Biomedical Engineering Reference
In-Depth Information
other biological constituents are detected by AFM, but early detection of diseases
such as the aging cartilage and osteoarthritis can be achieved with the help of the
AFM (
Stoltz et al. 2009
).
3.2
Bioapplications of Scanning Tunneling Microscopy
Scanning tunneling microscopy (STM) is also able to offer images of surfaces
z
.x;y/ with atomic resolution, including images of individual surface atoms. The
STM technique relies on measuring the tunneling current between a sharp metallic
tip separated from a conductive sample by nanometric distances. By applying a bias
V between the tip and the sample, the electrons in the tip with energy E and mass
m tunnel through a vacuum barrier of height >Eand arrive at the sample (see
Fig.
3.5
). At low voltages, the tunneling current density (
Hansma and Tersoff 1987
)
is approximated as
i
D
V
surf
.E
F
/ exp
f
Œ
2m.
E/
z
=
„g/
V
surf
.E
F
/ exp.
1:025
1=2
z
/ (3.17)
where
surf
.E
F
/ is the density of states (DOS) of the sample at the Fermi edge and
z
is the tip-sample distance, i.e., the barrier width. The current drops with one
order of magnitude if the width
z
of the barrier, with a height of 5 eV, changes
with only 0.1 nm. In STM mode, the current variation is generally 2-3%, which
implies that the gap changes with about 0.001 nm. Under these circumstances, the
tunneling current maps the surface DOS. These considerations are accurate only in
one dimension. In three dimensions, surface mapping by means of the tip-surface
tunneling current is much more complex, and the full Tersoff-Hamann model must
be applied to describe the STM (
Hansma and Tersoff 1987
). The main findings of
this theory is that the tunneling current,
Z
E
F
CeV
I.V/
/
.r;E/dE;
(3.18)
E
F
barrier
tunneling
current
φ
E
F
E
F
-
V
tip
sample
z
Fig. 3.5
The STM energy diagram
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