Biomedical Engineering Reference
In-Depth Information
8.2.1 Controlled Shrinking by e-Beam or Laser Heating
First a brief recount of the history will be discussed on the methods developed at
Delft using a transmission electron microscope (TEM). It was first discovered by
Jianghua Chen and Henny Zandbergen in mid 2002 in the SiO
2
/Si
3
N
4
membranes
prepared by Arnold Storm in Cees Dekker's group at Delft, that under the electron
beam (or e-beam) of a TEM, a pore often spontaneously shrinks all the way to
complete closure. It was quickly realized that this is a typical surface-tension driven
mass flow effect: the electron beam fluidizes the amorphous materials which flows
under the force of surface tension. As shown in Fig.
8.1
, the pore shrinks as a
function of time during the TEM imaging process. The TEM images in Fig.
8.1
are
time-lapsed (from left to right). The average diameter of the pore decreases almost
linearly with time. The TEM used was a commercial transmission electron micro-
scope (TEM), Philips CM-30UT, operated at an accelerating voltage of 300 kV.
A good level of vacuum in the microscope was maintained, at ~ 10
7
mbar, such
that one can rule out the pore closing as being an effect of carbon contamination.
Several features of the observed effect can be understood using the Laplace's
law of surface tension. As shown in a simplified model in Fig.
8.2
, once the glassy
silicon oxide material is fluidized by high energy electrons in a TEM, the material
Fig. 8.1 (
Top
) Time-lapsed TEM images of a SiO
2
nanopore; (
Lower
) The average diameter of
the pore vs. time with the pore region being exposed to the electron beam under a TEM [
6
]. Used
with permission. Copyright Nature 2004
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