(258h) Controlling the Absorption Spectra of Transition Metal Doped Nanostructures

Rare earth ions have played a prominent role in solid state lighting and optical displays due to their ability to emit at specific wavelengths. While this has traditionally been seen at a benefit, a new class of materials is needed in with adaptive luminescence is needed for long device lifetimes. Previously, organic materials have been used due to the ability of a single molecule to emit multiple wavelengths based on the phase of the material and the applied stress. However, for electronics applications it is difficult for these materials to repeatable convert between liquid and solid phase. Additionally, these materials are known to be carcinogenic and should be avoided in consumer applications. Alternatively, transition metals have been shown to have a wide range of emission and absorption spectra indicating the potential for a wide range of applications. Unfortunately, it is difficult to predict the appearance of the material based solely on the dopant as the energy level spacing is a function of the crystal field of the host lattice, as is the case for Cr²⁺ produce the red and green appearance in rubies and emeralds, respectively. In this work, transition metal doped thin nanostructures are used for adaptive luminescence, due to their wide absorption range, with the spectra tuned via external stimuli.

Specifically, 5-10 nm thick thin films of ZnO, TiO₂, and SiO₂ will be synthesized via common sol-gel techniques. Various first row transition metals, i.e., Cr, Mn, and Co, are readily incorporated into the film at various concentrations through the incorporation into the sol-gel solution film deposition. High temperature annealing was performed after film deposition to convert each material to the final oxide form, including anatase and rutile TiO₂. The three host materials where chosen to ensure different absorption spectra for the dopant based on the crystal field strength whereas a film below 10 nm prevents the ion shielding based on the distance from the interface. Nanoparticles approximately 20 nm in diameter and similar compositions were synthesized as an alternative material to the thin films. The dopant ions are seen to be readily incorporated into the polycrystalline thin films are lower concentrations based on X-ray diffraction and photoemission spectroscopy techniques. Absorption and photoluminescence measurements were conducted on all the various materials before and after annealing to quantify the shift in optical properties, indicative of the role of external forces on the luminescent particle. Based on these characterization methods it is possible to extract the local crystal field strength in order to predict the extent of absorption and emission shifts that are possible under stronger applied fields, such as electric and magnetic fields.