Formation Of Color Centers Research Articles

Radiation effects on the performance of flexible perovskite solar cells for space applications

Solar cells for space applications are required to be tolerant to harsh environmental conditions. Especially, tolerance against radiation and charged particles is mandatory. Here we study the effect of low-energy (<< 1 MeV) proton radiation to evaluate the radiation tolerance of flexible perovskite solar cells (PSCs). Low-energy protons are more likely to be stopped in the shallower regions of solar cells, thereby causing greater performance degradation than high-energy protons. Flexible PSCs with layer sequence PET/ITO/PEDOT:PSS/perovskite/PCBM/BCP/metal were fabricated and were irradiated with 100 keV protons (fluence from ~ 3 × 1010 to ~ 3 × 1012 protons/cm2, equating several years in space). Flexible PSCs exhibited a good radiation tolerance and did not show color center formation, revealing their outstanding resistance against low-energy proton radiation. This can be credited to the combined effect of intrinsically large carrier diffusion length exceeding the thin absorber film thickness and the defect tolerance of perovskite crystals.

UV-induced defect formation in cubic ZrO2. Optical demonstration of Y, Yb and Er dopants interacting with photocarriers

The UV-induced photocoloration of cubic zirconia (c-ZrO2) stabilized by Y2O3, Yb2O3 and Er2O3 and treated as a wide bandgap metal oxide (MO2) heavily doped with M3+ ions has been examined by diffuse reflectance spectroscopy. Using the narrow f-f absorption bands of optically labeled Yb3+ and Er3+ as the M3+ dopants, the interaction of these dopant ions with photocarriers is demonstrated spectroscopically against a non-resolved broadband background. The latter occurs as a result of an offset of specific f-f absorption bands following conversion of M3+ ions into optically silent M2+ ions by the trapping of photoelectrons or by a change of the local symmetry of the M3+ surroundings caused by the UV-induced formation of color centers.

Enhanced radiation resistance of ytterbium-doped silica fiber by pretreating on a fiber preform

We propose a new pretreatment method on fiber preforms by loading deuterium (D2), pre-irradiation, combined with thermal annealing. The effect of the pretreatment condition on the optical loss at 1200 nm and a laser slope efficiency of ytterbium-doped silica fibers (YDFs) before and after γ-radiation was comparatively investigated. The related mechanism was revealed by combining the optical absorption, continuous wave electron paramagnetic resonance (CW-EPR), Raman, and Fourier transform infrared (FTIR) spectroscopies. A laser experiment shows that the radiation resistance of YDFs can be significantly improved by the pretreatment on a fiber preform. Also, this new pretreatment method has no obvious negative impact on the laser performance of non-irradiated YDFs. Furthermore, the vacuum experiment confirms that the YDF obtained by this method, named pretreated YDF, may have a long-term radiation stability when it is used in a vacuum environment (such as space). CW-EPR spectra show that the formation of color centers was effectively inhibited in pretreated YDF, which is correlated with the decrease of color center precursor and the existence of the deuterium radical as confirmed by Raman and FTIR spectra.

Фотолюминесценция центров окраски германий-вакансия в полученных химическим газофазным осаждением алмазных частицах

Hot Filament Chemical Vapor Deposition technique was used to synthesize diamond particles with germanium-vacancy color centers on a germanium substrate. The formation of color centers occurred during the growth of diamond particles due to the incorporation of germanium atoms formed as a result of a crystalline germanium wafer etching with atomic hydrogen. The conditions of Chemical Vapor Deposition process which affect the photoluminescence of color centers of germanium vacancy in diamond particles, are considered. The highest photoluminescence intensity of germanium-vacancy color centers was achieved for diamond particles obtained on a substrate at a surface temperature close to the melting temperature of germanium. The photoluminescence spectra of the diamond particles also showed lines presumably associated with the optical centers which contain tungsten.

Formation of color centers at X-ray irradiation of ZnSe single crystals

In the present work we study the dose dependences of X-ray luminescence and X-ray conductivity at different temperatures and different angles of incidence of the exciting radiation and the registration of luminescence. Low-energy X-ray quanta (with energies up to 20 keV) do not create new structural defects in ZnSe crystals, but can only recharge existing ones. It was established from the dose dependences of the luminescence intensity of the 630-nm and 970-nm bands that X-rays do not create the color centers in ZnSe crystals, even those that are temperature unstable, which absorb the luminescent radiation. This fact is confirmed by the absence of conduction current change at additional optical intensive irradiation of ZnSe samples by optical quanta with wavelengths that coincide with luminescent quanta.

Gamma irradiation effect towards photoluminescence and optical properties of Makrofol DE 6-2

The optical, Raman and Photoluminescence properties of Makrofol DE 6-2 polycarbonates were explored in detail. A set of polymeric Makrofol samples were subjected to γ-ray source at various doses ranging between 1 and 200 kGy. UV–Vis spectra depicted that Makrofol DE 6-2 has a high absorption maximum band at about 230 nm and decreases sharply at about 285 nm. The optical energy band gap was calculated using different methods, Ineffective Thickness Method (ITM), Derivation of Ineffective Thickness Method (DITM), Absorption Spectrum Fitting (ASF) and Derivation of Absorption Spectrum Fitting (DASF) methods. All used methods revealed an observable decrease in the optical energy band gap values with increasing gamma irradiation doses for both the indirect and direct transitions, whereas the refractive index showed the reverse trend. Also, the photoluminescence intensity decreases with increasing gamma dose up to 200 kGy due to formation of defects. Peaks at 884, 1364, 1394 and 1854 cm−1 are registered corroborative its structure in the Raman spectrum. Its intensity showed a decrease with increasing the gamma dose which may be attributed to the formation of non-irradiative levels as a result of degradation. Color change of the polymer from transparent to faint yellowish after γ-irradiation of doses higher than 1 kGy was observed which may be due to formation of color centers appeared in the range 305–315 nm. The present results should be taken into account before utilization of Makrofol DE 6–2 in photoelectric devices and medical applications related to gamma radiation.

Colour centre generation in diamond for quantum technologies

Abstract Effective methods to generate colour centres in diamond and other wide band-gap materials are essential to the realisation of solid state quantum technologies based on such systems. Such methods have been the subject of intensive research effort in recent years. In this review, we bring together the various techniques used in the generation and positioning of colour centres in diamond: ion implantation, delta-doping, electron irradiation, laser writing and thermal annealing. We assess the roles and merits of each of these techniques in the formation of colour centres for different quantum technologies and consider future combinations of the techniques to meet the requirements of the most demanding applications.

High-dose temperature-dependent neutron irradiation effects on the optical transmission and dimensional stability of amorphous fused silica

The primary concern for implementing amorphous fused silica (a-SiO2) fiber optic sensors in a nuclear environment is the radiation-induced attenuation (RIA) of the light signal due to the formation of radiation-induced color centers. In addition, Bragg grating sensors drift under irradiation due to radiation-induced compaction of the a-SiO2 structure. This work provides new data regarding RIA and radiation-induced compaction of a-SiO2 samples irradiated to a fast neutron fluence of 2.4 × 1021 n/cm2 at temperatures of 95, 298, and 688 °C. Results show that RIA may be approaching saturation for the range of photon energies evaluated in this paper and that the hydroxyl content has a significant impact on RIA when the irradiation temperature is increased to 688 °C. A model was developed for predicting radiation-induced compaction, and the resulting signal drift for Bragg grating sensors, as a function of neutron fluence and temperature.

Down-conversion luminescence of Ce-Yb ions in YF3

The single-phase Y1-x-yCexYbyF3 solid solution powders were prepared by melt crystallization technique. The 5d-4f interconfigurational luminescence of Ce3+ ions and 2F5/2-2F7/2 luminescence of Yb3+ ions in YF3 crystalline powder doped with Ce3+/Yb3+ ions pair upon 266 nm excitation were studied. The energy transfer from Ce3+ to Yb3+ after 266 nm excitation has complex character and deals with the UV excitation induced dynamic processes and color centers formation in YF3:Ce3+ powders. It was shown that UV light irradiation leads to in absorbing in 525–650 nm spectral range which is not observed for Ce3+-Yb3+ containing samples. This speaks for additional channels of Ce3+ and Yb3+ excitation as a result of recombination of free charge carriers originated from excited state absorption by Ce3+ ions and trapped by Yb3+ ions. The efficiency of energy transfer from Ce3+ to Yb3+ was 22,6% for YF3:Ce(0.05 mol. %):Yb(10.0 mol.%) solid solution with 0,91% down-conversion luminescence external quantum yield.

Visible photoluminescence of color centers in lithium fluoride detectors for low-energy proton beam Bragg curve imaging and dose mapping

In the last few years, the visible photoluminescence (PL) of radiation-induced laser-active F2 and F3+ aggregate color centers (CCs) in lithium fluoride, LiF, has been successfully used for advanced diagnostics of low-energy proton beams. A systematic study of the optical properties of LiF crystals and thin films exposed in different irradiation geometries to a proton beam of nominal energy 7 MeV is under way. The ionization induces the stable formation of primary and aggregate CCs in the crystalline lattice, among which the F2 and F3+ defects possess broad emission bands in the red and green spectral ranges, respectively, when optically pumped at wavelengths close to 450 nm. Their PL intensity has been found to be linearly proportional to the dose absorbed by the host material over at least three orders of magnitude, so that LiF solid-state dosimeters based on spectrally-integrated PL reading can be envisaged. A simple defect formation model that takes into account the energy released in the material, crystals and films, together with the saturation of CC concentration at high doses, allows obtaining both the full Bragg curve reconstruction with its dose distribution with depth and the accumulated bi-dimensional transversal dose-mapping from the visible fluorescence images latently stored in LiF crystals and thin films, respectively. Results obtained by using a fluorescence microscope are presented and discussed with the aim of highlighting some quantitative aspects of the PL behavior of CCs both in LiF crystals and films, which are relevant for low-energy proton-beam Bragg curve imaging and dose mapping in a wide interval of doses up to the ones at which strong CC saturation occurs.

Electric-field-induced composite color centers formation and Yb ions valence state change in spark plasma sintered Yb:YAG transparent ceramics

Transparent Yb:YAG ceramics with submicron-scale grains were fabricated through the spark-plasma-sintering of co-precipitated Yb:YAG powders without the use of any additives. Yb valence state shift and composite color center formation were investigated by visible range absorption spectra、Emission spectra and Electron Paramagnetic Resonance (EPR) measurements. The underlying mechanisms by which an applied field influences the valence state transition and composite color center formation are discussed in detail. Absorption spectra measurements of Yb:YAG ceramics consolidated by SPS at 1100–1300 °C revealed absorption bands located at 380, 487, 524, 634, 646, and 654 nm. EPR characterization and photoluminescence spectrum confirmed the presence of a Yb2+ ion population with the 4f13 ground state electronic configuration by the 589 nm luminescence under 354 nm excitation, formed by the Yb3+ ion capturing an electron to form Yb2+-F+ composite color centers. It was determined that both the shift in Yb valence state and the formation of composite color centers were caused by the electric field employed to conduct the sintering process.

Supercontinuum generation in the absence and in the presence of color centers in NaCl and KBr

We study supercontinuum generation with femtosecond mid-infrared laser pulses in NaCl and KBr crystals when pumped at the vicinity of their zero group velocity dispersion points. Almost three octave-spanning SC spectra (covering the 0.7–5.4 μm and 0.85–5.4 μm ranges in 5 mm-thick NaCl and KBr samples, respectively) were produced by filamentation of 70 fs, 3.6 μm input pulses in continuously translated samples. In the static setup, we show that rapid formation of persistent color centers (within a few seconds at 500 Hz repetition rate) inside these materials results in a marked decrease (by 40%) of the overall energy transmittance and in significant narrowing of SC spectra. We also demonstrate that the effect of color centers on the SC spectrum could be adequately simulated numerically using a simple phenomenological model which considers color centers as impurities with a certain energy bandgap.

Kinetic model for color-center formation in TiO2 film using femtosecond laser irradiation

The authors investigate the formation of color centers (CC) using a femtosecond laser, which causes the darkening of TiO2 film surfaces and reduction of their electrical resistance under visible-light illumination. They propose a kinetic model to explain the CC formation from femtosecond laser irradiation. The kinetic model treats three components, which are conduction-band electrons, self-trapped excitons or polarons, and color centers. The CC formation coefficient is assumed to be expressed by the Arrhenius equation. The laser fluence dependence on the decrease of the electrical resistance is well explained by the kinetic model.

Manganese ion implanted ultrananocrystalline diamond films: Optical and electrical characterization

We report the optical and electrical properties of high-dose (1015–1017 ions/cm2) Mn-ion implanted ultrananocrystalline diamond (Mn-UNCD) films. Mn-ion implantation and post-annealing of UNCD films lead to the formation of Mn-related color centers, characterized in Mn-UNCD films by their zero phonon line emissions at 621.2 nm and phonon sidebands at 611.2 nm and 630.3 nm. Raman spectra of Mn-UNCD films indicated amorphization via high-dose Mn-ion implantation and that the annealing process results in graphitization of the films. The Mn-UNCD film implanted with the Mn-ion dose of 1017 ions/cm2 exhibits a conductivity of 122.25 (Ω cm)−1, as well as enhanced field electron emission (FEE) properties such as a turn-on field of 10.67 V/μm and a FEE current density of 0.85 mA/cm2.

Creating of luminescent defects in crystalline media by a scanning laser beam

Line-by-line two-dimensional step small-scale scanning irradiation of a transparent cubic LiF crystal was carried out by intense linearly polarized femtosecond laser radiation in the multiple filamentation mode. As a result, isolated longitudinal extended tracks consisting of induced color centers were formed in the medium. It was found that no transverse periodicity associated with the scanning step is observed in the arrangement of tracks formed by laser filaments. This is because inhomogeneities that stimulate filamentation are not contained in the laser beam itself but are formed randomly when the medium interacts with the first laser pulses and are supported and amplified by subsequent pulses. The efficiency of color center formation in crystals at normal laser beam incidence on the cube face depends periodically on the azimuth angle θ between the electric vector and the cube edge on the face, with the period of π/2. It was found that azimuthal dependences for defect formation (maximum at θ = π/4) and for carrier photogeneration (maximum at θ = 0) are in the antiphase. Calculations showed that the processes of self-focusing and filamentation controlled by the components of the third-order nonlinear susceptibility tensor are most effective at the orientation where θ = π/4. The experiment showed that at such an orientation, the critical power and the length of self-focusing decrease, and therefore, the density of the number of filaments in the beam section increases and, as a result, the average concentration of the color centers created by laser filaments increases.

Formation of GeV, SiV, and NV Color Centers in Single Crystal Diamond Needles Grown by Chemical Vapor Deposition

Herein the color centers formation in diamond crystallites of pyramidal shape grown by chemical vapor deposition (CVD) is described. The individual crystallites are extracted from polycrystalline films grown on silicon substrates. The silicon‐vacancy (SiV), germanium‐vacancy (GeV), and nitrogen‐vacancy (NV) centers are created by introducing corresponding precursors from substrate material, gaseous nitrogen, and thermally evaporated germanium added during CVD. Some amount of NV centers detected in all types of the diamond crystallites is assigned to uncontrollable presence of nitrogen in the CVD reactor. The low temperature photoluminescence spectra indicate presence of ensembles of SiV centers in diamond structure. The SiV centers are concentrated predominantly at apexes of the diamond crystallites located at substrate surface. Intense growth of CN radicals in plasma‐activated gas environment during CVD process is detected after nitrogen gas addition. The crystallites obtained with the nitrogen addition demonstrate significant variation of their pyramid angle. The photoluminescence spectra of crystallites grown with nitrogen addition demonstrate increase of both negatively and neutrally charged states of NV color center. The GeV centers are created via thermal evaporation of pure Ge during CVD growth. The photoluminescence spectra of the crystallites grown with Ge addition demonstrate presence of GeV color centers ensemble.

Радіаційна чутливість іонних кристалів. Одновимірна модель. Кристали з дефектами дипольного типу

Виникнення радіаційного забарвлення в іонних кристалах є результатом локалізації створених радіацією вільних носіїв заряду на дорадіаційних точкових дефектах кристалічної ґратки. Із накопиченням у кристалах центрів забарвлення вступають в дію зворотні процеси. Вільні носії заряду рекомбінують на центрах забарвлення, дорадіаційні дефекти відновлюються. Завдяки довготривалого опромінення кристалів встановлюється динамічна рівновага між процесами генерації центрів забарвлення та висвітлювальною дією рентгенівських променів. Запропоновано одновимірну модель іонних кристалів, у якій було розраховано параметри радіаційної чутливості кристалів флюоритів, легованих неізовалентними домішками. Імовірність генерації центрів забарвлення при розпаді електронно-діркової пари та імовірність висвітлювальної дії знижується зі зменшенням концентрації активатора. За високих температур незалежно від концентрації активатора в кристалі імовірність утворення центрів забарвлення під час розпаду електронно-діркової пари така ж, як імовірність їх руйнування. Це пояснено тим, що в обох випадках носії заряду взаємодіють із електронейтральними дипольними центрами. Зі зменшенням концентрації активатора зростає величина енергії, що витрачається на створення однієї комплементарної пари центрів забарвлення, а відповідно радіаційна чутливість кристала знижується.

From Fancy Blue to Red: Controlled Production of a Vibrant Color Spectrum of Fluorescent Diamond Particles

AbstractThe current study reports a breakthrough method for production of multicolor diamond particulates using a rapid thermal annealing (RTA) approach with precise temperature and time control, enabling annealing of diamond particulates up to 2100 °C without extensive graphitization. The RTA method generates conditions which allow formation of one‐, two‐, and three‐atom nitrogen complexes with vacancies in electron irradiated type Ib synthetic diamond, providing vibrant luminescence in the red, green, and blue spectral ranges, correspondingly. Controlled and highly reproducible formation of specific color centers previously not possible in type Ib synthetic diamond particles opens new opportunities for particulate diamond in a plethora of fluorescence imaging applications in biological and industrial fields.

Formation of Color Centers and Concentration of Defects in Boron Carbide Irradiated at Low Gamma Radiation Doses

In the present work, boron-carbide (B4C) samples (purity of 99.5% and density of 1.80 g/cm3) were irradiated by using gamma radiation from a 60Co gamma source. Gamma irradiation of the samples was carried out at doses 48.5, 97, 145.5 and 194 kGy. The samples were analysed using a UV-V Gary 50 Scan spectrophotometer. The effect of different irradiation doses on the defects created in the B4C samples was investigated. In the B4C samples, the formation processes for color centers depended on the gamma irradiation dose. The calculated activation energies at room temperature essential for the formation of F and F+ color centers ranged from 1.89 – 2.05 eV.

Effect of neutron irradiation on the structural, electrical and optical properties evolution of RPLD VO2 films

This study reports on the effect of neutron irradiation at different fluences on the properties of VO2 thin films. The irradiations were performed at NUR research reactor, Algiers at a temperature of about 40 °C, with fast neutron fluence (En > 1 MeV) up to 1.9 × 1018 n.cm−2. The induced defects have been investigated using structural, optical and electrical measurements. Both bulk sensitive characterization techniques, Raman and grazing incident angle X-ray diffraction (GIXRD) analysis, show that no structural transformation is induced by neutron irradiation, although strain induced defect production are generated throughout the films while surface sensitive techniques, X-ray photoelectron spectroscopy (XPS) and work function measurements, show that the charge carrier (electron) concentration at room temperature decreases after irradiation. Potentially due to fast neutron irradiation induced defects, mainly in the form of Frenkel pairs, swelling and color center formation occurs in VO2 thin films without amorphization. This is further corroborated by an increase of the room temperature resistivity through the irradiated films. Temperature-dependent electrical and optical transmission measurements confirm that the characteristic semiconductor-to-metal transition of the VO2 films is preserved upon irradiation. We therefore conclude that VO2 is an excellent candidate for thermal shielding and thermal management of small satellites.