CN1135588C - A method for improving the field electron emission performance of carbon nanotube film - Google Patents

Abstract

The invention provides a method for improving the field electron emission performance of a carbon nanotube film, which belongs to the field of field emission displays. Its characteristic is that the CNT thin film cathode prepared by transplantation adopts heat treatment process and plasma surface treatment process; while the CNT thin film cathode prepared by direct growth method only adopts plasma surface treatment process, and the process parameters of plasma surface treatment are power density 0.1-3W /cm ³ , the treatment time is 5-60 minutes, the current density of the CNT film can be increased by 3 times, the threshold intensity can be reduced by more than 3 times, and the electron emission point density can be increased by using _H2 or hydrogen-containing compounds. It can be improved by more than 3 orders of magnitude and the uniformity is obviously improved. The field emission performance of the CNT thin film cathode is comprehensively improved by organically combining the two treatment processes for the thin film cathode grown by the transplantation method.

Description

A method for improving the field electron emission performance of carbon nanotube film

technical field

The invention relates to a method for improving the field electron emission performance of a carbon nanotube (CNT) film, more precisely, the method of combining a heat treatment process and a plasma surface treatment process to make the field electron emission performance of a CNT film A way to significantly improve. It belongs to field emission display field.

Background technique

Field emission has become a very active field in recent years. It is agreed internationally that Field Emission Display (FED) has broad prospects and future in the flat panel display market. Compared with liquid crystal displays, the performance of field emission displays is comprehensively ahead, with the advantages of high brightness, better viewing angle effect, low power consumption, greatly reduced size, and simplified manufacturing process, etc., so it is known as the twenty-first A Century of Display Technology (K. Derbyshire, Solid State Technol. 38, 71 (1995); J. Roberston, Thin Solid Films 296, 62 (1997)). Among them, the cold cathode material is the core component of the field emission display. Metal Mo is easily polluted, and the adsorption or reaction of impurities on its surface can increase the surface work function of Mo and reduce the electron field emission performance. At the same time, the mechanical properties of Mo are poor, which shortens the service life of the material. Si is a rather brittle material that also conducts heat poorly. Obviously, with the rapid rise of field emission displays, traditional field emission materials have become inadequate. Today, the production of FED is based on the Spindt (Mo tipped cone) method applied by the French PixTech company. However, this method requires large-area precision micromachining, and the cost has increased significantly, thus limiting its competitiveness. At present, the most popular cold cathode material in the field emission field is CNT (D.Normile, "Nanotubes Generate Full-Color Displays", Science, 286, 2056, (1998); W.B.Choi, N.S.Lee, W.K.Yi, Y.W.Jin , Y.S.Choi, L.T.Han, D.S.Chung, H.Y.Kim, J.H.Kang, J.H.You, and J.M.Kim, SID 00 Digest, 324, (2000)). CNTs usually have a diameter of several tens of nanometers and a length of several microns. Such a shape makes it possible for CNTs to generate a large enough field enhancement factor under a certain electric field strength, thereby obtaining good electron emission performance. At the same time, CNT has high strength, good thermal conductivity and chemical stability, so it is a very ideal field emission cold cathode material.

There are usually two methods for the preparation of CNT cathode film arrays: one is the direct growth method; the other is the transplantation method. The CNTs grown by the direct growth method have good bonding force to the substrate and high purity, but the growth temperature is generally high, which makes it difficult to grow CNTs on glass substrates (practical requirements), and is not conducive to large-scale growth. Low efficiency and high cost. The advantages of the implantation method are high efficiency, low cost, and the ability to prepare large-area cathodes, so many large companies in the world, such as ISES (S.Uemura, SID 00Digest, 320-323 (2000)), Samsung (W.B.Choi, SID 00 Digest, 324-327 (2000)), and ERSO/ITRI (F.Y.Chuang, SID 00 Digest, 329-331 (2000)) both adopt the transplantation method to prepare the cathode of the field emission display. The transplantation method is an ideal technique to make FED practical and industrialized. At present, both methods will pursue the goal of low working field strength, high current density, high point density, uniform emission and so on. The application prospect of transplantation method is more attractive.

Contents of the invention

The object of the present invention is to provide a simple and effective method to improve the field electron emission performance of CNT thin films, which is embodied in the reduction of the threshold electric field of electron emission, the improvement of emission current density, the improvement of point density of electron emission and the emission The uniformity is improved in four areas.

The method provided by the invention is mainly aimed at the CNT film cathode prepared by the transplantation method. First, remove the organic additives during the transplantation process through a heat treatment process, reduce the content of amorphous carbon (a-C) and graphitic carbon (G-C) in the CNT, and form a good mechanical and electrical contact between the CNT and the substrate, and then pass the plasma The bulk surface treatment process further cleans the surface and reduces the surface work function, so that the field electron emission performance of the CNT film is significantly improved. That is to say, the method for improving the field emission electron performance of the CNT film provided by the present invention is realized by matching the two processes of heat treatment and plasma surface treatment.

The implantation method for preparing CNT thin film cathode mentioned in the present invention refers to: firstly, prepare CNT powder by various methods, and then transfer CNT powder to different substrate materials by various methods to obtain CNT thin film cathode. Methods for preparing CNT powder include DC arc discharge, CVD method of glow discharge in various frequency bands, laser flash evaporation (Laser Ablation), hot wire CVD method, evaporation method, catalytic pyrolysis method, etc.; transfer methods include screen printing method, spin coating-exposure-development method, etc.; substrate materials include glass, metal, alloy, silicon wafer, ceramics and other materials. The thickness of the film is 10-60μ. The key to improving the field electron emission performance of the CNT film is to purify and improve the dispersibility of the CNT. Usually, the transplantation method will add organic glue and other substances harmful to field emission to the CNT before transplantation, so the primary purpose of the treatment process is to completely remove such additives. The treatment process of the present invention is mainly carried out in the manner of matching process parameters in two ways, that is, heat treatment process and plasma surface treatment process. The former can be carried out at 300-700°C in O ₂ -N ₂ or H ₂ -N ₂ , the flow of O ₂ or H ₂ accounts for 3%-20% of the total flow, the purpose is to remove additives, reduce aC and GC Content, form good mechanical contact and electrical contact between CNT and substrate. The purpose of plasma surface treatment is to further purify, so that the near-surface part of CNT can achieve the best purification effect, reduce the surface work function, and thus improve the dispersion of surface CNT. Plasma surface treatment can be realized by various plasma generating equipment, such as capacitive coupling type, inductive coupling type, microwave plasma and other equipment. The process conditions are as follows: plasma power density 0.1-3W/cm ³ , plasma sheath voltage 100-500V; working pressure 0.1-5 Torr; treatment time 5-60 minutes. The gas used is one of hydrogen (H ₂ ) or gaseous hydrogen-containing compounds such as ammonia (NH ₃ ), methane (CH ₄ ), ethylene (C ₂ H ₄ ), and acetylene (C ₂ H ₂ ), The flow rate is 10-100 ml/min.

For the CNT thin film cathode prepared by the direct growth method, the heat treatment process can be omitted and only the plasma surface treatment process among the present invention is used to improve the field electron emission performance of the CNT thin film, because the CNT thin film cathode prepared by the direct growth method does not contain organic Impurities, no heat treatment is required to volatilize organic impurities. The rest of the plasma surface treatment process is the same as the CNT thin film cathode grown by the above-mentioned implantation method.

The present invention improves the field electron emission performance of the CNT film prepared by the transplantation method or the direct growth method by combining the heat treatment process and the surface treatment process, which is mainly reflected in the same field strength, which can increase the current density of the CNT film by 3 times, the threshold field strength is reduced by more than 3 times, the point density of electron emission can be increased by more than 3 orders of magnitude and the uniformity is obviously improved. If only the heat treatment process can only increase the current density, the threshold field strength cannot be significantly reduced and the point density can be increased; if only the plasma treatment process is used, the threshold field strength can be reduced and the point density can be increased to a certain extent, but the current density cannot be significantly increased . The invention comprehensively improves the field emission performance of the CNT film cathode prepared by the transplantation method through the organic combination of the two treatment processes.

Description of drawings

Fig. 1 is a schematic diagram of a field electron emission performance testing device for a CNT thin film cathode treated by the method provided by the present invention. Among them: 1-transparent glass sheet; 2-indium tin oxide electrode; 3-low voltage phosphor; 4-insulating isolation column; 5-CNT thin film; 6-substrate material.

Fig. 2 adopts the current density-electric field strength curve of the field electron emission of the CNT film cathode after the present invention's processing and the CNT film cathode of the present invention's processing, wherein the abscissa is the electric field strength (V/μm), and the ordinate is the electric current Density (mA/cm ² ). ▲ and ■ respectively represent the CNT film samples treated by the method of the present invention and without any treatment.

Fig. 3 is the photoluminescent photo of fluorescent screen under two different treatment conditions, and Fig. 3 (a) is the photoluminescent photo of fluorescent screen of cathode field emission of CNT thin film without any treatment, and Fig. 3 (b) is the CNT processed by the method of the present invention Phosphor screen luminescent photographs of field emission from thin film cathodes.

Detailed ways

Example 1

The field electron emission performance of the CNT thin film cathodes treated by the invention and not treated by the invention were tested. The CNT film cathode used is prepared by screen printing method, the substrate material is metallic nickel (Ni), the thickness of the CNT film is about 40 μm, and the effective area is 10 mm×10 mm. The tested samples are CNT films without any treatment and CNT films subjected to heat treatment and plasma surface treatment according to the method of the present invention. The heat treatment process conditions used in this embodiment are 600°C; H ₂ -N ₂ protective gas, the flow rate of H ₂ is 0.2 liter/min, the flow rate of N ₂ is 2 liter/min; the treatment time is 10 minutes. The plasma surface treatment equipment used in this embodiment is a capacitively coupled plasma etching equipment. The process conditions are: plasma power density 0.275W/cm ³ , plasma sheath voltage 215V; hydrogen flow, gas flow rate 20ml/min; working pressure 0.4 Torr; treatment time 20 minutes; room temperature. An electric field is generated by applying a voltage to the indium tin oxide anode. Under a certain electric field, electrons escape from the surface of the film and bombard the phosphor with a certain energy, thereby generating fluorescence. The display of fluorescence can intuitively reflect the uniformity of electron emission. The test of the field electron emission performance of the above-mentioned CNT film cathode shows that: when the electric field strength is 10.4V/μm, the electron field emission current density of the CNT film cathode treated by the method of the present invention is 6.84mA/cm ² , which is the same as The CNT thin film cathode without any treatment under the electric field strength has improved nearly 3 times (2.58mA/cm ² ); the threshold field strength is 1.375V/μm (in this embodiment, the threshold field strength is defined as the same area of the test sample Under the conditions, when the electric field strength applied when 1μA current is detected), it is more than 3 times lower than that of the CNT film cathode without any treatment (4.725V/μm); at the same time, the point density and uniformity of emission are significantly improved, As shown in Figure 3. Figure 3 is a group of photoluminescence photos of fluorescent screens under different treatment conditions, and Fig. 3(a) is the photoluminescent photos of fluorescent screens without any treatment under the electric field strength of 6.68V/μm, and the density of emission points is about 10 ³ /cm ² , Fig. 3(b) is the luminescent photo of the fluorescent screen of the cathode field emission of the CNT thin film treated by the method of the present invention under the same electric field strength, the emission point density is about 10 ⁶ /cm ² , and the point density increases The field electron emission performance of the CNT thin film cathode has reached the practical requirements.

Example 2

The field electron emission performance of the CNT thin film cathodes treated by the invention and not treated by the invention were tested. The CNT film cathode used is prepared by screen printing method. The substrate material is glass with a layer of metal Ti grown. The growth of metal Ti adopts electron beam evaporation method with a thickness of 500nm. The thickness of the CNT film is about 40μm and the effective area is 10mm. ×10mm. The tested samples are CNT films without any treatment and CNT films subjected to heat treatment and plasma surface treatment according to the method of the present invention. Processing condition is the same as embodiment 1. The treatment effect is similar to Example 1.

Example 3

The field electron emission performance of the CNT thin film cathodes treated by the invention and not treated by the invention were tested. The CNT film cathode used is prepared by screen printing method, the substrate material is silicon wafer (Si), the thickness of the CNT film is about 60 μm, and the effective area is 10mm×10mm. The tested samples are CNT films without any treatment and CNT films subjected to heat treatment and plasma surface treatment according to the method of the present invention. The heat treatment process conditions adopted in this embodiment are 550°C; O ₂ -N ₂ protective gas, O ₂ flow rate is 0.1 liter/min, N ₂ flow rate is 2 liter/min; treatment time is 10 minutes. The plasma surface treatment equipment used in this embodiment is a capacitively coupled plasma etching equipment. The process conditions are: plasma power density 0.275W/cm ³ , plasma sheath voltage 215V; ammonia gas flow, gas flow rate 30ml/min; working pressure 0.8 Torr; treatment time 30 minutes; room temperature. The treatment effect is similar to Example 1.

Example 4

The field electron field emission performance of the CNT thin film cathodes treated by the invention and not treated by the invention were tested. The CNT film cathode used is prepared by the direct growth method, the substrate material is silicon (Si), the thickness of the CNT film is about 10 μm, and the effective area is 5mm×5mm. The tested samples are CNT films without any treatment and CNT films treated with plasma surface according to the method of the present invention. The plasma surface treatment equipment used in this embodiment is a capacitively coupled plasma etching equipment. The process conditions are: plasma power density 0.3W/cm ³ , plasma sheath voltage 170V; hydrogen flow, gas flow rate 30ml/min; working pressure 1 Torr; treatment time 10 minutes; room temperature. The field emission current density of the plasma-treated CNT film cathode is slightly increased, the threshold electric field is reduced to 1/2 of that of the untreated sample, and the emission point density is increased by nearly two orders of magnitude. All the other are with embodiment 1.

Claims (2)

1. A method for improving the field electron emission performance of a carbon nanotube film, characterized in that the CNT film cathode prepared by the transplantation method adopts a heat treatment process and a plasma surface treatment process; the temperature of the heat treatment process is 300-700 ℃, O ₂ -N ₂ or H ₂ -N ₂ atmosphere; the power density of the plasma surface treatment process is 0.1-3W/cm ³ ; the plasma sheath voltage is 100-500V; the working pressure is 0.1-5 Torr; The time is 5-60 minutes; the gas used is H ₂ or a gaseous hydrogen-containing compound, such as ammonia, methane, ethylene, and acetylene, and the flow rate is 10-100 ml/min. 2. The method for improving the field electron emission performance of carbon nanotube thin films according to claim 1, characterized in that the heat treatment is carried out in O ₂ -N ₂ atmosphere O ₂ flow rate 0.1 liter/min, N ₂ flow rate 2 liters /min, the flow of H ₂ is 0.2 liters/min and the flow of N ₂ is 2 liters/min when _carried out in H 2 -N ₂ atmosphere; the processing time is 10 minutes.

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