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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Decision making based on optical excitation transfer via near-field interactions between quantum dots

Optical near-field interactions between nanostructured matters, such as quantum dots, result in unidirectional optical excitation transfer when energy dissipation is induced. This results in versatile spatiotemporal dynamics of the optical excitation, which can be controlled by engineering the dissipation processes and exploited to realize intelligent capabilities such as solution searching and decision making. Here, we experimentally demonstrate the ability to solve a decision making problem on the basis of optical excitation transfer via near-field interactions by using colloidal quantum dots of different sizes, formed on a geometry-controlled substrate. We characterize the energy transfer behavior due to multiple control light patterns and experimentally demonstrate the ability to solve the multi-armed bandit problem. Our work makes a decisive step towards the practical design of nanophotonic systems capable of efficient decision making, one of the most important intellectual attributes of the human brain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Fabricating Single Crystal Quantum Dot Solids

The central goal of this research program was to establish fundamental synthesis and processing principles to enable the fabrication of single crystal quantum dot solids (QDS) with programmable structure (i.e., hexagonal and square) and composition. Our approach towards that goal leveraged access to and experience with unique in-situ, multi-probe characterization techniques to understand and ultimately control the fundamental relationship between processing conditions and nucleation and growth of QDS. The program integrated synthesis and fabrication (Hanrath) with in-situ TEM analysis (Kourkoutis) and computational modeling (Clancy) to gain atomic-level insights into the underlying physical phenomena governing assembly and attachment and to guide the development of optimized processing methods. We have established a foundational understanding of physicochemical interactions of self-assembly at a fluid interface, superlattice structure transformation pathways, the critical role of disorder during the initial dimerization of colloidal quantum dot monomers, and the residual strain distribution within the inter-dot epitaxial bridge which hampers the formation of novel electronics states of the quantum dot solids. Collectively, these insights have contributed towards advancing the mechanistic understanding of the complex choreography of assembly and attachment as well as understanding what currently limits further advances in high-fidelity quantum dot solids and tiles.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Morphology of Passivating Organic Ligands around a Nanocrystal

Semiconductor nanocrystals are an enticing group of materials for a variety of novel optoelectronic devices, since many of their properties, such as the electronic gap and conductivity, can be controlled. Much of this control is achieved via the organic ligand shell, through control of the size of the nanocrystal and the distance to other objects. In this work, we simulate ligand-coated CdSe nanocrystals using atomistic molecular dynamics, allowing for the resolution of novel structural details about the ligand shell. We show that the ligands on the surface can lie flat to form a highly anisotropic “wet hair” layer as opposed to the “spiky ball” appearance typically considered. We discuss how this can give rise to a dot-to-dot packing distance of one ligand length since the thickness of the ligand shell is reduced to approximately one-half of the ligand length for the system sizes considered here; these distances imply that energy and charge transfer rates between dots and nearby objects will be enhanced due to the thinner-than-expected ligand shell. Our model suggests a non-linear scaling of ligand shell thickness as the ligands transition from “spiky” to “wet hair”. We confirm this scaling using transmission electron microscopy on a PbS nanoarray, confirming that this theory gives a qualitatively correct picture of the ligand shell thickness of colloidal quantum dots.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Single photon sources with near unity collection efficiencies by deterministic placement of quantum dots in nanoantennas

Deterministic coupling between photonic nodes in a quantum network is an essential step toward implementing various quantum technologies. The omnidirectionality of free-standing emitters, however, makes this coupling highly inefficient, in particular if the distant nodes are coupled via low numerical aperture (NA) channels such as optical fibers. This limitation requires placing quantum emitters in nanoantennas that can direct the photons into the channels with very high efficiency. Moreover, to be able to scale such technologies to a large number of channels, the placing of the emitters should be deterministic. In this work, we present a method for directly locating single free-standing quantum emitters with high spatial accuracy at the center of highly directional bullseye metal–dielectric nanoantennas. We further employ non-blinking, high quantum yield colloidal quantum dots for on-demand single-photon emission that is uncompromised by instabilities or non-radiative exciton recombination processes. Taken together, this approach results in a record-high collection efficiency of 85% of the single photons into a low NA of 0.5, setting the stage for efficient coupling between on-chip, room temperature nanoantenna-emitter devices and a fiber or a remote free-space node without the need for additional optics.

36 MATERIALS SCIENCE↗
DOE OSTI2022

No One Size Fits All: Semiconductor Nanocrystal Sizing Curves

Although colloidal quantum dots or nanocrystals (NCs) are now long past their infancy, there remain basic issues that must be addressed to fully establish their fundamental, size-dependent properties. This is especially true for model systems such as CdSe where confinement effects are responsible for changes in their optical response and where significant effort has been invested in developing theories of their underlying electronic structure. Surprisingly, there still appears to be a lack of consensus on one of the most fundamental metrics for NCs, their sizing curves. As shown here, these are empirical functions that link the size of isotropically shaped semiconductor NCs (such as the diameter, d, for spherical NCs and edge length, l, for cube-shaped ones) to corresponding changes in optical response.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Circularly Polarized Optical Stark Effect in CdSe Colloidal Quantum Wells

Colloidal quantum wells, or nanoplatelets, exhibit large, circularly polarized optical Stark effects under sub-band-gap femtosecond illumination. The optical Stark effect is measured for CdSe colloidal quantum wells of several thicknesses and separately as a measure of pump photon energy, pump fluence, and temperature. These measurements show that optical Stark effects in colloidal quantum wells shift the absorption features up to 5 meV, at the intensities up to 2.9 GW.cm -2 and large detuning (>400 meV) of the pump photon energy from the band edge absorption. Optical Stark shifts are underpinned by large transition dipoles of the colloidal quantum wells (mu = 15-23 D), which are larger than those of any reported colloidal quantum dots or epitaxial quantum wells. The rapid (<500 fs), narrow band blue shift of the excitonic features under circular excitation indicates the viability of these materials beyond light emission such as spintronics or all-optical switching.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Diffusion-Limited Kinetics of Isovalent Cation Exchange in III–V Nanocrystals Dispersed in Molten Salt Reaction Media

The goal of this work is to determine the kinetic factors that govern isovalent cation exchange in III–V colloidal quantum dots using molten salts as the solvent and cation source. We focus on the reactions of InP + GaI 3 → In 1 – x Ga x P and InAs + GaI 3 → In 1–x Ga x As to create technologically important ternary III–V phases. We find that the molten salt reaction medium causes the transformation of nearly spherical InP nanocrystals to tetrahedron-shaped In 1–x Ga x P nanocrystals. Furthermore, we determine that the activation energy for the cation exchange reaction is 0.9 eV for incorporation of Ga into InP and 1.2 eV for incorporation of Ga into InAs, both much lower than the measured values in bulk semiconductors. Next, we use powder XRD simulations to constrain our understanding of the structure of the In 1–x Ga x P nanocrystals. Together our results reveal several important features of molten salt-mediated cation exchange and provide guidance for future development of these materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Efficient Light Harvesting Biotic–Abiotic Nanohybrid System Incorporating Atomically Thin van der Waals Transition Metal Dichalcogenides

Inspired by nature’s light harvesting complex architectures, which bring together, in one unit, several active nanocomponents to harvest light and to flow the harvested energy in precise directions and places, we designed and assembled a biotic/abiotic stepwise energy transfer nanohybrid system that is based on an atomically thin, two-dimensional (2D) van der Waals semiconductor, colloidal quantum dots, and a light harvesting protein. We utilized the van der Waals semiconductor as a 2D platform for bottom-up assembly, connecting nanocomponents via electrostatic self-assembly and layer-by-layer polyelectrolyte deposition. Through bandgap engineering of the self-assembled active components, we created conditions for efficient directional stepwise energy transfer, from optically excited quantum dots, to proteins and finally to the 2D van der Waals semiconductor, here molybdenum diselenide (MoSe 2 ). A field effect transistor incorporating the biotic/abiotic nanohybrid was demonstrated to exhibit enhanced spectral responsivity when compared to the pristine device (2D van der Waals semiconductor only). Furthermore, this is because the biotic/abiotic nanohybrid exhibits enhanced absorption cross section and enhanced light harvesting through the addition of quantum dots and proteins next to MoSe 2 , which in turn leads to enhanced exciton generation in MoSe 2 via energy transfer from quantum dots to proteins and finally to MoSe 2 .

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Self-Assembled Framework Enhances Electronic Communication of Ultrasmall-Sized Nanoparticles for Exceptional Solar Hydrogen Evolution

Colloidal quantum dots (QDs) have demonstrated great promise in artificial photosynthesis. However, the ultrasmall size hinders its controllable and effective interaction with cocatalysts. To improve the poor interparticle electronic communication between free QD and cocatalyst, we design here a self-assembled architecture of nanoparticles, QDs and Ptnanoparticles, simply jointed together by molecular polyacrylate to greatly enhance the rate and efficiency of interfacial electron transfer (ET). The enhanced interparticle electronic communication is confirmed by femtosecond transient absorption spectroscopy and X-ray transient absorption. Taking advantage of the enhanced interparticle ET with a time scale of ~65 ps, 5.0 mL assembled CdSe/CdS QDs/cocatalysts solution produces 94 ± 1.5 mL (4183 ± 67 µmol) molecular H 2 in 8 h, giving rise to an internal quantum yield of ~65% in the first 30 min and a total turnover number of >16,400,000 per Pt-nanoparticle. This study demonstrates that self-assembly is a promising way to improve the sluggish kinetics of interparticle ET process, which is the key step for advanced H 2 photosynthesis.

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

Peptoid-directed assembly of CdSe nanoparticles

The high information content of proteins drives their hierarchical assembly and complex function, including the organization of inorganic nanomaterials. Peptoids offer an organic scaffold very similar to proteins, but with a wider solubility range and easily tunable side chains and functional groups to create a variety of self-assembling architectures with atomic precision. If we could harness this paradigm and understand the factors that govern how they direct nucleation and assembly of inorganic materials to design order within such materials, new dimensions of function and fundamental science would emerge. In this work, peptoid tubes and sheets were explored as platforms to assemble colloidal quantum dots (QDs) and clusters. We have successfully synthesized CdSe QDs with difunctionalized capping ligands containing both carboxylic acid and thiol groups and mixed them with maleimide containing peptoids, to create an assembly of the QDs on the peptoid surface via a covalent linkage. This conjugation was seen to be successful with peptoid tubes, sheets and CdSe QDs and clusters. The particles were seen to have a high preference for the peptoid surface but non-specific interactions with carboxylic acid groups on the peptoids limited control over QD density via maleimide conjugation. Replacing the carboxylic acid groups with methoxy ethers, however, allowed for control over QD density as a function of maleimide concentration. 1 H NMR analysis demonstrated that binding of QDs to peptoids involved a subset of surface ligands bound through the carboxylate functional group, allowing the distal thiol to engage in a covalent linkage to the maleimide. Overall, we have shown the compatibility and control of CdSe-peptoid interactions via a covalent linkage with varying peptoid structures and CdSe particles to create complex hybrid structures.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Alternate InP synthesis with aminophosphines: solution–liquid–solid nanowire growth

Indium phosphide nanowires are important components in high-speed electronics and optoelectronics, including photodetectors and photovoltaics. However, most syntheses either use high-temperature and costly vapor-phase methodology or highly toxic and pyrophoric tris(trimethylsilyl)phosphine. To expand on the success of the aminophosphine-based InP colloidal quantum dot synthesis, we developed a synthesis for thin (~11 nm) zinc blende InP nanowires at 180 °C using indium tris(trifluoroacetate) and tris(diethylamino)phosphine. A flat nanoribbon morphology was identified by transmission electron and atomic force microscopy analysis, with the stoichiometric (110) lattice plane exposed. Nanowire growth proceeded through a solution–liquid–solid mechanism from in situ-formed indium metal nanoparticles. Molecular byproducts of tris(oleylamino)phosphine oxide and N-oleyltrifluoroacetamide observed by 31 P and 19 F NMR spectroscopy inform a proposed mechanism of indium reduction by the aminophosphine. Morphological control over the nanowire product was achieved by varying the phosphorus injection to control the aspect ratio, the In : P ratio to toggle between nanowires and multipods, and the pre-hot injection evacuation step to favor a quantum dot product. Furthermore, replacing the indium precursor with indium tris(trifluoromethanesulfonate) was found to make bulk zinc blende InP nanowires with an average diameter of >250 nm and tens of microns in length.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Time-Resolved Line Shapes of Single Quantum Emitters via Machine Learned Photon Correlations

Solid-state single-photon emitters (SPEs) are quantum light sources that combine atomlike optical properties with solid-state integration and fabrication capabilities. SPEs are hindered by spectral diffusion, where the emitter’s surrounding environment induces random energy fluctuations. Timescales of spectral diffusion span nanoseconds to minutes and require probing single emitters to remove ensemble averaging. Photon correlation Fourier spectroscopy (PCFS) can be used to measure time-resolved single emitter line shapes, but is hindered by poor signal-to-noise ratio in the measured correlation functions at early times due to low photon counts. Here, we develop a framework to simulate PCFS correlation functions directly from diffusing spectra that match well with experimental data for single colloidal quantum dots. We use these simulated datasets to train a deep ensemble autoencoder machine learning model that outputs accurate, noiseless, and probabilistic reconstructions of the noisy correlations. Using this model, we obtain reconstructed time-resolved single dot emission line shapes at timescales as low as 10 ns, which are otherwise completely obscured by noise. This enables PCFS to extract optical coherence times on the same timescales as Hong-Ou-Mandel two-photon interference, but with the advantage of providing spectral information in addition to estimates of photon indistinguishability. Further, our machine learning approach is broadly applicable to different photon correlation spectroscopy techniques and SPE systems, offering an enhanced tool for probing single emitter line shapes on previously inaccessible timescales.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2013

Photoconductivity of composite structures based on porous SnO{sub 2} sensitized with CdSe nanocrystals

The introduction of CdSe nanocrystals (colloidal quantum dots) into a porous SnO{sub 2} matrix brings about the appearance of photoconductivity in the structures. Sensitization is a consequence of charge exchange between the quantum dots and the matrix. Photoconductivity spectral measurements show that the nanocrystals embedded into the matrix are responsible for the optical activity of the structure. The photoconductivity of the structures sensitized with different-sized quantum dots is studied in the temperature range from 77 to 300 K. It is shown that the maximum photoconductivity is attained by introducing nanocrystals of the minimum size (2.7 nm). The mechanisms of charge-carrier transport in the matrix and the charge-exchange kinetics are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Luminescence properties of Si-containing porous matrix–PbS nanoparticle systems

The luminescence properties of systems that contain lead-sulfide nanoparticles deposited onto substrates fabricated from porous silicon, oxidized porous silicon, and porous (tin-oxide)–(silicon-oxide) layers are studied. It is shown that the structure and composition of the matrix induce a strong effect on the luminescence spectra of colloidal quantum dots, defining their emission wavelength.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Probing Nanocrystal Optical Properties in the Limit of Single Nanocrystals

The Bawendi lab has been at the forefront of the synthesis and characterization of the fundamental photo-physics of quantum confined systems for the past three decades. During this DoE project, our progress has been enabled by both our expertise in single nanocrystal (NC) photoluminescence (PL) spectroscopy as well as our extensive knowledge of the synthetic optimization of various NC semiconductors - all of this forming an efficient feedback loop between characterization and synthesis. Our research in this project has focused on the development and application of novel single emitter optical tools for the characterization of colloidal quantum dots (CQDs) which are semiconducting NCs with size-tunable quantum confined electronic structure. CQDs have had a significant commercial impact as display technologies. They are also commercially available as fluorescent biological markers. CQDs are being investigated as a potential material for photovoltaic or optical sensing applications and as emerging optical quantum materials. We have focused on the fundamental understanding of the optical properties of NCs through the development of novel spectroscopic methods at the single emitter level, including understanding how synthetic and environmental heterogeneities influence NC properties. A major advance in our research has been the development of novel photon resolved correlation spectroscopies that allow us to extract the physics of photo-excited single NCs with unprecedented time and spectral resolution from the photon stream of individual NCs.

36 MATERIALS SCIENCE↗
NASA NTRS1995

Metal colloids and semiconductor quantum dots: Linear and nonlinear optical properties

One aspect of this project involves a collaborative effort with the Solid State Division of ORNL. The thrust behind this research is to develop ion implantion for synthesizing novel materials (quantum dots wires and wells, and metal colloids) for applications in all optical switching devices, up conversion, and the synthesis of novel refractory materials. In general the host material is typically a glass such as optical grade silica. The ions of interest are Au, Ag, Cd, Se, In, P, Sb, Ga and As. An emphasis is placed on host guest interactions between the matrix and the implanted ion and how the matrix effects and implantation parameters can be used to obtain designer level optical devices tailored for specific applications. The specific materials of interest are: CdSe, CdTe, InAs, GaAs, InP, GaP, InSb, GaSb and InGaAs. A second aspect of this research program involves using porous glass (25-200 A) for fabricating materials of finite size. In this part of the program, we are particularly interested in characterizing the thermodynamic and optical properties of these non-composite materials. We also address how phase diagram of the confined material is altered by the interfacial properties between the confined material and the pore wall.

Henderson, D. O.↗