Page 108 - Science at the nanoscale

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Surfaces at the Nanoscale
98
has been observed directly with Scanning Tunneling Microscopy
(STM), and is discussed in Section 8.3.1.
Surface adsorption is another common way of reducing the sur- ch05
face energy. In this case, a foreign species sticks onto the sur-
face (i.e. adsorb), forming bonds or just weak electrostatic or van
der Waals interactions with the surface atoms. A good example
is the adsorption of hydroxyl groups (-OH) at the surface dan-
gling bonds of silicon wafers after treatment with Piranha solu-
2
tion, thus making the surface hydrophilic. In Chapter 7, we will
see that surface adsorption using specific molecules has been vig-
ilantly applied by chemists in the size-controlled synthesis and
isolation of nanoparticles from solutions.
During the preparation or processing of nanostructures, there
are several dynamic mechanisms that can occur to reduce the
overall surface energy of the system. In the most common situ-
ation, several nano-sized objects will associate together through
chemical or physical attraction at the interfaces. This agglomer-
ation into larger associations or clusters does not alter the indi-
vidual properties of the nanostructures. It will, however, give
rise to difficulty in re-dispersing the associated clusters in solu-
tions. Due to the huge surface energy incurred, attempts to pre-
pare nanostructures without appropriate stabilisation measures
are very likely to result in agglomerate formation. Hence, effec-
tively preventing agglomeration is one of the main considerations
in the preparation and handling of nanomaterials.
Individual nanostructures will merge into larger structures in
order to reduce the overall surface area in two other mechanisms,
sintering and Ostwald ripening. Sintering often occurs at elevated
temperatures, during which atoms at surfaces or grain bound-
aries undergo solid-state diffusion, evaporation-condensation
or dissolution-precipitation processes. The individual nanos-
tructures thus change their shapes when they combine with
each other, and this often results in a polycrystalline material
(Fig. 5.6(a)). Such a process has in fact been advantageously used
in the ceramic and powder metallurgy industries. 3
Ostwald ripening, on the other hand, will eventually produce a
single uniform structure with the larger nanostructures growing

2 Piranha is a mixture of sulphuric acid and hydrogen peroxide, used as a common
etchant in the microelectronics industry.
3 J. S. Reed, Introduction to Principles of Ceramic Processing, Wiley, New York, 1988.