Session F17: Focus Session: Two-dimensional Materials Design

New two dimensional compounds: beyond graphene

In the field of nanosciences, the quest for materials with reduced dimensionality is only at its beginning. While a lot of effort has been put initially on graphene, the focus has been extended in the last past years to functionalized graphene, boron nitride, silicene, and transition metal dichalcogenides in the form of single layers. Although these two-dimensional compounds offer a larger range of properties than graphene, there is a constant need for new materials presenting equivalent or superior performances to the ones already known. Here I will present an approach that we have used to discover potential new two-dimensional materials. This approach corresponds to perform datamining in the Inorganic Crystal Structure Database using simple geometrical criterias, and allowed us to identify nearly 40 new materials that could be exfoliated into two-dimensional sheets. Then, their electronic structure (density of states and bandstructure) was obtained with density functional theory to predict whether the two-dimensional material is metallic or insulating, as well as if it undergoes magnetic ordering at low temperatures. If time allows, I will also present some of our recent results concerning the electronic structure of transition metal dichalcogenides bilayers.   

New-class of Semiconducting 2D materials: Tin Dichalcogenides (SnX2)

Recent studies have focused on a new generation of atomically thin films of semiconducting materials. A broad family of two-dimensional (2D) semiconducting transition metal dichalcogenides (MX2) have been fabricated and investigated in monolayer, bilayer and few layer form. In this work, we investigated the electronic, optical and elastic properties of single and few layer and bulk SnX2 (X$=$ S, Se) both theoretically and experimentally. Using density functional theory (DFT) we carried out stability analysis through phonon and electronic, optical and elastic structure calculations. Single-few layer SnX2s are mechanically exfoliated and Raman and photoluminescence (PL) measurements are taken. UV-Vis absorption spectrum together with PL measurements and DFT calculations yield an indirect gap of $\sim$ 2.5 eV for SnS2 structures (bulk). Tunability of the energy band gap and indirect-direct gap transitions are investigated by controlling the number of layers and applied stress. Lowering the number of layers decreases the indirect gap (0.1-0.3 eV), but indirect-direct gap transition occurs when layer-layer distance is reduced. Due to flexibility in engineering the electronic and optical properties, SnX2 compounds are promising materials for future optoelectronic nanoscale applications.   

Computational design of p-type contacts for MoS$_{2}$-based electronic devices

The excellent physical and semiconducting properties of transition metal dichalcogenide (TMDC) monolayers make them promising materials for many applications. A well-known example is MoS$_{2}$, which has gained significant attention as a channel material for next-generation transistors. While n-type MoS$_{2}$ field-effect transistors (n-FETs) can be fabricated with relative ease, fabrication of p-FETs remains a challenge as the Fermi-level of elemental metals used as contacts are pinned close to the conduction band, leading to large p-type Schottky barrier heights (SBHs). Using \textit{ab initio} computations, we design and propose efficient hole contacts utilizing high work function oxide-based hole injection materials, with the aim of advancing p-type MoS$_{2}$ device technology. Our calculations will highlight the possibility to tune and lower the p-type SBH at the metal/semiconductor interface by controlling the structural properties of oxide materials. Taken together, our results provide an interesting platform for experimental design of next-generation MoS$_{2}$-based electronic and optoelectronic devices.   

Computational Design of 2D materials for Energy Applications

Since the successful synthesis of graphene, tremendous efforts have been devoted to two-dimensional monolayers such as boron nitride (BN), silicene and MoS$_{2}$. These 2D materials exhibit a large variety of physical and chemical properties with unprecedented applications. Here we report our recent studies of computational design of 2D materials for fuel cell applications which include hydrogen storage, CO$_{2}$ capture, CO conversion and O$_{2}$ reduction.   

Systematic Enumeration of sp$^{3}$ Nanothreads and Computational Study of their Properties

A novel 1D allotrope of carbon arise from slow decompression of solid benzene in high-pressure (GPa) cells, wherein columns of benzene, guided by ``topochemical'' constraint as six-valent 1D super-atoms, rehybridize into a crystal of sp$^{3}$ nanothreads. We exhaustively enumerate the allowed hexavalent bonding topologies and discovered several new low-energy allotropes not previously described. The intermediate conformational nature of these systems -- stiffer than a polymer, more reconfigurable than a nanowire or nanotube -- allows the translational repeat unit for interatomic connectivity (``topological unit cell'') to deviate from the crystallographic unit cell. A topological unit of 12 carbons accommodates thirty-seven distinct chemically stable nanothreads, fourteen of which are within 80 meV/carbon of the most stable member. Careful optimization of aperiodic helicity reveals the most stable structures to be chiral; several are stiffer (per carbon atom) than bulk diamond. They have the large gaps of saturated hydrocarbons, but reasonable routes exist towards producing semiconducting variants. We generalize Euler's rules for ring counting to cover these systems, and propose a naming convention that can be generalized to handle a broad range of pregenitor molecules.   

A Pentagon Based Carbon Sheet

A new two-dimensional (2D) meta-stable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling, is proposed. State-of-the-art theoretical calculations confirm that the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K. Due to its unique atomic configuration penta-graphene has an unusual negative Poisson's ratio (NPR) and ultra-high ideal strength that can even outperform graphene. Furthermore, unlike graphene that needs to be functionalized for opening a band gap, penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 eV - close to that of ZnO and GaN. Equally important, when rolled up, penta-graphene can form a pentagon-based nanotube. The resulting penta-carbon nanotubes are semiconducting regardless of their chirality. When stacked in different patterns, dynamically and thermally stable 3D twin structures of T12-carbon are generated with band gaps even larger than that of T12-carbon. The versatility of penta-graphene and its derivatives are expected to have broad applications in nanoelectronics and nanomechanics.   

The size and shape dependence of graphene domain on the band gap of h-BN

This talk will report the structure and electronic characteristics of graphene domains embedded in a hexagonal boron-nitride sheet (h-BN) with the goal of band gap tuning in mind. Different shapes (triangular, circular, rectangular, and irregular structures) and sizes of graphene domains will be studied. The structural stability of these hybrid materials will be studied using a new generation of the semi-empirical Hamiltonian (SCED-LCAO) developed recently [arXiv:1408.4931]. It is found that the lattice mismatch between graphene domains and the h-BN generates large strain, leading to a reduction or a symmetry breaking of the hexagonal lattice of h-BN. The extent of the strain depends on the shape and the size of the domain, as well as on the distribution of B atoms around the graphene domains. This effect also creates impurity states in the band gap of h-BN and changes the band gap. The interplay between the shape and size of graphene domains, the local strain around the domains and the nature of the impurity states on the band gap of h-BN will be discussed.   

Plenty of motion at the bottom: atomically thin, free-standing liquid gold

Recent experiments have shown that, in addition to covalent materials, also metals like iron can display an atomically thin free-standing solid phase. This phase can be created when existing hole in free-standing graphene is patched by metal atoms. Here we investigate such a metal patch by density-functional -based molecular dynamics simulations when the patching material is gold. Simulations show that while at room temperature gold displays a solid phase, at elevated temperatures it displays an atomically thin liquid phase. This stability of free-standing gold to remain planar even upon two-dimensional diffusion is outright remarkable and has its origins in relativistic effects.   

Dirac Loops in Carbon Allotropes

We propose a family of structures that have ``Dirac loops'': closed lines in momentum space with Dirac-like quasiparticles, on which the density of states vanishes linearly with energy. The structures all possess the planar trigonal connectivity present in graphene, but are three dimensional. We discuss the consequences of their multiply-connected Fermi surface for transport, including the presence of three dimensional Integer Quantum Hall effect. In the presence of spin-orbit coupling, we show that those structures may have topological surface states. We discuss the feasibility of realizing the structures as an allotrope of carbon.   

Effects of interlayer Sn-Sn lone pair interaction on the band gap of bulk and nanosheet SnO

Effects of interlayer lone-pair interactions on the electronic structure of SnO are firstly explored by the density-functional theory. Our comprehensive study reveals that the band gap of SnO opens as increase in the interlayer Sn-Sn distance. The effect is rationalized by the character of band edges which consists of bonding and anti-bonding states from interlayer lone pair interactions. The band edges for several nanosheets and strained double-layer SnO are estimated. We conclude that the double-layer SnO is a promising material for visible-light driven photocatalyst for hydrogen evolution.   

Phosphorene Oxide: Stability and electronic properties of a novel 2D material

Phosphorene, the monolayer form of the (black) phosphorus, was recently exfoliated from its bulk counterpart. Phosphorene oxide, by analogy to graphene oxide, is expected to have novel chemical and electronic properties, and may provide an alternative route to synthesis of phosphorene. In this letter, we investigate physical and chemical properties of the phosphorene oxide including its formation by the oxygen adsorption on the bare phosphorene. Analysis of the phonon dispersion curves finds stoichiometric and non-stoichiometric oxide configurations to be stable at ambient conditions, thus suggesting that the oxygen asorption may not degrade the phosphorene. The nature of the band gap of the oxides depends on the degree of the functionalization of phosphorene; indirect gap is predicted for the non-stoichiometric configurations whereas a direct gap is predicted for the stoichiometric oxide. Application of the mechanical strain and external electric field leads to tunability of the band gap of the phosphorene oxide. In contrast to the case of the bare phosphorene, dependence of the diode-like asymmetric current-voltage response on the degree of stoichiometry is predicted for the phosphorene oxide.   

Chemical scissors cut phosphorene and their novel electronic properties

Phosphorene, a recently fabricated two-dimensional puckered honeycomb structure of black phosphorus, showed promising properties for applications in nano-electronics. In this work, we report a chemical scissors effect on phosphorene, using first principles density-functional methods. It was found that chemical species, such as H, OH, F, and Cl, can act as scissors to cut phosphorene. Phosphorus nanochains and nanoribbons can be obtained using such chemical scissors. The scissors effect results from the strong bonding between the chemical species and phosphorus atoms. Other species such as O, S and Se fail to cut phosphorene due to their weak bonding with phosphorus. The electronic structures of the produced P-chains reveal that the hydrogenated P-chain is an insulator; however, the pristine P-chain is a one-dimensional Dirac material, in which the charge carriers are massless fermions travelling at an effective speed of light approximately 8x10$^{5}$ m/s. The obtained zigzag phosphorene nanoribbons show either metallic or semiconducting behaviors, depending on the treatment of the edge phosphorus atoms. \\[4pt] [1] X.-H. Peng, Q. Wei, ``Chemical scissors cut phosphorene nanostructures,'' Material Research Express, in press.