In situ confinement of perovskite nanocrystals for efficient blue light-emitting LEDs

 

Metal halide perovskites have emerged as promising semiconductor materials for the fabrication of next-generation light-emitting diodes (LEDs) owing to their outstanding luminescent properties, tunable band gaps, high color purity, and low-cost fabrication processes. Perovskite LEDs (PeLEDs) have undergone rapid development, achieving external quantum efficiencies exceeding 30% for green and red emissions. 

However, for blue light emitters, the performance of PeLEDs has not yet matched that of organic LEDs (OLEDs) or metal chalcogenide quantum-dot LEDs (QD-LEDs). Blue PeLEDs still lag behind in terms of efficiency, operational lifetime, or both. Improving the performance of blue PeLEDs requires perovskite materials with both high crystallinity and nanoscale grain sizes.

High crystallinity with a low defect density suppresses non-radiative recombination losses and material degradation, whereas small grains enhance radiative efficiency through charge confinement and limited carrier diffusion. However, simultaneously achieving highly crystalline and nanoscale-confined perovskite nanocrystals via in situ synthesis on substrates remains a major challenge.

Researchers from China and the Netherlands have developed a simple in situ polymerization strategy to produce highly crystalline and size-confined Cs₀.₇EA₀.₃PbBr₃ perovskite nanocrystals (EA = ethylamine). The polymer network formed in situ from oligo(ethylene glycol) methyl ether acrylate (OEGA) dynamically restricts the excessive growth of nanocrystals during crystallization, reducing their size from more than 250 nm to 11 nm while achieving a high photoluminescence quantum yield of 83%.

Owing to its strong coordination affinity, OEGA interacts effectively with the perovskite precursors, moderating the rapid initial growth of perovskite seeds and thereby improving crystallinity. The fully confined nanocrystals exhibit a cubic phase at room temperature with reduced octahedral distortions, substantially mitigating non-radiative losses caused by electron-phonon coupling. Consequently, the resulting blue PeLEDs achieve an external quantum efficiency of 21.8% at 491 nm, placing them among the best-performing blue PeLEDs reported to date.

This work demonstrates a viable in situ nanocrystal confinement strategy that provides deeper insight into the role of ligand engineering in perovskite nanocrystal synthesis, thereby advancing the development of efficient blue PeLEDs and related optoelectronic technologies.

For further information go to Nature 

Synthesis of 2D diamonds

 

Diamond is renowned for its exceptional hardness, high thermal conductivity, and low chemical reactivity. However, it exhibits low fracture toughness and poor electrical conductivity compared with many metallic materials and nonmetallic inorganic solids. For decades, it has been theorized that two-dimensional (2D) diamond may exhibit enhanced properties relative to bulk diamond because of its two-dimensional nature, including greater mechanical strength, extraordinary carrier mobility, and a tunable band gap.

Researchers from institutions in China successfully synthesized high-quality two-dimensional diamonds by heating graphene layers with a near-infrared (NIR) laser. They employed a high-pressure, high-temperature (HPHT) process to irreversibly transform graphene layers into 2D diamond. A polished rhenium (Re) metal foil was used as both the substrate and the laser-energy absorber to facilitate the heating process. The researchers succeeded in synthesizing 2D diamonds with thicknesses ranging from the equivalent of a graphene bilayer (~1 nm) to several hundred nanometers.

The 2D diamonds were characterized by Raman spectroscopy, whose spectra exhibited a well-defined characteristic peak at 1332 cm⁻¹ with a full width at half maximum (FWHM) of approximately 3.6 cm⁻¹, indicating their high crystalline quality. Photoluminescence measurements demonstrated that these diamonds are excellent candidates for quantum computing and sensing applications. 

Furthermore, the researchers found that the band gap can be tuned within the range of 1.4 to 1.9 eV, depending on the proportion of sp³ species in the sample, which varied from 71.3% to 89.9%. Finally, they found that the diamonds remain stable at temperatures above 1000 °C.

This study demonstrates the successful synthesis of 2D diamond and reveals properties that differ from those of bulk (3D) diamond. The results indicate that 2D diamonds possess significant potential for applications in nanoelectronics and optoelectronics.

For further information go to nature communications