Environmental Engineering Reference
In-Depth Information
It would be interesting to compare thermal contact resistance
among the three types of graphene structure. The single-interface
junction always gives lower thermal contact resistance than its
double-interface counterparts. This is easy to understand because
an extra interface exerts additional phonon scattering. The double-
interfacejunctionandtheQDstructuresharetwosimilarinterfaces,
however, their thermal contact resistance differs significantly. The
QDhasobviouslylargerthermalcontactresistancethanthedouble-
interface junction. This is unexpected in the classical picture. When
thermal resistances are connected in series, they are additive. If
using the pristine
n
-ZGNR as reference system, introducing an
interface produces additional thermal contact resistance, which
is given by the thermal contact resistance difference between
the single-interface junction and the reference system. Thus, an
interface in fact acts as a thermal resistor. Two of them are placed
in series in the double-interface junction and QD. The increase of
thermal contact resistance with respect to the reference system
in the double-interface junction/QD, however, is not equal to but
lower/higher than two times that in the single-interface junction.
The conventional additive law fails, since quantum interference
effect plays an important role in transport [4]. Transport properties
cannotbe determined simply by the numberof interfaces.
We further analyze the phonon transmission function to under-
stand the differences in thermal conductance and thermal contact
resistance. Transport systems composed of 8-ZGNR and 18-ZGNR is
selected for our following discussion without losing generality. As
shown in Fig. 3.16a, phonon transmission function of the single-
interface junction is slightly that of the narrow GNR, but largely
lower than that of the wide GNR (data not shown here). This
resultindicatesthatphononsfromthenarrowGNRexperiencelittle
scattering when transporting across the interface into the wide
GNR, while phonons from the wide GNR are largely scattered as
they transport into the narrow GNR. In other words, the average
transmission per mode is strongly direction-dependent, due to
the asymmetry structure of the single-interface junction. This can
qualitatively explain the different transport properties in the three
types of graphene structures. Comparing with the single interface
junction, the double-interface junction and the QD have one more
