Environmental Engineering Reference
In-Depth Information
The average bond length of diamond-like Si is 0.235 nm. For
naked surface SiNWs, our simulations show shortening of Si-
Si bonds (by 0.004 nm) in the surface layer compared with
the diamond-like case due to surface reconstruction. By contrast,
the inner part of the SiNWs retains the diamond lattice. This
remarkablesurfacelatticecontractioneffecthasalsobeenobserved
for the Si clusters by tight-binding MD (TBMD) simulations [50]
and by experiment [51]. Compared to naked SiNWs, passivation
by hydrogen saturates the dangling bonds and circumvents the
surfacereconstruction.Forthecalculatedfully-hydrogenatedSiNWs
(i.e., H-SiNWs), no lattice contraction is observed while very small
lattice expansions (by 0.001 nm) is only found in the surface layer
due to hydrogen bond effect, consistent well with TBMD results
[52]. Therefore, the bond length of SiNWs in the surface layer is
remarkably different between naked surface and H-surface. The
bond length of H-surface is higher than that of naked surface,
although the inner case is most similar to the bulk Si. Thus the
particular surface structures of SiNWs may have an important role
in the vibrationalmodes andthereby the thermal conductivity.
Figure 5.6 shows plots of the phonon density of state (PDOS)
spectra for Si atom in the inner and surface layers of naked SiNW
and H-SiNW (the cross-sectional area is 2.36 nm
2
), respectively.
Both 3.69 nm
2
and 5.31 nm
2
SiNWs have similar PDOS spectra.
Here PDOS is computed from the Fourier transform of the velocity
autocorrelation function in MD simulation [53]. For inner Si atoms,
the PDOS curve does not change much with terminated hydrogen
layers, as expected from bulk-like behavior. By contrast, for naked
SiNWs, the high frequency optical modes of surface Si atoms exhibit
a significant blueshift relative to that of inner parts due to the
surface lattice contraction effect—a shorter bond length implies a
larger force constant and a higher vibration frequency. Compared
with PDOSof naked SiNWs, passivation by hydrogen can induce and
increase the overlaps of PDOS between surface and inner atoms in
H-SiNW, which isbenefit to the thermal conductivity. Therefore, the
hydrogenation-inducedincreaseinthephononthermalconductivity
is attributed to the specific changes in surface atom vibrational
modes.
