Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 44

electrodes

I

I

water drop
solution
containing
DNA

V

V

nylon

Fig. 1.43 The gap electrode configuration for circular DNA in water

I (mA)

hopping

6

4

hump
(tunneling)

2

thermionic emission-diffusion

1 1.5 2

V (V)

Fig. 1.44 Conduction mechanisms in circular DNA in water

300 meV. However, a hump located at V h D 1:5 V in the curve, and present in
nearly all samples, cannot be explained by the above equation (Fig. 1.44 ).
As pointed out above, the transport in DNA and in general in any organic
molecule is the result of several different charging mechanisms. The hump is
explained by the potential barrier that the charges must tunnel through inside the
DNA molecules. For a rectangular barrier, the tunneling current is given by

I Š exp.2' T eV / 1=2 ;

(1.42)

where T is the barrier energy, which has values between 2.5 and 4.5 eV in different
samples, consistent with the gap between HOMO and LUMO.
At low voltages, the effects of the contact between DNA and gold become visible
in the I V characteristic, and the dominant transport mechanism is the thermionic
emission-diffusion process expressed by the formula