Perspective for the Journal of Nanoparticle Research
October 16, 1998
Revised November 27, 1998
Nanoparticle Patterns
Aristides A. G. Requicha
Laboratory for Molecular Robotics
University of Southern California
941 West 37th Place
Los Angeles, CA 90089-0781
E-mail: requicha@usc.edu
Telephone: (213) 740-4502
Fax: (213) 740-7512
The very existence of this new journal attests to the remarkable successes of nanoparticle research. Today it is possible to synthesize particles with a variety of properties — for example, they may be metalic, semiconductor, or magnetic — and in sizes that typically range from one nanometer to the tens of nm. Recently, reliable and accurate techniques for positioning individual nanoparticles on a substrate surface have been demonstrated. The particles are moved by mechanically pushing them, using the tip of an Atomic Force Microscope (AFM) as a robot manipulator [Junno et al. 1995, Baur et al. 1997, Ramachandran et al. 1988, Resch et al. 1998a]. Nanomanipulation with the AFM has been demonstrated with various substrates and particles, and appears to have wide applicability. It is typically done at room temperature, in ambient air, with commercially-available instruments and tips, and using relatively simple techniques for sample preparation. It requires, however, custom software, because commercial systems are intended primarily for imaging and have poor or no support for nanomanipulation.
A new field, called nanorobotics, is thus beginning to emerge. Coupled with current and future technology for nanoparticle synthesis, it opens new avenues of research involving the assembly of arbitrarily-shaped patterns of nanoparticles. This Perspective discusses a few of these research directions that seem especially promising. Patterns of nanoparticles may be useful in themselves, or they may serve as templates for constructing other structures. We present examples of both below. Although regular, symmetric patterns of nanoparticles can be constructed by self-assembly, many (perhaps most) of the applications require asymmetric shapes. We focus on these, because they cannot be built by self-assembly without some form of patterning or masking, which are difficult operations at the nanoscale.
Imagine a rectangular grid of equally-spaced points on a plane. We refer to these points as nodes. Along a row of this grid either place a nanoparticle at a node or leave the node empty. If we interpret the presence of a particle as a binary 1 and its absence as a 0, a row of the grid encodes a binary sequence. Therefore, this scheme can be used to store digital data. Suppose that we place 15 nm particles on a grid with 100 nm spacing. (We do this routinely in our lab.) The corresponding bit density is 10 Gbit/cm2, which is two orders of magnitude higher than the current compact disk (CD) density. By using smaller particles and closer spacing, even higher densities are achievable. In addition, this NanoCD is editable, simply by moving the particles. Similar data densities may be achieved by other techniques such as electron-beam lithography. However, nanoparticles are advantageous because they are much more uniform in size and properties than corresponding structures built by E-beam lithography.
Electronic circuits in which current flows through the transfer of a single electron are generating much scientific and technological interest today, because they are extremely small and embody interesting quantum-mechanical phenomena. A single-electron field-effect transistor can be built by laying down source, drain and gate electrodes by lithographic techniques, and then constructing a channel between source and drain as a sequence of nanoparticles. Single electrons tunnel through the junctions between the particles under control of the gate voltage. If the particles have sizes on the order of a few nanometers, the transistor operates at room temperature. This has not yet been demonstrated experimentally, although work at Lund University [Junno et al 1998] and in our lab shows that nanoparticles can be manipulated with an AFM to form a channel. However, the particles in these experiments were not small enough for room-temperature operation. (Single-electron, room-temperature transistors have been demonstrated, but they were not built with nanoparticles [Matsumoto et al 1996].)
We have demonstrated in our lab that small groups of nanoparticles can be linked and moved as a whole [Resch et al. 1998b]. If this work can be extended so as to link and manipulate larger numbers of particles, it may be possible to construct relatively rigid structures of arbitrary (planar) shapes. These, in turn, may serve as components for building complex nanoelectromechanical systems (NEMS) from the bottom up, by robotic assembly. Linking of gold nanoparticles arranged on a self-assembled regular grid has been achieved experimentally, by using dithiols [Andres et al 1996]. This suggests that asymmetric patterns can also be connected, provided that the distances between particles are approximately equal to the length of the connecting elements. In very recent work in our lab, we have demonstrated such linkage. Extensions to 3-D may be possible by working on successive layers, much like in macroscopic rapid prototyping by stereolithography and similar techniques. Thus far, only extremely simple 3-D constructions have been reported.
The contact imprinting techniques developed by Chou and others [Chou et al 1996] are capable of producing large numbers of nanostructures in a simple, parallel operation, by pressing a template against a suitable substrate. Assembly of nanoparticle patterns may be an effective procedure for manufacturing the required templates, since it can produce features as small as a few nm, with low size variability. The patterns may be used directly, or as a mask for producing the actual template by deposition or by etching.
Nanoparticle patterns can also be used as a resist, as shown by recent research at the University of Konstanz, Germany [Burmeister et al 1998]. They started with a self-assembled regular pattern and deposited material so as to fill the space between the particles. By etching the particles away, they obtained the complement of the original pattern. In a similar vein, a Japanese/English group used regular nanoparticle patterns as templates in a process that involves both etching and growth [Tada et al 1988]. First they deposited nanoparticles on a Si substrate. Then they etched the Si. As the substrate was etched away, reaction products condensated around the nanoparticles. Using this technique, they were able to produce regular patterns of pillars with diameters on the order of a few nm. In both cases, it should be possible to perform similar operations for arbitrary patterns constructed by nanomanipulation.
Nanomanipulation with AFMs or other Scanning Probe Microscopes is a serial process, with low throughput. However, multi-tip arrays are being developed at several laboratories [Miller et al. 1997, Minne et al. 1996]. Current arrays still have small numbers of tips, but the technology should scale up because it is based on semiconductor fabrication processes. Initial investigations indicate that there are simple and massively parallel algorithms for performing useful operations at high speed with such tip arrays [Requicha 1999].
In summary, in this Perspective I have attempted to show that nanorobotic assembly of arbitrary patterns of nanoparticles is a rich and largely unexplored area of research, with interesting potential applications. And I hope I have been able to convey some of the excitment that we feel daily in the lab, pushing forward along many of these fronts.
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