Figure 155 Scanning electron micrograph of selfassembled Si nanowires grown by vaporliquidsolid epitaxy after T Picraux et al

Carbon nanotubes are currently the focus of considerable attention because of the many remarkable properties of this new structural state of carbon. Figure 15-6 shows a schematic of a CNT that is composed of carbon atoms arranged in a stable tube configuration. It is a highly stable state of matter, very similar in concept to fullerenes like C60 (buckyballs). The structure can be envisioned as a graphite sheet (where the carbon atoms form hexagonal rings), which is rolled in a tube a few nanometers in diameter, as shown in Figure 15-6a. In rolling the tube and joining itself, the carbon rings forming the graphite structure can align in different offset configurations, characterized by their chirality. Depending on the chirality, CNTs can be metallic, semiconducting, or insulating, all the components required in conventional semiconductor IC technology (interconnects, transistors, and dielectrics). Field effect transistors have been fabricated from CNTs, and basic logic functions demonstrated by researchers at IBM and other research laboratories, as shown in Figure 15-6b. The extreme sensitivity of the conductivity of the nanotube to an attached atom or molecule to the wall or tip of the nanotube, also makes CNTs very attractive as sensors, the subject of considerable current research. The primary challenge faced in the evolution of this technology is the directed growth of CNTs with the desired chirality, and positioning on a semiconductor surface, suitable for large-scale manufacturing.

Figure 15-6. (a) Different states of carbon, including diamond, graphite, C60, and a carbon nanotube (right) (from Richard Smalley's image gallery, http://smalley.rice.edu/smalley.cfm); (b) carbon nanotube inverter formed from p- and n-channel FETs (from IBM, with permission).

Figure 15-6. (a) Different states of carbon, including diamond, graphite, C60, and a carbon nanotube (right) (from Richard Smalley's image gallery, http://smalley.rice.edu/smalley.cfm); (b) carbon nanotube inverter formed from p- and n-channel FETs (from IBM, with permission).

Perhaps the ultimate limit of size scaling are devices comprised of a small number of molecules, forming the basis of electronic systems realized with molecular devices, or molecular electronics (moltronics). Figure 15-7 shows a schematic diagram of a nanoscale contact to a molecular device, through which current is passed. Here the molecular device is an organic chain to which different side groups or molecules are attached to realize a desired functionality. The molecular chain structure shown in the lower half of the figure, studied by Mark Reed (Yale) and James Tour (Rice), showed "negative differential conductance (NDC)" in the current voltage characteristics, that is, a decreasing current with increasing voltage. From a circuit standpoint, NDC appears as a negative resistance, which leads to signal amplification and the possibility of bistable behavior because the circuit does not like to reside in the regime, which is the basis for elementary switching devices. Elementary molecular electronic architectures have been demonstrated by HP Research Laboratories using crossbar-type logic.

Figure 15-7. A molecular "junction" (left), and the corresponding molecular device contacted by external leads.

Figure 15-7. A molecular "junction" (left), and the corresponding molecular device contacted by external leads.

A very attractive feature of molecular systems is the possibility of bottom-up or self-assembly of functional systems. Such templated self-assembly is of course the basis of biological systems, which have exquisite complexity and functionality as well as self-replication and self-repair. Such "biomimetic" approaches to molecular circuits would represent an inexpensive alternative to the exponentially increasing cost of top-down nanofabrication, which is currently driving fab costs into the billions of dollars. However, at present there is no clearcut manufacturing approach to self-assembly in the near term.

Another difficulty in understanding and using molecular electronic structures is the need to separate the intrinsic behavior of a molecular device from the contacts themselves. In conventional devices, contacts are nearly ideal, providing a connection to the other devices and the external world through interconnects, and not affecting the intrinsic performance of devices except through well-controlled parasitic contact resistances. As the number of devices per chip scales exponentially, the number of contacts and interconnects per device increases even faster, and from an architecture standpoint, system performance is increasingly dominated by the interconnects and not the devices themselves. In a nanoscale devices, the contacts may in fact dominate the performance of the device, and at a minimum they are an integral part of the device. This problem is particularly evident in molecular electronic devices, as the schematic of Figure 15-7 indicates (where the contact is much larger than the device). This challenge remains one of the many issues to be solved in evolving molecular electronics in the future.

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