K-Λ crossover transition in the conduction band of monolayer MoS2 under hydrostatic pressure

Pressure-Driven One-Dimensional Superlattices in Monolayer Crystals on a Vicinal Surface

A 1D superlattice can be imposed on monolayer materials by the periodic atomic steps of a vicinal crystalline surface. By means of the rotational anisotropy of second harmonic generation, we demonstrate that the periodic modulation of monolayer WS2 by the diamond (230) surface gives rise to an orientation-dependent breaking of crystal symmetry, a fundamental prerequisite for generation of a one-dimensional superlattice from a two-dimensional lattice, when subjected to external pressure. These observations contrast with the case of monolayer WS2 on the diamond (100) surface, which largely retains its 3-fold rotational symmetry when pressure is applied. Vicinal surfaces offer a potentially versatile approach to the nanoscale periodic in-plane modulation of layered crystals and engineering their electronic and optical functionality.

Design of stimuli-responsive transition metal dichalcogenides

Stimuli-responsive systems are an emerging class of materials in fields as diverse as electronics, optoelectronics, cancer detection, drug delivery, or sensing. Especially focusing on nanomaterials, 2D transition metal dichalcogenides have recently attracted the scientific community's attention due to their remarkable intrinsic stimuli-responsive behaviour upon external stimuli such as pH, light, voltage, or certain pathogens. This significant response can be further enhanced by forming mixed-dimensional heterostructures and by molecular functionalization, capitalizing on chemistry to manipulate and boost their intrinsic stimuli-responsive properties. Furthermore, thanks to the endless possibilities of chemistry, a new class of smart materials based on the combination of stimuli-responsive molecular systems with transition metal dichalcogenides has recently been synthesized. In these materials, the physical properties of the 2D layers are reversibly modified by the switchable molecules, not only enhancing their stimuli-responsive behaviour but also providing memory to the hybrid. Therefore, this review explores the recent breakthroughs in the chemical design of smart transition metal dichalcogenides with built-in responsiveness.

Quasi-One-Dimensional Zigzag Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution from Water

Covalent organic frameworks (COFs) have potential applications in a wide range of fields. However, it remains a critical challenge to constrain their covalent expansions in the one-dimensional (1D) direction. Here, we developed a general approach to fabricate 15 different highly crystalline COFs with zigzag-packed 1D porous organic chains through the condensation of V-shaped ditopic linkers and X-shaped tetratopic knots. Appropriate geometrical combinations of a wide scope of linkers and knots with distinct aromatic cores, linkages, and functionalities offer a series of quasi-1D COFs with dominant pore sizes of 7-13 Å and surface areas of 116-784 m2 g-1. Among them, nitrogen (N)-doped 1D COFs with site-specific doping of heteroatoms favor a tunable control of band structures and conjugations and thus allow a remarkable hydrogen evolution rate up to 80 mmol g-1 h-1 in photocatalytic water splitting. This general strategy toward programming function in porous crystalline materials has the potential to tune the topologically well-defined electronic properties through precisely periodic doping.

Novel two-dimensional HfSi2N4 monolayer with excellent bandgap modulation and electronic properties modulation

The bandgap modulation and electronic properties modulation of two-dimensional HfSi2N4 monolayer induced by strain, electric field and atomic adsorption are studied by first principles. The HfSi2N4 monolayer was found to be dynamically, thermally, and mechanically stable at equilibrium, and it is a direct semiconductor with a bandgap of 1.87 eV. The bandgap of the HfSi2N4 monolayer can be precisely modulated by strain. Under the action of strain, HfSi2N4 monolayer not only transforms from direct semiconductor to indirect semiconductor, but also improves the absorption of visible light. An external electric field in the 0-0.5 eV/Å range can also modulate the bandgap of HfSi2N4 monolayer from 1.87 eV to 0 eV, and most importantly, at an external electric field of 0.5 eV/Å, HfSi2N4 monolayer shows the characteristics of spin gapless semiconductor. The calculated adsorption energy shows that the structures of H, O and F atoms adsorbed by HfSi2N4 monolayer can all exist stably. The bandgap of the configuration after adsorption of O and F atoms is significantly reduced compared with that of HfSi2N4 monolayer. Furthermore, the HfSi2N4 monolayer after adsorption of H and F atoms is transformed into a magnetic semiconductor. METHOD: All calculations were performed using Vienna ab initial simulation package, The electronic structure, mechanical properties, electronic properties and other properties were carried out using generalized gradient approximation (GGA-PBE), supplemented by HSE06 and GGA + U. The total-energy and force convergence are less than 10-6 eV and 0.001 eV/Å, respectively. The vacuum on the z-axis is selected 20 Å. The vdW interactions were corrected using the Grimme scheme (DFT-D3).

Tunable Photocarrier Dynamics in CuS Nanoflakes under Pressure Modulation

Two-dimensional materials with a unique layered structure have attracted intense attention all around the world due to their extraordinary physical properties. Most importantly, the internal Coulomb coupling can be regulated, and thus electronic transition can be realized by manipulating the interlayer interaction effectively through adding external fields. At present, the properties of two-dimensional materials can be tuned through a variety of methods, such as adding pressure, strain, and electromagnetic fields. For optoelectronic applications, the lifetime of the photogenerated carriers is one of the most crucial parameters for the materials. Here, we demonstrate effective modulation of the optical band gap structure and photocarrier dynamics in CuS nanoflakes by applying hydrostatic pressure via a diamond anvil cell. The peak differential reflection signal shows a linear blueshift with the pressure, suggesting effective tuning of interlayer interaction inside CuS by pressure engineering. The results of transient absorption show that the photocarrier lifetime decreases significantly with pressure, suggesting that the dissociation process of the photogenerated carriers accelerates. It could be contributed to the phase transition or the decrease of the phonon vibration frequency caused by the pressure. Further, Raman spectra reveal the change of Cu-S and S-S bonds after adding pressure, indicating the possible occurrence of structural phase transition. Interestingly, all of the variation modes are reversible after releasing pressure. This work has provided an excellent sight to show the regulation of pressure on the photoelectric properties of CuS, exploring CuS to wider applications that can lead toward the realization of future excitonic and photoelectric devices modulated by high pressure.

Two-Channel Indirect-Gap Photoluminescence and Competition between the Conduction Band Valleys in Few-Layer MoS2

MoS2 is a two-dimensional layered transition metal dichalcogenide with unique electronic and optical properties. The fabrication of ultrathin MoS2 is vitally important, since interlayer interactions in its ultrathin varieties will become thickness-dependent, providing thickness-governed tunability and diverse applications of those properties. Unlike with a number of studies that have reported detailed information on direct bandgap emission from MoS2 monolayers, reliable experimental evidence for thickness-induced evolution or transformation of the indirect bandgap remains scarce. Here, the sulfurization of MoO3 thin films with nominal thicknesses of 30 nm, 5 nm and 3 nm was performed. All sulfurized samples were examined at room temperature with spectroscopic ellipsometry and photoluminescence spectroscopy to obtain information about their dielectric function and edge emission spectra. This investigation unveiled an indirect-to-indirect crossover between the transitions, associated with two different Λ and K valleys of the MoS2 conduction band, by thinning its thickness down to a few layers.

Robustness of Trion State in Gated Monolayer MoSe2 under Pressure

Quasiparticles consisting of correlated electron(s) and hole(s), such as excitons and trions, play important roles in the optical phenomena of van der Waals semiconductors and serve as unique platforms for studies of many-body physics. Herein, we report a gate-tunable exciton-to-trion transition in pressurized monolayer MoSe2, in which the electronic band structures are modulated continuously within a diamond anvil cell. The emission energies of both the exciton and trion undergo large blueshifts over 90 meV with increasing pressure. Surprisingly, the trion binding energy remains constant at 30 meV, regardless of the applied pressure. Combining ab initio density functional theory calculations and quantum Monte Carlo simulations, we find that the remarkable robustness of the trion binding energy originates from the spatially diffused nature of the trion wave function and the weak correlation between its constituent electron-hole pairs. Our findings shed light on the optical properties of correlated excitonic quasiparticles in low-dimensional materials.

Broken-gap type-III band alignment in monolayer halide perovskite/antiperovskite oxide van der Waals heterojunctions

The integration of halide perovskites with other functional materials provides a new platform for applications beyond photovoltaics, which has been realized in experiments. Here, through first-principles methods, we explore the possibility of constructing halide perovskite/antiperovskite oxide van der Waals heterostructures (vdWHs) for the first time with monolayers Rb₂CdCl₄ and Ba₄OSb₂ as representative compounds. Our calculation results reveal that the Rb₂CdCl₄/Ba₄OSb₂ vdWHs have negative binding energies and their most stable stacking possesses a rare type-III band alignment with a broken gap, which is highly promising for tunnel field-effect transistor (TFET) applications. Moreover, their electronic features can be further tuned by applying strain or an external electric field. Specifically, compressive strain can enlarge the tunneling window, while tensile strain can realize a type-III to type-II band alignment transformation. Therefore, our work provides fundamental insights into the electronic properties of Rb₂CdCl₄/Ba₄OSb₂ vdWHs and paves the way for the design and fabrication of future halide perovskite/antiperovskite-based TFETs.

Orbital distortion and electric field control of sliding ferroelectricity in a boron nitride bilayer

Recent experiments confirm that two-dimensional boron nitride (BN) films possess room-temperature out-of-plane ferroelectricity when each BN layer is sliding with respect to each other. This ferroelectricity is attributed to the interlayered orbital hybridization or interlayer charge transfer in previous work. In this work, we attempt to understand the sliding ferroelectricity from the perspective of orbital distortion of long-pair electrons. Using the maximally localized Wannier function method and first-principles calculations, the out-of-planep_zorbitals of BN are investigated. Our results indicate that the interlayer van der Waals interaction causes the distortion of the Np_zorbitals. Based on the picture of out-of-plane orbital distortion, we propose a possible mechanism to tune the ferroelectric polarization by external fields, including electric field and stress field. It is found that both the polarization intensity and direction can be modulated under the electric field. The polarization intensity of the system can also be controlled by stress field perpendicular to the plane. This study will provide theoretical help in the device design based on sliding ferroelectrics.

Converting a Monolayered NbSe2 into an Ising Superconductor with Nontrivial Band Topology via Physical or Chemical Pressuring

Two-dimensional transition metal dichalcogenides possessing superconductivity and strong spin-orbit coupling exhibit high in-plane upper critical fields due to Ising pairing. Yet to date, whether such systems can become topological Ising superconductors remains to be materialized. Here we show that monolayered NbSe₂ can be converted into Ising superconductors with nontrivial band topology via physical or chemical pressuring. Using first-principles calculations, we first demonstrate that a hydrostatic pressure higher than 2.5 GPa can induce a p-d band inversion, rendering nontrivial band topology to NbSe₂. We then illustrate that Te-doping can function as chemical pressuring in inducing nontrivial topology in NbSe_2-xTe_x with x ≥ 0.8, due to a larger atomic radius and stronger spin-orbit coupling of Te. We also evaluate the upper critical fields within both approaches, confirming the enhanced Ising superconductivity nature, as experimentally observed. Our findings may prove to be instrumental in material realization of topological Ising superconductivity in two-dimensional systems.

Tunneling Magnetoresistance Transition and Highly Sensitive Pressure Sensors Based on Magnetic Tunnel Junctions with a Black Phosphorus Barrier

Black phosphorus is a promising material to serve as a barrier for magnetic tunnel junctions (MTJs) due to weak van der Waals interlayer interactions. In particular, the special band features of black phosphorus may endow intriguing physical characteristics. Here we study theoretically the effect of band gap tunability of black phosphorus on the MTJs with the black phosphorus barrier. It is found that the tunneling magnetoresistance (TMR) may transition from a finite value to infinity owing to the variation in the band gap of black phosphorus. Combined with the latest experimental results of the pressure-induced band gap tunability, we further investigate the pressure effect of TMR in the MTJs with a black phosphorus barrier. The calculations show that the pressure sensitivity can be quite high under appropriate parameters. Physically, the high sensitivity originates from the TMR transition phenomenon. To take advantage of the high pressure sensitivity, we propose and design a detailed structure of highly sensitive pressure sensors based on MTJs with a black phosphorus barrier, whose working mechanism is basically different from that of convential pressure sensors. The present pressure sensors possess four advantages and benifits: (1) high sensitivity, (2) good anti-interference, (3) high spatial resolution, and (4) fast response speed. Our study may advance new research areas for both the MTJs and pressure sensors.

Interlayer Excitons in Transition Metal Dichalcogenide Semiconductors for 2D Optoelectronics

Optoelectronic materials that allow on-chip integrated light signal emitting, routing, modulation, and detection are crucial for the development of high-speed and high-throughput optical communication and computing technologies. Interlayer excitons in 2D van der Waals heterostructures, where electrons and holes are bounded by Coulomb interaction but spatially localized in different 2D layers, have recently attracted intense attention for their enticing properties and huge potential in device applications. Here, a general view of these 2D-confined hydrogen-like bosonic particles and the state-of-the-art developments with respect to the frontier concepts and prototypes is presented. Staggered type-II band alignment enables expansion of the interlayer direct bandgap from the intrinsic visible in monolayers up to the near- or even mid-infrared spectrum. Owing to large exciton binding energy, together with ultralong lifetime, room-temperature exciton devices and observation of quantum behaviors are demonstrated. With the rapid advances, it can be anticipated that future studies of interlayer excitons will not only allow the construction of all-exciton information processing circuits but will also continue to enrich the panoply of ideas on quantum phenomena.

Electronic Band Tuning and Multivalley Raman Scattering in Monolayer Transition Metal Dichalcogenides at High Pressures

Transition metal dichalcogenides (TMDs) possess spin-valley locking and spin-split K/K' valleys, which have led to many fascinating physical phenomena. However, the electronic structure of TMDs also exhibits other conduction band minima with similar properties, the Q/Q' valleys. The intervalley K-Q scattering enables interesting physical phenomena, including multivalley superconductivity, but those effects are typically hindered in monolayer TMDs due to the large K-Q energy difference (ΔE_KQ). To unlock elusive multivalley phenomena in monolayer TMDs, it is desirable to reduce ΔE_KQ, while being able to sensitively probe the valley shifts and the multivalley scattering processes. Here, we use high pressure to tune the electronic properties of monolayer MoS₂ and WSe₂ and probe K-Q crossing and multivalley scattering via double-resonance Raman (DRR) scattering. In both systems, we observed a pressure-induced enhancement of the double-resonance LA and 2LA Raman bands, which can be attributed to a band gap opening and ΔE_KQ decrease. First-principles calculations and photoluminescence measurements corroborate this scenario. In our analysis, we also addressed the multivalley nature of the DRR bands for WSe₂. Our work establishes the DRR 2LA and LA bands as sensitive probes of strain-induced modifications to the electronic structure of TMDs. Conversely, their intensity could potentially be used to monitor the presence of compressive or tensile strain in TMDs. Furthermore, the ability to probe K-K' and K-Q scattering as a function of strain shall advance our understanding of different multivalley phenomena in TMDs such as superconductivity, valley coherence, and valley transport.

Interlayer Coupling and Pressure Engineering in Bilayer MoS2

Controlling the interlayer coupling by tuning lattice parameters through pressure engineering is an important route for tailoring the optoelectronic properties of two-dimensional materials. In this work, we report a pressure-dependent study on the exciton transitions of bilayer MoS2 exfoliated on a diamond anvil surface. The applied hydrostatic pressure changes from ambient pressure up to 11.05 GPa using a diamond anvil cell device. Raman, photoluminescence, and reflectivity spectra at room temperature are analyzed to characterize the interlayer coupling of this bilayer system. With the increase of pressure, the indirect exciton emission disappears completely at about 5 GPa. Importantly, we clearly observed the interlayer exciton from the reflectivity spectra, which becomes invisible at a low pressure around 1.26 GPa. This indicates that the interlayer exciton is very sensitive to the hydrostatic pressure due to the oscillator strength transfer from the direct transition to the indirect one.

Strong Substrate Strain Effects in Multilayered WS2 Revealed by High-Pressure Optical Measurements

The optical properties of two-dimensional materials can be effectively tuned by strain induced from a deformable substrate. In the present work we combine first-principles calculations based on density functional theory and the effective Bethe-Salpeter equation with high-pressure optical measurements to thoroughly describe the effect of strain and dielectric environment onto the electronic band structure and optical properties of a few-layered transition-metal dichalcogenide. Our results show that WS2 remains fully adhered to the substrate at least up to a -0.6% in-plane compressive strain for a wide range of substrate materials. We provide a useful model to describe effect of strain on the optical gap energy. The corresponding experimentally determined out-of-plane and in-plane stress gauge factors for WS2 monolayers are -8 and 24 meV/GPa, respectively. The exceptionally large in-plane gauge factor confirms transition metal dichalcogenides as very promising candidates for flexible functionalities. Finally, we discuss the pressure evolution of an optical transition closely lying to the A exciton for bulk WS2 as well as the direct-to-indirect transition of the monolayer upon compression.

Engineering Interlayer Electron-Phonon Coupling in WS2/BN Heterostructures

In van der Waals (vdW) heterostructures, the interlayer electron-phonon coupling (EPC) provides one unique channel to nonlocally engineer these elementary particles. However, limited by the stringent occurrence conditions, the efficient engineering of interlayer EPC remains elusive. Here we report a multitier engineering of interlayer EPC in WS₂/boron nitride (BN) heterostructures, including isotope enrichments of BN substrates, temperature, and high-pressure tuning. The hyperfine isotope dependence of Raman intensities was unambiguously revealed. In combination with theoretical calculations, we anticipate that WS₂/BN supercells could induce Brillouin-zone-folded phonons that contribute to the interlayer coupling, leading to a complex nature of broad Raman peaks. We further demonstrate the significance of a previously unexplored parameter, the interlayer spacing. By varying the temperature and high pressure, we effectively manipulated the strengths of EPC with on/off capabilities, indicating critical thresholds of the layer-layer spacing for activating and strengthening interlayer EPC. Our findings provide new opportunities to engineer vdW heterostructures with controlled interlayer coupling.

Recent Advances on Tuning the Interlayer Coupling and Properties in van der Waals Heterostructures

2D van der Waals (vdW) heterostructures are receiving increasing research attention due to the theoretically amazing properties and unprecedented application potential. However, the as-synthesized heterostructures are generally underperforming due to the weak interlayer coupling, which inspires the researchers to find ways to modulate the interlayer coupling and properties, realizing the tailored performance for actual applications. There have been a lot of publications regarding the controllable regulation of the structures and properties of 2D vdW heterostructures in the past few years, while a review work summarizing the current advances is not yet available, though it is significant. This paper conducts a state-of-the-art review regarding the current research progress of performance modulation of vdW heterostructures by different techniques. First, the general synthesis methods of vdW heterostructures are summarized. Then, different performance modulation techniques, that is, mechanical-based, external fields-assisted, and particle beam irradiation-based methods, are discussed and compared in detail. Some of the newly proposed concepts are described. Thereafter, applications of vdW heterostructures with tailored properties are reviewed for the application prospects of the topic around this area. Moreover, the future research challenges and prospects are discussed, aiming at triggering more research interest and device applications around this topic.

Tuning of Optical Behavior in Monolayer and Bilayer Molybdenum Disulfide Using Hydrostatic Pressure

Researchers have shown great interest in two-dimensional crystals recently, because of their thickness-dependent electronic and optical properties. We have investigated the Raman and photoluminescence spectra of free-standing monolayer and bilayer MoS₂, as a function of pressure. As the enforcement of layer interaction, an electronic and a crystal phase transition were revealed at ∼6 GPa and ∼16 GPa, respectively, in bilayer MoS₂, while no phase transition in the monolayer is observed. The electronic phase transition at ∼6 GPa is supposed to be a direct interband changing to an indirect Λ-K interband transition, and the new structure shown at ∼16 GPa is not metallized and supposed to be a transformation from stacking faults due to layer sliding like 2H_c to 2H_a. The different pressure-induced features of monolayer MoS₂, compared with bilayer MoS₂, can help to get a better understanding about the importance of interlayer interaction on modifying the optical properties of MoS₂ and other fundamental understanding of 2D materials.

Pressure-Enhanced Ferromagnetism in Layered CrSiTe3 Flakes

Despite recent advances in layered ferromagnets, ferromagnetic interactions in these materials are rather weak. Here, we report pressure-enhanced ferromagnetism in layered CrSiTe₃ flakes revealed by high-pressure magnetic circular dichroism measurements. Below ∼3 GPa, CrSiTe₃ undergoes a paramagnetic-to-ferromagnetic phase transition at ∼32 K, and the field-induced spin-flip in the ferromagnetic phase produces nearly zero hysteresis loops, demonstrating soft ferromagnetism. Above ∼4 GPa, a soft-to-hard ferromagnetic transition occurs, signaled by rectangular-shaped hysteresis loops with finite coercivity and remanent magnetization. Interestingly, as pressure increases, the Curie temperature and coercivity dramatically increase up to ∼138 K and 0.17 T at 7.8 GPa, respectively, in contrast to ∼36 K and 0.02 T at 4.6 GPa. It indicates a remarkable influence of pressure on exchange interactions, which is consistent with DFT calculations. The effective interaction between magnetic couplings and external pressure offers new opportunities in pursuit of high-temperature layered ferromagnets.

Robust Interlayer Exciton in WS2/MoSe2 van der Waals Heterostructure under High Pressure

The van der Waals (vdW) heterostructures have rich functions and intriguing physical properties, which has attracted wide attention. Effective control of excitons in vdW heterostructures is still urgent for fundamental research and realistic applications. Here, we successfully achieved quantitative tuning of the intralayer exciton of monolayers and observed the transition from intralayer excitons to interlayer excitons in WS₂/MoSe₂ heterostructures, via hydrostatic pressure. The energy of interlayer excitons is in a "locked" or "superstable" state, which is not sensitive to pressure. The first-principles calculation reveals the stronger interlayer interaction which leads to enhanced interlayer exciton behavior in WS₂/MoSe₂ heterostructures under external pressure and reveals the robust peak of interlayer excitons. This work provides an effective strategy to study the interlayer interaction in vdW heterostructures and reveals the enhanced interlayer excitons in WS₂/MoSe₂, which could be of great importance for the material and device design in various similar quantum systems.

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.

Investigation of Hot Carrier Cooling Dynamics in Monolayer MoS2

The hot carrier cooling dynamics in the C-excitonic state of monolayer MoS₂ is slowed down by the hot phonon bottleneck and Auger heating effects, as exploited by ultrafast transient absorption spectroscopy. The hot carrier cooling process, determined by the hot phonon bottleneck, can be prolonged through rising the excitation photon energy or increasing the absorbed photon flux. By inducing the Auger heating effect under higher absorbed photon flux, the hot carrier lifetime also increases at the low excitation photon energy. When these two effects are combined under higher excitation photon energy and higher absorbed photon flux, the hot phonon bottleneck is gradually weakened because of Auger recombination. In addition, the similar hot carrier phenomenon can be observed in A/B excitonic states owing to the same physical mechanism. Our work establishes a solid photophysics foundation for 2D transition-metal dichalcogenide applications in advanced energy conversion, optical quantum communication, quantum technology, etc.

Band Engineering of Large-Twist-Angle Graphene/h-BN Moiré Superlattices with Pressure

Graphene interfacing hexagonal boron nitride (h-BN) forms lateral moiré superlattices that host a wide range of new physical effects such as the creation of secondary Dirac points and band gap opening. A delicate control of the twist angle between the two layers is required as the effects weaken or disappear at large twist angles. In this Letter, we show that these effects can be reinstated in large-angle (∼1.8°) graphene/h-BN moiré superlattices under high pressures. A graphene/h-BN moiré superlattice microdevice is fabricated directly on the diamond culet of a diamond anvil cell, where pressure up to 8.3 GPa is applied. The band gap at the primary Dirac point is opened by 40-60 meV, and fingerprints of the second Dirac band gap are also observed in the valence band. Theoretical calculations confirm the band engineering with pressure in large-angle graphene/h-BN bilayers.

Benchmark Investigation of Band-Gap Tunability of Monolayer Semiconductors under Hydrostatic Pressure with Focus-On Antimony

In this paper, the band-gap tunability of three monolayer semiconductors under hydrostatic pressure was intensively investigated based on first-principle simulations with a focus on monolayer antimony (Sb) as a semiconductor nanomaterial. As the benchmark study, monolayer black phosphorus (BP) and monolayer molybdenum disulfide (MoS2) were also investigated for comparison. Our calculations showed that the band-gap tunability of the monolayer Sb was much more sensitive to hydrostatic pressure than that of the monolayer BP and MoS2. Furthermore, the monolayer Sb was predicted to change from an indirect band-gap semiconductor to a conductor and to transform into a double-layer nanostructure above a critical pressure value ranging from 3 to 5 GPa. This finding opens an opportunity for nanoelectronic, flexible electronics and optoelectronic devices as well as sensors with the capabilities of deep band-gap tunability and semiconductor-to-metal transition by applying mechanical pressure.

Pressure Effect on Electronic and Excitonic Properties of Purely J-Aggregated Monolayer Organic Semiconductor

Different from monolayer inorganic semiconductors, such as transition metal dichalcogenides, monolayer organic semiconductors derived from perylene have attracted much attention because of their strong absorption and bright photoluminescence (PL). Pressure has proved to be an effective tool in probing the exciton behavior in monolayer semiconductors. Here, by studying the high-pressure behavior of purely J-aggregated monolayer organic semiconductors experimentally and theoretically, we find a red shift of PL spectra due to a decrease of band gap, which is consistent with fluorescent images taken under pressure. The PL center dominates the perylene group and the band edges are flat, indicating Frenkel exciton in the monolayer organic semiconductor under ambient conditions. With increasing pressure, the band edges become more dispersive, suggesting the exciton transform to Wannier-Mott exciton, which is commonly observed in inorganic semiconductors.

Anomalous Behavior of 2D Janus Excitonic Layers under Extreme Pressures

Newly discovered 2D Janus transition metal dichalcogenides layers have gained much attention from a theory perspective owing to their unique atomic structure and exotic materials properties, but little to no experimental data are available on these materials. Here, experimental and theoretical studies establish the vibrational and optical behavior of 2D Janus S-W-Se and S-Mo-Se monolayers under high pressures for the first time. Chemical vapor deposition (CVD)-grown classical transition metal dichalcogenides (TMD) monolayers are first transferred onto van der Waals (vdW) mica substrates and converted to 2D Janus sheets by surface plasma technique, and then integrated into a 500 µm size diamond anvil cell for high-pressure studies. The results show that 2D Janus layers do not undergo phase transition up to 15 GPa, and in this pressure regime, their vibrational modes exhibit a nonmonotonic response to the applied pressures (dω/dP). Interestingly, these 2D Janus monolayers exhibit unique blueshift in photoluminescence (PL) upon compression, which is in contrast to many other traditional semiconductor materials. Overall theoretical simulations offer in-depth insights and reveal that the overall optical response is a result of competition between the ab-plane (blueshift) and c-axis (redshift) compression. The overall findings shed the very first light on how 2D Janus monolayers respond under extreme pressures and expand the fundamental understanding of these materials.

A review on role of tetra-rings in graphene systems and their possible applications

Inspired by the success of graphene, various two-dimensional (2D) non-hexagonal graphene allotropes having sp²-bonded tetragonal rings in free-standing (hypothetical) form and on different substrates have been proposed recently. These systems have also been fabricated after modifying the topology of graphene by chemical processes. In this review, we would like to indicate the role of tetra-rings and the local symmetry breaking on the structural, electronic and optical properties of the graphene system. First-principles computations have demonstrated that the tetragonal graphene (TG) allotrope exhibits appreciable thermodynamic stability. The band structure of the TG nanoribbons (TGNRs) strongly depends on the size and edge geometry. This fact has been supported by the transport properties of TGNRs. The optical properties and Raman modes of this graphene allotrope have been well explored for characterisation purposes. Recently, a tight-binding model was used to unravel the metal-to-semiconductor transition under the influence of external magnetic fluxes. Even the introduction of transition metal atoms into this non-hexagonal network can control the magnetic response of the TG sheet. Furthermore, the collective effect of B-N doping and confinement effect on the structural and electronic properties of TG systems has been investigated. We also suggest future directions to be explored to make the synthesis of T graphene and its various derivatives/allotropes viable for the verification of theoretical predictions. It is observed that these doped systems act as a potential candidate for carbon monoxide gas sensing and current rectification devices. Therefore, all these experimental, numerical and analytical studies related to non-hexagonal TG systems are extremely important from a basic science point of view as well as for applications in sensing, optoelectronic and photonic devices.

On the impact of the stress situation on the optical properties of $WSe_2$ monolayers under high pressure

We have studied the optical properties of $WSe_2$ monolayers (ML) by means of photoluminescence (PL), PL excitation (PLE) and Raman scattering spectroscopy at room temperature and as a function of hydrostatic pressure up to ca. 12 GPa. For comparison the study comprises two cases: A single $WSe_2$ ML directly transferred onto one of the diamonds of the diamond anvil cell and a $WSe_2$ ML encapsulated into hexagonal boron nitride (hBN) layers. The pressure dependence of the A and B exciton, as determined by PL and PLE, respectively, is very different for the case of the bare $WSe_2$ ML and the $hBN/WSe_2-ML/hBN$ heterostructure. Whereas for the latter the A and B exciton energy increases linearly with increasing pressure at a rate of 3.5 to 3.8 meV/GPa, for the bare $WSe_2$ ML the A and B exciton energy decreases with a coefficient of -3.1 and -1.3 meV/GPa, respectively. We interpret that this behavior is due to a different stress situation. For a single ML the stress tensor is essentially uniaxial with the compressive stress component in the direction perpendicular to the plane of the ML. In contrast, for the substantially thicker $hBN/WSe_2-ML/hBN$ heterostructure the compression is hydrostatic. The results from an analysis of the pressure dependence of the frequency of Raman active modes comply with the interpretation of having a different stress situation in each case. Reviewed by: A. San Miguel, Institut Lumière Matière, Université de Lyon, France; Edited by: J. S. Reparaz

Probing Evolution of Local Strain at MoS2-Metal Boundaries by Surface-Enhanced Raman Scattering

Strain usually exists in two-dimensional (2D) materials and devices, and its presence drastically modulates their properties. When 2D materials interface with noble metals, local strain and surface plasmon can couple at the metal-2D material boundaries, delivering a lot of intriguing phenomena. Current studies are mostly focused on the explanations of these strain-related phenomena based on a static point of view. Although strain can typically be relaxed in many environments, the time evolution of strain at metal-2D material interfaces remains largely unknown. In this work, we investigate the evolution of local strain at Ag-MoS₂ boundaries by surface-enhanced Raman scattering. With the split of MoS₂ Raman peaks as an indicator of local strain, it is found that the originally localized strain at Ag-MoS₂ boundaries evolves and relaxes with time into a delocalized strain in MoS₂ plane. The time to start the strain relaxation depends on the number of layers of MoS₂ flakes, suggesting that the relaxation may result from the mechanical instability of the interface between the topmost MoS₂ layer and the underlying materials. The relaxation occurs in a certain period of time, i.e., ∼70 days for 1L and ∼30 days for 3L. Accompanying the strain relaxation, surface sulfurization of Ag also occurs, a process that reduces the strength of locally enhanced electric field. Our results not only provide a deep understanding of strain evolution at metal-MoS₂ interfaces but also shed light on the optimization of MoS₂-based device fabrications.