Wang et al. Nanoscale Research Letters 2012, 7:246 http://www.nanoscalereslett.com/content/7/1/246 charge carriers transport can be influenced greatly by the density and spacing of the ZnO nanorod arrays. Wang et al. report the field emission properties of ZnO arrays are correlative with the rod density [16]. Subsequently, Spencer et al. prove theoretically that varying the spacing will affect the sensing property of ZnO nanorods using the density functional theory [17]. Therefore, ZnO nanostruc- ture arrays fabricated following a designed pattern, with a high degree of control in density and spacing is highly de- sirable. The development of nanofabrication techniques and equipments such as electron beam lithography (EBL), nanoimprint lithography, laser interference lithography, and nanosphere lithography provide us the potential op- portunity to fulfill this goal [18]. However, precise control of these geometric parameters is rather challenging in practice. The e-beam lithography is the most reliable technique which can define the exact positions of the nanostructures with high precision. Therefore, the spacing distance between the patterned ZnO seed areas can be easily controlled in nanometer scales by using the EBL method. On the other hand, the hydrothermal growth technique of ZnO is versatile, large scale, and not confined to the inorganic substrate due to its low growth temperature as mentioned above. There- fore, combining these two methods together to fabricate patterned ZnO nanorod arrays with controllable geomet- ric parameters is possible and attractive. In this paper, patterned ZnO nanorod arrays with well-defined positions and spacing distance are fabri- cated onto Si substrate by a combination of EBL and hydrothermal growth process. The influences of spacing distance and growth time on the morphology of the nanorod arrays are also investigated. Finally, the trans- port properties of our ZnO nanorod arrays with suitable spacing distance and density are investigated. Page 2 of 7 Methods Schematic diagram of the experimental procedures for the patterned ZnO nanorod arrays are shown in Figure 1. Si wafer is cleaned sequentially in ethanol and acetone for 20 min, respectively. A layer of polymethyl methacrylate (PMMA) is cast on the Si substrate at 2, 000 rpm (rounds per minute) for 40 s by spin coating methods as shown in Figure 1a. Then, the patterns are designed on PMMA by using the EBL methods as shown in Figure 1b. Reactive ion etching treatment (25 mTorr, 50 sccm, 50 W, 5 s) is used to remove the residual PMMA on the exposure Si substrate. ZnO seed layer (thickness about 40 nm) is deposited using magnetron sputtering method (Figure 1c), and after strip, the PMMA in acetone, patterned ZnO seed areas, are obtained on the Si substrate. (Figure 1d). The hydrothermal method is used for the growth of ZnO nanorods on the patterned areas as shown in Figure 1e. For details, the Si substrate is immersed verti- cally in the nutrient solution of 0.035 M zinc nitrate [Zn(NO3)2 6H2O] and 0.65 M NH3. The reaction temperature is kept at 80°C with different reaction times. The morphologies of the samples are characterized by scanning electron microscopy (SEM) (JEOL, JSM-5600LV, Akishima, Tokyo, Japan), high-resolution transmission elec- tron microscopy (high-resolution transmission electron mi- croscopy (HRTEM) (JEOL JEM 2010). The current–voltage characteristics measurements are carried out on the probe station (Lake Shore) equipped with a Keithley 4,200 semi- conductor characterization system (Cleveland, OH, USA). The light used in our experiment is an UV light source (λ = 350 nm) with an average power of 0.8 mW. Results and Discussion Figure 2 are the SEM images of ZnO nanorod arrays patterned by EBL method. Figure 2a,c,e is the ZnO dot Figure 1 Schematics of the experimental procedures of patterned ZnO nanorod arrays. (a) Spincoating PMMA on Si substrate; (b) Pattern fabrication by EBL method; (c) Magnetron sputtering ZnO seed layer; (d) Strip PMMA in acetone solution; (e) Hydrothermal growth ZnO nanorods on the patterned areas.