Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
  • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
  • G02B6/122—Basic optical elements, e.g. light-guiding paths
  • G02B6/1225—Basic optical elements, e.g. light-guiding paths comprising photonic band-gap structures or photonic lattices
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F2/00—Processes of polymerisation
  • C08F2/46—Polymerisation initiated by wave energy or particle radiation
  • C08F2/48—Polymerisation initiated by wave energy or particle radiation by ultraviolet or visible light
  • C08F2/50—Polymerisation initiated by wave energy or particle radiation by ultraviolet or visible light with sensitising agents
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
  • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
  • G02B6/122—Basic optical elements, e.g. light-guiding paths
  • G02B6/1221—Basic optical elements, e.g. light-guiding paths made from organic materials

Definitions

• the present invention relates to semiconductor or polymer structures and to methods of formation thereof.
• the invention relates to nanoscale semiconductor or polymer structures, more particularly but not exclusively organic and/or light-emitting semiconductor nanostructures.
• Embodiments of the invention relate to organic semiconductor liquid crystal structures.
• nanometre scale structures in transparent insulating thin films.
• Methods of fabricating such structures include lithographic patterning and etching, for example using electron beam lithographic techniques.
• Other techniques include nanoembossing where a nanostructure is fabricated by embossing a thin film using a stamp.
• Photonic crystals are structured materials with a periodic variation in refractive index. The length of the period is of the order of the optical wavelength. This can allow the creation of photonic band-gaps (PBGs). PBGs are spectral regions over which light propagation is forbidden.
• a problem with known techniques for fabricating photonic structures is that they are typically not compatible with low cost mass-production techniques.
• the availability of easily formed nano-photonic structures on light-emitting and semiconducting materials, which are compatible with roll-to-roll and other large scale manufacturing processing, may provide the disruptive factor required to deliver next generation photonic materials and devices to markets in displays, optical communications, quantum information processing, sensors, etc.
• 2D PCs require high contrast between regions of low and high refractive index and may be fabricated by etching holes in inorganic semiconductor materials such as GaAs and silicon. Very high performance devices are obtained but the nanostructures are fabricated using electron or ion beams, which is expensive and slow. Line defects and nanocavities are made in the PC by omitting a number of holes, shifting their position or changing their size. The Purcell effect has been observed with extremely high Q factors (>10 6 ) (S. Noda, Nature Photon., 1 449 (2007).
• LCs polymer dispersed liquid crystals
• 3D as well as 2D structures are obtained, these materials have a low refractive index contrast and are not light emitting.
• 1 D and 2D photonic structures are commonly used to provide distributed feedback for organic lasers.
• Substrate etching and mechanical nano-imprinting have been used to pattern the materials.
• Embossing and nanoimprinting techniques have frequently been used to pattern surface features on organic semiconductors. Patterns may be created by mechanical deformation of the organic thin film using a mold with topographic features. Very good resolution is obtained but the masters are expensive.
• Photolithography has been used to pattern photopolymerisable organic semiconductors but involves a wet etching step to develop structures.
• Organic semiconductors have also been patterned by laser ablation, e.g. to form channels in pentacene thin film transistors.
• a method of forming a photonic crystal structure by means of polymerisable material comprising the steps of: forming a layer of a polymerisable material, the polymerisable material being arranged to experience photoinduced mass transfer upon irradiation with optical radiation; and exposing the polymerisable material to said optical radiation at locations corresponding to a photonic crystal structure to be patterned thereby to form said structure by photoinduced mass transfer.
• Embodiments of the invention have the advantage that a patterned light emitting organic structure may be formed in a single step of exposure of the PLC material to light. Consequently, in some embodiments a patterned light emitting organic structure may be formed in a rapid, materials and energy efficient manner and at relatively low cost. [0011] Embodiments of the invention are useful in telecommunications, quantum information processing and display devices such as emitters including lasers and radiation detectors, OLEDs, backlights and any other suitable devices.
• the period of the periodic variation of refractive index of a PC may be in the range of from around 120 nm to around 3000 nm, optionally from around 350 nm to around 2000 nm, further optionally from around 400 nm to around 1800 nm.
• the polymerisable material comprises a polymerisable liquid crystal (PLC) material.
• PLC polymerisable liquid crystal
• the polymerisable material may comprise a polymerisable light emitting liquid crystal material.
• the polymerisable material may comprise a polymerisable light emitting nematic liquid crystal material.
• the polymerisable material may comprise a semiconducting material.
• the polymerisable material may comprise an amorphous material.
• the method does not require a further step of polymerisation of the polymerisable material thereby to form said photonic crystal structure.
• the step of exposing the polymerisable material to said optical radiation causes polymerisation of said polymerisable material thereby to form said photonic crystal structure.
• the step of exposing the polymerisable material to said optical radiation may be performed with the polymerisable material at room temperature.
• the step of exposing the polymerisable material to said optical radiation may be performed with the polymerisable material at a temperature in the range from around 15 0 C to around 100 0 C, optionally from around 2O 0 C to around 7O 0 C.
• the method may further comprise the step of annealing the structure following irradiation at an elevated temperature, the elevated temperature being optionally in the range from around 15 0 C to around 100 0 C, further optionally from around 2O 0 C to around 7O 0 C.
• the photonic crystal structure may comprise a light emitting polymer, the structure having a plurality of laterally spaced apart portions corresponding to said structure to be patterned wherein the light emitting polymer has a thickness different from portions of the structure between the laterally spaced apart portions.
• the laterally spaced apart portions may have a thickness greater than the portions of the structure between the laterally spaced apart portions.
• the portions of the structure between the laterally spaced apart portions may have a thickness of substantially zero.
• the laterally spaced apart portions may have a thickness lower than the portions of the structure between the laterally spaced apart portions.
• the laterally spaced apart portions may have a thickness of substantially zero.
• the laterally spaced apart portions are preferably substantially periodically spaced apart.
• the structure to be patterned may comprise a plurality of spaced apart ridge members.
• the structure to be patterned may comprise a plurality of spaced apart post members.
• the post members may be spaced apart in a plurality of non-parallel directions.
• the structure to be patterned may comprise a plurality of spaced apart wells.
• the wells may be spaced apart in a plurality of non-parallel directions.
• the wells may be arranged to provide a periodic structure having a period corresponding to a photonic bandgap whereby optical radiation of a prescribed one or more wavelengths is substantially prevented from propagating laterally through the structure along a direction in which one or more wells are provided.
• the polymerisable material may comprise a light-emitting material and the one or more wavelength may correspond to one or more wavelengths emitted by the light emitting material.
• An emission wavelength of the light-emitting material may overlap an edge of the photonic band-gap.
• the wells are periodically spaced apart.
• a prescribed one or more wells of the structure may not be formed or may be displaced whereby an emitter of optical radiation may be provided.
• the emitter may correspond to a nanocavity micro-emitter.
• the method may comprise the step of providing a phase mask and exposing the mask to optical radiation thereby to expose the layer of polymerisable material to optical radiation thereby to pattern the photonic crystal structure.
• the step of exposing the polymerisable material to said optical radiation at locations corresponding to a photonic crystal structure to be patterned comprises the step of providing an optical interference pattern using holographic or other methods.
• the photopolymerisable material may comprise a reactive mesogen having the formula B-S-A-S-B, wherein A is a chromophore, S is a spacer, and B is an end group which is susceptible to photopolymerisation.
• A is a chromophore of general formula -(Ar-FI) n -Ar wherein Ar is an aromatic diradical or a heteroaromatic diradical bonded linearly or substantially linearly to adjoining diradicals, or a single bond; Fl is a 9,9-dialkyl substituted fluorene diradical joined to adjoining diradicals at the 2 and 7 positions; the Ar and Fl diradicals may be chosen independently in each of the n subunits of the chromophore; and 1 ⁇ n ⁇ 10; S is a spacer; and B is an endgroup which is susceptible to polymerisation.
• the phonotic crystal structure may comprise a material having a periodic variation in refractive index.
• the variation in refractive index may have a period of from around 120 nm to around 3000 nm. [0047] The variation in refractive index may have a period of from around 350 nm to around 2000 nm, preferably from around 400 nm to around 1800 nm, more preferably around 400 nm to around 1600 nm, optionally around 400 nm to around 1000 nm, optionally around 1550 nm.
• Suitable photopolymerisable material such as a photopolymerisable insulating liquid crystal material or non liquid crystal material of sufficiently high refractive index.
• Non light emitting and non- semiconducting materials may be useful.
• passive photonic crystal materials may be useful.
• a photonic crystal structure comprising a photopolymerised material patterned by photoinduced mass transfer.
• a method of forming a light emitting and/or semiconducting polymer structure comprising the steps of: forming a layer of a polymerisable liquid crystal (PLC) material, the PLC material being arranged to experience photoinduced mass transfer upon irradiation with optical radiation; exposing the PLC material to said optical radiation at locations corresponding to a structure to be patterned thereby to cause said photoinduced mass transfer.
• PLC polymerisable liquid crystal
• Figure 1 shows a chemical structure of a compound used for photoembossing surface structures according to embodiments of the present invention
• Figure 2 shows an arrangement for exposing a layer of material to optical radiation thereby to pattern a nanostructure
• Figure 3 shows (a) a 3D image of a typical grating formed using a 1 micron phase mask and (b) a surface profile of the grating parallel to a direction of the grating wave- vector;
• Figure 4 shows a distributed feedback laser device structure
• Figure 5 shows a process of in-situ photopolymerisation of a liquid crystal
• Figure 6 shows a phase mask and the interference pattern produced on transmission through it according to an embodiment of the invention
• Figure 7 shows a relative orientation of a reactive mesogen and stripes of a stripe structure during a course of patterning of the structure
• Figure 8 shows a photonic bandgap structure containing a defect.
• LC semiconducting liquid crystal
• PC photonic crystal
• the transition temperatures of the material are:
• the central core or chromophore is light-emitting and semiconducting. It has diene end-groups, which are crosslinkable by irradiation with ultraviolet light. Different crosslinking groups can be alternately used.
• sample preparation and exposure were carried out in a glove box under nitrogen atmosphere with humidity of less than 1 ppm and an oxygen concentration less than 30 ppm.
• a cleaned 25 x 25 mm 2 glass substrate was used in some examples, optionally covered with a layer of poly(3,4- ethylenedioxythiophene) (PEDOT) around 40 nm thick.
• the PEDOT layer can be used to improve the film forming quality of the LC.
• PLC films of different thicknesses were applied by spin-coating the compound dissolved in a chlorobenzene solvent. A typical spin speed is 900 rpm for 30 s.
• the compound concentrations varied between 0.02-0.03 mg/ ⁇ l and higher concentrations can also be used for thicker films.
• the thickness of a film prepared with the standard settings 900 rpm, 0.03 mg PV237 in 1 ⁇ l chlorobenzene was 80 ⁇ 5 nm.
• spin-coating all films were heated at 5 ⁇ €/min to 65 0 C, where they were cured for 15 min and then cooled down to RT again at ⁇ ' €/min.
• the samples were patterned and crosslinked by irradiation with ultraviolet light from a HeCd laser at 325 nm through a +1/-1 phase mask of period 530 nm or 1 micron.
• FIG. 2 An arrangement for patterning and cross-linking according to an embodiment of the invention is shown in Figure 2.
• a beam of laser radiation 10 is incident upon a phase mask 20.
• a phase mask 20 formed from a material transparent to the laser radiation employed is provided.
• the mask 20 has a variation in thickness arranged to retard light passing through the mask at periodic locations thereby to form an interference pattern on a layer 55 of PLC material to be patterned.
• the PLC layer 55 was formed on a substrate 50 oriented in a plane parallel to that of the phase mask 20 and provided in close proximity thereto.
• the phase mask 20 acts as a precision diffraction grating giving an interference pattern of intensity modulated light.
• the period of the intensity pattern is equal to either the period of the phase mask or half the period.
• the phase mask 20 is very delicate and is sensitive to the vibrations or misalignment.
• the mask alignment with respect to the polarisation direction of the laser was judged by eye with a probable error of ⁇ 3°.
• Two polarisation configurations were used: the polarisation of the laser was either parallel or perpendicular to the wave vector of the phase mask 20.
• the mask 20 was placed directly on top of the PLC layer 55 and the mask 20 and substrate 50 pressed together as they were attached to a hot stage below with several screws, whose heads were pressing the mask down.
• the mask 20 was then exposed to laser radiation thereby to cause photoinduced mass transfer of PLC material of PLC layer 55.
• a laser radiation energy of 600 J/cm 2 was required in order to produce a patterned liquid crystal (LC) film that was insoluble to the solvent used for deposition during subsequent processing.
• LC liquid crystal
• the substrate 50 was moved with respect to the mask 20 by means of an xy- stage so than a larger area than that of the laser beam could be irradiated.
• the speed of travel of the xy-stage was kept between 0.48 and 0.52 mm/s.
• a doughnut shaped laser beam was generated, 4 mm in diameter.
• the laser power of the beam was measured before the crosslinking was started and varied between 52-53 mW.
• the mask's attenuation of the initial beam as well as the splitting into two beams were also considered in the fluence calculation.
• Crosslinking times were adapted accordingly so that all areas were crosslinked with at least 600 J/cm 2 or higher.
• the phase mask 20 was a 530 nm phase mask (by Ibsen Photonics) fabricated to match the wavelength of the HeCd laser. It was formed from fused silica and split the beam in 5 mW for the 0 th order and 22 mW for - 1 and +1 order respectively. For the fluence calculations 22 mW was used. In this particular example no higher orders were present.
• a 1 ⁇ m mask was also used.
• the working wavelength was unknown but was also in the ultraviolet region.
• 52 mW initial laser power about 32-38 mW were measured for the 0 th order and 5 mW for -1 and +1 order.
• the crosslinking time was calculated with 32 mW as laser power to ensure that the layer would be fully crosslinked.
• Figure 3 shows (a) a 3D image of a typical grating formed using a 1 micron phase mask and (b) a surface profile of the grating parallel to a direction of the grating wave- vector.
• the process of photo-embossing described above has the advantage that patterning of a film of a PLC material may be performed by mass transfer upon irradiation with patterned light, such as patterned ultra-violet (UV) light.
• patterned light such as patterned ultra-violet (UV) light.
• surface relief amplitudes of up to 140 nm were obtained even though the as-deposited films (with smooth surface) were only around 80 nm in thickness. Grating periods as small as 260 nm were achieved in some examples.
• the photo-embossing process may be performed in a single step with no post- annealing or wet etch required, in some embodiments.
• the method is highly suited to high-throughput manufacturing technologies.
• the patterned films are cross-linked during the course of exposure to laser irradiation at the patterning stage, providing robust, environmentally stable structures.
• Photoinduced mass transfer is understood to be due to monomer diffusion from troughs to peaks in intensity of incident optical radiation. It is distinct from laser ablation, where material is physically removed from a surface by laser radiation.
• the method may be used to produce an organic laser device by patterning a series of stripes of light emitting semiconductor polymer material. Light- emitting dopants could be incorporated to tailor the wavelength of emission. [0078] A spacing between the stripes (or 'grating period') may be selected in order to provide distributed feedback (DFB) along a length of the material, which may provide a laser cavity 170. Two-dimensional structures could also be used to provide distributed feedback in both in-plane directions.
• DFB distributed feedback
• the grating period, ⁇ may be selected according to the equation:
• n eff 2n eff A
• m is an integer (the 'order' of the DFB structure)
• ⁇ is the wavelength of light emitted by the light emitting polymer material
• n eff is the refractive index of the polymer material at wavelength ⁇ . It is to be understood that n ett is around 2 for the PLC materials discussed herein. Other values of n eff can be obtained by changing the chemical structure of the material or the lasing wavelength.
• ⁇ may be any suitable wavelength.
• ⁇ may be in the range from around 400 nm to around 3 microns ( ⁇ m).
• ⁇ is in the range from around 400 nm to around 2000 ⁇ m.
• ⁇ is in the range from around 400 nm to around 1600 ⁇ m.
• m 1 (first order structure)
• ⁇ is typically in excess of 120 nm.
• ⁇ is the outcoupling efficiency and n is the refractive index.
• This limit occurs because light emitted at a wavelength greater than the critical angle is waveguided in the thin film.
• a suitable DFB laser structure is shown in Figure 4 (not to scale). It can be seen that a structured layer 155 of an organic light emitting LC material has been formed on a glass substrate 150 by photo-induced mass transfer as described above. Optical pumping of the layer 155 with laser irradiation may be used to cause light emission by the layer 155.
• a thickness of the layer 155 varies between t1 at a peak in thickness and t2 at a trough where t1 is greater than t2.
• t2 is substantially zero.
• the layer 155 is substantially in the form of a series of pillars or stripes spaced apart along a length of the cavity 170.
• the laser device structure of Figure 4 may be optically pumped. Alternatively or in addition it may electrically pumped.
• the DFB structure can also be incorporated into an OLED to improve the outcoupling efficiency.
• Figure 5 shows a chemical structure of a compound 105 used for photoembossing surface structures according to embodiments of the present invention (a) before exposure to optical radiation and (b) after exposure to optical radiation. Following exposure to optical radiation the reactive mesogens (RMs) 105 crosslink to form a stable polymer structure.
• RMs reactive mesogens
• the RMs 105 each comprise a chromophore group C coupled to and sandwiched between two spacer groups S, each spacer group having a polymerisable group PG at or close to a free end thereof.
• Figure 6(a) is a schematic illustration of a phase mask 20 during a course of exposure to laser radiation. It is to be understood that the two first order beams of optical radiation emerge through the phase mask 20 with zero order ideally completely suppressed.
• Figure 6(b) shows a calculated intensity variation at a surface exposed to optical radiation transmitted through the phase mask of Figure 6(a) under ideal conditions where no zero order beams are transmitted. It can be seen that the optical radiation generates a stripe pattern directly on the exposed surface.
• Figure 6(c) shows a calculated intensity for a 'real' phase mask having some zero order beams. It can be seen that some 3D peaks in light intensity are caused to occur on the exposed surface, leading to a deviation from the ideal exposure conditions of FIG. 6(a) for some embodiments of the invention in which an intensity of optical radiation exposing the surface is substantially uniform along a length of each stripe.
• polarised laser radiation may be employed to expose the RMs 105. It is found that longitudinal axes of RM molecules tend to become aligned parallel to a direction of electric polarisation of optical radiation incident upon the PLC material. Thus, a direction of orientation of the RM molecules may be controlled by controlling the polarisation of incident laser radiation.
• the RM molecules also show anisotropic viscosity, their viscosity being lower along a direction parallel to the longitudinal axis.
• the electric field is oriented parallel to a wavevector of the stripe pattern as shown in Figure 7(b), i.e. normal to a longitudinal axis of the stripes and in the plane of the stripe pattern
• more rapid mass transport occurs along the direction parallel to the wavevector of the stripe pattern compared with the situation in which the electric field is oriented normal to the wavevector, parallel to the longitudinal axis of stripes of the stripe pattern as shown in Figure 7(a).
• stripe patterns are generally deeper when the electric field vector of the laser radiation is oriented parallel to the wavevector.
• Figure 8 shows a structure in which a layer 255 of PLC material has been exposed to optical radiation thereby to form a 2D array of 'holes' in the layer 255.
• the holes are spaced apart with a period corresponding to that required to form a photonic band gap at a prescribed wavelength. Similar periods are required for the first DFB structure discussed above. For a wavelength of 600 nm and an effective index of 1.8, a spacing of 166 nm is estimated. Finite element modelling is required to obtain the best depth, shape and relative size of hole. It is understood that good results are expected when the film thickness is equal to wavelength/2.
• Region 285 may be used to generate optical radiation with optical or electrical excitation. It is to be understood that light emitted by region 285 may be prevented from travelling laterally through the layer 255 by the array of holes 282. Thus, a relatively high intensity beam of light is emitted from the region 285 in a direction normal to a plane of the layer 255.
• Such a structure has the advantage that a source of radiation of relatively high finesse may be produced.
• the region 285 is produced by exposing the region 285 to light to cause crosslinking of the PLC material provided in that area. Subsequently, the film is exposed to light to pattern the holes 282 in region 255 in the film by photo- induced mass transport (or mass transfer).
• the region 285 where the holes are missing may also be referred to as a 'nanocavity'.
• inverse structures may also be formed in order to provide a photonic band gap (PBG) structure.
• region 285 may be surrounded by discrete spaced apart pillars instead of a material having holes 282 therein.
• the pillars may be of a size and spacing corresponding to that of the holes 282.
• PCs organic 2D photonic crystals
• conjugated light-emitting/ semiconducting polymers have never been photo-embossed.
• luminescent LCs As a result of their high conjugation, luminescent LCs according to embodiments of the present invention have a high refractive index ⁇ n average > 2) at the emission peak of a thiophene-fluorene nematic, (K.L Woon, Liq. Cryst. 32, 1 191 (2005)). This is significantly bigger than the minimum value of 1.6. predicted to give a TE-like full PBG (opt Express2007).
• the material is luminescent and emits TE modes only, since the LCs lie in the plane of the thin film. TM modes need not be considered, which lowers the constraints to achieve a full PBG.
• the photoluminescence quantum efficiency (PLQE) of a thin film of the material used in the structure of Figure 3(a) is 0.27.
• Other nematic LCs with PLQE >0.50 have been synthesised.
• relatively deep nanostructures with a relatively small period are useful.
• polymeric LC material emitting in the region 500-600 nm were used, thereby requiring a sub-200 nm grating period.
• a second approach is to use F ⁇ rster energy transfer to effectively excite the emission of dopants at longer wavelengths. Light-emitting semiconductor nanocrystals may be used as dopants.
• Defects or nanocavities may be introduced by perturbating the PC lattice locally, e.g. by omitting a hole (which may be an air hole) as discussed above.
• quantum information processing requires the quantum state of light to be stored for a sufficiently long time to enable quantum operations. Slowing and trapping light are ways to achieve this.
• One of the current limitations of integrated optical signal processing is an easily integrated light source. Lasers generally emit a large amount of undesired spontaneous emission before they start to lase, which degrades their efficiency. Therefore, so-called “thresholdless lasers,” that emit almost no spontaneous emission, are feasible in a nanocavity, where emission is suppressed in all modes apart from the cavity mode. Our materials present an extremely simple and unique way to fabricate defects.
• an area of diffraction-limited size of a thin film layer is cross-linked before irradiation through the phase mask. (The nonlinear response of photopolymerisation may allow an even smaller area to be fixed.)
• a further advantage of our materials is that they are easily aligned with very high order parameters (S>0.9). Photoalignment methods can be used where the alignment direction is patternable. The transition dipole moments of the LC emitters are oriented along the rod axis, so that they all emit in a single in-plane direction when they are uniformly aligned. This gives the added advantage that the cavity needs only to apply a 1 D rather than 2D PC structure to completely suppress emission in the plane.
• Photo-embossing i.e. photo-induced mass transfer
• Similar photoinduced gratings have previously been generated in transparent thin films. These materials are extensively researched for optical filters, holography, antireflection surfaces as well as resonant cavities. They are passive unlike nematic semiconductor structures described herein which show efficient light-emission and semiconducting properties. Unlike grating formation in passive materials, nanostructures described herein may be formed at room temperature with sub-micron periods and do not require heating to develop a latent image. A photoinduced birefringence is obtained during their formation suggesting some similarities with grating formation in azo materials.
• Photoembossed surface relief structures have been formed in non-semiconducting photopolymers primarily used for holography.
• the photopolymer layer is irradiated with spatially modulated UV light, creating radicals in the illuminated areas.
• the free radicals are captured in the glassy matrix but there is little polymerisation because monomer diffusion to the reactive sites is restricted. After exposure the sample is heated above a threshold temperature, where the sample changes from a solid to a more mobile, liquid-like state.
• the polymerisation of the monomer in the exposed areas changes the chemical potential and provides a driving force for the monomers to diffuse from the unexposed to the exposed areas.
• the mass transport creates a volume increase in these regions and, consequently, a surface relief structure.
• Surface relief gratings can only be made in photopolymers at low spatial frequencies ( ⁇ 10 6 rrV 1 ).
• aspect ratios feature height to width
• the photoembossing of surface relief structures has also been demonstrated by polymerisation of a nematic liquid crystals monomer blend, which is transparent and insulating.
• the material was locally irradiated in the nematic phase to produce a latent image which developed into a topological feature by annealing above the clearing point.
• Surface structures can also be reversibly formed in materials containing azobenzene moieties or other photoisomerisable groups.
• gratings are formed by mass transport even in the glassy state.
• the formation of gratings has also some unusual polarisation effects; large amplitudes are obtained by interference of two orthogonal circularly polarised beams even though there is no intensity modulation.
• Several mechanisms for photoinduced mass migration of azobenzene molecules have been suggested e.g. the gradient force of the optical electric field, isomerisation-driven free volume expansion, the mean field theory of anisotropic intermolecular interaction etc.
• a 2D phase is used to create a 2D array of nanopillars surrounded by air or the inverse structure described above with reference to Figure 8. Pillars of relatively large amplitude are preferred in some embodiments and therefore almost complete transfer of material from the un-irradiated regions to the exposed regions of the film is preferred.
• PLC materials suitable for use in embodiments of the invention include, for example, a light emitting or charge transporting polymer comprising a polymer formed from reactive mesogens having the formula:
• A is a chromophore of general formula -(Ar-FI) n -Ar- wherein Ar is an aromatic diradical or a heteroaromatic diradical bonded linearly or substantially linearly to adjoining diradicals, or a single bond; Fl is a 9,9-dialkyl substituted fluorene diradical joined to adjoining diradicals at the 2 and 7 positions; the Ar and Fl diradicals may be chosen independently in each of the n subunits of the chromophore; and 1 ⁇ n ⁇ 10; S is a spacer; and B is an endgroup which is susceptible to polymerisation.
• the endgroup B may be susceptible to photopolymerisation.
• the polymer may be formed by photopolymerisation.
• the endgroup (B) comprises a diene and the diene may be selected from the group consisting of 1 ,4 dienes, 1 ,5 dienes and 1 ,6 dienes.
• the diene functionalities are separated by an aliphatic linkage.
• the diene functionalities may be separated by an inert linkage, which is optionally selected from the group consisting of ether and amine linkages.
• the light emitting or charge transporting polymer is a light emitting electroluminescent polymer.
• the light emitting or charge transporting polymer is a hole transporting polymer.
• the light emitting or charge transporting polymer is an electron transporting polymer.
• the light emitting or charge transporting polymer is substantially photoinititor free.
• the invention also envisages a process for forming a light emitting or charge transporting polymer comprising photopolymerisation of a reactive mesogen having the formula:
• A is a chromophore of general formula-(Ar-FI) n -Ar- wherein Ar is an aromatic diradical or a heteroaromatic diradical bonded linearly or substantially linearly to adjoining diradicals, or a single bond; Fl is a 9,9-dialkyl substituted fluorene diradical joined to adjoining diradicals at the 2 and 7 positions; the Ar and Fl diradicals may be chosen independently in each of the n subunits of the chromophore; and 1 ⁇ n ⁇ 10; S is a spacer; and B is an endgroup which is susceptible to photopolymerisation.
• the endgroup (B) optionally comprises a diene, and the diene may be selected from the group consisting of 1 ,4 dienes, 1 ,5 dienes and 1 ,6 dienes.
• the diene functionalities are separated by an aliphatic linkage.
• the diene functionalities may be separated by an inert linkage, which is optionally selected from the group consisting of ether and amine linkages.
• the photopolymerisation of the reactive mesogen optionally results in a light emitting electroluminescent polymer, a charge transporting polymer or an electron transporting polymer.
• the invention also envisages a device comprising a polymer layer comprising a light emitting or charge transporting polymer comprising a polymer formed from reactive mesogens, as hereinbefore defined.
• said device is one of an electronic device, a light emitting device, a display device, an organic light emitting device, a lighting element, a backlight and a laser.
• the invention also provides a material for forming a light emitting or charge transporting polymer comprising a reactive mesogen having the formula:
• A is a chromophore of general formula-(Ar-FI) n -Ar- wherein Ar is an aromatic diradical or a heteroaromatic diradical bonded linearly or substantially linearly to adjoining diradicals, or a single bond; Fl is a 9,9-dialkyl substituted fluorene diradical joined to adjoining diradicals at the 2 and 7 positions; the Ar and Fl diradicals may be chosen independently in each of the n subunits of the chromophore; and 1 ⁇ n ⁇ 10; S is a spacer; and B is an endgroup which is susceptible to polymerisation.
• the endgroup B may be susceptible to photopolymerisation.
• said material may be formed by photopolymerisation.
• the endgroup (B) comprises a diene and the diene may be selected from the group consisting of 1 ,4 dienes, 1 ,5 dienes and 1 ,6 dienes.
• the diene functionalities are separated by an aliphatic linkage.
• the diene functionalities may be separated by an inert linkage, which is optionally selected from the group consisting of ether and amine linkages.
• the light emitting or charge transporting polymer is a light emitting electroluminescent polymer, a hole transporting polymer or an electron transporting polymer.
• the light emitting or charge transporting polymer is substantially photoinititor free.
• the invention also envisages a backlight or display which includes a polymer formed from reactive mesogens having the formula:
• A is a chromophore of general formula-(Ar-FI) n -Ar- wherein Ar is an aromatic diradical or a heteroaromatic diradical bonded linearly or substantially linearly to adjoining diradicals, or a single bond; Fl is a 9,9-dialkyl substituted fluorene diradical joined to adjoining diradicals at the 2 and 7 positions; the Ar and Fl diradicals may be chosen independently in each of the n subunits of the chromophore; and 1 ⁇ n ⁇ 10; S is a spacer; and B is an endgroup which is susceptible to polymerisation.
• said polymer has 2 ⁇ n ⁇ 7.

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• 2009
  • 2009-05-13 GB GB0908206A patent/GB0908206D0/en not_active Ceased
• 2010
  • 2010-05-13 WO PCT/GB2010/050787 patent/WO2010131046A1/en active Application Filing

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