Magnetic behavior and microstructure of Finemet-type ribbons in both,surface and bulk

Different kinds of magnetic anisotropies have been induced during the nanocrystallization process of Co- and Ni-rich amorphous ferromagnetic (Finemet) ribbons using diverse procedures like the application of a constant stress or an axial magnetic field during the annealing process. Magnetization measurements have evidenced the anisotropy of the treated samples. The main goal of this work has been the structural and microstructural analysis of the treated ribbons using X-ray Diffraction (XRD) and Atomic Force Microscopy (AFM), detecting substantial differences in the crystallization state and grain size of the samples depending on the treatment that was carried out. Moreover, AFM measurements revealed in all the treated samples a strong nanocrystallization of the surface without evidences of amorphous matrix, which contrast with XRD measurements that have shown a high content of amorphous phase in the bulk of the ribbons. Magneto-optical Kerr effect measurements have been performed with the aim to elucidate the complex magnetic behavior that is expected for the surface of the ribbons, measuring surface hysteresis loops that show much higher coercive field values than in the bulk.     

AFM investigations during the nanostructure formation in FeZrB alloys

We present a study by intermittent contact atomic force microscopy technique, using the corresponding line profile analysis, performed in as-cast amorphous and Joule heated nanocrystalline Fe₉₁Zr₇B₂ ribbons. The average grain size of the nanocrystalline phases estimated from AFM images, (≈10–15 nm), correlates well with respect to that obtained from neutron powder diffraction. The differences in the relative amount of amorphous to nanocrystalline phases estimated from both techniques, suggest that the onset of the crystallisation process begins at the ribbon surface.     

Synthesis and characterization of manganese sulphide thin films deposited by spray pyrolysis

Manganese sulphide (MnS) thin films have been deposited onto glass substrate by a low cost spray‐pyrolysis technique at 220 °C. The as‐deposited MnS thin films have been characterized using scanning electron microscopy (SEM), energy dispersive X‐ray (EDX) spectroscopy, atomic force microscopy (AFM), X‐ray diffraction, UV visible spectroscopy and photo electrochemical (PEC) measurement. The SEM and AFM images showed that the MnS thin films were well covered onto the substrate surface. The as‐deposited raw thin film was found to be amorphous in nature and perfectly crystalline phase after annealing the sample. Optical band gap of the MnS thin films was found to vary from 3.1 to 3.21 eV and the band gap decreases with the increase in film thickness. Optical constants such as refractive index, extinction coefficient have been evaluated using reflectance and absorbance data. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and morphology of nanosized zeolite L

AFM is a powerful tool for imaging nanoscale surface features; it provides two and three dimensional crystal structure images and other information about actual surface of zeolite crystallites. In this paper, nanosized zeolite L is synthesized in different crystallization times and a study of crystal growth of zeolite L is reported using atomic force microscopy (AFM). X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques are used for characterization of the as synthesized samples. TEM and two‐dimensional AFM images indicate that the zeolite particles are in a nano‐range and they have hexagonal structure. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Generating ordered Si nanocrystals via atomic force microscopy

We describe two successful routes for generating ordered arrays of Si nanocrystals by using atomic force microscopy (AFM) and amorphous silicon thin films (200–400 nm) on Ti/Ni coated glass substrates. First, we show that field-enhanced metal-induced solid phase crystallization at room temperature can be miniaturized to achieve highly spatially localized (below 100 nm) current-induced crystallization of the amorphous silicon films using a sharp tip in AFM. In the second route, resistive nano-pits are formed at controlled positions in the amorphous silicon thin films by adjusting (lowering and/or stabilizing) the exposure currents in the AFM process. Such templated substrates are further used to induce localized growth of Si nanocrystals in plasma-enhanced chemical vapor deposition process. In both cases the crystalline phase is identified in situ as features of enhanced current in current-sensing AFM maps.     

Surface morphology and lattice distortion of heteroepitaxial GaInP on InP

We examine the relation between surface morphology and lattice distortions of a number of Ga_xIn_1-xP epilayers (x ≈ 0.04) grown on InP (001) substrates using atomic force microscopy (AFM) and high resolution X-ray diffractometry (HRXRD). The heteroepitaxial layers (thickness ≈ μm) were grown by low pressure metalorganic vapour phase epitaxy (MOVPE) at temperatures from 600 to 660°C. AFM images over a scale of 75 μm by 75 μm show no surface corrugations for samples grown at 640°C. For other growth temperatures between 600 and 660°C, unidirectional corrugations aligned along the [1 0] direction or a cross-hatched pattern can be seen on the sample surfaces with a typical interline separation of 10 μm. Another feature revealed by AFM is the presence, for some samples, of weak parallel corrugations along the direction making a 54° angle with the [1 0] direction. HRXRD spectra recorded first with the [110] and then with the [1 0] direction in the plane of incidence reveal an asymmetric relaxation in the sample plane accompanied by a broadening of the epilayer X-ray peak for diffraction with the [110] direction in the plane of incidence. The lattice distortion changes from tetragonal to orthorhombic as the surface corrugations appear along the [1 0] direction. Proton microprobe scans indicate that these samples are compositionally uniform so that the asymmetry is not the result of lateral variations of the alloy composition. Finally, low temperature photoluminescence from these samples shows good agreement between observed and calculated peak positions on the basis of the strains obtained from HRXRD measurements.     

Microstructure and optical properties of (Pb,Ca)TiO3 films grown on Si(1 0 0) substrates by a simple sol-gel process

Amorphous and polycrystalline (Pb_0.76Ca_0.24)TiO₃ (PCT) thin films deposited on an Si(1 0 0) substrate have been prepared by a simple sol-gel process. The microstructure and surface morphologies of the thin films have been studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The polycrystalline PCT film on the Si(1 0 0) substrate has a tetragonal perovskite structure with grain size from 60 to 110 nm. AFM reveals smooth surfaces and root mean square (rms) roughness of 0.17 and 4.4 nm for amorphous and polycrystalline films, respectively. The refractive index n and extinction coefficient k of the amorphous and polycrystalline thin films was obtained by spectroscopic ellipsometry as a function of the photon energy in the range from 2.0 to 5.4 eV. The maximum n and direct bandgap energies of amorphous and polycrystalline thin films were 2.66 and 4.11 eV, 2.64 and 3.84 eV, respectively.     

Electrosynthesis and characterization of [Cu(Taab)(H2O)2](L)2 non-crystalline thin films

Molecular-material thin films of diaqua tetrabenzo (b,f,j,n) {1,5,9,13} tetraazacyclohexadecine copper (II) bisanthraflavates and base organic molecules L have been electrodeposited on Corning 7059 glass slices, quartz substrates with tin oxide film and (100) single-crystalline silicon (c-Si) wafers. The surface morphology and structure of the deposited films were studied by atomic force microscopy (AFM), energy-dispersive spectrophotometry (EDS), ultraviolet-visible (UV-Vis) and Fourier-transform infrared (FT-IR) spectroscopies. The IR-spectra show that compounds obtained by chemical synthesis have the same absorption bands as the amorphous thin films obtained by electrosynthesis. The conductivities and the electrical-conduction mechanisms in the thin films were also investigated. Cubic nonlinear optical (NLO) characterizations of the film samples were performed with the Z-Scan technique, with some samples exhibiting remarkably high nonlinear activity.     

Characterization of mixed phase silicon by Raman spectroscopy

We have examined the common methods for determination of the crystallinity of mixed phase silicon thin films from the TO–LO phonon band in Raman spectra. Spectra are decomposed into contributions of amorphous and crystalline phase and empirical formulas are used to obtain crystallinity either from the integral intensities (peak areas) or from magnitudes (peak maxima). Crystallinity values obtained from Raman spectra excited by Ar⁺ laser green line (514.5 nm) for a special sample with a profile of structure from amorphous to fully microcrystalline were compared with surface crystallinity obtained independently from atomic force microscopy (AFM). Analysis of the Raman collection depth in material composed of grains with absorption depth 1000 nm in an amorphous matrix (absorption depth 100 nm), was used to explain reasons for systematic difference between surface and Raman crystallinities. Recommendations are given for obtaining consistent results.     

Study of the surface heterogeneity of topographic and electric nature by atomic force microscopy by the example of triglycine sulfate

Triglycine sulfate crystals with an ideal (010) cleavage plane are used as model objects to reveal problems in interpreting atomic force microscopy (AFM) images of surfaces with nonuniform charge distribution. Specific microrelief features of two types are found: lenslike formations with different contrast and rounded protrusions/valleys of different size but fixed height. An analysis of their evolution with a change in temperature and under an electric field and mechanical impacts has made it possible to separate relief elements from the crystal domain structure. The interpretation proposed is confirmed by the multimode AFM data. The specific features of the images of dynamic domains and aged domains (which cannot undergo polarization reversal) are studied. The domain-wall width found in the AFM measurements depends on the technique used and the specificity of probe-surface interaction; it varies from 9 to 2000 nm. The most reliable data on the domain-wall width in triglycine sulfate crystals are provided by piezoelectric force microscopy, according to which the wall width does not exceed 30 nm.     

Fracture toughness evaluation of precursor-derived Si–C–N ceramics using the crack opening displacement approach

Difficulties in the fabrication of representative test specimens and in the application of suitable test procedures e.g., in the measurement of the test parameters viz., load, crack length etc., have so far limited the intrinsic fracture mechanical characterization of the precursor-derived ceramics (PDC). The present work reports the evaluation of the crack tip toughness (K_I0) of Si–C–N ceramics, using the novel crack opening displacement (COD) approach. The fully dense PDC test specimens synthesized from a poly(ureamethylvinyl)silazane precursor covered material structures ranging from partly organic amorphous to inorganic nano-crystalline states. Critically loaded cracks were achieved using either a special loading fixture or with Vickers indentation. Crack tip CODs were measured with the atomic force microscopy (AFM). Fractography of the fracture surfaces were performed using topographic, frictional and phase contrast AFM. The measured K_I0 values ranged from 0.6 to 1.2 MPa m^1/2. The net change in crack resistance was affected by the stripping of OFC-hydrogen atoms in the amorphous materials and by the segregated turbostratic graphite phase in the phase-separated materials. Nano-scale crack deflection observed even in the amorphous materials indicated the presence of structural and compositional inhomogeneities within the amorphous network.     

TEM investigation on the structure of amorphous silicon monoxide

Commercially available powder samples of silicon monoxide have been investigated by methods of transmission electron microscopy: electron scattering, electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Pair distribution functions (PDFs) as well as EEL spectra can be shown to be a composition of the PDF and EEL spectra of elemental silicon and amorphous SiO₂. The distribution of the elements silicon and oxygen calculated from ESI images proof the silicon monoxide to be inhomogeneous, i.e. it consists of amorphous silicon and amorphous SiO₂. The phase separated regions measure ≈3–4 nm. One maximum in the PDF at 2.95 Å does not stem from either a-Si or a-SiO₂, and it is assigned to atomic configurations at the interphase boundary layer between Si and SiO₂. The portion of the interphase domain in the total composite material is estimated to be between 20% and 25%.     

Fourier analysis of scratches generated on m‐GaN substrates during polishing

Generation of scratches on surface of m‐plane GaN substrates due to polishing was studied by atomic force microscopy (AFM). For epi‐ready substrates AFM images confirm a flat surface with the atomic step roughness while a lot of scratches are visible in AFM images for partially polished GaN substrates. The Fourier analysis of AFM images show that scratches propagate easier along {c‐plane} and {a‐plane} directions on m‐plane GaN surface. This observation is an evidence of anisotropy of mechanical properties of GaN crystals in the micro‐scale. This anisotropy is directly correlated with the symmetry and atomic arrangement of m‐plane GaN.     

Detailed structural study of low temperature mixed-phase Si films by X-TEM and ambient conductive AFM

Microcrystalline silicon (μc-Si:H) thin films prepared by plasma enhanced chemical vapour deposition (PECVD) at 37 °C has been studied by cross-sectional TEM and ambient conductive AFM. We have succeeded in the combined measurement of topography and local conductivity under standard ambient conditions, overcoming the surface native oxide by more sensitive (pA range) current detection. We observed the columnar structure of the amorphous phase in the TEM micrograph and related it to the surface corrugation of the same size detected by AFM.     

Third-order nonlinearity in Ag-nanoparticles embedded 56GeS2–24Ga2S3–20KBr chalcohalide glasses

Ag-nanoparticles were embedded in the 56GeS₂–24Ga₂S₃–20KBr chalcohalide glasses with varied doses by ion implantation. Ag-nanoparticles were observed in the AFM images of all implanted samples. The third-order optical nonlinearity of Ag-implanted chalcohalide glasses was investigated by the femtosecond Z-scan measurements. The results indicated an enhancement of third-order optical nonlinearity due to the formation of Ag-nanoparticles in glasses. The relation between implanting dose and third-order optical nonlinearity was strongly related to the intrinsic local field and interaction of Ag-nanoparticles.     

Stress-enhanced ion diffusion at the vicinity of a crack tip as evidenced by atomic force microscopy in silicate glasses

The slow advance of a crack in soda-silicate glasses was studied at nanometer scale by in-situ and real-time atomic force microscopy (AFM) in a well-controlled atmosphere. An enhanced diffusion of sodium ions in the stress-gradient field at the sub-micrometric vicinity of the crack tip was revealed through several effects: growth of nodules in AFM height images, changes in the AFM tip-sample energy dissipation. The nodules patterns revealed a dewetting phenomenon evidenced by ‘breath figures’. Complementary chemical micro-analyses were done. These experimental results were explained by a two-step process: (i) a fast migration (typical time: few milliseconds) of sodium ions towards the fracture surfaces as proposed by Langford et al. [J. Mat. Res. 6 (1991) 1358], (ii) a slow backwards diffusion of the cations as evidenced in these AFM experiments (typical time: few minutes). Measurements of the diffusion coefficient of that relaxing process were done at room temperature. Our results strengthen the theoretical concept of a near-surface structural relaxation due to the stress-gradient at the vicinity of the crack tip. The inhomogeneous migration of sodium ions might be a direct experimental evidence of the presence of sodium-rich channels in the silicate structure [A. Meyer et al., Phys. Rev. Let. 93 (2004) 027801].     

The effect of ion exchange on glass surface morphology and composition

The surface morphology and K⁺, Na⁺ concentration distributions of plate glass were investigated by atomic force microscope (AFM) and electron probe microanalyzer (EPMA) before and after ion exchange. The AFM images indicate that the surface microcrack of chemical strengthened glass can be closed to a certain extent and the surface smoothness can be improved. The EPMA results indicate that the relationship between the weight gain and time, temperature follows Fick’s law, which are same as that of the thickness of ion exchanged layer, the total exchanged amount. Consequently, a simple and accurate weighing method was first proposed to estimate the ion exchange state.     

Role of the silica source on the geopolymerization rate

The synthesis of geopolymer-silica composites was achieved at room temperature to determine the role of the silica source (quartz or amorphous silica) on the polycondensation rate and the mechanical properties of synthesized materials. Then, samples with a composition range from 100% quartz to 100% amorphous silica were formed, compared and characterized by XRD, infrared spectroscopy, thermal analysis, SEM, and compression tests. The results give evidence that the increase of amorphous silica in the mixture favors the polycondensation reaction (i.e., “geopolymerization”) to form consolidated materials whereas quartz led to heterogeneous materials without cohesion. These facts are explained by the modification of the Si/Al ratio in the geopolymer matrix due to the increase of quartz in the mixture.     

Observing the twinkling fractal nature of the glass transition

A fundamental understanding of the nature and structure of the glass transition in amorphous materials is currently seen as a major unsolved problem in solid-state physics. A new conceptual approach to understanding the glass transition temperature (T_g) of glass-forming liquids called the twinkling fractal theory (TFT) has been proposed in order to solve this problem. The main idea underlying the TFT is the development of dynamic rigid percolating solid fractal structures near T_g, which are said to be in dynamic equilibrium with the surrounding liquid. This idea is coupled with the concept of the Boltzmann population of excited vibrational states in the anharmonic intermolecular potential between atoms in the energy landscape. Solid and liquid clusters interchange or “twinkle” at a cluster size dependent frequency ω_TF, which is controlled by the population of intermolecular oscillators in excited energy levels. The solid-to-liquid cluster transitions are in accord with the Orbach vibrational density of states for a particular fractal cluster g(ω) ~ ω^df − 1, where the fracton dimension d_f = 4/3. To an observer, these clusters would appear to be “twinkling.” In this paper, experimental evidence supporting the TFT is presented. The twinkling fractal characteristics of amorphous, atactic polystyrene have been captured via atomic force microscopy (AFM). Successive two-dimensional height AFM images reveal that the percolated solid fractal clusters exist for longer time scales at lower temperatures and have lifetimes that are cluster size dependent. The computed fractal dimensions, ≈ 1.88, are shown to be in excellent agreement with the theory of the fractal nature of percolating clusters in accord with the TFT. The twinkling dynamics of polystyrene within its glass transition region are captured with time-lapse one-dimensional AFM phase images. The autocorrelation cluster relaxation function was found to behave as C(t) ~ t^− 4/3 and the cluster lifetimes τ versus width R were found to be in excellent agreement with the TFT via τ ~ R^1.42. This paper provides compelling new experimental evidence for the twinkling fractal nature of the glass transition.     

Stability of GaAs oxide under metalorganic molecular beam epitaxy process

Well-defined oxide of GaAs can be used as a mask material for selective-area metalorganic molecular beam epitaxy (MOMBE) of GaAs. In this study, the reaction between triethylgallium (TEG) and the GaAs oxide layer was studied using a quadrupole mass spectrometer (QMS) and an atomic force microscope (AFM). Results of the QMS observation showed that TEG was reflected on the GaAs oxide surface until the start of desorption of the GaAs oxide, and the GaAs oxide layer was desorbed from the wafer after a large time delay from the start of TEG supply. AFM images showed that many holes appeared on the GaAs oxide surface during the desorption of the GaAs oxide. The effect of incident TEG upon the stability of the GaAs oxide mask is discussed.