Time-Resolved Studies and Nanostructure Formation in Sb2Te3 Films Using Femtosecond Lasers.

Abstract

Antimony telluride (Sb2Te3) is an important material with a wide range of applications in thermoelectrics, data storage devices and topological insulator research. Our work on femtosecond laser studies of Sb2Te3 films has significance for insights into femtosecond laser interaction with Sb2Te3 above the damage threshold, as well as providing a new pathway for novel fabrication of highly-ordered nanostructured Sb2Te3. These new developments are made possible by careful control of the laser scanning conditions, opening the way to future nanoscale studies and materials applications. The pump-probe scheme for the time-resolved studies employed a novel asynchronous optical sampling (ASOPS) technique, which has distinctive advantages over the traditional mechanical-delay scheme including superior stability of beam alignment during scans, faster data acquisition rates, and the ability to monitor a much wider range of dynamics up to ten nanoseconds. With ASOPS, it is shown that a sequence of connected processes can be studied in Sb2Te3 films, from coherent optical phonons and acoustic echoes at picosecond timescale, through thermal transport at nanosecond timescale. In particular, the coherent phonons were used, for the first time, to monitor the element segregation in Sb2Te3 films under high-fluence pump laser irradiation conditions. These results are important for the ultrafast spectroscopy research community: they highlight the need for careful interpretation of coherent phonon spectra in tellurides, which are susceptible to fs laser damage. Femtosecond laser irradiation of Sb2Te3 above ~6 mJ/cm2 was also found to produce highly-ordered nanotracks with a periodicity an order of magnitude below the laser wavelength. A variety of characterization techniques identified these nanotracks as single crystalline Sb2Te3 nanowires separated by polycrystalline phases including a large amount of the insulating Sb2O3. Laser irradiation in different gas environments revealed the sensitivity of the Sb2Te3 surface morphology to the surrounding gas species, especially O2, highlighting the critical role of the ambient environment interactions with Sb2Te3 for nanostructure formation in the thin films. These results provide valuable experimental input for the future analysis of the generation mechanism of these nanostructures. Additionally, the results open up new opportunities for fabrication of in-plane Sb2Te3 nanowires for planar applications.