Introduction - Implications of Emerging Micro-and Nanotechnologies - page 7

has turned to quantum dot structures that have all three dimensions in this regime.
In some sense, these are “designer” atoms and molecules that can be engineered
to provide the needed functionality. Another example is the wavelength of visible
light, which is 400 to 800 nanometers. When periodic structures are created in
optical materials at these dimensional scales by varying the dielectric constant,
the propagation of light can be strongly influenced in analogy to electrons in
semiconductors. While these properties are only now being explored, the possi-
bilities include confining and steering light down to unprecedented small scales
and creating low loss-optical devices such as near-thresholdless lasers.
The second factor is a consequence of the large surface areas and unique
chemical reactivity of nanostructures. This is the basis for much of the excitement
at the juncture of nano- and biotechnologies. The information stored in the ge-
nome and the exquisite selectivity of biochemical interactions based on chemical
recognition and matching are examples of nanoscale properties where the inter-
faces play a determining role. Nanoparticles have size-dependent chemical and
electronic structure, reactivity, etc. that can be exploited to produce improved
catalysts as well as electronic, magnetic, optical, and biomaterials.
Materials constituted of nano particles are different from bulk materials and
different from molecules. An easy characterization is to say that nanoscale ob-
jects contain a large (more than a simple molecule) but countable (for example a
box of 100 atoms on a side containing 1 million atoms) number of atoms. With
our increasing ability to fabricate structures with well-defined nanoscale features,
new materials are emerging that promise both evolutionary and unexpected new
properties. Another major thrust of nanoscale research is integration, where the
aim is to preserve the unique properties of nanoscale structures as they are incor-
porated into macroscopic objects.
Nanotechnology is generally anticipated to require a fundamentally different
approach to fabrication than microtechnology. Whereas microscale structures are
typically formed by top-down techniques such as patterning, deposition, and
etching, the practical formation of structures at nanoscale dimensions will require
an additional component—bottom-up self-assembly. This is the process whereby
structures are built up from atomic- or molecular-scale units into larger and
increasingly complex structures—as is widely used by biological systems. In
practice some combination of top-down (lithographic) and bottom-up (self-as-
sembly) techniques likely will be necessary for the efficient manufacturing and
integration of nanoscale systems. Many tools now exist for investigating struc-
ture and properties at the nanoscale, including scanning tunneling probes, elec-
tron microscopies, and various diffraction techniques. An important development
in nanoscale tools occurred in 1981 with the introduction of the scanning tunnel-
ing microscope for imaging individual atoms on surfaces. This development,
which earned Bennig and Rohrer the Nobel Prize in Physics, allowed the imaging
and manipulation of single atoms and set the stage for an entire family of scan-
ning microscopy, with atomic force microscopy being the most widely used.