Electroabsorption Spectra of Quantum Dots of PbS and Analysis by the Integral Method

Electroabsorption (E-A) spectra of semiconductor quantum dots (QDs) of PbS have been analyzed by the integral method, which is powerful not only to determine the change in electric dipole moment and/or polarizability following absorption precisely but also to confirm the weak absorption bands buried under other strong absorption bands. In the results, two weak absorption bands which induce blue-shift and red-shift, respectively, with application of electric fields have been newly confirmed in PbS QDs to be located just above the intense first exciton band. The energy separation between these two bands becomes smaller, as the applied field strength becomes stronger, suggesting that the interaction between the states excited by these transitions becomes weaker by application of electric fields. The QD size dependence has been reported both for the transition energy and for the magnitude of the change in electric dipole moment and molecular polarizability following the absorption of each band. The assignment of the absorption bands has also been discussed

Integral Method Analysis of Electroabsorption Spectra and Its Application to Quantum Dots of PbSe

The integral method is proposed to analyze the electroabsorption (E-A) spectra, since the change in the electric dipole moment and/or polarizability following absorption can be determined precisely and the bands buried under strong absorption bands can be confirmed. This method, where not only the observed E-A spectra but also each of their first and second integral spectra are fitted using the absorption and their derivative and integral spectra, has been successfully applied to the E-A spectra of semiconductor quantum dots of PbSe. In the results, one absorption band, which is not identified in the absorption spectrum because of the extremely weak intensity and also showing a remarkable blue shift in the presence of an electric field because of the large difference in polarizability between the ground state and the excited state, has been confirmed to be located between the first and second strong exciton bands. The size dependence of PbSe QDs of the peak position of the newly confirmed band as well as the magnitude of the change in electric dipole moment and polarizability following the absorption of each absorption band is also reported, based on the analysis by the integral method

Effects of Nanosecond Pulsed Electric Fields on the Intracellular Function of HeLa Cells As Revealed by NADH Autofluorescence Microscopy

The fluorescence lifetime of the endogenous fluorophore of reduced nicotinamide adenine dinucleotide (NADH) in HeLa cells is affected by the application of nanosecond pulsed electric fields (nsPEFs). In this study, we found that after nsPEF application, the fluorescence lifetime became longer and then decreased in a stepwise manner upon further application, irrespective of the pulse width in the range of 10–50 ns. This application time dependence of the NADH fluorescence lifetime is very similar to the time-lapse dependence of the NADH fluorescence lifetime following the addition of an apoptosis inducer, staurosporine. These results, as well as the membrane swelling and blebbing after the application of nsPEFs, indicate that apoptosis is also induced by the application of nsPEFs in HeLa cells. In contrast to the lifetime, the fluorescence intensity remarkably depended on the pulse width of the applied nsPEF. When the pulse width was as large as 50 ns, the intensity monotonically increased and was distributed over the entire cell as the application duration became longer. As the pulse width of the applied electric field became smaller, the magnitude of the field-induced increase in NADH fluorescence intensity decreased; the intensity was reduced by the electric field when the pulse width was as small as 10 ns. These results suggest that the mechanism of electric-field-induced apoptosis depends on the pulse width of the applied nsPEF

Anisotropic Electric Field Effect on the Photoluminescence of CH₃NH₃PbI₃ Perovskite Sandwiched between Conducting and Insulating Films

Photoluminescence (PL) of a nanocrystalline film of methylammonium lead iodide perovskite (MAPbI₃) sandwiched between an electrode of a fluorine-doped tin oxide (FTO) layer and an insulating film of poly­(methyl methacrylate) is found to increase and decrease significantly with the application of an external electric field (<i><b>F</b></i>_ext), depending on the direction of the applied field, based on the measurements of electrophotoluminescence (E-PL) spectra, i.e., field-induced change in PL spectra. The field-induced change in PL intensity is confirmed to originate from the field-induced change in the number of free carriers which induce radiative recombination, based on temporally resolved E-PL measurements. We propose that an internal field (<i><b>F</b></i>_int) exists even without application of <i><b>F</b></i>_ext. The anisotropic behavior of the effect of <i><b>F</b></i>_ext on PL is interpreted in terms of a synergy effect of <i><b>F</b></i>_int and <i><b>F</b></i>_ext; both fields are additive with the applied field direction from Ag to FTO electrode (positive direction) or subtractive with the opposite applied field direction (negative direction), where FTO is the positive electrode, resulting in an increased or decreased total electric field as well as quenching or enhancement of PL, respectively. The PL lifetime in the nanosecond region increased and decreased with an application of an electric field in the positive and negative directions, respectively, which is attributed to a field-induced change in the concentration of free carriers

External Electric Field Effects on Excited-State Intramolecular Proton Transfer in 4′‑<i>N</i>,<i>N</i>‑Dimethylamino-3-hydroxyflavone in Poly(methyl methacrylate) Films

The external electric field effects on the steady-state electronic spectra and excited-state dynamics were investigated for 4′-<i>N</i>,<i>N</i>-(dimethylamino)-3-hydroxyflavone (DMHF) in a poly­(methyl methacrylate) (PMMA) film. In the steady-state spectrum, dual emission was observed from the excited states of the normal (N*) and tautomer (T*) forms. Application of an external electric field of 1.0 MV·cm^–1 enhanced the N* emission and reduced the T* emission, indicating that the external electric field suppressed the excited-state intramolecular proton transfer (ESIPT). The fluorescence decay profiles were measured for the N* and T* forms. The change in the emission intensity ratio N*/T* induced by the external electric field is dominated by ESIPT from the Franck–Condon excited state of the N* form and vibrational cooling in potential wells of the N* and T* forms occurring within tens of picoseconds. Three manifolds of fluorescent states were identified for both the N* and T* forms. The excited-state dynamics of DMHF in PMMA films has been found to be very different from that in solution due to intermolecular interactions in a rigid environment

Inhomogeneous Photoluminescence Characteristic in Carbon Nanodots and Electrophotoluminescence Measurements

Photoluminescence (PL) spectra, time-resolved PL spectra, and PL decay profiles have been observed for carbon nanodots (CDs) with different excitation wavelengths in an embedded solid film and in solution. PL excitation spectra have been also observed with different monitoring wavelengths. Then, it is found in both solid film and solution that not only the location of the PL spectra but also the peak of the excitation spectra show a significant red shift, as the excitation and monitoring wavelengths become longer, respectively, indicating that the emitting states of the excitation-dependent PL are the real state to which direct absorption occurs from the ground state, not the transient trapped states produced by photoexcitation. It is shown that not only the excitation-dependent PL but also the excitation-independent PL with a peak at ∼375 nm exist. The lifetimes of both PL emissions are very sensitive to the surroundings. Multiple emitting states that give excitation-dependent PL are ascribed to the inhomogeneous properties in prepared carbon nanodots, which is supported by the fluorescence lifetime image measurements. Electrophotoluminescence spectrum, that is, the electric-field-induced change in PL spectrum, has also been observed for the excitation-independent PL of CDs embedded in a poly­(methyl methacrylate) film, and the magnitude of the change in electric dipole moment and molecular polarizability following emission has been determined

Electroabsorption Studies of Multicolored Lead Halide Perovskite Nanocrystalline Solid Films

Electroabsorption (E-A) spectra of lead halide perovskites, that is, CH₃NH₃PbI_3–<i>x</i>Br_<i>x</i> (<i>x</i> = 0–3), which show large spectral shift depending on the ratio between iodine and bromide, have been measured. By analyzing E-A spectra with the integral method, spectral shape of the absorption spectra for the first exciton band and binding energy of exciton have been determined. Magnitudes of the change in electric dipole moment and molecular polarizability following excitation into the exciton state have been also evaluated in these perovskites. The binding energy of electron and hole in exciton of these materials as well as the magnitude of change in electric dipole moment following exciton absorption is roughly the same, suggesting that the difference of the photoenergy conversion in photovoltaic cells using these materials comes from the difference in light harvesting effect and difference in carrier mobility, not from the difference in carrier generation efficiency. The frequency-dependent third-order nonlinear susceptibility χ⁽³⁾ has also been calculated, based on the E-A spectra

Stark Spectroscopy of Absorption and Emission of Indoline Sensitizers: A Correlation with the Performance of Photovoltaic Cells

Electric field effects on photoexcitation dynamics and electronic properties of highly efficient indoline sensitizers, DN488, D205, and DN182, embedded in PMMA films have been examined by using electroabsorption (E-A) and electrophotoluminescence (E-PL) spectroscopic techniques and time-resolved photoluminescence (PL) decay measurements in the presence of electric fields. Photovoltaic performances have been also measured for devices constructed using these sensitizers. Then, field-induced quenching of PL and field-induced change in PL decay profile were observed, and it was found that these field effects, which depend on the sensitizers investigated herein, are well correlated with the trend of power conversion efficiencies of the corresponding photovoltaic cells. Electric dipole moment and molecular polarizability of these sensitizers both in the ground state (S₀) and in the excited state have been calculated at the level of B3LYP/6-31G­(d), and the differences of these physical parameters between S₀ and the excited state thus obtained have been compared with the ones determined from the E-A and E-PL spectra. The present study of Stark spectroscopy of indoline dyes provides new insights for the exciton dissociation property and carrier mobility of organic dyes, which are important factors to understand the operation mechanism in dye-sensitized solar cells

Field-Induced Fluorescence Quenching and Enhancement of Porphyrin Sensitizers on TiO₂ Films and in PMMA Films

Three highly efficient porphyrin sensitizersYD2, YD2-oC8, and YD30, either sensitized on TiO₂ films or embedded in PMMA filmswere investigated using electrophotoluminescence (E-PL) spectra. Under both thin-film conditions, on application of an external electric field we observed the quenching of fluorescence of push–pull porphyrins (YD2 and YD2-oC8) and a slightly enhanced fluorescence of the reference porphyrin without an electron donor group (YD30). A nonfluorescent state with charge separation (CS) is proposed to be involved in both YD2 and YD2-oC8 systems so that the electron injection becomes accelerated in the presence of a strong electric field. In contrast, the retardation of the nonradiative process not involving a CS state was the reason for the field-induced enhancement of fluorescence of YD30. The extent of fluorescence quenching of YD2-oC8 was greater than that of YD2 on TiO₂ films, indicating that the <i>ortho</i>-substituted long alkoxyl chains play a key role to accelerate the consecutive electron injection involving the CS state. Our E-PL results indicate that a field-induced variation of fluorescent intensity is related to the efficiency of conversion of solar energy and that further improvement of the performance of devices containing push–pull porphyrin dyes is achievable under an applied electric field