Author: Azim, M Bakhtiar
Two-dimensional Transitional Metal Dichalcogenides (TMDs) such as MX2 (M= Mo, W; X= S, Se) have gained tremendous attention for use in optoelectronic applications because of their high carrier mobility and indirect-direct band gap transition for thin layers resulting in light emission. Moreover, monolayer TMDs have exceptional other properties such as piezoelectricity, gate-induced superconductivity, and tunable band structure. Mechanical exfoliation, hydrothermal method, electrochemical exfoliation, chemical vapor deposition (CVD) etc. are the most widely used methods for preparing monolayer TMDs. Among these methods, CVD is regarded as the most promising approach because it can produce large area crystal growth and uniform monolayers. The challenges associated with other methods are either small flake size or low quality with lower carrier mobility limitingperformancein electronic devices. CVD grown TMDs tend to show weak, non-uniform photoluminescence. If we want to use pristine TMDs for optoelectectronics applications, we can use different chemical reagents such as strong acid vapor for passivating surface of pristine TMDs which eventually leads to enhanced photoluminescence. In this study, we first demonstrate growth of monolayer triangular WS2 crystals using a 3-heating zone furnace using a bottom-up CVD process. The average lateral crystal size is ~20-25 µm and the largest crystal size is ~75 µm. Although, several research groups have reported WS2 growth using WO3 and S precursors, specific parameters such as precursor amount, growth substrate, growth pressure and flow rate, temperature, use of gases (e.g. N2, Ar, Ar+H2), growth time, use of promoter (e.g. PTAS, NaCl, KBr), pre-surface treatment of substrate etc. can vary widely from lab to lab,affecting the growth morphology, mechanism, light emission, Raman spectra. Atomic Force Microscopy (AFM) measurements indicate that the thickness of the monolayer WS2 is ~1 nm. We also performed SEM imaging to investigate surface morphology of monolayer WS2 and EDX to perform elemental analysis of monolayer WS2. X-ray Photoelectron Spectroscopy (XPS) has been performed for pristine WS2to reveal its chemical states. Photoluminescence spectroscopy revealed a sharp emission peak at ~626 nm confirming indirect (bulk) to direct band-gap (monolayer) transition in the monolayer. On the other hand, the PL intensity for bi/tri-layer is relatively weak compared to monolayer. Moreover, we investigate the effect of surface passivation using chemical reagents such as H2SO4-vapor for modifying light emission property of pristine WS2 for using in next generation optoelectronics. After H2SO4-vapor treatment, we achieved light emission at ~634 nm corresponding to red-shift with enhanced trion emission. Edges of H2SO4-vapor treated sample shows enhanced biexciton compared to pristine-WS2. We are able to achieve maximum 10-fold PL enhancement from our H2SO4-vapor treated sample and, on an average, we got ~5 fold enhancement. H2SO4-vapor treatment has not been used before for surface passivation. We also studied the laser power dependence PL of pristine and H2SO4-vapor treated monolayer WS2where it shows that with increasing laser power, pristine and H2SO4-vapor treated monolayer WS2shows enhanced PL specially at the crystal edges. In addition, we also focused on investigating photoemission from pristine and H2SO4-vapor treated monolayer WS2along certain lines which eventually shows PL distribution within a specific flake.
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Thesis advisor: Adachi, Michael
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