Optical, magnetic, and electronic anisotropies in low-dimensional materials

Optical spectroscopic techniques with linearly polarized light are used to characterize the nature of optical, magnetic, and electronic anisotropies within a variety of materials. Optical anisotropy studies coupled with magnetic torque measurements probe the origin of broadband photoluminescence (PL) reported from graphene oxide (GO) and demonstrate the two-dimensional character of electron-hole pairs for both absorptive and emissive processes, supporting theories in which the PL arises fundamentally from a gapping of the parent compound, graphene. Strong in-plane polarization memory of GO requires that we generalize prior optical anisotropy models that assume separable absorption and emission processes to account for preservation of the incident polarization in the emission due to electronic correlations. Subpicosecond time-resolved PL measurements on GO and reduced GO (rGO) indicate that, in contrast to prior suggestions, the PL spectra of GO and rGO are inhomogeneously broadened. We observe substantial energy redistribution and relaxation among the emitting states within the first few picoseconds, leading to a progressive red shift of the emission spectrum. Blue shifts that arise in time-integrated spectra upon photothermal reduction are easily understood within this dynamical context without invoking a modified distribution of dipole-coupled states. Rather, reduction increases the nonradiative electron-hole recom- bination rate and curtails the red-shifting process, which is consistent with an increase in quenching through the introduction of larger and/or more numerous sp2 clusters. In-plane polarization memory measurements show similar energetic signatures of electron-hole correlations at all levels of reduction that are established on a subpicosecond time scale and develop little thereafter. Polarized PL studies on 9-(3,5-di(naphthalene-l-yl)phenyl)anthracene (α,α-A) thin films confirm a predominantly isotropic ordering of anthracene molecules at film deposition temperatures -9.75°C - 64.4°C, indicating a negative refractive index anisotropy, Δn, which contrasts with the positive Δn measured via ellipsometry. Polarized Raman scattering on single wall carbon nanotube (SWNT) films formed using urea-functionalized SWNTs con- firms strong alignment, where the alignment direction varies across the film surface. In- plane polarized PL studies on single-layer molybdenum disulfide (MoS2) with plasmonic nano-antennae verifies a polarization-dependent enhancement of the PL and additionally identifies spatial imaging of MoS2 PL as a robust technique for identifying single-layer regions