Enhanced Field Emission from Vertically Oriented Graphene by Thin Solid Film Coatings

Recent progress and a coordinated national research program have brought considerable effort to bear on the synthesis and application of carbon nanostructures for field emission. at the College of William and Mary, we have developed field emission arrays of vertically oriented graphene (carbon nanosheets, CNS) that have demonstrated promising cathode performance, delivering emission current densities up to 2 mA/mm2 and cathode lifetime \u3e 800 hours. The work function ( & phis;) of CNS and other carbonaceous cathode materials has been reported to be &phis;∼4.5-5.1 eV. The application of low work function thin films can achieve several orders of magnitude enhancement of field emission.;Initially, the intrinsic CNS field emission was studied. The mean height of the CNS was observed to decrease as a function of operating time at a rate of ∼0.05 nm/h (I 1 ∼ 40 muA/mm2). The erosion mechanism was studied using a unique UHV diode design which allowed line-of-site assessment from the field emission region in the diode to the ion source of a mass spectrometer. The erosion of CNS was found to occur by impingement of hyperthermal H and O neutrals and ions generated at the surface oxide complex of the Cu anode by electron stimulated desorption. Techniques for minimizing this erosion are presented. ; The Mo2C (&phis;∼3.7 eV) beading on CNS at previously reported carbide formation temperatures of ∼800 ?? C was circumvented by physical vapor deposition of Mo and vacuum annealing at ∼300??C which resulted in a conformal Mo2C coating and stable field emission of 1 ∼ 50 muA/mm2. For a given applied field, the emission current was \u3e 102 greater than uncoated CNS.;ThO2 thin film coatings were presumed to be even more promising because of a reported work function of &phis; ∼2.6 eV. The fundamental behavior of the initial oxidation of polycrystalline Th was studied in UHV (p \u3c 1x10-11 Torr), followed by studies of thin film coatings on Ir and thermionic emission characteristics. Although a work function of 3.3 eV was determined by a RichardDushman plot, activation of the thin film was not achieved at T \u3c 1700??C. Rather, the deposited ThO2 film decomposed, surface diffused and aggregated into stable ThO2(111) crystallites.;Thin film ThO2 coatings deposited on CNS initially demonstrated excellent field emission (up to ∼2 muA/mm2) and apparently activated spontaneously without significant thermal energy. Fowler-Nordheim plots suggested a work function of &phis; ∼2.6 eV. Undesired beading and ThO2 surface diffusion away from active emission sites resulted in rapidly deteriorating performance at higher field emission currents. Techniques that should provide a more stable ThO2/CNS conformal coating are presented.;The impact of thin films of Mo2C and ThO2on the magnitude of field emission from carbon nanosheets (CNS) was substantial. For a given field emission current density, J ∼2 muA/mm 2, the necessary applied field for uncoated CNS was ∼12 V/mum, but only ∼8 V/mum when coated with Mo2C (&phis;∼3.7 eV) and ∼5 V/mum when coated with ThO2 (&phis;∼2,6 eV). The mechanism for enhanced emission and the stability of the coatings are discussed, with special focus on the activation of ThO2 thin films. The major limitation observed in these studies has been the difference in surface energy of the graphene and the coatings which resulted in a tendency for the films to bead and separate from active emission sites at elevated currents. Suggested techniques to prevent this unwanted surface diffusion are presented

High performance aluminum–cerium alloys for high-temperature applications

Light-weight high-temperature alloys are important to the transportation industry where weight, cost, and operating temperature are major factors in the design of energy efficient vehicles. Aluminum alloys fill this gap economically but lack high-temperature mechanical performance. Alloying aluminum with cerium creates a highly castable alloy, compatible with traditional aluminum alloy additions, that exhibits dramatically improved high-temperature performance. These compositions display a room temperature ultimate tensile strength of 400 MPa and yield strength of 320 MPa, with 80% mechanical property retention at 240 °C. A mechanism is identified that addresses the mechanical property stability of the Al-alloys to at least 300 °C and their microstructural stability to above 500 °C which may enable applications without the need for heat treatment. Finally, neutron diffraction under load provides insight into the unusual mechanisms driving the mechanical strength

Identification and HLA-Tetramer-Validation of Human CD4(+) and CD8(+) T Cell Responses against HCMV Proteins IE1 and IE2

Human cytomegalovirus (HCMV) is an important human pathogen. It is a leading cause of congenital infection and a leading infectious threat to recipients of solid organ transplants as well as of allogeneic hematopoietic cell transplants. Moreover, it has recently been suggested that HCMV may promote tumor development. Both CD4+ and CD8+ T cell responses are important for long-term control of the virus, and adoptive transfer of HCMV-specific T cells has led to protection from reactivation and HCMV disease. Identification of HCMV-specific T cell epitopes has primarily focused on CD8+ T cell responses against the pp65 phosphoprotein. In this study, we have focused on CD4+ and CD8+ T cell responses against the immediate early 1 and 2 proteins (IE1 and IE2). Using overlapping peptides spanning the entire IE1 and IE2 sequences, peripheral blood mononuclear cells from 16 healthy, HLA-typed, donors were screened by ex vivo IFN-γ ELISpot and in vitro intracellular cytokine secretion assays. The specificities of CD4+ and CD8+ T cell responses were identified and validated by HLA class II and I tetramers, respectively. Eighty-one CD4+ and 44 CD8+ T cell responses were identified representing at least seven different CD4 epitopes and 14 CD8 epitopes restricted by seven and 11 different HLA class II and I molecules, respectively, in total covering 91 and 98% of the Caucasian population, respectively. Presented in the context of several different HLA class II molecules, two epitope areas in IE1 and IE2 were recognized in about half of the analyzed donors. These data may be used to design a versatile anti-HCMV vaccine and/or immunotherapy strategy

Spatiotemporal characterization of the laser-induced plasma plume in simulated Martian conditions

With ChemCam on NASA's Curiosity rover, SuperCam on NASA's Perseverance rover, and MarSCoDe on the Zhurong rover of the China National Space Administration, there are currently three instruments on Mars that employ laser-induced breakdown spectroscopy (LIBS) to analyze the chemical composition of Martian rocks and soils. With more than 880,000 LIBS measurements on Mars by ChemCam alone, the LIBS technique has been proven to be uniquely qualified for the in-situ robotic exploration of planetary surfaces. Since the laser-induced plasma exhibits a complex spatiotemporal evolution that has a significant impact on the recorded LIBS spectra, fundamental investigations of the plasma propagation and the spatial distributions of the plasma emissions can provide important insight that can help to improve the analysis of Martian LIBS spectra. Here we present first results from our LIBS plasma imaging setup, which allows us to spatially and temporally resolve the spectral emissions from the laser-induced plasma in simulated Martian atmospheric conditions. Investigating a calcium sulfate sample, we find that the commonly applied assumption of a nearly isothermal and homogeneous plasma with a colder outer layer is not sufficient to describe the laser-induced plasma on Mars. Instead, different spectral features show unique spatial distributions that suggest a strong influence of the outgoing shock wave. After plasma formation, the plasma center is found to rapidly become colder and more rarefied than the outer plasma regions. Molecular emissions are found to originate in this cold plasma center

Detecting sulfur on the Moon: The potential of VUV-LIBS

Laser-induced breakdown spectroscopy (LIBS) receives increasing interest in the field of space exploration. Some volatile elements important to planetary research are, however, challenging to detect with conventional LIBS because their strongest emission lines lie in the vacuum-UV (VUV) spectral range below 200nm. We demonstrate the potential of LIBS detection in the VUV range from 125 to 190 nm on one of the relevant challenging elements, S, in a lunar context. With our laboratory set-up, we detected S in lunar analogues at concentrations ranging from 0.5 to 4.0 at\% in a time-integrated configuration. The experiments were conducted in high vacuum ($10^{-3}$ Pa) with laser parameters realistic for a space instrument (1064 nm, 25 mJ, 6 ns). The results indicate improved detection capabilities for S in a lunar geological context as compared to LIBS in the more common UV/VIS/NIR spectral range. We found limits of detection in the order of 0.5at S for all investigated sample series. Normalization by total intensity and by an internal standard, in our case by the emission signal of the O I triplet at 130 nm, was tested as well, but did not yield a noticeable improvement for our data. Besides S, we were also able to detect Al, Fe, O, Si and Ti from the lunar analogue matrix

A Systematic, Unbiased Mapping of CD8⁺ and CD4⁺ T Cell Epitopes in Yellow Fever Vaccinees

Examining CD8+ and CD4+ T cell responses after primary Yellow Fever vaccination in a cohort of 210 volunteers, we have identified and tetramer-validated 92 CD8+ and 50 CD4+ T cell epitopes, many inducing strong and prevalent (i.e., immunodominant) T cell responses. Restricted by 40 and 14 HLA-class I and II allotypes, respectively, these responses have wide population coverage and might be of considerable academic, diagnostic and therapeutic interest. The broad coverage of epitopes and HLA overcame the otherwise confounding effects of HLA diversity and non-HLA background providing the first evidence of T cell immunodomination in humans. Also, double-staining of CD4+ T cells with tetramers representing the same HLA-binding core, albeit with different flanking regions, demonstrated an extensive diversification of the specificities of many CD4+ T cell responses. We suggest that this could reduce the risk of pathogen escape, and that multi-tetramer staining is required to reveal the true magnitude and diversity of CD4+ T cell responses. Our T cell epitope discovery approach uses a combination of (1) overlapping peptides representing the entire Yellow Fever virus proteome to search for peptides containing CD4+ and/or CD8+ T cell epitopes, (2) predictors of peptide-HLA binding to suggest epitopes and their restricting HLA allotypes, (3) generation of peptide-HLA tetramers to identify T cell epitopes, and (4) analysis of ex vivo T cell responses to validate the same. This approach is systematic, exhaustive, and can be done in any individual of any HLA haplotype. It is all-inclusive in the sense that it includes all protein antigens and peptide epitopes, and encompasses both CD4+ and CD8+ T cell epitopes. It is efficient and, importantly, reduces the false discovery rate. The unbiased nature of the T cell epitope discovery approach presented here should support the refinement of future peptide-HLA class I and II predictors and tetramer technologies, which eventually should cover all HLA class I and II isotypes. We believe that future investigations of emerging pathogens (e.g., SARS-CoV-2) should include population-wide T cell epitope discovery using blood samples from patients, convalescents and/or long-term survivors, who might all hold important information on T cell epitopes and responses.Fil: Stryhn, Anette. Universidad de Copenhagen; DinamarcaFil: Kongsgaard, Michael. Universidad de Copenhagen; DinamarcaFil: Rasmussen, Michael. Universidad de Copenhagen; DinamarcaFil: Harndahl, Mikkel Nors. Universidad de Copenhagen; DinamarcaFil: Østerbye, Thomas. Universidad de Copenhagen; DinamarcaFil: Bassi, Maria Rosa. Universidad de Copenhagen; DinamarcaFil: Thybo, Søren. Universidad de Copenhagen; DinamarcaFil: Gabriel, Mette. No especifíca;Fil: Hansen, Morten Bagge. Universidad de Copenhagen; DinamarcaFil: Nielsen, Morten. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas; ArgentinaFil: Pravsgaard Christensen, Jan. Universidad de Copenhagen; DinamarcaFil: Randrup Thomsen, Allan. Universidad de Copenhagen; DinamarcaFil: Buus, Soren. Universidad de Copenhagen; Dinamarc