Manipulation and assembly of ZnO nanowires with single holographic optical tweezers system Jing Li* and Gang Du Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230027, China *Corresponding author: [email protected] Received 24 October 2013; revised 15 December 2013; accepted 17 December 2013; posted 18 December 2013 (Doc. ID 200114); published 14 January 2014
ZnO nanowires, characterized with high melting points, are hard to assemble together with laser fusion. In order to build micro–nano structures with ZnO nanowires, a polymer film with a low melting point and high optical transparency is introduced as a substrate for ZnO nanowires to be deposited. A holographic optical tweezers system is used not only to manipulate ZnO nanowires, but also to melt the polymer film for the fixation of ZnO nanowires. By this method, micro–nano structures composed of ZnO nanowires are produced, which can be utilized as subwavelength optical waveguides. © 2014 Optical Society of America OCIS codes: (350.4855) Optical tweezers or optical manipulation; (090.1760) Computer holography; (120.4610) Optical fabrication. http://dx.doi.org/10.1364/AO.53.000351
1. Introduction
Semiconductor nanowires, such as ZnO nanowires, having good electro-optical properties and versatile functions as subwaveguides, gas sensors, lasers, etc. [1–4], are ideal building blocks for functional micro–nanoscale electronic and photonic structures. The methods for manipulating those nanowires include fluidics [5], electric fields [6], optical tweezers [7–10], etc. Among these methods, optical tweezers can control the positions of nanowires more precisely. There have been several different ways with optical tweezers to manipulate semiconductor nanowires, such as using single optical tweezers to trap nanowires aligned along the optical axis [7], manipulating CuO nanowires with line optical tweezers [8], employing counterpropagating optical line tweezers to manipulate metal–oxide nanowires [9], and using holographic optical line tweezers to manipulate nanowires [10]. The way of using holographic optical
1559-128X/14/030351-05$15.00/0 © 2014 Optical Society of America
tweezers demonstrates greater flexibility, and more dynamic and precise control. Once positioned with holographic optical tweezers, the semiconductor nanowires need to be assembled into desired structures. Several techniques for assembly have been proposed, such as using electrostatic interactions to fix nanowires [9], assembling SPM-like probes from steptavidin-coated silica microspheres and biotin-coated CdS rods [11], and adding another optical process system to fuse nanowires [10]. The first technique requires a complex process of forming negatively and positively charged layers on the surfaces of nanowires, silica particles, and glass covers; the second one requires the surface treatment of nanomaterials with biomaterials. With these two methods, the optical properties of nanowires may be influenced by the coatings. The last one needs an additional optical system for the fusing process and is suitable for semiconductor nanowires with lower melting points. Since ZnO nanowires have high melting points, it is difficult to fuse them together with a high-power continuous-wave (cw) laser. Moreover, the melting points of ZnO nanowires are close to the boiling points, which makes 20 January 2014 / Vol. 53, No. 3 / APPLIED OPTICS
351
controlling the melting and evaporating processes more difficult. Usually, most polymers are carbon based and have relatively low melting points. Some of them have high optical transparency and low refractive indices, which can work as a substrate to be melted by a laser to fix ZnO nanowires located on it and form micro– nano structures. In our experiments, the widely used biaxially oriented polypropylene (BOPP) film is chosen as a substrate, and has a low melting point and does not affect the quality of the transmitted beams. A holographic optical tweezers system is used not only to trap and manipulate ZnO nanowires, but also to melt the BOPP film and fix ZnO nanowires on it.
The software is programmed with C++ and Matlab languages. The GS-based algorithm is employed to design holograms [13]; the generated holograms are displayed on the SLM. The holographic optical line tweezers are composed of multi-individual tweezers, which are distributed along the nanowire with the same intensity and equal distance. The number of traps and the distance between traps can be adjusted. The nanowire can be trapped transversely by at least two tweezers located at the two ends of the nanowire. The direction of the holographic optical line tweezers can also be full dynamically controlled. The built holographic optical tweezers system can translate and rotate the nanowire to the desired position.
2. Experimental Setup
3. Experiments and Results
Figure 1 shows our holographic optical tweezers system [12,13]; a linearly polarized cw Nd:YAG laser operating at a wavelength of 532 nm, with Gaussian intensity profile, is used as the light source. After the laser beam is expanded and reflected, the beam illuminates a spatial light modulator (SLM). The SLM (Model P512-0532 from BNS Co.) can provide an output phase ranging from 0 to 2π at a wavelength of 532 nm and has at least 50 phase grayscales. The modulated beams are compressed by a telescope and strongly focused by an inverted oil-immersion objective (1.25NA 100×) to form optical tweezers. The sample cell is fixed on an xyz-piezo stage. A dichroic mirror is used to separate the modulated beams and CCD imaging beams. A CCD camera is used for realtime observation and recording the process of the experiments. The laser beam is filtered before entering the CCD camera. Software is developed for the generation of holographic optical line tweezers to trap nanowires and control the three-dimensional dynamic motion of nanowires, as well as to melt the polymer film.
ZnO nanowires that are 5–20 μm long and 200 nm in diameter are synthesized using a hydrothermal growth method [14]. The nanowires are dispersed by sonication. The BOPP film with a thickness of 20 μm is pasted on the inner surface of the cover slip with acrylic ester glue. The cover slip (base) is placed on a slide glass (top) to build a sample cell. The sample is an ethanol–water solution containing suspended ZnO nanowires. Since a high refractive index (1.9) and larger transverse cross section of a ZnO nanowire will increase the scatter force of the optical tweezers, and the gradient force will rise as the trapping position is closer to the bottom of the sample cell [15,16], as a result, the ZnO nanowire is stably trapped near the surface of the sample cell by holographic optical line tweezers. By moving the piezo stage up or adding a holographic lens to the hologram, the nanowire can be positioned on the surface of the polymer film without being stuck to the film. (Once the holographic optical tweezers are removed, the nanowire will freely diffuse in solution.) Then the number of holographic
Fig. 1. Schematic of a holographic optical tweezers system. 352
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Fig. 2. Series of microscopy images shows that a ZnO nanowire is translated, rotated, and fixed with holographic optical tweezers. (a)–(c) The nanowire is translated; the white arrow in (a) denotes a protruding line on the film regarded as a reference position. (d)–(f) The nanowire is rotated about 90 deg around its geometric center. (g) The film is heated by three holographic optical tweezers distributed along the nanowire. (h) The nanowire is fixed on the film.
optical tweezers is decreased to two or three, and the laser power is increased to the maximum output (i.e., 1 W). At this time the holographic optical tweezers act as trapping tools, as well as heating sources. Due to the high refractive index of the ZnO nanowire, a portion of the light will be reflected at the surface of the nanowire and contribute to the melting process. The holographic optical tweezers work for more than several seconds; finally, the nanowire will be fixed on the polymer film. This process is demonstrated
by a series of microscopy images shown in Fig. 2. In Figs. 2(a)–2(c), a ZnO nanowire is translated by holographic optical line tweezers. A defect in the film, i.e., a microprotruding line at the bottom of each image, can be regarded as a reference position, and it is denoted by a white arrow in Fig. 2(a). Starting from the position shown in Fig. 2(d), the ZnO nanowire is rotated about 90 deg around its geometric center as Figs. 2(e) and 2(f) demonstrate. Figure 2(g) shows that the membrane is heated by three traps
Fig. 3. Three nanowires are trapped and controlled simultaneously.
Fig. 4. Micro–nano structures consist of ZnO nanowires fixed on the polymer: (a) structure formed by connecting the ends of nanowires, and (b) structure with a nanowire aligned along the trap’s optical axis. 20 January 2014 / Vol. 53, No. 3 / APPLIED OPTICS
353
Fig. 5. Arrows 1 and 2 indicate the positions of the light source and the observation point, respectively. (a) Built micro–nano structure. (b) Light propagation in the micro–nano structure.
distributed along the ZnO nanowire. In Fig. 2(h), the ZnO nanowire has been fixed on the membrane. Multinanowires can be trapped, moved, and rotated simultaneously in our optical system. Figure 3 shows that three nanowires are controlled simultaneously. The micro–nano structures consist of ZnO nanowires fixed on the polymer shown in Fig. 4. In Fig. 4(a), the ends of nanowires are connected to form a structure. Figure 4(b) shows a structure with a nanowire aligned along the trap’s optical axis. In this case, single holographic optical tweezers act on one end of the nanowire, which leads to the trap’s optical axis alignment of the nanowire and fixation of it on the film. Since each nanowire in the structures is in contact with the other or located in each other’s near field, light or evanescent light will be transmitted from one nanowire to another. Figure 5(b) demonstrates light propagation in the micro–nano structure built by the above-mentioned method and shown in Fig. 5(a). In Fig. 5(b), the position of the light source is indicated by arrow 1, and light emitted at the output end of a ZnO nanowire indicated by arrow 2 is observed. Therefore, the built nanostructures can be utilized as subwavelength optical waveguides for their potential applications in optical communication, information processing, optical sensing, and the integration of optical logic circuits [4]. 4. Conclusions
Using a single holographic optical tweezers system, micro–nano structures assembled on a polymer are built. Both manipulation of ZnO nanowires and melting of the polymer film to fix ZnO nanowires are performed. The holographic optical line tweezers can dynamically control nanowires and fix nanowires on the polymer transversely, and single holographic optical tweezers can fix nanowires near-vertically on the polymer, which makes more complex structures possible. The experimental observation of light propagation demonstrates that the built micro–nano structures can effectively work as subwavelength optical waveguides. In the experiments, it is found that the ZnO nanowires are more easily fixed if there are micro–nano protruding structures on the surface of the polymer film. It prompts that building micro–nano structures 354
APPLIED OPTICS / Vol. 53, No. 3 / 20 January 2014
with polymer substrate may be a better way to guide the assembling process of ZnO nanowires. Another improvement is adding slightly dark pigments to the polymer, which will lead to enhanced absorption of laser energy and be advantageous to the polymer melting process. Since the whole experiments are performed under the single holographic optical tweezers system, the experimental method is simpler than in the previous reports [7–10]. This work provides a way for constructing micro–nano devices with high melting-point semiconductor nanowires. We are grateful to the National Natural Science Foundation of China (grant no. 91023049), the 973 Programs (grant no. 2012CB937500), and the Program for the scientific instrument development of the Chinese Academy of Sciences (grant no. Y2201265) for financial support of this work. References 1. D. Park and K. Yong, “Photoconductivity of vertically aligned ZnO nanoneedle array,” J. Vac. Sci. Technol. B 26, 1933–1936 (2008). 2. Z. L. Wang and J. Song, “Piezoelectric nanogenerators based on zinc oxide nanowire arrays,” Science 312, 242–246 (2006). 3. L. K. van Vugt, S. Rühle, and D. Vanmaekelbergh, “Phasecorrelated nondirectional laser emission from the end facets of a ZnO nanowire,” Nano Lett. 6, 2707–2711 (2006). 4. C. Zhang, Y. S. Zhao, and J. Yao, “Optical waveguides at micro/ nanoscale based on functional small organic molecules,” Phys. Chem. Chem. Phys. 13, 9060–9073 (2011). 5. Y. Huang, X. Duan, Q. Wei, and C. M. Lieber, “Directed assembly of one-dimensional nanostructures into functional networks,” Science 291, 630–633 (2001). 6. X. Duan, Y. Huang, Y. Cui, J. Wang, and C. M. Lieber, “Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices,” Nature 409, 66–69 (2001). 7. P. J. Pauzauskie, A. Radenovic, E. Trepagnier, H. Shroff, P. Yang, and J. Liphardt, “Optical trapping and integration of semiconductor nanowire assemblies in water,” Nat. Mater. 5, 97–101 (2006). 8. T. Yu, F.-C. Cheong, and C.-H. Sow, “The manipulation and assembly of CuO nanorods with line optical tweezers,” Nanotechnology 15, 1732–1736 (2004). 9. A. van der Horst, A. I. Campbell, L. K. van Vugt, D. A. M. Vanmaekelbergh, M. Dogterom, and A. van Blaaderen, “Manipulating metal-oxide nanowires using counterpropagating optical line tweezers,” Opt. Express 15, 11629–11639 (2007). 10. R. Agarwal, K. Ladavac, Y. Roichman, G. Yu, C. M. Lieber, and D. G. Grier, “Manipulation and assembly of nanowires with holographic optical traps,” Opt. Express 13, 8906–8912 (2005). 11. L. Ikin, D. M. Carberry, G. M. Gibson, M. J. Padgett, and M. J. Miles, “Assembly and force measurement with SPM-like
probes in holographic optical tweezers,” New J. Phys. 11, 023012 (2009). 12. L.Shi,J. Li,T.Tao,and X.Wu, “Rotation of nanowires withradially higher-order Laguerre–Gaussian beams produced by computergenerated holograms,” Appl. Opt. 51, 6398–6402 (2012). 13. T. Tao, J. Li, Q. Long, and X. Wu, “3D trapping and manipulation of micro-particles using holographic optical tweezers with optimized computer-generated holograms,” Chin. Opt. Lett. 9, 120010 (2011).
14. Y.-H. Ni, X.-W. Wei, J.-M. Hong, and Y. Ye, “Hydrothermal preparation and optical properties of ZnO nanorods,” Mater. Sci. Eng. B 121, 42–47 (2005). 15. E.-L. Florin, A. Pralle, E. H. K. Stelzer, and J. K. H. Hörber, “Photonic forcemicroscope calibration by thermal noise analysis,” Appl. Phys. A 66, S75–S78 (1998). 16. L. P. Ghislain, N. A. Switz, and W. W. Webb, “Measurement of small forces using an optical trap,” Rev. Sci. Instrum. 65, 2762–2768 (1994).
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ZnO nanowires, characterized with high melting points, are hard to assemble together with laser fusion. In order to build micro–nano structures with ZnO nanowires, a polymer film with a low melting point and high optical transparency is introduced as a substrate for ZnO nanowires to be deposited. A holographic optical tweezers system is used not only to manipulate ZnO nanowires, but also to melt the polymer film for the fixation of ZnO nanowires. By this method, micro–nano structures composed of ZnO nanowires are produced, which can be utilized as subwavelength optical waveguides. © 2014 Optical Society of America OCIS codes: (350.4855) Optical tweezers or optical manipulation; (090.1760) Computer holography; (120.4610) Optical fabrication. http://dx.doi.org/10.1364/AO.53.000351
1. Introduction
Semiconductor nanowires, such as ZnO nanowires, having good electro-optical properties and versatile functions as subwaveguides, gas sensors, lasers, etc. [1–4], are ideal building blocks for functional micro–nanoscale electronic and photonic structures. The methods for manipulating those nanowires include fluidics [5], electric fields [6], optical tweezers [7–10], etc. Among these methods, optical tweezers can control the positions of nanowires more precisely. There have been several different ways with optical tweezers to manipulate semiconductor nanowires, such as using single optical tweezers to trap nanowires aligned along the optical axis [7], manipulating CuO nanowires with line optical tweezers [8], employing counterpropagating optical line tweezers to manipulate metal–oxide nanowires [9], and using holographic optical line tweezers to manipulate nanowires [10]. The way of using holographic optical
1559-128X/14/030351-05$15.00/0 © 2014 Optical Society of America
tweezers demonstrates greater flexibility, and more dynamic and precise control. Once positioned with holographic optical tweezers, the semiconductor nanowires need to be assembled into desired structures. Several techniques for assembly have been proposed, such as using electrostatic interactions to fix nanowires [9], assembling SPM-like probes from steptavidin-coated silica microspheres and biotin-coated CdS rods [11], and adding another optical process system to fuse nanowires [10]. The first technique requires a complex process of forming negatively and positively charged layers on the surfaces of nanowires, silica particles, and glass covers; the second one requires the surface treatment of nanomaterials with biomaterials. With these two methods, the optical properties of nanowires may be influenced by the coatings. The last one needs an additional optical system for the fusing process and is suitable for semiconductor nanowires with lower melting points. Since ZnO nanowires have high melting points, it is difficult to fuse them together with a high-power continuous-wave (cw) laser. Moreover, the melting points of ZnO nanowires are close to the boiling points, which makes 20 January 2014 / Vol. 53, No. 3 / APPLIED OPTICS
351
controlling the melting and evaporating processes more difficult. Usually, most polymers are carbon based and have relatively low melting points. Some of them have high optical transparency and low refractive indices, which can work as a substrate to be melted by a laser to fix ZnO nanowires located on it and form micro– nano structures. In our experiments, the widely used biaxially oriented polypropylene (BOPP) film is chosen as a substrate, and has a low melting point and does not affect the quality of the transmitted beams. A holographic optical tweezers system is used not only to trap and manipulate ZnO nanowires, but also to melt the BOPP film and fix ZnO nanowires on it.
The software is programmed with C++ and Matlab languages. The GS-based algorithm is employed to design holograms [13]; the generated holograms are displayed on the SLM. The holographic optical line tweezers are composed of multi-individual tweezers, which are distributed along the nanowire with the same intensity and equal distance. The number of traps and the distance between traps can be adjusted. The nanowire can be trapped transversely by at least two tweezers located at the two ends of the nanowire. The direction of the holographic optical line tweezers can also be full dynamically controlled. The built holographic optical tweezers system can translate and rotate the nanowire to the desired position.
2. Experimental Setup
3. Experiments and Results
Figure 1 shows our holographic optical tweezers system [12,13]; a linearly polarized cw Nd:YAG laser operating at a wavelength of 532 nm, with Gaussian intensity profile, is used as the light source. After the laser beam is expanded and reflected, the beam illuminates a spatial light modulator (SLM). The SLM (Model P512-0532 from BNS Co.) can provide an output phase ranging from 0 to 2π at a wavelength of 532 nm and has at least 50 phase grayscales. The modulated beams are compressed by a telescope and strongly focused by an inverted oil-immersion objective (1.25NA 100×) to form optical tweezers. The sample cell is fixed on an xyz-piezo stage. A dichroic mirror is used to separate the modulated beams and CCD imaging beams. A CCD camera is used for realtime observation and recording the process of the experiments. The laser beam is filtered before entering the CCD camera. Software is developed for the generation of holographic optical line tweezers to trap nanowires and control the three-dimensional dynamic motion of nanowires, as well as to melt the polymer film.
ZnO nanowires that are 5–20 μm long and 200 nm in diameter are synthesized using a hydrothermal growth method [14]. The nanowires are dispersed by sonication. The BOPP film with a thickness of 20 μm is pasted on the inner surface of the cover slip with acrylic ester glue. The cover slip (base) is placed on a slide glass (top) to build a sample cell. The sample is an ethanol–water solution containing suspended ZnO nanowires. Since a high refractive index (1.9) and larger transverse cross section of a ZnO nanowire will increase the scatter force of the optical tweezers, and the gradient force will rise as the trapping position is closer to the bottom of the sample cell [15,16], as a result, the ZnO nanowire is stably trapped near the surface of the sample cell by holographic optical line tweezers. By moving the piezo stage up or adding a holographic lens to the hologram, the nanowire can be positioned on the surface of the polymer film without being stuck to the film. (Once the holographic optical tweezers are removed, the nanowire will freely diffuse in solution.) Then the number of holographic
Fig. 1. Schematic of a holographic optical tweezers system. 352
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Fig. 2. Series of microscopy images shows that a ZnO nanowire is translated, rotated, and fixed with holographic optical tweezers. (a)–(c) The nanowire is translated; the white arrow in (a) denotes a protruding line on the film regarded as a reference position. (d)–(f) The nanowire is rotated about 90 deg around its geometric center. (g) The film is heated by three holographic optical tweezers distributed along the nanowire. (h) The nanowire is fixed on the film.
optical tweezers is decreased to two or three, and the laser power is increased to the maximum output (i.e., 1 W). At this time the holographic optical tweezers act as trapping tools, as well as heating sources. Due to the high refractive index of the ZnO nanowire, a portion of the light will be reflected at the surface of the nanowire and contribute to the melting process. The holographic optical tweezers work for more than several seconds; finally, the nanowire will be fixed on the polymer film. This process is demonstrated
by a series of microscopy images shown in Fig. 2. In Figs. 2(a)–2(c), a ZnO nanowire is translated by holographic optical line tweezers. A defect in the film, i.e., a microprotruding line at the bottom of each image, can be regarded as a reference position, and it is denoted by a white arrow in Fig. 2(a). Starting from the position shown in Fig. 2(d), the ZnO nanowire is rotated about 90 deg around its geometric center as Figs. 2(e) and 2(f) demonstrate. Figure 2(g) shows that the membrane is heated by three traps
Fig. 3. Three nanowires are trapped and controlled simultaneously.
Fig. 4. Micro–nano structures consist of ZnO nanowires fixed on the polymer: (a) structure formed by connecting the ends of nanowires, and (b) structure with a nanowire aligned along the trap’s optical axis. 20 January 2014 / Vol. 53, No. 3 / APPLIED OPTICS
353
Fig. 5. Arrows 1 and 2 indicate the positions of the light source and the observation point, respectively. (a) Built micro–nano structure. (b) Light propagation in the micro–nano structure.
distributed along the ZnO nanowire. In Fig. 2(h), the ZnO nanowire has been fixed on the membrane. Multinanowires can be trapped, moved, and rotated simultaneously in our optical system. Figure 3 shows that three nanowires are controlled simultaneously. The micro–nano structures consist of ZnO nanowires fixed on the polymer shown in Fig. 4. In Fig. 4(a), the ends of nanowires are connected to form a structure. Figure 4(b) shows a structure with a nanowire aligned along the trap’s optical axis. In this case, single holographic optical tweezers act on one end of the nanowire, which leads to the trap’s optical axis alignment of the nanowire and fixation of it on the film. Since each nanowire in the structures is in contact with the other or located in each other’s near field, light or evanescent light will be transmitted from one nanowire to another. Figure 5(b) demonstrates light propagation in the micro–nano structure built by the above-mentioned method and shown in Fig. 5(a). In Fig. 5(b), the position of the light source is indicated by arrow 1, and light emitted at the output end of a ZnO nanowire indicated by arrow 2 is observed. Therefore, the built nanostructures can be utilized as subwavelength optical waveguides for their potential applications in optical communication, information processing, optical sensing, and the integration of optical logic circuits [4]. 4. Conclusions
Using a single holographic optical tweezers system, micro–nano structures assembled on a polymer are built. Both manipulation of ZnO nanowires and melting of the polymer film to fix ZnO nanowires are performed. The holographic optical line tweezers can dynamically control nanowires and fix nanowires on the polymer transversely, and single holographic optical tweezers can fix nanowires near-vertically on the polymer, which makes more complex structures possible. The experimental observation of light propagation demonstrates that the built micro–nano structures can effectively work as subwavelength optical waveguides. In the experiments, it is found that the ZnO nanowires are more easily fixed if there are micro–nano protruding structures on the surface of the polymer film. It prompts that building micro–nano structures 354
APPLIED OPTICS / Vol. 53, No. 3 / 20 January 2014
with polymer substrate may be a better way to guide the assembling process of ZnO nanowires. Another improvement is adding slightly dark pigments to the polymer, which will lead to enhanced absorption of laser energy and be advantageous to the polymer melting process. Since the whole experiments are performed under the single holographic optical tweezers system, the experimental method is simpler than in the previous reports [7–10]. This work provides a way for constructing micro–nano devices with high melting-point semiconductor nanowires. We are grateful to the National Natural Science Foundation of China (grant no. 91023049), the 973 Programs (grant no. 2012CB937500), and the Program for the scientific instrument development of the Chinese Academy of Sciences (grant no. Y2201265) for financial support of this work. References 1. D. Park and K. Yong, “Photoconductivity of vertically aligned ZnO nanoneedle array,” J. Vac. Sci. Technol. B 26, 1933–1936 (2008). 2. Z. L. Wang and J. Song, “Piezoelectric nanogenerators based on zinc oxide nanowire arrays,” Science 312, 242–246 (2006). 3. L. K. van Vugt, S. Rühle, and D. Vanmaekelbergh, “Phasecorrelated nondirectional laser emission from the end facets of a ZnO nanowire,” Nano Lett. 6, 2707–2711 (2006). 4. C. Zhang, Y. S. Zhao, and J. Yao, “Optical waveguides at micro/ nanoscale based on functional small organic molecules,” Phys. Chem. Chem. Phys. 13, 9060–9073 (2011). 5. Y. Huang, X. Duan, Q. Wei, and C. M. Lieber, “Directed assembly of one-dimensional nanostructures into functional networks,” Science 291, 630–633 (2001). 6. X. Duan, Y. Huang, Y. Cui, J. Wang, and C. M. Lieber, “Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices,” Nature 409, 66–69 (2001). 7. P. J. Pauzauskie, A. Radenovic, E. Trepagnier, H. Shroff, P. Yang, and J. Liphardt, “Optical trapping and integration of semiconductor nanowire assemblies in water,” Nat. Mater. 5, 97–101 (2006). 8. T. Yu, F.-C. Cheong, and C.-H. Sow, “The manipulation and assembly of CuO nanorods with line optical tweezers,” Nanotechnology 15, 1732–1736 (2004). 9. A. van der Horst, A. I. Campbell, L. K. van Vugt, D. A. M. Vanmaekelbergh, M. Dogterom, and A. van Blaaderen, “Manipulating metal-oxide nanowires using counterpropagating optical line tweezers,” Opt. Express 15, 11629–11639 (2007). 10. R. Agarwal, K. Ladavac, Y. Roichman, G. Yu, C. M. Lieber, and D. G. Grier, “Manipulation and assembly of nanowires with holographic optical traps,” Opt. Express 13, 8906–8912 (2005). 11. L. Ikin, D. M. Carberry, G. M. Gibson, M. J. Padgett, and M. J. Miles, “Assembly and force measurement with SPM-like
probes in holographic optical tweezers,” New J. Phys. 11, 023012 (2009). 12. L.Shi,J. Li,T.Tao,and X.Wu, “Rotation of nanowires withradially higher-order Laguerre–Gaussian beams produced by computergenerated holograms,” Appl. Opt. 51, 6398–6402 (2012). 13. T. Tao, J. Li, Q. Long, and X. Wu, “3D trapping and manipulation of micro-particles using holographic optical tweezers with optimized computer-generated holograms,” Chin. Opt. Lett. 9, 120010 (2011).
14. Y.-H. Ni, X.-W. Wei, J.-M. Hong, and Y. Ye, “Hydrothermal preparation and optical properties of ZnO nanorods,” Mater. Sci. Eng. B 121, 42–47 (2005). 15. E.-L. Florin, A. Pralle, E. H. K. Stelzer, and J. K. H. Hörber, “Photonic forcemicroscope calibration by thermal noise analysis,” Appl. Phys. A 66, S75–S78 (1998). 16. L. P. Ghislain, N. A. Switz, and W. W. Webb, “Measurement of small forces using an optical trap,” Rev. Sci. Instrum. 65, 2762–2768 (1994).
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