Wang et al. Nanoscale Research Letters 2012, 7:246 http://www.nanoscalereslett.com/content/7/1/246 Page 4 of 7 Figure 3 The SEM image of ZnO dot area and nanorod arrays with growth time 1.5 and 2.0 h. (a) The SEM image of ZnO nanorod arrays with growth time of 1.5 h on 200 × 200 nm seed areas; (b) seed areas are 500 × 500 nm; and (c) seed areas are1 × 1 μm; (d) The SEM image of ZnO nanorod arrays with growth time of 2.0 h on 200 × 200 nm seed areas; (e) 500 × 500 nm seed areas; and (f) 1 × 1 μm seed areas. patterned areas used for the 1-h reaction time as shown in Figure 2. The patterns on 200 × 200 nm seed areas cannot be distinguished due to the longer reaction time, which make the nanorods bundles much longer to close- ness with each other (Figure 3a). For the 500 × 500 nm seed areas as shown in Figure 3b, the nanorod arrays are vertical aligned on the Si substrate with 2 μm spacing distance. We should note that the ZnO nanorods at the edge of the pattern have a tendency to grow outward, whereas the nanorods at the central part of the patterns grow vertically to the substrate. Such effect appears more serious for the seed areas with a larger area size, which results in the hemispherical growth of ZnO nanorod array from the patterned areas just as shown in Figure 3c. With the growth time increased to 2 h, the patterns for 200 × 200 nm and 500 × 500 nm ZnO seed areas both dis- appeared as shown in Figure 3d,e. For the 1 × 1 μm ZnO seed areas with 4 μm spacing distance higher density of patterned ZnO nanorod arrays are obtained as display in Figure 3f. In addition, the diameters of the ZnO nanor- ods near the edge of the pattern are larger than that in- side the pattern which can be seen in Figure 3f. The dependence of the diameters of the nanorods may be due to the effects of the density of neighboring nanorods which can hinder nanorods growth; and also, the nanor- ods at the edge have a greater precursor chemical source supply than are present for the high density ZnO nanor- ods in the central regions. Figure 4a,b is the transmission electron microscopy (TEM) and HRTEM images of individual ZnO nanorod, respectively. It shows the single nanorod with diameter less than 100 nm, and well-defined lattice fringe separ- ation with 0.26 nm. It is also in agreement with the selected area electron diffraction results (the inset of Figure 4b) which indicates that the nanorod have a high quality single-crystalline structure and growth orienta- tion along the [0001] direction. The photoelectric transport properties of the patterned ZnO nanorod arrays are then investigated through the microprobe station equipment. Figure 5a is the sche- matics of the apparatus we used in our experiment. In this system, Au electrodes on the top of the nanorods and the Si substrate are designed through one-step de- position method on the 1.5-h growth time of the pat- terned ZnO nanorods in the 500 × 500 nm seed areas with 2-μm spacing distance. Figure 5b shows typical current voltage (I-V) characteristics of the fabricated ZnO nanorod structures measured in dark and under UV light illumination conditions. With applied bias (from −2.0 to +2.0) voltages, the curve exhibits asymmet- rical nonlinear behavior with lower current value as sev- eral nA in dark conditions. Under UV light illumination, the current is increased greatly, and the curve exhibits from asymmetrical to symmetrical nonlinear I-V behav- ior. At 2 V applied bias voltage, the dark current is about 4.0 × 10−10 A, and the photocurrent is 3.0 × 10−6 A; the