Development of a novel micro-spray-assembly process for multilayer ultra-thin film formation

A novel micro-spray-assembly process and an automatic device to fabricate multilayer ultra-thin film are introduced. Employing self-assembly monolayer (SAM) technique, ultra-thin film can be assembled by utilizing the micro-spray-assembly device. The thickness and roughness of each monolayer can be controlled by varying various materials attributes, i.e., deposition time, ionic strength, pH value, molecular concentration and by selecting different manufacturing parameters of the automatic device such as spraying rate, size of micro-drop, N₂ flow rate, temperature of N₂ flow.

ATR FT-IR absorption enhancement of a thin film under the photon-tunneling condition

A simple, yet very powerful technique for the spectral acquisition of an extremely thin film with enhanced absorption was explored. An infrared absorption of an extremely thin film confined between media of high refractive indices was greater than that of its bulk when the spectrum was acquired under the attenuated total reflection (ATR) condition with parallel (p) polarized radiation. The absorption enhancement was not observed under perpendicular (s) polarized radiation. Theoretical investigations indicated that the absorption enhancement was proportional to the integration of the mean square evanescent field within the film. The field integration under p-polarized radiation increased, while that under s-polarized radiation decreased as the thickness of the confined film became thinner. The maximum enhancement was observed when the film was sufficiently thinner than the penetration depth. The phenomena were experimentally investigated, and the results agreed very well with theoretical predictions.

The 'wimple' : rippled deformation of a fluid drop caused by hydrodynamic and surface forces during thin film drainage

It is well-known that hydrodynamic pressures in a thin draining liquid film can cause inversion of the curvature of a drop or bubble surface as it approaches another surface, creating a so-called “dimple”. Here it is shown that a more complicated rippled shape, dubbed a “wimple”, can be formed if a fluid drop that is already close to a solid wall is abruptly pushed further toward it. The wimple includes a central region in which the film remains thin, surrounded by a ring of greater film thickness that is bounded at the outer edge by a barrier rim where the film is thin. This shape later evolves into a conventional dimple bounded by the barrier rim, which then drains in the normal way. During the evolution from wimple to dimple, some of the fluid in the thicker part of the film ring flows toward the central region before eventually draining in the opposite direction. Although the drop is pressed toward the wall, the central part of the drop moves away from the wall before approaching it again. This is observed even when the inward push is too small to create a wimple.

The influence of surface forces on thin film drainage between a fluid drop and a flat solid

An experiment is described in which a mica surface is driven towards a mercury drop immersed in aqueous electrolyte. Under appropriate conditions, hydrodynamic pressure in the aqueous film creates a classical dimple in the mercury drop. The use of optical interferometry and video recording to monitor the shape of the drop and the thickness of the aqueous film with sub-nanometre resolution yields a high density of precise data showing the formation and evolution of the dimple as the film drains. Variation of electrical potential applied to the mercury phase allows control of the surface forces acting between the drop and the mica surface, so that the effect of surface forces on the film drainage process is highlighted. It is found that the film thickness at the centre of the dimple and the lateral extent of the dimple are not significantly affected by surface forces. On the other hand, the minimum film thickness at the edge of the dimple is sensitive even to weak surface forces. Since this minimum film thickness is a major determinant of the film drainage rate, it is shown that surface forces have an important effect on the overall drainage process.

Molecular organization and viscosity of a thin film of molten polymer between two surfaces as probed by force measurements

Measurements are presented of the force between two molecularly smooth mica surfaces immersed in liquid poly(dimethylsiloxane) (Dow Corning 200 of nominal viscosity 50 cS) over a range of film thicknesses from 3 to 200 nm. There is a repulsion, attributed to conformational restrictions, when the polymer molecules are confined to a gap less than about 15 nm thick. In extremely thin films (<5 nm) the force is an oscillatory function of thickness with a repeat spacing corresponding to the width of the polymer molecule, which suggests that the polymer segments are arranged in layers near the solid surfaces. Dynamic force measurements show that the polymer has a viscosity equal to its bulk value even in very thin films, but a region next to each surface, only about one radius of gyration thick, does not flow. Saturation of the polymer with water destabilizes the film when it is very thin.

Understanding water and ion transport behaviour and permeability through poly(amide) thin film composite membrane

Molecular dynamics (MD) together with the adaptive biasing force (ABF) and metadynamics free energy calculation methods was used to investigate the permeation properties of salt water through poly(amide) thin film composite reverse osmosis membranes. The thin films were generated by annealing an amorphous cell of poly(amide) chains through an MD method. The MD results showed they have typical structural properties of the active layer of thin film composite membranes and comparable water diffusivity (2.13×10^-5cm²/s for the film with a density of 1.06g/cm³) and permeability (9.27×10^-15cm³cm/cm²sPa) to experimental data. The simulations of water permeation through the films under different transmembrane pressures revealed the behaviours of water molecules in the thin films and the dynamic regimes of water permeation, including Brownian diffusion, flush and jump diffusion regimes. The intermolecular interactions of water and ions with poly(amide) chains showed a strong dependence on the local structure of films. The attraction between water and ploy(amide) molecules can be up to 8.5kcal/mol in dense polymer regions and 5kcal/mol in the pores of about 3nm. The ABF and metadynamics simulations produced the profiles of free energy potential of water and ions along the depth of the thin films, which provided important information for quantitatively determining the barrier energy required for water permeation and rejection of ions. The thin film with a density of 1.06g/cm³ and a thickness of 6nm offers a rejection to Na⁺ but a slight absorption of Cl^- (0.25kcal/mol) at 0.3-0.4nm distance to its surface. Water molecules must overcome 63kcal/mol energy to move to the centre of the film. The dependences of the barrier energy and the water-polymer interaction energy on the local free volume size in the thin film were analysed. The simulations of water permeation under high transmembrane pressures showed a nonlinear response of the concentration and distribution of water molecules in the film to the imposed pressure. Compaction of the film segments close to the porous substrate and water congestion in dense regions significantly influenced the water permeation when the membrane was operated under pressures of more than 3.0MPa.

Graphene quantum dots directly generated from graphite via magnetron sputtering and the application in thin-film transistors

This work presents a novel method to prepare graphene quantum dots (GQDs) directly from graphite. A composite film of GQDs and ZnO was first prepared using the composite target of graphite and ZnO via magnetron sputtering, followed with hydrochloric acid treatment and dialysis. Morphology and optical properties of the GQDs were investigated using a number of techniques. The as-prepared GQDs are 4-12 nm in size and 1-2 nm in thickness. They also exhibited typical excitation-dependent properties as expected in carbon-based quantum dots. To demonstrate the potential applications of GQDs in electronic devices, pure ZnO and GQD-ZnO thin-film transistors (TFTs) using ZrOx dielectric were fabricated and examined. The ZnO TFT incorporating the GQDs exhibited enhanced performance: an on/off current ratio of 1.7 × 107, a field-effect mobility of 17.7 cm2/Vs, a subthreshold swing voltage of 90 mV/decade. This paper provides an efficient, reproducible and eco-friendly approach for the preparation of monodisperse GQDs directly from graphite. Our results suggest that GQDs fabricated using magnetron sputtering method may envision promising applications in electronic devices.

Comparison of mechanical behaviour of thin film simulated by discrete dislocation dynamics and continuum crystal plasticity

3D finite element simulations of 9-grain multicrystalline aggregates are performed within the framework of the classical continuum crystal plasticity and discrete dislocation dynamics. The results are processed in a statistical way by ensemble averaging. The comparison is made at three levels: macroscopic stress–strain curves, average stress values per grain, local values of stress and plastic strain. The comparison shows that some similarities are observed in the stress and strain distributions in both simulations approaches. But there are also large discrepancies caused by the discrete nature of plasticity in DDD. The DDD simulations provide higher stress levels in the aggregate due to the small number of dislocation sources and to the stress field induced by individual dislocations.

Measurement of aqueous film thickness between charged mercury and mica surfaces : a direct experimental probe of the Poisson-Boltzmann distribution

A stable aqueous electrolyte film is formed between a mercury drop and a flat mica surface due to electrical double-layer repulsion when a negative potential is applied to the mercury. Film thickness has been measured as a function of applied potential while keeping the film pressure constant. By making measurements in this way, it is possible to map the data directly according to the Poisson-Boltzmann equation. An excellent fit to the data is obtained, providing direct evidence for this classical equation and its use as the basis of the Gouy-Chapman model of the diffuse double layer in electrolyte solutions.

Thin film drainage : hydrodynamic and disjoining pressures determined from experimental measurements of the shape of a fluid drop approaching a solid wall

Accurate measurements of the shape of a mercury drop separated from a smooth flat solid surface by a thin aqueous film reported recently by Connor and Horn (Faraday Discuss. 2003, 123, 193-206) have been analyzed to calculate the excess pressure in the film. The analysis is based on calculating the local curvature of the mercury/aqueous interface, and relating it via the Young-Laplace equation to the pressure drop across the interface, which is the difference between the aqueous film pressure and the known internal pressure of the mercury drop. For drop shapes measured under quiescent conditions, the only contribution to film pressure is the disjoining pressure arising from double-layer forces acting between the mercury and mica surfaces. Under dynamic conditions, hydrodynamic pressure is also present, and this is calculated by subtracting the disjoining pressure from the total film pressure. The data, which were measured to investigate the thin film drainage during approach of a fluid drop to a solid wall, show a classical dimpling of the mercury drop when it approaches the mica surface. Four data sets are available, corresponding to different magnitudes and signs of disjoining pressure, obtained by controlling the surface potential of the mercury. The analysis shows that total film pressure does not vary greatly during the evolution of the dimple formed during the thin film drainage process, nor between the different data sets. The hydrodynamic pressure appears to adjust to the different disjoining pressures in such a way that the total film pressure is maintained approximately constant within the dimpled region.

Electrophoretic deposition of YSZ thin-film electrolyte for SOFCs utilizing electrostatic-steric stabilized suspensions obtained via high energy ball milling

This manuscript describes a facile alternative route to make thin-film yttria-stabilized zirconia (YSZ) electrolyte by liquid-phase assisted electrophoretic deposition utilizing electrostatic-steric stabilized YSZ suspension followed by sintering. Very fine YSZ particles in ball-milled suspension facilitate their sustained dispersion through electrostatic mechanism as evidenced by their higher zeta potentials. Binder addition into the ball-milled suspension is also demonstrated to contribute complementary steric hindrance effects on suspension stability. As the consequence, the film quality and sinterability improve in the sequence of film made from non ball-milled suspension, film made from ball-milled suspension and film made from ball-milled suspension with binder addition. The specific deposition mechanisms pertaining to each suspension are also postulated and discussed below. A very thin dense electrolyte layer of ∼10 μm can be achieved via electrophoretic deposition route utilizing ball-milled suspension and binder addition. This in turn, makes the electrolyte resistance a more negligible part of the overall cell resistance. Further on, we also tested the performance of SOFC utilizing as-formed 10 μm YSZ electrolyte i.e. YSZ-NiO|YSZ|LSM (La0.8Sr0.2MnO3-δ), whereby a maximum power density of ∼850 mW cm−2 at 850 °C was demonstrated.

Oxygen reduction reaction activity of La-based perovskite oxides in alkaline medium : a thin-film rotating ring-disk electrode study

In this work, LaMO₃ and LaNi_0.5M_0.5O₃ (M = Ni, Co, Fe, Mn and Cr) perovskite oxide electrocatalysts were synthesized by a combined ethylenediaminetetraacetic acid-citrate complexation technique and subsequent calcinations at 1000 °C in air. Their powder X-ray diffraction patterns demonstrate the formation of a specific crystalline structure for each composition. The catalytic property of these materials toward the oxygen reduction reaction (ORR) was studied in alkaline potassium hydroxide solution using the rotating disk and rotating ring-disk electrode techniques. Carbon is considered to be a crucial additive component because its addition into perovskite oxide leads to optimized ORR current density. For LaMO₃ (M = Ni, Co, Fe, Mn and Cr)), in terms of the ORR current densities, the performance is enhanced in the order of LaCrO₃, LaFeO₃, LaNiO₃, LaMnO₃, and LaCoO3. For LaNi_0.5M_0.5O₃, the ORR current performance is enhanced in the order of LaNi_0.5Fe_0.5O₃, LaNi_0.5Co_0.5O₃, LaNi_0.5Cr_0.5O₃, and LaNi_0.5Mn_0.5O₃. Overall, LaCoO₃ demonstrates the best performance. Most notably, substituting half of the nickel with cobalt, iron, manganese, or chromium translates the ORR to a more positive onset potential, suggesting the beneficial catalytic effect of two transition metal cations with Mn as the most promising candidate. Koutecky–Levich analysis on the ORR current densities of all compositions indicates that the four-electron pathway is favored on these oxides, which are consistent with hydroperoxide ion formation of <2%.