CN111128728A

CN111128728A - A stretchable transistor and method of making the same

Info

Publication number: CN111128728A
Application number: CN201911289202.1A
Authority: CN
Inventors: 冯雪; 孟艳芳; 马寅佶
Original assignee: Tsinghua University
Current assignee: Tsinghua University
Priority date: 2019-12-13
Filing date: 2019-12-13
Publication date: 2020-05-08
Anticipated expiration: 2039-12-13
Also published as: CN111128728B

Abstract

The invention provides a stretchable transistor and a preparation method thereof. The preparation method includes the following steps: after heating a flexible substrate, processing a metal electrode thereon, and then cooling to obtain a stretchable electrode-covered substrate; The graphene grown on the copper foil is prepared by chemical vapor deposition, and the graphene grown on the back of the copper foil is retained, and the copper foil is etched away to obtain a graphene roll. The olefin roll is transferred and stacked onto the resulting substrate as a semiconductor layer; a stretchable transistor is constructed. The invention adopts the method of annealing the flexible substrate to realize that the metal electrode prepared on it still has conductivity under stretching; the graphene that does not need back etching is used to make a multi-layer stacked graphene roll as the semiconductor layer, which has the advantages of The extraordinary tensile properties and high transparency effectively solve the problems that the traditional stretchable transistor structure design cannot effectively utilize the space, and the stretchable organic semiconductor preparation is not environmentally friendly and unhealthy.

Description

Stretchable transistor and preparation method thereof

Technical Field

The invention relates to the field of flexible electronic devices, in particular to a stretchable transistor and a preparation method thereof.

Background

The flexible electronic technology is characterized in that different material systems and different functional components are integrated on a flexible substrate in a large area and on a large scale to form a flexible information device and system capable of stretching/bending deformation, the flexible information device and system has the characteristics of low modulus, light weight, and multiple degrees of freedom and flexibility in structure and function, the rigid physical form of a traditional electronic system is subversively changed, and the life style of human is greatly changed. The rapid development of flexible electronics brings innovation of materials and improvement of processing, flexibility, advanced evolution, integration and miniaturization of sensors, medical treatment, health monitoring and artificial intelligence are achieved, and application and popularization of new technologies are greatly promoted.

In flexible electronics, the stretchable transistor always occupies a place due to the advantages of high sensitivity, multi-parameter monitoring, intellectualization and the like. In the prior art, the stretchable transistor is generally realized by using a structural design, namely, a rigid island-flexible connector configuration is designed, and the transistor basic units are connected on the island through the stretchable connector, but the structure is not favorable for realizing effective space utilization. Or, the stretchable transistor is made of a stretchable material, that is, a stretchable organic semiconductor (generally a polymer) is coated on a stretchable substrate by spin coating, but the processing process involves a relatively toxic organic reagent, which is not environment-friendly and is not good for the health of operators.

Disclosure of Invention

Aiming at the problems in the prior art, the invention provides a stretchable transistor and a preparation method thereof.

The invention provides a preparation method of a stretchable transistor, which comprises the following steps:

(1) heating the flexible substrate, processing a metal electrode on the flexible substrate, and cooling to obtain a stretchable substrate covered with the electrode;

(2) preparing graphene growing on a copper foil by adopting a chemical vapor deposition method, reserving the graphene originally growing on the back of the copper foil, etching the copper foil to obtain a graphene roll, and transferring and stacking a plurality of layers of graphene rolls onto the substrate obtained in the step (1) to be used as a semiconductor layer;

(3) the stretchable transistor is constructed.

The stretchable transistor of the present invention comprises a stretchable semiconductor layer disposed on a stretchable substrate + electrode. The invention processes the metal electrode in the expansion state by heat treatment of the flexible substrate, and the metal electrode appears undulate and fold after cooling, thus realizing the electrical property maintenance in the stretching state. For the stretchable semiconductor layer, except for the organic semiconductor, graphene is used in the prior art, but the degree of freedom of the graphene in stretching is limited due to a two-dimensional grid structure with carbon-carbon single-double bonds alternating, so that the stretchability of the graphene is very low, and cracks can occur under small strain, while the invention adopts a special graphene material, namely graphene roll, to obtain the semiconductor layer with the stretching rate of up to 120%.

Further, in the step (1), the flexible substrate is Polydimethylsiloxane (PDMS).

Further, the step (1) specifically includes: and heating the cured flexible substrate at 125-135 ℃ for 5-8 hours, then evaporating the metal electrode on the cured flexible substrate, and naturally cooling to room temperature.

In a preferred embodiment of the invention, the preparation of the stretchable substrate + electrode comprises: uniformly stirring a PDMS precursor (prepolymer) and a curing agent in a mass ratio of 10:1, pouring the mixture into a flat mold, and curing for 1-3 hours at 50-80 ℃; after complete curing, heating at 130 ℃ for 5-8 hours, and then evaporating to form metal; and finally naturally cooling to room temperature.

Further, the step (2) specifically includes: preparing graphene growing on a copper foil by adopting a chemical vapor deposition method, coating polymethyl methacrylate (PMMA) on the upper surface of the graphene, putting the obtained product into an etching solution, taking out the copper foil after the copper foil is completely etched, transferring the copper foil to the substrate obtained in the step (1), removing the PMMA by using acetone, and repeating the steps for multiple times to obtain a graphene roll which is stacked on the substrate obtained in the step (1) in a multi-layer mode.

According to the method, the graphene on the back surface of the copper foil does not need to be etched like the traditional transfer graphene, and due to the fact that the graphene on the back surface of the copper foil is discontinuous, the micro channels which can be penetrated by etching liquid exist, and even if the graphene on the back surface is not etched, the copper foil can still be etched after the etching liquid penetrates into the micro channels.

Further, the number of times of repetition is 3 times until three layers of the graphene roll are transferred.

In a preferred embodiment of the present invention, the preparation of the stretchable semiconductor layer comprises:

first using piranha solution (H)₂SO₄And H₂O₂Mixture) cleaning copper foil for 15min, soaking the copper foil in deionized water, drying with nitrogen, adding into quartz tube with air exhausted, and allowing the inner pressure of the quartz tube to reach 5 × 10^-3When the temperature is Torr, H is introduced₂Simultaneously heating the quartz tube to 1000 ℃ for 30min, and continuously introducing H₂(flow rate 10sccm), CH (hydrogen chloride) with a flow rate of 5sccm is introduced₄Gas to continuously increase the graphene, and stopping introducing CH after 30min₄The quartz tube is in H₂Cooling the solution to room temperature in the flow to obtain graphene growing on the copper foil;

spin-coating a PMMA solution (mass fraction is 4.6%, rotation is carried out at 3000rpm for 1min) on the upper surface of the obtained graphene, then placing the whole graphene which is coated with PMMA and grows on a copper foil into an ammonium persulfate solution (1M) to enable the graphene to be suspended, transferring the graphene to clean water to wash away residual ammonium persulfate solution after the copper foil is dissolved, transferring the graphene to a stretchable substrate and an electrode, and removing PMMA on the graphene by using acetone after drying; this is repeated until three such graphene rolls are transferred.

Further, the step (3) further comprises preparing an electric double layer of the ionic gel grid electrode, wherein the ionic gel liquid consists of an ionic liquid, a monomer and a photoinitiator, the ionic liquid is 1-ethyl-3-methylimidazole-bis (trifluoromethanesulfonic acid) imidazole, the monomer is a polyethylene glycol diacrylate monomer and 2-hydroxy-2-methyl propyl phenyl ketone, and the photoinitiator is 2-methyl propyl phenyl ketone.

Further, the mass ratio of the ionic liquid to the monomer to the photoinitiator is (88-91): (7.5-9.5): (1.5-2.5), more preferably 90:8: 2.

The invention also provides a stretchable transistor prepared by the preparation method.

The stretchable transistor of the present invention includes a stretchable electrode-coated substrate, a semiconductor layer composed of a plurality of layers of the graphene roll, and a gate electrode of an ionic gel, the semiconductor layer being disposed on the stretchable electrode-coated substrate.

The invention adopts a flexible substrate annealing method to realize that the metal electrode prepared on the flexible substrate still has conductivity under stretching; the graphene which does not need back etching is adopted to be made into a multilayer stacked graphene roll as a semiconductor layer, so that the graphene roll has supernormal tensile property and higher transparency, and the problems that the traditional stretchable transistor structure design cannot effectively utilize space, and the stretchable organic semiconductor is not environment-friendly and unhealthy in preparation are effectively solved.

Drawings

FIG. 1 is a photomicrograph of PDMS coated with a metal electrode according to example 1 of the present invention after being stretched by 5% compared to PDMS coated with a metal electrode without heat treatment according to comparative example 1;

FIG. 2 is an SEM photograph showing that the PDMS coated with a metal electrode according to example 1 of the present invention and the PDMS coated with a metal electrode not heat-treated according to comparative example 1 are both stretched by 5%;

FIG. 3 is a graph of current-voltage test at two ends under different tensions for PDMS coated with metal electrodes in example 1 of the present invention and PDMS coated with metal electrodes without heat treatment in comparative example 1 (I-V curve);

FIG. 4 is a graph of the mechanical properties of each semiconductor layer under different tensile strains;

FIG. 5 is a graph showing the mechanical properties of each semiconductor layer under different bending strains;

FIG. 6 is an I-V curve and a transfer curve for different tensile strains for the single layer graphene transistor of comparative example 2 and the three layer graphene roll-up transistor of example 1;

fig. 7 is an I-V curve and a transfer curve of the single-layer graphene transistor of comparative example 2 and the three-layer graphene roll-up transistor of example 1 under different bending strains.

Detailed Description

The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention. The examples do not show the specific techniques or conditions, according to the technical or conditions described in the literature in the field, or according to the product specifications. The reagents or instruments used are conventional products available from regular distributors, not indicated by the manufacturer.

Example 1

The present embodiment provides a stretchable transistor, and a method for manufacturing the stretchable transistor includes the following steps:

(1) uniformly stirring a PDMS precursor (prepolymer) and a curing agent in a mass ratio of 10:1, pouring the mixture into a flat mold, curing the mixture for 1h at 60 ℃, heating the mixture for 7h at 130 ℃ after the mixture is completely cured, plating a 10nm/50nm chrome-gold electrode by using a thermal evaporation method, naturally cooling the chrome-gold electrode to room temperature, and photoetching the chrome-gold electrode by using an ultraviolet exposure machine;

(2) first using piranha solution (H)₂SO₄And H₂O₂Mixture) cleaning copper foil (10cm × 10cm, 25 μm, Sigma) for 15min, soaking the copper foil in deionized water, drying with nitrogen, adding into quartz tube with air exhausted, and allowing the quartz tube to reach internal pressure of 5 × 10^-3When the temperature is Torr, H is introduced₂Simultaneously heating the quartz tube to 1000 ℃ for 30min, and continuously introducing H₂(flow rate 10sccm), CH (hydrogen chloride) with a flow rate of 5sccm is introduced₄Gas to continuously increase the graphene, and stopping introducing CH after 30min₄The quartz tube is in H₂Cooling the solution to room temperature in the flow to obtain graphene growing on the copper foil;

spin-coating a PMMA solution (mass fraction is 4.6%, rotation is carried out at 3000rpm for 1min) on the upper surface of the obtained graphene, then placing the whole graphene which is coated with PMMA and grows on a copper foil into a 1M ammonium persulfate solution to enable the graphene to be suspended, transferring the graphene to clean water to wash away residual ammonium persulfate solution after the copper foil is dissolved, transferring the graphene to the substrate and the electrode prepared in the step (1), drying, removing PMMA on the graphene by using acetone, and obtaining a single-layer graphene roll transferred to the substrate; repeating the operation of the step (2) until three layers of graphene rolls are transferred and stacked on the substrate obtained in the step (1), and photoetching the graphene rolls by using an ultraviolet exposure machine;

(3) mixing ionic liquid 1-ethyl-3-methylimidazole-bis (trifluoromethyl sulfonic acid group) imidazole, monomer polyethylene glycol diacrylate monomer (PEGDA), 2-hydroxy-2-methyl propyl phenyl ketone and photoinitiator 2-methyl propyl phenyl ketone (HOPP) according to a mass ratio of 90:8:2 to obtain ionic gel liquid; and (3) enclosing a substrate (the substrate stacked with the photoetching graphene roll obtained in the step (2)) into a groove shape by using a transparent adhesive tape, adding an ionic gel liquid in the middle, exposing the substrate for 10 seconds by using a mask under ultraviolet light, wherein an initiator HOMP generates a free radical to react with acrylate under the ultraviolet light to initiate the polymerization of a monomer PEGDA, and the part which is not transparent has no polymerization reaction and can be washed away by using deionized water.

Example 2

This example provides a stretchable transistor, which is prepared by transferring only two graphene sheets onto a substrate in step (2), unlike example 1.

Comparative example 1

This comparative example provides a transistor, which is prepared by a method different from that of example 1 in that PDMS is directly plated with a metal electrode without being subjected to heat treatment in step (1).

Comparative example 2

The present comparative example provides a transistor, and the preparation method thereof is different from that of example 1 in that after graphene grown on a copper foil is obtained in step (2), both the copper foil and the graphene on the back surface of the copper foil are etched away, and only a single layer of the graphene etched on the back surface is transferred onto a substrate.

Comparative example 3

The present comparative example provides a transistor, and the preparation method thereof is different from that of example 1 in that after graphene grown on a copper foil is obtained in step (2), both the copper foil and the graphene on the back surface of the copper foil are etched away, and then three layers of the graphene etched on the back surface are transferred onto a substrate.

Performance testing

A. Substrate + electrode performance testing

The metal electrode-plated PDMS obtained in step (1) of example 1 and the metal electrode-plated PDMS obtained in comparative example 1 were stretched by 5%, and their micrographs and SEM images were measured, respectively, and the results are shown in fig. 1 and 2; and subjected to a two-terminal current-voltage test under different tensions, the results of which are shown in fig. 3.

As can be seen from FIG. 1, after the PDMS which is not subjected to heat treatment is plated with gold, cracks appear due to the large modulus difference; the right plot shows that the PDMS is less flat and smooth than the left plot, with slight undulations, but no cracks.

As can be seen from FIG. 2, after the PDMS which is not subjected to heat treatment is plated with gold, cracks appear due to the large modulus difference; the right plot shows that the PDMS is less flat and smooth than the left plot, with slight undulations, but no cracks.

As can be seen from FIG. 3, after the PDMS which is not subjected to heat treatment in the left image is plated with gold, cracks appear due to the large modulus difference, and the conductivity is almost lost when the tensile strength is less than 5%; in the right figure, the PDMS is plated with gold after heat treatment to form a corrugated structure, which will not break under tension, thus maintaining conductivity under tension.

B. Performance testing of semiconductor layers

The semiconductor layers in example 1, example 2, comparative example 2, and comparative example 3 (i.e., three graphene rolls without back etching, two graphene rolls without back etching, single graphene roll with back etching, and three graphene rolls with back etching) were transferred onto silicone rubber (with a tensile rate higher than PDMS), and the semiconductor layers were made into strips of 500 μm and 2000 μm by photolithography, and the electrical properties of the semiconductor layers under tensile and bending strains were compared by measuring the resistances at both ends under different tensile and bending degrees at normal temperature and pressure.

The mechanical properties of the semiconductor layers under different tensile strains are shown in fig. 4, and it can be seen that when the strain is parallel to the current direction, the highest tensile rate of the three-layer graphene roll without back etching is achieved while maintaining the conductivity (resistance below 1M Ω), which is up to 120%, the two-layer graphene roll without back etching is 100%, the three-layer graphene roll with back etching is 90%, and far exceeds the single-layer graphene with back etching (7%).

The mechanical properties of the semiconductor layers under different bending strains are shown in fig. 5, and it can be seen that when the strain is perpendicular to the current direction, the same result as when the strain is parallel to the current direction is obtained, the highest elongation rate under the condition of maintaining the conductivity (resistance below 1M Ω) of the three-layer graphene roll without back etching is up to 120%, the two-layer graphene roll without back etching is 110%, the three-layer graphene with back etching is 100%, and the elongation rate is far higher than that of the single-layer graphene with back etching (7%).

C. Transistor performance testing

Fig. 6 shows I-V curves and transfer curves for the single-layer graphene transistor (a) of comparative example 2 and the three-layer graphene roll-up transistor (b) of example 1 at different tensile strains. At strain of 5%, the resistance increased by 40%, but the single layer graphene resistance increased by a factor of 10, and the three layer graphene coil possessed the best resistance retention until the elongation exceeded 25%. But single layer graphene only loses conductivity (meaning breaks) when elongated by 7%. Transistors made of three-layer graphene have small changes in transfer curves under tension, while single-layer graphene loses conductivity when the tension is only 7%.

Fig. 7 shows I-V curves and transfer curves of the single-layer graphene transistor (a) of comparative example 2 and the three-layer graphene roll-up transistor (b) of example 1 at different bending strains. The electric-force performance of the single-layer graphene transistor and the three-layer graphene coil transistor under bending strain is similar to that under tensile strain: single layer graphene transistors lose conductivity at small strains, while three layer graphene roll-up transistors can still maintain good electrical performance at higher strains.

Although the invention has been described in detail hereinabove with respect to a general description and specific embodiments thereof, it will be apparent to those skilled in the art that modifications or improvements may be made thereto based on the invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.

Claims

1. a preparation method of a stretchable transistor, is characterized in that, comprises the following steps:

(1) After the flexible substrate is heated, metal electrodes are processed on it, and then cooled to obtain a stretchable electrode-covered substrate;

(2) using chemical vapor deposition method to prepare the graphene grown on the copper foil, retain the original graphene grown on the back of the copper foil, etch away the copper foil, make the graphene roll, combine the multilayered Described graphene roll is transferred and stacked on the substrate obtained in step (1), as semiconductor layer;

(3) Constructing the stretchable transistor.

2 . The preparation method according to claim 1 , wherein, in the step (1), the flexible substrate is polydimethylsiloxane. 3 .

3 . The preparation method according to claim 2 , wherein the step (1) specifically comprises: heating the cured flexible substrate at 125-135° C. for 5-8 hours, and then heating the cured flexible substrate on the The metal electrodes were evaporated and cooled to room temperature naturally.

4. preparation method according to claim 1, is characterized in that, described step (2) specifically comprises: adopting chemical vapor deposition method to prepare the graphene grown on copper foil, then described graphene upper surface is coated Coating with polymethyl methacrylate, then putting the obtained product into the etching solution, taking out after the copper foil is all etched, and transferring it to the substrate obtained in step (1), then removing the polymethyl methacrylate with acetone, Repeat the above steps several times.

5. The preparation method according to claim 4, wherein the number of repetitions is 3 times until three layers of the graphene rolls are transferred.

6 . The preparation method according to claim 1 , wherein the step (3) further comprises preparing an ion gel grid electric double layer, wherein the ion gel liquid is composed of an ionic liquid, a monomer and a photoinitiator, 7 . The ionic liquid is 1-ethyl-3-methylimidazole-bis(trifluoromethanesulfonic acid group) imidazole, and the monomers are polyethylene glycol diacrylate monomer and 2-hydroxy-2 methyl acetophenone, and the photoinitiator is 2-methyl acetophenone.

7. The preparation method according to claim 6, wherein the mass ratio of the ionic liquid, the monomer and the photoinitiator is (88-91):(7.5-9.5):(1.5-2.5).

8 . A stretchable transistor, characterized in that, it is prepared by the preparation method according to any one of claims 1 to 7 .

9 . The stretchable transistor according to claim 8 , comprising a stretchable electrode-covered substrate, a semiconductor layer and a gate electrode of ion gel, the semiconductor layer being composed of multiple layers of the graphite. 10 . A olefin roll is formed and disposed on the stretchable electrode-covered substrate.