Optical properties of La2/3Ca1/3MnO3 thin films and the effects of embedded noble metal nanospheres
Andrews, Keller R.
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Vanadium dioxide (VO2) exhibits a solid-solid phase transition at 341 K. This transition involves a slight shift in the shape of the crystal lattice of VO2 due to a change in internal energy. This energy change can come from many external sources (light, heat, strain, electromagnetic fields, etc.), but the change in the crystal structure caused by the change in energy exhibits several interesting shifts in the physical properties of VO2. In particular, the dielectric function (which controls optical properties) changes as this material undergoes its insulator-metal transition (IMT). However, VO2 has been extensively studied and has been shown to have optical and thermal limitations, so other materials with IMTs need to be studied for applications outside the reach of VO2. One such material is a perovskite structured compound, lanthanum calcium manganese oxide (La1- xCaxMnO3; hereafter, LCMO). LCMO has a number of magnetic phases and displays useful magnetic properties. One of the most important of these magnetic properties is colossal magneto resistance, which is maximized with a calcium fraction of x = 1=3. The optical properties around the IMT of LCMO thin films have not been well studied, especially the properties of thin films containing noble metal nanostructures, which are known to affect the physical and optical properties of VO2 around its IMT temperature. Thus, we developed a sol-gel method for the fabrication of LCMO thin films that enables us to incorporate nanostructures into thin films. To create LCMO thin films, we based our sol-gel precursor on a non-polar solvent that allowed us to move 100 nm nanospheres of gold and (separately) silver suspended in a citrate buffer solution into the precursor. This precursor was then used to create thin films through a process consisting of spin-coating, sintering, and annealing that was optimized to give the smoothest films possible and approximately 100 nm thick. Films were created with and without nanostructures in order to study both the transmission of LCMO films (with x = 1=3) as well as the effect of noble metal nanospheres incorporated in the thin film, in terms of both on the critical IMT temperature (TIMT) and the transmission. Our studies found a higher TIMT than most others in the LCMO literature, possibly due to strain caused by fabricating the thin films on a c-cut sapphire substrate. The inherent lattice mismatch between the hexagonal sapphire crystalline structure and the perovskite LCMO crystalline structure produces a constant strain that affects the internal energy of the film, and potentially shifts TIMT. Incorporating the nanospheres only changes TIMT slightly and improves (i.e., increases) the contrast of the infrared transmission, especially at a wavelength of 2,000 nm.