The understanding of two-dimensional (2D) materials has grown tremendously, especially for isolated monolayers. Recently, complex structures formed by stacking 2D materials have attracted considerable attention. This is in part due to the fact that the properties of monolayers are known to be influenced by their surroundings. Consequently, monolayer properties are predicted to be affected by “heterostructuring”. A study involving high-charge-carrier-density effects and dynamics is presented here for monolayers of WSe2 on different substrates and heterostructures comprised of 2D h-BN and WSe2. The influence of h-BN as well as the bilayer stacking order on the spectral and dynamical properties of WSe2- monolayer emission is discussed for the low-density regime and evidenced for high-density effects such as exciton-exciton annihilation and the Mott transition.
This paper presents a review of the controlled growth of transition metal dichalcogenide (TMD) heterostructures, and the elucidation of the role of underlying two dimensional (2D) materials on temporal degradation of transition metal dichalcogenides (TMDs). Chemical vapor deposition (CVD)-growth is carried out to achieve localized, patterned, single crystalline or polycrystalline monolayers of TMDs, including MoS<sub>2</sub>, WS<sub>2</sub>, WSe<sub>2</sub> and MoSe<sub>2</sub>, as well as their heterostructures. The localized growth of TMDs has an important implication for nonlinear optics applications. Extensive material characterization is performed to illuminate the role of dissimilar 2D substrates in the prevention of interior defects in TMDs. This characterization provides a detailed observation of the oxidation rates and behaviors of TMDs, which corroborate the role of underlying 2D layers in the prevention of in-air oxidation in TMDs. The epitaxial growth is demonstrated to create TMDs on hBN and graphene, as well as vertical/lateral heterostructures of TMDs, uniquely forming in-phase 2D heterostructures.
We introduce a novel method for low substrate temperature carbon nanotube (CNT) deposition utilizing photo-chemical
vapor deposition (PCVD). Aluminum and nickel catalyst layers are deposited on thermally oxidized silicon substrates for
CNT growth. The catalyst layers of varying thicknesses are deposited by electron beam evaporation. Different catalyst
annealing temperatures and pressures are investigated. The CNT deposition is carried out immediately following the
annealing process. The presence of light source during CNT deposition assists in fragmentation of the CCl<sub>4</sub> precursor
molecules used, thereby permitting a lower substrate temperature during growth. We have successfully deposited CNTs
at substrate temperatures as low as 400 °C by this technique.