Process for production of layer element containing at least one monomolecular layer of an amphiphilic molecule and one fullerene

A layer element carries on a base at least one regularly arranged monomolecular layer containing a fullerene and a polymeric amphiphilic compound, in particular a polymethacrylic ester derivative. It is suitable for purposes of nonlinear optics.

BACKGROUND OF THE INVENTION 
The present invention relates to a layer element containing at least one 
regularly arranged monomolecular layer of an amphiphilic molecule and one 
fullerene and to a process for the production of this layer element using 
the transfer method of Langmuir-Blodgett. This method is described e.g. in 
European Patent Application 0 432 619 and in U.S. Pat. No. 4,772,856. 
It is already known that C.sub.60 -fullerene forms rigid films on water/air 
interfaces. It is not possible to transfer these films, as is customary in 
the Langmuir-Blodgett method, to a perpendicularly immersed substrate. It 
is likely that at least in part multilayers are formed on the water 
surface. The transfer characteristics of fullerene LB films is improved by 
mixing the fullerene with the amphiphilic compound icosanoic acid 
(Takayoshi Nakamura et al, Langmuir 1992, Vol. 8, No. 1). Unfortunately, 
according to in-house tests with omegatricosenoic acid, the amount of 
fullerene which can be incorporated in the monomolecular layer is limited 
to about 50% by weight, and the thermal stability of the films thus 
produced is low. Owing to their strongly delocalized electron systems, 
fullerenes exhibit nonlinear optical properties similar to those of 
conjugated polymers. They have high .chi.(3) values which makes them 
ideally suitable for apparatuses for optical frequency tripling. For 
fullerene-containing LB layers, this is true all the more, the higher the 
fullerene content. A certain thermal minimum stability of the layers is 
necessary if they are to be used more widely. 
Accordingly, the object was to describe a process which allows the 
fullerene content in an LB layer to increase and the thermal stability of 
such layer systems to improve. 
SUMMARY OF THE INVENTION 
A process for the production of a layer element has now been found in which 
a mixture of a fullerene and an amphiphilic compound is spread on a water 
surface and transferred to a base by the Langmuir-Blodgett method. In this 
process, the amphiphilic compound used is a polymeric compound, for 
example polyglutamate and trimethylsilylcellulose. The amphiphilic 
polymethacrylic acid derivative used is preferably derived from a 
monomeric methacrylic ester of the formula I 
##STR1## 
in which k is an integer from 0 to 10, 
R.sup.2 is a C.sub.1 -C.sub.24 -alkyl group and 
R.sup.3 is a C.sub.8 -C.sub.24 -alkyl group with the proviso that the 
groups 
R.sup.2 and R.sup.3 contain a different number of carbon atoms. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The mixture of fullerene and amphiphilic polymers used preferably contains 
at least 10% by weight, in particular at least 40% by weight, of 
fullerene. In the case of polymers derived from monomers of the formula I, 
preferably 55-70% by weight of fullerene is used for preparing the mixture 
of amphiphilic polymer and fullerene, so that the monomolecular layers of 
the layer element also contain at least in part 55-70% by weight of 
fullerene (balance of polymeric amphiphilic compound). The fullerene used 
can be the compound C.sub.60 and/or the compound C.sub.70. 
The invention furthermore relates to a layer element containing at least 
one regularly arranged monomolecular layer of a fullerene-containing 
amphiphilic molecule on a free base for layer elements. The composition of 
a monomolecular layer of the layer element is the same as the composition 
of the mixture of fullerene and polymeric amphiphilic compounds used for 
its production. The specific feature of the layer element according to the 
invention is that the amphiphilic molecule is a polymeric compound, in 
particular a polymethacrylic ester derivative, preferably a polymer formed 
by polymerization of I. If its fullerene content is at least 50%, the 
layer element is suitable for optical frequency-tripling. Increasing 
fullerene concentration results in a more than linear increase of 
.chi.(3). A high .chi.(3) is desirable for optical applications. Suitable 
layer elements preferably receive at least two fullerene-containing 
monomolecular layers. Preference is given to at least 50 
fullerene-containing layers, preferably to 100-200 of such layers. 
Suitable bases are any desired solid, preferably dimensionally stable, 
substrates made of various materials. The substrates serving as bases can, 
for example, be trsnsparent or not transparent, electrically conducting or 
insulating. The substrate can be hydrophobic or hydrophilic. The surface 
of the substrate to which the LB layer is applied can have been made 
water-repellent. The substrate surface to be coated should be as pure as 
possible so as not to interfere in the formation of a thin, ordered layer. 
In particular, the presence of surface-active substances on the substrate 
surface to be coated can impair the layer production. Before applying the 
LB films, it is possible to provide the substrate serving as base on the 
surface to be coated first with an intermediate layer in order to improve, 
for example, adhesion of the film to the substrate. 
Examples of materials which can be used for the substrate are metals such 
as gold, platinum, nickel, palladium, aluminum, chromium, niobium, 
tantalum, titanium, steel, and the like. Other suitable materials for the 
substrates are plastics, such as, for example, polyester, for example 
polyethylene terephthalate or polybutylene terephthalate, polyvinyl 
chloride, polyvinylidene chloride, polytetrafluoroethylene, polyethylene 
or polypropylene. 
Likewise, semiconductors, such as silicon or germanium, glass, silicon 
dioxide, ceramic materials or cellulose products are also suitable 
materials for the substrates. 
If necessary, the surface of glass and other hydrophilic substrates can be 
made water-repellent in a manner known per se, for example by reaction 
with alkylsilanes or hexamethyldisilazane. The selection of the substrate 
materials depends primarily on the intended use of the layer elements 
produced from the films according to the invention. For optical elements, 
it is customary to use transparent, light-transmitting substrates as the 
base. If the layer elements according to the invention are used, for 
example, in electrical engineering or in electrochemical processes, 
materials which serve as substrates are in particular electrically 
conductive materials, such as metals or metallic films, for example on 
plastic sheets. 
Depending on the intended use, the substrates which serve as bases for the 
films according to the invention can have any desired form. For example, 
they can be in the form of films, sheets, plates, tapes or else in the 
form of cylinders or can be selected from any other desired forms. In 
general, their bases will be flat, planar substrates, such as, for 
example, films, sheets, plates, tapes, and the like. The substrate surface 
to be coated is preferably smooth, such as is customary for the production 
of LB films. In the case of flat, planar substrates, the films according 
to the invention can be applied to one or both substrate surfaces. 
A characteristic feature of the Langmuir-Blodgett method is that 
water-insoluble molecules (predominantly amphiphilic monomer/polymer 
substances) are spread from a solution (concentration: 1 mg/ml) on the 
water surface. The solvent evaporates; an ordered two-dimensional film is 
built up by compression. Successive immersion of the substrate (e.g. 
silicon) through the water/air gap interface makes it possible to build up 
multilayers of defined thickness with a thickness increment of about 2 to 
6 nm. 
In particular, a mixture of the fullerene and the amphiphilic compound is 
dissolved in a volatile organic solvent to form a solution, the solution 
is spread on the water surface, the organic solvent is vaporized, whereby 
a film remains on the water surface, the film is compressed and 
transferred onto the solid base by immersing into the water and/or 
withdrawing from the water the solid base through the water surface 
containing the film. 
Compared with the conventional low-molecular-weight amphiphilic compounds, 
such as omega-tricosenoic acid or arachidic acid, the polymeric 
amphiphilic compounds used lead to films having improved transferability, 
lower defect density and high thermal stability. 
The fullerene/polymer mixed layers can be transferred to a substrate in 
particular by means by the Y coating customary in the LB method. 
If the layer elements according to the invention contain a 
light-transmitting base, they are suitable as optical limiters, since the 
light transmission in the fullerene layers decreases with increasing light 
current. 
If the layer elements according to the invention comprise an electrically 
conductive base (for example an ITO layer vapor-deposited on a base), they 
can be used for the production of light-emitting diodes. After mounting a 
cover electrode on the uppermost layer and applying a voltage between base 
and electrode, the fullerene-containing layers can be made to emit 
electroluminescent radiation.

EXAMPLE 1 
First, the monomer of the formula I where k is 0 and in which R.sup.2 was a 
n-octadecyl group and R.sup.3 an ethyl group was synthesized. Polymer (II) 
was obtained therefrom by homopolymerization. The formula of II is shown 
below. 
FIG. 1 shows the isotherms of mixtures of II with fullerenes (75% by weight 
of C.sub.60, 25% by weight of C.sub.70) in a II-fullerene weight ratio of 
3:1 ("2") and 1:1 ("3"). Isotherm 1 is that of the pure compound II, and 
isotherm 4 is that of pure fullerene. The shape of the isotherms clearly 
shows the increasing influence of polymer (II) with simultaneously 
decreasing fullerene content. 
EXAMPLE 2 
FIG. 2 shows the absorption spectra of a mixed film of the transferred 
layer comprising 50% by weight of polymer II and 50% by weight of 
fullerene for various layer thicknesses. The layer thicknesses are 40 nm, 
100 nm and 200 nm, which corresponds to 20, 50 and 100 monolayers. 
The increase in absorption with increasing thickness is a measure of the 
coating quality of the multilayer films. There is good fulfillment of the 
linear relationship between the absorption of all three bands in the UV at 
340, 265 and 220 nm and the film thickness. This shows that coating can be 
carried out reproducibly and the microscopic environment of the fullerene 
molecules in the LB film remains unchanged with an increasing number of 
monolayers. 
FIG. 3 shows the absorption spectra of a mixed film containing 50 
monolayers each for various fullerene concentrations. 
There is a more than linear increase in absorption with increase in 
fullerene content. This is due to an increase in monolayer thickness with 
increasing fullerene content. FIG. 5 shows the increase in layer thickness 
as a function of fullerene concentration determined from measurements of 
small-angle X-ray scattering. 
EXAMPLE 3 
Frequency-tripling measurements were carried out on samples having a 
fullerene content in the transferred layer of 0, 10, 30, 50 and 70% by 
weight. The balance was composed of polymer II. The fullerene mixture from 
Example 1 was used. In centrosymmetric materials, such as 
buckminsterfullerene, induced polymerization is described by 
EQU p=.epsilon..sub.0 .chi..sup.(3) E.sup.3 
in which .chi..sup.(3) is the third-order nonlinear susceptibility, E is 
the electric field applied of the laser pulse and .epsilon..sub.0 is the 
electric field constant. The experimental set up of an apparatus for 
determining third-order nonlinear susceptibility is known to one skilled 
in the art, for example from P. N. Prasad, D. J. Williams, Introduction to 
Nonlinear Optical Effects in Molecules and Polymers, John Wiley, 1991, p. 
202. 
The harmonic signal was recorded in the transmission mode as a function of 
the angle of rotation of the sample relative to the beam axis. The 
parallel reference branch serves for compensating the fluctuations between 
pulses of the laser intensity. Scaling of the harmonic intensity is 
effected by measuring the quartz glass substrate without film. 
For an exact determination of the nonlinear optical susceptibilities, the 
refractive index at the fundamental and harmonic frequency must be known. 
The layer thickness was determined by small-angle X-ray scattering. It 
increases with increasing fullerene content (cf. FIG. 5). Since the 
refractive indices at 1,064 nm and 154 nm are difficult to determine 
directly, a Kramers-Kronig analysis was carried out. This method allows 
the refractive index to be ascertained from the absorption spectrum as 
long as the refractive index can be determined at a frequency, for example 
ellipsometrically at 633 nm (balance: Polymer II). FIG. 4 shows the 
dependence of the .sub..chi..sup.(3) coefficient from three-photon 
resonance on the fullerene content. 
The .sub..chi..sup.(3) coefficients have an error of about 10% and are, 
within the margin of error, on a straight line in agreement with the shape 
of curve expected by theory. Upon extrapolation to 100% by weight, 
2.times.10.sup.-10 esu are obtained for the .sub..chi..sup.(3) value of 
the pure fullerene layer. 
##STR2##