VK Centres Research Articles

Investigating the electrical and optical characteristics of lead-free all-inorganic Cs3Sb2Br9 single crystals

In recent years, halide perovskites have emerged as promising materials for electronic devices. While lead-based perovskites have been extensively studied, exploration of lead-free variants has been limited. Here, we highlight the innovation of observing negative photoconductivity (NPC) induced by light, followed by self-recovery, in lead-free Cs3Sb2Br9 perovskite single crystals. Our study reveals the self-trapping of holes at the Vk center, corresponding to the Br2− dimer, within the midband states of this vacancy-ordered perovskite. This leads to the entrapment of photogenerated charge carriers by charged defect states denoted as Vk, creating an internal electrical field that counteracts the externally imposed field, resulting in NPC. We demonstrate the innovation through the construction of a prototype photodetector with notable sensitivity, featuring high responsivity (6.77 mA/W), detectivity (2.83 × 1012 Jones), and a dark-to-light current ratio of approximately 10. This recognition of retroactive photocurrent in optically active perovskite materials holds promise for advancing highly sensitive detectors.

X‐Ray Luminescence and Thermally Stimulated Processes in Cesium Iodide Crystal

X‐ray luminescence spectra, thermally stimulated luminescence, and thermally stimulated conductivity of undoped cesium iodide (CsI) crystal are investigated in order to reach better understanding of the factors which govern the emission processes in this material. X‐ray luminescence spectra are recorded in the temperature range from 15 to 293 K. The low energy band at 2.2 eV emerging at heating above 120 K is assigned to the emission of residual impurities. Other two bands peaking at 3.6 and 4.3 eV at 15 K are attributed to the intrinsic emission of CsI due to the self‐trapped excitons (STEs). The parameters of the peaks observed in the thermally stimulated luminescence and conductivity of CsI crystal are calculated. Investigations of the thermally stimulated processes in CsI lead to the conclusion that the increase of luminescence of 4.3 eV observed above 70 K is due to release of trapped electrons, which subsequently interact with Vk centers and form on‐center STEs. The considerable ionic conductivity observed above 250 K can be explained by the influence of uncontrolled impurities of divalent metal atoms as well as Na+ ions.

The color centers in halite induced by Martian dust activities

The electrostatic discharge (ESD) induced by Martian dust activities (dust storms, dust devils, and grain saltation) has been recently proposed as a powerful geological agent to modify the structure and chemistry of Martian salts. Here, we performed new experiments to simulate gas discharge in Martian dust activity in a Mars chamber to investigate the modifications of chloride exposed to a medium strength ESD process. The structural, chemical and spectral features of NaCl before and after ESD exposure were systematically characterized with multiple analysis methods. On the basis of the experimental results, we found that 1) the colorless NaCl precursor subjected to ESD exposure subsequently displayed violet/blue color, indicating the presence of color centers; 2) the color centers (VK centers and Na colloids) in NaCl cause strong and characteristic spectral features in the visible to near-infrared reflectance spectra and Raman spectra; and 3) the relative density of color centers (VK centers and Na colloids) in NaCl gradually changes with increasing ESD duration. Our results also suggest that preferential Cl was released within NaCl crystals during the ESD process, and new phases ClO3−, ClO4−, and CO32− formed, which is consistent with previous studies (Wang et al., 2020b; Wu et al., 2018). We interpreted that preferential Cl release within NaCl crystals should be associated with the decomposition of VK centers and the sputtering process. We also proposed that the chemically active Cl species released from solid NaCl crystals is an essential part of the overall reaction pathway of perchlorate formation on Mars. Chloride exposed to the radiation environment at the Martian surface may also contain color centers, shedding light on the detection of normally featureless chlorides on Mars.

Photoconductive mechanism of IR-sensitive iodized PbSe thin films via strong hole-phonon interaction and minority carrier diffusion.

Photoconductive PbSe thin films are highly important for mid-infrared imaging applications. However, the photoconductive mechanism is not well understood so far. Here we provide additional insight on the photoconductivity mechanism using transmission electron microscopy, x-ray photoelectron microscopy, and electrical characterizations. Polycrystalline PbSe thin films were deposited by a chemical bath deposition method. Potassium iodide (KI) was added during the deposition process to improve the photoresponse. Oxidation and iodization were performed to sensitize the thin films. The temperature-dependence Hall effect results show that a strong hole-phonon interaction occurs in oxidized PbSe with KI. It indicates that about half the holes are trapped by KI-induced self-trapped hole centers (Vk center), which results in increasing dark resistance. The photo Hall effect results show that the hole concentration increases significantly under light exposure in sensitized PbSe, which indicates the photogenerated electrons are compensated by trapped holes. The presence of KI in the PbSe grains was confirmed by I 3d5/2 core-level x-ray photoelectron spectra. The energy dispersive x-ray spectra obtained in the scanning transmission electron microscope show the incorporation of iodine during the iodization process on the top of PbSe grains, which can create an iodine-incorporated PbSe outer shell. The iodine-incorporated PbSe releases electrons to recombine with holes in the PbSe layer so that the resistance of sensitized PbSe is about 800 times higher than that of PbSe without the iodine-incorporated layer. In addition, oxygen found in the outer shell of PbSe can act as an electron trap. Therefore, the photoresponse of sensitized PbSe is from the difference between the high dark resistance (by KI addition and iodine incorporation) and the low resistance after IR exposure due to electron compensation (by electron traps at grain boundary and electron-hole recombination in KI hole traps).

Formation and migration of VK centers from localized excitons in NaCl:I− and NaCl:I−,Tl+ crystals

We have investigated the temperature dependence of the decay profiles of the localized exciton luminescence related to the I− center in single-doped NaCl:I− and co-doped NaCl:I−,Tl+ crystals. At the temperature region below 160 K, the localized exciton luminescence exhibits almost single-exponential decay profiles regardless of single- or co-doped crystals, reflecting the radiative and nonradiative transitions inside the exciton localized at the I− center. Above 200 K, the decay profiles change to non-single-exponential decay profiles even in the single-doped NaCl:I− crystal. The non-single-exponential decay profiles in NaCl:I− relate to the formation of the VK center. The non-single-exponential decay profiles in the co-doped crystal are faster than those in the single-doped crystal. The faster decay profiles of the localized exciton luminescence in the co-doped crystal are attributed to the energy transfer process through migration of the VK center to the Tl+ center. The decay profiles are analyzed on the basis of the rate equations considering the formation and migration of the VK center, which are discussed through thermally activated processes.

Luminescence properties of TlAlF4 crystal

A single crystal of TlAlF4 is grown by a self-seeded vertical Bridgman-method. The measured powder X-ray diffraction pattern is compared with the standard JCPDS pattern and found to be in good agreement. The crystal structure and cell parameters are calculated and refined by Rietveld refinement analysis. The optical absorption spectrum is measured and the characteristic spectral properties of Tl+ bands are studied. The temperature dependent photoluminescence (PL) excitation and emission spectra are measured in the temperature range of 10–300 K. The doublet structure of Tl+ ion A excitation bands (A1, A2) are observed and correlated with absorption bands. The excitation under both bands yields the same broad emission band in the range of 330–500 nm with the maximum peak at around 390 nm in room temperature. At low temperatures, this emission band is well resolved into two emission bands with maximum at 371 and 404 nm for separate excitations of 227 and 236 nm, respectively. The schematic configuration coordinate energy level diagram for emission of Tl+ ion is also proposed. Low temperature thermally stimulated luminescence (TSL) were carried out and analysed the trap depth (E) and frequency factor values. The mechanisms of the processes, responsible for the emission band and formation of defects pairs of the type of Tl0–Vk center have been discussed in detail manner.

Hydrogen Bonding and Broad-Band Emission in Hybrid Zinc Halide Phosphors.

In solid-state lighting (SSL) applications, hybrid zinc halide phosphors are a promising family because they meet specific criteria such as high color rendering, low cost, and nontoxicity. However, contrary to hybrid lead halide phosphors, their quantum efficiencies are low and the origin of this luminescence remains unclear. To unravel this origin and provide new insights into enhancement of this emission, four hybrid zinc halides have been investigated. These four compounds exhibit similar crystal structures but different photoluminescence properties. We show that photoemission requires the formation of Vk centers, which can be promoted by specific hydrogen bonding. We anticipate that the selection of a specific environment for the zinc halide units could lead to a promising family of low-cost and environmentally friendly phosphors for SSL.

Asynchronous Photoexcited Electronic and Structural Relaxation in Lead-Free Perovskites.

Vacancy-ordered lead-free perovskites with more-stable crystalline structures have been intensively explored as the alternatives for resolving the toxic and long-term stability issues of lead halide perovskites (LHPs). The dispersive energy bands produced by the closely packed halide octahedral sublattice in these perovskites are meanwhile anticipated to facility the mobility of charge carriers. However, these perovskites suffer from unexpectedly poor charge carrier transport. To tackle this issue, we have employed the ultrafast, elemental-specific X-ray transient absorption (XTA) spectroscopy to directly probe the photoexcited electronic and structural dynamics of a prototypical vacancy-ordered lead-free perovskite (Cs3Bi2Br9). We have discovered that the photogenerated holes quickly self-trapped at Br centers, simultaneously distorting the local lattice structure, likely forming small polarons in the configuration of Vk center (Br2- dimer). More significantly, we have found a surprisingly long-lived, structural distorted state with a lifetime of ∼59 μs, which is ∼3 orders of magnitude slower than that of the charge carrier recombination. Such long-lived structural distortion may produce a transient "background" under continuous light illumination, influencing the charge carrier transport along the lattice framework.

Formation of radiophotoluminescence centers at room temperature in Ag-doped alkali halides

Radiophotoluminescence (RPL) is an emission phenomenon resulting from the light excitation of RPL centers formed by hole and electron transfer upon irradiation with γ- or X-rays. The RPL intensity is proportional to the absorption dose, and therefore, RPL is applied in dosimetry. Many materials showing RPL have been investigated. Amongst them, Ag-doped alkali halides are the earliest known compounds with RPL, but the mechanism underlying RPL center formation has not been well understood. In this study, we investigated the RPL center formation in Ag-doped alkali halides by combining fluorescence and electron spin resonance (ESR) spectroscopy. An emission peak attributed to Ag0 and an ESR signal attributed to the holes trapped at the Cl− sites were observed in the spectrum of the irradiated Ag-doped NaCl sample. Therefore, we suggest that in Ag-doped NaCl, electrons are trapped at the Ag+ sites and form Ag0, while holes are trapped at the localization centers of Cl− and are expected to form VK centers. Conversely, an emission peak attributed to Ag0 was observed in the spectrum of the irradiated Ag-doped KCl sample. In addition, ESR signals related to Ag0 and Ag2+ were observed in the spectrum of the non-irradiated Ag-doped KCl sample. These results suggest that electrons and holes are trapped at the Ag+ sites to form Ag0 and Ag2+.

Luminescence characteristics of silver and rare earth co-doped KCl single crystals grown from melt using Czochralski technique

Single crystals of KCl:Ag, KCl:Ag,Eu and KCl:Ag,Ce were grown from melt using Czochralski technique and investigated for thermally (TSL) and optically stimulated luminescence characteristics (OSL) with main focus on dosimetric applications. The XRD of crystals showed good crystalline nature with a face centered cubic KCl structure. SEM and EDAX analysis confirmed the distribution of dopants in the host lattice. Thermo luminescence (TL) emission of KCl:Ag after irradiating with 60Co gamma rays showed two TL glow peaks around 100 °C and 150 °C. An enhanced TL intensity of 3.5 times and OSL intensity of 1.5 times were observed by co-doping KCl:Ag with Ce. The result is further confirmed by taking photoluminescence and TL emission spectra of the samples. This increased TL and OSL response for KCl:Ag,Ce crystal is attributed to the formation of Vk centers formed due to the doping of Ag in KCl and the F center formed by Ce2+ on irradiation. Dose response of KCl:Ag,Ce showed linearity up to 6 Gy indicating its use in the personal dosimetry applications.

Radiation creation of cation defects in alkali halide crystals: Review and today's concept (Review Article)

Irradiation of alkali halide crystals creates pairs of Frenkel defects both in anion and cation sublattices. However, the particular nonimpact creation mechanisms (related to the decay of different electronic excitations) of cation Frenkel pairs are still unclear. At helium temperatures, there is yet no direct evidences of the creation of stable (long-lived) elemental cation defects. On the other hand, a number of complex structural defects containing cation vacancies and/or interstitials, were detected after irradiation of alkali halides at higher temperatures. Besides already proved mechanism related to the association of H and VK centers into trihalide molecules, the following possibilities of cation interstitial-vacancy pair creation are analyzed as well: (i) a direct decay of cation or anion excitons, (ii) the transformation of anion Frenkel pairs, formed at the decay of anion excitons or e-h recombination, into cation ones.

A three-state model for describing the temperature variation of the scintillation properties of Cs2HfCl6

The temperature dependence of the scintillation intensity of Cs2HfCl6 was measured and found to increase monotonically from 10 to 300 K, whereas no significant dependence on temperature was observed for the scintillation decay. A simple three-state model in which two excited levels are slightly different in their energies and have similar radiative rates was able to describe the experimental trends. The model is consistent with the experimental observation of two kinds of Vk centers and with theoretical predictions of two types of self-trapped excitons reported in the literature.

Analysis of electron and hole trap states in novel storage phosphors: Undoped, Eu-doped, and Ce-doped CsCaCl3 ceramics

The electron and hole trap states of undoped, Eu-doped, and Ce-doped CsCaCl3 were analyzed using the absorption spectra and the TSL glow curves after the optical bleaching. The trap depths were estimated using the initial rise method. In CsCaCl3, F centers were the electron trap centers, and Vk centers stabilized by defects or impurities (referred to as Vk′ centers) were one type of the hole trap centers. The recombination of F–Vk′ centers occurred around 400–420 K. In addition to Vk′ centers, V2 centers, V3 centers, and V3 centers perturbed by dopant ions were also the hole trap centers. In Eu-doped CsCaCl3 ceramics, Eu-related hole centers were found, and their trap depths were estimated to be 1.55 and 1.63 eV. For the Ce-doped CsCaCl3 ceramics, Ce4+ ions were observed as hole trap centers. Only Ce-doped CsCaCl3 had charge-compensating defects, which contributed toward TSL glow curves. In all the samples, retrapping electrons or holes was observed and Ce-doped CsCaCl3 was considered to have the highest retrapping efficiency. In addition, the trapped electrons or holes in Eu-doped CsCaCl3 ceramics are more stabilized than those in undoped and Ce-doped CsCaCl3 ceramics.

Luminescence and Charge Trapping in Cs2HfCl6 Single Crystals: Optical and Magnetic Resonance Spectroscopy Study

Single crystals of the undoped Cs2HfCl6 were grown using the vertical Bridgman method. Photoluminescence spectra and decays were measured in the extended temperature range of 8–500 K and approximated by a phenomenological model. Self-trapped exciton and defect-related emissions were clearly distinguished with the onset of thermal quenching above the room temperature. Electron paramagnetic resonance experiment revealed the formation of two configurations of Vk center (Cl2– molecular ion) at low temperatures. One of the Vk centers is created by a hole self-trapping at two neighboring Cl ions. In the temperature range 70–80 K, this state is thermally disintegrated and reappears in another configuration formed most probably by an interstitial Cl ion (an analog of the H-type center in alkali halides).

Analysis of self-trapped hole mobility in alkali halides and metal halides

The small radius hole polarons (self-trapped holes (STH) known also as the Vk centers) are very common color centers observed in numerous alkali halides and alkaline-earth halides. Their mobility controls the rate of secondary reactions between electron and hole defects and thus radiation stability/sensitivity of materials. We have analysed here the correlation between the temperatures at which hole polarons start migration in a series of alkali halides (fluorites, chlorides, bromides, iodides) and the lattice displacement around quasi-molecule. These results are especially important for identification of the self-trapped holes, for example, in novel scintillating materials such as SrI2, as well as in a large family of perovskite halides and more complex halide materials.

Carrier Self-trapping and Luminescence in Intrinsically Activated Scintillator: Cesium Hafnium Chloride (Cs2HfCl6)

There has been renewed interest in the scintillation properties of Cs2HfCl6 (CHC) primarily due to its favorable characteristics such as nonhygroscopicity, cubic structure, and fairly high light yield without any intentional doping. In this work, we report on its electronic and optical properties using first-principles calculations. As a large gap insulator (band gap ∼ 6 eV), CHC favors the formation of localized charge carriers, viz., Vk centers and electron polarons. The [HfCl6]-octahedra play a central role in trapping both types of localized carriers that leads to several low energy excitonic structures or self-trapped excitons (STEs). The observed emission spectrum of CHC is compared with our modeled STE structures and their emission energies. We find that Zr present as an unintentional impurity has high solubility in CHC and may be responsible for a secondary emission peak observed around 480 nm. Finally, we mention the electronic structure of the bromide and iodide analogues of CHC as well as the m...

Stability and Polaronic Motion of Self-Trapped Holes in Silver Halides: Insight through DFT+U Calculations

Polarons and their associated transport properties are a field of great current interest both in chemistry and physics. To further our understanding of these quasi-particles, we have carried out first-principles calculations of self-trapped holes (STHs) in the model compounds AgCl and AgBr, for which extensive experimental information exists. Our calculations confirm that the STH solely stabilizes in AgCl but with a binding energy of only 165 meV, an order of magnitude smaller than that found for the Vk center in KCl. Key contributions to this stabilization energy come from the local relaxation along breathing (a1g) and Jahn–Teller (eg) modes in the AgCl64– unit. To study the transfer of the STH among silver sites, we (i) use first-principles calculations to obtain the hopping barrier of the STH to first and second neighbors, involving eight distinct paths, using first-principles and (ii) construct a simple model, based on Slater–Koster parameters, that highlights the similarity of polaron transfer with m...

The role of oxygen in the photostimulated luminescence process of europium doped potassium chloride

A recent suggestion that europium doped potassium chloride (KCl:Eu2+) has the potential to significantly advance the state-of-the-art in radiation therapy dosimetry has generated a renewed interest in a classic storage phosphor material. The purposes of this work are to investigate the role of oxygen in the photostimulated luminescence (PSL) process and to determine if both increased PSL yield and improved temporal stability could be realized in KCl:Eu2+ by incorporating oxygen in the material fabrication process. Regardless of synthesis atmosphere, air or pure nitrogen, PSL amplitude shows a maximum at 1.0 mol % Eu. Depending on europium concentration, dosimeters fabricated in air exhibit stronger PSL by a factor of 2–4 compared to those made in N2. There is no change in PSL stimulation spectrum while noticeable shifts in both photoluminescence and PSL emission spectra are observed for air vs. nitrogen. Almost all charge-storage centers are spatially correlated, suggesting oxygen's stabilization role in the PSL process. However, oxygen alone does not improve material's temporal stability in the first few hours post irradiation at room temperature, probably because a significant portion of radiation-induced holes are stored in the Vk centers which are mobile.

Theoretical studies of the dependence of EPR g-factors on local structure for the trigonal Er3+–VK centres in KMgF3 and KZnF3

The dependence of the EPR g-factors on the local structural parameter for a 4f11 configuration ion Er3+ in a trigonal crystal-field has been studied by diagonalizing the 364×364 complete energy matrices. Our studies indicate that the EPR spectra of the trigonal Er3+–VK centers in KMgF3 and KZnF3 may be attributed to the translation of the cubic Kramers doublet Γ7. Furthermore, the EPR g-factors of the trigonal Er3+–VK centers may be interpreted reasonably by the shifts ΔZ≈0.340Å and ΔZ≈0.303Å of the Er3+ ions toward the charge compensator VK along the C3 axis for the KMgF3:Er3+ and the KZnF3:Er3+ systems respectively.

Energy transfer mechanism in CsI:Eu crystal

This paper studies the scintillation process in CsI:Eu crystal exposed to the pulse electron irradiation (E=0.25MeV, t1/2=15ns and W=0.003J/cm2). It has been proved that the energy transfer from the lattice to Eu2+ ions in CsI:Eu occurs through the re-absorption of STE emission. The proposed model rests on the following experimental facts: (1) the activator emission at 2.68eV rises gradually after the decay of the excitation pulse even at temperature lower than 90K when Vk centers are immobile; (2) the rise time of 2.68eV emission and the decay time of STE emission have the same temperature dependences at T=78–300K; (3) the excitation spectrum of 2.68eV emission overlaps the emission spectrum of STE.