Browsing by Author "Abbasian, Hamed"
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Item Open Access Modeling of semiconductor devices based on quantum wells: quantum cascade laser as an example(Bilkent University, 2018-02) Abbasian, HamedIt has been two decades since the quantum-cascade lasers (QCLs) have emerged in 1994 for the first time. As time goes on, QCLs reach to higher points scientifically and economically and the usage of QCLs devices continually grows in optoelectronic device market because of their potential applications in various areas in mid- and far-infrared regions. Moreover, their performance is still improving. QCLs lase based on electron transition between intersubbands and tunneling through potential barriers where electron transition causes photon emission. This takes place in conduction band; that is why QCLs are considered as unipolar semiconductor lasers. The frequencies of emitted photons depend on the location of the allowed energy levels which can be controlled by carefully choosing consecutive wells and barriers with suitable widths. In the present thesis, the transfer matrix method is employed to obtain transmission coe cient and wave functions of electron inside an arbitrary potential profile which is crucial for characterizing semiconductor devices based on quantum well. The obtained wave functions are used to get quantities necessary for characterizing QCL resulted from the potential profile.Item Open Access Theoretical study of hydrogen adsorption in Ti-decorated capped carbon nanotube(Taylor and Francis Ltd., 2017) Niaz, Shanawer; Abbasian, Hamed; Badar, Manzoor Ahmad; Anwar-ul-Haq, Muhammad; Karayel, ArzuWe present ab initio study using dispersion-corrected density functional theory calculations to investigate the hydrogen interaction with Ti-coated, one end closed, single-walled carbon nanotube (SWCNT). Our results demonstrate that a single Ti atom binds up to five hydrogen molecules on SWCNT cap top, whereas adsorption of four hydrogen molecules is energetically more favourable. The analyses fromadsorption energy profile, highest occupied molecular orbital–lowest unoccupied molecular orbital gap and Mulliken charge distribution show contrast in first hydrogen molecule adsorption compared with the rest of four configurations. This is clearly due to the strongly different bonding nature of first hydrogen adsorption among others, between hydrogen molecules and Ti-coated SWCNT. These results not only support our understanding of adsorption nature of hydrogen in Ti-coated SWCNTs but also suggest new directions for smart storage techniques.