As-grown Graphene Research Articles

Facile fabrication of biosensors based on Cu nanoparticles modified as-grown CVD graphene for non-enzymatic glucose sensing

Copper nanoparticles modified as-grown monolayer and bilayer graphene on copper substrates have been used for non-enzymatic electrochemical glucose sensing. The high quality, continuous, full coverage graphene on copper substrate act as a good electrochemical electrode which obviating the tedious transfer processes. The present sensor shows excellent performance for glucose detection including a wide linear range of 0.02–2.3 mM, a low detection limit (1.39 μM, S/N = 3), high sensitivity (379.31 μA mM−1 cm−2), fast response time (＜5 s), good selectivity to the general coexisted interferences, reliable stability, etc. We believe this kind of electrode has a great potential for practical glucose sensing applications and also promise for other biomolecules detection.

Enhanced Thermoelectric Performance of As-Grown Suspended Graphene Nanoribbons.

Conventionally, graphene is a poor thermoelectric material with a low figure of merit (ZT) of 10-4-10-3. Although nanostructuring was proposed to improve the thermoelectric performance of graphene, little experimental progress has been accomplished. Here, we carefully fabricated as-grown suspended graphene nanoribbons with quarter-micron length and ∼40 nm width. The ratio of electrical to thermal conductivity was enhanced by 1-2 orders of magnitude, and the Seebeck coefficient was several times larger than bulk graphene, which yielded record-high ZT values up to ∼0.1. Moreover, we observed a record-high electronic contribution of ∼20% to the total thermal conductivity in the nanoribbon. Concurrent phonon Boltzmann transport simulations reveal that the reduction of lattice thermal conductivity is mainly attributed to quasi-ballistic phonon transport. The record-high ratio of electrical to thermal conductivity was enabled by the disparate electron and phonon mean free paths as well as the clean samples, and the enhanced Seebeck coefficient was attributed to the band gap opening. Our work not only demonstrates that electron and phonon transport can be fundamentally tuned and decoupled in graphene but also indicates that graphene with appropriate nanostructures can be very promising thermoelectric materials.

Adhesion energy of as-grown graphene on nickel substrates via StereoDIC-based blister experiments

The interfacial interactions between as-grown graphene (G) and its metal substrates play a crucial role in large-scale graphene transfer and governing anticorrosion properties. However, few studies have been conducted on quantifying the adhesion energy of as-grown graphene, especially, in systems such as nickel/graphene (Ni/G) with strong interfacial interactions. In this study, for the first time, a novel full-field three dimensional (3D) blister test via stereo-digital image correlation (StereoDIC) was developed to characterize the interfacial interaction between as-grown graphene on nickel (Ni) and copper (Cu) substrates, respectively. To minimize effects of surface roughness and residue stress on adhesion energy, the graphene growth processes on both Cu and Ni substrates were carefully controlled to assure nearly identical growth processes in terms of temperature, growth time, heating and cooling rates. The adhesion energy of as-grown graphene on Ni was measured to be 6.775±0.556Jm−2, which is more than 7 times higher than that of as-grown graphene on Cu. Instead of van der Waals bonds, the Ni/G interface exhibited adhesion that is close to covalent bonding. We believe that this full-field 3D blister test could be further extended for evaluating strong interfacial interactions between graphene and other metal substrates.

Ultrafast Growth of Uniform Multi-Layer Graphene Films Directly on Silicon Dioxide Substrates.

To realize the applications of graphene in electronics, a large-scale, high-quality, and uniform graphene film should first be placed on the dielectric substrates. Challenges still remain with respect to the current methods for the synthesis graphene directly on the dielectric substrates via chemical vapor deposition, such as a low growth rate and poor quality. Herein, we present an ultrafast method for direct growth of uniform graphene on a silicon dioxide (SiO2/Si) substrate using methanol as the only carbon source. A 1 × 1 cm2 SiO2/Si substrate square was almost fully covered with graphene within 5 min, resulting in a record growth rate of ~33.6 µm/s. This outcome is attributed to the quick pyrolysis of methanol, with the help of trace copper atoms. The as-grown graphene exhibited a highly uniform thickness, with a sheet resistance of 0.9–1.2 kΩ/sq and a hole mobility of up to 115.4 cm2/V·s in air at room temperature. It would be quite suitable for transparent conductive electrodes in electrophoretic displays and may be interesting for related industrial applications.

Structural properties of graphene on copper substrates

Introduction. Studies of the substrate influence on graphene properties are relevant due to the high sensitivity of this two-dimensional material to the smallest effects. At the same time, studies dedicated to the separation of direct substrate influence and residual synthesis effects on graphene properties are not described in the literature in sufficient detail. Materials and methods. This paper presents the results of a study of the structural properties of as-grown and transferred graphene on copper substrates by Raman spectroscopy and atomic force microscopy. Results and discussion. Using Raman spectroscopy, we found that the mechanical strain in the as-grown grapheme is increased, while negligibly small in transferred graphene, the strain coefficients being of 0.22– 0.33 and ∼ 0 %, respectively. Using atomic force microscopy, we showed that this effect does not relate to the copper surface irregularities: the average roughness of the substrate surface for as-grown graphene was of 20.4 nm, while for the transferred graphene it was of 62.0 nm, the height difference being of almost an order of magnitude greater for the latter. This contradiction was explained in terms of residual synthesis effects present in case of as-grown graphene. Conclusion. Taking into account the analyzed substrate effects is important both for the refinement of methods for the initial characterization of as-grown graphene, and for practical applications requiring graphene transfer to corrugated and perforated substrates, membranes and objects of complex shape.

All-Dry Transfer of Graphene Film by van der Waals Interactions.

We report a method that uses van der Waals interactions to transfer continuous, high-quality graphene films from Ge(110) to a different substrate held by hexagonal boron nitride carriers in a clean, dry environment. The transferred films are uniform and continuous with low defect density and few charge puddles. The transfer is effective because of the weak interfacial adhesion energy between graphene and Ge. Based on the minimum strain energy required for the isolation of film, the upper limit of the interfacial adhesion energy is estimated to be 23 meV per carbon atom, which makes graphene/Ge(110) the first as-grown graphene film that has a substrate adhesion energy lower than that of typical van der Waals interactions between layered materials. Our results suggest that graphene on Ge can serve as an ideal material platform to be integrated with other material systems by a clean assembly process.

Direct growth of few-layer graphene on AlN-based resonators for high-sensitivity gravimetric biosensors.

We present the successful growth of few-layer graphene on top of AlN-based solidly mounted resonators (SMR) using a low-temperature chemical vapour deposition (CVD) process assisted by Ni catalysts, and its effective bio-functionalization with antibodies. The SMRs are manufactured on top of fully insulating AlN/SiO2 acoustic mirrors able to withstand the temperatures reached during the CVD growth of graphene (up to 650 °C). The active AlN films, purposely grown with the c-axis tilted, effectively excite shear modes displaying excellent in-liquid performance, with electromechanical coupling and quality factors of around 3% and 150, respectively, which barely vary after graphene integration. Raman spectra reveal that the as-grown graphene is composed of less than five weakly coupled layers with a low density of defects. Two functionalization protocols of the graphene are proposed. The first one, based on a covalent binding approach, starts with a low-damage O2 plasma treatment that introduces a controlled density of defects in graphene, including carboxylic groups. After that, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride/N-hydroxysuccinimide (EDC/NHS) chemistry is used to covalently bind streptavidin molecules to the surface of the sensors. The second functionalization protocol is based on the non-covalent bonding of streptavidin on hydrophobic graphene surfaces. The two protocols end with the effective bonding of biotinylated anti-IgG antibodies to the streptavidin, which leaves the surface of the devices ready for possible IgG detection.

Metal-Catalyst-Free Growth of Patterned Graphene on SiO2 Substrates by Annealing Plasma-Induced Cross-Linked Parylene for Optoelectronic Device Applications.

A metal-catalyst-free method for the direct growth of patterned graphene on an insulating substrate is reported in this paper. Parylene N is used as the carbon source. The surface molecule layer of parylene N is cross-linked by argon plasma bombardment. Under high-temperature annealing, the cross-linking layer of parylene N is graphitized into nanocrystalline graphene, which is a process that transforms organic to inorganic and insulation to conduction, while the parylene N molecules below the cross-linking layer decompose and vaporize at high temperature. Using this technique, the direct growth of a graphene film in a large area and with good uniformity is achieved. The thickness of the graphene is determined by the thickness of the cross-linking layer. Patterned graphene films can be obtained directly by controlling the patterns of the cross-linking region (lithography-free patterning). Graphene-silicon Schottky junction photodetectors are fabricated using the as-grown graphene. The Schottky junction shows good performance. The application of direct-grown graphene in optoelectronics is achieved with a great improvement of the device fabrication efficiency compared with transferred graphene. When illuminated with a 792 nm laser, the responsivity and specific detectivity of the detector measured at room temperature are 275.9 mA/W and 4.93 × 109 cm Hz1/2/W, respectively.

A Gas Sensing Channel Composited with Pristine and Oxygen Plasma-Treated Graphene.

Oxygen plasma treatment has been reported as an effective way of improving the response of graphene gas sensors. In this work, a gas sensor based on a composite graphene channel with a layer of pristine graphene (G) at the bottom and an oxygen plasma-treated graphene (OP-G) as a covering layer was reported. The OP-G on top provided oxygen functional groups and serves as the gas molecule grippers, while the as-grown graphene beneath serves as a fast carrier transport path. Thus, the composite channel (OP-G/G) demonstrated significantly improved response in NH3 gas sensing tests compared with the pristine G channel. Moreover, the OP-G/G channel showed faster response and recovering process than the OP-G channel. Since this kind of composite channel is fabricated from chemical vapor deposited graphene and patterned with standard photolithography, the device dimension was much smaller than a gas sensor fabricated from reduced graphene oxide and it is favorable for the integration of a large number of sensing units.

Ultrasensitive Gas Sensors Based on Vertical Graphene Nanowalls/SiC/Si Heterostructure.

Gas sensors, which play an important role in the safety of human life, cover a wide range of applications including intelligent systems and detection of harmful and toxic gases. It is known that graphene is an ideal and attractive candidate for gas sensing due to its high surface area and excellent mechanical, electrical, optical, and thermal properties. However, in order to fully realize its potential as a commercial gas sensor, demand for a graphene-based device of low-limit detection, high sensitivity, and fast response time needs to be met. Here, we demonstrate a metal/insulator/semiconductor (MIS) based gas sensor consisting of as-grown epitaxial graphene nanowalls (EGNWs)/silicon carbide (SiC)/silicon (Si) structure. The unique edge dominant three-dimensional (3D) EGNWs based MIS device achieved an extraordinarily low limit of detection (0.5 ppm) and unprecedented sensitivity (82 μA/ppm/cm2 for H2) with a fast response of shorter than 500 ms. These unique properties of our MIS device are attributed to the abundance of vertically oriented nanographitic edges and structural defects that act as extra-favorable adsorption sites and exhibit fast electron-transfer kinetics through the edges. Our experimental findings can pave the way for the realization of high-performance 3D graphene-based gas sensor devices.

Step-by-step monitoring of CVD-graphene during wet transfer by Raman spectroscopy.

Transfer acts as a crucial bridge between the chemical vapor deposition (CVD) synthesis of large-scale graphene and its applications, but the quality evolution of a graphene film during transfer remains unclear. Here we use scanning Raman spectroscopy to monitor as-grown graphene during each step of wet transfer including floating on etchant solution, loaded onto a target substrate, and with additional annealing. Results show that the etchant solution results in strong compressive strain and p-type doping to floating graphene, but both are significantly reduced after the sample is loaded and rinsed especially for the doping. An annealing treatment increases the compressive strain in graphene but hardly its doping level. Moreover, when a poly(methyl methacrylate) (PMMA) layer is used to assist the transfer, it does not only increase the p-type doping of floating graphene but also lowers the crystalline quality of annealed graphene. Therefore, to obtain graphene with better quality, besides the attempts of improving CVD synthesis for its larger domain sizes, universal and easy-to-use polymer-free transfer techniques must be developed as well.

Influence of surface energy and elastic strain energy on the graphene growth in chemical vapor deposition

Polycrystalline metal substrates such as copper (Cu) have been intensively used to grow graphene in chemical vapor deposition (CVD) technique. It has been observed that crystal orientations affect the quality of graphene produced to some degree. The existence of crystal orientations caused graphene to grow randomly on top of Cu which also resulted on the formation of polycrystalline graphene. Despite this, the influence of crystal orientations on the quality of graphene produced are not yet fully understood. There are two possible factors that could affect graphene growth from crystal orientation point of view; surface energy and elastic strain energy. The understanding towards these both factors might beneficial to control the quality of graphene. This paper thus aims to highlight the influence of surface energy and elastic strain energy on the graphene growth in CVD. Substrate used were single crystal Cu with (111), (110) and (100) orientations. The graphene was grown inside a closed reaction chamber with the presence of argon (Ar), hydrogen (H2) and methane (CH4) gases with partial pressure ratio of 0.6: 0.2: 0.2 at 1 atm, 1000 ˚C in 30 minutes. The quality of the as-grown graphene was identified using Raman spectroscopy. The Raman spectra show the existence of graphene peak for all the Cu substrates. The calculation of ID/IG ratio revealed that the Cu (100) possessed the lowest amount of defects compared to Cu(110) and Cu(111). While I2D/IG ratio fluctuated between 0.22 to 0.34 suggested that the crystal orientation does not control the thickness of graphene layer at these reaction conditions. The usage of higher CH4 partial pressure produced a thicker graphene. It is assumed that the thickness of the graphene exceeded the critical thickness thus elastic strain energy becomes the dominant factor in controlling graphene growth. Larger lattice mismatch causes major defects on graphene and this result has been shown in graphene grown on top of Cu(111). These findings thus would give a new insight to tailor the high-quality large-area graphene growth in CVD.

Concomitant enhancement of electron-phonon coupling and electron-electron interaction in graphene decorated with ytterbium

The interplay between electron-electron interaction and electron-phonon coupling has been one of the key issues in graphene as it can provide information on the origin of enhanced electron-phonon coupling in graphene by foreign atoms. In ytterbium-decorated graphene on SiC substrate, electron-phonon coupling exhibits strong enhancement compared to that of as-grown graphene. Based on angle-resolved photoemission study, the presence of ytterbium is also found to result in the decrease of Fermi velocity, revealing the enhancement of electron-electron interaction within the Fermi liquid theory. Our finding on the concomitant enhancement of electron-electron interaction and electron-phonon coupling suggests a possibility of the interplay between the two representative many-body interactions in graphene decorated with foreign atoms.

Facile Fabrication of Large-Area Atomically Thin Membranes by Direct Synthesis of Graphene with Nanoscale Porosity.

Direct synthesis of graphene with well-defined nanoscale pores over large areas can transform the fabrication of nanoporous atomically thin membranes (NATMs) and greatly enhance their potential for practical applications. However, scalable bottom-up synthesis of continuous sheets of nanoporous graphene that maintain integrity over large areas has not been demonstrated. Here, it is shown that a simple reduction in temperature during chemical vapor deposition (CVD) on Cu induces in-situ formation of nanoscale defects (≤2-3 nm) in the graphene lattice, enabling direct and scalable synthesis of nanoporous monolayer graphene. By solution-casting of hierarchically porous polyether sulfone supports on the as-grown nanoporous CVD graphene, large-area (>5 cm2 ) NATMs for dialysis applications are demonstrated. The synthesized NATMs show size-selective diffusive transport and effective separation of small molecules and salts from a model protein, with ≈2-100× increase in permeance along with selectivity better than or comparable to state-of-the-art commercially available polymeric dialysis membranes. The membranes constitute the largest fully functional NATMs fabricated via bottom-up nanopore formation, and can be easily scaled up to larger sizes permitted by CVD synthesis. The results highlight synergistic benefits in blending traditional membrane casting with bottom-up pore creation during graphene CVD for advancing NATMs toward practical applications.

Van der Waals Epitaxy of Antimony Islands, Sheets, and Thin Films on Single-Crystalline Graphene.

Antimony (Sb) nanostructures, including islands, sheets, and thin films, of high crystallinity were epitaxially grown on single-crystalline graphene through van der Waals interactions. Two types of graphene substrates grown by chemical vapor deposition were used, the as-grown graphene on Cu(111)/ c-sapphire and the transferred graphene on SiO2/Si. On the as-grown graphene, deposition of ultrathin Sb resulted in two growth modes and associated morphologies of Sb. One was Sb islands grown in Volmer-Weber (VW) mode, and the other was Sb sheets grown in Frank-van der Merve (FM) mode. In contrast, only Sb islands grown in VW mode were found in a parallel growth experiment on the transferred graphene. The existence of Sb sheets on the as-grown graphene was attributed to the remote epitaxy between Sb and Cu underneath the graphene. In addition, Sb thin films were grown on both the as-grown and transferred graphene substrates. Both films indicated high quality, and no significant difference can be found between these two films. This work unveiled two epitaxial alignments between Sb(0001) and graphene, namely, Sb [101̅0]∥graphene [10] for Sb islands and Sb [21̅1̅0]∥graphene [10] for Sb sheets. For Sb thin films on graphene, the epitaxial alignment followed that of Sb islands, implying that Sb thin films originated from the continued growth of Sb islands. Last, Raman spectroscopy was used to probe the state of graphene under ultrathin Sb. No strain, doping, or disorder was found in the graphene postgrowth of Sb. The knowledge of the interface formation between ultrathin Sb and graphene provides a valuable foundation for future research on van der Waals heterostructures between antimonene and graphene.

Synthesis of high quality graphene on capped (1 1 1) Cu thin films obtained by high temperature secondary grain growth on c-plane sapphire substrates

We propose a novel growth technique, in which graphene is synthesized on capped Cu thin films deposited on c-plane sapphire. The cap is another sapphire plate which is just laid upon the Cu thin film, in direct contact with it. Thanks to this ‘contact cap’, Cu evaporation can be suppressed at high temperature and the 400 nm-thick Cu films can be annealed above 1000 °C, resulting in (1 1 1)-oriented grains of millimeter size. Following this high temperature annealing, graphene is grown by chemical vapor deposition during the same pump-down operation, without removing the contact cap. The orientation and doping type of the as-grown graphene were first studied, using low energy electron diffraction, as well as high resolution angle-resolved photoemission spectroscopy. In particular, the orientation relationships between the graphene and copper thin film with respect to the sapphire substrate were precisely determined. We find that the graphene sheets exhibit a minimal rotational disorder, with ~90% of the grains aligned along the copper high symmetry direction. Detailed transport measurements were also performed using field-effect transistor structures. Carrier mobility values as high as 8460 cm2 V−1 s−1 have been measured on top gate transistors fabricated directly on the sapphire substrate, by etching the Cu film from underneath the graphene sheets. This is by far the best carrier mobility value obtained to date for graphene sheets synthesized on a thin film-type metal substrate.

Graphene Grown on Ni Foam: Molecular Sensing, Graphene-Enhanced Raman Scattering, and Galvanic Exchange for Surface-Enhanced Raman Scattering Applications

The growth of graphene on an irregular three-dimensional (3D) Ni structure is demonstrated to be an interesting platform for molecular sensing, graphene-enhanced Raman scattering, and surface-enhanced Raman scattering (SERS) applications after galvanic exchange of Ag+ ions. Raman, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX), optical images, and diffuse reflectance demonstrate that graphene grows in a multilayer fashion with different stacking configurations. Statistics performed employing Raman show that the as-grown graphene can be classified into two main stacking configurations: AB (or Bernal stacking) and rotated graphene, which are separated by a full width at half-maximum (fwhm) threshold of ∼30 cm–1 corresponding to the 2D Raman band. The rotated stacking senses low concentrations of methylene blue (MB), whereas the AB stacking seems to be much less sensitive upon molecular adsorption. The galvanic exchange of Ag leads to agglomerates preferentially formed on top of gra...

Direct growth of high crystallinity graphene from water-soluble polymer powders

The use of solid-state carbon sources is effective to produce graphene by safe and low-cost chemical vapor deposition (CVD) process. Water-soluble polymers are generally environmentally friendly and have great potential on large-scale green production of graphene. Here, we systematically study the growth of graphene from water-soluble polymers on copper foils. Two different conversion ways are adopted to investigate the growth mechanism of graphene from water-soluble polymers. We find that the metal-binding functional group hydroxyl strongly influences the vaporization of water-soluble polymers on Cu foils, which hinders the formation of graphene films by rapid thermal treatment. In direct CVD process using water-soluble polymer powders as precursors, oxygenated functional groups in polymers can enhance the crystallinity of as-grown graphene in contrast to solid hydrocarbons without containing oxygen (e.g. polyethylene). Large and continuous graphene films of high quality are synthesized from polyvinyl alcohol and polyethylene glycol. Nitrogen doping in graphene can be easily realized by using nitrogen-containing water-soluble polymers (e.g. polyvinyl pyrrolidone).

Magnetic Dipole Resonance and Coupling Effects Directly Enhance the Raman Signals of As-Grown Graphene on Copper Foil by over One Hundredfold

Large-area graphene is commonly prepared through chemical vapor deposition (CVD); in situ and nondestructive methods for its characterization are desirable. In this paper, we demonstrate a practical method—exploiting magnetic dipole resonance and coupling effects—with which the Raman signals of graphene on copper (Cu) foil can be directly, faithfully, and greatly enhanced. The magnetic dipole resonance of a silicon nanoparticle (SiNP) can effectively couple its electromagnetic field with the Cu foil to induce an enormous electric field located solely at the position of the SiNP on the graphene. The coupled electromagnetic field can lead to hot spots of high electric field intensity (E2/E02 = 123.2) within the graphene. Even when we positioned only a few SiNPs upon the graphene/Cu foil, we obtained a Raman signal enhancement (ca. 206 times) much greater than that of graphene transferred onto a 300 nm oxide film (ca. 12 times). From a series of experiments comparing the Raman signals of graphene before and ...

Self-Assembly of Metal Oxide Nanoparticles in Liquid Metal toward Nucleation Control for Graphene Single-Crystal Arrays

Summary We have achieved the spontaneous assembly of alumina nanoparticles on a liquid metal surface and further synthesized a very-large-scale graphene single-crystal array. The effective electrical field established as a result of the electrical charge of the alumina nanoparticles in the liquid metal ensures the ordered self-assembly of the alumina islands and lays the foundation for precise spatial control of graphene nucleation, leading to the formation of graphene single-crystal arrays. In addition, controlling the density of the alumina in the liquid metal can precisely tune the periodicity of the arrays. This simple strategy, which is unlike the usual solid/aqueous solution system, opens up new territory for nanoparticle assembly. Focusing on the origin of the nanoparticle interaction that drives the self-assembly of ordered structure in the liquid metal system is a promising avenue for triggering more interesting applications.