Novel ZnO Nanorod Flexible Strain Sensor and Strain Driving

Dec 10, 2010 - Novel strain sensor based on ZnO bridging nanorods has been fabricated on Kapton substrate by a single-step hydrothermal reaction, and ...
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Novel ZnO Nanorod Flexible Strain Sensor and Strain Driving Transistor with an Ultrahigh 107 Scale “On”-“Off” Ratio Fabricated by a Single-Step Hydrothermal Reaction Nishuang Liu, Guojia Fang,* Wei Zeng, Hao Long, Longyan Yuan, and Xingzhong Zhao Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Department of Electronic Science and Technology, School of Physics and Technology, Wuhan UniVersity, Wuhan 430072, People’s Republic of China ReceiVed: September 2, 2010; ReVised Manuscript ReceiVed: October 29, 2010

Novel strain sensor based on ZnO bridging nanorods has been fabricated on Kapton substrate by a singlestep hydrothermal reaction, and fully packaged by a polydimethylsioxane layer. Via introducing a metastable contact that can be controlled by strain, flexible strain sensors with high sensitivity and a strain driving transistor with an ultrahigh 107 scale “on”-“off” ratio have been demonstrated. However, high performance devices only come from the samples fabricated with lower nutrient solution concentration and are only sensitive to tensile strain. We found out that the response behavior strongly depends on the device structure. The I-V characteristic is highly sensitive to strain due to the change in Schottky barrier height, as well as the change of contact area between ZnO bridging nanorods induced by the metastable morphology under strain. 1. Introduction Zinc oxide (ZnO), which is a direct wide band gap (3.37 eV at room temperature) semiconductor with a large exciton binding energy of 60 meV, has attracted a wide range of interest in science and technology. Moreover, one-dimensional ZnO nanostructures are especially attractive because of their unique properties such as high surface-to-volume ratio and carrier confinement in two dimensions, which could improve device performance. Because of the unique optical, electronic, mechanical, and piezoelectric properties, ZnO nanostructures-based field effect transistors,1 ultraviolet (UV) laser,2 UV photodetectors,3 light emitting diodes,4 piezoelectric transducer,5 and solar cells6 have been reported. Especially, via utilizing semiconducting-pizoelectric coupled properties of ZnO,7 nanogenerators,8,9 piezoelectric field effect transistors,10 piezoelectric diodes,11 and piezoelectric humidity/chemical sensors12 were demonstrated. As for nanoscale strain and pressure measurements, a series of sensors based on nanowires13,14 and carbon nanotubes (CNTs)15-17 have been fabricated recently. In particular, ZnObased piezoelectric strain sensors are expected to have a higher sensitivity,18 as compared to the currently most intensively studied piezoelectric strain sensor based on Si nanowires and CNTs. Additionally, considering the similar working mode, ZnO piezoelectric field effect transistors also can be regarded as a type of piezoelectric strain sensor.10 However, on the basis of the literature reports, the control of strain on current, which also can be regarded as a form of sensitivity, is still not good enough. For example, for common ZnO piezoelectric strain sensor, an “on”-“off” ratio less than 10 can be observed mostly.18,19 Even for piezoelectric-potential-controlled polarity-reversible Schottky diodes based on ZnO wires, the “on”-“off” ratio can only be ∼120 at the most.20 Maybe due to the same working mechanism, the working current of ZnO piezoelectric field effect transistors could only be restricted to a small range,10,21 which blocks its practical application. However, it has been reported that the * To whom correspondence should be addressed. Tel: +86 (0)27 68752147. Fax: +86 (0)27 68752569. E-mail: [email protected].

“on”-“off” ratio can be very high for many nanoscale switches, which are operated by an electrostatic force to switch between “on” and “off” depending on mechanical contact.22,23 Up to now, most of the nanoscale piezoelectric strain sensors are composed of single nanowire with firm mechanical contact. Although firm mechanical contact is very important for working stability or analyzing the function mechanism, it blocks the possibility of achieving ultrahigh “on”-“off” ratio. Therefore, it must be very useful to improve the “on”-“off” ratio or sensitivity via introducing a metastable contact, which can be controlled by strain. In fact, this kind of mechanically strain controllable devices has already been achieved to stretch individual molecules.24 However, to the best of our knowledge, no literature about a nanoscale piezoelectric strain sensor with this kind of metastable contact, which can be controlled by strain, has ever been reported until now. On the other hand, how to put nanostructures into devices is still a challenge. The most popular choice is the conventional “pick and place” method.18,20 In this way, nanostructures are flaked away from their initially grown substrates, and then are dispersed randomly on an insulating substrate. At last, sophisticated techniques such as electron beam lithography or focused ion beam are required to make metallic contacts to the nanostructures. Although this way offers a means to put nanostructures into a working device, it is time-consuming and inefficient, which blocks its way to practical applications. Therefore, it must be helpful to develop a more effective method to fabricate nanoscale devices. In this Article, via introducing a metastable contact that can be controlled by strain, we demonstrate a flexible ZnO bridging nanorod strain sensor with an ultrahigh 107 scale “on”-“off” ratio, which is fabricated on Kapton substrate by a single-step hydrothermal reaction. With the combined effect from a ZnO seed layer and an inactive layer for nanorod growth, ZnO nanorods could grow laterally and align between the two electrodes.25-28 When the growth process is terminated, the integration of ZnO nanorods into a function device can be achieved in the meantime. In addition, a 107 scale “on”-“off” ratio can be observed in our sample with an applied strain only

10.1021/jp108352b  2011 American Chemical Society Published on Web 12/10/2010

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Figure 1. (a) Schematic diagram for the fabricated ZnO bridging nanorod strain sensor device. (b) Schematic of the measurement system to test the performance of the fabricated strain sensor device.

Figure 2. FESEM images of typical ZnO nanorod strain sensor grown with nutrient solution concentrations of (a) 50 and (b) 25 mM. (c) HRTEM picture of a single ZnO nanorod.

up to 1.4% under 2 V bias voltage. Moreover, due to the wide transformation range of current under strain, it indicates the possible application of strain driving transistor. Yet high performance devices only come from the samples fabricated with lower nutrient solution concentration and are only sensitive to tensile strain. We found out that the response behavior strongly depends on the device structure. On the basis of these results, we proposed a working principle for our new strain sensor in this study. 2. Experimental Section The schematic of the strain sensor device is shown in Figure 1a. The typical Kapton substrate has a length of ∼3 cm, width of ∼8 mm, and thickness of 0.25 mm. Before the ZnO nanorods were grown, a 300 nm ZnO seed layer was deposited on the substrate using a radio frequency magnetron sputtering deposition system. To prepare the two contact electrodes pattern, a conventional photolithography followed by lift-off techniques was used. Next, we sputtered a metal (Sn) layer on the patterned ZnO seed layer for preventing the local growth of ZnO and serving as metal contacts in the meantime. The two contact electrodes were 5 mm wide with 5 µm spacing. The nutrient solution for ZnO nanorod growth was an aqueous solution of 25 (or 50) mM zinc nitrate [Zn(NO3)2 · 6H2O] and hexamethylenetetramine. The reaction was kept at 90 °C for 2 h. The fabricated ZnO nanorod strain sensors were removed from the solution, rinsed with distilled water, and dried in air. Silver paste was applied at both ends of the two contact electrodes to make a good contact between electrodes and external electric leads. Also, a copper tape was put on the silver paste to make the contact of electrodes between external electric leads tight enough. To prevent the ZnO nanorods from contamination or corrosion, a thin layer of polydimethylsiloxane (PDMS) was used to package the entire device by spin coating, and the PDMS layer is much thinner than the Kapton substrate. Next, the entire device was annealed at 80 °C for 10 h. At last, a flexible and well-packaged ZnO bridging nanorod strain device has been fabricated. The morphology of the as-grown ZnO bridging nanorod strain sensor was characterized by field emission scanning electron

microscopy (FESEM, FEI XL-30). High-resolution transmission electron microscopy (HRTEM JEOL JEM 2010) was then used to characterize the crystallographic properties of the as-grown ZnO nanorods. The experimental configuration for strain sensor measurements is shown schematically in Figure 1b. One end of the strain sensor was fixed on an optical air table via a sample holder, and the other free end of the strain sensor was drived by a mechanical stage with movement resolution of 1 µm, which brings a compressive or tensile strain. A Keithley 4200 semiconductor parameter analyzer was then used to measure current-voltage (I-V) characteristics of the fabricated ZnO nanorod strain sensors under different strain. All of the measurements were carried out at room temperature in ambient condition. 3. Results and Discussion Figure 2a and b shows the physical structures of typical ZnO nanorod strain sensors grown with nutrient solution concentrations of 50 and 25 mM. It can be seen that ZnO nanorods grew laterally between the two opposite electrodes with a good alignment. Also, the hexagonal cross section of nanorods implies that the c axis of ZnO nanorods is along its length direction. The position-controlled growth of the nanorods implies that the nucleation leading to the growth of nanorods takes place only at the open area exposed to the edge of ZnO seed layer. From these images, we can observe that ZnO nanorods grew longer with nutrient solution of higher concentration. It indicates that we can control the physical structure of the ZnO nanorod strain sensor accurately by control the nutrient solution or the gap between two electrodes. Moreover, it is the key point to fabricate a highly sensitive strain sensor, which will be discussed in the later part of this study. The HRTEM image (Figure 2c) indicates that the ZnO nanorods are structurally uniform and contain no defects such as dislocations or stacking. The lattices spacing of 0.52 nm corresponds to a d-spacing of (002) crystal planes, indicating the growth of crystalline ZnO nanorods is along the c-axis direction. Considering the fact that the Young’s modulus of the thin PDMS layer (E ) 360-870 kPa) is much smaller than that of

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Figure 3. (a) I-V characteristics of a strain sensor device with low “on”-“off” ratio at different strain. The stretch strain is considered as positive. I-V characteristics of a strain sensor device with ultrahigh “on”-“off” ratio at different stretch strain in (b) linear and (c) logarithmic coordinates. (d) Transfer characteristics of our strain driving transistor under different bias voltage.

the Kapton substrate (E ) 2.5 GPa), we do not believe that the PDMS layer alters the mechanical properties of the Kapton substrate at any significant level. So the strain in the ZnO nanorod strain sensor is purely induced by the bending of the Kapton substrate at any direction. The strain in the ZnO nanorod strain sensor is approximately equal to the strain of the mechanical stage where it was placed on the outer surface of the Kapton substrate. Considering the extremely small diameter of the ZnO nanorods in comparison with the thickness of the Kapton substrate, the axial strain εzz along the length of the ZnO nanorods is approximately:18

εzz ) 3

aD z 1l l l

(

)

(1)

where z is the vertical distance measured from the fixed end of the Kapton substrate to the middle of the ZnO nanorod sensor; a is the half-thickness of the Kapton substrate; l is the length of Kapton substrate from the fixed end to the free end; and D is the deformation of the free end of the Kapton substrate, which has a positive or negative sign depending upon whether the Kapton substrate is under tensile or compressive strain, respectively. From eq 1, we can see that the strain εzz has a linear relationship with the maximum deformation D. Considering the fact that the length of the substrate is much larger than the length of the channel where ZnO bridging nanorods grow, we believe that the strain in the ZnO nanorod sensor is uniform to an excellent approximation in practice.18

At first, we have measured the I-V characteristic of the ZnO nanorod sensor device with or without strain. Also, we found out that the I-V curves exhibited almost symmetrical nonlinear behaviors for all of our 30 samples under any condition. This phenomenon may be due to the almost same contact and growth condition at the two sides of the channel. So we just show the positive side of the I-V curves for simplicity in the later part of this work. After testing all of our 30 samples, we found out that the fabricated ZnO nanorod strain sensors can be divided into two categories: sensor with low and ultrahigh “on”-“off” ratios. It is interesting that only the samples fabricated with nutrient solution concentration of 25 mM have ultrahigh “on”-“off” ratio. We believe that this giant difference in the response behaviors of strain sensors must come from the tiny difference between the individual structure of them, which has been shown in Figure 2a and b. The detailed mechanism will be discussed in the later part of this work. We found out that the ratio of the device with ultrahigh “on”-“off” ratio turns out to be at about 107 scale, while the ratio of the device with low “on”-“off” ratio turns out only to be less than 10. Typical I-V characteristics of a sample from the devices with low “on”-“off” ratio under various strains are shown in Figure 3a. The I-V curves all shift downward either with tensile strain or with compressive strain. The I-V curve fully recovered when the strain was relieved. This behavior is quite common and is very similar to other literature.18-20 Therefore, in this study, we only focused on the devices with an ultrahigh “on”-“off” ratio, which has never been reported yet.

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Figure 4. (a) Current response of the strain sensor device grown with nutrient solution concentration of 25 mM to different stretch strain. (b) Schematic band diagram showing band bending at ZnO nanorod-nanorod junction caused by adsorption. (c) Fitting the ln I-V data of the strain sensor device with ultrahigh “on”-“off” ratio at different stretch strain using the thermionic emission-diffusion theory. The red lines are the theoretical fit of ln I-V1/4. (d) Logarithm plot of the current under different bias voltage as a function of strain.

After careful measurement, we found out an interesting phenomenon that the devices with ultrahigh “on”-“off” ratio are only sensitive to the tensile strain, but very insensitive to the compressive strain. The response to the compressive strain is quite weak, just as the device with low “on”-“off” ratio. This phenomenon is also related to the individual structure of the strain sensor. So we mainly focused on the giant response to the tensile strain here. The I-V curves of a quite sensitive device under various tensile strains are displayed in Figure 3b. It can be seen that the I-V curves shift downward with different tensile strain. The I-V curve also fully recovered when the strain was relieved. The response of current to the strain is so huge that the I-V curves seem as a typical output curve of transistor. For a more visual illustration, the I-V curves in logarithmic coordinates are shown in Figure 3c. In addition, the transfer characteristics of our strain driving transistor under different bias voltage are shown in Figure 3d. It can be clearly seen from the image that the “on”-“off” ratio can reach 0.94 × 107 with an applied strain of 1.4% under 2 V bias voltage. Because of the mA scale “on” state current and 10-10 A scale “off” state current, we can even use this strain driving transistor to drive a light emitting diode. Usually, to compare the performance of a strain sensor in practical application, a gauge factor is introduced. Also, the gauge is defined to be the slope of the normalized current (I)-strain (ε) curve, [∆I(ε)/I(0)]/∆ε.18 The gauge factor of our ZnO strain sensor device shown in Figure 3b is as high as 6.7 × 108, which exceeds the gauge factor reported in most literatures.17,18 This fantastic result indicates

great possibility of application in the field of micro- and nanoelectronmechanical (MEMS and NEMS) systems. Moreover, due to the single direction response behavior, we believe that it indicates great promising practical application. For example, we can put many sensors at different directions in a detector system, and then we can determine the direction and magnitude of applied strain, when an external perturbation arises. Furthermore, the stability and response of the sensor device was carefully studied. Figure 4a shows the time-resolved current response of the sensor to different tensile strain repeatedly. It can be seen that the current decreases with more tensile strain and increases with less tensile strain, which is consistent with the behavior observed in Figure 3b and c. As shown in the figure, the current can reach almost the same value in each cycle under the same strain, and the current can fully recover to the original state when the strain was relieved, indicating that the ZnO bridging nanorod sensor device had high reproducibility and good stability. Restricted by the experiment condition, the shortest interval time we can achieve is 100 ms, and it can be clearly seen that the response time is absolutely less than 100 ms. To explain the working mechanism, we must make the electric transportation mechanism clear at first. The nonlinear I-V characteristics are commonly observed in measuring semiconductor devices.29,30 Generally, the nonlinearity is caused by the Schottky barriers formed between the semiconductor and the metal electrodes in the semiconductor device, and the shape of the I-V curve depends on the heights of the Schottky barriers

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formed at the source and drain due to different interface properties.30,31 However, for our sample, metal Sn and n-type ZnO (work function of 4.3 and 5.1 eV) can form ohmic contact.32 Therefore, we believe the nonlinear behavior must come from the Schottky contact between the ZnO bridging nanorods. Additionally, it is well-known that oxygen molecules would adsorb on the ZnO surface by capturing free electrons, creating a surface depletion layer with low conductivity.33,34 As a result, the energy band would bend on the surface, and this surface band bending caused by adsorption is given by:34

φB )

( )

q2ND W 2εZnO

(2)

where q is the electronic charge, εZnO is the dielectric constant of ZnO, ND is donor density, and W is depletion width. Next, the energy band diagram of the ZnO bridging nanorod device is shown in Figure 4b. For our ZnO nanorod bridge device, the electrons have to overcome the nanorod-nanorod junction barrier when tunneling from one nanorod to another. Although there is much literature discussing the electric transportation mechanism of this kind of nanostructures bridging or network device, no specific theory is proposed to describe the I-V characteristic of such device.35,36 However, we found out that the energy band diagram shown in Figure 4b is similar to that of isotype heterojunction or semiconductor grain boundaries, of which the I-V characteristic can model as two Schottky diodes back-to-back.37,38 Considering the similar energy band structure, we believe that using this model to analyze the I-V characteristics of our device is reasonable. Therefore, the structure of our device can be modeled as being composed of two Schottky diodes with the same barrier height connected back-to-back. At a fixed applied bias V, the voltage drop occurs mainly at the reverse biased Schottky barrier φs (eV), and it is denoted by Vs. Because the voltage at the forward biased Schottky barrier is almost neglectable, we assume Vs ≈ V. Considering that our devices all worked at room temperature and the ZnO nanorods had a low doping, we believe that the dominant transport property at the barrier is thermionic emission and diffusion, while the contribution of tunneling can be ignored.18,31 Therefore, in our experiment, the current through the reverse bias Schottky barrier is as follows based on classic thermionic emission-diffusion theory31 (for V . 3kT/q ≈ 77 mV):

( )

I ) SA**T2 exp -

φs exp kT

(

4

√q7ND(V + Vbi - kT/q)/8π2εs3 kT

)

Figure 5. Schematic of the two types of strain sensor devices with shorter and longer ZnO nanorods under different strain.

which has been reported in other literature,18 while in the second stage, the value of ln I falls much faster and no longer has a linear relationship with the applied strain. It can be seen from eq 3 that there are two variables that exist for our sample. The first variable is the Schottky barrier φs. When the strain sensor is working under no strain, the Schottky barrier φs here equals the surface band bending φB of course. Yet then the situation changes, when the strain sensor is working under strain. It has been reported that the SBH (Schottky barrier height) change for ZnO under strain can be viewed as a combination of band structure change and piezoelectric effects.18 The effect of the band structure change may be equivalently characterized by a change in semiconductor (ZnO) electron affinity under strain, which is denoted as ∆φs-bs.18 The change in SBH by piezoelectric polarization is given approximately by:39

∆φs-pz )

(

σpz 1 1+ D 2qsd

)

-1

(4)

where σpz is the area density of piezoelectric polarization changes (in units of electron charge), D is a two-dimensional density of interface states at the Fermi level in the semiconductor band gap at the Schottky barrier, and d is the width of the depletion layer. Associated with the states in the band gap at interface is a two-dimensional screening parameter qs ) (2πq2/ k0)D, where q is the electronic charge and k0 is the dielectric constant of the ZnO. Therefore, the total change in SBH of ZnO sensor can be expressed as18

(3)

where S is the contact area of the Schottky barrier, A** is the effective Richardson constant, q is the electron charge, k is the Bolzman constant, ND is the donor impurity density, Vbi is the build in potential at the barrier, and εs is the permittivity of ZnO. The ln I-V curves shown in Figure 4c qualitative indicate that variation of ln I has a linear relationship with V1/4 for reverse biased Schottky barrier instead of with V as for forward biased Schottky barrier. Therefore, we believe that eq 3 can be used to precisely fit the experimentally observed ln I-V curve. Additionally, Figure 4d shows the logarithm plot of the current under different bias voltage as a function of applied strain. Also, the figure can be divided into two stages. In the first stage, the change of ln I varies approximately linear with the applied strain,

∆φs ) ∆φs-bs + ∆φs-pz

(5)

The second variable is the contact area of the Schottky barrier, which is the key point of the ultrahigh “on”-“off” ratio. The schematic of the strain sensor devices with strain and without strain is shown in Figure 5. As shown in the figure, the contact area does not change much for the ZnO strain sensor with longer nanorods. However, the contact area changes a lot even with little deformation (only under tensile strain) for the ZnO strain sensor with shorter nanorods. The contact area shows more change under tensile strain than under compressive strain. Especially, the contact area can almost turn into zero with an appropriate tensile deformation. We believe that it is the reason

ZnO Nanorod Flexible Strain Sensor for the ultrahigh “on”-“off” ratio and single direction response behavior in this study. Therefore, on the basis of the above analysis, we believe that the behavior of the device is modulated by strain due to the change in Schottky barrier height and contact area between ZnO bridging nanorods. As we described above, the ln I-strain curves can be divided into two stages, and the curves are approximately linear in the first stage. In addition, it has been reported that ln I has an almost linear relationship with the applied strain for ZnO nanowire piezotronic strain sensor due to the change in SBH.18 So we believe that the change in SBH induced by strain is the main function mechanism for our device, when the deformation is relative slight, or the device is composed of longer ZnO nanorods. This ordinary strain response behavior is similar to other piezoelectric strain sensor. When the deformation is relatively large for the device composed of shorter ZnO nanorod, the change of contact area becomes the main function mechanism, and it is also the reason for our incredible highly sensitive strain sensor. 4. Conclusions A novel fully packaged strain sensor based on hydrothermally grown ZnO bridging nanorods has been demonstrated. The ZnO bridging nanorod strain sensors have excellent stability, fast response, ultrahigh 107 scale “on”-“off” ratio, and high gauge factor of up to 6.7 × 108. However, the high performance devices only come from the samples fabricated with lower nutrient solution concentration and are only sensitive to the tensile strain. Therefore, we believe that the response behavior is strongly dependent on the device structure, which is also the reason for the ultrahigh “on”-“off” ratio and single direction responsebehavior.Besidesthecommonsemiconducting-pizoelectric coupled mechanism, the change of contact area between ZnO bridging nanorods induced by the metastable morphology under strain also plays an important role in the function mechanism. Our results imply that ZnO bridging nanorods devices synthesized by hydrothermal approach on a flexible substrate are promising candidates for strain and stress measurements in biomedical sciences and MEMS devices. Acknowledgment. This work was partially supported by the Special Fund of Ministry of Education for Doctor’s Conferment Post (20070486015), the National High Technology Research, Development Program of China (2009AA03Z219), the National Natural Science Foundation of China (11074194), and the National Basic Research Program (2011CB933300) of China. References and Notes (1) Arnold, M.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659–663. (2) Huang, M.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897–1899. (3) Soci, C.; Zhang, A.; Xiang, B.; Dayeh, S. A.; Aplin, D. P. R.; Park, J.; Bao, X. Y.; Lo, Y. H.; Wang, D. Nano Lett. 2007, 7, 1003–1009. (4) Park, W. I.; Yi, G. C. AdV. Mater. 2004, 16, 87–90. (5) Buchine, B. A.; Hughes, W. L.; Degertekin, F. L.; Wang, Z. L. Nano Lett. 2006, 6, 1155–1159.

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