Transport in nanoribbon interconnects obtained from graphene grown by chemical vapor deposition

We study graphene nanoribbon (GNR) interconnects obtained from graphene grown by chemical vapor deposition (CVD). We report low- and high-field electrical measurements over a wide temperature range, from 1.7 to 900 K. Room temperature mobilities range from 100 to 500 cm(2)·V(-1)·s(-1), comparable to GNRs from exfoliated graphene, suggesting that bulk defects or grain boundaries play little role in devices smaller than the CVD graphene crystallite size. At high-field, peak current densities are limited by Joule heating, but a small amount of thermal engineering allows us to reach ～2 × 10(9) A/cm(2), the highest reported for nanoscale CVD graphene interconnects. At temperatures below ～5 K, short GNRs act as quantum dots with dimensions comparable to their lengths, highlighting the role of metal contacts in limiting transport. Our study illustrates opportunities for CVD-grown GNRs, while revealing variability and contacts as remaining future challenges.     

Top-gated chemical vapor deposition grown graphene transistors with current saturation

Graphene transistors are of considerable interest for radio frequency (rf) applications. In general, transistors with large transconductance and drain current saturation are desirable for rf performance, which is however nontrivial to achieve in graphene transistors. Here we report high-performance top-gated graphene transistors based on chemical vapor deposition (CVD) grown graphene with large transconductance and drain current saturation. The graphene transistors were fabricated with evaporated high dielectric constant material (HfO(2)) as the top-gate dielectrics. Length scaling studies of the transistors with channel length from 5.6 μm to 100 nm show that complete current saturation can be achieved in 5.6 μm devices and the saturation characteristics degrade as the channel length shrinks down to the 100-300 nm regime. The drain current saturation was primarily attributed to drain bias induced shift of the Dirac points. With the selective deposition of HfO(2) gate dielectrics, we have further demonstrated a simple scheme to realize a 300 nm channel length graphene transistors with self-aligned source-drain electrodes to achieve the highest transconductance of 250 μS/μm reported in CVD graphene to date.     

石墨烯的化学气相沉积法制备

化学气相沉积(CVD)法是近年来发展起来的制备石墨烯的新方法,具有产物质量高、生长面积大等优点,逐渐成为制备高质量石墨烯的主要方法.通过简要分析石墨烯的几种主要制备方法(胶带剥离法、化学剥离法、SiC外延生长法和CVD方法)的原理和特点,重点从结构控制、质量提高以及大面积生长等方面评述了CVD法制备石墨烯及其转移技术的研究进展,并展望了未来CVD法制备石墨烯的可能发展方向,如大面积单晶石墨烯、石墨烯带和石墨烯宏观体的制备与无损转移等.     

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.     

Morphologies of carbon micro-coils grown by chemical vapor deposition

The carbon micro-coils were obtained by the Ni-catalyzed pyrolysis of acetylene. The carbon micro-coils with various coiling morphology: regular double coils, coils built up by circular or flat fibers, super helix coils, single coils, etc. can be observed. The carbon coils with various coil diameters and coil pitches were obtained by controlling reaction conditions, such as reaction temperature, source gas flow rate of sulfur-impurity, acetylene or hydrogen.     

Layer-by-layer growth of graphene layers on graphene substrates by chemical vapor deposition

We demonstrate a synthesis of graphene layers on graphene templates prepared by the mechanical exfoliation of graphite crystals using a developed chemical vapor deposition (CVD) apparatus that has a furnace with three temperature zones and can regulate the temperatures separately in each zone. This results in individual control over the decomposition reaction of the carbon feedstock and the growth of graphene layers by activated carbon species. CVD growth using multi-temperature zones provides wider temperature windows appropriate to grow graphene layers. We observed that graphene layers proceed by a layer-by-layer growth mode using an optical microscopy, an atomic force microscopy, and Raman spectroscopy. This result suggests that a graphene growth technique using the CVD apparatus is a potential approach for making graphene sheets with precise control of the layer numbers.     

High-quality hexagonal ZnO crystals grown by chemical vapor deposition

High-quality zinc oxide (ZnO) crystals were grown on a (0001) sapphire substrate by chemical vapor deposition at 830 °C under atmospheric pressure. The hexagonal crystals had an average diameter of about 150 μm, and a thickness of about 15 μm, as observed under a polarizing microscope. The large (0002) facet was flat, regular, and neat. In the X-ray diffraction pattern, strong (0002) and weak (0004) peaks indicate that the crystals had a wurtzite structure. The crystalline quality was characterized by Raman scattering, and the E₂(high), E₂(low), and A_l(LO) modes confirm the high quality of the ZnO crystals. Photoluminescence (PL) spectra of the crystals had a strong and sharp ultraviolet emission peak at 379 nm. The PL mechanism was also discussed.     

Shear modulus of monolayer graphene prepared by chemical vapor deposition

We report shear modulus (G) and internal friction (Q(-1)) measurements of large-area monolayer graphene films grown by chemical vapor deposition on copper foil and transferred onto high-Q silicon mechanical oscillators. The shear modulus, extracted from a resonance frequency shift at 0.4 K where the apparatus is most sensitive, averages 280 GPa. This is five times larger than those of the multilayered graphene-based films measured previously. The internal friction is unmeasurable within the sensitivity of our experiment and thus bounded above by Q(-1) ≤ 3 × 10(-5), which is orders-of-magnitude smaller than that of multilayered graphene-based films. Neither annealing nor interface modification has a measurable effect on G or Q(-1). Our results on G are consistent with recent theoretical evaluations and simulations carried out in this work, showing that the shear restoring force transitions from interlayer to intralayer interactions as the film thickness approaches one monolayer.     

Gas sensing properties of graphene synthesized by chemical vapor deposition

The gas sensing properties of graphene synthesized by a chemical vapor deposition (CVD) method are investigated. Synthesis of graphene is carried out on a copper substrate using a methane and hydrogen gas mixture by a CVD process at the atmospheric pressure. The graphene films are transferred to different substrates after wet etching of the copper substrates. The Raman spectra reveal that the graphene films made on SiO₂/Si substrates are of high quality. The reflectance spectra of graphene were measured in UV/Visible region of the spectrum. Theoretically calculated reflectance spectra based on Fresnel's approach indicates that the CVD graphene has a single layer. The gas sensing properties of graphene were tested for different reducing gasses as a function of measurement temperature and gas concentration. It is found that the gas sensing characteristics such as response time, recovery time, and sensitivity depend on the target gas, gas concentration, test temperature, and the ambient gas composition. The cross sensitivity of few combinations of reducing gasses such as, NH₃, CH_4, and H₂ was also investigated.     

Structure of multiwalled carbon nanotubes grown by chemical vapor deposition

Carbon nanotubes have been grown by chemical vapor deposition at 650°C in an argon atmosphere using a butane-propane mixture and a nickel catalyst and have been characterized by scanning and transmission electron microscopy and Raman spectroscopy. The results indicate that the multiwalled nanotubes have an imperfect graphite-like structure with a conical supramolecular configuration. A phenomenological technique is proposed for statistical analysis of the state of carbon nanotubes in measurements of the intensity of the defect zone D in their Raman spectra.     

Wafer scale homogeneous bilayer graphene films by chemical vapor deposition

The discovery of electric field induced band gap opening in bilayer graphene opens a new door for making semiconducting graphene without aggressive size scaling or using expensive substrates. However, bilayer graphene samples have been limited to μm(2) size scale thus far, and synthesis of wafer scale bilayer graphene poses a tremendous challenge. Here we report homogeneous bilayer graphene films over at least a 2 in. × 2 in. area, synthesized by chemical vapor deposition on copper foil and subsequently transferred to arbitrary substrates. The bilayer nature of graphene film is verified by Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. Importantly, spatially resolved Raman spectroscopy confirms a bilayer coverage of over 99%. The homogeneity of the film is further supported by electrical transport measurements on dual-gate bilayer graphene transistors, in which a band gap opening is observed in 98% of the devices.     

Silicon epilayers grown by chemical vapor deposition from high-purity silane

Structurally perfect, high-purity silicon epilayers up to 20 μm in thickness, with a carrier concentration n = 10¹² cm^?3 are grown through thermal decomposition of silane. Experimental evidence is presented that the concentration of “electrically active” impurities in high-purity silane can be evaluated from the electrical parameters of Si epilayers grown from it.     

Narrow gap a-SiGe:H grown by hot-wire chemical vapor deposition

We have improved the quality of our narrow bandgap a-SiGe:H grown by hot-wire chemical vapor deposition (HWCVD) by decreasing our W filament diameter and our substrate temperature. We now grow a-SiGe:H with Tauc bandgaps below 1.5 eV having a photoresponse equal to or better than our plasma enhanced CVD grown alloys. We enhanced the transport properties—as measured by the photoconductivity frequency mixing technique—relative to previous HWCVD results. These improved alloys do not necessarily show an improvement in the degree of structural heterogeneity on the nanometer scale as measured by small-angle X-ray scattering. Decreasing both the filament temperature and substrate temperature produced a film with relatively low structural heterogeneity while photoluminescence showed an order of magnitude increase in defect density for a similar change in the process.     

Immobilization of carbon nanotubes on functionalized graphene film grown by chemical vapor deposition and characterization of the hybrid material

We report the surface functionalization of graphene films grown by chemical vapor deposition and fabrication of a hybrid material combining multi-walled carbon nanotubes and graphene (CNT–G). Amine-terminated self-assembled monolayers were prepared on graphene by the UV-modification of oxidized groups introduced onto the film surface. Amine-termination led to effective interaction with functionalized CNTs to assemble a CNT–G hybrid through covalent bonding. Characterization clearly showed no defects of the graphene film after the immobilization reaction with CNT. In addition, the hybrid graphene material revealed a distinctive CNT–G structure and p–n type electrical properties. The introduction of functional groups on the graphene film surface and fabrication of CNT–G hybrids with the present technique could provide an efficient, novel route to device fabrication.     

Double-walled boron nitride nanotubes grown by floating catalyst chemical vapor deposition

One-dimensional nanostructures exhibit quantum confinement which leads to unique electronic properties, making them attractive as the active elements for nanoscale electronic devices. Boron nitride nanotubes are of particular interest since, unlike carbon nanotubes, all chiralities are semiconducting. Here, we report a synthesis based on the use of low pressures of the molecular precursor borazine in conjunction with a floating nickelocene catalyst that resulted in the formation of double-walled boron nitride nanotubes. As has been shown for carbon nanotube production, the floating catalyst chemical vapor deposition method has the potential for creating high quality boron nitride nanostructures with high production volumes.     

Few-layer graphene direct deposition on Ni and Cu foil by cold-wall chemical vapor deposition

We report an alternative synthesis process, cold-wall thermal chemical vapor deposition (CVD), is replied to directly deposit single-layer and few-layer graphene films on Ar plasma treated Ni and Cu foils using CH4 as carbon source. Through optimizing the process parameters, large scale single-layer graphene grown on Ni foil is comparable to that grown on Cu foil. The graphene films were able to be transferred to other substrates such as SiO2/Si, flexible transparent PET and verified by optical microscopy, Raman microscopy and scanning electron microscopy. The sheet resistance and transmission of the transferred graphene films on PET substrate were also discussed.     

石墨烯薄膜的前驱气体预热化学气相沉积快速制备方法

石墨烯具有优异的光学、电学和力学等性能而备受人们关注,但是目前石墨烯材料受产量、尺寸和均匀性等因素的限制,以至于在终端产品上还没有形成真正的应用。主要阐述了一种利用前驱气体预热化学气相沉积法(PT-CVD)快速制备大面积单层石墨烯薄膜方法,实现石墨烯薄膜批量制备和大面积转移在300mm×300 mm面积的聚对苯二甲酸乙二醇酯衬底上,并获得在400~800nm光波段下大于95%的光透过率和(146±15)Ω/sq的方块电阻。分别利用扫描电镜、共聚焦拉曼光谱仪、紫外-可见分光光度计和四探针设备等检测了前驱气体预热化学气相沉积法制备的石墨烯薄膜的均匀性、透过率和方块电阻等参数特性。最后,通过解决了石墨烯微纳米线路结构和真空贴合等关键技术实现了石墨烯真实多点电容式触控面板,并成功运用在5.5寸手机终端产品应用上。     

Electrical characterization of graphene synthesized by chemical vapor deposition using Ni substrate

In this work, the electrical characterization of graphene films grown by chemical vapor deposition (CVD) on a Ni thin film is carried out and a simple relation between the gate-dependent electrical transport and the thickness of the films is presented. Arrays of two-terminal devices with an average graphene film thickness of 6.9 nm were obtained using standard fabrication techniques. A simple two-band model is used to describe the graphene films, with a band overlap parameter E(0) = 17 meV determined by the dependence of conductivity on temperature. Statistical electrical measurement data are presented for 126 devices, with an extracted average background conductivity σ = 0.91 mS, average carrier mobility μ = 1300 cm(2) V(-1) s(-1) and residual resistivity ρ = 1.65 kΩ. The ratio of mobility to conductivity is calculated to be inversely proportional to the graphene film thickness and this calculation is statistically verified for the ensemble of 126 devices. This result is a new method of graphene film thickness determination and is useful for films which cannot have their thickness measured by AFM or optical interference, but which are electrically contacted and gated. This general approach provides a framework for comparing graphene devices made using different fabrication methods and graphene growth techniques, even without prior knowledge of their uniformity or thickness.     

Effects of polycrystalline cu substrate on graphene growth by chemical vapor deposition

Chemical vapor deposition of graphene on Cu often employs polycrystalline Cu substrates with diverse facets, grain boundaries (GBs), annealing twins, and rough sites. Using scanning electron microscopy (SEM), electron-backscatter diffraction (EBSD), and Raman spectroscopy on graphene and Cu, we find that Cu substrate crystallography affects graphene growth more than facet roughness. We determine that (111) containing facets produce pristine monolayer graphene with higher growth rate than (100) containing facets, especially Cu(100). The number of graphene defects and nucleation sites appears Cu facet invariant at growth temperatures above 900 °C. Engineering Cu to have (111) surfaces will cause monolayer, uniform graphene growth.