Wenbin Li

Giant piezoelectricity of monolayer group IV monochalcogenides: SnSe, SnS, GeSe, and GeS

We predict enormous, anisotropic piezoelectric effects in intrinsic monolayer group IV monochalcogenides (MX, M=Sn or Ge, X=Se or S), including SnSe, SnS, GeSe, and GeS. Using first-principle simulations based on the modern theory of polarization, we find that their piezoelectric coefficients are about one to two orders of magnitude larger than those of other 2D materials, such as MoS2 and GaSe, and bulk quartz and AlN which are widely used in industry. This enhancement is a result of the unique “puckered” C2v symmetry and electronic structure of monolayer group IV monochalcogenides. Given the achieved experimental advances in the fabrication of monolayers, their flexible character, and ability to withstand enormous strain, these 2D structures with giant piezoelectric effects may be promising for a broad range of applications such as nano-sized sensors, piezotronics, and energy harvesting in portable electronic devices.

Patterning of graphene

Two-dimensional atomic sheets of carbon (graphene, graphane, etc.) are amenable to unique patterning schemes such as cutting, bending, folding and fusion that are predicted to lead to interesting properties. In this review, we present theoretical understanding and processing routes for patterning graphene and highlight potential applications. With more precise and scalable patterning, the prospects of integrating flat carbon (graphene) with curved carbon (nanotubes and half nanotubes) and programmable graphene folding are envisioned.

Piezoelectricity in two-dimensional group-III monochalcogenides

It is found that several layer-phase group-III monochalcogenides, including GaS, GaSe, and InSe, are piezoelectric in their monolayer form. First-principles calculations reveal that the piezoelectric coefficients of monolayer GaS, GaSe, and InSe (2.06, 2.30, and 1.46 pm·V™1) are of the same order of magnitude as previously discovered two-dimensional (2D) piezoelectric materials such as boron nitride (BN) and MoS2 monolayers. This study therefore indicates that a strong piezoelectric response can be obtained in a wide range of two-dimensional materials with broken inversion symmetry. The co-existence of piezoelectricity and superior photo-sensitivity in these monochalcogenide monolayer semiconductors means they have the potential to allow for the integration of electromechanical and optical sensors on the same material platform.

Modelling of stacked 2D materials and devices

Tremendous developments in the synthesis and understanding of two-dimensional materials such as graphene, phosphorene, BN and MoS2 have spurred great interest in exploiting their heterostructures in devices. Here, we present a compact review of the modeling and simulation of stacked two-dimensional materials and devices. Particular emphasis is placed on the intriguing phenomena enabled by different stacking geometries and material choices, and the fundamental physical mechanisms behind them, which enable new device concepts and applications to be developed which would have been difficult to achieve with other material platforms.

Signature of Metallic Behavior in the Metal-Organic Frameworks M3(hexaiminobenzene)2 (M = Ni, Cu)

The two-dimensionally connected metal-organic frameworks (MOFs) Ni3(HIB)2 and Cu3(HIB)2 (HIB = hexaiminobenzene) are bulk electrical conductors and exhibit ultraviolet-photoelectron spectroscopy (UPS) signatures expected of metallic solids. Electronic band structure calculations confirm that in both materials the Fermi energy lies in a partially filled delocalized band. Together with additional structural characterization and microscopy data, these results represent the first report of metallic behavior and permanent porosity coexisting within a metal-organic framework.

Ferroelasticity and domain physics in two-dimensional transition metal dichalcogenide monolayers.

Monolayers of transition metal dichalcogenides can exist in several structural polymorphs, including 2H, 1T and 1T′. The low-symmetry 1T′ phase has three orientation variants, resulting from the three equivalent directions of Peierls distortion in the parental 1T phase. Using first-principles calculations, we predict that mechanical strain can switch the relative thermodynamic stability between the orientation variants of the 1T′ phase. We find that such strain-induced variant switching only requires a few percent elastic strain, which is eminently achievable experimentally with transition metal dichalcogenide monolayers. Calculations indicate that the transformation barrier associated with such variant switching is small (<0.2 eV per chemical formula unit), suggesting that strain-induced variant switching can happen under laboratory conditions. Monolayers of transition metal dichalcogenides with 1T′ structure therefore have the potential to be ferroelastic and shape memory materials with interesting domain physics.

Deformation-driven diffusion and plastic flow in amorphous granular pillars.

We report a combined experimental and simulation study of deformation-induced diffusion in compacted quasi-two-dimensional amorphous granular pillars, in which thermal fluctuations play a negligible role. The pillars, consisting of bidisperse cylindrical acetal plastic particles standing upright on a substrate, are deformed uniaxially and quasistatically by a rigid bar moving at a constant speed. The plastic flow and particle rearrangements in the pillars are characterized by computing the best-fit affine transformation strain and nonaffine displacement associated with each particle between two stages of deformation. The nonaffine displacement exhibits exponential crossover from ballistic to diffusive behavior with respect to the cumulative deviatoric strain, indicating that in athermal granular packings, the cumulative deviatoric strain plays the role of time in thermal systems and drives effective particle diffusion. We further study the size-dependent deformation of the granular pillars by simulation, and find that different-sized pillars follow self-similar shape evolution during deformation. In addition, the yield stress of the pillars increases linearly with pillar size. Formation of transient shear lines in the pillars during deformation becomes more evident as pillar size increases. The width of these elementary shear bands is about twice the diameter of a particle, and does not vary with pillar size.

Scalable synthesis of a sulfur nanosponge cathode for a lithium–sulfur battery with improved cyclability

Although lithium–sulfur batteries exhibit a high initial capacity, production costs and lack of cyclability are major limitations. Here we report a liquid-based, low-cost and reliable synthesis method of a lithium–sulfur composite cathode with improved cyclability. An open network of Conductive Carbon Black nanoparticles (Cnet) is infused with a sulfur network (Snet) to form sponge-like networks (Cnet + Snet). Initially, Snet is open to the outside, allowing liquid electrolyte to infiltrate and impart Snet Li+ conductivity. During lithiation, Cnet could accommodate the volume expansion of Snet largely without losing electrical contact. During delithiation, the carbon nanoparticles would preferably flocculate on the outer surface due to polysulfide dissolution and depletion of sulfur, to form a passivation layer that still allows Li+ exchange, but prevents more polysulfides from escaping, thus slowing the leaching of polysulfides into the bulk electrolyte liquid. The plausibility of a carbonaceous passivation layer was checked using an extra carbon deposition layer to achieve an improved performance of ∼400 mA h g−1 after 250 cycles under a high rate 2.0 C. A 763 mA h g−1 discharge specific capacity of this sulfur nanosponge cathode (abbreviated as “SULFUN”) was obtained after 100 cycles under a rate of 0.2 C. Discharge capacities of 520 mA h g−1 and 290 mA h g−1 were attained after 300 and 500 cycles, respectively, making this cathode material attractive for rechargeable battery applications.

Envelope function method for electrons in slowly-varying inhomogeneously deformed crystals

We develop a new envelope-function formalism to describe electrons in slowly-varying inhomogeneously strained semiconductor crystals. A coordinate transformation is used to map a deformed crystal back to a geometrically undeformed structure with deformed crystal potential. The single-particle Schrödinger equation is solved in the undeformed coordinates using envelope function expansion, wherein electronic wavefunctions are written in terms of strain-parametrized Bloch functions modulated by slowly varying envelope functions. Adopting a local approximation of the electronic structure, the unknown crystal potential in the Schrödinger equation can be replaced by the strain-parametrized Bloch functions and the associated strain-parametrized energy eigenvalues, which can be constructed from unit-cell level ab initio or semi-empirical calculations of homogeneously deformed crystals at a chosen crystal momentum. The Schrödinger equation is then transformed into a coupled differential equation for the envelope functions and solved as a generalized matrix eigenvector problem. As the envelope functions are slowly varying, a coarse spatial or Fourier grid can be used to represent the envelope functions, enabling the method to treat relatively large systems. We demonstrate the effectiveness of this method using a one-dimensional model, where we show that the method can achieve high accuracy in the calculation of energy eigenstates with relatively low cost compared to direct diagonalization of the Hamiltonian. We further derive envelope function equations that allow the method to be used empirically, in which case certain parameters in the envelope function equations will be fitted to experimental data.