Single-step approach to sensitized luminescence through bulk-embedded organics in crystalline fluorides

Excitation Wavelength Engineering through Organic Linker Choice in Luminescent Atomic/Molecular Layer Deposited Lanthanide-Organic Thin Films

We demonstrate multiple roles for the organic linker in luminescent lanthanide-organic thin films grown with the strongly emerging atomic/molecular layer deposition technique. Besides rendering the hybrid thin film mechanically flexible and keeping the lanthanide nodes at a distance adequate to avoid concentration quenching, the organic moieties can act as efficient sensitizers for the lanthanide luminescence. We investigate six different aromatic organic precursors in combination with Eu3+ ions to reveal that by introducing different nitrogen species within the aromatic ring, it is possible to extend the excitation wavelength area from the UV range to the visible range. This opens new horizons for the application space of these efficiently photoluminescent thin-film materials.

Atomic layer deposition of CoF2, NiF2 and HoF3 thin films

The present study describes atomic layer deposition (ALD) processes and characterization of CoF₂, NiF₂, and HoF₃ thin films. For CoF₂ deposition CoCl₂(TMEDA) (TMEDA = N,N,N',N'-tetramethylethylenediamine) and NH₄F were used as precursors. CoF₂ deposition was studied at 180-275 °C, resulting in a growth per cycle (GPC) of 0.7 to 1.2 Å. All the films consist of tetragonal CoF₂ according to XRD. The impurity contents were measured with ToF-ERDA and less than 1 at% of N and Cl were detected in the films, indicating effective reactions. In addition, the F/Co ratio is close to 2 as measured by the same method. The saturation of the GPC with respect to precursor pulses and purges was verified at 250 °C. The common feature of ALD metal fluoride films - remarkable roughness - is encountered also in this process. However, the films became smoother as the deposition temperature was increased. CoF₂ deposition was also demonstrated on graphite substrates. NiF₂ deposition was studied at 210-250 °C by using Ni(thd)₂ and TaF₅ or a new fluoride source NbF₅ as the precursors. Tetragonal NiF₂ was obtained, but the oxygen and hydrogen contents in the films were remarkable, up to ∼11 at%, as measured by ToF-ERDA. This was observed also when the films were in situ capped with YF₃. NbF₅ was shown to be a potential fluoride precursor by combining it with Ho(thd)₃ to deposit HoF₃ films. Orthorhombic HoF₃ was obtained at deposition temperatures of 200-275 °C. The films deposited at 235-275 °C are pure, and the Nb contents in films deposited at 250 and 275 °C are only 0.21 and 0.15 at%. The main impurity in both films is oxygen, but the contents are only 1.5 and 1.6 at%. The saturation of the GPC with respect to precursor pulses was verified at 250 °C. The GPC is ∼1 Å.

Aromatic sensitizers in luminescent hybrid films

Atomic layer deposition offers a unique set of design possibilities due to the vast range of metal and organic precursors that can be used and combined. In this work, we have combined lanthanides with aromatic aids as strongly absorbing sensitizers to form highly luminescent thin films. Terephthalic acid is used as a base sensitizer, absorbing shorter wavelengths than 300 nm. The absorption range is extended towards the near-UV and blue range by increasing the aromatic system and adding functional groups that have strong red-shifting effects. While terbium and europium provide green and red emission, yttrium allows emission from the sensitizer itself spanning the whole color range from purple, blue and green to red. Many organic dye molecules show very high luminescence quantum yields and several of the molecules and materials investigated in this work show bright luminescence.

Quinizarin: a large aromatic molecule well suited for atomic layer deposition

Atomic layer deposition (ALD) is a remarkable synthesis tool due to the vast array of materials that can be deposited and the complexity of structures that can be designed. The low-temperature layer-by-layer approach even allows organic and inorganic components to be combined as hybrid or composite materials. The technique is then called molecular layer deposition (MLD). This opens the door for deposition of advanced optical materials using highly absorbing aromatic molecules. Unfortunately, most large aromatic molecules are difficult to sublime or have insufficient reactivity. This is a major barrier for ALD when designing with the use of organic components for dye-sensitized solar cells, luminescence, visible light photochemistry, chemical sensors and organic electronics. In this work, we introduce a well-known orange dye molecule, quinizarin. This molecule has a large conjugated aromatic system with strong absorption of visible light and shows strong luminescence both in solutions and as a complex together with aluminium ions. Interestingly, quinizarin also shows surprisingly good properties for film deposition due to reactive -OH groups and low sublimation temperature (130 °C). Strongly coloured pink hybrid films were deposited with trimethylaluminium and quinizarin at 175 °C with a growth rate of 0.28 nm per cycle. These films were not luminescent although their optical absorption spectra are similar to those of the corresponding solution. An attempt was made to dilute quinizarin through partial replacement with pentaerythritol as a multilayer structure or simultaneous co-pulsing, although this also did not produce luminescent films. The low sublimation temperature, good reactivity and large conjugated system of quinizarin open the way for exploration of solid-state hybrid and organic films based on this molecule along many different technological pathways.