Polyaniline (PAn) has emerged as a promising conducting polymer (Fig. 1) due to its chemical stability, optoelectronic properties and ease of synthesis (MacDiarmid et al ; <BR> <BR> Synthetic Metals Vol 125, pages 11-22 (2002) ). However, the processing of PAn into desired forms useful for practical applications has been hampered by the insoluble and infusible nature of polyaniline. The lack of processability in PAn was recently addressed by modified synthetic protocols using functionalised aniline monomers and by using surfactant acid as protonating agents in emulsion and dispersion polymerisations that give a solubilised form (Pud et al, Progress in Polymer Science, Vol 28, pages 1701-1753, (2003)).

A major challenge is to increase the conductivity of PAn. One approach is to coat PAn onto flexible substrates and to apply strain to induce conductivity in the range <BR> <BR> of 300-2500 S/cm (Theophilou et al ; United States patent 5217650 (1993) ). However the requirement for stretchable substrates make it impractical to generate conductivity in this way with common substrates such as silicon wafers, glass, sapphire and the like.

A method of secondary doping using aromatic phenols was established by <BR> <BR> MacDiarmid (Synthetic Metals, Vol 69, pages 85-92,159-160 (1995) ) to increase the conductivity. In this method an aromatic phenol induces a coil to chain transformation in the polymer chains, thereby introducing high conductivity.

US5403913 describes a conductivity of-170 S/cm after doping with phenol in vapour phase or in solution. The conductivity obtained using these methods of doping with phenol was not as high as for the stretched films. Another disadvantage is the thickness of the films (0.5 to 100 um) used for secondary doping which

prevent effective doping of the bulk, confining doping to the surfaces. Recently Ormecon GMbH has produced a PAn dispersion named 7301-026-002 (ttp//www. zipperling. de/index. html). While this material is said to exhibit a conductivity of 200 S/cm for a 340 nm thick film this conductivity is too small for applications requiring metallic conductivity.

In summary, the problems with polyaniline that have limited many commercial applications are one or more of (1) Non processability or insolubility making it difficult to process; (2) Low conductivity; (3) The difficulty in developing thin films or coatings; and (4) The decline in conductivity over time (ageing).

This invention is therefore directed to providing an improved process for producing thin films of polyaniline which will address these issues.

Statements of Invention According to the invention there is provided a process for preparing a polyaniline conducting material comprising the steps of :- preparing an aniline-surfactant complex; polymerising the complex with a polymerising agent to form a gel; forming a coating from the gel ; and secondary doping the coating with a dopant.

In one embodiment the coating is secondary doped in at least two stages. In one embodiment the concentration of secondary dopant in the solution is increased from one stage to a following stage.

In one case the secondary dopant comprises a phenolic compound. The phenolic compound may be selected from any one or more of phenol, mono-substituted phenolic compounds and di-substituted phenolic compounds wherein the substituents are selected from any one or more of hydroxyl, halogen, alkyl or alkoxy groups. The phenolic compound may comprise an aromatic phenol which may be selected from any one or more of m-cresol, 3-ethylphenol, 2-chlorophenol, 2-flurophenol, and phenol.

In one embodiment the secondary dopant is dissolved in a solvent. The solvent may be selected from any one or more of water and an organic solvent. The organic solvent may be selected from any one or more of tetrahydrofuran, chloroform, dichloromethane, acetone, toluene, ethanol and benzene.

In one embodiment secondary doping comprises exposing the coating to a dopant comprising a diluted phenolic compound at least 2 times, the concentration of the phenolic compound increasing each time. The secondary doping may comprise exposing the coating to a dopant comprising a diluted phenolic compound at least 10 times. The concentration of the phenolic compound may increase from 1% to 50% between a first and a final stage secondary doping.

In one case the concentration of the phenolic compound increases from 1% to 20% between a first and a final stage secondary doping.

In one embodiment the coating is dipped in the secondary dopant solution for a period for from 2 to 10 seconds in each stage.

After secondary doping, the solvent may be evaporated from the coating.

After doping, the coating may be maintained at room temperature for up to 24 hours.

In one embodiment the coating, after secondary doping, is dried under high vacuum.

In another aspect the invention provides a process for preparing a polyaniline conducting material comprising the steps of :- preparing an aniline-surfactant complex; polymerising the complex with a polymerising agent to form a gel ; and forming a coating from the gel.

In one embodiment the concentration of the aniline-surfactant complex is in the range of 0.01 to 0. 1 moles/litre. The surfactant may be a surfactant acid which may be selected from any one or more of more of alkyl benzene sulfonic acid comprising dodecylbenzene, octadecylbenzenesulfonic acid, decylbenzenesulfonic acid, and hexylbenzene sulfonic acid.

In one embodiment the concentration of the polymerising agent is in the range of from 0.1 moles/litre to 0.15 moles/litre. The concentration of the polymerising agent is determined by the concentration of the aniline (preferably for 0.1 moles/litre of aniline).

In one case the polymerising agent is selected from any one or more of oxidising agent comprising ammonium persulfate, potassium persulfate, iron (III) chloride and potassium dichromat.

In one embodiment the gel is applied onto a substrate. The substrate may be a flexible and/or non-flexible substrate. The substrate may be selected from any one or more of silicon wafer, glass slide, quartz, sapphire, polymer film and stable metals.

In one embodiment the coating comprises a film.

The substrate may comprise a fibre which may be selected from fibres of silicon, glass, quartz and polymer.

The invention also provides polyaniline materials prepared by a process of the invention.

The invention further provides polyaniline thin films prepared by a process of the invention. The films may be patterned.

According to another aspect the invention provides a composite film comprising a polyaniline film prepared by a process of the invention.

In a further aspect the invention provides polyaniline materials having a conductivity layer with a conductivity of greater than 500 S/cm. The conductivity layer may have a thickness in the range of 2 nm to 1,000 nm.

The polyaniline materials may be in the form of a film which can be patterned.

The polyaniline materials may be in the form of a fibre.

The invention also provides polyaniline thin films having a thickness in the range of 2 nm to 000 nm and conductivity in the range of 500 S/cm to 5000 S/cm.

The invention also provides polyaniline thin films having a thickness in the range of 10 nm to 500 nm and conductivity in the range of 500 S/cm to 3000 S/cm.

In another aspect the invention provides a sensor comprising a polyaniline material of the invention.

In a further aspect the invention provides a diode comprising a polyaniline thin film of the invention.

The invention also provides an electronic device comprising a polyaniline material of the invention.

Most of the films prepared exhibited conductivity of at least 500 S/cm. In some cases 2500 S/cm was reached.

Brief description of the drawings The invention will be more clearly understood from the following description thereof given by way of example only with reference to the accompanying drawings in which:- Fig. I shows the chemical structure of conductive PAn in the form of a polyemeraldine salt, n is greater than 10; Fig. 2 shows UV-spectra of (a) precursor film of DBSA doped aniline on a glass slide, and (b) secondary doped highly conductive PAn on a glass slide; Fig. 3 are photographs of thin films of polyemeraldine salt dip coated on (a) a glass slide and (b) to (d) dip coated on patterned glass slides and later the patterns were removed; Fig. 4 are photographs of (a) precursor polyaniline thin films processed on glass with conductivity 1 S/cm, (b) and (c) highly conductive polyaniline with conductivities (-1000 S/cm); Fig. 5 is a graph showing the temperature dependent resistivity data of secondary doped PAn film on silicon wafer with gold ohmic contacts;

Fig. 6 are scanning electron micrographs showing the morphology of the secondary doped PAn thin films at a magnification of (a) 4,500, and (b) 100,000. The film shows smooth morphology without any microscopic features; Fig. 7 is a graph showing the temperature dependent conductivity profile of a highly conductive PAn thin film of thickness 45 nm upon (a) cooling (b) warming. The film was high vacuum dried (-10-6 mtorr) and the contacts were provided by gold.

Fig. 8 is a graph showing Ln (conductivity) vs. T-l/2 at cooling and heating cycle for the sample described in Fig. 6. The graph is a straight line, which is the form expected for variable range hopping mechanisms of conduction.

Fig. 9 is a graph of magnetoresistance (MR) data at low temperatures showing a positive quadratic behaviour of the film of Fig 7 ;.

Fig. 10 is a graph showing the temperature dependent conductivity profile of a highly conductive PAn thin film of thickness 240 nm which has not been vacuum dried and where the contacts are provided by silver; Fig. I 1 is a graph showing the temperature dependent conductivity profile of a highly conductive PAn thin film of thickness 50 nm. The film was high vacuum dried (-10-6 mtorr) and the contacts were provided by gold; Fig. 12 is a graph showing the temperature dependent conductivity profile of a highly conductive PAn thin film of thickness 35nm. The film was high vacuum dried (-10-6 mtorr) and the contacts were provided by gold; and

Fig. 13 is a graph showing the temperature dependent conductivity profile of a highly conductive PAn thin film of thickness 72nm.

Detailed description The present invention provides a method for preparing coatings such as thin films of PAn with high conductivities in the range of greater than 500, typically 500-3000 S/cm with thickness in the range of 2 nm to 1, 000 nm, typically 20 to 500 nm. Thin films of surfactant acid doped PAn with a conductivity of 1-10 S/cm are prepared on rigid substrates such as silicon wafers and glass slides using an in situ gelation process. In this process PAn is doped in a primary doping step using for example a surfactant acid. The films are then subjected to secondary doping.

The PAn films/coatings prepared have several advantages over known PAn films.

The thin films of the invention may be prepared on any substrate such as plastic films, silicon wafers and glass. The films have very high conductivity in the range of 500-3000 S/cm. They are ultra thin (approximately 20 to 500 nm) and transparent.

They have very good temperature dependant conductivity profiles.

The PAn of the invention has also been used to prepare composite films. Thin films were generated with high dielectric properties by combining the polyaniline with magnetic nanoparticles, for example composite polyaniline films with magnetic particles of magnetite (Fe304) or Cobalt have been prepared. Such films have potential use in electromagnetic shielding. In particular, such composites may exhibit permeability and permittivity over an extended frequency range which qualifies them as left handed materials with a negative refractive index and may be used, for example for sub-wavelength focussing. (Pendry and Smith, Physics Today 2004 Vol.

57 issue 6 from page 37).

PAn films of the invention which were not subjected to a secondary coating stage were found to have improved quality over known PAn films. As shown in Figs 3 and 4 (a) the film is green in colour and has a conductivity in the range 1 to 10 S/cm.

This is a relatively low conductivity, however the film is of very high quality and can be coated on to any stable substrate. Stability is as good as other conductive polyanilines. These films have valuable potential uses.

The polyaniline coatings/films prepared have multiple applications. They may be used as highly sensitive sensors for various materials, where a decrease in conductivity could be obtained. They may also be used as a transporting layer in organic light emitting diodes.

The invention will be more clearly understood by the following examples. The examples presented are illustrative only and various changes and modifications within the scope of the present invention will be apparent to those skilled in the art.

In situ gelation Thin films of surfactant acid doped PAn with a conductivity of 1 S/cm were fabricated on rigid substrates such as silicon wafers and glass slides using an in situ gelation process. Polymerisation was conducted in an aqueous solution of aniline- surfactant complex to form a solid gel. Thin films of PAn can be processed on any substrate prior to gelation by dip coating or spin coating. In the case of dodecylbenzene sulfonic acid (DBSA)-aniline complex, polymerisation induced gel formation was observed only over a specific concentration range of 0. 01 to 0.1 moles/litre. Deviation from this concentration range resulted in either the solubilisation or precipitation of PAn. In a typical synthesis, sodium salt of the surfactant (100 mL 0.025 molar) was converted to DBSA, the acid form by the addition of hydrochloric acid (3.4 mL, 1 N). To this 0.18 mL of aniline was added drop-by-drop and thoroughly mixed to form an aniline-DBSA complex. To the above turbid solution 0.62 g of ammonium persulfate was added as polymerising

agent and the reaction was conducted at 23°C. After 15 to 25 minutes the solution turned light green. The solution was stirred using a glass rod at this stage for 3Q seconds. Slowly the solution exhibited a gradual increase in viscosity. Gelation was observed within 20 minutes in the final stage of the polymerisation and resulted in a solid green gel, which is a conductive form of PAn called polyemeraldine salt. The viscous solution phase observed prior to gelation can be utilized to dip-coat or spin coat good quality films on various substrates. The UV-visible spectrum of polyemeraldine salt coated on a glass slide is shown in Fig. 2 (a). A photograph of a thin film of polyemeraldine salt on a glass slide is shown in Fig. 3 and Fig 4 (a). A thin film of PAn can be deposited on both flexible and non-flexible substrates.

The in situ processed thin films of PAn were washed with water and subjected to multi-step secondary doping from a solution of aromatic phenol in a suitable organic solvent and dried under high vacuum (-10-6 mtorr).

Photographs of thin films of polyemeraldine salt secondary doped with m-cresol on a glass slide and a silicon wafer are shown in Figs. 4 (b) and 4 (c) respectively.

The dilution of secondary dopants in a suitable solvent assists effective doping and also prevent the dissolution of PAn thin films.

Properties In Figs 3 and 4 (a) the film is green in colour and has a conductivity in the range I to 10 S/cm. This is a relatively low conductivity, however the film is of very high quality and can be coated on to any stable substrate. Stability is as good as other conductive polyanilines. A UV-Visible spectrum of green polyaniline film is similar to that reported in the literature for conductive polyaniline form, emeraldine salt.

Fast interconversion between emeraldine base and emeraldine salt using mild acid and base conditions is also possible. In addition, the film can be processed on patterned surfaces

In Figs 4 (b) and 4 (c) the film is of a yellow colour and has a conductivity of greater than 500 S/cm. Such films are highly sensitive to basic conditions. The film is a good sensor for organic and inorganic bases. The film is very stable and conductivity is retained after the removal of residual m-cresol by vacuum drying. In addition, the ageing effect is minimised.

A UV-Visible spectrum of surfactant doped material which is of Green form as shown in Fig 2 (a) and the spectrum for m-cresol doped forms (Fig. 2 (b))-yellow.

Example I A thin film of PAn doped with dodecyl benzene sulfonic acid was fabricated using the in situ gelation process described above on a silicon wafer which has a insulating silicon oxide layer on its surface and a patterned gold contacts of 1 mm square separated by 10 mm. The above film was immersed in water for 5 minutes to remove the excess dodecylbenzene sulfonic acid and then dried at room temperature for 24 hours. A conductivity of 2 S/cm is measured at this stage using a four point probe method at room temperature.

A 5% solution of m-cresol was prepared in tetrahydrofuran and the PAn film was dipped into the solution for 2-10 seconds and taken out. The tetrahydrofuran was allowed to evaporate at room temperature leaving a deposit of m-cresol spread over the PAn film. The film was not disturbed for 30 minutes.

The process was repeated with 10%, 15%, 20%, 25% and 35% of m-cresol solutions in tetrahydrofuran. Finally the film was kept in 50% m-cresol solution for 12 hours and dried under vacuum.

The thickness of the film was 34 3 nm and the conductivity measured using a four point probe method was 2500 S/cm at room temperature. Temperature dependant resistivity data of the PAn film is shown in Fig. 12.

Example 2 A thin film of PAn doped with dodecylbenzene sulfonic acid was fabricated using the in situ gelation process described above on a silicon wafer which has a ninsulating silicon oxide layer on its surface. The above film was immersed in water for 5 minutes to remove the excess dodecylbenzene sulfonic acid and then dried at room temperature for 24 hours. A conductivity of 1 S/cm was measured using a four point probe method at room temperature.

A 5% solution of m-cresol is prepared in tetrahydrofuran and the above PAn film was dipped into the solution for 2-10 seconds and taken out. The tetrahydrofuran was allowed to evaporate at room temperature leaving a deposit of m-cresol spread over the film. The film was not disturbed for 30 minutes.

The process was repeated with 5%, 10%, 15%, 20%, 25% and 30% of m-cresol solutions in tetrahydrofuran. Finally the film was kept in 50% m-cresol solution for 12 hours, dried under vacuum. The thickness of the film was 62+ 3 nm and the conductivity measured using a four point probe method with evaporated gold contacts on top of the film was 1560 S/cm at room temperature.

Example 3 A thin film of PAn doped with dodecylbenzene sulfonic acid was fabricated using the in situ gelation process described above on a on a glass. The film was immersed in water for 5 minutes to remove excess dodecylbenzene sulfonic acid and then dried at room temperature for 24 hours. A conductivity of 2.5 S/cm was measured using a four point probe method.

A 5% solution of m-cresol was prepared in tetrahydrofuran and the above PAn film is dipped into the solution for 2-10 seconds and taken out. The tetrahydrofuran was allowed to evaporate at room temperature leaving a deposit of m-cresol spread over the film. The film was not disturbed for 30 minutes.

The process was repeated with 8%, 10%, 15%, 20% and 35% of m-cresol solutions in tetrahydrofuran. Finally the film was kept in 50% m-cresol solution for 12 hours, dried under vacuum. The thickness of the film was 240 3 nm and the conductivity measured using a four point probe method with evaporated gold contacts on top of the film was 1250 S/cm at room temperature.

Example 4 A thin film of PAn doped with dodecylbenzene sulfonic acid was coated on the glass fibres of diameter 1-10 mm using the in situ gelation process described above. The fibres were immersed in water for 5 minutes to remove the excess dodecylbenzene sulfonic acid and then dried at room temperature for 24 hours. A conductivity of 0.7 S/cm was measured using a four point probe method at room temperature.

A 2% solution of m-cresol is prepared in tetrahydrofuran and the above fibres were dipped into the solution for 2-10 seconds and taken out. The tetrahydrofuran was allowed to evaporate at room temperature leaving a deposit of m-cresol spread over the film. The film was not disturbed for 40 minutes.

The process was repeated with 5%, 10%, 15%, 20%, 25% and 30% of m-cresol solutions in tetrahydrofuran. Finally the fibres were allowed to remain at room temperature for 2 hours and then dried under high vacuum for 30 hours.. The thickness of the coating was 40 3 nm and the conductivity measured using a four point probe method with evaporated gold contacts on top of the film was 500 S/cm at room temperature.

The process can be conducted on any fibres or nanofibres of glass, silicon and polymer or on stable metal wires or nanowires such as gold and platinum.

Example 5 A thin film of PAn doped with dodecylbenzene sulfonic acid was coated on the pre- patterned glass slides using the in situ gelation process described above. The glass slides were immersed in water for 5 minutes to remove the excess dodecylbenzene sulfonic acid and then dried at room temperature for 24 hours. A conductivity of 4 S/cm was measured using a four point probe method at room temperature.

A 3 % solution of m-cresol is prepared in tetrahydrofuran and the above fibres were dipped into the solution for 2-10 seconds and taken out. The tetrahydrofuran was allowed to evaporate at room temperature leaving a deposit of m-cresol spread over the film. The film was not disturbed for 40 minutes.

The process was repeated with 5%, 10%, 15%, 20%, 25% and 30% of m-cresol solutions in tetrahydrofuran. Finally the fibres were allowed to remain at room temperature for 5 hours and then dried under high vacuum for 25 hours.. The pre- patterns were removed to obtain polyaniline lines of width land 2 mm and lengh I em to 2 cm. The thickness of the coating was 54 2 nm and the conductivity measured using a four point probe method with evaporated gold contacts on top of the film was 1200 S/cm at room temperature.

Example 6 The electrical properties of four random samples were examined. Figs. 10 to 13 shows the temperature dependence of conductivity using a closed-cycle helium refrigerator (minimum temperature of 10 K). The thickness of the films were obtained using a Zygo white-light surface profiler.

Sample 1 : PAn film was processed on a Si wafer consisting 25 nm Si02 top layer and gold contacts. The secondary doping was conducted using m-cresol. The sample was dried at room temperature. The transport measurements were carried out without

drying the sample under high vacuum. Contacts were given directly by silver paint.

The measurement was too noisy and was not reproducible.

Sample 2 to Sample 4: PAn film was processed on a Si wafer consisting 25 nm Si02 top layer. The secondary doping was conducted using m-cresol and the films were dried in high vacuum. Then the gold contacts were evaporated on the film through a mask.

Sample 1 880 S/cm (240 nm) is without drying at high vacuum (Fig. 10) Sample 2 1300 S/cm (50 nm) (Fig. 11) Sample 3 2500 S/cm (35 nm) (Fig. 12) Sample 4 730 S/cm (72 nm) (Fig. 13) Film Morphology The morphology of the secondary doped PAn thin films were investigated using Scanning electron microscopy. The film showed smooth morphology without any microscopic features. (Fig. 6 (a) and 6 (b)).

In situ UV-Visible spectra of secondary doping A UV-Visible spectra of a thin film of PAn-DBSA is taken initially (Fig. 2a). The film is them dipped in a dilute solution of m-cresol in THF for 30 seconds and was taken out. After 30-45 minutes in room temperature, the volatile THF is evaporated from the surface. The in situ absorption spectra of this PAn thin film consisting a surface layer of m-cresol was monitored at a time interval of 5 minutes. The spectra is shown in Fig. 2b.

The highly conductive polyaniline materials of the invention may be used in a wide range of applications such as biosensors, molecular devices, conducting photoresists, optical switches, smart windows, transistors, rechargable batteries and the like. They may be used to provide antistatic and/or anticorrosion coatings.

The invention is not limited to the embodiments hereinbefore described which may be varied in detail.