Reverse Switching Phenomena in Hybrid Organic-Inorganic Thin Film Composite Material Kallol Mohanta, Jose Rivas, and Ranjith Krishna Pai* Nanostructured Hybrid Functional Materials (NHFM), International Iberian Nanotechnology Laboratory, Avenida Mestre Jose ́ Veiga, 4715-330 Braga, Portugal * S Supporting Information ABSTRACT: A systematic approach was followed to develop a new hybrid organic-inorganic composite material with intriguing electrical and fluorescence properties into one ultrathin film system. Providing facile and cost-effective synthesis, this method utilizes a double decomposition reaction to introduce electric and fluorescence as an intrinsic property into one ultrathin film system, through dihydrolipoic acid- coated core/shell CdSe/ZnS quantum dots. Scanning tunneling microscope was used to asses, at the microstructured level, electrical properties of the composite material. Thin film composite devices exhibit higher conductivity with the application of a lower electrical field and inversely show lower conductivity when applying higher electrical bias point. The prospect of this feature solely lies in band gap engineering inherent to the device structure and geometric properties. The merits of such a device, paired with the ease of chemical functionalization provided by the water-soluble quantum dots, make the obtained hybrid organic-inorganic thin film composite material a viable candidate to be used as sensors, optolectronic devices, as well as pathogenic detectors. ■ INTRODUCTION Hybrid organic-inorganic thin films have attracted a great deal of interest among materials scientists due to their wide-range applications in optoelectronic devices and biological sen- sors. 1-5 Double decomposition reaction 6 (DDR) has been demonstrated to be a promising method for combining different types of materials such as anionic and cationic bipolar species, 7 inorganic nanoparticles, 8,9 polyelectrolytes, proteins, and polymers 10 into one ultrathin film system. In addition, the DDR method is applied in many applications due to its low cost and simplicity in organic-inorganic composite preparation and its ability in achieving good reproducibility for the composite. 9,11 Hybrid organic-inorganic thin film composite materials are not simply physical mixtures. They can be broadly defined as molecular or nanocomposites with organic and inorganic components, intimately mixed where at least one of the components domains has a dimension ranging from a few angstroms to several nanometers. 12 Consequently, not only are the properties of hybrid composite materials the sum of the individual contributions of both phases but also the role of their inner interfaces could be predominant. 13 Spherical shell arrays consisting of organic-inorganic nanoparticles have shown size- dependent di ffraction properties due to their periodic modulation of the dielectric function, which can inhibit the propagation of certain frequencies of light through specific crystal orientations. 14,15 It may be noted that the spherical shell arrays with organic-inorganic nanoparticles exhibit quite different surface topographies, which may lead to considerable differences in optical and surface properties. 16 To fabricate such highly ordered organic-inorganic arrays with feature sizes down to the submicrometer length scale, composite thin film preparation involving vaterite as templates have been recognized to be a cost-efficient bottom-up strategy. 10,17 Vaterite is the least thermodynamically stable form of calcium carbonate (CaCO 3 ). It appears as 1-10 μm spherulitic crystals composed of nanoparticles 20-30 nm in size. 18 Vaterite can be manufactured in the laboratory by mixing concentrated solutions of calcium and carbonate containing salts. Vaterite has attracted particular attention due to high specific area, low density, high solubility, and high dispersion compared with other forms of calcium carbonate. Although stable under dry conditions, vaterite transforms easily and irreversibly into a thermodynamically stable polymorph of CaCO 3 (calcite), thus limiting its applications. Stability can be achieved by the incorporation of polyelectrolytes into the vaterite crystal lattice. 8,10 There has been increased attention to the fabrication of patterned CaCO 3 thin films due to the importance of patterning for both scientific and industrial applications. Recently, fabrication of spherical vaterite consisting of CdSe/ ZnS nanoparticles with interesting anomalous optical properties has been reported. 8,10 However, the development of mild, facile, and low-cost solution approaches toward such micro- structured composite films made of desirable materials with Received: October 2, 2012 Revised: December 5, 2012 Published: December 11, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 124 dx.doi.org/10.1021/jp309750p | J. Phys. Chem. C 2013, 117, 124-130