Strain engineering of 2D semiconductors and graphene: from strain fields to band-structure tuning and photonic applications

Anomalous Phonon Softening with Inherent Strain in Wrinkled Monolayer WSe2

Local deformation is a control knob to dynamically tune the electronic band structure of 2D semiconductors. This study demonstrates the local strain-dependent phonon properties of monolayer tungsten diselenide, which are investigated by using the scanning tunneling microscopy-based tip-enhanced Raman spectroscopy. The anomalous appearance and softening of the Raman-inactive out-of-planeA 2 ' ' ( Γ ) $A_2^{^{\prime\prime}}( \Gamma )$ mode are first revealed, which exhibits equivalent behavior to other principal phonons of tungsten diselenide. Local strain calculations unveiled the linear proportionalities ofA 2 ' ' ( Γ ) $A_2^{^{\prime\prime}}( \Gamma )$ phonon nature on strain and it facilitates the derivation of Grüneisen parameter by experimental and theoretical approaches. Additionally, the origins of the anomalous appearance of the Raman-inactiveA 2 ' ' ( Γ ) $A_2^{^{\prime\prime}}( \Gamma )$ mode are clearly proved in both classical physics and quantum mechanics. Especially quantum mechanical calculations have precisely described strain-induced selection rule relaxation by polarizability changes. The first discovery provides a fundamental understanding of the strain-dependent phonon properties, as well as suggesting a new distinct strain indicator,A 2 ' ' ( Γ ) $A_2^{^{\prime\prime}}( \Gamma )$ mode, for strain engineering.

Intrinsic Mechanical Effects on the Activation of Carbon Catalysts

The mechanical effects on carbon-based metal-free catalysts (C-MFCs) have rarely been explored, despite the global interest in C-MFCs as substitutes for noble metal catalysts. Stress is ubiquitous, whereas its dedicated study is severely restricted due to its frequent entanglement with other structural variables, such as dopants, defects, and interfaces in catalysis. Herein, we report a proof-of-concept study by establishing a platform to continuously apply strain to a highly oriented pyrolytic graphite (HOPG) lamina, simultaneously collecting electrochemical signals. Notably, we establish, for the first time, the correlation between the surface strain of graphitic carbon and its activation effect on the oxygen reduction reaction (ORR). Our results indicate that while in-plane and edge carbon sites in HOPG could not be further activated by applying tensile strain, a strong and repeatable dependence of catalytic activity on tensile strain was observed when the structure incorporated in-plane defects, leading to a significant ∼35.0% improvement in ORR current density with the application of ∼0.6% tensile strain. Density functional theory (DFT) simulations reveal that appropriate strain on specific defects can optimize the adsorption of reaction intermediates, and the Stone-Wales defect on graphene is correlated with the observed mechanical effect. This work elucidates fundamental principles of strain effects on the catalytic activity of graphitic carbon toward ORR and may lay the groundwork for the development of carbon-based mechano-electrocatalysis.

Dynamic bandgap modulation in CsPbBr3 perovskite nanocrystals through reversible ammonia intercalation

Modulation of the electronic states of a semiconductor is an intriguing area of research because of its interesting applications. In general, physical methods are used to reversibly manipulate the bandgap of semiconductors. Herein, we have used a simple molecule, ammonia, and allowed it to intercalate inside the crystal lattice of CsPbBr3 perovskites to alter the band positions. The molecular intercalation of ammonia induces strain in the crystal structure of perovskite, which widens the bandgap. Ammonia intercalation results in fall-off of the visible absorption and emission of the CsPbBr3 perovskites and a new absorption emerges in the ultraviolet region. Interestingly, with time, the deintercalation takes place, as a result of the population in the antibonding orbitals formed due to the mixing of s orbital of the Pb and p orbital of N in the intercalated NH3. The deintercalation of gaseous ammonia results in the narrowing of the bandgap which results in the regaining of the visible absorption. Together with the density functional theory calculations, herein, we demonstrate the reversible bandgap modulation in CsPbBr3 perovskite nanocrystals. Aspects discussed here can give directions to develop newer methodologies to tune the band positions of semiconductors by the intercalation of the right molecules inside their crystal lattice.

Mechanical and electromechanical properties of 2D materials studied via in situ microscopy techniques

Two-dimensional (2D) materials with van der Waals stacking have been reported to have extraordinary mechanical and electromechanical properties, which give them revolutionary potential in various fields. However, due to the atomic-scale thickness of these 2D materials, their fascinating properties cannot be effectively characterized in many cases using conventional measurement techniques. Based on typical microscopy techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM), a range of in situ microscopy techniques have been developed to systematically quantify the mechanical and electromechanical properties of 2D materials. This review highlights the advancements of in situ microscopy techniques for studying elasticity and fracture, adhesion and separation, structural superlubricity, as well as c-axis piezoresistivity and rotation angle-related transport of 2D materials. The methods and results of various microscopy experiments, including nanoindentation using AFM, pressurized bubble tests, self-retraction experiments, pull-to-peel methods and so on, are compared, and their respective advantages and limitations are discussed. Finally, we summarize the current challenges in these microscopy techniques and outline development opportunities.

Patterning and nanoribbon formation in graphene by hot punching

Large area graphene patterning is critical for applications. Current graphene patterning techniques, such as electron beam lithography and nano imprint lithography, are time consuming and can scale unfavorably with sample size. Resist-based masking and subsequent dry plasma etching can lead to high roughness edges with no alignment to the underlying graphene crystal orientations. In this study, we present hot punching as a novel and feasible method for patterning of chemical vapor deposition (CVD) graphene sheets supported by a polyvinylalcohol (PVA) layer. Additionally, we observe the effect of such hot punching on graphene supported by PVA via optical microscopy, Raman spectroscopy, AFM, and TEM, including wrinkling, strain and the formation of nanoribbons with crystallographically aligned and smooth edges due to fracturing. We present hot punching as a facile technique for the production of arrays of such nanoribbons.

Electrical Contacts to Graphene by Postgrowth Patterning of Cu Foil for the Low-Cost Scalable Production of Graphene-Based Flexible Electronics

Numerous studies have focused on graphene owing to its potential as a next-generation electronic material, considering its high conductivity, transparency, superior mechanical stiffness, and flexibility. However, cost-effective mass production of graphene-based electronics based on existing fabrication methods, such as graphene transfer and metal formation, remains a challenge. This study proposes a simple and efficient method for creating electrical contacts with graphene. The method involves patterning a Cu foil after graphene growth, enabling the low-cost scalable production of graphene-based flexible electronics. The fabricated graphene devices exhibited linear current-voltage characteristics, indicating good electrical contact between the postgrowth-patterned Cu electrodes and graphene. The proposed postgrowth patterning method allows for the fabrication of Cu-contacted graphene devices on large areas and various flexible substrates, including ultrathin and stretchable films (<10 μm). The feasibility of the proposed method for electronic devices was demonstrated by implementing gas and flexible force sensors. The proposed approach advances the field of graphene-based electronics and holds potential for practical applications in various electronic devices, paving the way for scalable, cost-effective, and flexible technology solutions.

Effect of Strain on the Photocatalytic Reaction of Graphitic Carbon Nitride: Insight from Single-Molecule Localization Microscopy

Strain engineering in two-dimensional nanomaterials holds significant potential for modulating the lattice and band structure, particularly through localized strain, which enables modulation at specific regions. Despite the remarkable effects of local strain, the relationships among local strain, spatial correlation of photogenerated charge carriers, and photocatalytic performance remain elusive. The current study coupled single-molecule localization microscopy with coordinate-based colocalization (CBC) analysis to explain these relationships. The methodology involved mapping the spatial distributions of photoinduced oxidation and reduction reaction sites across graphitic carbon nitride (g-C3N4) nanosheets, quantifying and spatially resolving their spatial correlation, and also evaluating their photocatalytic activity. The study examined 65 individual g-C3N4 nanosheets, revealing interparticle and intraparticle heterogeneity, which was classified based on their CBC score distributions. Among the 65 g-C3N4 nanosheets, type A nanosheets predominated (45 out of 65) and demonstrated both correlated and noncorrelated subregions along some wrinkles. In contrast, type B nanosheets (20 out of 65) were primarily characterized by noncorrelated subregions with minimal correlated localizations. The coexistence of both noncorrelated and correlated subregions inferred the structure of the wrinkles as folding wrinkles, which have larger tensile-strained areas than rippling wrinkles. Folding wrinkles promote colocalization through the formation of type I band alignment at tensile-strained subregions. This band alignment also enhances photocatalytic activity through a funneling effect and improved light absorption, leading to higher specific activity in correlated subregions compared to noncorrelated ones. The role of strain-induced band alignment in modulating the spatial correlation of the photoredox reaction and the photocatalytic performance at the subregion level is highlighted.

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.

Mixed-Chalcogen 2D Silver Phenylchalcogenides (AgE1-xExPh; E = S, Se, Te)

Alloying is a powerful strategy for tuning the electronic band structure and optical properties of semiconductors. Here, we investigate the thermodynamic stability and excitonic properties of mixed-chalcogen alloys of two-dimensional (2D) hybrid organic-inorganic silver phenylchalcogenides (AgEPh; E = S, Se, Te). Using a variety of structural and optical characterization techniques, we demonstrate that the AgSePh-AgTePh system forms homogeneous alloys (AgSe1-xTexPh, 0 ≤ x ≤ 1) across all compositions, whereas the AgSPh-AgSePh and AgSPh-AgTePh systems exhibit distinct miscibility gaps. Density functional theory calculations reveal that chalcogen mixing is energetically unfavorable in all cases but comparable in magnitude to the ideal entropy of mixing at room temperature. Because AgSePh and AgTePh have the same crystal structure (which is different from AgSPh), alloying is predicted to be thermodynamically preferred over phase separation in the case of AgSePh-AgTePh, whereas phase separation is predicted to be more favorable than alloying for both the AgSPh-AgSePh and AgSPh-AgTePh systems, in agreement with experimental observations. Homogeneous AgSe1-xTexPh alloys exhibit continuously tunable excitonic absorption resonances in the ultraviolet-visible range, while the emission spectrum reveals competition between exciton delocalization (characteristic of AgSePh) and localization behavior (characteristic of AgTePh). Overall, these observations provide insight into the thermodynamics of 2D silver phenylchalcogenides and the effect of lattice composition on electron-phonon interactions in 2D hybrid organic-inorganic semiconductors.

Controlled tuning of HOMO and LUMO levels in supramolecular nano-Saturn complexes

Optoelectronics usually deals with the fabrication of devices that can interconvert light and electrical energy using semiconductors. The modification of electronic properties is crucial in the field of optoelectronics. The tuning of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and their energy gaps is of paramount interest in this domain. Herein, three nano-Saturn supramolecular complex systems are designed, i.e., Al12N12@S-belt, Mg12O12@S-belt, and B12P12@S-belt, using S-belt as the host and Al12N12, Mg12O12, and B12P12 nanocages as guests. The high interaction energies ranging from -22.03 to -63.64 kcal mol-1 for the complexes demonstrate the stability of these host-guest complexes. Frontier molecular orbital (FMO) analysis shows that the HOMO of the complexes originates from the HOMO of the host, and the LUMO of the complexes originate entirely from the LUMO of the guests. The partial density of states (PDOS) analysis is in corroboration with FMO, which provides graphical illustration of the origin of HOMO and LUMO levels and the energy gaps. The shift in the electron density upon complexation is demonstrated by the natural bond orbital (NBO) charge analysis. For the Al12N12@S-belt and B12P12@S-belt complexes, the direction of electron density shift is towards the guest species, as indicated by the overall negative charge on encapsulated Al12N12 and B12P12. For the Mg12O12@S-belt complex, the overall NBO charge is positive, elaborating the direction of overall shift of electronic density towards the S-belt. Electron density difference (EDD) analysis verifies and corroborates with these findings. Noncovalent interaction index (NCI) and quantum theory of atoms in molecules (QTAIM) analyses signify that the complexes are stabilized via van der Waals interactions. Absorption analysis explains that all the complexes absorb in the ultraviolet (UV) region. Overall, this study explains the formation of stable host-guest supramolecular nano-Saturn complexes along with the controlled tuning of HOMO and LUMO levels over the host and guests, respectively.

Effective Strain Engineering of 2D Materials via Metal Deposition

2D materials, especially their monolayers, have garnered significant attention due to their unique electrical, optical and mechanical properties. Strain engineering is an effective way to modulate these properties. However, challenges remain in preventing slip or decoupling between the 2D material and the substrate due to the inherent weak van der Waals interactions. In this study, metal films are employed to apply strain to 2D materials. The high surface energy of metals helps to provide higher interaction forces, thereby improving strain transfer efficiency. Biaxial compressive and uniaxial tensile strain can be applied to monolayer MoS2, with the highest modulation rate of 542 and 161.7 meV/%, respectively, as characterized by photoluminescence (PL) spectra. Furthermore, this new approach can be broader to other 2D materials, such as WS2 or WSe2, allowing for precise control over strain manipulation. The work introduces a promising new approach for efficient and controllable strain engineering of 2D materials.

Width-dependent continuous growth of atomically thin quantum nanoribbons from nanoalloy seeds in chalcogen vapor

Nanoribbons (NRs) of atomic layer transition metal dichalcogenides (TMDs) can boost the rapidly emerging field of quantum materials owing to their width-dependent phases and electronic properties. However, the controllable downscaling of width by direct growth and the underlying mechanism remain elusive. Here, we demonstrate the vapor-liquid-solid growth of single crystal of single layer NRs of a series of TMDs (MeX2: Me = Mo, W; X = S, Se) under chalcogen vapor atmosphere, seeded by pre-deposited and respective transition metal-alloyed nanoparticles that also control the NR width. We find linear dependence of growth rate on supersaturation, known as a criterion for continues growth mechanism, which decreases with decreasing of NR width driven by the Gibbs-Thomson effect. The NRs show width-dependent photoluminescence and strain-induced quantum emission signatures with up to ≈ 90% purity of single photons. We propose the path and underlying mechanism for width-controllable growth of TMD NRs for applications in quantum optoelectronics.

Electric Field-Enhanced SERS Detection Using MoS2-Coated Patterned Si Substrate with Micro-Pyramid Pits

This study utilized semiconductor processing techniques to fabricate patterned silicon (Si) substrates with arrays of inverted pyramid-shaped micro-pits by etching. Molybdenum trioxide (MoO3) was then deposited on these patterned Si substrates using a thermal evaporation system, followed by two-stage sulfurization in a high-temperature furnace to grow MoS2 thin films consisting of only a few atomic layers. During the dropwise titration of Rhodamine 6G (R6G) solution, a longitudinal electric field was applied using a Keithley 2400 (Cleveland, OH, USA) source meter. Raman mapping revealed that under a 100 mV condition, the analyte R6G molecules were effectively confined within the pits. Due to its two-dimensional structure, MoS2 provides a high surface area and supports a surface-enhanced Raman scattering (SERS) charge transfer mechanism. The SERS results demonstrated that the intensity in the pits of the few-layer MoS2/patterned Si SERS substrate was approximately 274 times greater compared to planar Si, with a limit of detection reaching 10-5 M. The experimental results confirm that this method effectively resolves the issue of random distribution of analyte molecules during droplet evaporation, thereby enhancing detection sensitivity and stability.

Strain-induced bandgap engineering in 2D ψ-graphene materials: a first-principles study

High mechanical strength, excellent thermal and electrical conductivity, and tunable properties make two-dimensional (2D) materials attractive for various applications. However, the metallic nature of these materials restricts their applications in specific domains. Strain engineering is a versatile technique to tailor the distribution of energy levels, including bandgap opening between the energy bands. ψ-Graphene is a newly predicted 2D nanosheet of carbon atoms arranged in 5,6,7-membered rings. The half and fully hydrogenated (hydrogen-functionalized) forms of ψ-graphene are called ψ-graphone and ψ-graphane. Like ψ-graphene, ψ-graphone has a zero bandgap, but ψ-graphane is a wide-bandgap semiconductor. In this study, we have applied in-plane and out-of-plane biaxial strain on pristine and hydrogenated ψ-graphene. We have obtained a bandgap opening (200 meV) in ψ-graphene at 14% in-plane strain, while ψ-graphone loses its zero-bandgap nature at very low values of applied strain (both +1% and -1%). In contrast, fully hydrogenated ψ-graphene remains unchanged under the influence of mechanical strain, preserving its initial characteristic of having a direct bandgap. This behavior offers opportunities for these materials in various vital applications in photodetectors, solar cells, LEDs, pressure and strain sensors, energy storage, and quantum computing. The mechanical strain tolerance of pristine and fully hydrogenated ψ-graphene is observed to be -17% to +17%, while for ψ-graphone, it lies within the strain span of -16% to +16%.

Effect of triangular pits on the mechanical behavior of 2D MoTe2: a molecular dynamics study

CONTEXT

Among two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) stand out for their remarkable electronic, optical, and chemical properties. Their atomic thinness also imparts flexibility, making them ideal for flexible and wearable devices. However, our understanding of the mechanical characteristics of molybdenum ditelluride (MoTe2), particularly with defects such as pits, remains limited. Such defects, common in grown TMDs, degrade the mechanical properties and affect electronic and magnetic behaviors. This study uses molecular dynamics (MD) simulations of uniaxial and biaxial tensile loading performed on monolayer molybdenum ditelluride sheets of 2H phase containing triangular pits of varying vertex angles to investigate their fracture properties and visualize their crack propagation. From the stress-strain relationship, Young's modulus, fracture strain, ultimate tensile strength, and toughness for comparative analysis were calculated.

METHOD

Tensile loading simulations were performed in molecular dynamics (MD) software LAMMPS, using the Stillinger-Weber (SW) interatomic potential, under strain rate 108 s-1 at room temperature (300 K). From the stress-strain relationship obtained, we calculated Young's modulus, fracture strain, ultimate tensile strength, and toughness. Results showed that variations in pit edge length, angle, and perimeter significantly affected these properties in monolayer MoTe2. Regulated alteration of pit angle under constant simulation conditions resulted in improved uniaxial mechanical properties, while altering pit perimeters improved biaxial mechanical properties. Stress distribution was visualized using OVITO software. MoTe2 with pit defects was found to be more brittle than its pristine counterpart. This study provides foundational knowledge for advanced design strategies involving strain engineering in MoTe2 and similar TMDs.

Strain-tunable electronic anisotropy of the AlSb double-layer honeycomb structure

Two-dimensional (2D) materials show promising applications in nanoelectronic devices due to their excellent physical and chemical properties, large specific surface area, and good flexibility. 2D AlSb, a representative of a new class of two-dimensional materials with a double-layer honeycomb (DLHC) structure was recently obtained in experiments and was reported to be a direct band gap semiconductor. Strain engineering is an effective way of tuning the properties of 2D materials. Here, by first-principles calculations, the strain effects on the electronic structure of AlSb were investigated. An interesting inversion of band order near the bottom of the conduction band can be induced by applying uniaxial strain, which will introduce a large electronic anisotropy. In addition, under tensile strain along the armchair direction larger than 5%, band inversion occurs, indicating possible topological insulator properties of AlSb. Small carrier effective mass and strain tunable electronic anisotropy pave the way for the application of AlSb in future nanoelectronic devices.

Electrical contact property and control effects for stable T(H)-TaS2/C3B metal-semiconductor heterojunctions

Metal-semiconductor heterojunctions are the basis for developing new electronic devices. Here, T(H)-TaS2/C3B metal-semiconductor heterostructures are constructed by different phase T- and H-TaS2 monolayers combined with the C3B monolayer. The calculated corrected binding energies, phonon band structures, elastic constants, and molecular dynamics simulations indicated that both heterojunctions are highly stable, meaning that T(H)-TaS2/C3B heterojunctions possibly exist in experiments. The electronic property calculations showed that the intrinsic T(H)-TaS2/C3B heterojunction is an n(p)-type Schottky contact with a low Schottky barrier height (SBH), which is very important for the design of high-performance field-effect transistors. The electronic properties of the T(H)-TaS2/C3B heterojunctions can be controlled by varying the vertical strain and external electric field; however, the strain only resulted in a small change in the heterojunction SBH. Nevertheless, under external electrical field control, the T-TaS2/C3B heterojunction could manage a transition from an n-type Schottky contact to an n-type Ohmic contact and the H-TaS2/C3B heterojunction could be altered from a p-type Schottky contact to a p-type Ohmic contact. These findings provide theoretical insights into the electronic and electrical contact properties of the T(H)-TaS2/C3B heterojunction, which could be beneficial for developing n-type MOS and p-type MOS transistors.

Ag+-Induced Assembly of Pt Clusters for Photocatalytic Hydrogen Production

Cluster-assembled nanowires provide a unique strategy for the preparation of high-performance nanostructures. However, existing preparations are limited by complex processes and harsh reaction conditions. Here, Ag+ ions were utilized as a novel structure-directing agent to generate the self-assembly of Pt clusters to form ultrafine nanowires with a diameter of less than 5 nm. Electrospray ionization mass spectrometry (ESI-MS) and extended X-ray absorption fine structure (EXAFS) characterizations demonstrated that every Ag+ bridged two [Pt3(CO)3(μ2-CO)3]n2- clusters through coordination and formed a sandwich-like structure of [Pt3(CO)3(μ2-CO)3]nAg[Pt3(CO)3(μ2-CO)3]m3-. As a result, multiple sandwich-like structures of [Pt3(CO)3(μ2-CO)3]nAg[Pt3(CO)3(μ2-CO)3]m3- were established by Ag+ to form Pt nanowire superstructures {[Pt3(CO)6]nAg[Pt3(CO)6]mAg[Pt3(CO)6]x}∞ (abbreviated as Ag-Pt NWS). Our results demonstrate that the Pt nanowire superstructures showed promising cocatalytic performance for photocatalytic H2 production with the involvement of Ag+, which promises a desirable way to develop advanced functional nanomaterials.

Texture-Induced Strain in a WS2 Single Layer to Monitor Spin-Valley Polarization

Nanoscale-engineered surfaces induce regulated strain in atomic layers of 2D materials that could be useful for unprecedented photonics applications and for storing and processing quantum information. Nevertheless, these strained structures need to be investigated extensively. Here, we present texture-induced strain distribution in single-layer WS2 (1L-WS2) transferred over Si/SiO2 (285 nm) substrate. The detailed nanoscale landscapes and their optical detection are carried out through Atomic Force Microscopy, Scanning Electron Microscopy, and optical spectroscopy. Remarkable differences have been observed in the WS2 sheet localized in the confined well and at the periphery of the cylindrical geometry of the capped engineered surface. Raman spectroscopy independently maps the whole landscape of the samples, and temperature-dependent helicity-resolved photoluminescence (PL) experiments (off-resonance excitation) show that suspended areas sustain circular polarization from 150 K up to 300 K, in contrast to supported (on un-patterned area of Si/SiO2) and strained 1L-WS2. Our study highlights the impact of the dielectric environment on the optical properties of two-dimensional (2D) materials, providing valuable insights into the selection of appropriate substrates for implementing atomically thin materials in advanced optoelectronic devices.

Manipulating 2D Materials through Strain Engineering

This review explores the growing interest in 2D layered materials, such as graphene, h-BN, transition metal dichalcogenides (TMDs), and black phosphorus (BP), with a specific focus on recent advances in strain engineering. Both experimental and theoretical results are delved into, highlighting the potential of strain to modulate physical properties, thereby enhancing device performance. Various strain engineering methods are summarized, and the impact of strain on the electrical, optical, magnetic, thermal, and valleytronic properties of 2D materials is thoroughly examined. Finally, the review concludes by addressing potential applications and challenges in utilizing strain engineering for functional devices, offering valuable insights for further research and applications in optoelectronics, thermionics, and spintronics.

Two-Dimensional Semiconductors for State-of-the-Art Complementary Field-Effect Transistors and Integrated Circuits

As the trajectory of transistor scaling defined by Moore's law encounters challenges, the paradigm of ever-evolving integrated circuit technology shifts to explore unconventional materials and architectures to sustain progress. Two-dimensional (2D) semiconductors, characterized by their atomic-scale thickness and exceptional electronic properties, have emerged as a beacon of promise in this quest for the continued advancement of field-effect transistor (FET) technology. The energy-efficient complementary circuit integration necessitates strategic engineering of both n-channel and p-channel 2D FETs to achieve symmetrical high performance. This intricate process mandates the realization of demanding device characteristics, including low contact resistance, precisely controlled doping schemes, high mobility, and seamless incorporation of high- κ dielectrics. Furthermore, the uniform growth of wafer-scale 2D film is imperative to mitigate defect density, minimize device-to-device variation, and establish pristine interfaces within the integrated circuits. This review examines the latest breakthroughs with a focus on the preparation of 2D channel materials and device engineering in advanced FET structures. It also extensively summarizes critical aspects such as the scalability and compatibility of 2D FET devices with existing manufacturing technologies, elucidating the synergistic relationships crucial for realizing efficient and high-performance 2D FETs. These findings extend to potential integrated circuit applications in diverse functionalities.

Strain and Substrate-Induced Electronic Properties of Novel Mixed Anion-Based 2D ScHX2 (X = I/Br) Semiconductors

Exploration of compounds featuring multiple anions beyond the single-oxide ion, such as oxyhalides and oxyhydrides, offers an avenue for developing materials with the prospect of novel functionality. In this paper, we present the results for a mixed anion layered material, ScHX2 (X: Br, I) based on density functional theory. The result predicted the ScHX2 (X: Br, I) monolayers to be stable and semiconducting. Notably, the electronic and mechanical properties of the ScHX2 monolayers are comparable to well-established 2D materials like graphene and MoS2, rendering them highly suitable for electronic devices. Additionally, these monolayers exhibit an ability to adjust their band gaps and band edges in response to strain and substrate engineering, thereby influencing their photocatalytic applications.

Broken-gap energy alignment in two-dimensional van der Waals heterostructures for multifunctional tunnel diodes

Two-dimensional (2D) materials are promising platforms for future nanoelectronic technologies as they provide the building blocks for atomically thin devices, including switches, amplifiers, and oscillators. When 2D materials are layered on top of each other, forming van der Waals heterostructures (vdWHs), they can provide unique properties not possessed by the individual layers. Here we consider the vdWHs HfS2/MoTe2, HfS2/WTe2, 1T-HfS2/WTe2, TiS2/WSe2, TiS2/ZnO, and TiSe2/WTe2 as potential Esaki (or tunnel) diodes that can be incorporated into electronic devices. In this work, the strongly constrained and appropriately normed (SCAN) meta-generalised-gradient approximation (meta-GGA) functional is employed for the structural properties, whereas the Heyd-Scuseria-Ernzerhof (HSE) functional is used for the electronic properties. We establish that the band alignments in these systems form broken-band heterojunctions. We show that the electronic properties of the systems can be effectively modulated by applying lateral strain or an external electric field. Importantly, we demonstrate that the band gap of the vdWHs can be widened by up to 0.65 eV by applying an electric force field of -1 to +1 eV Å-1. This work demonstrates a set of 6 vdWHs with properties suitable for application as 2D Esaki tunnel diodes, 4 of which could be applied as multifunctional devices. These materials not only offer new device properties, but their small dimensions allow for the creation of ultrathin devices.

Liquid Phase Exfoliation of Few-Layer Non-Van der Waals Chromium Sulfide

Exfoliation of 2D non-Van der Waals (non-vdW) semiconductor nanoplates (NPs) from inorganic analogs presents many challenges ahead for further exploring of their advanced applications on account of the strong bonding energies. In this study, the exfoliation of ultrathin 2D non-vdW chromium sulfide (2D Cr2S3) by means of a combined facile liquid-phase exfoliation (LPE) method is successfully demonstrated. The morphology and structure of the 2D Cr2S3 material are systematically examined. Magnetic studies show an obvious temperature-dependent uncompensated antiferromagnetic behavior of 2D Cr2S3. The material is further loaded on TiO2 nanorod arrays to form an S-scheme heterojunction. Experimental measurements and density functional theory (DFT) calculations confirm that the formed TiO2@Cr2S3 S-scheme heterojunction facilitates the separation and transmission of photo-induced electron/hole pairs, resulting in a significantly enhanced photocatalytic activity in the visible region.

Electrically Controlled High Sensitivity Strain Modulation in MoS2 Field-Effect Transistors via a Piezoelectric Thin Film on Silicon Substrates

Strain can modulate bandgap and carrier mobilities in two-dimensional (2D) materials. Conventional strain-application methodologies relying on flexible/patterned/nanoindented substrates are limited by low thermal tolerance, poor tunability, and/or scalability. Here, we leverage the converse piezoelectric effect to electrically generate and control strain transfer from a piezoelectric thin film to electromechanically coupled 2D MoS2. Electrical bias polarity change across the piezo film tunes the nature of strain transferred to MoS2 from compressive (∼0.23%) to tensile (∼0.14%) as verified through Raman and photoluminescence spectroscopies and substantiated by density functional theory calculations. The device architecture, on silicon substrate, integrates an MoS2 field-effect transistor on a metal-piezoelectric-metal stack enabling strain modulation of transistor drain current (130×), on/off ratio (150×), and mobility (1.19×) with high precision, reversibility, and resolution. Large, tunable tensile (1056) and compressive (-1498) strain gauge factors, electrical strain modulation, and high thermal tolerance promise facile integration with silicon-based CMOS and micro-electromechanical systems.

Enhancing Layer-Engineered Interlayer Exciton Emission and Valley Polarization in van der Waals Heterostructures via Strain

Layer-engineered interlayer excitons from heterostructures of transition-metal dichalcogenides (TMDCs) exhibit a rich variety of emissive states and intriguing valley spin-selection rules, the effective modulation of which is crucial for excitonic physics and related device applications. Strain or high pressure provides the possibility to tune the energy of the interlayer excitons; however, the reported emission intensity is substantially quenched, which greatly limits their practical application in optoelectronic devices. Here, via applying uniaxial strain based on polyvinyl alcohol (PVA) encapsulation technique, we report enhanced layer-engineered interlayer exciton emission intensity with largely modulated emission energy in WSe2/WS2 heterobilayer and heterotrilayer. Both momentum-direct and momentum-indirect interlayer excitons were observed, and their emission energies show an opposite shift tendency upon applied strain, which agrees with our DFT calculations. We further demonstrate that intralayer and interlayer exciton states with low phonon interactions can be modulated through the mechanical strain applied to the PVA substrate at low temperatures. Due to strain-induced breaking of the 3-fold rotational symmetry, we observe the enhanced valley polarization of interlayer excitons. Our study contributes to the understanding and modulation of the optical properties of interlayer excitons, which could be exploited for optoelectronic device applications.

Enhanced Electromechanical Response Due to Inhomogeneous Strain in Monolayer MoS2

2D transition metal dichalcogenides (TMDs) exhibit exceptional resilience to mechanical deformation. Applied strain can have pronounced effects on properties such as the bandgaps and exciton dynamics of TMDs, via deformation potentials and electromechanical coupling. In this work, we use piezoresponse force microscopy to show that the inhomogeneous strain from nanobubbles produces dramatic, localized enhancements of the electromechanical response of monolayer MoS2. Nanobubbles with diameters under 100 nm consistently produce an increased piezoresponse that follows the features' topography, while larger bubbles exhibit a halo-like profile, with maximum piezoresponse near the periphery. We show that spatial filtering enables these effects to be eliminated in the quantitative determination of effective piezoelectric or flexoelectric coefficients. Numerical strain modeling reveals a correlation between the hydrostatic strain gradient and the effective piezoelectric coefficient in large MoS2 nanobubbles, suggesting a localized variation in electromechanical coupling due to symmetry reduction induced by inhomogeneous strain.

Strain distribution in WS2 monolayers detected through polarization-resolved second harmonic generation

Two-dimensional (2D) graphene and graphene-related materials (GRMs) show great promise for future electronic devices. GRMs exhibit distinct properties under the influence of the substrate that serves as support through uneven compression/ elongation of GRMs surface atoms. Strain in GRM monolayers is the most common feature that alters the interatomic distances and band structure, providing a new degree of freedom that allows regulation of their electronic properties and introducing the field of straintronics. Having an all-optical and minimally invasive detection tool that rapidly probes strain in large areas of GRM monolayers, would be of great importance in the research and development of novel 2D devices. Here, we use Polarization-resolved Second Harmonic Generation (P-SHG) optical imaging to identify strain distribution, induced in a single layer of WS2 placed on a pre-patterned Si/SiO2 substrate with cylindrical wells. By fitting the P-SHG data pixel-by-pixel, we produce spatially resolved images of the crystal armchair direction. In regions where the WS2 monolayer conforms to the pattern topography, a distinct cross-shaped pattern is evident in the armchair image owing to strain. The presence of strain in these regions is independently confirmed using a combination of atomic force microscopy and Raman mapping.

Improved Strain Transfer Efficiency in Large-Area Two-Dimensional MoS2 Obtained by Gold-Assisted Exfoliation

Strain engineering represents a pivotal approach to tailoring the optoelectronic properties of two-dimensional (2D) materials. However, typical bending experiments often encounter challenges, such as layer slippage and inefficient transfer of strain from the substrate to the 2D material, hindering the realization of their full potential. In our study, using molybdenum disulfide (MoS2) as a model 2D material, we have demonstrated that layers obtained through gold-assisted exfoliation on flexible polycarbonate substrates can achieve high-efficient strain transfer while also mitigating slippage effects, owing to the strong interfacial interaction established between MoS2 and gold. We employ differential reflectance and Raman spectroscopy for monitoring strain changes. We successfully applied uniaxial strains of up to 3% to trilayer MoS2, resulting in a notable energy shift of 168 meV. These values are comparable only to those obtained in encapsulated samples with organic polymers.

Constructing Tunable Electrides on Monolayer Transition Metal Dichalcogenides

Electrides have emerged as promising materials with exotic properties due to the presence of localized electrons detached from all atoms. Despite the continuous discovery of many new electrides, most of them are based on atypical compositions, and their applications require an inert surface structure to passivate reactive excess electrons. Here, we demonstrate a different route to attain tunable electrides. We first report that monolayer transition metal dichalcogenides (TMDCs) exhibit weak electride characteristics, which is the remainder of the electride feature of the transition metal sublattice. By introducing chalcogen vacancies, the enhanced electride characteristics are comparable to those of known electrides. Since the precise tailoring of the chalcogen vacancy concentration has been achieved experimentally, we proposed that TMDCs can be used to build electrides with controllable intensities. Furthermore, we demonstrate that the electride states at the chalcogen vacancy of monolayer TMDCs will play an important role in catalyzing hydrogen evolution reactions.

Compact computer controlled biaxial tensile device for low-temperature transport measurements of layered materials

A biaxial tensile device for the transport study of layered materials is described. The device is mounted on the standard 24 pin zero force connector and can be moved between various setups. The compact design of the device makes it suitable for a wide range of studies. In our case, it is placed inside a 50 mm diameter chamber in the cryocooler and is used in the temperature range 9-310 K. A sample is glued in the center of a polyimide cruciform substrate, the ends of which are connected to a tension system driven by four computer-controlled stepper motors providing tensile force up to 30 N. Computer simulation results and their experimental verification show that tensile strain along one axis depends on the tensile load along the perpendicular direction, and this dependence turns out to be relatively strong and exceeds 40%. The operation of the device is demonstrated by studying the effect of deformation on the electrical conductivity of the layered compound 2H-NbS2.

Strain and Electric Field Engineering of G-ZnO/SnXY (X, Y = S, Se) S-Scheme Heterostructures for Photocatalyst and Electronic Device Applications: A Hybrid DFT Calculation

Using hybrid density functional theory calculations, we systematically study the biaxial strain and electric field modulated electronic properties of g-ZnO/SnS2, g-ZnO/SnSe2, and g-ZnO/SnSSe S-scheme van der Waals heterostructures (vdWHs). g-ZnO/SnS2 and g-ZnO/SnSSe are found to be promising photocatalysts for water splitting with high solar-to-hydrogen efficiencies, even under acidic, alkaline, and high-stress conditions. The strain effect on the bandgaps of g-ZnO/SnXY is explained in detail according to the correlation between geometry structure and orbital hybridization of SnXY, which could help understand the strain-induced band structure evolutions in other SnXY (X, Y = S, Se)-based vdWHs. It is surprising that under an external electric field, g-ZnO/SnS2, g-ZnO/SnSe2, and g-ZnO/SnSSe can offer the occupied nearly free-electron (NFE) states. In many materials, NFE states are usually unoccupied and is not conducive to the charge transport. The NFE state in g-ZnO/SnSe2 is the most sensitive to the electric field and might be promising electron transport channel in nanoelectronic devices. g-ZnO/SnSe2 might also have application potential in gas sensors and high-temperature superconductors.

Strain Engineering of Ion-Coordinated Nanochannels in Nanocellulose

Expanding the interlayer spacing plays a significant role in improving the conductivity of a cellulose-based conductor. However, it remains a challenge to regulate the cellulose nanochannel expanded by ion coordination. Herein, starting from multiscale mechanics, we proposed a strain engineering method to regulate the interlayer spacing of the cellulose nanochannels. First-principles calculations were conducted to select the most suitable ions for coordination. Large-scale molecular dynamics simulations were performed to reveal the mechanism of interlayer spacing expansion by the ion cross-linking. Combining the shear-lag model, we established the relationship between interfacial cross-link density and interlayer spacing of an ion-coordinated cellulose nanochannel. Consequently, fast ion transport and current regulation were realized via the strain engineering of nanochannels, which provides a promising strategy for the current regulation of a cellulose-based conductor.

Tuning instability in suspended monolayer 2D materials

Monolayer two-dimensional (2D) materials possess excellent in-plane mechanical strength yet extremely low bending stiffness, making them particularly susceptible to instability, which is anticipated to have a substantial impact on their physical functionalities such as 2D-based Micro/Nanoelectromechanical systems (M/NEMS), nanochannels, and proton transport membrane. In this work, we achieve quantitatively tuning instability in suspended 2D materials including monolayer graphene and MoS2 by employing a push-to-shear strategy. We comprehensively examine the dynamic wrinkling-splitting-smoothing process and find that monolayer 2D materials experience stepwise instabilities along with different recovery processes. These stepwise instabilities are governed by the materials' geometry, pretension, and the elastic nonlinearity. We attribute the different instability and recovery paths to the local stress redistribution in monolayer 2D materials. The tunable instability behavior of suspended monolayer 2D materials not only allows measuring their bending stiffness but also opens up new opportunities for programming the nanoscale instability pattern and even physical properties of atomically thin films.

Emerging Spintronic Materials and Functionalities

The explosive growth of the information era has put forward urgent requirements for ultrahigh-speed and extremely efficient computations. In direct contrary to charge-based computations, spintronics aims to use spins as information carriers for data storage, transmission, and decoding, to help fully realize electronic device miniaturization and high integration for next-generation computing technologies. Currently, many novel spintronic materials have been developed with unique properties and multifunctionalities, including organic semiconductors (OSCs), organic-inorganic hybrid perovskites (OIHPs), and 2D materials (2DMs). These materials are useful to fulfill the demand for developing diverse and advanced spintronic devices. Herein, these promising materials are systematically reviewed for advanced spintronic applications. Due to the distinct chemical and physical structures of OSCs, OIHPs, and 2DMs, their spintronic properties (spin transport and spin manipulation) are discussed separately. In addition, some multifunctionalities due to photoelectric and chiral-induced spin selectivity (CISS) are overviewed, including the spin-filter effect, spin-photovoltaics, spin-light emitting devices, and spin-transistor functions. Subsequently, challenges and future perspectives of using these multifunctional materials for the development of advanced spintronics are presented.

Probing the Strain Direction-Dependent Nonmonotonic Optical Bandgap Modulation of Layered Violet Phosphorus

Recent theoretical investigations have well-predicted strain-induced nonmonotonic optical band gap variations in low-dimensional materials. However, few two-dimensional (2D) materials are experimentally confirmed to exhibit nonmonotonic optical band gap variation under varying strain. Here, a strain-induced nonmonotonic optical bandgap variation in violet phosphorus (VP) nanosheets is observed, as evidenced by photoluminescence spectroscopy, which is reported in a few other 2D materials in knowledge. The optical bandgap variations are characterized to show the modulation rates of 41 and -24 meV/% with compression and tensile strains, respectively. Remarkably, first-principle calculations predict and clarify the nonmonotonic modulation accurately, highlighting its relationship with the strain direction-dependent asymmetric distribution of conduction band minimum wavefunctions, demonstrating that this unique nonmonotonic optical bandgap modulation is determined by the distinctive crystal structure of VP. This work provides a deep insight into the design of 2D materials toward optoelectronic and photoelectrochemical applications via strain engineering.

Strain Engineering of Twisted Bilayer Graphene: The Rise of Strain-Twistronics

The layer-by-layer stacked van der Waals structures (termed vdW hetero/homostructures) offer a new paradigm for materials design-their physical properties can be tuned by the vertical stacking sequence as well as by adding a mechanical twist, stretch, and hydrostatic pressure to the atomic structure. In particular, simple twisting and stacking of two layers of graphene can form a uniform and ordered Moiré superlattice, which can effectively modulate the electrons of graphene layers and lead to the discovery of unconventional superconductivity and strong correlations. However, the twist angle of twisted bilayer graphene (tBLG) is almost unchangeable once the interlayer stacking is determined, while applying mechanical elastic strain provides an alternative way to deeply regulate the electronic structure by controlling the lattice spacing and symmetry. In this review, diverse experimental advances are introduced in straining tBLG by in-plane and out-of-plane modes, followed by the characterizations and calculations toward quantitatively tuning the strain-engineered electronic structures. It is further discussed that the structural relaxation in strained Moiré superlattice and its influence on electronic structures. Finally, the conclusion entails prospects for opportunities of strained twisted 2D materials, discussions on existing challenges, and an outlook on the intriguing emerging field, namely "strain-twistronics".

Probing the tunable multi-cone band structure in Bernal bilayer graphene

Bernal bilayer graphene (BLG) offers a highly flexible platform for tuning the band structure, featuring two distinct regimes. One is a tunable band gap induced by large displacement fields. Another is a gapless metallic band occurring at low fields, featuring rich fine structure consisting of four linearly dispersing Dirac cones and van Hove singularities. Even though BLG has been extensively studied experimentally, the evidence of this band structure is still elusive, likely due to insufficient energy resolution. Here, we use Landau levels as markers of the energy dispersion and analyze the Landau level spectrum in a regime where the cyclotron orbits of electrons or holes in momentum space are small enough to resolve the distinct mini Dirac cones. We identify the presence of four Dirac cones and map out topological transitions induced by displacement field. By clarifying the low-energy properties of BLG bands, these findings provide a valuable addition to the toolkit for graphene electronics.

Core/Shell-Like Localized Emission at Atomically Thin Semiconductor-Au Interface

Localized emission in atomically thin semiconductors has sparked significant interest as single-photon sources. Despite comprehensive studies into the correlation between localized strain and exciton emission, the impacts of charge transfer on nanobubble emission remains elusive. Here, we report the observation of core/shell-like localized emission from monolayer WSe2 nanobubbles at room temperature through near-field studies. By altering the electronic junction between monolayer WSe2 and the Au substrate, one can effectively adjust the semiconductor to metal junction from a Schottky to an Ohmic junction. Through concurrent analysis of topography, potential, tip-enhanced photoluminescence, and a piezo response force microscope, we attribute the core/shell-like emissions to strong piezoelectric potential aided by induced polarity at the WSe2-Au Schottky interface which results in spatial confinement of the excitons. Our findings present a new approach for manipulating charge confinement and engineering localized emission within atomically thin semiconductor nanobubbles. These insights hold implications for advancing the nano and quantum photonics with low-dimensional semiconductors.

Nanoimprint-induced strain engineering of two-dimensional materials

The high stretchability of two-dimensional (2D) materials has facilitated the possibility of using external strain to manipulate their properties. Hence, strain engineering has emerged as a promising technique for tailoring the performance of 2D materials by controlling the applied elastic strain field. Although various types of strain engineering methods have been proposed, deterministic and controllable generation of the strain in 2D materials remains a challenging task. Here, we report a nanoimprint-induced strain engineering (NISE) strategy for introducing controllable periodic strain profiles on 2D materials. A three-dimensional (3D) tunable strain is generated in a molybdenum disulfide (MoS2) sheet by pressing and conforming to the topography of an imprint mold. Different strain profiles generated in MoS2 are demonstrated and verified by Raman and photoluminescence (PL) spectroscopy. The strain modulation capability of NISE is investigated by changing the imprint pressure and the patterns of the imprint molds, which enables precise control of the strain magnitudes and distributions in MoS2. Furthermore, a finite element model is developed to simulate the NISE process and reveal the straining behavior of MoS2. This deterministic and effective strain engineering technique can be easily extended to other materials and is also compatible with common semiconductor fabrication processes; therefore, it provides prospects for advances in broad nanoelectronic and optoelectronic devices.

First principles predictions of structural, electronic and topological properties of two-dimensional Janus Ti2N2XI (X = Br, Cl) structures

Motivated by the report of the giant Rashba effect in ternary layered compounds BiTeX, we consider two Janus structured compounds Ti2N2XI (X = Br, Cl) of the same ternary family exhibiting a 1 : 1 : 1 stoichiometric ratio. Broken inversion symmetry in the Janus structure, together with its unique electronic structure exhibiting anti-crossing states formed between Ti-d states and strong spin-orbit coupled I-p states, generates large Rashba cofficients of 2-3 eV Å for these compounds, classifying them as strong Rashba compounds. The anti-crossing features of the first-principles calculated electronic structure also result in non-trivial topology, combining two quantum phenomena - Rashba effect and non-trivial topology - in the same materials. This makes Janus TiNI compounds candidate materials for two-dimensional composite quantum materials. The situation becomes further promising by the fact that the properties are found to exhibit extreme sensitivity and tunability upon application of uniaxial strain.

Large second-order susceptibility from a quantized indium tin oxide monolayer

Due to their high optical transparency and electrical conductivity, indium tin oxide thin films are a promising material for photonic circuit design and applications. However, their weak optical nonlinearity has been a substantial barrier to nonlinear signal processing applications. In this study, we show that an atomically thin (~1.5 nm) indium tin oxide film in the form of an air/indium tin oxide/SiO2 quantum well exhibits a second-order susceptibility χ2 of ~1,800 pm V-1. First-principles calculations and quantum electrostatic modelling point to an electronic interband transition resonance in the asymmetric potential energy of the quantum well as the reason for this large χ2 value. As the χ2 value is more than 20 times higher than that of the traditional nonlinear LiNbO3 crystal, our indium tin oxide quantum well design can be an important step towards nonlinear photonic circuit applications.

Systematic DFT Modeling van der Waals Heterostructures from a Complete Configurational Basis Applied to γ-PC/WS2

Periodic boundary conditions in density functional theory (DFT)-based modeling of bilayer van der Waals heterostructures introduce an artificial lock to a metastable configuration. Depending on the initial supercell, geometric optimization may reach local energy minima at a fixed twist-angle in a restricted strain-space. In this work, an algorithm was introduced for generating a complete scope of ways to combine two monolayer unit cells into a common supercell. In its application to γ-PC/WS2, 18,123 bilayer supercells were derived, for which the constituting monolayers possessed isotropic strains, anisotropic strains, or intralayer shear strains. Based on analysis, 45 isotropically strained configurations were carefully chosen for optimization by DFT. Geometric and energetic features and band structures were revealed and compared, following the variations at different strains and twist-angles. As such, this case study brought to resolution the impacts of supercell construction on DFT's outcomes and the merits of in-depth screening of the different options. Repetitions and extensions to the demonstrated approach may be applied to characterize van der Waals heterostructures and derivatives in the future.

Proposing TODD-graphene as a novel porous 2D carbon allotrope designed for superior lithium-ion battery efficiency

The category of 2D carbon allotropes has gained considerable interest due to its outstanding optoelectronic and mechanical characteristics, which are crucial for various device applications, including energy storage. This study uses density functional theory calculations, ab initio molecular dynamics (AIMD), and classical reactive molecular dynamics (MD) simulations to introduce TODD-Graphene, an innovative 2D planar carbon allotrope with a distinctive porous arrangement comprising 3-8-10-12 carbon rings. TODD-G exhibits intrinsic metallic properties with a low formation energy and stability in thermal and mechanical behavior. Calculations indicate a substantial theoretical capacity for adsorbing Li atoms, revealing a low average diffusion barrier of 0.83 eV. The metallic framework boasts excellent conductivity and positioning TODD-G as an active layer for superior lithium-ion battery efficiency. Charge carrier mobility calculations for electrons and holes in TODD-G surpass those of graphene. Classical reactive MD simulation results affirm its structural integrity, maintaining stability without bond reconstructions at 2200 K.

Technology and Integration Roadmap for Optoelectronic Memristor

Optoelectronic memristors (OMs) have emerged as a promising optoelectronic Neuromorphic computing paradigm, opening up new opportunities for neurosynaptic devices and optoelectronic systems. These OMs possess a range of desirable features including minimal crosstalk, high bandwidth, low power consumption, zero latency, and the ability to replicate crucial neurological functions such as vision and optical memory. By incorporating large-scale parallel synaptic structures, OMs are anticipated to greatly enhance high-performance and low-power in-memory computing, effectively overcoming the limitations of the von Neumann bottleneck. However, progress in this field necessitates a comprehensive understanding of suitable structures and techniques for integrating low-dimensional materials into optoelectronic integrated circuit platforms. This review aims to offer a comprehensive overview of the fundamental performance, mechanisms, design of structures, applications, and integration roadmap of optoelectronic synaptic memristors. By establishing connections between materials, multilayer optoelectronic memristor units, and monolithic optoelectronic integrated circuits, this review seeks to provide insights into emerging technologies and future prospects that are expected to drive innovation and widespread adoption in the near future.

Ultrafast acousto-optic modulation at the near-infrared spectral range by interlayer vibrations

The acousto-optic modulation over a broad near-infrared (NIR) spectrum with high speed, excellent integrability, and relatively simple scheme is crucial for the application of next-generation opto-electronic and photonic devices. This study aims to experimentally demonstrate ultrafast acousto-optic phenomena in the broad NIR spectral range of 0.77-1.1 eV (1130-1610 nm). Hundreds of GHz of light modulation are revealed in an all-optical configuration by combining ultrafast optical spectroscopy and light-sound conversion in 10-20 nm-thick bismuth selenide (Bi2Se3) van der Waals thin films. The modified optical transition energy and the line shape in the NIR band indicate phonon-photon interactions, resulting in a modulation of optical characteristics by the photoexcited interlayer vibrations in Bi2Se3. This all-optical, ultrafast acousto-optic modulation approach may open avenues for next-generation nanophotonic applications, including optical communications and processing, due to the synergistic combination of large-area capability, high photo-responsivity, and frequency tunability in the NIR spectral range.

Uniform, Strain-Free, Large-Scale Graphene and h-BN Monolayers Enabled by Hydrogel Substrates

Translation of the unique properties of 2D monolayers from non-scalable micron-sized samples to macroscopic scale is a longstanding challenge obstructed by the substrate-induced strains, interface nonuniformities, and sample-to-sample variations inherent to the scalable fabrication methods. So far, the most successful strategies to reduce strain in graphene are the reduction of the interface roughness and lattice mismatch by using hexagonal boron nitride (h-BN), with the drawback of limited uniformity and applicability to other 2D monolayers, and liquid water, which is not compatible with electronic devices. This work demonstrates a new class of substrates based on hydrogels that overcome these limitations and excel h-BN and water substrates at strain relaxation enabling superiorly uniform and reproducible centimeter-sized sheets of unstrained monolayers. The ultimate strain relaxation and uniformity are rationalized by the extreme structural adaptability of the hydrogel surface owing to its high liquid content and low Young's modulus, and are universal to all 2D materials irrespective of their crystalline structure. Such platforms can be integrated into field effect transistors and demonstrate enhanced charge carrier mobilities in graphene. These results present a universal strategy for attaining uniform and strain-free sheets of 2D materials and underline the opportunities enabled by interfacing them with soft matter.

Structural, electronic and thermoelectric properties of monolayer TiSe2

CONTEXT AND RESULTS

In this work the first-principles calculations of the structural, electronic and thermoelectric properties of monolayer TiSe2 are presented. The optimized lattice parameter of monolayer TiSe2 shows excellent agreement with the experimental value. The computed band structure and density of states calculations predict metallic nature of monolayer TiSe2 with overlapping of 0.44 eV between the lowest conduction band and top valance band at high symmetry point M. The position of pseudogap formed by Ti-3d orbitals near the Fermi level confirms the mechanical stability of monolayer TiSe2. Due to the influence of positive strain (tensile strain), the Ti-Se bond length increases and the layer height decreases. The applied tensile strain changes the metallic nature of TiSe2 into a semiconductor with opening of bandgap. It has also been observed that the positions of conduction band minima and valance band maxima change with strain. The charge analysis shows that charge transfer from Ti to Se atom increases when tensile strain is applied, while an opposite trend is observed with compression. The computed thermoelectric coefficients i.e. Seeback coefficient, power factor and figure of merit are in good agreement with the experimental data. The temperature dependence of these coefficients is also reported.

COMPUTATIONAL METHOD

The density functional theory based calculations are reported employing the PBE-GGA ansatz using the plane wave-pseudopotential method embodied in the Quantum ESPRESSO package. The self-consistent field calculations are performed over a dense Monkhorst-Pack net of 12 × 12 × 1 k-points. The energy convergence criteria for the self-consistent field calculation were set to 10-6 Ry/atom with a cutoff energy of 90 Ry. The thermoelectric properties are computed by combining the band structure calculations with the Boltzmann transport equation using Boltztrap2 peckage.

2D Semi-Metallic Hafnium Ditelluride: A Novel Nonlinear Optical Material for Ultrafast and Ultranarrow Photonics Applications

2D semi-metallic hafnium ditelluride material is used in several applications such as solar steam generation, gas sensing, and catalysis owing to its strong near-infrared absorbance, high sensitivity, and distinctive electronic structure. The zero-bandgap characteristics, along with the thermal and dynamic stability of 2D-HfTe2, make it a desirable choice for developing long-wavelength-range photonics devices. Herein, the HfTe2 -nanosheets are prepared using the liquid-phase exfoliation method, and their superior nonlinear optical properties are demonstrated by the obtained modulation depth of 11.9% (800 nm) and 6.35% (1560 nm), respectively. In addition, the observed transition from saturable to reverse saturable absorption indicates adaptability of the prepared material in nonlinear optics. By utilizing a side polished fiber-based HfTe2 -saturable absorber (SA) inside an Er-doped fiber laser cavity, a mode-locked laser with 724 fs pulse width and 56.63 dB signal-to-noise ratio (SNR) is realized for the first time. The generated laser with this SA has the second lowest mode-locking pump threshold (18.35 mW), among the other 2D material based-SAs, thus paving the way for future laser development with improved efficiency and reduced thermal impact. Finally, employing this HfTe2 -SA, a highly stable single-frequency fiber laser (SNR ≈ 74.56 dB; linewidth ≈ 1.268 kHz) is generated for the first time, indicating its promising ultranarrow photonic application.

Dynamical control of nanoscale light-matter interactions in low-dimensional quantum materials

Tip-enhanced nano-spectroscopy and -imaging have significantly advanced our understanding of low-dimensional quantum materials and their interactions with light, providing a rich insight into the underlying physics at their natural length scale. Recently, various functionalities of the plasmonic tip expand the capabilities of the nanoscopy, enabling dynamic manipulation of light-matter interactions at the nanoscale. In this review, we focus on a new paradigm of the nanoscopy, shifting from the conventional role of imaging and spectroscopy to the dynamical control approach of the tip-induced light-matter interactions. We present three different approaches of tip-induced control of light-matter interactions, such as cavity-gap control, pressure control, and near-field polarization control. Specifically, we discuss the nanoscale modifications of radiative emissions for various emitters from weak to strong coupling regime, achieved by the precise engineering of the cavity-gap. Furthermore, we introduce recent works on light-matter interactions controlled by tip-pressure and near-field polarization, especially tunability of the bandgap, crystal structure, photoluminescence quantum yield, exciton density, and energy transfer in a wide range of quantum materials. We envision that this comprehensive review not only contributes to a deeper understanding of the physics of nanoscale light-matter interactions but also offers a valuable resource to nanophotonics, plasmonics, and materials science for future technological advancements.