Direct observation of ferromagnetic spin polarization in gold nanoparticles

Silver nanoparticles linked by a Pt-containing organometallic dithiol bridge: study of local structure and interface by XAFS and SR-XPS

Silver nanoparticles (AgNPs) functionalized with an organometallic bifunctional thiol containing Pt(ii) centers, generated in situ from trans-trans-[thioacetyl-bistributylphosphine-diethynylbiphenyl-diplatinum(ii)], were synthesized with different sulphur/metal molar ratios (i.e. AgNPs-1 and AgNPs-2) with the aim to obtain nanosystems of different mean size and self-organization behaviour. AgNPs spontaneously self-assemble, giving rise to 2D networks, as previously assessed. In this work a deeper insight into the chemico-physical properties of these AgNPs is proposed by means of synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS) and X-ray absorption fine structure spectroscopy (XAFS) techniques. The results are discussed in order to probe the interaction at the interface between a noble metal and a thiol ligand at the atomic level and the aim of this study is to shed light on the chemical structure and self-organization details of nanosystems. The nature of the chemical interaction between the dithiol ligand and the Ag atoms on the nanoparticle surface was investigated by combining SR-XPS (S2p, Ag3d core levels) and XAS (S and Ag K-edges) analysis. UV-visible absorption and emission measurements were also carried out on all samples and compared with TD-DFT calculations so as to get a better understanding of their optical behavior and establish the nature of the excitation and emission processes.

Magnetoresistance of granular Pt-C nanostructures close to the metal-insulator transition

We investigate the electrical and magneto-transport properties of Pt-C granular metals prepared by focused electron beam induced deposition. In particular, we consider samples close to the metal-insulator transition obtained from as-grown deposits by means of a low-energy electron irradiation treatment. The temperature dependence of the conductivity shows a σ ∼lnT behavior, with a transition to σ ∼ √T at low temperature, as expected for systems in the strong coupling tunneling regime. The magnetoresistance is positive and is described within the wavefunction shrinkage model, normally used for disordered systems in the weak coupling regime. In order to fit the experimental data, spin-dependent tunneling has to be taken into account. In the discussion we attribute the origin of the spin-dependency to the confinement effects of Pt nano-grains embedded in the carbon matrix.

Magnetism in nanoparticles: tuning properties with coatings

This paper reviews the effect of organic and inorganic coatings on magnetic nanoparticles. The ferromagnetic-like behaviour observed in nanoparticles constituted by materials which are non-magnetic in bulk is analysed for two cases: (a) Pd and Pt nanoparticles, formed by substances close to the onset of ferromagnetism, and (b) Au and ZnO nanoparticles, which were found to be surprisingly magnetic at the nanoscale when coated by organic surfactants. An overview of theories accounting for this unexpected magnetism, induced by the nanosize influence, is presented. In addition, the effect of coating magnetic nanoparticles with biocompatible metals, oxides or organic molecules is also reviewed, focusing on their applications.

Giant magnetic susceptibility of gold nanorods detected by magnetic alignment

We have determined the magnetic properties of single-crystalline Au nanorods in solution using an optically detected magnetic alignment technique. The rods exhibit a large anisotropy in the magnetic volume susceptibility (Δχ(V)). Δχ(V) increases with decreasing rod size and increasing aspect ratio and corresponds to an average volume susceptibility (χ(V)), which is drastically enhanced relative to bulk Au. This high value of χ(V) is confirmed by SQUID magnetometry and is temperature independent (between 5 and 300 K). Given this peculiar size, shape, and temperature dependence, we speculate that the enhanced χ(V) is the result of orbital magnetism due to mesoscopic electron trajectories within the nanorods.

Spin polarization and quantum spins in Au nanoparticles

The present study focuses on investigating the magnetic properties and the critical particle size for developing sizable spontaneous magnetic moment of bare Au nanoparticles. Seven sets of bare Au nanoparticle assemblies, with diameters from 3.5 to 17.5 nm, were fabricated with the gas condensation method. Line profiles of the X-ray diffraction peaks were used to determine the mean particle diameters and size distributions of the nanoparticle assemblies. The magnetization curves M(H_a) reveal Langevin field profiles. Magnetic hysteresis was clearly revealed in the low field regime even at 300 K. Contributions to the magnetization from different size particles in the nanoparticle assemblies were considered when analyzing the M(H_a) curves. The results show that the maximum particle moment will appear in 2.4 nm Au particles. A similar result of the maximum saturation magnetization appearing in 2.3 nm Au particles is also concluded through analysis of the dependency of the saturation magnetization M_P on particle size. The M_P(d) curve departs significantly from the 1/d dependence, but can be described by a log-normal function. Magnetization can be barely detected for Au particles larger than 27 nm. Magnetic field induced Zeeman magnetization from the quantum confined Kubo gap opening appears in Au nanoparticles smaller than 9.5 nm in diameter.

Magnetism in nanocrystalline gold

While bulk gold is well known to be diamagnetic, there is a growing body of convincing experimental and theoretical work indicating that nanostructured gold can be imparted with unconventional magnetic properties. Bridging the current gap in experimental study of magnetism in bare gold nanomaterials, we report here on magnetism in gold nanocrystalline films produced by cluster deposition in the aggregate form that can be considered as a crossover state between a nanocluster and a continuous film. We demonstrate ferromagnetic-like hysteretic magnetization with temperature dependence indicative of spin-glass-like behavior and find this to be consistent with theoretical predictions, available in the literature, based on first-principles calculations.

Magnetic disorder in diluted FexM100-x granular thin films (M=Au, Ag, Cu; x < 10 at.%)

Nanogranular thin films of Fe7Au93, Fe7Ag93 and Fe9Cu91 have been sputtered onto Si(100) substrates with the aim of studying the magnetic interactions. X-ray diffraction shows a major noble metal matrix with broad peaks stemming from (111) textured fcc-Au, Ag and Cu. The noble metal forms a nanogranular environment, as confirmed by transmission electron microscopy, with mean particle sizes below 10 nm. The high magnetoresistance (>6%) reveals the existence of Fe nanoparticles. X-ray absorption near edge spectroscopy confirms the presence of a bcc-Fe atom arrangement and some dissolved Fe atoms in the matrix, and XMCD shows the polarization of Au by the Fe nanoparticles. DC-magnetization displays a field-dependent irreversibility produced by the freezing of magnetic nanoparticles into a superspin-glass state. The hysteresis loops remain unsaturated at 5 K and 45 kOe. The coercivity displays a sharp temperature decrease towards a minimum below 50 K, levelling off at higher values, reaching Hc = 200 Oe at 300 K. Annealing of FeAu results in a double-peak zero field cooled magnetization and a slight decrease of the coercivity. The interpretation of the results supports the presence of Fe nanoparticles embedded in the major noble matrix, with some diluted Fe atoms/clusters.

Competing effects in the magnetic polarization of non-magnetic atoms

The magnetic polarization of the Lu 5d states through the Ho(1-x)Lu(x)(Fe(1-y)Al(y))(2) series has been studied by means of x-ray magnetic circular dichroism. A combined study of the dichroic signals performed at the Fe K-, Ho L(2)- and Lu L(2,3)-edges gives a complete picture of the polarization scheme at the conduction band. The results show that in the presence of competing localized magnetic moments, μ(Fe)(3d) and μ(Ho)(4f), the dichroic signal at the Lu site is mainly due to the Fe atoms, the effect of the magnetic rare-earth being negligible. Estimation of the spin and orbital components of the Lu(5d) induced magnetic moment have been obtained by applying the magneto-optical sum rules derived for x-ray magnetic circular dichroism.

Strong paramagnetism of gold nanoparticles deposited on a Sulfolobus acidocaldarius S layer

Magnetic properties of Au nanoparticles deposited on an archaeal S layer are reported. X-ray magnetic circular dichroism and superconducting quantum interference device magnetometries demonstrate that the particles are strongly paramagnetic, without any indication of magnetic blocking down to 16 mK. The average magnetic moment per particle is M(part)=2.36(7) μ(B). This contribution originates at the particle's Au 5d band, in which an increased number of holes with respect to the bulk value is observed. The magnetic moment per Au atom is 25 times larger than any measured in other Au nanoparticles or any other configurations up to date.

Decorating polypyrrole nanotubes with au nanoparticles by an in situ reduction process

Au nanoparticle-decorated polypyrrole nanotubes (defined as PPy/Au nanocomposites) are prepared by an in situ reduction process. Polypyrrole (PPy) nanotubes are prepared by a self-degraded template method, and Au nanoparticles are deposited in situ by the reduction of HAuCl(4) . The size and uniformity of the Au nanoparticles that decorate the PPy nanotubes can be controlled by adjusting the experimental conditions, such as the stabilizers used and the reaction temperature. The morphologies and optical properties of the nanocomposites have been characterized by scanning electron microscopy, transmission electron microscopy, UV-vis, and FT-IR spectroscopy. Conductivity measurements show that the conductivities of the nanocomposites decrease with a decrease of temperature, and the conductivity-temperature relationship obeys the quasi-one dimensional variable range hopping model.

Magnetism in gold nanoparticles

Gold nanoparticles currently elicit an intense and very broad research activity because of their peculiar properties. Be it in catalysis, optics, electronics, sensing or theranostics, new applications are found daily for these materials. Approximately a decade ago a report was published with magnetometry data showing that gold nanoparticles, most surprisingly, could also be magnetic, with features that the usual rules of magnetism were unable to explain. Many ensuing experimental papers confirmed this observation, although the reported magnetic behaviours showed a great variability, for unclear reasons. In this review, most of the experimental facts pertaining to "magnetic gold" are summarized. The various theories put forth for explaining this unexpected magnetism are presented and discussed. We show that despite much effort, a satisfying explanation is still lacking and that the field of hypotheses should perhaps be widened.

Coexistence of ferromagnetism and superconductivity in YBCO nanoparticles

Nanoparticles of superconducting YBa(2)Cu(3)O(7-δ) were synthesized via a citrate pyrolysis technique. Room temperature ferromagnetism was revealed in the samples by a vibrating sample magnetometer. Electron spin resonance spectra at selected temperatures indicated that there is a transition from the normal to the superconducting state at temperatures below 100 K. The M-T curves with various applied magnetic fields showed that the superconducting transition temperatures are 92 K and 55 K for the air-annealed and the post-annealed samples, respectively. Compared to the air-annealed sample, the saturation magnetization of the sample by reheating the air-annealed one in argon atmosphere is enhanced but its superconductivity is weakened, which implies that the ferromagnetism maybe originates from the surface oxygen defects. By superconducting quantum interference device measurements, we further confirmed the ferromagnetic behavior at high temperatures and interesting upturns in field cooling magnetization curves within the superconducting region are found. We attributed the upturn phenomena to the coexistence of ferromagnetism and superconductivity at low temperatures. Room temperature ferromagnetism of superconducting YBa(2)Cu(3)O(7-δ) nanoparticles has been observed in some previous related studies, but the issue of the coexistence of ferromagnetism and superconductivity within the superconducting region is still unclear. In the present work, it will be addressed in detail. The cooperation phenomena found in the spin-singlet superconductors will help us to understand the nature of superconductivity and ferromagnetism in more depth.

Measurement of a pauli and orbital paramagnetic state in bulk gold using x-ray magnetic circular dichroism spectroscopy

We show that bulk gold (Au) exhibits temperature-independent paramagnetism in an external magnetic field by x-ray magnetic circular dichroism spectroscopy at the Au L(2) and L(3) edges. Using the sum-rule analysis, we obtained a magnetic moment of 1.3 × 10(-4) μB/atom in an external magnetic field of 10 T and a paramagnetic susceptibility of 8.9 × 10(-6) for the 5d orbit. The induced paramagnetism in bulk Au is characterized by a large (≈ 30%) orbital contribution. This orbital component was retained even when Au atoms formed nanoparticles, playing an important role in stabilizing the spontaneous spin polarization in the Au nanoparticles.

Ferromagnetism exhibited by nanoparticles of noble metals

Gold nanoparticles with average diameters in the range 2.5-15 nm, prepared at the organic/aqueous interface by using tetrakis(hydroxymethyl)phosphonium chloride (THPC) as reducing agent, exhibit ferromagnetism whereby the saturation magnetization M(S) increases with decreasing diameter and varies linearly with the fraction of surface atoms. The value of M(S) is higher when the particles are present as a film instead of as a sol. Capping with strongly interacting ligands such as alkane thiols results in a higher M(S) value, which varies with the strength of the metal-sulfur bond. Ferromagnetism is also found in Pt and Ag nanoparticles prepared as sols, and the M(S) values vary as Pt>Au>Ag. A careful study of the temperature variation of the magnetization of Au nanoparticles, along with certain other observations, suggests that small bare nanoparticles of noble metals could indeed possess ferromagnetism, albeit weak, which is accentuated in the presence of capping agents, specially alkane thiols which form strong metal-sulfur bonds.

Structural reordering in monolayers of gold nanoparticles during transfer from water surface to solid substrate

Structural reordering in monolayers of gold nanoparticles during transfer from water surface to solid substrate has been studied by synchrotron x-ray scattering techniques. Grazing incidence diffraction (GID) and grazing incidence x-ray off-specular scattering (GIXOS) measurements were performed as a function of time to track the in-plane and out-of-plane structural reordering in the transferred monolayers. GID measurements show shift in the in-plane particle-particle correlation peak toward the lower in-plane momentum transfer value, signifying possible expansion of triangular lattice formed on the water surface. However, GIXOS data and supportive microscopy measurements clearly show compactification in the in-plane structure and associated out-of-plane movements. A model that assumes the possibility of a two-dimensional short-range structural reordering from triangular to square-like lattice as a function of time could explain all the data. The observed change in the electron densities of the nanoparticles before and after the structural reordering matches well with the expected change in the calculated electron densities of the nanoparticles arranged in triangular (pretransition) and square-like (post-transition) symmetry.

Layer-by-layer assembly of thiol-capped au nanoparticles on a water surface and their deposition on H-terminated Si(001) by the Langmuir-Blodgett method

A monolayer of dodecanethiol-encapsulated Au nanoparticles when compressed laterally transforms into layer-by-layer assemblies on water surface. These layer-by-layer assemblies of Au nanoparticles have been deposited on H-terminated Si(001) substrates by using one down-up cycle (two strokes) in the Langmuir-Blodgett (LB) method. The transformation from monolayer to layer-by-layer assembly on a water surface is irreversible; i.e., if the compressed film is decompressed the layer-by-layer structure cannot regenerate the monolayer structure. Unlike layer-by-layer growth, only odd numbers of layers grow from the monolayer on the H-terminated Si(001) substrates by using different numbers of down-up cycles. Z-type LB deposition occurs only in the first down-up cycle of the hydrophobic substrate, whereas Y-type LB deposition takes place in the successive cycles. Such layer-by-layer assemblies of Au nanoparticles, which are made on bare silicon surfaces and where thickness can be controlled at the nanoscale level, are very promising for their novel applications in the field of nanoscience.

Magnetic surface nanostructures

Recent trends in the emerging field of surface-supported magnetic nanostructures are reviewed. Current strategies for nanostructure synthesis are summarized, followed by a predominantly theoretical description of magnetic phenomena in surface magnetic structures and a review of experimental research in this field. Emphasis is on Fe- or Co-based nanostructures in various low-dimensional geometries, which are studied as model systems to explore the effects of dimensionality, atomic coordination, chemical bonds, alloying and, most importantly, interactions with the supporting substrate on the magnetism. This review also includes a discussion of closely related systems, such as 3d element impurities integrated into organic networks, surface-supported Fe-based molecular magnets, Kondo systems or 4d element nanostructures that exhibit emergent magnetism, thereby bridging the traditional areas of surface science, molecular physics and nanomagnetism.

Local structure relaxation, quantum trap depression, and valence charge polarization induced by the shorter-and-stronger bonds between under-coordinated atoms in gold nanostructures

Relativistic density functional theory calculations have been conducted to examine the effect of atomic under-coordination on the crystal structure, binding energy, and electron configuration of cuboctahedral and Marks decahedral gold clusters. Trend consistency between calculations and experimental observations confirmed the predictions made using BOLS correlation theory, suggesting that the shorter-and-stronger bonds between under-coordinated atoms induce local structure relaxation, potential well depression, and the associated local charge and energy densification, as well as the polarization of the otherwise conducting s-electrons (valence charge) by the densely- and tightly-trapped core electrons of which the binding energy shifts positively to deeper energies. Findings are in good agreement with scanning tunneling microscopy/spectroscopy results from monomers, dimers, chain ends, and nanostructures of gold and other metals.

Tuning the electronic coupling and magnetic moment of a metal nanoparticle dimer in the nonlinear dielectric-response regime

We show that the electronic coupling and magnetic moment of a silver nanoparticle dimer can be readily tuned by applying an electric field in the nonlinear dielectric-response regime. For a given interparticle separation, the electronic coupling becomes tunable as soon as the system crosses over from the linear to nonlinear regime. Remarkably, this transition takes place for modest strengths of the electric field. Further increase of the field strength may close the HOMO-LUMO gap of the dimer due to Stark shifts, accompanied by the emergence of a net magnetic moment from two nonmagnetic building blocks. These findings, obtained within density functional theory, exhibit the delicate coupling between the electronic and magnetic degrees of freedom and point to new approaches to gain multifunctionality of nanoparticle aggregates.

Magnetic Anisotropic Energy Gap and Strain Effect in Au Nanoparticles

We report on the observation of the size effect of thermal magnetization in Au nanoparticles. The thermal deviation of the saturation magnetization departs substantially from that predicted by the Bloch T(3/2)-law, indicating the existence of magnetic anisotropic energy. The results may be understood using the uniaxial anisotropy Heisenberg model, in which the surface atoms give rise to polarized moments while the magnetic anisotropic energy decreases as the size of the Au nanoparticles is reduced. There is a significant maximum magnetic anisotropic energy found for the 6 nm Au nanoparticles, which is associated with the deviation of the lattice constant due to magnetocrystalline anisotropy.

Electronic and magnetic properties of ultrathin Au/Pt nanowires

We have reported the synthesis of Au(25)Pt(75) and Au(48)Pt(52) alloyed ultrathin nanowires with average widths of less than 3 nm via a wet chemistry approach at room temperature. Using a combination of techniques, including scanning transmission electron microscopy equipped with X-ray energy dispersive spectroscopy, ultraviolet-visible spectroscopy, and X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopies, we identified the stoichiometry-dependent heterogeneous crystalline structures, as well as electronic structures with respect to the charge transfer between Pt and Au within both nanowires. In particular, we observed d-charge depletion at the Au site and the d-charge gain at the Pt site in Au(48)Pt(52) nanowires, which accounted for its ferromagnetic magnetic behavior, in contrast to the paramagnetism and diamagnetism appearing respectively in bulk Pt and Au.

Evidence for ligand-induced paramagnetism in CdSe quantum dots

We report evidence that paramagnetism in CdSe QDs can be induced via manipulation of the surface chemistry. Using SQUID magnetometry and X-ray absorption spectroscopy, we demonstrate that the paramagnetic behavior of the CdSe QDs can be varied by changing the ligand end-group functionality of the passivating layer. Contrary to previous reports, no evidence for ferromagnetism was observed. The results suggest that the paramagnetism is induced via pi back-bonding between Cd 4d orbtials and ligands with empty pi* orbitals.

s-Electron ferromagnetism in gold and silver nanoclusters

Ferromagnetic (FM) ordering in transition-metal systems (solids, surface layers, nanoparticles) arises from partially filled d shells. Thus, recent observations of FM Au nanoclusters was unexpected, and an explanation has remained elusive. Here we report first-principles density-functional spin-polarized calculations for Au and Ag nanoclusters. We find that the highest-occupied level is highly degenerate and partially filled by s electrons with spins aligned according to Hund's rule. The nanoclusters behave like "superatoms", with the spin-aligned electrons being itinerant on the outer shell of atoms.

Magnetic potassium clusters in a nanographite host system

The magnetism and structure for potassium clusters physisorbed in nanographite-based nanoporous host are investigated. Potassium clusters are subjected to a slight charge transfer to a host nanographite and modify the magnetism of the edge-state spins of nanographite. The localized spins of 4s electrons in the potassium cluster interact with each other through a strong antiferromagnetic interaction. Finite size effect and structural disorder around the cluster surface are responsible for anomalous spin fluctuations in the magnetism.