Environmental Engineering Reference
In-Depth Information
Figure 1.4
SchematicpictureforNEMDsimulationofSiNWs.Thelongitudi-
naldirectionissetalong
x
-axis.Thegreenarrowspointtheboundaryatoms.
Atoms in red and blue boxes are put in contact with the heat bath at high
temperature
T
H
and low temperature
T
L
, respectively.
particles at two ends [30] or scaling the velocity of the particles
at two ends [31]. This approach has the advantage that the slowly
converging quantity
heat flux
is now known exactly and need not to
be calculated. As a result, total simulation time can be reduced as
only the relatively fast converging quantity
temperature
needs to be
calculatedinthesimulation.However,thedrawbackofthisapproach
is obvious: it is not consistent with the actual physical picture of
heat conduction. Moreover, the heat flux is artificially introduced
and there is no fundamental theoretical basis for it. As a result, the
dynamics of the whole system is not completely governed by the
Hamiltonian.
Therefore, the first approach in NEMD simulation is employed
throughout this chapter. Figure 1.4 shows the schematic picture
for NEMD simulation of silicon nanowires (SiNWs). Here we set
longitudinal direction along
x
-axis, and atoms in the same layers
means they have the same
x
-coordinate. Fixed boundary condition
(BC) is imposed on the boundary layers (indicated by arrows in
Fig. 1.4) at two ends of SiNWs and the surface atoms, either fixed
or free BC. Next to the boundary layers, several layers of SiNWs
are put in contact with the heat bath (inside the rectangular box
in Fig. 1.4), which can reproduce the canonical ensemble (i.e.,
constant temperature). To simulate the heat source and sink in
real experiment, two heat bathes at high temperature
T
H
and low
temperature
T
L
are used.
