Metal Nanoparticles and Self-Assembly into Electronic Nanostructures
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Nanotechnology is a catchword that evokes excitement in researchers and captures the imagination of laymen. The excitement in the research community stems from the fact that theoretically predicted variations in the physical properties of solid objects as their dimensions approach a few nanometers (1 nm = 10− 9 m)—viz. lowering of melting point, Coulomb charging, novel magnetic, optical phenomena, etc.—have been verified experimentally. This experimental verification has been possible because of rapid strides made over the last two decades, both in techniques for synthesis and in tools for characterization of individual nanoscale objects. These advances have also opened up a vast array of potential technological applications and have occurred just as the limits of photolithography-based solid-state technology are being reached. Coupled with the revolutionary impact of solid-state electronic devices on our lives, advances in synthesis, manipulation, and characterization of nanomaterials have made nanotechnology—and in particular nanoelectronics—a cynosure of public interest.
There are two alternative approaches for fabricating nanoelectronic devices: the “top–down” and the “bottom–up” approaches. The top–down approach is similar to current photolithographic techniques used to produce microelectronic devices. It consists of “chiseling” nanometer-scale features in bulk materials. Using such techniques as e-beam lithography and x-ray phase shift lithography, this approach can now produce nanoscale features (< 50 nm) and has the decided advantage of being compatible with current microelectronic processing methods and design concepts. However, this approach suffers from two important drawbacks: 1) processing costs rise exponentially as feature size decreases; and 2) the surfaces and interfaces produced by this approach exhibit atomic-scale imperfections, which critically degrade device performance as feature size approaches nanometer dimensions. The bottom–up approach consists of “building” the device or circuit by assembling it from preformed nanoscale “bricks.” Bottom–up processing, involving the serial manipulation of nanoscale objects, is technologically impractical. However, it is often possible to induce nanoscale objects to assemble themselves into desired structures. It is such biologically inspired self-assembly which holds the greatest promise.
There are a number of interesting nanoelectronic building blocks, viz. metal nanocrystals and nanowires (both magnetic and nonmagnetic), semiconductor nanocrystals and nanowires, and carbon nanotubes. In this chapter, we focus solely on nonmagnetic metal nanocrystals. Two characteristics of metal nanoparticles are critical for assessing their usefulness in nanoelectronic applications: 1) the ease with which bare metal particles cold weld on contact to form hard aggregates; and 2) the tendency of metal particles to oxidize in an atmospheric environment especially in the presence of water molecules. The first characteristic means that the surface of a metal nanoparticle must be passivated by attachment of a monolayer of capping ligands or surfactant molecules before any attempt is made to assemble these particles into a uniform nanostructure. We will refer to such encapsulated particles as molecularly protected nanoparticles (MPNs). The second characteristic means that only noble metals such as Au and Ag will form nanoparticles that are not rapidly oxidized when exposed to an atmospheric environment. Linear alkanethiol molecules readily react in solution with both Au and Ag nanoparticles to form stable MPNs. These MPNs can be manipulated as stable physical species in a variety of organic solvents. The ability to manipulate Au and Ag MPNs in organic liquids, to synthesize macroscopic quantities of these particles with diameters in the 2–20 nm range, and to control the interparticle spacing in arrays of these particles by changing the length of the alkanethiol molecules coating the metal core make Au and Ag MPNs ideal building blocks for the self-assembly of nanoelectronic devices. To date, most studies of metal MPNs are of Au nanoparticles encapsulated by an alkanethiol monolayer.