Local Surface Morphology Research Articles

Photon emission spectroscopy of thin MgO films with the STM: from a tip-mediated to an intrinsic emission characteristic

Electron injection from the tip of a scanning tunnelling microscope (STM) has been used to stimulate local photon emission from the surface of a thin MgO film grown on Mo(001). Depending on the excitation energy, several emission regimes have been identified on the basis of the energy and the spatial distribution of the emitted photons. At low excitation bias, tip-induced plasmons are excited in the tip–sample gap, carrying little information on the oxide film. With increasing tip bias, radiative electron transitions between field-emission resonances dominate the photon response from the MgO surface. Intrinsic optical modes of the oxide material, such as radiative decays of electron–hole pairs, are only observed when operating the STM in the field emission regime, where the direct correlation between the photon signal and the local surface morphology gets partially lost due to the increased tip–sample distance.

Nano-scale Tribological Behavior of Polycrystalline Silicon Structural Films in Ambient Air

ABSTRACTDynamic friction, wear volumes and wear morphology have been studied for sliding wear in polysilicon in ambient air at μN normal loads using on-chip micron-scale test specimens. With increasing number of wear cycles, the friction coefficients show two distinct types of behavior: (i) an increase by a factor of two and a half to a steady-state regime after peaking at three times the initial value of about 0.10 ± 0.04, with no failure after millions of cycles; (ii) an increase by a factor larger than three followed by failure after ∼105 cycles. Additionally, the average nano-scale wear coefficient sharply increased in the first ∼105 cycles up to about 10−4 and then decayed by an order of magnitude over the course of several million cycles. For both modes of behavior, abrasive wear is the governing mechanism, the difference being attributed to variations in the local surface morphology (and wear debris) between the sliding surfaces. The oxidation of worn polysilicon surfaces only affects the friction coefficient after periods of inactivity (>30 min).

Threading dislocations in domain-matching epitaxial films of ZnO

The structures of high-quality ZnO epitaxial films grown by pulsed-laser deposition on sapphire (0001) without an oxygen gas flow were investigated by X-ray diffraction and transmission electron microscopy. The great disparity of X-ray diffraction line widths between the normal and in-plane reflections reveals the specific threading dislocation geometry of ZnO. Most threading dislocations are pure edge dislocations. From a combination of scattering and microscopic results, it is found that threading dislocations are not uniformly distributed in the ZnO films, but the films consist of columnar epitaxial cores surrounded by annular regions of edge threading dislocations in large density. The local surface morphology and capacitance signal obtained from atomic force and scanning capacitance microscopes indicate that the aggregation of threading dislocations leads to high interface traps at the annular regions.

Surface smoothing of amorphous silicon thin films: Kinetic Monte Carlo simulations

Based on the established comprehensive model including deposition radicals either diffusing on the surfaces or incorporating into the film with different activation barriers dependent on the local surface morphology, we perform a systematical study of the surface smoothing of the hydrogenated amorphous silicon $(a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H})$ films under a wide range of temperature by the kinetic Monte Carlo (KMC) simulation. An important transition has been found, i.e., the temperature-dependent transition from rough to smooth and from smooth to rough again, which is responsible for the surface smoothing of $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ films. These intriguing findings are in excellent agreement with recent experiments. It is predicted that effects of the deposition flux on the surface smoothing are nonmonotonously dependent on the temperature.

Surface Smoothening Mechanism of Amorphous Silicon Thin Films

An important concern in the deposition of thin hydrogenated amorphous silicon () films is to obtain smooth surfaces. Herein, we combine molecular-dynamics simulations with first-principles density functional theory calculations to elucidate the smoothening mechanism of plasma deposited thin films. We show that the deposition precursor may diffuse rapidly on the film surface via overcoordinated surface Si atoms and incorporate into the film preferentially in surface valleys, with activation barriers for incorporation dependent on the local surface morphology. Experimental data on smoothening and precursor diffusion are accounted for.

The Measurement of Solvation Forces with Application to Biological Systems

M. J. Higgins,* T. Uchihashi,* M. Polcik,* J. E. Sader,** S.P. Jarvis* *Centre for Research on Adaptive Nanostructures and Nanodevices, University of Dublin, Trinity College, Dublin 2, Ireland **Department of Mathematics and Statistics, University of Melbourne, Victoria 3010, Australia Directly measuring forces in aqueous environments with a mechanical probe of lateral dimensions comparable to that of a single molecule provides an invaluable insight into the processes controlling if and how a molecule approaches another molecule or a membrane. In the immediate vicinity of the molecule, continuum models break down and the aqueous environment will often form a discrete layered structure depending on the nature of the molecule. The absence or presence of such structure may be fundamental in influencing the promotion or inhibition of protein adsorption, biological function and membrane recognition. In order to perform such measurements it has been necessary to combine a number of innovative techniques with a standard atomic force microscopy (AFM). For high-resolution imaging we use the highly sensitive frequency modulation detection scheme (FM-AFM) in liquid [1, 2] and have subsequently used the method of Sader and Jarvis to extract quantitative force values from frequency shift measurements [3]. To implement FM-AFM successfully in liquid we have used magnetically activated dynamic mode (MAD-mode!) where a small magnetic particle is attached to the end of the cantilever (Fig. 1) and an external magnetic field applied via a current carrying coil. To increase the sensitivity of the measurement to the interaction local to the tip apex we have used a high aspect ratio multiwalled carbon nanotube probe (Fig. 2). This reduces the hydrodynamic squeeze damping between the surface and the bulk of the tip. The nanotube is attached in a specially designed field emission scanning electron microscope, which permits us some control over both the length and direction of the probe. We have directly measured localised density changes in ultra-pure water at small tip-sample separations and have combined this with nanometer resolution images of the surface. Careful characterisation of the nanotube probe using electron microscopy after the measurement has allowed us to normalise interaction forces as a function of probe radius [4]. Using self-assembled monolayers with varying end groups, we have been able to investigate the role of local surface chemistry and morphology on measured water structure and tip-surface interaction. This has been extended to observations of quantified differences in localised structures in the presence of ions with the structures being dependent on the choice of ion. Further, we have investigated whether oscillatory forces corresponding to water structure are present at the surface of lipid bilayers (Fig.3). We aim to understand the effects of various physiological conditions on water structure, the relevant length scales over which hydration forces interact and the influence of water structure on biological processes.

Mapping and analysis of microscopic Seebeck coefficient distribution

Understanding the local Seebeck coefficient distribution is helpful for the development of novel thermoelectric materials. Seebeck coefficient distributions of both zone melted and hot pressed samples of I-doped Bi2Te3 based alloys were measured using microscopic Seebeck coefficient mapping method. Seebeck coefficient differences up to 40–50 μV/K were found between different locations on the same sample. There is no visible relationship between the microscopic Seebeck coefficient distribution and the local surface morphology and element distributions. It is suggested that the local Seebeck coefficient variations were mainly originated from the lattice defects for the zone melted sample and also due to the grain orientation for the hot pressed sample.

Use of laser scanning confocal microscopy for characterizing changes in film thickness and local surface morphology of UV-exposed polymer coatings

Laser scanning confocal microscopy (LSCM) has been used to characterize the changes in film thickness and local surface morphology of polymer coatings during the UV degradation process. With the noninvasive feature of LSCM, one can obtain thickness information directly and nondestructively at various exposure times without destroying the specimens or deriving the thickness values from IR measurement by assuming uniform film ablation. Two acrylic polymer coatings were chosen for the study, and the physical and chemical changes of the two systems at various exposure times were measured and analyzed. Those measurable physical changes caused by UV exposure include film ablation, formation of pits and other surface defects, and increases in surface roughness. It was found in both coatings that changes in measured film thickness by LSCM were not correlated linearly to the predicted thickness loss using the changes in the CH band obtained by the Fourier Transform Infrared (FTIR) spectroscopy measurements in the later degradation stages. This result suggested it was not a uniform film ablation process during the UV degradation. At later stages, where surface deformation became severe, surface roughness and profile information using LSCM were also proven to be useful for analyzing the surface degradation process

Contact angle measurements: an empirical diagnostic method for evaluation of thin film solar cell absorbers (CuInS2)

An empirical diagnostic method for the evaluation of solar cell grade CuInS2 absorbers has been developed. The method involves the measurement of the contact angle between water and the CuInS2 absorber before fabrication of a solar cell. The contact angle is expected to depend upon local inhomogeneity, chemical composition and surface morphology of the CuInS2 absorber. The variation of these factors on the surface is supported with scanning electron micrographs, chemical analyses, laser scanning photocurrent mapping of various CuInS2 absorbers and measurements of the solar cell performance. The contact angle has been found to be different at different places on the CuInS2 surface. Empirically, it was found that for high conversion efficiency solar cells (>8–10.5%), the contact angle on CuInS2 absorbers ranges between 53° and 63°. For low conversion efficiency solar cells (<6%), it is between 48° and 50°. Therefore, it is seen that contact angle measurements on CuInS2 absorbers can be used to assess the quality of CuInS2 absorbers prior to solar cell fabrication.

Dynamics of pattern formation during low-energy ion bombardment of Si(0 0 1)

Sputtering of surfaces by collimated, low-energy ion beams results in spontaneous pattern formation in many systems. In order to explore the mechanisms that control the pattern formation, we have used in situ light scattering to measure the evolution of sputtered Si(0 0 1) surfaces. The results are interpreted within a linear instability model originally proposed by R.M. Bradley and J.M.E. Harper [J. Vac. Sci. Technol. A 6 (1988) 2390] that includes the dependence of the sputter yield on the local surface morphology.

Scanning localized viscoelastic image using a quartz crystal resonator combined with an atomic force microscopy

A quartz crystal resonator and an atomic force microscopy were applied for the measurement of local viscoelasticity and surface morphology. For the reduction of signal noise from the quartz crystal resonator, we designed an oscillation circuit based on a referential quartz crystal method. A polystyrene bead-coated quartz crystal was used as a signal quartz crystal, and a bare Au quartz crystal was used as a reference. By approaching the cantilever of atomic force microscopy to the surface of working quartz crystal, the differential resonant frequency in the two quartz crystals showed changes by the interaction between the tip and the quartz crystal. The changes of differential resonant frequency in the two quartz crystals were influenced by the local viscoelasticity. The image resolution of differential resonant frequency was observed under 80 nm for the polystyrene bead-coated quartz crystal.

Atomistic simulation of chemical vapor deposition of (111)-oriented diamond film using a kinetic Monte Carlo method

Chemical vapor deposition (CVD) of (111)-oriented diamond film is modeled using a kinetic Monte Carlo atomic scale method. The method is parameterized by the rates of the accompanying surface chemical reactions and permits one of these reactions to take place at each simulation step. The effect of local surface structure and morphology on the rates of surface reaction is examined. Film growth at two different chemical compositions of the feed gas and two substrate temperatures is studied in order to determine the effect of these process parameters on (a) the quality of the deposed film and (b) the rate of deposition. The quality of the film is judged by concentration of the point defects (vacancies and H atoms embedded in the film) and by surface roughness. The results obtained show that the parameters that increase the deposition rate, primarily the substrate temperature and the concentration of CH4 in the feed gas, also increase the defect content and surface roughness.

Ballistic electron emission spectroscopy of magnetic multilayers (abstract)

The giant magnetoresistance observed in magnetic multilayers arises from spin-dependent scattering and transmission of electrons at the Fermi energy. We will describe a method for the measurement of these quantities in a “CPP” geometry at electron energies both above and below the Fermi energy. Initial results will also be presented. The measurements employ ballistic electron emission spectroscopy (BEES) to detect the ballistic electron current transmitted through a multilayer as a function of magnetic field and electron energy. The experiments are similar in concept to the “spin–valve transistor,”1 except that the injector is the tip of a scanning tunneling microscope. This allows the injection energy to be varied over a wide range, and spectra can be correlated with the local surface morphology on a nanometer scale. Spectral broadening due to sample inhomogeneities is also eliminated. We anticipate that BEES measurements and complementary scanning tunneling spectroscopy will provide information that is easily compared with calculations of the multilayer band structure and the electron transmittance versus energy.

An investigation of the spatial association between snow depth and topography in a Prairie agricultural landscape using digital terrain analysis

The association of snowcover depth with topography and land cover was examined for a 1.5 km 2 agricultural site in the Canadian Prairies. Topography was quantified by a suite of general geomorphometric variables that were measured directly by the analysis of a raster digital elevation model (DEM) with a 10 m grid cell size. These variables expressed local surface morphology and relative topographic position. The major land covers represented in the study area were wheat stubble and summerfallow. Point observations of snow depth at the site were obtained near the time of peak snow accumulation and during melt. Initial analysis was made of scatterplots and correlations between snow depth and individual topographic variables. These correlations were weak but illustrated that, at the local scale, snow depth patterns cannot be adequately modelled through simple bivariate relationships with topographic variables. The correlations also showed that snow depth is more strongly related to relative topographic position variables than to variables expressing local surface morphology. Subsequent analysis involved a divisive subdivision of the study site into six terrain units defined in terms of several relative topographic position and land cover variables. The units were used to model the spatial patterns of snow depth that occurred during the observed snowcover situations. Although the terrain classification only reduced the total snow depth variance by approximately 30%, it delineated the major patterns of snow depth observed over the study area and elucidated the spatial relationships between snowcover depth and landscape attributes.

Ｔｉ‐６Ａｌ‐４Ｖ合金の疲労強度のバラツキに対するショットピーニングによる抑性効果

When Ti-6Al-4V alloy was polished by using diamond paste or #400 emery paper, fatigue cracks initiated at the surface and its fatigue strength scattered widely. It was considered that the scatter of fatigue strength depended on the local surface morphology. On the other hand, when the fatigue crack initiated at the subsurface with shot-peening, the scatter of fatigue strength became narrowed remarkably. The fatigue strength of the shot-peened specimen, however, decreased again in the long-life region, and no fatigue limit was recognized even more than 107 cycles. This phenomenon might depend on the residual stress distribution.

Phosphorylation-site mutagenesis of the growth-associated protein GAP-43 modulates its effects on cell spreading and morphology.

The 43-kD growth-associated protein (GAP-43) is a major protein kinase C (PKC) substrate of axonal growth cones, developing nerve terminals, regenerating axons, and adult central nervous system areas associated with plasticity. It is a cytosolic protein associated with the cortical cytoskeleton and the plasmalemma. Membrane association of GAP-43 is mediated by palmitoylation at Cys3Cys4. In vitro and in vivo, phosphorylation by PKC exclusively involves Ser41 of mammalian GAP-43 (corresponding to Ser42 in the chick protein). To identify aspects of GAP-43 function, we analyzed the actions of wild-type, membrane-association, and phosphorylation-site mutants of GAP-43 in nonneuronal cell lines. The GAP-43 constructs were introduced in L6 and COS-7 cells by transient transfection. Like the endogenous protein in neurons and their growth cones, GAP-43 in nonneuronal cells associated with the cell periphery. GAP-43 accumulated in the pseudopods of spreading cells and appeared to interact with cortical actin-containing filaments. Spreading L6 cells expressing high levels of recombinant protein displayed a characteristic F-actin labeling pattern consisting of prominent radial arrays of peripheral actin filaments. GAP-43 had dramatic effects on local surface morphology. Characteristic features of GAP-43-expressing cells were irregular cell outlines with prominent and numerous filopodia. The effects of GAP-43 on cell morphology required association with the cell membrane, since GAP-43(Ala3Ala4), a mutant that failed to associate with the cell cortex, had no morphogenetic activity. Two GAP-43 phosphorylation mutants (Ser42 to Ala42 preventing and Ser42 to Asp42 mimicking phosphorylation by PKC) modulated the effects of GAP-43 in opposite ways. Cells expressing GAP-43(Asp42) spread extensively and displayed large and irregular membranous extensions with little filopodia, whereas GAP-43(Ala42) produced small, poorly spreading cells with numerous short filopodia. Therefore, GAP-43 influences cell surface behavior and phosphorylation modulates its activity. The presence of GAP-43 in growing axons and developing nerve termini may affect the behavior of their actin-containing cortical cytoskeleton in a regulatable manner.