Magnetospectroscopy on WS2 single layer crystals
Scrace, Thomas A., Jr.
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This dissertation is a magneto-optical study of two-dimensional tungsten disulfide (WS2) single layer crystals. Tungsten disulfide is a semiconductor from the group named transition metal dichalcogenides (TMDs). Single layer TMDs have characteristics that exhibit new physics, when compared to their bulk counterparts. The locking of valley and spin quantum numbers, as well as a bandgap in the visible region of the electromagnetic spectrum, makes TMDs suitable for opto-electronic device applications. We used various experimental techniques, including magneto-photoluminescence and reflectivity, to study the properties of WS2. The first set of experimental results in this dissertation focuses on the circularly polarized photoluminescence emission of the trion state in the presence of a spontaneously polarized two-dimensional electron gas, at zero magnetic field, using linearly polarized excitation. The circular polarization of the emission changes at a rate, dP/dB, of 2 %/T, as the magnetic field applied perpendicular to the sample plane is varied from B = -7 T to 7 T. The rate dP/dB is not affected by an increase in the sample temperature as well. The accompanying theory predicts that there exists a series of electron droplets, with varying densities, throughout the sample. The droplets with electron densities below a critical density value will cause the fully polarized state of the two-dimensional electron gas to be the ground state of the system. When this happens, there is spontaneous circular polarization of the photoluminescence of the X--2DEG feature. The second set of experimental results in this dissertation is a study of three different WS2 single layer crystals with varying electron densities. The study of the circular polarization of the photoluminescence emission as a function of magnetic field gives results that show all three samples have the same rate of change, dP/dB (2 %/T). This is surprising because sample 1 (from the first set of results) shows a spontaneous circular polarization of the photoluminescence emission at zero magnetic field, while the other two samples do not. The effective Zeeman splitting was also studied as a function of magnetic field. These results show that all three samples have a different rate, dEz/dB, with changing magnetic field, with sample 3 having the largest and sample 1 the smallest. The accompanying theory predicts that sample 3 has the highest electron density while sample 1 has the lowest. This study was an extension of the previous results on sample 1 and strengthens our prediction that it has a low electron density, smaller than the critical value, to emit photons with spontaneous circular polarization at zero magnetic field.