Shape Tuning of Type II CdTe-CdSe Colloidal Nanocrystal Heterostructures through Seeded Growth

Shape Tuning of Type II CdTe-CdSe Colloidal Nanocrystal Heterostructures through Seeded Growt

Centimeter-Sized Na-Doped CsPb₂Br₅ Single Crystals with Efficient Self-Trapped Exciton Emission

We first report on the aqueous solution growth of centimeter-sized Na-doped CsPb2Br5 single crystals using a precursor solution with high NaBr concentration. The obtained Na-doped CsPb2Br5 single crystal has a tetragonal structure with a slight lattice contraction of 0.34% for Cs0.88Na0.12Pb2Br5. In comparison with the previously reported pure CsPb2Br5 single crystal, the Na-doped CsPb2Br5 single crystal shows interesting self-trapped exciton (STE) emission with emission peak at 689 nm, full width at half-maximum of 246 nm, and average photoluminescence lifetime of 429 ns. The STE emission feature of Na-doped CsPb2Br5 single crystal was further evidenced by the wavelength-dependent photoluminescence excitation spectra and intensity-dependent photoluminescence spectra measurements. By increasing the Na doping concentration, the photoluminescence quantum yields of Na-doped CsPb2Br5 single crystal can be greatly improved to a maximum value of 28.5% for Cs0.88Na0.12Pb2Br5. In addition, the methodology was also extended to fabricate Li-doped CsPb2Br5 single crystal with STE emission. The available Na-doped or Li-doped CsPb2Br5 single crystals are promising candidates for both fundamental research and the exploration of photonic applications

Illustrating the Key Role of Hydrogen Bonds in Fabricating Pure-Phase Two-Dimensional Perovskites

Two-dimensional (2D) layered perovskites consisting of multiple quantum wells are emerging as functional materials to achieve high-performance and stable optoelectronic devices. Pure-phase 2D perovskites provide a platform to investigate their fundamental properties; however, the deposition of pure-phase films remains a scientific challenge. Herein, we reveal the critical role of hydrogen bonds in fabricating pure-phase 2D perovskites. We demonstrate that the phenylalkylammonium molecules exhibit different hydrogen bonding abilities with formamidinium (FA) by varying their alkyl chain lengths. The stronger hydrogen bonding-assisted FA localization at the corner of [PbBr6]4– octahedral layer plays a key role in the crystallization of n = 2 pure-phase perovskites. On the basis of these understandings, we demonstrate deep-blue electroluminescence with an emission peak at 442 nm and a narrow line width of 13 nm, showing a peak external quantum efficiency of 0.19%. This finding opens up a new avenue for domain distribution control of 2D perovskites

In Situ Aggregation of ZnSe Nanoparticles into Supraparticles: Shape Control and Doping Effects

The ability to tune the size, shape, and properties of supraparticles is of great importance for fundamental study as well as their promising applications. We previously developed a method to synthesize monodisperse ZnSe supraparticles via “in situ aggregation” of ZnSe nanoparticles through a simple hot-injection method. In the present work, we show that the “in situ aggregation” strategy can be extended to tune the shapes of ZnSe supraparticles, and introduce novel functional magnetic and luminescence properties. Shape control is manipulated with oleic acid as ligands, which balances the attractive interparticles van der Waals forces and steric repulsive forces from the ligands. With the increase of oleic acid concentration, a morphology change from microspheres to asymmetrical multimer and three-dimensional nanoflowers was observed. “Doping” preformed Fe₃O₄ nanoparticles into ZnSe supraparticles endow them with magnetic properties. The magnetism of these Fe₃O₄@ZnSe supraparticles depends on the dosage of dopant. Doping of preformed CdS nanocrystals was also studied, resulting in emissive hybrid CdS@ZnSe supraparticles with diameters of 50–100 nm. It is noted that the doping of Fe₃O₄ and CdS nanoparticles show differing morphologies. The differences can be explained by variance in the lattice mismatches which leads to differing potentials for crystal growth

Quantitative Determination of Charge Accumulation and Recombination in Operational Quantum Dots Light Emitting Diodes via Time-Resolved Electroluminescence Spectroscopy

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

Balanced Carrier Injection and Charge Separation of CuInS₂ Quantum Dots for Bifunctional Light-Emitting and Photodetection Devices

The ligand exchange of 6-mercaptohexanol on the surface CuInS2 quantum dots not only improves their solution processability in alcoholic solvents such as methanol, ethanol, and N,N-dimethylformamide but also modulates their electrical band gap and thus the charge injection and extraction at the charge transport interfaces. Bifunctional light-emitting and photodetection devices based on these alcohol-soluble CuInS2 quantum dots are realized adopting an inverted structure with ZnO as the electron transport layer and poly­[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4′-(N-(4-butylphenyl)­diphenylaminel)] and poly­(3,4-ethylenedioxythiophene):polystyrenesulfonate as the hole transport layers. The optimized device with selected active layer thickness exhibits red emission at 647 nm with a maximum luminance of 1600 cd/m2 under forward bias and works as a photodetector at zero bias with a maximum responsibility of 0.53 mA/W and detectivity of 2.5 × 1010 jones. Furthermore, with interface engineering of the polyethylenimine ethoxylated (PEIE) layer at the electron transport side, more balanced charge injection is achieved, leading to reducing electroluminescence roll-off effect. The insulating PEIE layer also blocks the current leakage, giving rise to reduced dark current and improved detectivity of 3.5 × 1010 jones. The effective bidirectional charge transfer achieved under simplified device design using the alcohol-soluble quantum dots brings a new candidate for multifunctional devices

A Photoinitiator-Grafted Photoresist for Direct In Situ Lithography of Perovskite Quantum Dots

Precise pixel control of quantum dots (QDs) offers unparalleled opportunities for various display applications, such as the OLED and Micro-LED. However, precise selective patterning of QDs is still a challenge due to the lack of a design methodology. Therefore, the aim of this study was thus to develop a photoinitiator-grafted oligomer for “on demand” control of active free radicals to improve the line edge roughness in QD patterning. This photosensitive oligomer was constructed by grafting the photosensitive benzophenone structure onto a phenolic resin oligomer, thus resulting in the confinement of active free radicals and highly selective photolithography. As a proof of concept, we have demonstrated high-quality QD patterns with high resolution and low edge roughness by using direct in situ photolithography. This work opens an avenue for the precise design and synthesis of QD photoresists, improving the precision of QD patterning for display applications

Single-Photon-Camera-Based Time and Spatially Resolved Electroluminescence Spectroscopy for Micro-LED Analysis

To investigate the operational mechanisms of micrometer-sized light-emitting diodes (micro-LEDs), we here demonstrate a transient methodology of time and spatially resolved electroluminescence spectroscopy (TSR-EL) to measure the spatial distribution of light emission from LED devices. By combining a single-photon camera (SPC) with the time-gated sampling method, we derived the time and spatially resolved electroluminescence intensity with increasing time. Benefiting from the high sensitivity of the SPC, this methodology can detect ultralow electroluminescence (EL) at the delay stage from the device operated around the turn-on voltage. Furthermore, we investigated the spatial light distribution of a typical quantum dots light-emitting diode (QLED) under different applied voltages and varied temperatures. It was found that the EL emission of the QLED device became more uniform with increasing temperature and applied voltage. Moreover, the methodology of TSR-EL is versatile to investigate other LEDs such as organic light-emitting diodes (OLEDs), micro-LEDs, etc

Quantitative Determination of Charge Accumulation and Recombination in Operational Quantum Dots Light Emitting Diodes via Time-Resolved Electroluminescence Spectroscopy

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

Size Dependent Specific Heat Capacity of PbSe Nanocrystals

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