Variation Of Photoluminescence Research Articles

Inter-Oligomer Interaction Influence on Photoluminescence in Cis-Polyacetylene Semiconductor Materials.

Semiconducting conjugated polymers (CPs) are pivotal in advancing organic electronics, offering tunable properties for solar cells and field-effect transistors. Here, we carry out first-principle calculations to study individual cis-polyacetylene (cis-PA) oligomers and their ensembles. The ground electronic structures are obtained using density functional theory (DFT), and excited state dynamics are explored by computing nonadiabatic couplings (NACs) between electronic and nuclear degrees of freedom. We compute the nonradiative relaxation of charge carriers and photoluminescence (PL) using the Redfield theory. Our findings show that electrons relax faster than holes. The ensemble of oligomers shows faster relaxation compared to the single oligomer. The calculated PL spectra show features from both interband and intraband transitions. The ensemble shows broader line widths, redshift of transition energies, and lower intensities compared to the single oligomer. This comparative study suggests that the dispersion forces and orbital hybridizations between chains are the leading contributors to the variation in PL. It provides insights into the fundamental behaviors of CPs and the molecular-level understanding for the design of more efficient optoelectronic devices.

Optical Signal Investigation of Monolayer MoS2 Grown Via Glass-Assisted CVD On Patterned Surfaces

Enhancing photoluminescence (PL) in single-layer transition metal dichalcogenides has garnered significant interest, particularly for advancing high-performance 2D electronics and optoelectronics. The combination of surface engineering and contemporary growth methods has provided a platform for investigating optical signals. In this study, we present variations in PL and Raman signals of single-layer MoS2 flakes grown conformally using the glass-assisted CVD method on square-patterned surfaces with varying well depths. PL spectroscopy revealed a systematic and pronounced enhancement in intensities as the valley thickness decreased from 285 nm to 225 nm. Conversely, for the hill regions of the samples, the PL intensity initially increased with decreasing valley thickness and then decreased, despite the hill regions having a constant thickness of 300 nm. On the other hand, PL maps did not exhibit a systematic dependence of intensities on the hill-valley thickness distinction, contrary to expected results based on literature data for similar materials on flat surfaces. The origin of the intensity oscillations was attributed to possible mechanisms, including thickness-dependent interference and strain-related exciton funneling effects. Additionally, Raman measurements revealed irregular variations in intensity in hill regions, dependent on the thicknesses of the underlying SiO2 layers. Furthermore, we observed that the sizes of the flakes increased as the well depths of the underlying patterned surface decreased. This phenomenon might be attributed to alterations in the carrier gas flow pattern and varying temperature gradients between the hills and valleys. These results hold substantial potential to open new avenues for the integration of 2D transition metal dichalcogenides into on-chip electronic and optoelectronic devices.

Quantifying photoluminescence variability in monolayer molybdenum disulfide films grown by chemical vapour deposition

Monolayer molybdenum disulfide (MoS2) is a promising candidate for inclusion in optoelectronic technologies, owing to its two-dimensional (2D) nature and resultant novel photoluminescence (PL). Chemical vapour deposition (CVD) is an important method for the preparation of large-area films of monolayer MoS2. The PL character of as-prepared monolayer MoS2 must be well understood to facilitate detailed evaluation of any process-induced effects during device fabrication. We comparatively explore the PL emission from four different commercially available CVD-grown MoS2 monolayer films. We characterize the samples via Raman and PL spectroscopy, using both single-spot and mapping techniques, while atomic force microscopy (AFM) is applied to map the surface structure. Via multipeak fitting, we decompose the PL spectra into constituent exciton and trion contributions, enabling an assessment of the quality of the MoS2 monolayers. We find that the PL character varies significantly from sample to sample. We also reveal substantial inhomogeneity of the PL signal across each individual MoS2 film. We attribute the PL variation to non-uniform MoS2 film morphologies that result from the nucleation and coalescence processes during the CVD film development. Understanding the large variability in starting PL behaviour is vital to optimize the optoelectronic properties for MoS2-based devices.

Investigation of structural and optical properties of Ge/Al2O3 multilayer thin films fabricated using electron beam evaporation

The present work reports the structural, optical, and luminescence properties of Ge/Al2O3 multilayers prepared using electron beam evaporation followed by conventional furnace annealing at 600 and 700 °C. XRD and Raman spectroscopy results reveal the formation of germanium nanocrystals (Ge NCs) in Al2O3 matrix after annealing. The estimated band gap of the pristine sample is 0.78 eV. The band gap of film annealed at 600 °C is 1.34 eV, which is decreased with an increase in annealing temperature. The obtained band gap confirms the size-dependent quantum confinement effect in Ge/Al2O3 samples. A broad band at 624 nm is observed in photoluminescence (PL) spectra from the multilayer structure. PL spectra show an enhancement in the peak intensity with an increase in annealing temperature. The average decay lifetime of charge carriers for the pristine and annealed samples is in the nanosecond range. The origin of PL emission and variation of band gap with annealing temperature is discussed in detail. The correlation of optical results with structural properties, and the possible use of Ge NCs embedded in Al2O3 matrix in optoelectronic devices are reported.

Iridium(III) solvent complex-based electrogenerated chemiluminescence and photoluminescence sensor array for the discrimination of bases in oligonucleotides

Development of rapid and sensitive method for the discrimination of bases in oligonucleotides is of great importance in clinical diagnosis. Here, we demonstrate the first case of single iridium(III) solvent complex-based electrogenerated chemiluminescence (ECL) and photoluminescence (PL) sensor array for the discrimination of bases in oligonucleotides. One iridium (III) solvent complex ([Ir(ppy)2(DMSO)Cl], ppy = 2-phenylpyridine, probe 1) was designed as both ECL and PL probe while five bases (guanine, adenine, cytosine, thymine and uracil) were chosen as analytes. Two-element sensor array was built for the discrimination of five bases based on the fingerprint response of probe 1 to bases via coordination interactions. The combination of unique ECL and PL variations with principal component analysis was applied for the quantitative analysis of five bases in a linear range of 1.0 μM–10 μM and for the effective discrimination of individual base, the mixture of bases and oligonucleotides. Moreover, the sensor array was successfully applied to discriminate different mismatched ss-DNAs from HIV gene (a fully-matched ss-DNA), even at single-base difference. This work demonstrates that the sensor array using single iridium (III) solvent complex is a promising approach for the discrimination of bases with good sensitivity and simpleness in clinical diagnosis.

Yellow‐Emissive Carbon Dots with High Solid‐State Photoluminescence

AbstractCarbon dots (CDs) present an enticing prospect for a variety of optical applications relying on their high photoluminescence (PL) quantum yield (QY). Herein, the synthesis, optical properties, structural characterizations, density‐functional theory (DFT) calculations, and potential applications of yellow‐emissive CDs (Y‐CDs) with ultra‐high PL QY are reported. Solvothermal treatment of citric acid and urea in toluene, followed by column chromatography, produces Y‐CDs exhibiting excitation‐independent PL emission at 553 nm with a high solution PL QY of 92%. A variety of optical and structural characterizations and DFT theoretical calculations are implemented to confirm the general structure and fluorescence origin of Y‐CDs, conjugated sp2‐carbon domains (fused rings) with edge groups. Significantly, transparent Y‐CDs/acrylic resin films with strong solid‐state emissions are fabricated. The Y‐CD films exhibit a high fluorescence with PL QY of 98%, good PL stability (no PL variation under continuous irradiation for 180 h), and large Stokes shift (129 nm). The potential applications of Y‐CDs for luminescent solar concentrators as well as yellow phosphors for lighting are also demonstrated. These findings thus promote the development of high‐performance CDs and their optoelectronic applications.

Multimodal Microscale Imaging of Textured Perovskite-Silicon Tandem Solar Cells.

Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficiencies beyond the limits of single-junction cells. However, the local light-matter interactions of the perovskite material embedded in this pyramidal multijunction configuration, and the effect on device performance, are not well understood. Here, we characterize the microscale optoelectronic properties of the perovskite semiconductor deposited on different c-Si texturing schemes. We find a strong spatial and spectral dependence of the photoluminescence (PL) on the geometrical surface constructs, which dominates the underlying grain-to-grain PL variation found in halide perovskite films. The PL response is dependent upon the texturing design, with larger pyramids inducing distinct PL spectra for valleys and pyramids, an effect which is mitigated with small pyramids. Further, optimized quasi-Fermi level splittings and PL quantum efficiencies occur when the c-Si large pyramids have had a secondary smoothing etch. Our results suggest that a holistic optimization of the texturing is required to maximize light in- and out-coupling of both absorber layers and there is a fine balance between the optimal geometrical configuration and optoelectronic performance that will guide future device designs.

Multimodal Nanoscopic Study of Atomic Diffusion and Related Localized Optoelectronic Response of WS2/MoS2 Lateral Heterojunctions.

The atomic diffusion in transition metal dichalcogenides (TMDs) van der Waals heterojunctions (HJs) strongly modifies their optoelectronic properties in the nanoscale. However, probing such localized properties challenges the spatial resolution and the sensitivity of a variety of analytic tools. Herein, a multimodal nanoscopy (based on tip enhanced Raman spectroscopy (TERS) and photoluminescence (TEPL)) combined with the Kelvin probe force microscopy (KPFM) method was used to probe such nanoscale localized optoelectronic properties induced by atomic diffusion. Chemical vapor deposition (CVD)-grown lateral bilayer (2L) WS2/MoS2 HJs were imaged with a spatial resolution better than 40 nm via TERS and TEPL mapping by using intrinsic Raman and photoluminescence (PL) peaks. The contact potential difference (CPD), capacitance, and PL variation in a nanoscale vicinity of the HJ interface can be correlated to the local stoichiometry variation determined by TERS. The diffusion coefficients of W and Mo were obtained to be ∼0.5 × 10-12 and ∼1 × 10-12 cm2/s, respectively, by using Fick's second law. The obtained results would be useful to further understand the localized optoelectronic response of the TMDs HJs.

Luminescent Yb3+,Er3+-Doped α-La(IO3)3 Nanocrystals for Neuronal Network Bio-Imaging and Nanothermometry.

Dual-light emitting Yb3+,Er3+-codoped α-La(IO3)3 nanocrystals, known to exhibit both second harmonic signal and photoluminescence (PL), are evaluated as optical nanoprobes and thermal sensors using both conventional microscopes and a more sophisticated micro-PL setup. When loaded in cortical and hippocampal neurons for a few hours at a concentration of 0.01 mg/mL, a visible PL signal arising from the nanocrystals can be clearly detected using an epifluorescent conventional microscope, enabling to localize the nanocrystals along the stained neurons and to record PL variation with temperature of 0.5% K−1. No signal of cytotoxicity, associated with the presence of nanocrystals, is observed during the few hours of the experiment. Alternatively, a micro-PL setup can be used to discriminate the different PL lines. From ratiometric PL measurements, a relative thermal sensitivity of 1.2% K−1 was measured.

Reversible temperature-dependent photoluminescence in semiconductor quantum dots for the development of a smartphone-based optical thermometer.

Photoluminescence (PL) intensity-based non-contact optical temperature sensors are in great demand due to their non-contact nature, rapid response, sensitivity, as well as thermal and chemical stability at different environmental conditions. However, herein, reversible temperature dependent PL emission quenching properties of chemically synthesized Mn2+-doped ZnS QDs (MZQDs) have been advantageously utilized for achieving the development of a smartphone-based optical thermometer. The temperature dependent variations of PL have been studied by taking MZQDs in various forms, such as in aqueous dispersion, powder form, and a polymer-encapsulated thin film. The origin of the PL quenching of MZQD in the polymer film has been cross-verified through temperature-dependent electrical conductivity measurement and the movement of charge carriers has also been confirmed by the first-principles DFT simulation. Through thermal cycling experiments on QD-encapsulated polymer film and by utilizing an indigenously-developed Android App based on color coordinates, a novel smartphone-based colorimetric imaging method for the measurement of temperature has been demonstrated in this work. The synthesized smart QDs might be suitable candidates for temperature sensing and the colorimetric thermometer probe may be utilized in various photonics applications as a smart optical sensor for daily life applications.

The role of luminescence self-absorption in photocatalytic properties of self-assembled ZnS nanocrystals

In this work, we used a template-free method to synthesize zinc sulfide (ZnS) materials. The photocatalytic performances of the samples were evaluated, and the physical natures of the ZnS nanocrystals were determined by photoluminescence (PL). Interestingly, the temperature-dependent PL variations of the samples were very different in connection with the synthesis condition. Combined with a series of catalytic experiments, it suggested that the intrinsic self-absorption of PL and photon recycling play key roles in photocatalysis. This work gives a new idea for understanding the photocatalysis and presents a new strategy to design ZnS related photocatalysts for further applications.

Full Optical Contactless Thermometry Based on LED Photoluminescence

In this work, we report a contactless temperature measurement method based on the photoluminescence (PL) of an AlInGaP light-emitting diode (LED) structure under open-circuit conditions. The proposed method has been evaluated between 90 and 460 K, but a broader temperature range is possible. The system is based on the temperature-dependent PL variation of a commercial AlInGaP LED designed to emit at 630 nm at room $T$ . The sample is placed inside a temperature-controlled cryostat equipped with a transparent glass window. After exciting the sample by means of a green 532-nm laser, the emitted PL spectrum has been collected by an optical fiber and acquired by means of a charge-coupled device (CCD) array spectrometer. In the first part of the work, the most effective calibration is discussed as a function of the properties of the sampled spectrum; both PL emission peak and spectral width have been considered to calculate the optimal estimated temperature. The effect of spectrometer resolution on temperature estimation has been evaluated by comparing and analyzing the results from two spectrometers with different pixel densities. The effects of excitation intensity and angular positioning have also been evaluated. Finally, uncertainty due to calibration, together with measurement accuracy and precision, has been calculated over 50 acquisitions resulting on average in ±1, ±0.7, and ±0.3 K, respectively, on a temperature range of 300 K.

Electro-Photoluminescence Color Change for Deformable Visual Encryption.

Although structural coloring and photoluminescence (PL) have been investigated for radiation-responsive color change, electroluminescence (EL) has not been used for the radiation-responsive system. An electro-photoluminescence (EPL) color change is presented here. The phosphors in the alternating current electroluminescence (ACEL) act simultaneously as electro-luminophores and photo-luminophores. The EPL chromaticity is systematically investigated depending on the ACEL frequency and UV intensity. It is found that the PL variation depending on UV intensity is the mechanism of the EPL color change. It is revealed that EL and PL can be controlled independently in the low electric field so that the EPL chromaticity can be adjusted by a linear combination of the EL color and the PL color. The EPL color-changing device is used as a deformable visual encryption system and a soft skin for a soft robotic rover, imitating the concealment and signaling functions in nature.

Photoluminescent crystalline-assembly of gold nanoclusters: Facile preparation with the mediation of hydroxyl-terminated hyperbranched polyethylenimine and its reversible response to CO2 adsorption and desorption

Stimuli-responsive metal nanoclusters (NCs) are intriguing materials due to their wide applications in photoluminescence (PL) sensors, bioimaging and catalysis. CO2 gas as a green stimulus could arouse the PL variation of certain metal NCs only under high-pressure condition. Metal NCs responding to the normal-pressure CO2 have not been reported yet. In this work, hydroxyl-terminated hyperbranched polyethylenimine (HPEI-OH) was used to mediate the preparation of water-soluble Au NC assemblies (Au NC-As) with crystallization-induced emission (CIE) nature. HPEI-OH played a multifunctional role, such as reducing Au ions, endowing water solubility to Au NC-As, packing thiolated Au NCs closely and enabling CO2-responsiveness to Au NC-As. Controlling the preparation pH at 2.0 or 3.2, red or yellow-emitting Au NC-As with quantum yield of ca. 4.6% or 5.8% were achieved. The as-prepared CIE-type Au NC-As possessed pH-responsive characteristic and strong PL emission was achieved under acidic condition. The CIE nature rendered not only strong PL emission to Au NC-As, but also strong photoelectron emission verified by the X-ray photoelectron spectroscopy. The introduction of Zn2+ ions extended the application range of Au NC-As from acidic condition till to weak basic condition. Under neutral condition, the formed complex of Zn2+/Au NC-As showed reversible PL response to the normal-pressure CO2/N2 cycling through adsorption and desorption process. Finally, the application of the complex in measuring the CO2 concentration of carbonated beverages was presented.

Distinguish the Quenching and Degradation of CH3NH3PbI3 Perovskite by Simultaneous Absorption and Photoluminescence Measurements

Organometal halide perovskites have been one of the most promising alternative semiconductors in photovoltaic and light-emitting devices due to their excellent optoelectronic properties. However, the photophysical processes in these hybrid perovskites are still not fully understood. Photoluminescence (PL) of perovskite materials has been one of the most popular methods to investigate the photophysical processes in these materials. However, the PL signal only provides information of the radiative recombination of the charge carriers, which is not sufficient to understand the full picture of the photophysics. For example, both PL quenching and degradation of the materials cause decrease of PL intensity, which cannot be distinguished by PL. In this work, by simultaneously monitoring the absorption and PL of CH3NH3PbI3 perovskite during the photoexcitation with the assistance of microscopic technology, we followed the PL variation and the absorption change at the same time, allowing us to directly distinguish...

Effect of layer and stacking sequence in simultaneously grown 2H and 3R WS2 atomic layers

In two-dimensional layered materials, layer number and stacking order have strong effects on the optical and electronic properties. Tungsten disulfide (WS2) crystal, as one important member among transition metal dichalcogenides, has been usually prepared in a layered 2H prototype structure with space group P63/mmc () in spite of many other expected ones such as 3R. Here, we report simultaneous growth of 2H and 3R stacked multilayer (ML) WS2 crystals in large scale by chemical vapor deposition and effects of layer number and stacking order on optical and electronic properties. As revealed in Raman and photoluminescence (PL) measurements, with an increase in layer number, 2H and 3R stacked ML WS2 crystals show similar variation of PL and Raman peaks in position and intensity. Compared to 2H stacked ML WS2, however, 3R stacked one always exhibits the larger red (blue) shift of Raman (A1g) peak and the appearance of PL A, B and I peaks at lower energies. Thereby, PL and Raman features depend on not only layer number but also stacking order. In addition, circularly polarized luminescence from two prototype WS2 crystals under circularly polarized excitation has also been investigated, showing obvious spin or valley polarization of these CVD-grown multilayer WS2 crystals.

Process-dependent mechanical and optical properties of nanostructured silicon carbonitride thin films

Amorphous hydrogenated silicon carbonitride (a-SiCN:H) thin films were grown using electron cyclotron resonance chemical vapour deposition using a mixture of methane, nitrogen, and silane as precursors. The origin of the variation of macroscopic properties such as hardness (H), elastic modulus (E), photoluminescence (PL), and the optical band gap was investigated through their correlation with the microscopic features of a-SiCN:H thin films as a function of the process parameters, including the deposition temperature and methane gas flow rate. From a microstructural perspective, the thin films were investigated using x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, elastic recoil detection, transmission electron microscopy, and x-ray diffraction. It is verified that an increase of the substrate temperature resulted in the substitution of hydrogen atoms mainly by carbon atoms, causing the density of the silicon carbide-related structures to increase in the amorphous structure of the a-SiCN:H thin films. Hardness and elastic modulus were found to increase with the deposition temperature and decreased with an increase of the methane gas flow during the deposition, resulting in higher carbon content in the films. The observed changes are ascribed to the reduced density of the weak hydrogen terminated bonds and the variation of the relative bond density of Si–C to Si–N bonds. In addition, the thin films were depth profiled using a slow positron beam to investigate the role of vacancies. The observed increase of the positronium formation with increasing deposition temperature was found to correlate with the variation of PL, where an enhancement of the visible emission originating from carbon-related defects was observed. A set of optimized process parameters to fabricate a-SiCN:H thin films with improved visible emission and hardness properties is suggested.

Tuneable fluorescent, recombination pathways of biogenic Au[sbnd]SiO2 nanospheres from rice husk: An optical study

This work explores the optical properties of amorphous silica (ASO) and gold incorporated silica (AuSiO2) prepared from Rice husk (RH) which is a biomass and also an environment waste. X-ray diffraction study shows the amorphous nature of prepared SiO2 and diffraction peaks related to gold for Au:SiO2 samples. Optical absorption study reveals the surface plasmon resonance (SPR) band centred at 525 nm. Photoluminescence (PL) study expose the light emitting nature of prepared SiO2 and Au:SiO2 at different excitation wavelengths. Variations in PL of SiO2 while incorporating Au is attributed to the structural modifications and metal enhanced fluorescence. Time resolved photoluminescence (TRPL) conducted at different excitation and emission wavelengths show the three exponential emission decay of SiO2 and Au:SiO2 samples.

A- and B-exciton photoluminescence intensity ratio as a measure of sample quality for transition metal dichalcogenide monolayers

The photoluminescence (PL) in monolayer transition metal dichalcogenides (TMDs) is dominated by the recombination of electrons in the conduction band with holes in the spin-orbit split valence bands, and there are two distinct emission features referred to as the A-peak (ground state exciton) and B-peak (higher spin-orbit split state). The intensity ratio of these two features varies widely, and several contradictory interpretations have been reported. In this work, we analyze the room temperature PL from MoS2, MoSe2, WS2, and WSe2 monolayers and identify the underlying cause of observed variations in emission profile. We determine that PL variations arise from differences in the non-radiative recombination associated with defect densities. Therefore, the relative intensities of the A- and B-emission features can be used to qualitatively assess the non-radiative recombination and a low B/A ratio is indicative of low defect density and high sample quality. We also performed polarization-resolved PL measurements. Emission from TMD monolayers is governed by unique optical selection rules which make them promising materials for valleytronic operations. We observe a notably higher valley polarization in the B-exciton relative to the A-exciton. The high polarization is a consequence of the shorter B-exciton lifetime resulting from rapid relaxation of excitons from the B-exciton to the A-exciton of the valence band. Our work clarifies disparities reported in the literature relating to the emission profile and provides a straightforward means to assess sample quality.

Laser exposure induced alteration of WS2 monolayers in the presence of ambient moisture

Photoluminescence (PL) emergence in monolayer transition metal dichalcogenides (TMDs) such as WS2, has been one of the key attractions of such materials. However, there have been many observational contradictions in PL measurements presented in the past literature. This work addresses such issues. Firstly, the observational changes of the flakes’ PL patterns under exposure to various intensities of radiant exposure via laser sources are presented. These experiments show that these changes are a function of radiant exposure. Interestingly, it is observed that PL loss is accompanied by a change of the profile height for WS2 monolayers. In order to explore the fundamental mechanism for PL and height variations, laser irradiation was applied to monolayer WS2 flakes with varying radiant exposure to obtain PL maps, under the absence and presence of oxygen, H2O and nitrogen molecules in the atmosphere. It was seen that, after relatively high radiant exposure (>15 mJ µm−2), the PL pattern loss occurs only in the presence of atmospheric H2O molecules (45% humidity) and is also accompanied by an increase in height. Compositional analysis determined that this height increase was due to the substitution of surface S atoms with sulphate groups. This discovery represents an important step forward in understanding the necessary precautions when investigating optical properties of 2D TMDs in atmospheric conditions, and highlights the need for precise evaluation of the thresholds for radiant exposure at which specific reactions begin to occur. This knowledge is crucial for efficient and effective control of ambient operating conditions for optical characterisation of monolayer WS2 and TMDs in general.