Phonon Dispersion and Band Gap Engineering of Strained and Isotopically Enriched Atomically Thin Transition Metal Dichalcogenide Semiconductors

Transition metal dichalcogenide (TMD) materials consist of strong intra-layer covalently bonded and weak inter-layer van der Waals bonded layers, which enable the preparation of atomically thin devices by either mechanical exfoliation or chemical vapor deposition (CVD). As a certain group of TMDs MX2 (M=Mo, W; X=S, Se, Te) are scaled to the thickness of one three-atom-thick layer, the electronic dispersion undergoes a transition from an indirect to a direct band gap due to quantum confinement effects. The mechanical flexibility can be also enhanced by reducing layer number in TMDs. In view of these extraordinary properties, TMDs have been proposed as good candidate active materials for optoelectronic and photonic devices. However, the mechanisms of tuning electronic band gap energy and phonon dispersion are still unclear. In additional, the quantum emission behavior in atomically thin TMDs remains to be better understood. This thesis examines the mechanisms of strain and isotope effect on optoelectronic and photonic transport in atomically thin WSe2. A two-order of magnitude enhancement in the photoluminescence (PL) emission intensity in uniaxially strained single crystalline CVD grown WSe2bilayers is demonstrated using a new encapsulation four-point bending technique. A strain transfer model is developed considering 3-dimensional Poisson effect. Adding confidence to the high levels of elastic strain achieved, mode dependent Grüneisen parameters were obtained by interpreting the Raman spectra. Additionally, a larger optical band gap in isotopically pure bilayer WSe2 compared to the naturally abundant sample is reported in this thesis due to the isotope effect on the indirect excitons. The X-ray diffraction characterization reveals a slightly shorter interlayer van der Waals bond length in isotopic WSe2 bilayer with heavier atomic mass. Furthermore, a new route of creating locally defined single photon emitters on nominally bilayer WSe2 is investigated. The bilayer WSe2 is continuous over 1 cm2 and transferred onto silicon oxide tip arrays to create engineered defects due to local strain. The seconder order photon correlation measurement confirms quantum emission arising from engineered defect sites. The results are promising for integration of quantum emission sites into scaled device architectures.