THE SELF-ASSEMBLY PROCESS OF COBALT NANOPARTICLES ON HOPG 1Institut
M. Varón1, Ll. Balcells2, B. Martínez2 and V. Puntes1
Català de Nanotecnologia, Campus UAB, 08193 Bellaterra, Spain. 2Institut de Ciència de Materials de Barcelona(ICMAB-CSIC) Campus UAB, 08193 Bellaterra, Spain.
HOPG DEPOSITION
INTRODUCTION
Optical Microscopy
Colloidal dispersed nanoparticles (NP) self assemble into complex structures when they are segregated from the solvent either by evaporation or precipitation. Different micro and macroscopic structures formed by nanoparticles are observed as a result of the balance between electrostatic forces, surface tension, entropy, topography, substrate affinity, among others, and evidently, the size, shape and concentration of the particles. In the case of magnetic nanoparticles, the magnetic properties arise from the competition between short and long-range interactions. These competing interactions favor parallel alignment of distant spins, forming magnetic domains in 'bulk' magnets. In addition, the
SEM-EDAX(EDS)
dipolar magnetic interactions, add a new term in the interactions balance. Up to this point , studies have been developed onto TEM substrates (copper grids coated
Co
with carbon) Our aim is to study the self-assembly onto more technological interesting substrates. Substrate
Highly Ordered Pyrolytic Graphite (HOPG) • Exfoliable • Smooth and clean surface • Oriented surfaces may influence the assembly of nanoparticles • Hydrophobic substrate (compatible with Oleic Acid)
Magnetic Measurements Magnetic measurements show the magnetic character of the sample and a smaller coercivity in the case of the NPstructures probably due to the demagnetizing character of the dipolar interactions.
0.0006 10K
0.002
10K
0.0004
0.001
0.0002 M (emu)
M (emu)
RT 0
RT
0
-0.0002
-0.001
-0.0004
SYNTHESIS
-0.002 -3 10 4
-2 10 4
-1 10 4
0 H (Oe)
1 104
2 10 4
-0.0006 4 4 4 4 -4 10 -3 10 -2 10 -1 10
3 10 4
1 10
4
2 10
4
3 10
4
4 10
4
5 10-5
0.00035
Synthesis of cobalt nanoparticles was performed under argon atmosphere and high
0 H (Oe)
Co rice-like structures
Co Nanoparticles 0.0003
4 10
-5
3 10
-5
0.00025
containing different surfactants. This produces a temporarily discrete homogeneous
M (emu)
100Oe M (emu)
temperature. Involves the rapid decomposition of cobalt carbonyl in a coordinated solvent
0.0002 0.00015
100Oe
2 10-5
0.0001
1 10
-5
-5
5 10
0
nucleation employed for the production of monodisperse metallic passivated nanoparticles.
0 0
50
100
150 T (K)
200
250
300
0
50
100
150 T (K)
200
250
300
AFM-MFM Measurements
Co ~ 10 nm
4.0µm
Small magnetic particles can be considered as a single domain, and the particle behaves as a single magnetic dipole. The magnetic dipole of the particles will be free to rotate (SUPERPARAMAGNETISM) or blocked in the anisotropy direction (FERROMAGNETISM). Superparamagnetic nanoparticles
AFM
AFM
MFM
MFM shows that the observed structures are magnetic. No magnetic nanostructures are observed inside the microstructures (Black correspond to attraction)
Application of different Magnetic Fields Optical Microscopy
Ferromagnetic nanoparticles
On a strong magnetic field ~ 8 nm
~ 12 nm
Magnet
On a weak magnetic field
~ 16 nm
We work with small and medium size particles, which start presenting ferromagnetism but without being the dominant phenomenon. Therefore, it allows easier modification of other parameters.
Magnets
We work at room temperature. In a homogeneous magnetic field
CONCLUSIONS
ACKNOWLEDGMENTS
Cobalt nanoparticles can self assemble into large wires depending on the substrate.
Inorganic Nanoparticles Group
The orientation of the wires can be manipulated with magnetic fields during the
Magnetic Materials and Functional Oxides group
evaporation process.
Ministerio de Educación y ciencia. MAT2006-13572-C02-02