An enhancing factor: the enhancement of the electric properties of a dye molecule (IR26) by indium-tin oxide nanoparticles (ITO NPs, see picture) has been shown by measuring the near-infrared two-photon-excited transient absorption spectra. The dye molecule was excited much more efficiently in the presence of an ITO NP layer.
Excited state dynamics is a central issue in discussing the origin of the high efficiency of thermally activated delayed fluorescence (TADF) molecules. Here we will present experimentally a complexity of the excited state dynamics of the extremely high efficient TADF molecules, 4CzIPN. By means of pump‐probe transient absorption spectroscopy, we will show the sign of intramolecular charge delocalization formed in the excited state. This specific excited state originates from the intramolecular charge resonance within carbazole moieties, which occurs through the dimerization of the moieties. On the other hand, the counter molecules of 4CzIPN, i.e. 2CzPN, forms only a localized excited state. The role of charge delocalized state for TADF efficiency will be discussed.
In addition to the process of photogeneration of electrons and holes in photocatalyst materials, the competitive process of trapping of these charge carriers by defects which can both enhance photocatalytic activity by promoting electron-hole separation or can deteriorate the activity by serving as recombination centers is also very crucial to the performance of the photocatalyst [1]. Similarly, addition of cocatalyst is also known to enhance the photocatalytic activity of a material by promoting efficient charge separation. In this work we report on the charge carrier dynamics in a visible-light responsive (oxy) nitride photocatalyst: LaTi2ON with and without the presence of CoOx cocatalyst. For femtosecond diffuse reflectance measurements, an amplified 150 fs Ti:Sa laser with OPA for pump (λexc= 500nm & 580nm) and white light continuum and IR light for probes are used [1]. Synthesis of LaTiO2N and LaTiO2N/CoOx powder are described elsewhere [2]. Figure 1a shows the effect of change of excitation wavelength on the dynamics of the energetically shallow trapped charge carriers of LaTiO2N powder (with and without CoOx) probed at 880 nm. As is evident from the figure, shifting the excitation wavelength to longer λexc 580 nm, the kinetics of the relaxation from the shallow to deep traps slows down for unloaded LaTiO2N. This hints at the existence of energetically distributed trapped states in unloaded LaTiO2N powder which play an interesting role in the charge carrier kinetics making it excitation wavelength dependent. Possibly, direct trap filling of deep traps for higher λexc= 580nm slows down the relaxation process of charge carriers from shallow trap states. Kinetics of mobile conduction band electrons, which are observed by the IR probe, however remains unaffected to excitation wavelength changes. Interestingly, co-catalyst loading is found to have a suppressing effect on this slowing down of this relaxation of shallow trapped electrons to deep-trapped states as well as a decrease in TA intensity of the shallow trapped carriers. As can be seen in Figure 1b, the larger the amount of the cocatalyst, the faster is the decay and smaller the TA intensity. Thus, it is evident that cocatalyst loading has an impact on the carrier dynamics but, a better knowledge of the cocatalyst distribution, size etc. is necessary to strongly comment on the observation. At this point we lack this information. However, based on literature we can very loosely comment at this stage that probably due to charge transfer from the photocatalyst to the cocatalayst (TA data implies in <1ps which is very fast), the decay becomes less sensitive to λexc. Infact, a previous study on Co modified single crystalline LaTiO2N reports ~27% quantum efficiency at 440nm [2]. CoOx deposition was found to prolong the lifetime of mobile carriers (2000 cm-1) to a time scale of 1 second. Acknowledgement: This work is supported by “Research Project for Future Development: Artificial Photosynthetic Chemical Process (ARPChem)” (METI, Japan: 2012-2022). Reference [1] Tamaki, Y. Hara, K. Katoh, R. Tachiya, M. Furube, A., J. Phys. Chem. C, 2009, 113, 11741. [2] Zhang, Y, Yamakata, A, Maeda, K, Moriya, Y, Takata, T, Kubota, J, Teshima, K, Oishi, S, Domen, K., J. Am. Chem. Soc. 2012, 134, 8348.
Nanoparticles (NPs) of noble metals such as gold and silver are well-known to have strong plasmon bands in the visible region and show unique optical characteristics because of an enhanced near-field electric field at the NP surface. This near field can sense the environmental properties through optical measurements of light absorption and scattering. The plasmon resonance condition changes when the refractive index of the surrounding medium changes, which can be measured as an absorption band shift. Even a single molecule can be detected through surface-enhanced Raman scattering (SERS) when the molecule is located within the enhanced electric field. SERS enhancement factors of > 10 4 for a spherical NP and > 10 10 for a NP dimer are reported. [1][2][3][4][5][6][7][8] The SERS enhancement factor is proportional to E 4 , where E is the electromagnetic field enhancement factor.We have recently reported that indium-tin oxide (ITO) NPs have a unique plasmon property in the near-infrared (NIR) range and the peak wavelength can be easily tuned by controlling the amount of Sn doping. [9] The plasmon oscillation is generated by conductive electrons in the conduction band through doping and not by the free electrons of the metal. Because of the reduced electron densities in ITO (n % 10 21 cm À3 ) compared to metals (n % 10 23 cm À3 ) and other different properties of the conductive electrons, such as the effective mass and mobility, it is still not understood how large the electric enhancement factor of ITO NPs is and whether this property can be used to enhance any optical transition near the NPs, although there are recent articles on a doped metal oxide with a tuneable plasmon band in the NIR region. [10,11] In this work, we have applied transient absorption spectroscopy to a dye-coated ITO NP film, where dye absorption is present at half the wavelength of the plasmon resonance so that two-photon-induced transient absorption can be expected. Since a two-photon process occurs only in a strong electric field, we can selectively observe the near field generated by ITO NPs.A hexane solution of 11 nm spherical ITO NPs was prepared by a method reported previously. [9] ITO NPs doped with 10 % Sn were synthesized and used for this study because they show the strongest plasmon absorption within a doping range of 3 to 30 % of Sn. The solution gave a plasmon absorption peak at 1720 nm as shown in Figure 1. Spincoating was applied to produce a NP film on a glass substrate. The film thickness was estimated to be about 100-150 nm using the optical density of the film, the known absorption coefficient of the ITO NPs (around 0.5 10 8 m À1 cm À1 ), and SEM measurements for several selected cross-sections of the film, indicating that the film consisted of 10-15 layers of NPs. The film showed a plasmon band that shifted to a longer wavelength at 2140 nm relative to that in solution because of plasmon coupling.A near-infrared laser dye, IR26 (Exciton), was coated homogeneously on the ITO film from an acetonitrile solution. Since ITO NPs ar...
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