Perovskite light-emitting diodes (PeLEDs) have showed significant progress in recent years; the external quantum efficiency (EQE) of electroluminescence in green and red regions has exceeded 20%, but ...the efficiency in blue lags far behind. Here, a large cation CH
CH
NH
is added in PEA
(CsPbBr
)
PbBr
perovskite to decrease the Pb-Br orbit coupling and increase the bandgap for blue emission. X-ray diffraction and nuclear magnetic resonance results confirmed that the EA has successfully replaced Cs
cations to form PEA
(Cs
EA
PbBr
)
PbBr
. This method modulates the photoluminescence from the green region (508 nm) into blue (466 nm), and over 70% photoluminescence quantum yield in blue is obtained. In addition, the emission spectra is stable under light and thermal stress. With configuration of PeLEDs with 60% EABr, as high as 12.1% EQE of sky-blue electroluminescence located at 488 nm has been demonstrated, which will pave the way for the full color display for the PeLEDs.
Regulating the fluorescent properties of organic small molecules in a controlled and dynamic manner has been a fundamental research goal. Although several strategies have been exploited, realizing ...multi-color molecular emission from a single fluorophore remains challenging. Herein, we demonstrate an emissive system by combining pyrene fluorophore and acylhydrazone units, which can generate multi-color switchable fluorescent emissions at different assembled states. Two kinds of supramolecular tools, amphiphilic self-assembly and γ-cyclodextrin mediated host-guest recognition, are used to manipulate the intermolecular aromatic stacking distances, resulting in the tunable fluorescent emission ranging from blue to yellow, including a pure white-light emission. Moreover, an external chemical signal, amylase, is introduced to control the assembly states of the system on a time scale, generating a distinct dynamic emission system. The dynamic properties of this multi-color fluorescent system can be also enabled in a hydrogel network, exhibiting a promising potential for intelligent fluorescent materials.
Dynamic control over molecular emission, especially in a time‐dependent manner, holds great promise for the development of smart luminescent materials. Here we report a series of dynamic multicolor ...fluorescent systems based on the time‐encoded locking and unlocking of individual vibrational emissive units. The intramolecular cyclization reaction driven by adding chemical fuel acts as a chemical lock to decrease the conformational freedom of the emissive units, thus varying the fluorescence wavelength, while the resulting chemically locked state can be automatically unlocked by the hydrolysis reaction with water molecules. The dynamic molecular system can be driven by adding chemical fuels for multiple times. The emission wavelength and lifetime of the locking states can be readily controlled by elaborating the molecular structures, indicating this strategy as a robust and versatile way to modulate multi‐color molecular emission in a time‐encoded manner.
Multicolor molecular emission is controlled in a dynamic timing manner by introducing a temporal chemical lock. The temporal life‐time of the systems can be encoded by molecularly engineering the structural information.
Due to their environmental sustainability and high efficiency, proton‐exchange‐membrane fuel cells (PEMFCs) are expected to be an essential type of energy source for electric vehicles, energy ...generation, and the space industry in the coming decades. Here, the recent developments regarding shape‐controlled nanostructure catalysts are reviewed, with a focus on the stability of high‐performance Pt‐based catalysts and related mechanisms. The catalysts, which possess great activity, are still far from meeting the requirements of their applications, due to stability issues, especially in membrane electrode assemblies (MEAs). Thus, solutions toward the comprehensive performance of Pt‐based catalysts are discussed here. The research trends and related theories that can promote the application of Pt‐based catalysts are also provided.
Recent developments regarding high‐performance nanostructured catalysts, especially in durability, are summarized. Their durability severely limits the application of shape‐controlled catalysts. Thus, the solutions to improve their durability are systematically introduced and their feasibility is also analyzed, with reference to the design of a new generation of Pt‐based catalysts.
Underground rock structures are frequently subjected to water erosion and dynamic disturbance simultaneously. Herein, in order to study the coupled effects of water and high strain rate on the ...mechanical behavior and microstructures of sandstone, we conducted a series of dynamic unconfined compressive tests on oven-dried and water-saturated sandstone core samples using a split Hopkinson pressure bar. Test results revealed that, three macroscopic final patterns, namely unbroken, axial split and pulverization, were observed. At given strain rates, the presence of water weakens the dynamic peak stress and the dissipated energy density (the ratio of energy dissipation to residual axial strain) of rock sample but enhances the elastic modulus regardless of the failure pattern. Additionally, the saturated sample owns a higher rate dependence of dynamic strength compared to the dry one under the explored range of strain rate (43.9–156.7 s−1), which indicates that water-weakening on rock strength gradually attenuates with the increase of strain rate. Interestingly, thin sections analysis microscopically showed that the failure of dry samples is characterized by the intra-granular fracturing in larger quartz grains while that of saturated samples by the inter-granular fracturing. The underlying mechanisms were interpreted with two micro-mechanical damage models.
•Dynamic compression tests are performed on dry and saturated rocks.•Water presence weakens the peak stress and the dissipated energy density but enhances the elastic modulus.•The rate dependence in strength of saturated samples is greater than that of dry samples.•The micro-damage mechanisms of dry and saturated samples under dynamic loading are different.
Water content has a pronounced influence on the properties of rock materials, which is responsible for many rock engineering hazards, such as landslides and karst collapse. Meanwhile, water injection ...is also used for the prevention of some engineering disasters like rock-bursts. To comprehensively investigate the effect of water content on mechanical properties of rocks, laboratory tests were carried out on sandstone specimens with different water contents in both saturation and drying processes. The Nuclear Magnetic Resonance technique was applied to study the water distribution in specimens with variation of water contents. The servo-controlled rock mechanics testing machine and Split Hopkinson Pressure Bar technique were used to conduct both compressive and tensile tests on sandstone specimens with different water contents. From the laboratory tests, reductions of the compressive and tensile strength of sandstone under static and dynamic states in different saturation processes were observed. In the drying process, all of the saturated specimens could basically regain their mechanical properties and recover its strength as in the dry state. However, for partially saturated specimens in the saturation and drying processes, the tensile strength of specimens with the same water content was different, which could be related to different water distributions in specimens.
Freeze-thaw-fatigue (FTF) testing was carried out on Tibet marble at lab-scale by considering various Freeze-Thaw (FT) cycles and multi-level cyclic loading. The aim is to investigate the evolution ...of deformation and damage related parameters as well as the hysteresis behavior. The test results show that an increase of FT cycles and fatigue load level both accelerate the damage rate of marble. A rock suffering more FT cycles shows a much larger axial strain rate, radial strain rate and increasing Poisson's ratio. The relation of axial strain rate, radial strain rate and Poisson's ratio rate versus fatigue load level can be fitted by an exponential function. A warning level is defined according to the evolution of radial strain and Poisson's ratio which can inform before dilation starts. Moreover, the hysteresis behavior of stress-strain is investigated. It has been proven, that the proposed two indexes, “advance ratio” and “lag ratio” are effective precursors for rock failure prediction if subjected to FTF actions. The drastic drop of the “advance ratio” and the increase of the “lag ratio” are the indicators for forthcoming failure. The recommended warning limits for “advance ratio” and “lag ratio” for the tested marble are 5% and 70%, respectively.
In underground rock engineering, rock structures at great depth are commonly subjected to water infusion, static geo-stress as well as the dynamic disturbances. It is thus essential to understand the ...influence of water saturation on the dynamic behavior of rock under static pre-stress. In this paper, coupled static-dynamic loading tests were conducted on oven-dried and water-saturated sandstone specimens with 0–62.5 MPa axial pre-stresses. Test results show that, for each pre-stress, the dynamic strength of both dry and saturated specimens increases with the increase in strain rate. The rate dependence of saturated specimens is greater. Under a similar loading condition (i.e., same pre-stress and similar incident energy), compared with dry rocks, saturated ones possess lower dynamic strength, consume less energy before failure, dissipate more energy during post-peak stage, and produce a larger proportion of finer particles. Moreover, the comparison of failure patterns between dry and saturated specimens under different pre-stresses suggests that the critical energy of each failure pattern for saturated specimen is lower than that for dry ones. This would be responsible for the more frequent occurrence of many dynamic geo-hazards at depth under hydrated condition.
To deeply understand the rock failure characteristics under actual engineering condition, in which static geo-stress and dynamic disturbance usually act simultaneously, impact tests were conducted on ...sandstone subjected to axial static pre-stresses varying from 0 to 75 MPa by a modified split Hopkinson pressure bar. The fracturing process of specimens was recorded by a high speed camera. Dynamic parameters of sandstone, such as strain rate, dynamic strength and energy partition were acquired. Fracture mechanisms of pulverized specimens were identified by the method combining the displacement trend line and digital image correlation technique. Moreover, fragments of failed specimens were sieved to obtain the fragment size distribution. Test results revealed that, under the same incident energy, the dynamic compressive strength increases first, then decreases slowly and at last drops rapidly with the increase of pre-stress, and reaches the maximum under 24.4% of uniaxial compressive strength due to the closure of initial defects. Four final patterns were observed, namely intact, axial split, rock burst, and pulverization. The rock burst only occurs when the pre-stress lies in the elastic deformation stage or initial stable crack growth stage and the incident energy is intermediate. For pulverized specimens, the fracture mechanism is transformed into shear/tensile equivalent from tensile-dominated mixed mode as the pre-stress increases. Specimens with 75 MPa pre-stress release strain energy during failure process, contrary to specimens with lower pre-stresses absorbing energy from outside. The crushing degree of pulverized specimens exhibits a positive correlation with the pre-stress as a consequence of higher damage development in rock.
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