R-3 Repository :: Browsing by Author "Thomann, Isabell"

Browsing by Author "Thomann, Isabell"

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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.
Direct plasmon-driven photoelectrocatalysis
(2016-05-18) Robatjazi, Hossein; Thomann, Isabell
Harnessing the energy from hot charge carriers is an emerging research area with the potential to improve energy conversion technologies. This thesis present a novel plasmonic photoelectrode architecture carefully designed to drive photocatalytic reactions by efficient, non-radiative 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 with heterogeneous particle size distribution to adsorbed water molecules. Also, the quantum efficiency of hot electron injection for different particle diameters has been investigated to elucidate potential quantum effects while keeping the plasmon resonance frequency unchanged. This thesis also present a simple strategy to prepare free-standing through-hole ultrathin alumina membranes (UTAMs) for efficient sub-100 nm nanoarray fabrication that, in contrast to past works, can be generalized to any substrate and material. The potential of developed strategy for nanoarray fabrication has been demonstrated through fabrication of centimeter-scale of dense plasmonic nanoarray of sub-100 nm nanodots on very susceptible and rough substrates. Subsequently, the fabricated nanoarray has been employed for direct plasmon-driven photoelectrocatalysis of water.
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.
Femtosecond Carrier Dynamics in Metal/Quasi-2D MoS2 Nanostructures
(2016-04-25) Doiron, Chloe; Thomann, Isabell
Plasmonic nanoparticles and quasi-2D (Q2D) transition metal dichalcogenides (TMDs) have been identified as promising materials for solar-to-fuel energy conversion. Plasmonically active materials are interesting because large absorption cross-sections and non-radiative decay of plasmons that can excite hot electrons for injection into semiconducting materials. Q2D MoS$_2$ is known to be highly catalytically active for driving the hydrogen evolution reaction (HER). Combined together plasmonically active particles and MoS$_2$ can act as a hybrid antenna/catalyst nanostructures with both high absorption and catalytic activity. We performed femtosecond transient absorption spectroscopy measurements of Au/MoS$_2$ hybrid nanostructures finding ultrafast signatures of hot electron generation in the form of "anomalous" sub-100 fs lifetime signals indicative of electron-electron scattering. Coherent generation of acoustic phonon modes was also observed allowing for estimation of the peak electron temperature during excitation. Near field scanning probe microscopy measurements showed the presence of hot spots that may be responsible for hot electron generation observed in Au/MoS$_2$ hybrid nanostructures.
Label-free Imaging of Thyroid and Parathyroid Glands Using Coherent Anti-Stokes Raman Scattering (CARS) Microscopy
(2015-04-28) Weng, Sheng; Kelly, Kevin; Wong, Stephen; Thomann, Isabell; Kono, Junichiro
Thyroid and parathyroid glands play a vital role in regulating the body's metabolism and calcium levels. Surgical removal of the glands is the main treatment for both thyroid cancer and parathyroid adenoma. In thyroidectomy and parathyroidectomy, it's very important to differentiate thyroid, parathyroid, and the other tissues around the neck. Traditionally, physicians use ultrasound guided fine needle aspiration (FNA) to evaluate thyroid nodules, but up to 30% of FNA results are “inconclusive”. The sestamibi scan can localize parathyroid adenoma, but currently it only has 50% accuracy. Here we applied the emerging CARS technique to image both thyroid and parathyroid tissues, which has potential to be used in real-time in vivo examination of different structures. We also developed algorithms to differentiate different cellular structures based on CARS images. When incorporated with a fiber optic endoscope in the future, CARS imaging technique can help surgeons identify cancerous thyroid tissue intraoperatively, preserve good parathyroid glands during thyroidectomy and find parathyroid adenoma during parathyroidectomy.
Photoinduced force mapping of plasmonic nanostructures
(American Chemical Society, 2016) Tumkur, Thejaswi U.; Yang, Xiao; Cerjan, Benjamin; Halas, Naomi J.; Nordlander, Peter; Thomann, Isabell; Laboratory for Nanophotonics; Rice Quantum Institute
The ability to image the optical near-fields of nanoscale structures, map their morphology and concurrently obtain spectroscopic information, all with high spatiotemporal resolution, is a highly sought-after technique in nanophotonics. As a step towards this goal, we demonstrate the mapping of electromagnetic forces between a nanoscale tip and an optically excited sample consisting of plasmonic nanostructures, with an imaging platform based on atomic force microscopy. We present the first detailed joint experimental-theoretical study of this type of photo-induced force microscopy. We show that the enhancement of near-field optical forces in gold disk dimers and nanorods follows the expected plasmonic field enhancements, with strong polarization sensitivity. We then introduce a new way to evaluate optically-induced tip-sample forces by simulating realistic geometries of the tip and sample. We decompose the calculated forces into in-plane and out-of-plane components and compare the calculated and measured force enhancements in the fabricated plasmonic structures. Finally, we show the usefulness of photo-induced force mapping for characterizing the heterogeneity of near-field enhancements in precisely e-beam fabricated nominally alike nanostructures - a capability of widespread interest for precise nanomanufacturing, SERS and photocatalysis applications.
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]
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.

Browsing by Author "Thomann, Isabell"

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