Luminescence Process Research Articles

Computational Studies of Molecular Materials for Unconventional Energy Conversion: The Challenge of Light Emission by Thermally Activated Delayed Fluorescence.

In this paper we describe the mechanism of light emission through thermally activated delayed fluorescence (TADF)—a process able to ideally achieve 100% quantum efficiencies upon fully harvesting the energy of triplet excitons, and thus minimizing the energy loss of common (i.e., fluorescence and phosphorescence) luminescence processes. If successful, this technology could be exploited for the manufacture of more efficient organic light-emitting diodes (OLEDs) made of only light elements for multiple daily applications, thus contributing to the rise of a sustainable electronic industry and energy savings worldwide. Computational and theoretical studies have fostered the design of these all-organic molecular emitters by disclosing helpful structure–property relationships and/or analyzing the physical origin of this mechanism. However, as the field advances further, some limitations have also appeared, particularly affecting TD-DFT calculations, which have prompted the use of a variety of methods at the molecular scale in recent years. Herein we try to provide a guide for beginners, after summarizing the current state-of-the-art of the most employed theoretical methods focusing on the singlet–triplet energy difference, with the additional aim of motivating complementary studies revealing the stronger and weaker aspects of computational modelling for this cutting-edge technology.

Ultrafast Radiative Relaxation Processes in Multication Cross-Luminescence Materials

Scintillators with ultrafast response are of demand in medical imaging for time-of-flight positron emission tomography, in high-energy physics calorimetry, and in other applications. There are several fast luminescence processes in solid-state materials, which could be good candidates for fast timing applications. The development of new ultrafast silicon photomultipliers with increased vacuum-ultraviolet to ultraviolet (VUV-UV) sensitivity has renewed interest to one of those, cross-luminescence (CL), emitted in the VUV-UV region with subnanosecond decay time. Previous investigations have shown that CL is mostly situated in the VUV region in binary K and Ba compounds, but ternary fluorides with a complex valence band structure can provide ultrafast emissions in UV. According to the band structure calculations for K2SiF6, its complex valence band is composed of the F 2p, Si 2s, and Si 2p states. Transitions between the valence subbands and the outermost K 3p core band may extend CL from the VUV-UV to the visible region. This motivated us to perform spectroscopic study of phase-pure K2SiF6 micropowders with a high degree of crystallinity. The stationary (10 keV) and pulsed (100 keV, $\delta t \approx 55$ ps) electron beams were applied in the studies of relaxation processes of K2SiF6 in the temperature range of 77–300 K. At 300 K, the dynamics of ultrafast processes was studied at the FemtoMAX beamline (MAX IV Lab) providing 10-keV X-ray pulses of 100-fs duration. As a result, we revealed multiple CL bands extending from 9 to 4 eV. A nonexponential decay in the subnanosecond time range was revealed for the CL bands at 4.5 and 5.25 eV under both pulsed electron-beam and X-ray excitations. This opens prospects for a band structure engineering approach in developing new ultrafast scintillators with intrinsic emissions. This approach is based on a targeted modification of material composition to influence the density of states in complex valence bands and to provide a favorable shift of CL to lower energies.

A Double-Band Emitter with Ultranarrow-Band Blue and Narrow-Band Green Luminescence.

Understanding the origin and mechanisms of luminescence is a crucial point when it comes to the development of new phosphors with targeted luminescence properties. Herein, a new phosphor belonging to the substance class of alkali metal lithosilicates with the generalized sum formula Cs4−x−y−zRbxNayLiz[Li3SiO4]4:Eu2+ is reported. Single crystals of the cyan‐emitting UCr4C4‐type phosphor show a peculiar double‐band luminescence with one ultranarrow emission band at 473 nm and a narrow emission band at 531 nm under excitation with UV light (λ exc=408 nm). Regarding occupation of the channels by the light metal ions, investigations of single‐crystal XRD data led to the assumption that domain formation with distinct lithium‐ and sodium‐filled channels occurs. Depending on which of these channels hosts the activator ion Eu2+, a green or blue emission results. The herein‐presented results shed new light on the luminescence process in the well‐studied UCr4C4‐type alkali metal lithosilicate phosphors.

Study of the microscopic mechanism of Ir(ppy)$_{3}$ regulating exciton splitting and luminescence process in Rubrene

To explore the microscopic mechanism of singlet exciton splitting (${\\rm~S}_{1}+{\\rm~S}_{0}\\to~{\\rm~T}_{1}+~{\\rm~T}_{1}$, STT) and luminescence in Rubrene, a phosphorescent material Ir(ppy)$_{3}$ with strong spin-orbit coupling (SOC) and green emission was selected and mixed into Rubrene thin films with different proportions to fabricate a series of luminescent devices. By measuring the magneto-electroluminescence (MEL) and current-luminescence ($I$-$B$) curves of the devices under different temperatures and currents, we found that the MEL profiles of light-emitting devices with different mixing ratios at room temperature show an STT fingerprint characteristic curve of magnetic field modulation. MEL amplitude first increases and then decreases with increased mixing ratio, whereas luminescence intensity increases monotonously. This is different from conventional Rubrene doped devices (such as mCP: $y$%Rubrene) which show STT increases with increasing concentration but with decreasing luminescence . By analyzing the singlet and triplet energy levels and emission spectra of Ir(ppy)$_{3}$ and absorption spectra of Rubrene, it can be seen that aside from Rubrenes molecular spaces influence on the STT process, intersystem crossing (ISC) caused by the strong SOC of Ir(ppy)$_{3}$ and energy transfer processes between the T$_{1}$ exciton of Ir(ppy)$_{3}$ and the S$_{1}$ exciton of Rubrene are also included in the devices. The combined action of these three micro-mechanisms leads complex MEL and luminescence changes in the device, and the devices current density and working temperature also have a good regulatory effect on them. Obviously, this study helps with understanding of the microscopic process and its evolution mechanism based on Rubrene optoelectronic devices.

Upconversion luminescence of Er<SUP align="right">3+</SUP> doped Y<SUB align="right">2O<SUB align="right">3 and Gd<SUB align="right">2O<SUB align="right">3 nanophosphors

The luminescence process and mechanism of Er3+ doped Y2O3 and Gd2O3 nanophosphors are investigated with focus on their upconversion luminescence properties. Here the strong up-conversion (UC) emissions of both Y2O3:Er3+ nanoparticles prepared by combustion with EDTA and Gd2O3:Er3+ nanospheres synthesised by multistep chemical reaction using urea have been measured at room or low temperatures. The downward-conversion luminescence spectra in visible and near infrared region have been assigned from the transitions in the energy level system of Er3+ion. For Y2O3:Er3+nanoparticles, the upconversion luminescent intensity depends on excitation power at 980 nm have properly indicated the two photon process which are the calculated slope values of main transitions in Er3+ions: 2H11/2-4I15/2, 4S3/2-4I15/2and 4F9/2-4I15/2 are 2.07, 1.72, 1.87 (0.5% Er) and 2.32, 1.82 and 2.07 (Er 1.0%) respectively. The integral intensity of the red band is smaller than that of the green band in the upconversion emission of nanoparticles Y2O3 doped with Er3+ with 0.50, 1.00 and 1.50%. However it was the integral intensity of red emission bands that dominated in Gd2O3: 1% Er3+. The lifetimes of the band in green colour peaked at 562 nm upon excitation at 380 nm in Y2O3:Er3+ with 0.50, 1.00 and 1.50%, and have shown the two sequent time decay processes, the shorter and the longer, both in millisecond scale, 0.14 ms (0.5%); 0.28 ms (1.00%) and 0.13 ms (1.50%) and 4.49 ms (0.5%); 6.89 ms (1.00%) and 5.16 ms (1.50%), respectively. For the nanoparticles Gd2O3: 1% Er the intensity decrease of the upconversion emission peaked at 562 nm with only a single process with a decay time of 1.24 ms. Both nanophosphors Y2O3 and Gd2O3 doped with Er3+ are promising materials to develop multifunctional units for medical applications.

Study of persistent luminescence and thermoluminescence in SrSi2N2O2:Eu2+,M3+ (M = Ce, Dy, and Nd).

The process of persistent luminescence or glow-in-the-dark, the delayed emission of light of irradiated substances, has long fascinated researchers, who have made efforts to explain the underlying physical phenomenon as well as put it to practical use. However, persistent luminescence is an elusive and difficult process, both in terms of controlling or altering its properties, as well as providing a quantitative description. In this paper, we used SrSi2N2O2:Eu2+ as a model persistent phosphor, characterized by the broad distribution of structural defects and exhibiting long-lasting Eu2+ luminescence that is visible for a few minutes after switching off UV light. We investigated the persistent luminescence process by two complementary methods, namely, thermoluminescence and temperature-dependent persistent luminescence decay measurements. Analysis of experimental data allowed us to determine the depth distribution of traps, and allowed us to distinguish two different mechanisms by which the emission is delayed. The first, the temperature-dependent mechanism, is related to trap activation, while the second, temperature-independent mechanism is related to carrier migration. Finally, we employed the strategy of the co-doping of the phosphor SrSi2N2O2:Eu2+,M3+ (M = Ce, Nd, Dy) to modify the persistent luminescence properties.

Focus on photonics, spectrometry, and spectroscopy

Focus on photonics, spectrometry, and spectroscopy

Sm3+-doped PVDF–SiO2 hybrid for greenish-blue light emission

Sm3+-doped PVDF–SiO2, a polymer–phosphor hybrid material, was synthesized by a sol–gel method. The X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy were used to study the chemical states and vibrational modes in Sm3+-doped PVDF–SiO2, respectively. The optical properties of Sm3+-doped PVDF–SiO2 were studied using absorption and photoluminescence spectra. The energy band gap was estimated from the absorption spectrum. Sm3+–ligand bond in the material was identified by estimating the bonding parameter from the absorption peaks. This study will serve to a better understanding of the luminescence process in Sm3+-doped PVDF–SiO2 hybrid.

Each phenyl group performs its own functions on luminescence: phenyl substituted effect in tetraphenylpyrazine

Three phenyl groups of TPP derivatives affect the luminescence process in various modes. TPPs show AIE thanks to the ortho substituted groups, while meta and para groups can modulate their luminescence in different extents.

Luminescence of uranium(VI) after liquid-liquid extraction from HCl by Aliquat® 336 in n-dodecane:1-decanol by time-resolved laser-induced luminescence spectroscopy

Abstract To investigate the extraction of uranium(VI) in HCl media by Aliquat® 336 in 1:99 (v:v) 1-decanol:n-dodecane mixture, our objective is to identify the complexe(s) in the organic phase by time-resolved laser-induced luminescence spectroscopy (TRLS). The extraction mechanism is supposed to involve the formation of [ U O 2 C l 4 2 − ⋅ ( R 4 N + ) 2 ] $[U{O_2}Cl_4^{2 - } \cdot {({R_4}{N^ + })_2}]$ in the organic phase. The occurrence of such a species leads to the presence of the UO 2 Cl 4 2 − ${\rm{U}}{{\rm{O}}_2}{\rm{Cl}}_4^{2 - }$ species in the organic solution, which luminescence shows particular features. The luminescence spectra and decay time evolutions are obtained in the organic phase as a function of HCl concentration in the aqueous phase (0.5–6 M). The extraction of UO 2 Cl 4 2 − ${\rm{U}}{{\rm{O}}_2}{\rm{Cl}}_4^{2 - }$ is confirmed by the particular spectrum of uranium(VI) in the organic phase, and the typical splitting of the luminescence bands, due to the crystal field effect, is clearly evidenced. The stoichiometry is verified using luminescence intensity variation as a function of the activity of Cl−, and extraction constants are calculated both using the specific interaction theory and Pitzer model. A decomposition of the spectrum of the extracted complex in the organic phase is also proposed. The decay time variation as a function of temperature allows estimating the activation energy of the luminescence process of the extracted complex.

Reduced Binding Energy and Layer-Dependent Exciton Dynamics in Monolayer and Multilayer WS2.

The exciton dynamics in WS2 from monolayer to four-layer was investigated by using fluorescence lifetime imaging measurement (FLIM). The transition process of negatively charged trions is measured and detected using a fluorescence detection method. Compared with neutral excitons, negatively charged trions have a longer fluorescence lifetime. Further exploration illustrated that the fluorescence lifetime of both neutral excitons and trions get longer when the thickness increased. When WS2 was added from monolayer to four-layer, lifetimes of direct transition excitons and trions tended to increase over 10 and 2.5 times, separately, whereas the lifetime of indirect transition excitons tended to be reduced by nearly 2.5 times. This layer-dependent signature is ascribed to the reduced binding energy in thicker WS2 at room temperature, which is verified by density theory functional calculation. Although the direct transition exciton dominates the whole fluorescence decay process, it is influenced by trions and dark excitons. Based on the FLIM results, we proposed four main exciton transition channels during the fluorescence luminescence process. Such layer-dependent transition channel conception helps to control the fluorescence lifetime, which determines the efficiency of the carriers' separation.

Photoluminescence properties and energy transfer of apatite-type Sr3LaNa(PO4)3 F:Ce3+, Tb3+ phosphors

A series of apatite-type Sr3LaNa(PO4)3F:Ce3+, Tb3+ phosphors were synthesized via high temperature solid-state reaction method. X-ray diffraction (XRD), scanning electron microscopy (SEM), the photoluminenscence excitation (PLE) spectra and photoluminescence (PL) spectra were used to characterize samples. And systematic study on micro-morphology, spectral luminescent properties and process of energy transfer in Sr3LaNa(PO4)3F:Ce3+, Tb3+ phosphors.The results show that Sr3LaNa(PO4)3F:Ce3+, Tb3+ phosphors with strontium apatite crystal structure, the morphology is irregular. The optimum concentration of Ce3+ in Sr3LaNa(PO4)3F:Ce3+ was decided to be 0.04, the emission peaks at 340 nm is due to the 5d → 2F5/2 and 2F7/2 transitions of Ce3+. The PL spectra of Sr3LaNa(PO4)3F:Tb3+ phosphor excited by 253.7 nm presents green-emitting light centered at 543 nm. Effective energy transfer from the sensitizer Ce3+ to activator Tb3+ can be expected in the Ce3+ and Tb3+ co-doped phosphors Sr3La0.96−yNa(PO4)3F:0.04Ce3+,yTb3+. Based on the spectral analysis, The energy transfer efficiency from Ce3+ to Tb3+ reaches up to 78.6%. The energy transfer mechanism from Ce3+ to Tb3+ was via the exchange interaction. In Ce3+ and Tb3+ co-doped Sr3La0.96−yNa(PO4)3F:0.04Ce3+, yTb3+ phosphors, the chromatic coordinate was observed with increasing Tb3+ co-doping rate on moving from (0.2867, 0.2411) at y = 0.00 to (0.3267, 0.5845) at y = 0.40.

Avalanche-like behavior of up-conversion luminescence by nonlinear coupling of pumping rates.

Here we report and discuss the avalanche-like up-conversion behavior in absence of the avalanche. We experimentally observed significant changes in the slope of the curve for the intensity dependence of up-conversion luminescence of erbium ions in the green band (520-560 nm) on the pump intensity of the diode laser. Such changes are typical for the photon avalanche. However, the concentration of erbium ions is insufficient for an efficient exchange of energy between them, and the excitation of a photon avalanche is not possible. Using a simple three-level approximation of the up-conversion process model, we have shown that the observed avalanche-like luminescence process can also occur in the absence of a photon avalanche due to the nonlinear relation between the efficiency of two pumping channels of erbium ions caused by the intensity dependence of the pump spectrum.

Bleaching studies on Al-hole ([AlO4/h]0) electron spin resonance (ESR) signal in sedimentary quartz

Electron spin resonance (ESR) dating of sediments using quartz is most commonly used for older sediments (>100 ka), since large residuals render the ESR signal unsuitable for dating young sediments. The multiple-centre approach (utilising both Ti and [AlO4/h]0 signals) is usually used to test the resetting of the signals used for ESR dating. Here we work towards a better understanding of, and correction for, the residual signal in ESR samples of sedimentary quartz. We undertook multiple-centre ESR measurements using quartz [AlO4/h]0 and Ti signals on young aeolian samples of different grain sizes which have been independently dated using optically stimulated luminescence (OSL). Our results demonstrate that [AlO4/h]0 signal yields residuals indicating equivalent doses of about 500 Gy, substantially older than expected for the known OSL equivalent doses in the range of 8–37 Gy. The decay of [AlO4/h]0 signal as function of bleaching time can be represented by an exponential function. We investigate the dependence of the residual magnitude of the ESR signal as a function of the previous given dose and observe an exponential increase in the residual signal with dose. Such observations are consistent with the results of luminescence process modelling conducted for a model comprising two luminescence centres and several traps, one of which is a so-called deep disconnected trap that cannot be emptied during optical stimulation. We propose that bleaching occurs through an electron-hole recombination process with electrons released from optically sensitive traps. In addition to our new insights into the bleaching mechanisms of the [AlO4/h]0 ESR signal, we discuss the implications for the procedures used for performing residual dose corrections in ESR dating. We recommend that modern analogues be used in addition to laboratory-bleached samples when performing residual dose corrections.

Luminescence and scintillation properties of strontium hafnate and strontium zirconate single crystals

The transparent single crystals of high-melting strontium hafnate (SHO) and strontium zirconate (SZO) have been grown by the floating zone method with vertical mirror arrangement. The X-ray diffraction analysis revealed orthorhombic perovskite structures for both compounds. Evaporation of Sr-containing species during the growth was proved by micro x-ray fluorescence analysis and led to slightly Sr-poor composition in the studied materials. The overall scintillation efficiency of SHO was found to be comparable with that of Bi4Ge3O12 standard scintillator, while that of SZO was considerably lower. The emission spectrum of both compounds was red-shifted when compared to the previously intensively studied powder counterparts and the photoluminescence decay curves became accelerated with pronounced nonexponentiality. The phenomenological model applied to the temperature dependences of the luminescence data revealed that in single crystals the leading luminescence process is either strongly modified with respect to powders or is of a different origin. The scintillation light yield of both materials was determined to be only several tens of photons per MeV under alpha-ray excitation due to strong afterglow, most probably related to ionization of excited state of the luminescence center.

Statistical treatment of Photoluminescence Quantum Yield Measurements

The photoluminescence quantum yield (PLQY) is an important measure of luminescent materials. Referring to the number of emitted photons per absorbed photons, it is an essential parameter that allows for primary classification of materials and further is a quantity that is of utmost importance for many detailed analyses of luminescent systems and processes. Determining the PLQY has been discussed in literature for many years and various methods are known. Absolute values can be measured directly using an appropriate setup. As this relies on the correct evaluation of photon-counts, it is a very sensitive method. Hence, systematic errors that can occur are discussed widely. However, of course those measurements also contain random uncertainties, which remain mainly unconsidered. The careful evaluation of both systematic and statistical errors of the PLQY is the only way to gain confidence in its absolute value. Here, we propose a way of evaluating the statistical uncertainty in absolute PLQY measurements. This relies on the combination of multiple measurements and the subsequent calculus of the weighted mean. The statistical uncertainty is then obtained as the standard deviation of the mean. This method not only quantifies the impact of statistical influences on the measurements, but also allows simple analysis of time-dependent systematic errors during the measurement and the identification of outliers.

Simulation, synthesis, characterisation and dyeing properties of a fluorescent hemicyanine dye

AbstractIn this study, a hemicyanine dye, trans‐4‐(4‐(diethylamino)styryl)‐1‐ethylpyridinium bromide (DYE‐BD), was synthesised and simulated. The maximum absorption wavelength of DYE‐BD was calculated with the same basis set by using the Gaussian 09 software based on different methods, while time‐dependent density functional theory was applied to simulate the maximum emission wavelength of the designed dye. The changes in enthalpy and Gibbs free energy were simulated at the b3lyp/6‐31G(d) level, and energy changes in the luminescence process were assessed using mathematical methods. Furthermore, DYE‐BD was used to colour acrylic fabric following the typical dyeing procedure for cationic dyes. The photostability of the synthesised dye was analysed using quantum theory and mathematical methods. The synthesised dye, analysed using the Ecological Structure Activity Relationships class program, had no toxicity to aquatic life, while the acrylic fabrics dyed with DYE‐BD met the fluorescent orange requirements for background and combined performance materials according to the EN ISO 20471:2013 standard.

Luminous and thermal characteristics of Er3+/Yb3+ co-doped WO3: a heat-sensitive micro-particle

Er3+/Yb3+ co-doped WO3 micro-particles were prepared by sol-gel method. The green upconversion (UC) luminescence and the photothermal characteristics have been investigated. The center wavelength of the green luminescence shifted to short wavelength with the increase of pumping power. For the W2 sample (98.9W2O3-0.1Er2O3-1Yb2O3), the laser-induced temperature can rise up to ∼85 °C under 975 nm excitation. These results indicated that an optical heating of the nanoparticles can be realized, and the temperature rise of the nanoparticles can be investigated via the UC luminescence process. These researches will be meaningful to explore high-quality photothermal materials.

Exciton Luminescence of Wse2 Bilayers

WSe2 films with thickness of two monolayers are deposited on SiO2/Si substrates using the top-down technology. The thickness and the composition of the films are confirmed by measurements of the Raman spectra, photoluminescence, and interference contrast in three RGB channels. The luminescence of WSe2 bilayers is studied at temperatures decreased down to 5 K. It is shown that at low temperatures, the fine structure of the emission spectrum near the direct intrinsic absorption edge is determined by the recombination of A excitons (in the ground or excited state) and various complexes (trions and excitons bound on defects). The emission spectrum near the indirect fundamental absorption edge is described by exciton luminescence processes in which the exciton energy and momentum are transferred to phonons corresponding to the Λ point of the Brillouin zone. A possible contribution of the electron-hole liquid to the emission spectrum is discussed.

Ultrafast and low-power optoelectronic infrared-to-visible upconversion devices

Photon upconversion with transformation of low-energy photons to high-energy photons has been widely studied and especially applied in biomedicine for sensing, stimulation, and imaging. Conventional upconversion materials rely on nonlinear luminescence processes, suffering from long decay lifetime or high excitation power. Here, we present a microscale, optoelectronic infrared-to-visible upconversion device design that can be excited at low power (1–100  mW/cm2). By manipulating device geometry, illumination position, and temperature, the device luminescence decay lifetime can be tuned from tens to hundreds of nanoseconds. Based on carrier transportation and circuit dynamics, theoretical models are established to understand the transient behaviors. Compared with other mechanisms, the optoelectronic upconversion approach demonstrates the shortest luminescence lifetime with the lowest required excitation power, owing to its unique photon–electron conversion process. These features are expected to empower the device with essential capabilities for versatile applications as high-performance light emitters.