Ultrafast Carrier Dynamics of Pristine and K Doped Hematite

Kumar, Manoj (2018) Ultrafast Carrier Dynamics of Pristine and K Doped Hematite. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Among all the metal oxides, iron oxides are subject to a lot of research in the last few decades. Hematite (α-Fe₂O₃) is one of the most important iron oxide. It can be used as pigments, magnetic material and as photoanodes. In this respect, we present the study of carrier dynamics of hematite (α-Fe₂O₃) along with K-doped hematite. Ultrafast pump-probe spectroscopy is a powerful technique for the study of carrier dynamics of a material. Using this technique, carrier dynamics of an optically excited system undergoing different processes, on the picosecond to femtosecond timescale can be studied. We will use this technique to study the carrier dynamics of hematite and K-doped hematite. We started our study with the characterization of the given materials. The doped and undoped hematite were characterized using UV-Visible spectroscopy, Raman spectroscopy and Scanning electron microscopy. The UV-Vis absorption spectrum of doped and undoped hematite showed strong absorption in 220-700 nm region and 750-900 nm region which indicates that they have high absorption in near UV and Visible range. The band gap of hematite was calculated using its UV-Vis spectra and was found to be ~ 1.78 eV. In the Raman analysis, all the characteristic peaks of hematite reported earlier were observed in the Raman spectra. The Raman peaks were slightly suppressed for with doping. The morphology of the samples was studied using SEM. The size of undoped α-Fe₂O₃ nanoparticles was found to be around 100 nm and the size of the doped nanoparticles was found to be around 40 nm. The carrier dynamics of doped and undoped α-Fe₂O₃ was studied using the pump-probe technique. The transient transmission profile of these materials was used to obtain information about the processes that undergo in the system when it is excited. For both doped and undoped samples, the decay profiles were best fitted with a triple exponential with a time constant 0.41, 5.30 and 87.16 ps. The three time constants represent three different processes thermalization, intraband energy relaxation, and carrier recombination. These decay processes were found to be varying with pump fluence which can be attributed to the increased number of carriers in the semiconductor with pump fluence.The effect of K-doping on the performance of hematite was also studied briefly, and it was found the doping has a significant effect on two out of three processes (Intraband energy relaxation and carrier recombination).

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