Abstract:
This invention relates to a photoluminescent layer in the optical and adjoining spectral regions based on a solid solution of organic dyes. The photoluminescent layer consists of organic dye molecules with a low dye concentration and a matrix material of metal oxides, with the matrix material having a slightly sub-stoichiometric oxygen content. A method and a device for producing the photoluminescent layer are described.

Description:
FIELD OF THE INVENTION 
     This invention relates to a photoluminescent composition (e.g., a layer) as well as related articles, methods, and devices. 
     BACKGROUND 
     A more or less effective luminescence conversion has already been used for some time in various fields, for example in radiation detector technology. In general, functional units that are used for luminescence conversion are based on absorption/emission processes. Utilized is the fact that there is a shift of luminescence to longer wavelengths compared to absorption in most cases, for energetic reasons. This phenomenon can be used, for example, for spectral matching of detector sensitivity to a radiation source. Furthermore, the property of luminescence radiation no longer to be bound to the direction of the incident radiation is of interest, since concentration of radiation in a medium can be realized by total reflection at the interfaces. 
     A recent example is the production of “white” light by way of partial conversion of the radiation from a blue luminescent diode. The LUCOLED (P. Schlotter, R. Schmidt, J. Schneider, Appl. Phys. A 64, 417 (1997)) utilizes this principle. A portion of the high-energy blue luminescent radiation is absorbed by a suitable layer in the beam direction and is emitted again shifted toward lower energies, so that a white color impression is produced by additive mixing. 
     DE 196 25 622 A1 describes such a light-radiating semiconductor component with a semiconductor body emitting radiation and with a luminescence conversion element. The semiconductor body has a sequence of semiconductor layers that emits electromagnetic radiation with a wavelength λ of ≦520 nm, and the semiconductor conversion element converts radiation of a first spectral subregion of the radiation emitted by the semiconductor body from radiation originating from a first wavelength region into radiation of a second wavelength region, so that the semiconductor component emits radiation from a second spectral subregion of the first wavelength region and radiation of the second wavelength region. Thus, for example, radiation emitted by the semiconductor body is absorbed with spectral selectivity by the luminescence conversion element and is emitted in the longer-wavelength region (in the second wavelength region). In this method, organic dye molecules are imbedded in an organic matrix. 
     DE 196 38 667 A1 also discloses a semiconductor component with a semiconductor body emitting radiation and a luminescence conversion element that emits mixed-color light, with the luminescence conversion element having a luminous inorganic substance, in particular a phosphor. 
     Besides spectral suitability with regard to the corresponding application, such a layer has two principal requirements: The photoluminescence quantum yield must be high, usually clearly greater than 50%, and its stability must permit long service lives, usually more than 10,000 hours. 
     The basic concept for realizing such a layer with organic dyes consists of separating and immobilizing molecules in a matrix so that they behave like monomers with optical properties similar to a liquid solution, particularly with high quantum yield. Polymers and sol-gel layers are known as matrices. 
     Mixed layers that were produced from the organic dyes 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) and SiO 2  by co-vaporization onto quartz substrates under high vacuum are described in H. Fröb, K. Kurpiers, K. Leo, CLEO&#39;98, San Francisco/CA, May 1998, 210; 1998 OSA Technical Digest Series Vol. 6, published by Optical Society of America (“ The Fröb publication ”). The concentration range studied was 0.65–100 vol. %. It was observed that the absorption and emission spectra for decreasing concentrations gradually approach those in a liquid solution, and for the lowest concentration a photoluminescence quantum yield of about 50% is achieved at room temperature ( FIG. 6 , corresponding to FIG. 2 of the Fröb publication). 
     A device used for this purpose is described by M. A. Herman, H. Sitter, Molecular Beam Epitaxy, Ch. 2 (Sources of Atomic and Molecular Beams), Springer 1989, pp. 29–59. A dye vaporizer and a metal oxide vaporizer are provided in a vacuum chamber, whose vapor beam is aimed at a substrate, with the dye vaporizer being cup-shaped and consisting, viewed from the inside toward the outside, of a quartz cuvette, a graphite block, a heater, a shield, and a jacket, with a thermocouple being placed between the quartz cuvette and the graphite block in the bottom center of the cup. 
       FIG. 7  shows the normalized absorption and emission of 30-nm thick layers for a pure and a diluted dye layer. It is important that the spectra of the diluted layers can be fitted to those of monomers with their typical vibrational progression. It is found that the line width remains constant for all low concentrations; its enlargement compared to that observed in liquid solution is not surprising, considering the inhomogeneous conditions of the surroundings of the molecule. 
     The authors hold a weakening Förster transfer because of the increasing mean molecular separation responsible for the increase of quantum yields toward lower concentrations, and they expect a maximum at about 0.1 vol. %, but of course without experimental confirmation of this. No predictions are made about the lifetime, with regard to which all organic conversion layers so far have foundered. 
     SUMMARY OF THE INVENTION 
     The invention relates to a photoluminescent composition (e.g., a layer) as well as related articles, methods, and devices. 
     It is important for the solution of the problem that organic dye molecules are imbedded in an inorganic, amorphous or nanocrystalline matrix. The use of a silicon or metal suboxide in the vaporization is especially crucial for the optical stability of the photoluminescent layer. During the deposition of the silicon or metal suboxide in mixed vaporization of the components under high vacuum on the substrate, the suboxide reacts with residual gaseous oxygen of the high vacuum, with a slightly sub-stoichiometric oxygen content being reached by the matrix material under suitable vapor deposition conditions (characterized by the ratio of oxygen partial pressure to rate of vapor deposition). It is characteristic of the sub-stoichiometric oxygen content that with a matrix material of SiO x  or TiO x , x is between 1.95 and 2. A precise adjustment of the dye vapor deposition rate is necessary. For a low dye concentration, a definite adjustment to a low dye deposition rate (down to &lt;10 −5  nm/s) is critically important. In certain embodiments, a temperature-regulated dye vaporizer is used for this purpose. 
     The vaporizer pursuant to the invention differs from the dye vaporizers known from the state of the art in the fact that the cover in the cup-shaped opening of the dye vaporizer constricted to a cut-out hole is connected to the quartz cuvette and is displaced toward the dye, so that the cut-out hole in the cover has a temperature like that of the heated quartz cuvette. 
     In some embodiments, the configuration of the photoluminescent layer makes it possible by extremely low dye surface densities, for example, to provide luminescence standards with almost ideal point sources of light for appropriately equipped microscopes (for example optical near-field microscopes, confocal luminescence microscopes) for the determination of resolving power and optical transmission functions or tests for the determination of the optical properties of individual molecules. 
     The benefits produced by the invention in particular, lie in the fact that a material is available that satisfies practical requirements with regard to optical stability with an average number of excitation/de-excitation cycles per molecule greater than 10 11 , that can be applied to very diverse substrates by dry technology (mixed vaporization of the components under high vacuum), and that at the same time has the highest known concentration of dyes in solutions without having photoluminescence quantum yields limited by aggregation or by Förster transfer. 
     Embodiments advantageously provide high optical stability for a photoluminescent layer based on a solid solution of organic dyes, as measured by the numerical midpoint of the excitation/de-excitation cycles per molecule before a fixed value of the decline of photoluminescence of the overall system. 
    
    
     
       The invention is explained in detail below with reference to examples of embodiment. The drawings show: 
         FIG. 1  an illustration of the photoluminescence quantum yield of 30-nm thick layers with various dye concentrations 
         FIG. 2  an illustration of the change of photoluminescence with high-intensity irradiation 
         FIG. 3  an illustration of the luminescence of SiO x  layers with equal amounts of the dye MPP with different dye concentration. 
         FIG. 4  a dye vaporizer pursuant to the invention in cross section 
         FIG. 5  an Arrhenius plot for calibrating the dye vaporizer 
         FIG. 6  an illustration of the photoluminescence quantum yield of PTCDA-SiO 2  mixed layers at room temperature 
         FIG. 7  normalized absorption and emission of 30-nm thick layers for pure and diluted PTCDA layers 
     
    
    
     DETAILED DESCRIPTION 
     A photoluminescent layer is described. The layer is photoluminescent in the optical and adjoining spectral regions. The layer is typically a solid solution of organic dye molecules within a silicon oxide or metal oxide. The layer can be applied (e.g., vapor deposited) on a substrate. 
     Applications of the layer include using the layer to provide white light, using the layer to input or output light to or from a waveguide, using the layer as a radiation detector, or using the layer as a point source for testing near-field microscopes or the like. In general, the layer is applied on a substrate for particular applications. 
     EXAMPLE 1 
     In Example 1,3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) was incorporated in an SiO x  matrix, where 1.95&lt;x&lt;2. The layer is produced by thermal vaporization at operating pressures of about 10 −4  Pa produced by a turbomolecular pump, with SiO having been vapor-deposited at a deposition rate of 10 −2  nm/s for-the production of the matrix, which reacts on the substrate with residual gaseous oxygen to give SiO x . The quartz resonators used in this multiple-source vapor deposition for the independent control of deposition rate and layer thickness are shielded from the other sources. To be able to measure even very small deposition rates, the measuring head for PTCDA is at a small distance from the vaporizer; this is possible with no problems because of the comparatively low vaporization temperature (typically 300–400° C.). For extremely small rates of vapor deposition, a temperature-regulated dye vaporizer was developed that permits stable rates down to &lt;10 −5  nm/s for a period of at least one hour. 
     Radiationless energy transmission to nonradiating traps is the limiting factor for luminescence quantum yield. To reach a quantum yield similar to that in liquid solution, volume concentrations of about 0.1% are necessary in the present system ( FIG. 1 ). Compared to the data given in H. Fröb, M. Kurpiers, K. Leo, CLEO &#39;98, San Francisco/CA, May 1998, 210, 1998 OSA Technical Digest Series Vol. 6, published by Optical Society of America, both a lower concentration was achieved and the quantum yields were determined and corrected with greater accuracy. 
     Results of studies of the optical stability of the layer are shown in  FIG. 2 . To achieve adequately high excitation densities, a confocal microscope was used (excitation wavelength 532 nm); the luminescence was detected. After an initially severe non-exponential decline, a state is reached that can be described by a lifetime with about 10 11  excitation cycles per molecule, a value that is about 2 orders of magnitude above the best known in such systems. 
     One possible application is found as a photoluminescent layer in a system similar to the LUCOLED (P. Schlotter, R. Schmidt, J. Schneider, Appl. Phys. A 64, 417 (1997)). Applied to luminous densities occurring in luminescent diodes, service lives of the order of magnitude of 10 5  hours would be expected, based on the data in  FIG. 2 . 
     EXAMPLE 2 
     Production is analogous to that in Example 1, using N,N-dimethylperylene-3,4,9,10-bisdicarboximide (MPP), and the same effects are observed relative to the context of the invention: Increase of the photoluminescence quantum yield with decreasing concentration ( FIG. 3 ) and optical stability in the aforementioned sense of about 10 11  excitation cycles per molecule. The fact that the quantum yield becomes maximum at comparatively higher concentrations is due to the smaller absorption strength of MPP compared to that of PTCDA. 
     EXAMPLE 3 
     Production is analogous to that in Example 1, with the difference that (a) the vapor deposition rate of PTCDA is extremely low, typically &lt;10 −5  nm/s, and (b) the PTCDA vapor jet to the substrate is released by suitable diaphragms for only a very short time. Assuming that the procedure is performed extremely cleanly and exactly, dye molecules in this way can be placed enclosed by matrix material, with an average lateral molecular spacing of more than 100 nm being achievable. An optical near-field microscope at this time can achieve a resolving power of better than 50 nm; with a cover layer of 5 nm SiO x  over the dye layer there is thus a test that permits determining the point transmission function by a direct path, or with which optical properties of individual molecules can be determined. 
       FIG. 4  shows a dye vaporizer that is placed in a vacuum chamber with a metal oxide vaporizer to carry out the procedure. The vapor jet of each vaporizer is aimed at a substrate. Diaphragms can be placed between vaporizers and substrate to interrupt the vapor deposition. The dye vaporizer shown in  FIG. 4 , viewed from the inside to the outside, consists of a quartz cuvette  1 , a graphite block  2 , a heater  3 , a shield  4 , and a water-cooled copper jacket  5 . There is a thermocouple  7  in the bottom center of the cup between the quartz cuvette  1  and the graphite block  2 . There is a cover constricted to a cut-out hole in the cup-shaped opening of the dye vaporizer which is connected to the quartz cuvette  1  and is displaced toward the dye  6 , so that the cut-out hole in the cover has a temperature like that of the heated quartz cuvette  1 . 
     This dye vaporizer provides the capability of definitely setting an extremely low dye vapor deposition rate of &lt;10 −5  nm/s, since such rates are not accessible to direct measurement. Such low rates of deposition are achieved by using the temperature-regulated dye vaporizer with high temperature distribution homogeneity in the quartz cuvette  1 , with a small heated cut-out hole in the cover of the quartz cuvette  1 , and extrapolation based on calibration with an Arrhenius plot ( FIG. 5 ).