Photoluminescence Measurements Research Articles

Argon plasma-enhanced UV light emission from ZnO submicrowires grown by hydrothermal method

In this work, the optical, morphological, and structural improvements of vertically aligned, hydrothermally grown ZnO submicrowires (SMWs) treated with a 250 W, 250 V RF argon plasma (Ar) during different exposure times were investigated by scanning and transmission electrons microscopy, X-ray diffraction, and micro-Raman and photoluminescence (PL) spectroscopies. In two steps, the SMWs were synthesized in an aqueous medium at low temperatures. The plasma-treated samples showed significantly improved room-temperature PL compared to untreated samples. All the treated samples exhibited a substantial increase of the near band edge UV emission intensity and a decrease of the deep level emission in the visible. The samples treated for 4 minutes presented the best UV/vis intensity ratio of ∼ 1568 (an increase of ⁓174-fold with respect to the untreated sample). The analysis of the UV band in terms of the first and second phonon replica of the excitonic emission indicated that the Ar plasma treatment favors the multiple phonon-exciton coupling, with partial correlation with the observed overall increase of the UV/visible intensity ratio. From the PL measurements at low temperatures, the presence of excitons bound to H donors in the ZnO structure was inferred. The possible reasons for the Ar plasma-induced enhancement of the UV emission from the treated SMWs are discussed in terms of previous work, the observed morphology and changes expected to occur at the SMW surfaces due to Ar ion impacts from the plasma.

Ligand-assisted reprecipitation synthesis of highly luminescent and stable CsPbBr3@SiO2 nanoparticles with formation kinetics analysis

Cesium lead bromide (CsPbBr3) perovskite nanocrystals are becoming a popular alternative to chalcogenide quantum dots because of their bright green fluorescence and high color purity. However, owing to the poor stability caused by their highly ionic nature and the dynamic binding of long-chain capping ligands, their practical applications are limited. Although (3-aminopropyl)triethoxysilane (APTES) is a frequently used insulating material for wrapping CsPbBr3nanocrystals, it often causes surface etching. To address this issue, we introduced oleic acid into the anti-solvent toluene to inhibit the etching effect of APTES using a modified room-temperature ligand-assisted reprecipitation process. We utilized in situ time-dependent photoluminescence measurements to study the formation kinetics of CsPbBr3nanocrystals and determine the optimal ligands ratio. This innovative approach enables precise control over CsPbBr3@SiO2nanoparticles synthesis, yielding uniformly shaped nanocrystals with a silica shell, a consistent size around 10.17 ± 1.6 nm, and enhanced photoluminescence quantum yields ranging from 90% and 100%. The photoluminescence lifetimes of our CsPbBr3@SiO2nanoparticles were significantly prolonged owing to a reduction in non-radiative recombination. This boosts their stability in thermal and polar solvent environments, making them superior candidates for use in photonic devices.

P-Doped Carbon Nanotubes As Light Absorbers and Electron Donors in Photovoltaics

Doping is a ubiquitous and powerful tool used to alter a material’s electronic properties. Conventional Si solar cells would not exist without it, and it has been used to increase the efficiency of organic and inorganic photovoltaics alike. While doped carbon nanotubes (CNTs) have been the subject of numerous spectroscopic studies, few investigations have examined their performance in photovoltaic devices. Here, we incorporate chemically p-doped CNTs into photovoltaic heterojunctions with C60. In a conventional CNT–C60 heterojunction device, photocurrent can be produced when photogenerated excitons in the CNT layer diffuse to the CNT–C60 heterointerface. The offset in conduction band energy drives dissociation of these excitons via photoelectron transfer from CNT to C60, which yields separated charges that can be collected. Our objective here was to determine the following: 1) does p-doping increase or decrease the efficiency with which excitons dissociate into electrons and holes? 2) Do p-dopants trap excitons and inhibit their diffusion to the heterointerface? 3) How efficiently can a photogenerated trion (exciton coupled to an injected hole) be dissociated to produce a photocurrent?To answer these questions, we fabricated polymer wrapped (6,5) CNT–C60 heterojunctions over a range of CNT thickness with various levels of p-doping introduced by triethyloxonium hexachloroantimonate treatment. We measured photocurrent to quantify relative photoelectron transfer efficiency for both excitons and trions and compared this to photoluminescence and transient absorbance measurements on doped CNT films. We found that p-doping decreases photocurrent because injected holes bleach the CNTs’ S11 transition and decrease the amount of light absorbed. Meanwhile, the photogenerated exciton to collected electron transfer efficiency remains unchanged for doping densities up to 50μm-1. In contrast, photoluminescence quantum yield drops by over 90% at an equivalent doping density. We attribute this disparity to the fast electron transfer at the CNT–C60 interface, which occurs on the order of femtoseconds. We also found that moderate doping can promote photocurrent at the trion peak, with a photogenerated trion to collected electron transfer efficiency about 1/4 that of the photogenerated exciton to collected electron transfer efficiency. Photocurrent at the trion absorption peak is invariant with reverse bias up to 0.3 V, despite the charged nature of the trion, supporting evidence for the immobile nature of trions. These results indicate that p-CNTs are a viable material for use in photovoltaics, albeit with weaker absorption, and that photocurrent from trions is possible, although with lower quantum efficiency. Figure 1

(Invited) Chiral Metamaterials for Polarized Photoluminescence

Metasurfaces offer compact routes to spatial and polarization control of luminescence from nearby emitters, enabling applications ranging from document security to stereoscopic displays to sensing.Most of the time, light-emitters are distributed evenly across the surface. This talk will discuss an alternate approach, where we use direct-write electron beam lithography to pattern semiconductor nanocrystals into targeted shapes and locations, creating a highly tailored metasurface. We use a combination of nanoscale fabrication, Fourier-space photoluminescence measurements, and computational methods to study the circularly polarized photoluminescence from these designer structures.We will discuss the influence of patterning on the resulting optical properties, including the effects of spatial averaging and dipole orientation. We will then show an optimization strategy to produce highly polarized light at chosen angles of outcoupling. The final part of the talk will discuss adding layers of complexity, including the chirality of the underlying pattern, and creating light-emitting solids from intrinsically chiral semiconductor nanocrystals.

(Invited) Photoluminescence Dynamics in PbS/CdS Core/Shell Nanocrystal Films: Carrier Recombination and Energy Transfer

Colloidal semiconductor nanocrystals (NCs) exhibit highly controllable size-dependent optical properties due to the quantum confinement effect. Among other semiconductor NCs, the lead chalcogenides, especially PbS, are one of the most studied and promising NC systems. They are the material of choice for studies of fundamental phenomena as well as for the development of solution processed infrared optoelectronic devices including LEDs, lasers, solar cells, and photodetectors. Compared to organically passivated NCs, the core-shell approach (more specifically PbS/CdS core-shell NCs) offers not only the improved stability and higher photoluminescence (PL) quantum efficiency but also an opportunity of the core/shell interface nano-engineering resulting in suppression of Auger recombination, reduction of the exciton radiative lifetime and an improved lasing threshold. However, despite more than a decade of detailed studies there are conflicting reports on PL dynamics in PbS/CdS core-shell NCs, and there is no accepted model explaining the observed PL lifetimes ranging from hundred nanoseconds to tens of microseconds. In this work we use time-resolved PL measurements and demonstrate that low-temperature PL dynamics in PbS/CdS core shell NCs show strong dependence on the PL detection wavelengths. The presented experimental results allow us to distinguish between processes of energy transfer and exciton recombination. We propose a model, which considers energy transfer between NCs and various mechanisms of exciton recombination (non-radiative, radiative, spin-flip, diffusion limited, etc.).

Boosted Photocatalytic Activities of Ag2CrO4 through Eu3+-Doping Process.

Ag2CrO4 is a representative member of a family of Ag-containing semiconductors with highly efficient visible-light-driven responsive photocatalysts. The doping process with Eu3+ is known to effectively tune their properties, thus opening opportunities for investigations and application. Here, we report the enhancement of the photocatalytic activity and stability of Ag2CrO4 by introducing Eu3+cations. The structural, electronic, and photocatalytic properties of Ag2CrO4:xEu3+ (x = 0, 0.25, 0.5, 1%) synthesized using the coprecipitation method were systematically discussed, and their photodegradation activity against rhodamine B (RhB), ciprofloxacin hydrochloride monohydrate (CIP), and 4-nitrophenol (4-NP) was evaluated. Structural analyses reveal a short-range symmetry breaking in the Ag2CrO4 lattice after Eu3+ doping, influencing the material morphology, size, and electronic properties. XPS analysis confirmed the incorporation of Eu3+ and alteration of the surface oxygen species. Furthermore, photoluminescence measurements indicated that the doping process was responsible for reducing recombination processes. The sample doped with 0.25% Eu3+ exhibited superior photocatalytic performance compared to pure Ag2CrO4. Scavenger experiments revealed an increase in the degradation via •OH reactive species for the sample doped with 0.25% Eu3+. DFT calculations provided atomic-scale insights into the structural and electronic changes induced by the Eu3+ doping process in the Ag2CrO4 host lattice. This study confirms that Eu3+ doping alters the band structure, enabling different degradation paths and boosting the separation/transfer of photogenerated charges, thereby improving the overall photocatalytic performance.

Fabrication of luminescent disc-shaped microstructures via wet-chemical etching of hybrid sol–gel layers for potential photonic applications

Sol–gel materials based on SiO2 and TiO2 precursors are attractive as a new platform for planar photonics. Particularly interesting are those based on organically modified silica (ORMOSIL), which may improve the luminescent properties of organic dyes. However, their microstructurization remains a challenge as it requires optimization of various technological stages. Here, we report the structurization of thin layers based on ORMOSIL precursor and titanium(IV) ethoxide (TET) containing luminescent rhodamine B (RhB) dye. Films were fabricated using sol–gel synthesis and dip-coating method. Depending on the time of annealing performed at 200 °C, layers with different thicknesses (300–760 nm) and refractive indices (RI) (1.51–1.68) were obtained. Combining photolithography and wet-chemical etching processes made it possible to fabricate well-separated sol–gel waveguides and discs of different diameters. The etching time in buffered hydrofluoric acid (BHF) affected the depth of the etched luminescent microstructures. Additionally, it was found that a longer layer’s annealing time increased the etching selectivity of the substrate over the sol–gel layer. This enabled the obtaining of under-etched sol–gel goblet microstructures. Selected samples were investigated using scanning electron microscopy (SEM). UV–Vis photoluminescence measurements showed that long heat treatment also influenced the emission spectrum’s shape. The stability of the films under ambient conditions was established using spectroscopic ellipsometry. It was proven that films heat-treated at 200 °C did not change their properties during storage time of around 2 months. Relatively high RI, luminescent properties, and structurization potential make these microstructures interesting for application in integrated photonic devices, e.g., light amplifiers or sensing systems.

Long-Range Self-Hybridized Exciton-Polaritons in Two-Dimensional Ruddlesden-Popper Perovskites.

Lead halide perovskites have emerged as platforms for exciton-polaritonic studies at room temperature, thanks to their excellent photoluminescence efficiency and synthetic versatility. In this work, we find proof of strong exciton-photon coupling in cavities formed by the layered crystals themselves, a phenomenon known as the self-hybridization effect. We use multilayers of high-quality Ruddlesden-Popper perovskites in their 2D crystalline form, benefiting from their quantum-well excitonic resonances and the strong Fabry-Pérot cavity modes resulting from the total internal reflection at their smooth surfaces. Optical spectroscopy reveals bending of the cavity modes typical for exciton-polariton formation, and absorption and photoluminescence spectroscopy shows splitting of the excitonic resonance and thickness-dependent peak positions. Strikingly, local optical excitation with energy below the excitonic resonance of the flakes in photoluminescence measurements unveils the coupling of light to in-plane polaritonic modes with directed propagation. These exciton-polaritons exhibit high coupling efficiencies and extremely low loss propagation mechanisms, which are confirmed by finite difference time domain simulations. Thus, we prove that mesoscopic 2D Ruddlesden-Popper perovskite flakes represent an effective but simple system to study the rich physics of exciton-polaritons at room temperature.

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.

Harnessing waste Pd-based powders contaminated with ZrO2 as efficient photoelectrocatalysts for glycerol oxidation

Pd-based electrocatalysts synthesized by mechanical alloying were contaminated with ZrO2 due to an excessive process time that wore down the ZrO2 milling media balls. Instead of disposing the materials and taking into account the synergistic properties that Pd-based materials and ZrO2 could present since ZrO2 is an active oxide with photocatalytic activity, these samples were characterized through physicochemical and electrocatalytic tests, to finally be used as non-reported Pd-ZrO2-based (photo)electrocatalysts for the glycerol oxidation reaction (GOR). XRD measurements confirmed the metals alloy formation and the presence of zirconium oxide to obtain the bimetallic composites PdCo/ZrO2, PdNi/ZrO2, and PdRu/ZrO2, as well as the trimetallic PdRuCo/ZrO2 and PdRuNi/ZrO2. Physicochemical analysis showed that PdRuNi/ZrO2 presented the smallest crystallite size (14.55 nm) and the highest surface area (8.76 m2/g). Photoluminescence measurements also show that PdRuNi/ZrO2 presented the lowest emission signal, which indicates a lower charge carrier’s recombination rate. XPS measurements demonstrate the presence of different metal oxide species in alkaline medium responsible for electrostatic interactions that improve the glycerol adsorption onto the metallic centers. Electrochemical tests showed high stability for all the composites; however, the analysis of the capacitive region allowed us to calculate the electrochemical surface area (ECSA), PdRuNi/ZrO2 gave the highest value (above 7 cm−2). Confirming the results mentioned above, PdRuNi/ZrO2 showed the highest activity to carry out the electrochemical oxidation of glycerol. Regarding photoactivity, the Pd-ZrO2-based composites presented a glycerol oxidation increase up to 17 % when they were light-irradiated through a photoelectrocatalytic arrangement. PdRuNi/ZrO2 seems to be a promising material to perform the electro- but also the photoelectro- GOR in alkaline medium.

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.

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.

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).

Leveraging Compatible Iridium(III) Complexes to Boost Performance of Green Solvent‐Processed Non‐Fullerene Organic Solar Cells

AbstractIn organic solar cells (OSCs), the short exciton lifetime poses a significant limitation to exciton diffusion and dissociation. Extending exciton lifetime and suppressing recombination are crucial strategies for improving the OSC performance. Herein, an effective approach is proposed by introducing the phosphorescent emitter, tris(2‐(4‐(tert‐butyl)phenyl)‐5‐fluoropyridine)Iridium(III), with long‐lived triplet exciton lifetime in OSCs. This research reveals that the steric structure of fac‐Ir(tBufppy)3 exhibits excellent compatibility with both the donor PM6 and acceptor BTP‐eC9, maintaining efficiencies of over 90% even with a 30% third component loading. Moreover, a 10% addition of fac‐Ir(tBufppy)3 mitigates excessive aggregation in the acceptor BTP‐eC9, optimizing the active layer morphology and improving the fill factor. Transient absorption spectroscopy and transient photoluminescence measurements demonstrate that the introduction of fac‐Ir(tBufppy)3 significantly extends exciton lifetimes and suppresses recombination, which increases the short‐circuit current (JSC). Ultimately, employing the non‐halogenated solvent o‐xylene for processing, an impressive power conversion efficiency (PCE) of 18.54% is achieved in devices based on PM6:10%fac‐Ir(tBufppy)3:BTP‐eC9, surpassing the efficiency of binary PM6:BTP‐eC9 devices (17.41%). This work provides a promising approach to further improve the PCEs in binary OSCs by introducing a phosphorescent iridium(III) complex as the third component.

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.