Enhancement Of Photoluminescence Intensity Research Articles

Luminescent carbon dots encapsulating in Eu3+ doped gahnite spinel nanocomposite for boosting thermal sensing, advanced level III detection and intelligent anti-counterfeiting applications

The exceptional optical properties of carbon dots (CDs) position them as a highly promising category of multifunctional carbon-based nanomaterials. When hydrothermally prepared CDs are incorporated into europium-doped zinc aluminate ZnAl2O4 nanophosphors (ZAO:Eu3+ NPs) synthesized via ultrasonication method, the resulting CDs@ZAO:Eu3+ nanocomposite (NCs) exhibits a broad excitation band, with a prominent peak at approximately 393 nm, perfectly matching the emission wavelength range (500–750 nm) of near-ultraviolet (n-UV) LED chips. Significantly, the CDs (5 wt %)@ZAO:Eu3+ NCs exhibit a remarkable 12.5 folds enhancement in photoluminescence (PL) intensity compared to ZAO:Eu3+ alone, thanks to the Förster Resonance Energy Transfer (FRET) between CDs and Eu3+ ions. This enhancement in luminescence could stem from the CDs capturing electrons and facilitating energy transfer to the luminescent Eu3+ ions. Utilizing CDs to enhance fluorescence presents a straightforward and eco-friendly approach for boosting luminescence properties. Impressively, even under elevated temperatures reaching 420 K, the thermal stability of CDs (5 wt %)@ZAO:Eu3+ NCs remains outstanding, retaining 90 % of its initial luminescence intensity. This highlights its exceptional thermal stability, characterized by a high activation energy of 0.27 eV. With an impressive internal quantum efficiency (IQE) of 87.01 %, the optimized composite also achieves remarkable colour purity (CP), reaching 99.5 %. The relative sensitivity was 1.36 %, at 300 K. Thus, CDs@ZAO:Eu3+ is a promising material for non-contact optical thermometry applications. Utilizing the optimized NCs through a simple powder dusting technique effectively develops latent fingerprints (LFPs) on various surfaces, revealing fingerprints (FPs) under 365 nm UV light. Additionally, the study investigates diverse pore parameters including number, position, size, shape, and inter-spacing. Furthermore, optimized NCs are utilized to create a transparent anti-counterfeiting (AC) ink, offering effective AC features through its red emission under 365 nm UV light.

Enhanced Spontaneous Emission through High‐k Modes in CsPbBr3 Perovskite Hyperbolic Metamaterials

AbstractNovel optical sources require fast decay rates, making hyperbolic metamaterials (HMMs) an increasingly attractive option. HMMs are well‐known for their remarkable anisotropy, and leverage hyperbolic dispersion to enhance the decay rate of a fluorophore placed on top of them. This study tackles the complex task of embedding a fluorophore into an HMM, successfully overcoming challenges related to surface roughness, thickness imperfections, and layer washing effects. Specifically, CsPbBr3 perovskite nanocrystals (NCs)‐based HMM are fabricated, by alternating silver/nanocrystals (Ag/NCs) layers. Through a systematic investigation of the photophysical response following the deposition of each bilayer, compelling evidence of the achievement of hyperbolic dispersion is provided. Specifically, the impact of “high‐k” modes is isolated, which is distinctive to the HMM architecture. Therefore, the longstanding debate regarding the number of bilayers needed to achieve hyperbolic dispersion is conclusively resolved. The research demonstrates a nearly twofold increase in the decay rate and a threefold enhancement in photoluminescence intensity. These findings are further supported by theoretical Purcell factor calculations. This study marks a pioneering advancement in the field of bulk dye‐embedded HMMs, laying the groundwork for the development of advanced optical sources such as “resonant gain HMMs”.

Stable FAPbI3 Perovskite Solar Cells via Alkylammonium Chloride-Mediated Crystallization Control.

α-Phase formamidinium lead iodide (FAPbI3) perovskite solar cells (PSCs) have garnered significant attention, owing to their remarkable efficiency. Methylammonium chloride (MACl), a common additive, is used to control the crystallization of FAPbI3, thereby facilitating the formation of the photoactive α-phase. However, MACl's high volatility raises concerns regarding its stability and potential impact on the stability of the device. In this study, we partially substituted MACl with n-propylammonium chloride (PACl), which has a long alkyl chain, to promote the oriented crystallization of FAPbI3, ultimately forming an δ-phase-free perovskite. The FAPbI3 film containing PACl demonstrates an enhanced photoluminescence intensity and lifetime. Additionally, PACl's presence at grain boundaries acts as a protective layer for the PSCs. Consequently, we achieved a power conversion efficiency (PCE) of 22.4% and exceptional stability. It maintains over 95% of initial PCE for 100 days in an N2 glovebox, over 85% after 100 h of maximum power point tracking, and over 80% after 60 °C thermal aging.

Effect of DC poling on transparency and photoluminescence in Pr:PMN-PT transparent ceramics

Electric poling could significantly influence the transparency and the photoluminescence (PL) intensities in transparent ferroelectric PL ceramics. This work applied direct current (DC) poling on a highly transparent 0.75PMN-0.25PT:0.015Pr3+ ceramic, observed the domain configurations in a direction perpendicular to poling direction. Lamellar domains perpendicular to electric field were observed after poling and decreased the transmittance by changing the propagation path of incident light. Besides, the enhancement of PL intensities by poling was found as high as 62 %, mainly due to the following reasons: the change of light propagation path by the lamellar domains is supposed to increase the absorption of exciting light; and the reduced crystal symmetry may increase the PL efficiency. This work revealed the effects of DC poling on transparency and PL properties in transparent ferroelectric PL ceramics.

Unraveling Exciton-Plasmon Coupling and the PIRET Mechanism in Decorated Silicon Nanowires.

Exciton-plasmon coupling is a fascinating physical phenomenon that has been investigated in various metal semiconductor systems. Intentionally chosen silicon nanowires (SiNWs) systems act as a host material for providing exciton as well as silicon oxide as a thin dielectric. A clear blue-shift in photoluminescence (PL) peak and a significant increase in visible range absorption were observed for metal nanoparticle (MNP) decorated SiNWs (D-SiNWs) which signifies the presence of exciton-plasmon coupling. A further investigation reveals that the possibility of the occurrence of the plasmon-induced resonance energy transfer (PIRET) mechanism is higher. The PL intensity enhancement in Au-decorated SiNWs is higher (∼38 times) in comparison to that in Pt due to the presence of a strong and localized electric field of plasmons near the interface of metal and semiconductors. Moreover, splitting in PL for gold-decorated SiNWs might be due to the presence of dipole-quadrupole coupling along with dipole-dipole coupling, which further increases the strength of the PIRET mechanism.

Static and dynamic strain-driven photoluminescence tuning in rare-earth doped perovskite oxide thin films

Recently, there have been significant interest in the wide potential of photoluminescence (PL) response of rare-earth doped perovskite oxide thin films in the field of optoelectronics. This work reports an effective method to modulate the PL properties of 0.8 mol%Sm3+-doped BaTiO3 (BTO:Sm) thin films through static and dynamic strain. BTO:Sm films were epitaxially grown on LaAlO3 (LAO), 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT), MgO, and Mica single-crystal substrates by pulsed laser deposition technique. Compared with the BTO:Sm/PMN-PT heterostructure which experiences minimal lattice mismatch effect, the PL intensity of BTO:Sm/LAO and BTO:Sm/MgO is enhanced by 818.1% and 1014.7%, respectively, due to static tensile/compressive strain (-0.429% and 0.461%) during the epitaxial growth process. In the flexible BTO:Sm/Mica heterostructure, dynamic strain is introduced through mechanical bending. Compared with the unbent state, the u-type and n-type bending modes increase the relative changes in PL intensity (ΔI/I) by 599.0% and 768.5%, respectively. Particularly, BTO:Sm/Mica also exhibits a remarkable optical transmittance exceeding 75% (500–2500 nm). The enhancement of PL intensity is closely related to the adjustment of lattice strain, which changes the crystal field strength of the BTO:Sm and the radiative transition probability. This work provides valuable insights for designing future optoelectronic devices and environmental monitoring equipment. It has the potential to advance photonics and sensor technologies, paving the way for innovation and exploration in these fields.

Study of structural and optical properties of Gd doped ZnO nanostructures synthesized by one-step, mass-scale productive method

In the present work, one-step, mass-scale productive solid state reaction method has been employed for the preparation of gadolinium (Gd) doped ZnO (GZOx: x = 0, 2, 4, 6, 8 wt%) nanostructures. The prepared GZOx samples have been studies their structural, morphological, elemental diffusion profile and optical properties using P-XRD, FE-SEM, EDS, FTIR, and photoluminescence (PL) spectroscopy. The powder X-ray diffractometer (P-XRD) results clearly indicate strong and sharp diffraction peaks with broadening that confirms the formation of hexagonal nanocrystalline structure of ZnO and Gd doped ZnO. The hexagonal morphology formation of the grains after doping of Gd contents into the ZnO nanostructures system is seen from the FE-SEM micrographs and the sizes of the grains are found to close to the results obtained from XRD patterns. The presence of Gd contents in ZnO matrix confirms its doping by EDS spectrum of 4 wt% Gd doped ZnO nanostructures. Fourier transform infrared (FTIR) results confirm the presence of functional groups and chemical bonding of ZnO at room temperature. Investigation of optical emission is interesting and the PL spectra reveal two emission peaks, one in UV-region located in the range of ∼385 to 395 nm and other is defects related emission centred at ∼450, ∼468, ∼503 and ∼562 nm in visible region for all the samples. The enhanced PL intensity is observed in the higher doped Gd contents in ZnO nanostructures and can be attributed to the presence of Gd ions into ZnO lattice. The detailed structural and luminescence analysis has been carried out and correlated. The study of structural and optical properties GZOx samples suggest the luminescent optoelectronic applications.

Compositional Transformation and Impurity‐Mediated Optical Transitions in Co‐Evaporated Cu2AgBiI6 Thin Films for Photovoltaic Applications

AbstractQuaternary copper‐silver‐bismuth‐iodide compounds represent a promising new class of wide‐bandgap (2 eV) semiconductors for photovoltaic and photodetector applications. In this study, vapor phase co‐evaporation is utilized to fabricate Cu2AgBiI6 thin films and photovoltaic devices. The findings show that the properties of vapor‐deposited films are highly dependent upon processing temperature, exhibiting increased pinhole density and transforming into a mixture of quaternary, binary, and metallic phases depending on the post‐deposition annealing temperature. This change in phase is accompanied by an enhancement in photoluminescence (PL) intensity and charge‐carrier lifetime, along with the emergence of an additional absorption peak at high energy (≈3 eV). Generally, increased PL is a desirable property for a solar absorber material, but this change in PL is ascribed to the formation of CuI impurity domains, whose defect‐mediated optical transition dominates the emission properties of the thin film. Via optical pump terahertz probe spectroscopy, it is revealed that CuI impurities hinder charge‐carrier transport in Cu2AgBiI6 thin films. It is also revealed that the predominant performance limitation in Cu2AgBiI6 materials is the short electron‐diffusion length. Overall, the findings pave the way for potential solutions to critical issues in copper‐silver‐bismuth‐iodide materials and indicate strategies to develop environmentally compatible wide‐bandgap semiconductors.

Rapid PL Enhancement in Cd doped Alq3 Nanowires

Aluminium tris (8-hydroxyquinoline) (Alq3) is a popular coordination complex extensively used in organic luminescent devices as the emissive layer. Metal doping in Alq3 and its nanoscale synthesis are two strategies which are widely adopted to enhance the functional properties of Alq3. In the present work, Cadmium (Cd) doped Alq3 films having 1-dimensional morphology have been successfully synthesised using thermal vapor transport method. These films have been subjected to different analytical studies to study their functional properties. Scanning Electron Microscopy (SEM) images reveal the dendritic nature of Cd-doped Alq3 nanowires while Energy Dispersive X-ray Spectroscopy (EDAX) analysis confirms the presence of Cd in Cd-doped Alq3 nanowires. X-ray Diffraction (XRD) study reveals the polycrystalline nature of Cd-doped Alq3 nanowires. X-ray Photoelectron Spectroscopy (XPS) analysis of Cd-doped Alq3 nanowires samples indicates the replacement of some Al atoms with Cd atoms in Alq3 matrix. UV–visible spectroscopy study of the Cd-doped Alq3 nanowires shows enhancement in the optical absorbance with increasing Cd concentration while Photoluminescence (PL) spectroscopy analysis shows a 7-fold enhancement in PL intensity for 0.5 mol% Cd-doping. This rapid enhancement in the PL is explained on the basis of electronic charge transfer from Cd ions to Alq3 molecules.

Enhancement of Optical Properties and Stability in CsPbBr3 Using CQD and TOP Doping for Solar Cell Applications

Cesium lead bromide (CsPbBr3) nanocrystals exhibit remarkable optoelectronic properties and exceptional stability. As a result, they have garnered significant interest for their potential applications in various fields, including solar cells, light-emitting devices, photodetectors, and lasers. Despite its resistance to moisture, oxygen, and heat compared to other perovskite materials, CsPbBr3 still faces challenges maintaining its structural and optical stability over extended periods. This study proposes a robust solution to enhance and improve simultaneously the photoluminescence intensity and stability of CsPbBr3 nanocrystals. The solution involves doping the perovskite precursor with green-synthesized carbon quantum dots (CQDs) and tri-n-octyl phosphine (TOP). The results indicate that the photoluminescence intensity of the perovskite nanocrystals (NCs) is sensitive to varying CQD ratios. A high photoluminescence intensity enhancement of 45% was achieved at the optimal CQDs ratio. The synthesized perovskite NCs/CQDs also demonstrated improved stability by adding TOP into the mixture. After storage in the air for 45 days, the mixed perovskite NCs maintained their performance, which was almost unchanged. Solar cell devices based on the modified perovskite NCs showed a power conversion of 7.74%. The devices also demonstrated a significant open-circuit voltage (VOC), with the most successful device achieving a VOC of 1.193 V, an Isc of 10.5748 mA cm−2, and a fill factor (FF) of 61%. This study introduces a cost-effective method for producing high-quality all-inorganic optoelectronic devices with enhanced performance and stability.

Conjugated polymer-perovskite quantum dot (MDMO-PPV:CsPbBr3) nanocomposites: Miscibility, nano-structures, and properties

All-inorganic cesium lead bromide (CsPbBr3) quantum dots (QDs) have received a surge of attention in the field of light-emitting diode (LED) display and lighting. Hence, it is interesting to study the composite film composed of CsPbBr3 and light-emitting MDMO-PPV matrix polymer. In this study, we investigate the phase behavior among the components, MDMO-PPV, toluene (solvent), and oleic acid and oleylamine (the surface ligands for QDs) based on the Flory-Huggins theory with the group contribution method for the first time. Here we find that the MDMO-PPV and ligand molecules are immiscible whereas MDMO-PPV and toluene are partially miscible. Then through the x-ray diffraction (XRD) patterns, we demonstrate that CsPbBr3 QDs form a nanoscale domain with ∼33–52 nm crystallites in the MDMO-PPV matrix. Furthermore, the scanning electron microscope (SEM) images display that CsPbBr3 QDs can be highly aggregated at MDMO-PPV:CsPbBr3= 50:50 composition. Then, through the ultraviolet-visible (UV–vis) and photoluminescence (PL) spectra, the enhancement of PL intensity is observed at ∼30–50 wt% CsPbBr3. Finally, the electrochemical impedance spectra indicate that the composite film exhibits less resistance (∼3.2×104 Ω) than the pure MDMO-PPV film (∼1.4×107 Ω), suggesting that the MDMO-PPVCsPbBr3 composite approach is promising for electrochemical and optoelectronic applications.

Disentangling doping and strain effects at defects of grown MoS2 monolayers with nano-optical spectroscopy.

The role of defects in two-dimensional semiconductors and how they affect the intrinsic properties of these materials have been a widely researched topic over the past few decades. Optical characterization techniques such as photoluminescence and Raman spectroscopies are important tools to probe the physical properties of semiconductors and the impact of defects. However, confocal optical techniques present a spatial resolution limitation lying in a μm-scale, which can be overcome by the use of near-field optical measurements. Here, we use tip-enhanced photoluminescence and Raman spectroscopies to unveil the nanoscale optical properties of grown MoS2 monolayers, revealing that the impact of doping and strain can be disentangled by the combination of both techniques. A noticeable enhancement of the exciton peak intensity corresponding to trion emission quenching is observed at narrow regions down to a width of 47 nm at grain boundaries related to doping effects. Besides, localized strain fields inside the sample lead to non-uniformities in the intensity and energy position of photoluminescence peaks. Finally, two distinct MoS2 samples present different nano-optical responses at their edges associated with opposite strains. The edge of the first sample shows a photoluminescence intensity enhancement and energy blueshift corresponding to a frequency blueshift for E2g and 2LA Raman modes. In contrast, the other sample displays a photoluminescence energy redshift and frequency red shifts for E2g and 2LA Raman modes at their edges. Our work highlights the potential of combining tip-enhanced photoluminescence and Raman spectroscopies to probe localized strain fields and doping effects related to defects in two-dimensional materials.

(Invited) Deformation of 2D Semiconductors By Interfaces with Organic Materials

2D semiconducting materials gather attention for realizing atomically thin optoelectronic devices and a platform for emerging quantum phenomena. Especially monolayer 2D materials are fascinating because of their ultrathin crystalline structure with various physicochemical characteristics such as direct-band gap and asymmetric crystalline structures. In this presentation, I will show our recent results related to the deformation of transition metal chalcogenides (TMDCs) morphology by interfaces with organic materials.The mechanical exfoliation method is a simple method to prepare TMDCs and has been widely applied in many research studies. However, the exfoliated TMDCs on substrates show a bunch of bulk flakes and very few target monolayers. In the worst case, the exfoliated monolayer is surrounded by bulk flakes and is difficult to apply for fabricating devices. We developed a method of selectively removing bulk flakes, furthermore, isolating monolayer TMDCs on substrates. The method uses ultrasonication in organic solvents with a consideration of surface chemistry. Surprisingly, bulk flakes are selectively removed, and the adjacent monolayers can be effectively isolated on substrates. [1]Monolayer TMDCs show emission properties because of their direct-band gap nature. The emission strength, e.g., photoluminescence (PL) intensity, is often low. The limitation factor includes various parameters, for example, the over-carrier concentration is one limiting factor in sulfur-based compounds (MoS2 and WS2). So far, p-type doping to reduce the electron concentration is demonstrated to enhance their PL intensity, however, the method is not applicable to selenium-based compounds, such as MoSe2 and WSe2. In addition, a universal method to enhance PL intensity for the selenium-based compounds is still few. We found a general PL enhancement method for both sulfur- and selenium-based compounds, MoS2, WS2, MoSe2, and WSe2, by applying a hydrocarbon, paraffin. The method simply uses paraffin on the surface of TMDCs. I would show the details of the results and methods in the presentation. [2]REF:[1] T. Nakamoto, et al., " Selective isolation of mono to quad layered 2D materials via sonication-based solution engineering", ChemRxiv, 2022, DOI: 10.26434/chemrxiv-2022-lgp8l[2] T. Nakahara, et al., " Spontaneous crystal fluctuation in hydrocarbon polymer–coated monolayer MoS2, MoSe2, WS2, and WSe2 with strong photoluminescence enhancement", ChemRxiv, 2023, DOI: 10.26434/chemrxiv-2023-gxzqw

Influence of Annealing Temperature on the Structural and Luminescence Features and Thermoluminescence Trap Parameters of Ca2Al2SiO7: Ce3+ Phosphors

Cerium-doped Ca2Al2SiO7 phosphors were synthesized via the combustion technique and annealed at 1073 K, 1173 K, and 1373 K. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), photoluminescence (PL), and thermoluminescence (TL) studies were performed to explore the variation of structural, morphological, luminescence, and TL properties with annealing temperature. The XRD plot showed an increase in crystallinity with an increase in annealing temperature. Similarly, the enhancement of PL intensity with an increase in annealing temperature supports the increased crystallinity of the phosphor samples. Detailed analysis of traps and trap parameters was carried out using the principle of TL. Samples were irradiated with different γ-ray doses, and, on heating, luminescence was observed. Thus, TL glow curves were drawn and deconvoluted using computerized glow curve deconvolution (CGCD) and Chen’s peak shape method to study the traps and trap parameters. With increasing annealing temperature, an increase in TL intensity was observed. Regardless of annealing temperature, every sample displayed a linear dose–response for doses ranging from 3 Gy to 500 Gy. TL investigations suggested that the produced phosphors are suitable candidates for gamma dosimetry applications.

Fabrication of InGaN/GaN multiple quantum well embedded on nanoporous GaN mirror under different acid electrolytes

Large-size (2 inch) InGaN/GaN multiple quantum wells (MQW) embedded on nanoporous (NP)-GaN distributed Bragg reflectors (DBR) were fabricated by electrochemical (EC) etching at different acid electrolytes (e.g. HF, oxalic, and HNO3 acids). The sample obtained in HNO3 electrolyte presents the highest reflectance and the strongest photoluminescence (PL) intensity, whereas the sample fabricated by the HF etching has uneven surface, leading to very low reflectance. Compared with the as-deposited sample, the HNO3 etching leads to a 5-fold enhancement of PL intensity of etched sample, probably resulting from two factors which reflection of NP-GaN DBR mirror enhanced the light output, and amplify spontaneous emission (ASE) via a resonant cavity increased PL performance.

Utilizing internal resonant scattering to improve photoluminescence energy outcoupling of CsPbBr3/Cs4PbBr6 perovskite phosphor

Although stable perovskite phosphors, i.e., CsPbBr3/Cs4PbBr6, were extensively reported and made great progress, achieving strong photoluminescence (PL) energy outcoupling remains challenging owing to their short Stoke’s shift with serious PL energy absorption. This study utilized internal resonant scattering (IRS) to improve the PL energy outcoupling (PEO) of CsPbBr3/Cs4PbBr6 perovskite phosphor, which was realized by the nanopore-packing perovskite particles (NP-PPs) structure inside a phosphor. The nano-porous matrix hybrid solid–liquid interfacial reaction was developed to construct NP-PPs in a phosphor, and it is optimized to achieve a denser NP-PP structure with an enhanced PEO effect (2.8-fold enhanced PL intensity). Numerical results revealed that the NP-PP structure transforms the PL energy absorption mode to the IRS mode, most of the scattering PL energy evades NP-PPs instead of propagating inside. The optimized phosphor was applied to white devices, and we finally achieved an ultrahigh luminous efficacy of 131.32 lm/W based on green perovskite phosphors. This study provides a general guide for efficient perovskite phosphors utilizing the nano-porous matrix to enable the IRS effect.

Passivation of CdTe/MgCdTe double heterostructure by dielectric thin films deposited using atomic layer deposition

This work reports a study of the passivation effects of different dielectric thin films deposited on monocrystalline CdTe/MgCdTe double heterostructures (DHs) using atomic layer deposition (ALD). Enhanced photoluminescence intensity was observed in all DHs with surface passivation, and increased photogenerated carrier lifetime was observed in DHs with HfO2, TiO2, Al2O3, and TiN passivated CdTe/MgCdTe DHs. These results have demonstrated effective suppression of the undesired surface recombination of CdTe/MgCdTe DHs by dielectric thin films deposited via ALD.

Plasmon-enhanced fluorescence of gold nanoparticle/graphene quantum dots for detection of Cr3+ ions

Graphene quantum dots (GQDs), fascinating semiconductors with stable photoluminescence (PL), have important potential applications in the fields of biology, medicine, and new semiconductor devices. However, it is still challenging to overcome the weak PL intensity. Here, we report a strategy for selective resonance enhancement of GQD fluorescence using gold nanoparticles (AuNPs) as plasmas. Interestingly, the addition of low concentration AuNP makes AuNP/GQDs exhibit significant fluorescence enhancement of 2.67 times in the visible range. The addition of high concentration AuNP leads to the formation of an excitation peak at 421 nm and selectively enhances certain radiation modes. We concluded that the main reason for the selective enhancement of PL intensity in high concentration AuNP is the transfer of generous hot electrons at high energy states from AuNP to GQD and relaxation to the ground state. The electron resonance of low concentration AuNP transfers to GQD and relaxes to lower energy levels, exhibiting an overall enhancement of PL intensity. We apply it for detection of the heavy metal ion Cr3+, and verify that it has a correlation coefficient of 97.36%. We believe AuNP/GQDs can be considered excellent candidates for heavy metal detection and high fluorescence bio-imaging.

A systematic investigation of structural and optical properties of Li ion implanted MgTiO3 thin films

Effects of lithium (Li) ions implantation on structural, optical, and luminescent properties of MgTiO3 (MTO) thin films were investigated in the present report. The films were initially deposited at a substrate temperature of 200 °C, which were then annealed at 700 °C for crystallization. Crystalline MTO films were implanted with 30 keV Li ions at various fluences ranging from 1 × 1014 to 1 × 1015 ions/cm2. The crystallinity of films decreases upon Li ions implantation at a fluence of 1 × 1014 ions/cm2. In contrast, a further increase in fluence to 1 × 1015 ions/cm2 results in an improvement in crystallinity. Annealed and implanted films show lesser transmittance than the as-deposited films. As-deposited film exhibits a bandgap of 3.79 eV, which increases to 4.28 eV upon annealing the films. Subsequently, with an increase in implantation fluence, the shrinkage in optical bandgap from 4.26 to 4.09 eV is observed. Moreover, the annealed films exhibit a refractive index of ∼2.2 which is higher as compared to as-deposited and implanted MTO samples. X-ray photoelectron spectroscopy was carried out to investigate the surface states related to Mg, Ti, and O in the samples and to analyze the variation of defects’ concentration with implantation. MTO thin films show luminescent centres in near ultraviolet and visible region. The annealing results in the enhancement of photoluminescence intensity which decreases on implantation, and a drastic quenching of intensity was observed at a fluence of 1 × 1015 ions/cm2. The average decay lifetime of as-deposited film is 9.1 ns. Annealing results in an increase in the average decay lifetime, while implantation leads to the reduction of average decay lifetime of MTO films.

Photoluminescence upconversion in monolayer WSe2 activated by plasmonic cavities through resonant excitation of dark excitons

Anti-Stokes photoluminescence (PL) is light emission at a higher photon energy than the excitation, with applications in optical cooling, bioimaging, lasing, and quantum optics. Here, we show how plasmonic nano-cavities activate anti-Stokes PL in WSe2 monolayers through resonant excitation of a dark exciton at room temperature. The optical near-fields of the plasmonic cavities excite the out-of-plane transition dipole of the dark exciton, leading to light emission from the bright exciton at higher energy. Through statistical measurements on hundreds of plasmonic cavities, we show that coupling to the dark exciton leads to a near hundred-fold enhancement of the upconverted PL intensity. This is further corroborated by experiments in which the laser excitation wavelength is tuned across the dark exciton. We show that a precise nanoparticle geometry is key for a consistent enhancement, with decahedral nanoparticle shapes providing an efficient PL upconversion. Finally, we demonstrate a selective and reversible switching of the upconverted PL via electrochemical gating. Our work introduces the dark exciton as an excitation channel for anti-Stokes PL in WSe2 and paves the way for large-area substrates providing nanoscale optical cooling, anti-Stokes lasing, and radiative engineering of excitons.