Hydrogen-excluded graphene synthesis via atmospheric pressure chemical vapor deposition

The morphology of graphene synthesized via atmospheric pressure chemical vapor deposition (APCVD) process was investigated with respect to the hydrogen introduction in each process step. A pristine monolayer graphene was obtained in the condition where hydrogen was excluded in all the steps. The study of growth mechanism of this hydrogen-excluded APCVD process suggests that hydrogen plays a critical role in determining the rate-limiting step, which further determines whether or not a monolayer graphene can be achieved, irrespective to the roughness of the surface. Particularly, the dominant kinetic regime changed, depending on the introduction of hydrogen in the growth step. Finally, electric properties of the graphene via the hydrogen-excluded APCVD process were characterized and compared with the one via the low pressure CVD process, along with the characterization of etch pits in a graphene-passivated etch test. The resulted better performance of the former graphene in both cases suggests that this method can be considered as an alternative but easier route for the synthesis of monolayer graphene.     

Large scale atmospheric pressure chemical vapor deposition of graphene

We demonstrate that large scale high quality graphene synthesis can be performed using atmospheric pressure chemical vapor deposition (CVD) on Cu and illustrate how this procedure eliminates major difficulties associated with the low pressure CVD approach while allowing straightforward expansion of this technology to the roll-to-roll industrial scale graphene production. The detailed recipes evaluating the effects of copper foil thicknesses, purity, morphology and crystallographic orientation on the graphene growth rates and the number of graphene layers were investigated and optimized. Various foil cleaning protocols and growth conditions were evaluated and optimized to be suitable for production of large scale single layer graphene that was subsequently transferred on transparent flexible polyethylene terephthalate (PET) polymer substrates. Such “ready to use” graphene–PET sandwich structures were as large as 40″ in diagonal and >98% single layer, sufficient for many commercial and research applications. Synthesized large graphene film consists of domains exceeding 100 μm. Some curious behavior of high temperature graphene etching by oxygen is described that allows convenient visualization of interdomain boundaries and internal stresses.     

Scalable synthesis of pyrrolic N-doped graphene by atmospheric pressure chemical vapor deposition and its terahertz response

Atmospheric pressure chemical vapor deposition is employed to synthesize the N-doped graphene, which is mainly composed of pyrrolic type N bonding configuration with a controllable doping concentration from ∼1.6% to ∼6.4%. Transmission electron microscope, X-ray photoelectron spectroscopy, and Raman spectrometer are used for characterizing the pyrrolic N-doped graphene. X-ray photoelectron spectra confirm the dominant pyrrolic N bonding configuration, which is consistent with the Raman spectra compared with the pristine graphene. THz time-domain spectroscopy, four-probe DC electrical measurements, and visible spectroscopy are also utilized to analyze doping concentration qualitatively.The investigation suggests that THz wave is sensitive to the dopant concentration, which is relevant to the conductivity, while the visible light is insensitive to the dopant concentration. Our results further extend the synthesis method of N-doped graphene and the new type doped graphene might have potential applications in electrochemistry, electronics, photonics, and THz devices.     

Growth of large-sized graphene thin-films by liquid precursor-based chemical vapor deposition under atmospheric pressure

Large-sized thin-films composed of single- and few-layered graphene have been synthesized by chemical vapor deposition (CVD) on copper foils under atmospheric pressure using ethanol or pentane as the precursor. Confocal Raman measurements, transmission electron microscopy and scanning tunneling microscopy show that the majority part of the obtained films exhibit hexagonal graphene lattice. Optical microscopy and electrical measurements confirm the continuity of these films. It is also found that the CVD-grown graphene films with ethanol as the precursor exhibit lower defect density, higher electrical conductivity, and higher hall mobility than those grown with pentane as the precursor. This liquid-precursor-based atmospheric pressure CVD synthesis provides a new route for simple, inexpensive and safe growth of graphene thin-films.     

Monolayer graphene growth using additional etching process in atmospheric pressure chemical vapor deposition

The synthesis of monolayer graphene is the key to graphene’s practical applications. Herein we report a facile and scalable technique to grow monolayer graphene on Cu, Ni, Co, and Fe surfaces using an etching-aided chemical vapor deposition (CVD) process. The growth was performed using an additional step of hydrogen etching in atmospheric pressure CVD after stopping the carbon supply. The etching of formed multi-layer graphene for Cu substrates assists the formation of monolayer graphene. The etching of excessive dissolved carbon for Ni, Co, and Fe substrates really helps to suppress the troublesome carbon precipitation which is believed to cause the non-uniform thickness of the produced graphene. We believe this technique is not only limited to Cu, Ni, Co, and Fe surfaces but also can be extend to other metal substrates such as Pt, Au, Pd, and Ru if choosing appropriate carbon precursors. We also found out that by varying the time of hydrogen exposure both monolayer and bilayer graphene were successfully synthesized on Ni surfaces. Metal substrates with high carbon solubility in them seem to hold great advantages in the layer-controlled synthesis of graphene. Our findings open a new pathway for an efficient growth of monolayer graphene and will facilitate graphene research.     

绝缘衬底上化学气相沉积法生长石墨烯材料

利用化学气相沉积法,在Si衬底、蓝宝石衬底和SiC衬底上生长石墨烯材料,研究石墨烯的表面形貌、缺陷、晶体质量和电学特性。原子力显微镜、光学显微镜和拉曼光谱测试表明,Si₃N₄覆盖层可以有效抑制3C-SiC缓冲层的形成;低温生长有利于保持材料表面的平整度,高温生长有利于提高材料的晶体质量。5.08 cm蓝宝石衬底上石墨烯材料,室温下非接触Hall测试迁移超过1000 cm²·V^-1·s^-1,方块电阻不均匀性为2.6%。相对于Si衬底和蓝宝石衬底,SiC衬底上生长石墨烯材料的表面形态学更好,缺陷更低,晶体质量和电学特性更好,迁移率最高为4900 cm²·V^-1·s^-1。     

Promoter-assisted chemical vapor deposition of graphene

The synthesis of graphene by chemical vapor deposition (CVD) is a promising approach for producing graphene for novel applications. Especially graphene synthesis on Copper substrates has resulted in high quality, large area graphene growth. This method, however, exhibit limitations in achievable graphene quality due to the low catalytic activity of the growth substrate and occurring catalyst deactivation at high graphene coverage. We here study the effect of adding a material to promote graphene growth on Cu. Catalytic materials such as Nickel and Molybdenum were found to affect the graphene quality and growth rate positively. The origin for this enhancement is a decrease of the energy barrier of catalytic methane decomposition through a process of distributed catalysis. This process can also help overcome the issue of catalyst deactivation and increase film continuity. These findings not only provide aroute for improving the CVD synthesis of graphene but also answer fundamental questions about graphene growth.     

Low-temperature synthesis of graphene on Cu using plasma-assisted thermal chemical vapor deposition

Plasma-assisted thermal chemical vapor deposition (CVD) was carried out to synthesize high-quality graphene film at a low temperature of 600°C. Monolayer graphene films were thus synthesized on Cu foil using various ratios of hydrogen and methane in a gaseous mixture. The in situ plasma emission spectrum was measured to elucidate the mechanism of graphene growth in a plasma-assisted thermal CVD system. According to this process, a distance must be maintained between the plasma initial stage and the deposition stage to allow the plasma to diffuse to the substrate. Raman spectra revealed that a higher hydrogen concentration promoted the synthesis of a high-quality graphene film. The results demonstrate that plasma-assisted thermal CVD is a low-cost and effective way to synthesis high-quality graphene films at low temperature for graphene-based applications.     

Photo-thermal chemical vapor deposition of graphene on copper

We demonstrate the synthesis of single-layer graphene films on copper by photo-thermal chemical vapor deposition (PTCVD) realized using a rapid thermal processing system typically used in CMOS processing. Influence of the temperature on the low-pressure (10 mbar) graphene synthesis using methane precursor was characterized by analyzing the crystalline quality, thickness and electronic properties of the films. Using a growth time of only 60 s, for graphene fabricated at 950 °C the sheet resistance and mobility show equivalent quality compared to thermal CVD graphene. Moreover, μ-Raman mapping reveals very low defect density and high 2D to G band ratio similar to the fingerprint of exfoliated single-layer graphene. The synthesis process was found to exhibit a threshold at around 900 °C at which (and below) the single-layer graphene film does not contain adlayer flakes typically observed in high temperature CVD graphene on copper. Our study shows that PTCVD can be used for the high throughput fabrication of high-quality single-layer graphene on copper and is therefore a promising method while pursuing cost-effective graphene fabrication.     

Photo-thermal chemical vapor deposition growth of graphene

The growth of graphene on Ni using a photo-thermal chemical vapor deposition (PT-CVD) technique is reported. The non-thermal equilibrium nature of PT-CVD process resulted in a much shorter duration in both heating up and cooling down stages, thus allowing for a reduction in the overall growth time. Despite the reduced time for synthesis compared to standard thermal chemical vapor deposition (T-CVD), there was no decrease in the quality of the graphene film produced. Furthermore, the graphene formation under PT-CVD is much less sensitive to cooling rate than that observed for T-CVD process. Growth on Ni also allows for the alleviation of hydrogen blister damage that is commonly encountered during growth on Cu substrates and a lower processing temperature. To characterize the film’s electrical and optical properties, we further report the use of pristine PT-CVD grown graphene as the transparent electrode material in an organic photovoltaic device (OPV) with poly(3-hexyl)thiophene (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) as the active layer where the power conversion efficiency of the OPV cell is found to be comparable to that reported using pristine graphene prepared by conventional CVD.     

Growth of graphene on Cu by plasma enhanced chemical vapor deposition

The growth of graphene on Cu substrates by plasma enhanced chemical vapor deposition (PE-CVD) was investigated and its growth mechanism was discussed. At a substrate temperature of 500 °C, formation of graphene was found to precede the growth of carbon nanowalls (CNWs), which are often fabricated by PE-CVD. The growth of graphene was investigated in various conditions, changing the plasma power, gas pressures, and the substrate temperature. The catalytic nature of Cu also affects the growth of monolayer graphene at high substrate temperatures, while the growth at low temperatures and growth of multilayer graphene are dominated mostly by radicals generated in the plasma.     

Synthesis of large-area graphene on molybdenum foils by chemical vapor deposition

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 193 cm² V^?1 s^?1. Our experiments show that the Mo substrate has the similar simplicity and large tolerance to processing conditions as Cu.     

石墨烯化学气相沉积法可控制备的催化反应体系研究

石墨烯的化学气相沉积(CVD)法制备是一个复杂的多相催化反应过程。如何从催化反应角度理解此过程中的诸多科学问题对石墨烯的精准结构控制以及石墨烯产品的标准化和实用化至关重要。结合最新研究进展,系统分析了CVD反应体系所包含的碳源、反应气氛、催化金属及其内部碳杂质、反应中间碳物种对石墨烯生长的热、动力学以及石墨烯宏、微观结构特性(层数、质量、形状、晶畴等)的影响机制和调控策略,挖掘不同石墨烯CVD反应体系背后的共性科学规律。简介了增强型CVD技术的发展,展望和建议基元反应步骤和含碳反应中间物种对于石墨烯控制制备的重要作用,为未来无缺陷、超洁净、低成本、快速、宏量化的大面积石墨烯薄膜及特定石墨烯衍生结构的定制化制备提供参考。     

Fabrication of high-quality graphene by hot-filament thermal chemical vapor deposition

To the best of our knowledge, the previously reported graphene fabricated using catalytic chemical vapor deposition techniques contained a high defect density, which will hinder its opto-electronic properties. In this work, the effects of two crucial parameters, namely deposition time and hydrogen flow rate on the growth of graphene using a hot-filament thermal chemical vapor deposition technique were systematically studied. Fabrications were conducted at substrate and filament temperatures of 1000 °C and 1750 °C, respectively. Very low I_D/I_G ratios (≪0.1) were obtained for all the samples, which reflected the formation of high-quality graphene deposited on Cu foils. A quasi-static equilibrium copper vapor inside an alumina tube was found to be an important factor to obtain a low defect density graphene. A growth mechanism was then proposed, where the cuprous oxide (Cu₂O) acted as a nucleation site for graphene growth.     

The synthesis of graphene nanowalls on a diamond film on a silicon substrate by direct-current plasma chemical vapor deposition

Graphene nanowalls have been synthesized on diamond by direct-current plasma enhanced chemical vapor deposition (CVD) on silicon substrates pre-seeded with diamond nanoparticles in gas mixtures of methane and hydrogen. Switching from diamond CVD to graphene CVD is done by increasing the methane concentration and decreasing the plasma power without breaking the vacuum. Graphene nanowalls stand on the CVD diamond film to form a 3-dimensional network. Scanning electron microscopy, high-resolution transmission electron microscopy, UV and visible Raman scattering and electrochemical cyclic voltammetry measurements are used to characterize the multi-layer turbostratic graphitic carbon nanostructure and demonstrate its electrochemical durability with a low background current in a wide electrochemical potential window.     

Role of residual polymer on chemical vapor grown graphene by Raman spectroscopy

Although various applications extensively utilize polymer-assisted graphene transfer step, the role of residual polymer on graphene was not well-understood. Here, we report the effect of poly (methyl methacrylate) (PMMA) on chemical vapor deposition-grown hexagonal graphene via Raman spectroscopy. Analysis of bare-, PMMA-covered supported, and PMMA-covered suspended graphene exhibits that their G and 2D band positions are progressively downshifted in that order. Mapping of spatial G and 2D band shifts into doping and strain contributions shows that PMMA residue exerts moderate 0.15% tensile strain on graphene/substrate, as compared to that of bare graphene. During this tensile strain, residual PMMA-covered graphene maintains its doping level as much as bare graphene does.     

Diamond synthesis by chemical vapor deposition: The early years

In the two decades following World War II there was a great surge in interest in high-pressure diamond synthesis, especially in Sweden, the United States and the Soviet Union. It is less well known that during this time major efforts were also made in low-pressure, metastable growth of diamond. All of these efforts, both high and low pressure, were characterized by great secrecy and a considerable lack of transparency. General Electric made the first public announcement of successful high pressure-high temperature diamond synthesis in 1955.The first reports of low-pressure diamond syntheses in the open literature were in 1962 (Eversole; Union Carbide Corporation) in 1968 (Angus et al.; Case Western Reserve University, Cleveland, and also in 1968 (Deryagin et al.; Physical Chemistry Institute, Moscow). In addition to skepticism about the veracity of these claims, a common view was that even if true, growth rates would always be far too slow to be of interest. Also, the apparent violation of thermodynamic laws was a continuing theme by many (but not all) during this time period. These attitudes changed dramatically in the early 1980s when the National Institute for Research in Inorganic Materials (NIRIM) in Japan (Kamo, Matsumoto, Sato, Setaka) announced diamond growth rates in the micron per hour range.The role of hydrogen in diamond synthesis was suggested in 1966 by Lander and Morrison at Bell Labs. An understanding of the critical role of atomic hydrogen grew incrementally, through sequential growth-cleaning cycles (Angus, Gardner) and culminating in its using during growth by the Moscow group (Deryagin, Fedoseev, Polanskaya, Spitsyn, Varnin, et al.).     

Growing graphene on polycrystalline copper foils by ultra-high vacuum chemical vapor deposition

We show that monolayer graphene can be grown isothermally on polycrystalline copper foils via ultra-high vacuum chemical vapor deposition (UHV-CVD), using acetylene as a carbon precursor. The growth is self-limiting, yielding monolayer graphene with a quality comparable to that of graphene grown by atmospheric- or low-pressure chemical vapor deposition. Copper sublimation, a typical concern for UHV-CVD, is shown to be suppressed by growing graphene domains. Further, the roughness of the copper surface after growth is similar to that of copper foils after growth processes at higher pressures. A dependency of the growth kinetics on the surface orientation of the copper grains is observed and a growth model including all stages of growth is presented and discussed. Similar to observations at higher growth pressures, the graphene domains possess sigmoidal growth, however the overall growth behavior is more complicated with two subsequent growth modes. The role of hydrogen is investigated and shows that, contrary to reports for higher growth pressures, dissolved hydrogen in the copper foil plays an essential role for graphene growth whereas ambient hydrogen does not have a noticeable influence.     

Growth and properties of few-layer graphene prepared by chemical vapor deposition

The structure and the electrical, mechanical and optical properties of few-layer graphene (FLG) synthesized by chemical vapor deposition (CVD) on a Ni-coated substrate were studied. Atomic resolution transmission electron microscope (TEM) images show highly crystalline single-layer parts of the sample changing to multi-layer domains where crystal boundaries are connected by chemical bonds. This suggests two different growth mechanisms. CVD and carbon segregation participate in the growth process and are responsible for the different structural formations found. Measurements of the electrical and mechanical properties on the centimeter scale provide evidence of a large scale structural continuity: (1) in the temperature dependence of the electrical conductivity, a non-zero value near 0 K indicates the metallic character of electronic transport; (2) Young’s modulus of a pristine polycarbonate film (1.37 GPa) improves significantly when covered with FLG (1.85 GPa). The latter indicates an extraordinary Young modulus value of the FLG-coating of TPa orders of magnitude. Raman and optical spectroscopy support the previous conclusions. The sample can be used as a flexible and transparent electrode and is suitable for use as special membranes to detect and study individual molecules in high-resolution TEM.