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Metal halide perovskite nanocrystals are promising materials for a diverse range of applications, such as light-emitting devices and photodetectors. We demonstrate the bandgap tunability of strongly emitting CH 3 NH 3 PbBr 3 nanocrystals synthesized at both room and elevated (60 °C) temperature through the variation of the precursor and ligand concentrations. We discuss in detail the role of two ligands, oleylamine and oleic acid, in terms of the coordination of the lead precursors and the nanocrystal surface. The growth mechanism of nanocrystals is elucidated by combining the experimental results with the principles of nucleation/growth models. The proposed formation mechanism of perovskite nanocrystals will be helpful for further studies in this field and can be used as a guide to improve the synthetic methods in the future. The development of perovskite nanocrystals is limited by poor mechanistic understanding of their growth. Here, the authors systematically study the ligand-assisted reprecipitation synthesis of CH 3 NH 3 PbBr 3 nanocrystals, revealing the effect of precursor and ligand concentrations on bandgap tunability.

In the regime of deep strong light–matter coupling, the coupling strength exceeds the transition energies of the material 1 – 3 , fundamentally changing its properties 4 , 5 ; for example, the ground state of the system contains virtual photons and the internal electromagnetic field gets redistributed by photon self-interaction 1 , 6 . So far, no electronic excitation of a material has shown such strong coupling to free-space photons. Here we show that three-dimensional crystals of plasmonic nanoparticles can realize deep strong coupling under ambient conditions, if the particles are ten times larger than the interparticle gaps. The experimental Rabi frequencies (1.9 to 3.3 electronvolts) of face-centred cubic crystals of gold nanoparticles with diameters between 25 and 60 nanometres exceed their plasmon energy by up to 180 per cent. We show that the continuum of photons and plasmons hybridizes into polaritons that violate the rotating-wave approximation. The coupling leads to a breakdown of the Purcell effect—the increase of radiative damping through light–matter coupling—and increases the radiative polariton lifetime. The results indicate that metallic and semiconducting nanoparticles can be used as building blocks for an entire class of materials with extreme light–matter interaction, which will find application in nonlinear optics, the search for cooperative effects and ground states, polariton chemistry and quantum technology 4 , 5 . Photons and plasmons hybridize into polaritons in three-dimensional crystals of plasmonic nanoparticles, leading to deep strong light–matter coupling and the breakdown of the Purcell effect.

Understanding chirality, or handedness, in molecules is important because of the enantioselectivity that is observed in many biochemical reactions 1 , and because of the recent development of chiral metamaterials with exceptional light-manipulating capabilities, such as polarization control 2 – 4 , a negative refractive index 5 and chiral sensing 6 . Chiral nanostructures have been produced using nanofabrication techniques such as lithography 7 and molecular self-assembly 8 – 11 , but large-scale and simple fabrication methods for three-dimensional chiral structures remain a challenge. In this regard, chirality transfer represents a simpler and more efficient method for controlling chiral morphology 12 – 18 . Although a few studies 18 , 19 have described the transfer of molecular chirality into micrometre-sized helical ceramic crystals, this technique has yet to be implemented for metal nanoparticles with sizes of hundreds of nanometres. Here we develop a strategy for synthesizing chiral gold nanoparticles that involves using amino acids and peptides to control the optical activity, handedness and chiral plasmonic resonance of the nanoparticles. The key requirement for achieving such chiral structures is the formation of high-Miller-index surfaces ({ hkl }, h ≠ k ≠ l ≠ 0) that are intrinsically chiral, owing to the presence of ‘kink’ sites 20 – 22 in the nanoparticles during growth. The presence of chiral components at the inorganic surface of the nanoparticles and in the amino acids and peptides results in enantioselective interactions at the interface between these elements; these interactions lead to asymmetric evolution of the nanoparticles and the formation of helicoid morphologies that consist of highly twisted chiral elements. The gold nanoparticles that we grow display strong chiral plasmonic optical activity (a dis-symmetry factor of 0.2), even when dispersed randomly in solution; this observation is supported by theoretical calculations and direct visualizations of macroscopic colour transformations. We anticipate that our strategy will aid in the rational design and fabrication of three-dimensional chiral nanostructures for use in plasmonic metamaterial applications. Chirality can be ‘encoded’ into gold nanoparticles by introducing chiral amino acids or peptides during the growth process, leading to the formation of helicoid morphologies.

Two-dimensional (2D) structures have been shown to possess interesting and potentially useful properties. Because of their isotropic structure, however, clusters tend to assemble into 3D architectures. Here we report the assembly of polyoxometalate clusters into layered structures that feature uniform hexagonal pores and in-plane electron delocalization properties. Because these structures are 2D and visually reminiscent of graphene, they are referred to as ‘clusterphenes’. A series of multilayer and monolayer clusterphenes have been constructed with 13 types of polyoxometalate cluster. The resulting clusterphenes were shown to exhibit substantially improved stability and catalytic efficiency towards olefin epoxidation reactions, with a turnover frequency of 4.16 h −1 , which is 76.5 times that of the unassembled clusters. The catalytic activity of the clusterphenes derives from the electron delocalization between identical clusters within the 2D layer, which efficiently reduces the activation energy of the catalytic reaction. Polyoxometalate clusters have been assembled into two-dimensional ‘clusterphene’ layers that are held together by coordination to lanthanide ions and electrostatic interactions with quaternary ammonium cations. The resulting materials resemble graphene sheets on account of their uniform hexagonal pores and are shown to catalyse epoxidation reactions due to their in-plane electron delocalization.

Most bulk-scale graphene is produced by a top-down approach, exfoliating graphite, which often requires large amounts of solvent with high-energy mixing, shearing, sonication or electrochemical treatment 1 – 3 . Although chemical oxidation of graphite to graphene oxide promotes exfoliation, it requires harsh oxidants and leaves the graphene with a defective perforated structure after the subsequent reduction step 3 , 4 . Bottom-up synthesis of high-quality graphene is often restricted to ultrasmall amounts if performed by chemical vapour deposition or advanced synthetic organic methods, or it provides a defect-ridden structure if carried out in bulk solution 4 – 6 . Here we show that flash Joule heating of inexpensive carbon sources—such as coal, petroleum coke, biochar, carbon black, discarded food, rubber tyres and mixed plastic waste—can afford gram-scale quantities of graphene in less than one second. The product, named flash graphene (FG) after the process used to produce it, shows turbostratic arrangement (that is, little order) between the stacked graphene layers. FG synthesis uses no furnace and no solvents or reactive gases. Yields depend on the carbon content of the source; when using a high-carbon source, such as carbon black, anthracitic coal or calcined coke, yields can range from 80 to 90 per cent with carbon purity greater than 99 per cent. No purification steps are necessary. Raman spectroscopy analysis shows a low-intensity or absent D band for FG, indicating that FG has among the lowest defect concentrations reported so far for graphene, and confirms the turbostratic stacking of FG, which is clearly distinguished from turbostratic graphite. The disordered orientation of FG layers facilitates its rapid exfoliation upon mixing during composite formation. The electric energy cost for FG synthesis is only about 7.2 kilojoules per gram, which could render FG suitable for use in bulk composites of plastic, metals, plywood, concrete and other building materials. Flash Joule heating of inexpensive carbon sources is used to produce gram-scale quantities of high-quality graphene in under a second, without the need for a furnace, solvents or reactive gases.

The continuous development of total synthesis chemistry has allowed many organic and biomolecules to be produced with known synthetic history–that is, a complete set of step reactions in their synthetic routes. Here, we extend such molecular-level precise reaction routes to nanochemistry, particularly to a seed-mediated synthesis of inorganic nanoparticles. By systematically investigating the time−dependent abundance of 35 intermediate species in total, we map out relevant step reactions in a model size growth reaction from molecularly pure Au 25 to Au 44 nanoparticles. The size growth of Au nanoparticles involves two different size−evolution processes (monotonic LaMer growth and volcano-shaped aggregative growth), which are driven by a sequential 2-electron boosting of the valence electron count of Au nanoparticles. Such fundamental findings not only provide guiding principles to produce other sizes of Au nanoparticles (e.g., Au 38 ), but also represent molecular-level insights on long-standing puzzles in nanochemistry, including LaMer growth, aggregative growth, and digestive ripening. Synthetic nanochemistry currently lacks the molecular step-by-step routes afforded to organic chemistry by total synthesis. Here, the authors track the seeded growth of atom-precise gold nanoclusters using mass spectrometry, revealing that the clusters evolve through a series of intermediates in two-electron steps.

Enriching the library of chiral plasmonic nanoparticles that can be chemically mass-produced will greatly facilitate the applications of chiral plasmonics in areas ranging from constructing optical metamaterials to sensing chiral molecules and activating immune cells. Here we report on a halide-assisted differential growth strategy that can direct the anisotropic growth of chiral Au nanoparticles with tunable sizes and diverse morphologies. Anisotropic Au nanodisks are employed as seeds to yield triskelion-shaped chiral nanoparticles with threefold rotational symmetry and high dissymmetry factors. The averaged scattering g -factors of the l - and d -nanotriskelions are as large as 0.57 and − 0.49 at 650 nm, respectively. The Au nanotriskelions have been applied in chiral optical switching devices and chiral nanoemitters. We also demonstrate that the manipulation of the directional growth rate enables the generation of a variety of chiral morphologies in the presence of homochiral ligands. Expanding the library of chiral plasmonic nanoparticles will foster the development of chiroptical applications. Here, the authors apply halide-assisted differential growth to convert Au nanodisks into triskelion-shaped chiral nanoparticles with threefold rotational symmetry.

Phase has emerged as an important structural parameter — in addition to composition, morphology, architecture, facet, size and dimensionality — that determines the properties and functionalities of nanomaterials. In particular, unconventional phases in nanomaterials that are unattainable in the bulk state can potentially endow nanomaterials with intriguing properties and innovative applications. Great progress has been made in the phase engineering of nanomaterials (PEN), including synthesis of nanomaterials with unconventional phases and phase transformation of nanomaterials. This Review provides an overview on the recent progress in PEN. We discuss various strategies used to synthesize nanomaterials with unconventional phases and induce phase transformation of nanomaterials, by taking noble metals and layered transition metal dichalcogenides as typical examples. Moreover, we also highlight recent advances in the preparation of amorphous nanomaterials, amorphous–crystalline and crystal phase-based hetero-nanostructures. We also provide personal perspectives on challenges and opportunities in this emerging field, including exploration of phase-dependent properties and applications, rational design of phase-based heterostructures and extension of the concept of phase engineering to a wider range of materials. Properties of nanomaterials respond to changes in the material’s phase, as well as changes in size, composition and morphology. This Review discusses the most recent developments in phase engineering of nanomaterials to afford conventional and unconventional crystal phases, amorphous phases and amorphous–crystalline heterophases.

The chirality of (nano)structures is paramount in many phenomena, including biological processes, self-assembly, enantioselective reactions, and light or electron spin polarization. In the quest for new chiral materials, metallo-organic hybrids have been attractive candidates for exploiting the aforementioned scientific fields. Here, we show that chiral carbon nanoparticles, called carbon nanodots, can be readily prepared using hydrothermal microwave-assisted synthesis and easily purified. These particles, with a mean particle size around 3 nm, are highly soluble in water and display mirror-image profile both in the UV–Vis and in the infrared regions, as detected by electronic and vibrational circular dichroism, respectively. Finally, the nanoparticles are used as templates for the formation of chiral supramolecular porphyrin assemblies, showing that it is possible to use and transfer the chiral information. This simple (and effective) methodology opens up exciting opportunities for developing a variety of chiral composite materials and applications. A promising and efficient route to chiral materials involves the transfer of chirality across length scales. Here, the authors use chiral molecular precursors to synthesize chiral carbon nanodots, which in turn can template the formation of chiral supramolecular assemblies.

Optical characteristics of luminescent materials, such as emission profile and lifetime, play an important role in their applications in optical data storage, document security, diagnostics, and therapeutics. Lanthanide-doped upconversion nanoparticles are particularly suitable for such applications due to their inherent optical properties, including large anti-Stokes shift, distinguishable spectroscopic fingerprint, and long luminescence lifetime. However, conventional upconversion nanoparticles have a limited capacity for information storage or complexity to prevent counterfeiting. Here, we demonstrate that integration of long-lived Mn 2+ upconversion emission and relatively short-lived lanthanide upconversion emission in a particulate platform allows the generation of binary temporal codes for efficient data encoding. Precise control of the particle’s structure allows the excitation feasible both under 980 and 808 nm irradiation. We find that the as-prepared Mn 2+ -doped nanoparticles are especially useful for multilevel anti-counterfeiting with high-throughput rate of authentication and without the need for complex time-gated decoding instrumentation. Luminescent materials that are capable of binary temporal coding are desirable for multilevel anti-counterfeiting. Here, the authors engineer nanoparticles that produce binary color codes on different timescales by combining the long-lived luminescence of Mn 2+ with the relatively short-lived emission of lanthanides.