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Examples of progress in fabricating branched multi-terminal multi-wall and single-wall carbon nanotube junctions as predicted by nanotechnology simulations, ...

Journal of Nanoparticle Research 5: 395–400, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

Brief communication

Branched carbon nanotube junctions predicted by computational nanotechnology and fabricated through nanowelding D. Srivastava1 , M. Menon2 and P.M. Ajayan3 NASA Ames Research Center, MS 229-1, Moffett Field, CA 94035-1000, USA; 2 Department of Physics and Center for Computational Sciences, University of Kentucky, Lexington, KY 40506-0055, USA; 3 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590, USA

1

Received 27 January 2003; accepted in revised form 3 February 2003

Key words: computational nanotechnology, carbon nanotube junctions, molecular electronics, quantum mechanics, numerical simulations

Abstract Examples of progress in fabricating branched multi-terminal multi-wall and single-wall carbon nanotube junctions as predicted by nanotechnology simulations, using template growth and nanowelding techniques, respectively, have been briefly reviewed in this report. It is argued that similar general progress in computational nanotechnologydriven fabrication of applications in other nanomaterials such as nanotubes, fullerenes, nanowires, quantum dots, DNA molecules, and nanoparticle-based systems and devices are also feasible. This is because, at nanometer length scale, system sizes have shrunk sufficiently small such that it is feasible to simulate the structural, stability, and physical and chemical characteristics with very high accuracy predictive simulations. Introduction A real progress in the nanotechnology of materials, electronics and devices with in last few years has been due to the discovery of many innovative fabrication methods. The atomically precise materials such nanotubes, fullerenes, nanowires, quantum dots, dendrimers, etc. are fabricated with better process control and explicit characterization. Many successful nanoscale engineering and manipulation efforts then go to the next step of fabricating novel nanoscale structures, which are closer to applications and are derived from the originally discovered materials listed above. Examples of this second category include, growing nanowires and nanotubes on a variety of substrates and templates, using surface curvature and mechanical distortions to do side-wall chemistry or functionalization of nanotubes and fullerenes, and doping of dendrimers to achieve functionalities never imagined before. The role of computational nanotechnology,

i.e., physics- and chemistry-based modeling and simulation of nanomaterials, devices, and applications, has been significant in advancing these frontiers. It turns out that at nanoscale, materials, devices, and even systems sizes have shrunk sufficiently small so that it is possible to describe their behavior at a fairly accurate level. The simulation technologies have also become predictive in nature and many novel concepts and designs have been first proposed by modeling and simulations followed by their fabrication, realization, or verification in experiments. In coming years, the computer power available for typical simulations will continue to increase, and length scale of nanotechnology-based materials, devices and systems will continue to decrease. This means that it will be possible to do even higher fidelity quantum-mechanics-based simulations of nanoscale systems and applications which are not possible today. The growth and developments in computational nanotechnology thus could play increasingly important

396 roles in the over-all development of nanotechnologybased novel materials, structures, devices, and applications. Computer aided design (CAD) and prototyping tools, based on large scale atomistic and quantummechanics-based simulations, may become as standard and as popular as the currently used CAD tools in the large scale engineering and structural applications. An example of computational nanotechnology predicting the structure and design of new materials structure, which has then recently resulted in the fabrication of the same, is the topic of this brief review. In general, single-wall carbon nanotubes (SWCNTs) are a rolled-up graphene sheet (a single layer of C atoms arranged in a hexagonal lattice structure) with various chiralities (Iijima, 1991; Meyyappan & Srivastava, 2003). The electronic structure of these tubes can be either metallic or semiconducting, depending on the nature of the rolling direction of the sheet into a cylindrical or tubular structure. The possibilities of connecting nanotubes of different diameters and chiralities (rolling directions) to form two-terminal nanotube hetero-junctions were then proposed (Lambin et al., 1995; Chico et al., 1996; Charlier et al., 1996; Saito et al., 1996) from modeling and simulation-based studies. These are proposed as the basis of CNT-based molecular electronic device or switching components. The simplest way to connect two dissimilar nanotubes is found to be via the introduction of pairs of heptagons and pentagons in an otherwise perfect hexagonal graphene sheet structure (Lambin et al., 1995; Chico et al., 1996). The resulting junction still contains three-fold coordination for all carbon atoms, and the hetero-junction between a semiconducting and a metallic CNT can act like a rectifying diode or a switch. Such two-terminal CNT heterojunctions or rectifying diodes were first proposed through simulation-based studies of the stability and electronic properties of such structures (Lambin et al., 1995; Chico et al., 1996; Charlier et al., 1996; Saito et al., 1996). The characteristic angle at which these hetero-junctions were supposed to have been formed was observed in as produced samples of single-wall nanotubes (Han et al., 1998). Such junctions could also be made in experiments by pushing single-wall nanotube with atomic force microscope (AFM) tip (Yao et al., 1999). The experimental investigation of electronic characterization at the junction showed the predicted rectifying behavior as well (Yao et al., 1999). There are two ways to go about creating more than two-terminal nanotube hetero-junctions.

First, connecting different nanotubes, like in the above, through molecularly perfect but topologically defectmediated junctions. Second, laying down crossed nanotubes over each other and simply forming the physically contacted or touching junctions. The differences in the two approaches are in the nature of the junctions forming the device or the interconnection. In the first case, nanotubes are chemically or ‘weld’-connected through bonding networks forming a stable junction that could possibly give rise to a variety of switching, logic, and transistor applications in the category of monomolecular electronic or computing devices (Joachim, 2001). In the second case, the junction is merely through a physical contact and will be amenable to changes in the nature of the contact. The main applications in the second category will be in electromechanical switches for cross-bar type memory or sensor applications (Rueckes, 2000). The formation of three- or more terminal ‘weld’-connected CNT junctions is briefly described below. Simulation-based predictions of the possibilities of three- or more terminal junctions of nanotubes have been a different challenge altogether. Soon after the discovery of nanotubes in 1991 (Iijima, 1991), the multi-terminal CNT junctions, in a ‘Jungle Gym’ type structure (Chernozatonskii, 1992) or via a nodal fullerene (Scusaria, 1992) were proposed as simple model structures in as early as 1992. The three-terminal branched CNT T- and Y-junctions shown in Figures 1 and 2, were the first rigorously proposed structures for three-terminal molecular electronic devices, such as rectifying, tunnel junctions, or transistor type applications (Menon & Srivastava, 1997; 1998). Large scale quantum molecular dynamics simulations with a non-orthogonal tight-binding method, together with carefully placed pentagons and heptagons in otherwise all hexagonal graphene lattice, showed that the T- and Y-junctions, if fabricated, (a) will be structurally stable, and (b) they could form the basis of first all ‘carbon’ (CNT) based three-terminal tunnel junctions for molecular electronics device applications (Menon & Srivastava, 1997; 1998). Inspired by the above computational simulation work in 1997–1998, multi-wall CNT (MWCNT) Y-junctions were first fabricated in a template-based CVD growth of CNTs in nanoscale anodic templates. The Y-branching in the template led to controlled branching of thousands of grown MWCNT junctions in the template (Li et al., 1999; Papadopoulos et al., 2000). Since then, many groups using different techniques

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Figure 1. A CNT Y-junction predicted from computal nanotechnology is shown with (inset) a qualitatively similar MWCNT Y-junction that was fabricated in a template-based growth of CNT junctions (Papadapoulos et al., 2000).

have produced three- or more terminal MWCNT junctions in different scenarios (Satishkumar et al., 2000; Gan et al., 2001; Deepak et al., 2001; Gao et al., 2002; Zhu et al., 2002). Simulations leading the way, results also have showed their use in analog-rectifying and logic devices and experiments have observed similar current–voltage characteristics when biased similarly as in simulations (Andriotis et al., 2001a,b; 2002). MWCNT Y-junctions were the first example of the possibility of carbon-based nanoscale three-terminal devices. However, the experiments were not able to scale down the technology of setting three independent electrical contacts on such fabricated threeterminal devices. These were placed in configurations where only two-terminal measurements were made, (see for example Figure 1) which showed that under two-terminal scenario the Y-junctions have rectifying behavior. Computer simulations, on the other hand,

are not limited by the experimental inability to put independent electrical contacts on the three terminals. Simulation results have shown that, under such configurations, a gate voltage from the third terminal can be used to modulate the current–voltage behavior through the other two terminals. The possibilities of analog logic gates, based on the modulation possibilities without any gate isolation, have been explored. A significant experimental progress has thus been already made to turn into reality the simulation-based proposals of multi-terminal nanotube hetero-junctions for molecular electronic device applications (Andriotis et al., 2001a,b; 2002). More recently, however, another significant experimental milestone has been achieved. The chemically or ‘weld’-connected three- or four-terminal junctions of SWCNTs also have been fabricated in experiments (Terrones et al., 2002). The experiment shows

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Figure 2. SWCNT T- and X-junctions as predicted by high fidelity computational nanotechnology-based simulations.

the feasibility of connecting a SWCNT with another SWCNT at a variety of angles at the junction by an engineering type ‘nanowelding’ technique. The SWCNTs, to be connected, are placed over each other, and a simultaneous local e-beam-induced irradiation and annealing in a tunneling electron microscope (TEM) set-up causes the two nanotubes to form a spot-weld junction at the contact point. Both T- and Y-junctions of SWCNTs have been fabricated with this approach, and it is feasible that more control and variation would be feasible in the next generation of this engineeringled experiments. For example, in Figure 3, we show SWCNT T-, Y-, and X-junctions fabricated by local e-beam heating or welding of two CNTs physically placed over each other in a desired T, Y, or crossing configuration.

This is an example of significant progress in nanotechnology where high fidelity physics- and chemistry-based computer simulations can be shown to be predictive in nature. As typical computing power and simulation capabilities continue to increase, algorithms and implementations have become more refined and accurate, many more such areas and examples will occur where computer simulation aided rapid prototyping (CAP) of yet to be fabricated nanoscale systems will be a norm and not an exception. From architectural view point, a three-dimensional tree or branched network of chemically or ‘weld’-connected CNTs (Srivastava et al., 2001) has been now proposed as a next challenge to experimentalists that could define a bio-mimetic architecture for nanoscale computing and sensing systems with functionality similar to that of dendritic neurons in a biological systems. Considering the above in a broader framework about the reach and capabilities of general computational nanotechnology-based investigations, it turns out that the above possibilities are not limited only to CNTs and/or fullerene-based nanomaterials and the above applications. The other properties of CNTs and fullerenes such as mechanical characterization of individual nanotubes and nanotube–polymer composites, the site-specific chemistry and functionalization of nanotubes, electromechanical response characteristics of fullerenes and nanotubes, and gas adsorption and flow through nanotubes have also been recently simulated to aggressively propose new applications. The details of these investigations and role of computational nanotechnology in driving this progress have been summarized in Srivastava et al. (2001), Meyyappan and Srivastava (2002). Additionally, the underlying physics- and chemistrybased theoretical techniques and highly optimized computational algorithms remain same, as one goes from one type of nanomaterials and devices to another type. For example, the above described computational approaches are applicable to other nanoscale materials, devices, and systems as well. Examples of these include nanowires and clusters, metal and semiconductor nanoparticles, quantum dots, DNA molecular strands, and small protein molecules in solution phase environment, etc. The basic feature is that, the length scale of these proposed nanomaterials systems have shrunk to sizes that are very well accessible to predictive high fidelity computer simulations. On one hand, it is possible to simulate the structure, stability, and physical or chemical characteristics of such systems

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Figure 3. SWCNT T-, Y-, and X-junctions that are fabricated in experiments via spot nanowelding technique of Terrones et al. (2002).

to propose novel materials previously not discovered in experiments, and on the other hand equally accurate computations of thermal and electrical transport through some of these systems have given rise to possibilities of novel device applications as well. Acknowledgements This submission (D.S.) is supported by NASA contract DTTS59-99-D-00437 to CSC. P.M.A. acknowledges NEDO international joint research grant on nano-integration system. M.M. acknowledges support from NSF (NER-0165121, ITR-0266277), DOE (00-63857), NASA (02-465679) and University of Kentucky Center for Computational Sciences. References Andriotis A.N., M. Menon, D. Srivastava & L. Chernozatonskii, 2001. Phys. Rev. Lett. 87, 6602. Andriotis A.N., M. Menon, D. Srivastava & L. Chernozatonskii, 2001. Appl. Phys. Lett. 79, 266. Andriotis A.N., M. Menon, D. Srivastava & L. Chernozatonskii, 2002. Phys. Rev. B65, 165416. Charlier J.-C., T.H. Ebbesen & P. Lambin, 1996. Phys. Rev. B53, 11108.

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