Twist-Angle and Thickness-Ratio Tuning of Plasmon Polaritons in Twisted Bilayer van der Waals Films.

Stacking bilayer structures is an efficient way to tune the topology of polaritons in in-plane anisotropic films, e.g., by leveraging the twist angle (TA). However, the effect of another geometric parameter, the film thickness ratio (TR), on manipulating the plasmon topology in bilayers is elusive. Here, we fabricate bilayer structures of WTe2 films, which naturally host in-plane hyperbolic plasmons in the terahertz range. Plasmon topology is successfully modified by changing the TR and TA synergistically, manifested by the extinction spectra of unpatterned films and the polarization dependence of the plasmon intensity measured in skew ribbon arrays. Such TR- and TA-tunable topological transitions can be well explained based on the effective sheet optical conductivity by adding up those of the two films. Our study demonstrates TR as another degree of freedom for the manipulation of plasmonic topology in nanophotonics, exhibiting promising applications in biosensing, heat transfer, and the enhancement of spontaneous emission.

Layer-Dependent Pressure Effect on the Electronic Structure of 2D Black Phosphorus.

Through infrared spectroscopy, we systematically study the pressure effect on electronic structures of few-layer black phosphorus (BP) with layer number ranging from 2 to 13. We reveal that the pressure-induced shift of optical transitions exhibits strong layer dependence. In sharp contrast to the bulk counterpart which undergoes a semiconductor to semimetal transition under ∼1.8 GPa, the band gap of 2 L increases with increasing pressure until beyond 2 GPa. Meanwhile, for a sample with a given layer number, the pressure-induced shift also differs for transitions with different indices. Through the tight-binding model in conjunction with a Morse potential for the interlayer coupling, this layer- and transition-index-dependent pressure effect can be fully accounted. Our study paves a way for versatile van der Waals engineering of two-dimensional BP.

Tunable Terahertz Plasmons in Graphite Thin Films.

Tunable terahertz plasmons are essential for reconfigurable photonics, which have been demonstrated in graphene through gating, though with relatively weak responses. Here we demonstrate strong terahertz plasmons in graphite thin films via infrared spectroscopy, with dramatic tunability by even a moderate temperature change or an in situ bias voltage. Meanwhile, through magnetoplasmon studies, we reveal that massive electrons and massless Dirac holes make comparable contributions to the plasmon response. Our study not only sets up a platform for further exploration of two-component plasmas, but also opens an avenue for terahertz modulation through electrical bias or all-optical means.

From Anomalous to Normal: Temperature Dependence of the Band Gap in Two-Dimensional Black Phosphorus.

The temperature dependence of the band gap is crucial to a semiconductor. Bulk black phosphorus is known to exhibit an anomalous behavior. Through optical spectroscopy, here we show that the temperature effect on black phosphorus band gap gradually evolves with decreasing layer number, eventually turns into a normal one in the monolayer limit, rendering a crossover from the anomalous to the normal. Meanwhile, the temperature-induced shift in optical resonance also differs with different transition indices for the same thickness sample. A comprehensive analysis reveals that the temperature-tunable interlayer coupling is responsible for the observed diverse scenario. Our study provides a key to the apprehension of the anomalous temperature behavior in certain layered semiconductors.

Acid-Base-Triggered Structural Transformation of a Polyoxometalate Core Inside a Dodecahedrane-like Silver Thiolate Shell.

Self-assembly of metavanadate and organosilver(I) salts leads to a novel dodecahedrane-like [Ag30 ((t) BuS)20 ](10+) silver(I) thiolate nanocage that tightly wraps an unusual C2h polyoxovanadate anion. The polyoxovanadate core undergoes transformation to a D3d configuration upon acidification, and reverts back to its original C2h structure upon addition of base. Chromism was observed for the silver(I) thiolate cluster during the configurational change of the central polyoxovanadate core; the color of the solution changes reversibly from green to dark yellow. This work represents the first reported example of chromic polyoxometalate-templated silver(I) thiolate shells that respond to external acid-base stimuli. It also represents an important advance in providing crystallographic proof that structural transformations occur in a nanoscale core-shell cluster.

[High Precision Spectral Calibration Method of Fourier Interferometric Spectrometer].

The Fourier interferometric spectrometer (FIS) acquires the interference data information of the spectrum and during the spectrum data processing, a series of spectrum reconstruction will be performed on the interference information to obtain the final spectrum information data. The spectral calibration is the key step to spectrum reconstruction of FIS, which directly determines accuracy and availability of the spectrum results. This paper introduces the basic ideas and calibration accuracy about the spectral calibration for the FIS and puts forward a new spectral calibration method based on calculating the precise value of the total optical path difference (TOPD). The TOPD of FIS is difficult to be precisely measured, but it is the core and key to the spectral calibration. In order to calculate the precise TOPD, this paper proposes the idea how to traverse the TOPD and analyzes the spectrum drift. During the calibration, all the possible values of the TOPD participate in the spectrum reconstruction flow to carry out spectrum recovery and analysis. Ultimately the TOPD with the minimum spectrum drift will be achieved, namely solution value of the TOPD. This method can accurately resolve the TOPD of the FIS and then calibrate the spectrum with high accuracy. In addition, the paper introduces the detailed and complete spectral calibration flow and obtains the center wavelength value of every band and wavenumber resolution. Moreover, the paper designs the main parameters of the typical FIS and generates its simulation interference data. Using the above method to calibrate the simulation data, the analysis and verification of the spectral calibration results proves that the calibration precision of wavenumber resolution achieves 0.000 25 cm⁻¹ or above.

Tunable anisotropic van der Waals films of 2M-WS2 for plasmon canalization.

In-plane anisotropic van der Waals materials have emerged as a natural platform for anisotropic polaritons. Extreme anisotropic polaritons with in-situ broadband tunability are of great significance for on-chip photonics, yet their application remains challenging. In this work, we experimentally characterize through Fourier transform infrared spectroscopy measurements a van der Waals plasmonic material, 2M-WS2, capable of supporting intrinsic room-temperature in-plane anisotropic plasmons in the far and mid-infrared regimes. In contrast to the recently revealed natural hyperbolic plasmons in other anisotropic materials, 2M-WS2 supports canalized plasmons with flat isofrequency contours in the frequency range of ~ 3000-5000 cm-1. Furthermore, the anisotropic plasmons and the corresponding isofrequency contours can be reversibly tuned via in-situ ion-intercalation. The tunable anisotropic and canalization plasmons may open up further application perspectives in the field of uniaxial plasmonics, such as serving as active components in directional sensing, radiation manipulation, and polarization-dependent optical modulators.

ZnO QDs/GO/g-C3N4 Preparation and Photocatalytic Properties of Composites.

Using an ultrasound-assisted chemical technique, ZnO quantum dot and ZnO composites were created. The optical characteristics and structural details of these composites were examined using TEM, XRD, XPS, FT-IR, UV-vis, and BET. The results revealed that both the ZnO quantum dot composite and ZnO composite exhibited outstanding optical properties, making them suitable for photocatalytic reactions. In order to analyze the photocatalytic performance, a degradation experiment was conducted using Rhodamine B solution as the simulation dye wastewater. The experiment demonstrated that the degradation of Rhodamine B followed the first-order reaction kinetics equation when combined with the photocatalytic reaction kinetics. Moreover, through cyclic stability testing, it was determined that the ZnO QDs-GO-g-C3N4 composite sample showed good stability and could be reused. The degradation rates of Rhodamine B solution using ZnO-GO-g-C3N4 and ZnO QDs-GO-g-C3N4 reached 95.25% and 97.16%, respectively. Furthermore, free-radical-trapping experiments confirmed that ·O2- was the main active species in the catalytic system and its photocatalytic mechanism was elucidated. The photocatalytic oxidation of ZnO quantum dots in this study has important reference value and provides a new idea for the subsequent research.

Synthesis and Catalytic Degradation of PEF, ENR, and CIP by g-C3N4/TCNQ/Eu Composite.

By using melamine as a precursor for the copolymerization process, g-C3N4 and g-C3N4/TCNQ/Eu complexes with various amounts of doping were created. These complexes were then examined using XRD, FT-IR, SEM, TEM, XPS, PL, UV-vis, and I-T. The degradation rates of pefloxacin (PEF), enrofloxacin (ENR), and ciprofloxacin (CIP) were 91.1%, 90.8%, and 93.2% under visible light (λ > 550 nm). The photocatalytic performance of the composite was analyzed, and the best effect was obtained for CIP photocatalysis when Eu doping was 3 mg at 20 °C and pH 7. Kinetic analysis showed that there was a linear relationship between the sample and the photocatalytic time, and the degradation rate was about 5 times that of g-C3N4. The cyclic stability of the g-C3N4/TCNQ/Eu composite sample was found to be good through repeated experiments. UPLC-MS visualizes the degradation process of CIP. The extremely low stability of piperazine ring induced subsequent degradation, followed by the fracture of quinolone ring promoting the complete decomposition of CIP.

Layer-dependent exciton polarizability and the brightening of dark excitons in few-layer black phosphorus.

The evolution of excitons from 2D to 3D is of great importance in photo-physics, yet the layer-dependent exciton polarizability hasn't been investigated in 2D semiconductors. Here, we determine the exciton polarizabilities for 3- to 11-layer black phosphorus-a direct bandgap semiconductor regardless of the thickness-through frequency-resolved photocurrent measurements on dual-gate devices and unveil the carrier screening effect in relatively thicker samples. By taking advantage of the broadband photocurrent spectra, we are also able to reveal the exciton response for higher-index subbands under the gate electrical field. Surprisingly, dark excitons are brightened with intensity even stronger than the allowed transitions above certain electrical field. Our study not only sheds light on the exciton evolution with sample thickness, but also paves a way for optoelectronic applications of few-layer BP in modulators, tunable photodetectors, emitters and lasers.

HgCdTe/black phosphorus van der Waals heterojunction for high-performance polarization-sensitive midwave infrared photodetector.

New-generation infrared detectors call for higher operation temperature and polarization sensitivity. For traditional HgCdTe infrared detectors, the additional polarization optics and cryogenic cooling are necessary to achieve high-performance infrared polarization detection, while they can complicate this system and limit the integration. Here, a mixed-dimensional HgCdTe/black phosphorous van der Waals heterojunction photodiode is proposed for polarization-sensitive midwave infrared photodetection. Benefiting from van der Waals integration, type III broken-gap band alignment heterojunctions are achieved. Anisotropy optical properties of black phosphorous bring polarization sensitivity from visible light to midwave infrared without external optics. Our devices show an outstanding performance at room temperature without applied bias, with peak blackbody detectivity as high as 7.93 × 1010 cm Hz1/2 W-1 and average blackbody detectivity over 2.1 × 1010 cm Hz1/2 W-1 in midwave infrared region. This strategy offers a possible practical solution for next-generation infrared detector with high operation temperature, high performance, and multi-information acquisition.

Prediction of hyperbolic exciton-polaritons in monolayer black phosphorus.

Hyperbolic polaritons exhibit large photonic density of states and can be collimated in certain propagation directions. The majority of hyperbolic polaritons are sustained in man-made metamaterials. However, natural-occurring hyperbolic materials also exist. Particularly, natural in-plane hyperbolic polaritons in layered materials have been demonstrated in MoO3 and WTe2, which are based on phonon and plasmon resonances respectively. Here, by determining the anisotropic optical conductivity (dielectric function) through optical spectroscopy, we predict that monolayer black phosphorus naturally hosts hyperbolic exciton-polaritons due to the pronounced in-plane anisotropy and strong exciton resonances. We simultaneously observe a strong and sharp ground state exciton peak and weaker excited states in high quality monolayer samples in the reflection spectrum, which enables us to determine the exciton binding energy of ~452 meV. Our work provides another appealing platform for the in-plane natural hyperbolic polaritons, which is based on excitons rather than phonons or plasmons.

Plasmons in the van der Waals charge-density-wave material 2H-TaSe2.

Plasmons in two-dimensional (2D) materials beyond graphene have recently gained much attention. However, the experimental investigation is limited due to the lack of suitable materials. Here, we experimentally demonstrate localized plasmons in a correlated 2D charge-density-wave (CDW) material: 2H-TaSe2. The plasmon resonance can cover a broad spectral range from the terahertz (40 µm) to the telecom (1.55 µm) region, which is further tunable by changing thickness and dielectric environments. The plasmon dispersion flattens at large wave vectors, resulted from the universal screening effect of interband transitions. More interestingly, anomalous temperature dependence of plasmon resonances associated with CDW excitations is observed. In the CDW phase, the plasmon peak close to the CDW excitation frequency becomes wider and asymmetric, mimicking two coupled oscillators. Our study not only reveals the universal role of the intrinsic screening on 2D plasmons, but also opens an avenue for tunable plasmons in 2D correlated materials.

Ultra-fast single-crystal polymerization of large-sized covalent organic frameworks.

In principle, polymerization tends to produce amorphous or poorly crystalline materials. Efficiently producing high-quality single crystals by polymerization in solvent remains as an unsolved issue in chemistry, especially for covalent organic frameworks (COFs) with highly complex structures. To produce µm-sized single crystals, the growth time is prolonged to >15 days, far away from the requirements in practical applications. Here, we find supercritical CO2 (sc-CO2) accelerates single-crystal polymerization by 10,000,000 folds, and produces two-dimensional (2D) COF single crystals with size up to 0.2 mm within 2~5 min. Although it is the fastest single-crystal polymerization, the growth in sc-CO2 leads to not only the largest crystal size of 2D COFs, but also higher quality with improved photoconductivity performance. This work overcomes traditional concept on low efficiency of single-crystal polymerization, and holds great promise for future applications owing to its efficiency, industrial compatibility, environmental friendliness and universality for different crystalline structures and linkage bonds.

Van der Waals thin films of WTe2 for natural hyperbolic plasmonic surfaces.

A hyperbolic plasmonic surface supports highly directional propagating polaritons with extremely large density of states. Such plasmon polaritons have been realized in artificially structured metasurfaces. However, the upper bound of the achievable plasmon wave vector is limited by the structure size, which calls for a natural hyperbolic surface without any structuring. Here, we experimentally demonstrate a natural hyperbolic plasmonic surface based on thin films of WTe2 in the light wavelength range of 16 to 23 microns by far infrared absorption spectroscopy. The topological transition from the elliptic to the hyperbolic regime is further manifested by mapping the isofrequency contours of the plasmon. Moreover, the anisotropy character and plasmon frequency exhibit prominent temperature dependence. Our study demonstrates the first natural platform to host 2D hyperbolic plasmons, which opens exotic avenues for the manipulation of plasmon propagation, light-matter interaction and light emission in planar photonics.

The optical conductivity of few-layer black phosphorus by infrared spectroscopy.

The strength of light-matter interaction is of central importance in photonics and optoelectronics. For many widely studied two-dimensional semiconductors, such as MoS2, the optical absorption due to exciton resonances increases with thickness. However, here we will show, few-layer black phosphorus exhibits an opposite trend. We determine the optical conductivity of few-layer black phosphorus with thickness down to bilayer by infrared spectroscopy. On the contrary to our expectations, the frequency-integrated exciton absorption is found to be enhanced in thinner samples. Moreover, the continuum absorption near the band edge is almost a constant, independent of the thickness. We will show such scenario is related to the quanta of the universal optical conductivity of graphene (σ0 = e2/4h), with a prefactor originating from the band anisotropy.

The discovery of dynamic chiral anomaly in a Weyl semimetal NbAs.

The experimental discovery of Weyl semimetals offers unprecedented opportunities to study Weyl physics in condensed matters. Unique electromagnetic response of Weyl semimetals such as chiral magnetic effect has been observed and presented by the axial θ E · B term in electromagnetic Lagrangian (E and B are the electric and magnetic field, respectively). But till now, the experimental progress in this direction in Weyl semimetals is restricted to the DC regime. Here we report experimental access to the dynamic regime in Weyl semimetal NbAs by combining the internal deformation potential of coupled phonons with applied static magnetic field. While the dynamic E · B field is realized, it produces an anomalous phonon activity with a characteristic angle-dependence. Our results provide an effective approach to achieve the dynamic regime beyond the widely-investigated DC limit which enables the coupling between the Weyl fermions and the electromagnetic wave for further study of novel light-matter interactions in Weyl semimetals.

Strain-tunable van der Waals interactions in few-layer black phosphorus.

Interlayer interactions in 2D materials, also known as van der Waals (vdWs) interactions, play a critical role in the physical properties of layered materials. It is fascinating to manipulate the vdWs interaction, and hence to "redefine" the material properties. Here, we demonstrate that in-plane biaxial strain can effectively tune the vdWs interaction of few-layer black phosphorus with thickness of 2-10 layers, using infrared spectroscopy. Surprisingly, our results reveal that in-plane tensile strain efficiently weakens the interlayer coupling, even though the sample shrinks in the vertical direction due to the Poisson effect, in sharp contrast to one's intuition. Moreover, density functional theory (DFT) calculations further confirm our observations and indicate a dominant role of the puckered lattice structure. Our study highlights the important role played by vdWs interactions in 2D materials during external physical perturbations.

Highly Efficient Charge Collection in Bulk-Heterojunction Organic Solar Cells by Anomalous Hole Transfer and Improved Interfacial Contact.

The dilute donor-fullerene bulk heterojunction (BHJ) has been proven to be an efficient architecture of organic solar cells. However, the hole-extraction pathway and the origin of the high open-circuit voltage ( VOC) in this peculiar architecture remains elusive. Direct evidence is provided here that the photogenerated holes can be extracted via the acceptor phase even under the operating conditions. Meanwhile VOC is found to be closely correlated with the surface composition at the MoO3/BHJ interface. Extending these findings into device optimization, more than 37% enhancement is achieved in a prototype BHJ device. These results evoke renewed insight into the underlying physics in organic solar cells.

Largely Tunable Band Structures of Few-Layer InSe by Uniaxial Strain.

Because of the strong quantum confinement effect, few-layer γ-InSe exhibits a layer-dependent band gap, spanning the visible and near infrared regions, and thus recently has been drawing tremendous attention. As a two-dimensional material, the mechanical flexibility provides an additional tuning knob for the electronic structures. Here, for the first time, we engineer the band structures of few-layer and bulk-like InSe by uniaxial tensile strain and observe a salient shift of photoluminescence peaks. The shift rate of the optical gap is approximately 90-100 meV per 1% strain for four- to eight-layer samples, which is much larger than that for the widely studied MoS2 monolayer. Density functional theory calculations well reproduce the observed layer-dependent band gaps and the strain effect and reveal that the shift rate decreases with the increasing layer number for few-layer InSe. Our study demonstrates that InSe is a very versatile two-dimensional electronic and optoelectronic material, which is suitable for tunable light emitters, photodetectors, and other optoelectronic devices.