Session A56: Deformation of 2D Materials and Interfaces

Live

The properties of 2D materials can be tuned through alloying and phase and strain engineering. I will present a novel approach, combining phase/strain engineering with shape programming, to form 3D objects by patterned alloying of 2D transition metal dichalcogenide (TMD) monolayers. Conjugately, monolayers can be compositionally patterned using non-flat substrates. For concreteness, we focus on the TMD alloy MoSe_2cS_2(1-c); i.e., MoSeS. These 2D materials down-scale shape/composition programming to nanoscale objects/patterns, provide control of both bending and stretching deformations, are reversibly actuatable with electric fields, and possess the extraordinary and diverse properties of TMDs. Utilizing a first principles-informed continuum model, we demonstrate how a variety of shapes/composition patterns can be programmed and reversibly modulated across length scales. I will also demonstrate the mechanical actuation of 2D TMD alloy films through application of electric fields.   

Live

Recent developments in 2D incommensurate atomic heterostructures reveal a vast phase space of complex systems rich in exotic phenomena and opportunities for control. These developments include cutting-edge computational tools such as Mismatched Interface Theory (MINT)[1] and other continuum theories that enable accurate modeling of charge transfer, strain, spin-orbit interactions, and magnetism of incommensurate interfaces that were previously inaccessible to traditional ab initio techniques. We combine these advances with the open access materialsproject.org to develop a versatile interface database tool that predicts charge transfer, strain, and other crucial parameters of an interface between two arbitrary materials.

[1] Gerber, E., Yao, Y., Arias, T. A. & Kim, E.-A. Phys. Rev. Lett. 124, 106804 (2020)   

Live

Two-dimensional (2D) semiconducting materials have attracted extensive research interests for applications in optoelectronics, spintronics, photovoltaics, and catalysis. Realizing their potential in such applications requires a good understanding of the effects of defects, dopants, and impurities on the properties of these systems. We perform density functional theory (DFT) calculations to accurately compute formation energies, charge transition levels, and electronic properties of dopants, defects, and complexes in the technologically important 2D semiconductor materials focusing on the metal chalcogenides MoS₂, WSe₂, and SnS. We investigate the dependence of computed defect properties on different levels of theory, utilizing a correction scheme to ensure appropriate electrostatic boundary conditions for charged defects in 2D materials. Some defects induce structural distortions, e.g., Jahn-Teller and other lattice reconstructions, which alter the electronic properties. We identify dopants which bind with intrinsic defects to form complexes, passivating the dopants and rendering them less effective. Finally, we demonstrate how theoretical predictions based on DFT and beyond-DFT approaches such as GW and BSE can help inform the interpretation of experimental results.   

Live

The stacking type of bilayer graphene plays a major role in determining its electronic spectrum. AB-BA domain walls contain SP stacking solitons and twisted bilayer gives rise to the flat bands and superconductivity. With the use of a non-planar substrate, a bilayer can be bent, introducing slips in the stacking pattern. These non-AB non-twisted patches of bilayer can be prevented from slipping back into AB by a larger AB bilayer “anchor” on the other side of the bend. Classical molecular dynamics relaxation is performed to account for possible intralayer lattice distortions. Two registries are shown to be stable with their electronic spectrum calculated in the tight-binding model. The first one is a 1nm wide strip of 0.4A away-from-AA stacking and the second is a 5nm wide strip of SP stacking, previously only found in AB-BA domain walls. Finally, we created a map of all stackings one might be able to obtain within this approach.   

Live

Strain engineering is a powerful tool for controlling the electronic properties of condensed matter systems. Prior work has focused primarily on applying strain to a wide variety of bulk crystals in order to tune their electronic states, including those with strong correlations and/or non-trivial topology. However, so far comparatively little experimental effort has been focused on controlling the properties of two-dimensional van der Waals (vdW) materials and heterostructures with strain. Here, we develop new experimental techniques to integrate in situ strain tuning with low temperature, high magnetic field electrical transport measurements of arbitrary vdW heterostructure devices. We fabricate devices on a thin silicon wafer and affix them onto a three-piezostack strain apparatus. By applying a voltage to the piezostack, we can continuously exert either compressive or tensile strain to the vdW heterostructure resting on the silicon wafer. We will discuss ongoing efforts to utilize this new technique to dynamically control the electronic properties of various vdW heterostructures with strain.   

Live

The discovery of superconductivity in twisted bilayer graphene, associated to flat bands close to the Fermi level, has raised a lot of excitement. Such highly correlated states have also been observed in many layers of multilayer stacked rhombohedral graphene (RG). However, it is observed less frequently than multilayer stacked Bernal graphene (BG), so the conditions under which RG becomes more stable than BG should be determined. Here we show, using first principles calculations, that applying shear stress induces long-range rhombohedral order. The experimental conditions under which RG can be obtained are presented in a stress-angle phase diagram [1]. Then we calculate the energy difference of bulk RG and BG using different functionals, and show that the electronic temperature plays a crucial role. The stability is also studied for a finite amount of layers, and the low energy states are characterized. Our work clarifies inconsistencies in the literature, and sets the basis to add factors like doping or an electric field in first principles calculations.

[1] J. P. Nery, M. Calandra, and F. Mauri, Nano Lett. 20, 5017 (2020).   

Live

The low-energy part of the vibrational spectrum of van-der-Waals layered materials is characterised by two fundamental sets of normal-mode vibrations, where the layers oscillate as rigid units, either parallel (shear or C modes) or perpendicular (layer-breathing or LB modes) to each other. Their frequencies depend on the number of layers and can be used to characterise the layered materials by Raman or infrared spectroscopy. We present here a general approach to predict the fan diagram of optically-active C and LB modes of any layered material with any number of layers. Based on symmetry arguments, we describe the evolution of the point group as a function of the number of layers and of the spacegroup of the corresponding bulk system. We then combine group theory with a tensorial one-dimensional mechanical model to compute vibrational normal modes and identify which ones are Raman and/or infrared active. This procedure allows us to seamlessly provide the fan diagram of optically-active modes for any multilayer stack of any layered material. We implement this method and algorithms in an open tool that we make available online on the Materials Cloud portal, to assist any researcher in the prediction and interpretation of such diagrams.   

Live

Modifications of the geometric structure of pristine graphene should be a promising method to expand the possibility of application. We study the electronic properties of graphene with structural defects in the shape of truncated triangles arranged periodically in the framework of the density-functional theory. First, we consider systems in which defects are arranged in the same direction. In their energy band structures, one or more nearly flat bands appear at the Fermi energy. It is also found that the ground state of the system is ferromagnetic. Next, we consider systems in which additional triangular defects with the opposite direction are arranged. There are two defects in each unit cell and sides of regular triangles face each other. We find that all sysytems in this study can be semiconductors and the band gaps are tunable by changing the sizes of the defect and the unit cell. Interestingly, nearly flat bands sometimes appear at the valence band top and the conduction band bottom. We also discuss spatial distributions of the valence band top state and conduction band bottom state.   

Live

Hexagonal boron nitride (h-BN) nanomesh, a two-dimensional insulating monolayer, grown on the (111) surface of rhodium exhibits an intriguing hexagonal corrugation pattern with a lattice constant of 3.2 nm. Despite numerous experimental and theoretical studies, structural details such as the corrugation amplitude have been difficult to determine quantitatively due to the differences in chemical and electronic environments in the strongly bound pore regions and the weakly bound wire regions of the corrugated structure. For reliable results it is important to probe the structure with a method that is intrinsically sensitive to the position of the atomic cores rather than the electron density of states.
In this contribution, we determine the corrugation of h-BN nanomesh from angle- and energy-resolved photoelectron diffraction measurements with chemical state resolution. By combining the results from angle and energy scans and comparing them to multiple-scattering simulations true adsorbate-substrate distance can be measured with high precision, avoiding pitfalls of apparent topography observed in scanning probe techniques.

L. H. de Lima et al., 2D Mater. 7, 035006 (2020)   

Live

2D materials are at the core of nano/micro-electro-mechanical systems (MEMS/NEMS). However, surface corrugations are unavoidable in 2D materials because of operational inefficiencies involved in the synthesis procedures. So far, most related works focus on single planar 2D materials, i.e., Graphene. The existing Transient Metal Dichalcogenide (TMD) analyses do not include torque implications on the frictional force and different variables simultaneously. In this work, we have combined the two aspects, i.e., surface irregularities with the need for tuning the mechanical properties. We have studied tuning the frictional properties of Molybdenum Disulfide (MoS₂) by probing it with reactive molecular dynamics (rMD) modeling, which is further analyzed by density functional theory (DFT) code. We analyzed BADER charge and molecular orbitals by DFT code. We considered multiple cases, i.e., varying number of the layers (1-3), the number of dents (2-8), the radius of each dent (12-24 Å), and the pattern of the dents (in-line and zigzag). We hypothesize the frictional force experienced by the probe is affected by the substrate's geometry, both adjacent to and along the line of contact of probe. The torque plots validate our hypothesis. This approach is useful for any other 2D materials.   

Live

Revealing and exploiting novel electronic properties in atomically thin 2D materials such as graphene and transition metal dichalcogenides have progressed dramatically in recent years. A challenge is to correlate structure and electronic properties, especially in heterostructures. Here, we focus on a prototypical Van der Waals hetero-interface of graphene and monolayer WSe₂, where both are prepared epitaxially on a SiC substrate. We present angle-resolved photoelectron spectroscopy (ARPES) and synchrotron X-ray diffraction studies, which show that WSe₂ is compressed by exactly the amount needed to realize a perfect epitaxial matching to the graphene lattice. Combined APRES and XPS directly reveal electron transfer from graphene to WSe₂.   

Live

Auxetic two-dimensional materials provide a promising platform for multiple applications at the nanoscale. Utilizing a hypothesis-based data-driven approach we identify multiple materials with remarkable in-plane auxetic behavior in a family of buckled 2D materials with the stoichiometry MX (M = V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag, and X= Se, Cl, Br, I). The desirable auxetic behavior originates from the interplay between the buckled structure and the weak metal-metal interaction determined by their electronic structures. The Poisson's ratio is sensitive to magnetic order and the amount of uniaxial stress applied. A transition from positive Poisson's ratio to negative Poisson's ratio for a subgroup of MX compounds under large uniaxial stress is predicted.   

Live

We investigate phonon spectra in MoS₂ twisted bilayers and find substantial renormalization: over a range of small twist angles, the phonon spectra evolve rapidly due to ultra-strong coupling between different phonon modes and the atomic reconstruction of the moiré pattern. These phonon spectra also reveal three regimes of atomic reconstructions. We develop a new low-energy continuum model for phonons that overcomes the outstanding challenge of calculating phonon properties of large moiré supercells and successfully captures essential experimental observations. We show how simple optical spectroscopy experiments can provide information on strain and lattice distortions in moiré crystals for nanometer-sized supercells. Our newly developed theory provides a comprehensive and unified understanding of structural and optical properties of moiré superlattices.