In recent years nanoscale science and technology have grown rapidly. Nanochemistry, in particular, presents a unique approach to building devices with a molecular-scale precision. One can envision the advantages of nanodevices in medicine, computing, scientific exploration, and electronics, where nanochemistry offers the promise of building objects atom by atom. The main challenges to full utilization of nanochemistry center on understanding new rules of behavior, because nanoscale systems lie at the threshold between classical and quantum behavior and exhibit behaviors that do not exist in larger devices.
Although nanochemical control was proposed decades ago, it was only recently that many of the tools necessary for studying the nanoworld were developed. These include the scanning tunneling microscope (STM), atomic force microscope (AFM), high resolution scanning and transmission electron microscopies, x rays, ion and electron beam probes, and new methods for nanofabrication and lithography.
Studies of nanochemical systems span many areas, from the study of the interactions of individual atoms and how to manipulate them, how to control chemical reactions at an atomic level, to the study of larger molecular assemblies, such as dendrimers, clusters, and polymers. From studies of assemblies, significant new structures—such as nanotubes, nanowires, three-dimensional molecular assemblies, and lab-on-a-chip devices for separations and biological research—have been developed.
The ultimate frontier of nanochemistry is the chemical manipulation of individual atoms. Using the STM, single atoms have been assembled into larger structures, and researchers have observed chemical reactions between two atoms on a surface. The use of atoms as building blocks opens new routes to novel materials and offers the ability to create the smallest features possible in integrated circuits (IC) and to explore areas like quantum computing. Until now the ever-decreasing size of IC circuitry has been well described by Moore's law, but further shrinkage of circuit size will halt by 2012 because of quantum mechanical effects. Quantum computing provides a way to circumvent this apparent roadblock and use these quantum effects to advantage. Atomic-scale devices, although promising, present major challenges in how to achieve spatial control and stability.
Dendrimers are highly branched three-dimensional nanoscale molecular objects of the same size and weight as traditional polymers. However, dendrimers are synthesized in a stepwise fashion, allowing for extremely precise control of their size and geometry (see Figure 1, a molecular model of a dendrimer). In addition, the chemical reactivity and properties of their periphery and core can be controlled easily and independently. Dendrimers are already being used in molecular recognition, nanosensing, light harvesting, and optoelectrochemical devices. Because they are built up layer by layer and the properties of any individual layer can be controlled through selection of the monomer, they are ideal building blocks in nanochemistry for the creation of more complex three-dimensional structures.
Nanocrystals and Clusters
Nanocrystals are crystals of nanometer dimensions, usually consisting of aggregates of a few hundred to tens of thousands of atoms combined into a cluster. Nanocrystals have typical dimensions of 1 to 50 nanometers (nm),
and thus they are intermediate in size between molecules and bulk materials and exhibit properties that are also intermediate . For example, the small size of semiconductor quantum "dots" leads to a shifted light emission spectrum through quantum confinement effects—with the magnitude of the shift being determined by the size of the nanocrystal. Nanocrystals are of great interest because of their promise in high density data storage and in optoelectronic applications, as they can be efficient light emitters. Nanocrystals have also found applications as biochemical tags, as laser and optical components, for the preparation of display devices, and for chemical catalysis .
Recently, hollow carbon tubes of nanometer dimensions have been prepared and studied. These nanotubes constitute a new form of carbon, configurationally equivalent to a graphite sheet rolled into a hollow tube (see Figure 2, a molecular model of a carbon nanotube). Carbon nanotubes may be synthesized, with sizes ranging from a few microns to a few nanometers and with thicknesses of many carbon layers down to single-walled structures. The unique structure of these nanotubes gives them advantageous behavior relative to properties such as electrical and thermal conductivity, strength, stiffness, and toughness. Carbon nanotubes can also be functionalized with molecular recognition agents so that they may bind specifically to discrete molecular targets, allowing them to be used as high resolution AFM probes, as channels for materials separation, and as selective gates for molecular sensing.
Like nanotubes, nanowires are very small rods of atoms, but nanowires are solid, dense structures, much like a conventional wire. Controlling the atom (material) used for building the wire, as well as its impurity doping , allows for control of its electrical conduction properties. Ultimately, chemists wish to fabricate and control nanowires that are a single atom or molecule in diameter, thus creating an unprecedented laboratory for studying how small structures affect electron transfer within the wire and between the wire and external agents. Clearly, nanowires offer the potential for creating very small IC components.
Nanocomposites encompass a large variety of systems composed of dissimilar components that are mixed at the nanometer scale. These systems can be one-, two-, or three-dimensional; organic or inorganic; crystalline or amorphous. A critical issue in nanocomposite research centers on the ability to control their nanoscale structure via their synthesis . The behavior of nanocomposites is dependent on not only the properties of the components, but also morphology and interactions between the individual components, which can give rise to novel properties not exhibited by the parent materials. Most important, the size reduction from microcomposites to nanocomposites yields an increase in surface area that is important in applications such as mechanically reinforced components, nonlinear optics, batteries, sensors, and catalysts.
Lab on a Chip
Lab-on-a-chip devices are designed to carry out complex chemical processes at an ultrasmall scale, for example, synthesizing chemicals efficiently; carrying out biological, chemical, and clinical analyses; performing combinatorial chemistry; and conducting separations and analysis on a single, miniaturized device. When the amount of material in a sample is small or when it is highly toxic or dangerous, lab-on-a-chip devices offer an ideal way to complete complex chemical manipulations with extremely small sample sizes. Further, because the volumes used to carry solutions are extremely small, even very small sample amounts can be present in reasonable concentrations. Lab-on-a-chip technology has been aggressively pursued in biotechnology, where better ways to separate and analyze DNA and proteins are of great interest. It has also sparked great interest in the analysis of dangerous materials where it can be used, for example, by law enforcement or the military to analyze explosives and biological or chemical agents, while maintaining low risks.
Nano-electro-mechanical systems have also generated significant interest in the creation of tiny devices that can use electrochemical energy to carry out mechanical tasks, for example, nanomotors. One can envision that the coupling of chemical energy to mechanical transducers will enable the construction of devices that may be applied in medicine to treat illnesses, explore dangerous areas, or just reach places that larger-scale devices cannot. Research in this area focuses on understanding the preparation of nanoscale components to build such devices as well as the interactions between the components, especially the coupling between the electrochemical and mechanical components. In addition, a new understanding of effects such as friction and wear is required as the nanoscale components obey a different set of rules than their macroscopic counterparts.
THE SCANNING TUNNELING MICROSCOPE AND ATOMIC FORCE MICROSCOPE
The scanning tunneling microscope (STM) and the atomic force microscope (AFM) are very high resolution microscopes that allow scientists to obtain high resolution images of surfaces with atomic or molecular resolution. Both microscopes work by scanning a very sharp tip on a surface and measuring current (STM) or intermolecular forces (AFM) between the tip and the surface.
In 1965 Gordon Moore, cofounder of Intel, predicted the trend that the number of transistors on integrated circuits (IC) was going to follow. The number of transistors in IC has grown exponentially, with the implication that the size of such transistors has decreased in a similar fashion. His law stills hold, but it is predicted that the law will not be applicable by the year 2012 as further shrinking of transistors will not be possible after that time.
see also Computational Chemistry; Fullerenes; Materials Science.
Diego J. Díaz
Paul W. Bohn
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