Geoffrey A. Ozin

Periodic mesoporous organosilicas with organic groups inside the channel walls

Surfactant-mediated synthesis methods have attracted much interest for the production of inorganic mesoporous materials, which can, on removal of the surfactant template, incorporate polymeric, organic, inorganic and organometallic ‘guests’ in their pores. These materials—initially made of silica, but now also available in the form of other oxides, sulphides, phosphates and metals—could find application in fields ranging from catalysis, adsorption and sensing technology to nanoelectronics. The extension of surfactant-mediated synthesis to produce inorganic–organic hybrid material (that is, materials that contain organic groups as an integral part of their framework structure) promises access to an even wider range of application possibilities. Such hybrid materials have been produced in the form of amorphous silicates (xerogels) that indeed display unique properties different to those of the individual components, but their random networks with broad pore-size distributions severely limit the shape and size selectivity of these materials. Mesoporous hybrid materials with periodic frameworks have been synthesized, but the organic groups are all terminally bonded to the pore surface, rather than incorporated into the pore walls. Here we describe a periodic mesoporous organosilica containing bridge-bonded ethene groups directly integrated into the silica framework. We are able to solvent-extract and ion-exchange the surfactant templates to create a stable and periodic mesoporous ethenesilica with high surface area and ethene groups that are readily accessible for chemical reaction. Recent syntheses of similar periodic mesoporous organosilicas and the ability to incorporate a variety of bridging organic and organometallic species raise the prospect of being able to fuse organic synthesis and inorganic materials chemistry to generate new materials with interesting chemical, mechanical electronic, optical and magnetic properties.

Large-scale synthesis of a silicon photonic crystal with a complete three-dimensional bandgap near 1.5 micrometres

Photonic technology, using light instead of electrons as the information carrier, is increasingly replacing electronics in communication and information management systems. Microscopic light manipulation, for this purpose, is achievable through photonic bandgap materials, a special class of photonic crystals in which three-dimensional, periodic dielectric constant variations controllably prohibit electromagnetic propagation throughout a specified frequency band. This can result in the localization of photons, thus providing a mechanism for controlling and inhibiting spontaneous light emission that can be exploited for photonic device fabrication. In fact, carefully engineered line defects could act as waveguides connecting photonic devices in all-optical microchips, and infiltration of the photonic material with suitable liquid crystals might produce photonic bandgap structures (and hence light-flow patterns) fully tunable by an externally applied voltage. However, the realization of this technology requires a strategy for the efficient synthesis of high-quality, large-scale photonic crystals with photonic bandgaps at micrometre and sub-micrometre wavelengths, and with rationally designed line and point defects for optical circuitry. Here we describe single crystals of silicon inverse opal with a complete three-dimensional photonic bandgap centred on 1.46 µm, produced by growing silicon inside the voids of an opal template of close-packed silica spheres that are connected by small ‘necks’ formed during sintering, followed by removal of the silica template. The synthesis method is simple and inexpensive, yielding photonic crystals of pure silicon that are easily integrated with existing silicon-based microelectronics.

Synthesis of Ligand-Free Colloidally Stable Water Dispersible Brightly Luminescent Lanthanide-Doped Upconverting Nanoparticles

The synthesis using the thermal decomposition of metal trifluoroacetates is being widely used to prepare oleate-capped lanthanide-doped upconverting NaYF(4):Er(3+)/Yb(3+) nanoparticles (Ln-UCNPs). These nanoparticles have no inherent aqueous dispersibility and inconvenient postsynthesis treatments are required to render them water dispersible. Here, we have developed a novel and facile approach to obtain water-dispersible, ligand-free, brightly upconverting Ln-UCNPs. We show that the upconversion luminescence is affected by the local environment of the lanthanide ions at the surface of the Ln-UCNPs. We observe a dramatic difference of the integrated upconverted red:green emission ratio for Ln-UCNPs dispersed in toluene compared to Ln-UCNPs dispersed in water. We can enhance or deactivate the upconversion luminescence by pH and H/D isotope vibronic control over the competitive radiative and nonradiative relaxation pathways for the red and green excited states. Direct biofunctionalization of the ligand-free, water-dispersible Ln-UCNPs will enable myriad new opportunities in targeting and drug delivery applications.

Colloidal Synthesis of 1T-WS2 and 2H-WS2 Nanosheets: Applications for Photocatalytic Hydrogen Evolution

In recent years, a lot of attention has been devoted to monolayer materials, in particular to transition-metal dichalcogenides (TMDCs). While their growth on a substrate and their exfoliation are well developed, the colloidal synthesis of monolayers in solution remains challenging. This paper describes the development of synthetic protocols for producing colloidal WS2 monolayers, presenting not only the usual semiconducting prismatic 2H-WS2 structure but also the less common distorted octahedral 1T-WS2 structure, which exhibits metallic behavior. Modifications of the synthesis method allow for control over the crystal phase, enabling the formation of either 1T-WS2 or 2H-WS2 nanostructures. We study the factors influencing the formation of the two WS2 nanostructures, using X-ray diffraction, microscopy, and spectroscopy analytical tools to characterize them. Finally, we investigate the integration of these two WS2 nanostructured polymorphs into an efficient photocatalytic hydrogen evolution system to compare their behavior.

Challenges and advances in the chemistry of periodic mesoporous organosilicas (PMOs)

The merger of materials synthesis, organic synthesis, and supramolecular chemistry has lead to a plethora of hybrid organic–inorganic materials with control over the molecular organization, nanoscale periodicity, and macroscopic morphology. Self-assembly of polymeric precursors can be directed by micelle templates, or through microphase separation of block copolymers. This has provided a pathway to new materials whose hierarchical structure determines material properties and function. Periodic mesoporous organosilicas (PMOs), which are composed of bridge-bonded silsesquioxanes organized into a mesoporous architecture, have emerged as promising materials for nanotechnology applications. This article provides an overview of PMOs and describes the challenges, problems and our predictions for the future of these intriguing solid-state materials.

A New Model for Aluminophosphate Formation: Transformation of a Linear Chain Aluminophosphate to Chain, Layer, and Framework Structures

Simple transformations of an unbranched parent chain lead to a diverse range of aluminophosphates with 1D chain, 2D porous layer, and 3D open-framework structures. A section of this chain, which has been hydrolyzed in one position leading to ring-opening, is shown schematically below. Reorientation of the chain followed by renewed condensation leads to, for example, two edge-sharing four-rings. In this case water acts as a catalyst; thus the amount of water present during the synthesis determines the degree of hydrolysis and therefore the resulting structure type.

Non-aqueous supramolecular assembly of mesostructured metal germanium sulphides from (Ge4S10)4- clusters

Microporous materials have found extensive application as catalysts, ion-exchange media and sorbents. The discovery of mesoporous silica has opened the path to selective catalysis and separation of large molecules and to the synthesis of inorganic–organic composite materials, polymer mesofibres and semiconducting quantum dots. Various oxide-based mesoporous materials, such as TiO2, ZrO2, SnO2, Al2O3, Nb2O5 and GeO2, have been reported. A challenge for materials research is now to expand the scope of mesoporous materials beyond the oxides. Only a few non-oxide mesostructured composites, such as CdS, SnS2 and CdSe, have been reported; they are usually synthesized by ad hoc hydrothermal methods or from aqueous solutions containing ill-defined species, and are often not well characterized. Herewe report the rational synthesis of a new family of metal germanium sulphide mesostructured materials prepared by a non-aqueous surfactant-templated assembly of adamantanoid [Ge4S10]4− cluster precursors. In the presence of quaternary alkylammonium surfactants, [Ge4S10]4− anions in formamide solution self-organize with metal cations (Co2+, Ni2+, Cu+ and Zn2+) to create well ordered hexagonal metal germanium sulphide mesostructures, some having fibre-like morphologies with channels running down the long axis of the fibre. Materials of this genre could prove effective in applications as diverse as detoxification of heavy metals in polluted water streams, sensing of sulphurous vapours, and the formation of semiconductor quantum ‘anti-dot’ devices.

Multicolor Silicon Light-Emitting Diodes (SiLEDs)

We present highly efficient electroluminescent devices using size-separated silicon nanocrystals (ncSi) as light emitting material. The emission color can be tuned from the deep red down to the yellow-orange spectral region by using very monodisperse size-separated nanoparticles. High external quantum efficiencies up to 1.1% as well as low turn-on voltages are obtained for red emitters. In addition, we demonstrate that size-separation of ncSi leads to drastically improved lifetimes of the devices and much less sensitivity of the emission wavelength to the applied drive voltage.

Opal Circuits of Light—Planarized Microphotonic Crystal Chips

We present a novel technique coined directed evaporation-induced self-assembly (DEISA) that enables the formation of planarized opal-based microphotonic crystal chips in which opal crystal shape, size, and orientation are under synthetic control. We provide detailed synthetic protocols that underpin the DEISA process and formulate directed self-assembly strategies that are suited for the fabrication of opal architectures with complex form and designed optical functionality. These developments bode well for the utilization of opal-based photonic crystals in microphotonic crystal devices and chips.

Materials chemistry for low-k materials

The microelectronics industry is constantly trying to reinvent itself, to find new technological solutions to keep pace with the trend of increasing device densities in ultra-large-scale integrated (ULSI) circuits. Integral in this development has been the replacement of the conventional Al/SiO2 metal and dielectric materials in multilevel interconnect structures. Higher-conductivity Cu has now successfully replaced Al interconnects, but there is still a need for new low dielectric constant (k) materials, as an interlayer dielectric.