Energetics at the Edge: Direct Optical Mapping of Bulk and Interfacial Electronic Structure in CdSe Quantum Dots using Broadband Electronic Sum Frequency Generation Microspectroscopy

Understanding and controlling the electronic structure of nanomaterials is the key to tailoring their use in a wide range of practical applications. Despite this need, many important electronic states are invisible to conventional optical measurements and are typically identified indirectly based on their inferred impact on luminescence properties. This is especially common and important in the study of nanomaterial surfaces and their associated defects. Surface trap states play a crucial role in photophysical processes yet remain remarkably poorly understood. Here we demonstrate for the first time that broadband electronic sum frequency generation (eSFG) microspectroscopy can directly map the optically bright and dark states of nanoparticles, including the elusive below gap states. This new approach is applied to model cadmium selenide (CdSe) quantum dots (QDs), where the energies of interfacial trap states have eluded direct optical characterization for decades. Our eSFG measurements show clear signatures of electronic transitions both above the band gap, which we assign to previously reported one- and two-photon transitions associated with the CdSe core, as well as broad spectral signatures below the bandgap that are attributed to interfacial trap states. In addition to the core states, this analysis reveals two distinct distributions of below gap states providing the first direct optical measurement of both shallow and deep trapping sites on this system. Finally, chemical modification of the surfaces via oxidation results in the relative increase in the signals originating from the interfacial trap states. Overall, our eSFG experiments provide an avenue to directly map the entirety of QD bulk and interfacial electronic structure, which is expected to open up opportunities to study how these materials are grown in situ and how surface states can be controlled to tune functionality

Elucidation of Perovskite Film Micro-Orientations Using Two-Photon Total Internal Reflectance Fluorescence Microscopy

The emergence of efficient hybrid organic–inorganic perovskite photovoltaic materials has brought about the rapid development of a variety of preparation and processing techniques designed to maximize their performance. As processing methods continue to emerge, it is important to understand how the optical properties of these materials are affected on a microscopic scale. Here, polarization-resolved two-photon total internal reflectance microscopy (TIRFM) was used to probe changes in transition dipole moment orientation as a function of thermal annealing time in hybrid organic–inorganic lead-iodide-based perovskite (CH₃NH₃PbI₃) thin films on glass. These results show that as thermal annealing time is increased the distribution of transition moments pointing out-of-plane decreases in favor of forming areas with increased in-plane orientations. It was also shown through the axial sensitivity of TIRFM that the surface topography is manifested in the signal intensity and can be used to survey aspects of morphology in coincidence with the optical properties of these films

MAFG-driven astrocytes promote CNS inflammation

Multiple sclerosis is a chronic inflammatory disease of the CNS$^{1}$. Astrocytes contribute to the pathogenesis of multiple sclerosis$^{2}$, but little is known about the heterogeneity of astrocytes and its regulation. Here we report the analysis of astrocytes in multiple sclerosis and its preclinical model experimental autoimmune encephalomyelitis (EAE) by single-cell RNA sequencing in combination with cell-specific Ribotag RNA profiling, assay for transposase-accessible chromatin with sequencing (ATAC-seq), chromatin immunoprecipitation with sequencing (ChIP-seq), genome-wide analysis of DNA methylation and in vivo CRISPR-Cas9-based genetic perturbations. We identified astrocytes in EAE and multiple sclerosis that were characterized by decreased expression of NRF2 and increased expression of MAFG, which cooperates with MAT2α to promote DNA methylation and represses antioxidant and anti-inflammatory transcriptional programs. Granulocyte-macrophage colony-stimulating factor (GM-CSF) signalling in astrocytes drives the expression of MAFG and MAT2α and pro-inflammatory transcriptional modules, contributing to CNS pathology in EAE and, potentially, multiple sclerosis. Our results identify candidate therapeutic targets in multiple sclerosis

Ground-based near-UV observations of 15 transiting exoplanets: constraints on their atmospheres and no evidence for asymmetrical transits

Transits of exoplanets observed in the near-UV have been used to study the scattering properties of their atmospheres and possible star-planet interactions. We observed the primary transits of 15 exoplanets (CoRoT-1b, GJ436b, HAT-P-1b, HAT-P-13b, HAT-P-16b, HAT-P-22b, TrES-2b, TrES-4b, WASP-1b, WASP-12b, WASP-33b, WASP-36b, WASP-44b, WASP-48b, and WASP-77Ab) in the near-UV and several optical photometric bands to update their planetary parameters, ephemerides, search for a wavelength dependence in their transit depths to constrain their atmospheres, and determine if asymmetries are visible in their light curves. Here, we present the first ground-based near-UV light curves for 12 of the targets (CoRoT-1b, GJ436b, HAT-P-1b, HAT-P-13b, HAT-P-22b, TrES-2b, TrES-4b, WASP-1b, WASP-33b, WASP-36b, WASP-48b, and WASP-77Ab). We find that none of the near-UV transits exhibit any non-spherical asymmetries, this result is consistent with recent theoretical predictions by Ben-Jaffel et al. and Turner et al. The multiwavelength photometry indicates a constant transit depth from near-UV to optical wavelengths in 10 targets (suggestive of clouds), and a varying transit depth with wavelength in 5 targets (hinting at Rayleigh or aerosol scattering in their atmospheres). We also present the first detection of a smaller near-UV transit depth than that measured in the optical in WASP-1b and a possible opacity source that can cause such radius variations is currently unknown. WASP-36b also exhibits a smaller near-UV transit depth at 2.6 sigma. Further observations are encouraged to confirm the transit depth variations seen in this study.NASA's Planetary Atmospheres programme; Virginia Space Grant Consortium Graduate Research Fellowship Program; National Science Foundation [DGE-1315231]; University of Arizona Astronomy Club; Steward Observatory TAC; Lunar and Planetary LaboratoryThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]