Adsorption of perfluoroalkyl substances on deep eutectic solvent-based amorphous metal-organic framework: Structure and mechanism.

Perfluoroalkyl substances (PFASs) are a class of emerging organic pollutants characterized by high toxicity, environmental persistence, and widespread detection in water sources. The removal of PFASs from water is a matter of global concern, given their detrimental impact on both the environment and public health. Many commonly used PFAS adsorbents demonstrate limited adsorption capacities and/or slow adsorption kinetics. Therefore, there is an urgent need for the development of efficient adsorbents. For the first time, this work systematically investigated the performance of a deep eutectic solvent (DES)-based amorphous metal-organic framework (MOF) for the adsorption of PFASs with different carbon-chain lengths under the state of the mixture in aquatic environments. The adsorption mechanism was probed by a suite of adsorption kinetics studies, adsorption isotherm profiling, spectral characterization, and ab initio molecular dynamics (AIMD) simulations, revealing that PFAS adsorption is driven by synergistic capturing effects including acid/base coordination, CF-π (carbon-fluorine-π), hydrogen bonding, and hydrophobic interactions. Furthermore, the adsorption processes of short-chain and long-chain targets were found to involve different rate-controlling steps and interaction sites. Hydrophobic interactions facilitated the swift arrival of long-chain PFASs at the coordinatively interacting sites between carboxyl termini and Lewis acid Zr unsaturated sites, thanks to their lower reaction barriers. On the other hand, the adsorption of short-chain PFASs primarily relied on a Zr hydroxyl-based ligand exchange force, which would take place at Brønsted acid sites. The existence of massive structural disorder in amorphous UiO-66 led to the development of larger pores, thus improving the accessibility of abundant adsorption sites and facilitating adsorption and diffusion. The presence of multiple types of interactions and flexible structure in defect-rich amorphous UiO-66 significantly increased the exposure of functional groups to the adsorbates. Additionally, this material possessed outstanding regeneration efficiency and outperformed other MOF-based adsorbents with high affinity for targets. It enhances our understanding of the adsorption performances and mechanisms of amorphous materials toward PFASs, thereby paving the way for designing more efficient PFAS adsorbents.

Interference tunable second harmonic generation for two-dimensional materials in layered structures.

We experimentally study the tunability of second harmonic generation (SHG) from a two-dimensional (2D) material in a 2D material/dielectric film/substrate layered structure. Such tunability arises from two interferences: one is between the incident fundamental light and its reflected light, and the other is between the upward second harmonic (SH) light and the reflected downward SH light. When both interferences are constructive, the SHG is maximally enhanced; it becomes attenuated if either of them is destructive. The maximal signal can be obtained when both interferences are perfectly constructive, which can be realized by choosing a highly reflective substrate and an appropriate thickness for a dielectric film that has a large difference in its refractive indices at the fundamental and the SH wavelengths. Our experiments demonstrate variations of three orders of magnitude in the SHG signals from a monolayer MoS2/TiO2/Ag layered structure.

Selective excitation of four-wave mixing by helicity in gated graphene.

Gapless Dirac fermions in monolayer graphene give rise to an abundance of peculiar physical properties, including exceptional broadband nonlinear optical responses. By tuning the chemical potential, stacking order, and photonic structures, the effective modulation of nonlinear optical phenomena in graphene has been demonstrated in recent years. Here, we demonstrate that optical helicity can be used as an extra tuning knob for four-wave mixing in gated graphene. Our results reveal the helicity selection rule for four-wave mixing in monolayer graphene, revealing nearly perfect circular polarization. Corresponding theoretical interpretations of the helicity selection rule that are also applicable to other nonlinear optical processes and materials are presented.

Modulating the optical and electrical properties of MAPbBr3 single crystals via voltage regulation engineering and application in memristors.

Defect density is one of the most significant characteristics of perovskite single crystals (PSCs) that determines their optical and electrical properties, but few strategies are available to tune this property. Here, we demonstrate that voltage regulation is an efficient method to tune defect density, as well as the optical and electrical properties of PSCs. A three-step carrier transport model of MAPbBr3 PSCs is proposed to explore the defect regulation mechanism and carrier transport dynamics via an applied bias. Dynamic and steady-state photoluminescence measurements subsequently show that the surface defect density, average carrier lifetime, and photoluminescence intensity can be efficiently tuned by the applied bias. In particular, when the regulation voltage is 20 V (electrical poling intensity is 0.167 V µm-1), the surface defect density of MAPbBr3 PSCs is reduced by 24.27%, the carrier lifetime is prolonged by 32.04%, and the PL intensity is increased by 112.96%. Furthermore, a voltage-regulated MAPbBr3 PSC memristor device shows an adjustable multiresistance, weak ion migration effect and greatly enhanced device stability. Voltage regulation is a promising engineering technique for developing advanced perovskite optoelectronic devices.

Microfluidic generation of cholesteric liquid crystal droplets with an integrative cavity for dual-gain and controllable lasing.

The integration of one more gain media in droplet microlasers with morphology-dependent modes, which can be employed in optofluidic systems as multi-wavelength lasing sources, is highly attractive and demands new cavity design and fabrication approaches. Here, cholesteric liquid crystal (CLC) droplets with an integrative triple-emulsion cavity are fabricated via glass-capillary-based microfluidic technologies and dual-gain lasing with variable modes, flexibly configured by the combination and incorporation of gain dyes and CLCs into both the core and shell. The distributed feedback (DFB) mode, formed by the feedback from the self-assembled helix periodic structure of CLCs, the whispering gallery (WG) mode, and the hybrid, is selectively excited by controlling the spatial coupling between the pump beam and the droplet with gain. With the merits of dual-gain and controllable lasing, a prototype dual-wavelength-ratiometric thermometer with self-calibration capability is expected to be developed. Furthermore, the anisotropic CLC core is substituted with an isotropic fluid and the gain from the CLC shell is additionally removed, DFB lasings in both shell and core are absent, and only Bragg-shell reflection-based hybrid modes are excited for lasing. The CLC droplet microlasers with an integrative cavity are expected to provide a new route to future lab-on-chip (LOC) applications.

Giant nonreciprocal second-harmonic generation from antiferromagnetic bilayer CrI3.

Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. The van der Waals magnet chromium triiodide (CrI3) has been shown to be a layered antiferromagnetic insulator in its few-layer form1, opening up opportunities for various functionalities2-7 in electronic and optical devices. Here we report an emergent nonreciprocal second-order nonlinear optical effect in bilayer CrI3. The observed second-harmonic generation (SHG; a nonlinear optical process that converts two photons of the same frequency into one photon of twice the fundamental frequency) is several orders of magnitude larger than known magnetization-induced SHG8-11 and comparable to the SHG of the best (in terms of nonlinear susceptibility) two-dimensional nonlinear optical materials studied so far12,13 (for example, molybdenum disulfide). We show that although the parent lattice of bilayer CrI3 is centrosymmetric, and thus does not contribute to the SHG signal, the observed giant nonreciprocal SHG originates only from the layered antiferromagnetic order, which breaks both the spatial-inversion symmetry and the time-reversal symmetry. Furthermore, polarization-resolved measurements reveal underlying C2h crystallographic symmetry-and thus monoclinic stacking order-in bilayer CrI3, providing key structural information for the microscopic origin of layered antiferromagnetism14-18. Our results indicate that SHG is a highly sensitive probe of subtle magnetic orders and open up possibilities for the use of two-dimensional magnets in nonlinear and nonreciprocal optical devices.

Chiral selection rules for multi-photon processes in two-dimensional honeycomb materials.

We examine the chirality-dependent optical selection rules in two-dimensional monolayer materials with honeycomb lattices, and, based on symmetry argument, we generalize these rules to multi-photon transitions of arbitrary orders. We also present the phase relations between incident and outgoing photons in such processes. The results agree nicely with our experimental observations of second- and third-harmonic generation. In particular, we demonstrate that the phase relation of chiral second-harmonic generation can serve as a handy tool for imaging domains and domain boundaries of these monolayers. Our results can benefit future studies on chirality-related optical phenomena and opto-electronic applications of such materials.

Epitaxial growth of a 100-square-centimetre single-crystal hexagonal boron nitride monolayer on copper.

The development of two-dimensional (2D) materials has opened up possibilities for their application in electronics, optoelectronics and photovoltaics, because they can provide devices with smaller size, higher speed and additional functionalities compared with conventional silicon-based devices1. The ability to grow large, high-quality single crystals for 2D components-that is, conductors, semiconductors and insulators-is essential for the industrial application of 2D devices2-4. Atom-layered hexagonal boron nitride (hBN), with its excellent stability, flat surface and large bandgap, has been reported to be the best 2D insulator5-12. However, the size of 2D hBN single crystals is typically limited to less than one millimetre13-18, mainly because of difficulties in the growth of such crystals; these include excessive nucleation, which precludes growth from a single nucleus to large single crystals, and the threefold symmetry of the hBN lattice, which leads to antiparallel domains and twin boundaries on most substrates19. Here we report the epitaxial growth of a 100-square-centimetre single-crystal hBN monolayer on a low-symmetry Cu (110) vicinal surface, obtained by annealing an industrial copper foil. Structural characterizations and theoretical calculations indicate that epitaxial growth was achieved by the coupling of Cu <211> step edges with hBN zigzag edges, which breaks the equivalence of antiparallel hBN domains, enabling unidirectional domain alignment better than 99 per cent. The growth kinetics, unidirectional alignment and seamless stitching of the hBN domains are unambiguously demonstrated using centimetre- to atomic-scale characterization techniques. Our findings are expected to facilitate the wide application of 2D devices and lead to the epitaxial growth of broad non-centrosymmetric 2D materials, such as various transition-metal dichalcogenides20-23, to produce large single crystals.

Doping-Induced Second-Harmonic Generation in Centrosymmetric Graphene from Quadrupole Response.

For centrosymmetric materials such as monolayer graphene, no optical second-harmonic generation (SHG) is generally expected, because it is forbidden under the electric-dipole approximation. Yet we observe a strong, doping-induced SHG from graphene, with its highest strength comparable to the electric-dipole-allowed SHG in noncentrosymmetric 2D materials. This novel SHG has the nature of an electric-quadrupole response, arising from the effective breaking of inversion symmetry by optical dressing with an in-plane photon wave vector. More remarkably, the SHG is widely tuned by carrier doping or chemical potential, being sharply enhanced at Fermi-edge resonances but vanishing at the charge neutral point that manifests the electron-hole symmetry of massless Dirac fermions. This striking behavior in graphene, which should also arise in graphenelike Dirac materials, expands the scope of nonlinear optics and holds the promise of novel optoelectronic and photonic applications.

A Photoelectric-Stimulated MoS2 Transistor for Neuromorphic Engineering.

The von Neumann bottleneck has spawned the rapid expansion of neuromorphic engineering and brain-like networks. Synapses serve as bridges for information transmission and connection in the biological nervous system. The direct implementation of neural networks may depend on novel materials and devices that mimic natural neuronal and synaptic behavior. By exploiting the interfacial effects between MoS2 and AlOx, we demonstrate that an h-BN-encapsulated MoS2 artificial synapse transistor can mimic the basic synaptic behaviors, including EPSC, PPF, LTP, and LTD. Efficient optoelectronic spikes enable simulation of synaptic gain, frequency, and weight plasticity. The Pavlov classical conditioning experiment was successfully simulated by electrical tuning, showing associated learning behavior. In addition, h-BN encapsulation effectively improves the environmental time stability of our devices. Our h-BN-encapsulated MoS2 artificial synapse provides a new paradigm for hardware implementation of neuromorphic engineering.

Gate Switching of Ultrafast Photoluminescence in Graphene.

The control of optical properties by electric means is the key to optoelectronic applications. For atomically thin two-dimensional (2D) materials, the natural advantage lies in that the carrier doping could be readily controlled through the electric gating effect, possibly affecting the optical properties. Exploiting this advantage, here we report the gate switching of the ultrafast upconverted photoluminescence from monolayer graphene. The luminescence can be completely switched off by the Pauli-blocking of one-photon interband transition in graphene with an on/off ratio exceeding 100, which is remarkable compared to other 2D semiconductors and 3D bulk counterparts. The chemical potential and pump fluence dependences of the luminescence are nicely described by a two-temperature model, including both the hot carrier dynamics and carrier-optical phonon interaction. This gate switchable and background-free photoluminescence can open up new opportunities for graphene-based ultrafast optoelectronic applications.

High-Performance Wafer-Scale MoS2 Transistors toward Practical Application.

Atomic thin transition-metal dichalcogenides (TMDs) are considered as an emerging platform to build next-generation semiconductor devices. However, to date most devices are still based on exfoliated TMD sheets on a micrometer scale. Here, a novel chemical vapor deposition synthesis strategy by introducing multilayer (ML) MoS2 islands to improve device performance is proposed. A four-probe method is applied to confirm that the contact resistance decreases by one order of magnitude, which can be attributed to a conformal contact by the extra amount of exposed edges from the ML-MoS2 islands. Based on such continuous MoS2 films synthesized on a 2 in. insulating substrate, a top-gated field effect transistor (FET) array is fabricated to explore key metrics such as threshold voltage (V T ) and field effect mobility (µFE ) for hundreds of MoS2 FETs. The statistical results exhibit a surprisingly low variability of these parameters. An average effective µFE of 70 cm2 V-1 s-1 and subthreshold swing of about 150 mV dec-1 are extracted from these MoS2 FETs, which are comparable to the best top-gated MoS2 FETs achieved by mechanical exfoliation. The result is a key step toward scaling 2D-TMDs into functional systems and paves the way for the future development of 2D-TMDs integrated circuits.

Stacking symmetry governed second harmonic generation in graphene trilayers.

Crystal symmetry plays a central role in governing a wide range of fundamental physical phenomena. One example is nonlinear optical second harmonic generation (SHG), which requires inversion symmetry breaking. We report a unique stacking-induced SHG in graphene trilayers, whose individual monolayer sheet is centrosymmetric. Depending on layer stacking sequence, we observe a strong optical SHG in a Bernal ABA-stacked non-centrosymmetric trilayer, while it vanishes in a rhombohedral ABC-stacked one, which preserves inversion symmetry. This highly contrasting SHG due to the distinct stacking symmetry enables us to map out the ABA and ABC crystal domains in an otherwise homogeneous graphene trilayer. The extracted second-order nonlinear susceptibility of the ABA trilayer is surprisingly large, comparable to the best known two-dimensional semiconductors enhanced by excitonic resonance. Our results reveal a novel stacking order-induced nonlinear optical effect, as well as unleash the opportunity for studying intriguing physical phenomena predicted for stacking-dependent ABA and ABC graphene trilayers.

Enhanced light-matter interactions in graphene-covered dielectric magnetic mirrors.

Enhanced interactions of light with graphene on the surface of a lossless dielectric magnetic mirror (DMM) are studied theoretically and experimentally in the visible range, where the DMM is composed of truncated dielectric photonic crystals (PCs). The absorption of graphene on the DMM was enhanced by about 4-fold for the spectral range within the forbidden gap of PCs over a wide range of incidence angles for both transverse electric and transverse magnetic polarizations compared with that of free-standing graphene. Moreover, the enhanced local electric field on the DMM surface led to much better detection efficiencies of the photocurrent, Raman spectroscopy and enhanced third-harmonic generation of graphene. These results offer a new way to achieve an enhanced interaction of light with graphene and develop new compact graphene-based devices.

Magnetic Nanoparticle-Assisted Tunable Optical Patterns from Spherical Cholesteric Liquid Crystal Bragg Reflectors.

Cholesteric liquid crystals (CLCs) exhibit selective Bragg reflections of circularly polarized (CP) light owing to their spontaneous self-assembly abilities into periodic helical structures. Photonic cross-communication patterns could be generated toward potential security applications by spherical cholesteric liquid crystal (CLC) structures. To endow these optical patterns with tunability, we fabricated spherical CLC Bragg reflectors in the shape of microshells by glass-capillary microfluidics. Water-soluble magnetofluid with Fe3O4 nanoparticles incorporated in the inner aqueous core of CLC shells is responsible for the non-invasive transportable capability. With the aid of an external magnetic field, the reflection interactions between neighboring microshells and microdroplets were identified by varying the mutual distance in a group of magnetically transportable and unmovable spherical CLC structures. The temperature-dependent optical reflection patterns were investigated in close-packed hexagonal arrangements of seven CLC microdroplets and microshells with inverse helicity handedness. Moreover, we demonstrated that the magnetic field-assisted assembly of microshells array into geometric figures of uppercase English letters "L" and "C" was successfully achieved. We hope that these findings can provide good application prospects for security pattern designs.

Kinetic Nature of Grain Boundary Formation in As-Grown MoS2 Monolayers.

Grain boundaries in as-grown polycrystalline MoS2 monolayers are revealed by second-harmonic-generation microscopy. Through the anisotropic polarization pattern and phase interference at the grain boundary, grain edge termination and boundary types are identified. Statistical analysis on hundreds of grains shows that grain-boundary formation is driven by kinetics and can be nicely described by the edge attachment growth model.