Biomedical Engineering Reference
In-Depth Information
Fig. 1.5 Al
2
O
3
nanopore formation kinetics [
90
], reprinted with permission (a) Intensity profiles
of the focused electron probes used during nanopore formation in Al
2
O
3
thin films. Intensity is
normalized with respect to peak intensity of the 3.9 nm
FWHM
probe (Inset) TEM image of a
3.2 nm probe showing spatial intensity distribution (b) Nanopore sputtering kinetics illustrating
pore diameter vs. time for the various probe sizes examined. Distinct expansion rates were
observed delineating the three stages of pore formation, I Pore Nucleation (not shown), II Rapid
Expansion and III Controlled Growth
confirms that an irregular density of exposed Al-O groups exists at the pore surface
which in turn corresponds to an irregular surface charge distribution in a hydrated
Al
2
O
3
nanopore. This irregular charge distribution is expected to strongly impact
DNA translocation kinetics. Prolonged exposure to the electron beam resulted in a
polycrystalline structure with preferred
phases only [
90
]. These results
provide evidence that the surface charge in the nanopore can be engineered based
on critical electron doses. Electron beam induced crystallization was not observed
in Si
3
N
4
and SiO
2
systems [
89
].
a
and/or
g
1.3.3.3 Variations in the Nanopore Stoichiometry
Wu et al. demonstrated that the electron beam inadvertently modifies the stoichi-
ometry of the nanopore [
97
,
98
]. Nanopores formed in ~60 nm thick SiO
2
/SiN/SiO
2
membranes through electron beam sputtering, induced the formation of Si rich
clusters in the vicinity of the nanopore as confirmed through electron energy loss
spectroscopy (EELS) and energy filtered TEM (EFTEM). O and N were preferen-
tially sputtered away in this case. Electron beam induced sputtering also has a
dramatic impact on the composition of Al
2
O
3
based nanopores. EELS analysis
confirmed that Al-rich regions were formed at and near the pore edge due to
the preferential desorption of O [
90
]. Compositional variations were calculated
by the
k-factor
method [
24
], and revealed that the O to Al ratio in the local nanopore
region decreased from 1.5 before pore formation to ~0.6 after pore formation. This
result confirms that the sputtering process preferentially desorbs oxygen, leaving
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