Nonradiative Channels Research Articles

Theoretical Rationalization of the Dual Photophysical Behavior of C60+

Interest in fullerenes has been renewed recently in astrophysics as a consequence of their detection in circumstellar environments. In particular, C60+ was detected in the diffuse interstellar medium and its presence has been related to some diffuse interstellar bands (DIBs) whose origin was previously unknown. A single recent laboratory experiment ( J. Phys. Chem. A 2017, 121, 7356-7361) shows that upon laser excitation at 785 nm, C60+ in neon matrixes exhibits a radiative decay at 965 nm, while UV photoexcitation does not lead to any significant luminescence. To rationalize this original dual photophysical behavior, we have performed time-dependent density functional theory (TD-DFT) calculations on C60+ to investigate the potential energy surfaces of the relevant electronic states, completed by the simulations of vibrationally resolved absorption and emission spectra. The proposed photophysical pathways shed light on the experimental measurements: The near-IR laser excitation populates the 11th doublet excited state (D11) that decays to the lowest first bright excited state D5, from which photoluminescence is predicted. Indeed, D5 is largely separated from the lower electronic states (D0-D4). Thus, D5 behaves effectively as the first excited state, while the D0-D4 set of states act as the electronic ground state. In addition, there are no low-lying conical intersections between D5 and lower excited states energetically accessible upon near-IR excitation that can provide efficient nonradiative decay channels for this state, leaving radiative decay as the most likely deactivation pathway. However, a sloped conical intersection between D5 and D4 was located around 2.9 eV above D0. While it is too high in energy to be accessible upon near-IR excitation, it provides a funnel for efficient nonradiative decay down to the ground state (D0) accessible upon UV light excitation. Thus, the photophysics of C60+ is controlled by the ability to access this funnel: Upon near-IR excitation, the system fluoresces because the funnel for nonradiative decay cannot be reached, while UV irradiation provides a different route by opening up a radiationless decay channel via this funnel, accounting for the absence of fluorescence.

Photophysical Constants of the Tetraoxa[8]Circulene Molecule

The rate constants of photophysical processes (radiation rate constant, internal conversion rate constant, and quantum fluorescence yield) have been calculated for the tetraoxa[8]circulene molecule by the INDO/S, TDDFT, and CC2 methods. It is confirmed that the doubly degenerate triplet level Т2 and the level Т1 are located below the first singlet excited level S1. According to the selection rules for the angular momentum, the spin-orbit interaction between the levels S1 and T1 (S1 and T2) is equal to zero. Therefore, the internal conversion is the only nonradiative channel in this molecule. The results of calculations also demonstrate that values of the internal conversion rate constant and the quantum fluorescence yield obtained by the INDO/S method are incorrect.

Wave Function Control of Charge-Separated Excited-State Lifetimes.

Control of excited-state processes is crucial to an increasing number of important device technologies that include displays, photocatalysts, solar energy conversion devices, photovoltaics, and photonics. However, the manipulation and control of electronic excited-state lifetimes and properties continue to be a challenge for molecular scientists. Herein, we present the results of ground-state and transient absorption spectroscopies as they relate to magnetic exchange control of excited-state lifetimes. We describe a novel mechanism for controlling these excited-state lifetimes that involves varying the magnetic exchange interaction between a stable organic radical and the unpaired electrons present in the open-shell configuration of a charge-separated excited state. Specifically, we show that the excited-state lifetime can be controlled in a predictable manner based on an a priori knowledge of the pairwise magnetic exchange interactions between excited-state spins. These magnetic exchange couplings affect the excited-state electronic structure in a manner that introduces variable degrees of spin forbiddenness into the nonradiative decay channel between the excited state and the electronic ground state.

Optical properties of porous Sm[formula omitted]-doped yttrium orthophosphate nanoparticles tailored by co-precipitation route

Sm3+ doped yttrium orthophosphate nanoparticles with porous structure was synthesized via simple co-precipitation method. The structure, the morphology and the elemental composition of the tailored nanoparticles were investigated using transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDAX). Under the influence of trisodium citrate, Sm3+ doped YPO4 phosphor with porous structure could be synthesized. Under our reaction conditions, Sm3+ doped YPO4 nanophosphor crystallize in the hexagonal structure, as confirmed by the powder XRD patterns. The formation of porous structure turned out to be an essential step for the peculiar luminescent properties. The porosity undoubtedly enlarges the surface of the semiconductor/air interface, resulting in suppression of the non-radiative recombination channels. The tailored porous yttrium orthophosphate nanoparticles significantly displayed potentiality in the field of optical displays and also demonstrated the applicability in the biological field.

Tetraphenylpyrazine Based AIE Luminogens: Unique Excited State Decay and Its Application in Deep‐Blue Light‐Emitting Diodes

AbstractCarbazole‐modified tetraphenylpyrazine (TPP) derivatives TPP‐Cz, TPP‐PhCz, TPP‐2Cz, and TPP‐2PhCz are prepared with different patterns by inserting a π‐bridge and adopting two‐armed forms to produce aggregation‐induced emission (AIE)‐type deep‐blue emitters for organic light‐emitting diode (OLED) applications. Systematic spectral analysis and detailed theoretical calculation of these TPP derivatives reveal their special excited state behaviors. Unlike the traditional AIE unit, which features enhanced emission from solution to aggregate with the nonradiative decay channel blocked, a tenfold increase in the radiative decay rate of these TPP‐based AIE luminogens (AIEgens) is observed from the solution to film states. The formation of a stable quinone conformation of the TPP center and its peripheral phenyl group in the excited state is the main reason for this behavior. The distribution changes in the total reorganization energy are further integrated, and a feasible molecular design strategy is proposed for the TPP family. Nondoped OLEDs are fabricated, and the AIEgens exhibit good performance with valuable blue emission (x + y < 0.3). The TPP‐Cz‐based optimized devices exhibit more stable external quantum efficiencies and an ideal roll‐off with unchanged Commission Internationale de l'Elcairage (CIE) coordinates of (0.16, 0.11). These findings demonstrate the great potential of TPP with its unique excited state characteristics as a deep‐blue AIEgen.

On the Nature of Interplay among Major Flexibility Channels in Molecular Rotors

As a part of our interest in the excited-state dynamics of flexible materials, we have undertaken a theoretical investigation to the photo-induced reactions of 2-[4-(dimethylamino)benzylidene]malononitrile (BMN) by a combination of the density functional theory, its extended time-dependent (TD-DFT) single reference, and ab initio molecular dynamic (MD) simulations. The results showed that double-bond twisting and the neighbor single-bond twisting togetherness in the excited singlet state is the most important nonradiative deactivation channel to the ground state. Double- and single-bond twisting insert clear intersections among the potential energy surfaces of the singlet states (especially S1/S0) leading to fluorescence quenching. Furthermore, effects of molecular dynamic simulations on molecular properties in the femtosecond to picosecond time domain are studied to validate the results. In agreement with the experimental results, the findings conclude the existence of a flexible geometry-dependent single emission band. Such a study may give information on how the molecule could be externally modified/fixed to yield a desired effect, i.e., more fluorescence or more nonradiative decay.

The origin of the solvent dependence of fluorescence quantum yields in dipolar merocyanine dyes.

Fluorophores with high quantum yields are desired for a variety of applications. Optimization of promising chromophores requires an understanding of the non-radiative decay channels that compete with the emission of photons. We synthesized a new derivative of the famous laser dye 4-dicyanomethylen-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM), i.e., merocyanine 4-(dicyanomethylene)-2-tert-butyl-6-[3-(3-butyl-benzothiazol-2-ylidene)1-propenyl]-4H-pyran (DCBT). We measured fluorescence lifetimes and quantum yields in a variety of solvents and found a trend opposite to the energy gap law. This motivated a theoretical investigation into the possible non-radiative decay channels. We propose that a barrier to a conical intersection exists that is very sensitive to the solvent polarity. The conical intersection is characterized by a twisted geometry which allows a subsequent photoisomerization. Transient absorption measurements confirmed the formation of a photoisomer in unpolar solvents, while the measurements of fluorescence quantum yields at low temperature demonstrated the existence of an activation energy barrier.

Plasmon resonance energy transfer and research progress in plasmon-enhanced photocatalysis

Plasmon resonance energy transfer refers to the coherent energy transfer via dipole-dipole coupling from surface plasmons to adjacent exciton nanosystems such as semiconductor quantum dots or dye molecules. The plasmon resonance energy transfer is a non-radiative plasmon decay pathway, which can also act as an available channel to extract the plasmon-harvested energy. In addition, hot electron relaxation (non-radiative channel) and scattering (radiative channel) are also the dissipation pathways of surface plasmon resonances. The plasmon-harvested energy can be effectively transferred to other nanosystems or converted into other energy forms through these correlated dissipation pathways. In this paper, the underlying mechanism and dynamics of the plasmon resonance energy transfer as well as the related energy and charge transfer processes (such as near field enhancement and coupling, far field scattering, plasmon-induced hot electron transfer) are introduced. The recent research progress of the plasmon-enhanced photocatalysis by energy and charge transfer is reviewed.

Non-radiative decay of an eumelanin monomer: to be or not to be planar.

In order to understand the photoprotection process in eumelanin, it is imperative to understand the photoprocesses in its monomers. Eumelanin is a polymeric structure made of di-hydroxyindole (DHI) as the basic motif. There have been studies on the ground and low-lying excited state potential energy surfaces (PESs) and a few of the important modes have been identified in the eumelanin monomer. However, to properly depict the fate of a molecule after an initial photoexcitation, it is important to look at the dynamical picture and in this work, we have studied the excited state dynamics of a eumelanin monomer. We observe that there are planar OH elongation modes that can be important for the de-excitation mechanism. There are also non-planar modes that distort the phenyl ring of the DHI molecule. In the excited state dynamics, it is the non-planar modes that are more relevant to the non-radiative deactivation process of the monomer. We further notice that due to the non-planarity in the molecules along the deactivation path, the excited states involved in the processes are mixed ππ* and πσ* in nature. Furthermore, it is noticed that this deactivation process occurs well below a ps timescale.

Terahertz Spectroscopy of Two-Dimensional Semimetal in Three-Layer InAs/GaSb/InAs Quantum Well

The features of terahertz photoluminescence and magnetoabsorption in magnetic fields up to 16 T in threelayer InAs/GaSb/InAs quantum wells with a band structure corresponding to a “two-dimensional semimetal” are studied. The comparison of the positions and amplitudes of the photoluminescence lines with the theoretical calculations of oscillator strengths for interband and intraband transitions performed using the eight-band Kane Hamiltonian indicates the existence of a nonradiative recombination channel associated with the overlap of the conduction and valence bands. The energies of interband and intraband Landau level transitions observed in the magnetoabsorption spectra are in good agreement with theoretical calculations, which confirms the predicted band structure.

Photostimulated control of luminescence quantum yield for colloidal Ag2S/2-MPA quantum dots.

In this paper, we present the results on photoinduced formation of colloidal Ag2S quantum dots with sizes of 1.5–3 nm passivated by 2-mercaptopropionic acid (Ag2S/2-MPA) in the presence of ethylene glycol. The synthetized colloidal Ag2S/2-MPA QDs have NIR recombination luminescence with its maximum near 800 nm. The control of absorption and luminescence properties of the QDs is achieved by photoactivation. It is shown that photoexposure of colloidal solution of Ag2S/2-MPA QDs leads to an increase in the QD size and monodispersity along side with the growth of the luminescence quantum yield from 1% to 7.9%. Enhancement of the luminescence quantum yield is accompanied by an increase in the average luminescence lifetime up to 190 ns, which is due to the blocking of the nonradiative recombination channel with the radiative recombination rate being (3–5.5) × 105 s−1. It is shown that the purification of the Ag2S/2-MPA solution by a dialysis membrane from regenerated cellulose leads to an increase in the sample monodispersity, as well as stops the photoinduced growth of QDs, and also reduces the degradation of their photoluminescence.

Ultrahigh-efficiency aqueous flat nanocrystals of CdSe/CdS@Cd1-xZnxS colloidal core/crown@alloyed-shell quantum wells.

Colloidal semiconductor nanoplatelets (NPLs) are highly promising luminescent materials owing to their exceptionally narrow emission spectra. While high-efficiency NPLs in non-polar organic media can be obtained readily, NPLs in aqueous media suffer from extremely low quantum yields (QYs), which completely undermines their potential, especially in biological applications. Here, we show high-efficiency water-soluble CdSe/CdS@Cd1-xZnxS core/crown@shell NPLs formed by layer-by-layer grown and composition-tuned gradient Cd1-xZnxS shells on CdSe/CdS core/crown seeds. Such control of shell composition with monolayer precision and effective peripheral crown passivation, together with the compact capping density of short 3-mercaptopropionic acid ligands, allow for QYs reaching 90% in water, accompanied by a significantly increased photoluminescence lifetime (∼35 ns), indicating the suppression of nonradiative channels in these NPLs. We also demonstrate the controlled attachment of these NPLs without stacking at the nanoscale by taking advantage of their 2D geometry and hydrophilicity. This is a significant step in achieving controlled assemblies and overcoming the stacking process, which otherwise undermines their film formation and performance in optoelectronic applications. Moreover, we show that the parallel orientation of such NPLs achieved by the controlled attachment enables directed emission perpendicular to the surface of the NPL films, which is highly advantageous for light extraction in light-emitting platforms.

A photo-switchable supramolecular hyperbranched polymer with aggregation-induced emission based on host-guest interaction

In this work, we report a simple strategy by mixing a coumarin appended tetraphenylethylene derivative (TPEC) and γ-cydodextrin(γ-CD) in aqueous solution, based on the host-guest interaction between TPEC and γ-CD to construct a dual-modality supramolecular hyperbranched polymer. The host-guest interaction between γ-CD and TPEC efficiently restricted the intramolecular rotation and the non-radiative relaxation channel, minimizing energy loss, thereby resulting in the strong emission of TPEC in dilute solution. Moreover, we achieved the conversion between non-covalent supramolecular hyperbranched polymers and its corresponding covalent supramolecular hyperbranched polymers under the irradiation of UV light, which can be reversibly controlled through the irradiation at the wavelengths of 254 nm and 365 nm. Furthermore, both non-covalent supramolecular hyperbranched polymers and covalent supramolecular hyperbranched polymers can self-assemble into spherical structures. This methodology provides a new idea for exploring a new type of supramolecular hyperbranched polymers with aggregation-induced emission.

Functionalizing plasmonic nanoparticles through adding a shell to improve electronic properties of c-Si thin-film solar cells

Embedding plasmonic nanoparticles (p-NPs) inside the solar cell’s active layer is capable of enhancing active layer optical absorption; however, such inclusion has some detrimental effects on the electronic properties of the solar cells. In addition, p-NPs are highly catalytic, their presence enables other non-radiative decay channels besides generation of electron–hole pairs, and the electrons usually get absorbed by these p-NPs. This results in the unfavorable fact that the potential enhancement in the carrier generation rate and the generated current is no longer in line with the enhancement in the optical absorption. In this paper, we propose to functionalize p-NPs by adding a dielectric or semiconductor shell, to passivate the p-NP without deteriorating scattering and/or plasmonic effects. Ag@SiO2 and Ag@TiO2 core–shell p-NPs have been intensely studied using extensive computational electromagnetic simulations to model the spectral response of the active layer’s optical absorption as well as electronic properties as a function of both shell composition and thickness. It is shown that a 5 nm TiO2 shell is apt to optically passivate the p-NP without any reduction in optical absorption, while improving the short circuit current density (J sc ) of the thin-film solar cell by 33.3%.

Optical Properties of Neodymium Doped Magnesium Zinc Sulfo- Phopshate Glass: Impact of Gold Nanoparticles Inclusion

Gold (Au) nanoparticles (NPs) incorporated in magnesium zinc sulfo-phosphate glass with the molar composition of (58.5P_2O_5–20MgO–20ZnSO_4–1.5Nd_2O_3–xAu NPs (where x = 0.0, 0.1, 0.2, 0.3, and 0.4 mol%) were prepared via melt quench technique. The effect of Au NPs as incorporated in Nd^3+ doped glasses were studied using X‒ray diffraction (XRD), High Resolution-Transmission Electron Microscope (HR‒TEM), UV‒Visible-NIR absorption spectrometer and photoluminescence spectrometer. The amorphous nature of glass is confirmed via XRD measurements and existence of Au NPs inside the glass is reveal by HR-EM. The typical absorption bands of Nd^3+ are attained which positioned around 352, 430, 459, 474, 512, 525, 581, 626, 682, 744, 801and 875 nm. Up-conversion (UC) photoluminescence (PL) demonstrated six prominent emission bands located around 420, 445, 460, 484, 515 and 545 nm corresponding transition 2P_1/2--> 4I_9/2, 2D_5/2-->4I_11/2, 2P_1/2-->4I_11/2, 2D_5/2-->4I_13/2, 2K_15/2,4G_7/2-->4I_11/2,4I_9/2 and 2D_5/2-->4I_15/2 respectively. Glass containing 0.1 mol% of Au NPs shows utmost PL enhancement and the emission decrease beyond 0.1 mol% of Au NPs depicts re-absorption of SPR and increase of non-radiative channel. The improvement of PL intensity attributed to local field effect of Au NPs that altered the environment in proximity Nd^3+ ions. The proposed glass could be potential for improvement of solid state laser design.

Using density based indexes to characterize excited states evolution.

With the aim of offering new computational tools helping in the description of photochemical reactions and phenomena occurring at the excited state, we present in this work the capability of a density based index (Π) in locating decay channels from higher to lower excited states. The Π index, previously applied to disclose non-radiative decay channels from the first excited state to the ground state, is very simple in its formulation and can be evaluated, practically with no extra computational cost, and coupled to any quantum method able to provide excited states densities. Indeed, this index relies only on the knowledge of energetics and electron densities of the different electronic states involved in the decay. In the present work, we show the proficiency of the Π index in the general case of decay between excited states by applying it to two model systems well characterized both theoretically and experimentally. In both cases, this descriptor was successful in spotting the regions where excited states are more likely to decay, thus suggesting its potential interest for further application in the design of new compounds. © 2018 Wiley Periodicals, Inc.

MEH-PPV photophysics: insights from the influence of a nearby 2D quencher

The effect of 2D quenching on single chain photophysics was investigated by spin coating 13 nm thick films of polystyrene lightly doped with MEH-PPV onto CVD grown graphene and observing the changes in several photoluminescent (PL) observables. With 99% of the PL quenched, we found a 60% drop in the PL lifetime, along with a significant blue-shift of the PL emission due to the preferential quenching of emission at longer wavelengths. During photo-bleaching, the blue spectral shift observed for isolated polymers was eliminated in the presence of the quencher up until 70% of the polymer was photo-bleached. Results were interpreted using a static disorder induced conjugation length distribution model. The quencher, by opening up a new non-radiative decay channel, ensures that excitons do not have sufficient time to migrate to nearby lower energy chromophores. The reduction of energy transfer into the lowest-energy chromophores thus reduces their rate of photo-bleaching. Finally, the difference between the quenched and non-quenched spectra allows the rate of energy transfer along the polymer backbone to be estimated at ∼2 ns−1.

Monolayer and thin h–BN as substrates for electron spectro-microscopy analysis of plasmonic nanoparticles

The influence of four substrates [thin Si3N4, few-layer graphene (FLG), thin h–BN, and monolayer h–BN] on plasmon resonances of metallic nanoparticles was studied using electron energy loss spectroscopy. The h–BN monolayer is an excellent substrate for the study of plasmonic particles due to its large bandgap, negligible charging under electron irradiation, and negligible influence on the plasmon resonance full width at half maximum and peak positions. These effects were evidenced in experiments with gold nanotriangles focusing on dipolar modes. Nanotriangles on h–BN exhibit the lowest influence from the substrate compared to Si3N4 and FLG. In a dataset containing 23 triangles of similar sizes, the dipolar mode was found to have smaller redshifts, sharper peak widths, and higher resonance quality factors on h–BN, showing that it has nearly no effect on the plasmon absorption properties, provided that it is free from carbon contamination. However, light emission (cathodoluminescence) decreases as a function of electron irradiation for triangles on h–BN, even though the electron energy loss signal stays unchanged. This indicates the creation of non-radiative decay channels.

Improved performance of GaAsSb/AlGaAs nanowire ensemble Schottky barrier based photodetector via in situ annealing

In this work, we report on the p-i GaAsSb/AlGaAs nanowires (NWs) ensemble device exhibiting good spectral response up to 1.1 μm with a high responsivity of 311 A W−1, an external quantum efficiency of 6.1 × 104%, and a detectivity of 1.9 × 1010 Jones at 633 nm. The high responsivity of the NWs has been attributed to in situ post-growth annealing of GaAsSb axial NWs in the ultra-high vacuum. The enabling growth technology is molecular beam epitaxy for the Ga-assisted epitaxial growth of these NWs on Si (111) substrates. Room temperature Raman spectra, as well as temperature dependent micro-photoluminescence peak analysis indicated suppression of band tail states and non-radiative channels due to annealing. A similar improvement in in situ annealed p-i GaAsSb NW ensemble with an AlGaAs passivating shell was inferred from a reduction in the Schottky barrier height as well as the NW resistance compared to the as-grown NW ensemble. These results demonstrate in situ annealing of nanowires to be an effective pathway for improving the optoelectronic properties of the NWs and the device thereof.

Nonradiative decay and absorption rates of quantum emitters embedded in metallic systems: Microscopic description and their determination from electronic transport

We investigate nonradiative decay and absorption rates of two-level quantum emitters embedded in a metal at low temperatures. We obtain the expressions for both nonradiative transition rates and identify a unique, experimentally accessible way to obtain both nonradiative transition rates via electronic transport in the host metallic system. Our findings not only provide a microscopic description of the nonradiative channels in metals, but they also allow one to identify, determine, and differentiate them from other decay channels, which is crucial to the understanding and controlling of the light-matter interactions at the nanoscale.