Samples of particles after a surfactant exchange were stored at temperatures of −18◦C, room temperature (∼21◦C),48◦C,80◦C,121◦C,180◦C and300◦C.
For temperatures of −18◦C, room temperature (∼ 21◦C) and 48◦C a passivating oxide layer was formed. For the higher temperatures the particles oxidized completely. No clear dependence of the oxidation on the covering surfactant could be found in the investigated time spans starting with particles about 2 hours to half a day old after fabrication.
For these long time spans only the diffusion of oxygen in the nanoparticles could be ob-served. The curves of the decrease of the saturation magnetic moment for all temperatures for four surfactants are displayed in figures 7.12, 7.13, 7.14 and 7.15 exemplary.
7.7 Conclusion
Figure 7.22: TOPO - all temperatures, sat-uration magnetic moment normalized.
Figure 7.23: Oleylamine - all temperatures, saturation magnetic moment normalized.
Figure 7.24: Oleic acid - all temperatures,
saturation magnetic moment normalized. Figure 7.25: 1-pyrenebutyric acid - all temperatures, saturation magnetic moment normalized.
cobalt nanoparticles in dependence of temperature
After finishing the oxidation experiment, TEM images of the particles stored at room tem-perature and stored at 180◦Cwere taken. It was found that the particles that were stored at 180◦Cfor two hours had undergone a change in their crystal structure and their shape from a round shape to a more irregular round shape. The result is displayed in figure 8.1.
An image of the original particles prior to oxidation is displayed next to the image for com-parison in figure 8.2. The particles oxidized at room temperature are displayed in figure 8.3. The particles stored at room temperature show no change in their crystal structure.
The size distribution of the particles after oxidation shows that the average diameter has increased in both cases. For octadecylamine the diameter has grown from hDi= 10.91 nm
± 2.38 nm at the beginning to hDi= 13.69 nm ± 2.73 nm during the storage at 180◦C.
The difference of these diameters denotes the increase of the average diameter by volume expansion and therefore an oxide shell thickness of 2.78 nm. In the case of the particles oxidized at room temperature, the diameter has increased to hDi= 11.62 nm ±1.19 nm, which is equivalent to a diameter increase by volume expansion of 0.71 nm.
The particles from the experiment of the surfactant exchange based on the particles fab-ricated with oleylamine were stored at180◦Cwith a similar result. Images of the particles before and after the experiment are displayed in figures 8.7 and 8.8. The particle diam-eter has increased from hDi= 11.86 nm ± 2.63 nm to hDi= 16.81 nm ± 3.38 nm, which denotes anincrease of the diameter by volume expansion of 4.95 nm; the size distribution has broadened, see figures 8.9 and 8.10.
The heated particles of the first surfactant exchange have been investigated by HRTEM
1. The HRTEM images of the particles stored at room temperature displayed in figure 8.11 and the ones stored at 180◦Cshown in image 8.12 show clearly that the particles are not hollow, but consist of several crystallites, which are differently orientated and show therefore a different contrast in the electron beam.
If the measured values for the average diameter hDi and the measured volume expan-sion are compared to a calculated values based on the initially measured average diameter, depicted in table 8.1, it is visible that the calculated volume expansion fits to the mea-sured data. For the particles fabricated with TOPO where the surfactant was exchanged to dodecylamine an average diameter hDi= 10.91 nm ± 2.38 nm was measured. After complete oxidation at a storage of 2 hours at 180◦Cthe average diameter had increased to hDi= 13.69 nm ±2.73 nm. The corresponding calculated average diameter for completely
1Images taken by Inga Ennen at TU Wien
Figure 8.1: Particles stored at180◦Cfor 2h.
Figure 8.2: Particles directly after surfactant exchange.
Figure 8.3: Particles stored at room temperature.
Figure 8.4: Size distribution of fresh particles,hDi= 10.91 nm ±1.35 nm, N=208.
Figure 8.5: Size distribution of particles stored at 180◦C, hDi= 13.69 nm ± 2.73 nm, N=246.
Figure 8.6: Size distribution of particles stored at RT, hDi= 11.59 nm ±1.02 nm, N=99.
Figure 8.7: Particles before oxidation.
Figure 8.8: Particles stored at180◦Cfor 2h.
Figure 8.9: Size distribution of fresh
parti-cles,hDi= 11.86 nm ±2.64 nm, N=255. Figure 8.10: Size distribution of particles stored at180◦C,hDi= 16.81 nm±3.38 nm, N=122.
Figure 8.11: Particles stored at room temperature.
Figure 8.12: Particles stored at 180◦Cfor 2h.
oxidized particles ishDi= 15.27 nm±3.33 nm, which is greater than the measured values, but still within the error range. The particles fabricated with oleylamine have an average diameterhDi= 11.86 nm±2.63 nm directly after fabrication. After being stored at180◦C for two hours these particles are oxidized completely as well and the average diameter has increased to hDi= 16.81 nm±3.38 nm. The calculated diameter for completely oxidized particles ishDi= 16.60 nm±3.68 nm. In this case the measured and calculated values fit very well.
octadecylamine oleylamine fresh particleshDi [nm] 10.91 ±2.38 11.86 ±2.63 oxid. room temperaturehDi [nm] 11.59 ±1.19
-oxid. 180◦ChDi [nm] 13.69 ±2.73 16.81 ±3.38
diameter increase RT measured [nm] 0.71
-diameter increase180◦Cmeasured [nm] 2.78 4.95 complete oxidation calculated hDi [nm] 15.27 ±3.33 16.60 ±3.68
diameter increase calculated [nm] 4.36 4.74
Table 8.1: Change of the effective magnetic volume and corresponding Co core radius;
diameter increase.
first 30 minutes after fabrication
In the previous measurements, where the particles were initially measured after an expo-sure time to air between one hour and half a day after fabrication, no correlation between the covering surfactant and the oxidation behaviour was found. From [45] and the pre-vious investigations it was deduced, that an influence of the surfactant can be discovered only during the first 30 minutes of oxidation at room temperature. Therefore the sample preparation and especially the transportation had to be modified. The particles were again prepared in an argon atmosphere to prevent oxidation as described in chapter 2. After the particle suspension was dropped onto the piece of wafer under argon (instead of in air) to shield the particles from oxygen, the gas was removed and the solvent evaporated fast in vacuum. For the transport to the laboratory, the flask with the sample was flooded with argon again and sealed, to keep out oxygen. Te sample was removed from the flask in the laboratory, placed on the probe and inserted into the helium gas stream.
The time the sample was exposed to air during installation was measured and was in all cases less than one minute.
9.1 AGM oxidation setup
Figure 9.1: Oxidation setup of the AGM.1
To achieve detailed information about the oxidation of the particles during the first thirty minutes, the AGM setup was modified in a way that was based upon the low temperature setup displayed in figure 3.16 to allow a controlled exposure to air of the sample. The glas tube with an aluminum holder which is connected to the cryostate in the low temperature
1vgl. Seyr (1980), S. 134