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Optical Transitions in Organic and Inorganic Semiconductors
Published in Juan Bisquert, The Physics of Solar Energy Conversion, 2020
Another technique to detect transient phenomena associated with free carriers is the time-resolved microwave conductivity (TRMC) method. This technique is based on the measurement of the relative change of the microwave power reflected from a semiconductor that is caused by a small increase in the conductivity (Savenije et al., 2013). However, TRMC operates in the GHz frequencies and the trap states in the band gap of nanocrystalline semiconductors may contribute to the observed response. Carriers in band gap localized states, discussed in Section 8.4, also induce light absorption in the long wavelength region of the spectrum. Figure 19.10 illustrates different mechanisms whereby the electrons in traps in TiO2 provide transitions that contribute to the optical absorption. These effects result in absorption features in the middle infrared at energies much lower than the band gap, 0.2 eV, corresponding to a wavelength of 1 μm, as shown in Figure 19.11. The same difference in spectra can be obtained either from UV generation of carriers or from the electrons injected at negative bias. In the case of ZnO nanostructures, electrons accumulated within the conduction band cause a bleach of the excitonic band (Subramanian et al., 2003).
Photocatalytic Phenomena
Published in Debasish Sarkar, Nanostructured Ceramics, 2018
The main function of electron-hole pair is charge transfer and interaction with adsorbate species. But, a major percentage of charge recombination reduces the population of free electrons or holes and reduces the photocatalytic efficiency, and sometimes the rate of reaction may be reduced up to 90% [6]. Usually, the quantification of the charge dynamics is carried out by ultra-fast time-resolved absorption spectroscopy, identifying all the reactive species, trapped holes and electrons, and quasi-free electrons, besides the determination of the rate of recombination of electron and hole using a decay profile [7]. Time-resolved microwave conductivity technique is preferentially employed for the measurement of charge carrier mobility of nano-structured semiconducting materials.
New Tools for Facing New Challenges in Radiation Chemistry
Published in Paul R. Bolton, Katia Parodi, Jörg Schreiber, Applications of Laser-Driven Particle Acceleration, 2018
Uli Schmidhammer, Jun Ma, Mehran Mostafavi
The potential of time-resolved emission spectroscopy was shown early [Sauer 1988]. The emitting excited states can be produced directly by the particle radiation, by geminate recombination, by other ground state reactions or by excited state energy transfer processes. With streak cameras, single shot acquisition of kinetics is possible with a typical instrument-limited time resolution of 5 picoseconds. On larger time scales, electron paramagnetic resonance is used to study the transient radicals [Trifunac 1980, Lund 2014]. The real part of time-resolved microwave conductivity probes the microwave absorption influenced by mobility and concentration of radiation induced transients with nanosecond resolution, while the knowledge of the imaginary part of the conductivity gives access to the product of yield and the change in molecular polarizability [Bird 2014, Prins 2007]. Electrochemical detection was recently coupled to the spectroscopic analysis to probe the concentration of the redox intermediates with high sensitivity and a time resolution of 100 μs [Shahdo 2013].
Perspective on structure-property relationship of room temperature single-component liquid crystals
Published in Liquid Crystals, 2022
Govindaswamy Shanker, K. R. Sunil Kumar, Bishwajit Paul
In continuation with the radial distribution of side chain, a new molecular design approach in which the alkyl chains are coupled to the hexabenzocoronene core via a paraphenylene moiety. The phase transition of the coronene SLC-61 (Figure 44) monitored by DSC, which represents two transition peaks during heating/cooling cycles. It is attributed that the relatively broad DSC peaks observed at ca. 20 and 80°C represent conformational changes within the discotic mesophase and, notably with no evidence of isotropic peak below 400°C. Interestingly, SLC-61 measured from the pulse-radiolysis time-resolved microwave conductivity technique at RT (22 °C) and at lowest (−78 °C), and highest temperature (192 °C), show no abrupt changes in LC phases indicative of a wide range of temperature stability. In conclusion, this compound is a combination of high charge carrier mobility, room-temperature liquid-crystallinity, and pronounced solubility in organic solvents, making it attractive for applications as a conducting layer in molecular electronic devices [97].