Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 32

polar region

nonpolar
region

rotation
towards
polar region

gold surface

Fig. 1.30 The principle for aligning millions of carbon nanotubes by self-assembly

coated with nonpolar or polar groups. Because the CNTs in liquid suspension are
attracted by the polar regions only, millions of CNTs align in less than 10 s, as
illustrated in Fig. 1.30 . The CNTs are rotated by the electrostatic attraction force in
the direction of the polar region and are immobilized only in this region (Fig. 1.30 ).
Graphene is the most recent nanomaterial and has already attracted attention
due to its unusual physics and potential applications, as demonstrated by awarding
the Nobel Prize for physics in 2010 to A. Geim and K. Novoselov ( Geim and
Novoselov 2007 ) for groundbreaking research in this domain. Graphene is a
monolayer sheet of graphite, with a thickness as small as 0.34 nm, consisting
of carbon atoms in a sp 2 hybridization state, in which each atom is covalently
bonded to three other atoms forming a honeycomb lattice. This lattice can be
understood as consisting from two interpenetrating triangular sublattices. Graphene
is at the origin of many carbon-based materials. For instance, graphite is formed
by stacking millions of graphene layers, and single-walled carbon nanotube forms
when graphene rolls up along a certain direction.
Graphene is a planar crystal and a natural 2D gas of charged particles. In its basic
configuration, graphene is deposited on a 300-nm-thick SiO 2 layer grown on top of
an n C silicon substrate. Only in this case it can be seen at an optical microscope.
The wavelength at which graphene can be seen and its type, monolayer or bilayer
graphene flakes, is determined by filtering a white light source and depends strongly
on the SiO 2 thickness ( Blake et al. 2007 ): for a 300-nm-thick SiO 2 layer graphene is
optimally discriminate in green light, while blue light is most favorable for a SiO 2
layer with a thickness of 200 nm.
In the basic configuration mentioned above, containing a doped Si substrate,
silicon is acting as gate which, upon applying a gate voltage V g , controls the surface
charge density n according to the expression ( Novoselov et al. 2004 )

n D " 0 " d V g = te :

(1.30)

In ( 1.30 ), " 0 and " d are the dielectric permittivities of air and SiO 2 , respectively,
and t denotes the thickness of the SiO 2 layer. The carriers induced by the gate
voltage exemplify the electrical doping effect, which is analogous to the chemical