Graphene for field emission cathode and method for preparing the same

Abstract

The application provides graphene for a field emission cathode and a preparation method thereof, and the preparation method is as follows: a PI film with a film thickness of 0.1-0.15 mm is provided, the PI film is placed on a laser induction device, a laser power of 7.2-10.8 W is preset, a scanning speed is 300 mm / s, the PI film is ablated by using laser direct writing, and graphene is prepared. Compared with the graphene produced by using the laser direct writing method, the preparation process is simple, and the cycle is short. The graphene prepared by the application has a smaller opening field strength, a larger maximum current density, and a larger field enhancement factor, namely, the preparation method is more convenient, and the field emission performance is also better.

Description

Technical Field

[0001] This invention relates to the field of field emission cathode materials technology, specifically to a field emission cathode graphene and its preparation method. Background Technology

[0002] Field emission refers to the phenomenon of electrons being emitted from a solid surface under the influence of a strong electric field. Good cathode materials make electrons easier to emit and the emission process more stable. The main application areas of field emission include: (1) Displays: Field emission displays (FEDs) have the characteristics of high brightness, high contrast and wide viewing angle, and are a new type of flat panel display. (2) Electron sources: In the fields of electron microscopes and electron probes, field emission can provide high-energy electron beams for material analysis and processing. (3) Vacuum electronic devices: Such as vacuum microelectronic devices and cold cathode devices, field emission serves as an electron emission source in these devices, with the advantages of low energy consumption and high efficiency.

[0003] Graphene is a type of graphene containing sp 2 Novel two-dimensional carbon nanofibers with carbon mesh structures possess excellent electrical, thermal, and mechanical properties, as well as abundant electron tunneling edges, making them ideal materials for field emission cathodes. Many methods exist for graphene preparation, including mechanical exfoliation, redox methods, electrophoresis, and chemical vapor deposition (CVD). However, mechanical exfoliation, redox methods, and electrophoresis suffer from drawbacks such as low yields, poor process controllability, and difficulties in electronic integration. In contrast, CVD is an effective method for large-area, low-cost preparation of graphene. The traditional CVD process for preparing graphene is as follows... Figure 1As shown. First, copper foil is cut to a certain size to serve as the graphene growth substrate. To remove surface impurities and oxides, the copper foil is soaked in glacial acetic acid, acetone, isopropanol, and deionized water for 5 minutes each. The treated copper foil is then placed in a quartz tube in the center of the heating zone of a tube furnace and rapidly heated to 1030°C in an atmosphere of 300 sccm (1 sccm = 1 mL / min) of Ar and a certain flow rate of H2, maintaining this temperature for 30 minutes. Then, CH4 is introduced to grow graphene. The flow rates of CH4 and H2, as well as the growth time, can be adjusted as needed depending on the morphology of the grown graphene. After graphene growth is complete, the introduction of H2 and CH4 is stopped, and the tube furnace is cooled to room temperature. A polymethyl methacrylate (PMMA)-assisted wet transfer method is used to transfer the grown graphene from the copper foil to an oxide-containing silicon wafer substrate, completing the preparation. Most researchers prepare graphene using chemical vapor deposition (CVD), first using the traditional method to prepare graphene films, islands, and buffered graphene layers, among other morphologies, and then investigating the impact of graphene morphology on its field emission performance. Experimental results show that compared to monolayer graphene films, the turn-on field strength of graphene islands and buffered graphene layers is reduced by approximately 6 V / μm, while the maximum field emission current density is also increased—the best results achieved by most studies. However, regardless of the specific adjustments, the preparation of graphene using CVD remains problematic due to its complex process, long cycle time, demanding environmental requirements, and relatively high turn-on field strength, which significantly hinders the field emission performance and industrial production of graphene. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides graphene for field emission cathodes and its preparation method. Compared to the laser direct writing method used to produce graphene, this invention offers a simpler preparation process with a shorter cycle time. Furthermore, the graphene prepared by this invention exhibits a lower on-state field strength, a higher maximum current density, and a larger field enhancement factor. In other words, while maintaining a convenient preparation method, it also provides superior field emission performance.

[0005] To achieve the above objectives, the specific solution adopted by the present invention is as follows: On the one hand, the present invention provides a method for preparing graphene for field emission cathodes, which mainly includes the following steps: S1. Provide a PI film with a thickness of 0.1–0.15 mm, and place the PI film on a laser-inducing device. S2. With a preset laser power of 7.2–10.8 W and a scanning speed of 300 mm / s, graphene is prepared by laser direct writing ablation of PI film.

[0006] Furthermore, the thickness of the PI film is 0.125 mm, and the laser power is 7.2 W.

[0007] Furthermore, graphene exhibits an upright fibrous structure with a three-dimensional tip emission morphology.

[0008] Furthermore, the graphene field emitter motor can simultaneously achieve voltages higher than 2 × 10⁻⁶ at a turn-on field strength below 0.3 V / μm. 4 μA / cm 2 Current density and 10 4 A stabilization field enhancement factor of several orders of magnitude.

[0009] On the other hand, the present invention provides graphene for field emission cathode, which is prepared by the above-described method.

[0010] Beneficial effects:

[0011] (1) This invention employs laser direct writing technology to prepare graphene by ablating PI films with a carbon dioxide laser, and for the first time applies this technology to the field of field emission. By irradiating the PI film surface with a laser, heat accumulation causes a rapid rise in the local surface temperature. This localized high temperature breaks down the CO, C=O, and NC bonds in the PI film, allowing gases such as CO and C2H2 to escape. These gases help to isolate oxygen, reduce oxidation, and facilitate the formation of fibrous structures. During the recombination of aromatic groups, sp... 3 Hybridized carbon atoms transform into sp 2 Hybridized carbon atoms form the basic structure of graphene.

[0012] (2) The present invention has an emission current density of 10 μA / cm 2 At that time, the graphene prepared with 7.2W power can obtain an on-state field strength of 0.272V / μm, and an ultra-high field enhancement factor of 14391 was obtained in the field emission experiment. Attached Figure Description

[0013] Figure 1 This is a diagram illustrating the process of preparing graphene using chemical vapor deposition in existing technologies.

[0014] Figure 2 This is a process diagram of the preparation of graphene using laser direct writing in this invention.

[0015] Figure 3 Raman spectroscopy of samples obtained in Examples 1-12 and Comparative Examples 1-6.

[0016] Figure 4 SEM images of graphene samples prepared in Examples 2, 5, 8, 11, Comparative Example 2, and Comparative Example 5.

[0017] Figure 5 The graphs show the JE characteristics of the graphene samples prepared in Examples 2, 5, 8, 11, Comparative Example 2, and Comparative Example 5.

[0018] Figure 6 The FN diagrams are of the graphene samples prepared in Examples 2, 5, 8, 11, Comparative Example 2, and Comparative Example 5. Detailed Implementation

[0019] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0020] This invention provides graphene for field emission cathodes and its preparation method. Please refer to [reference needed]. Figure 2 Its preparation method mainly includes the following steps: S1. Provide a PI film with a thickness of 0.1–0.15 mm, and place the PI film on a laser-inducing device. S2. With a preset laser power of 7.2–10.8 W and a scanning speed of 300 mm / s, graphene is prepared by laser direct writing ablation of PI film.

[0021] Example 1

[0022] A method for preparing graphene for field emission cathodes mainly includes the following steps:

[0023] S1. Provide a PI film with a thickness of 0.1 mm, and place the PI film on the laser-inducing device.

[0024] S2. With a preset laser power of 7.2W and a scanning speed of 300mm / s, graphene was prepared by laser direct writing ablation of PI film.

[0025] Example 2

[0026] The difference between Example 2 and Example 1 is that in step S1, the thickness of the PI film is 0.125 mm.

[0027] Example 3

[0028] The difference between Example 3 and Example 1 is that in step S1, the thickness of the PI film is 0.15 mm.

[0029] Example 4

[0030] The difference between Example 4 and Example 1 is that in step S2, the laser power is 8.4W.

[0031] Example 5

[0032] The difference between Example 5 and Example 2 is that in step S2, the laser power is 8.4W.

[0033] Example 6

[0034] The difference between Example 6 and Example 3 is that in step S2, the laser power is 8.4W.

[0035] Example 7

[0036] The difference between Example 7 and Example 1 is that in step S2, the laser power is 9.6W.

[0037] Example 8

[0038] The difference between Example 8 and Example 2 is that in step S2, the laser power is 9.6W.

[0039] Example 9

[0040] The difference between Example 9 and Example 3 is that in step S2, the laser power is 9.6W.

[0041] Example 10

[0042] The difference between Example 10 and Example 1 is that in step S2, the laser power is 10.8W.

[0043] Example 11

[0044] The difference between Example 11 and Example 2 is that in step S2, the laser power is 10.8W.

[0045] Example 12

[0046] The difference between Example 12 and Example 3 is that in step S2, the laser power is 10.8W.

[0047] Comparative Example 1

[0048] The difference between Comparative Example 1 and Example 1 is that the laser power in step S2 is 4.8W.

[0049] Comparative Example 2

[0050] The difference between Comparative Example 2 and Example 2 is that the laser power in step S2 is 4.8W.

[0051] Comparative Example 3

[0052] The difference between Comparative Example 3 and Example 3 is that the laser power in step S2 is 4.8W.

[0053] Comparative Example 4

[0054] The difference between Comparative Example 4 and Example 1 is that the laser power in step S2 is 6.0W.

[0055] Comparative Example 5

[0056] The difference between Comparative Example 5 and Example 2 is that the laser power in step S2 is 6.0W.

[0057] Comparative Example 6

[0058] The difference between Comparative Example 6 and Example 3 is that the laser power in step S2 is 6.0W.

[0059] Comparative Example 7

[0060] The difference between Comparative Example 7 and Example 1 is that the thickness of the PI film in step S1 is 0.05 mm.

[0061] Comparative Example 8

[0062] The difference between Comparative Example 8 and Example 1 is that the thickness of the PI film in step S1 is 0.075 mm.

[0063] Comparative Examples 7 and 8, due to their thinner PI films, were prone to curling upon heating, and were even punctured when the power was too high, resulting in poor uniformity of the prepared samples. Therefore, Comparative Examples 7 and 8 will not be analyzed further in subsequent performance analyses.

[0064] Raman spectroscopy was performed on the samples prepared in Examples 1-12 and Comparative Examples 1-6, and the results are as follows: Figure 3 As shown, all samples exhibited the three intrinsic peaks of graphene (D peak, G peak, and 2D peak), proving that graphene was indeed generated through laser direct writing. (1350 cm⁻¹) -1 The D peak at 1580 cm⁻¹ represents the degree of disorder and defects in vertical graphene and is considered to be the disorder vibration peak of vertical graphene. -1 The G peak at [location] indicates the formation of graphitized structures and is considered a characteristic peak of vertical graphene, usually represented by the I peak. D / I G The strength ratio indicates the degree of defects in graphene; a smaller ratio indicates fewer defects, more perfect crystallization, and higher quality. 2700cm -1 The 2D peak at a certain point is generally related to the number of graphene layers, and is usually represented by I. 2D / I G The strength ratio characterizes the number of graphene layers; the larger the ratio, the fewer the layers.

[0065] We performed Raman spectroscopy on each group of samples at three different locations. All samples prepared in the examples and comparative examples exhibited high D peaks in their Raman spectra. High D peaks indicate the presence of numerous defects and edges in the graphene, suggesting that amorphous carbon easily accumulates at the bottom and surface of the prepared graphene samples. In addition, the 2D peaks of each group of samples were very low, indicating that the graphene sheets have a multilayer structure. The I peaks of the graphene prepared in Examples 1-12 and Comparative Examples 1-6... D / I G The values ​​are shown in Table 1.

[0066] Table 1. I of the samples obtained in Examples 1-12 and Comparative Examples 1-6 D / I G value Example 1 0.1 7.2 0.993 Example 2 0.125 7.2 0.845 Example 3 0.15 7.2 1.136 Example 4 0.1 8.4 0.972 Example 5 0.125 8.4 0.705 Example 6 0.15 8.4 1.158 Example 7 0.1 9.6 1.012 Example 8 0.125 9.6 0.886 Example 9 0.15 9.6 1.323 Example 10 0.1 10.8 1.061 Example 11 0.125 10.8 0.844 Example 12 0.15 10.8 1.251 Comparative Example 1 0.1 4.8 0.979 Comparative Example 2 0.125 4.8 0.827 Comparative Example 3 0.15 4.8 1.120 Comparative Example 4 0.1 6.0 0.969 Comparative Example 5 0.125 6.0 0.796 Comparative Example 6 0.15 6.0 1.136

[0067] As shown in Table 1, under the same power, the ratio of the D peak to the G peak was lower in the 0.125 mm thick film compared to the other groups, indicating that the graphene had fewer defects and edges, resulting in better quality and greater benefits for subsequent experiments. Therefore, we ultimately chose a 0.125 mm thick PI film as the substrate for our subsequent experiments.

[0068] The surface morphology of samples prepared in Examples 2, 5, 8, 11, Comparative Example 2, and Comparative Example 5 on a 0.125 mm thick PI film was characterized using SEM. Figure 4 As shown, the graphene prepared in Comparative Example 2 (under low power, i.e., 4.8 W laser power) was observed to be foamy; the graphene prepared in Comparative Example 5 (under medium power, i.e., 6 W laser power) had an upper fiber and a lower foamy morphology; the graphene prepared in Example 2 (under high power, i.e., 7.2 W laser power) was vertically fibrous; the graphene prepared in Examples 5, 8, and 11 were all vertically fibrous, similar in structure to the graphene prepared in Example 2, i.e., the morphology of the prepared graphene remained basically fixed when the laser power was greater than 7.2 W. This is because the physical formation of graphene follows a fluid dynamics process; as the laser radiation energy increases, the carbon-containing substrate transforms from a sheet-like graphene structure into a nanofiber structure.

[0069] Field emission refers to the phenomenon of electrons being released from the cathode surface under the influence of a strong electric field, belonging to cold cathode emission. Free electrons within a metal require a certain amount of work to escape, called the work function of the metal. Therefore, in metallic conductors, free electrons typically move within a certain electron potential well. When a metal is used as the cathode and a certain voltage is applied between the anode and cathode, a potential barrier is formed on the cathode surface. When the applied voltage is large, the barrier width decreases, and free electrons can be released from the metal through the quantum effect of barrier penetration. The emission phenomenon was first discovered in 1897. However, the theoretical model of field emission was not derived until 1928 by R.F. Fowler and L.W. Nordheim, based on the theory of free electrons in metallic materials and the quantum tunneling effect. According to this theory, they derived the Fowler-Nordheim equation (FN equation), successfully establishing a functional relationship between the current density, the work function of the material, and the applied electric field strength in field emission, laying the theoretical foundation for subsequent research on field emission.

[0070] According to the FN formula: In the formula, E is the average electric field between the anode and cathode plates, and J is the current density, where A = 1.5414 × 10⁻⁶. - 6 AeVV -2 B = 6.83 × 10 3 eV -3 / 2 Vμm -1 The work function φ is determined by the band structure of the material itself, which is the Fowler-Nordheim formula. E in equation (1) 2 Moving the equation to the left side and taking the logarithm of both sides, we obtain the following formula:

[0071] This gives us ln(J / E) 2 The relationship between 1 / E and the work function φ is shown in the curve, i.e., the FN curve. The relationship between them is linear, and the slope K of the curve is related to the field enhancement factor β and the work function φ. Therefore, the field enhancement factor β can be obtained by calculating the slope of the FN curve, and the value of β is:

[0072] ln(J / E) in the FN curve 2 The relationship between 1 / E and 1 / E is linear, and this relationship, along with the field enhancement factor, is used to evaluate the field emission performance of the material.

[0073] Field emission performance tests were performed on some samples. In the field emission test, indium tin oxide (ITO) coated glass was used as the anode, and vertical graphene with a copper foil substrate was fixed to a stainless steel plate using conductive tape as the cathode. A high-voltage DC power supply was connected to provide a negative high voltage. All test samples were 1 cm in diameter circles with a 5 mm anode-cathode spacing. After fixing the samples (the method was as follows: the back of the sample was wiped clean, it was fixed to the cathode plate of the field emission test using conductive tape, and then the conductive tape was folded upwards to connect with the graphene prepared on the front of the sample, so that the graphene and the cathode plate formed a conductive path), the vacuum pump was started. When the vacuum reached 5 × 10⁻⁶ mm, the vacuum was reduced to 5 × 10⁻⁶ mm. -4 When the voltage is below Pa, field emission characteristics are tested. During the test, the distance between the two electrodes remains constant, and the voltage between the two electrodes is gradually increased from zero, thereby generating a current between the two electrodes and gradually increasing it. The voltage and current signals are recorded.

[0074] Define current density to reach 10 μA / cm 2 The corresponding electric field strength is the turn-on electric field. The field emission JE characteristic curves of graphene prepared in Examples 2, 5, 8, 11, Comparative Example 2, and Comparative Example 5 are shown below. Figure 5 As shown in the comparison, it is clear that the graphene prepared in Example 2 has a higher maximum current density than the other groups, with a maximum current density of 26242.04 μA / cm². 2 Furthermore, the opening electric field was 0.272 V / μm, which was lower than that of the other groups. The reason for this was that, compared with the samples prepared in Comparative Example 2 and Comparative Example 5, the sample prepared in Example 2 began to show an upright fibrous structure with sharp emission sites, which is conducive to electron emission.

[0075] Considering the unique structure of graphene, including its two-dimensional structure and the three-dimensional structure of carbon fibers, a composite conductive network is formed. This increases the electron transport channels between graphene and the electrodes. Furthermore, graphene itself possesses extremely high conductivity. Therefore, this structure can effectively improve the conductivity between graphene and the electrodes, thereby enhancing the field emission performance of the device. Consequently, the conductivity of the samples prepared in Examples 2, 5, 8, 11, Comparative Example 2, and Comparative Example 5 was measured. The turn-on field strength, maximum current density, conductivity, and resistivity of each group are shown in Table 2.

[0076] Table 2 shows the turn-on field strength, maximum current density, conductivity, and resistivity of graphene obtained from some of the examples and comparative examples.

[0077] As shown in Table 2, Comparative Examples 2 and 5 have relatively high turn-on field strengths. This is because these two groups of samples still have a near-two-dimensional planar morphology, and their emitted electrons reach 10 μA / cm². 2 Achieving a high current density requires a higher voltage, and without a sufficient number of field emission tips, it is difficult to obtain a large current density. The sample in Example 2 exhibits the highest current density while simultaneously possessing the highest conductivity and lowest resistivity, fully demonstrating that a large current density is inseparable from its excellent electrical properties. As the laser power increases, the fiber structure does not fundamentally change; however, due to the increased fiber density, the screen effect creates some resistance, causing the turn-on field strength to not continue decreasing, and the current density to not increase indefinitely, but rather tending to stabilize. Through comparison, it can be observed that as the laser power increases, the turn-on field strength continuously decreases, and the maximum current density continuously increases, but eventually stabilizes within a certain range. Therefore, it is necessary to control the laser within a reasonable range; too high or too low a range will affect the magnitude of the turn-on field strength.

[0078] The FN diagrams of the graphene samples prepared in Examples 2, 5, 8, 11, Comparative Example 2, and Comparative Example 5 are shown below. Figure 6 As shown, the field enhancement factor β can be calculated by combining formula (3). The field enhancement factors of the samples prepared in Examples 2, 5, 8, 11, Comparative Example 2 and Comparative Example 5 are shown in Table 3.

[0079] Table 3 shows the field enhancement factors of graphene prepared in some examples and comparative examples. Example 2 14391 Example 5 12808 Example 8 11978 Example 11 11149 Comparative Example 2 5304 Comparative Example 5 6270

[0080] As shown in Table 3, the field enhancement factor prepared in the embodiments of the present invention is 11149-14391, and the field enhancement factor of the sample prepared in Example 2 is significantly increased. This is attributed to the morphology and structure of the fibrous graphene and its high density on the substrate.

[0081] The above analysis shows that when the laser power is sufficiently high, it induces fibrous graphene with a three-dimensional tip emission morphology and excellent field emission performance. However, it is crucial to control the laser power within a suitable range; excessively high or low power will negatively impact the field emission performance of the graphene.

[0082] Meanwhile, in existing research, this invention compares the turn-on field strength, maximum current density, and field enhancement factor of commonly used cathode materials in the prior art (graphene and carbon fiber prepared by conventional methods) with those of the graphene prepared by this invention. The results are shown in Table 4.

[0083] Table 4 shows the turn-on field strength, maximum current density, and field enhancement factor of graphene, carbon fiber, and graphene prepared by conventional methods, as well as graphene prepared according to this invention. Graphene in existing technologies 5.55-7.95 7.3-40.3 336-1605 Carbon fiber in existing technology 0.95-1.74 960-1410 9880-19856 Graphene prepared by this invention 0.272-0.320 18955-26242 11149-14391

[0084] As shown in Table 4, the optimal turn-on field strength of graphene prepared by conventional methods is generally 5.0-7.0 V / μm, with a relatively low maximum current density and a field enhancement factor typically around 10. 3 The field emission performance of carbon fiber is orders of magnitude higher than that of graphene. Furthermore, the graphene prepared in this invention can simultaneously achieve field emission performance higher than 2 × 10⁻⁶ at a turn-on field strength below 0.3 V / μm. 4 μA / cm 2 Current density and 10 4 With a stabilization field enhancement factor of several orders of magnitude, the graphene prepared in this invention exhibits excellent field emission characteristics.

[0085] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention in any way. All equivalent transformations or modifications made in accordance with the essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing graphene for a field emission cathode, characterized in that, The main steps include the following: S1. Provide a PI film with a thickness of 0.1~0.15mm, and place the PI film on a laser-inducing device. S2. The preset laser power is 7.2~10.8W, the scanning speed is 300mm / s, and the PI film is directly written and ablated by laser to prepare graphene. The graphene has an upright fibrous structure and a three-dimensional tip emission morphology.

2. The method for preparing graphene for a field emission cathode according to claim 1, characterized in that, The PI film has a thickness of 0.125 mm and a laser power of 7.2 W.

3. The method for preparing graphene for a field emission cathode according to claim 2, characterized in that, The graphene field emitter motor can simultaneously generate voltages higher than 2 × 10⁻⁶ at a turn-on field strength below 0.3 V / μm. 4 μA / cm 2 Current density and 10 4 A stabilization field enhancement factor of several orders of magnitude.

4. A type of graphene for field emission cathodes, characterized in that, It is prepared by the method according to any one of claims 1-3, wherein the graphene has an upright fibrous structure and a three-dimensional tip emission morphology.

Images