Use of X-ray K-edge Tomography and Interferometry Imaging Techniques for the Studies of Brominated Flame Retardants

The work presented in this dissertation is based on the studies of flame retardancy performance of various formulations consisting of brominated flame retardants (BFRs: Saytex 8010 and Green Armor) and their synergist, antimony trioxide (Sb2O3) in high impact polystyrene (HIPS). Chemical flame retardants are incorporated in polymers to improve their flame inhibition for optimal applications in electrical and electronic devices, furniture, printers and more. These flame retardant polymer blends are studied using the Underwriters Laboratory vertical burn test (UL 94) and X-ray imaging techniques such as X-ray K-edge absorption tomography and X-ray grating interferometry. The UL 94 burn test is initially performed to assess the flammability behavior of flame retardant samples before X-ray imaging methods of burnt and pristine polymer blends. Because the UL 94 test bars are formulated with varying concentrations of a brominated flame retardant (Saytex 8010® or Green Armor®) and a synergist, Sb2O3 into a high impact polystyrene (HIPS), samples pass or fail the UL 94 plastics flammability test based on the burn time and other factors. Then, the X-ray imaging techniques are used to reveal internal features for the flame retardant performance during the burn. The Underwriters Laboratory 94 test bars are imaged with X-ray K-edge absorption tomography between 12 to 32 keV to assess the bromine and antimony concentration gradient across char layers of partially burnt samples. X-ray grating interferometry on partially burnt samples shows gas bubbles and dark-field scattering ascribed to residual blend inhomogeneity. In addition, X-ray single-shot grating interferometry is used to record X-ray movies of test samples during heating intended to mimic the UL 94 plastics flammability test. Key features such as char layer, gas bubble formation, micro-cracks, and dissolution of the flame retardant in the char layer regions are used in understanding the efficiency of the flame retardant and synergist. The samples that pass the UL 94 test have a thick, highly visible char layer, low bromine and antimony concentration in the char layer as well as an interior rich in gas bubbles. Growth of gas bubbles from flame retardant thermal decomposition is noted in the X-ray phase contrast movies

Controlling light-matter interactions with two-dimensional semiconductors at cryogenic temperatures

Efficient interactions between solid-state systems and photons are the basis for the emerging quantum technologies. An outstanding challenge in facilitating light-matter interactions can be solved by the introduction of an optical resonator. By trapping the photons in a small volume, the interaction time with the solid-state system is increased in such quantum optics experiments. This thesis focuses on light-matter interactions with two-dimensional semiconductor transition-metal dichalcogenides. Strongly bound electron-hole pairs (excitons) in transition-metal dichalcogenides exhibit large oscillator strength and offer a valley degree-of-freedom in the band structure that interacts with specific polarization of light, this makes them an excellent prospect for quantum optics experiments. In this work, we realize light-matter interactions with monolayer tungsten diselenide in two experiments using different optical resonators. In the first experiment, we investigated a hybrid system combining localized plasmons, on the surface of gold nanodisks, with a monolayer tungsten diselenide. The coherent interference of excitons with plasmons yield an asymmetric optical response, known as Fano line-shape, in the limit of weak light-matter coupling. This optical response of the hybrid system is corroborated with a three-level model. In addition, magnetic field-induced valley-dependent exciton energy splitting is harnessed to achieve chiral reflection. The second experiment develops a modular tunable cavity at cryogenic temperatures. A major technological challenge involving scalable cryogenic experiments is mechanical vibrations. After a thorough understanding of the functionality of a closed-cycle cryostat, a variety of vibration-reduction techniques were applied to develop an open cavity setup. In comparison to the vibrations on the standard closed-cycle cryostat, we attain 50-fold reduction to reach the root-mean-square stability of less than 100 pm over the entire period of a cooling cycle. This enables the operation of a high-finesse cavity at low temperatures. Subsequently, the versatility of the platform was demonstrated in a controlled experiment with monolayer tungsten diselenide. Exciton-polaritons were observed in the high cooperativity strong-coupling regime.Effiziente Wechselwirkungen zwischen Festkörpersystemen und Photonen bilden die Grundlage für die aufstrebenden Quantentechnologien. Die zentrale Herausforderung bei der Kontrolle von Wechselwirkungen zwischen Licht undMaterie kann durch die Verwendung eines optischen Resonators gelöst werden. Durch Einfangen der Photonen in einem kleinen Volumen wird die Wechselwirkungszeit mit dem Festkörpersystem in solchen quantenoptischen Experimenten erhöht. Diese Arbeit konzentriert sich auf Wechselwirkungen zwischen Licht und Materie in zweidimensionalen halbleitenden Übergangsmetalldichalkogeniden. Stark gebundene Elektron-Loch-Paare (Exzitonen) in Übergangsmetalldichalkogeniden weisen eine große Oszillatorstärke auf und bieten einen weiteren Pseudospin-Freiheitsgrad in der Bandstruktur, der mit spezifischer Polarisation des Lichts interagiert. In dieser Arbeit realisieren wir in zwei Experimenten Licht-Materie-Wechselwirkungenmit monolagigem Wolframdiselenid mit unterschiedlichen optischen Resonatoren. Im ersten Experiment untersuchten wir ein Hybridsystem, das lokalisierte Plasmonen auf der Oberfläche von Goldnanoscheiben mit monolagigem Wolframdiselenid kombiniert. Kohärente Interferenz von Exzitonen mit Plasmonen ergibt eine asymmetrische spektrale Antwort - als Fano-Linienformbekannt - im Grenzfall schwacher Licht-Materie-Kopplung. Diese optische Antwort des Hybridsystems wird durch ein Drei-Niveau-Modell bestätigt. Zusätzlich wird die magnetfeldinduzierte Energieaufspaltung der Exzitonen genutzt, um chirale Reflexion zu ermitteln. Das zweite Experiment entwickelt einen modular abstimmbaren optischen Resonator bei kryogenen Temperaturen. Eine große technologische Herausforderung bei skalierbaren kryogenen Experimenten sind mechanische Schwingungen. Basierend auf einem gründlichen Verständnis der Funktionalität eines Kryostaten mit geschlossenem Kreislauf wurden verschiedene Techniken zur Schwingungsreduzierung implementiert, um einen Aufbau mit offenen optischen Resonatoren zu entwickeln. Im Vergleich zu Vibrationen des Standardsystems wurde eine 50-fache Verminderung erreicht, und somit über den gesamten Zeitraum eines Kühlzyklus eine mittlere Stabilität von weniger als 100 pm gewährleistet. Dies ermöglicht den Betrieb eines optischen Resonators mit hoher Finesse bei niedrigen Temperaturen. Anschließend wurde die Vielseitigkeit des Systems in einemkontrollierten Experimentmit monolagigem Wolframdiselenid demonstriert. Exziton-Polaritonen wurden im Regime starker Kopplung mit hoher Kooperativität beobachtet

Scanning Tunneling Spectroscopy on Graphene Nanostructures

This thesis describes investigations on graphene nanostructures by the means of scanning tunneling microscopy (STM)and spectroscopy (STS) in ultra high vacuum at low temperature (5.5 K), focused on their electronic structure on the local scale. The experiments are based on structurally highly perfect epitaxial graphene on Ir(111) [gr/Ir(111)], but extend the range towards new graphene based nanomaterials. The first topic comprises the development of new nanomaterials which keep the structural coherency of epitaxial graphene on Ir(111) at a reduced electronic substrate interaction, in particular concerning graphene's quasi-relativistic Dirac particles. Therefor, we present the first study on graphene quantum dots (GQDs) on silver (gr/Ag). In STS, we observe the Ag(111) surface state on 15 ML of Ag on Ir(111), study its behavior in the presence of graphene, and discuss its role in the observation of Dirac electron confinement on GQDs. We find the surface state suppressed in 1 ML of Ag on Ir(111). In a next step we present an experimental advancement towards a system, where the metallic surface states are completely absent, namely oxygen covered Ir(111) [O/Ir(111)]. In an STS study, we discover new oxygen superstructures on iridium under graphene and two types of charge effects in the GQDs' local density of states (LDOS). We present the first unambiguous experimental observation of Dirac electron confinement on GQDs. We calculate the Dirac dispersion relation on the basis of our experimental data and confirm the efficient decoupling by DFT calculations and the direct observation of a Dirac feature in point spectroscopy and characteristic electron scattering processes. In addition to the benefit for the observation of Dirac confinement, our findings gain universal insight into the decoupling of graphene's electronic system from the metallic substrate by oxygen intercalation. The studies are extended towards the unoccupied surface state spectrum at high energies in form of image potential states (IPSs). For the first time we experimentally prove the size dependence of IPSs due to confinement on GQDs acting as a quantum well. We explain the occurrence of a strongly pronounced state, which is not the ground state, by an interplay of the LDOS and momentum conservation during tunneling. The positions of the IPSs can be tuned by chemical gating, which means the experimental realization of a quantum well tunable in both width and depth. We discuss the benefit of a direct measurement of the local workfunction for the determination of the local doping level in graphene intercalation compounds. In a next step we propose a route how to experimentally access the binding situation at the boundaries of GQDs on Ir(111), using the advanced technique of Inelastic Electron Tunneling Spectroscopy (IETS). Finally, we observe metallic features in the LDOS which are related to one dimensional defects in an extended monolayer of epitaxial graphene on Ir(111)

Functional optical surfaces by colloidal self-assembly: Colloid-to-film coupled cavities and colloidal lattices

Future developments in nanophotonics require facile, inexpensive and parallelizable fabrication methods and need a fundamental understanding of the spectroscopic properties of such nanostructures. These challenges can be met through colloidal self-assembly where pre-synthesized colloids are arranged over large areas at reasonable cost. As so-called colloidal building blocks, plasmonic nanoparticles and quantum dots are used because of their localized light confinement and localized light emission, respectively. These nanoscopic colloids acquires new hybrid spectroscopic properties through their structural arrangement. To explore the energy transfer between these nanoscopic building blocks, concepts from physical optics are used and implemented with the colloidal self-assembly approach from physical chemistry. Through an established synthesis, the nanocrystals are now available in large quantities, any they receive the tailored spectroscopic properties through directed self-assembly. Moreover, the tailored properties of the colloids and the use of stimuli-responsive polymers allow a functionality that goes beyond current developments. The basics developed in this habilitation thesis can lead to novel functional devices in the field of smart sensors, dynamic light modulators, and large-area quantum devices.:1 Abstract 2 2 State of the art 4 2.1 Metallic and semiconductive nanocrystals as colloidal building blocks 4 2.2 Concept of large-scale colloidal self-assembly 7 2.3 Functional optical nanomaterials by colloidal self-assembly 9 2.4 Scope 13 2.5 References 14 3 Single colloidal cavities 20 3.1 Nanorattles with tailored electric field enhancement 20 4 Colloidal -to-film coupled cavities 31 4.1 Template-assisted colloidal self-assembly of macroscopic magnetic metasurfaces 31 4.2 Single particle spectroscopy of radiative processes in colloid-to-film-coupled nanoantennas 50 4.3 Active plasmonic colloid-to-film coupled cavities for tailored light-matter interactions 65 5 Colloidal polymers 74 5.1 Direct observation of plasmon band formation and delocalization in quasi-infinite nanoparticle chains 74 6 Colloidal lattice 84 6.1 Hybridized guided-node resonances via colloidal plasmonic self-assembled grating 84 6.2 Mechanotunable surface lattice resonances in the visible optical range by soft lithography templates and directed self-assembly 94 6.3 Tunable Circular Dichroism by Photoluminescent Moiré Gratings 103 7 Conclusion and perspective 112 8 Appendix 113 8.1 Further publications during the habilitation period 113 8.2 Curriculum vitae of the author 116 9 Acknowledgments 117 10 Declaration 118Zukünftige Entwicklungen in der Nanophotonik erfordern einfache, kostengünstige und parallelisierbare Herstellungsmethoden und benötigen ein grundlegendes Verständnis der spektroskopischen Eigenschaften solcher Nanostrukturen. Diese Herausforderungen können durch kolloidale Selbstorganisation erfüllt werden, bei der kostengünstige und zuvor synthetisierte Kolloide großflächig angeordnet werden. Als sogenannte kolloide Bausteine werden wegen ihrer lokalisierten Lichtfokussierung unterhalb der Beugungsbegrenzung plasmonische Nanopartikel sowie wegen ihrer lokalisierten Lichtemission Quantenpunkte verwendet. Diese nanoskopischen Kolloide werden in dieser Habilitationsschrift verwendet und durch Selbstanordnung in ihre gewünschte Nanostruktur gebracht, die neue hybride Eigenschaften aufweist. Um den Energietransfer zwischen diesen nanoskopischen Bausteinen zu untersuchen, werden Konzepte aus der physikalischen Optik verwendet und mit dem kolloidalen Selbstorganisationskonzept aus der physikalischen Chemie großflächig umgesetzt. Durch eine etablierte Synthese sind die Nanokristalle nun in großen Mengen verfügbar, wobei sie durch gerichtete Selbstorganisation die gewünschten spektroskopischen Eigenschaften erhalten. Darüber hinaus ermöglicht die Verwendung von stimulierbaren Polymeren eine Funktionalität, die über die bisherigen Entwicklungen hinausgeht. Die in dieser Habilitationsschrift entwickelten Grundlagen können bei der Entwicklung neuartiger Funktionsgeräte im Bereich für intelligente Sensorik, dynamischer Lichtmodulatoren und großflächiger Quantengeräte genutzt werden.:1 Abstract 2 2 State of the art 4 2.1 Metallic and semiconductive nanocrystals as colloidal building blocks 4 2.2 Concept of large-scale colloidal self-assembly 7 2.3 Functional optical nanomaterials by colloidal self-assembly 9 2.4 Scope 13 2.5 References 14 3 Single colloidal cavities 20 3.1 Nanorattles with tailored electric field enhancement 20 4 Colloidal -to-film coupled cavities 31 4.1 Template-assisted colloidal self-assembly of macroscopic magnetic metasurfaces 31 4.2 Single particle spectroscopy of radiative processes in colloid-to-film-coupled nanoantennas 50 4.3 Active plasmonic colloid-to-film coupled cavities for tailored light-matter interactions 65 5 Colloidal polymers 74 5.1 Direct observation of plasmon band formation and delocalization in quasi-infinite nanoparticle chains 74 6 Colloidal lattice 84 6.1 Hybridized guided-node resonances via colloidal plasmonic self-assembled grating 84 6.2 Mechanotunable surface lattice resonances in the visible optical range by soft lithography templates and directed self-assembly 94 6.3 Tunable Circular Dichroism by Photoluminescent Moiré Gratings 103 7 Conclusion and perspective 112 8 Appendix 113 8.1 Further publications during the habilitation period 113 8.2 Curriculum vitae of the author 116 9 Acknowledgments 117 10 Declaration 11

Production and processing of graphene and related materials

© 2020 The Author(s). 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 resourceconsuming, 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

Production and processing of graphene and related materials

© 2020 The Author(s). 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 resourceconsuming, 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

Production and processing of graphene and related materials

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 sp2 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 sp2 carbon network by π–π 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, can be functionalized and we summarize emerging approaches to covalently and noncovalently functionalize MoS2 both in the liquid and on substrate. Section IX describes some of the most popular characterization techniques, ranging from optical detection to the measurement of the electronic structure. Microscopies play an important role, although macroscopic techniques are also used for the measurement of the properties of these materials and their devices. Raman spectroscopy is paramount for GRMs, while PL is more adequate for non-graphene LMs (see section IX.2). Liquid based methods result in flakes with different thicknesses and dimensions. The qualification of size and thickness can be achieved using imaging techniques, like scanning probe microscopy (SPM) or transmission electron microscopy (TEM) or spectroscopic techniques. Optical microscopy enables the detection of flakes on suitable surfaces as well as the measurement of optical properties. Characterization of exfoliated materials is essential to improve the GRM metrology for applications and quality control. For grown GRMs, SPM can be used to probe morphological properties, as well as to study growth mechanisms and quality of transfer. More generally, SPM combined with smart measurement protocols in various modes allows one to get obtain information on mechanical properties, surface potential, work functions, electrical properties, or effectiveness of functionalization. Some of the techniques described are suitable for 'in situ' characterization, and can be hosted within the growth chambers. If the diagnosis is made 'ex situ', consideration should be given to the preparation of the samples to avoid contamination. Occasionally cleaning methods have to be used prior to measurement