Evolution Of Photoluminescence Research Articles

Light-induced transformation of all-inorganic mixed-halide perovskite nanoplatelets: ion migration and coalescence

When exposed to light, the colloidal perovskite nanoplatelets (NPLs) in the film can fuse into larger grains, and this phenomenon was thought to be closely related to ion migration. However, the available CsPbBr3 NPLs are not conducive to directly distinguishing this hypothesis. Herein, we prepare mixed-halide perovskite CsPbBr2.7I0.3 NPLs by a ligand-assisted reprecipitation method and investigate the photoluminescence evolution of NPLs under laser irradiation. At a low-irradiation intensity, 4.5-monolayer NPLs exhibit blue-shifted photoluminescence peaks due to the migration of iodide ions. Under higher laser fluence, a new photoluminescence component appears in the long wavelength region after the spectral blue shift, which is attributed to the coalescence of NPLs according to transmission electron microscopy analysis. A similar spectral evolution is also observed in 8-monolayer NPLs, while only the spectral blue shift caused by ion migration is detected in cuboidal CsPbBr2.7I0.3 nanocrystals. The use of strong bonding ligands can inhibit the fusion process of the NPLs, but not to impede ion migration, suggesting that fusion requires ligand detachment rather than ion migration. Similar suppression effects can be achieved in a vacuum atmosphere. Moreover, we demonstrate that mixed-halide NPLs can be used to realize anti-counterfeiting applications with superior photosensitivity.

Pressure-induced emission in 0D metal halide (EATMP)SbBr5 by regulating exciton-phonon coupling

Zero-dimensional (0D) hybrid metal halides are considered as promising light-emitting materials due to their unique broadband emission from self-trapped excitons (STEs). Despite substantial progress in the development of these materials, the photoluminescence quantum yields (PLQY) of hybrid Sb–Br analogs have not fully realized the capabilities of these materials, necessitating a better fundamental understanding of the structure-property relationship. Here, we have achieved a pressure-induced emission in 0D (EATMP)SbBr5 (EATMP = (2-aminoethyl)trimethylphosphanium) and the underlying mechanisms are investigated using in situ experimental characterization and first-principles calculations. The pressure-induced reduction in the overlap between the STE states and ground states (GSs) results in the suppression of phonon-assisted non-radiative decay. The photoluminescence (PL) evolution is systematically demonstrated to be controlled by the pressure-regulated exciton-phonon coupling, which can be quantified using Huang-Rhys factor S. Through detailed studies of the S-PLQY relation in a series of 0D hybrid antimony halides, we establish a quantitative structure-property relationship that regulating S value toward 21 leads to the optimized emission. This work not only sheds light on pressure-induced emission in 0D hybrid metal halides but also provides valuable insights into the design principles for enhancing the PLQY in this class of materials.

Metallic photoluminescence of plasmonic nanoparticles in both weak and strong excitation regimes

Abstract The luminescent nature of plasmonic nanoparticles (NPs) has been intensively investigated in recent years. Plasmon-enhanced electronic Raman scattering and the radiation channels of metallic photoluminescence (PL) involving conventional carrier recombinations and emergent particle plasmons are proposed in the past few decades but largely limited to weak excitation regimes. Here, we systematically examine the PL evolution of plasmonic NPs under different excitation power levels. The spectral resonances and chromaticity of PL are investigated within and beyond the scope of geometry conservation. Results indicate the nature of PL in plasmonic NPs could be a process of graybody radiation, including one factor of plasmonic emissivity in the weak excitation regime and the other factor of blackbody radiation in the strong excitation regime. This comprehensive analysis provides a fundamental understanding of the luminescent nature of plasmonic NPs and highlights their potential applications in transient temperature detection at the nanometer scale.

Anisotropic carrier dynamics and laser-fabricated luminescent patterns on oriented single-crystal perovskite wafers

Perovskite materials and their applications in optoelectronics have attracted intensive attentions in recent years. However, in-depth understanding about their anisotropic behavior in ultrafast carrier dynamics is still lacking. Here we explore the ultrafast dynamical evolution of photo-excited carriers and photoluminescence based on differently-oriented MAPbBr3 wafers. The distinct in-plane polarization of carrier relaxation dynamics of the (100), (110) and (111) wafers and their out-of-plane anisotropy in a picosecond time scale were found by femtosecond time- and polarization-resolved transient transmission measurements, indicating the relaxation process dominated by optical/acoustic phonon interaction is related to photoinduced transient structure rearrangements. Femtosecond laser two-photon fabricated patterns exhibit three orders of magnitude enhancement of emission due to the formation of tentacle-like microstructures. Such a ultrafast dynamic study carried on differently-oriented crystal wafers is believed to provide a deep insight about the photophysical process of perovskites and to be helpful for developing polarization-sensitive and ultrafast-response optoelectronic devices.

Synthesis-Kinetics of Violet- and Blue-Emitting Perovskite Nanocrystals with High Brightness and Superior Stability toward Flexible Conversion Layer.

The low photoluminescence (PL) efficiency and unstable features of small blue-emitting CsPbX3 nanocrystals (NCs) greatly limit their applications in optoelectronics field. Herein, the synergistic and post-treatment kinetics are studied to create highly bright and anomalous stable violet (peak position of ≈408nm) and blue (peak position of ∼ 466nm) emitting perovskite NCs. Ligand and ion exchange mechanism are systematic studied by the evolution of absorption, PL, and fluorescence lifetime to evaluate ligand bonding, defect engineering, and non-radiative recombination. Didodecyl dimethyl mmonium chloride (DDAC) and CuX2 post-synergistictreatment created DDAC-CsPbCl3-CuCl2 and DDAC-CsPbCl3-CuBr2 NCs that remained the phase composition, morphology, and size of CsPbCl3 NCs. The PL efficiencies are drastically increased to 42 and 85% for violet- and blue-emitting NCs, respectively. The stability test indicated that the NCs enable against various harsh conditions (e.g., ultraviolet light irradiation and heat-treatment). The NCs retained their initial PL efficiency after 2 months under ambient conditions and UV light irradiation. These NCs also exhibited high stability after heat-treatment at 120°C. The emitting NCs embedded in flexible films still revealed bright PL and high stability, suggesting current results provide a new avenue for the application in the field of optoelectronics.

Time-dependent multicolor evolution of photoluminescence and afterglow in lanthanide-doped gallate

The general stimulus-responsive color modulation method requires both strict control of microstructures and continuous adjustment of specific stimuli conditions to externally manipulate emission color for scientific research and applications. Herein, by hierarchically utilizing the energy transfer processes from traps to luminescent centers, a possible mechanism for the dynamic multicolor evolution in a single material is proposed. This mechanism explains the fundamental principle of the dynamic photochromic behavior and afterglow color variation. A series of Sr3Ga4O9:Ln3+ (SGO, Ln = Dy, Tb, Pr, Sm) phosphors with dynamic photoluminescence (PL) and afterglow color that change over time under a fixed stimulus were developed using the mechanism as a basis for experimental design. The PL dynamically transformed from orange, green or red to blue under 254 nm irradiation in seconds and reversed from blue to orange, green or red after the excitation source was removed. Finally, based on distinctive luminescence properties, their potential application in optical encryption and optical anticounterfeiting was also methodically investigated.

Ultra-sensitive luminescent ratiometric thermometry based on matrix energy transfer in Dy3+ doped CaWO4 phosphors

In this paper, we report the photoluminescence (PL) properties of the WO42− group and Dy3+ ions for a new highly sensitive luminescent ratiometric thermometry in Dy3+ doped CaWO4 (CaWO4:Dy) phosphors. Under 259 nm excitation, the violet emission from the WO42− group, as well as the blue and yellow emissions of Dy3+ from the transitions of thermally coupled energy levels (TCLs) of 4I15/2 and 4F9/2, were both investigated, which ascribed to an energy transfer from WO42− to Dy3+. The thermal-enhanced PL of Dy3+ and the thermal-quenched PL of WO42− were detected with increasing temperature. By taking advantage of the opposite temperature-dependent PL evolution, a new thermometry strategy was proposed based on the luminescence intensity ratio (LIR) between Dy3+ and WO42− having a quantitative relationship with temperature. The maximum absolute and relative temperature sensing sensitivities were as high as 449.52*10−4 K−1 and 21.03% K−1 at 300 K, respectively, which were superior to those reported previously for traditional TCLs-based LIR thermometry. This study demonstrated the potential of CaWO4:Dy phosphors in temperature sensing and provided guidance for constructing an effective ratiometric thermometry strategy with excellent and reliable sensing performance.

Nonlinear Photoluminescence in Gold Thin Films

Promising applications in photonics are driven by the ability to fabricate crystal-quality metal thin films of controlled thickness down to a few nanometers. In particular, these materials exhibit a highly nonlinear response to optical fields owing to the induced ultrafast electron dynamics, which is however poorly understood on such mesoscopic length scales. Here, we reveal a new mechanism that controls the nonlinear optical response of thin metallic films, dominated by ultrafast electronic heat transport when the thickness is sufficiently small. By experimentally and theoretically studying electronic transport in such materials, we explain the observed temporal evolution of photoluminescence in two-pulse correlation measurements that we report for crystalline gold flakes. Incorporating a first-principles description of the electronic band structure, we model electronic transport and find that ultrafast thermal dynamics plays a pivotal role in determining the strength and time-dependent characteristics of the nonlinear photoluminescence signal, which is largely influenced by the distribution of hot electrons and holes, subject to diffusion across the film as well as relaxation to lattice modes. Our findings introduce conceptually novel elements ruling the nonlinear optical response of nanoscale materials, while suggesting additional ways to control and leverage hot carrier distributions in metallic films.

Exciton fine structure of a single highly anisotropic CsPbBr3 nanocrystal

We measured the photoluminescence (PL) of single CsPbBr3 nanocrystals (NCs) that have a highly anisotropic shape and orthorhombic crystal phase. As the thickness of these NCs is much more smaller than the other two dimensions, they are also called nanoplatelets (NPLs). We obtain PL spectra characterized by doublets separated in energy by about 2 meV in average and showing orthogonal and linearly polarized polar lines. We identified these doublets as the two bright-exciton states of the exciton fine structure contained in the plane of the NPLs. By a comparison between theory and experiments, we were able to obtain fundamental parameters as tetragonal and orthorhombic crystal field. We measured and analysed the time-resolved PL evolution as a function of temperature of small ensemble of NPLs. We thus succeed at framing the experimental value of the bright-dark exciton splitting (5-7meV) that is slightly smaller than the theoretical value.

Toward the energy optimization of smart lighting systems through the luminous potential of photoluminescence

Lighting-related energy consumption has recently increased due to the significant expansion of buildings’ floor area and the growing demand for lighting coming from the occupants. With the aim of mitigating the global warming phenomenon by reducing the impacts of fossil fuels-powered systems, the lighting sector has faced a continuous renovation through years, up to the modern and efficient light emitting diodes (LEDs) solutions. Moving from the basic principle of phosphor-converted LEDs, consisting in the combination of phosphor materials to the radiation emitted by a LED chip, the present research investigates the response of photoluminescent pigments with different absorption and emission spectra as a function of the specific spectral distribution of the excitation source. The evolution of photoluminescence is later investigated during both the charging and the decay phase. The performance of each “pigment–lamp” pair is characterized using a spectroradiometer and a specially designed experimental equipment. Results demonstrate how a yellow-emitting material is more suitable than others for lighting purposes, providing for the highest luminance values (up to 2.61 cd/m2) and the longest decay time (>5 h). Moreover, the fastest activation of luminescence due to UV radiation (after 20–60 min of exposure) does not guarantee the most persistent and intense performance of the samples.

Properties of Compacts from Mixtures of Calcium Fluoride Micro- and Nanopowders

In this work, compacts from mechanical mixtures of CaF2 micron powder (MCP) with CaF2 nanopowder (NP) additives were produced, with mass ratios of the mixture components ranging from 10:0.125 to 10:1, respectively, using magnetic pulse (MP) and static pressing (SP) methods. The effects of pressure (Pp) and pressing temperature (Tp), concentration and phase composition of the additive on the density and color of compacts were studied, taking into account the properties of the initial components of the mixtures. The evolution of pulsed cathodoluminescence (PCL) spectra and photoluminescence (PL) of compacts from pure powders and their mixtures depending on Pp, Tp and characteristics of initial CaF2 NP was also studied. A new near-infrared (NIR) band associated with fluoride vacancies was discovered with a maximum at ~765 nm in PCL spectra of compacts produced by MP at a temperature of 425 °C. A blue band at 435 nm associated with impurity oxygen vacancies in the CaF2 lattice was found in PL spectra compacts of pure NP and powder mixtures. The density of compacts of pure NP and MCP reached 89% of the theoretical density, and the density of compacts of mixtures did not exceed 78%. The defective structure and phase composition of the CaF2 NP had a decisive effect on the luminescent properties of compacts from mixtures of micro- and nanopowders.

Optical characteristics of bilayer decoupling MoS2 grown by the CVD method.

Study of exciton recombination process is of great significance for the optoelectronic device applications of two-dimensional transition metal chalcogenides (TMDCs). This research investigated the decoupling MoS2 structures by photoluminescence (PL) measurements. First, PL intensity of the bilayer MoS2 (BLM) is about twice of that of the single layer MoS2 (SLM) at low temperature, indicating no transition from direct bandgap to indirect bandgap for BLM due to the decrease of interlayer coupling which can be shown by Raman spectra. Then, the localized exciton emission appears for SLM at 7 K but none for BLM, showing different exciton localization characteristics. The PL evolution with respect to the excitation intensity and the temperature further reveal the filling, interaction, and the redistribution among free exciton states and localized exciton states. These results provide very useful information for understanding the localized states and carrier dynamics in BLM and SLM.

Deep investigation of phonon induced photoluminescence evolution of YAG:Cr transparent ceramics at a wide temperature range

• In this study, we focused mainly on the vibronic coupling induced luminescence behavior of Cr 3+ doped YAG deep-red transparent ceramics, and two highlights have been listed as below. • Room-temperature PL excitation spectrum was experimentally recorded, which was further theoretically simulated to reveal the underlying electron-phonon coupling interaction with giving different Huang-Rhys factor values. • The dominant phonon energy involving in the vibronic coupling process was determined to be 57 meV which was consistent with that of the fitting work on temperature-dependent PL integrated intensity.

Defect-control electron transport behavior of gallium nitride/silicon nonplanar-structure heterojunction

Compared with a traditional heterojunction, a nonplanar-structure heterojunction can reduce the problems caused by a lattice mismatch through a three-dimensional stress release mechanism, which will be helpful for promoting the performance and stability of related devices. In this paper, we report our study on the electron transport behavior of a gallium nitride (GaN)/silicon (Si) heterojunction with nonplanar-structure interface, which was prepared through growing GaN on a hierarchical structure, Si nanoporous pillar array (Si-NPA). To clarify the electron transport mechanism and promote the device performance, annealing treatment in ammonia atmosphere was carried out to as-prepared GaN/Si-NPA. The formation of the heterojunction was verified by the typical rectification behavior observed in both as-prepared and annealed samples. After annealing treatment, a lower turn-on voltage, a smaller reverse saturation current density, a larger forward current density and a higher reverse breakdown voltage were obtained, which indicate the promotion of the heterojunction performance. By comparatively studying the spectrum evolution of photoluminescence before and after annealing treatment, the underlying mechanism is clarified as the variation of the type and density of point defects such as gallium vacancy (V Ga), oxygen substitutional impurity (ON), and their complex defect V Ga−ON in GaN. The results illustrate an effective defect-control strategy for optimizing the performance of GaN/Si heterojunction optoelectronic devices.

Defect-induced distinct exciton-exciton interactions in WS2 monolayers

The optoelectronic properties of atomically thin transition metal dichalcogenides (TMDs) are largely influenced by defect populations (DPs). In this work, we fabricate WS2 monolayers with different DPs by varying the fabrication methods and further reveal their distinct exciton-exciton interactions. Steady-state photoluminescence (PL) experiments show that the monolayer with the lowest DP shows optimal PL intensity at low excitation power; however, it is overtaken and significantly surpassed by monolayers with higher DPs at high excitation powers. Excitation-power-dependent experiments demonstrate that these monolayers exhibit distinct PL saturation behaviors with the threshold power differing by four orders of magnitude. Combined with in situ PL imaging and time-resolved PL experiments, we attribute such PL evolution discrepancies to the different DPs within these monolayers, which largely influence the exciton diffusion behavior and subsequently bring about distinct nonradiative exciton-exciton annihilations (EEAs). Valley polarization experiments are further employed to re-examine the DPs of these monolayers. This work reveals the distinct PL behaviors and underlying exciton dynamics in TMD monolayers with different DPs, which can largely facilitate the engineering of relevant high-performance devices for practical applications.

Deep blue photoluminescence and optical gain from sodium-doped carbon dots

The photoluminescence (PL) from carbon dots can be dramatically modified through the engineering of surface oxygen-containing functional groups. Despite the recent surge of interest in the non-metal doping of carbon dots, the photo-physical properties of metal-doped carbon dots remain unexplored. Here, we report the highly efficient deep blue PL and amplified spontaneous emission from sodium (Na)-ion-doped carbon dots. Based on the structure- and temperature-dependent PL and the temporal evolution of PL, we reveal that the introduction of Na ions into carbon dots not only passivate the electronegative oxygen related defects but also create new Na–O related defect centers in the band gap, leading to deep blue PL with dramatically reduced bandwidth and significantly enhanced quantum yield. We establish a luminescence model with four energy levels to interpret the PL dynamics of Na+-doped carbon dots. We demonstrate for the first time the ASE from such carbon dots, which exhibit a net optical gain coefficient of 47.8 cm−1 at 440 nm. Our results open new horizons for improving the PL of carbon dots and pave the way for realizing optoelectronic devices based on carbon dots.

Photoluminescence Lightening: Extraordinary Oxygen Modulated Dynamics in WS2 Monolayers.

Transition metal dichalcogenide monolayers exhibit ultrahigh surface sensitivity since they expose all atoms to the surface and thereby influence their optoelectronic properties. Here, we report an intriguing lightening of the photoluminescence (PL) from the edge to the interior over time in the WS2 monolayers grown by physical vapor deposition method, with the whole monolayer brightened eventually. Comprehensive optical studies reveal that the PL enhancement arises from the p doping induced by oxygen adsorption. First-principles calculations unveil that the dissociation of chemisorbed oxygen molecule plays a significant role; i.e., the dissociation at one site can largely promote the dissociation at a nearby site, facilitating the photoluminescence lightening. In addition, we further manipulate such PL brightening rate by controlling the oxygen concentration and the temperature. The presented results uncover the extraordinary surface chemistry and related mechanism in WS2 monolayers, which deepens our insight into their unique PL evolution behavior.

Omnibearing Interpretation of External Ions Passivated Ion Migration in Mixed Halide Perovskites.

Fundamental understanding of ion migration inside perovskites is of vital importance for commercial advancements of photovoltaics. However, the mechanism for external ions incorporation and its effect on ion migration remains elusive. Herein, taking K+ and Cs+ co-incorporated mixed halide perovskites as a model, the impact of external ions on ion migration behavior has been interpreted via multiple dimensional characterization aspects. The space-effect on phase segregation inhibition has been revealed by the photoluminescence evolution and in situ dynamic cathodoluminescence behaviors. The plane-effect on current suppression along grain boundary has been evidenced via visualized surface current mapping, local current hysteresis, and time-resolved current decay. And the point-effect on activation energy incremental for individual ions has been also probed by cryogenic electronic quantification. All these results sufficiently demonstrate the passivated ion migration results in the eventually improved phase stability of perovskite, of which the origin lies in various ion migration energy barriers.

Spherical red-emitting X1-Y2SiO5:Eu and α-Y2Si2O7:Eu phosphors with high color purity: The evolution of morphology, phase and photoluminescence upon annealing

Spherical silicate phosphor has widespread application in display or lighting industry. By using spherical silica particles as the core, we successfully prepared shell-thickness-controllable core/shell spherical particles, which can then be controllably converted to spherical X1-Y2SiO5:Eu (YSO:Eu) and α-Y2Si2O7:Eu (YPS:Eu) particles upon annealing. Even after 1000–1200 °C annealing, both X1-YSO:Eu and α-YPS:Eu particles still had the spherical shape and narrow size distribution. The shell thickness and annealing temperature synergistically controlled the phase evolution upon annealing. Thinner shell of around 30 nm, having extremely high chemical reaction activity, promoted the formation of spherical X1-YSO:Eu particles at 1000 °C and α-YPS:Eu particles at 1100 °C. Shell of around 85 nm produced spherical X1-YSO:Eu particles at 1100 °C and α-YPS:Eu particles at 1200 °C. Thicker shell also promoted the formation of spherical particles, but these particles usually had the mixed silicate phases. The phase-dependent photoluminescence properties were investigated in detail. Both spherical X1-YSO:Eu and α-YPS:Eu phosphors emitted reddish-orange color, with nearly 100% color purity and dominant wavelength of around 601 nm, which would absorb great attention for the application in display or lighting devices.

Graphene-induced lasing mode tailoring in GaN floating microring cavity

Due to its unique properties, such as high electron and thermal conductivities, graphene has attracted much attention, especially in its applications to optoelectronic devices. Herein, graphene is used in the tailoring of the lasing mode from a floated GaN microring cavity. The lasing performances of the GaN cavities are investigated, including its lasing intensity, full width at half maximum (FWHM) and mode properties for samples with or without the presence of graphene. We show that clear side-mode suppression, and an improvement in the photoluminescence (PL), is obtained for samples containing graphene. For samples covered with graphene, the threshold values are reduced by half and the PL intensity is enhanced by over 20 times. Based on observations of the PL evolution in terms of spontaneous emission, the side-mode suppression phenomenon can be attributed to electrons transfer induced PL modulation. Our work reveals a promising strategy for fabricating high-performance optoelectronic device.