JP3724302B2 - Preparation of ultra-thin films of sparingly soluble molecules - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention belongs to the technical field of thin film production, and particularly relates to a novel method for producing an ultrathin film of a hardly soluble molecule such as fullerene.
[0002]
[Prior art]
A thin film (ultra-thin film) with a thickness of from a single molecule to a micrometer level is used extensively as a research tool for studying the surface structure and physical properties or exploring the reaction mechanism, and is practically an optical component. It is also important because it is applied in a wide range of fields as various functional elements such as electronic devices. For example, fullerene epitaxial thin films have been energetically studied from both the fundamental and application aspects because of their unique and ideal molecular film structure.
[0003]
Conventionally, the most commonly used method for producing a thin film is a vacuum deposition method. However, it is difficult to perform the vacuum deposition method using a complicated and expensive apparatus including a vacuum means and a heating means. Furthermore, the object which can produce a thin film by a vacuum evaporation method is restricted to what is stable under a vacuum (reduced pressure) and heating.
[0004]
In addition to a dry process such as vacuum deposition, a simple wet process (wet process) on the device also allows organic molecules to be adsorbed electrochemically onto a substrate such as a metal single crystal in solution. There is also a thin film manufacturing method based on the above. However, the weak point of such a wet process is adsorption from a solution, so that it is impossible to produce a thin film that does not dissolve in the solution, and at present, the solvent is limited to an aqueous system due to problems such as adsorption of the solvent itself. It is that you are.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide a new technique capable of easily forming a thin film (ultra-thin film) from various hardly soluble molecules (compounds) by a wet process.
[0006]
[Means for Solving the Problems]
As a method for producing a thin film by a wet process, the present inventor can achieve the above object by transferring a thin film previously produced at a gas-liquid interface to a metal substrate modified with a specific substance. The present invention has been found out and the present invention has been derived.
[0007]
Thus, according to the present invention, a step of developing a poorly soluble molecule thin film by developing a poorly soluble molecule in the liquid constituting the gas-liquid interface on the gas-liquid interface, A step of transferring onto a metal substrate modified with an electrochemically detachable substance, and applying a potential in solution to the modified metal substrate onto which a thin film has been transferred, so that the modifying substance is removed from the substrate. There is provided a method for producing a thin film of a sparingly soluble molecule, characterized by comprising a step of electrochemically desorbing.
[0008]
The thin film production method of the present invention is preferably carried out when the liquid constituting the gas-liquid interface is generally water. According to a preferred embodiment of the thin film preparation method of the present invention, the electrochemically detachable substance is halogen or sulfur.
[0009]
A preferred example to which the thin film preparation method of the present invention is applied is a case where the hardly soluble molecule is fullerene, the electrochemically detachable substance is iodine, and the metal substrate is gold.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a method for producing a thin film of various insoluble molecules (compounds) by a wet process as a Langumuir film or a Langumuir-Blodgett film (hereinafter referred to as “Langumuir-Blodgett” film) produced at a gas-liquid interface. , Referred to as LB film) on the surface of the metal single crystal. Hereinafter, embodiments of the present invention will be described along the respective steps constituting the thin film manufacturing method of the present invention.
[0011]
Step of forming a thin film on the gas-liquid interface In order to carry out the method of the present invention, first, a molecule that is sparingly soluble in the liquid constituting the gas-liquid interface is developed on the gas-liquid interface. A thin film of soluble molecules is formed. This step is performed according to a LB film manufacturing method well known in the art.
[0012]
That is, a solution having an appropriate concentration in which a hardly soluble molecule to be formed into a thin film is dissolved in an appropriate solvent (developing solvent: a solvent that dissolves the hardly soluble molecule and is incompatible with the liquid constituting the gas-liquid interface). Is dripped onto the gas-liquid interface to develop the hardly soluble molecule. After deployment, the solution is left to stand for a certain period of time to completely evaporate the development solvent, and then the area of the gas-liquid interface where the molecules are developed is reduced (compression of the membrane). A target thin film (LB film) can be obtained by stabilization.
[0013]
The liquid that constitutes the gas-liquid interface is generally water. Therefore, water (ultra-pure) is dissolved in an organic solvent such as hexane or chloroform using a poorly soluble (water-insoluble) molecule as a developing solvent. Dripping and spreading on the water surface. However, the method of the present invention is similarly applied to the case where a non-aqueous solvent (organic solvent) is used as a liquid constituting the gas-liquid interface, and a thin film is formed by a LB film preparation method from molecules that are hardly soluble in these solvents. it can.
[0014]
In addition, the thin film formed by the above operation is generally a monomolecular film, but is not necessarily limited to a monomolecular film. When a monomolecular layer or more is formed, an appropriate solvent is used later. May be washed away, or a multi-layer may be formed depending on the purpose. Thus, the term “thin film” or “ultra-thin film” as used in connection with the present invention refers to a film having a thickness of a monomolecular layer to a multi-molecular layer level of several μm.
[0015]
Transfer process onto substrate In the thin film manufacturing method of the present invention, the thin film (LB film) obtained by the above-described process is then transferred onto a metal substrate. This operation is generally performed by a horizontal deposition method or a vertical dipping method well known in the art.
[0016]
That is, a metal substrate placed almost parallel to the gas-liquid interface is gently brought into contact with, attached to, and attached to the thin film (LB film) (horizontal adhesion method: see FIG. 1A), or a thin film (LB) at the interface. After the metal substrate is immersed in the liquid in which the film is formed, the LB film is attached by moving in the vertical direction (vertical immersion method). Here, the thin film manufacturing method of the present invention is characterized in that an LB film is brought into contact with the surface of a metal substrate modified with an electrochemically detachable substance and transferred to the substrate.
[0017]
An electrochemically detachable substance is a substance that adsorbs to a metal substrate to modify the metal and can be detached from the metal substrate when a voltage of a specific value is applied in the solution. Preferable examples of the electrochemically detachable substance include halogen or sulfur. In addition, a thiol compound or the like can also be used.
[0018]
The metal used as the substrate is not particularly limited, and is selected from metals that can be adsorbed depending on the combination of the poorly soluble molecule to be prepared for the thin film and the electrochemically detachable substance. . For example, when the poorly soluble molecule forming the thin film is fullerene, iodine is preferable as the electrochemically detachable substance, and gold is preferable as the metal substrate.
[0019]
In accordance with the method of the present invention, a thin film of poorly soluble molecules is transferred directly to the metal surface as described later by transferring the hardly soluble molecule thin film to the metal substrate through the electrochemically detachable substance. Compared to the case, a thin film with significantly improved uniformity can be manufactured.
[0020]
Electrochemical desorption step The modified metal substrate from which the thin film has been transferred is then desorbed of a modifying substance such as halogen from the substrate by applying a voltage in a solution. The desired thin film (ultra thin film) is obtained.
[0021]
Specifically, this step is performed by placing the metal substrate from which the thin film (LB film) has been transferred into an electrochemical cell. That is, while using the substrate as one electrode, a counter electrode and a reference electricity are installed, and a voltage (potential) is applied and controlled in an electrolyte solution to which an appropriate electrolyte is added as required, and current-potential characteristics Is measured to confirm the desorption of the modifying substance from the substrate.
[0022]
This step is preferably performed in an electrochemical STM cell capable of STM (scanning tunneling microscope) observation, thereby directly observing the molecular arrangement structure (including defect structure) of the resulting thin film, In addition, a lot of knowledge about intermolecular interactions can be obtained.
[0023]
According to the method of the present invention comprising the steps described above, a thin film (ultra-thin film) can be produced from various hardly soluble molecules (compounds) that have been difficult in the conventional wet process. Examples of molecules that can be prepared by the method of the present invention include organic molecules such as fullerene, adamantane, and various aromatic compounds (perylene, phthalocyanine, porphyrin, anthraquinone, etc.), or metal complexes such as metal clusters and cyclams. However, the present invention is not limited to these examples. Furthermore, it is also possible to produce thin films of poorly soluble and unstable compounds (for example, proteins and various polymers) that could not be produced by vacuum deposition.
[0024]
Furthermore, according to the method for preparing an ultrathin film of the present invention, it is difficult to apply a specific voltage to a metal substrate on which a thin film of poorly soluble molecules is adsorbed via an electrochemically detachable substance. Compared to the case where a thin film of soluble molecules is directly transferred and adsorbed, a thin film with extremely improved uniformity can be obtained. This is because by modifying the surface of the substrate with halogen or the like, the interaction between the sparingly soluble molecule and the substrate becomes weak, so that the molecule moves around the substrate surface (see FIG. 1B) and is set to a predetermined potential. This is probably because the substrate-modifying substance is slowly replaced with a hardly soluble molecule on the substrate to form a highly uniform molecular film that is epitaxially controlled by the substrate (see FIG. 1C). As described above, in the method of the present invention, a desired thin film can be obtained while controlling the adsorption rate of the hardly soluble molecule by potential operation.
[0025]
According to the method of the present invention, after transferring the poorly soluble molecule thin film (LB film) formed at the gas-liquid interface to the solid-liquid interface, by controlling the adsorption / desorption of the modifying substance and the hardly soluble molecule by potential operation, An ultra-thin film is produced that is epitaxially controlled by a solid (substrate) rather than a gas-liquid interface structure.
[0026]
Furthermore, according to the thin film preparation method of the present invention, even a poorly soluble molecule can be converted into an LB film and transferred to a solid-liquid interface, and electrochemically controlled, thereby allowing not only an epitaxial film but also an intermolecular interaction. Therefore, it is considered that film formation based on two-dimensional self-organization also occurs. This is also inferred from the fact that a C ₇₀ thin film, which is said to be less likely to form an epitaxial arrangement in a single phase, can be produced by the method of the present invention. As described above, the method of the present invention makes it possible to produce a thin film of a molecule (compound), which was considered impossible or difficult to form a thin film according to conventional knowledge.
[0027]
【Example】
In the following, the case of producing a fullerene thin film according to the present invention will be described as an example. However, this example is merely illustrative and does not limit the present invention.
_{C 60, C 70} as prepared <br/> fullerenes of the fullerene thin film, and using a _C 60 _{/ C 70} mixture. The fullerene-benzene solution was developed on an LB trough (ultra pure water) to form a fullerene molecular film at the gas / liquid interface. From the result of the surface pressure-area curve, the surface pressure (10-20 mNm ⁻¹ ) considered to be a monomolecular film was formed, and the LB film was transferred to the cleaned substrate by the horizontal adhesion method. As the substrate, an unmodified single crystal Au (111) surface or a single crystal Au (111) surface modified with iodine (hereinafter sometimes referred to as I / Au (111)) was used. Au (111) or I / Au (111) from which the LB film was transferred was set in an electrochemical STM cell, and STM observation was performed while measuring current-voltage characteristics in a 0.1 M HClO ₄ aqueous solution. In the STM observation, the probe was measured in a constant current mode using an electropolished tungsten tip.
[0028]
Current - voltage characteristics and STM observation <br/> Figure 2, I / Au (111) was transferred to a substrate made of a plane taken the C ₆₀ thin film of current - shows the potential characteristics (cyclic voltammogram). The horizontal axis represents voltage (V) with respect to the reference electrode (SCE), and the vertical axis represents current value (μA / cm ² ).
[0029]
To the separated is removed slowly iodine that modified the substrate (Au) and held for approximately -0.05 (V) the potential at the first scan, as shown in solid lines in FIG. 2 is replaced with C ₆₀ molecules, C ₆₀ An epitaxial thin film was formed, and as shown by the broken line in the figure, it was confirmed that it was stably present in perchloric acid in a wide potential region of +0.75 to −0.25 (V).
[0030]
Figure 3 is a STM photograph of _{C 60} thin film prepared by taking transferred LB film of _{C 60} on the substrate [I / Au (111)] in accordance with the present invention. The observed fullerene film is a molecular film highly arranged in hexagonal, and the structure arranged along the Au atomic lattice (in phase arrangement) and the arrangement arranged 30 degrees apart (2√3 × 2√3R30 degree arrangement) It consists of two structures. The same structure as these two has been confirmed even in an epitaxial film fabricated by vacuum deposition, and it can be seen that the structure is controlled epitaxially by the substrate, not by the structure at the gas-liquid interface.
[0031]
Figure 4 shows, for comparison, transferred directly up method, i.e., is a STM photograph of C ₆₀ films made adsorbed directly without Au (111) substrate having modification with iodine molecule comprising a large defect structure There are many disordered arrangements.
[0032]
Further, for the C ₆₀ / C ₇₀ mixture, the LB film was transferred to an I / Au (111) substrate, and a thin film was similarly produced. This was also an epitaxial molecular film filled in hexagonal like the C ₆₀ thin film. In the STM observation of this mixed film, two points with clearly different brightness (height) are mixed at random compared to FIG. 3, and the bright (high) point is about 0.1 nm higher than the dark (low) point. It was confirmed. Therefore, C ₇₀ molecules in a state where the higher has caused a major axis (standing-Stay up-sequence), low more is considered to be a C ₆₀ molecule. A similar mixed image is also observed for epitaxial films fabricated by vacuum deposition.
[0033]
In contrast, similarly the LB film I / Au (111) low sequence of two-dimensional surface is transferred up iodine to the substrate at C ₇₀ thin film fabricated by electrochemical desorption also C _70, partially Although epitaxial growth was confirmed, many missing regions and discontinuous structures were also observed. This was consistent with the fact that a stable monomolecular film was not formed at the _C70 gas-liquid interface.
[0034]
【The invention's effect】
According to the present invention, it is simpler than the vacuum deposition method, and therefore, an ultra-thin film having a very high uniformity can be produced from a wide range of molecules (compounds) including a hardly soluble molecule by a low cost method.
[Brief description of the drawings]
FIG. 1 is a schematic view showing a state of hardly soluble molecules and the like in an ultrathin film production method of the present invention.
Figure 2 shows a cyclic voltammogram of iodine modified Au substrate took transferred C ₆₀ film in accordance with the present invention.
FIG. 3 is a scanning tunneling micrograph of a C ₆₀ thin film prepared by performing electrochemical substitution on an iodine modified Au substrate according to the present invention.
FIG. 4 is a scanning tunneling micrograph of a C ₆₀ thin film prepared by transferring directly to an Au substrate.

Claims (4)

A step of developing a poorly soluble molecule thin film by developing a poorly soluble molecule in a liquid constituting the gas-liquid interface on the gas-liquid interface; A step of transferring onto the metal substrate modified with the substance, and applying a voltage in solution to the modified metal substrate onto which the thin film has been transferred, thereby electrochemically desorbing the modifying substance from the substrate A method for producing a thin film of a sparingly soluble molecule comprising a step. 2. The method for producing a thin film according to claim 1, wherein the liquid constituting the gas-liquid interface is water. 3. The method for producing a thin film according to claim 2, wherein the electrochemically detachable substance is halogen or sulfur. 4. The thin film production method according to claim 3, wherein the hardly soluble molecule is fullerene, the electrochemically detachable substance is iodine, and the metal substrate is gold.

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