Organic–Inorganic Sol–Gel Composites Incorporating Semiconductor Nanocrystals for Optical Gain Applications By Miri Kazes, Tsiala Saraidarov, Renata Reisfeld, and Uri Banin* Chemically synthesized colloidal semiconductor nanocrystals (NCs) are chromophores with broadly tunable linear and nonlinear optical properties controlled by their size, shape, and composition. [1–3] These fluorescent NCs were investigated as potential tunable optical gain and laser materials. [4,5] In this context, quantum rods were found to provide improved optical gain characteristics compared to quantum dots (QDs). [6–9] Much of the work was carried out for close-packed NC films, which display poor optical quality and less favorable gain properties. [9,10] Moreover, the poor thermal conductivity of such films limits the temperature range of gain operation. There is, therefore, a need for additional matrices that incorporate NCs and provide platforms with high optical quality and improved thermal resilience that can be integrated into optical devices. Organic polymers can be chemically compatible with the organically capped NCs, but suffer from UV sensitivity and limited thermal range. [11–13] Synthesis of II-VI NCs directly within glasses has been demonstrated. [14–19] However, there are fundamental limitations to this preparation strategy that prevent the formation of complex structures such as rods, wires, and semiconductor heterostructures. Colloidal solution based synth- esis of such complex nanocrystals has evolved tremendously in recent years yielding high quality quantum dots, rods, and wires, as well as core/shell and multishell structures with a high level of control. [2,20–22] Combining these sophisticated nanocrystals with sol-gel processing is needed to create new complex glass-based optical materials. Semiconductor NCs incorporated into sol-gel derived metal oxide matrices such as the sol-gel titania matrix have been exploited as optical gain media, exhibiting tunable amplified spontaneous emission (ASE). [23,24] In addition, room tempera- ture microcavity lasing has been demonstrated. [6,8,25–29] Although sol-gel derived matrices provide thermally resilient host structures, exposure to water and short-chain alcohols result in structural and photophysical degradation of the NCs. [30] Recently, we developed hybrid organic-inorganic sol-gel composites based on zirconia, silica, epoxy-siloxane, and polyurethane–siloxane that form optically transparent, crack-free films and allow the incorporation of high volume fractions of various organic or inorganic materials. [31] Due to strong chemical bonding between the organic and inorganic parts, this hybrid material displays good mechanical properties, flexibility, and chemical and thermal stability. Here, we describe a hybrid host material based on two metal-alkoxide derived ormosils (silica- urethane and zirconia-epoxy siloxane) for incorporation of highly emitting CdSe-based core/shell QDs and quantum rods (QRs). Thick (up to 200 mm) crack-free films are obtained. Optical gain measurements under nanosecond excitation were performed and tunable ASE was demonstrated for a range of temperatures. QRs in this matrix showed stable ASE at room temperature. This constitutes a general approach for the incorporation of colloidal NCs into a composite polymer-sol-gel matrix for optical applications. The CdSe/ZnS QDs used in this study were grown using the methods of colloidal NC synthesis utilizing high temperature pyrolysis of organometallic precursors in coordinating organic solvents. [2,32] The core/shell configuration was chosen since the growth of a few monolayers of a ZnS shell on the CdSe core enhances dramatically the QDs’ fluorescence quantum yield from a few percents to typically 40–70%. [22,33,34] The QD/composite film was prepared by drop-casting from a solution mixture of the QDs and matrix precursors. The film was cured in an oven at 100 8C for a few days. The resultant film was 35 mm thick and had an optical density of 0.38 at the excitation wavelength of 532 nm. Figure 1a shows the normalized absorption and photoluminescence (PL) spectra of a 5 nm diameter CdSe/ZnS QD sample in a toluene solution (dotted line) and of a planar film of the same QD sample incorporated into the hybrid organic-inorganic sol-gel matrix (solid line). The well-defined absorption spectra and the narrow PL emission clearly indicate the good size distribution and homogeneity of the QD sample. Note that the spectrum of the QDs/composite film is nearly identical to the QD solution spectra confirming that there is no structural change or damage during film preparation. The slight red-shift in the PL spectrum was assigned to reabsorption owing to the high density of QDs in the film. Notably, the PL quantum yield of the film was maintained at the same value as in the solution (65%, measured using an integrating sphere). This indicates that the preparation procedure does not compromise the optical properties of the QDs. These indications show that the QD/composite film is of excellent optical quality. To further establish the generality of the preparation procedure of the NC hybrid organic-inorganic sol-gel composite to other NC systems, CdSe/CdS/ZnS QRs were also incorporated. QRs are more susceptible to intraparticle ripening effects [32] and indeed experiments with CdSe/ZnS QRs exhibited ripening during the sol-gel curing stage, manifested by a change in absorption and COMMUNICATION [*] M. Kazes, Dr. T. Saraidarov, Prof. R. Reisfeld, Prof. U. Banin Institute of Chemistry The Center for Nanoscience and Nanotechnology The Hebrew University of Jerusalem Jerusalem 91904 (Israel) E-mail: banin@chem.ch.huji.ac.il DOI: 10.1002/adma.200802883 Adv. Mater. 2009, 21, 1–5 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1