Upconversion Photoluminescence Research Articles

Mn2+ Mediated Spin‐Exchange Enables CsPbBr3 Nanoplatelets to Function as Highly Efficient Triplet Sensitizers for Triplet‐Triplet Annihilation Upconversion

AbstractPerovskite nanoplatelets (NPls) have emerged as a new generation of nanomaterials that are promising for optoelectronic applications due to their large absorption and fast yet efficient emission. Particularly, they are also potential triplet sensitizers for triplet‐triplet annihilation upconversion (TTA‐UC), provided that the fast intrinsic decay of exciton can be circumvented. Herein, doping the CsPbBr3 NPls with Mn2+ (Mn2+: CsPbBr3 NPls) can efficiently convert fast‐decay triplet exciton to a long‐lived excited state of Mn2+ through Dexter‐type spin‐allowed energy transfer, enabling simultaneous large absorption and long‐lived excited state are demonstrated. Additionally, similar to the exciton‐to‐Mn2+ energy transfer, Mn2+‐to‐triplet state of annihilator energy transfer is also spin‐allowed and thus highly efficient. These pave the way for using Mn2+: CsPbBr3 NPls as triplet sensitizers. The superior triplet sensitization ability of Mn2+: CsPbBr3 NPls is demonstrated by using 9,10‐diphenylanthracene molecules (DPA) as representative annihilators. 93% quenching of Mn2+ emission and considerably accelerated decay of Mn2+ are observed after the addition of a trace amount of DPA. TTA‐UC measurement demonstrates that benefitted from the long‐lived excited state of Mn2+, the normalized upconversion photoluminescence quantum yield is ≈10% with an energy threshold being as low as ≈0.35 W cm−2. The results shed new light on using Mn2+: CsPbBr3 NPls as a triplet sensitizer.

Comparative study of luminescence in alkali‐metal‐based yttrium fluoride nanophosphor for biophotonic applications

AbstractAlkali‐metal‐based yttrium fluoride host (AY1‒xF4:xEu3+; A: Li/Na/K) nanophosphors were developed using co‐precipitation method. Phase analysis and emission spectral behavior were established for the said nanophosphors. The highest emission observed for NaYF4 doped with Eu3+ was then optimized to achieve maximum photoluminescence (PL) emission for varied dopant amount. The downconversion and upconversion PL studies revealed pure red emission under n‐ultraviolet and infrared excitations. The CIE coordinates are observed to be lie in the red hue of the CIE diagram. The PL decay curves reported to study lifetime of the emitting state and to identify quantum efficiency using Auzel's model for the said nanophosphor. Thermal stability was also evaluated from temperature‐sensitive luminescent spectra. The studies conducted on AYF4:.05Eu3+ (A: Li/Na/K) nanophosphors contemplates that NaYF4 nanophosphors doped with Eu3+ ions are best suited for biophotonic applications as the later one show relatively high emission.

Unexpected Performance of a Bifunctional Sensitizer/Activator Component for Photon Energy Management via Upconversion.

We here report on the observation of upconverted photoluminescence (UC-PL) from the blue-light-emitting 9,10-diphenylanthracene (DPA) mixed with the yellow-light-absorbing bifunctional sensitizer/activator component of (3,3,7,8,12,13,17,18-octaethylporphyrin-22,24-diid-2-one) PtII (PtOEP-K). Yellow-to-blue UC-PL (0.680 eV spectral upshift) is achieved at room temperature under ultralow power continuous incoherent photoexcitation (220 μW/cm2) despite the absence of triplet energy transfer (TET) between PtOEP-K and DPA. Under selective CW-laser photoexcitation of PtOEP-K in DPA:PtOEP-K, a 2.5% UC-PL quantum yield is obtained; that is an improvement exceeding by more than 3 orders of magnitude the UC-PL quantum yield of TTA-UC material combinations wherein no TET is operative. The PL response of DPA:PtOEP-K to varying laser fluence suggests that bimolecular annihilation reactions between triplet-excited PtOEP-K facilitate the UC-PL activation in DPA. These findings pave the way toward low-complexity strategies for the reduction of transmission losses in solar energy technologies through an innovative wavelength upshifting protocol involving excitonic materials.

Upconversion nanoparticles-modified aptasensors for highly sensitive mycotoxin detection for food quality and safety.

Mycotoxins, highly toxic and carcinogenic secondary metabolites produced by certain fungi, pose significant health risks as they contaminate food and feed products globally. Current mycotoxin detection methods have limitations in real-time detection capabilities. Aptasensors, incorporating aptamers as specific recognition elements, are crucial for mycotoxin detection due to their remarkable sensitivity and selectivity in identifying target mycotoxins. The sensitivity of aptasensors can be improved by using upconversion nanoparticles (UCNPs). UCNPs consist of lanthanide ions in ceramic host, and their ladder-like energy levels at f-orbitals have unique photophysical properties, including converting low-energy photons to high-energy emissions by a series of complex processes and offering sharp, low-noise, and sensitive near-infrared to visible detection strategy to enhance the efficacy of aptasensors for novel mycotoxin detection. This article aims to review recent reports on the scope of the potential of UCNPs in mycotoxin detection, focusing on their integration with aptasensors to give readers clear insight. We briefly describe the upconversion photoluminescence (UCPL) mechanism and relevant energy transfer processes influencing UCNP design and optimization. Furthermore, recent studies and advancements in UCNP-based aptasensors will be reviewed. We then discuss the potential impact of UCNP-modified aptasensors on food safety and present an outlook on future directions and challenges in this field. This review article comprehensively explains the current state-of-the-art UCNP-based aptasensors for mycotoxin detection. It provides insights into potential applications by addressing technical and practical challenges for practical implementation.

High‐Efficiency Single‐Photon Upconversion Photoluminescence of Perovskite Quantum Dots via Intragap State Regulation

AbstractLead halide perovskite quantum dots (PQDs) have shown remarkable single‐photon upconversion photoluminescence (SPUC‐PL) bringing them a brilliant perspective as the active media for biological imaging, optiag, and upconversion photovoltaics. However, the mechanism of SPUC in PQDs is still in dispute, and the specific strategy for optimizing PQDs toward a higher SPUC performance has yet to be established, which impedes the application of PQDs‐based SPUC‐PL in practice. In this work it is revealed that the PL quantum yield is declined by surface trap states, while the SPUC efficiency is promoted by Urbach tail states. These two types of intragap states can be chemically manipulated by engineering the surface ligand and the growth kinetics of perovskite nanocrystals, bringing on the improvement of light emission efficiency and the promotion of subgap low‐energy photon absorption, respectively. As a proof of concept, utilizing the synergistic effect of the proposed chemical manipulation method, the SPUC‐PL efficiency of PQDs is boosted by more than 40%, which gives rise to a spectral window of optical refrigeration gain as broad as ≈130 meV.

Upconversion photoluminescence and optical temperature probe of Er3+/Yb3+ co-doped LAZT glass prepared by containerless aerodynamic levitation method

xEr3+/yYb3+ co-doped La2O3-Al2O3-ZrO2-TiO2 ([30-(30/88) × (x + y)]La2O3-[58-(58/88) × (x + y)]Al2O3-4ZrO2-8TiO2: x mol% Er2O3/y mol% Yb2O3, x = 2 mol%, and y = 4, 8, 12, 16, 20 mol%, abbreviated as: LAZT: xEr/yYb) glasses were synthesized using a containerless aerodynamic levitation technique. The maximum phonon energy, fluorescence performance and optical temperature measurement performance of the prepared glasses were studied. Raman test shows that the maximum phonon energy of all glasses is less than 355 cm−1, in which the low phonon energy can provide an excellent internal environment for the induction of fluorescence effects. Under excitation at 980 nm, all samples exhibit noticeable red emission peak centering at 662 nm and feeble green emission peak centering at 549 nm at room temperature. The luminous intensity reaches the highest value at x = 2 mol% and y = 12 mol%, at which the concentration quenching phenomenon occurs. The fluorescence mechanism of LAZT: xEr/yYb involves a double-photon process, in which the energy transfer process (ET) accounts for the main part and the excited state absorption process (ESA) accounts for the secondary part. The fluorescence intensity ratio (FIR) can be used for temperature sensing, and the absolute sensitivity FIR reaches maximum value of 1.3 × 10−2 K−1 at 79 K. These results indicate that the LAZT: 2Er/12Yb glass is expected to become a candidate material for non-contact optical temperature probe.

Novel CQDs/BiOI Microspheres with Gradient Oxygen Vacancies for Efficient Photocatalytic Performance Boosting by the Synergism of Reactive Oxygen and Iodine Species

AbstractThe synergy of reactive oxidative species (ROSs: ⋅OH, ⋅O2− and 1O2) and reactive iodine species (RISs: ⋅I and ⋅I2−) is conclusive to driving organic pollutant degradation in various aquatic environments. Over the past years, rational design of Vis‐NIR responsive photocatalyst with enhanced photocatalytic performance has attracted drawing attention. Herein, CQDs/BiOI composites with up‐conversion photoluminescence (UCPL) properties and oxygen vacancies was constructed for tetracycline (TC) and oxytetracycline (OTC) degradation. The apparent kinetic constant of the 4‐CQDs/BiOI is 2.13 and 2.38 times on pure BiOI. The excellent photodegradation efficiency can be attributed to the enhanced light absorption originated from the up‐conversion property and improved charge separation efficiency. Furthermore, the iodine free radicals (⋅I) and superoxide free radicals (⋅O2−) played a crucial role in the photodegradation procedure. I− from BiOI could be partially oxidized to form ⋅I. At the same time, the electron capture traps formed by Oxygen vacancies can promote the production of ⋅O2−. Remarkably, the removal efficiency of TC and OTC was still maintained at 80 % by 4‐CQDs/BiOI under different water quality conditions due to the synergy of ⋅O2− with ⋅I. This work sheds light on a new treatment for the synergistic degradation of pollutants in complex water by RISs and ROSs.

Modulation of photoluminescence properties by poling and quenching strategy in rare earth ions doped Na1/2Bi1/2TiO3-based ceramics

With the continuous development of the society, optical ceramics are more and more widely used in daily life. In this work, the photochromic (PC) and photoluminescence (PL) properties of Na1/2Bi1/2TiO3 (NBT)-based ceramics doped with different rare-earth elements (Er, Eu, and Ho) are investigated. It is found that NBT-Er\\Eu and NBT-Er\\Ho ceramics have good up-conversion PL properties under 980 nm laser excitation. After poling and quenching treatments, the PL intensity of NBT-Er\\Eu ceramics increases by 1.5 and 11.7 times, respectively, and the NBT-Er\\Ho ceramics increases by 5.1 and 3.6 times, respectively. Besides, the rare-earth ions doped NBT-based ceramics exhibit fast PC response and self-bleaching behavior. In addition, multicolor luminescence behavior (from bright green to orange-yellow) is obtained in NBT-Er\\Eu ceramics, suggesting the potential application of this material in the field of optical information storage and optical anti-counterfeiting. Therefore, this work not only provides a new PC material exhibiting fast PC response and self-bleaching behavior, but also develops new universal strategies to modulate PL properties without changing the composition.

Amphi-Luminescent MoS2 nanostructure for photocatalytic splitting of water and removal of Methylene Blue

Chemical dyes used in the textile industries are one of the major pollutants in water. Methylene blue (MB) is a commonly seen dye that creates hazardous health problems. In this article the photocatalytic degradation of MB by the nanocatalyst MoS2 (Nano-MoS2) and carbon dot (C Dots) incorporated MoS2 (Nano-CD-MoS2) is reported. The photocatalytic degradation of MB is analyzed based on the electron-hole recombination rate of the catalyst. Photoluminescence emission exhibited by the catalyst is used as a key indicator to probe the electron-hole recombination rate. Nano-MoS2 was synthesized hydrothermally at 180 0C for 8 h from ammonium tetra thiomolybdate (ATTM). C Dot was prepared following a green root from ash guard extract which later mixed with Nano-MoS2 and kept in an autoclave at a temperature 140 °C for 4 h to get Nano-CD-MoS2. The photoluminescence (PL) and photocatalytic behavior of Nano-MoS2 and Nano-CD-MoS2 and their application for water splitting and water purification are reported. The incorporation of graphene and artificial C Dot into MoS2 nanostructures are reported to increase the conductivity and active edge sites of MoS2 that enhances the photocatalytic action. Since green C Dots are eco-friendly and easily synthesizable than artificial C Dots, as a novel study, this article investigated the influence of green C Dots on the PL and photocatalytic performance of nanosized MoS2. Nano-MoS2 and Nano-CD-MoS2 exhibited both upconversion and downconversion PL; accordingly the nanostructures were termed as amphi-luminescent. The amphi-luminescence property widens the photon absorption range and hence enhances the catalytic degradation of dyes. Nano-MoS2 which exhibited lesser intensity of amphi-luminescence emission compared to Nano-CD-MoS2 showed better results in degradation of MB. C Dots may bind with the valence band electrons of MoS2, resulting in the reduction of dangling bonds. Dangling bonds can trap photo-induced excitons to hinder the rate of electron-hole recombination. So, fast electron-hole recombination occurs in Nano-CD-MoS2 than Nano-MoS2. Fast electron-hole recombination supports radiative electron-hole recombination while suppresses the non-radiative energy transfer of electrons and causes high PL intensity. However, according to the energy level diagram, Nano-MoS2 with minimal electron-hole recombination rate is more favorable for O2/O2–,.OH/ OH– and.OH/H2O reactions that facilitate MB degradation. Photocatalytic activity of catalysts were confirmed by measuring the photocurrent from a simple custom-made two-electrode water photolysis cell where the nanocatalysts were dispersed in electrolyte. Lead and steel rods were used as electrodes. Multimeter was used to measure current. Nano-MoS2 exhibited better performance with a maximum photocurrent of 141 µA. Influence of green C Dots in energy levels, PL and photocatalysis of MoS2 and mechanisms of PL and degradation of MB are thoroughly investigated in this article.

The Impact of the Amorphous-to-Crystalline Transition on the Upconversion Luminescence in Er3+-Doped Ga2O3 Thin Films

Gallium oxide (Ga2O3) is an emerging wide bandgap semiconductor promising a wide range of important applications. However, mass production of high-quality crystalline Ga2O3 still suffers from limitations associated with poor reproducibility and low efficiency. Low-temperature-grown amorphous Ga2O3 demonstrates comparable performance with its crystalline counterparts. Lanthanide Er3+-doped Ga2O3 (Ga2O3: Er) possesses great potential for developing light-emitting devices, photodetectors, solid-state lasers, and optical waveguides. The host circumstance can exert a crystal field around the lanthanide dopants and strongly influence their photoluminescence properties. Here, we present a systematical study of the impact of amorphous-to-crystalline transition on the upconversion photoluminescence in Ga2O3: Er thin films. Through controlling the growth temperature of Ga2O3: Er films, the upconversion luminescence of crystalline Ga2O3: Er thin film is strongly enhanced over 100 times that of the amorphous Ga2O3: Er thin film. Moreover, the variation of photoluminescence reflects the amorphous-to-crystalline transformation of the Ga2O3: Er thin films. These results will aid further designs of favorable optoelectronic devices integrated with lanthanide-doped Ga2O3 thin films.

Designing Photochromic Materials La2MgSnO6:Er,Fe with Dynamic Luminescence Modulation for Dual‐mode Optical Information Reading

AbstractInorganic photochromic luminescent materials hold immense promise as potential candidates for optical information storage applications. Unfortunately, static and single reading mode of photochromic luminescent materials in bright and dark environments pose a security threat to the stored information. Here, a novel material La2MgSnO6:Er3+,Fe3+ is developed, possessing both reversible photochromic and adjustable upconversion/downconversion photoluminescent (PL) properties. Upon exposure to 275 nm ultraviolet (UV) light, the sample changes in color from white to brown. And this transformation rapidly reverses when exposed to 365 nm UV light, resulting in a corresponding alteration in luminescence intensity. Notably, when combined with photochromic, the 980 nm excited upconversion PL maintains stable luminescence, while the 365 nm excited downconversion PL exhibits significant intensity changes. Inspired by this unique feature, a dark‐field selective dual‐mode reading is proposed that depends on the excitation wavelength, including static decrypt in long‐time cases and dynamic reading in time‐limited situations. These findings have the potential to revolutionize high‐security data storage applications and drive advancements in the field.

Structural and optical properties of ZnGa2−x(YbXCrGe)xO4 (X = Ho and Er) up-conversion photoluminescence materials

ZnGa2−x(YbXCrGe)xO4 (X = Ho3+, Er3+) compounds are prepared using the solid-state-reaction method. Alteration in the lattice parameters and strain signifies the substitutional doping of elements at the octahedrally coordinated Ga3+ ions. Down-shifting PL properties are observed at the excitation wavelength of 275 nm. NIR wavelength (980 nm) induced up-conversion PL is observed in the Yb and Ho and/or Er doped samples. The prominent green-colored emission is observed in Yb3+ and Ho3+ doped sample and intense red-colored emission is achieved in the Yb3+ and Er3+ doped sample. The existence of up-conversion photoluminescence is mechanistically discussed by considering the energy transfer processes among the Zn2+, Ga3+, Yb3+, Ge3+, Cr3+, Er3+, and Ho3+ ions.

Ionic-confining-assisted multiple-mode tunable light emitting of carbon nanodots

Multiple-mode light emitting in one material system is tremendous desirable for various applications. Herein, we have developed an ionic crystal confining strategy to prepare the carbon nanodots (CDs) with quadruple-mode light emitting and demonstrated their promising application in information anti-counterfeiting via optical logic gates. In detail, the quadruple-mode light emitting of CDs (QM-CDs) are prepared with the citric acid and urea with NaOH matrix via microwave-assisted polymerization. Mechanically, the high-density ionic bonds stemmed from the NaOH ionic-crystal can effectively inactivate nonradiative defect centers, stabilize triplet excitons, increase absorption cross-section and boost resonance energy transfer, evoking the outstanding solid-state fluorescence, upconversion photoluminescence (UCPL), phosphorescence, and chemiluminescence simultaneously. Meanwhile, the isolated NaOH matrix can promote the confining aggregation of CDs, leading to the redshifted emission wavelength and endowing the light emitting with tunable wavelength. With the excitability of logic functions between the light emitting and controlled excitation, the programmable multiple-inputs logic gates are established with the QM-CDs, enabling the application in high-throughput logical operations and information anti-counterfeiting. This research provides a new insight into the synthesize of multiple-mode luminescent CDs, and thus promote the applications of nanomaterials in intelligent encryption and anti-counterfeiting.

Unveiling the Radiative Electron-Hole Recombination of MoS2 Nanostructures at Extreme pH Conditions.

Nanostructures of MoS2 are in wide research for optoelectronic, energy and biological applications. Opto-electronic and biological applications requires the tuning of photoluminescence properties of MoS2 nanostructures. In this article, nanosized MoS2 is hydrothermally synthesized, and photoluminescence at extreme pH conditions (pH 1 and 13) is examined. As the photoluminescence gives a key to probe the radiative electron-hole recombination, here, photoluminescence emissions are used as an indicator to suggest the pattern of electron-hole recombination in the material at extreme pH conditions. Raman spectroscopy, dynamic light scattering, Scanning electron microscopic image and energy dispersive x-ray analysis are done for material confirmation. At pH 1 and 13 as-synthesized nanostructured MoS2 exhibited both upconversion and downconversion photoluminescence. The intensity of photoluminescence is varied with respect to pH. Excitation-dependent photoluminescence mechanisms and preliminary understanding on the ratio of quantum yields and life span of excited state of as-synthesized nanostructured MoS2 are unveiled here.

Resonant Multiple-Phonon Absorption Causes Efficient Anti-Stokes Photoluminescence in CsPbBr3 Nanocrystals.

Lead halide perovskite nanocrystals, such as CsPbBr3, exhibit efficient photoluminescence (PL) up-conversion, also referred to as anti-Stokes photoluminescence (ASPL). This is a phenomenon where irradiating nanocrystals up to 100 meV below gap results in higher energy band edge emission. Most surprising is that ASPL efficiencies approach unity and involve single-photon interactions with multiple phonons. This is unexpected given the statistically disfavored nature of multiple-phonon absorption. Here, we report and rationalize near-unity anti-Stokes photoluminescence efficiencies in CsPbBr3 nanocrystals and attribute them to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals. Moreover, the developed microscopic mechanism has immediate and important implications for applications of ASPL toward condensed phase optical refrigeration.

Biaxial strain tuned upconversion photoluminescence of monolayer WS2

Monolayer tungsten disulfide (1L-WS2) is a direct bandgap atomic-layered semiconductor material with strain tunable optical and optoelectronic properties among the monolayer transition metal dichalcogenides (1L-TMDs). Here, we demonstrate biaxial strain tuned upconversion photoluminescence (UPL) from exfoliated 1L-WS2 flakes transferred on a flexible polycarbonate cruciform substrate. When the biaxial strain applied to 1L-WS2 increases from 0 to 0.51%, it is observed that the UPL peak position is redshifted by up to 60 nm/% strain, while the UPL intensity exhibits exponential growth with the upconversion energy difference varying from − 303 to − 120 meV. The measured power dependence of UPL from 1L-WS2 under biaxial strain reveals the one photon involved multiphonon-mediated upconversion mechanism. The demonstrated results provide new opportunities in advancing TMD-based optical upconversion devices for future flexible photonics and optoelectronics.

Superior energy storage density and bright upconversion emission in Er-modified KNN-based multifunctional ferroelectric ceramics

Developing highly tunable multifunctional performances in one ferroelectric system is in line with the integration trend of electronic equipment. Herein, rare earth erbium (Er) modulated 0.95K0.5Na0.5NbO3–0.05Bi(Li0.5Nb0.5)O3–x%Er photoluminescent–ferroelectric energy storage multifunctional ceramics are prepared. All the compositions exhibit a similar grain size and compact microstructure. In addition, the optimized Er dopant enhances the enhanced relaxor characteristic and reduces oxygen vacancies. The introduction of Er3+ into perovskite lattice contributes to the observed photoluminescence behavior. Therefore, high effective energy storage density (Wrec) of 7.17 J/cm3, energy storage efficiency (η) of 65.4%, and strong green/red upconversion photoluminescence are obtained in x = 0.2 sample. This work opens up a paradigm to develop multifunctional ferroelectric ceramics for application in electro-optical devices.

Enhancement of ultraviolet upconversion photoluminescence of Y7O6F9:Pr, Gd induced by Li+ codoping☆

Ultraviolet upconversion photoluminescence materials have great potential in various fields, but the improvement of the upconversion efficiency is challenging. Codoping of Li+ is considered as an effective strategy and widely used to improve the photoluminescence properties of phosphors. In this paper, Li+ is introduced into a Y7O6F9:Pr3+,Gd3+ system. The effect of Li + codoping on the phase purity, crystal structure, microstructure, downshifting and upconversion photoluminescence as well as the decay dynamic of the phosphors was studied. It is revealed that the overall photoluminescence efficiency and the energy transfer efficiency from Pr3+ to Gd3+ are greatly promoted. The downshifting and upconversion photoluminescence increase by 2.58 and 10 times as 6 mol% of Li+ is codoped. The photoluminescence decay dynamic study shows that the 3P0 state decays slower in the Li + -containing phosphor than the Li+-free one. The improvement of the photoluminescence properties is due to the increase of the crystallinity and the reduce of the quenching center.

Enhanced photocatalytic gaseous formaldehyde degradation using N-CQDs/OVs-TiO2 composite under visible light: Unraveling the synergistic effects of N-CQDs and oxygen vacancies

Limited utilization of photogenerated charge carriers in titanium dioxide under visible light have hinder its application development. To address this challenge, a novel N-doped carbon quantum dots (N-CQDs) and oxygen vacancies (OVs) synergistically decorated on TiO2 (P25) was synthesized through a facile one-step hydrothermal method. Under visible light irradiation, the first order reaction rate constants of formaldehyde (HCHO) photocatalytic oxidation by OVs-TiO2 and N-CQDs/OVs-TiO2 was significantly higher than that of pristine P25, with 10.1 and 16.7 folds increase, respectively. Characterization results confirmed the generation of OVs on the surface of N-CQDs/TiO2 composite. The optical and electrochemical experiments suggested the electron capture center effect of OVs and the properties of N-CQDs in unique up-converted photoluminescence, efficient charge separation, as well as significant adsorption in visible light region. In addition, the work function also clarified that photoelectrons could transfer from N-CQDs to OVs-TiO2. Furthermore, different relative humidity and electron paramagnetic resonance (EPR) experiments demonstrated that the hydroxyl radical (•OH) was the dominant reactive radical in HCHO photodegradation. The •O2− could also enhance the photodegradation efficiency of HCHO. This work provides an in-depth understanding on the complementary roles of N-CQDs and OVs and is helpful for designing metallic oxide photocatalysts for volatile organic compounds removal.

Upconversion photoluminescence of monolayer WSe2 with biaxial strain tuning.

Mechanical strain can be used to tune the optical properties of monolayer transition metal dichalcogenides (1L-TMDs). Here, upconversion photoluminescence (UPL) from 1L-WSe2 flakes is tuned with biaxial strain induced by cruciform bending and indentation method. It is found that the peak position of UPL is redshifted by around 24 nm as the applied biaxial strain increases from 0% to 0.51%. At the same time, the UPL intensity increases exponentially for the upconversion energy difference that lies within a broad range between -157 meV to -37 meV. The observed linear and sublinear power dependence of UPL emission in 1L-WSe2 with and without biaxial strain at three different excitation wavelengths of 784 nm, 800 nm, and 820 nm indicates the multiphonon-assisted one-photon upconversion emission process. The results of strain-dependent UPL emission from 1L-TMDs pave a unique path to the advances in photon upconversion applications and optoelectronic devices.