Visible Excitation Research Articles

Capturing the Long-Lived Photogenerated Electrons in Au/TiO2 upon UV or Visible Irradiation by Time-Resolved Infrared Spectroscopy

Gold nanoparticle modifications for TiO2 (Au/TiO2) can extend the absorption wavelength from UV to visible (Vis) and enhance the photocatalytic performance, thus fueling increasing attention as an emerging photocatalysis strategy. To explore the plasmon-enhanced photocatalytic mechanism and directly unveil the intrinsic properties of Au/TiO2, the decay kinetics of photoelectrons upon UV (355 nm) or Vis (532 nm) excitation are monitored by means of nanosecond time-resolved infrared spectroscopy, which is a unique tool offering observations without interference of the holes. Under UV irradiation, the longer lifetime of photoelectrons observed in Au/TiO2 compared to that in bare TiO2 provides unambiguous evidence for the enhanced charge separation by AuNPs. Under Vis irradiation, the long-lived (hundreds of microseconds) electrons produced by injection from AuNPs into TiO2 upon plasmon excitation are here detected for the first time. Moreover, the effects of TiO2 phase composition and the amount of AuNPs loa...

Generation of Microsecond Charge-Separated Excited States in Rhenium(I) Diimine Complexes: Driving Force Is the Dominant Factor in Controlling Lifetime.

A transition-metal-based donor-(linker)-acceptor system can produce long-lived charge transfer excited states using visible excitation wavelengths. The ground- and excited-state photophysical properties of a series of [ReCl(CO)3(dppz-(linker)-TPA)] complexes, with varying donor and acceptor energies, have been systematically studied using spectroscopic techniques (both vibrational and electronic) supported by computational chemistry. The long-lived excited state is 3ILCT in nature for all complexes studied, characterized through transient absorption and emission, transient resonance Raman (TR2), and time-resolved infrared (TRIR) spectroscopy and TDDFT calculations. Modulation of the donor and acceptor energies results in changes of the 3ILCT lifetime by 1 order of magnitude, ranging from 6.1(±1) μs when a diphenylamine donor is used to 0.6(±0.2) μs when a triazole linker and triphenylamine donor is used. The excited-state lifetime may be rationalized by consideration of the driving force within the framework of Marcus theory and appears insensitive to the nature of the linker.

Superlattice in collapsed graphene wrinkles

Topographic corrugations, such as wrinkles, are known to introduce diverse physical phenomena that can significantly modify the electrical, optical and chemical properties of two-dimensional materials. This range of assets can be expanded even further when the crystal lattices of the walls of the wrinkle are aligned and form a superlattice, thereby creating a high aspect ratio analogue of a twisted bilayer or multilayer – the so-called twisted wrinkle. Here we present an experimental proof that such twisted wrinkles exist in graphene monolayers on the scale of several micrometres. Combining atomic force microscopy and Raman spectral mapping using a wide range of visible excitation energies, we show that the wrinkles are extremely narrow and their Raman spectra exhibit all the characteristic features of twisted bilayer or multilayer graphene. In light of a recent breakthrough – the superconductivity of a magic-angle graphene bilayer, the collapsed wrinkles represent naturally occurring systems with tuneable collective regimes.

π-Expanded Thioxanthones - Engineering the Triplet Level of Thioxanthone Sensitizers for Lanthanide-Based Luminescent Probes with Visible Excitation.

Bright lanthanide based probes for optical bioimaging must rely on the antenna principle, where the lanthanide-centred excited state is formed by a complex sensitization process. Efficient sensitization of lanthanide-centred emission occurs via triplet states centred on the sensitizing chromophore. Here, the triplet state of thioxanthone chromophores is modulated by extending the π-system. Three thioxanthone chromophores-thioxanthone, benzo[c]thioxanthone, and naphtho[2,3-c]thioxanthone were synthesised and characterised. The triplet state energies and lifetimes is found to change as expected, and two dyes are found to be suitable sensitizers for europium(iii) luminescence. Reactive derivatives of thioxanthone and benzo[c]thioxanthone were prepared and coupled to a 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) lanthanide binding pocket. The photophysics and the performance in optical bioimaging of the resulting europium(iii) complexes were investigated. It is concluded that while the energetics favour efficient sensitization, the solution structure does not. While it was found that the complexes are too lipophilic to be efficient luminescent probes for optical bioimaging, we successfully demonstrated bioimaging using europium(iii) luminescence following 405 nm excitation.

KGaP2O7:Mn4+ deep red emitting phosphor: Synthesis, structure, concentration and temperature dependent photoluminescence characteristics

Mn4+-activated KGaP2O7 red emitting phosphor was successfully synthesized via traditional solid-state reaction method in air. The structural features of KGaP2O7:xMn4+ material were investigated by means of powder XRD & Raman, morphology from SEM, and element analysis, corresponding mapping through EDAX. In conjunction, concentration and temperature dependent spectroscopic properties were analyzed from UV–Vis reflectance, excitation, emission and lifetime decay curves. Powder XRD and photoluminescence profiles revealed that GaO6 polyhedra offer Mn4+ ions to substitute the Ga3+ sites in the crystal lattice resulting in a pure single phase structure with an efficient red emission at 702 nm (Mn4+:2Eg→4A2g) under 452 nm excitation. The emission decay curves exhibited non-exponential nature with lifetimes shortening when Mn4+ concentration is increased. Electric dipole-dipole interaction is identified to be responsible for the concentration quenching beyond 0.07 Mn4+ via energy transfer between Mn4+ ions. The temperature-dependent emission intensity for KGaP2O7:0.07Mn4+ exhibited 66% at 423 K (150 °C) displaying good thermal stability and activation energy (Ea) of ∼0.186 eV. From UV–Vis reflectance spectra using Tanabe-Sugano diagram, crystal splitting factor (Dq), two Racah parameters (B & C), Dq/B, C/B and energy of states are calculated to evaluate nephelauxetic ratio (β). In addition, the effect of Mn4+ content (concentration quenching) and temperature (thermal quenching) on emission intensity are discussed based on the Tanabe-Sugano energy diagram and configurational coordinate scheme of Mn4+. Therefore, KGaP2O7:Mn4+ phosphor can serve as a potential red emitting phosphor under blue light excitation and can also be useful to enhance plants growth.

Preparation and photoluminescence behavior of Mn-doped nano-ZnO

The Mn-doped nano-ZnO powder was successfully synthesized via co-precipitation and vacuum annealing and the scanning electron microscopy (SEM), X-ray diffracton (XRD), ultraviolet visible absorption (Uv–vis) and photoluminescence (PL) of as-synthesized samples were characterized. The SEM and XRD show that the morphology of the synthesized samples was wurtzite nanosheets and nanoparticles at low doping concentration and with increasing Mn-doping concentration, mainly became to nanoparticle with a size of about 50nm. All of the ultraviolet absorption edges of Mn-doped ZnO have blue shift from Uiv-vis spectra and PL spectra show that Mn-doping significantly enhance the visible excitation band compared with pure ZnO, which indicates that Mn-doping not only produces elemental energy levels, but also affects the intrinsic defect energy levels of ZnO.

Optical and NIR luminescence spectral studies: Nd3+-doped borosilicate glasses

Nd3+-doped borosilicate glasses with molar composition 20B2O3+ (45-x) SiO2 +20K2CO3+5Na2CO3+10LiF + xNd2O3 were prepared by conventional melt-quenching technique. Structural and optical properties of the proposed glasses were studied using XRD, FTIR, UV–Visible absorption, excitation and NIR emission spectroscopy. Judd-Ofelt theory was applied to the absorption spectrum of 1.0 mol% Nd3+-doped borosilicate glass to estimate the JO intensity parameters (Ω2, Ω4, Ω6). From this theory, various radiative properties like radiative transition probabilities (AT), branching ratios (βr) and stimulated emission cross section (σemi) were determined and reported. The measured lifetimes for theF3/24 level were decreased with increase in Nd3+ concentrations. Moreover, the optical energy gap was calculated for direct and indirect optical transitions. The higher stimulated emission cross section (5.84622 × 10−20 cm2), optical gain (142.64 × 10−25 cm2s), quantum efficiency (77%), and low threshold (1.71 W/m2) indicate that 1.0 mol% Nd3+-doped borosilicate glass could be potential gain medium for solid state NIR laser application at 1.06 μm.

Practical application of visible-induced luminescence and use of parasitic IR reflectance as relative spatial reference in Egyptian artifacts

Egyptian blue (CaCuSi4O10) is one of the most ancient artificial pigments, widely used in ancient times. Its peculiarity is an exceptional infrared emission upon visible excitation, allowing an easy and non-invasive diagnostic through the so-called visible-induced luminescence (VIL) technique. Usually, it requires total absence of infrared parasitic light, highlighting areas in which the pigment is present even in traces. In this report, we propose the introduction of a small portion of IR parasitic light as spatial reference for locating Egyptian blue on analyzed object. In VIL modality, the contemporary reflectance transformation imaging (RTI) and 3D photogrammetric model reconstruction were performed with the final 3D rebuilding of surface, morphology, and pigment distribution. We demonstrated the possibility to perform VIL and 3D photogrammetry without opening the conservation case that is extremely important by a conservation point of view, avoiding any microclimatic alteration, compatibly with the minimum invasiveness (absence of contact and displacement of the object).

Eosin Thermoluminescence in Polyvinyl Alcohol Films After Doubl Vis-IR Laser Excitation in a Wide Temperature Range

We developed an experimental technique of double stationary visible (l = 532 nm) and pulsed infrared (l = 10.6 μm) laser excitation of the electron-vibrational singlet and triplet states of eosin molecules in polyvinyl alcohol. After infrared excitation, delayed fluorescence (τdf ≈ 0.15 s) and thermoluminescence (τТL = 4.7 s) of eosin molecules in polymer matrix occur. The rate constants of these processes depend on the temperature and the presence of water in the polymer. The kinetics of low-temperature processes of thermoluminescence generation and quenching after IR laser pulse is studied. Two characteristic stages of the processes with different activation energies of thermoluminescence are determined: film heating in the 80–270 K temperature range (57.4 J/mol) and ice melting in the polymer matrix within the 270–330 K temperature interval (13.3 kJ/mol).

Molecular Photoelectrode for Water Oxidation Inspired by Photosystem II.

In artificial photosynthesis, the sun drives water splitting into H2 and O2 or converts CO2 into a useful form of carbon. In most schemes, water oxidation is typically the limiting half-reaction. Here, we introduce a molecular approach to the design of a photoanode that incorporates an electron acceptor, a sensitizer, an electron donor, and a water oxidation catalyst in a single molecular assembly. The strategy mimics the key elements in Photosystem II by initiating light-driven water oxidation with integration of a light absorber, an electron acceptor, an electron donor, and a catalyst in a controlled molecular environment on the surface of a conducting oxide electrode. Visible excitation of the assembly results in the appearance of reductive equivalents at the electrode and oxidative equivalents at a catalyst that persist for seconds in aqueous solutions. Steady-state illumination of the assembly with 440 nm light with an applied bias results in photoelectrochemical water oxidation with a per-photon absorbed efficiency of 2.3%. The results are notable in demonstrating that light-driven water oxidation can be carried out at a conductive electrode in a structure with the functional elements of Photosystem II including charge separation and water oxidation.

Molecular Structure, DFT, Vibrational Spectra with Fluorescence Effect, Hirshfeld Surface, Docking Simulation and Antioxidant Activity of Thiazole Derivative

Abstract2‐(4‐methylphenoxy)‐N‐(4‐(4‐bromophenyl) thiazol‐2‐yl) acetamide compound was obtained via a multistep synthesis sequence processes, which is characterized by 1H NMR, 13C NMR and its three‐dimensional structure has been confirmed by single crystal X‐ray diffraction method. The supramolecular structure of the molecule revealed the stability of crystal packing with diverse intermolecular interactions. Density functional theory calculations (DFT) at B3LYP 6–311++G(d,p) level has been used to predict the molecular geometry and were in good agreement with the experimental data. Moreover, the theoretical calculations were carried out to appraise the electronic structure, vibrational spectra, natural bond orbital analysis, molecular electrostatic potential, frontier molecular orbitals and global reactivity descriptors. Raman spectra with fluorescence effect was examined with visible and near IR excitation wavelengths. Hirshfeld surface and energy framework analysis revealed the important intermolecular contacts and interaction energies. The antioxidant activity of this synthesized compound was predicted by in silico docking study on human peroxiredoxin 5 protein. The potential antioxidant activity was investigated by 1,1‐diphenyl‐1‐picrylhydrazyl (DPPH) free radical screening and compared with standard drug ascorbic acid.

SERS study of fluorescent and non-fluorescent flavonoids: what is the role of excitation wavelength on SERS optical response?

Study of colorful compounds using Raman spectrometry can reveal crucial issues especially if fluorescent background is present. Fluorescence effect in flavonoids is shifted to higher wavelengths by the excited state intramolecular proton transfer. Therefore, new peaks arise in the visible spectral range. In this work, four flavonoids (flavone, 3-hydroxyflavone, chrysin and quercetin) are studied in the form of pure compounds in solid state and after adsorption on gold surface. Four excitation wavelengths 532, 633, 785, and 1064 nm are tested for normal Raman and surface-enhanced Raman scattering spectra of used flavonoids. Some of these flavonoids namely chrysin and quercetin exhibited high fluorescence background in normal Raman spectra and, therefore, it was not possible to obtain their good quality spectra using visible excitations. SERS spectrometry allowed detecting and identifying small amount of the colorful compounds and significantly reducing fluorescence background. Characteristic spectral bands were observed using SERS spectrometry at spectra of both fluorescent and non-fluorescent flavonoids.

Selective photocurrent generation in HfO2 and carbon nanotube hybrid nanocomposites under Ultra-Violet and visible photoexcitations

We report on the photocurrent generation in HfO2-carbon nanotube (CNT) nanocomposites under Ultra-Violet (UV) and visible excitations at zero bias. Cubic phase HfO2 nanoparticles have been combined with multi-walled carbon nanotubes in this work. The cubic phase of HfO2 has been stabilized by oxygen vacancies which act as luminescent band gap states. In a broad UV–visible range of below band gap photoexcitation, a photocurrent is generated which was found to be most efficient under UV illumination. We discuss the possible mechanism in terms of a CNT assisted charge transfer involving optically active surface states of the HfO2 nanoparticles. The abrupt generation and relaxation responses of the photocurrent on/off cycles along with a constant steady state current as high as 200 nA for 1 mg of the nanocomposite, has potential in energy harvesting and other applications requiring stable charge retention.

Efficient conversion of broad UV–visible light to near-infrared emission in Mn4+/Yb3+ co-doped CaGdAlO4 phosphors

Abstract Mn4+/Yb3+ co-doped CaGdAlO4 are prepared to achieve efficient spectral conversion. XRD Rietveld refinement analysis is performed to obtain the structural information of the synthesized materials. The results indicate the Al3+ and Gd3+ sites are substituted by the Mn4+ and Yb3+ ions, respectively. Intense NIR emission within the most sensitive response region of c-Si solar cells is observed under UV–visible excitation, indicating the existence of efficient energy transfer from Mn4+ to Yb3+ ions. The dipole-dipole interaction is verified to be the dominant energy transfer mechanism among the Mn4+-Yb3+ pair. The linear dependence of energy transfer rate on the Yb3+ concentration indicates that one NIR photon is generated with the absorption of one UV–visible photon. The broad excitation spanning from 250 to 600 nm monitored at NIR emission demonstrates that the Mn4+ ions in the CaGdAlO4 host can be efficient sensitizers for Yb3+ ions to enhance the performance of c-Si cells.

Eu3+/Yb3+ co-doped gadolinium oxysulfide upconverting nanorods: Morphological, physicochemical and optical evaluation

In this study, we introduce a novel route to produce down and upconverting Eu3+/Yb3+ co-doped oxysulfide nanorods, which display strong red emission at 620 nm under Vis (460 nm) or NIR (976 nm) wavelength excitation. An in-depth analysis of the synthesis parameters such as lanthanides concentration, type of nucleating agent, reaction temperature, and the reaction pressure was performed to determine their influence in the morphology and photoluminescence properties. It was demonstrated that as a nucleating agent, ethanolamine is a good substitute of the commonly used triethylamine providing the advantage of using lower toxicity reagents. The Transmission Electron Microscopy (TEM), Infrared (FTIR) and Zeta Potential analysis showed that the chemical surface of nanorods evolve during thermal and sulfidation processes without producing morphological changes. The intensity of light emission during downconversion (DC) and upconversion (UC) phenomena was found to be doping ions concentration dependent. The highest DC light emission was found at a Yb3+/Eu3+ concentration ratio of 0.25, while the most intense UC luminescence was found at 4. Conversely, quenching of DC and UC was observed when using Yb3+/Eu3+ concentration ratios of 4 and 0.25 respectively, indicating that luminescent centers are strongly affected in both processes by the chemical environment but also for the ions doping ratio. It was demonstrated that two-photon absorption is the primary mechanism for the red emission in the UC process.

Ultrasonication‐Induced Self‐Assembled Fixed Nanogap Arrays of Monomeric Plasmonic Nanoparticles inside Nanopores

AbstractMonomeric gold (Au) and silver (Ag) nanoparticle (NP) arrays are self‐assembled uniformly into anodized aluminium oxide (AAO) nanopores with a high homogeneity of greater than 95%, using ultrasonication. The monomeric metal NP array exhibits asymmetric plasmonic absorption due to Fano‐like resonance as interpreted by finite‐difference time‐domain (FDTD) simulation for the numbers up to 127 AuNPs. To examine gap distance‐dependent collective‐plasmonic resonance, the different dimensions of S, M, and L arrays of the AuNP diameters/the gap distances of ≈36 nm/≈66 nm, ≈45 nm/≈56 nm, and ≈77 nm/≈12 nm, respectively, are prepared. Metal NP arrays with an invariable nanogap of ≈50 nm can provide consistent surface‐enhanced Raman scattering (SERS) intensities for Rhodamine 6G (Rh6G) with a relative standard deviation (RSD) of 3.8–5.4%. Monomeric arrays can provide an effective platform for 2D hot‐electron excitation, as evidenced by the SERS peak‐changes of 4‐nitrobenzenethiol (4‐NBT) adsorbed on AgNP arrays with a power density of ≈0.25 mW µm‐2 at 514 and 633 nm. For practical purposes, the bacteria captured by 4‐mercaptophenylboronic acid are found to be easily destroyed under visible laser excitation at 514 nm with a power density of ≈14 mW µm‐2 for 60 min using Ag due to efficient plasmonic‐electron transfer.

Simultaneous detection of different probe molecules using silver nanowires as SERS substrates.

Metallic silver nanowires with high yield were synthesized using a modified seed mediated approach at room temperature. Ribbon-like nanostructures were obtained when the concentration of NaOH was lower and further increase of NaOH transformed it into long nanowires. These nanowires possess high aspect ratio, with length and diameter ~6.5 μm and 17 nm respectively. The surface enhanced Raman scattering activity of these nanowires was tested with three different probe molecules viz., crystal violet, malachite green and nile blue chloride using visible (514.4 nm) and near-infrared (784.8 nm) excitation lines. The minimum detection limits for crystal violet and nile blue chloride molecules were found to be down to 10-7 M with good linear responses, as evidenced by values of correlation coefficients, indicating their potential for a variety of applications such as sensing. Principal component analysis was performed with the surface enhanced Raman spectra in order to discriminate the dye molecules and their mixture, simultaneously. The first two principal components, which provided 69.80 and 27.93% of the total data variance, could be conveniently represented as a two dimensional PCA score plot. The score plot showed clear clustering of probe molecules and their mixture. The relative contribution of wavenumbers to each of the two principal components was identified by plotting the PCA loading matrix. These results further promote possibilities of quantification of multiplexed SERS detection and analysis.

Residual stress in underlying silicon at the fixed end of SiO2 microcantilevers — A micro-Raman study

In this work, nature of the residual stress developed in the convex corners created in underlying Si at the fixed end of SiO2 microantilevers (MCs) fabricated by wet chemical etching method was investigated using micro-Raman spectroscopy with visible excitation. It revealed the presence of tensile stress near the sharp edge of the convex corner and is attributed to the localized stress generated in the neighborhood of the discontinuities, acting as stress concentration region. Residual stress estimated by micro-Raman technique across the convex corner was also validated by FEM simulations. Micro-Raman also revealed the presence of tensile stress on the etched Si surface, which is explained on the basis of stress induced by native oxide shells covering the etched features.

Synthesis, X-ray crystal structure, DFT calculations, spectroscopic characterization and redox behaviour of a rhodium(III) complex of an anthracene–pyridylhydrazone ligand

A pyridylhydrazone incorporating an anthracene moiety, designated as HLAnc, has been synthesized in order to examine its coordination behaviour towards rhodium(III). The complex [RhLAnc(PPh3)2Cl2], incorporating a four-membered metallacycle, has been isolated and authenticated by a single-crystal X-ray diffraction study. The complex shows interesting redox and optoelectronic properties, and to better understand these, theoretical investigations have been performed using density functional theory (DFT) and time-dependent DFT. The visible excitation for the complex arises from primarily mixed singlet-manifold 1ILCT and 1LLCT transitions.

Chemical characterization and source identification of organic matter in eroded sediments: Role of land use and erosion intensity

Soil erosion is a key variable in the biogeochemical cycle of carbon (C) on the Earth's surface. However, questions remain about the roles of land use and erosion intensity on the composition, source, and fate of soil C eroded from terrestrial to fluvial systems. In this study, chemical characteristics of eroded water-extractable organic matter (WEOM) in soils and sediments, as well as subsequent source identification, were inferred from UV–Visible absorption and fluorescence excitation emission matrix (EEM)-parallel factor analysis (PARAFAC) in study sites that include land uses and gully banks, experiencing three levels of erosion intensity in a semi-arid region of China. 13C and 15N isotopic signatures along with elemental ratios were also successfully employed to explore the source of bulk soil organic matter (SOM) in sediments. We found a much greater contribution of condensed aromatic structures and hydrophobic fraction of soluble organic compounds in forest soils compared to croplands at eroding sites, where these variables were greater than those of depositional sites. The results from fluorescence analysis in soil materials showed that erosion intensity has a negligible influence on WEOM quality. The EEM-PARAFAC with fluorescence indices indicated that biological production of soil substrates can also play a key role in the dynamics of WEOM induced by soil erosion. Our results from an isotopic mixing model analysis showed that gully bank soil was the primary sources of sedimentary SOM in all regions with different erosion intensities (mean probability estimate (MPE) 100% for the region with light erosion intensity, 36.18% for the region with high erosion intensity, and 99.25% for the region with extremely high erosion intensity). However, orchard and grassland were also the main contributors for the SOM in sediments in regions with high erosion intensity, accounting for MPE 29.93% and 33.89%, respectively. Our findings demonstrate that land use and erosion intensity have significant effect on nature of eroded OM.