Environmental Engineering Reference
In-Depth Information
Figure 5.8 plots the PDOS spectra for Si and nitrogen atoms in
N-SiNWs.Giventhesurfacelatticedeformation,thePDOSspectraof
N-SiNWs are quite different from that of H-SiNW (i.e., 0% surface
nitrogenation). With the degree of nitrogenation increasing, the
Si-Si vibrational peak near 16 THz is depressed and blue shifted
whereas a high frequency peak near 30 THz, which corresponds
to Si-N vibrational mode (830 cm
−
1
890 cm
−
1
[56]), appears.
In addition, when the surface nitrogenation ratio increases from
0% to 50%, the PDOS from Si atoms and nitrogen atoms show a
depression in the low-frequency part (Fig. 5.9), resulting from the
lattice mismatch and difference in the atomic mass between the Si
coreandN-ad-layer.Theremarkablereductioninthelow-frequency
PDOS mainly cause the decrease in the thermal conductivity for
N-SiNWs. Therefore, the surface nitrogenation induced attenuation
in thermal conductivity is attributed to the remarkable phonon
scattering by surface defects [46] (the presence of lattice mismatch
and passivated atoms near the surface), which dampen some of the
lattice vibrationalmodes [23].
−
5.5 Conclusions and Remarks
In summary, the phonon thermal conductivities of surface hydro-
genatedandnitrogenatedthinSiNWsinvestigatedusingequilibrium
MDsimulations,respectively,havebeenreviewed.Surfacehydrogen
passivation can increase the thermal conductivity compared to
that of naked SiNWs without surface passivation. Hydrogenation
can saturate the dangling bonds and reduce the lattice mismatch
between the inner part and the surface of SiNWs, and hence
increase the overlaps of the phonon spectra between surface and
inner atoms. Such specific changes in the lattice structures and
vibrational modes of surface Si atoms cause the increase of thermal
conductivity in hydrogenated SiNWs. In addition, surface nitrogen
passivation can significantly reduce the thermal conductivity of
SiNWs. In Particular, 50% surface nitrogenation on SiNWs can
lower the thermal conductivity by about 75% with respect to
that of fully hydrogenated SiNWs. This reduction in the thermal
conductivity arises from the suppression of some lattice vibra-