Origin of extraordinary luminescence shift in graphene quantum dots with varying excitation energy: An experimental evidence of localized sp2 carbon subdomain

We report the extraordinary photoluminescence (PL) behavior of high-quality graphene quantum dots (GQDs) prepared with a graphite intercalation compound method: the alternating PL peak shifts (redshift-blueshift-redshift) with increasing the excitation wavelength. The PL peak position is redshift in the excitation wavelength range 250–270 nm, subsequently blueshift in 270–290 nm, and again redshift in the range 290–330 nm. This unconventional optical response of the GQDs is attributed to two types of energy states: intrinsic states formed by localized sp2 carbon subdomains and extrinsic states created by various oxygen-functional groups. The PL behavior of GQDs shows different tendencies according to the size and amount of oxygen-functional groups. Furthermore, since the extrinsic states of GQDs can be inactivated under acidic condition, its extraordinary PL behavior of high-quality GQDs with varying excitation wavelength largely recedes. On the other hand, we clearly observed these alternating PL shift behaviors when localized sp2 carbon subdomains with intrinsic emission are artificially formed by reducing oxygen-functional groups in graphene oxide quantum dots, which have mostly the extrinsic PL originated from the energy states of various oxygen-functional groups. The results offer experimental evidence for the existence of localized sp2 carbon subdomains and provide key information that clarifies the electronic structure of GQDs and their derivatives.