Size dependence of the magnetic moments of exposed nanoscale iron particles

The magnetic moments in exposed, mass-selected, nanoscale Fe clusters in the size range 1.89–2.20 nm (300–475 atoms), deposited onto graphic in situ have been measured by X-ray magnetic circular dichroism. The smallest clusters possess moments that are enhanced by around 4% for m_spin and 80% for m_orb and decrease towards the bulk value with increasing size. The larger clusters show an in-plane anisotropy that is consistent with the anisotropy in the orbital moment. The smallest clusters are, within experimental error, magnetically isotropic. The anisotropy constant in the 475-atom clusters is significantly higher than the bulk value.     

Growth and characterization of Au,Ni and Au–Ni nanoclusters on 6H-SiC(0001) carbon nanomesh

The nucleation and thermal stability of Au, Ni, and Au–Ni nanoclusters on 6H-SiC(0001) carbon nanomesh as well as the interaction between Au–Ni bimetallic clusters and reactive gases have been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Both Au and Ni atoms grow as three-dimensional (3D) clusters. Annealing the Au/carbon nanomesh surface up to 1150 °C leads to complete desorption of the Au clusters, while interfacial reaction occurs between Ni clusters and the substrate surface when the Ni clusters are subjected to the same annealing process. The nucleation of Au–Ni clusters depends critically on the deposition sequence. Au atoms preferentially nucleate on the existing Ni clusters, leading to the formation of bimetallic clusters with Au enriched on the surface. If the deposition sequence is reversed, a part of Ni atoms nucleate between the Au clusters. The thermal stability of the Au–Ni clusters resembles that of the Ni/carbon nanomesh surface, irrespective of the deposition sequence. XPS characterization reveals that Ni atoms in Au–Ni bimetallic clusters are oxidized upon exposure to 5.0 × 10^? 7 mbar O₂ for 5 min at room temperature while negligible structure change can be detected when the bimetallic clusters are exposed to CO gas under the similar conditions.     

Tantalum induced butterfly-like clusters on Si (111)-7 × 7 surface: STM/STS study at low coverage

The adsorption of the small amounts of tantalum on Si (111)-7 × 7 reconstructed surface is investigated systematically using scanning tunneling microscopy and tunneling spectroscopy combined with first-principles density functional theory calculations. We find out that the moderate annealing of the Ta covered surface results in the formation of clusters of the butterfly-like shape. The clusters are sporadically distributed over the surface and their density is metal coverage dependent. Filled and empty state STM images of the clusters differ strongly suggesting the existence of covalent bonds within the cluster. Tunneling spectroscopy measurements reveal small energy gap, showing semiconductor-like behavior of the constituent atoms. The cluster model based on experimental images and theoretical calculations has been proposed and discussed. Presented results show that Ta joins the family of adsorbates, that are known to form magic clusters on Si (111)-7 × 7, but its magic cluster has the structural and electronic properties that are different from those reported before.     

Scanning tunneling microscopy at multiple voltage biases of stable “ring-like” Ag clusters on Si(111)–(7 × 7)

Since more than twenty years it is known that deposition of Ag onto Si(111)–(7 × 7) leads under certain conditions to the formation of so-called “ring-like” clusters, that are particularly stable among small clusters. In order to resolve their still unknown atomic structure, we performed voltage dependent scanning tunneling microscopy (STM) measurements providing interesting information about the electronic properties of clusters which are linked with their atomic structure. Based on a structural model of Au cluster on Si(111)–(7 × 7) and our STM images, we propose an atomic arrangement for the two most stable Ag “ring-like” clusters.     

Experimental and simulated performance of lithium niobate 1–3 piezocomposites for 2 MHz non-destructive testing applications

Lithium niobate piezocomposites have been investigated as the active element in high temperature resistant ultrasonic transducers for non-destructive testing applications up to 400 °C. Compared to a single piece of lithium niobate crystal they demonstrate shorter pulse length by 3×, elimination of lateral modes, and resistance to cracking. In a 1–3 connectivity piezocomposite for high temperature use (200–400 °C), lithium niobate pillars are embedded in a matrix of flexible high temperature sealant or high temperature cement.In order to better understand the design principles and constraints for use of lithium niobate in piezocomposites experiments and modelling have been carried out. For this work the lithium niobate piezocomposites were investigated at room temperature so epoxy filler was used. 1–3 connectivity piezocomposite samples were prepared with z-cut lithium niobate, pillar width 0.3–0.6 mm, sample thickness 1–4 mm, pillar aspect ratio (pillar height/width) 3–6, volume fraction 30 and 45%. Operating frequency was 1–2 MHz.Experimental measurements of impedance magnitude and resonance frequency were compared with 3-D finite element modelling using PZFlex. Resonance frequencies were predicted within 0.05 MHz and impedance magnitude within 2–5% for samples with pillar aspect ratio ⩾3 for 45% volume fraction and pillar aspect ratio ⩾6 for 30% volume fraction. Laser vibrometry of pulse excitation of piezocomposite samples in air showed that the lithium niobate pillars and the epoxy filler moved in phase. Experiment and simulation showed that the thickness mode coupling coefficient k_t of the piezocomposite was maintained at the lithium niobate bulk value of approximately 0.2 down to a volume fraction of 30%, consistent with calculations using the (Smith and Auld, 1991 [1]) model for piezocomposites.     

Arrays of Ru nanoclusters with narrow size distribution templated by monolayer graphene on Ru

Ru nanoclusters self-assemble over macroscopic sample areas during vapor deposition of Ru on monolayer graphene (MLG) on Ru(0001). The Ru nanoclusters form arrays with a mean lateral cluster diameter of ~ 20 Å, cluster heights of 1 or 2 ML, and a size distribution that remains nearly constant with increasing coverage. Combined scanning tunneling microscopy and density functional theory (DFT) show that the clusters are templated by the MLG/Ru(0001) moiré unit cell and identify the preferred binding site of the clusters as the low fcc region of the moiré. Cross-sectional transmission electron microscopy (TEM) and high-resolution TEM contrast simulations experimentally demonstrate that the interaction of the Ru clusters with the underlying MLG/Ru(0001) leads to a local lifting of the graphene layer of the template. DFT calculations confirm this mechanism of interaction of the Ru clusters with the strongly coupled MLG/Ru(0001). Weakening of the graphene-support coupling via oxygen intercalation is shown to have a major effect on the assembly of Ru nanocluster arrays. With a preferred binding site lacking on decoupled graphene, the Ru nanoclusters grow significantly larger, and clusters with 1 to 4 ML height can coexist.     

Vapor–solid–solid growth of Ge nanowires from GeMn solid cluster seeds

We describe the fabrication of Ge nanowires during a single co-deposition step of Ge and Mn at high temperature. In these experimental conditions, a phase separation occurs and two different phases Ge and Ge_1 ? xMn_x are formed with Ge_1 ? xMn_x in the shape of small clusters distributed randomly in the Ge matrix. Because of the high deposition temperature, a new Ge_1 ? xMn_x phase with low eutectic point is stabilized; this phase is different from the one (commonly Ge₃Mn₅) stabilized at lower temperature. During the growth process at 350 °C, the crystalline clusters remain solid but they are highly mobile and can float at the surface, serving as seeds to direct the growth of crystalline Ge nanowires from the vapor. The sketch steps of NWs formation are first the phase separation with formation of specific Ge_1 ? xMn_x critical nuclei with low eutectic point and second the growth of Ge NWs directed by the Ge_1 ? xMn_x solid cluster seeds. Ge NWs growth is forced along particular crystalline axis by the cluster seeds that lower the interfacial energy Ge/Ge_1 ? xMn_x and the energy formation of the germanium crystal stabilizes the cluster position at the tip of the NWs. The density of NWs can be tuned by varying the nominal Mn concentration since this density is related to the number of clusters with the specific Ge_1 ? xMn_x phase (with low eutectic point). The single step MBE process presented here has the main advantage to fully avoid any incorporation of unintentional impurity into Ge nanowires (apart from Mn atoms) and could be applied to several other systems. This work also provides new insights into the vapor–solid–solid growth mechanisms of Ge NWs.     

Structural and electronic properties of group III Rich In0.53Ga0.47As(001)

The structural and electronic properties of group III rich In_0.53Ga_0.47As(001) have been studied using scanning tunneling microscopy/spectroscopy (STM/STS). At room temperature (300 K), STM images show that the In_0.53Ga_0.47As(001)–(4 × 2) reconstruction is comprised of undimerized In/Ga atoms in the top layer. Quantitative comparison of the In_0.53Ga_0.47As(001)–(4 × 2) and InAs(001)–(4 × 2) shows the reconstructions are almost identical, but In_0.53Ga_0.47As(001)–(4 × 2) has at least a 4× higher surface defect density even on the best samples. At low temperature (77 K), STM images show that the most probable In_0.53Ga_0.47As(001) reconstruction is comprised of one In/Ga dimer and two undimerized In/Ga atoms in the top layer in a double (4 × 2) unit cell. Density functional theory (DFT) simulations at elevated temperature are consistent with the experimentally observed 300 K structure being a thermal superposition of three structures. DFT molecular dynamics (MD) show the row dimer formation and breaking is facilitated by the very large motions of tricoodinated row edge As atoms and z motion of In/Ga row atoms induced changes in As–In/Ga–As bond angles at elevated temperature. STS results show there is a surface dipole or the pinning states near the valence band (VB) for 300 K In_0.53Ga_0.47As(001)–(4 × 2) surface consistent with DFT calculations. DFT calculations of the band-decomposed charge density indicate that the strained unbuckled trough dimers being responsible for the surface pinning.     

Scanning tunneling microscopy on iron-chalcogenide superconductor Fe(Se, Te) single crystal

We have investigated the iron-chalcogenide superconductor Fe(Se, Te) using a low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) technique. STM topography at 4.9 K shows clear regular square arrangements of spots with the lattice spacing ～0.37 nm, from which what we observe are attributed to Se or Te atomic plane. In the topography, brighter and darker atomic spots are randomly distributed, which are most probably due to Te and Se atoms, respectively. For the FeTe compound, the topography exhibits clusters of the bright spots probably arising from separated iron atoms distributing over several Te lattice sites. The STS measurements clarify the existence of the large-size gap with 2Δ = 0.4–0.6 eV.     

First-principles study of small palladium clusters on NiAl(1 1 0) alloy surface

Theoretical calculations focused on the geometry, stability, electronic and magnetic properties of small palladium clusters Pd_n (n=1–5) adsorbed on the NiAl(1 1 0) alloy surface were carried out within the framework of density functional theory (DFT). In agreement with the experimental observations, both Ni-bridge and Al-bridge sites are preferential for the adsorption of single palladium atom, with an adsorption energy difference of 0.04 eV. Among the possible structures considered for Pd_n (n=1–5) clusters adsorbed on NiAl(1 1 0) surface, Pd atoms tend to form one-dimensional (1D) chain structure at low coverage (from Pd₁ to Pd₃) and two-dimensional (2D) structures are more stable than three-dimensional (3D) structures for Pd₄ and Pd₅. Furthermore, metal-substrate bonding prevails over metal–metal bonding for Pd cluster adsorbed on NiAl(1 1 0) surface. The density of states for Pd atoms of Pd/NiAl(1 1 0) system are strongly affected by their chemical environment. The magnetic feature emerged upon the adsorption of Pd clusters on NiAl(1 1 0) surface was due to the charge transfer between Pd atoms and the substrate. These findings may shade light on the understanding of the growth of Pd metal clusters on alloy surface and the construction of nanoscale devices.     

Small silver clusters at paramagnetic defects of silica surfaces: A density functional embedded-cluster study

We studied the interaction of small Ag_n clusters (n = 1–4) with paramagnetic defect centers of a dehydroxylated silica surface using an all-electron scalar relativistic density functional method. The surface and adsorption complexes on it were modeled with an accurate quantum mechanics/molecular mechanics (QM/MM) scheme of embedding QM clusters in an elastic polarizable environment, described at the molecular mechanics level (MM). We analyzed two types of frequent point defects as sites for trapping and growing of Ag clusters: a silicon atom with a dangling bond (E′ center), ≡ Si?, and a non-bridging oxygen center (NBO), ≡ Si–O?. The Ag clusters interact with these paramagnetic centers forming strong covalent metal-defect bonds. The high adsorption energy allows one to consider the NBO and E′ sites as traps of single Ag atoms and as centers of cluster growth. We also explored the effect of adsorption on observable electronic properties of the silver clusters and of the defects of the silica surface.     

In situ STM observation during InAs growth in nano holes at 300 °C

We have successfully confirmed that In atoms were favored to congregate inside hole structures, during In and As₄ irradiations, by a STMBE system which was a scanning tunneling microscope located inside a molecular beam epitaxy growth chamber. After forming 1.5 monolayer of InAs wetting layer (WL) on a GaAs(001) surface, we applied voltage at a particular site on the WL during As₄ irradiation at 300 °C, creating hole structures (widths: 33–66.1 nm, depths: 4.9–9.7 nm). With the In and As₄ irradiations, spontaneously, In atoms on the WL were congregated inside the holes, decreasing the volume of the hole structures. It was found that InAs growth rates inside the hole structures were 23.1–217 times larger than that at the WL growth region near the holes.     

Local electronic structure of the Si(111)-(7 × 7) surface interacting with Ag atoms

Spatially resolved scanning tunneling spectroscopy was used for the investigation of changes in electronic structure of the Si(111)-(7 × 7) surface which are induced by the presence of a silver atom, a dimer and a cluster in a half-unit cell of the surface reconstruction. Tunnel spectra were measured at room temperature at significant positions in half-unit cells. The presence of one or two Ag atoms moving quickly in a half-unit cell significantly influences only filled states in spectra, while empty states remain practically unchanged and are still dominated by silicon adatom states. Corresponding features identified in the filled states depend on the number of Ag atoms in a half-unit cell and localization of these new states reflects the dynamic behavior of the Ag atom in a half-unit cell. A cluster of silver atoms in the half cell exhibits pronounced maxima in both filled and empty states of the spectrum.     

Effect of annealing temperature on the characteristics of ZnO thin films

Effect of annealing temperature on characteristics of sol–gel driven ZnO thin film spin-coated on Si substrate was studied. The UV–visible transmittance of the sol decreased with the increase of the aging time and drastically reduced after 20 days aging time. Granular shape of ZnO crystallites was observed on the surface of the films annealed at 550, 650, and 750 °C, and the crystallite size increased with the increase of the annealing temperature. Consequently nodular shape of crystallites was formed upon increasing the annealing temperature to 850 °C and above. The current–voltage characteristics of the Schottky diodes fabricated with ZnO thin films with various annealing temperatures were measured and analyzed. It is found that, ZnO films showed the Schottky characteristics up to 750 °C annealing temperature. The Schottky diode characteristics were diminished upon increasing the annealing temperature above 850 °C. XPS analysis suggested that the absence of oxygen atoms in its oxidized state in stoichiometric surrounding, might be responsible for the diminished forward current of the Schottky diode when annealed above 850 °C.     

Hydrogen chemisorption on Si(111)√ 3×√ 3R30∘-B passivated surface studied by thermal desorption and scanning tunneling microscopy

Atomic H chemisorption on the Si(111)√ 3×√ 3R30^°-B surface has been studied by thermal desorption spectroscopy (TDS) and scanning tunneling microscopy (STM). The B-modified Si surface is known to be inert towards adsorbates, since the surface dangling bonds of Si adatoms are passivated by B atoms sitting in sub-surface sites. However, it was found that even on a perfectly passivated surface, H is adsorbed on the surface by destroying the original √ 3 × √ 3 structure. STM observations revealed that H exposure led to the creation of defects at surface sites, and H was subsequently adsorbed as Si-monohydride at these sites. H exposure also caused cluster island formation at the top surface. The islands are composed of hydrogenated amorphous Si atoms or B-hydrogen complexes.     

Microstructure of multilayer interface in an Al matrix composite reinforced by TiNi fiber

A multilayer interface was formed in the Al matrix composite which was reinforced by 30% volume fraction of TiNi fiber. The composite was fabricated by pressure infiltration process and the interface between the TiNi fiber and Al matrix was investigated by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). When the TiNi fiber was pre-oxidized in the air at 773 K for 1 h, three layers have been found and characterized in the interface: TiNi–B₂ layer near the TiNi fiber, Ti–Al compound layer with Ti and granular TiO₂ near the Al matrix, and Ti–Ni compound layer between TiNi–B₂ and Ti–Al compound layers. The effect of the multilayer interface on the mechanical properties of the composite was also discussed. The result showed that the uniaxial tensile strength of the composite at room temperature was 318 MPa, which was very close to the theoretical calculation value of 326 MPa. Moreover, the composite with good ductility exhibited a typical ductile-fracture pattern.     

Real time dynamics of Si magic clusters mediating phase transformation: Si(111)-(1 × 1) to (7 × 7) reconstruction revisited

Using Scanning Tunneling Microscope (STM), we show that the surface undergoes phase transformation from disordered “1 × 1” to (7 × 7) reconstruction which is mediated by the formation of Si magic clusters. Mono-disperse Si magic clusters of size ~ 13.5 ± 0.5 Å can be formed by heating the Si(111) surface to 1200 °C and quenching it to room temperature at cooling rates of at least 100 °C/min. The structure consists of 3 tetra-clusters of size ~ 4.5 ? similar to the Si magic clusters that were formed from Si adatoms deposited by Si solid source on Si(111)-(7 × 7) [1]. Using real time STM scanning to probe the surface at ~ 400 °C, we show that Si magic clusters pop up from the (1 × 1) surface and form spontaneously during the phase transformation. This is attributed to the difference in atomic density between “disordered 1 × 1” and (7 × 7) surface structures which lead to the release of excess Si atoms onto the surface as magic clusters.     

Anomalous scaling behaviour of cobalt cluster size distributions on graphite,epitaxial graphene and carbon-rich (6√3 × 6√3)R30°

The influence of the underlying interface on adsorption of cobalt (Co) is investigated by comparing the nucleation and growth of Co at room temperature on three carbon (C) surfaces, i.e. highly oriented pyrolytic graphite (HOPG), epitaxial graphene/SiC(0001) (hereafter abbreviated as EG) and precursor of EG i.e. C-rich (6√3 × 6√3)R30°/SiC(0001) (hereafter abbreviated as 6√3). On all three surfaces, Co adopts Volmer–Weber growth mode via formation of three-dimensional dome-shaped nanoclusters. Co clusters formed on 6√3 surface are smaller but denser than Co/HOPG or Co/EG. Scaling analysis reveals a critical nucleus size, i* = 1 (atom) and the smallest stable cluster (i* + 1) would be a dimer. Co/HOPG and Co/EG have the same order of magnitude for their cluster densities and sizes. Scaling analyses however show that the i* for Co/EG (i* = 3) is larger than Co/HOPG (i* = 0) and in this respect the smallest stable cluster would be tetramer and monomer respectively. This difference is attributed to the influence of an interface situated between graphene and SiC bulk. It appears that EG is more inert than HOPG towards the adsorption of Co and may act as a better substrate to host Co clusters.     

H adsorption at Ag/Si interfaces in epitaxially grown Ag(1 1 1) films on Si(1 1 1)7 × 7 substrates

The interaction of atomic H with Ag(1 1 1)/Si(1 1 1)7 × 7 surfaces was studied by thermal desorption (TD) spectroscopy and scanning tunneling microscopy (STM) at room temperature. TD spectroscopy revealed an intense peak from mono H–Si bonds, even though the Si surface was covered by the Ag atoms. This peak was not observed from Ag-coated SiO₂/Si substrates. STM observation showed no clear change of the Ag surface morphology resulting from H exposure. All these results indicate that the atomic H adsorbs at neither the Ag surfaces nor Ag bulk sites, but at the Ag/Si interface by diffusing through the Ag film.