Introduction

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.

Experimental Section

The W18O49 nanowires were synthesized on ITO glass substrates by thermal evaporation of tungsten trioxide powders without catalysts or additives [20]. 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

The Typical XRD pattern of the as-prepared nanowires is shown in Fig. 1. All the characteristic peaks can be indexed to monoclinic W18O49 phase with the lattice constants a = 18.280 Å,b = 3.775 Å,c = 13.980 Å and β = 115.20°, which is in good agreement with the JCPDS, No. 05-0392. The sharp peaks confirm the high crystallinity of the material.

Figure 1
figure 1

XRD pattern of the W18O49 nanowires. The lines mark the expected position for W18O49 phase (JCPDS 05-0392)

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Figure 2a, 2b respectively, show a schematic illustration and a SEM image of the nanowire photoconductor in a configuration of field effect transistor (FET). Under dark condition, the current–voltage (Isd–Vsd) characteristics of the FET at different gate voltages (from +20 to −20 V with a 10 V step) are shown in Fig. 2c. The conductivity of W18O49 nanowire increases with increasing the gate voltage, which indicates that the nanowire is an n-type semiconductor. The n-type conduct behavior in nominally undoped tungsten oxide can be attributed to the presence of oxygen vacancies [18].

Figure 2
figure 2

a Schematic view of the W18O49 nanowire photoconductor. b SEM image of a single-W18O49 nanowire device. c Ids–Vds characteristics of a typical W18O49 nanowire FET

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Optoelectronic characteristics of the device were investigated under 312 nm UV illumination with an intensity of 1.0 mW/cm2. As shown in Fig. 3a, the nanowire is highly insulating in the dark. The conductivity of the nanowire increases from 2 nS in the dark to 37 nS under the UV light illumination, which shows its potential application as UV photodetector. The current flowing between Au electrodes without the nanowires connecting has been measured to exclude the possible contribution of the electrodes and the substrate.

Figure 3
figure 3

a Photocurrent as a function of bias voltage for a single W18O49 nanowire under 312 nm UV light with an intensity of 1.0 mW/cm2 and dark state, respectively. b Photocurrent under 312 nm UV light illumination with different light intensities. The bias voltage is 0.5 V

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The photoconductance of the W18O49 nanowire is dependent on light intensity. Figure 3b shows the photocurrent as a function of the light intensity for a single nanowire irradiated with the 312 nm UV light. The photocurrent (Ip) can be expressed by a simple power law [21]:

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.

As a critical parameter for photoconductors, the gain G was defined as the number of electrons collected by electrodes due to the excitation by one photon. G can be expressed as

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.

The response of photoconductivity is very important for a photodetector. Figure 4a shows the response of the device to 312 nm light at a bias of 0.5 V. The real-time by ON/OFF switching was measured with an intensity of 1.6 mW/cm2. The measured photocurrent shows a rapidly increase to 60 nA upon exposure to UV light with 1.6 mW/cm2 and decreases back to the initial value when the UV light was turned off. The change on the photocurrent shows excellent stability and reversibility. The enhanced conductivity under UV light illumination is attributed to the photogenerated carriers in the semiconducting nanowire. As shown in Fig. 4b, detailed data analysis reveals a rise time (tr) and fall time (tf) of 35 and 100 s, respectively. It is worth mentioning that the time constant for the rise time is always faster than the fall time, which is believed that traps and other defect states were involved in the process.

Figure 4
figure 4

a Photoresponse of a single W18O49 nanowire upon pulsed illumination by a 312 nm wavelength UV light with an intensity of 1.6 mW/cm2. The bias voltage is 0.5 V. b A typical response of the photoconductivity. The arrows indicate the points of 10 and 90% peak value used for calculating the rise time tr and fall time tf

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For their large surface-to-volume ratio, chemisorption on the surface of nanowires may play an important role on the conductivity. To study the adsorption effect, we investigated the response of the W18O49 nanowires in air and vacuum under dark condition. Due to the presence of oxygen vacancies, as-synthesized nanowires are usually n-type semiconductors as demonstrated in Fig. 2. These vacancies serve as active sites for adsorption of ambient oxygen, which can create a depletion layer in the near-surface region of the nanowires by capturing free electrons, and result in a decrease of conductivity of the nanowires [6]. The conductivity of the W18O49 nanowire device increases obviously in vacuum compared to that measured in air under the same bias voltage, as shown in Fig. 5. In vacuum, some oxygen molecules could be desorbed from the surface of the nanowire, and some captured free electrons can be released from the near-surface depletion region, leading to an increase of the conductivity. Therefore, the ambient would be an important factor to the photodetector of the W18O49 nanowires.

Figure 5
figure 5

The photocurrent response of a single-W18O49 nanowire device in vacuum and atmosphere under dark condition

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Conclusion

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.