Browsing by Author "Singha, Achintya"

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    Probing lattice dynamics in ST 12 phase germanium nanowires by Raman spectroscopy
    (AMER INST PHYSICS, 2021-12-06) Raha, Sreyan; Srivastava, Divya; Biswas, Subhajit; Garcia-Gil, Adrià; Karttunen, Antti J.; Holmes, Justin D.; Singha, Achintya; Bose Institute; Inorganic Materials Modelling; University College Cork; Department of Chemistry and Materials Science
    Germanium (Ge) plays a crucial role in setting up important functionalities for silicon-compatible photonics. Diamond cubic germanium is an extensively studied semiconductor, although its other exotic forms, like BC8, ST8, ST12 phases, may possess distinct electronic properties. We have fabricated stable ST12-Ge nanowires via a self-seeded bottom-up three phase growth in a confined supercritical toluene environment. Here, we report on the direct evidence of the presence of the ST12 phase by a combination of Raman spectroscopy and first-principles calculations using density functional theory (DFT). It is important to remark that the DFT calculation predicts all the Raman active optical phonon modes of the P 4321 structure, and it is in very good agreement with the experimental results. The phonon dynamics as a function of temperature is investigated through Raman measurements at temperatures varying from 80 to 300 K. First-order temperature coefficients for all the observed Raman modes are estimated from the linear temperature dependence of the phonon shifts. A complete set of isobaric Grüneisen parameters is reported for all Raman modes of ST12-Ge nanowire, and the values are lower compared to the same for Si, dc-Ge bulk, and Ge nanowire. These results have important implications for understanding thermal properties of ST12-Ge nanowire.
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    Tailoring phonon modes of few-layered MoS2 by in-plane electric field
    (2020-04-30) Mitra, Sreemanta; Srivastava, Divya; Singha, Shib Shankar; Dutta, Saurav; Satpati, Biswarup; Karppinen, Maarit; Ghosh, Arindam; Singha, Achintya; Department of Chemistry and Materials Science; Inorganic Materials Modelling; Inorganic Materials Chemistry; Bose Institute; University of Calcutta; Saha Institute of Nuclear Physics; Indian Institute of Science Bangalore
    We discuss the effect of the in-plane electric field on the Raman spectroscopy for few-layered MoS2. The characteristic Raman modes of MoS2 show gradual red shift, while the intensity increases by 45–50% as the electric field is increased, showing a large electro-optical effect. Structural analysis suggests that our few-layered MoS2 belongs to P6/m2 space group with broken inversion symmetry. We attribute this gradual red shift to this broken symmetry-driven piezoelectricity in MoS2, which generates tensile strain along the perpendicular direction when the electric field is applied. The enhancement of the effect upon reversing the electric field direction adds credence to our interpretation. Our first principal density-functional theory calculation further substantiates the claim. This optical probing of the electromechanical coupling may lead to applications as a nonextensive technique for electric field/strain sensors in the nanoelectronics devices.