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Building crystals out of crystals: Synthesis, structure and magnetic properties of iron oxide nanoparticles and self-assembled mesocrystals
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is focused on the fabrication and characterization of self-assembled arrays of magnetic iron oxide (Fe3O4, γ-Fe2O3 and Fe1-xO) nanoparticles. The synthesis of spherical and cubic iron oxide nanocrystals, with sizes between 5 and 30 nm and narrow size distributions, is demonstrated, along with a rigorous morphological characterization of the cubic nanoparticles. The transformation of core|shell Fe1-xO|Fe3-δO4 particles into single-phase Fe3-δO4 particles is studied in detail. It is found that anti-phase boundaries in the particles result in the emergence of anomalous magnetic properties i.e. exchange bias, and a reduced saturation magnetization compared to that of bulk Fe3O4. Cubic nanocrystals are assembled into arrays possessing an exceptionally high degree of translational ordering and a high degree of crystallographic alignment. A combination of electron microscopy and small-angle X-ray scattering is used in the characterization of the 3D nanostructures. The directional (anisotropic) interactions in the 3D structures are modeled in an attempt to find a link between the nanocrystal morphology and the corresponding mesostructure. Here, the cohesive van der Waals energy is estimated for a system of nanocubes with a variable truncation. The assembly of nanocubes in magnetic fields of various strengths is systematically investigated. A perturbed mesocrystal growth habit is observed at intermediate fields, whereas at high field strengths, the assembly is dominated by ferrohydrodynamic instabilities. Last, magnetometry is used to study the collective magnetic properties of self-assembled nanocrystals. The magnetic susceptibility in a weak magnetic field is studied as a function of film thickness and particle size. An increase in the tendency to form ferromagnetic couplings  with decreasing film thickness can be established. This 2D to 3D crossover of the magnetic properties of the nanoparticle arrays can be related to a change in the magnetic vortex states.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry (MMK), Stockholm University , 2013. , 112 p.
Keyword [en]
iron oxide, nanoparticle, synthesis, self-assembly, characterization, electron microscopy, scattering, magnetic properties
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-93791ISBN: 978-91-7447-768-9 (print)OAI: oai:DiVA.org:su-93791DiVA: diva2:648779
Public defence
2013-10-18, Ahlmannsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2013-09-26 Created: 2013-09-16 Last updated: 2013-10-08Bibliographically approved
List of papers
1. Anomalous Magnetic Properties of Nanoparticles Arising from Defect Structures: Topotaxial Oxidation of Fe1-xO|Fe3-δO4 Core|Shell Nanocubes to Single-Phase Particles
Open this publication in new window or tab >>Anomalous Magnetic Properties of Nanoparticles Arising from Defect Structures: Topotaxial Oxidation of Fe1-xO|Fe3-δO4 Core|Shell Nanocubes to Single-Phase Particles
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2013 (English)In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 7, no 8, 7132-7144 p.Article in journal (Refereed) Published
Abstract [en]

Here we demonstrate that the anomalous magnetic properties of iron oxide nanoparticles are correlated with defects in their interior. We studied the evolution of microstructure and magnetic properties of biphasic core|shell Fe1–xO|Fe3−δO4 nanoparticles synthesized by thermal decomposition during their topotaxial oxidation to single-phase nanoparticles. Geometric phase analysis of high-resolution electron microscopy images reveals a large interfacial strain at the core|shell interface and the development of antiphase boundaries. Dark-field transmission electron microscopy and powder X-ray diffraction concur that, as the oxidation proceeds, the interfacial strain is released as the Fe1–xO core is removed but that the antiphase boundaries remain. The antiphase boundaries result in anomalous magnetic behavior, that is, a reduced saturation magnetization and exchange bias effects in single-phase nanoparticles. Our results indicate that internal defects play an important role in dictating the magnetic properties of iron oxide nanoparticles.

Keyword
iron oxide, core|shell, nanoparticles, defects, antiphase boundary, electron microscopy, exchange bias
National Category
Materials Chemistry Nano Technology Chemical Sciences
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-93790 (URN)10.1021/nn402487q (DOI)000323810600076 ()
Note

AuthorCount: 5;

Funding Agencie:

Swedish Research Council (VR);  Knut and Alice Wallenberg (KAW) Foundation  3DEM-NATUR 

Available from: 2013-09-16 Created: 2013-09-16 Last updated: 2017-12-06Bibliographically approved
2. Shape Induced Symmetry in Self-Assembled Mesocrystals of Iron Oxide Nanocubes
Open this publication in new window or tab >>Shape Induced Symmetry in Self-Assembled Mesocrystals of Iron Oxide Nanocubes
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2011 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 11, no 4, 1651-1656 p.Article in journal (Refereed) Published
Abstract [en]

Grazing incidence small-angle scattering and electron microscopy have been used to show for the first time that nonspherical nanoparticles can assemble into highly ordered body-centered tetragonal mesocrystals. Energy models accounting for the directionality and magnitude of the van der Waals and dipolar interactions as a function of the degree of truncation of the nanocubes illustrated the importance of the directional dipolar forces for the formation of the initial nanocube clusters and the dominance of the van der Waals multibody interactions in the dense packed arrays.

Keyword
Nanoparticles, self-assembly, structure, GISAXS
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-58020 (URN)10.1021/nl200126v (DOI)000289341500045 ()
Funder
Swedish Research Council
Available from: 2011-05-26 Created: 2011-05-26 Last updated: 2017-12-11Bibliographically approved
3. Structural diversity in iron oxide nanoparticle assemblies as directed by particle morphology and orientation
Open this publication in new window or tab >>Structural diversity in iron oxide nanoparticle assemblies as directed by particle morphology and orientation
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2013 (English)In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 5, no 9, 3969-3975 p.Article in journal (Refereed) Published
Abstract [en]

The mesostructure of ordered arrays of anisotropic nanoparticles is controlled by a combination of packing constraints and interparticle interactions, two factors that are strongly dependent on the particle morphology. We have investigated how the degree of truncation of iron oxide nanocubes controls the mesostructure and particle orientation in drop cast mesocrystal arrays. The combination of grazing incidence small-angle X-ray scattering and scanning electron microscopy shows that mesocrystals of highly truncated cubic nanoparticles assemble in an fcc-type mesostructure, similar to arrays formed by iron oxide nanospheres, but with a significantly reduced packing density and displaying two different growth orientations. Strong satellite reflections in the GISAXS pattern indicate a commensurate mesoscopic superstructure that is related to stacking faults in mesocrystals of the anisotropic nanocubes. Our results show how subtle variation in shape anisotropy can induce oriented arrangements of nanoparticles of different structures and also create mesoscopic superstructures of larger periodicity.

National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-90208 (URN)10.1039/c3nr33282a (DOI)000317859400053 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

AuthorCount:11;

Available from: 2013-05-28 Created: 2013-05-28 Last updated: 2017-12-06Bibliographically approved
4. 2D to 3D crossover of the magnetic properties in ordered arrays of iron oxide nanocrystals
Open this publication in new window or tab >>2D to 3D crossover of the magnetic properties in ordered arrays of iron oxide nanocrystals
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2013 (English)In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 5, no 3, 953-960 p.Article in journal (Refereed) Published
Abstract [en]

The magnetic 2D to 3D crossover behavior of well-ordered arrays of monodomain gamma-Fe2O3 spherical nanoparticles with different thicknesses has been investigated by magnetometry and Monte Carlo (MC) simulations. Using the structural information of the arrays obtained from grazing incidence small-angle X-ray scattering and scanning electron microscopy together with the experimentally determined values for the saturation magnetization and magnetic anisotropy of the nanoparticles, we show that MC simulations can reproduce the thickness-dependent magnetic behavior. The magnetic dipolar particle interactions induce a ferromagnetic coupling that increases in strength with decreasing thickness of the array. The 2D to 3D transition in the magnetic properties is mainly driven by a change in the orientation of the magnetic vortex states with increasing thickness, becoming more isotropic as the thickness of the array increases. Magnetic anisotropy prevents long-range ferromagnetic order from being established at low temperature and the nanoparticle magnetic moments instead freeze along directions defined by the distribution of easy magnetization directions.

National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-88309 (URN)10.1039/c2nr33013j (DOI)000313803000019 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

AuthorCount:13;

Available from: 2013-03-19 Created: 2013-03-12 Last updated: 2017-12-06Bibliographically approved

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