Page 139 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Methods for Structural and Chemical Characterization of Nanomaterials 125

to coating the sample is mounted on a ring stand using nonconducting
carbon tape. In many instances, as with most common forms of SEMs,
it is necessary that the measurement be done under high vacuum. This
requires that the sample be dry in order to prevent off-gassing during
the measurement. However, recent advances in the design of SEM meas-
urement chambers has led to the development of environmental (ESEM)
and cryogenic SEMs, for imaging wet and frozen or fixed samples,
respectively. As opposed to conventional SEM, in ESEM the sample
may be both wet and does not need to be conductive. It is therefore
desirable for delicate biological samples. In cryo-SEM and cryo-TEM the
sample is frozen or fixed using liquid nitrogen and transferred to a cryo-
preparation chamber that is held in vacuum. In the case of SEM meas-
urements, a thin conductive coating is usually applied to allow
high-resolution imaging or microanalysis in the SEM. Transfer to the
SEM/TEM chamber is via an interlocked airlock and onto a cold stage
module fitted to the SEM/TEM stage.
Materials that are appropriate for TEM analysis are constrained to
very thin samples (1000–2000 Å), but do not require the presence of a
conducting layer as in conventional SEM. However, a high vacuum is
required and is accompanied by the aforementioned constraints.
Nanoparticles are particularly viable for study using TEM as they are
appropriately thin and may be imaged using TEM support grids. TEM
support grids are fine mesh supports that are commonly made of copper,
which may be covered with a range of materials (e.g., carbon, Formvar,
, etc.). Particles are either deposited through evaporation or
holey, SiO 2
through electrostatic attraction using positively charged grids. The
grid/sample must be allowed to dry prior to imaging to prevent off-
gassing once the sample is placed in the vacuum. When examining par-
ticle samples it is important to avoid aggregation during the drying
step, which will inhibit analysis of individual particles.
Application in the particular case of nanoparticles. High-resolution TEM (HRTEM)
may be employed to provide extremely valuable information about the
atomic structure of nanoparticles. For instance, Marın-Almazo et al.
[2005] used HRTEM to determine the atomic arrangement of rhodium
nanoparticles (d 1.8 nm). The authors found that for these nanoparti-
cles the [111] and [200] inter-planar distances corresponded to large
minerals indicating that no distortion of the network existed. However,
for the smaller clusters (below 1.5 nm) some range stacking faults, dis-
locations, and twins were identified (as illustrated in Figure 4.13).
In another study by Yan et al. [2005] it was found that by coupling XRD
analysis (Sherer equation) and TEM imaging, it was possible to deter-
mine the structure and size of ultra-small gold nanoparticles (d = 1 nm).
TEM can also be a powerful characterization technique for studying the
dispersion and chemistry of nanoparticles in the environment. For