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Results for “synthesis (scalable processing)”

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 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Tailoring the energy distribution and loss of 2D plasmons

Here, the ability to tailor the energy distribution of plasmons at the nanoscale has many applications in nanophotonics, such as designing plasmon lasers, spasers, and quantum emitters. To this end, we analytically study the energy distribution and the proper field quantization of 2D plasmons with specific examples for graphene plasmons. We find that the portion of the plasmon energy contained inside graphene (energy confinement factor) can exceed 50%, despite graphene being infinitely thin. In fact, this very high energy confinement can make it challenging to tailor the energy distribution of graphene plasmons just by modifying the surrounding dielectric environment or the geometry, such as changing the separation distance between two coupled graphene layers. However, by adopting concepts of parity-time symmetry breaking, we show that tuning the loss in one of the two coupled graphene layers can simultaneously tailor the energy confinement factor and propagation characteristics, causing the phenomenon of loss-induced plasmonic transparency.

2D plasmons↗
DOE OSTI2015

Weyl points in photonic-crystal superlattices

In this paper, we show that Weyl points can be realized in all-dielectric superlattices based on three-dimensional (3D) layered photonic crystals. Our approach is based on creating an inversion-breaking array of weakly-coupled planar defects embedded in a periodic layered structure with a large omnidirectional photonic band gap. Using detailed band structure calculations and tight-binding theory arguments, we demonstrate that this class of layered systems can be tailored to display 3D linear point degeneracies between two photonic bands, without breaking time-reversal symmetry and using a configuration that is readily-accessible experimentally. These results open new prospects for the observation of Weyl points in the near-infrared and optical regimes and for the application of Weyl-physics in integrated photonic devices.

14 SOLAR ENERGY↗
DOE OSTI2015

Magnetic field dependence of singlet fission in solutions of diphenyl tetracene

Magnetic field effects provide a convenient and specific probe of singlet exciton fission within optoelectronic devices. Here, we demonstrate that this tool may also be applied to screen potential fission material candidates in solution. We characterize the phenomenon in diphenyl tetracene (DPT), which shows strong fluorescence modulation and the expected field dependence in its transient decay as a function of concentration. Furthermore, solution measurements may also be used to test for the presence of an intermediate charge transfer state, but we observe no changes to the field dependence of DPT singlet exciton fission in toluene relative to chloroform.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Transverse-electric Brewster effect enabled by nonmagnetic two-dimensional materials

It was discovered in the 19th century, the Brewster effect is known to occur for transverse-magnetic waves in regular optical dielectrics; however, it is believed to arise for transverse-electric (TE) waves only in systems with magnetic responses, i.e., nonunity effective relative permeability. This paper introduces a scheme to realize the TE Brewster effect in a homogeneous dielectric interface without magnetic responses, by adding ultrathin two-dimensional (2D) materials such as graphene. In particular, the effect remains even for waves approaching normal incidence, spanning from terahertz to visible frequencies. In contrast to the conventional Brewster effect, the graphene-assisted TE Brewster effect is asymmetric, and can be achieved only when the incidence is from the higher-refractive-index side. Moreover, graphene layers can tailor a total-internal-reflection dielectric interface into zero reflection, accompanied by perfect absorption. This control over TE waves enabled by ultrathin 2D materials may lead to a variety of applications, such as atomically thin absorbers, polarizers, and antireflection coating.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Band-Gap Reduction in ( Bi Cr O 3 ) m / ( Bi Fe O 3 ) n Superlattices: Designing Low-Band-Gap Ferroelectrics

Ferroelectric BiFeO 3 is promising for photovoltaic applications, especially in regard to the exploitation of ferroelectric photovoltaic effects for charge separation. However, its large band gap limits efficient sunlight absorption. Here, we demonstrate a new strategy to effectively tune the band gap of tetragonal BiFeO 3 via superlattice structuring with the ferroelectric BiCrO 3 . The (BiCrO 3 ) m /(BiFeO 3 ) n superlattices are found to exhibit conventional ferroelectric properties, but low fundamental band gaps, smaller than either of the parent materials. First-principles calculations reveal that the unexpected band-gap reduction is induced by charge reconstruction due to lattice strain, octahedral distortion, and polarization discontinuity at the BiCrO3-BiFeO 3 interfaces. Ultimately, these results provide a new strategy, in the form of superlattice structuring, which could open the door to the creation of efficient ferroelectric photovoltaics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Metamaterial broadband angular selectivity

We reveal how broadband angular selectivity can be achieved with stacks of one-dimensionally periodic photonic crystals, each consisting of alternating isotropic layers and effective anisotropic layers, where each effective anisotropic layer is constructed from a multilayered metamaterial. We demonstrate that by simply changing the structure of the metamaterials, the selective angle can be tuned to a broad range of angles; and, by increasing the number of stacks, the angular transmission window can be made as narrow as desired. As a proof of principle, we realize the idea experimentally in the microwave regime. The angular selectivity and tunability we report here can have various applications such as in directional control of electromagnetic emitters and detectors.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Boundary scattering of phonons: Specularity of a randomly rough surface in the small-perturbation limit

Scattering of normally incident longitudinal and transverse acoustic waves by a randomly rough surface of an elastically isotropic solid is analyzed within the small-perturbation approach. In the limiting case of a large correlation length L compared with the acoustic wavelength, the specularity reduction is given by 4η 2 k 2 ,where η is the rms roughness and k is the acoustic wave vector, which is in agreement with the well-known Kirchhoff approximation result often referred to as Ziman's equation [J. M. Ziman, Electrons and Phonons (Clarendon Press, Oxford, 1960)]. In the opposite limiting case of a small correlation length, the specularity reduction is discovered to be proportional to η 2 k 4 L 2 , with the fourth power dependence on frequency as in Rayleigh scattering. Numerical calculations for a Gaussian autocorrelation function of surface roughness connect these limiting cases and reveal a maximum of diffuse scattering at an intermediate value of L . This maximum becomes increasingly pronounced for the incident longitudinal wave as the Poisson's ratio of the medium approaches 1/2 as a result of increased scattering into transverse and Rayleigh surface waves. The results imply that thermal transport models using Ziman's formula are likely to overestimate the heat flux dissipation due to boundary scattering, whereas modeling interface roughness as atomic disorder is likely to underestimate scattering.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Ab initio study of electron-phonon interaction in phosphorene

The monolayer of black phosphorus, or “phosphorene,” has recently emerged as a two-dimensional semiconductor with intriguing highly anisotropic transport properties. Existing calculations of its intrinsic phonon-limited electronic transport properties so far rely on the deformation potential approximation, which is in general not directly applicable to anisotropic materials since the deformation along one specific direction can scatter electrons traveling in all directions. We perform a first-principles calculation of the electron-phonon interaction in phosphorene based on density functional perturbation theory and Wannier interpolation. Our calculation reveals that (1) the high anisotropy provides extra phase space for electron-phonon scattering, and (2) optical phonons have appreciable contributions. Both effects cannot be captured by the deformation potential calculations. Furthermore, our simulation predicts carrier mobilities ~170 cm 2 /Vs for both electrons and holes at 300 K, and a thermoelectric figure of merit $zT$ of up to 0.14 in $p$-type impurity-free phosphorene at 500 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Reconstructing phonon mean-free-path contributions to thermal conductivity using nanoscale membranes

Knowledge of the mean-free-path distribution of heat-carrying phonons is key to understanding phonon-mediated thermal transport. We show that thermal conductivity measurements of thin membranes spanning a wide thickness range can be used to characterize how bulk thermal conductivity is distributed over phonon mean free paths. A noncontact transient thermal grating technique was used to measure the thermal conductivity of suspended Si membranes ranging from 15–1500 nm in thickness. A decrease in the thermal conductivity from 74–13% of the bulk value is observed over this thickness range, which is attributed to diffuse phonon boundary scattering. Because of the well-defined relation between the membrane thickness and phonon mean-free-path suppression, combined with the range and accuracy of the measurements, we can reconstruct the bulk thermal conductivity accumulation vs. phonon mean free path, and compare with theoretical models.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Extending the range of validity of Fourier's law into the kinetic transport regime via asymptotic solution of the phonon Boltzmann transport equation

We derive the continuum equations and boundary conditions governing phonon-mediated heat transfer in the limit of a small but finite mean-free path from the asymptotic solution of the linearized Boltzmann equation in the relaxation time approximation. Our method uses the ratio of the mean-free path to the characteristic system length scale, also known as the Knudsen number, as the expansion parameter to study the effects of boundaries on the breakdown of the Fourier description. We show that, in the bulk, the traditional heat conduction equation using Fourier's law as a constitutive relation is valid at least up to second order in the Knudsen number for steady problems and first order for time-dependent problems. Yet, this description does not hold within distances on the order of a few mean-free paths from the boundary; this breakdown is a result of kinetic effects that are always present in the boundary vicinity and require solution of a Boltzmann boundary layer problem to be determined. Matching the inner, boundary layer solution to the outer, bulk solution yields boundary conditions for the Fourier description as well as additive corrections in the form of universal kinetic boundary layers; both are found to be proportional to the bulk-solution gradients at the boundary and parametrized by the material model and the phonon-boundary interaction model (Boltzmann boundary condition). Our derivation shows that the traditional no-jump boundary condition for prescribed temperature boundaries and the no-flux boundary condition for diffusely reflecting boundaries are appropriate only to zeroth order in the Knudsen number; at higher order, boundary conditions are of the jump type. We illustrate the utility of the asymptotic solution procedure by demonstrating that it can be used to predict the Kapitza resistance (and temperature jump) associated with an interface between two materials. All results are validated via comparisons with low-variance deviational Monte Carlo simulations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Variational approach to extracting the phonon mean free path distribution from the spectral Boltzmann transport equation

The phonon Boltzmann transport equation (BTE) is a powerful tool for studying nondiffusive thermal transport. Here, we develop a new universal variational approach to solving the BTE that enables extraction of phonon mean free path (MFP) distributions from experiments exploring nondiffusive transport. By utilizing the known Fourier heat conduction solution as a trial function, we propose a direct approach to calculating the effective thermal conductivity from the BTE. We reveal this technique on the transient thermal grating experiment, which is a useful tool for studying nondiffusive thermal transport and probing the MFP distribution of materials. We obtain a closed form expression for a suppression function that is materials dependent, successfully addressing the nonuniversality of the suppression function used in the past, while providing a general approach to studying thermal properties in the nondiffusive regime.

42 ENGINEERING↗