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At least 1,045 records · Page 58
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Twinned nanostructure of VO 2 thin films grown on r-cut sapphire

Due to strong electron-phonon interactions, strain engineering is a powerful tool to control quantum critical phenomena in strongly correlated oxides. Exploring this possibility requires understanding the nanoscale structure of quantum materials and the role it plays in forming the strain landscape. In this work, we used a combination of x-ray nanoimaging and reciprocal space mapping to study the nanostructure of the archetypal Mott insulator VO 2 featuring an insulator-to-metal and structural phase transitions. We found that VO 2 thin films grown on r-cut sapphire consist of two intertwined crystal lattices, permanently inclined with respect to each other. This persistent pattern of twin domains stands out from the symmetry breaking induced by the structural phase transition and conceivably originates from the post-growth strain relaxation process. We propose a model explaining the formation of twin domains and the emergence of anisotropy in the film nanostructure. Our work suggests using miscut substrates to suppress either one or the other twin that can serve as a new tool to control strain in VO 2 films.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Transport of hot carriers in plasmonic nanostructures

Plasmonic hot carrier devices extract excited carriers from metal nanostructures before equilibration and have the potential to surpass semiconductor light absorbers. However their efficiencies have so far remained well below theoretical limits, which necessitates quantitative prediction of carrier transport and energy loss in plasmonic structures to identify and overcome bottlenecks in carrier harvesting. Here, we present a theoretical and computational framework, nonequilibrium scattering in space and energy (NESSE), to predict the spatial evolution of carrier energy distributions that combines the best features of phase-space (Boltzmann) and particle-based (Monte Carlo) methods. Within the NESSE framework, we bridge first-principles electronic structure predictions of plasmon decay and carrier collision integrals at the atomic scale, with electromagnetic field simulations at the nano- to mesoscale. Finally, we apply NESSE to predict spatially-resolved energy distributions of photoexcited carriers that impact the surface of experimentally realizable plasmonic nanostructures at length scales ranging from tens to several hundreds of nanometers, enabling first-principles design of hot carrier devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Ultimate Suppression of Thermal Transport in Amorphous Silicon Nitride by Phononic Nanostructure.

Engineering the thermal conductivity of amorphous materials is highly essential for the thermal management of future electronic devices. Here, we demonstrate the impact of ultrafine nanostructuring on the thermal conductivity reduction of amorphous silicon nitride (a-Si3N4) thin films, in which the thermal transport is inherently impeded by the atomic disorders. Ultrafine nanostructuring with feature sizes below 20 nm allows us to fully suppress contribution of the propagating vibrational modes (propagons), leaving only the diffusive vibrational modes (diffusons) to contribute to thermal transport in a-Si3N4. A combination of the phonon-gas kinetics model and the Allen-Feldmann theory reproduced the measured results without any fitting parameters. The thermal conductivity reduction was explained as extremely strong diffusive boundary scattering of both propagons and diffusIons. These findings give rise to substantial tunability of thermal conductivity of amorphous materials, which enables us to provide better thermal solutions in microelectronic devices.

Tambo, Naoki↗
DOE OSTI2023

Gradient nanostructured steel with superior tensile plasticity

Nanostructured metallic materials with abundant high-angle grain boundaries exhibit high strength and good radiation resistance. While the nanoscale grains induce high strength, they also degrade tensile ductility. We show that a gradient nanostructured ferritic steel exhibits simultaneous improvement in yield strength by 36% and uniform elongation by 50% compared to the homogenously structured counterpart. In situ tension studies coupled with electron backscattered diffraction analyses reveal intricate coordinated deformation mechanisms in the gradient structures. The outermost nanolaminate grains sustain a substantial plastic strain via a profound deformation mechanism involving prominent grain reorientation. This synergistic plastic co-deformation process alters the rupture mode in the post-necking regime, thus delaying the onset of fracture. The present discovery highlights the intrinsic plasticity of nanolaminate grains and their significance in simultaneous improvement of strength and tensile ductility of structural metallic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Optical properties of hybrid quantum-well–dots nanostructures grown by MOCVD

The deposition of In{sub x}Ga{sub 1–x}As with an indium content of 0.3–0.5 and an average thickness of 3–27 single layers on a GaAs wafer by metalorganic chemical vapor deposition (MOCVD) at low temperatures results in the appearance of thickness and composition modulations in the layers being formed. Such structures can be considered to be intermediate nanostructures between ideal quantum wells and quantum dots. Depending on the average thickness and composition of the layers, the wavelength of the photoluminescence peak for the hybrid InGaAs quantum well–dots nanostructures varies from 950 to 1100 nm. The optimal average In{sub x}Ga{sub 1–x}As thicknesses and compositions at which the emission wavelength is the longest with a high quantum efficiency retained are determined.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nonradiative Energy Transfer in Hybrid Nanostructures with Varied Dimensionality

A composite nanostructure based on quasi-one-dimensional InP nanowires with an InAsP nanoinsert, grown on a Si(111) substrate by the method of molecular-beam epitaxy, and CdSe/ZnS zero-dimensional colloidal quantum dots is reported for the first time. The nonradiative resonance energy transfer between components of the hybrid nanostructure, namely, between the colloidal quantum dots and the nanoinsert, is experimentally confirmed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Two-Stage Conversion of Silicon to Nanostructured Carbon by the Method of Coordinated Atomic Substitution

Abstract—: A fundamentally new method of obtaining epitaxial layers of nanostructured carbon on silicon substrates has been considered. Epitaxial growth in the case of seemingly incompatible lattices has been achieved by converting the crystal by the method of coordinated substitution of atoms, in which the overall structure of the bonds between the atoms is not destroyed. In the first stage of conversion, the first half of the silicon atoms are concertedly replaced by the carbon atoms due to the reaction of silicon with CO gas, during which an epitaxial layer of SiC–3C cubic silicon carbide is obtained. In the second stage of conversion, the remaining half of silicon atoms is concertedly replaced by carbon atoms due to the reaction of SiC with CF{sub 4} gas. Carbon structures with different properties from nanodiamonds to nanotubes and carbon nanoonions have been obtained depending on the orientation of the silicon surface, pressure of the reagent gas, and temperature and growth time. A key feature of this method is that the substrate orders the resulting structures using the original chemical bonds between the atoms in silicon. The term “concertedly” means that new chemical bonds are formed simultaneously and concertedly with the destruction of old bonds. Data on electron diffraction and analysis of Raman and ellipsometric spectra of the obtained samples of nanostructured carbon on silicon substrates have been presented. Two competing growth mechanisms have been discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Evolution of Multiwalled Carbon Nanotubes and Related Nanostructures during the Formation of Alumomatrix Composite Materials

Composite materials based on AMg2 aluminum alloy reinforced by additions of 0.05 wt % multiwalled carbon nanotubes (MWCNTs) or Al/MWCNT hybrid nanostructures have been obtained by powder metallurgy techniques. The evolution of MWCNTs and Al/MWCNT hybrid nanostructures at various stages of powder composite material formation has been studied by Raman spectroscopy. It is established that Al/MWCNT hybrid filler is subjected to lower damage and less favors the formation of Al{sub 4}C{sub 3} phase during isothermal consolidation stage as compared to the case of nonmodified MWNTs.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Nanostructure-Driven Ion Transport in PCBM-Based Polymer Electrolytes

Nanostructure-Driven Ion Transport in PCBM-Based Polymer ElectrolytesChe-Nan Sun1, Thomas A. Zawodzinski1,2, Fei Ren3, Jong Kahk Keum1 and Jihua Chen1, (1)Oak Ridge National Laboratory, (2)The University of Tennessee, (3)Temple University Polyethylene oxide or PEO is an extensively-examined candidate for solid polymer electrolyte materials of lithium ion batteries, and its composite electrolytes has promising ion conductivities.[1-3] Oxide nanoparticles with sizes of 5-10 nm are often introduced into these polymer-based composite electrolytes in order to suppress their room-temperature crystallite formation.1-9 The size, geometry and surface functionality of the added particles were known to largely affect the structure and performance of the blended electrolytes.5,10 In this study, we examined a functionalized-fullerene-based composite electrolytes, providing details in their self-assembled nanostructures, modulus, hardness, as well as temperature-dependent ion-conducting behaviors. To the best of our knowledge, no fullerene-based, lithium conducting, composite electrolyte has been reported previously. Herein we used a bench-mark fullerene derivative, phenyl-C61-butyric acid methyl ester (PCBM) as a model fullerene compound and performed impedance spectroscopy, equivalent circuit modeling, nanoscale elemental mapping (in transmission electron microscope), wide-angle X-ray diffraction, as well as nanoindentation to shed light on a 6-fold enhancement in low temperature (less than 50oC) ion conductivity of PEO - lithium bis(trifluoromethanesulfonyl) imide (LiTFSI)-PCBM electrolytes, along with the underlying changes in nanomorphology , mechanical properties, and crystal structures. Based on a previous density functional theory (DFT) calculation, 11 the interaction energies Ei among PEO polymers is estimated to be 2.58 kcal mol-1 per monomer, the Ei between PCBM and PEO is 3.50 kcal mol-1 per monomer (PCBM is taken as 1 repeat unit), and the Ei among PCBMs themselves is 6.01 kcal mol-1 per monomer. This explains that at very low PCBM weight percentage, without sufficient PCBM-PCBM contacts, it is more energetically favorable for fullerenes to disperse into PEO matrix. However, with higher PCBM concentration, the fullerenes will efficiently pack with each other into domains with gradually increased dimensions. Quantification of PCBM domains is performed by line scan analysis of energy filtered TEM (EFTEM) images. Upon the addition of PCBM, the average domain sizes gradually increase from 3.4±1 nm ( 0% PCBM), to 4.6±1 nm (10% PCBM) and 4.9±2 nm (20% PCBM), and finally to 7.5±5 nm (40% PCBM ). (A precise determination of PCBM domain dimension is not possible when the domain size is less than 3 nm, due to the lack of EFTEM contrast in these samples). We attribute the observed ion conductivity improvement to those nanomorphological variation in PCBM-PEO-LiTFSi systems.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Defect passivation in nanostructured silica for stable birefringence in polarization optics

Nanostructured materials with anisotropic microstructure exhibit direction-dependent refractive index, known as form birefringence. One of the main advantages of these nanostructured materials is that the ease of fabrication significantly reduces the cost of production. However, their birefringence is affected by humidity as well as time. The hypothesis is that defects, particularly silanols, are responsible. Here, in this work, we present the use of hexamethyldisilazane (HMDS) vapor as a passivation strategy. Using spectroscopic ellipsometry and Raman scattering, the data show that HMDS successfully passivates silanol groups and promotes O-Si-O bond formation, resulting in improved stability. The results demonstrate a simple method to enhance the reliability of all-silica polarization optics offering potential advancements for high-power laser components.

Mireles, Marcela [Univ. of Rochester, NY (United S↗
DOE OSTI2015

Optical and infrared properties of glancing angle-deposited nanostructured tungsten films

For this study, nanotextured tungsten thin films were obtained on a stainless steel (SS) substrate using the glancing-angle-deposition (GLAD) method. It was found that the optical absorption and thermal emittance of the SS substrate can be controlled by varying the parameters used during deposition. Finite-difference time-domain (FDTD) simulations were used to predict the optical absorption and infrared (IR) reflectance spectra of the fabricated samples, and good agreement was found between simulated and measured data. FDTD simulations were also used to predict the effect of changes in the height and periodicity of the nanotextures. These simulations show that good control over the absorption can be achieved by altering the height and periodicity of the nanostructure. These nanostructures were shown to be temperature stable up to 500°C with the addition of a protective HfO 2 layer. Finally, applications for this structure are explored, including a promising application for solar thermal energy systems.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Ultra-intense femtosecond laser interactions with aligned nanostructures

The interaction of ultrafast laser pulses of relativistic intensity with high aspect ratio nanostructures can efficiently and volumetrically heat matter to an ultra-high-energy-density regime encountered in the center of stars and within the core of fusion capsules compressed by the world’s largest lasers. It also generates gigantic quasi-static electromagnetic fields that accelerate particles to very high energy. Here, we present an overview of the physics and applications of these dense relativistic plasmas that can be created with pulses of relatively modest energy from lasers that can operate at a high repetition rate. Recent nanowire array experiments produced near-solid density plasmas with an extreme degree of ionization (e.g., Au +72 ), converted ultrafast pulses of laser light into intense x-ray flashes with record efficiency, and accelerated ions to MeV energies, efficiently driving micro-scale fusion reactions that generate flashes of quasi-monoenergetic neutrons. These plasmas also serve as a platform for advancing the understanding of atomic processes in extreme environments and open a new pathway to laser-driven fusion energy. The irradiation of nanostructures at intensities of > 1 x 10 22 W cm –2 is predicted to lead to an extreme ultra-high energy density plasma regime characterized by terabar pressures that is virtually unexplored.

Optics↗
DOE OSTI2012

Carbon Nanosheets and Nanostructured Electrodes in Organic Photovoltaic Devices (CRADA Final Report)

Carbon nanosheet thin films were employed as nanostructured electrodes in organic solar cells. Due to the nanostructured texture of the carbon nanosheet electrodes, there was an increase in performance over standard ITO electrodes with very thick active layers. ZnO deposited via atomic layer deposition (ALD) was used as a hole blocking layer to provide for carrier selectivity of the carbonnanosheets.

14 SOLAR ENERGY↗
DOE OSTI2011

High-efficiency photovoltaics based on semiconductor nanostructures

The objective of this project was to exploit a variety of semiconductor nanostructures, specifically semiconductor quantum wells, quantum dots, and nanowires, to achieve high power conversion efficiency in photovoltaic devices. In a thin-film device geometry, the objectives were to design, fabricate, and characterize quantum-well and quantum-dot solar cells in which scattering from metallic and/or dielectric nanostructures was employed to direct incident photons into lateral, optically confined paths within a thin (~1-3um or less) device structure. Fundamental issues concerning nonequilibrium carrier escape from quantum-confined structures, removal of thin-film devices from an epitaxial growth substrate, and coherent light trapping in thin-film photovoltaic devices were investigated. In a nanowire device geometry, the initial objectives were to engineer vertical nanowire arrays to optimize optical confinement within the nanowires, and to extend this approach to core-shell heterostructures to achieve broad-spectrum absorption while maintaining high open-circuit voltages. Subsequent work extended this approach to include fabrication of nanowire photovoltaic structures on low-cost substrates.

14 SOLAR ENERGY↗
DOE OSTI2013

Plasmon Mapping in Metallic Nanostructures and its Application to Single Molecule Surface Enhanced Raman Scattering: Imaging Electromagnetic Hot-Spots and Analyte Location

A major component of this proposal is to elucidate the connection between optical and electron excitation of plasmon modes in metallic nanostructures. These accomplishments are reported: developed a routine protocol for obtaining spatially resolved, low energy EELS spectra, and resonance Rayleigh scattering spectra from the same nanostructures; correlated optical scattering spectra and plasmon maps obtained using STEM/EELS; and imaged electromagnetic hot spots responsible for single-molecule surface-enhanced Raman scattering (SMSERS).

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Optoacoustic Microscopy for Investigation of Material Nanostructures-Embracing the Ultrasmall, Ultrafast, and the Invisible

The goal of this grant was the development of a new type of scanning acoustic microscope for nanometer resolution ultrasound imaging, based on ultrafast optoacoustics (>GHz). In the microscope, subpicosecond laser pulses was used to generate and detect very high frequency ultrasound with nanometer wavelengths. We report here on the outcome of the 3-year DOE/BES grant which involved the design, multifaceted construction, and proof-of-concept demonstration of an instrument that can be used for quantitative imaging of nanoscale material features – including features that may be buried so as to be inaccessible to conventional lightwave or electron microscopies. The research program has produced a prototype scanning optoacoustic microscope which, in combination with advanced computational modeling, is a system-level new technology (two patents issues) which offer novel means for precision metrology of material nanostructures, particularly those that are of contemporary interest to the frontline micro- and optoelectronics device industry. For accomplishing the ambitious technical goals, the research roadmap was designed and implemented in two phases. In Phase I, we constructed a “non-focusing” optoacoustic microscope instrument (“POAM”), with nanometer vertical (z-) resolution, while limited to approximately 10 micrometer scale lateral recolution. The Phase I version of the instrument which was guided by extensive acoustic and optical numerical modeling of the basic underlying acoustic and optical physics, featured nanometer scale close loop positioning between the optoacoustic transducer element and a nanostructured material sample under investigation. In phase II, we implemented and demonstrated a scanning version of the instrument (“SOAM”) where incident acoustic energy is focused, and scanned on lateral (x-y) spatial scale in the 100 nm range as per the goals of the project. In so doing we developed advanced numerical simulations to provide computational models of the focusing of multi-GHz acoustic waves to the nanometer scale and innovated a series fabrication approaches for a new type of broadband high-frequency acoustic focusing microscope objective by applying methods on nanoimprinting and focused-ion beam techniques. In the following, the Phase I and Phase II instrument development is reported as Section II. The first segment of this section describes the POAM instrument and its development, while including much of the underlying ultrafast acoustic physics which is common to all of our work for this grant. Then, the science and engineering of the SOAM instrument is described, including the methods of fabricating new types of acoustic microlenses. The results section is followed by reports on publications (Section III), Participants (Section IV), and statement of full use of the allocated grant funds (Section V).

36 MATERIALS SCIENCE↗
DOE OSTI2011

International Symposium on Clusters and Nanostructures (Energy, Environment, and Health)

The international Symposium on Clusters and Nanostructures was held in Richmond, Virginia during November 7-10, 2011. The symposium focused on the roles clusters and nanostructures play in solving outstanding problems in clean and sustainable energy, environment, and health; three of the most important issues facing science and society. Many of the materials issues in renewable energies, environmental impacts of energy technologies as well as beneficial and toxicity issues of nanoparticles in health are intertwined. Realizing that both fundamental and applied materials issues require a multidisciplinary approach the symposium provided a forum by bringing researchers from physics, chemistry, materials science, and engineering fields to share their ideas and results, identify outstanding problems, and develop new collaborations. Clean and sustainable energy sessions addressed challenges in production, storage, conversion, and efficiency of renewable energies such as solar, wind, bio, thermo-electric, and hydrogen. Environmental issues dealt with air- and water-pollution and conservation, environmental remediation and hydrocarbon processing. Topics in health included therapeutic and diagnostic methods as well as health hazards attributed to nanoparticles. Cross-cutting topics such as reactions, catalysis, electronic, optical, and magnetic properties were also covered.

Jena, Puru↗
DOE OSTI2011

Nanostructure Determination by Co-Refining Models to Multiple Datasets

The results of the work are contained in the publications resulting from the grant (which are listed below). Here I summarize the main findings from the last period of the award, 2006-2007: • Published a paper in Science with Igor Levin outlining the “Nanostructure Problem”, our inability to solve structure at the nanoscale. • Published a paper in Nature demonstrating the first ever ab-initio structure determination of a nanoparticle from atomic pair distribution function (PDF) data. • Published one book and 3 overview articles on PDF methods and the nanostructure problem. • Completed a project that sought to find a structural response to the presence of the so-called “intermediate phase” in network glasses which appears close to the rigidity percolation threshold in these systems. The main result was that we did not see convincing evidence for this, which drew into doubt the idea that Ge x Se 1-x glasses were a model system exhibiting rigidity percolation.

77 NANOSCIENCE AND NANOTECHNOLOGY↗