Synthesis Of Monolayer Graphene Research Articles

A Novel and Large-Scale Rapid Green Synthesis of Few-Layer and Multi-Layer Graphene

Mono-layer graphene has shown path-breaking potential in almost all areas that are useful to humankind. However, research in the last decade indicated that few-layer graphene (FLG) (3-10 layers) and multi-layer graphene (MLG) (>10 layers) could also be utilized for a wide range of applications. The synthesis of mono-layer graphene at a large scale is quite complex. Producing FLG and MLG at a kilogram scale is equally challenging. A vast amount of literature on different routes to prepare FLG and MLG is available. However, most of the preparation routes involve the use of hazardous chemicals, copious amounts of water, and thermal treatments and are time taking. Also, these processes need bigger machinery to convert the lab-scale methods into continuous large-scale production. On the contrary, in this work we report a novel process of producing FLG and MLG along with trace quantities of graphene (< 3 layers) developed using a kitchen mixer and a modified mixing jar (3 litres). The highlights of this process include the use of no hazardous chemicals/solvents, no washing or thermal treatment needed, much less time (30 minutes), and large quantity (250-500 grams) per batch. Our study successfully converted natural graphite flakes (NGF) into FLG and MLG by carrying out rapid liquid phase shear mixing using a green solvent. The shear mixing and the green solvent convert the NGF into FLG and MLG. The quantity of FLG and MLG getting produced is time-dependent. It has been optimized by thoroughly characterizing FLG and MLG using techniques like particle size analyser, X-ray diffraction, Raman spectroscopy, and electron microscopy. The conversion of NGF to FLG and MLG is also affirmed by studying changes in properties using tools like thermogravimetry, BET, and UV-visible spectroscopy. The developed process is rapid, efficient, economical, scalable, continuous, and, most importantly, chemical free.The graphenaceous powder produced unlike graphite can be easily coated on different substrates using appropriate binders and thus makes it useable for energy storage applications. Also the fine particle size of graphite makes is an ideal material to be used as lubricant, both in solid and liquid mediums. The developed material can be explored for its applications in several other areas which require FLG/MLG.

Cr cation-anchored carbon nanosheets: synthesis, paramagnetism and ferromagnetism

Since the successfully synthesis of monolayer graphene, carbon-based materials have attracted wide and extensive attentions from researches. Due to the excellent transport capacity and conductivity, they are promising to be applied in electronic devices, even substituting the silicon-based electronic devices, optoelectronics and spintronics. Nevertheless, due to the non magnetic feature, many efforts have been devoted to endow carbon materials magnetism to apply them in the spintronic devices fabrication. Herein, a strategy of Cr cation solely anchored on two-dimensional carbon nanosheets by Cr–N bonds is developed, which introduces magnetism in carbon nanosheets. By extended x-ray absorption fine structure characterization, Cr cations are demonstrated to be atomically dispersed with Cr–N3 coordination. And after Cr–N3 anchored, carbon nanosheets exhibit ferromagnetic features with paramagnetic background. The magnetization varies with Cr content and reaches the maximum (Cr: 2.0%, 0.86 emu g−1) under 3 T at 50 K. The x-ray magnetic circular dichroism and first-principle calculations indicate that the magnetism is caused by the Cr3+ component of the anchored Cr cations. This study sets a single cation anchoring carbon as a suitable candidate for future spintronics.

Open-atmosphere flame synthesis of monolayer graphene

An open-atmosphere, unconfined setup comprising a novel multiple inverse-diffusion flame (m-IDF) burner modified with extended precursor tubes is employed to synthesize graphene on substrates. Growth conditions of mono-, bi-, and few-layer graphene (MLG, BLG, and FLG, respectively) are investigated, with systematic variation of parameters such as substrate temperature, methane-to-hydrogen volume flow rate ratio (JCH4:JH2), growth duration, post-flame flow profiles, substrate material, precursor species (e.g., CH4, C2H2, C2H4), and in-situ post-growth hydrogen annealing. Graphene growth on copper is observed for a wide range of temperatures from 850 °C to 1000 °C, with high-quality BLG created at a substrate temperature of 1000 °C with JCH4:JH2 of 1:100 for 5 min growth duration. A sequential in-situ post-growth hydrogen annealing treatment, where the hydrocarbon precursor flow is terminated but the hydrogen m-IDFs are maintained, is found to be effective for etching adlayers of graphene. As such, BLG is reduced to MLG by increasing the post-growth hydrogen annealing duration at 1000 °C to 10 min. In-situ gas-phase Raman measurements characterize the evolution of the gas-phase precursor species in the synthesis flow field. CH2 is determined to be the main gas-phase carbon species needed near the substrate to form graphene in our flame synthesis system.

Scalable synthesis of nanoporous atomically thin graphene membranes for dialysis and molecular separations via facile isopropanol-assisted hot lamination.

Scalable graphene synthesis and facile large-area membrane fabrication are imperative to advance nanoporous atomically thin membranes (NATMs) for molecular separations. Although chemical vapor deposition (CVD) allows for roll-to-roll high-quality monolayer graphene synthesis, facile transfer with atomically clean interfaces to porous supports for large-area NATM fabrication remains extremely challenging. Sacrificial polymer scaffolds commonly used for graphene transfer typically leave polymer residues detrimental to membrane performance and transfers without polymer scaffolds suffer from low yield resulting in high non-selective leakage through NATMs. Here, we systematically study the factors influencing graphene NATM fabrication and report on a novel roll-to-roll manufacturing compatible isopropanol-assisted hot lamination (IHL) process that enables scalable, facile and clean transfer of CVD graphene on to polycarbonate track etched (PCTE) supports with coverage ≥99.2%, while preserving support integrity/porosity. We demonstrate fully functional centimeter-scale graphene NATMs that show record high permeances (∼2-3 orders of magnitude higher) and better selectivity than commercially available state-of-the-art polymeric dialysis membranes, specifically in the 0-1000 Da range. Our work highlights a scalable approach to fabricate graphene NATMs for practical applications and is fully compatible with roll-to-roll manufacturing processes.

Graphene growth with no intended carbon precursor feeding into the LPCVD process: causes, solutions, and effects

In this research, we have investigated the unintended graphene nucleation problem and its damaging effects on monolayer graphene synthesis in low-pressure chemical vapor deposition (LPCVD) process. This problem is the growth of graphene on the copper surface with no carbon feedstock. A new source of undesired carbon species was identified which has not been addressed so far. The hydrogen-rich heating stage was diagnosed as the onset of the unintended nucleation for the first time owing to the determinant catalytic role of hydrogen in this stage. It was found out that this problem leads to uncontrollable growth of multilayer graphene, growth of defective graphene film and also inhibition of the reliable synthesis of monolayer graphene. We managed to grow enhanced-quality monolayer graphene by developing some innovative solutions to the problem containing a general solution based on the hydrogen effects in the heating stage. The results reveal a significant decrease in the unintended nucleation density from ∼2000 to almost zero domains per 100 × 100 μm2 copper area. Furthermore, Raman, HRTEM and SAED analysis confirm the defect-free growth of monolayer graphene after employing the solutions. These findings could pave the way for the reliable synthesis of high-quality monolayer graphene as well as large-sized graphene domains.

Direct Synthesis of Armchair Graphene Nanoribbons on Ge(001)/Si(001) Using CVD

Direct synthesis of graphene nanoribbons on dielectric or semiconducting substrates offers a scalable route for integration of graphene-based devices into a conventional silicon-based technology.(1) So-far, wafer-scale synthesis of armchair graphene nanoribbons has been demonstrated on Ge (001) using chemical vapor deposition.(2) However, direct synthesis of graphene or graphene nanoribbons has not yet been achieved on CMOS compatible Si(001) due to formation of stable SiC at temperatures > 1000 K, which are needed to achieve synthesis of nanoribbons with smooth, armchair edges. A promising and realistic way to overcome this challenge is to synthesize graphene nanoribbons on CMOS compatible Ge(001)/Si(001) substrates. So far the synthesis of monolayer graphene on Ge(001)/Si(001) substrates has been demonstrated by CVD.(3, 4) In this study, we synthesized graphene nanoribbons on 3 µm epitaxial Ge(001)/Si(001) substrates, by ambient pressure CVD using methane as the carbon precursor. We show that growth kinetics of graphene nanoribbons on Ge(001)/Si(001) are comparable to that on Ge(001). By tuning the methane flow and growth time we are able to synthesize graphene nanoribbons ranging from 100 nm to 1 µm in length with high aspect ratios, whilst avoiding Si diffusion from the bulk. For instance, by restricting methane to < 6000 ppm and growth time to < 3h, graphene nanoribbons with widths < 10 nm can be synthesized with aspect ratios as high as 70. Such nanoribbons grown on Ge(001) have been shown to exhibit technologically relevant band-gaps as well as exceptional charge transport properties.(5) Furthermore, we also investigated the possible role of threading dislocations in Ge epilayer on nucleation or growth of nanoribbons and show that nanoribbons readily grow over the threading dislocations. Lastly, we studied the evolution of surface roughness with the nucleation density of nanoribbons. Our study provides valuable insight into the mechanism of graphene nanoribbon growth on Ge(001)/Si(001) substrates. From this information, it is expected that unidirectional graphene nanoribbons with rational placement and control over width poly-dispersity can be synthesized on Ge(001)/Si(001) platform akin to Ge(001), which has been demonstrated recently using seed-mediated growth.(6) This provides a scalable way for wafer scale integration of graphene nanoribbon arrays on Si and provides motivation for further research into this direction. References A. Khan et al., Direct CVD Growth of Graphene on Technologically Important Dielectric and Semiconducting Substrates. Advanced Science 5, (2018).R. M. Jacobberger et al., Direct oriented growth of armchair graphene nanoribbons on germanium. Nature Communications 6, (2015).I. Pasternak et al., Graphene growth on Ge(100)/Si(100) substrates by CVD method. Scientific Reports 6, (2016).M. Lukosius et al., Metal-Free CVD Graphene Synthesis on 200 mm Ge/Si(001) Substrates. Acs Applied Materials & Interfaces 8, 33786-33793 (2016).R. M. Jacobberger, M. S. Arnold, High-Performance Charge Transport in Semiconducting Armchair Graphene Nanoribbons Grown Directly on Germanium. Acs Nano 11, 8924-8929 (2017).A. J. Way, R. M. Jacobberger, M. S. Arnold, Seed-Initiated Anisotropic Growth of Unidirectional Armchair Graphene Nanoribbon Arrays on Germanium. Nano Letters 18, 898-906 (2018). Figure 1

Controlled synthesis of monolayer graphene with a high quality by pyrolysis of silicon carbide

Monolayer graphene with high quality was synthesized by pyrolysis of 6H-silicon carbide single crystal in a medium frequency induction heating furnace. The obtained graphene was systematically studied via Raman spectroscopy, atomic force microscopy (AFM), and high-resolution transmission electron microscopy (HRTEM). The characterized results showed that monolayer graphene with high quality was obtained when the growth temperature, Ar flow, and growth pressure was 2000 °C, 150 sccm, and 15 Torr, respectively. Our results showed that growth temperature, Ar flow, and growth pressure had great influence on the quality of the obtained monolayer graphene. Compared with the conventional monolayer graphene preparation methods, our method is simple, controllable, and repeatable, which greatly expands the application prospect of graphene.

In situ synthesis of monolayer graphene on silicon for near-infrared photodetectors.

Direct integration of monolayer graphene on a silicon (Si) substrate is realized by a simple thermal annealing process, involving a top copper (Cu) layer as the catalyst and an inserted polymethylmethacrylate (PMMA) as the carbon source. After spin-coating the PMMA carbon source on the Si substrate, the Cu catalyst was deposited on PMMA/Si by electron beam evaporation. After that, graphene was directly synthesized on Si by decomposition and dehydrogenation of PMMA and the catalyzation effect of Cu under a simple thermal annealing process. Furthermore, under an optimized growth condition, monolayer graphene directly formed on the Si substrate was demonstrated. Utilizing the as-grown graphene/Si heterojunction, near-infrared photodetectors with high detectivity (∼1.1 × 1010 cm Hz1/2 W−1) and high responsivity (50 mA W−1) at 1550 nm were directly fabricated without any post-transfer process. The proposed approach for directly growing graphene on silicon is highly scalable and compatible with present nano/micro-fabrication systems, thus promoting the application of graphene in microelectronic fields.

Highly uniform monolayer graphene synthesis via a facile pretreatment of copper catalyst substrates using an ammonium persulfate solution.

The demand for large-area, high-quality synthesis of graphene with chemical vapor deposition (CVD) has increased for the realization of next-generation transparent and flexible optoelectronic applications. In conventional CVD processes, various synthesis parameters can strongly affect the quality of the resultant graphene. In particular, surface engineering of a copper catalyst substrate is one of the most promising pathways for achieving high-quality graphene with excellent reproducibility. For this purpose, simple wet chemical etching of a catalyst substrate without toxic fume byproducts or metal ion residues is desired. Here, we suggest a facile method for preparing a pretreated copper catalyst substrate for highly uniform, large-area CVD graphene growth. This pretreatment method involves a wet copper etchant, ammonium persulfate (APS) solution, and gentle ultrasonication (100 W), which do not produce unwanted or toxic fume byproducts during their reaction. Moreover, this approach does not leave metal ion residue on the cleaned copper substrates that serves as residual nucleation sites and leads to multilayer graphene growth. To evaluate the quality of the synthesized monolayer graphene on the cleaned copper catalyst substrates, we used various characterization techniques, such as Raman spectroscopy and sheet resistance, optical transmittance, and FET characterization.

Interfacial monolayer graphene growth on arbitrary substrate by nickel-assisted ion implantation

Direct synthesis of monolayer graphene on arbitrary substrate (such as SiO2, Al2O3, glass, and Si3N4) is demonstrated through a universal and controllable approach, i.e., carbon ion implantation technique. By tuning the implantation energy to precisely implant carbon ions into the thin Ni film, which is pre-deposited on the objective substrate, followed by post-annealing and fast-cooling processes, monolayer graphene films are directly synthesized on the arbitrary objective substrate. Micro-Raman spectroscopy, STM, and TEM are cooperatively utilized to verify that the synthesized graphene is monolayer with high quality. Moreover, field-effect transistors are fabricated with the directly synthesized monolayer graphene on SiO2/Si substrate to reveal the corresponding electrical properties. This study provides an avenue for direct growth of graphene on arbitrary substrate, which offers more flexibility in the experimental conditions, especially the experimental atmosphere. In addition, involving the ion implantation technique may pave the way for wafer-scale graphene synthesis, thus benefitting the application of graphene in micro-/nano-electronic field.

High-quality graphene synthesis on amorphous SiC through a rapid thermal treatment

Synthesizing high-quality graphene by catalytic transformation from amorphous silicon carbide (a-SiC) through a rapid thermal treatment (RTT) method is reported. SiO2/Si substrates are coated by a-SiC films followed by Cu and Ni films deposited sequentially. The samples are then thermally annealed by RTT for the synthesis of high-quality graphene in only 3 min. The synergistic effect of Cu and Ni catalyst is observed. We conjecture that the inserted copper film acts not only as a catalyst or substrate for graphene growth but also as a barrier for carbon diffusion to facilitate the synthesis of monolayer graphene, while the nickel film acts as another catalyst and forms Cu-Ni alloy to lower the catalytic temperature. In this paper, we present a simple and time-saving way in preparation of high-quality graphene and put forward a brief theoretical model for the growth of graphene.

Large-area, high-quality monolayer graphene from polystyrene at atmospheric pressure

Graphene films have been attracting great interest owing to their unique physical properties. In this paper, we develop an efficient method to prepare large-area monolayer graphene (97.5% coverage) by atmospheric pressure chemical vapor deposition on Cu foils using polystyrene in a short time (3 min). Raman spectroscopy, transmission electron microscopy and scanning electron microscopy are employed to confirm the thickness and uniformity of the graphene films. Graphene films on glass substrates show high optical transmittance and electrical conductivity. Magnetic transport studies demonstrate that the as-grown monolayer graphene exhibits a high carrier mobility of 3395 cm2 V−1 s−1 at 25 K. On the basis of the analysis, it is concluded that our method is a simple, safe and versatile approach for the synthesis of monolayer graphene.

Nanoparticle mass detection by single and multilayer graphene sheets: Theory and simulations

The discovery of graphene opened up a new field of research and led to the development of various nanosensors and actuators that form new classes of nano electro mechanical systems (NEMSs) called graphene based nano resonators (GBNRs). Recently, GBNRs have been used in mass detection applications. In such applications and others, single-layer graphene sheets play a fundamental role. However, the synthesis of monolayer graphene is usually challenging and expensive. This makes the commercial use of multilayer graphene systems more attractive and necessary. Moreover, it is now more possible to control the number of layers in large-size graphene systems, and thus the theoretical design tools could help the engineers develop more efficient graphene-based sensors for everyday applications. This paper introduces a non-local elasticity theory for the general configurations of carbon atoms that could form carbon nanotubes (CNTs), nanosheets (graphene) and nanospheres (fullerenes) by means of the laminated plate theory. The problem is solved by the generalized differential quadrature element method (GDQEM). Then, a molecular dynamics approach is used in conjunction with the operational modal analysis (OMA) to perform a nano metric modal analysis. Several comparisons are presented to validate the introduced approaches; and finally, the effects of the number of layers and of nanoparticle mass and position on the frequency shift and sensitivity of the designed sensors are studied. It is demonstrated that the frequency shift decreases slightly and the sensitivity increases by increasing the number of graphene layers. It is also demonstrated that, depending on the positions of nanoparticles in a senor, various mode shapes of a graphene sheet could be excited; and consequently, the shift of the first natural frequency may not be appropriate for particles far from the middle of graphene sheet.

A fast transfer-free synthesis of high-quality monolayer graphene on insulating substrates by a simple rapid thermal treatment.

The transfer-free synthesis of high-quality, large-area graphene on a given dielectric substrate, which is highly desirable for device applications, remains a significant challenge. In this paper, we report on a simple rapid thermal treatment (RTT) method for the fast and direct growth of high-quality, large-scale monolayer graphene on a SiO2/Si substrate from solid carbon sources. The stack structure of a solid carbon layer/copper film/SiO2 is adopted in the RTT process. The inserted copper film does not only act as an active catalyst for the carbon precursor but also serves as a "filter" that prevents premature carbon dissolution, and thus, contributes to graphene growth on SiO2/Si. The produced graphene exhibits a high carrier mobility of up to 3000 cm(2) V(-1) s(-1) at room temperature and standard half-integer quantum oscillations. Our work provides a promising simple transfer-free approach using solid carbon sources to obtain high-quality graphene for practical applications.

Influence of Nickel Catalyst Film Thickness and Cooling Condition for Synthesis of Monolayer Graphene by Thermal Chemical Vapor Deposition at 800 ºC

Influence of Nickel Catalyst Film Thickness and Cooling Condition for Synthesis of Monolayer Graphene by Thermal Chemical Vapor Deposition at 800 ºC

Redistribution of carbon atoms in Pt substrate for high quality monolayer graphene synthesis

The two-dimensional material graphene shows its extraordinary potential in many application fields. As the most effective method to synthesize large-area monolayer graphene, chemical vapor deposition has been well developed; however, it still faces the challenge of a high occurrence of multilayer graphene, which causes the small effective area of monolayer graphene. This phenomenon limits its applications in which only a big size of monolayer graphene is needed. In this paper, by introducing a redistribution stage after the decomposition of carbon source gas to redistribute the carbon atoms dissolved in Pt foils, the number of multilayer flakes on the monolayer graphene decreases. The mean area of monolayer graphene can be extended to about 16 000 μm2, which is eight times larger than that of the graphene grown without the redistribution stage. A Raman spectrograph is used to demonstrate the high quality of the monolayer graphene grown by the improved process.

The Preparation of BN-Doped Atomic Layer Graphene via Plasma Treatment and Thermal Annealing

We report a new method for the codoping of boron and nitrogen in a monolayer graphene film. After the CVD synthesis of monolayer graphene, BN-doped graphene is prepared by performing power-controlled plasma treatment and thermal annealing with borazine. BN-doped graphene films with various doping levels, which were controlled by altering the plasma treatment power, were found with Raman and electrical measurements to investigate exhibit p-doping behavior. Transmission electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy were used to demonstrate that the synthesized BN-doped graphene films have a sp2 hybridized hexagonal structure. This approach to tuning the distribution and doping levels of boron and nitrogen in monolayer sp2 hybridized BN-doped graphene is expected to be very useful for applications requiring large-area graphene with an opened band gap.

High-quality monolayer graphene synthesis on Pd foils via the suppression of multilayer growth at grain boundaries.

The segregation of carbon from metals in which carbon is highly soluble, such as Ni (≈1.1 atom% at 1000 °C), is a typical method for graphene growth; this method differs from the surface-catalyzed growth of graphene that occurs on other metals such as Cu (<0.04 atom%). It has not been established whether strictly monolayer graphene could be synthesized through the traditional chemical vapor deposition route on metals where carbon is highly soluble, such as Pd (≈3.5 atom%). In this work, this issue is investigated by suppressing the grain boundary segregation using a pretreatment comprising the annealing of the Pd foils; this method was motivated by the fact that the typical thick growths at the grain boundaries revealed that the grain boundary functions as the main segregation channel in polycrystalline metals. To evaluate the high crystallinity of the as-grown graphene, detailed atomic-scale characterization with scanning tunneling microscopy is performed.

Synthesis of transfer-free graphene by solid phase reaction process in presence of a carbon diffusion barrier

Synthesis of transfer-free high quality graphene on an arbitrary substrate by metal assisted graphitization with control carbon diffusion process has lot of significance for device applications. Here, we demonstrate the synthesis of monolayer graphene using common triblock co-polymer (EOnPOmEOn) as carbon source by solid phase reaction. In the synthesis process, we used Ni and NiO thin film as catalyst and carbon diffusion barrier, respectively on the top of polymer deposited SiO2/Si substrate. The long carbon chain of triblock co-polymer pluronic F127(EO106PO70EO106) can be graphitized in presence of Ni layer, where carbon diffusion to the top surface is controlled by NiO thin layer. The effects of carbon diffusion, thickness of polymer layer and annealing process are studied to obtain high quality monolayer graphene. Our findings reveal that the NiO/Ni stacked, thickness of polymer layer and reaction process in H2 atmosphere strongly influence the quality of synthesized graphene.

Synthesis of Monolayer Graphene Having a Negligible Amount of Wrinkles by Stress Relaxation

For the chemical vapor deposition (CVD) of graphene, the grain growth of the catalyst metal and thereby surface roughening are unavoidable during the high temperature annealing for the graphene synthesis. Considering that nanoscale wrinkles and poor uniformity of synthesized graphene originate from the roughened metal surface, improving surface flatness of metal thin films is one of the key factors to synthesize high quality graphene. Here, we introduce a new method for graphene synthesis for fewer wrinkle formation on a catalyst metal. The method utilizes a reduced graphene oxide (rGO) interfacial layer between the metal film and the wafer substrate. The rGO interlayer releases the residual stress of the metal thin film and thereby suppresses stress-induced metal grain growth. This technique makes it possible to use much thinner nickel films, leading to a dramatic suppression of RMS roughness (~3 nm) of the metal surface even after high temperature annealing. It also endows excellent control of the graphene thickness due to the reduced amount of total carbon in the thin nickel film. The synthesized graphene layer having negligible amount of wrinkles exhibits excellent thickness uniformity (91% coverage of monolayer) and very high carrier mobility of ~15,000 cm(2)/V·s.