Variation Of Photoluminescence Intensity Research Articles

Photoluminescence measurement of ruby excited by duty ratio-controlled pulse light-emitting diodes (LED) light source.

Photoluminescence (PL) spectra from ruby were obtained using a highly stable LED light source, employing pulse width modulation technique for excitation. The temporal variation in PL intensity caused by the increasing temperature of the LED used for excitation can be mitigated by adjusting the duty ratio (%) of the pulsed LED light to below 10% for cooling the LED. Stable PL spectra measurements were achieved with a duty ratio of less than 10% using a duty ratio-controlled pulsed LED light source, as temperature fluctuations in LED light intensity are minimized at duty ratios less than 10%. Furthermore, fluctuations in the measured PL intensity were diminished by setting the frequency of the pulsed LED light source to greater than 1kHz. This method enables more reliable, cost-effective, and stable PL measurements for material characterization in semiconductors, photonics, and nanotechnology.

Charged Exciton Generation by Curvature-Induced Band Gap Fluctuations in Structurally Disordered Two-Dimensional Semiconductors.

Curvature is a general factor for various two-dimensional (2D) materials due to their flexibility, which is not yet fully unveiled to control their physical properties. In particular, the effect of structural disorder with random curvature formation on excitons in 2D semiconductors is not fully understood. Here, the correlation between structural disorder and exciton formation in monolayer MoS2 on SiO2 was investigated by using photoluminescence (PL) and Raman spectroscopy. We found that the curvature-induced charge localization along with band gap fluctuations aid the formation of the localized charged excitons (such as trions). In the substrate-supported region, the trion population is enhanced by a localized charge due to the microscopic random bending strain, while the trion is suppressed in the suspended region which exhibits negligible bending strain, anomalously even though the dielectric screening effect is lower than that of the supported region. The redistribution of each exciton by the bending strain leads to a huge variation (∼100-fold) in PL intensity between the supported and suspended regions, which cannot be fully comprehended by external potential disorders such as a random distribution of charged impurities. The peak position of PL in MoS2/SiO2 is inversely proportional to the Raman peak position of E12g, indicating that the bending strain is correlated with PL. The supported regions exhibit an indirect portion that was not shown in the suspended regions or atomically flat substrates. The understanding of the structural disorder effect on excitons provides a fundamental path for optoelectronics and strain engineering of 2D semiconductors.

Observation of Strong Anisotropic Interlayer Excitons.

Anisotropic interlayer excitons had been theoretically predicted to exist in two-dimensional (2D) anisotropy/isotropy van der Waals heterojunctions. However, experimental results consolidating the theoretical prediction and exploring the related anisotropic optoelectronic response have not been reported so far. Herein, strong photoluminescence (PL) of anisotropic interlayer excitons is observed in a symmetric anisotropy/isotropy/anisotropy heterojunction exemplified by 3L-ReS2/1L-MoS2/3L-ReS2 using monolayer (1L) MoS2 and trilayer (3L) ReS2 as components. Sharp interlayer exciton PL peaks centered at ∼1.64, ∼1.61, and ∼1.57 eV are only observed at low temperatures of ≤120 K and become more pronounced as the temperature decreases. These interlayer excitons exhibit strong anisotropic PL intensity variations with periodicities of 180° as functions of the incident laser polarization angles. The polarization ratios of these interlayer excitons are calculated to be 1.33-1.45. Our study gives new insight into the manipulation of excitons in 2D materials and paves a new way for a rational design of novel anisotropic optoelectronic devices.

Probing Spin Dynamics of 2D Excitons with Twisted Light

We propose a mechanism of intravalley spin-flip scattering in spin-valley coupled two dimensional systems by transferring momentum of light into exciton center of mass using optical vortex (OV) beams. By varying the dispersion of light using the topological charge of OV beam, we demonstrate a unique approach to control the intra-valley spin-flip scattering rate of excitons. From our photoluminescence measurements, we demonstrate that the intra-valley scattering rate in W-based TMDs can be tuned externally by OV beams. Variation of photoluminescence intensity with topological charges shows a crossover temperature (> 150 K), indicating competitions among time scales involving radiative recombination, spin-flip scattering, and thermal relaxations. Our proposed technique utilizing a structured light beam can open up a new approach to explore the physics of excitons in 2D systems.

Interrelation of micro-strain, energy band gap and PL intensity in Ce doped ZnS quantum structures

Three-dimensional (3-D) quantum structures (QSs) of ZnS and Cerium (Ce) doped ZnS were synthesized via chemically affordable sol-gel process. Influence of Ce doping and thus induced micro-strain on the structural, morphological, and optical characteristics was explored. XRD confirmed the formation of single-phase zinc blende ZnS. Estimated average crystal size corresponding to highest intensity XRD peak (111) varied within 1.65–4.65 nm which are comparable with Bohr radius of ZnS. Due to the size mismatch between Ce and Zn, micro-strain and vacancies were found to be developed in host matrix of ZnS. Thermodynamic calculations validated an expansion and contraction in lattice parameter due to Ce doping. FTIR spectra confirmed the presence of different functional groups related to Zn and S. Photoluminescence (PL) emissions observed at 420, 461, 509 and 560 nm are related to the defect states such as interstitial sulfur, zinc interstitial, sulfur vacancies and zinc vacancies respectively. Rise of another emission peak in doped ZnS at 600 nm was due to 5d → 4f energy level transitions in Ce3+ ions. Evolved micro-strain profile, PL intensity and energy band gap variation were analogous to each other with respect to doping concentration. Microscopic images confirmed the structural transformation to cuboidal shaped ZnS QSs with increase in doping concentration. EDX and XPS supported the elemental analysis along with oxidation states of the available elements such as Zn, S and Ce.

Variation of photoluminescence intensity depending on the timing of electric field application during isothermal flash sintering for 3mol%Y2O3–ZrO2 polycrystal

The shrinkage process and the change in photoluminescence (PL) intensity during isothermal flash sintering were investigated for 3 mol% Y2O3–ZrO2 (3YSZ) polycrystals doped with 300 ppm Bi2O3 as a PL impurity centre. The PL intensity of 3YSZ is enhanced by isothermal flash sintering. However, the enhancement of the PL intensity was revealed to become smaller when flash-treatment was applied for thermally pre-sintered 3YSZ polycrystal. To maximize the efficiency of electric field on the enhancement of the PL intensity, the electric field needs to be applied throughout the sintering from the initial stage to the final densification. The necessity for continuing to apply the electric field until the densification is completed is due to the fact that the recovery of oxygen vacancies, the origin of the F+ band, must be suppressed.

A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission

A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission

INTERACTION OF TIO2 NANOPARTICLES AND PROTEINS OF AN IMMUNE COMPLEX IN PHOTOLUMINESCENCE BASED BIOSENSOR

Interaction between nanostructured semiconductor thin layers and immune complex based proteins is a fundamental issue of the formation of nanobiointerface in the various biosensors, in particular, optical biosensors. In this work, the main aspects of the interaction between photoluminescent TiO2 nanoparticles and Bovine leucosis virus (BLV) protein gp51, used as a model protein, during the formation of photoluminescence based immunosensor have been discussed. The antigens of gp51 were immobilized on the surface of a nanostructured TiO2 thin film formed on the glass substrates. As a result, an increase of the photoluminescence (PL) signal intensity and PL peak shift from 517 nm to 499 nm were observed. An incubation of TiO2/gp51 structure in a solution containing anti-gp51 antibodies resulted in the backward PL peak shift from 499 nm to 516 nm and decrease of the PL intensity. The main reason of the changes in the PL spectra (i.e. PL maxima shifts and PL intensity variations) as a result of BLV protein gp51 adsorption on the surface TiO2 thin film is an electrostatic interaction between negatively charged surface of TiO2 and positively charged atoms and groups provided by the adsorbed gp51 protein due to the presence of partial uncompensated charges within the proteins.

Investigation of carrier migration from WS2 monolayer to substrate by photoluminescence

Carriers in atomically thin two-dimensional (2D) semiconductors like WS2 monolayers are prone to interact with their surroundings, which may have a significant impact on both the electric and optical properties of 2D semiconductors. In this work, abnormal dependence of photoluminescence (PL) intensity on excitation power densities is observed in mechanically exfoliated WS2 monolayers on SiO2/Si substrates, which is manifested as a sudden intensity drop of exciton and trion emissions at certain power density followed by much slowed intensity increase. As-exfoliated and wetted WS2 monolayers exhibit similar abnormal variation of PL intensity but different power densities that trigger the intensity drop. The sudden drop of PL intensity is ascribed to the modification to the contact status of the monolayer with the substrate, caused by the interplay of the trapped air or water with the monolayer and the substrate upon laser irradiation, which therefore alters the carries transport rate from the monolayer to the substrate. A rate equation model has been developed to delicately deal with the carrier recombination and transport processes in a steady state, and the variation dependence of exciton, trion and free charge populations on the cross-plane transport rate is demonstrated, which consistently explains the abnormal PL intensity dependence on the excitation power density. The important role of carrier transport in the PL performance has been verified in this work, and further understanding of the carrier interactions in atomical thin 2D semiconductors has been achieved.

Effects of Implant Temperature, Backside Contamination and Scribe Lines on Room Temperature Photoluminescence Measurements on Silicon

To investigate the effect of implantation temperature on the damage to a Si lattice, room temperature photoluminescence (PL) spectra were measured from highly-channeled MeV 11B+ implanted Si wafers with different implant temperatures (25 °C and 450 °C) after annealing at 950 °C for 3 min in N2. Small pieces from the wafers were used for PL characterization. The implanted Si wafer piece at the elevated temperature resulted in higher overall PL intensity under both excitation wavelengths suggesting lesser lattice damage than the Si wafer implanted at room temperature. Unexpectedly large PL intensity variations were observed from PL area mapping of both wafer pieces. In addition, strange behaviors of localized PL intensity variations, with opposite trends between 670 nm and 827 nm excitation PL measurements were observed near the scribe lines and sample number markings on the backside. The PL intensity was increased under 670 nm excitation while it was decreased under 827 nm excitation. Possible reasons for this strange behavior were discussed based on experimental results and analysis. PL measurement is verified to be very sensitive to the surface condition, interface, bulk and backside conditions of the Si. For reliable PL measurement, backside contamination and scratches, such as scribe lines, should be avoided.

Variations in Photoluminescence Intensity of a Quantum Dot Assembly Investigated by Its Adsorption on Cubic Metal–Organic Frameworks

In this study, a novel method is proposed for investigating energy transfer between quantum dots (QDs), especially of the same type, in an ensemble by employing cubic crystals of a commonly used metal–organic framework-5 (MOF-5). Three types of QDs were synthesized by standard methods and ligand-exchanged with weakly bound pyridine derivatives; thereafter, they were instantaneously adsorbed on MOF-5 in a monolayer by simply mixing both the materials. When two types of QDs were simultaneously loaded, energy transfer from the green- to red-emitting QDs as reflected by an increase in the photoluminescence (PL) intensity of the long-wavelength component was clearly observed. Conversely, when the same type of QDs were loaded on MOF-5, the increase in the PL intensity was lower than that expected corresponding to the amount of QDs and the deviation between the expected and measured values was different based on the type of QDs. These behaviors are discussed in relation to the occurrence of the Förster resonance energy transfer between the same species in close distance and the existence of temporarily or permanently dark QDs.

ДОСЛІДЖЕННЯ ОСОБЛИВОСТЕЙ ТЕХНОЛОГІЇ СИНТЕЗУ КВАНТОВИХ ТОЧОК CdTe В КОЛОЇДНИХ РОЗЧИНАХ МЕТОДАМИ ФОТОЛЮМІНЕСЦЕНТНОЇ СПЕКТРОСКОПІЇ ТА РЕФРАКТОМЕТРІЇ ПОВЕРХНЕВОГО ПЛАЗМОННОГО РЕЗОНАНСУ

Effect of activation current variation in the synthesis of quantum dots (QD) of cadmium telluride in colloidal solutions on the change of emission spectra of photoluminescence (PL), the average size of QD and the change in the angular position of refractometric characteristics of surface plasmon resonance (SPR) was studied. The activation current was changed from 0.1 to 0.6 A in steps of 100 mA. For comparison, also performed the synthesis of cadmium telluride QD in colloidal solutions with a change in the duration of the synthesis in the range from 1 to 5 minutes in increments of 1 minute. The average QD size of cadmium telluride was determined by the peak values of the measured PL spectra of the corresponding colloidal solutions according to the modified Bruce formula. The response of the SPR sensor constructed according to the Kretschman scheme was defined as the angular shift of the minimum of the measured refractometric characteristics of the SPR when replacing deionized water with the studied colloidal solutions. The results of the study showed that the increase in the activation current in contrast to the increase in the duration of the synthesis led not only to a decrease in PL intensity, but also to its increase at low currents, which may be due to "electropolishing" of the quantum dot surface. Both technologies for the synthesis of quantum dots were characterized by the presence of slow growth processes, which was associated with the thermodynamics of nucleation processes and manifested in the form of saturation regions on the graphs of PL peak and minimum refractometric characteristics of SPR from synthesis duration and activation current. At different durations of quantum dot synthesis, a dependence (correlation coefficient -0.9992) was established between the response of the SPR sensor and the spectral position of the PL maximum, which was approximated by a linear function with a matching coefficient R 2 = 0.998. The results of the study showed that the use of variation of activation current in stabilizing the duration of synthesis is the dominant condition for creating high-emission light-emitting and light-converting semiconductor systems based on cadmium telluride QD, because with variation of activation current PL intensity was 431.9 abs., 4 times higher than for the case of variation in the duration of synthesis, namely 181.2 abs.one. The results of the study can be used to optimize the technology of manufacturing QD of cadmium telluride with predetermined characteristics.

Revealing Interplay of Defects in SnO2 Quantum Dots for Blue Luminescence and Selective Trace Ammonia Detection at Room Temperature.

Defect chemistry in SnO2 is well established for resistive sensors but remains to be elusive for photoluminescence (PL) sensors. It demands a comprehensive understanding of the role of cationic and oxygen defects as well as the creation of abundant such defects to provide a selective PL signal. To accomplish it, SnO2 quantum dots (QDs ∼ 2.4 nm) are prepared without a capping agent along with other dimensions. Then, the relationship of defects with the blue-emission PL is unfolded by electron energy loss spectroscopy, lifetime measurements, X-ray absorption, and Raman spectroscopic measurements. The defects acting as Lewis acid sites are utilized for selective ammonia detection. Huge enhancements of the obscured blue luminescence at 2.77 and 2.96 eV from the SnO2 QDs are observed because of interaction with ammonia. The linear variation of PL intensities with analyte concentrations and the recovery of the sensor are elaborated with detection up to 5 ppm. The interplay of defects in SnO2 is further established theoretically for site-specific interactions with ammonia by density functional theory (DFT) calculations. Thus, the unique mechanism revealed for the superlative performance of the PL sensor with uncapped SnO2 QDs provides a novel platform for defect-engineering-based optoelectronic applications.

Photoluminescence evolutions and physical mechanism discussions of Sr1.95-(Ca/Ba) MgSi2O7:Eu2+ phosphors

Abstract With Ca and Ba substitutions for Sr, the photoluminescence evolution of Sr2MgSi2O7:0.05Eu2+ has been investigated and discussed. With spectra and crystal structure analysis, the photoluminescence intensity variation, the emission band shift, the chromaticity coordinate evolution and the thermal stability changes have been revealed. The corresponding physical mechanisms have been discussed. With Ca/Ba substitution, the emission spectra show entire red/blue shift; while the photoluminescence intensity are both quenched. The crystal structure and lattice parameter variations were refined. Based on the reflection spectra measurements, the forbidden band gap of the series of Sr1.95-x(Ca/Ba)xMgSi2O7:0.05Eu2+ phosphors were calculated. Both for Ca and Ba substitution, the forbidden band gap is elevated. The regular emission band red/blue shifts are ascribed to the increased/decreased crystal splitting of the Eu2+ 5d electron. Based on the estimation of the crystal splitting intensity and the calculation of the forbidden band gap, the Eu2+ 5d electron transition process and the Ca/Ba substitution tuning process are illustrated. The thermal stability of Sr2MgSi2O7: Eu2+ is promoted with Ca/Ba substitution. The thermal quenching mechanism of this phosphor can be confirmed to self-ionization. This work proves cation substitution is a suitable route to tune the photoluminescence property of the phosphors. The revealed physical mechanisms are in favor of understanding the tuning process of the phosphors.

Tailoring of stoichiometry and band-tail emission in PLD a-SiC thin films by varying He deposition pressure

The present work focuses on the effect of He pressure (vacuum, 10−2–5 mbar) on structural and optical properties of amorphous silicon carbide (a-SiC) thin films, deposited by pulsed laser deposition (PLD) technique onto fused silica. A stoichiometric change occurs in PLD a-SiC film, with increasing He deposition pressure, transforming a near stoichiometric SiC material to C-rich SiC films till 1 mbar of He pressure followed by drastic decrease in carbon (C) content at 5 mbar. The optical band gap of the films estimated using Tauc plot from UV–Vis–NIR transmittance spectra showed initial increase from 2.09 to 2.61 eV with increasing He pressure from 10−6 to 1 mbar, then decrease to 2.11 eV at 5 mbar. A broad band photoluminescence (PL) peak featured in each of the a-SiC films at room temperature. The PL spectra exhibited blue shift in peak energy from 1.8 to 2.2 eV with increase in atomic percentage of C from 49.4 to 62.4%. The origin of the PL exhibited in these films is clearly due to band tail emissions from amorphous SiC structures which has been explained by the observed variation of PL intensity and corresponding FWHM with band gap as well as Urbach energy.

Development of the PANI/MWCNT Nanocomposite-Based Fluorescent Sensor for Selective Detection of Aqueous Ammonia.

The present work reported the polyaniline (PANI) and multiwalled carbon nanotube (MWCNT)-based nanocomposite as a sensing material for the determination of aqueous ammonia by the enhanced fluorescence method. The excitation wavelength-dependent photoluminescence (PL) intensity has shown dual emission peaks at 340 and 380 nm that correspond to two different excitation energy states. The pH-based PL intensity and zeta potential variation were analyzed to optimize the suitable medium for aqueous ammonia sensing. Zeta potential was found to shift from 4 to −21 mV upon changing the pH of the the solution from acidic to alkaline medium. The fluorescence intensity of PANI/MWCNTs was found to increase upon increasing the pH from 3.0 to 6.0 (acidic region) and exhibits a plateau upon further increasing the pH from 7.0 to 12 (basic region). The PANI/MWCNT composite has shown a linear response to aqueous ammonia concentration varying from 25 to 200 μM with a correlation coefficient (R2) of 0.99 and a limit of detection of 15.19 μM. The presence of relevant interference molecules and physiological ions had no influence on the detection of aqueous ammonia. Field-level study demonstrated that the level of aqueous ammonia can be determined selectively by using the PANI/MWCNT composite for various applications. The mechanism for the selective detection of aqueous ammonia is deliberated in detail.

Annealing impact on emission and phase varying of Nd-doped Si-rich-HfO2 films prepared by RF magnetron sputtering

HfO2 films doped with Nd and Si atoms were produced by RF magnetron sputtering in argon plasma atmosphere. The effect of annealing treatment on the morphology, crystal structure and light emission of the films was investigated by means of the scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). The thermal treatment was performed in the temperature range of TA = 800–1100 °C in horizontal furnace with continuous nitrogen flow. For annealed Si–HfO2:Nd films, the SEM study revealed the formation of the grains with the mean size of about 20–60 nm that show the tendency to enlarge with the TA rise. Besides, the phase separation was observed and tetragonal HfO2 and SiO2 phases were detected by the XRD method for the films annealed at TA > 950 °C. The PL study revealed that both Nd3+ ions and host defects contribute to PL emission whereas their relative contribution depends on the TA and on the crystal phase of host matrix. The highest PL intensity of Nd3+ ions via 4f inner electronic shell levels was detected for TA = 950 °C. The variation of PL intensity of Nd3+ ions was correlated with the change of PL intensity of the band caused by the host defects. These latter participate in the energy transfer towards Nd3+ ions. This statement was confirmed by XPS data, as well as by the shape of PL spectra. It was shown that the bright emission via Nd3+ ions can be achieved for those located in the tetragonal HfO2 matrix.

Achieving white-light/tunable emissions via the controllable energy transfer in the single Ba9La2Si6O24:Eu3+,Bi3+ phosphor for UV converted white LEDs

Nowadays, ultraviolet (UV) and blue converted phosphors have played a key role in the solid-state lighting field, and the white light is usually produced by mixing multiple phosphors. In this work, we report a type of UV converted single white phosphor, i.e., Ba9La2Si6O24:Eu3+,Bi3+.Various techniques, such as powder X-ray diffraction (XRD), scanning electron microscopic (SEM), photoluminescence (PL) spectra, and quantum efficiency (QE), have been used to characterize the samples. In Bi3+ doped samples, upon excitation within the wavelength range of 250–380 nm can lead to three Bi3+ emission bands (i.e., 358 nm, 415 nm, and 516 nm), but only one emission band peaking at 415 nm appears when the excitation wavelength exceeds 380 nm. Structural analysis and discussion on the PL intensity variation reveal there are three Bi3+ luminescent centers at two different La3+ sites and one Ba2+ site. The Bi3+ concentration-induced luminescent quenching is dominated by the d-d interaction. In view of the spectral overlap between the Bi3+ emissions and the Eu3+ excitation spectra, the energy transfer from Bi3+ to Eu3+ of the Ba9La2Si6O24:Eu3+,Bi3+ is studied, and it is dominated by a dipole-quadrupole (d-q) interaction. Moreover, upon excitation at 365 nm, the emissions broadly tuning from blue, white and to red are achieved by varying the Eu3+ content. For the white Ba9La2Si6O24:Eu3+,Bi3+ sample, high temperature PL results show that it does not have severe degradation of emission intensity after increasing the temperature from 25 to 300 °C, exhibiting a good thermal stability. To show the potential application in the lighting fields, we fabricate a white light LEDs device by depositing this white phosphor on a 365 nm UV LED chip. As a result, the values, i.e., a color rendering index (CRI) of 95, luminous efficiency of 80 lm/W, and color temperature (CT) of ~4215 K at chromaticity (0.382, 0.398), are achieved.

Twin domains modulate light-matter interactions in metal halide perovskites

Despite the extensive insights gained in how the microstructure impacts the device performance of metal halide perovskites (MHPs), little is known about the effect of the ferroelastic twin domains on the optoelectronic properties of MHPs. In this work, the effect of the ferroelastic twin domains on the photoluminescence (PL) behavior of CH3NH3PbI3 is investigated by correlating measurements from multiple microscopies. PL spectra and the confocal PL lifetime maps reveal no difference in wavelength of emitted light and decay dynamics between the neighboring domains, whereas PL intensity is different. We propose that the PL intensity variation is induced by the difference in light-matter interactions between neighboring domains. These results suggest that the effect of ferroelastic twin domains on the intrinsic PL behavior is negligible. We expect that this work will stimulate researchers to further explore the impact of twin domains on the photophysical properties of MHPs.

Synthesis and photoluminescence of silver and europium codoped germinate tellurite oxyfluoride glasses

Germinate tellurite oxyfluoride glasses of composition TeO2-GeO2-WO3-Li2O3-EuF3-AgCl (TGEUAC) were synthesized by melt quenching and heat-treated for the growth of silver nanoparticles (AgNPs). The presence of surface Plasmon resonance (SPR) absorption band of silver species with Eu3+ ions absorptions peaks for differently heat-treated glass samples were measured by the absorption spectrum. The green, red and deep- red photoluminescence intensity variation of Eu3+ ions under the influence of AgNPs has been observed. Maximum three-fold enhancement of TGEUAC-6 sample for green, red and mainly deep- red emission of Eu3+ caused by SPR of silver nanoparticles has also been observed. On the other hand, emission quenching of after long heat-treated of other samples was attributed to back energy transfer from Eu3+ ions to silver species or local field saturation. The emission intensities characterized through the CIE diagram and energy transfer mechanism of the Ag NPs to Eu3+ ions in glasses are discussed for potential application of optical devices.