“… 35 The optical absorption coefficient was obtained by the equation
where ε 1 ( ω ) and ε 2 ( ω ) are frequency dependent real and imaginary parts of dielectric function, ω is photon frequency, μ 0 is the permeability of free space. 36 The real part of the dielectric function ε 1 ( ω ) can be evaluated from the imaginary part ε 2 ( ω ) by the famous Kramers–Kronig relationship. 37 Direct optical band gap of the samples was obtained from the equation ( αhν ) n = A ( hν − E opt ), where A is a constant and n denotes the transition type as follows: n = 2 for direct allowed, 2/3 for direct forbidden, 1/2 for indirect allowed and 1/3 for indirect forbidden transitions.…”