CN115636389B - Fabrication method and application of monomolecular junction based on controllable nano-gap of piezoelectric sheet - Google Patents

Abstract

The invention discloses a preparation method of a single-molecule junction based on a controllable nano gap of a piezoelectric sheet, which comprises the following steps: the piezoelectric sheet, the driving electrode, the insulating layer and the gold wire with the pre-circular cutting; the piezoelectric sheet is a substrate/substrate; the upper and lower surfaces of the piezoelectric sheet are provided with driving electrodes; the insulating layer is positioned on the upper layer of the driving electrode; the gold wire with the pre-circular cutting is fixed on the insulating layer; when a driving voltage is applied to the driving electrode, the piezoelectric sheet is deformed transversely/horizontally, and the gold wires fixed on the insulating layer are correspondingly stretched, so that the size of the nanogap is precisely controlled. The invention constructs an in-situ adjustable on-chip metal nano gap with angstrom-level adjustment precision by using a piezoelectric sheet expanding in the horizontal direction, and further develops the in-plane controllable crack; technical support is provided for manufacturing on-chip devices with integration potential by taking single molecules as building blocks.

Description

Fabrication method and application of monomolecular junction based on controllable nano-gap of piezoelectric sheet

technical field

The invention belongs to the technical field of nanometer electrodes, and in particular relates to a method for preparing a monomolecular junction based on a controllable nanometer gap of a piezoelectric sheet.

Background technique

As the feature size of the semiconductor process shrinks to less than ten nanometers, the leakage current caused by the tunneling effect between electrodes will become a key technical challenge restricting the further miniaturization of devices. Molecules have the advantages of low synthesis cost, structural diversity, and self-assembly. Using the properties of individual molecules to construct molecular devices with special functions has become a research hotspot in molecular electronics. A prerequisite for the realization of single-molecule functional devices is the ability to connect microscopic molecules to macroscopic circuits to measure and control the charge transport through molecules. Therefore, the preparation of molecular junctions with electrode-molecule-electrode structures has become a top priority.

The key to constructing unimolecular junctions is to connect electrodes and molecules stably and efficiently through chemical bonds. Usually, the electrodes of the unimolecular junction are mainly acted by metals, forming a metal-monimolecular-metal junction. The processing technology of the metal point electrode is complex, and it is difficult to precisely control the shape of the electrode and the number of molecules to be measured.

At present, the main methods to obtain unimolecular junctions are mechanically controllable split junction technology, scanning probe microscopy technology, electrochemical deposition technology of nano-gap electrodes and electromigration technology. However, in these methods, the mechanically controllable fission junction technology can provide continuously adjustable nanometer gaps between needle-shaped electrodes, and has a high degree of mechanical stability. However, the mechanically controllable fission junction technology device needs to use piezoelectric ceramics or stepping motors to change the distance between electrodes by bending the substrate, which cannot achieve on-chip integration.

Therefore, how to provide an on-chip controllable nano-splitting monomolecular junction preparation method is an urgent problem to be solved by those skilled in the art.

Contents of the invention

In view of this, the present invention provides a single-molecule junction preparation method based on the controllable nanogap of the piezoelectric sheet. By using the horizontal expandable piezoelectric sheet to construct an in-situ adjustable on-chip metal nanogap with angstrom-level modulation resolution, it can be further developed into in-plane fracture and connection, wherein the nanogap can be repeatedly broken and connected millions of times, realizing an on-chip split junction with freely adjustable gap size; using single molecules as building blocks to manufacture high-yield on-chip molecular devices with integration potential provides technical support.

To achieve the above object, the present invention adopts the following technical solutions:

A method for preparing single-element junctions based on the controllable nano-gap of piezoelectric sheets, including: piezoelectric sheets, driving electrodes, insulating layers, and gold wires with pre-ring cuts;

Wherein, the piezoelectric sheet is a substrate/substrate; the upper and lower surfaces of the piezoelectric sheet are provided with driving electrodes;

The insulating layer is located on the upper layer of the driving electrodes; the gold wires with pre-cutting are fixed on the insulating layer;

When a driving voltage is applied to the driving electrodes, the piezoelectric sheet sandwiched between the driving electrodes will undergo lateral/horizontal deformation, and the gold wires fixed on the insulating layer will be stretched accordingly, precisely controlling the size of the nano-gap.

Preferably, the preparation method of the gold wire with pre-circumcision is as follows:

1) Carry out circumcision in the middle part of the gold wire 1 to form a groove, and cut the gold wire 1 into a relatively symmetrical hourglass shape;

2) fixing the gold wires treated in step 1) on the piezoelectric sheet coated with insulating glue;

3) placing the second gold wire on the upper layer of the piezoelectric sheet, and placing one end of the second gold wire at the center of the groove close to the first gold wire;

4) dropping an electrolyte on the piezoelectric substrate near the groove to immerse the end position of the second gold wire and the groove portion of the first gold wire;

5) Applying a low potential on the second gold wire to make it a cathode for electrochemical reaction, and applying a high potential to the first gold wire to make it an anode for etching by electrochemical reaction; with the gradual corrosion of the anode, the diameter of the groove decreases.

Preferably, the groove diameter after circumcision in step 1) is 10-15 μm; and the groove diameter is reduced to ≤1 μm after chemical etching in step 5).

And/or, step 2) the piezoelectric substrate is a piezoelectric ceramic substrate; the gold wires are fixed on the piezoelectric substrate by epoxy resin;

And/or, the electrolyte in step 4) is an aqueous solution containing 0.01 mole per milliliter of chloroauric acid and boron chloride at a ratio of 1:1.

Preferably, the piezoelectric sheet is a longitudinally polarized rectangular piezoelectric ceramic sheet.

Preferably, the driving electrode is a silver film, and the silver film is respectively plated on the upper and lower surfaces of the piezoelectric sheet.

Preferably, the gold wire is fixed on the insulating layer by bonding.

Preferably, the tensile fracture process of the gold wire is monitored by measuring the current passing through the gold wire with a current signal analyzer.

Preferably, when the driving voltage increases, the groove part of the gold wire will be subjected to tension, and the cross section of the groove will decrease until it breaks to form a nano-gap.

Preferably, the minimum diameter of the groove part of the gold wire is 1% of the initial diameter, and the length of the suspended part is twice the initial diameter, the total stretching length of the nanobridge is 0.1% of the length of the suspension part, and the maximum strain force is generated in the thinnest part of the groove.

Another object of the present invention is to provide an application for the preparation of unimolecular junctions based on the controllable nano-splitting of piezoelectric sheets; the application of the unimolecular junctions obtained by the above-mentioned preparation method based on the controllable nano-splitting of piezoelectric sheets in the preparation of molecular splitting arrays.

Through the above-mentioned technical solution, compared with the prior art, it can be seen that the present invention has at least the following technical effects:

1) The present invention considers the requirements of in-plane integration and adjustable gap, and proposes a method for realizing on-chip adjustable nano-gap through laterally expandable piezoelectric sheets;

2) In principle, the present invention can make tens of thousands of parallel electrode pairs on the substrate/substrate, which provides technical support for making molecular junction arrays;

3) The separation distance of each pair of electrodes in the scheme designed by the present invention can be adjusted independently by changing the initial relative position of the electrode pair, so the gap size modulation accuracy and the total gap size variation range can be fine-tuned according to the needs of practical applications;

4) The gap on the chip can be adjusted and maintained at sub-nanometer resolution, providing applications in extreme optics, such as tip-enhanced Raman spectroscopy and localized surface plasmons.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the accompanying drawings that need to be used in the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without creative work.

Fig. 1 is a schematic diagram of the working principle based on the controllable nano-gap of the piezoelectric sheet in Embodiment 1 of the present invention.

Fig. 2 is a scanning electron microscope image of different electrode spacings under different driving voltages in Example 2 of the present invention.

3 is a schematic diagram of an adjustable on-chip nanogap based on electron beam lithography according to Embodiment 3 of the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some of the embodiments of the present invention, but not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

Example 1

This embodiment provides a method for preparing single-element junctions based on the controllable nano-gap of piezoelectric sheets, including: piezoelectric sheets, driving electrodes, insulating layers, and gold wires with pre-ringing;

Wherein, the piezoelectric sheet is a substrate/substrate; the upper and lower surfaces of the piezoelectric sheet are provided with driving electrodes;

The insulating layer is located on the upper layer of the driving electrodes; the gold wire with pre-ring cut is fixed on the insulating layer;

When the driving voltage is applied to the driving electrodes, the piezoelectric sheet sandwiched between the driving electrodes will deform laterally/horizontally, and the gold wires fixed on the insulating layer will be stretched accordingly, precisely controlling the size of the nano-gap.

In order to further optimize the above-mentioned technical scheme, the preparation method of the gold wire with pre-circumcision is as follows:

1) Carry out circumcision in the middle part of the gold thread 1, and cut the gold thread 1 into a relatively symmetrical hourglass shape;

2) fixing the gold wire 1 treated in step 1) on the insulating piezoelectric sheet;

3) Place the gold wire 2 on the upper layer of the piezoelectric sheet, and place one end of the gold wire 2 in the center of the groove close to the gold wire 1;

4) Drop the electrolyte on the piezoelectric sheet near the groove to immerse the end position of the gold wire two and the groove part of the gold wire one;

5) Apply a low potential on the gold wire 2 to make it a cathode for electrochemical reactions, and apply a high potential to the gold wire 1 to make it an anode for etching through electrochemical reactions; with the gradual corrosion of the anode, the diameter of the groove decreases.

In order to further optimize the above-mentioned technical scheme, step 1) the diameter of the groove after circumcision is 10-15 μm;

And/or, step 2) the piezoelectric sheet is a piezoelectric ceramic sheet; the gold wire is fixed on the piezoelectric sheet through epoxy resin;

And/or, step 4) the electrolyte is an aqueous solution containing 0.01 mole per milliliter of chloroauric acid and boron chloride at a ratio of 1:1.

In order to further optimize the above technical solution, the piezoelectric sheet is a longitudinally polarized rectangular piezoelectric ceramic sheet.

In order to further optimize the above technical solution, the driving electrode is a silver film, and the silver film is respectively plated on the upper and lower surfaces of the piezoelectric sheet.

In order to further optimize the above technical solution, the gold wire is fixed on the insulating layer by bonding.

In order to further optimize the above technical solution, the gold wire is monitored by measuring the current passing through the gold wire through a current signal analyzer during the tensile fracture process.

In order to further optimize the above technical solution, when the driving voltage increases, the groove part of the gold wire will be under tension, and the cross section of the groove will decrease until it breaks, forming a nano-gap.

In order to further optimize the above-mentioned technical scheme, the minimum diameter of the groove part of the gold wire is 1% of the initial diameter, and the length of the suspended part is twice the initial diameter, the total stretching length of the nanobridge is 0.1% of the length of the suspended part, and the maximum strain force is generated in the thinnest part of the groove.

Furthermore, the initial diameter of the gold wire is 100 μm, the minimum diameter of the groove is ≤1 μm, and the length of the suspended part is 200 μm, the total stretching length of the nanobridge is 200 nm, and the maximum strain force is generated in the thinnest part of the groove, and the maximum strain force is 1.35×10 ¹⁰ N/m ² .

Figure 1 is a working principle diagram based on the controllable nano-gap of the piezoelectric sheet.

Figure a shows the working principle of the on-chip split junction technology; a longitudinally polarized piezoelectric ceramic is used as the substrate. When a driving voltage (V _d ) is applied to the substrate, a lateral deformation occurs, which in turn stretches the gold wire fixed above it to create separated electrode pairs. The elongation of the gold wire is monitored by observing the current at both ends of the gold wire.

Figure b shows the enlarged area of the suspended part of the gold wire. The gold wire with grooves is fixed on the substrate by two drops of black glue, and the initial spacing distance is d. The grooved portion will be elongated until it eventually breaks when the underlying substrate reaches a certain lateral deformation.

Figure c shows the stress distribution around the simulated groove when the gold wire is stretched, specifically the stress distribution when the gold wire is stretched using the COMSOL software package.

Figure d is a schematic diagram of the etching process to reduce the diameter of the gold wire in the groove part.

Example 2

This embodiment is further verified on the basis of embodiment 1:

The ability to control the precision of gap size modulation is very important for building molecular devices. In this embodiment, a 16-bit voltage output module (0V-10V) is used to control the voltage applied to the piezoelectric sheet, that is, the lateral deformation of the piezoelectric ceramic sheet. This voltage is amplified by a linear amplifier and applied to the piezoelectric sheet to produce transverse tensile deformation. The deformation of the piezoelectric ceramic is linearly proportional to the applied driving voltage. Under the maximum driving voltage (V _max ), the maximum lateral deformation is 0.1% of the total length (L) of the piezoelectric sheet. Under the applied driving voltage ( _Vd ), the elongation (Δd) of the wire between two vinyl-fixed gold wire points (d) can be estimated as:

Δd=(L×0.1%)×(V _d /V _max )×(d/L)=V _d /V _max ×0.1%×d (1)

Then the minimum step size of the applied drive voltage (V _min ) is 1mV determined by the output module, so the minimum/maximum elongation of the wire can be calculated as:

Δd _min = V _min /V _max × 0.1% × d = 1mV/10V × 0.1% × d = 10 ^-7 × d (2)

Δd _max = V _max /V _max × 0.1% × d = 10 ^-3 × d (3)

It can be seen in accompanying drawing 2: scanning electron microscope images of gold electrode pairs with different distances under different driving voltages.

Among them, a-c are the SEM images of the electrode separation when the driving voltages of 1V, 4V and 10V are applied in the coarse adjustment mode, and the gap sizes of about 0.4μm, 2.4μm and 5.2μm are obtained, respectively. Scale bar: 2 μm. Arrows indicate the direction of increasing gap size. At this time, the initial distance d between the two black glues fixing the gold wire is about 8mm.

Figures d-f show that in fine regulation mode, when driving voltages of 1V, 4V and 10V are applied, gap sizes of about 82nm, 124nm and 232nm are obtained, respectively. Scale bar: 200 nm.

Based on Equation (2), the minimum step size of electrode separation (Δd _min ) is linearly dependent on the initial distance (d) between two fixed points, which provides adjustable precision for gap size control.

In the fine mode, the gold wire is fixed by two drops of black glue (for example, d≈1mm) that are close to each other. According to the formula (2), Δd _min =10 ^-7 ×d≈0.1nm can be obtained, which means that the gap size can be precisely adjusted with sub-nanometer precision. In the coarse adjustment mode, the gold wire is fixed by two drops of black glue far apart (for example, d≈10mm). According to the formula (3), Δd _max =10 ^-3 ×d≈10μm can be obtained, that is to say, we can obtain the micron-level gap variation range.

Example 3

This embodiment is to make electrode array on the basis of embodiment 1-2

Examples 1-2 prove that fixing metal electrodes on piezoelectric ceramics can obtain gap sizes from sub-nanometer to micron, and have adjustable control precision.

However, it remains a great challenge to generate thousands of such electrode arrays, which are essential to realize highly integrated functional devices. In order to make an electrode array, in Embodiment 3, a nanoscale metal bridge is made on a piezoelectric ceramic sheet by photolithography. As the voltage (V _d ) applied to the piezoelectric ceramic sheet, ie, the substrate, increases, the nanobridge fixed on the substrate will be stretched until it finally breaks, as shown in Figure 3a.

To fabricate the samples, a solution of poly(pyromellitic dianhydride-co-4,4’-diphenylamine oxide)amic acid (PAA) was first spin-coated on the upper layer of the piezoceramic top electrode, which can electrically insulate the gold nanostructures from the piezoceramic, followed by heat treatment to cure the insulating layer. Next, using standard photolithography and reactive ion etching techniques, suspended nanobridges (d ~ 2 μm) with symmetrical grooves (about 80 nm wide and about 40 nm thick) were obtained on the insulating layer, as shown in Figure 3b. Figures 3c–3e show enlarged SEM images of the groove positions under different driving voltages. It shows that when no driving voltage is applied, unstretched nanobridges on the insulating layer can be observed (Fig. 3c). When a driving voltage of 5 V was applied to the piezoelectric sheet, nanobridge breakage and gaps with a size of approximately 25 nm were observed (Fig. 3d). When the driving voltage is increased to 10V, the gap size expands to about 42nm.

Figure 3a shows the working principle of the tunable on-chip nanogap based on e-beam lithography.

Figure 3b is the SEM image of the sample, scale bar: 2 μm.

Figure 3c shows the suspended nanobridge located on the upper layer of the substrate, without driving voltage (V _b =0V), scale bar: 200nm.

Figure 3d shows that the nanobridge is stretched and a nanogap (~25nm) is observed when a driving voltage ( _Vb = 5V) is applied to the underlying piezoelectric substrate, scale bar: 200nm.

Figure 3e The nanogap increases to ~42nm when a driving voltage (~10V) is applied. Scale bar: 200 nm.

The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention will not be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (8)

1. A preparation method based on a piezoelectric sheet controllable nano-gap, comprising: a piezoelectric sheet, a driving electrode, an insulating layer, and a gold wire with a pre-ring cut; Wherein, the piezoelectric sheet is a substrate/substrate; the upper and lower surfaces of the piezoelectric sheet are provided with driving electrodes; The insulating layer is located on the upper layer of the driving electrodes; the gold wires with pre-cutting are fixed on the insulating layer; When a driving voltage is applied to the driving electrodes, the piezoelectric sheet sandwiched between the driving electrodes will be deformed laterally/horizontally, and the gold wires fixed on the insulating layer will be correspondingly stretched to accurately control the size of the nano-gap; wherein, the nano-gap is mechanically stretched by the lateral/horizontal deformation of the piezoelectric sheet substrate to break the gold wires fixed on the surface to form controllable splits; And, the preparation method of the gold wire with pre-ring cutting is as follows: 1) Carry out circumcision in the middle part of the gold wire 1 to form a groove, and cut the gold wire 1 into a relatively symmetrical hourglass shape; 2) fixing the gold wires treated in step 1) on the piezoelectric substrate coated with insulating glue; 3) placing the second gold wire on the upper layer of the piezoelectric substrate, and placing one end of the second gold wire at the center of the groove close to the first gold wire; 4) dropping an electrolyte on the piezoelectric substrate near the groove to immerse the end position of the second gold wire and the groove portion of the first gold wire; 5) Applying a low potential to the second gold wire to make it a cathode for electrochemical reaction, and applying a high potential to the first gold wire to make it an anode for etching by electrochemical reaction; as the anode corrodes gradually, the diameter of the groove decreases. 2. The method for preparing a monomolecular junction based on a controllable nano-gap of a piezoelectric sheet according to claim 1, wherein the diameter of the groove after the circular cutting in step 1) is 10-15 μm; and after step 5), the diameter of the groove is reduced to ≤1 μm; And/or, step 2) the piezoelectric substrate is a piezoelectric ceramic substrate; the gold wires are fixed on the piezoelectric substrate by epoxy resin; And/or, the electrolyte in step 4) is a 1:1 aqueous solution of chloroauric acid and boron chloride at 0.01 mole per milliliter. 3 . The method for preparing monomolecular junctions based on piezoelectric sheet controllable nano-gap according to claim 1 , wherein the piezoelectric sheet is a longitudinally polarized rectangular piezoelectric ceramic sheet. 4 . 4. The method for preparing a monomolecular junction based on a controllable nanogap of a piezoelectric sheet according to claim 1, wherein the driving electrode is a silver film, and the silver film is respectively plated on the upper and lower surfaces of the piezoelectric sheet. 5 . The method for preparing a monomolecular junction based on a controllable nano-gap of a piezoelectric sheet according to claim 1 , wherein the gold wire is fixed on the insulating layer by bonding. 5 . 6 . The method for preparing a monomolecular junction based on a controllable nano-gap of a piezoelectric sheet according to claim 1 , wherein the gold wire is monitored by measuring the current passing through the gold wire during the tensile fracture process by a current signal analyzer. 7 . 7. The method for preparing monomolecular junctions based on piezoelectric sheet controllable nanogap according to claim 6, characterized in that, when the driving voltage increases, the groove part of the gold wire will be subjected to tension, and the cross section of the groove will decrease until it breaks to form a nanogap. 8. The single molecular junction preparation method based on the controllable nanogap of the piezoelectric sheet according to claim 7, wherein the minimum diameter of the groove part of the gold wire is 1% of the initial diameter, and the length of the suspended part is twice the initial diameter, the total stretching length of the nanobridge is 0.1% of the length of the suspended part, and the maximum strain force is generated in the thinnest part of the groove.