The effect of temperature on the growth of carbon nanotubes on copper foil using a nickel thin film as catalyst

Growth of carbon nanotubes (CNTs) on bulk copper foil substrates has been achieved by sputtering a nickel thin film on Cu substrates followed by thermal chemical vapor deposition. The characteristics of the nanotubes are strongly dependent on the Ni film thickness and reaction temperature. Specifically, a correlation between the thin film nickel catalyst thickness and the CNT diameter was found. Two hydrocarbon sources investigated were methane and acetylene to determine the best conditions for growth of CNTs on copper. These results demonstrate the effectiveness of this simple method of directly integrating CNTs with highly conductive substrates for use in applications where a conductive CNT network is desirable.     

Direct growth of aligned carbon nanotubes on bulk metals

There are several advantages of growing carbon nanotubes (CNTs) directly on bulk metals, for example in the formation of robust CNT-metal contacts during growth. Usually, aligned CNTs are grown either by using thin catalyst layers predeposited on substrates or through vapour-phase catalyst delivery. The latter method, although flexible, is unsuitable for growing CNTs directly on metallic substrates. Here we report on the growth of aligned multiwalled CNTs on a metallic alloy, Inconel 600 (Inconel), using vapour-phase catalyst delivery. The CNTs are well anchored to the substrate and show excellent electrical contact with it. These CNT-metal structures were then used to fabricate double-layer capacitors and field-emitter devices, which demonstrated improved performance over previously designed CNT structures. Inconel coatings can also be used to grow CNTs on other metallic substrates. This finding overcomes the substrate limitation for nanotube growth which should assist the development of future CNT-related technologies.     

预置镍化学气相沉积法制备定向碳纳米管(英文)

采用化学气相沉积法(CVD),在溅射了镍薄膜的硅基底上制备了定向碳纳米管薄膜。对镍薄膜的氨气预处理过程及其机理进行了研究。结果发现预处理后的岛状区域随着薄膜厚度的增加而增加,纳米粒子区域的变化则与之相反。对5nm的镍薄膜进行预处理能获得细化和均匀分布的纳米粒子,有利于定向碳纳米管的生长。碳纳米管的生长过程及其细微结构与温度有很大关系。碳源的分解、碳原子在催化剂内部的扩散以及催化剂粒子的团聚三者之间的竞争决定了碳纳米管的生长情况。本文分析了碳纳米管的顶部生长模式及该模式下催化剂粒子的形态变化。     

Influence of co-catalyst on growth of carbon nanotubes using alcohol catalytic CVD method

This paper describes the influence of a co-catalyst on growth of carbon nanotubes (CNTs) by alcohol catalytic chemical vapour deposition (ACCVD) method. Silicon wafers covered with thermal oxide or polycrystalline diamond thin film were used as substrates. Ni thin film supported with Al, Cu or Ti was used as a catalyst. The films were deposited by pulsed laser deposition technique. Comparison of the various types of the co-catalyst (Al, Cu, Ti) leads to the conclusion that Cu co-catalyst is suitable for producing very thin single wall carbon nanotubes (SWCNTs) and combination of Al and Ni provide a good condition to the catalytic growth of CNTs. In addition, we observed also the influence of the various diffusion barriers (thermal oxide and polycrystalline diamond) on growth of CNTs. Prepared samples were analysed by Raman spectroscopy (RS) and scanning electron microscopy (SEM).     

Direct growth of horizontally aligned carbon nanotubes between electrodes and its application to field-effect transistors

This paper presents direct growth of horizontally-aligned carbon nanotubes (CNTs) between two predefined various inter-spacing up to tens of microns of electrodes (pads) and its use as CNT field-effect transistors (CNT-FETs). Using the conventional photolithography technique followed by thin film evaporation and lift off, the catalytic electrodes (pads) were prepared, consisting of Pt, Al and Fe triple layers on SiO2/Si substrate. The grown CNTs were horizontally-aligned across the catalytic electrodes on the modified gold image furnace hot stage (thermal CVD) at 800 degrees C by using an alcohol vapor as the carbon source. Scanning and transmission electron microcopies (SEM/TEM) were used to observe the structure, growth direction and density of CNTs, while Raman spectrum analysis was used to indicate the degree of amorphous impurity and diameter of CNTs. Both single- and multi-wall CNTs with diameters of 1.1-2.2 nm were obtained and the CNT density was controlled by thickness of Fe catalytic layer. Following horizontally-aligned growth of CNTs, the electrical properties of back-gate CNT-FETs were measured and showd p-type conduction behaviors of FET.     

Enhancement of field emission property from hills-like carbon nanotubes film

Improved field emission property of carbon nanotubes (CNTs) is achieved by using NiTi alloy film as catalyst under optimized condition. The NiTi alloy films are prepared by magnetron co-sputtering process and the CNTs films are synthesized by thermal chemical vapor deposition. With the increase of the Ni/Ti ratio from 19 at.% to 95 at.%, the CNTs density increases from discrete cluster to dense network, and the optimized field emission property of CNTs film is found at the medium density. However, the field emission property is significantly enhanced when the Ni/Ti ratio is about 76 at.%, and it is supposed to attribute to the combined effect of the hills-like surface enhancement and the intrinsic emission properties of CNTs.     

Selective growth of carbon nanotubes using catalyst poisoning and geometric trench

This work suggests catalyst poisoning and geometric patterned approaches to selectively grow multiwall carbon nanotubes. Ferromagnetic particles as a catalyst for CNTs growth vanish when they are deposited over an aluminum thin film. Additionally, geometric features, such as trenches or cavities, are revealed to be capable of selectively ceasing the growth of CNTs even though catalytic thin films were covered on entire samples by an atmospheric thermal chemical vapor deposition technique.     

Magnetic properties of carbon nanotubes filled with ferromagnetic metals

Carbon nanotubes (CNTs) encapsulating Fe nanowires were prepared by the chemical vapor deposition (CVD) method using ferrocene as a precursor. The influence of the addition of Pt to an Fe catalyst, which is required for growing CNTs by CVD, on the magnetic properties of the resulting CNTs was examined from the viewpoint of enhancing coercivity. Our results showed that the addition of a Pt layer on the Fe catalyst deposited on a substrate increased the coercivity of the Fe-filled CNT. This increase is due to changes in the easy magnetization axis of the Fe nanowires in the CNTs. This result indicates that the magnetic properties of the Fe-filled CNTs can be tuned by the controlling the growth conditions, which is suitable for applications in areas such as magnetic recording media and medicine.     

辅助气体对RF-PECVD制备碳纳米管薄膜形貌的影响

采用射频等离子体增强化学气相沉积(RF-PECVD)技术,以Ni为催化剂,经600℃裂解C2H2在Si基底上制备出定向碳纳米管薄膜。采用扫描电子显微镜(SEM)表征了刻蚀后Ni颗粒与沉积的碳纳米管薄膜的形貌。研究了辅助气体对等离子体预处理催化剂与碳纳米管生长的影响。结果表明:辅助气体(H2与N2)流量比对催化剂颗粒尺寸、分布以及碳纳米管生长有显著影响;合适的气体流量比有利于减少碳纳米管薄膜的杂质颗粒,促进其定向生长。预处理过程中气体流量比H2:N2=20:5时,预处理后催化剂Ni颗粒分布密度大、粒径小且分布范围窄,适合碳纳米管均匀着床;沉积生长碳纳米管薄膜时,H2:N2=20:15可得到纯度高、定向性好的碳纳米管。     

Horizontally oriented carbon nanotubes coated with nanocrystalline carbon

Single-walled carbon nanotubes (CNTs) and multi-walled CNTs of length 2-5 mm were grown from Fe/Mo nanoparticles and Fe thin film catalyst, respectively, by thermal chemical vapor deposition. Following CNT growth, the CNTs were in-situ coated with nanocrystalline carbon shells of thickness 100-1500 nm. Horizontally oriented CNTs with carbon shells in the direction of the feeding gas were visible under a regular optical microscope. They were easily manipulated by optical manipulators, and CNT probes can thus be fabricated.     

Root growth of multi-wall carbon nanotubes by MPCVD

This work examines the relationships among the growth and interlayer reactions of carbon nanotubes (CNTs) to develop an effective process for controlling the nanostructure, orientation and characteristics of CNTs. Vertically oriented CNTs were successfully synthesized by microwave plasma chemical vapor deposition (MPCVD) with CH₄/H₂ as source gases. Additionally, the Ti and SiO₂ barrier layers and the Co catalyst were used in an experiment on the growth of CNTs on the Si wafer. Then, the SiO₂ barrier layer was deposited by low-pressure chemical vapor deposition (LPCVD). The Ti barrier layer and Co catalyst films were deposited on the Si wafer by physical vapor deposition (PVD). The deposited nanostructures were characterized by scanning and transmission electron microscopy, the results of which reveal that the deposited MWCNTs were grown under the influence of a catalyst on Si substrates with or without a barrier layer, by MPCVD. Vertically grown, dense MWCNTs attached to a catalytic film demonstrate that various MWCNTs penetrated the root particles. The diameter of the root particles, of approximately in the order of 100 nm, is larger than those of the tube, 10–15 nm. The well-known model of the growth of CNTs includes base- and tip-root growth. The interaction between the catalytic film and the supporting barrier layer is suggested to determine whether the catalytic particles are driven up or pinned down on the substrate during the growth.     

Coating of carbon nanotubes with amorphous carbon nitride thin films and characterization of long-term emission stability

The effects of amorphous carbon nitride (CN) thin films that were coated on carbon nanotubes (CNTs) and their thermal treatment were investigated, in terms of the chemical bonding and morphologies of the CNTs and their field emission properties. CNTs were directly grown on conical tip-type tungsten substrates via the inductively coupled plasma-chemical vapor deposition (ICP-CVD) system, and the CNTs were coated with CN films using the RF magnetron sputtering system. The CN-coated CNTs were thermally treated using the rapid thermal annealing (RTA) system by varying the temperature (300-700 °C). The morphologies, microstructures, and chemical compositions of the CN-coated CNTs were analyzed as a function of the thickness of the CN layers and the RTA temperatures. The field emission properties of the CN/CNT hetero-structured emitters, and the fluctuation and long-term stability of the emission currents were measured and compared with those of the conventional non-coated CNT-emitter. The results showed that the electron emission capability of CNT was noticeably improved by coating a thin CN layer on the surface of the CNT. This was attributed to the low work function and negative electron affinity nature of the CN film. The CN-coated CNT-emitter had a more stable emission characteristic than that of the non-coated one. In addition, the long-term emission stability of the CN-coated emitter was further enhanced by thermal treatment, which was verified by x-ray photoelectron spectroscopy (XPS) analysis.     

Supercapacitors based on pillared graphene nanostructures

We describe the fabrication of highly conductive and large-area three dimensional pillared graphene nanostructure (PGN) films from assembly of vertically aligned CNT pillars on flexible copper foils for applications in electric double layer capacitors (EDLC). The PGN films synthesized via a one-step chemical vapor deposition process on flexible copper foils exhibit high conductivity with sheet resistance as low as 1.6 ohms per square and possessing high mechanical flexibility. Raman spectroscopy indicates the presence of multi walled carbon nanotubes (MWCNT) and their morphology can be controlled by the growth conditions. It was discovered that nitric acid treatment can significantly increase the specific capacitance of the devices. EDLC devices based on PGN electrodes (surface area of 565 m2/g) demonstrate enhanced performance with specific capacitance value as high as 330 F/g extracted from the current density-voltage (CV) measurements and energy density value of 45.8 Wh/kg. The hybrid graphene-CNT nanostructures are attractive for applications including supercapacitors, fuel cells and batteries.     

Growth and characterization of bamboo-shaped carbon nanotubes using nanocluster-assembled ZnO:Co thin films as catalyst

Bamboo-shaped carbon nanotubes (CNTs) had been successfully fabricated by a plasma enhanced chemical vapor deposition method, in which nanocluster-assembled ZnO:Co thin film was used as catalyst. It was found that bamboo-shaped CNTs were generally grown in a direction perpendicularly to the substrate surface with the tops of CNTs dominated by the droplet-like catalyst covered by the carbon layer. The diameter of CNTs was ranged from 20-50 nm. High resolution of TEM image showed that the typical CNT had a multi-walled structure with an inner core presented. The ordered graphite layers were inclined to an axis of CNT about 18 degrees and the interlayer space of a CNT was about 0.35 nm. Two peaks in Raman spectrum at 1586 cm(-1) and 1372 cm(-1) were identified as G-band and D-band for graphite, respectively. The results showed that catalyst based on ZnO:Co thin films could be used for the growth of CNTs with bamboo-shaped structure.     

Improved electrochemical capacitive characteristics by controlling the carbon nanotube morphology and electrolyte solution concentration

Carbon nanotubes (CNTs) have been used as the electrochemical double layer in capacitor (EDLC) electrodes. CNTs were synthesized using thermal chemical vapor deposition (CVD) at a growth temperature of 750 °C by flowing C₂H₂. The surface morphology of the synthesized CNTs could be controlled with or without Al film deposition between the stainless (SUS) sheet and Fe catalyst film. Electrochemical measurements were performed in a three-electrode arrangement. H₂SO₄ with different concentrations was used as the electrolyte solution. The relation between the specific capacitance and the surface morphology of the CNTs and the electrolyte concentration were investigated. The results showed that the electrode formed using vertically aligned CNTs with higher electrolyte concentration exhibited higher specific capacitance.     

The Pattern Growth of Carbon Nanotubes by Self-assembled Monolayers Techniques

The well controllable selective growth of carbon nanotubes (CNTs)on the desired area is an important issue for their future applications. In this study, a novel method for selective growth of CNTs was proposed by using the technology of self-assembly monolayers (SAMs) and the Fe-assisted CNTs growth. The Si wafers with the a: Si/Si3N4 layer patterns were first prepared by low pressure chemical vapor deposition (LPCVD)and lithography techniques to act as the substrates for selective deposition of SAMs. The selectivity of SAMs from APTMS solution (N-(2-aminoethyl)-3-aminopropyltrimethoxsilane) is based on its greater reactivity of head group on a-Si than Si3N4 films. The areas of pattern with SAMs will first chelate the Fe3 ions by their diamine-terminated group. The Fe3 ions were then consolidated to become Fe-hydroxides in sodium boron hydride solution to form the Fe-hydroxides pattern. Finally, the Fe-hydroxides pattern was pretreated in H plasma to become a well-distributed Fe nano-particles on the surface, and followed by CNTs deposition using Fe as catalyst in a microwave plasma-chemical vapor deposition (MP-CVD) system to become the CNTs pattern. The products in each processing step, including microstructures and lattice images of CNTs, were characterized by contact angle measurements, scanning electron microscopy(SEM), XPS, Auger spectroscopy, transmission electron microscopy (TEM) and high resolution TEM (HRTEM)deposition. The results show that the main process parameters include the surface activation process and its atmosphere, consolidation time and temperature, H plasma pretreatment. The function of each processing step will be discussed.     

Nonlinear dielectric thin films for high-power electric storage with energy density comparable with electrochemical supercapacitors

Although batteries possess high energy storage density, their output power is limited by the slow movement of charge carriers, and thus capacitors are often required to deliver high power output. Dielectric capacitors have high power density with fast discharge rate, but their energy density is typically much lower than electrochemical supercapacitors. Increasing the energy density of dielectric materials is highly desired to extend their applications in many emerging power system applications. In this paper, we review the mechanisms and major characteristics of electric energy storage with electrochemical supercapacitors and dielectric capacitors. Three types of in-house-produced ferroic nonlinear dielectric thin film materials with high energy density are described, including (Pb(0.97)La(0.02))(Zr(0.90)Sn(0.05)Ti(0.05))O(3) (PLZST) antiferroelectric ceramic thin films, Pb(Zn(1/3)Nb(2/3))O(3-)Pb(Mg(1/3)Nb(2/3))O(3-)PbTiO(3) (PZN-PMN-PT) relaxor ferroelectric ceramic thin films, and poly(vinylidene fluoride) (PVDF)-based polymer blend thin films. The results showed that these thin film materials are promising for electric storage with outstandingly high power density and fairly high energy density, comparable with electrochemical supercapacitors.     

The Pattern Growth of Carbon Nanotubes by Self-assembled Monolayers Techniques

The well controllable selective growth of carbon nanotubes (CNTs)on the desired area is an important issue for their future applications. In this study, a novel method for selective growth of CNTs was proposed by using the technology of self-assembly monolayers (SAMs) and the Fe-assisted CNTs growth. The Si wafers with the a ： Si/Si3N4 layer patterns were first prepared by low pressure chemical vapor deposition (LPCVD)and lithography techniques to act as the substrates for selective deposition of SAMs. The selectivity of SAMs from APTMS solution (N-(2-aminoethyl)-3-aminopropyltrimethoxsilane) is based on its greater reactivity of head group on a-Si than Si3N4 films. The areas of pattern with SAMs will first chelate the Fe3 ions by their diamine-terminated group. The Fe^3 ions were then consolidated to become Fe-hydroxides in sodium boron hydride solution to form the Fe-hydroxides pattern. Finally, the Fe-hydroxides pattern was pretreated in H plasma to become a well-distributed Fe nano-particles on the surface, and followed by CNTs deposition using Fe as catalyst in a microwave plasma-chemical vapor deposition (MP-CVD) system to become the CNTs pattern. The products in each processing step, including microstructures and lattice images of CNTs, were characterized by contact angle measurements, scanning electron microscopy (SEM), XPS, Auger spectroscopy, transmission electron microscopy (TEM) and high resolution TEM (HRTEM) deposition. The results show that the main process parameters include the surface activation process and its atmosphere, consolidation time and temperature, H plasma pretreatment. The function of each processing step will be discussed.     

Preconcentration of volatile organics on self-assembled, carbon nanotubes in a microtrap

This paper reports the self-assembly of carbon nanotubes (CNTs) on the inside wall of a steel capillary to fabricate a microtrap for the adsorption/desorption of trace organics. The microtrap functioned as a nanoconcentrator and an injector for gas chromatography (GC). The CNTs were deposited as a thin film by catalytic chemical vapor deposition from either CO or C2H4 as the precursor. The sorbent film synthesized from C2H4-CVD (CVD = chemical vapor deposition) had higher CNT density and thus was a stronger sorbent. In general, the CNT microtraps showed high-capacity adsorption and fast quantitative desorption, and the process showed excellent precision. This study demonstrates that CNT films can be deposited quite easily in a steel capillary for use in different analytical applications, and CNT films can perform as efficiently as packed-bed carbon sorbents.     

Substrate considerations for graphene synthesis on thin copper films

Chemical vapor deposition on copper substrates is a primary technique for synthesis of high quality graphene films over large areas. While well-developed processes are in place for catalytic growth of graphene on bulk copper substrates, chemical vapor deposition of graphene on thin films could provide a means for simplified device processing through the elimination of the layer transfer process. Recently, it was demonstrated that transfer-free growth and processing is possible on SiO(2). However, the Cu/SiO(2)/Si material system must be stable at high temperatures for high quality transfer-free graphene. This study identifies the presence of interdiffusion at the Cu/SiO(2) interface and investigates the influence of metal (Ni, Cr, W) and insulating (Si(3)N(4), Al(2)O(3), HfO(2)) diffusion barrier layers on Cu-SiO(2) interdiffusion, as well as graphene structural quality. Regardless of barrier choice, we find the presence of Cu diffusion into the silicon substrate as well as the presence of Cu-Si-O domains on the surface of the copper film. As a result, we investigate the choice of a sapphire substrate and present evidence that it is a robust substrate for synthesis and processing of high quality, transfer-free graphene.