Oak Ridge National Laboratory

About: Oak Ridge National Laboratory is a facility organization based out in Oak Ridge, Tennessee, United States. It is known for research contribution in the topics: Neutron & Ion. The organization has 31868 authors who have published 73724 publications receiving 2633689 citations. The organization is also known as: ORNL.

Porous Graphene as the Ultimate Membrane for Gas Separation

Abstract: We investigate the permeability and selectivity of graphene sheets with designed subnanometer pores using first principles density functional theory calculations. We find high selectivity on the order of 10(8) for H(2)/CH(4) with a high H(2) permeance for a nitrogen-functionalized pore. We find extremely high selectivity on the order of 10(23) for H(2)/CH(4) for an all-hydrogen passivated pore whose small width (at 2.5 A) presents a formidable barrier (1.6 eV) for CH(4) but easily surmountable for H(2) (0.22 eV). These results suggest that these pores are far superior to traditional polymer and silica membranes, where bulk solubility and diffusivity dominate the transport of gas molecules through the material. Recent experimental investigations, using either electron beams or bottom-up synthesis to create pores in graphene, suggest that it may be possible to employ such techniques to engineer variable-sized, graphene nanopores to tune selectivity and molecular diffusivity. Hence, we propose using porous graphene sheets as one-atom-thin, highly efficient, and highly selective membranes for gas separation. Such a pore could have widespread impact on numerous energy and technological applications; including carbon sequestration, fuel cells, and gas sensors.

A Survey of Techniques for Approximate Computing

Abstract: Approximate computing trades off computation quality with effort expended, and as rising performance demands confront plateauing resource budgets, approximate computing has become not merely attractive, but even imperative. In this article, we present a survey of techniques for approximate computing (AC). We discuss strategies for finding approximable program portions and monitoring output quality, techniques for using AC in different processing units (e.g., CPU, GPU, and FPGA), processor components, memory technologies, and so forth, as well as programming frameworks for AC. We classify these techniques based on several key characteristics to emphasize their similarities and differences. The aim of this article is to provide insights to researchers into working of AC techniques and inspire more efforts in this area to make AC the mainstream computing approach in future systems.

van der Waals Epitaxy of MoS2 Layers Using Graphene As Growth Templates

Abstract: We present a method for synthesizing MoS2/Graphene hybrid heterostructures with a growth template of graphene-covered Cu foil. Compared to other recent reports,(1, 2) a much lower growth temperature of 400 °C is required for this procedure. The chemical vapor deposition of MoS2 on the graphene surface gives rise to single crystalline hexagonal flakes with a typical lateral size ranging from several hundred nanometers to several micrometers. The precursor (ammonium thiomolybdate) together with solvent was transported to graphene surface by a carrier gas at room temperature, which was then followed by post annealing. At an elevated temperature, the precursor self-assembles to form MoS2 flakes epitaxially on the graphene surface via thermal decomposition. With higher amount of precursor delivered onto the graphene surface, a continuous MoS2 film on graphene can be obtained. This simple chemical vapor deposition method provides a unique approach for the synthesis of graphene heterostructures and surface funct...

Signatures of moiré-trapped valley excitons in MoSe 2 /WSe 2 heterobilayers

Abstract: The formation of moire patterns in crystalline solids can be used to manipulate their electronic properties, which are fundamentally influenced by periodic potential landscapes. In two-dimensional materials, a moire pattern with a superlattice potential can be formed by vertically stacking two layered materials with a twist and/or a difference in lattice constant. This approach has led to electronic phenomena including the fractal quantum Hall effect1–3, tunable Mott insulators4,5 and unconventional superconductivity6. In addition, theory predicts that notable effects on optical excitations could result from a moire potential in two-dimensional valley semiconductors7–9, but these signatures have not been detected experimentally. Here we report experimental evidence of interlayer valley excitons trapped in a moire potential in molybdenum diselenide (MoSe2)/tungsten diselenide (WSe2) heterobilayers. At low temperatures, we observe photoluminescence close to the free interlayer exciton energy but with linewidths over one hundred times narrower (around 100 microelectronvolts). The emitter g-factors are homogeneous across the same sample and take only two values, −15.9 and 6.7, in samples with approximate twist angles of 60 degrees and 0 degrees, respectively. The g-factors match those of the free interlayer exciton, which is determined by one of two possible valley-pairing configurations. At twist angles of approximately 20 degrees the emitters become two orders of magnitude dimmer; however, they possess the same g-factor as the heterobilayer at a twist angle of approximately 60 degrees. This is consistent with the umklapp recombination of interlayer excitons near the commensurate 21.8-degree twist angle7. The emitters exhibit strong circular polarization of the same helicity for a given twist angle, which suggests that the trapping potential retains three-fold rotational symmetry. Together with a characteristic dependence on power and excitation energy, these results suggest that the origin of the observed effects is interlayer excitons trapped in a smooth moire potential with inherited valley-contrasting physics. This work presents opportunities to control two-dimensional moire optics through variation of the twist angle. The trapping of interlayer valley excitons in a moire potential formed by a molybdenum diselenide/tungsten diselenide heterobilayer with twist angle control is reported.