Strain Engineering Of 2D Materials Research Articles

Laser-assisted synthesis and modification of 2D materials.

Two-dimensional materials with unique physical, electronic, and optical properties have been intensively studied to be utilized for the next-generation electronic and optical devices, and the use of laser energy in the synthesis and modification of 2D materials is advantageous due to its convenient and fast fabrication processes as well as selective, controllable, and cost-effective characteristics allowing the precise control in materials properties. This paper summarizes the recent progress in utilizations of laser technology in synthesizing, doping, etching, transfer and strain engineering of 2D materials, which is expected to provide an insight for the future applications across diverse research areas.

Towards efficient strain engineering of 2D materials: A four-points bending approach for compressive strain

Strain engineering, as a powerful strategy to tune the optical and electrical properties of two-dimensional (2D) materials by deforming their crystal lattice, has attracted significant interest in recent years. 2D materials can sustain ultra-high strains, even up to 10%, due to the lack of dangling bonds on their surface, making them ideal brittle solids. This remarkable mechanical resilience, together with a strong strain-tunable band structure, endows 2D materials with a broad optical and electrical response upon strain. However, strain engineering based on 2D materials is restricted by their nanoscale and strain quantification troubles. In this study, we have modified a homebuilt three-points bending apparatus to transform it into a four-points bending apparatus that allows for the application of both compressive and tensile strains on 2D materials. This approach allows for the efficient and reproducible construction of a strain system and minimizes the buckling effect caused by the van der Waals interaction by adamantane encapsulation strategy. Our results demonstrate the feasibility of introducing compressive strain on 2D materials and the potential for tuning their optical and physical properties through this approach.

Wetting and strain engineering of 2D materials on nanopatterned substrates.

The fascinating realm of strain engineering and wetting transitions in two-dimensional (2D) materials takes place when placed on a two-dimensional array of nanopillars or one-dimensional rectangular grated substrates. Our investigation encompasses a diverse set of atomically thin 2D materials, including transition metal dichalcogenides, hexagonal boron nitride, and graphene, with a keen focus on the impact of van der Waals adhesion energies to the substrate on the wetting/dewetting behavior on nanopatterned substrates. We find a critical aspect ratio of the nanopillar or grating heights to the period of the pattern when the wetting/dewetting transition occurs. Furthermore, energy hysteresis analysis reveals dynamic detachment and re-engagement events during height adjustments, shedding light on energy barriers of 2D monolayer transferred on patterned substrates. Our findings offer avenues for strain engineering in 2D materials, leading to promising prospects for future technological applications.

Strain engineering of two-dimensional materials for energy storage and conversion applications

Two-dimensional (2D) materials have garnered much interest due to their exceptional optical, electrical, and mechanical properties. Strain engineering, as a crucial approach to modulate the physicochemical characteristics of 2D materials, has been widely used in various fields, especially for energy storage and conversion. Herein, the recent progress in strain engineering of 2D materials is summarized for energy storage and conversion applications. The fundamental understanding of strain in 2D materials is first described. Then, some synthetic methods for modulating the properties of 2D materials via strain engineering are introduced. Further, the applications of strain engineering of 2D materials in energy storage, photocatalysis, and electrocatalysis are discussed. Finally, the challenges and perspectives on strain engineering of 2D materials are also outlined.

Directional dependence of band gap modulation via uniaxial strain in MoS2 and TiS3

Strain is widely employed to modulate the band structures of two-dimensional (2D) van der Waals (vdW) materials. Such band engineering with strain applied along different crystallographic directions, however, is less explored. Here, we investigate the band gap modulation of layered chalcogenides, MoS2 and TiS3, and the dependence of their band gaps on the directions of applied strain, using first-principles calculations. The band gap transition in MoS2 is found to reduce in energy linearly as a function of increasing tensile strain, with a weakly directional-dependent gradient, varying by 4.6 meV/% (from −52.7 ± 0.6 to −57.3 ± 0.1 meV/%) from the zigzag to armchair directions. Conversely, the band gap in TiS3 decreases with strain applied along the a lattice vector, but increases with strain applied in the perpendicular direction, with a non-linear strain-band gap relationship found between these limits. Analysis of the structure of the materials and character of the band edge states under strain helps explain the origins of the stark differences between MoS2 and TiS3. Our results provide new insights for strain engineering in 2D materials and the use of the direction of applied strain as another degree of freedom.

Improved strain engineering of 2D materials by adamantane plasma polymer encapsulation

Two-dimensional materials present exceptional crystal elasticity and provide an ideal platform to tune electrical and optical properties through the application of strain. Here we extend recent research on strain engineering in monolayer molybdenum disulfide using an adamantane plasma polymer pinning layer to achieve unprecedented crystal strains of 2.8%. Using micro-reflectance spectroscopy, we report maximum strain gauge factors of −99.5 meV/% and −63.5 meV/% for the A and B exciton of monolayer MoS2, respectively, with a 50 nm adamantane capping layer. These results are corroborated with photoluminescence and Raman measurements on the same samples. Taken together, our results indicate that adamantane polymer is an exceptional capping layer to transfer substrate-induced strain to a 2D layer and achieve higher levels of crystal strain.

Recent Progress in Strain Engineering on Van der Waals 2D Materials: Tunable Electrical, Electrochemical, Magnetic, and Optical Properties.

Strain engineering is a promising way to tune the electrical, electrochemical, magnetic, and optical properties of 2Dmaterials, with the potential to achieve high-performance 2D-material-based devices ultimately. This review discusses the experimental and theoretical results from recent advances in the strain engineering of 2D materials. Some novel methods to induce strain are summarized and then the tunable electrical and optical/optoelectronic properties of 2D materials via strain engineering are highlighted, including particularly the previously less-discussed strain tuning of superconducting, magnetic, and electrochemical properties. Also, future perspectives of strain engineering are given for its potential applications in functional devices. The state of the survey presents the ever-increasing advantages and popularity of strain engineering for tuning properties of 2D materials. Suggestions and insights for further research and applications in optical, electronic, and spintronic devices are provided.

Interfacial friction of vdW heterostructures affected by in-plane strain

Interfacial properties of van der Waals (vdW) heterostructures dominate the durability and function of their booming practical and potential applications such as opoelectronic devices, superconductors and even pandemics research. However, the strain engineering modulates of interlayer friction of vdW heterostructures consisting of two distinct materials are still unclear, which hinders the applications of vdW heterostructures, as well as the design of solid lubricant and robust superlubricity. In the present paper, a molecular model between a hexagonal graphene flake and a rectangular SLMoS2 sheet is established, and the influence of biaxial and uniaxial strain on interlayer friction is explored by molecular dynamics. It is found that the interlayer friction is insensitive to applied strains. Strong robustness of superlubricity between distinct layers is owed to the structure’s intrinsic incommensurate characteristics and the existence of Moiré pattern. In engineering practice, it is of potential importance to introduce two distinct 2D materials at the sliding contact interface to reduce the interfacial friction of the contact pair and serve as ideal solid lubricants. Our research provides a further basis to explore the nanotribology and strain engineering of 2D materials and vdW heterostructures.

A Versatile Approach to Create Nanobubbles on Arbitrary Two‐Dimensional Materials for Imaging Exciton Localization

AbstractStrained 2D materials with lattice deformation have the optimal band structure, lattice vibration, and thermal conductivity and various methods have been proposed to introduce strain into 2D materials. However, the creation of localized strain in arbitrary 2D materials in predesigned areas is difficult and challenging. Herein, a versatile approach to creating on‐demand nanobubbles on five different 2D materials using a functional atomic force microscopy (AFM) tip is described. Strain‐induced redshifts are observed from the Raman scattering and photoluminescence (PL) spectra of the 2D materials in the region with the nanobubble arrays. In addition, the localized exciton state is observed from the periphery of a steep WS2 nanobubble by high‐resolution nano‐photoluminescence and is supported by theoretical simulation. These results demonstrate a programmable and reliable method to create localized strain in different 2D materials and pave the way for nanoscale strain engineering of 2D materials to cater to different applications.

Straining of atomically thin WSe2 crystals: Suppressing slippage by thermal annealing

The atomically thin two-dimensional (2D) transition-metal dichalcogenide (e.g., MoS2) material can withstand large strains up to 11% to change its energy band structure, thereby further tuning its optical, electrical, and other physical properties. However, the slippage of 2D materials on substrate hammers the further strain tuning of the properties of 2D materials. Hereby, a facile three points approach combined with a dry transfer method that can apply uniaxial strain to two-dimensional materials is provided. The slippage of WSe2 on polycarbonate (PC) substrate can be suppressed by thermally annealing WSe2/PC in low pressure Ar atmosphere above 100 °C for 3 h. Straining cycle evolution experiments revealed that the thermal annealing of (1L) WSe2 could suppress slippage from the surface of the PC. The spectral gauge factor of 1L WSe2 is found to be around -60 meV/%. After thermal treatment, WSe2/PC stacking can survive in DI water for at least 24 h without the degradation of the spectral gauge factor. Dome structures are formed after thermal treatments with the interplay of the viscoelasticity and surface tension of the PC and the 0.4% tensile strain on WSe2, and the RMS roughness of WSe2/PC increased from 820 to 1292 pm, indicating that there could be larger lateral friction force to suppress slippage following thermal annealing. Our findings enrich the strain engineering of 2D materials and their device applications.

Large-Scale Ultrafast Strain Engineering of CVD-Grown Two-Dimensional Materials on Strain Self-Limited Deformable Nanostructures toward Enhanced Field-Effect Transistors.

Strain engineering of 2D materials is capable of tuning the electrical and optical properties of the materials without introducing additional atoms. Here, a method for large-scale ultrafast strain engineering of CVD-grown 2D materials is proposed. Locally nonuniform strains are introduced through the cooperative deformation of materials and metal@metal oxide nanoparticles through cold laser shock. The tensile strain of MoS2 changes and the band gap decreases after laser shock. The mechanism of the ultrafast straining is investigated by MD simulations. MoS2 FETs were fabricated, and the field-effect mobility of devices could be increased from 1.9 to 44.5 cm2 V-1 s-1 by adjusting the strain level of MoS2. This is currently the maximum value of MoS2 FETs grown by CVD with SiO2 as the dielectric. As a large-scale and ultrafast manufacturing method, laser shock provides a universal strategy for large-scale adjustment of 2D material strain, which will help to promote the manufacturing of 2D nanoelectronic devices.

Non‐Uniform Strain Engineering of 2D Materials

Abstract2D materials are elastic substances that can sustain high strain. While the response of these materials to spatially uniform strain is well studied, the effects of spatially non‐uniform strain are understood much less. In this review, we examine the response of two different 2D materials, transition metal dichalcogenides and graphene, under non‐uniform strain. First, we analyze pseudo‐magnetic fields formed in graphene subjected to highly localized non‐uniform strain. Second, we discuss the effect of non‐uniform strain on excitons in non‐uniformly strained TMDC. We show that while transport or “funneling” of excitons is relatively inefficient, a different process, a strain‐related conversion of excitons to trions is dominant. Finally, we discuss the effects of uniform and non‐uniform strain in a graphene‐based phononic crystal. We find that uniform strain can be used to broadly tune the frequency of the phononic bandgap by more than 350 % and non‐uniform strain smears that bandgap.

Deep Elastic Strain Engineering of 2D Materials and Their Twisted Bilayers.

Conventionally, tuning materials' properties can be done through strategies such as alloying, doping, defect engineering, and phase engineering, while in fact mechanical straining can be another effective approach. In particular, elastic strain engineering (ESE), unlike conventional strain engineering mainly based on epitaxial growth, allows for continuous and reversible modulation of material properties by mechanical loading/unloading. The exceptional intrinsic mechanical properties (including elasticity and strength) of two-dimensional (2D) materials make them naturally attractive candidates for potential ESE applications. Here, we demonstrated that using the strain effect to modulate the physical and chemical properties toward novel functional device applications, which could be a general strategy for various 2D materials and their heterostructures. We then show how ultralarge, uniform elastic strain in free-standing 2D monolayers can permit deep elastic strain engineering (DESE), which can result in fundamentally changed electronic and optoelectronic properties for unconventional device applications. In addition to monolayers and van der Waals (vdW) heterostructures, we propose that DESE can be also applied to twisted bilayer graphene and other emerging twisted vdW structures, allowing for unprecedented functional 2D material applications.

Strain regulated interlayer coupling in WSe2/WS2 heterobilayer

Strain engineering can effectively modify the materials lattice parameters at atomic scale, hence it has become an efficient method for tuning the physical properties of two-dimensional (2D) materials. The study of the strain regulated interlayer coupling is deserved for different kinds of heterostructures. Here, we systematically studied the strain engineering of WSe2/WS2 heterostructures as well as their constituent monolayers. The measured Raman and photoluminescence spectra demonstrate that the strain can evidently modulate the phonon energy and exciton emission of monolayer WSe2 and WS2 as well as the WSe2/WS2 heterostructures. The tensile strain can tune the electronic band structure of WSe2/WS2 heterostructure, as well as enhance the interlayer coupling. It is further revealed that the photoluminescence intensity ratio of WS2 to WSe2 in our WSe2/WS2 heterobilayer increases monotonically with tensile strain. These findings can broaden the understanding and practical application of strain engineering in 2D materials with nanometer-scale resolution.

(Invited) Experimental Quantum Transport in Strained Graphene and SWCNTs - Towards Noems

We measure ballistic charge conductivity in strained suspended graphene and observe the theoretically predicted [1] strain-induced scalar and vector potentials. To do so, we built an experimental platform for in-situ quantum transport strain engineering in 2D materials. This instrumentation also permits low temperature (0.3 K) transport in 0 to 9 Tesla magnetic fields. The tunable uniaxial strain (up to 3%) is completely decoupled from the gate-tunable charge density, permitting quantitative understanding of strain effects. We show slippage-free mechanical clamping of high aspect-ratio graphene crystals, where atomically ordered edges are unnecessary for quantitative straintronics. We study in detail transport in a ballistic graphene channel whose length is 90 nm and width is 600 nm. By applying strain up to 0.6 % in this device, we observe that the strain-induced scalar potential shifts its low energy band structure downward by up to 30 meV. We show precise control of the gauge vector potential which reversibly suppresses the conductance. We discuss our progress towards strain engineering in SWCNTs, and calculations showing its potential in valley filtering and tuning electron-hole asymmetric quantum transistors.We conclude with an overview of another project to integrate suspended bilayer graphene devices in planar optical cavities towards achieving fully tunable nano-opto-electro-mechanical (NOEMS) carbon systems. To do so, we discuss a nitrocellulose-based dry-stamping method to manipulate and suspend individual 2D materials [2]. Using this method we assembled optical cavities able to a widely tune Raman scattering and absorption in graphene. [1] A. C. McRae, G. Wei, and A. R. Champagne, Phys. Rev. Applied 11, 054019 (2019). [2] I. G. Rebollo, F. C. Rodrigues-Machado, W. Wright, G. J. Melin, A. R. Champagne, ArXiv:2011.14166 Figure 1

Thermomechanical Nanostraining of Two-Dimensional Materials.

Local bandgap tuning in two-dimensional (2D) materials is of significant importance for electronic and optoelectronic devices but achieving controllable and reproducible strain engineering at the nanoscale remains a challenge. Here, we report on thermomechanical nanoindentation with a scanning probe to create strain nanopatterns in 2D transition metal dichalcogenides and graphene, enabling arbitrary patterns with a modulated bandgap at a spatial resolution down to 20 nm. The 2D material is in contact via van der Waals interactions with a thin polymer layer underneath that deforms due to the heat and indentation force from the heated probe. Specifically, we demonstrate that the local bandgap of molybdenum disulfide (MoS2) is spatially modulated up to 10% and is tunable up to 180 meV in magnitude at a linear rate of about −70 meV per percent of strain. The technique provides a versatile tool for investigating the localized strain engineering of 2D materials with nanometer-scale resolution.

Strain engineering of two-dimensional multilayered heterostructures for beyond-lithium-based rechargeable batteries

Beyond-lithium-ion batteries are promising candidates for high-energy-density, low-cost and large-scale energy storage applications. However, the main challenge lies in the development of suitable electrode materials. Here, we demonstrate a new type of zero-strain cathode for reversible intercalation of beyond-Li+ ions (Na+, K+, Zn2+, Al3+) through interface strain engineering of a 2D multilayered VOPO4-graphene heterostructure. In-situ characterization and theoretical calculations reveal a reversible intercalation mechanism of cations in the 2D multilayered heterostructure with a negligible volume change. When applied as cathodes in K+-ion batteries, we achieve a high specific capacity of 160 mA h g−1 and a large energy density of ~570 W h kg−1, presenting the best reported performance to date. Moreover, the as-prepared 2D multilayered heterostructure can also be extended as cathodes for high-performance Na+, Zn2+, and Al3+-ion batteries. This work heralds a promising strategy to utilize strain engineering of 2D materials for advanced energy storage applications.

Microheater Actuators as a Versatile Platform for Strain Engineering in 2D Materials.

We present microfabricated thermal actuators to engineer the biaxial strain in two-dimensional (2D) materials. These actuators are based on microheater circuits patterned onto the surface of a polymer with a high thermal expansion coefficient. By running current through the microheater one can vary the temperature of the polymer and induce a controlled biaxial expansion of its surface. This controlled biaxial expansion can be transduced to biaxial strain to 2D materials, placed onto the polymer surface, which in turn induces a shift of the optical spectrum. Our thermal strain actuators can reach a maximum biaxial strain of 0.64%, and they can be modulated at frequencies up to 8 Hz. The compact geometry of these actuators results in a negligible spatial drift of 0.03 μm/°C, which facilitates their integration in optical spectroscopy measurements. We illustrate the potential of this strain engineering platform to fabricate a strain-actuated optical modulator with single-layer MoS2.

(Invited) Experimental Quantum Transport in Strain Engineered Graphene - Towards NOEMS

We measure ballistic charge conductivity in strained suspended graphene and observe the theoretically predicted [1] strain-induced scalar and vector potentials. To do so, we built an experimental platform for quantum transport strain engineering in 2D materials. This instrumentation permits low temperature (0.3 K- 70 K) transport in 0 to 9 Tesla magnetic fields. The tunable uniaxial strain (up to 3%) is completely decoupled from the gate-tunable charge density, permitting quantitative understanding of strain effects. We show slippage-free mechanical clamping of high aspect-ratio graphene crystals, where atomically ordered edges are unnecessary for quantitative straintronics. We study in detail transport in a ballistic graphene channel whose length is 90 nm and width is 600nm. By applying strain up to 0.6 % in this device, we observe that the strain-induced scalar potential shifts its low energy band structure downward by up to 30 meV. We show precise control of the gauge vector potential which reversibly suppresses the conductance by up to 14 %.We conclude with a brief overview of other projects aimed at integrating both RF dynamic strain control and optically resonant cavities in suspended bilayer graphene devices to achieve fully tunable nano-opto-electro-mechanical (NOEMS) carbon systems. We report for instance a nitrocellulose-based dry-stamping method to manipulate and suspend individual 2D materials. Using this method we assembled optical cavities able to a widely tune light absorption in graphene. [1] A. C. McRae, G. Wei, and A. R. Champagne, Phys. Rev. Applied 11, 054019 (2019) Figure 1

Radial buckle delamination around 2D material tents

Microscale tents, formed when transferring two-dimensional (2D) materials over nanoparticles or nanopillars, or when indenting a suspended 2D material drumhead with an atomic force microscope (AFM) tip, emerge to be a useful structure for the strain engineering of 2D materials. In the periphery of the tents, where the 2D materials are supported by the substrate, radial buckle delamination can often be observed, yet the formation mechanism and the profile characteristics remain unclear. Here, we suggest that the tent-induced buckles result from the 2D material-substrate interface sliding radially inward, and their profiles and extent are controlled by the interface adhesion and friction. We experimentally characterized that the crest curvature of the buckles is proportional to a characteristic length that compares the elastic bending energy of the 2D material with its adhesion energy to the substrate. We then obtain theoretical predictions for the extent of those buckles by exact closed-form solutions to Föppl–von Kármán (FvK) equations under both near-threshold and far-from-threshold conditions. Our results are highly analytical, provide a direct means to estimate the interfacial shear and adhesive properties of the 2D material-substrate system based on simple topological characterizations of buckles. Our theoretical understandings also establish a fundamental base for the rational design of 2D material tents.