A reversible single-molecule switch based on local cation regulation

Abstract

The application discloses a reversible single-molecule switch based on local cation regulation and belongs to the technical field of molecular electronics. The application is based on the crack junction technology of an electrochemical scanning tunneling microscope. The distribution of local cations in an interface double electric layer is changed by controlling an electric potential, so that the state of a carboxylic acid molecule is influenced. The contact action of the carboxylic acid molecule and a gold needle tip is adjusted, and a single-molecule switch is realized. The electric potential is-0.5-0V. The application proves the influence of local cations in an electrochemical interface on carboxyl molecules, and opens up a new way for realizing the practical application of a reversible single-molecule switch through a gate electrode.

Description

Technical Field

[0001] This invention belongs to the field of molecular electronics technology, specifically relating to a reversible single-molecule switch based on local cation regulation. Background Technology

[0002] With the increasing demand for miniaturization of electronic devices, the use of molecules with similar chemical properties as functional elements has become a rapidly developing branch of nanoscience. Since molecular switches play a crucial role in information storage, logic data processing, and signal processing, constructing reliable molecular switches is an important step. Over the past three decades, a common approach to designing single-molecule switches has been to use stimulus-responsive molecular frameworks, utilizing external stimuli such as light, mechanics, pH, chemical reactants, magnetism, and electricity to alter molecular conformation or spin / redox states. However, this typically requires complex organic synthesis to obtain stimulus-responsive molecular frameworks, and currently, molecular switches using this method have reportedly small on / off states, and most external stimulation methods cannot be applied to fully electrically driven circuit components. On the other hand, molecular-metal contact interactions can also significantly affect electron transport. Controlling molecular-electrode contact interactions in situ while maintaining the molecular backbone is challenging. Electrochemical methods can effectively alter the charge state of the electrode surface by applying a potential, modulating the distribution of ions in the interfacial double layer, and thus affecting the interaction between carboxyl groups and the metal electrode. Based on this, by electrochemically adjusting the local ion type of the outer Helmholtz layer in the electrode interface double layer, the connection between the electrode and the carboxylic acid molecule can be controlled, which provides a new approach for realizing reversible high-performance single-molecule switches. Summary of the Invention

[0003] The purpose of this invention is to provide a reversible single-molecule switch based on local cation regulation. By controlling the ion type in the Helmholtz layer outside the electrode interface through potential regulation, the contact interaction between the molecule and the needle tip can be controlled to achieve a sudden change in conductivity, thus realizing a single-molecule switch.

[0004] To achieve the above objectives, the present invention proposes the following technical solution:

[0005] A method based on localized cations (Na) + K + Ca 2+ Mg 2+ A reversible single-molecule switch regulated by electrochemical scanning tunneling microscopy (EC-STM-BJ) is based on metallic gold as a substrate and a carboxyl-containing compound as the target molecule. By controlling the potential to change the distribution of local cations in the interfacial double layer, the state of carboxylic acid molecules is affected, thereby regulating the contact between carboxylic acid molecules and gold needle tips to achieve a single-molecule switch.

[0006] The potential is -0.5 to 0V.

[0007] Furthermore, the solution containing carboxylic acid molecules is prepared in an aqueous perchlorate solution with 4-methylthiobenzoic acid as the target molecule, and the cation in the solution is Na. + K + Ca 2+ or Mg 2+ .

[0008] Furthermore, the purity of the gold is ≥99.999%.

[0009] This invention also provides a method for preparing the reversible single-molecule switch based on local cation regulation, which is based on electrochemical scanning tunneling microscopy split junction technology, using metallic gold as a substrate and a carboxyl-containing compound as the target molecule. By controlling the potential to change the distribution of local cations in the interfacial electric double layer, the state of carboxylic acid molecules assembled on the gold substrate is affected, thereby regulating the contact between the carboxylic acid molecules and the gold needle tip to achieve a single-molecule switch; the potential is -0.5-0V.

[0010] Furthermore, the specific preparation method of the reversible single-molecule switch based on local cation regulation is as follows:

[0011] A gold substrate was installed at the bottom of the electrolytic cell, and a gold needle tip was inserted into the scanning head of a scanning tunneling microscope (STM). The gold (111) substrate was used as the working electrode, and platinum was used as the counter electrode and reference electrode. A solution containing carboxylic acid molecules was poured into the electrolytic cell, and the potential was controlled at -0.5V and 0V respectively. The test was started when the whole device was stable.

[0012] The scanning tunneling microscope was set to a bias voltage of 50 mV. The gold needle tip was controlled to approach the gold (111) substrate to reach a preset current value of 8 nA. Then, a pulse voltage was applied to make soft contact between the needle tip and the substrate, and the needle tip moved away from the gold substrate at a speed of 20 nm / s. During this period, the sampling card collected conductivity-distance curves at 20 kH. By repeating the above process thousands of times, a large number of conductivity-distance curves could be obtained to plot a one-dimensional conductivity graph, and then the conductivity value of the single-molecule junction could be obtained. By cyclically jumping between two potentials, the circuit between the molecule and the gold needle tip was opened and closed, realizing a stable single-molecule switch, that is, a reversible single-molecule switch based on local cation regulation was obtained.

[0013] The main process of this invention is as follows:

[0014] (1) By controlling the piezoelectric ceramic, the needle tip is continuously brought closer to the gold substrate. After reaching the preset current value of 8nA, it is extended forward a certain distance to ensure that the gold needle tip is in contact with the gold substrate. Then the needle tip will move away from the substrate. During the process of moving away, the needle tip and the substrate will form atomic contact. As the distance of withdrawal increases, the number of contacting atoms will decrease, eventually becoming a single-atom contact. During this process, the conductivity-distance curve is recorded, and the appearance of conductivity steps can be observed. Finally, the collected curves are statistically processed.

[0015] (2) Utilizing the anchoring groups in the solution molecules that bond with gold, metal-molecule-metal junctions can be formed. Initially, a large number of molecules may be attached to the gold electrodes at both ends. As the piezoelectric ceramic controls the gold needle tip to move away from the gold substrate, the molecules attached to the gold electrodes at both ends will gradually become three molecules, two molecules, one molecule, until finally completely broken. During this process, the conductivity as a function of distance is recorded, and a conductivity step can be clearly observed. The collected conductivity-distance curves are statistically analyzed and plotted into a one-dimensional conductivity graph. The conductivity value of the single-molecule junction can be obtained by Gaussian fitting. When the solution does not contain the analyte molecule, no conductivity step or conductivity peak can be observed. In this method, it should be noted that: 1. The molecule under study must be able to bond with the metals at both ends; 2. The molecule under study must be a carboxylic acid molecule.

[0016] (3) Repeating step (2) at different potentials can yield a large amount of data. The ion distribution in the outer Helmholtz layer of the electrode interface double layer is different at different potentials. Below the zero charge potential (PZC), the cations localized on the electrode surface will have a strong coordination interaction with the carboxylic acid groups of the molecules, thereby inhibiting the interaction between the carboxylic acid molecules and the gold needle tip; while above PZC, the carboxylic acid is deprotonated, and the resulting -COO - It interacts strongly with gold, thus forming a molecular junction. Therefore, by controlling the potential at -0.5V, no single-molecule junction conductivity peak was observed; however, a significant conductivity peak was observed at 0V. Reversible changes in the conductivity peak can be achieved through these two potential cycles, realizing the "on" and "off" function of the single-molecule junction conductivity. Furthermore, this single-molecule switch exhibits excellent stability and a large on / off ratio.

[0017] The present invention also provides an application of the reversible single-molecule switch based on local cation regulation in information storage, logic data processing, and signal processing.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] This invention utilizes EC-STM-BJ technology as its core, adjusting the state of carboxylic acid molecules by applying a potential to alter the distribution of localized cations in the electric double layer. This influences the interaction between the carboxylic acid groups and the gold needle tip, thereby controlling the electron transport pathway. Furthermore, the molecules used in this method are simple and readily available. This invention demonstrates the effect of localized cations at the electrochemical interface on carboxyl molecules, opening a new avenue for the practical application of reversible single-molecule switching via gate electrodes. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the experimental apparatus of the present invention;

[0022] Figure 2 This is a schematic diagram of the reversible single-molecule switch principle controlled by local cations in Example 1 of the present invention;

[0023] Figure 3 This is a one-dimensional conductivity diagram of 4-methylthiobenzoic acid at different potentials for different cations in Example 1 of the present invention;

[0024] Figure 4 This is a one-dimensional conductivity diagram of 4-methylthiobenzoic acid, terephthalic acid, and 3-methylthiopropionic acid at different potentials in Example 1 of the present invention;

[0025] Figure 5 This is a cyclic diagram showing the reversible switching of conductivity peaks between high and low conductivity states when the potentials of -0.5V and 0V alternate. Detailed Implementation

[0026] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0027] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0028] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0029] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This application specification and embodiments are merely exemplary.

[0030] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0031] All raw materials used in this invention were purchased from the market.

[0032] This invention is based on the potential-dependent nature of single-molecule junctions. The technical solution employed is as follows: EC-STM-BJ is used to control the potential and alter the distribution of localized cations in the interfacial electric double layer, thereby affecting the state of carboxylic acid molecules and regulating their contact with the gold needle tip, thus achieving single-molecule switching. EC-STM-BJ technology involves applying a potential between a gold substrate and a gold needle tip, then allowing the gold needle tip to continuously collide with and move away from the gold substrate to construct a large number of molecular junctions. The electrical properties of these molecular junctions at different potentials are measured, obtaining a large amount of reliable data in a short time for statistical analysis to obtain conductivity values. Using this testing platform, conductivity values ​​at a series of potentials are measured, and stable single-molecule switching is achieved by cyclically jumping between two potentials to open and close the circuit between the two gold electrodes.

[0033] In an embodiment of the present invention, the metal is a gold needle tip and a gold (111) substrate, and the main molecule is a carboxylic acid molecule.

[0034] The technical solution of the present invention will be further described and explained below with reference to the accompanying drawings and specific embodiments.

[0035] Example 1

[0036] 1. Preparation: Electrolytic cell, 100mL volumetric flask, O-ring, and several clean reagent bottles.

[0037] Place the reagent bottle, electrolytic cell, 100mL volumetric flask, and O-ring into a beaker, pour in the "piranha" solution (concentrated sulfuric acid: 30% hydrogen peroxide, volume ratio = 3:1), and soak for at least 1 hour. After soaking, pour the waste liquid into a waste liquid container, and then rinse with plenty of ultrapure water (at least 10 times). Place the beaker containing the prepared materials on an electric heater (the volumetric flask does not need to be heated), boil it three times with ultrapure water, and finally dry it in a 105℃ oven until ready for use.

[0038] 2. Preparation of molecular solutions

[0039] Add 1.68 mg of 4-methylthiobenzoic acid and 612.30 mg of sodium perchlorate to a 100 mL volumetric flask, then dilute to volume with ultrapure water. After complete dissolution, the resulting solution contains 50 mM sodium perchlorate and 0.1 mM target molecule (carboxylic acid molecule). Then, use a pipette to transfer 10 mL of the solution to a previously cleaned 10 mL reagent bottle for later use.

[0040] 3. Preparation and treatment of gold needle tips

[0041] Take a 1.5cm piece of gold wire with a purity ≥99.999% and a diameter of 0.25mm. Straighten the gold wire with flat tweezers. Place scissors at a certain angle against the gold wire, and immediately cut and quickly remove the scissors. Observe the resulting gold needle tip under a microscope. A perfect needle tip has a smooth cut surface and a very sharp tip. After the needle tip is prepared, place it in a soldering iron containing hot melt adhesive (polymethyl styrene) at 175℃ for insulation encapsulation to reduce the influence of Faraday current. Finally, insert the processed gold needle tip into the STM scanning head.

[0042] 4. Preparation and treatment of gold (111) substrate

[0043] The gold wire was placed in a crucible and melted into a gold ingot by heating with an oxyhydrogen flame. After the gold ingot cooled, it was wrapped in three layers of filter paper and hammered until it became a thin sheet with a thickness of 0.5 cm. After hammering, a 10×15 mm gold sheet was cut out with scissors to serve as the substrate for fixing the Au(111) single crystal electrode. Then, a gold wire (purity ≥99.999%, Φ=0.5 mm) was taken and the Au(111) single crystal electrode was fabricated using the Clavilier method. After the preparation was completed, eight Au(111) facets were visible to the naked eye. Under a microscope, a better facet was selected and fixed onto the prepared gold substrate. Before each experiment, the Au(111) substrate needs to be electrolyzed in 0.5M dilute sulfuric acid solution with a 5V DC current for 10s, then rinsed with ultrapure water, then soaked in 0.5M dilute hydrochloric acid for 10s, and then rinsed with a large amount of ultrapure water. Repeat the above steps twice. Finally, let it stand in dilute hydrochloric acid for 2min, then rinse with ultrapure water and anneal with a butane gun.

[0044] 5. Preparation and assembly of electrolytic cells

[0045] To withstand various experimental environments, the electrolytic cell is made of acid- and corrosion-resistant, non-deformable polytetrafluoroethylene (PTFE). To ensure a perfect fit between the gold substrate and the electrolytic cell and prevent solution leakage during experiments, a PTFE O-ring is added to the cell. Above the O-ring, a Pt wire is wound around the edge of the cell as a counter electrode. During the experiment, the gold substrate is placed on a steel sheet, then secured with the electrolytic cell mounting clamp, and finally fixed with screws. A Pt wire reference electrode is then added to the apparatus.

[0046] 6. Testing and Data Processing

[0047] 200 μL of molecular solution was transferred into the electrolytic cell using a pipette, with the potential controlled at -0.5 V. The STM bias voltage was set to 50 mV, and the gold needle tip current was set to 8 nA. A shielding cover was then placed over the device to reduce the impact of noise on the measurement results. Testing began once the entire apparatus had stabilized. A schematic diagram of the apparatus is shown below. Figure 1 The gold needle tip is controlled to approach and impact the substrate, then moved away from the substrate at a speed of 20 nm / s. Current-distance curves are then acquired at 20 kHz. This process is repeated until thousands of pull-up curves are obtained, which are then processed and plotted as conductivity maps. The potential is then increased to 0 V, and thousands more pull-up curves are obtained and processed using the same method. This cycle of measurement is repeated three times.

[0048] 7. Working Principle

[0049] The main principle of this invention is to utilize the localized cations in the Helmholtz layer (OHP) outside the electrochemical interface to regulate the state of carboxyl molecules assembled on the Au(111) substrate (i.e., protonation or deprotonation, carboxyl-metal ion coordination) to control the contact interaction between carboxylic acid molecules and the gold needle tip, thereby controlling the formation of molecular knots and realizing a single-molecule switch. Specifically, by controlling the electrochemical potential to make the Au(111) substrate electrode surface positively or negatively charged, anions or cations in the solution will aggregate in the OHP due to Coulomb's effect. Figure 2 As shown, when the Au(111) substrate surface carries a negative charge below zero potential, locally hydrated protons can protonate carboxylic acid molecules adsorbed on the Au(111) substrate, while locally hydrated metal cations (Na+) can... + K + Ca 2+ Mg 2+The carboxyl group can coordinate with the carboxylic acid group of the molecule, thus hindering the interaction between the carboxylic acid group and the gold needle tip, preventing the formation of a molecular knot and thus preventing electron transport, corresponding to the "off" state. Conversely, when the electrode surface is positively charged above the zero charge potential, the localized hydroxyl groups in the OHP can deprotonate the carboxylic acid molecule, and the electrostatic repulsion can destroy the carboxyl-metal-ion coordination, thereby allowing a large number of carboxylic acid groups to interact and combine with the gold needle tip to form a molecular knot, corresponding to the "on" state of electron transport.

[0050] 8. Loop Testing

[0051] Option 1: Following the steps above, control the electrode potential at -0.5V and 0V respectively, and perform single-molecule conductivity measurements in a solution containing 0.1mM 4-methylthiobenzoic acid + 50mM sodium perchlorate. Statistically analyze the thousands of data points collected each time. Figure 3 As shown, a one-dimensional conductivity graph of the 4-methylthiobenzoic acid molecular junction with gold as the electrode can be obtained. At -0.5V, the cations adsorbed in the Helmholtz layer outside the electric double layer coordinate with the carboxyl groups, thus inhibiting the formation of the 4-methylthiobenzoic acid molecular junction, and no conductivity peak appears. However, when the potential increases to 0V, the carboxylic acid molecule is predominantly deprotonated, which can interact with the gold needle tip to form a molecular junction for electron transport, thus allowing a significant conductivity peak to be observed.

[0052] Option 2: Replace sodium perchlorate in the molecular solution of Option 1 with potassium perchlorate, calcium perchlorate, and magnesium perchlorate (all at 50 mM), and test different cations (K). + Mg 2+ Ca 2+ The change in the conductivity of 4-methylthiobenzoic acid under certain conditions is shown in the following results. Figure 3 Similar to sodium ions, the conductivity peaks were found to repeat at 0 V and disappear at -0.5 V, with a conductivity value of 10. -2.85±0.1 G0.

[0053] Option 3: Replace the 4-methylthiobenzoic acid in the molecular solution of Option 1 with terephthalic acid and 3-methylthiopropionic acid (concentration of 0.1 mM) for further testing. Each solution is cycled three times. Figure 4 As shown. From Figure 4 As can be seen, the conductivity peaks all repeat at 0V and disappear repeatedly at -0.5V. Due to the different molecular structures, the conductivity of terephthalic acid is 10. -3.34±0.1 The conductivity of G0,3-methylthiopropionic acid is 10. -2.2±0.2 G0, and the on / off ratio of 3-methylthiothiopropionic acid reaches 10. 3 .

[0054] Figure 5The figure shows the reversible switching cycle diagram of conductivity peaks between high and low conductivity states when the potential alternates between -0.5V and 0V in Scheme 3. As can be seen from the figure, the three carboxylic acid molecules show obvious conductivity peaks at 0V, while there are no obvious peaks in the detection range of -0.5V. This phenomenon occurs in all three cycles, indicating that the local cation reversible single-molecule switch has good stability.

[0055] The results above show that the same results can be obtained by using different cations and different raw materials containing carboxylic acid molecules, under the control potential of -0.5V and 0V. This proves that the present invention has a certain degree of universality in the application of reversible single-molecule switches with local cation regulation.

[0056] Comparative Example 1

[0057] Similar to Example 1, the difference is that the carboxylic acid group in the molecule was replaced with a thiomethyl group, and the potential was controlled at -0.5V and 0V. The results showed that the conductivity peak did not disappear and did not exhibit potential dependence, indicating that the carboxylic acid group in the interfacial molecule is key to achieving reversible single-molecule switching with localized cation regulation.

[0058] Comparative Example 2

[0059] Similar to Example 1, the difference lies in the concentration of 4-methylthiobenzoic acid molecules in the aqueous solution was changed to 0.001 mM during the molecular solution preparation process. It was observed that this potential-dependent characteristic of the conductivity peak could be observed at both 0.001 mM and 0.1 mM (the saturated solubility of the molecule at room temperature). Further reducing the molecular concentration (i.e., below 0.001 mM) resulted in the absence of the conductivity peak due to insufficient molecules. This indicates that within the carboxylic acid concentration range sufficient for effective unimolecular junction formation, the reversible unimolecular switching phenomenon regulated by localized cations exists.

[0060] Comparative Example 3

[0061] Same as Example 1, except that no metal cation Na is added during the preparation of the molecular solution. + K + Mg 2+ Ca 2+ The perchlorate was added, and when the same concentration of perchloric acid was added and the potential was controlled at -0.5V, the conductivity peak did not disappear. At the same time, when the potential was increased to 0V, the conductivity peak still existed and did not show potential dependence. This indicates that the metal cations in the solution are the key to realizing the reversible single-molecule switch of local cation regulation.

[0062] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A reversible single-molecule switch based on localized cation regulation, characterized in that, Based on the electrochemical scanning tunneling microscope cleaving junction technology, taking metal gold as the substrate and taking the compound containing carboxyl as the target molecule, the molecular solution containing the target molecule and the hydrated metal cation arranged on the substrate is controlled by controlling the substrate surface potential, the carboxyl-metal cation coordination state of the carboxyl molecule assembled on the gold substrate is regulated by using the local cation in the electrochemical interface outer Helmholtz layer, the contact between the target molecule and the gold needle tip is controlled, the formation of the molecular junction is controlled, and the single-molecule switch is realized. The potential is -0.5-0V.

2. The local cation regulation based reversible monomolecular switch according to claim 1, wherein, The hydrated metal cation comprises Na + , K + , Ca 2+ , or Mg 2+ .

3. The local cation regulation based reversible monomolecular switch according to claim 1, wherein, The target molecule includes 4-methylthiobenzoic acid, terephthalic acid and 3-methylthiopropionic acid.

4. The local cation regulation based reversible monomolecular switch according to claim 1, wherein, The purity of the gold is greater than or equal to 99.999%.

5. A method for preparing a local cation regulation based reversible monomolecular switch according to any one of claims 1 to 4, characterized in that, Based on the electrochemical scanning tunneling microscope cleaving junction technology, taking metal gold as the substrate and taking the compound containing carboxyl as the target molecule, the contact between the target molecule and the gold needle tip is regulated by changing the distribution of the local cation in the interface double electric layer through controlling the potential, the single-molecule switch is realized, and the potential is -0.5-0V.

6. The production method according to claim 5, wherein The preparation method is specifically as follows: The gold substrate is installed at the bottom of the electrolytic cell, the gold needle tip is inserted on the scanning head of the scanning tunneling microscope, the gold substrate is taken as the working electrode, platinum is taken as the counter electrode and the reference electrode, the solution containing the carboxylic acid molecule is poured into the electrolytic cell, the potentials are controlled to be -0.5V and 0V respectively, the on-off path between the molecule and the gold needle tip is achieved through the cycle jump between the two potentials, the stable single-molecule switch is realized, and the reversible single-molecule switch based on the local cation regulation is obtained.

7. The production method according to claim 6, wherein The bias voltage of the scanning tunneling microscope is set to 50mV.

8. The preparation method according to claim 6, characterized in that, The current value is set to 8nA.

9. The application of the reversible single-molecule switch based on the local cation regulation in information storage, logic data processing and signal processing according to any one of claims 1-4.

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