School of Materials Science and Engineering, Nanyang Technological University, Singapore.
1.3. Definitions of Graphene and Graphene Related Items
Accordingly, there have been various terms for graphene and graphene related materials, which also follow Ref. [2].
Graphene is defined as a single-layer atomic carbon sheet, with a perfect hexagonal configuration of sp2 carbon atoms, which is either isolated free-standing or deposited on a substrate. The lateral dimensions can vary in range from several nanometers to micrometers for free-standing graphene and even to the macroscale for deposits on substrates.
Graphene layer means a single-layer atomic carbon sheet with hexagonal configuration of sp2 carbon atoms, which is only present within a given carbon material structure, having either 3D ordering (graphitic) or other (e.g., turbostratic or rotationally faulted) characteristics. Therefore, it is a building block (unit) of a certain structure.
Turbostratic carbon is used to describe three-dimensional configurations, with sp2-bonded carbon atoms, but without clearly defined atomic carbon layers. The name is a combination of the words “turbo” (rotated) and “strata” (layer), therefore it is also known as rotationally faulted.
Bilayer graphene and trilayer graphene stand for 2D or sheet-like layered materials that contain two (2) and three (3) stacked graphene layers, respectively, which could be further specified as “AB-stacked bilayer graphene” or “rotationally faulted trilayer graphene”.
Multilayer graphene (MLG) is similar to the above graphenes, but the number of atomic carbon sheets is between 2 and 10, which are well-defined and tightly stacked with an extended lateral dimension.
Few-layer graphene (FLG) is actually a group of multilayer graphene, with 2–5 layers of graphene.
Graphite nanoplates, graphite nanosheets and graphite nanoflakes include 2D graphite materials with stackings of ABA or ABCA, with a thickness and/or lateral dimension of less than 100 nm.
Exfoliated graphite refers to multilayer structured items, which are made by partially exfoliating graphite into thin multilayer sheets, while still retaining the 3D crystal stacking of graphite, by using thermal, chemical or mechanical methods.
Graphene nanosheet refers to a single atomic carbon sheet, with carbon atoms arranged hexagonally, with sp2 hybridization, which is either free-standing or deposited on substrates. According to the definition of nanomaterials, it should have a lateral dimension of < 100 nm. However, graphene nanosheet or nanosheets have been widely used in the literature to represent all 2D graphene materials, and are used similarly in this book.
Graphene microsheet refers to a single-layer atomic carbon sheet with sp2-bonded carbon atoms arranged hexagonally, which is either free-standing or deposited on substrates, with a lateral dimension ranging from 100 nm to 100 μm.
Graphene nanoribbon is a single-layer atomic carbon strip, with sp2-bonded carbon atoms having a hexagonal configuration, which is either free-standing or deposited on substrates. The width should be less than 100 nm.
Graphene quantum dots (GQD) is an alternative name for graphene nanosheets or few-layer graphene nanosheets, with lateral dimensions of < 10 nm (or average of 5 nm).
Graphene oxide (GO) stands for all the graphenes, which are chemically derived through oxidation and exfoliation, in which the basal planes contain a large amount of oxygen, in the form of oxygen containing functional groups.
Graphite oxide is a bulk solid made by oxidizing graphite through functionalization of the basal planes, with the interlayer spacing expanded to a certain degree. It is also known as expanded graphite.
Reduced graphene oxide (rGO) is the product of graphene oxide that is reduced by using various reducing methods, such as chemical, thermal, microwave, photo-chemical, photo-thermal and microbial/bacterial; the graphene oxide is reduced by eliminating oxygen or oxygen containing functional groups. rGO has characteristics similar to that of graphene.
Graphenization is used to describe the processes that are used to prepare graphene layers, either in 2D or 3D forms. Two related terms are (i) carbonization, meaning the conversion of organic materials into a carbonaceous solid, and (ii) graphitization, referring to the crystallization of carbonaceous materials into 3D ordered structures.
Graphene materials, graphene-based materials, graphene nanomaterials, graphene-family nanomaterials are also used to imply more generally the collection of the above discussed 2D materials that contain the word graphene.
Graphenic carbon materials represent the broadest class of carbonaceous solids that are formed by elemental carbon with bounding of sp2-hybridization, in either 2D or 3D forms.
There are also numerous ways that have been used in the open literature to represent newly emerged materials based on or related to graphene, in one way or another. Examples are briefly discussed below.
Graphene oxide nanosheet is graphene oxide (monolayer) with a lateral dimension of < 100 nm, according to the definition of nanotechnology. However, in the literature, graphene oxide nanosheet or graphene oxide nanosheets are also used to stand for those with a lateral size of > 100 nm.
Few-layer graphene nanoribbons belong to the category of graphene nanoribbons, with 2–5 atomic carbon layers.
Multilayer graphene oxide film refers to a multilayer structure with restacking of graphene oxide monolayer sheets that are deposited on substrates. A similar term is graphene film.
There are also some terms and usages that appear in the literature but do not follow the above discussed principles in the nomenclature, which are presented as follows.
Graphite layer is a term that is not recommended because graphite is usually used to refer to the 3D crystals. Therefore, graphene layer is a more appropriate term to replace graphite layer.
Graphene nanosheet is similar to graphene oxide nanosheet or reduced graphene nanosheet. Although it is not recommended to use this term in Ref. [2], its usage can be quite frequently found in the literature, while it will be used in this book.
Graphene nanoplates, graphene nanoplatelets: these terms are used for some industry products with microscale lateral dimension, but are not recommended in our scientific nomenclature for reasons given above. “Graphene” does not need the prefix “nano” to indicate thinness, and instead “nano” used in this way should indicate the lateral dimension.
Graphitic is also not recommended in Ref. [2], which is not adopted in this book.
Graphene materials can also be incorporated or hybridized in various ways with various other components, thus leading to additional terms, such as folded, wrinkled, activated, decorated or functionalized [8]. One group of such materials, i.e., hybrids with inorganic components, will be discussed in detail in Chapter 2 and Chapter 3 in this book.