Photoluminescence Measurements Research Articles

EELS-Monitored AC-STEM-Fabrication of Sub-Nanometer Pores and Features in Hexagonal Boron Nitride and Correlated Photoluminescence and Ionic Transport Measurements

EELS-Monitored AC-STEM-Fabrication of Sub-Nanometer Pores and Features in Hexagonal Boron Nitride and Correlated Photoluminescence and Ionic Transport Measurements

Enhancing GaN Nanowires Performance Through Partial Coverage with Oxide Shells.

Core-shell gallium nitride (GaN)-based nanowires offer noteworthy opportunities for innovation in high-frequency opto- and microelectronics. This work delves deeply into the physical properties of crystalline GaN nanowires with aluminum and hafnium oxide shells. Particular attention is paid to partial coverage of nanowires, resulting with exceptional properties. First, the crystal lattice relaxation is observed by X-ray diffraction, photoluminescence, and Raman spectroscopy measurements. A high potential of partial coverage for optoelectronic applications is revealed with photo- and cathodoluminescence spectra along with an exploration of their temperature dependency. Next, the study focuses on understanding the mechanisms behind the observed enhancement of the luminescence efficiency. It is confirmed that nanowires are effectively protected against photoadsorption using partial coatings. This research advances the frontiers of nanotechnology, investigating the benefits of partial coverage, and shedding light on its complex interaction with cores.

Raman spectroscopy and photoluminescence study of PN junction p-graphene/n-GaAs.

Single layer graphene (SLG) was synthesized via high-quality chemical vapor deposition (CVD) on high-quality copper and subsequently transferred onto SiO2 and on n-GaAs substrates with varying doping electron concentrations (n = 1016, 1017, 5 × 1017, 1018, and 5 × 1018cm-3). The n-GaAs substrates were grown by molecular beam epitaxy. The optical properties of the SLG were investigated through photoluminescence (PL) and Raman spectroscopy measurements. Carrier concentration n or p and Fermi energy (EF) values in SLG were determined both before and after the transfer onto n-GaAs, and these findings were validated through PL studies. The Raman spectroscopy results indicated an increase in the transfer of electrons from n-GaAs to SLG as the doping electron density in n-GaAs increased. PL analysis revealed a significant change in the bandgap energy (Eg) of n-GaAs due to bandgap narrowing and the Burstein-Moss shift. Our data enable us to determine the energy band diagrams. Upon aligning the energy bands, an increase in transferred carrier density is accompanied by changes in Fermi energies and an increase in the potential barrier (∆U). The increase in ∆U is of significant interest to ensure that charges are directed more efficiently toward the cell's electrical contacts in the case of photovoltaic application. There, they can contribute significantly to the generated electric current, thereby enhancing the performance of a cell. Our results can provide insights into the interaction in graphene-based heterostructures and aid in selecting the best parameters for developing new advanced devices.

Photoluminescence studies of different types of feldspars and the implications to the dating application using a Raman system

In recent years, the infrared photoluminescence (PL) of feldspar has been explored for geological dating because it offers a great increase in signal intensity with increasing radiation dose. However, a comprehensive understanding of the PL emission spectra of different types of feldspar minerals remains poorly understood and the uncommonly available instruments have hindered its research. In this study, we investigated PL characteristics (e.g., emission spectra and the dependence of PL emission on irradiation dose) of single-crystal alkali and plagioclase feldspars using a commercial Raman microscope instrument configured to excite samples with 532 and 785 nm lasers. The results show that despite being sample-dependent, 2–3 PL bands peaking at ∼2.10 eV (590 nm), 1.75 eV (710 nm), and 1.43 eV (865 nm) under 532 nm laser excitation, and 0–2 peaks ranging from ∼1.44 eV (863 nm) to ∼1.34 eV (925 nm) under 785 nm laser excitation, are broadly observed. These variations in PL bands may be attributed to subtle differences in chemical composition and crystal structure among the feldspar minerals. The sensitivity of PL emission to irradiation dose varies greatly depending on feldspar types and peak positions. The PL intensities of the peaks at ∼1.43 eV (865 nm) are particularly sensitive in K-feldspar, demonstrating their potential applicability for dating applications. Additionally, solar bleaching experiments demonstrate that PL signals of these peaks can be effectively reset by 1.5 h of solar exposure. The dose-response curves obtained using 860–870 nm PL signals of K-feldspar conform to a relationship of a single saturating exponential function between the signal and irradiation dose. Furthermore, this study demonstrates that a commonly available Raman system can be utilized for PL measurements of single grains.

CsPbBr3 Perovskite Crack Platelet Nanocrystals and Their Biexciton Generation.

Lead halide perovskite nanocrystals have been extensively studied in recent years as efficient optical materials for their bright and color-tunable emissions. However, these are mostly confined to their 3D nanocrystals and limited to the anisotropic nanostructures. By exploring the Cs-sublattice-induced metal(II) ion exchange with Pb(II), crack CsPbBr3 perovskite platelet nanocrystals having polar surfaces in all three directions are reported here, which remained different than reported standard square platelets. The crack platelets are also passivated with halides to enhance their brightness. Further, as these crack and passivated crack platelets have defects and polar surfaces, the exciton and biexciton generation in these platelets is investigated using femtosecond photoluminescence and transient absorption measurement at ambient as well as cryogenic temperatures, correlated with time-resolved single-particle photoluminescence spectroscopy, and compared with standard square platelets having nonpolar facets. These investigations revealed that the crack platelets and passivated crack platelets possess enhanced biexciton emission compared to square platelets due to the presence of polar surfaces in all three directions. These results provide insights into not only the design of the anisotropic nanostructures of ionic nanocrystals but also the possibility of tuning the single exciton to biexciton generation efficiency, which has potential applications in optoelectronic systems.

Spectator Exciton Effects in Nanocrystals III: Unveiling the Stimulated Emission Cross Section in Quantum Confined CsPbBr3 Nanocrystals.

Quantifying stimulated emission in semiconductor nanocrystals (NCs) remains challenging due to masking of its effects on pump-probe spectra by excited state absorption and ground state bleaching signals. The absence of this defining photophysical parameter in turn impedes assignment of band edge electronic structure in many of these important fluorophores. Here we employ a generally applicable 3-pulse ultrafast spectroscopic method coined the "Spectator Exciton" (SX) approach to measure stimulated-emission efficiency in quantum confined inorganic perovskite CsPbBr3 NCs, the band edge electronic structure of which is the subject of lively ongoing debate. Our results show that in 5-6 nm CsPbBr3 NCs, a single exciton bleaches more than half of the intense band edge absorption band, while the cross section for stimulated emission from the same state is nearly 6 times weaker. Discussion of these findings in light of several recent electronic structure models for this material proves them unable to simultaneously explain both measures, proving the importance of this new input to resolving this debate. Along with femtosecond time-resolved photoluminescence measurements on the same sample, SX results also verify that biexciton interaction energy is intensely attractive with a magnitude of ∼80 meV. In light of this observation, our previous suggestion that biexciton interaction is repulsive is reassigned to hot phonon induced slowdown of carrier relaxation leading to direct Auger recombination from an excited state. The mechanism behind the extreme slowing of carrier cooling after several stages of exciton recombination remains to be determined.

Incorporation and Interaction of Co‐Doped Be and Mg in GaN Grown by Metal‐Organic Chemic Vapor Deposition

Despite recent advances in growth and characterization of GaN:Be, reliable conductive p‐type GaN:Be remains elusive. In this work, GaN is co‐doped with Be and Mg to improve the incorporation and ionization efficiency of both Be and Mg. Be and Mg are found to interact in complex ways rendering GaN:Be,Mg semi‐insulating. Rather than improving Be and Mg incorporation efficiency, there is an apparent mutual inhibition of incorporation when Be and Mg are co‐dopants. Furthermore, photoluminescence measurements indicate that the BeGa acceptor is reduced in particular, relative to the total [Be]. The same effect is not observed for Mg. From this, it is concluded that Mg preferentially incorporates into Ga lattice sites over Be, and excess Be that cannot incorporate into substitutional sites instead occupies interstitial sites. The interstitial Be acts as donor defect, which compensate Mg acceptors. This provides an explanation for the observed reduction in BeGa‐related luminescence intensity (without an associated decrease in [Be]) in GaN:Be with significant [Mg] content.

Reversible photoluminescence shift in imidazolium l-tartrate crystal triggered by acoustic shock waves

Abstract Imidazolium l-tartrate crystal has been grown by employing the slow evaporation technique using de-ionized water as the solvent. An impact study on the exposure of shock pulses on the surface of the material has been carried out and the influence of shock waves on its photo luminance has been investigated. In the present work, in order to carry out the experiment, a shock wave of Mach number 1.5 has been utilized which has been generated by a semi-automated Reddy Tube. Imidazolium tartrate crystal is made into a fine powder and split into four identical parts to be used for further characterization. A series of shock waves such as 25, 50, and 75 are impacted on the respective samples while keeping one of the samples as the control. The powder X-ray diffraction analysis reveals that the observed increase in peak intensity and peak shifting is due to the increase in the number of shock pulses from 25 to 75. FTIR is performed to analyze the presence of functional groups in the material before and after shock exposure. Photoluminescence measurements are also carried out for the pre- and post-shocked samples to determine the nature of optical emission with respect to various shock pulse counts. The above experimental analyzes confirm that the title sample undergoes a reversible photoluminescence shift induced by shock waves.

Understanding the interplay of defects, oxygen, and strain in 2D materials for next-generation optoelectronics

Two-dimensional transition metal dichalcogenides are leading materials for next-generation optoelectronics, but fundamental problems stand enroute to commercialization. These problems include, firstly, the widely debated defect- and strain-induced origins of intense low-energy broad luminescence peaks (L-peaks) observed at low temperatures. Secondly, the role of oxygen in tuning the properties via chemisorption and physisorption is intriguing but challenging to understand. Thirdly, our physical understanding of the benefits of hexagonal boron nitride (hBN) encapsulation is inadequate. Using a series of samples, we decouple the contributions of oxygen, defects, adsorbates, and strain on the optical properties of monolayer MoS2. The defect origin of the L-peak is confirmed by temperature- and power-dependent photoluminescence (PL) measurements, with a dramatic redshift of ∼130 meV for oxygen-assisted chemical vapour deposition (O-CVD) samples compared with exfoliated samples. Anomalously, the O-CVD samples show high A-exciton PL at room temperature (cf exfoliated), but reduced PL at low temperatures, attributed to the strain-induced direct-to-indirect bandgap crossover in low-defect O-CVD MoS2. These observations are consistent with our density functional theory calculations and are supported by Raman spectroscopy. In the exfoliated samples, the charged O adatoms are identified as thermodynamically favourable defects, and create in-gap states. The beneficial effect of encapsulation originates from the reduction of charged O adatoms and adsorbates. This experimental–theoretical study uncovers the type of defects in each sample, enables an understanding of the combined effect of defects, strain, and oxygen on the band structure, and enriches our understanding of the effects of encapsulation. This work proposes O-CVD as a method for creating high-quality materials for optoelectronics.

Surface Engineering of Anodic WO3 Layers by In Situ Doping for Light-Assisted Water Splitting.

This study presents a novel approach to fabricating anodic Co-F-WO3 layers via a single-step electrochemical synthesis, utilizing cobalt fluoride as a dopant source in the electrolyte. The proposed in situ doping technique capitalizes on the high electronegativity of fluorine, ensuring the stability of CoF2 throughout the synthesis process. The nanoporous layer formation, resulting from anodic oxide dissolution in the presence of fluoride ions, is expected to facilitate the effective incorporation of cobalt compounds into the film. The research explores the impact of dopant concentration in the electrolyte, conducting a comprehensive characterization of the resulting materials, including morphology, composition, optical, electrochemical, and photoelectrochemical properties. The successful doping of WO3 was confirmed by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, photoluminescence measurements, X-ray photoelectron spectroscopy (XPS), and Mott-Schottky analysis. Optical studies reveal lower absorption in Co-doped materials, with a slight shift in band gap energies. Photoelectrochemical (PEC) analysis demonstrates improved PEC activity for Co-doped layers, with the observed shift in photocurrent onset potential attributed to both cobalt and fluoride ions catalytic effects. The study includes an in-depth discussion of the observed phenomena and their implications for applications in solar water splitting, emphasizing the potential of the anodic Co-F-WO3 layers as efficient photoelectrodes. In addition, the research presents a comprehensive exploration of the electrochemical synthesis and characterization of anodic Co-F-WO3, emphasizing their photocatalytic properties for the oxygen evolution reaction (OER). It was found that Co-doped WO3 materials exhibited higher PEC activity, with a maximum 5-fold enhancement compared to pristine materials. Furthermore, the studies demonstrated that these photoanodes can be effectively reused for PEC water-splitting experiments.

Long Excited-State Lifetimes in Three-Coordinate Copper(I) Complexes via Triplet-Triplet Energy Transfer to Pyrene-Decorated Isocyanides.

There has been much effort to improve excited-state lifetimes in photosensitizers based on earth-abundant first-row transition metals. Copper(I) complexes have gained significant attention in this field, and in most cases, sterically driven approaches are used to optimize their lifetimes. This study presents a series of three-coordinate copper(I) complexes (Cu1-Cu3) where the excited-state lifetime is extended by triplet-triplet energy transfer. The heteroleptic compounds feature a cyclohexyl-substituted β-diketiminate (CyNacNacMe) paired with aryl isocyanide ligands, giving the general formula Cu(CyNacNacMe)(CN-Ar) (CN-dmp = 2,6-dimethylphenyl isocyanide for Cu1; CN-pyr = 1-pyrenyl isocyanide for Cu2; CN-dmp-pyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl isocyanide for Cu3). The nature, energies, and dynamics of the low-energy triplet excited states are assessed with a combination of photoluminescence measurements at room temperature and 77 K, ultrafast transient absorption (UFTA) spectroscopy, and DFT calculations. The complexes with the pyrene-decorated isocyanides (Cu2 and Cu3) exhibit extended excited-state lifetimes resulting from triplet-triplet energy transfer (TTET) between the short-lived charge-transfer excited state (3CT) and the long-lived pyrene-centered triplet state (3pyr). This TTET process is irreversible in Cu3, producing exclusively the 3pyr state, and in Cu2, the 3CT and 3pyr states are nearly isoenergetic, enabling reversible TTET and long-lived 3CT luminescence. The improved photophysical properties in Cu2 and Cu3 result in improvements in activity for both photocatalytic stilbene E/Z isomerization via triplet energy transfer and photoredox transformations involving hydrodebromination and C-O bond activation. These results illustrate that the extended excited-state lifetimes achieved through TTET result in newly conceived photosynthetically relevant earth-abundant transition metal complexes.

Grain boundaries are not the source of Urbach tails in Cu(In,Ga)Se2 absorbers

The presence of Urbach tails in Cu(In,Ga)Se2 (CIGSe) absorbers has been identified as a limiting factor for the performance of the CIGSe solar cells. The tail states contribute to both radiative and non-radiative recombination processes, ultimately leading to a reduction in the open-circuit voltage and, consequently, decreasing the overall efficiency of CIGSe devices. Urbach tails result from structural and thermal disorders. The Urbach tails can be characterized by the Urbach energy, which is associated with the magnitude of the tail states. Within polycrystalline CIGSe absorbers, grain boundaries can be considered as structural disorder and, therefore, can potentially contribute to the Urbach tails. In fact, it has been proposed that the band bending at grain boundaries contribute significantly to the tail states. This study focuses on examining the correlation between Urbach tails and the band bending at the grain boundaries. The Urbach energies of the CIGSe samples are extracted from photoluminescence (PL) measurements, which reveal that the introduction of Sodium (Na) into the material can lead to a reduction in the Urbach energy, and an even further decrease can be achieved through the RbF post-deposition treatment. The band bending at the grain boundaries is investigated by Kelvin probe force microscopy measurements. A thorough statistical analysis of more than 340 grain boundaries does not show any correlation between Urbach tails and grain boundaries. We measure small band bending values at the grain boundaries, in the range of the thermal energy (26 meV at room temperature). Furthermore, our intensity dependent PL measurements indicate that Urbach tails are, at least in part, a result of electrostatic potential fluctuations. This supports the model that the introduction of alkali elements mainly decreases the magnitude of electrostatic potential fluctuations, resulting in a subsequent reduction in the Urbach energy.

Application of Sm3+ doped Gd2O3–Y2O3–ZnO–B2O3 glass for development of X-ray imaging scintillator

The X-ray scintillator materials are mostly made from single crystal. The limitations are high productive cost, and the process of producing is complex. There are numerous studies about developing new scintillators by using glass that are easier preparation and lower cost. This study tries to produce a new series of borate glass doped with Sm3+ in 20Gd2O3:10Y2O3:10ZnO: (60-x)B2O3:xSm2O3 formula with varying concentrations of Sm2O3. The samples were measured and analyzed by physical, optical, luminescence and scintillation properties. The results showed that the density increased after increasing the Sm2O3 concentration. The strong emission intensity was observed from photoluminescence and radioluminescence measurements, and the concentration quenching was observed at 0.5 and 1.0 mol% of Sm2O3 respectively. The strongest emission peak of Sm3+ ion is located at 600 nm that corresponds to f-f transition of 4G5/2 to 6H7/2 level. Moreover, we found that the Sm3+ luminescence at 600 nm emission wavelength increases (up to 0.5% mol), while the Gd3+ emission wavelength at 313 nm decreases, attributed to the energy transfer (ET) from Gd3+ ions to Sm3+ ions. X-ray imaging was also observed by using Synchrotron X-rays. These results suggest that the developed glasses could be applied to X-ray scintillators material, especially imaging applications.

Application of Sm3+ doped Gd2O3-Y2O3-ZnO-B2O3 Glass for Development of X-ray Imaging Scintillator

The X-ray scintillator materials are mostly made from single crystal. The limitations are high productive cost, and the process of producing is complex. There are numerous studies about developing new scintillators by using glass that are easier preparation and lower cost. This study tries to produce a new series of borate glass doped with Sm3+ in 20Gd2O3:10Y2O3:10ZnO:(60-x)B2O3:xSm2O3 formula with varying concentrations of Sm2O3. The samples were measured and analyzed by physical, optical, luminescence and scintillation properties. The results showed that the density increased after increasing the Sm2O3 concentration. The strong emission intensity was observed from photoluminescence and radioluminescence measurements, and the concentration quenching was observed at 0.5 and 1.0 mol% of Sm2O3 respectively. The strongest emission peak of Sm3+ ion is located at 600 nm that corresponds to f-f transition of 4G5/2 to 6H7/2 level. Moreover, we found that the Sm3+ luminescence at 600 nm emission wavelength increases (up to 0.5% mol), while the Gd3+ emission wavelength at 313 nm decreases, attributed to the energy transfer (ET) from Gd3+ ions to Sm3+ ions. X-ray imaging was also observed by using Synchrotron X-rays. These results suggest that the developed glasses could be applied to X-ray scintillators material, especially imaging applications.

La:ZnO nanoparticles: an investigation on structural, optical, and microwave properties

This paper presents the utilization of ethylene glycol monomethyl ether (EGME) during the synthesis of ZnO and La:ZnO with two tasks as a solvent and a fuel source within the gel combustion technique. The use of EGME for this purpose provides one-step production of the nanoparticles (NPs) and saves a considerable amount of time. The detailed characterization of the nanoparticles was carried out by X-ray diffractometer (XRD), scanning electron microscope (SEM), particle size analyzer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Vector Network Analyzer (VNA) measurements, respectively. The NPs exhibited a hexagonal wurtzite structure with good crystallinity and a porous spongy morphology. The photoluminescence emission maxima of the synthesized NPs appeared at 500, 560, and 676 nm, upon excitation by the 372 nm of excitation. La:ZnO NPs showed significantly better photoluminescent characteristics than La-free ZnO forms. When excited at the same wavelength, La-free ZnO, 3%, and 7% La:ZnO exhibited 92, 45, and 35 μs average decay times, respectively. Finally, the microwave properties of the relative complex permittivity and permeability characteristics were also investigated and discussed in detail, which were derived from the scattering parameters of S11 and S21 in the X band regime.

Strain-induced modulation of the band structure of GaAs/GaSb/GaAs core-dual-shell nanowires

The amalgamation of high-strained core/shell geometries in nanowire (NW) heterostructures with bandgap engineering enables the systems with novel transport and optical properties. Herein, the lattice-mismatch-high construction GaAs/GaSb/GaAs core-dual-shell (CDS) NWs were grown by molecular beam epitaxy (MBE), with a focus on exploring the impact of strain on their bandgap characteristics. Interestingly, the high stain between GaSb and GaAs leads to the surface roughening that favors Stranski-Krastanov (SK) mode growth, and coherent GaSb islands on GaAs NWs form plastically relaxed mounds at the edges of the NW sidewall facets. Strain properties of GaAs/GaSb/GaAs CDS NWs have been investigated systematically by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Raman spectra. It is clearly revealed that the presence of different strains, with the strain diminished of GaSb shell layer in the large-scale NWs and amplified in the small-scale NWs. The optical properties of the GaAs/GaSb/GaAs CDS NWs were investigated by power- and temperature-dependent photoluminescence (PL) measurements, which reveal the emission properties with the exciton localization phenomenon. Our results on the unique optical properties of localized excitons in GaAs/GaSb/GaAs core-shell nanostructures open the way to the design of stable optoelectronic devices.

The photoluminescence studies of Gd3+/Tb3+ co-doped xerogels and nano-glass-ceramics dedicated for green-emitting devices

In this work, we reported the results of photoluminescent and structural investigations of Gd3+/Tb3+ co-doped silicate xerogels as well as derivative SiO2-LaF3 nano-glass-ceramics. The molar ratio of appropriate acetates used during the synthesis, i.e., La(AcO)3:Gd(AcO)3:Tb(AcO)3 was fixed to x:(0.95-x):0.05. Based on XRD measurements, the crystallization of hexagonal LaF3 nanophase was verified without any admixture of GdF3 phase, even for samples with La3+:Gd3+ molar ratio ≤1. The optical characterization of fabricated Gd3+/Tb3+ co-doped sol-gel materials involved the registration and analysis of photoluminescence excitation (PLE) and emission spectra (PL), as well as the decay curves for the 6P7/2 (Gd3+) and the 5D4 (Tb3+) excited levels. Upon excitation at λex = 275 nm wavelength, the fabricated sol-gel samples revealed the series of emission lines assigned to the electronic transitions from both dopants, i.e., Gd3+ and Tb3+, indicating apparently the occurrence of Gd3+/Tb3+ energy transfer process (ET). The photoluminescence measurements comprehensibly proved the competitiveness between Gd3+ and Tb3+ ions to be entered into LaF3 fluoride nanocrystal lattice, formed during annealing of as-prepared xerogels. Based on the recorded PL spectra for fabricated sol-gel samples, some chromatic parameters were calculated, i.e., chromaticity coordinates (CIE), correlated color temperatures (CCT), and color purities (CP). The prepared Gd3+,Tb3+ co-doped samples are able to emit green light with CCT values in a range from 5967 K to 6291 K, and high CP reached 70 %, which predispose them for use as efficient constituents for developing green-emitting devices.

Substitution of Europium (III) in Hydroxyapatite Lattice for Luminescence Applications: Integrating Experimental and Theoretical Insights

Europium(III) (Eu3+/Eu(III)) substitution in the hydroxyapatite (HAp) lattice imparts luminescence properties, enabling its application in diagnostics and bioimaging. Despite numerous experimental studies, there has been a lack of comprehensive discussion on the substitution behavior and local structural stability of Eu3+ within HAp. In this study, experimental and first-principles investigations were integrated to explore and understand the substitution of Eu3+ in HAp. Optimization of various HAp models revealed the dependency of Eu3+ substitution site on the charge compensation mechanism. Eu3+ prefers the Ca(2) site when OH is converted to O and the Ca(1) site in the presence of a Ca vacancy. Phonon calculations showed that temperature influences charge compensation and site preference, favoring OH to O compensation and Eu3+ in Ca(2) at higher temperatures and Ca vacancy and Eu3+ in Ca(1) at lower temperatures. Electronic structure simulations explained that the luminescence in Eu(III)-substituted HAp primarily arises from charge transfer between Eu3+ and O atoms, supported by experimental photoluminescence measurements. Correspondence between simulations and experimental photoluminescence data emphasizes the reliability of the computational approach in this study. The integration of insights from experimental and simulation results enhances the understanding of substitution of Eu3+ in the HAp lattice, providing valuable guidance for designing and optimizing HAp-based luminescent and optical materials.

Emission color tuning and enhancement of the upconversion emission in NaBiF4:Yb3+,Er3+ phosphors by controlling the Na/F ratio

NaBiF4:Yb3+(20 mol%),Er3+(2mol%) phosphors (NBF:Yb,Er) were synthesized by a simple precipitation method. Several NBF:Yb,Er samples were made with different Na/F ratios (from 2/16 to 6/16 and 6/24). XRD and SEM measurements indicated that all the NBF:Yb,Er phosphors presented an hexagonal phase and morphology of microbars with sizes of 1–2 μm, respectively. According to the photoluminescence measurements, these phosphors emitted red/green emission bands at 541/655 nm and their emission intensity increased ⁓10.5 and 21 times for the green and red bands, respectively, after increasing the Na/F ratio from 2/16 to 4/16. In addition, the emission color was tuned from deep-green to yellow. Moreover, the Na/F ratio was decreased from 6/16 to 6/24, this in turn, changed the color emission from deep-green to orange. This last change of color occurred because the red emission was mainly favored by the following two cross relaxation (CR) processes CR1: 4I11/2 (Er3+)+ 4F7/2 (Er3+)→4F9/2 (Er3+)+ 4F9/2 (Er3+) and CR2: 4S3/2 (Er3+)+ 2F7/2 (Yb3+)→4I13/2 (Er3+)+ 2F5/2 (Yb3+). The second CR2 process also promoted the NIR emission at 1527 nm. Additionally, the lifetimes for the green and red bands increased from 393 to 420 μs and from 428 to 518 μs with the Na/F ratio, respectively. Thus, we presented here a new strategy to enhance the photoluminescence intensity and to control the emission-color/lifetimes without using extra co-dopants (as previously reported in literature).

Photocurrent interference spectroscopy of solids and characterization of semiconductor solar-cell materials

Photocurrent measurements are widely used to study the intrinsic optoelectronic properties of semiconductors, such as photocarrier generation efficiency and carrier mobility, as well as evaluate the performance of optoelectronic devices, such as solar cells and photodetectors. Interferometric spectroscopy precisely measures optical properties and gathers optical spectra information on semiconductors. Consequently, photocurrent-based interferometric measurements, with high signal-to-noise ratios, high resolution, and broad frequency bandwidths, can probe the energy distributions of low-density defects and impurities and investigate charge transport in materials and devices. Here, we demonstrate that photocurrent interference spectroscopy reveals the intrinsic properties of solar-cell materials: bulk crystals of GaAs and halide perovskite, and thin films of halide perovskite and quantum dot. We show that homodyne interference spectroscopy of photocurrent can monitor low-density localized states in semiconductors and that it can be used in combination with other spectroscopy techniques, such as photoluminescence measurements, to provide a deep understanding of photocurrent generation processes. Furthermore, we show that heterodyne interference spectroscopy of photocurrent can be used to investigate the frequency dependence of material parameters, such as the dielectric constant, absorption coefficient, and reflectance. As an application, we used interference spectroscopy of photocurrent to show the impact of multiexcitons on the photoabsorption and photocarrier generation processes in quantum dot solar cells. Finally, we used it to reveal the distinctive spectral characteristics at the band edge of a halide perovskite, which is considered to be an exceptional solar-cell material with high energy conversion efficiency.