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Size dependence of the stability, electronic structure, and optical properties of silicon nanocrystals with various surface impurities
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 12, 125402Article in journal (Refereed) Published
Abstract [en]

We present a comprehensive, ground-state density functional theory study of the size dependence of the optical and electronic properties and the stability of spherical silicon nanocrystals (NCs) with different impurities on the surface. We vary the size of the NCs from 1.0 to 3.5 nm, considering single-bonded (CH3, F, Cl, OH) and double-bonded (O, S) impurities and bridged oxygen. We show that the density of states (DOS) and absorption indices of the NCs with single-bonded impurities are very similar to each other and the fully hydrogenated NCs, except for the 1.0-nm NCs, where a slight difference is present. In the case of the NCs with double-bonded impurities, the DOS and absorption indices exhibit a significant difference, compared to the fully hydrogenated NCs, for sizes up to 2.5 nm. We argue that this difference arises from the difference in the contribution from the impurity to the states around the gap, which can considerably change the character of the states. We demonstrate that the double-bonded impurities contribute significantly to the states around the gap, compared to the single-bonded impurities, causing changes in the symmetry of these states. This observation was further supported by analyzing the changes of the Fourier transform of the charge densities of the highest occupied and lowest unoccupied eigenstate. We also show that the formation energies of NCs with bridged oxygen and fluorine are the lowest, regardless of the size. Furthermore, we show that high hydrogen concentration can be used to suppress the addition of oxygen and fluorine on the surface of the Si NCs.

Place, publisher, year, edition, pages
2015. Vol. 91, no 12, 125402
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-248082DOI: 10.1103/PhysRevB.91.125402ISI: 000350500100005OAI: oai:DiVA.org:uu-248082DiVA: diva2:798510
Available from: 2015-03-26 Created: 2015-03-26 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Theory and Modelling of Functional Materials
Open this publication in new window or tab >>Theory and Modelling of Functional Materials
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The diverse field of material research has been steadily expanding with a great help from computational physics, especially in the investigation of the fundamental properties of materials. This has driven the computational physics to become one of the main branches of physics, allowing for density functional theory (DFT) to develop as one of the cornerstones of material research. Nowdays, DFT is the method of choice in a great variety of studies, from fundamental properties, to materials modelling and searching for new materials. In this thesis, DFT is employed for the study of a small part of this vast pool of applications. Specifically, the microscopic characteristics of Zn1-xCdxS alloys are studied by looking into the evolution of the local structure. In addition, the way to model the growth of graphene on Fe(110) surface is discussed. The structural stability of silicon nanocrystals with various shapes is analysed in detail, as well.

DFT is further used in studying different properties of semiconductor nanocrystals. The size evolution of the character of the band gap in silicon nanocrystals is investigated in terms of changes in the character of the states around the band gap. The influence of various surface impurities on the band gap, as well as on the electronic and optical properties of silicon nanocrystals is further studied. In addition, the future use of silicon nanocrystals in photovoltaic devices is examined by studying the band alignment and the charge densities of silicon nanocrystals embedded in a silicon carbide matrix. Furthermore, the electronic and optical properties of different semiconductor nanocrystals is also investigated. In the case of the CdSe/CdS and CdS/ZnS core-shell nanocrystals the influence of the nanocrystal size and different structural models on their properties is analysed. For silicon nanocrystal capped with organic ligands, the changes in the optical properties and lifetimes is thoroughly examined with changes in the type of organic ligand.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 93 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1247
Keyword
nanocrystals, graphene, alloys, density functional theory, optical properties, electronic properties, core-shell structures, semiconductors
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-248513 (URN)978-91-554-9231-1 (ISBN)
Public defence
2015-05-27, Å10132 (Häggsalen), Ångström Laboratory, Lägerhydddsvägen 1, Uppsala, 13:30 (English)
Opponent
Supervisors
Available from: 2015-05-05 Created: 2015-03-30 Last updated: 2015-07-07

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Kocevski, VanchoEriksson, OlleRusz, Jan

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