Browsing by Author "Bahauddin, Shah Mohammad"

Now showing 1 - 7 of 7
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    Broadband Absorption Engineering to Enhance Light Absorption in Monolayer MoS2
    (American Chemical Society, 2016) Bahauddin, Shah Mohammad; Robatjazi, Hossein; Thomann, Isabell; Laboratory for Nanophotonics; Rice Quantum Institute
    Here we take a first step toward tackling the challenge of incomplete optical absorption in monolayers of transition metal dichalcogenides for conversion of photon energy, including solar, into other forms of energy. We present a monolayer MoS2-based photoelectrode architecture that exploits nanophotonic light management strategies to enhance absorption within the monolayer of MoS2, while simultaneously integrating an efficient charge carrier separation mechanism facilitated by a MoS2/NiOx heterojunction. Specifically, we demonstrate two extremely thin photoelectrode architectures for solar-fuel generation: (i) a planar optical cavity architecture, MoS2/NiOx/Al, that improves optical impedance matching and (ii) an architecture employing plasmonic silver nanoparticles (Ag NPs), MoS2/Ag NPs/NiOx/Al, that further improves light absorption within the monolayer. We used a combination of numerical simulations, analytical models, and experimental optical characterizations to gain insights into the contributions of optical impedance matching versus plasmonic near-field enhancement effects in our plasmonic photoelectrode structures. By performing three-dimensional electromagnetic simulations, we predict structures that can absorb 37% of the incident light integrated from 400 to 700 nm within a monolayer of MoS2, a 5.9× enhanced absorption compared to that of MoS2 on a sapphire (Al2O3) substrate. Experimentally, a 3.9× absorption enhancement is observed in the total structure compared to that of MoS2/Al2O3, and photoluminescence measurements suggest this enhancement largely arises from absorption enhancements within the MoS2 layer alone. The results of these measurements also confirm that our MoS2/NiOx/Al structures do indeed facilitate efficient charge separation, as required for a photoelectrode. To rapidly explore the parameter space of plasmonic photoelectrode architectures, we also developed an analytical model based on an effective medium model that is in excellent agreement with results from numerical FDTD simulations.
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    Direct Plasmon-Driven Photoelectrocatalysis
    (American Chemical Society, 2015) Robatjazi, Hossein; Bahauddin, Shah Mohammad; Doiron, Chloe; Thomann, Isabell; Laboratory for Nanophotonics (LANP); Rice Quantum Institute
    Harnessing the energy from hot charge carriers is an emerging research area with the potential to improve energy conversion technologies. Here we present a novel plasmonic photoelectrode architecture carefully designed to drive photocatalytic reactions by efficient, nonradiative plasmon decay into hot carriers. In contrast to past work, our architecture does not utilize a Schottky junction, the commonly used building block to collect hot carriers. Instead, we observed large photocurrents from a Schottky-free junction due to direct hot electron injection from plasmonic gold nanoparticles into the reactant species upon plasmon decay. The key ingredients of our approach are (i) an architecture for increased light absorption inspired by optical impedance matching concepts, (ii) carrier separation by a selective transport layer, and (iii) efficient hot-carrier generation and injection from small plasmonic Au nanoparticles to adsorbed water molecules. We also investigated the quantum efficiency of hot electron injection for different particle diameters to elucidate potential quantum effects while keeping the plasmon resonance frequency unchanged. Interestingly, our studies did not reveal differences in the hot-electron generation and injection efficiencies for the investigated particle dimensions and plasmon resonances.
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    Modeling Transition Region Hot Loops on the Sun: The Necessity of Rapid, Complex Spatiotemporal Heating and Nonequilibrium Ionization
    (IOP Publishing, 2024) Bahauddin, Shah Mohammad; Bradshaw, Stephen J.
    The study examines the heating profile of hot solar transition region loops, particularly focusing on transient brightenings observed in IRIS 1400 Å slit-jaw images. The findings challenge the adequacy of simplistic, singular heating mechanisms, revealing that the heating is temporally impulsive and requires a spatially complex profile with multiple heating scales. A forward-modeling code is utilized to generate synthetic Interface Region Imaging Spectrograph (IRIS) emission spectra of these loops based on HYDRAD model output, confirming that emitting ions are out of equilibrium. The modeling further indicates that density-dependent dielectronic recombination rates must be included to reproduce the observed line ratios. Collectively, this evidence substantiates that the loops are subject to impulsive heating and that the components of the transiently brightened plasma are driven far from thermal equilibrium. Heating events such as these are ubiquitous in the transition region, and the analysis described above provides a robust observational diagnostic tool for characterizing the plasma.
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    Photon Management Strategies in Two Dimensional Molybdenum Disulfide
    (2016-07-28) Bahauddin, Shah Mohammad; Thomann, Isabell
    Here we take a first step toward tackling the challenge of incomplete optical absorption in monolayers of transition metal dichalcogenides for conversion of photon energy, including solar, into other forms of energy. We present a monolayer MoS2-based photoelectrode architecture that exploits nanophotonic light management strategies to enhance absorption within the monolayer of MoS2, while simultaneously integrating an efficient charge carrier separation mechanism facilitated by a MoS2/NiOx heterojunction. Specifically, we demonstrate two extremely thin photoelectrode architectures for solar-fuel generation: (i) a planar optical cavity architecture, MoS2/NiOx/Al, that improves optical impedance matching and (ii) an architecture employing plasmonic silver nanoparticles (Ag NPs), MoS2/Ag NPs/NiOx/Al, that further improves light absorption within the monolayer. We used a combination of numerical simulations, analytical models, and experimental optical characterizations to gain insights into the contributions of optical impedance matching versus plasmonic near-field enhancement effects in our plasmonic photoelectrode structures. By performing three-dimensional electromagnetic simulations, we predict structures that can absorb 37% of the incident light integrated from 400 to 700 nm within a monolayer of MoS2, a 5.9× enhanced absorption compared to that of MoS2 on a sapphire (Al2O3) substrate. Experimentally, a 3.9× absorption enhancement is observed in the total structure compared to that of MoS2/Al2O3, and photoluminescence measurements suggest this enhancement largely arises from absorption enhancements within the MoS2 layer alone. The results of these measurements also confirm that our MoS2/NiOx/Al structures do indeed facilitate efficient charge separation, as required for a photoelectrode. To rapidly explore the parameter space of plasmonic photoelectrode architectures, we also developed an analytical model based on an effective medium model that is in excellent agreement with results from numerical FDTD simulations. [Bahauddin, S. M., Robatjazi, H., and Thomann, I., ACS Photonics 3.5 (2016): 853-862.123]
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    The origin of reconnection-mediated transient brightenings in the solar transition region
    (Springer Nature, 2021) Bahauddin, Shah Mohammad; Bradshaw, Stephen J.; Winebarger, Amy R.
    The ultraviolet emission from the solar transition region is dominated by dynamic, low-lying magnetic loops. The enhanced spatial and temporal resolution of the solar observation satellite Interface Region Imaging Spectrograph (IRIS) has made it possible to study these structures in fine detail. IRIS has observed ‘transient brightenings’ in these loops, associated with strong excess line broadenings1,2 providing important clues to the mechanisms that heat the solar atmosphere. However, the physical origin of the brightenings is debated. The line broadenings have been variously interpreted as signatures of nanoflares3, magneto-hydrodynamic turbulence4, plasmoid instabilities5 and magneto-acoustic shocks6. Here we use IRIS slit-jaw images and spectral data, and the Atmospheric Imaging Assembly of the Solar Dynamics Observatory spacecraft, to show that the brightenings are consistent with magnetic-reconnection-mediated impulsive heating at field-line braiding sites in multi-stranded transition-region loops. The spectroscopic observations present evidence for preferential heating of heavy ions from the transition region and we show that this is consistent with ion cyclotron turbulence caused by strong currents at the reconnection sites. Time-dependent differential emission measure distributions are used to determine the heating frequency7,8,9 and to identify pockets of faintly emitting ‘super-hot’ plasma. The observations we present and the techniques we demonstrate open up a new avenue of diagnostics for reconnection-mediated energy release in solar plasma.
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    Ultrathin AAO Membrane as a Generic Template for Sub-100 nm Nanostructure Fabrication
    (American Chemical Society, 2016) Robatjazi, Hossein; Bahauddin, Shah Mohammad; Macfarlan, Luke H.; Fu, Sidan; Thomann, Isabell; Laboratory for Nanophotonics; Rice Quantum Institute; Rice Center for Quantum Materials
    Anodic aluminum oxide (AAO) templates are emerging as a platform for simple, cost-effective, high-throughput top-down nanofabrication of regular arrays of nanostructures. Thus far, however, AAO pattern transfer has largely been restricted to smooth and chemically inert surfaces, mostly Silicon substrates. Here, we present a more generalizable strategy for preparing free-standing through-hole ultrathin alumina membranes (UTAMs) and transferring them to both smooth and rough substrates, thereby enabling the fabrication of centimeter-scale arrays of nanostructures with sub-100 nm feature diameters on almost arbitrary substrates. To validate the utility of our procedures, we transferred UTAMs to surfaces relevant for photocatalytic applications and prepared plasmonic photocathodes consisting of dense arrays of size-controlled sub-100 nm Au and Ni nanodots on top of chemically noninert NiOx thin films. To demonstrate the functionality of the fabricated structures, we used a plasmonic photocathode consisting of an array of sub-50 nm Au nanodots on NiOx/Al substrates to drive direct, plasmon-enhanced photoelectrocatalysis and found excellent device performance. We also successfully decorated very rough fluorine-doped tin oxide substrates with an array of high-density sub-100 nm nanodots. Our results extend the opportunities for AAO masks to serve as generic templates for novel applications that were previously prohibited by lack of methods to transfer to the required substrate.
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    Understanding the energy balance of TR structures observed by IRIS in non-equilibrium emission
    (2019-04-18) Bahauddin, Shah Mohammad; Bradshaw, Stephen J
    The corona, the outer atmosphere of the Sun, is a multi-million degree plasma, nearly three orders magnitude hotter than the visible surface. The exact mechanism by which the corona is heated is still the subject of debate, but possibilities include magnetic reconnection and magnetohydrodynamic waves. Studying the thin boundary layer connecting the cooler chromosphere to the hotter corona, named the TR, is an important step toward understanding mass and energy transport from the chromosphere to the corona. Thus, spectral emissions from the cool (< 1 MK) loop-like structures in this region are in need of extensive study and analysis. Because observations lack sufficient spatial resolution, this type of structure was called the “unresolved fine structure”, which is now considered resolved by the Interface Region Imaging Spectrograph (IRIS). In the active TR of the Sun, IRIS has observed loop-like structures with intermittent brightenings which are thought to originate from impulsive heating. In this thesis, the author present evidence of magnetic field line braiding and reconnection mediated brightenings of TR loops using IRIS slit-jaw images and spectral data, complemented by the EUV channels of the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). The set of observables used to characterize the brightenings consists of diagnostics of temperature, density, line broadening, and Doppler-shift on a pixel-by-pixel basis. The characterization scheme is extended by accumulating time dependent differential emission measure (DEM) distributions to define the nature of the spatial heating profile and frequency. A field-aligned hydrodynamic simulation and a forward modeling code, designed to generate synthetic observations from numerical experiments for comparison with real data, are employed. Non-equilibrium ionization is included in the computation of synthetic spectra. In addition, the relatively high-density TR plasma requires the inclusion of density-dependent dielectronic recombination rates to calculate the ion populations and the emission line intensities. We show that the observations and the numerical experiments are consistent with reconnection mediated impulsive heating at the braiding sites of multi-stranded TR loops. The combination of observation and numerical analysis will provide the building blocks of time-dependent 3D models of these loops and their contribution to active region emission which will, in turn, help us to understand the energy balance of these structures and may shed light on the long standing coronal heating problem: “Why is the Sun’s corona so much hotter than the surface?”.