WO2006034561A1

WO2006034561A1 - High-efficient small-aperture light converter

Info

Publication number: WO2006034561A1
Application number: PCT/BY2004/000023
Authority: WO
Inventors: Sergey V. Gaponenko; Ulrike Woggon; Mikhail V. Artemyev; Leonid I. Gurinovich; Nikolay V. Gaponenko; Igor S. Molchan; Andrey A. Lutich
Original assignee: The State Scientific Institution 'institute Of Molecular And Atomic Physics Of The National Academy Of Science Of Belarus'
Priority date: 2004-09-27
Filing date: 2004-09-27
Publication date: 2006-04-06
Also published as: EA010503B1; EA200700629A1

Abstract

Provided is a design of a new type of devices constructed on the basis of known basic mechanisms of photoluminescence excitation in quantum-sized semiconductor nanocrystals, and factors that enhance the emission efficiency owing to modified density of photonic states and energy transfer processes in the doped nanocrystal systems. This idea has been actually embodied in photonic structures of the type « microporous xerogel/doped nanocrystals/mesoporous anodic alumina», which provides the films obtained with new modified properties related to luminescence and absorption spectra.

Description

High-Efficient Small-Aperture Light Converter

FIELD OF THE INVENTION

The present invention relates to optical devices, and more particularly to optical devices comprising semiconductor quantum-size structures, which exhibit spatial electronic confinement, and dielectric photonic crystals, which exhibit spatial photonic confinement in the optical range of the electromagnetic spectrum. The present invention can be used in optoelectronic devices employed as light detectors, emitters, and converters.

DESCRIPTION OF THE PRIOR ART The development of contemporary electronic industry is focused on high-technology microelectronics, which puts tougher requirements on materials and engineering. The need for reducing materials and energy consumption, as well as the need for breakthrough in performance encourages manufacturers to consider nanotechnology. Research in this field has resulted in the appearance of new areas thereof, both applied (such as nanooptics, nanoelectronics, and nanoengineering) and basic (such as physics of quantum-size effects, single atom and single molecule photonics, quantum chemistry, etc.). The most fruitful among them is the search for new technologies in quantum-size semiconductor structure optics. For such structures the usual size decrease in one or more dimensions results in completely new phenomena, so-called quantum-size effects. Some of the studied phenomena have already been applied in novel devices, such as single-mode semiconductor injection laser with multiple quantum well heterostructures and nonlinear optical filters based on matrices with embedded quantum dots.

In the area of microelectronic engineering there is a gap between consumer demands and technical capabilities of large-scale production. Bridging this gap is a paramount task both in design and in technology. As regards optical imaging devices with long-terhi operation (displays, information panels, indicators in alarm systems, etc.), the main consumer qualities are ergonomics and maintenance costs versus manufacturing costs. Lowering said relative maintenance costs can in turn be achieved by increasing the general conversion efficiency as well as by decreasing production costs. This can be realized both by employing novel materials and by using innovative design.

Typically, light conversion devices are based on employing new phosphors and/or new means of excitation thereof. For the most part, in such attempts certain benefits of a device in one aspect are accompanied by considerable drawbacks in another aspect. For instance, the problem of low emission rate in light conversion surfaces can be addressed by using organic dyes. However, organic luminescent materials introduce a number of unavoidable problems such as photochemical instability, invariability of spectral properties, low optical density, sensitivity to outdoor influence, etc. Due to above mentioned drawbacks organic-based phosphors do not possess sufficient compatibility with silicon-based photodetectors. the latter being most widely used type of photodevices. There is also an alternative approach to creating advanced devices. For a vast majority of existing expensive or unique devices it is far more efficient to upgrade the device in order to improve some of its properties. The same is true for devices in large-scale production, because it is expensive to redesign the whole fabrication process and replace all the devices already in use. This approach is illustrated in the known problem of increasing ultraviolet-range i sensitivity of commercial silicon-based optical detectors. The additional process of sensitizing coating deposition is highly cost-effective and does not disrupt a well-coordinated manufacturing process. However, the problem of choosing a proper sensitizing material brings us back to the aforementioned problem of new phosphors. The instability problems associated with organic luminescent materials in light-converting coatings [[1] G.Blasse, B.C.Grabmaier. Luminescent materials. Berlin: Springer-Verlag, 1994] can be eliminated. Prior methods have been published on the organic luminescent coating such as Aluminium-Tris-Quinolate [[2] Patent WO 9727503 Organic Luminescent Coating For Light Detectors] which was applied to a light detector for converting UV to green light to improve the efficiency of the light detector, or on a complex light detector consisted of a plate-like light-absorbing body and at least one optical waveguide connected thereto [[3] Patent US 5132530 Light Detector Based On Fluorescent Dyes], wherein both parts contain a fluorescent dye, whereby the irradiated light is converted into a fluorescence radiation that is guided by total reflection to a light-sensitive semiconductor element. These methods have the common disadvantage of being based on a degradation of the organic molecules luminescence, a converting of radiation only to green light and a disturbed interference from surfaces of the coverings thus limiting the device performance rather than on a quantum size nanocrystal network such as the one we describe here. Another prior method using of creating a multiplied layer structures consist of a UV sensitive sheet of plastics material incorporating a fluorescent dye Perspex Green 6609 with visible non-transmissive filter fabricated during of photo device producing [[4] Patent GB 2200987 Ultraviolet Radiation Detector]. However, in this method, the filter is opaque in the visible region between 405 nm and 660 nm. Further, the green fluorescent dye is not a suitable material which can be used for commercial Si photodiodes having maximum sensitivity in the red, it is also much less stable than semiconductor inorganic materials.

Therefore, there is a need for an efficient, stable, and cost-effective light conversion device based on new physical phenomena and materials.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a method for enhancement of efficiency of optical converters by means of artificially concentrating the output emission therefrom in the required direction.

It is also an object of the present invention to create an efficient broadband short-to-long- wavelength spectral optical converter possessing a narrow output angular diagram.

It is a further object of the present invention to enhance efficiency of optical converters without increasing the cost and/or deteriorating the quality thereof by means of using efficient, highly stable and inexpensive inorganic materials.

It is yet another object of the present invention to provide highly efficient semiconductor optical devices, more particularly light-emitting devices and optical photodctectors. comprising the said optical converter. In accordance with one aspect of the present invention, there is disclosed an optical spectral converter. Said converter comprises a film of transparent unidirectionally arrayed material (a two-dimensional photonic structure), wherein the cavities are filled with wavelength- converting substance. The wavelength-converting substance has the form of xerogel containing nanosize nanostrυctures (nanocrystals or nanoclusters), which exhibit strong quantum-size effects.

The xerogel can be chosen from the family Of Al₂O₃, In₂O₃, TiO₂, SiO₂ gels.

Nanocrystals can be chosen from the series of H-VI, I-VII, IH-V semiconductor compounds, preferably CdS, CdSe, ZnS, ZnSe, or a combination thereof forming a core-shell structure, e.g., CdSe/ZnS. The nanocrystals are doped by metal ions, which can be chosen from among Mn²⁺, Eu³⁺, Tb³⁺, Sm³⁺, e.g., ZnSe:Mn²⁺/ZnS.

The film of transparent unidirectionally arrayed material can be one of the following photonic crystal types: porous membrane; mesotube monolayer; artificial opal. The said photonic crystal is fabricated from the substance chosen from among the oxides SiO₂, Al₂O₃, TiO₂.

In accordance with another aspect of the present invention, there is disclosed an optical-range photosensitive device with a thin-film wavelength converter. Said converter comprises a film of transparent unidirectionally arrayed material (a two-dimensional photonic structure), wherein the cavities are filled with wavelength-converting substance. The wavelength- converting substance has the form of xerogel containing nanosize nanostructures (nanocrystals or nanoclusters), which exhibit strong quantum-size effects.

Nanocrystals can be chosen from the series of H-VI, I-VII, III-V semiconductor compounds, preferably CdS, CdSe, ZnS, ZnSe, or a combination thereof forming a core-shell structure, e.g., CdSe/ZnS. The nanocrystals are doped by metal ions, which can be chosen from among Mn²⁺, Eu³⁺, Tb³⁺, Sm³⁺, e.g., ZnSe:Mn²⁺/ZnS.

The film of transparent unidirectionally arrayed material can be one of the following photonic crystal types: porous membrane; mesotube monolayer; artificial opal. The said photonic crystal is fabricated from the substance chosen from among the oxides SiO₂, Al₂O₃,, TiO₂. Said optical photosensitive device comprises a said optical converter and a conventional optical detector, preferably a photodiode or phototransistor, in which case the whole device is fabricated on a silicon substrate. Alternatively, the said optical detector may be a photoresistor, a solar cell, or a charge-coupled device (CCD) detector.

In accordance with yet another aspect of the present invention, there is disclosed a method of increasing the sensitivity of a photodetector in short wavelength range (blue to ultraviolet spectral range). The method consists in depositing an additional thin-film optical coating (conversion coating) onto a photosensitive area of the photodetector. Said coating comprises a film of transparent unidirectionally arrayed material (a two-dimensional photonic structure), wherein the cavities are filled with wavelength-converting substance. The wavelength- converting substance has the form of xerogel chosen from among Al₂O₃, In₂O₃, TiO₂, SiO₂ gels and containing nanosize nanocrystals chosen from the series of H-VI, I-VII, IH-V semiconductor compounds, preferably CdS, CdSe, ZnS, ZnSe, or a combination thereof forming a core-shell structure (e.g., CdSe/ZnS), and doped by metal ions chosen from among Mn^2", Eu³⁺, Tb³⁺, Sm³⁺ (e.g., ZnSe:Mn²⁺/ZnS). The film of transparent unidirectionally arrayed material can be one of the following photonic crystal types: porous membrane; mesotube monolayer; artificial opal. Said optical photodetector can be a commercial semiconductor device of any type, such as a photodiode, a phototransistor, a photoresistor, a CCD detector, or a solar cell. The commercial photodetector can be silicon-based.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of exemplary embodiments of the invention as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. FIG. 1 is a schematic illustration of various types of nanostructures: microporous silicon (Λ), mesotubes (b), synthetic opals (c), and the same said structures with pores filled with a gel (d. e,f) containing semiconductor quantum-sized nanocrystals.

FIG. 2 is a plot of the angular scattering diagram relative to the direction of the pores for a thin film of porous alumina for a light ray incident at different angles φ. FIG. 3 is a plot of the absorption and photoluminescence (PL) spectra of semiconductor quantum-sized ZnSe:Mn/ZnS nanocrystals in a thin polymer film, measured at room temperature.

FIG. 4 is a plot of the luminescence indicatrix of a thin film of mesoporous alumina containing in its pores quantum-sized Mn-doped CdS nanocrystals (α), together with the luminescence indicatrix of the same nanocrystals inside a polymer film on a smooth silicon substrate (b).

FIG. 5 is a view of the spectral converter for conversion of radiation from blue and UV spectral range into longer-wavelength spectral range, according to the present invention.

FIG. 6 is a view of a highly efficient indicator panel with narrow output angular diagram, according to the present invention.

FIG. 7 is a view of a photodetector with increased photosensitivity in the near-UV spectral range.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

During the past decade persistent interest has been displayed in the field of synthesis of films doped with optically active centers. Particular attention has been drawn to the sol-gel method of luminescent film fabrication. A specific feature of the sol-gel method is the possibility of luminescent film formation made of xerogels in various mesoporous matrices [[5]

N.V.Gaponenko. Synthesis and Optical Properties of Films Formed by the Sol-Gel Method in

Mesoporous Matrices. Journal of Applied Spectroscopy, V. 69, No. 1 (2002) 1 — 20]. Among the most suitable mesoporous materials one can pick out films of porous anodic alumina, which consist of hexagonally packed self-organized cells with vertically arranged mesopores in the middle of the cells [[6] G.E.Thompson and G.C.Wood. Nature, V. 290 (1981 ) 230—

232]. The unique features of such "microporous xerogel/mesoporous anodic alumina" film structure, such as its comparatively high transmittance, its possibility to be formed on a variety of substrates, wide-range modulation of the refractive index of the xerogel formed in the pores, possibility to vaiy the film thickness and porosity, etc. [[7] Patent US 5580825

1996 Process for making multilevel interconnections of electronic components. Vladimir A. Labunov, Vitaly A. Sokol, Vladimir M. Parkun, Alia I. Vorob'yova (BY). Int. Technology Exchange Corp.; [δ] Patent US 4859288 1989 Porous anodic aluminum oxide films. Robin C. Furneaux, William R. Rigby (GB). Alcan International Limited], require insight in the various factors possibly enhancing the photoluminescence (PL) of the active centers. During fabrication of such structure the thickness of a photoluminescent layer, which is defined by a previously grown anodic alumina film, can be equal several tens of micrometers, i.e., it can fall in the intermediate scale range between thin films and bulk materials. The luminescence of nanocrystals in the films formed in mesoporous matrices of anodic alumina and porous silicon, is of significant interest due to high photochemical stability and better quantum yield compared to traditional films, which are rare earth-doped and formed with the sol-gel method on monocrystalline or porous silicon [[9] A.M.Dorofeev, N.V.Gaponenko, V.P.Bondarenko, E.E.Bachilo, N.M.Kazuchits, A.A.Leshok, G.N.Troyanova, N.N.Vorozov, V.E.Borisenko, H.Gnaser, W.Bock, P.Becker and H.Oechsner. J. Appl. Phys., 77 (1995) 2679—2683; [10] W.Henley, Y.Koshka, J.Lagowski, J.Siejka. J. Appl. Phys., 87 (2000) 7848—7852; [1 1 ] N.V.Gaponenko, J.A.Davidson, B.Hamilton, P.Skeldon, G.E.Thompson, X.Zhou. Appl. Phys. Lett., 76 (2000) 1006—1008] or in ion-implanted oxide films [[10], [12] J.C.Pivin, N.V.Gaponenko, I.S.Molchan, R.Kudrawiec, J.Misiewicz, L.Bryja, G.E.Thompson, P.Skeldon. J. Alloys and Compounds, 341 (2002) 272—274].

It is generally accepted that samples of porous anodic alumina can exhibit properties of a tvvo- dimensional photonic crystal [[13] H.Masuda, M.Ohya, H.Ason, M.Nakao, M.Montomi, and

T.Tamamura. Jpn. J. Appl. Phys., 38 (1999) L1403— L1405; [14] R.Kudrawiec,

A.Podhorodecki, N.Mirowska, J.Misiewicz, I.S.Molchan, N.V.Gaponenko, A.A.Lutich, and

S.V.Gaponenko. Photoluminescence Investigation of Europium-Doped Alumina, Titania and

Indium Sol-Gel-Derived Films in Porous Anodic Alumina. Mat. Sci. Eng. B, 105 (2003) 53 — 56; [15] N.V.Gaponenko, I.S.Molchan, S.V.Gaponenko, A.V.Mudryi, A.A.Lyutich,

J.Misiewicz, and R.Kudrawiec. Luminescence of the Eu³⁺ and Tb^J+ Ions in the Structure

Microporous Xerogel/Mesoporous Anodic Aluminum Oxide. Journal of Applied

Spectroscopy, V. 70, No. 1 (2003) 59 — 64]. In this case porous anodic alumina gives rise to a synthesis technique of film structures providing control over spontaneous emission of phosphors located inside the pores as was observed with some samples of synthetic opals saturated with luminescent dyes [[16] E.P.Petrov, V.N.Bogomolov, I.I.Kalosha, and

S.V.Gaponenko. Phys. Rev. Lett., 81 (1998) 77—80].

Multiple scattering and interference of the waves scattered on numerous pore walls results in the increase of the effective "mean free path" of the photons /^*, which by many times exceeds the geometrical sample thickness / in the direction of the incident external radiation. Since the constituent materials in the "microporous xerogel/mesoporous anodic alumina" film structure have relatively high refractive indices, it can be said that the mentioned effect can be manifested. In this case the intensity of absorbed radiation W equals

W = W₀[I - (I - R) exp(-k!')), where WQ is the intensity of incident external radiation, R the reflectance at air/matrix boundary, k the absorptance, /^* the effective "mean free path" of the photons. and owing to the condition /^*>/; W can exceed the intensity of light absorbed in a bulk sample with the same values of A: and / by one order of magnitude or more Another effect of multiple light scattering in porous anodic alumina, which represents an assembly of monodisperse cylindrical particles, is the attenuation of incident short-wave radiation as it propagates through the porous structure, which was observed experimentally [15] and confirmed in theoretical calculations [[17] V.G.Vereshchagin, R.A.Dynich, and A.N.Ponyavina. Application of the Rayleigh-Gans Approximation in the Model of the Amplitude-Pase Screen for a Monolayer of Cylindrical Particles. Journal of Applied Spectroscopy, V. 63, No. 6 (1996) 897—900; [18] V.G.Vereshchagin, R.A.Dynich, and A.N.Ponyavina. Effective Optical Parameters of Porous Dielectric Structures. Optics and Spectroscopy, V. 84, No. 3 (1998) 427 — 431]. Scattering-induced attenuation of short-wave radiation during its propagation along the pore channels of anodic alumina can be also used for the design of scattering band-cut filters formed, e.g., on a quartz substrate [15].

One more reason of PL intensity increase of nanoparticles is the modification of spontaneous emission due to spatial and spectral redistribution of the electromagnetic mode density. As it is known, spontaneous emission of light in not just an inherent property of a substance but rather stimulated transitions resulting from zero-point modes of electromagnetic field. Therefore the probability of spontaneous transition is proportional to the density of electromagnetic modes (density of photonic states) in the medium surrounding the luminescent center (i.e. an excited ion, molecule, or nanocrystal). In inhomogeneous media the density of modes is redistributed across the spectrum and angles [[19] S.M.Barnett and R.Loudon. Phys. Rev. Lett., 77 (1996) 2444 — 2446], which results in a corresponding change of the spectral and angular distribution of spontaneous emission, as well as a change in the lifetime of the excited state. Therefore in samples of porous anodic alumina, which represent a two-dimensional photonic crystal, the density of states is reduced along the plane normal to the axial direction of the pores (the xy plane in FIG. 1) and is enhanced along the said axial direction of the pores (the ∑ axis in FIG. 1). Since experiments usually register radiation within a small solid angle, angular redistribution of radiation can cause considerable increase of intensity as registered by a photodetector or through visual means. In fact, the "microporous xerogel/mesoporous anodic alumina" film structures ([15]) fabricated according to the present invention, having a thickness of 5-30 micrometers, doped by terbium and europium, indeed exhibit noticeable green and red PL in the temperature range of 10-300K when excited by UV-radiation from argon laser, deuterium or xenon lamp [[20] N.V.Gaponenko, I.S.Molchan, O.V.Sergeev, G.E.Thompson, A.Pakas, P.Skeldon, R.Kudrawiec, L.Bryja, J.Misiewicz. J.C.Pivin, B.Hamilton, and E.A.Stepanova. J. Electrochcm. Soc, V. 149, No. 2 (2002) H49— H52; [21] I.S.Molchan, N.V.Gaponenko, R.Kudrawiec, J.Misiewicz, L.Bryja, G. E. Thompson, P.Skeldon. J. Alloys Comp., 341 (2002) 272—274].

Experimental measurement of the luminescence indicatrix for semiconductor nanocrystals in porous matrices carried out by the present authors also indicates that luminescence of centers embedded into a two-dimensional photonic crystal is indeed anisotropic and is characterized by nearly double increase of luminescence intensity along the pore axial direction as compared to a reference thin-film sample.

The choice of semiconductor nanocrystals exhibiting quantum-size effects as an optically active material for luminescence center formation is determined by the presence of a number of unique optical properties related to spatial electron confinement. The spatial confinement modifies the energy spectrum for the electrons and the probabilities of transition from one state to another. This leads to the optical manifestation of quantum-size effects. The absorption and luminescence spectra, as well as the lifetime of the excited state is determined by the spatial configuration and size of quantum-sized structures rather than by the chemical composition thereof. The characteristic size of such structures in the direction of confinement is of the order of one to several tens of nanometers, so the structures are called nanostructures.

One of the simplest yet most illustrative quantum-size effects is the dependence of absorption and luminescence spectra of nanoparticles on the size thereof. The spectral shift to shorter wavelengths directly results from Heisenberg's uncertainty relation

Ap Ax > h I 4π, where Ax is the uncertainty of the particle coordinate, Ap the uncertainty of the particle momentum, h the Planck's constant, from where it follows that the less uncertainty there is in particle position Δx, the more uncertainty there is in its momentum Ap. As regards a particle in a potential well of the size Ax = a the uncertainty relation means that the minimum energy of the particle in a well equals

4π - Smo where £_min is the minimum energy of the particle, m its mass, a is the crystallite size, h the Planck's constant.

Therefore the smaller the crystallite size is, the greater the minimum kinetic energy of the quasiparticles (electrons, holes and excitons) that are born in the crystallite when a photon is absorbed, i.e., the greater is the shift of the absorption spectrum to shorter wavelengths.

Taking this into account, it appears possible to design a new type or class of devices constructed on the basis of already known mechanisms of photoluminescence excitation in quantum-sized semiconductor nanocrystals, and factors that enhance the emission efficiency owing to modified density of photonic states and energy transfer processes in the doped nanocrystal systems. This idea has been actually embodied in photonic structures of the type "microporous xerogel/doped nanocrystals/mesoporous anodic alumina", which provides the films obtained with new modified properties related to luminescence and absorption spectra.

Presently the synthesis technology of nanostructures having enhanced properties in the optical range of an electromagnetic spectrum are represented by two major approaches. These are synthesis of quantum-sized semiconductor nanocrystals and synthesis of photonic crystals. The properties of both taken separately are sufficiently investigated and they alone provide advantageous effects.

Among the properties of photonic crystals that are of interest to device engineering one can name the ability to modify the photonic mode density in the area of space occupied by a photonic crystal. This means that by means of changing the photonic crystal topology it is possible to purposely modify the direction and mode structure of the electromagnetic radiation interacting therewith. Three types of such photonic structures have been realized practically, as depicted in FIG. 1 : microporous membrane (a), mesotubes (b), and synthetic opal (c). A thin film structured according to one of said types of photonic crystals is thus capable of redistributing the flow of light with any cross-section in a predetermined direction (as seen in FIG. 2). Porous structures can have their pores filled with various compositions, for instance, with products of sol-gel synthesis (sols, gels, or xerogels) in order to adjust the mode structure by means of changing the medium/pore refractive index contrast.

Among a wide range of compounds used for nanocrystal synthesis, it is worth to note H-VI semiconductors, which serve as a base for fabrication of quantum-sized nanoparticles with controlled optical characteristics. Owing to strong spatial confinement of charge carriers in such nanocrystals, their absorption and luminescent spectra depend on the particle size rather than on its material, which allows to vary their optical properties. The absorptance being high in the short-wavelength range (α>10⁴), semiconductor nanocrystals are efficient phosphors in the visible wavelength range. The greatest quantum yield (60% or more) has been obtained for core-shell nanocrystals, said core doped with atomic ions forming luminescence centers and surrounded by a shell of similar semiconductor, which impedes non-radiative processes related to the exterior matrix [[22] D.V.Talapin, A.L.Rogach, A.Kornowski, M.Haase, and H.Weller. Highly Luminescent Monodisperse CdSe and CdSe/ZnS Nanocrystals Synthesized in a Hexadecylamine-Trioctylphosphine Oxide-Trioctylphospine Mixture. Nanoletters, V. 1 , No. 4 (2001) 207-21 1 ; [23] A.A.Bol and A.Meijerink. Long-lived Mn²⁺ emission in nanocrystaline ZnS:Mn²⁺. Physical Review B (Condensed Matter), V. 58, No. 24 (1998) R15997-R16000; [24] D.M.Hofman, A.Hofstaetter, U.Leib, B.K.Meyer, and G.Counio. EPR and ENDOR investigations on CdS:Mn nanocrystals. Journal of Crystal Growth. 184/185 (1998) 398-387]. For example, using the processes of energy transfer from charge carriers excited in the core to the Mn²⁺ ions, which form a recombination channel with the emission of long-wavelength photons, it is possible to separate the absorption and emission bands in ZnSe:Mn²⁺/ZnS nanocrystals as shown in FIG. 3. Thus there is a possibility of conversion of short-wave radiation into long-wave radiation with minimal losses. Semiconductor nanocrystals can technically be placed inside various media (solutions, gels, polymer films, or glasses) without impairment of their physical and chemical qualities as well as optical properties.

The idea of the efficiency increase of light-emitting or light-converting devices according to the present invention is to concentrate the flow of light of the spontaneous emission from a source in one predetermined direction corresponding to a main photonic mode by placing the luminescent centers inside the cavities (pores) of a photonic crystal, the luminescence being excited at wavelength from other (more particularly, shorter-wave) spectral range. For instance, vertically aligned mesoscopic pores in the anodic alumina film are filled with xerogel according to the sol-gel technology, said xerogel containing quantum-sized ZnSe:Mn ⁺/ZnS nanocrystals doped with manganese ions, by means of multiple centrifugation of the corresponding sols and subsequent thermal treatment. A structure possessing a highly directional output angular diagram and providing at least double increase in intensity at the direction normal to the sample surface (see FIG. 4) is thus formed. Based on the aforementioned technology, a line of optical devices can be designed, including displays, concentrators, converters, photodetectors, etc., which possess enhanced optical properties in a wide spectral range.

At the first stage of the fabrication process according to the present invention a porous film of material according to the construction design (silicon, alumina, glass, etc.) is manufactured. with the required porosity period according to the chosen working wavelength range. The technology is chosen according to the relevant technical and cost requirements (chemical etching, anodic treatment, epitaxial growth, colloidal synthesis, etc.). The manufactured film can be deposited on a working surface or remain on technological optically transparent substrates (such as glass, quartz, sapphire, etc.). Mesoporous films located on transparent substrates can be utilized as a standalone device such as an optical concentrator. At the second stage of the fabrication process according to the present invention quantum- sized nanocrystals are synthesized of any type of semiconductor compounds (II-VI, IH-V, I- VII; IV - Si, Ge, etc.) with or without shell, doped with Mn²⁺ ions (or ions of other elements, e.g., rare earth ions Eu³⁺, Tb³⁺, Sm³⁺, etc.). Said nanocrystals are nanoparticles with the such average size as to achieve a quantum-size short-wavelength shift of the edge of absorption band sufficient to separate the said absorption band from the luminescence band. By means of controlling the composition of the core and choosing the dopant element the desired spectral ranges for excitation and emission bands can be achieved. The synthesized nanocrystals are passivated and dried to a powdery form. At the third stage of the fabrication process according to the present invention nanocrystals are injected into sol-precursors of the xerogels based on titanium oxides TiOz, aluminum oxides Al₂O₃, indium oxides In₂O₃, silicon oxides SiO₂, etc., having a high transmittance in the ultraviolet range. Previously synthesized nanocrystals placed inside transparent xerogel matrices (or polymer or glass matrices) can be utilized as a standalone device, e.g., as a spectral converter of optical radiation or a UV-sensitizing coating for silicon-based and other photodetectors.

At the forth and final stage of the fabrication process according to the present invention the previously manufactures mesoporous films are saturated with previously obtained xerogel containing previously synthesized nanocrystals by means of centrifugation (dipping, capillary wetting, etc.) in one or several cycles until the whole volume of the film pores is filled. An external planar layer of gel is then formed to provide an optical gate for output radiation. After that the film is dried in the open air. To increase the mechanical durability properties the mesoporous film can be mounted on an additional substrate made of any optically transparent material, such as an anodic alumina film on quartz or sapphire substrates. The universal nature and a wide range of optical properties of output radiation of the present invention make it possible to create a complete line of highly directional display panels of various colors from near-monochromatic to white operating with a single illumination source and suitable for mass production.

Non-limiting embodiments of the present invention will be described below with reference to the drawings.

First Embodiment.

The spectral optical converter, constructed according to the first aspect of the present invention and schematically depicted in FIG. 5 comprises a film 1 of transparent uniformly arrayed mesoporous material, more particularly anodic alumina, saturated with a microporous xerogel, more particularly that based on indium oxide, containing emission centers 2, more particularly quantum-sized II-VI semiconductor nanocrystals doped with metal ions, more particularly manganese ions, and additionally comprising a substrate 3 made of quartz or glass and encased in a setting 4.

In what follows, an exemplary fabrication process of said film filled with said xerogel containing said nanocrystals is described.

The mesoporous anodic alumina film can be fabricated through the next way.

The starting substrates represented glass, quartz or sapphire wafers, one surface of which contained adhesive Ta sublayer (20 A) and Al layer (5... 15 μm) formed by magnetron sputtering. Fabrication of porous anodic alumina was realized by electrochemical anodizing of Al layer in 1.2 M phosphoric acid (PA) or 0.3 M oxalic acid (OA) solutions by a two-step method (see [[25] A.P.Li, F.Muller, U.Gosele. Polycrystalline and monocrystalline pore arrays with large interpore distance in anodic alumina, Electrochem. Solid-State Lett., V. 3, No. 3 5 (2000) 131 — 134] for the details). Anodizing was performed at a temperature maintained at 20 ⁰C at a constant voltage 120 V (PA) or 40 V (OA), the resulting current was registered by an amperemeter. The first anodizing was performed as long as a few minutes elapsed in steady-state regime. Further, the formed layer of anodic alumina was removed in an aqueous solution of 6 wt. % phosphoric acid and 1.8 wt. % chromic acid at a temperature 80...90 ⁰C.

10 After rinsing in running and distilled water, the second anodizing was carried out in the same conditions. The moment of finish of anodizing of Al layer was checked by decreasing the current in comparison to steady-state regime. At that moment, the voltage was lowered down to 100 V (PA) or 35 V (OA) to anodize Ta sublayer. Anodizing of Ta sublayer was carried out until the current was decreased down to value of 0.05 of that of at steady-state regime.

15 Further, the pore widening was performed by chemical etching of porous anodic alumina in 50 vol. % phosphoric acid (PA) for 1 h or in 10 vol. % phosphoric acid (OA) for 1 h, the temperature was kept at 25 ⁰C. The preparation of porous anodic alumina was finished by rinsing in distilled water for 30 min and drying in air at 200 ⁰C for 10 min. The resulting structure parameters of porous anodic alumina was as follows: pore diameters of 150 (PA) or

>0 40 nm (OA), interpore distances 300 (PA) or 100 nm (OA).

Prior to infiltration of porous anodic alumina with sols, the samples were dried at a temperature 200 ⁰C for 20 min to remove physically adsorbed water. Deposition of sol was performed by spin-on route at 2700...3000 rpm for 30 sec followed by drying in air at 200 ⁰C for 20 min.

!5 The nanocrystals can be made as follows (see [[26] Jae Hun Chung, Chil Seong Ah, and Du- Jeon Jang. Formation and Distinctive Decay Times of Surface- and Lattice-Bound Mn^"+ Impurity Luminescence in ZnS Nanoparticles. J. Phys. Chem. B, 105 (2001) 4128-4132] for the details). Source materials Na₂S 9H₂O, Zn(NO₃)₂-6H₂O, and Mn(NO₃)r6H₂O were used as purchased from the Aldrich Chemical (Milwaukee, WI). For the preparation of undoped ZnS

O nanoparticles dispersed in water (free sample), 30 mL of 2-mM Na₂S-9H₂O was added to 30 mL of 2-mM Zn(NOs)₂-OH₂O aqueous solution, which was preadjusted to pH 10.3. For the synthesis of 2% (in mole) Mn²⁺-doped ZnS nanoparticles suspended in water (doped sample). 30 mL of 2-mM Na₂S-9H₂O aqueous solution was added to 30-mL aqueous basic (pH 10.3) solution of 2-mM Zn(NO₃)₂-6H₂O and 40-μM Mn(NO₃)r6H₂O with stirring. 2% Mn²⁺-doped

5 and ZnS-passivated ZnS nanoparticles (doped and passivated sample) were prepared by adding 2.5 mL of 40-mM Zn(NO₃)₂-6H₂O aqueous solution and 2.5 mL of 40-mM Na₂S-W₂O aqueous solution to 10 mL of the 2% Mn²⁺-doped sample at pH 10.3. The average diameters of free and doped nanoparticles were estimated to be about 6 nm by using a transmission electron microscope (JEOL, JEM2000).

0 Synthesis of sol can be made as follows. Ti(OC₂Hj)₄ (Aldrich) was used as a precursor. 10.820 g Ti(OC₂H₅)₄ was added to 100 ml of 96 % ethanol. Precipitated Ti(OH)₄ as a result of hydrolysis was transferred to a colloidal phase by adding concentrated hydrochloric acid until pH=l .

The saturation of the mesoporous film with xerogel containing nanocrystals can be carry out by means of multiple centrifugation of the corresponding sols and subsequent thermal t trrpeaϋttmmpenntt. The device specification; dimentions - 20x20 mm², input wavelength - 250...450 nm, output wavelength - 550...800 nm, operation angles - ±30 degrees, maximum input light power - upto 10² W/cm², quantum efficient - 60...80%, predictable lifetime - more 10 000 hrs.

Second Embodiment. The optical display panel constructed according to the second aspect of the present invention is schematically depicted in FIG. 6. The said panel comprises a film 1 of transparent uniformly arrayed macroporous material, more particularly alumina, manufactured according to the example in the First Embodiment, saturated with a xerogel, more particularly that based on titanium oxide, containing emission centers 2, more particularly quantum-sized nanocrystals, and additionally comprising a substrate 3 made of a transparent material, more particularly quartz, glass, or sapphire.

Third Embodiment.

The optical photosensitive device, constructed according to the third aspect of the present invention and schematically depicted in FIG. 7 comprises a photodetector 5, more particularly a semiconductor photodiode having a photosensitive area 6 and an optical converter located over said photosensitive area 6, said converter manufactured according to the example in the First Embodiment and comprising a film 1 of transparent uniformly arrayed macroporous material, saturated with a xerogel, more particularly that based on aluminum oxide, containing emission centers 2, more particularly quantum-sized nanocrystals of the core-shell type, said core doped with metal ions, more particularly manganese ions, and additionally comprising electrical contacts 4 and a substrate 3 made of quartz.

Claims

1. Optical spectral converter device, comprising a film of transparent unidirectionally structured nanoporous material, the pores in said material containing a substance capable of wavelength conversion of incident optical radiation, wherein said substance comprising a xerogel, said xerogel containing quantum-sized nanostructures, nanocrystals or nanoclusters including, said nanostructures possessing strong quantum-size effects in a quantum dot.

2. The converter device according to claim 1, wherein the said xerogel is chosen from among the gels Al₂O₃, In₂O₃, TiO₂, SiO₂.

3. The converter device according to claim 1 or 2, wherein the nanocrystals are made from a material chosen from among the semiconductor compounds H-VI, I-VII, TII-V.

4. The converter device according to claim 3, wherein the nanocrystals are made from a material chosen from among the compounds CdS, CdSe, ZnS, ZnSe or a combination thereof in core-shell structures CdSe/ZnS, ZnSe:Mn/ZnS.

5. The converter device according to claim 3 or 4, wherein the nanocrystals are doped with metal ions chosen from among the elements Mn²⁺, Eu³⁺, Tb³⁺, Sm³⁺.

6. The converter device according to claims 1-5, wherein said film of transparent unidirectionally structured material is one of the types of photonic crystal structure, including porous membrane, mesotube monolayer, or synthetic opal, manufactured from the oxides chosen from the group SiO₂, Al₂O₃. TiO₂.

7. The converter device according to claims 1-6, wherein said film of transparent unidirectionally structured material, the pores in said material containing a distributed substance, is manufactured in the form of images by means of photoresist centrifugation, photolithography, template etching and photoresist removal.

8. Optical photoreceptive device, comprising a photosensitive detector operating in a predetermined optical range and a thin-film optical converter device, said optical converter device comprising a film of transparent unidirectionally structured nanoporous material, the pores in said material containing a substance capable of wavelength conversion of incident optical radiation, wherein said substance comprising a xerogel, said xerogel containing quantum-sized nanostructures, nanocrystals or nanoclusters including, said nanostructures possessing strong quantum-size effects in a quantum dot.

9. The device according to claim 8, wherein the said xerogel is chosen from among the gels Al₂O₃, In₂O₃, TiO₂, SiO₂.

10. The device according to claim 8 or 9, wherein the nanocrystals are made from a material chosen from among the semiconductor compounds H-VI, I-VII, IH-V.

1 1. The device according to claim 9, wherein the nanocrystals are made from a material chosen from among the compounds CdS, CdSe, ZnS, ZnSe or a combination thereof in core- shell structures CdSe/ZnS, ZnSe:Mn/ZnS.

12. The device according to claim 10 or 1 1 , wherein the nanocrystals arc doped with metal ions chosen from among the elements Mn²⁺, Eu^J+, Tb^J+, Sm³⁺.

13. The device according to claims 8-12, wherein said film of transparent unidirectionally structured material is one of the types of photonic crystal structure, including porous membrane, mesotube monolayer, or synthetic opal.

14. The device according to claims 8-13, wherein said optical photosensitive detector is a photodiode.

15. The device according to claims 8-13, wherein said optical photosensitive detector is a phototransistor.

16. The device according to claim 14 or 15, wherein said optical photosensitive detector is silicon-based.

17. The device according to claims 8-13, wherein said optical photosensitive detector is a photoresistor.

18. The device according to claims 8-13, wherein said optical photosensitive detector is a CCD detector made as a matrix or as a line.

19. The method of enhancing the photodetector sensitivity in the short- wavelength optical range, blue and ultraviolet including, said method comprising the application of an additional thin-film coating to photosensitive area of said photodetector, said coating comprising a film of transparent unidirectionally structured nanoporous material, the pores in said material containing a substance capable of wavelength conversion of incident optical radiation, wherein said substance comprising a xerogel, said xerogel containing quantum-sized nanostructures, nanocrystals or nanoclusters including, said nanostructures possessing strong quantum-size effects in a quantum dot.

20. The method according to claim 19, wherein the said xerogel is chosen from among the gels Al₂O₃, In₂O₃, TiO₂, SiO₂.

21. The method according to claim 19 or 20, wherein the nanocrystals are made from a material chosen from among the semiconductor compounds H-VI, I-VII, IH-V.

22. The method according to claim 21 , wherein the nanocrystals are made from a material chosen from among the compounds CdS, CdSe, ZnS. ZnSe or a combination thereof in core- shell structures CdSe/ZnS, ZnSe:Mn/ZnS.

23. The method according to claim 21 or 22, wherein the nanocrystals are doped with metal ions chosen from among the elements Mn²⁺, Eu³⁺, Tb³⁺, SnT³⁺.

24. The method according to claims 19-23, wherein said film of transparent unidirectionally structured material is one of the types of photonic crystal structure, including porous membrane, mesotube monolayer, or synthetic opal.

25. The method according to claim 23, wherein said photodetector operating in a predetermined optical spectral range is a commercial photodetector chosen from the group of devices including a photodiode, a phototransistor, a photoresistor, a CCD detector, or a solar cell.

26. The method according to claim 23, wherein said commercial photodetector is silicon- based.