Gernot Wirnsberger

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.

Novel synthetic pathways to layered iron(hydro) oxyhydroxide-surfactant composites

Mesostructured lamellar iron(hydro)oxyhydroxide–surfactant composites have been prepared using novel synthetic methods based on the hydrolysis chemistry of FeIII. By carefully adjusting the reaction conditions, composites with inorganic walls from ca. 19–26 A can be synthesised in a controllable manner.

Intermolecular interactions of inorganic and organic molecules embedded in zeolite-type materials probed by near-infrared Fourier transform Raman spectroscopy

Abstract Near-infrared Fourier transform Raman spectroscopy represents an excellently suited tool to investigate spectroscopically inorganic and organic molecules occluded in zeolite-type materials as well as interactions between them. Two examples are presented: First, insertion compounds of iodine in various microporous SiO 2 modifications (deca-dodecasil 3R, all-silica theta-1 and silicalite-1) are discussed. Intermolecular interaction between the inserted molecules is prevented by occlusion of iodine in the cages of deca-dodecasil 3R, but is allowed in the insertion compounds of hosts with higher pore dimensionalities. The intermolecular coupling is confirmed by an appreciable reduction of the Raman shifts, as observed similarly for liquid and amorphous iodine. The second example deals with pyridine and n -alkylamines ( n -propyl-, n -butyl- or n -pentylamine) occluded during synthesis in all-silica ferrierite. Raman spectra reveal for all compounds, regardless of the n -alkylamine used, an interaction between the n -alkylamine and neighboring pyridine molecules, with both amines being located in the ten-membered ring channels. For this reason, it is proposed that bimolecular complexes, consisting of an n -alkylamine weakly bound to a pyridine molecule act as structure-directing agents during synthesis.

Probing the Limit of Weak Host–Guest Interactions: Insertion Compounds of Mercury(II) Halides with Microporous SiO2 Hosts

Mercury(II) halides HgX(2) (X=Cl, Br, I) were inserted into the voids of the crystalline microporous SiO(2) modifications deca-dodecasil 3R (short term: DDR), silica-theta-1 (TON), silica-ferrierite (FER) and silicalite-1 (MFI) by vapour phase loading. The properties of the occluded guest species were studied by X-ray absorption spectroscopy (X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis), UV/Vis spectroscopy, and IR and Raman spectroscopy. The methods reveal the presence of HgX(2) molecules in the insertion compounds. The interactions between these electroneutral guest molecules and the electroneutral surrounding SiO(2) framework are weak. In addition, no indication of any significant guest-guest interaction between the embedded molecules was found, in contrast to the analogous iodine insertion compounds, where these become more important with increasing pore dimensionality (G. Wirnsberger et al., Angew. Chem. 1996, 108, 2951-2953; Angew. Chem. Int. Ed. Engl. 1996, 35, 2777). Analysis of the HgL(3) EXAFS confirms a coordination number of two for Hg and gives HgX bond lengths of 2.26 +/- 0.02, 2.38 +/- 0.02 and 2.57 +/- 0.02 A for the trapped HgCl(2), HgBr(2) and HgI(2) molecules, respectively. These values are very close to those of the corresponding molecules in the vapour phase and are the shortest determined for HgX(2) molecules in solid-state compounds to date (a comparably short distance only appears in the recently reported [Cu(2-pyrazinecarboxylato)(2)HgI(2)] x HgI(2) with d(Hg[bond]I)=2.577(2) A; Dong et al., Angew. Chem. 2000, 112, 4441-4443; Angew. Chem. Int. Ed. 2000, 39, 4271). Thus, there emerges a picture of almost unperturbed HgX(2) molecules, similar to those in the vapour phase or in non-coordinating solvents, in a solid crystalline matrix of high temperature stability, a very unusual state of matter. Despite the weakness of the host-guest interactions, investigations on small crystallites of the HgX(2)-TON composites using a Raman microscope show a strong polarization dependence, providing evidence for an orientational alignment of the HgX(2) molecules inside the one-dimensional pore system of this host. For these reasons, the host matrices used in this study can be viewed as orienting solid solvents, coordinating only very weakly to the inserted HgX(2) guest molecules, but exhibiting a strong geometrical template function for their alignment. The concept of using electroneutral SiO(2) modifications as host components for a modular construction of new host-guest compounds thus allows the designed construction of ordered guest assemblies, with the pore systems of the rigid host matrices acting as space-confining and ordering templates for the guest components.