Chemistry Reference
In-Depth Information
Self-assembly structures at interfaces.
Semiconductors.
Depending on the sensitivity and experimental conditions, the methods of molecular
microscopy are many and varied. The applications of these microscopes are also extensive,
as with crystal structures and the three-dimensional configurations of macromolecules.
The greatest application of microscopy is in the case of surfaces and the study of
molecules at the surfaces. Generally, the study of surfaces is dependent on under-
standing not only the reactivity of the surface but also the underlying structures that
determine reactivity. Understanding the effects of different morphologies may lead
to a process for the enhancement of a given morphology, and hence, to improved
reaction selectivities and product yields.
Atoms or molecules at the surface of a solid have fewer neighbors as compared
with atoms in the bulk phase, which is analogous to the liquid surface; therefore,
the surface atoms are characterized by an unsaturated, bond-forming capability and,
accordingly, are quite reactive. Until a decade ago, electron microscopy and other simi-
larly sensitive methods provided information about the interfaces. There were always
some limitations inherent in all these techniques, which needed improvement.
A few decades ago, the best electron microscope images of globular proteins
were virtually little more than shapeless blobs. However, these days, due to relentless
technical advances, electron crystallography is capable of producing images at reso-
lutions close to those attained by x-ray crystallography or multidimensional nuclear
magnetic resonance (NMR). In order to improve upon some of the limitations of
the electron microscope, newer methods were needed. A decade ago, a new proce-
dure for molecular microscopy was invented and will be delineated herein. The new
scanning probe microscopes not only provide new kind of information than hitherto
known from x-ray diffraction, for example, but these also open up a new area of
research (such as in nanoscience and nanotechnology).
The basic method of these scanning probe microscopes (SPMs; Birdi, 2002a) was
essentially to be able to move a tip over the substrate surface with a sensor (probe)
with molecular sensitivity (nanometer) in both the longitudinal and height direction
(Figure 10.1). This may be compared with the act of sensing with a finger over a sur-
face, or more akin to the old-fashioned record player with a metallic needle (a probe
for converting mechanical vibrations to music sound) on a vinyl record.
Scanning probe microscopy was invented by Binnig and Rohrer (Nobel Prize,
1986) (Birdi, 2002a). Scanning tunneling microscope (STM) was based on scanning
a probe (metallic tip) since it is a sharp tip just above the substrate, while monitoring
Sensor
Sample
FIGure 10.1
Basic principle of a scanning probe microscope (SPM).
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