Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 27

cracks nanowire array

metal

SiO 2

Si

Fig. 1.24 Fabrication of a metal nanowire array by self-assembly

eliminate the OH impurities, process in which tensile stress develops in the film.
As a result, the film cracks, the nanoscale cracks being able to extend up to the
Si substrate. Finally, a metal is deposited in the cracks, and wet etching is used to
remove the oxide. The geometry of the cracks depends on the type of the generated
stress. For instance, a parallel array of cracks, and thus a parallel array of metal
nanowires, as displayed in Fig. 1.24 , is obtained due to an uniaxial stress.

1.2.3 Nanomaterials

The strength of bionanoelectronics resides in the fact that one or more dimensions
of the nanoscale materials, termed as nanomaterials, are of the same size as bacteria,
viruses, or biomolecules, such as DNA, from which the entire living matter is made
( Gruner 2006 ). In what follows, we present briefly the main nanomaterials. The
interaction of these nanomaterials with living matter is the main subject of the topic.
Nanoparticles are frequently used in bionanoelectronics, especially for targeted
drug delivery and sensing of biomolecules. Nanoparticles having few nm in
diameter can be metallic, semiconducting, or even isolators, but their physical
properties depend strongly on their size and differ dramatically from those of the
bulk material from which they are made. A typical example is the transformation
of the conduction band of bulk metals in discrete energy levels in a metallic
nanoparticle, which resembles a quantum dot described in Sect. 1.1 . The discrete
nature of electronic states in a metallic nanosized particle is described by the average
spacing between adjacent quantum levels, called Kubo gap:

ı D 4E F =3n; (1.29)

where n is the number of electrons in the nanoparticle and E F denotes the Fermi
energy level. The nanoparticle behaves as a metal, i.e., the Kubo gap is not apparent,
if k B T>ı. For instance, an Ag particle with a 3-nm diameter is metallic at ambient
temperature because k B T Š 25 meV for T D 300 K, while ı D 10 meV. Almost all
metal nanoparticles (Au, Ag, Pd, Ni, Cu) are fabricated via evaporation of the bulk
metal counterpart in vacuum.