Large-Area Epitaxial Growth of Transition Metal Dichalcogenides.

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Giant nonlinear optical wave mixing in a van der Waals correlated insulator.

Optical nonlinearities are one of the most fascinating properties of two-dimensional (2D) materials. While tremendous efforts have been made to find and optimize the second-order optical nonlinearity in enormous 2D materials, opportunities to explore higher-order ones are elusive because of the much lower efficiency. Here, we report the giant high odd-order optical nonlinearities in centrosymmetric correlated van der Waals insulator manganese phosphorus triselenide. When illuminated by two near-infrared femtosecond lasers, the sample generates a series of profound four- and six-wave mixing outputs. The near-infrared third-order nonlinear susceptibility reaches near the highest record values of 2D materials. Comparative measurements to other prototypical nonlinear optical materials [lithium niobate, gallium(II) selenide, and tungsten disulfide] reveal its extraordinary wave mixing efficiency. The wave mixing processes are further used for nonlinear optical waveguide with multicolor emission. Our work highlights the promising prospect for future research of the nonlinear light-matter interactions in the correlated 2D system and for potential nonlinear photonic applications.

Two-Dimensional Exciton Oriented Diffusion via Periodic Potentials.

Excitonic devices operate based on excitons, which can be excited by photons as well as emitting photons and serve as a medium for photon-carrier conversion. Excitonic devices are expected to combine the advantages of both the high response rate of photonic devices and the high integration of electronic devices simultaneously. However, because of the neutral feature, exciton transport is generally achieved via diffusion rather than using electric fields, and the efficient control of exciton flux directionality has always been difficult. In this work, a precisely designed one-dimensional periodic nanostructure (1DPS) is used to introduce periodic strain field along with resonant mode to the WS2 monolayer, achieving exciton oriented diffusion with a 7.6-fold exciton diffusion coefficient enhancement relative to that of intrinsic, while enhancing the excitonic emission intensity by a factor of 10 and reducing exciton saturation threshold power by 2 orders of magnitude. Based on the analysis of the density functional theory (DFT) and the finite-element method (FEM), we attribute the anisotropy of exciton diffusion to exciton funneling induced by periodic potentials, which do not require excessive potential height difference for an efficient oriented diffusion. As a result of resonant emission, the exciton diffusion is dragged into the nonlinear regime owing to the high exciton density close to saturation, which improves the exciton diffusion coefficient and diffusion anisotropy more appreciably.

Critical-Layered MoS2 for the Enhancement of Supercontinuum Generation in Photonic Crystal Fibre.

Supercontinuum generation (SCG) from silica-based photonic crystal fibers (PCFs) is of highly technological significance from microscopy to metrology, but has been hindered by silica's relatively low intrinsic optical nonlinearity. The prevailing approaches of filling PCF with nonlinear gases or liquids can endow fibre with enhanced optical nonlinearity and boosted SCG efficiency, yet these hybrids are easily plagued by fusion complexity, environmental incompatibility or transmission mode instability. Here this work presents a strategy of embedding solid-state 2D MoS2 atomic layers into the air-holes of PCF to efficiently enhance SCG. This work demonstrates a 4.8 times enhancement of the nonlinear coefficient and a 70% reduction of the threshold power for SCG with one octave spanning in the MoS2-PCF hybrid. Furthermore, this work finds that the SCG enhancement is highly layer-dependent, which only manifests for a real 2D regime within the thickness of five atomic layers. Theoretical calculations reveal that the critical thickness arises from the trade-off among the layer-dependent enhancement of the nonlinear coefficient, leakage of fundamental mode and redshift of zero-dispersion wavelength. This work provides significant advances toward efficient SCG, and highlights the importance of matching an appropriate atomic layer number in the design of functional 2D material optical fibers.

All-2D-Materials Subthreshold-Free Field-Effect Transistor with Near-Ideal Switching Slope.

The Boltzmann Tyranny, set by thermionic statistics, dictates the lower limit of switching slope (SS) of a MOSFET to be 60 mV/dec, the fundamental barrier for low-dissipative electronics. The large SS leads to nonscalable voltage, significant leakage, and power consumption, particularly at short channels, making transistor scaling an intimidating challenge. In recent decades, an array of steep-slope transistors has been proposed; none is close to an ideal switch with ultimately abrupt switching (SS ∼ 0 mV/dec) between the binary logic states. We demonstrated an all-2D-materials van-der-Waals-heterostructure (vdW)-based FET that exhibits ultrasteep switching (0.33 mV/dec), a large on/off current ratio (∼107), and an ultralow off current (∼0.1 pA). The "Subthreshold-Free" operation achieved by the collective behavior of functional materials enables FET switching directly from the OFF-state to the ON-state with entirely eliminated subthreshold region, behaving as the ideal logic switch. Two-inch wafer-scale device fabrication is demonstrated. Boosted by device innovation and emerging materials, the research presents an advancement in achieving the "beyond-Boltzmann" transistors, overcoming one of the CMOS electronics' most infamous technology barriers that have plagued the research community for decades.

Interfacial epitaxy of multilayer rhombohedral transition-metal dichalcogenide single crystals.

Rhombohedral-stacked transition-metal dichalcogenides (3R-TMDs), which are distinct from their hexagonal counterparts, exhibit higher carrier mobility, sliding ferroelectricity, and coherently enhanced nonlinear optical responses. However, surface epitaxial growth of large multilayer 3R-TMD single crystals is difficult. We report an interfacial epitaxy methodology for their growth of several compositions, including molybdenum disulfide (MoS2), molybdenum diselenide, tungsten disulfide, tungsten diselenide, niobium disulfide, niobium diselenide, and molybdenum sulfoselenide. Feeding of metals and chalcogens continuously to the interface between a single-crystal Ni substrate and grown layers ensured consistent 3R stacking sequence and controlled thickness from a few to 15,000 layers. Comprehensive characterizations confirmed the large-scale uniformity, high crystallinity, and phase purity of these films. The as-grown 3R-MoS2 exhibited room-temperature mobilities up to 155 and 190 square centimeters per volt second for bi- and trilayers, respectively. Optical difference frequency generation with thick 3R-MoS2 showed markedly enhanced nonlinear response under a quasi-phase matching condition (five orders of magnitude greater than monolayers).

Correlated Charge Density Wave Insulators in Chirally Twisted Triple Bilayer Graphene.

Electrons residing in a flat-band system can play a vital role in triggering spectacular phenomenology due to relatively large interactions and spontaneous breaking of different degeneracies. In this work, we demonstrate chirally twisted triple bilayer graphene, a new moiré structure formed by three pieces of helically stacked Bernal bilayer graphene, as a highly tunable flat-band system. In addition to the correlated insulators showing at integer moiré fillings, commonly attributed to interaction induced symmetry broken isospin flavors in graphene, we observe abundant insulating states at half-integer moiré fillings, suggesting a longer-range interaction and the formation of charge density wave insulators which spontaneously break the moiré translation symmetry. With weak out-of-plane magnetic field applied, as observed half-integer filling states are enhanced and more quarter-integer filling states appear, pointing toward further quadrupling moiré unit cells. The insulating states at fractional fillings combined with Hartree-Fock calculations demonstrate the observation of a new type of correlated charge density wave insulators in graphene and points to a new accessible twist manner engineering correlated moiré electronics.

Understanding epitaxial growth of two-dimensional materials and their homostructures.

The exceptional physical properties of two-dimensional (2D) van der Waals (vdW) materials have been extensively researched, driving advances in material synthesis. Epitaxial growth, a prominent synthesis strategy, enables the production of large-area, high-quality 2D films compatible with advanced integrated circuits. Typical 2D single crystals, such as graphene, transition metal dichalcogenides and hexagonal boron nitride, have been epitaxially grown at a wafer scale. A systematic summary is required to offer strategic guidance for the epitaxy of emerging 2D materials. Here we focus on the epitaxy methodologies for 2D vdW materials in two directions: the growth of in-plane single-crystal monolayers and the fabrication of out-of-plane homostructures. We first discuss nucleation control of a single domain and orientation control over multiple domains to achieve large-scale single-crystal monolayers. We analyse the defect levels and measures of crystalline quality of typical 2D vdW materials with various epitaxial growth techniques. We then outline technical routes for the growth of homogeneous multilayers and twisted homostructures. We further summarize the current strategies to guide future efforts in optimizing on-demand fabrication of 2D vdW materials, as well as subsequent device manufacturing for their industrial applications.

Evidence of abnormal hot carrier thermalization at van Hove singularity of twisted bilayer graphene.

Interlayer twist evokes revolutionary changes to the optical and electronic properties of twisted bilayer graphene (TBG) for electronics, photonics and optoelectronics. Although the ground state responses in TBG have been vastly and clearly studied, the dynamic process of its photoexcited carrier states mainly remains elusive. Here, we unveil the photoexcited hot carrier dynamics in TBG by time-resolved ultrafast photoluminescence (PL) autocorrelation spectroscopy. We demonstrate the unconventional ultrafast PL emission between the van Hove singularities (VHSs) with a ∼4 times prolonged relaxation lifetime. This intriguing photoexcited carrier behavior is ascribed to the abnormal hot carrier thermalization brought by bottleneck effects at VHSs and interlayer charge distribution process. Our study on hot carrier dynamics in TBG offers new insights into the excited states and correlated physics of graphene twistronics systems.

Probing structural superlubricity of two-dimensional water transport with atomic resolution.

Low-dimensional water transport can be drastically enhanced under atomic-scale confinement. However, its microscopic origin is still under debate. In this work, we directly imaged the atomic structure and transport of two-dimensional water islands on graphene and hexagonal boron nitride surfaces using qPlus-based atomic force microscopy. The lattice of the water island was incommensurate with the graphene surface but commensurate with the boron nitride surface owing to different surface electrostatics. The area-normalized static friction on the graphene diminished as the island area was increased by a power of ~-0.58, suggesting superlubricity behavior. By contrast, the friction on the boron nitride appeared insensitive to the area. Molecular dynamic simulations further showed that the friction coefficient of the water islands on the graphene could reduce to <0.01.

WS2 ribbon arrays with defined chirality and coherent polarity.

One-dimensional transition metal dichalcogenides exhibiting an enhanced bulk photovoltaic effect have the potential to exceed the Shockley-Queisser limit efficiency in solar energy harvest within p-n junction architectures. However, the collective output of these prototype devices remains a challenge. We report on the synthesis of single-crystalline WS2 ribbon arrays with defined chirality and coherent polarity through an atomic manufacturing strategy. The chirality of WS2 ribbon was defined by substrate couplings into tunable armchair, zigzag, and chiral species, and the polarity direction was determined by the ribbon-precursor interfacial energy along a coherent direction. A single armchair ribbon showed strong bulk photovoltaic effect and the further integration of ~1000 aligned ribbons with coherent polarity enabled upscaling of the photocurrent.

Strong chiroptical nonlinearity in coherently stacked boron nitride nanotubes.

Nanomaterials with a large chiroptical response and high structural stability are desirable for advanced miniaturized optical and optoelectronic applications. One-dimensional (1D) nanotubes are robust crystals with inherent and continuously tunable chiral geometries. However, their chiroptical response is typically weak and hard to control, due to the diverse structures of the coaxial tubes. Here we demonstrate that as-grown multiwalled boron nitride nanotubes (BNNTs), featuring coherent-stacking structures including near monochirality, homo-handedness and unipolarity among the component tubes, exhibit a scalable nonlinear chiroptical response. This intrinsic architecture produces a strong nonlinear optical response in individual multiwalled BNNTs, enabling second-harmonic generation (SHG) with a conversion efficiency up to 0.01% and output power at the microwatt level-both excellent figures of merit in the 1D nanomaterials family. We further show that the rich chirality of the nanotubes introduces a controllable nonlinear geometric phase, producing a chirality-dependent SHG circular dichroism with values of -0.7 to +0.7. We envision that our 1D chiral platform will enable novel functions in compact nonlinear light sources and modulators.

GHz-rate 57-fs acousto-optic mode-locking fiber laser based on cascaded all-fiber pulse compression.

We demonstrate a compact ultrafast fiber laser system that can deliver 1.87 GHz pulse train at 1550 nm with a pulse energy of 52 pJ and an ultrashort pulse duration of 57 fs. While an acousto-optic mode-locking fiber laser was used as the seed light source at GHz rate, a stage of Er-doped fiber amplifier boosted the laser power to ∼320 mW, giving a pulse energy of ∼170 pJ. Then, a pulse compression setup was constructed, providing a high compression ratio of ∼10 with a total efficiency of ∼32%. In the cascaded compression configuration, multiple fiber samples with alternately normal and anomalous dispersion were fused together, providing efficient nonlinear spectral broadening while suppressing excessive pulse broadening over propagation. This GHz-rate ultrafast fiber laser, with compact configuration, broad optical spectrum, and high time-resolving ability could be used as the seed light source for constructing high-rate, high-power ultrafast laser systems and may find a few applications in optical measurements and microwave photonics.

Bevel-edge epitaxy of ferroelectric rhombohedral boron nitride single crystal.

Within the family of two-dimensional dielectrics, rhombohedral boron nitride (rBN) is considerably promising owing to having not only the superior properties of hexagonal boron nitride1-4-including low permittivity and dissipation, strong electrical insulation, good chemical stability, high thermal conductivity and atomic flatness without dangling bonds-but also useful optical nonlinearity and interfacial ferroelectricity originating from the broken in-plane and out-of-plane centrosymmetry5-23. However, the preparation of large-sized single-crystal rBN layers remains a challenge24-26, owing to the requisite unprecedented growth controls to coordinate the lattice orientation of each layer and the sliding vector of every interface. Here we report a facile methodology using bevel-edge epitaxy to prepare centimetre-sized single-crystal rBN layers with exact interlayer ABC stacking on a vicinal nickel surface. We realized successful accurate fabrication over a single-crystal nickel substrate with bunched step edges of the terrace facet (100) at the bevel facet (110), which simultaneously guided the consistent boron-nitrogen bond orientation in each BN layer and the rhombohedral stacking of BN layers via nucleation near each bevel facet. The pure rhombohedral phase of the as-grown BN layers was verified, and consequently showed robust, homogeneous and switchable ferroelectricity with a high Curie temperature. Our work provides an effective route for accurate stacking-controlled growth of single-crystal two-dimensional layers and presents a foundation for applicable multifunctional devices based on stacked two-dimensional materials.

Remote epitaxy of single-crystal rhombohedral WS2 bilayers.

Compared to transition metal dichalcogenide (TMD) monolayers, rhombohedral-stacked (R-stacked) TMD bilayers exhibit remarkable electrical performance, enhanced nonlinear optical response, giant piezo-photovoltaic effect and intrinsic interfacial ferroelectricity. However, from a thermodynamics perspective, the formation energies of R-stacked and hexagonal-stacked (H-stacked) TMD bilayers are nearly identical, leading to mixed stacking of both H- and R-stacked bilayers in epitaxial films. Here, we report the remote epitaxy of centimetre-scale single-crystal R-stacked WS2 bilayer films on sapphire substrates. The bilayer growth is realized by a high flux feeding of the tungsten source at high temperature on substrates. The R-stacked configuration is achieved by the symmetry breaking in a-plane sapphire, where the influence of atomic steps passes through the lower TMD layer and controls the R-stacking of the upper layer. The as-grown R-stacked bilayers show up-to-30-fold enhancements in carrier mobility (34 cm2V-1s-1), nearly doubled circular helicity (61%) and interfacial ferroelectricity, in contrast to monolayer films. Our work reveals a growth mechanism to obtain stacking-controlled bilayer TMD single crystals, and promotes large-scale applications of R-stacked TMD.

Association between programmed cell death ligand-1 expression in patients with cervical cancer and apparent diffusion coefficient values: a promising tool for patient´s immunotherapy selection.

OBJECTIVE: To investigate the associations between apparent diffusion coefficient (ADC) values extracted from three different region of interest (ROI) position approaches and programmed cell death ligand-1 (PD-L1) expression, and evaluate the performance of the nomogram established based on ADC values and clinicopathological parameters in predicting PD-L1 expression in cervical cancer (CC) patients. METHODS: Through retrospective recruitment, a training cohort of 683 CC patients was created, and a validation cohort of 332 CC patients was prospectively recruited. ROIs were delineated using three different methods to measure the mean ADC (ADCmean), single-section ADC (ADCss), and the minimum ADC of tumors (ADCmin). Logistic regression was employed to identify independent factors related to PD-L1 expression. A nomogram was drawn based on ADC values combined with clinicopathological features, its discrimination and calibration performances were estimated using the area under the curve (AUC) of receiver operating characteristic and calibration curve. The clinical benefits were evaluated by decision curve analysis. RESULTS: The ADCmin independently correlated with PD-L1 expression. The nomogram constructed with ADCmin and other independent clinicopathological-related factors: FIGO staging, pathological grade, parametrial invasion, and lymph node status demonstrated excellent diagnostic performance (AUC = 0.912 and 0.903, respectively), good calibration capacities, and greater net benefits compared to the clinicopathological model in both the training and validation cohorts. CONCLUSION: ADCmin independently correlated PD-L1 expression, and the nomogram established with ADCmin and clinicopathological independent prognostic factors had a strong predictive performance for PD-L1 expression, thereby serving as a promising tool for selecting cases eligible for immunotherapy. CLINICAL RELEVANCE STATEMENT: The minimum ADC can serve as a reliable imaging biomarker related to PD-L1 expression; the established nomogram combines the minimum ADC and clinicopathological factors that can assist clinical immunotherapy decisions.

Eight In. Wafer-Scale Epitaxial Monolayer MoS2.

Large-scale, high-quality, and uniform monolayer molybdenum disulfide (MoS2) films are crucial for their applications in next-generation electronics and optoelectronics. Epitaxy is a mainstream technique for achieving high-quality MoS2 films and is demonstrated at a wafer scale up to 4-in. In this study, the epitaxial growth of 8-in. wafer-scale highly oriented monolayer MoS2 on sapphire is reported as with excellent spatial homogeneity, using a specially designed vertical chemical vapor deposition (VCVD) system. Field effect transistors (FETs) based on the as-grown 8-in. wafer-scale monolayer MoS2 film are fabricated and exhibit high performances, with an average mobility and an on/off ratio of 53.5 cm2 V-1 s-1 and 107, respectively. In addition, batch fabrication of logic devices and 11-stage ring oscillators are also demonstrated, showcasing excellent electrical functions. This work may pave the way of MoS2 in practical industry-scale applications.

Giant Photoluminescence Enhancement of Monolayer WSe2 Using a Plasmonic Nanocavity with On-Demand Resonance.

Monolayer transition metal dichalcogenides (TMDs) are considered promising building blocks for next-generation photonic and optoelectronic devices, owing to their fascinating optical properties. However, their inherent weak light absorption and low quantum yield severely hinder their practical applications. Here, we report up to 18000-fold photoluminescence (PL) enhancement in a monolayer WSe2-coupled plasmonic nanocavity. A spectroscopy-assisted nanomanipulation technique enables the assembly of a nanocavity with customizable resonances to simultaneously enhance the excitation and emission processes. In particular, precise control over the magnetic cavity mode facilitates spectral and spatial overlap with the exciton, resulting in plasmon-exciton intermediate coupling that approaches the maximum emission rate in the hybrid system. Meanwhile, the cavity mode exhibits high radiation directivity, which overwhelmingly directs surface-normal PL emission and leads to a 17-fold increase in the collection efficiency. Our approach opens up a new avenue to enhance the PL intensity of monolayer TMDs, facilitating their implementation in highly efficient optoelectronic devices.

Microalgal-bacterial treatment of ice-cream wastewater to remove organic waste and harvest oil-rich biomass.

The diversity of microalgae and bacteria allows them to form beneficial consortia for efficient wastewater treatment and nutrient recovery. This study aimed to evaluate the feasibility of a new microalgal-bacterial combination in the treatment of ice cream wastewater for biomass harvest. The bacterium Novosphingobium sp. ICW1 was natively isolated from ice cream wastewater and the microalga Vischeria sp. WL1 was a terrestrial oil-producing strain of Eustigmatophyceae. The ice cream wastewater was diluted 4 folds for co-cultivation, which was relatively less inhibitory for the growth of Vischeria sp. WL1. Four initial algal-bacterial combinations (v:v) of 150:0 (single algal cultivation), 150:1, 150:2, and 150:4 were assessed. During 24 days of co-cultivation, algal pigmentation was dynamically changed, particularly at the algal-bacterial combination of 150:4. Algal growth (in terms of cell number) was slightly promoted during the late phase of co-cultivation at the combinations of 150:2 and 150:4, while in the former the cellular oil yield was obviously elevated. Treated by these algal-bacterial combinations, total carbon was reduced by 67.5 ~ 74.5% and chemical oxygen demand was reduced by 55.0 ~ 60.4%. Although single bacterial treatment was still effective for removing organic nutrients, the removal efficiency was obviously enhanced at the algal-bacterial combination of 150:4. In addition, the harvested oils contained 87.1 ~ 88.3% monounsaturated fatty acids. In general, this study enriches the biotechnological solutions for the sustainable treatment of organic matter-rich food wastewater.

Axially Chiral Nonbenzenoid Nanographene with Second Harmonic Generation Property.

Chiral nanographenes (NGs) have garnered significant interest as optoelectronic materials in recent years. While helically chiral NGs have been extensively studied, axially chiral NGs have only witnessed limited examples, with no prior reports of axially chiral nonbenzenoid NGs. Herein we report an axially chiral nonbenzenoid nanographene featuring six pentagons and four heptagons. This compound, denoted as 2, was efficiently synthesized via an efficient Pd-catalyzed aryl silane homocoupling reaction. The presence of two bulky 3,5-di-tert-butylphenyl groups around the axis connecting the two nonbenzenoid PAH (AHR) segments endows 2 with atropisomeric chirality and high racemization energy barrier, effectively preventing racemization of both R- and S-enantiomers at room temperature. Optically pure R-2 and S-2 were obtained by chiral HPLC separation, and they exhibit circular dichroism (CD) activity at wavelengths up to 660 nm, one of the longest wavelengths with CD responses reported for the chiral NGs. Interestingly, racemic 2 forms a homoconfiguration π-dimer in the crystal lattice, belonging to the I222 chiral space group. Consequently, this unique structure renders crystals of 2 with a second harmonic generation (SHG) response, distinguishing it from all the reported axially chiral benzenoid NGs. Moreover, R-2 and S-2 also exhibit SHG-CD properties.