Browsing by Author "Ohta, Nobuhiro"

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Anisotropic Electric Field Effect on the Photoluminescence of CH3NH3PbI3 Perovskite Sandwiched between Conducting and Insulating Films
(2017-10-19)
Awasthi, Kamlesh
;
Wang, Chi-Yung
;
Fathi, Amir
;
Narra, Sudhakar
;
Diau, Eric Wei-Guang
Photoluminescence (PL) of a nanocrystalline film of methylammonium lead iodide perovskite (MAPbI(3)) 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 (F-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 (F-int) exists even without application of F-ext. The anisotropic behavior of the effect of F-ext on PL is interpreted in terms of a synergy effect of F-int and F-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.
Application of Fluorescence Lifetime Imaging (FLIM) to Measure Intracellular Environments in a Single Cell
(2017-01-01)
Nakabayashi, Takakazu
;
Awasthi, Kamlesh
;
Ohta, Nobuhiro
;
應用化學系
;
應用化學系分子科學碩博班
Fluorescence lifetime imaging (FLIM) has now been used in many bioscience fields, which comes from the quantification of fluorescence lifetime. The procedure for obtaining lifetime images is very similar to that used in fluorescence microscopy. However, obtaining reliable lifetime images requires an understanding of the theory of fluorescence lifetime, principle of FLIM systems, and evaluation procedure of intracellular environments. In this chapter, the materials, methods, and notes on FLIM measurements have been described, in conjunction with a brief explanation of the background of FLIM.
Characterization of endogenous fluorescence in nonsmall lung cancerous cells: A comparison with nonmalignant lung normal cells
(1970-01-01)
Awasthi, Kamlesh
;
Chang, Feng-Lin
;
Hsieh, Pei-Ying
;
Hsu, Hsin-Yun
;
Ohta, Nobuhiro
Monitoring fluorescence properties of endogenous fluorophores such as nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) in normal and cancerous cells provide substantial information noninvasively on biochemical and biophysical aspects of metabolic dysfunction of cancerous cells. Time-resolved spectral profiles and fluorescence lifetime images of NADH and FAD were obtained in human lung nonsmall carcinomas (H661 and A549) and normal lung cells (MRC-5). Both fluorophores show the fast and slowly decaying emission components upon pulsed excitation, and fluorescence spectra of NADH and FAD show blue- and red-shifts, respectively, during their decay. All identified lifetime components of NADH and FAD were found to be shorter in cancerous cells than in normal cells, no matter how they were measured under different extra-cellular conditions (cells suspended in cuvette and cells attached on glass substrate), indicating that the changes in metabolism likely altered the subcellular milieu and potentially also affected the interaction of NADH and FAD with enzymes to which these cofactors were bound. The intensity ratio of NADH and FAD of cancerous cells was also shown to be larger than that of normal cells.
Differentiation between normal and cancer cells with autofluorescence lifetime microscopy and nanosecond pulsed electric field effects
(2019-01-01)
Ohta, Nobuhiro
;
Awasthi, Kamlesh
;
Hsu, Hsin-Yun
;
Hsieh, Pei-Ying
;
交大名義發表
Fluorescence lifetime images of the endogenous nicotinamide adenine dinucleotide (NADH), which is well known as autofluorescence chromophores, were obtained for rat normal fibroblast cells (WFB) and H-ras oncogene-transfected WFB cancer cells (W31) and for human normal lung fibroblast (MRC-5) and human lung large carcinoma (HCI-H661). In both cases, the average lifetime of the NADH autofluorescence was shorter in cancer cells than in normal cells, indicating that the difference in metabolism between healthy and cancer cells might alter the level of coenzymes such as NADH. It is also shown that application of nanosecond pulsed electric field (nsPEF) induces apoptosis in these cell, resulting in the morphological changes and lengthening the autofluorescence lifetime of NADH. Furthermore, we found that nsPEF more efficiently affected cancer cells than normal cells in cell viability, suggesting the possibility of drug-free cancer therapy by nsPEF.
Effect of an External Electric Field on the Fluorescence of a pi-Conjugated Polymer Film of P3HT Sandwiched between FTO and PMMA Published as part of The Journal of Physical Chemistry virtual special issue "Hai-Lung Dai Festschrift"
(2019-05-23)
Awasthi, Kamlesh
;
Chiou, Chin-Shiun
;
Iimori, Toshifumi
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
We report the photoexcitation dynamics of poly(hexylthiophene-2,5-diyl) (P3HT) as a solid film sandwiched between conducting films of a fluorine-doped tin-oxide-coated glass substrate (FTO) and an insulator film of poly(methyl methacrylate) (PMMA), from measurements of the effects of an external electric field on the absorption and photoluminescence (PL) spectra of P3HT. An Ag film was coated on the PMMA film; the FTO and Ag films served as electrodes to apply an external electric field to P3HT. According to the effects of a quadratic electric field, the lifetime of the emitting state of P3HT increased in the presence of that external field, resulting in an enhanced fluorescence quantum yield. An enhancement and quenching of fluorescence were observed also on applying an external electric field having the opposite polarity to the P3HT film between FTO and Ag films, which results from a field-induced change of both the population of the emitting state following photoexcitation and the rate of nonradiative decay at the emitting state. The dependence of the field-induced change of photoluminescence on the polarity of the applied external electric field between the FTO and Ag films was interpreted as an influence of an internal field located in the P3HT film sandwiched between FTO and PMMA films. These results of the effects of an electric field on the PL of a P3HT film sandwiched between FTO and PMMA films are compared with the effects on PL of P3HT sandwiched between a semiconductor film of TiO2 or a Sb2S3 film and a PMMA film.
Effects of Nanosecond Pulsed Electric Field on Intracellular NADH Autofluorescence: A Comparison between Normal and Cancer Cells
(2017-06-01)
Awasthi, Kamlesh
;
Nakabayashi, Takakazu
;
Li, Liming
;
Ohta, Nobuhiro
;
應用化學系
Intracellular fluorescence lifetime and intensity images of the endogenous fluorophore of nicotinamide adenine dinucleotide (NADH) have been observed before and after application of nanosecond pulsed electric field (nsPEF) in normal and cancer cells, that is, in Wistar-KingAptekman rat fetus fibroblast (WFB) cells and W31 cells, which are the malignant transformed cells from WFB. The application of nsPEF induces a change both in intensity and lifetime of NADH, indicating that the intracellular function is affected by application of nsPEF in both normal and cancer cells. The application of nsPEF induces an increase in the fluorescence lifetime of NADH and a morphological change, which is attributed to the induction of apoptosis by nsPEF. The field effect on the intensity and lifetime clearly depends on the pulse width, and magnitude of the field-induced increase in the fluorescence lifetime of NADH has a tendency to increase with a decreasing pulse width. It is also found that apoptosis can be induced only in cancer cells using a suitable nsPEF, showing a possibility that ultrashort pulsed electric field is applicable for drugfree cancer therapy.
Effects of Nanosecond Pulsed Electric Fields on the Intracellular Function of HeLa Cells As Revealed by NADH Autofluorescence Microscopy
(2016-09-01)
Awasthi, Kamlesh
;
Nakabayashi, Takakazu
;
Ohta, Nobuhiro
;
應用化學系
;
應用化學系分子科學碩博班
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.
Electric Field Effects on Photoluminescence of Polyfluorene Thin Films: Dependence on Excitation Wavelength, Field Strength, and Temperature
(2009-07-09)
Mehata, Mohan Singh
;
Hsu, Chain-Shu
;
Lee, Yuan-Pern
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Ohta, Nobuhiro
;
應用化學系
Electroabsorption (E-A) and electrophotoluminescence (E-F) spectra of thin films of blue-light-emitting poly(9,9-dioctylfluorene) (PFO) have been measured at temperatures ranging from 25 to 295 K to examine both the optical property and excitation dynamics of these films in the presence of external electric fields of 0-1.2 MV cm(-1). Electric field effects on excitation dynamics depend on not only applied field strength but also excitation wavelength and temperature. For photoexcitation at 344 or 402 nm, E-F spectra observed with low applied fields show only the Stark shift, whereas fluorescence quenching is induced only by strong electric fields. For photoexcitation at a shorter wavelength of 298 nm, on the other hand, field-induced quenching of fluorescence is observed even with a weak electric field at any temperature. The presence of a nonradiative process from highly excited states that is effectively affected by electric fields is suggested.
Electric-Field-Induced Enhancement/Quenching of Photoluminescence of pi-Conjugated Polymer S3-PPV: Excitation Energy Dependence
(2010-05-20)
Mehata, Mohan Singh
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Hsu, Chain-Shu
;
Lee, Yuan-Pern
;
Ohta, Nobuhiro
;
應用化學系
The effects of electric field on absorption and photoluminescence (PL) of films of sulfide-substituted PPV derivative S3-PPV, poly[2-(phenyl)-3-(4'-(3,7-dimethyloctyloxy)phenyl)-1,4-phenylenevinylene-co-2-(11'-decyl sulfanylundecanyloxy)-5-methoxy-1,4-phenylene vinylene], were investigated. Electroabsorption (E-A) and electrophotoluminescence (E-PL) responses of S3-PPV show the Stark shifts, indicating a significant alternation in the molecular polarizability (Delta(alpha) over bar) associated with the optical transitions. Field-induced enhancement or quenching is also observed for PL of S3-PPV, depending on the photoexcitation energy, whereas the shape of the PL spectra is independent of the excitation wavelength. The field effects on the decay profiles of PL indicate that the quenching results from a diminished population of the emitting states on excitation at 300 nm, whereas the PL is enhanced on excitation at 471 nm because the emitting state has an increased lifetime. The efficiency of field-assisted generation of electron-hole pairs produced through excitons monotonically increases with increasing excitation energy, and the nonradiative decay rate in the emitting state is diminished by electric fields in S3-PPV. The photoirradiation of S3-PPV in ambient air resulted in rapid degradation of the polymer film.
Electroabsorption and Electrophotoluminescence of Poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene)
(2012-07-19)
Mehata, Mohan Singh
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Hsu, Chain-Shu
;
Lee, Yuan-Pern
;
Ohta, Nobuhiro
;
應用化學系
Electric-field-induced changes of the absorption, photoluminescence, and photoluminescence excitation spectra and the photoluminescence decay profile have been measured for poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene). The magnitudes of the change of electric dipole moment and molecular polarizability of this compound have been determined for excitation into the first excited state having a strong absorption, and a large charge-separated character, which is relevant for photovoltaic devices, is confirmed. The photoluminescence of this compound is quenched by application of an electric field, and the quenching efficiency increases with decreasing excitation wavelength. Field-assisted dissociation is considered to occur from the photoexcited Franck-Condon state, resulting in the field-induced decrease in the population of the emitting state, which induce the quenching of photoluminescence.
"Electroabsorption and Electrophotoluminescence of Poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene)"
(1970-01-01)
Mehata, Mohan Singh
;
Hsu, Chain-Shu
;
Lee, Yuan-Pern
;
Ohta, Nobuhiro
;
應用化學系
"Electric-field-induced changes of the absorption, photoluminescence, and photoluminescence excitation spectra and the photoluminescence decay profile have been measured for poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene). The magnitudes of the change of electric dipole moment and molecular polarizability of this compound have been determined for excitation into the first excited state having a strong absorption, and a large charge-separated character, which is relevant for photovoltaic devices, is confirmed. The photoluminescence of this compound is quenched by application of an electric field, and the quenching efficiency increases with decreasing excitation wavelength. Field-assisted dissociation is considered to occur from the photoexcited Franck-Condon state, resulting in the field-induced decrease in the population of the emitting state, which induce the quenching of photoluminescence."
Electroabsorption spectra of push-pull porphyrins in solution and in solid films
(2015-01-01)
Awasthi, Kamlesh
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Hsu, Hung-Yu
;
Chiang, Hung-Chu
;
Mai, Chi-Lun
;
Yeh, Chen-Yu
Polarized electroabsorption (E-A) spectra of highly efficient porphyrin sensitizers (YD2 and YD2-oC8) have been measured in benzene solution. Polarized E-A spectra of these push-pull porphyrins embedded in poly(methyl methacrylate) films or sensitized on TiO2 films are also observed. Based on the analysis of the E-A spectra, the magnitude of the electric dipole moment both in the ground state and in the lowest excited state have been evaluated in solution and in solid films. The electric dipole moment in the excited state of these compounds is very large on TiO2 films, suggesting the interfacial charge transfer on TiO2 surface following photoexcitation of porphyrin dyes. The electric dipole moment in the excited state evaluated from the E-A spectra is very different from the one evaluated from the electrophotoluminescence spectra on TiO2, suggesting that the strong local field of TiO2 films is applied to the fluorescing dyes attached to TiO2 films.
Electroabsorption Studies of Multicolored Lead Halide Perovskite Nanocrystalline Solid Films
(2018-06-01)
Awasthi, Kamlesh
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Du, Kai-Bo
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Wang, Chi-Yung
;
Tsai, Chao-Lin
;
Hamada, Morihiko
Electroabsorption (E-A) spectra of lead halide perovskites, that is, CH3NH3PbI3-xBrx (x = 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 S. 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 chi((3)) has also been calculated, based on the E-A spectra.
Enhanced charge transfer character of photoexcited states of dye sensitizer on the N719/TiO2 interface as revealed by electroabsorption spectra
(2014-08-15)
Awasthi, Kamlesh
;
Hsu, Hung-Yu
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
;
應用化學系
Electroabsorption (E-A) spectra of (Bu4N)(2)[Ru(dcbpyH)(2)(NCS)(2)] (N719) sensitized on TiO2 nanoparticles are very different from those of the N719 solid film. In contrast with N719 fixed in a solid form, N719 adsorbed on TiO2 shows a significant electric field effect on absorbance, indicating the strong interaction in the surface between N719 and TiO2. The magnitude of the change in electric dipole moment following absorption of N719 sensitized on TiO2 is much greater than that of the sold film, suggesting that the interfacial charge transfer occurs following photoexcitation of N719 adsorbed on TiO2 surface. (C) 2014 Elsevier B.V. All rights reserved.
Enhanced Dissociation of Hot Excitons with an Applied Electric Field under Low-Power Photoexcitation in Two-Dimensional Perovskite Quantum Wells
(2019-08-15)
Kattoor, Vidya
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Awasthi, Kamlesh
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Jokar, Efat
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Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
The dependence of photoluminescence (PL) on excitation power and the effect of an external electric field have been studied for a two-dimensional (2D) perovskite (C4H9NH3)(2)PbI4 thin-film sample. The efficiency of dissociation of hot excitons to produce free carriers was enhanced with a small excitation power because the relaxation of hot excitons to cold emissive excitons was slow, indicating that the thermal energies of hot carriers can be utilized in solar cells under weak photoirradiation. The dissociation was notably enhanced with an applied electric field, resulting in efficient field-induced quenching of the PL. The present results shed light on an application of 2D perovskite materials to photovoltaic (PV) devices with dim radiation, e.g., for indoor PV applications; the concept of electric field-assisted solar cells might be applicable to next-generation solar cells.
External Electric Field Effects on Excited-State Intramolecular Proton Transfer in 4 '-N,N-Dimethylamino-3-hydroxyflavone in Poly(methyl methacrylate) Films
(2015-09-17)
Furukawa, Kazuki
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Hino, Kazuyuki
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Yamamoto, Norifumi
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Awasthi, Kamlesh
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Nakabayashi, Takakazu
The external electric field effects on the steady-state electronic spectra and excited-state dynamics were investigated for 4\'-N,N-(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.
Field-Induced Fluorescence Quenching and Enhancement of Porphyrin Sensitizers on TiO2 Films and in PMMA Films
(2013-11-28)
Hsu, Hung-Yu
;
Chiang, Hung-Chu
;
Hu, Jyun-Yu
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Awasthi, Kamlesh
;
Mai, Chi-Lun
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 ortho-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.
Fluorescence enhancement induced by quadratic electric-field effects on singlet exciton dynamics in poly(3-hexylthiophene) dispersed in poly(methyl methacrylate)
(2019-03-14)
Iimori, Toshifumi
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Awasthi, Kamlesh
;
Chiou, Chin-Shiun
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
The dynamics of the exciton generated by photoexcitation of a regioregular poly(3-hexylthiophene) (P3HT) polymer dispersed in a poly(methyl methacrylate) (PMMA) matrix was examined using electro-photoluminescence (E-PL) spectroscopy, where electric field effects on the photoluminescence (PL) spectra were measured. The quadratic electric-field effect was investigated using the modulation technique, with field-induced changes in the PL intensity monitored at the second harmonic of the modulation frequency of the applied electric field. Absorption and PL spectra indicated the formation of both ordered crystalline aggregates and amorphous regions of P3HT polymer chains. Although previous studies of electric field effects on -conjugated polymers have generally shown that the PL intensity is decreased by electric fields, we report that the PL intensity of P3HT and PL lifetime increased with the quadratic electric-field effect. The magnitude of the change in PL intensity was quantitatively explained in terms of the field-induced decrease in the nonradiative decay rate constants of the exciton. We proposed that a delayed PL, originating from charge carrier recombination, was enhanced in the presence of electric fields. The rate constant of the downhill relaxation process of the exciton, which originated from the relaxation in distributed energy levels due to an inherent energetic disorder in P3HT aggregates, was implied to decrease in the presence of electric fields. The radiative decay rate constant and PL quantum yield of P3HT dissolved in solution, which were evaluated from the molar extinction coefficient and the PL lifetime, were compared with those of P3HT dispersed in a PMMA matrix.
Influence of External Electric Fields on Photoluminescence and Charge Carrier Dynamics of pi-Conjugated Polymer P3HT in Multilayer Films with Heterojunctions to TiO2 and Sb2S3
(2018-11-01)
Iimori, Toshifumi
;
Awasthi, Kamlesh
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Chiou, Chin-Shiun
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
pi-conjugated polymers are actively studied as a promising component for developing inexpensive dye sensitized solar cells (DSSCs). DSSCs utilizing pi-conjugated polymers as p-type organic semiconductor and metal chalcogenides as sensitizers have attracted interest due to high absorbance and stability in comparison to DSSCs using conventional molecular dyes. Here, we report the study of the excited state and charge carrier dynamics in poly(hexylthiophene) (P3HT) with junction structures of FTO/TiO2/P3HT/PMMA/Ag and FTO/TiO2/Sb2S3/P3HT/PMMA/Ag, where fluorine-doped tin oxide (FTO) and silver (Ag) are used as electrodes and poly(methyl methacrylate) (PMMA) is used as an insulating film. We measured electroabsorption spectra and the effects of external electric fields on photoluminescence (PL) of P3HT in these two different systems. Two PL bands emitted from different emitting states are observed in both of the systems. As the quadratic electric field effect, fluorescence quantum yield and lifetime become larger and longer in both of the PL bands, respectively, in the presence of external electric fields, indicating that nonradiative decay processes are depressed by applied electric fields in both of the systems. We also measured the linear electric field effects on PL of P3HT, and PL enhancement and quenching of PL intensity as well as lengthening and shortening of PL lifetime were observed, depending on the direction of the applied electric fields. It is shown that the mechanism of the linear field effect is different between the two emitting states in both of the systems; i.e., the population of the emitting state is affected by application of electric field for one PL band but not for the other PL band. The anisotropy of the field-induced change in PL with respect to the field direction of the applied electric fields can be explained in terms of the synergy effect between internal electric field which exists in the P3HT film and externally applied electric field on the charge separation of hot excitons generated following photoexcitation to free electron and hole (polaron) and on nonradiative decay rate of the PL emitting states.
Inhomogeneous Photoluminescence Characteristic in Carbon Nanodots and Electrophotoluminescence Measurements
(2018-03-22)
Hamada, Morihiko
;
Okoshi, Kenji
;
Rana, Shailesh
;
Awasthi, Kamlesh
;
Iimori, Toshifumi
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 similar to 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.