GROWTH, VALLEYS MANIPULATION OF MX2 (M=MO; W, X=S; SE) AND THEIR HETEROSTRUCTURE

Abstract

The current electronic industry is mainly dominated by silicon, which is an intrinsically three-dimensional (3D) semiconducting material. The technology is improving rapidly and the silicon-based microprocessor is about to achieve its ultimate limit in the near future in terms of device performance and size. In this regard, search for alternative materials had been started from the last few decades and likely to remain in the foreseeable future. In 2004, the discovery of first truly two-dimensional (2D) material, namely graphene, was regarded as a major breakthrough and was considered to be an alternative material for the development of next generation electronic devices. However, the closed band gap of graphene is a serious issue and have not yet been addressed without compromising on its other excellent properties, particularly its electronic mobility. The graphene discovery, however, opened up a new window and many different 2D materials including insulator, metal, and semiconductors are discovered in this journey so far. Among the 2D materials, the semiconducting monolayer transition metal dichalcogenides (TMDCs) possess huge potential for next generation electronic devices owing to their direct band gap and reasonable mobility. Moreover, their non-centrosymmetric nature, strong spin-orbit coupling (SOC) together with the time reversal symmetry render them an ideal setting for studying nonlinear optics and the fascinating valley physics. Furthermore, the artificial vertical stacking of various bandgap monolayer TMDCs forms type-II band alignment rendering them for PN-junctions and energy harvesting devices. However, in order to achieve high performance devices, the growth of high quality, large area and continuous film of different band gap monolayer TMDCs materials such as MoS2, WS2, MoSe2 and WSe2 is still a major concern. The superior optical and electrical properties of mono and few layers MoS2, makes it a promising candidate for next generation flexible nanoelectronics and optoelectronics devices. However, controlled growth, scalability, uniform and continuous large area with a repeatable synthesis of mono and few layers MoS2 are highly desirable. In chapter 2, a two-step synthetic approach has been adopted for the preparation of few layers MoS2 films. MoS2 films were prepared with different deposition times (45 s, 75 s, 100 s, 2.5 min and 10 min) at the same laser power, 200 mJ/cm2 by pulse laser deposition (PLD) on a sapphire substrate. The samples were further annealed at 850 oC in a sulfur-rich environment. The optical properties of the annealed samples were moderately improved. Significantly, the MoS2 film deposited at 45 s exhibited a superior photoluminescence response with full width half maximum (FWHM) value of ~72.24 meV, as compared to the previous report of few layers MoS2 by physical vapor deposition. Moreover, Raman frequency difference (Δω) between A1g and E12g was found to be 23.6 cm-1 indicating the film is approximately three layer. Compared to sulfur compounds, the low reactivity of selenium makes it more challenging to synthesize good quality continuous single layered selenides required for practical applications. In chapter 3, we present the synthesis of monolayer MoSe2 by selenization of pulsed laser deposited MoO3 film on SiO2/Si and sapphire substrates. The laser energy was carefully varied to optimize the growth of highly uniform continuous monolayer film. The morphological characterizations including optical microscope, field emission scanning electron microscope and atomic force microscope results clearly demonstrate that the synthesized film is monolayer, continuous and homogeneous. The Raman and photoluminescence maps of the continuous film exhibit uniform brightness indicating the uniform and homogeneous nature of the film. Moreover. The full width half maximum values of A1g and A exciton peaks were found to be 3.7 cm-1 and 24 nm, respectively, which show the excellent optical quality of the grown film. These results imply the potential use of grown film and the synthetic approach in device fabrication. The covalently bonded in-plane heterostructure (HS) of monolayer transition metal dichalcogenides (TMDCs) possesses huge potential for high-speed electronic devices in terms of valleytronics. In chapter 4, high-quality monolayer MoSe2-WSe2 lateral HSs are grown by the PLD-assisted selenization method. The sharp interface of the lateral HS is verified by morphological and optical characterizations. Intriguingly, photoluminescence spectra acquired from the interface show rather clear signatures of pristine MoSe2 and WSe2 with no intermediate energy peak related to intralayer excitonic matter or formation of MoxW(1-x)Se2 alloys, thereby confirming the sharp interface. Furthermore, the discrete nature of laterally attached TMDC monolayers, each with doubly degenerated but nonequivalent energy valleys marked by (KM, K’M) for MoSe2, and (KW, K’W) for WSe2 in k space, allows simultaneous control of the four valleys within the excitation area without any crosstalk effect over the interface. As an example, KM and KW valleys or K’M and K’W valleys are simultaneously polarized by controlling the helicity of circularly polarized optical pumping, where the maximum degree of polarization is achieved at their respective band edges. The current work provides the growth mechanism of laterally sharp HSs and highlights their potential use in valleytronics. Furthermore, the non-centrosymmetric nature of monolayer MX2 together with time reversal symmetry gives rise to the non-zero Berry curvature Ω(k) which serves as an effective internal magnetic field flux. The contrasting effect of Ω(k) on K and K’ valleys (energy extremum) dictates the charge carriers of respective valleys to move in opposite transverse direction in the presence of an external electric field. This fascinating phenomenon is called valley Hall effect. Recently, the Hall effect for neutral exciton (e-h pair), namely exciton Hall effect (EHE), has also been observed in mechanically exfoliated monolayer MoS2 by locally inducing a chemical potential through laser illumination. Here, it is worth mentioning that unlike electric potential, a chemical potential drives both electrons and holes in the same direction, and is, therefore, essential for the realization of Hall effect of composite particles. In chapter 5, we briefly investigate the exciton transport in monolayer WS2 by using polarization and spatially resolved photoluminescence (PL) spectroscopy. In this regard, the stripes of various widths were prepared by laser trimming of WS2 grown via CVD. The excellent quality of WS2 was verified by optical and morphological characterizations. Interestingly, the excitons at K and K’ valleys were found to move in the contrasting transverse direction due to the intrinsic Berry curvature showing the signal of EHE. Moreover, we observed that the diffusion length of valley excitons increases with the decreasing width of the stripe, implying that chemical potential could simply be altered by modifying the geometry of MX2.