Photoemission study of Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0)

An angle-resolved photoemission study for Ag nanofilm grown on pseudomorphic metastable-fcc-phase Fe(1 0 0) has been done in order to investigate in detail the quantized electronic structures. From the low-energy electron-diffraction and angle-resolved photoemission spectra, it is found that the present Ag nanofilms were grown in the direction of [1 1 1] on pseudomorphic fcc Fe(1 0 0) substrates. The angle-resolved photoemission spectra of Ag nanofilms grown on pseudomorphic fcc Fe(1 0 0) exhibit the features derived from Shockley-type surface state and additional fine-structures derived from the quantized state of Ag sp valence electron. The experimental nanofilm-thickness dependence of binding energies of these quantized states is compared with the theoretical calculation based on the phase accumulation model, taking into account the phase shifts of electron reflection at both interfaces of the Ag nanofilm. From these results, we discuss the quantized electronic structure in Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0).     

The initial growth structure of Ni1-xFex (x=0.6–0.8) films dc-biased plasma-sputter-deposited on Ni/MgO(0 0 1), and on Fe/MgO(0 0 1)

Cross sectional and plane-view transmission electron microscopy (X- and PV-TEM) were used to investigate the initial growth phase of 5, 10, 20 and 40 nm thick Ni_1-xFe_x (x=0.6–0.8) films, prepared on MgO(0 0 1) covered with a buffer layer of Fe or Ni as well as on naked MgO(0 0 1). The 100 nm thick buffer layers of Fe and Ni were pre-grown on MgO(0 0 1). All of Ni_0.20Fe_0.80, Ni_0.40Fe_0.60, Fe and Ni films could be epitaxially grown at 250°C by dc-biased plasma sputtering at 2.9 kV in pure Ar gas.The films of Ni_0.20Fe_0.80 and Ni_0.40Fe_0.60 were grown in their own stable phase, bcc and fcc on MgO(0 0 1), respectively. However, Ni_0.20Fe_0.80 film could be grown in fcc phase pseudomorphic with Ni(0 0 1) up to 20 nm thick on Ni/MgO(0 0 1), while Ni_0.40Fe_0.60 film in bcc phase pseudomorphic with Fe(0 0 1) up to 10 nm thick on Fe/MgO(0 0 1). With increasing thickness, their growth phases transformed into their own stable phases. Whether or not the pseudomorphic phase may be induced and what its critical thickness may be should depend primarily on the lattice misfit between the crystal planes in contact. The growth mode of Ni_0.40Fe_0.60 film was investigated more in details to be compared with the simulations of the average strain energy versus thickness and with those of the critical thickness of the pseudomorphic films versus the lattice misfit between the contacted crystal planes.     

Angle-resolved photoemission study of Ag nanofilms grown on fcc transition-metal (111) substrates

A comparative study of the electronic structures of Ag nanofilms on the pseudomorphic metastable fcc Fe(111) and bulk-like fcc Co(111) substrates has been carried out to investigate their quantized electronic structures. The photoemission spectra of both Ag nanofilms exhibit the fine structures derived from the quantized sates of Ag sp valence electron. The nanofilm-thickness dependences of the binding energy of the quantized states are reproduced by the calculated results based on the phase accumulation model. From the angle-resolved photoemission measurements, the effective masses of the quantized electronic states along the direction parallel to the nanofilm surface were directly determined. We discuss the electronic hybridization effect between quantized states in Ag nanofilm and 3d-derived electronic states in transition-metal substrates.     

Angle-resolved photoemission study for double Ag nanofilm structures

We have carried out an angle-resolved photoemission study for Ag/Cu/Ag/Cu(1 1 1) system in order to investigate the electronic coupling between the two quantum-well (QW) states in the double Ag nanofilm structures. It is found that the outer nanofilm thickness dependence of QW state in double Ag nanofilm structures can be explained as the electronic coupling through the thin Cu barrier layer between the QW states in the inner and outer Ag nanofilms. It is also found that the coupling strength depends on the Cu barrier thickness. From these results, we discuss the electronic coupling between the two QW states in the double Ag nanofilm structures.     

A high (0 0 1)-oriented CoPt/Ag film deposited on glass substrate

The magnetic properties and structures of CoPt_15 nm/Ag_0−100 nm films deposited by DC magnetron sputtering on glass substrates have been studied. The (0 0 1) texturing was improved by introducing an Ag underlayer. As the Ag underlayer thickness was 100 nm, a nearly perfect (0 0 1)-textured CoPt film was obtained.     

Structural and magnetic properties of BCC Fe/Co (0 0 1) superlattices

In thin layered Fe/Co (0 0 1), grown on MgO (0 0 1), both Fe and Co crystallize in the body-centered cubic (BCC) structure, as seen in a series of superlattices where the layer thickness of the components is varied from two to twelve atomic monolayers. These superlattices have novel magnetic properties as observed by magnetization and polarized neutron reflectivity measurements. There is a significant enhancement of the magnetic moments of both Fe and Co at the interfaces. Furthermore, the easy axis of the system changes from [1 0 0] for films of low cobalt content to [1 1 0] for a Co content exceeding 33%. No indication of a uniaxial anisotropy component is found in any of the samples. The first anisotropy constant (K₁) of BCC Co is found to be negative with an estimated magnitude of 110 kJ/m³ at 10 K. In all cases, the magnetic moments of Fe and Co have parallel alignment.     

Role of electrolyte pH on structural and magnetic properties of Co–Fe films

Co–Fe films were electrodeposited on polycrystalline Titanium substrates from the electrolytes with different pH levels. X-ray diffraction (XRD) was used to study the crystal structure of the films. The XRD patterns showed that the films grown at the pH levels of 3.70 and 3.30 have a mixed phase consisting of face-centred cubic (fcc) and body-centred cubic, while those grown at pH=2.90 have only fcc structure. It was observed that the film composition, by energy dispersive x-ray spectroscopy, contain around 88 at% Co and 12 at% Fe for all films investigated in this study. Morphological observations indicated that all films have grainy structure with the slight change of grain size depending on the electrolyte pH. Magnetoresistance measurements, made at room temperature, showed that all films exhibited anisotropic magnetoresistance, which is affected by the electrolyte pH. From the magnetic measurements made by vibrating sample magnetometer, the saturation magnetization increases as the electrolyte pH decreases. Furthermore, all films have in-plane easy-axis direction of magnetization.     

Structural and magnetic properties of thin epitaxial Fe films on (1 1 0) GaAs prepared by metalorganic chemical vapor deposition

Iron films have been grown on (1 1 0) GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition at substrate temperatures (T_s) between 135°C and 400°C. X-ray diffraction (XRD) analysis showed that the Fe films grown at T_s between 200°C and 330°C were single crystals. Amorphous films were observed at T_s below 200°C and it was not possible to deposit films at T_s above 330°C. The full-width at half-maximum of the rocking curves showed that crystalline qualities were improved at T_s above 270°C. Single crystalline Fe films grown at different substrate temperature showed different structural behaviors in XRD measurements. Iron films grown at T_s between 200°C and 300°C showed bulk α-Fe like behavior regardless of film thickness (100–6400 Å). Meanwhile, Fe films grown at 330°C (144 and 300 Å) showed a biaxially compressed strain between substrate and epilayer, resulting in an expanded inter-planar spacing along the growth direction. Magnetization measurements showed that Fe films (>200 Å) grown at 280°C and 330°C were ferromagnetic with the in-plane easy axis along the [1 1 0] direction. For the thinner Fe films (⩽200 Å) regardless of growth temperature, square loops along the [1 0 0] easy axis were very weak and broad.     

Growth and characterization of CoSi2 films on Si (1 0 0) substrates

In the present work, a special solid phase epitaxy method has been adapted for the preparation of CoSi₂ film. This method includes an epitaxial growth of Co films on Si (1 0 0) substrate, and in situ annealing of the Co/Si films in vacuum. It has been found that at the substrate temperature of 360°C, fcc cobalt film grows epitaxially on the Si (1 0 0) surface. The crystallographic orientation relations between fcc Co film and Si substrate determined from the electron diffraction result are: (0 0 1) Co//(0 0 1) Si, [1 0 0] Co//[1 1 0]Si. Upon annealing at temperatures range from 500 to 600°C, Co film reacts with Si substrate and transforms into CoSi₂. The CoSi₂ films prepared by this way are characterized by XTEM, XPS and AFM.     

In situ magneto-optical Kerr effect study of uncovered Fe films on ZnSe(0 0 1)

The magnetic properties of uncovered Fe/ZnSe/GaAs(1 0 0) ultrathin films have been determined in situ by magneto-optical Kerr effect (MOKE). Fe films up to 10 monolayers (ML) thick were deposited on c(2×2) Zn-rich ZnSe/GaAs(0 0 1) surfaces at 180 °C. We have studied the thickness dependence of the in-plane lattice parameter of the Fe films and of the MOKE hysteresis loops in the longitudinal geometry, at 150 K, under magnetic fields up to 0.1 T applied along the [1 1 0] and [1-1 0] directions of the ZnSe(0 0 1). Reflection high energy electron diffraction show that in the low thickness regime the Fe films present an in-plane structural anisotropy characterized by an expansion along the [1 1 0] direction. Hysteretic loops were obtained only starting from ∼5 ML Fe. We found the onset of an uniaxial magnetic anisotropy with [1 1 0] magnetic easy axis at 7 ML Fe.     

Heteroepitaxial growth of thin InAs layers on GaAs(1 0 0) misoriented substrates: A structural and morphological comparison

Thin InAs epilayers were grown on GaAs(1 0 0) substrates exactly oriented and misoriented toward [1 1 1]A direction by atmospheric pressure metalorganic vapor phase epitaxy. InAs growth was monitored by in situ spectral reflectivity. Structural quality of InAs layers were studied by using high-resolution X-ray diffraction. No crystallographic tilting of the layers with respect to any kind of these substrates was found for all thicknesses. This result is discussed in terms of In-rich growth environment. InAs layers grown on 2° misoriented substrate provide an improved crystalline quality. Surface roughness of InAs layers depend on layer thickness and substrate misorientation.     

Effect of substrate temperature on few-layer graphene grown on Al2O3 (0 0 0 1) via direct carbon atoms deposition

Few-layer graphene (FLG) was grown on Al₂O₃ (0 0 0 1) substrates at different temperatures via direct carbon atoms deposition by using solid source molecular beam epitaxy (SSMBE) method. The structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that the FLG started to form at the substrate temperature of 700 °C. When the substrate temperature increased to 1300 °C, the quality of the FLG was the best and the layer number was estimated to be less than 5. At higher substrate temperature (1400 °C or above), the crystalline quality of the FLG would be deteriorated. Our experiment results demonstrated that the substrate temperature played an important role on the FLG layer formation on Al₂O₃ (0 0 0 1) substrates and the related growth mechanism was briefly discussed.     

Perpendicular magnetization in Fe/Ni bilayers on GaAs(0 0 1)

Following the concept of spin-injection into a semiconductor-based device, a ferromagnetic element (like a GMR multilayer structure) can be used as a spin filter. A high spin-polarization of the electrons can be realized by the preparation of a monocrystalline multilayer structure consisting of ultrathin films of a high magnetic polarization. In the case of ultrathin films, the manipulation of the easy-axis of magnetization is possible, by changing the anisotropy terms contributing to the effective anisotropy of the structure. We report on the structural and magnetic properties of Ni/Fe and Fe/Ni bilayers epitaxially grown on GaAs(0 0 1). By a proper choice of Fe and Ni sequences (Fe/Ni/GaAs) and their thickness (up to 3 ML of Fe on the top of Ni), the rotation of magnetization from the in-plane to the out-of-plane direction was achieved.     

Synthesis and characterization of Ag/PVA nanorods by chemical reduction method

Silver (Ag) nanorods with the average length of 280 nm and diameters of around 25 nm were synthesized by a simple reduction process of silver nitrate in the presence of polyvinyl alcohol (PVA) and investigated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission-electron microscopy (TEM) and UV–vis spectrum. It was found out that both temperature and reaction time are the important factors in determining the morphology and aspect ratios of nanorods. TEM images showed the prepared Ag nanorods have a face centered shape (fcc) with fivefold symmetry consisting of multiply twinned face centered cubes as revealed in the cross-section observations. The fivefold axis, i.e. the growth direction, normally goes along the (1 1 1) zone axis direction of the basic fcc Ag-structure. Preferred crystallographic orientation along the (1 1 1), (2 0 0) or (2 2 0) crystallographic planes and the crystallite size of the Ag nanorods are briefly analyzed.     

Surface vibrational thermodynamics from ab initio calculations for fcc(1 0 0)

We present vibrational dynamics and thermodynamics for the (1 0 0) surfaces of Cu, Ag, Pd, Pt and Au using a real space approach. The force field for these systems is described by density functional theory. The changes in the vibrational dynamics and thermodynamics from those in bulk are confined mostly to the first-layer. A substantial enhancement of the low-frequency end of the acoustic branch was found and is related to a loosening of the bond at the surface. The thermodynamics of the first-layer also show significant differences (higher heat capacity, lower free energy and higher mean vibrational square amplitudes) from what obtains in bulk. Comparing these results with those calculated using embedded-atom method potentials, we discovered that for Ag(1 0 0) and Cu(1 0 0), the two methods yield very similar results while for Pd(1 0 0), Pt(1 0 0) and Au(1 0 0) there are substantial differences.     

Atomic structure of two-dimensional binary surface alloys: Si(111)-√21 × √21 superstructure

The atomic structures of Au and Ag co-adsorption-induced √21 × √21 superstructure on a Si(111) surface, i.e., (Si(111)-√21 × √21-(Au, Ag)), where the Si(111)-5 × 2-Au surface is used as a substrate, have been investigated using reflection high-energy positron diffraction (RHEPD) and photoemission spectroscopy. From core-level spectra, we determined the chemical environments of Ag and Au atoms present in the Si(111)-√21 × √21-(Au, Ag) surface. From the rocking curve and pattern analyses of RHEPD, we found that the atomic coordinates of the Au and Ag atoms were approximately the same as those of the Au and Ag atoms in other Si(111)-√21 × √21 surfaces with different stoichiometries. On the basis of the core-level and RHEPD results, we revealed the atomic structure of the Si(111)-√21 × √21-(Au, Ag) surface.     

Beryllium overlayers on Mo(1 1 2) and Mo(0 1 1) surfaces

Low-energy electron diffraction, Auger electron spectroscopy and contact potential difference methods have been used to study formation of Be overlayers on the Mo(1 1 2) and (0 1 1) surfaces in the temperature range from T = 78 K up to the beginning of Be desorption. At a coverage θ = 1, where θ is defined as the ratio between concentrations of adatoms and surface substrate atoms, overlayers were found to be pseudomorphic on both the substrates. Various types of close-packed Be structures were observed at θ > 1. Annealing of the Mo samples covered with Be caused not only overlayer ordering, but also a partial solution of Be in the near-surface layer of Mo and creation of a surface alloy. It is inferred that the work function changes, positive on Mo(1 1 2) up to 0.4 eV and negative on Mo(0 1 1) down to ? 0.6 eV, are caused mainly by changes in surface roughness while the contribution of polarization of the BeMo adsorption bond seems to be only minor.     

The growth of Ag films on a TiO2(110)-(1×1) surface

We report here the growth of Ag film and its thermal stability on the TiO₂(1 1 0)-(1×1) surface using combination techniques of low-energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At a surface temperature as low as 125 K, a 2D growth of Ag films seems to occur for submonolayer coverages up to ∼0.8 ML. Annealing of low temperature grown Ag films to 500 K for coverage of 1–2.4 ML would result in the formation of metastable Ag layers with rest of Ag forming 3D needle-like islands on top of this Ag film.     

Initial stage of CdTe on Si(1 0 0) grown by MBE

The initial stage of CdTe growth on silicon has been investigated using angle-resolved photoemission and scanning tunneling microscopy (STM). In order to study initial stage of CdTe on Si, we have desorbed CdTe by annealing at 600 °C so that only one monolayer of Te remains on the Si(1 0 0) substrate. Te/Si(1 0 0)2×1 superstructure has been observed by LEED. Photoemission spectra indicate that Te atoms bond with the Si dangling bond. Atomically resolved STM images reveal that the Te atoms form dimers. It is observed that buckling direction of Te-dimer changes and the dimmers are broken in the site of some dimmer rows. It can be explained that the large lattice mismatch cause the switching of the buckling direction and the breaking of Te-dimer resulted surface relaxation.     

Strong structure sensitivity in the partial oxidation of styrene on silver single crystals

In contrast to the formation of styrene oxide on Ag(1 1 1), phenylacetaldehyde and phenylketene dominate the partial oxidation of styrene on Ag(1 1 0), even though the reactions follow the same mechanism on both surfaces. The origin of this difference is that on Ag(1 1 0) the activation energy for transformation of the oxametallacycle to the combustion intermediate is much lower than on the (1 1 1) surface, so that ring-closure of the oxametallacycle to form styrene oxide is short circuited. Also the combustion intermediate appears more stable on Ag(1 1 0) than on Ag(1 1 1).