1. Introduction

Tin dioxide, a typical n-type semiconductor with a wide direct band gap of 3.6 eV has been developed for decades. Recently, one dimensional (1D) SnO₂ nanowires (NWs) are known to have many unique optoelectronic properties, which have attracted intensive attentions because of a large surface-to-volume ratio of 1D nanostructures [1]. Due to the high quantum efficiency of SnO₂ NWs in the UV region, there are plenty potential applications in many practical devices, such as gas sensors including H₂S, CO, NO₂ etc. [2–4]. SnO₂ NWs have also been used in photodetectors and solar cells [5–7]. The upward band-bending owing to oxygen defects at the SnO₂ NWs surface forms a low-conductivity depletion layer at the surface. Once electron-hole pairs are photogenerated, holes drift to the surface through the electric field, leaving unpaired electrons inside, thereupon the recombination probability of electrons and holes reduces, and the lifetime increases [8,9]. Because NWs have a high surface-to-volume ratio, the surface of NWs can influence the conductivity drastically. Recently, some metals, such as Pd and Ag, on semiconductor NWs have been proposed to improve the sensitivity of devices [10–12]. The metal particles with a large work function (Ag ~4.7 eV, Pd ~5.2 eV and Au ~5.1 eV) on the nanowire were coated by vapor deposition [10,11] and sputtering method [12]. After the decoration of metal nanoparticles, a Schottky junction will be created and resulting in the formation of charge depletion regions in the nanowire. For the detection of light illumination [12], the increasing surface electric field will enhance the spatial separation of electrons and holes. Therefore, the photocurrent gain is amplified. Especially, on illumination with photon energy larger than the energy gap of SnO₂ (~3.6 eV), the conductivity of the SnO₂ NWs increases greatly. For gas detection [10,11], the surface state of metal nanoparticles will be affected by the gas molecules and so is the conduction channel of SnO₂ NW. Thereby, the sensitivity of gas sensor is improved. Here, we provide an alternative approach, which is based on the functionalization of NW surface by p-type nanoparticles. It is expected that the p-type nanoparticles on the surface will create a larger charge depletion region than that of Schottky junction owing to the existence of p-n heterojunction. The larger surface electric field will greatly improve the spatial separation of photoexcited electrons and holes, thereby the photocurrent gain can be further enhanced. In this paper, to illustrate our working principle, we report the photoresponse of a single SnO₂ NW decorated with p-type NiO nanoparticles. It is found that the photocurrent gain can be greatly enhanced by up to 3500. This result can be well interpreted based on the formation of p-n heterojunction on the decorated surface. Our study shown here may open up a new possibility for the creation of novel photodetectors with high efficiency.

2. Experiment

The growth of SnO₂ NWs is based on vapor-liquid-solid process (VLS). In the synthesizing process, a Au layer with 10 nm in thickness is first deposited on M-plane (100) sapphire (Al₂O₃) to serve as the catalyst. Then, Sn powder was placed on a ceramic boat and put into a furnace with Argon flow as gas carriers with a flow rate of 200 sccm. The temperature was increased from room temperature up to 1000 °C at a rate of 100 °C/min. The vaporized Sn will react with oxygen in air and form SnO₂, and then blown onto the Au catalyst layer. When SnO₂ dissolves into the Au layer, the supersaturation of the alloy SnO₂ -Au droplet will serve as the nucleation site, and SnO₂ nanowires are obtained. Figure 1(a) shows the field emission scanning electron micrograph (FE-SEM) image of the morphology of SnO₂ nanowires, which was performed by a JSM-6500F FE-SEM system. It clearly reveals that the as-growth SnO₂ nanowires have a length of about 40 μm and diameters ranging between 70 nm and 200 nm. Figure 1(b) shows the X-Ray diffraction (XRD) of SnO₂ nanowires, which can be employed to identify the structure of synthesized product. The XRD analysis was carried out using a diffractometer (Panalytical X’pert PRO) with Kα line as incident radiation. Most of the peaks can be perfectly indexed according to the tetragonal rutile structure of SnO₂. According to all previous reports, SnO₂ NWs grown by VLS method possess n-type conduction [5].

 

Fig. 1 (a) Scanning electron microscope (SEM) image of as-grown SnO₂ nanowires. (b) XRD pattern of as-grown SnO₂ nanowires. (c) SEM image of a single SnO₂ nanowire device. (d) I-V characteristics of the pristine SnO₂ nanowire and the NiO nanoparticles decorated nanowire.

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After the fabrication, the two terminal tungsten (W) contacts of SnO₂ NWs devices were made by focused-ion-beam (FIB) deposition on the insulating SiO_x (500nm) / Si substrates. The typical SEM image of a single SnO₂ NW device is illustrated in Fig. 1(c). The distance of the channel between two terminals is 10 µm, and the diameter of the NW is about 100 nm. A treatment of thermal annealing at 500 °C for 60 s after contact deposition was necessary to minimize the contact resistance. As shown in Fig. 1(d), the I-V curves of the NWs show a well-behaved Ohmic characteristic. The Ni thin film of about several nm was first deposited on the SnO₂ NW by thermal evaporation, then the sample was annealed at 400 °C for 5 hrs under O₂ environment to form NiO nanoparticles. Under the similar growth condition, the pure NiO film exhibits a p-type conduction with a carrier concentration of about 10¹⁷ cm⁻³ by Hall measurement. Figure 2 shows the Raman spectrum of NiO nanoparticles. The four bands correspond to one-phonon LO modes (at ~570 cm⁻¹), two-phonon 2TO modes (at ~730 cm⁻¹), TO + LO (at ~906 cm⁻¹) and 2LO (at ~1090 cm⁻¹) modes, respectively [13]. For the photocurrent (PC) measurement, a He-Cd laser working at 325 nm was used as the excitation light source. A measurement system (Keithley 236) was utilized to supply the dc voltage (0.1 V) and to record the photocurrent.

 

Fig. 2 Raman spectrum of NiO nanoparticles.

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3. Results and discussion

Figure 3 (a) shows the decoration of NiO nanoparticles on the surface of SnO₂ NW, which indicates that the diameter of the particle size is about 10 nm. As shown in Fig. 1 (d), the dark current of the NiO nanoparticles decorated NW is lower than the pure NW owing to the formation of p-n junction, which creates a low-conducivity depletion layer on the surface of the NW. Figure 3 (b) shows the phtocurrent of the two samples with a bias of 0.1 V and under the illumination of a He-Cd laser (325 nm) with an excitation intensity of 8.9, 21, 83, 270 W/m², respectively. The optical switch effect as light is on/off in NiO nanoparticles decorated SnO₂ NW was illustrated in Fig. 3 (c). Quite interestingly, when the NiO nanoparticles deposited on the NW, the photocurrent can be greatly enhanced.

 

Fig. 3 (a) SEM image of NiO nanoparticles decorated SnO₂ nanowire. (b) Photoresponse of the single SnO₂ NW with and without NiO nanoparticles decoration under a bias of 0.1 V and under the illumination of a He-Cd laser (325nm) with different excitation intensity of 8.9, 21, 83, 270 W/m². (c) The optical switch effect of NiO nanoparticles decorated SnO₂ nanowire under the illumination of He-Cd laser (325 nm) with excitation intensity of 83 W/m²

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The current gain (Γ) of the photoresponse, determining the efficiency of electrons induced by photon and collected during the PC measurement can be obtained by [14]

where $\Delta i$ is the current difference between PC and dark current, q is the electron charge, $h\nu$ is the photon energy of incident light, P is the power of photon that the nanowire has absorbed, which means $P=I\times l\times d$, and I is the exciting intensity illuminating the pattern, l and d are the length and the width of the nanowire, respectively, and η is the quantum efficiency, which is set to be 1 for simplicity. Under the illumination of a He-Cd laser with wavelength of 325 nm, the calculated gain for the pure SnO₂ NWs can reach up to 950. Surprisingly, the gain of NiO nanoparticles decorated NWs can be enhanced by up to 3500. It has been shown that the phototcurrent gain of Au-SnO₂ NW composite systwm can be enhanced by about two times [12]. Therefore, our result shows that the p-n heterojunctions can serve as a better enhancement system than the Schottky junctions under the similar light wavelength and intensity.

According to the previous reports, the high current gain in SnO₂ has been attributed to the presence of oxygen vacancies. The existence of oxygen vacancies on the surface of SnO₂ NWs causes the free electrons accumulation on the surface [15]. Consequently, the low-conductivity depletion region near the surface referred to space charge regions (SCRs) is formed because of the existence of upward band-bending. Once electron-hole pairs are photogenerated, the photoinduced holes drift to the surface readily due to the built-in electric □eld, leaving unpaired electrons inside, thus being spatially separated. And the spatial separation of electrons and holes also reduces the electron-hole recombination rate, therefore, the electron lifetime increases and the photoresponse is enhanced. According to the simulation of Garrido et al., the PC induced by the modulation of surface SCRs causes Γ following an inverse power law versus excitation intensity, i.e., $\Gamma \propto I^{-\kappa }$, and the exponent κ is between 0.5 and 0.9 [16]. As shown in Fig. 4 , the gain logarithmic plot versus intensity shows a power law $\kappa ~0.78$, which is in well agreement with the theoretical prediction.

 

Fig. 4 The gain logarithmic plot versus intensity of the pristine SnO₂ NW and NiO nanoparticles decorated NW under the illumination of He-Cd laser (325 nm).

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The mechanism of the PC enhancement after the NiO nanoparticles deposition is discussed as follows. The band diagram of NiO and SnO₂ was shown in Fig. 5(a) . In thermal equilibrium, the Fermi energy E_F must be equal in both p-NiO and n-SnO₂. The area with upward band bending will be formed. In our study, the formation of p-n heterojuction will increase the surface electric field and so as the width and height of the space charge region, which is larger than the region of oxygen-vacancies-related defects effect on the pristine NW. Figure 5(b) illustrates the enhancement of space charge region by the NiO nanoparticles decoration. Therefore, the spatially separated electrons and holes created by incident photon will be enhanced and the photocurrent gain is also improved. When carriers are excited by photons, a variation of the conductive volume can be produced in the SCRs, and the $\Delta i$ can be expressed as [16]

 

Fig. 5 (a) Schematic diagram of the band alignment of NiO and SnO₂. (b) The enhanced space charge region results from NiO naoparticles decoration.

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where

$\Delta {\Psi }_0$ is the barrier height, ε is the permittivity, $N_d$ is the doping level, $V_T=kT/q$,and A is Richardson constant $=1.2\times {10}^{6}A/m^2{K}^2$. Equation (2) indicates the current difference ($\Delta i$) between photocurrent and dark current. The derivation was mainly based on the change of space charge regions (SCRs) induced by the incident photon [16]. According to previous report, the increase in barrier height will increase both the gain Γ and the exponent κ [12]. Indeed, Fig. 4 shows the exponent κ of inverse power law changes from 0.78 to 0.91 after the NiO nanoparticles decoration. This result is consistent with the proposed model as mentioned above. Furthermore, the computer simulation according to Eq. (2) shown in Fig. 6 indicates that the barrier height of NiO nanoparticles decorated NW is about 0.90 eV and 0.69 eV for the pristine NW, which is also in good agreement with our prediction.

 

Fig. 6 Computer simulation of gain versus intensity and barrier heights were obtained by Eq. (2).

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4. Conclusions

In summary, we have demonstrated that the sensitivity of photoresponse in a n-type SnO₂ NW can be greatly improved by p-type NiO nanoparticles decoration. The underlying mechanism is attributed to the formation of p-n heterojunction between decorated nanoparticles and NWs, which enhances the spatial separation of photogenerated electrons and holes. It prolongs the photoinduced electron lifetime and improves the photocurrent gain. In addition, it is stressed here that the drastic improvement of photocurrent gain also takes the advantage of a large surface to volume ratio of 1D structure, which greatly enhances the effect of p-n heterojunction. Our approach should be very useful for creating novel high efficiency photodetectors.