U.S. patent application number 12/675285 was filed with the patent office on 2010-08-19 for graphite layers.
This patent application is currently assigned to UNIVERSITAT BIELEFELD. Invention is credited to Armin Golzhauser, Andrey Turchanin.
Application Number | 20100209330 12/675285 |
Document ID | / |
Family ID | 39988102 |
Filed Date | 2010-08-19 |
United States Patent
Application |
20100209330 |
Kind Code |
A1 |
Golzhauser; Armin ; et
al. |
August 19, 2010 |
Graphite Layers
Abstract
The present invention relates to a method for preparing graphite
layers, comprising the step of heating at least one monolayer with
low-molecular weight aromatics and/or low-molecular weight
heteroaromatics crosslinked in the lateral direction under vacuum
or inert gas to a temperature of >800 K, and to graphite layers
which are obtainable by this method.
Inventors: |
Golzhauser; Armin;
(Bielefeld, DE) ; Turchanin; Andrey; (Bielefeld,
DE) |
Correspondence
Address: |
MCDONNELL BOEHNEN HULBERT & BERGHOFF LLP
300 S. WACKER DRIVE, 32ND FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
UNIVERSITAT BIELEFELD
Bielefeld
DE
|
Family ID: |
39988102 |
Appl. No.: |
12/675285 |
Filed: |
September 3, 2008 |
PCT Filed: |
September 3, 2008 |
PCT NO: |
PCT/EP08/07203 |
371 Date: |
February 25, 2010 |
Current U.S.
Class: |
423/448 |
Current CPC
Class: |
C01B 2204/02 20130101;
B82Y 40/00 20130101; C01B 2204/04 20130101; C01B 32/21 20170801;
B82Y 30/00 20130101; C01B 32/184 20170801 |
Class at
Publication: |
423/448 |
International
Class: |
C01B 31/04 20060101
C01B031/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 3, 2007 |
DE |
102007041820.7 |
Claims
1. A method for preparing graphite layers, comprising the step of
heating at least one monolayer with low-molecular weight aromatics
and/or low-molecular weight heteroaromatics, which are crosslinked
in the lateral direction, under vacuum or inert gas to a
temperature of greater than 800 K.
2. The method according to claim 1, wherein the monolayer is
composed of aromatics selected from the group consisting of phenyl,
biphenyl, terphenyl, naphthalene and anthracene, and/or of
heteroaromatics selected from the group consisting of bipyridine,
terpyridine, thiophene, bithienyl, terthienyl and pyrrole.
3. The method according to claim 1, wherein the low-molecular
weight aromatics and/or low-molecular weight heteroaromatics have
anchor groups.
4. The method according to claim 3, wherein the anchor groups are
selected from the group consisting of carboxy, thio,
trichlorosilyl, trialkoxysilyl, phosphonate, hydroxamic acid and
phosphate groups.
5. The method according to claim 3, wherein the anchor groups are
covalently bonded to the laterally crosslinked monolayer composed
of low-molecular weight aromatics and/or low-molecular weight
heteroaromatics by means of a spacer with a length of 1 to 10
methylene groups.
6. The method according to claim 1, wherein the laterally
crosslinked monolayer is physisorbed or chemisorbed on a substrate,
or is present in a free-standing manner.
7. The method according to claim 6, wherein the substrate has
recesses in some areas.
8. The method according to claim 6, wherein the substrate is
selected from the group consisting of gold, silver, titanium,
zirconium, vanadium, chromium, manganese, tungsten, molybdenum,
platinum, aluminium, iron, steel, silicon, germanium, indium
phosphide, gallium arsenide and oxides or alloys or mixtures
thereof, as well as graphite, indium tin oxide (ITO) and silicate
or borate glasses.
9. The method according to claim 1, wherein the monolayer composed
of low-molecular weight aromatics and/or low-molecular weight
heteroaromatics carries functional groups on its surface, the
groups being selected from halogen atoms, carboxy, trifluoromethyl,
amino, nitro, cyano, thiol, hydroxy or carbonyl groups.
10. The method according to claim 1, wherein the monolayer is
composed of biphenyl units, and the anchor groups of thio
groups.
11. The method according to claim 1, wherein the monolayer has a
layer thickness in the range from 0.3 nm to 3 nm.
12. The method according to claim 1, wherein heating is carried out
under vacuum in a pressure range from 10.sup.-7 mbar to 10.sup.-12
mbar.
13. The method according to claim 1, wherein heating is carried out
under inert gas.
14. The method according to claim 1, wherein heating of the
laterally crosslinked monolayer is carried out at a temperature of
greater than 1600 K.
15. A graphite layer obtainable by a method according to claim 1.
Description
[0001] The present invention relates to a method for preparing
graphite layers, comprising the step of heating at least one
monolayer with low-molecular weight aromatics and/or low-molecular
weight heteroaromatics, which are crosslinked in the lateral
direction, under vacuum or inert gas to a temperature of >800 K,
and to graphite layers obtainable by this method.
[0002] Thin graphite layers are known in the art and are either
obtained by peeling from a high-purity graphite crystal, by
pyrolysis of silicon carbide, by heating C.sub.2H.sub.6 to Pt(111),
or from graphene oxides. However, these methods for preparing
graphite layers are subject to severe limitations and can only be
applied to a limited extent.
[0003] Thus, it is the object of the present invention to provide a
novel method for preparing ultra-thin graphite layers, which also
is to enable a targeted, lateral structuring of a substrate surface
in the nanometer range. In particular, the method serves to enable
the preparation of structures of graphite layers in the nanometer
range on different substrates.
[0004] This object is solved by the embodiments characterized in
the claims.
[0005] In particular, a method for preparing graphite layers is
provided, comprising the step of heating at least one monolayer
with low-molecular weight aromatics and/or low-molecular weight
heteroaromatics, which are crosslinked in the lateral direction,
under vacuum or inert gas to a temperature of >800 K.
[0006] According to the present invention, the term "graphite
layer" means an electrically conductive layer largely composed of
carbon, which consists of several atomic layers, preferably 1 to 3
atomic layers, and may optionally have dopants. Preferably, the
graphite layer according to the present invention is a layer
composed of carbon, which consists of 1 to 3 atomic layers and may
as well be referred to as graphene layer. The layer thickness of
the graphite layer according to the present invention is preferably
below 2 nm.
[0007] The monolayer with low-molecular weight aromatics and/or
low-molecular weight heteroaromatics crosslinked in the lateral
direction, or laterally crosslinked monolayer, can be prepared by
crosslinking low-molecular weight aromatics and/or low-molecular
weight heteroaromatics, which preferably have anchor groups.
Preferably, the laterally crosslinked monolayer is prepared by
treating a (non-crosslinked) monolayer of low-molecular weight
aromatics and/or low-molecular weight heteroaromatics, which
preferably have anchor groups, with high-energy radiation.
[0008] In the method according to the present invention, the
monolayer, which can in particular be crosslinked by being treated
with high-energy radiation, is preferably composed of aromatics
selected from the group consisting of phenyl, biphenyl, terphenyl,
naphthalene and anthracene, and/or of heteroaromatics selected from
the group consisting of bipyridine, terpyridine, thiophene,
bithienyl, terthienyl and pyrrole.
[0009] Crosslinking of the monolayer in the lateral direction
preferably is carried out with high-energy radiation. In
particular, crosslinking of the monolayer in the lateral direction
can be achieved by treatment with electron radiation, plasma
radiation, X-ray radiation, .beta.-radiation, .gamma.-radiation,
VUV radiation, EUV radiation or UV radiation.
[0010] The low-molecular weight aromatics and/or low-molecular
weight heteroaromatics preferably have anchor groups. If the
low-molecular weight aromatics and/or low-molecular weight
heteroaromatics have anchor groups, the monolayer of low-molecular
weight aromatics and/or low-molecular weight heteroaromatics can be
bonded to a plurality of substrates as a monolayer in a simple
manner. Bonding of the crosslinked monolayer to a substrate can be
achieved by physisorption (i.e. with a bond energy of approx. 0.5
eV/atom or <41.9 kJ/mol) or by chemisorption (i.e. with a bond
energy greater than 0.5 eV/atom or .ltoreq.41.9 kJ/mol), for
example by forming covalent bonds. The anchor groups can be
selected from the group consisting of carboxy, thio,
trichlorosilyl, trialkoxysilyl, phosphonate, hydroxamic acid and
phosphate groups. The anchor groups can be covalently bonded to the
monolayer composed of low-molecular weight aromatics and/or
low-molecular weight heteroaromatics, which are crosslinked in the
lateral direction, by means of a spacer with a length of 1 to 10
methylene groups.
[0011] The skilled person is capable of suitably tailoring the
nature of the anchor group to the respective desired substrate
material. For example, trichlorosilane or trialkoxysilane, such as
trimethoxysilane, triethoxysilane, etc., are particularly suitable
as anchor groups for oxidized silicon surfaces. Alcohol groups can
be used for anchoring for hydrogenated silicon surfaces. For gold
and silver surfaces, thio groups are possible anchor groups, and
for oxidized metal surfaces such as iron or chromium, phosphonic
acids, carboxylic acids, or hydroxamic acids are suitable.
[0012] The laterally crosslinked monolayer, which can be prepared
by treating a monolayer composed of low-molecular weight aromatics
and/or low-molecular weight heteroaromatics with high-energy
radiation, can be chemisorbed or physisorbed on a substrate, or be
free-standing or not bonded to a substrate surface.
[0013] A laterally crosslinked monolayer, which is chemisorbed or
covalently bonded on a substrate, can be prepared by applying a
monolayer composed of low-molecular weight aromatics and/or
low-molecular weight heteroaromatics, which preferably have anchor
groups, to a substrate, and by being treated with high-energy
radiation.
[0014] If desired, the laterally crosslinked monolayer can be
transferred to another substrate by means of a suitable transfer
medium. For example, a laterally crosslinked monolayer applied to a
gold substrate, silicon substrate or silicon nitride substrate can
be transferred to another substrate, preferably a thermally stable
substrate, such as silicon oxide, aluminium oxide, glass, platinum,
iridium, tungsten or molybdenum, by means of a suitable transfer
medium.
[0015] A laterally crosslinked monolayer, which is free-standing or
not bonded to a substrate surface, can be prepared by applying a
monolayer composed of low-molecular weight aromatics and/or
low-molecular weight heteroaromatics, which preferably have anchor
groups, to a substrate, by being treated with high-energy radiation
and by cleaving the bonds between the crosslinked monolayer and a
substrate, preferably by cleaving a covalent bond between anchor
groups of the crosslinked monolayer and a substrate. A skilled
person is capable of selecting suitable conditions for cleaving the
bond between the crosslinked monolayer and the substrate. For
example, the bond between a crosslinked monolayer composed of
biphenylthiol and gold as a substrate can be cleaved by treatment
with iodine vapor.
[0016] In another embodiment, a laterally crosslinked monolayer,
which is free-standing or not bonded to a substrate surface, can be
prepared by applying a monolayer composed of low-molecular weight
aromatics and/or low-molecular weight heteroaromatics to a
sacrificial layer or intermediate layer on a substrate, by being
treated with high-energy radiation and by dissolving the
sacrificial layer or intermediate layer between the crosslinked
monolayer and a substrate. A skilled person is capable of selecting
suitable materials for such sacrificial layers disposed between the
crosslinked monolayer and the substrate. For example, a silicon
nitride layer as a sacrificial layer between a crosslinked
monolayer and a substrate, such as silicon, can be removed by
treatment with hydrofluoric acid.
[0017] Cleavage of the bonds between a laterally crosslinked
monolayer and a substrate and dissolving of a sacrificial layer or
intermediate layer between a laterally crosslinked monolayer and a
substrate can e.g. be carried out as in Advanced Materials 2005,
17, 2583-2587, but are not limited thereto. By means of these
techniques, it is e.g. possible to prepare a laterally crosslinked
monolayer, which is free-standing or not bonded to a substrate
surface, which monolayer can be used for the preparation of
graphite layers in the inventive step of heating at least one
monolayer with low-molecular weight aromatics and/or low-molecular
weight heteroaromatics, which are crosslinked in the lateral
direction, under vacuum or inert gas to a temperature of >800
K.
[0018] The substrate can have recesses in some areas. For example,
it is possible for the substrate to have holes, depressions or
grooves. The substrate may as well be lattice-shaped or have a
lattice-like shape. Preferably, the substrate is composed of a
thermally stable material, such as tungsten. A graphite layer
prepared according to the present invention can be supported on a
substrate or merely rest on a substrate, and overspan or cover
recesses in the substrate at least in some areas.
[0019] According to one embodiment of the present invention, it is
possible for the laterally crosslinked monolayer to be prepared by
crosslinking low-molecular weight aromatics and/or low-molecular
weight heteroaromatics, which preferably have anchor groups, on a
first substrate, such as gold, silicon or silicon nitride, to
subsequently transfer the laterally crosslinked monolayer to a
second, thermally stable substrate, such as silicon oxide,
aluminium oxide, glass, platinum, iridium, tungsten or molybdenum,
and to then heat it to a temperature of >800 K under vacuum or
inert gas on this thermally stable substrate.
[0020] However, it is also possible for the laterally crosslinked
monolayer to be prepared by crosslinking low-molecular weight
aromatics and/or low-molecular weight heteroaromatics, which
preferably have anchor groups, on a first substrate, such as gold,
silicon, silicon nitride, platinum or iridium, to then heat the
laterally crosslinked monolayer to a temperature of >800 K under
vacuum or inert gas on said substrate, and to subsequently transfer
it to a second substrate, such as silicon oxide or glass.
[0021] In a particularly preferred embodiment of the present
invention, the laterally crosslinked monolayer is prepared by
crosslinking low-molecular weight aromatics and/or low-molecular
weight heteroaromatics, which preferably have anchor groups, on a
substrate, such as silicon oxide, and the laterally crosslinked
monolayer is heated to a temperature of >800 K under vacuum or
inert gas on said substrate in order to form an electrically
conductive graphite layer on silicon oxide. In this way, it is e.g.
possible to prepare an electrically conductive layer on insulator
surfaces, such as glass. Such arrangements can be applied in
monitors and/or solar cells, for example.
[0022] The substrate can be selected from the group consisting of
gold, silver, titanium, zirconium, vanadium, chromium, manganese,
tungsten, molybdenum, platinum, aluminium, iron, steel, silicon,
germanium, indium phosphide, gallium arsenide and oxides or alloys
or mixtures thereof, as well as graphite, indium tin oxide (ITO)
and silicate or borate glasses.
[0023] In one embodiment of the present invention, the monolayer
composed of the low-molecular weight aromatics and/or low-molecular
weight heteroaromatics can carry functional groups on its surface,
the groups being selected from halogen atoms, carboxy,
trifluoromethyl, amino, nitro, cyano, thiol, hydroxy or carbonyl
groups. The low-molecular weight aromatic and/or low-molecular
weight heteroaromatic molecules or units, which make up the
monolayer, can be chemically coupled to an underlying substrate
surface or be covalently bonded thereto by means of an anchor
group.
[0024] In a particularly preferred embodiment of the present
invention, a monolayer composed of biphenyl units can be covalently
bonded to a corresponding substrate surface, in particular of gold
or silver, via thio groups as anchor groups.
[0025] The monolayer, which can be crosslinked by being treated
with high-energy radiation, has a thickness of only several atomic
layers, wherein a layer thickness in the range from 0.3 nm to 3 nm
is preferred.
[0026] If the surface of a substrate material is atomically flat
and homogeneous, i.e. it does not have any edge dislocation or
defects, then the graphite layer is also atomically flat,
homogeneous and free of defects and forms an almost ideally smooth
film on a substrate surface. The graphite layer adapts to the
morphology of the substrate. In this way, objects having
three-dimensional surface morphologies can be covered with a
graphite layer of defined thickness as well.
[0027] According to a preferred embodiment of the present
invention, the inventive method for preparing graphite layers
comprises the steps of: [0028] providing a substrate, [0029]
optionally modifying the surface of the substrate, [0030] applying
a monolayer of low-molecular weight aromatics and/or low-molecular
weight heteroaromatics to a surface of the substrate using covalent
bonding via anchor groups or physisorption, [0031] treating the
obtained substrate with high-energy radiation at least in some
areas such that the monolayer composed of low-molecular weight
aromatics and/or low-molecular weight heteroaromatics, which is
bonded to the surface of the substrate in a covalent manner or by
physisorption, is covalently crosslinked in the lateral direction
(at the areas treated with high-energy radiation), [0032]
optionally transferring the laterally crosslinked monolayer from
the above-described substrate to another, preferably thermally
stable substrate, and [0033] heating the laterally crosslinked
monolayer, which is prepared by treating the monolayer composed of
low-molecular weight aromatics and/or low-molecular weight
heteroaromatics with high-energy radiation at least in some areas,
under vacuum or inert gas to a temperature of >800 K.
[0034] The term "thermally stable substrate" means a substrate that
is stable in the step of heating the laterally crosslinked
monolayer under vacuum and that remains substantially unchanged. A
thermally stable substrate is stable at >800 K, preferably
>1000 K, particularly preferably >1600 K, and more preferably
>2000 K, and does basically not change. Most preferably, the
thermally stable substrate is thermally stable even at >3000 K
and remains substantially unchanged in the heating step.
[0035] The prepared graphite layer can directly remain on a
substrate or, for example by dissolving a sacrificial layer
disposed between the prepared graphite layer and a substrate, be
disposed on a substrate in analogy with the aforementioned.
Furthermore, it is possible for the prepared graphite layer to be
disposed on a substrate in a free-standing manner. In particular,
this can be realized by employing a free-standing, laterally
crosslinked monolayer in the step of heating the laterally
crosslinked monolayer under vacuum or inert gas to a temperature of
>800 K.
[0036] The applied monolayer is covalently crosslinked in the
lateral direction when being treated with high-energy radiation,
preferably X-ray radiation, .beta.-radiation, .gamma.-radiation,
VUV radiation, EUV radiation or UV radiation, so that a physisorbed
or covalently bonded, thin and stable layer is created on the
substrate surface. By crosslinking in the lateral direction, the
monolayers composed of the low-molecular weight aromatics and/or
low-molecular weight heteroaromatics obtain high mechanical and
chemical stability and effectively protect the underlying substrate
surface against damage or corrosive substances. In addition, a
laterally crosslinked monolayer, which is physisorbed or covalently
bonded to the surface of a substrate via anchor groups and which
can be prepared by treating a monolayer composed of low-molecular
weight aromatics and/or low-molecular weight heteroaromatics with
high-energy radiation, is thermally stable and exhibits excellent
adhesion to a suitable substrate.
[0037] In one embodiment of the method according to the present
invention, crosslinking can be carried out using lateral
structuring by means of fine-focussed, ionizing electron, ion or
photon radiation. The focusing and scanning of the beam across the
areas to be structured can be performed by electron-optical or
ion-optical elements, such as in electron beam lithography with
scanning electron microscopes or in lithography with focused ions
(FIB). Preferably, the structuring can also be carried out by means
of local probe processes. Here, the focusing of electrons, ions or
photons is ensured by the smallness of the electron, ion or photon
source (local probe). The local probe is then guided across the
areas to be structured in distances between 0.1 nm and 1000 nm.
Particularly suitable local probes for electrons include the tips
of scanning tunneling microscopes (STM), atomic force microscopes
(AFM) and atomically defined field emitter tips, which e.g. have
been produced by the method of Muller et al. (Ultramicroscopy 50,
57 (1993)). The latter are particularly suitable as local probes
for structuring with larger distances (>10 nm) between probe and
sample, and can also be used as field ion sources. Fine tips made
of glass or another photon-conducting material, as are used in
near-field optical microscopes (SNOM), are suitable for structuring
with photons. In all local probe methods, the local probe is
positioned directly over the areas to be exposed by means of a
positioning device, for example one made of piezoceramic
elements.
[0038] If, instead of applying a monolayer of low-molecular weight
aromatics and/or low-molecular weight heteroaromatics, for example
saturated or physisorbed molecules or units or molecules or units
that are covalently bonded to a substrate surface via an anchor
group are applied, said molecules or units including e.g.
cyclohexyl, bicyclohexyl, tercyclohexyl, partially or completely
hydrogenated naphthalene or anthracene, or partially or completely
hydrogenated heteroaromatics, then dehydrogenation to the
corresponding aromatics or heteroaromatics can occur in addition to
crosslinking in the lateral direction in the treatment with
high-energy radiation. If nitro groups are bonded to the surface of
the monolayer composed of low-molecular weight aromatics and/or
low-molecular weight heteroaromatics, then in the method according
to the present invention, these nitro groups can as well be
converted to amino groups in the region of action of the
crosslinking radiation.
[0039] In a further embodiment of the method according to the
present invention, the treatment with high-energy radiation can be
carried out using a shadow mask such that only spatially defined
areas of the monolayer applied to the substrate surface are exposed
or irradiated, whereby a structured surface is formed on the
substrate with protected and unprotected areas, i.e. the exposed
areas are protected and the unexposed areas are unprotected. The
product produced according to the present invention can thus also
be used as a negative resist.
[0040] The physisorbed or covalently bonded or chemisorbed
low-molecular weight aromatics and/or low-molecular weight
heteroaromatics, which are preferably bonded via anchor groups,
(i.e. which are not part of the laterally crosslinked monolayer),
present at unexposed or non-irradiated areas, can subsequently be
removed. This can be done e.g. by a thermal treatment, by treatment
with a suitable solvent, or by treatment with a suitable desorbent.
By means of the above-described structuring methods using
high-energy radiation, it is possible to create a laterally
crosslinked monolayer with a structuring or patterning in the
nanometer range. If the adsorbed or covalently bonded low-molecular
weight aromatics and/or low-molecular weight heteroaromatics, which
are preferably bonded via anchor groups, present at unexposed or
non-irradiated areas, are subsequently removed and the resulting
substrate is afterwards heated under vacuum or inert gas to a
temperature of >800 K, then graphite layers or graphene layers
with a structuring or patterning in the nanometer range can be
created.
[0041] In another embodiment, it is possible to not remove the
adsorbed or covalently bonded low-molecular weight aromatics and/or
low-molecular weight heteroaromatics, which are preferably bonded
via anchor groups, present at unexposed or non-irradiated areas,
and to directly heat the resulting substrate under vacuum or inert
gas to a temperature of >800 K. In this approach, the adsorbed
or covalently bonded low-molecular weight aromatics and/or
low-molecular weight heteroaromatics, which are preferably bonded
via anchor groups, are thermally desorbed and the laterally
crosslinked aromatics and/or heteroaromatics forming the
crosslinked monolayer are converted to graphite layers or graphene
layers having a structuring or patterning in the nanometer range.
Thereby, it has been possible for the first time to create graphite
layers or graphene layers with a targeted structuring or patterning
in the nanometer range on a substrate.
[0042] For irradiation with electrons, a large-area illuminating
electron source can be used, e.g. a "flood gun" or a construction
as described in FIG. 2 of Hild et al., Langmuir, 14, 342-346
(1998). The electron energies used can be adapted to the respective
organic films and their substrates over a broad range, preferably
from 1 to 1000 eV. For example, electron radiation of 50 eV or 100
eV can be used for crosslinking 1,1'-biphenyl-4-thiol on gold.
[0043] For lateral structuring, a large-area illuminating electron
source in combination with a shadow mask can be used, so that only
the open areas are exposed to the electrons. Also suitable for
lateral structuring are focused electron beams, which can be
positioned over the areas to be crosslinked by a scanning electron
microscope. Moreover, electron sources such as field emitter tips,
from which electrons are emitted in a small angular range, can be
directly used if they are positioned over the areas to be
crosslinked by means of suitable displacement elements (step
motors, piezotranslators).
[0044] For large-area crosslinking by means of electromagnetic
radiation (e.g. X-ray radiation, UV radiation), light sources
available in the prior art can be used. For lateral structuring,
masks suitable for the respective wavelength range or scanning by
means of suitable light guides are possible.
[0045] The surface of a substrate can be cleaned or chemically
modified before the monolayer is applied. Cleaning can be carried
out by simple rinsing of the surface with water or organic
solvents, such as ethanol, acetone or dimethylformamide, or by
treatment with an oxygen plasma generated by UV radiation. If the
monolayers with anchor groups, such as phophonic acid, carboxylic
acid, or hydroxamic acid groups, are to be applied to oxidized
metal surfaces, prior controlled oxidation of the metal surface is
advantageous. This can be done by treatment of the metal surface
with oxidizing agents, such as hydrogen peroxide, Caro's acid, or
nitric acid. A further possibility for modifying a substrate
surface is to apply a first organic monolayer with terminal
reactive groups, such as amino, hydroxy, chloro, bromo, carboxy or
isocyanate groups, to which the monolayer actually to be
crosslinked is chemically coupled by means of suitable functional
groups in a second step.
[0046] The application of the monolayer of low-molecular weight
aromatics and/or low-molecular weight heteroaromatics to a
substrate can e.g. be carried out by dipping, casting, spin-coating
methods, by adsorption from dilute solution, or by vacuum vapor
deposition.
[0047] According to an embodiment of the method of the present
invention, heating of the at least one monolayer with laterally
crosslinked low-molecular weight aromatics and/or heteroaromatics
is carried out under vacuum. The vacuum applied in the method for
preparing graphite layers is selected such that oxidation and/or
contamination with undesired foreign atoms of the laterally
crosslinked monolayer and the resulting graphite layer can be
effectively prevented in the heating step. Therefore, in the method
for preparing graphite layers, a pressure of <1 mbar is applied,
wherein a pressure of <10.sup.-2 is preferred and a pressure of
<10.sup.-3 is particularly preferred. In the method for
preparing graphite layers, a vacuum in a pressure range from
10.sup.-2 mbar to 10.sup.-12 mbar has turned out to be particularly
suitable, wherein a vacuum in a pressure range from 10.sup.-7 mbar
to 10.sup.-12 mbar (ultrahigh vacuum) is particularly well
suitable.
[0048] In another embodiment of the method according to the present
invention, heating of the at least one monolayer with laterally
crosslinked low-molecular weight aromatics and/or heteroaromatics
is carried out under inert gas, wherein within the scope of the
present invention the term "inert gas" also means mixtures of an
inert gas and hydrogen. The inert gas can be any suitable inert gas
or its mixture with hydrogen. Preferably, argon or a mixture of
argon and hydrogen is used.
[0049] Heating of the laterally crosslinked monolayer, which can be
formed by treating a monolayer composed of low-molecular weight
aromatics and/or low-molecular weight heteroaromatics with
high-energy radiation, is carried out at a temperature of >800
K. Heating of the laterally crosslinked monolayer is preferably
done at a temperature of >1000 K, and a temperature of >1300
K is particularly preferred. Even more preferably, the temperature
is >1600 K. If heating is carried out at higher temperatures
(i.e. at temperatures of >1600 K), it is preferred to not use
molybdenum or tungsten as the substrate, since they react with the
graphite layers according to the present invention at those
temperatures, thus forming carbides.
[0050] According to a further preferred embodiment of the method of
the present invention, heating of the laterally crosslinked
monolayer, which can be formed by treating a monolayer composed of
low-molecular weight aromatics and/or low-molecular weight
heteroaromatics with high-energy radiation, can be carried out at a
temperature of >2000 K, in particular at a temperature of
>2500 K or a temperature of >3000 K.
[0051] The upper temperature limit for the step of heating the
laterally crosslinked monolayer, which can be formed by treating a
monolayer composed of low-molecular weight aromatics and/or
low-molecular weight heteroaromatics with high-energy radiation, is
determined by the sublimation temperature of carbon and the melting
temperature or decomposition temperature of the substrate used.
[0052] Furthermore, according to the present invention, a graphite
layer or graphene layer is provided that can be obtained by means
of the above-defined method of the present invention. The graphite
layer that can be prepared according to the method of the present
invention is preferably electrically conductive.
[0053] The graphite layer according to the present invention
preferably has a layer thickness of <2 nm. Preferably, the
graphite layer prepared according to the present invention is an
electrically conductive graphite layer.
[0054] By use of the method according to the present invention, it
is possible, in particular by means of lithographic techniques, to
prepare laterally crosslinked monolayers of arbitrary shape and
size, which can be converted to graphite layers of arbitrary shape
and size.
[0055] By selection of suitable process parameters for the lateral
crosslinking (e.g. energy and dose) and the temperature and/or
pressure, chemical purity (e.g. the number of foreign atoms) and
structural defects in the graphite layers can be controlled.
[0056] The use of chemically functionalized monolayers enables the
preparation of chemically doped and/or chemically functionalized
graphite layers.
[0057] The fields of application of the graphite layers according
to the present invention are nanoscopic electrical conductors,
conductive membranes in miniaturized pressure sensors (e.g.
nanomicrophones), pads in electron microscopy, doping of graphene
for adjusting the electrical conductivity and chemical
functionalization of graphene, for example for substance separation
(e.g. as a membrane or filter).
[0058] The present invention will be explained in more detail by
the following examples.
EXAMPLES
A) Preparation of a Laterally Crosslinked Monolayer by Treatment of
a Monolayer Composed Low-Molecular Weight Aromatics and/or
Low-Molecular Weight Heteroaromatics with High-Energy Radiation
Example A1
[0059] A monolayer of 4-biphenylthiol on gold is prepared by
placing a silicon wafer with a 100 nm-thick vapor-deposited gold
layer in a 1 mmolar ethanolic solution of 4-biphenylthiol for one
hour. Subsequently, the wafer is taken out, rinsed several times
with ethanol p.a. and dried in a stream of nitrogen. To crosslink
the layers, the wafer with the monolayer is irradiated in a vacuum
chamber (p=10.sup.-5 to 10.sup.-9 mbar) with a "Leybold flood gun"
(model 869000) with electrons of energy 100 eV and a dose of 40,000
.mu.C/cm.sup.2. After removal from the vacuum chamber, the layer
can be immediately used for its intended application or be
chemically functionalized further.
Example A2
[0060] After a surface of stainless steel has been cleaned several
times with conventional organic detergent solutions and rinsed
several times with deionized water, a monolayer of
terphenyl-4-phosphonic acid is prepared by treating the cleaned
surface with a 1 mmolar solution of terphenyl phosphonic acid in
dimethylformamide. After 12 hours, the monolayer is formed and the
steel substrate is rinsed once each with pure dimethylformamide and
with deionized water. Subsequently, the monolayer is irradiated and
crosslinked as in Example 1. Here, the electron energy can be
increased up to 200 eV. A dose of 30,000 .mu.C/cm.sup.2 is
sufficient for complete crosslinking.
Example A3
[0061] A silicon gear with a diameter of 500 .mu.m is placed in a
mixture of 3 parts of 30% hydrogen peroxide and 1 part conc.
sulfuric acid for 1 min. Subsequently, it is rinsed with deionized
water and placed in a 1 mmolar solution of 4-trichlorosilylbiphenyl
in tetrahydrofuran. After two hours, the gear is taken out, rinsed
with tetrahydrofuran, dried in a stream of nitrogen and subjected
to the same irradiation and crosslinking procedure as in Example 1.
A stable and continuously crosslinked surface layer is obtained,
which effectively protects the gear against mechanical
abrasion.
Example A4
[0062] By analogy with Example 1, monolayers of
4,4'-nitrobiphenylthiol are prepared on a gold surface. Before
irradiation, the layer is covered with a metallic shadow mask.
After irradiation, carried out as in Example 1, the nitro groups at
the exposed spots have been converted to amino groups and the layer
is crosslinked at those spots. The remaining areas of the layer,
which are covered by the mask, remain unchanged. The amino groups
formed by the irradiation can e.g. be acylated by subsequent
treatment with an isocyanate, acid chloride or acid anhydride from
solution in an organic solvent, such as tetrahydrofuran or
acetone.
B) Preparation of Graphite Layers by Heating of a Laterally
Crosslinked Monolayer under Vacuum to a Temperature of >800
K
Example B1
[0063] A laterally crosslinked monolayer of
4'[(3-trimethoxysilyl)propoxy]-[1,1'-biphenyl]-4-carbonitrile,
which is prepared on silicon nitride, is transferred to a platinum
grid and heated to 1500 K at 10.sup.-5 mbar.
Example B2
[0064] A laterally crosslinked monolayer of 4-biphenylthiol, which
is prepared on gold, is transferred to a platinum substrate and
heated to 1800 K at 10.sup.-8.
Example B3
[0065] A laterally crosslinked monolayer of 4-biphenylthiol, which
is prepared on gold, is transferred to an iridium substrate and
heated to 2000 K at 10.sup.-6.
[0066] After heating to the respectively indicated temperature
under the corresponding pressure, the graphite layers formed in
Examples B1 to B3 exhibit the desired properties, such as a
thickness of <2 nm and electrical conductivity.
* * * * *