Structural and optical properties of lanthanide oxides grown by atomic layer deposition (Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb)

Atomic/molecular layer deposition of europium-organic thin films on nanoplasmonic structures towards FRET-based applications

We present a novel atomic/molecular layer deposition (ALD/MLD) process for europium-organic thin films based on Eu(thd)3 and 2-hydroxyquinoline-4-carboxylic acid (HQA) precursors. The process yields with appreciably high growth rate luminescent Eu-HQA thin films in which the organic HQA component acts as a sensitizer for the red Eu3+ luminescence, extending the excitation wavelength range up to ca. 400 nm. We moreover deposit these films on nanoplasmonic structures to achieve a twentyfold enhanced emission intensity. Finally, we demonstrate the FRET-type energy transfer process for our Eu-HQA coated nanoplasmonic structures in combination with commercial Alexa647 fluorophor, underlining their potential towards novel bioimaging applications.

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.

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.

Structure and Electrical Behavior of Hafnium-Praseodymium Oxide Thin Films Grown by Atomic Layer Deposition

Crystal structure and electrical properties of hafnium-praseodymium oxide thin films grown by atomic layer deposition on ruthenium substrate electrodes were characterized and compared with those of undoped HfO2 films. The HfO2 reference films crystallized in the stable monoclinic phase of HfO2. Mixing HfO2 and PrOx resulted in the growth of nanocrystalline metastable tetragonal HfO2. The highest relative permittivities reaching 37-40 were measured for the films with tetragonal structures that were grown using HfO2:PrOx cycle ratio of 5:1 and possessed Pr/(Pr + Hf) atomic ratios of 0.09-0.10. All the HfO2:PrOx films exhibited resistive switching behavior. Lower commutation voltages and current values, promising in terms of reduced power consumption, were achieved for the films grown with HfO2:PrOx cycle ratios of 3:1 and 2:1 and showing Pr/(Pr + Hf) atomic ratios of 0.16-0.23. Differently from the undoped HfO2 films, the Pr-doped films showed low variability of resistance state currents and stable endurance behavior, extending over 104 switching cycles.

Rational Development of Guanidinate and Amidinate Based Cerium and Ytterbium Complexes as Atomic Layer Deposition Precursors: Synthesis, Modeling, and Application

Owing to the limited availability of suitable precursors for vapor phase deposition of rare-earth containing thin-film materials, new or improved precursors are sought after. In this study, we explored new precursors for atomic layer deposition (ALD) of cerium (Ce) and ytterbium (Yb) containing thin films. A series of homoleptic tris-guanidinate and tris-amidinate complexes of cerium (Ce) and ytterbium (Yb) were synthesized and thoroughly characterized. The C-substituents on the N-C-N backbone (Me, NMe2 , NEt2 , where Me=methyl, Et=ethyl) and the N-substituents from symmetrical iso-propyl (iPr) to asymmetrical tertiary-butyl (tBu) and Et were systematically varied to study the influence of the substituents on the physicochemical properties of the resulting compounds. Single crystal structures of [Ce(dpdmg)3 ] 1 and [Yb(dpdmg)3 ] 6 (dpdmg=N,N'-diisopropyl-2-dimethylamido-guanidinate) highlight a monomeric nature in the solid-state with a distorted trigonal prismatic geometry. The thermogravimetric analysis shows that the complexes are volatile and emphasize that increasing asymmetry in the complexes lowers their melting points while reducing their thermal stability. Density functional theory (DFT) was used to study the reactivity of amidinates and guanidinates of Ce and Yb complexes towards oxygen (O2 ) and water (H2 O). Signified by the DFT calculations, the guanidinates show an increased reactivity toward water compared to the amidinate complexes. Furthermore, the Ce complexes are more reactive compared to the Yb complexes, indicating even a reactivity towards oxygen potentially exploitable for ALD purposes. As a representative precursor, the highly reactive [Ce(dpdmg)3 ] 1 was used for proof-of-principle ALD depositions of CeO2 thin films using water as co-reactant. The self-limited ALD growth process could be confirmed at 160 °C with polycrystalline cubic CeO2 films formed on Si(100) substrates. This study confirms that moving towards nitrogen-coordinated rare-earth complexes bearing the guanidinate and amidinate ligands can indeed be very appealing in terms of new precursors for ALD of rare earth based materials.

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

Luminescent materials enable warm white LEDs, molecular tagging, enhanced optoelectronics and can improve energy harvesting. With the recent development of multi-step processes like down- and upconversion and the difficulty in sensitizing these, it is clear that optimizing all properties simultaneously is not possible within a single material class. In this work, we have utilized the layer-by-layer approach of atomic layer deposition to combine broad absorption from an aromatic molecule with the high emission yields of crystalline multi-layer lanthanide fluorides in a single-step nanocomposite process. This approach results in complete energy transfer from the organic molecule while providing inorganic fluoride-like lanthanide luminescence. Sm³⁺ is easily quenched by organic sensitizers, but in our case we obtain strong fluoride-like Sm³⁺ emission sensitized by strong UV absorption of terephthalic acid. This design allows combinations of otherwise incompatible species, both with respect to normally incompatible synthesis requirements and in controlling energy transfer and quenching routes.

HSA functionalized Gd2O3:Eu3+ nanoparticles as an MRI contrast agent and a potential luminescent probe for Fe3+, Cr3+, and Cu2+ detection in water

PVP capped Gd₂O₃:Eu³⁺ (PVP@Gd₂O₃:Eu³⁺) and HSA functionalised PVP@Gd₂O₃:Eu³⁺ (HSA@PVP@Gd₂O₃:Eu³⁺) NPs as fluorescent detection probe for metal ion detection and MRI contrast agent.

Modulating the Interface Chemistry and Electrical Properties of Sputtering-Driven HfYO/GaAs Gate Stacks by ALD Pulse Cycles and Thermal Treatment

In the current work, a detailed exploration on the cleaning effect of intrinsic oxide existing at the GaAs/HfYO interface by using an atomic-layer-deposition-derived trimethylaluminum (ALD TMA) precursor as functions of TMA pulse cycles and postannealing temperature has been evaluated via X-ray photoemission spectroscopy (XPS) measurements and electrical characterization. According to XPS analyses, it can be noted that the intrinsic As oxides, Ga oxides, and As⁰ are effectively reduced from the HYO/GaAs gate stack after ALD TMA treatment with 20 pulse cycles. Meanwhile, optimized electrical parameters, such as the largest permittivity (k), the lowest hysteresis, and the minimum leakage density (J _g), have also been obtained for the HfYO/GaAs gate stack with 20 pulse cycles of ALD TMA. Based on the optimized pulse cycles of 20 ALD TMA, postannealing temperature-dependent interface quality and electrical performance of GaAs-based devices based on the HfYO/GaAs gate stack have also been investigated. The HfYO/GaAs/Al metal-oxide semiconductor capacitor annealed at 300 °C with optimized pulse cycles of 20 displays the greatest dielectric constant of 38, the minimum J _g of 3.28 × 10^-6 A cm^-2, and a small hysteresis of 0.01 V. Meanwhile, the leakage current transport mechanism at low temperature (77-327 K) has been discussed systematically.

Sensors for optical thermometry based on luminescence from layered YVO4: Ln3+ (Ln = Nd, Sm, Eu, Dy, Ho, Er, Tm, Yb) thin films made by atomic layer deposition

Below the Earth’s crust, temperatures may reach beyond 600 K, impeding the batteries used to power conventional thermometers. Fluorescence intensity ratio based temperature probes can be used with optical fibers that can withstand these conditions. However, the probes tend to exhibit narrow operating ranges and poor sensitivity above 400 K. In this study, we have investigated single and dual layered YVO₄: Ln³⁺ (Ln = Nd, Sm, Eu, Dy, Ho, Er, Tm, Yb) thin films (100–150 nm) for use in fluorescence intensity ratio based temperature sensors in the 300–850 K range. The type of lanthanide emission can be fine-tuned by adjusting the thickness of each layer, and the layered structure allows for emission from otherwise incompatible lanthanide pairs. This novel multi-layered approach enables high sensitivity over a broad temperature range. The highest relative sensitivity was achieved for a dual layered YVO₄: Eu³⁺/YVO₄: Dy³⁺ sample, exhibiting a maximum sensitivity of 3.6% K⁻¹ at 640 K. The films were successfully deposited on all tested substrates (silicon, iron, aluminum, glass, quartz, and steel), and can be applied homogenously to most surfaces without the use of binders. The films are unaffected by water, enabling non-contact temperature sensing in water, where IR thermometers are not an option.

Luminescent YbVO4 by atomic layer deposition

Thin films of YbVO₄ displaying UV to NIR down conversion have been deposited by atomic layer deposition and crystallized through a flux aided process.