Function Of Excitation Wavelength Research Articles

High thermal stability of warm white emitting single phase GdPO4: Dy3+/ Sm3+ phosphor for UV excited wLEDs

Highly thermally stable warm white light emission has been obtained from a single component phosphor; GdPO4: Dy3+/Sm3+. A series of GdPO4: Sm3+ (1%)/Dy3+ (x%) (x = 0.1, 0.5, 1 and 2) phosphors have been synthesized by the co-precipitation method. The crystal structure and phase purity of the samples obtained were verified by X-ray diffraction. Photoluminescence spectra, luminescence lifetimes are used to study the effect of Dy3+ concentration and the energy transfer from Dy3+ to Sm3+ as a function of excitation wavelength. The nearest-neighbor contributed to the non-radiative energy transfer from Dy3+ to Sm3+ in GdPO4, under 273 and 363 nm excitation. A warm white light was obtained from all phosphors excited at 273 nm and for 1 and 2% of Dy3+ in GdPO4:Sm3+ under 363 nm excitation. Temperature-dependent luminescence properties are determined for GdPO4: Sm3+ (1%), Dy3+ (1%) under 273 nm and 363 nm excitation. The studied phosphor has a good luminescence thermal stability and the determined activation energy for thermal quenching is 1.26 and 0.75 eV for 273 nm and 363 nm excitation wavelengths, respectively. The effect of the Dy3+ concentration as an activator ion responsible for the white emission light, at a fixed concentration of Sm3+ as a red emission compensator in the Dy3+ emission spectrum, was discussed. The low color temperature and high thermal stability indicate that the studied material is a promising solid-state emitting phosphors for the UV chip excited warm wLED applications.

Raman study of layered breathing kagome lattice semiconductor Nb3Cl8

Niobium chloride (Nb3Cl8) is a layered two-dimensional semiconducting material with many exotic properties including a breathing kagome lattice, a topological flat band in its band structure, and a crystal structure that undergoes a structural and magnetic phase transition at temperatures below 90 K. Despite being a remarkable material with fascinating new physics, the understanding of its phonon properties is at its infancy. In this study, we investigate the phonon dynamics of Nb3Cl8 in bulk and few layer flakes using polarized Raman spectroscopy and density-functional theory (DFT) analysis to determine the material’s vibrational modes, as well as their symmetrical representations and atomic displacements. We experimentally resolved 12 phonon modes, five of which are A 1g modes while the remaining seven are Eg modes, which is in strong agreement with our DFT calculation. Layer-dependent results suggest that the Raman peak positions are mostly insensitive to changes in layer thickness, while peak intensity and full width at half maximum are affected. Raman measurements as a function of excitation wavelength (473–785 nm) show a significant increase of the peak intensities when using a 473 nm excitation source, suggesting a near resonant condition. Temperature-dependent Raman experiments carried out above and below the transition temperature did not show any change in the symmetries of the phonon modes, suggesting that the structural phase transition is likely from the high temperature P 1 phase to the low-temperature R phase. Magneto-Raman measurements carried out at 140 and 2 K between −2 and 2 T show that the Raman modes are not magnetically coupled. Overall, our study presented here significantly advances the fundamental understanding of layered Nb3Cl8 material which can be further exploited for future applications.

Cover Feature: Symmetrically Functionalized Copper and Silver Corrole‐Bis‐Tetracyanobutadiene Push‐Pull Conjugates: Efficient Population of Triplet States via Charge Transfer (Chem. Eur. J. 41/2023)

Chemistry–art–culture combo: Rangoli, an ancient art from the Indian subcontinent, creates designs on the floor by using dry powders. A rangoli of the structure of the investigated compound is depicted here to link chemistry to art to culture. An efficient population of triplet excited states via the intermediate charge-transfer state formed as a function of excitation wavelength is demonstrated by using ultrafast pump–probe spectroscopy in these symmetrically functionalized push–pull systems. More information can be found in the Research Article by M. Sankar, F. D'Souza, and co-workers (DOI: 10.1002/chem.202301341). Artists: I. Yadav, N. Rana, R. Jangra and S. Rathi.

Symmetrically Functionalized Copper and Silver Corrole-Bis-Tetracyanobutadiene Push-Pull Conjugates: Efficient Population of Triplet States via Charge Transfer.

Copper and silver tritolylcorroles (TTC) are symmetrically functionalized to carry two tetracyanobutadiene (TCBD) entities via [2+2] cycloaddition-retroeletrocyclization reaction involving ethynyl functionalized corroles with an electron acceptor, tetracyanoethylene (TCNE) in excellent yields, as the first examples of corrole-TCBD push-pull systems. The strong push-pull effect resulted in charge polarization in the ground state resulting in a considerable hypsochromic shift of the spectrum extending it into the near-IR region. Electrochemical studies coupled with computational studies revealed considerable interactions between the two TCBD entities via the corrole π-system and the degree of such interactions was found to depend on the metal ion present in the corrole cavity. Energy considerations suggested charge transfer (CT) from the S2 or vibrationally hot S1 state but not the relaxed S1 state in the case of CuTTC(TCBD)2 while CT to occur from all these states in the case of AgTTC(TCBD)2 . Additionally, the high-energy CT states populate the low-lying triplet states. Systematic femtosecond pump-probe studies provided the ultimate proof for the occurrence of excited CT as a function of excitation wavelength followed by the efficient population of the triplet states. The present study brings out the significance of charge transfer in efficiently populating the triplet states in rather unusual copper and silver corroles carrying two TCBD entities.

Critical Crystallographic Transition in Violation of Kasha’s Rule of Size-Specific ZnO Quantum Dots

Investigation of the intrinsic relationship between crystallography and electronic properties may provide an avenue to unravel the photophysical aspects of semiconductor nanostructures. Since the 1980s, semiconducting materials at the nanoscale dimensions have been of immense scientific interest because of the unique quantum characteristics beyond a particular size limit. Prudent advances in the synthetic strategies of naked ZnO quantum dots have offered the opportunity to imbue the structure–property relationship of nanostructured systems. Seven size-specific ZnO nanospheres have been synthesized through alkaline hydrolysis of the precursor salt in the presence of alcohol in the reaction medium. The photoluminescence behavior of colloidal dispersion of ZnO nanoparticles has been studied as a function of excitation wavelength with a periodic interval of 10 nm in the range of 290–430 nm. It has been observed that changes in the excitation wavelength shift the emission spectrum in a regular manner independent of the size of ZnO particles under investigation. The empirical proposition in the framework of Kasha’s rule has been established as a milestone of spectroscopy, being validated in tens of thousands of fluorophore systems; meanwhile, although obscure, anomalous cases have also been observed both in molecular and nanoscale systems. Crystallographic analysis has shown that there occurs a structural transition that determines the localization of electronic energy levels in the experimental size range of the ZnO particles. Therefore, the salient feature of physical significance is that crystallographic transition appears to exhibit the excitation wavelength-dependent shift of the photoluminescence maximum and is therefore the violation of Kasha’s rule for the ZnO quantum dots.

Coherent exciton coupling in fluorescent protein dimers probed by two-photon polarization ratio

Coherent excitonic coupling of two chromophores in a dimer of a GFP-analogue, VenusA206, was discovered previously by using circular dichroism and a combination of photon antibunching and fluorescence correlation spectroscopy. To further elucidate this effect, here we use a new approach based on the measurements of two-photon polarization ratio (Ω) as a function of excitation wavelength. Theoretically, Ω depends on the angle between the vectors of absorption transition dipole moment and permanent dipole moment change upon excitation. Generally, excitonic coupling of two chromophores should provide a change of this angle. We measured Ω for the tandem dimer of Venus (tdV) with two chromophores per protein chain and, as a control - for the tandem dimer with one of the chromophores knocked out by point mutation (tdVx). Compared to tdVx, we observe in tdV a significant increase of Ω in the red side of the pure electronic transition and a significant decrease of Ω in the blue side. At the same time, the two-photon absorption spectra of the two systems virtually coincide. Our observations are consistent with a model where only a minority of dimers are coupled coherently and the two permanent dipoles are oriented in an oblique head-to-head arrangement with an angle ∼400 between them. For multiphoton microscopy applications, it is interesting to investigate if the coherently coupled GFP dimer can provide a cooperative enhancement of the two-photon cross section, i.e., absorb stronger than two uncoupled monomers. This possibility can come from the “motional narrowing” of excitonic transition and will be discussed.

Confinement-Driven Photophysics in Hydrazone-Based Hierarchical Materials.

Confinement-imposed photophysics was probed for novel stimuli-responsive hydrazone-based compounds demonstrating a conceptual difference in their behavior within 2D versus 3D porous matrices for the first time. The challenges associated with photoswitch isomerization arising from host interactions with photochromic compounds in 2D scaffolds could be overcome in 3D materials. Solution-like photoisomerization rate constants were realized for sterically demanding hydrazone derivatives in the solid state through their coordinative immobilization in 3D scaffolds. According to steady-state and time-resolved photophysical measurements and theoretical modeling, this approach provides access to hydrazone-based materials with fast photoisomerization kinetics in the solid state. Fast isomerization of integrated hydrazone derivatives allows for probing and tailoring resonance energy transfer (ET) processes as a function of excitation wavelength, providing a novel pathway for ET modulation.

Confinement‐Driven Photophysics in Hydrazone‐Based Hierarchical Materials

AbstractConfinement‐imposed photophysics was probed for novel stimuli‐responsive hydrazone‐based compounds demonstrating a conceptual difference in their behavior within 2D versus 3D porous matrices for the first time. The challenges associated with photoswitch isomerization arising from host interactions with photochromic compounds in 2D scaffolds could be overcome in 3D materials. Solution‐like photoisomerization rate constants were realized for sterically demanding hydrazone derivatives in the solid state through their coordinative immobilization in 3D scaffolds. According to steady‐state and time‐resolved photophysical measurements and theoretical modeling, this approach provides access to hydrazone‐based materials with fast photoisomerization kinetics in the solid state. Fast isomerization of integrated hydrazone derivatives allows for probing and tailoring resonance energy transfer (ET) processes as a function of excitation wavelength, providing a novel pathway for ET modulation.

Vibrational energy levels and predissociation lifetimes of the A2Σ+ state of SH/SD radicals by photodissociation spectroscopy.

Photo-predissociation of SH and SDradicals in the A2Σ+ state is investigated using the high-n Rydberg atom time-of-flight technique. By measuring the photoproduct translational energy distributions as a function of excitation wavelength, contributions from overlapping A2Σ+ (v') ← X2Π (v″) transitions can be separated, and the H/D + S(3PJ) photofragment yield (PFY) spectra are obtained across various rovibrational levels (SH v' = 0-7 and SDv' = 0-8) of the A2Σ+ ← X2Π bands. The upper A2Σ+ state vibrational levels v' = 5-7 of SH and v' = 3-8 of SDare determined for the first time. The PFY spectra are analyzed with the simulation program PGOPHER [C. M. Western, J. Quant. Spectrosc. Radiat. Transfer 186, 221 (2016)], which gives vibrational origins and linewidths of the rovibrational levels of the A2Σ+ state. The linewidths (≥1.5cm-1) of the SH A2Σ+ v' = 3-7 and SDA2Σ+ v' = 2-8 states are characterized for the first time in this work, demonstrating that these levels undergo rapid predissociation with lifetimes on the order of picosecond. The lifetimes of the SDA2Σ+ v' = 0, N' = 1 and 2 levels are determined to be 247 ± 50ns and 176 ± 60ns by pump-probe delay measurements, respectively. The experimentally measured lifetimes are in reasonable agreement with the theoretical predictions.

Disentangling Electronic Spectra of Linear and Cyclic Hydrogenated Carbon Cluster Cations, C2n+1H+ (n = 3-10).

Electronic spectra are measured for protonated carbon clusters (C2n+1H+) containing between 7 and 21 carbon atoms. Linear and cyclic C2n+1H+ isomers are separated and selected using a drift tube ion mobility stage before being mass selected and introduced into a cryogenically cooled ion trap. Spectra are measured using a two-color resonance-enhanced photodissociation strategy, monitoring C2n+1+ photofragments (H atom loss channel) as a function of excitation wavelength. The linear C7H+, C9H+, C11H+, C13H+, C15H+, and C17H+ clusters, which are predicted to have polyynic structures, possess sharp 11Σ+ ← X̃1Σ+ transitions with well-resolved vibronic progressions in C-C stretch vibrational modes. The vibronic features are reproduced by spectral simulations based on vibrational frequencies and geometries calculated with time-dependent density functional theory (ωB97X-D/cc-pVDZ level). The cyclic C15H+, C17H+, C19H+, and C21H+ clusters exhibit weak, broad transitions at a shorter wavelength compared to their linear counterparts. Wavelengths for the origin transitions of both linear and cyclic isomers shift linearly with the number of constituent carbon atoms, indicating that in both cases, the clusters possess a common structural motif.

3D Upconversion Barcodes for Combinatory Wireless Neuromodulation in Behaving Animals.

Upconversion techniques offer all-optical wireless alternatives to modulate targeted neurons in behaving animals, but most existing upconversion-based optogenetic devices show prefixed emission that is used to excite just one channelrhodopsin at a restricted brain region. Here, a hierarchical upconversion device is reported to enable spatially selective and combinatory optogenetics in behaving rodent animals. The device assumes a multiarrayed optrode format containing engineered upconversion nanoparticles (UCNPs) to deliver dynamic light palettes as a function of excitation wavelength. Three primary emissions at 477, 540, and 654nm are selected to match the absorption of different channelrhodopsins. The UCNPs are barcode assembled to multiple nanomachined optical pinholes in a microscale pipette device to allow remotely addressable, spectrum programmable, and spatially selective optical interrogation of complex brain circuits. Using the unique device, the basolateral amygdala and caudoputamen circuits are selectively modulated and the associated fear or anxiety behavior in freely behaving rodents is successfully differentiated. It is believed that the 3D barcode upconversion device would be a great supplement to current optogenetic toolsets and opens up new possibilities for sophisticated neural control.

High performance FIR thermometry on the basis of the redshift of CTB by dual-wavelength alternative excitation in Eu3+:YVO4.

Compared with the forbidden 4f transition of rare earth ions, the strong absorption of the charge transfer band (CTB) enabled fluorescence thermometry to have high luminescence efficiency. Based on the temperature induced redshift of CTB, a high performance fluorescence intensity ratio (FIR) thermometry performed by dual-wavelength alternative excitation was studied. By way of the rising and falling edges of CTB in Eu3+ doped YVO4, monochrome sensitivity as a function of excitation wavelength was studied in the range of 303-783 K. The excitation wavelength with the highest positive monochrome sensitivity was determined, as well as that with the negative one. The optimum FIR temperature sensing strategy is proposed, and the theoretical highest relative sensitivity (Sr) is calculated to be 1.86% K-1, with the lowest uncertainty (ΔT) of 0.1 K at 783 K.

Fluorescence Emission of Self-assembling Amyloid-like Peptides: Solution versus Solid State.

Analysis of the intrinsic UV‐visible fluorescence exhibited by self‐assembling amyloid‐like peptides in solution and in solid the state highlights that their physical state has a profound impact on the optical properties. In the solid state, a linear dependence of the fluorescence emission peaks as a function of excitation wavelength is detected. On the contrary, an excitation‐independent emission is observed in solution. The present findings constitute a valuable benchmark for current and future explanations of the fluorescence emission by amyloids.

Heterometallic Actinide-Containing Photoresponsive Metal-Organic Frameworks: Dynamic and Static Tuning of Electronic Properties.

Acquiring fundamental knowledge of properties of actinide-based materials is a necessary step to create new possibilities for addressing the current challenges in the nuclear energy and nuclear waste sectors. In this report, we established a photophysics-electronics correlation for actinide-containing metal-organic frameworks (An-MOFs) as a function of excitation wavelength, for the first time. A stepwise approach for dynamically modulating electronic properties was applied for the first time towards actinide-based heterometallic MOFs through integration of photochromic linkers. Optical cycling, modeling of density of states near the Fermi edge, conductivity measurements, and photoisomerization kinetics were employed to shed light on the process of tailoring optoelectronic properties of An-MOFs. Furthermore, the first photochromic MOF-based field-effect transistor, in which the field-effect response could be changed through light exposure, was constructed. As a demonstration, the change in current upon light exposure was sufficient to operate a two-LED fail-safe indicator circuit.

Heterometallic Actinide‐Containing Photoresponsive Metal‐Organic Frameworks: Dynamic and Static Tuning of Electronic Properties

AbstractAcquiring fundamental knowledge of properties of actinide‐based materials is a necessary step to create new possibilities for addressing the current challenges in the nuclear energy and nuclear waste sectors. In this report, we established a photophysics–electronics correlation for actinide‐containing metal‐organic frameworks (An‐MOFs) as a function of excitation wavelength, for the first time. A stepwise approach for dynamically modulating electronic properties was applied for the first time towards actinide‐based heterometallic MOFs through integration of photochromic linkers. Optical cycling, modeling of density of states near the Fermi edge, conductivity measurements, and photoisomerization kinetics were employed to shed light on the process of tailoring optoelectronic properties of An‐MOFs. Furthermore, the first photochromic MOF‐based field‐effect transistor, in which the field‐effect response could be changed through light exposure, was constructed. As a demonstration, the change in current upon light exposure was sufficient to operate a two‐LED fail‐safe indicator circuit.

Photoluminescent ZrO2:Tb3+ thin films synthesized by USP technique using a metal-organic precursor

Photoluminescent ZrO2 and ZrO2:Tb3+ thin films were synthesized from metal-organic precursor, zirconium acetylacetonate [Zr(C5H7O2)4], using Ultrasonic Spray Pyrolysis method. These thin films were deposited on Corning glass substrates at several deposition temperatures. ZrO2 and ZrO2:Tb3+ (X a/o) films were characterized by x-ray diffraction to identify the crystalline structure; results showed tetragonal phase of zirconia; with a crystalline size of approximately 4 nm according to Scherrer’s formula. Also, SEM micrographs revealed that surface morphology of these films is very flat. Chemical composition microanalysis showed presence of oxygen and zirconium as major species. Analysis by Infrared spectroscopy demonstrates that 900 and 760 nm bands correspond to presence of ZrO2. In addition, these films showed optical transmittances from 75 to 98 % in the visible range. Photoluminescent features varied as a function of excitation wavelength; when excited at λ ex = 286 nm strong green emission is observed, which is associated to electronic transitions 5D4 → 7Fn (n = 6, 5, 4, 3) corresponding to Tb3+ ions. Excitation with λ ex = 336 nm exhibits spectra with simultaneous emissions from host lattice and Tb3+ ions; here, observed color was blue-green.

Excitation induced enhancement of spectral response and energy transfer mechanisms in Fe/Sm modified ZnO phosphors

The present study aims to analyze the tunability of photonic emissions as a function of excitation wavelengths in Fe/Sm co-doped ZnO phosphors. We have investigated the up-conversion (UC) and down-conversion (DC) luminescences in detail along with possible channels for energy transfer and their local electronic structures. These phosphors are polycrystalline with a hexagonal wurtzite structure, and the co-doping of Fe/Sm ions leads the 3D-pyramid like morphology of the ZnO to transform in flower-shaped nanorods. Further, from the UV–Vis spectra, it is found that bandgap contracts due to the formation of defects. The DC emission spectra (λex = 325 nm) show an enhancement of polychromatic emission as a function of the Sm concentration and tune from orange to red along with the transition from the warm to cool region in the Commission International de l'Eclairage 1931 XY spectral chromaticity coordinates. X-ray absorption spectra confirm the presence of Fe2+/Fe3+ ions and Sm3+ ions. Using the decay kinetics and transfer efficiencies, the energy transfer between the host defect levels and dopant ions is explained with the help of the energy level diagram. The UC emission spectra (λex = 980 nm) exhibit monochromatic red emission along with a strong near-infrared emission lying in the cool region with 100% color purity. These phosphors are expected to find applications in solid-state lighting applications, optoelectronics, and biomedical engineering, etc.

Fluorescence Characterization of Standard, Mutant and Sweet Corn.

This work resulted in the development of a method based on fluorescence spectroscopy to differentiate between three corn varieties, standard, mutant and sweet, and to characterize the corn variety present in finished products. This was achieved by recording fluorescence emission spectra as a function of excitation wavelength. For a standard, non-transgenic and non-sweet corn, the maximum of the first peak is around 412-414nm at the excitation wavelength equal to 280nm and shifts to the longer emission wavelengths as the excitation wavelength increases. Also, the second peak is located at 535nm or is slightly higher (537 to 540nm) and does not vary for excitation wavelengths from 280 to 360nm. For mutant corn, the position of the first peak is located at 420nm and above for λex = 280nm, while the second peak starts at 525-530nm (depending on the mutant) and never reaches 535nm. Finally, for a sweet corn, the position of the first fluorescence emission peak is around 430nm. If the sweet corn is non-hybrid, the position of the second emission peak is at 535nm. A hybrid sweet corn has its second peak around 530nm. Thus, fluorescence emission at 530 is characteristic of corn that has undergone natural or artificial genetic transformation. Finally, we found simple mathematical equations to calculate the percentage of amylopectin and amylose in a given corn.

Beyond Hydrogen Evolution: Solar-Driven, Water-Donating Transfer Hydrogenation over Platinum/Carbon Nitride

Hydrogen-rich organic molecules such as alcohols are widely used as hydrogen donors in transfer hydrogenation. Nevertheless, water as a more abundant and ecofriendly hydrogen source has hardly been used due to the high difficulty in splitting water molecules. Herein, we designed a photocatalytic water-donating transfer hydrogenation (PWDTH) technique, in which hydrogen was extracted from water under light illumination and then in situ added to different unsaturated bonds (C═C, C═O, N═O) for chemical synthesis. Platinum-loaded carbon nitride (Pt/CN) was used as the model catalyst for this cascade reaction, which is beyond its normal applications for water splitting. This approach was highly accessible to efficiency optimization, either by modifying CN for extended light absorption and enhanced charge transfer or by alloying Pt with another metal for better catalytic activities. Remarkably, a quantum efficiency of up to 21.8% was achieved for nitrobenzene hydrogenation under 380 nm irradiation, which is 3 times higher than that obtained in the single water-splitting reaction, indicating that the PWDTH can be more rewarding than hydrogen evolution for solar energy harvesting. Deep insights into the underlying mechanism were provided by detailed measurements and interpretations of femtosecond transient absorption spectra, action spectra (quantum efficiency as a function of excitation wavelength), and reaction kinetic profiles under varied conditions including the variation of light intensities, temperatures, and water isotopes. The mild reaction conditions, simple processing, and broad substituent group tolerance endow this approach with a high potential toward a general solar-to-chemical conversion technique.

Resonant Multi-phonon Raman scattering of black phosphorus

Black phosphorus (BP) has been attracting intense interest due to its unique anisotropic properties. The investigations on phonon dispersion and electronic band structure could expand the understanding of the properties of BP and promote its application on next generation nano-electronic devices. As the fingerprint of materials, Raman spectroscopy can provide the information of their phonon dispersion and electronic band structure. According to the Raman selection rule, Raman process involving multiple (two or more) phonons can be used to probe the phonon density of states within the whole Brillouin zone. However, the intensity of high-order Raman modes is much lower than that of the first-order Raman mode. To break through the limit of low intensity, here, we measured the resonant Raman spectroscopy of BP excited by several wavelength lasers and observed rich information about high-order Raman modes in the spectral range of 680–930 cm<sup>–1</sup>. To further investigate high-order Raman modes and avoid the birefringence effects from optical anisotropy on Raman intensity, we employ a special polarization configuration to obtain resonant Raman spectra and Raman intensity as a function of excitation wavelength. All the observed high-order Raman modes are certainly assigned, according to the phonon dispersion and symmetry analysis of related phonons. This indicates the great contribution of phonons within the Brillouin zone to the second- and third-order Raman scattering. This work proposes a general and systematical method to investigate high-order Raman modes, and paves ways for the researches of phonon dispersion and resonance Raman spectroscopy in other anisotropic materials.