The Worldwide Graphene Flake Production

Alan P. Kaulinga, Andressa T. Seefeldta, Diego P. Pisonia, Roshini C. Pradeepa, Ricardo

Bentinia, Ricardo V.B. Oliveiraa, K. S. Novoselovb, A. H. Castro Netoa,*

aCentre for Advanced 2D Materials (CA2DM), Faculty of Science, National University of Singapore

(NUS); bNational Graphene Institute and School of Physics & Astronomy, University of Manchester.

Abstract

There are hundreds of companies around the globe claiming to produce “graphene”. These

“graphene” products show a large variation in their physical properties. Hence, there is a

urgent need to establish reliable protocols and tests that would allow the assessment of such

products for their physical properties, as well as suitability for a particular application. We

developed a systematic and reliable protocol to test graphene quality using electron

microscopy, atomic force microscopy, Raman spectroscopy, elemental analysis, x-ray

photoelectron spectrometry, scanning electron microscopy, and transmission electron

microscopy that we used to study graphene flakes from 60 producers around the world. We

establish the statistical nature of the liquid phase exfoliation (LPE) of graphite and show that

the current classification of graphene flakes used in the market today is erroneous. We

propose a new classification in terms of distribution functions for number of layers and flake

size. We show unequivocally that the quality of the graphene produced in the world today is

rather poor, not optimal for most applications and that most companies are producing

graphite micro-platelets instead. We believe that this is possibly the main reason for the slow

development of graphene applications, which usually require customized solution in terms of

the properties of the graphene crystals. We argue that the creation of stringent standards for

graphene characterization and production, which take into account both the physical

properties of the material, as well as the requirements from the particular application, is the

only way forward to create a healthy and reliable worldwide graphene market.

Keywords: graphene, industrial chemistry, synthetic methods.

*Corresponding Author (AH Castro Neto)

Centre for Advanced 2D Materials (CA2DM), Faculty of Science National University of

Singapore, Block S16, Level 6, 6 Science Drive 2, Singapore 117546

Email: phycastr@nus.edu.sg

Phone: +65 6601 2575

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1. Introduction

The International Organization for Standardization (ISO) has defined a nanomaterial

as a “… material with any external dimension in the nanoscale (length range approximately

from 1 nm to 100 nm) or having internal structure or surface structure in the nanoscale”[1]. It

is also accepted that two-dimensional (2D) materials are “substances with a thickness of a

few nanometers or less”. Hence, according to these definitions 2D materials are

nanomaterials. Graphene is the best-known 2D material and the first one to be isolated in a

laboratory. Only recently[2] ISO has established the nomenclature for graphene as a single

layer, mono-crystal, of carbon atoms organized in a hexagonal lattice (lattice symmetry

p3m1, point group D3)[3].

Graphene was isolated in 2004[4] and because of its extraordinary structural, physical

and chemical properties, the industrial interest in exploring graphene applications in many

different areas, from inks to transistors, grew exponentially over the last decade[5]. At the

same time, different routes for production and synthesis of graphene are available with

different degrees of success. The original method used for graphene production via direct

mechanical exfoliation of graphite with adhesive tape has been very successful in rendering

high quality material for scientific purposes. However, this method is not scalable for

industrial use.

The most popular method for creating large area continuous graphene film has been

the chemical vapor deposition (CVD). CVD uses hydrocarbon gas as feeding stock and is

capable of producing polycrystalline films that can be square meters in size [6, 7]. This method

is a bottom-up approach since it uses simpler molecules to produce continuous films.

Although CVD growth is widely used, it has limited success due to the presence of extended

defects and voids that jeopardize the film’s structural stability and spoil its exceptional

physical properties [7, 8]. However, a common use for CVD graphene is in applications such

as touch panels and displays [9]. The production of graphene via the CVD method is a topic

on its own right and will be covered elsewhere.

Another route for the large scale graphene production, is a top-down approach,

which starts with graphite and exfoliates it by mechanical, chemical or electrochemical

means to graphene flakes. Two main methods frequently used to produce graphene flakes

are:

Oxidation of graphite producing graphene oxide (GO) that is partially de-

oxidized to produce reduced graphene oxide (rG0) [10-12];

Liquid phase exfoliation (LPE) of graphite[13].

2

Oxidation via the Hummers and Offeman’s method[14], and its variations, generically

exposes graphite to a solution of potassium permanganate, sodium nitrate, sulfuric acid and

water. In this process, oxygen atoms attach to the carbon scaffold in the form of epoxy,

carboxyl and hydroxyl groups (usually 45% of oxygen content). By the own nature of the

oxidation process, this graphene oxide (GO) has a high density of defects. Thus, GO is not a

crystal but an amorphous material. As a result GO shows poor electrical and thermal

conductivities, compared to crystalline graphene. One can partially eliminate some of the

oxygen groups by a reduction reaction (for instance, by treating GO with hydrazine hydrate

while maintaining the solution at 100 Celsius for 24 hours). Although there can be substantial

reduction in the oxygen content (around 23%)[15], the carbon framework becomes “scarred”

by the previous oxidation procedure with the presence of structural defects in the form of

vacancies, Stone-Wales defects, etc. The reduced graphene oxide (rGO), which is also

amorphous, performs electrically and thermally poorly when compared to a graphene crystal

but, however, better than GO[12]. The presence of lattice defects is easily detectable by

Raman spectroscopy since the symmetry of the lattrice is broken and the vibrational

frequencies of the carbon atoms are affected. In our studies of graphene producers we have

discarded GO and rGO samples. At the same time, we would like to point out that from our

experience a large number of the samples on the market labelled as graphene are actually

GO and rGO.

2. Liquid phase exfoliation of graphite

In order to understand the mechanism behind the liquid phase exfoliation (LPE) one

has to recall that graphite is a layered material and essentially can be seen as individual

graphene crystals stacked one on top of each other. Depending on the relative orientation of

graphene crystals in the graphite stack there are several types of graphite, such as ABA,

ABC, turbostratic, etc[16]. Furthermore, graphite as a mineral can have different morphologies

and contain different types of impurities (usually metals) depending on its geological

environment. These variations in the structure of graphite determine the most probable

cleaving positions along the crystals. In LPE, the graphite crystals undergoes mechanical

impact through shear stress and sonication in various solution (either water, solvents and

surfactants or organic solvents[13]). The chemicals help to stabilize the individual graphene

stacks after the breaking of the graphite crystal. The advantage of this method is that the

carbon network remains intact, that is, crystallinity is preserved. However, the exfoliation

process never produces 100% monolayer graphene due to the randomness of the cleaving

positions. Hence, when graphene is striped off from graphite by LPE, the product is a

3

statistical distribution of graphene stacks, that is, stacks with a certain number N of graphene

layers. Furthermore, lateral breaking also occurs and exfoliation also produces a statistical

distribution of crystallites with a given size L.

It has been established by ISO2 that, for practical purposes, a stack of graphene

crystals with more than 10 layers at room temperature (T ~ 300 K) can be treated as bulk

graphite. In order to understand the reasoning behind this conclusion one has to recall that,

in first place, a stack is formed because graphene crystals interact with each other via

attractive covalent and van der Waals forces[17]. In order to separate the individual crystals

from each other, and produce finite stacks with fewer layers, one has to break the bond

between graphene crystals by providing energy (mechanical, thermal, chemical, etc) to the

stack. The thermal energy, in the form of lattice vibrations, randomizes the electron motion

and destroys the coherence between crystals along the stack. Hence, at high temperatures

the graphene crystals behave independently from each other while at low temperatures they

are coupled forming a graphitic structure. The total thermal energy of electrons in a stack

with N layers is of order N kBT (kB is the Boltzmann constant). When the thermal energy is

larger than the interaction between crystals (E ~ 0.3 eV)[18] the random thermal electronic

motion in the graphene crystal destroys the interaction between adjacent crystals, that is,

each crystal behaves independently from each other. The condition, N kBT ~ E, sets the

energy scale below which a stack of N graphene crystals behaves as N independent

graphene mono-crystals. At room temperature (T ~ 300 K) one easily finds that N ~ E/ kBT ~

10 is the condition for independence between layers. For applications where the temperature

of the material is higher, that is, above room temperature, this condition is relaxed and even

stacks with larger number of graphene layers will behave as a collection of independent

graphene monolayers.

The physical properties of a stack of graphene layers can be very sensitive to the

number of layers. Consider, for instance, the energy required to bend a stack with N layers,

the so-called bending stiffness, or bending rigidity[18, 19], which is given by: Y a3 N3 , where Y

(= 1012 N/m) is the in-plane Young modulus of graphene and a (= 0.34 nm) is the inter-

crystal distance. Thus, the bending rigidity is a fast growing function of the number of layers,

that is, it increases with N3 (for one layer, the bending energy is of the order of 1 eV). Notice

that a graphene bilayer is almost one order of magnitude harder to bend than the monolayer.

The electrical conductivity, on the other hand, is proportional to the electronic density of

states, which for a stack of N crystals, grows like N and, hence, grows slower with the

number of layers (the stack behaves as a set of N resistors in parallel). If one assumes that,

the graphene stack obeys the Wiedemann–Franz law[20] we would conclude that the thermal

conductivity of a graphene stack also grows with N.

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The same general considerations are valid in regards to the dependence of physical

properties with the size of the flakes, L. The bending energy behaves as[16] 1/L2 while the

electric and thermal conductivity behave as 1/L. Hence, broadly speaking, the number of

layers, N, and the size of the crystal, L, are fundamental for the understanding of the

physical properties of graphene stacks. The smaller the number of layers and the larger the

size of the crystal, the closer the stack is from behaving as monolayer graphene.

There is a direct analogy between LPE and the production of oil derivatives from

crude oil (Figure 1). In a refinery’s distillation column, the lighter products such as

hydrocarbon gas, naphtha, gasoline, kerosene, float to the top, while the heavier ones such

as tar, heavy gas oil, lubricating oil, Diesel, stay in the bottom. In the chemical reactor of LPE

the lighter products such as stacks with very few layers float to the top of the solution while

the heavier products such as graphite remain at the bottom of the reactor (there are many

practical ways how to achieve it, including gravimetric, centrifugation, etc). It is possible to

extract each product and repeat the process several times to get higher concentration of

monolayers but this, of course, impacts on cost. Just as in the case of oil, the distribution of

stacks is continuous along the column of the reactor.

Figure 1. Liquid phase exfoliation schematic process.

3. Graphene classification

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One can find in the literature several different ways to classify graphene samples.

The most popular uses the following classification:

Very Few Layers graphene (vFLG): 1 to 3 layers;

Few Layers graphene (FLG): 2 to 5 layers;

Multilayer graphene (MLG): 2 to 10 layers;

Graphene nanoplatelet (GNP): more than 10 layers.

However, recently the ISO launched the first standard related with 2 dimensional

materials nomenclature where the classification is as follows[2]:

Graphene: single layer of carbon atoms;

Bilayer graphene: two well-defined stacked graphene layers;

Few-layer graphene: consisting of three to ten well-defined stacked graphene

layers;

Graphene nano-platelet: thickness between 1 nm to 3 nm and lateral

dimensions ranging from approximately 100 nm to 100 µm

Notice that these definitions are rather arbitrary from the physical point of view.

Moreover, the terminology “graphene nano-platelets” is a misnomer since from the physical

point of view GNP is essentially fine graphite. From the previous discussion, it is clear that

any characterization of graphene flakes requires a statistical definition and cannot be

arbitrary.

The lack of standards for graphene has been stalling the development of graphene

applications due to the bad quality of the material sold in the open market. There is a lot of

confusion and misinformation in the graphene application’s market leading to unreliability

and senseless pricing schemes. The creation of a universal protocol, to classify graphene

samples according to a standard procedure, will help to develop real-world applications

because standardization of measurements is vital to allow quantitative comparison of data

originated from different laboratories and users. Standardization is not a way to force

companies to make a product in a strict way and compromising innovation. A standard

creates the capacity of comparing materials from different producers in a consistent manner

and provide buyers with a reliable product. In fact, standardization helps producers to

improve their products and help users and developers with a clear way to analyse what they

are purchasing. With this aim, it is important to develop a protocol based on scientifically

accepted techniques reported in the literature.

In this study, we analyse graphene samples (but not GO or rGO) from 60 companies

from the Americas, Asia and Europe. We use standard methods that can be found in

6

research laboratories around the world. Atomic force microscopy (AFM) measures the

thickness of graphene stacks [4, 21, 22]. Optical microscopy provides us with the flake size [23, 24].

Raman spectroscopy provides information on the structural integrity of the sample as well as

indicates the presence of GO and/or rGO.[25-28] X-ray photoelectron spectroscopy (XPS)

measures the carbon content (purity).[29, 30] Scanning electron microscopy (SEM) and

transmission electron microscopy (TEM) providing supporting information on the sample

morphology[31-34].

4. Sample Preparation and Characterization

We developed a protocol and workflow for testing graphene that is consistent with

the scientific literature and the common practice in laboratories worldwide. Figure 2

illustrates the procedure.

No

Graphene Solution/Slurry

Graphene Powder

Vehicle removal

Raman

Characterization

Water IP NMP

Spin-coating on Sio2/Si wafer

G, GO, or rGO present?

Yes

No

Soluble?

Yes Yes

Yes

No No

Soluble? Soluble?

OM AFM Raman SEM XPS

Data Analysis

+ TEM (if possible)

CHNS BET

Discard

Discard Purification (1st water / 2nd ethanol)

Figure 2. Flowchart for G/GO/rGO systematically characterization.

There are three main steps: In Step 1 (Figure 3), we identify what is the nature of the

sample, namely, if we have graphene, GO or rGO using Raman spectroscopy. If the sample

comes in the form of a solution, we filter the solution and dry it before evaluation. We discard

GO or rGO and focus solely on pure graphene. In Step 2, subsequently to the sample

solubility test we produce a graphene solution by mixing a standard amount of the sample

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(1mg) with 1mL of a solvent and disperse the solution using soft sonication for 60 min. In

step 3, the solution with graphene is dispersed on top of a silicon oxide surface (300 nm

thickness) in a spin coating machine for a standard amount of time and rotation speed.

Finally, the silicon wafer dries overnight in a dry cabinet before it is evaluated by the different

characterization techniques (the flowchart and the sample preparation are concomitant).

Figure 3. Three steps used in the preparation of samples for characterization.

Optical Microscopy. Graphene flakes can be visualized with the use of a high

magnification (objective x100) optical microscope. This technique can be used to measure

the distribution of the lateral sizes and shape of the flakes[24]. The incident light is chosen to

be perpendicular to the silicon wafer plane with the electric field polarization in-plane [35]. At

least 1,000 flakes are measured from 4 different quadrants and the relevant information

regarding lateral size is obtained. The contrast is enhanced by the use of the suitable image

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filters. A typical procedure (Figure 4) is to select the green channel (based on RGB

coordinates) of the image and convert it to a binary base (black and white image). After the

image conversion to a binary base, an automated identification of the graphene flakes is

used and their lateral size is measured and the size distribution chart can be generated

accordingly.

Figure 4. Typical optical microscopy image of the graphene flakes, its binary base and the

size distribution chart.

Raman Spectroscopy. Raman spectroscopy provides a quick and facile structural

and quality characterisation of the produced material[7], as an important non-destructive tool

for the characterization process.[36] During the Raman spectroscopy analysis, the incident

laser light excites the in-plane vibrations, stretching the sp2 bonds. The lattice vibrations

have a large polarizability and, by symmetry, each hexagonal carbon ring contributes

constructively to the interference pattern[26]. The emission spectra response for crystalline

graphene occurs in the forms of two main peaks in the emission spectra at laser

wavelengths[37] of 1575 cm-1 and 1355 cm-1, due to the stretching and breathing modes[38].

In the case of graphene, the Raman spectra fingerprints allows the differentiation

between monolayer, bilayers and few layers as reflected by changes in the Raman bands

(G, D and 2D). Samples that are thicker than 5 layers are hard to distinguish from bulk

graphite[27]. It is also possible to measure features such as uniaxial and biaxial strain since

the Raman peaks are very sensitive to lattice deformations [28, 39]. The shape of the G and 2D

peaks provides information on the sample quality. The Raman spectra acquisition is done in

10 different areas with 5 spectra per area, with a laser spot smaller than 3 µm. For graphene

and rGO, the rates between the Raman peaks (D, G, and 2D) also used to access the

structural quality (Figure 5).

9

Raman Spectra of a Graphene sample with chemical information is collected using a

Raman microscope with a 100 x lens (NA=1.0), at an excitation wavelength of 532 nm. The

data acquisition controlled by the software package and the harsh between the intensity of G

and 2D bands obtained directly from the spectra. The Raman laser spot is directly focused

on the region of interest at the sample and different spectra are collected, referring to the

quality of the flakes on that region. The different results are statistically representative since

the analysis is performed in 50 different regions for each sample. The optical micrograph on

Figure 5 exemplifies the focusing spots for the laser. The corresponding spectra are

presented on Figure 5A-C.

(A)

(B) (C)

Figure 5. Scanning Raman spectroscopy of a commercial graphene sample in three different

regions labelled A, B, and C.

Atomic Force Microscopy. Atomic force microscopy (AFM) has proven to be

particularly valuable for the study of flake thickness. In contact or tapping modes the sensor

makes physical contact with the flakes[21] and gives reliable readings of the thickness[4]. AFM

measurements allow an easy identification of single-layer graphene flakes and can provide

an estimation of the dead layer thickness between the flake and the silicon oxide surface [4].

However, it is difficult to find single crystals only by scanning the surface randomly [22] since

many of the flakes can be wrinkled and/or folded[40]. In Figures 5 and 6, we depict the steps

used during the AFM measurements as well as how to use the obtained data to measure the

thickness of each sample. AFM images were recorded using a tapping imaging mode with a

10

G

D2D

silicon tip on silicon nitride cantilever (T:0.6 µm, fo:1,400 kHz, L:27 µm, K:18 N/m, W: 32 µm).

We evaluate at least 500 particles in 10 different areas within the wafer. To avoid the

influence of unwanted features from scan lines (e.g. noise, bow or tilt) the areas without

particles of the AFM images must be flattened. This procedure will modify the data by

deleting the low frequency noise and removing tilt from the images. Combining the

measurements performed in different images is possible to obtain the average thickness

distribution for each sample.

Figure 6. (a) AFM image of a commercial graphene sample and cross-sectional (red) profile

line. (b) Flakes height (thickness) as a function of position in the cross-sectional profile line.

Figure 7. AFM histogram of number of layers of commercial samples with (a) high and (b)

low graphene contents.

On the one hand, a fundamental application for the AFM is the use of commutative

percentage to determine the sample average until a certain number of layers, in our case

the average below 50% and 90% (Figure 8). From this information, 50% and 90% of the

11

sample population, respectively, has less than a given number of layers, becoming possible

to obtain the D50 and D90 as the average population of the sample.

Figure 8. Cumulative distribution curve of number of layers in the measured flakes.

On the other hand, an important information is the graphene content in percentage

revealing the quality of flakes with less than 10 layers. This “graphene content” is nothing but

the number of graphene flakes divided by the total number of measured flakes multiplied by

100. To define this value as the average number of layers of the graphene population:

Graphene content%=number of graphene flakestotalnumber of flakes

∗100 Equation (1)

AFM allows for the measurement of the number of layers and their size

simultaneously. Nevertheless, this type of measurement is extremely time consuming. From

the physical standpoint, there is no reason to believe that the number of layers in a flake and

their size are relate to each other since these two properties depend on many factors

associated with the process used to break graphite apart. As previously stressed, one has to

have in mind that these processes are all statistical in nature and have a limited degree of

control. Hence, we chose to use AFM to measure only the number of graphene layers in a

flake while their lateral size measured by optical means.

Elemental Analysis. Elemental analysis is also fundamental in order to evaluate the

quality of samples[41]. The CNH (Carbon, Nitrogen and Hydrogen) determination is based on

the dynamic combustion of the sample (heated up to 1200 oC under O2 atmosphere). After

combustion, the resulted gases (N2, CO2, H2O and SO2) are carried by a helium flow through

a gas chromatography (GC) column, which provides the separation of the combusted gases

using temperature programmed desorption (TDP).

12

D90D50

X-ray photoelectron spectrometry. X-ray photoelectron spectroscopy (XPS) is

used to further analyse the chemical composition of the sample. During the sample

evaluation, spectral features are dependent on the chemical environment experienced by the

carbon atoms. Within these, one can measure the sp2 and sp3 bond content of each

sample[29, 42]. Oxygen content and other elements can be observed during the chemical

evaluation[30]. Figure 9, shows an example of an XPS spectra for a sample where the carbon

C1s core level is evaluated. The result indicates a low degree of oxidation, only the

contribution as a small amount of carboxylates is evident (O-C=O). The photoelectron

spectra is collected using the following conditions: the base pressure of 1x10-9 Torr, the

working pressure 5x10-9 Torr (generally better than 1x10-8 Torr), X-ray source mono Al Kα h

=1486.71 eV, 5mA, 15 kV (75 Watt), with normal angle between sample surface and

detector. In Figure 9, for instance, a high-resolution scan from the carbon 1s core level for a

sample dominated by a single peak is assigned to the sp2 carbon framework (284.02eV) and

showing a low oxidation level at ~10%. The deconvolution of the spectrum shows carbon-

hydrogen bonded groups in a sp3 hybridised state (284.9eV) as well as carbon-oxygen

bonded groups (288eV)[30]. The largest contribution comes from the sp2 C framework from

the bulk of the material (C=C and benzene-like CH terminating the framework).

Figure 9. XPS spectra of a commercial graphene sample.

Scanning Electron Microscopy. Scanning Electron Microscopy (SEM) is a rapid,

non-invasive, and effective manner for imaging the shape and morphology of flakes. SEM

has a much higher spatial resolution than optical microscopy, but efficiently scans only small

13

areas of the wafer. Therefore, SEM has low statistical information and only qualitative value.

Imaging graphene flakes with a low beam voltage electron microscope allows the acquisition

of images with high resolution. As the acceleration voltage decreases, the number of

secondary electrons generated from the substrate reduce[31], enhancing the contrast and

revealing more surface details such as wrinkles, ruptures, folds and voids, and even

impurities[32]. The contrast usually is due to factors such as roughness, presence of edges,

difference in work function and conductivity, and combination of these effects. Since the

samples are thin, the image contrast is very low as the electron bean can easily penetrate

the sample. Therefore, the secondary electrons collected have high influence of the

substrate[43]. The edges of graphene flakes appear with a high brightness forming obvious

white lines along every graphene boundary[32]. The absence of charging during the SEM

imaging indicates that the network of the graphene samples is electrically conductive[11]. The

samples, in powder form, dispersed on a surface of a carbon tape on the top of a stainless-

steel stub was imaged with a 4 kV beam and variable magnification. In Figure 10 we show a

SEM image revealing flakes with random aggregates of thin sheets closely associated with

each other and forming a disordered structure[11]. SEM images for various arrays of

multilayer flakes after exfoliation process allows the evaluation of the lateral size and a clear

view of the tearing of the crystallites[13]. Each SEM image was collected under the same

conditions (except by the increased magnification).

14

Figure 10. SEM micrographs of commercial graphene samples.

Transmission Electron Microscopy. Transmission Electron Microscopy (TEM)

helps to study freely suspended flakes. As explained previously, the bending rigidity is highly

dependent on the number of layers. Thick stacks are rigid and look like “bricks” under TEM.

Thin graphene stacks are soft and show typical characteristics of soft materials such as folds

and creases. Hence, TEM provides an indirect means to confirm the quality of the samples

as measured by AFM. During the TEM evaluations in bright-field mode samples are more

visible as phase contrast at a sufficiently defocus[44]. Strong diffraction spots with six fold

rotational symmetry show that the electrons are incident at the normal axis to the carbon

basal plane[33]. As for preparation, the graphene flakes solution is dispersed on a 300-mesh

standard holey-carbon-film-covered copper grid. The experiments are carried out at 300kV

acceleration voltage. There is no evidence of electron-beam-induced damage to the

samples[45] that are exposed to the electron beam for long period of time during data

collection[46].

(a)

(b) (c) (d) (e)

15

Figure 11. TEM micrographs of commercial graphene samples. Slides (a)-(e) are described

in the text.

Figure 11 shows TEM micrographs of commercial graphene samples. In this figure is

shown (a) HRTEM image with a multilayer graphene sample where it is possible to measure

the interlayer thickness (inset), (b) Low magnification image from a flake (it appears

transparent and folded over on the edges [13, 47]), (c) Low magnification image for a cranked

flake (the edges tend to scroll and there are folded regions in which a graphene sheet

became detached suggesting that for a monolayer graphene sheet, a fold exhibits only one

dark line. However, scrolls and multiple folds can give rise to any number of dark lines even

for a monolayer. Additionally it is directly possible to distinguish between monolayer

graphene and thicker samples analysing the electron diffraction pattern[34]. (d) Chipped

edges for a delaminated flake high magnification image revealing not only the chipped edges

but also the contrast variation intensities within the image and few layer small flake in high

magnification. (e) The contrast from a very few layers flake is found to be weak as indicated

by the fact that it is barely distinguishable from the entire carbon coating surface of the

image[45].

In Figure 12 we show different TEM images of flakes: (a) Normal incidence diffraction

pattern for graphene yielding the hexagonal structure profile from carbon [48] from a few

layers flake; (b) Winding edge; (c) Chipped edge, and (d) Single layer. The hexagonal

16

pattern is illustrative of the hexagonal carbon honeycomb arrangement of graphite [49] (Figure

2, insertion).

17

(a) (b)

(c) (d)

Figure 12. TEM micrographs of commercial graphene samples. Slides (a)-(d) are described

in the text.

18

The ordered graphite lattices are clearly visible; the graphitic laminar structure can be

resolved in the ordered region and the well-defined diffraction spots confirm the crystalline

structure of the flakes obtained[50]. We are able to use TEM to visualise and to resolve the

layer and interlayer arrangements of the carbon atoms in graphene flakes. There is no need

of pre-treatment before evaluate the samples by TEM. As mentioned previously, and in

analogy to the SEM experiments, TEM provides only qualitative information but serves to

confirm the results obtained by the quantitative methods.

4. Results

In Figure 13, we present the result for the graphene content, as defined in equation

(1), from 60 companies in North America, Europe, Asia and Australia. As one can clearly

see, the majority of the companies are producing less than 10% graphene content and no

company is currently producing above 50% graphene content. This result may come as a

surprise given the widely advertised graphene “fever” of the last decade. However, it also

helps to understand why graphene applications are not commonplace yet. In terms of

material properties, graphene and graphite are very different and cannot be interchanged in

many important applications such as coatings, composites, batteries, etc.

19

Figure 13. Graphene content per number of companies.

A more in depth analysis shows that the processes currently used are not efficient in

reducing the number of layers. In Figure 14, we show D50 and D90 for the companies studied.

Notice how D50 is essentially scattered randomly while D90 is peaked at a small number of

layers. It is clear that the majority of the companies are producing fine graphite instead of

graphene. We stress that at the naked eye it is not possible to detect these differences

because we are dealing with a nano-material. Only through nanotechnology tools and well-

defined protocols as the ones established in this study on can determine the quantity and

quality of the graphene produced.

Furthermore, it is worrisome that producers are labelling black powders as graphene,

and selling for top dollar, while in reality they contain mostly cheap graphite. This kind of

activity gives a bad reputation to the whole industry and has a negative impact on serious

developers of graphene applications. Only through standardization and following protocols

for characterization as proposed here, the graphene industry can evolve reliably.

We would like to mention that some applications do not require graphene and few

layer graphene would be sufficient or work even better than graphene itself. However, proper

labelling and characterisation is always required for the product developer to make an

intelligent choice of a material.

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Figure 14. Number of companies related with the number of layers from AFM (D50 and D90).

In Figure 15, we show the results for the companies that produce graphene as we

define previously (10 layers or less). We see that no company produce pure monolayers,

which is consistent with our previous discussion on the statistical nature of the production

process. We find that most of these companies produce 4 layers on average.

Figure 15. Average graphene content number of layers per number of companies.

21

Interestingly enough, in terms of the size of the flakes one can see in Figure 16 that

most companies produce flakes with a few microns in size. Even companies that produce

40% or more of graphene content cannot produce flakes with more than 5 micrometres in

size. This surprising result seems to indicate that the flake size is almost independent of the

process used since, given the large number of companies studied; one assumes that a large

number of different processes are used. Since we do not have direct information on the

processes used, it is difficult to speculate on the significance of this result.

Figure 16. Average flake lateral size per number of companies.

Carbon content analysis shows that in many cases there is substantial contamination

of the samples. Ideally we would expect 100% carbon content, nevertheless, as show in

Figure 17, a large number of companies produce material a with low carbon content.

Contamination has many possible sources but most likely, it arises from the chemicals used

in the processes.

22

Figure 17. Carbon content per number of companies (Elemental Analysis).

Contamination also affects the number of sp2 bonds. Crystalline graphene should

have 100% sp2 bonds. However, as shown in Figure 18, we were not able to find, in any of

the companies studied, a sample with more than 60% sp2 bonds. It is reported[50] that even

adsorbed hydrogen atoms can produce sp3 defects and that the presence of transition

metals can have an even stronger effect. It is obvious from the studies that the degree of

contamination is high and can be detrimental in many important applications of graphene.

For instance, the presence of intermetallics in graphene samples can significantly lower the

efficiency and performance of graphene electrodes in batteries. In order to use some of

these samples for batteries one would require washing the samples in acid in order to get rid

of the unwanted metals. However, a procedure such as this one would significantly increase

the cost of graphene production.

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Figure 18. C-C sp2 content per number of companies (XPS).

5. Conclusions

Our extensive studies of graphene production worldwide indicate that there is almost

no high quality graphene, as defined by ISO, in the market yet. The lack of properly

characterised, high quality material has been stalling the development of applications that

depend fundamentally on graphene such as advanced coatings and composites, high

performance batteries and supercapacitors, etc. In hindsight, this is not a surprising result

given that graphene is a nano-material and its characterization depends on nano-technology

tools that are not readily available, or are too expensive, to ordinary producers and

developers.

Our expectation is that the current work will serve to speed up the process of

standardization of graphene within ISO as there is a huge market need for that. Furthermore,

we hope that it will stimulate graphene producers worldwide to improve their methods and

approaches in order to produce a better, properly characterised product that will lead to

better performance of applications. It is clear that this is of their interest since as more

applications are found there will be an increasing demand for high quality graphene.

We would also like to mention, that different applications would require different grade of

graphene. For instance, the best performance in composite materials applications would be

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achieved with large flakes which are 2-3 monolayer thick.[51] At the same time, the best

electrodes for neurological applications are prepared from monolayer flakes, etc. [Ref.

Barcelona people]. We envisage that each particular application would require fine-tuning of

the properties of the graphene material (in terms of its thickness and size distributions, basal

plane and edge functionalization, etc), which again requires careful characterisation.

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