ZnWO4 is a direct band gap material while ZnMoO4 is an indirect band gap material. While characterizing ZnW(x)Mo(1-x)O4, which value of 'n' should be taken in Tauc's relation i.e., αħω = B (ħ ω – Eg)^n ? How could we determine whether the mixed solid solution has an indirect or a direct band gap?
Dear Avi,
What you can is to plot the derivative of ln(a.h.v), as a function of (a.h.v). Here is the devellopement:
the initial tauc equation is:
(a.h.v)=Bx(h.v-Eg)n
but you can use it without the B constant:
(a.h.v)=(h.v-Eg)n
meaning that ln(a.h.v)=n x ln(h.v-Eg)
and the derivative :
d(ln(a.h.v))/d(hv) = n x 1/(h.v-Eg)
This plot allows you to determine the gap, you should observe a discontinuity when you plot the derivative as a function of h.v. Then, knowing Eg, you can determine n, by doing one more linearisation:
ln (d(ln(a.h.v))/d(hv)) = ln n - ln(h.v-Eg)
Regards,
Emile HAYE
Thanks Emile Haye. I'll try plotting the curve as suggested by you and get back, if I have any doubts.
I have done the DRS of the solid sample so have got the %R Vs wavelength plot. Could anyone tell me how to calculate in this particular case. Is only alpha of Tauc's relation to be replaced by F(R∞) and rest we could go with Emile's approach of plotting ln( F(R∞) .h.v), as a function of ( F(R∞) .h.v) to determine Eg and 'n'?
I have tried plotting graphs with [F(R)hv]^2 (direct band gap) vs hv and [F(R)hv]^1/2 (indirect) for the mixed compound ZnW0.5Mo0.5O4. The resultant bang gap comes around 3.2 in both cases. Could anyone suggest how could I know the exact value of 'n' to be used? Should it be 2 (for indirect) or 1/2 (for direct) as per derivative of ln ([F(R)hv]) Vs [F(R)hv] plot as explained by Emile Haye?
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