Todd H. Brintlinger

The Effect of Preparation Conditions on Raman and Photoluminescence of Monolayer WS2

We report on preparation dependent properties observed in monolayer WS2 samples synthesized via chemical vapor deposition (CVD) on a variety of common substrates (Si/SiO2, sapphire, fused silica) as well as samples that were transferred from the growth substrate onto a new substrate. The as-grown CVD materials (as-WS2) exhibit distinctly different optical properties than transferred WS2 (x-WS2). In the case of CVD growth on Si/SiO2, following transfer to fresh Si/SiO2 there is a ~50 meV shift of the ground state exciton to higher emission energy in both photoluminescence emission and optical reflection. This shift is indicative of a reduction in tensile strain by ~0.25%. Additionally, the excitonic state in x-WS2 is easily modulated between neutral and charged exciton by exposure to moderate laser power, while such optical control is absent in as-WS2 for all growth substrates investigated. Finally, we observe dramatically different laser power-dependent behavior for as-grown and transferred WS2. These results demonstrate a strong sensitivity to sample preparation that is important for both a fundamental understanding of these novel materials as well as reliable reproduction of device properties.

NRL Materials Testing Facility

The Naval Research Laboratory performs basic research on high power railgun electric launchers. The program uses a 1.5-MJ, 2.5 km/s launch velocity railgun located in NRLs Materials Testing Facility. The railgun consists of an 11-MJ capacitive energy store configured as 22, 0.5-MJ modules. Each bank module has an independently triggered thyristor switch, series inductor, and crowbar diode and is joined to the railgun breech with coaxial cables. Individual bank timing and charge levels can be set to produce up to 1.5 MA peak current and 4-5 ms long current pulses. The 6-m long railgun used a nominally 5 cm bore diameter with steel or copper rails and epoxy laminate insulators. The muzzle contains a Tungsten-Copper arc horn to minimize damage from residual drive current upon launch. Aluminum armatures with acrylic bore riders are used for the launch package. Launch data is recorded digitally and analyzed using in-house computer codes. The system design and operation will be discussed.

EM Gun Bore Life Experiments at Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5-MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical finite element analysis models of railgun operation. Results of several experiments on rail wear will be discussed.

Defective by design: vanadium-substituted iron oxide nanoarchitectures as cation-insertion hosts for electrochemical charge storage

Vanadium-substituted iron oxide aerogels (2 : 1 Fe : V ratio; VFe2Ox) are synthesized using an epoxide-initiated sol–gel method to form high surface-area, mesoporous materials in which the degree of crystallinity and concentration of defects are tuned via thermal treatments under controlled atmospheres. Thermal processing of the X-ray amorphous, as-synthesized VFe2Ox aerogels at 300 °C under O2-rich conditions removes residual organic byproducts while maintaining a highly defective γ-Fe2O3-like local structure with minimal long-range order and vanadium in the +5 state. When as-synthesized VFe2Ox aerogels are heated under low partial pressure of O2 (e.g., flowing argon), a fraction of vanadium sites are reduced to the +4 state, driving crystallization to a Fe3O4-like cubic phase. Subsequent thermal oxidation of this nanocrystalline VFe2Ox aerogel re-oxidizes vanadium +4 to +5, creating additional cation vacancies and re-introducing disordered oxide domains. We correlate the electrochemical charge-storage properties of this series of VFe2Ox aerogels with their degree of order and chemical state, as verified by X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. We find that the disordered O2-heated VFe2Ox aerogel yields the highest Li+- and Na+-insertion capacities among this series, approaching 130 mA h g−1 and 70 mA h g−1, respectively. Direct heat-treatment of the VFe2Ox aerogel in flowing argon to yield the partially reduced, nanocrystalline form results in significantly lower Li+-insertion capacity (77 mA h g−1), which improves to 105 mA h g−1 by thermal oxidation to create additional vacancies and structural disorder.

EM gun bore life experiments at the Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5 MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical Finite Element Analysis (FEA) models of railgun operation. Results of several experiments on rail wear will be discussed.

NRL materials testing facility

The Naval Research Laboratory Materials Testing Facility performs basic research on high-power railguns. The laboratory houses a 6-m-long, 5-cm-bore railgun capable of launching 0.5-kg projectiles at up to 2.5 km/s. The railgun is powered by an 11-MJ capacitor bank comprising twenty-two 0.5-MJ modules. The crow-barred banks can drive up to 1.5 MA in the railgun. The railgun core consists of steel rails with copper backers and epoxy laminate insulators. Aluminum armatures with acrylic bore riders are used for the launch package. Launch data are recorded digitally and analyzed using in-house computer codes. The system design is presented along with typical data.

Chemically exfoliating large sheets of phosphorene via choline chloride urea viscosity-tuning

Exfoliation of two-dimensional phosphorene from bulk black phosphorous through chemical means is demonstrated where the solvent system of choice (choline chloride urea diluted with ethanol) has the ability to successfully exfoliate large-area multi-layer phosphorene sheets and further protect the flakes from ambient degradation. The intercalant solvent molecules, aided by low-powered sonication, diffuse between the layers of the bulk black phosphorus, allowing for the exfoliation of the multi-layer phosphorene through breaking of the interlayer van der Waals bonds. Through viscosity tuning, the optimal parameters (1:1 ratio between the intercalant and the diluting solvent) at which the exfoliation takes place is determined. Our exfoliation technique is shown to produce multi-layer phosphorene flakes with surface areas greater than 3 μm2 (a factor of three larger than what has previously been reported for a similar exfoliation method) while limiting exposure to the ambient environment, thereby protecting the flakes from degradation. Characterization techniques such as optical microscopy, Raman spectroscopy, ultraviolet-visible spectroscopy, and (scanning) transmission electron microscopy are used to investigate the quality, quantity, and thickness of the exfoliated flakes.

Characterizing Multi-layer Pristine Graphene, Its Contaminants, and Their Origin Using Transmission Electron Microscopy

1. Materials Sci. and Tech. Division, U.S. Naval Research Laboratory, Washington, DC, USA 20375 2. Electronics Sci. and Tech. Division, U.S. Naval Research Laboratory, Washington, DC, USA 20375 3. Chemistry Division, U.S. Naval Research Laboratory, Washington, DC, USA 20375 4. NRC Postdoctoral Associate, U.S. Naval Research Laboratory, Washington, DC, USA 20375 5. Dept. of Physics and Nanoscience Technology Ctr., Univ. of Central Florida, Orlando, FL USA 32816 *current address: Dept. of Mat. Sci. and Eng., McMaster Univ., Hamilton, Ontario, Canada L9H 4L7

S)TEM Characterization of Chemically Exfoliated Black Phosphorus

Phosphorene is the 2-dimensional form of black phosphorus and a close relative of graphene. It has a nonzero fundamental band gap that gives rise to semiconductor properties, which makes it highly desirable for numerous applications in optoelectronics [1] and as a replacement channel for conventional semiconductor devices [2]. However, difficulties in isolating large area single-, few-, or multi-layer sheets are an impediment to realizing the aforementioned applications. We are investigating multiple routes for optimal production of phosphorene sheets via chemical intercalation combined with mechanical agitation. Utilizing solvent systems of increasing viscosity, i.e. chloroform (0.57 cP), ethanol (1.1 cP), choline chloride-urea/ethanol (2.84 cP), and EMI-BF4 (66 cP), we have obtained flakes of differing thickness and sizes. Following our solvent treatments to obtain micro-scale flakes, we characterize the structure and composition with both high-resolution and aberration-corrected scanning transmission electron microscopy (HRTEM and ac-STEM, a JEOL JEM2200FS and Nion UltraSTEMX 200 at the U.S. Naval Research Laboratory, at 200 kV and 60 kV, respectively), as seen in Fig. 1. Flake size, quality, and quantity as a function of the solvent system will be presented.

Nanopatteming in GeTe phase change materials using heated atomic force microscope tips

Patterning surfaces leads to the creation of physiochemical heterogeneities (i.e. surface energy, chemical reactivity, conductivity, topography, etc) which are important to the design of complex components used in modern electronics. The ability to control patterns and write and rewrite circuits is critical for adaptive learning in future electronic devices.