Here, we present detailed results of Raman and photoluminescence (PL) characterization of monolayers of MoS2 grown by chemical vapor deposition (CVD) on SiO2/Si substrates after thermal annealing at 150 °C, 200 °C, and 250 °C in an argon atmosphere. In comparison to the as-grown monolayers, annealing in the temperature range of 150–250 °C brings about significant changes in the band edge luminescence. It is observed that annealing at 150 °C gives rise to a 100-fold increase in the PL intensity and produces a strong band at 1.852 eV attributed to a free-to-bound transition that dominates over the band edge excitonic luminescence. This band disappears for the higher annealing temperatures. The improvement in PL after the 200 °C anneal is reduced in comparison to that obtained after the 150 °C anneal; this is suggested to arise from a decrease in the non-radiative lifetime caused by the creation of sulfur di-vacancies. Annealing at 250 °C degrades the PL in comparison to the as-grown sample because of the onset of disorder/decomposition of the sample. It is clear that the PL features of the CVD-grown MoS2 monolayer are profoundly affected by thermal annealing in Ar atmosphere. However, further detailed studies are needed to identify, unambiguously, the role of native defects and/or adsorbed species in defining the radiative channels in annealed samples so that the beneficial effect of improvement in the optical efficiency of the MoS2 monolayers can be leveraged for various device applications.
Efficient unconditionally stable FDTD method is developed for the electromagnetic analysis of dispersive media. Toward this purpose, a quadratic complex rational function (QCRF) dispersion model is applied to the alternating-direction-implicit finite-difference time-domain (ADI-FDTD) method. The 3-D update equations of QCRF-ADI-FDTD are derived using Maxwell's curl equations and the constitutive relation. The periodic boundary condition of QCRF-ADI-FDTD is discussed in detail. A 3-D numerical example shows that the time-step size can be increased by the proposed QCRF-ADI-FDTD beyond the Courant-Friedrich-Levy (CFL) number, without numerical instability. It is observed that, for refined computational cells, the computational time of QCRF-ADI-FDTD is reduced to 28.08 % of QCRF-FDTD, while the L2 relative error norm of a field distribution is 6.92 %.
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