Environmental Engineering Reference
In-Depth Information
Monte Carlo simulations [16], Boltzmann transport equation [17,
18], non-equilibrium Green function approach [19, 20], and first-
principles methods [21, 22]. Moreover, reduced thermal conduc-
tivity has been observed in Core-Shell nanowires [23], isotope-
doping nanowires [24], vacancy-defected nanowires [20], surface-
decorated nanowires [25], surface-disordered nanowires [26], and
superlatticed nanowires [27, 28]. Such findings provide new
strategies to search for high-performance Si-based TE materials by
reducing the thermal conductivity.
SiNWs have a large surface-to-volume ratio. At nanometer scale,
the surface-to-volume ratio increases with decreasing size, leading
to increased significance of the surface in SiNWs. The properties
of SiNWs differ remarkably from those of bulk Si due to the
surfaceeffect.Inaddition,thelargesurface-to-volumeratioofSiNWs
provides plenty of room for surface functionalization/modification
[29]. SiNWs can be passivated by various radicals, such as hydrogen
(H), oxygen, nitrogen (N), phosphorus, fluorine (F) or hydroxyl
(OH) groups, which have been observed in experiments [3, 30-33]
and studied by computational modeling [29, 34-43]. For example,
theoretical study has shown that nitrogen atoms on the surface of
Si nanostructures can stabilize the structures and clean the gap
states;thisisimportantforvisibleluminescenceinSinanomaterials
[34]. Additionally, first-principles calculations done by us [38]
revealed the formation of N-Si-N-Si chains due to nitrogen atoms
adsorbed on the (110) facets of [112] SiNWs, which sequentially
induces metallization (Fig. 5.1). Their calculations were conducted
using density functional theory (DFT) withthe generalized gradient
approximate(GGA)thatisavailableintheVASPmethod.Theenergy
cutoff was set at 420 eV, and the GGA was used with the PW91
exchange-correlation functional. The distance between neighboring
wires was greater than 0.9 nm to reduce the cell-to-cell interaction,
and a 1
×
×
6 k-point grid was used for the SiNWs. Recent
experimental study demonstrated an effective doping of SiNWs
induced by surface passivant [3]. The stability and doping level
tunability of SiNWs with alkene adsorption were also observed by
us [41], as shown in Fig. 5.2. In their study, DFT calculations were
also used within the local density approximation and the projected
augmented wave method.
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