Lattice Vacancy Induced Energy Renormalization of Photonic Quasiparticles in Two-Dimensional Semiconductors.

Coulomb interactions among dense charges and quasiparticle energy renormalization are at the center of quantum science because they significantly reshape the fundamental electronic and photonic properties of materials. While lattice vacancies are ubiquitous in solid materials, their physical effect on the Coulomb interaction among quasiparticles is normally weak and negligible. Here we show that in atomically thin semiconductors the presence of lattice vacancies emerges as an important but unexplored origin for the nontrivial renormalization of quasiparticle binding energies, due to the subtle modification of overall dielectric functions at low dimensionality. Such a renormalization effect leads to unusual reduction in the energy scales of photonic quasiparticles and red shifts of photoluminescence as the density of lattice vacancies increases. With strict configurative form factors derived, a dielectric screening model is also established for the generalized trilayer systems to capture the fine modification in the energy scales of quasiparticles and to elucidate the dielectric functions versus realistic Bohr lengths. This finding highlights the essential but commonly neglected role of lattice vacancies and deciphers the longstanding enigma of unpredictable photoluminescent line shifts in low-dimensional systems.

Achieving Simplified and Tunable Flexibility in Carborane-Based Emitters for Quantitative Vapochromic VOC Sensing.

Photoluminescence (PL) sensing of volatile organic compounds (VOCs) represents a convenient and economic detection method toward air pollutants. However, tetraphenylethylene (TPE)-based and recent carborane (Cb)-based sensors retained multiple sites that are responsive to VOC stimulation, making quantitative PL sensing rather challenging. Rendering the simplified and tunable flexibility in the PL sensors is key to achieve the quantitative target. In this work, we proposed a dimeric model of Cb-based emitters to deal with flexibility. Three emissive dibenzothiophene (DBT)-alkynylated carboranes (Cb-1/2/3) were designed and synthesized. Among them, Cb-3 contributed green and green-yellow emission in the crystals, as well as yellow and orange emission in the VOC-incorporated films, together unfolding its vapochromic properties. Crystallographic studies revealed that Cb-3 molecules were invariably dimerized in an interlocked fashion and the redshift in PL was caused by the successive through-space conjugation of DBT moieties. Theoretical calculations verified the thermodynamics stability of Cb-3 dimers and suggested that DBT could individually rotate different angles under the simulation of VOCs. Based on the above findings, we introduced DBT-alkynylated carboranes to detect the VOCs and established linear relationships between the photon energy at the PL maxima and the concentrations of benzene and tetrahydrofuran (THF) vapors. Aside from the successful implementation of quantitative vapochromic sensing, the fast response (6 s) and recovery (3∼5 s), as well as the good reusability, were also evidenced in the sensing of THF vapors.

All-Optical Switching Based on Sub-Bandgap Photoactivation of Charge Trapping in Metal Halide Perovskites.

Controllable optical properties are crucial for the application of light-emitting materials in optical devices. In this work, controllable photoluminescence in metal halide perovskite crystals is realized via photoactivation of their defects. It is found that under continuous excitation, the photoluminescence intensity of a CH3 NH3 PbBr3 crystal can be fully controlled by sub-bandgap energy photon illumination. Such optically controllable emission behavior is rather general as it is observed also in CsPbBr3 and other perovskite materials. The switching mechanism is assigned to reversible light-induced activation/deactivation of nonradiative recombination centers, the presence of which relates to an excess of Pb during perovskite synthesis. Given the success of perovskites in photovoltaics and optoelectronics, it is believed that the discovery of green luminescence controlled by red illumination will extend the application scope of perovskites toward optical devices and intelligent control.

Moisture-Dependent Blinking of Individual CsPbBr3 Nanocrystals Revealed by Single-Particle Spectroscopy.

All-inorganic metal halide perovskite nanocrystals (NCs) have been exceptional candidates for high-performance solution-processed optoelectronic and photonic devices compared with organometal halide perovskite NCs due to their superior stability. However, the interactions between all-inorganic perovskite NCs and moisture, which is an acknowledged detrimental factor, are still under debate, and detailed investigations to uncover such fundamentals remain to be performed. Herein, with wide-field fluorescence microscopy, the burst photoluminescence blinking responses of CsPbBr3 NCs were observed in ambient air, and moisture rather than oxygen was verified to be the key factor that leads to the enhanced PL intensity and reduced OFF duration. This behavior is rationalized through an effective passivation effect of the adsorbed water molecules on the surface halide vacancies on CsPbBr3 NCs. This work validates that ∼40% humidity atmospheres are helpful for better utilizing the all-inorganic perovskites, which is evidence of their promising prospect for application.

Ultrasensitive detection of local acoustic vibrations at room temperature by plasmon-enhanced single-molecule fluorescence.

Sensitive detection of local acoustic vibrations at the nanometer scale has promising potential applications involving miniaturized devices in many areas, such as geological exploration, military reconnaissance, and ultrasound imaging. However, sensitive detection of weak acoustic signals with high spatial resolution at room temperature has become a major challenge. Here, we report a nanometer-scale system for acoustic detection with a single molecule as a probe based on minute variations of its distance to the surface of a plasmonic gold nanorod. This system can extract the frequency and amplitude of acoustic vibrations with experimental and theoretical sensitivities of 10 pm Hz-1/2 and 10 fm Hz-1/2, respectively. This approach provides a strategy for the optical detection of acoustic waves based on molecular spectroscopy without electromagnetic interference. Moreover, such a small nano-acoustic detector with 40-nm size can be employed to monitor acoustic vibrations or read out the quantum states of nanomechanical devices.

Controllable Introduction of Surface Defects on CH3NH3PbI3 Perovskite.

One of the unique characteristics of semiconductors is the strong dependence of their properties on crystal defects and doping. However, due to the species diversity and low density, it is very difficult to control the type and concentration of the defects. In perovskite materials, crystal defects are randomly formed during the fast crystallization process, causing large heterogeneity of the samples. Here, in this work, we report a controllable method to introduce surface defects on CH3NH3PbI3 perovskite materials via the interaction with 1,4-benzoquinone (BQ) molecules on the gas and solid interface. After the adsorption of BQ molecules on the perovskite surface, surface defects can be generated by photoinduced chemical reactions. The concentration of the defects can thus be controlled by precisely regulating the laser irradiation time. The concentration of the defects can be characterized by a gradually decreased PL intensity and lifetime and was found to influence the atmospheric response and the subsequent acetone-induced degradation of the materials. These results demonstrate that crystal defects in perovskite materials can be controllably introduced, which provides a possible way to fully understand the correlation between the nature and chemical structure of these defects.

Characterizing Academic Help-seeking Moods for Enrollment Performance of Institutional Online Student.

Few could have anticipated the sudden and dramatic impact of COVID-19 on all aspects of life, including online academic help-seeking of institutional education. Academic help-seeking is a quite prevalent phenomenon that supports students to learn knowledge and improve academic performance. This study is aiming to understand learners and associate their performances via exploiting academic help-seeking moods with online learning of institutional education setting. Adopting the relevant theories, we propose a novel research model and identify three different online help-seeking moods, which are namely goal-directed seeker, exploratory seeker and avoidant seeker. Goal-directed seekers are described with preference for more challenging assignments and more posting on the platform discussion board frequently. Exploratory seekers hold the highest achievements during all help-seeking moods. Avoidant seekers are well-distinguished by holding the lowest frequency of posting among all moods and the most average time spent on the platform. Students have collective preferences for assignment submission in each help-seeking mood, and deviation from those preferences increases their probability of dropping academic grade significantly. To the best of our knowledge, this research is the first work that characterizes the help-seeking moods and associates moods with the enrollment performance for online education of institutional student.

Rational Building of Nonblinking Carbon Dots via Charged State Recovery.

Carbon dots (CDs), as emerging luminescent nanomaterials, possess excellent but complex properties, and thus, they have attracted immense attention for their applications. Further practical application of CDs has been hindered by their limited photostability and photoluminescence intermittency. In this study, we demonstrated that an antioxidant (Trolox) can dramatically enhance the photostability and minimize the photoblinking of CDs without affecting their native spectral characteristics. Significant photoluminescence enhancement and stabilization were observed with the addition of Trolox in ensemble level. Meanwhile, strikingly stable emissions from individual CDs have been observed in the presence of Trolox in single-particle-level experiments. Our observations revealed that the charged state of CDs can be effectively recovered to a neutral state by Trolox via electron transfer. These results prove that the combination of antioxidants and CDs is a powerful means to improve their fluorescence robustness, which is crucial for applications that demand long-lived, nonblinking emission.

Electric-Field-Induced Ion Migration Behavior in Methylammonium Lead Iodide Perovskite.

Ionic movement inside organometal halide perovskites (OMHP) materials has been widely reported to be linked with stability issues in the perovskite-based optoelectronic devices. However, the dynamic processes of the ionic movement and how they influence the devices are still not well-understood. In this work, we applied an external electric field to the CH3NH3PbI3 crystal and simultaneously monitored the PL behaviors. Two successive PL responses were observed in the same location of the crystal. First, an irreversible PL quenching was observed caused by the photo-annealing effect under an electric field accompanied by a permanent morphology change. The annealed area also showed reversible PL variation, which was attributed to the activation-deactivation of the radiative recombination centers induced by the migration of the iodine ions. Such results can help us gain a deep insight into how the ionic movements in OMHPs influence the performance of the perovskite-based optoelectronic devices under working conditions.

Photostable and Uniform CH3NH3PbI3 Perovskite Film Prepared via Stoichiometric Modification and Solvent Engineering.

Solution-processed organometal halide perovskites (OMHPs) have been widely used in optoelectronic devices, and have exhibited brilliant performance. One of their generally recognized advantages is their easy fabrication procedure. However, such a procedure also brings uncertainty about the opto-electric properties of the final samples and devices, including morphology, stability, coverage ratio, and defect concentration. Normally, one needs to find a balanced condition, because there is a competitive relation between these parameters. In this work, we fabricated CH3NH3PbI3 films by carefully changing the ratio of the PbI2 to CH3NH3I, and found that the stoichiometric and solvent engineering not only determined the photoluminescence efficiency and defects in the materials, but also affected the photostability, morphology, and coverage ratio. Combining solvent engineering and the substitution of PbI2 by Pb(Ac)2, we obtained an optimized fabrication condition, providing uniform CH3NH3PbI3 films with both high photoluminescence efficiency and high photostability under either I-rich or Pb-rich conditions. These results provide an optimized fabrication procedure for CH3NH3PbI3 and other OMHP films, which is crucial for the performance of perovskite-based solar cells and light emitting devices.

Controlled Fluorescence Enhancement of DNA-Binding Dye Through Chain Length Match between Oligoguanine and TOTO.

Fluorescent DNA-binding dyes are extensively employed as probe and biosensing in biological detection and imaging. Experiments and theoretical calculations of thiazole orange homodimeric (TOTO) dye binding to a single-strand DNA (ssDNA), poly(dG)n (n = 2, 4, 6, 8), reveal that the n = 6 complex shows about 300-fold stronger fluorescence than n = 2, 4 and a slightly stronger one than n = 8 complexes, which is benefited from the length match between TOTO and poly(dG)6. The machine learning, based on molecular dynamics trajectories, indicates that TOTO is featured by the dihedral angle along its backbone and its end-to-end distance, in which the latter one defines the stretch and hairpin structures of TOTO, respectively. The time-dependent density functional theory calculations on the low-lying excited states show that the stretched TOTO with π-π end-stacking binding mode can bring about strong fluorescence with localized π-π* transitions. For the n = 2, 4, and 8 complexes, the linear scaling quantum mechanics calculations indicate that the dominant hairpin TOTO with intercalative binding modes have relatively larger binding energies, leading to fluorescence quenching by intramolecular charge transfer. Our results may provide an insight for modulating the DNA-dye binding modes to tune the degree of charge transfer and designing fluorescent probes for the recognition of specific DNA sequences.

Deactivation/Activation of Quenching Defects in CH3NH3PbI3 Perovskite by Direct Electron Injection/Extraction.

Organometal halide perovskites (OMHPs) have emerged as advisible materials for application in optoelectronic devices over the past decade. However, a variety of complex slow responses in OMHPs under an external electric field have been observed, and the mechanisms for these responses remain a topic of intense debate. In this work, with an external voltage applied to the CH3NH3PbI3 crystal, reversible photoluminescence (PL) enhancement and quenching behaviors respectively near the anode and the cathode were observed under wide-field fluorescence microscopy. Further experiments attribute the reversible PL enhancing responses to the electron injection effect increasing the radiative recombination, while PL quenching was attributed to be due to the electron extraction effect increasing the nonradiative recombination. The control of PL by external applied voltage indicates brilliant carrier mobility in the CH3NH3PbI3 crystal and also reminds us to focus on the effect of hole/electron injection on the materials which may limit the performance of perovskite-based optoelectronic devices.

Physical insights into protection effect of conjugated polymers by natural antioxidants.

Conjugated polymers (CPs) known as organic semiconductors have been broadly applied in photovoltaic and light emitting devices due to their easy fabrication and flexibility. However, one of the bottlenecks limiting the application of CPs is their poor photostability upon continuous excitation which is one of the crucial parameters of CPs. How to improve the photostability of CPs is always one of the key questions in this field. In this work, we found that the photostability of poly(3-hexylthiophene-2,5-diyl) (P3HT) molecules can be largely improved by addition of vitamin E (VE) in bulk solution, solid films and single molecules. In solution and films, VE can not only significantly retard the photodegradation of P3HT but also enhance the fluorescence intensity. For individual P3HT molecules, with increasing VE concentrations, the on-time duration increases and the off-time duration becomes shorter. VE as natural antioxidants can not only donate electrons to the long-lived charged species but also quench the triplet states of CPs via energy transfer accelerating the depopulation process back to the ground state. The short duration time of the charged species and the triplet states provides higher fluorescence intensity. Furthermore, VE can also directly react with singlet oxygen or other reactive oxygen species (ROS) preventing them from reacting with CPs. These results not only provide an efficient strategy for improving the photostability of conjugated polymers in solution and films, but also shed light on better understanding the photophysics of conjugated polymers at single-molecule level.

A Novel Application of Educational Management Information System based on Micro Frontends.

With the launch of the Education Informatization 2.0 action plan by the Ministry of Education, a large number of college information systems have been born in China. Most of these systems are single page web applications (SPA) based on traditional MVC structures. Due to the complex logic and high coupling between educational businesses, developers need to write a lot of code. The education information system has many businesses and high coupling between businesses that the system often face problems such as bloated frontend businesses, iterative system updates, and difficult incremental function developments. Combined with the idea of service-oriented architecture, this paper proposes a micro frontends solution and applies it to the new generation of graduate information platform of East China Normal University, which has better agile development capabilities. From the aspects of service separation, efficient development, and incremental upgrade, this paper verifies that the architecture can well adapt to the needs of future educational management information system. The design of the micro frontends provides a new idea for the development of a new generation of education information system.

Inhibition of Phase Segregation in Cesium Lead Mixed-Halide Perovskites by B-Site Doping.

The emergence of all-inorganic halide perovskites has shown great potential in photovoltaic and optoelectronic devices. However, the photo-induced phase segregation in lead mixed-halide perovskites has severely limited their application. Herein, by real-time monitoring the photoluminescence (PL) spectra of metal mixed-halide perovskites under light irradiation, we found that the photo-induced phase transition can be significantly inhibited by B-site doping. For pristine mixed-halide perovskites, an intermediate phase of CsPbBrxI3-x can only be stabilized under low excitation power. After introducing Sn2+ ions, such intermediate phase can be stabilized in nitrogen atmosphere under high excitation power and phase segregation can be started after the exposure in oxygen due to oxidization of Sn2+. Replacing Sn2+ by Mn2+ can further improve the intermediate phase's tolerance to oxygen proving that B-site doping in perovskites structure by Sn2+ or Mn2+ could effectively minimize the light-induced phase segregation and promote them to serve as promising candidates in photovoltaic and light-emitting devices.

Amino Acid Encoding Methods for Protein Sequences: A Comprehensive Review and Assessment.

As the first step of machine-learning based protein structure and function prediction, the amino acid encoding play a fundamental role in the final success of those methods. Different from the protein sequence encoding, the amino acid encoding can be used in both residue-level and sequence-level prediction of protein properties by combining them with different algorithms. However, it has not attracted enough attention in the past decades, and there are no comprehensive reviews and assessments about encoding methods so far. In this article, we make a systematic classification and propose a comprehensive review and assessment for various amino acid encoding methods. Those methods are grouped into five categories according to their information sources and information extraction methodologies, including binary encoding, physicochemical properties encoding, evolution-based encoding, structure-based encoding, and machine-learning encoding. Then, 16 representative methods from five categories are selected and compared on protein secondary structure prediction and protein fold recognition tasks by using large-scale benchmark datasets. The results show that the evolution-based position-dependent encoding method PSSM achieved the best performance, and the structure-based and machine-learning encoding methods also show some potential for further application, the neural network based distributed representation of amino acids in particular may bring new light to this area. We hope that the review and assessment are useful for future studies in amino acid encoding.

Identification of the Band Gap Energy of Two-dimensional (OA)2(MA)n-1PbnI3n+1 Perovskite with up to 10 Layers.

Two-dimensional (2D) perovskites are attracting broad attention for their stability and wavelength tunability. However, random crystallization of sample preparation makes it difficult to obtain 2D perovskites with pure structure, especially when the number of layers is large. Herein, we prepared 2D perovskite (C8H17NH3)2(MA)n-1PbnI3n+1 with different layers (n = 1-10). For the first time, we experimentally identified the band gap energy Eg of 2D perovskite (C8H17NH3)2(MA)n-1PbnI3n+1 with layers up to 10 by investigating specific pieces of crystal with pure emission spectra using fluorescence microscopy. Intriguingly, the relationship between Eg and n perfectly fits an exponential function rather than the pure quantum confinement effect in good agreement with the theoretical calculation based on first principles. Our results suggest that the band gap of the 2D perovskite is determined not only by quantum confinement effect, but other factors including chemical components also give significant contribution.

Surface plasmon resonance enhanced direct Z-scheme TiO2/ZnTe/Au nanocorncob heterojunctions for efficient photocatalytic overall water splitting.

Solar-driven photocatalytic overall water splitting is regarded as one of the ideal strategies to generate renewable hydrogen energy without the initiation of environmental issues. However, there are still a few remaining challenges to develop wide-light-absorption and stable photocatalysts for the simultaneous production of H2 and O2 in pure water without sacrificial reagents. Herein, we report the design and preparation of Z-scheme TiO2/ZnTe/Au nanocorncob heterojunctions by homogeneously decorating Au nanoparticles onto the surface of core-shell TiO2/ZnTe coaxial nanorods for highly efficient overall water splitting. With the appropriate band structure of TiO2/ZnTe heterojunctions and the surface plasmon resonance enhancement of Au nanoparticles, the well-designed TiO2/ZnTe/Au nanocorncob heterojunctions can synergistically make effective utilization of broad-range solar light illunimation and enhance the separation efficency of electron-hole pairs, as evidenced by UV-Vis absorption and time-resolved photoluminescence spectroscopy. Photoelectrochemical characterization confirms that the water-splitting reaction on TiO2/ZnTe/Au nanocorncobs is mainly carried out via a two-electron/two-electron transfer process with an intermediate product of H2O2. As a result, the TiO2/ZnTe/Au nanocorncob photocatalyst can generate H2 and O2 with a stoichiometric ratio of 2 : 1 under light irradiation without any sacrificial agents, exhibiting a high H2 production rate of 3344.0 µmol g-1 h-1 and a solar-to-hydrogen (STH) efficiency of 0.98%. Moreover, the TiO2/ZnTe/Au nanocorncob heterojunctions show high stability and well-preserved morphological integrity after long-term photocatalytic tests. This study provides a prototype route to produce clean hydrogen energy from only sunlight, pure water, and rationally-designed heterojunction photocatalysts.

Predicting protein-ligand binding residues with deep convolutional neural networks.

BACKGROUND: Ligand-binding proteins play key roles in many biological processes. Identification of protein-ligand binding residues is important in understanding the biological functions of proteins. Existing computational methods can be roughly categorized as sequence-based or 3D-structure-based methods. All these methods are based on traditional machine learning. In a series of binding residue prediction tasks, 3D-structure-based methods are widely superior to sequence-based methods. However, due to the great number of proteins with known amino acid sequences, sequence-based methods have considerable room for improvement with the development of deep learning. Therefore, prediction of protein-ligand binding residues with deep learning requires study. RESULTS: In this study, we propose a new sequence-based approach called DeepCSeqSite for ab initio protein-ligand binding residue prediction. DeepCSeqSite includes a standard edition and an enhanced edition. The classifier of DeepCSeqSite is based on a deep convolutional neural network. Several convolutional layers are stacked on top of each other to extract hierarchical features. The size of the effective context scope is expanded as the number of convolutional layers increases. The long-distance dependencies between residues can be captured by the large effective context scope, and stacking several layers enables the maximum length of dependencies to be precisely controlled. The extracted features are ultimately combined through one-by-one convolution kernels and softmax to predict whether the residues are binding residues. The state-of-the-art ligand-binding method COACH and some of its submethods are selected as baselines. The methods are tested on a set of 151 nonredundant proteins and three extended test sets. Experiments show that the improvement of the Matthews correlation coefficient (MCC) is no less than 0.05. In addition, a training data augmentation method that slightly improves the performance is discussed in this study. CONCLUSIONS: Without using any templates that include 3D-structure data, DeepCSeqSite significantlyoutperforms existing sequence-based and 3D-structure-based methods, including COACH. Augmentation of the training sets slightly improves the performance. The model, code and datasets are available at https://github.com/yfCuiFaith/DeepCSeqSite .

Photo-induced dual passivation via Usanovich acid-base on surface defects of methylammonium lead triiodide perovskite.

Post-fabrication defect passivation of organometal halide perovskites has become an efficient way to improve their photophysical properties, but the underlying mechanisms are still in debate. In this work, we used p-benzoquinone (p-BQ) to generate surface defects on methylammonium lead triiodide perovskite (MAPbI3), and found that a Usanovich acid-base (O2, acetone or acetonitrile) treatment can effectively passivate those defects and lead to photoluminescence (PL) enhancement. The passivation effect arose from partial neutralization of defect charges via electron transfer between passivation reagents and relevant defects. O2 accepted photo-generated electrons, formed negatively charged oxygen species and attached to the I vacancy site to reduce its PL quenching efficiency by neutralising the defects positive charge. Likewise, acetone accepted photo-generated holes, formed positively charged species and partially neutralised the defects negative charge. The reduced trapping ability of defects caused PL enhancement. In addition, the observed photo-catalysed oxidation of acetone by O2 on the crystal surface supported the single electron transfer mechanism, and showed the potential of MAPbI3 as a photo-catalyst.