Nonradiative Transition Process Research Articles

Synthesis of ZrO2:Pr3+,Gd3+ nanocrystals for optical thermometry with a thermal sensitivity above 2.32% K-1 over 270 K of sensing range.

Nowadays, there is enthusiastic effort to develop luminescent thermometers used for remote and high-sensitivity temperature readout over a wide sensing range. Herein, Pr3+ and Gd3+ co-doped ZrO2 nanocrystals are designed, prepared and investigated by XRD, Raman spectroscopy, XPS, TEM, EDS, DRS, PLE and PL spectroscopy. Upon 275 nm irradiation, the PL spectrum of ZrO2:Pr3+,Gd3+ is found to be composed of a narrow emission peak at 314 nm (Gd3+ 6P7/2-8S7/2), a broad defect-related emission band at 400 nm, and several emission peaks in the wavelength region of 585-700 nm (Pr3+ 1D2-3H4, 3P0-3H6, and 3P0-3F2), which exhibit different thermal responses owing to the effects of the various non-radiative relaxation processes and trap energy levels. Accordingly, the luminescence intensity ratio (LIR) between the Pr3+ 1D2-3H4 and Gd3+ 6P7/2-8S7/2 transitions demonstrates excellent relative sensing sensitivity values ((2.32 ± 0.01)% K-1-(8.32 ± 0.05)% K-1) and low temperature uncertainties (0.08 K-0.28 K) over a wide temperature sensing range of 303 K to 573 K, which are remarkably better than those of many other luminescence thermometers. What is discussed in the present study may be conducive to broadening the research region of RE3+ doped luminescence thermometric phosphors, especially for materials with rich 4f-4f transition lines and defect-related luminescence.

Fabrication of ternary ZrO2-Al2O3-YAG:Ce ceramic phosphors for white light-emitting diodes

Ternary ZrO2-Al2O3-YAG:Ce (ZA-YAG:Ce) phosphor ceramics with a different volume ratio of ZrO2 addition were fabricated through vacuum solid-state reaction. From the EDS analysis result, it was discovered that the three phases can co-exist with each other independently after sintering. The emission peak of Ce3+ redshifts from 535 nm to 560−570 nm with the increasing of ZrO2, which is argued to be related to the non-radiative transition process between the 5d1 level of Ce3+ and oxygen vacancy levels. After packaged with blue chips, the w-LED based on ZA-YAG:Ce ceramic shows an enhanced CRI of 64 compared to Al2O3-YAG:Ce and a high LE of 92.36 lm/W, which is greatly promoted compared to the previous report of ZrO2-YAG:Ce. The result has indicated the great application prospect of ZA-YAG:Ce ceramic phosphors in warm w-LEDs.

Employing Lactam Copolymerization Strategy to Effectively Achieve Pure Organic Room‐Temperature Phosphorescence in Amorphous State

AbstractPure organic, amorphous room‐temperature phosphorescent (RTP) materials have aroused great attention because of their easy preparation and wide applications. Herein, several metal‐free, amorphous, lactam phosphorescent copolymers are prepared. Copolymers of N‐vinylpyrrolidone (NVP) and N‐vinylcaprolactam (NVCL) with small amounts of monomers exhibit diverse RTP emissions. The rigid polymer matrices of poly(N‐vinylpyrrolidone) (PVP) and poly(N‐vinylcaprolactam) (PNVCL) effectively help to inhibit nonradiative transition process of phosphors, which enhances the RTP emission. Furthermore, outstanding biocompatibility, low cost, and highly hydrophilic character make these PVP‐ and PNVCL‐based RTP copolymer materials potentially applicable in various fields.

Luminescent materials comprised of wood-based carbon quantum dots adsorbed on a Ce0.7Zr0.3O2 solid solution: synthesis, photoluminescence properties, and applications in light-emitting diode devices

In this study, a luminescent material of wood-based carbon quantum dots (CQDs)/Ce0.7Zr0.3O2 with a regular, uniform shape, and a high color purity was synthesized by a facile method. CexZr1−xO2 solid-solution oxides (x = 0.7) were prepared by a hydrothermal method, and CQDs were adsorbed on the as-obtained Ce0.7Zr0.3O2 nanoparticles. The crystal structure was identified by powder X-ray diffraction and Rietveld refinement. In addition, the effect of adsorbed CQDs on the luminescence properties of Ce0.7Zr0.3O2 was systematically investigated by Raman spectroscopy, elemental analysis, electron spin resonance spectroscopy, and photoluminescence spectroscopy, as well as decay curve and quantum yield (QY) measurements. Herein, a promising strategy of surface engineering, that is, reduction of the surface defect concentration in Ce0.7Zr0.3O2 by the adsorption of CQDs to decrease the nonradiative transition processes at defect sites was described. As expected, the adsorption of CQDs decreased the Ce–O bond length and the surface defect concentration of Ce0.7Zr0.3O2, thereby effectively decreasing the nonradiative transition processes and suppressing fluorescence quenching. Hence, CQDs/Ce0.7Zr0.3O2 with a perfect cubic structure exhibits a performance improvement in terms of the luminescence and QY value. Furthermore, a light-emitting diode device was fabricated using as-synthesized CQDs/Ce0.7Zr0.3O2, emitting apparent yellow light with a high color purity (93.3%), with International Commission on Illumination coordinates of (0.5239, 0.4531), and a low correlated color temperature (2271 K) under a driven current of 60 mA, suggesting that CQDs/Ce0.7Zr0.3O2 exhibits potential for practical applications.

Chemical vapor deposition synthesis of Ge doped ZnO nanowires and the optical property investigation

Ge-doped ZnO nanowires with high quality were prepared with simple chemical vapor deposition method. The morphology, crystal structure and elemental distribution were characterized using SEM (scanning electron microscopy), HRTEM (high resolution transmission electron microscopy) and EDS (energy dispersive spectrometry) mapping approaches. The optical properties were investigated with combination of room-temperature CL (cathode ray luminescence) and variable temperature micro-PL methods. It is found for the first time that a tapered Ge doped nanowire exhibits a thickness-dependent spectrum shift under a cathode-ray excitation, which was interpreted by the electron-lattice interaction model and the band-filling effect of semiconductors. The exciting power dependent emission spectra (lux-luminescent characteristics) at varied temperatures reveal a varied radiative recombination mechanism. At 83 K, free excitons contribute to the emission spectrum, while at 300 K while at 300 K multi-levels related non-radiative transition process largely contributed. The optical property indicates an excellent crystallization of the nanowires with a small amount of defects.

Luminescence Decay of Colloidal Quantum Dots and Stretched Exponential (Kohlrausch) Relaxation Function

The room temperature non-exponential luminescence decay of colloidal quantum dots is investigated theoretically in an attempt to identify the underlying physical mechanisms responsible for the shape of the decay. It is shown that a stretched exponential functional form of the luminescence decay can be understood in terms of long-range resonance energy transfer from quantum dots to acceptors (molecules, quantum dots, or anharmonic molecular vibrations) or by contact quenching by quenchers (surface traps, molecules or quantum dots) distributed statistically (Poisson distribution) on the surface of the dots. These non-radiative transition processes are assigned to different ranges of the stretching parameter β.

Anomalous Effect of Intramolecular Charge Transfer on the Light Emitting Properties of BODIPY.

A new way to approaching highly emissive BODIPY dyes in both solution and solid state is achieved by introducing intramolecular charge transfer (ICT). A hybrid excited state that shows behaviors of both the ICT excited state and local excited state is discovered to be beneficial in avoiding the disturbance from the rotation vibration of the flexible phenyl substituents. Thus, the nonradiative transition process is suppressed, and the fluorescence efficiency goes up. By modification of the excited state, we realize emission enhancement of crystalline BODIPY derivatives with a PLQY from 0.08 to 0.27, and the compound expresses a good potential application prospect in organic semiconductors.

Luminescence Decay of Colloidal Quantum Dots and Stretched Exponential (Kohlrausch) Relaxation Function

AbstractThe room temperature non-exponential luminescence decay of colloidal quantum dots is investigated theoretically in an attempt to identify the underlying physical mechanisms responsible for the shape of the decay. It is shown that a stretched exponential functional form of the luminescence decay can be understood in terms of long-range resonance energy transfer from quantum dots to acceptors (molecules, quantum dots, or anharmonic molecular vibrations) or by contact quenching by quenchers (surface traps, molecules or quantum dots) distributed statistically (Poisson distribution) on the surface of the dots. These non-radiative transition processes are assigned to different ranges of the stretching parameter β.

Fluorescence via Reverse Intersystem Crossing from Higher Triplet States in a Bisanthracene Derivative

To elucidate the high external quantum efficiency observed for organic light-emitting diodes using a bisanthracene derivative (BD1), non-radiative transition processes as well as radiative ones are discussed employing time-dependent density functional theory. It has been previously reported that the observed high external quantum efficiency of BD1 cannot be explained by the conventional thermally activated delayed fluorescence involving T1 exciton nor triplet-triplet annihilation. The calculated off-diagonal vibronic coupling constants of BD1, which govern the non-radiative transition rates, suggest a fluorescence via higher triplets (FvHT) mechanism, which entails the conversion of a high triplet exciton generated during electrical excitation into a fluorescent singlet exciton. This mechanism is valid as long as the relaxation of high triplet states to lower states is suppressed. In the case of BD1, its pseudo-degenerate electronic structure helps the suppression. A general condition is also discussed for the suppression of transitions in molecules with pseudo-degenerate electronic structures.

On the origin of stretched exponential (Kohlrausch) relaxation kinetics in the room temperature luminescence decay of colloidal quantum dots.

The non-exponential room temperature luminescence decay of colloidal quantum dots is often well described by a stretched exponential function. However, the physical meaning of the parameters of the function is not clear in the majority of cases reported in the literature. In this work, the room temperature stretched exponential luminescence decay of colloidal quantum dots is investigated theoretically in an attempt to identify the underlying physical mechanisms associated with the parameters of the function. Three classes of non-radiative transition processes between the excited and ground states of colloidal quantum dots are discussed: long-range resonance energy transfer, multiphonon relaxation, and contact quenching without diffusion. It is shown that multiphonon relaxation cannot explain a stretched exponential functional form of the luminescence decay while such dynamics of relaxation can be understood in terms of long-range resonance energy transfer to acceptors (molecules, quantum dots, or anharmonic molecular vibrations) in the environment of the quantum dots acting as energy-donors or by contact quenching by acceptors (surface traps or molecules) distributed statistically on the surface of the quantum dots. These non-radiative transition processes are assigned to different ranges of the stretching parameter β.

Amorphous 2-Bromocarbazole Copolymers with Efficient Room-Temperature Phosphorescent Emission and Applications as Encryption Ink

The development of metal-free organic room-temperature phosphorescent (RTP) materials is promising but challenging, because spin–orbit coupling is less efficient without heavy metals such as platinum and palladium. Here, we present a novel amorphous copolymer composed of 2-bromocarbazole phosphor and acrylamide on its side chains, which can engender blue–purple phosphorescence emission with high quantum yield at room temperature. The polymer matrices of acrylamide and the hydrogen bonding in the polymeric chain system can effectively help to inhibit nonradiative transition process and, hence, strengthen the phosphorescent emission. The molar ratio of the 2-bromocarbazole phosphor and acrylamide remarkably influences the RTP emission intensities and quantum yields of the polymers. The high amount of phosphor will weaken the rigidity of the polymers and the shielding effect from oxygen, thus leading to a decrease in their RTP emission, while a low concentration of the phosphor will also weaken their RTP emi...

The ratiometric dual-fluorescence of near infrared absorbing aminocyanine dyes: A mechanistic study

A systematic spectral analysis is presented for a heptaaminocyanine dye (Cy7-NCY) and pentaaminocyanine dye (Cy5-NHY). The dyes displayed near infrared and ratiometric dual-fluorescence in viscous environments. Computational studies on these dyes revealed the origin of dual-fluorescence and the nature of the difference in viscous-sensitivity for these dyes. In low-viscous solvents, the amino group can be described as a rotor and was decoupled with the polymethine chain by photo-excitation, displaying an emission peak with long wavelength. In high-viscous solutions, on the contrary, the amino group is almost parallel to the parent polymethine chain and the lone-pair of the amino group is in an appropriate orientation for efficient overlap with the polymethine chain, resulting in an emission peak with short wavelength. The overall balance of the rotating ability for the substituted amino group and the polymethine chain affects the competition between radiative and non-radiative transition processes in a viscous environment.

Ce3+-Sensitized GdPO4:Tb3+ Nanorods: An Investigation on Energy Transfer, Luminescence Switching, and Quantum Yield

Herein we report the enhanced green emission from Tb3+-doped GdPO4 nanorods sensitized with Ce3+. The increase in the rate of nonradiative transition processes in sensitizer due to efficient energy transfer to activator is realized from steady-state and dynamic luminescence studies. Luminescence quenching due to cross relaxation is least significant up to 20 at. % Ce3+ and 7 at. % Tb3+ concentration. The quantum yield of the sample with maximum luminescence, i.e., GdPO4:Tb3+ (5 at. %)/Ce3+ (7 at. %), is found to be 28%. Also, samples are readily redispersible in water and could be easily incorporated in polymer-based films that show strong green light emission under UV excitation. The luminescence switching (ON and OFF) behavior is examined using alternately an oxidizing agent (KMnO4) and then a reducing agent (ascorbic acid) through a redox reaction (Ce3+/Ce4+). Both GdPO4:Tb3+ and GdPO4:Tb3+/Ce3+ are observed to be paramagnetic.

Seven-photon ultraviolet upconversion emission of Er3+ induced by 1,540-nm laser excitation

The multi-photon ultraviolet upconversion emission properties and synergistic effect are investigated in BaSr2Y6O12:Er3+ phosphor. The deep-ultraviolet emissions centered at 274, 297 and 324-nm are observed under the 1,540-nm excitation, which results from a seven-, six- and six-photon upconversion process, respectively. A synergistic effect is found, which shows that the red emission intensity under 351- and 1,540-nm dual excitation is 4.7 % time stronger than the sum of red emission intensities under the 351 and 1,540-nm single excitation. This phenomenon is attributed to the 4I13/2 and 4I11/2 levels of Er3+ from non-radiative transition process under the 351-nm excitation are excited again to 4F9/2 level by absorbing 1,540-nm photon in the 351- and 1,540-nm dual-excitation process.

Nonradiative Transition Processes between the Relaxed Excited States of Ag-Centers Doped in KI Crystals

Decay kinetics and intensities of the luminescence bands for Ag - centers doped in KI single crystals have been investigated over a wide temperature range between 5 and 310 K. The Ag - centers doped in KI exhibit a broad and largely Stokes-shifted C * luminescence band at about 1.60 eV in addition to the A' and C' luminescence bands, whose peaks are 2.65 and 3.67 eV, respectively. The luminescence intensities of these bands change with temperature while complementing each other. Decay times of the luminescence decrease in the temperature range where the luminescence diminishes with increasing temperature. The temperature evolutions of the decay times and the intensities of the luminescence are analyzed on the basis of a configuration coordinate model including the thermally activated nonradiative transitions among the relaxed excited states. From the analysis results, the parameters characterizing the respective nonradiative transitions are determined.

Theory of Excited State Decays and Optical Spectra: Application to Polyatomic Molecules

General formalism of absorption and emission spectra, and of radiative and nonradiative decay rates are derived using a thermal vibration correlation function formalism for the transition between two adiabatic electronic states in polyatomic molecules. Displacements, distortions, and Duschinsky rotation of potential energy surfaces are included within the framework of a multidimensional harmonic oscillator model. The Herzberg-Teller (HT) effect is also taken into account. This formalism gives a reliable description of the Q(x) spectral band of free-base porphyrin with weakly electric dipole-allowed transitions. For the strongly dipole-allowed transitions, e.g., S(1) --> S(0) and S(0) --> S(1) of linear polyacenes, anthracene, tetracene, and pentacene, the HT effect is found to enhance the radiative decay rates by approximately 10% compared to those without the HT effect. For nonradiative transition processes, a general formalism is presented to extend the application scope of the internal conversion theory by going beyond the promoting-mode approximation. Numerical calculations for the nonradiative S(1) --> S(0) decay rate of azulene well explain the origin of the violation of Kasha's rule. When coupled with first-principles density functional theory (DFT) calculations, the present approach appears to be an effective tool to obtain a quantitative description and detailed understanding of spectra and photophysical processes in polyatomic molecules.

Vibration correlation function formalism of radiative and non-radiative rates for complex molecules

General radiative and non-radiative rates formalisms are derived using the vibration correlation function method for the transition from the excited singlet to ground singlet states by considering the Duschinsky rotation and Herzberg–Teller effects at finite temperature. For the non-radiative transition process, the conventional assumption of (single) “promoting-mode” is abandoned and a promoting-mode free formula is presented. Using this new rate formalism, we re-examine the well-established photophysical properties of anthracene. Both the calculated radiative and non-radiative rates are in good agreement with the available experimental measurements and previous theoretical values. Furthermore, we rationalize the exotic aggregation induced emission phenomenon in 9-[(o-Aminophenyl)phenylmethylene]-9H-fluorene molecule: the roles of low-frequency phenyl ring twist motions and their Duschinsky mode mixings are found to be crucial, especially for the temperature dependence. The present rate theory can quantitatively describe the excited-states dynamic processes in large molecules and is a powerful tool for the design of new high-efficiency light-emitting materials.

Photoacoustic spectra on Mn-doped zinc silicate powders by evacuated sealed silica tube method

The non-radiative transition processes on non-doped Zn2SiO4 and Zn2SiO4:Mn powders with various Mn concentrations were studied by photoacoustic (PA) spectroscopy. Zn2SiO4:Mn x powders were prepared by dry reaction within an evacuated silica glass tube. As the result of photoluminescence (PL) measurement, the increase of PL intensity for green emission on samples doped with Mn between 1% and 6% and the concentration quenching for luminescence on samples doped with Mn between 7% and 12% were confirmed. For the green luminescence on zinc silicate doped with Mn phosphor, the PL decay behavior is assumed to be due to tunneling directly from excited states of electron traps to the excited states of Mn-ion. The Mn content dependence of PL intensity for green emission is well interpreted by tunneling theory and the results of PA spectra, that is, the green emission is assisted by tunneling from non-radiative levels of Mn-ion to luminescencet level as 4T1(4G).

Defect centers and optical absorption edge of degenerated semiconductor ZnO thin films grown by a reactive plasma deposition by means of piezoelectric photothermal spectroscopy

Undoped and Ga-doped (3 wt %) n-type ZnO thin films were grown by a reactive plasma deposition method on glass substrates at 200 °C under an oxygen flow rate from 0 to 50 SCCM. In this paper, we report on the defect and band edge related signals in the optical absorption spectra for ZnO thin film by using a piezoelectric photothermal (PPT) spectroscopy, which is effective in observing a nonradiative transition process. The PPT peak around 2.5 eV was observed only for the undoped ZnO samples grown under a low oxygen flow rate. This signal is considered to be related to the oxygen vacancies, because it disappears with the increase of the oxygen flow rates. No corresponding peak was found for the Ga-doped samples. This result indicates that Ga doping inhibits the generation of the oxygen vacancies, and it agrees with that from the first-principle electronic band structure calculations. We have also carried out the theoretical calculation for the optical absorption edge of degenerated ZnO as a function of the carrier concentration. Burstein-Moss effect and band-gap-narrowing effect in ZnO should be considered in the case of high carrier concentration. Comparing the experimental results with the theoretical predictions, we found out that the proposed PPT edge energy coincides well with the Fermi level EF.

Nonradiative Transition Processes Observed from Photoacoustic Spectra of ZnO Thin Films Fabricated by Pulsed Laser Deposition

Photoacoustic (PA) spectroscopy is a powerful tool for evaluating nonradiative transition processes in semiconductors. By this technique, we evaluated the nonradiative transition process (NRTP) in ZnO thin films that were fabricated by pulsed laser deposition on R-sapphire and C-sapphire substrates in vacuum and in oxygen ambient. The samples each exhibited a specific PA spectrum reflecting the characteristics based on different crystalline orientations and qualities. The relationships between a NRTP and a radiative transition process (RTP) were studied using photoluminescence and PA spectroscopy to investigate the effect of oxygen vacancy (VO) compensation in films. (1120) ZnO/R-sapphire indicated that the NRTP in the UV region and the RTP in the green-yellow region dominate strongly over transition processes. On the other hand, (0001) ZnO/C-sapphire had opposite domination relationships in the RTP and the NRTP to those of (1120) ZnO/R-sapphire. We concluded that the decrease of the NRTP that originated from the compensation of VO strongly affected the RTP in (0001) ZnO/C-sapphire and the NRTP in (1120) ZnO/R-sapphire.