212 research outputs found
Unusual negative formation enthalpies and atomic ordering in isovalent alloys of transition metal dichalcogenide monolayers
Common substitutional isovalent semiconductor alloys usually form disordered
metastable phases with positive excess formation enthalpies ({\Delta}H). In
contrast, monolayer alloys of transition metal dichalcogenides (TMDs) MX2 (M =
Mo, W; X = S, Se) always have negative {\Delta}H, suggesting atomic ordering,
which is, however, not yet experimentally observed. Using first-principles
calculations, we find that the negative {\Delta}H of cation-mixed TMD alloys
results from the charge transfer from weak Mo-X to nearest strong W-X bonds and
the negative {\Delta}H of anion-mixed TMD alloys comes from the larger energy
gain due to the charge transfer from Se to nearest S atoms than the energy cost
due to the lattice mismatch. Consequently, cation-mixed and anion-mixed alloys
should energetically prefer to have Mo-X-W and S-M-Se ordering, respectively.
The atomic ordering, however, is only locally ordered but disordered in the
long range due to the symmetry of TMD monolayers, as demonstrated by many
energetically degenerate structures for given alloy compositions. Besides, the
local ordering and disordering effects on the macroscopic properties such as
bandgaps and optical absorptions are negligible, making the experimental
observation of locally ordered TMD alloys challenging. We propose to take the
advantage of microscopic properties such as defects which strongly depend on
local atomic configurations for experiments to identify the disordering and
local ordering in TMD alloys. Finally, quaternary TMD alloys by mixing both
cations and anions are studied to have a wide range of bandgaps for
optoelectronic applications. Our work is expected to help the formation and
utilization of TMD alloys.Comment: 25 pages, 6 figure
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Optimization of Nano-Carbon Materials for Hydrogen Sorption
Research undertaken has added to the understanding of several critical areas, by providing both negative answers (and therefore eliminating expensive further studies of unfeasible paths) and positive feasible options for storage. Theoretical evaluation of the early hypothesis of storage on pure carbon single wall nanotubes (SWNT) has been scrutinized with the use of comprehensive computational methods (and experimental tests by the Center partners), and demonstrated that the fundamentally weak binding energy of hydrogen is not sufficiently enhanced by the SWNT curvature or even defects, which renders carbon nanotubes not practical media. More promising direction taken was towards 3-dimensional architectures of high porosity where concurrent attraction of H2 molecule to surrounding walls of nano-scale cavities can double or even triple the binding energy and therefore make hydrogen storage feasible even at ambient or somewhat lower temperatures. An efficient computational tool has been developed for the rapid capacity assessment combining (i) carbon-foam structure generation, (ii) accurate empirical force fields, with quantum corrections for the lightweight H2, and (iii) grand canonical Monte Carlo simulation. This made it possible to suggest optimal designs for carbon nanofoams, obtainable via welding techniques from SWNT or by growth on template-zeolites. As a precursor for 3D-foams, we have investigated experimentally the synthesis of VANTA (Vertically Aligned NanoTube Arrays). This can be used for producing nano-foams. On the other hand, fluorination of VANTA did not show promising increase of hydrogen sorption in several tests and may require further investigation and improvements. Another significant result of this project was in developing a fundamental understanding of the elements of hydrogen spillover mechanisms. The benefit of developed models is the ability to foresee possible directions for further improvement of the spillover mechanism
Assessing carbon-based anodes for lithium-ion batteries: A universal description of charge-transfer binding
Many key performance characteristics of carbon-based lithium-ion battery
anodes are largely determined by the strength of binding between lithium (Li)
and sp2 carbon (C), which can vary significantly with subtle changes in
substrate structure, chemistry, and morphology. Here, we use density functional
theory calculations to investigate the interactions of Li with a wide variety
of sp2 C substrates, including pristine, defective, and strained graphene;
planar C clusters; nanotubes; C edges; and multilayer stacks. In almost all
cases, we find a universal linear relation between the Li-C binding energy and
the work required to fill previously unoccupied electronic states within the
substrate. This suggests that Li capacity is predominantly determined by two
key factors -- namely, intrinsic quantum capacitance limitations and the
absolute placement of the Fermi level. This simple descriptor allows for
straightforward prediction of the Li-C binding energy and related battery
characteristics in candidate C materials based solely on the substrate
electronic structure. It further suggests specific guidelines for designing
more effective C-based anodes. The method should be broadly applicable to
charge-transfer adsorption on planar substrates, and provides a
phenomenological connection to established principles in supercapacitor and
catalyst design.Comment: accepted by Physical Review Letter
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