JPH07261021A - Optical material - Google Patents

Abstract

PURPOSE:To obtain a new material having a superlattice structure comprising a heterostructure of an org. layer and inorg. layer 4 and significant optical characteristics. CONSTITUTION:This material contains at least one group of a hetero structure 5 comprising an org. layer 4 of CuPc or the like between two inorg. layers 3 such as TiOx. These layers are formed in such a manner that the electron affinity and the band gap of the inorg. layer 3 are larger than those of the org. layer 4, the thickness of the org. layer 4 is the size of at most two molecules, and that the excited state of electrons in the molecules can be controlled by changing the kinds and thickness of the inorg. layers 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical material having a superlattice structure using an organic thin film and an inorganic thin film, an optical filter,
Used for wavelength converters, optical switches, etc.

[0002]

2. Description of the Related Art A so-called superlattice is an artificially formed layered lattice. In 1969, Esaki and Tsu
Since the proposal of a structure in which each layer is made of an inorganic semiconductor and the thickness of each layer is made into a nano size has been proposed by [L. Esa
ki and R. Tsu, IBM Research
Note, RC-2418 (1969)], and has been put to practical use as a material device having a photoelectron control function through a vast amount of research. On the other hand, with respect to the superlattice made of such an inorganic semiconductor, research on the formation of an organic thin film has started, and in 1990, So et al. Reported an organic superlattice [F. F. So, S. R. Forrest,
Y. Q. Shi, and W. H. Steier, Ap
pl. Phys. Lett. , 56, 674, (199
0)].

[0003]

By the way, nano-sized superlattices using inorganic semiconductors have been analyzed considerably due to their old history. Research on the lattice, including its physical phenomena, has just begun. Under these circumstances, the inventors of the present application have been investigating the optical characteristics of a superlattice structure having a heterostructure of an organic substance and an inorganic substance. Various proposals have been made for the superlattice used [J. Takada, H .; Awaji, M .; Kos
hioka, A .; Nakajima, and W.A. A.
Nevin, Appl. Phys. Lett. , 61,
2185 (1992), J. Takada, et a
l, Procending Ls of The 3r
d IUMRS on Advanced Mater
ials, Tokyo, (1993)]. Then, the present application aims to further develop such a proposal and to provide a novel optical material having outstanding optical characteristics which has not been obtained in the past.

[0004]

According to a first aspect of the present invention, there is provided at least one set of heterostructures in which an organic layer made of an organic material is sandwiched between two inorganic layers made of an inorganic material. The material is laminated so as to include, and the electron affinity and the band gap of the inorganic layer are larger than those of the organic layer, and the thickness of the organic layer is at most 2 molecule size or less.

The optical material according to claim 2 of the present invention comprises:
The thickness of the organic layer according to claim 1 is defined as the thickness when the energy value of the absorption peak in a predetermined band is measured with the thickness as a variable, and the two values sandwiching the organic layer are defined. The thickness and material of the inorganic layer are appropriately adjusted so as to have a required energy value.

The optical material according to claim 3 of the present invention is
The material appropriately adjusted in claim 2 is a material having a different dielectric constant.

The optical material according to claim 4 of the present invention is
The organic layer according to claim 1 or 2 is phthalocyanine having a thickness of 2.6 nm or less, and the inorganic layer is made of TiO _x .

The optical material according to claim 5 of the present invention is
The inorganic layer in claim 4 is amorphous TiO _x , wherein x = about 1.5.

[0009]

Since the organic layer constituting the present optical material includes a heterostructure sandwiched by inorganic layers having a larger electron affinity and bandgap than the organic layer, an organic layer having a narrow bandgap when absorbing light. The exciton motion is confined in the layer. In addition, since the thickness of the organic layer is at most 2 molecule size or less, the excited state of molecules in the organic layer is affected by the material and thickness of the inorganic layer, and artificial adjustment can be made by appropriately adjusting these materials and thickness. It becomes an optical material whose optical characteristics can be controlled. In addition, since an organic layer having low mechanical strength can be sandwiched by an inorganic layer having high strength, the mechanical strength can be increased, which is practical.

[0010]

EXAMPLES Examples of optical materials according to the present invention will be described below with reference to the drawings. FIG. 1 shows an optical material according to the present invention. The optical material 1 shown in FIG. 1 has a superlattice structure in which an inorganic layer 3 made of an inorganic substance and an organic layer 4 made of an organic substance are sequentially laminated on a substrate 2 made of, for example, quartz or silicon. The thicknesses of the inorganic layer 3 and the organic layer 4 are so-called nanometer-scale thin film layers. The basic structure of the optical material 1 is the organic layer 4
Is a heterostructure 5 sandwiched between two inorganic layers 3 and FIG. 1 shows a case where two sets of the heterostructure 5 are formed.

The electron affinity and the band gap of the inorganic layer 3 are larger than those of the organic layer 4. For example, when TiO _x is used as the inorganic layer 3,
The band gap is about 3.2 eV, and when phthalocyanine is used as the organic layer 4, the band gap is about 2.0 eV. Therefore, in this case, the inorganic layer 3 is not sensitive to visible light, but the organic layer 4 is a material sensitive to visible light.

The inorganic layer 3 may have a crystalline structure or an amorphous one, and the substrate 2 is kept at a relatively low temperature (for example, 200 ° C.) in order to form the organic layer 4 with particularly good crystallinity. In that case, TiO _x becomes amorphous.
The excited electronic state in the optical material 1 is the inorganic layer 3
The material can be appropriately changed depending on the type and thickness of the material, and in particular, the optical characteristics can be artificially controlled by the values of the electron affinity and the dielectric constant of the inorganic layer 3.

The phthalocyanine materials are metal phthalocyanine MPc and metal-free phthalocyanine P.
There is cH ₂ , but either may be used. Examples of the metal phthalocyanine include copper phthalocyanine CuPc, VOPc, AlClPc, ZnPc, NiPc, and C.
oPc, FePc, MnPc, etc. are used.

The thickness of the inorganic layer 3 is, for example, in the range of 0.3 to 100 nm when TiO _x is used, and when the copper phthalocyanine CuPc is used as the organic layer 4, CuPc molecules are present at the interface 6. 2.6 nm, which is equivalent to the thickness of two molecular sizes when it is oriented vertically
The following is set to, for example, 2 nm.

As a manufacturing apparatus for manufacturing the optical material 1 having the above structure, for example, a multi ion source type ion cluster beam (ICB) apparatus is used, and the inorganic layer 3 is made of silicon, titanium or the like. When it is composed of the oxide of, the reaction vapor deposition method is used.
That is, the substrate 2 is heated to 200 ° C. in an environment of 2 × 10 ⁻⁴ Torr in an oxygen atmosphere, and Ti is 8 mm in diameter.
The inorganic layer 3 is placed in a carbon crucible having an injection hole formed therein and evaporated by opening and closing a shutter attached to the evaporation source.
To form. Here, the temperature of the substrate is set to 200 ° C. because the crystallinity is the best when the α-type CuPc layer is formed as the organic layer 4, and thus the inorganic layer 3 is amorphous. became. When a TiO _x layer is formed,
According to the measurement by X-ray photoelectron spectroscopy, it was about x = 1.5. Further, the silicon oxide formed by using Si as an evaporation source is close to silicon dioxide in terms of optical characteristics.

After that, CuPc crystals are vapor-deposited under a vacuum of 3 × 10 ⁻⁶ Torr at a growth rate of about 0.03 nm / s, and the thickness thereof is about 2.6 nm or less and at most 2 molecular size or less. Thus, the organic layer 4 is grown. This thickness can be adjusted by appropriately selecting the size of the injection hole of the crucible containing the phthalocyanine crystal to be vapor-deposited, the growth rate, the growth time, and other manufacturing parameters. Hereinafter, the steps of forming the inorganic layer 3 and the organic layer 4 described above are sequentially repeated to manufacture the optical material 1 having the structure shown in FIG. The interface 6 of the optical material 1 having a nanoscale superlattice structure formed in this way is
X-ray diffraction (XRD), secondary ion mass spectrometry (SIM
S), atomic force microscope (AFM), and transmission electron microscope (TEM), the roughness was the best,
1 nm or less was achieved.

Next, a comparative experiment for clarifying the characteristics of the optical material 1 will be described with reference to FIGS. 2 and 3. FIG. 2 shows a sample in which CuPc was produced using quartz as the substrate 2, and a sample in which CuPc was used as the organic layer 4 and sandwiched between the inorganic layer 3 of SiO ₂ or TiO _x (this thickness is fixed at 2 nm). 2 is a characteristic obtained by plotting the energy value E _s1 (unit: eV) of the absorption peak near the Q band of 690 nm of CuPc in this system when the thickness of CuPc is changed.

In the figure, SiO ₂ /
In the CuPc / SiO ₂ sample, the exciton energy does not depend on the CuPc thickness, but the Ti plotted in ●.
In the sample of O _x / CuPc / TiO _x , CuPc is 2.
It can be seen that the exciton energy decreases at 6 nm or less. The thickness of this critical point corresponds to approximately two molecular sizes, assuming that the CuPc molecular surface is oriented perpendicular to the interface 6. Here, the characteristics are different as described above when the material of the inorganic layer 3 is SiO ₂ and TiO _x is that when the thickness of the inorganic layer 3 is about 2 nm, all CuP is formed in the case of TiO _x.
It seems that the c molecules are in direct contact with TiO _x , and when the inorganic layer 3 is TiO _x , the excited electrons of the inorganic layer are more likely to seep out due to the difference in electron affinity and dielectric constant compared to SiO _2. There is That is, C as the organic layer 4
It is considered that this exudation action is very likely to occur when uPc has a size of 2 molecules or less.

Next, the thickness of CuPc was fixed to the above-mentioned level of two molecules, and the three-layer structure film TiO _x / CuPc / TiO _{x in} which the thickness of TiO _x sandwiching the CuPc was changed was prepared on a quartz substrate. Is the energy value E _s1 (unit: e) of the absorption peak in the same manner as in FIG. 2 with the thickness of TiO _x as a variable.
V) was measured. From the figure, it can be seen that the energy value E _s1 rapidly decreases as the thickness of TiO _x increases. Here, the limit of infinite thickness of TiO _x is the energy of excitons having a Bohr radius determined by this dielectric constant, and the limit of zero thickness of TiO _x is the exciton energy of the CuPc molecule.

Thus, the excitons of CuPc are TiO 2.
_{If x} is on the nanometer scale, its Bohr radius will be limited by the thickness of TiO _x . That is,
It is considered that the excitons of the organic layer 4 are two-dimensional below a certain thickness because the extent of the excitons in the direction perpendicular to the interface 6 is restricted, and three-dimensional above that.

The present optical material 1 produced in this way
Can produce artificial composite structures that do not exist in nature on the nanometer scale, as compared to bulk material synthesis. By the way, in the inorganic substance that constitutes the present optical material 1, the electron wave function is likely to spread throughout the crystal, and its excitons are hydrogen atom-like Wannier-type enough to include a plurality of atoms. In matter, the molecules are mainly bound by weak van der Waals forces, and their excitons are considered to be the Frenkel-type with no spread. For this reason, when the organic layer 4 is photoexcited, its excitons can confine its motion by the barrier of the inorganic layer 3, but since the exciton size is in the molecular size order, it is easy to produce a remarkable effect. is not.

However, as in the present optical material 1, CuPc /
In the case of a TiO _x superlattice, the inorganic layer 3 does not serve as a barrier against excited electrons in the organic layer 4, but it affects the Bohr radius of excitons. Therefore, by using the organic layer 4 having the above-mentioned thickness and appropriately changing the type (for example, a material having a different dielectric constant) or the thickness of the inorganic layer 3 sandwiching the organic layer 4, the excited electronic state of the molecule can be obtained. Will be able to control artificially. Since the Bohr radius is proportional to the value of the dielectric constant, it is effective to use the inorganic layer 3 having a large dielectric constant. In particular, the organic layer 4 is made to have a size of at most 2 molecules and the inorganic layer 3 is made of Ti.
When a material such as O _x having a larger electron affinity and energy gap and a larger dielectric constant than the organic layer 4 is used, the excited electrons of the inorganic layer 3 are easily exuded and a remarkable effect as an optical material can be obtained. it can.

Although copper phthalocyanine CuPc is used as the organic layer 4 and TiO _x is used as the inorganic layer 3 in the above-mentioned embodiment, the present invention is not limited to this.

[0024]

As described above, according to the present invention, the excited state of the molecule can be changed depending on the type and thickness of the inorganic layers sandwiching the organic layer. can do. Also, since the organic layer with low mechanical strength is sandwiched by the inorganic layer with high mechanical strength,
It becomes a highly practical optical material with improved mechanical strength.

[Brief description of drawings]

FIG. 1 is a cross-sectional view illustrating an optical material according to the present invention.

FIG. 2 is a characteristic diagram showing the dependence of exciton energy (E _s1 ) on the thickness of a CuPc layer.

FIG. 3 is a characteristic diagram showing the dependence of exciton energy (E _s1 ) on the thickness of TiO _x .

[Explanation of symbols]

 1 ... Optical material 2 ... Substrate 3 ... Inorganic layer 4 ... Organic layer 5 ... Heterostructure 6 ... Interface

Claims (5)

[Claims] 1. A material laminated so as to include at least one set of a heterostructure in which an organic layer made of an organic substance is sandwiched between two inorganic layers made of an inorganic substance. An optical material having a band gap larger than those of an organic layer and a thickness of the organic layer being at most 2 molecule size or less. 2. The thickness of the organic layer is defined as a thickness when the energy value of an absorption peak in a predetermined band is measured with the thickness as a variable, and the organic layer is sandwiched between the organic layers. The optical material according to claim 1, wherein the thickness and the material of the two inorganic layers are appropriately adjusted so as to have a required energy value. 3. The optical material according to claim 2, wherein the material appropriately adjusted is a material having a different dielectric constant. 4. The optical material according to claim 1, wherein the organic layer is phthalocyanine having a thickness of 2.6 nm or less, and the inorganic layer is made of TiO _x . 5. The TiO _x of which the inorganic layer is amorphous, wherein x = about 1.5.
The optical material described.