High Energy Luminescence Research Articles

Confinement of methylammonium lead bromide nanocrystals in metal–organic frameworks as a stable scintillator for high-performance X-ray imaging

The development of perovskite nanocrystals (NCs) based composites with enhanced stability and exceptional photoelectric properties is of great significance for their application in scintillators for X-ray imaging, but it remains challenging. Herein, we demonstrate a novel and stable scintillator, achieved through a host–guest strategy involving the confinement of perovskite NCs, e.g., MAPbBr3 NCs, in metal–organic frameworks (MOFs). The host MOF, bio-MOF-100 (denoted as bMOF), facilitates the in-situ space-confined formation of MAPbBr3 NCs while providing quantum confinement, surface passivation, and protection to these guests. This host–guest strategy results in high exciton binding energy, bright and efficient luminescence, and significantly enhancing their stability against various factors such as air, heat, UV, solvents and X-ray irradiation. Consequently, this advancement prompts the transition of their application scenarios from the colloidal state to the solid state. The bMOF⊃MAPbBr3 scintillator exhibits a linear response to X-ray with a detection limit determined to be 170 nGyair s−1. Moreover, the bMOF⊃MAPbBr3–PMMA scintillator screen demonstrates high-quality X-ray imaging with an impressive spatial resolution of 14.7 lp mm−1. This work provides a new avenue for the development of perovskite NCs-based host–guest composites and paves the way for the broader implementation in solid-state optoelectronic applications.

Raman scattering study of the rare-earth binary ferroborate Nd0.75Dy0.25Fe3(BO3)4 single crystal

We report comprehensive Raman scattering measurements on a single crystal of binary ferroborate Nd0.75Dy0.25Fe3(BO3)4 in the temperature range of 7–295 K with 532 nm (18797 cm−1) laser excitation. The performed analysis of the polarized Raman spectra revealed the bands assigned to phonon, magnetic, and electronic excitations. The temperature evolution of these quasiparticle excitations has allowed us to ascertain the intricate coupling and interplay between lattice, magnetic, and electronic degrees of freedom. Analysis of the measured Raman spectra made it possible to identify all A1 and E phonon modes predicted by the group-theoretical analysis. The splitting energies between the LO and TO components of the polar E phonons were determined. Below the magnetic ordering temperature of the Fe sublattice, TN, we have revealed a multiple peaked two-magnon excitation. Analyzing the temperature evolution of low-frequency modes in the spectra, we also identified modes that are associated with electronic transitions between the crystal field levels of the Nd3+ ions with ground-state 4I9/2 and of the Dy3+ ions with ground-state 6H15/2 multiplets. In addition to the already known temperatures of magnetic transitions, analysis of the temperature behavior of low-frequency phonon and electronic excitations made it possible to establish a temperature T* = 100 K, presumably associated with local distortions of the crystal lattice. The presence of this temperature is confirmed by our ultrasonic study. A group of intense bands observed in the frequency range 1700–2200 cm−1 has been associated with the mixed low-lying electronic Raman transitions 4I9/2→4I11/2 and the high-energy luminescence ones 4G5/2+2G7/2→4I9/2 for the Nd3+ ion.

Dual phase two-color emission observed in van der Waals GaTe planes

After the eagerly study on two-dimensional (2D) electronic devices made by layered semiconductors, light-emitting devices based on 2D semiconductors have also flourished in recent years due to their special characters of flexibility, flatness surface, and thickness-tunable band gap modulation, etc. Among the 2D materials, III-VI layer compounds with hexagonal structure like GaSe and InSe are essentially direct semiconductors that own high band-edge luminescence efficiency and wide thickness tunable range from bulk to thin few-layer scale. In this work, dual-phase two-color emissions of multilayered gallium monotelluride positioned at 1.588 and 1.652 eV are simultaneously detected by micro-photoluminescence (μPL) measurement at 300 K. The lower-energy peak may originate from the hexagonal GaTe (H-GaTe) phase while the higher-energy luminescence might come from monoclinic GaTe (M-GaTe) phase and which are verified by micro-thermoreflectance (μTR) and high-resolution transmission electron microscopy (HRTEM). Micro-time-resolved photoluminescence (TRPL) and area fluorescence lifetime mapping (AFLM) of the multilayer GaTe indicated that the luminescence decay time constant of the H-GaTe is larger than that of the M-GaTe for verification of the dominant phase in multilayered GaTe is the monoclinic phase. In-plane structural and optical anisotropy of the multilayered GaTe were characterized by using polarized micro-Raman and polarized μPL measurements. The results clearly indicate that the monoclinic-phase GaTe possesses higher polarized extinction ratio with respect to that of the hexagonal GaTe for fully extinction of the Raman vibration mode and band-edge emission in the dual-phase gallium telluride. The existence of dual crystalline phases in multilayer GaTe may render it possible for fabrication of two-color emission device available for near-infrared (NIR) optical communication use.

The Effect of Shallow Impurities on the Light Absorption by the Nanocrystals CdS

The energy of quantum transitions of exciton-impurity states in a heterosystem with nanocrystals is calculated. The absorption spectra associated with the indicated transitions are analyzed. It is qualitatively shown that broad luminescence bands in the red region are related to the impurity acceptor states, and the high narrow ones in the violet region to pure excitonic states and excitonic states interacting with the ion of a donor. The satisfactory quantitative agreement of our calculations with experimental data for high energy luminescence bands is obtained.

Valence band inversion and spin-orbit effects in the electronic structure of monolayer GaSe

Two-dimensional monochalcogenides (MX) have been identified as a unique and promising class of layered materials in recent years. The valence band of single-layer MX, as predicted by theory, is inverted into a bow-shaped (often referred to as an inverted sombrero) and relatively flat dispersion, which is expected to give rise to strongly correlated effects. The inversion leads to an indirect band gap, which is consistent with photoluminescence (PL) experiments, but PL provides no direct evidence of the band inversion in the valence band. Here we demonstrate for a hexagonal MX crystal, gallium selenide (GaSe), using a combination of angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), that the valence band of monolayer (ML) GaSe exhibits a robust inversion of the valence dispersion at the \ensuremath{\Gamma} point forming a bow-shaped dispersion with a depth of 120 \ifmmode\pm\else\textpm\fi{} 10 meV between the double valence band maximum along the \ensuremath{\Gamma}$K$ direction. We also demonstrate that the deeper-lying bands detected in the ARPES spectrum are consistent with DFT calculations only if spin-orbit coupling is considered. The presented ARPES evidence that spin-orbit coupling leads to the splitting of two fourfold-degenerate states into four Kramers doublets is of significance for PL measurements, as the change in energy of the second highest valence state at the \ensuremath{\Gamma} point has a measurable effect on the PL energies in high-energy luminescence. We predict the optical absorption coefficients for the principal transitions in ML GaSe using a four-band $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model parametrized from first-principles theory with spin-orbit effects considered.

Vapor-Transport Synthesis and Annealing Study of Zn x Mg1-x O Nanowire Arrays for Selective, Solar-Blind UV-C Detection.

This work uniquely reports the synthesis of ZnxMg1–xO nanowires and submicron columns by utilizing a traditional carbothermal reduction process toward forming ZnO nanowire ultraviolet detectors, while simultaneously utilizing Mg3N2 as the source of Mg. To investigate the relationship between Mg content in the ZnO lattice and the cutoff wavelength for high spectral responsivity, the nanowires were annealed in a series of designed conditions, whereas chemical, nanostructural, and optoelectronic characteristics were compared before and after treatment. Postanneal scanning electron micrographs revealed a reduction of the average ensemble nanowire dimensions, which was correlated to the modification of ZnO lattice parameters stemming from Zn2+ dissociation and Mg2+ substitution (confirmed via Raman spectroscopy). The analysis of cathodoluminescence spectra revealed a blueshift of the peak alloy band-edge emission along with a redshift of the ZnO band-edge emission; and both were found to be strong functions of the annealing temperature. The conversion of Zn2SiO4 to Mg2SiO4 (in O2) and MgSiO3 (in Ar) was found to correspond to transformations (shifting and scaling) of high-energy luminescence peaks and was confirmed with X-ray diffraction analysis. The tunability of the cutoff photodetection wavelength was evaluated as the nanowire arrays exhibited selective absorption by retaining elevated conduction under high-energy UV-C irradiation after thermal treatment but exhibiting suppressed conductivity and a single order of magnitude reduction in both spectral responsivity (Rλ) and photoconductive gain (G) under UV-A illumination. Noise analysis revealed that the variation of detectivity (D*) depended on the regime of ultraviolet irradiation, and that these variations are related to thermal noise resulting from oxygen-related defects on both nanowire and substrate surfaces. These results suggest a minor design tradeoff between the noise characteristics of thermally treated ZnMgO nanowire array UV detectors and the tunability of their spectral sensitivity.

Polarization-induced confinement of continuous hole-states in highly pumped, industrial-grade, green InGaN quantum wells

We investigate industrial-grade InGaN/GaN quantum wells (QWs) emitting in the green spectral region under high, resonant pumping conditions. Consequently, an ubiquitous high energy luminescence is observed that we assign to a polarization field Confined Hole Continuum (CHC). Our finding is supported by a unique combination of experimental techniques, including transmission electron microscopy, (time-resolved) photoluminescence under various excitation conditions, and electroluminescence, which confirm an extended out-of-plane localization of the CHC-states. The larger width of this localization volume surpasses the QW thickness, yielding enhanced non-radiative losses due to point defects and interfaces, whereas the energetic proximity to the bulk valence band states promotes carrier leakage.

Impact ionization in N-polar AlGaN/GaN high electron mobility transistors

The existence of impact ionization as one of the open questions for GaN device reliability was studied in N-polar AlGaN/GaN high electron mobility transistors. Electroluminescence (EL) imaging and spectroscopy from underneath the device gate contact revealed the presence of hot electrons in excess of the GaN bandgap energy even at moderate on-state bias conditions, enabling impact ionization with hole currents up to several hundreds of pA/mm. The detection of high energy luminescence from hot electrons demonstrates that EL analysis is a highly sensitive tool to study degradation mechanisms in GaN devices.

Photoluminescence spectroscopy and energy-level analysis of metal-organic-deposited Ga2O3:Cr3+ films

The aims of this study are (i) to demonstrate the synthesis of Cr3+-activated β-Ga2O3 films by metal-organic deposition and (ii) to report the temperature-dependent photoluminescence (PL) properties of such films from 20 to 300 K. An activation energy of ∼0.9 eV for the Cr3+ ions in β-Ga2O3 is determined from a plot of PL intensity vs calcination temperature. The red-line emission doublet R1 and R2 at ∼1.8 eV and the broad emission band with a peak at ∼1.7 eV are ascribed to the Cr3+ ions in the β-Ga2O3 host. The energies of the excited states, i.e., 2E, 4T2, 2T2, 4T1, and 4T1, in Cr3+ are determined from the experimental PL and PL excitation spectra using a newly developed analysis model. The high-energy luminescence tail of the broad 4T2 → 4A2 emission band can be explained by the hot-carrier effect of the photoexcited electrons in the 4T2 state. The relative intensities of the R-line emission doublet can also be explained very well by the population and depopulation of the electron numbers in the E¯ (R1) and 2A¯ (R2) states. PL properties, such as the temperature-dependent PL intensity, peak energy, and spectral width, are analyzed in detail.

Re-charging the luminescence states in CdSe/ZnS quantum dots at the conjugation to Osteopontin antibodies

The paper presents the original study of photoluminescence (PL) and Raman scattering spectra of core–shell CdSe/ZnS quantum dots (QDs) covered by the amine-derivatized polyethylene glycol (PEG) with luminescence interface states. First commercially available CdSe/ZnS QDs with emission at 640nm (1.94eV) covered by PEG polymer have been studied in nonconjugated states. PL spectra of nonconjugated QDs are characterized by a superposition of PL bands related to exciton emission in a CdSe core and to the hot electron–hole recombination via high energy luminescence states. The study of high energy PL bands in QDs at different temperatures has shown that these PL bands are related to luminescence interface states at the CdSe/ZnS or ZnS/polymer interface. Then CdSe/ZnS QDs have been conjugated with biomolecules—the Osteopontin antibodies. It is revealed that the PL spectrum of bioconjugated QDs changed essentially with decreasing hot electron–hole recombination flow via luminescence interface states. It is shown that the QD bioconjugation process to Osteopontin antibodies is complex and includes the covalent and electrostatic interactions between them. The variation of PL spectra due to the bioconjugation is explained on the basis of electrostatic interaction between the QDs and biomolecule dipoles that stimulates re-charging QD interface states. The study of Raman scattering of bioconjugated CdSe/ZnS QDs has confirmed that the antibody molecules have the electric dipoles. It is shown that CdSe/ZnS QDs with luminescence interface states are promising for the study of bioconjugation effects with specific antibodies and can be a powerful technique in biology and medicine.

Water-soluble amino derivatives of free-base dppz – syntheses and DNA binding studies

The syntheses of two water soluble dipyrido-[3,2-a:2',3'-c]-phenazine analogues containing one or two appended amino/amide chains are reported. Steady state optical studies on the two new compounds reveal high-energy dppz-based luminescence in water and non-aqueous solvents. Optical titrations with duplex DNA show that the luminescence is quenched on the addition of DNA. Binding curves constructed from absorption and emission changes indicate that, while one of the compounds display negligible binding properties, the other binds DNA with relatively high affinity (>10(5) M(-1)). Isothermal calorimetry experiments, designed to investigate the higher binding compound in more detail, reveals that its interaction with CT-DNA is actually biphasic with one tight (>10(5) M(-1)) and one weaker binding site (~10(5) M(-1)). In both cases binding is entropically driven. Further calorimetry studies involving the interaction of the new compound with a variety of polynucleotides were carried out. To aid comparisons, similar experiments involving a previously reported bipyridyldiylium derivative of dppz were also carried out. These studies reveal that the bipyridyldiylium derivative binds all these sequences monophasically with relatively low affinities (~10(4) M(-1)). However, while the amino/amide chain appended derivative binds to Poly(dA).poly(dT) monophasically with relatively low affinities, it binds all the other polynucleotide studied biphasically, with affinities ranging from <10(6) M(-1) to >10(8) M(-1). The ITC data reveals that for both compounds thermodynamic signatures for binding are dependent on the sequence being bound. In both cases, the data for Poly(dA).poly(dT) is particularly anomalous. An analysis of the data shows that binding is selective, with affinities at flexible sequences being several orders of magnitude higher than those at more rigid sequences.

Photophysical Properties of Chlorotriethylphosphinegold(I)

Spectroscopic and quantum mechanical studies of the Et3PAuCl complex were performed to characterize the effect of aurophilicity on the optical properties. When excited with UV light at low temperature, the crystalline complex produced a deep luminescence in both the blue (high-energy) and red (low-energy) regions of the spectrum. The intensity of the low-energy luminescence was markedly reduced in the powdered state and quenched in the solution state. Time-dependent density functional theory (TD-DFT) calculations on electronic structures of both the ground and excited states of aggregates <TEX>$[Et_3PAuCl]_n$</TEX> (n = 1 - 3) indicated that the low-energy luminescence was attributable to Au-centered transitions, which are significantly affected by aurophilic interactions. By contrast, the high-energy luminescence appeared to be independent of the state of the complex and was strongly associated with the charge transfer from Cl to Au.

Impurity Luminescence in ${\rm BaF} _{2}$:${\rm Y} ^{3+}$ and ${\rm BaF} _{2}$:${\rm Yb}^{3+}$ Crystals

This paper presents both investigations of high-energy luminescence with maximum at 7.5 eV in BaF2:Y3+ and BaF2:Yb3+ crystals and theoretical calculations of cross-luminescence spectrum in crystals of barium fluoride with interstitial ions of fluorine. The dependencies of the 7.5 eV emission intensity as function of different impurities mol. concentration are presented. The intensity of this emission increases with concentration of rare-earth impurity, but in the case of doping by Yb3+ and Y3+ this intensity is less than that in the case of La3+ doping. This feature (as well as the nonlinear dependence) can be due to the creation of clusters, i.e., aggregation of impurity ions and F--interstitials. The intensity of the 7.5 eV luminescence in a BaF2:Y3+ crystal after annealing at 1000 K increased by three times but it remained less than in the case of La3+ doping. Theoretical calculations of the core hole electronic and spatial structure and cross-luminescence spectrum were performed in the [Ba6F12] cluster. This cluster contains interstitial ion of fluorine, which is surrounded by six ions of Ba2+. In the case of using Hartree-Fock method the calculated luminescence spectrum of BaF2 crystal with interstitial fluorine shows cross-luminescence band with main maximum at 5.2 eV and high-energy band at 11.7 eV. In the case of using TD DFT method the energies of both cross-luminescence transitions and of transition between interstitial ion and core levels are in satisfactory agreement with experimental data, being all overestimated by approximately the average value of 0.8 eV.

Structural and Photophysical Properties of Adducts of [Ru(bipy)(CN)4]2- with Different Metal Cations: Metallochromism and Its Use in Switching Photoinduced Energy Transfer

We show in this paper how the 3MLCT luminescence of [Ru(bipy)(CN)4]2-, which is known to be highly solvent-dependent, may be varied over a much wider range than can be achieved by solvent effects, by interaction of the externally directed cyanide ligands with additional metal cations both in the solid state and in solution. A series of crystallographic studies of [Ru(bipy)(CN)4]2- salts with different metal cations Mn+ (Li+, Na+, K+, mixed Li+/K+, Cs+, and Ba2+) shows how the cyanide/Mn+ interaction varies from the conventional "end-on" with the more Lewis-acidic cations (Li+, Ba2+) to the more unusual "side-on" interaction with the softer metal cations (K+, Cs+). The solid-state luminescence intensity and lifetime of these salts is highly dependent on the nature of the cation, with Cs+ affording the weakest luminescence and Ba2+ the strongest. A series of titrations of the more soluble derivative [Ru(tBu2bipy)(CN)4]2- in MeCN with a range of metal salts showed how the cyanide/Mn+ association results in a substantial blue-shift of the 1MLCT absorptions, and 3MLCT energies, intensities, and lifetimes, with the complex varying from essentially non-luminescent in the absence of metal cation to showing strong (phi = 0.07), long-lived (1.4 micros), and high-energy (583 nm) luminescence in the presence of Ba2+. This modulation of the 3MLCT energy, over a range of about 6000 cm-1 depending on the added cation, could be used to reverse the direction of photoinduced energy transfer in a dyad containing covalently linked [Ru(bipy)3]2+ and [Ru(bipy)(CN)4]2- termini. In the absence of a metal cation, the [Ru(bipy)(CN)4]2- terminus has the lower 3MLCT energy and thereby quenches the [Ru(bipy)3]2+-based luminescence; in the presence of Ba2+ ions, the 3MLCT energy of the [Ru(bipy)(CN)4]2- terminus is raised above that of the [Ru(bipy)3]2+ terminus, resulting in energy transfer to and sensitized emission from the latter.

High pressure study of ZnSe:Cr2+ crystals: the origin of the 1.25 eV luminescence

High pressure studies of Cr2+ luminescence in ZnSe crystals were performed at low temperatures. Inaddition to the mid-infrared luminescence transitions from the first excited5E level, higher energy luminescence was also observed. These two luminescencebands exhibit similar pressure coefficients but of opposite signs, which identifythe origin of the higher energy band as the transitions from the second excited3T1 level. At higher pressures, increased covalence effects are observed, which result inthe appearance of an additional peak in the luminescence due to changes in the spin–orbit coupling strength.

Cathodoluminescence characterization of hydrothermal ZnO crystals

Hydrothermal ZnO crystals were grown using 3N NaOH, 1N KOH and 0.5N Li 2CO 3 alkali solvents. The crystals have been studied by means of cathodoluminescence (CL), revealing high crystalline quality. The different growth sectors are identified by CL imaging. The incorporation of impurities and non-radiative recombination centers depends on the growth sector. The surface introduces band tailing producing self-absorption of the high energy luminescence photons. The broad visible luminescence band is composed of at least three different bands, green, orange and red, which are non-equally distributed, depending on the growth sector. Also a strong dependence of the visible luminescence on the penetration depth of the excitation — the e beam — is observed, which suggests that the concentration of the deep levels responsible for the visible luminescence is at the sample surface to a depth of 2 μm.

Influence of dual incorporation of In and N on the luminescence of GaInNAs/GaAs single quantum wells

The optical properties of above- and below-band-edge transitions have been investigated by incorporating In atoms into GaNAs/GaAs single quantum wells. The experimental results show that with increasing In concentration the interband luminescence is improved and the luminescence intensity below the band edge in GaInNAs/GaAs decreases significantly. An interpretation is given that N atoms are preferable to form a covalent bond with In than with Ga atoms in a GaInNAs alloy, due to the compensation of the atomic-size difference between In and N atoms on the GaAs substrate. The photoreflectance spectra of the GaInNAs/GaAs single quantum well support the assignment of an intrinsic mechanism to the high-energy luminescence peak.

Nanoscale Hexagonal Gallium Nitride from Single Molecule Precursors: Microstructure and Crystallite Size Dependent Photoluminescence

High quality nanoscale, phase-pure hexagonal gallium nitride crystallites have been synthesized by using the thermally induced explosion of the molecular precursor (Et 3 N)Ga(N 3 ) 3 . The method allows the control of the particle size from 2 to 24 nm. X-ray diffraction and Rietveld simulations revealed a platelet-like shape of the larger particles in contrast to the rather spherical smaller crystallites. An increasing blue shift of the high energy luminescence with decreasing crystallite size was found (up to 4.4 cV at 5 K).

In-situ photoluminescence studies of porous silicon in liquids

Photoluminescence of porous silicon in HF-based liquids has been revisited. When the porous layer is kept wet after fabrication, the luminescence is initially red and weak. Upon exposure to the photoluminescence-excitation light, a brighter luminescence gradually builds up and shifts to higher energies. If the sample is removed from the liquid, the blue shift is decreased, but the intensity of the luminescence then appears much larger than previously. In the liquid, the evolution rate is faster and the spectral shift is slightly larger for more energetic excitation light. Under intense light, one observes a saturation of the ageing rate and of the photoluminescence intensity, which can be ascribed to Auger recombination. The evolution of the luminescence under illumination is explained by photoelectrochemical etching of porous silicon by HF. The etching creates small-size structures responsible for the high-energy luminescence. These structures are destroyed upon drying, due to capillary forces. A quantitative interpretation of the evolution rate is attempted on the basis of optical absorption of porous silicon. The screening capability of the liquid is suggested as a possible origin for quenching of the luminescence in the electrolyte. © 1997 Elsevier Science S.A.

Nanocrystalline Gallium Nitride from Gallium Azide Precursors

ABSTRACTHigh quality nanoscale, phase-pure hexagonal gallium nitride (GaN) crystallites have been synthesized using the thermally induced explosion of the molecular precursor (Et3N)Ga(N3)3. The method allows the control of the particle size from 2 to 24 nm. X-ray diffraction and Rietveld simulations revealed a platelet-like shape of the larger particles in contrast to the rather spherical smaller crystallites. An increasing blue-shift of the high energy luminescence with decreasing crystal size was found (up to 4.4 eV at 5 K). A preliminary analysis of the surface chemistry of has been carried out based on silylation and deuteration experiments and IR spectroscopy, indicating hydrophilic, O-H and N-H terminated particles.