W18O49 Nanowires as Ultraviolet Photodetector
© The Author(s) 2009
Received: 26 October 2009
Accepted: 16 November 2009
Published: 27 November 2009
Photodetectors in a configuration of field effect transistor were fabricated based on individual W18O49 nanowires. Evaluation of electrical transport behavior indicates that the W18O49 nanowires are n-type semiconductors. The photodetectors show high sensitivity, stability and reversibility to ultraviolet (UV) light. A high photoconductive gain of 104 was obtained, and the photoconductivity is up to 60 nS upon exposure to 312 nm UV light with an intensity of 1.6 mW/cm2. Absorption of oxygen on the surface of W18O49 nanowires has a significant influence on the dark conductivity, and the ambient gas can remarkably change the conductivity of W18O49 nanowire. The results imply that W18O49 nanowires will be promising candidates for fabricating UV photodetectors.
Nowadays, ultraviolet (UV) photodetectors play very important roles in many fields such as missile tracking, ozone monitoring, flame detection, imaging techniques and lightwave communications [1–3]. One-dimensional nanostructures of high-performance oxides have attracted considerable attention as a class of potential optoelectronic materials. So far, UV nano-photodetectors based on ZnO nanowires, SnO2 nanowires and Ga2O3 nanowires have been investigated, and some remarkable characteristics such as wavelength selectivity and photoresponse have been revealed [4–7].
As a kind of important transition metal oxides, tungsten oxides have been extensively researched for their distinctive properties including electrochromism, photochromism, gaschromism and photosensitivity [8–13]. Among the sub-stoichiometric phases of WO x , monoclinic W18O49 has attracted much attention for their photoluminescence, gas sensing and field emission properties [14–19]. However, to our knowledge, the photoconductivity characteristics of the W18O49 nanostructures have not been reported until now.
In this paper, we report a systematic study on UV photoconductivity characteristics of single W18O49 nanowires. The conductivity of W18O49 nanowires is extremely sensitive to UV light exposure, allowing us to reversibly switch the photoconductors between “OFF” and “ON” states with Ilight/Idark ratios of two orders of magnitude, excellent stability and reproducibility. The results indicate that the W18O49 nanowires are a potential candidate for applications in high sensitivity nano-photodetector and nano-photoelectronic switch.
The W18O49 nanowires were synthesized on ITO glass substrates by thermal evaporation of tungsten trioxide powders without catalysts or additives . To fabricate single-nanowire detectors, seven parallel Ti/Au (10 nm/150 nm) electrodes spaced about 2 μm apart were fabricated with photolithography on a p++-type Si substrate with a 500 nm SiO2 layer. The as-prepared W18O49 nanowires were dispersed in deionized water by ultrasonic. An ac voltage with a frequency of 1 MHz and a peak to peak voltage Vp–p = 16 V was applied between the two electrodes when a droplet of the W18O49 nanowires suspension was dropped to cover the electrodes area using a micropipette. A fast thermal annealing at 300°C was carried out in N2 atmosphere for 2 min to form the ohmic contacts between the electrodes and nanowire.
The as-prepared nanowires were characterized by X-ray powder diffraction (XRD) on a Rigaku RINT 2400 X-ray diffractometer with Cu Kα radiation. Agilent B1500a measurement system was used for electrical measurements. Spectroline E-series Ultraviolet hand lamps were used as the UV light sources. All measurements were performed at room temperature.
Results and Discussion
where P is the intensity of UV illumination. The non-unity exponent is a result of the complex process of electron–hole generation, trapping and recombination within the material. To change the power of illumination, the conductance can increase by 10 times without damaging the nanowire. Because the UV light intensity is relatively low, no saturation photocurrent can be observed as shown in Fig. 3b. It suggests that the hole-traps present on the surface of the nanowire haven’t absolutely been released at low light intensity, leading to unsaturation of the photocurrent.
where Ne is the number of electrons collected in unit time,Np is the number of absorbed photons in unit time, τ is carrier lifetime, and ttran is the transit time between the electrodes. Take the obtained photocurrent value under 312 nm UV light with an intensity of 1.0 mW/cm2 and the exposal area about 4 × 10−9 cm2 of the nanowire into Eq. (2), the corresponded gain of the nanowire photoconductor is about 104.
In summary, the photoconductor devices were fabricated based on the single W18O49 nanowires. The photoelectrical properties have been characterized systematically and shown the highest light sensitivity at UV light. A simple power-law dependence on UV light intensity was observed at room temperature. The W18O49 nanowire photodetectors exhibit superior performance in sensitivity and reversibility. Absorption of oxygen on the surface of the W18O49 nanowires can significantly influence their conductivity. The results will open up some new possibilities of using W18O49 nanowires for fabricating nanodevices such as high-performance UV detectors, optical keys and optical memory.
This work was supported by the National Natural Science Foundation of China and the Teaching and Research Award Program for Outstanding Young Teachers in High Education Institutions of MOE, China.
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