Caption: (Left) A cross section schematic of a multiple ZnO nanorod LED. (Right) A top view of multiple ZnO nanorods.

We show that with a simple process involving integration of existing n-type ZnO nanowires with hole-conducting polymers, light emission can be achieved in a ZnO nanowire/semiconductor polymer matrix. The hybrid p-n junction consisting of the hole-conducting polymer poly (3,4-ethylene-dioxythiophene)-poly(styrene-sulfonate) (PEDOT/PSS) and n-ZnO nanorods grown on a n-GaN layer on sapphire.

Spin-coating of polystyrene was used to electrically isolate neighboring nanorods and a top layer of transparent conducting indium-tin-oxide (ITO) was used to contact the PEDOT/ PSS. Multiple peaks were observed in the electroluminescence spectrum from the structure under forward bias, including ZnO band-edge emission at ~383 nm as well as peaks at 430, 640 and 748 nm. The threshold bias for UV light emission was <3 V, corresponding to a current density of 6.08 A.cm-2 through the PEDOT/ PSS at 3 V. This is one approach to utilizing ZnO nanorods in a system which uses hole injection from the polymer, rather than requiring p-n junction ZnO nanorods.

Figure Caption: I-V curve and light intensity of a ZnO NWs/PEDOT diode as the function of current of vertical ZnO NWs/PEDOT LED

Hydrogen Sensing

There is currently great interest in the development of hydrogen sensors for applications involving leak detection in hydrogen fuel storage systems and fuel cells for space craft. One of the main demands for such sensors is the ability to selectively detect hydrogen at room temperature in the presence of air. In addition, for most of these applications, the sensors should have very low power requirements and minimal weight. One important aspect is to increase their sensitivity for detecting gases such as hydrogen at low concentrations or temperatures, since typically an on-chip heater is used to increase the dissociation efficiency of molecular hydrogen to the atomic form and this adds complexity and power requirements.

ZnO nanorods were grown by nucleating on a Al₂O₃ substrate coated with Au islands. Selected area diffraction patterns showed the nanorods to be single-crystal. To make ZnO nanowire gas sensors, the nanorods were coated with Pd, Pt, Au, Ni, Ag or Ti thin films (~100Å thick) deposited by sputtering for detecting different gases. For hydrogen sensing, Pt or Pt was used. Contacts to the multiple nanorods were formed using a shadow mask and sputtering of Al/Ti/Au electrodes. The separation of the electrodes was ~30 um.

Figure Caption: (Left) A photograph of multiple ZnO nanowires. (Right) A phtograph of a packaged multiple nanowire hydrogen sensor.

Au wires were bonded to the contact pad for current –voltage (I-V) measurements performed at 25°C in a range of different ambients (N₂, O₂ or 10-500 ppm H₂ in N₂). The I-V characteristics from the multiple nanorods were linear with typical currents of 0.8 mA at an applied bias of 0.5V. The I–V characteristics were the same when measured in vacuum as in air, indicating that the sensors are not sensitive to humidity. The power requirements for the sensors were very low.

The time dependence of relative resistance change of either metal-coated or uncoated multiple ZnO nanorods as the gas ambient was switched from N₂ to 500 ppm of H₂ in air and then back to N₂ as time proceeds were measured at a bias voltage of 0.5V.

Figure Caption: Sensitivity of a packaged multiple nanowire hydrogen sensor.

pH Sensing

ZnO nanorod surfaces are also very sensitive to the liquid ambient, which respond electrically to variations of the pH in electrolyte solutions introduced via an integrated microchannel. The ion-induced changes in surface potential are readily measured as a change in conductance of the single ZnO nanorods and suggest that these structures are very promising for a wide variety of sensor applications. An integrated microchannel across the single ZnO nanowire was made from SYLGARD@ 184 polymer. Entry and exit holes in the ends of the channel were made with a small puncher (diameter less than 1mm) and the film immediately applied to the nanorod sensor. The pH solution was applied using a syringe autopipette (2-20ul).

Figure Caption: Photograph of a single ZnO naowire microfludic pH sensor

The device current at a bias of 0.5 V varied corresponding to a series of solutions whose pH was varied from 2-12. The conductance of the rods was higher under UV illumination but the percentage change in conductance is similar with and without illumination. The nanorod exhibited a linear change in conductance between pH 2-12 of 8.5 nS/pH in the dark and 20 nS/pH when illuminated with UV (365nm) light. The nanorod shows stable operation with a resolution of ~0.1 pH over the entire pH range, showing the remarkable sensitivity to relatively small changes in concentration of the liquid.

Figure Caption: Conductance of a single ZnO naowire as a function of solution pH value