Fe Clusters Research Articles

Structures and stability of metal-doped GenM (n = 9, 10) clusters

The lowest-energy structures of neutral and cationic GenM (n = 9, 10; M = Si, Li, Mg, Al, Fe, Mn, Pb, Au, Ag, Yb, Pm and Dy) clusters were studied by genetic algorithm (GA) and first-principles calculations. The calculation results show that doping of the metal atoms and Si into Ge9 and Ge10 clusters is energetically favorable. Most of the metal-doped Ge cluster structures can be viewed as adding or substituting metal atom on the surface of the corresponding ground-state Gen clusters. However, the neutral and cationic FeGe9,10,MnGe9,10 and Ge10Al are cage-like with the metal atom encapsulated inside. Such cage-like transition metal doped Gen clusters are shown to have higher adsorption energy and thermal stability. Our calculation results suggest that Ge9,10Fe and Ge9Si would be used as building blocks in cluster-assembled nanomaterials because of their high stabilities.

Adsorption behavior of Fe atoms on a naphthalocyanine monolayer on Ag(111) surface**Project supported by the National Natural Science Foundation of China (Grant Nos. 61390501, 51325204, and 11204361), the National Basic Research Program of China (Grant Nos. 2011CB808401 and 2011CB921702), the National Key Scientific Instrument and Equipment Development Project of China (Grant No. 2013YQ1203451), the National Supercomputing Center in Tianjin, China,

Adsorption behavior of Fe atoms on a metal-free naphthalocyanine (H2Nc) monolayer on Ag(111) surface at room temperature has been investigated using scanning tunneling microscopy combined with density functional theory (DFT) based calculations. We found that the Fe atoms were adsorbed on the centers of H2Nc molecules and formed Fe–H2Nc complexes at low coverage. DFT calculations show that Fe sited in the center of the molecule is the most stable configuration, in good agreement with the experimental observations. After an Fe–H2Nc complex monolayer was formed, the extra Fe atoms self-assembled to Fe clusters of uniform size and adsorbed dispersively at the interstitial positions of Fe–H2Nc complex monolayer. Therefore, the H2Nc monolayer grown on Ag(111) could be a good template to grow dispersed magnetic metal atoms and clusters at room temperature for further investigation of their magnetism-related properties.

Protective role of bacillithiol in superoxide stress and Fe-S metabolism in Bacillus subtilis.

Glutathione (GSH) serves as the prime thiol in most organisms as its depletion increases antibiotic and metal toxicity, impairs oxidative stress responses, and affects Fe and Fe–S cluster metabolism. Many gram-positive bacteria lack GSH, but instead produce other structurally unrelated yet functionally equivalent thiols. Among those, bacillithiol (BSH) has been recently identified in several low G+C gram-positive bacteria. In this work, we have explored the link between BSH and Fe–S metabolism in Bacillus subtilis. We have identified that B. subtilis lacking BSH is more sensitive to oxidative stress (paraquat), and metal toxicity (Cu(I) and Cd(II)), but not H2O2. Furthermore, a slow growth phenotype of BSH null strain in minimal medium was observed, which could be recovered upon the addition of selected amino acids (Leu/Ile and Glu/Gln), supplementation of iron, or chemical complementation with BSH disulfide (BSSB) to the growth medium. Interestingly, Fe–S cluster containing isopropylmalate isomerase (LeuCD) and glutamate synthase (GOGAT) showed decreased activities in BSH null strain. Deficiency of BSH also resulted in decreased levels of intracellular Fe accompanied by increased levels of manganese and altered expression levels of Fe–S cluster biosynthetic SUF components. Together, this study is the first to establish a link between BSH and Fe–S metabolism in B. subtilis.

Magnetic properties of iron cluster/chromium matrix nanocomposites.

A custom-designed apparatus was used for the fine-tuned co-deposition of preformed Fe clusters into antiferromagnetic Cr matrices. Three series of samples with precisely defined cluster sizes, with accuracy to a few atoms, and controlled concentrations were fabricated, followed by a complete characterization of structure and magnetic performance. Relevant magnetic characteristics, reflecting the ferromagnetic/antiferromagnetic coupling between Fe clusters and the Cr matrix, i.e., blocking temperature, coercivity field, and exchange bias were measured and their dependence on cluster size and cluster concentration in the matrix was analyzed. It is evident that the blocking temperatures are clearly affected by both the cluster size and their concentration in the Cr matrix. In contrast the coercivity shows hardly any dependence on size or inter-cluster distance. The exchange bias was found to be strongly sensitive to the cluster size but not to the inter-cluster distances. Therefore, it was concluded to be an effect that is purely localized at the interfaces.

Influence of Cr on the nanoclusters formation and superferromagnetic behavior of Fe-Cr-Nb-B glassy alloys

High resolution imaging and electron diffraction confirm that in the as-quenched state the structure of Fe79.7−xCrxNb0.3B20 (x = 11–13 at. %) melt-spun ribbons is completely amorphous, independent of the Cr content. Energy-dispersive X-ray spectroscopy mapping emphasizes clearly the presence of Fe and Cr clusters varying from approximately 1 to 2–3 nm in size with the increase of Cr content from 11 to 13 at. %. The Fe and Cr atoms segregate the atomic scale to form nanometer sized clusters, influencing strongly the macroscopic magnetic behavior. The Curie temperature of the system, TCsystem, confirmed by the magnetic susceptibility versus temperature measurements, gives the strength of the magnetic interactions between clusters. The inter-cluster interactions are much stronger for lower contents of Cr, the microstructure is less uniform, and TCsystem increases from 290 K for 13 at. % Cr to 330 K for 11.5 at. % Cr. The whole system transforms to a ferromagnetic state through interactions between the clusters. Zero-field cooling and field cooling curves confirm the cluster behavior with a blocking temperature, Tb, of about 250 K. Above Tb, the ribbons behave as a superferromagnetic system, whilst below the blocking temperature a classical ferromagnetic behavior is observed.

Low- and high-temperature magnetism of Cr and Fe nanoclusters in iron-chromium alloys

Low-energy magnetic states and finite-temperature properties of Cr nanoclusters in bulk bcc Fe and Fe nanoclusters in bulk Cr are investigated using density functional theory (DFT) and the Heisenberg-Landau Hamiltonian based magnetic cluster expansion (MCE). We show, by means of noncollinear magnetic DFT calculations, that magnetic frustration caused by competing ferromagnetic and antiferromagnetic interactions either strongly reduces local magnetic moments while keeping collinearity or generates noncollinear magnetic structures. Small Cr clusters generally exhibit collinear ground states. Noncollinear magnetic configurations form in the vicinity of small Fe clusters if antiferromagnetic Fe-Cr coupling dominates over ferromagnetic Fe-Fe interactions. MCE predictions broadly agree with DFT data on the low-energy magnetic structures, and extend the DFT analysis to larger systems. Nonvanishing cluster magnetization caused by the dominance of Fe-Cr over Cr-Cr antiferromagnetic coupling is found in Cr nanoclusters using both DFT and MCE. Temperature dependence of magnetic properties of Cr clusters is strongly influenced by the surrounding iron atoms. A Cr nanocluster remains magnetic until fairly high temperatures, close to the Curie temperature of pure Fe in the large cluster size limit. Cr-Cr magnetic moment correlations are retained at high temperatures due to the coupling of interfacial Cr atoms with the Fe environment. Variation of magnetization of Fe-Cr alloys as a function of temperature and Cr clusters size predicted by MCE is assessed against the available experimental data.

Hierarchical BiOCl microflowers with improved visible-light-driven photocatalytic activity by Fe(III) modification

Hierarchical BiOCl microflowers have been synthesized by one-step solvothermal method, which are constructed from many thin nanosheets by exposing the highly active highly active {001} facets. Considering the wide band gap of BiOCl microflowers (3.4eV), the amorphous Fe(III) clusters were grafted on the surfaces through a simple impregnation method to extend the photocatalytic activity to the visible light region. The obtained products were characterized by X-ray diffraction, scanning electron and transmission microscopy, X-ray photoelectron spectroscopy, UV–vis diffuse reflectance spectroscopy, nitrogen adsorption–desorption, and electrochemical measurements. It is found that the Fe(III) clusters are just deposited on the surfaces rather than doped in the lattices of BiOCl. The morphologies and crystal structures of BiOCl microflowers thus remain unchanged after modification of Fe(III) clusters. The photocatalytic activity for the decomposition gaseous acetaldehyde can be significantly improved by the modification of Fe(III) clusters under visible light irradiation. The surface Fe(III) clusters play an important role in the interfacial charge transfer for the visible light absorption. Furthermore, together with the strong oxidative holes in the valence band of BiOCl, the Fe(III) clusters can serves as the redox reaction centers for the multi-electron reduction of oxygen molecules, resulting in the full decomposition of acetaldehyde into CO2.

A versatile apparatus for the fine-tuned synthesis of cluster-based materials.

In this paper, a custom-designed experimental setup for the fine-tuned synthesis of various cluster-based materials is presented. Providing custom-designed deposition stages and special sample holders it offers a high degree of control over the sample characteristics such as the cluster size, cluster amounts, and sample homogeneity in combination with high sample purity. The system is capable of producing thin films of pure clusters and various cluster-matrix combinations with cluster sizes ranging from single atoms up to aggregates of several thousand atoms. Two custom-designed deposition stages are available, one utilizes the full cluster beam, yielding micrograms of pure clusters within a few hours and the second one uses mass-separated clusters (mass-resolution between 2% and 10%), yielding nanograms of pure clusters in the same timescale. Furthermore, at the second deposition stage, a variety of matrix materials can be co-deposited at a controlled sample temperature between 153 K and 673 K. In order to prove the capabilities of the apparatus, a series of experiments with Fe clusters embedded in Ag matrices with different volume fractions of clusters were carried out. Energy dispersive X-ray spectroscopy measurements revealed that the amount of deposited clusters as well as the sample homogeneity can be controlled with an outstanding accuracy of 10%. Additional measurements of the magnetic properties indicated the presence of separated clusters for volume fractions of clusters around 2 volume percent (vol. %), while above this concentration (10 vol. %) a partial aggregation of the clusters was observed. It was also shown that the resulting thin films are nearly oxygen free, which ascertains that oxygen sensitive materials can be safely handled in this newly developed apparatus.

Structure of the key species in the enzymatic oxidation of methane to methanol.

Methane monooxygenase (MMO) catalyses the O2-dependent conversion of methane to methanol in methanotrophic bacteria, thereby preventing the atmospheric egress of approximately one billion tons of this potent greenhouse gas annually. The key reaction cycle intermediate of the soluble form of MMO (sMMO) is termed compound Q (Q). Q contains a unique dinuclear Fe(IV) cluster that reacts with methane to break an exceptionally strong 105kcalmol(-1) C-H bond and insert one oxygen atom. No other biological oxidant, except that found in the particulate form of MMO, is capable of such catalysis. The structure of Q remains controversial despite numerous spectroscopic, computational and synthetic model studies. A definitive structural assignment can be made from resonance Raman vibrational spectroscopy but, despite efforts over the past two decades, no vibrational spectrum of Q has yet been obtained. Here we report the core structures of Q and the following product complex, compound T, using time-resolved resonance Raman spectroscopy (TR(3)). TR(3) permits fingerprinting of intermediates by their unique vibrational signatures through extended signal averaging for short-lived species. We report unambiguous evidence that Q possesses a bis-μ-oxo diamond core structure and show that both bridging oxygens originate from O2. This observation strongly supports a homolytic mechanism for O-O bond cleavage. We also show that T retains a single oxygen atom from O2 as a bridging ligand, while the other oxygen atom is incorporated into the product. Capture of the extreme oxidizing potential of Q is of great contemporary interest for bioremediation and the development of synthetic approaches to methane-based alternative fuels and chemical industry feedstocks. Insight into the formation and reactivity of Q from the structure reported here is an important step towards harnessing this potential.

Electronic structure, stability and magnetic properties of small M1–4(M = Fe, Co, Ni) clusters encapsulated inside a (BN)48 cage

The geometrical structure and magnetic properties of M1–4(M=Fe, Co and Ni) clusters within a (BN)48 cage were calculated at the BPW91/LanL2DZ level. The small M1–4 clusters generally prefer an off-centered position near the hexagonal rings in the (BN)48 cages. The (BN)48 cages can increase the stability of these small magnetic clusters while protecting the magnetic nature of M and M2 clusters.

On the interplay between geometrical structure and magnetic anisotropy: a relativistic density-functional study of mixed Pt–Co and Pt–Fe trimers and tetramers in the gas-phase and supported on graphene

The structural and magnetic properties of mixed Pt–Co and Pt–Fe trimers and tetramers in the gas-phase and supported on a free-standing graphene layer have been calculated using density-functional theory. The influence of the strong magnetic moments of the 3d atoms on the Pt atoms and the influence of the strong spin–orbit coupling contributed by the Pt atoms on the 3d atoms have been studied in detail. All mixed trimers form isocele triangles in the gas-phase. On a graphene layer the structure is influenced by the strong binding of the 3d atoms, leading to an asymmetric configuration for Pt-rich and more symmetric structures for 3d-rich clusters. The magnetic anisotropy energy defined as the energy difference for easy and hard magnetization directions varies between 5 and 13 meV/atom for the free trimers, but is strongly reduced to values between 0.7 and 6.6 meV/atom for the graphene-supported clusters. The saddle-point energy representing the barrier against magnetization reversal is on average 3 meV/atom for free trimers, it is reduced to 2 meV/atom for the more symmetric PtCo(Fe)2 clusters, and to only about 0.3 meV/atom for the asymmetric Pt2Co(Fe) cluster on graphene. For the mixed tetramers the strong magnetism stabilizes a flat geometric structure, except for Pt3Co which forms a distorted trigonal pyramid. The geometry of the graphene-supported tetramers is very different due to the requirement of a good match to the substrate. Large magnetic anisotropy energies are found for free Pt3Co where the change of the magnetization direction also induces a transition from a high- to a low-moment magnetic isomer. For all other free tetramers the magnetic anisotropy energy ranges between 3 to 5 meV/atom only, it is further reduced to 0.4 to 3.8 meV/atom for the graphene-supported tetramers. The reduction is strongest for Pt3Fe/graphene because of the asymmetric structure of the adsorption complex. The barriers against magnetization reversal range between only 0.3 meV/atom for Pt3Fe/graphene and about 3 meV/atom for PtFe3 and Pt3Co. Altogether our results demonstrate a strong correlation between the geometric and magnetic degrees of freedom and the necessity to base investigations of the magnetic anisotropy of nanostructures on a simultaneous optimization of the total energy with respect to all geometric and magnetic parameters.

Formation of bcc and fcc during the coalescence of free and supported Fe and Ni clusters.

The formation of bcc and fcc during the coalescence of free and supported Fe and Ni clusters has been studied by molecular dynamics simulation using an embedded atom method. Structural evolution of the clusters, coalesced under varying temperature, Ni content and substrate conditions, was explored by interatomic energy, snapshots, pair distribution functions and bond order parameters. The results show that the formation of bcc and fcc is strongly related to Ni content, substrate and coalescence temperature. Free clusters coalesced at 1200 K form bcc at lower Ni contents with fcc forming at higher Ni concentrations and no observable coexistence of bcc and fcc. Differences in coalescence at 1000 K result from the coexistence of bcc and fcc within the Ni range of 50-70%. Free clusters supported on disordered Ni substrates were shown to transform from spherical morphology to islands of supported clusters with preferred epitaxial orientation. The Ni content required to form bcc and fcc coexistence on supported clusters at 1000 K decreased to 30-50% Ni. Free clusters possessing bcc and fcc generally stacked along the bcc (110) and fcc (111) facets, whereas supported clusters stacked along the (111) bcc and (100) fcc planes. Structural transformation was induced by clusters containing greater numbers of atoms. Spread over the substrate enhanced interatomic energy, order substrates affect the epitaxial growth direction and increase the melting points of the supported clusters. This study can be used to predict the nature of fcc and bcc formation in Fe-Ni films.

Structural Heterogeneity of the Melt-spun (Fe, Co)-Si-B-P-Cu Alloy with Excellent Soft Magnetic Properties

A structural heterogeneity of a newly developed soft magnetic Fe81.2Co4Si0.5B9.5P4Cu0.8 melt-spun alloy has been studied by transmission electron microscopy (TEM) and electron diffraction. Hollow-cone dark-field TEM imaging revealed that the density of coherent scattering regions in the as-quenched Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy is as high as that in the Fe85.2Si2B8P4Cu0.8 hetero-amorphous alloy (NANOMET®). According to the aberration-corrected high-resolution TEM, crystalline atomic clusters, typically of ∼1nm in diameter, are densely distributed in an amorphous matrix of Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy, while nanobeam electron diffraction detected weak Bragg reflections from Fe clusters with the body centered cubic structure. These results unambiguously reveal structural heterogeneity of the as-quenched Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy, which is similar to that observed in the NANOMET®.

Low-valent iron: an Fe(I) ate compound as a building block for a linear trinuclear Fe cluster.

A low-valent trinuclear iron complex with an unusual linear Fe(I)-Fe(II)-Fe(I) unit is presented. It is accessed in a rational approach using a salt metathesis reaction between a new anionic Fe(I) containing heterocycle and FeCl2. Its electronic structure was studied by single crystal XRD analysis, EPR and Mössbauer spectroscopy, and magnetic susceptibility measurements.

Fe原子在Al基底上的扩散及薄膜生长条件对磁特性的影响

利用蒙特卡罗方法模拟Fe原子在Al基底上的沉积、扩散，以及Fe的团簇形貌的生长演化。研究了薄膜生长条件对Fe薄膜的分形形貌及磁特性的影响。模拟结果表明，随着温度的升高，Fe微观形貌经历了一个从分散成长、分形成长到凝聚成长的变化过程；而Fe薄膜的形貌极大地影响薄膜的磁特性，具有更紧凑的分形形貌的Fe薄膜具有更高的居里温度。 A Monte Carlo technique has been developed for simulating the deposition and diffusion of Fe atoms on Al (100) surfaces, and the growth of Fe clusters. The effects of growth conditions on the Fe fractal morphology and the magnetic properties are investigated. The simulated results indicate that with increasing temperature of substrate, the surface morphology evolves from dispersed growth, fractal growth to dense growth, and finally to another dispersed growth. It is also found that the morphology of Fe film greatly affects the magnetic properties, and ramified atomic aggregate with a more compact morphology has a higher Curie temperature.

Instability of Ferromagnetic Nanoclusters in Fe Implanted Amorphous SiO2

A 460nm thick amorphous SiO2 layer, formed on a Si (100) surface by air-annealing the Si substrate at 1100oC for 24h, was implanted with 57Fe to a fluence of 1 x 1016/cm2 at room temperature and annealed at temperatures up to 1000oC. The implanted and annealed samples were studied by conversion electron Mössbauer spectroscopy (CEMS) and magnetization measurements.The CEMS spectra up to an annealing temperature of 600oC showed the presence of a singlet due to dispersed Fe ions and paramagnetic doublets with hyperfine parameters characteristic of Fe2+ and Fe3+. The spectrum after the 1000oC annealing was dominated (> 80%) by ferromagnetic sextets, the main components of which were sextets with a hyperfine field of 320(20) kOe and 264(20) kOe, showing the formation of Fe0 clusters, in agreement with previous observations. Magnetization measurements (m(H)) on the sample after the 1000oC annealing showed a small hysteresis at 4K and saturation magnetization with zero hysteresis at room temperature, reached with application of small external field.The CEMS measurement on this sample was repeated after storing the sample under ambient conditions for a period of 6 months. The spectrum showed complete disappearance of the ferromagnetic sextets and the presence only of paramagnetic doublets due to Fe2+. Evidently progressive oxidation of the Fe clusters had occurred. Magnetization results confirm the paramagnetic transformation of the Fe clusters.

The growth of Fe clusters over graphene/Cu(111)

The growth of Fe clusters up to nine atoms over graphene/Cu(111) is investigated within the density functional theory. Graphene is weakly physisorbed on Cu(111) through van der Waals force. The structures of Fe clusters over graphene/Cu(111) grow differently compared to gas-phase Fe clusters where Fe clusters are predicted to form towards a pyramid-like structure on graphene/Cu(111). The graphene is negatively charged upon the adsorption of Fe clusters as a result of charge transfer from Fe to graphene. Despite the fact that the electronic structure of graphene is affected by Fe clusters, magnetic moment of Fe clusters over graphene/Cu(111) remains relatively high. This suggests that graphene can be a potential substrate for supporting Fe clusters towards applications in magnetism and catalysis.

Adsorption and dissociation of ammonia on small iron clusters

The stepwise dehydrogenation of NHx on small iron clusters is investigated from the density functional theory (DFT) calculations. The results indicate that the fewer the H atoms of the NHx (x = 0–3) species, the higher the adsorption energies of NHx on the same Fe cluster are. Also the larger the cluster size, the stronger the adsorption for the same NHx species is. The catalytic activity of small Fe clusters for NH3 dehydrogenation is comparatively studied, and the rate-limiting steps for the reactions are addressed. It is found that the rate-limiting steps for the dehydrogenation of NHx on the Fe and Fe3 are that for the NH decomposition, and those on the Fe2 and Fe4 are that for the NH2 dissociation. The barriers for the rate-limiting steps are 1.06, 1.49, 1.43 and 1.51 eV for the dehydrogenation of NHx on the Fe, Fe2, Fe3 and Fe4 clusters, respectively. The results suggest that the small Fe clusters can be regarded as the potential candidates for NH3 dehydrogenation reactions and can serve as a reference for further investigations on the catalytic activity of small Fe clusters supported by various catalytic materials.

Structure and magnetic properties of Fe nanoparticles embedded in a Cr matrix

Abstract The structure of 2 nm diameter (340 atoms) Fe nanoparticles embedded in a Cr matrix was determined using X-ray Absorption Fine Structure (EXAFS) and the magnetic properties studied by Superconducting Quantum Interference Device (SQUID) magnetometry. The thin films were produced by the co-deposition of pre-formed gas-phase Fe clusters synthesised by a gas aggregation source with an atomic vapour of Cr produced by an MBE source. The behaviour was studied as a function of Fe nanoparticle volume fraction in the range 5–20% and was compared to previous results on ferromagnetic nanoparticles in antiferromagnetic matrices. EXAFS showed that the atomic structure in the Cr-embedded Fe nanoparticles is the same as the bulk bcc structure. Whereas alloying between the nanoparticles and matrix material has previously been shown to be very pronounced for Co nanoparticles in antiferromagnetic Mn, it was found that any alloying between Fe nanoparticles and Cr matrix material is limited. For dilute samples of Fe nanoparticles in Cr the measured saturation magnetisation ( M S ) was 1 µ B /Fe atom, which is significantly less than the bulk M S value of 2.22 µ B /Fe atom indicating that the surface of Fe nanoparticles is either antiferromagnetic or non-magnetic. An increase in the volume fraction produces an increase in the value of M S and at a volume fraction of 20%, M S exceeds the value of bulk Fe showing that some Cr spins provide a ferromagnetic contribution. After field cooling below 30 K, all films show Exchange Bias (EB) and an increase of coercivity, which are both much larger for the most concentrated sample. The Cr spins at the surface of the Fe particles play a key role in determining the overall magnetic behaviour.

Magnetic properties of Fe nano-clusters stabilized at grain boundaries of Yb films

The temperature dependence of the magnetic properties of Fe nano-sized clusters in (5.0 at. % Fe)Yb films, prepared by vapor co-deposition, has systematically been investigated using zero-field and in-field Mössbauer spectroscopy as well as AC and DC magnetization measurements. Room temperature Mössbauer spectra reveal two types of Fe clusters, with non-cubic symmetry, located at Yb grain boundaries. Spectra, taken at 4.2 K, show a complex distribution of magnetic hyperfine fields, which can be related to inhomogeneous inter-cluster interactions. In addition, zero-field and in-field Mössbauer experiments indicate that the interactions within the Fe-clusters are predominantly ferromagnetic-like and their average magnetic moments are about 80 μB. As suggested by Mössbauer and AC magnetization data, the Fe-clusters are superparamagnetic above ca. 20 K, while their magnetic behavior below this temperature is that of cluster-glass-like systems with weak inter-cluster interactions. The details of the freezing process as a function of frequency and magnetic field are discussed and magnetic anisotropy constants for these Fe clusters are estimated to be Keff ∼ 8–12 × 105 J m−3.