Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 45

Fig. 1.45 The DNA thin film
Schottky contact diode

gold

DNA film

n- Si

I Š exp

;

n' b eV
nk B T

(1.43)

where b is the potential barrier between electrodes and sample and n Š 18 is a
system-dependent constant.
So there are three conduction mechanisms which can be retrieved in a single
experiment on circular DNA dispersed in water. In Fig. 1.44 , we have displayed
the regions of the current-voltage dependence in which each of these conduction
mechanisms dominate. From fitting data analysis, the b parameter is determined to
be in the 400-500-meV range.
DNA thin films sandwiched between a n-doped Si substrate and a gold top
contact display rectifying characteristics. The thickness of the thin film containing
DNA extracted from wheat leaf tissues is about 19 nm, and the Au/DNA/n-Si
structure behaves like a Schottky diode with an ideality factor of 1.22 and a barrier
height of 0.78 eV ( Sonmezolu et al. 2010 ). So, in the case of DNA thin films, the
thermionic transport mechanism is dominant, and a Schottky-like diode with good
performances can be obtained.
We can see from the above examples that there are two basic methods to connect
biomolecules to metallic electrodes. One method is the top contact, as in the case of
STM tip-(nanoparticle-DNA) electrodes in Fig. 1.40 or the DNA Schottky diode in
Fig. 1.45 . The other method consists of the nanogap electrodes, as in the case of the
DNA molecule in Fig. 1.36 formed from 30 base pairs of poly(G)-poly(C), which
displays a semiconducting behavior.
Nanogap electrodes are mostly used when one or few molecules must be
connected to metallic contacts. Because the nanogap electrode has a planar con-
figuration, this can be used for further integration with other electronic devices.
However, it is quite difficult to fabricate gap widths of 1-8 nm, but many methods
recently reviewed in Li et al. ( 2010 ) are presently able to produce a nanogap
electrode or an array of nanogap electrodes. For example, FIB is a maskless method
able to create nanogaps with dimensions between 10 and 20 nm in combination with
EBL and optical lithography. The two electrodes of the nanogap are the drain and
source of a transistor, and between them a CNT, a nanowire, or graphene can be
deposited. For the subject treated in this topic, nanogap electrodes are bridged by
biological molecules. Conversely, nanogap electrodes can be fabricated over CNTs
or nanowires, which are used as channels for ballistic field-effect transistors (FETs)
where the two arms of the nanogap act as drain and source. In these cases, nanogap