NRC Postdoctoral Research Associate

Washington, DC, United States

NRC Postdoctoral Research Associate

Washington, DC, United States
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Lock E.H.,U.S. Navy | Hernandez S.C.,NRC Postdoctoral Research Associate | Anderson T.J.,U.S. Navy | Schmucker S.W.,NRC Postdoctoral Research Associate | And 8 more authors.
Surface and Coatings Technology | Year: 2014

Graphene transfer is critical for successful graphene-device integration. Even though multiple transfer approaches have been developed, an optimal solution is still not available. In this paper, we demonstrate dry transfer of graphene transfer Cu foil to polystyrene using mild heat and pressure. Two different printing systems are evaluated: a wafer bonder and a more cost-effective heat press. Chemical, morphological, structural and electrical characterization of the samples before and after transfer is performed. The results suggest that differences in the operation of the printing devices can be correlated to uniformity of transferred graphene. Regardless of the printing approach the surface resistivity of the polymer was decreased by 16 orders of magnitude. © 2013.

Yoon W.,U.S. Navy | Boercker J.E.,U.S. Navy | Lumb M.P.,U.S. Navy | Lumb M.P.,George Washington University | And 5 more authors.
2015 IEEE 42nd Photovoltaic Specialist Conference, PVSC 2015 | Year: 2015

In this work, we report dark current reduction in solution-processed PbS nanocrystal-metal Schottky junction solar cells via improved LiF passivation of the interface between PbS nanocrystal films and the metal electrode. For the optimized LiF interfacial layer, the dark saturation current density (J0) is decreased by at least one order of magnitude, resulting in very high open-circuit voltage (Voc) of 692±7 mV under one sun illumination for ∼1.4 eV PbS nanocrystals. Using different size of PbS nanocrystals and therefore different bandgaps, we also demonstrate Voc (mV)=553Eg/q-59 as a function of the PbS nanocrystal bandgap (Eg). For different types of junctions employed for solution-processed PbS nanocrystal solar cells, we plot the bandgap-voltage offsets (Eg/q-Voc) under open-circuit conditions, showing strong dependence of the Voc on the Eg regardless of the types of junction used. Similar dependence is also found in solution-processed and sintered CdTe nanocrystal solar cells. These results suggest that suppressing the non-radiative recombination contributions to the dark current, such as improved passivation of nanocrystal surfaces, is more critical to improve the Voc in nanocrystal solar cells, rather than optimizing the device architecture with varying the n-type semiconducting materials. © 2015 IEEE.

Whitener Jr. K.E.,NRC Postdoctoral Research Associate | Sheehan P.E.,U.S. Navy
Diamond and Related Materials | Year: 2014

The promise of graphene, a two-dimensional hexagonal form of elemental carbon, as a revolutionary material has sparked a flurry of research into its optical, electronic, thermal, and mechanical properties. The most famous method of isolating graphene sheets, introduced by Novoselov et al. in 2004 [1], uses adhesive tape to mechanically cleave graphite crystals into successively thinner platelets. This micromechanical cleavage is time-consuming and produces an abundance of few- and multilayer graphene along with single-layer material. In addition, the area of the graphene sheet obtained by this method is limited by the initial size of the graphite crystal. These limitations of micromechanical cleavage, along with the explosion of interest in graphene in general, have led researchers to devise a number of alternative methods for graphene synthesis. In this review, we discuss different synthetic methods for obtaining graphene along with their advantages and disadvantages and then introduce current avenues of research in this rapidly expanding field. © 2014 Elsevier B.V.

Hatami K.,University of Oklahoma | Esmaili D.,University of Oklahoma | Esmaili D.,NRC Postdoctoral Research Associate
Geosynthetics International | Year: 2015

One main concern related to the performance of unsaturated soils during the construction and service life of earthen structures is loss of matric suction due to the seasonal variations of gravimetric water content (GWC), ground water infiltration and possible development of excess pore water pressure. In addition to reducing the soil shear strength, loss of matric suction as a result of wetting could also reduce the soil-reinforcement interface shear strength in comparison with the as-built value at a lower GWC. This paper presents the results of small-scale pullout and interface tests on a woven geotextile reinforcement material in different marginal soils in order to quantify the difference in the soil-geotextile interface shear strength as a function of GWC for practical applications. A moisture reduction factor [MRF µ(ω)] is used to account for the reduction in the soil-geotextile interface shear strength as a function of matric suction over a range of GWC values that includes the dry and wet sides of the soil optimum gravimetric water content (GWCopt) or optimum moisture content (OMC). It was observed that the interface shear strength of geotextile reinforcement in marginal soils could be significantly lower (e.g. by as much 50%) at only 2% wet of optimum (i.e. OMC+2%) in comparison with OMC-2%, which is assumed to represent the as-built condition. © 2015 Thomas Telford Ltd.

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