Hi, You can use Tauc plot method as described in the link (https://en.wikipedia.org/wiki/Tauc_plot) and articles (https://doi.org/10.1016%2F0025-5408%2868%2990023-8 , and https://doi.org/10.1080%2F14786437008221061) . At the lower edge of the absorption peak, you need to extrapolate linear portion of the plot (αhν)^1/r vs hv on the hv-axis. Note that you need to decide on the value of exponent 'r' based on the nature of transition i.e. direct or indirect.

Thank you Dr Bhaveshkumar, but how can determine the nature of transition i.e. direct or indirect of the clusters ?

I am interested