Co-multiplexing spectral and temporal dimensions based on luminescent materials.

Optical multiplexing is a pivotal technique for augmenting the capacity of optical data storage (ODS) and increasing the security of anti-counterfeiting. However, due to the dearth of appropriate storage media, optical multiplexing is generally restricted to a single dimension, thus curtailing the encoding capacity. Herein, the co-multiplexing spectral and temporal dimensions are proposed for optical encoding based on photoluminescence (PL) and persistent-luminescence (PersL) at four different wavelengths. Each emission color comprises four luminescence modes. The further multiplexing of four wavelengths leads to the maximum encoding capacity of 8 bits at each pixel. The wavelength difference between adjacent peaks is larger than 50 nm. The well-separated emission wavelengths significantly lower the requirements for high-resolution spectrometers. Moreover, the information is unable to be decoded until both PL and PersL spectra are collected, suggesting a substantial improvement in information security and the security level of anti-counterfeiting.

Snapshot multi-frame parallel spectral holographic microscopy based on a reconfigurable optical comb.

We present a snapshot multi-frame parallel holographic microscopy system through a reconfigurable optical comb source, which consists of a digital micromirror device (DMD) based spectrum filter system and a spectroscopic Michelson interferometric system. The proposed system allows arbitrarily tuning comb spacing and comb number, and the capturing of multi-frame images without overlap in one exposure. As a result, high-quality spectral holograms can be obtained with less acquisition time. The performance of the system is detailed in the experiment and 45-wavelengths holographic imaging for perovskite micro-platelets is conducted, which proves the system has the ability to realize high-performance four-dimensional (4D) imaging.

Structural characterizations on the degradation of 2D organic-inorganic hybrid perovskites and its enlightenment to improved stability.

Despite the demonstrated high-efficiency of solar cells and light-emitting devices based on two-dimensional (2D) perovskites, intrinsic stability of the 2D perovskites is yet far from satisfactory. In this work, we find the 2D (BA)2PbI4perovskite crystals rapidly degrade in the ambient conditions and the photoluminescence (PL) nearly completely quenches in 6 d. Moreover, the PL shoulder band due to defects and absorption band of PbI2gradually rise during degradation, suggesting the precipitation of PbI2. Besides, rod structures are observed in the degraded crystals, which are attributed to the formation of one-dimensional (1D) (BA)3PbI5perovskites. And the degradation can be largely retarded by decreasing the humidity during storage. Therefore, a chemical reaction for the degradation of (BA)2PbI4is proposed, revealing the interactions between water molecules and undercoordinated defects are very critical for understanding the degradation. Enlightened by these findings, dimethyl itaconate (DI) treatment is developed to passivate the defects and block the intrusion of moisture to improve the stability of the (BA)2PbI4. After storage in the ambient environment for 16 d, the DI treated (BA)2PbI4only shows a slight surface degradation without formation of any nanorod-like structures, and the PL intensity retains about 70%. Therefore, our systematic study provides a comprehensive understanding on the degradation dynamics of 2D perovskites, which will promote future development of intrinsically stable 2D perovskites.

Mechanism of Photoinduced Phase Segregation in Mixed-Halide Perovskite Microplatelets and Its Application in Micropatterning.

Photoinduced phase segregation (PPS) is considered as a dominant factor that greatly deteriorates the performances of mixed-halide perovskite devices. However, the mechanism of PPS is still under fierce debate. Herein, CsPb(Brx/Cl1-x)3 microplatelets (MPs) with homogeneous and heterogeneous surfaces are obtained by controlling the growth conditions. Under continuous irradiation, a new photoluminescence (PL) band at 516 nm gradually appears in the heterogeneous MPs, accompanied with the decreased emission of the mixed phase at 480 nm, revealing the occurrence of PPS, while the photoirradiation only leads to slight PL dimming without PPS in the homogeneous MPs. The direct correlation between PPS and the structural heterogeneity indicates that the localized electric field-induced drift (LEFD) of halide ions/carriers is responsible for the PPS. In situ microfluorescence images evidence that the migration of halide ions is directed by the structural heterogeneity-induced localized electric field. Our refined model not only consolidates that PPS can be suppressed by eliminating the defects but also reveals that PPS can be directed by the distribution of defects. Therefore, a fluorescence micropatterning technique is developed based on PPS.

Microsteganography on all inorganic perovskite micro-platelets by direct laser writing.

Direct laser writing (DLW) is a mask-free and cost-efficient micro-fabrication technology, which has been explored to pattern structures on perovskites. However, there is still a lack of research on DLW methods for microsteganography. Herein, we developed a sophisticated DLW condition to pattern on CsPbBr3 perovskite micro-platelets (MPs). In addition to the reversible PL quenching caused by photo-induced ion migration, permanent nonradiative centers are also produced by the DLW treatment. Therefore, the patterned information is retained after long-term storage. Meanwhile, the mild DLW condition only results in a faint trace, which is almost invisible under a regular optical microscope. Thus, the patterned information is hidden unless applying an excitation source, which paves the way for applications in microsteganography and anti-counterfeiting. As a proof-of-concept, different patterns are drawn on the CsPbBr3 MPs by DLW, which are only observable under a fluorescence microscope.

Power-Free and Self-Cleaning Solar Light Detector Based on the Temperature-Sensitive Structural Color and Photothermal Effect.

In this work, photothermal materials are integrated with a temperature-sensitive hydrogel and structural color for visually detecting solar intensity. Inspired by the functional performance of beetles, the photothermal layer is constructed by depositing candle soot on a film of Cu nanoparticles, while the temperature-sensitive colored hydrogel is fabricated by self-assembling colloidal photonic crystals on poly(N-isopropylacrylamide) (PNiPAM). The deposition of candle soot not only improves the photothermal performance but also leads to a superhydrophobic surface with a self-cleaning function. The photothermal layer absorbs sunlight and converts it into heat, which is then transferred to the hydrogel. The structural color of the hydrogel changes due to the heat-induced volume shrinkage. As the solar intensity increases from 0.62 to 1.27 kW/m2, the structural color conspicuously changes from red to orange, yellow, green, cyan, and blue, with reflection peaks shifting from 640 to 460 nm accordingly. The color change is highly apparent, which can be easily observed by the naked eye, suggesting that the solar intensity can be easily detected by reading out the structural color. This power-free and self-cleaning solar sensor can work for a long period without maintenance, which is suitable for a wide application prospect, such as smart home and agriculture.

Fluorescent dynamics of CsPbBr3 nanocrystals in polar solvents: a potential sensor for polarity.

During synthesis, device processes, and applications of perovskite nanocrystals (NCs), there are usually inevitable interactions between perovskite NCs and polar solvents. To elaborately control the properties of perovskite NCs, investigating the effects of solvent polarity on perovskite NCs is thus highly important. Herein, fluorescent variations induced by different solvents into CsPbBr3 NCs solution are systematically studied. In this report, it is found that when CsPbBr3 NCs are treated with polar solvents, the fluorescence intensity decreases with a general redshift of fluorescence peak position. Moreover, the fluorescence quenching and peak position shift amplitude monotonously increase with the solvent polarity. Absorption spectra and fluorescent lifetime suggest that, with addition of polar solvents, the surface of NCs are destroyed and defect states are generated, leading to the fluorescent variations. Besides, dielectric constant of the solvent also increases with polarity, which may weaken the quantum confinement effect and decrease the exciton binding energy. We find the fluorescence may slightly blue shift if the emission of free carrier is strong enough with certain solvents, such as dimethylsulfoxide (DMSO). We also find the fluorescence intensity generally deceases to a stable state in 2 min, indicating quick interactions between CsPbBr3 NCs and solvents. However, water continuously quenches the fluorescence of CsPbBr3 NCs up to 72 h due to the poor miscibility between water and n-hexane. This work not only provides a comprehensive understanding on the fluorescent dynamics of CsPbBr3 NCs in polar solvents but also affords a potential fluorescent indicator for solvent polarity.