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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Hydrogen-Bonded Organic Framework Enables Phase-Pure Layered Tin Perovskite Nanowires for Room-Temperature Lasing

Room-temperature lasing is a key milestone in the development of miniaturized optoelectronic and photonic devices. We present a simple approach to synthesize phase-pure quasi-2D layered tin perovskite nanowires with varying quantum well thicknesses (n = 1 to 4). By incorporating a new organic spacer capable of forming a hydrogen-bonded organic framework, this method promoted anisotropic crystal growth and enhanced lattice rigidity. Furthermore, introducing molecular intercalants enabled controlled crystallization into well-defined nanowires that function as Fabry−Pérot cavities. Cavities made from n = 2 to 4 perovskites support efficient and robust nearinfrared, room-temperature optically pumped lasing with the threshold as low as 75.8 μJ/cm 2 , cavity quality factor over 3000, and negligible degradation over 106 pulses. A cleaved coupled nanolaser was fabricated as a proof-of-concept device for photonic applications.

Cavities↗
DOE OSTI2024

Quantum barriers engineering toward radiative and stable perovskite photovoltaic devices

Efficient photovoltaic devices must be efficient light emitters to reach the thermodynamic efficiency limit. Here, we present a promising prospect of perovskite photovoltaics as bright emitters by harnessing the significant benefits of photon recycling, which can be practically achieved by suppressing interfacial quenching. We have achieved radiative and stable perovskite photovoltaic devices by the design of a multiple quantum well structure with long (~3 nm) organic spacers with oleylammonium molecules at perovskite top interfaces. Our L-site exchange process (L: barrier molecule cation) enables the formation of stable interfacial structures with moderate conductivity despite the thick barriers. Compared to popular short (~1 nm) Ls, our approach results in enhanced radiation efficiency through the recursive process of photon recycling. This leads to the realization of radiative perovskite photovoltaics with both high photovoltaic efficiency (in-lab 26.0%, certified to 25.2%) and electroluminescence quantum efficiency (19.7 % at peak, 17.8% at 1-sun equivalent condition). Furthermore, the stable crystallinity of oleylammonium-based quantum wells enables our devices to maintain high efficiencies for over 1000 h of operation and >2 years of storage.

14 SOLAR ENERGY↗
DOE OSTI2021

Tuning Hybrid exciton–Photon Fano Resonances in Two-Dimensional Organic–Inorganic Perovskite Thin Films

As easy-to-grow quantum wells with narrow excitonic features at room temperature, two-dimensional (2D) Ruddleson–Popper perovskites are promising for realizing novel nanophotonic devices based on exciton–photon interactions. Here, we demonstrate a distinct hybrid exciton–photon Fano resonance in (C 4 H 9 NH 3 ) 2 PbI 4 thin films prepared via spin coating. Using a classical coupled-oscillator model and finite-difference time-domain simulations, we link the Fano interference to the coupling of the exciton with the Rayleigh-like scattering of the film microstructure. Combining colloidal plasmonic cavities with the 2D perovskite films, we demonstrate tuning of the Fano resonance. In combination with silver nanoparticles, the exciton–photon Fano interference couples to the in-plane plasmonic modes with indications of Rabi splitting. By creating a nanoparticle on mirror geometry, we address the out-of-plane excitonic component, reaching an intermediate coupling regime. Furthermore, these structures suggest possible photonic targets for biomolecular self-assembly applications.

2D perovskites↗
DOE OSTI2024

Frontiers in Magnetic Materials

Magnetism is crucial to many modern technologies, a driver for condensed matter physics research and one of the most remarkable and diverse properties of matter. We propose to develop understanding of novel magnetism and magnetic related behavior in materials and use this to accelerate the discovery of forefront magnetic materials. The approach is via the connection of magnetic properties to specific structures and materials. Topics that will be addressed are (1) Metallic magnetic materials with unusually low carrier concentrations and/or moments (2) Magnetism arising from unusual chemistry including 4d and 5d magnetism and (3) Materials with strong spin-fluctuations, which can lead to quantum criticality, spin-fluctuation induced superconductivity and other novel quantum behavior. These topics overlap, for example, the 4d ruthenates include ferromagnets (perovskite SrRuO 3 ), extremely high ordering temperature antiferromagnets (honeycomb lattice SrRu 2 O 6 ) and well as quantum materials with strong spin fluctuations (layered perovskite Sr 2 RuO 4 and Sr 3 Ru 2 O 7 ). We will use of density calculations to connect magnetic properties with chemistry and structure and employ phenomenological theories to extend these results to properties that are not directly given by direct first principles methods and we will conduct tests to explore the limitations of density functional approximations and new functionals.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Ultrafast Excitonic Response in Two-Dimensional Hybrid Perovskites Driven by Intense Midinfrared Pulses

Two-dimensional organic-inorganic hybrid perovskites (2DHPs) are natural quantum-well-like materials, in which strong quantum and dielectric confinement effects due to the organic spacers give rise to tightly bound excitons with large binding energy. To examine the mutual interactions between the organic spacer cations and the inorganic charge-residing octahedral framework in 2DHPs, here we perform femtosecond pump-probe spectroscopy by direct vibrational pumping of the organic spacers, followed by a visible-to-ultraviolet probe covering their excitonic resonances. Measurements on prototypical lead-bromide based 2DHP compounds, (BA) 2 ⁢PbBr 4 and (BA) 2 ⁢(FA)⁢Pb 2 ⁢Br 7 (BA+ = butylammonium; FA + = formamidinium), reveal two distinct regimes of the temporal response. The first regime is dominated by a pump-induced transient expansion of the organic spacer layers that reduces the exciton oscillator strength, whereas the second regime arises from pump-induced lattice heating effects primarily associated with a spectral shift of the exciton energy. In addition, vibrational excitation enhances the biexciton emission, which we attribute to a stronger intralayer exciton confinement as well as vibrationally induced exciton detrapping from defect states. Finally, our study provides fundamental insights regarding the impact of organic spacers on excitons in 2DHPs, as well as the excited-state dynamics and vibrational energy dissipation in these structurally diverse materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Long-range exciton transport and slow annihilation in two-dimensional hybrid perovskites

Two-dimensional hybrid organic-inorganic perovskites with strongly bound excitons and tunable structures are desirable for optoelectronic applications. Exciton transport and annihilation are two key processes in determining device efficiencies; however, a thorough understanding of these processes is hindered by that annihilation rates are often convoluted with exciton diffusion constants. Here we employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton diffusion and annihilation in twodimensional perovskites, unraveling the key role of electron-hole interactions and dielectric screening. The exciton diffusion constant is found to increase with quantum-well thickness, ranging from 0.06 ± 0.03 to 0.34 ± 0.03 cm 2 s –1 , which leads to long-range exciton diffusion over hundreds of nanometers. The exciton annihilation rates are more than one order of magnitude lower than those found in the monolayers of transition metal dichalcogenides. The combination of long-range exciton transport and slow annihilation highlights the unique attributes of two-dimensional perovskites as an exciting class of optoelectronic materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Efficient Energy Funneling in Quasi‐2D Perovskites: From Light Emission to Lasing

Abstract Quasi‐2D Ruddlesden–Popper halide perovskites with a large exciton binding energy, self‐assembled quantum wells, and high quantum yield draw attention for optoelectronic device applications. Thin films of these quasi‐2D perovskites consist of a mixture of domains having different dimensionality, allowing energy funneling from lower‐dimensional nanosheets (high‐bandgap domains) to 3D nanocrystals (low‐bandgap domains). High‐quality quasi‐2D perovskite (PEA) 2 (FA) 3 Pb 4 Br 13 films are fabricated by solution engineering. Grazing‐incidence wide‐angle X‐ray scattering measurements are conducted to study the crystal orientation, and transient absorption spectroscopy measurements are conducted to study the charge‐carrier dynamics. These data show that highly oriented 2D crystal films have a faster energy transfer from the high‐bandgap domains to the low‐bandgap domains (<0.5 ps) compared to the randomly oriented films. High‐performance light‐emitting diodes can be realized with these highly oriented 2D films. Finally, amplified spontaneous emission with a low threshold 4.16 µJ cm −2 is achieved and distributed feedback lasers are also demonstrated. These results show that it is important to control the morphology of the quasi‐2D films to achieve efficient energy transfer, which is a critical requirement for light‐emitting devices.

Lei, Lei↗
DOE OSTI2023

Mechanistic origins of excitonic properties in 2D perovskites: Implications for exciton engineering

As the field of 2D halide perovskites (HPs) matures, state-of-the-art techniques to measure important properties, such as the band gap (E g ) and exciton binding energy (E b ), continue to produce inconsistent values. Here, we tackle this long-standing problem by obtaining direct measurements of E g and E b for 31 unique HP structures. The E b values are lower than in previous literature reports and lower than expected from standard theory that assumes excitons are screened by optical-frequency dielectric constants. These low E b values are shown to be a consequence of unique screening effects, such as superlattice screening and phonon screening. Here, we find a strikingly strong correlation between E b and E g and provide design principles to a priori tune E g and E b to their optimal values. As such, this work offers a blueprint for E g -E b engineering of low-dimensional semiconductors as an even more useful replacement for simply band-gap engineering.

14 SOLAR ENERGY↗
DOE OSTI2023

Two-Factor Phase Separations in Mixed-Halide Quasi-2D Perovskite LEDs: Dimensionality and Halide Segregations

Quasi-2D halide perovskites have attracted much interest as a promising material for light-emitting diodes (LEDs) due to their tunability in quantum confinement and halide alloy formation to modulate the energy bandgap and emission color. However, two-factor phase separations with respect to heterogeneous quantum-well thicknesses and halide segregation are still crucial issues in quasi-2D perovskite LEDs, leading to low external quantum efficiencies (EQEs) and color shifts. In this work, we compare quasi-2D perovskite films using different cations to unveil the key contributions from the chemical design of organic cations. While mixing halide ions in conventional quasi-2D perovskite films induces micrometer-scale heterogeneity, new extended and twisted conjugated cations suppress the two-factor phase separations, leading to high EQEs of over 25% and controllable emission wavelengths across red and near-infrared regions. The fundamental insights in this work will provide guidance for advancing materials design and device performance in the future.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Insights into the Growth Orientation and Phase Stability of Chemical-Vapor-Deposited Two-Dimensional Hybrid Halide Perovskite Films

Chemical vapor deposition (CVD) offers a large-area, scalable, and conformal growth of perovskite thin films without the use of solvents. Low-dimensional organic–inorganic halide perovskites, with alternating layers of organic spacer groups and inorganic perovskite layers, are promising for enhancing the stability of optoelectronic devices. Moreover, their multiple quantum-well structures provide a powerful platform for tuning excitonic physics. Here, in this work, we show that the CVD process is conducive to the growth of 2D hybrid halide perovskite films. Using butylammonium (BA) and phenylethylammonium (PEA) cations, the growth parameters of BA 2 PbI 4 and PEA 2 PbI 4 and mixed halide perovskite films were first optimized. These films are characterized by well-defined grain boundaries and display characteristic absorption and emission features of the 2D quantum wells. X-ray diffraction (XRD) and a noninteger dimensionality model of the absorption spectrum provide insights into the orientation of the crystalline planes. Unlike BA 2 PbI 4 , temperature-dependent photoluminescence measurements from PEA 2 PbI 4 show a single excitonic peak throughout the temperature range from 20 to 350 K, highlighting the lack of defect states. These results further corroborate the temperature-dependent synchrotron-based XRD results. Furthermore, the nonlinear optical properties of the CVD-grown perovskite films are investigated, and a high third harmonic generation efficiency is observed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Suppressing phase disproportionation in quasi-2D perovskite light-emitting diodes

Electroluminescence efficiencies and stabilities of quasi-two-dimensional halide perovskites are restricted by the formation of multiple-quantum-well structures with broad and uncontrollable phase distributions. Here, we report a ligand design strategy to substantially suppress diffusion-limited phase disproportionation, thereby enabling better phase control. We demonstrate that extending the π-conjugation length and increasing the cross-sectional area of the ligand enables perovskite thin films with dramatically suppressed ion transport, narrowed phase distributions, reduced defect densities, and enhanced radiative recombination efficiencies. Consequently, we achieved efficient and stable deep-red light-emitting diodes with a peak external quantum efficiency of 26.3% (average 22.9% among 70 devices and cross-checked) and a half-life of ~220 and 2.8 h under a constant current density of 0.1 and 12 mA/cm 2 , respectively. Our devices also exhibit wide wavelength tunability and improved spectral and phase stability compared with existing perovskite light-emitting diodes. These discoveries provide critical insights into the molecular design and crystallization kinetics of low-dimensional perovskite semiconductors for light-emitting devices.

14 SOLAR ENERGY↗
DOE OSTI2023

Hot Carrier Dynamics in Low-Dimensional Systems

This program aimed on a fundamental level to understand the nature of hot carrier transport and thermalization in both hybrid metal halide perovskites and type-II inorganic semiconductors; as well as, to assess the role of confinement, electron phonon interactions, and the dynamic nature of these systems under external perturbations.

14 SOLAR ENERGY↗
DOE OSTI2023

Optical anisotropy of CsPbBr3 perovskite nanoplatelets

Abstract The two-dimensional CsPbBr 3 nanoplatelets have a quantum well electronic structure with a band gap tunable with sample thicknesses in discreet steps based upon the number of monolayers. The polarized optical properties of CsPbBr 3 nanoplatelets are studied using fluorescence anisotropy and polarized transient absorption spectroscopies. Polarized spectroscopy shows that they have absorption and emission transitions which are strongly plane-polarized. In particular, photoluminescence excitation and transient absorption measurements reveal a band-edge polarization approaching 0.1, the limit of isotropic two-dimensional ensembles. The degree of anisotropy is found to depend on the thickness of the nanoplatelets: multiple measurements show a progressive decrease in optical anisotropy from 2 to 5 monolayer thick nanoplatelets. In turn, larger cuboidal CsPbBr 3 nanocrystals, are found to have consistently positive anisotropy which may be attributed to symmetry breaking from ideal perovskite cubes. Optical measurements of anisotropy are described with respect to the theoretical framework developed to describe exciton fine structure in these materials. The observed planar absorption and emission are close to predicted values at thinner nanoplatelet sizes and follow the predicted trend in anisotropy with thickness, but with larger anisotropy than theoretical predictions. Dominant planar emission, albeit confined to the thinnest nanoplatelets, is a valuable attribute for enhanced efficiency of light-emitting devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Single-photon superradiance in individual caesium lead halide quantum dots

The brightness of an emitter is ultimately described by Fermi’s golden rule, with a radiative rate proportional to its oscillator strength times the local density of photonic states. As the oscillator strength is an intrinsic material property, the quest for ever brighter emission has relied on the local density of photonic states engineering, using dielectric or plasmonic resonators. By contrast, a much less explored avenue is to boost the oscillator strength, and hence the emission rate, using a collective behaviour termed superradiance. Recently, it was proposed that the latter can be realized using the giant oscillator-strength transitions of a weakly confined exciton in a quantum well when its coherent motion extends over many unit cells. Here we demonstrate single-photon superradiance in perovskite quantum dots with a sub-100 picosecond radiative decay time, almost as short as the reported exciton coherence time. The characteristic dependence of radiative rates on the size, composition and temperature of the quantum dot suggests the formation of giant transition dipoles, as confirmed by effective-mass calculations. The results aid in the development of ultrabright, coherent quantum light sources and attest that quantum effects, for example, single-photon emission, persist in nanoparticles ten times larger than the exciton Bohr radius.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Multifunctional Polar 2D Lead Iodide Perovskites Exhibiting Persistent Spin Texture

Ferroelectric Rashba semiconductors are a rare class of multifunctional materials promising for spin–orbitronics due to the possibility of electrically switchable spin textures. Hybrid organic–inorganic metal halide perovskites offer exceptional tunability that can be harnessed to target noncentrosymmetry; however, the complex interactions between organic and inorganic components are not rationally understood. Here, we use an asymmetric spacer cation with a strong molecular dipole moment (2-fluorobenzylammonium, 2FBZ) and increase the intrinsic quantum well thickness to n > 1 to synthesize three new polar 2D lead iodide perovskites (2FBZ) 2 (A) n-1 Pb n I 3n+1 with A = methylammonium (MA) or formamidinium (FA) and n = 2, 3. Single-crystal structure analysis reveals the C 2v symmetry of these crystal structures and substantial structural distortions, which enable Rashba and Dresselhaus band splitting and persistent spin texture, confirmed by DFT calculations. Measurements of the resulting symmetry-dependent properties, such as second harmonic generation, switchable photovoltaic effect, ferroelectric polarization, and low temperature photoluminescence show switchable ferroelectric semiconducting properties. Furthermore, these results introduce a general chemical design strategy toward polar symmetry by exploiting the complex interplay between asymmetric spacer cations and A-site cations in quasi-2D (n > 1) halide perovskites to realize new classes of multifunctional materials for future spin–orbitronic applications.

Cations↗
DOE OSTI2021

Mixed halide bulk perovskite triplet sensitizers: Interplay between band alignment, mid-gap traps, and phonons

Photon upconversion, particularly via triplet-triplet annihilation (TTA), could prove beneficial in expanding the efficiencies and overall impacts of optoelectronic devices across a multitude of technologies. The recent development of bulk metal halide perovskites as triplet sensitizers is one potential step toward the industrialization of upconversion-enabled devices. In this work, we investigate the impact of varying additions of bromide into a lead iodide perovskite thin film on the TTA upconversion process in the annihilator molecule rubrene. We find an interplay between the bromide content and the overall device efficiency. In particular, a higher bromide content results in higher internal upconversion efficiencies, enabled by more efficient charge extraction at the interface, likely due to a more favorable band alignment. However, the external upconversion efficiency decreases, as the absorption cross section in the near infrared is reduced. The highest upconversion performance is found in our study for a bromide content of 5%. This result can be traced back to a high absorption cross section in the near infrared and higher photoluminescence quantum yield in comparison to the iodide-only perovskite, as well as an increased driving force for charge transfer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Coherent Random Lasing in Subwavelength Quasi‐2D Perovskites

Abstract Quasi‐2D lead halide perovskites have garnered increasing interest as lasing gain media. Relatively simple fabrication, high refractive index, and unique quantum well structure encourage their use in traditional cavity lasers and cavity‐free systems called random lasers (RLs). Despite tremendous advances reported thus far, coherent random lasing in quasi‐2D perovskite subwavelength films has not been reported. Consequently, coherent optical feedback mechanisms in quasi‐2D perovskite systems are still unexplored. Here, this work reports the observation of coherent random lasing in subwavelength quasi‐2D perovskite films. Statistical analysis of spectral measurements reveals Lévy‐like intensity fluctuations, replica symmetry breaking confirms random lasing, and the coherent modes are studied with spectral and spatial correlation techniques. The observed coherent lasing modes are found to be extended states that arise from the random crystal grain structure during fabrication and span the entire pump volume. These modes out‐compete diffusive lasing due to their coherence.

47 OTHER INSTRUMENTATION↗
DOE OSTI2026

2D Semiconductor Nanosheets Supported on Colloidal Quantum Cubes

Two-dimensional (2D) colloidal nanocrystals bridge the physics of epitaxial quantum wells with the synthetic scalability of solution-processed nanomaterials, making them an attractive platform for future LEDs, lasers, and photodetectors. However, their strongly anisotropic 2D morphology complicates the formation of close-packed films required for electrically interfaced devices. Here, we address this limitation by introducing semiconductor quantum cubes (QCs) in which 2D CdSe nanosheets are conformally grown on six faces of CdS cubic scaffolds. This architecture preserves excitonic physics of quasi-2D nanosheets while leveraging the mechanical rigidity and self-assembly of nanocubes, enabling close-packed films with improved electronic coupling and electrical conductivity. By extending CdS/CdSe QCs into a CdS/CdSe/CdS core/shell/shell structure, we realize 2D quantum wells that exhibit unusual multiexciton behavior. In particular, CdS/CdSe/CdS QCs show repulsive exciton–exciton interactions, which decouple CdSe facets into quasi-independent emitters. This leads to the suppressed Auger recombination, long multiexciton lifetimes, and broadband optical gain. The demonstrated combination of unusual multiexciton photophysics and favorable film self-assembly characteristics in QCs presents opportunities for solution-processed optoelectronic devices.

perovskites↗