Imaging and Manipulation of Biomolecules - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 115

I

I

z

V V

z

I

x

x

Fig. 3.6 Schematic representation of the STM operation modes: ( a ) constant current mode and
( b ) constant height mode

depends on the local DOS (LDOS) in the sample,
.r;E/ D X

2 ı.E E i /;

i j ‰ i . r / j

(3.19)

at the Fermi edge. In ( 3.18 ), ‰ i . r / is the 3D wave function of the sample at energy
E i . From this equation, it follows that the normalized conductance obtained from
the I V curve, and defined as .dI=dV/=.I=V/, is proportional to the LDOS in
the sample.
STM has two principal modes of operation, displayed in Fig. 3.6 . In the constant
current mode, the tunneling current is detected when the biased tip is located near
the surface. The tip scans the surface of the sample, and any variation in the
tunneling current is assessed by a feedback loop, which modifies the height z in
order to keep the current constant. On the contrary, in the constant height mode,
information on the surface is obtained by directly measuring the current as the tip
scans the surface at a fixed height.
STM systems are working at room temperature and in normal atmospheric
pressure, and are integrated with complicated signal processing units needed for
calibrating the instrument and for removing the thermal noise. Other STM systems
work at low temperatures (350 mK), in ultrahigh vacuum (UHV), and in high
magnetic fields (11 T) because these extreme conditions grant an extremely high
resolution and a clean environment necessary to investigate nanostructures and
quantum processes. Examples of systems currently studied with STM in UHV and at
low temperatures include liquid crystals, single molecules, DNA sequencing, LDOS
of metals, superconductors, and semiconductors. STM can analyze the magnetic
properties of some materials if equipped with magnetic tips, for instance, Fe tips.