Tsinghua University (Beijing, CN) and Hon Hai Precision Industry Co., Ltd. (Tu-Cheng, Taipei Hsien, TW) share U.S. Patent 7,771,698 for a laser-based method for growing an array of carbon nanotubes.
According to inventors Zhuo Chen, Yang Wei, Kai-Li Jiang and Shou-Shan Fan their method for making/growing an array of carbon nanotubes includes the steps of: (a) providing a substrate; (b) forming a light absorption film made of a light absorption material on the substrate; (c) forming a catalyst film on the light absorption film; (d) introducing a mixture of a carrier gas and a carbon source gas by flowing the mixture over/across the catalyst film; (e) focusing a laser beam on the light absorption film to locally heat the catalyst to a predetermined/reaction temperature; and (f) growing an array of the carbon nanotubes from the substrate.
FIG. 1 is a flow chart of a method for making an array of carbon nanotubes
FIG. 2 shows a Scanning Electron Microscope (SEM) image of the array of carbon nanotubes formed by the method of FIG. 1.
Referring to FIG. 1, a method for making an array of carbon nanotubes includes the steps of: (a) providing a substrate; (b) forming a film of light absorption material (i.e., a light absorption film) on the substrate; (c) forming a catalyst film on the light absorption film; (d) introducing a gas mixture of a carrier gas and a carbon source gas as a gas flow across/adjacent the catalyst film; (e) focusing a laser beam on the light absorption film to locally heat the catalyst to a predetermined temperature; and (f) growing an array of the carbon nanotubes from the substrate.
In step (a), the substrate is made of a heat-resistant material, which can tolerate a high reaction temperature. It is to be understood that depending on different applications, the material of the substrate could be selected, e.g., from a group consisting of silicon, silicon dioxide and metal for semiconductor electronical devices and/or glass for flat displays.
Step (b) includes the steps of: (b1) applying a carbonaceous material layer onto the substrate; (b2) gradually heating the substrate with the carbonaceous material layer to about 300.about.450.degree. C. for a time within a range of about 60.about.90 minutes in an atmosphere of N.sub.2 and/or another inert gas and baking the substrate with the carbonaceous material thereon for about 15.about.20 minutes; and (b3) cooling down the substrate with the carbonaceous material thereon to room temperature and thereby forming/yielding a light absorption layer on the substrate.
In step (b1), the carbonaceous material layer can be made of materials having merits of good electrical conductivity, strong adhesion with the substrate, and compatibility with high vacuum environment. Quite usefully, the carbonaceous material is a commercial colloidal graphite, as used for CRTs. The carbonaceous material can, beneficially, be spin-coated on the substrate at a rotational speed of about 1000.about.5000 rpm. Quite suitably, the rotational speed for spin coating is about 1500 rpm. In step (b2), the baking process is, at least in part, to eliminate any impurities in the carbonaceous material layer, such as the macromolecule material in the commercial graphic inner coating (GIC). The thickness of the formed light absorption layer is in the approximate range from 1 to 20 micrometers.
In step (c), the catalyst film can be uniformly disposed on the light absorption layer by means of chemical vapor deposition, thermal deposition, electron-beam deposition, or sputtering. The catalyst can, opportunely, be made of iron, gallium nitride, cobalt, nickel, or any combination alloy thereof. In one useful embodiment, a catalyst-ethanol solution is spin-coated (.about.1500 rpm) on the GIC layer to form the catalyst film. The catalyst-ethanol solution is a mixture solution of ethanol and one or more metallic nitrate compounds selected from a group consisting of magnesium nitrate (Mg(NO3)2.6H2O), iron nitrate (Fe(NO3)3.9H2O), cobalt nitrate (Co(NO3)2.6H2O), nickel nitrate (Ni(NO3) 2.6H2O) and any combination thereof. Quite usefully, the catalyst-ethanol solution includes about 0.01.about.0.5 Mol/L magnesium nitrate and about 0.01.about.0.5 Mol/L iron nitrate. The thickness of the formed catalyst film is in the approximate range from 1 to 100 nanometers.
In step (d), a carbon source gas, which is combined with a carrier gas, is introduced as a gas flow across/adjacent the catalyst film. The carbon source gas acts as a primary source of carbon for growing the carbon nanotubes. In one useful embodiment, the carbon source gas and the carrier gas, in open air, are directly introduced by a nozzle to an area adjacent to the catalyst film. That is, the method can be operated without a closed reactor and/or without being under a vacuum. The carrier gas can be a nitrogen (N2) gas or a noble gas. The carbon source gas can be ethylene (C2H4), methane (CH4), acetylene (C2H2), ethane (C2H6), or any combination thereof. Quite suitably, the carrier gas is argon (Ar), and the carbon source gas is acetylene. A ratio of the carrier gas flow-rate to the carbon source gas flow-rate can be adjusted in the range from 5:1 to 10:1. Quite usefully, the argon flow-rate is 200 sccm (Standard Cubic Centimeter per Minute), and the acetylene flow-rate is 25 sccm.
In step (e), the laser beam can be generated by a laser beam generator (e.g., a carbon dioxide laser or an argon ion laser, etc.). A power of the laser beam generator is in the approximate range above about 0 W (Watt) (i.e., a measurable amount of power) to .about.5 W. Quite usefully, a carbon dioxide laser of 470 mW is used for generating the laser beam. The laser beam generator further includes at least one lens for focusing laser beams generated by the laser beam generator. It is to be understood that the focused laser beam could be employed to directly irradiate the catalyst film to heat the catalyst to a predetermined temperature along a direction vertical/orthogonal or oblique to the substrate (i.e., the surface of the substrate upon which the array is grown). That is, the method can be operated in open air without heating the entire substrate to meet a reaction temperature for synthesizing carbon nanotubes.
In step (f), due to catalyzing by the catalyst film, the carbon source gas supplied through the catalyst film is pyrolized in a gas phase into carbon units (C.dbd.C or C) and free hydrogen (H.sub.2). The carbon units are absorbed on a free surface of film of the catalyst and diffused into the film of the catalyst. When the film of the catalyst is supersaturated with the dissolved carbon units, carbon nanotube growth is initiated. As the intrusion of the carbon units into the film of the catalyst continues, an array of carbon nanotubes is formed. The additional hydrogen produced by the pyrolized reaction can help reduce the catalyst oxide and activate the catalyst. As such, the growth speed of the carbon nanotubes is increased, and the height of the array of the carbon nanotubes is enhanced.
It is noted that the catalyst film can be heated by laser irradiating. At the same time, the light absorption layer can absorb laser energy and further promote heating the catalyst film. Thus, the predetermined temperature for locally heating the catalyst film by laser beam can be less than 600.degree. C. Moreover, the present method can heat the catalyst film to a predetermined reaction temperature within less time. Further, the carbon source gases are directly introduced to area near the catalyst film. As such, the predetermined reaction temperature and the concentration of the carbon source gases can achieve the requirements for CNTs growth in open air, without a closed reactor under a vacuum.
Referring to FIG. 2, an array of carbon nanotubes manufactured by the present method is shown. The array of carbon nanotubes is synthesized by irradiating the focused laser beam on the catalyst film for about 30 seconds. A diameter of the focused laser beam is in the approximate range from 50 to 200 micrometers. The formed array of carbon nanotube manifests a hill-shaped morphology. The diameter of the hill is in the approximate range from 100 to 200 micrometers. The maximum height of the hill is in the approximate range from 10 to 20 micrometers. The diameter of each of carbon nanotubes is in the approximate range from 10 to 30 nanometers.
In step (a), the substrate is made of a heat-resistant material, which can tolerate a high reaction temperature. It is to be understood that depending on different applications, the material of the substrate could be selected, e.g., from a group consisting of silicon, silicon dioxide and metal for semiconductor electronical devices and/or glass for flat displays.
Step (b) includes the steps of: (b1) applying a carbonaceous material layer onto the substrate; (b2) gradually heating the substrate with the carbonaceous material layer to about 300.about.450.degree. C. for a time within a range of about 60.about.90 minutes in an atmosphere of N.sub.2 and/or another inert gas and baking the substrate with the carbonaceous material thereon for about 15.about.20 minutes; and (b3) cooling down the substrate with the carbonaceous material thereon to room temperature and thereby forming/yielding a light absorption layer on the substrate.
In step (b1), the carbonaceous material layer can be made of materials having merits of good electrical conductivity, strong adhesion with the substrate, and compatibility with high vacuum environment. Quite usefully, the carbonaceous material is a commercial colloidal graphite, as used for CRTs. The carbonaceous material can, beneficially, be spin-coated on the substrate at a rotational speed of about 1000.about.5000 rpm. Quite suitably, the rotational speed for spin coating is about 1500 rpm. In step (b2), the baking process is, at least in part, to eliminate any impurities in the carbonaceous material layer, such as the macromolecule material in the commercial graphic inner coating (GIC). The thickness of the formed light absorption layer is in the approximate range from 1 to 20 micrometers.
In step (c), the catalyst film can be uniformly disposed on the light absorption layer by means of chemical vapor deposition, thermal deposition, electron-beam deposition, or sputtering. The catalyst can, opportunely, be made of iron, gallium nitride, cobalt, nickel, or any combination alloy thereof. In one useful embodiment, a catalyst-ethanol solution is spin-coated (.about.1500 rpm) on the GIC layer to form the catalyst film. The catalyst-ethanol solution is a mixture solution of ethanol and one or more metallic nitrate compounds selected from a group consisting of magnesium nitrate (Mg(NO3)2.6H2O), iron nitrate (Fe(NO3)3.9H2O), cobalt nitrate (Co(NO3)2.6H2O), nickel nitrate (Ni(NO3) 2.6H2O) and any combination thereof. Quite usefully, the catalyst-ethanol solution includes about 0.01.about.0.5 Mol/L magnesium nitrate and about 0.01.about.0.5 Mol/L iron nitrate. The thickness of the formed catalyst film is in the approximate range from 1 to 100 nanometers.
In step (d), a carbon source gas, which is combined with a carrier gas, is introduced as a gas flow across/adjacent the catalyst film. The carbon source gas acts as a primary source of carbon for growing the carbon nanotubes. In one useful embodiment, the carbon source gas and the carrier gas, in open air, are directly introduced by a nozzle to an area adjacent to the catalyst film. That is, the method can be operated without a closed reactor and/or without being under a vacuum. The carrier gas can be a nitrogen (N2) gas or a noble gas. The carbon source gas can be ethylene (C2H4), methane (CH4), acetylene (C2H2), ethane (C2H6), or any combination thereof. Quite suitably, the carrier gas is argon (Ar), and the carbon source gas is acetylene. A ratio of the carrier gas flow-rate to the carbon source gas flow-rate can be adjusted in the range from 5:1 to 10:1. Quite usefully, the argon flow-rate is 200 sccm (Standard Cubic Centimeter per Minute), and the acetylene flow-rate is 25 sccm.
In step (e), the laser beam can be generated by a laser beam generator (e.g., a carbon dioxide laser or an argon ion laser, etc.). A power of the laser beam generator is in the approximate range above about 0 W (Watt) (i.e., a measurable amount of power) to .about.5 W. Quite usefully, a carbon dioxide laser of 470 mW is used for generating the laser beam. The laser beam generator further includes at least one lens for focusing laser beams generated by the laser beam generator. It is to be understood that the focused laser beam could be employed to directly irradiate the catalyst film to heat the catalyst to a predetermined temperature along a direction vertical/orthogonal or oblique to the substrate (i.e., the surface of the substrate upon which the array is grown). That is, the method can be operated in open air without heating the entire substrate to meet a reaction temperature for synthesizing carbon nanotubes.
In step (f), due to catalyzing by the catalyst film, the carbon source gas supplied through the catalyst film is pyrolized in a gas phase into carbon units (C.dbd.C or C) and free hydrogen (H.sub.2). The carbon units are absorbed on a free surface of film of the catalyst and diffused into the film of the catalyst. When the film of the catalyst is supersaturated with the dissolved carbon units, carbon nanotube growth is initiated. As the intrusion of the carbon units into the film of the catalyst continues, an array of carbon nanotubes is formed. The additional hydrogen produced by the pyrolized reaction can help reduce the catalyst oxide and activate the catalyst. As such, the growth speed of the carbon nanotubes is increased, and the height of the array of the carbon nanotubes is enhanced.
It is noted that the catalyst film can be heated by laser irradiating. At the same time, the light absorption layer can absorb laser energy and further promote heating the catalyst film. Thus, the predetermined temperature for locally heating the catalyst film by laser beam can be less than 600.degree. C. Moreover, the present method can heat the catalyst film to a predetermined reaction temperature within less time. Further, the carbon source gases are directly introduced to area near the catalyst film. As such, the predetermined reaction temperature and the concentration of the carbon source gases can achieve the requirements for CNTs growth in open air, without a closed reactor under a vacuum.
Referring to FIG. 2, an array of carbon nanotubes manufactured by the present method is shown. The array of carbon nanotubes is synthesized by irradiating the focused laser beam on the catalyst film for about 30 seconds. A diameter of the focused laser beam is in the approximate range from 50 to 200 micrometers. The formed array of carbon nanotube manifests a hill-shaped morphology. The diameter of the hill is in the approximate range from 100 to 200 micrometers. The maximum height of the hill is in the approximate range from 10 to 20 micrometers. The diameter of each of carbon nanotubes is in the approximate range from 10 to 30 nanometers.
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