Production and processing of graphene and related materials
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Production and processing of graphene and related materials, Claudia Backes et al., 2020, 2D Materials, 7 (2), DOI: http://dx.doi.org/10.1088/2053-1583/ab1e0auntranslatedDownload Item:
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. SectionIV 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 sectionV.
Transfer technologies are the focus of sectionVI. 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. SectionVII 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.
Sponsor
Grant Number
European Commission
696656
European Commission
785219
Publisher:
IOP PublishingCollections
Series/Report no:
2D Materials;7, 2Availability:
Full text availableKeywords:
Processing of layered materials, Inks of layered materials, Characterization of layered materials, Functionalization of layered materials, Synthesis of graphene and related materials, Growth of layered materialsDOI:
http://dx.doi.org/10.1088/2053-1583/ab1e0aMetadata
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