Synthesis of Cobalt Np and their Self-Assembly on HOPG by victor puntes

MÀSTER EN NANOTECNOLOGIA PROGRAMA OFICIAL DE POSTGRAU D’ENGINYERIA

MEMÒRIA DEL TREBALL DE MASTER Especialitat Nanociència i Nanotecnologia

Synthesis of Cobalt Nanoparticles and their Self-Assembly on Highly Oriented Pyrolitic Graphite (HOPG)

Autora: Miriam Varón Izquierdo Director: Victor Puntes Departament/Institut del director: Inorganic Nanoparticles Group/Institut Català de Nanotecnologia Data: 20/02/2009

CONTENTS

1. ABSTRACT.................................................................................................... 1 2. INTRODUCTION............................................................................................ 3 2.1. MAGNETIC PROPERTIES OF SOLIDS.................................................. 3 2.2. MAGNETIC NANOPARTICLES............................................................... 4 2.3. SELF-ASSEMBLY ................................................................................... 8 2.4. CHARACTERIZATION .......................................................................... 12 2.4.1. Optical Microscope.......................................................................... 12 2.4.2. Transmission Electron Microscopy.................................................. 12 2.4.3. Scanning Electron Microscope - EDX ............................................. 13 2.4.4. SQUID Magnetometry ..................................................................... 13 2.4.5. Atomic Force Microscope (AFM)/Magnetic Force Microscope (MFM) .................................................................................................................. 14 3. RESULTS and DISCUSSION ...................................................................... 17 3.1. SYNTHESIS OF COBALT NANOPARTICLES ...................................... 17 3.2. Deposition onto HOPG .......................................................................... 18 3.3. Regarding the formation mechanism ..................................................... 22 4. CONCLUSIONS........................................................................................... 29 5. REFERENCES............................................................................................. 33 5. ACKNOWLEDGMENTS .............................................................................. 35 6. ANNEX I....................................................................................................... 37

1. ABSTRACT Superparamagnetic single crystal Cobalt nanoparticles (Co NPs), with single magnetic domain, of 6 nm and 8 nm in diameter evaporated onto Highly Pyrolytic Oriented Graphite (HOPG), spontaneously self-assemble into super structures with an elongated shape. These structures have been studied by optical and scanning electron microscopies (SEM), atomic and magnetic force microscopy (AFM and MFM), electron dispersive x-ray (EDX) analysis and SQUID magnetometry. It was proposed that the weak dipolar interactions between superparamagnetic dipoles of the Co NPs are responsible for the formation of these structures. This could be explained because dipolar magnetic interactions are strong enough to influence the general process of self assembly dominated by Van der Waals forces between nanoparticles and between nanoparticles and substrate, and also due to the evaporation dynamics of the experiment.

2. INTRODUCTION 2.1. MAGNETIC PROPERTIES OF SOLIDS Depending on their response towards external applied fields, materials can be classified as diamagnetic, paramagnetic, ferromagnetic, ferrimagnetic and antiferromagnetic, according to the arrangement of their magnetic dipoles in the absence or presence of an external magnetic field. Diamagnetic material: It is a material with no unpaired electrons in the orbital shells (magnetic dipoles) resulting in no net magnetic moment in the absence of an external field. The magnetization of a diamagnet responds in the opposite direction to the external field. Paramagnetic material: Some of the atoms or ions in the material have a net magnetic moment due to the unpaired electrons in partially filled orbitals, even in the absence of an applied magnetic field. When a magnetic field is applied, the dipoles tend to align with the applied field, resulting in a net magnetic moment in the direction of the applied field (attraction). Ferromagnetic material: A ferromagnetic material has unpaired electrons (magnetic dipoles) resulting in a permanent magnetic dipole (both in presence or absence, after being magnetized of an external field) and exhibits a long arrangement at the atomic level. Ferrimagnetic and antiferromagnetic materials: Like ferromagnetic ones, these materials have permanent magnetic dipoles. The magnetic structure is composed of two magnetic sublattices (called A and B). In ferrimagnets the magnetic moment of A and B are not equal and result in a net magnetic moment. If the A and B sublattices are exactly equal but opposite, the net moment is zero and this type of magnetic ordering is called antiferromagnetism.

Superparamagnetic material: A superparamagnetic material is composed of small ferromagnetic clusters (crystallites), being so small that these can randomly flip direction under thermal fluctuations. As a result, the material as a whole is not magnetized except under an externally applied magnetic field (in that respect, it is like paramagnetism).

2.2. MAGNETIC NANOPARTICLES A magnetic nanoparticle is a structured material formed at least by hundreds or thousands of atoms. These atoms are organized within a crystalline structure that determines their size and magnetic properties. A magnetic nanoparticle has a net moment that is the sum of the spin of all atoms. Therefore, the magnetic moment depends of the size and shape of the particle (Figure 2.1). The magnetic moment of the particle aligns spontaneously in one direction, corresponding to the easy magnetization axis and the direction that minimizes the anisotropy energy. Some materials possess any kind of anisotropy that affects the behavior of the magnetization.

Spherical Particles Diameter (nm) Volume (nm3) Magnetic Moment (J/T) 3

14.1

2.0 e-20

65.4

9.3 e-20

268

3.8 e-19

524

7.4 e-19

1770

2.5 e-18

Figure 2.1: Magnetic moment for cobalt nanoparticles depending on the size.

Inside a material the spins form domains, where the individual moments of the atoms are aligned with one another. The regions separating different magnetic domains are called domain walls where the magnetization rotates coherently

from the direction in one domain to that in the next domain. The existence of magnetic domains is a result of the energy minimization. The formation of domains is highly related with the size of the particle. In one hand, in big particles the energy considerations suits the formation of domains. On the other hand, when the size of the particle decreases the number of domains also decreases and becomes a single domain. If the particle size is reduced, there is a critical volume below which it costs more energy to create a domain wall than to support the external magnetostatic energy (stray field) of the single-domain state. This critical diameter typically lies in the range of a few tens of nanometers, depends on the material and it is influenced by the contribution from various anisotropic energy terms.

Hysteresis curves Ferromagnetism

Superparamagnetism Superparamagnetic

Figure 2.2: Different magnetic responses in the hysteresis curves when varying the particle size.

Magnetic nanoparticles have been the subject of extensive research because they have the potential to be utilized in several applications such us ultra-highdensity recording media,1,2 contrast agent in magnetic resonance imaging,3 drug delivery4 and as single electron transistors.1,2 The development, characterization and exploitation of novel materials based on the assembly of molecular components (such us nanoparticles) is highly active and rapid expanding field. Their properties arise from the large amount of atoms present on the surface of the nanoparticle and from the finite number of atoms in each crystalline core. It was in magnetic materials that such finite-size effects were recognized, with more subtle effects noted, later in non-magnetic metals such as semiconductors. The focus in this area is still on building solids with tailored magnetic properties and to make a breakthrough in the principal

challenge facing work in low dimension structures; i.e., how to make the transition from individual nanoscale behaviour to bulk macroscopic phenomena and properties. Recently it has been shown that nanocrystals can self assemble in 2D and 3D arrays with long range translational and even orientational order. Nanocrystals can be organized into close packed arrays simply by evaporating the solvent from a hydrophobic sterically stabilized dispersion, provided that the size distribution of the particles is sufficiently narrow (i.e., a standard deviation about the mean diameter of less than 10%). Thus, these arrays may reproduce the nanoscale properties in macroscopic materials through homogeneity and periodicity, and elicit collective electronic, magnetic, and optical behavior resulting from the relative positioning of the nanocrystals in the array. In ferromagnetic materials, the dipolar magnetic interactions between particles add a new term in the energy balance, and they are not screened or affected by the surrounding materials (solvent, substrate, surfactants, etc.). In particular, as the mean dipolar energy increases with the particle volume, the thermal energy at RT becomes comparable to the dipolar energy of two sticking Co NPs with diameter size of about 12 nm. A collective behavior due to dipolar interactions has been observed in the low susceptibility measurements corresponding to a highly ordered fine particles system. Magnetic metal particles experience strong Van der Waals attractions which, combined with their magnetic dipole interactions, makes the stabilization of these systems very challenging. Highly ordered structures of nanocrystals such as two or three dimensional assemblies are also interesting because these nanocrystals could act as building blocks in future devices for potential applications in nanoelectronics and spintronics. In this work it was observed the power of spontaneous SA when multiple forces are balanced (including isotropic and anisotropic) which reminds the observed complex biological self organization able to reconstruct a living cell after duplication of its components during mitosis. This is relevant, not only to build up super-structures for specific applications, like compact monolayers for 6

magnetic recording media, but also to elucidate the behaviour of magnetic nanoparticles in complex liquid media, that are often out of equilibrium, as in biology. Thus, a minimal amount of solvent has to be present even in the late stages of the SA, at whose moment the liquid film breaks into droplets that later evaporate. Even if this has an impact on the SA, it is clearly not the driving mechanism in this case nor in the other observed formation of super-crystals of nanoparticles. Synthesis Various methods have been developed for the synthesis of colloidal nanocrystals.5-7 In particular, the thermal decomposition of metal carbonyls is known to produce well-defined metallic nanocrystals.8 The properties of the individual particles as well as their mutual interactions determine important features of the nanoparticles systems. Since magnetic properties are highly dependent on the size, shape, crystallinity and surface state of the nanocrystals, their controlled synthesis with a narrow size distribution and uniform shape remains an important issue. By systematically increasing the particle size, TEM images show an abrupt transition from SA monolayers to randomly oriented linear aggregates and branched chains or networks,8 witnessing the rise of stability and intensity of the magnetic moment (Figure 3.1). organometallic reagents

ATOMS

Nucleation

Growth of particle by condensation

Figure 2.3: General scheme of nanoparticles synthesis by thermal descomposition

In this work, spherical Co NPs in the epsilon (Îľ) crystalline face coated with oleic acid were synthesized. Synthesis of Îľ-Co NPs was performed under argon atmosphere at high temperature, which involved the thermal decomposition of di-cobalt octa-carbonyl Co2(Co)8 in a mixture of a hot organic solvent (dichlorobenzene) and surfactants (oleic acid and trioctylphosphine oxide), according to a method previously reported.8,9

Reagents •

Organometallic reagent:

•

Surfactants: Oleic Acid (OA) (hydrophobic), Trioctylphosphine oxide

Co2(CO)8

(TOPO)

•

Organic Media:

Dichlorobenzene (DCB)

Figure 2.4: General scheme of cobalt nanoparticles

2.3. SELF-ASSEMBLY One of the most important reasons for the study of SA is its central role in life. For example, the components of a cell replicate and assemble into another cell during mitosis.

SA is a spontaneous organization of components into patterns and structures without the human intervention, and is common throughout nature and technology. This is one of the few practical strategies for making ensembles of nanostructures and it will therefore be an essential part of nanotechnology. The concept of SA is increasingly used in many disciplines with a different nuances in order to break natureâ&#x20AC;&#x2122;s code of SA.10 SA structures are highly present in the nature. Peptide chains fold to form proteins and enzymes, single-stranded DNA finds its complementary strand and forms a double-stranded helix, and phospholipids align themselves in order to make up cell walls. The process of SA is facilitated by both specific molecular interactions and the drive to minimize the interaction energy between molecules. In addition, it is the only practical approach for building a wide variety of nanostructures.

Figure 2.5: A) A self-assembled peptide amphiphile nanofibers. B) A Self-assembly of gold nanoparticles.

Dispersed colloidal nanoparticles (NPs) self-assemble into complex structures when segregated from the solvent either by evaporation or precipitation. Different micro and macroscopic structures like opals, fractals, mixed structures and other forms based in NPs have been observed11 as a result of the balance between electrostatic forces, surface tension, entropy, topography, substrate affinity and, most importantly, the size, shape and concentration of the

particles.12 This is a key issue for applications and for fundamental investigations. Different effects involved in the SA process of magnetic nanoparticles such as proximity (matrix), long-range interactions, long-range order, domain walls, hysteresis, collective behavior, percolation, correlated local and global perturbations, among others, are subjects of current investigation. In this context, magnetic dipolar interactions that are not screened or interfered by the medium are an interesting piece of puzzle to understand and control such processes. Making sure that the components assemble themselves correctly is not an easy task, because the forces at work are so small. Self-assembling particles can get trapped in undesirable conformations, making defects that are impossible to avoid.

Figure 2.6.: Example to understand the SA process.

Most synthesized particles contain a surfactant layer, which maintain them separated

some

nanometers.

This

means

that

interactions

between

nanoparticles, even in these dense assemblies, is negligible and their mutual interactions are, therefore, dominated by long-range dipole-dipole interactions. For nanostructured materials built from magnetic nanometric units, dipolar interactions play an important role in determining cooperative phenomena at the molecular scale and the final properties of the material.13 Dipolar materials display a for arranging themselves into highly non homogeneous structures. This is a consequence of the very strong anisotropy of the dipole-dipole interaction, which couples the orientations of the dipole moments with that of

the interparticle vector that is different from the isotropic Van der Waals interaction. For the past few years, various methods have been developed for the synthesis and the SA of magnetic nanocrystals. For example, Gao et al.14 produced uniform 3D super-structures of spherical cobalt nanocrystals by the interplay between dipolar interaction and applied magnetic field. Furthermore, Yin et al.15 reported three different kinds of self-assemblies of cobalt nanocrystals forming different structures attributed to the magnetic properties of cobalt nanoparticles with different size. Zeng et al.13 showed how SA of mixtures of magnetic nanoparticles make it possible to produce materials with excellent magnetic properties. Legrand et al.16 reported the formation of large-scale 3D superlattices of cobalt nanocrystals, applying a magnetic field perpendicular to the substrate during the deposition onto HOPG substrates. Sevchenko et al.17,18 obtained faceted triangular and hexagonal platelets, pyramids and tetrahedron with sizes between 10-30 Âľm, consisting of individual monodisperse FePt nanocrystals aligned in a 3D superlattice. Other examples include the formation of micrometric supercubes from nanometric iron cubes,19 all of them yielding different structures. In this study it was observed the influence of the substrate during the evaporation process, the NP-Substrate interactions and the formation of SA super-structures of magnetic NPs in the absence of an applied magnetic field. The study of the SA processes of cobalt nanoparticles coated with oleic acid onto HOPG and comparing with the SA on a carbon coated TEM grid, gives us the opportunity to study in detail the balance between NP-NP and NP-substrate interactions in the particular case where both particles and substrate are highly hydrophobic.

The

micro

self-assembled

structures

resulting

from

the

evaporation of a solution of superparamagnetic (SPM) Co NPs onto a HOPG substrate create a framework to study the magnetic properties of the material since self-assembled structures reflect information coded (as shape, surface properties, charge, polarizability, magnetic dipole, mass, etc.) residing in its individual components.

2.4. CHARACTERIZATION

2.4.1. Optical Microscope The optical microscope, often referred to as the "light microscope", is a type of microscope which uses visible light and a system of lenses to magnify images of small samples (micron sizes). An optical microscope Zeiss observer z1m was used in our studies.

2.4.2. Transmission Electron Microscopy The transmission electron microscope (TEM) operates on the same basic principles as the light microscope but uses electrons instead of light. What you can see with a light microscope is limited by the wavelength of light. TEM overcomes the limitations from the wavelength of light by using electrons as "light source" making it possible to get a resolution a thousand times better than with a light microscope. Objects can be observed down to a few angstroms (1010

m).

TEM is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as they pass through. An image is formed from the interaction of the electrons transmitted through the specimen, which is magnified and focused onto an imaging device, such as a fluorescent screen, as is commonly used in most TEMs, on a layer of photographic film, or to be detected by a sensor such as a CCD camera. TEM images show contrast in color due to the absorption of electrons in the material, the thickness of the film and the composition of the material. A Transmission Electron Microscopy JEOL 1010 with a Bioscan (Gatan) digital imaging system is used throughout our studies.

Figure 2.7: Transmission Electron Microscopy scheme

2.4.3. Scanning Electron Microscope - EDX The scanning electron microscope (SEM) is a type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity. X-rays, which are also produced by the interaction of electrons with the sample, may also be detected in an SEM equipped for energy-dispersive X-ray spectroscopy. The morphology and chemical composition of the Rice-Grain like structures were characterized with an Environmental Scanning Electron Microscope. The images were taken on an ESEM Quanta 200 FEI, XTE 325/D8395 with a beam energy of 10 kV and 15 kV.

Figure 2.8: Scanning Electron Microscopy scheme

2.4.4. SQUID Magnetometry SQUIDs, or superconducting quantum interference devices, measure extremely small magnetic fields; They are very sensitive vector magnetometers, with noise levels as low as 3 fT·Hz−0.5 in commercial instruments and 0.4 fT·Hz−0.5 in experimental devices. A SQUID magnetometer (MPMS, Quantum Design) was used to record the magnetic signal (in-plane and out-of-plane) of a Co NPs monolayer. In order to

check if the particles were oxidized, and the effect of that oxidation on the magnetic behaviour, hysteresis loops were performed after 1-T field cooling at 50 K, a temperature lower than the Neel temperature for cobalt oxide (CoO) (corresponding to the ordering of the antiferromagnetic phase). CoOâ&#x20AC;&#x201C;Co coupling would result in either an exchange coupling, which would increase the anisotropy and thus the coercivity, and/or exchange bias, which would induce an x-shift in the loops after field cooling.

Figure 2.9: SQUID Magnetometer scheme

2.4.5. Atomic Force Microscope (AFM)/Magnetic Force Microscope (MFM) The Atomic Force Microscope (AFM) or scanning force microscope (SFM) is a very high-resolution type of scanning probe microscope with demonstrated resolution of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. The AFM is one of the foremost tools for imaging, measuring and manipulating matter at the nanoscale. The information is gathered by "feeling" the surface with a mechanical probe. Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic) command enable the very precise scanning. The AFM consists of a microscale cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to the Hooke law.

Normally, the deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes.

Figure 2.10: Atomic Force Microscopy scheme

The MFM is a type AFM. Unlike typical AFM, magnetic materials are used for the sample and tip, so that not only the atomic force but also the magnetic interaction is detected. Many kinds of magnetic interactions are measured by MFM, including magnetic dipolar interaction.

3. RESULTS and DISCUSSION 3.1. SYNTHESIS OF COBALT NANOPARTICLES A concentrated solution of Co2(CO)8 (0.54 g in 3 ml of anhydrous DCB) was injected into a mixture of OA in DCB in anhydrous DCB (~15 ml) at reflux temperature

(181

Â°C).

The

decomposition

and

nucleation

occurs

instantaneously upon injection. The lifetime of atoms in solution is short leading to the simultaneous formation of many small metal clusters. The surfactant is present in the reaction mixture at concentrations of about 1%. Mixture of OA and TOPO were used. The control over the temperature and the surfactant concentration modifies the strength of the metallic particleâ&#x20AC;&#x201C;organic molecule bonding. Thus, by controlling the precursor/surfactant ratio, the reaction temperature and the injection time, the size of the spherical particles can be tailored and varied between 3 and 17 nm. This method produces macroscopic quantities of Co single crystals that are nearly monodisperse. As shown in Figure 3.1, Co NPs of 6, 8, 14 and 17 nm were obtained and characterized by TEM. A

Figure 3.1: TEM images of Co nanoparticles with sizes of 6 nm (A), 8 nm (B), 14 nm (C) and 17 nm (D).

Given that the majority studies of self-assembly of such magnetic nanoparticles have been developed onto carbon coated TEM substrates, the objective of this work was the study the SA onto a more technological interesting substrate. HOPG presents a set of favorable properties: smooth, flat and clean surface and hydrophobic character that make it compatible with OA and DCB that increases the affinity between the solvent and the substrate. The Co NPs are spherical (no shape anisotropy) and with epsilon (cubic) crystal anisotropy (six easy magnetization axes). In these assemblies, dipolar interactions are strong, as they are systems of strong magnetic moment and low and degenerated anisotropy. In this context, gravity force for a 10 nm Co nanoparticle is 0.04 fN, while the force associated to the Brownian motion at 300 K is 4 fN20.

3.2. DEPOSITION ONTO HOPG The deposition of NPs onto a HOPG substrate was performed by drop casting. Were deposited a few drops of the initial solution (1016 particles per ml) on a 5x5 mm of freshly cleaved HOPG. The excess of the solution was removed with the aid of an absorbent paper, leaving the entire surface evenly coated. The substrate was covered with a petri dish during 30 minutes to facilitate a slow initial evaporation under a DCB atmosphere. After this time the substrate was uncovered and the evaporation process continued until total dryness of the solvent. Spherical Co NPs self-assembled during evaporation from a colloidal solution onto graphite. While the NPs on the substrate were still mobile, they tend to arrange according their lowest energy sites. Equilibrium structures of NPs are determined by a balance between Van der Waals attractive forces on one side and vibrational entropy of the particles and steric repulsion of surfactant molecules on the other side. Dipolar magnetic interactions can significantly modify the balance to promote SA of NPs into nanometric or microscopic objects of different shape and size.

Slow evaporation at room temperature of the DCB (181 ºC) resulted to a spontaneous formation of micrometric cobalt rice-like structures in the case of 6 nm NPs (Figure 3.2) and wire-like structures in the case of 8 nm NPs (Figure 3.3), which were first observed by optical microscopy.

A 30

Counts of NP

2020 µm

D is ta n c e (µ m )

Figure 3.2: A) Optical microscope image of 4-8 μm cobalt like-rice structures. Insert is a zoom view of the image. B) Size distribution analysis of cobalt like-rice structures.

Figure 3.3: Optical microscope image of cobalt like-wire structures. Insert is a zoom view of the image.

The size distribution analysis of cobalt like-rice structures reveals a relative narrow size distribution, around 1-2 μm and a length that varies between 4-8 μm (figure 3.2B). The observed (Gaussian and narrow) size distribution recalls the LaMer description of homogeneous nucleation and growth of monodisperse NPs in solution known as the “burst nucleation” of a supersaturated solution.21 When the size of the particles was increased the formation of chains of particles were observed, due to the fact that are moving to the ferromagnetic regime. It is

well known that for large ferromagnetic NPs, dipolar interactions favour formation of chains, which are observed in the wire-like structures on the HOPG substrate. SEM images (Figures 3.4 and 3.5) show the cobalt structures in white color. The corresponding EDX analysis of the structures indicates that they are formed by CoNP. The background of the picture corresponds to the HOPG substrate.

Figure 3.4: Back Scattered SEM images of 4-8 Îźm cobalt like-rice structures (A and B). EDX analysis of the cobalt like-rice structures (C) and the HOPG substrate (D).

Figure 3.5: A) SEM image of 4-8 Îźm cobalt wire-like structures. B) EDX analysis of the cobalt like-rice structures.

Figure 3.4 shows a distribution of random orientations of the micrometric grains. The fact that the majority of the structures are isolated suggest that these assemblies did not form first in solution and then sedimented. Rather, it indicates that these structures formed on the substrate at the last stages of evaporation. In areas where the density of structures is higher, they appeared as intersected structures rather than collapsed. These observations together with their flat top surface suggest growth from the substrate. It could be proposed that the particles self-assemble in solution at very high concentrations (prior to deposition) when the majority of the solvent has been evaporated but still loosely bind to the substrate, and later the structure is transferred to the substrate when the evaporation is completed. However, this simple scenario was discarded by the work of Liangshi et al.,22 in which different structural patterns and dynamics were obtained with different substrates, indicating that, apart from concentration increasing, substrate affinity also plays a crucial role.23 This issue can be further corroborated by investigating the effect of NPs mobility reduction by depositing them onto substrates with different affinities. In the work of Puntes et al.,24 a similar sample was evaporated onto a (111) single silicon crystal functionalized with an amine self-assembled monolayer obtaining disordered Co monolayers. Amines are known to strongly bind to Co and, therefore, reduce particle mobility onto such a substrate. These results suggest that in order to form ordered structures a certain mobility is needed once the nanoparticle reaches the substrate or the growing crystal. The mobility of the particles is related to its size.33 Therefore, in the case of bimodal size distributions, particles self-assemble with similar ones and the smaller ones appear to be beneath the larger, as a result of the different mobility. Regarding the wire-like structures, we can observe that they are not uniform structures. SEM images at high magnifications show us that they are constituted by small substructures forming the chain. The use of high boiling solvents allows slow evaporation at high temperature. This added thermal energy permits the particles to diffuse to their lowest energy sites during evaporation, producing a well-defined superlattice structure.

3.3. REGARDING THE FORMATION MECHANISM It is obvious that SA does not occur either in the dispersed colloid nor in the colloid vessel walls (these colloids are stable for months to years). Besides, a minimal amount of solvent is needed in order to start self-assembling, which lasts until there is no more solvent to allow particles to move and then, the structure freezes. It is also likely that particles feel an effective attraction by the substrate. Therefore, as particles arrive near the substrate, they temporally attach on it before going back to the solution. When the concentration is high enough, the mobility of the particles on the substrate surface is reduced and then SA starts, due to both a reduction of unavailable positions for the NP and an increase in the self-assembled structure stability. Smooth grains supports the idea that particles are able to move during structure formation, i.e., there is still solvent left in contact with the substrate, which is consistent with the previous observations and the flat tops observed by AFM (Figure 3.6)

4.0Âľm Figure 3.6: AFM image of rice-like structures integrated by cobalt NP.

SA requires the components to be able to move with respect to one another. Their steady state positions balance both the attraction and repulsion. Because SA requires that the components be mobile, it usually takes place in fluid phases and on smooth surfaces. The environment can modify the interactions between the components. If components stick together irreversibly when they collide, they form a glass rather than a crystal or other regular structure. SA

requires that the components equilibrate between aggregated and nonaggregated states, or adjust their position relative to one another once they aggregate. These facts, together with a high affinity of the particles to attach on the substrate, favor 2D over 3D SAs enough to construct flat objects with their larger surface in contact with the substrate. This balance between NP-NP and NP-substrate affinity may be modified by varying the hydrophobic/hydrophilic character of the substrate. In our case, both particles and substrate are highly hydrophobic, although the presence of surfactant molecules results in a steric repulsion between particles and a lower NP-NP affinity than NP-substrate, favoring flat structures. Same experiments onto freshly cleaved mica (a clean and flat, but hydrophilic substrate) did not yield any particular SA structure (Figure 3.7).

Figure 3.7: Optical image of cobalt nanoparticles deposited on MICA (scale bar 20µm).

Isolated medium size Co NPs (6 nm) are superparamagnetic at RT. Figure 3.9 shows susceptibility curves obtained after a zero field cooling (ZFC)-field cooling (FC) process in a magnetic field of 100 Oe. A maximum in the ZFC branch is observed around the blocking temperature TB≈130 K. The broadness of the peak indicates that NPs present a wide size distribution function. A second peak is observed in both the ZFC and FC branches at T≈45 K. We tentatively assign this peak to the presence of some cobalt oxide at the NP surface that has the Neèl transition close to this temperature. Despite the existence of the protecting oleic acid layer, a small degree of oxidation attributed to the surface can not be avoided when samples are exposed to air, as previously described.9 In fact, AC susceptibility measurements (Figure 3.8) do not show any frequency dependence of the position of this peak thus, 23

indicating that it corresponds to a true thermodynamic transition. The paramagnetic upturn observed in the low temperature regime may be due to the existence of very small Co NPs with a very low blocking temperature or to the existence of paramagnetic Co atoms coordinated to OA or TOPO.

Figure 3.8: Temperature dependence of AC susceptibility for 6 nm colloidal NP

Figure 3.9: ZFC-FC measurements. A) Dispersed 6 nm Co nanoparticles and B) riceshape grains

Figure 3.10 shows the M(H) curves of dispersed NP at RT and at T=10 K. At RT, M(H) exhibits the typical features of superparamagnetic behavior and can be described by using the Langevin paramagnetic function M(x)=coth x-1/x, being x=µH/kBT, for a classical Heisenberg spin of moment µ in a magnetic field H. The values obtained for µ are around 104 µB, that assuming the density and moment per Co atom corresponding to bulk fcc Co,25 allows an estimation of the particle size of about 6 nm, similar to that obtained from TEM that also includes the antiferromagnetic (AFM) cobalt oxide layer at the surface of the particles. At T=10 K M(H) curve exhibits a well developed hysteresis loop with a coercive field HC≈ 2 kOe. It is also worth mentioning that after a FC in a high

field the loop is off-set from zero indicating the existence of an exchange bias field (HEX≈ 0.5 kOe) likely generated by the CoO layer at the surface of the Co NP.26 On the other hand, the remanence to saturation ratio is about ½ in agreement with the Stoner-Wolfart model for non interacting particles.

10K

Self-Assem bled Colloida l NP

0 .5

M/Ms

0.5

RT C olloid a l N P

-0.5

-0 .5

S e lf-A s se m b le d

-1 -30

-20

-10

H (kO e)

-1 4 -2 1 0

-1 1 0

0 H (O e )

1 10

2 10

Figure 3.10: Hysteresis loop for colloidal and self-assembled Co NP. A) low temperature (10ºK) and B) room temperature (RT)

The magnetic characterization of self assembled NP is shown in Figures 3.9. In this figure we depict the ZFC-FC susceptibility curves. They look very much alike to that of the dispersed NP. A broader peak is observed at the blocking temperature TB. It is worth mentioning that in this case TB is slightly smaller that for dispersed NP, which is contrary to that found in other cases in which a small increase of TB use to be observed due to interparticle interactions. Figure 3.10 shows M(H) curves a RT and T=10 K. As in de case of dispersed NP, at RT M(H) exhibits the typical features of superparamagnetic behavior and can be described by using the Langevin paramagnetic function with similar values of the magnetic moment per NP (µ≈104 µB). Nevertheless at low temperature the hyteresis loop is quite different, the coercive field has been drastically reduced to HC≈ 380 Oe as well as the exchange bias field HEX≈ 100 Oe. These effects are usually attributed to interparticle interactions. NPs in self assembled objects are tightly packed and interparticle interactions may strongly affect the magnetic behavior. In particular, dipolar interactions between NP can frustrate the orientation of the moments along the NP’s easy axis at low temperature promoting the appearance of a collective glassy behavior that drastically reduces the coercive field. The existence of strong magnetic interactions

between NPs in self assembled nanoobjects is also evidenced by the remanence to saturation ratio that is clearly smaller than ½. At 10

K smaller coercivity was observed in the case of the Co structures

probably due to the demagnetizing character of the dipolar interactions27-29. The hysteresis observed at 10 K is only in part due to the particles themselves, as it is larger than that observed in diluted nanoparticle colloids, indicating that interactions are partially responsible for the coercivity. In disordered systems, dipolar interactions tend to reduce the coercivity, whereas in ordered systems the dipolar interactions increase it. For an isolated Co nanoparticle with diameter of 6 nm and anisotropy KCo(fcc) = 8 x 104 J/m3 the estimated blocking temperature TBSP = KV/25kB (where V is the volume) should be about 40 K. Considering the extremely short-range character of exchange interactions and the fact that the cores are isolated by at least twice the surfactant thickness, the possible coupling of Co through such interactions can be ruled out. However when such a particle is in the vicinity of other particles, interparticle dipolar

μ0 H dip =

interactions

develop.

The

maximum

dipolar

field,

μ0 N μat μ0 μat R 3 = (where μ0 is the permeability of vacuum, N is 2π r 3 2π a 3 (2 R + l )3

the number of atoms per particle, μat is the atomic magnetic moment, R is the particle radius, a is the apparent atomic radius, l/2 is the surfactant thickness), created along the axis between two particles in contact amounts to about 0.7 T. The energy of 6 nm Co particle (particle moment Nμat ≈ 1.10-4 μB) in a field of 0.7 T is much higher than room temperature. The above oversimplified considerations suggest possibility for thermally (enough) stable magnetic moments of the particles during the solvent evaporation and hence for their self assembling. Thus, the intuitive picture of self organization scenario is in the Figure 3.11. Since the dipolar interactions favor formation of chains of particles, “nucleus” of such chains will spontaneously appear during the initial stage of solvent evaporation and they will further grow by attracting particles which are in 26

the vicinity of the chains ends. At the same time, other particles which are positioned far from the chain ends won’t be able to join the chain. Instead they will try to move towards the nearest particle of the chain, orienting their magnetic moments in opposite direction. Thus chains with progressively shorter lengths will be eventually attached to the original “nucleus” resulting in 3-D “riceshaped” objects. Naturally, the coercivity and the remanence of such “riceshaped”

objects

formed

dipolar

forces,

being

macroscopic

“antiferomagneets”, will be close to zero. Of course, the interplay of several parameters (like particle concentration, particle magnetic moment, solvent viscosity and evaporation conditions) will all play a role in determining whether the particles will self assemble into “rice” or chains, confine in layers or they will not assemble at all.

Low field

High

Figure 3.11: A) Scheme of the self-organization process. B) For large ferromagnetic nanoparticles dipolar interactions favor formation of chains of particles.

Magnetic Force Microscopy (MFM) measurements show that the observed Co structures are magnetic (Figure 3.12). No magnetic structures are observed inside the microstructures (black corresponds to attraction). Neither variation of the contrast is observed varying the orientation of the MFM tip magnetization corroborating the absence of remanence and coercivity. One also has to bear in mind that before the MFM scan, where the tip approaches the sample at approximately 50 nm, the tip is scanned at 5 nm to record the surface structure. Thus, the sensing magnet of the tip is first scanned very close to the particles in what remains a write (AFM)â&#x20AC;&#x201C;read (MFM) process.

Figure 3.12: MFM image of some of these structures showing the magnetic character at room temperature.

4. CONCLUSIONS There have been numerous experimental studies of closed-packed magnetic NPs arrays, yet proof of dipolar ferromagnetism has been eluded because the patterns have not previously been observed experimentally. In previous works, ferromagnetic-like (collective) behavior in magnetization measurements of a 2D self-assembled monolayer of 9 and 12 nm diameter Co NPs has been observed.27-29 They demonstrated the formation of micron-sized magnetically ordered regions and transverse domain walls, with a spatially varying local order parameter. Dipolar ferromagnetism was first predicted by Luttinger and Tisza,30 who found a ferromagnetic ground state for a face-centered-cubic (fcc) lattice of point dipoles. We have previously observed that monodisperse Cobalt nanoparticles with sizes that lead to a blocking temperature close to RT (therefore showing significant dipolar interactions) tend to display correlated magnetic areas in a domain-like type structure. In the present work, it is studied how such structures are formed and how those dipolar interactions drive the SA into elongated super-structures with a narrow size distribution, from micrometric rice-grain-like structures to micrometric wire-like structures. The properties of the individual particles as well as their mutual interactions determine important features of the nanoparticle systems, such as storage capacity and switching field. A Full understanding of nanoparticle SA would allow the design and construction of complex modular materials with customized properties and would bring material science to a higher stage of potentiality. A complete magnetic characterization of both individual particles and self assembled structures as a function of magnetic field and temperature is reported here.

The results also indicate new and unique features of purely dipolar ferromagnets. There is a small local order parameter even at RT, 200 K higher than the temperature where the apparent coercivity drops to zero. The minimal hysteretic losses and the ability to tune the ordering temperature with particle size may be beneficial for designing nanocomposites. At the same time, if a

dipolar ferromagnetic state exists it may prompt further investigation of twodimensional critical behavior, and of spin waves, which would be important for high-density patterned media. NPs coated with organic surfactants are able to self organize into ordered arrays due to the interplay between Van der Waals attractive forces and steric repulsion. When the NPs are magnetic, magnetostatic forces due to dipolar interactions, which are anisotropic, may play a relevant role influencing and then prevailing over isotropic interactions. When magnetic NPs have a stable enough magnetic moment (note that the superparamagnetic behaviour of an isolated particle may decrease when dipole-dipole interactions appear) depending on the relation between magnetostatic forces, and dispersive forces, different self-assembled arrays may be generated. For instance, when magnetostatic forces prevail over dispersion forces, particles form chains. In contrast, when magnetic interactions are weaker than the other isotropic interactions,

close-packed

structures

are

formed.31,32 Van

der

Waals

interactions and steric forces can be modulated by using different types of surfactant while dipolar magnetic interactions can be modified by changing the size of the magnetic NPs. In the case of these studies, the aim is to modulate the contribution of dipolar magnetic interactions to the self assembling process onto HOPG by using Co NPs of different sizes. Smaller NPs, around 4 nm in diameter, do not show any ordered SA. Medium size NPs around 6-8 nm in diameter self-assemble into regular ellipsoidal grains of about 6 µm in length (for diameters ≈ 6 nm) and chains of about 2 µm wide and variable length (for diameters ≈ 8 nm). Finally large NPs of around 13-14 nm in diameter do not show any type or regular arrangement. The focus of this work is addressed to medium size NPs since only they exhibit clear evidence of SA. Even when isolated NPs are superparamagnetic, above TB, groups of NPs can have a non-zero remanence and local ferromagnetic ordering. Thus, once the SA has started, NPs close to the growing objects will experience significant dipolar interactions. The theoretical SA process to generate objects similar to that found in our case can be found in reference 3131. Dipolar interactions 30

between NPs would initially favor parallel alignment of the moments forming a monolayer. Nearby NPs will experience a field favoring parallel alignment with the monolayer moment. Once a second small layer is formed on the top of the first one, the magnetic moment direction of the second layer of NPs is locked in by the large in-plane Lorentz cavity field. Additional particles arriving at the surface of the bilayer will experience a ferromagnetic field from the layer below and antiferromagnetic from the first layer. In this way, the formation of the ellipsoidal objects, due to the existence of dipolar magnetic interactions, can be understood. Obviously, the appearance of such objects and their size depends critically on the balance between the different forces acting on the NPs assembly. In fact we have observed that a small variation of the NPs size (variation of the NPs mean diameter from â&#x2030;&#x2C6; 6 nm to â&#x2030;&#x2C6; 8 nm) ends up in the formation of chains instead of ellipsoidal objects. Some of the chain structures seem an intermediate state where the grains started thinning and aligning nucleating a wire, leaving curly elongated structures, suggesting that the chain forming process is based on the coalescence of elongated structures, all consistent on the increase of magnetization per particle.

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5. ACKNOWLEDGMENTS I want to tank Dr. Victor Puntes for his valuable discussions and experimental support. Also, to the members of the comitee for their time in reading this work. The Inorganic Nanoparticles Group lab-mates, for the support and discussions. Finally, I want to thank the Magnetic Materials and Functional Oxides group of the Institut de Ci猫ncia de Materials de Barcelona (ICMAB) for all the magnetic measurements and discussion of results. This work is supported by the Misnisterio de Educaci贸n y Ciencia (MAT200613572-C02-02).

6. ANNEX I We have studied three cases doing the deposition of 8 nm nanoparticles under different magnetic fields. A) The HOPG on top of a strong magnetic field (NdFeB)

B) The HOPG on top of a On a weak magnetic field (ferrite magnet)

C) The HOPG in a homogeneous magnetic field (between two ferrite magnets)

Picture of the edge of the surface, near the magnet.

Picture of the center of the surface, where there are homogenous field.

These experiments confirm the magnetic character of the particles and that cobalt NPs can self assemble into large wires depending on the substrate. The orientation of the wires can be manipulated with magnetic fields during the evaporation process.