Regulating off-centering distortion maximizes photoluminescence in halide perovskites

Metal halide perovskites possess unique atomic and electronic configurations that endow them with high defect tolerance and enable high-performance photovoltaics and optoelectronics. Perovskite light-emitting diodes have achieved an external quantum efficiency of over 20%. Despite tremendous progress, fundamental questions remain, such as how structural distortion affects the optical properties. Addressing their relationships is considerably challenging due to the scarcity of effective diagnostic tools during structural and property tuning as well as the limited tunability achievable by conventional methods. Here, using pressure and chemical methods to regulate the metal off-centering distortion, we demonstrate the giant tunability of photoluminescence (PL) in both the intensity (>20 times) and wavelength (>180 nm/GPa) in the highly distorted halide perovskites [CH₃NH₃GeI₃, HC(NH₂)₂GeI₃, and CsGeI₃]. Using advanced in situ high-pressure probes and first-principles calculations, we quantitatively reveal a universal relationship whereby regulating the level of off-centering distortion towards 0.2 leads to the best PL performance in the halide perovskites. By applying this principle, intense PL can still be induced by substituting CH₃NH₃⁺ with Cs⁺ to control the distortion in (CH₃NH₃)_1-xCs_xGeI₃, where the chemical substitution plays a similar role as external pressure. The compression of a fully substituted sample of CsGeI₃ further tunes the distortion to the optimal value at 0.7 GPa, which maximizes the emission with a 10-fold enhancement. This work not only demonstrates a quantitative relationship between structural distortion and PL property of the halide perovskites but also illustrates the use of knowledge gained from high-pressure research to achieve the desired properties by ambient methods.

Pressure-Regulated Dynamic Stereochemical Role of Lone-Pair Electrons in Layered Bi2O2S

Lone-pair electrons (LPEs) ns² in subvalent 14 and 15 groups lead to highly anharmonic lattice and strong distortion polarization, which are responsible for the groups' outstanding thermoelectric and optoelectronic properties. However, their dynamic stereochemical role in structural and physical properties is still unclear. Here, by introducing pressure to tune the behavior of LPEs, we systematically investigate the lone-pair stereochemical role in a Bi₂O₂S. The gradually suppressed LPEs during compression show a nonlinear repulsive electrostatic force, resulting in two anisotropic structural transitions. An orthorhombic-to-tetragonal phase transition happens at 6.4 GPa, caused by the dynamic cation centering. This structural transformation effectively modulates the optoelectronic properties. Further compression beyond 13.2 GPa induces a 2D-to-3D structural transition due to the disappearance of the Bi-6s² LPEs. Therefore, the pressure-induced LPE reconfiguration dominates these anomalous variations of lattice, electronic, and optical properties. Our findings provide new insights into the materials optimization by regulating the characters of LPEs.

A novel bacteriocin BM1029: physicochemical characterization, antibacterial modes and application

AIM

Bacteriocins with antimicrobial activity are considered as potential natural bio-preservatives to control the growth of food spoilage bacteria. The aim of this work was to characterize a novel bacteriocin BM1029 discovered from Lactobacillus crustorum MN047 and evaluate its antibacterial mechanism.

METHODS AND RESULTS

Bacteriocin BM1029 was purified by cation-exchange chromatography and reversed-phase chromatography. Antibacterial activity assay showed that BM1029 is antagonistic against both Gram-positive and Gram-negative bacteria. Furthermore, it was found that BM1029 showed low haemolysis with high stability to the pretreatment with different temperatures, pH and surfactants. Moreover electron microscopy and flow cytometry suggested that BM1029 inhibit indicator strains by damaging the cell envelope integrity. Cell cycle assay suggested that BM1029 arrested cell cycle in R-phase.

CONCLUSION

The novel bacteriocin BM1029 showed high bactericidal activity against Escherichia coli and Staphylococcus aureus through a cell envelope-associated mechanism.

SIGNIFICANCE AND IMPACT OF THE STUDY

Application of BM1029 inhibited the growth of indicator strains on beef meat storage at 4°C suggesting that this bacteriocin is promising to be used as a novel preservative in food processing and preservation.

Pressure-Suppressed Carrier Trapping Leads to Enhanced Emission in Two-Dimensional Perovskite (HA)2 (GA)Pb2 I7

A remarkable PL enhancement by 12 fold is achieved using pressure to modulate the structure of a recently developed 2D perovskite (HA)₂ (GA)Pb₂ I₇ (HA=n-hexylammonium, GA=guanidinium). This structure features a previously unattainable, extremely large cage. In situ structural, spectroscopic, and theoretical analyses reveal that lattice compression under a mild pressure within 1.6 GPa considerably suppresses the carrier trapping, leading to significantly enhanced emission. Further pressurization induces a non-luminescent amorphous yellow phase, which is retained and exhibits a continuously increasing band gap during decompression. When the pressure is released to 1.5 GPa, emission can be triggered by above-band gap laser irradiation, accompanied by a color change from yellow to orange. The obtained orange phase could be retained at ambient conditions and exhibits two-fold higher PL emission compared with the pristine (HA)₂ (GA)Pb₂ I₇ .

Antiperovskites with Exceptional Functionalities

ABX₃ perovskites, as the largest family of crystalline materials, have attracted tremendous research interest worldwide due to their versatile multifunctionalities and the intriguing scientific principles underlying them. Their counterparts, antiperovskites (X₃ BA), are actually electronically inverted perovskite derivatives, but they are not an ignorable family of functional materials. In fact, inheriting the flexible structural features of perovskites while being rich in cations at X sites, antiperovskites exhibit a diverse array of unconventional physical and chemical properties. However, rather less attention has been paid to these "inverse" analogs, and therefore, a comprehensive review is urgently needed to arouse general concern. Recent advances in novel antiperovskite materials and their exceptional functionalities are summarized, including superionic conductivity, superconductivity, giant magnetoresistance, negative thermal expansion, luminescence, and electrochemical energy conversion. In particular, considering the feasibility of the perovskite structure, a universal strategy for enhancing the performance of or generating new phenomena in antiperovskites is discussed from the perspective of solid-state chemistry. With more research enthusiasm, antiperovskites are highly anticipated to become a rising star family of functional materials.

Frequency and power stabilization of a terahertz quantum-cascade laser using near-infrared optical excitation

We demonstrate a technique to simultaneously stabilize the frequency and output power of a terahertz quantum-cascade laser (QCL). This technique exploits frequency and power variations upon near-infrared illumination of the QCL with a diode laser. It does not require an external terahertz optical modulator. By locking the frequency to a molecular absorption line, we obtain a long-term (one-hour) linewidth of 260 kHz (full width at half maximum) and a root-mean-square power stability below 0.03%. With respect to the free-running case, this stabilization scheme improves the frequency stability by nearly two orders of magnitude and the power stability by a factor of three.

Chemistry Design Towards a Stable Sulfide-Based Superionic Conductor Li4 Cu8 Ge3 S12

Sulfide-based superionic conductors with high ionic conductivity have been explored as candidates for solid-state Li batteries. However, moisture hypersensitivity has made their manufacture complicated and costly and also impeded applications in batteries. Now, a sulfide-based superionic conductor Li4 Cu8 Ge3 S12 with superior stability was developed based on the hard/soft acid-base theory. The compound is stable in both moist air and aqueous LiOH aqueous solution. The electrochemical stability window was up to 1.5 V. An ionic conductivity of 0.9×10-4  S cm with low activation energy of 0.33 eV was achieved without any optimization. The material features a rigid Cu-Ge-S open framework that increases its stability. Meanwhile, the weak bonding between Li+ and the framework promotes ionic conductivity. This work provides a structural configuration in which weak Li bonding in the rigid framework promotes an environment for highly conductive and stable solid-state electrolytes.

[Clinical features of primary tracheobronchial pulmonary amyloidosis]

Objective: To analyze the clinical features of primary tracheobronchial pulmonary amyloidosis (PTBA). Methods: The records of 11 patients with PTBA diagnosed by pathology from January 2002 to June 2018 in Xiangya Hospital of Central South University were retrospectively reviewed, including clinical manifestations, laboratory and imaging examination, bronchoscopic manifestations and therapies. Meanwhile, PTBA was staged based on the severity and extent of lesion under bronchoscopy. Results: The most common clinical symptoms were cough and expectoration followed by shortness of breath and hemoptysis. Chest computed tomography (CT) revealed pulmonary infection (4/11), pulmonary nodules and masses (5/11), interstitial lesions (2/11), tracheal bronchial wall thickening (5/11) or airway stenosis (4/11). Bronchoscopy showed mucosal hypertrophy (8/11), nodular bulge (3/11), and luminal stenosis (6/11). According to the lesion involvement, 1 case only involved the lungs, 10 cases involved the trachea and/or bronchus, with (8/10) or without (2/10) lung lesions. According to the bronchoscopic staging, 2 cases (2/10) in the stage Ⅰ, lesions were limited; 2 cases (2/10) in the stage Ⅱ, lesions were diffuse; 6 cases (6/10) in the stage Ⅲ, diffuse lesions with stenosis. Conclusions: PTBA is a rare disease of the respiratory system, with unspecific clinical manifestations and diverse pulmonary imaging findings; airway mucosal hypertrophy and nodular bulging can be seen under bronchoscopy. Patients with advanced stage may present with airway stenosis.

Wideband, high-resolution terahertz spectroscopy by light-induced frequency tuning of quantum-cascade lasers

Near-infrared optical excitation enables wideband frequency tuning of terahertz quantum-cascade lasers. In this work, we demonstrate the feasibility of the approach for molecular laser absorption spectroscopy. We present a physical model which explains the observed frequency tuning characteristics by the optical excitation of an electron-hole plasma. Due to an improved excitation configuration as compared to previous work, we observe a single-mode continuous-wave frequency coverage of as much as 40 GHz for a laser at 3.1 THz. This represents, for the same device, a ten-fold improvement over the usually employed tuning by current. The method can be readily applied to a large class of devices.

Isolation, identification and characterization of lignin-degrading bacteria from Qinling, China

AIMS

Lignin is an aromatic heteropolymer forming a physical barrier and it is a big challenge in biomass utilization. This paper first investigated lignin-degradation bacteria from rotten wood in Qinling Mountain.

METHODS AND RESULTS

Nineteen potential strains were selected and ligninolytic enzyme activities were determined over 84 h. Strains that had higher enzyme activities were selected. Further, the biodegradation of wheat straw lignin and alkali lignin was evaluated indicating that Burkholderia sp. H1 had the highest capability. It was confirmed by gel permeation chromatography and field emission scanning electron microscope that alkali lignin was depolymerized into small fragments. The degraded products were analysed using gas chromatography-mass spectrometry. The total ion chromatograph of products treated for 7 days showed the formation of aromatic compounds, an important intermediate from lignin degradation. Interestingly, they disappeared in 15 days while the aldehyde and ester compounds increased.

CONCLUSIONS

The results suggest that the lignin-degrading bacteria are abundant in rotten wood and strain H1 has high potential to break down lignin.

SIGNIFICANCE AND IMPACT OF THE STUDY

The diversity of lignin-degrading bacteria in Qinling Mountain is revealed. The study of Burkholderia sp. H1 expands the range of bacteria for lignin degradation and provides novel bacteria for application to lignocellulosic biomass.

Pressure-induced dramatic changes in organic-inorganic halide perovskites

Organic-inorganic halide perovskites have emerged as a promising family of functional materials for advanced photovoltaic and optoelectronic applications with high performances and low costs. Various chemical methods and processing approaches have been employed to modify the compositions, structures, morphologies, and electronic properties of hybrid perovskites. However, challenges still remain in terms of their stability, the use of environmentally unfriendly chemicals, and the lack of an insightful understanding into structure-property relationships. Alternatively, pressure, a fundamental thermodynamic parameter that can significantly alter the atomic and electronic structures of functional materials, has been widely utilized to further our understanding of structure-property relationships, and also to enable emergent or enhanced properties of given materials. In this perspective, we describe the recent progress of high-pressure research on hybrid perovskites, particularly regarding pressure-induced novel phenomena and pressure-enhanced properties. We discuss the effect of pressure on structures and properties, their relationships and the underlying mechanisms. Finally, we give an outlook on future research avenues in which high pressure and related alternative methods such as chemical tailoring and interfacial engineering may lead to novel hybrid perovskites uniquely suited for high-performance energy applications.

Hidden Interface Driven Exchange Coupling in Oxide Heterostructures

A variety of emergent phenomena have been enabled by interface engineering in complex oxides. The existence of an intrinsic interfacial layer has often been found at oxide heterointerfaces. However, the role of such an interlayerin controlling functionalities is not fully explored. Here, we report the control of the exchange bias (EB) in single-phase manganite thin films with nominallyuniform chemical composition across the interfaces. The sign of EB depends on the magnitude of the cooling field. A pinned layer, confirmed by polarized neutron reflectometry, provides the source of unidirectional anisotropy. The origin of the exchange bias coupling is discussed in terms of magnetic interactions between the interfacial ferromagnetically reduced layer and the bulk ferromagnetic region. The sign of EB is related to the frustration of antiferromagnetic coupling between the ferromagnetic region and the pinned layer. Our results shed new light on using oxide interfaces to design functional spintronic devices.

NaxMV(PO4)3 (M = Mn, Fe, Ni) Structure and Properties for Sodium Extraction

NASICON (Na⁺ super ionic conductor) structures of Na_xMV(PO₄)₃ (M = Mn, Fe, Ni) were prepared, characterized by aberration-corrected STEM and synchrotron radiation, and demonstrated to be durable cathode materials for rechargeable sodium-ion batteries. In Na₄MnV(PO₄)₃, two redox couples of Mn³⁺/Mn²⁺ and V⁴⁺/V³⁺ are accessed with two voltage plateaus located at 3.6 and 3.3 V and a capacity of 101 mAh g^-1 at 1 C. Furthermore, the Na₄MnV(PO₄)₃ cathode delivers a high initial efficiency of 97%, long durability over 1000 cycles, and good rate performance to 10 C. The robust framework structure and stable electrochemical performance makes it a reliable cathode materials for sodium-ion batteries.

Enhanced Structural Stability and Photo Responsiveness of CH3 NH3 SnI3 Perovskite via Pressure-Induced Amorphization and Recrystallization

An organic-inorganic halide CH₃ NH₃ SnI₃ perovskite with significantly improved structural stability is obtained via pressure-induced amorphization and recrystallization. In situ high-pressure resistance measurements reveal an increased electrical conductivity by 300% in the pressure-treated perovskite. Photocurrent measurements also reveal a substantial enhancement in visible-light responsiveness. The mechanism underlying the enhanced properties is shown to be associated with the pressure-induced structural modification.

Conducting Interface in Oxide Homojunction: Understanding of Superior Properties in Black TiO2

Black TiO2 nanoparticles with a crystalline core and amorphous-shell structure exhibit superior optoelectronic properties in comparison with pristine TiO2. The fundamental mechanisms underlying these enhancements, however, remain unclear, largely due to the inherent complexities and limitations of powder materials. Here, we fabricate TiO2 homojunction films consisting of an oxygen-deficient amorphous layer on top of a highly crystalline layer, to simulate the structural/functional configuration of black TiO2 nanoparticles. Metallic conduction is achieved at the crystalline-amorphous homointerface via electronic interface reconstruction, which we show to be the main reason for the enhanced electron transport of black TiO2. This work not only achieves an unprecedented understanding of black TiO2 but also provides a new perspective for investigating carrier generation and transport behavior at oxide interfaces, which are of tremendous fundamental and technological interest.

Suppression of superconductivity and structural phase transitions under pressure in tetragonal FeS

Pressure is a powerful tool to study iron-based superconductors. Here, we report systematic high-pressure transport and structural characterizations of the newly discovered superconductor FeS. It is found that superconductor FeS (tetragonal) partly transforms to a hexagonal structure at 0.4 GPa, and then completely transforms to an orthorhombic phase at 7.4 GPa and finally to a monoclinic phase above 9.0 GPa. The superconducting transition temperature of tetragonal FeS was gradually depressed by pressure, different from the case in tetragonal FeSe. With pressure increasing, the S-Fe-S angles only slightly change but the anion height deviates farther from 1.38 Å. This change of anion height, together with the structural instability under pressure, should be closely related to the suppression of superconductivity. We also observed an anomalous metal-semiconductor transition at 6.0 GPa and an unusual increased resistance with further compression above 9.6 GPa. The former can be ascribed to the tetragonal-orthorhombic structural phase transition, and the latter to the electronic structure changes of the high-pressure monoclinic phase. Finally, a phase diagram of tetragonal FeS as functions of pressure and temperature was mapped out for the first time, which will shed new light on understanding of the structure and physics of the superconducting FeS.