Electronic and optical properties of tapered tetrahedral semiconductor nanocrystals.

The quantum confinement effect resulting from size reduction drastically alters the electronic structure and optical properties of optoelectronic materials. Quantum confinement in nanomaterials can be efficiently controlled by morphology variation combined characteristics of nanomaterials, such as their size, shape, and spatial organization. In this study, considering indium arsenide (InAs) in tetrahedral semiconductors as an example, we demonstrated the controllable morphology evolution of InAs nanostructures by tuning the growth conditions. We used the atomistic pseudopotential method to investigate the morphology-dependent electronic and optical properties of InAs nanostructures: tapered and uniform nanostructures, including the absorption spectra, single-particle energy levels, distribution and overlap integral of band-edge states, and exciton binding energies. Compared with uniform nanomaterials, a weaker quantum confinement effect was observed in the tapered nanomaterials, because of which tapered InAs nanostructures have a smaller bandgap, larger separation of photoinduced carriers, and smaller exciton binding energy. The absorption spectra of InAs nanostructures also exhibit strong morphology dependence. Our results indicate that morphology engineering can be exploited as a potential approach for modulating the electronic and optoelectronic properties of nanomaterials.

Colloidal Synthesis of Ternary Copper Halide Nanocrystals for High-Efficiency Deep-Blue Light-Emitting Diodes with a Half-Lifetime above 100 h.

Currently, the blue perovskite light-emitting diodes (PeLEDs) suffer from a compromise in lead toxicity and poor operation stability, and most previous studies have struggled to meet the crucial blue NTSC standard. In this study, electrically driven deep-blue LEDs (∼445 nm) based on zero-dimensional (0D) Cs3Cu2I5 nanocrystals (NCs) were demonstrated with the color coordinates of (0.16, 0.07) and a high external quantum efficiency of ∼1.12%, comparable with the best-performing blue LEDs based on lead-halide perovskites. Encouraged by the remarkable stability of Cs3Cu2I5 NCs against heat and environmental oxygen/moisture, the proposed device was operated in a continuous current mode for 170 h, producing a record half-lifetime of ∼108 h. The device stability was further verified by an aggressive thermal cycling test (300-360-300 K) and a 35-day storage test. Together with the eco-friendly features and facile colloidal synthesis technique, the 0D Cs3Cu2I5 NCs can be therefore regarded as a promising candidate for deep-blue LEDs applications.

Diverse electronic properties of 2D layered Se-containing materials composed of quasi-1D atomic chains.

The two-dimensional (2D) atomically thin layered materials have attracted significant attention for constructing next-generation integrated electronic and optoelectronic devices. A special class of 2D materials composed of quasi one-dimensional (1D) atomic chains that show intriguing properties are less studied. Here, two Se-containing 2D layered materials α-Se and Sb2Se3 that have quasi-1D atomic chains are investigated via first-principles electronic structure calculations. Results shows that the electronic properties of n-monolayers (n-MLs) stacked α-Se and Sb2Se3 exhibit distinct layer-dependence electronic properties. The band gap of 2D α-Se remarkably decreases with increasing thickness, whereas the band gap of 2D Sb2Se3 show negligible change with thickness. The evolution of lattice phonon frequencies with thickness also show similar distinction. The underpinnings of the diverse electronic properties are attributed to the different electronic coupling among the layers of α-Se and Sb2Se3 that results in different van der Waals interactions among chains/layers. Our study demonstrates the rich diversity in the properties of 2D layered materials composed of lower-dimensional structural motifs.

Switching of carbohydrate nanofibers for regulating cell proliferation.

We report switchable, fluorescent carbohydrate nanofibers formed through the self-assembly of aromatic rod amphiphiles with a combination of mannose epitopes and thermoresponsive oligoether dendrons. The carbohydrate nanofibers undergo reversible switching between carbohydrate-exposed and hidden states on their surface in response to a thermal signal, and have the ability to regulate cell proliferation.

JAMIP: an artificial-intelligence aided data-driven infrastructure for computational materials informatics.

Materials informatics has emerged as a promisingly new paradigm for accelerating materials discovery and design. It exploits the intelligent power of machine learning methods in massive materials data from experiments or simulations to seek new materials, functionality, and principles, etc. Developing specialized facilities to generate, collect, manage, learn, and mine large-scale materials data is crucial to materials informatics. We herein developed an artificial-intelligence-aided data-driven infrastructure named Jilin Artificial-intelligence aided Materials-design Integrated Package (JAMIP), which is an open-source Python framework to meet the research requirements of computational materials informatics. It is integrated by materials production factory, high-throughput first-principles calculations engine, automatic tasks submission and monitoring progress, data extraction, management and storage system, and artificial intelligence machine learning based data mining functions. We have integrated specific features such as an inorganic crystal structure prototype database to facilitate high-throughput calculations and essential modules associated with machine learning studies of functional materials. We demonstrated how our developed code is useful in exploring materials informatics of optoelectronic semiconductors by taking halide perovskites as typical case. By obeying the principles of automation, extensibility, reliability, and intelligence, the JAMIP code is a promisingly powerful tool contributing to the fast-growing field of computational materials informatics.

Stable and luminescent halide perovskite fabricated in water.

Lead bromide perovskite nanoparticles are fabricated in the water, which has been recognized previously as a severe source of damage to halide perovskite materials and devices. The perovskite nanoparticles exhibit a high photoluminescence quantum yield and excellent material stability.

Imaging of the Atomic Structure of All-Inorganic Halide Perovskites.

All-inorganic halide perovskites are promising materials for optoelectronic applications. The surface or interface structure of the perovskites plays a crucial role in determining the optoelectronic conversion efficiency, as well as the material stability. A thorough understanding of surface atomic structures of the inorganic perovskites and their contributions to their optoelectronic properties and stability is lacking. Here we show a scanning tunneling microscopy investigation on the atomic and electronic structure of CsPbBr3 perovskite. Two different surface structures with a stripe and an armchair domain are identified, which originates from a complex interplay between Cs cations and Br anions. Our findings are further supported and correlated with density functional theory calculations and photoemission spectroscopy measurements. The stability evaluation of photovoltaic devices indicates a higher stability for CsPbBr3 in comparison with MAPbBr3, which is closely related to the low volatility of Cs from the perovskite surface.

Stacking Effects on Electron-Phonon Coupling in Layered Hybrid Perovskites via Microstrain Manipulation.

Organic-inorganic hybrid halide perovskites (ABX3), especially layered 2D perovskites, have been recognized as promising semiconductors due to their tunable crystal structure and unique optoelectronic properties. A-site cations, as spacers, allow various metal halide assemblies, but the stacking pattern and the influence of their collective behavior on the properties of the resultant materials remain ambiguous. Here, the cation-stacking effects in the 2D perovskite single crystals, with a focus on the electron-phonon interaction, are investigated. We reveal the different photoluminescence from the surface region and the interior of the crystal, which is due to the residual strain induced by A-site cation stacking. We also examine the cation-stacking effects on the electron-phonon interaction, which is further employed to tailor the optoelectronic properties of the resultant 2D crystals. By reducing the microstrain, we reduce the electron-phonon coupling to improve the mobility and their stability against electric field in the corresponding crystals. Our study suggests a way to manipulate the optoelectronic properties in 2D perovskite materials by rational design of cation stacking.

Strain engineering in perovskite solar cells and its impacts on carrier dynamics.

The mixed halide perovskites have emerged as outstanding light absorbers for efficient solar cells. Unfortunately, it reveals inhomogeneity in these polycrystalline films due to composition separation, which leads to local lattice mismatches and emergent residual strains consequently. Thus far, the understanding of these residual strains and their effects on photovoltaic device performance is absent. Herein we study the evolution of residual strain over the films by depth-dependent grazing incident X-ray diffraction measurements. We identify the gradient distribution of in-plane strain component perpendicular to the substrate. Moreover, we reveal its impacts on the carrier dynamics over corresponding solar cells, which is stemmed from the strain induced energy bands bending of the perovskite absorber as indicated by first-principles calculations. Eventually, we modulate the status of residual strains in a controllable manner, which leads to enhanced PCEs up to 20.7% (certified) in devices via rational strain engineering.