Laser induced oxidation and optical properties of stoichiometric and non-stoichiometric Bi₂Te₃ nanoplates

Bi-Te nanoplates (NPs) grown by a low pressure vapor transport method have been studied by Raman spectroscopy, atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDS), and Auger electron spectroscopy (AES). We find that the surface of relatively thick (more than tens of nanometers) Bi₂Te₃ NPs is oxidized in the air and forms a bump under heating with moderate laser power, as revealed by the emergence of Raman lines characteristic of Bi₂O₃ and TeO₂ and characterization by AFM and EDS. Further increase of laser power burns holes on the surface of the NPs. Thin (thicknesses less than 20 nm) NPs with stoichiometry different from Bi₂Te₃ were also studied. Raman lines from non-stoichiometric NPs are different from those of stoichiometric ones and display characteristic changes with the increase of Bi concentration. Thin NPs with the same thickness but different stoichiometries show different color contrast compared to the substrate in the optical image. This indicates that the optical absorption coefficient in thin Bi-Te NPs strongly depends on their stoichiometry.