Note: Descriptions are shown in the official language in which they were submitted.
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~OECHST AKT~ENGESELLSC~AFT ~OE 90/F 208 Dr. DS/PL
~escription
Process for the preparation of inorqani~ ~ cro~tructure~
from Langmuir-Blodgett films
Coated supports are gaining increasing importance in
industrial technology. Thus, for example, optical wave-
guide systems or filters for optical purposes are coated
with thin films, which, owing to their low critical
surface tension, are also suitable for improving the
friction properties of these materials, for the prepar
ation of protective layers and for further relPvant
applications. ~he preparation of monomolecular layers and
the construction of system~ composed of monomolecular
layers of inorganic metals or metal ions i5 important for
studying surface reaction~, such a~ catalysis or corro-
sion, but in particular also for ~he construction of
magnetic information stores and for optical applications.
The most simple method seems to be to start with mono-
molecular layer3 as subunits for the construction of
complex systems. These layers are obtained by transer of
monomolecular films from a liquid surface to a suitable
solid support. On a water ~urface, films of this type,
for example composed of molecule~ of a fatty acid having
20 carbon atoms (arachidic acid, C~3-(CH2)18-COO~), can be
easily formed and tightly packed. To thi~ end, a solution
of the organic acid i~ added dropwise to a water ~urface
which i~ limited by rigid barriers and a movable float.
The solvent evaporateq, and the remaining molecule~ are
compressed and tightly packed by a defined force acting
on the float. Since the molecule have a hydrophilic (for
example a carboxyl group) and a hydrophobic portion (for
example a hydrocarbon chain) and are very ~paringly
soluble in water, they are oriented at the water sur~ace.
The hydrophilic groups remain in the water, and the
hydrophobic chains point upwards.
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To transfer ("apply") a tightly packed ~angmuir-Blodgett
film from the water surface to a solid support, the
latter is immersed and again withdrawn. When it is
withdrawn, the hydrophilic group~ of the film adhere to
the hydrophilic 6upport surface, and the sheet i8 thus
covered with a monomolecular layer. In contrast, a
hydrophobic ~ubstrate i8 already coated during the
initial immersion. In each further dipping process
(immersion and withdrawal), two further monomolecular
layers are transferred. The interaction betw~en the
hydrophilic groupæ or hydrophobic end groups causes each
layer transferred to adhere to the previous one~
For years, Langmuir-Blodgett (LB) films were only con-
sidered to be of interest because of their monomolecular
layer structures. In 1967 Kuhn (Kuhn, Naturwissenschaften
1967, 54, 429) caused a ~ensation with his new strategy
of arranging multilayers in which "clever" molecules were
disposed in an inert matrix of fatty acid molecules. This
was the starting point for a large num~er of functionally
designed LB films. Polymerization, polycondensation,
redox reactions and in-situ synthe~es in this ordered
state were developed (~uaudel-Teixier, Rosilio, Barraud,
Thin Solid Films, 1980, 68, 7). In the organized solid
state, reactivity i8 closely related to structure, 80
that it i8 of great interest to prepare films of this
type selectively for the desired areas of application.
Molecular arrangements of this type have new interesting
properties, which in some cases differ con0iderably from
those of the starting ~ubstances.
The heat treatment of Langmuir-Blodgett films on the
water surface in order to prepare high-quality calcium
arachidate layers i8 know~ (Xato, Oh~hi~a, Suzuki, Thin
Solid Films, 178, 37-~5). In this procedure, insoluble
monolayeræ are compres3ed on the air/water interface,
subjected to a heat treatment and then applied to a
support. This is ~uppo~ed to lead to virtually defect-
free calcium arachidate LB films.
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The heat treatment of layers composed of various metals
has been described many times in the li erature. In this
process, thin films composed of metals or inorganic
compounds (for example metals of rare earths, gallium
arsenides) are applied to a support, for example by vapor
deposition, and then subjected to a heat treatment,
resulting in the formation of a variety of intermetallic
phases or othex formations. The temperatures appliPd in
these processes are between 200 and 700C. (J. Vac. Sci,
Technol. A, 7 (5), 3016-3022; Mater. Res. Soc. Symp.
Proc. 144, 526-530, CA 112 (14): 129936 v; J. Les~-Common
Met. 151, 263-9; CA ~ 111 (12)s 105999 w).
However, these processes do not en~ure the preparation of
layer~ having a defined molecular structure. Therefore,
there was still the object of finding a process which
enables metal films having a defined film thickness in
the order of magnitude of molecules to be prepared
selectively, in which the metal, depending on the parti-
cular application, can be present in oxide or atomic form
on the support. The present process achieves this ob~ect.
It is based on the finding that in multilayers composed
of salts of organic acids the organic component can be
selectively desorbed from the individual layers by a heat
treatment, as a rasult of which the inorganic component
remains on the support.
The process according to the i~vention makes it po~sible
to produce inorganic microstructure~ composed of
Langmuir-Blodgett films by transferring one or more
monomolecular layers of salts of organic acid~ to a
support while maintaining their order and then de~orbing
the organic component by heating, as a result of which
the metal ion remains on the support surface. The ad-
jacent layers applied to the support can, as desired, be
composed of identical or different salts.
The proce~s according to the invention comprises in
principle three steps:
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a) construction of a layered structure
b) heat treatment of the multilayer
c) optional aftertreatment of the multilayer.
The construction of a layered structure comprising
several monomolecular layers can be effected by the
Langmuir-Blodgett method in a very simple manner. Ti~htly
packed mono- or multilayers can be prepared by spreading
salts of organic acids on a water surface or else by
spreading organic acids on a ~ubphase in which metal ions
are dissolved (Thin Solid Films, 146 (1987) L15-L17),
compression of this layer, followed by tran~fer to a
support. ~he organic component of the salt aan be, for
example, an arachidic acid, stearic acid, palmitic acid
or other fatty acid radical, or another amphiphilic acid,
for example a charged phospholipid. Inorganic components
which can be used are in general all metals; preferably
all transition metals, in particular iron, cobalt,
cadmium, chromium, aopper, nickel, manganese, platinum,
silver, gold and rhodium, but also sodium, potas~ium,
calcium, strontium and barium. Furthermore the starting
materials used can also be polymer salts ~he layers
suitable for the process accordinq to the invention can
be prepared by adsorption processes or by the Langmuir-
Blodgett method. The term "~angmuir-Blodgett films" is in
general understood in this context to mean thin, ideally
monomolecular, layers or multilayers of defined struc-
ture.
The layered structures should be such that the di~tribu-
tion of metal ions and organic componenta alternates
along the surface normal. To this end, a monolayer of an
organic acid is spread on a water surface. ~he aqueous
subphase has a p~ of more than 5, preferably in the range
from 5 to 9, as a result of which a proton di~sociate~
from the organic acid and the metal ions are transferred
~5 to the subphase by addition of an inorganic ~ lt ~for
example chloride, bromide, iodide, nitrate, and the
like). When a divalent metal i8 incorporated, ltS
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concentration in the subpha~e is preferably 10-4 to 10-3 M
and in the case of trivalent or higher valent ions the
concentration can be many time~ les~, while in the ca~e
of monovalent metals it ~hould be more than 10-4, prefer-
ably more than 10-2 M. The ~alt can be tran~ferred by the
LB method to a support whi~h can be made, for example, of
glass or silicon, multilayers in which the layers com
pxiqing aliphatic chains are intercalated by a metal
layer being produced by repeated immersion and with-
drawal. By changing the ~ubphase between the individual
dipping processes, it is also possible to con~truct
intercalating layers compri~ing different metal ions. In
this manner, the process according to the invention makes
it possible to produce microstructures composed of
different metals or metal ion~.
~he multilayer can be melted or sublimed by subsequent
heat treatment. Selection of a suitable temperature
allows desorption of the organic component (at 100 to
350C), while the inorganic ~omponent remains on the
support surface. The temperatures to be used in each case
are dependent on the corresponding ion and counterion.
The ~ollowing two mechanisms of thermodesorption of
multilayers have been disclosed to date:
-a) When multilayerq are heated to 100-160C, fir~t
droplets are formedO When the temperature is further
increased, the surface breakæ up more and more with
further formation of droplets, the organic component
is desorbed and what remains are cluster~ of the
corre~ponding metal or the corresponding metal ions.
(Example 1 )
-b) When the multilayer i3 heated, the layer doe~ not
break up and no fo~mation o~ droplet~ takes place.
The organic component is desorbed and the metal or
the metal ion remain on the support as a coherent
layer. ~Example 1 ~.
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Thus, depending on the selection of the inorganic and
organic components, the process according to the inven-
tion makes it possible to produce micro~truc~ures of
different design. Using the example of desorption of iron
stearate, it has bee~ shown that plate-like olids linked
to one another and containing iron(III) which differ from
those formed b~ thermodesorption of cadmium arachidate
can be formed.
The diameter of the cadmium clusters i~ usu~lly in the
range from 0~5 to 2 ~m at a spacing of 3 to 5 ~m
(Example 2). The clusters formed are in this case not
linked to one another.
The distribution of ~he microstructures can be controlled
by adjusting preparation parameters such as desorption
kinetics, nucleation or layer thickness.
The thermodesorption temperatures used in each ca-~e
depend on the organic and inorganic components used. The
bond between metal and acid radical mu~t not be too weak
nor too strong in order to en~ure desorption at suffi-
ciently low temperatures and thus the microstructureformation of the metal ions. The advantage of the process
according to the invention is in particular that rela-
tively mild temperatures are used. The microstructures
themselves which predominantly contain the metal com-
ponent but hardly any carbon are only de~orbed attemperatures above 350C or even higher, depending on the
metal ion used.
By treating the layers thus formed with hot hydrogen
(T = 500C), it is possible to change the oxidation
states of the metal ions. U~ing the exzmple of iron
stearate, it could be shown by Mo~bauer spectroscopy that
after thermodesorption of the stearate radical first
iron(III) ions are present which are reduced by the
hydrogen t~eatment, so that afterwards the iron atom~ are
also present in the oxidation state "0". By varying the
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treatment time and temperature, it is pos~ible to control
the reduction ratio. (Working Example 3).
The process according to the invention makes it possible
to construct inorganic microstructures at relatively low
temperatures, which have a high degree of order and can
additionally be structured laterally by means o~ photo-
lithography. Possible aspects of economic interest are in
particular in the area of applications in optics, data
storage, catalyst technology but al80 in magnetometry.
Working examples
Example 1:
XPS (X-ray-induced photoelectron spectroscopy) measure-
ments on transferred ~B films before and after thermo-
desorption provided information on the composition of the
lS chemical elements of the layers.
The measurements were carried out on a cadmium arachidate
(Figure la) and an iron(III) stearate (Figure lb) multi-
layer. Coating was carried out by the con~entional ~B
method; the support used was hydrophilic silicon having
a natural silicon dioxide layer of about 20-50 A.
The cadmium arachidate film comprising 7 monolayers was
applied after spreading arachidic acid onto Millipore
water to~ether with 10 3 M CdCl2 at a pH of 7 ~6 mg of
NaHCO3/l of water) at 20C~ followed by compres3ion at a
constant pressure of 30 mN/m. The iron(IIX) stearate
multilayer (17 monolayers) was formed from stearic acid
on a subphase containing 4x10-5 M FeCl3, pH 5.5, 40C and
under a pressure of 30 mN~m. In both cases, the xate of
application was 10 mm/min.
TheXPSmeasurementsbeforethermodesorption (Figure la,b)
showed with both samples tha~ ~he organic componen~ (high
C signal) and the inorganic metal ion (high cadmium
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or iron intensity) had been transferred to the substrate.
The measurements after thermodesorption ~Figure la, b,
the samples were heated to 250C for 30 minute~) show
that the organic component i8 almost completely de~orbed
(considerably lower C intensity), the metal ion however
remains on the support tdistinct Cd, Fe signals). In the
sample containing cadmium, the support signal (Si) i8
clearly visible after de60rption (Figure la). This can
only be explained by the fact that large portions of the
~upport are uncovered and the cadmium ions are present on
the support in the form of clusters which are not linked
toqether. In the iron sample, the support signal is
hardly visible, since the silicon support is covered by
a coherent iron layer (probably in the form of iron
oxide), making it impossible ~o detect any electrons
having the binding energy of silicon.
Bxampl~ 2:
The melting of a cadmium arachidate layer (7 monolayers
on silicon, preparation conditions as in Example 1) was
monitored under a ~omarski microscope. The breakup of the
layer and the subsequent droplet formation accompanied by
desorption could be observed. At a heating rate of
0.7 K/s, followed by cooling to room temperature, the
brig~t cadmium clusters (Figure 2) having a diameter of
0.5 to ~ ~m can be seen.
In the iron stearate samples neither droplet nor cluster
formation was observed.
Fxample 3:
In order to determine the oxidation state of the iron
ions, the thermode~orbed iron multilayers were
investigated by conversion electxon Mo~bauer spectro~
scopy. Figure 3 shows such a measurement. The ~ample
(15 monolayer~) was prepared as described in Example 1,
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except that up to 95 % of enriched Fe57(III) chloride was
u~ed. Desorption was carried ou~ at 510C under a hydro-
gen atmosphere (1 hour). This gave a proportion of 36 %
of Fe3+, 24 % of FeZ~ ion~ and 40 % of Fe ~metallic iron).
Figure l:
XPS spectra (intensity plotted versus bonding energy) of
a silicon support, originally coated with
a) 7 monolayers of cadmium arachidate,
b) 17 monolayers of iron(III) stearate
before (25C) and after thermodesorption (heated at 250C
for 30 minutes and then cooled to room temperature).
Figure 2:
Photograph under a Nomarski microscope of a film compris-
ing 7 monolayers of cadmium arachidate on silicon, heated
at a rate of 0.7 K/s up to 350C and then cooled to xoom
temperature.
Figure 3:
Conversion electron Ma~bauer spectrum of a multilayer
originally comprising 15 monolayer~ of iron(III) stear-
ate, followed by thermodesorption (1 hour) at 510C undera hydrogen atmosphere.
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