Browsing by Author "Jocić, Milan"

Now showing 1 - 3 of 3
  • No Thumbnail Available
    Item
    Ab Initio Construction of Symmetry-adapted k·p Hamiltonians for the Electronic Structure of Semiconductors
    Jocić, Milan; Vukmirović, Nenad
    While k·p Hamiltonians are frequently used for the description of electronic states in quantum nanostructures, a method is lacking to obtain them in their symmetrized form directly from ab initio band structure calculations of bulk material. We developed a method for obtaining the parameters and the symmetry-adapted form of the k·p Hamiltonian from the output of an ab initio band structure calculation. The method consists of (i) evaluation of momentum matrix elements between the wave functions obtained from band structure calculation; (ii) identification of the unitary transformation that transforms these wave functions to the symmetry-adapted basis; (iii) transformation of the k·p Hamiltonian to the symmetry-adapted basis. We illustrate the methodology by obtaining k·p Hamiltonians that describe the band structure of zinc-blende CdSe and then we use the Hamiltonians obtained to calculate the electronic states in CdSe quantum wells. Excellent agreement between density functional theory and k·p is obtained for the electronic structure, even for quite thin wells.
  • No Thumbnail Available
    Item
    Ab-initio calculations of temperature dependent electronic structures of inorganic halide perovskite materials
    Jocić, Milan; Vukmirović, Nenad
    Despite wide interest in halide perovskite materials, it is still challenging to accurately calculate their electronic structure and its temperature dependence. In this work, we present ab-initio calculations of the temperature dependence of the electronic structure of CsPbX3 materials (X = Cl, Br or I) in the cubic form and of the zero temperature electronic structure of the orthorhombic phase of these materials. Phonon-induced temperature dependent band energy renormalization was calculated within the framework of Allen-Heine-Cardona theory, where we exploited the self-consistent procedure to determine both the energy level shifts and their broadenings. The phonon spectrum of the materials was obtained using the self-consistent phonon method since standard density functional perturbation theory calculations in harmonic approximation yield phonon modes with imaginary frequencies due to the fact that the cubic structure is not stable at zero temperature. Our results suggest that low energy phonon modes mostly contribute to phonon-induced band energy renormalization. The calculated values of the band gaps at lowest temperature where the material exhibits a cubic structure are in good agreement with experimental results from the literature. The same is the case for the slope of the temperature dependence of the band gap for the CsPbI3 material where reliable experimental data are available in the literature. We also found that phonon-induced temperature dependence of the band gap is most pronounced for the conduction band minimum and valence band maximum, while other bands exhibit a weaker dependence.
  • Thumbnail Image
    Item
    Electronic Properties of Silver–Bismuth Iodide Rudorffite Nanoplatelets
    Danilović, Danijela; Milosavljević, Aleksandar R.; Sapkota, Pitambar; Dojčilović, Radovan; Tošić, Dragana; Vukmirović, Nenad; Jocić, Milan; Djoković, Vladimir; Ptasinska, Sylwia; Božanić, Dušan K.
    Silver-bismuth iodide (Ag-Bi-I) rudorffites are chemically stable and non-toxic materials that can act as a possible lead-free replacement for methylammonium lead halides in optoelectronic applications. We report on a simple route for fabricating Ag-Bi-I colloidal nanoplatelets approximately 160 nm in lateral dimensions and 1-8 nm in thickness via exfoliation of Ag-Bi-I rudorffite powders in acetonitrile. The valence band electronic structure of isolated Ag-Bi-I nanoplatelets was investigated using synchrotron radiation to perform X-ray aerosol photoelectron spectroscopy (XAPS). The ionization energy of the material was found to be 6.1 +/- 0.2 eV with respect to the vacuum level. UV-vis absorption and photoluminescence spectroscopies of the Ag-Bi-I colloids showed that the optical properties of the nanoplatelets originate from I 5p to Bi 6p and I 5p to I 5p transitions, which is further confirmed by density functional theory (DFT) calculations. Finally, calculations based on the DFT and k . p theoretical methods showed that the quantum confinement effect is very weak in the system studied.