WO2014029518A2

WO2014029518A2 - Direct attachment of organic moieties to an inorganic surface

Info

Publication number: WO2014029518A2
Application number: PCT/EP2013/061552
Authority: WO
Inventors: Jozef MICHL; Florian Von Wrochem; Eva KALETOVA; Lubomir POSPISIL
Original assignee: Sony Corporation; Institute Of Organic Chemistry And Biochemistry Academy Of Sciences Of The Czech Republic; Sony Deutschland Gmbh
Priority date: 2012-08-23
Filing date: 2013-06-05
Publication date: 2014-02-27
Also published as: WO2014029518A3

Abstract

The present invention relates to methods and the use of organometallic compounds for attaching organic moieties to an inorganic surface. It further relates to stable assemblies of organic moieties, such as monomolecular layers, on an inorganic surface as well as to devices comprising such stable assemblies.

Description

Direct attachment of organic moieties to an inorganic surface

At present, small organic molecules are finding increasing use in molecular electronics as active electronic components. Self-assembled monomolecular layers (SAMs) on the surface of a metal are a fundamental structure in the organization of such devices. They are formed by a layer of molecules whose "head" terminus exhibits specific affinity for the surface of a substrate. The opposite terminus, often at the end of a long aliphatic chain, can carry a functional group, such as -OH, -NH₂, or -COOH. The most frequently used molecules are alkanethiols, whose alkyl chain is attached through the S-H head group to a noble metal substrate, for which a sulfur atom has high affinity. Very often the interaction is through a semicovalent bond, such as a charge transfer bond, between an atom of sulfur and an atom of gold (Love et al., Chem. Rev. 2005, 105, 1103-1170). Thiol molecules adsorb on atoms of gold very easily from solutions (e.g., in ethanol), and the resulting highly uniform and densely packed monomolecular layers can posses a variety of chemical properties depending on the functional groups at the opposite end.

As an alternative to thiolates, the attachment of individual organic moieties to gold-coated substrates can occur through a sulfur atom of thiocyanates (Ciszek et al., /. Am. Chem. Soc. 2004, 126, 13172). Atoms of other elements have been utilized to form a SAM by adsorption from solution or vapor, such as selenium (Huang et al, Langmuir 1998, 14, 4802), tellurium (Weidner et al., J. Phys. Chem. C 2007, 111, 11627), silicon (Owens et al, J. Phys. Chem. B 2003, 107, 3177), or a carbon atom in acetylenes (Zhang et al, J. Am. Chem. Soc, 2007, 129, 4876), arenediazonium salts (Shewchuk a McDermott, Langmuir 2009, 25, 4556) and isocyanates (Stapleton et al., Langmuir 2005, 21, 11061).

The steric demands posed by simple alkyl chains attached to a substrate through sulfur or silicon atoms are conducive to the formation of essentially impenetrable wide domains composed of densely packed layers of long alkane chains, somewhat inclined away from the surface normal. The attachment of the organic group through a silicon atom (Owens et al., . Phys. Chem. B 2003, 107, 3177) utilizes monoalkylsilanes and the deposition takes place from the vapor phase.

The SAM-forming attachment of molecules to the surfaces of gold and other metals through an atom of divalent sulfur is facile and well documented (Love et al., Chem. Rev. 2005, 105, 1103; Ulman, Chem Rev. 1996, 96, 1533 and Ultrathin Organic Films, Academic Press: San Diego, 1991). This type of attachment offers numerous advantages, such as easy formation under ordinary laboratory conditions (ambient pressure and temperature), but also has some disadvantages, such as limited long-term resistance of thiolates to aerial oxidation (Joseph et al., Chem Mater. 2009, 21, 1670; Willey et al., Surface Science 2005, 576, 188; Huang et al., Langmuir. 1998, 14, 4802) and somewhat marginal electron transport across the partially polar bond between the electronegative sulfur atom and a metal atom, which limits the use of these SAMs, for instance in molecular electronics, which is based on the attachment of individual molecules to metal surfaces (substrates). Contacts provided by thiol or oxy-acid anchoring groups have relatively high electrical resistances, which introduce barriers to electron or hole transfer across the electrode-molecule interface. Accordingly, a direct connection between the metal surface and the hydrocarbon backbone would be beneficial to reduce contact resistances and injection barriers. A further disadvantage of SAMs of thiols, especially those on noble metals, is that they are thermally unstable with respect to both desorption and oxidation. This could be overcome by an attachment with a higher chemisorption energy to the substrate.

The inventors have previously found that monolayers are formed on a metal surface upon treatment with solutions of the tin (Sn)-containing organometallic compounds trialkylstannyl tosylate, trifluoroacetate or triflate under ambient conditions (Khobragade et al., Langmuir, 2010, 26, 8483; WO 2011/124187 Al). This discovery was preceded by previous accidental observations of attachment of organomercury salts to the surface of gold (Zheng et al., 7. Am. Chem. Soc, 2004, 126, 4540; Mulcahy et al., . Phys. Chem. C, 2009, 113, 20698; Mulcahy et al., Phys. Chem. C, 2010, 114, 14050) and even earlier investigations of adsorption of organoplatinum complexes to platinum (Lee and Whitesides, Acc. Chem. Res., 1992, 25, 266). The use of the (Sn)-containing organometallic compounds allowed the production of monolayers, in which the organic moiety is bound to the metal surface via direct interaction between the metal surface and at least one carbon atom of the organic moiety.

Thus, the use of organometallics for the direct transfer of organic residues/moieties to metal surfaces promises to complement the established application of thiols and analogous compounds that produce monolayers in which carbon is attached to the gold surface through a mediating non-carbon atom, such as sulfur.

It was an object of the present invention to provide methods for directly attaching organic moieties/residues to an inorganic (e.g. metal) surface, wherein the metal atoms of the used organometallic compounds are removed during or after adsorption of the organic moiety to the inorganic surface. It was another object to provide for methods allowing the production of "pure" and more dense monomolecular layers of organic moieties directly attached to inorganic surfaces. It was a yet another object of the present invention to identify further organometallic compounds that can be used for the direct attachment of organic moieties to inorganic surfaces.

The objects of the present invention are solved by a method for attaching at least one organic moiety to an inorganic surface, said method comprising the steps of:

(a) providing a substrate having an inorganic surface;

(b) treating said inorganic surface with a solution comprising an organometallic compound, said organometallic compound comprising a metal atom and said organic moiety attached to said metal atom; and

(c) removing said metal atom by electrochemical stripping or thermal removal, wherein said metal atom is selected from the group consisting of mercury, tin, aluminum, lithium, cadmium, bismuth, lead and thallium, preferably mercury, tin, aluminum and lithium, wherein said at least one organic moiety is bound to said inorganic surface via direct interaction between said inorganic surface and at least one carbon atom of said organic moiety, and

wherein said inorganic surface is a metal surface, a metal oxide surface or a semiconductor surface, preferably a metal surface.

The objects of the present invention are also solved by a method for attaching at least one organic moiety to an inorganic surface, said method comprising the steps of: (a) providing a substrate having an inorganic surface;

(b) treating said inorganic surface with a solution comprising an organometallic compound, said organometallic compound comprising a metal atom and said organic moiety attached to said metal atom;

wherein said metal atom is selected from the group consisting of tin, aluminum, lithium, cadmium, bismuth, lead and thallium, preferably tin, aluminum and lithium,

wherein said at least one organic moiety is bound to said inorganic surface via direct interaction between said inorganic surface and at least one carbon atom of said organic moiety, and

Preferably, the above (second) method does not comprise an additional step of removing said metal atom (e.g. by electrochemical stripping or thermal removal), as the metal atom leaves spontaneously during the adsorption of the organic moiety to the inorganic surface. Preferably, when said metal atom is tin, said organometallic compound used in the above (second) method is Bu₂Sn(OTs)2.

According to the present invention, the organic moiety is bound to the inorganic surface via direct interaction between the surface and at least one carbon atom (e.g. two carbon atoms) of the organic moiety, i.e. the attachment of the organic moiety to the inorganic (e.g. metal) surface does not involve an atom of another element, such as sulfur. The attachment of the organic moiety via more than one carbon atoms (e.g. two carbon atoms) implies that the adsorbed organic moiety may have lost at least one additional hydrogen atom as compared to its original state (for example, an alkyl may be actually adsorbed as an alkylidene and so on). Such scenarios are explicitly included in the present invention.

In one embodiment, said metal surface is a surface of a metal selected from the group consisting of gold, platinum, palladium, copper, silver, zinc, indium, nickel and alloys thereof, preferably gold, platinum, palladium, copper, silver and alloys thereof, more preferably gold.

In one embodiment, said metal surface is an electrode. Preferably, said organic moiety (or organic residue/group) comprises at least two carbon atoms.

In one embodiment, said organic moiety is selected from the group consisting of unsubstituted or substituted alkyls, unsubstituted or substituted alkenyls, unsubstituted or substituted alkynyls, unsubstituted or substituted aryls, carbon nanotubes, graphene sheets and combinations of any of the foregoing, wherein, preferably, said organic moiety comprises a C2-C20 alkyl.

In one embodiment, the organic moiety comprises at least one heteroatom, which, preferably, makes the organic moiety polar, dipolar, electron-donating, electron-accepting or functional in terms of selective binding of molecular species.

In one embodiment, the organic moiety comprises at least one functional group, such as -OH, -NH₂, or COOH, wherein, preferably, said at least one functional group is located at an end of the organic moiety opposite to the at least one carbon atom of the organic moiety interacting with said inorganic, preferably metal, surface. Such "terminal" functional groups may be used to functionalize the organic moiety or a monomolecular layer of the organic moiety, e.g. for use in industrial, biological or medical applications. In one embodiment, the "terminal" functional groups are used for the attachment of biological molecules, such as enzymes, antibodies, receptors, and tags used for applications in molecular biology and biotechnology, such as biotin, myc, His6, GST and MBP.

In one embodiment, said organic moiety is a C₂-C₂0 alkyl chain.

The term "organometallic compound", as used herein, is meant to refer to compounds containing at least one carbon-metal bond. The organometallic compound used in accordance with the present invention may further comprise one or more so called leaving groups (herein referred to as X), which are electronegative. A person skilled in the art is aware of numerous different leaving groups that can be used in accordance with the present invention. Particularly preferred leaving groups are triflate, trifluoroacetate, tosylate and halides, such as chlorides, bromides, iodides and fluorides. Triflate is the salt of trifluoromethanesulfonic acid (also referred to as OTf), trifluoroacetate is the salt of trifluoroacetic acid (also referred to as OCOCF3) and tosylate is the salt of p-toluenesulfonic acid (also referred to as 4- methylbenzenesulfonic acid or OTs). The leaving group(s) is/are split off during the adsorption of the organic moiety, e.g. during the formation of the monomolecular layer of said organic moiety. A leaving group is, however, not always needed in accordance with the present invention. Preferred general formulae of the organometallic compound used in accordance with the present invention are R5M, R4MX, R₃MX2, R2MX₃, RMX4 for a pentavalent metal M, such as Bi; R₄M, R₃MX, R₂MX₂, RMX₃ for a tetravalent metal M, such as Sn or Pb; R₃M, R₂MX, RMX₂ for a trivalent metal M, such as Al or Tl; R₂M, RMX for a divalent metal M, such as Hg or Cd; and RM for a monovalent metal, such as Li (wherein R refers to an organic moiety).

In case the organometallic compound comprises more than one organic moiety attached to said metal atom, these organic moieties may be the same or different.

In one embodiment, when said metal atom is mercury, said organometallic compound has the general formula (R)Hg-X;

when said metal atom is tin, said organometallic compound has the general formula (R)_nSn(CH₃)(4__n) with n being an integer from 1 to 4, (R)_nSn(CH₃)(₃__n)-X with n being an integer from 1 to 3, or (R)_nSn(CH₃)₍₂-_n)-X2 with n being 1 or 2;

when said metal atom is aluminum, said organometallic compound has the general formula (R)_nAl(CH₃)(₃__n) with n being an integer from 1 to 3, (R)_nAl(CH₃)(2-n₎-X with n being 1 or 2, or RA1X₂; and,

when said metal atom is lithium, said organometallic compound has the general formula (R)Li;

wherein R is said organic moiety and X is a leaving group.

Corresponding formulae for organometallic compounds comprising the metal atom cadmium, bismuth, lead or thallium are also included in the present invention.

In one embodiment, said leaving group is selected from the group consisting of triflate, trifluoroacetate, tosylate and halides.

In one embodiment, the methods according to the present invention further comprise the step of cleaning said inorganic surface prior to said step of "treating said inorganic surface with a solution comprising an organometallic compound" (i.e. step (b)). In one embodiment, said cleaning step is performed by hydrogen flame or butane flame annealing, in another embodiment, said cleaning step is performed by immersing said substrate having an inorganic surface into a cleaning solution at an elevated temperature (e.g. 85 to 95 °C). Preferably, said cleaning solution is a 3: 1 mixture of sulfuric acid and hydrogen peroxide ("piranha solution").

Preferably, said step of "treating said inorganic surface with a solution comprising an organometallic compound" (i.e. step (b)) is performed by immersing said inorganic surface into said solution. In one embodiment, said step is performed for 10 min to 24 hours, preferably 30 min to 15 hours, more preferably 1 to 7 hours.

In one embodiment, said solution comprising an organometallic compound further comprises a solvent, preferably selected from the group consisting of solvents based on hydrocarbons, ethers, such as tetrahydrofuran (THF), halogenated alkanes, carbonitriles and nitro compounds, wherein, preferably, said solvent has been pre-dried.

In one embodiment, said solution comprises said organometallic compound in a concentration of from 10 M to 1 x 10^"6 M, preferably 1 M to 1 x 10^"5 M, more preferably 1 x 10^"1 M to 1 x 10^"5 M, even more preferably 1 x 10^~2 M to 1 x 10^~6 M.

In one embodiment, the methods according to the present invention further comprise the step of rinsing said inorganic surface after said step of "treating said inorganic surface with a solution comprising an organometallic compound" (i.e. step (b)). For said rinsing step, a solvent selected from the group consisting of solvents based on hydrocarbons, ethers, such as tetrahydrofuran (THF), halogenated alkanes, carbonitriles and nitro compounds, is used, wherein, preferably, said solvent has been pre-dried. Preferably, said solvent used in the rinsing step is the same solvent as used in step (b).

In one embodiment, the methods according to the present invention further comprise the step of drying said inorganic (e.g. metal) surface after said step of "treating said inorganic surface with a solution comprising an organometallic compound" (i.e. step (b)), and, optionally, said rinsing step. Preferably, said drying step is performed by means of a stream of nitrogen or of another inert gas. The term "electrochemical stripping", as used herein, refers to a technique, in which remaining metal atoms are removed by electrochemical treatment, e.g. oxidative anodic stripping. More particularly, a potential is applied at which the metal atoms (e.g. Hg atoms) that are present are oxidized into metal ions (e.g. Hg ions), which then diffuse into the electrolyte solution and are ultimately washed away. A reductive stripping process may be used when the metal oxides are to be removed. A person skilled in the art knows how to perform such electrochemical treatment.

The terms "thermal removal" and "thermal annealing", as used herein, refer to a technique in which remaining metal atoms/oxides are removed by dry heating under vacuum. In one embodiment, the temperature used for the thermal removal/annealing is in the range of from 80°C to 100°C, preferably in the range of from 85°C to 100°C, more preferably in the range of from 90°C to 100°C. The duration of the thermal annealing step will largely depend on the precise kinetics of the removal reaction. In one embodiment, the duration of the thermal stripping/annealing step is between 1 min and 3 hours, preferably between 10 min and 2 hours. In a particular embodiment, the temperature is 90°C and the duration is 1 hour. Without wishing to be bound by any theory, the inventors believe that the thermal annealing triggers the diffusion of the surface-bound metal atoms of said organometallic compound (e.g. Hg) into the bulk metal surface (e.g. Au).

The objects of the present invention are also solved by a stable assembly of at least one organic moiety on an inorganic surface, produced by a method as defined above, wherein said inorganic surface is a metal surface, a metal oxide surface or a semiconductor surface, preferably a metal surface.

Thus, the methods as defined above can also be referred to as methods for producing such stable assemblies.

The objects of the present invention are also solved by a stable assembly of at least one organic moiety on an inorganic surface,

wherein said at least one organic moiety is bound to said inorganic surface via direct interaction between said inorganic surface and at least one carbon atom of said organic moiety, wherein said inorganic surface is a metal surface, a metal oxide surface or a semiconductor surface, preferably a metal surface, and wherein said stable assembly is substantially free of metal atoms selected from the group consisting of mercury, tin, aluminum, lithium, cadmium, bismuth, lead and thallium, preferably mercury, tin, aluminum and lithium.

"Substantially free of metal atoms selected from the group consisting of mercury, tin, aluminum, lithium, cadmium, bismuth, lead and thallium, preferably mercury, tin, aluminum and lithium" means that said stable assembly contains less than 3%, preferably less than 2%, more preferably less than 1%, even more preferably less than 0.5% of metal atoms of the organometallic compound relative to the number of organic moieties adsorbed to the inorganic surface, as determined by spectroscopic methods, such as X-ray photoemission spectroscopy (XPS). In one embodiment, said stable assembly does not contain any metal atoms derived from the organometallic compound within the resolution of spectroscopic methods. In one embodiment, no metal atoms derived from the organometallic compound are detectable by XPS. When some tin-based organometallic compounds (e.g. Bu₂Sn(OTs)₂) and, possibly, aluminum-, lithium-, cadmium-, bismuth-, lead- and thallium-based organometallic compounds are used, no additional removal step (e.g. removal by electrochemical stripping or thermal annealing) is necessary because these organometallic compounds do not leave any metal atoms on the inorganic (e.g. metal) surface upon adsorption of the organic moiety (step (b)).

In one embodiment, said stable assembly is a monomolecular layer of said organic moiety on said inorganic surface.

In one embodiment, said metal surface is as defined above. In one embodiment, said organic moiety is as defined above.

In one embodiment, the monomolecular layer, preferably chemisorbed monomolecular layer, (produced) according to the present invention has a thickness of 0.1 to 5 nm (1 A to 50 A), preferably 0.1 to 3 nm (1 A to 30 A).

The objects of the present invention are also solved by a device comprising a stable assembly as defined above, e.g. a monomolecular layer, wherein, preferably, said device is selected from the group consisting of electronic devices, opto-electronic devices, such as OLEDs and OTFTs, medical devices and (bio-)sensor devices.

The objects of the present invention are also solved by the use of an organometallic compound for attaching at least one organic moiety to an inorganic surface,

wherein said inorganic surface is a metal surface, a metal oxide surface or a semiconductor surface, preferably a metal surface, and

wherein said organometallic compound comprises a metal atom selected from the

group consisting of aluminum, lithium, cadmium, bismuth, lead and thallium, preferably aluminum and lithium, and said organic moiety attached to

said metal atom,

wherein,

when said metal atom is aluminum, said organometallic compound has the general formula (R)_nAl(CH₃)(3__n) with n being an integer from 1 to 3, (R)nAl(CH₃)(2-_n)-X with n being 1 or 2, or RA1X₂; and,

wherein R is said organic moiety and X is a leaving group.

The objects of the present invention are further solved by the use of an organometallic compound for forming a monomolecular layer of an organic moiety on an inorganic surface, wherein said inorganic surface is a metal surface, a metal oxide surface or a semiconductor surface, preferably a metal surface, and

wherein said organometallic compound comprises a metal atom selected from the

group consisting of aluminum, lithium, mercury, cadmium, bismuth, lead and thallium, preferably aluminum, lithium and mercury, and said organic moiety attached to said metal atom,

wherein,

when said metal atom is aluminum, said organometallic compound has the general formula (R)_nAl(CH₃)(3__n) with n being an integer from 1 to 3, (R)_nAl(CH₃)(₂-_n)-X with n being 1 or 2, or RA1X₂;

when said metal atom is lithium, said organometallic compound has the general formula (R)Li; and, when said metal atom is mercury, said organometallic compound has the general formula (R)Hg-X;

wherein R is said organic moiety and X is a leaving group.

In one embodiment, said metal surface is as defined above.

In one embodiment, said organic moiety is as defined above.

The inventors of the present invention have surprisingly found that when attaching organic moieties to an inorganic (e.g. metal) surface by means of an organometallic compound, it is possible to remove the metal atom of said organometallic compound (of mercury, tin, aluminum, lithium, cadmium, bismuth, lead and thallium) after adsorption of the organic moieties by one of two specific techniques, electrochemical stripping (e.g. oxidative anodic stripping) and thermal annealing. Furthermore, the inventors have found that when some tin- based organometallic compounds (e.g. Bu₂Sn(OTs)₂) and, possibly, aluminum-, lithium-, cadmium-, bismuth-, lead- and thallium-based organometallic compounds are used as the organometallic compound, no removal step is necessary, since these organometallic compounds do not leave any metal atoms on the inorganic surface upon adsorption of the organic moiety.

Stable assemblies, such as chemisorbed monomolecular layers, (produced) according to the present invention exhibit an increased chemical stability against a variety of chemical agents (including aerial oxidation) and provide improved electron transport due to their direct attachment to the inorganic (e.g. metal) surface, as compared to thiol-based attachment to a metal surface, which relies on an interaction between an electronegative sulfur atom and a metal atom of the metal surface. It is anticipated that the direct connection between the metal surface and the hydrocarbon backbone will allow a lower electrical resistance in devices, as this kind of contact will reduce contact resistances and injection barriers. Due to the direct attachment to the inorganic (e.g. metal) surface, monomolecular layers produced according to the present invention are also thinner than their respective thiol-based counterparts. Because the metal atoms of the organometallic compounds are removed during or after adsorption of the organic moieties, chemisorbed monomolecular layers of the present invention are substantially pure (i.e. metal-free) and densely packed, showing molecular densities comparable to those of alkanethiol monolayers. Furthermore, the stable assemblies of at least one organic moiety on an inorganic (e.g. metal) surface according to the present invention have an increased thermal stability (i.e. they can withstand higher temperatures), both with respect to desorption and oxidation, as compared to state of the art assemblies, offering a significant advantage in terms of the processing conditions for surface functionalization and ligand attachment used in common device fabrication processes (e.g. CMOS). Whereas the desorption temperature for alkanethiol layers is at about 105°C, alkane layers produced according to the present invention exhibit a desorption temperature above 150°C, which might be even higher for organic molecules that are more stable than alkanes. The monolayers or molecular adsorbates prepared according to the present invention provide a direct connection between highly conducting or semiconducting periodic carbon structures, including functionalized carbon nanotubes and graphene sheets, to metal electrodes. This will reduce contact instabilities to the electrodes (e.g. source, drain or gate in field effect transistors), reduce contact resistances and hence increase operational frequencies and device reliability.

Monolayers (produced) according to the present invention can be used as corrosion protection agents for inorganic (e.g. metal) surfaces used in industrial or in medical applications. They can also be used to control the interaction of a surface with the liquid environment, for example in case of biocompatible implants for medical applications. The advantageous features of monomolecular layers (produced) according to the present invention, such as the higher thermal and mechanical stability or the chemical stability towards different types of solvents, can be exploited to form a new generation of monomolecular layers that are ultrathin, that do not change the topological features of the surface, and that can present terminal chemical groups being compatible with the environment required by each individual application (e.g. biocompatible in case of biological or medical applications, corrosion protective, solvent-compatible or lubricant-compatible in case of industrial applications).

Monolayers (produced) according to the present invention can also be used for bio- recognition in biological and medical applications. In this case, the strength of the layers would consist in the tight attachment of functional and receptor terminal groups (e.g., biotin, antibodies, receptors and others) to the substrate, capable in selectively binding bio-molecules at the surface. This could be used, e.g., for identification and diagnosis by optical means.

The charge injection between metals and organic semiconductors can be tuned by the energy level alignment at the interface between the organic material and the metal electrode. This can be affected by dipolar molecules organized as a surface layer at the interface, as described in detail in EP 2 278 636 Al, which is incorporated herein by reference. The dipole that exists at a metal/molecule interface ("interface dipole") can be divided into two parts, one part resulting from the metal-molecule interaction and the other part resulting from the intrinsic (permanent) dipole of the molecule itself. The interface dipole can be changed significantly upon adsorption of molecules having an intrinsic dipole moment oriented perpendicular to the metal surface. In recent years, self-assembled monolayers (SAMs) of polar organic molecules have been used for the purpose of tuning the interface dipole at metal/organic interfaces. Molecules used for this purpose are typically rod-shaped and terminated with functional groups enabling chemisorption. In close analogy to EP 2 278 636 Al, compounds could be linked to the electrodes via organometallic compounds as disclosed by the present invention instead of using standard thiol, dithiocarbamate or other alternative metal-molecule anchor groups. The molecular structures will then possess dipolar backbones including an intrinsic dipole moment (e.g. as the terminal functional groups). Such molecular backbones can be aromatic, aliphatic, or can include dipolar groups represented by adatoms having strong differences in electronegativity. Examples include CF₃(CH₂)_n- and NC(CH₂)_n- with n being an integer between 2 and 20.

A major problem in today's graphene-based electronics is the reliable formation of contacts between a graphene sheet acting as a channel and the source and drain electrodes in field effect transistor (FET) devices. The contact between source, drain and the graphene sheet is often created by the direct metal evaporation on a pre-deposited graphene sheet, which involves a complex lithographic and alignment processes. The contacts are often unreliable and exhibit high contact impedance, reducing the bandwidth and operation frequency of the FET device. This problem can be overcome with the present invention, by modification of graphene with appropriate organometallic ligands at its two-dimensional edges and by reacting these ligand groups with the metal contacts of interest (e.g. source and drain). Technically, this can be carried out by immobilization of the graphene sheets on pre-patterned FET electrode structures. The procedures described in the present invention will allow a direct chemical contact of the graphene sheet to the metal electrodes, yielding more reliable contacts, lower contact resistance and higher operating frequencies. Of course, this approach can be extended to any contact between metal and graphene other than source and drain contacts, which exists in a graphene-based electronic circuit.

An analogous problem is the reliable formation of contacts between carbon nanotubes (CNTs) and e.g. source and drain electrodes in field effect transistor (FET) devices. The problems to be overcome are practically identical to those in graphene-based electronics - however, the dimensionality of the problem is different, as CNTs are approximately one-dimensional entities. In this case, the modification of CNTs with appropriate organometallic ligands could be carried out either at the open ends of the CNT (possessing dangling bonds) or by modification of the outer CNT boundaries, e.g. by a short alkyl chain.

The physical limits encountered in the miniaturization of CMOS devices and the exponential increase in the production cost of future CMOS-based electronics triggered a growing research effort in the area of molecular electronics. For the realization of organic-based memories and field-programmable gate arrays, the development of molecular rectifiers as well as molecular switches is an issue of fundamental importance. By definition, a rectifier (diode) is a two-terminal device with a unipolar current-voltage characteristic. The first concept of a unipolar molecular diode, based on a donor-insulator-acceptor (D-σ-Α) model, was proposed by Aviram and Ratner. According to this model, a recombination of opposite charges previously transferred from the electrodes into the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of two decoupled units within the rectifier (D⁺ - σ - A^" zwitterion state) takes place. Other authors described different mechanisms for molecular rectification, such as single electroactive units asymmetrically positioned between two electrodes. In this case, the alignment of the HOMO and LUMO of the phenyl ring with the Fermi levels of the contacts determines the asymmetry of the current- voltage characteristics. There are further mechanisms that could be exploited for rectification, many of those described in the literature.

In view of molecular switches, which are the basic element for information storage in memories, mechanisms such as redox activity, doping, conformational switching, ion transfer, bond formation and breaking (also hydrogen bonds), intra-molecular or inter-molecular charge transfer, polarization changes (ferroelectric switching), conjugation changes and other mechanisms could be exploited. All these switching mechanisms could be operated by defined chemical groups or substituents that are introduced into the molecular units by chemical synthesis.

The above described switching or rectifying units can subsequently be provided in the form of organometallic compounds as defined above and connected to the electrodes in accordance with the present invention. The resulting devices could be either based on monolayers, bilayers, or multilayers that are themselves composed of such active compounds.

For the fabrication of μιη- to nm-scale molecular junctions for electronics applications, the formation of ultra-thin self-assembled monolayers in a solution deposition process represents one of the most promising assembly strategies. Moreover, the control of the packing density and tilt angle of the molecular backbone within the monolayer can be exploited to obtain reproducible current- voltage characteristics.

By these means, a strategy for the formation of thin-film electronics based on monolayers of active organic moieties connected to the electrodes by the methods according to the present invention or of single molecule -based electronics where single molecules are addressed in an analog way by nanometer-sized electrodes is provided.

More examples for applications in the field of molecular electronics are provided in EP 2 112 692 Al and in EP 2011 195 943, which are incorporated herein by reference.

Reference is now made to the figures, wherein

Figure 1 shows the kinetics of monolayer formation (thickness) for Ci₈H₃₇HgOTs (A,T) and

C1₈H₃7SH (■, · ) on gold substrates cleaned by piranha solution or by hydrogen flame annealing (A; R stands for C1₈H₃7) and the kinetics of monolayer formation (thickness) for C4H₉HgOTs ( · ) and CisH₃₇HgOTs ( A ) in THF on gold substrates cleaned by piranha solution (B); Figure 2 shows the kinetics of monolayer formation (contact angle) for CziHgHgOTs ( · ) and

Ci₈H₃₇HgOTs (A) in THF on gold substrates cleaned by piranha solution (A), the contact angles of water on Ci₈H₃₇HgOTs and CigH₃₇SH based monolayers on gold substrates cleaned by piranha solution or by hydrogen flame annealing (B) and the contact angle of water on gold cleaned by hydrogen flame annealing as compared to untreated gold (C);

Figure 3 shows ATR-FTIR (infrared) spectra of Ci₈H₃₇SH and Ci₈H₃₇HgOTs based monolayers adsorbed on gold substrates cleaned by piranha solution or by hydrogen flame annealing (A; R stands for Ci₈H₃₇) as well as of monolayers based on C₄H₉HgOTs (e) and Ci₈H₃₇HgOTs (f) in THF on gold substrates cleaned by piranha solution (B);

Figure 4 shows an x-ray photoelectron spectrum (XPS) of a CisH₃₇HgOTs based monolayer adsorbed on gold and the structural formula of Ci₈H₃₇HgOTs;

Figure 5 shows Au 4d and Hg 4d XPS of monolayers formed by treatment of a gold surface with Ci₈H₃₇HgOTs; data: circles, fitted curves: red and blue, background: black;

Figure 6 shows a scanning tunnel microscopy (STM) constant-current image (60 x 60 nm²) of a monolayer formed by treatment of an Au(l l l) surface with octadecylmercury tosylate (Ci₈H₃₇HgOTs), wherein the striped phase is found in the upper part of the image and the square phase in the lower part;

Figure 7 is a graph showing the stability of monolayers based on Ci₈H ₇SH and CisH₃₇HgOTs on gold surface (cleaned by piranha solution (P) or hydrogen flame annealing (A)) after 20 - 23 h immersion at room temperature, followed by rinsing and drying; R is the percentage of the monolayer remaining on the gold surface, calculated as the ratio of the final to the initial integrated intensity of the 2800-3000 cm^"1 bands; A = dry CH₂CI₂, B = wet CH2CI2, C = n-hexane, D = ethanol, E = water, F = 0.1 M H₂S0₄, G = 0.1 M NaOH, H = 1 mM KMn0₄, 1 = 30% H₂0₂, J = 10 mM NaBH₄, and K = 7 days in laboratory air (A) and the stability of adsorbed monolayers of n-BuHgOTs and CisH₃₇HgOTs to overnight immersion in various reagents at room temperature, followed by rinsing and drying; R is the percentage of the monolayer remaining on the gold surface, calculated as the ratio of the final to the initial integrated intensity of the 2800-3000 cm ¹ bands (B); Figure 8 shows differential pulse polarography (DPP) data of an Au surface coated with a monolayer based on CisH3₇HgOTs; raw data and base line (dashed) are shown in A; base line subtracted, first and second (dashed) scan are shown in B;

Figure 9 shows current-time transients at 0.3 V against the Ag|AgCl|3M LiCl electrode; a double logarithmic plot, recorded on a Ci₈H₃₇HgOTs modified Au surface is shown in A; a semi-logarithmic plot, recorded on a clean Au surface is shown in B; the straight lines have a slope of -1 ;

Figure 10 shows the results of cyclic voltammetry in solution of 2 mM K.3[Fe(CN)6] in 0.1 M KNO₃ on an Au plate, bare or modified with a monolayer deposited from Ci₈H₃₇HgOTs, or Ci₈H₃₇SH;

Figure 11 shows the imaginary component of Au electrode admittance in 0.2 mM Ci₈H₃₇HgOTs in acetonitrile and 0.1 M «-Bu₄PF₆; AC frequency: 64 Hz; amplitude: 5 mV; voltage scan start: -1.0 V;

Figure 12 shows complex impedance of an Au electrode in 0.2 mM CisH3₇HgOTs in acetonitrile measured at the potential 0.430 V against Ag|AgCl|3M LiCl at the maximum in AC polarograms; experimental points were fitted to an equivalent circuit;

Figure 13 shows the Hg(4f) XP spectrum (A) and the C(ls) XP spectrum (B) of a Ci₈H₃₇HgOTs based monolayer before and after electrochemical treatment;

Figure 14 shows (A) the XP-spectrum of a n-butyl(tosyloxy)mercury monolayer in the Hg 4f region, as-deposited (T = 24°C) and after thermal annealing (T = 95°C) and (B) the XP- spectrum in the C Is region of the as-deposited (T = 24°C) and annealed (T = 95°C) monolayers, showing that the thermal treatment does not affect the alkyl monolayer quality;

Figure 15 shows (A) the PM-IRRAS-spectrum for ra-butyl(tosyloxy)mercury monolayers on Au in the 2800-3000 cm^"1 region and (B) an analogous spectrum for butanethiol monolayers; Figure 16 shows (A) the removal of the Hg layer from rc-butyl(tosyloxy)mercury SAMs on Au by thermal annealing, as monitored with the Hg 4 XPS elemental area as a function of the temperature and (B) the temperature evolution of the C Is and O XPS Is elemental area, showing a progressive desorption of carbon at temperatures >100°C;

Figure 17 shows the kinetics of monolayer formation (thickness) for various organotin compounds on gold;

Figure 18 shows the contact angle of water on adsorbed monolayers based on di-n- butyldimethylstannane (1), di-«-butylstannane di-p-toluenesulfonate (2), di-«-butyldi(methyl- <¾)stannane (3), tetra-n-butyltin (4), n-octadecyltrimethyltin (5) or octadecane- 1 -thiol (6) on an Au surface (0 = control = untreated Au surface);

Figure 19 shows ATR-FTIR (infrared) spectra of adsorbed monolayers based on various organotin compounds;

Figure 20 shows cyclic voltammograms (A) and impedance response (B) of Fe(CN)6 ^{3~ 4~} of monolayers produced from the organotin precursors di-w-butyldimethylstannane (1) and di-n- butylstannane di-/ toluenesulfonate (3) in comparison to clean gold;

Figure 21 is a graph showing the chemical stability of monolayers based on di-n- butylstannane di-/?-toluenesulfonate on an Au surface against various reagents after a 18 h (dark grey) and a 44 h (light grey) period, wherein A is 0.1 M NaOH, B is 0.1 M H₂S0₄, C is 30 % H₂0₂, D is H₂0, E is 1 mM KMn0₄, F is 10 mM NaBH₄, G is w-hexane, H is wet CH₂C1₂ and I is dry CH₂C1₂;

Figure 22 is a graph showing the aging of a monolayer based on di-w-butylstannane di-/>- toluenesulfonate on an Au surface under laboratory conditions;

Figure 23 is a graph showing the thermal degradation of a monolayer based on di-n- butylstannane di-/?-toluenesulfonate on gold at 80, 140 and 200 °C under argon atmosphere;

Figure 24 shows XP spectra of di-ra-butyldimethylstannane and di-n-butylstannane di-/>- toluenesulfonate based monolayers on the surface of gold coated mica; Figure 25 summarizes the results obtained for organometallic compounds comprising Al, Li or Mg.

The present invention is now further described by means of the following examples, which are meant to illustrate, but not to limit the present invention:

Example 1 : Monolayers based on Hg compounds

Synthesis of organomercuric compounds rc-Octadecylmercuric tosylate, rc-C^H^H OTs Octadecylmercury(II) bromide (680 mg, 1.28 mmol) was dissolved in ethanol (99.5%, 100 mL) and silver (-toluene sulfonate (356 mg, 1.28 mmol) was added in one portion. The reaction mixture was stirred at room temperature overnight and filtered over cellite. The solid was washed with ethanol (99.5%, 30 mL). The solvent was evaporated, and the product was recrystallized from petroleum ether. Yield 480 mg (60 %).

M.p. 74 °C. Elemental analysis: calcd.: 48.02 % C, 7.09 % H, 5.13 % S, 32.08 % Hg; found 47.90 % C, 6.98 % H, 5.10 % S, 32.42 % Hg. 1H NMR (CDC1₃) _ 0.88 (t, J = 7 Hz, CH₃, 3H), 1.20 - 1.42 (m, s CH₂, 30H), 1.70 (quintet, J = 8 Hz, CH₂, 2H), 2.23 (d,t J_HCH_G = 213 Hz, JHCCH = 8 Hz, CH₂Hg, 2H), 2.29 (s, CH₃ -Ar, 3H), 7.13, 7.15, 7.48, 750, AB system, Ar). re-Butylmercuric tosylate. n-BuHgOTs (rc- HqHgOTs) Among the various possibilities, the method used was chosen because it yields a product that is relatively easy to purify. The starting compound was a mixture of w-butylmercuric chloride and bromide, prepared according to a literature procedure (Marvel, C. S.; Gould, V. L. J. Am. Chem. Soc. 1922, 44, 153). The mixture of these two compounds can be easily purified by crystallization from anhydrous ethanol furnishing a product in the form of blistering flakes. This was used as the starting material in the next step. To 1 g of this mixture of n-butylmercuric chloride and bromide dissolved in 50 mL of anhydrous ethanol (heating is sometimes necessary) an excess (1 g) of silver tosylate was added at once. The reaction mixture was stirred at 80 °C for 2 h, and then overnight at ambient temperature. Precipitated silver halides were removed by filtration through a celite pad and the filtrate was evaporated to dryness. The resulting crude product was dissolved in dichloromethane (50 mL), filtered through a layer of silica, and evaporated under reduced pressure. Resulting solids were dissolved in dichloromethane (~5 mL for 1 g of solids) and a mixture of hexanes was added gradually until mixture became cloudy. After 12 h at -20° C, the product was collected in the form of white wool, filtered, and dried in air.

Elemental analysis: Calcd.: 30.80 % C, 3.76 % H, 7.48% S. Found: 30.98 % C, 3.90 % H, 7.78% S. H NMR (CDC1₃): 0.92 (t, 8Hz, 3H, CH₃); 1.37 (m, 2H, CH₂); 1.63 (m, 2H, CH₂ ); 2.31 (m, 2H, CH₂ ); 2.39 (s, 3H, CH₃); 7.25, 7.27, 7.78, 7.80 (AB sys , 4H, ArH). ¹³C NMR (CDC1₃): 13.61 (CH₃); 21.67 (CH₂ ); 27.96 (CH₂ ); 30.19 (CH₂ ); 30.53 (CH₃); 126.64 (3,5- Ar-C); 129.58 (2,6-Ar-C); 139.33 (4-Ar-C); 142.56 (1-Ar-C).

Adsorbed monolayer formation

Glass substrates coated with 2 000 A of gold were purchased from Platypus Technologies and were cleaned by immersion in piranha solution (3: 1 sulfuric acid / hydrogen peroxide) at 90 °C for 30 s, rinsed with copious amounts of 18.2 ΜΩ water and absolute ethanol, and dried under a stream of nitrogen. Alternatively, substrates were flame annealed prior to use with a hydrogen torch for ca 30 s. Samples for XPS and STM measurements were prepared on gold deposited on mica. Atomically flat Au(l 11) surfaces were obtained by flame annealing of the substrates. Monolayers from octadecylmercuric tosylate were deposited by immersion of a gold substrate in a 1 x 10^"5 M solution in dry THF for ~2 h. rc-Butylmercuric tosylate monolayers were formed by immersion of a gold substrate in a 1 x 10^"3 M solution in dry THF for -1.5 h. After removal from solution, the gold substrates were rinsed three times with THF and dried under a stream of nitrogen prior to analysis.

Electrochemistry

Electrochemical measurements were performed using an AutoLab PGSTAT30 potentiostat/galvanostat equipped with a frequency response module (Metrohm AutoLab, The Netherlands). A four-electrode electrochemical cell was constructed for measurements of voltammetry and electrochemical impedance spectroscopy in a small area of a gold-covered plate. The cell was moved to different locations on the Au plate for improved statistics. The working electrode was Au(l l l) deposited on a glass substrate, with a total area of 0.64 cm². It was mechanically attached to a microscope table equipped with two manipulating screws enabling the choice of a particular spot on the gilded plate. The cell containing the solution was a glass tube mounted on a holder that can be vertically moved with a fine thread screw. The lower part of the cell tube was fitted to a piece of Teflon with an O-ring. Two reference electrode wires were mounted through the Teflon body. The DC reference electrode was an Ag|AgCl wire. The high-frequency reference electrode was a Pt wire coupled to the DC reference electrode via a 0.1 μ¥ condenser. The distance between the two reference electrodes and the working electrode was approximately 1 mm. The auxiliary electrode was a Pt net mounted on a Pt wire and immersed in the cell tube through its upper end, which was closed by a septum. The area of the net was sufficiently larger than the working electrode area to meet the requirement for obtaining a cell impedance that corresponds to the impedance of the working electrode. Prior to measurements the cell tube assembly was moved down, gently pressed against the Au plate, and filled with the test solution. Oxygen was removed from the solution by passing a stream of argon. The entire setup showed no liquid leaks and a good reproducibility of each area tested. The adsorption and the compactness of adsorbed layers were followed by an established method, using inhibition of electron transfer of the couple [Fe(CN)₆]^{4 / ~} in aqueous 0.1 M KC1 or KN0₃. Cyclic voltammetry was measured at a 0.007 V/s scan rate for the applied DC potential. Inhibition or desorption was followed by changes of the kinetic parameters of the redox exchange of [Fe(CN)6]^{4_ 3~}. Rate constants were evaluated by digital simulation using finite difference elements. Electrochemical impedance spectroscopy was measured in the range 100 kHz to 0.8 Hz using a 5 mV amplitude of the AC signal. The size of data collection was 120 points with a logarithmic distribution over the whole frequency range. Faradaic charge transfer resistance, reflecting quantitatively the inhibition efficiency, was evaluated by a simulation program Nova 1.8 supplied by the manufacturer of the electrochemical instrument. Layers showing a high degree of inhibition were evaluated by voltammetry, whereas at low inhibition the impedance method proved to be more sensitive. The time dependence of the double layer capacity C of Au and glassy carbon (GCE) electrodes was measured using small electrode discs (0.5 mm diameter) sealed in a glass capillary. Phase-sensitive AC polarography used a sine wave of 64 Hz frequency and 10 mV amplitude. The change of C-E dependence during the adsorption process was measured in 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile. All solution components were pre-dried.

Ellipsometry All measurements were made using a variable-angle Stokes ellipsometer (Gaertner Scientific) with a 633 nm HeNe laser with the incident angle adjusted to 70°. Optical constants of the gold substrates were taken for all freshly cleaned substrates. An index of refraction of 1.47 was assumed for the films. Ellipsometry measurements were taken at a minimum of five different areas on each sample.

Contact angle measurement

A static contact angle for 18.2 ΜΩ water was found with a CAMlOl instrument (KSV Instruments) using a 1 to 2 drop of water. Measurements were taken at a minimum of five different areas for each sample.

Infrared spectroscopy

FTIR-ATR spectra (800 scans, 4 cm^"1 resolution) were recorded in the range of 650-4000 cm^"1 using a Nicolet 6700 FT-IR spectrometer (Thermo Electron Corporation) with a liquid-N₂- cooled MCT detector. The data were collected with p-polarized light at 45° incidence using a Seagull variable-angle accessory (Harrick Scientific Inc.) and a Ge hemisphere (12.5 mm diameter). Prior to each measurement, the Ge crystal was cleaned with ethanol and a reference spectrum of the crystal in contact with air was measured.

X-ray photoelectron spectroscopy

XPS was recorded with a Kratos Axis Ultra instrument using a monochromated Al K_a emission source (1486.6 eV) operated at 15 kV and 180 W. The photoelectrons were collected by the spectrometer in normal emission geometry. With an X-ray monochromator and a pass energy of 40 eV for the analyzer, an instrumental energy resolution of -0.5 eV was achieved. The energy scale is referenced to the Au 4f₇/₂ line at a binding energy (BE) of 84.0 eV. For all samples, a survey spectrum and high resolution spectra of the Hg 4d, S 2p, C Is, O Is and Au 4f regions were acquired. The spectra were fitted using a linear background for all elements except Au, where the Shirley background was used. Voigt functions employing a 50:50 Lorentz-Gaussian ratio, including a slight asymmetry factor (instrumental) were used as fit functions. The line shape parameters were determined by least squares fitting to carbon or sulfur core level lines of known reference samples. Monolayer stability

The IR spectrum of each self-assembled monolayer was recorded. For stability measurements, the gold slide was immersed at room temperature for 17 to 20 h in CH2CI2, w-hexane, ethanol, water, 0.1 M H₂S0₄, 0.1 M NaOH, 1 mM KMn0₄, 30% H₂0₂, or 10 mM NaBH₄, removed from solution, rinsed thoroughly with either an appropriate solvent (CH2CI2, hexane, or ethanol) or with copious amounts of 18.2 ΜΩ Η₂0 and absolute ethanol in the case of water solutions, and dried under a stream of nitrogen before an IR spectrum was recorded. The samples were also exposed to the ambient laboratory atmosphere for 7 days and then rinsed with absolute ethanol and dried before the IR spectrum was measured. Prior to the IR measurements, the gold slides were rinsed with absolute ethanol and dried under a stream of nitrogen.

Results

Monolayer formation and ellipsometric characterization

Figure 1A shows the gradual increase of the ellipsometric thickness of the surface layers formed from 10^"5 moll^"1 solutions of CisF^HgOTs in THF and C1₈H₃7SH in ethanol. In both instances, the adsorption is self-limiting, and the growth of the adsorbed layer stops after about two hours at an ellipsometric thickness of -13 A for Ci8H37HgOTs and ~21 A for C1₈H₃7SH, regardless of whether the gold surface is cleaned with a hydrogen flame or a piranha solution. This is significantly less than the thickness of corresponding thiol-based layers. Figure IB shows the gradual increase of the ellipsometric thickness of the surface layers formed from 10^~3 M solutions of C4H₉HgOTs or Ci₈H₃₇HgOTs in THF. The adsorption is self-limiting, and the growth of the adsorbed layer stops after about 1.5 h at an ellipsometric thickness of ~3.5 A in the case of C4HgHgOTs, and after about two hours at an ellipsometric thickness of -13 A in the case

Contact angle goniometry

Contact angles of water on monolayers from Ci₈H₃₇HgOTs are 85° ± 3° when the gold substrates are cleaned by piranha solution (Figure 2A). They are higher than the 71 ± 4° angles measured on the initial gold substrate cleaned with piranha solution. The angle is much lower than the 100° observed for a monolayer from Ci₈H₃₇OTs adsorbed on gold cleaned by piranha solution. The angle is also much lower than the 113° observed for a monolayer from CisH₃₇SH adsorbed on gold cleaned by piranha (Figure 2B).

Infrared spectroscopy (IR)

Single-reflection attenuated total reflectance (ATR) IR spectra contain the typical peaks of alkyl chains (Figure 3) and those of the tosylate residue are absent. The spectra show only vibrations attributable to the CH₃ and CH₂ groups: stretching at 2854, v_s(CH₂), 2870, v_s(CH₃), 2925, v_as(CH₂), and 2959, v_as(CH₃), (all in cm^"1).

X-Ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectra show the presence of Hg and C and the absence of S at the gold surface in a fully grown CigH₃₇HgOTs monolayer, hence the tosylate residue is lost during the monolayer self-assembly process from Ci₈H₃₇HgOTs. The results are presented in Figures 4 and 5.

Scanning tunnelling microscopy (STM)

The lateral arrangement and periodicity of the alkylmercury tosylate derived adsorbates on Au(l l l) were also studied by STM. The considerable length of the alkyl chains and the expected high tunnelling resistance across the molecular layer forced us to use very low tunnelling currents for STM imaging (2 pA at a bias voltage of 400 mV). Two different monolayer phases were found (Figure 6). In one of these, the chains assume a lower density, with striped islands on Au(l l l) terraces. The other is a square phase, superposed on a wavy corrugation of the Au substrate. The square phase is characterized by a unit cell with unit vector length d of 0.64 ± 0.02 nm. The wave-like surface reconstruction of the Au surface shows a short periodicity of about 5-10 nm with a height amplitude of ~2.3 A, shorter than that of the known Au(l 11) herringbone reconstruction.

Chemical stability Chemical stability of the n-BuHgOTs derived monolayers was examined by monitoring the loss of their IR absorbance between 2800 and 3000 cm^"1 as a function of time after exposure either to the laboratory atmosphere for a week or to various solvents and reagents overnight, and is generally the same as that of monolayers obtained with

(Figure 7B). The stability is generally slightly lower than that of monolayers obtained with Ci₈H₃₇SH (Figure 7A). Only the resistance to oxidants is significantly higher. Even after the monolayers are partially or fully desorbed, no new absorption bands appear. There is not much difference between the two gold cleaning procedures, and the alkylmercury derived monolayers are at least as stable as the monolayers produced from trialkylstannyl precursors (see Example 2).

Electrochemistry

The electrochemical stability range is similar for monolayers derived from Ci₈H₃₇HgOTs (-

I, 38 to +1 ,40 V) and Ci₈H₃₇SH (-1,33 to +1 ,40 V). Cyclic voltammetry with aqueous [Fe(CN)₆]³7[Fe(CN)₆]^4" as a redox probe shows that the layer deposited from Ci₈H₃₇HgOTs, like those obtained from trialkylstannyl compounds, does not block the electrode much and reduces the cyclic voltammetric peak height to about one half (Figure 10). The blocking is incomplete even after an overnight immersion, whereas a 2-h immersion is sufficient to produce a monolayer of Ci8H3₇SH that suppresses the electrochemical response completely.

Successive scans by differential pulse polarography (DPP) (Bard and Faulkner, Electrochemical Methods. Fundamentals and Applications, 2^nd edition, J. Wiley, New York, 2001, p. 286-293) yielded mercury oxidation signals of gradually decreasing size (Figure 8), suggesting that elemental mercury was being removed. Determinations averaged over four different spots on a plate yielded a peak potential value of +135 ±5 mV and a peak current of

I I .9 ± 4.4 nA at an area of 4.02 mm².

When potential steps from -0.1 to +0.3 V were applied to an Au surface covered with a monolayer produced with CisHgOTs and immersed into aqueous 0.1 M KNO₃ (immersed area 0.32 cm²), transient currents i decayed in time (t) as lit (Figure 9), demonstrating that the current transient is not controlled by diffusion from the bulk of the solution and that the decay corresponds to the oxidation of surface confined Hg atoms. The similar hyperbolic i-t dependence for an adsorbed species has been described before ( oper, Z, Phys. Chem. 2003, 217, 547). Numerical integration yielded a charge of 48 C, which contains faradaic ( ¾) and double layer (Qc) contributions. Repetition of the experiment with a clean Au surface of the same area yielded Qc = 12.9 _C. The charging current decayed exponentially in time (Figure 9). From <2F = 35.1 mC, the surface concentration of Hg atoms is 1.14x l0^~9 mol/cm², or 1 Hg atom per 0.15 nm².

Electrochemical stripping of mercury

When the stripping potential of 0.3 V against the Ag|AgCl|3M LiCl electrode, estimated from the DPP results, was applied to the whole electrode for 1 h, the DPP peak of mercury disappeared, but XPS indicated that 65 % of the elemental Hg remained on the surface. Repeated scans of E from -1.0 V to +1.4 V lead to a gradual decrease of the double layer capacitance C between -I V and +1 V to a very low value, suggestive of 100% coverage by a compact layer (Figure 11). The decrease is especially apparent at positive potentials, whereas at negative potentials the initial capacitance is already smaller. The capacitance values show that outside this range Ci₈H₃₇HgOTs is still adsorbed to some extent, in a form of a less compact adsorbate.

The C - E plots contain a sharp AC peak at 0.5 V against the Ag|AgCl|3M LiCl electrode, absent at early and late times. Complex impedance at the potential of the peak maximum as a function of the applied frequency (Figure 12) is a semicircle connected to a mass transfer line, implying that the removal of mercury is a kinetically controlled process.

The results obtained on gold electrodes are quite unlike those obtained on a glassy carbon electrode, expected to be inert. On carbon the admittance data show no maximum and no substantial time evolution of the adsorption zones upon repeated voltage scans. Admittance at positive potentials never reaches very low and potential independent values.

The results shown in Figure 11 suggested an improved protocol for possible electrochemical removal of Hg from the surface. In a series of experiments Au plates were immersed for 2 h in a CigH₃₇HgOTs solution. The plate was then transferred to an electrochemical cell filled with 0.1 M TBAPF₆ and a potential of 1.0 V was applied for 1 h. XPS analysis indicated that the content of Hg on the surface dropped to about 20% of the original value. An even higher degree of Hg removal was achieved by initially immersing the Au plate in the electrochemical cell containing 10^"5 M Ci₈H₃₇HgOTs and in 0.1 M TBAPF₆ in THF and allowing the monolayer formation to take place at 1.0 V for a period of 4 h. The plate was then rinsed with THF and kept at a potential of 1.0 V in the solution of 0.1 M TBAPFg in acetonitrile. This procedure yielded an adsorbed layer containing less than 1% of the original content of Hg although C(ls) peaks were still present (Figure 13).

Thermal annealing of mercury

The w-alkyl SAMs were grown by soaking Au(l 1 1) substrates (100 nm of Au evaporated on mica, subsequently flame annealed) in a 1 mM THF solution of w-butyl(tosyloxy)mercury for 3h. At this stage, dense n-butyl monolayers showing a clear spectroscopic signature of Hg are formed. This self-assembly procedure is followed by the thermal annealing of the dry samples in vacuum at 90°C for lh. It is believed to trigger the diffusion of the initial surface Hg into the bulk Au und hence its complete removal, leaving a dense, metal-free n-butyl monolayer.

The nature of the as-prepared and thermally treated SAMs was verified by X-ray photoemission spectroscopy (XPS), I reflection adsorption spectroscopy (IRRAS), and contact angle goniometry. As shown in Figure 14A, XPS spectra of the Hg 4f region, acquired upon self-assembly, demonstrate the presence of elementary Hg° in amounts equal to those known for sulfur in densely packed alkanethiol SAMs (corresponding to the (Λ/3 X V3)R30° phase). The oxidation state of Hg is extracted from the chemical shift of the Hg 7/2 core level, showing a binding energy of 99.7 eV, which is in good agreement with previous reports from elemental Hg layers on Au (99.8 eV and 99.6 eV), as well as from solid Hg (99.9 eV).

The C \s signal (Figure 14B) confirms that exclusively aliphatic carbon (284.5 eV) covers the surface, even though the alkyl concentration is higher than expected when considering the model stoichiometry of C₄H₉HgOTs and the measured surface density of Hg. Based on the concentration of sulfonate, found at 167.4 eV, it appears that only about half of the OTs groups leave the surface upon self-assembly. The remaining OTs groups persist within the SAM, most probably coordinated to water molecules by hydrogen bonding (as inferred by the presence of oxygen in the XPS spectra), but can be removed by an additional rinsing step (see Figure 4). PM-IRRAS spectra in the C-H stretching region (Figure 15A) show the characteristic vibrational modes also known from alkyl chains in alkanethiol monolayers. The IR lines are significantly broader for n-butyl-Hg than for the butanethiol reference (Figure 15B), and the proportion of the peaks resembles that of butanethiol SAMs except for the symmetric C-H stretching mode of the methyl at 2876/cm, which appears strongly suppressed. The higher FWHM of the IR lines can generally be related to a higher degree of disorder in n-butyl than in butanethiol monolayers, whereas the lower intensity of the symmetric methyl stretching mode can be explained in terms of a transition dipole for the C¾ mode being oriented more parallel to the surface plane, i.e. more perpendicular to the surface normal than in case of butanethiol SAMs.

Remarkably, by thermal annealing of the sample at 90°C the elemental Hg is completely removed from the surface, as demonstrated in the Hg 4f spectrum in Figure 14A. Neither the XPS C Is signal (Figure 14A) nor the IRRAS spectrum (Figure 15) show changes upon the thermal annealing procedure, proving that the alkyl chains in the SAM remain structurally unaffected during annealing.

In order to get insight into the dynamics of the Hg removal process, a series of XPS scans in the C \s, O \s and Hg 4f regions was acquired under ultra high vacuum conditions while the temperature of the sample was ramped up from 20°C to 280°C. This is shown in Figure 16A, where a steep decrease in the atomic density of Hg clearly marks the threshold for Hg removal at a temperature ranging from 80°C to 90°C. It is interesting to note that also carbon undergoes a gradual lowering in the atomic density (Figure 16B), occurring however at higher temperatures and at a significantly slower rates. The C 1* intensity linearly decreases with temperature in the range from 100°C to 240°C, where it stabilizes at ½ of the initial carbon concentration. At temperatures above 240°C, pyrolysis of the n-alkyl layer occurs, expressed in a shift in the binding energy of the C 1 s component.

The inventors assume that the Hg layer - initially intercalated between Au surface and self- assembled ra-butyl layer - is ultimately removed at temperatures of 90°C and higher by thermal diffusion into the bulk Au substrate. This leaves the «-butyl chains directly attached to the Au surface, a fact that is supported by the relatively high stability of ra-butyl chains on Au as determined in thermodesorption data. Example 2: Monolayers based on Sn compounds

Synthesis of organotin compounds

Di-n-butyldimethylstannane was prepared from commercial dimethyldichlorostannane and n- butylmagnesium chloride according to a published procedure (Seyferth, D. J. Org. Chem. 1957, 22, 1509). It was isolated by distillation. Its boiling point and all spectroscopic data were in agreement with those published.

Di-w-butylstannane di-p-toluenesulfonate was prepared from commercial di-n-butyltin oxide and p-toluenesulfonic acid according to a published procedure (Seyferth, D.; Stone, F. G. A. J. Am. Chem. Soc. 1957, 79, 515). Its analytical and spectroscopic data were in agreement with those published.

Di-n-butyldifmethyl-ia stannane A solution of methyl-^-magnesium iodide in diethylether (1.0 M, 14.2 mL; 14.2 mmol) was added dropwise to a stirred solution of dibutyldichlorostannane (1.5 g; 4.94 mmol) in 50 mL of THF under inert atmosphere at RT. The reaction mixture was stirred 2 h at RT and then quenched by addition of 1 mL of water. THF was evaporated on vacuum evaporator and 50 mL of CH2CI2 was added to the residue. The mixture was extracted twice with 10 mL of water, dried over Na₂SC>4, filtered and evaporated on vacuum evaporator. The crude product was distilled at 6.0 Torr/90-92 °C) to give 4.25 g of the desired product (86.2 %).

Anal. Calcd for Ci₀ ¹Hi₈ ²H₅Sn: C, 44.64; H, 9.20. Found: C, 44.65; H, 9.00. (The content of hydrogen was calculated and found as for a compound containing only ^XH, i.e. 24 protons.) ^LH NMR (400 MHz, CDC1₃): 0.81 (t, 4H, ³J_HH = 8.0 Hz, CH₂-Sn); 0.89 (t, 6H, ³J_HH = 8.0 Hz, CH₃-CH₂), 1.29 (m, 4H, CH₂), 1.46 (m, 4H, CH₂). ¹³C APT (100 MHz, CDC1₃): 10.17 (s, CH₂-Sn), 13.86 (s, CH₃-CH₂), 27.26 (s, CH₂), 29.22 (s, CH₂). Signals of deuterated methyls were overlapped by noise.

Tetra-»-butyltin was purchased from ABCR GmbH & Co. KG (99 %, AB 182177, CAS Number: 1461-25-2). ra-Octadecyltrimethyltin was prepared from commercial tt-octadecylmagnesium chloride and trimethylstannyl chloride according to a published procedure ( hobragade, D et al. Langmuir 2010, 11, 8483). Its analytical and spectroscopic data were in agreement with those published.

Formation and analysis of self-assembled monolayers

Gold-coated glass surfaces (Platypus Technologies) were cleaned in piranha solution (3:1 sulphuric acid and hydrogen peroxide) at 90 °C, rinsed with H₂0 (18.2 ΜΩ) and absolute ethanol, and dried under argon. Self-assembled monolayers were produced by immersing the plates a into a 1 x 10^"4 M or 1 x 10^"5 M solution of the organotin compound in dry THF under argon or air atmosphere for 5 h. The plates were then thoroughly rinsed with dry THF and dried under nitrogen. Monolayers of 1-octadecanethiol used for comparison were formed by immersing the gold coated glasses in a 1 x 10^"5 M solution of 1-octadecanethiol in absolute ethanol under air atmosphere. Kinetics experiments were carried out in the time range from 10 min to 18 h in the dark.

All monolayers were tested by using a Variable Angle Stokes Ellipsometer (Geartner Scientific), Contact Angle Meter CAM 101 (KSV Instruments) and Infrared Spectrometer (Nicolet 6700 FT-IR/ATR, Thermo Electron Instruments). All measurements of the ellipsometric thickness, contact angle, and IR absorbance were repeated at least 4 times for each immersing time. The monolayers contained defect-free interface and were investigated by electrochemical blocking tests, by X-ray photoelectron spectroscopy (XPS) and by examinations of stability in various solutions. The di-n-butyldimethylstannane and di-n- butylstannane di-/ toluenesulfonate based monolayers were tested by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using a homebuilt four-electrode system. Electrochemical blocking properties were examined with Fe(CN)6^3_/4~ in 0.1 M KNO₃ solution. Electrochemical measurements were performed using an AutoLab PGSTAT302N potentiostat (Metrohm AutoLab).

The stability of monolayers produced from di-ra-butylstannane di-^-toluenesulfonate was investigated under a variety of conditions. The influence of various reagents at 20 °C and thermal desorption of the monolayer at 80, 140 and 200 °C under argon was observed. Moreover, the progressive aging of this monolayer during 4 weeks in the laboratory atmosphere was studied. The structures of monolayers were examined using X-ray photoelectron spectroscopy (XPS). The specimens of monolayers for XPS were prepared on gold surface of mica (Glimmer V3, Piano GmbH). The surface of Au substrates was cleaned by butane flame annealing and then was immersed in 5 x 10^"4 M solution of di-n-butyldimethylstannane and di-ra-butylstannane di-/?-toluenesulfonate under ambient atmosphere for 4 h.

Results

The kinetics of spontaneous formation of self-assembled monolayers on a gold surface immersed in a 1 x 10^"4 M or 1 x 10^~5 M solutions of various organotin compounds in ambient atmosphere were followed in the time interval from 10 min to 18 h. The growth of the di-ra- butyldimethylstannane and di-ra-butylstannane di-p-toluenesulfonate based monolayers stopped after about 5 h and that of the other tested compounds after about 3 h (Figure 17). The ellipsometric thickness of the di-n-butyldimethylstannane and di-w-butylstannane di-p- toluenesulfonate based monolayers was ~6 A with contact angles in the range 88 - 93°. The thickness of monolayers based on di-n-butyldi(methyl-ii_?)stannane and tetra-«-butyltin was ~7 A with contact angles in the range 86 - 95° and 97 - 103°, respectively. The thickness of the monolayer based on n-octadecyltrimethyltin was ~7.5 A with a contact angle in the range 94 - 99°. In contrast, the ellipsometric thickness of the 1 -octadecanethiol monolayer was around 22 A and the contact angle was -1 13°. Figure 18 shows the contact angles obtained for the films based on various organotin compounds and, in addition, on a cleaned gold surface after 5 h in dry THF (0).

FTIR-ATPv spectra of the monolayers are shown in Figure 19. The spectra of di-n- butyldimethylstannane and di-ra-butylstannane di-/?-toluenesulfonate based monolayers on gold show vibrations attributable to the CH₃ and CH₂ groups. The v_s(CH₂), v_s (CH₃), v_as (CH₂) and Vas (CH₃) bands are observed at 2854, 2870, 2925 and 2960 cm^"1. The v_s (CH₃) band is more distinct for both organotin substrates prepared under argon. The v_s (CH₂), v_s (CH₃), v_as (CH₂) and v_as (CH₃) bands for monolayer made based on the other test compounds are observed at 2854, 2871, 2926 and 2957 cm^"1.

The di-n-butyldimethylstannane and di-ra-butylstannane di-/?-toluenesulfonate based monolayers on Au-coated glasses and clean bulk gold, obtained by immersion for 5 h in 1 x 10^"4 M solution of the organotin compound, were analyzed electrochemically by CV and EIS. Figure 20 shows the cyclic voltammograms (A) and impedance spectra (B) as responses to 2 mM [Fe(CN)6]^3" in 0.1 M K O3 at sample interface. The electrochemical diagrams report the poor electrode blocking properties.

The stability of a di-n-butylstannane di-/?-toluenesulfonate based monolayer produced on an Au surface was investigated under a variety of conditions by monitoring the loss of I absorbance between 2800 and 3000 cm^"1 as a function of time after exposure either to the laboratory atmosphere (20°C, air humidity and daylight) during four weeks, and after exposure to various solvents at laboratory temperature for 14 and 44 h. The results are summarized in Figures 21 and 22. Moreover, a monolayer based on di-«-butylstannane di-p- toluenesulfonate was examined under argon after 1 h each at 80, 140 and 200 °C. Percentages of adsorbed monolayer remaining on the Au surface after this treatment are shown in Figure 23.

X-ray photoelectron spectra are shown in Figure 24.

Example 3 : Monolayers based on other organometallic compounds

Figure 25 summarizes the results obtained with organometallic compounds comprising Al, Li or Mg.

Monomolecular layers were formed with organoaluminum and organolithium compounds, but not with a organomagnesium compound, as shown by ellipsometry and ATR-FTIR spectrometry.

The features of the present invention disclosed in the specification, the claims, and/or in the accompanying drawings may, both separately and in any combination thereof, be material for realizing the invention in various forms thereof.

Claims

1. Method for attaching at least one organic moiety to an inorganic surface, said method comprising the steps of:

(a) providing a substrate having an inorganic surface;

2. Method for attaching at least one organic moiety to an inorganic surface, said method comprising the steps of:

(a) providing a substrate having an inorganic surface;

3. Method according to claim 1 or 2, wherein said metal surface is a surface of a metal selected from the group consisting of gold, platinum, palladium, copper, silver, zinc, indium, nickel and alloys thereof, preferably gold, platinum, palladium, copper, silver and alloys thereof, more preferably gold.

4. Method according to any of the foregoing claims, wherein said organic moiety is selected from the group consisting of unsubstituted or substituted alkyls, unsubstituted or substituted alkenyls, unsubstituted or substituted alkynyls, unsubstituted or substituted aryls, carbon nanotubes, graphene sheets and combinations of any of the foregoing, wherein, preferably, said organic moiety comprises a C2-C2₀ alkyl.

5. Method according to any of the foregoing claims, wherein,

when said metal atom is mercury, said organometallic compound has the general formula (R)Hg-X;

when said metal atom is tin, said organometallic compound has the general formula (R)_nSn(CH₃)(4_n₎ with n being an integer from 1 to 4, (R)_nSn(CH₃)(3__n)-X with n being an integer from 1 to 3, or (R)_nSn(CH3)(2-_n)-X2 with n being 1 or 2;

when said metal atom is aluminum, said organometallic compound has the general formula (R)_nAl(CH3)(3_n) with n being an integer from 1 to 3, (R)_nAl(CH3)(2-_n)-X with n being 1 or 2, or RA1X₂; and,

wherein R is said organic moiety and X is a leaving group.

6. Method according to claim 5, wherein said leaving group is selected from the group consisting of triflate, trifluoroacetate, tosylate and halides.

7. Method according to any of claims 1 and 3 to 6, wherein said thermal removal is performed by heating under vacuum, wherein, preferably, the temperature used for said thermal removal is in the range of from 80°C to 100°C.

8. Stable assembly of at least one organic moiety on an inorganic surface, produced by the method according to any of the foregoing claims, wherein said inorganic surface is a metal surface, a metal oxide surface or a semiconductor surface, preferably a metal surface.

9. Stable assembly of at least one organic moiety on an inorganic surface, wherein said at least one organic moiety is bound to said inorganic surface via direct interaction between said inorganic surface and at least one carbon atom of said organic moiety, wherein said inorganic surface is a metal surface, a metal oxide surface or a semiconductor surface, preferably a metal surface, and

wherein said stable assembly is substantially free of metal atoms selected from the group consisting of mercury, tin, aluminum, lithium, cadmium, bismuth, lead and thallium, preferably mercury, tin, aluminum and lithium.

10. Stable assembly according to claim 8 or 9, wherein said stable assembly is a monomolecular layer of said organic moiety on said inorganic surface.

11. Stable assembly according to any of claims 8 to 10, wherein said metal surface is a surface of a metal selected from the group consisting of gold, platinum, palladium, copper, silver, zinc, indium, nickel and alloys thereof, preferably gold, platinum, palladium, copper, silver and alloys thereof, more preferably gold.

12. Stable assembly according to any of claims 8 to 11, wherein said organic moiety is selected from the group consisting of unsubstituted or substituted alkyls, unsubstituted or substituted alkenyls, unsubstituted or substituted alkynyls, unsubstituted or substituted aryls, carbon nanotubes, graphene sheets and combinations of any of the foregoing, wherein, preferably, said organic moiety comprises a C2-C2₀ alkyl.

13. Device comprising a stable assembly according to any of claims 8 to 12, wherein, preferably, said device is selected from the group consisting of electronic devices, optoelectronic devices, medical devices and (bio-) sensor devices.

14. Use of an organometallic compound for attaching at least one organic moiety to an inorganic surface,

wherein said organometallic compound comprises a metal atom selected from the group consisting of aluminum, lithium, cadmium, bismuth, lead and thallium, preferably aluminum and lithium, and said organic moiety attached to

said metal atom,

wherein,

when said metal atom is aluminum, said organometallic compound has the general formula (R)_nAl(CH3)(3_n₎ with n being an integer from 1 to 3, (R)_nAl(CH3)(_2-„)-X with n being 1 or 2, or RA1X₂; and,

wherein R is said organic moiety and X is a leaving group.

15. Use of an organometallic compound for forming a monomolecular layer of an organic moiety on an inorganic surface,

wherein said organometallic compound comprises a metal atom selected from the

wherein,

when said metal atom is aluminum, said organometallic compound has the general formula (R)_nAl(CH3)(3_n) with n being an integer from 1 to 3, (R)_nAl(CH3)(_2-n)-X with n being 1 or 2, or RA1X₂;

when said metal atom is lithium, said organometallic compound has the general formula (R)Li; and,

wherein R is said organic moiety and X is a leaving group.

16. Use according to claim 14 or 15, wherein said leaving group is selected from the group consisting of triflate, trifluoroacetate, tosylate and halides.

17. Use according to any of claims 14 to 16, wherein said metal surface is a surface of a metal selected from the group consisting of gold, platinum, palladium, copper, silver, zinc, indium, nickel and alloys thereof, preferably gold, platinum, palladium, copper, silver and alloys thereof, more preferably gold.

18. Method according to any of claims 14 to 17, wherein said organic moiety is selected from the group consisting of unsubstituted or substituted alkyls, unsubstituted or substituted alkenyls, unsubstituted or substituted alkynyls, unsubstituted or substituted aryls, carbon nanotubes, graphene sheets and combinations of any of the foregoing, wherein, preferably, said organic moiety comprises a C2-C2₀ alkyl.