Synergy between plasmonic nanocavities and random lasing modes: a tool to dequench plasmon quenched fluorophore emission.

Metal nanoparticles (NPs) can be employed to modify the emission level of a dye emitter by tailoring the spectral overlap of the optical gain and localized surface plasmon resonance (LSPR). In the case of plasmonic random lasers, tuning the spectral overlap by manipulating metal NPs changes the scattering properties of the system, which is crucial in random lasers (RLs). In order to overcome this drawback, the emitter gain spectrum across the LSPR is tuned by appropriately choosing various dye emitters. A system with Au nanoislands (NIs) randomly distributed on the surface of vertically aligned ZnO nanorods on a glass substrate coated with three different dye emitters has been employed to study the metal-gain interaction as a function of spectral overlap. It is observed that the photoluminescence is quenched in the presence of Au NIs for all the three dye emitters; however, the degree of quenching is found to be directly proportional to the extent of spectral overlap of the LSPR and the fluorophore emission spectrum, with the resonantly coupled systems exhibiting higher random lasing thresholds. However, a dequenching of the emission is observed under spectrally off-resonant conditions, leading to a lower threshold RL. The effect of tailoring of the metal-gain interaction on the coherent and incoherent intensity components of RL emission is studied to elucidate the contrasting results of photoluminescence and RL emission. As the optical gain shifts away from the LSPR peak, the RL emission is dominated by the coherent intensity. The speckle-like field distributions of the RL modes couple to the plasmonic nanocavities along with a reduced absorption loss for the off-resonant case, leading to an enhanced stimulated emission. Hence, a synergy between random laser modes, plasmonic nanocavities and optimum spectral overlap has been utilized as a tool to dequench the plasmon quenched fluorophore emission.

Superior white electroluminescent devices using nitrogen-doped carbon dots/TiO2nanorods heterostructures.

Nitrogen-doped carbon dots (NCDs), exhibiting strong yellow emission in aqueous solution and solid matrices, have been utilized for fabricating heterostructure white electroluminescence devices. These devices consist of nitrogen-doped carbon dots as an emissive layer sandwiched between an organic hole transport layer (PEDOT:PSS) and an array of rutile TiO2nanorods, acting as an electron transport layer. Under an applied forward bias of 5 V, the device exhibits broadband electroluminescence covering the wavelength range of 390-900 nm, resulting in pure white light emission characteristics at room temperature. The result demonstrates the successful fabrication of all solution-processed, low-cost, eco-friendly NCDs-based LEDs with CIE (Commission Internationale d'Éclairage) coordinate of (0.31, 0.34) and color rendering index (CRI) > 90, which are close to ideal white light emission characteristics. The device functionalities are achieved based on defect-related NIR emission from TiO2nanorods array and visible emission from nitrogen-doped carbon dots. This result paves a new opportunity to develop low-cost, solution-processed nitrogen-doped carbon dots based on warm White light emitting diodes with high CRI for large-area display and lighting applications.

Role of Efficient Charge Transfer at the Interface between Mixed-Phase Copper-Cuprous Oxide and Conducting Polymer Nanostructures for Photocatalytic Water Splitting.

Photocatalytic hydrogen generation from water splitting is regarded as a sustainable technology capable of producing green solar fuels. However, the low charge separation efficiencies and the requirement of lowering redox potentials are unresolved challenges. Herein, a multiphase copper-cuprous oxide/polypyrrole (PPy) heterostructure has been designed to identify the role of multiple oxidation states of metal oxides in water reduction and oxidation. The presence of a mixed phase in PPy heterostructures enabled an exceptionally high photocatalytic H2 generation rate of 41 mmol h-1 with an apparent quantum efficiency of 7.2% under visible light irradiation, which is a 7-fold augmentation in contrast to the pure polymer. Interestingly, the copper-cuprous oxide/PPy heterostructures exhibited higher charge carrier density, low resistivity, and 6 times higher photocurrent density compared to Cu2O/PPy. Formation of a p-p-n junction between polymer and mixed-phase metal oxide interfaces induce a built-in electric field which influences directional charge transfer that improves the catalytic activity. Notably, photoexcited charge separation and transfer have been significantly improved between copper-cuprous oxide nanocubes and PPy nanofibers, as revealed by femtosecond transient absorption spectroscopy. Additionally, the photocatalyst demonstrates excellent stability without loss of catalytic activity during cycling tests. The present study highlights a superior strategy to boost photocatalytic redox reactions using a mixed-phase metal oxide in the heterostructure to achieve enhanced light absorption, longer charge carrier lifetimes, and highly efficient photocatalytic H2 and O2 generation.

Vacancy-Mediated Anomalous Emission Characteristics of Size-Confined Semiconducting CoTe2.

Transition-metal tellurides (TMTs) are promising materials for "post-graphene age" nanoelectronics and energy storage applications owing to their industry-standard compatibility, high electron mobility, large spin-orbit coupling (SOC), etc. However, tellurium (Te) having a larger ionic radius (Z = 52) and broader d-bands endows TMTs with semimetallic nature, restricting their application in photonic and optoelectronic domains. In this work, we report the optical properties of the quantum-confined semiconducting phase of cobalt ditelluride (CoTe2) for the first time, exhibiting excellent two-color band photoabsorption attributes covering the UV-visible and near-infrared regions. Furthermore, novel excitonic resonances (X) of size-varying CoTe2 nanocrystals and quantum dots (QDs) are indicated by their temperature-dependent emission characteristics, which are attributed to the splitting of band edge states via confinement. On the other hand, the sudden rupture of the large-area CoTe2 nanosheets via ultrasonication incorporates Co vacancy-mediated localized trap states within the band gap, which is attributed to the superior room-temperature photoluminescence (PL) quantum yield of QDs and further corroborated using Raman analysis and atomistic density functional theory (DFT) simulations. Most interestingly, the excitonic peak of CoTe2 QDs reveals a unique positive-to-negative thermal quenching transition phenomenon, owing to the thermal activation of nonradiative surface trap states. These results introduce an exciting approach for the defect-mediated color-saturated light emission that paves the way for solution-processed telluride-based QD light-emitting diodes.

Piezo-phototronic mediated enhanced photodetection characteristics of plasmonic Au-g-C3N4/CdS/ZnO based hybrid heterojunctions on a flexible platform.

We have studied the piezo-phototronic induced enhancement in the photo-response of CdS/ZnO heterojunctions attached with plasmonic Au nanoparticle loaded 2D-graphitic carbon nitride (g-C3N4). The hybrid g-C3N4/CdS/ZnO heterojunction favours the charge carrier separation through the formation of a step-like band alignment. Furthermore, the integration of plasmonic Au loaded g-C3N4 nanosheets on the conventional CdS/ZnO heterojunction facilitates improved visible light absorption properties. The heterojunction device on a flexible platform under the application of a strain (∼0.017%) exhibits ∼102 times higher photoresponse over the control sample at a constant bias of ∼2 V. The variation in the photo-response under different bending conditions has been explained in terms of the improved charge transport through the modified energy bands at the interface of ZnO. The improved piezo-phototronic properties originated from the plasmonic properties of Au loaded g-C3N4 and the piezoelectric characteristics of c-axis oriented ZnO films may be used for future flexible photonic devices.