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Theory and Modelling of Functional Materials
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
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 [en]
nanocrystals, graphene, alloys, density functional theory, optical properties, electronic properties, core-shell structures, semiconductors
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-248513ISBN: 978-91-554-9231-1 (print)OAI: oai:DiVA.org:uu-248513DiVA: diva2:800747
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
List of papers
1. Transition between direct and indirect band gap in silicon nanocrystals
Open this publication in new window or tab >>Transition between direct and indirect band gap in silicon nanocrystals
2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 24, 245401- p.Article in journal (Refereed) Published
Abstract [en]

Using ground-state density functional theory we study the transition from indirect to direct band gap in hydrogen-terminated silicon nanocrystals (NCs) as a function of decreasing diameter. The studied range, from 1.0 to 4.6 nm diameter of nanocrystals, with spherical and Wulff-shape NCs, covers the transition from nano-to bulk regime. A change in the symmetry of the lowest unoccupied state as a function of decreasing NC diameter is observed, gradually increasing the oscillator strength of transitions from the highest occupied to the lowest unoccupied state. Real space and Fourier space characteristics of highest occupied and lowest unoccupied states are explored in detail and linked to a smooth transition from nano-to bulk regime.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-203530 (URN)10.1103/PhysRevB.87.245401 (DOI)000319909600003 ()
Available from: 2013-07-16 Created: 2013-07-15 Last updated: 2017-12-06Bibliographically approved
2. Size dependence of the stability, electronic structure, and optical properties of silicon nanocrystals with various surface impurities
Open this publication in new window or tab >>Size dependence of the stability, electronic structure, and optical properties of silicon nanocrystals with various surface impurities
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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-248082 (URN)10.1103/PhysRevB.91.125402 (DOI)000350500100005 ()
Available from: 2015-03-26 Created: 2015-03-26 Last updated: 2017-12-04Bibliographically approved
3. Band alignment switching and the interaction between neighbouring silicon nanocrystals embedded in a SiC matrix
Open this publication in new window or tab >>Band alignment switching and the interaction between neighbouring silicon nanocrystals embedded in a SiC matrix
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, 165429-165435 p.Article in journal (Refereed) Published
Abstract [en]

We present results from density functional theory of the electronic properties of silicon nanocrystals (Si NCs) embedded in a silicon carbide (SiC) matrix, considering different combinations of various NCs and host matrix sizes. We show that the NC and the host matrix form a type-II band alignment, with the states at the top of the valence band being in the Si NC and the states at the bottom of the conduction band in the host matrix. Moreover, this band alignment can be interchanged with introducing oxygen at the interface. We demonstrate that the charge densities of some valence band states can overlap with the charge densities of the neighbouring NCs. We also demonstrate that this leakage of states is significant when the distance between the neighbouring NCs is less than ~1.6 nm.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-248553 (URN)10.1103/PhysRevB.91.165429 (DOI)000353460000007 ()
Available from: 2015-03-31 Created: 2015-03-31 Last updated: 2017-12-04Bibliographically approved
4. First-principles study of the influence of different interfaces and core types on the properties of CdSe/CdS core-shell nanocrystals
Open this publication in new window or tab >>First-principles study of the influence of different interfaces and core types on the properties of CdSe/CdS core-shell nanocrystals
2015 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, 10865Article in journal (Refereed) Published
Abstract [en]

With the expanding field of nanoengineering and the production of nanocrystals (NCs) with higher quality and tunable size, having reliable theoretical calculations to complement the experimental results is very important. Here we present such a study of CdSe/CdS core-shell NCs using density functional theory, where we focus on dependence of the properties of these NCs on core types and interfaces between the core and the shell, as well as on the core/shell ratio. We show that the density of states and the absorption indices depend rather weakly on the type of interface and core type. We demonstrate that the HOMO wavefunction is mainly localised in the core of the nanocrystal, depending primarily on the core/shell ratio. On the other hand the LUMO wavefunction spreads more into the shell of the nanocrystal, where its confinement in the core is almost the same in each of the studied structural models. Furthermore, we show that the radiative lifetimes decrease with increasing core sizes due to changes in the dipolar overlap integral of the HOMO and LUMO wavefunctions. In addition, the electron-hole Coulomb interaction energies follow a similar pattern as the localisation of the wavefunctions, with the smaller NCs having higher Coulomb interaction energies.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-248551 (URN)10.1038/srep10865 (DOI)000355652900001 ()
Available from: 2015-03-31 Created: 2015-03-31 Last updated: 2017-12-04Bibliographically approved
5. Influence of dimensionality and interface type on optical and electronic properties of CdS/ZnS core-shell nanocrystals: a first-principles study
Open this publication in new window or tab >>Influence of dimensionality and interface type on optical and electronic properties of CdS/ZnS core-shell nanocrystals: a first-principles study
Show others...
2015 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, no 16, 164701Article in journal (Refereed) Published
Abstract [en]

Semiconducting nanocrystals (NCs) have become one of the leading materials in a variety of applications, mainly due to their size tunable band gap and high intensity emission. Their photoluminescence properties can be notably improved by capping the nanocrystals with a shell of another semiconductor, making core-shell structures. We focus our study on the CdS/ZnS core-shell nanocrystals, that are closely related to extensively studied CdSe/CdS NCs, albeit exhibiting rather different photoluminescence properties. We employ density functional theory to investigate the changes in the electronic and optical properties of these nanocrystals with size, core/shell ratio and interface structure between the core and the shell. We show that the band gap depends on the size of the NCs and the core/shell ratio. We suggest that the differences in the density of states and absorption are mainly governed by the core/shell ratio. We present that both the LUMO and the HOMO wavefunctions are localised in the core of the NCs, with the distribution of the LUMO wavefunction being more sensitive to the size and the core/shell ratio. We also demonstrate that the Coulomb interaction energies closely follow the behaviour of the localisation of the HOMO and LUMO wavefunctions, and are decreasing with increasing NC size. Furthermore, we investigated the electronic and optical properties of the NCs with different interfaces between the core and the shell, and different core types. We find that the different interfaces and core types have rather small influence on the band gaps and the absorption indices, as well as on the confinement of the HOMO and LUMO wavefunctions. In addition, we compare these results with the previous results for CdSe/CdS NCs, reflecting the different PL properties of these two types of NCs. We argue that the difference in their Coulomb interaction energies is one of the main reasons for their distinct PL properties.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-248552 (URN)10.1063/1.4933058 (DOI)000364235800046 ()26520537 (PubMedID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationStandUpeSSENCE - An eScience Collaboration
Available from: 2015-03-31 Created: 2015-03-31 Last updated: 2017-12-04Bibliographically approved
6. Electronic and optical properties of silicon nanocrystals capped with organic ligands studied using first-principles theory
Open this publication in new window or tab >>Electronic and optical properties of silicon nanocrystals capped with organic ligands studied using first-principles theory
(English)Article in journal (Other academic) In press
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-248696 (URN)
Available from: 2015-04-07 Created: 2015-04-07 Last updated: 2015-07-07
7. Formation and Structure of Graphene Waves on Fe(110)
Open this publication in new window or tab >>Formation and Structure of Graphene Waves on Fe(110)
Show others...
2012 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 2, 026101- p.Article in journal (Refereed) Published
Abstract [en]

A very rich Fe-C phase diagram makes the formation of graphene on iron surfaces a challenging task. Here we demonstrate that the growth of graphene on epitaxial iron films can be realized by chemical vapor deposition at relatively low temperatures, and that the formation of carbides can be avoided in excess of the carbon-containing precursors. The resulting graphene monolayer creates a novel periodically corrugated pattern on Fe(110). Using low-energy electron microscopy and scanning tunneling microscopy, we show that it is modulated in one dimension forming long waves with a period of similar to 4 nm parallel to the [001] direction of the substrate, with an additional height modulation along the wave crests. The observed topography of the graphene/Fe superstructure is well reproduced by density functional theory calculations, and found to result from a unique combination of the lattice mismatch and strong interfacial interaction, as probed by core-level photoemission and x-ray absorption spectroscopy.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-179022 (URN)10.1103/PhysRevLett.109.026101 (DOI)000306324100012 ()
Funder
Swedish Research CouncilEU, European Research Council
Available from: 2012-08-06 Created: 2012-08-06 Last updated: 2017-12-07Bibliographically approved
8. Microscopic description of the evolution of the local structure and an evaluation of the chemical pressure concept in a solid solution
Open this publication in new window or tab >>Microscopic description of the evolution of the local structure and an evaluation of the chemical pressure concept in a solid solution
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 22, 224105- p.Article in journal (Refereed) Published
Abstract [en]

Extended x-ray absorption fine-structure studies have been performed at the Zn K and Cd K edges for a series of solid solutions of wurtzite Zn1-xCdxS samples with x = 0.0, 0.1, 0.25, 0.5, 0.75, and 1.0, where the lattice parameter as a function of x evolves according to the well-known Vegard's law. In conjunction with extensive, large-scale first-principles electronic structure calculations with full geometry optimizations, these results establish that the percentage variation in the nearest-neighbor bond distances are lower by nearly an order of magnitude compared to what would be expected on the basis of lattice parameter variation, seriously undermining the chemical pressure concept. With experimental results that allow us to probe up to the third coordination shell distances, we provide a direct description of how the local structure, apparently inconsistent with the global structure, evolves very rapidly with interatomic distances to become consistent with it. We show that the basic features of this structural evolution with the composition can be visualized with nearly invariant Zn-S-4 and Cd-S-4 tetrahedral units retaining their structural integrity, while the tilts between these tetrahedral building blocks change with composition to conform to the changing lattice parameters according to the Vegard's law within a relatively short length scale. These results underline the limits of applicability of the chemical pressure concept that has been a favored tool of experimentalists to control physical properties of a large variety of condensed matter systems.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-229446 (URN)10.1103/PhysRevB.89.224105 (DOI)000338282500002 ()
Available from: 2014-08-08 Created: 2014-08-07 Last updated: 2017-12-05Bibliographically approved

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