Pawel Gluchowski

Laser induced white lighting of graphene foam

Laser induced white light emission was observed from porous graphene foam irradiated with a focused continuous wave beam of the infrared laser diode. It was found that the intensity of the emission increases exponentially with increasing laser power density, having a saturation level at ca. 1.5 W and being characterized by stable emission conditions. It was also observed that the white light emission is spatially confined to the focal point dimensions of the illuminating laser light. Several other features of the laser induced white light emission were also discussed. It was observed that the white light emission is highly dependent on the electric field intensity, allowing one to modulate the emission intensity. The electric field intensity ca. 0.5 V/μm was able to decrease the white light intensity by half. Origins of the laser-induced white light emission along with its characteristic features were discussed in terms of avalanche multiphoton ionization, inter-valence charge transfer and possible plasma build-up processes. It is shown that the laser-induced white light emission may be well utilized in new types of white light sources.

Mechanisms of Tenebrescence and Persistent Luminescence in Synthetic Hackmanite Na8Al6Si6O24(Cl,S)(2)

Synthetic hackmanites, Na8Al6Si6O24(Cl,S)2, showing efficient purple tenebrescence and blue/white persistent luminescence were studied using different spectroscopic techniques to obtain a quantified view on the storage and release of optical energy in these materials. The persistent luminescence emitter was identified as impurity Ti(3+) originating from the precursor materials used in the synthesis, and the energy storage for persistent luminescence was postulated to take place in oxygen vacancies within the aluminosilicate framework. Tenebrescence, on the other hand, was observed to function within the Na4(Cl,S) entities located in the cavities of the aluminosilicate framework. The mechanism of persistent luminescence and tenebrescence in hackmanite is presented for the first time.

Synthesis, structure and optical properties of two novel luminescent polar dysprosium metal–organic frameworks: [(CH3)2NH2][Dy(HCOO)4] and [N2H5][Dy(HCOO)4]

We report the synthesis, crystal structures, Raman, infrared, electron absorption and emission spectra of two novel luminescent MOFs, [(CH3)2NH2][Dy(HCOO)4] and [N2H5][Dy(HCOO)4]. These data show that [(CH3)2NH2][Dy(HCOO)4] and [N2H5][Dy(HCOO)4] crystallize in polar space groups of Pna21 and Pca21 symmetry, respectively. The polar properties of [N2H5][Dy(HCOO)4] come from the arrangement of template cations that possess an internal dipole moment as well as from the symmetry of the framework. In [(CH3)2NH2][Dy(HCOO)4], the Dy(III)–formate framework is centrosymmetric and the polar properties of this compound arise from the arrangement of the DMA+ cations. Vibrational studies reveal that the position of the ν(NN) band, which is very sensitive to coordination changes, is characteristic for hydrazine molecules forming onium ions. Optical studies show that both compounds exhibit emission of Dy(III) ions, in spite of full concentration of dysprosium ions in the studied compounds. Our data also show that radiative decay times, 57 μs for [(CH3)2NH2][Dy(HCOO)4] and 96 μs for [N2H5][Dy(HCOO)4], are very long compared to lifetimes observed in inorganic compounds. We have also preformed Judd–Ofelt analysis of the optical absorption spectrum of the hydrazinium analogue that provides information on various optical parameters such as electric-dipole oscillator strengths, Judd–Ofelt parameters Ωλ, branching ratios and spontaneous emission probabilities.

Persistent Luminescence of Tenebrescent Na8Al6Si6O24(Cl,S)2: Multifunctional Optical Markers.

Na8Al6Si6O24(Cl,S)2 materials were prepared with a solid state reaction. The products were studied using X-ray powder diffraction, reflectance measurements as well as X-ray fluorescence, conventional and persistent luminescence, nuclear magnetic resonance, and electron paramagnetic resonance spectroscopies. All materials containing sulfur showed purple tenebrescence, which persisted 2 days in a lit room at room temperature. Considerable blue persistent luminescence peaking at 460 nm and lasting for 1 h was obtained, as well. Persistent luminescence was obtained with irradiation at 365 nm, while tenebrescence required 254 nm. The materials show great promise as low-cost multifunctional optical markers.

Persistent luminescence warm-light LEDs based on Ti-doped RE2O2S materials prepared by rapid and energy-saving microwave-assisted synthesis

Persistent luminescence (PeL) materials find many applications nowadays, such as in emergency lighting, medical imaging, and enhancement of solar cell devices. However, the majority of PeL materials are synthesized by time- and energy-consuming processes such as conventional solid-state methods. Here, a series of Ti-doped RE2O2S materials (RE: La, Gd, and Y) were successfully prepared by an energy-saving microwave-assisted solid-state synthesis. This versatile method allows obtaining oxysulfide materials starting from primary precursors (e.g. oxides and elemental sulfur) in a one-step 25 minute process. The PeL materials were characterized using synchrotron radiation X-ray powder diffraction (SR-XPD), scanning electron microscopy (SEM), X-ray absorption spectroscopy (SR-XAS), and infrared absorption spectroscopy (FTIR), and the spectroscopic properties were studied using photoluminescence spectroscopy (PL) and thermoluminescence (TL). SR-XAS at the sulfur K-edge of RE2O2S suggests that elemental sulfur (S0) is oxidized in a first step forming sulfate-like (SVI) species, then is gradually reduced to form oxysulfide (S2−). The RE2O2S:Ti,Mg2+ materials show a broad emission band in the orange-red spectral region, assigned to the transitions of Ti3+/TiIV ions. The persistent luminescence of the Gd and Y2O2S:Ti,Mg2+ materials is long-lasting, being visible for ca. 5 h with the naked eye. Lastly, these photonic materials were shown to act as emitting layers for PeL warm white-LEDs, presenting great potential for applications such as self-sustained security lighting.

An Approach in the Structural and Spectroscopic Analysis of Yb3+-Doped YAG Nano-ceramics by Conjugation of TEM-EDX and Optical Techniques

We show our approach in the structural and spectroscopic analysis of Yb3+-doped YAG nano-ceramics prepared by the low temperature-high pressure sintering technique (LTHP) by conjugation of both TEM-EDX and optical techniques. Pressure sintering dependences of absorption, emission and decays are analyzed and interpreted. The sample pressurized at 8 GPa for sintering is characterized by the highest transparency and confirms the Y3Al5O12 garnet structure of the grains of ∼ 21 nm average size. Yb3+ ion distribution has been analyzed by both TEM-EDX evaluation in grains and grain boundaries and spectroscopy of Yb3+ pairs of small population from the cooperative luminescence phenomenon. EDX analysis at the TEM scale provides unambiguous results on a clear tendency of almost uniform Yb3+ distribution. An important new observation has been made at 4 K and room temperature with the \(^{2}F_{7/2} \rightarrow ^{2}F_{5/2}\) 0-phonon absorption line at 975.7 nm in addition of the 0-phonon line of the YAG structure of grains at 968 nm similar to that of bulky YAG single crystals. We have discussed the origin of this new 0-phonon line relaxing only by non-radiative transitions and conclude that this line might be assigned to Yb3+ distorted sites on the grain surfaces.