Results for “emission lifetime” — Page 14

DOE OSTI2022

Application of a Novel Heat Pump Model for Estimating Economic Viability and Barriers of Heat Pumps in Dairy Applications in the United States

Heat pumps represent an important opportunity for energy savings and decarbonization. This work investigates the techno-economic performance of high-temperature heat pumps (HTHPs) for use in the U.S. dairy industry. The studied heat pump performs a 50 °C temperature lift on a waste heat stream of cleaning water and applies the upgraded heat stream to a fluid milk pasteurization process. This work involved the creation of a HTHP model that estimated the coefficient of performance (COP), internal rate of return (IRR), net present value (NPV), and payback period (PBP), and emissions saved for a heat pump replacing a natural gas boiler. Capital costs, operations, and maintenance (O&M) cost, heat pump lifetime, electricity prices, natural gas prices, and a cost of carbon were varied to perform a parametric study on the factors affecting the break-even price of HTHPs. The results show that HTHP economics are highly sensitive to COP and energy price environment, and less sensitive to capital and O&M cost variance, leading to a large scatter of positive and negative NPVs based on U.S. location. PBPs demonstrate a defined threshold, based on energy price environment, below which favorable two-to-three-year PBPs predominate. This work is focused on the U.S. dairy industry, but international application in relation to fossil vs. electricity price regimes. Heat pumps have seen wider adoption in regions with a high ratio of fossil energy to electricity prices ($/MMBTU vs. $/kWh). The U.S. has plentiful natural gas resulting in lower fossil energy prices which has reduced heat pump adoption. This paper identifies potential first mover industries for HTHP adoption and their associated price regimes even in regions with lower ratios of fossil energy to electricity prices that exist many places globally.

ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATION,↗

DOE OSTI2022

Superradiance and Exciton Delocalization in Perovskite Quantum Dot Superlattices

Achieving superradiance in solids is challenging due to fast dephasing processes from inherent disorder and thermal fluctuations. Perovskite quantum dots (QDs) are an exciting class of exciton emitters with large oscillator strength and high quantum efficiency, making them promising for solid-state superradiance. However, a thorough under-standing of the competition between coherence and dephasing from phonon scattering and energetic disorder is currently unavailable. Here, we present an investigation of exciton coherence in perovskite QD solids using temperature-dependent photoluminescence line width and lifetime measurements. Our results demonstrate that excitons are coherently delocalized over 3 QDs at 11 K in superlattices leading to superradiant emission. Scattering from optical phonons leads to the loss of coherence and exciton localization to a single QD at temperatures above 100 K. Further, at low temperatures, static disorder and defects limit exciton coherence. These results highlight the promise and challenge in achieving coherence in perovskite QD solids.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2020

Spectrally reconfigurable quantum emitters enabled by optimized fast modulation

The ability to shape photon emission facilitates strong photon-mediated interactions between disparate physical systems, thereby enabling applications in quantum information processing, simulation and communication. Spectral control in solid state platforms such as color centers, rare earth ions, and quantum dots is particularly attractive for realizing such applications on-chip. Here we propose the use of frequency-modulated optical transitions for spectral engineering of single photon emission. Using a scattering-matrix formalism, we find that a two-level system, when modulated faster than its optical lifetime, can be treated as a single-photon source with a widely reconfigurable photon spectrum that is amenable to standard numerical optimization techniques. To enable the experimental demonstration of this spectral control scheme, we investigate the Stark tuning properties of the silicon vacancy in silicon carbide, a color center with promise for optical quantum information processing technologies. We find that the silicon vacancy possesses excellent spectral stability and tuning characteristics, allowing us to probe its fast modulation regime, observe the theoretically-predicted two-photon correlations, and demonstrate spectral engineering. Our results suggest that frequency modulation is a powerful technique for the generation of new light states with unprecedented control over the spectral and temporal properties of single photons.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2021

Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer

We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y x Sc 3-x N@C 80 (x = 0–3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S 1 –T 1 gap decrease in this row. For Sc 3 N@C 80 , we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin–lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with 14 N, 45 Sc, and 89 Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y 3 N to Sc 3 N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Electronic structure modulation of MoS2 by substitutional Se incorporation and interfacial MoO3 hybridization: Implications of Fermi engineering for electrocatalytic hydrogen evolution and oxygen evolution

The design of earth-abundant electrocatalysts that can facilitate water splitting at low overpotentials, provide high current densities, and enable prolonged operational lifetimes is central to the production of sustainable fuels. The distinctive atomistic and electronic structure characteristics of the edges of MoS2 imbue high reactivity toward the hydrogen evolution reaction. MoS2 is nevertheless characterized by significantly high overpotentials as compared to platinum. Here, we demonstrate that modulation of the electronic structure of MoS2 through interfacial hybridization with MoO3 and alloying of selenium on the anion sublattice allows for systematic lowering of the conduction band edge and raising of the valence band edge, respectively. The former promotes enhanced electrocatalytic activity toward hydrogen evolution, whereas the latter promotes enhanced activity toward the oxygen evolution reaction. Such alloyed heterostructures prepared by sol-gel reactions and hydrothermal selenization expose a high density of edge sites. The alloyed heterostructures exhibit low overpotential, high current density, high turnover frequency, and prolonged operational lifetime. The mechanistic origins of catalytic activity have been established based on electronic structure calculations and x-ray absorption and emission spectroscopy probes of electronic structure, which suggest that interfacial hybridization at the MoO3 interface yields low-lying conduction band states that facilitate hydrogen adsorption. In contrast, shallow Se 4p-derived states give rise to a raised effective valence band maximum, which facilitates adsorption of oxygen intermediates and engenders a low overpotential for the oxygen evolution reaction. The findings illustrate the use of electronic structure modulation through interfacial hybridization and alloying to systematically improve electrocatalytic activity.

Parija, Abhishek↗

DOE OSTI2024

β -delayed neutron spectroscopy of Co 70 , 72 ground-state and isomeric-state decays

Here, the β-decaying states of 70,72 Co were studied at the National Superconducting Cyclotron Laboratory using the VANDLE neutron time-of-flight array. The (6 - ,7 - )⁢β-decaying state in 70 Co is near-spherical with a lifetime of 113 ± 7 ms, and the low-spin (1 + ,2 + )⁢β-decaying state is postulated to be the prolate deformed ground state with a lifetime of 508 ± 7 ms. Both decay predominantly to the bound states of 70 Ni. For the first time neutron-emissions from neutron unbound states from both the (6 - ,7 - ) and (1 + ,2 + )⁢β decays were measured. Even with the low statistics data, we were able to disentangle the neutron emission from both decays, which enabled a determination of β-decay strength above the neutron separation energy of 70 Ni. Neutron emission probabilities were measured to be 7.1 ± 1.5% and 9.4 ± 1.7%, respectively, for the (6 - ,7 - ) and (1 + ,2 + ) decays. The decay pattern of the 70 Co is driven by neutron f 5/2 to proton f 7/2 Gamow-Teller transformation. The observed population of neutron unbound states is attributed to the conversion of p 1/2 and p 3/2 neutrons to p 3/2 and p 1/2 protons excited across the Z = 28 closed shell.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Isolation of Single Donors in ZnO

The shallow donor in zinc oxide (ZnO) is a promising semiconductor spin qubit with optical access. Single indium donors are isolated in a commercial ZnO substrate using plasma focused ion beam (PFIB) milling. Quantum emitters are identified optically by spatial and frequency filtering. The indium donor assignment is based on the optical bound exciton transition energy and magnetic dependence. The emission stability of these single donors in terms of both intensity and frequency, alongside their transition linewidths less than twice the lifetime limit, highlight the promise of single In donors as optically accessible spin qubits. Here, the optical stability of single donors after FIB fabrication is promising for optical device integration required for scalable quantum technologies based on single donors in direct band gap semiconductors.

42 ENGINEERING↗

DOE OSTI2021

The Future of Nuclear Energy: Electrochemical Reprocessing of Fuel Takes Center Stage

Nuclear power plants use energy-dense fuel and provide dependable baseload energy without generating greenhouse gas emissions. Despite these advantages, the long-term management of used nuclear fuel (UNF) remains a key challenge due to its lifetime (hundreds of thousands of years) and radiotoxicity. The components of UNF that contribute the most to this challenge are the actinide elements. A potential solution to this issue is to separate these radioisotopes from the bulk of the UNF and recycle them as fuel in advanced nuclear reactors. These separations can be achieved using electrochemical reprocessing, which employs electrochemical conversion and electrodeposition in a high-temperature molten salt electrolyte medium to separate the actinides from UNF. In this work, we review the current status, fundamental challenges, and future prospects of electrochemical reprocessing as they relate to UNF recycling.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2024

Spectroscopic search for optical emission lines from dark matter decay

We search for narrow-line optical emission from dark matter decay by stacking dark-sky spectra from the Dark Energy Spectroscopic Instrument (DESI) at the redshift of nearby galaxies from DESI’s Bright Galaxy and Luminous Red Galaxy samples. Our search uses regions separated by 5 to 20 arcsec from the centers of the galaxies, corresponding to an impact parameter of approximately 50 kpc. No unidentified spectral line shows up in the search, and we place a line flux limit of 10 −19 ergs/s/cm 2 /arcsec 2 on emissions in the wavelength range of 2000–9000⁢ Å. This places the tightest constraints yet on the two-photon decay of dark matter in the mass range of 5 to 12 eV, with a particle lifetime exceeding 3 × 10 25 s. Furthermore, this detection limit also implies that the line surface brightness contributed from all dark matter along the line of sight is at least 2 orders of magnitude lower than the measured extragalactic background light (EBL), ruling out the possibility that narrow optical-line emission from dark matter decay is a major source of the EBL.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2019

Combining Fluorescence and Phosphorescence to Achieve Very Long Lifetime and Efficient White OLEDs (Final Technical Report)

The objective of our project was to develop a high efficiency, long lived WOLED using a hybrid approach to harvesting singlet and triplet excitons, as described above. WE met out GONOGO metric at the end of year one, achieving an efficiency of nearly 50 lm/W for high quality white light emission. We faced an unforeseen problem in achieving this high efficiency level at high brightness. Our hybrid devices showed a significant roll off in device efficiency as the current is raised, such that our efficiency dropped to ca. 10 lm/W at 1000 nits. This roll off precluded us form achieving our send year targets with regards to efficiency at high brightness and long device lifetime. We spent the majority of our efforts in year two trying to decrease or eliminate this efficiency roll off, but this roll off prevented us from reaching our end of program target of high efficiency at high brightness with long device lifetime. Toward the end of the sixth quarter of the program we began to investigate a new approach, which involved a tandem WOLED, with a hybrid blue/red OLED stacked on a phosphorescent green OLED. The resultant emission from the stack will be white and the tandem structure relaxes the constraints on the materials, which we believe is key issue in the high roll off in efficiency.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Global Impacts of Marine Methanethiol Emissions and Chemistry in the Atmosphere

Oceanic emissions of dimethyl sulfide (DMS) have long been known to influence aerosol particle composition, cloud condensation nuclei (CCN) concentration, and Earth’s radiative budget. However, the impact of oceanic emissions of methanethiol (MeSH), a sulfur compound produced by the same oceanic precursor as DMS, has been relatively less explored. The gas-phase oxidation of MeSH has a higher effective yield of SO 2 and a shorter oxidative lifetime compared to DMS, highlighting the relevance of this pathway for the modeled representation of particle formation, growth, and CCN abundance in the marine atmosphere. Here, we use the global chemical transport model GEOS-Chem to explore possible scenarios representative of specific environmental conditions and MeSH emission schemes based on previous experimental studies. We further implement and test previously reported chemical mechanisms for MeSH oxidation, along with additional improvements, highlighting key uncertainties and sensitivities for regional and global sulfur budgets. We place our results in the context of recent modeling updates to DMS chemistry and cloud processing, which further impact SO 2 production in the marine atmosphere in parallel with MeSH oxidation. Within the overall marine sulfur budget, our findings highlight that MeSH plays a significant role in SO 2 production in the marine atmosphere, contributing to regional surface layer concentration increases of up to 40–60%. These results point to the importance of MeSH for efforts aimed at improving the modeled representation of sulfur spatiotemporal patterns relevant to air quality predictions and climate impact assessments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Towards a Circular Economy for PET Bottles in the US

The United States generates the most plastic waste of any country. Along with that GHG emissions from the global plastic economy are expected to increase to 15% of the global carbon budget by 2050. It is imperative that plastic recycling is made a reality to reduce both plastic pollution in the environment and GHG emissions. A portfolio of end-of-life strategies must be implemented to minimize environmental impacts and retain valuable plastic material, but it is challenging to compare options that generate products with different utility and lifetime. Plastic use reduction, reuse and recycling are thus increasingly important, but making informed policy and research decisions within this space can be challenging given the diverse range of available solutions. The novel analysis framework, Plastic Parallel Pathways Platform (4P) has been equipped with consequential life cycle assessment, techno-economic analysis, and a plastic circularity indicator to estimate the greenhouse gas (GHG) emissions, circularity, and cost of polyethylene terephthalate (PET) down-cycling to lower-quality resin, closed-loop recycling to food-grade PET bottles, up-cycling to fiber-reinforced plastic (FRP), and conversion to non-plastic products (electricity, oil) on a United States economy-wide basis. Integrating system dynamics into this robust plastics model that already incorporates techno-economics, circularity, and environmental impacts will enable identification of key bottlenecks between manufacturers, waste sorters, and reclaimers that currently prevent rapid decarbonization of the plastics economy. System dynamics (SD) explore the evolution of activities and technologies based on changed macro parameters such as plastic demand and supply, market shifts, and cross-sectoral interactions. This project particularly aims to explore the interplay between, waste collection, plastic waste sorting, recycling, and manufacturing, as well as the effect of plastic bale quality and plastic reuse initiatives on the surrounding process stages. This functionality will facilitate combinatory analysis in which a portfolio of end-of-life pathways are assessed simultaneously, with the exact makeup of that portfolio affected by parameters such as technology scales, resource constraints, and waste mitigation efforts. Integrating SD with the 4P framework enables analyzing the effect of increased revenue and reinvestment into improving process efficiencies, sorting and collection quantities. Through that, market effects of increased recycled resin availability can be studied for the plastics systems model for the US. The results will help identify technical or economic bottlenecks that currently limit efforts to decarbonize the U.S. plastics economy.

circular economy↗

DOE OSTI2024

Towards a Circular Economy for PET Bottles in the U.S. - 4P Model

The United States generates the most plastic waste of any country. Along with that GHG emissions from the global plastic economy are expected to increase to 15% of the global carbon budget by 2050. It is imperative that plastic recycling is made a reality to reduce both plastic pollution in the environment and GHG emissions. A portfolio of end-of-life strategies must be implemented to minimize environmental impacts and retain valuable plastic material, but it is challenging to compare options that generate products with different utility and lifetime. Plastic use reduction, reuse and recycling are thus increasingly important, but making informed policy and research decisions within this space can be challenging given the diverse range of available solutions. The novel analysis framework, Plastic Parallel Pathways Platform (4P) has been equipped with consequential life cycle assessment, techno-economic analysis, and a plastic circularity indicator to estimate the greenhouse gas (GHG) emissions, circularity, and cost of polyethylene terephthalate (PET) down-cycling to lower-quality resin, closed-loop recycling to food-grade PET bottles, up-cycling to fiber-reinforced plastic (FRP), and conversion to non-plastic products (electricity, oil) on a United States economy-wide basis. Integrating system dynamics into this robust plastics model that already incorporates techno-economics, circularity, and environmental impacts will enable identification of key bottlenecks between manufacturers, waste sorters, and reclaimers that currently prevent rapid decarbonization of the plastics economy. System dynamics (SD) explore the evolution of activities and technologies based on changed macro parameters such as plastic demand and supply, market shifts, and cross-sectoral interactions. This project particularly aims to explore the interplay between, waste collection, plastic waste sorting, recycling, and manufacturing, as well as the effect of plastic bale quality and plastic reuse initiatives on the surrounding process stages. This functionality will facilitate combinatory analysis in which a portfolio of end-of-life pathways are assessed simultaneously, with the exact makeup of that portfolio affected by parameters such as technology scales, resource constraints, and waste mitigation efforts. Integrating SD with the 4P framework enables analyzing the effect of increased revenue and reinvestment into improving process efficiencies, sorting and collection quantities. Through that, market effects of increased recycled resin availability can be studied for the plastics systems model for the US. The results will help identify technical or economic bottlenecks that currently limit efforts to decarbonize the U.S. plastics economy.

carbon↗

DOE OSTI2024

Towards a Circular Economy for PET Bottles in the U.S. - 4P Framework

The United States generates the most plastic waste of any country. Along with that GHG emissions from the global plastic economy are expected to increase to 15% of the global carbon budget by 2050. It is imperative that plastic recycling is made a reality to reduce both plastic pollution in the environment and GHG emissions. A portfolio of end-of-life strategies must be implemented to minimize environmental impacts and retain valuable plastic material, but it is challenging to compare options that generate products with different utility and lifetime. Plastic use reduction, reuse and recycling are thus increasingly important, but making informed policy and research decisions within this space can be challenging given the diverse range of available solutions. The novel analysis framework, Plastic Parallel Pathways Platform (4P) has been equipped with consequential life cycle assessment, techno-economic analysis, and a plastic circularity indicator to estimate the greenhouse gas (GHG) emissions, circularity, and cost of polyethylene terephthalate (PET) down-cycling to lower-quality resin, closed-loop recycling to food-grade PET bottles, up-cycling to fiber-reinforced plastic (FRP), and conversion to non-plastic products (electricity, oil) on a United States economy-wide basis. Integrating system dynamics into this robust plastics model that already incorporates techno-economics, circularity, and environmental impacts will enable identification of key bottlenecks between manufacturers, waste sorters, and reclaimers that currently prevent rapid decarbonization of the plastics economy. System dynamics (SD) explore the evolution of activities and technologies based on changed macro parameters such as plastic demand and supply, market shifts, and cross-sectoral interactions. This project particularly aims to explore the interplay between, waste collection, plastic waste sorting, recycling, and manufacturing, as well as the effect of plastic bale quality and plastic reuse initiatives on the surrounding process stages. This functionality will facilitate combinatory analysis in which a portfolio of end-of-life pathways are assessed simultaneously, with the exact makeup of that portfolio affected by parameters such as technology scales, resource constraints, and waste mitigation efforts. Integrating SD with the 4P framework enables analyzing the effect of increased revenue and reinvestment into improving process efficiencies, sorting and collection quantities. Through that, market effects of increased recycled resin availability can be studied for the plastics systems model for the US. The results will help identify technical or economic bottlenecks that currently limit efforts to decarbonize the U.S. plastics economy.

circular economy↗

DOE OSTI2023

Nanoscale imaging of quantum dot dimers using time-resolved super-resolution microscopy combined with scanning electron microscopy

Abstract Time-resolved super-resolution microscopy was used in conjunction with scanning electron microscopy to image individual colloidal CdSe/CdS semiconductor quantum dots (QD) and QD dimers. The photoluminescence (PL) lifetimes, intensities, and structural parameters were acquired with nanometer scale spatial resolution and sub-nanosecond time resolution. The combination of these two techniques was more powerful than either alone, enabling us to resolve the PL properties of individual QDs within QD dimers as they blinked on and off, measure interparticle distances, and identify QDs that may be participating in energy transfer. The localization precision of our optical imaging technique was ∼3 nm, low enough that the emission from individual QDs within the dimers could be spatially resolved. While the majority of QDs within dimers acted as independent emitters, at least one pair of QDs in our study exhibited lifetime and intensity behaviors consistent with resonance energy transfer from a shorter lifetime and lower intensity donor QD to a longer lifetime and higher intensity acceptor QD. For this case, we demonstrate how the combined super-resolution optical imaging and scanning electron microscopy data can be used to characterize the energy transfer rate.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Excitation energy transfer in proteoliposomes reconstituted with LH2 and RC-LH1 complexes from Rhodobacter sphaeroides

Abstract Light-harvesting 2 (LH2) and reaction-centre light-harvesting 1 (RC-LH1) complexes purified from the photosynthetic bacterium Rhodobacter (Rba.) sphaeroides were reconstituted into proteoliposomes either separately, or together at three different LH2:RC-LH1 ratios, for excitation energy transfer studies. Atomic force microscopy (AFM) was used to investigate the distribution and association of the complexes within the proteoliposome membranes. Absorption and fluorescence emission spectra were similar for LH2 complexes in detergent and liposomes, indicating that reconstitution retains the structural and optical properties of the LH2 complexes. Analysis of fluorescence emission shows that when LH2 forms an extensive series of contacts with other such complexes, fluorescence is quenched by 52.6 ± 1.4%. In mixed proteoliposomes, specific excitation of carotenoids in LH2 donor complexes resulted in emission of fluorescence from acceptor RC-LH1 complexes engineered to assemble with no carotenoids. Extents of energy transfer were measured by fluorescence lifetime microscopy; the 0.72 ± 0.08 ns lifetime in LH2-only membranes decreases to 0.43 ± 0.04 ns with a ratio of 2:1 LH2 to RC-LH1, and to 0.35 ± 0.05 ns for a 1:1 ratio, corresponding to energy transfer efficiencies of 40 ± 14% and 51 ± 18%, respectively. No further improvement is seen with a 0.5:1 LH2 to RC-LH1 ratio. Thus, LH2 and RC-LH1 complexes perform their light harvesting and energy transfer roles when reconstituted into proteoliposomes, providing a way to integrate native, non-native, engineered and de novo designed light-harvesting complexes into functional photosynthetic systems.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2022

Impact of doped barriers on the recombination coefficients of c -plane InGaN/GaN single quantum well light-emitting diodes

Differential carrier lifetime measurements were performed on c-plane InGaN/GaN single quantum well (QW) light-emitting diodes (LEDs) of different QW indium compositions as well as with and without doped barriers. Mg-doped p-type and Si-doped n-type barriers close to the QW were used to reduce the net internal electric field in the QW, thereby improving the electron–hole wavefunction overlap on the LEDs. LEDs with doped barriers show short lifetimes and low carrier densities in the active region compared to the reference LEDs. The recombination coefficients in the ABC model were estimated based on the carrier lifetime and quantum efficiency measurements. The improvement in the radiative coefficients in the LEDs with doped barriers coupled with the blueshift of the emission wavelengths indeed indicates an enhancement in wavefunction overlap and a reduction of quantum confined Stark effect as a result of the reduced internal electric field. However, doped barriers also introduce non-radiative recombination centers and thereby increase the Shockley–Read–Hall (SRH) coefficient, although the increment is less for LEDs with high indium composition QWs. As a result, at high indium composition (22%), LEDs with doped barriers outperform the reference LEDs even though the trend is reversed for LEDs with lower indium composition (13.5%). In conclusion, despite the trade-off of higher SRH coefficients, doped barriers are shown to be effective in reducing the internal electric field and increasing the recombination coefficients.

42 ENGINEERING↗

DOE OSTI2020

Engineering of Lead Selenide Quantum Dot Based Devices and Core/Shell Heterostructures

Near infrared (NIR) emitting colloidal quantum dots (QDs) such as PbSe are interesting materials for implementation in various optoelectronic devices such as solar cells, photodetectors, and radiation detectors. Material properties like size tunable emission wavelengths, facile solution processability, the possibility of carrier multiplication and Auger assisted up-conversion, and high Z number provide PbSe QDs with unique advantages over many currently available commercial materials used in these devices. In this dissertation, after an introduction to QDs in general, we first focus on synthesis of PbSe-based heterostructured QDs for study via optical spectroscopy. PbSe/CdSe QDs synthesized via cation exchange reaction were used as seeds to study the proper conditions for further CdSe or CdS shell growth. Optical spectroscopy studies show that growth of thicker CdSe shells leads to tunable visible emission, while growth of thick CdS shells can greatly increase the photoluminescence quantum yield (PLQY) observed from the intermediate CdSe shell. NIR PL lifetimes can be greatly enhanced by the growth of CdS shells, due to strong delocalization of the hole wavefunction within the PbSe core, while the electron is delocalized throughout the QD including in the CdS shell, resulting in radiative lifetimes in the tens of microseconds. Two pulse delay measurements show that Auger assisted up conversion is also taking place in these engineered QD dot heterostructures. In the sections that follow, we look at QD devices, first with a focus on novel devices fabricated from PbSe QDs, using an amine synthesis method that allows for facile in-solution ligand exchange. QD film mobilities are measured for PbSe QDs capped with various short ionic ligands. Carrier densities in PbSe QD films are calculated from C-V measurements, showing for the first time that carrier densities can be modulated by simply changing the capping ligand. vii Radiation detectors utilizing PbSe QDs capped with KI and NH4I ligands show tangible response to incident alpha radiation. Current output response to alpha radiation is dependent on strength of radiation source and amount of bias voltage applied. In the final section a critical review of hybrid layered 2D-QD photodetectors is presented. In a typical example, PbSe and PbS QDs act as the absorbing layer in phototransistor devices which utilize layered 2D materials such as graphene and transition metal dichalcogenides as transport layers. Resulting devices display high gains and improved directivities compared to QD only and 2D only devices.

36 MATERIALS SCIENCE↗