Quantum size effect and anomalous optical property of Bi2Se3 thin films on mica

Abstract

Bismuth selenide (Bi2Se3) is a promising 2D quantum nanomaterial for thermoelectric and optoelectronic devices; however, several growth techniques lack crystalline uniformity and scalability. Through multiple corroborating methods, we study the optical properties of Bi2Se3 in conjunction with its structural properties for a uniquely comprehensive description. The Raman spectra of films of Bi2Se3 prepared using a radio frequency (RF) source of magnetron sputtering reveal the characteristic Eg2 and A1g2 peaks of Bi2Se3 at ∼120 and 175 cm−1, which undergo spectral blue shifts as film thickness is increased due to the quantum size effect. Numerical analysis of the width of the Raman peaks suggests that the films are crystalline with nanocrystal sizes of 13–22 nm, confirmed by Scherrer's analysis of complementary x-ray diffraction. A strong optical absorption was observed in the visible–near infrared (NIR) range with an optical direct bandgap Eg determined from visible–NIR spectra within a 1.8–1.4 eV range, depending on film thickness and nanoparticulate structure. The interplay between film thickness and surface roughness strongly influences the optical response of Bi2Se3, where thinner, rougher films exhibit enhanced scattering and Urbach tailing, while thicker, smoother films display sharper absorption edges and bulk-like behavior. The deposition of nanometer Bi2Se3 films using RF magnetron sputtering constitutes a promising scalable platform for quantum thermoelectric and optoelectronic devices.