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.     

Thermal conductivity measurements of suspended graphene with and without wrinkles by micro-Raman mapping

The thermal conductivity (κ) of suspended graphene membranes made by chemical vapor deposition (CVD) was measured by micro-Raman mapping. Cracks and wrinkles present in these suspended graphene membranes were identified by micro-Raman mapping, and κ values and their statistics were obtained on membranes free of such imperfections in a single mapping. Based on this new technique, an average κ value of 1875?±?220?W?m(-1)?K(-1) at 420?K was measured on 26 suspended graphene membranes that were free of wrinkles, ～27% higher than the average value measured from 12 graphene membranes with wrinkles. These results suggest that the variation in published thermal conductivity values for suspended graphene samples could, at least in part, be due to the presence or absence of wrinkles.     

Molecular absorption and photodesorption in pristine and functionalized large-area graphene layers

We studied the photodesorption behavior of pristine and nitric acid (HNO(3)) treated graphene layers fabricated by chemical vapor deposition (CVD). The decrease in electrical conductivity and a negative shift of the Dirac point in graphene layers illuminated with ultraviolet light are caused by molecular photodesorption, while the UV illumination does not degrade the carrier mobility of graphene layers. When graphene layers were treated with concentrated HNO(3), the photodesorption-induced current decrease became less significant than for pristine graphene layers. We suggest this is due to the passivation of oxygen-bearing functionalities to CVD grown graphene structural defects by HNO(3) functionalization, which prevents the further absorption of gas molecules. Our results provide a new strategy for stabilizing the electrical performance of CVD grown large-area graphene layers for applications ranging from nanoelectronics to optoelectronics.     

3D Graphene Fibers Grown by Thermal Chemical Vapor Deposition

3D assembly of graphene sheets (GSs) is important for preserving the merits of the single‐atomic‐layered structure. Simultaneously, vertical growth of GSs has long been a challenge for thermal chemical vapor deposition (CVD). Here, vertical growth of the GSs is achieved in a thermal CVD reactor and a novel 3D graphene structure, 3D graphene fibers (3DGFs), is developed. The 3DGFs are prepared by carbonizing electrospun polyacrylonitrile fibers in NH₃ and subsequently in situ growing the radially oriented GSs using thermal CVD. The GSs on the 3DGFs are densely arranged and interconnected with the edges fully exposed on the surface, resulting in high performances in multiple aspects such as electrical conductivity (3.4 × 10⁴–1.2 × 10⁵ S m^?1), electromagnetic shielding (60 932 dB cm² g^?1), and superhydrophobicity and superoleophilicity, which are far superior to the existing 3D graphene materials. With the extraordinary properties along with the easy scalability of the simple thermal CVD, the novel 3DGFs are highly promising for many applications such as high‐strength and conducting composites, flexible conductors, electromagnetic shielding, energy storage, catalysis, and separation and purification. Furthermore, this strategy can be widely used to grow the vertical GSs on many other substrates by thermal CVD.     

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.     

Thermal conductivity of isotopically modified graphene

In addition to its exotic electronic properties graphene exhibits unusually high intrinsic thermal conductivity. The physics of phonons--the main heat carriers in graphene--has been shown to be substantially different in two-dimensional (2D) crystals, such as graphene, from in three-dimensional (3D) graphite. Here, we report our experimental study of the isotope effects on the thermal properties of graphene. Isotopically modified graphene containing various percentages of 13C were synthesized by chemical vapour deposition (CVD). The regions of different isotopic compositions were parts of the same graphene sheet to ensure uniformity in material parameters. The thermal conductivity, K, of isotopically pure 12C (0.01% 13C) graphene determined by the optothermal Raman technique, was higher than 4,000?W?mK(-1) at the measured temperature T(m)~320?K, and more than a factor of two higher than the value of K in graphene sheets composed of a 50:50 mixture of 12C and 13C. The experimental data agree well with our molecular dynamics (MD) simulations, corrected for the long-wavelength phonon contributions by means of the Klemens model. The experimental results are expected to stimulate further studies aimed at a better understanding of thermal phenomena in 2D crystals.     

Effect of anchor and functional groups in functionalized graphene devices

The electrical properties of chemically derived graphene and graphene grown by chemical vapor deposition (CVD), until now, have been inferior to those of mechanically exfoliated graphene. However, because graphene is easier to produce in large quantities through CVD or growth from solid carbon sources, it has a higher potential for use in future electronics applications. Generally, modifications to the pristine lattice structure of graphene tend to adversely affect the electrical properties by shifting the doping level and changing the conductivity and the mobility. Here we show that a small degree of graphene surface functionalization, using diazonium salts with electron-withdrawing and electron-donating functional groups, is sufficient to predominantly induce p-type doping, undiminished mobility, and higher conductivity at the neutrality point. Molecules without a diazonium anchor group desorb easily and do not have a significant effect on the electronic properties of graphene devices. We further demonstrate the variability between identically fabricated pristine devices, thereby underscoring the caution needed when characterizing graphene device behaviors lest conclusions be drawn based on singular extremes.      

Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition

Integration of individual two-dimensional graphene sheets into macroscopic structures is essential for the application of graphene. A series of graphene-based composites and macroscopic structures have been recently fabricated using chemically derived graphene sheets. However, these composites and structures suffer from poor electrical conductivity because of the low quality and/or high inter-sheet junction contact resistance of the chemically derived graphene sheets. Here we report the direct synthesis of three-dimensional foam-like graphene macrostructures, which we call graphene foams (GFs), by template-directed chemical vapour deposition. A GF consists of an interconnected flexible network of graphene as the fast transport channel of charge carriers for high electrical conductivity. Even with a GF loading as low as ～0.5 wt%, GF/poly(dimethyl siloxane) composites show a very high electrical conductivity of ～10 S cm(-1), which is ～6 orders of magnitude higher than chemically derived graphene-based composites. Using this unique network structure and the outstanding electrical and mechanical properties of GFs, as an example, we demonstrate the great potential of GF/poly(dimethyl siloxane) composites for flexible, foldable and stretchable conductors.     

Control of thickness uniformity and grain size in graphene films for transparent conductive electrodes

Large-scale and transferable graphene films grown on metal substrates by chemical vapor deposition (CVD) still hold great promise for future nanotechnology. To realize the promise, one of the key issues is to further improve the quality of graphene, e.g., uniform thickness, large grain size, and low defects. Here we grow graphene films on Cu foils by CVD at ambient pressure, and study the graphene nucleation and growth processes under different concentrations of carbon precursor. On the basis of the results, we develop a two-step ambient pressure CVD process to synthesize continuous single-layer graphene films with large grain size (up to hundreds of square micrometers). Scanning electron microscopy and Raman spectroscopy characterizations confirm the film thickness and uniformity. The transferred graphene films on cover glass slips show high electrical conductivity and high optical transmittance that make them suitable as transparent conductive electrodes. The growth mechanism of CVD graphene on Cu is also discussed, and a growth model has been proposed. Our results provide important guidance toward the synthesis of high quality uniform graphene films, and could offer a great driving force for graphene based applications.     

基于化学气相沉积石墨烯/PEDOT-PSS共混复合材料的导电薄膜

采用液相剥离技术制备基于化学气相沉积（CVD）石墨烯/聚乙撑二氧噻吩-聚苯乙烯磺酸（PEDOT-PSS）共混复合材料的导电薄膜。采用原子力显微镜和SEM表征石墨烯/PEDOT-PSS复合薄膜的微观结构;通过紫外吸收谱、X射线光电子能谱、FTIR等分析技术探索该共混复合薄膜的导电机制。结果表明：石墨烯/PEDOT-PSS导电薄膜具有优异的电学特性,其方阻约为8 Ω/□;表面分析表明,石墨烯与PEDOT主链中五元噻吩发生π-π共轭效应,由此引起两作用体之间电子云密度的变化,该变化增加了PEDOT主链的载流子离域化程度,有利于PEDOT导电性的提升,同时提高石墨烯的载流子浓度,使CVD石墨烯/PEDOT-PSS复合薄膜的导电能力增强。     

Thermal conductivity of CVD diamond films

Diamond films 60 and 170 µm in thickness were grown by PACVD (plasma-assisted chemical vapor deposition) under similar conditions. The thermal diffusivity of these freestanding films was measured between 100 and 300 K using AC calorimetry. Radiation heat loss from the surface was estimated by analyzing both the amplitude and the phase shift of a lock-in amplifier signal. Thermal conductivity was calculated using the specific heat data of natural diamond. At room temperature, the thermal conductivity of the 60 and 170 m films is 9 and 16 W-cm^–1. K^–1 respectively, which is 40–70% that of natural diamond, The temperature dependence of thermal conductivity of the CVD diamond films is similar to that of natural diamond, Phonon scattering processes are considered using the Debye model, The microsize of the grain boundary has a significant effect on the mean free path of phonons at low temperatures. The grain in CVD diamond film is grown as a columnar structure, Thus, the thicker film has the larger mean grain size and the higher thermal conductivity. Scanning electron microscopy (SEM) and Raman spectroscopy were used to study the microstructure of the CVD diamond films. In this experiment, we evaluated the quality of CVD diamond film of the whole sample by measuring the thermal conductivity.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Thermal properties of chemical vapour-deposition SiC-C nanocomposites

The relationship between the thermal properties and the microstructure of chemical vapour-deposition (CVD) SiC-C nanocomposites, covering the entire composition range from SiC to C, was investigated after measuring thermal conductivity and thermal expansion. The samples were prepared under deposition temperatures (T _dep) of 1673 and 1773 K and total gas pressure (P_tot) of 40 kPa. The thermal conductivity of CVD SiC-C nanocomposites decreased as C content increased. For the deposits containing 24.3 to 71 mol % C prepared atT _dep = 1773 K, some parts of the C phase formed a layered structure having its plane parallel to the deposition surface. This arrangement reduced the thermal conductivity in the direction perpendicular to the deposition surface to a much lower value. The CVD C and CVD C-SiC containing < 1.5 mol % SiC showed strong anisotropic thermal expansion. However, the thermal expansion of CVD SiC-C nanocomposites having a C content up to about 70 mol % was isotropic and nearly equal to that of CVD SiC. The low preferred orientation and the low modulus of elasticity of the C phase may be reasons for these results.     

Scalable fabrication of self-aligned graphene transistors and circuits on glass

Graphene transistors are of considerable interest for radio frequency (rf) applications. High-frequency graphene transistors with the intrinsic cutoff frequency up to 300 GHz have been demonstrated. However, the graphene transistors reported to date only exhibit a limited extrinsic cutoff frequency up to about 10 GHz, and functional graphene circuits demonstrated so far can merely operate in the tens of megahertz regime, far from the potential the graphene transistors could offer. Here we report a scalable approach to fabricate self-aligned graphene transistors with the extrinsic cutoff frequency exceeding 50 GHz and graphene circuits that can operate in the 1-10 GHz regime. The devices are fabricated on a glass substrate through a self-aligned process by using chemical vapor deposition (CVD) grown graphene and a dielectrophoretic assembled nanowire gate array. The self-aligned process allows the achievement of unprecedented performance in CVD graphene transistors with a highest transconductance of 0.36 mS/μm. The use of an insulating substrate minimizes the parasitic capacitance and has therefore enabled graphene transistors with a record-high extrinsic cutoff frequency (> 50 GHz) achieved to date. The excellent extrinsic cutoff frequency readily allows configuring the graphene transistors into frequency doubling or mixing circuits functioning in the 1-10 GHz regime, a significant advancement over previous reports (～20 MHz). The studies open a pathway to scalable fabrication of high-speed graphene transistors and functional circuits and represent a significant step forward to graphene based radio frequency devices.     

Anomalous size dependence of the thermal conductivity of graphene ribbons

We investigated the thermal conductivity K of graphene ribbons and graphite slabs as the function of their lateral dimensions. Our theoretical model considered the anharmonic three-phonon processes to the second-order and included the angle-dependent phonon scattering from the ribbon edges. It was found that the long mean free path of the long-wavelength acoustic phonons in graphene can lead to an unusual nonmonotonic dependence of the thermal conductivity on the length L of a ribbon. The effect is pronounced for the ribbons with the smooth edges (specularity parameter p > 0.5). Our results also suggest that, contrary to what was previously thought, the bulk-like three-dimensional phonons in graphite make a rather substantial contribution to its in-plane thermal conductivity. The Umklapp-limited thermal conductivity of graphite slabs scales, for L below ～30 μm, as log(L), while for larger L, the thermal conductivity approaches a finite value following the dependence K(0) - A × L(-1/2), where K(0) and A are parameters independent of the length. Our theoretical results clarify the scaling of the phonon thermal conductivity with the lateral sizes in graphene and graphite. The revealed anomalous dependence K(L) for the micrometer-size graphene ribbons can account for some of the discrepancy in reported experimental data for graphene.     

Graphene epitaxy by chemical vapor deposition on SiC

We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. (1) CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ～1800 cm(2)/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.     

Heterogeneous graphene nanostructures: ZnO nanostructures grown on large-area graphene layers

In this work, the synthesis and characterization of three-dimensional hetergeneous graphene nanostructures (HGN) comprising continuous large-area graphene layers and ZnO nanostructures, fabricated via chemical vapor deposition, are reported. Characterization of large-area HGN demonstrates that it consists of 1-5 layers of graphene, and exhibits high optical transmittance and enhanced electrical conductivity. Electron microscopy investigation of the three-dimensional heterostructures shows that the morphology of ZnO nanostructures is highly dependent on the growth temperature. It is observed that ordered crystalline ZnO nanostructures are preferably grown along the <0001> direction. Ultraviolet spectroscopy and photoluminescence spectroscopy indicates that the CVD-grown HGN layers has excellent optical properties. A combination of electrical and optical properties of graphene and ZnO building blocks in ZnO-based HGN provides unique characteristics for opportunities in future optoelectronic devices.     

Chemical vapor deposition-derived graphene with electrical performance of exfoliated graphene

While chemical vapor deposition (CVD) promises a scalable method to produce large-area graphene, CVD-grown graphene has heretofore exhibited inferior electronic properties in comparison with exfoliated samples. Here we test the electrical transport properties of CVD-grown graphene in which two important sources of disorder, namely grain boundaries and processing-induced contamination, are substantially reduced. We grow CVD graphene with grain sizes up to 250 μm to abate grain boundaries, and we transfer graphene utilizing a novel, dry-transfer method to minimize chemical contamination. We fabricate devices on both silicon dioxide and hexagonal boron nitride (h-BN) dielectrics to probe the effects of substrate-induced disorder. On both substrate types, the large-grain CVD graphene samples are comparable in quality to the best reported exfoliated samples, as determined by low-temperature electrical transport and magnetotransport measurements. Small-grain samples exhibit much greater variation in quality and inferior performance by multiple measures, even in samples exhibiting high field-effect mobility. These results confirm the possibility of achieving high-performance graphene devices based on a scalable synthesis process.     

基于激光选区熔化成形Ni-Cu合金模板的Ni-Cu-石墨烯复合材料的制备

采用优化的SLM成形参数,用激光选区熔化(SLM)增材制造技术制备了三维Ni-Cu合金.使用三维Ni-Cu合金基底材料用化学气相沉积法(CVD)制备Ni-Cu合金/石墨烯复合材料,研究了 CVD法生长反应温度对石墨烯结构的影响并分析其原因.结果表明,石墨烯层的厚度随着反应温度的提高而减小.与未生长石墨烯的样品相比,在1...     

Enhanced electrical conductivity in polystyrene nanocomposites at ultra-low graphene content

We compared the electrical conductivity of multiwalled-carbon-nanotube/polystyrene and graphene/polystyrene composites. The conductivity of polystyrene increases from ～6.7 × 10(-14) to ～3.49 S/m, with an increase in graphene content from ～0.11 to ～1.1 vol %. This is ～2-4 orders of magnitude higher than for multiwalled-carbon-nanotube/polystyrene composites. Furthermore, we show that the conductivity of the graphene/polystyrene system can be significantly enhanced by incorporation of polylactic acid. The volume-exclusion principle forces graphene into the polystyrene-rich regions (selective localization) and generates ～4.5-fold decrease in its percolation threshold from ～0.33 to ～0.075 vol %.     

Fast and facile graphene oxide grafting on hydrophobic polyamide fabric via electrophoretic deposition route

Fabric surface coating is deemed as the major route to fabricate functional fabrics, and interface stability is a critical factor affecting the performance of fabric. Here, electrophoretic deposition (EPD) is employed for fast and facile modification of hydrophobic polyamide fabric with graphene oxide (GO) nanosheets embedded in polymeric networks. For better grafting, polyethyleneimine is utilized to modify the surface of the fabric substrate, endowing more polar groups and resulting in reasonable interface properties of graphene oxide and fabric substrate. GO nanosheets are uniformly deposited on modified fabric via EPD method and then reduced by green hot-press processing. The modified fabric shows excellent electrical conductivity (electrical conductivity?>?3.3 S/m), thermal conductivity (0.521 W/m·K), and UV protection performance (UPF?>?500, UVA?<?0.2%). Meanwhile, the contact angle test of fabric reveals that the addition of graphene significantly improved the hydrophobicity of the fabric.