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- Backes, Claudia, et al.
(författare)
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Production and processing of graphene and related materials
- 2020
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Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 7:2
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Tidskriftsartikel (refereegranskat)abstract
- We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach. The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step. Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V. Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resource-consuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer. There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown. Section VIII discusses advances in GRM functionalization. A broad range of organic molecules can be anchored to the sp(2) basal plane by reductive functionalization. Negatively charged graphene can be prepared in liquid phase (e.g. via intercalation chemistry or electrochemically) and can react with electrophiles. This can be achieved both in dispersion or on substrate. The functional groups of GO can be further derivatized. Graphene can also be noncovalently functionalized, in particular with polycyclic aromatic hydrocarbons that assemble on the sp(2) carbon network by pi-pi stacking. In the liquid phase, this can enhance the colloidal stability of SLG/FLG. Approaches to achieve noncovalent on-substrate functionalization are also discussed, which can chemically dope graphene. Research efforts to derivatize CNMs are also summarized, as well as novel routes to selectively address defect sites. In dispersion, edges are the most dominant defects and can be covalently modified. This enhances colloidal stability without modifying the graphene basal plane. Basal plane point defects can also be modified, passivated and healed in ultra-high vacuum. The decoration of graphene with metal nanoparticles (NPs) has also received considerable attention, as it allows to exploit synergistic effects between NPs and graphene. Decoration can be either achieved chemically or in the gas phase. All LMs,
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| 8. |
- Xia, Chao
(författare)
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Characterizations of as grown and functionalized epitaxial graphene grown on SiC surfaces
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The superior electronic and mechanical properties of Graphene have promoted graphene to become one of the most promising candidates for next generation of electronic devices. Epitaxial growth of graphene by sublimation of Si from Silicon Carbide (SiC) substrates avoids the hazardous transfer process for large scale fabrication of graphene based electronic devices. Moreover, the operation conditions can potentially be extended to high temperatures, voltages and frequencies. This thesis is focused on characterizations of as grown and functionalized epitaxial graphene grown on both Si-face and C-face SiC. Synchrotron radiation-based techniques are employed for detailed investigations of the electronic properties and surface morphology of as grown and functionalized graphene.Large area and homogeneous monolayer (ML) graphene has been possible to grow on SiC(0001) substrates by sublimation, but efforts to obtain multilayer graphene of similar quality have been in vain. A study of the transport behavior of silicon atoms through carbon layers was therefore performed for the purpose to gain a better understanding of the growth mechanism of graphene on Si-face SiC. It showed that a temperature of about 800°C is required for Si intercalation into the interface to take place. Intercalation of Si was found to occur only via defects and domain boundaries which probably is the reason to the limited growth of multilayer graphene. Annealing at 1000-1100°C induced formation of SiC on the surface and after annealing above 1200°C Si started to de-intercalate and desorb/sublimate.Different alkali metals were found to affect graphene grown in SiC quite differently. Li started to intercalate already at room temperature by creating cracks and defects, while K, Rb and Cs were found unable to intercalate into the graphene/SiC interface. Effects induced by the alkali metal Na on graphene grown on both Si-face and C-face SiC were therefore studies. For the Si-face, partial intercalation of Na through graphene was observed on both 1 ML and 2 ML areas directly after Na deposition. Annealing at a temperature of about 75°C strongly promoted Na intercalation at the interface. The intercalation was confirmed to start at domain boundaries between 1 ML and 2 ML areas and at stripes/streaks on the 1 ML areas. Higher annealing temperature resulted in desorption of Na from the sample surface. Also for C-face graphene, a strong n-type doping was observed directly after Na deposition. Annealing at temperatures from around 120 to 300 °C was here found to result in a considerable π-band broadening, interpreted to indicate penetration of Na in between the graphene layers and at the graphene SiC interface.The thermal stability of graphene based electronic devices can depend on the choice of contact material. Studies of the stability and effects induced by two commonly used metals (Pt and Al) on Si-face graphene were carried out after deposition and after subsequent annealing at different temperatures. Both Al and Pt were found to be good contact materials at room temperature. Annealing at respectively ~400 ºC and ~ 800 ºC was found to trigger intercalation of Al and Pt into the graphene/SiC interface, and induce quasi-free-standing bilayer electronic properties. Contacts of Pt can thus withstand higher temperatures than Al contacts. For Al inhomogeneous islands of different ordered phases were observed to form on the surface during annealing, while this was not the case for Pt. The initial single π-band structure was in the Al case restored after annealing at ~1200 ºC although some Al remained detectable from the sample. For Pt, the bilayer graphene electronic properties induced by intercalation were thermally stable up to 1200ºC. In the case of Al the stability and effects induced on C-face graphene were also investigated for comparison, and significant differences were revealed. An ordered Al-Si-C compound was found to form after annealing at temperatures between ca. 500ºC and 700ºC. Formation of this compound was accompanied with a large reduction of graphene in the surface region. Annealing at temperatures above 800°C resulted in a gradual decomposition of this compound and regrowth of graphene. No Al signal could be detected after annealing C-face graphene at 1000°C.Graphene grown on C-face SiC has attracted high interest since its mobility has been reported to be one order of magnitude higher compared to Si-face graphene. C-face graphene has moreover been claimed to be fundamentally different compared to Si-face graphene. A rotational disorder between adjacent graphene layers has been suggested that effectively decouples the graphene layers and result in monolayer electronic properties of multilayer C-face graphene. The domain/grain size is typically much smaller for C-face graphene and the number of graphene layers less uniform than on Si-face graphene. Using LEEM and micro-LEED we showed that there is no rotational disorder between adjacent layers within the domains/grains but that they had different azimuthal orientations. Using nano-APRES, we recently also revealed that multilayer Cface graphene show multiple π-bands and Bernal stacking, similar to multilayer Si-face graphene.
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