High-sensitivity humidity sensor based on a single SnO(2) nanowire.

Nanowires (NWs) and nanobelts (NBs) are considered as ideal building blocks for constructing nanosized devices due to high surface to volume ratio and their special physical and chemical properties resulting from the reduced sizes. Until now, many semiconductor NWs/NBs have been successfully applied in nanodevices, including nanolasers,1 nanogenerators,2 and various chemical and biological nanosensors.3,4 Well-known humidity control is very important for many fields in technology and our daily life. In the past years, many detection techniques have been explored from old wet and dry bulb thermometry to modern capacitive, resistive, and thermal conductive moisture detectors. In order to further promote the sensitivity, selectivity, chemical and thermal stability, intensive efforts have been put in the exploration of a humidity sensor based on nanostructured materials such as carbon nanotubes,5 metal oxide nanoparticles,6 and NW films.7 Being an important n-type semiconductor with a wide band gap (Eg ) 3.6 eV at 300 K), SnO2 possesses many unique optical and electrical properties: remarkable receptivity variation in gaseous environment, high optical transparency in the visible range (up to 97%), low resistivity (10-4 to 106 Ω‚cm-1), and excellent chemical stability. These properties make SnO2 NWs/NBs well suited for chemical sensors and transparent conducting electrodes. To date, many nanodevices based on SnO2 NWs/NBs have been fabricated, including field effect transistors (FET),8 field emissions,9 UV sensors,10 and gas sensors.3a,11 In this communication, we present a new type of SnO2 nanodevice, a humidity detector using a single SnO2 NW as the sensing unit. This new type of SnO2 NW-based sensor has fast and sensitive response to relative humidity (RH) in air from a wide range of environments at room temperature (30 °C). In addition, it has relatively good reproducibility, and its linear response to RH makes it to calibrate. Single-crystalline SnO2 NWs to be used as humidity sensors were synthesized by chemical vapor deposition (CVD) using Au nanoparticles as catalysts in a homemade synthetic apparatus.12 Experimental details are available in Supporting Information. Figure 1a is a typical SEM image of as-synthesized NWs with high yield. The XRD pattern indicates that the NWs are rutile structured SnO2 with a good crystallinity (Figure S1). The diameter of SnO2 NWs ranges from 50 to 300 nm, and the length of SnO2 NWs is up to tens of micrometers. Low-magnification TEM image (upper inset of Figure 1a) and corresponding EDS analysis (Figure S2) show that a Au nanoparticle exists at the tip of the SnO2 NW, which is the representative characteristic of the vapor-liquid-solid (VLS) growth mechanism.13 The selective area electron diffraction (SAED, lower inset of Figure 1a) pattern taken from the body of the SnO2 NW reveals that the as-synthesized SnO2 NW is single crystalline and grows along the [001] direction. Cathodoluminescence (CL) analysis is a suitable technique to determine the crystalline quality and the presence of defect structure in nanocrystals. Room-temperature CL spectrum (Figure S3) of SnO2 NWs indicates that there is a broad blue luminescent peak centered at around 470 nm (2.64 eV), but near band edge (NBE) emission (expected around 320 nm) was not detected. According to previous studies,14 the luminescence in the range of 400-600 nm of SnO2 is attributed to several possible luminescence centers, such as oxygen vacancies, defects in the surface, and/or impurities in the NWs. As for blue emission around 470 nm, the electron transition mediated by oxygen vacancies in the band gap is responsible, too.14c The SnO2 NW-based humidity sensor is based on a FET nanodevice. A single SnO2 NW of 250 nm in diameter was placed between two Au electrodes of 100 nm thickness and deposited with Pt by focused ion beam (FIB) microscopy as the top electrode to improve contact (see inset of Figure 1b). In order to measure the current signals through the SnO2 NW, two Au electrodes are connected with a support chip by a Au wire binding technique. Figure 1b shows the I-V curves of the SnO2 NW-based sensing element in different static air of 5-85% RH at 30 °C. In any RH atmospheres, I-V curves of the device exhibit good linear behavior, which proves a good ohmic contact between the SnO2 NW and Au electrodes. At the same time, it is clearly seen that the resistance of the SnO2 NW decreases promptly with the increase of RH in air. The resistance of the SnO2 NW in dry air (5%) is calculated to be about 2.80 × 107 Ω, which is 14 times that (2.00 × 106 Ω) in † Xiamen University. ‡ Georgia Institute of Technology. Figure 1. (a) Typical SEM image of SnO2 NWs. Insets are corresponding TEM image (upper) and SAED pattern (lower) of a single SnO2 NW. (b) I-V curves of a single SnO2 NW in different static RH atmosphere from dry air (5%) to 85% RH air at 30 °C. Insets are schematic illustration (upper) and corresponding SEM image (lower) of a SnO2 NW-based nanodevice. Published on Web 04/26/2007