Showing papers on "Photoluminescence published in 2005"
TL;DR: Time-resolved pump-probe spectroscopy revealed that a photoinduced electron transfer from MDMO-PPV to nc-ZnO occurs in these blends on a sub-picosecond time scale and produces a long-lived charge-separated state.
Abstract: Bulk heterojunction photovoltaic devices based on blends of a conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as electron donor and crystalline ZnO nanoparticles (nc-ZnO) as electron acceptor have been studied. Composite nc-ZnO:MDMO-PPV films were cast from a common solvent mixture. Time-resolved pump-probe spectroscopy revealed that a photoinduced electron transfer from MDMO-PPV to nc-ZnO occurs in these blends on a sub-picosecond time scale and produces a long-lived (milliseconds) charge-separated state. The photovoltaic effect in devices, made by sandwiching the active nc-ZnO:MDMO-PPV layer between charge-selective electrodes, has been studied as a function of the ZnO concentration and the thickness of the layer. We also investigated changing the degree and type of mixing of the two components through the use of a surfactant for ZnO and by altering the size and shape of the nc-ZnO particles. Optimized devices have an estimated AM1.5 performance of 1.6% with incident photon to current conversion efficiencies up to 50%. Photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy have been used to gain insight in the morphology of these blends.
859 citations
TL;DR: A range of remarkable characteristics of ZnO nanostructures are presented, organized into sections describing the mechanical, electrical, optical, magnetic, and chemical sensing properties.
Abstract: This article provides a comprehensive review of the current research activities that focus on the ZnO nanostructure materials and their physical property characterizations. It begins with the synthetic methods that have been exploited to grow ZnO nanostructures. A range of remarkable characteristics are then presented, organized into sections describing the mechanical, electrical, optical, magnetic, and chemical sensing properties. These studies constitute the basis for developing versatile applications of ZnO nanostructures.
758 citations
TL;DR: The fluorescence quantum yield of Cy5 molecules attached to gold nanoparticles via ss DNA spacers via ssDNA spacers is measured for Cy5-nanoparticle distances between 2 and 16 nm to show distance dependent quantum efficiency.
Abstract: The fluorescence quantum yield of Cy5 molecules attached to gold nanoparticles via ssDNA spacers is measured for Cy5-nanoparticle distances between 2 and 16 nm. Different numbers of ssDNA per nanoparticle allow to fine-tune the distance. The change of the radiative and nonradiative molecular decay rates with distance is determined using time-resolved photoluminescence spectroscopy. Remarkably, the distance dependent quantum efficiency is almost exclusively governed by the radiative rate.
742 citations
TL;DR: The observation of the strong-coupling regime between the excitonic transition of a single GaAs quantum dot and a discrete optical mode of a microdisk microcavity is reported on.
Abstract: We report on the observation of the strong-coupling regime between the excitonic transition of a single GaAs quantum dot and a discrete optical mode of a microdisk microcavity. Photoluminescence is performed at various temperatures to tune the quantum dot exciton with respect to the optical mode. At resonance, we observe a clear anticrossing behavior, signature of the strong-coupling regime. The vacuum Rabi splitting amounts to 400 microeV and is twice as large as the individual linewidths.
722 citations
TL;DR: In this article, three Ir(III) complexes used as principal dopants in organic electrophosphorescent diodes have very high photoluminescence quantum efficiency (ηPL) in a solid-state film.
Abstract: We demonstrate that three Ir(III) complexes used as principal dopants in organic electrophosphorescent diodes have very high photoluminescence quantum efficiency (ηPL) in a solid-state film. The green emitting complex, fac-tris(2-phenylpyridinato)iridium(III) [Ir(ppy)3], the red-emitting bis[2-(2′-benzothienyl)pyridinato-N,C3′] (acetylacetonato)iridium(III) [Btp2Ir(acac)], and the blue complex bis[(4,6-difluorophenyl)pyridinato-N,C2](picolinato)iridium(III) (FIrpic) were prepared as codeposited films of varying concentration with 4,4′-bis(N-carbazolyl)-2,2′-biphenyl, a commonly used host material. The maximum ηPL values for Ir(ppy)3, Btp2Ir(acac), and FIrpic were, respectively, 97%±2% (at 1.5mol%), 51%±1% (at 1.4mol%), and 78%±1% (at 15mol%). Furthermore, we also observed that the maximum ηPL of FIrpic reached 99%±1% when doped into the high triplet energy host, m-bis(N-carbazolyl)benzene, at an optimal concentration of 1.2mol%.
688 citations
TL;DR: The optical band gap of ZnO thin films deposited on fused quartz by metal-organic chemical-vapor deposition was studied in this article, where X-ray diffraction measurements showed that samples deposited at low temperatures (<450°C) consisted of amorphous and crystalline phases.
Abstract: The optical band gap of ZnO thin films deposited on fused quartz by metal-organic chemical-vapor deposition was studied. The optical band gap of as-grown ZnO blueshifted from 3.13to4.06eV as the growth temperature decreased from 500to200°C. After annealing, the optical band gap shifted back to the single-crystal value. All the ZnO thin films studied show strong band-edge photoluminescence. X-ray diffraction measurements showed that samples deposited at low temperatures (<450°C) consisted of amorphous and crystalline phases. The redshift of the optical band gap back to the original position after annealing was strong evidence that the blueshift was due to an amorphous phase. The unshifted photoluminescence spectra indicated that the luminescence was due to the crystalline phase of ZnO, which was in the form of nanocrystals embedded in the amorphous phase.
662 citations
TL;DR: Applications are made to absorption and low-temperature emission in thin films of regioregular poly(3-hexylthiophene), with excellent agreement between theory and experiment obtained for a spatial correlation length of only 3-4 molecules.
Abstract: Absorption and emission in polymer aggregates is studied theoretically, taking into account excitonic (intermolecular) coupling, exciton-phonon (EP) coupling, and disorder, all treated on equal footing within a generalized Holstein Hamiltonian with numerically generated eigenmodes and energies. The disorder is modeled as a Gaussian distribution of molecular transition frequency offsets of width sigma and spatial correlation length l(0). Both herringbone (HB) and lamellar aggregate morphologies are considered. The emission spectral line shape is shown to undergo marked changes in response to increasing disorder, with the intensity of the ac-polarized 0-0 emission peak generally increasing relative to the replica intensities (0-1,0-2,[ellipsis (horizontal)]) as sigma increases and/or as l(0) decreases. This is contrary to the behavior of the b-polarized component of the 0-0 intensity, which, in HB aggregates, decreases with increasing disorder. Comparisons are made to analogous trends in oligomer aggregates. Analytical results are obtained in the strong EP coupling regime appropriate for conjugated polymers while treating the disorder perturbatively. A method for uniquely determining sigma and l(0) from the emission and absorption spectra is presented. Applications are made to absorption and low-temperature emission in thin films of regioregular poly(3-hexylthiophene), with excellent agreement between theory and experiment obtained for a spatial correlation length of only 3-4 molecules.
573 citations
TL;DR: The state-of-the-art in infrared colloidal quantum dots, with an emphasis on applications and devices, can be found in this paper, where a progress report summarizes the state of the art in infrared quantum dots and devices.
Abstract: Colloidal nanocrystals are quantum-size-effect tunable; offer an abundance of available surface area for electronic and chemical interactions; and are processible from organic or aqueous solution onto substrates rigid or flexible, smooth or rough, flat or curved, inorganic or organic (including biological), crystalline or amorphous, conducting, semiconducting, or insulating. With the benefit of over a decade's progress in visible-light-emitting colloidal-quantum-dot synthesis, physical chemistry, and devices, significant progress has recently been made in infrared-active colloidal quantum dots and devices. This progress report summarizes the state-of-the-art in infrared colloidal quantum dots, with an emphasis on applications and devices. The applications of interest surveyed include monolithic integration of fiber-optic and free-space-communications photonic components with electronic substrates such as silicon and glass; in-vivo biological tagging in infrared spectral bands in which living tissue is optically penetrable to a depth of 5–10 cm; solar and thermal photovoltaics for energy conversion; and infrared sensing and imaging based on non-visible, including thermal, signatures. The synthesis and properties of quantum dots are first reviewed: photoluminescence quantum efficiencies greater than 50 % are achievable in solution, and stable luminescent dots are available in organic and aqueous solvents. Electroluminescent devices based on solution processing have been reported with external quantum efficiencies approaching 1 %. Photoconductive devices have been realized with 3 % internal quantum efficiencies, and a photovoltaic effect was recently observed. Electro-optic modulation achieved by either field- or charge-induced modification of the rate of optical absorption has been demonstrated based both on interband and intersubband (intraband) transitions. Optical gain from these processible materials with a threshold of 1 mJ cm–2 and an optical net modal gain coefficient of 260 ± 20 cm–1 have been reported.
527 citations
TL;DR: The chemical process used to terminate the surfaces of the silicon quantum dots changes the internal electronic structure and thus plays an important role in the resultant emission wavelength and radiative lifetime, and ultimately determines the solubility.
Abstract: For silicon quantum dots to be used in biomedical applications it is essential that they have a substantial photoluminescence quantum yield in the visible region, have a fast radiative recombination rate, and are water soluble and hydrophilic to prevent aggregation and precipitation in a biological environment. The chemical process used to terminate the surfaces of the silicon quantum dots changes the internal electronic structure and thus plays an important role in the resultant emission wavelength and radiative lifetime, and ultimately determines the solubility. [18] Silicon quantum dots with an oxide surface passivation typically display a dipole-forbidden yellow-red emission with radiative lifetimes of 10 3 –10 6 s. [18, 26] This slow rate of recombination limits the use of oxide-passivated silicon quantum dots in biological imaging. However, silicon quantum dots with a hydrogen or carbon surface passivation have electric-dipole-allowed direct band gap transitions that lead to blue photoluminescence with fast recombination rates of 10 8 –10 9 s. [18, 20]
515 citations
TL;DR: In this paper, spherical SiO2 particles have been successfully coated with YVO4:Eu3+ phosphor layers through a Pechini sol−gel process, and the resulted YVO 4:Eeu3+@SiO2 core−shell phosphors were characterized by X-ray diffraction (XRD), Fourier-transform IR spectroscopy, scanning electron microscopy, Xray photo-electron spectra, transmission electron microscope, UV/vis absorption spectra.
Abstract: Spherical SiO2 particles have been successfully coated with YVO4:Eu3+ phosphor layers through a Pechini sol−gel process. The resulted YVO4:Eu3+@SiO2 core−shell phosphors were characterized by X-ray diffraction (XRD), Fourier-transform IR spectroscopy, scanning electron microscopy, X-ray photoelectron spectra, transmission electron microscopy, UV/vis absorption spectra, general and time-resolved photoluminescence spectra, as well as kinetic decays. The XRD results demonstrate that the YVO4:Eu3+ layers begin to crystallize on the SiO2 particles after annealing at 400 °C, and the crystallinity increases with raising the annealing temperature. The obtained core−shell phosphors have perfect spherical shape with narrow size distribution (average size ca. 500 nm), nonagglomeration, and smooth surface. The thickness of the YVO4:Eu3+ shells on SiO2 cores could be easily tailored by varying the number of deposition cycles (60 nm for two deposition cycles). The Eu3+ shows a strong photoluminescence (PL) (dominated b...
507 citations
TL;DR: In this paper, the introduction of defects into nanoscale ZnO, their comprehensive analysis using a combination of techniques (powder X-ray diffraction (PXRD), Xray absorption spectroscopy/extended X ray absorption fine structure (XAS/EXAFS), electron paramagnetic resonance (EPR), magic-angle spinning nuclear magnetic resonance (MAS-NMR), Fourier transform infrared (FTIR), UV-vis, and photoluminescence (PL) spectroscopies coupled with ab-initio calculations), and the investigation of
Abstract: Zinc oxide in the form of nanoscale materials can be regarded as one of the most important semiconductor oxides at present. However, the question of how chemical defects influence the properties of nanoscale zinc oxide materials has seldom been addressed. In this paper, we report on the introduction of defects into nanoscale ZnO, their comprehensive analysis using a combination of techniques (powder X-ray diffraction (PXRD), X-ray absorption spectroscopy/extended X-ray absorption fine structure (XAS/EXAFS), electron paramagnetic resonance (EPR), magic-angle spinning nuclear magnetic resonance (MAS-NMR), Fourier-transform infrared (FTIR), UV-vis, and photoluminescence (PL) spectroscopies coupled with ab-initio calculations), and the investigation of correlations between the different types of defects. It is seen that defect-rich zinc oxide can be obtained under kinetically controlled conditions of ZnO formation. This is realized by the thermolysis of molecular, organometallic precursors in which ZnO is pre-organized on a molecular scale. It is seen that these precursors form ZnO at low temperatures far from thermodynamic equilibrium. The resulting nanocrystalline ZnO is rich in defects. Depending on conditions, ZnO of high microstructural strain, high content of oxygen vacancies, and particular content of heteroatom impurities can be obtained. It is shown how the mentioned defects influence the electronic properties of the semiconductor nanoparticles.
TL;DR: In this paper, N−F−codoped TiO2 powders were further characterized by ultraviolet−visible (UV−Vis) absorption spectroscopy and photoluminescence (PL) spectra.
Abstract: N−F−codoped TiO2 (NFT) powders, prepared by spray pyrolysis (SP), were further characterized by ultraviolet−visible (UV−Vis) absorption spectroscopy and photoluminescence (PL) spectra. The UV−Vis spectra indicated that the NFT powders could absorb not only ultraviolet light like pure TiO2 powder but also part of the visible-light spectrum (λ < 550 nm). The PL spectra provided confirmation that four electronic energy states exist between the valence band and conduction band of N−F−codoped TiO2 that were attributed to F center, F+ center, an origin-unidentified energy state, and an impurity energy state formed by doped N atoms. Acetaldehyde decomposition was used as a probe reaction to evaluate the photocatalytic properties of these NFT powders. As a result, we found that the photocatalytic activity of the NFT powder prepared at the SP temperature of 1173 K was superior to that of commercial P25 under both UV and Vis irradiation. Moreover, trichloroethylene and toluene were selected as the other two target ...
TL;DR: The complex of trioctylphosphine with Se atoms on the surface of CdSe NCs is found responsible for the trap emission band that is red-shifted relative to the photoluminescence band edge.
Abstract: We report effects of various organic and inorganic ligands on optical properties of CdSe nanocrystals (NCs) by changes in their photoluminescence and absorbance spectra. Surface ligand loss occurring during dilution and purification of solutions of CdSe NCs leads to a decrease of photoluminescence intensity. The complex of trioctylphosphine with Se atoms on the surface of CdSe NCs is found responsible for the trap emission band that is red-shifted relative to the photoluminescence band edge.
TL;DR: In this article, size-dependent blue shifts of photoluminescence and absorption spectra revealed the quantum confinement effect and the band gap enlargement was in agreement with the theoretical calculation based on the effective mass model.
TL;DR: In this article, the ionization of the 5d electron to conduction band states is shown to be the genuine quenching mechanism for Eu2+ 5d−4f emission on Ba, Sr, or Ca sites in compounds.
Abstract: The thermal quenching of Eu2+ 5d–4f emission on Ba, Sr, or Ca sites in compounds is often attributed to a large displacement between the ground state and excited state in the configuration coordinate diagram. This work will demonstrate that the ionization of the 5d electron to conduction band states is the genuine quenching mechanism. A model is proposed to explain why in some types of compounds the quenching temperature decreases when going from the Ba variant via the Sr variant to the Ca variant and in other types of compounds the reverse behaviour occurs. The nature of the bottom of the conduction band plays an important role in this.
TL;DR: This work reports the use of targeted p- and n-type chemical perturbations to manipulate high-T(C) ferromagnetism in Mn( 2+):ZnO and Co(2+): ZnO in predictable and reproducible ways and demonstrates a clear correlation between nitrogen and high- T(C).
Abstract: We report the use of targeted $p$- and $n$-type chemical perturbations to manipulate high-${T}_{\mathrm{C}}$ ferromagnetism in ${\mathrm{Mn}}^{2+}\ensuremath{\mathbin:}\mathrm{ZnO}$ and ${\mathrm{Co}}^{2+}\ensuremath{\mathbin:}\mathrm{ZnO}$ in predictable and reproducible ways. We demonstrate a clear correlation between nitrogen and high-${T}_{\mathrm{C}}$ ferromagnetism for ${\mathrm{Mn}}^{2+}\ensuremath{\mathbin:}\mathrm{ZnO}$ and an inverse correlation for ${\mathrm{Co}}^{2+}\ensuremath{\mathbin:}\mathrm{ZnO}$, both as predicted by recent theoretical models. These chemical perturbations reveal rich possibilities for exerting external control over high-${T}_{\mathrm{C}}$ spin ordering in diluted magnetic semiconductors.
TL;DR: In this paper, an angle-resolved magneto-photoluminescence microscope was developed to investigate the anisotropic electron-spin interactions of single nitrogen-vacancy (N-V) centers at room temperature.
Abstract: Experiments on single nitrogen–vacancy (N–V) centres in diamond, which include electron spin resonance1, Rabi oscillations2, single-shot spin readout3 and two-qubit operations with a nearby13C nuclear spin4, show the potential of this spin system for solid-state quantum information processing. Moreover, N–V centre ensembles can have spin-coherence times exceeding 50 μs at room temperature5. We have developed an angle-resolved magneto-photoluminescence microscope apparatus to investigate the anisotropic electron-spin interactions of single N–V centres at room temperature. We observe negative peaks in the photoluminescence as a function of both magnetic-field magnitude and angle that are explained by coherent spin precession and anisotropic relaxation at spin-level anti-crossings. In addition, precise field alignment unmasks the resonant coupling to neighbouring ‘dark’ nitrogen spins, otherwise undetected by photoluminescence. These results demonstrate the capability of our spectroscopic technique for measuring small numbers of dark spins by means of a single bright spin under ambient conditions.
TL;DR: It is observed that heterostructures consisting of a spin-coated poly(p-phenylene vinylene) (PPV)-based derivative and evaporated C60 are ill-defined because of diffusion of C60 into the polymer, leading to an overestimation of the exciton diffusion length.
Abstract: Exciton diffusion and photoluminescence quenching in conjugated polymer/fullerene heterostructures are studied by time-resolved photoluminescence. It is observed that heterostructures consisting of...
TL;DR: Luminescence from individual gold nanorods is investigated and it is found that their emission characteristics closely resemble surface plasmon behavior.
Abstract: Light emission resulting from two-photon excited gold nanoparticles has been proposed to originate from the radiative decay of surface plasmon resonances. In this vein, we investigated luminescence from individual gold nanorods and found that their emission characteristics closely resemble surface plasmon behavior. In particular, we observed spectral similarities between the scattering spectra of individual nanorods and their photoluminescence emission. We also measured a blueshift of the photoluminescence peak wavelength with decreasing aspect ratio of the nanorods as well as an optically tunable shape-dependent spectrum of the photoluminescence. The emission yield of single nanorods strongly depends on the orientation of the incident polarization consistent with the properties of surface plasmons.
TL;DR: In this article, the expected excitation power dependencies for any upconversion emission band of an acceptor ion were investigated theoretically when the excitation takes place on a sensitizer ion and subsequent energy transfer up-conversion from the sensitizers to the acceptors is exclusively responsible for the excitations of the acceptor.
Abstract: The expected excitation power dependencies for any upconversion emission band of an acceptor ion is investigated theoretically when the excitation takes place on a sensitizer ion and subsequent energy transfer upconversion from the sensitizer to the acceptor ion is exclusively responsible for the excitation of the acceptor ion. Under these limitations it is shown that emission from a state that requires $k$ energy transfer upconversion steps will have a slope of $k$ in the low-power regime when the luminescence intensity is plotted in a double-logarithmic representation versus absorbed pump intensity. In the high-power regime, any emission band will show a slope of 1, irrespective of the number of energy transfer steps from the sensitizer to the acceptor ions that are involved. The theoretical results are verified experimentally by data on three different inorganic systems with different types of sensitizer and acceptor ions: rare earth (RE) ions as well as transition metal (TM) ions. The active ions in the systems that are studied experimentally are RE/RE, RE/TM, and TM/TM, where the first dopant indicates the sensitizer ion and the second dopant indicates the upconverting ion. These different classes of sensitizer and upconverter ions all agree with the theoretical predictions put forward by the model. Thus providing confidence in the applicability (within the boundary conditions put forward here) of the model described.
TL;DR: In this article, a shift in deep-level emission from green to yellow was observed with reduced Zn pressure during the growth of ZnO films, and it was suggested that the green emission is related to donor-deep acceptor (Zn vacancy VZn−) and the yellow-to-donor-deep-acceptor (oxygen vacancy, Oi−).
Abstract: The properties of ZnO films grown by molecular-beam epitaxy are reported. The primary focus was on understanding the origin of deep-level luminescence. A shift in deep-level emission from green to yellow is observed with reduced Zn pressure during the growth. Photoluminescence and Hall measurements were employed to study correlations between deep-level/near-band-edge emission and carrier density. With these results, we suggest that the green emission is related to donor-deep acceptor (Zn vacancy VZn−) and the yellow to donor-deep acceptor (oxygen vacancy, Oi−).
TL;DR: Temperature-dependent and time-resolved PL studies reveal rich spectral features and show that an exciton-exciton interaction is critical to lasing up to 75 K, while an excitation-phonon process dominates at higher temperatures.
Abstract: The mechanism of lasing in single cadmium sulfide (CdS) nanowire cavities was elucidated by temperature-dependent and time-resolved photoluminescence (PL) measurements. Temperature-dependent PL studies reveal rich spectral features and show that an exciton-exciton interaction is critical to lasing up to 75 K, while an exciton-phonon process dominates at higher temperatures. These measurements together with temperature and intensity dependent lifetime and threshold studies show that lasing is due to formation of excitons and, moreover, have implications for the design of efficient, low threshold nanowire lasers.
TL;DR: In this paper, the two-photon-induced photoluminescence properties of single gold nanorods were investigated by scanning near-field spectroscopy, and the process was found to be initiated by a sequential onephoton absorption for creating a pair of an electron and a hole in the sp and d bands.
Abstract: We investigated the two-photon-induced photoluminescence properties of single gold nanorods by scanning near-field spectroscopy. The process was found to be initiated by a sequential one-photon absorption for creating a pair of an electron and a hole in the sp and d bands. Photoluminescence is then radiated when the electron near the Fermi surface recombines with the hole near the X and L symmetry points. The polarization characteristics of emitted photons from the X and L regions were found to be different. These characteristics can be understood by the crystalline structure and the band structure of the gold nanorod. We found characteristic spatial oscillatory features along the long axis of the nanorods in photoluminescence excitation images. The images were well reproduced by density-of-states maps of the nanorods calculated with Green's dyadic method and were attributed to the spatial characteristics of the wave functions of the plasmon modes inside the nanorods.
TL;DR: In this article, it was shown that most of the observed anomalous Raman modes correspond to wurtzite-ZnO silent modes allowed by the breakdown of the translational crystal symmetry induced by defects and impurities.
Abstract: Anomalous Raman modes have been reported in several recent papers dealing with doped- and undoped-ZnO layers grown by different methods. Most of these anomalous Raman modes have been attributed to local vibrational modes of impurities or defects. However, we will show that most of the observed modes correspond to wurtzite-ZnO silent modes allowed by the breakdown of the translational crystal symmetry induced by defects and impurities.
TL;DR: In this paper, a 32-fold increase in spontaneous emission rate of InGaN/GaN quantum well at 440 nm was observed by employing surface plasmons (SPs) probed by time-resolved photoluminescence spectroscopy.
Abstract: We observed a 32-fold increase in the spontaneous emission rate of InGaN/GaN quantum well (QW) at 440 nm by employing surface plasmons (SPs) probed by time-resolved photoluminescence spectroscopy. We explore this remarkable enhancement of the emission rates and intensities resulting from the efficient energy transfer from electron-hole pair recombination in the QW to electron vibrations of SPs at the metal-coated surface of the semiconductor heterostructure. This QW-SP coupling is expected to lead to a new class of super bright and high-speed light-emitting diodes (LEDs) that offer realistic alternatives to conventional fluorescent tubes.
TL;DR: New strategies for the rational design of NLO molecular assemblies are inferred on the basis of the acquired insights into the nature of fundamental excited-state dynamics and the nonlinear optical (NLO) response involved.
Abstract: To investigate the effect of branching on linear and nonlinear optical properties, a specific series of chromophores, epitome of (multi)branched dipoles, has been thoroughly explored by a combined theoretical and experimental approach. Excited-state structure calculations based on quantum-chemical techniques (time-dependent density functional theory) as well as a Frenkel exciton model nicely complement experimental photoluminescence and one- and two-photon absorption findings and contribute to their interpretation. This allowed us to get a deep insight into the nature of fundamental excited-state dynamics and the nonlinear optical (NLO) response involved. Both experiment and theory reveal that a multidimensional intramolecular charge transfer takes place from the donating moiety to the periphery of the branched molecules upon excitation, while fluorescence stems from an excited state localized on one of the dipolar branches. Branching is also observed to lead to cooperative enhancement of two-photon absorption (TPA) while maintaining high fluorescence quantum yield, thanks to localization of the emitting state. The comparison between results obtained in the Frenkel exciton scheme and ab initio results suggests the coherent coupling between branches as one of the possible mechanisms for the observed enhancement. New strategies for the rational design of NLO molecular assemblies are thus inferred on the basis of the acquired insights.
TL;DR: The effect of preheating temperature on structural and optical properties of ZnO thin films is discussed in this paper, where the optical band gap energy is evaluated to be 3.24∼3.26 eV and photoluminescence shows the ultraviolet emission at near band edge and broad green-yellow radiation at 490∼620 nm.
TL;DR: In this paper, different shapes of ZnO microcrystals have been achieved controllably by a capping-molecule-assisted hydrothermal process using ammonia, citric acid, and polyvinyl alcohol (PVA) as the capping molecules.
Abstract: Different shapes of ZnO microcrystals have been achieved controllably by a capping-molecule-assisted hydrothermal process. The flowerlike, disklike, and dumbbell-like ZnO microcrystals of hexagonal phase have been obtained respectively using ammonia, citric acid (CA), and poly(vinyl alcohol) (PVA) as the capping molecules. Only a very strong UV emission at ∼380 nm is observed in the photoluminescence (PL) spectra of the three kinds of ZnO microcrystals, indicative of their high crystal quality. The formation mechanisms for the hydrothermally synthesized microcrystals in different morphologies have been phenomenologically presented.
TL;DR: The high local electric fields at the junction between the suspended and supported parts of a single carbon nanotube molecule to produce unusually bright infrared emission under unipolar operation allows us to observe emission from higher excited states not seen by photoexcitation.
Abstract: We used the high local electric fields at the junction between the suspended and supported parts of a single carbon nanotube molecule to produce unusually bright infrared emission under unipolar operation. Carriers were accelerated by band-bending at the suspension interface, and they created excitons that radiatively recombined. This excitation mechanism is ∼1000 times more efficient than recombination of independently injected electrons and holes, and it results from weak electron-phonon scattering and strong electron-hole binding caused by one-dimensional confinement. The ensuing high excitation density allows us to observe emission from higher excited states not seen by photoexcitation. The excitation mechanism of these states was analyzed.