Synthesis of conducting transparent few-layer graphene directly on glass at 450 °C

International audiencePost-growth transfer and high growth temperature are two major hurdles that research has to overcome to get graphene out of research laboratories. Here, using a plasma-enhanced chemical vapour deposition process, we demonstrate the large-area formation of continuous transparent graphene layers at temperatures as low as 450 °C. Our few-layer graphene grows at the interface between a pre-deposited 200 nm Ni catalytic film and an insulating glass substrate. After nickel etching, we are able to measure the optical transmittance of the layers without any transfer. We also measure their sheet resistance directly and after inkjet printing of electrical contacts: sheet resistance is locally as low as 500 Ω sq-1. Finally the samples equipped with printed contacts appear to be efficient humidity sensors

Impact of Amorphous-C/Ni Multilayers on Ni-Induced Layer Exchange for Multilayer Graphene on Insulators

Layer exchange growth of amorphous carbon (a-C) is a unique technique for fabricating high-quality multilayer graphene (MLG) on insulators at low temperatures. We investigated the effects of the a-C/Ni multilayer structure on the quality of MLG formed by Ni-induced layer exchange. The crystal quality and electrical conductivity of MLG improved dramatically as the number of a-C/Ni multilayers increased. A 600 °C-annealed sample in which 15 layers of 4-nm-thick a-C and 0.5-nm-thick Ni were laminated recorded an electrical conductivity of 1430 S/cm. This value is close to that of highly oriented pyrolytic graphite synthesized at approximately 3000 °C. This improvement is likely related to the bond weakening in a-C due to the screening effect of Ni. We expect that these results will contribute to low-temperature synthesis of MLG using a solid-phase reaction with metals

Conversion of self-assembled monolayers into nanocrystalline graphene: Structure and electric transport

Graphene-based materials have been suggested for applications ranging from nanoelectronics to nanobiotechnology. However, the realization of graphene-based technologies will require large quantities of free-standing two-dimensional (2D) carbon materials with tuneable physical and chemical properties. Bottom-up approaches via molecular self-assembly have great potential to fulfil this demand. Here, we report on the fabrication and characterization of graphene made by electron-radiation induced cross-linking of aromatic self-assembled monolayers (SAMs) and their subsequent annealing. In this process, the SAM is converted into a nanocrystalline graphene sheet with well defined thickness and arbitrary dimensions. Electric transport data demonstrate that this transformation is accompanied by an insulator to metal transition that can be utilized to control electrical properties such as conductivity, electron mobility and ambipolar electric field effect of the fabricated graphene sheets. The suggested route opens broad prospects towards the engineering of free-standing 2D carbon materials with tuneable properties on various solid substrates and on holey substrates as suspended membranes.Comment: 30 pages, 5 figure

Fabrication and characterization of graphene-superconductor devices

Graphene is the first single-atom-thick two-dimensional material and exhibits a large set of interesting properties. This thesis consists of two parts. The first regards the growth of large-area graphene using chemical vapor deposition (CVD). Graphene is grown using CVD on copper catalyst showing high quality with charge carrier mobility exceeding 3000 cm2/Vs.Wet chemical etching is used to transfer graphene to insulating substrates. Cu is removed using either diluted HNO3 or diluted HCl with a small amount of added H2O2. To allow for faster transfer and avoid consuming copper, a hydrogen-bubbling method is developed to delaminate graphene from Cu. Graphene transferred this way shows properties similar to those of graphene transferred using wet etching.To avoid transfer-related issues, graphene is grown non-catalytically directly on insulating substrates such as SiO2, Al2O3, and Si3N4. The grain size is only ~10 nm due to the lack of catalytic activity during growth. Such graphene shows inferior electronic properties with mobility in the order of ~tens of cm2/Vs. Despite that, sheet resistance around kΩ, the possibility to grow several layer thick films, and optical properties similar to those of pristine graphene make it an interesting material.A method for cleaning graphene mechanically using atomic force microscopy (AFM) is developed. By appropriate choice of the applied force, atomically smooth (roughnes

Fabrication and characterization of graphene-superconductor devices

Graphene is the first single-atom-thick two-dimensional material and exhibits a large set of interesting properties. This thesis consists of two parts. The first regards the growth of large-area graphene using chemical vapor deposition (CVD). Graphene is grown using CVD on copper catalyst showing high quality with charge carrier mobility exceeding 3000 cm2/Vs.Wet chemical etching is used to transfer graphene to insulating substrates. Cu is removed using either diluted HNO3 or diluted HCl with a small amount of added H2O2. To allow for faster transfer and avoid consuming copper, a hydrogen-bubbling method is developed to delaminate graphene from Cu. Graphene transferred this way shows properties similar to those of graphene transferred using wet etching.To avoid transfer-related issues, graphene is grown non-catalytically directly on insulating substrates such as SiO2, Al2O3, and Si3N4. The grain size is only ~10 nm due to the lack of catalytic activity during growth. Such graphene shows inferior electronic properties with mobility in the order of ~tens of cm2/Vs. Despite that, sheet resistance around kΩ, the possibility to grow several layer thick films, and optical properties similar to those of pristine graphene make it an interesting material.A method for cleaning graphene mechanically using atomic force microscopy (AFM) is developed. By appropriate choice of the applied force, atomically smooth (roughnes

Near Room-temperature Synthesis of Transfer-free Graphene Films

Materials Science EngineeringGraphene is a single layer of only carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice and is a basic building block for graphitic materials of all other dimensionalities and is the basis for understanding of physical or chemical properties of the various carbon-based materials. In graphene lattice, the carbon bonds are sp2 hybridized, where the in-plane σ bond is one of the strongest bonds in materials and the out-of-plane π bond, which contributes to a delocalized network of electrons, is responsible for the electron conduction of graphene and provides the weak interaction between graphene and substrates. In addition, the energy dispersion at K and K’ points in the first Brillouin zone is linear, which closely resembles the Dirac spectrum for massless fermions. With these unique structural characteristics and the band structure, graphene has shown exceptional physical properties, which have attracted enormous research interest in both scientific and engineering fields. One of the most remarkable properties of graphene is that the charge carriers behave as Dirac fermions, which give rise to extraordinary effects such as mobility up to 200,000 cm2V-1s-1, ballistic transport distances of up to a micron at room temperature, half-integer quantum Hall effect. Graphene also possesses the excellent mechanical strength, such as the breaking strength of ~ 42 Nm-1 and the Young’s modulus of 1.0 TPa. Its thermal conductivity is measured with a value of ~ 5,000WmK-1. In addition, graphene is highly transparent, with absorption of ~ 2.3% towards visible light. For applying these outstanding properties of graphene to various fields, the development of various methods has stimulated a vast amount of research in recent years and thus there are four different methods; the mechanical or chemical exfoliation of graphite, sublimation of SiC, and CVD growth on metal substrates. Among these, large-area graphene films are currently best synthesized via the CVD process onto polycrystalline metal surfaces and this method is the most promising method for realization of graphene-based flexible optoelectronic display technology. However, even in the CVD process, there are several problems for direct device applications, such as additional transfer process, introduction of high process temperature and high process costs. Therefore, in this study, we describe a very low-temperature and transfer-free approach to controllably deposit graphene films onto desired substrates, which we refer to as Diffusion-Assisted Synthesis (DAS) method. Our synthesis methodology exploits the properties of a ‘diffusion couple’, wherein a Ni thin film is deposited first on the substrate, and solid carbon (graphite powders) is then deposited on top of the Ni, and allowed to diffuse along the Ni layer to create a thin graphene film at the Ni-substrate interface. First we conducted our DAS process on the hard substrate, such as SiO2 layers, at temperatures below 260 °C. In this case, the as-synthesized graphene films are wrinkle-free and smooth over large areas. Interestingly, we find that the morphologies of regions covered with mono- and bi-layer graphene resemble those of the grains, and the multi-layer graphene ridges, the grain boundaries in the Ni thin films. The electrical properties of graphene layers on SiO2/Si obtained at low-temperature (T ≤ 260 °C) have been evaluated with back-gated graphene-based field-effect transistor (FET) devices and by using transmission line model method. The estimated hole mobility is ~667cm2V-1s-1 at room temperature in ambient conditions and the sheet resistance is found to be ~1,000Ω per square, suggesting that the as-synthesized graphene films are of reasonable quality. We also find that graphene films obtained range from 25 °C to 260 °C have similar structural quality, but the surface coverage of graphene on SiO2 shows a strong dependence on the growth temperature. Furthermore, we have explored the possibility of using our approach to grow graphene in air instead of inert Ar atmospheres. Surprisingly, we find that the surface morphology, areal coverage and Raman structure of the graphene films grown in Ar as well as in air are similar. In addition, we studied the characteristics of the DAS-graphene grown on SiO2/Si substrates at high-temperature growth regime (300 °C ≤ T ≤ 600 °C). In this study, we observe the formation of nanocrystalline graphene layers by precipitation and the morphologies of graphene films are largely independent of process temperature, time and microstructure of poly-Ni films in this process regime. Also we find that the layers contain no graphene ridges at all. From above experimental results and theoretical estimation using Fisher model and DFT calculations, we propose a mechanism for the growth of graphene layers in the DAS process as follows: (1) the resulting C atoms from solid carbon source are transported across the Ni film primarily along the grain boundaries to the Ni-SiO2 interface at low-temperatures and (2) upon reaching the Ni-SiO2 interface, C atoms precipitate out as graphene at the grain boundaries and (3) excess C atoms reaching the graphene ridges, diffuse laterally along the graphene-Ni (111) interface and lead to the growth of graphene over large areas, driven by the strong affinity of C atoms to self-assemble and expand the sp2 lattice. Finally, we demonstrated the applicability of our approach to prepare large-area graphene on the soft material substrates, such as PDMS, PMMA, and glass. To this purpose, we use T ≤ 160 °C and do not anneal the Ni thin films so as to minimize thermal degradation of the substrates. In contrast to graphene on SiO2, the graphene films on plastic and glass substrates are continuous over large areas at all temperatures, possibly due to the decrease in distance between grain boundaries. The as-grown layers on the soft material substrates are nanocrystalline graphene.ope

Low temperature direct growth of graphene patterns on flexible glass substrates catalysed by a sacrificial ultrathin Ni film

Direct deposition of graphene on substrates would avoid costly, time consuming and defect inducing transfer techniques. In this paper we used ultrathin films of Ni, with thickness ranging from 5 to 50 nm, as a catalytic surface on glass to seed and promote chemical vapor deposition (CVD) of graphene. Different regimes and dynamics were studied for various parameters including temperature and reaction time. When a critical temperature (700 °C) was reached, Ni films retracted and holes formed that are open to the glass surface, where graphene deposited. After CVD, the residual Ni could be etched away and the glass substrate with graphene regained maximum transparency (>90%). The fact that we could achieve low growth temperatures indicates the potential of the technique to widen the range of substrate materials over which graphene can be directly deposited. We demonstrated this by depositing graphene patterns on ultrathin, 100 μm thick, sheet of glass with low strain point (670 °C), particularly suitable for flexible electronic and optoelectronic devices.Peer ReviewedPostprint (published version

Graphene Photonics and Optoelectronics

The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential to be in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultra-wide-band tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light emitting devices, to touch screens, photodetectors and ultrafast lasers. Here we review the state of the art in this emerging field.Comment: Review Nature Photonics, in pres