Chemistry Reference
In-Depth Information
diameters are suciently small or the bonding between constituent atoms is
too weak.
19
2.2.2 Electrical Properties (Electron Transport)
A reduction in the dimension to below a certain critical size (i.e. electron de
Broglie wavelength) will result in a change of electronic structure and thus
the electrical properties of nanomaterials. As the critical dimension of an
individual device becomes smaller, the electron transport properties of its
components become an important issue to study.
19
Studies from a number
of groups indicate that some metal nanowires might undergo a transition to
become semiconducting as their diameters are reduced below certain values.
For instance, two-probe measurements by Dresselhaus and co-workers
33
on
arrays of single-crystalline Bi nanowires indicated that these nanowires
underwent a metal-to-semiconductor transition at a diameter of
d
n
3
r
4
n
g
|
7
52 nm.
Two-probe measurements performed by Choi and co-workers
34
B
on
individual single-crystalline Bi nanowires of
40 nm in diameter showed
that these nanowires were semiconductors or insulators because their
resistances increased with decreasing temperature. As a result of quantum
confinement, it was proposed that the external conduction sub-band and
valence sub-band of this system moved in opposite directions to open up a
band gap in this particular confinement along the long axis of a wire and by
surface imperfection.
19
Gold is another metal whose electron-transport
properties have been extensively studied in the form of short nanowires as
thin as a single, linear chain of atoms.
35
Because these wires are extremely
short in length (usually a few atoms across, sometimes also referred to as
point contacts), their conductance has been shown to be in the ballistic
regime with the transverse momentum of electrons becoming discrete.
19
The transport phenomena (e.g. conductance quantization in units of
2e
2
h
1
) observed in this kind of 1D system are found to be independent of
material.
36
As for semiconductors, recent measurements on a set of nano-
scale electronic devices indicated that GaN nanowires as thin as 17.6 nm
could still function properly as a semiconductor.
37,38
In a related study,
transport measurements by Heath and co-workers
39
suggested that Si
nanowires with a thickness of 15 nm had become insulating.
The effects of size on electrical conductivity of nanostructures and nano-
materials are complex. The mechanism can be generally grouped into four
categories: surface scattering including grain boundary scattering, quantized
conduction including ballistic conduction, Coulomb charging and tunnel-
ing, widening and discrete band gap, and charge of microstructure. In
addition, increased perfection (reduced impurities, structural defects and
dislocations) would affect the electrical conductivity of nanostructured
materials.
20
Scaling down in microelectronics is very important to realize high per-
formance electronics. The ongoing miniaturization technology in electronics
is approaching the point where the fundamental issues are expected to limit
B
.
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