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

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