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  • 101. Li, Shu-yi
    et al.
    Zamulko, Sergiy
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Univ Oslo, Norway.
    Ross, Nils
    Larsen, Jes K.
    Platzer-Bjorkman, Charlotte
    Optical properties of Cu2ZnSn(SxSe1-x)(4) solar absorbers: Spectroscopic ellipsometry and ab initio calculations2017In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 110, no 2, article id 021905Article in journal (Refereed)
    Abstract [en]

    Dielectric functions of Cu2ZnSn(SxSe1-x)(4) thin film absorbers with varied x were determined by spectroscopic ellipsometry and ab initio calculations. From the combination of experimental and theoretical studies, the fundamental interband transition energy E-0 (similar to 1-1.5 eV) and the next following transition energy E-1 (similar to 2-3 eV) were identified and found to blue-shift with increasing sulfur anion content, while keeping the energy separation E-1 - E-0 almost constant, similar to 1.4 eV from experiments, and 1 eV from theory. In addition, the average dielectric responses were found to decrease with sulfur anion content from both theoretical and experimental results. The Tauc optical bandgap value E-g determined on samples prepared on Mo and soda lime glass substrate showed a positive linear relationship between x and bandgap E-g. The bandgap bowing factor determined from the theoretical data is 0.09 eV.

  • 102. Malyi, Oleksandr I.
    et al.
    Bostroem, Mathias
    Kulish, Vadym V.
    Thiyam, Priyadarshini
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Parsons, Drew F.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Univ Oslo, Ctr Mat Sci & Nanotechnol, Norway.
    Volume dependence of the dielectric properties of amorphous SiO22016In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 10, p. 7483-7489Article in journal (Refereed)
    Abstract [en]

    Using first principles calculations, the analysis of the dielectric properties of amorphous SiO2 (am-SiO2) was performed. We found that the am-SiO2 properties are volume dependent, and the dependence is mainly induced by the variation of nanoporosity at the atomic scale. In particular, both ionic and electronic contributions to the static dielectric constants are functions of volume with clear trends. Moreover, using the unique parameterization of the dielectric function provided in this work, we predict dielectric functions at imaginary frequencies of different SiO2 polymorphs having similar band gap energies.

  • 103. Malyi, Oleksandr I.
    et al.
    Kulish, Vadym V.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling. University of Oslo, Norway.
    In search of new reconstructions of (001) alpha-quartz surface: a first principles study2014In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 4, no 98, p. 55599-55603Article in journal (Refereed)
    Abstract [en]

    Using Born-Oppenheimer molecular dynamics (BOMD) simulations and "static" density functional theory (DFT) calculations, the stability of cleaved and reconstructed alpha-SiO2(001) surfaces was studied. We found reconstructions ("dense", 2 x 2 reoptimized "dense", and 3 x 3 reoptimized "dense") which minimize the surface energy. The analysis of the surface energies shows that the cleaved surface reconstructs to the 2 x 2 reoptimized "dense" surface having a surface energy around 10% smaller than the "dense" surface. The results suggest that the optimization of Si-Si and Si-O distances at top surface layers plays the key role in stabilizing the 2 x 2 "dense" surface over the well-known "dense" surface.

  • 104. Malyi, Oleksandr I.
    et al.
    Thiyam, Priyadarshini
    Boström, Mathias
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Univ Oslo, Norway.
    A first principles study of CO2 adsorption on alpha-SiO2(001) surfaces2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 31, p. 20125-20133Article in journal (Refereed)
    Abstract [en]

    In this work, using first principles calculations, an analysis of CO2 interaction with cleaved and reconstructed alpha-SiO2(001) surfaces was performed. We showed that CO2 could strongly interact with a cleaved surface forming CO3-like configurations. Here, the binding energy per CO2 molecule depends strongly on CO2 surface coverage and can reach -2.35 eV. Despite this, even with CO2 molecules, the cleaved surface has a substantially higher surface energy than that of the reoptimized "dense'' surface. This observation is also consistent with molecular dynamics simulations. Because of this, for thermodynamically stable system, the interaction of CO2 molecules with a alpha-SiO2(001) surface should be treated as the physisorption of CO2 molecules on the reoptimized "dense'' surface with the binding energy varying from -0.26 eV for single CO2 molecule adsorption to -0.32 eV per molecule for monolayer coverage.

  • 105. Mohn, P.
    et al.
    Persson, Clas
    Blaha, P.
    Schwarz, K.
    Novak, P.
    Eschrig, H.
    Correlation induced paramagnetic ground state in FeAl2001In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 8719, no 19Article in journal (Refereed)
    Abstract [en]

    Experimentally the intermetallic compound FeAl is known to be nonmagnetic, whereas conventional density functional theory calculations within the local density approximation always yield a ferromagnetic ground state with a magnetic moment at the Fe site of about 0.7/mu (B). We show that a correlation ;correction within the LDA + U scheme yields a nonmagnetic ground state for U greater than or equal to 3.7 eV using two. different implementations. The disappearance of the magnetic ground state occurs since Fe-t(2g) and Fe-e(g) manifolds are affected differently by a common U. For large values of U a magnetic solution reappears as expected for strong correlation.

  • 106. Ninham, Barry W.
    et al.
    Boström, Mathias
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling. University of Oslo, Norway.
    Brevik, Iver
    Buhmann, Stefan Y.
    Sernelius, Bo E.
    Linköping University, Sweden.
    Casimir forces in a plasma: possible connections to Yukawa potentials2014In: European Physical Journal D: Atomic, Molecular and Optical Physics, ISSN 1434-6060, E-ISSN 1434-6079, Vol. 68, no 10, p. 328-Article in journal (Refereed)
    Abstract [en]

    We present theoretical and numerical results for the screened Casimir effect between perfect metal surfaces in a plasma. We show how the Casimir effect in an electron-positron plasma can provide an important contribution to nuclear interactions. Our results suggest that there is a connection between Casimir forces and nucleon forces mediated by mesons. Correct nuclear energies and meson masses appear to emerge naturally from the screened Casimir-Lifshitz effect.

  • 107.
    Olsen, Vegard S.
    et al.
    Univ Oslo, Ctr Mat Sci & Nanotechnol, Dept Phys, POB 1048, N-0316 Oslo, Norway..
    Bazioti, Calliope
    Univ Oslo, Ctr Mat Sci & Nanotechnol, Dept Phys, POB 1048, N-0316 Oslo, Norway..
    Baldissera, Gustavo
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Azarov, Alexander
    Univ Oslo, Ctr Mat Sci & Nanotechnol, Dept Phys, POB 1048, N-0316 Oslo, Norway..
    Prytz, Oystein
    Univ Oslo, Ctr Mat Sci & Nanotechnol, Dept Phys, POB 1048, N-0316 Oslo, Norway..
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Svensson, Bengt G.
    Univ Oslo, Ctr Mat Sci & Nanotechnol, Dept Phys, POB 1048, N-0316 Oslo, Norway..
    Kuznetsov, Andrej Y.
    Univ Oslo, Ctr Mat Sci & Nanotechnol, Dept Phys, POB 1048, N-0316 Oslo, Norway..
    Vines, Lasse
    Univ Oslo, Ctr Mat Sci & Nanotechnol, Dept Phys, POB 1048, N-0316 Oslo, Norway..
    Effects of Substrate and Post-Deposition Annealing on Structural and Optical Properties of (ZnO)(1-x)(GaN)x Films2019In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 256, no 6, article id 1800529Article in journal (Refereed)
    Abstract [en]

    The structural and optical properties of magnetron sputtered thin films of (ZnO)(1-x)(GaN)(x) deposited on zinc oxide, sapphire, and silicon oxide are studied as a function of strain accumulation and postdeposition anneals at 600-800 degrees C. For the experimental conditions studied, we found that different amounts of tensile strain accumulated in the samples practically does not affect the strong bandbowing effect, that is, optical bandgap, observed in the asdeposited alloys. In its turn, postdeposition annealing results in a reduction of the tensile strain and dislocation density in the films, as measured by both X-ray diffraction and transmission electron microscopy, corroborating an increase in the crystal quality. In addition, the grain size is found to increase with annealing temperature, for example, mean values of 20 nm up to 50 nm were measured for the alloys with x=0.15. Meanwhile, the fullwidth at half maximum of the (0002) X-ray diffraction reflection increases with annealing temperature, but with only a small increase in bandgap energies for the x=0.15 sample. However, this observation was explained combining the experimental data and firstprinciples calculations based on density functional theory, showing that the increase in the amount of Ga-N bonds lowers the total energy of the system. As such, we conclude that the thermal treatments increase the Ga-N ordering, resulting in several contributions or a widening of the diffraction peaks.

  • 108.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Anisotropic hole-mass tensor of CuIn1-xGax(S,Se)(2): Presence of free carriers narrows the energy gap2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 93, no 7Article in journal (Refereed)
    Abstract [en]

    Analysis of the band edges of CuIn1-xGax (S, Se)(2) with x= 0.0, 0.5, and 1.0 reveals that (i) CuIn1-xGaxS2 has larger effective electron m(c) and hole m(v) masses than CuIn1-xGaxSe2; (ii) whereas Ga content affects band curvatures only slightly, the sign of the crystal- field split Delta(cf) as well as the spin-orbit coupling affect my strongly; (iii) the optical response epsilon(2) (h omega) is comparable for all six compounds; and (iv) band filling of electrons (n= 10(19) cm(-3)) and holes (p=n) narrows the band gap by E-g approximate to E-g(0)- 0.1 eV and generates an optical band gap of E-g(opt) approximate to E-g+ 0.2 eV.

  • 109.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Electronic and optical properties of Cu2ZnSnS4 and Cu2ZnSnSe42010In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 107, no 5Article in journal (Refereed)
    Abstract [en]

    The electronic structure as well as the optical response of kesterite and stannite structures of Cu2ZnSnS4 and Cu2ZnSnSe4 are analyzed by a relativistic full-potential linearized augmented plane wave method. The energy dispersion of the conduction-band edge reveals larger effective electron mass of the two Cu2ZnSnS4 compounds (m(c1)approximate to 0.18m(0)) compared with Cu2ZnSnSe4 (m(c1)approximate to 0.07m(0)). Whereas the effective electron mass tensor is fairly isotropic, the effective hole masses show strong anisotropy. The fundamental band-gap energy is estimated to be E-g approximate to 1.5 eV for Cu2ZnSnS4 and E-g approximate to 1.0 eV for Cu2ZnSnSe4. The larger band gap results in a smaller high-frequency dielectric constant: epsilon(infinity)approximate to 6.7 for Cu2ZnSnS4 whereas epsilon(infinity)approximate to 8.6 for Cu2ZnSnSe4. The characteristic anisotropy of the dielectric function epsilon(omega) in the stannite compounds allows for a complementary identification of the crystalline structure type. Overall, however, all four compounds show similar atomic-resolved density-of-states, dielectric function, and optical absorption coefficient alpha(omega).

  • 110.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Impurity induced band-gap narrowing in p-type CuIn1-xGax(S,Se)(2)2009In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 517, no 7, p. 2374-2379Article in journal (Refereed)
    Abstract [en]

    Green's functions modelling of the impurity induced effects in p-type CuIn1-xGaxS2 and CuIn1-xGaxSe2 (x = 0.0, 0.5, and 1.0) reveals that: (i) the critical active acceptor concentration for the metal non-metal transition occurs at Nc approximate to 10(17)-10(18) cm(-3) for impurities with ionization energy of E-A approximate to 30-60 meV. (ii) For acceptor concentrations N-A>N-C, the hole gas of the metallic phase affects the band-edge energies and narrows the energy gap E-g=E-g(0)=Delta E-g. The energy shift of the valence-band maximum Delta E-v1 is roughly twice as large as the shift of the conduction-band minimum Delta E-c1. (iii) Delta E-v1 depends strongly on the non-parabolicity of the valence bands. (iv) Sulfur based compounds and Ga-rich alloys have the largest shifts of their band edges. (v) A high active acceptor concentrations of N-A = 10(20) cm(-3) implies a band-gap narrowing in the order of Delta E-g approximate to 0.2 eV, thus E-g=E-g(0)-0.2 eV, and an optical band gap of E-g(opt)approximate to E-g(0)-0.1 eV.

  • 111.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Thin-film ZnO/CdS/CuIn1-xGaxSe2 solar cells: Anomalous physical properties of the CuIn1-xGaxSe2 absorber2006In: Brazilian journal of physics, ISSN 0103-9733, E-ISSN 1678-4448, Vol. 36, no 3B, p. 948-951Article in journal (Refereed)
    Abstract [en]

    Thin-film ZnO/CdS/CuIn1-xGaxSe2 solar cells are manufactured with similar to 20% solar-cell conversion efficiency. The CuIn1-xGaxSe2 (CIGS) absorber exhibits rather different physical properties compared with conventional semiconductors (e.g. Si, GaAs and ZnSe). For instance, (i) the valence-band maximum in CIGS consists of cation-d-anion-p hybridized states. (ii) Cation vacancies have low formation energies and high mobility at room temperature. (iii) The most stable surface of CuIn1-xGaxSe2 is the reconstructed, and otherwise polar, (112) surface. (iv) Solar cells with absorbers containing grain-boundaries outperform cells with crystalline absorbers. In this work, the fundamental physical properties of CIGS, like the electronic structure, the defect formation energies, as well as surface properties are discussed from a theoretical perspective.

  • 112. Persson, Clas
    et al.
    Ahuja, R.
    da Silva, A. F.
    Johansson, Börje
    First-principle calculations of optical properties of wurtzite AlN and GaN2001In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 231, no 3, p. 407-414Article in journal (Refereed)
    Abstract [en]

    The imaginary part of the dielectric function of wurtzite AIN and GaN has been calculated in the long wavelength limit, using two different first-principle electronic structure methods. The first method is a full-potential linearized augmented plane wave method and the second is a full-potential linear muffin-tin orbital method. From the Kramers-Kronig dispersion relations the real part of the dielectric function has been obtained, taking into account a quasi-particle band-gap correction according to Bechstedt and Del Sole. Absorption due to optical phonons is treated as a delta function in the imaginary part of the dielectric function. Both the longitudinal as well as the transverse components of the dielectric function are presented, showing that the anisotropy is small in these materials. Although we use different correlation potentials in the two methods, the results are similar, We compare our calculated dielectric functions with spectroscopic ellipsometry and reflectance spectra measurements.

  • 113. Persson, Clas
    et al.
    Ahuja, R.
    da Silva, A. F.
    Johansson, Börje
    First-principle calculations of the dielectric function of zinc-blende and wurtzite InN2001In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 13, no 40, p. 8945-8950Article in journal (Refereed)
    Abstract [en]

    The imaginary part of the dielectric function of zinc-blende and wurtzite InN has been calculated using a full-potential linearized augmented plane wave method. We show that the exchange potential of Engel and Vosko gives an insulating ground state for both structures. The real part of the dielectric function has been obtained from the Kramers-Kronig dispersion relations, assuming a quasiparticle band-gap correction according to Bechstedt and Del Sole. We have found that it is necessary to have a good account of the band gap in order to derive the low-frequency optical properties. We present the longitudinal as wen as the transverse components in wurtzite InN, showing that the anisotropy is small.

  • 114.
    Persson, Clas
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Ahuja, Rajeev
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Full band calculation of doping-induced band-gap narrowing in p-type GaAs2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6403, no 3, p. art. no.-033201Article in journal (Refereed)
    Abstract [en]

    We have calculated the energy shift of the fundamental band gap as a function of dopant concentration in p-type GaAs, using a perturbation theory based on the zero-temperature Green's function formalism within the random-phase approximation. The electronic structure of the intrinsic crystal was obtained from a relativistic, full-potential band-structure calculation, with the exchange potential of Engel and Vosko [Phys. Rev. B 47, 13164 (1993)]. We have found that this potential gives accurate band curvatures at the band edges. The nonparabolicity of the uppermost valence bands has a reducing effect on the energy shifts of both the conduction and valence bands for high acceptor, concentrations. The resulting narrowing of the band gap is in good agreement with photoluminescence measurements. Thus, it is crucial to take into account the full band structure to calculate the band-gap narrowing accurately for high acceptor concentrations.

  • 115.
    Persson, Clas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ambrosch-Draxl, Claudia
    A full-band FPLAPW plus k center dot p-method for solving the Kohn-Sham equation2007In: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 177, no 3, p. 280-287Article in journal (Refereed)
    Abstract [en]

    We have implemented a full-band k . p-approach into a full-potential linearized augmented plane wave (FPLAPW) code in order to more efficiently-and still accurately - calculate the electronic and optical properties of periodic crystalline solids within the Kohn-Sham singleelectron formalism. The validity of this full-band k - p-method is discussed as well as the convergence of the eigenvalues and eigenvectors with respect to basis set and k-mesh, with applications to the semiconductor ZnO and the metal Al. Moreover, the accuracy of the FPLAPW + k - p-method for computing the band structure and the dielectric function is demonstrated for the more complex materials YBa2Cu3O7 and poly(paraphenylene). For these structures, the full-band k - p-approach reduces the computational time by as much as 90%.

  • 116.
    Persson, Clas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Chen, Rongzhen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Zhao, Hanyue
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Kumar, Mukesh
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Huang, Dan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Electronic Structure and Optical Properties from First-Principles Modeling2015In: Copper Zinc Tin Sulfide-Based Thin-Film Solar Cells, John Wiley & Sons, 2015, p. 75-105Chapter in book (Other academic)
    Abstract [en]

    This chapter explores the electronic and optical properties of the kesterite and stannite phases of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) by means of first-principles modeling. It analyzes the optical properties of CZT(S,Se), in terms of the dielectric function and the optical absorption coefficient. The chapter demonstrates the application of density functional theory (DFT) in conjunction with the Kohn-Sham (KS) equation, utilizing the generalized gradient approximation (GGA), the screened hybrid functional, and the single-electron excitation GW approach, as implemented in the Wien2k and VASP software packages. In the theoretical research on CZT(S,Se) several methods and implementations have been utilized, and it is not surprising to find somewhat deviating results for calculations using similar computational approaches. The chapter briefly discusses these methods. It also describes the crystal and electronic structures of kesterite and stannite phases of CZTS and CZTSe.

  • 117.
    Persson, Clas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    da Silva, A. F.
    Strong polaronic effects on rutile TiO2 electronic band edges2005In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 86, no 23Article in journal (Refereed)
    Abstract [en]

    Thin TiO2 films are prepared by dc magnetron sputtering as well as by the sol-gel technique, and the optical band edge absorption alpha(omega) is obtained from transmission spectroscopy. The electronic structure and optical properties are studied employing a first-principle linearized augmented plane-wave method within the local density approximation (LDA), improved by an on-site Coulomb self-interaction potential (LDA+U-SIC). We show that the correction potential, the polaronic screening, and the spin-orbit interaction are crucial for determining the TiO2 effective electron and hole masses. The dielectric function and the high-frequency constant epsilon(0 <

  • 118. Persson, Clas
    et al.
    da Silva, A. F.
    Ahuja, R.
    Johansson, Börje
    Effective electronic masses in wurtzite and zinc-blende GaN and AlN2001In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 231, no 3, p. 397-406Article in journal (Refereed)
    Abstract [en]

    The effective electron and hole masses are fundamental quantities of semiconductors, used in numerous analyses of experiments and theoretical investigations. We present calculations of the band structure near the band edges in intrinsic GaN and AIN, both for the wurtzite and the zinc-blende polytypes. We have utilized a full-potential linearized augmented plane wave method within the density functional theory and with two different exchange-correlation potentials. The lattice parameters have been determined by a minimization of the total energy, whereupon the crystal-field splitting, the spin-orbit splitting, and the effective electron and hole masses have been calculated. The calculated effective masses are in good agreement with available experimental values. We show the importance of performing a fully relativistic calculation. For instance, the hole mass in cubic AIN is a very large and negative quantity if the spin-orbit coupling is excluded, whereas the fully relativistic calculation gives a relatively small and positive value.

  • 119. Persson, Clas
    et al.
    da Silva, A. F.
    Johansson, Börje
    Metal-nonmetal transition in p-type SiC polytypes2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6320, no 20Article in journal (Refereed)
    Abstract [en]

    The critical concentration for the metal-nonmetal transition has been calculated fur the polytypes 3C, 4H, and 6H of SiC, duped with Al, Ga, B, and Sc. Three different computational methods have been utilized: the first is Mott's original model, the second is an extended Mott-Hubbard model, and the third is based on the total energy of the metallic and nonmetallic phases. All three methods show similar results. For comparison, calculations fur p-type Ge and Si materials have also been performed.

  • 120.
    Persson, Clas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Dong, C. L.
    Vayssieres, L.
    Augustsson, A.
    Schmitt, T.
    Mattesini, M.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Nordgren, J.
    Chang, C. L.
    Ferreira da Silva, A.
    Guo, J. H.
    X-ray absorption and emission spectroscopy of ZnO nanoparticle and highly oriented ZnO microrod arrays2006In: Microelectronics Journal, ISSN 0026-2692, Vol. 37, no 8, p. 686-689Article in journal (Refereed)
    Abstract [en]

    The electronic structures of ZnO nanoparticles and microrod arrays are studied by 0 Is X-ray absorption spectroscopy (XAS) and O K alpha X-ray emission spectroscopy (XES). We show that the present LDA +U-SIC calculation approach is suitable to correct the LDA self-interaction error of the cation d-states. The atomic eigenstates of 3d in zinc and 2p in oxygen are energetically close, which induces strong Zn-3d-O-2p hybridization. This anomalous valence band cation-d-anion-p hybridization is affected when the localization of the Zn 3d-states is taken into account. Experimentally, the XES spectra show energy dependence in the spectral shape revealing selected excitations to the Zn 3d, 4s and 4p states, hybridized with 0 2p states. Strong anisotropic effects are observed for the highly oriented ZnO rods, but not for the isotropic spherical nanoparticles. The nanostructured ZnO has primarily bulk XAS and XES properties.

  • 121.
    Persson, Clas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ferreira da Silva, A.
    Linear optical response of zinc-blende and wurtzite III-N (III = B, Al, Ga, and In)2007In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 305, no 2, p. 408-413Article in journal (Refereed)
    Abstract [en]

    We present calculated dielectric function epsilon(omega) and linear optical absorption alpha(omega) of the cubic and hexagonal structures of BN, AIN, GaN, and InN, employing an all-electron linearized augmented plane wave method within the local density approximation (LDA) plus quasi-particle (QP) correction. We show that this LDA+QP approach yields accurate high-frequency dielectric constant Moreover, by employing a modelled electron-optical phonon interaction we calculate the static dielectric function epsilon(0).

  • 122.
    Persson, Clas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Jayakumar, O. D.
    Sudakar, C.
    Sudarsan, V.
    Tyagi, A. K.
    Morphology and Magnetic Coupling in ZnO:Co and ZnO:Ni Co-Doped with Li2011In: Acta Physica Polonica. A, ISSN 0587-4246, E-ISSN 1898-794X, Vol. 119, no 2, p. 95-98Article in journal (Refereed)
    Abstract [en]

    Zn0.95Co0.05O and Zn0.97Ni0.03O nanorods, prepared by a solvothermal method, show intriguing morphology and magnetic properties when co-doped with Li. At low and moderate Li incorporation (below 10 and 3 at.% Li in the Co- and Ni-doped samples, respectively) the rod aspect ratio is increased and room temperature ferromagnetic properties are enhanced, whereas the ferromagnetic coupling in Zn0.97Ni0.03O is decreased for Li concentrations > 3 at.%. First-principles theoretical analyses demonstrate that Li co-doping has primarily two effect 3 in bulk Zn1-xMxO (with M = Co or Ni). First, the Li-on-Zn acceptors increase the local magnetic moment by depopulating the M 3d minority spin-states. The magnetic coupling is Ruderman-Kittel-Kasuya-Yosida-like both without and with Li co-doping. Second, Li-on-Zn prefer to be close to the M atoms to compensate the M-O bonds and to locally depopulate the 3d states, and this will help forming high aspect nanostructures. The observed room temperature ferromagnetism in Li co-doped Zn1-xMxO nanorods can therefore be explained by the better rod morphology in combination with ionizing the magnetic M atoms.

  • 123. Persson, Clas
    et al.
    Lindefelt, U.
    Sernelius, B. E.
    Plasma-induced band edge shifts in 3C-, 2H-, 4H-, 6H-SiC and Si2000In: Solid-State Electronics, ISSN 0038-1101, E-ISSN 1879-2405, Vol. 44, no 3, p. 471-476Article in journal (Refereed)
    Abstract [en]

    Plasma-induced energy shifts of the conduction band minimum and of the valence band maximum have been calculated for 3C-, 2H-, 4H-, 6H-, 6H-SiC and Si. The resulting narrowing of the fundamental band gap and of the optical band gap are presented. The method utilized is based on a zero-temperature formalism within the random phase approximation. Electron-electron, hole-hole, electron-hole, electron-optical phonon and hole-optical phonon interactions have been taken into account. The calculations are based on band structure data from a relativistic, full-potential band structure calculation.

  • 124.
    Persson, Clas Marcus
    et al.
    KTH, Superseded Departments, Chemistry.
    Kumpulainen, Atte J.
    KTH, Superseded Departments, Chemistry.
    Surface forces measured in sugar surfactant solutions between two hydrophobic thiolated silica spheres2004In: Colloids and Surfaces A: Physicochemical and Engineering Aspects, ISSN 0927-7757, E-ISSN 1873-4359, Vol. 233, no 1-3, p. 43-49Article in journal (Refereed)
    Abstract [en]

    The force acting between two hydrophobic surfaces (hexadecyl-thiolated gold surfaces) across two different non-ionic sugar surfactant solutions above cmc was investigated. The two surfactants studied were n-decyl-beta-D-glucopyranoside (Glu) and n-decyl-beta-D-maltopyranoside (Mal). No long-range double-layer force was detected between the surfactant covered surfaces showing that they are non-ionic as expected. At distances from 20 nm down to a separation of 3 nm the ever-present van der Waals force creates a net attraction between the surfaces. At 3 nm the surfaces are in monolayer-monolayer contact and the system has a secondary force minimum. At shorter separations the surfactant monolayer starts to interact and a steric force develops counteracting the depletion of surfactants from the gap. As the compressive load reaches FIR approximate to 1.5 mN/m the surfaces are in hydrophobic-hydrophobic contact, where the system has its primary energy minimum. The nature of the pressure-induced depletion of surfactants from the contact zone showed a strong dependence on the approach velocity. This reveals that the time scale of the surfactants to reach their equilibrium situation under external compression is of several seconds. The surfactant with the glucoside head group offered a greater resistance against being forced out (15% higher). This was attributed to the larger adsorbed amount of Gin at the hydrophobic solid-liquid interface. It was further concluded that the forces generated from the surfactants were enough to prevent the system from coagulating but that flocculation will occur in the secondary force minimum (primary energy minimum). The hydrodynamic force was studied by changing the approach velocity of the surfaces. The position of the slipping plane coincided with the layer thickness of the adsorbed monolayer of surfactant at moderate approach velocities. The adhesion force increased with the time the surfaces were in contact and no limiting value was reached. This was attributed to surfactants remaining in the gap slowly being removed from the contact zone, thereby, increasing its hydrophobic character.

  • 125.
    Persson, Clas Marcus
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Nur, O.
    Willander, M.
    De Andrada e Silva, E. A.
    Da Silva, A. F.
    Electronic band-edge structure, effective masses, and optical absorption of Si1-xGex using an extended FPLAPW/VCA/LDA+U computational method2006In: Brazilian journal of physics, ISSN 0103-9733, E-ISSN 1678-4448, Vol. 36, no 2 A, p. 447-450Article in journal (Refereed)
    Abstract [en]

    Electronic band-edge structure and optical properties of Si 1-xGex are investigated theoretically emloying a full-potential linearized augmented plane wave (FPLAPW) method. The exchange-correlation potential in the local density approximation (LDA) is corrected by an on-site Coulomb potential (i.e., within the LDA.+USIC approach) acting asymmetrically on the atomic-like orbitals in the muffin-tin spheres. The electronic structure of the Si1-xGex is calculated self-consistently, assuming a Td symmetrized Hamiltonian and a linear behavior of the valence-band eigenfunctions for Si, SiGe, and Ge with respect to Ge composition x, assuming randomly alloyed crystal structure, i.e., a "virtual-crystal like" approximation (VGA). We show that this approach yields accurate band-gap energies, effective masses, dielectric function, and optical properties of Si1-xGex. We perform absorption measurements showing the band-gap energy for x &lt; 0.25.

  • 126.
    Persson, Clas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Mirbt, S.
    Improved electronic structure and optical properties of sp-hybridized semiconductors using LDA+U-SIC2006In: Brazilian journal of physics, ISSN 0103-9733, E-ISSN 1678-4448, Vol. 36, no 2A, p. 286-290Article in journal (Refereed)
    Abstract [en]

    We propose the local density approximation (LDA) plus an on-site Coulomb self-interaction-like correction (SIC) potential for describing sp-hybridized bonds in semiconductors and insulators. We motivate the present LDA+U-SIC scheme by comparing the exact exchange (EXX) hole with the LDA exchange hole. The LDA+U-SIC method yields good band-gap energies E-g and dielectric constants (omega approximate to 0) of Si, Ge, GaAs, and ZnSe. We also show that LDA consistently underestimates the Gamma-point effective electron m, and light-hole M-lh masses, and the underlying reason for this is a too strong light-hole-electron coupling within LDA. The advantages of the LDA+USIC approach are a computational time of the same order as the ordinary LDA, the orbital dependent LDA+USIC exchange-correlation interaction is asymmetric analogously to the EXX potential, and the method can be used for materials and compounds involving localized d- and f-orbitals.

  • 127.
    Persson, Clas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Platzer-Bjorkman, Charlotte
    Malmstrom, Jonas
    Torndahl, Tobias
    Edoff, Marika
    Strong valence-band offset bowing of ZnO1-xSx enhances p-type nitrogen doping of ZnO-like alloys2006In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 97, no 14Article in journal (Refereed)
    Abstract [en]

    Photoelectron spectroscopy, optical characterization, and density functional calculations of ZnO1-xSx reveal that the valence-band (VB) offset E-v(x) increases strongly for small S content, whereas the conduction-band edge E-c(x) increases only weakly. This is explained as the formation of local ZnS-like bonds in the ZnO host, which mainly affects the VB edge and thereby narrows the energy gap: E-g(x=0.28)approximate to E-g(ZnO)-0.6 eV. The low-energy absorption tail is a direct Gamma(v)->Gamma(c) transition from ZnS-like VB. The VB bowing can be utilized to enhance p-type N-O doping with lower formation energy Delta H-f and shallower acceptor state in the ZnO-like alloys.

  • 128. Persson, Clas
    et al.
    Sernelius, B. E.
    da Silva, A. F.
    Ahuja, R.
    Johansson, Börje
    Effective electron and hole masses in intrinsic and heavily n-type doped GaN and AlN2001In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 13, no 40, p. 8915-8922Article in journal (Refereed)
    Abstract [en]

    We have investigated the electronic structure near the band edges in intrinsic and heavily n-type doped GaN and AlN. Both the wurtzite and the zincblende polytypes have been considered. The electronic structures of the intrinsic materials were obtained from a full-potential linearized augmented plane wave calculation. We show the importance of performing a fully relativistic calculation. For instance, the hole mass in cubic AIN is a very large and negative quantity if spin-orbit coupling is excluded, whereas the fully relativistic calculation gives a relatively small and positive value. The electron-phonon coupling was taken into account according to the Frohlich Hamiltonian for large polarons, resulting in effective polaron masses. The effects on the effective electron masses due to doping were investigated by using a Green's function formalism within the random phase approximation and with a local-field correction according to Hubbard.

  • 129. Persson, Clas
    et al.
    Sernelius, B. E.
    da Silva, A. F.
    Araujo, C. M.
    Ahuja, Rajeev
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Optical and reduced band gap in n- and p-type GaN and AlN2002In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 92, no 6, p. 3207-3216Article in journal (Refereed)
    Abstract [en]

    We present a full band calculation of the doping-induced energy shifts of the conduction-band minimum and the valence-band maximum for n- and p-type GaN and AlN. Both wurtzite and zinc-blende structures have been considered. The resulting optical and reduced band-gap energies are presented as functions of the ionized impurity concentration in the heavily doped regime. The computational method is based on a zero-temperature Green's function formalism within the random phase approximation and with the local-field correction of Hubbard. The calculation goes beyond the spherical approximation of the energy bands by using energy dispersions and overlap integrals from a first-principle, full-potential band-structure calculation. Inclusion of the spin-orbit interaction is crucial for describing the uppermost valence bands properly, and we show that the nonparabolicity of the valence bands influences the energy shifts strongly, especially the shift of the optical band gap. With the full band structure, we can explain the results of photoluminescence measurements by Yoshikawa [J. Appl. Phys. 86, 4400 (1999)].

  • 130.
    Persson, Clas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhao, Y. J.
    Lany, S.
    Zunger, A.
    n-type doping of CuInSe2 and CuGaSe22005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 3Article in journal (Refereed)
    Abstract [en]

    The efficiency of CuInSe2 based solar cell devices could improve significantly if CuGaSe2, a wider band gap chalcopyrite semiconductor, could be added to the CuInSe2 absorber layer. This is, however, limited by the difficulty of doping n-type CuGaSe2 and, hence, in its alloys with CuInSe2. Indeed, wider-gap members of semiconductor series are often more difficult to dope than lower-gap members of the same series. We find that in chalcopyrites, there are three critical values of the Fermi energy E-F that control n-type doping: (i) E-F(n,pin) is the value of E-F where the energy to form Cu vacancies is zero. At this point, the spontaneously formed vacancies (=acceptors) kill all electrons. (ii) E-F(n,comp) is the value of E-F where the energy to form a Cu vacancy equals the energy to form an n-type dopant, e.g., Cd-Cu. (iii) E-F(n,site) is the value of E-F where the formation of Cd-on-In is equal to the formation of Cd-on-Cu. For good n-type doping, E-F(n,pin), E-F(n,comp), and E-F(n,site) need to be as high as possible in the gap. We find that these quantities are higher in the gap in CuInSe2 than in CuGaSe2, so the latter is difficult to dope n-type. In this work, we calculate all three critical Fermi energies and study theoretically the best growth condition for n-type CuInSe2 and CuGaSe2 with possible cation and anion doping. We find that the intrinsic defects such as V-Cu and In-Cu or Ga-Cu play significant roles in doping in both chalcopyrites. For group-II cation (Cd, Zn, or Mg) doping, the best n-type growth condition is In/Ga-rich, and maximal Se-poor, which is also the optimal condition for stabilizing the intrinsic In-Cu/Ga-Cu donors. Bulk CuInSe2 can be doped at equilibrium n-type, but bulk CuGaSe2 cannot be due to the low formation energy of intrinsic Cu-vacancy. For halogen anion doping, the best n-type materials growth is still under In/Ga-rich, and maximal Se-poor conditions. These conditions are not best for halogen substitutional defects, but are optimal for intrinsic In-Cu/Ga-Cu donors. Again, CuGaSe2 cannot be doped n-type by halogen doping, while CuInSe2 can.

  • 131. Persson, Clas
    et al.
    Zunger, A.
    Anomalous grain boundary physics in polycrystalline CuInSe2: The existence of a hole barrier2003In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 91, no 26Article in journal (Refereed)
    Abstract [en]

    First-principles modeling of grain boundaries (GB) in CuInSe2 semiconductors reveals that an energetic barrier exists for holes arriving from the grain interior (GI) to the GB. Consequently, the absence of holes inside the GB prevents GB electrons from recombining. At the same time, the GI is purer in polymaterials than in single crystals, since impurities segregated to the GBs. This explains the puzzle of the superiority of polycrystalline CuInSe2 solar cells over their crystalline counterpart. We identify a simple and universal mechanism for the barrier, arising from reduced p-d repulsion due to Cu-vacancy surface reconstruction. This discovery opens up possibilities for the future design of superior polycrystalline devices.

  • 132.
    Persson, Clas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zunger, A.
    Compositionally induced valence-band offset at the grain boundary of polycrystalline chalcopyrites creates a hole barrier2005In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 87, no 21Article in journal (Refereed)
    Abstract [en]

    First-principles calculations of model grain boundaries (GBs) in CuInSe2 and CaGaSe2 show that cation-terminated GBs have a valence-band offset with respect to the grain interior (GI). This offset repels holes from the GBs, thus depriving electrons there from recombination at the GB defects. Anion-terminated GBs have no such valence offset. CuGaSe2 has, in addition, a conduction-band offset at the GB/GI interface, attracting electrons to the GBs. These features explain how polycrystalline chalcopyrite solar cells could outperform their crystalline counterparts.

  • 133. Persson, Clas
    et al.
    Zunger, A.
    Deep nitrogen-induced valence- and conduction-band states in GaAs1-xNx2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, no 3Article in journal (Refereed)
    Abstract [en]

    Most studies of the anomalous electronic properties of the GaAs1-N-x alloy have focused on near-edge states, but x-ray spectroscopic experiments [V. N. Strocov et al., Phys. Status Solidi B 233, RI (2002)] have now revealed anomalous properties deep inside the valence and conduction bands. Indeed, whereas N p character is found in GaN in the energy region near the valence-band maximum (VBM), when similar to3% N is introduced into GaAs one finds that N p character exists about 2-3 eV below the VBM and as two narrow peaks just above the conduction-band minimum. First-principles calculations of the N p character in Ga32As31N and GaN show that the valence resonances are due the fact that the VBM of GaAs0.97N0.03 itself lies > 2 eV above that of GaN. Thus, there is no need to involve an N--> As charge transfer to explain the data. This conclusion is further confirmed by our calculated valence--> conduction-band absorption spectra. We also show that the broken-symmetry core-hole calculations are necessary to explain, within the local density approximation (LDA), the energy of the N 1s<--valence-band transitions. As to the conduction-band peaks observed experimentally in the x-ray absorption, our LDA calculations do not reveal the sharp peaks seen in the measured spectra, leading to the possibility that the latter may not be related to substitutional N.

  • 134. Persson, Clas
    et al.
    Zunger, A.
    s-d coupling in zinc-blende semiconductors2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, no 7Article in journal (Refereed)
    Abstract [en]

    Most zinc blende semiconductors have a single anion-like s state near the bottom of the valence band, found in density-of-states (DOS) calculations, and seen in photoemission. Here, we discuss the case where two s-like peaks appear, due to strong s-d coupling. Indeed, away from the k=0 Brillouin zone center, cation d states and anion s states can couple in zinc blende symmetry. Depending on the energy difference DeltaE(sd)=E-s(anion)-E-d(cation), this interaction can lead to either a single or two s-like peaks in the DOS and photoemission. We find four types of behaviors. (i) In GaP, GaAs, InP, and InAs, DeltaE(sd) is large, giving rise to a single cation d peak well below the single anion s peak. (ii) Similarly, in CdS, CdSe, ZnS, ZnSe, and ZnTe, we see also a single s peak, but now the cation d is above the anion s. In both (i) and (ii) the s-d coupling is very weak. (iii) In GaN and InN, the local density approximation (LDA) predicts two s-like peaks bracketing below and above the cation d-like state. Correcting the too low binding energies of LDA by LDA+SIC (self-interaction correction) still leaves the two s-like peaks. The occurrence of two s-like peaks represents the fingerprint of strong s-d coupling. (iv) In CdTe, LDA predicts a single s-like peak just as in case (ii) above. However, LDA+SIC correction shifts down the cation d state closer to the anion s band, enhancing the s-d coupling, and leading to the appearance of two s-like peaks. Case (iv) is a remarkable situation where LDA errors cause not only quantitative energetic errors, but actually leads to a qualitative effect of a DOS peak that exists in LDA+SIC but is missing in LDA. We predict that the double-s peak should be observed in photoemission for GaN, InN, and CdTe.

  • 135.
    Roman, L. S.
    et al.
    Departamento de Física, Universidade Federal do Paraná.
    Valaski, R.
    Departamento de Física, Universidade Federal do Paraná.
    Canestraro, Carla Daniele
    Departamento de Física, Universidade Federal do Paraná.
    Magalhães, E. C. S.
    Instituto de Física, Universidade Federal da Bahia.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ahuja, Rajeev
    Department of Physics, Uppsala University.
    da Silva Jr., E. F.
    Departamento de Física, Universidade Federal de Pernambuco.
    Pepe, I.
    Instituto de Física, Universidade Federal da Bahia.
    Ferreira da Silva, Antonio
    Instituto de Física, Universidade Federal da Bahia.
    Optical band-edge absorption of oxide compound SnO22006In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 252, no 15, p. 5361-5364Article in journal (Refereed)
    Abstract [en]

    Tin oxide (SnO2) is an important oxide for efficient dielectrics, catalysis, sensor devices, electrodes and transparent conducting coating oxide technologies. SnO2 thin film is widely used in glass applications due to its low infra-red heat emissivity. In this work, the SnO2 electronic band-edge structure and optical properties are studied employing a first-principle and fully relativistic full-potential linearized augmented plane wave (FPLAPW) method within the local density approximation (LDA). The optical band-edge absorption alpha(omega) of intrinsic SnO2 is investigated experimentally by transmission spectroscopy measurements and their roughness in the light of the atomic force microscopy (AFM) measurements. The sample films were prepared by spray pyrolysis deposition method onto glass substrate considering different thickness layers. We found for SnO2 qualitatively good agreement of the calculated optical band-gap energy as well as the optical absorption with the experimental results.

  • 136. Shin, D. W.
    et al.
    Dong, C. L.
    Mattesini, M.
    Augustsson, A.
    Mao, S.
    Chang, C. L.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Nordgren, J.
    Wang, S. X.
    Guo, J. H.
    Size dependence of the electronic structure of copper nanoclusters in SiC matrix2006In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 422, no 4-6, p. 543-546Article in journal (Refereed)
    Abstract [en]

    We studied the size dependence of the electronic structure of nanocrystalline copper embedded in silicon carbide by means of soft Xray absorption spectroscopy. Changes in the local electronic states of copper nanoclusters, including the shift in binding energy and the reduction of s-p-d hybridization, occurred. The experimental result was compared with the ab initio self-consistent, real-space, multiple-scattering calculation. The calculation was in good agreement with the trend found in our experimental results. We concluded that the reduction of d-d interaction and the concomitant changes in s-p-d hybridization in copper nanoclusters arise due to the surface effect.

  • 137. Siebentritt, Susanne
    et al.
    Igalson, Malgorzata
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Lany, Stephan
    The electronic structure of chalcopyrites-bands, point defects and grain boundaries2010In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 18, no 6, p. 390-410Article in journal (Refereed)
    Abstract [en]

    We summarize the progress made recently in understanding the electronic structure of chalcopyrites. New insights into the dispersion of valence and conduction band allow conclusions on the effective masses of charge carriers and their orientation dependence, which influences the transport in solar cell absorbers of different orientation. Native point defects are responsible for the doping and thus the band bending in solar cells. Results of optoelectronic defect spectroscopy are reviewed. Native defects are also the source for a number of metastabilities, which strongly affect the efficiency of solar cells. Recent theoretical findings relate these effects to the Se vacancy and the In-Cu antisite defect. Experimentally determined activation energies support these models. Absorbers in chalcopyrite solar cells are polycrystalline, which is only possible because of the benign character of the grain boundaries. This can be related to an unusual electronic structure of the GB.

  • 138.
    Souza Dantas, Nilton
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Arwin, Hans
    Nzulu, Gabriel
    de Oliveira Rappl, Paulo Henrique
    Ferreira da Silva, Antonio
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Optical characterization of rock salt Pb1-xSnx Te alloys2008In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 205, no 4, p. 837-840Article in journal (Refereed)
    Abstract [en]

    The optical properties in terms of dielectric function ϵ (ħω) = ϵ1(ħω) + 2(ħω) of rocksalt Pb1–xSnx Te alloys (0 ≤ x ≤ 1) are investigated experimentally by spectroscopic ellipsometry and theoretically by first-principles electronic band structure calculation. We find a strong optical response in the 0.5–2.0 eV region arising from optical absorption around the LW-line of the Brillouin zone. The response peak of the imaginary part of the dielectric functions at E = 1.6–1.8 eV shifts towards lower energies for high Sn compositions as a consequence of narrower W-point band-gap Eg(W) for the Sn-rich alloys.

  • 139.
    Souza Dantas, Nilton
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ferreira da Silva, Antonio
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Electronic band -edge properties of rock salt PbY and SnY (Y = S, Se, and Te)2008In: Optical materials (Amsterdam), ISSN 0925-3467, E-ISSN 1873-1252, Vol. 30, no 9, p. 1451-1460Article in journal (Refereed)
    Abstract [en]

    The electronic band-edges of lead chalcogenides PbY and tin chalcogenides SnY (where Y = S, Se, and Te) are investigated by the means of a full-potential linearized augmented plane wave (FPLAPW) method and the local density approximation (LDA). All six chalcogenide binaries have similar electronic structures and density-of-states, but there are differences in the symmetry of the band-edge states at and near the Brillouin zone L-point. These differences give the characteristic composition, pressure, and temperature dependences of the energy gap in Pb1-xSnxY alloys.We find that: (1) SnY are zero-gap semiconductors E-g = 0 if the spin-orbit (SO) interaction is excluded. The reason for this is that the conduction band (CB) and the valence band (VB) cross along the Q equivalent to LW line. (2) Including the SO interaction splits this crossing and creates a direct gap along the Q-line, thus away from the L symmetry point. Hence, the fundamental band gap E-g in SnY is induced by the SO interaction and the energy gap is rather small Eg approximate to 0.2-0.3 eV. At the L-point, the CB state has symmetric L-4(+) and the VB state is antisymmetric L-4(-) thereby the L-point pressure coefficient partial derivative E-g(L)/partial derivative p is a positive quantity. (3) PbY have a direct band gap at the L-point both when SO coupling is excluded and included. In contrast to SnY, the SO interaction decreases the gap energy in PbY. (4) Including the SO interaction, the LDA yields incorrect symmetries of the band-edge states at the L-point; the CB state has L-4(+) and the VB state has L-4(-) symmetry. However, a small increase of the cell volume corrects this LDA failure, producing an antisymmetric CB state and a symmetric VB state, and thereby also yields the characteristic negative pressure coefficient partial derivative E-g(L)/partial derivative p in agreement with experimental findings. (5) Although PbY and SnY have different band-edge physics at their respective equilibrium lattice constants, the change of the band-edges with respect to cell volume is qualitatively the same for all six chalcogenides. (6) Finally, in the discussion of the symmetry of the band edges, it is important to clearly state the chosen unit cell origin; a shift by (a/2,0,0) changes the labeling L-4(+) double left right arrow L-4(-) of the irreducible representations.

  • 140.
    Thiyam, Priyadarshini
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Boström, Mathias
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Parsons, D. F.
    Brevik, I.
    Sernelius, Bo E.
    Finite-size-dependent dispersion potentials between atoms and ions dissolved in water2014In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 106, no 5, p. 53002-Article in journal (Refereed)
    Abstract [en]

    A non-expanded theory is used for dispersion potentials between atoms and ions dissolved in a medium. The first-order dispersion interaction between two atoms in an excited state must account for the fact that the two atoms are coupled via the electromagnetic field and must include effects from background media, retardation and finite size. We show that finite-size corrections when two particles are close change the dispersion interactions in water by several orders of magnitude. We consider as four illustrative examples helium atoms, krypton atoms, phosphate ions, and iodide ions. We demonstrate that, due to large cancellation effects, retardation dominates the interaction for helium atom pairs in an isotropic excited state down to the very small atom-atom separations where finite-size corrections are also important.

  • 141.
    Thiyam, Priyadarshini
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Fiedler, J.
    Buhmann, S. Y.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Brevik, I.
    Bostrom, M.
    Parsons, D. F.
    Ice Particles Sink below the Water Surface Due to a Balance of Salt, van der Waals, and Buoyancy Forces2018In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 27, p. 15311-15317Article in journal (Refereed)
    Abstract [en]

    According to the classical Archimedes' principle, ice floats in water and has a fraction of its volume above the water surface. However, for very small ice particles, other competing forces such as van der Waals forces due to fluctuating charge distributions and ionic forces due to salt ions and charge on the ice surface also contribute to the force balance. The latter crucially depends on both the pH of the water and the salt concentration. We show that a bulge in the air-water interface due to interaction of surface tension with the rising ice particle becomes significant when the particle radius is greater than 50-100 mu m. The role of these forces in governing the initial stages of ice condensation has never been considered. Here, we show that small ice particles can only form below an exclusion zone, from 2 nm (in high salt concentrations) up to 1 mu m (in pure water at pH 7) thick, under the water surface. This distance is defined by an equilibrium of upward buoyancy forces and repulsive van der Waals forces. Ionic forces due to salt and ice surface charge push this zone further down. Only after growing to a radius larger than 10 pm, will the ice particles eventually float toward the water surface in agreement with the simple intuition based on Archimedes' principle. Our result is the first prediction of observable repulsive van der Waals forces between ice particles and the water surface outside a laboratory setting. We posit that it has consequences on the biology of ice water as we predict an exclusion zone free of ice particles near the water surface which is sufficient to support the presence of bacteria.

  • 142.
    Thiyam, Priyadarshini
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Lima, E. R. A.
    Malyi, O. I.
    Parsons, D. F.
    Buhmann, S. Y.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. University of Oslo, Norway.
    Boström, M.
    Effects of van der Waals forces and salt ions on the growth of water films on ice and the detachment of CO2 bubbles2016In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 113, no 4, article id 43002Article in journal (Refereed)
    Abstract [en]

    We study the effect of salts on the thickness of wetting films on melting ice and interactions acting on CO2 bubble near ice-water and vapor-water interfaces. Governing mechanisms are the Lifshitz and the double-layer interactions in the respective three-layer geometries. We demonstrate that the latter depend on the Casimir-Polder interaction of the salt ions dissolved in water with the respective ice, vapor and CO2 interfaces, as calculated using different models for their effective polarizability in water. Significant variation in the predicted thickness of the equilibrium water film is observed for different salt ions and when using different models for the ions' polarizabilities. We find that CO2 bubbles are attracted towards the ice-water interface and repelled from the vapor-water interface.

  • 143.
    Thiyam, Priyadarshini
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Malyi, O. I.
    Persson, Clas
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Energy Processes.
    Buhmann, S. Y.
    Parsons, D. F.
    Boström, M.
    Effective lennard-jones parameters for CO2-CO2 dispersion interactions in water and near amorphous silica-water interfaces2015Conference paper (Refereed)
    Abstract [en]

    Different models for effective polarizability in water and the corresponding dispersion forces between dissolved molecules are explored in bulk water and near interfaces. We demonstrate that the attractive part of the Lennard-Jones parameters, i.e., the van der Waals parameter C6 (UvdW ≈ -C6/ρ6), is strongly modified when two carbon dioxide (CO2) molecules are near an amorphous silica-water and near a vapor-water interface. Standard simulation parameters for near-surface modeling are based on intermolecular forces in bulk media.

  • 144.
    Thiyam, Priyadarshini
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Parashar, Prachi
    Shajesh, K. V.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM). University of Oslo, Norway.
    Schaden, Martin
    Brevik, Iver
    Parsons, Drew F.
    Milton, Kimball A.
    Malyi, Oleksandr I.
    Bostrom, Mathias
    Anisotropic contribution to the van der Waals and the Casimir-Polder energies for CO2 and CH4 molecules near surfaces and thin films2015In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 92, no 5, article id 052704Article in journal (Refereed)
    Abstract [en]

    In order to understand why carbon dioxide (CO2) and methane (CH4) molecules interact differently with surfaces, we investigate the Casimir-Polder energy of a linearly polarizable CO2 molecule and an isotropically polarizable CH4 molecule in front of an atomically thin gold film and an amorphous silica slab. We quantitatively analyze how the anisotropy in the polarizability of the molecule influences the van der Waals contribution to the binding energy of the molecule.

  • 145.
    Thiyam, Priyadarshini
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM).
    Brevik, I.
    Sernelius, Bo E.
    Boström, Mathias
    Nonperturbative theory for the dispersion self-energy of atoms2014In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 90, no 5, p. 054502-Article in journal (Refereed)
    Abstract [en]

    We go beyond the approximate series expansions used in the dispersion theory of finite-size atoms. We demonstrate that a correct, and nonperturbative, theory dramatically alters the dispersion self-energies of atoms. The nonperturbed theory gives as much as 100% corrections compared to the traditional series-expanded theory for the smaller noble gas atoms.

  • 146.
    Thiyam, Priyadarshini
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. University of Oslo, Norway .
    Parsons, D. F.
    Huang, Dan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. College of Physical Science and Technology, China .
    Buhmann, S. Y.
    Bostrom, M.
    Trends of CO2 adsorption on cellulose due to van der Waals forces2015In: Colloids and Surfaces A: Physicochemical and Engineering Aspects, ISSN 0927-7757, E-ISSN 1873-4359, Vol. 470, p. 316-321Article in journal (Refereed)
    Abstract [en]

    The non-retarded van der Waals and Casimir-Polder forces on a CO2 molecule in water near a single surface and between surfaces are explored. We observe preferential adsorption and desorption of CO2 molecules depending on the material of the surfaces. We also find a potential mechanism of capture and storage of CO2 molecules in a geometry consisting of a cellulose surface coated by a thin film of water and then by air.

  • 147.
    Thiyam, Priyadarshini
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Sernelius, Bo E.
    Parsons, Drew F.
    Malthe-Sørenssen, Anders
    Boström, Mathias
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Intermolecular Casimir-Polder forces in water and near surfaces2014In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 90, no 3, p. 032122-Article in journal (Refereed)
    Abstract [en]

    The Casimir-Polder force is an important long-range interaction involved in adsorption and desorption of molecules in fluids. We explore Casimir-Polder interactions between methane molecules in water, and between a molecule in water near SiO2 and hexane surfaces. Inclusion of the finite molecular size in the expression for the Casimir-Polder energy leads to estimates of the dispersion contribution to the binding energies between molecules and between one molecule and a planar surface.

  • 148.
    Thomazi, Fabiano
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Roman, Lucimara Stolz
    da Silva, Antonio Ferreira
    Persson, Clas
    Optical absorption of rutile SnO2 and TiO22009In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 6, no 12, p. 2740-2742Article in journal (Refereed)
    Abstract [en]

    The electronic band-edge structure and optical response of rutile SnO2 and TiO2 are studied, employing a fully relativistic projected augmented wave method within the local density approximation (LDA). A quasi-particle model corrects the LDA band-gap energies, which is used to fit the on-site self-interaction correction (SIC) potential LDA+U-SIC. We show that inclusion of this k-dependent SIC-like potential correction, as well as inclusion of polaronic screening are of outermost importance for accurately determining the electronic structure and optical properties. For both SnO2 and TiO2, the calculated absorption coefficient alpha(omega) reveals a very small optical band-edge absorption in the photon energy region E-g <= h omega <= similar to E-g+Delta (Delta = 0.8 eV and 0.5 eV for SnO2 and TiO2, respectively). The main difference between SnO2 and TiO2 is the presence of the unoccupied low-energy 3d conduction-band states in TiO2 which yield flat conduction bands and a very strong optical absorption for h omega > E-g+Delta. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

  • 149. Tomas, S. A.
    et al.
    Palomino, R.
    Lozada, R.
    da Silva, E. F.
    de Vasconcelos, E. A.
    de Azevedo, W. M.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Pepe, I.
    David, D. G. F.
    da Silva, A. F.
    Optical and electronic characterization of the band structure of blue methylene and rhodamine 6G-doped TiO2 sol-gel nanofilms2005In: Microelectronics Journal, ISSN 0026-2692, Vol. 36, no 3-6, p. 570-573Article in journal (Refereed)
    Abstract [en]

    The optical properties and the band structure of TiO2 sol-gel thin films prepared by the sot-gel process and doped with methylene blue or rhodamine were studied by absorption, excitation and emission spectroscopy. The absorption spectra show two absorption bands, around 2.3 eV and above 3.0 eV suggesting that besides the excitation bands observed in rhodamine at 2.3 eV an efficient energy transfer occurs between the TiO2 conduction band and the molecular rhodamine excited states, yielding a strong emission band at 600 nm, while for methylene blue a strong hypsochromic shift of the absorption bands is observed. The optical absorption and band gap energy for the phase rutile of TiO2 films were calculated with a LDA+U/FPLAPW method, and a comparison of this calculation with experimental data of TiO2 films prepared by undoped sol-gel and sputtering is performed.

  • 150. Tomas, S. A.
    et al.
    Stolik, S.
    Palomino, R.
    Lozada, R.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ahuja, R.
    Pepe, I.
    da Siva, A. F.
    Optical properties of rhodamine 6G-doped TiO2 sol-gel films2005In: Journal de Physique IV: Colloque, ISSN 1155-4339, E-ISSN 1764-7177, Vol. 125, p. 415-417Article in journal (Refereed)
    Abstract [en]

    The optical properties of titania (TiO2) thin films prepared by the sol-gel process and doped with rhodamine 6G were studied by Photoacoustic Spectroscopy. Rhodamine 6G-doping was achieved by adding 0.01%, 0.02%, 0.05% y 0.1% mol rhodamine to a solution that contained titanium isopropoxide as precursor. Two absorption regions were distinguished in the absorption spectrum of a typical rhodamine 6G-doped TiO2 film. A shift of these bands occured as a function of rhodamine 6G-doping concentration. In addition, the optical absorption and band gap energy for rutile-phase TiO2 films were calculated employing the full-potential linearized augmented plane wave method. A comparison of these calculations with experimental data of TiO2 films prepared by sol-gel at room temperature was performed.

1234 101 - 150 of 157
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