Photoluminescence Intensity Ratio Research Articles

Tailoring optical properties and humidity sensing response of multilayered Tb(Sal)3Phen and Eu(DBM)3Phen complex nanofibres

In this work, we investigate the opto-humidity sensing and colour tuning in polyvinyl alcohol (PVA) polymeric electrospun nanofibres dispersed with Tb(Sal)3Phen (TSP) and Eu(DBM)3Phen (EDP) complexes. Fourier transform infrared spectroscopy, scanning electron microscopy, and photoluminescence analysis were used to thoroughly analyse the structural, morphological, and spectroscopic features of the synthesised nanofibres in single and stacked multilayer nanofibres. A spectroscopic analysis was performed on all configuration schemes. Under UV excitations, the synthesised TSP complex and EDP complex inhibited nanofibres yield green and red emission corresponding to 5D4→7Fj (j = 4, 5, 6) and 5D0→7Fj (j = 0, 1, 2, 3) transitions attributed to Tb3+ and Eu3+ ions, respectively. The EDP nanofibres presented better UV radiation absorption than TSP, which was attributed to higher absorptivity of DBM than Sal. Additionally, the photoluminescence intensity ratio of characteristic emission peaks i.e. 544 nm (5D4→7F5)/615 nm (5D0→7F2) is function of humidity exposure and excitation wavelength. The stacked multilayer nanofibres exhibit good response and recovery times, 35 and 52 s, negligible hysteresis, and cyclic stability.

Variable Photoluminescence Intensity Ratio with the Excitation Wavelength in Eu3+-Doped Perovskite-Type Alkaline Earth Zirconates─Possibility of a Unique Visualization of Ultraviolet Light.

Photoluminescence (PL) spectra arising from 4f-4f dipole transitions of Eu3+ doped at both the A and B sites of perovskite-type alkaline earth zirconates obtained at various excitation wavelengths were evaluated. Changes in the excitation wavelength caused obvious differences in the PL intensity ratio of the induced electric dipole (ED) 5D0 → 7F2 transition to the magnetic dipole (MD) 5D0 → 7F1 transition for Eu3+-doped SrZrO3 and CaZrO3, in which only the B site had a center of symmetry. Two charge transfer (CT) bands associated with the excitation of Eu3+ were observed in the spectra of these oxide samples, which arose from the difference in the electronic structure of the Eu-O coordination at the A and B sites. We conclude that simultaneous Eu3+ substitution at sites with and without a center of symmetry, which have different charge transfer energies, induced the observed novel PL changes with a changing excitation wavelength. Our results may enable the development of a new type of ultraviolet light detector.

Scalable Large-Area 2D-MoS2/Silicon-Nanowire Heterostructures for Enhancing Energy Storage Applications.

Two-dimensional (2D) transition-metal dichalcogenides have shown great potential for energy storage applications owing to their interlayer spacing, large surface area-to-volume ratio, superior electrical properties, and chemical compatibility. Further, increasing the surface area of such materials can lead to enhanced electrical, chemical, and optical response for energy storage and generation applications. Vertical silicon nanowires (SiNWs), also known as black-Si, are an ideal substrate for 2D material growth to produce high surface-area heterostructures, owing to their ultrahigh aspect ratio. Achieving this using an industrially scalable method paves the way for next-generation energy storage devices, enabling them to enter commercialization. This work demonstrates large surface area, commercially scalable, hybrid MoS2/SiNW heterostructures, as confirmed by Raman spectroscopy, with high tunability of the MoS2 layers down to the monolayer scale and conformal MoS2 growth, parallel to the silicon nanowires, as verified by transmission electron microscopy (TEM). This has been achieved using a two-step atomic layer deposition (ALD) process, allowing MoS2 to be grown directly onto the silicon nanowires without any damage to the substrate. The ALD cycle number accurately defines the layer number from monolayer to bulk. Introducing an ALD alumina (Al2O3) interface at the MoS2/SiNW boundary results in enhanced MoS2 quality and uniformity, demonstrated by an order of magnitude reduction in the B/A exciton photoluminescence (PL) intensity ratio to 0.3 and a reduction of the corresponding layer number. This high-quality layered growth on alumina can be utilized in applications such as for interfacial layers in high-capacity batteries or for photocathodes for water splitting. The alumina-free 100 ALD cycle heterostructures demonstrated no diminishing quality effects, lending themselves well to applications that require direct electrical contact with silicon and benefit from more layers, such as electrodes for high-capacity ion batteries.

Ratiometric fluorescence turn-off probe for highly selective recognition of 2-pentanone based on a Eu3+- functionalized MOF

Here, a Eu3+-functionalized UiO-66 was employed as a probe for the purpose of detecting 2-pentanone. Remarkably, the Eu/Zr-UiO-66 demonstrated a clear “turn-off” response to 2-pentanone with a high degree of selectivity and reusability. The ratio of photoluminescence intensity (I617/I355) in ethanol displayed a linear decrease with increasing 2-pentanone concentration within the range of 0–90 ppm. The present study reports the limit of detection (LOD) for 2-pentanone as 2.51 ppm, which is the first instance of a self-calibrating photoluminescence probe for 2-pentanone utilizing the bimetallic metal–organic framework. Furthermore, FRET was responsible for the fluorescence quenching of 2-pentanone. The sensor system exhibits notable advantages such as facile preparation, high selectivity and excellent repeatability, thereby offering a convenient and reliable means of detecting volatile organic compounds (VOCs) in routine applications.

Determination of Gallium Concentration in Silicon from Low‐Temperature Photoluminescence Analysis

Increasing the understanding of the electronic properties of gallium (Ga) in silicon (Si) used nowadays to manufacture p‐type Si solar cells is of key technological importance. In this contribution, the results of the effect of Ga concentration on the low‐temperature photoluminescence (PL) spectra in crystalline Si are reported. The Ga‐doped Si samples studied have negligible boron concentrations, which can complicate spectral analysis of the bound exciton (BE) lines. The split Ga BE ground state at T = 10 K is analyzed and the PL intensity ratios for the BE to free exciton peaks are compared. By comparing these to known Ga concentrations, derived from capacitance–voltage measurements, an all‐optical (PL) calibration curve for the quantification of Ga concentration in Si is established. The effects of both temperature and excitation power on the PL intensity ratios are also studied. By combining the temperature‐induced changes in the PL intensity ratios with the calibration curve at 10 K, a calibration function has been determined. It is found that the decay rates of the PL intensity ratios as a function of excitation power are independent of the chosen (split) BE peak. The major benefits of this method and its limitations are discussed.

Depth Analyses of Mg‐Acceptor and Si‐Donor Concentrations in GaN by Combining Stepwise Etching and Photoluminescence

The depth profiles of the Mg‐acceptor or Si‐donor concentration in GaN are visualized using stepwise etching and photoluminescence (PL) measurements. Low‐damage etching techniques are applied: multistep‐bias inductively coupled plasma reactive ion etching for p‐type GaN and electrochemical etching for n‐type GaN. To determine the concentration calibration curve, the PL intensity ratios of Mg‐neutral acceptor or Si‐neutral donor‐bound excitons to free excitons are plotted against Mg or Si atomic concentrations in etched GaN epitaxial layers having depth distributions of dopant concentrations, as estimated by secondary ion mass spectrometry. The quantitative accuracy of the calibration curve is ensured by the appropriate sample temperature and excitation density ranges for PL. The stepwise dry etching and the Mg acceptor quantification by PL using this calibration curve are applied to Mg‐ion‐implanted GaN after ultra‐high‐pressure annealing at 1300 °C for 1 min. The obtained depth profile of the Mg‐acceptor concentration is consistent with the Mg atomic concentration profile in the region deeper than the implantation peak, whereas the acceptor concentration in the shallow region is lower than the Mg atomic concentration. This analysis is useful for revealing the site substitution ratio of dopants in GaN materials partially involving inactive dopants.

High-entropy (La0.2Dy0.2Er0.2Yb0.2Y0.2)2Zr2O7 oxide, a potential thermal barrier coating material with photoluminescence property sensitive to pressure

In this work, high-entropy (La0.2Dy0.2Er0.2Yb0.2Y0.2)2Zr2O7 oxide (RZHEO-1600) has been synthesized by using a simple solid-state reaction method and investigated as a promising thermal barrier coating (TBC) material with potential optical pressure sensing application. Compared to oxide-yttria stabilized zirconium (YSZ) and single component zirconates, RZHEO-1600 exhibits excellent thermal stability at 1600 °C and a larger thermal expand coefficient 13.5 × 10−6 K−1 due to the high entropy effect. Its thermal conductivity (1.23–1.41 W m−1 K−1) and Vickers hardness (10 GPa) are lower than YSZ, while its fracture toughness (2.9 MPa m1/2) is much larger. The photoluminescence (PL) emissions of RZHEO-1600 excited by 514 nm laser are assigned to 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ ions. The centre positions of emission peaks initially at 560 and 677 nm are sensitive to pressure, which red shift linearly with pressure coefficients of 0.178 nm/GPa (−5.650 cm−1/GPa) and 0.223 nm/GPa (−4.932 cm−1/GPa), respectively. Interestingly, the PL intensity ratios I677/I658 and I560/I547 are found to be pressure sensitive, which continuously increase during the compression. The findings of this study could serve as a new guide for the development of novel high-entropy oxide TBC materials with optical sensing applications.

Grain-size effect on Cr3+ and F-centres photoluminescence in nanophase MgAl2O4 ceramics

Photoluminescence (PL) spectroscopy of transparent MgAl2O4 spinel ceramics with grain size between 100 and 300 nm was studied at 7 K temperature in the near-IR-VUV range of spectrum with synchrotron radiation excitation. The PL spectra were composed of optical transitions from spatially different regions of the ceramics, which analysis evidenced grain size effect on the emission line-shapes and intensities. In particular, emission of impurity Cr3+ ions, being structured in the crystalline bulk, became broad-band in the grain boundary regions, which was associated with respectively strong and weak local crystalline fields. It was observed that (i) excitons and F centres transfer energy to Cr3+ and (ii) Cr(2Eg)/Cr(4T2g) and F-centres/Cr3+ PL intensity ratios underwent a linear dependence on the grain size.

Calibration of Low‐Temperature Photoluminescence of Boron‐Doped Silicon with Increased Temperature Precision

Low‐temperature photoluminescence spectroscopy enables the determination of the dopant concentration of shallow impurities in silicon. This measurement method is therefore well suited for identifying and analyzing dopants (intentional impurities in silicon). A method is presented which allows the determination of the boron concentration in silicon in a range from to at temperatures from 4.2 to 20 K with increased temperature accuracy. This method requires only one calibration function for the photoluminescence intensity ratio of the boron‐bound exciton and the free exciton . The measurement temperature is obtained from the intrinsic silicon photoluminescence line of free excitons () using a fitting method, which distinguishes the and components of the free exciton peak. The determined calibration function is . The obtained exciton binding energy to boron, , agrees well with literature data.

Plasmon-enhanced deep ultraviolet Micro-LED arrays for solar-blind communications.

Localized surface plasmon resonance (LSPR)-enhanced deep ultraviolet (DUV) Micro-light emitting diodes (Micro-LEDs) using Al nanotriangle arrays (NTAs) are reported for improving the -3 dB modulation bandwidth. Through self-assembled nanospheres, the high-density Al NTAs arrays are transferred into the designated p-AlGaN region of the Micro-LEDs, realizing the effect of LSPR coupling. A 2.5-fold enhancement in photoluminescence (PL) intensity is demonstrated. Combined with the PL intensity ratio at 300 K and 10 K, internal quantum efficiency (IQE) may be increased about 15-20% by the plasmonic effect and the carrier lifetime decreases from 1.15 ns to 0.82 ns, suggesting that LSPR accelerates the spontaneous emission rate. Resulting from the improvement of the IQE, the electroluminescence intensity of Micro-LED arrays with LSPR is obviously increased. Meanwhile, the -3 dB bandwidth of 6 × 6 Micro-LED arrays is increased from 180 MHz to 300 MHz at a current density of 200 A/cm2. A potential way is proposed to further increase both the IQE and the modulation bandwidth of DUV Micro-LEDs.

Ultra-narrow deep-red emitting AlN:Sm2+phosphor with nano-branched construct for wide color gamut backlight display and high-pressure sensing applications

Rare-earth (RE) doped AlN are excellent candidate materials for electroluminescent devices, full color displays and white lighting technology. In this paper, a deep red Sm2+ doped AlN (AlN:Sm2+) phosphor was synthesized for the first time by a one-step direct nitridation method. Detailed XRD and EDS studies show the presence of samarium (Sm) ions the AlN, and XPS measurements indicate Sm ions are divalent. SEM and TEM studies show that the AlN:Sm2+ have a branched nanostructure, consisting of a primary stem and secondary short nano-branches. AlN:Sm2+ phosphor has a broad and strong excitation bands in the range of 300–600 nm, ultra-narrow deep red emission at 686 nm, near unity color purity, and good thermal stability (78.2% at 413 K). A blue-pumped warm white light emitting diode with high color rendering index (Ra∼87.5) and low correlated color temperature (CCT∼4875 K) was fabricated. Moreover, a super-wide color gamut (117.6% of the NTSC) can be achieved by using AlN:Sm2+ as the red component. Furthermore, photoluminescence (PL) and Raman spectra of AlN:Sm2+ were studied under hydrostatic pressure up to 25 GPa. The shift of the 5D0→7F0 emission band (dλ/dP≈0.13 nm/GPa) and the decrease of PL intensity ratio (5D0→7F0/5D0→7F1, dIR(0/1)/dP≈−5.6%/GPa) with applied pressure can be used for optical pressure sensor. Raman spectroscopy revealed a phase transition of AlN:Sm2+ from wurtzite to rocksalt phase at 19.9 GPa. The large doping of Sm2+ ions and unique intrinsic geometry in branched nanostructure co-affect its compressibility and structural stability under high pressure. The results indicate that AlN:Sm2+ phosphor has promising applications in backlight displays and optical pressure sensors due to their excellent luminescent properties.

Harvesting strong photoluminescence of physical vapor deposited GeSn with record high deposition temperature

In this work, strong photoluminescence (PL) from physical vapor deposited GeSn on Ge-buffered Si is harvested by pursuing high deposition temperature. High-quality Ge0.938Sn0.062 films are obtained through sputtering epitaxy at a record high temperature of 405 °C. The PL peak intensity of the sputtering-grown GeSn is enhanced by 21 times compared to that of GeSn with similar Sn content grown by molecular beam epitaxy at 150 °C. The PL intensity ratio between the sputtering-grown GeSn and Ge virtual substrate reaches a value of 18. Power-dependent and temperature-dependent PL characterizations demonstrate that band-to-band recombination dominates in the sputtering-grown GeSn film. The results indicate that high-temperature sputtering epitaxy, which has the merit of low cost and high-productivity potential, is promising for preparing high-quality GeSn films for optoelectronic applications.

Significant Quantum Efficiency Enhancement of InGaN Red Micro-Light-Emitting Diodes with a Peak External Quantum Efficiency of up to 6%

We demonstrate a significant quantum efficiency enhancement of InGaN red micro-light-emitting diodes (μLEDs). The peak external quantum efficiency (EQE) of the packaged 80 × 80 μm2 InGaN red μLEDs was largely increased to 6.0% at 12 A/cm2, representing the significant process in exploring the efficiency of InGaN red μLEDs. The improvement of the EQE is attributed to the significant enhancement of the quantum efficiency, which is confirmed by the electron–hole wavefunction overlap in the InGaN quantum well from the band gap simulation and the photoluminescence intensity ratio at room temperature/low temperature. Ultrasmall 5 × 5 μm2 InGaN red μLEDs were also obtained, which show a high peak EQE of 4.5%. This work demonstrates a simple approach to achieving highly efficient InGaN red μLEDs, which are very promising candidates for ultrasmall red μLEDs required by AR/VR displays.

Developing "smart windows" for optical storage and temperature sensing based on KNN transparent ceramics

Smart windows have attracted considerable attention due to their wide applications in optical data storage, switchable sunroof and temperature sensing. The development strategy for smart windows is focused on performance design, enhancement and integration. However, developing integrated multi-functional smart windows in a single material remains a challenge. In this work, we have successfully prepared (K0.5Na0.5)0.95Ba0.04Er0.01NbO3 (4Ba-1Er-KNN) transparent ceramics for potential applications of temperature detection and optical information storage in smart windows. With alternating ultraviolet (UV) illumination and 300 °C thermal stimulation, the prepared 4Ba-1Er-KNN ceramics can not only achieve non-destructive luminescence readout, but also exhibits an ultra-high photochromic (PC) contrast with rapid response time of 1 s. Furthermore, based on the up-conversion (UC) photoluminescence (PL) intensity ratio of Er3+: 2H11/2/4S3/2 thermally coupled levels, excellent low-temperature sensing performance with the maximum relative sensitivity of 0.023 K−1 at 213 K is obtained. The integration between UC PL, PC response and temperature sensing performance makes it possible to develop multi-functional smart windows.

Raman and photoluminescence intensity ratio: A tool for high temperature optical thermometry in Ba2Y2/3TeO6: Eu3+ double perovskites

Ratiometric thermometry based on lanthanide-activated phosphors is the most reliable method to measure the thermometric figures of merit due to its self-referencing ability. In this work, we use the rarely investigated Raman and photoluminescence intensity ratio (RPIR) method to evaluate the high-temperature sensitivity of a double perovskite phosphor Ba2Y2/3TeO6: 2.5% Eu3+ for the first time. The phosphor exhibits a maximum relative sensitivity of 0.52% K−1 at 573 K with a temperature resolution of 0.17 K. The study proved the RPIR method as a promising strategy for high-temperature optical thermometry using crystalline solids.

Ratiometric Fluorescence Probe Based on Deep-Red Emissive CdTe Quantum Dots and Eu3+ Hybrid for Oxytetracycline Detection

Oxytetracycline (OTC) is an environmental pollutant caused by the abuse of antibiotics, and its content in water is closely related to human health. Therefore, the development of a simple, rapid, and accurate method to detect OTC has become desirable. In this work, a ratiometric fluorescence probe based on deep red emissive CdTe quantum dots (QDs) modified by mercaptopropionic acid and Eu3+ is developed to accurately and rapidly detect OTC in water. After the addition of OTC, the photoluminescence intensity of CdTe QDs at 698 nm remains almost unchanged, while the peak at 617 nm intensifies within 40 s due to the coordination of Eu3+ with OTC. An excellent linear relationship is present between the photoluminescence intensity ratio of I617/I698 and the concentration of OTC. The limit of detection of the probe towards OCT is 5.4 nM. In addition, the probe shows good selectivity and anti-interference ability for OTC in the presence of other antibiotics, including other antibiotics, ions, and amino acids. The probe has been successfully applied to detect OTC in actual samples, demonstrating its potential for environmental application.

Methylmercury bioaccumulation in water flea Daphnia carinata by AIEgen

Mercury ion (Hg2+) is a toxic heavy metal ion and Hg2+ is convertible to methylmercury (MeHg) by many aquatic microorganisms, leading to bioaccumulation and biomagnification in aquatic organisms, which can interfere with brain development and function in humans. This study employs a newly developed AIEgen (Aggregation-induced emission fluorogen) to quantify and visualise the process of MeHg bioaccumulation in vivo on the species of water flea Daphnia carinata. Two approaches to MeHg absorption were taken, either by direct incubation in a MeHg solution or by indirect consumption of algae contaminated with MeHg. We analysed the relationship between the ratio of photoluminescence (PL) intensities (I585/I480) and MeHg concentration (CMeHg) and generated a master curve for determining MeHg concentration based on the measurement of PL intensities. Fluorescent image analysis showed the occurrence of MeHg in D. carinata to be mainly in the compound eyes, optic nerve and carapace. This study indicates that MeHg absorption can be quantified and visualised in the body of zooplankton, and the MeHg transfer to zooplankton is more likely through direct exposure than via indirect food intake. The accumulation of MeHg in the eye and the nervous system could be the cause of the high mortality of D. carinata exposed to MeHg in water.

Strain-Modulated Interlayer Charge and Energy Transfers in MoS2/WS2 Heterobilayer.

Excitonic properties in 2D heterobilayers are closely governed by charge transfer (CT) and excitonic energy transfer (ET) at van der Waals interfaces. Various means have been employed to modulate the interlayer CT and ET, including electrical gating and modifying interlayer spacing, but with limited extent in their controllability. Here, we report a novel method to modulate these transfers in the MoS2/WS2 heterobilayer by applying compressive strain under hydrostatic pressure. Raman and photoluminescence measurements, combined with density functional theory calculations, show pressure-enhanced interlayer interaction of the heterobilayer. Heterobilayer-to-monolayer photoluminescence intensity ratio (η) of WS2 decreases by five times up to ≈4 GPa, suggesting enhanced ET, whereas it increases by an order of magnitude at higher pressures and reaches almost unity. Theoretical calculations show that orbital switching and charge transfers in the heterobilayer's hybridized conduction band are responsible for the non-monotonic modulation of the transfers. Our findings provide a compelling approach toward effective mechanical control of CT and ET in 2D excitonic devices.

Low‐Temperature Photoluminescence Investigation of Light‐Induced Degradation in Boron‐Doped CZ Silicon

Light‐induced degradation (LID) in boron‐doped Czochralski grown (CZ) silicon is a severe problem for silicon devices such as solar cells or radiation detectors. Herein, boron‐doped CZ silicon is investigated by low‐temperature photoluminescence (LTPL) spectroscopy. An LID‐related photoluminescence peak is already found while analyzing indium‐doped p‐type silicon samples and is associated with the ASi–Sii defect model. Herein, it is investigated whether a similar peak is present in the spectra of boron‐doped p‐type CZ silicon samples. The presence of change in the photoluminescence signal intensity due to activation of the boron defect is investigated as well. Numerous measurements on boron‐doped samples are made. For this purpose, samples with four different boron doping concentrations are analyzed. The treatments for activation of the boron defect are based on the LID cycle. During an LID cycle, an additional peak or shoulder neither in the areas of the boron‐bound exciton transverse acoustic and nonphonon‐assisted peaks (BTA, BNP) nor in the area of the boron‐bound exciton transverse optical phonon‐assisted peak (BTO) is found. The defect formation also does not lead to a lower photoluminescence (PL) intensity ratio BTO(BE)/ITO(FE).

Electric‐Tunable Photoluminescence of 2D ErOCl for High‐Security Encryption of Programmable Information

AbstractHigh‐security encryption has always been important in economic and military fields as well as in daily life. 2D van der Waals (vdW) rare‐earth (RE) materials have advantages in photoluminescence (PL) modulation to achieve high‐security encryption because of their multiple sharp emission peaks, which will facilitate the multimode regulation for high‐security encryption of programmable information. Here, programmable information encryption has been achieved by applying 2D vdW ErOCl via the editable electric‐tunable PL. The correct information encoded in PL outputs can be tactfully decrypted from the PL intensity ratio of 2H11/2–4I15/2 and 4S3/2–4I15/2 transitions. This strategy for ASCII codes and images encryption with high‐security is demonstrated. This novel approach, PL modulation of 2D vdW RE material based on programmable electric inputs, will mark a new path to achieve high‐security encryption.