A METHOD AND APPARATUS FOR GENERATING A FLUX OF PHOTONS OF VARIABLE SPECTRAL COMPOSITION FROM A MATERIAL SURFACE WITH METALLIC CONDUCTIVITY, AND USES OF THE SAME

Abstract: A method of generating a flux of photons of variable spectral composition from a given material surface with metallic conductivity is characterised by bringing the surface into contact with an electrolyte containing donors or acceptors of electric charge, with the spectral composition of the flux of said photons being controlled via the applied polarisation.

Description: §._Elϋ χ _of_Elιa θGa_2f_ϊa l j i§ L bi£_seg.ctj:ai ComDoai .ian_f nom_a_Ma g.r:ial_Su.r.£§,ce. witli_ etallic_CgQductiy.i..i._.a.ad_Il§.g.£_Sf_til£_Sa.iiig The present invention relates to a method of generating a flux of photons of variable spectral composition from a given material surface with metallic conductivity, to apparatus for carrying out the method, and to uses of the method and apparatus. A method of the above named kind is for example known in connection with the common incandescent light bulb. Here the spectral composition can be varied by varying the current through the filament of the light bulb, thus varying the spectral distribution of the light emitted therefrom. However, incandescent light bulbs are almost exclusively used only for illuminating purposes, the variation in spectral distribution which can be obtained is relatively restricted and is accompanied by a pronounced change in the light yield. The principal object underlying the present invention is to provide a method and apparatus for generating a flux of photons of variable spectral composition which can be readily and inexpensively controlled, and which lends itself to use both as a source of coloured light and as an investigatory tool for providing information about the quality of material surfaces, the operation of electrochemical processes and the performance of certain electrical devices. In order to satisfy this object the invention provides a development of the method of the initially named kind which is characterised by bringing the surface into contact with an electrolyte containing donors or acceptors of electric charge, with the spectral composition of the flux of said photons being controllable via the applied polarisation. Also according to the present invention there is provided apparatus for generating a flux of photons of variable composition from a given material surface with metallic conductivity which is characterised by an electrolytic cell comprising an electrode having said ma-terial surface, an electrolyte in contact therewith, a second electrode in contact with said electrolyte and spaced from the first electrode and a reducing or oxidising agent; by a power source and by means to control and vary the electrode potential. The invention is based on the recognition that if electrons are injected into the surface of an electrode in an electrolytic cell with an appropriate energy above the Fermi level in the electrode, a very small proportion of the electrons, perhaps one in 10 , will drop down to a lower energy and will generate a light photon which, if it is sufficiently near the surface, will escape from the electrode and should be visible. Furthermore, the invention is based on the recognition that, for a material surface of metallic conduc ivity, i.e. having a charge carrier density of the order of 10 to 10 ^ per cubic centimeter, there will be a continuum of energy states present above the Fermi level so that electrons dropping down to the Fermi level will generate photons with a range of frequencies up to a maximum frequency corresponding to the energy difference between the potential at which electrons are injected into the material surface and the prevailing Fermi level. Moreover, the invention recognises that both the frequency maximum of the emitted light and the spectral distribution of it can be varied by varying the Fermi level in the material surface which is achieved by simply varying the polarisation or potential of the surface. Although the conversion efficiency is not very high because of the aforementioned factor of 1 to 10 one can nevertheless obtain a substantial yield of light with relatively low current densities and this is entirely sufficient for many practical purposes which will be described later. It will be appreciated that the ability to change the spectral composition of a beam of light simply by varying the polarisation of an electrode in an electrolyte is very beneficial, there being only very limited means otherwise available in practice for producing a change in the spectral composition, i.e. of the colour of a light source. Almost all other devices of this kind rely on the use of colour selectivefilters. An earlier publication originating from one of the present inventors admittedly describes the emission of light from a semiconductor placed in an electrolytic cell (R. Mclntyre, B. Smandek and H. Gerischer, Ber. Bunsenges. Phys. Chem. 89, 78 (1985)), in the published work the semiconductor surface did not however exhibit metallic conductivity and it was found that varying the applied potential did not result in any change in the spectral composition of the emitted light. In one embodiment the material used for the material surface is a metal. It is however also possible for the material which is used for the material surface to be a semiconductor material provided the latter is polarised such that majority carriers are accumulated at the surface, i.e. so that the semiconductor exhibits metallic conductivity. In a further alternative the material can be a conductive polymer. i one preferred embodiment of the method the electrolyte is a mixture of a salt and a reducing or oxidising agent which are both dissolved in a non-aqueous solvent, preferably an aprotic solvent. in this embodiment the reducing or oxidising agent form the donors or acceptors of electric charge. When using this method with a reducing agent, i.e. v/ith a donor of electric charge the polarisation applied to the material surface is modulated, preferably in accordance with a square wave between two different values. At the upper val.ue the Fermi level in the material is selected to be sufficiently high that free electrons located at the Fermi level can easily pass out of the material surface to a suitable acceptor in the interface region. The subsequent reduction step in the polarisation of the material surface caused by the applied modulation results in a rapid drop in energy of the Fermi level so that some of the electrons maintained in the interface region by the acceptors immediately transfer back into the material surface where they are able to drop to a lower energy level and where a few of them will generate light photons which are able to escape from the surface. The majority of electrons will however result in the formation of electron hole pairs which subsequently recombine and generate heat. In a variation of this embodiment in which an oxidising agent is used the light generation is due to the injection of holes into the material surface. The aprotic solvent is preferably chosen, in accordance with the invention, from the class comprising: acetonitrile, propylene carbonate, N,N-dimethyl forma ide, dimethylsulphoxide, hexamethylphosphoramide and V- -butyrolacetone, or a mixture thereof. The salt dissolved in the aprotic solvent which is responsible for the formation of the electrolyte is preferably chosen from the class comprising the perchlorate, tetrafluoroborate or hexafluorophosphate of tetraalkylammonium, or of alkali metal cations, or a mixture thereof. Suitable reducing or oxidising agents are conveniently chosen, in accordance with the invention, from the classes comprising the radical anions of benzophenone and of anthracene and the radical cations of thianthrene and of tetrachloroorthobenzoquinone respectively. The present invention also contemplates the use of the above described method and apparatus for determining the spatial distribution of the current density at the interface between the material surface of an electrode and an electrolyte. The invention thus recognises that the current density at a particular point of the surface will determine the quantity of light emitted from that point of the surface. Accordingly, it will be possible to take for example an electrode used in catalysis and to examine the current density at different points of the electrode surface by measuring the spatial distribution of the light emitted from the surface. This information will then provide important design information for improving the design of the electrode. The invention also contemplates the use of the above described method and apparatus for determining the quality of a material surface. In other words, the invention recognises that flaws in the material surface will also affect the spatial distribution of the light emitted by the surface. One very important application of this recognition is the checking of the metallic contacts on high quality integrated circuits. The integrated circuit can namely be subjected to a simple non-destructive test by immersing it in a suitable electrolytic cell and any faults in the quality of the very thin metallic tracks will manifest themselves in the spatial distribution of the light emitted therefrom. This can again be detected using a suitable vidicon camera and a magnifying system. If necessary a colour filter or electronic manipulation of the vidicon camera signals can be used to improve contrast so that faulty areas are emphasised. Furthermore, a comparison can be made electronically between a high quality master and sample integrated circuits. The invention can be used with advantage to detect defects in thin metallic or conductive films such as films deposited electrochemically, by evaporation, by sputtering, by CVD etc. The present invention also contemplates the use of the method and apparatus described above for obtaining information about energy levels and electrode materials in electrochemical processes. There are namely a wide range of industrial manufacturing processes which use special electrodes to promote the chemical reaction. However, it is very difficult to obtain reliable information on the potentials at which the electrodes should be operated in order to promote the desired chemical reactions and it is also difficult to decide which electrode material is most suitable for a specific purpose. The emission of light from the surface of the electrode as proposed herein, can however form an important tool providing information on the efficiency of a particular electrode material and information on energy levels in the interface region where the chemical reaction is to take place. A whole variety of other applications for the method and apparatus of the invention can be conceived ranging from the checking of the surface quality of semiconductor samples to checking the locations of donor or acceptor impurities in doped semiconductor samples and carrying out investigations on the effectiveness of catalytic converters, for example catalytic converters for exhaust gases. In addition, electrolytic cells in accordance with the invention lend themselves to use as a teaching aid to make it possible for students, in particular in schools and universities, to visualise through the colour changes the processes which are taking place and the effects of changing the Fermi level etc. one particularly important type of apparatus that can be realised using the present invention is a variable colour display. Such apparatus is characterised in that a plurality of working electrodes are provided and are disposed in an array; and in that means is provided for independently varying the electrode potential at each of said working electrodes to selectively vary the spectral composition of the light emitted by the individual electrode. It may even be possible, in accordance with the invention and using this type of embodiment, to realise large area, shallow, high resolution displays to replace the presently bulky tubes of television sets. The invention will now be described in more detail by way of example only and with reference to specific embodiments as illustrated in the accompanying drawings which show: Fig. la a sketch illustrating the physical situation at the interface between a material surface of metallic conductivity and an electrolyte in an electrolytic cell when operated in accordance with the invention, with the electrolyte con¬ taining donors of electric charge, Fig. lb a sketch similar to Fig. la illustrating the physical situation when the electrolyte contains acceptors of electric charge. Fig. 2 a schematic view of a first embodiment of n apparatus in accordance with the present invention, Fig. 3 a graph illustrating the variation in bias potential applied to the working electrode of the apparatus of Fig. 2, Fig. 4 a graph showing the variation in spectral composition achieved by varying the pola¬ risation applied to the working electrode of the cell of Fig. 2, Fig. 5 a schematic diagram of an alternative embodiment of an apparatus in accordance with the present invention, and Fig. 6 a schematic diagram of a variable colour display in accordance with the present invention . Referring first of all to Fig. la there can be seen a vertical line 10 which represents the interface between an electrolytic solution 11 and a working electrode 12. The vertical line 10 may also be thought of as the material surface of the working electrode 12. The electrolyte solution consists in this example of a mixture of a salt and a reducing agent which are dissolved in an aprotic solvent. For example the salt may be tetraalkylammonium perchlorate, the reducing agent may be benzophenone and the solvent may acetonitrile. The salt ensures that the solution has electrolytic properties, i.e. is conductive. A potential gradient is maintained in the solution between the electrode 12 and a counterelectrode not shown. The potential in front of the interface region is maintained at the value Er„ed, relative to a reference p c otential commonly referred to in scientific circles as the vacuum level. This potential is sufficient to ensure conversion of the benzophenone into its radical anions. The white distribution 13 represents the energy band distribution of non-ionised benzophenone radicals at the interface. The hatched distribution 14 represents the energy band distribution of the radical anions of benzophenone at the interface. If the Fermi level E„ is raised from the lower value indicated by the horizontal line 15 to the level E p represented by the horizontal broken line 16 this will mean that free electrons are present at the electrode surface at the Fermi energy level E„ which lies above the redox potential E , of the benzophenone reducing agent. Because benzophenone likes to accept an electron and form the radical anion electrons will cross from the electrode 12 to the benzophenone molecules and these will be negatively charged, i.e. will be converted into the radical anions. If the Fermi level in the electrode 12 is now reduced rapidly to the level 15 which lies belαwi the iredox energy E , by changing the potential applied to the electrode then electrons will pass from the radical anions of benzophenone back into the electrode where a small proportion of them (ca. one in 10 ) will drop to a lower energy level generating a light photon which, if it is at or near the surface of the electrode, can escape into the electrolyte and can be viewed with suitable apparatus. Indeed the effect is sufficiently pronounced that it can be observed with the naked eye in a darkened room. The maximum energy which the emitted light photon can have is the difference in electron volts between the redox energy E , and the lower Fermi level 15. This energy in electron volts is directly equated with the energy of the light photon equal to H v^ and it will be seen that the maximum energy thus defines a maximum light frequency VIUcLX The Fermi level E„ F 16 has to be selected so that it is just greater than the redox energy for the particular reducing agent that is selected. For maximum efficiency the level 16 should lie only just above this redox potential. In fact it is found that electrons cross most easily to the benzophenone radical at energies corresponding to the edge of the energy band distribution and leave it again at energies corresponding to the center of the energy band distribution. The Fermi level E p defined by the line 15 can however be selected at will by controlling the polarisation at the electrode. Accordingly it will be appreciated that the energy difference between the redox potential E , and the lower Fermi level 15 can be varied at will thus varying the maximum frequency V of the emitted photons. Because the electrode 12 has metallic conductivity there is effectively a continuum of energy states present above the Fermi level 15 so that one not only J obtains lig J ht of freq^uency V max but also light of lower frequencies. The Fermi level E p can, however, not be reduced to any arbitrary value because at a critical level for the particular electrolyte solution it will result in an electrochemical reaction of the aprotic solvent or the salt with the result that electric charge is transferred between the reducing agent and the aprotic solvent or the salt leading to chemical luminescence in the electrolyte, i.e. the generation of light within the electrolyte which predominates over any light which may be generated at the electrode. With conventionally available electrolytes the maximum potential difference between the redox energy and the Fermi level is about 4-6 V with non-aqueous or aprotic solvents. For aqueous solvents the available potential difference is much less and this is the reason that aqueous solvents are not preferred. Fig. lb shows the parallel situation when using an acceptor of electric charge in place of a donor. I.e. the reducing agent used as the donor in the arrangement of Fig. la is replaced by an oxidising agent, for t example thianthrene. In the cell the potential E„ is now set to ensure the formation of the oxidising radical cations of thianthrene by placing it at the level 16' which in this case lies below the Fermi level E... . When using oxidising agents the photon generation mechanism is due to the formation of holes in the electrode 12. Turning now to Fig. 2 there can be seen a highly schematic illustration of an electrolytic cell used in accordance with the invention. The cell comprises a beaker or container 17 of which only the cylindrical wall can be seen in Fig. 2. The working electrode 12 is placed at the bottom of the cell parallel to the bottom surface thereof. A second electrode 18, the so-called counterelectrode, is placed over the working electrode 12 spaced apart therefrom but parallel thereto. For the sake of illustration however, the second electrode has been shown pivoted through 90° so that the surface of the working electrode 12 is more clearly visible. In practice the second electrode will either be a very thin transparent electrode, for example of gold or tin dioxide, or a wire-mesh or apertured structure so that in all cases light emitted from the working electrode 12 can pass through the second electrode 18. The reference numeral 19 schematically illustrates a lens which is used to focus the surface of the working electrode 12 with appropriate magnification onto a vidicon camera 20 which is able to scan the surface of the working electrode and to produce output signals representative of the spatial distribution of light across the surface of the electrode. It will be appreciated that in practice the vidicon camera will normally view the surface of the working electrode 12 perpendicular thereto through the second electrode 18. For the purpose of illustration the lens and vidicon camera have however been angularly displaced as is apparent from Fig. 2. To the left of the working electrode there is shown a so-called Luggin-Haber capillary 21 which serves to ensure that the required potential exists in the electrolyte adjacent to the surface of the working electrode. This system is well known in the field of electrochemical cells and is described, for example, in the book "Electrochemical Methods, Fundamentals and Applications" by A. J. Bard and L. R. Faulkner, published by Wiley, Chichester, 1980. It suffices here to say that the Luggin-Haber capillary- comprises a capillary tube which contains in its lower section 22 the electrolyte of the electrolytic cell. The bulb 23 and upper part 24 of the capillary contains a reference electrolyte which is separated from the first electrolyte by a frit 25. The bulb 23 of the Luggin-Haber capillary contains an electrode which is connected to one input of an operational amplifier 27. The other input of the operational amplifier is connected to a voltage source and the output of the operational amplifier is connected to the second electrode 18. The operational amplifier has, in known manner, an infinite input impedance and effectively zero output impedance and operates with high voltage gain tα maintain the potential at the surface of the working electrode 12 constant. If the voltage changes from the reference level selected by the device 28 , which is the source of the desired potential and is illustrated here schematically as a variable signal source, then the operational amplifier will feed more current to the second electrode 18 to increase the potential at the working electrode 12 to the desired level. The circuit 28 together with the operational amplifier 27 forms a so-called potentiostat and is the device which is used to switch the potential or polarisation of the electrode 12 between the two levels 15 and 16. The actual switching takes place in a preferred embodiment in accordance with the graph of Fig. 3 from v/hich it will be seen that the so-called modulation potential applied to the working electrode 12 is a square wave which oscillates at a frequency between 100 MHz and the maximum possible according to the state of the art of electronic design. The important feature of the modulation potential - time curve is the rate (rise time) at which the potential can be changed from the value 15 to the value 16 and back again to the value 15. Photon production is highly dependent upon the rise time of the potential modulation and this factor is more important than the frequency of the modulation. Various electronic techniques can be employed in the circuit 28 to decrease rise time, for example by incorporating positive feedback around the operational amplifier or by using a charge injection method of supplying charges from an additional source of electrical energy such as a charged capacitor. The threshold energy of the photons depends upon the rate at which the potential can be modulated, faster rise times produce photons of higher energy. This effect adds to the dependence of the spectral distribution of light upon the limits of the potential modulation 15 and 16. Also, it has been observed that with rise times shorter than approximately 100 iαsec the threshold limit of the photon energy may exceed that mentioned before. The mechanism for the production of photons with energy in excess of E , - E p is not at present understood but may be a consequence of surface absorption of the reducing or oxidising agents in the solution or it may be due to reaction of two injected electrons combining their energies, or of two holes reacting in a similar manner. The square v/ave does not need to be a symmetrical square wave, i.e. the periods at which the electrode 12 is polarised at the level 15 may be longer than the level at v/hich it is polarised at the level 16. The provision of a square wave modulation is a preferred feature of the invention, with particular emphasis being placed on vertical leading and trailing edges. It has been noted that with rapid rise times a huge current initially flows and then reduces to a lower value. Since the majority of the energy is contained in the initial flow of current it is possible to ensure better energy transfer into the system if higher frequencies are used and indeed frequencies in the MHz-range, in particular in the range from 1 to 10 MHz and above, are preferred. With such high frequencies it i s important that the cell should have a low ohmic resistance. This is preferably achieved by reducing the separation between the working electrode and the second electrode to a relatively small distance. The distance should preferably be less than 1 mm and favourable results have been obtained with spacings of a few tenths of a millimeter. Indeed one cell has been constructed with an electrode separation of 4 microns and has been found to be particularly effective. The photons which are generated can most easily be seen or used if the second electrode is constructed as an apertured electrode, for example a fine grid or mesh. It is however possible, at least in theory, to make the second electrode solid and to view the light emitted from one or more sides of the gap between the two electrodes. Clearly it might also be possible to couple the photons generated out of the system using some form of light conductor, such as light conducting fibres. The ohmic resistance of the cell can also be reduced by increasing the proportion of the salt in the electrolyte. Also the size of the electrodes may be appropriately selected in many applications. By optimising the design of the system and by selecting fast rise times, high frequencies and low ohmic resistance of the cell it is readily possible to boost the yield of photons from the system to a useful and impressive level. In a comparison test using two different working electrodes, one of gold and the other of carbon, but with the electrolytic and electrical set-up remaining otherwise unchanged, it was found that there was a remarkable difference in the spectral composition of the photons emitted from the gold electrode and the photons emitted from the carbon electrode. It is believed that a characteristic photon spectral composition will be found for a variety of different materials and that the light emitted from a particular electrode may be regarded as being a finger print for a material of that composition. Thus, it should be possible to carry out chemical analysis, in particular of metals and alloys by generating photons in the manner proposed herein. It may even be possible to draw conclusions about the metallographic structure of the alloy under investigation from the flux of photons that is generated. This may for example be possible with reference to both the spatial distribution of photons emitted from the surface and also the spectral composition thereof. It is anticipated that a detailed wealth of information about the material under investigation can be obtained by varying the polarisation applied to the working electrode, i.e. by varying the" difference between the levels 15 and 16. Since the surface of the working electrode can be scanned readily with conventional electronic means, for example using video techniques, it should be possible to deal with the information contained in the emitted flux of photons in a purely electronic manner, for example using a computer to compare the photon signature from a particular surface with known signatures of other surfaces stored in a computers' memory. Fig. 4 shows a plot of light intensity against frequency for the flux of photons emitted from a single crystal platinum working electrode 12. The solid line 31 shows the spectral distribution which is obtained at a first potential swing between the levels 15 and 16 of 3 V, whereas the broken line 32 shows the different spectral distribution which is obtained if the potential swing between the levels 15 and 16 is reduced to approximately 2.2 V. It can be seen that the peak of the intensity distributions 31 and 32 lie at 500 and 700 nm respectively, so that a very substantial change in the colour of the emitted light has been achieved. Turning now to Fig. 5 there can be seen an alternative apparatus in accordance with the present invention in which a continuous flow of solvated electrons takes place in an aprotic electrolyte, as e.g. hexamethyl phosphoramide contaning tetrapropylammoniu tetrafluoroborate from the second electrode 18 to the working electrode 12. In this considerably simplified embodiment the working electrode 12 is maintained constantly at the potential 15 (or 15') whereas the second electrode 18 is constantly maintained at the potential 16 (or 16'). Again the optical magnifying system 19 and the vidicon camera 20 have been schematically illustrated. The cell of Fig. 5 can be driven by a simple DC source, such as a battery, and the polarisation can be controlled by a simple device such as a potentiometer 28'. The solvated electrons move by diffusion from the counterelectrode 18 across the electrolyte layer to the working electrode 12. Turning now to Fig. 6 there can be seen a preferred embodiment of the apparatus of the present invention which is constructed as a variable colour display. The light source of the display is formed by an array of independent working electrodes which, in the present embodiment, are formed by the end faces of a plurality of carbon fibres. For the sake of simplicity only nine such end faces (a,b,c,d,e,f,g,h,i) are shown in a 3x3 array. The array is equivalent to the working electrode of the previous embodiments and has thus been generally identified by the reference numeral 12. In front of the working electrode 12 there is positioned a metal plate 18 having an array of holes in its surface. For the sake of clarity of illustration part of the second electrode ' 18 has been cut away (although its outline is continued by broken lines) . It will be noted that each hole in the second electrode 18 is positioned in practice in front of an individual one of the working electrodes, in this embodiment in fron€ of the end faces of the individual carbon fibres. Because of this correspondence the holes shown in the array have been designated by the same small case letters as the working letters but provided with a prime. It should be appreciated that in practice an array having many more elements is envisaged and the working electrode 18 will be positioned much closer to the working electrodes than shown in the present drawing where the separation between them has been increased simply for clarity of illustration. The purpose of the holes is to allow the light generated at the individual working electrodes to be viewed through the second electrode 18, i.e. from the right in the drawing of Fig. 6. In order to be able to control the spectral composition of the light photons emitted from each working electrode a to i it is necessary to be able to independently control the potential applied thereto. For this purpose each carbon fibre is carried back out of the array to a respective adder 30a to 30i. Each adder 30a to 30i serves to add a potential (positive or negative) supplied from the computer 31 to a basic potential supplied by the potentiostat 27, 28 via the line 32. Clearly the line 32 is connected to each of the adders 30a to 30i. In the embodiment shown a separate line 33a to -33i leads from the computer 31 to each of the adders 30a to 30i. the display is thus controlled by the computer which is programmed to apply the relevant potential differences to the lines 33a to 33i, and thus to the adders 30a to 30i in order to achieve the desired colours/colour changes at the individual working electrodes 12a to 12g of the array. The computer may be provided with an input terminal 34 enabling for example coloured messages to be written in by hand. More normally however, the display will be controlled by the computer to produce a picture representative of data either supplied to the computer or processed thereby. It will be noted that the arrangement of the Luggin-Haber capillary 21 and the general electronic layout corresponds closely to that shown in Fig. 2. The computer could for example be arranged to receive a video signal from an aerial or other input and to control the display so as to reproduce the video signal in colour. it will be understood that the embodiment of Fig. 6 is purely schematic in nature. There is no requirement for the individual working electrodes to be formed by the carbon fibres, this embodiment has simply been selected because carbon fibres connected to and leading away from integrated circuits such as adders are already commercially available for other purposes. Indeed it would be quite possible for the working electrodes to be formed by the end faces of wires, or for them to be deposited on an insulating substrate by vapor deposition or any of the other existing techniques for depositing thin coatings, for example the techniques that are used to form integrated circuits. It is important however that the working electrodes are present in or on an insulating substrate so that the potential at the individual working electrodes can be individually controlled. It will also be understood that the second electrode 18 can be formed in a variety of ways and does not have to take the form of an apertured metal plate. The second electrode 18 could for example be formed by a thin metallic coating deposited on a glass plate, with the deposition being arranged so that a plurality of apertures are formed in the deposited coating. The coating could also be formed«by a mesh of wires, or indeed possibly by a semi-transparent coating such as is used for forming beam dividers and the like. For the sake of simplicity the drawing of Fig. 6 does not show the electrolyte or the walls of the cell in which the electrolyte is contained. It is however quite conceivable for the two large area walls of a shallow rectangular or square cell to be formed by the insulating substrate of the working electrode 12 on the one side and by a support for the second electrode on the other side. Furthermore, it is pointed out that there are a variety of different layouts for the computer and for the electronic circuitry which could be used. By way of example each working electrode of the array can be uniquely identified by an X and a Y coordinate and the computer may be organised to identify the adder associated with a particular working electrode to which a potential is to be supplied via its X and Y coordinates and to supply the respective potential difference to the so identified adder, for example through the use of suitably organised logic circuitry. If the working electrodes are realised by thin film techniques then it may be convenient to form the electronic circuit components associated therewith, for example the adders of the embodiment of Fig. 6, at the same time by standard integrated circuit techniques. It will be appreciated that the electrolyte used for the purposes of the present invention may be a solid or liquid electrolyte. Although preference has been expressed for non-aqueous solvents, it is also possible to use aqueous solvents which may indeed be considered advantageous in some respects.

Claims: 1. A method of generating a flux of photons of variable spectral composition from a given material surface with metallic conductivity, characterised by bringing the surface into contact with an electrolyte containing donors or acceptors of electric charge, wherein the spectral composition of the flux of said photons is controlled via the applied polarisation. 2. A method according to claim 1, wherein the material is a metal. 3. A method according to claim 1, wherein the material is a semiconductor and is polarised such that majority carriers are accumulated at the surface. 4. A method according to claim 1, wherein said material is a conductive polymer. 5. A method according to any one of the preceding claims, wherein the electrolyte is a mixture of a salt and a reducing or oxidising agent, both dissolved in a non-aqueous solvent, preferably an aprotic solvent. 6. A method in accordance with claim 5, wherein the aprotic solvent is chosen from the class comprising: acetonitrile, propylene carbonate, N,N-dirnethyI formamide, dimethylsulphoxide, hexamethylphosphoramide and Y" -butyrolacetone, or a mixture thereof. 7. A method in accordance with claim 5 or claim 6, wherein the salt is chosen from the class comprising the perchlorate, tetrafluoroborate or hexafluorophosphate of tetraalkyla moniu or of alkali metal cations or a mixture thereof. 8. A method in accordance with one of claims 5 to 7, wherein said reducing or oxidising agents are chosen from the classes comprising the radical anions of benzophenone and of anthracene, and radical cations of thianthrene and of tetrachloro-ortho-benzoquinone respectively. 9. Apparatus for generating a flux of photons of variable spectral composition from a given material surface with metallic conductivity, characterised by an electrolytic cell comprising a working electrode having said material surface, an electrolyte in contact therewith, a second electrode in contact with said electrolyte and spaced from the first electrode and a reducing or oxidising agent; by a power source and by means to control and vary the electrode potential. 10. Apparatus in accordance with claim 9, characterised in that said means for controlling and varying the electrode potential is adapted to apply a periodically varying potential to said working electrode and is preferably adapted to vary the frequency of said periodically varying potential and/or the rise time thereof. 11. Apparatus in accordance with claim 9, characterised in that the electrolytic cell provides a continuous flow of solvated electrons from said second electrode to said working electrode; and in that said means for controlling and varying the electrode potential is adapted to maintain a constant but selectively variable potential difference therebetween. 12. Apparatus in accordance with claim 9, characterised in that a plurality of working electrodes are provided and are disposed in an array; and in that means is provided for independently varying the electrode potential at each of said working electrodes to selectively vary the srjectral composition of the light emitted by the individual electrode, whereby to form a variable colour display. 13. The use of the method and apparatus of any one of the preceding claims 1 to 11 to determine the spatial distribution of the current density at the interface between the material surface of an electrode and an electrolyte. 14. The use of the method and apparatus of any one of the preceding claims 1 to 11 for determining the quality of a material surface. 15. The use of the method and apparatus of any one of the claims 1 to 11 for obtaining information about energy levels and of electrode materials in electrochemical processes. 16. The use of the method and apparatus of any one of the claims 1 to 11 for determining the composition of the material of the working electrode and/or its structure by comparing the measured photon signature with reference to photon signatures from a variety of reference materials.

Inventors: MCINTYRE ROBERT, ROE DAVID K, SASS JUERGEN, MCINTYRE, ROBERT, ROE, DAVID, K., SASS, JUERGEN
Applicants: MAX PLANCK GESELLSCHAFT, MAX-PLANCK-GESELLSCHAFT ZUR FOERDERUNG DER WISSENSC
Application year: 1986
Main classification: H05B33
IPC codes: H05B33