Reactivity at the film/solution interface of ex situ prepared bismuth film electrodes: A scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM) investigation

Bismuth film electrodes (BiFEs) prepared ex situ with and without complexing bromide ions in the modification solution were investigated using scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM). A feedback mode of the SECM was employed to examine the conductivity and reactivity of a series of thin bismuth films deposited onto disk glassy carbon substrate electrodes (GCEs) of 3 mm in diameter. A platinum micro-electrode (? = 25 μm) was used as the SECM tip, and current against tip/substrate distance was recorded in solutions containing either Ru(NH₃)₆³⁺ or Fe(CN)₆⁴⁻ species as redox mediators. With both redox mediators positive feedback approach curves were recorded, which indicated that the bismuth film deposition protocol associated with the addition of bromide ions in the modification solution did not compromise the conductivity of the bismuth film in comparison with that prepared without bromide. However, at the former Bi film a slight kinetic hindering was observed in recycling Ru(NH₃)₆³⁺, suggesting a different surface potential. On the other hand, the approach curves recorded by using Fe(CN)₆⁴⁻ showed that both types of the aforementioned bismuth films exhibited local reactivity with the oxidised form of the redox mediator, and that bismuth film obtained with bromide ions exhibited slightly lower reactivity. The use of SECM in the scanning operation mode allowed us to ascertain that the bismuth deposits were uniformly distributed across the whole surface of the glassy carbon substrate electrode. Comparative AFM measurements corroborated the above findings and additionally revealed a denser growth of smaller bismuth crystals over the surface of the substrate electrode in the presence of bromide ions, while the crystals were bigger but sparser in the absence of bromide ions in the modification solution.     

Local reactivity of diamond-like carbon modified PTFE membranes used in SO2 sensors

The local electrochemical activity of polytetrafluorethylene (PTFE) membranes coated with diamond-like carbon (DLC) was investigated using scanning electrochemical microscopy (SECM). During z-approach curves in the feedback mode of SECM unexpected local variations in the electron-transfer rate of [Fe(CN)₆]^3−/4− and [Ru(NH₃)₆]^3+/2+ were observed. This local heterogeneity of the electrochemical activity was further evaluated in a system adapted from SO₂ gas sensors. In this case, the Cu^2+/+ couple is used as dissolved reversible redox system. Reaction of SO₂ with Cu²⁺ yields Cu⁺ which is re-oxidized at the DLC-coated PTFE membrane. Gas permeation/tip-collection mode SECM experiments allowed visualizing the local pore distribution as sites where the SO₂ is permeating through the membrane and hence formation of Cu⁺ takes place.     

Iodide/triiodide electrochemistry in ionic liquids: Effect of viscosity on mass transport, voltammetry and scanning electrochemical microscopy

The electrochemistry of I⁻/I₃⁻ was studied in ionic liquids using a combination of cyclic voltammetry, chronoamperometry and scanning electrochemical microscopy (SECM). The electrolytes were 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C_nC₁Im][Tf₂N], ionic liquids (where n = 2, 4 and 8) and I⁻ was typically added at a concentration of approximately 11 mM. During cyclic voltammetry, two sets of peaks were observed in each ionic liquid due to oxidation and reduction of the I⁻/I₃⁻ redox couple and oxidation/reduction of the I₃⁻/I₂ redox couple. The diffusion coefficients of I⁻ and I₃⁻, as determined using chronoamperometry, increased with increasing temperature and decreased with increasing ionic liquid viscosity. The effect of ionic liquid viscosity on ultramicroelectrode (UME) voltammetry was also determined using the I⁻/I₃⁻ redox couple. Steady-state behaviour was observed at 1.3 μm UMEs at slow voltammetric scan rates and steady-state SECM feedback approach curves were also obtained at a 1.3 μm Pt SECM tips, provided that the tip approach speed was sufficiently low.     

Uses of scanning electrochemical microscopy for the characterization of thin inhibitor films on reactive metals: The protection of copper surfaces by benzotriazole

Scanning electrochemical microscopy (SECM) was used to study the film formation of benzotriazole towards corrosion of copper. SECM was operated in the feedback mode by using ferrocene-methanol as redox mediator, and the sample was left unbiased at all times to freely attain its open circuit potential in the test environment. Following exposure to aggressive electrolytes the anticorrosion abilities of the layers were characterized by image analysis and by an electrochemical method derived from the experimental approach curves. Changes in the shape of the approach curves were clearly observed during the inhibitor film formation process. They showed the transition from an active conducting behaviour towards ferrocinium reoxidation typical of unprotected copper, to a surface exhibiting insulating characteristics when the metal was covered by a surface film containing the inhibitor. This supports that SECM is a practical technique in the investigation of corrosion inhibitor performance. However, a consistent tendency for the characterization of inhibitor film formation using SECM measurements in the positive feedback mode for the copper-benzotriazole system was only found if the experiments were conducted when the inhibitor molecule was not present in the test solution. That is, inhibitor molecules were found to interact not only with the copper surface during the monitoring process, but with the SECM tip as well, this effect being significantly enhanced when chloride ions were present in the electrolyte. Finally, a procedure to image the chemical activity of copper surfaces partially covered with the inhibitor film with SECM is proposed.     

Application of the boundary element method numerical simulations for characterization of heptode ultramicroelectrodes in SECM experiments

Quantitative analysis of the scanning electrochemical microscope (SECM) experiments with heptode ultramicroelectrodes (UME) as a probe is performed by means of the boundary element method (BEM). The method is used to calculate the amperrometric steady-state response of the heptode UME in multiple electrode working mode, including the disk-ring competition mode, within the SECM feedback mode. SECM approach curves and line scans with the heptode UME are measured and analyzed by means of numerical simulations. Simulations are performed in the 3D space, thus enabling treatment of non-symmetrical systems, which the heptodes themselves and the line scan experiments are. The approach curves and the line scans are simulated using the real heptode geometry obtained from CCD camera images. The experimental parameters are determined and adjusted as a result of simulations. In particular, the focus was made on the analysis of such system imperfections as electrode/sample tilts, electrode scan heights and heterogeneous reaction rates at the UME. It is shown how the simulations featuring the real system geometry can be used further for the analysis of the SECM line scan and imaging experiments.     

Effect of oxides on the adhesion of Cu films deposited onto stainless steel by electron shower and thermal evaporation methods

When Cu films were deposited by thermal evaporation onto stainless steel substrates at 30°C, the oxygen gas in the vacuum chamber (1.5 x 10^-3 Torr) caused the adhesion of Cu films to increase from 3 to 5 MPa. Moreover, it increased further from 13 to 16 MPa when deposited at 300°C. The Cu film was not peeled off when deposited by the electron shower method and the epoxy resin failed (20 MPa), and this was independent of the addition of oxygen gas. As the chemical shift of Cu 2_p3/2 was observed at the interface between the Cu film and the substrate when oxygen gas was added, it is concluded that the adhesion is mainly determined by the chemical bonding, such as CuO and Cu₂O. The depth profile of Cu 2_p3/2 measured by X-ray photoelectron spectroscopy (XPS) using Ar etching showed apparent thermal diffusion of Cu into the substrate. But the Ar etching rate was decreased by Cu oxides at the interface. The amount of oxides depended on the substrate temperature and the deposition method for Cu film. Therefore, the depth profile of Cu measured by XPS did not represent the thermal diffusion of Cu into the substrate correctly. When the etching rate was modified, the diffusion of Cu was almost the same for different samples deposited at the same temperature, and the effect of the thermal diffusion on the adhesion was small. The adhesion on hydrated [Cr(OH)₃.0.4H₂O] and hydroxide [Cr(OH)₃] surfaces was lower than that on the oxide (Cr₂O₃) surface. In other words, the pretreatment of the substrates was very important to the adhesion.     

Organosilicon deep UV positive resist consisting of poly(p-disilanylenephenylene)

A new class of positive deep ultravoilet (UV) resists consisting of poly(p-disilanylenephenylene)s was developed, in which a disilanylene unit and a phenylene unit are connected alternatively in the polymer main chain. These resists had very high etching resistance against oxygen plasma. The lithographic applications of a double-layer resist system in which the poly(p-disilanylenephenylene) film was used as the top imaging layer were examined. As a result, very high resolution and high contrast were attained. The double-layer resist technique using organosilicon deep UV positive resist appears very promising for lithographic applications.     

FREE CONVECTIVE DRAFT INDUCED BY THERMAL AND CONCENTRATION GRADIENTS INSIDE AN ISOTHERMAL,VERTICAL CYLINDER†

Laminar, free convective flow through a vertical cylinder induced by the thermal and concentration buoyancy forces is investigated. The numerical studies involve development of a steady-state, two-dimensional heat and mass transfer model for the moist air core of the vertical tube. The stream function-vorticity method is employed to simplify the governing, coupled conservation equations which were then numerically solved by the successive over-relaxation (SOR) and alternating direction implicit ((ADI) methods.

A graphical correlation was found between dimensionless flow rate and dimensionless tube length as a function of the buoyancy force ratio N = Gr/Grc. Excellent agreement was obtained for the dimensionless flow rale results with those of Davis and Perona¹⁶ and Kageyama and Izumi¹³ for the case when only the-thermal buoyancy force is considered.

The combined buoyancy force from thermal and species diffusion provides larger local Nusselt Nu and local Sherwood Sh numbers relative to the case when just one buoyancy force is accounted for. Both local Nu and local Sh are seen to asymptotically approach a constant value as flow develops.     

Studying the localized deposition of Ag nanoparticles on self-assembled monolayers by scanning electrochemical microscopy (SECM)

The local deposition of Ag nanoparticles (NPs) on ω-mercaptoalkanoic acid, HS(CH₂)_nCO₂H, (n = 2, 10) self-assembled monolayers (SAMs) by scanning electrochemical microscopy (SECM) is reported. We found that the presence of a SAM had a pronounced effect on Ag deposition. Experiments were conducted by applying different potentials to an Au(1 1 1) substrate either in the presence of a constant concentration of Ag⁺ ions in solution (bulk deposition) or by generating a flux of Ag⁺ from an Ag microelectrode that was positioned close to the Au(1 1 1) substrate (SECM deposition). SECM was used for generating a controlled flux of silver ions by anodic dissolution of an Ag microelectrode close to the SAMs modified Au(1 1 1). We found that the shape of the NPs was affected by the length of the carbon-chain of the SAM. Tetrahedral NPs were obtained on bare Au(1 1 1) surfaces while rod like and cubic Ag NPs were deposited onto 3-mercaptopropanoic acid (MPA) and 11-mercaptoundecanoic acid (MUA) SAMs, respectively. The size and shape of the deposited NPs were influenced by the deposition potential.We conclude that the shape and distribution of locally deposited Ag NPs on Au(1 1 1) can be controlled by modification of the substrate with a SAM and through controlling the Ag⁺ flux generated by SECM.     

Dynamic output feedback covariance control of stochastic dissipative partial differential equations

In this work, we develop a method for dynamic output feedback covariance control of the state covariance of linear dissipative stochastic partial differential equations (PDEs) using spatially distributed control actuation and sensing with noise. Such stochastic PDEs arise naturally in the modeling of surface height profile evolution in thin film growth and sputtering processes. We begin with the formulation of the stochastic PDE into a system of infinite stochastic ordinary differential equations (ODEs) by using modal decomposition. A finite-dimensional approximation is then obtained to capture the dominant mode contribution to the surface roughness profile (i.e., the covariance of the surface height profile). Subsequently, a state feedback controller and a Kalman-Bucy filter are designed on the basis of the finite-dimensional approximation. The dynamic output feedback covariance controller is subsequently obtained by combining the state feedback controller and the state estimator. The steady-state expected surface covariance under the dynamic output feedback controller is then estimated on the basis of the closed-loop finite-dimensional system. An analysis is performed to obtain a theoretical estimate of the expected surface covariance of the closed-loop infinite-dimensional system. Applications of the linear dynamic output feedback controller to both the linearized and the nonlinear stochastic Kuramoto-Sivashinsky equations (KSEs) are presented. Finally, nonlinear state feedback controller and nonlinear output feedback controller designs are also presented and applied to the nonlinear stochastic KSE.     

System of coupled non-linear reaction diffusion processes at conducting polymer-modified ultramicroelectrodes

The theoretical analysis of the steady-state amperometric response for conducting polymer-modified ultramicroelectrodes is discussed. The effect of substrate diffusion in the solution adjacent to the polymer film on both the concentration profile and current response is also examined. Simple analytical expressions for substrate and mediator concentrations and current responses for all values of reaction/diffusion parameters are presented. The model is based on non-stationary system of coupled reaction/diffusion equations containing a non-linear term related to Michaelis-Menten kinetics of the enzymatic reactions. He's variational iteration method is used to give approximate analytical solutions of coupled non-linear reaction diffusion equations. A good agreement with available limiting case results is noticed.     

Transient behavior of viscoelastic fluid in an extruder

A simplified model is used for calculating the time-dependent velocity of polymeric fluid in an extruder. The flow properties of the fluid are characterized by a simple constitutive equation based on two parameters: a constant viscosity μ and a constant elasticity modulus G. It was found that the transient velocity fluctuates periodically, and the time t_t needed to restore the steady-state velocity from a disturbance varies with the ratio G/μ and the dimensionless group _ρH²G/μ², where ρ is the density of the fluid and H is the screw depth of the extruder.     

Optimization of etching parameters of a fluorinated polyimide using the RIBE technique

A parametric study of etching a new type of fluorinated polyimide (6FDA-ODA) has been carried out. This kind of material was especially elaborated for plasma etching (reactive ion etching, RIE); its behaviour was tested with the reactive ion beam etching (RIBE) technique which uses an (O⁺) reactive ion beam and allows independent control of the ion energy and current density of the beam. Etch rates were measured as a function of energy (E), current density (J) and incident angle (θ) of the beam with the sample normal. These rates were shown to present a maximum value (1000Åmin^-1) for an energy flux of about 3Wcm^-2 (E=6keV and J=0·5mAcm^-2) and decreased when θ increased. These results were then compared with etching rates obtained with chemically inert ions (Ar⁺): in this case, etching rates were five times lower than those measured with O⁺ ions. © 1998 Society of Chemical Industry     

Micropatterning of active enzyme with a high-resolution by scanning electrochemical microscopy coupled with a nanometer-sized carbon fiber disk tip

We developed a new method for fabrication of nanometer-sized carbon fiber disk electrodes and applied them to micropattern active horseradish peroxidase (HRP) with a high-resolution by scanning electrochemical microscopy (SECM). In order to pattern active HRP, except for active HRP micropatterns predesigned other regions on a HRP-immobilized substrate was deactivated by a reactive species generated at the electrode as the tip of SECM held at 1.7 V through oxidation of Br⁻ in 0.20 mol/L phosphate buffer (PB) containing 2.5 × 10⁻² mol/L KBr and 2.0 × 10⁻³ mol/L BQ (pH 7.0). The micropatterns of active HRP were characterized using the feedback mode of SECM in PB containing 2.0 × 10⁻³ mol/L BQ and 2.0 × 10⁻³ mol/L H₂O₂, when the tip potential was held at −0.2 V.     

The influence of oxidizing agents on etching and passivation of silicon in KOH solution

The effect of oxidizing agents (MnO₄⁻ and CrO₄²⁻) on the electrochemistry, etching kinetics and etching morphology of p-type silicon in KOH solution has been studied. Electrochemical results show that reduction of MnO₄⁻ occurs via hole injection into the valence band of oxide-covered silicon, while the rate of cathodic reduction of CrO₄²⁻ is very low. The current density measured with MnO₄⁻ at negative potential drops to a low value when the surface oxide is removed and chemical etching starts. It is suggested that during etching, a surface intermediate is formed which can interact chemically with the oxidizing agents. This is confirmed by optical absorption measurements. At potentials positive with respect to the open circuit potential both oxidizing agents modify the formation of surface oxide. In addition, both oxidizing agents influence the surface morphology by ‘preventing’ pyramid formation. These results are discussed in terms of a model previously suggested for chemical etching of silicon in alkaline solutions.     

Shielding effect of double microelectrode tips in a scanning electrochemical microscope

Electrochemical interaction of coaxial double microelectrodes, in which a ring microelectrode was surrounded by another ring microelectrode, was investigated. Mass-transfer reactions that occurred on both inner and outer microelectrodes interfered with each other and showed a “shielding” effect depending on potentials and geometries of microelectrodes. Application of the inner microelectrode of the double microelectrodes for a probing tip of a scanning electrochemical microscope (SECM) revealed that the shielding effect by the outer microelectrode affected the electrochemistry on the inner microelectrode in the vicinity of the substrate surface. The effect was intensified above the insulator but attenuated above the conductor as the microelectrodes approached in feedback mode of the SECM. Approach to a critical interelectrode distance also intensified the shielding effect in the substrate generation/tip collection mode. An SECM line-scan using a platinum/epoxy resin-model substrate was carried out to investigate the shielding effect on current sensitivity and lateral resolution of an SECM image.     

Effect of noble metal catalyst species on the morphology of macroporous silicon formed by metal-assisted chemical etching

Macroporous silicon with ordered pore intervals was fabricated by the site-selective chemical etching of a Si substrate using patterned noble-metal thin films as a catalyst. The morphology of the etched silicon surface and the etching rate was affected by the shape of deposits and metal catalyst species such as Pt-Pd, Au, and Pt. The etching rate increased in the following order: Au < Pt ≤ Pt-Pd. The pores of macroporous silicon prepared by using Pt-Pd catalyst were conical in shape because of the chemical dissolution of the surface of the macropores. On the other hand, by using Au catalyst, relatively straight pores with uniform diameter were formed in the direction of pore depth. The morphology of macroporous silicon was assumed to be affected by the difference in the shape of metal catalysts and the diffusion behaviour of injected positive holes at the silicon/metal interface.     

Investigation of diamond etching and growth by in situ scanning tunneling microscopy

Recently we published a new experimental set-up for the investigation of dynamic processes under diamond deposition conditions. In the present paper the first measurements of diamond etching and growth with this set-up are presented. Etching experiments at 500°C substrate temperature reveal no etching of the pre-grown diamond film used as sample, whereas at 530°C rapid etching of a (111)-faceted crystal and slow etching of a (100)-faceted crystal is visible. Similar, at 500°C no growth could be detected on a boron doped (100)-single crystal, but at 530°C growth was monitored with a rate comparable with that of the etching rate of the (100)-plane at the same substrate temperature (about 10 nm h⁻¹).     

Transient instability of the flow induced by an impulsively started rotating cylinder

The onset of instability induced by transient momentum diffusion in a boundary layer over an impulsively started rotating cylinder is examined. Kirchner and Chen (J. Fluid Mech. 40 (1970) 39) have conducted experiments and reported anomalously large Taylor numbers of up to 20×10⁶, far exceeding the well-known value of 1708 for Taylor vortices in steady flow. In this paper, we argue that it is inappropriate to treat the phenomenon as a steady-state wide-gap Couette flow, because the unstable boundary layer in their experiments was very thin. The instability in the fluid induced by momentum diffusion is an unsteady-state phenomenon analogous to transient thermal instability, whose mathematical equivalence for the steady-state cases have been established by Taylor (Philos. Trans. R. Soc. London A 223 (1923) 289). We find that the onset of instability can be predicted from a transient Taylor number defined as Ta=y⁵(∂u/∂y)²/ν²R_i. The maximum transient Taylor number is found to occur at a critical depth , where Ta_max=1.461U_i²(νt)^1.5/ν²R_i. Ta_max bears a theoretical critical value of 1100 from linear stability analysis. The experimental data of Kirchner and Chen (J. Fluid Mech. 40 (1970) 39) agree remarkably well with this value. The critical time can thus be predicted with good accuracy from a critical value of the maximum transient Taylor number of 1100. The theoretical critical dimension of the toroidal plume formed after the boundary layer becomes unstable is found to be , which agrees well with measurements. The average critical dimensionless wavenumber of the experiments of Kirchner and Chen (J. Fluid Mech. 40 (1970) 39) is found to be 3.05, which is very close to the theoretical value of 2.9.     

Gravity draining of a yield-stress fluid through an orifice

We study the draining of a yield-stress fluid from a vertical vessel having a hole or a tube at its bottom. In order to understand the basic process we first study the problem with a Newtonian fluid and show that the flow characteristics can be very well described by assuming that the flow is analogous to that through a straight conduit of given length. For a yield-stress fluid draining through a hole the behaviour is different: the flow stops when the pressure drop across the orifice falls to a finite value which increases as the yield stress of the fluid increases or the hole radius R decreases. All the data collapse onto a master curve when plotted in terms of dimensionless numbers involving a characteristic length which is a function of R. We deduce an empirical model for the flow characteristics in such a case. When a length of tube is added after the hole we show that the characteristics of the flow are similar to those for flow through a straight conduit with an equivalent length equal to the tube length plus a fixed additional length.