159 research outputs found
On the nature of tone
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Linguistics and Philosophy, 1990.Includes bibliographical references (leaves 327-337).by Zhiming Bao.Ph.D
On-surface synthesis of size- and shape-controlled two-dimensional Aun nanoclusters using a flexible fullerene molecular template
Idealizing Tauc Plot for Accurate Bandgap Determination of Semiconductor with UV-Vis: A Case Study for Cubic Boron Arsenide
The Tauc plot method is widely used to determine the bandgap of
semiconductors via UV-visible optical spectroscopy due to its simplicity and
perceived accuracy. However, the actual Tauc plot often exhibits significant
baseline absorption below the expected bandgap, leading to discrepancies in the
calculated bandgap depending on whether the linear fit is extrapolated to zero
or non-zero baseline. In this study, we show that both extrapolation methods
can produce significant errors by simulating Tauc plots with varying levels of
baseline absorption. To address this issue, we propose a new method that
involves idealizing the absorption spectrum by removing its baseline before
constructing the Tauc plot. Experimental verification of this method using a
gallium phosphide (GaP) wafer with intentionally introduced baseline
absorptions shows promising results. Furthermore, we apply this new method to
cubic boron arsenide (c-BAs) and resolve discrepancies in c-BAs bandgap values
reported by different groups, obtaining a converging bandgap of 1.835 eV based
on both previous and new transmission spectra. The method is applicable to both
indirect and direct bandgap semiconductors, regardless of whether the
absorption spectrum is measured via transmission or diffuse reflectance, will
become essential to obtain accurate values of their bandgaps
Photoacoustic Identification of Laser-induced Microbubbles as Light Scattering Centers for Optical Limiting in Liquid Suspension of Graphene Nanosheets
Liquid suspensions of carbon nanotubes, graphene and transition metal
dichalcogenides have exhibited excellent performance in optical limiting.
However, the underlying mechanism has remained elusive and is generally
ascribed to their superior nonlinear optical properties such as nonlinear
absorption or nonlinear scattering. Using graphene as an example, we show that
photo-thermal microbubbles are responsible for the optical limiting as strong
light scattering centers: graphene sheets absorb incident light and become
heated up above the boiling point of water, resulting in vapor and microbubble
generation. This conclusion is based on direct observation of bubbles above the
laser beam as well as a strong correlation between laser-induced ultrasound and
optical limiting. In-situ Raman scattering of graphene further confirms that
the temperature of graphene under laser pulses rises above the boiling point of
water but still remains too low to vaporize graphene and create graphene plasma
bubbles. Photo-thermal bubble scattering is not a nonlinear optical process and
requires very low laser intensity. This understanding helps us to design more
efficient optical limiting materials and understand the intrinsic nonlinear
optical properties of nanomaterials
Theory and Experiments of Pressure-Tunable Broadband Light Emission from Self-Trapped Excitons in Metal Halide Crystals
Hydrostatic pressure has been commonly applied to tune broadband light
emissions from self-trapped excitons (STE) in perovskites for producing white
light and study of basic electron-phonon interactions. However, a general
theory is still lacking to understand pressure-driven evolution of STE
emissions. In this work we first identify a theoretical model that predicts the
effect of hydrostatic pressure on STE emission spectrum, we then report the
observation of extremely broadband photoluminescence emission and its wide
pressure spectral tuning in 2D indirect bandgap CsPb2Br5 crystals. An excellent
agreement is found between the theory and experiment on the peculiar
experimental observation of STE emission with a nearly constant spectral
bandwidth but linearly increasing energy with pressure below 2 GPa. Further
analysis by the theory and experiment under higher pressure reveals that two
types of STE are involved and respond differently to external pressure. We
subsequently survey published STE emissions and discovered that most of them
show a spectral blue-shift under pressure, as predicted by the theory. The
identification of an appropriate theoretical model and its application to STE
emission through the coordinate configuration diagram paves the way for
engineering the STE emission and basic understanding of electron-phonon
interaction
Linguistics
Contains table of contents for Section 4, an introduction and abstracts on eight doctoral dissertations
Bifunctional metal phosphide FeMnP films from single source metal organic chemical vapor deposition for efficient overall water splitting
Developing stable and efficient bifunctional catalysts for overall water splitting into hydrogen and oxygen is a critical step in the realization of several clean-energy technologies. Here we report a robust and highly active electrocatalyst that is constructed by deposition of the ternary metal phosphide FeMnP onto graphene-protected nickel foam by metal-organic chemical vapor deposition from a single source precursor. FeMnP exhibits high electrocatalytic activity toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Utilizing FeMnP/GNF as both the anode and the cathode for overall water splitting, a current density of 10 mA cm−2 is achieved at a cell voltage of as low as 1.55 V with excellent stability. Complementary density functional theory (DFT) calculations suggest that facets exposing both Fe and Mn sites are necessary to achieve high HER activity. The present work provides a facile strategy for fabricating highly efficient electrocatalysts from earth-abundant materials for overall water splitting
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