The responses of individual ZnO nanowires to UV light demonstrate that the persistent photoconductivity (PPC) state is directly related to the electron-hole separation near the surface. Our results demonstrate that the electrical transport in these nanomaterials is influenced by the surface in two different ways. On the one hand, the effective mobility and the density of free carriers are determined by recombination mechanisms assisted by the oxidizing molecules in air. This phenomenon can also be blocked by surface passivation. On the other hand, the surface built-in potential separates the photogenerated electron-hole pairs and accumulates holes at the surface. After illumination, the charge separation makes the electron-hole recombination difficult and originates PPC. This effect is quickly reverted after increasing either the probing current (self-heating by Joule dissipation) or the oxygen content in air (favouring the surface recombination mechanisms). The model for PPC in individual nanowires presented here illustrates the intrinsic potential of metal oxide nanowires to develop optoelectronic devices or optochemical sensors with better and new performances.

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The effects of electron–hole separation on the photoconductivity of individual metal oxide nanowires

Single SnO 2 metal oxide nanowires are used at the nanoscale level as individual monocrystal for the electrical transduction of the gas interaction with these sensing materials. Electrical contact characteristics and resistance variations under different gas ambient are analyzed from two- and four-probes measurements of individual nanowires. These data have allowed the estimation of their resistivities and contact resistances. At the gas sensor working conditions, AC impedance spectroscopy technique has extensively been applied to analyze the interaction with the gas molecules and study the influence of the nanowire diameter size on the electrical transduction processes. CO and humidity behaviors are reported for single SnO 2 nanowires with CO detection threshold smaller than 5 ppm and measurement instability lower than 4%.

High response and stability in CO and humidity measures using a single SnO2 nanowire

We present a set of criteria to optimize photodetectors based on n-type metal oxide nanowires and a comparison methodology capable of overcoming the present lack of systematic studies dealing with such devices. The response of photoconductors is enhanced following different fabrication strategies, such as diminishing the distance between the electrical contacts, increasing the width of the photoactive area, or improving the electrical mobility of the nanomaterials. The validity of the theoretical background is verified by experimental results obtained with devices based on ZnO nanowires. The performances of our devices show that the normalized gain of single ZnO nanowire-based photodetectors exceeds those of thin films.

Toward a Systematic Understanding of Photodetectors Based on Individual Metal Oxide Nanowires

Two- and four-probe electrical measurements on individual tin oxide (SnO2) nanowires were performed to evaluate their conductivity and contact resistance. Electrical contacts between the nanowires and the microelectrodes were achieved with the help of an electron- and ion-beam-assisted direct-write nanolithography process. High contact resistance values and the nonlinear current–bias (I–V) characteristics of some of these devices observed in two-probe measurements can be explained by the existence of back-to-back Schottky barriers arising from the platinum–nanowire contacts. The nanoscale devices described herein were characterized using impedance spectroscopy, enabling the development of an equivalent circuit. The proposed methodology of nanocontacting and measurements can be easily applied to other nanowires and nanometre-sized materials.

https://iopscience.iop.org/article/10.1088/0957-4484/17/22/009/pdf

Fabrication and electrical characterization of circuits based on individual tin oxide nanowires

A simple and useful experimental alternative to field-effect transistors for measuring electrical properties free electron concentration nd, electrical mobility , and conductivity in individual nanowires has been developed. A combined model involving thermionic emission and tunneling through interface states is proposed to describe the electrical conduction through the platinum-nanowire contacts, fabricated by focused ion beam techniques. Current-voltage I-V plots of single nanowires measured in both two- and four-probe configurations revealed high contact resistances and rectifying characteristics. The observed electrical behavior was modeled using an equivalent circuit constituted by a resistance placed between two back-to-back Schottky barriers, arising from the metal-semiconductor-metal M-S-M junctions. Temperature-dependent I-V measurements revealed effective Schottky barrier heights up to BE=0.4 eV.

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Electrical properties of individual tin oxide nanowires contacted to platinum electrodes

Fabrication of metallic contacts to nanometre-sized materials using a focused ion beam (FIB)

A procedure to contact single metal-oxide nanowires and nanoparticles of different materials using a Dual Beam FIB has been developed. The quality of the electrical contacts has been evaluated and the possibility of extracting the electrical parameters of these nanowires and nanoparticles demonstrated. The response of some of them to different gases is presented, proving the feasibility of fabricating metal-oxide nanosensors.

M. Abid

Papers

Fabrication of metallic contacts to nanometre-sized materials using a focused ion beam (FIB)

Characterization of metal-oxide nanosensors fabricated with focused ion beam (FIB)