EP2069234A2 - Growth of carbon nanotubes using metal-free nanoparticles - Google Patents

Growth of carbon nanotubes using metal-free nanoparticles

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Publication number
EP2069234A2
EP2069234A2 EP07815688A EP07815688A EP2069234A2 EP 2069234 A2 EP2069234 A2 EP 2069234A2 EP 07815688 A EP07815688 A EP 07815688A EP 07815688 A EP07815688 A EP 07815688A EP 2069234 A2 EP2069234 A2 EP 2069234A2
Authority
EP
European Patent Office
Prior art keywords
metal
substrate
nanoparticle
nanoparticles
carbon
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP07815688A
Other languages
German (de)
French (fr)
Inventor
Caroline Whelan
Santiago Cruz Esconjauregui
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Katholieke Universiteit Leuven
Interuniversitair Microelektronica Centrum vzw IMEC
Original Assignee
Nanocyl SA
Interuniversitair Microelektronica Centrum vzw IMEC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nanocyl SA, Interuniversitair Microelektronica Centrum vzw IMEC filed Critical Nanocyl SA
Publication of EP2069234A2 publication Critical patent/EP2069234A2/en
Withdrawn legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/158—Carbon nanotubes
    • C01B32/16—Preparation
    • C01B32/162—Preparation characterised by catalysts
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B01J21/08—Silica
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/14—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of germanium, tin or lead
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/20—Carbon compounds
    • B01J27/22—Carbides
    • B01J27/224—Silicon carbide
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures

Definitions

  • the present invention relates to the growth of carbon nanotubes. More particularly, the present invention relates to the growth of carbon nanotubes using metal-free nanoparticles.
  • Carbon nanotubes in general exhibit very good electronic and mechanical properties. Therefore, CNTs are expected to find a large diversity of industrial applications. One of these applications could be the use as both passive and active components in nano-electronic devices.
  • the most commonly accepted growth mechanism for CNTs is based on catalytic decomposition of a carbon source on a surface of a metal nanoparticle which acts as catalyst in the CNT synthesis.
  • the hydrocarbon source decomposes on front-exposed surfaces of the metal nanoparticle thereby releasing hydrogen and carbon, which dissolves in the nanoparticle.
  • the dissolved carbon then diffuses through the metal nanoparticle and is precipitated to initiate formation of CNTs.
  • One of the key issues in the growth mechanisms described in the prior art is the need for a metal catalyst particle to initiate the carbon nanotube growth.
  • a disadvantage thereof is that the metal catalyst particles can lead to the presence of impurities in the grown CNTs. Before the CNTs can be used in many applications, these impurities have to be removed.
  • a variety of chemical and thermal oxidative treatments are usually required to remove the unwanted metal impurities from the CNTs. For example, a multi-step purification procedure may be used which involves the use of nitric acid reflux and thermal oxidation.
  • Catalyst-free growth of CNTs has been achieved previously by using laser ablation and arc discharge CNT growth.
  • these methods require very high temperatures, i.e. temperatures of above 3000 0 C. Due to these high required temperatures, these methods are not suitable for in-situ CNT growth and consequently require an ex-situ approach.
  • these methods may give low production yields compared to CVD methods that can be performed at relatively low temperatures (450-1100 0 C), can be in-situ or ex- situ, and give mass production yields.
  • nanoletters, 2002 Vol. 2, No. 10, 1043-1046 (Derycke et al.) catalyst-free CNT growth has been reported to occur on SiC(111 ) above 1500 0 C.
  • the catalyst-free growth of CNTs is in this document achieved by repetitive annealing a carbon face of hexagonal silicon carbide in vacuum at predefined temperature ranges.
  • the CNTs are produced without the use of a metal catalyst but these CNTs grow with their axis parallel to the surface, or in other words aligned to the substrate, and cannot be considered feasible for mass production of CNTs.
  • carbon nanotips are grown on a silicon substrate without the use of a catalyst by using plasma-enhanced hot filament chemical vapor deposition using a mixture of methane, ammonia and hydrogen as reaction gas.
  • the carbon nanotips formation is realized by first growing a carbon film on the silicon substrate during a time period of an hour. A combination of further growth of the carbon film and ion bombardment by applying a negative bias of 430 V to the silicon ' substrate produces glow discharge and makes growth of the carbon nanotips possible.
  • the above objective is accomplished by a method according to the present invention.
  • the present invention provides a method for forming at least one carbon nanotube.
  • the method comprises:
  • a method according to embodiments of the present invention leads to formation of carbon nanotubes which do not comprise metal impurities.
  • the temperature of the substrate may be kept between 800 0 C and 1000 0 C.
  • providing at least one metal-free catalyst nanoparticle to the Chemical Vapor Deposition reactor may be performed by:
  • the at least one carbon nanotube can be formed on a substrate.
  • decomposing the carbon source gas may be performed by using a hot filament, by using a plasma, or by using a combination of a hot filament and a plasma.
  • the hot filament may be a metallic filament such as a W filament or a
  • the hot filament may have a temperature suitable for decomposing or cracking the carbon source gas.
  • the filament may be kept at a temperature of 950 0 C.
  • providing at least one metal-free catalyst nanoparticle to a Chemical Vapor Deposition reactor may be performed by providing at least one semiconductor comprising nanoparticle, e.g. a Si or Ge comprising nanoparticle.
  • the at least one Si comprising nanoparticle may, for example, be a SiC, a SiO 2 or a pure silicon nanoparticle.
  • the at least one Ge comprising nanoparticle may, for example, be a GeO 2 or a pure Ge nanoparticle. According to embodiments of the invention, providing at least one metal-free catalyst nanoparticle on a substrate is performed by:
  • metal-free catalyst material e.g. a semiconductor material catalyst material
  • Annealing may be performed at temperatures of between 500 0 C and 800 0 C.
  • the at least one metal-free catalyst nanoparticle may have a diameter of between 0.4 nm and 100 nm or of between 0.4 nm and 50 nm.
  • the method may furthermore comprise, before providing at least one metal-free catalyst nanoparticle on the substrate, providing a barrier layer on the substrate for preventing interaction, e.g. chemical interaction, of the at least one metal-free catalyst nanoparticle with the substrate.
  • the method may furthermore comprise pre-treating the at least one metal-free catalyst nanoparticle before providing it to the Chemical Vapor Deposition reactor.
  • a pre-treatment may be removal of a native oxide (e.g. SiO 2 in case of Si nanoparticles) by means of, for example a HF dip (e.g. a dip in 2% HF for 5 minutes).
  • the carbon source gas may be a hydrocarbon gas having one (C1 ) up to three (C3) carbon atoms.
  • the carbon source gas may, for example, be CH 4 , C 2 H 4 , C 2 H 2 or C 3 H 6 .
  • the carbon source gas may be CO.
  • the CVD reactor in which the method according to embodiments of the invention is performed may comprise an inert gas and hydrogen.
  • the inert gas may, for example, be nitrogen.
  • the flow of gasses in the CVD reactor may, for example, be 4 l/min N 2 , 2 l/min H 2, 0.5 l/min C 2 H 2 or 0.1 l/min C 2 H 2 .
  • the present invention provides a carbon nanotube grown from a metal-free catalyst nanoparticle. It is an advantage that these nanotubes are free from metal impurities.
  • the present invention provides the use of a metal-free catalyst nanoparticle to grow a carbon nanotube. It is an advantage that these nanotubes are free from metal impurities.
  • Figure 1 illustrates a method for forming metal-free CNT onto Si particles according to embodiments of the present invention.
  • FIG. 2 and Figure 3 schematically illustrate a reactor which can be used for growing metal-free CNTs on a substrate according to embodiments of the present invention.
  • Figure 4 illustrate a Scanning Electron Microscopy picture after growth of CNTs onto Si nanoparticles according to embodiments of the present invention.
  • the present invention provides a method for forming at least one carbon nanotube (CNT).
  • the method comprises:
  • the at least one carbon nanotube may be formed on a substrate.
  • the at least one metal-free catalyst nanoparticle may be provided on a substrate and the substrate with the at least one metal-free catalyst on it may then be transferred to the CVD reactor for the growth of CNTs.
  • the CVD method used for growing CNTs may be thermal CVD or Plasma enhanced CVD (PE-CVD).
  • PE-CVD Plasma enhanced CVD
  • the method according to embodiments of the invention can be applied for growing CNTs according to a "base growth” principle or a “tip growth principle. Occurring of a particular kind of growth principle depends on interactions between the catalyst nanoparticle and the underlying substrate.
  • base growth also referred to as “rooth growth” refers to a growth mechanism where the nanoparticles used to initiate the CNT growth stay located at the substrate during growth.
  • tip growth also referred to as “top down growth” refers to a growth mechanism where the CNTs growth having the CNT situated at the surface during growth and the catalyst nanoparticle on top of the CNT.
  • non-metal containing nanoparticles refers to nanoparticles comprising a material different from a metal and suitable to be used as a catalyst nanoparticle for initiating the growth of CNTs.
  • the nanoparticles may comprise a semiconductor material such as silicon or germanium.
  • the nanoparticles may be silicon comprising nanoparticles and may, for example, comprise pure Si, SiO 2 or SiC or may be germanium comprising nanoparticles and may, for example, comprise pure Ge or GeO 2 .
  • the nanoparticles may be pure Si nanoparticles or pure Ge nanoparticles. Whenever in the description of the present invention reference is made to catalyst nanoparticles it has to be understood that non-metal containing catalyst nanoparticles are meant.
  • a method according to embodiments of the invention allows synthesis of CNTs which do not comprise metal impurities because the growth starts from suitable non-metal containing catalyst nanoparticles onto which the CNT growth according to embodiments of the invention can take place. Hence, no purification process is required after formation of the CNTs.
  • a method according to embodiments of the invention is suitable to be used for massive CNT growth and can be used in high production yield applications.
  • the size of the catalyst nanoparticles may have an impact on the final diameter of the CNTs formed or, in other words, may determine the final diameter of the CNTs.
  • the catalyst nanoparticles suitable to be used for growing CNTs according to a method of embodiments of the present invention may have a diameter in the range of between 0.4 nm and 100 nm or between 0.4 nm and 50 nm.
  • a method for growing CNTs will be described by means of
  • a substrate 10 is provided (see Figure 1 ).
  • the term “substrate” may include any underlying material or materials that may be used, or upon which CNTs may be grown.
  • the term “substrate” may include a semiconductor substrate such as e.g. a doped or undoped silicon, gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), indium phosphide (InP), germanium (Ge), or silicon germanium (SiGe) substrate.
  • GaAs gallium arsenide
  • GaAsP gallium arsenide phosphide
  • InP indium phosphide
  • Ge germanium
  • SiGe silicon germanium
  • the "substrate” may include, for example, an insulating layer such as a SiO 2 or an Si 3 N 4 layer in addition to a semiconductor substrate portion.
  • the term “substrate” also includes glass, plastic, ceramic, silicon-on-glass, silicon-on-sapphire substrates.
  • substrate is thus used to define generally the elements for layers that underlie a layer or portions of interest, in particular for the present invention the
  • the substrate 10 may be a semicondcutor wafer, e.g. a Si wafer or a
  • a major surface of the substrate 10 should be inert with respect to CNT growth or should be such that it does not interact with the catalyst nanoparticles formed on it. Therefore, according to embodiments of the invention a barrier layer 11 may be provided onto the substrate before catalyst nanoparticles are formed on it (see further).
  • a thin layer 12 of non-metal material is provided, e.g. deposited onto a major surface of the substrate 10.
  • This layer 12 may, for example, comprise seumiconductor material such as Si or Ge.
  • this thin layer 12 may be a uniformly deposited thin layer, such as a poly-Si (polycrystalline Silicon), amorphous silicon or silicon dioxide layer deposited by commonly used deposition techniques such as, for example, CVD (Chemical Vapor Deposition).
  • the thickness of the thin layer 12 may be less than 15 nm and may, for example, be between 0.4 nm and 5 nm.
  • the thin layer 12 may also be a non-uniform sub-atomic layer deposited by e.g. ALD (Atomic Layer Deposition). Alternatively spin-on and dip coating techniques may be used to deposit a uniform thin layer 12.
  • ALD atomic layer Deposition
  • spin-on and dip coating techniques may be used to deposit a uniform thin layer 12.
  • a barrier layer 11 can be deposited onto the substrate 10 before the deposition of the thin layer 12 (see Figure 1 ).
  • the barrier layer 11 may, for example, be used to prevent reaction of the material of the thin layer 12, e.g. semiconductor layer such as Si or Ge layer, and/or formed nanoparticles with the substrate 10 underneath.
  • the barrier layer 11 may, for example, be a Si 3 N 4 layer or any other suitable layer that prevents reaction of the material of the thin layer 12 with the substrate 10.
  • an annealing step may be performed to break up the thin layer 12 and to form nanoparticles 14 (see step 13 in Figure 1 ).
  • the formed nanoparticles 14 may have a diameter of between 0.4 nm and 100 nm and may, for example, have a diameter of between 0.4 nm and 50 nm.
  • Figure 1 illustrates the formation of the nanoparticles 14.
  • the thickness of the deposited thin layer 12 as well as the temperature and time of the annealing step may be controlled or well-chosen.
  • the optimal temperature and time to create the nanoparticles 14 depends on the type and the thickness of the deposited thin layer 12 of metal-free material.
  • the temperature for annealing may range between 500 0 C and 800 0 C.
  • the anneal step may be performed in a reactor. In the reactor, gases such as nitrogen and/or hydrogen can be used as ambient gases.
  • nanoparticles 14 which may, for example, comprise pure semiconductor material, e.g. pure Si, may be formed in a thin dielectric layer, e.g. a SiO 2 layer, which is provided, e.g. deposited for example by CVD, onto the substrate 10, for example onto a semiconductor wafer, e.g. Si wafer.
  • a low energy Si ion implantation step may be performed on the SiO 2 layer followed by an annealing step to create Si nanocrystals.
  • a dissolving treatment e.g. HF treatment (e.g. HF vapor or dilute solution), can then be applied to remove the SiO 2 such that Si nanoparticles 14 which are suitable for use as initiators of CNT growth are left on the substrate 10.
  • the substrate 10 onto which the nanoparticles 14 are formed or deposited may be formed of a porous material.
  • suitable porous materials to be used with embodiments of the present invention may be zeolites and porous low-k materials (commonly used in semiconductor processing and commercially available).
  • Using porous material or in other words using a substrate 10 having inner pores makes it possible to deposit the thin layer 12 not only on the major surface of the substrate 10 but also within these inner pores of the substrate 10. This significantly increases the surface area onto which nanoparticles 14 can be formed. As a result the amount of CNTs which can be formed by the method according to embodiments of the invention may also significantly increase.
  • a thin layer 12 of, for example, semiconductor material, e.g. Si, the layer being continuous or non continuous, may be deposited onto a major surface of the porous substrate 10 and on the surface of the inner pores of the porous substrate 10.
  • nanoparticles 14 may be formed on the major surface of the substrate 10 and in the inner pores of the substrate 10. These nanoparticles 14 can then be used as catalysts to grow CNTs.
  • no substrate is used but bulk catalyst nanoparticles may be provided to grow CNTs.
  • the bulk nanoparticles should be such that a carbon source gas is able to flow in between neighbouring nanoparticles such that, when the bulk catalyst nanoparticles are provided in a reactor, CNTs can be grown onto the catalyst nanoparticles (see further).
  • the nanoparticles 14, e.g. semiconductor nanoparticles such as Si or Ge nanoparticles can be pre- treated before growth of CNTs is started.
  • An example of such a pre-treatment may be removal of a native oxide (e.g. Si ⁇ 2 in case of Si nanoparticles) by means of, for example a HF dip (e.g. a dip in 2% HF for 5 minutes).
  • the substrate 10 on which the nanoparticles 14 are formed, or according to alternative embodiments the bulk nanoparticles are transferred to a suitable reactor chamber of a reactor such as a Chemical Vapor Deposition (CVD) reactor to grow the CNTs 16 (see step 15 in Figure 1 ).
  • the CVD reactor can, for example, be a Plasma Enhanced CVD reactor or a Thermal CVD reactor.
  • a carbon source gas is decomposed or cracked by heating it. Cracking the carbon source gas leads to formation of different carbon fragments such that these fragments can be recombined on the catalyst nanoparticles to form a CNT. Recombination thus takes place at a surface of the formed nanoparticles 14, e.g. semiconductor comprising nanoparticles such as Si or Ge comprising nanoparticles.
  • Heating the carbon source gas may, according to embodiments of the invention, be done by using a hot filament, by using a plasma or by using a combination of a hot filament and a plasma.
  • this hot filament may be located in the reactor chamber such that cracked or decomposed carbon species do not recombine before they have reached the catalyst nanoparticles so as to grow CNTs (see further).
  • the hot filament may be a metallic filament and can comprise W (Tungsten) or Ta (Tantalum) and is kept at high temperatures. The height of the temperature depends on the carbon source used and needs to be high enough to crack the carbon source. For example, the temperature of the hot filament may be 950°C or higher.
  • the temperature of the catalyst nanoparticles 14 and/or the substrate 10 on which the catalyst nanoparticles 14 are formed may be in the range of between 800 0 C up to 1000 0 C.
  • any suitable carbon source gas known by a person skilled in the art may be used.
  • the carbon source gas may be a hydrocarbon source and may be a hydrocarbon gas having one (C1 ) up to three (C3) carbon atoms.
  • suitable hydrocarbon gases to be used for CVD assisted CNT growth may be CH 4 , C 2 H 4 , C 2 H 2 or C 3 H 6 .
  • alternative carbon sources such as carbon oxide (CO) can also be used as a carbon source gas.
  • the amount of carbon source gas used in the reactor chamber determines the growth, morphology and properties of CNTs formed.
  • the amount of carbon gas and/or the amount of cracked carbon fragments in the reactor chamber should be sufficient, i.e. high enough, to achieve CNT growth but on the other hand should be low enough so as to avoid formation of amorphous carbon onto the catalyst nanoparticles as in that case no CNT growth will occur.
  • the CVD reactor chamber may furthermore comprise an inert gas and hydrogen.
  • the inert gas may, for example, be nitrogen.
  • the total flow of gasses in the CVD reactor during the step of forming CNTs 16 may be around 4 l/min N 2 , 2 l/min H 2 and 0.01 up to 1 l/min carbon gas such as e.g. C 2 H 2 .
  • a suitable gas flow can be 4 l/min N 2 , 2 l/min H 2 and 0.1 l/min carbon gas such as C 2 H 2.
  • Examples of a simplified reactor which may be used to perform CNT growth according to embodiments of the present invention is schematically illustrated in Figure 2 and Figure 3. The difference between Figure 2 and Figure 3 is the location of the hot filament 2.
  • the reactor comprises a quartz tube 6 in which the substrate 10 comprising the nanoparticles 14, e.g. semiconductor comprising nanoparticles such as Si or Ge comprising nanoparticles, is placed.
  • a furnace 3 is situated at the outside of the quartz tube 6 and is used to create an optimal reaction temperature within the quartz tube 6. With optimal temperature is meant a temperature at which CNT growth can take place.
  • a hot filament 2 is placed at the entrance or gas inlet 1 of the reactor such that the carbon source, e.g. carbon source gas is cracked into fragments which may then recombine on the nanoparticles 14 to form CNTs 16.
  • the hot filament 2 may be located above the substrate 10.
  • a simple release process such as, for example, chemical dissolution of the substrate 10 can be done.
  • a silicon wafer was provided as a substrate 10 to grow the CNTs 16 on.
  • a Si 3 N 4 barrier layer 11 was deposited in a vacuum reactor.
  • a thin layer 12 of 5 nm poly-Si was deposited. Without breaking the vacuum the sample was annealed in conditions such that the thin layer 12 broke into nanoparticles 14.
  • the anneal step to break up the poly-Si layer 12 into Si nanoparticles 14 was performed at 530 0 C during a time period of 20 minutes.
  • the obtained Si nanoparticles 14 had a diameter of approximately 5 nm. 2.
  • the substrate 10 with the nanoparticles 14 on was then placed in a standard HF solution (2% HF) for a couple of minutes (e.g. 5 minutes) at room temperature so as to remove a possibly present native oxide formed after the nanoparticles have been exposed to air.
  • the substrate 10 comprising the Si nanoparticles 14 was placed in a CVD reactor at 900 0 C for 5 min.
  • the reactor gases were N 2 and H 2 at a ratio of 4 l/min N 2 to 4 l/min H 2 .
  • the Si nanoparticles 14 were found to be suitable for growing CNTs 16 according to a method of embodiments of the present invention.
  • the nanoparticles 14 being suitable is meant that they can act as a template or precursor for CNT formation, in other words, that they can be used to initiate CNT growth.
  • C 2 H 2 gas was added to the reactor at a flow of 0,5 l/min.
  • N 2 and H 2 were also present in the reactor chamber during CNT growth at a ratio of 4 l/min N 2 to 2 l/min H 2 .
  • the substrate temperature was in the range of between 800°C and 1000 0 C 1 for example 900°C.
  • a W or Ta filament 2 situated at the entrance 1 of the gas inlet of the reactor, was heated such that incoming C 2 H 2 gas was cracked into different carbon fragments such as C-C, C-H, CH 3 * radicals, as well as stable species like CH 4 or C 2 H 2 .
  • the filament temperature was around 950 0 C (filament current was 6!4 - 6% A).
  • Figure 4 illustrates a scanning electron microscopy (SEM) picture of CNTs grown onto Si catalyst nanoparticles. It can be seen from Fig. 4 that, in the experiment conducted, nanotubes are grown at a ⁇ m scale distance.
  • the substrates 10 with nanoparticles 14 on were etched in HF (2%) for 1 min. at room temperature in order to remove a possibly present native oxide from the Si nanoparticles 14.
  • the samples were placed in the CVD reactor chamber with temperatures ranging between 600 0 C and 900 0 C, under reducing atmosphere in N 2 : H 2 (4:2 l/min.) for 5 minutes at atmospheric pressure.
  • a W wire was used as a hot filament 2 and was located above the substrate 10 comprising the catalyst nanoparticles 14.
  • the gas composition used for this experiment was N 2 :H 2 :C at a ratio 4:2:0.1 l/min.
  • the Carbon source used was either one of acetylene, ethylene or methane.
  • the CNTs 16 were grown for half an hour at atmospheric pressure.
  • FIG. 5 and Figure 6 illustrate SEM pictures after growth of CNTs onto the Si nanoparticles according to embodiments of the present example. Massive growth of CNTs 16 was observed, i.e. CNTs were grown much closer to each other when compared to Figure 4. A more intensive growth was observed in the area where the Si nanoparticles are closer to the hot filament 2 (see upper row of CNTs 16 in Figure 5).

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Abstract

The present invention provides a method for forming at least one carbon nanotube (16) by using metal-free catalyst nanoparticles (14), for example Si or Ge comprising nanoparticles. The method uses the step of decomposing a carbon source gas to form carbon fragments which then recombine at the metal-free catalyst nanoparticles (14) to grow carbon nanotubes (16). The method according to embodiments of the invention leads to carbon nanotubes (16) which do not comprise metal impurities.

Description

GROWTH OF CARBON NANOTUBES USING METAL-FREE
NANOPARTICLES
Technical field of the invention
The present invention relates to the growth of carbon nanotubes. More particularly, the present invention relates to the growth of carbon nanotubes using metal-free nanoparticles.
Background of the invention
Carbon nanotubes (CNTs) in general exhibit very good electronic and mechanical properties. Therefore, CNTs are expected to find a large diversity of industrial applications. One of these applications could be the use as both passive and active components in nano-electronic devices.
The most commonly accepted growth mechanism for CNTs is based on catalytic decomposition of a carbon source on a surface of a metal nanoparticle which acts as catalyst in the CNT synthesis. According to this growth mechanism, the hydrocarbon source decomposes on front-exposed surfaces of the metal nanoparticle thereby releasing hydrogen and carbon, which dissolves in the nanoparticle. The dissolved carbon then diffuses through the metal nanoparticle and is precipitated to initiate formation of CNTs.
One of the key issues in the growth mechanisms described in the prior art is the need for a metal catalyst particle to initiate the carbon nanotube growth. A disadvantage thereof is that the metal catalyst particles can lead to the presence of impurities in the grown CNTs. Before the CNTs can be used in many applications, these impurities have to be removed. A variety of chemical and thermal oxidative treatments are usually required to remove the unwanted metal impurities from the CNTs. For example, a multi-step purification procedure may be used which involves the use of nitric acid reflux and thermal oxidation.
Catalyst-free growth of CNTs has been achieved previously by using laser ablation and arc discharge CNT growth. However, these methods require very high temperatures, i.e. temperatures of above 30000C. Due to these high required temperatures, these methods are not suitable for in-situ CNT growth and consequently require an ex-situ approach. Furthermore, these methods may give low production yields compared to CVD methods that can be performed at relatively low temperatures (450-11000C), can be in-situ or ex- situ, and give mass production yields. In Nanoletters, 2002 Vol. 2, No. 10, 1043-1046 (Derycke et al.), catalyst-free CNT growth has been reported to occur on SiC(111 ) above 15000C. The catalyst-free growth of CNTs is in this document achieved by repetitive annealing a carbon face of hexagonal silicon carbide in vacuum at predefined temperature ranges. The CNTs are produced without the use of a metal catalyst but these CNTs grow with their axis parallel to the surface, or in other words aligned to the substrate, and cannot be considered feasible for mass production of CNTs.
In Applied Surface Science 245 (2005) 21-25 (Wang et al.) carbon nanotips are grown on a silicon substrate without the use of a catalyst by using plasma-enhanced hot filament chemical vapor deposition using a mixture of methane, ammonia and hydrogen as reaction gas. The carbon nanotips formation is realized by first growing a carbon film on the silicon substrate during a time period of an hour. A combination of further growth of the carbon film and ion bombardment by applying a negative bias of 430 V to the silicon ' substrate produces glow discharge and makes growth of the carbon nanotips possible.
Summary of the invention
It is an object of embodiments of the present invention to provide a good method for growing carbon nanotubes on a substrate. The above objective is accomplished by a method according to the present invention.
It is an advantage of a method according to embodiments of the invention that the carbon nanotubes grown by this method do substantially not comprise metal impurities. A method according to embodiments of the invention may use Chemical
Vapour Deposition. The present invention provides a method for forming at least one carbon nanotube. The method comprises:
- providing at least one metal-free catalyst nanoparticle to a Chemical Vapor Deposition reactor, - forming reactive carbon fragments by decomposing a carbon source gas in the Chemical Vapor Deposition reactor, and
- recombining the reactive carbon fragments on top of the at least one metal- free catalyst nanoparticle to grow the at least one carbon nanotube.
A method according to embodiments of the present invention leads to formation of carbon nanotubes which do not comprise metal impurities.
During decomposing the carbon source gas and growing the at least one carbon nanotube, the temperature of the substrate may be kept between 8000C and 10000C.
According to embodiments of the invention, providing at least one metal-free catalyst nanoparticle to the Chemical Vapor Deposition reactor may be performed by:
- providing at least one metal-free catalyst nanoparticle on a substrate, and
- transferring the substrate with the at least one metal-free nanoparticle on it to the Chemical Vapor Deposition reactor. According to these embodiments, the at least one carbon nanotube can be formed on a substrate.
According to embodiments of the invention, decomposing the carbon source gas may be performed by using a hot filament, by using a plasma, or by using a combination of a hot filament and a plasma. The hot filament may be a metallic filament such as a W filament or a
Ta filament. The hot filament may have a temperature suitable for decomposing or cracking the carbon source gas. For example when a hot filament is used for decomposing the carbon source gas, the filament may be kept at a temperature of 9500C. According to embodiments of the invention, providing at least one metal-free catalyst nanoparticle to a Chemical Vapor Deposition reactor may be performed by providing at least one semiconductor comprising nanoparticle, e.g. a Si or Ge comprising nanoparticle. The at least one Si comprising nanoparticle may, for example, be a SiC, a SiO2 or a pure silicon nanoparticle.
The at least one Ge comprising nanoparticle may, for example, be a GeO2 or a pure Ge nanoparticle. According to embodiments of the invention, providing at least one metal-free catalyst nanoparticle on a substrate is performed by:
- providing a thin layer of metal-free catalyst material, e.g. a semiconductor material catalyst material, onto the substrate, and
- annealing the thin layer of metal-free material so as to break it up and form the at least one metal-free catalyst nanoparticle.
Annealing may be performed at temperatures of between 5000C and 8000C.
According to embodiments of the invention, the at least one metal-free catalyst nanoparticle may have a diameter of between 0.4 nm and 100 nm or of between 0.4 nm and 50 nm.
According to embodiments of the invention, the method may furthermore comprise, before providing at least one metal-free catalyst nanoparticle on the substrate, providing a barrier layer on the substrate for preventing interaction, e.g. chemical interaction, of the at least one metal-free catalyst nanoparticle with the substrate.
According to further embodiments of the invention, the method may furthermore comprise pre-treating the at least one metal-free catalyst nanoparticle before providing it to the Chemical Vapor Deposition reactor. An example of such a pre-treatment may be removal of a native oxide (e.g. SiO2 in case of Si nanoparticles) by means of, for example a HF dip (e.g. a dip in 2% HF for 5 minutes).
According to embodiments of the invention, the carbon source gas may be a hydrocarbon gas having one (C1 ) up to three (C3) carbon atoms. The carbon source gas may, for example, be CH4, C2H4, C2H2 or C3H6. According to other embodiments of the invention, the carbon source gas may be CO. The CVD reactor in which the method according to embodiments of the invention is performed may comprise an inert gas and hydrogen. The inert gas may, for example, be nitrogen.
The flow of gasses in the CVD reactor may, for example, be 4 l/min N2, 2 l/min H2, 0.5 l/min C2H2 or 0.1 l/min C2H2.
In a further aspect, the present invention provides a carbon nanotube grown from a metal-free catalyst nanoparticle. It is an advantage that these nanotubes are free from metal impurities.
In yet a further aspect, the present invention provides the use of a metal-free catalyst nanoparticle to grow a carbon nanotube. It is an advantage that these nanotubes are free from metal impurities.
Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.
Although there has been constant improvement, change and evolution of devices in this field, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient, stable and reliable devices of this nature.
The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.
Brief description of the drawings
All drawings are intended to illustrate some aspects and embodiments of the present invention. Not all alternatives and options are shown and therefore the invention is not limited to the content of the given drawings. Like numerals are employed to reference like parts in the different figures. Figure 1 illustrates a method for forming metal-free CNT onto Si particles according to embodiments of the present invention.
Figure 2 and Figure 3 schematically illustrate a reactor which can be used for growing metal-free CNTs on a substrate according to embodiments of the present invention.
Figure 4, Figure 5 and Figure 6 illustrate a Scanning Electron Microscopy picture after growth of CNTs onto Si nanoparticles according to embodiments of the present invention.
In the different figures, the same reference signs refer to the same or analogous elements.
Description of illustrative embodiments
The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.
It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments. Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.
Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.
In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.
The invention will now be described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured according to the knowledge of persons skilled in the art without departing from the true spirit or technical teaching of the invention, the invention being limited only by the terms of the appended claims. The present invention provides a method for forming at least one carbon nanotube (CNT). The method comprises:
- providing at least one metal-free catalyst nanoparticle to a Chemical Vapor Deposition (CVD) reactor, - forming reactive carbon fragments by decomposing a carbon source gas in the Chemical Vapor Deposition reactor, and
- recombining the reactive carbon fragments on top of the at least one metal- free catalyst nanoparticle to grow the at least one carbon nanotube.
According to embodiments of the invention, the at least one carbon nanotube may be formed on a substrate. According to these embodiments the at least one metal-free catalyst nanoparticle may be provided on a substrate and the substrate with the at least one metal-free catalyst on it may then be transferred to the CVD reactor for the growth of CNTs.
The CVD method used for growing CNTs may be thermal CVD or Plasma enhanced CVD (PE-CVD).
The method according to embodiments of the invention can be applied for growing CNTs according to a "base growth" principle or a "tip growth principle. Occurring of a particular kind of growth principle depends on interactions between the catalyst nanoparticle and the underlying substrate. The term "base growth", also referred to as "rooth growth" refers to a growth mechanism where the nanoparticles used to initiate the CNT growth stay located at the substrate during growth. The term "tip growth", also referred to as "top down growth" refers to a growth mechanism where the CNTs growth having the CNT situated at the surface during growth and the catalyst nanoparticle on top of the CNT.
Furthermore, the term "non-metal containing" nanoparticles refers to nanoparticles comprising a material different from a metal and suitable to be used as a catalyst nanoparticle for initiating the growth of CNTs. According to embodiments of the invention, any non-metal containing nanoparticles can be used. According to embodiments of the invention, the nanoparticles may comprise a semiconductor material such as silicon or germanium. For example, the nanoparticles may be silicon comprising nanoparticles and may, for example, comprise pure Si, SiO2 or SiC or may be germanium comprising nanoparticles and may, for example, comprise pure Ge or GeO2. According to specific embodiments of the invention, the nanoparticles may be pure Si nanoparticles or pure Ge nanoparticles. Whenever in the description of the present invention reference is made to catalyst nanoparticles it has to be understood that non-metal containing catalyst nanoparticles are meant.
A method according to embodiments of the invention allows synthesis of CNTs which do not comprise metal impurities because the growth starts from suitable non-metal containing catalyst nanoparticles onto which the CNT growth according to embodiments of the invention can take place. Hence, no purification process is required after formation of the CNTs.
Furthermore, a method according to embodiments of the invention is suitable to be used for massive CNT growth and can be used in high production yield applications.
In general, the size of the catalyst nanoparticles may have an impact on the final diameter of the CNTs formed or, in other words, may determine the final diameter of the CNTs. The catalyst nanoparticles suitable to be used for growing CNTs according to a method of embodiments of the present invention may have a diameter in the range of between 0.4 nm and 100 nm or between 0.4 nm and 50 nm. Hereinafter, a method for growing CNTs will be described by means of
Figure 1. It has to be understood that the sequence of steps described hereinafter is not intended to limit the invention in any way.
In a first step, a substrate 10 is provided (see Figure 1 ). In embodiments of the present invention, the term "substrate" may include any underlying material or materials that may be used, or upon which CNTs may be grown. According to embodiments, the term "substrate" may include a semiconductor substrate such as e.g. a doped or undoped silicon, gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), indium phosphide (InP), germanium (Ge), or silicon germanium (SiGe) substrate. The "substrate" may include, for example, an insulating layer such as a SiO2 or an Si3N4 layer in addition to a semiconductor substrate portion. Thus the term "substrate" also includes glass, plastic, ceramic, silicon-on-glass, silicon-on-sapphire substrates. The term "substrate" is thus used to define generally the elements for layers that underlie a layer or portions of interest, in particular for the present invention the
CNTs to be grown. According to a specific embodiment of the present invention, the substrate 10 may be a semicondcutor wafer, e.g. a Si wafer or a
Ge wafer. According to embodiments of the invention, a major surface of the substrate 10 should be inert with respect to CNT growth or should be such that it does not interact with the catalyst nanoparticles formed on it. Therefore, according to embodiments of the invention a barrier layer 11 may be provided onto the substrate before catalyst nanoparticles are formed on it (see further).
A thin layer 12 of non-metal material, also referred to as metal-free material, is provided, e.g. deposited onto a major surface of the substrate 10. This layer 12 may, for example, comprise seumiconductor material such as Si or Ge. In case of, for example, Si comprising material, this thin layer 12 may be a uniformly deposited thin layer, such as a poly-Si (polycrystalline Silicon), amorphous silicon or silicon dioxide layer deposited by commonly used deposition techniques such as, for example, CVD (Chemical Vapor Deposition). The thickness of the thin layer 12 may be less than 15 nm and may, for example, be between 0.4 nm and 5 nm. According to embodiments of the invention, the thin layer 12 may also be a non-uniform sub-atomic layer deposited by e.g. ALD (Atomic Layer Deposition). Alternatively spin-on and dip coating techniques may be used to deposit a uniform thin layer 12.
If needed, a barrier layer 11 can be deposited onto the substrate 10 before the deposition of the thin layer 12 (see Figure 1 ). The barrier layer 11 may, for example, be used to prevent reaction of the material of the thin layer 12, e.g. semiconductor layer such as Si or Ge layer, and/or formed nanoparticles with the substrate 10 underneath. The barrier layer 11 may, for example, be a Si3N4 layer or any other suitable layer that prevents reaction of the material of the thin layer 12 with the substrate 10.
After deposition of the thin layer 12, an annealing step may be performed to break up the thin layer 12 and to form nanoparticles 14 (see step 13 in Figure 1 ). The formed nanoparticles 14 may have a diameter of between 0.4 nm and 100 nm and may, for example, have a diameter of between 0.4 nm and 50 nm. Figure 1 illustrates the formation of the nanoparticles 14. To control the size, more particularly to control the diameter of the nanoparticles 14, the thickness of the deposited thin layer 12 as well as the temperature and time of the annealing step may be controlled or well-chosen. The optimal temperature and time to create the nanoparticles 14 depends on the type and the thickness of the deposited thin layer 12 of metal-free material. For example, the temperature for annealing may range between 5000C and 8000C. The anneal step may be performed in a reactor. In the reactor, gases such as nitrogen and/or hydrogen can be used as ambient gases.
According to an alternative embodiment, nanoparticles 14 which may, for example, comprise pure semiconductor material, e.g. pure Si, may be formed in a thin dielectric layer, e.g. a SiO2 layer, which is provided, e.g. deposited for example by CVD, onto the substrate 10, for example onto a semiconductor wafer, e.g. Si wafer. After deposition of the thin SiO2 layer a low energy Si ion implantation step may be performed on the SiO2 layer followed by an annealing step to create Si nanocrystals. A dissolving treatment, e.g. HF treatment (e.g. HF vapor or dilute solution), can then be applied to remove the SiO2 such that Si nanoparticles 14 which are suitable for use as initiators of CNT growth are left on the substrate 10.
According to still other embodiments of the invention, the substrate 10 onto which the nanoparticles 14 are formed or deposited, may be formed of a porous material. Examples of suitable porous materials to be used with embodiments of the present invention may be zeolites and porous low-k materials (commonly used in semiconductor processing and commercially available). Using porous material or in other words using a substrate 10 having inner pores makes it possible to deposit the thin layer 12 not only on the major surface of the substrate 10 but also within these inner pores of the substrate 10. This significantly increases the surface area onto which nanoparticles 14 can be formed. As a result the amount of CNTs which can be formed by the method according to embodiments of the invention may also significantly increase. In case of such porous substrates 10, a thin layer 12 of, for example, semiconductor material, e.g. Si, the layer being continuous or non continuous, may be deposited onto a major surface of the porous substrate 10 and on the surface of the inner pores of the porous substrate 10. After performing of an annealing step as described above to create nanoparticles, nanoparticles 14 may be formed on the major surface of the substrate 10 and in the inner pores of the substrate 10. These nanoparticles 14 can then be used as catalysts to grow CNTs. According to yet another alternative embodiment, no substrate is used but bulk catalyst nanoparticles may be provided to grow CNTs. The bulk nanoparticles should be such that a carbon source gas is able to flow in between neighbouring nanoparticles such that, when the bulk catalyst nanoparticles are provided in a reactor, CNTs can be grown onto the catalyst nanoparticles (see further).
According to embodiments of the invention, the nanoparticles 14, e.g. semiconductor nanoparticles such as Si or Ge nanoparticles, can be pre- treated before growth of CNTs is started. An example of such a pre-treatment may be removal of a native oxide (e.g. Siθ2 in case of Si nanoparticles) by means of, for example a HF dip (e.g. a dip in 2% HF for 5 minutes).
After formation of the non-metal containing nanoparticles 14, the substrate 10 on which the nanoparticles 14 are formed, or according to alternative embodiments the bulk nanoparticles, are transferred to a suitable reactor chamber of a reactor such as a Chemical Vapor Deposition (CVD) reactor to grow the CNTs 16 (see step 15 in Figure 1 ). The CVD reactor can, for example, be a Plasma Enhanced CVD reactor or a Thermal CVD reactor. In the CVD reactor a carbon source gas is decomposed or cracked by heating it. Cracking the carbon source gas leads to formation of different carbon fragments such that these fragments can be recombined on the catalyst nanoparticles to form a CNT. Recombination thus takes place at a surface of the formed nanoparticles 14, e.g. semiconductor comprising nanoparticles such as Si or Ge comprising nanoparticles.
Heating the carbon source gas may, according to embodiments of the invention, be done by using a hot filament, by using a plasma or by using a combination of a hot filament and a plasma. When using a hot filament for decomposing the carbon source gas, this hot filament may be located in the reactor chamber such that cracked or decomposed carbon species do not recombine before they have reached the catalyst nanoparticles so as to grow CNTs (see further). The hot filament may be a metallic filament and can comprise W (Tungsten) or Ta (Tantalum) and is kept at high temperatures. The height of the temperature depends on the carbon source used and needs to be high enough to crack the carbon source. For example, the temperature of the hot filament may be 950°C or higher.
During formation of reactive carbon fragments by decomposing the carbon source gas and during subsequent CNT growth, the temperature of the catalyst nanoparticles 14 and/or the substrate 10 on which the catalyst nanoparticles 14 are formed may be in the range of between 8000C up to 10000C.
According to embodiments of the invention, any suitable carbon source gas known by a person skilled in the art may be used. For example the carbon source gas may be a hydrocarbon source and may be a hydrocarbon gas having one (C1 ) up to three (C3) carbon atoms. Examples of suitable hydrocarbon gases to be used for CVD assisted CNT growth may be CH4, C2H4, C2H2 or C3H6. According to other embodiments, alternative carbon sources such as carbon oxide (CO) can also be used as a carbon source gas. The amount of carbon source gas used in the reactor chamber determines the growth, morphology and properties of CNTs formed. The amount of carbon gas and/or the amount of cracked carbon fragments in the reactor chamber should be sufficient, i.e. high enough, to achieve CNT growth but on the other hand should be low enough so as to avoid formation of amorphous carbon onto the catalyst nanoparticles as in that case no CNT growth will occur.
The CVD reactor chamber may furthermore comprise an inert gas and hydrogen. The inert gas may, for example, be nitrogen. As an example the total flow of gasses in the CVD reactor during the step of forming CNTs 16 may be around 4 l/min N2, 2 l/min H2 and 0.01 up to 1 l/min carbon gas such as e.g. C2H2. A suitable gas flow can be 4 l/min N2, 2 l/min H2 and 0.1 l/min carbon gas such as C2H2. Examples of a simplified reactor which may be used to perform CNT growth according to embodiments of the present invention is schematically illustrated in Figure 2 and Figure 3. The difference between Figure 2 and Figure 3 is the location of the hot filament 2. The reactor comprises a quartz tube 6 in which the substrate 10 comprising the nanoparticles 14, e.g. semiconductor comprising nanoparticles such as Si or Ge comprising nanoparticles, is placed. A furnace 3 is situated at the outside of the quartz tube 6 and is used to create an optimal reaction temperature within the quartz tube 6. With optimal temperature is meant a temperature at which CNT growth can take place. In the example given in Figure 2, a hot filament 2 is placed at the entrance or gas inlet 1 of the reactor such that the carbon source, e.g. carbon source gas is cracked into fragments which may then recombine on the nanoparticles 14 to form CNTs 16. According to other embodiments and as illustrated in Figure 3, the hot filament 2 may be located above the substrate 10. In the latter case, more extensive growth of CNTs may be obtained because in that case the cracked carbon species do not have to travel a long way to reach the catalyst nanoparticles 14, and thus have a lower chance, with respect to the case illustrated in Figure 2, of recombining before having assisted in CNT growth.
To release the CNTs 16 formed on the substrate 10, for, for example, bulk production of CNTs 16, a simple release process such as, for example, chemical dissolution of the substrate 10 can be done.
Hereinafter, some examples will be described. It has to be understood that these are only for the ease of understanding the present invention and are not intended to limit the invention in any way.
Examples
1. Nanoparticles preparation
A silicon wafer was provided as a substrate 10 to grow the CNTs 16 on. Onto the silicon substrate 10, first a Si3N4 barrier layer 11 was deposited in a vacuum reactor. Onto the Si3N4 barrier layer 11 a thin layer 12 of 5 nm poly-Si was deposited. Without breaking the vacuum the sample was annealed in conditions such that the thin layer 12 broke into nanoparticles 14. The anneal step to break up the poly-Si layer 12 into Si nanoparticles 14 was performed at 5300C during a time period of 20 minutes. The obtained Si nanoparticles 14 had a diameter of approximately 5 nm. 2. Catalyst nanoparticle pre-treatment
The substrate 10 with the nanoparticles 14 on was then placed in a standard HF solution (2% HF) for a couple of minutes (e.g. 5 minutes) at room temperature so as to remove a possibly present native oxide formed after the nanoparticles have been exposed to air. Immediately after removal of the native oxide, the substrate 10 comprising the Si nanoparticles 14 was placed in a CVD reactor at 9000C for 5 min. The reactor gases were N2 and H2 at a ratio of 4 l/min N2 to 4 l/min H2. The Si nanoparticles 14 were found to be suitable for growing CNTs 16 according to a method of embodiments of the present invention. By the nanoparticles 14 being suitable is meant that they can act as a template or precursor for CNT formation, in other words, that they can be used to initiate CNT growth.
3. CNT growth After formation of the Si catalyst nanoparticles 14 in the CVD reactor,
C2H2 gas was added to the reactor at a flow of 0,5 l/min. N2 and H2 were also present in the reactor chamber during CNT growth at a ratio of 4 l/min N2 to 2 l/min H2. The substrate temperature was in the range of between 800°C and 10000C1 for example 900°C. During CNT growth a W or Ta filament 2, situated at the entrance 1 of the gas inlet of the reactor, was heated such that incoming C2H2 gas was cracked into different carbon fragments such as C-C, C-H, CH3 * radicals, as well as stable species like CH4 or C2H2. The filament temperature was around 9500C (filament current was 6!4 - 6% A). Figure 4 illustrates a scanning electron microscopy (SEM) picture of CNTs grown onto Si catalyst nanoparticles. It can be seen from Fig. 4 that, in the experiment conducted, nanotubes are grown at a μm scale distance.
4. Massive CNT growth Before performing CNT growth, the substrates 10 with nanoparticles 14 on were etched in HF (2%) for 1 min. at room temperature in order to remove a possibly present native oxide from the Si nanoparticles 14. The samples were placed in the CVD reactor chamber with temperatures ranging between 6000C and 9000C, under reducing atmosphere in N2: H2 (4:2 l/min.) for 5 minutes at atmospheric pressure.
A W wire was used as a hot filament 2 and was located above the substrate 10 comprising the catalyst nanoparticles 14. A flow of 0.1 l/min. of acetylene, ethylene or methane in addition to the other gases (N2: H2) was flown over the hot filament 2 such that the Carbon source gas was cracked.
The gas composition used for this experiment was N2:H2:C at a ratio 4:2:0.1 l/min. The Carbon source used was either one of acetylene, ethylene or methane. The CNTs 16 were grown for half an hour at atmospheric pressure.
Figure 5 and Figure 6 illustrate SEM pictures after growth of CNTs onto the Si nanoparticles according to embodiments of the present example. Massive growth of CNTs 16 was observed, i.e. CNTs were grown much closer to each other when compared to Figure 4. A more intensive growth was observed in the area where the Si nanoparticles are closer to the hot filament 2 (see upper row of CNTs 16 in Figure 5).
It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope of this invention as defined by the appended claims.

Claims

1.- A method for forming at least one carbon nanotube (16), the method comprising: providing at least one metal-free catalyst nanoparticle (14), and growing a carbon nanotube (16) from the metal-free catalyst nanoparticle
(14).
2.- A method according to claim 1 , wherein the at least one metal-free catalyst nanoparticle (14) is provided to a Chemical Vapor Deposition reactor, and wherein growing a carbon nanotube (16) comprises - forming reactive carbon fragments by decomposing a carbon source gas in the Chemical Vapor Deposition reactor, and
- recombining the reactive carbon fragments on top of the at least one metal-free catalyst nanoparticle (14) so as to grow the at least one carbon nanotube (16).
3.- A method according to claim 2, wherein the carbon source gas is a hydrocarbon gas having one (C1 ) up to three (C3) carbon atoms.
4.- A method according to claim 3, wherein the carbon source gas is CH4,
02H4, C2H2 or C3H6.
5.- A method according to claim 2, wherein the carbon source gas is CO.
6.- A method according to any of claims 2 to 5, wherein providing at least one metal-free catalyst nanoparticle (14) to the Chemical Vapor Deposition reactor is performed by:
- providing at least one metal-free catalyst nanoparticle (14) on a substrate (10), and - transferring the substrate (10) with the at least one metal-free nanoparticle (14) on it to the Chemical Vapor Deposition reactor.
7.- A method according to claim 6, wherein providing at least one metal-free catalyst nanoparticle (14) on a substrate (10) is performed by:
- providing a layer of metal-free material (12) onto the substrate (10), and
- annealing the layer (12) so as to form the at least one metal-free catalyst nanoparticle (14).
8.- A method according to claim 7, wherein annealing is performed at temperatures of between 5000C and 8000C.
9.- A method according to any of claims 6 to 8, wherein the method furthermore comprises, before providing the at least one metal-free catalyst nanoparticle (14) on the substrate (10), providing a barrier layer
(11 ) on the substrate (10) for preventing interaction of the at least one metal-free catalyst nanoparticle (14) with the substrate (10).
10.- A method according to any of claims 6 to 9, wherein, during decomposing the carbon source gas and growing the at least one carbon nanotube (16), the temperature of the substrate (10) is kept between 8000C and
10000C.
11.- A method according to any of claims 2 to 10, wherein decomposing the carbon source gas is performed by using a hot filament (2), by using a plasma, or by using a combination of both.
12.- A method according to claim 11 , wherein decomposing the carbon source gas is performed by using a hot filament (2) at a temperature of 9500C.
13.- A method according to any of the previous claims, wherein providing at least one metal-free catalyst nanoparticle (14) is performed by providing at least one semiconductor comprising nanoparticle (14).
14.- A method according to claim 13, wherein the at least one semiconductor comprising nanoparticle (14) is a SiC, a Siθ2, a pure silicon nanoparticle, a Geθ2 or a pure Ge nanoparticle.
15.- A method according to any of the previous claims, wherein the at least one metal-free catalyst nanoparticle (14) has a diameter of between 0.4 nm and 100 nm.
16.- A method according to any of the previous claims, wherein the method furthermore comprises pre-treating the at least one metal-free catalyst nanoparticle (14) before growing the carbon nanotube (16).
17.- A carbon nanotube (16) grown from a metal-free catalyst nanoparticle (14).
18.- Use of a metal-free catalyst nanoparticle (14) to grow a carbon nanotube
(16).
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