High-quality Monolayer Graphene Research Articles

8-Inch Wafer Scale, Transfer-Free, High-Quality Monolayer Graphene Growth on Ti-Buffered Si (001) Substrates At 100 ºC via Plasma-Assisted Chemical Vapor Deposition

8-Inch Wafer Scale, Transfer-Free, High-Quality Monolayer Graphene Growth on Ti-Buffered Si (001) Substrates At 100 ºC via Plasma-Assisted Chemical Vapor Deposition

Scalable and High-Quality Monolayer Graphene Transfer onto Polymer Membranes Assisted by Camphor

Scalable and High-Quality Monolayer Graphene Transfer onto Polymer Membranes Assisted by Camphor

Memristors with Monolayer Graphene Electrodes Grown Directly on Sapphire Wafers.

The development of the memristor has generated significant interest due to its non-volatility, simple structure, and low power consumption. Memristors based on graphene offer atomic monolayer thickness, flexibility, and uniformity and have attracted attention as a promising alternative to contemporary field-effect transistor (FET) technology in applications such as logic and memory devices, achieving higher integration density and lower power consumption. The use of graphene as electrodes in memristors could also increase robustness against degradation mechanisms, including oxygen vacancy diffusion to the electrode and unwanted metal ion diffusion. However, to realize this technological transformation, it is necessary to establish a scalable, robust, and cost-effective device fabrication process. Here we report the direct growth of high-quality monolayer graphene on sapphire wafers in a mass-producible, contamination-free, and transfer-free manner, using a commercially available metal-organic chemical vapor deposition (MOCVD) system. By taking advantage of this approach, graphene-electrode based memristors are developed, and all the processes used in the device fabrication incorporating graphene electrodes can be performed at wafer scale. The graphene electrode-based memristor demonstrates promising characteristics in terms of endurance, retention, and ON/OFF ratio. This work presents a possible and viable route to achieving robust graphene-based memristors in a commercially and technologically sustainable manner, paving the way for the realization of more powerful and compact integrated graphene electronics in the future.

Transparent shape memory polyimide enables OLED for smart deformation

An innovative process for producing flexible transparent graphene electrodes has been proposed. The electrochemically polished copper-foil was used as the metal substrate to prepare high-quality monolayer graphene. Monolayer graphene was transferred from copper-foil to transparent shape memory polyimide (TSMPI) intactly by novel flow-casting transfer technology, and the smart, ultra-clean and smooth flexible graphene electrode (pG@TSMPI) was fabricated. The light transmittance of pG@TSMPI at 550 nm is 86%, the average sheet resistance is 310 Ω/sq, and the surface roughness (Ra) is 18 nm. Smart white OLED was manufactured using pG@TSMPI as transparent anode, and its luminescence performance was comparable to that of ITO-based device. Depending on the shape memory effect of TSMPI substrate, the smart OLED has the superiorities of variable stiffness and reconfigurable shape, which can transform from 2D structure into complex 3D structure.

Chemical Vapor Deposition Growth of Graphene on 200 mm Ge(110)/Si Wafers and Ab Initio Analysis of Differences in Growth Mechanisms on Ge(110) and Ge(001).

For the fabrication of modern graphene devices, uniform growth of high-quality monolayer graphene on wafer scale is important. This work reports on the growth of large-scale graphene on semiconducting 8 inch Ge(110)/Si wafers by chemical vapor deposition and a DFT analysis of the growth process. Good graphene quality is indicated by the small FWHM (32 cm-1) of the Raman 2D band, low intensity ratio of the Raman D and G bands (0.06), and homogeneous SEM images and is confirmed by Hall measurements: high mobility (2700 cm2/Vs) and low sheet resistance (800 Ω/sq). In contrast to Ge(001), Ge(110) does not undergo faceting during the growth. We argue that Ge(001) roughens as a result of vacancy accumulation at pinned steps, easy motion of bonded graphene edges across (107) facets, and low energy cost to expand Ge area by surface vicinals, but on Ge(110), these mechanisms do not work due to different surface geometries and complex reconstruction.

In Situ Growth of Graphene on Polyimide for High-Responsivity Flexible PbS-Graphene Photodetectors.

Graphene is an ideal material for flexible optoelectronic devices due to its excellent electrical and optical properties. However, the extremely high growth temperature of graphene has greatly limited the direct fabrication of graphene-based devices on flexible substrates. Here, we have realized in situ growth of graphene on a flexible polyimide substrate. Based on the multi-temperature-zone chemical vapor deposition cooperated with bonding a Cu-foil catalyst onto the substrate, the growth temperature of graphene was controlled at only 300 °C, enabling the structural stability of polyimide during growth. Thus, large-area high-quality monolayer graphene film was successfully in situ grown on polyimide. Furthermore, a PbS-graphene flexible photodetector was fabricated using the graphene. The responsivity of the device reached 105 A/W with 792 nm laser illumination. The in-situ growth ensures good contact between graphene and substrate; therefore, the device performance can remain stable after multiple bending. Our results provide a highly reliable and mass-producible path for graphene-based flexible devices.

Graphene as effective anti-fading agent for the protection of artworks

Since all art materials in time are prone to degradation due to environmental conditions, a solution is needed to delay or even to prevent such an interference. Graphene, the pioneer of 2D materials, seems to pave the way to this end either as a protective layer on artworks or as an inclusion in paints, dyes, pigments acting as protective additive, respectively. Regarding the former, the proposed invention provides a method for applying a protective graphene coating onto a surface, such a two- or three-dimensional artwork or other coloured surface. The steps followed within this method are: Deposition of Chemical Vapor Deposition (CVD) produced graphene onto a supporting substrate in order to produce a graphene/substrate composite; Removal the graphene membrane from the supporting substrate; and Deposition the graphene membrane onto the surface of interest All the pre-mentioned steps are performed following the roll-to-roll method (Choi et al., 2015), with the assistance of a tailor-made laminator. However, the direct deposition of graphene onto artworks may not be straightforward due to the wet transfer and hot lamination conditions that can be catastrophic for fragile painted substrates. The novel strategy designed exploits the high-quality CVD monolayer graphene and uses a pressure-sensitive adhesive film (PSAF) in the transfer process that has been demonstrated to enable graphene wetting on the final target-substrate (Kim et al., 2015), thus maximizing the contact area and van der Waals interactions. The low work of adhesion between graphene and the PSAF, compared to the corresponding between graphene and the destination surface, promotes the transfer that can be performed at mild pressure and temperature conditions, without using a surface modifier or adhesives (Martins et al. 2013). Additionally, the proposed roll-to-roll method does not affect the mechanical integrity of the typical paper substrates used in graphic arts. As for the protective additive case, graphene is employed in the form of an aqueous dispersion in media such as paints, dyes, inks, pigments and/or varnishes used for artistic purposes in order to create indelible colours to capable to prevent degradation, attributed to UV radiation, moisture and oxidizing agents. The method presents a new approach for obtaining aqueous dispersions for the hydrophobic graphene and thus, protecting artworks and painted indoor/outdoor wall surfaces taking advantage of the optical and barrier properties of graphene.

Roles of CuO and Cu2O in graphene growth on a copper substrate

In chemical vapor deposition, oxygen plays an important role during graphene growth, especially in the case of copper substrates. However, currently, there is inconsistent information on the role of oxygen in graphene growth. In this study, the mechanism of pre-oxidation on copper substrate to reduce the nucleation density was explored, and the effects of the oxidation products (CuO and Cu2O) introduced on copper substrates under different oxygen partial pressures were discussed. CuO and Cu2O were found to be beneficial in promoting graphene nucleation and growth. The Cu2O surface exhibited greater C absorption activity than the Cu2O/Cu(111) step, which was completely opposite to that of CuO on the Cu(111) substrate. A uniform Cu2O layer with a thickness of < 10 nm was prepared to promote graphene growth on the Cu(111) substrate; thereby, a monolayer single-crystalline graphene was achieved. This work provides new data for the preparation of high-quality monolayer graphene on copper substrate.

(Invited) Recent Progress in Large Area MOCVD Growth of 2D Materials

Metal-organic chemical vapor deposition (MOCVD) has developed to a mature production technology for state-of-the-art (opto)electronic devices such as LEDs, lasers, solar cells and transistors. Current multi-wafer MOCVD systems allow for scalable processes with precise control of growth parameters ensuring excellent uniformity of deposition rates, dopant concentrations and layer thicknesses. All of these properties are of crucial importance for reproducibly obtaining high-quality ultra-thin films required for next-generation nano-electronic devices. One of the most promising materials for the fabrication of such devices are layered transition-metal dichalcogenides (TMDCs), e.g. molybdenum disulfide (MoS2) or tungsten disulfide (WS2), graphene (Gr) or hBN. Due to their 2D structure, these materials possess unique optical, electrical and magnetic characteristics, and monolayers are considered as material system for several future applications. Moreover, the properties and functionality of a single monolayer can be even further improved by forming heterostructures of several two-dimensional materials. Such complex structures which require control on the atomic level strongly limit the number of available fabrication options, especially for volume production, leaving MOCVD as the most reliable industrial deposition method.In this work, we present wafer-scale MOCVD of high-quality monolayers of MoS2, WS2, hBN and graphene (Gr) on single-crystalline C-plane (0001) sapphire deposited in commercial AIXTRON reactors. Our upscaling effort from 2 inch wafer up to growth on 200 mm wafer supports the industrial demand. Molybdenum hexacarbonyl, tungsten hexacarbonyl and di-tert-butyl sulfide are employed as precursors. By optimizing growth conditions, fully-coalesced MoS2 and WS2 samples without parasitic carbon-related Raman peaks were obtained. XPS analysis indicates only Mo-S and W-S bonds for MoS2 and WS2 samples, respectively, with no indications of any side reactions with either the substrate, impurities or oxygen and demonstrates a high layer quality suitable for research on all kind of devices including advanced transistors and memresistive devices. Samples were characterized using Raman/photoluminescence spectroscopy, SEM and AFM to understand the correlation of growth parameter and material properties. As an example a WS2 growth duration series at 700°C temperature was performed. The intensity ratio of about 2 for the Raman E2g/A1g peaks for the optimized WS2 samples suggests deposition of monolayer WS2 with some admixture of bilayer, as confirmed by AFM. The WS2 monolayer film growth has been successfully upscaled from 2” to 200mm sapphire wafers. hBN thin films have been also grown from borazine precursor both on 2” and 200 mm sapphire substrates. Thickness uniformity of 9 nm hBN films was 3% on 200 mm wafers, as determined by XRR. High-quality monolayer graphene layers grown from CH4/H2 on 2” and 200mm sapphire wafers demonstrated good performance by Raman (2D/G peak ratio >1.5 and low D/G peak ratio (<0.25), as well as low sheet resistance (~1300-1500 Ohm/sq). The grown films had smooth surface morphology with the expected graphene wrinkles visible by AFM). These results demonstrate the applicability of AIXTRON systems for the growth of TMDC, hBN and graphene films on large size substrates. The growth could be easily upscaled from 2” to 200 mm wafer growth with only minor adjustment of the growth parameters. More details on the growth process and material properties as well as results on hBN/Gr, TMDC/Gr and TMDC/hBN heterostructures will be discussed in detail. Figure 1

Proximity Effect of Epitaxial Iron Phthalocyanine Molecules on High-Quality Graphene DevicesSupported by the National Natural Science Foundation of China (Grant No. 11674150), the Key-Area Research and DevelopmentProgram of Guangdong Province (Grant No. 2019B010931001), the Guangdong Innovative and Entrepreneurial Research Team Program (Grant No. 2016ZT06D348), the SUSTech CRF, and the Hong Kong RGC (Grant Nos. 16300617 and C6012-17E).

Depositing magnetic insulators on graphene has been a promising route to introduce magnetism via exchange proximity interaction in graphene for future spintronics applications. Molecule-based magnets may offer unique opportunities because of their synthesis versatility. Here, we investigate the magnetic proximity effect of epitaxial iron phthalocyanine (FePc) molecules on high-quality monolayer and bilayer graphene devices on hexagonal boron nitride substrates by probing the local and nonlocal transport. Although the FePc molecules introduce large hole doping effects combined with mobility degradation, the magnetic proximity gives rise to a canted antiferromagnetic state under a magnetic field in the monolayer graphene. On bilayer graphene and FePc heterostructure devices, the nonlocal transport reveals a pronounced Zeeman spin-Hall effect. Further analysis of the scattering mechanism in the bilayer shows a dominated long-range scattering. Our findings in graphene/organic magnetic insulator heterostructure provide a new insight for use of molecule-based magnets in two-dimensional spintronic devices.

Fabrication of Graphene Nanomesh FET Terahertz Detector.

High sensitivity detection of terahertz waves can be achieved with a graphene nanomesh as grating to improve the coupling efficiency of the incident terahertz waves and using a graphene nanostructure energy gap to enhance the excitation of plasmon. Herein, the fabrication process of the FET THz detector based on the rectangular GNM (r-GNM) is designed, and the THz detector is developed, including the CVD growth and the wet-process transfer of high quality monolayer graphene films, preparation of r-GNM by electron-beam lithography and oxygen plasma etching, and the fabrication of the gate electrodes on the Si3N4 dielectric layer. The problem that the conductive metal is easy to peel off during the fabrication process of the GNM THz device is mainly discussed. The photoelectric performance of the detector was tested at room temperature. The experimental results show that the sensitivity of the detector is 2.5 A/W (@ 3 THz) at room temperature.

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.

Tellurium-assisted and space-confined growth of graphene single crystals

Low-temperature synthesis of large-area graphene films by chemical vapor deposition (CVD) method has attracted extensive research interests owing to their extraordinary potential applications. Current reports on the low-temperature graphene growth usually involve catalytic metal substrates, which exceedingly impedes the direct use of graphene in nanoelectronic fabrications. In this project, we design a space-confined growth chamber utilizing two SiO2/Si chips. The bottom chip loading pre-transferred graphene flakes accommodates the growth of second monolayer graphene, with a tellurium (Te) vapor assisted growth process. Polymethylmethacrylate (PMMA) residues introduced from the transferred graphene substrate is employed as the solid carbon source. Further, the numerical simulation results prove that the high concentration of active carbons in the confined space plays a vital role in this growth process. Density functional theory (DFT) calculations reveal that, Te atoms prefer to bond with the edge carbons of small graphene flakes and serve as an effective catalyst, which helps to grow small graphene flakes by reducing their energy of formation. The successful growth can provide the high-quality monolayer graphene grains at the low temperature (∼350 °C). This facile strategy promises the mass production of graphene at mild conditions and low costs, which contributes to the green chemistry in future.

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.

Monolayer Graphene Grown on Nanoscale Pt Films Deposited on TiO2 Substrates for Micro- and Nanoelectromechanical Systems

This study presents chemical vapor deposition (CVD) growth of high-quality monolayer graphene on 100 nm-thick Pt thin films deposited on TiO2-coated silicon wafers. Conventional graphene growth on ...

High Quality Graphene Thin Films Synthesized by Glow Discharge Method in A Chemical Vapor Deposition System Using Solid Carbon Source.

Arc discharge is traditionally used to synthesize randomly arranged graphene flakes. In this paper, we substantially modify it into a glow discharge method so that the discharge current is much more reduced. The H2 and/or Ar plasma etching of the graphitic electrode (used to ignite the plasma) is hence much gentler, rendering it possible to grow graphene in thin film format. During the growth at a few mbar, there is no external carbon gas precursor introduced. The carbon atoms and/or carbon containing particles as a result of the plasma etching are emitted in the chamber, some of which undergo gas phase scattering and deposit onto the metallic catalyst substrates (Cu-Ni alloy thin films or Cu foils) as graphene sheets. It is found that high quality monolayer graphene can be synthesized on Cu foil at 900 °C. On Cu-Ni, under the same growth condition, somewhat more bilayer regions are observed. It is observed that the material quality is almost indifferent to the gas ratios, which makes the optimization of the deposition process relatively easy. Detailed study on the deposition procedure and the material characterization have been carried out. This work reveals the possibility of producing thin film graphene by a gas discharge based process, not only from fundamental point of view, but it also provides an alternative technique other than standard chemical vapor deposition to synthesize graphene that is compatible with the semiconductor planar process. As the process uses solid graphite as a source material that is rich in the crust, it is a facile and relatively cheap method to obtain high quality graphene thin films in this respect.

Molecular dynamics simulation of graphene sinking during chemical vapor deposition growth on semi-molten Cu substrate

Copper foil is the most promising catalyst for the synthesis of large-area, high-quality monolayer graphene. Experimentally, it has been found that the Cu substrate is semi-molten at graphene growth temperatures. In this study, based on a self-developed C–Cu empirical potential and density functional theory (DFT) methods, we performed systematic molecular dynamics simulations to explore the stability of graphene nanostructures, i.e., carbon nanoclusters and graphene nanoribbons, on semi-molten Cu substrates. Many atomic details observed in the classical MD simulations agree well with those seen in DFT-MD simulations, confirming the high accuracy of the C–Cu potential. Depending on the size of the graphene island, two different sunken-modes are observed: (i) graphene island sinks into the first layer of the metal substrate and (ii) many metal atoms surround the graphene island. Further study reveals that the sinking graphene leads to the unidirectional alignment and seamless stitching of the graphene islands, which explains the growth of large single-crystal graphene on Cu foil. This study deepens our physical insights into the CVD growth of graphene on semi-molten Cu substrate with multiple experimental mysteries well explained and provides theoretic references for the controlled synthesis of large-area single-crystalline monolayer graphene.

Transfer-Free, Large-Scale, High-Quality Monolayer Graphene Grown Directly onto the Ti (10 nm)-buffered Substrates at Low Temperatures

Graphene has attracted the interest of many researchers due to various its advantages such as high mobility, high transparency, and strong mechanical strength. However, large-area graphene is grown at high temperatures of about 1,000 °C and must be transferred to various substrates for various applications. As a result, transferred graphene shows many defects such as wrinkles/ripples and cracks that happen during the transfer process. In this study, we address transfer-free, large-scale, and high-quality monolayer graphene. Monolayer graphene was grown at low temperatures on Ti (10nm)-buffered Si (001) and PET substrates via plasma-assisted thermal chemical vapor deposition (PATCVD). The graphene area is small at low mTorr range of operating pressure, while 4 × 4 cm2 scale graphene is grown at high working pressures from 1.5 to 1.8 Torr. Four-inch wafer scale graphene growth is achieved at growth conditions of 1.8 Torr working pressure and 150 °C growth temperature. The monolayer graphene that is grown directly on the Ti-buffer layer reveals a transparency of 97.4 % at a wavelength of 550 nm, a carrier mobility of about 7,000 cm2/V×s, and a sheet resistance of 98 W/□. Transfer-free, large-scale, high-quality monolayer graphene can be applied to flexible and stretchable electronic devices.

Raman spectroscopy estimation of the carrier concentration and the value of strain in monolayer graphene films grown on 4H-SiC

Comprehensive study of high-quality monolayer graphene samples grown by thermal destruction of the Si-face of the 4H-SiC substrate was carried out. Analysis of the data obtained by Raman spectroscopy and angle-resolved photoemission spectroscopy suggest the need to use the Fermi velocity in the graphene layer under study to obtain a correct estimate of the electron concentration and strain values using Raman data. This statement is valid not only for graphene on the SiC substrate, but for graphene on any other substrate as well, since the Fermi velocity in graphene depends on the dielectric constant of the substrate.

Wafer-Scale Synthesis of Graphene on Sapphire: Toward Fab-Compatible Graphene.

The adoption of graphene in electronics, optoelectronics, and photonics is hindered by the difficulty in obtaining high-quality material on technologically relevant substrates, over wafer-scale sizes, and with metal contamination levels compatible with industrial requirements. To date, the direct growth of graphene on insulating substrates has proved to be challenging, usually requiring metal-catalysts or yielding defective graphene. In this work, a metal-free approach implemented in commercially available reactors to obtain high-quality monolayer graphene on c-plane sapphire substrates via chemical vapor deposition is demonstrated. Low energy electron diffraction, low energy electron microscopy, and scanning tunneling microscopy measurements identify the Al-rich reconstruction ° of sapphire to be crucial for obtaining epitaxial graphene. Raman spectroscopy and electrical transport measurements reveal high-quality graphene with mobilities consistently above 2000 cm2 V-1 s-1 . The process is scaled up to 4 and 6 in. wafers sizes and metal contamination levels are retrieved to be within the limits for back-end-of-line integration. The growth process introduced here establishes a method for the synthesis of wafer-scale graphene films on a technologically viable basis.