Introduction

Nanostructures have unique properties due to the quantum confinement effects arising from the reduced dimensions. One-dimensional nanostructures have received more and more attention in recent years following the discovery of carbon nanotubes [1, 2] and related nanowires [3]. Carbon nanotubes are structures made by rolling up graphite sheets into seamless cylinders with diameters in nanometer scale. The structural change from graphite sheets into carbon nanotubes introduces quantum confinement along the direction of the tube circumference. This confinement allows electron transport only along the direction of tube axis, which makes carbon nanotube a good one-dimensional electron system. It is interesting to note that the combination of the quantum confinement effect and the electronic structure of graphite can change the properties of carbon nanotubes more than other materials: A single carbon nanotube can be metallic or semiconductor depending on its diameter or its chirality—a notation on how the nanotube is rolled up [4]. This property gives us great hope to realize intramolecular devices by engineering the atomic structure of carbon nanotubes and fusing them together into one molecule. Several theoretical and experimental investigations have shown the junction of a metallic tube and a semiconducting tube could work as a Schottky diode [5-7].

Although this kind of intramolecular junction is of great interest in molecular devices, it is difficult to find a practical method to fabricate the junctions in a controllable way. Comparing with carbon nanotubes, the properties of non-carbon nanotubes and nanowires are normally easier to control because they are primarily determined by their chemical composition instead of geometry. For example, BN is insulator independent of its chirality and diameter, and silicon nanowires are always semiconductors like their bulk counterpart. Similar quantum confinement effects, however, would exist for all these different one-dimensional structures. Construction of nanoscale heterostructures by combining these one-dimensional objects will provide great potential for future quantum devices. Although a rich variety of one-dimensional nanostructures has been discovered in recent years, a general technology to build the one-dimensional heterostructure has not been well established. Unlike the two-dimensional heterostructures such as thin-film semiconductor superlattices that can be precisely engineered by techniques such as molecular beam epitaxy (MBE), phase formation in one-dimensional nanostructures is not straightforward and is hard to control by simple physical deposition. Nevertheless, some very exciting progress has been made in synthesizing heterogeneous nanotubes, nanowires, and their junctions. And some have already show promises in real applications. For example, the method of making carbon nanotube and titanium carbide junction has been used to improve the contact in nanotube field-effect transistors [8, 9].

The heterogeneous one-dimensional nanostructures composed of nanotubes and nanowires can generally be divided

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 6: Pages (61-76)

into two categories according to the direction of nanowire axes relative to the interfaces that separate different materials in the heterogeneous nanostructures. In the first category, the axes of the nanowires and nanotubes are parallel to the interface planes. The phase separation is in the radial direction. Most nanotubes or nanowires in this category have homogeneous phase distribution along the whole length. The nanostructures in this category are also called composite nanotubes or nanowires (different from nanotube or nanowire composite which normally refers to a material made by dispersing nanotubes or nanowires into a matrix material). A representative structure of this category is a "nanocable" that has a core nanowire sheathed by one or more layers of different materials [10]. Composite nano-tubes and carbon nanotubes with a second material filled into their hollow cores also belong to this category. In the second category, the axes of the nanotubes and nanowires are normal to the heterointerfaces. This category includes heterojunctions connecting nanotubes and solid nanowires [8, 11] or nanowire superlattices that are composed of alternating semiconductor nanowire segments of different chemical composition [12-14]. Because of the small cross section of nanotubes and nanowires, the junctions formed from them are in nanometer scale. They are therefore called "nanojunctions." The nanojunctions represent a new kind of structure that could have great potential applications in nanoscale electronics and optoelectronics. This chapter will give a review of the progress on fabrication and characterization of the heterogeneous one-dimensional nanostructures of both categories.

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