Fundamentals of Hierarchical Nanostructures - Hierarchical Nanostructures for Energy Devices - page 12

the performance enhancement. Therefore, the aforementioned unique
electrical properties observed in nanomaterials are becoming very important
in micro/nano electronics fabrication development. There are several
appealing features for this ''bottom-up'' approach to nanoelectronics. 19
First, the size of the nanowire building blocks can be readily tuned to
sub-100 nm and smaller, which should lead to a high density of devices on
a chip. Second, the material systems for the nanowires are essentially
unlimited, which should give researchers great flexibility to select the
right materials for the desired device functionality. It is obvious that great
progress has been made along the direction of using nanowire building
blocks for various device applications. Nevertheless, one has to admit that
ultimately achieving the goal of ''bottom-up'' manufacturing in the future
will still require substantial work, including, for example, the development
of 3D hierarchical assembly processes, as well as the improvement of
material synthesis.

d n 3 r 4 n g | 7

2.2.3 Phonon Transport (Thermo-electric Properties)
While electron transport in nanomaterials has been extensively studied,
investigation of phonon transport in nanostructures was relatively less
studied until very recently even though electron transport and phonon
transport are closely connected. As the dimension of a 1D nanostructure is
reduced to the range of phonon mean free paths (MFPs), the thermal con-
ductivity will be reduced due to scattering by boundaries. 19 Theoretical
studies suggested that as the diameter of a silicon nanowire became smaller
than 20 nm, the phonon dispersion relation might be modified (as a result of
the phonon confinement) such that the phonon group velocities would be
significantly less than the bulk value. 40,41 Molecular dynamics (MD) simu-
lations also showed that the thermal conductivities of Si nanowires could be
two orders of magnitude smaller than that of bulk silicon in the temperature
range from 200 K to 500 K. 42 The reduced thermal conductivity is desirable
in applications such as thermoelectric cooling and power generation, but is
not preferable for other applications such as electronics and photonics.
Dresselhaus and co-workers 43,44 have theoretically predicted that the
thermoelectric figure of merit could be substantially enhanced for thin
nanowires by carefully tailoring their diameters, compositions, and carrier
concentrations. This prediction still needs to be validated experimentally
by measuring the thermal conductivities, Seebeck coecients, carrier
mobilities, and electrical conductivities of different nanowire systems. Good
thermoelectrical systems include nanowires made of Bi, BiSb alloy and
Bi 2 Te 3 . 45,46 Recently, Si/SiGe superlattice nanowires, 47 Si nanowires 48 and
rough Si nanowires 49 have expanded the list of potential high performance
thermoelectric materials with high ZT values. As shown in Figure 2.2,
Si nanowires added another interesting twist to this direction of work,
noting the possible interfacial phonon scattering within such highly
complex 1D nanostructures.

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