Controlled assembly of zinc oxide nanowires using dielectrophoresisDingqu Wang, Rong Zhu, Zhaoying Zhou, and Xiongying Ye Citation: Applied Physics Letters 90, 103110 (2007); doi: 10.1063/1.2711756 View online: http://dx.doi.org/10.1063/1.2711756 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Determination of the epitaxial growth of zinc oxide nanowires on sapphire by grazing incidence synchrotron x-raydiffraction Appl. Phys. Lett. 90, 181929 (2007); 10.1063/1.2735956 Room temperature fast synthesis of zinc oxide nanowires by inductive heating Appl. Phys. Lett. 90, 093101 (2007); 10.1063/1.2709618 Room-temperature nanowire ultraviolet lasers: An aqueous pathway for zinc oxide nanowires with low defectdensity J. Appl. Phys. 98, 094305 (2005); 10.1063/1.2113418 Quantum efficiency of ZnO nanowire nanolasers Appl. Phys. Lett. 87, 043106 (2005); 10.1063/1.2001754 Horizontal growth and in situ assembly of oriented zinc oxide nanowires Appl. Phys. Lett. 85, 3244 (2004); 10.1063/1.1803951

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Controlled assembly of zinc oxide nanowires using dielectrophoresisDingqu Wang,a� Rong Zhu, Zhaoying Zhou, and Xiongying YeState Key Laboratory of Precision Measurement Technology and Instruments, Department of PrecisionInstruments and Mechanology, Tsinghua University, Beijing 100084, China

�Received 29 June 2006; accepted 1 February 2007; published online 7 March 2007�

A structure similar to a field effect transistor with two isolated top electrodes comprising the sourceand drain and a lower substrate electrode as the gate was used for the dielectrophoresis-basedassembly of zinc oxide nanowires. The results reveal that the assembly of nanowires is significantlyaffected by the gap distance between the two top electrodes as well as the magnitude and frequencyof the applied electric field. Gate assisted assemblies using direct current and alternating currentdielectrophoresis were also investigated and determined to improve the assembly effect ofnanowires. © 2007 American Institute of Physics. �DOI: 10.1063/1.2711756�

Quasi-one-dimensional nanostructures, such as nano-tubes, nanowires, and nanobelts, have attracted extensive at-tentions both in scientific and technological communities dueto their unique electrical and mechanical properties.1,2 Re-cently the research emphasis has partially shifted from thestudies of the intrinsic properties of these nanostructures tothe exploration of the possibility of making nanoscale elec-tromechanical systems.3,4 One of the most important issuesregarding the fabrication of nanoelectromechanical systemsis the assembly of nanostructures with microelectromechani-cal structures �such as microelectrodes� to form a functionalstructure. Therefore a number of assembling methods havebeen developed which include electric dielectrophoresis,5–12

fluidic alignment,13,14 chemically selective deposition,15

atomic force microscope manipulation,16 and optical assistedalignment.17 Of these methods, the assembly technique basedon dielectrophoresis forces has been widely used because ofits simple manipulation and high efficiency. It has been re-ported that the density of aligned nanotubes5,6 and galliumnitride nanowires11 mainly depended on the magnitude of theapplied electric field. In addition, Chen et al.6 found that thenanotube orientation strongly depended on the magnitudeand the frequency of the electric field. In this letter, we usedthe dielectrophoresis technique to assemble zinc oxide �ZnO�nanowires onto electrodes. We then further extended themethod to control the deposition site of the nanowires byusing a substrate electrode.

The ZnO nanowires used in this work were semicon-ducting nanostructures synthesized by thermal evaporationof ZnO powders under controlled conditions without thepresence of catalyst. They were pure, structurally uniform,single crystalline, and most were free from dislocation.18 Thesynthesized nanowires, about 1–15 �m in length and10–50 nm in diameter, were freed from being bound up inbundles by ultrasonically dissolving the bundles in ethanolsolvent for several minutes.

A structure similar to a field effect transistor �FET� withtwo isolated top electrodes and a lower substrate electrodewas employed to study the electric field assisted assembly ofthe nanowires. The fabrication process of the FET electrodestructure, shown in Fig. 1�a�, started with the preparation ofa phosphorus-doped �P-doped� silicon substrate ��1.5–4.0�

�1016 cm−3� that was covered by a 40 nm SiO2 layerformed by thermal oxidization. A 2000 nm thick lower pres-sure chemical vapor deposition polycrystalline silicon layerwas then deposited on the SiO2 layer and heavily P doped�2.5�1019 cm−3�. Afterwards a Cr/Au film was sputtered.Finally, the polycrystalline silicon and Cr/Au were maskedand etched to form the top electrodes that were a pair ofcomblike electrodes with multiple face-to-face narrow fin-gers. The doped silicon substrate acted as the lower elec-trode. Using standard transistor terminology, we defined thetwo top electrodes as the source and drain and the bottomsubstrate as the gate. The three electrodes were connected tothe power supply via three individual probes on a probestation, as schematically shown in Fig. 1�b�.

The assembly experiment was conducted by dispensinga drop of the nanowire-containing solvent onto the wafer andapplying an alternating current �ac� electric field to thesource and drain electrodes while the gate was floating. Thecircuit is shown schematically in Fig. 1�b�.

The scanning electron microscope �SEM� images of theassembled ZnO nanowires are shown in Fig. 2. These re-sulted from applying different ac electric fields with voltagesof 1 and 20 V peak to peak �Vpp� and frequencies of 0.1 and10 MHz, respectively. It can be seen from Figs. 2�a� and 2�b�that at a voltage of 1Vpp only a single nanowire was as-sembled. However, when the voltage was increased to 20Vpp,

a�Electronic mail: dqwang99@mails.tsinghua.edu.cn

FIG. 1. �Color online� �a� Fabrication process of microelectrodes. The struc-tured films from bottom to top are Si, SiO2, polycrystalline silicon, andCr/Au. �b� Schematic drawing of the wiring scheme �not to scale�. Poly-crystalline silicon and Cr/Au were used as the top-layer electrode material.Si substrate was used as the bottom electrode. VG �Gate voltage� is the dcvoltage applied on the bottom electrode to control the deposition site ofnanowires. �c� A SEM image of comblike electrode pair with multiple nar-row fingers. The scale bar corresponds to 20 �m.

APPLIED PHYSICS LETTERS 90, 103110 �2007�

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the quantity of the assembled nanowires also increased, ascan be clearly seen in Figs. 2�c� and 2�d�. Thus it can beconcluded that the density of the assembled nanowires growsin proportion to the applied electric field. That is, the higherthe applied voltage, the higher the density of the assemblednanowires. From Fig. 2, we can also conclude that the fre-quency of the applied electric field dominated the degree ofalignment of the assembled nanowires. As the frequency in-creased, the nanowire assembly became tighter, straighter,and more uniformly aligned.

We also studied the gap distance between the top elec-trodes to determine its effect on nanowire assembly. Two topelectrodes consisting of four finger pairs with different gapdistances ranging from 2 to 10 �m were used to assembleZnO nanowires, as shown in Fig. 3�a�. A voltage of 10Vpp ata frequency of 20 MHz was applied to the drain electrode�right� relative to the grounded source electrode �left�, whilethe gate electrode was allowed to float. Figures 3�b�–3�e�are enlarged SEM images of the assembled nanowires at dif-ferent gaps. They reveal that a closer gap distance betweenthe electrodes resulted in larger quantities of assemblednanowires.

The assembly experiment was further extended to inves-tigate the effect of the gate electrode on the depositionposition of the nanowires using direct current �dc� dielectro-phoresis. Different dc electric fields ranging from

−10 to 10 V were applied to the gate electrode, while a fixeddc electric field of 10 V was applied to the drain electrode�right� relative to the grounded source electrode �left�.

The experimental results are shown in Fig. 4. A remark-able effect of gate voltage on the deposition position of thenanowires was observed. When the gate was applied a volt-age of 10 V, the nanowires were almost all deposited on thedrain electrode �right� rather than the source electrode �left�.When the gate voltage decreased to 5 V, the site of the nano-wire assembly moved to the left and the nanowires depositedon both the source and drain electrodes. When the gate volt-age further decreased to the range from 0 to −10 V, thenanowires were all deposited on the source electrode. Fur-ther, the deposition site of the nanowires changed from left toright along the source electrode pad as the gate voltagechanged from 0 to −10 V. With the gate at −10 V most ofthe nanowires were concentrated near the electrode gap.

The above gate-assisted assemblies were reproducible.The site-selective assembly behaviors of the nanowires areprobably attributed to the electric field controlled by the gateelectrode. A dielectrophoretic force can be formulated as5

FDEP =1

4v Re� �p − �m

�p + 2�m� � �E�2, �1�

E = Esd + Eg, �2�

where Esd is the electric field generated by the voltage be-tween source and drain, Eg is the electric field generated bythe gate voltage, v is the dimensional constant of the particle�nanowire�, and �m and �p are the permeability of the me-dium �solvent� and particle, respectively. When the gate volt-age varies, the electric field Eg will vary accordingly, whichconsequently leads to the change of the electric field distri-bution in the solvent and moves the deposition site of thenanowires. A complete analysis of the dynamics of nano-wires in the process of dielectrophoresis is quiet complexbased on their dielectrophoretic force FDEP. However, it isreasonable that the nanowires were likely to move awayrather than stay at the site where dielectrophoretic forces inthe direction of electrode surfaces were larger and fluctuatinggreatly. Conversely, the nanowires could be stably situated atthe position where the dielectrophoretic forces in the direc-

FIG. 2. SEM images of ZnO nanowires assembled between electrodes byusing ac dielectrophoresis under different magnitudes and frequencies ofapplied electric field. The magnitudes and frequencies were �a� 1Vpp at0.1 MHz, �b� 1Vpp at 10 MHz, �c� 20Vpp at 0.1 MHz, and �d� 20Vpp at10 MHz. The scale bars correspond to 1 �m.

FIG. 3. �Color online� SEM images of assembled ZnO nanowires betweenelectrodes with different gap distances. An ac voltage of 10Vpp at a fre-quency of 20 MHz was applied to the right electrode relative to the leftgrounded electrode. �a� The left four fingers are all part of the left electrodeand the right four fingers are all part of the right electrode. The four gapdistances are 2, 4, 6, and 10 �m respectively and are enlarged in �b�–�e�.The scale bars are 10 �m in �a� and 2 �m in �b�–�e�.

FIG. 4. �Color online� Gate assisted deposition of ZnO nanowires by usingdc dielectrophoresis. ��a�–�e�� SEM images of the deposition results at gatevoltages VG ranging from −10 to 10 V. The blue frames indicate the depo-sition sites of the nanowires. �f� Dependence of the deposition site of nano-wires �x axis� on the gate voltage VG �y axis�. The bars represent the depo-sition positions of nanowires with respect to different gate voltages.

103110-2 Wang et al. Appl. Phys. Lett. 90, 103110 �2007�

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tion of electrode surfaces were low and there was littlechange.

Gate-assisted assembly was further investigated to deter-mine its effectiveness to trap and align nanowires by using acdielectrophoresis. The source electrode was grounded, andan alternating electric field of 15Vpp at a frequency of10 MHz was applied to the drain electrode while a dc volt-age was applied to the gate. Figure 5 shows the SEM imagesof the nanowire assemblies that resulted from applied gatevoltages of −10, 0, and 10 V. The nanowires were welltrapped and aligned between the two top electrodes at a gatevoltage of −10 V, and comparatively disordered at gate volt-ages of 0 and 10 V. As a result, the gate voltage of −10 Vwas apparently an optimal condition for trapping and align-ing the nanowires between two top electrodes during acdielectrophoresis.

In summary, we demonstrated an approach to assembleZnO nanowires onto a micromachined electrode pair with dcand ac dielectrophoresis that utilized an additional substrategate electrode to control the assembly position of nanowires.The experiment demonstrated that a dc voltage of −10 V wasan optimal gate voltage for trapping and aligning the nano-wires between the top electrode pair during ac dielectro-phoresis. We also found that the density and the alignmentdegree of assembled nanowires mainly depended on themagnitude and the frequency of the applied ac electric field.In addition, it was demonstrated that the assembly was

highly dependent on the gap distance between the top elec-trodes. The proposed method and corresponding results willhave potential applications in fabricating nanoelectrome-chanical systems.

The authors want to thank Zhonglin Wang at the GeorgiaInstitute of Technology, U.S.A., for providing samples ofnanowires for this work and also for his helpful experimentrelated discussions. This work was partly supported byNational Natural Science Foundation of China Project�50575113� and Program for NCET.

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FIG. 5. SEM images of gate assisted assembly using ac dielectrophoresis.The gate electrode was applied a dc voltage of −10 V in �a�, 0 V in �b�, and10 V in �c�. The drain electrode �right� was wired to the function generatorthat supplied an ac voltage of 15Vpp at a frequency of 10 MHz, and thesource electrode �left� was grounded.

103110-3 Wang et al. Appl. Phys. Lett. 90, 103110 �2007�

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