Hot Filament Chemical Vapor Deposition: Enabling the Scalable Synthesis of Bilayer Graphene and Other Carbon Materials

The hot filament chemical vapor deposition (HFCVD) technique is limited only by the size of the reactor and lends itself to be incorporated into continuous roll-to-roll industrial fabrication processes. We discuss the HFCVD reactor design and the interplay between the reactor parameters, such as filament and substrate temperatures, filament-to-substrate distance, and total pressure. Special attention is given to the large-area synthesis of bilayer graphene on copper, which is successfully grown by HFCVD with transmittance greater than 90% in the visible region and no gaps. We also discuss the HFCVD synthesis of carbon nanotubes, microcrystalline diamond, and nanocrystalline diamond

Study on the optical and electrical properties of tetracyanoethylene doped bilayer graphene stack for transparent conducting electrodes

We report the optical and electrical properties of chemically-doped bilayer graphene stack by tetracyanoethylene, a strong electron acceptor. The Tetracyanoethylene doping on the bilayer graphene via charge transfer was confirmed by Raman spectroscopy and Infrared Fourier transform spectroscopy. Doped graphene shows a significant increase in the sheet carrier concentration of up to 1.520 × 1013 cm−2 with a concomitant reduction of the sheet resistance down to 414.1 Ω/sq. The high optical transmittance (ca. 84%) in the visible region in combination with the low sheet resistance of the Tetracyanoethylene-doped bilayer graphene stack opens up the possibility of making transparent conducting electrodes for practical applications

Grain size-dependent thermal conductivity of polycrystalline twisted bilayer graphene

Abstract We report the room temperature thermal conductivity of polycrystalline twisted bilayer graphene (tBLG) as a function of grain size measured by employing a noncontact optical technique based on micro-Raman spectroscopy. Polycrystalline tBLG sheets of different grain sizes were synthesized on copper by hot filament chemical vapor deposition. The thermal conductivity values are 1305 ± 122 , 971 ± 73 , and 657 ± 42 W m − 1 K − 1 for polycrystalline tBLG with average grain sizes of 54, 21, and 8 nm, respectively. Based on these thermal conductivity values, we also estimated the grain boundary conductance, 14.43 ± 1.21 × 10 10 W m − 2 K − 1 , and the thermal conductivity for single crystal tBLG, 1510 ± 103 W m − 1 K − 1 . Our results show that the relative degradation of thermal conductivity due to grain boundaries is smaller in bilayer than in monolayer graphene. Molecular dynamics simulations indicate that interlayer interactions play an important role in the heat conductivity of polycrystalline bilayer graphene. The quantitative study of the grain size dependent thermal conductivity of polycrystalline bilayer graphene is valuable in technological applications as well as for fundamental scientific understanding

Ingeniously enhanced ferromagnetism in chemically-reduced 2D Ti3C2TX MXene

Chemical reduction is a facile and cost-effective technique for the modulation of the physical and chemical properties of nanomaterials. Herein, we demonstrate an enhancement of the magnetic behavior of Ti3C2Tx MXene after its chemical reduction via L-ascorbic acid treatment. Small ferromagnetic loops have been observed below 50 K for Ti3C2Tx prepared by hydrofluoric acid (HF) etching of Al layers from Ti3AlC2. Such a ferromagnetic ordering of spins was significantly enhanced via a chemical reduction of Ti3C2Tx with L-ascorbic acid. Ferromagnetic hysteresis loops were observed for reduced Ti3C2Tx (r-Ti3C2Tx) up to 150 K indicating a significant upshift of the paramagnetic to the ferromagnetic transition temperature, pushing towards room temperature. The enhancement of ferromagnetism and upshift of the ferromagnetic transition temperature could be attributed to the localized unpaired electron in Ti-3d orbital of the r-Ti3C2Tx crystal and increase in the number of unsaturated Ti atoms upon L-ascorbic acid treatment. Chemical reduction via L-ascorbic acid treatment shows a promising pathway towards the modulation and enhancement of magnetism in various MXene materials for the development of 2D metallic soft ferromagnets and spintronic devices

A graphene integrated highly transparent resistive switching memory device

We demonstrate the hybrid fabrication process of a graphene integrated highly transparent resistive random-access memory (TRRAM) device. The indium tin oxide (ITO)/Al2O3/graphene nonvolatile memory device possesses a high transmittance of >82% in the visible region (370-700 nm) and exhibits stable and non-symmetrical bipolar switching characteristics with considerably low set and reset voltages (<±1 V). The vertical two-terminal device shows an excellent resistive switching behavior with a high on-off ratio of ∼5 × 103. We also fabricated a ITO/Al2O3/Pt device and studied its switching characteristics for comparison and a better understanding of the ITO/Al2O3/graphene device characteristics. The conduction mechanisms in high and low resistance states were analyzed, and the observed polarity dependent resistive switching is explained based on electro-migration of oxygen ions

A Novel Approach to the Layer-Number-Controlled and Grain-Size-Controlled Growth of High Quality Graphene for Nanoelectronics

Thermal chemical vapor decomposition of methane on copper is the most widely employed technique for high quality and large area graphene growth. Graphene growth by this technique tends to be limited to a monolayer owing to the surface mediated self-limiting growth mechanism, and hence, it is difficult to obtain continuous bilayers and multilayers. We report the layer-controlled growth of high quality and large area polycrystalline graphene on copper foil in a hot filament chemical vapor deposition (HFCVD) reactor and demonstrate that graphene can be grown with a controlled grain size in the range of 5–16 μm. The field effect hole mobilities of the largest grain monolayer and bilayer graphene and the thickest few-layer graphene are 4310 ± 348, 2745 ± 276, and 2472 ± 185 cm²V^–1s^–1, respectively, which are comparable to that of high quality polycrystalline graphene and suitable for future nanoelectronics. The results of the systematic parametric study hereby presented indicate that graphene adlayers grow on top of the previously grown layer(s) and show that the HFCVD growth of graphene overcomes the difficulties to growing layer-controlled-graphene on copper. Copper vapor together with carbon radicals produced at the filaments appear to enable the adlayer graphene formation. The versatility and capability of the HFCVD for facile and inexpensive growth of graphene make it a superior technique for industrial applications

Grain size-dependent thermal conductivity of polycrystalline twisted bilayer graphene

Abstract We report the room temperature thermal conductivity of polycrystalline twisted bilayer graphene (tBLG) as a function of grain size measured by employing a noncontact optical technique based on micro-Raman spectroscopy. Polycrystalline tBLG sheets of different grain sizes were synthesized on copper by hot filament chemical vapor deposition. The thermal conductivity values are 1305 ± 122 , 971 ± 73 , and 657 ± 42 W m − 1 K − 1 for polycrystalline tBLG with average grain sizes of 54, 21, and 8 nm, respectively. Based on these thermal conductivity values, we also estimated the grain boundary conductance, 14.43 ± 1.21 × 10 10 W m − 2 K − 1 , and the thermal conductivity for single crystal tBLG, 1510 ± 103 W m − 1 K − 1 . Our results show that the relative degradation of thermal conductivity due to grain boundaries is smaller in bilayer than in monolayer graphene. Molecular dynamics simulations indicate that interlayer interactions play an important role in the heat conductivity of polycrystalline bilayer graphene. The quantitative study of the grain size dependent thermal conductivity of polycrystalline bilayer graphene is valuable in technological applications as well as for fundamental scientific understanding