Environmental Engineering Reference
In-Depth Information
n
-ZGNR and
m
-ZGNR (
n
<
m
is assumed without loss of generality),
are selected to construct single-interface junction, double-interface
junction and QD, which are labeled as “Z
n
-Z
m
”“Z
m
-Z
n
-Z
m
”“Z
n
-Z
m
-
Z
n
” for convenience. Then
n
and
m
representatively correspond to
thewidthsofthenarrowandwidepartsofthestructures.Thelength
of the central GNR is fixed to be as long as shown in Fig. 3.15a. Our
calculations show that further increasing its length has negligible
influence on the calculated thermal conductance.
Previous studies reveal that varying the ribbon width strongly
affects the performance of graphene-based electronic nanodevices.
For instance, the room temperature on/off ratio of GNR FET
induced by gate voltage increases exponentially as the ribbon width
decreases [56]. We find that thermal-transport behavior has a more
complicated dependence on the ribbon width. Varying the width
of wide and narrow part affects thermal transport in a distinctly
different way. As presented in Fig. 3.15b, thermal conductance of
the single-interface and double-interface junctions is insensitive
the width change in the wide part. This is also the case for the
QD structure except when the width of the wide center part close
to that of the two narrow neighboring parts. The wide part in
the QD structure acts as a protrusion along the GNR. Once the
protrusion disappears, thermal conductance will show a noticeable
increase. In contrast, thermal conductance of all the three types of
graphene structures strongly dependent on the width of the narrow
part. Thermal conductance of the single-interface and double-
interfacejunctionsisalmostlinearlyproportionaltothewidthofthe
narrow part, while that of the QD approximately shows a quadratic
dependence,aspresentedinFig.3.15b.Sincethenumberofphonon
transport channels is limited by the narrow part, decreasing its
width will remarkably reduce thermal conductance of the whole
system.
Due to interface scattering, thermal conductance of the three
graphene structures is always lower than that of the corresponding
pristine narrow GNR (i.e.,
n
-ZGNR) as demonstrated in Fig. 3.15b.
ThermalconductanceofGNRsincreasesapproximatelylinearlywith
the ribbon width. Due to this feature of strong width-dependence,
thermal conductance itself is not suitable to measure the thermal-
transport ability of a transport system. Instead, to quantify the
