Accurate control of individual metallic nanowires by light-induced dielectrophoresis: Size-based separation and array-spacing regulation
Introduction
In the bottom-up paradigm for functional nanosystems [1], [2], the controllable manipulation or assembly of synthesized-nanowires has increasingly become crucial for the promotion of nanotechnology engineering. One of the key issues is to force the nanowires, individual or arrays, precisely aligning or positioning in artificial ways. Toward this end, a variety of technical approaches have been proposed to assemble nanowires into ordered arrays or films [3], [4], [5], [6]. By these methods, the nanowires handled with large quantities during the assembly are hard to manipulate individually or in parallel fashions, which lowers the accuracy of single-nanowire's orientation and location. Straightway, micro-probes or grippers integrated in the electron microscopes are utilized to mechanically poked or stirred individual nanowires via cautiously stepping the manipulator [7], [8]. In this approach, the operation is extremely of low efficiency and the direct physical-contact dramatically increases the risk of material damage. Furthermore, non-contact modes are supposed to provide more gentle ways for controlling the alignment of individual nanowires, including optical tweezers [9], [10], magnetic [11], [12] and dielectrophoretic (DEP) [13], [14], [15], [16], [17] approaches. Generally, the concurrent assembly of multiple-nanowires is still unrealized by using sophisticated optical-tweezers system, and the position or posture of the assembled nanowires is usually strictly restricted in most dielectrophoretic cases. And, by using the magnetic approach, it is unable to drive the nanostructures composed of non-ferromagnetism materials.
Optoelectronic tweezers [18], which combines both the flexibility of optical tweezers and the versatility of DEP, is supposed to be a novel technique for the real-time manipulation of individual micro-scale objects [18], [19], [20], [21]. For nanostructures, nanowires of different materials were dynamically separated and large-scale nanowire-bundles were assembled by Jamshidi [22] for the first time. Additionally, nano-particles were directly written into patterns by the light-beams [23]. Later, bundled and dispersed carbon nanotubes were patterned and sorted by Lee [24], and the parallel manipulation of individual carbon nanotubes was demonstrated by Pauzauskie [25]. Currently, micro- and nano-particles were simultaneously separated and concentrated into patterns by Liang [26]. Nevertheless, there remains challenge for the accurate control of homogeneous one-dimensional nanostructures by utilizing this technique due to the nano-scale observation-limit and significant thermal fluctuations. Especially, positioning and managing the individual-nanowire or uniform-arrays in specific locations, translating and separation of different-sized nanowires were still unrealized in these works. By achieving these, it would properly be helpful to the domination of self-assembly and constructing of nanowire-based devices.
In this paper, single silver nanowire or nanowire-arrays were precisely positioned and controlled by using a regular geometric light-spot based on a light-induced dielectrophoretic (LIDEP) system. Initially, the posture and motion details of the nanowire were experimentally studied during the positioning and translating operations. Then, an acceleration-displacement measurement (ADM) method was proposed to measure the limiting velocities of different-sized nanowires, which offers safe and accurate parameters to ensure the reliability of subsequent manipulations. Thus, nanowires of different sizes could be individually separated, and uniform nanowire-arrays were instantly assembled. Furthermore, the array spacing was automatically adjusted in real time by changing ac frequencies due to the rebalance of DEP force, electrostatic repulsion and fluidic viscous force on each nanowire. The realization of operations on nanowire-separation or array-regulation at individual-resolution would be beneficial to improve the manipulation accuracy, purify the nanowire-bundles, and facilitate the automation of nanostructure assembly.
Section snippets
Light-induced dielectrophoretic forces
The schematic of LIDEP chip for nanowire-manipulation is shown in Fig. 1a. Based on the dipole approximation, the DEP force nanowires feel can be expressed as [27]where r and l are the radius and length of the nanowires, respectively, E is the strength of the electric field, and ɛm is the permittivity of the medium. fCM represents the Clausius–Mossotti factor, which is expressed aswhere , k = m, p, . ɛp is the permittivity of
LIDEP device fabrication
Initially, 50-nm n+ α-Si:H and 1.5-μm intrinsic α-Si:H served as photoconductive layer were consecutively deposited on the bottom indium-tin-oxide (ITO) glass by plasma enhanced chemical vapor deposition (PECVD) method. The doped n+ α-Si:H layer with an electrical conductivity value of 2.87 × 10−1 S m−1 formed ohmic contact on the ITO film, and made the intrinsic α-Si:H more adhesive to the substrate. The conductivity value of the intrinsic α-Si:H layer is 5 × 10−3 S m−1 when the electron–hole pairs
Accurate positioning of single nanowires
In the liquid layer, the nanowire undergone Brownian motion would move and fluctuate randomly spanning tens of square micro-meters within seconds [22]. As ac signal excited, an induced dipole arises in the nanowire due to electric polarization and causes its major axis aligning parallel to the electric field. Meanwhile, the projected light-spot, which dramatically decreases the impedance of illuminated photoconductive layer, emerges a spatial non-uniform electric field in the liquid (Fig. 1a).
Conclusion
The regular geometric light-patterns introduced into a light-induced dielectrophoretic system can precisely position the nanowires, single or multiple, at the illuminated center. Hence, a variety of operations on nanowire-manipulation can be experimentally performed, including positioning, translating, limiting-velocity characterization, size-based separation, nanowire-array spacing regulation and rearrangement. In these operations, individual nanowires vary its posture and velocity
Acknowledgements
This research work was supported by National Natural Science Foundation of China (51375089 and 51175083), National Basic Research Program of China (2011CB707601), the Specialized Research Fund for the Doctoral Program of Higher Education (20110092110003).
Ke Chen, born in 1985, is currently a Ph.D. candidate in School of Mechanical Engineering, Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, Southeast University, China. His research interests include micro-fabrication, microfluidics, and nanostructure manipulation.
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Ke Chen, born in 1985, is currently a Ph.D. candidate in School of Mechanical Engineering, Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, Southeast University, China. His research interests include micro-fabrication, microfluidics, and nanostructure manipulation.
Yunlin Quan, gborn in 1990, is currently doing his Master degree in School of Mechanical Engineering, Southeast University, China. His research interests include MEMS and microfluidics.
Chunfeng Song, born in 1983, received his Ph.D. degree from Southeast University, Nanjing, China, in 2013. He is now a technical fellow of SMEE in Shanghai, China.
Nan Xiang, born in 1986, is currently a Ph.D. candidate in School of Mechanical Engineering, Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, Southeast University, China. His research interests include micro-fabrication, microfluidics, and biological separation.
Di Jiang, born in 1987, is currently a Ph.D. candidate in School of Mechanical Engineering, Southeast University, China. Her research interests include computational fluid mechanics and microfluidic fabrication.
Dongke Sun, born in 1982, received his Ph.D. degree from School of Materials Science and Engineer, Southeast University, Nanjing, China, in 2013. He is now a postdoctoral fellow at Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments. His research field of interest focuses on computational fluid mechanics.
Juekuan Yang, received his Ph.D. degree in Southeast University, China, in 2004. He worked at Vanderbilt University in USA as a postdoc research associate from 2008 to 2011. He is now an associate professor in the School of Mechanical Engineering, Southeast University. His research interests include contact thermal resistance measurement between individual nanostructures, and thermoelectric properties investigation of Boron-based one-dimensional nanostructures.
Hong Yi, received his Ph.D. degree in Tsinghua University, China, in 1990. He is now a professor in the School of Mechanical Engineering, Southeast University. His research interests include mechanical manufacture and automation, MEMS and nano-fabrication.
Zhonghua Ni, received his Ph.D. degree in Southeast University, China, in 2001. He worked at Vanderbilt University in USA as a visiting scholar in 2011. He is now a professor in the School of Mechanical Engineering, Southeast University. His research interests include medical device fabrication, MEMS and nano-fabrication.