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MoS2-CZTS: a 2D-3D nanocomposite to enhance the photocatalytic performance in degradation of methylene blue dye

Cu2ZnSnS4 nanocrystals (CZTS NCs), MoS2 nanosheet (NS) and MoS2-CZTS nanocomposite (NC) have been synthesized using solvothermal route. Structural characterization of samples have been done by XRD, Raman spectroscopy, HR-TEM. Samples have optically characterized by UV-Vis absorption, photoluminescence (PL) and time co-related single photon counting (TCSPC) study. The XRD, HR-TEM and Raman spectroscopy established tetragonal kesterite phase for both CZTS NCs and MoS2-CZTS NC. Enhancement of efficiency of CZTS NCs to degrade methylene blue (MB) dye, illuminated by visible light, have been observed by loading 1 wt.% MoS2 NS and found to be ~100% in only 15 minutes. This is due to efficent transfer of charge carriers at p-CZTS and n-MoS2 heterojunction interface, confirmed by quenching of PL intensity and decrease in average lifetime of carriers.

Effect of halide-mixing on tolerance factor and charge-carrier dynamics in (CH3NH3PbBr3−xClx) perovskites powders

This work demonstrates a route to making mixed halide perovskite powders at room temperature by the anti-solvent-assisted crystallization method. Although, mixed halide CH3NH3PbBr3−xClx perovskites have been prepared by different methods, however, to the best of our knowledge the anti-solvent-assisted crystallization method is employed here for the first time to prepare mixed halide CH3NH3PbBr3−xClx perovskite powders. Solution-processed methyl ammonium lead tribromide CH3NH3PbBr3 (x = 0) and different amounts of chloride (Cl) containing mixed halide perovskites (CH3NH3PbBr3−xClx) were prepared for compositions of x = 0.5, 1, 1.25, 1.75. It reveals that bulk CH3NH3PbBr3−xClx samples are highly crystalline and exists in pure single cubic phase with an increased tolerance factor as compared to pure CH3NH3PbBr3. The CH3NH3PbBr3 perovskite has space-group Pm-3 m and a cell parameter of 5.930 A (volume = 206 A). The synthesis route adopted here gives access to hybrid perovskites powders with high Cl content and hence enables the band gap to be precisely tuned over a range from 2.26 to 2.49 eV. The powder samples display the subtle shifts in the emission spectra and the photoluminescence kinetics exhibits a decrease in average lifetime by increasing the Cl contents due to the presence of trap states in the structures that encourage non-radiative recombination of charge carrier. Conventionally, the CH3NH3PbBr3-based inverted solar cell architecture is prepared via mixing of the CH3NH3Br and PbBr2 precursors. In contrast, herein, the precursor solutions are directly prepared from the CH3NH3PbBr3 powder and the active layer of the inverted perovskite solar cells are then spin coated using this solution. The high Voc value of the fabricated solar cells potentially makes it a promising candidate for tandem photovoltaic, photocatalytic water splitting, and semi-transparent photovoltaic applications.

Temperature features of non-radiative energy transfer in hybrid associates of CdS/TGA quantum dots with methylene blue molecules

The paper presents the study results of non-radiative resonance energy transfer of electronic excitation from colloidal CdS quantum dots passivated with thioglycolic acid (CdS/TGA QDs) with an average size of 2.9 nm to methylene blue (MB+) molecules in temperature ranging from 300 to 80 K. In an aqueous MB+ and colloidal CdS/TGA QDs mixture, the increase in (MB+)2 dimers fraction and shift of the absorption band peaks of monomers (MB+) from 662 to 676 nm and dimers (MB+)2 from 606 to 590 nm were found. It is concluded that the hybrid associates of CdS/TGA+MB+ and CdS/TGA+(MB+)2 are formed. Luminescence quenching at CdS/TGA QDs band and MB+ luminescence enhancement under increase in MB+ molecules concentration were observed. A decrease in average lifetime of CdS/TGA QDs luminescence from 25 to 4 ns was found for MB+ concentration of 42 μ M. The Stern–Volmer constant, determined from the data on luminescence quenching, is 0.36 ∗ 106 M− 1. It is concluded that there is non-radiative resonance energy transfer (FRET) from the luminescence center of CdS/TGA quantum dots to MB+ molecule. Decreasing in temperature value to 80 K led to an increase in the luminescence intensity by 12 times. At the same time, the average lifetime of luminescence for pure CdS/TGA QDs increases from 25 to 72 ns. The Stern–Volmer constant increased to 0.64 ∗ 106 M− 1. It is concluded that its increase is due to an increase in average lifetime of CdS/TGA QD luminescence. In the framework of FRET theory, the rate constant of non-radiative energy transfer was estimated for an even distribution of CdS/TGA quantum dots and MB molecules in solution. Its is much smaller (6.7 ∗ 104 s− 1) than the experimentally determined value (2.5 ∗ 108 s− 1). This fact confirms the conclusion about the formation of CdS/TGA+MB hybrid system with effective FRET.

Structural and spectroscopic studies of a nanostructured silicon-perovskite interface.

While extensively investigated in thin film form for energy materials applications, this work investigates the formation of APbBr3 structures (A = CH3NH3+ (MA), Cs+) in silicon and oxidized silicon nanotubes (SiNTs) with varying inner diameter. We carefully control the extent of oxidation of the nanotube host and correlate the relative Si/Si oxide content in a given nanotube host with the photoluminescence quantum efficiency (PLQE) of the perovskite. Complementing these measurements is an evaluation of average PL lifetimes of a given APbBr3 nanostructure, as evaluated by time-resolved confocal photoluminescence measurements. Increasing Si (decreasing oxide) content in the nanotube host results in a sensitive reduction of MAPbBr3 PLQE, with a concomitant decrease in average lifetime (τave). We interpret these observations in terms of decreased defect passivation by a lower concentration of oxide species surrounding the perovskite. In addition, we show that the use of selected nanotube templates leads to more stable perovskite PL in air over time (weeks). Taken in concert, such fundamental observations have implications for interfacial carrier interactions in tandem Si/perovskite photovoltaics.

New polymer-matrix nanocomposites based on SWCNTs and PVK-PPV copolymer: Synthesis, functionalization and characterization

An original nanocomposite material was prepared by in-situ grafting of a PVK-PPV copolymer onto SWCNTs. The new composites were elaborated with various SWCNTs weight concentrations into homemade PVK-PPV copolymer. Different experimental analyses were performed to investigate their morphological features and their optical and vibrational spectroscopy behaviors as a function of the concentration of carbon nanotubes. Π-staking and covalent interactions and functionalization process between copolymer and SWCNT undergo dramatic effect in vibrational and photoemissives properties of copolymer matrix. Transmission (TEM) and scanning electron microscopy (SEM) were used. Then, resonant Raman scattering and steady-state and time-resolved photoluminescence (PL) and (TRPL) measurements were used to study the evolution of the spectroscopic characteristics and optical properties of PVK-PPV/SWCNTs composites. As results, we have observed PL quenching effect and a decrease of average life time. These indicated the processing of a charge transfer that leads to exciton dissociations to the SWCNTs matrix. Such results, also supported by modelling based on DFT method, have given a strong evidence of the functionalization and the charge transfer between the SWCNTs and the PVK-PPV copolymer and predicted a structure-properties correlation.

Detection of Pentosidine Cross-Links in Cell-Secreted Decellularized Matrices Using Time Resolved Fluorescence Spectroscopy.

Hyperglycemia-mediated, nonenzymatic collagen cross-links such as pentosidine (PENT) can have deleterious effects on cellular interactions with the extracellular matrix (ECM). Present techniques to quantify PENT are limited, motivating the need for improved methods to study the accumulation and contribution of PENT toward diabetic clinical challenges such as impaired bone healing. Current methods for studying PENT are destructive, laborious, and frequently employ oversimplified collagen films that lack the complexity of the native ECM. The primary goal of this study was to evaluate the capacity of time-resolved fluorescence spectroscopy (TRFS) to detect PENT in cell-secreted ECMs possessing enhanced compositional complexity. To demonstrate an application of this method, we assessed the response of human mesenchymal stem cells (MSCs) to cross-linked substrates to explore the role of detected PENT on osteogenic differentiation. We exposed MSC-secreted decellularized matrices (DMs) to 0.66 M ribose for 2 weeks and used TRFS to detect the accumulation of PENT. Ribose treatment resulted in a 30 nm blue shift in peak fluorescence emission and a significant decrease in average lifetime compared to that of control DMs (4.4 ± 0.3 ns vs 3.5 ± 0.09 ns). Evaluation of samples with high performance liquid chromatography (HPLC) confirmed that changes in observed fluorescence were due to PENT accumulation. A strong correlation was found between TRFS parameters and the HPLC measurement of PENT, validating the use of TRFS as an alternative method of PENT detection. Osteoblastic gene expression was significantly reduced in MSCs seeded on ribose DMs at days 7 and 14. However, no significant differences in calcium deposition were detected between control and ribose DMs. These data demonstrate the efficacy of nondestructive fluorescence spectroscopy to examine the formation of nonenzymatic collagen cross-links within biomimetic culture platforms and showcase one example where an improved biomimetic substrate can be used to probe cell-ECM interactions in the presence of collagen cross-links.

Spectroscopic and photoluminescence characterization of Dy(3+) in Sr0.5Ca0.5TiO3 phosphor.

The spectroscopic and photoluminescence characteristics of trivalent dysprosium (Dy(3+))-doped Sr0.5Ca0.5TiO3 phosphor materials synthesized via solid-state reaction method were studied. The X-ray diffraction profile confirmed the orthorhombic perovskite structure of the prepared samples. Judd-Ofelt analysis was carried out to obtain the intensity parameters and predicted radiative properties of Sr0.5Ca0.5TiO3:2wt%Dy(3+). The photoluminescence spectrum of Dy(3+)-doped Sr0.5Ca0.5TiO3 showed three emission peaks at 481, 574 and 638 nm corresponding to (4)F9/2 →(6)H15/2, (4)F9/2 →(6)H13/2 and (4)F9/2 →(6)H11/2 transitions respectively. The variation of luminescence intensity with different excitation wavelengths and Dy(3+) concentrations is discussed. The decay profiles of (4)F9/2 excited levels of Dy(3+) ions show bi-exponential behaviour and also a decrease in average lifetime with increase in Dy(3+) concentration. Yellow to blue luminescence intensity ratio, CIE chromaticity co-ordinates and correlated color temperature were also calculated for different concentrations of Dy(3+)-doped Sr0.5Ca0.5TiO3 phosphor at different λex.

Photoinduced Surface Oxidation and Its Effect on the Exciton Dynamics of CdSe Quantum Dots

With increased interest in semiconductor nanoparticles for use in quantum dot solar cells there comes a need to understand the long-term photostability of such materials. Colloidal CdSe quantum dots (QDs) were suspended in toluene and stored in combinations of light/dark and N2/O2 to simulate four possible benchtop storage environments. CdSe QDs stored in a dark, oxygen-free environment were observed to better retain their optical properties over the course of 90 days. The excited state lifetimes, determined through femtosecond transient absorption spectroscopy, of air-equilibrated samples exposed to light exhibit a decrease in average lifetime (0.81 ns) when compared to samples stored in a nitrogen/dark environment (8.3 ns). A photoetching technique commonly used for controlled reduction of QD size was found to induce energetic trap states to CdSe QDs and accelerate the rate of electron–hole recombination. X-ray absorption near edge structure (XANES) analysis confirms surface oxidation, the extent of whi...

Investigating Fluorescence Quenching of ZnS Quantum Dots by Silver Nanoparticles

Water dispersible zinc sulfide quantum dots (ZnS QDs) with an average diameter of 2.9 nm were synthesized in an environment friendly method using chitosan as stabilizing agent. These nanocrystals displayed characteristic absorption and emission spectra having an absorbance edge at 300 nm and emission maxima (λ emission) at 427 nm. Citrate-capped silver nanoparticles (Ag NPs) of ca. 37-nm diameter were prepared by modified Turkevich process. The fluorescence of ZnS QDs was significantly quenched in presence of Ag NPs in a concentration-dependent manner with K sv value of 9 × 109 M−1. The quenching mechanism was analyzed using Stern–Volmer plot which indicated mixed nature of quenching. Static mechanism was evident from the formation of electrostatic complex between positively charged ZnS QDs and negatively charged Ag NPs as confirmed by absorbance study. Due to excellent overlap between ZnS QDs emission and surface plasmon resonance band of Ag NPs, the role of energy transfer process as an additional quenching mechanism was investigated by time-resolved fluorescence measurements. Time-correlated single-photon counting study demonstrated decrease in average lifetime of ZnS QDs fluorescence in presence of Ag NPs. The corresponding Forster distance for the present QD–NP pair was calculated to be 18.4 nm.