Oxidation of surfactant stabilized magnetic cobalt nanoparticles [Elektronische Ressource] / Britta Vogel. Fakultät für Physik

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• Physik

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• Publié par: universitat_bielefeld
• Publié le: 01 janvier 2011
• Nombre de lectures: 34
• Poids de l'ouvrage: 9 Mo

Extrait

Dissertation
Oxidation of
surfactant stabilized
magnetic cobalt nanoparticles
vorgelegt von
Britta Vogel
July 18, 2011
Universität Bielefeld
Fakultät für PhysikFor AleksErklärung
Hiermit erkläre ich, dass ich die vorliegende Arbeit selbständig verfasst und keine anderen
als die angegebenen Hilfsmittel verwendet habe.
Bielefeld, 18. Juli 2011
Gutachter:
Prof. Dr. Andreas Hütten
Prof. Dr. Dario Anselmetti
Datum des Einreichens der Arbeit: 22. Juli 2011Contents
0 Introduction 1
1 Theoretical Background 3
1.1 Magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1 Forms of magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.2 Magnetism on nanoscales . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2 Formation of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2.1 Wet chemical approach . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2.2 Kinetical description of particle formation . . . . . . . . . . . . . . . 10
1.2.3 Thermodynamical description of particle formation . . . . . . . . . . 11
2 Synthesis of cobalt nanoparticles and oxidation 13
2.1 Synthesis of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.1 Purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2 Surfactants and surfactant exchange . . . . . . . . . . . . . . . . . . . . . . 14
2.2.1 Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2.2 Surfactant exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.3 Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3 Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3.1 Oxidation of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3.2 Mathematical description of the oxidation process. . . . . . . . . . . 20
3 Devices 24
3.1 TEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2 SEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.3 STEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3.1 Image modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3.2 Lens aberrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.4 EDX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.5 FIB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.5.1 LMIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.5.2 Interactions of electrons and ions with the sample . . . . . . . . . . . 33
3.5.3 Ion etching and sputtering . . . . . . . . . . . . . . . . . . . . . . . . 34
3.5.4 Gas assisted ion beam etching . . . . . . . . . . . . . . . . . . . . . . 34
3.5.5 Gas assisted ion beam deposition . . . . . . . . . . . . . . . . . . . . 34
3.5.6 Sample manipulation with the micromanipulator . . . . . . . . . . . 35
3.6 XRD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.7 AGM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.7.1 Function of an AGM . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.7.2 Low temperature setup . . . . . . . . . . . . . . . . . . . . . . . . . 38
viContents
3.8 IR-spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4 Nanoparticles - characteristics 41
4.1 Cobalt nanoparticles - overview . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.2 Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.3 Crystallinity of Co particles . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.4 Oxidation of particles prepared with different surfactants . . . . . . . . . . . 45
5 Surfactants and surfactant exchange 47
5.1 Basic particles prepared with trioctylphosphin oxide (TOPO) . . . . . . . . 47
5.2 Surfactant exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.2.1 Used surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
5.2.2 Amine group as head molecule . . . . . . . . . . . . . . . . . . . . . 51
5.2.3 Carboxyl group as head molecule . . . . . . . . . . . . . . . . . . . . 56
5.2.4 Size reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2.5 Inter particle distance . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.2.6 Convolution of the surfactant molecules . . . . . . . . . . . . . . . . 64
5.2.7 Change of magnetic properties . . . . . . . . . . . . . . . . . . . . . 65
5.3 Comparison - surfactant exchange on TOPO particles . . . . . . . . . . . . 68
5.4 Basic particles prepared with oleylamine . . . . . . . . . . . . . . . . . . . . 70
5.4.1 Used surfactants and size reduction . . . . . . . . . . . . . . . . . . . 70
5.4.2 Conclusion for self-assembly changes by surfactant exchange . . . . . 71
5.4.3 Change of magnetic properties . . . . . . . . . . . . . . . . . . . . . 72
5.5 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6 Oxidation at room temperature 74
6.1 Calculation of the effective magnetic particle volume and radius . . . . . . . 77
6.2 Second surfactant exchange on TOPO . . . . . . . . . . . . . . . . . . . . . 80
6.3 Surfactant exchange based on oleylamine . . . . . . . . . . . . . . . . . . . . 81
6.4 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
7 Temperature dependence 84
◦7.1 Particles stored at a temperature of −18 C . . . . . . . . . . . . . . . . . . 84
◦7.2 Particles stored at a temperature of 48 C . . . . . . . . . . . . . . . . . . . 88
◦7.3 Particles stored at a temperature of 80 C . . . . . . . . . . . . . . . . . . . 88
◦7.4 Particles stored at a temperature of 121 C . . . . . . . . . . . . . . . . . . . 91
◦7.5 Particles stored at a temperature of 180 C . . . . . . . . . . . . . . . . . . . 94
◦7.6 Particles stored at a temperature of 300 C . . . . . . . . . . . . . . . . . . . 94
7.7 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8 Temperature effects on crystallinity and shape 100
9 Oxidation measurements during the first 30 minutes after fabrication 106
9.1 AGM oxidation setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
9.2 Used surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
9.3 Effects of the surfactant exchange . . . . . . . . . . . . . . . . . . . . . . . . 108
9.3.1 Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
9.4 Oxidation during the first thirty minutes after fabrication . . . . . . . . . . 110
viiContents
9.4.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
9.5 Crystallinity of particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
9.6 Oxide shells - conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
10 Oxidation behaviour in dependence on shape 124
10.1 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
10.2 Finite elements simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
10.3 Volume consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
10.4 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
10.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
11 Oxidation of nanoparticle clusters 131
11.1 Older Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
11.2 Newer Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
11.3 Effective oxygen diffusion in clusters . . . . . . . . . . . . . . . . . . . . . . 141
11.3.1 Oxygen diffusion process in the case of one nanoparticle . . . . . . . 141
11.3.2 Oxidation process in the case of nanoparticle clusters . . . . . . . . . 144
11.3.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
12 Summary 152
.0.4 Conference Contributions . . . . . . . . . . . . . . . . . . . . . . . . VIII
.0.5 Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII
viii0 Introduction
“There is plenty of room at the bottom” was the name of a talk given by Richard Feynman
at the annual meeting of the American Physical Society in 1959.
Today the main idea is still more topical than ever. Research in the field of nanoscale
structures is booming in modern physics because nanomaterials exhibit outstanding prop-
erties. The surface to bulk ratio is extremely large and, therefore, surface effects gain in
importance, leading to unique magnetic, electronic and optical effects [1,2,3]. The boom
has been made possible by the fast development of technology during the last few decades,
allowing the visualization of structures in the nanometer range. Improved methods to
fabricate nanoparticles have contributed to this development as well. Nanoparticles and
physics of nanostructures already play an important role in economy. Products built of
or containing nanoparticles are widely spread although the risks and advantages have not
been evaluated completely yet [4].