The electronic structure of two-dimensional gallium selenide crystals containing a small number of layers was investigated theoretically and experimentally. The electronic band structure of the layered GaSe crystal was investigated by the first-principles density functional theory calculations. The $GW$ approximation was used for the correction of the band-gap values. A dependence of the band-gap value on the number of tetralayers has been demonstrated. For the thin crystal with several tetralayers, the band gap becomes larger compared to the bulk crystal. The thin layers of GaSe have been experimentally produced by the ultrasonication of GaSe particles in water suspensions in the presence of Pluronic F127 surfactant. Their thickness was from one to a few tetralayers, according to the transmission electron microscopy studies. The optical absorption spectra demonstrate the well-resolved bands, shifted toward the blue relative to those of the bulk GaSe. Their origin is caused by the presence of GaSe structures with one to a few tetralayers.

• Received 11 March 2011

DOI:https://doi.org/10.1103/PhysRevB.84.085314