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All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Simultaneous measurements of kinetic and scalar energy spectrum time evolution in the Richtmyer–Meshkov instability upon reshock

The Richtmyer–Meshkov instability (Richtmyer, Commun. Pure Appl. Maths , vol. 13, issue 2, 1960, pp. 297–319; Meshkov, Fluid Dyn. , vol. 4, issue 5, 1972, pp. 101–104) of a twice-shocked gas interface is studied using both high spatial resolution single-shot (SS) and lower spatial resolution, time-resolved, high-speed (HS) simultaneous planar laser-induced fluorescence and particle image velocimetry in the Wisconsin Shock Tube Laboratory's vertical shock tube. The initial condition (IC) is a shear layer with broadband diffuse perturbations at the interface between a helium–acetone mixture and argon. This IC is accelerated by a shock of nominal strength Mach number $M = 1.75$ , and then accelerated again by the transmitted shock that reflects off the end wall of the tube. An ensemble of experiments is analysed after reshock while the interface mixing width grows linearly with time. The kinetic and scalar energy spectra and the terms of their evolution equation are calculated and compared between SS and HS experiments. The inertial range scaling of the scalar power spectrum is found to follow Gibson's relation (Gibson, Phys. Fluids , vol. 11, issue 11, 1968, pp. 2316–2327) as a function of Schmidt number when the effective turbulent Schmidt number is used in place of the material Schmidt number that controls equilibrium scaling. Further, the spatially integrated scalar flux follows similar behaviour observed for the kinetic energy in large eddy simulation studies by Zeng et al. ( Phys. Fluids , vol. 30, issue 6, 2018, 064106) while the spatially varying scalar flux exhibits back scatter along the centre of the mixing layer and forward energy transfer in the spike and bubble regions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Phase Imaging: A Compressive Sensing Approach

Since Wolfgang Pauli posed the question in 1933, whether the probability densities |Ψ(r)|² (real-space image) and |Ψ(q)|² (reciprocal space image) uniquely determine the wave function Ψ(r) [1], the so called Pauli Problem sparked numerous methods in all fields of microscopy [2, 3]. Reconstructing the complete wave function Ψ(r) = a(r)e-iφ(r) with the amplitude a(r) and the phase φ(r) from the recorded intensity enables the possibility to directly study the electric and magnetic properties of the sample through the phase. In transmission electron microscopy (TEM), electron holography is by far the most established method for phase reconstruction [4]. Requiring a high stability of the microscope, next to the installation of a biprism in the TEM, holography cannot be applied to any microscope straightforwardly. Recently, a phase retrieval approach was proposed using conventional TEM electron diffractive imaging (EDI). Using the SAD aperture as reciprocal-space constraint, a localized sample structure can be reconstructed from its diffraction pattern and a real-space image using the hybrid input-output algorithm [5]. We present an alternative approach using compressive phase-retrieval [6]. Our approach does not require a real-space image. Instead, random complimentary pairs of checkerboard masks are cut into a 200 nm Pt foil covering a conventional TEM aperture (cf. Figure 1). Used as SAD aperture, subsequently diffraction patterns are recorded from the same sample area. Hereby every mask blocks different parts of gold particles on a carbon support (cf. Figure 2). The compressive sensing problem has the following formulation. First, we note that the complex-valued reciprocal-space wave-function is the Fourier transform of the (also complex-valued) real-space wave-function, Ψ(q) = F[Ψ(r)], and subsequently the diffraction pattern image is given by |Ψ(q)|2 = |F[Ψ(r)]|2. We want to find Ψ(r) given a few differently coded diffraction pattern measurements yn = |F[HnΨ(r)]|2, where the matrices Hn encode the mask structure of the aperture. This is a nonlinear inverse problem, but has been shown to be solvable even in the underdetermined case [6]. Since each diffraction pattern yn contains diffraction information from selected regions of the same sample, the differences in each pattern contain local phase information, which can be combined to form a full estimate of the real-space wave-function[7]. References: [1] W. Pauli in “Die allgemeinen Prinzipien der Wellenmechanik“, ed. H Geiger and W Scheel, (Julius Springer, Berlin). [2] A. Tonomura, Rev. Mod. Phys. 59 (1987), p. 639. [3] J. Miao et al, Nature 400 (1999), p. 342. [4] H. Lichte et al, Annu. Rev. Mater. Res. 37 (2007), p. 539. [5] J. Yamasaki et al, Appl. Phys. Lett. 101 (2012), 234105. [6] P Schniter and S Rangan. Signal Proc., IEEE Trans. on. 64(4), (2015), pp. 1043. [7] Supported by the Chemical Imaging, Signature Discovery, and Analytics in Motion initiatives at PNNL. PNNL is operated by Battelle Memorial Inst. for the US DOE; contract DE-AC05-76RL01830.

Schneider, Sebastian↗
DOE OSTI2017

Compressive Classification for TEM-EELS

Acquiring atomic-resolution images of beam sensitive materials or under dynamic conditions (e.g. operando STEM) is very difficult and requires precise dose fractioning to minimize the dose delivered to the sample and acquire the data more rapidly than dynamics present. Typical methods of low-dose STEM imaging include adjusting the beam current, dwell-time, and probe Scanning transmission electron microscopy (STEM) is used routinely to obtain atomic-resolution images of a small selection of radiation tolerant materials under stable conditions.size. All three of these determine the dose over a spatial area in STEM. Recently, compressive sensing [1] has been applied to electron microscopy. The computational imaging method of inpainting [2] has been proposed as a method to reduce dose and increase acquisition speed for STEM. Inpainting is a technique to recover missing pixels in an image. By skipping pixels (randomly) during acquisition, a fourth dose-control mechanism is realized [3]. A few implementations of randomly-subsampled STEM exist (e.g., [4]). To make the concepts of compressive sensing accessible to the microscopy community we begin with an example using atomic resolution images of a GaAs crystal (see Figure 1). Because the crystal is periodic it has a sparse representation in reciprocal space. In fact, a sparse representation is a key component of compressive sensing. If an image is sub-sampled randomly we find that the Fourier transform remains sparse, but becomes increasing noisy as less and less data is acquired. Yet, by finding the peaks in the reciprocal image and removing the noise it is possible to recover atomic resolution images with only 1% of the pixels. Figure 1 shows the reconstruction process for GaAs sub-sampled at 50%, 10%, 5%, and 1%. Periodic images form a special class for which the optimal sampling approach (according to CS theory) is a random selection of pixels. For non-periodic images, such as those arising in in-situ STEM, methods such as dictionary learning are necessary [2,5]. The data can be compressed in time for TEM [5] or pixels can be skipped randomly for STEM. There are several issues that arise in developing a random scan for STEM [4] and a coded aperture for TEM. The remainder of the talk will address these issues. References: [1] E Candes and M Wakin. Signal Processing Magazine, IEEE. 2(25) (2008), pp. 21. [2] A Stevens, Pu Y, Sun Y, et al. Artificial Intelligence & Statistics (AISTATS), 2017. [3] A Stevens, H Yang, L Carin et al. Microscopy 63(1), (2014), pp. 41. [4] L Kovarik, A Stevens, A Liyu et al. Appl. Phys. Lett. 109, 164102 (2016). [5] A Stevens, L Kovarik, P Abellan et al. Adv. Structural and Chemical Imaging 1(10), (2015), pp. 1. [6] Supported by the Chemical Imaging, Signature Discovery, and Analytics in Motion Initiatives at PNNL. PNNL is operated by Battelle Memorial Inst. for the US DOE; contract DE-AC05-76RL01830.

Hao, Weituo↗
DOE OSTI2017

Cryo-STEM Tomography with Inpainting

Traditionally, microscopists have worked with the Nyquist-Shannon theory of sampling, which states that to be able to reconstruct the image fully it needs to be sampled at a rate of at least twice the highest spatial frequency in the image. This sampling rate assumes that the image is sampled at regular intervals and that every pixel contains information that is crucial for the image (it even assumes that noise is important). Images in general, and especially low dose S/TEM images, contain significantly less information than can be encoded by a grid of pixels (which is why image compression works). Mathematically speaking, the image data has a low dimensional or sparse representation. Through the application of compressive sensing methods [1,2,3] this representation can be found using pre-designed measurements that are usually random for implementation simplicity. These measurements and the compressive sensing reconstruction algorithms have the added benefit of reducing noise. This reconstruction approach can be extended into higher dimensions, whereby the random sampling in each 2-D image can be extended into: a sequence of tomographic projections (i.e. tilt images); a sequence of video frames (i.e. incorporating temporal resolution and dynamics); spectral resolution (i.e. energy filtering an image to see the distribution of elements); and ptychography (i.e. sampling a full diffraction image at each location in a 2-D grid across the sample). This approach has been employed experimentally for materials science samples requiring low-dose imaging [2], and can be readily applied to biological samples. Figure 1 shows the resolution possible in a complex biological system, mouse pancreatic islet beta cells [4], when tomogram slices are reconstructed using subsampling. Reducing the number of pixels (1/6 pix and 1/3*1/3) shows minimal degradation compared to the reconstructions using all pixels (all data and 1/3 tilt). Although subsampling 1/6 of the tilts (1/6 of overall dose) degrades the reconstruction to the point that the cellular structures cannot be identified. Using 1/3 of both the pixels and the tilts provides a high quality image at 1/9 the overall dose even for this most basic and rapid demonstration of the CS methods. Figure 2 demonstrates the theoretical tomogram reconstruction quality (vertical axis) as undersampling (horizontal axis) is increased; we examined subsampling pixels and tilt-angles individually and a combined approach in which both pixels and tilts are sub-sampled. Note that subsampling pixels maintains high quality reconstructions (solid lines). Using the inpainting algorithm to obtain tomograms can automatically reduce the dose applied to the system by an order of magnitude. Perhaps the best way to understand the impact is to consider that by using inpainting (and with minimal hardware changes), a sample that can normally withstand a dose of ~10 e/Å2 can potentially be imaged with an “equivalent quality” to a dose level of 103 e/Å2. To put this in perspective, this is approaching the dose level used for the most advanced images, in terms of spatial resolution, for inorganic systems. While there are issues for biological specimens beyond dose (structural complexity being the most important one), this sampling approach allows the methods that are traditionally used for materials science to be applied to biological systems [5]. References: [1] A Stevens, H Yang, L Carin et al. Microscopy 63(1), (2014), pp. 41. [2] L Kovarik, A Stevens, A Liyu et al. Appl. Phys. Lett. 109, 164102 (2016) [3] A Stevens, L Kovarik, P Abellan et al. Adv. Structural and Chemical Imaging 1(10), (2015), pp. 1. [4] MD Guay, W Czaja, MA Aronova et al. Scientific Reports 6, 27614 (2016) [5] Supported by the Chemical Imaging, Signature Discovery, and Analytics in Motion Initiatives at PNNL. PNNL is operated by Battelle Memorial Inst. for the US DOE; contract DE-AC05-76RL01830.

Stevens, Andrew↗
DOE OSTI2017

Numerical Analysis of Experiments on Thermally Induced Dissociation of Methane Hydrates in Porous Media

Numerical simulation is essential for the prediction and evaluation of hydrocarbon reservoir performance. Numerical simulators developed for the description of the behavior of hydrates under production and the corresponding flow of fluids and heat accounting for all known processes are powerful, but they need validation through comparison to field or experimental data in order to instill confidence in their predictions. In this study, we analyze by means of numerical simulation the results of an experiment of methane hydrate dissociation by thermal stimulation in unconsolidated porous media heated through the vessel walls. The physics captured by the model include multicomponent heat and mass transfer, multiphase flow through porous media, and the phase behavior of the CH 4 + H 2 O system involved in methane hydrate formation and dissociation. The set of governing equations consists of the mass and energy conservation equations coupled with constitutive relationships, i.e., the dissolution of gas in H 2 O, relative permeability and capillary pressure models, composite thermal conductivity models, and methane hydrate phase equilibria. The model geometry describes accurately the hydrate reactor used in a recent experimental study investigating methane hydrate dissociation behavior [Chong et. al. Appl. Energy 2016, 177, 409-421]. The cumulative gas production is estimated and validated against three tests of experimental data involving different boundary temperatures, showing a good agreement between observations and numerical predictions. The predicted evolution of the spatial distributions of different phases over time shows that hydrate dissociation progresses inward from the reactor boundary to the center, methane gas accumulates to the top of the reactor because of buoyancy, and water migrates down to the bottom of the reactor because of gravity. A sharp hydrate dissociation front is predicted, and the estimated location of hydrate dissociation front suggests a linear relationship with the square root of time. A sensitivity analysis on the thermal conductivity of sand under fully saturated conditions is conducted to elucidate its effect on the gas production behavior. In addition, the energy efficiency ratio computed from the simulation of this boundary-wall heating technique varies from 14.0 to 16.2. Deviations between observations and predictions of the evolution of the temperature profile are attributed to initial heterogeneous distribution of the hydrate phase in the hydrate reactor.

42 ENGINEERING↗
DOE OSTI2017

Measurement of Fructose–Asparagine Concentrations in Human and Animal Foods

The food-borne bacterial pathogen, Salmonella enterica, can utilize fructose-asparagine (F-Asn) as the sole carbon and nitrogen source. F-Asn is the product of an Amadori rearrangement following the non-enzymatic condensation of glucose and asparagine. Heating converts F-Asn via complex Maillard reactions to a variety of molecules which contribute to the color, taste, and aroma of heated foods. Among these is acrylamide, which is present in some foods, especially in fried potato products. The F-Asn utilization pathway in Salmonella, specifically FraB, is a potential drug target because inhibition of this enzyme would lead to intoxication of Salmonella in the presence of F-Asn. However, F-Asn would need to be packaged with the FraB inhibitor, or available in human foods. To determine if there are foods that have sufficient F-Asn, we measured F-Asn concentrations in a variety of human and animal foods. The 399 pmol/mg F-Asn found in mouse chow is sufficient to intoxicate a Salmonella fraB mutant in mouse models of salmonellosis, and several human foods were found to have this concentration or higher (fresh apricots, lettuce, asparagus, and in canned peaches). Much higher concentrations (11,000 to 35,000 pmol/mg dry weight) were found in heat-dried apricots, apples, and asparagus. This report clarifies the origins of F-Asn as a nutrient source for Salmonella, and identifies foods that could be used in concert with a FraB inhibitor as a therapeutic agent for Salmonella.

Wu, Jikang↗
DOE OSTI2019

Probing Electric Double-Layer Composition via in Situ Vibrational Spectroscopy and Molecular Simulations.

At an electrode, ions and solvent accumulate to screen charge, leading to a nanometer-scale electric double layer (EDL). The EDL guides electrode passivation in batteries, while in (super)capacitors, it determines charge storage capacity. Despite its importance, quantification of the nanometer-scale and potential-dependent EDL remains a challenging problem. Here, we directly probe changes in the EDL composition with potential using in situ vibrational spectroscopy and molecular dynamics simulations for a Li-ion battery electrolyte (LiClO 4 in dimethyl carbonate). The accumulation rate of Li + ions at the negative surface and ClO 4 - ions at the positive surface from vibrational spectroscopy compares well to that predicted by simulations using a polarizable APPLE&P force field. The ion solvation shell structure and ion-pairing within the EDL differs significantly from the bulk, especially at the negative electrode, suggesting that the common rationalization of interfacial electrochemical processes in terms of bulk ion solvation should be applied with caution.

25 ENERGY STORAGE↗
DOE OSTI2017

Spin-orbit torque-driven skyrmion dynamics revealed by time-resolved X-ray microscopy

Magnetic skyrmions are topologically protected spin textures with attractive properties suitable for high-density and low-power spintronic device applications. Much effort has been dedicated to understanding the dynamical behaviours of the magnetic skyrmions. However, experimental observation of the ultrafast dynamics of this chiral magnetic texture in real space, which is the hallmark of its quasiparticle nature, has so far remained elusive. Here, we report nanosecond-dynamics of a 100nm-diameter magnetic skyrmion during a current pulse application, using a time-resolved pump-probe soft X-ray imaging technique. We demonstrate that distinct dynamic excitation states of magnetic skyrmions, triggered by current-induced spin-orbit torques, can be reliably tuned by changing the magnitude of spin-orbit torques. Our findings show that the dynamics of magnetic skyrmions can be controlled by the spin-orbit torque on the nanosecond time scale, which points to exciting opportunities for ultrafast and novel skyrmionic appl ications in the future.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Impact of gas background on XFEL single-particle imaging

Abstract Single-particle imaging (SPI) using X-ray free-electron Lasers (XFELs) offers the potential to determine protein structures at high spatial and temporal resolutions without the need for crystallization or vitrification. However, the technique faces challenges due to weak diffraction signals from single proteins and significant background scattering from gases used for sample delivery. A recent observation of a diffraction pattern from an isolated GroEL protein complex Ekeberg T et al. (Light Sci Appl 13:15, 2024.https://doi.org/10.1038/274s41377-023-01352-7) had similar numbers of signal and background photons. Ongoing efforts aim to reduce the background created by sample delivery, with one approach replacing most of the used gas with helium Yenupuri T et al. (Sci Rep 14:4401, 2024.https://doi.org/10.1038/s41598-024-54605-9). In this study, we investigate the effects of a reduced background on the resolution limits for SPI of isolated proteins under different experiment conditions. As a test case, we used GroEL, and we used experimentally derived parameters for our simulations. We observe that background significantly impacts the achievable resolution, particularly when the signal strength is comparable to the background. This is best exemplified at 6.0 keV, where a background reduction by a factor of 10 leads to a resolution improvement from 1.9 to 1.2 nm, for a dataset of$$10^4$$patterns.

Science & Technology - Other Topics↗
DOE OSTI2014

Development of an ELISA microarray assay for the sensitive and simultaneous detection of ten biodefense toxins.

Plant and microbial toxins are considered bioterrorism threat agents because of their extreme toxicity and/or ease of availability. Additionally, some of these toxins are increasingly responsible for accidental food poisonings. The current study utilized an ELISA-based protein antibody microarray for the multiplexed detection of ten biothreat toxins, botulinum neurotoxins (BoNT) A, B, C, D, E, F, ricin, shiga toxins 1 and 2 (Stx), and staphylococcus enterotoxin B (SEB), in buffer and complex biological matrices. The multiplexed assay displayed a sensitivity of 1.3 pg/mL (BoNT/A, BoNT/B, SEB, Stx-1 and Stx-2), 3.3 pg/mL (BoNT/C, BoNT/E, BoNT/F) and 8.2 pg/mL (BoNT/D, ricin). All assays demonstrated high accuracy (75-120 percent recovery) and reproducibility (most coefficients of variation < 20%). Quantification curves for the ten toxins were also evaluated in clinical samples (serum, plasma, nasal fluid, saliva, stool, and urine) and environmental samples (apple juice, milk and baby food) with overall minimal matrix effects. The multiplex assays were highly specific, with little crossreactivity observed between the selected toxin antibodies. The results demonstrate a multiplex microarray that improves current immunoassay sensitivity for biological warfare agents in buffer, clinical, and environmental samples.

Jenko, Kathryn↗
DOE OSTI2016

Effect of water on the structure of a prototype ionic liquid

The influence of water on the structure of a prototype ionic liquid (IL) 1-octyl-3-methyimidazolium tetrafluoroborate (C8mimBF4) is examined in the IL-rich regime using high-energy x-ray diffraction (HEXRD) and molecular dynamics (MD) simulations. A many-body polarizable force field APPLE&P was developed for C8mimBF4 water mixture. It predicts structure factors of pure IL and IL-water mixture in excellent agreement with the HEXRD experiments. The MD results provide detailed insights into the structural changes from the partial structure factors, 2-D projections of the simulation box and 3-D distribution functions. Water partitioning with IL and its competition with BF4- for complexing the imidazolium rings was examined. The added water molecules occupy a diffuse coordination shell around the imidazolium ring but are not present around the alkyl tail. The strong coordination of the fluorine atoms of the BF4- anions to the imidazolium ring is not significantly changed by the addition of water. These results are consistent with the very small differences in the average structure between the pure IL and the mixture.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

A contribution to spectroscopic diagnostics and cathode sheath modeling of micro-hollow gas discharge in argon

In this paper, the hydrogen Balmer beta line shape from a micro-hollow gas discharge (MHGD) in argon with traces of hydrogen is used for simultaneous diagnostics of plasma and cathode sheath (CS) parameters. For this purpose, a simple model of relevant processes responsible for the line broadening is introduced and applied to the Balmer beta profile recorded from a MHGD generated in the microhole (diameter 100 {mu}m at narrow side and 130 {mu}m at wider side) of a gold-alumina-gold sandwich in the pressure range (100-900 mbar). The electron number density N{sub e} in the range (0.4-4.5) x 10{sup 20} m{sup -3} is determined from the width of the central part of the Balmer beta line profile, while, from the extended wings of the Balmer beta profile, induced by dc Stark effect, the next three parameters are determined: the average value E{sub a} of electric field strength in the CS in the range (16-95 kV/cm), the electric field strength E{sub 0} at the cathode surface in the range (32-190 kV/cm), and the CS thickness z{sub g} in the range (18-70 {mu}m). All four MHGD parameters, N{sub e}, E{sub a}, E{sub 0}, and z{sub g}, compare reasonably well with results of the modeling experiment by M. J. Kushner [J. Phys. D: Appl. Phys. 38, 1633 (2005)]. The results for N{sub e} are compared with other emission experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Areal density evolution of isolated surface perturbations at the onset of x-ray ablation Richtmyer-Meshkov growth

Isolated defects on inertial confinement fusion ignition capsules are a concern as defects taller than a few hundred nanometers are calculated to form jets of high-Z material, which enter the main fuel. If this mixing of high-Z material is not controlled, a serious degradation in thermonuclear burn can occur. A path towards controlling the growth of defects on the outer surface of plastic capsules is currently under development, but requires accurate predictions of defect evolution driven by the early time ablative Richtmyer-Meshkov (RM) effect. The chief uncertainty is the Equation of State (EOS) for polystyrene and its effect on ablative RM. We report on measurements of the growth of isolated defects made at the onset of ablative RM oscillations driven by x-ray ablation to differentiate between EOS models used in design calculations. Experiments at the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] used on-axis area backlighting radiography and x-ray framing cameras to determine bump areal densities at discrete times. Bumps 12 and 14 {mu}m tall and 33 {mu}m FWHM were found to grow to 2 x their initial areal density by 3 ns after the start of the drive laser pulse. Shock speed measurements established target conditions resulting from the ablation process. The tabular LEOS 5310 [D. Young and E. Corey, J. Appl. Phys. 78, 3748 (1995)] model shows good agreement with measured shock speeds and bump growth whereas the QEOS model [R. More et al., Phys. Fluids 31, 3059 (1988)] over predicts shock speed and under predicts bump growth by 6x. Differences in ablative RM behavior were also found for x-ray ablation compared to laser ablation, which result in an overestimation (or non-existence) of oscillation frequency for x-ray ablation as predicted by theory.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Anomalous evolution of Ar metastable density with electron density in high density Ar discharge

Recently, an anomalous evolution of argon metastable density with plasma discharge power (electron density) was reported [A. M. Daltrini, S. A. Moshkalev, T. J. Morgan, R. B. Piejak, and W. G. Graham, Appl. Phys. Lett. 92, 061504 (2008)]. Although the importance of the metastable atom and its density has been reported in a lot of literature, however, a basic physics behind the anomalous evolution of metastable density has not been clearly understood yet. In this study, we investigated a simple global model to elucidate the underlying physics of the anomalous evolution of argon metastable density with the electron density. On the basis of the proposed simple model, we reproduced the anomalous evolution of the metastable density and disclosed the detailed physics for the anomalous result. Drastic changes of dominant mechanisms for the population and depopulation processes of Ar metastable atoms with electron density, which take place even in relatively low electron density regime, is the clue to understand the result.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Effect of molecular weight on the efficiency of poly(N-vinylcarbazole)-based polymer light-emitting diodes

Polymer light-emitting diodes based on poly(N-vinylcarbazole) (PVK) with molecular weights M{sub W} of 1.1 x 10{sup 6} and {approx}7.5 x 10{sup 4} are compared. For devices without an electron transport layer (ETL), the high M{sub W} PVK yields higher external quantum efficiency (0.67% vs 0.18%), but for devices with an ETL, the low M{sub W} PVK yields higher efficiency (1.13% vs 0.83%). This intriguing difference is believed to result from higher energetic disorder in the high M{sub W} polymer and different recombination zone-quenching metal electrode distances, in agreement with Konezny et al. [Appl. Phys. Lett. 97, 143305 (2010)].

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Deep traps in nonpolar m-plane GaN grown by ammonia-based molecular beam epitaxy

Deep level defects in nonpolar m-plane GaN grown by ammonia-based molecular beam epitaxy were characterized using deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) and compared with polar c-plane GaN that was grown simultaneously in the same growth run. Significant differences in both the levels present and their concentrations were observed upon comparison of both growth orientations. DLTS revealed electron traps with activation energies of 0.14 eV, 0.20 eV, and 0.66 eV in the m-plane material, with concentrations that were {approx}10-50 x higher than traps of similar activation energies in the c-plane material. Likewise, DLOS measurements showed {approx}20 x higher concentrations of both a C{sub N} acceptor-like state at E{sub C} - 3.26 eV, which correlates with a high background carbon concentration observed by secondary ion mass spectroscopy for the m-plane material [A. Armstrong, A. R. Arehart, B. Moran, S. P. DenBaars, U. K. Mishra, J. S. Speck, and S. A. Ringel, Appl. Phys. Lett. 84, 374 (2004)], and the V{sub Ga}-related state level at E{sub C} - 2.49 eV, which is consistent with an enhanced yellow luminescence observed by photoluminescence. The findings suggest a strong impact of growth dynamics on the incorporation of impurities and electrically active native point defects as a function of GaN growth plane polarity.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Effect of e-beam irradiation on graphene layer grown by chemical vapor deposition

We have grown graphene by chemical vapor deposition (CVD) and transferred it onto Si/SiO{sub 2} substrates to make tens of micron scale devices for Raman spectroscopy study. The effect of electron beam (e-beam) irradiation of various doses (600 to 12 000 {mu}C/cm{sup 2}) on CVD grown graphene has been examined by using Raman spectroscopy. It is found that the radiation exposures result in the appearance of the strong disorder D band attributed the damage to the lattice. The evolution of peak frequencies, intensities, and widths of the main Raman bands of CVD graphene is analyzed as a function of defect created by e-beam irradiation. Especially, the D and G peak evolution with increasing radiation dose follows the amorphization trajectory, which suggests transformation of graphene to the nanocrystalline and then to amorphous form. We have also estimated the strain induced by e-beam irradiation in CVD graphene. These results obtained for CVD graphene are in line with previous findings reported for the mechanically exfoliated graphene [D. Teweldebrhan and A. A. Balandin, Appl. Phys. Lett. 94, 013101 (2009)]. The results have important implications for CVD graphene characterization and device fabrication, which rely on the electron microscopy.

36 MATERIALS SCIENCE↗