Optics: measuring and testing – By shade or color – With color transmitting filter
Reexamination Certificate
2000-12-21
2003-04-22
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By shade or color
With color transmitting filter
C435S004000, C356S364000, C250S341100
Reexamination Certificate
active
06552791
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a nondestructive apparatus and method for determining the molecular orientation and the orientation of the unit cell of organic systems which are anisotropic or have anisotropic behavior, and particularly in thin films.
BACKGROUND OF THE INVENTION
Over the past ten years, interest in organic materials (or organic molecules) has increased enormously in view of the prospect of developing a new generation of electronic and photonic devices based on said materials.
Examples of organic-based devices which are currently of great interest are thin-film transistors [1] and light-emitting diodes [2], but the application potential is extremely broad.
With respect to the conventional technology of inorganic semiconductors, such as silicon and gallium arsenide, organic molecular materials offer the fundamental advantage that they can be easily processed as thin films and therefore can be compatible with large area flexible substrates. Their thermal and mechanical stability is another characteristic that makes them excellent candidates for applications in optoelectronic devices.
It has been demonstrated that high control of molecular order in films is absolutely necessary in order to control the basic physical phenomena that determine the operation of optoelectronic devices. Furthermore, control of the orientation of highly anisotropic systems is important, since the optical and transport properties are significantly anisotropic along the various crystalline directions.
Various growth techniques (such as for example Langmuir-Blodgett, vacuum sublimation, supersonic beams, et cetera) have been proved to be suitable for growing monomolecular layers of an organic compound. In said ultrathin films, thickness is controlled on the magnitude order of one unit of molecular length.
Molecular orientation on the substrate depends not only on the intrinsic molecular properties and on intermolecular interactions but also on the nature of the molecule-substrate interaction and on the growth kinetics. Said kinetics is controlled by a series of experimental parameters, such as the growth rate, the temperature of the substrate, and the thickness of the film.
The absolutely critical dependence of structural order and molecular orientation on the growth conditions makes it indispensable, in order to obtain highly orientated organic thin films, to be able to simply and rapidly measure the orientation of the molecules in the film.
Furthermore, since the film may be subsequently manipulated in order to vary the molecular orientation and therefore the optoelectronic properties, the real-time noninterfering evaluation of said manipulation is useful both for quality and for process control.
Known methods for evaluating molecular orientation comprise anisotropy of ultrasonic speed, X-ray diffraction, optical double refraction, optical dichroism, small-angle light scattering [U.S. Pat. No. 4,264,207], polarized reflectance [U.S. Pat. 5,365,067], photoluminescence anisotropy, and polarized photoluminescence.
These techniques usually require complicated measurements and highly critical modeling. Specific technical skill and a long time for making the measurements are also necessary.
Photoluminescence anisotropy and polarized photoluminescence have been used in order to measure molecular orientation in thin films of polymers and in films of organic molecules [3, 4, 5].
Furthermore, measurement of photoluminescence intensity as excitation polarization varies is used to monitor molecular orientation in [U.S. Pat. No. 4,521,111].
However, prior to the present invention it was not known how to determine the molecular orientation in a film from a single non-polarized luminescence measurement.
The aim of the present invention is to overcome the drawbacks of currently known methods for determining molecular orientation in thin films, by providing a method and an apparatus for detecting the orientation of organic molecules in thin films by means of a simple and rapid measurement of nonpolarized luminescence and without critical modeling.
Another object of the present invention is to provide a method and an apparatus which allow to detect whether the molecules are orientated in a thin film parallel to the substrate or at right angles thereto without performing polarized photoluminescence or photoluminescence anisotropy measurements.
Another object of the present invention is to provide a method and an apparatus for monitoring in real time, during the growth or manipulation of the film, the molecular orientation in said film with respect to the substrate or surface of the film.
Another object of the present invention is to provide a method and an apparatus for monitoring molecular orientation in the film noninvasively and noninterferingly.
Another object of the present invention is to provide a method and an apparatus which allow to optimize in real time the growth conditions of the film, in order to obtain specific structural characteristics.
SUMMARY OF THE INVENTION
This aim and these and other objects which will become better apparent from the description that follows are achieved, according to the present invention, by means of a method which comprises the steps of a) stimulation of the luminescence of the molecular layer; b) measurement of the consequent nonpolarized luminescence spectrum; c) spectral and vibrational analysis of said luminescence spectrum in order to determine the orientation of the molecules.
Luminescence can be obtained with any method, particularly with photoluminescence excitation or with an electroluminescence excitation. For the sake of simplicity, but without intending to limit the invention, reference is made in the remainder of the description exclusively to photoluminescence.
The invention furthermore provides an apparatus for detecting noninvasively and noninterferingly and, if necessary, in real time during dynamic growth or manipulation phenomena, the orientation of the molecules according to the above method. Said apparatus comprises the following elements: a) an exciter source suitable to induce photoluminescence of the specimen; b) an optical system for collecting the photoluminescence generated by the specimen; c) an analyzer of photoluminescence spectra with appropriate optical and spectral characteristics; d) an interface with the user, suitable to indicate the orientation of the molecules, optionally providing the relative weights of the various orientations detected with respect to the dominant orientation. Conveniently, the apparatus according to the present invention furthermore comprises e) a system for varying in real time the growth or manipulation conditions, suitable to act according to the result of the determination of the molecular orientation and achieve specific structural characteristics.
The inventors of the present invention have in fact found that the orientation of the molecules on the substrate is revealed by measuring a nonpolarized photoluminescence spectrum and observing the active vibronic transitions.
REFERENCES:
patent: 4264207 (1981-04-01), Batyrev et al.
patent: 4521111 (1985-06-01), Paulson, Jr.
patent: 4651011 (1987-03-01), Ors et al.
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patent: 5365067 (1994-11-01), Cole et al.
patent: 5532488 (1996-07-01), Ishibashi et al.
patent: 5598005 (1997-01-01), Wang et al.
patent: 6151115 (2000-11-01), Naulleau
Feldmann J, Guss W, Lemmer U, Gobel E O, Taliani C, Mohn H, Muller W, Haussler P, and Ter Meer H-U: “Photoluminescence studies of C60 single crystals.” Molecular Crystals and Liquid Crystals, vol. 256, 1994, pp. 757-762, XP001010830 Switzerland p. 758, line 15-line 24 p. 761, line 1-p. 762, line 20 figure 1.
Muccini Michele
Taliani Carlo
Consiglio Nationale Delle Ricerche
Davis Willie
Josif Albert
Modiano Guido
O'Byrne Daniel
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