Editorial Recent Development in Nanoscale ... - IEEE Xplore

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IEEE TRANSACTIONS ON AUTOMATION SCIENCE AND ENGINEERING, VOL. 3, NO. 3, JULY 2006

Editorial Recent Development in Nanoscale Manipulation and Assembly

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HE development of nanotechnology in the past decade has made the engineering of nanodevices and nanosystems possible. Eventually, nanometer-scale devices or systems will become prevalent in all aspects of our lives. For instance, implantable nanoelectromechanical systems (NEMS) or artificial cells are envisioned to someday to replace cells damaged by today’s incurable diseases and to extend human life expectancy. There are many nanoscale materials with unique mechanical, electrical, optical, and chemical properties which have a variety of potential applications in nanodevices, nanosensors, and NEMS. For instance, carbon nanotubes (CNTs) consist of materials that have drawn a lot of attention during the last decade. In order to make these nanomaterials useful in nanotechnology applications, the ability to manipulate them in a controllable manner is very critical. Unfortunately, manipulation of nanomaterials thus far has proven to be difficult because there is only a few existing techniques to bring nanoentities together and to join them into patterns as specified by a designer. Therefore, nanomanipulation and nanoassembly are one of the important challenges in realizing the miniaturization of devices and machines potentially down to atomic and molecular sizes. Research in manipulating nanoentities is still in its infancy since the physical and chemical phenomenon at this scale have not been completely understood, intelligent automatic precision manipulation strategies are not developed, and the specific tools for explicit applications have not been defined or designed clearly. It is a major aim of this focus section to bring the state-of-the-art research results in nanomanipulation and nanoassembly, especially concentrating on the outcome produced by researchers with an engineering background. We believe this will allow engineering experts, particularly those with automation background and interests, to understand some of the fundamental problems with nanomanipulation and nanoassembly, and perhaps provide solutions to address these difficulties in the future. It is our aim to provide a forum for experts in nanoscience and nanoengineering to disseminate their recent advances and their views on the future perspectives of research and direct cross-fertilization of various subdisciplines of nanotechnology. We have solicited very high quality paper submissions from top experts worldwide to encompass topics in nanoscale manipulation and assembly. Through a rigorous review process, nine papers have been selected to be included in the focus section. “Drift Compensation for Automatic Nanomanipulation with Scanning Probe Microscopes” discusses a new solution for the one of the most important prob-

Digital Object Identifier 10.1109/TASE.2006.878878

lems in scanning probe microscopy-based nanomanipulation and drift compensation. “CAD Guided Automated Nano-Assembly Using Atomic Force Microscopy” presents a new approach to integrate computer-aided design with the atomic force microscopy-based nanoassembly. “Development of an Automated Microspotting System for Rapid Dielectrophoretic Fabrication of Bundled Carbon Nanotube Sensors” provides a new method for using microfluidic and dielectrophoretic forces for nanoassembly. “Toward Nanotube Linear Servomotors” discusses a new nanoactuation mechanism potentially being used for nanomanipulation and assembly. Furthermore, “Automated CAD/CAM Based Nanolithography Using a Custom Atomic Force Microscope,” “Task-Based and Stable Tele-Nanomanipulation in a Nanoscale Virtual Environment,” and “A Novel Design and Analysis of a 2-DOF Compliant Parallel Micromanipulator for Nanomanipulation” present new results on the planning and control of nanomanipulation and nanoassembly processes. In addition, “Understanding and Harnessing Biomimetic Molecular Machines for NEMS Actuation Materials” and “Microscale Hybrid Devices Powered by Biological Flagellar Motors” discuss the possibilities of nanoscale manipulation by bioentities. The papers in this focus section are strongly connected by the major theme to present the problems encountered in nanoscale manipulation and assembly research and provide some up-to-date solutions to some of these problems. Special attention was paid to select papers that focus on integrating nanomechatronics technology to solve problems caused by physical and chemical phenomena at the nanoscale. Physical phenomena at the nanoscale, modeling of nanoforces, precision manipulation control, teleoperated nanoscale object manipulation, real-time graphical display and force sensing at the nanoscale world, and user interface issues for nanomanipulation are some of the technical topics discussed. The focus section has also introduced less conventional methods of manipulating nanoentities, such as electrophoresis and laser-trapping techniques. Although this focus section gives a relatively thorough discussion on state-of-the-art nanoscale manipulation and assembly methods, we should note here that there are other available and successful nanomanipulation and nanoassembly methods. Currently, there are several techniques for building general nanoscale structures, which can be classified into “top-down” and “bottom-up” methods. A typical “top-down” method for nanofabrication is the E-beam nanolithography method. As an extension of lithography, nanolithography uses an electron beam instead of ultraviolet light to expose photoresist. Due to the short wavelength of an E-beam, nanolithography can generate patterns a few nanometers in size.

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However, E-beam nanolithography has to operate under a scanning electron microscope (SEM) which is an expensive piece of equipment. Moreover, E-beam nanolithography is a very slow process because the photoresist has to be exposed point-by-point sequentially. Usually, E-beam lithography is used to generate extremely fine resolution masks, which are then used in a traditional lithographic process in order to fabricate a complete device. Although a single carbon nanotube-based device can be fabricated by using E-beam nanolithographically-made electrodes, the device (e.g., a sensing element or a transistor) location cannot be precisely controlled due to the random distribution of carbon nanotubes during the deposition. This makes it almost impossible to fabricate nanodevices massively in the desired locations using E-beam nanolithography. There are some other nanolithographical methods available. For example, focused ion-beam (FIB) nanolithography works similar to E-beam nanolithography. The major difference is that the exposing source is an ion beam instead of an E-beam. Sometimes, a focused ion-beam method requires an expensive transmission mask. Recently, atomic force microscopy (AFM)-based nanolithography has drawn attention because of its less demanding operating conditions. Instead of using an E-beam, the AFM-based nanolithography uses the tunnelling current between the sharp tip and the substrate surface to expose the photoresist. The feature size is only limited by the AFM tip apex which is usually less than a few nanometers. Similar to the E-beam lithography, AFM-based nanolithgraphy is also a slow serial process. In addition, FIB and AFM techniques also do not solve the problem of having randomly distributed nanomaterials during the deposition (or formation) process. Self-assembly is a typical “bottom-up” method. During self-assembly, molecules join together through chemical bonding or small particles aggregate together through the van der Waals force or an electrical charge force to form regular or symmetric patterns, such as a monolayer, or a line on a surface [1]. However, many potential nanostructures and nanodevices have asymmetric patterns, which cannot be manufactured using self-assembly. Therefore, the “bottom-up” method has to be in a controlled manner in order to achieve the device fabrication level. The recent studies of dip-pen lithography [2], AFM tip-induced oxidation [3], molecular imprinting [4], and direct growth across electrodes [5] are trying to address this limitation. Although CNT-based transistors have been fabricated by direct growth of CNTs across electrodes [6], the yield is very low and the growth is very difficult to control. Neither a “top-down” nor a “bottom-up” method can be considered as a sufficient way for fabrication of a complete device. Using the aforementioned methods, the device materials have to be fabricated in situ either from the bulk material or through catalytic growth or an oxidation process. However, some nanoscale materials, such as CNTs and nanowires are very difficult to fabricate in situ and in a designed manner. They are usually fabricated from processes such as laser ablation or chemical vapor deposition (CVD). In order to make these nanomaterials useful in nanotechnology applications, the ability to manipulate them in a controllable manner is very critical. Only when these materials can be easily transported, repositioned, and removed, are they going to be widely used in device fabrication

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Fig. 1. SEM image showing the bridging of Au/Cr microelectrodes with bundled CNTs by dielectrophoretic force.

instead of stagnating at theoretical and experimental studies. Therefore, the combination of “top-down” and “bottom-up” methods together with manipulation of nano-objects in a controlled manner has to be utilized in order to manufacture nanostructures and nanodevices. Three nanomanipulation techniques seem to be very promising in fabricating nanodevices and nanosystems: dielectrophoretic-based nanomanipulation, SEM-based nanomanipulation, and AFM-based nanoassembly. These techniques may eventually enable systematic engineering and fabrication of nanodevices and systems. Several groups have demonstrated successful manipulation of CNTs, and metallic and inorganic nanowires using the dielectrophoretic force in the past few years. K. Yamamoto et al. have demonstrated the use of dielectrophoretic force to align CNTs with an ac electric field [7]. L. A. Nagahara et al. have successfully demonstrated the use of dielectrophoretic force to manipulate individual CNTs to form electrical contacts between two metal nanoelectrodes, which is a very promising demonstration for parallel assembly of CNTs [8]. V. T. S. Wong and W. J. Li have recently demonstrated the formation of bulk carbon nanotubes between microelectrodes using dielectrophoretic forces for practical sensing applications [9] as shown in Fig. 1. Other than carbon nanotubes, metallic and inorganic nanowires have also been manipulated successfully by P. A. Smith et al. [10] and X. Duan et al. [11]. To a certain extent, these methods have their shortcomings in terms of repeatability and ability in eliminating uncertainties. For instance, a single CNT or nanowire cannot be guaranteed to be aligned along the electrode gap, and once it has been aligned, it cannot be removed. Another promising method for nanomanipulation of nanoscale materials is to build a small nanomanipulator inside the vacuum capsule of an SEM. Piezoelectric vacuum manipulators constructed inside the SEM have the ability to manipulate objects along the three linear degrees of freedom (DOF) using an AFM tip as the end effectors [12]. Several kinds of manipulation of CNTs were performed using this kind of device [13], [14]. The obvious advantage of this method is that multiend effectors can be built inside the SEM to achieve more DOF. Their operation can be monitored in real time. However,

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the expense of an SEM, ultrahigh vacuum condition and space limitation inside the SEM vacuum capsule also impede the wide application of this method. In terms of fabrication at device level, the SEM-based nanomanipulation is behind that of AFM-based nanolithogaphy due to these limitations. The third promising method for manufacturing nanodevices is the AFM-based nanoassembly or nanomanipulation. When researchers first employed the scanning tunnelling microscope (STM) or AFM to image sample surfaces at the atomic scale resolution, the capability of modifying and manipulating sample surfaces of the interaction of the scanning probe microscopy (SPM) tip (electric field and force generated by the STM and the contact force exerted on the sample surface by the AFM) was obviously observed. Many researchers have attempted to control, modify, and manipulate the surface using SPM in a precise and selective fashion since the late 1980s. The early attempts at surface modification experiments involved unstructured deposition or removal of atoms or clusters of atoms from a sample surface using an STM, although the results were inconsistent and in an uncontrolled manner. As more and more attempts at surface modification in the nanometer scale have been conducted, researchers from various disciplines gained much experience in manipulating atoms and molecules and aimed to perform precise position control and manipulation of atoms and molecules that would be required for the assembly of nanodevices. One of the famous examples was demonstrated by Eigler and Schweizer at IBM Almaden Laboratory in 1990 [15]. They positioned 35 Xenon atoms to spell out the “IBM” pattern on a Nickel (111) surface using a Tungsten STM tip under an ultrahigh vacuum and at very low temperature (about 4 K). Inspired by the preliminary results of atomic manipulation obtained in the late 1980s and early 1990s, more research groups from various disciplines (including physics, chemistry, biotechnology, computer science, robotics, etc.) in the world have joined the exploration of the nanomanipulation since the mid 1990s. The application of nanomanipulation has been extended to nonconducting materials and even objects in fluids, such as living cells, DNA, proteins, etc. This led to the trend of using the AFM as the major nanomanipulation tool in recent years. The AFM-based nanomanipulation is more complicated and difficult than the AFM-based nanolithography. That is, it is not necessary to relocate the nano-objects on a surface during a nanolithographical process, while nano-objects have to be manipulated from one place to another by the AFM tip during nanomanipulation. In the most recently available AFM-based manipulation methods, the manipulation paths are ascertained either manually using haptic devices [16], [17] or in an interactive way between the users and the AFM images [18], [19]. The main problem with these schemes is their lack of real-time visual feedback. Since the nano-objects can be easily lost or moved to wrong destinations during the AFM-based manipulation process, the result of each operation has to be verified by a new image scan before the next operation starts. This scan-design-manipulation-scan cycle is very time consuming and inefficient. In order to improve the efficiency of AFM-based nanomanipulation, real-time visual feedback becomes very necessary. An AFM-based nanomanipulation system assisted by an aug-

Fig. 2. Pushing particles on a glass surface to form patterns. The nano-objects are latex particles 100 nm in diameter. The work area is 10 m 10m.

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Fig. 3. Pushing DNA on a polycarbonate surface (scanning range of 3 m). (a) Image of DNA before pushing. (b) The real-time display on the augmented reality during pushing. (c) A new scanning image after several pushing operations.

mented reality interface has been developed in [20]. During the nanomanipulation of nanoparticles, not only can the operator feel the real-time three dimensional (3-D) interaction forces but can also observe the real-time changes of the nanoenvironment. The real-time visual feedback is achieved by locally updating the image file based on real-time force information, the nano-objects’ behavior model, as well as the interaction model among the tip, objects, and the sample surface. Under the assistance of the augmented reality system, the manipulation of nanoparticles becomes very straightforward. The experimental result shown in Fig. 2 is conducted by pushing more than 100 nanoparticles to form complex patterns within a half hour. As an extension of [20], the real-time visual feedback has improved in [21] from manipulating nanoparticles to nanowires and nanorods. A further extension of the real-time visual feedback during manipulation to manipulate flexible materials such as DNA molecules has been achieved in [22]. An example of DNA manipulation is shown in Fig. 3, in which Fig. 3(a) shows the DNA molecules in their original shapes; and Fig. 3(b) shows the manipulation of DNA molecules displayed in the augmented reality environment; Fig. 3(c) shows an AFM image after manipulation. It can be seen that several kinks have been created by slightly pushing the DNA molecules or bundles and the kinks created in the augmented reality environment are relatively identical to the real results. The real-time visual feedback during manipulation has significantly improved the effectiveness of the AFM-based nanomanipulation system for the assembly of nanostructures. Although the real-time visual feedback provides more convenience for the operator, the control of tip position during nanomanipulation is still a major issue because of the defor-


Fig. 4. Manipulating a CNT onto a pair of gold electrodes. (a) The AFM image before manipulation. There are two CNTs in the image. (b) The image from a new AFM scan after manipulation. One is pushed onto the electrodes to form the connection, and the other one is pushed away from the electrode.

mation of the cantilever caused by the manipulation force. The softness of the conventional cantilevers also causes the failure when manipulating a relatively large and sticky nano-object because the tip can easily slip over the nano-object. In order to overcome these problems, an active AFM probe has been used in [23] to change the cantilever’s flexibility or rigidity through different control strategies in imaging and manipulation modes, respectively. During the imaging mode, the active probe is controlled to bend up with respect to the interaction force between the tip and samples, thus making the tip response faster and, therefore, to increase the imaging speed. During the manipulation mode, the active probe is controlled to bend down with respect to the interaction force between the tip and the samples and, thus, to increase its nominal rigidity to avoid tip slipping over nano-objects. The tip position control becomes very accurate because the active prove behaves like a rigid probe during manipulation and causes less deformation of the cantilever. The control signal serves as the force signal for haptic feedback since the interaction force between the tip and the object is proportional to the control signal. Combining with the dielectrophoretic-based nanomanipulation method, a single CNT-based nanodevice has been fabricated successfully for the first time by AFM-based nanomanipulation [24]. In this method, the individual CNTs are deposited close to the electrodes by dielectrophoresis. Using the AFM-based nanorobotic system, the desired CNT is manipulated to lie across the electrodes, and any unwanted nano-objects may also be pushed away from the electrodes as shown in Fig. 4. The AFM-based manipulation by an operator is still a very slow process. In order to increase the efficiency and accuracy of the AFM-based nano-assembly, an automated CAD-guided nanoassembly is desirable. In the macroworld, CAD-guided automated manufacturing has been widely studied. However, it is not a trivial extension from the macroworld to the nanoworld. In the nanoenvironments, the nano-objects, which include nanoparticles, nanorods, nanowires, nanotubes, etc., are usually distributed on a substrate randomly. Therefore, the nanoenvironment and the available nano-objects have to be modeled in order to design a feasible nanostructure. In order to generate a feasible path to manipulate nano-objects, obstacle avoidance has to be considered. Turns around obstacles should also be

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avoided since turns may cause failure during manipulation. Chen and et al. have developed an automated CAD-guided nanoassembly method using an AFM-based nanorobotic system, which will be discussed in this issue. In summary, there are currently several nanofabrication techniques available but it is still very difficult to manufacture nanodevices in a massively parallel way due to the yield, control, and speed issues. It has been shown that a single nanomanufacturing method is usually not sufficient to fabricate a nanodevice. Combined methods are often used to increase the yield and repeatability. For example, by combining the dielectropheretic force with the AFM-based nanomanipulation, a single CNTbased device can be fabricated reproducibly. Among all of the methods aforementioned, AFM-based nanomanipulation shows a very strong potential for nanofabrication especially when it is combined with other techniques. Recent developments in AFMs also make this method even more promising. For example, the fast-scan AFM will make the real-time visual feedback more reliable and the fabrication process more efficient. In order to further improve nanofabrication efficiency, more attention should be drawn to the study of automated AFM-based nanofabrication in the future. As a result, the AFM-based nanomanipulation and nanoassembly method can be integrated with other schemes, such as electrophoretic force-based assembly and self-assembly. They will provide an effective and efficient method to assemble nanoscale devices and systems. NING XI, Guest Editor Robotics and Automation Laboratory Department of Electrical and Computer Engineering Michigan State University East Lansing, MI 48824 USA WEN J. LI, Guest Editor Centre for Micro and Nano Systems The Chinese University of Hong Kong Hong Kong SAR, China

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Ning Xi (S’89–M’95) received the B.S. degree in electrical engineering from the Beijing University of Aeronautics and Astronautics, Beijing, China, in 1982, the M.S. degree from Northeastern University, Boston, MA, in 1989, and the D.Sc. degree in systems science and mathematics from Washington University, St. Louis, MO, in 1993. Currently, he is the John D. Ryder Professor of Electrical and Computer Engineering in the Department of Electrical and Computer Engineering at Michigan State University, East Lansing. His research interests include robotics, manufacturing automation, micro/nanosystems, and intelligent control and systems. Dr. Xi received the Best Paper Award at the IEEE/RSJ International Conference on Intelligent Robots and Systems in 1995 and the Best Paper Award at the 1998 Japan-USA Symposium on Flexible Automation. He was awarded the first Early Academic Career Award by the IEEE Robotics and Automation Society in 1999. He is also a recipient of the National Science Foundation CAREER Award.

Wen J. Li (S’96–M’98) received the B.S. and M.S. degrees in aerospace engineering from the University of Southern California, Los Angeles, and the Ph.D. degree in microelectromechanical systems (MEMS) from the University of California, Los Angeles. Currently, he is with the Department of Automation and Computer-Aided Engineering, The Chinese University of Hong Kong (CUHK), Hong Kong, China, where he heads the Centre for Micro and Nano Systems. Before joining CUHK, he held R&D positions at the NASA Jet Propulsion Laboratory, Pasadena, CA and The Aerospace Corporation, El Segundo, CA. Dr. Li’s research group won best paper awards from premier conferences of the IEEE Nanotechnology Council (IEEE-NANO) and the IEEE Robotics and Automation Society (IEEEICRA) in 2003 for their work on microcell grippers and nanosensors. The group’s work on nanotube sensors has recently gained significant international interest and, hence, he presented several keynotes and invited talks in the U.S., Taiwan, Canada, and Europe, in 2005. He has served as a Guest Editor for the IEEE TRANSACTIONS ON AUTOMATION SCIENCE AND ENGINEERING, and the IEEE/ASME TRANSACTIONA ON MECHATRONICS. He has been appointed to serve as the General Chair of IEEE NANO 2007 by the IEEE Nanotechnology Council. He is also a Distinguished Overseas Scholar of the Chinese Academy of Sciences.