Cation Excitons Research Articles

Construction of Z-scheme photocatalyst with p-n heterojunction of molecular semiconductors for enhanced photocatalytic water oxidation

Here, the donor-acceptor organic composites of n-type perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and p-type cobalt phthalocyanine (CoPc) semiconductors are firstly synthesized and optimized for photocatalytic oxygen evolution (POE) as catalyst. The POE rate of 12.30 mmol·g−1·h−1 with the apparent quantum yield of 4.00 % at 475 nm can be achieved under visible light (> 420 nm), as compared with 1.58 mmol.g−1.h−1 of PTCDA. Under the full UV-Vis light (> 320 nm), the rate is up to 17.65 mmol·g−1·h−1. The systematical experiments clearly reveal the generation of dual-channel energy transfer of fluorescence resonance from CoPc to PTCDA and electron transfer from oxygen groups in PTCDA to Co atom in CoPc under UV-Vis light irradiation. Consequently, the formation of Z-scheme p/n heterojunction in CoPc/PTCDA composite greatly enhances the POE performance due to the lifetime increase of metal-ligand charge transfer exciton in CoPc (44.83 vs. 2.95 ps) and perylene cationic exciton (20.13 vs. 2.13 ps).

Spectroscopic ellipsometry characterization of PbWO4 single crystals

Optical characterization of PbWO4 single crystals grown by Czochralski method was achieved in virtue of spectroscopic ellipsometry experiments carried out in the energy region of 1.0–5.6 eV at room temperature. Tetragonal scheelite structure with lattice parameters of a = b = 5.4619 Å and c = 12.0490 Å was determined for the bulk crystal utilizing from XRD analysis. Analyses of the ellipsometry data presented the photon energy dependencies of complex dielectric function of the crystal. The real part of the dielectric function exhibited increasing behavior with energy in the below 4.1 eV and then decreased immediately. Zero frequency refractive index and dielectric constant were determined to be 2.02 and 4.08, respectively, using Wemple and DiDomenico oscillator model. High frequency dielectric constant was calculated as 4.30 by Spitzer-Fan model. Optical band gap of PbWO4 was found to be 3.24 eV from the dielectric relaxation time spectrum. Moreover, existence of two critical points with energies of 3.70 and 4.58 eV was revealed from the analyses of extinction coefficient and second derivative of the dielectric function. These levels were considered to be due to creation of cation exciton (Pb2+ 6s2 − Pb2+ 6s6p) and transitions in the [WO4]2− group.

Relaxation of electronic excitations in K2GeF6 studied by means of time-resolved luminescence spectroscopy under VUV and pulsed electron beam excitation

Highly crystalline K2GeF6 micropowders were prepared by co-precipitation in the hydrofluoric acid-water-alcohol solution and studied by means of the cathodoluminescence and time-resolved luminescence spectroscopy under synchrotron radiation excitation at 7 K. The nature of various intrinsic emissions was revealed. The luminescence band at 510 nm detected under excitation by photons with the energy above 9 eV is related to the radiative decay of self-trapped excitons. Fast emission bands with ~400 ps decay time found in the spectral range from VUV to visible (8–2.4 eV) are assigned to the cross-luminescence transitions from K 3p hole states and intraband luminescence transitions due to the presence of the Ge 4s, 4p valence states. The creation of cation excitons was detected in the energy range 18–20 eV. Their non-radiative decay results in the formation of hole states in the Ge energy bands, which finally recombine with electrons from the above lying valence states and provide fast intraband luminescence. Important electronic band structure parameters including the energy gap width of 11 eV, cation exciton formation energy 18.1 eV and ionization energy of cation states 20.0 eV were determined from the luminescence excitation spectra.

Luminescence of divalent lanthanide doped BaBrI single crystal under synchrotron radiation excitations

Luminescence excitation spectra of BaBrI single crystals doped by divalent lanthanide ions are studied using synchrotron radiation excitations from the MAX IV 1.5 GeV storage ring. The energy of the edge and the formation of core cation exciton as well as the energy threshold of the multiplications of electronic excitations is found. It was clearly established the energy transfer from intrinsic luminescence centers to Sm2+ and Eu2+ ions.

Evolution of Anion and Cation Excitons in Alkali Halide Crystals

The manifestations of the existence of free anion excitons, the processes of their self-trapping, and the coexistence of mobile and self-trapped excitons (STEs) in wide-gap alkali halide crystals are reviewed. The radiative channel of decay of anion excitons, yielding luminescence, and a particular type of nonradiative channel with the creation of elementary Frenkel defects (FDs) are considered. We analyzed the criteria for the efficiency of this channel for defect formation, possible mechanisms for the decay of self-trapped excitons with the production of neutral and charged anion Frenkel defects, and the processes of multiplication of electronic excitations in alkali halide crystals. Particular attention is paid to the decay of cation excitons, including from the point of view of the possibility of the low-temperature creation of elementary Frenkel defects in the cation sublattice of alkali halide crystals.

Эволюция анионных и катионных экситонов в щелочно-галоидных кристаллах (О б з о р)

AbstractThe manifestations of the existence of free anion excitons, the processes of their self-trapping, and the coexistence of mobile and self-trapped excitons (STEs) in wide-gap alkali halide crystals are reviewed. The radiative channel of decay of anion excitons, yielding luminescence, and a particular type of nonradiative channel with the creation of elementary Frenkel defects (FDs) are considered. We analyzed the criteria for the efficiency of this channel for defect formation, possible mechanisms for the decay of self-trapped excitons with the production of neutral and charged anion Frenkel defects, and the processes of multiplication of electronic excitations in alkali halide crystals. Particular attention is paid to the decay of cation excitons, including from the point of view of the possibility of the low-temperature creation of elementary Frenkel defects in the cation sublattice of alkali halide crystals.

Low-temperature creation of Frenkel defects via hot electron-hole recombination in highly pure NaCl single crystals

The creation spectrum of stable F centres (being part of F-H pairs of Frenkel defects) by synchrotron radiation of 7–40 eV has been measured for highly pure NaCl single crystals at 12 K using a highly sensitive luminescent method. It is shown that the efficiency of F centre creation in a closely packed NaCl is low at the decay of anion or cation excitons (7.8–8.4 and 33.4 eV, respectively) or at the recombination of relaxed conduction electrons and valence holes. Only the recombination of nonrelaxed (hot) electrons with holes provides the energy exceeding threshold value EFD, which is sufficient for the creation of Frenkel defects at low temperature.

Optical properties of KPb_2Cl_5 and RbPb_2Cl_5 single crystals in the far ultraviolet spectral region

Electronic structure properties of undoped single crystals of KPb2Cl5 (KPC) and RbPb2Cl5 (RPC) have been studied using low-temperature (T=8°K) far ultraviolet (3.7–24 eV) synchrotron radiation spectroscopy. Dispersions of optical functions have been calculated. The electronic transitions at 4.4–5.0 eV were ascribed to the excitation of cationic excitons. The energy region of 5–9 eV was associated with the transitions from the valence band to the conduction band: 6S10→6P11 at 6.28 eV, 6S10→6P32 at 5.94 eV. The complex band at 7.87 eV was attributed to the excitation of anionic excitons. The 19–24 eV peaks were associated with the 5d→6p transitions in Pb2+ ions.

Vacuum ultraviolet and X-ray emission spectroscopy of anion and cation excitons in oxide crystals

The results of a combined experimental investigation into the radiative relaxation of anion and cation excitons during selective vacuum ultraviolet-(up edge) and soft X-ray (core) excitation of berylliumcontaining BeO, Be2SiO4 and La2Be2O5 crystals are presented. They indicate that the relaxation of anion and cation excitons in multi-component oxides occurs in the same local fragments of the crystal lattice, due to their higher predisposition (in comparison with other fragments) to short-lived distortions. It has been found that in La2Be2O5 crystals characterized by a considerable difference in the ionic radii of cations (beryllium and lanthanum), the beryllium-oxygen tetrahedra do not participate in exciton relaxation processes.

High-energy frenkel cation exciton and specific features of its self-trapping in the CdI2-PbI2 crystal system

It has been shown using atomic-force microscopy that the PbI2 impurity is embedded in the CdI2 crystal lattice in the form of nanocrystalline inclusions. The model of a high-energy cation exciton related to the 3P2 state of a free Pb2+ ion has been considered for the impurity absorption (excitation) band at 3.23 eV. The resonance narrow photoluminescence bands with the split absorption band at 3.12 and 3.20 eV have been compared with the emission of a free Frenkel exciton. It has been demonstrated that, in the temperature range 25–45 K, there arises a self-trapped exciton state, and the main role in its formation is played by the bending vibrations of the CdI2 crystal lattice. The potential barrier separating the self-trapped state from the free exciton is 23 meV. The photoluminescence band at 2.4 eV is assigned to the emission of the self-trapped high-energy cation exciton of PbI2 in the CdI2 crystal lattice.

Combined luminescence and X-ray emission study of self-trapped excitons in oxides

The experimental study of relaxation of anion and cation excitons at the selective VUV-(valence) and soft X-ray (inner-shell) excitations for binary BeO crystal and multicomponent oxide crystals Be2SiO4, Al2Be3Si6O18 and Y2SiO5 is presented. Results show that the relaxation during the time-scale of decay of short-living anion and cation excitations leads to creation of self-trapped excitons at the corresponding low-symmetry (tetrahedral) local structural units of crystalline lattice. This can be caused by increase of dynamical instability of such a local structural units (in comparison with high symmetry ones), which facilitates self-trapping processes in these systems.

Color center creation in LiF crystals irradiated with 5- and 10-MeV Au ions

The peculiarities of defect creation in LiF crystals by irradiation with 5- and 10-MeV Au ions have been studied using optical spectroscopy. The crystals were irradiated at room temperature with fluences from 1×1012 to 2×1014ions/cm2 at various beam current densities (1–200nA/cm2). The short ion range in LiF allowed measurements for higher absorbed energy densities and high defect volume concentration. The efficiency of single F center and Fn and larger aggregate creation depends drastically both on ion fluence and flux (beam current). Color center creation is less effective for 5-MeV Au ions than for 10-MeV Au ions, which produce δ electrons with energies sufficient for creation of cation excitons (∼62eV). The peculiarities of color center accumulation in LiF crystals under MeV Au ion irradiation are considered taking into account the nonlinear kinetics of interstitials (H centers).

Creation of nanosize defects in LiF crystals under 5- and10−MeVAu ion irradiation at room temperature

Investigation of radiation defects induced by the irradiation of LiF crystals with 5- or $10\text{\ensuremath{-}}\mathrm{MeV}$ Au ions (fluences of ${10}^{11}--2\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\text{ions}∕{\mathrm{cm}}^{2}$; flux varies by 2 orders of magnitude) at room temperature has been performed using the methods of optical absorption and high-temperature $(400--750\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ thermoactivation spectroscopy. The creation efficiency of color centers $(F,{F}_{2},{F}_{3},\dots{})$ and colloids drastically depends on both the fluence and ion flux (beam current). Besides impurity (magnesium) colloids with the absorption band peaked at $4.4--4.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, the broad absorption band at $2.3--3.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ related to intrinsic Li colloids is reliably distinguished. The creation efficiency of Li colloids by $5\text{\ensuremath{-}}\mathrm{MeV}$ Au ions is lower than that by $10\text{\ensuremath{-}}\mathrm{MeV}$ ions, which form $\ensuremath{\delta}$ electrons with higher energies sufficient for the creation of cation excitons $(\ensuremath{\sim}62\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. The cation exciton decays, in turn, with the formation of a group of spatially close $F$ centers. At a high ion flux, the next bombarding ions hit the same crystal region with a small time delay $(10--100\phantom{\rule{0.3em}{0ex}}\mathrm{s})$ and also form, after similar intermediate processes, the groups of $F$ centers that participate in the formation of stable agglomerates of several ${F}_{3}$ or even more complex centers, which serve as stable (up to $620\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) seeds for nanosize Li colloids. The peculiarities of the formation, enlargement, and annealing of intrinsic colloids in LiF crystals are considered, invoking a formal analog with the processes in photographic materials based on silver halides.

Defect creation caused by the decay of cation excitons and hot electron–hole recombination in wide-gap dielectrics

Insufficient radiation resistance of construction materials is the Achilles heel for thermonuclear energetics. In wide-gap dielectrics, Frenkel defects are created not only because of the knock-out (impact) mechanism but also because of the decay of the electronic excitations formed during the irradiation (i.e. due to nonimpact mechanisms). The processes of the defect creation at the irradiation of highly pure LiF single crystals at 6–8K by 1–30-keV electrons, X-rays, or synchrotron radiation of 12–70eV have been investigated. The annealing processes of these defects in a temperature range up to 200K have been studied as well. In LiF, creation has been revealed for the following: (1) F–H pairs caused by the decay of anion excitons or by the recombination of electrons and holes, (2) F′–H–VK and F–I–VK defect groups at the decay of cation excitons (62eV), or (3) 20-keV electron irradiation. The mechanism of defect creation at the recombination of hot holes and hot electrons has been discussed for α-SiO2 crystals with an energy gap between the subbands of a valence band. One of the possible ways to suppress this mechanism (“luminescent defence”) is doping a material by luminescent impurities able to capture a part of the energy of hot carriers before their relaxation and recombination (e.g. in MgO:Cr).

Electronic, optical and luminescent properties of PbFCl single crystal

The first-principles calculation of energy band structure and density of states of the PbFClcrystal based on the plane waves and pseudopotential method in the frame of thegeneralized gradient approximation (GGA) were performed. Further, calculation with theGW approximation was used to correct the bandgap. The so-calledGW approximation represents the lowest expansion of the self-energy,Σ, equal to the Greenfunction G times the screenedCoulomb interaction W. The calculated result shows that the direct bandgap is located at the Z pointin the Brillouin zone with the gap of 3.5 eV in the GGA frame and 5.0 eV afterGW correction. X-ray photoemission spectroscopy for the valence band (VB-XPS) was carriedout to compare with the calculated density of states in the valence band, suggesting thatthe calculated result is in agreement with the experimental result. The reflection,excitation and emission spectra were measured using a synchrotron radiation lightsource in the energy region of 4–9.5 eV. The absorption and emission of the cationexciton was observed at 5.0 and 3.8 eV, respectively. The components of Pb 6pstates and Pb 6s states located at the bottom of the conduction band and the topof the valence band, respectively, prove the possibility of the cation exciton ofPb2+.Violet band emission peaking at 3.1 eV was also observed, which may be due to the self-trapped excitonor the Pb2+ ion close to the Schottky-like defect.

Luminescence spectra of PbFCl single crystal under the excitation of synchrotron radiation VUV–UV

The luminescence spectra of PbFCl single crystal under the excitation of synchrotron radiation vacuum ultraviolet–ultra violet (VUV–UV) were investigated. The emission spectra of PbFCl single crystal show a broad violet band peaking at 400 nm having decreasing intensity with the rising temperature being from 20 to 293 K under the excitation of VUV–UV. A weak ultraviolet band peaking at 325 nm, which can be ascribed to the cation exciton emission of Pb 2+, appears at 20 K under the excitation at wavelength shorter than 247 nm, but is completely quenched above 60 K. The violet band emission due to energy transfer from cation exciton was observed, which became more effective above 60 K. The luminescence in the violet band may be due to STE luminescence or is probably associated with the Pb 2+ ion close to the Schottky-like defect.

CATION AND ANION ELECTRONIC EXCITATIONS IN MgO AND BaF2 CRYSTALS UNDER EXCITATION BY PHOTONS UP TO 75 eV

The creation of primary and secondary cation excitations was investigated for MgO and BaF 2 crystals using the energy-and time-resolved luminescence method. It was shown that the primary cation excitations in MgO created by photons of 50–65 eV decay with the formation of anion free excitons and electron–hole pairs. The formation of secondary cation holes by hot photoelectrons in the Ba 2+ 5p shell causes the increase of cross-luminescence intensity whereas the creation of neither primary nor secondary cation excitons causes the appearance of cross-luminescence in BaF 2.

Decay of cation excitons in sodium halide crystals

The reflection spectra as well as the excitation spectra of intrinsic emissions have been measured for NaF and NaBr crystals using photons of 6 to 36 eV at 7 to 10K. The decay processes of cation excitons with the creation of anion electronic excitations have been analyzed for a number of alkali halide crystals, where the ratio for the formation energy of cation excitons and the energy gap varies by several times.

Physics of halide scintillators

The paper reviews the results obtained within the last 5–6 years in the field of the physics of some halide crystals which have led to a better understanding of their characteristics. The luminescence and scintillation characteristics of caesium iodides, widely used in the existing detectors, as well as the properties of some other selected materials are discussed in view of their possible applications. For all the systems concerned, the emission important for their practical applications arises from the radiative decay of excitons of various types. The following features are considered in more detail: (1) the excited states structure, the mechanism of excitation of the localized exciton emission in the impurity-induced absorption bands, the processes of defects creation, and the scintillation mechanism in CsI : Tl and CsI : Na scintillators; (2) the luminescence of on-centre excitons in CsI and RbI crystals; (3) the luminescence of free cation excitons in the CsPbX 3-type aggregates thermally created in CsX : Pb single crystals (X=Cl, Br); (4) the model of the excited states responsible for the cation exciton luminescence of lead halides.

Dependence of the efficiency of various emissions on excitation density in BaF 2 crystals

The dependence of the intensity of the emission of self-trapped excitons, crossluminescence (CL) and intraband luminescence on the excitation density in BaF 2 by electron pulses (300 keV , 3 ns ) has been studied. The energy input above 2×10 19 eV cm −3 causes quenching of 5.5 eV CL due to the recombination of conduction electrons with the outermost-core holes resulting in the formation of cation excitons and the subsequent energy transfer from a cation exciton to a valence electron of a neighbouring anion. The excitation spectra for various fast (τ<5 ns) and inertial emissions have been measured using synchrotron radiation at 9 K .