Lithography colloidal

A number of methods can produce ordered colloidal monolayers, the simplest of which is drop-casting on a flat substrate where the degree of order is governed by the attractive capillary forces between partides during slow solvent evaporation [46, 70, 71, 76-78]. This process occurs by nucleation around an ordered region followed by growth by convective transport. The nucleation-growth mechanism requires a very 

A number of assembly methods employ self-assembly of nanoparticles at the air-water interface. Monolayers (and subsequent multilayers) can be formed at the interface due particle interaction and transferred to a solid substrate by controlled dip-coating and vertical deposition methods similar to Langmuir-Blodgett film deposition [81-86]. Regular monolayers of polymer colloids can also be assembled via an electrohydrodynamic route, whereby electrophoretically deposited particles between two electrodes can be manipulated to cluster in the presence of an electric field. On application of an AC or DC field, contrary to electrostatic norms, the like-charged particles are observed to coalesce producing large close-packed 2D crystalline domains [87]. 

As monodispersed colloidal polymers are generally spherical, a number of methods have been developed to diversify the colloidal crystal shape to produce a larger range of nanoarrays. Colloidal crystal sensor devices and polymer materials 

In this section, we will describe a different approach, which utilizes adsorption of charged colloidal particles onto an oppositely charged surface. This leads to an electrostatic attraction between the particles and the surface and electrostatic repulsion between the particles, resulting in a quite 

The electrostatic screening realized in an electrolyte can be used to control the particle-particle interactions. This interaction potential, R(r), is characterized by the Debye length which in the simplest mean field models describes the electrostatic screening of ionic particles interacting via Coulomb forces in a dielectric continuum, viz V(r) oc exp(— r). It is convenient to 

Step 2 The surface is etched by directed Ar+ sputtering, which removes the Pt, which is not shadowed by adsorbed PS particles. The etching is continued 10 nm into the ceria layer (with about half the etch-rate) to ensure that all exposed Pt is removed. 

Step 3 The PS particles are removed by an O2 plasma treatment (2 min at 250 W, and SOOmTorr). In Chemical Properties we present XPS results, which describe the effect of O2 plasma treatment regarding removal of the PS particles by O2 plasma and oxidation of Pt. It should be noted that it is not possible to dissolve the PS particles in acetone after the Ar+ etching process, which is believed to be due to ion-induced crosslinking of the polymer chains during ion etching (111), making them resistant to normal solvents for PS. The radial distribution function, g r), from the initial colloidal adsorption step is preserved throughout the nanofabrication procedure. 

The general interest in catalysis is typically particles of 10nm in diameter. This requirement introduces additional difficulties in the colloidal lithography process. The size of nanoparticles resulting from the colloidal lithography 

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GustavssonM, Fredriksson H, Kasemo B, Jusys Z, Jun C, Behm RJ. 2004. Nanostructured platinum-on-carbon model electrocatalysts prepared by colloidal lithography. J Electroanal Chem 568 371-377. 

Michel, R., et al. (2002), A novel approach to produce biologically relevant chemical patterns at the nanometer scale Selective molecular assembly patterning combined with colloidal lithography, Langmuir, 18, 8580-8586. 

Hanarp, P., et al. (1999), Nanostructured model biomaterial surfaces prepared by colloidal lithography, Nanostruct. Mater., 12(1), 429 132. 

Denis, F. A., et al. (2004), Nanoscale chemical patterns fabricated by using colloidal lithography and self-assembled monolayers, Langmuir, 20, 9335-9339. 

One of the attractive ways to make such model structures is based on lithographic techniques. We devote the next section to two important lithographic fabrication model catalyst methods, namely, electron-beam lithography (EBL) and colloidal lithography (CL). Lithography can be defined as a patterning process whereby an initial pattern is designed as some type of dataset which is subsequently written on the surface of a substrate as an array or ordered... 

Colloidal lithography Self-assembled mask Parallel, fast, multi-component structures, no resist, solvents, etc. Large particles, no long-range order Smaller colloidal particles, new mask removal methods... 

Fig. 4.6. General lithographic fabrication procedure, applicable to both electron-beam lithography and colloidal lithography...

We consider below recent progress in EBL and colloidal lithography (CL) to make well-defined planar model catalysts. The former method has been used for almost a decade in various model studies in catalysis mainly by us and Somorjai and coworkers at Berkeley [66-71], and must be considered as an old lithographic method, at least in comparison with the many other methods discussed above. Colloidal lithography, however, represents a new method that brings together ideas from surface chemistry of self-assembly and lithographic methods in terms of process versatility and cleanliness. These two methods represent slow serial (EBL) and fast parallel (CL) fabrication of model nanocatalysts. 

Fig. 4.12. Pt/ceria catalysts made by colloidal lithography using suspensions of d = 107 nm polystyrene particles with three different NaCI concentrations, resulting in different surface coverages and interparticle separations. NN refers to the nearest neighbor distance, as measured by the main peak in the radial distribution function, gir), shown in the inset of each SEM picture (from [92])...

Colloidal lithography allows for fabrication of advanced structures, such as 3D structures similar to those shown in Fig. 4.4. Here, we describe three different strategies to make advanced Pt/ceria structures. 

Fig. 4.19. Diffraction pattern and bright field TEM images of as-prepared 120-nm Pt particles (10-nm thick) on Si02 fabricated with colloidal lithography. The images were taken by Tomas Liljenfors, Chalmers University of Technology...

Fig. 4.21. Normalized XPS spectra of the Pt 4f region for (1) 130-nm Pt dots on ceria, (2) Pt film exposed to all the steps in the colloidal lithography process, (3) untreated Pt film, as reference. The samples were analyzed (a) as-prepared (b) after heating in vacuum to 400° C and cooled to room temperature (c) after reduction in H2 plasma (SOOmTorr, 250 W, 2min). The binding energies were referenced to adventitious carbon at 284.8eV (from [92])...

Figure 4.29 shows 120-nm wide and 10-nm thick Pt particles made by colloidal lithography on a 40-nm thick oxidized TEM membrane, i.e., with Si02 as the Pt support. Even though a thinner membrane 3delds better phase contrasts in TEM, atomic-scale resolution of particle-support boundary sites will normally require cross-sectional analysis. Using colloidal lithography, it is evident that a much narrower size distribution is obtained than by evaporation. 

Catalytic Reaction Studies with Model Catalysts Made by Colloidal Lithography... 

CO oxidation experiments were performed on Pt/alumina and Pt/ceria model catalysts, prepared by colloidal lithography. The samples were prepared without any of the additional plasma or UV-ozone pretreatments steps described in Preparation Procedures. Figure 4.38 shows T50 (the temperature at which 50% of reactant conversion is reached) and E (the apparent activation energy) as a function of CO oxidation cycle (ramping up and down in temperature). It is seen that both T50 and E initially shifts up during... 

Fig. 4.38. The effects of various pretreatments (oxidative and reductive) on CO oxidation on a 40-nm Pt/ceria model catalyst prepared by colloidal lithography as measured by the temperature of 50% of CO conversion and the apparent activation energy from the Arrhenius plot. CO reduction was made in 0.5% CO for Ih at 573K, H2 oxidation (a-treatment) was done at a = Ph2/(.Ph.2 + P02) = 0.33 at 573 K for 1 h, and finally /3 = CO oxidation (/3-treatment) was done in the O-rich regime (oxidative conditions), /3 = Pco/ Pco + P02) = 0.2 with 0.3% CO and 1.2% O2 at temperatures between 300 and 673 K. It is seen that reduction leads to a lower Tbo and activation energy, while sustained CO oxidation leads to an increase of the activation energy, which is not recovered by reductive treatments. The latter is explained in terms of strong-metal-support interactions (SMSI) and particle reshaping (see text)...

Here, we give an example where the electrochemical and electrocatalytic properties of nanostructured Pt catalysts supported on glassy carbon, prepared with colloidal lithography, were tested with oxidation of preadsorbed CO, so-called CO-stripping, as a test reaction [99]. The Pt nanoparticles were prepared as described in Fig. 4.11, except that the polystyrene particles were removed by UV-ozone treatment. The particles had an average diameter of 122 11 nm and were about 40 nm high. There was no indication (by AFM... 

Fig. 4.40. 107-nm Pt/CeOx model catalysts prepared by colloidal lithography both before and after a H2 oxidation reaction (Ph2/(Ph2 + P02) = 0.67) at 973K for 2h (nonflammable mixture in Ar)... 

Two fabrication processes for OSITs are proposed, and the resulting OSITs are schematically shown in Figure 10.4. One process is the shadow evaporation technique [16-18,25,26,28], and the other is colloidal lithography [31]. 

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