Environmental Engineering Reference
In-Depth Information
shown the basic theory and numerical implementation of the NEGF
method. Some representative applications of the NEGF method will
be shown to demonstrate the powerfulness of this theoretical tool
in exploring quantum thermal-transport phenomena. Graphene-
based transport systems are very suitable for such purpose
because of their simplicity and rich experimental data. The NEGF
study will focus on thermal transport in quasi-1D systems whose
size perpendicular to the transport direction is finite. This, to
some extent, differs from most experiments whose samples have
dimensions of macroscopic scale. However, results of wide enough
systems can be used to compare with those experiments. An
additional advantage of the quasi-1D study is that by systematically
enlarging the system size, size effects and quantum confinement
effects, which for experiments are challenging to explore, can be
investigated.
At first we will systematically investigate thermal transport in
GNRs and discuss the size effects and influence of edge shape
on thermal-transport properties. Then we will discuss the origin
of extremely high thermal conductivity in graphene. The NEGF
calculations show that it is the very long phonon mean free path
rather than extraordinary high ballistic thermal conductance that
gives rise to superiorly high thermal conductivity in graphene.
Finally we will systematically study thermal-transport properties
of graphene-based nanodevices. The obtained structure-property
relationsofferguidelinestothedesignofgraphene-basedelectronic,
thermal, and thermoelectric nanodevices.
3.4.1
Graphene Nanoribbons
AsthefirstapplicationexamplefortheNEGFmethod,weinvestigate
thermal-transport properties of pristine GNRs. GNRs are the build-
ing blocks of graphene-based nanoelectronics. The experimental
production of high-quality GNRs with controlled shape and width is
nowfeasible,forinstance,bygraphitenanotomyprocess,whichuses
sharp diamond knife to cut graphite into graphite nanoblocks that
are then exfoliated into GNRs [35], or by longitudinal unzipping of
CNTs [36, 37]. Compared to extensive study on electronic transport,
much less attention has been paid for understanding thermal
