Iron nanoparticles by inert gas condensation
Xing, Lijuan
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Chapter 1
Introduction
T
his Chapter presents a brief introduction of magnetic
nanoparticles. The finite‐size effects, which dominate the
magnetic behavior of individual nanoparticles, are discussed to
illustrate the evolution of magnetic structure and property with
particle size variation. Furthermore, an introduction of iron
nanoparticles and the applications is given briefly. Wulff
constructions based on a minimized energy solution can help to
determine the stable shape for nanocrystals. At the end, we
introduce the Kirkendall effect to illustrate the post‐fabrication
evolution of nanoparticles.
1.1 Magnetic Nanoparticles
Magnetic nanoparticles are developing into one of the most vital and fast growing topics in the field of nanotechnology due to their wide range of applications, including catalysis1,2, data storage3,4, environmental/groundwater remediation5–7,
and biomedicine8–14 ( e.g. contrast enhancement in magnetic resonance imaging
(MRI), drug delivery, and hyperthermia). It is well established that magnetic properties of nanoparticles, e.g. magnetic anisotropy, specific magnetic moment, Curie temperature, and coercivity, normally differ from those of the corresponding bulk materials15. As the dimension of materials is downscaled, the ratio of the
surface area to the volume increases, leading to more atoms resident on the surface. For nanoparticles with a size of 1-100 nm, the significantly large surface-to-volume ratio can lead to novel physical and chemical properties compared to their bulk counterparts. In ultrafine magnetic nanostructures, the magnetization configuration might be different due to the discreteness of the crystalline lattice16. In the near
surface region, the magnetic moment per atom changes dramatically with the distance from the surface owing to the discreteness resulting from reduced coordination at the surface, which therefore leads to an improved moment of NPs17.
1.1.1 Finite-Size Effects
Two features dominate the magnetic properties of nanoparticles: finite-size effects and surface effects. Finite-size effects result from, for instance, the quantum confinement of the electrons, whereas surface effects are typically related to the symmetry breaking of the crystal structure at the boundary of each particle8. Size
effects, which dominate the magnetic behavior of individual nanoparticles, will be discussed briefly herein to illustrate the size dependence of magnetic structure and properties. As the particle size decreases, two limits can be defined, single-domain limit (DC) and superparamagnetic limit (DS), at which point the magnetic structures and properties of nanoparticles experience a significant change. Figure 1.1 schematically illustrates the variation of domain structures with particle size, and the coercivity evolution is also indicated qualitatively. In general, the coercivity of the superparamagnetic nanoparticles is zero above the blocking temperature. As the particle size increases beyond the superparamagnetic limit, the coercivity starts to increase dramatically until supersaturated at the single domain limit, and then gradually drops to the bulk value in the multi-domain region. The blocking temperature of superparamagnetic nanoparticles, at which point the transition between superparamagnetism and blocked state occurs, is proportional to the
nanoparticle volume and the magnetic anisotropy energy density of the nanoparticle material8,18, where the latter is a measure of the magnetic ‘hardness’ of the
nanoparticle material.
Figure 1.1 Schematic illustration of the domain structure variation with particle size,
indicating the single‐domain limit, superparamagnetic limit, and the change of coercivity18.
1.1.1.1 Single-domain limit
Bulk ferromagnetic materials or large magnetic particles are composed of small regions of uniform magnetization, i.e. magnetic domains, separated by domain walls. The presence of these domains can be attributed to the balance between two energy terms: the magnetostatic energy, which increases proportionally to the volume of the material, and the domain-wall energy, which increases proportionally to the interface area between domains. In 1930, Frenkel and Dorfman first predicted that a particle of ferromagnetic materials with a size below a critical dimension would possess a single-domain structure, within which the magnetic moments of free electrons are aligned parallel and contribute to a large net magnetization18. Since then, great efforts have been devoted to the study of
magnetism at the nanoscale. Results have shown that the decrease of the dimensions of the ferromagnetic materials can indeed generate the transformation from multi-domain to single-domain behavior along with an increase in coercivity19–21. As the particle size is reduced to a critical volume where it takes
more energy to generate a domain wall than to support the external magnetostatic energy (stray field) of the single domain state, the spins of free electrons within the nanoparticle will be aligned by ferromagnetic coupling into one direction acting as a single-domain magnet. The critical diameter for a spherical nanoparticle, Dc, below which the single-domain state can be established, is given by18
2 0 36 S C M AK D (1.1)
where A is the exchange constant, K is the effective anisotropy constant that measures the energy per unit volume required to flip the magnetization direction,
μ0 is the vacuum permeability, and MS is the saturation magnetization. It was found that ferromagnetic crystals have a critical size of a few tens of nanometers, which depends on the material with the additional influence from various anisotropy energy terms8.
For multi-domain nanoparticles, the alignment of the magnetization direction within each domain is controlled by both the domain wall motion and the anisotropy energy KV (where V describes the domain volume). For single-domain nanoparticles, where domain walls are absent, the reversal of the spin orientation is realized by spin rotation instead of domain wall motion. The latter contributes to the very high coercivity observed for small nanoparticles22. Shape anisotropy also
influences the critical dimension of the single-domain limit. The spherical particles, in principle, have a smaller critical size compared to their counterparts with a higher shape anisotropy.
1.1.1.2 Superparamangetic limit
As the particle size decreases further below the single-domain limit, the energy barrier KV continues to reduce due to a decreasing volume, generally at a disadvantage in the competition with thermal energy kBT (kB is the Boltzmann constant, and T is the temperature). As manifested in Figure 1.1, the decrease of the particle size below the single-domain limit is accompanied by the reduction of coercivity23,24. When a critical size D
S is reached, where kBT overtakes KV , the thermal excitations helps to overcome the single-energy barrier so that the total magnetization behaves as a super-spin inside each particle25 showing a high
saturation magnetization (MS) close to the value of their larger ferromagnetic counterparts with no coercivity.
The magnetization reversal behavior of magnetic nanoparticles is sensitive to temperature. The characteristic time of the thermal fluctuation of the particle magnetization τN is given by Néel relaxation time18
0exp N B KV k T (1.2)
where τ0 is a pre-factor constant approximately 10-9 s18. The measurement time scale is defined as τM which is around 100 s for a typical laboratory experimental condition. If the relaxation time τN is shorter than the measurement time τM, the rapid fluctuations of the particle moments mimics superparamagnetism. While in the opposite situation, where the relaxation time τN is larger than τM, thermal fluctuations do not allow the accomplishment of the magnetization reversal resulting in a so-called “blocked” state18,25. The temperature at which τ
M = τN is
defined by the blocking temperature TB, which is given by18
0 ln M B B k KV T . (1.3)
The blocking temperature thus depends on the effective anisotropy constant, the particle size, the applied magnetic field, and the measurement time.
1.1.2 Iron Nanoparticles
Iron is the fourth most plentiful element in the earth’s crust and the most ubiquitous one of the transition metals. Nowadays, much effort has been devoted to the study of magnetic materials and applications, and iron is among the most attractive candidates. As a ferromagnetic material, iron has a high saturation magnetization (σS=222 Am2kg-1 at 0 K, and σS=218 Am2kg-1 at 293 K). The latter combined with a high enough Curie temperature (TC=1043 K) allows the majority applications to be at room temperature26. Moreover, iron is a very soft magnetic material with a
low magnetocrystalline anisotropy. As for all magnetic nanoparticles, a sufficiently small size results in the superparamagnetic behavior of iron nanoparticles. Due to its low magnetocrystalline anisotropy, the critical size for the superparamagnetic limit is larger than cobalt, which has the second-highest room temperature saturation magnetization only to iron. Therefore, larger-size superparamagnetic
iron nanoparticles can be prepared with a higher moment that is not possible to be realized by any of the other magnetic metals26.
1.1.3 Applications of Iron Nanoparticles
Iron nanoparticles can be used in a variety of fields, e.g. magnetic recording media, catalysis, as well as biomedical applications including magnetic labels for highly efficient bioseparation/drug delivery, highly sensitive biodetection, and hyperthermia treatment27. The implementable customization makes iron a
competitive candidate as a magnetic material. Iron nanoparticles have been commercially used as magnetic recording media26, where high-aspect-ratio iron
particles behave as very hard permanent magnets with large shape anisotropy, contributing to a high-capacity and reliable data storage methods26. The application
of iron as a catalyst starts with its functioning in the Fischer-Tropsch synthesis28,29,
followed by the further study of alternative iron nanoparticles that have a catalytic activity several times that of a conventional material30.
The potential biomedical applications of iron nanoparticles will be discussed to explore what advantages the zero-valent state can potentially offer as compared to the typically used iron oxides31–33. For drug delivery, a magnetic gradient is
applied to afford a directed force on the particles26. Since the exerted force is
proportional to the magnetization, the high-moment nanoparticles are favorable. Iron nanoparticles, as compared with any of its oxides, can maintain superparamagnetism at a larger size, therefore offering higher particle moments that contributes to better functioning in directed drug delivery.
In magnetic resonance imaging (MRI), superparamagnetic iron oxide particles are now commercially used as contrast enhancers, and the applied high magnetic field results in the saturation of the adopted superparamagnetic nanoparticles. Zero-valent iron, which exhibits a saturated magnetization double that of its magnetic oxides, can therefore provide higher image contrasts34–36.
Hyperthermia37,38 treatment is based upon the idea of localized and selective
heating of diseased tissue, which can be realized by employing magnetic nanoparticles within an alternating magnetic field of suitable amplitude and frequency. Superparamagnetic nanoparticles, with a high saturation magnetization combined with low anisotropy, can serve as an ideal option for efficient heating mediators. Note that since for biomedical applications iron particles are present in aqueous, salt-containing systems, a probable drawback of iron is its reactivity. As a result, a protective coating is necessary to provide an effective barrier against oxygen and other oxidizing agents26.
1.2 Wulff Construction
The formation of a nanoparticle shape can be explained by the minimum energy solution, i.e. the stable shapes always possess the lowest surface energy. The analysis of nanocrystals, compared to liquid droplets or amorphous nanoparticles, is more complicated due to the various stabilities of different facets resulting from the different densities of surface atoms, charges, and ligands. Provided that all the facets of a crystal are indistinctive and equally favored, a spherical shape will be preferred because of its lowest surface to volume ratio as is shown in Figure 1.2. The other extreme situation is a sharp-edged polyhedron which is preferred with only one stable facet, e.g. cubes with {100} facets or octahedrons with {111} facets (see Figure 1.2). However, most of the nanocrystals lie in between these two extremes, and the lowest energy shape depends on the relative stability of the facets. Nanocrystals will adopt a truncated shape, gradually close to spherical, when the diversity in the facets stability is reduced.
Figure 1.2 The lowest energy structure of a nanocrystal exhibits dependence on the relative
stability of its facets39.
The total surface energy of a system is the sum of each facet area weighted by the corresponding surface energy. The latter is given by39
i i
A
G
(1.4)where γ is the surface energy of a facet and gives the corresponding surface area. The standard process to create the Wulff shape is to plot the surface energy as a
function of the relative angles the normals of the surface planes make and then take the inner envelope formed by these planes. Figure 1.3 illustrates the simplified situation where the three dimensional (3D) structure can be represented by two dimensional (2D) projection (here for Figure 1.3 (a) along the [110] direction) such that only the {100} and {111} planes present on the surface and then from the cubic symmetry of the structure still the overall 3D nanoparticle shape can be reconstructed as shown by Figure 1.3 (b). In principle, if high index planes would be present, this will result in increasing complexity of the overall particle shape.
Figure 1.3 Illustration of the Wulff construction for a cubic crystal structure where {100}
and {111} planes dominate: (a) [110] projection of the particle; (b) the corresponding 3D structure, with green denoted the {100} facets, and yellow the {111} facets40.
1.3 Kirkendall Effect
The transformation of nanoparticles to hollow or porous structures through nanoscale Kirkendall effect was first reported in 200441, and the void formation
during oxidation was attributed to the difference in diffusion rates between two components in a diffusion couple. We show in Figure 1.4 (b) the formation of voids at the metal-oxide interface observed after 40-month exposure to ambient air, together with an overview image of the as-deposited nanoparticles shown in Figure 1.4 (a). A close-up of the particle morphology is shown in the right column of Figure 1.4 (b) where the voids are labeled with arrows.
The Kirkendall effect is a classical phenomenon in metallurgy42, which
describes the nonequilibrium mutual diffusion process through an interface of two metals with the occurrence of vacancy diffusions to compensate for the unequality of material flow. Figure 1.5 schematically illustrates the process of the Kirkendall effect in the bulk phase diffusion couple A-B. As indicated, there is a difference between the diffusion rates of the two materials, resulting in an unequal transport of A and B to the other side, which is then accompanied by vacancy formation and diffusion. This effect can give rise to the concentration of voids located near the bond interface within material showing faster-diffusion43–45.
Figure 1.4 Formation of voids at the interface between the iron core and the oxide shell.
(a) Overview image of as‐deposited core‐shell structured iron nanoparticles. (b) Overview image of the same sample after 40‐month exposure to ambient air with a close‐up of particles in the right column showing the formation of voids.
Coming to the nanoscale, the Kirkendall effect can also be observed in nanoscale size compound systems, typically in oxides, sulfides, and nitrides, where oxidation, sulfidation, or nitridation takes place with a faster outward diffusion rate of metal cations through the compound interface46. The difference in the outward
diffusion of metal cations and the inward diffusion of anions gives rise to an inward flux of vacancies to balance the diffusivity difference. The process of nanoscale Kirkendall effect is accompanied by the thickening of the compound layer and the loss of the core material density.
Figure 1.5 Schematic illustration of the Kirkendal effect where nonequilibrium lattice
diffusion at the interface takes place. JA, JB and JV represent the diffussion fluxes of metal
A, B, and vacancies, respectively47.
Figure 1.6 shows a general model for the formation of a hollow structure induced by the Kirkendall effect in nanoscale particles. It then holds that A exhibits a faster diffusion flux than B. In the initial stage (Figure 1.6 (a)), the exchange of material through the interface is realized by bulk interdiffusion assisted by the inward diffusion of vacancies. The voids located close to the interface result from the coalescence of the inward diffused vacancies, and grow larger as the interdiffusion proceeds. Further coalescence of voids can form larger ones, as described in Figure 1.6 (b), blocking the connections for lattice diffusion. New bridges have to be established subsequently as fast transport paths for the remaining material A, which can then redistribute itself at the open surface of the compound layer by surface diffusion. As the diffusion continues, A is gradually exhausted with more vacancies generated. Finally smooth and uniform hollow compound nanocrystals can be obtained.
Figure 1.6 Generalized model for the formation of hollow structures based on a
combination of Kirkendall effect with surface diffusion process48.
1.4 Motivation and Thesis Outline
Magnetic nanoparticles are considered promising candidates for applications in magnetic recording, environmental remediation, and biomedicine. Iron, as the fourth most plentiful element with a high saturation magnetization, is among the most attractive magnetic materials. Its low magnetocrystalline anisotropy contributes to lager size superparamagnetic nanoparticles with a higher moment that is not possible to be realized by any of the other materials, making iron nanoparticles more efficient in tumor diagnostic and therapeutic. Iron nanoparticles can be produced by various methods, and the cluster source adopted in our work is a combination of magnetron sputtering and inert gas aggregation technique, giving solvent-free and monodispersed iron nanoparticles with higher diversity of crystal motifs than it is achieved with chemical techniques. The downscaling of nanoparticles results in a sharp increase of atoms resident on the surface, and the variant particle shapes lead to the different crystallographic surfaces that enclose the particle, contributing to different chemical and physical properties.
Magnetic properties of nanoparticles show strong sensitivity to particle size and shape, whereas the standard magnetic characterization techniques, as for example the superconducting quantum interference devices, give only the magnetic properties of nanoparticles as ensemble of the nanostructures or equivalently the macro-performance. In the present thesis, magnetic structures of individual iron nanoparticles are probed with magnetic force microscopy, which provides the possibility for the subsequent determination of the precise single-domain and superparamagnetic limit of iron.
Chapter 2 introduces the experimental techniques employed in this thesis.
Iron nanoparticles with tunable sizes are synthesized by inert gas condensation with a magnetron sputtering source. Transmission electron microscopy is used to determine the nanoparticle size, structure, and composition. Magnetic force microscopy is employed to probe the magnetic structure and the magnetization orientation of iron nanoparticles.
Chapter 3 is a study devoted to the precise structure and constitution of iron
nanoparticles, as well as the particle shape evolution with size variation. The nanoparticles are constituted by a single crystal iron core, as a combined analysis of high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) indicate, and a polycrystalline shell (Fe3O4 and/or
γ-Fe2O3). The particle size and shape show strong dependence on the gas environment
in the cluster source. Based on Wulff construction analysis, iron nanoparticles adopt different morphologies depending on the ratio of the growth rate along the <100> and the <110> directions.
Chapter 4 gives a method to decrease the minimum particle size that can be
obtained by introducing a backing plate in between the target and the magnetron. The magnetic field distribution above the target surface can be modified by the variant target-magnetron spacing, resulting in smaller particles with a thicker backing plate. Particle morphologies evolves from faceted to close-to-spherical polyhedral shapes with increasing spacer thickness.
Chapter 5 describes the determination of the ferromagnetism of iron
nanoparticles with a size ~50-70 nm using magnetic force microscopy. Indeed, the reversal of the magnetic contrasts observed with opposite tip polarities confirms the ferromagnetism of the measured nanoparticles. Moreover, the bright and dark contrasts agree with the single-domain structure, and the specific magnetization orientation can be probed by theoretical analysis of the magnetic force microscopy data.
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