Colloidal synthesis offers a route to nanoparticles (NPs) with controlled composition and structural features. This Perspective describes the use of polyvinylpyrrolidone (PVP) to obtain such nanostructures. PVP can serve as a surface stabilizer, growth modifier, nanoparticle dispersant, and reducing agent. As shown with examples, its role depends on the synthetic conditions. This dependence arises from the amphiphilic nature of PVP along with the molecular weight of the selected PVP. These characteristics can affect nanoparticle growth and morphology by providing solubility in diverse solvents, selective surface stabilization, and even access to kinetically controlled growth conditions. This Perspective includes discussions of the properties of PVP-capped NPs for surface enhanced Raman spectroscopy (SERS), assembly, catalysis, and more. The contribution of PVP to these properties as well as its removal is considered. Ultimately, the NPs accessed through the use of PVP in colloidal syntheses are opening new applications, and the concluding guidelines provided herein should enable new nanostructures to be accessed facilely.

We describe the simple, scalable, single-step, and polar-solvent-free synthesis of high-quality colloidal CsPbX (X=Cl, Br, and I) perovskite nanocrystals (NCs) with tunable halide ion composition and thickness by direct ultrasonication of the corresponding precursor solutions in the presence of organic capping molecules. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) revealed the cubic crystal structure and surface termination of the NCs with atomic resolution. The NCs exhibit high photoluminescence quantum yields, narrow emission line widths, and considerable air stability. Furthermore, we investigated the quantum size effects in CsPbBr and CsPbI nanoplatelets by tuning their thickness down to only three to six monolayers. The high quality of the prepared NCs (CsPbBr ) was confirmed by amplified spontaneous emission with low thresholds. The versatility of this synthesis approach was demonstrated by synthesizing different perovskite NCs.

Lead halide perovskite nanocrystals (NCs) are receiving a lot of attention nowadays, due to their exceptionally high photoluminescence quantum yields reaching almost 100% and tunability of their optical band gap over the entire visible spectral range by modifying composition or dimensionality/size. We review recent developments in the direct synthesis and ion exchange-based reactions, leading to hybrid organic-inorganic (CH 3 NH 3 PbX 3 ) and all-inorganic (CsPbX 3 ) lead halide (X = Cl, Br, I) perovskite NCs, and consider their optical properties related to quantum confinement effects, single emission spectroscopy and lasing. We summarize recent developments on perovskite NCs employed as an active material in several applications such as light-emitting devices, solar cells and photodetectors, and provide a critical outlook into the existing and future challenges.

High-quality hybrid halide perovskite nanocrystals are fabricated through a simple, versatile, and efficient two-step process involving a dry step followed by a ligand-assisted liquid-phase exfoliation step. The emission wavelength of the resulting nanocrystals can be tuned either through composition by varying the halide content or by reducing their thickness.

The inexorable rise of carbon dioxide level in the atmosphere, already exceeding 400 ppm, highlights the need for reduction of CO 2 emissions. Harvesting solar energy to drive reverse chemical reactions to fuel combustion offers a possible solution. The produced chemical fuels (e.g. hydrogen, methane, or methanol) are also a convenient means of energy storage, not available in photovoltaic cells. This Review is focused on the heterogeneous photocatalytic water splitting and on CO 2 reduction with nanostructured semiconductors, metals, and their hybrids. The stages of light absorption, charge separation and transfer, and surface reactions are discussed, together with possible energy-loss mechanisms and means of their elimination. Many novel materials have been developed in this active field of research, and this Review describes the concepts underpinning the continued progress in the field. The approaches which hold promise for substantial improvement in terms of efficiency, cost, and environmental sustainability are discussed in the second part. These include emerging materials (carbon dots and nitrides, bimetallic catalysts, perovskite oxides, 2D materials), more complex architectures of the photocatalyst (Z-scheme, self-assembly), and mechanisms (defect engineering, hot electron injection, redox mediators). The concluding part provides an outlook for the future directions in the field.

We demonstrate that the intrinsic properties of monolayer graphene allow it to act as a more effective saturable absorber for mode-locking fiber lasers when compared to multilayer graphene. The absorption of monolayer graphene can be saturated at lower excitation intensity compared to multilayer graphene, graphene with wrinkle-like defects, or functionalized graphene. Monolayer graphene has a remarkably large modulation depth of 65.9%, whereas the modulation depth of multilayer graphene is greatly reduced due to nonsaturable absorption and scattering loss. Picosecond ultrafast laser pulses (1.23 ps) can be generated using monolayer graphene as a saturable absorber. Due to the ultrafast relaxation time, larger modulation depth and lower scattering loss of monolayer graphene, it performs better than multilayer graphene in terms of pulse shaping ability, pulse stability, and output energy.

We describe a simple and efficient methodology for the aqueous synthesis of stable, uniform, and size tunable Au@Ag core-shell nanoparticles (NPs) that are stabilized by citrate ions. The synthetic route is based on the stepwise Ag reduction on preformed Au NPs. The final size of the core-shell NPs and therefore their optical properties can be modulated at least from 30 to 110 nm by either tuning the Ag shell thickness or changing the size of the Au core. The optical properties of the Au@Ag core-shell NPs resemble those of pure Ag NPs of similar sizes, which was confirmed by means of Mie extinction calculations. We additionally evaluated the surface-enhanced raman scattering (SERS) enhancing properties of Au@Ag core-shell NP colloids with three different laser lines (532, 633, and 785 nm). Importantly, such core-shell NPs also exhibit a higher SERS efficiency than Ag NPs of similar size under near-infrared excitation. The results obtained here serve as a basis to select Au@Ag core-shell NPs of specific size and composition with maximum SERS efficiency at their respective excitation wavelengths for SERS-based analytical and bioimaging applications.

The colloidal synthesis and assembly of semiconductor nanowires continues to attract a great deal of interest. Herein, we describe the single-step ligand-mediated synthesis of single-crystalline CsPbBr perovskite nanowires (NWs) directly from the precursor powders. Studies of the reaction process and the morphological evolution revealed that the initially formed CsPbBr nanocubes are transformed into NWs through an oriented-attachment mechanism. The optical properties of the NWs can be tuned across the entire visible range by varying the halide (Cl, Br, and I) composition through subsequent halide ion exchange. Single-particle studies showed that these NWs exhibit strongly polarized emission with a polarization anisotropy of 0.36. More importantly, the NWs can self-assemble in a quasi-oriented fashion at an air/liquid interface. This process should also be easily applicable to perovskite nanocrystals of different morphologies for their integration into nanoscale optoelectronic devices.