Site-specific stamping of graphene micro-patterns over large areas using flexible stamps

Site-specific stamping has the potential of becoming a low-cost, high-throughput method for depositing specific-shaped graphene micro-patterns over large areas on a wide variety of substrates. The use of an approach involving flexible stamps presented here represents an important advance towards reaching that potential. This approach entails lithographic creation (dry etching) of high-quality micro-pillar arrays of highly oriented pyrolytic graphite (HOPG) over large areas. This is followed by embedding the micro-pillar arrays in polydimethylsiloxane (PDMS), and detaching them from the HOPG base. This results in flexible stamps containing embedded HOPG micro-pillar arrays with freshly cleaved stamping surfaces. The flexible HOPG/PDMS stamps are then brought into contact with substrate surfaces to site-specifically stamp graphene or few-layer graphene (FLG) arrays over large areas. The freshly cleaved nature of the micro-pillar surfaces in the flexible stamps, the low elastic modulus of the flexible stamps and the elimination of sidewall deposits on the micro-pillars allow for more uniform stamping, relative to the use of stiff HOPG stamps from earlier studies. This approach has the potential to expand the substrate choice for graphene or FLG stamping to include curved and/or flexible substrates that could have an impact on the burgeoning field of flexible/stretchable electronics.     

Remote catalyzation for direct formation of graphene layers on oxides

Direct deposition of high-quality graphene layers on insulating substrates such as SiO(2) paves the way toward the development of graphene-based high-speed electronics. Here, we describe a novel growth technique that enables the direct deposition of graphene layers on SiO(2) with crystalline quality potentially comparable to graphene grown on Cu foils using chemical vapor deposition (CVD). Rather than using Cu foils as substrates, our approach uses them to provide subliming Cu atoms in the CVD process. The prime feature of the proposed technique is remote catalyzation using floating Cu and H atoms for the decomposition of hydrocarbons. This allows for the direct graphitization of carbon radicals on oxide surfaces, forming isolated low-defect graphene layers without the need for postgrowth etching or evaporation of the metal catalyst. The defect density of the resulting graphene layers can be significantly reduced by tuning growth parameters such as the gas ratios, Cu surface areas, and substrate-to-Cu distance. Under optimized conditions, graphene layers with nondiscernible Raman D peaks can be obtained when predeposited graphite flakes are used as seeds for extended growth.     

Ion implantation assisted synthesis of graphene on various dielectric substrates

Direct synthesis of high-quality graphene on dielectric substrates is of great importance for the application of graphene-based electronics and optoelectronics. However, high-quality and uniform graphene film growth on dielectric substrates has proven challenging due to limited catalytic ability of dielectric substrates. Here, by employing a Cu ion implantation assisted method, high-quality and uniform graphene can be directly formed on various dielectric substrates including SiO2/Si, quartz glass, and sapphire substrates. The growth rate of graphene on the dielectric substrates was significantly improved due to the catalysis of Cu. Moreover, during the graphene growth process, the Cu atoms gradually evaporated away without involving any metal contamination. Furthermore, an interesting growth behavior of graphene on sapphire substrate was observed, and the results show the graphene domains growth tends to grow along the sapphire flat terraces. The ion implantation assisted approach could open up a new pathway for the direct synthesis of graphene and promote the potential application of graphene in electronics.     

Copper/silver nanoparticle incorporated graphene films prepared by a low-temperature solution method for transparent conductive electrodes

A facile one-step co-reduction and low-temperature solution process was developed to prepare Cu–graphene (Cu–G), Ag–graphene (Ag–G), and Cu–Ag–graphene (Cu–Ag–G) composite films on glass substrates. Scanning electron microscope and transmission electron microscope images show that Cu/Ag nanoparticles are either distributed on the surface of graphene nanosheets or covered by graphene. The conductivity and transparency of these films were studied, and the results show that incorporation of Cu and Ag nanoparticles into graphene films can improve film conductivity. Ag nanoparticles are more effective in improving film conductivity. The conductivity and transparency of the composite films can be balanced by introducing the optimum amount of Cu or Ag nanoparticles. The conductivity and transparency of Cu–Ag–G films with optimum metal nanoparticle concentration are as good as those of Ag–G composite films. The Cu–Ag–G films meet the requirements of low-cost, high-conductivity, and transparent films that can be used as electrode materials. Thus, the proposed low-temperature solution process is a new route to preparing low-cost transparent and conductive electrodes on various substrates, including glass and flexible polymer substrates.     

Direct growth of special-shape graphene on different templates by remote catalyzation of Cu nanoparticles

A novel method avoiding the complex transfer process is proposed to directly grow low-defect and few-layer graphene on different insulating substrates(SiO_2, Al_2O_3, etc.) by remote catalyzation of Cu nanoparticles(NPs) using ambient pressure chemical vapor deposition(APCVD). The insulating substrates with special structure are used as templates to grow wrapped graphene sheets with special shapes.Hollow graphene species are obtained by removing the substrates. The prime feature of the proposed method is using Cu NPs as catalyst rather than metal foils. The Cu NPs play an important role in the remote catalyzation during the nucleation of graphene. This method can improve the quality and relatively decrease the growth temperature of the graphene on the insulating substrates, which displays the great potential of APCVD direct growth of graphene on dielectric substrates for electronic and photovoltaic applications.     

Soft lithographic printing of patterns of stretched DNA and DNA/electronic polymer wires by surface-energy modification and transfer

Aligned and stretched lambda DNA is directed to specific locations on solid substrates. Surface-energy modification of glass substrates by using patterned polydimethylsiloxane (PDMS) stamps is used to direct DNA onto the surface-energy-modified micrometer-scale pattern through molecular combing. As an alternative, patterned and nonpatterned PDMS stamps modified with polymethylmethacrylate (PMMA) are utilized to direct the stretched DNA to the desired location and the results are compared. The DNA is elongated through molecular combing on the stamp and transfer printed onto the surfaces. PMMA-modified stamps show a more defined length of the stretched DNA, as compared to bare PDMS stamps. A combination of these two methods is also demonstrated. As an application example, transfer printing of DNA decorated with a semiconducting conjugated polyelectrolyte is shown. The resulting patterned localization of stretched DNA can be utilized for functional nanodevice structures, as well as for biological applications.     

Surface crystallographic structure insensitive growth of oriented graphene domains on Cu substrates

Cu-based chemical vapor deposition method can produce large-area graphene films, usually polycrystalline films with grain boundaries as the main defects. One way to reduce grain boundaries is to grow oriented graphene domains (OGDs), which can ultimately perfectly integrate. In contrast to previously reported methods of limiting OGD growth on Cu (1 1 1), we find that OGDs can grow on Cu substrates with a large surface crystallographic structure tolerance. Density functional theory calculations show that this is due to the single lowest energy state of graphene nucleation. The growth temperature is crucial. It must be high enough (1045 °C) to suppress mis-OGD nucleation, but not too high (1055 °C) to deteriorate OGD growth. Mis-OGD nucleation can also be caused by C impurity in Cu grains, which can be depleted by thermal pretreatment of the substrate in an oxidizing atmosphere. On the other hand, OGD growth is not sensitive to the atmosphere at growth stage within the range that we have tested.     

Facile one-step transfer process of graphene

Chemical vapour deposition (CVD) is emerging as a popular method for growing large-area graphene on metal substrates. For transferring graphene to other substrates the technique generally used involves deposition of a polymer support with subsequent etching of the metal substrate. Here we report a simpler one-step transfer process. Few-layer graphene (FLG) grown on a Cu substrate were transferred to a silanized wafer by just pressing them together. Hydrogen bonding between the hydroxyl group on FLG and the amine group on silane molecules facilitate the transfer.     

Ice-assisted electron beam lithography of graphene

We demonstrate that a low energy focused electron beam can locally pattern graphene coated with a thin ice layer. The irradiated ice plays a crucial role in the process by providing activated species that locally remove graphene from a silicon dioxide substrate. After patterning the graphene, the ice resist is easily removed by sublimation to leave behind a clean surface with no further processing. More generally, our findings demonstrate that ice-assisted e-beam lithography can be used to pattern very thin materials deposited on substrate surfaces. The procedure is performed in situ in a modified scanning electron microscope. Desirable structures such as nanoribbons are created using the method. Defects in graphene from electrons backscattered from the bulk substrate are identified. They extend several microns from the e-beam writing location. We demonstrate that these defects can be greatly reduced and localized by using thinner substrates and/or gentle thermal annealing.     

Replication of a thin polydimethylsiloxane stamp and its application to dual-nanoimprint lithography for 3D hybrid nano/micropatterns

This paper presents the fabrication of a thin and flexible polydimethylsiloxane (PDMS) stamp with a thickness of a few tens of um and its application to nanoimprint lithography (NIL). The PDMS material generally has a low elastic modulus and high adhesive characteristics. Therefore, after being treated, the thin PDMS stamp is easily deformed and torn, adhering to itself and other materials. This paper introduces the use of a metal ring around the flange of a thin PDMS stamp to assist with the handling of this material. A PDMS stamp with a motheye pattern in nanometer scale was inserted between a substrate and a microstamp with concave patterns in micrometer scale. Subsequently, three-dimensional (3D) hybrid nano/micropatterns were fabricated by pressing these two stamps and curing the resist. The fabricated hybrid patterns were measured and verified in both the microscale and nanoscale. The process, termed "dual NIL," can be applied to the fabrication of optical components or bio-sensors that require repetitive nanopatterns on micropatterns.     

Substrate considerations for graphene synthesis on thin copper films

Chemical vapor deposition on copper substrates is a primary technique for synthesis of high quality graphene films over large areas. While well-developed processes are in place for catalytic growth of graphene on bulk copper substrates, chemical vapor deposition of graphene on thin films could provide a means for simplified device processing through the elimination of the layer transfer process. Recently, it was demonstrated that transfer-free growth and processing is possible on SiO(2). However, the Cu/SiO(2)/Si material system must be stable at high temperatures for high quality transfer-free graphene. This study identifies the presence of interdiffusion at the Cu/SiO(2) interface and investigates the influence of metal (Ni, Cr, W) and insulating (Si(3)N(4), Al(2)O(3), HfO(2)) diffusion barrier layers on Cu-SiO(2) interdiffusion, as well as graphene structural quality. Regardless of barrier choice, we find the presence of Cu diffusion into the silicon substrate as well as the presence of Cu-Si-O domains on the surface of the copper film. As a result, we investigate the choice of a sapphire substrate and present evidence that it is a robust substrate for synthesis and processing of high quality, transfer-free graphene.     

Study on thermal properties of graphene foam/graphene sheets filled polymer composites

The polymer composites composed of graphene foam (GF), graphene sheets (GSs) and pliable polydimethylsiloxane (PDMS) were fabricated and their thermal properties were investigated. Due to the unique interconnected structure of GF, the thermal conductivity of GF/PDMS composite reaches 0.56 W m⁻¹ K⁻¹, which is about 300% that of pure PDMS, and 20% higher than that of GS/PDMS composite with the same graphene loading of 0.7 wt%. Its coefficient of thermal expansion is (80–137) × 10⁻⁶/K within 25–150 °C, much lower than those of GS/PDMS composite and pure PDMS. In addition, it also shows superior thermal and dimensional stability. All above results demonstrate that the GF/PDMS composite is a good candidate for thermal interface materials, which could be applied in the thermal management of electronic devices, etc.     

Silica coating onto graphene for improving thermal conductivity and electrical insulation of graphene/polydimethylsiloxane nanocomposites

Graphene possess extremely high thermal conductivity, and they have been regarded as prominent candidates to be used in thermal management of electronic devices. However, addition of graphene inevitably causes dramatic decrease in electrical insulation, which is generally unacceptable for thermal interface materials(TIMs) in real electronic industry. Developing graphene-based nanocomposites with high thermal conductivity and satisfactory electrical insulation is still a challenging issue. In this study,we developed a novel hybrid nanocomposite by incorporating silica-coated graphene nanoplatelets(Silica@GNPs) with polydimethylsiloxane(PDMS) matrix. The obtained Silica@GNP/PDMS composites showed satisfactory electrical insulation(electrical resistivity of over 10~(13)Ωcm) and high thermal conductivity of 0.497 W m-1K-1, increasing by 155% compared with that of neat PDMS, even higher than that of GNP/PDMS composites. Such high thermal conductivity and satisfactory electrical insulation is mainly attributed to the insulating silica-coating, good compatibility between components, strong interfacial bonding, uniform dispersion, and high-efficiency heat transport pathways. There is great potential for the Silica@GNP/PDMS composites to be used as high-performance TIMs in electronic industry.     

Bond-detach lithography: a method for micro/nanolithography by precision PDMS patterning

We have discovered a micro/nanopatterning technique based on the patterning of a PDMS membrane/film, which involves bonding a PDMS structure/stamp (that has the desired patterns) to a PDMS film. The technique, which we call "bond-detach lithography", was demonstrated (in conjunction with other microfabrication techniques) by transferring several micro- and nanoscale patterns onto a variety of substrates. Bond-detach lithography is a parallel process technique in which a master mold can be used many times, and is particularly simple and inexpensive.     

Direct formation of wafer scale graphene thin layers on insulating substrates by chemical vapor deposition

Direct formation of high-quality and wafer scale graphene thin layers on insulating gate dielectrics such as SiO(2) is emergent for graphene electronics using Si-wafer compatible fabrication. Here, we report that in a chemical vapor deposition process the carbon species dissociated on Cu surfaces not only result in graphene layers on top of the catalytic Cu thin films but also diffuse through Cu grain boundaries to the interface between Cu and underlying dielectrics. Optimization of the process parameters leads to a continuous and large-area graphene thin layers directly formed on top of the dielectrics. The bottom-gated transistor characteristics for the graphene films have shown quite comparable carrier mobility compared to the top-layer graphene. The proposed method allows us to achieve wafer-sized graphene on versatile insulating substrates without the need of graphene transfer.     

Rayleigh imaging of graphene and graphene layers

We investigate graphene and graphene layers on different substrates by monochromatic and white-light confocal Rayleigh scattering microscopy. The image contrast depends sensitively on the dielectric properties of the sample as well as the substrate geometry and can be described quantitatively using the complex refractive index of bulk graphite. For a few layers (<6), the monochromatic contrast increases linearly with thickness. The data can be adequately understood by considering the samples behaving as a superposition of single sheets that act as independent two-dimensional electron gases. Thus, Rayleigh imaging is a general, simple, and quick tool to identify graphene layers, which is readily combined with Raman scattering, that provides structural identification.     

Micro-contact printing of polydiacetylene liposomes using hydrophilic stamps

Micron-sized polydiacetylene (PDA) liposome patterns have been fabricated on titanium (Ti) substrates using a micro-contact printing (micro-CP) technique. Two types of stamps (PDMS and agarose) and inking methods ("soaking" and "dropping") are used for micro-CP, and we compare their effect on the morphology of the PDA patterns. The size and morphology of the patterned PDA liposomes are analysized by optical and fluorescence microscopies and atomic force microscopy (AFM). When the agarose stamp is inked by the "dropping" method, PDA patterns are most efficiently transferred to the Ti substrate. However, the thickness of the transferred PDA patterns is not homogeneous, with the edge of the transferred pattern being thicker than its center. In contrast, when the PDMS stamp is used for micro-CP, the center of the pattern is thicker than the edge. Red fluorescence patterns are readily obtained by heat treatment of the PDA-immobilized solid substrate. The intensity of the fluorescence of the samples is consistent with the results of optical microscopy and AFM experiments.     

Few-layer graphene direct deposition on Ni and Cu foil by cold-wall chemical vapor deposition

We report an alternative synthesis process, cold-wall thermal chemical vapor deposition (CVD), is replied to directly deposit single-layer and few-layer graphene films on Ar plasma treated Ni and Cu foils using CH4 as carbon source. Through optimizing the process parameters, large scale single-layer graphene grown on Ni foil is comparable to that grown on Cu foil. The graphene films were able to be transferred to other substrates such as SiO2/Si, flexible transparent PET and verified by optical microscopy, Raman microscopy and scanning electron microscopy. The sheet resistance and transmission of the transferred graphene films on PET substrate were also discussed.     

Large physisorption strain in chemical vapor deposition of graphene on copper substrates

Graphene single layers grown by chemical vapor deposition on single crystal Cu substrates are subject to nonuniform physisorption strains that depend on the orientation of the Cu surface. The strains are revealed in Raman spectra and quantitatively interpreted by molecular dynamics (MD) simulations. An average compressive strain on the order of 0.5% is determined in graphene on Cu(111). In graphene on Cu (100), MD simulations interpret the observed highly nonuniform strains.     

Comprehensive study of graphene grown by chemical vapor deposition

Graphene was grown on Cu foil by chemical vapor deposition with CH₄ as carbon source, and then was transferred onto various substrates for device applications. The structural and optical properties of graphene were investigated, comprehensively. Raman spectra indicate as-grown and transferred graphene films are homogenous monolayer graphene. Optical microscopy and scanning electron microscopy images reveal wrinkle-free and smooth surface of transferred graphene, confirming the high quality of graphene. In addition, the transferred graphene on glass exhibits excellent transmittances in the visible region (89.3 % at ~500 nm). Therefore, the results present the controllable approaches to achieve as-grown and transferred high quality graphene for the fabrication of multiple nanoelectronic devices.