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J. Phys. Chem. C 2009, 113, 5010–5013
Enhanced Performance of ZnO Nanocomposite Transistor by Simple Mechanical Compression Ji-Hyuk Choi, Jyoti Prakash Kar, Dahl-Young Khang, and Jae-Min Myoung* Information and Electronic Materials Research Laboratory, Department of Materials Science and Engineering, Yonsei UniVersity, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea ReceiVed: December 4, 2008; ReVised Manuscript ReceiVed: January 22, 2009
A simple yet effective method is developed to enhance the electrical properties of ZnO nanowire-polymer nanocomposite. While the as-cast nanocomposite devices showed no detectable current output due to the absence of continuous wire-to-wire contacts, devices pressed at 300 or 600 kPa exhibit significantly enhanced device performances. Such a drastic performance improvement would be due to the generation of more direct contacts between nanowires, leading to continuous electrical pathways for the device. The approach introduced here would be a general way to get percolation threshold in various nanocomposites, without changing the loading content of filler materials. 1. Introduction Recently, nanocomposites, within which nanowires are dispersed forming a network structure, have emerged as new electronic materials with the intrinsic properties originating from nanowires retained.1-6 Networked nanocomposites based on carbon nanotubes (CNTs) or silcon nanowires (Si NWs) have already demonstrated their performance as active channels or electrodes in a variety of devices with the promise of significant improvement in the performance for large-area electronics applications.1-7 Compared to typical composite materials constructed by conventional fillers, nanocomposites containing nanowires show the intended properties even with extremely low contents of them, because nanowires are easily percolated due to their high aspect ratio and surface-to-volume ratio. Polymer composites with carbon nanotube or grapheme, which are representative 1-D and 2-D nanofillers, respectively, exhibit a percolation threshold of ∼0.1 vol % for electrical conductivity.1-6,8,9 Such a low threshold enables far less usage of expensive filler materials. Another merit of nanocomposites lies in their easy processability; that is, one can apply processing techniques developed for the host matrices of nanocomposite, instead of direct manipulation of those nanoscale entities. Especially, polymer-based nanocomposites can easily be produced using well-developed solution processes such as drop casting, spin coating, ink jet printing, etc.10-12 This is one of the key features that make polymer nanocomposites one of the prospective candidates for future applications such as large-area electronics, organic electronics, and smart materials. In addition, transparency, flexibility, low-temperature processability, and compatibility with other organic devices render the study of the polymer nanocomposites mandatory. With the many advantages mentioned above, however, there are still challenging issues in the practical applications of nanocomposites, such as contact resistance between nanowires and homogeneous dispersion in the host matrix. Therefore, there is a growing need to explore methods to lower the contact resistance between nanowires in polymer matrix, which is * Corresponding author. Tel.: +82 2 2123 2843. Fax: +82 2 365 2680. E- mail:
[email protected].
expected to have a profound impact on the future of nanowirebased nanocomposite for electronic applications. In this study, we present a simple, yet effective, method for improving contacts between nanowires in polymer-based nanocomposite. Simple mechanical compression of the nanocomposite performed by PDMS mold under different pressures was found to greatly decrease the contact resistance between nanowires embedded in polymer matrices, thus leading to the increase in output current. 2. Experimental Details The ZnO nanowires were grown vertically on c-plane Al2O3 substrates using metal-organic chemical vapor deposition (MOCVD). Diethylzinc, precursor of Zn, was carried by Ar (6 N) gas, and oxygen (6 N) was used as an oxidizer. The details for the ZnO nanowire growth can be found elsewhere.13 The poly(4-vinyl phenol), PVP, solution was prepared with cross-linking agent poly(melamine-co-formaldehyde) in propylene glycol monomethyl ether acetate (PGMEA). The ZnO nanowires removed from the growth substrate were subsequently dispersed into the PVP solution. The amount of the ZnO nanowires in PVP solution was varied from 1 to 6 wt % relative to that of PVP in solution. The dispersion was sonicated for 1 h at room temperature before use. The source and drain electrodes, Al (50 nm), were deposited by sputtering on highly doped silicon substrate with a thermally grown 300 nm thick oxide. The nanowire/PVP dispersion was drop cast on the channel region of electrodes-patterned SiO2/Si substrate. After the sample was dried at room ambient for ∼1 day, it was loaded onto a homemade press machine with a flat slab of poly(dimethylsiloxane) (PDMS) (Sylgard 184) rubber on it. The sample was heated to 200 °C while maintaining the compression at specified pressures, 300 or 600 kPa, for 30 min. The gradual thermal cross-linking of PVP is expected to occur by heating, thus firmly setting the newly formed networks of nanowires by compression intact after the process. The morphologies of the synthesized ZnO nanowire arrays were examined by field emission scanning electron microscopy (FESEM, Hitach S-4200). The crystal structure of the synthesized NWs was analyzed using X-ray diffraction (XRD, Rigaku DMAX-2500) and high-resolution transmission electron mi-
10.1021/jp810669c CCC: $40.75 2009 American Chemical Society Published on Web 03/02/2009
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Figure 2. (a) Representative XRD pattern for the ZnO nanowires grown on c-plane sapphire substrate. The inset image shows the nanowires grown vertically on the substrate. (b) Representative HRTEM image and SAED pattern (inset) of a single ZnO nanowire whose growth direction is along [0001].
Figure 1. (a) Schematic illustration of experimental procedures. (b) ZnO nanowires-PVP nanocomposite before (top) and after (bottom) compression. Note that the nanowires in the composite form continuous electrical conduction pathways when compressed.
croscopy (HRTEM, JEOL JEM-2100F). The surface morphologies and roughness of the nanocomposotes were observed by an atomic force microscope (AFM, NanoScope IVa Digital Instruments). The electrical characteristics of those nancomposite devices were characterized using the HP-4145B semiconductor parameter analyzer. 3. Results and Discussion Figure 1a schematically illustrates experimental procedures developed in this work. First, the solution of PVP and ZnO nanowires was formulated and drop cast on the substrate that has prepatterned electrodes structure. Then the nanocomposite devices were pressed to 300 or 600 kPa at 200 °C for 30 min. Figure 1b shows the expected change of nanowire configuration upon compression. When compressed, a connected continuous network of nanowires is formed between two metal electrodes. The simple mechanical compression leads to electrically conducting or semiconducting nanocomposites, otherwise insulating ones due to the lack of conducting pathways. Note here that the drop casting of pure nanowire dispersion, i.e., without polymer, led to nanowires preferentially aggregated around the rim of the initial drop, due to the well-known ringstain effect caused by capillary flow.14 Nanowire dispersion with polymer, however, resulted in the nonsegregated uniform distribution of nanowires throughout the composite. Even though the mechanical pressing employed here would be effective only
at the initial stage of compression, due to the gradual crosslinking of PVP at high temperature, it was found that there is a profound effect of the pressing on forming continuous electrical conducting pathways among dispersed nanowires in the composite. Once fully cured, the PVP matrix fixes the configuration of nanowires in it and thus helps maintain those electrical paths intact later on. Figure 2a shows the XRD spectra of ZnO nanowires grown on c-plane sapphire. The intensities are plotted on a logarithmic scale. The XRD peaks in Figure 2a are assigned to ZnO (0002), Al2O3 (0006), and ZnO (0004), respectively, from left to right. This, together with the strong (0002) peaks, indicates that the ZnO nanowires grown on c-plane sapphire have c-axis preferred orientations as well as good vertical alignment with respect to the normal of substrate surfaces. The SEM image, inset of Figure 2a, shows the vertically aligned ZnO nanowire arrays grown on c-plane Al2O3 substrates. The diameter and length of the grown ZnO nanowires were ∼120 nm and ∼4 µm, respectively, and they are uniformly distributed on the growth substrate with the density of ∼10/µm2. The HRTEM image and selected-area electron diffraction (SAED) pattern of the grown nanowire are shown in Figure 2b and in the inset, respectively, and they demonstrate that the ZnO nanowire has a single crystalline ZnO having wurtzite structure with the growth direction along [0001], without dislocations or stacking faults. In order to gain insight into controlling the density of ZnO nanowires in the composites, the areal density of nanowires was measured by SEM after burning off the polymer matrix at high temperature. After drop casting and drying of the composite solution on Si substrate, the composite was calcined at 600 °C for 30 min in air to remove polymeric materials. Figure 3a shows the plot of nanowire density as a function of its concentration in the starting dispersion, and representative SEM images are shown in the inset. As expected, the higher the nanowire concentration, the more the nanowires were found on the substrate. Although this data should not be taken as actual areal density in nanocomposite, because it resulted from the collapse of 3-dimensionally distributed nanowires inside the composite
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Figure 3. (a) Areal number density of nanowires (no./µm2) as a function of nanowire concentration. The insets show the images of the nanowires prepared at 1 wt % (left) and 6 wt % (right), after calcinations at 600 °C. (b) Cross-sectional SEM micrographs of nanocomposite before and after compression at 200, 400, and 600 kPa, respectively (scale bar ) 2 µm). (c) Areal number density of nanowires as a function of pressure from 0 to 600 kPa. (d) Optical microscopy image of naocomposite FET around the channel region and AFM topography image (inset: height scale is 30 nm) of nancomposite after compression at 600 kPa.
by calcination, it is likely that the mechanical compression may lead to similar configuration of nanowires. Based on this, the ZnO nanowires concentration of 6 wt % (∼11/µm2) was found to be good enough to form the continuously networked pathways between two measuring electrodes by mechanical compression. The effectiveness of compression to the enhanced nanowiresbased transistor performance, which is induced by simple pressing, can be seen from the results shown in Figure 3b, c. Based on a series of sectional microscopy images of the composite layer, the areal number density of nanowires was counted and plotted. It can be clearly seen that the external mechanical pressure leads to the higher density of nanowires, which, in turn, means that more electrical pathways or direct contacts among nanowires are formed in the composite layer. Note also that the pressure range used in this work is enough for the manipulation of nanocomposite structure, and the nanowire density is well controlled with pressure. As will be shown in the following sections, this morphology change leads to a dramatic enhancement of the nanowire-based device performance. Figure 3d shows an optical microscopy image of the nancomposite device pressed at 600 kPa and an AFM image of the compressed composite surface in the inset. The channel length and width of the device were approximately 10 and 100 µm, respectively. As shown in the optical image, the density of the nanowires is enough to form the continuous electrical paths. Drop-cast composite film showed very bumpy topology (data not shown here). Upon compression, however, the surface became flat as can be seen in the inset AFM image. The electrical characteristics of these nancomposite devices were characterized in air at room temperature. The transistor with nanocomposite as an active material showed n-type
Figure 4. Output curves of the FET device pressed at (a) 300 kPa and (b) 600 kPa, respectively. The insets in (a) and (b) show the transfer characteristic for each device.
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TABLE 1: Comparison of the ZnO Nanocomposite Device Performances density (no./µm2)
pressure (kPa)
Vth (V)
gm (nS)
Ion/Ioff
µe (cm2/V · s)
∼11 ∼11
300 600
-2.3 -2.7
0.38 11.2
∼18 ∼3.3 × 103
∼9.5 × 10-4 ∼3.3 × 10-2
conduction, and the characteristics of devices are shown in Figure 4a, b. The as-cast device, not having undergone the pressurization step, did not show any detectable currents under the source-drain bias (Vds) of 10 V, because nanowires were separated far from each other by the insulating polymer matrix intervening between them (data not shown here). The output characteristics of the nanocomposite device that was compressed at 300 kPa is shown in Figure 4a, and the drain current is very small and exponentially increases with increasing Vds, which implies that the externally induced compression produces some pathways for electrical conduction all over the composite film. But the electrical resistance originating from contacts between nanowires is still prevailing. The threshold voltage, on/off current ratio, and maximum transconductance (gm) are estimated to be about -2.6 V, 18, and 0.38 nS, respectively. To calculate the linear regime device mobility (µe), we evaluated the slope of the transfer (Id - Vg) curve and used the conventional formula µe ) (dIds/dVg)/(0VdsW/Lt), where is the dielectric constant, 0 is the permittivity of free space, t is the thickness of SiO2, L is channel length, and W is the channel width. µe was calculated to be ∼9.5 × 10-4 cm2/V · s. The output characteristics of the device compressed at 600 kPa is shown in Figure 4b. Compared to the device pressed at 300 kPa (Figure 4a), the drain current has increased by ∼50 times. The nanocomposite device, pressed at still higher pressure of 800 kPa, showed similar performance to that pressed at 600 kPa (data not shown here). Also, the device pressed at 600 kPa exhibited much higher on/off current ratio, maximum transconductance, and mobility of ∼3.3 × 103, 11.2 nS, and ∼3.3 × 10-2 cm2/V · s, respectively, as shown in Table 1. Compared to a single nanowire transistor, the transistor made from nanocomposite did not exhibit a clean saturation regime and high mobility, which is possibly due to the increased carrier scattering by the complex nanowire network path and grain boundary at nanowire junctions.15 Also, it was found that there is limited electrical contact between nanowires and source/drain electrodes, possibly due to the very thin residual insulating polymer layer between them. 4. Conclusion In summary, we developed a simple method for tuning the nanocomposite properties from insulator to semiconductor based onamechanicalcompressionofcomposite.Thenanowire-polymer
composite was turned from insulating into semiconducting state by applying external compression. The externally applied pressure is believed to form continuous electrical pathways among the dispersed nanowires. This method would be a very useful and general way to get a percolation threshold in various composites for applications such as sensors, transistors, and photovoltaics, etc., without changing the loading concentration of filler materials. Acknowledgment. This work was supported by the “System IC 2010” Project of Korea Ministry of Commerce, Industry and Energy (2008-8-1511) and the academic-industrial cooperation program funded by Hynix Semiconductor (2008-8-0306). This work was also supported by the Second Stage of Brain Korea 21 Project in 2008. References and Notes (1) Moniruzzaman, M.; Winey, K. I. Macromolecules 2006, 39, 5194. (2) Pradhan, B; Batabyal, S. K.; Pal, A. J. J. Phys. Chem. B 2006, 110, 8274. (3) Bo, X. Z.; Lee, C; Y; Strano, M. S.; Goldfinger, M.; Nockolls, C.; Blanchet, G. B. Appl. Phys. Lett. 2005, 86, 182102. (4) Snow, E. S.; Novak, J. P.; Campbell, P. M.; Park, D. Appl. Phys. Lett. 2003, 82, 2145. (5) Hu, L.; Hecht, D. S.; Gruner, G. Nano Lett. 2004, 4, 2513. (6) Artukovic, E.; Kaempgen, M.; Hecht, D. S.; Roth, S.; Gruner, G. Nano Lett. 2005, 5, 757. (7) Menard, E.; Lee, K. J.; Khang, D. Y.; Nuzzo, R. G.; Rogers, J. A. Appl. Phys. Lett. 2004, 84, 5398. (8) Ramanathan, T.; Abdala, A. A.; Stankovich, S.; Dikin, D. A.; Herrera-alonso, M.; Piner, R. D.; Adamson, D. H.; Schniepp, H. C.; Chen, X.; Ruoff, R. S.; Nguyen, S. T.; Aksay, I. A.; Prud’homme, R. K.; Brinson, L. C. Nat. Nanotech. 2008, 3, 327. (9) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Nature 2007, 448, 457. (10) Grunlan, J. C.; Liu, L.; Kim, Y. S. Nano Lett. 2006, 6, 911. (11) Sung, J.; Jo, P. S.; Shin, H.; Huh, J.; Min, B. G.; Kim, D. H.; Park, C. AdV. Mater. 2008, 20, 1505. (12) Beecher, P.; Servati, P.; Rozhin, A.; Colli, A.; Scardaci, V.; Pisana, S.; Hasan, T.; Flewitt, A. J.; Robertson, J.; Hsieh, G. W.; Li, F. M.; Nathan, A. A.; Ferrarib, C.; Milne, W. I. J. Appl. Phys. 2007, 102, 043710. (13) Jeong, M. C.; Oh, B. Y.; Lee, W.; Myoung, J. M. J. Cryst. Growth 2004, 268, 149. (14) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827. (15) Gao, P.; Brent, J. L.; Buchine, B. A.; Weinstraub, B.; Wang, Z. L.; Lee, J. L. Appl. Phys. Lett. 2007, 91, 142108.
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