Browsing by Author "Rana, Shailesh"

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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.
Switching between spectral broadening and narrowing of the exciton absorption band of a CH3NH3PbI3 film on altering the polarity of an applied electric field
(2020-06-22)
Awasthi, Kamlesh
;
Kala, Kannankutty
;
Rana, Shailesh
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
Electric field-induced broadening and narrowing of an exciton absorption band have been observed to depend on the polarity of the applied electric field for methylammonium lead triiodide perovskite (MAPbI(3)) as a thin film sandwiched between a conducting film of fluorine-doped tin oxide (FTO) and an insulating film of poly(methyl methacrylate) (PMMA). The width of the absorption band increased or decreased when the direction of the applied field was toward FTO or PMMA, respectively, indicating that the spectral broadening and narrowing become switchable on altering the direction of the applied electric field. When titanium oxide (TiO2) in a compact layer was introduced between the MAPbI(3) and FTO layers, the linear electric field effect that depended on the polarity was either not observed or decreased significantly although the quadratic field effects on the exciton absorption band were similarly observed with or without the TiO2 layer.
Temperature-Dependent Electroabsorption and Electrophotoluminescence and Exciton Binding Energy in MAPbBr(3) Perovskite Quantum Dots
(2019-08-15)
Rana, Shailesh
;
Awasthi, Kamlesh
;
Bhosale, Sumit S.
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
Organic inorganic lead halide perovskite nanocrystals have attracted much attention as promising materials for the development of solid-state light-emitting devices, but the existence of free or bound excitons or the formation of trap states remains under debate. We recorded temperature-dependent electroabsorption (E-A) and electrophotoluminescence (E-PL) spectra, that is, electric field-induced change in absorption and photoluminescence spectra, for methylammonium lead tribromide (MAPbBr(3)) colloidal perovskite nanocrystals, that is, quantum dots (QD), doped in a poly(methyl methacrylate) film in the temperature range of 40-290 K. Based on the results, the binding energy of the exciton (electron hole pair) was estimated. The exciton binding energy of QD of MAPbBr(3) estimated from the absorption and E-A spectra (similar to 17 meV) is nearly the same as that of a MAPbBr(3) polycrystalline thin solid film, while the exciton binding energy estimated from the temperature-dependent PL spectra (similar to 70 meV) is much greater than that estimated from the absorption profile. The frequency dependence of the E-A intensity observed at 40 and 290 K for the modulated applied electric field indicates a slow ion migration in nanocrystals, which follows the modulation of the applied electric field at a frequency less than 500 Hz. The observed E-A spectra were analyzed with an integral method on assuming the Stark effect; the magnitudes of the changes in electric dipole moment and polarizability following photoexcitation were determined at each temperature from 40 to 290 K. E-PL spectra show that the PL of QD of MAPbBr(3) is quenched on the application of an external electric field; the extent of quenching is much greater for trap emission than for exciton emission. Exciton phonon scattering, which is responsible for the line broadening of the PL spectra, is also discussed based on the temperature-dependent PL spectra.