Raman Spectral Band Oscillations in Large Graphene Bubbles

High-quality-factor viscoelastic nanomechanical resonators from moiré superlattices

The moiré superlattice, created by stacking van der Waals layered materials with rotational misalignments, exhibits a multitude of emergent correlated phenomena ranging from superconductivity to Mott insulating states. In addition to exotic electronic states, the intricate networks of incommensurate lattices may give rise to polymer-like viscoelasticity, which combines the properties of both elastic solids and viscous fluids. This phenomenon may enrich the dynamics of nanomechanical resonators, in which viscoelasticity has not played a role thus far. Here, we report on a controllable hysteretic response of the nanomechanical vibrations in twisted bilayer graphene membranes, which we attribute to viscoelasticity. Accompanying this hysteretic response, we measure unusually large mechanical quality factors Q reaching a remarkably high value of  ~1900 at room temperature. We interpret the enhancement of Q as a signature of dissipation dilution, a phenomenon of considerable interest that has recently been harnessed in quantum optomechanical systems. Viscoelasticity features a "lossless" potential that overcomes the corrugation registry and reinforces such a dissipation dilution. Our work introduces the moiré superlattice as a promising system for viscoelasticity engineering through rotating angles and for observing emergent nanoelectromechanical couplings.

Local Strain Engineering of Two-Dimensional Transition Metal Dichalcogenides Towards Quantum Emitters

Two-dimensional transition metal dichalcogenides (2D TMDCs) have received considerable attention in local strain engineering due to their extraordinary mechanical flexibility, electonic structure, and optical properties. The strain-induced out-of-plane deformations in 2D TMDCs lead to diverse excitonic behaviors and versatile modulations in optical properties, paving the way for the development of advanced quantum technologies, flexible optoelectronic materials, and straintronic devices. Research on local strain engineering on 2D TMDCs has been delved into fabrication techniques, electronic state variations, and quantum optical applications. This review begins by summarizing the state-of-the-art methods for introducing local strain into 2D TMDCs, followed by an exploration of the impact of local strain engineering on optical properties. The intriguing phenomena resulting from local strain, such as exciton funnelling and anti-funnelling, are also discussed. We then shift the focus to the application of locally strained 2D TMDCs as quantum emitters, with various strategies outlined for modulating the properties of TMDC-based quantum emitters. Finally, we discuss the remaining questions in this field and provide an outlook on the future of local strain engineering on 2D TMDCs.

Biaxial Strain Transfer in Monolayer MoS2 and WSe2 Transistor Structures

Monolayer transition metal dichalcogenides are intensely explored as active materials in 2D material-based devices due to their potential to overcome device size limitations, sub-nanometric thickness, and robust mechanical properties. Considering their large band gap sensitivity to mechanical strain, single-layered TMDs are well-suited for strain-engineered devices. While the impact of various types of mechanical strain on the properties of a variety of TMDs has been studied in the past, TMD-based devices have rarely been studied under mechanical deformations, with uniaxial strain being the most common one. Biaxial strain on the other hand, which is an important mode of deformation, remains scarcely studied as far as 2D material devices are concerned. Here, we study the strain transfer efficiency in MoS2- and WSe2-based flexible transistor structures under biaxial deformation. Utilizing Raman spectroscopy, we identify that strains as high as 0.55% can be efficiently and homogeneously transferred from the substrate to the material in the transistor channel. In particular, for the WSe2 transistors, we capture the strain dependence of the higher-order Raman modes and show that they are up to five times more sensitive compared to the first-order ones. Our work demonstrates Raman spectroscopy as a nondestructive probe for strain detection in 2D material-based flexible electronics and deepens our understanding of the strain transfer effects on 2D TMD devices.

Manipulation of Moiré Superlattice in Twisted Monolayer-multilayer Graphene by Moving Nanobubbles

In the heterostructure of two-dimensional (2D) materials, many novel physics phenomena are strongly dependent on the Moiré superlattice. How to achieve the continuous manipulation of the Moiré superlattice in the same sample is very important to study the evolution of various physical properties. Here, in minimally twisted monolayer-multilayer graphene, we found that bubble-induced strain has a huge impact on the Moiré superlattice. By employing the AFM tip to dynamically and continuously move the nanobubble, we realized the modulation of the Moiré superlattice, like the evolution of regular triangular domains into long strip domain structures with single or double domain walls. We also achieved controllable modulation of the Moiré superlattice by moving multiple nanobubbles and establishing the coupling of nanobubbles. Our work presents a flexible method for continuous and controllable manipulation of Moiré superlattices, which will be widely used to study novel physical properties in 2D heterostructures.

Optically Triggered Emergent Mesostructures in Monolayer WS2

The ultrahigh surface area of two-dimensional materials can drive multimodal coupling between optical, electrical, and mechanical properties that leads to emergent dynamical responses not possible in three-dimensional systems. We observed that optical excitation of the WS2 monolayer above the exciton energy creates symmetrically patterned mechanical protrusions which can be controlled by laser intensity and wavelength. This observed photostrictive behavior is attributed to lattice expansion due to the formation of polarons, which are charge carriers dressed by lattice vibrations. Scanning Kelvin probe force microscopy measurements and density functional theory calculations reveal unconventional charge transport properties such as the spatially and optical intensity-dependent conversion in the WS2 monolayer from apparent n- to p-type and the subsequent formation of effective p-n junctions at the boundaries between regions with different defect densities. The strong opto-electrical-mechanical coupling in the WS2 monolayer reveals previously unexplored properties, which can lead to new applications in optically driven ultrathin microactuators.

Recent Progress in Two-Dimensional Material Exfoliation Technology and Enlightenment for Geological Sciences

Mechanical exfoliation technology is vital for the development of two-dimensional (2D) materials. This technology has also facilitated the verification of the performance of electronic and optical devices made from 2D materials. In this Perspective, we provide an overview of exfoliation techniques and highlight key physical properties. Additionally, we explored the chemical instability of certain 2D materials and proposed practical solutions to enhance their stability. Furthermore, we discuss the advantages of suspended 2D materials, which demonstrate improved compatibility and properties compared to nonsuspended materials. A particularly intriguing aspect of this Perspective is the exploration of the similarities between the Earth's crust and 2D materials, offering insights into the formation mechanisms of geological phenomena. In this context, 2D materials may serve as simulators for studying geological processes. We hope that this Perspective stimulates further research into exfoliation technology and the physical/chemical properties of 2D materials while providing new inspiration for earth science investigations.

Ferroelectric Domain and Switching Dynamics in Curved In2Se3: First-Principles and Deep Learning Molecular Dynamics Simulations

Despite its prevalence in experiments, the influence of complex strain on material properties remains understudied due to the lack of effective simulation methods. Here, the effects of bending, rippling, and bubbling on the ferroelectric domains are investigated in an In2Se3 monolayer by density functional theory and deep learning molecular dynamics simulations. Since the ferroelectric switching barrier can be increased (decreased) by tensile (compressive) strain, automatic polarization reversal occurs in α-In2Se3 with a strain gradient when it is subjected to bending, rippling, or bubbling deformations to create localized ferroelectric domains with varying sizes. The switching dynamics depends on the magnitude of curvature and temperature, following an Arrhenius-style relationship. This study not only provides a promising solution for cross-scale studies using deep learning but also reveals the potential to manipulate local polarization in ferroelectric materials through strain engineering.

Interference micro/nanolenses of salts for local modulation of Raman scattering

Micro/nanolenses play a crucial role in optics and spectroscopy, but the effect of interference patterns within each lens has been largely unexplored. Herein, we investigate modulation of Raman scattering by the interference within a single micro/nanolens of a hygroscopic salt. Lenses having two different diameter (d) ranges, d > 2 μm and d ∼1 μm, are placed on a silicon substrate, followed by collection of a Raman intensity map of the silicon peak. Lenses with d > 2 μm show dark and bright circular fringes in the Raman map, resembling the Newton's rings formed by optical interference. In the smaller lenses (d ∼1 μm), the map yields only a single peak at the center, representing either an intensity maximum or minimum. In both diameter ranges, whether the Raman intensity is enhanced or suppressed is determined by interference conditions, such as wavelength of the excitation laser or thickness of the SiO2 layer. The interference in salt micro/nanolenses finds applications in local modulation of Raman scattering of a nanoscale object, as demonstrated in individual single-walled carbon nanotubes decorated with the salt lenses.

Laser-Induced Periodic Surface Structures on Layered GaSe Crystals: Structural Coloring and Infrared Antireflection

We study structural and morphological transformations caused by multipulse femtosecond-laser exposure of Bridgman-grown ϵ-phase GaSe crystals, a van der Waals semiconductor promising for nonlinear optics and optoelectronics. We unveil, for the first time, the laser-driven self-organization regimes in GaSe allowing the formation of regular laser-induced periodic surface structures (LIPSSs) that originate from interference of the incident radiation and interface surface plasmon waves. LIPSSs formation causes transformation of the near-surface layer to amorphous Ga2Se3 at negligible oxidation levels, evidenced from comprehensive structural characterization. LIPSSs imprinted on both output crystal facets provide a 1.2-fold increase of the near-IR transmittance, while the ability to control local periodicity by processing parameters enables multilevel structural color marking of the crystal surface. Our studies highlight direct fs-laser patterning as a multipurpose application-ready technology for precise nanostructuring of promising van der Waals semiconductors, whose layered structure restricts application of common nanofabrication approaches.

Hoop compression driven instabilities in spontaneously formed multilayer graphene blisters over a polymeric substrate

The blistering of elastic membranes is prone to elastic-solid as well as substrate-based mechanical instabilities. The solid-based instabilities have been well-explored in the mechanically indented blisters of elastic membranes over the rigid/solid substrates, but an integrated study illustrating the underlying mechanism for the onset of solid as well as substrate-based instabilities in the spontaneous blistering of a 2D material is still lacking in the literature. In this article, an extensive experimental as well as analytical analysis of the spontaneous blister-formation in the multilayer graphene (MLG) flakes over a polymeric substrate is reported, which elucidates the involved mechanism and the governing parameters behind the development of elastic-solid as well as viscoelastic-substrate based instabilities. Herein, a 'blister-collapse model' is proposed, which infers that the suppression of the hoop compression, resulting from the phase-transition of the confined matter, plays a crucial role in the development of the instabilities. The ratio of blister-height to flake-thickness is a direct consequence of the taper-angle of the MLG blister and the thickness-dependent elasticity of the upper-bounding MLG flake, which shows a significant impact on the growth-dynamics of the viscous fingering pattern (viscoelastic-substrate based instability) under the MLG blister.

Strain and Interference Synergistically Modulated Optical and Electrical Properties in ReS2/Graphene Heterojunction Bubbles

Two-dimensional (2D) material bubbles, as a straightforward method to induce strain, represent a potentially powerful platform for the modulation of different properties of 2D materials and the exploration of their strain-related applications. Here, we prepare ReS₂/graphene heterojunction bubbles (ReS₂/gr heterobubbles) and investigate their strain and interference synergistically modulated optical and electrical properties. We perform Raman and photoluminescence (PL) spectra to verify the continuously varying strain and the microcavity induced optical interference in ReS₂/gr heterobubbles. Kelvin probe force microscopy (KPFM) is carried out to explore the photogenerated carrier transfer behavior in both strained ReS₂/gr heterobubbles and ReS₂/gr interfaces, as well as the oscillation of surface potential caused by optical interference under illumination conditions. Moreover, the switching of in-plane crystal orientation and the modulation of optical anisotropy of ReS₂/gr heterobubbles are observed by azimuth-dependent reflectance difference microscopy (ADRDM), which can be attributed to the action of both strain effect and interference. Our study proves that the optical and electrical properties can be effectively modulated by the synergistical effect of strain and interference in a 2D material bubble.

Polymer curing assisted formation of optically visible sub-micron blisters of multilayer graphene for local strain engineering

The local or global straining techniques are used to modulate the electronic, vibrational and optical properties of the two-dimensional (2D) materials. However, manipulating the physical properties of a 2D material under a local strain is comparatively more challenging. In this work, we demonstrate an easy and efficient polymer curing assisted technique for the formation of optically visible multilayer graphene (MLG) blisters of different shapes and sizes. The detailed spectroscopic and morphological analyses have been employed for exploring the dynamics of the confined matter inside the sub-micron blisters, which confirms that the confined matter inside the blister is liquid (water). From further analyses, we find the nonlinear elastic plate model as an acceptable model under certain limits for the mechanical analyses of the MLG blisters over the (poly)vinyl alcohol (PVA) polymer film to estimate the MLG-substrate interfacial adhesion energy and confinement pressure inside the blisters. The findings open new pathways for exploiting the technique for the formation of sub-micron blisters of the 2D materials for local strain-engineering applications, as well as the temperature-controlled release of the confined matter.

Spatially Separated Superconductivity and Enhanced Charge-Density-Wave Ordering in an IrTe2 Nanoflake

The interplay among collective electronic states like superconductivity (SC) and charge density wave (CDW) is of significance in transition metal dichalcogenides. To date, a consensus on the relationship between SC and CDW has not been established in IrTe₂. Here we use the Au-assisted exfoliation method to cleave IrTe₂ down to 10 nm. A striking feature is the concurrence of phase separation in a single piece of nanoflake, i.e., the superconducting (P3̅m1) and CDW (P3̅) phases. In the former area, the dimensional fluctuations suppress the CDW ordering and induce SC at 3.5 K. The CDW area at the phase boundary shows enhanced T_CDW at 605 K (T_CDW = 280 K in the bulk phase), which is accompanied by a unique wrinkle. Detailed analyses suggest that the strain-induced bond breaking of Te-Te dimers favors the CDW. Our works provide compelling evidence of competition between SC and CDW in IrTe₂.

Mechano-ferroelectric coupling: stabilization enhancement and polarization switching in bent AgBiP2Se6 monolayers

Two-dimensional (2D) van der Waals (vdW) ferroelectrics are core candidates for the development of next-generation non-volatile storage devices, which rely highly on ferroelectric stability and feasible approaches to manipulate the ferroelectric polarization and domain. Here, based on density functional theory calculations, we demonstrate that the bending deformation can not only manipulate the polarization direction and domain size of AgBiP₂Se₆ monolayers but also significantly improve the ferroelectric stability. The ordered polarization in the bent AgBiP₂Se₆ monolayers can be well maintained at a temperature of 200 K in molecular dynamics simulations; by contrast, it is broken at only 100 K for their freestanding counterparts. These phenomena can be attributed to synergic effects from the asymmetric strain energy induced by a strain gradient and a reduced migration barrier of Ag ions from convex to concave surfaces. More interestingly, a ferroelectric bubble can be induced in the monolayer under biaxial compression strain. This mechano-ferroelectric coupling represents a new mechanism and feasible route towards stabilization and polarization flip in 2D ferroelectrics.

Structural Defects Modulate Electronic and Nanomechanical Properties of 2D Materials

Two-dimensional materials such as graphene and molybdenum disulfide are often subject to out-of-plane deformation, but its influence on electronic and nanomechanical properties remains poorly understood. These physical distortions modulate important properties which can be studied by atomic force microscopy and Raman spectroscopic mapping. Herein, we have identified and investigated different geometries of line defects in graphene and molybdenum disulfide such as standing collapsed wrinkles, folded wrinkles, and grain boundaries that exhibit distinct strain and doping. In addition, we apply nanomechanical atomic force microscopy to determine the influence of these defects on local stiffness. For wrinkles of similar height, the stiffness of graphene was found to be higher than that of molybdenum disulfide by 10-15% due to stronger in-plane covalent bonding. Interestingly, deflated graphene nanobubbles exhibited entirely different characteristics from wrinkles and exhibit the lowest stiffness of all graphene defects. Density functional theory reveals alteration of the bandstructures of graphene and MoS₂ due to the wrinkled structure; such modulation is higher in MoS₂ compared to graphene. Using this approach, we can ascertain that wrinkles are subject to significant strain but minimal doping, while edges show significant doping and minimal strain. Furthermore, defects in graphene predominantly show compressive strain and increased carrier density. Defects in molybdenum disulfide predominantly show tensile strain and reduced carrier density, with increasing tensile strain minimizing doping across all defects in both materials. The present work provides critical fundamental insights into the electronic and nanomechanical influence of intrinsic structural defects at the nanoscale, which will be valuable in straintronic device engineering.

Construction of Position-Controllable Graphene Bubbles in Liquid Nitrogen with Assistance of Low-Power Laser

Graphene bubbles (GBs) are of significant interest owing to their distinguished electrical, optical, and magnetic properties. GBs can also serve as high-pressure reaction vessels to numerous chemical reactions. However, previous strategies to produce GBs are relatively elaborate and random. Therefore, their potential applications are severely restricted. Here, a facile and effective protocol is proposed to construct position-controllable GBs in liquid nitrogen (LN) with the assistance of laser and graphene wrinkles. Specifically, a film of graphene mounted on a SiO₂ substrate (G@SiO₂) is subjected to irradiation by a low-power laser in LN and then many GBs emerge from the surface of G@SiO₂. Most impressively, the domain where GBs arise is the position of the laser beam spot. Hence, we demonstrated that the high collimation of laser facilitates the position definition of GBs. The microscopic results indicate that some GBs split into three parts when they were subjected to irradiation by an electron. Meanwhile, some GBs degenerate into pores with a diameter of 500 nm when they are exposed to air. To grasp the properties of GBs in depth, the molecular dynamics (MD) simulations are performed, and the corresponding results indicate that temperature has very little impact on the GBs' shape. A phase transition process of the substance inside GBs is also revealed. Moreover, a two-dimensional (2D) solid nitrogen is discovered by MD simulations. The simplicity of our protocol paves the way to engineer high-pressure microreaction vessels and fabricate porous graphene membranes.

Cell-Like Behaviors of Dynamic Graphene Bubbles with Fast Water Transport

Ultrafast water transport in graphitic nanoenvironment is fundamentally important in the research of biomimetic membranes for potential applications in separation and energy. Yet, the form of graphitic nanostructures has not been fully explored with only carbon nanotubes and graphene nanochannels reported. Here, we fabricated dynamic graphene bubbles via strain engineering of chemical vapor deposition (CVD)-grown graphene on metal substrates. These graphene bubbles could switch between an inflated state and a deflated state continuously with the control of environmental moisture flow. It is demonstrated that water vapors transport through graphene wrinkles and condense inside graphene bubbles. The water transport rates across these graphene bubbles were calculated via dynamic Newton rings, which is comparable to that of carbon nanotubes and aquaporin. The discovery of dynamic graphene bubbles hosting the ability of fast water transport is helpful for an advanced understanding of the nanofluidic phenomenon and its future applications.

Dynamically-enhanced strain in atomically thin resonators

Graphene and related two-dimensional (2D) materials associate remarkable mechanical, electronic, optical and phononic properties. As such, 2D materials are promising for hybrid systems that couple their elementary excitations (excitons, phonons) to their macroscopic mechanical modes. These built-in systems may yield enhanced strain-mediated coupling compared to bulkier architectures, e.g., comprising a single quantum emitter coupled to a nano-mechanical resonator. Here, using micro-Raman spectroscopy on pristine monolayer graphene drums, we demonstrate that the macroscopic flexural vibrations of graphene induce dynamical optical phonon softening. This softening is an unambiguous fingerprint of dynamically-induced tensile strain that reaches values up to ≈4 × 10⁻⁴ under strong non-linear driving. Such non-linearly enhanced strain exceeds the values predicted for harmonic vibrations with the same root mean square (RMS) amplitude by more than one order of magnitude. Our work holds promise for dynamical strain engineering and dynamical strain-mediated control of light-matter interactions in 2D materials and related heterostructures.

Here, the authors use Raman spectroscopy on circular graphene drums to demonstrate dynamical softening of optical phonons induced by the macroscopic flexural motion of graphene, and find evidence that the strain in graphene is enhanced under non-linear driving.