Lynn Endicott

Coherent optical phonon spectroscopy studies of femtosecond-laser modified Sb2Te3 films

We performed time-resolved reflectivity measurements to monitor changes in optical phonon modes in Sb2Te3 thin films under femtosecond laser irradiation. We found that a phonon mode at 3.64 THz appears after high-fluence laser irradiation, in addition to the phonon modes of Sb2Te3. We determined that the additional mode is due to Te segregation as a result of laser-induced decomposition of the Sb2Te3 film. This experiment clearly illustrates the irreversible effects of femtosecond laser irradiation during the measurement of coherent optical phonon dynamics in Sb2Te3.

Femtosecond laser-induced nanostructure formation in Sb2Te3

We report femtosecond laser-induced nanotracks in highly absorbing Sb2Te3. Groups of nanotracks are observed with widths ∼50 nm and periodicity ∼130 nm, their area of coverage extending with the increase of laser fluence. We demonstrate that under a narrow range of laser fluences and laser irradiation times, long highly aligned nanotracks can be formed in Sb2Te3. The results suggest a promising avenue for laser nanostructuring of chalcogenide thermoelectrics, with implications for high efficiency thermoelectric energy conversion.

High-quality II-VI films grown on amorphous substrates using tunable tetradymite templates

We demonstrate the growth of highly oriented CdSe and ZnTe films at ∼300 °C on amorphous substrates such as glass and flexible polyimide using ultrathin tetradymite buffer layers composed of SbxBi2-xTe3 alloys lattice-matched to the film overgrowth. This leads to significant improvement of the crystallinity, roughness, grain size, and pit density of the II-VI overlayer along with enhancement of the optoelectronic properties. For example, photoluminescence emission is observed at ∼1.74 eV for optimized CdSe films, the same as in a single crystal reference. An in-plane carrier diffusion length of ∼500 nm is inferred from transient optical data. The use of tetradymite buffer layers to control II-VI compound deposition on non-crystalline substrates is a promising route for large area optoelectronic applications such as photovoltaic, light-emission, or infrared detector devices.

Origins of enhanced thermoelectric power factor in topologically insulating Bi0.64Sb1.36Te3 thin films

In this research, we report the enhanced thermoelectric power factor in topologically insulating thin films of Bi0.64Sb1.36Te3 with a thickness of 6–200 nm. Measurements of scanning tunneling spectroscopy and electronic transport show that the Fermi level lies close to the valence band edge, and that the topological surface state (TSS) is electron dominated. We find that the Seebeck coefficient of the 6 nm and 15 nm thick films is dominated by the valence band, while the TSS chiefly contributes to the electrical conductivity. In contrast, the electronic transport of the reference 200 nm thick film behaves similar to bulk thermoelectric materials with low carrier concentration, implying the effect of the TSS on the electronic transport is merely prominent in the thin region. The conductivity of the 6 nm and 15 nm thick film is obviously higher than that in the 200 nm thick film owing to the highly mobile TSS conduction channel. As a consequence of the enhanced electrical conductivity and the suppressed bip...

Ordered horizontal Sb{sub 2}Te{sub 3} nanowires induced by femtosecond lasers

Nanowires are of intense interest on account of their ability to confine electronic and phononic excitations in narrow channels, leading to unique vibronic and optoelectronic properties. Most systems reported to date exhibit nanowire axes perpendicular to the substrate surface, while for many applications (e.g., photodetectors and sensors), a parallel orientation may be advantageous. Here, we report the formation of in-plane Sb 2Te3 nanowires using femtosecond laser irradiation. High-resolution scanning transmission electron microscopy imaging and element mapping reveal that an interesting laser-driven anion exchange mechanism is responsible for the nanowire formation. This development points the way to the scalable production of a distinct class of nanowire materials with in-plane geometry.

Ordered horizontal Sb2Te3 nanowires induced by femtosecond lasers

Nanowires are of intense interest on account of their ability to confine electronic and phononic excitations in narrow channels, leading to unique vibronic and optoelectronic properties. Most systems reported to date exhibit nanowire axes perpendicular to the substrate surface, while for many applications (e.g., photodetectors and sensors), a parallel orientation may be advantageous. Here, we report the formation of in-plane Sb 2Te3 nanowires using femtosecond laser irradiation. High-resolution scanning transmission electron microscopy imaging and element mapping reveal that an interesting laser-driven anion exchange mechanism is responsible for the nanowire formation. This development points the way to the scalable production of a distinct class of nanowire materials with in-plane geometry.