RESUMO
The risk of information leaks increases as images become a crucial medium for information sharing. There is a great need to further develop the versatility of image encryption technology to protect confidential and sensitive information. Herein, using high spatial redundancy (strong correlation of neighboring pixels) of the image and the in situ encryption function of a quantum dot functionalized encryption camera, in situ image encryption is achieved by designing quantum dot films (size, color, and full width at half maximum) to modify the correlation and reduce spatial redundancy of the captured image during encryption processing. The correlation coefficients of simulated encrypted image closely apporach to 0. High-quality decrypted images are achieved with a PSNR of more than 35 dB by a convolutional neural network-based algorithm that meets the resolution requirements of human visual perception. Compared with the traditional image encryption algorithms, chaotic image encryption algorithms and neural network-based encryption algorithms described previously, it provides a universal, efficient and effective in situ image encryption method.
RESUMO
The quantum-dot light-emitting diode (QLED) is a new generation light emission source that holds great promise for display and lighting applications. Understanding the dynamics of electrons and holes in QLEDs during their operation is crucial for future QLED optimization, but a time-resolved technology capable of characterizing electrons is still lacking. To tackle this challenge, we develop a unique electrically pumped transient absorption (E-TA) spectroscopy to probe the density of electrons in the QD layer with a nanosecond time resolution. The E-TA result provides a comprehensive understanding of the electron dynamics in QLEDs by quantifying the electron injection time after external voltage on, electron release time after external voltage off, and equilibrated electron density (Ne) in the QD layer during device operation. By combining E-TA technology with time-resolved electroluminescence and transient current measurements, we present a comprehensive overview of the dynamics of both electrons and holes in a QLED during operation.
RESUMO
The development of efficient perovskite light-emitting diodes (PeLEDs) relies strongly on the fabrication of perovskite films with rationally designed structures (grain size, composition, surface, etc.). Therefore, an understanding of structure-performance relationships is of vital importance for developing high-performance perovskite devices, particularly for devices with in-situ fabricated perovskite nanocrystal films. In this study, we reveal the vertical structure of an in-situ fabricated quasi-two-dimensional perovskite film. By combining time-of-flight secondary ion mass spectrometry, energy dispersive spectroscopy, grazing incidence wide-angle X-ray scattering (GIWAXS), and low-temperature photoluminescence spectra, we illustrate that the resulting in-situ fabricated DPPA2Csn-1Pbn(Br0.3I0.7)3n+1 (DPPA+: 3,3-diphenylpropylammonium) film has a gradient structure with a very thin layer of ligands on the surface, predominantly small-n domains at the top, and predominantly large-n domains at the bottom owing to the solubility difference of the precursors. In addition, GIWAXS measurements show that the domain of n = 2 on the top layer has an ordered in-plane alignment. Based on the understanding of the film structure, we developed an in-situ fabrication process with ligand exchange to achieve efficient pure red PeLEDs at 638 nm with an average external quantum efficiency (EQE) of 7.4%. The optimized device had a maximum luminance of 623 cd/m2 with a peak EQE of 9.7%.
RESUMO
Specific heat capacity is one of the most fundamental thermodynamic properties of materials. In this work, we measured the specific heat capacity of PbSe nanocrystals with diameters ranging from 5 to 23 nm, and its value increases significantly from 0.2 to 0.6 J g-1 °C-1. We propose a mass assignment model to describe the specific heat capacity of nanocrystals, which divides it into four parts: electron, inner, surface, and ligand. By eliminating the contribution of ligand and electron specific heat capacity, the specific heat capacity of the inorganic core is linearly proportional to its surface-to-volume ratio, showing the size dependence. Based on this linear relationship, surface specific heat capacity accounts for 40-60% of the specific heat capacity of nanocrystals with size decreasing. It can be attributed to the uncoordinated surface atoms, which is evidenced by the appearance of extra surface phonons in Raman spectra and ab initio molecular dynamics (AIMD) simulations.
RESUMO
High-performance solution-processed perovskite light-emitting diodes (PeLEDs) have emerged as a good alternative to the well-established technology of epitaxially grown AIIIBV semiconductor alloys. Colloidal cesium lead halide perovskite nanocrystals (CsPbX3 NCs) exhibit room-temperature excitonic emission that can be spectrally tuned across the entire visible range by varying the content of different halogens at the X-site. Therefore, they present a promising platform for full color display manufacturing. Engineering of highly efficient PeLEDs based on bromide and iodide perovskite NCs emitting green and red light, respectively, does not face major challenges except low operational stability of the devices. Meanwhile, mixed-halide counterparts demonstrating blue luminescence suffer from the electric field-induced phase separation (ion segregation) phenomenon described by the rearrangement (demixing) of mobile halide ions in the crystal lattice. This phenomenon results in an undesirable temporal redshift of the electroluminescence spectrum. However, to realize spectral tuning and, at the same time, address the issue of ion segregation less mobile Cd2+ ion could be introduced in the lattice at Pb2+-site that leads to the band gap opening. Herein, we report an original synthesis of CsPb0.88Cd0.12Br3 perovskite NCs and study their structural and optical properties, in particular electroluminescence. Multilayer PeLEDs based on the obtained NCs exhibit single-peak emission centered at 485 nm along with no noticeable change in the spectral line shape for 30 min which is a significant improvement of the device performance.
RESUMO
The aging phenomenon is commonly observed in quantum-dot light emitting diodes (QLEDs), involving complex chemical or physical processes. Resolving the underlying mechanism of these aging issues is crucial to deliver reliable electroluminescent devices in future display applications. Here, we report a reversible positive aging phenomenon that the device brightness and efficiency significantly improve after device operation, but recover to initial states after long-time storage or mild heat treatment, which can be termed as warming-up effects. Steady and transient equivalent circuit analysis suggest that the radiative recombination current dramatically increases but electron leakage from the quantum dots (QDs) to hole transport layer becomes more accessible during the warming-up process. Further analysis discloses that the notable enhancement of device efficiency can be ascribed to the filling of shell traps in gradient alloyed QDs. This work reveals a distinct positive aging phenomenon featured with reversibility, and further guidelines would be provided to achieve stable QLED devices in real display applications.
RESUMO
The shell growth of large ZnSe/ZnS nanocrystals( is of great importance in the pursuit of pure-blue emitters for display applications, however, suffers from the challenges of spectral blue-shifts and reduced photoluminescence quantum yields. In this work, the ZnS shell growth on different-sized ZnSe cores is investigated. By controlling the reactivity of Zn and S precursors, the ZnS shell growth can be tuned from defect-related strain-released to defect-free strained mode, corresponding to the blue- and red-shifts of resultant nanocrystals respectively. The shape of strain-released ZnSe/ZnS nanocrystals can be kept nearly spherical during the shell growth, while the shape of strained nanocrystals evolutes from spherical into island-like after the critical thickness. Furthermore, the strain between ZnSe core and ZnS shell can convert the band alignment from type-I into type-II core/shell structure, resulting in red-shifts and improved quantum yield. By correlating the strain effects with interfacial defects, a strain-released shell growth model is proposed to obtain large ZnSe/ZnS nanocrystals with isotropic shell morphology.
RESUMO
The operational stability of the blue quantum dot light-emitting diode (QLED) has been one of the most important obstacles to initialize its industrialization. In this work, we demonstrate a machine learning assisted methodology to illustrate the operational stability of blue QLEDs by analyzing the measurements of over 200 samples (824 QLED devices) including current density-voltage-luminance (J-V-L), impedance spectra (IS), and operational lifetime (T95@1000 cd/m2). The methodology is able to predict the operational lifetime of the QLED with a Pearson correlation coefficient of 0.70 with a convolutional neural network (CNN) model. By applying a classification decision tree analysis of 26 extracted features of J-V-L and IS curves, we illustrate the key features in determining the operational stability. Furthermore, we simulated the device operation using an equivalent circuit model to discuss the device degradation related operational mechanisms.
RESUMO
Light-to-heat conversion has been intensively investigated due to the potential applications including photothermal therapy and solar energy harvesting. As a fundamental property of materials, accurate measurement of light-to-heat conversion efficiency (LHCE) is of vital importance in developing advanced materials for photothermal applications. Herein, we report a photothermal and electrothermal equivalence (PEE) method to measure the LHCE of solid materials by simulating the laser heating process with electric heating process. The temperature evolution of samples during electric heating process was firstly measured, enabling us to derive the heat dissipation coefficient by performing a linear fitting at thermal equilibrium. The LHCE of samples can be calculated under laser heating with the consideration of heat dissipation coefficient. We further discussed the effectiveness of assumptions by combining the theoretical analysis and experimental measurements, supporting the obtained small error within 5% and excellent reproducibility. This method is versatile to measure the LHCE of inorganic nanocrystals, carbon-based materials and organic materials, indicating the applicability of a variety of materials.
RESUMO
The operational stability is a current bottleneck facing the quantum dot light-emitting diodes (QLEDs). In particular, the device working around turn-on voltage suffers from unbalanced charge injection and heavy power loss. Here, we investigate the operational stability of red emissive CdSe QLEDs operated at different applied voltages. Compared to the rising luminance at higher voltages, the device luminance quickly decreases when loaded around the turn-on voltage, but recovers after unloading or slight heat treatment, which is termed fatigue effects of operational QLED. The electroluminescence and photoluminescence spectra before and after a period of operation at low voltages show that the abrupt decrease in device luminance derives from the reduction of quantum yield in quantum dots. Combined with transient photoluminescence and electroluminescence measurements, as well as equivalent circuit model analysis, the electron accumulation in quantum dots mainly accounts for the observed fatigue effects of a QLED during the operation around turn-on voltage. The underlying mechanisms at the low-voltage working regime will be very helpful for the industrialization of QLED.
RESUMO
The use of amino-terminated siloxanes as coupling interface for perovskite single crystals (PSCs)/silicon integrated devices has been demonstrated to be an effective method toward CMOS compatible optoelectronics; however, it suffers from the coupling stability against the hydrophilicity of the exposed terminal amino groups. In this work, a phenyl-terminated interfacial molecule, anilino-methyl-triethoxysilane (AMTES), is proposed to achieve the effectively galvanic coupling between PSCs and silicon, which can not only improve the device environmental reliability but also lower the surface energy of the silicon substrate so as to facilitate the epitaxial growth of PSCs. Benefiting from the interfacial coupling of AMTES, the obtained MAPbI3 SC/silicon integrated device possesses highly efficient multiwavelength photodetection properties across the X-ray and NIR range, which exhibits a specific detectivity D* of 3.84 × 1013 cm Hz1/2 W-1 in the visible-NIR region and an X-ray sensitivity of 1.18 × 104 µC Gyair-1 cm-2 with the lowest detection limit of 49.6 nGyair s-1. The ultra wide -3 dB bandwidth of 67,300 Hz and the linear dynamic range (LDR) of 112 dB also prove its impressive dynamic response capabilities. Moreover, the AMTES modified integrated device almost maintains 96% of the initial photodetection performance even after keeping in the atmosphere environment for 28 days. This work opens a new avenue for interfacial engineering toward the development of on-chip PSC integrated silicon optoelectronic devices.
RESUMO
In this work, we report the quantitative determination of charge accumulation and recombination in an operated QLED using time-resolved electroluminescence (TREL) spectroscopy. As a supplement technique, time-resolved current (TRC) measurement was introduced and simulated using equivalent circuit model with a series resistance, a parallel resistance, and a capacitance. By modeling the key processes in a typical TREL spectra, the stages of delay, rising, and decay can be correlated to the charge accumulations, charge injection and recombination, and charge release and recombination, respectively. In particular, the rising stage can be described using a modified Langevin recombination model. The electroluminescence recombination rate can be derived by fitting the rising stage curves in the TREL spectra, providing an intrinsic parameter of the emissive materials. In all, this work provides a methodology to quantitatively determine the charge accumulation and recombination of an operational QLED device.
RESUMO
The band-edge structure of halide perovskites, derived from the hybridization of atomic orbitals, plays a fundamental role in determining their optical and electronic properties. Several important concepts have been frequently discussed to describe the influence of band-edge structure on their optoelectronic properties, including Urbach tail, Rashba splitting, and exciton binding energy. In this Perspective, we provide a fundamental understanding of these concepts, with the focus on their dependence on composition, structure, or dimensionality. Subsequently, the implications for material optimization and device fabrication are discussed. Furthermore, we highlight the Rashba effect on the exciton fine structure in perovskite nanocrystals (PNCs), which explains the unique emissive properties. Finally, we discuss the potential influence of band-edge properties on the light emission process. We hope that this Perspective can inspire the investigation of band-edge properties of halide perovskites for light-emitting diodes, lasers, and spin electronics.
RESUMO
Ultraviolet (UV) visualization has extensive applications in military and civil fields such as security monitoring, space communication, and wearable equipment for health monitoring in the internet of things (IoT). Due to their remarkable optoelectronic features, perovskite materials are regarded as promising candidates for UV light detecting and imaging. Herein, we report for the first time the excitation-dependent perovskite/polymer films with dynamically tunable fluorescence ranging from green to magenta by changing the UV excitation from 260 to 380 nm. And they still render dynamic multi-color UV light imaging with different polymer matrixes, halogen ratios, and cations of perovskite materials. The mechanism of its fluorescence change is related to the chloride vacancies in perovskite materials. A patterned multi-color ultraviolet visualization pad is also demonstrated for visible conversion of the UV region. This technique may provide a universal strategy for information securities, UV visualizations, and dynamic multi-color displays in the IoT.
Assuntos
Compostos de Cálcio , Filmes Cinematográficos , Halogênios , Óxidos , PolímerosRESUMO
Photolithography has shown great potential in patterning solution-processed nanomaterials for integration into advanced optoelectronic devices. However, photolithography of perovskite quantum dots (PQDs) has so far been hindered by the incompatibility of perovskite with traditional optical lithography processes where lots of solvents and high-energy ultraviolet (UV) light exposure are required. Herein, we report a direct in situ photolithography technique to pattern PQDs based on the photopolymerization catalyzed by lead bromide complexes. By combining direct photolithography with in situ fabrication of PQDs, this method allows to directly photolithograph perovskite precursors, avoiding the complicated lift-off processes and the destruction of PQDs by solvents or high-energy UV light, as PQDs are produced after lithography exposure. We further demonstrate that the thiol-ene free-radical photopolymerization is catalyzed by lead bromide complexes in the perovskite precursor solution, while no external initiators or catalysts are needed. Using direct in situ photolithography, PQD patterns with high resolution up to 2450 pixels per inch (PPI), excellent fluorescence uniformity, and good stability, are successfully demonstrated. This work opens an avenue for non-destructive direct photolithography of high-efficiency light-emitting PQDs, and potentially expands their application in various integrated optoelectronic devices.
RESUMO
The complementary integration of perovskite single crystals (PSCs) and silicon-based circuitry provides a feasible way to combine their superiority toward efficient multiwavelength photodetection and imaging readout; however, it suffers from distinct lattice mismatch as well as the ambiguous coupling interface effect. Herein, we develop a vacuum-assisted vapor deposition strategy to realize an ultrauniform aminosiloxane interface-modified silicon wafer, which enables the monolithic epitaxial growth of PSCs with the highest mechanical coupling strength up to 340,000 N m-2 achieved so far. According to the molecular coupling engineering development with different aminosiloxanes, we achieve a highly efficient multiwavelength-responsive integrated photodetector, possessing specific photodetectivity values of 4.36 × 1012 jones and 4.55 × 1011 jones within the visible and NIR regions, respectively, as well as the lowest X-ray detection limit of 42.6 nGyair s-1. Moreover, a particularly wide -3dB cut-off frequency of 6350 Hz as well as a 120 dB linear dynamic range (LDR) also endows the integrated device with excellent dynamic photodetection capability. This work provides an efficacious approach in the integration technology for PSC-based optoelectronic applications.
RESUMO
PQDs are promising color converters for micro-LED applications. Here we report the micropore filling fabrication of high resolution patterned PQDs with a pixel size of 2 µm using a template with SU8 micropores.
RESUMO
Halide perovskites are considered to be next-generation semiconductor materials with bright prospects to advance the technology of photonics and optoelectronics. Because of the intrinsic ionic feature, the interactions between perovskites and water induce serious stability issues, which has been one of the fundamental problems hindering the practical application of perovskites. The degradation of halide perovskites upon water exposure has been intensively studied, resulting in chemical insights into key processes, including hydration, phase transformation, decomposition, and dissolution. In this Perspective, we try to illustrate what happens when halide perovskites meet with water. We summarize the research progress regarding the understanding of these processes and discuss the principle of strategy design toward improved stability against water. In addition to the instability-related interactions, we also discuss the aqueous solution of perovskite precursors for fabricating perovskite-based functional materials. Hopefully, this Perspective can inspire more fundamental studies on the interactions between perovskites and water, such as spectroscopy and simulation, crystal structure and material characterizations, and solution chemistry and crystallization.