WO1996036082A1

WO1996036082A1 - Electronic devices based on discotic liquid crystals

Info

Publication number: WO1996036082A1
Application number: PCT/GB1996/000744
Authority: WO
Inventors: Neville Boden; Bijan Movaghar; Jonathan Clements
Original assignee: The University Of Leeds
Priority date: 1995-05-13
Filing date: 1996-03-28
Publication date: 1996-11-14
Also published as: GB9509729D0; AU5279396A; JPH11505068A; EP0827635A1

Abstract

This invention relates to electronic devices including discotic liquid crystals, either doped or undoped, and more specifically to discotic liquid crystals positioned between a pair of opposing first and second electrodes. The various arrangements described may be used in a variety of electronic devices such as temperature sensors, field effect transistors, light sensing devices, non-linear optical devices, chemical and radiation sensing devices and fluorescent devices.

Description

ELECTRONIC DEVICES BASED ON DISCOTIC LIQUID CRYSTALS

Statement of Invention

The invention relates to electronic devices, and aspects thereof, which exploit the unique properties offered by discotic liquid crystals (DLC).

In particular, the electronic devices are those which benefit from functioning at the molecular level, that is to say the transfer and/or detection of charge at the molecular level. Such devices typically include, but are not limited to, devices where enhanced sensitivity is of advantage.

Examples of electronic devices in accordance with the invention include, but are not limited to the following devices.

a) a molecular-scale current rectifier (diode) which has potential applications as a temperature sensor. b) a field effect transistor (FET) device which has potential application as: a radiation detector, a chemical/molecular detector, in the imaging of radiation on a molecular scale, as a temperature sensor, and in electronic photography etc. Providing this device with an array of micro-fabricated electrodes will lead to applications as a charge transfer device, a molecular registering device, a device for storage of information, and for electronic imaging. c) a doped field effect transistor (FET) device with potential application as a sensitive chemical/molecular detector, or light/radiation detector etc. Provision of this device with micro-fabricated electrodes will lead to potential applications in charge transfer devices, molecular registering devices, information storage devices and photo imaging. d) a high mobility doped discotic liquid crystal field effect transistor (FET) device, possessing high charge carrier mobility characteristics, for application in light sensing devices and non-linear optical devices. e) a high resistance device sensitive to chemicals and light radiation. f) a fluorescent switch or optical modulator device structure, for application in television screen displays and devices which detect the arrival of charge.

General Background

The search for organic materials suitable for electronic devices dates back to the 1950's. Organic materials possess a variety of advantages over the conventional use of silicon based semiconductor technologies. Among these advantages there is included the ability to control accurately the thickness of semiconductor deposition down to a thickness of only several molecular layers. In addition, the infinite variety of organic molecules available allow chemists to tailor properties of the organic material to the particular application they require, for example to exhibit phosphorescence, fluorescence, magnetic and electrical properties.

Early investigations into the application of organic materials for thin film electronic devices involved Langmuir-Blodgett (LB) thin film technology and also self-assembly (SA) monolayer systems. The growth of interest in these technologies has accompanied the increasing interest in finding new and improved electronic devices. It is likely that ordered organic materials will become increasingly important in the development of organic/inorganic hybrid semiconductor technology.

The LB technique provides for a unique approach to achieve supra-molecular architectures of layered assemblies of suitably designed organic molecules including macro-molecules. Originally LB technology was designed to build layered assemblies of simple long chain aliphatic carboxylic acids and their salts. Later it was developed into a sophisticated method to produce supramolecular architectures useful to test molecularly controlled processes of heat transfer, charge carrier motion, energy conservation and molecular recognition. Although the method and unique opportunities it offers for construction of molecular assemblies has found wide spread acceptance among physical chemists interested in the basic mechanisms of inter- molecular interactions of organic molecules, it has found little practical application.

This is in part due to the inherent instability of the layered assemblies particularly when composed of low molecular weight amphiphilic molecules. Reorganisation processes, due to temperature instability, lead to a gradual destruction of the layered structure even when kept at room temperature for a few days.

The use of polymeric organic molecules in LB film applications has been developed to overcome the problems of molecular reorganisation. Initially, the polymerisation of functional monomers in a form of layered assemblies was tried. For example, the topo-chemical polymerisation of diacetylenes offer the clean and complete conversion of multi layers composed of monomers to produce polymers without any resulting change in the texture and morphology.

An alternative to the polymerisation of preformed layers of monomers is, the spreading and transfer of amphiphilic polymers along the lines of the LB method. Here, homo- or co-polymers of amphiphilic monomers are spread to the air-water interface, compressed and transferred. In most cases the transfer is only possible if a solid analogue phase of the side chains is formed.

Wegner¹ developed further the technology of applying LB techniques to polymeric compounds in his work on the Phthalocyaninatopolysiloxanes, which he refers to as "hairy-rods". Wegner¹ has applied this hairy rod technology to prototype electronic devices, in particular pH measuring devices.

It is of note that although polymeric LB films are intrinsically more stable, the use of polymeric organic materials in LB film applications has not been widely exploited for the production of electronic devices. In part, this is because such film applications do not have the desirable properties of self healing, nor do they have the desirable wetting properties required for providing a good contact between the film and a conducting material. However, liquid-crystalline states, located between the crystalline and isotropic liquid states, are interesting in view of their relevance to advanced materials. In particular, they offer several potential advantages over devices made from standard solid and polymeric LB type films, such as:

a) The wetting properties of liquid crystals ensure efficient molecular contact with electrodes, thus avoiding the necessity to use evaporative techniques to apply a film onto an electrode surface. This wetting property greatly simplifies construction of electronic devices.

b) The high degree of internal order combined with the semi-fluidity of liquid crystals give them unique "self-healing" properties if a disturbance should occur in their molecular order. Recently, Closs ² has compared the photoconductivity of discotic and smectic liquid crystals. In his studies, he showed that the photo-current produced was higher in the highly ordered mesophase than in either the polyciystalline or isotropic state. In addition his studies demonstrated that the discotic liquid crystal phase exhibited an electronic charge transport rate comparable to the transport rate in a single crystal of anthracene.

The highly ordered columnar arrangement in discotic liquid crystals has attracted much interest and has led to a number of studies to examine their electronic characteristics.

Shimizu³, has studied the photocurrent rectification behaviour of mesogenic 5,10,15,20-tetrakis (4-n-pentadecylphenyl)ρo hyrin and has demonstrated that sandwiching a mesogen layer between two Indium-Tin Oxide electrodes resulted in a marked photocurrent rectification in the D_h phase.

Bengs⁴ investigated the photoconductivity in an homologous series of hexa- alkoxytriphenylenes. Under irradiation, all samples showed photoconductivity in the mesophase whereas in the isotropic phase, the photocurrents dropped to zero. All of the observed effects, ie the phase dependence of the photocurrent, the increasing values of photo- current with decreasing length of side chains, and the higher photoconductivity during cooling process, he explained in terms of the transition temperatures, the inter-columnar distances, and the orientation behaviour.

Adams⁵ has studied the photoconduction in the mesophase of hexahexylthiotriphenylene. In his work, Adams discovered that the charge carrier mobility in the helical columnar (H) phase was far greater than in the D_h phase, and of similar magnitude to the charge carrier mobilities in an organic single crystal.

UK patent document number 2 223 493 describes a discotic liquid crystalline material which is doped with a radical salt. This document goes on to describe that the discotic liquid crystalline phase possesses anisotropic electronic properties when either undoped or doped with less than 0.5 moles per mole of discotic liquid crystal.

Problems with the prior art

Devices based on inorganic semiconducting materials such as silicon and germanium were amongst the original materials used in semiconducting electronic devices. Among the advantages of manufacturing devices from inorganic materials such as silicon include their robustness and reliability. In addition inorganic semiconductors can be "cycled" many times without failure.

However, devices based on inorganic semiconductors have a number of drawbacks. Due to their high melting point they can only be fabricated in the solid state and as such do not readily allow atomic tailorability. Ordered thin films however can be produced using the technique of "Molecular Beam Epitaxy" (MBE).

Furthermore, inorganic materials such as silicon and germanium can only be chemically functionalised with difficulty due to their inert nature. This drawback inhibits development of materials possessing novel physical and chemical properties which may find utility in electronic devices which exhibit characteristics such as phosphorescence, fluorescence, magnetic properties etc. A further drawback of inorganic materials used in electronic devices is the requirement to use conducting adhesive material to join the electrode or gate with the inorganic semiconductor itself. The presence of an "adhesive layer" limits the extent of 3-dimensional miniaturisation of electronic devices due to the problems of constructing such devices.

A complication with electronic devices based on LB thin film technology arises from the grainy texture of the layered assemblies. This grainy texture arises during the course of making LB films, in which the amphiphilic molecules are spread on the air-water interface of a Langmuir trough. During this process, the LB film can rapidly form islands of two dimensional crystals which are compressed into a two dimensional continuous solid by action of a floating barrier.

A further drawback with LB thin film technology is that it relies on evaporative methodologies to apply the organic film onto the electrode surface. Evaporative methodologies tend to generate defects.

Object

It is therefore an object of the invention to provide devices in which a semiconducting layer can be applied to an electrode surface in a controlled manner so that stable thin films can be produced.

It is a further object of the invention to provide devices which include a stable semiconducting layer, preferably an organic semiconducting layer, which can be readily functionalised chemically to exhibit a variety of physical or chemical characteristics, for example, altering the radiation absorption characteristics, fluorescence, phosphorescence etc. It will be apparent to a man skilled in the art that the said variety of physical or chemical characteristics will be determined by the amount and type of functionalisation and thus the above list is not intended to be exhaustive but rather it is intended to be exemplary.

It is yet a further object of the invention to provide devices which contain a material, preferably a liquid crystaline material, which has wetting properties which provide an effective and simple method for making contact with an electrode and avoid the use of adhesive bonding layers and evaporative methodologies.

It is further object of the invention to provide devices which include a semiconducting organic material which possesses strong "self-organising forces" which are less prone to destruction and can self-heal.

It is also an object of the invention to provide devices which include a semiconducting organic material which produces good bonding characteristics to the electrode or gate surface.

It is an object of the invention to provide devices which include a semiconducting organic material which has internal order, for example a discotic liquid crystal, which can act as a conducting molecular wire.

It is an object of the invention to provide devices which include a semiconducting organic material, for example, a discotic liquid crystal, in which, in certain embodiments, the columnar channels form an effective insulator relatively preventing passage of current in a direction perpendicular to the direction of the channels. It is an object of the invention to provide an efficient rectifying device which is temperature sensitive at high temperatures.

It is an object of the invention to provide an FET device which may find application as a radiation detector, a chemical/molecular detector, in the imaging of radiation on a molecular scale, in temperature sensing and electronic photography.

It is yet a further object of the invention to provide a doped field effect transistor (FET) device with potential application as a sensitive chemical/molecular detector, or light/radiation detector, or indeed any other form of detector which is consistent with this construction.

It is a yet further object of the invention to provide a doped device which includes a high charge carrier mobility semiconductor. It is thought that this particular device will have application in light sensing devices and non-linear optical devices.

It is a yet further object of the invention to provide a high resistance device which is sensitive to chemicals and light radiation.

It is still a further object of the invention to provide a fluorescent switch or optical modulator device structure, which has potential application in television display screens, and devices which detect the arrival of charge, ie scintillation counters.

Consistory Clauses

According to a first aspect of the invention there is provided a device which comprises:

opposing outer first and second electrodes, which electrodes have therebetween a film of discotic liquid crystal material and further wherein said first electrode comprises a low work function material and said second electrode comprises a high work function material.

In use, the device is connected to an electrical circuit, ideally externally, and a considerably larger current flows in the circuit when a relatively positive voltage is applied to said high work function electrode than if a relatively positive voltage is applied to said low work function electrode. The magnitude of this effect is very sensitive to temperature. Particularly, but not exclusively at high temperatures.

It will be apparent from the above to a person skilled in the art that this device may find application in temperature sensing devices.

In a preferred embodiment of the invention said discotic liquid crystal is a 2,3,6,7,10,11 hexa-alkoxytriphenylene material and preferably 2,3,6,7,10,11 hexa-hexyloxytriphenylene (HAT6) material.

In a preferred embodiment said low work function material is aluminium, calcium, barium or indeed any other material classified, or known as, low work function material.

In a further preferred embodiment said high work function material is Indium - Tin Oxide, silicon or gold or indeed any other material classified, or known as, high work function materials. According to a second aspect of the invention there is provided a field effect transistor device (FET) which comprises opposing outer first and second electrodes and having therebetween a discotic liquid crystal (DLC) material in contact with said first electrode; and a thin layer of conducting and/or semiconducting material in contact with second electrode and said DLC material.

In a preferred embodiment the semiconductor will be silicon with a hydrogen passivated surface.

In the presence of a gate potential we achieve a unique gate-field dependent 2-dimensional electronic band-structure in the thin layer (semiconductor or metal).

In use a gate voltage can be applied across said opposing outer first and second electrodes and a source-drain current can be detected across said thin layer, perpendicular to said gate voltage.

It will be apparent from the above to a person skilled in the art, that this device may find application in a range of electronic devices, for example, as a radiation detection device. In this application the absorption of radiation in said DLC material will lead to variations in the periodic charge modulation in the surface of said thin layer, leading to large variations in the source-drain current.

A further application may be in the field of chemical or molecular detection. In this application the molecules in said DLC material will sensitise the complex conductivity of the DLC and produce a significant variation in the source-drain current. This is again because such molecules in said DLC material will perturb the periodic charge modulation in the surface of said thin layer, leading to large variations in the source-drain current.

In addition, other applications of the device include imaging of radiation on a molecular scale, temperature sensing (particularly high temperature sensing), and electronic photography.

It will be understood that the above examples are given for the purpose of comprehension only and are not intended to limit the application of the device.

In a preferred embodiment said first and second electrodes are metal.

In a preferred embodiment said thin layer is metal or doped semiconductor.

In a further preferred embodiment said first electrode comprises a plurality of microfabricated electrodes.

It will be apparent from the last preferred embodiment to a person skilled in the art that this device will find a number of applications. For example as an electronic imaging device or a multistage memory device.

The two dimensional organisation of the molecular columns in said DLC material gives rise to a #W-ts.-two-dimensional periodic charge modulation near the surface of said thin layer. Thus, this unique switchable band structure can also be viewed as a molecular scale charge transfer device, since the electrodes are of molecular dimensions. Essentially, electrical contact to the surface of said thin layer is made by an array of molecular wires (the molecular stacks in said DLC material). Electrons or holes generated by doping said thin layer or by applying "light" are modulated by the periodic potential generated by said molecular wires in said DLC material. This periodic and regular modulation can be interrupted by external fields such as light sources in the range [0.1 to leV] and radiation (as described above) or molecules adsorbed in the electrode (as described above) to produce a change in said current. Said voltage can also be pulsed and/or in conjunction with a radiation beam used to inject charge into said molecular wires and in turn into said thin layer and keep this charge, so to speak, as a memory for a given length of time, then the charge can be removed and detected by a change in said source-drain current.

Microfabricating said electrodes and or said source-drain current will allow us to achieve local addressability on a micron and submicron scale. This means we can store and return charge into said molecular wires with practically no sideways diffusion in different segments of the wire. The anisotropy of the discotic system is lO O⁴ for electronic transport.

In a preferred embodiment of the invention said DLC material is 2,3,6,7,10,11 hexa-alkoxytriphenylene and preferably HAT6 material.

According to a third aspect of the invention there is provided a device which comprises: two opposing outer first and second electrodes, having therebetween a p- doped DLC material in contact with the said first electrode; and an n-doped semiconducting material in contact with said second electrode and said p- doped DLC material.

In use a voltage is applied across said first and second electrodes, and a source-drain current is detechable across said n-doped semiconducting material. Application of said voltage across said first and second electrodes separates the dopant counter ions and holes within said DLC material, and produces an "image" of the charge in said n-doped semiconductor material. Said gates of this device are not only of molecular dimensions but also have the unique property of charge separation within the molecular wires of the columnar structure of said DLC material.

It will be apparent from the above to a man skilled in the art that this device may find a number of applications. For example, the device may find application as a radiation detector. In this application, the absorption of radiation inducing changes in the source-drain current. Other applications include a molecular or chemical sensing device, absorption of molecules into the DLC materials altering the charge profile at said DLC material/said semiconducting material interface and affecting the source-drain current.

In a preferred embodiment of the invention the device can be used to emit light at the junction between said p-doped DLC materials and said n-doped semiconducting materials. Each emitter is a molecular wire.

In a preferred embodiment said first electrode comprises a plurality of micro- fabricated electrodes. Provision of a plurality of micro-fabricated electrodes enables local addressability on a micron and sub-micron scale.

It will be apparent from the last embodiment that this device will find application in a number of applications. For example, as a molecular scale information storage device, in transistor applications, and electronic imaging devices.

In a preferred embodiment of the invention two distinct source-drain currents can be applied to the device across said n-doped semiconducting material perpendicular to said voltage, for example, one source-drain current can be detected at the interface of said p-doped DLC material and said n-doped semiconducting material, and another source-drain current can be detected at the interface of said n-doped semiconducting material and said second electrode.

In a preferred embodiment of the invention said DLC material is a 2,3,6,7,10,11 hexa-alkoxytriphenylene material and preferably HAT6 material.

In yet a further embodiment of the invention a plurality of, ideally, alternating semiconducting materials may be provided between said electrodes and in addition, a plurality of source-drain currents may be provided, ideally, each source-drain current is provided at or adjacent to the interface of said alternating materials.

According to a fourth aspect of the invention there is provided a device which comprises:

two opposing outer first and second electrodes having therebetween a p-doped DLC thin film material in contact with said first electrode; and a high mobility semiconducting material in contact with second electrode and said p-doped DLC material.

In use a voltage is applied across said opposing first and second electrodes and a source-drain current is applied across said high mobility semiconducting material along an axis perpendicular to said voltage.

This device exploits the novel feature that on application of said voltage, positive charges are injected into said high mobility semiconducting material, and these charges carry current in the source-drain field, far away from the counter-ions which remain in said DLC material. This device is quite unlike devices based upon ordinary doped high mobility semiconducting materials, where in use, the counter-ions remain in said high mobility semiconducting layers and act as scatterers reducing the mobility of the flow of the source- drain current.

It will be apparent from the above to a man skilled in the art that the device offers the following features: a) An ultra-fast response in said source-drain current to changes in said voltage. This is because the device reduces the number of scattering centres in the bulk of said high mobility semiconducting material. b) A very thin DLC semiconducting material less than 100 A which acts as a quantum system for the injection of holes. The holes would then occupy sub-bands of the quantised system. c) More charge can be induced because of the higher capacitance. d) Greater light sensitivity because of space charge.

It will also be apparent to a man skilled in the art from the above that the device will find application in a number of devices, for example as a non linear optical device.

In a preferred embodiment of the invention there is provided a six terminal device which accommodates two source-drain currents across said high mobility semiconducting material.

The first source-drain current is detected at the interface between said p- doped DLC thin film material and said high mobility semiconducting material. The second source-drain current is detected at the interface between said high mobility semiconducting material and said second outermost electrode.

In a preferred embodiment there is provided a relatively insulating material between said p-doped DLC thin film material and said high mobility semiconducting material.

In a preferred embodiment there is provided a relatively insulating material between said high mobility semiconducting material and said second outermost electrode.

In a preferred embodiment said p-doped DLC thin film material is a 2,3,6,7,10,11 hexa-alkoxytriphenylene material and preferably HAT6 material.

According to a fifth aspect of the invention, there is provided a device which comprises: a thin film of DLC material possessing two metal electrodes at the surface of said material and a third electrode within the bulk of said DLC material.

In use a voltage applied between one of said surface electrodes and said bulk electrodes would create a current which reacts sensitively to either absorbed molecules on the surface or light penetrating the DLC layer.

It would be apparent to a person skilled in the art that this device would find application as a sensitive chemical detector and/or a sensitive radiation detector, or in the field of electronic photography.

In a preferred embodiment said surface and said bulk electrodes are metal. In a preferred embodiment said surface electrodes are 5μm long, lμm deep, and <l m apart.

In a preferred embodiment the arrangement of said electrodes is repeated periodically.

It is a property of organic compounds that they can be modified to exhibit optical properties, for example, fluorescence. It will therefore be appreciated that the synthesis of DLC materials which possess fluorescent properties may lead to their application in electronic devices, such as a fluorescent switch or an Optical Modulator Device Structure.

According to a sixth aspect of the invention there is therefore provided a device which comprises: two opposing outermost first and second electrodes, which first electrode is substantially transparent to electromagnetic radiation, and containing therebetween an undoped DLC thin film material in contact with said first electrode and a p-doped semiconducting material in contact with said second electrode and said undoped DLC thin film material; which DLC thin film material can exhibit fluorescent characteristics.

In use incident light passes through said transparent first electrode and creates excitons in said DLC thin film material which results in fluorescence.

A voltage pulse is applied across said first and second electrodes with said transparent first electrode held at a relatively negative potential. Said voltage pulse causes positively charged holes to flow from said p-doped semiconducting material into said undoped fluorescing DLC thin film material. The application of said voltage pulse lowers the fluorescing intensity because excitons generated in this region by the external light source transfer their energy to the charged molecules in said undoped DLC thin film via the Fδrster mechanism. Said charged molecules possess charge as they carry with them said positively charged holes.

In a preferred embodiment of the invention said p-doped semiconducting material is a p-doped DLC material.

In this embodiment the diffusion of counter-ions, for example, aluminium trichloride , from said p-doped DLC material into said fluorescent DLC material can be reversed by the application of a voltage.

In a further preferred embodiment of the invention, said p-doped semiconducting material is an inorganic p-doped semiconductor which has been conventionally optimised to provide easy injection of holes into said fluorescent DLC material.

It will be apparent from the above to the skilled person that DLC based fluorescent active devices (F-active) possess several advantages over conventional fluorescent active (F-active) devices, for example:

DLC based F-active devices contain one-dimensional self-healing properties with relatively large charge carrier mobilities (μ~10^"3cm²/Vs).

In addition, DLC based F-active devices exhibit efficient diffusion and trapping (τ_t) of excitons along the columns (D ~10³cm²/s and τ_t<10^"8s) because of the low dielectric constant ε -2-4 and the efficient Fδrster transfer rate - lO¹²^¹. Furthermore, DLC based F-active devices exhibit charge separation in said p-doped layer which provides an efficient hole injection mechanism.

It will be apparent to a skilled person that the device according to the sixth aspect of the invention will find application as a fluorescent switch and/or Optical Modulator Device Structure in, for example, television screen displays and devices which detect the arrival of charge, eg scintillation counters.

In all of the above devices the term material preferably comprises a layer or film of material.

Diagrams

The invention will now be described by way of example only with reference to the following figures, wherein:

Figure 1 shows the structure of a typical discotic liquid crystal and the structure of a 2,3,6,7,10,11 hexa-alkoxytriphenylene (HAT6)

Figure 2 shows the device according to the first aspect of the invention.

Figure 3 shows Current v Voltage graph for the said first aspect of the invention.

Figure 4 shows the effect of temperature on the current at various voltages for said first aspect of the invention.

Figure 5 shows the device according to the second aspect of the invention. Figure 6 shows the device according to the third aspect of the invention.

Figure 7 shows the device according the fourth aspect of the invention.

Figure 8 shows the device according to the fifth aspect of the invention.

Figure 9 shows the device according to the sixth aspect of the invention.

Figure 10 shows the effect of temperature on the conductivity at various voltages for HAT6.

Figure 11 shows how charge on discotic columns can be stored on a molecular scale using an ultra thin insulating (oxide) layer attached to a metallic gate.

Figure 12 shows schematically a typical potential curve seen by electrons in the 2-dimensional channel under the periodic array of discotic columns. The application of gate voltage can change the depth of the potential and then switch the electronic band-structure.

Figure 1 shows the disordered stacked arrangement of the disc-shaped molecules within a discotic liquid crystal. The structure can be described as a two-dimensional lattice of disordered stacks.

This structure imparts novel properties which can be applied to devices: a) the electronic band structure of organic solids combined with liquid¬ like properties. b) the ability to prepare thin (as thin as a few molecular layers), large area arrays of ordered molecules on surfaces. c) highly anisotropic charge carrier mobilities. d) self-healing of defects in the liquid crystalline state.

Figure 2 shows the device according to the first aspect of the invention. It comprises a high work function electrode 2 and a low work function electrode 3, and sandwiched therebetween is a thin film 1 of DLC material. Figure 2 also shows an external circuit for applying a potential difference across the electrodes.

Figure 3 shows a current against voltage plot obtained using the device shown in Figure 2. The graph in Figure 3 clearly shows rectification in that a current only flows when a positive voltage is applied to the high work function electrode. No current flows in the external circuit when a negative voltage is applied to the high work function electrode.

Figure 4 shows a plot of the variation in current with temperature at voltages of 20, 50 and 100 volts when a positive voltage is applied to the high work function electrode of the device shown in Figure 2. The plot at 100 volts shows a dramatic increase in current at 340K as HAT6 enters the liquid crystalline (D_h) phase, and also the dramatic fall in current as the phase changes from liquid crystalline (D_h) to liquid (I). It is at these phase transitions that the device will demonstrate the highest sensitivity to temperature, however throughout the D_h phase the device shows high sensitivity to temperature as shown by the steeply curved portion of the graph represented by triangles

Referring to Figure 5 there is shown a device according to the second aspect of the invention, which shows a Field Effect Transistor device which comprises electrodes 5 and 6, a discotic liquid film material 4 and a layer 7 of metal or doped semi-conductor. Figure 5 also shows a voltage across electrodes 5 and 6, and a source-drain current across layer 7. Figure 5 also indicates how an image of charge is created in the layer 7.

Referring to Figure 6 there is shown a device according to the third aspect of the invention, which shows a field effect transistor device which comprises electrodes 10 and 11, a p-doped discotic liquid layer 8 and an n-doped semiconducting layer 9. Figure 6 also shows a gate voltage across electrodes 10 and 11, and also a source-drain current across layer 9.

In the top right hand corner of Figure 6 is also shown an embodiment of the invention in which electrode 10 is comprised of a plurality of microfabricated gates 12.

Referring to Figure 7 there is shown a device according to the fourth aspect of the invention which shows a high mobility FET comprising electrode 15 and 16, a layer of p-doped DLC material 14, and a layer of high mobility semiconducting material 13.

According to an embodiment of the device, Figure 7 also shows relatively insulating layers 17 and 18.

Figure 7 also shows a voltage across electrodes 15 and 16.

According to a further embodiment of the device, Figure 7 also shows the application of two source-drain currents across the high mobility semiconducting material 13.

Figure 8 shows a device according to the fifth aspect of the invention. Figure 8 shows a DLC layer 19, with electrodes 20 and 21 implanted at the surface and electrode 22 implanted in the bulk.

Figure 9 shows a device according to the sixth aspect of the invention. Figure 9 shows an fluorescent undoped DLC layer 23 and a p-doped layer 24 which could comprise either DLC material or inorganic semiconducting material. Figure 9 also shows said first transparent electrode 25 and second electrode 26. The region δ represents the 'skin-depth' which is the distance that the electromagnetic radiation (λ_r) enters said fluorescent undoped DLC material.

Figure 10 shows the effect of temperature on the conductivity of HAT6 at various voltages. These results show that this material is a very good insulator at these temperatures.

Referring to Figures 11 and 12, discotic liquid crystals form high quality insulating films and can replace, in part, the traditional insulating barrier in the gate of a field effect transistor [FET]. The self assembled array of molecular wires address the two-dimensional electron gas formed in the inversion layer. The molecular columns contact the semiconductor surface through a surface passivation layer, or if need be passivation "layers". In the case of a Silicon transistor, the passivation of the surface states would be achieved by a bonded monolayer of Hydrogen. The coupling between the electrons in the channel and the molecular columns gives rise to a net potential which is in the range of 0.05 to 0.2 eN depending on molecule and gate potential applied. This potential is considerably larger than the thermal energy at room temperature. The energy to trap an electron underneath a column in the active channel is of the order 0.2 to 0.5 eN. By changing the gate voltage applied to the discotic film, it is possible to increase the depth of the potential well in the channel by up to 0.1 eV in HAT-n and possibly more using materials with lower band gaps. In this way are in a unique position to engineer a class of FET devices in which the electronic band- structure can be switched from on state to another with the potentiality of arriving to complete localisation of the "band". The band structure determines, in particular, the speed of electronic motion (mobility), the saturation velocities (maximum speed), and the magnitude of the band gaps.

Applications: 1) Switching channel conductance FET

2) Memory device with potential for molecular scale charge storage

3) A sensing device for gases and molecules with potential for molecular scale imaging of molecules.

The sensing device uses the fact that adsorbed molecules significantly change the complex conductance characteristics of the discotic film and in this way modify the potential which allows the channel to form and the electrons in the inversion layer to flow from source to drain. The molecules are therefore detected as changes in the source drain current. Selectivity can be achieved by using specially designed molecular side-chains which respond specifically to particular gases and adsorbed molecules. The specific response results from the strength of the inducing conduction change along and or across the molecular columns. References

(1) G. Wegner Ber. Bunsenges. Phys. Chem., 1991, Vol 95, 1326.

(2) F. Closs et al, Liquid Crystals, 1993, Vol 14, 629.

(3) Y. Shimizu et al, J Chem. Soc, Chem. Commun, 1993, 656. (4) H. Bengs et al, Liquid Crystals, 1993, Vol 15, 565.

(5) D. Adams et al, Nature, 1994, Vol 371, 141.

Claims

1. A device comprising opposing outer first and second electrodes, which electrodes have therebetween a film of discotic liquid crystal (DLC) material and further wherein said first electrode comprises a low work function material and said second electrode comprises a high work function material.

2. A device according to claim 1 which is connected to an electrical circuit.

3. A device according to claims 1 or 2 wherein said discotic liquid crystal is 2, 3, 6, 7, 10, 11, hexa-alkoxytriphenylene.

4. A device according to any preceding claim wherein said discotic liquid crystal is 2, 3, 6, 7, 10, 11 hexa-hexyloxytriphenylene.

5. A device according to any preceding claim wherein said low work function material is aluminum or calcium or barium.

6. A device according to any preceding claim wherein high work function material is indium-tin oxide or silicon or gold.

7. A device comprising a opposing outer first and second electrodes and having therebetween a discotic liquid crystal (DLC) material in contact with said first electrode and a conducting and/or semiconducting material in contact with said second electrode.

8. A device according to claim 7 wherein said conducting and/or semiconducting material is also in contact with said DLC material.

9. A device according to claim 7 or 8 wherein said conducting and/or semiconducting material is surface passivated.

10. A device according to claim 9 wherein said material is silicon with a hydrogen passivated surface.

11. A device according to claims 7 to 10 wherein a gate voltage is applied across said electrodes and a source-drain current is detected across said conducting and/or semiconducting material.

12. A device according to claims 7 to 11 wherein said first electrode comprises a plurality of microfabricated electrodes.

13. A device according to claims 7 to 12 wherein said conducting and/or said semiconducting material comprises a doped semiconductor.

14. A device according to claims 7 to 13 wherein said DLC material is p- doped and said conducting and/or semiconducting material is an n-doped semiconducting material.

15. A device according to claim 14 wherein p-doped DLC material is in contact with said n-doped semiconducting material.

16. A device according to claims 14 or 15 which is further adapted to emit light at the junction between said p-doped DLC material and said n-doped semiconducting material.

17. A device according to claims 14 to 16 adapted such that two source- drain currents can be detected, one at the interface of said p-doped DLC material and said n-doped semiconducting material, and another at the interface of said n-doped semiconducting material and said second electrode.

18. A device according to claims 7 to 17 wherein said conducting and/or semiconducting material is a high mobility semiconducting material.

19. A device according to claim 18 wherein said high mobility semiconducting material is in contact with said second electrode and said DLC material.

20. A device according to claim 18 or 19 wherein said DLC material is p- doped DLC material.

21. A device according to claims 18 to 20 wherein the device is adapted to have a voltage applied across said electrodes and a source-drain current detected across said high mobility semiconductor material.

22. A device according to claims 18 to 21 wherein two source-drain currents are detected across said high mobility semiconducting material.

23. A device according to claim 22 wherein a first source-drain current is detected at the interface between said p-doped DLC material and said high mobility semiconducting material and the second source-drain current is detected at the interface between said high mobility semiconducting material and said second electrode.

24. A device according to claims 18 to 23 wherein an insulating material is provided between said p-doped DLC material and said high mobility semiconducting material.

25. A device according to claims 18 to 24 wherein insulating material is provided between said high mobility semiconducting material and said second electrode.

26. A device according to claims 7 to 25 wherein said first electrode is transparent to electromagnetic radiation.

27. A device according to claim 26, when dependent upon claim 7, wherein said conducting and/or semiconducting material is a p-doped semiconducting material.

28. A device according to claim 27 wherein said p-doped semiconducting material is in contact with said second electrode and said DLC material.

29. A device according to claims 26 and 28 wherein said DLC material in the form a thin film of material.

30. A device according to claims 26 to 29 wherein said DLC material exhibits florescent characteristics.

31. A device according to claims 26 to 30 which is further adapted such that a voltage pulse can be applied across said electrodes.

32. A device according to claims 27 to 31 wherein said p-doped semiconductor material is a p-doped DLC material.

33. A device according to claims 27 to 31 wherein said p-doped semiconducting material is an inorganic p-doped semiconductor.

34. A device according to claim 33 when dependent on claim 30 wherein said inorganic p-doped semiconductor has been optimised to provide easy injection of holes in said fluorescent DLC material.

35. A device comprising a thin film of DLC material possessing two metal electrodes at the surface of said material and a third electrode within the bulk of said DLC material.

36. A device according to claim 35 which is adapted to have a voltage applied between one of said surface electrodes and said bulk electrode.

37. A device according to claims 35 or 36 wherein said electrodes are metallic.

38. A device according to claims 35 to 37 wherein a plurality of surface electrodes are provided and arranged in predetermined manner.

39. A device according to claims 7 to 38 wherein a plurality of semiconducting materials may be provided between said electrodes.

40. A device according to claim 39 wherein said semiconducting materials are arranged in alternating fashion.

41. A device according to claims 39 or 40 wherein a plurality of source- drain currents may be detected.

42. A device according to claim 41 wherein each source-drain current is detected at, or adjacent, the interface of said alternating material.

43. A device according to claims 7 to 42 wherein said DLC material is 2, 3, 6, 7, 10, 11 hexa-alkoxytriphenylene.

44. A device according to claims 7 to 42 wherein said DLC material is 2, 3, 6, 7, 10, 11 hexa-hexyloxytriphenylene.