gradient coils probe
field coils glas tube to cryostate sample
Figure 3.16: AGM z-axis probe (only parallel) with glas tube.
up in an exsiccator. It is important to be cautious not to touch the sample with anything that could lead to magnetic contamination such as stainless steel tweezers. The abrasion is sufficient enough to affect the measurement. Therefore nonmagnetic titanium or plastic tweezers have to be used to handle the samples.
3.8 Infrared spectroscopy
IR spectroscopy is used to identify a substance and its structure. For measurement of an IR spectrum, the substance is filled in a cuvette. A beam of infrared radiation is passed through the sample and the absorption is measured for each wavelength. Infrared light spectrum covers wavelengths from 0.8µm to 500 µm. This corresponds to wavenumbers from 12500cm−1 to 20cm−1 including near and far infrared. Wavenumbers< 4000cm−1 down to about30cm−1are commonly used [60].
Figure 3.17: Schematic diagram of a double-beam infrared spectrophotometer [60].
A molecule can absorb energy from an electromagnetic field of a light wave if one of its vibrational frequencies matches the frequency of the incident wave. The position, width and intensity of the absorption bands are characteristic for a molecule. The intensity Iof the outgoing light wave is reduced compared to the incoming light intensityI0depending on the thicknessd, the concentrationcand the extinction coefficientεof the sample material.
It can be described by the Beer-Lambert law
I=I0e−εcd . (3.4)
The transmissivity T is given by
T = I
I0 . (3.5)
The whole molecule, in the case of small molecules, or parts of the molecule, in the case of large molecules, are excited into higher quantum states. These can be electronic, vibra-tional and rotavibra-tional energy levels. A state can be only excited if the vibration is infrared active, which means if a dipole moment is induced. Stretching vibrations 8, bending and deformation vibrations 9 and libration as well as mixing forms can occur.
For larger molecules the increasing number of atoms causes an increase in the number of vibrations. Symmetry and degeneracy on the other hand reduce the quantity of vibrations.
In these large molecules many groups act isolated, so that vibrations are localized only in parts of the molecule. Several functional groups have rather unique vibration transitions.
This means compounds with groups in common show absorption bands in the same region.
To identify a substance or gain information about its structure the obtained spectrum is compared to reference spectra [61, 62, 60, 63].
8symmetric and antisymmetric stretching vibration, breathing vibration
9symmetric and antisymmetric: wagging vibration, rocking vibration, torsional vibration, twisting vibra-tion
4 Cobalt Nanoparticles - synthesis and characteristics
Cobalt nanoparticles offer a wide range of possibilities for application.
The influence of surfactants on particle properties during fabrication is described in the first part of this chapter. Examples for size distribution, shape and crystallinity are introduced.
In table 4.1 the saturation magnetisation Msand superparamagnetic limit rspfor dif-ferent phases of cobalt, iron, iron oxide and two iron-cobalt alloys are given.
Cobalt exhibits an saturation magnetisation lower than iron but greater than iron ox-ide. According to the lower magnetisation of iron oxide, which is currently widely used in nanoparticles for several applications, e.g. Fe3O4 particles in commercial ferrofluids for use in stepper motors [8], cobalt offers an opportunity to fabricate nanoparticles with a higher saturation magnetisation and a smaller volume due to a higher magnetic moment. A high magnetic moment joined with a small volume is desireable for many applications. An example for such an application is the use in cells without size-related rejection combined with good reaction to outer fields [11]. As another example, the high coercivity makes it interesting for application in magnetic recording media [64].
material Ms[emucm3] rsp[nm] at RT
bcc-Fe 1740 8.0
fcc-Co 1420 7.9
hcp-Co 1400 3.9
Fe50Co50 1910 11.8 Fe70Co30 1933 8.9
Fe3Co 1993 8.5
Fe3O4 415 14.0
Fe2O3 380 17.45
Table 4.1: Saturation magnetisation and superparamagnetic limit [19, 20, 21, 22, 23, 24]
The magnetic properties of nanoparticles dependend on the microstructural order within the particle. Hcp-Co and fcc-Co differ in their radius for the superparamagnetic limit.
The cobalt nanoparticles (Co NP) were stabilized during their fabrication with surfactant molecules as described in section 2.2.3. Their surface is covered with amphiphilic molecules afterwards and it is known that nanoparticles oxidize despite their stabilizing surfactant shell. It bears an interesting question to what extend the oxidation of the nanoparticles is influenced by the coverage of the surface by these molecules. A surfactant, for example with a carboxylic headgroup binding to a metallic nanoparticle surface leads to an initial oxidation of one monolayer of Co atoms at the particle surface [18].
The (microstructural) parameters of the synthesized particles differ depending on sev-eral parameters during synthetization such as used surfactants, surfactant and precursor concentration, temperature, solvents, precursors, time interval during injection of the pre-cursor solution into the surfactant solution as well as largeness of the batch. Fluctuations of one or more of the previously mentioned parameters during the steps of particle forma-tion have an important influence as well [29, 25, 38]. An increase of the heating rate, as described in [38], leads to an increase in the nucleation rate and results in smaller particles.
The parameters have to be kept constant for reproductible results.
To gain information concerning the influence of the surfactants on the oxidation process, it is necessary to investigate particles with comparable parameters such as same diameter, same shape and same crystal structure [and similar concentrations], where only the stabi-lizing surfactants are varied [18].
It is not possible to prepare particles with several different surfactants, which are similar in all these properties because of the influence of the surfactants itself during particle forma-tion [29, 3, 38]. To overcome this problem, the method of a surfactant exchange is chosen to examine particles with different surfactants.
The nanoparticles fabricated in our chemistry lab were characterized in size distribution, shape, self-assembly and crystallinity 1 by charting TEM images. The average diameter and variance of a batch of NPs was estimated by measuring diameters of several particles of a TEM brightfield image with the program AnalySIS by Soft Imaging Systems GmbH. A histogram of the obtained particles was compiled and fitted by a gaussian curve2 with the program Igor Pro 5.05A by WaveMetrics,Inc.. The crystallographic phase is determined by evaluation of HRTEM - data, FFT and XRD curves.
HRTEM analysis makes it possible to gain information of the structure within a single particle or of a few particles while XRD analysis results in an average information of a large amount of particles.
4.1 Cobalt nanoparticles - overview
The stabilizing surfactants were varied in particle fabrication. A short overview over typical outcomes of some of the syntheses of particles stabilized directly with different surfactants is displayed. For the particles displayed in figure 4.1 oleylamine was used for stabilization instead of trioctylphosphinoxid (TOPO). These particles have an average diameter hDi= 11.59 nm ± 1.53 nm. Apart from spherical particles a large amount of flat triangles, truncated triangles, hexagons and nearly circular discs are visible. These flat particles exhibit a nearly uniform contrast. The particles self assemble into larger fields, where larger particles are flanked by smaller ones. The particles depicted in figure 4.2 were stabilized with TOPO and have an average diameter hDi= 15.56 nm ± 1.85 nm. Here only a few flat triangles and truncated triangles can be found. The contrast in the thicker circular particles that are lying upon another indicates that these particles are rather discs than spheres. These particles form mostly bead-on-a-string like structures, that seem to entangle and even cross each other. The particles displayed in image 4.3 were stabilized with a combination of oleylamine and oleic acid and exhibit an average diameterhDi= 5.99 nm ± 1.11 nm. Larger and smaller spherical particles are visible as well as larger discs
1if assignable
2ffitgauss=y0+Aexp −x−xw02
,y0=0
4.1 Cobalt nanoparticles - overview
Figure 4.1: Particles stabilized with oleylamine, average diameterhDi= 11.59 nm ± 2.35 nm, N=248.
Figure 4.2: Particles stabilized with TOPO, average diameter hDi= 15.56 nm±3.42 nm, N=274.
Figure 4.3: Particles stabilized with oleylamine and oleic acid, average diameterhDi= 5.99 nm±1.23 nm, N=842.
that are aligned mostly upright flat side to flat side next to each other in small rows on the carbon foil of the TEM grid. The discs are mostly circular as can be seen in horizontal stacks. The height of the discs was estimated by measuring a smaller amount of upright discs. Generally these particles have a height between one half radius and one radius. A few triangles, that are slightly smaller than the circular discs, can be found.
It can be clearly seen that the use of different surfactant molecules results in different di-ameters, standard deviations of the particle diameter distribution, morphologies that occur in the batch and differences in the self assembly and therefore in the particle interaction and magnetic properties.