Browsing by Author "Hamada, Morihiko"

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Electroabsorption Studies of Multicolored Lead Halide Perovskite Nanocrystalline Solid Films
(2018-06-01)
Awasthi, Kamlesh
;
Du, Kai-Bo
;
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.
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.
Time-Gated Imaging on Live Cancer Cells Using Silicon Quantum Dot Nanoparticles with Long-Lived Fluorescence
(2017-06-01)
Tu, Chang-Ching
;
Awasthi, Kamlesh
;
Chen, Kuang-Po
;
Lin, Chih-Hsiang
;
Hamada, Morihiko
In this work we demonstrate time-gated confocal fluorescence imaging on live cancer cells immunostained by antibody-conjugated silicon quantum dot nano particles (SiQD-NPs) and organic dyes, for simultaneous detection of two biological targets and removal of background autofluorescence. With almost all radiative recombinations occurring through oxide-related defect states located on the SiQD surface, the SiQD-NPs have very long photoluminescence lifetimes of about 25 its, in contrast to the nanosecond range lifetimes of other commonly used biological fluorophores. This drastic lifetime difference enables a time-gated imaging method here, in which the time-resolved photon distribution of each pixel of a fluorescence image is measured by using a time-correlated single-photon counting technique. Then, by integrating the photon histogram of each pixel over respective time windows, the long-lived component of the fluorescence image comprising only the fluorescence emitted from the SiQD-NPs is separated from all other short-lived signals resulting from the organic dyes and the cell endogenous luminescence. For instance, the membrane and nucleus of a single cancer cell or two types of cancer cells, immunostained with the SiQD-NPs and the organic dyes, respectively, can be clearly distinguished from each other by time-gating, which otherwise cannot be accomplished by conventional multiplexing due to spectral overlap in the wavelength domain.