For the optical and the electrical properties of

nanostructured semiconductor materials, the index of refraction plays a significant
role in determining the optical and the electrical properties of the crystals
and is an important consideration in manufacture the optoelectronic devices as
well as the solar cells.111,112 The complex refractive index can be expressed as113

,                                                    (1)

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n and k represent the real and imaginary components, i.e., the refractive index
and the extinction coefficient of the complex refractive index, respectively.
The index of refraction variation of ZnO nanorods with cobalt doping is seldom reported.
Doping dependent index of refraction variation enables ZnO nanorods to be used
for making different waveguides. Apart from this, if the alternate layers of
doped and undoped ZnO nanorods exhibit positive and negative thermo-optic
coefficients, then the combined structure will be least affected by the
temperature variations.114 Therefore, it is necessary to investigate the
temperature dependence of refractive index values in cobalt-doped and undoped
ZnO nanorods. In our previous study14 of 0-9% cobalt doped ZnO nanorods, we observed a
red-shift in the optical bandgap of the ZnO due to thermal relaxation under
thermal stress,  which provides higher
modulation in the refractive index around the band edge. We observed that the index
of refraction value increased from 1.99 to 2.12 as the cobalt concentration
increased from 0% to 9%. Similarly, Caglar88 reported increase in index of refraction from 2.20 to
2.67 with increase in cobalt doping from 0 to 5%. Similar increase in
refractive index with cobalt doping is also reported by Khan et al.115 The observed variation of refractive index with
cobalt doping can be explained on the basis of the contribution from both
lattice contraction and the disorder due to doping. The increase in the
absorption coefficient indicates the decrease in the loss energy with cobalt content.
The decrease in the loss energy points out that the charge carriers absorb more
energy giving large absorption coefficient and boost in refractive index. Hence,
the variation in the index of refraction and the extinction coefficient
can be attributed to the optical absorption and a variety of impurities and
defects in ZnO nanorods. Therefore, it is reasonable to expect that the structural disorders and defects in the cobalt-doped
ZnO nanostructures are caused by the incorporation of cobalt during doping.
Thus, cobalt doping significantly enhances the index of refraction of the ZnO

Dietrich et al.116 investigated that refractive
index depends entirely on the deviation
in lattice parameter concerning the bulk value and concluded that lattice
contraction would cause the change in the
refractive index. If lattice contraction is the primary
cause of the variation of refractive
index, a continuous decrease in the refractive index must be observed with increasing cobalt
concentration, as lattice constant
decreases in our work. However, our results14 do not show such a trend.