Brian J. Scott

Mesostructured materials for optical applications: from low-k dielectrics to sensors and lasers

Recent advances on the use of mesoporous and mesostructured materials for electronic and optical applications are reported. The focus is on materials which are processed by block-copolymer templating of silica under weakly acidic conditions and by employing dip- and spin-coating as well as soft lithographic methods to bring them into a well-defined macroscopic shape. Several chemical strategies allow the mesostructure architecture to be used for electronic/optical applications: Removal of the block-copolymers results in highly porous, mechanically and thermally robust materials which are promising candidates for low dielectric constant materials. Since the pores are easily accessible, these structures are also ideal hosts for optical sensors, when suitable are incorporated during synthesis. For example, a fast response optical pH sensor was implemented on this principle. As-synthesized mesostructured silica/block-copolymer composites, on the other hand, are excellently suited as host systems for laser dyes and photochromic molecules. Laser dyes like rhodamine 6G can be incorporated during synthesis in high concentrations with reduced dimerization. This leads to very-low-threshold laser materials which also show a good photostability of the occluded dye. In the case of photochromic molecules, the inorganic-organic nanoseparation enables a fast switching between the colorless and colored form of a spirooxazine molecule, attributed to a partitioning of the dye between the block-copolymer chains. The spectroscopic properties of these dye-doped nanocomposite materials suggest a silica/block-copolymer/dye co-assembly process, whereby the block-copolymers help to highly disperse the organic dye molecules.

pH Sensing with mesoporous thin films

Optically clear thin mesoporous films with covalently attached fluorescein entities are shown to exhibit very fast response pH sensing.

Dye‐Doped Mesostructured Silica as a Distributed Feedback Laser Fabricated by Soft Lithography

Self-assembled block co-polymer templated mesostructured silica (nanocomposites) produced under acidic conditions have recently attracted attention as potential optical materials. They have several desirable properties such as easy processing that allows for a variety of useful structures for optical applications such as thin films, fibers, monoliths, hierarchical ordering, and micropatterns produced by soft lithography. Further, these materials can be readily doped with a wide range of components such as organometallic complexes, semiconducting nanocrystals, semiconducting polymers, and dyes. Such host/ guest nanocomposites combine the high stability of the inorganic host framework with the diversity of guest dopants, leading to versatile properties that are currently being explored to produce novel optical materials. Previously, dye-doped mesostructures have demonstrated their utility as potential laser materials by displaying amplified spontaneous emission (ASE). Further, their unique architectures, e.g., organic/inorganic phase separation on the nanometer scale, has allowed for higher active dye doping by suppressing concentration quenching. While characterizing ASE is a useful way to demonstrate that a material can amplify light, to make a laser it is necessary to incorporate the gain material into a cavity with resonant feedback. A simple way to produce a high Q laser cavity is to dip-coat an optical fiber or form a microdisk through photolithography. Although these structures are easy to fabricate, they have the disadvantage that they emit light into a ring instead of a well-defined beam. One way to make an in-plane laser with a well-defined output beam is to reflect light in a waveguide through the incorporation of a periodic modulation of refractive index or gain so that light is Bragg reflected. Lasers that operate by this type of grating-induced coupling are known as distributed feedback (DFB) lasers. The lasing wavelength of a DFB laser is close to the Bragg wavelength, kBragg = 2neffK (neff is the effective refractive index of the waveguide and K is the period of the grating), and can be tuned by changing either neff or K. Single-mode DFB lasers are made when light is reflected through modulation of gain or by incorporating a phase shift. Dual-mode DFB lasers are obtained when Bragg reflection occurs through modulation of the refractive index with one mode just below and the other just above kBragg. Recently, DFB lasers have been made by etching gratings into a low-refractive-index substrate and then spin casting a conjugated polymer or evaporating small luminescent molecules over the grating. The gratings were made by holographic lithography or precision photolithography and reactive ion etching, techniques that require expensive optical and clean room equipment. Over the past several years, alternative techniques to traditional lithography that are much less expensive and easier to perform, such as embossing, ink jet printing, and soft lithography, have been developed. Soft lithography uses elastomeric molds (stamps) to pattern materials. This technique can be used to make patterned structures with dimensions ranging in size from ~30 nm to over a centimeter and can be performed on non-planar substrates. Conventional lithography is used to make the mold, but once the mold is made many replica structures can be made from it. In this letter we report the use of soft lithography to fabricate an optically pumped rhodamine-6G-doped mesostructured silica DFB laser that has a moderately low lasing threshold (~55 kW/cm) and emission linewidths with a full width at half maximum (FWHM) of only 0.3 nm. Figure 1 illustrates the fabrication steps for the DFB resonators. A typical master was fabricated by first spin-coating a standard photoresist, e.g., AZ 4110 (Clariant), onto a silicon wafer. The photoresist was first exposed in a conventional mask aligner through a photomask with a waveguide stripe pattern on it. Then, the photoresist was exposed to a holographic grating pattern that was generated by interfering two beams from a 325 nm wavelength He:Cd laser. The grating

Synthesis and luminescence properties of mesostructured thin films activated by in-situ formed trivalent rare earth ion complexes

Incorporation of trivalent rare earth ions and 1,10-phenanthroline into mesostructured block-copolymer/silica thin films produces spectrally pure emission from in-situ formed rare earth ion complexes.

A thermodynamic analysis of the Cal–Ad method with respect to gas–solid calorimetry

Abstract A recent article claims that the Cal–Ad method cannot discriminate multiple acid sites in the analysis of the interaction of pyridine (dilute pyridine in n -hexane) with HZSM-5. The Cal–Ad technique measures the enthalpy of a base-displacement reaction, while gas–solid calorimetry is a measure of an adduct formation between pyridine and the solid. A Born–Haber cycle with the appropriate values can demonstrate that the enthalpies derived from gas–solid calorimetry are an average of the two enthalpies of the interaction of pyridine with the two acid sites observed with the Cal–Ad method.