Dear Akibul,

Raman spectroscopy is employed in order to gain more scientific insight about crystal disorder and chemical modification due to functional groups.

Generally, Graphene layer shows the G (at ~1585 cm-1) band which shifts to left, this small left-shift is so-called blue-shift.

I hope this is the satisfactory answer.

Azhar Ali you are saying correct however, one thing more is that such shifting in Raman band is due to the presence of oxygen vacancies generated residual stresses created defects and micro-strains