Photoluminescence and upconversion properties of lanthanide-based atomic and molecular layer deposited thin films

Photoluminescence of trivalent lanthanide (Ln) ions is highly relevant to applications such as solar cells, light emitting devices and biological imaging. Organic molecules may potentially be utilized to enhance and tune the luminescence of Ln3+ ions when properly combined into Ln-organic hybrids. Such on-demand tailored materials could pave the way to various next-generation applications, especially if the materials could be produced in high-quality thin-film form. However, the conventional thin-film techniques of Ln-organic materials lack the ability to combine the well-controlled deposition and the tunability of the luminescence proper-ties to the needs of various applications. To address these challenges, efforts to apply the strongly emerging atomic/molecular layer deposition (ALD/MLD) method for lanthanide-organic thin films started recently. While ALD/MLD offers well controlled process development and growth of thin films, the tunability of the luminescence properties of such films has been critically difficult to achieve. In this thesis, the tunability issue was one of the central research questions, and it was addressed by developing a number of ALD/MLD processes with novel organic components. Within the scope of the thesis, the following organic precursors were inves-tigated for the first time in the context of ALD/MLD: pyridine-3-carboxylic acid (PDA), cytosine (Cyt), 1,3,5-triazine-2,4,6-triol (TZO), and 2-hyrdoxyquinoline-4-carboxylic acid (HQA). Among these, PDA and Cyt were found particularly interesting as they allowed the remarkable tuning of the absorption and excitation properties of the lanthanide-organic thin films by shifting the excitation wavelength from the typical 250 nm up to 365 nm. More careful selection during this work led to the development of Eu-HQA thin films, which can be excited through an exceptionally wide excitation wavelength range from ultraviolet light of 185 nm up to visible light of 400 nm. Luminescent thin films that can be excited with visible light are of particular interest to biological imaging applications. Therefore, the new Eu-HQA thin films were tested for the Förster resonance energy transfer mechanism, which is used in various bioimaging and detection techniques. As another way to tune the luminesce properties challenged in the thesis, different Ln3+ were combined into a single thin-film. Here, the combination of Eu3+, Tb3+, and Er3+ yielded interesting thin films with photoluminescence emissions that could be controlled between green, red and white light. On the other hand, using Er3+ and Ho3+ provided a promising means to achieve upconversion emission, through which near-infrared light can be converted to visible light. These latter results could provide a route to enhance the performance of solar cells that suffer from weak absorption of infrared light.