Abstract |
The discovery of graphene has created a new class of materials, the so-called two-dimensional (2D) materials, which exhibit strong optical properties. Transition metal dichalcogenides (TMDs) have the form of MX2 (where M=Mo or W and X=S, Se or Te). As the thickness decreases, these crystals as monolayers exhibit a direct band gap at the K point of the Brillouin zone, in contrast to their bulk counterparts. Their tunable direct gap and ultrathin form makes 2D materials the forefront of research for many optoelectronic applications.
Here, monolayers of molybdenum diselenide (MoSe2) are investigated after photochemical doping with spectroscopic methods such as photoluminescence (PL), differential reflectivity (DR) and Raman spectroscopy. This doping process is called photochlorination and includes UV laser irradiation at chlorine (Cl2) environment. This photoinduced doping method seems to result in an e-density reduction (p-type doping) on 1-L MoSe2, as predicted from theory.
The PL can be significantly enhanced after photochlorination and it is dominated by the neutral exciton emission as a consequence of the adsorption of electron-withdrawing chlorine adatoms that strongly suppress the electron concentration. Differential reflectivity measurements performed in the same monolayer are in remarkable agreement with the micro-PL data.
Additionally, the optical response of tin disulfide (SnS2) monolayers was also studied in this thesis. It was confirmed (with photoluminescence and differential reflectivity) that SnS2 is an indirect band gap semiconductor, even as a monolayer. It was shown that the intensity of the out-of-plane mode (A1g) is thickness dependent with Raman spectroscopy. Thus a linear equation of the Raman intensity ratio (Isample/Isubstrate) as a function of the number of layers was constructed using AFM and Raman spectroscopy.
The results of the MoSe2 photochlorination take us one step closer to fully clarify the mechanism of this doping process. Having control of the carrier density as well as improving the optoelectronic properties of 2D materials is very important for the fabrication of TMD-based field effect transistors (FETs), photovoltaic, light emitting diodes (LEDs) etc.
|