Phonons in Bi2S3nanostructures: Raman scattering and first-principles studies

Bi${}_{2}$S${}_{3}$ 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 Bi${}_{2}$S${}_{3}$ nanostructures probed by micro-Raman scattering spectroscopy and first-principles calculations. Bi${}_{2}$S${}_{3}$ nanowires are synthesized using a physical vapor transport method via a vapor-liquid-solid mechanism on silicon substrates. Oriented Bi${}_{2}$S${}_{3}$ 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. A Raman scattering study is conducted for as-prepared Bi${}_{2}$S${}_{3}$ nanostructures, in which 33, 38, 46, and 53 cm${}^{\ensuremath{-}1}$ 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 Bi${}_{2}$S${}_{3}$ predict a series of Raman modes, in good agreement with our experiments. Phonon-dispersion curves of Bi${}_{2}$S${}_{3}$ 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.