Flexible pressure sensor based on two-dimensional quantum tunneling force-sensing mechanism and its preparation method

Abstract

This invention discloses a flexible pressure sensor based on a two-dimensional quantum tunneling force-sensing mechanism. The sensor consists of a top electrode, a middle dielectric layer, a bottom electrode, and a flexible substrate. The top electrode is a conformal carbon nanofilm, the middle dielectric layer is hexagonal boron nitride (HBN), and the bottom electrode is a single-layer graphene. The conformal carbon nanofilm consists of microstructured graphene nanowalls and a flexible substrate. Its operating mechanism is the change in two-dimensional interlayer quantum tunneling current caused by microstructural deformation under pressure. The flexible pressure sensor fabricated in this invention exhibits ultra-high sensitivity, a low detection limit, and a fast response time in the low-pressure range (<100 Pa).

Description

Technical Field

[0001] The present invention relates to a flexible pressure sensor based on a two-dimensional quantum tunneling force-sensitive mechanism and a preparation method thereof, belonging to the technical field of flexible tactile sensors. Background Art

[0002] Graphene has garnered widespread attention due to its exceptional electronic, mechanical, physical, and thermal properties, making it a common foundational material for pressure sensor devices. Hexagonal boron nitride (HBN), an insulating isomorph of graphene, has a lattice mismatch with graphene of only 1.7%, and rivals graphene in mechanical strength and thermal conductivity. Hybrid devices combining graphene and HBN are increasingly being researched, with research focused on tunable band gaps between graphene and HBN layers, the conductivity of graphene on HBN substrates, and the applications of this heterostructure. The graphene-HBN heterojunction is being applied in pressure sensors. HBN acts as a protective layer to protect graphene from oxidation and contamination. Furthermore, the current in this structure is generated by carrier tunneling, which increases exponentially with decreasing interatomic distance. However, current pressure sensors fabricated using this heterostructure lack flexibility and high sensitivity.

[0003] Piezoresistive sensors have attracted widespread attention due to their simple structure, easy readout, and high sensitivity. Among them, the most commonly used permeation conversion mechanism is to prepare a composite pressure sensor by physically mixing an insulating polymer with a conductive filler. When subjected to pressure, the resistance change caused by the reduction in the distance between the conductive fillers is used to measure the pressure. There are two main types of existing piezoresistive sensors: one is a piezoresistive sensor based on the permeation effect, and the other is a piezoresistive sensor based on microstructural deformation. However, the resistance change of the piezoresistive sensor based on the permeation effect depends entirely on the force deformation of the polymer matrix material, and the piezoresistive sensor based on microstructural deformation will produce hysteresis due to the viscosity of the elastomer. Therefore, it is very difficult for the resistance of these two piezoresistive sensors to meet both fast response and signal amplification capabilities.

[0004] The quantum tunneling effect describes signal changes at the quantum scale. When used in pressure sensors, it can induce very large electrical signal changes within a very small pressure range. Quantum tunneling effects in pressure sensors typically occur in composite materials, but low filler concentrations limit sensor performance. Furthermore, filler loading can be easily influenced by other factors, exceeding the percolation threshold. Consequently, the resulting tunneling current changes are small and limited in range, resulting in low sensitivity and other performance levels. Summary of the Invention

[0005] The present invention provides a flexible pressure sensor based on a two-dimensional quantum tunneling force-sensitive mechanism, which can realize ultra-high sensitivity detection of a pressure sensor with a two-dimensional quantum tunneling effect.

[0006] To achieve the above objectives, the present invention employs a flexible pressure sensor based on a two-dimensional quantum tunneling force-sensing mechanism. The sensor comprises a top electrode, an intermediate dielectric layer, a bottom electrode, and a flexible substrate. The top electrode is a conformal carbon nanofilm, the intermediate dielectric layer is HBN, and the bottom electrode is a single-layer graphene. The conformal carbon nanofilm comprises graphene nanowalls with microstructures and a flexible substrate. The microstructures include micropyramids, microhemispheres, microprisms, microstrips, and microhemispheres.

[0007] Preferably, the flexible substrate layer is PET; and the flexible base is PDMS.

[0008] The present invention also provides a method for preparing a flexible pressure sensor, comprising the following steps: sputtering a gold electrode on a flexible substrate layer, transferring a pre-grown single-layer graphene onto the gold electrode, and then transferring a pre-grown HBN onto the single-layer graphene to form a flexible substrate layer containing a heterogeneous structure; growing graphene nanowalls on a microstructured mold, and transferring the graphene nanowalls to a flexible substrate to obtain a conformal carbon nanofilm; and encapsulating the prepared conformal carbon nanofilm and the flexible substrate layer containing the heterogeneous structure to obtain a flexible pressure sensor.

[0009] Furthermore, the process involves spin-coating a protective photoresist layer onto the flexible substrate layer, where the graphene and HBN have been transferred, and then photolithography to preserve the force-sensitive and electrode regions on the flexible substrate. To reduce external interference with the tunneling current, a photoresist mask is used to preserve the force-sensitive regions, improving device stability.

[0010] Furthermore, the number of layers of the HBN transfer is 1 to 9 layers.

[0011] Furthermore, transferring graphene (Gr) and HBN to the flexible substrate layer specifically includes the following steps: preparing an etching solution, cutting a copper foil grown with graphene or HBN and placing it in the etching solution for 5 hours to remove the copper foil to form a graphene or HBN film; the etching solution includes water, hydrochloric acid and hydrogen peroxide; spin-coating polymethyl methacrylate (PMMA) as a support layer and curing it on a hot plate, transferring the graphene or HBN film to clean water for soaking for use, using the prepared flexible substrate to scoop the graphene or HBN film in the clean water to the intermediate dielectric layer area, placing it on a hot plate for drying, and then soaking it in an acetone solution to remove the PMMA support layer, and finally blowing it dry with high-purity nitrogen.

[0012] Furthermore, the graphene and HBN are both grown by using a PE-CVD method; and the copper foil on which the graphene or HBN is grown is transferred by using a wet etching process.

[0013] Furthermore, the method of growing graphene nanowalls on an inverted pyramid structure mold and transferring the graphene nanowalls to a flexible substrate specifically includes the following steps: ultrasonically cleaning a silicon wafer in deionized water, acetone, and alcohol for 15 minutes to remove surface impurities; performing photolithography on a clean silicon wafer to form a square pattern, spin-coating photoresist, and then drying the wafer at 100° C. on a hot plate for 10 minutes. After drying, the wafer is exposed using a binary exposure machine and developed with a developer to form a square pattern, thereby exposing periodic micrometer-scale square holes in the silicon dioxide layer; performing dry etching on the exposed square holes using an etching gas on the silicon wafer after photolithography to remove the surface silicon dioxide layer, and then performing anisotropic etching on the silicon in the holes using a wet etching solution to obtain a mold for an array inverted pyramid structure; growing graphene nanowalls on the inverted pyramid mold using a plasma-enhanced chemical vapor deposition method, and transferring the graphene nanowalls using polydimethylsiloxane (PDMS).

[0014] Furthermore, the etching gas is trifluoromethane, the etching solution includes potassium hydroxide, isopropyl alcohol and deionized water, and the etching temperature is 80°C.

[0015] Furthermore, a heat shrink film is used to encapsulate the conformal carbon nanofilm and the flexible substrate layer containing the heterogeneous structure.

[0016] The flexible pressure sensor fabricated by the present invention utilizes a Gr-HBN-Gr heterostructure, applying the interlayer quantum tunneling effect of two-dimensional materials to the pressure sensor. This leverages the superior performance of graphene and HBN materials and the exponential tunneling current variation of the heterostructure. The heterostructure not only provides excellent protection for the graphene bottom electrode but also amplifies the relative variation in current, thereby enhancing the sensor's sensitivity and response speed. The graphene and HBN heterostructure, combined with a conformal carbon nanofilm with a microstructure, can regulate the quantum tunneling effect between two-dimensional atomic layers. The conformal carbon nanofilm with the microstructure increases the contact area while enhancing the tunneling response, further amplifying the signal. The local field enhancement effect of the microstructure promotes quantum tunneling between the graphene and HBN layers, enhancing the device's detection capability in the micropressure range (<100 Pa). This results in the flexible pressure sensor of the present invention exhibiting ultra-high sensitivity, a low detection limit, and a fast response time. The HBN layer in the heterojunction can effectively reduce the initial current to obtain a low-noise current level, which can improve the sensor's ability to perceive micro-pressure; at the same time, in order to reduce the interference of the external environment on the tunneling current, the present invention uses a photoresist mask to retain the force-sensitive area to improve the stability of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic structural diagram of a flexible pressure sensor based on a two-dimensional quantum tunneling force-sensing mechanism provided by an embodiment of the present invention;

[0018] Figure 2 Schematic diagram of the flexible pressure sensor in Example 1 of the present invention;

[0019] Figure 3 Graph showing sensitivity test results of the flexible pressure sensor in Example 1 of the present invention;

[0020] Figure 4 1 is the IV curve of the flexible pressure sensor under different pressures in Example 1 of the present invention;

[0021] Figure 5 : This is the resolution test result of the flexible pressure sensor in Example 1 of the present invention;

[0022] Figure 6 This is a graph showing the detection results of the response time of the flexible pressure sensor in Example 1 of the present invention.

[0023] Among them, 1-PDMS flexible substrate, 2-graphene nanowall, 3-electron, 4-HBN layer, 5-single layer graphene, 6-PET flexible substrate layer. DETAILED DESCRIPTION

[0024] In order to better understand the essence of the present invention, the present invention is further described below in conjunction with specific embodiments and drawings.

[0025] Example 1

[0026] The present invention provides a flexible pressure sensor based on a two-dimensional quantum tunneling force-sensitive mechanism, which consists of a top electrode, an intermediate dielectric layer, a bottom electrode and a PET flexible substrate layer 6; the top electrode is a conformal carbon nanofilm, the intermediate dielectric layer is an HBN layer 4, and the bottom electrode is a single-layer graphene 5 and is led out through an Au electrode line. The conformal carbon nanofilm is composed of a graphene nanowall 2 with a microstructure and a PDMS flexible substrate 1; the microstructure includes micro-pyramids, micro-hemispheres, micro-prisms, micro-strips and micro-hemispheres, etc. The working mechanism of the flexible pressure sensor is the change in the two-dimensional interlayer quantum tunneling current caused by the deformation of the microstructure under pressure. Figure 1 Shown is a flexible pressure sensor with a micro-pyramid structure.

[0027] Because graphene and boron nitride heterojunctions require extremely high pressure to induce changes in tunneling current when used directly in pressure sensors, such sensors have low sensitivity. The top electrode of the present invention utilizes a micro-nanostructure design, modulating the tunneling current region by varying the contact area between the top electrode and the intermediate dielectric layer. The device's sensitivity is also enhanced by utilizing the field enhancement effect of the pyramidal tip of the top electrode.

[0028] The bottom electrode (single-layer graphene film) and the middle dielectric layer (HBN film) of the flexible pressure sensor prepared by the present invention are bonded by van der Waals force, and the heterostructure formed is the basic condition for the formation of tunneling current; and the top electrode (graphene nanowall with conformal structure obtained by CVD growth transfer) can promote electrons 3 from the top electrode through the HBN barrier to the bottom electrode through the change of contact area and field enhancement effect, thereby regulating the tunneling response under different pressures; the thickness of the middle HBN determines the barrier height, and different HBN layers determine the different current change rates of the pressure sensor. Figure 2 As shown in Figure A, in the initial state, the pyramid tip on the graphene nanowall 2 has a local field enhancement effect, and a large amount of charge is accumulated at the tip, which has a significant enhancement effect on the quantum tunneling effect and greatly enhances the device's ability to detect tiny forces; Figure 2 As shown in Figure B, the pyramid microstructure deforms under pressure, causing the contact area between the top electrode and the HBN layer to increase, making the device highly sensitive. While the contact area between the top electrode and the HBN layer increases, the distance between the top electrode and the bottom electrode, and between the HBN layer and the single-layer graphene atomic layer decreases. The intensity of the quantum tunneling effect region will be significantly enhanced, resulting in huge current changes, thereby greatly improving the sensitive response and detection limit of the device.

[0029] Example 2

[0030] Taking the flexible pressure sensor composed of the graphene nanowall 2 having a micro-pyramid structure as an example, the preparation steps of the flexible pressure sensor of the present invention are described:

[0031] S1. Laser cutting and pretreatment of the PET flexible substrate: The PET flexible substrate was laser cut to the appropriate size and an electrode line mask was prepared. After the mask was applied, Au was sputtered using magnetron sputtering to form the bottom electrode lines.

[0032] S2. Transfer graphene and HBN onto a PET flexible substrate.

[0033] Prepare an etching solution. Cut the copper foil with graphene or HBN growth on it and place it in the etching solution for 5 hours. After the copper foil is removed, a graphene or HBN film is formed. The etching solution consists of 350ml of water, 10ml of hydrochloric acid, and 5ml of hydrogen peroxide. The graphene or HBN growth on the copper foil is transferred using a wet etching process. Both graphene and HBN are grown using chemical vapor deposition (PE-CVD).

[0034] Spin-coat polymethyl methacrylate (PMMA) as a support layer and solidify it on a hot plate. Transfer the graphene or HBN film to clean water and soak it for later use. Use the PET flexible substrate prepared in step S1 to move it to the middle dielectric layer area, place it on a hot plate to dry, and then soak it in acetone solution to remove PMMA. Finally, blow it dry with high-purity nitrogen.

[0035] When transferring graphene and HBN to the PET flexible substrate, the single-layer graphene is transferred first, and then the HBN is transferred onto the single-layer graphene.

[0036] S3. Spin-coat a photoresist protective layer on the PET flexible substrate transferred in step S2.

[0037] A laser writer was used to create a custom mask, retaining the force-sensitive and electrode areas on the PET flexible substrate. The remaining areas were then covered with photoresist to eliminate interference. The PET flexible substrate was spin-coated with photoresist, dried, and then exposed using a binary exposure machine. The developer solution was then used to develop the area, retaining the force-sensitive areas.

[0038] S4. Preparation of conformal carbon nanofilms.

[0039] S41. Pre-treat the silicon wafer: Since dust particles on the surface of the silicon wafer will affect the formation of the photoresist film, before starting photolithography, the silicon wafer is first ultrasonically cleaned in deionized water, acetone and alcohol for 15 minutes to remove surface impurities.

[0040] S42. Perform photolithography to form a square pattern on a clean silicon wafer:

[0041] After spin coating photoresist on a clean silicon wafer, it is placed on a hot plate and dried at 100°C for 10 minutes. After drying, it is exposed using a binary exposure machine and developed with a developer to form a square pattern, exposing periodic micron-scale square holes in the silicon dioxide layer.

[0042] S43. Etch the silicon wafer after photolithography to obtain a mold of an array inverted pyramid structure: Use an RIE dry etcher to dry-etch the exposed square holes on the silicon wafer after photolithography to remove the surface silicon dioxide layer. The etching gas is trifluoromethane (CHF3), and then use acetone and alcohol to clean the photoresist on the sample surface.

[0043] The silicon within the holes is then anisotropically etched using a wet etchant containing deionized water, potassium hydroxide, and isopropyl alcohol at 80°C, resulting in a mold with an inverted pyramid array structure. The addition of isopropyl alcohol improves the orderliness of the pyramid structure. After etching, the mold is rinsed with deionized water and dried under high-purity nitrogen to obtain the mold with the inverted pyramid array structure.

[0044] S44. A plasma-enhanced chemical vapor deposition (PE-CVD) method is used to grow graphene nanowalls on an inverted pyramid mold, and the graphene nanowalls are transferred using polydimethylsiloxane (PDMS) to obtain a conformal carbon nanofilm.

[0045] S5. Using heat shrink film to bond the conformal carbon nanowall film to the middle dielectric layer and the bottom electrode can keep the device in a good initial state.

[0046] The conformal carbon nanofilm of the present invention is produced using a photolithographic process, ensuring microstructural consistency and order. It can produce microstructures as small as 5 microns or multi-level response structures intersecting 5 and 10 microns, enhancing device sensitivity. Graphene nanowalls with high conductivity can be grown using PE-CVD. PDMS is an optically transparent elastomeric material formed by a coupling reaction between the active end groups of macromolecular polydimethylsiloxane and a curing agent, completing the curing process. The conformal carbon nanofilm formed by PDMS and graphene nanowalls provides the top electrode with excellent flexibility and biocompatibility.

[0047] Example 3

[0048] The sensitivity, resolution and response time of the flexible pressure sensor prepared in Example 2 were tested respectively, and its IV curve under different pressures was tested. The test results are shown in FIG. Figures 3 to 6 shown.

[0049] 1. Sensitivity detection.

[0050] The sensitivity S is calculated using formula (1):

[0051] S=(ΔI / I0) / ΔP (1)

[0052] Where ΔI is the change in current, I0 is the initial current, and ΔP is the change in pressure on the device.

[0053] The pressure loading is completed by F1 grade milligram weight and micro pressure gauge, and the current response is measured by 2450 source meter. By fitting the relationship between the current change and pressure under different pressures of the sensor, two sensitivity curves are obtained, such as Figure 3 As shown, in the range of 0-100Pa, the sensitivity of the flexible pressure sensor in Example 2 is as high as 80815kPa -1 , when the pressure is greater than 100Pa, the sensitivity is about 2704kPa -1 The sensitivity range of current flexible pressure sensors is mainly in the range of 10 3 kPa -1 Within.

[0054] 2. IV curves under different pressures.

[0055] The device is loaded with 0mg, 2mg, 5mg, 10mg and 20mg of pressure respectively and its IV curve is measured between -1V and 1V. Figure 4 The medium current showed exponential growth, further verifying the tunneling response of the device.

[0056] 3. Resolution detection.

[0057] A 500 mg weight was loaded on the device, followed by a 2 mg weight. Data was collected using a 2450 source meter (U = 0.1 V). Figure 5 As shown in Figure 3, the device can still detect a tiny pressure signal of 2 mg under a preload of 500 mg.

[0058] 4. Response time detection.

[0059] A 2g weight was loaded onto the device and then quickly removed. Data was collected using a 2450 source meter (U = 0.1V) at a rate of 2012 points per second, or one point every 0.5ms. The results are shown below. Figure 6 The display device response time is less than 0.5ms and the recovery time is less than 2ms.

[0060] In summary, the pressure sensor of the present invention has an exponential tunneling current response due to the small initial dark current between the two-dimensional HBN counter electrodes and the changes in the contact area and the pressure in the contact area under pressure loading. The sensitivity is as high as 80815kPa-1 in the range of 0 to 100Pa. The field enhancement effect of the needle tip gives the device an extremely low detection limit, which can detect the response under a pressure of 0.1Pa, and the response time is short.

[0061] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are included in the scope of the claims of the present invention to be approved.

Claims

1. A flexible pressure sensor based on a two-dimensional quantum tunneling force-sensing mechanism, characterized by: It consists of a top electrode, an intermediate dielectric layer, a bottom electrode and a flexible substrate layer; the top electrode is a conformal carbon nanofilm, the intermediate dielectric layer is HBN, and the bottom electrode is a single-layer graphene; the conformal carbon nanofilm is composed of graphene nanowalls with microstructures and a flexible substrate; the microstructure is one of micropyramids, microhemispheres, microprisms and microstrips; and the microstructure deforms under pressure.

2. The flexible pressure sensor according to claim 1, characterized in that: The flexible substrate layer is PET; the flexible base is PDMS.

3. A preparation method for preparing the flexible pressure sensor according to claim 1 or 2, characterized in that: The following steps are involved: Sputtering a gold electrode on the flexible substrate layer, transferring a pre-grown single-layer graphene onto the gold electrode, and then transferring a pre-grown HBN onto the single-layer graphene to form a flexible substrate layer containing a heterogeneous structure; Growing graphene nanowalls on a microstructured mold and transferring the graphene nanowalls to a flexible substrate to obtain a conformal carbon nanofilm; the microstructured mold is an inverted pyramid structure mold; The prepared conformal carbon nanofilm is packaged with a flexible substrate layer containing a heterogeneous structure to obtain a flexible pressure sensor.

4. The method for preparing the flexible pressure sensor according to claim 3, characterized in that: The method also includes spin coating a photoresist protective layer on the flexible substrate layer where the graphene and HBN transfer is completed, and photolithography retains the force-sensitive area and the electrode area on the flexible substrate.

5. The method for preparing the flexible pressure sensor according to claim 3, characterized in that: The number of layers of the HBN transfer is 1 to 9 layers.

6. The method for preparing the flexible pressure sensor according to claim 3, wherein: Transferring graphene and HBN to the flexible substrate layer specifically includes the following steps: An etching solution is prepared, and a copper foil with graphene or HBN grown thereon is cut and placed in the etching solution for 5 hours to remove the copper foil, thereby forming a graphene or HBN film; the etching solution includes water, hydrochloric acid, and hydrogen peroxide; Polymethyl methacrylate (PMMA) is spin-coated as a support layer and cured on a hot plate. The graphene or HBN film is transferred to clean water and soaked for use. The graphene or HBN film in the clean water is scooped into the intermediate dielectric layer area using a prepared flexible substrate. The film is then placed on a hot plate for drying and then soaked in an acetone solution to remove the PMMA support layer. Finally, the film is blown dry with high-purity nitrogen.

7. The method for preparing the flexible pressure sensor according to claim 6, characterized in that: The graphene and HBN are both grown by chemical vapor deposition (CVD); the copper foil on which the graphene or HBN is grown is transferred by a wet etching process.

8. The method for preparing the flexible pressure sensor according to claim 3, wherein: The step of growing graphene nanowalls on a microstructure mold and transferring the graphene nanowalls to a flexible substrate specifically includes the following steps: The silicon wafer was ultrasonically cleaned in deionized water, acetone, and alcohol for 15 minutes each to remove surface impurities. A square pattern was formed on the clean silicon wafer by photolithography, and photoresist was spin-coated on the wafer. The wafer was then dried on a hot plate at 100°C for 10 minutes. After drying, the wafer was exposed using a binary exposure machine and developed with a developer to form a square pattern, exposing the periodic micrometer-scale square holes in the silicon dioxide layer. After the photolithography is completed, the exposed square holes are dry-etched with an etching gas to remove the surface silicon dioxide layer, and then the silicon in the holes is anisotropically etched with a wet etching solution to obtain a mold with an array inverted pyramid structure. Graphene nanowalls were grown on an inverted pyramid mold using a plasma-enhanced chemical vapor deposition method and transferred via polydimethylsiloxane (PDMS).

9. The method for preparing the flexible pressure sensor according to claim 8, characterized in that: The etching gas is trifluoromethane, the etching solution includes potassium hydroxide, isopropyl alcohol and deionized water, and the etching temperature is 80°C.

10. The method for preparing the flexible pressure sensor according to claim 3, characterized in that: The conformal carbon nanofilm and the flexible substrate layer containing the heterostructure are encapsulated using a heat shrink film.

Images