Time-resolved Photoluminescence Studies Research Articles

Evidence for plasmonic hot electron injection induced superior visible light photocatalysis by g-C3N4 nanosheets decorated with Ag–TiO2(B) and Au–TiO2(B) nanorods

Hot electrons produced by the surface plasmon decay can be efficiently utilized to drive photochemical reactions on a semiconductor surface. Considering this aspect of plasmonics, we have evaluated the visible light photocatalytic performance of Ag–TiO2(B)–C3N4 and Au–TiO2(B)–C3N4 heterostructures in the degradation of rhodamine B (RhB) and phenol. The synergetic effect of plasmonic hot electron injection and interfacial charge transfer in the heterostructures lead to 6–9 fold enhancement in the photodecomposition rate of RhB (phenol) over TiO2(B) and C3N4. Time-resolved photoluminescence study shows fast charge transfer through the integrated network of the heterostructure. The photocurrent is measured at 470 nm, 510 nm and 575 nm, near the plasmonic excitations of Ag and Au as well as under white light illumination (400–800 nm). Plasmonic systems show more than 6-fold enhancement in photocurrent over bare TiO2(B) under illumination near monochromatic plasmonic excitation. The overall photocurrent resulting from white light illumination is 2-fold stronger than that under plasmonic excitation. The increase is due to the contribution from Ti3+ excitation, hot electron injection, and charge transfer from TiO2(B) to C3N4. We propose that Ti3+ states in TiO2(B) provide channels for direct hot electron transfer from metal to semiconductor facilitating charge separation for participation in photocatalysis.

Charge carrier dynamics study and morphology optimization in solvent annealed CH3NH3PbI3 perovskite for air processed stable solar cell application

We report room temperature excitation intensity dependent charge carrier dynamics in solvent-annealed organo-metal halide perovskite semiconductor CH3NH3PbI3 thin film using steady state and time resolved photoluminescence studies. The average lifetime of these solvent annealed films does not vary significantly with excitation fluence confirming the formation of almost defect free perovskite crystallites. The photocarrier dynamics are well described by a simple rate equation. The local reduction of shallow trap densities as observed in PL dynamics was correlated with scanning electron microscopy (SEM) and X-ray spectroscopy. The observed photophysical investigation and morphology study suggests that anti-solvent method as a potential approach for CH3NH3PbI3 perovskite solar cell fabrication under ambient condition. A prototype all air processed anti-solvent method based stable perovskite device yielded the efficiency of 7.54% with JSC value of 14.21 mA/cm2 and VOC of 810 mV.

Time-resolved spectroscopy of Fe3+d-d transition in bulk ZnSe polycrystal

We report temperature dependent time-resolved photoluminescence studies of Fe3+ in ZnSe polycrystals over 10–300 K temperature ranges fabricated by the post growth thermal diffusion technique. The d-d transitions of Fe3+ assigned to the 4T2(G)→6A1(S) and 4T1(G)→6A1(S) transitions are clearly identified at 528 and 627 nm, respectively, at 10 K. The observed emission peaks in the near bandgap region are red-shifted from 439 to 463 nm as the temperature increases, resulting in a temperature coefficient of 10.21×10−4 eV/K and Debye temperature = 336 K fitted by the Varshni equation. The radiative lifetime at 528 nm by time-resolved photoluminescence is evaluated as τrad = 774 ± 4 ps, activation energy (ΔEa) = 717.2 cm−1, and relaxation time rate (1/W0) = 3.1 ps.

Physicochemical properties of chimie douce derived, digestively ripened, ultra-small (r

Ultra-small and uniform sized ZnO quantum dots (QDs) have been synthesized by using a chimie-douce, scalable and eco-friendly method. The physicochemical properties of digestively ripened ZnO QDs have been explored. Uniform sized colloidal QDs are achieved by employing a size control approach called digestive ripening (DR). In DR, monodispersed colloidal nanoparticles are obtained from polydispersed colloidal nanoparticles through the addition of substantial amount of a suitably chosen surfactant (having a suitable ligand). This is performed in the presence of a suitable solvent. Capping with the surfactant (triethanolamine) results in a change in the zeta potential of QDs (increased from 7 ± 3 mV to +22 ± 3 mV) which is responsible for the observed DR and a decrease in particle size from ˜3.5 ± 2 nm to ˜2.8 ± 0.5 nm also noticed. Hard-Soft-Acid-Base surface chemical interactions taking place between ZnO QDs and the surfactant (triethanolamine) are responsible for the formation of digestively ripened ZnO QDs. The as formed ZnO QDs are highly monodispersed in nature and the optical absorption studies indicate that band gap of ZnO QDs found to be Eg˜3.6 eV, indicating strong confinement. The photoluminescence (PL) studies clearly indicates that the major contribution to the photoluminescence is defects present of ZnO QDs-surface and the existence of oxygen vacancy defects on the ZnO QDs-surface are confirmed with PL studies. Time-resolved photoluminescence studies indicate that lifetimes of photo-generated carriers in digestively ripened (DRd) QDs is ˜12 ns and coating with digestive ripening agent reduces the surface-related emission and improves the radiative transitions in DRd ZnO QDs. Spectroscopic studies indicate that formation of highly pure ZnO QDs. These highly monodisperse, stable and ZnO QDs are prospectively useful in contemporary applications such as nanoscale electronic devices and photocatalysis.

Consequence of surfactant coating on the Raman active modes and highly symmetric blue-emission decay dynamics of cubic phase MnSe quantum dots

In this work, we discuss Raman active vibrational and photoluminescence (PL) features of hydrothermally derived cubic phase, manganese selenide (MnSe) quantum dots (QDs) coated with markedly different coating agents, such as, cetyl trimethyl ammonium bromide (CTAB), thioglycolic acid (TGA) and dextran molecules. The homogeneously grown QDs, upon adequate coating, offered a size variation between 5 and 11 nm as evident from imaging through transmission electron microscopy (TEM). The Raman spectra of the QDs have revealed the most prominent peak positioned at ∼680 nm and is ascribed to the third overtone of the longitudinal optical (LO(Γ)) mode. Moreover, the numerous, mixed modes which existed for the uncoated QDs, were found to be suppressed after adequate surface passivation due to surfactant coating. The prominent blue-violet emission response of the MnSe QDs, was seen to be highly symmetric and positioned at ∼428 nm. Furthermore, time resolved PL studies have predicted bi-exponential decay characteristics with a fast component (τ1 ∼ 0.10 ns) that remains nearly unaffected, but the slow component (τ2) has varied with the nature of coating agent. In particular, the dextran coated MnSe QDs exhibited a maximal τ2 value (∼5.5 ns) due to complete surface passivation of the QDs. The influence of surfactants on the vibrational and PL emission parameters of a magneto-fluorescent candidate, like MnSe, would bring new insights for their relevance in nano-photonics and nano-biotechnology to act as active probes for imaging, sensing and targeted therapeutics.

Perovskite Solar Cells Yielding Reproducible Photovoltage of 1.20 V

High photovoltages and power conversion efficiencies of perovskite solar cells (PSCs) can be realized by controlling the undesired nonradiative charge carrier recombination. Here, we introduce a judicious amount of guanidinium iodide into mixed-cation and mixed-halide perovskite films to suppress the parasitic charge carrier recombination, which enabled the fabrication of >20% efficient and operationally stable PSCs yielding reproducible photovoltage as high as 1.20 V. By introducing guanidinium iodide into the perovskite precursor solution, the bandgap of the resulting absorber material changed minimally; however, the nonradiative recombination diminished considerably as revealed by time-resolved photoluminescence and electroluminescence studies. Furthermore, using capacitance-frequency measurements, we were able to correlate the hysteresis features exhibited by the PSCs with interfacial charge accumulation. This study opens up a path to realize new record efficiencies for PSCs based on guanidinium iodide doped perovskite films.

Correlation of antibacterial and time resolved photoluminescence studies using bio-reduced silver nanoparticles conjugated with fluorescent quantum dots as a biomarker

This report lays the foundation stone for the real time detector to control and monitor antibiotic resistant bacteria’s. To start with, firstly, the green synthesis of silver nanoparticles were done by the bio-reduction of silver nitrate using different concentrations of medicinal plant extract taken from the leaves of Azadirachta indica (neem). The well-known standard surface plasmon resonance (SPR) wavelength of 430 nm and the characteristic brown color of the solution confirms the formation of eco-friendly silver nanoparticles. The silver nanoparticle size in the range of 10–20 nm was observed from TEM measurements for the sample showing best SPR which was further selected for conjugation with QDs. Secondly glutathione capped ZnS:Mn QDs were synthesized and conjugated with silver nanoparticles (0.09–0.1 mg/mL) and then their zone of inhibition studies were done using antibiotic resistant gram positive as well as gram negative bacteria’s i.e. Staphylococcus aureus and Escherichia coli respectively. Lastly, time resolved photoluminescence studies of the conjugated system were done to correlate zone of inhibition (antibiotic sensitivity) with silver nanoparticle concentration in both the cases of a gram positive bacteria and gram negative bacteria. Luminescence intensity of glutathione capped QDs conjugated silver nanoparticles decreases with increasing concentration of silver nanoparticles in tune with the increasing zone of inhibition (antibiotic sensitivity). This is very important for making a fluorescence based detector to monitor antibiotic resistant bacteria’s.

Linear and V-shaped carbazole-based molecules functionalized by cyano acceptors and diversified donors: Synthesis, single- and two-photon related photophysical properties

A series of novel linear and V-shaped carbazole-based molecules functionalized by cyano acceptors and diversified donors (carbazole, diphenylamino, dimethylamino, methoxy, and ferrocene) were synthesized and characterized by hydrogen-1 nuclear magnetic resonance, carbon-13 nuclear magnetic resonance, Fourier transform infrared spectroscopy, and high-resolution mass spectrometry. Their linear and nonlinear optical properties including UV–visible absorption, single-photon excited fluorescence, two-photon absorption, and frequency up-converted fluorescence, were systematically investigated in various solvents. The time-resolved photoluminescence studies using time-correlated single photon counting revealed their fluorescence lifetimes in THF. The solvent polarity exerts little effect on their absorption properties, while the emission properties (maximum fluorescence wavelength, Stokes shift, fluorescence quantum yield) display solvent polarity dependencies. Donor-acceptor functionalities and molecular dimensionality are considered to be the crucial structural factors that influence their optical properties. Pumped by a femtosecond laser (690–990 nm, 80 MHz, 140 fs), all the target molecules emit intense frequency up-converted fluorescence except FC1 and FC2 which contain one or two ferrocene donors. The two-photon absorption cross-sections in THF are in the range of 631–3014 GM. The density functional theory calculations were also conducted to unravel their electronic structures and to further understand the structure-property relationships. To our knowledge, this study is the first to be concerned with the single- and two-photon related photophysical properties of these D-π-A-π-D′ and D′-π-A-π-D-π-A-π-D′ carbazole-based molecules (D = donor, A = acceptor, π = conjugated bridge).

Perovskite Solar Cells Yielding Reproducible Photovoltage of 1.20 V.

High photovoltages and power conversion efficiencies of perovskite solar cells (PSCs) can be realized by controlling the undesired nonradiative charge carrier recombination. Here, we introduce a judicious amount of guanidinium iodide into mixed-cation and mixed-halide perovskite films to suppress the parasitic charge carrier recombination, which enabled the fabrication of >20% efficient and operationally stable PSCs yielding reproducible photovoltage as high as 1.20 V. By introducing guanidinium iodide into the perovskite precursor solution, the bandgap of the resulting absorber material changed minimally; however, the nonradiative recombination diminished considerably as revealed by time-resolved photoluminescence and electroluminescence studies. Furthermore, using capacitance-frequency measurements, we were able to correlate the hysteresis features exhibited by the PSCs with interfacial charge accumulation. This study opens up a path to realize new record efficiencies for PSCs based on guanidinium iodide doped perovskite films.

Optical responses of BSA protein under re-entrant condensation in presence of trivalent ions

Fluorescence behavior of globular protein like BSA is studied under the re-entrant condensation in the presence of trivalent salts like YCl3 and LaCl3. BSA shows a re-entrant behaviour in solution where a specific turbid phase is formed between the two critical concentrations of the trivalent salts. Optical behaviors of proteins are explored by UV–Vis, photoluminescence and time resolved photoluminescence (TRPL) spectroscopy, whereas hydrodynamic size is obtained from the dynamic light scattering (DLS) technique. BSA shows maximum emission intensity at ≈338 nm for an excitation at 278 nm, however, the value of the peak intensity of emission gradually increases with the increase in salt concentration and becomes maximum at the turbid phase but for higher salt concentration as the re-dissolution takes place, the emission intensity again decreases. TRPL study suggests that the decay time of BSA under re-entrant condensation is correlated with the salt concentration, whereas DLS study shows that the hydrodynamic size gradually increases up to a certain salt concentration where turbid phase is formed and then again decreases for higher salt concentrations. The variation of emission intensity of BSA nearly follows the modification of diffusion coefficient or hydrodynamic size and the phase transition behaviors.

Harvesting near- and far-field plasmonic enhancements from large size gold nanoparticles for improved performance in organic bulk heterojunction solar cells

Abstract The high stability and strong coupling nature of gold nanoparticles (Au-NPs) than other metal counter parts have attracted the solar cell industry to pursue enhanced performances. Herein, we report on the improved performance of polymer bulk hetero-junction (BHJ) solar cells by the incorporation of large-size Au-NPs in the hole transport layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). To examine the reproducibility of the enhancement parameters, two different donor photoactive materials have been adapted and the role of larger-size (>70 nm, i.e. 71, 80, 87, 103 nm) Au-NPs in BHJ solar cells have been studied extensively. Significantly, when employing Au-NPs smaller than 80 nm, near-field coupling (localized surface plasmon resonance; LSPR) was prevalent, while the infusion of Au-NPs with sizes greater than 87 nm resulted in far-field scattering enhancement as the dominant effect, which was clearly determined using time resolved photo luminescence studies. The superior power conversion efficiency of 5.35% and 8.58% was achieved with PBDTTT-C: PC61BM and PTB7: PC71BM BHJs respectively, by employing 87 nm Au-NPs due to the balanced contribution of near- and far-field plasmonic effects, improved vertical coverage and better interfacial properties. This study illustrates that 87 nm Au-NPs is the maximum size to attain the improved efficiency, above which the rate of enhancement reduces dramatically.

Exploring the Carrier Dynamics in Zinc Oxide–Metal Halide-Based Perovskite Nanostructures: Toward Reduced Dielectric Loss and Improved Photocurrent

Metal halide-based perovskites have emerged as a potential candidate for optoelectronic applications because of their impressive performance achieved by tuning the optical/electrical properties through tailoring the perovskite nanostructures. Herein, we report the synthesis of composite nanostructures by incorporation of ZnO (∼6 nm) into the CsPbBr3 (CPB) perovskite framework, which shows significant enhancement of photocurrent because of efficient interfacial charge separation and reduced dielectric loss. Detailed steady-state and time-resolved photoluminescence studies have been carried out to understand charge transfer dynamics in the CsPbBr3/ZnO nanostructure composite system. Femtosecond transient absorption and broadband dielectric spectroscopy studies were carried out to determine the charge carrier relaxation and transfer mechanism. The redox energy-level diagram suggests that the photoexcited electron from the conduction band (CB) of CPB can be transferred to the CB of ZnO NP because of thermodyn...

Design of a Pyrene Scaffold Multifunctional Material: Real-Time Turn-On Chemosensor for Nitric Oxide, AIEE Behavior, and Detection of TNP Explosive.

A dual-emission pyrene-based new fluorescent probe (N-(4-nitro-phenyl)-N′-pyren-1-ylmethyl-ene-ethane-1,2-diamine (PyDA-NP)) displays green fluorescence for nitric oxide (NO) sensing, whereas it exhibits blue emission in the aggregated state. The mechanism of nitric oxide (NO/NO+) sensing is based on N-nitrosation of aromatic secondary amine, which was not interfered by reactive oxygen species and reactive nitrogen species. The aggregation-induced enhancement of emission (AIEE) behaviors of the PyDA-NP could be attributed to the restriction of intramolecular rotation and vibration, resulting in rigidity enhancement of the molecules. The AIEE behavior of the probe was well established from fluorescence, dynamic light scattering, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, optical fluorescence microscopy, and time-resolved photoluminescence studies. In a H2O/CH3CN binary mixture (8:2 v/v), the probe showed maximum aggregation with extensive (833-fold) increases in fluorescence intensity and high quantum yield (0.79). The aggregated state of the probe was further applied for the detection of nitroexplosives. It displayed efficient sensing of 2,4,6-trinitrophenol (TNP), corroborating mainly the charge-transfer process from pyrene to a highly electron-deficient TNP moiety. Furthermore, for the on-site practical application of the proposed analytical system, a contact-mode analysis was performed.

Effect of Cu-doping on the photoluminescence and photoconductivity of template synthesized CdS nanowires

Cu-doped CdS nanowires were synthesized within the pores of anodic alumina membrane as template via electrochemical deposition. Morphological, structural, optical and photoconductive properties of the synthesized nanowires were studied. Morphological and structural properties were studied by using SEM with EDS (point & line mapper mode) and X-Ray Diffractometer. Scanning electron microscopy with point EDS and line EDS mapping confirms the deposition of pure CdS and Cu-doped CdS throughout the template length respectively. XRD investigations shows the presence of polycrystalline phase and cubic structure of pristine CdS nanowires. Optical properties were studied by UV–visible absorption spectroscopy and time resolved photoluminescence. A gradual decrease in energy band gap from 2.38 eV to 2.10 eV is observed for Cu doped CdS nanowires. Time resolved photoluminescence studies shows the red shift in emission wavelength and increase of excited state lifetimes with Cu doping and also with increase in its concentration in CdS nanowires. Current-voltage characteristics of pure and doped CdS nanowires studied at varying incident light intensity show that the Cu doped nanowires show negative photoconductivity.

Time resolved photoluminescence studies of degradation in GaInP/GaAs/Ge solar cells after 1MeV electron irradiation

Time resolved photoluminescence (TRPL) is a powerful method to character the behaviors of carriers as it has high time resolution that could reflect the reactions of carriers within nanoseconds. For solar cells, minority carrier lifetime is the most important parameter. TRPL has been used to measure the lifetime for uniform materials. However, for homojunction solar cells, doping distribution and carrier drift make the spectroscopy analysis much difficult. Thus one dimension numerical calculations are used to study the time-dependent photoluminescence (TRPL) decay of GaAs sub-cell in GaInP/GaAs/Ge solar cells. Calculation shows that both lifetime of minorities and light intensities could determine the line shape of TRPL. The bimolecular recombination under high injection modifies the curve from single-exponential to non-single-exponential one. For TRPL of homojunction solar cell, the photoluminescence decay process is not synchronized in all parts, and the decay is fast in emitter but slower in base regions. To get the lifetimes of minority carriers after 1MeV electron irradiation, carrier generation parameter G0 should be fitted by numerical method firstly. The damage factor Kτ=4.8×10-15 cm2/ns is fitted from TRPL results. Photoluminescence spectra (PL) are also used to get Kτ=5.5×10-15 cm2/ns which is similar with the value obtained from TRPL.

Tunable and High Photoluminescence Quantum Yield from Self‐Decorated TiO2 Quantum Dots on Fluorine Doped Mesoporous TiO2 Flowers by Rapid Thermal Annealing

AbstractHerein a novel approach is reported to achieve tunable and high photoluminescence (PL) quantum yield (QY) from the self‐grown spherical TiO2 quantum dots (QDs) on fluorine doped TiO2 (F‐TiO2) flowers, mesoporous in nature, synthesized by a simple solvothermal process. The strong PL emission from F‐TiO2 QDs centered at ≈485 nm is associated with shallow and deep traps, and a record high PL QY of ≈5.76% is measured at room temperature. Size distribution and doping of F‐TiO2 nanocrystals (NCs) are successfully tuned by simply varying the HF concentration during synthesis. During the post‐growth rapid thermal annealing (RTA) under vacuum, the arbitrary shaped F‐TiO2 NCs transform into spherical QDs with smaller sizes and it shows dramatic enhancement (≈163 times) in the PL intensity. Electron spin resonance (ESR) and X‐ray photoelectron spectroscopy (XPS) confirm the high density of oxygen vacancy defects on the surface of TiO2 NCs. Confocal fluorescence microscopy imaging shows bright whitish emission from the F‐TiO2 QDs. Low temperature and time resolved PL studies reveal that the ultrafast radiative recombination in the TiO2 QDs results in highly efficient PL emission. A highly stable, biologically inert, and highly fluorescent TiO2 QDs/flowers without any capping agent demonstrated here is significant for emerging applications in bioimaging, energy, and environmental cleaning.

Enhancement of direct and indirect exciton emissions in few-layer WSe2 at high temperatures

We present a temperature-dependent and time-resolved photo-luminescence (PL) study for few-layer tungsten diselenide (WSe2) that is exfoliated from the bulk crystals. The PL intensities of direct and indirect exciton emissions in monolayer and bilayer WSe2 decrease while temperature increases. However, abnormal enhancement of PL emission from direct and indirect excitons in ≥3-layer WSe2 is observed at high temperatures, ranging from 300 K to 400 K. A crossover of Λ → Γ and Κ → Γ indirect transitions occurs in few-layer WSe2 at elevated temperature and this phenomenon is verified by comparing the temperature-dependent shift of indirect exciton PL peak and ab initio band structure calculations. The enhancement of high-temperature PL from direct and indirect excitons can be attributed to the rapid intervalley transfer of thermally activated carriers from Λ point to K point and the crossover of Λ → Γ and Κ → Γ indirect transitions. The distinct behavior of temperature dependence of the PL intensities at a high temperature between bilayer and trilayer WSe2 reflects their band structure discrepancy. This work provides a thorough understanding of the origin of the observed indirect optical transition in few-layer WSe2 and suggests a possible means of improving the luminescence efficiency of multilayer WSe2.

Photoluminescence Quenching in Self-Assembled CsPbBr3 Quantum Dots on Few-Layer Black Phosphorus Sheets.

An ordered self-assembly of CsPbBr3 quantum dots (QDs) was generated on the surface of few-layer black phosphorus (FLBP). Strong quenching of the QD fluorescence was observed, and analyzed by time-resolved photoluminescence (TR-PL) studies, DFT calculations, and photoconductivity measurements. Charge transfer by type I band alignment is suggested to be the cause of the observed effects.

Excitonic Energy Transfer within InP/ZnS Quantum Dot Langmuir-Blodgett Assemblies.

Interparticle energy transfer offers great promise to a diverse range of applications ranging from artificial solar energy harvesting to nanoscale rulers in biology. Here, we assembled InP/ZnS core/shell quantum dot monolayers via the Langmuir–Blodgett technique and studied the effect of ZnS shell thickness on the excitonic energy transfer within these core/shell quantum dots. Three types of InP-based core/shell quantum dot Langmuir–Blodgett assemblies with different ZnS shell thicknesses were assembled. The structural and optical properties of colloidal quantum dots reveal the successful multiple ZnS shell growth, and atomic force microscopy studies show the smoothness of the assembled monolayers. Time-resolved photoluminescence (PL) and fluorescence lifetime imaging microscopy (FLIM) studies of the thick-shell QD monolayer reveal narrower lifetime distribution in comparison with the thin-shell QD monolayer. The interparticle excitonic energy transfer was studied by spectrally resolved PL traces, and higher energy transfer was observed for the thin-shell InP/1ZnS QD monolayer. Finally, we calculated the average exciton energy and indicated that the energy transfer induced exciton energy shift decreased significantly from 95 to 27 meV after multiple ZnS shell growth.

Evolution of Nitrogen-Related Defects in Graphitic Carbon Nitride Nanosheets Probed by Positron Annihilation and Photoluminescence Spectroscopy

Defects play a pivotal role in the device performance of a photocatalytic, light-emitting, or photovoltaic system. Herein, graphitic carbon nitride (g-C3N4) nanosheets are prepared at different calcination temperatures, and the evolution of defects in the system is studied by positron annihilation spectroscopy (PAS) and photoluminescence (PL) spectroscopy. Steady-state PL spectra show that free and defect-bound excitonic emission peaked at 2.78, 2.58, and 2.38 eV are dominant with above-band-gap excitation. Time-resolved PL studies reveal a significant enhancement of excitonic lifetime from 17.4 ns for free exciton to 27.4 ns in case of defect-bound exciton. We provide a direct correlation between the defects observed by PAS and those of the excitonic lifetime found from PL studies. Below-band-gap excitation activates defect emission, and it is characterized by a short carrier lifetime (∼0.14 ns). An excitation power-dependent PL study with 405 nm laser shows a progressive red shift and narrowing of the e...