Gold oxide layers

Monolayers can be transferred onto many different substrates. Most LB depositions have been perfonned onto hydrophilic substrates, where monolayers are transferred when pulling tire substrate out from tire subphase. Transparent hydrophilic substrates such as glass [18,19] or quartz [20] allow spectra to be recorded in transmission mode. Examples of otlier hydrophilic substrates are aluminium [21, 22, 23 and 24], cliromium [9, 25] or tin [26], all in their oxidized state. The substrate most often used today is silicon wafer. Gold does not establish an oxide layer and is tlierefore used chiefly for reflection studies. Also used are silver [27], gallium arsenide [27, 28] or cadmium telluride wafer [28] following special treatment. 

Nitric acid reacts with all metals except gold, iridium, platinum, rhodium, tantalum, titanium, and certain alloys. It reacts violentiy with sodium and potassium to produce nitrogen. Most metals are converted iato nitrates arsenic, antimony, and tin form oxides. Chrome, iron, and aluminum readily dissolve ia dilute nitric acid but with concentrated acid form a metal oxide layer that passivates the metal, ie, prevents further reaction. 

The BLM layer uses a glue layer of chromium or titanium. These metals stick well to other metals and most dielectrics, but they are not solderable. Copper, nickel, and silver have been used as the solder-wetting layer for BLM in appHcations involving 95% lead—5% tin solders. Gold is commonly used as the oxidation layer on account of its resistance to oxidation and its excellent solderabiUty. 

We said in Chapter 21 that all metals except gold have a layer, no matter how thin, of metal oxide on their surfaces. Experimentally, it is found that for some metals the junction between the oxide films formed at asperity tips is weaker in shear than the metal on which it grew (Fig. 25.4). In this case, sliding of the surfaces will take place in the thin oxide layer, at a stress less than in the metal itself, and lead to a corresponding reduction in x to a value between 0.5 and 1.5. 

All metal surfaces are reactive, including the noble ones. Therefore, under ambient conditions, they all have chemisorbed layers on their surfaces. These vary greatly from metal to metal in thickness, from atomic monolayers, to microns, or more. The oxide layer on gold is very thin, for example, whereas it is quite thick on copper or lead. 

During the last 5 years, there have been several reports of multiblock copolymer brushes by the grafting-from method. The most common substrates are gold and silicon oxide layers but there have been reports of diblock brush formation on clay surfaces [37] and silicon-hydride surfaces [38]. Most of the newer reports have utilized ATRP [34,38-43] but there have been a couple of reports that utilized anionic polymerization [44, 45]. Zhao and co-workers [21,22] have used a combination of ATRP and nitroxide-mediated polymerization to prepare mixed poly(methyl methacrylate) (PMMA)Zpolystyrene (PS) brushes from a difunctional initiator. These Y-shaped brushes could be considered block copolymers that are surface immobilized at the block junction. 

The oxidation of N02 was reinvestigated by Piela etal. [31, 32] at gold, platinum, glassy carbon, and oxide layer... 

It was observed that the oxide layer formed on gold and platinum surfaces hinders the oxidation process. According to the authors, the electrooxidation on pure electrode surfaces can be described by the following reaction sequence ... 

Oxide layers on gold electrode surfaces are interesting, mainly due to their electrocat-alytic properties. 

Most metals undergo corrosion of some form except for the so-called noble metals of gold, platinum, and palladium. Metals such as aluminum and zinc have an even greater tendency than iron to oxidize, but the oxide layer on these metals forms an impervious protective layer. This protects the metal below from further oxidation. Iron (III) oxide is highly porous and as a result rust does not protect the underlying metal from further corrosion. 

Aluminum Oxide Moisture Sensor. This type of sensor is a capacitor, formed by depositing a layer of porous aluminum oxide onto a conductive substrate, and then coaling the oxide with a thin film of gold The conductive base and the gold layer become the capacitor s electrodes. Water vapor penetrates the gold layer and is absorbed by the porous oxidation layer The number of water molecules absorbed determines the electrical impedance of the capacity, which is. m turn, a measure of water vapor pressure. 

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