Substrate In Air Research Articles

Effects of contact time, humidity, and surface roughness on the adhesion hysteresis of polydimethylsiloxane

Dynamic contact mechanics experiments have been performed on small polydimethylsiloxane (PDMS) lenses and several substrates in both ambient air and in dry nitrogen. The experimental results are analyzed with the Johnson-Kendall-Roberts theory. While the theory adequately describes the approach data, it is unable to account for the large hysteresis observed upon retraction. Adhesion hysteresis is shown to scale with the roughness of the substrate, the hydrophilicty of the substrate, the time of contact, and the ambient humidity. The experimental results also demonstrate that this method is sensitive to changes in the surface energy of the substrate. The cumulative adhesion hysteresis is quantified and is shown to be largest for rough, hydrophilic substrates in relatively high humidity and smallest for smooth substrates in dry nitrogen. The origin of the hysteresis is analyzed by considering favorable interfacial bonding resulting from water-mediated bonding between the substrate and oxygen atoms in the PDMS backbone or other polar species on the polymer surface. Capillary forces are also postulated to contribute to the cumulative adhesion hysteresis.

Point contact current–voltage measurements on individual organic semiconductor grains by conducting probe atomic force microscopy

Conducting probe atomic force microscopy (CP-AFM) was used to make point contact current–voltage (I–V) measurements on individual microscopic grains of the organic semiconductor sexithiophene (6T). The 6T grains ranged from 1 to 6 molecules (2–14 nm) in thickness, 1–2 μm in length and width, and were deposited by thermal evaporation onto SiO2 substrates previously patterned with 200 nm wide Au wires. Au-coated AFM probes were used to image the substrates in air to identify individual 6T grains which grew in contact with a wire. The same probes were used to record the I–V characteristics of single grains. Analysis of the differential resistance as a function of probe wire separation yielded typical grain resistivities of 100 Ω cm and contact resistances of ∼100 MΩ. Over the 0–3 V range probed, the shape of the I–V curves can be attributed to a combination of the nonlinear I–V characteristics of the Au-6T junctions and the ohmic response of the grain. In general, we have shown that CP-AFM is a reliable method for correlating electrical transport properties with microscopic morphology in organic semiconductors.

Broadband form birefringent quarter-wave plate for the mid-infrared wavelength region

We discuss the design, fabrication and optical performance of a broadband form-birefringent quarter-wave plate for the 3.5 to 5 aem wave-length region. Rigorous coupled wave analysis (RCWA) was used to design the requisite subwavelength grating for silicon substrates in ambient air. Fabricated samples yield a measured phase retardation of 89ø to 102ø over the desired wavelength range.

Simultaneous topographical and amperometric imaging of surfaces in air: towards a combined scanning force–scanning electrochemical microscope (SF–SECM)

The first simultaneous images of electrochemical activity and surface topography of a substrate in air are reported using a combined scanning force microscope (SFM) and scanning electrochemical microscope (SECM): SF–SECM. In the SF–SECM, the amperometric response of a Pt-coated SFM tip connected as a working electrode is monitored as the probe is scanned over the surface, in contact mode, enabling topographical and electrochemical maps of the surface to be generated simultaneously. This methodology is illustrated through studies of the surface of a track-etched porous membrane (pore diameter 100 nm), hydrated with a solution containing 5×10 −3 mol dm −3 potassium hexachloride iridate(III) (electroactive mediator) in excess supporting electrolyte. The optimal conditions for the preparation of the most effective and durable Pt thin film-coated probe, essential for the production of a long-lasting conducting tip, are also determined.

Oxidation of aluminium nitride

The oxidation kinetics of polycrystalline aluminium nitride substrates in air at temperatures in the range 1150 to 1750°C have been studied by measuring the weight increase in the oxidized samples. At the lowest temperature, the oxide layer was not continuous on the AlN surface and the oxidation kinetics followed a linear rate law with an activation energy of 175 kJ mol−1. At all the higher temperatures, the growth kinetics followed a parabolic rate law with an activation energy of 395 kJ mol−1. Samples oxidized at these higher temperatures were covered with a dense oxide layer having a fine-grained microstructure.

Microstructure Observations of (Nd, Ce)2CuO4 Superconducting Thin Films by Using Cross-Sectional HRTEM and the Mechanism f Film Growth.

“Electron-doped”; (Nd, Ce)2CuO4 (NCCO) superconductor thin films were prepared by post-annealing of metallic double-layer films of Nd(Ce) and Cu deposited onto (001) surface of SrTiO3 (STO) substrates in air at 600 to 1100°C. Cross-sectional high-resolution transmission electron microscopy (HRTEM) was used to observe the microstructure of the films. The formation of NCCO crystallites was observed at the film/STO interface, when the post-annealing temperature was higher than 700°C In spite of epitaxial growth of NCCO at the interface, thin films annealed at 700 to 900°C were polycrystalline. Single-crystalline NCCO films with epitaxial relation to STO substrate were obtained by postannealing between 900 and 1000°C. However, in the film annealed at 1050°C, many planar faults and also some buffer layers at the interface were observed in the NCCO crystals. The film annealed at 1100°C contained no NCCO crystal and a secondary phase was observed. From these findings the mechanism of the film growth is discussed.

光第二高調波発生（ＳＨＧ）法によるＧｅ／Ｇｅ‐ｏｘｉｄｅ界面の評価

The reflected optical second harmonic generation from the interface between Ge (111) and germanium oxide layers has been measured in dry nitrogen atmosphere as a function of the sample rotation angle around its surface normal. The germanium oxide layer was produced either by oxidizing the Ge (111) substrate in air at room temperature (natural oxidation) or by oxidizing the Ge (111) substrate in pure oxygen gas at 823 K (thermal oxidation). A difference was found in the SH intensity in p-in/p-out polarization configuration between the Ge/native-oxide interface and the Ge/ thermal-oxide interface. By XPS measurement, we found that GeO and GeO2 were formed at the Ge (111) /nativeoxide interface and at the Ge (111) /thermal-oxide interface, respectively. We conclude that the difference in the interface structure has led to the difference in the SH intensity pattern.

Dynamic Surface Force Measurement. 2. Friction and the Atomic Force Microscope

The mechanism and geometry of force measurement with the atomic force microscope are analyzed in detail. The effective spring constant to be used in force measurement is given in terms of the cantilever spring constant. Particular attention is paid to possible dynamic effects. Theoretical calculations show that inertial effects may be neglected in most regimes, the exception being when relatively large colloidal probes are used. Model calculations of the effects of friction show that it can cause hysteresis in the constant compliance region and a shift in the zero of separation. Most surprising, friction can cause a significant diminution of the measured precontact force, and, if it actually pins the surfaces, it can change the sign of the calibration factor for the cantilever deflection, which would cause a precontact attraction to appear as a repulsion. Measurements are made of the van der Waals force between a silicon tip and a glass substrate in air. The evidence for friction and other dynamic effects is discussed. Interferometry is used to characterize the performance of the piezoelectric drive motor and position detector used in the atomic force microscope. It is shown that hysteresis in the former, and backlash in the latter, preclude a quantitative measurement of friction effects. The experimental data appear to underestimate the van der Waals attraction at high driving velocities, in qualitative agreement with the model friction calculations.

Structural and electrical properties of cathodic sputtered thin chromium films

Films with a thickness of about 150–1500 nm are deposited on ceramic substrates in Ar, N 2 , air and Ar+O 2 (30%) atmospheres using the cathodic sputtering technique. The formation of chromium oxides and nitrides in the films during reactive deposition is confirmed by X-ray diffraction investigations. The temperature-dependent electrical resistivity is measured in the temperature range of 4–500 K, and the temperature coefficient of resistivity 1/ ρ ×(d ρ /d T ) is determined. The variations in electrical properties of the films are compared with the variations in structure and chemical composition, and it is shown that the greatest resistance with TCR close to zero can be obtained in films containing granules with metallic and non-metallic conductivity .

Preparation of Atomically Smooth Germanium Substrates for Infrared Spectroscopic and Scanning Probe Microscopic Characterization of Organic Monolayers

The suitability of antimony-doped germanium crystals for use as infrared attenuated total reflection wave guides and conducting substrates for scanning tunneling microscopic imaging of absorbed organic monolayers has been evaluated. We have used repetitive ion bombardment followed by resistive heating of a germanium crystal under ultrahigh vacuum conditions to produce large, atomically flat regions on a Ge(111) crystal face. Through the combined use of STM, Auger spectroscopy and low-energy electron diffraction, we have demonstrated that monolayer quantities of tellurium electrochemically deposited onto the Ge surface dramatically reduce the rate of surface oxidation and permit the use of germanium crystals as STM substrates in air. The first STM images acquired in air with Ge as the conductive substrate are reported. No degradation of IR spectral quality occurs when using the Te-covered, Sb-doped Ge crystals as ATR elements. These findings suggest that Te-coated, atomically flat, low-resistivity Ge substrates are suitable for both IR spectroscopic and STM imaging of organic monomolecular films.

Mechanical and chemical consequences of the residual stresses in plasma sprayed hydroxyapatite coatings

The residual stresses in thick hydroxyapatite coatings, deposited by plasma spraying, have been determined experimentally using Raman piezo-spectroscopy. The stress dependence of the centre position of the 980cm −1 Raman band, owing to the symmetric stretching of the phosphate ion, PO 4 3−, has been established and found to be 2.47cm −1 GPa −1. Using this calibration, the residual stresses in hydroxyapatite coatings deposited onto Ti-6AI-4V substrates in air have been found to be 100 MPa (tensile), whereas those deposited in a vacuum have been found to be 60 MPa (compressive). Although desirable from a mechanical point of view, it is shown that coatings under residual compression are thermodynamically more stable and, hence, the dissolution of the ionic species, necessary in the exchange between bone and hydroxyapatite coating, can be impeded. It is calculated that for the coatings under examination the stresses have an effect comparable with almost an order of magnitude change of the [OH −] concentration. The analysis explains the dissolution behaviour of hydroxyapatite coatings subject to cyclic stress reported previously.

Nanopatterning by biological templating and laser direct writing in thin laser deposited films

We report on three alternative approaches to laser-supported sub-micro- and nanopatterning of ultrathin films. The inherent non-linearities of direct writing techniques can be used to fabricate structures far below the optical resolution limit. Due to the non-linear dependence of light absorption and reaction rate on film thickness and temperature (a) the lateral self-limitation of the oxidation reaction in ultrathin titanium films on a transparent heat insulating substrate in air can occur. The polarization of sharp conducting tips results in (b) near-field enhancement of the optical field in nanometer regions that allows diminishing of the effective size of the radiation source. The third approach (c) is based on the nanometer-period pattern transfer from a biomolecular template to a Pt/C overlayer deposited by cross-beam pulsed laser deposition.

Corrosion fatigue behavior of a steel with sprayed coatings

This paper describes the corrosion fatigue behavior and fracture mechanisms of a steel with different sprayed coatings. Rotating bending fatigue tests were conducted in 3% NaCl solution using specimens of a medium carbon steel with sprayed coatings of a ceramic (Cr2O3), a cermet (WC-12%Co) and two metals (Ni-11 % P and Al-2% Zn). The corrosion fatigue process was basically the same for ceramic, cermet, and Ni-11 % P sprayed specimens. That is, the corrosive media could be supplied from the specimen surface to the substrate through cracks initiated during fatigue cycling and/or pores in the coatings, and thus corrosion pits were generated followed by subsequent crack initiation and growth in the substrate. The corrosion fatigue strength of ceramic sprayed specimens was slightly improved compared to that of the substrate steel because the under-coating (Ni-5%A1) could impede the penetration of the corrosive media although the ceramic coating had a poor resistance to cracking under cyclic loading. Cermet sprayed specimens also exhibited improved corrosion fatigue strength because of the high resistance to cracking and the low volume fraction of pores of the coating. In Ni-11 % Psprayed specimens, cracks were initiated in the coating even at low stress levels; thus the corrosion fatigue strength was the same as that of the substrate. Anodic dissolution took place in Al-2 % Zn coating because the coating was electrochemically poor, and thus the substrate was cathodically protected. Therefore, the corrosion fatigue strength of Al-2 % Zn sprayed specimens was enhanced to as high as the fatigue strength of the substrate in room air. Based on the experimental results, a dual-layer coating consisting of WC-12%Co and Al-2%Zn was fatigue tested. The coating was effective at low stress levels and exhibited long life under conditions where corrosion fatigue strength was critical.

Kinetics of field-induced oxidation of hydrogen-terminated Si(111)

Using an atomic force microscope (AFM) with a highly doped Si tip (0.01–0.02 Ω cm) we induced the oxidation of hydrogen-terminated Si(111) surfaces by scanning the surface with an applied bias Uox between sample tip (negative) and substrate (positive) in air of 60% relative humidity. The resulting oxide height was measured relative to the unoxidized substrate immediately afterwards using the same setup in the AFM mode. Typical oxide thicknesses of some nm were achieved. p-type as well as n-type samples were investigated and the oxidation time at each point was varied by varying the scan speed of the tip during oxidation. Some of the salient results are as follows: (i) There is a definite threshold voltage Uth such that no oxidation takes place below Uth; (ii) Uth depends on doping and is lowest (2.7 V) for n+- and highest (5.4 V) for p+-type material; (iii) Uth is independent of scan speed and thus oxidation time for speeds between 0.25 and 7 μm/s. Assuming that the oxidation time is inversely proportional to the tip scan speed during oxidation we tried to fit the data to current models of oxide growth (parabolic growth law, linear-quadratic growth law, and Mott–Cabrera mechanism). None of them fits the experimental data. Instead, we find that the results are well described by a power law of the form Z=(Uox−Uth) α0 (tox/t0)γ where Z and t are the oxide thickness and the oxidation time, respectively, and α0 is a factor that depends on experimental conditions such as doping, tip shape, and humidity. The exponent γ has a value of 1/4 to within experimental uncertainty.

Molecular Beam Epitaxial Growth of In0.15Ga0.85As Quantum Wells on (110) GaAs Surfaces

We report on molecular beam epitaxial (MBE) growth of high-quality In0.15Ga0.85As quantum wells (QWs) on (110) GaAs surfaces. The (110) surfaces are prepared by cleaving (001) GaAs substrates in air. The optimized growth condition is searched by changing the substrate temperature for the growth of In0.15Ga0.85As layers. We have found that the substrate temperature around 430°C is crucial to obtain the good surface morphology without facet structures and the sharp PL peak of an In0.15Ga0.85As QW.

The effect of metal type and multi-layering on friction surfacing

Friction surfacing was attempted with a stainless steel, aluminium and brass consumables on a mild-steel substrate in the open air and in flowing nitrogen. Stainless steel formed a strongly-bonded thick layer but friction surfacing with both aluminium and brass was not possible. Both brass and aluminium consumables failed to form a heated layer in contact with the mild steel, high thermal conductivity of either metal being the probable cause of failure to perform friction surfacing. Nitrogen ventilation caused cooling of the heated layer between the stainless steel and the mild-steel substrate and lowered the quality of the coating. Multi-layer surfacing by stainless steel consumables was successful, with at least three strongly-bonded layers possible, provided that the surfaces are free of oily contaminants.

Demonstration of nanometer recording with a scanning probe microscope

Feasibility of ultrahigh density information data storage has been newly demonstrated by using an atomic force microscope (AFM). Ultrasmall gold dots ranging from 10 to 100 nm in diameter could be successfully recorded on Si substrate in air by adjusting and applied pulsed voltage of AFM probe (SiO 2 birdbeak-type cantilever) coated with gold thin film. It indicates that the technique has a potential to realize a storage density of more than 100 Gb/in 2 and even 1 Tb/in 2.

Electrical transients generated by the peel of a pressure sensitive adhesive tape from a copper substrate

Contact electrification during the formation of adhesive bonds can lead to transient electrical signals and even electrical discharges when these bonds are subsequently broken. We present detailed measurements of the electrical current generated by peeling a pressure sensitive adhesive from an electropolished Cu substrate in air and in vacuum. The magnitude of this current depends strongly upon the peel speed. At constant peel speed, transient fluctuations in the rate of interfacial failure along the peel front produce corresponding current fluctuations. Electrical breakdown events during peel produce sharp spikes in the current signal as well as long wavelength electromagnetic radiation and visible/UV emissions with characteristic spectral features. We use these signals to characterize the breakdown events in various atmospheres. Current measurements are a potentially new time-resolved probe of micromechanical and electrical processes accompanying peeling.

Lateral Structure of a Grafted Polymer Layer in a Poor Solvent

The authors have used atomic force microscopy (AFM) to study the lateral structure of end grafted poly(styrene) chains on a silicon substrate in air. As the grafting density is decreased, the chains are found to clump together into islands, resulting in a highly inhomogeneous monomer density with considerable portions of the substrate exposed to air. The transition from a stable homogeneous film to the clump formation is in agreement with recent theoretical simulations of polymer brushes in different solvent conditions. The islands are found to be interconnected by thin arms, which can be visualized as isolated chains stretching out on the substrate.

Formation of nanometer-sized Au dots on Si substrate in air

Nanometer-sized Au (gold) dots on a Si substrate were formed by field evaporation using an atomic force microscope (AFM) in air. Theoretical and experimental studies show that field evaporation is possible in the AFM metal probe/vacuum/thin insulator/conductor configuration. Theoretically, the field evaporation is generated by further application of an additional bias voltage less than 30–40 V in a thin insulator. Experiments demonstrated that ultrasmall gold dots were formed on a natural SiO2/Si substrate by the applied voltage less than 10 V, and ultrasmall gold dots of 15 nm in diameter were made on the insulator.