Strain engineering, efficient excitonic photoluminescence, and exciton funnelling in unmodified MoS2 nanosheets

Strongly Enhanced Electronic Bandstructure Renormalization by Light in Nanoscale Strained Regions of Monolayer MoS2/Au(111) Heterostructures

Understanding and controlling the photoexcited quasiparticle (QP) dynamics in monolayer (ML) transition metal dichalcogenides (TMDs) lays the foundation for exploring the strongly interacting, nonequilibrium two-dimensional (2D) QP and polaritonic states in these quantum materials and for harnessing the properties emerging from these states for optoelectronic applications. In this study, scanning tunneling microscopy/spectroscopy (STM/scanning tunneling spectroscopy) with light illumination at the tunneling junction is performed to investigate the QP dynamics in ML MoS2 on an Au(111) substrate with nanoscale corrugations. The corrugations on the surface of the substrate induce nanoscale local strain in the overlaying ML MoS2 single crystal, which result in energetically favorable spatial regions where photoexcited QPs, including excitons, trions, and electron-hole plasmas, accumulate. These strained regions exhibit pronounced electronic bandstructure renormalization as a function of the photoexcitation wavelength and intensity as well as the strain gradient, implying strong interplay among nanoscale structures, strain, and photoexcited QPs. In conjunction with the experimental work, we construct a theoretical framework that integrates nonuniform nanoscale strain into the electronic bandstructure of a ML MoS2 lattice using a tight-binding approach combined with first-principle calculations. This methodology enables better understanding of the experimental observation of photoexcited QP localization in the nanoscale strain-modulated electronic bandstructure landscape. Our findings illustrate the feasibility of utilizing nanoscale architectures and optical excitations to manipulate the local electronic bandstructure of ML TMDs and to enhance the many-body interactions of excitons, which is promising for the development of nanoscale energy-adjustable optoelectronic and photonic technologies, including quantum emitters and solid-state quantum simulators for interacting exciton polaritons based on engineered periodic nanoscale trapping potentials.

Visualizing and Controlling of Photogenerated Electron-Hole Pair Separation in Monolayer WS2 Nanobubbles under Piezoelectric Field

The piezoelectric properties of two-dimensional semiconductor nanobubbles present remarkable potential for application in flexible optoelectronic devices, and the piezoelectric field has emerged as an efficacious pathway for both the separation and migration of photogenerated electron-hole pairs, along with inhibition of recombination. However, the comprehension and control of photogenerated carrier dynamics within nanobubbles still remain inadequate. Hence, this study is dedicated to underscore the importance of in situ detection and detailed characterization of photogenerated electron-hole pairs in nanobubbles to enrich understanding and strategic manipulation in two-dimensional semiconductor materials. Utilizing frequency modulation kelvin probe force microscopy (FM-KPFM) and strain gradient distribution techniques, the existence of a piezoelectric field in monolayer WS2 nanobubbles was confirmed. Combining w/o and with illumination FM-KPFM, second-order capacitance gradient technique and in situ nanoscale tip-enhanced photoluminescence characterization techniques, the interrelationships among the piezoelectric effect, interlayer carrier transfer, and the funneling effect for photocarrier dynamics process across various nanobubble sizes were revealed. Notably, for a WS2/graphene bubble height of 15.45 nm, a 0 mV surface potential difference was recorded in the bubble region w/o and with illumination, indicating a mutual offset of piezoelectric effect, interlayer carrier transfer, and the funneling effect. This phenomenon is prevalent in transition metal dichalcogenides materials exhibiting inversion symmetry breaking. The implication of our study is profound for advancing the understanding of the dynamics of photogenerated electron-hole pair in nonuniform strain piezoelectric systems, and offers a reliable framework for the separation and modulation of photogenerated electron-hole pair in flexible optoelectronic devices and photocatalytic applications.

Preparation and Modeling of Graphene Bubbles to Obtain Strain-Induced Pseudomagnetic Fields

It has been both theoretically predicted and experimentally demonstrated that strain can effectively modulate the electronic states of graphene sheets through the creation of a pseudomagnetic field (PMF). Pressurizing graphene sheets into bubble-like structures has been considered a viable approach for the strain engineering of PMFs. However, the bubbling technique currently faces limitations such as long manufacturing time, low durability, and challenges in precise control over the size and shape of the pressurized bubble. Here, we propose a rapid bubbling method based on an oxygen plasma chemical reaction to achieve rapid induction of out-of-plane deflections and in-plane strains in graphene sheets. We introduce a numerical scheme capable of accurately resolving the strain field and resulting PMFs within the pressurized graphene bubbles, even in cases where the bubble shape deviates from perfect spherical symmetry. The results provide not only insights into the strain engineering of PMFs in graphene but also a platform that may facilitate the exploration of the strain-mediated electronic behaviors of a variety of other 2D materials.

Locally Strained 2D Materials: Preparation, Properties, and Applications

2D materials are promising for strain engineering due to their atomic thickness and exceptional mechanical properties. In particular, non-uniform and localized strain can be induced in 2D materials by generating out-of-plane deformations, resulting in novel phenomena and properties, as witnessed in recent years. Therefore, the locally strained 2D materials are of great value for both fundamental studies and practical applications. This review discusses techniques for introducing local strains to 2D materials, and their feasibility, advantages, and challenges. Then, the unique effects and properties that arise from local strain are explored. The representative applications based on locally strained 2D materials are illustrated, including memristor, single photon emitter, and photodetector. Finally, concluding remarks on the challenges and opportunities in the emerging field of locally strained 2D materials are provided.

Local Nanostrain Engineering of Monolayer MoS2 Using Atomic Force Microscopy-Based Thermomechanical Nanoindentation

Strain engineering in two-dimensional materials (2DMs) has important application potential for electronic and optoelectronic devices. However, achieving precise spatial control, adjustable sizing, and permanent strain with nanoscale resolution remains challenging. Herein, a thermomechanical nanoindentation method is introduced, inspired by skin edema caused by mosquito bites, which can induce localized nanostrain and bandgap modulation in monolayer molybdenum disulfide (MoS2) transferred onto a poly(methyl methacrylate) film utilizing a heated atomic force microscopy nanotip. Via adjustment of the machining parameters, the strains of MoS2 are manipulated, achieving an average strain of ≤2.6% on the ring-shaped expansion structure. The local bandgap of MoS2 is spatially modulated using three types of nanostructures. Among them, the nanopit has the largest range of bandgap regulation, with a substantial change of 56 meV. These findings demonstrate the capability of the proposed method to create controllable and reproducible nanostrains in 2DMs.

Large band gap quantum spin Hall insulators in plumbene monolayer decorated with amidogen, hydroxyl and thiol functional groups

Two-dimensional Quantum Spin Hall (QSH) insulators featuring edge states that are topologically protected against back-scattering are arising as a novel state of quantum matter. One of the major obstacles to finding QSH insulators operable at room temperature is the insufficiency of suitable materials demonstrating the QSH effect with a large bulk band gap. Plumbene, the latest group-IV graphene analogous material, shows a large SOC-induced band gap opening but the coupling between topological states at different momentum points makes it a topologically trivial insulator. Pristine plumbene can be chemically functionalized to transform it from a conventional insulator to a topologically non-trivial insulator with a considerable bulk band gap. In this work, three new QSH phases in plumbene have been theoretically predicted through functionalization with amidogen (-NH₂), hydroxyl (-OH) and thiol (-SH) groups. The derived electronic properties show non-trivial topological states in plumbene with very high bulk band gaps ranging from 1.0911 eV to as high as 1.1515 eV. External strain can be used to further enhance and tune these bulk gaps, as demonstrated in this work. We also propose a H-terminated SiC (0001) surface as a suitable substrate for the practical implementation of these monolayers to minimize lattice mismatch and maintain their topological order. The robustness of these QSH insulators against strain and substrate effects and the large bulk gaps provide a promising platform for potential applications of future low dissipation nanoelectronic devices and spintronic devices at room temperature.

Strain relaxation in monolayer MoS2 over flexible substrate

In this communication, we demonstrate uniaxial strain relaxation in monolayer (1L) MoS2 transpires through cracks in both single and double-grain flakes. Chemical vapour deposition (CVD) grown 1L MoS2 has been transferred onto polyethylene terephthalate (PET) and poly(dimethylsiloxane) (PDMS) substrates for low (∼1%) and high (1-6%) strain measurements. Both Raman and photoluminescence (PL) spectroscopy revealed strain relaxation via cracks in the strain regime of 4-6%. In situ optical micrographs show the formation of large micron-scale cracks along the strain axis and ex situ atomic force microscopy (AFM) images reveal the formation of smaller lateral cracks due to the strain relaxation. Finite element simulation has been employed to estimate the applied strain efficiency as well as to simulate the strain distribution for MoS2 flakes. The present study reveals the uniaxial strain relaxation mechanism in 1L MoS2 and paves the way for exploring strain relaxation in other transition metal dichalcogenides (TMDCs) as well as their heterostructures.

Plasmon-exciton couplings in the MoS2/AuNP plasmonic hybrid structure

The understanding and engineering of the plasmon-exciton coupling are necessary to control the innovative optoelectronic device platform. In this study, we investigated the intertwined mechanism of each plasmon-exciton couplings in monolayer molybdenum disulfide (MoS₂) and plasmonic hybrid structure. The results of absorption, simulation, electrostatics, and emission spectra show that interaction between photoexcited carrier and exciton modes are successfully coupled by energy transfer and exciton recombination processes. Especially, neutral exciton, trion, and biexciton can be selectively enhanced by designing the plasmonic hybrid platform. All of these results imply that there is another degree of freedom to control the individual enhancement of each exciton mode in the development of nano optoelectronic devices.

Strain-Induced Performance Enhancement of a Monolayer Photodetector via Patterned Substrate Engineering

Two-dimensional (2D) materials exhibit tremendous potential for applications in next-generation photodetectors. Currently, approaches aiming at enhancing the device's performance are limited, mainly relying on complex hybrid systems such as heterostructures and sensitization. Here, we propose a new strategy by constructing patterned nanostructures compatible with the conventional silicon substrate. Using CVD-grown monolayer MoS₂ on the periodical nanocone arrays, we demonstrate a high-performance MoS₂ photodetector via manipulating strain distribution engineered by the substrate at the nanoscale. Compared to the pristine MoS₂ counterpart, the strained MoS₂ photodetector exhibits a much enhanced performance, including a high signal-to-noise ratio over 10⁵ and large responsivity of 3.2 × 10⁴ A W^-1. The physical mechanism responsible for the enhancement is discussed by combining Kelvin probe force microscopy with theoretical simulation. The enhanced performances can be attributed to the improved light absorption, the fast separation of photo-excited carriers, and the suppression of dark currents induced by the designed periodical nanocone arrays. This work depicts an alternative method to achieve high-performance optoelectronic devices based on 2D materials integrated with semiconductor circuits.

Holey Substrate-Directed Strain Patterning in Bilayer MoS2

Key properties of two-dimensional (2D) layered materials are highly strain tunable, arising from bond modulation and associated reconfiguration of the energy bands around the Fermi level. Approaches to locally controlling and patterning strain have included both active and passive elastic deformation via sustained loading and templating with nanostructures. Here, by float-capturing ultrathin flakes of single-crystal 2H-MoS₂ on amorphous holey silicon nitride substrates, we find that highly symmetric, high-fidelity strain patterns are formed. The hexagonally arranged holes and surface topography combine to generate highly conformal flake-substrate coverage creating patterns that match optimal centroidal Voronoi tessellation in 2D Euclidean space. Using TEM imaging and diffraction, as well as AFM topographic mapping, we determine that the substrate-driven 3D geometry of the flakes over the holes consists of symmetric, out-of-plane bowl-like deformation of up to 35 nm, with in-plane, isotropic tensile strains of up to 1.8% (measured with both selected-area diffraction and AFM). Atomistic and image simulations accurately predict spontaneous formation of the strain patterns, with van der Waals forces and substrate topography as the input parameters. These results show that predictable patterns and 3D topography can be spontaneously induced in 2D materials captured on bare, holey substrates. The method also enables electron scattering studies of precisely aligned, substrate-free strained regions in transmission mode.

Strain Engineering of Low-Dimensional Materials for Emerging Quantum Phenomena and Functionalities

Recent discoveries of exotic physical phenomena, such as unconventional superconductivity in magic-angle twisted bilayer graphene, dissipationless Dirac fermions in topological insulators, and quantum spin liquids, have triggered tremendous interest in quantum materials. The macroscopic revelation of quantum mechanical effects in quantum materials is associated with strong electron-electron correlations in the lattice, particularly where materials have reduced dimensionality. Owing to the strong correlations and confined geometry, altering atomic spacing and crystal symmetry via strain has emerged as an effective and versatile pathway for perturbing the subtle equilibrium of quantum states. This review highlights recent advances in strain-tunable quantum phenomena and functionalities, with particular focus on low-dimensional quantum materials. Experimental strategies for strain engineering are first discussed in terms of heterogeneity and elastic reconfigurability of strain distribution. The nontrivial quantum properties of several strain-quantum coupled platforms, including 2D van der Waals materials and heterostructures, topological insulators, superconducting oxides, and metal halide perovskites, are next outlined, with current challenges and future opportunities in quantum straintronics followed. Overall, strain engineering of quantum phenomena and functionalities is a rich field for fundamental research of many-body interactions and holds substantial promise for next-generation electronics capable of ultrafast, dissipationless, and secure information processing and communications.

Strain regulated interlayer coupling in WSe2/WS2heterobilayer

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/WS₂heterostructures 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 WSe₂and WS₂as well as the WSe₂/WS₂heterostructures. The tensile strain can tune the electronic band structure of WSe₂/WS₂heterostructure, as well as enhance the interlayer coupling. It is further revealed that the photoluminescence intensity ratio of WS₂to WSe₂in our WSe₂/WS₂heterobilayer increases monotonically with tensile strain. These findings can broaden the understanding and practical application of strain engineering in 2D materials with nanometer-scale resolution.

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.

Tuning the physical properties of ultrathin transition-metal dichalcogenides via strain engineering

Transition-metal dichalcogenides (TMDs) have become one of the recent frontiers and focuses in two-dimensional (2D) materials fields thanks to their superior electronic, optical, and photoelectric properties. Triggered by the growing demand for developing nano-electronic devices, strain engineering of ultrathin TMDs has become a hot topic in the scientific community. In recent years, both theoretical and experimental research on the strain engineering of ultrathin TMDs have suggested new opportunities to achieve high-performance ultrathin TMDs based devices. However, recent reviews mainly focus on the experimental progress and the related theoretical research has long been ignored. In this review, we first outline the currently employed approaches for introducing strain in ultrathin TMDs, both their characteristics and advantages are explained in detail. Subsequently, the recent research progress in the modification of lattice and electronic structure, and physical properties of ultrathin TMDs under strain are systematically reviewed from both experimental and theoretical perspectives. Despite much work being done in this filed, reducing the distance of experimental progress from the theoretical prediction remains a great challenge in realizing wide applications of ultrathin TMDs in nano-electronic devices.

Dynamic Exciton Funneling by Local Strain Control in a Monolayer Semiconductor

The ability to control excitons in semiconductors underlies numerous proposed applications, from excitonic circuits to energy transport. Two dimensional (2D) semiconductors are particularly promising for room-temperature applications due to their large exciton binding energy and enormous stretchability. Although the strain-induced static exciton flux has been observed in predetermined structures, dynamic control of exciton flux represents an outstanding challenge. Here, we introduce a method to tune the bandgap of suspended 2D semiconductors by applying a local strain gradient with a nanoscale tip. This strain allows us to locally and reversibly shift the exciton energy and to steer the exciton flux over micrometer-scale distances. We anticipate that our result not only marks an important experimental tool but will also open a broad range of new applications from information processing to energy conversion.

Breaking the absorption limit of Si toward SWIR wavelength range via strain engineering

Silicon has been widely used in the microelectronics industry. However, its photonic applications are restricted to visible and partial near-infrared spectral range owing to its fundamental optical bandgap (1.12 eV). With recent advances in strain engineering, material properties, including optical bandgap, can be tailored considerably. This paper reports the strain-induced shrinkage in the Si bandgap, providing photosensing well beyond its fundamental absorption limit in Si nanomembrane (NM) photodetectors (PDs). The Si-NM PD pixels were mechanically stretched (biaxially) by a maximum strain of ~3.5% through pneumatic pressure-induced bulging, enhancing photoresponsivity and extending the Si absorption limit up to 1550 nm, which is the essential wavelength range of the lidar sensors for obstacle detection in self-driving vehicles. The development of deformable three-dimensional optoelectronics via gas pressure-induced bulging also facilitated the realization of unique device designs with concave and convex hemispherical architectures, which mimics the electronic prototypes of biological eyes.

Heterogeneous deformation of two-dimensional materials for emerging functionalities

Atomically thin 2D materials exhibit strong intralayer covalent bonding and weak interlayer van der Waals interactions, offering unique high in-plane strength and out-of-plane flexibility. While atom-thick nature of 2D materials may cause uncontrolled intrinsic/extrinsic deformation in multiple length scales, it also provides new opportunities for exploring coupling between heterogeneous deformations and emerging functionalities in controllable and scalable ways for electronic, optical, and optoelectronic applications. In this review, we discuss (i) the mechanical characteristics of 2D materials, (ii) uncontrolled inherent deformation and extrinsic heterogeneity present in 2D materials, (iii) experimental strategies for controlled heterogeneous deformation of 2D materials, (iv) 3D structure-induced novel functionalities via crumple/wrinkle structure or kirigami structures, and (v) heterogeneous strain-induced emerging functionalities in exciton and phase engineering. Overall, heterogeneous deformation offers unique advantages for 2D materials research by enabling spatial tunability of 2D materials' interactions with photons, electrons, and molecules in a programmable and controlled manner.

Large-Area and Strain-Reduced Two-Dimensional Molybdenum Disulfide Monolayer Emitters on a Three-Dimensional Substrate

Atomically thin membranes of two-dimensional (2-D) transition-metal dichalcogenides (TMDCs) have distinct emission properties, which can be utilized for realizing ultrathin optoelectronic integrated systems in the future. Growing a large-area and strain-reduced monolayer 2-D material on a three-dimensional (3-D) substrate with microstructures or nanostructures is a crucial technique because the electronic band structure of TMDC atomic layers is strongly affected by the number of stacked layers and strain. In this study, a large-area and strain-reduced MoS₂ monolayer was fabricated on a 3-D substrate through a two-step growth procedure. The material characteristics and optical properties of monolayer TMDCs fabricated on the nonplanar substrate were examined. The growth of monolayer MoS₂ on a cone-shaped sapphire substrate effectively reduced the tensile strain induced by the substrate by decreasing the thermal expansion mismatch between the 2-D material and the substrate. Monolayer MoS₂ grown on the nonplanar substrate exhibited uniform strain reduction and luminescence intensity. The fabrication of monolayer MoS₂ on a nonplanar substrate increased the light extraction efficiency. In the future, large-area and strain-reduced 2-D TMDC materials grown on a nonplanar substrate can be employed as novel light-emitting devices for applications in lighting, communication, and displays for the development of ultrathin optoelectronic integrated systems.

Centimeter-Scale Periodically Corrugated Few-Layer 2D MoS2 with Tensile Stretch-Driven Tunable Multifunctionalities

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers exhibit superior optical, electrical, and structural properties unattainable in any traditional materials. Many of these properties are known to be controllable via external mechanical inputs, benefiting from their extremely small thickness coupled with large in-plane strain limits. However, realization of such mechanically driven tunability often demands highly complicated engineering of 2D TMD layer structures, which is difficult to achieve on a large wafer scale in a controlled manner. Herein, we explore centimeter-scale periodically corrugated 2D TMDs, particularly 2D molybdenum disulfide (MoS₂), and report their mechanically tunable multifunctionalities. We developed a water-assisted process to homogeneously integrate few layers of 2D MoS₂ on three-dimensionally corrugated elastomeric substrates on a large area (>2 cm²). The evolution of electrical, optical, and structural properties in these three-dimensionally corrugated 2D MoS₂ layers was systematically studied under controlled tensile stretch. We identified that they present excellent electrical conductivity and photoresponsiveness as well as systematically tunable surface wettability and optical absorbance even under significant mechanical deformation. These novel three-dimensionally structured 2D materials are believed to offer exciting opportunities for large-scale, mechanically deformable devices of various form factors and unprecedented multifunctionalities.

Electronic-dimensionality reduction of bulk MoS2 by hydrogen treatment

A reduction in the electronic-dimensionality of materials is one method for achieving improvements in material properties. Here, a reduction in electronic-dimensionality is demonstrated using a simple hydrogen treatment technique. Quantum well states from hydrogen-treated bulk 2H-MoS2 are observed using angle resolved photoemission spectroscopy (ARPES). The electronic states are confined within a few MoS2 layers after the hydrogen treatment. A significant reduction in the band-gap can also be achieved after the hydrogen treatment, and both phenomena can be explained by the formation of sulfur vacancies generated by the chemical reaction between sulfur and hydrogen.