John B. McManus

Bio: John B. McManus is an academic researcher from Trinity College, Dublin. The author has contributed to research in topics: Acoustic wave & Ion beam. The author has an hindex of 9, co-authored 22 publications receiving 442 citations.

Production and processing of graphene and related materials

Abstract: © 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.

Ultrafast Carrier Dynamics and Bandgap Renormalization in Layered PtSe 2

Abstract: Carrier interactions in 2D nanostructures are of central importance not only in condensed-matter physics but also for a wide range of optoelectronic and photonic applications. Here, new insights into the behavior of photoinduced carriers in layered platinum diselenide (PtSe2 ) through ultrafast time-resolved pump-probe and nonlinear optical measurements are presented. The measurements reveal the temporal evolution of carrier relaxation, chemical potential and bandgap renormalization in PtSe2 . These results imply that few-layer PtSe2 has a semiconductor-like carrier relaxation instead of a metal-like one. The relaxation follows a triple-exponential decay process and exhibits thickness-dependent relaxation times. This occurs along with a band-filling effect, which can be controlled based on the number of layers and may be applied in saturable absorption for generating ultrafast laser pulses. The findings may provide means to study many-body physics in 2D materials as well as potentially leading to applications in the field of optoelectronics and ultrafast photonics.

Defect sizing, separation, and substrate effects in ion-irradiated monolayer two-dimensional materials

Abstract: Precise and scalable defect engineering of two-dimensional (2D) nanomaterials is acutely sought after in contemporary materials science. Here, we present defect engineering in monolayer graphene and molybdenum disulfide (${\mathrm{MoS}}_{2}$) by irradiation with noble gas ions at 30 keV. Two ion species of different masses were used in a gas field ion source microscope: helium (${\mathrm{He}}^{+}$) and neon (${\mathrm{Ne}}^{+}$). A detailed Raman spectroscopy study was performed and a defect activation model applied with marked differences between the ion systems at a given dose. We propose that disparities between the ion systems are explained by different defect yields and defect sizes. Expanding on existing models, we suggest that the average defect size is smaller for supported than freestanding graphene and that the rate of defect production is larger. We infer that low-energy secondary atoms from the substrate play a significant role in defect production, creating smaller defects relative to those created by the primary ion beam. Furthermore, a similar model was also applied to supported ${\mathrm{MoS}}_{2}$, another promising member of the 2D material family. Defect yields for both ions were obtained for ${\mathrm{MoS}}_{2}$, demonstrating their different interaction with the material and facilitating comparison with other irradiation conditions in the literature.

Wafer-Scale Fabrication of Recessed-Channel PtSe 2 MOSFETs With Low Contact Resistance and Improved Gate Control

Abstract: Wafer-scale fabrication of PtSe2 MOSFETs was demonstrated by photolithography on Pt films directly selenized at 400 °C. Taking advantage of the unique property of PtSe2 to transition from a semiconductor to a semimetal as its thickness increases beyond a few monolayers, channel recess was adapted for improving gate control while keeping the contact resistance below 0.01 $\Omega \cdot \text {cm}$ . The wafer-scale fabrication resulted in uniform device characteristics so that average instead of best results was reported. For example, the drain currents at ${V}_{\text {GS}} = -10$ V, ${V}_{\text {DS}} = -1$ V were $25~\pm ~5$ , 57 ± 8, and $618~\pm ~17~\mu \text{A}/\mu \text{m}$ for 4-, 8-, and 12-nm-thick PtSe2, respectively. The corresponding peak transconductances were 0.20 ± 0.1, 0.60 ± 0.05, and $1.4~\pm ~0.1~\mu \text{S}/\mu \text{m}$ . The forward-current cutoff frequency of 12-nm-thick PtSe2 MOSFETs was 42 ± 5 MHz, whereas the corresponding frequency of maximum oscillation was 180 ± 30 MHz. These results confirmed the application potential of PtSe2 for future-generation thin-film transistors.

PtSe2 grown directly on polymer foil for use as a robust piezoresistive sensor

Abstract: Robust strain gauges are fabricated by growing PtSealt;subagt;2alt;/subagt; layers directly on top of flexible polyimide foils. These PtSealt;subagt;2alt;/subagt; layers are grown by low-temperature, thermally-assisted conversion of predeposited Pt layers. Under applied flexure the PtSealt;subagt;2alt;/subagt; layers show a decrease in electrical resistance signifying a negative gauge factor. The influence of the growth temperature and film thickness on the electromechanical properties of the PtSealt;subagt;2alt;/subagt; layers is investigated. The best-performing strain gauges fabricated have a superior gauge factor to that of commercial metal-based strain gauges. Notably, the strain gauges offer good cyclability and are very robust, surviving repeated peel tests and immersion in water. Furthermore, preliminary results indicate that the stain gauges also show potential for high-frequency operation. This host of advantageous properties, combined with the possibility of further optimization and channel patterning, indicate that PtSealt;subagt;2alt;/subagt; grown directly on polyimide holds great promise for future applications.

Principles Of Fluorescence Spectroscopy

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Superconductivity in the intercalated graphite compounds C6Yb and C6Ca

Abstract: This thesis concerns the discovery of superconductivity in the intercalated graphite compounds C6Yb and C6Ca. A novel technique for synthesis of these intercalates has been developed, and is presented in detail. These two materials are shown to superconduct at 6.5K and 11.5K respectively. The superconductivity is demonstrated by measurements of the magnetisation and resistivity. Initial measurements of the superconducting transition of these materials as a function of pressure shows an increase in the transition with increasing pressure.