Photochromic Centers Research Articles

KSr2Nb5O15-based composite ceramics: The atomic-scale anisotropic origin of photochromism and influences of second phase concentrations on optical properties

The compositing modification strategy overcame the contradiction between the concentration and depth of photochromic centers and effectively enhanced the photochromic and luminescence modulation abilities of KSr2Nb5O15-based ceramics. However, the compositing concentration influences and potential competitive mechanism are unclear. In this work, the influences of Nb2O5 composite concentration on the microstructures and the photochromic and luminescence modulation properties were systematically investigated. The overconcentration generated the aggregation of the second phase, which brought the competitive mechanism during the photochromic reaction and deteriorated the property. Moreover, to further strengthen the anti-counterfeiting ability, the textured composite ceramic was prepared by a template grain growth method. In addition to the grain-oriented microstructures and anisotropic optical properties, an atomic-scale explanation of the anisotropies of the defect distribution around interfaces was given. This work further explores the heterojunction modification mechanism and the atomic-scale origin of anisotropic photochromism and paves the way to develop and modify photochromic ceramics by interface design.

The photo-thermochromic properties and mechanism of PZNNT ferroelectric ceramics

The photo-thermochromic effect of perovskite-structured 0.83Pb(Zr1/2Ti1/2)O3-0.11Pb(Zn1/3Nb2/3)O3-0.06Pb(Ni1/3Nb2/3)O3 (PZNNT) ferroelectric ceramics was discovered. Its phase, microstructure, chemical states, and optical properties were investigated. The results show that the visible-range reflectivity of the PZNNT ceramics was reduced after near-UV light irradiation, and the optical state can be nondestructively recovered after thermal treatment at 200 °C for 10 min. The kinetic process of the photochromic reaction follows an exponential curve, but the thermodynamic process of the thermochromic reaction is linear. The photo-thermochromic effects exhibited good cyclic stability. The oxygen vacancy VO·· and the small polarons Ti3+ might be the potential photochromic centers of PZNNT ceramics. A multi-photo-thermochromic mechanism coexisted in PZNNT ceramics. This work opens up a paradigm for the development of multifunctional ferroelectric ceramics.

Synthesis of Redox-Active Photochromic Phenanthrene Derivatives.

A phenanthrene unit has been functionalized by several methylthiophene units in order to bring it a photochromic behavior. These compounds were characterized by NMR, absorption and emission spectroscopies, theoretical calculations as well as cyclic voltammetry. The association of a phenanthrene group with a photochromic center could open the door to a new generation of organic field-effect transistors.

Synthesis of a Negative Photochrome with High Switching Quantum Yields and Capable of Singlet-Oxygen Production and Storage.

A dimethyldihydropyrene (DHP) photochromic unit has been functionalized by donor (triphenylamine group) and acceptor (methylpyridinium) substituents. This compound was characterized by NMR, absorption and emission spectroscopies as well as cyclic voltammetry, and its properties were rationalized by theoretical calculations. The incorporation of both electron-donor and -withdrawing groups at the photochromic center allows i) an efficient photo-isomerization of the system when illuminated at low energy (quantum yield: Φc-o =13.3 % at λex =660 nm), ii) the reversible and quantitative formation of two endoperoxyde isomers when illuminated under aerobic conditions at room temperature, and iii) the storage and production of singlet oxygen. The photo-isomerization mechanism was also investigated by spin-flip TD-DFT (SF-TD-DFT) calculations.

Novel strategy for the enhancement of anti-counterfeiting ability of photochromic ceramics: Sm3+ doped KSr2Nb5O15 textured ceramics with anisotropic luminescence modulation behavior

Sm3+ doped KSr2Nb5O15 (KSN-Sm) textured ceramics with anisotropic photochromic and luminescence modulation behaviors provided a new strategy for the enhancement of anti-counterfeiting ability. The KSN-Sm textured ceramics were fabricated by the tape casting technology, which exhibited obvious grain-orientation, with Lotgering factor f(00l) of 0.62. The textured sample possessed evident difference of reflectivity, photochromic, luminescent and luminescence modulation properties among various grain-orientated directions. The difference of luminescent emission intensity was over than 30 % and the luminescence modulation ratios △Rt are 75.3 % and 63.3 % along paralleled and vertical [00l] orientations, respectively. These optical anisotropies were attributed to the different refractive indexes, distributions of photochromic centers and energy transfer rates at various orientations. This work is hopeful to achieve the multidirectional data recording and enhancement of anti-counterfeiting ability of photochromic ceramics by the anisotropic properties of textured ceramics.

The REE-Induced Absorption and Luminescence in Yellow Gem-Quality Durango-Type Hydroxylapatite from Muránska Dlhá Lúka, Slovakia

In talc-magnesite veins in serpentinite near Muránska Dlhá Lúka (MDL), Slovakia, yellow euhedral to subhedral crystals apatite of a gem quality occur. It has a composition of hydroxylapatite with F− varying between 0.29 and 0.34 apfu, Cl− in range of 0.02–0.05 apfu and calculated OH− content between 0.62–0.68 apfu. Moreover, [CO3]2− molecules were identified by FTIR and Raman spectroscopy. MDL apatite contains only up to 0.003 apfu As5+ and Si4+ substituting for P5+; Ca is substituted by small amount of Na, Fe2+, Mn2+ (all up to 0.006 apfu), and Rare Earth Elements (REE—in total up to 0.017 apfu). Compared to trace-element composition of similar apatites from Durango, Mexico, the REE content in MDL apatite is around ten times lower with Nd > Ce >> La, its chondrite-normalized REE pattern has almost a horizontal slope and larger negative Eu anomaly. The MDL apatite is richer in Mn, Pb and Li, but poorer in As, V, Th and U. The concentrations of Sr and Y are similar. In the optical absorption spectra, the most prominent bands are at 585–590 nm (Nd3+) and between 600 and 800 nm (Mn2+, Ce3+-SiO3− photochromic center and Nd3+). The photoluminescence spectrum of MDL apatite shows bands between 550 and 620 nm (Dy3+, Sm3+, Pr3+ and also Mn2+) which likely enhance its yellow color. MDL hydroxylapatite likely formed from fluids derived from granitic rocks as evidenced by the chondrite-normalized REE patterns, Li, Mn and Y concentrations. The Sr content reflects the host-rock serpentinite composition. Fluids for its crystallization were likely derived from Muráň complex orthogneisses by the Alpine deformation and recrystallization in greenschist to lower amphibolite facies.

Charge Transfer and F-Type Centers in Alkaline-Earth Fluorides Activated by Cd2+ or Zn2+ Ions

Photoluminescence spectra of impurity centers containing anion vacancy Cd+ $${{{v}}_{a}}$$ or Zn+ $${{{v}}_{a}}$$ in alkali-earth fluorides have been studied. A comparison with luminescence spectra of photochromic centers (PCs), which can be considered as perturbed F centers, shows that luminescence of centers Cd+ $${{{v}}_{a}}$$ or Zn+ $${{{v}}_{a}}$$ is similar to that of F centers; therefore, it is caused by charge transfer from an impurity ion to an anion vacancy.

Перенос заряда и центры F типа в щелочноземельных фторидах, активированных ионами Cd-=SUP=-2+-=/SUP=- или Zn-=SUP=-2+-=/SUP=-

The photoluminescence spectra of impurity centers containing the anionic vacancy Cd+va or Zn+va in alkaline-earth fluorides are studied. A comparison with the luminescence spectra of photochromic centers (РC), which can be considered as perturbed F centers, shows that the luminescence of the Cd+va or Zn+va centers is similar to the luminescence of F centers and, therefore, is associated with charge transfer from an impurity ion to an anionic vacancy.

Photochromic effect of transparent lead-free ferroelectric KSr2Nb5O15 ceramics

Transparent KSr2Nb5O15 (KSN) lead-free ferroelectric ceramics have been synthesized via modified pressureless sintering method. A significant photochromic effect was observed for the transparent KSN ceramics prepared without rare-earth dopant modification. The piezoelectric properties depend on the grain orientations were investigated. The optical transmittance of the KSN ceramics is greater than 40% in the wavelength range of 530–800 nm. After NUV irradiation, the absorbance was enhanced by more than 40% in a broad visible range (more than 79%). The absorbance returned to the initial value after a thermal bleaching process. The results of the cycling tests and response experiments showed the stability and saturation of the photochromic effect. In addition, the possible photochromic mechanism of the KSN ceramics is discussed and the photochromic centers are identified. This transparent KSN ceramics exhibits an obvious photochromic effect and is a potential candidate materials for optical data storage and information recording applications.

Are the Photochromic Centers in Alkaline-Earth Fluorides Analogous to DX Centers in Semiconductors?

Are the Photochromic Centers in Alkaline-Earth Fluorides Analogous to DX Centers in Semiconductors?

Effect of heat treatment on the luminescence properties of natural apatite

Regarding gemstone identification, the effect of heat treatment under reduction condition to optical and luminescence properties of natural green apatite (Ca5(PO4)3OH, F, Cl) from Madagascar has been investigated. Although electron microprobe analysis (EPMA) shows a luminescence defect of rare earth element such as Ce (0.47%), Pr (0.22%) and Nd (0.15%), however original sample does not show luminescence properties. With consecutive heating process, the color alteration from green to colorless of samples, and also the enhancement of luminescence properties of Ce3+ in natural apatite was observed. Mechanism of fluorescence enhancement and color changes during heating process were discussed using optical spectra and emission spectra. Optical spectra of heated apatite point out that green and blue color assigned to the existence of a photochromic center of Ce3+-SiO3− radical and SO3−, respectively. Consequently, the vanishing of these both bands resulted in the colorless heated apatite at 800 °C. Heated apatite showed emission band at 365 nm because of the deactivation of electron from 4f of Ce3+ to 5d. Optical spectra reveal that the correlation of luminescence enhancement and the fading of yellow tint of heated apatite by means of the descending of Ce3+ to Ce4+ charge transfer process. The ultimate contribution of this work is the finding of the difference of luminescence properties between non-heated and heated apatite which can be used as an effective tool for gem identification. This identification tool is remarkable because it is rapid and non-destructive.

Divalent rare-earth ions Pr, Sm, Нo, Er, Tm, and Yb in crystals of alkaline-earth fluorides

The absorption spectra of radiation-colored CaF2, SrF2, and BaF2 crystals activated by trivalent Pr, Sm, Нo, Er, Tm, and Yb (rare-earth, RE) ions are studied. It is shown that ionizing radiation reduces the impurity ions to the divalent state. The temperature resistance of divalent RE ions of radiation-colored CaF2 crystals correlates with the chemical stability of the compounds with divalent RE ions. The photochromic centers are produced in CaF2-Pr crystal colored by radiation at room temperature and heated to 200°C.

Luminescence of photochromic centers in calcium fluoride crystals doped with Lu3+ ions

We report data on the luminescence spectra associated with photochromic centers in X-ray irradiated calcium fluoride crystals doped with Lu ions. Irradiation in low energy photochromic centers absorption band excites emission, which can be identify with transitions into photochromic centers. Ab initio calculation of absorption spectrum of photochromic center agrees rather well with experimental data.

Instability of photoinduced optical absorption of Bi12SiO20: Al crystals

The results of the experimental investigation of the instability of the establishment and relaxation of a photochromic effect in aluminum-doped Bi12SiO20 crystals have been presented. The oscillating and nonmonotonic kinetic dependences of the photoinduced optical absorption have been observed. The absorption oscillations are associated with the competition of the formation and destruction of [AlSiO4]0 photochromic centers.

Effect of cationic impurities on the formation of radiation defects in alkaline earth fluorides

A new model of photochromic centers formed by ionizing radiation or the additive coloration of alkaline earth fluorides doped with rare-earth ions (La, Ce, Gd, Tb, Lu, and Y) is proposed, based on original experimental results and the literature data.

Photophysics of single nitrogen-vacancy centers in diamond nanocrystals

A study of the photophysical properties of nitrogen-vacancy (NV) color centers in diamond nanocrystals of size of 50~nm or below is carried out by means of second-order time-intensity photon correlation and cross-correlation measurements as a function of the excitation power for both pure charge states, neutral and negatively charged, as well as for the photochromic state, where the center switches between both states at any power. A dedicated three-level model implying a shelving level is developed to extract the relevant photophysical parameters coupling all three levels. Our analysis confirms the very existence of the shelving level for the neutral NV center. It is found that it plays a negligible role on the photophysics of this center, whereas it is responsible for an increasing photon bunching behavior of the negative NV center with increasing power. From the photophysical parameters, we infer a quantum efficiency for both centers, showing that it remains close to unity for the neutral center over the entire power range, whereas it drops with increasing power from near unity to approximately 0.5 for the negative center. The photophysics of the photochromic center reveals a rich phenomenology that is to a large extent dominated by that of the negative state, in agreement with the excess charge release of the negative center being much slower than the photon emission process.

Application of a volume holographic grating in a CaF2 crystal for measuring linear displacements with nanoscale accuracy

A holographic method for measuring linear displacements based on the use of a highly stable volume scale hologram recorded in an additively colored calcium fluoride crystal with photochromic color centers is proposed and experimentally approved. The essence of this method lies in measuring and analyzing harmonic signals formed during linear displacement of crystal with a volume hologram in an external interference field. A physical model of the formation of harmonic signals in photodetectors when measuring displacements is considered, and a mathematical method for calculating linear displacements by plotting a Lissajous figure is substantiated. A laboratory breadboard of a device for measuring linear displacements in a range of 10 mm, limited by the aperture of crystal with a recorded 8.7-mm-thick hologram, is designed. When using a scale hologram with a period of 2.18 μm and a 632.8-nm He-Ne laser for reading this hologram, the error in measuring displacements by this method is 9 nm at a resolution of 3 nm.

Photochromism in Calcium and Strontium Fluoride Crystals Doped With Rare-Earths Ions

This paper reports the absorption spectra of photochromic centers in CaF2 and SrF2 crystals doped with Ce3+ and Gd3+ impurities and thermal decay of the centers in temperature range 80-500 K. The ionized photochromic color centers are generated in crystals under low-temperature X-rays irradiation. These centers are transformed into photochromic color centers upon heating of crystals. All color centers decay at temperature about 500 K.

Ab Initio Investigation of the Electronic Properties of Coupled Dithienylethenes

The design of more efficient photochromes does not stand as the only challenge in the field of molecular switches; more complex and refined structures, allowing one to pave the way toward multiply addressable structures, are highly sought after. In this framework, compounds containing several dithienylethenes have been recently proposed. In the present Letter, we investigate the spectral properties of photochromes made of two dithienylethene units linked through conjugated bridges to form meta and para switches. All combinations of closed and open forms have been modeled by using (time-dependent) density functional theory. With this ab initio tool, the coupling between the photochromic centers could be assessed. Such calculations are a step necessary to understand the parameters guiding the formation of the fully closed forms.

Thermal decay of photochromic color centers in CaF2, SrF2, and BaF2 crystals doped by La and Y impurities

The absorption spectra of photochromic centers in CaF2, SrF2, and BaF2 crystals doped by La and Y impurities and thermal decay of the centers in the temperature range 80–600 K are investigated. Under low-temperature x-ray irradiation, ionized photochromic color (PC+) centers are generated in La- and Y-doped CaF2 crystals and in a La-doped SrF2 crystal. It is revealed that, upon heating of the CaF2-LaF3 crystal, PC+ centers are transformed into photochromic color (PC) centers. In the SrF2-YF3 crystal irradiated at room temperature, photochromic color centers are generated as well. All color centers decay at a temperature of approximately 600 K. After irradiation of the BaF2-YF3 crystal at a temperature of 80 K, absorption bands are observed at energies of 2.25 and 3.60 eV, which are related to neither PC centers nor PC+ centers.