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COVER ARTICLE

Efficient and stable blue perovskite light emitting diodes based on defect passivation
Wu Hai-Yan, Tang Jian-Xin, Li Yan-Qing
2020, 69 (13): 138502. doi: 10.7498/aps.69.20200566
Abstract +
Solution-processable metal halide perovskites materials have many advantages, such as adjustable band gap, high photoluminescence quantum yield (PLQY), high color purity, high carrier mobility, low temperature solution process, excellent charge transport property and so on. These make them potential application in the display field. In the past few years, the device performance of perovskite light emitting devices (PeLEDs) have been greatly improved by manipulating the perovskite microstructures through various strategies, such as stoichiometry control, dimensional engineering, defect passivation and so on. At present, except for blue PeLEDs, the external quantum efficiencies (EQEs) over 20% have been achieved for green, red, and near-infrared PeLEDs. The low efficiency of blue PeLEDs is retarding their potential applications in full-color display and solid-state lighting. The main reasons in blue PeLEDs are the poor film coverage of blue perovskite materials and the spectral instability during device operation. In order to improve the quality of perovskite film and device performance, the quasi two-dimensional perovskite materials phenylethylammonium cesium lead bromide chloride (PEAxCsPbBr3–yCly) are used as the main perovskite emission material, by partially replacing Br with Cl to enlarge their bandgap to achieve the blue emission. The Lewis base polyethyleneglycol (PEG) is introduced to passivate the surface trapping defects and improve perovskite film coverage. The potassium bromide (KBr) is introduced to reduce perovskite grain size, suppress mobile ion migration and exhibit excellent spectral stability. Dual additives PEG and KBr are incorporated into the quasi-2D blue perovskite for inhibiting the nonradiative losses by passivating the traps in the perovskite films. Eventually, the PEAxCsPbBr3–yCly + PEG + KBr based blue PeLEDs with the emission peak of 488 nm are accompanied, which maximum brightness, current efficiency, and external quantum efficiency reached 1049 cd·m–2, of 5.68 cd·A–1, and of 4.6%, respectively, with high color purity (the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates is (0.0747, 0.2570)) and the narrow full width at half maximum (FWHM) of 20 nm. Compare to the devices without additives, the efficiency has increased by nearly 3 times. Furthermore, the devices also show better spectral stability and operation lifetime. This work provides an effective method of blue PeLEDs toward the practical applications.

EDITOR'S SUGGESTION

Quantum entanglement in coherent feedback system based on the cascaded four wave mixing processes
Zhong Yin-Yin, Pan Xiao-Zhou, Jing Jie-Tai
2020, 69 (13): 130301. doi: 10.7498/aps.69.20200042
Abstract +
Based on the cascaded four wave mixing processes, a coherent-feedback control system is constructed by utilizing a linear beam splitter as the feedback controller. Considering the loss of optical propagation in the coherent feedback loop and the absorption effect of Rb vapor cells to beams, we theoretically investigate the entanglement properties of this system under different feedback ratio, gain and phase by calculating the covariance matrix of system and applying the positivity under partial transpose (PPT) criterion to all possible bipartitions. The result shows that the genuine tripartite entanglement exists in the coherent feedback control system, but the entanglement structure of system will be destroyed by the excessive feedback. In addition, when the phase is π, we find that the tripartite entanglement can be enhanced by changing the gains and the reflectivity of the beam splitter in the range of 0.1 to 0.4. The results pave the way for manipulating multipartite entanglement by coherent feedback control and have potential application in quantum communication.

EDITOR'S SUGGESTION

Density fluctuations and sub-Poisson distribution in the ultracold Fermi gas of 6Li
Wang Juan, Zhang Xiao-Tian, Wu Ze-Mao, Deng Shu-Jin, Wu Hai-Bin
2020, 69 (13): 136701. doi: 10.7498/aps.69.20200603
Abstract +
In this paper, we study the spatial noise fluctuations of the density distribution of non-interacting 6Li ultracold Fermi gases. For ideal ultracold Fermi gases, the Fermi-Dirac statistics governs its quantum distribution. The suppression of density fluctuations at low temperature, due to Pauli exclusion principle, is observed in a large cloud of fermions. To clearly reveal the density noise fluctuations of the ideal Fermi gases, other noises, such as the background noise, imaging laser noise, CCD photon counting noise, are greatly suppressed. The noise fluctuation shows a sub-Poissonian statistics in excess of 10,000 atoms per spin state. The dependence of the spatial atom noise fluctuation on the quantum degeneracy is also investigated by changing the temperature of the degenerated Fermi gases. The Fermi gases with lower temperature exhibit larger suppression of the noise fluctuations. The results may have great applications in measuring the temperature of strongly correlated many-body physics and observing the phase transition of incompressible quantum phases.

EDITOR'S SUGGESTION

Multi-channel few-mode multicore fiber based surface plasmon resonance biosensor with open air-hole
Xiao Shi-Yan, Jia Da-Gong, Nie An-Ran, Yu Hui, Ji Zhe, Zhang Hong-Xia, Liu Tie-Gen
2020, 69 (13): 137802. doi: 10.7498/aps.69.20200353
Abstract +
Based on the structural characteristics of the few-mode multicore fiber (FM-MCF), a multi-channel FM-MCF surface plasmon resonance (SPR) biosensor with open air-hole is presented. Due to the air-hole distribution of the FM-MCF, the six outer air-holes naturally become open air-holes, i.e. groove sensing channels, fabricated by chemical etching. Then, compared with D-shape structure, tapered structure of fiber and air-hole of photonic crystal fiber (PCF), the open groove structure is easy to accommodate the liquid analyte. In order to obtain better sensing performance, a sensing model of the presented FM-MCF SPR biosensor with sensitive dielectric layer is established and numerical simulations are performed using the finite element method. In the simulations, the effect of core-hole distance, coating thickness, sensing dielectrics, transmission modes in optical fiber on the sensing performance as well as the role of multi-channel are analyzed. The simulation results show that when the air-hole is tangent to the core (d = 0 μm), the FM-MCF SPR biosensor has the better performance because the core-hole distance d determines the leakage intensity of the evanescent wave. As the evanescent field excited by high-order mode (LP11ax mode) is stronger than that by fundamental mode (LP01x mode), the performance of biosensors for SPR excitation by using high-order mode is better than by using fundamental mode. Meanwhile when the coating thickness of gold, silver and indium tin oxides (ITOs) is 40 nm, 30 nm and 100 nm respectively, the FWHM of loss spectrum reaches a minimum value, which means that the presented biosensor has the better performance in this sense. For the case of different sensing dielectrics, it is observed that the resonance wavelength of gold and silver film are in the visible wavelength range, while the ITO is at near-infrared wavelength. Then it is useful for our biosensor to simultaneously detect many liquid analytes in one SPR transmittance spectrum. In addition, the calculation results also show that when one of the groove channels is coated with 100 nm ITO for the LP11ax mode, the FM-MCF SPR biosensor has a highest sensitivity of 20824.66 nm/RIU and refractive index (RI) resolution is 4.8 × 10–6 RIU with the surrounding RI changing from 1.33 to 1.39, in which the RI of bovine serum albumin (BSA) solution, human Immunoglobulin G and C-reactive protein can be detected. Moreover, when the outer groove channels of our biosensor are coated with gold, silver and ITO film with different thickness, many biological liquid analytes can be detected separately or the same biological liquid analyte can be detected jointly, which reveals that the control flexibility of the groove sensing channel and the diversity of the detection analytes .

REVIEW

  

EDITOR'S SUGGESTION

Progress in perovskite solar cells based on different buffer layer materials
Chen Yong-Liang, Tang Ya-Wen, Chen Pei-Run, Zhang Li, Liu Qi, Zhao Ying, Huang Qian, Zhang Xiao-Dan
2020, 69 (13): 138401. doi: 10.7498/aps.69.20200543
Abstract +
Based on the excellent optoelectronic properties of organic-inorganic hybrids perovskite materials, the power conversion efficiency of perovskite solar cells (PSCs) is rapidly increasing. However, factors that restrict the performance of PSCs still exist, such as interface and stability problems. Problems, such as band mismatching, carrier recombination and chemical reaction between interfaces, could be alleviated by introducing a buffer layer (BL) with a proper band structure between different layers. Moreover, stability as well as charge separation and collection could also be efficiently improved in PSCs. In this paper, an overview of the most contemporary strategies of BLs was provided. The passivation mechanism of BLs at different interfaces are highlighted and discussed in detail. Furthermore, the performances of recently developed BLs in PSCs are compared. Finally, we elaborate on the remaining challenges and future directions for the development of BLs to achieve high-efficiency and high-stability PSCs.

SPECIAL TOPIC—Dielectric materials and physics

  

COVER ARTICLE

Effect of manganese doping on ferroelectric and piezoelectric properties of KNbO3 and (K0.5Na0.5)NbO3 lead-free ceramics
Xu Ze, Lou Lu-Yao, Zhao Chun-Lin, Tang Hao-Cheng, Liu Yi-Xuan, Li Zhao, Qi Xiao-Mei, Zhang Bo-Ping, Li Jing-Feng, Gong Wen, Wang Ke
2020, 69 (12): 127705. doi: 10.7498/aps.69.20200277
Abstract +
Potassium sodium niobate ((K0.5Na0.5)NbO3)-based lead-free piezoelectric ceramics are excellent ferroelectric materials and have been demonstrated to have many practical applications. Recent studies have revealed that chemical doping plays a crucial role in optimizing the electromechanical coupling properties of (K0.5Na0.5)NbO3-based piezoelectric ceramics. In this paper, MnO2 is doped into potassium niobate (KNbO3) and (K0.5Na0.5)NbO3 piezoelectric ceramics prepared by the conventional solid-state reaction method. The influences of doped Mn cation on KNbO3 and (K0.5Na0.5)NbO3 piezoelectric ceramics including microstructure and macroscopic electrical properties are systematically investigated. The doping effects of Mn cation on the KNbO3 and (K0.5Na0.5)NbO3 piezoelectric ceramics are significantly different from each other. For the Mn-doped KNbO3 piezoelectric ceramics, the sizes of ferroelectric domains are reduced. Meanwhile, the diffused orthorhombic-tetragonal phase transition is observed, which is accompanied by reducing dielectric loss and Curie temperature, and broadening vibration peaks in Raman spectrum. It is known that the oxygen vacancy can be formed to compensate for the charges created by the acceptor doping of Mn into the B site of perovskite, and thus forming a defect dipole with the acceptor center. From the ferroelectric measurement, a double hysteresis loop (P-E curve) and a recoverable electric-field-induced strain due to the formation of defect dipole are observed. On the contrary, for the Mn-doped (K0.5Na0.5)NbO3 piezoelectric ceramics, the sizes of ferroelectric domains are not reduced. Meanwhile, the Curie temperature and vibration peaks in Raman spectrum are not changed. A rectangular hysteresis loop (P-E curve) and an unrecoverable electric-field-induced strain are observed in the ferroelectric measurement. The difference between these systems might originate from the greater ionic disorder and lattice distortion in (K0.5Na0.5)NbO3 piezoelectric ceramics. The difference in ionic radius between Na+ and K+ can affect the migration and distribution of oxygen vacancies, which makes it difficult to form stable defect dipoles in the Mn-doped (K0.5Na0.5)NbO3 piezoelectric ceramics. The results will serve as an important reference for preparing high-performance (K0.5Na0.5)NbO3-based piezoelectric ceramics via chemical doping.

YOUNG SCIENTISTS' FORUM

  

YOUNG SCIENTISTS' FORUM

Liquid phase epitaxial layer by layer dipping assembly of metal-organic framework thin films and their physical property
Wang Da-Wei, Gu Zhi-Gang, Zhang Jian
2020, 69 (12): 126801. doi: 10.7498/aps.69.20200274
Abstract +
Metal-organic framework (MOF) is a new kind of inorganic-organic hybrid porous ordered crystal material, which is connected by metal nodes and organic ligands through coordination bond. Because of its large specific surface area, high stability, diverse structure and adjustable function, MOF has received wide attention. The improvements in preparation and functionalization of MOF thin films expand their application fields. In this paper, the method for assembly of surface coordinated metal-organic framework thin films (SURMOF) by liquid phase expitaxial layer-by-layer dipping method is introduced, and the physical properties of some SURMOFs in optics, electricity and other aspects are summarized, and the application prospect of SURMOF is prospected as well.

EDITOR'S SUGGESTION

Application of photon-added two-mode squeezed vacuum states to phase estimation based on Mach-Zehnder interferometer
Wang Shuai, Sui Yong-Xing, Meng Xiang-Guo
2020, 69 (12): 124202. doi: 10.7498/aps.69.20200179
Abstract +
Quantum metrology is to estimate accurately the value of an unknown parameter with the assistance of the quantum effects, in order to break through the standard quantum limit, even reach the Heisenberg limit. In this work, we study the performance of a general photon-added two-mode squeezed vacuum state that is taken as a detection state of a Mach-Zehnder interferometer. Based on quantum Fisher information, within the constraint on the total mean photon number, symmetric and asymmetric photon addition cannot improve the ultimate phase sensitivity. However, for a given initial squeezing parameter, on this occasion, the symmetric and asymmetric photon addition can improve the ultimate phase sensitivity. Compared with the asymmetric photon-added two-mode squeezed vacuum state, the symmetric one can well improve the ultimate phase sensitivity. This may be because it is always better to implement the symmetric photon addition rather than the asymmetric one in order to increase the mean photon number of the resulting state. On the other hand, via parity detection, the symmetric and asymmetric photon-added two-mode squeezed vacuum state can indeed improve the phase sensitivity of a Mach-Zehnder interferometer for a given initial squeezing parameter. Based on the parity detection, within a constraint on the mean photon number, although the two-mode squeezed vacuum state can give the better phase sensitivity at the optimal phase shift (φ = 0), the phase sensitivity offered by the symmetric and asymmetric photon-added two-mode squeezed vacuum states are both more stable around φ = 0 than by the two-mode squeezed vacuum state. In addition, we show that for the symmetric photon-added two-mode squeezed vacuum state, parity detection is an optimal detection only when the optimal phase shift approaches to zero. When the phase shift slightly deviates from zero, the parity detection is not an optimal detection scheme. Finally, for all values of the phase shift, our results also clearly show that the parity detection is not an optimal detection scheme for the asymmetric photon-added two-mode squeezed vacuum state serving as an interferometer state.

EDITOR'S SUGGESTION

Characterization of phase separation on AlGaN surfaces by in-situ photoluminescence spectroscopy and high spatially resolved surface potential images
Liu Bo-Yang, Song Wen-Tao, Liu Zheng-Hui, Sun Xiao-Juan, Wang Kai-Ming, Wang Ya-Kun, Zhang Chun-Yu, Chen Ke-Bei, Xu Geng-Zhao, Xu Ke, Li Da-Bing
2020, 69 (12): 127302. doi: 10.7498/aps.69.20200099
Abstract +
AlGaN is a key material for deep ultraviolet optoelectronic and electronic devices. With the increase of the Al composition ratio, the phase separation on the surface, caused by small-scale compositional fluctuations, is prone to affecting the performance of the device. In order to explore the mechanism of the phase separation on a nanoscale, the AlGaN wafers with different quantities of Al compositions are investigated by the confocal photoluminescence spectroscopy and the single-pass Kelvin force probe microscopy. The composition ratios of Al for the three samples are about 0.3, 0.5, and 0.7, respectively. The single-pass Kelvin force probe microscopy based on dual-frequency phase-locking is used to obtain high spatially resolved (about 10 nm) surface potential images. In the area where the phase separation phenomenon is obvious in the photoluminescence spectrum, the sharp change of the surface potential can be observed at the irregular steps and the edges of the surface pits. The potential changes can be ascribed to the inhomogeneous composition distribution. In the area where the topography turns into step flow, the surface pits shrink and merge. No obvious surface potential domain boundaries appear at the steps nor on the edges of the surface pits. Meanwhile, the phase separation phenomenon in the photoluminescence spectrum almost disappears. Our experiments show that the steps and the edges of the surface pits on AlGaN surfaces are main reasons for small-scale compositional fluctuations and the phase separation in the spectrum. Combining with in-situ confocal photoluminescence spectra, high spatially resolved surface potential image by single-pass Kelvin force probe microscopy is an effective method to characterize the phase separation on AlGaN surface on a nanoscale.

EDITOR'S SUGGESTION

Fast structured illumination three-dimensional color microscopic imaging method based on Hilbert-transform
Qian Jia, Dang Shi-Pei, Zhou Xing, Dan Dan, Wang Zhao-Jun, Zhao Tian-Yu, Liang Yan-Sheng, Yao Bao-Li, Lei Ming
2020, 69 (12): 128701. doi: 10.7498/aps.69.20200352
Abstract +
As a wide-field microscopy, structured illumination microscopy (SIM) enables super-resolution and three-dimensional (3D) imaging. It has recently received lots of attention due to the advantages of high spatial resolution, short image recording time, and less photobleaching and phototoxicity. The SIM has found numerous important applications in time-lapse imaging of living tissues and cellular structures in the field of biomedical science. Color information is an important physical quantity describing the characteristics of living creatures and reflects the differences in its microstructure and optical property to some extent. Although HSV (hue, saturation, value) color space based structured illumination full-color 3D optical sectioning technique can recover the full color information on the surface of the samples without color distortion. However, for each optical sectioning, three raw images with fixed phase shift are required to calculate the sectioning images by the rootmean square (RMS) algorithm. This will dramatically increase the data acquisition time and data storage space, especially for a large-scaled sample that needs image stitching strategy. The image processing progress operated in HSV color space need to run the RMS algorithm three times in each channel of HSV space for every section, and transform the images between RGB (red-green-blue) space and HSV space twice. This will absolutely extend the data processing time and put forward higher requirements for computer hardware and software for data storage and processing. To this end, in this paper, a fast 3D color optical sectioning SIM algorithm based on Hilbert-transform is proposed. The Hilbert-transform has proved to be a powerful tool in digital signal and image processing and has successfully applied to the SIM. Here, only two raw images with structured illumination are needed to reconstruct a full-color optical sectioned image for each slice. This fast 3D color sectioning method has the advantage of insensitivity to phase-shift error and has better adaptability to noise, high quality color sectioning images can be obtained under the phase-shift error or noise disturbed environment. The image acquisition data are reduced by 1/3 and the color optical sectioning reconstruction time is saved by about 28%, this new method effectively improves the efficiency and speed for 3D color imaging and will bring a wider application range for SIM.
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