Effect mechanism of nitrogen injection into fire-sealed-zone on residual-coal re-ignition under stress in goaf

Coal is the one of foundations of energy and economic structure in China,while the unsealing of coal mine fres would cause a great risk of coal re-ignition.In order to explore the infuence of pressure-bearing state on the re-ignition characteristics for residual coal,the uniaxial compression equipped with a temperature-programmed device was built.The scanning electron microscope,synchronous thermal analyzer and Fourier transform infrared absorption spectrometer was applied to investigate the microscopic structure and thermal efect of the coal samples.Moreover,the microscopic efect of uniaxial stress on coal re-ignition is revealed,and the re-ignition mechanism is also obtained.As the uniaxial stress increasing,the number,depth and length of the fractures of the pre-treated coal increases.The application of uniaxial stress causes the thermal conductivity to change periodically,enhances the inhibition of injecting nitrogen on heat transfer and prolonges the duration of oxidation exothermic.The content of oxygen-containing functional groups has a high correlation with apparent activation energy,and coal samples at 6 MPa is more probability to re-ignite while the fre zone is unsealed.Uniaxial stress could control the re-ignition mechanism by changing the structure of fractures and pores.The side chains and functional groups of coal structure are easier to be broken by thermal-stress coupling.The higher the·OH content,the more difcult coal samples would be re-ignited.The research results would lay a solid theoretical foundation for the safe unsealing of closed fre-areas underground,tighten the common bond between the actual industry and the experimental theory in closed fre-areas underground,and provide the theoretical guidance for coal re-ignition preventing.

Optical properties of two-dimensional perovskites

The optical properties of two-dimensional(2D)perovskites recently receive numerous research focus thanks to the strong quantum and dielectric confinement effects.In addition to the strong excitonic effect at room temperature,2D perovskites also have appealing features that their optical properties can be flexibly tuned by alternating organic or inorganic layers.Particularly,2D chiral perovskites and 2D perovskites based heterostructures are emerging as new platforms to extend their functionalities.To optimize performance of 2D perovskites-based optoelectronic devices,it is critical to understand the fundamentals and explore the strategies to engineer their optical properties.This review begins with an introduction to the excitons and self-trapped excitons of 2D perovskites.Subsequently,inorganic/organic layer effects on optical properties and 2D perovskites based heterostructures are discussed.We also discussed the nonlinear optical properties of 2D perovskite.We are looking forward to that this review can stimulate more efforts to understand and optimize the optical properties of 2D perovskites.

Chiral plasmonics and enhanced chiral light-matter interactions

Chirality, which describes the broken mirror symmetry in geometric structures, exists macroscopically in our daily life as well as microscopically down to molecular levels. Correspondingly, chiral molecules interact differently with circularly polarized light exhibiting opposite handedness(left-handed and right-handed). However, the interaction between chiral molecules and chiral light is very weak. In contrast, artificial chiral plasmonic structures can generate "super-chiral" plasmonic near-field, leading to enhanced chiral light-matter(or chiroptical) interactions. The "super-chiral" near-field presents different amplitude and phase under opposite handedness incidence, which can be utilized to engineer linear and nonlinear chiroptical interactions. Specifically,in the interaction between quantum emitters and chiral plasmonic structures, the chiral hot spots can favour the emission with a specific handedness. This article reviews the state-of-the-art research on the design, fabrication and chiroptical response of different chiral plasmonic nanostructures or metasurfaces. This review also discusses enhanced chiral light-matter interactions that are essential for applications like chirality sensing, chiral selective light emitting and harvesting. In the final part, the review ends with a perspective on future directions of chiral plasmonics.

Nonlinear optics of two-dimensional transition metal dichalcogenides

Nonlinear optics(NLO)of transition metal dichalcogenides(TMDs)is promising for the on-chip photonic and optoelectronic applications.In this review,we will survey the current progress of NLO in TMDs.First,we will brief the basic theory of the NLO in TMDs.Second,several important nonlinear processes in TMDs such as harmonic generation,four-wave mixing,saturable absorption,and two-photon absorption will be presented and their potential applications are also discussed.Third,the main strategies to tune,modulate,and enhance the NLO in TMDs are reviewed,including the excitonic effect,symmetry modulation,optical cavity enhancement,valley selection,edge state,and material phase.Finally,we give an outlook regarding some important issues and directions of NLO in TMDs.

Identifying self-trapped excitons in 2D perovskites by Raman spectroscopy[Invited]

Two-dimensional(2D)perovskites exhibit broadband emission due to strong exciton–phonon coupling-induced self-trapped excitons and thus would find important applications in the field of white-light emitting devices.However,the available identifying methods for self-trapped excitons are currently rather limited and complex.Here,we identify the existence of self-trapped excitons by Raman spectroscopy in both excited and non-excited states.Under excited states,the shifting of the Raman peak indicates the presence of the lattice distortion,which together with the extra Raman scattering peak reveals the presence of self-trapped excitons.Our work provides an alternative simple method to study self-trapped excitons in 2 D perovskites.

Te-seeded growth of few-quintuple layer Bi2Te3 nanoplates

We report on a Te-seeded epitaxial growth of ultrathin Bi2Te3 nanoplates （down to three quintuple layers （QL）） with large planar sizes （up to tens of micrometers） through vapor transport. Optical contrast has been systematically investigated for the as-grown Bi2Te3 nanoplates on the SiO2/Si substrates, experimentally and computationally. The high and distinct optical contrast provides a fast and convenient method for the thickness determination of few-QL Bi2Te3 nanoplates. By aberration-corrected scanning transmission electron microscopy, a hexagonal crystalline structure has been identified for the Te seeds, which form naturally during the growth process and initiate an epitaxial growth of the rhombohedral- structured Bi2Te3 nanoplates. The epitaxial relationship between Te and Bi2T% is identified to be perfect along both in-plane and out-of-plane directions of the layered nanoplate. Similar growth mechanism might be expected for other bismuth chalcogenide layered materials.

Bose-Einstein condensation of exciton polariton in perovskites semiconductors

Polariton-hybridization of light-matter oscillations can emerge from various quasiparticles,such as phonon,plasmon,exciton and magnon.Particularly,exciton polaritons are bosonic quasiparticles with half-light,half-matter nature,which are originated from strong coupling between excitons and microcavity photons.The half-light nature results in extremely small effective mass,making it feasible to achieve high temperature even room-tempera-ture Bose-Einstein condensation(BEC).Meanwhile,the half-matter nature leads to strong nonlinear interaction,which is missing between photons and can promote the polaritons relaxation to ground state and give rise to low threshold polariton lasing,compared to photonic lasing.The exciton polaritons are of great importance in applications of quantum simulation,topological quantum optics,ultrafast optical switch and low threshold lasers.

Site-controlled interlayer coupling in WSe_(2)/2D perovskite heterostructure

Interlayer excitons(IXs) formed in transition metal dichalcogenides(TMDs)/two-dimensional(2 D) perovskite heterostructures are emerging as new platforms in the research of excitons. Compared with IXs in TMD van der Waals heterostructures, IXs can be robustly formed in TMDs/2 D perovskite heterostructures regardless of the twist angle and thermal annealing process. Efficient control of interlayer coupling is essential for realizing their functionalities and enhancing their performances. Nevertheless, the study on the control of interlayer coupling strength between TMD and 2 D perovskites is elusive. Therefore, we realize the control of interlayer coupling between monolayer WSe_(2) and(iso-BA)_(2)PbI_(4) with SiO_(2) pillars in situ. An abnormal 10-nm blue shift and 2.5 times photoluminescence intensity enhancement were observed for heterostructures on the pillar, which was contrary to the red shift observed in TMD heterobilayers. We attributed the abnormal blue shift to the enhanced interlayer coupling arising from the reduced gap between constituent layers. In addition, IXs became more dominant over intralayer excitons with enhanced coupling. The interlayer coupling could be further engineered by tuning the height(h) and diameter(d)of pillars. In particular, an additional triplet IX showed up for the pillar with an h/d ratio of 0.6 due to the symmetry breaking of monolayer WSe_(2). The symmetry breaking also induced an anisotropic response of IXs. Our study is beneficial for tuning and enhancing the performance of IX-based devices, exciton localization and quantum emitters.