Browsing by Subject "Density functional theory"

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  • Benatto, L.; Marchiori, C. F. N.; Talka, T.; Aramini, M.; Yamamoto, N. A. D.; Huotari, S.; Roman, L. S.; Koehler, M. (2020)
    The difference in aggregation size of the C-60 and C-70 fullerenes affect the photovoltaic performance of devices assembled in the so-called bilayer architecture with poly [2,7-(9,9- dioctyl- dibenzosilole)- alt-4,7- bis(thiophen-2-yl)benzo- 2,1,3- thiadiazole] (PSiF-DBT) as the electron donor material. Despite the better performance of the C-70 devices, which is related to the high absorption coefficient in the visible range and the superior charge transport properties, the short-circuit current variation upon annealing treatment at 100 degrees C is approximately twice bigger when the C-60 is the acceptor. We attribute this effect to the tendency of C-60 in form smaller aggregate domains relatively to the C-70. The increased roughness on the polymeric surface after annealing results in an enhanced donor/acceptor contact area and assists the fullerene diffusion deeper inside the polymeric layer. This effect leads to a better mixing between donor and acceptor species and create a interpenetrating layer close to the so-called bulk heterojunction. Since C-60 forms smaller aggregates, this mechanism is more pronounced for this molecule. Therefore, a significant variation in the performance of the C-60 devices is observed after this kind of treatment. Density Functional Theory calculations of the potential energy of interaction between two fullerene molecules and X-Ray measurements gives evidences to support this idea. In addition, combining spectrally resolved external quantum efficiency measurements with optical modeling our results also indicate the occurrence of the bilayer interfacial mixing for PSiF-DBT/C-60.
  • Kadribasic, Fedja; Mirabolfathi, Nader; Nordlund, Kai; Holmström, Eero; Djurabekova, Flyura (2018)
    A large body of astrophysical observations indicate that around 85% of the matter in the universe is not made of recognized standard model particles. Understanding the nature of this so-called dark matter is of fundamental importance to cosmology, astrophysics, and high-energy particle physics. We examine the response of commonly used semiconductor materials to low-mass WIMP interactions using numerical simulations based on classical interatomic potentials in these materials. These simulations, backed up by more precise density functional theory simulations and experiments, predict a nonlinear energy loss that never produces phonons due to the nonzero energy required to form crystallographic defects. We argue that such nonlinear effects related to defect formation in electron-volt-scale resolution semiconductor detectors allows for very effective directional sensitivity and possible statistical nuclear recoil discrimination to dark matter signals for masses below 1 GeV/c(2).
  • Wang, Xiuhong; Zhao, Junlei; Xu, Zongwei; Djurabekova, Flyura; Rommel, Mathias; Song, Ying; Fang, Fengzhou (2020)
    As a promisingmaterial for quantumtechnology, silicon carbide (SiC) has attracted great interest inmaterials science. Carbon vacancy is a dominant defect in 4H-SiC. Thus, understanding the properties of this defect is critical to its application, and the atomic and electronic structures of the defects needs to be identified. In this study, density functional theorywas used to characterize the carbon vacancy defects in hexagonal (h) and cubic (k) lattice sites. The zero-phonon line energies, hyperfine tensors, and formation energies of carbon vacancies with different charge states (2-, -, 0,+ and 2+) in different supercells (72, 128, 400 and 576 atoms)were calculated using standard Perdew-Burke-Ernzerhof and Heyd-Scuseria-Ernzerhof methods. Results show that the zero-phonon line energies of carbon vacancy defects are much lower than those of divacancy defects, indicating that the former is more likely to reach the excited state than the latter. The hyperfine tensors of VC+(h) and VC+(k) were calculated. Comparison of the calculated hyperfine tensor with the experimental results indicates the existence of carbon vacancies in SiC lattice. The calculation of formation energy shows that the most stable carbon vacancy defects in the material are VC2+(k), VC+(k), VC(k), VC-(k) and VC2-(k) as the electronic chemical potential increases.
  • Nordlund, K. (Elsevier Ltd., 2020)
    Frontiers of Nanoscience