Session A36: 2D Materials - Strain and Mechanical Properties

Effects of strain on second harmonic generation in alloyed transition metal dichalcogenide monolayers

First principles time-dependent density functional theory is used to calculate the second harmonic generation (SHG) spectra (χ⁽²⁾) of a variety of TMDs and their alloys. It is shown by comparison of calculations with and without the inclusion of the Bethe-Salpeter Equation (BSE) that when studying the effects of strain on SHG it is important to take excitonic effects into account. This can be attributed to the fact that χ⁽²⁾ is highly dependent on the dipole moments in the system which are significantly altered by the inclusion of electron-hole interactions. Changes in SHG spectra as a function of biaxial and uniaxial strain are calculated with a BSE implementation for the semiconducting TMDs, as well as several chalcogen and transition metal alloys with varying stoichiometric compositions. For all structures, significant changes in χ⁽²⁾ occur when strain is applied to the system, with the largest changes occurring in MoSe₂, WSe₂, and related alloys. These findings open new avenues of exploration for potential applications of these materials in strain-sensitive devices.   

Strain-enhanced p-doping in MoS2: first-principles study

MoS₂ is an important material for a variety of future electronic and optoelectronic applications. This material shows the atomically layered structure without any dangling bonds, good mobility, high current on/off ratio, large optical absorption. However, stable and controllable achievement of p-type MoS₂, which is prerequisite for most applications, is a still major issue. Using first-principles calculations, we address here the role of strain, for which no experimental evidence of its beneficial effect on the p-doping has been reported and no clear understanding of the mechanism to date. We theoretically predict that strain can enhance several p-dopants, and suppress the formation of sulfur vacancies that could be potential hole compensators.
  

Effects of electron-electron interaction in non-uniform strained graphene

Mechanical deformations on graphene affect electronic properties and open a route for new applications. Inhomogeneous strain has been predicted to produce band gap modulations [1], band filtering and quasi-bound states [2] in nanoscale size regions. Electronic confinement amplifies the role of electron interactions, a topic of renewed interest in graphene [3], and give access to phases of matter driven by interactions. Here, we report on the role of Hubbard interactions on electronic density and transport properties of a ZGNR with inhomogeneous strain due to a collective nanosphere substrate[4]. Size and geometrical arrangement of nanospheres, determine extent and density of the locally self-doped regions. A tight-binding model with a strain-induced scalar potential and a Hubbard U-term, is solved in the mean field approximation and reveals a redistribution of the electronic density and a lattice-dependent occupation. Magnetotransport calculations show similar features to transport through a quantum dots array. Results with pseudomagnetic field and magnetic correlations are also presented.
[1] V. Torres et al. PRB 95 (2017)
[2] R. Carrillo-Bastos et al. PRB 94 (2016)
[3] H. Ryu et al., Nano Lett. 17 (2017)
[4] Y. Zhang et al. arXiv:1703.05689 (2017)   

Measurement of mechanical properties of phase transformed 1T MoS2 using atomic force microscopy

It has been several years since graphene, which is the first 2D material, was revealed into the world. Still many of fundamental properties of 2D materials have not been clearly identified. Especially the experiment for measurement of mechanical properties of 2D materials is hard to design, fabricate or measure by traditional methods which have been adaptively developed for macro-scale experiments.

Molybdenum disulfide(MoS2) is an attractive material having interesting electronic properties and generally have 2H structure, which exhibits semiconducting behavior. The 2H structure can be transformed to 1T structure, which shows metallic behavior via some methods. There are several experimental results about mechanical properties of 2H MoS2. However, for 1T structure, only computational calculations exist due to its difficulty of the sample fabrication.

Here, we introduce our experimental process to make suspended 1T MoS2 on circular holes and its mechanical property measurement results. We tried to properly combine the existing techniques from previous research about 2D material and finally succeed to fabricate suspended 1T MoS2 on holes and its properties have been accurately characterized using the AFM indentation method.   

Rolled MoS2 : Fabrication and their properties

In this work, we rolled up molybdenum disulfide (MoS₂) monolayers and analyzed their properties. The MoS₂ monolayers were directly grown on SiO₂ substrates by using a chemical vapor deposition method. When Isopropyl alcohol solution was applied to the MoS₂, edges were lifted and rolled up. Structure and spectroscopic properties of the rolled MoS₂ were examined by atomic force microscopy, Raman and photoluminescence spectroscopy. Transport properties of the rolled-up MoS2 were also investigated down to liquid Helium temperature and discussed in this presentation.   

Stochastic Dynamics of Ripple Curvature Inversion in Freestanding Monolayers: A New Nanotechnology Source of Renewable Energy

The dynamic properties of ripples in freestanding monolayers have been exceedingly difficult to study with common experimental methods. Here, we present how to track the membrane in time using point-mode scanning tunneling microscopy. This method allows a direct measurement of the out-of-plane time trajectory and fluctuations at one point in space over long periods of time. We observe that individual ripples spontaneously invert their curvature from concave to convex. We have also successfully replicated spontaneous ripple curvature inversion using molecular dynamics simulations (LAMMPS) [see, Phys. Rev. Lett. 117, 126801 (2016)]. During curvature inversion thousands of atoms move together, in phase. Consequently, the Brownian motion is at times organized and this kinetic energy can be used to do work. The discovery of this novel motion represents a fundamental advance in our ability to harvest energy from thermal motion. For example, a single 10 nm by 10 nm ripple can produce 10 pW of power. Thus, a quartz wristwatch could be powered with a single 10 micron by 10 micron membrane.   

Programming Atomically Thin TMD Monolayers to Form 3D Shapes

Atomically- and near atomically-thin sheets can be programmed to form 3D shapes by spatially designing internal strains. We explore means of programming stretching and bending strains into 2D TMD monolayers, enabling 3D shape design and tunability of structural, mechanical, and optoelectronic properties. We present a general plate theory-based thermodynamic formalism for shape programming with elastic sheets and apply it via numerical simulation to various TMD systems and geometries. Our description is informed by input parameters computed from first-principles. It provides a quantitative framework for outlining paths toward designer 2D material systems with optimized shape and functional properties via the ability to program both stretching and bending strains locally. Potential applications are broad and include flexible / foldable electronics and nano-structured catalytic materials.   

Molecular Simulations of Shear Exfoliation of MoS2

Liquid-phase exfoliation (LPE) is a highly promising approach to large-scale production and dispersion of a wide variety of atomically thin layered materials (LMs). Among the three LPE approaches – electrochemical, sonication and shear – the last one appears to be the most efficient for synthesis of LMs. We perform molecular dynamics simulation (MD) to study optimal conditions for shear exfoliation of MoS₂. Different mixtures of water and 2-propanol are used to study shear exfoliation of MoS₂ under different shear rates. Results will be reported on the exfoliation yield as a function of shear stresses, temperature, and 2-proanol concentration in water.
  

Deterministic strain-induced arrays of quantum emitters in a two-dimensional semiconductor

The emergence of single quantum emitters in layered transition metal dichalcogenide semiconductors offers new opportunities to construct a scalable quantum architecture with a coherent light-matter interface. Here I will present results taking steps in this direction. First, using nanoscale strain engineering, we deterministically achieve a two-dimensional lattice of quantum emitters in an atomically thin semi- conductor. We create point-like strain perturbations in mono-and bi- layer WSe₂ which locally modify the band-gap, leading to efficient funnelling of excitons towards isolated strain-tuned quantum emitters. These emitters exhibit high-purity single photon emission that is stable and bright, yielding detected count rates up to 3 MHz. Next, we perform time-resolved photoluminescence, resonance fluorescence, and high-resolution photoluminescence excitation spectroscopy of these isolated, localized 2D excitons to characterize their dephasing mechanisms and unravel their origin. Finally, I will provide an outlook on investigations of the spin and valley coherence and prospects for integrated photonic chips incorporating quantum emitters in atomically-thin TMD semiconductors.   

Strain Field Enhancement of Kondo Effect in Graphene

The Kondo physics of screening of an impurity's magnetic moment by electrons in doped graphene has been predicted to exhibit peculiar features due to the linear density of states. However, experimental identification of this phenomenon remains controversial, mainly due to the interaction with substrates and measuring probes. Here, we propose to use mechanical out-of-plane deformations in graphene to recognize the unique fingerprints that the Kondo regime exhibits. The resulting inhomogeneous strain in the deformation is known to produce sublattice symmetry breaking that appears as alternating changes in the local density of states (LDOS). Since the Kondo effect is sensitive to the LDOS sampled by the impurity, these oscillating patterns are mirrored by dramatic changes in the Kondo temperature for impurities positioned at different lattice sites. The changes, which are suitable to be detected with local probes such as scanning tunneling microscope, are unique to the material and can be used to identify the setting of a Kondo regime.   

Kinks and antikinks of buckled graphene and their relation to the φ4 field model

Graphene kinks are topological excitations of long graphene membranes buckled in the transverse direction [1]. Their remarkable properties – the nanoscale size, stability, high propagation speed, and low energy dissipation – make them well positioned to be used in applications involving the nanoscale motion. Here we establish an analogy between the graphene kinks and classical φ⁴ model. Using molecular dynamics simulations, we investigated the dynamics of a buckled graphene nanoribbon with a single kink and with a kink-antikink pair. Several features of the φ⁴ model have been observed including the kink-antikink capture at low energies, kink-antikink reflection at high energies, and a bounce resonance. Our results pave the way towards the experimental observation of a rich variety of φ⁴ model predictions based on graphene.

[1] R. D. Yamaletdinov, V. A. Slipko and Y. V. Pershin, Phys. Rev. B 96, 094306 (2017)

  

Mechanical properties of a two-dimensional Covalent Organic Framework

Many atomically thin, two-dimensional crystals have emerged as materials with exceptional electronic, thermal, optical, and mechanical properties. These include members of the covalent organic framework (COF) family - porous materials ingeniously constructed from organic building units. While COFs show promise for gas storage applications, their mechanical properties beyond the elastic regime remain unknown. In this work, we combine first principles and reactive molecular dynamics simulations to characterize the mechanical properties of COF-1. We find that this material exhibits an anisotropic response to strain. In addition, for both pulling directions, we observe two regimes of response to external strain. By analyzing the trajectories, we find that softer and harder responses are related to reversible changes in the COF-1 microstructure as the strain increases. We also find that this process leads to a considerable increase in the material’s surface energy density. Finally, we report, for both armchair and zigzag orientations, the computed toughness, ultimate tensile strain, and ultimate tensile stress of 2D COF-1 membranes.