Ignacio Hernández and William P. Gillin

We have studied the spectroscopic properties of OLED materials Alq₃, Gaq₃ and Inq₃ (q = 8-hydroxyquinoline) under pressure. We discuss the results in terms of the influence of structural modifications, the isomeric state and the enhancement of the intermolecular interaction. As-grown Alq₃, Gaq₃, Inq₃ containing meridional (mer) isomer experience a red shift of nearly 90 nm (2400 cm⁻¹) in the 0−8 GPa range. Abrupt changes in the photoluminescence occur during compression at intermediate pressures for all materials. We assign them to a phase transition, its critical pressure depending on the central cation. All three samples experience an amorphization at P ~ 6 GPa, with associated changes in the spectroscopic properties. The pressure-induced phase transitions present hysteresis to ambient conditions. Photoluminescence lifetime decreases in all cases in the explored pressure range. In the case of facial isomer containing polymorphs of Alq₃, luminescence does not change its energy significantly. The most significant spectroscopic change observed in fac-isomer containing materials corresponds to γ-Alq₃, which presents a low energy component that gains relative importance when pressure is increased. We ascribe this phenomenon to the presence of sensitized mer isomer impurities.

Ignacio Hernández, R. H. C. Tan, J. M. Pearson, P. B. Wyatt and W. P. Gillin

We have performed a spectroscopic study of Er_xY_1-x[(p-CF₃-C₆F₄)₂PO₂]₃ aimed at understanding nonradiative de-excitation mechanisms. These fluorinated compounds have a long lifetime for the erbium ⁴I_13/2 → ⁴I_15/2 emission at λ ~ 1540 nm, but the lifetime increases with decreasing x. We have studied the lifetime as a function of morphology, temperature, and high hydrostatic pressure. We have demonstrated the occurrence of energy migration and calculated the corresponding activation energy. Moreover, using high pressure techniques, we provide evidence that cross-relaxation involving energy transfer from an excited erbium in the ⁴I_13/2 promoting a neighbor in the same state to ⁴I_9/2 is the dominant mechanism at ambient conditions for short erbium−erbium distances. The model explains the observed dynamics of excited states in the series and is tested against the Yb[(p-CF₃-C₆F₄)₂PO₂]₃ compound.

I. Hernandez, F. Rodriguez, H. D. Hochheimer 

A novel two-color photoluminescence (PL) is found in MnF₂ at room temperature under high pressure. Contrary to low-temperature PL, PL at room temperature is unusual in transition-metal concentrated materials like MnF₂, since the deexcitation process at room temperature is fully governed by energy transfer to nonradiative centers. We show that room-temperature PL in MnF₂ originates from two distinct Mn²⁺ emissions in the high-pressure cotunnite phase. The electronic structure and the excited-state dynamics are investigated by time-resolved emission and excitation spectroscopy at high pressure.

Ignacio Hernández and Fernando Rodríguez

This work presents a correlated structural and spectroscopic study on ball-milled MnF₂. The aims are to produce impurity-lean particles through particle-size reduction leading to room-temperature photoluminescence (PL) and to modify the electronic states of the emitting centres. Despite non-radiative centres being still present, the PL quenching temperature was increased nearly 80 K, from 120 to 200 K, following this method. Milled MnF₂ has particle sizes down to several nanometres, and structural changes from the initial α-TiO₂ structure to the α-PbO₂ phase. Milling favours the presence of adsorbed water on the nanoparticle surface. Time-resolved spectroscopy indicates that the nanoparticle PL consists of a significantly inhomogeneous broadened band with respect to the initial MnF₂ PL. The temperature dependence of the lifetime measured at different wavelengths of the emission spectrum indicates the presence of several PL centres, the population of which is controlled by exciton migration and trapping. The widespread occurrence of emitting centres is explained in terms of milling-induced strains, the coexistence of two different structural phases, and the presence of adsorbed water molecules.

I. Hernandez and F. Rodriguez

This work reports an effective way for inducing room temperature photoluminescence (PL) in Mn²⁺-doped BaF₂ and SrF₂ using high-pressure techniques. The aim is to understand the surprising PL behavior exhibited by Mn²⁺ at the cubal site of the fluorite structure. While Mn²⁺-doped CaF₂ shows a green PL with quantum yield close to 1 at room temperature, Mn²⁺-doped MF₂ (M=Ba,Sr) is not PL either at room temperature (SrF₂) or at any temperature (BaF₂) at ambient pressure. We associate the loss of Mn²⁺ PL on passing from CaF₂ to SrF₂ or BaF₂ with nonradiative multiphonon relaxation whose thermal activation energy decreases along the series CaF₂→SrF₂→BaF₂. A salient feature of this work deals with the increase of activation energy induced by pressure. It leads to a quantum yield enhancement, which favors PL recovery. Furthermore, the activation energy mainly depends on the crystal volume per molecule irrespective of the crystal structure or the local symmetry around the impurity. In this way, the relevance of the fluorite-to-cotunnite phase transition is analyzed in connection with the PL properties of the investigated compounds. The PL spectrum and the corresponding lifetime are reported for both structural phases as a function of pressure.

I. Hernandez and F. Rodriguez

This work investigates the photoluminescence (PL) properties of the cubic chloroperovskite NH₄MnCl₃. Like in most concentrated materials, the Mn²⁺ PL which is located at 2.10 eV at T = 10 K strongly depends on the temperature. Optical absorption (OA), emission, and excitation spectroscopy, as well as lifetime measurements, performed on NH₄MnCl₃ indicate that the PL is mainly intrinsic at T = 10 K and consists of a broad band located at 2.10 eV. Above this temperature, the PL gradually transforms to extrinsic PL due to exciton migration and subsequent trapping. Further temperature increase above 100 K yields transfer to killers of excitation which are responsible for the PL quenching, and hence the absence of PL at ambient conditions. The exciton traps are identified with perturbed Mn²⁺ sites with the effective activation energy of 52 meV, whilst the activation energy for energy transfer is 47 meV. The existence of these traps has been directly revealed by time-resolved spectroscopy. The detected intrinsic and extrinsic PL bands are displaced by 6 meV, which coincides with the activation energy difference between pure Mn²⁺ and trap Mn²⁺, as derived from temperature dependence studies of the lifetime τ(T). Interestingly, a PL band at 1.82 eV is observed above 60 K. This band, which was initially associated with deeper excitation traps, actually corresponds to precipitates of MnCl₂ inside NH₄MnCl₃. The correlation analysis performed on NH₄MnCl₃ using OA, PL, and lifetime data provides an estimate of the precipitate concentration of 0.3 mol%. The presence of two separated Mn²⁺ PL bands at different temperatures is a rather common phenomenon in concentrated materials such as AMnX₃ (A = NH₄, Rb; X = Cl, F), and has been interpreted in terms of exciton transfer to deeper traps. The present finding stresses the relevance of an adequate structural characterization in dealing with PL in concentrated materials.