A method of incorporating surface roughness into theoretical calculations of surface forces is presented. The model contains two chief elements. First, surface roughness is represented as a probability distribution of surface heights around an average surface height. A roughness-averaged force is determined by taking an average of the classic flat-surface force, weighing all possible separation distances against the probability distributions of surface heights. Second the model adds a repulsive contact force due to the elastic contact of asperities. We derive a simple analytic expression for the contact force. The general impact of roughness is to amplify the long range behaviour of noncontact (DLVO) forces. The impact of the elastic contact force is to provide a repulsive wall which is felt at a separation between surfaces that scales with the root-mean-square (RMS) roughness of the surfaces. The model therefore provides a means of distinguishing between "true zero," where the separation between the average centres of each surface is zero, and "apparent zero," defined by the onset of the repulsive contact wall. A normal distribution may be assumed for the surface probability distribution, characterised by the RMS roughness measured by atomic force microscopy (AFM). Alternatively the probability distribution may be defined by the histogram of heights measured by AFM. Both methods of treating surface roughness are compared against the classic smooth surface calculation and experimental AFM measurement.
The van der Waals forces between titanium dioxide surfaces produced by atomic layer deposition (ALD) at the isoelectric point have been measured and found to agree with the calculated interaction using Lifshitz theory. It is shown that under the right conditions very smooth ALD surfaces are produced. At pH values slightly below and above the isoelectric point, a repulsive diffuse double-layer repulsion was observed and is attributed to positive and negative charging of the surfaces, respectively. At high pH, it was found that the forces remained repulsive up until contact and no van der Waals attraction or adhesion was evident. The absence of an attraction cannot be explained by the presence of hydration forces.
The structure of the dye layer adsorbed on the titania substrate in a dye-sensitized solar cell is of fundamental importance for the function of the cell, since it strongly influences the injection of photoelectrons from the excited dye molecules into the titania substrate. The adsorption isotherms of the N719 ruthenium-based dye were determined both with a direct method using the depth profiling technique neutral impact collision ion scattering spectroscopy (NICISS) and with the standard indirect solution depletion method. It is found that the dye layer adsorbed on the titania surface is laterally inhomogeneous in thickness and there is a growth mechanism already from low coverage levels involving a combination of monolayers and multilayers. It is also found that the amount of N719 adsorbed on the substrate depends on the titania structure. The present results show that dye molecules in dye-sensitized solar cells are not necessarily, as presumed, adsorbed as a self-assembled monolayer on the substrate.
Atomic layer deposition is used with the aim of producing new model surfaces suitable for fundamental wet surface science investigations. Alumina surfaces are found to dissolve in aqueous solutions, although they can be passivated against dissolution by adsorption. Highly useful thick titania films can be produced by employing low temperatures during formation, whereas hafnia and zirconia films have a tendency to produce films that crystallize, and this increases the roughness of the films.Fundamental investigations of surfaces relevant to colloid and surface science usually require that model surfaces are employed as natural surfaces. This is because natural surfaces are often unsuitable as a result of excess surface roughness, chemical heterogeneity, or inappropriate geometry. A number of model surfaces are commonly employed and are desirable for a range of reasons. Muscovite mica can be cleaved to produce atomically smooth regions of significant area. Highly ordered pyrolytic graphite (HOPG) can be cleaved to produce regions that are atomically smooth. Silicon wafers have a natural or enhanced amorphous oxide layer and surface roughness of <0.5 nm root mean square (RMS). Silica glass surfaces that are of similar roughness can be formed into flats, spheres, and other useful geometries such as cylinders. Both silica and silicon surfaces can be easily functionalized using silane coupling chemistry. Also, gold can be produced as low-roughness films that are readily functionalized by thiol adsorption. It would be advantageous to increase the range of model surfaces available, as this would enable a wider range of properties to be investigated and provide analogs that are closer to those that are of technical interest. We have been employing atomic layer deposition (ALD) to grow films onto substrates, in an attempt to increase the range of materials that can be used for fundamental surface forces and adsorption studies.Atomic layer deposition 15 is a process whereby a material is grown in a layer-by-layer fashion onto a substrate. Typically, growth occurs in two steps, starting with a hydroxylated surface. In the first step, metal ions are introduced to the surface in the gas phase in the form of a reactive organometallic or halide compound, known as the precursor. The precursor reacts with the hydroxy groups on the surface forming a single layer of metal compound, which is bound to the surface through oxygen atoms. The remaining precursor is flushed from the system before water vapor is introduced. The water vapor displaces the remaining ligands of the bound precursor and hydroxylates the bound metal atoms. The process is then repeated, with each cycle of steps resulting in the addition of a single layer of metal oxide.The technique is appealing because the growth of the ALD material is conformal and therefore will follow the features of the substrate material. 3 In the right circumstances, it will do so without significantly adding to the roughness. Additionally, complex shapes, including porous materials and par...
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