Nanomedicine - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 135

wireless
signal

cantilever

vacuum

detected
current

Fig. 4.11 The nanotube radio

CNT

V

d

I F el

h

V G

Fig. 4.12 The tunneling nanoradio

The resonant mechanical oscillations of the CNT, which are produced by radio
waves with the same frequency, are detected by collecting the electron tunneling
current I across the air gap of width d formed between the CNT and another
electrode. The CNT resonant frequency can be tuned in a broadband range by the
electrostatic force F el acting between the CNT cantilever and a bottom electrode
on which a gate voltage V G is applied. For a gap of 5 nm, the current is of few
A at voltages not exceeding 10 V. If the CNT cantilever resonates at a high
frequency, around 300 MHz, we can detect only the average value of the current,
I m D T 1 R T

0 I.t/ dt ,whereT is the period of oscillations. The fraction I m =I 0 ,
where I 0 is the off-resonance current, for which the CNT does not vibrate is 0.91
for oscillation amplitudes, as small as d=10. Thus, the tunneling nanoradio is very
sensitive to the radio signal to be detected.
Moreover, the tuning mechanism is reversible and broadband. It is based on the
equivalence of the electrostatic actuation of CNT by the bottom gate electrode to an
increase of its effective mass. Considering that the mechanical resonance frequency
of a CNT with mass m, length L, moment of inertia I, and elastic modulus E is

r EI
m ;

ˇ 2
2L 2

f 0 D (4.4)

where ˇ is a constant parameter, a shift f D f f 0 in the resonance frequency
required to tune the radio can be caused by a change in the CNT effective mass.