Chemistry Reference
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Fig. 5.28
CL spectra
acquired at RT on core/shell
NWs (
red circles
) and SiC
NWs (
blue squares
)
5.5.3
Optical Properties
The optical properties of the NWs, with and without the oxide shell, were studied
by Cathodoluminescence (CL) spectroscopy. Figure
5.28
shows the comparison be-
tween typical room temperature CL spectra acquired on bundles of core/shell NWs
(red curve in Fig.
5.28
) and SiC NWs (blue curve,
10 in Fig.
5.28
). The com-
parison highlights that the emission from SiC NWs is very faint and broad, whereas
the core/shell structure favours a more efficient luminescence emission, with quite
sharp peaks at 2.3 eV due to the near-band-edge (NBE) recombination in the cubic
SiC core and at 2.7 eV related to the SiO
2
shell (Fabbri et al.
2010
,
2012
).
To explain why the presence of the silicon dioxide shell increases the radiative
recombination in the silicon carbide core, we can analyse the energy diagram of the
nanosystem. A type I band alignment (Pistol and Pryor
2008
) of 3C-SiC and SiO2 can
be hypothesized (Fig.
5.29
). The conduction and valence band-offsets have been ex-
perimentally found in (Bechstedt et al.
1997
) in the case of bulk material and they are
equal to
δ
E
c
=
×
2
.
9 eV respectively. In this framework, the carri-
ers generated by the electron beam in the shell diffuse into the core, and here recom-
bine according to the allowed transitions in 3C-SiC. The diffusion of the carriers could
be considered as an energy transfer from the shell to the core, an effect that has been
observed for semiconductor nanoparticles (Louis et al.
2006
) but not yet in NWs. In
our system, the amorphous shell results to be beneficial to enhance the luminescence
intensity of the crystalline core, preferentially the SiC NBE radiative recombination.
Besides the effectiveness as a carrier injector region, this could be partly related
to the fact that the shell can act as a passivation layer to reduce the non-radiative
recombination related to surface states, likewise in the case of entirely crystalline
core/shell systems (e.g. GaAs-based NWs (Sköld et al.
2005
; Jabeen et al.
2008
).
3
.
6 eV and
δ
E
v
=
