It is conventionally assumed that the growth of monodisperse colloidal nanocrystals requires a temporally discrete nucleation followed by monomer attachment onto the existing nuclei. However, recent studies have reported violations of this classical growth model, and have suggested that inter-particle interactions are also involved during the growth. Mechanisms of nanocrystal growth still remain controversial. Using in situ transmission electron microscopy, we show that platinum nanocrystals can grow either by monomer attachment from solution onto the existing particles or by coalescence between the particles. Surprisingly, an initially broad size distribution of the nanocrystals can spontaneously narrow. We suggest that nanocrystals take different pathways of growth based on their size- and morphology-dependent internal energies. These observations are expected to be highly relevant for other nanocrystal systems.

Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories

Metal halide perovskites represent a flourishing area of research, driven by both their potential application in photo-voltaics and optoelectronics, and for the fundamental science underpinning their unique optoelectronic properties. The advent of colloidal methods for the synthesis of halide perovskite nanocrystals has brought to the attention inter-esting aspects of this new type of materials, above all their defect-tolerance. This review aims to provide an updated survey of this fast-moving field, with a main focus on their colloidal synthesis. We examine the chemistry and the ca-pability of different colloidal synthetic routes to control the shape, size and optical properties of the resulting nano-crystals. We also provide an up to date overview of their post-synthesis transformations, and summarize the various so-lution processes aimed at fabricating halide perovskite-based nanocomposites. We then review the fundamental optical properties of halide perovskite nanocrystals, by focusing on their linear optical properties, on the effects of quantum confinement and, then, on the current knowledge of their exciton binding energies. We also discuss the emergence of non-linear phenomena such as multiphoton absorption, biexcitons and carrier multiplication. At last, we provide an outlook in the field, with the most cogent open questions and possible future directions.

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications and Their Optical Properties

Metal halide perovskites represent a flourishing area of research, which is driven by both their potential application in photovoltaics and optoelectronics and by the fundamental science behind their unique optoelectronic properties. The emergence of new colloidal methods for the synthesis of halide perovskite nanocrystals, as well as the interesting characteristics of this new type of material, has attracted the attention of many researchers. This review aims to provide an up-to-date survey of this fast-moving field and will mainly focus on the different colloidal synthesis approaches that have been developed. We will examine the chemistry and the capability of different colloidal synthetic routes with regard to controlling the shape, size, and optical properties of the resulting nanocrystals. We will also provide an up-to-date overview of their postsynthesis transformations, and summarize the various solution processes that are aimed at fabricating halide perovskite-based nanocomposites. Furthermore, we...

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties.

Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH3NH3PbBr3, CH(NH2)2PbBr3, and CsPbBr3. We observed hot fluorescence emission from energetic carriers with ~102-picosecond lifetimes in CH3NH3PbBr3 or CH(NH2)2PbBr3, but not in CsPbBr3. The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit.

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Screening in crystalline liquids protects energetic carriers in hybrid perovskites

Hybrid halide perovskites are now superstar materials leading the field of low-cost thin film photovoltaics technologies. Following the surge for more efficient and stable 3D bulk alloys, multilayered halide perovskites and colloidal perovskite nanostructures appeared in 2016 as viable alternative solutions to this challenge, largely exceeding the original proof of concept made in 2009 and 2014, respectively. This triggered renewed interest in lower-dimensional hybrid halide perovskites and at the same time increasingly more numerous and differentiated applications. The present paper is a review of the past and present literature on both colloidal nanostructures and multilayered compounds, emphasizing that availability of accurate structural information is of dramatic importance to reach a fair understanding of quantum and dielectric confinement effects. Layered halide perovskites occupy a special place in the history of halide perovskites, with a large number of seminal papers in the 1980s and 1990s. In ...

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Quantum and Dielectric Confinement Effects in Lower-Dimensional Hybrid Perovskite Semiconductors.

Wearable piezoresistive pressure sensors based on 3D graphene

Engineering the Exciton Dissociation in Quantum‐Confined 2D CsPbBr3 Nanosheet Films

Preparation of all-inorganic perovskite quantum dots-polymer composite for white LEDs application

Wearable rGO-Ag NW@cotton fiber piezoresistive sensor based on the fast charge transport channel provided by Ag nanowire

Flexible substrate consisted of PMMA-supported monolayer graphene with sandwiched Ag-nanoflowers (G/AgNFs/PMMA) for ultrasensitive, reproducible, and stable surface-enhanced Raman scattering (SERS) detection is reported. Graphene templated micro-current-assisted chemical reduction method was employed to support AgNFs growth, and uniform-distribution AgNFs with all directions nanotips can generate tremendous enhancement factor and intensive hotspots. The minimum detectable concentration (i.e., detection limit) for rhodamine 6G (R6G) in-situ detection by covering this as-synthesized G/AgNFs/PMMA flexible substrate can be as low as 10−14M. Moreover, graphene can effectively stabilize the SERS signals and protect AgNFs from oxidation, endowing this flexible substrate a long-term stability with maximum intensity deviation lower than 10%, for the quantitative measurements from spot-to-spot or substrate-to-substrate. In order to trial its practical applications with various real-world surfaces, the in-situ SERS detection of phenylalanine@apple, adenosine aqueous solution and methylene-blue@fish was performed by covering this G/AgNFs/PMMA flexible substrate. Clear Raman peaks can be obtained for all the selected samples with concentration of 10−10M and, importantly, good linear relationship between Raman intensity and molecular concentration indicates the potential application of the G/AgNFs/PMMA flexible substrate in quantitative determination. Thus, this high-efficiency and low-cost flexible SERS substrate may provide a new way for the molecular trace-detection in food security and environmental protection.

Hengwei Qiu

Papers

Wearable piezoresistive pressure sensors based on 3D graphene

Engineering the Exciton Dissociation in Quantum‐Confined 2D CsPbBr3 Nanosheet Films

Preparation of all-inorganic perovskite quantum dots-polymer composite for white LEDs application

Wearable rGO-Ag NW@cotton fiber piezoresistive sensor based on the fast charge transport channel provided by Ag nanowire

Reliable molecular trace-detection based on flexible SERS substrate of graphene/Ag-nanoflowers/PMMA