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10_CPL_01 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, incl...

10_CPL_01
This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Measuring the transport property of ZnO tetrapod using in situ nanoprobes Yudong Gu a,b, Jun Zhou b, Wenjie Mai b, Ying Dai d, Gang Bao c, Zhong Lin Wang b,* aDepartment of Advanced Materials and Nanotechnology, College of Engineering, Peking University, 100084 Beijing, China b School Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA cDepartment of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, USA d State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Institute of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China a r t i c l e i n f o Article history: Received 11 October 2009 In final form 10 November 2009 Available online 13 November 2009 a b s t r a c t The electrical transport characteristic of a ZnO tetrapod has been measured by in situ nanoprobes. By contacting the three legs of a tetrapod as the ‘gate’, source and drain, some control and tunability have been achieved in the output voltage/current, in analogous to the operation of a field effect transistor. A simple logic circuit has also been demonstrated. � 2009 Elsevier B.V. All rights reserved. Zinc oxide (ZnO) is a wide bandgap semiconductor with high excitation energy of 60 meV at room temperature, which has at- tracted a great attention for applications such as light emitting diode (LED) [1], gas sensor [2], strain sensor [3] ultraviolet (UV) photodetector [4] solar cell [5] power generation [6] and piezo- tronics [7]. Owing to the unique crystallographic structure and the polar surfaces, ZnO have been found to exhibit a wide range of structural morphology, such as nanowire [8] nanobelt [9] nanor- ing [10] nanohelix [11] nanocage [12] tetrapod [13] etc. Tetrapod is a structure that is created owing to the co-existence of two ZnO phases: cubic and wurtzite. The cubic phase ZnO is at the core, and four legs with [0 0 0 1] wurtsite structure extend along the h1 1 1i of the cubic structure [14]. This structure is created due to the fast growth of the wurtzite phase along [0 0 0 1] polar axis. The transport properties of ‘Y’ shape carbon nanotubes [15,16] and CdSe tetrapod [17] have been reported. In this Letter, using a 4-probe technique inside a scanning electron microscope, we have explored the transport property of a ZnO tetrapod by directly con- tacting metal probe at the 3 legs of the tetrapod. Using the geomet- rical shape offered by tetrapod, two legs as source and drain and the other as ‘gate’, the electrical transport characteristics can be tuned. In the current study, the ZnO tetrapods were synthesized by a vapor solid deposition process, as reported previously [18]. Col- lected ZnO tetrapods were ultrasonicated in ethanol solvent, and then dispersed on a silicon wafer that was covered by a 200 nm thick oxidation layer to prevent current leakage and dried on a hot- plate at 80 �C. The electrical characterization was carried out in the field emission scanning electron microscopy LEO 1550 (FE-SEM), which is equipped with a S100 Zyvex S100 manipulator system. The manipulator has four individual heads, each can move independently in X, Y, Z directions that are controlled by motors in course mode and piezoelectric actuators in fine mode. A tung- sten wire with a diameter of 250 microns was etched to form a tip at one end with �200 nm in radius using electrochemical meth- od and pre-cleaned in isopropyl alcohol (IPA) to remove surface oxidation layer for serving as probe to achieve electrical contact with the ZnO tetrapod. A Keithley 4200 semiconductor character- ization system (Keithley 4200-SCS) equipped with pre-amplifier was employed to apply the voltage and measure the current from each branch of the tetrapod. We first studied the I–V characteristic of ZnO tetrapod with the tungsten (W) tip directly in contacting with each of the branches as shown in Fig. 1. The I–V curve in Fig. 1 shows that the transport fol- lows a metal–semiconductor–metal (M–S–M) structure with back- to-back Schottky contacts at the two ends [19]. This is the result of direct contacting of W tips with the tetrapod without welding. In our ZnO-tetrapod devices, only three branches contacted with the substrate and one branch faced up because each pair of the two branches has an angle of about 109�. To improve the con- tact and largely reduce the contact resistance, we locally deposited a thin layer of Pt using a Nova 3D focused ion beam (FIB) micro- scope, and the deposition area was confined within 4 � 1 lm2, as shown in the inset SEM image in Fig. 2. The dual-beam FIB was operated at an acceleration voltage of 30 kV and Ga-ion current of 10 pA. The electron probe formed under such conditions is con- fined to minimize the Ga contamination. The incident angle, de- fined as the angle between the incident Ga-ion beam and the surface of the Si-substrate is about 90�. Linear I–V characteristics were received between each pair of the three branches (Fig. 2). The A–B, B–C, and A–C branch have nearly the same I–V character- istic. The resistance between each pair of branches was derived as 24 kX (A–B), 24 kX (A–C), 28 kX (B–C), in consistent with the data 0009-2614/$ - see front matter � 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.11.014 * Corresponding author. E-mail address: zlwang@gatech.edu (Z.L. Wang). Chemical Physics Letters 484 (2010) 96–99 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier .com/locate /cplet t Author's personal copy received previously. The resistance of each branch is listed in the Table 1. The resistance is proportional to the length, resistivity of the material, and the reverse of the sectional area. Each branch of the ZnO tetrapod is approximately in the shape of a truncated cone. The resistance is very sensitivity to the diameter of the ZnO-tetra- pod tip. Using a truncated cone-shape as an approximation for rep- resenting the tetrapod, the resistance of each branch is: R ¼ 4q p tanða=2Þ 1 D0 � 1 D � � ð1Þ where q is the resistivity of ZnO, D0 and D are the diameters of the branch at its thickest and thinnest parts, respectively, and a is the cone angle of the tetrapod branch. By measuring the D0, D, and a di- rectly from the SEM image, and deriving the resistance from the I–V curve, we get the conductivity of the ZnO tetrapod is 1.0–2.0 k Sm�1 (see Table 1). To test the effect of the interaction among the three terminals of the tetrapod, a DC voltage (Vg) is applied at one of the three branches as a ‘gate’, while the other two branches serve as source and drain (shown in Fig. 3a). The current flow through the source and drain is monitored by source measurement units (SMUs) of Keithley 4200-SCS. First, we measured current flowing through the source, drain and gate at different gate voltages. Fig. 3b–f show the ‘gating’ effect of the ZnO tetrapod. Different gating voltages (�0.1, �0.05, 0, 0.05 and 0.1 V) applied on the gate branch, and a proportional displacement of the current with Vg and Vs can be Fig. 2. I-V characteristics of three pairs of branches (AB, AC, BC) of a ZnO tetrapod that was deposited with Pt electrodes at the tips. Inset is an SEM image of the measured ZnO tetrapod. Fig. 1. Typical I-V characteristic of ZnO tetrapod measured by using tungsten (W) tips directly in contacting with the branches. Inset is an SEM image of the measured ZnO tetrapod. Fig. 3. (a) Schematic circuit for the measurement of ‘‘gating” effect of a ZnO tetrapod. The current flow through the source (Is), drain (Id) and gate (Ig) under different gating voltages of (b) �0.2 V, (c) �0.1 V, (d) 0 V, (e) 0.1 V and (f) 0.2 V. Table 1 Measured resistance the three branches of a tetrapod and the corresponding conductivity. The variation in the measured conductivity is due to the inaccuracy in quantifying the dimension of the pole especially at the tip side. Resistance Conductivity (kX) (Sm�1) Branch-A 10 2000 Branch-C 14 1500 Branch-B 14 917 Y. Gu et al. / Chemical Physics Letters 484 (2010) 96–99 97 Author's personal copy observed in Fig. 3. The current flow through each branch can be derived based on the Kirchhoff’s Voltage Law (KVL) and Kirchhoff’s Current Law (KCL), which give: Ig ¼ VgðRC þ RBÞ � V sRCR ð2Þ Is ¼ V sðRA þ RCÞ � VgRCR ð3Þ Id ¼ �VgRB þ V sRAR ð4Þ here R ¼ RARB þ RBRC þ RCRA, RA, RB, RC, are the resistance of each branch respectively, Vg is the gate voltage, and Vs is the source volt- age. From the formula above, we can see that the currents in the branches are linear proportional to (�RC)/R, (RA + RC)/R, and (�RA)/ R, respectively, at a given gate voltage. This result indicates that the tetrapod is effectively a current splitter. The current flowing through each branch at different gate volt- age was monitored (Fig. 4). Fig. 4b shows the current flowing through the drain terminal, at a constant source voltage, the cur- rent decreases linearly when the gate voltage increases. From the electrical characteristics of the ZnO tetrapod, it is clear that the current flowing through the source and drain can be modulated by the gate voltage, but the operation mode of the device is quite different from that of the conventional FET that is controlled by the carrier density and the thickness of the channel. The ZnO tetra- pod is a current splitter that can be controlled by the ‘gate’ voltage. The unique shape of the tetrapod can also be used as a logic component. Two of the branches (gate and source branch) are em- ployed as two inputs and the other one (drain branch) can be served as an output. A constant voltage (�0.1, �0.05, 0, 0.05, 0.1 V) is applied on the gate branch, and a sweeping voltage from �0.1 to 0.1 V is applied on the source branch, and the voltage of drain branch is monitored. The positive voltage can be described as logic state of ‘1’, and negative voltage including zero can be de- scribed as logic state of ‘0’. The output branch presents logic state of ‘1’ only when both the gate and source branch are at logic state ‘1’, otherwise the output will present logic state ‘0’, which has the same logic property of ‘AND’ logic component [20,21]. In summary, the electrical transport characteristic of a ZnO tet- rapod has been measured by in situ nanoprobes. By contacting the three legs of a tetrapod as the ‘gate’, source and drain, some control and tunability has been achieved in the output voltage/current, in analogous to the operation of a field effect transistor. A simple logic circuit has also been demonstrated. Acknowledgments This research was supported by DARPA (Army/AMOCOM/ REDSTONE AR, W31P4Q-08-1-0009), BESDOE (DE-FG-02— 07ER46394), Air Force Office (FA9550-08-1-0046), DARPA/ARO W911NF-08-1-0249, NSF. Yudong Gu thank the partial fellowship supported by the China Scholarship Council (CSC) (No. 20083019). References [1] A. Tsukazaki et al., Nat. Mater. 4 (2005) 42. [2] C.S. Lao, Q. Kuang, Z.L. Wang, M.C. Park, Y.L. Deng, Appl. Phys. Lett. 90 (2007) 262107. [3] J. Zhou et al., Nano Lett. 8 (2008) 3035. [4] J. Zhou et al., Appl. Phys. Lett. 94 (2009) 191103. [5] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nat. Mater. 4 (2005) 455. [6] Z.L. Wang, J.H. Song, Science 312 (2006) 242. [7] Z.L. Wang, Adv. Funct. Mater. 18 (2008) 3553. [8] M.H. Huang, Y.Y. Wu, H. Feick, N. Tran, E. Weber, P.D. Yang, Adv. Mater. 13 (2001) 113. [9] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [10] X.Y. Kong, Y. Ding, R.S. Yang, Z.L. Wang, Science 303 (2004) 1348. [11] P.X. Gao, Y. Ding, W. Mai, W.L. Hughes, C. Lao, Z.L. Wang, Science 309 (2005) 1700. Fig. 4. (a) Schematic circuit for the measurement of ‘‘gating” effect of a ZnO tetrapod. The current flowing through (b) drain, (c) source and (d) gate electrode at different gating voltages. 98 Y. Gu et al. / Chemical Physics Letters 484 (2010) 96–99 Author's personal copy [12] J. Carrasco, F. Illas, S.T. Bromley, Phys. Rev. Lett. 99 (2007) 235502. [13] Y. Dai, Y. Zhang, Q.K. Li, C.W. Nan, Chem. Phys. Lett. 358 (2002) 83. [14] Y. Ding, Z.L. Wang, T. Sun, J.S. Qiu, Appl. Phys. Lett. 90 (2007) 153510. [15] P.R. Bandaru, C. Daraio, S. Jin, A.M. Rao, Nat. Mater. 4 (2005) 663. [16] A.N. Andriotis, M. Menon, D. Srivastava, L. Chernozatonskii, Phys. Rev. Lett. 8706 (2001) 066802. [17] Y. Cui, U. Banin, M.T. Bjork, A.P. Alivisatos, Nano Lett. 5 (2009) 1519. [18] M.C. Newton, S. Firth, T. Matsuura, P.A. Warburton, J. Phys. Conf. Ser. 26 (2006) 251. [19] J. Zhou et al., Nano Lett. 8 (2008) 3973. [20] D. Wallin, H.Q. Xu, Appl. Phys. Lett. 86 (2005) 253510. [21] H.Q. Xu, Appl. Phys. Lett. 78 (2001) 2064. Y. Gu et al. / Chemical Physics Letters 484 (2010) 96–99 99
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