Wang et al. Nanoscale Research Letters 2012, 7:246 http://www.nanoscalereslett.com/content/7/1/246 current contrast ratio of photocurrent to dark is about 104. The nonlinear behaviors of ZnO nanorod have been reported in many literatures. Usually the nonlinear be- havior is caused by the Schottky barriers formed between the semiconductor and the metal electrodes, and the shape of I-V curve depends on the heights of the Schottky barriers between the interface of metal and semiconductors [20-22]. The nonlinear I-V characters as the reverse current of Schottky diodes properties are usually described by the thermionic field emission model which has been developed by Padovani and Stratton [23]. In this model of Schottky diodes, the logarithmic plot of the current I as a function of the bias V give approxi- mately a straight line of slope kqT E10 [24]. By fitting the I-V curves, we find that lnI is linear with V both for under dark and illumination conditions, just as shown in Figure 5c. Therefore, two back-to-back Schottky contacts are formed in Au-ZnO-Au structure. In this structure, the current is dominated by the reverse current of re- verse biased Schottky barrier. The asymmetrical I-V curves mean that the-two Schottky barriers heights are different in dark condition. In nanometer scales, the surfaces states on the semicon- ductor surface will play a crucial role in determining the electron transport properties of nanostructures. The pre- vious studies by our group and others have demonstrated that the surface states on the ZnO nanostructure surface can dominate the photoelectric properties of the ZnO nanorod arrays, and different surface states which caused by O2 adsorption or vacancies on the surfaces of semi- conductor nanowires will cause the asymmetrical behav- ior of I-V curves [25-30]. In this experiment, one end of the Au electrode is deposited on the top of the ZnO nanorods, and the other end is surrounding the pat- terned ZnO nanorods on the Si substrate. In the middle of the ZnO nanorod patterns, there might have no Au deposited because of the high density of the rod arrays. Therefore, different contact properties are formed be- tween the two ends of the Au electrode, accordingly dif- ferent surface states will formed between ZnO nanorod and Au electrodes. Different heights of the Schottky bar- riers at the interface of metal and semiconductor which dominate by the surface states will make the asymmet- rical I-V curves in dark condition. Under UV light irradi- ation, the photogenerated electrons and holes are quickly separated through the strong local electric fields formed at the reverse bias Schottky barrier, resulting in the current increased greatly. Since the barrier height and build-in electric field of Schottky diodes are different, the separation efficiency of photogenerated electron–hole pairs in the depletion layer will be different [25]. The higher build-in electric field can separate more photo- generated holes to the surface of the ZnO nanorod, and the Schottky barriers height will be degraded larger also. Page 6 of 7 As a result, the I-V curve changes from asymmetrical to symmetrical nonlinear I-V behavior under UV light illumination. Conclusions In summary, we have demonstrated an effective approach for controllable fabrication of ZnO nanorod arrays with dif- ferent geometric parameters through the combination of EBL and hydrothermal growth process. EBL is employed to fabricate the patterned ZnO seed layers with different areas and spacing distances with high precise, while a hydrother- mal growth method is used to control the density and morphologies of ZnO nanorod arrays. This combined nano- fabrication approach provide a possibility to integrate the patterned ZnO nanorod arrays into real devices. The verti- cal transport properties of the patterned ZnO nanorod arrays are investigated, and the I-V curve measurement indi- cates that the back-to-back Schottky contacts with different barrier heights are formed between the Au electrodes and ZnO nanorods in dark conditions. Under UV light illumin- ation, the patterned ZnO nanorod arrays show a high UV light sensitivity, and the response ratio is about 104. The controllable fabrication of patterned ZnO nanorod arrays and understanding their photo-electric transport properties are helpful to fabricate novel nanodevices based on them. Competing interests The authors declare that they have no competing interests. Authors’ contributions WSJ and SCC are the primary authors and carried out the experiments, characterization, and acquisition of data. CK participated in analysis and interpretation of data. DSX and ZZY participated in language modification. ZLD is the principal investigator who helped in analysis and interpretation of data, drafting of the manuscript, and revisions. All authors read and approved the final manuscript. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 20903034 and 10874040), and the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (No. 708062), and supported by Program for Changjiang Scholars and Innovative Research Team in University, No. PCS IRT1126. Received: 1 February 2012 Accepted: 26 March 2012 Published: 6 May 2012 References 1. Wang ZL: Nanobelts, nanowires, and nanodiskettes of semiconducting oxides—from materials to nanodevices. Adv Mater 2003, 15:432–436. 2. 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