Tribological carbon-based coatings: An AFM and LFM study

In this work some carbon-based coatings were studied by atomic force microscopy (AFM) and lateral force microscopy (LFM) techniques in order to evaluate their microstructure and friction properties at the micro and nanoscale. With this aim, four samples were prepared by magnetron sputtering: an amorphous carbon film (a-C), two nanocomposites TiC/a-C with different phase ratio (∼1:1 and ∼1:3) and a nanocrystalline TiC sample. Additionally, a highly oriented pyrolytic graphite (HOPG) and an amorphous hydrogenated carbon coating (a-C:H) were included to help in the evaluation of the influence of the roughness and the hydrogen presence respectively. The topography (roughness) of the samples was studied by AFM, whereas LFM was used to measure the friction properties at the nanoscale by two different approaches. Firstly, an evaluation of possible friction contrast on the samples was done. This task was performed by subtraction of forward and reverse images and lately confirmed by the study of lateral force profiles in both directions and the histograms of the subtraction images. Secondly, an estimation of the average friction coefficient over the analysed surface of each sample was carried out. To take into account the tip evolution/damaging, mica was used as a reference before and after each sample (hereafter called sandwich method), and samples-to-mica friction ratios were calculated. The LFM was shown to be a useful tool to characterise a mixture of phases with different friction coefficients. In general, the friction ratios seemed to be dominated by the amorphous carbon phase, as it was impossible to distinguish among samples with different proportions of the amorphous phase (friction ratios between 1.5 and 1.75). Nevertheless, it could be concluded that the differences in friction behaviour arose from the chemical aspects (nature of the phase and hydrogen content) rather than surface characteristics, since the roughness (Ra values up to 5.7 nm) does not follow the observed trend. Finally, the Ogletree method was employed in order to calibrate the lateral force and estimate the friction coefficient of our samples. A good agreement was found with macroscopic and literature values going from ∼0.3 for TiC to ∼0.1 for pure carbon.     

Competitive effects of metal dissolution and passivation modulated by surface structure: An AFM and EBSD study of the corrosion of alloy 22

Electron backscatter diffraction (EBSD) and atomic force microscopy (AFM) are used to correlate crystallographic grain orientation with corrosion rates of polycrystalline alloy 22 following immersion in 1 and 3 molar (M) hydrochloric acid. For each acid concentration, relative corrosion rates are simultaneously characterized for approximately 50 unique grain orientations. The results demonstrate that the corrosion rate anisotropies are markedly different in the two acid concentrations. In very aggressive acidic environments (3M HCl), where electrochemical impedance spectroscopy and spectroscopic ellipsometry data demonstrate that the passive oxide film of alloy 22 is completely dissolved, alloy dissolution rates scale inversely with the average coordination number of surface atoms for a given grain orientation, where highly correlated surfaces dissolve the slowest. Thus, similar to simple metallic systems, the corrosion rates scale with the surface plane-normal crystallographic orientations as {1 1 1} < {1 0 0} < {1 1 0}. Less intuitively, in milder corrosive environments (1M HCl), where the passive film of the alloy is still intact, the dissolution does not scale inversely with surface atomic density. Rather, corrosion rates scale with crystallographic orientations as {1 1 1} < {1 1 0} < {1 0 0}. This is attributed to the fact that facets most susceptible to corrosion (least coordinated) are also the most able to form protective oxides, so that the dissolution anisotropy is a result of the delicate balance between metal dissolution and oxide growth.     

Hydrodynamic interaction of AFM cantilevers with solid walls: An investigation based on AFM noise analysis

The European Physical Journal E - The noise power spectrum of the thermally activated motion of an AFM cantilever has been analyzed with respect to viscoelastic and hydrodynamic coupling between...     

An Unlikely Connection: Geochemistry and Nuclear Structure

Although geochemistry belongs to the earth sciences, historically it has interacted importantly with the physical sciences, in particular with astrophysics and nuclear physics. These interactions, which in traditional historiography have received little notice from either historians of physics or historians of geology, are the subjects of the present paper, which focuses on the period between 1915 and 1950. During the 1920s, geochemists established empirical regularities in the abundance data of the elements in rocks and meteorites, and from these they suggested that an improved knowledge of the atomic nucleus could be obtained. More significantly, geochemists supplied astrophysicists, cosmologists and nuclear physicists with important data that could not be obtained otherwise. The link between geochemistry and basic, nuclear physics is a historical reality. The paper explores parts of this link.     

Electronic structure and self-assembling processes in platinum metalloporphyrins: photoemission and AFM studies

The main goal of this paper is to investigate the electronic structure of valence band and core levels as well as surface topography of pristine tetraphenylporphyrin and Pt-based compounds Pt-TPP(p-COOH₃)₄, Pt-TPP(m-OCH₃)₄, PtCl₂-TPP(m-OCH₃)₄ thin films. The electronic structure of various Pt-based metalloporphyrins which were investigated in dependence on their chemical structure and spectra were measured by high-resolution X-ray photoelectron spectroscopy (XPS) of valence band and Pt4f, Pt4d, C1s, O1s, N1s core levels. Results of atomic force microscopy (AFM) studies of topography and self-assembling processes in thin films of porphyrines are presented and discussed.     

An electronspectroscopic study of adsorbate structure

The ultraviolet photoelectron spectra of xenon adsorbed on crystals of (100) nickel and (110) iron have been measured as a function of xenon coverage. It is shown that the total current in the xenon peaks is a measure of the xenon coverage but that the attenuation of the electrons from the metal substrate is dependent on the packing density of xenon on the surface. Previous measurements by Auger electron spectroscopy are compared and it is shown that the mean free path of 16 eV electrons from the d-bands of nickel and iron is 2.75 times that of 60 eV Auger electrons. The data allows the calculation according to a slab model of a mean thickness of a xenon monolayer. This is interpreted as a measure of the packing density of xenon localized on metal lattices of varying dimensions. The UPS spectrum for xenon on (110) iron shows a marked broadening of the xenon peaks as compared to (100) nickel. This is interpreted as due to variation in relaxation energy over adsorbate states on the iron surface arising from xenon atoms in offsite positions.     

Inverted structure perovskite solar cells: A theoretical study

We analysed perovskite CH₃NH₃PbI_3-xCl_x inverted planer structure solar cell with nickel oxide (NiO) and spiro-MeOTAD as hole conductors. This structure is free from electron transport layer. The thickness is optimized for NiO and spiro-MeOTAD hole conducting materials and the devices do not exhibit any significant variation for both hole transport materials. The back metal contact work function is varied for NiO hole conductor and observed that Ni and Co metals may be suitable back contacts for efficient carrier dynamics. The solar photovoltaic response showed a linear decrease in efficiency with increasing temperature. The electron affinity and band gap of transparent conducting oxide and NiO layers are varied to understand their impact on conduction and valence band offsets. A range of suitable band gap and electron affinity values are found essential for efficient device performance.     

Connection between light cone singularities and the asymptotic behaviour of the moments of the structure functions

On the basis of the Dyson-Jost-Lehmann representation a light-cone type expansion of the matrix elements of the current commutator can be justified. Suitable moments of the structure functions being defined, the connection between the asymptotic behaviour of the moments as Q² → ∞ and the behaviour of the expansion coefficients as x² → 0 is investigated. To solve this problem a restriction of the allowed class of generalized functions is needed. Then a one-to-one correspondence between both limits can be established.     

Relations between intergranular diffusion and structure: A molecular dynamics study

The high temperature (T > 0.5T_m) structure of the previously studied fcc (310)[001] Σ = 5 grain boundary is reinvestigated in order to determine the nature of the diffusion mechanism. The obtained results confirm our earlier conclusion that the grain boundary remains crystalline, but highly disordered, up to the melting point. In addition, we explored the diffusion mechanisms in the fcc (210)[001] Σ = 5 grain boundary. As expected, diffusion occurs mainly by the vacancy migration. The jump frequencies determined by the molecular dynamics simulation have been used to evaluate the tracer correlation factor and the anisotropy of the intergranular diffusion coefficient through a random walk model simulation of the vacancy migration.     

AFM and contact angle investigation of growth and structure of pp-HMDSO thin films

HMDSO was plasma polymerized on silicon wafer and polyethylene (PE) substrates. The chemical structure of the pp-HMDSO was analyzed with Fourier-transform infrared (FT-IR) spectroscopy. The morphological structure of the thin films deposited on the different substrates was investigated by means of atomic force microscopy (AFM), indicating different coverage mechanisms. In order to investigate the growth process of the pp-HMDSO, films of different thickness were also deposited, varying the plasma deposition time from 10 s to 1800 s. Thickness and structure of such deposits was detected with AFM. Finally, hydrophobic characteristics of the different samples were evaluated by means of contact angle measurements and correlated with the morphological characteristics.     

Nano- and microstructured polymer LB layers: A combined AFM/SIMS study

Ultra thin structured polymer films have been prepared by horizontal precipitation Langmuir-Blodgett (HP-LB) method of polymer blends. In particular we used mixtures of two incompatible polymers, poly-2-vinylpyridine and polystyrene, the former giving the necessary surface activity for LB film formation. Upon spreading at the air-water interface, the blend forms different surface structures depending on the relative amount of the two components. By adjusting the experimental parameters it is possible to obtain relatively regular structures in the submicron size range, which can be transferred on a solid substrate. These systems have been investigated by means of SIMS and atomic force microscopy.     

An AFM study of the effect of chemical treatments on the surface microstructure and adhesion properties of flax fibres

The mechanical properties of fibre-reinforced polymer composites are largely dependant on the adhesion between the matrix and the fibre. In order to enhance the interaction between flax fibres and unsaturated polyester resins, raw fibres were chemically modified using sodium hydroxide, sodium hydroxide plus acetic anhydride and formic acid-based treatments. The physical properties of the modified fibres were investigated by means of the atomic force microscopy. At first, the morphological analysis of the surfaces shows that after the chemical treatments, the fibres surface appear to be less heterogeneous in topology and smoother. Nonetheless, no significant roughness difference was found between the different treatments. Secondly, adhesion forces measurements were performed between a standard AFM silicon nitride tip and the fibres. The adhesion forces were found to vary according to the chemical treatment. The sodium hydroxide-based treatment was found to increase the adhesion force between the fibre and the AFM tip whereas the lowest adhesion force was found for the formic acid- based treated fibre. These results were attributed to the different hydrophilic character of the modified fibres. Due to the importance of the water layer adsorbed on the fibres, the adhesion forces between the AFM tip and the different samples are found to be mainly dominated by capillary forces in relation with the fibre's surface hydrophilicity.     

The Alpha Effect: The Connection between Cyclonic Events and Current Helicity

In a magnetized plasma, resistive diffusion of large-scale magnetic fields can be suppressed or even overcome by a turbulently generated electromotive force. For a plasma in which the turbulence is homogeneous and isotropic this EMF is characterized by the ensemble average = ?B0, where ?v and ?b represent the turbulent fields and B0 defines the large-scale field. Determination of the statistical properties of the turbulence that are required to generate a finite alpha effect, as it has become known, is one of the central subjects of dynamo theory. Parker has shown that helical velocity fluctuations possessing a net amount of kinetic helicity are capable of dynamo action. These "cyclonic events" produce electromagnetic fluctuations characterized by their own statistical properties. Within the context of "mean-field electrodynamics" we show that these fluctuations possess a net amount of current helicity, and find that a necessary condition for dynamo action is that the turbulent current helicity and the current helicity in the large-scale field be of opposite sign.     

Connection between nuclear moments of inertia and collective variables

Several frequently used formulae for calculating nuclear moments of inertia are shown to be associated with specific choices of collective variables.     

Effects of fluoride treatment on phosphoric acid-etching in primary teeth: An AFM observation

The aim of this study was to examine the effect of fluoride application on 37% phosphoric acid-etching by atomic force microscopy (AFM) in primary tooth samples based on a clinical protocol used in a pediatric dental hospital. Enamel samples were prepared from 36 exfoliated and non-carious primary teeth. Primary tooth samples were randomly assigned to one of the four groups based on the timing of acid-etching with 37% phosphoric acid after an acidulated phosphate fluoride (APF) pre-treatment. Group 1 received no fluoride application, Group 2 was pre-treated with fluoride and then received acid-etching 2 weeks later. One week separated the fluoride treatment and the acid-etching in Group 3, while Group 4 received acid-etching immediately after the fluoride treatment. The vestibular enamel surfaces of each primary tooth sample were scanned in air at a resolution of 512 × 512 pixels and a scan speed of 0.8 line/s. On the enamel surfaces of the primary teeth after APF pre-treatment, debris were observed although the teeth were smoother than they were prior to APF. As a result, it was concluded that APF treatment is responsible for decreased primary tooth surface roughness. The enamel surfaces etched for 20 s showed that acid-etching was effective not only in removing scratches and debris, but also for evaluating enamel rod characteristics. Primary tooth enamel surfaces after etching showed minute structures caused by the decreased hydroxyapatite nanoparticle space, compared to those before etching. Also, acid-etching showed significantly increased roughness effects (p < 0.0001, n = 9). Finally, as more time elapsed after APF pre-treatment, the roughness was decreased to a lesser degree (p = 0.005, n = 9). We suggest that primary teeth etching 2 weeks after APF pre-treatment used clinically in pediatric hospitals may be effective to obtain properly etched enamel surfaces.     

MFM and AFM study of thin cobalt films modified by fluorosilane

Polycrystalline cobalt films 100 nm thick were thermally evaporated on oxidized Si(100) substrates. Then 1H, 1H, 2H, 2H perfluorodecyltrichlorosilane (FDTS) films of various thicknesses, in the range of about 2 nm to 30 nm, were grown on cobalt surfaces by vapor phase deposition (VPD). The cobalt films modified by FDTS were investigated using magnetic force microscopy (MFM) and atomic force microscopy (AFM). MFM observation showed that the magnetic structure of the cobalt films modified by FDTS is composed of domains with a considerable component of magnetization perpendicular to the film surface. This in turn indicates that the cobalt films on oxidized Si(100) substrates crystallize in the hexagonal close-packed (HCP) phase and exhibit a texture with the hexagonal axis perpendicular to the film surface. The magnetic domains formed a maze structure. The domain width increased from typically 80–120 nm to 400–500 nm with increasing the thickness of FDTS films from about 2 nm to 30 nm. AFM imaging of the surfaces of FDTS films revealed the presence of an agglomerate morphology. The agglomerates varied in size from typically 30–70 nm to 150–300 nm as the film thickness was increased from about 2 nm to 30 nm.     

AFM and TEM study of hydrogenated sputtered Si/Ge multilayers

Multilayers of hydrogenated ultrathin (3 nm) amorphous a-Si and a-Ge layers prepared by sputtering have been studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM) to check the influence of annealing on their structural stability. The annealed multilayers exhibit surface and bulk degradation with formation of bumps and craters whose density and size increase with increasing hydrogen content and/or annealing temperature and time. Bumps are due to the formation of H₂ bubbles in the multilayer. The craters are bumps blown up very likely because of too high a gas pressure inside. The release of H from its bonds to Si and Ge occurs within cavities very likely present in the samples. The necessary energy is supplied by the heat treatment and by the recombination of thermally generated carriers. Results by energy filtered TEM on the interdiffusion of Si and Ge upon annealing are also presented.