Effects of ambient conditions on the quality of graphene synthesized by chemical vapor deposition

In this work, we investigated the effects of methane concentration and gas flow rate ratio between hydrogen and methane on the quality of graphene synthesized by chemical vapor deposition. It is found that a critical concentration of methane is needed to grow continuous graphene films, while discontinuous graphene flakes are formed at low methane concentrations. Under the condition without hydrogen, a graphene film in which monolayer areas are predominant is grown, whereas a great proportion of hydrogen causes thick graphene, which reduces the transmittance of the film. Our results present an instructive reference to the large-area synthesis of graphene for the potential applications in electronics.     

Direct growth of graphene on vertically standing glass by a metal-free chemical vapor deposition method

A new method to directly grow graphene on quartz glass substrate by atmospheric-pressure chemical vapor deposition (CVD) without using any catalyst was developed. The prime feature of this method is to build a vertical-glass model in the quartz tube to significantly increase the collision probability of the carbon precursors and reactive fragments between each other with the glass surface. The growth rate of high-quality graphene on glass remarkably increases compared with the conventional gas flow CVD technique. The optical transmittance and sheet resistance of the graphene glass can be readily adjusted by regulating growth time. When growth time is 35?min, the graphene glass presents an intriguing sheet resistance of about 1.48 kΩ sq^?1 at a transmittance of 93.08% and exhibits an excellent hydrophobic performance. The method is simple and scalable, and might stimulate various potential applications of transparent and conductive graphene glass in practical fields.     

Synthesis of graphene together with undesired CuxO nanodots on copper foils by low-pressure chemical vapor deposition

The synthesis of graphene on Cu foils has been carried out using a low-pressure chemical vapor deposition (LPCVD) process. Under certain growth conditions apart from the graphene flakes, undesired Cu_xO nanodots appear. The samples were characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy and X-ray photoemission spectroscopy. On the basis of the results, we investigated the effect of growth parameters such as pressure, methane-to-hydrogen ratio and cooling atmosphere on the growth rate, the composition, especially the cleanliness of graphene by scanning electron microscopy in detail. It is shown that the obtained film is quite sensitive on the preparation conditions and the appearance of Cu_xO nanodots is preventable.     

Synthesis of large-area, few-layer graphene on iron foil by chemical vapor deposition

We demonstrate a simple and controllable way to synthesize large-area, few-layer graphene on iron substrates by an optimized chemical vapor deposition (CVD) method using a mixture of methane and hydrogen. Based on an analysis of the Fe-C phase diagram, a suitable procedure for the successful synthesis of graphene on Fe surfaces was designed. An appropriate temperature and cooling process were found to be very important in the synthesis of highly crystalline few-layer graphene. Graphene-based field-effect transistor (FET) devices were fabricated using the resulting few-layer graphene, and showed good quality with extracted mobilities of 300–1150 cm²/(V·s).      

Structural, optical and electrical study of undoped GaN layers obtained by metalorganic chemical vapor deposition on sapphire substrates

We investigate optical, structural and electrical properties of undoped GaN grown on sapphire. The layers were prepared in a horizontal reactor by low pressure metal organic chemical vapor deposition at temperatures of 900 °C and 950 °C on a low temperature grown (520 °C) GaN buffer layer on (0001) sapphire substrate. The growth pressure was kept at 10,132 Pa. The photoluminescence study of such layers revealed a band-to-band emission around 366 nm and a yellow band around 550 nm. The yellow band intensity decreases with increasing deposition temperature. X-ray diffraction, atomic force microscopy and scanning electron microscopy studies show the formation of hexagonal GaN layers with a thickness of around 1 μm. The electrical study was performed using temperature dependent Hall measurements between 35 and 373 K. Two activation energies are obtained from the temperature dependent conductivity, one smaller than 1 meV and the other one around 20 meV. For the samples grown at 900 °C the mobilities are constant around 10 and 20 cm² V⁻¹ s^− 1, while for the sample grown at 950 °C the mobility shows a thermally activated behavior with an activation energy of 2.15 meV.     

Multi-walled carbon nanotube growth on oxidized silicon substrates using cyclohexane precursor by chemical vapor deposition

Randomly oriented multi-walled nanotubes (MWNTs) are grown by a thermal chemical vapor deposition (CVD) process from cyclohexane precursor on a 20% copper-80% nickel (Cu-Ni) catalyst on oxidized silicon substrates. This combination of precursor and catalyst, to our knowledge, has been employed for the first time to demonstrate growth of multi-walled carbon nanotubes. The effects of annealing, gas ambient and catalyst layer thickness on the morphology of the grown carbon layers are discussed. The low resistivity values of the MWNTs grown on oxidized silicon substrates are attractive for their potential use in photonic devices and display applications.     

Temperature effect on synthesis of different carbon nanostructures by sulfur-assisted chemical vapor deposition

The temperature effect on synthesizing different carbon nanostructures in the range of 820−1020 °C by sulfur-assisted chemical vapor deposition is investigated. When the growth temperature is no more than 900 °C (e.g. 820, 860, and 900 °C), carbon onions can be obtained, accompanying with some fishbone-like carbon nanofibers (CNFs), graphite sheet and carbon nanotubes (CNTs). When the growth temperature is increased to 940 °C or above (e.g. 980 and 1020 °C), the products are mainly CNTs. Furthermore, by comparing the nitrogen adsorption-desorption results of samples obtained with and without sulfur addition at each temperature, it is found that the specific surface area (SSA) of products can be remarkably enlarged after introducing small amount of sulfur during growth. This is favorable to their applications in areas like electrodes of supercapacitors, adsorbents, catalyst supports, and so on.     

Effect of heat treatment on the structure and stability of multiwalled carbon nanotubes produced by catalytic chemical vapor deposition technique

The multiwalled carbon nanotubes (MWCNTs) produced by catalytic chemical vapor deposition (CCVD) route were heat treated to 2500 °C to improve the structure, morphology and purity level. The process has lead to substantial reduction in the catalytic impurity along with an improved thermal stability and degree of graphitization of these tubes that can possibly lead to its better utilization in various applications. The structural changes following heat treatment have been correlated using various characterization techniques such as Raman spectroscopy, X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, thermo gravimetric analysis and electron paramagnetic resonance spectroscopy. The electrical and mechanical properties of the polymer composites prepared with heat treated MWCNT show improved properties over the one prepared by as produced MWCNT.     

Catalyst-free growth of oriented single-walled carbon nanotubes on mica by ethanol chemical vapor deposition

In this letter, it is reported for the first time that single-walled carbon nanotubes (SWNTs) can grow on mica substrate without additional catalyst by chemical vapor deposition (CVD) using ethanol as carbon source. The single-wall structure was characterized by Raman spectra and AFM (Atomic Force Microscopy) measurements. The growth of carbon nanotubes on mica surface contributes to the small amount of iron oxide in bare mica. The uniform dispersion and nanosized Fe particles formed from the reduction of iron oxide favor for the growth of SWNTs. Horizontally aligned superlong SWNTs arrays can be successfully generated on the mica surface, which is proved to be guided by the gas flow and under “kite growth mechanism”. The mica is a machinable material which can be easily cut and made a narrow slit on, thus the nanotubes can traverse the slit which can be in millimeter scale and long suspended SWNTs can be generated. This will provide an opportunity to manipulate individual SWNT for various purposes.     

Graphite formation on Ni film by chemical vapor deposition

Thin film formation of graphite by chemical vapor deposition using 2-methyl-1,2′-naphthyl ketone as a starting material was carried out on Ni film substrates. On Ni films directly deposited on quartz glass, the graphite films were obtained when the Ni film thickness was above 1 000 Å and above 5 000 Å at 700 °C and 1 000 °C, respectively. Depositions on thinner Ni film substrates comprise amorphous carbon (a-C) or graphite tubes which was owing to the thermal coagulation of the Ni film into droplets. On the other hand, graphite film was obtained on the Ni film with thickness 10 Å when a-C was inserted between the Ni film and the quartz glass. The coagulation of the Ni film is considered to be avoided by inserting a-C layer.     

化学气相沉积法快速生长定向纳米碳管

利用化学气相沉积法，采用二甲苯为碳源，二茂铁为催化剂，氮气作保护气，在石英基底上催化裂解生长定向纳米碳管，试验结果表明：在775℃，120min的条件下，可生长出长达200μm厚的定向纳米碳管薄膜；在775℃，反应时间为60min～120min时，纳米碳管的长度为100μm～200μm，而纳米碳管的直径变化不明显。而无氢气，较高的反应温度和连续的催化剂供给对快速生长定向纳米碳管有重要的影响。     

Effects of argon and oxygen flow rate on water vapor barrier properties of silicon oxide coatings deposited on polyethylene terephthalate by plasma enhanced chemical vapor deposition

Plasma polymer coatings were deposited from hexamethyldisiloxane on polyethylene terephthalate (PET) substrates while varying the operating conditions, such as the Ar and O₂ flow rates, at a fixed radio frequency power of 300 W. The water vapor transmission rate (WVTR) of the untreated PET was 54.56 g/m²/day and was decreased after depositing the silicon oxide (SiO_x) coatings. The minimum WVTR, 0.47 g/m²/day, was observed at Ar and O₂ flow rates of 4 and 20 sccm, respectively, with a coating thickness of 415.44 nm. The intensity of the peaks for the Si-O-Si bending at 800-820 cm^− 1 and Si-O-Si stretching at 1000-1150 cm^− 1 varied depending on the Ar and O₂ flow rates. The contact angle of the SiO_x coated PET increased as the Ar flow rate was increased from 2 to 8 sccm at a fixed O₂ flow rate of 20 sccm. It decreased gradually as the oxygen flow rate increased from 12 to 28 sccm at a fixed Ar carrier gas flow rate. The examination by atomic force microscopy revealed a correlation of the SiO_x morphology and the water vapor barrier performance with the Ar and O₂ flow rates. The roughness of the deposited coatings increased when either the O₂ or Ar flow rate was increased.     

Silicon-carbon-oxynitrides grown by plasma-enhanced chemical vapor deposition technique

In this paper we report some preliminary results about the growth at low temperature (493 K) of hydrogenated silicon-carbon-oxygen-nitrogen amorphous thin-film alloys (a-SiC_xO_yN_z:H) by means of capacitively-coupled radio-frequency (13.56 MHz) plasma-enhanced chemical vapor deposition using a mixtures of silane (SiH₄), propane (C₃H₈), nitrous oxide (N₂O) and ammonia (NH₃) precursor gases. Thin films of a-SiC_xO_yN_z:H were grown at different deposition conditions, obtaining growth speeds varying from 0.22 to 0.44 nm/s. The films were characterized by means of Fourier transform infra-red spectroscopy in order to investigate the internal bonding structure, by UV-VIS transmittance spectroscopy to check the optical properties and by mechanical profilometry to measure the film thickness and estimate the growth rate. The comparison of structural and optical properties of samples grown with and without NH₃ presence in the gas mixture showed that the ammonia addition allows a better control of nitrogen incorporation in the film structure, while increasing film transparency and reducing the growth rate.     

Epitaxial growth of lithium niobate film using metalorganic chemical vapor deposition

Lithium niobate films grown epitaxially on sapphire substrate were prepared using a thermal chemical vapor deposition method from the metalorganic compounds Li(C₁₁H₁₉O₂) and Nb(OC₂H₅)₅. The range of operating conditions for obtaining pure epitaxially grown LiNbO₃ without other oxides is within that for obtaining pure polycrystalline LiNbO₃ grown on silicon substrate. On analyzing the composition of the epitaxially grown LiNbO₃ film, the composition of the film was similar to that of the LiNbO₃ solid solution in the phase diagram of the Li-Nb composite oxide obtained for crystal growth from a molten solution.     

Hot-wire chemical vapor deposition and characterization of polycrystalline silicon thin films using a two-step growth method

A two-step growth method was proposed to reduce the amorphous incubation layer in the initial growth of polycrystalline silicon (poly-Si) films prepared by hot-wire chemical vapor deposition (HWCVD). In the two-step growth process, a thin seed layer was first grown on the glass substrate under high hydrogen dilution ratios (φ ≥ 0.9), and then a thick overlayer was subsequently deposited upon the seed layer at a lower φ value. The effect of various deposition parameters on the structural properties of poly-Si films was investigated by Raman spectroscopy and transmission electron microscopy. Moreover, the electrical properties, such as dark and photo conductivities, of poly-Si films were also measured. It was found that the Si incubation layer could be suppressed greatly in the initial growth of poly-Si with the two-step growth method. In the subsequent poly-Si film thickening, a lower φ value of the reactant gases can be applied to enhance the deposition rate. Therefore, a high-quality poly-Si film can be fabricated via a two-step growth method with a sufficient growth rate using HWCVD.     

Plasma deposition of fluorocarbon thin films using pulsed /continuous and downstream radio frequency plasmas

Fluorocarbon (FC) films deposited in continuous wave (cw) and pulsed difluoromethane radio frequency (r.f.) plasmas were characterized using Fourier transform infrared spectroscopy and atomic force microscopy. The effects of varying r.f. power, cw/pulsed discharge mode, and the distance of the substrate from the coil on the deposition rate, film structure, and surface roughness were investigated. These cw and pulsed deposition systems were characterized in-situ by means of optical emission spectroscopy. Emission intensities of H_α, H_β, H₂ and carbon-containing species in the coil region and downstream plasmas as a function of plasma parameters were measured. The hydrogen excitation temperature obtained from the relative emission intensities of H_α and H_β lines shows a clear dependence on the r.f. power and the substrate position. Correlations between film properties, gas-phase plasma diagnostic data, and film growth processes were discussed. Experimental results indicate that the film growth within the coil region in cw plasmas is controlled by the synergistic effect between energetic ions and low-energy species. The film growth in pulsed and downstream plasmas is controlled by the growth of coalesced nuclei via surface diffusion of adsorbed species, which results in the deposition of FC films with relatively rough surfaces.     

Crosslinking of copolymer thin films by initiated chemical vapor deposition for hydrogel applications

The ability of initiated chemical vapor deposition to finely tune crosslinking densities in copolymer thin films has been used to develop a functional, reactive hydrogel system. The system consists of poly[maleic anhydride-co-dimethyl acrylamide-co-di(ethylene glycol) divinyl ether] films covalently attached to silicon substrates using the coupling agent 3-aminopropylethoxydimethylsilane. The swelling of the films in water is pH-dependent, with a maximum swelling ratio of 11 at pH = 8. The hydrogel was also functionalized with 0.1 M cysteamine solutions in 2-propanol for 30 min to convert 97% of the anhydride functional groups to carboxylic acid and amide functionalities, confirmed by XPS and Fourier transform infrared spectroscopy. The functionalization yielded free thiol groups at the surface, which were used to attach CdSe/ZnS core-shell semiconductor nanoparticles to the hydrogels.     

General rules governing carbon nanomaterial growth directly on metal support by chemical vapor deposition

Growing carbon nanotubes and relative nanomaterials directly on metal support has attracted increasing attention due to its applications in nanoelectronics and composites. In order to reveal the effect of metal support on the catalytic property of the catalyst, several metal powders were selected as catalyst carrier for carbon nanomaterial growth by chemical vapor deposition (CVD). The results show that the interaction between metal support and catalyst plays a key role for the activity of the catalyst, while, the metal support itself has a relative little effect. By critically controlling their interactions, all metal powders are extrapolated to be used as catalyst supports for the carbon nanomaterial growth by CVD.     

化学气相沉积法制备硼掺杂玻璃炭材料(英文)

以甲烷和三氯化硼的混合气为反应气体,采用化学气相沉积法制备硼掺杂玻璃炭材料。利用X射线衍射仪、拉曼光谱仪、扫描和透射电子显微电镜对沉积产物的微观结构进行表征。结果表明,沉积产物是一种玻璃炭材料,但在其基体中均匀分布着约20 nm的碳化硼颗粒。由于硼元素强烈的催化石墨化作用,硼掺杂玻璃炭表现出完全不同于传统玻璃炭材料的石墨化行为。硼掺杂玻璃炭经高温热处理后,其结构发生剧烈变化而转变为片层炭结构,其转变过程可能遵循"固溶-析出"机制。     

Electrical characterization of Ge microcrystallites by atomic force microscopy using a conducting probe

We have studied the nucleation and growth of Ge microcrystallities on Si(100) or evaporated Cr substrates from an rf glow discharge decomposition of GeH₄ highly-diluted with H₂, where the crystallinity, the surface microroughness and the local electric transport of the films have been measured as a function of the film thickness. For the film growth thicker than ∼65 nm, Raman scattering spectra show that the evolution of the microcrystalline phase tends to be saturated. In the thickness range of 7-65 nm, the nucleation and/or microcrystalline grain formation with progressive film growth and corresponding significant difference in the electrical conductivity in the direction of the film thickness between the grains and their boundaries have been demonstrated from topographic and current images taken simultaneously by an atomic force microscope with a conducting probe.