Time-resolved Photoluminescence Spectroscopy Research Articles

Strongly polarized color conversion of isotropic colloidal quantum dots coupled to fano resonances.

Colloidal quantum dots (QDs) offer high color purity essential to high-quality liquid crystal displays (LCDs), which enables unprecedented levels of color enrichment in LCD-TVs today. However, for LCDs requiring polarized backplane illumination in operation, highly polarized light generation using inherently isotropic QDs remains a fundamental challenge. Here, we show strongly polarized color conversion of isotropic QDs coupled to Fano resonances of v-grooved surfaces compatible with surface-normal LED illumination for next-generation QD-TVs. This architecture overcomes the critically oblique excitation of surface plasmon coupled emission by using v-shapes imprinted on the backlight unit (BLU). With isotropic QDs coated on the proposed v-BLU surface, we experimentally measured a far-field polarization contrast ratio of ∼10. Full electromagnetic solution shows Fano line-shape transmission in transverse magnetic polarization allowing for high transmission as an indication for forward-scattering configuration. Of these QDs coupled to the surface plasmon-polariton modes, we observed strong modifications in their emission kinetics revealed by time-resolved photoluminescence spectroscopy and via dipole orientations identified by back focal plane imaging. This collection of findings indicates conclusively that these isotropic QDs are forced to radiate in a linearly polarized state from the patterned planar surface under surface-normal excitation. For next-generation QD-TVs, the proposed polarized color-converting isotropic QDs on such v-BLUs can be deployed in bendable electronic displays.

Unravelling material properties of halide perovskites by combined microwave photoconductivity and time-resolved photoluminescence spectroscopy

Simultaneous measurements of trPL and microwave photoconductivity facilitate a global fit of charge carrier density in perovskite materials. This enables the extraction of fundamental rate constants and the mobility ratio of electrons and holes.

Unraveling the excitonics of light emission from metal-halide perovskite quantum dots.

Metal halide semicondictor perovskites have been under intense investigation for their promise in light absorptive applications like photovoltaics. They have more recently experienced interest for their promise in light emissive applications. A key aspect of perovskites is their glassy, ionic lattice that exhibits dynamical disorder. One possible result of this dynamical disorder is their strong coupling between electronic and lattice degrees of freedom which may confer remarkable properties for light emission such as defect tolerance. How does the system, comprised of excitons, couple to the bath, comprised of lattice modes? How does this system-bath interaction give rise to novel light emissive properties and how do these properties give insight into the nature of these materials? We review recent work from this group in which time-resolved photoluminescence spectroscopy is used to reveal such insights. Based upon a fast time resolution of 3 ps, energy resolution, and temperature dependence, a wide variety of insights are gleaned. These insights include: lattice contributions to the emission linewidths, multiexciton formation, hot carrier cooling, excitonic fine structure, single dot superradiance, and a breakdown of the Condon approximation, all due to complex structural dynamics in these materials.

High-performance CsSnBr<sub>3</sub>/Si PN heterojunction photodetectors prepared by pulsed laser deposition epitaxy

Halide perovskite semiconductors have outstanding physical properties such as high light absorption coefficient, large carrier diffusion length, and high photoluminescence quantum efficiency, and demonstrate significant potential applications in optoelectronic devices such as photodetectors and solar cells. However, the toxicity and environmental instability associated with lead-based perovskites significantly limit their applications. An attractive solution is substituting tin for lead in perovskites and growing high-quality tin-based perovskite films. In this study, we adopt the pulsed laser deposition method to achieve the epitaxial growth of CsSnBr<sub>3</sub> films on silicon substrates. The morphologies, optical and electrical properties of the CsSnBr<sub>3</sub> films, as well as the CsSnBr<sub>3</sub>/Si heterojunction detectors, are comprehensively investigated with various characterization techniques, including XRD 2<i>θ</i>-<i>ω</i> and <i>φ</i> scans, atomic force microscope, scanning electron microscope, photoluminescence and time-resolved photoluminescence spectroscopy, and Hall electrical measurements. The results indicate that such a CsSnBr<sub>3</sub> film grows epitaxially onto the silicon substrate via a face-to-face mode. Interestingly, an unusual temperature-dependent bandgap increase is found to be due to the high electron effective mass of CsSnBr<sub>3</sub>. The CsSnBr<sub>3</sub> film shows the P-type semiconductor behavior with a high mobility of 122 cm²/(V·s), enabling the formation of an ideal Type-II heterojunction with the silicon substrate. The CsSnBr<sub>3</sub>/Si semiconductor heterojunction detector exhibits distinctive heterojunction PN diode characteristics in the dark and a pronounced photoresponse under illumination. At zero bias, the detector displays a switch ratio exceeding 10<sup>4</sup>, responsivity of 0.125 mA/W, external quantum efficiency of 0.0238%, detectivity (<inline-formula><tex-math id="Z-20240301150440">\begin{document}$D^* $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231645_Z-20240301150440.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231645_Z-20240301150440.png"/></alternatives></inline-formula>) of 2.1×10<sup>9</sup> Jones, response time 3.23 ms, and recovery time of 4.87 ms. Under a small bias of –1 V, the switch ratio decreases to 50, but responsivity and external quantum efficiency increase by 568 times. The detectors can maintain self-powered operation state with a high switch ratio of 10<sup>4</sup>, millisecond-level response time and millisecond-level recovery time. In conclusion, this work presents a self-powering, high-performance photodetector based on CsSnBr<sub>3</sub> epitaxial films integrated with silicon substrates.

Characterization of photoluminescent and Raman properties of ultramarine blue pigment variants with a novel multimodal approach

Ultramarine Blue (UB) pigment, derived from lapis lazuli, holds a significant place in the history of late medieval and Renaissance Europe, owing to its unusually bright colour and stability. Its prohibitive price, which equalled that of gold, meant that it was only used by estimated artists. In this work we present a non-invasive, multimodal approach to the characterization of the photoluminescent properties of different variants of the pigment. The ultimate goal of this research is to propose a protocol for the identification of UB in artworks thanks to the combination of Raman spectroscopy and time resolved-photoluminescence spectroscopy and imaging.

Temperature-dependent excited states for detecting reversible phase transitions in 2D lead(II) iodide perovskites.

Significant interest exists in water-tolerant 2D lead iodide perovskites owing to their stability and proven potential in photovoltaic and photonic applications. These materials have solid-state phase transitions that are accessible below 100 °C. Here, the study witnesses the multiple phase transitions of the last members of a series of organic-inorganic hybrid materials, [(CnH2n+1NH3)2PbI4], with even n as n = 14, 16, and 18, once again. By employing temperature-dependent steady-state photoluminescence (PL) and temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy in the temperature range of -18 to +90 °C and at -196 °C, we explore the thermal responses of these materials. The investigation reveals reversible phase transitions occurring between room temperature (RT) and elevated temperatures, impacting the optical properties and emitting colors of the perovskite compounds. The longer the alkyl chain, the higher the phase transition temperature, attributed to increased conformational disorder and enhanced perovskite symmetry. The decay constants for all compounds are very close in value, which confirms the underlying excited-state dynamics, pointing to contributions primarily from inorganic components across different phases. We anticipate that our results on the detection of phase transitions in 2D perovskites will not only motivate the use of these techniques for detecting phase transitions but also would help to understand their excited states in more details to selectively use them for solar cell and next-generation display technologies.

Electrically tunable non-radiative lifetime in WS2/WSe2 heterostructures.

Van der Waals heterostructures based on transition metal dichalcogenides (TMDs) have emerged as excellent candidates for next-generation optoelectronics and valleytronics, due to their fascinating physical properties. The understanding and active control of the relaxation dynamics of heterostructures play a crucial role in device design and optimization. Here, we investigate the back-gate modulation of exciton dynamics in a WS2/WSe2 heterostructure by combining time-resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS) at cryogenic temperatures. We find that the non-radiative relaxation lifetimes of photocarriers in heterostructures can be electrically controlled for samples with different twist-angles, whereas such lifetime tuning is not present in standalone monolayers. We attribute such an observation to doping-controlled competition between interlayer and intralayer recombination pathways in high-quality WS2/WSe2 samples. The simultaneous measurement of TRPL and TAS lifetimes within the same sample provides additional insight into the influence of coexisting excitons and background carriers on the photo-response, and points to the potential of tailoring light-matter interactions in TMD heterostructures.

Gamma-ray dose threshold for MAPbI3 solar cells.

In this work, we report on the effects observed in MAPbI3 polycrystalline films and solar cells under moderate gamma-ray doses of 3-21 kGy. We applied several instrumental techniques such as photoluminescence spectroscopy, time-resolved photoluminescence, Suns-VOC measurement, and impedance spectroscopy to characterize exposed samples. We observed a nonlinear dependency of such characteristics as PL intensity, career lifetime, ideality factor, and recombination resistance on the exposure dose. Small doses of 3-5 kGy annihilate some of the defect centers in the material, which results in improved carrier extraction and prolonged carrier lifetime, while with larger doses of 10 kGy and above, nonradiative recombination becomes predominant. In this way, we revealed a gamma-ray threshold for MAPbI3 films of around 10 kGy, above which it is not recommended to exploit this material. In space environment, yearly doses rarely exhibit 0.1 kGy (10 krad), and the MAPbI3 material has a sufficient margin of safety for space applications. Moreover, this unusual behaviour opens up the opportunity to use gamma-ray sources as an effective method to improve the quality of defective polycrystalline perovskite films before actual exploitation in an ionizing radiation-free environment.

Controlled synthesis of 2D-2D conductive metal-organic framework/g-C3N4 heterojunctions for efficient photocatalytic hydrogen evolution.

Designing photocatalysts with efficient charge separation and electron transport capabilities to achieve efficient visible-driven hydrogen production remains a challenge. Herein, 2D-2D conductive metal-organic framework/g-C3N4 heterojunctions were successfully prepared by an in situ assembly. Compared to pristine g-C3N4, the ratio-optimized Ni-CAT-1/g-C3N4 exhibits approximately 3.6 times higher visible-light H2 production activity, reaching 14 mmol g-1. Through investigations using time-resolved photoluminescence, surface photovoltage, and wavelength-dependent photocurrent action spectroscopies, it is determined that the improved photocatalytic performance is attributed to enhanced charge transfer and separation, specifically the efficient transfer of excited high-energy-level electrons from g-C3N4 to Ni-CAT in the heterojunctions. Furthermore, the high electrical conductivity of Ni-CAT enables rapid electron transport, contributing to the overall enhanced performance. This work provides a feasible strategy to construct efficient dimension-matched g-C3N4-based heterojunction photocatalysts with high-efficiency charge separation for solar-driven H2 production.

Photoluminescent Layered Crystal Consisting of Anderson-Type Polyoxometalate and Surfactant toward a Potential Inorganic-Organic Hybrid Laser.

The hybridization of inorganic and organic components is a promising strategy to build functional materials. Among several functions, luminescence is an important function which should be considered for practical usage. Inorganic-organic hybrid luminescent materials have been investigated as phosphors, sensors, and lasers. Organic luminescent centers such as dye molecules have often been hybridized with inorganic matrices. Polyoxometalate anions (POMs) are effective inorganic luminescent centers due to their luminescent properties and structural designability. However, most luminescent POM components are limited to lanthanide-based POMs. In this report, a photoluminescent inorganic-organic hybrid crystal based on a non-lanthanide POM was successfully synthesized as a single crystal. Anderson-type hexamolybdochromate ([CrMo6O18(OH)6]3-, CrMo6) anion exhibiting emission derived from Cr3+ was utilized with n-dodecylammonium ([C12H25NH3]+, C12NH3) surfactant cation to obtain a photoluminescent hybrid crystal. The grown single crystal of C12NH3-CrMo6 comprised a distinct layered structure consisting of inorganic CrMo6 layers and interdigitated C12NH3 layers. In the CrMo6 layers, the CrMo6 anions were associated with water molecules by hydrogen bonding to form a densely packed two-dimensional network. Steady-state and time-resolved photoluminescence spectroscopy revealed that the C12NH3-CrMo6 hybrid crystal exhibited characteristic emission from the CrMo6 anion. Preliminary lasing properties were also observed for C12NH3-CrMo6, which shows the possibility of using the C12NH3-CrMo6 hybrid crystal as an inorganic-organic hybrid laser.

Water-activation-facilitated photocatalytic CO2 reduction by a heterogeneous synergetic single-atomic Zn sites and Pd nanocrystals

Enhancing the photoreduction of carbon dioxide (CO2) to produce valuable chemicals, such as carbon monoxide (CO), is of significant importance. The bottleneck in developing advanced catalysts lies in the rational design of catalytic centers with efficient CO2 activation and hydrogen proton production. In this work, palladium nanoparticles (Pd NPs) were successfully modified on UiO-66-NH2-Zn SAs (UN-Zn SAs) by a photo-deposition method. The optimized photocatalyst, 4 Pd NPs-UN-0.7Zn SAs, demonstrated exceptional performance in converting CO2 to CO, exhibiting approximately 1.4-fold and 7.8-fold enhancements compared to UN-0.7Zn SAs and UN, respectively. A combination of time-resolved photoluminescence spectroscopy and electron paramagnetic resonance results unveiled that the introduction of Pd NPs effectively promoted the photogenerated charge separation and facilitated water activation to produce more hydrogen protons. The generated hydrogen protons were subsequently coupled with nearby CO2 that was activated by Zn SAs, thus achieving the facile water-activation-induced CO2 reduction process. Theoretical calculations further confirmed that the heterogeneous synergistic effects between Zn SAs and Pd NPs lowered the energy barrier for the rate-limiting step of CO2 conversion to COOH* , thereby achieving efficient photocatalytic CO2 reduction reactions. This work provides a new perspective on the development of efficient catalysts for the effective conversion of CO2.

Sub-nanosecond free carrier recombination in an indirectly excited quantum-well heterostructure

Nanometer-thick quantum-well structures are quantum model systems offering a few discrete unoccupied energy states that can be impulsively filled and that relax back to equilibrium predominantly via spontaneous emission of light. Here we report on the response of an indirectly excited quantum-well heterostructure, probed by means of time and frequency resolved photoluminescence spectroscopy. This experiment provides access to the sub-nanosecond evolution of the free electron density, indirectly injected into the quantum wells. In particular, the modeling of the time-dependent photoluminescence spectra unveils the time evolution of the temperature and of the chemical potentials for electrons and holes, from which the sub-nanosecond time-dependent electron density is determined. This information allows to prove that the recombination of excited carriers is mainly radiative and bimolecular at early delays after excitation, while, as the carrier density decreases, a monomolecular and non-radiative recombination channel becomes relevant. Access to the sub-nanosecond chronology of the mechanisms responsible for the relaxation of charge carriers provides a wealth of information for designing novel luminescent devices with engineered spectral and temporal behavior.

Energy Funneling in a Noninteger Two-Dimensional Perovskite.

Energy funneling is a phenomenon that has been exploited in optoelectronic devices based on low-dimensional materials to improve their performance. Here, we introduce a new class of two-dimensional semiconductor, characterized by multiple regions of varying thickness in a single confined nanostructure with homogeneous composition. This "noninteger 2D semiconductor" was prepared via the structural transformation of two-octahedron-layer-thick (n = 2) 2D cesium lead bromide perovskite nanosheets; it consisted of a central n = 2 region surrounded by edge-lying n = 3 regions, as imaged by electron microscopy. Thicker noninteger 2D CsPbBr3 nanostructures were obtained as well. These noninteger 2D perovskites formed a laterally coupled quantum well band alignment with virtually no strain at the interface and no dielectric barrier, across which unprecedented intramaterial funneling of the photoexcitation energy was observed from the thin to the thick regions using time-resolved absorption and photoluminescence spectroscopy.

Type-I CdSe@CdS@ZnS Heterostructured Nanocrystals with Long Fluorescence Lifetime.

Conventional single-component quantum dots (QDs) suffer from low recombination rates of photogenerated electrons and holes, which hinders their ability to meet the requirements for LED and laser applications. Therefore, it is urgent to design multicomponent heterojunction nanocrystals with these properties. Herein, we used CdSe quantum dot nanocrystals as a typical model, which were synthesized by means of a colloidal chemistry method at high temperatures. Then, CdS with a wide band gap was used to encapsulate the CdSe QDs, forming a CdSe@CdS core@shell heterojunction. Finally, the CdSe@CdS core@shell was modified through the growth of the ZnS shell to obtain CdSe@CdS@ZnS heterojunction nanocrystal hybrids. The morphologies, phases, structures and performance characteristics of CdSe@CdS@ZnS were evaluated using various analytical techniques, including transmission electron microscopy, X-ray diffraction, UV-vis absorption spectroscopy, fluorescence spectroscopy and time-resolved transient photoluminescence spectroscopy. The results show that the energy band structure is transformed from type II to type I after the ZnS growth. The photoluminescence lifetime increases from 41.4 ns to 88.8 ns and the photoluminescence quantum efficiency reaches 17.05% compared with that of pristine CdSe QDs. This paper provides a fundamental study and a new route for studying light-emitting devices and biological imaging based on multicomponent QDs.

Pt(II) Complexes with Tetradentate C^N*N^C Luminophores: From Supramolecular Interactions to Temperature-Sensing Materials with Memory and Optical Readouts.

A series of four regioisomeric Pt(II) complexes (PtLa-n and PtLb-n) bearing tetradentate luminophores as dianionic ligands were synthesized. Hence, both classes of cyclometallating chelators were decorated with three n-hexyl (n = 6) or n-dodecyl (n = 12) chains. The new compounds were unambiguously characterized by means of multiple NMR spectroscopies and mass spectrometry. Steady-state and time-resolved photoluminescence spectroscopy as well quantum chemical calculations show that the effect of the regioisomerism on the emission colour and on the deactivation rate constants can be correlated with the participation of the Pt atom on the excited state. The thermal properties of the complexes were studied by DSC, POM and temperature-dependent steady-state photoluminescence spectroscopy. Three of the four complexes (PtLa-12, PtLb-6 and PtLb-12) present an intriguing thermochromism resulting from the responsive metal-metal interactions involving adjacent monomeric units. Each material has different transition temperatures and memory capabilities, which can be tuned at the intermolecular level. Hence, dipole-dipole interactions between the luminophores and disruption of the crystalline packing by the alkyl groups are responsible for the final properties of the resulting materials.

Selective Metal-Free CO2 Photoreduction in Water Using Porous Nanostructures with Internal Molecular Free Volume.

The conversion of CO2 to a sole carbonaceous product using photocatalysis is a sustainable solution for alleviating the increasing levels of CO2 emissions and reducing our dependence on nonrenewable resources such as fossil fuels. However, developing a photoactive, metal-free catalyst that is highly selective and efficient in the CO2 reduction reaction (CO2RR) without the need for sacrificial agents, cocatalysts, and photosensitizers is challenging. Furthermore, due to the poor solubility of CO2 in water and the kinetically and thermodynamically favored hydrogen evolution reaction (HER), designing a highly selective photocatalyst is challenging. Here, we propose a molecular engineering approach to design a photoactive polymer with high CO2 permeability and low water diffusivity, promoting the mass transfer of CO2 while suppressing HER. We have incorporated a contorted triptycene scaffold with "internal molecular free volume (IMFV)" to enhance gas permeability to the active site by creating molecular channels through the inefficient packing of polymer chains. Additionally, we introduced a pyrene moiety to promote visible-light harvesting capability and charge separation. By leveraging these qualities, the polymer exhibited a high CO generation rate of 77.8 μmol g-1 h-1, with a high selectivity of ∼98% and good recyclability. The importance of IMFV was highlighted by replacing the contorted triptycene unit with a planar scaffold, which led to a selectivity reversal favoring HER over CO2RR in water. In situ electron paramagnetic resonance (EPR), time-resolved photoluminescence spectroscopy (TRPL), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) techniques, further supported by theoretical calculations, were employed to enlighten the mechanistic insight for metal-free CO2 reduction to exclusively CO in water.

Facile aqueous synthesis, structural and luminescence properties for CdTe@FeOOH (Core/Shell)

Seed-mediated growth, a straightforward technique with a low reaction temperature and low cost, was used to create multi-functional core/shell nanoparticles (CdTe@FeOOH). X-Ray Diffraction (XRD), High-resolution transmission electron microscopy (HTEM), Energy-dispersive x-ray (EDX), Fourier-transform infrared techniques (FT-IR), and Differential Thermal Analysis (DTA) were utilized to study the structure and composition of the as-prepared CdTe@FeOOH quantum dots (QDs). Photoluminescence (PL) properties of CdTe@FeOOH (Core/Shell) in an aqueous solution were studied by steady-state and time-resolved PL spectroscopy. The PL lifetime of CdTe@FeOOH QDs was reduced as a consequence of PL quenching. This results in the electron transfer mechanism being efficient, which reduces electron–hole recombination in the core–shell composite. The utilization of core–shell nanospheres of this nature presents numerous potentialities for applications in the field of photocatalysis.

Two- and three-photon absorption in bulk CuI

We report on photoluminescence emission in copper iodide bulk single crystals induced by two- and three-photon absorption around 1.525 eV. These non-linear optical processes are investigated utilizing density-dependent, steady-state, as well as time-resolved photoluminescence spectroscopy as a function of the excitation energy. Using an excitation energy that corresponds to half of the bandgap energy, the observed photoluminescence intensity dependence on the excitation power shows an almost parabolic behavior. By further reduction of the photon energy, a cubic contribution is observable, which increases with decreasing excitation energy. The experimentally observed behavior can be well described by taking into account two- and three-photon absorption. By a simultaneous analysis of the intensity behavior for all used excitation energies, we determined a ratio between the two- and three-photon absorption cross section on the order of σ0(3)/σ0(2)≈10−28 cm2s.

Construction of Co9S8/MoS2/Ni2P double S-scheme heterojunction for enhanced photocatalytic hydrogen evolution

Rational design of composite catalysts with efficient charge separation and transfer and good light collection ability is of great significance to achieve high photoelectrochemical conversion efficiency. In this paper, a Co9S8/MoS2/Ni2P dual S-scheme heterojunction photocatalyst was successfully synthesized by a simple sulfidation and hydrothermal method. The optimized Co9S8/MoS2/Ni2P composite with hierarchical structure, large surface area and numerous reactive active sites showed a good photocatalytic hydrogen evolution rate of 5.69 mmol g−1 h−1, which is 2.28, 4.12 and 10.74 times higher than that of pure Co9S8, MoS2 and Ni2P, respectively. A combination of time-resolved photoluminescence (TRPL) spectroscopy, valence band XPS (VB-XPS), ultraviolet photoelectron spectroscopy (UPS), and density-functional theory (DFT) calculations confirms that the formation of the Co9S8/MoS2/Ni2P double S-scheme greatly facilitates charge migration and spatial separation, and improves the oxidation and reduction ability. This work will provide insights into the development of PBA-based dual S-scheme heterojunction catalysts for photocatalytic hydrogen evolution.

Enhancing perovskite solar cells efficiency through cesium fluoride mediated surface lead iodide modulation

The presence of an excessive amount of lead iodide on the surface of perovskite solar cells (PSCs) is a significant contributing factor that adversely affects the stability of these devices when exposed to continuous light. To address this issue, we developed an effective strategy involving polishing PbI2 on a perovskite surface using CsF. In this study, we investigated the effects of CsF post-treatment on perovskite films and their photovoltaic properties. The results of the time-resolved photoluminescence and ultraviolet photoelectron spectroscopy tests reveal the significant positive impact of our passivation method based on CsF, which reduces the valence band offset between the perovskite and hole transport layers while simultaneously enhancing the carrier interface transport. PSCs treated with CsF exhibited a photoelectric conversion efficiency (PCE) of 24.25% and an increased fill factor (FF) of 81.72%, which surpassed those of the original PSCs (PCE = 22.12% and FF = 77.40%). Furthermore, after aging for over 2500 h at room temperature and in 30 ± 10% humidity, the PCE of the unpacked PSCs reduced to only 42% of the initial value. Furthermore, the devices treated with CsF maintained their impressive performance, with the PCE maintaining optimal levels at 91% of the initial efficiency.