Porous thin films

Fabrication of Porous Thin Film using the Langmuir-Blogdett (LB) Technique... 

Martinez-Ferrero, E., Sakatani, Y., Boissire, C., Grosso, D., Fuertes, A., Fraxedas, J., and Sanchez, C. (2007) Nanostructured titanium oxynitride porous thin films as efficient visible-active photocatalysts. Advanced Functional Materials, 17 (16), 3348—3354. 

A seminal contribution by Mansky et al. on the preparation of oriented thin films of PS-PBD block copolymers that were then subjected to ozonolysis degradation conditions to give porous thin films for use as nanolithography... 

In a follow up full publication to the work above, Harrison et al. showed that using porous thin films as described above, pattern transfer could be accomplished in both Si and Ge substrates as well [23]. While an entire three-inch Si wafer was patterned using this technique, practical limitations, such as in etch anisotropy, were reported. In this paper, an effective technique for confirming porosity in these films was demonstrated. After ozonolysis of these thin films, an overlayer of PS remained at the surface of these porous samples. By using a low power CF4 RIE followed by periodic SEM analysis, this layer could be slowly etched to reveal thus revealing the cylindrical pores (now observed as trenches). Also, the authors showed that the low power CF4 etch did not result in significant surface roughness. 

Kim, D. H. Anderson, M. A. Photoelectrocatalytic degradation of formic acid using a porous thin film electrode, Environ. Sci. Technol. 1994, 28, 479. 

The APM interaction is described in Section 3.3.4 while the TSM interaction is probably due to parasitic conduction through the solution. In one APM study, porous thin films were found to have an effect on conductivity trends, proposed in the study to be due to changes in solution conductivity in the porous regions... 

This chapter is intended to cover major aspects of the deposition of metals and metal oxides and the growth of nanosized materials from metal enolate precursors. Included are most types of materials which have been deposited by gas-phase processes, such as chemical vapor deposition (CVD) and atomic layer deposition(ALD), or liquid-phase processes, such as spin-coating, electrochemical deposition and sol-gel techniques. Mononuclear main group, transition metal and rare earth metal complexes with diverse /3-diketonate or /3-ketoiminate ligands were used mainly as metal enolate precursors. The controlled decomposition of these compounds lead to a high variety of metal and metal oxide materials such as dense or porous thin films and nanoparticles. Based on special properties (reactivity, transparency, conductivity, magnetism etc.) a large number of applications are mentioned and discussed. Where appropriate, similarities and difference in file decomposition mechanism that are common for certain precursors will be pointed out. 

The magnitude of the stress has been studied for a boehmite gel layer on a porous a-alumina support Voncken et al [1992] have found that among various stress measurement methods the cantilever principle is most suitable for studying porous thin films like gels. Using a laser displacement meter to detect the deflection of gel and support layers, they found that the tensile stress exerted on the drying membrane (gel)... 

EP is an elegant way of controlling alkali concentration at the surface of a working metal catalyst by varying its electric potential. The technique is implemented by depositing porous thin-film metal catalysts on solid electrolyte supports. Noteworthy features of EP systems are as follows ... 

Besides being used as adsorbent for gas molecules, both SWCNTs and MWC-NTs can be cast as a random network or a porous thin film on metal electrodes [57-59] or used as a three-dimensional scaffold [41,42] for biosensors. CNTs serve both as large immobilization matrices and as mediators to improve the electron transfer between the active enzyme site and the electrochemical transducer. Various enzymes, such as glucose oxidase and flavin adenine dinucleotide (FAD) can adsorb onto the CNT surface spontaneously and maintain their substrate-specific enzyme activity over prolonged times [57]. Recently, cells have been grown on CNT scaffolds which provide a three-dimensional permeable environment, simulating the natural extracellular matrix in a tissue [60-62]. 

Electrochemical promotion (EP) denotes electrically controlled modification of heterogeneous catalytic activity and/or selectivity. This recently discovered phenomenon has made a strong impact on modem electrochemistry/ catalysis/ and surface science. Although it manifests itself also using aqueous electrolytes/ the phenomenon has mainly been investigated in gas-phase reactions over metal and metal oxide catalysts. In the latter case, the catalyst, which is an electron conductor, is deposited in the form of a porous thin film on a solid electrolyte support, which is an ion conductor at the temperature of the catalytic reaction. Application of an electric potential on the catalyst/support interface or, which is equivalent, passing an electric current between catalyst and support, causes a concomitant change also in the properties of the adjacent catalyst/gas interface, where the catalytic reaction takes place. This results in an alteration of the catalytic behaviour, controllable with the applied potential or current. 

As mentioned earlier, thin films can be considered 2-D nanostructures. In the recent past, thin film-based technologies have been responsible for the design of an enormous variety of thin film-based electrochemical devices. 2-D nanostructures [91] composed of 0-D or 1-D materials are those that are associated with the interfacial properties of electrodes. In electrochemistry they are known as porous electrodes, and they sometimes possess an effective surface more than 1,000 times greater than the geometric area expected for a compact and homogeneous 2-D structured electrode, e.g., porous thin film-related electrodes [92-96]. 

Morphology control is indispensable in many of the advanced applications envisioned for functional mesoporous materials (54, 267). Permselective membranes, micro-spheres, or monoliths are important for sorption, separation, and chromatography purposes. Porous thin films or fibrous structures are relevant for electronics, optics, low fe-dielectrics, and sensing applications. Colloidal particles or nanospheres are preferred for biomedical systems to be used in drug delivery or magnetic resonance imaging (MRl) with contrast agents. 

The development of new materials possessing low dielectric constant (k) is crucial for applications in miniaturized integrated circuits (sub-lOOnm technology). Porous thin films with low k dielectric could provide solutions to the problems of signal delays caused by interconnect resistance-capacitance, signal crosstalk, and power consumption. Suggest a thin film material that would be suitable for low k dielectric applications. 

Membranes are one class of thin films. If membranes provide a restrictive barrier to the free motion of molecules between phases, other thin film configurations possess a wider range of potential applications. One important new application of porous thin films is in the microelectronics industry, where the increase in the miniaturisation of circuits on microchips requires insulators with improved performance compared to the material that is currently used, dense silicon oxide. The electrical insulating properties of a material improve with decreased dielectric constant, k, so values lower than those of dense silicon oxide (k=3.9-4.2) are required. The lowest values of k are found in a vacuum, and gases have low k values, so porous silica with reasonable mechanical properties provides a sensible solution. Indeed, low k values (k < 2.2) have been demonstrated for porous materials, which can readily be prepared as thin films. One non-templated mesoporous silica, MesoELK (ELK=Extra Low k), has been developed by Air Products for applications in computing that require low... 

In addition, since the SAW Is sensitive to minute perturbations occurring in thin films which are In Intimate contact with the surface, SAW devices can be used to monitor physical and chemical processes occurring In these overlayers. Based on this effect, SAW devices have recently found applications In the characterization of the properties of thin films (8-10). In this paper, we report on the utility of SAW devices to characterize (1) the surface area and pore size distribution of porous thin films based on Nj adsorption Isotherms and (2) diffusion coefficients (D) for thin polymer films based on absorption transients (l.e., mass absorbed as a function of time) as indicated by SAW velocity transients (l.e., SAW velocity... 

The results obtained with these films indicate that this SAW-based technique is a powerful tool for characterizing porous thin films. It is based on a standard industry practice involving the use of N2 adsorption isotherms. The main advantage of the technique is that the SAW device decreases the detection limit for the amount of adsorbed Nj by several orders of magnitude over conventional techniques used for this type of characterization (1 ). This increased sensitivity is critical for thin films, due to the small total surface area present. For example, conventional instruments typically require a total sample surface area on the order of 10,000 cm ... 

The kinetics of carrier hopping between silicon nanoparticles in contact in wet and dry porous thin films has been examined in some detail and provides an application of several of the expressions presented earlier. The point of departure is Equation (21) with A now given by... 

A shish-kebab model for tbe filament morphology was proposed. The electrospinning process was shown to be a means of creating porous thin films with structural gradients and controlled morphology that could enhance biocompatibility. 

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