Development of triode type RF plasma enhanced CVD equipment for low temperature growth of carbon nanotube

Triode-type radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) equipment has been developed in order to grow well-aligned carbon nanotubes on Si and glass substrates at 550 °C. The CVD equipment employs a grid electrode in addition to the cathode and anode electrodes. The grid electrode allows the growth of a well-aligned carbon nanotube with an inside and an outside diameter of 7 and 17 nm, respectively. Moreover, the patterning growth of the well-aligned CNT on a glass substrate was also demonstrated.     

Synthesis and Enhanced Field-Emission of Thin-Walled,Open-Ended,and Well-Aligned N-Doped Carbon Nanotubes

Thin-walled, open-ended, and well-aligned N-doped carbon nanotubes (CNTs) on the quartz slides were synthesized by using acetonitrile as carbon sources. As-obtained products possess large thin-walled index (TWI, defined as the ratio of inner diameter and wall thickness of a CNT). The effect of temperature on the growth of CNTs using acetonitrile as the carbon source was also investigated. It is found that the diameter, the TWI of CNTs increase and the Fe encapsulation in CNTs decreases as the growth temperature rises in the range of 780–860°C. When the growth temperature is kept at 860°C, CNTs with TWI = 6.2 can be obtained. It was found that the filed-emission properties became better as CNT growth temperatures increased from 780 to 860°C. The lowest turn-on and threshold field was 0.27 and 0.49 V/μm, respectively. And the best field-enhancement factors reached 1.09 × 10⁵, which is significantly improved about an order of magnitude compared with previous reports. In this study, about 30 × 50 mm² free-standing film of thin-walled open-ended well-aligned N-doped carbon nanotubes was also prepared. The free-standing film can be transferred easily to other substrates, which would promote their applications in different fields.     

Synthesis of well-aligned carbon nanotubes with open tips

Large amounts of well-aligned carbon nanotubes (CNTs) with open tips have been produced by pyrolysis of iron(II) phthalocyanine. The aligned CNTs have an average length about 10 μm and diameters ranging from 92 to 229 nm. Some of produced CNTs showed Y-junction structure due to the self-joint growth of two neighboring CNTs. The well-aligned CNTs indicated a bamboo-shaped multiwalled structure and fairly good crystallinity. The aligned CNTs follow tip growth mechanism.     

Synthesis of patterned carbon nanotube arrays for field emission using a two layer Sn/Ni catalyst in an ethanol flame

Patterned carbon nanotube (CNT) arrays on Si substrate have been fabricated by using a two layer Sn/Ni catalyst in a diffusion ethanol flame. Vertically well-aligned CNT arrays were achieved on a Si substrate without any catalyst pretreatment. The Sn underlayer activated the substrates for CNT growth with Ni as catalyst, and provided a good contact between CNTs and the substrate, which is useful for field emission. Since the adhesion of Sn/Ni nanoparticles to the substrate is very strong, the growth of the CNTs follows a base-growth mode. The thickness of the Sn underlayer largely determines the diameter and diameter distribution of the as-grown CNTs. The morphologies and field electron emission properties of CNT arrays grown on Si substrates with different thicknesses of Sn and growth times have been investigated. The variation of emission current density was less than 5% during a 4 h test under a field of 1.77 V/µm.     

Strong and ultra-flexible polymer-derived silicon carbonitride nanocomposites by aligned carbon nanotubes

We report the synthesis of flexible ceramic composites with a high tensile strength (536.33±7.23 MPa) using carbon nanotube sheet aligned by mechanically stretching process. The process is based on the infiltration and pyrolysis of liquid ceramic precursor into aligned carbon nanotube sheet. Mechanical properties and microstructure of the resultant composites are investigated. The resultant nanocomposites maintain well-aligned carbon nanotube morphology with high volume fraction (60%) and long pullout (15 µm), contributing to a high degree of load-transfer efficiency and toughening. Flexibility test reveals that such ceramic nanocomposites retain the original mechanical properties and microstructures after one thousand repetitions of 75% bending deformation, showing excellent compliance and durability. Applications requiring materials with high flexibility and mechanical properties can benefit from this research.     

Effect of temperature on lateral growth of ZnO grains grown by MOCVD

ZnO growth on sapphire by MOCVD using dimethylzinc and CO₂ as zinc and oxygen precursors was performed. A dense ZnO film with major (0 0 0 2) orientation can be prepared at 350 °C and above with high dimethylzinc flow rate. Result shows that the growth temperature suppresses the lateral growth of ZnO grains, promotes the coalescence of grains but reduces the crystal alignment. To further enhance the crystal alignment, a two-step temperature variation growth method is proposed. Using the two-step growth method, employing the initial growth at lower temperature followed by the growth at higher temperature, a densely packed ZnO film with larger grains and well-aligned (0 0 0 2) crystallographic orientation can be obtained. The effect of temperature on nucleation and growth rate, and its relation to the crystal alignment enhancement is also discussed.     

Synthesis of carbon nanotubes on metal alloy substrates with voltage bias in methane inverse diffusion flames

Vertically well-aligned multi-walled carbon nanotubes (MWNTs) with uniform diameters (∼15 nm) were grown on catalytic probes at high yield rates in an inverse diffusion flame (IDF) of a co-flow jet configuration using methane as fuel. Varied parameters investigated included: alloy composition (e.g. Fe, Ni/Cu, Ni/Cr/Fe), sampling positions within the flame structure, and voltage bias applied to the probe substrate. Spontaneous Raman spectroscopy was utilized to determine the local gas-phase temperature, as well as the concentrations of carbon-based precursor species (e.g. CO, C₂H₂) within the flame structure at specific locations of carbon nanotube (CNT) growth during synthesis. The variation of the aforementioned parameters strongly affects CNT formation, diameter, growth rate, and morphology.     

The use of camphor-grown carbon nanotube array as an efficient field emitter

Three-dimensional growth of well-aligned high-purity multiwall carbon nanotubes (CNTs) is achieved on silicon, nickel-coated silicon and cobalt-coated silicon substrates by thermal decomposition of a botanical carbon source, camphor, with different catalyst concentrations. Field emission study of as-grown nanotubes in a parallel-plate diode configuration suggests them to be an efficient emitter with a turn-on field of ∼1 V/μm (for 10 μA/cm²) and a threshold field of ∼4 V/μm (for 10 mA/cm²). Maximum current density lies in a range of 20-30 mA/cm² at 5.6 V/μm with significant reversibility. Prolonged stability test of camphor-grown CNT emitters suggests a life time of ∼5 months under continuous operation. A new feature, metal-assisted electron emission from CNTs, has been addressed. Isolated nanotubes used as a cold cathode in a field emission microscope reveal the pentagonal emission sites and hence the atomic structure of the nanotube tips.     

Well-aligned Nickel Nanochains Synthesized by a Template-free Route

Highly uniform and well-aligned one-dimensional Ni nanochains with controllable diameters, including 33, 78, and 120 nm, have been synthesized by applying an external magnetic field without any surface modifying agent. The formation can be explained by the interactions of magnetic dipoles in the presence of applied magnetic field. Magnetic measurements demonstrate that the shape anisotropy dominates the magnetic anisotropy. The demagnetization factor, ∆N, is in the range of 0.23–0.36.     

High coercivity magnetic multi-wall carbon nanotubes for low-dimensional high-density magnetic recording media

Fe-embedded multi-wall carbon nanotubes (MWCNTs) were fabricated using Fe-catalyst by the chemical deposition method. Microscopic characterizations showed that the well-aligned MWCNTs were ～ 80 mm in length, with outer diameter of 20–50 nm and inner diameter of 10–20 nm. Magnetic properties were characterized in temperatures of 5 K and 305 K, which revealed that the MWCNTs exhibited high coercivity of 2600 Oe at 5 K and 732 Oe at 305 K. These values are much higher than that of bulk iron (～ 0.9 Oe) and Fe/Co/Ni nanoparticles or nano-wire arrays (～ 200–500 Oe) at the room temperature. This high coercivity and the structure of single-domain Fe nanoparticles isolated by anti-ferromagnetic MWCNTs make it a promising candidate for low-dimensional high-density magnetic recording media.     

Highly-efficient fabrication of nanoscrolls from functionalized graphene oxide by Langmuir–Blodgett method

We report a Langmuir–Blodgett (LB) approach for highly-efficient fabrication of nanoscrolls from functionalized graphene oxide single sheets. Transmission electron microscope study reveals that the scroll has a tubular structure without caps at its ends. The scrolls align parallel to the moving barriers of the LB equipment and exhibit a loose-dense pattern during the LB compression process. We find that specific solvents can unwind the scrolled structures. Electrical characterization of the well-aligned scrolls was also performed. The method demonstrated here opens up a new avenue for high-yield fabrication of carbon nanoscrolls using functionalized graphene oxide as the building blocks.     

Mechanism growth of multiwalled carbon nanotubes on carbon black

The growth of multiwalled carbon nanotubes (MWCNTs) on carbon black has been studied using a combustion oxygen/acetylene flame method. Different types of carbon black and reaction temperatures were evaluated for the growth of carbon nanotubes. The reaction was stopped after different short duration times of deposition in an attempt to observe the growth of carbon nanotubes. The samples were characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and by Raman Spectroscopy. We have observed the transformation of the carbon black surface into graphitic sheets and the start formation of tubes from these graphitic sheets. The length of the tubes is increased but the diameter is decreased with increased deposition times. Carbon nanotubes with 10–20 nm of diameter and a length of about 50 µm are obtained after 1 min deposition. The growth of multiwalled carbon nanotubes on carbon black is a phenomenon that can not be fully explained by carbon nanotubes growth models currently known. Our results lead us to propose a mechanism for the solid-state transformation of the carbon black particles surface into nanotubes in the oxygen/acetylene flame.     

Prospective growth region for chemical vapor deposition synthesis of carbon nanotube on C–H–O ternary diagram

Chemical vapor deposition has become a standard process for synthesizing carbon nanotubes. Since the successful use of chemical vapor deposition for the first time, much effort has been expended into exploring various carbon sources that can be used to synthesize carbon nanotubes, such as methane, ethane, and ethanol. However, whole perspectives for suitable carbon sources have not been clear. In this study, we performed experiments in order to determine that the appropriate C–H–O components ratio in raw materials can be used to synthesize carbon nanotubes. We also examined a variety of raw materials in our newly developed round-trip-type vacuum furnace in order to determine whether they could be used to synthesize a carbon nanotube. We used Raman spectroscopy to identify the developed carbon nanotube, and we plotted the component ratios of effective and ineffective materials on a C–H–O ternary diagram; in this diagram, the growth region became highly apparent. It should be noted that for the growth of the carbon nanotube, this region should satisfy the equation O < C < (H + O) in molar ratio. Furthermore, it was observed that adjusting the component ratios by mixing raw materials did not cause an inconsistency in the growth region.     

Early stages of carbon film growth: carbide interface formation on molybdenum

In this study we present an investigation of the carbide formation and early stages of carbon film growth using a low energy carbon beam to supply the growth species. Carbon is supplied through electron beam evaporation of graphite and between 0.1 and 40 ML are deposited on a molybdenum substrate (substrate temperature 400 °C). Photoelectron spectroscopy in the ultraviolet and X-ray regime was employed to characterize the surface and observe the carbide and carbon film formation. Two regimes, with respect to the surface composition, can be identified: firstly, the carbide formation, and, secondly the growth of a pure carbon overlayer. A carbide interface with Mo₂C stoichiometry is created, and the formation of a pure amorphous carbon layer is observed for carbon coverages exceeding about 3.4 ML. But even after the onset of carbon film growth, the carbide interface growth is not terminated, and the extension of the carbide region into the bulk continues to increase. Diffusion through the carbide interface is still present and a dynamic rather than static interfacial layer exists. The diffusion of carbon through the metal carbide dominates the interface formation, which is also evidenced by a delayed onset of carbon film formation at 600 °C. Apart from the observation of interface formation, this experiment also enabled us to observe the valence band spectrum of molybdenum carbide (Mo₂C) for the first time. The sequential deposition of carbon was shown to be a suitable method to produce clean carbide surfaces.     

Synthesis of well-aligned millimeter-sized tetragon-shaped graphene domains by tuning the copper substrate orientation

We present a simple processing method for synthesizing well-aligned millimeter-sized tetragon-shaped graphene domains on a polycrystalline copper substrate via low-pressure chemical vapor deposition. The tetragonal shape is achieved simply by wet loading the copper substrate with processing conditions previously used for the growth of millimeter-sized hexagon-shaped graphene domains. Electron backscatter diffraction (EBSD) shows that the wet loaded copper substrate is uniformly textured with a surface plane between Cu (1 0 0) and Cu (1 1 0). The in-plane rotation of the crystalline orientation across the Cu grains is very small. However, the EBSD showed that the surface orientation of the dry loaded substrate is close to the (1 1 1) crystal plane. The different surface orientation of the wet and dry loaded samples is attributed to the different surface oxygen concentration, which changes the relative stability of the (0 0 1), (1 1 0), and (1 1 1) plane during copper sublimation and recrystallization. These results provide an approach to tune the surface crystal orientation and thus the shape and orientation of the graphene domains.     

High dispersion of γ-MnO2 on well-aligned carbon nanotube arrays and its application in supercapacitors

The well-aligned carbon nanotube arrays (ACNTs) were used as supporting material and the γ-MnO₂/ACNT electrode with high dispersion of γ-MnO₂ has been prepared by electrochemically induced deposition method. The crystal structure and morphology of the γ-MnO₂/ACNT electrode were investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The capacitive properties of γ-MnO₂/ACNT electrode were characterized by cyclic voltammetry and galvanostatic charge–discharge method. The specific capacitance of the γ-MnO₂/ACNT electrode is as high as 784 F g^− 1 based on γ-MnO₂ and 234 F g^− 1 based on γ-MnO₂/ACNT composites in 0.1 M Na₂SO₄ aqueous solution from 0 to 1 V when the charge–discharge current density is 1 mA cm^− 2. Additionally, the electrode shows excellent power characteristics, high electrochemical reversibility and excellent long-term charge–discharge cycle stability.     

Tailoring particle alignment and grain orientation during tape casting and templated grain growth

The quality of crystallographic alignment in textured ceramics produced by tape casting and templated grain growth (TGG) has been little studied despite its demonstrated impact on magnetic, piezoelectric, and optical properties. Physical and crystallographic alignment of anisotropic template particles is shown to be directly linked to the casting rate, gap height, and casting viscosity during tape casting. These parameters are shown to affect the shape and magnitude of the shear rate profile under the doctor blade during casting which in turn causes a gradient in the torque acting on anisotropic particles. The magnitude of the torque, the time the slurry is exposed to torque during casting, and the ratio of casting height to template diameter are demonstrated to enable the particle alignment process to be tailored to produce well-aligned template particles. Crystallographic alignment of the textured ceramic was quantified by grain misalignment angle (full width at half maximum, FWHM) and degree of orientation (r) and is directly correlated with the degree of torque during casting. High-quality alignment (FWHM = 4.5°; r = 0.13) was demonstrated in the model TGG system consisting of submicrometer alumina and 5 vol% 11 μm diameter template platelet particles.     

High quality double wall carbon nanotubes with a defined diameter distribution by chemical vapor deposition from alcohol

We have synthesized double wall carbon nanotubes (DWNTs) with few defects and little amorphous carbon by hot wall chemical vapor deposition (CVD) of alcohol. Catalysts for the DWNT growth were made from cobalt and molybdenum acetates. Scanning electron microscopy, transmission electron microscopy, multi frequency resonance Raman spectroscopy and optical absorption spectroscopy were used for characterization of the product with regard to DWNT yield, the nanotube diameter distribution, defect concentration and amorphous carbon content. Base pressures lower than 1 × 10⁻⁵ mbar in the CVD reactor considerably suppress defects in the DWNTs. Optimized growth conditions for DWNT formation are presented.     

Revealing the surface and bulk regimes of isothermal graphene nucleation and growth on Ni with in situ kinetic measurements and modeling

In situ optical diagnostics are used to reveal the isothermal nucleation and growth kinetics of graphene on Ni across a wide temperature range (560 °C < T < 840 °C) by chemical vapor deposition from single, sub-second pulses of acetylene. An abrupt, two-orders of magnitude change in growth times (∼100–1 s) is revealed at T = 680 °C. Above this temperature, sigmoidal kinetics are measured and attributed to autocatalytic nucleation and growth from carbon dissolved in the bulk of the Ni film. However, for T < 680 °C fast surface nucleation and growth occurring during the gas pulse appears responsible for the drastic alteration of the kinetics of subsequent dissolution-mediated growth. A simple and general kinetic model for isothermal graphene growth is developed that includes the nucleation phase and the effects of carbon solubility in metals, describes delayed nucleation, and allows the interpretation of the competition between surface- and bulk-nucleation and growth. The easily-implemented optical reflectivity diagnostics and the simple kinetic model described here allow a pathway to optimize the growth of graphene on metals with arbitrary carbon solubility.     

Transparent gas senor and photodetector based on Al doped ZnO nanowires synthesized on glass substrate

In this work high density, well-aligned Al doped ZnO (AZO) nanowires are hydrothermally synthesized on glass substrate at 99 °C. The Al content is ~1.57 at%. The PL spectrum shows that Al impurities caused an increase in the number of oxygen vacancies. The spectral response results show that the maximum responsivity and quantum efficiencies η of AZO NWs are 3.61 A/W and 84.9%, at an incident light wavelength of 360 nm. These AZO NWs have less humidity sensitivity, thus decreasing the effect of humidity effect on gas sensing. Low gas concentrations of 10 ppm ethanol and 10 ppm acetone can be detected with good responses of 24.5% and 21.2%, using the AZO NW sensor at 200 °C and with 0.1 V applied bias.