Phonons in Bi2S3 nanostructures: Raman scattering and first-principles studies

Bi2S3 has shown promise in thermoelectric and optoelectronic applications as well as biological and chemical sensors. We present here a comprehensive study on the lattice dynamics of Bi2S3 nanostructures probed by micro-Raman scattering spectroscopy and first-principles calculations. Bi2S3 nanowires are synthesized using a physical vapor transport method via a vapor-liquid-solid mechanism on silicon substrates. Oriented Bi2S3 nanosheets are also obtained on mica substrates. The structure of the nanowires is determined to be orthorhombic with a growth orientation of [110] by x-ray diffraction and high-resolution transmission electron microscopy. ARaman scattering study is conducted for as-prepared Bi2S3 nanostructures, in which 33, 38, 46, and 53 cm(-1) phonon modes are observed for the first time. We find several modes to be very sensitive to excitation wavelength and power. First-principles calculations of orthorhombic Bi2S3 predict a series of Raman modes, in good agreement with our experiments. Phonon-dispersion curves of Bi2S3 are also presented, and the effect of Born effective charges on the longitudinal-optical-transverse-optical splitting at the zone center is taken into account.