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The most attractive application of graphene in the RF area is low-noise amplifier (LNA) due to its amazing properties. But, design and fabrication of graphene LNA Monolithic Microwave Integrated Circuits (MMICs) are still blank. In this work, 200 nm gate-length chemical vapor deposition (CVD) monolayer graphene transistors were fabricated on a sapphire substrate. The graphene transistor shows an intrinsic gain of ${g} _{\text{m}}/{g} _{\text{ds}}= {1.1}$ , and high extrinsic maximum oscillation frequency with ${f} _{\text{max}}$ (48 GHz) > cutoff frequency ${f} _{\text{T}}$ (43 GHz). The graphene transistors show low extrinsic minimum noise figure of 2–7 dB in the measured frequency range of 6–50 GHz. A C-band graphene LNA MMIC was designed and fabricated and shows a maximum gain of 8.34 dB at 5.5 GHz, and minimum noise figure of 4.96 dB at 5.8 GHz. This work demonstrates the application potential of graphene for future RF high-speed electronics. [ABSTRACT FROM AUTHOR]

The rarely reported defects with crack-and-fold (CAF) style graphene grown on raw Cu foils substrates by chemical vapor deposition (CVD) were examined in this study. Factors for the formation of CAF defects were explored by performing different substrate treatments and varied hydrogen inflow. A clear relationship was found between CAF defects and Cu (110) facets by electron back-scattered diffraction measurement. CAF defects were affected by the interaction between graphene and the substrate. A model with a two-step process, including cracking and folding, is established to elucidate the formation of CAF defects. CAF defects can be reduced by performing substrate treatment, varying hydrogen flow and controlling the orientation of the Cu substrate. This study was performed to improve the quality of graphene grown by CVD for industrial production. [ABSTRACT FROM AUTHOR]

Inherent sulfur vacancies in molybdenum disulfide (MoS2) films grown via chemical vapor deposition have restricted the practical application of MoS2 in optoelectronic devices. The repair of sulfur vacancies is important for improving the quality of MoS2 and enhancing the performance of devices based on MoS2 films. Here we report a route that uses hydrogen (H2) etching to detect the repair effect of sulfur vacancies. The domains with and without sulfur annealing were etched via H2 under the same conditions, and the changes of surface microstructures and properties were compared. It was found that the tolerance of MoS2 to H2 etching was enhanced after sulfur annealing. Results of the photoluminescence spectrum further proved that H2 etching could effectively detect the repair effect of sulfur vacancies. As a practical application, the optimum condition of sulfur annealing was obtained through H2 etching. Our work promotes the practical application of MoS2 in the field of new-generation electronics and optoelectronics. [ABSTRACT FROM AUTHOR]

Ascorbic acid was used to modify to chemical vapor deposition (CVD)-grown graphene films transferred onto SiO substrate. Residual polymer (polymethyl methacrylate), Fe, Cl, HO, and O affected the electrical and thermal properties on graphene during the transfer or device fabrication processes. Exposure of transferred graphene to ascorbic acid resulted in significantly enhanced electrical properties with increased charge carrier mobility. All devices exhibited more than 30% improvement in room temperature carrier mobility in air. The carrier mobility of the treated graphene did not significantly decrease in 21 days. This result can be attributed to electron donation to graphene through the −OH functional group in ascorbic acid that is absorbed in graphene. This work provides a method to enhance the electrical properties of CVD-grown graphene. [ABSTRACT FROM AUTHOR]

Cu(111) is a suitable substrate for sixfold graphene domain synthesis, as confirmed theoretically and experimentally. However, an undulate striped structure, where stretched flower-like or approximate diamond-shaped graphene domains had formed, appeared on Cu(111) after annealing and growth in our study. Graphene domains were stretched along the undulate stripes. The Cu surface coated with graphene domains was flatter than the surrounding undulate striped structure. Oxygen plasma was used to remove the graphene coating, and the exposed Cu was also flat. We propose that slight steps formed on Cu(111) in the annealing process. The faster rate of graphene growth along these steps contributed to the stretching domain shape. Furthermore, the release of internal stress or the shrinking of Cu during cooling promotes the expansion step to form an undulate striped structure. However, the coated Cu step motion is limited by graphene. Consequently, the resulting surface is flat, thereby clearly indicating a graphene-Cu interaction. [ABSTRACT FROM AUTHOR]

Analysis of graphene domain synthesis explains the main graphene growth process. Size-limited graphene growth caused by hydrogen is studied to achieve efficient graphene synthesis. Graphene synthesis on Cu foils via the chemical vapor deposition method using methane as carbon source is limited by high hydrogen concentration. Results indicate that hydrogen affects graphene nucleation, the growth rate, and the final domain size. Considering the role of hydrogen as both activator and etching reagent, we build a model to explain the cause of this low graphene growth rate for high hydrogen partial pressure. A two-step method is proposed to control the graphene nucleation and growth rate separately. Half the time is required to obtain similar domain size compared with single-step synthesis, indicating improved graphene synthesis efficiency. The change of the partial pressure and transmission time between the two steps is a factor that cannot be ignored to control the graphene growth. [ABSTRACT FROM AUTHOR]

Abstract: We report the synthesis of large-area graphene films on Mo foils by chemical vapor deposition. X-ray diffraction indicates that the dissolution and segregation process governs the growth of graphene on Mo foils. Among all processing parameters investigated, the cooling rate is the key one to precisely control the thickness of graphene film. By optimizing the cooling rate between 1.5 and 10°C/s, we managed to achieve graphene films ranging from mono- to tri-layer. Their uniformity and thickness were confirmed by Raman spectroscopy and optical measurements. The carrier mobility of films reaches as high as 193cm2 V−1 s−1. Our experiments show that the Mo substrate has the similar simplicity and large tolerance to processing conditions as Cu. [Copyright &y& Elsevier]

Large-area and high-quality graphene grown on metal by chemical vapor deposition (CVD) has great potential applications in electronics and photo electronics. After more than 10 years of development, great progress has been made in the growth of graphene, and the role of various factors in the growth of graphene has been gradually clear. In this review, the role of hydrogen and oxygen in the formation and elimination of carbon-based impurities and silicon oxide particles and in the formation, observation, and control of grain boundary, point defects, and wrinkles is introduced. The content focuses on the specific experimental methods, results, and mechanism. Finally, the challenges of hydrogen and oxygen in the study of graphene growth on metal are introduced. This review can increase the theoretical knowledge and experimental design ability related to hydrogen and oxygen in CVD graphene grown on metal. • The concept of carbon-based impurity, silicon oxide particle, grain boundary, point defect and wrinkle, and their effects on the CVD graphene growth on metal are introduced in detail. • The role and the mechanism of hydrogen and oxygen in the study of carbon-based impurities, silicon oxide particles, grain boundaries, point defects and wrinkles are introduced. • We systematically share our experience and suggestions on the research of impurities and defects in CVD graphene growth on metal. [ABSTRACT FROM AUTHOR]

Wafer-scale single-crystal graphene with strong antioxidation is fundamentally important for their applications in electronics and optoelectronics. Although significant progress has been achieved in the chemical vapor deposition (CVD) growth of graphene, the production of wafer-scale graphene with high crystalline and excellent oxidation resistance still remains a challenge. Here, we report the epitaxial growth of 6-inch single-crystal graphene on twinned Pt (111) films with in-plane rotation of 60°(T-Pt) by ambient-pressure CVD. Our results show that the CVD-grown graphene on T-Pt exhibits fast growth rate and ultrahigh stability under the high-temperature air condition (>500 °C). The density functional theory (DFT) calculations reveal that the twinned Pt(111) surface does not change the preferential orientation of graphene nucleation, leading to highly aligned graphene domains on the T-Pt substrate. Moreover, the edge growth of graphene cannot be limited by the Pt twin boundaries (TBs), which is responsible for the fast growth of graphene single crystals. This work provides a reliable route to produce wafer-size single-crystal graphene monolayers with excellent oxidation resistance and clarifies the oriented growth mechanism of graphene domains on twinned Pt substrate. Here, we report an oriented multi-nuclei growth strategy toward 6-inch single-crystal graphene on twinned Pt (111) films with in-plane rotation of 60° (T-Pt) by ambient-pressure chemical vapor deposition. The graphene on Pt grows fast and has better oxidation resistance than graphene on Cu substrate. The density functional theory (DFT) calculations reveal that Pt-twinned structure does not change the preferential orientation of graphene nuclei, resulting in highly oriented graphene domains on the T-Pt substrate. This work provides a reliable approach to produce wafer-size single-crystal graphene monolayers with excellent oxidation resistance and clarifies the oriented growth mechanism of graphene domains on T-Pt substrate. [Display omitted] [ABSTRACT FROM AUTHOR]

AB-stacked bilayer graphene (BLG) has wide application prospects in the field of optoelectronic devices. Chemical vapor deposition (CVD) is an important method used to prepare BLG, but no definite stacking orientation is observed between the bilayers. Besides AB stacking, a number of disordered stacking orientations that could seriously affect the performance of BLG also exist. In this paper, we report a method to control the stacking orientation of reverse-transferred CVD-BLG via high pressure environment. We found that the orientation of BLG could be changed from weak-coupling stacking to AB stacking by increasing the temperature of the solvent in the autoclave. MoS 2 grains grown on graphene confirmed the stacking orientation variation of BLG. Reductions in the distance between layers in BLG under a high-pressure environment were considered to explain the observed orientation change. Reverse transfer allows the easy modification of the stacking orientation of BLG. The proposed high-pressure assistant method introduces a new route to prepare AB-stacked BLG and could be used to adjust the stacking direction of other two-dimensional materials. Increasing the pressure can change the distance between the two graphene layers, the distance reduced from d1 to d2 and the original stacking balance is modified. The stacking orientations of BLG can be determined by analyzing the orientation of MoS 2 crystals grown in SLG and BLG regions. After high pressure treatment, the orientation of MoS 2 crystals on the monolayer and bilayer is consistent. Image 1 [ABSTRACT FROM AUTHOR]

• The bilayer V-WS 2 was synthesized by liquid assisted precursor CVD. • The bilayer V-WS 2 was AB stacking. • The spectral changes of bilayer V-WS 2 with different doping concentrations were studied. • The effective doping of V atoms was confirmed by spectra and STEM. Doping is widely used in semiconductor devices as an effective means to improve TMD properties. At present, the doping of monolayer TMD has made a lot of progress. However, monolayer TMD has low carrier mobility which hinders the improvement of device performance. Compared with monolayer TMD, bilayer TMD has higher carrier mobility and state density. However, there are few researches on synthesis of doped bilayer TMD. Here, V-doped bilayer WS 2 films were grown by liquid precursor assisted chemical vapour deposition (LACVD). STEM characterization confirmed the effective doping of V atom in bilayer WS 2. The doping of bilayer TMD structure is helpful to improve the practical application of bilayer TMD in the field of optoelectronic devices. [ABSTRACT FROM AUTHOR]

We explored an easy technique to transfer directly large-area graphene grown by chemical vapor deposition (CVD) onto flexible substrates through substrate heating at appropriate temperatures. Polyethylene (PE) films were selected as the flexible substrate. The critical factor to ensure successful transfer was the good contact between the substrate and graphene. By heating the PE films merely approaching their glass-transition temperature, film morphology did not change. The stickiness of PE films further enabled the van der Waal forces to ensure close interaction between the viscoelastic substrate and graphene. This method did not require an intermediate transfer membrane such as polymethylmethacrylate (PMMA) that needs removal afterwards. Though this transfer method, monolayer and multilayer graphene films were transferred onto PE substrates with sheet resistance as low as 158 Ω/sq. All these results can be applied in existing and new graphene CVD applications on flexible substrates. [ABSTRACT FROM AUTHOR]

Single-crystal few-layer graphene films have potential applications in microelectronic and optoelectronic devices. Considerable efforts have been taken to prepare large-area bilayer graphene through chemical vapor deposition, but the growth of uniform graphene films with increased layers remains challenging. Herein, a hydrogen-modulation method for growing wafer-scale few-layer single-crystal graphene through chemical vapor deposition was proposed. By controlling the hydrogen flow rate at different stages, the dissolution amount and segregation process of carbon atoms in a CuNi substrate can be regulated separately. The hydrogen-modulation method fully exploits the effect of the dissolution–segregation mechanism, and the number of graphene layers can be controlled by modulating the hydrogen flow rate. Through the proposed method, the coverages of bilayer and trilayer graphene can reach more than 99% and 95%, respectively. The stacking and uniformity of few-layer graphene were evaluated by Raman and microstructure characterization, and bilayer and trilayer graphene were confirmed as AB and ABA stacks, respectively. Transmission line test structure and field effect transistor were prepared to verify the electrical properties of bilayer graphene. Results show that the square resistance of bilayer graphene is significantly lower than that of single-layer graphene, and it can also achieve high on-state current on field effect transistor. [Display omitted] [ABSTRACT FROM AUTHOR]

Microstructural defects (MSDs) in graphene are a primary negative factor for the crystal quality and electrical properties of graphene and there is little knowledge about their formation. Here we report the formation mechanism of MSDs in graphene domains during the CVD growth on Cu substrate. Hydrogen-etching results show that an etched graphene hole corresponds to the position of a MSD that appears not only at the nucleation sites of graphene. Based on density-functional calculations, a hidden growth mechanism of MSDs that is driven by defects/impurities (e.g., Cu clusters/particles) on substrate surfaces is proposed. Our model indicates that the number of MSDs in a graphene domain depends on the density of surface defects on the Cu substrate and we demonstrate that MSDs in graphene can be significantly reduced by the careful substrate polishing. [ABSTRACT FROM AUTHOR]

• The etching effect on graphene of hydrogen and oxygen was compared. • The etching effect of mixture of hydrogen and oxygen is weaker than that of oxygen. • The mechanisms of the etching effect of hydrogen and oxygen are different. Hydrogen and oxygen are the most commonly used gasses in the growth of chemical vapor deposition (CVD) graphene, which are very important for the growth of CVD graphene. In this work, a series of graphene etching and multistep growth experiments are designed to clarify the etching effect of hydrogen and oxygen to graphene on Cu. The etching effect to graphene in Ar (with oxygen impurities), mixture of H 2 and Ar, and growth atmosphere with different hydrogen concentration are studied in detail. The role of hydrogen and oxygen in graphene growth is further discussed finally. [ABSTRACT FROM AUTHOR]

The N-doped graphene domain is synthesized through co-growth of ammonia and methane by chemical vapor deposition (CVD). Results showed that the nitrogen concentration, defect density, and doping level increased with the decrease in growth temperature. Notably, the position of N1s main peak from X-ray photoelectron spectroscopy (XPS) showed a significant linear blueshift as the temperature decreased, indicating that the main doping nitrogen configuration gradually evolved from pyridinic N to pyrrolic N consistent with the theoretical explanation. In addition, the nitrogen configuration was not influenced by the NH 3 flow rate, which was mainly pyridinic N at the high temperature. With increasing NH 3 flow rate the redshift of the Raman peak was caused by the elongation of the C C bonds. This work presented a process to control the predominant bonding configuration of doping nitrogen by mainly choosing the growth temperature through CVD. [ABSTRACT FROM AUTHOR]

• Bilayer WS 2 was achieved by simple spin-coating and finely controlling the time of introducing sulfur. • The as-grown bilayer WS 2 exhibits good crystallinity with AA stacking. • Photodetector based on the bilayer WS 2 shows good performance. WS 2 is a promising material in the next generation integrated circuit applications and its electrical and optoelectronic characteristics is dependent on the number of layers. Here, a simple and scalable synthesis of WS 2 bilayers via chemical vapor deposition (CVD) is demonstrated by finely controlling the time of introducing sulfur. The optical and microstructural characterization of the as-grown bilayer WS 2 show a uniform single crystal quality. The photodetector based on bilayers WS 2 exhibits relatively good performance. [ABSTRACT FROM AUTHOR]

Abstract: Hexagonal single-crystal graphene domains were grown on copper (Cu) foil via chemical vapor deposition and were etched with hydrogen at 950°C from 7 to 60min at atmospheric pressure. Numerous trenches were observed on the initial graphene domains after etching, and the trench patterns were closely associated with the Cu crystal orientation. No trenches were found if the etching process was conducted before cooling down. Thus, the etching trenches were bound up with the wrinkles formed during the cooling down process. Then, the process of etching on the wrinkles was examined. This simple hydrogen etching technology proved that wrinkles and point defects existed even in hexagonal single-crystal graphene domains. This method could be a convenient way to detect the distribution and morphology of wrinkles in graphene. [Copyright &y& Elsevier]

Abstract: In this paper, the synthesis of high quality graphene on single-crystal Mo(110) films by chemical vapor deposition (CVD) of methane was reported for the first time. X-ray diffraction proves that carbon species had dissolved into the metal. The micro-Raman spectroscopy indicates that the thickness of Mo films, cooling rate and growing time play significant roles in the quality of as-grown graphene films. By optimizing the growth time (15min) and cooling rate (10°C/s), we achieved high quality graphene films with the small ratio I G/I 2D≈0.26 and the FWHM(2D)≈30.4cm−1 on 200nm-thick Mo films. Our experiments also suggest that graphene growth on Mo is a dissolution and segregation process as Ni. [Copyright &y& Elsevier]

• A simple synthesis method of scalable adlayer-free monolayer graphene on CuNi foil. • Graphene grown on Cu 80 Ni 20 has the advantage of much wider growth window than Cu. • Proposing a growth model of adlayer-free monolayer on Cu 80 Ni 20 substrate. Homogeneity is crucial to material applications. Although monolayer graphene was successfully prepared more than a decade ago, to date, the growth of adlayer-free monolayer with a large area remains challenging. Here, we report the synthesis of metre-size adlayer-free monolayer graphene film on Cu 80 Ni 20 foil via atmospheric pressure chemical vapour deposition. Experimental results show that the growth parameter window of adlayer-free monolayer graphene on Cu 80 Ni 20 is very wide, which makes the growth of adlayer-free monolayer graphene easy and controllable. A growth model of adlayer-free monolayer graphene on Cu 80 Ni 20 substrate is also proposed. [ABSTRACT FROM AUTHOR]

• The degradation of MoS 2 domains in atmospheric environment were studied. • MoS 2 domains were oxidized from edge to interior. • Moistures permeation and oxygen etching lead to MoS 2 degradation. • Isolating oxygen and moistures is beneficial to improve the stability of MoS 2. The stability of two-dimensional transition metal chalcogenides (TMDCs) is important for their reliable device applications in nanoelectronics and optoelectronics. Here, we report the degradation of MoS 2 grown by chemical vapor deposition (CVD) in atmospheric environment. Through high-pressure treatment and water immersion of molybdenum disulfide (MoS 2) samples, we found that moistures will permeate into the interface between MoS 2 and sapphire to accelerate oxidation. Therefore, isolating oxygen and moistures is beneficial to improve the long-term stability of TMDCs. Our study deepens the understanding of MoS 2 degradation process and provides an important basis for the stability improvement of TMDCs. [ABSTRACT FROM AUTHOR]

The positional relationship between the etched triangular pits and the MoS 2 domain depended entirely on the boundary structure of the original domain. Thus, a facile and rigorous route to distinguish the boundary structure via oxygen etching was proposed. • The controllable growth of MoS 2 with different boundary structures was achieved by adjusting Mo:S ratios. • A facile and rigorous route to distinguish the boundary structure via oxygen etching was proposed. • Combined etching and growth to study the boundary evolution of MoS 2 domains. • The schematic diagram of MoS 2 morphology evolution relative to different Mo:S ratios and time after oxygen etching was given. The boundary structure of 2D materials exhibits an important influence on their physical and chemical properties. We report the controllable growth of molybdenum disulfide with different boundary structures via chemical vapor deposition (CVD). A facile and rigorous route to distinguish the boundary structure via oxygen etching was proposed. The boundary control was realized by changing the substrate height to adjust the Mo:S ratios. Based on the stability differences of the boundary structures in oxygen, the microscopic boundary structures were characterized by oxygen etching. Our work combined growth and etching to develop a general route to identify the boundary structure of MoS 2. Results of this study provide a mean to determine the controllable growth of MoS 2 domains with different boundaries and to design the domains with special boundary structure efficiently. [ABSTRACT FROM AUTHOR]