Browsing by Author "Niaz, S."
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Item Open Access Comprehensive Ab Initio Study of Electronic, Optical, and Cohesive Properties of Silicon Quantum Dots of Various Morphologies and Sizes up to Infinity(American Chemical Society, 2016) Niaz, S.; Zdetsis, A. D.We present a comprehensive and integrated model-independent ab initio study of the structural, cohesive, electronic, and optical properties of silicon quantum dots of various morphologies and sizes in the framework of all-electron "static" and time-dependent density functional theory (DFT, TDFT), using the well-tested B3LYP and other properly chosen functional(s). Our raw ab initio results for all these properties for hydrogen-passivated nanocrystals of various growth models and sizes from 1 to 32 Å are subsequently fitted, using power-law dependence with judicially selected exponents, based on dimensional and other plausibility arguments. As a result, we can not only reproduce with excellent accuracy known experimental and well-tested theoretical results in the regions of overlap but also extrapolate successfully all the way to infinity, reproducing the band gap of crystalline silicon with almost chemical accuracy as well as the cohesive energy of the infinite crystal with very good accuracy. Thus, our results could be safely used, among others, as interpolation and extrapolation formulas not only for cohesive energy and band gap but also for interrelated properties, such as dielectric constant and index of refraction of silicon nanocrystals of various sizes all the way up to infinity.Item Open Access Mixed ab initio and semiempirical study of hydrogen-terminated finite germanium nanowires(Springer Berlin Heidelberg, 2018) Niaz, S.; Gülseren, Oğuz; Khan, M. A.; Ullah, I.We present a mixed ab initio and semiempirical method for the cohesive energy and electronic gap calculations of hydrogen passivated tetrahedral and clathrate germanium nanowires (∼ 1850 atoms) with acceptable accuracy, comparable to density functional theory results, and with a significantly lower computational cost. First, we find that the PM6 semiempirical method produce the most accurate geometries when compared with the DFT results; whereas other semiempirical methods such as AM1, PM3 and PM7 clearly underestimate (or overestimate). Second, we implement the DFT@PM6 mixed scheme for cohesive/binding energy and electronic gap calculations which shows promising results compared with reference values of DFT. However, the bulk energy gap and binding energy values from the quantum confinement fitting procedure slightly underestimate the results which can be easily overcome using suitable functional and basis set/ECP. Also, a comparison with previous work clearly shows that the calculated electronic gap for bulk germanium is extremely sensitive to the choice of framework. Further development in this research work is progressing.Item Open Access Systematic spatial and stoichiometric screening towards understanding the surface of ultrasmall oxygenated silicon nanocrystal(Elsevier, 2016-11) Niaz, S.; Zdetsis, A. D.; Koukaras, E. N.; Gülseren, O.; Sadiq, I.In most of the realistic ab initio and model calculations which have appeared on the emission of light from silicon nanocrystals, the role of surface oxygen has been usually ignored, underestimated or completely ruled out. We investigate theoretically, by density functional theory (DFT/B3LYP) possible modes of oxygen bonding in hydrogen terminated silicon quantum dots using as a representative case of the Si29 nanocrystal. We have considered Bridge-bonded oxygen (BBO), Doubly-bonded oxygen (DBO), hydroxyl (OH) and Mix of these oxidizing agents. Due to stoichiometry, all comparisons performed are unbiased with respect to composition whereas spatial distribution of oxygen species pointed out drastic change in electronic and cohesive characteristics of nanocrytals. From an overall perspective of this study, it is shown that bridge bonded oxygenated Si nanocrystals accompanied by Mix have higher binding energies and large electronic gap compared to nanocrystals with doubly bonded oxygen atoms. In addition, it is observed that the presence of OH along with BBO, DBO and mixed configurations further lowers electronic gaps and binding energies but trends in same fashion. It is also demonstrated that within same composition, oxidizing constituent, along with their spatial distribution substantially alters binding energy, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gap (up to 1.48 eV) and localization of frontier orbitals.