Optical recording medium and optical recording method

Details Optical recording medium and optical recording method Optical recording medium and optical recording method

2001-12-11

2003-08-19

Mulvaney, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

All metal or with adjacent metals

Composite; i.e., plural, adjacent, spatially distinct metal...

C428S064500, C428S403000, C430S270130

Reexamination Certificate

active

06607845

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a recording medium, represented by optical discs, using nanoparticles of an inorganic dielectric and nanoparticles of a metal chalcogenide. The terminology “nanoparticles” as used herein refers to ultrafine particles whose average particle size ranges from 1 nm to 50 nm.
BACKGROUND OF THE INVENTION
Optical recording materials have been improved in density and sensitivity. Laser light having a wavelength of 600 nm or longer has been employed for optical recording and reproduction, and optical recording media have conventionally been designed to exhibit their optimal performance within this wavelength region. It is well known that reduction of light wavelength to one-n′ th brings a recording density multiplied by n
2
. Therefore it has been keenly demanded to practically realize high-density recording with a short wavelength laser of about 400 nm. According as the wavelength is shortened for higher recording density, the demands for further increased density and sensitivity of recording media have been increasing.
Relevant technologies using ultrafine particles have hitherto been disclosed as follows. JP-A-5-62239 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses the use of ultrafine particles of semiconductors, such as Ge or Si, which are formed by sputtering and have a particle size distribution. This technique aims at multiple-wavelength recording by making use of difference in quantum size effect of ultrafine particles having different sizes in order to improve recording density without relying on reduction of laser wavelength or increase of numerical aperture (NA). It is therefore fundamentally different from the present invention which takes advantage of the overall phase change of nanoparticles of uniform size in an energy-irradiated area.
JP-A-10-261244 discloses an optical recording medium comprising a substrate having a fine uneven pattern on its surface and a recording layer formed on the patterned substrate by sputtering, in which the recording layer comprises a chalcogen compound having dispersed therein fine metal particles or fine noble metal particles or the recording layer comprises a dielectric material having dispersed therein composite fine particles of a noble metal and a chalcogen compound. Involving the additional step for forming a fine pattern, the technique requires a complicated production process and has poor practicability.
Although thin film formation by sputtering enjoys the merits of a dry process and allows freedom of film composition designing, it has difficulties in controlling the particle size, particle size distribution and structure of the formed particles and also difficulties in forming the particles as dispersed in a binder or a dielectric, as compared with a colloid process. Such difficulties lead to difficulty in improving distinguishability between recorded areas and non-recorded areas, size reduction of the recording area, and stability of the recording material.
As for a technique for preparing ultrafine particles, JP-A-12-54012 discloses a technique of forming magnetic nanocrystals of a metal, an intermetallic compound or an alloy by a reduction process. However, this disclosure is not relevant to the present invention contemplating preparation of a metal chalcogenide. In addition,
Appl. Phys. A,
vol. 69, pp. 369-373 (1999) reports a method of making ZnS nanoparticles, but it does not explicitly mention about applications thereof.
A thin film formed from a material that is included under the scope of the present invention can be formed by CVD as well as sputtering and reduction as noted above. For example, JP-A-2-195538 discloses making a dielectric layer containing ZnS and SiO
2
by sputtering. However, where a recording layer is formed by spread coating as in the present invention, it is inefficient to form dielectric layers over and beneath the recording layer by sputtering. Further, formation of an AgInTe
2
thin film by CVD is disclosed in JP-A-3-82593. This process requires a substrate to be kept at or above 100° C. Such a high temperature is hardly applicable to a substrate made of polymers such as polycarbonate. It is an additional problem that the film formation by CVD takes time.
The idea that an optical recording layer might be made of a heat-resistant matrix having an ultrafine particulate substance dispersed therein is taught in Japanese Patent 2908826, but the Patent has no mention of a specific process of making such a recording layer. Such a film structure has generally been made by a process comprising injecting a recording material into a heat-resistant matrix in a supersaturated state by sputtering and causing the injected material to precipitate by, for example, annealing. There is no literature teaching a process in which a recording material is made dispersible as a colloid by modifying the surface of ultrafine particles of the material. The Patent also refers to a wet process (a sol-gel process), but fails to disclose any specific process thereof. Accordingly, the description is no more than general one.
JP-A-3-231890 proposes firing after spraying or spin coating to make a recording layer comprising an InCuSe
2
alloy, which cannot be seen as practical in view of accuracy of film formation and heat resistance of a substrate.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a high-density and high-sensitivity recording medium capable of recording, reproducing and erasing information with laser beam irradiation in which a recording layer and dielectric layers are formed by spin coating or web coating with colloidal nanoparticles.
Another object of the invention is to provide a method for optically recording information on the recording medium.
Other objects and effects of the present invention will become apparent from the following description.
The above-described objects of the present invention have been achieved by providing the following recording media and recording method.
1) An optical recording medium comprising a substrate having thereon provided at least a first dielectric layer, a recording layer and a second dielectric layer in this order,
wherein at least one of said first dielectric layer and said second dielectric layer is formed by applying a colloidal dispersion which comprises inorganic dielectric nanoparticles having an average particle size of 1 to 50 nm and having a surface modified with an adsorptive compound, and
wherein said recording layer is formed by applying a colloidal dispersion which comprises metal chalcogenide nanoparticles having an average particle size of 1 to 20 nm and having a surface modified with an adsorptive compound. The recording layer shows a phase change upon being irradiated to change its optical constant, e.g., a reflectance. It is preferred that both the first and second dielectric layers comprise the inorganic dielectric nanoparticles.
2) The optical recording medium according to item 1) above, wherein said recording layer is formed by a process comprising:
preparing said metal chalcogenide nanoparticles by chemical synthesis in the form of a lipophilic colloidal dispersion; and
applying said lipophilic colloidal dispersion.
3) The optical recording medium according to item 1) or 2) above, wherein said metal chalcogenide nanoparticles comprise:
(A) at least one element selected from the elements of the groups 8, 1B and 2B and the elements in the 4th to 6th periods of the groups 3B, 4B and 5B; and
(B) at least one element selected from the elements of the group 6B.
4) The optical recording medium according to any one of items 1) to 3) above, wherein said surface-modified inorganic dielectric nanoparticles comprises at least ZnS nanoparticles which has a surface modified with an adsorptive compound.
5) The optical recording medium according to item 4) above, wherein at least one of said first dielectric layer and said second dielectric layer is formed by a process comprising:
preparing said surface-modified ZnS nanoparticles

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