Optically semi-transparent electrodes

For spectroelectrochemical and photoelectrochemical studies, optically semi-transparent electrodes have been fabricated by vapour deposition techniques on glass or quartz substrates (Chapter 12). Tin and indium oxides, platinum, and gold have been used. 

In the case of pure electrical measurements for substrates and contact materials Au, Ag, or Pt are preferred due to the p-type behaviour of as-prepared oligothiophenes. If simple band models are assumed for otnT and the contacts, materials like the noble metals with a workfunction of 5.3 eV (Au) or 5.6 eV (Pt) should lead to ohmic contacts whereas materials with low workfunction such as Al (4.28 eV) or Mg (3.66 eV) should form Schottky barriers. (For n-type behaviour, i.e. after n-doping or annealing in air, compare Section 4.2.2, the opposite is true.) Both types of contacts are necessary for electro-optical measurements. Here also one electrode has to be optically transparent. The most common material for the latter purpose is indium-oxide doped tin-oxide (ITO). This material is highly transparent and highly conductive but has the problem that the substrate always exhibits several spikes standing out of the surface. The other type of semi-transparent electrodes are ultra-thin metal films evaporated onto the organic film. 

In a typical spectroelectrochemical measurement, an optically transparent electrode (OTE) is used and the UV/vis absorption spectrum (or absorbance) of the substance participating in the reaction is measured. Various types of OTE exist, for example (i) a plate (glass, quartz or plastic) coated either with an optically transparent vapor-deposited metal (Pt or Au) film or with an optically transparent conductive tin oxide film (Fig. 5.26), and (ii) a fine micromesh (40-800 wires/cm) of electrically conductive material (Pt or Au). The electrochemical cell may be either a thin-layer cell with a solution-layer thickness of less than 0.2 mm (Fig. 9.2(a)) or a cell with a solution layer of conventional thickness ( 1 cm, Fig. 9.2(b)). The advantage of the thin-layer cell is that the electrolysis is complete within a short time ( 30 s). On the other hand, the cell with conventional solution thickness has the advantage that mass transport in the solution near the electrode surface can be treated mathematically by the theory of semi-infinite linear diffusion. 

Fig. 9.13. Construction of two optically transparent thin-layer cells, (a) With minigrid electrode (from Ref. 22 with permission) (b) With semi-transparent tin dioxide electrode, and usable in a flow system.

The pyroelectric detector contains a mono-crystal of deuterated triglycine sulfate (DTGS) or lithium tantalate (LiTaOj), sandwiched between two electrodes, one of which is semi-transparent to radiation and receives the impact of the optical beam. It generates electric charges with small temperature changes. The crystal is polarized proportionally to the radiation received and it acts as a capacitor. 

Harrison et al came to another conclusion [231,232]. They find radical cations, rt-diiners, and di-cations in their MIS device. In this device ITO was etched to form two 5 mm electrodes on the glass substrate. Onto this structure 100 nm 6T was evaporated under various conditions (see below). As insulator evaporated SiO was used. Finally, two semi-transparent gold strips were evaporated as gate electrodes. They form a right-angle with the ITO electrodes. In contrast to the above-mentioned experiment, four MIS diodes are formed in this way. With this device optical spectroscopy of the... 

Semi-infinite Ceiis Optically Transparent Electrodes (OTEs)... 

Spectroelectrochemistry at Optically Transparent Electrodes I. Electrodes Under Semi-infinite Diffusion Conditions, Theodore Kuwana and Nicholas Winograd... 

Fig. 10.2 - A cell for experiments with an optically transparent electrode under conditions of semi-infinite linear diffusion.

Spectroelectrochemistry at Optically Transparent Electrodes I. Electrodes under Semi-Infinite Diffusion Conditions, Theodore Kuwana and Nicholas Winograd Organometallic Electrochemistry, Michael D. Morris Faradaic Rectification Method and Its Applications in the Study of Electrode Processes, H.P. Agarwal... 

Spectroelectrochemistry at Optically Transparent Electrodes, II. Electrodes under Thin-Layer and Semi-Infinite Diffusion Conditions and Indirect Coulometric Iterations, William H. Heineman, Fred M. Hawkridge, and Henry N. Blount Polynomial Approximation Techniques for Differential Equations in Electrochemical Problems, Stanley Pons Chemically Modified Electrodes, Royce W. Murray... 

Spectroelectrochemistry at Optically Transparent Electrodes, n. Electrodes under Thin-Layer and Semi-Infinite Diffusion Conditions and Indirect Coulometric Iterations, William H. Heineman,... 

Figure 7 Nernst plot for spectropotentiostatic experiment on 0.87 mM [Tc" (diars)2Cy+, 0.5 M TEAR in DMF. Data at 403 nm from Figure 6 are used. Reprinted by courtesy of Marcel-Dekker, Inc. from Heineman WR, Hawkridge FM and Blount HN (1984) Spectroelectrochemistry at optically transparent electrodes. II. Electrodes under thin-layer and semi-infinite diffusion conditions and indirect coulometric titrations. In Bard AJ (ed) Electroanaiyticai Chemistry. A Series of Advances, Vol 13, pp 1-113. New York Marcel-Dekker.

Heineman WR, Hawkridge FM and Blount HN (1984) Spectroeiectrochemistry at optically transparent electrodes. II. Electrodes under thin-layer and semi-infinite... 

Kuwana T and Winograd N (1974) Spectroelectrochemis-try at optically transparent electrodes. I. Electrodes under semi-infinite diffusion conditions. In Bard AJ (ed) Electroanalytical Chemistry. A Series of Advances, Vol 7, pp 1-78. New York Marcel-Dekker. 

We have carried out an investigation of the electrical and electro-optical properties of a series of Schottky barrier diodes fabricated with polyacetylene sandwiched between two metal contact layers, one to form the Schottky barrier and the other (gold) to provide an ohmic contact [56]. This type of structure is straightforward to fabricate with an extrinsically-doped semiconductor and there have been several reports of such devices which use polyacetylene or other conjugated polymers [57-62]. The details of the device fabrication have been given in section 3.2, and we show in figure 10 the details of the typical structures that we have used for this work. We have worked with relatively thick films of polyacetylene, in the range 500 - 1(XX) nm, so as to avoid the possibility of short-circuits tetween top and bottom electrode, but we have kept the metal contact layers thin so that they are semi-transparent and allow optical transmission measurements. 

Developments in spectroelectrochem-istry based on TCO electrodes under semi-infinite diffusion conditions were reviewed by Kuwana and Winograd in 1974 [244]. Since 1974, significant developments in absorption spectroelectrochem-istry have occurred under semi-infinite diffusion conditions and in optically transparent thin-layer electrochemistry, several new TCO electrodes have heen characterized, and the indirect coulometric titration technique has been developed and apphed to biological systems [245]. 

The development of low-pressure synthesis methods for diamond, such as the chemical vapor deposition (CVD) technique, has generated enormous and increasing interest and has extended the scope of diamond applications. Highly efficient methods have been developed for the economical growth of polycrystalline diamond films on non diamond substrates. Moreover, these methods allow the controlled incorporation of an impurity such as boron into diamond, which in this case forms a ptype semiconductor. By doping the diamond with a high concentration of boron (B/C = O.Ol), conductivity can be increased, and semi-metallic behavior can be obtained, resulting in a new type of electrode material with all of the unique properties of diamond, such as hardness, optical transparency, thermal conductivity and chemical inertness [1,2]. 

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