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Nanowires are structures with an elongated shape and a large surface
to volume ratio.
This results in the appearance of atomic vibrational modes
specifically related to the
oscillation of surface atoms. Surface optical modes (SO)
are characteristic vibrations of the
free surface of polar semiconductors. This means that their amplitude is
maximum at the
surface and decays exponentially away from it. Therefore they are very
sensitive to
changes that take place at the surface of the materials: dielectric
environment, specific
morphology of the surface, adsorbates
etc.
We have investigated the dependence of the frequency of the surface
modes as a function
of NW density and NW environment:
R. Mata
et al. Phys. Rev. B 85, 035322 (2012)
The Raman spectra were obtained from locations of
the sample with different NW density.
Its frequency decreases as both density and
diameter of the wires decrease.
By means of macroscopic models we have analyzed
the influence of NW size and density on
the surface modes.
In the theoretical approach the density of
the nanowires is given through the filling factor F (proportion of material
with respect to surrounding air). It is shown that NWs are well
described as “isolated” for F<0.4,
while larger filling factors are better described by the
Maxwell-Garnett approximation, that takes
into account NW-NW interaction through an
effective-dielectric function of NWs and air.
The theoretical results indicate that
under certain conditions (F≈0.4, incident angle≠0) it is
possible to observe the anticrossing of two SO
modes. The frequency of these modes shifts when we change the dielectric
properties of the environment (change air by paraffin oil). The
changes can be appreciated in the figure below
(left, experimental results. Right,
calculated spectra).
J. Wang et. al. Phys.
Rev. B 85, 155432 , (2012)
The relative intensities of S1 and S2 are
very sensitive to NW density and scattering angle.
This last figure gives an idea of the
variation of the frequency and intensity of the modes.
The size of the symbols is proportional to
the intensity of the Raman peak.
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