A low leakage current tip applicable to STM-BJ and its preparation method

Abstract

The present invention relates to a low leakage current tip applicable to STM-BJ and a preparation method thereof, including: preparing an unencapsulated gold tip; immersing the unencapsulated gold tip in an ethanol solution containing mercapto silane, and obtaining a gold tip with mercapto silane adsorbed on the surface after sufficient soaking; rinsing the gold tip with mercapto silane adsorbed on the surface with deionized water, and then immersing the gold tip with mercapto silane adsorbed on the surface in a hydrochloric acid aqueous solution, and obtaining a gold tip coated with a hydroxyl molecular film after sufficient reaction; isolating a part of the gold tip coated with the hydroxyl molecular film, exposing the area to be encapsulated, and putting it into an atomic layer deposition system; introducing an oxygen source and a hafnium source into the atomic layer deposition system, and depositing hafnium dioxide on the area to be encapsulated of the gold tip coated with the hydroxyl molecular film to obtain a low leakage current tip.

Description

Technical Field

[0001] The present invention relates to the field of STM-BJ, and specifically refers to a low leakage current tip applicable to STM-BJ and a preparation method thereof. Background Art

[0002] Scanning tunneling microscope break junction technology (STM-BJ) is an important technical means for studying material materials. The scanning tunneling electron microscope is an important scientific instrument for studying the surface and interface of material materials. The scanning tunneling electron microscope controls the contact and disconnection of a reciprocating tip and a fixed substrate, so as to repeatedly construct molecular junctions between the tip and the substrate for studying the characteristics of the surface and interface of material materials. The tip used in scanning tunneling microscope technology is the key to achieving atomic-level precise manipulation and imaging. For the characterization of solid-liquid interfaces in some energy or electrochemical systems, the tip needs to be fully encapsulated with an insulating material to greatly reduce the influence of solution ions on leakage current.

[0003] Currently, the conventional method for preparing the tip is to electrochemically etch a gold wire with a 1:1 solution of hydrochloric acid and ethanol to obtain a gold tip. The prior arts "Three-State Single-Molecule Naphthalenediimide Switch: Integration of a Pendant Redox Unit for Conductance Tuning" (Li, Y et al., Angew. Chem. Int. Ed. 2015, 54, 13586-13589) and "Scanning tunneling spectroscopy in anionic liquid" (Zheng, Y et al., J. Am. Chem. Soc. 2006, 128, 6574-6575.) have revealed that when the test system contains electrolytes or polar solvents, the tip encapsulated with black wax can avoid the interference of background leakage current. The tip is slowly passed through the high-temperature molten black wax, and the cooled black wax forms a protective layer on the surface of the tip. The tip is protected by an extremely thin layer of black wax. A small amount of gold atoms can be exposed by slightly contacting the substrate during use, greatly reducing the contact area between the gold electrode and the solution, and thus reducing the background leakage current through the tip and the gold sheet of the substrate. However, the applicant has found that when the tip encapsulated with black wax is used to test polar or strongly acidic or alkaline solvents, the encapsulating material black wax on the tip surface is easily soluble in polar or strongly acidic or alkaline solvents, which limits the selection of the test system.

[0004] Therefore, there is an urgent need to develop a new tip encapsulation technology applicable to various systems for expanding the research system of STM-BJ. Summary of the Invention

[0005] In view of the problems existing in the above-mentioned prior art, the present invention aims to provide a low-leakage current tip applicable to STM-BJ and a preparation method thereof, which can effectively solve at least one of the problems existing in the above-mentioned prior art.

[0006] The technical solution of the present invention is as follows:

[0007] A preparation method of a low-leakage current tip applicable to STM-BJ, comprising:

[0008] Preparing an unencapsulated gold tip;

[0009] Immersing the unencapsulated gold tip in an ethanol solution containing mercapto silane, and fully soaking to obtain a gold tip with mercapto silane adsorbed on its surface;

[0010] Rinsing the gold tip with mercapto silane adsorbed on its surface with deionized water, and then immersing the gold tip with mercapto silane adsorbed on its surface in an aqueous hydrochloric acid solution, and fully reacting to obtain a gold tip coated with a hydroxyl molecular film;

[0011] Isolating a part of the gold tip coated with a hydroxyl molecular film to expose the area to be encapsulated, and putting it into an atomic layer deposition system;

[0012] Introducing an oxygen source and a hafnium source into the atomic layer deposition system, and depositing hafnium dioxide on the area to be encapsulated of the gold tip coated with a hydroxyl molecular film to obtain a low-leakage current tip.

[0013] Further, the preparation of the unencapsulated gold tip includes:

[0014] Soaking the gold wire and the glass container in piranha solution, then taking out the gold wire and the glass container, and soaking and boiling the gold wire and the glass container with deionized water for several times;

[0015] Etching the gold wire with an electrochemical device to obtain an etched tip;

[0016] Ultrasonically cleaning the etched tip in anhydrous ethanol and ultrapure water in sequence to obtain the unencapsulated gold tip.

[0017] Further, the etching of the gold wire with an electrochemical device includes:

[0018] Connecting the working electrode end of the electrochemical device to the gold wire, connecting both the electrode and the auxiliary electrode ends of the electrochemical device to a gold ring, using fuming hydrochloric acid and anhydrous ethanol with a volume ratio of 1:1 as the etching solution, and etching the gold wire by the potentiostatic method, and setting the voltage of the potentiostatic method to 2.3V.

[0019] Further, the ethanol solution containing mercapto silane is an ethanol solution containing 10 mM (3-mercaptopropyl) trimethoxysilane;

[0020] The unencapsulated gold needle tip is immersed in an ethanol solution containing mercapto silane. One end of the mercapto group of the mercapto silane is adsorbed on the surface of the unencapsulated gold needle tip, and the sufficient soaking time is 25 - 35 minutes.

[0021] Further, the gold needle tip with mercapto silane adsorbed on its surface is immersed in an aqueous hydrochloric acid solution. The methoxy group of the mercapto silane hydrolyzes under the action of hydrochloric acid to form hydroxyl groups, and adjacent hydroxyl groups further undergo dehydration condensation to form a hydroxyl molecular film on the gold surface. The sufficient reaction time is 4 - 6 hours.

[0022] Further, the pH value of the hydrochloric acid solution is 4.5 - 5.5.

[0023] Further, the oxygen source is water; and / or

[0024] The hafnium source is hafnium tetrakis(dimethylamino).

[0025] Further, the reaction conditions for depositing 5 - 20 nm of hafnium dioxide on the area to be encapsulated of the gold needle tip coated with the hydroxyl molecular film are as follows:

[0026] Controlling the temperature of the vacuum reaction chamber of the atomic layer deposition system to be 180 - 220 °C, the temperature of the hafnium source to be 70 - 90 °C, and the nitrogen pressure to be 1.5 - 2.5 bar.

[0027] Further, the deposition thickness of the hafnium dioxide is 5 - 20 nm.

[0028] Further provided is a low leakage current needle tip applicable to STM - BJ, which is obtained by the preparation method of a low leakage current needle tip applicable to STM - BJ described above.

[0029] Therefore, the present invention provides the following effects and / or advantages:

[0030] Through the specific step method of the present application, hafnium dioxide can be deposited on the needle tip surface, and depositing hafnium dioxide can reduce the leakage current of the needle tip.

[0031] By selecting the deposition thickness of hafnium dioxide in the present application, a balance relationship between the encapsulation thickness and the use effect can be obtained. By selecting a hafnium dioxide thickness of 12.5 nm to 15 nm, the effect of reducing the leakage current by hafnium dioxide can be maximized, and at the same time, it is convenient for a small amount of gold atoms to be exposed when the gold needle tip is used.

[0032] Compared with the original encapsulating material, black wax, hafnium dioxide in the present application is insoluble in strongly polar solvents or strong acid and strong base solvents. The advantages of high - K materials such as hafnium dioxide deposited by atomic layer deposition are that they are denser, have a high dielectric constant, are resistant to strong acids, strong bases, and strong polar solvents, and the properties of hafnium dioxide are superior to those of conventional alumina and silica.

[0033] It should be understood that the foregoing summary and the following detailed description of the present invention are exemplary and explanatory, and are intended to provide further explanation of the present invention as claimed. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic flowchart of one embodiment of the present invention.

[0035] Figure 2 It is a schematic diagram of experimental data of one embodiment of the present invention. Among them, (a) is the CV curve of the exposed gold needle tip after etching in an aqueous solution of K3[Fe(CN)6]; (b) is the CV curve of the needle tip directly deposited with hafnium dioxide atomic layer after etching. One line represents the gold needle tip deposited with 10 nm hafnium dioxide, and the other line represents the gold needle tip deposited with 150 nm hafnium dioxide. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] For the convenience of those skilled in the art to understand, the embodiments will now be further described in detail with reference to the accompanying drawings for the structure of the present invention:

[0037] Reference Figure 1 , a preparation method of a low leakage current needle tip applicable to STM-BJ, comprising:

[0038] S1. Prepare an unencapsulated gold needle tip;

[0039] Further, the preparation of the unencapsulated gold needle tip includes:

[0040] S1.1. Immerse the gold wire and the glass container in piranha solution, then take out the gold wire and the glass container, and soak and boil the gold wire and the glass container in deionized water for several times;

[0041] S1.2. Etch the gold wire with an electrochemical device to obtain an etched needle tip. Specifically, the etching of the gold wire with the electrochemical device includes:

[0042] Connect the working electrode end of the electrochemical device to the gold wire, connect the electrode and the auxiliary electrode ends of the electrochemical device to the gold ring, use fuming hydrochloric acid and absolute ethanol with a volume ratio of 1:1 as the etching solution, and etch the gold wire by the potentiostatic method. The voltage of the potentiostatic method is set to 2.3 V.

[0043] S1.3. Ultrasonically clean the etched needle tip in absolute ethanol and ultrapure water in sequence to obtain the unencapsulated gold needle tip.

[0044] In this embodiment, specifically, the gold wire and the glass container are soaked in piranha solution for about 6 hours, and then the gold wire and the container are taken out and soaked and boiled in deionized water three times. Then, an electrochemical device is connected, with the working electrode terminal connected to the gold wire, and the counter electrode and the auxiliary electrode terminals both connected to a gold ring. The etching solution is fuming hydrochloric acid and absolute ethanol with a volume ratio of 1:1. The tip of the needle is etched by the potentiostatic method, and the potential is controlled at 2.3V. Finally, the etching program starts, and when the current drops to the background, the program is stopped. The obtained etched needle tip is taken out and ultrasonically treated in absolute ethanol and ultrapure water in sequence to obtain an unencapsulated gold needle tip.

[0045] S2, immerse the unencapsulated gold needle tip in an ethanol solution containing mercapto silane, and after sufficient soaking, obtain a gold needle tip with mercapto silane adsorbed on its surface;

[0046] Specifically, the ethanol solution containing mercapto silane is an ethanol solution containing 10 mM (3-mercaptopropyl) trimethoxysilane;

[0047] S3, rinse the gold needle tip with surface adsorbed mercapto silane with deionized water, and then immerse the gold needle tip with surface adsorbed mercapto silane in an aqueous hydrochloric acid solution. After sufficient reaction, obtain a gold needle tip coated with a hydroxyl molecular film;

[0048] The unencapsulated gold needle tip is immersed in an ethanol solution containing mercapto silane. One end of the mercapto group of mercapto silane is adsorbed on the surface of the unencapsulated gold needle tip. The sufficient soaking time is 25 - 35 minutes. In this embodiment, the sufficient reaction time for the unencapsulated gold needle tip immersed in the ethanol solution containing mercapto silane is 30 minutes.

[0049] Specifically, when the gold needle tip with surface adsorbed mercapto silane is immersed in an aqueous hydrochloric acid solution, the methoxy group of mercapto silane hydrolyzes under the action of hydrochloric acid to form hydroxyl groups, and adjacent hydroxyl groups further undergo dehydration condensation to form a hydroxyl molecular film on the gold surface. The sufficient reaction time is 4 - 6 hours. In this embodiment, the sufficient reaction time for the gold needle tip with surface adsorbed mercapto silane immersed in an aqueous hydrochloric acid solution is 5 hours.

[0050] In other embodiments, the sufficient reaction time can be adjusted according to specific circumstances or selected according to industry experience. No limitation is made here.

[0051] The mercapto silane selected in this embodiment can adsorb on the gold surface at one end of the mercapto group, and the other end of the methoxy group will hydrolyze under the action of hydrochloric acid to form hydroxyl groups. Adjacent hydroxyl groups further undergo dehydration condensation to form a hydroxyl molecular film on the gold surface, which is beneficial to the adsorption of the precursor on the gold surface during ALD, making the deposited hafnium dioxide denser and achieving a better insulation effect.

[0052] Further, the pH value of the hydrochloric acid solution is 4.5 - 5.5, and the pH value of the hydrochloric acid solution used in this embodiment is 5. 30 minutes and 5 hours are only the time required for sufficient adsorption and reaction.

[0053] S4. Isolate a part of the gold needle tip coated with the hydroxyl molecular film, expose the area to be encapsulated, and place it in an atomic layer deposition system;

[0054] Specifically, in this step, the gold needle tip coated with the hydroxyl molecular film can be fixed on a glass slide with a high-temperature tape, expose the part to be encapsulated, and place it in the sample tray of the atomic layer deposition system (ALD).

[0055] S5. Introduce an oxygen source and a hafnium source into the atomic layer deposition system, and deposit hafnium dioxide on the area to be encapsulated of the gold needle tip coated with the hydroxyl molecular film to obtain a low leakage current needle tip.

[0056] Further, the oxygen source is water; and / or

[0057] The hafnium source is hafnium tetrakis(dimethylamino).

[0058] Further, the reaction conditions for depositing 5 - 20 nm of hafnium dioxide on the area to be encapsulated of the gold needle tip coated with the hydroxyl molecular film are:

[0059] Control the temperature of the vacuum reaction chamber of the atomic layer deposition system at 180 - 220 °C, the temperature of the hafnium source at 70 - 90 °C, and the nitrogen pressure at 1.5 - 2.5 bar.

[0060] Specifically, in this embodiment, the temperature of the vacuum reaction chamber of the atomic layer deposition system is controlled at 200 °C, the temperature of the hafnium source is 80 °C, and the nitrogen pressure is 2 bar.

[0061] Further, the deposition thickness of the hafnium dioxide is 5 - 20 nm.

[0062] In this step, by using hafnium tetrakis(dimethylamino) (TEMAH) as the hafnium source, water as the oxygen source, the temperature of the vacuum reaction chamber at 200 °C, the temperature of the hafnium source at 80 °C, and the nitrogen pressure at 2 bar in ALD, the thickness of the encapsulation layer can be controlled by adjusting the number of cycles, and the deposition thickness range is 5 - 20 nm.

[0063] Among them, parameters such as the temperature of the vacuum reaction chamber being 200 °C, the temperature of the hafnium source being 80 °C, and the nitrogen pressure being 2 bar are the parameters corresponding to the deposition of hafnium dioxide by the instrument itself, and are parameters reasonably selected by the inventor according to the actual situation and through the use of the hafnium source for deposition by ALD. The deposition thickness of hafnium dioxide is 5 - 20 nm. Hafnium dioxide within this range can play a role in reducing leakage current interference. After being greater than 12.5 nm, increasing the thickness has a significant effect on reducing the leakage current. Therefore, selecting a deposition thickness of 12.5 nm of hafnium dioxide can achieve the advantages of both effect and efficiency.

[0064] Furthermore, a low-leakage current tip applicable to STM-BJ is provided, which is obtained by the preparation method of a low-leakage current tip applicable to STM-BJ described above.

[0065] Experimental data

[0066] Reference Figure 2 , (a) CV curve of the etched bare gold tip in an aqueous solution of K3[Fe(CN)6]; (b) CV curve of the tip directly deposited with hafnium dioxide atomic layer after etching in an aqueous solution of K3[Fe(CN)6], where one line represents the gold tip deposited with 10 nm of hafnium dioxide, and the other line represents the gold tip deposited with 150 nm of hafnium dioxide. Reference Figure 2 (b) Applying the same voltage to the two tips, the tip deposited with 15 nm of hafnium dioxide has a smaller current than the tip deposited with 10 nm of hafnium dioxide, indicating that the encapsulation effect of 15 nm is better. At the same time, through the inventor's experimental summary, it is obtained that the gold tip deposited with 12.5 nm of hafnium dioxide achieves the best balance between the encapsulation thickness and the use effect. When the deposition thickness of hafnium dioxide reaches 12.5 nm, the influence of the encapsulation thickness on the use effect is small. The tip encapsulated with hafnium dioxide with a thickness greater than 20 nm is not conducive to exposing a small amount of gold atoms in subsequent use. Traditional black wax encapsulation is manual encapsulation, so the encapsulation thickness is inconsistent and the encapsulation effect is uncontrollable. In this application, the encapsulation thickness can be controlled by deposition, so as to obtain the best effect.

[0067] Comparison Figure 2 (a) and Figure 2 (b), it can be seen that the redox potential remains basically unchanged before and after encapsulation, but the peak current of the encapsulated 10 nm is reduced to about 1 / 3 of that of the unencapsulated. On the basis of the encapsulated 10 nm, the encapsulated 15 nm is further reduced to about 1 / 2, proving that hafnium dioxide can reduce the leakage current.

[0068] It is an experimental fact that black wax encapsulation is soluble in solvents such as TMB and is unstable in strong acid or alkaline solvents. Compared with the original encapsulating material, black wax, hafnium dioxide in this application does not dissolve in solvents with strong polarity or strong acid or alkaline solvents. The advantages of high-K materials such as hafnium dioxide deposited by atomic layer deposition are that they are denser, have a high dielectric constant, are resistant to strong acid, strong alkali, and strong polar solvents, and the properties of hafnium dioxide are superior to those of conventional alumina and silica.

[0069] It should be noted that in the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps not listed in the claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The present invention can be implemented by means of hardware including several different elements and by means of a suitably programmed computer. In a unit claim listing several devices, several of these devices can be embodied by the same item of hardware. The use of the words first, second, and third, etc. does not denote any order. These words can be interpreted as names.

[0070] Although the preferred embodiments of the present invention have been described, additional changes and modifications can be made by those skilled in the art once they learn the basic creative concept. Therefore, the appended claims are intended to be construed to cover the preferred embodiments as well as all changes and modifications falling within the scope of the present invention.

[0071] In the present invention, unless otherwise clearly specified and defined, terms such as "mounted", "connected", "coupled", "fixed", etc. shall be construed in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral body; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the internal communication of two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0072] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms should not be understood as necessarily referring to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without conflict, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

Claims

1. A preparation method of a low leakage current tip applicable to STM-BJ, characterized in that: Comprising: Preparing an unencapsulated gold needle tip; Immersing the unencapsulated gold needle tip in an ethanol solution containing mercapto silane, and fully soaking to obtain a gold needle tip with mercapto silane adsorbed on its surface; Rinsing the gold needle tip with mercapto silane adsorbed on its surface with deionized water, and then immersing the gold needle tip with mercapto silane adsorbed on its surface in an aqueous hydrochloric acid solution. After sufficient reaction, a gold needle tip coated with a hydroxyl molecular film is obtained. Among them, the mercapto group of mercapto silane is adsorbed on the gold surface, and the methoxy group of mercapto silane is hydrolyzed under the action of hydrochloric acid to form hydroxyl groups. The adjacent hydroxyl groups are further dehydrated and condensed to form a hydroxyl molecular film on the gold surface, which is beneficial to the adsorption of the precursor on the gold surface during ALD, making the deposited hafnium dioxide denser and achieving a better insulation effect. The time for sufficient reaction is 4 - 6 hours; Isolating a part of the gold needle tip coated with a hydroxyl molecular film, exposing the area to be encapsulated, and placing it in an atomic layer deposition system; Introducing an oxygen source and a hafnium source into the atomic layer deposition system, and depositing hafnium dioxide on the area to be encapsulated of the gold needle tip coated with a hydroxyl molecular film to obtain a low leakage current needle tip. Among them, the deposition thickness of the hafnium dioxide is 5 - 20 nm. The reaction conditions for depositing 5 - 20 nm of hafnium dioxide on the area to be encapsulated of the gold needle tip coated with a hydroxyl molecular film are: controlling the temperature of the vacuum reaction chamber of the atomic layer deposition system at 180 - 220 °C, the temperature of the hafnium source at 70 - 90 °C, and the nitrogen pressure at 1.5 - 2.5 bar.

2. The preparation method of a low leakage current tip applicable to STM-BJ according to claim 1, characterized in that: The preparation of the unencapsulated gold needle tip includes: Soaking the gold wire and the glass container with piranha solution, then taking out the gold wire and the glass container, and soaking and boiling the gold wire and the glass container with deionized water for several times; Etching the gold wire with an electrochemical device to obtain an etched needle tip; Ultrasonically cleaning the etched needle tip in anhydrous ethanol and ultrapure water in sequence to obtain the unencapsulated gold needle tip.

3. The preparation method of a low leakage current tip applicable to STM-BJ according to claim 2, characterized in that: The etching of the gold wire with an electrochemical device includes: Connecting the working electrode end of the electrochemical device to the gold wire, connecting both the electrode and the auxiliary electrode end of the electrochemical device to a gold ring, using a fuming hydrochloric acid and anhydrous ethanol mixture with a volume ratio of 1:1 as the etching solution, and etching the gold wire by the potentiostatic method. The voltage of the potentiostatic method is set at 2.3 V.

4. The preparation method of a low leakage current tip applicable to STM-BJ according to claim 1, characterized in that: The ethanol solution containing mercapto silane is an ethanol solution containing 10 mM (3-mercaptopropyl) trimethoxysilane; When the unencapsulated gold needle tip is immersed in the ethanol solution containing mercapto silane, one end of the mercapto group of mercapto silane is adsorbed on the surface of the unencapsulated gold needle tip, and the time for full soaking is 25 - 35 minutes.

5. The preparation method of a low leakage current tip applicable to STM-BJ according to claim 1, characterized in that: The pH value of the hydrochloric acid solution is 4.5 - 5.

5.

6. The preparation method of a low leakage current tip applicable to STM-BJ according to claim 1, characterized in that: The oxygen source is water; and / or The hafnium source is tetrakis(dimethylamino)hafnium.

7. A low leakage current tip applicable to STM-BJ, characterized in that: Obtained by the preparation method of a low leakage current needle tip for STM-BJ according to any one of claims 1 - 6.

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