Chapter 6 Producing Graphene: Methods and Sources
6.1 Chemical Methods of Producing Graphene Layers
graphite have existed for a long time. A number of such methods have been developed comparatively recently: individual graphene layers are released from highly oriented pyrolytic graphite (HOPG)4. Schematically, the process is shown in Fig. 6.2. The pro- cess includes: intercalation (i.e. a reversible inclusion of a molecule into layered struc- tures) of graphite with potassium (KC8) and graphite heating in the presence of K.
Potassium Graphite is otherwise known as graphite intercalation compound (GIC) in which potassium atoms are located between layers of graphitic carbon. Potassium Graphite is a very strong reducing agent used as a catalyst. The next step is growth of linear polymers in the presence of styrene or butadiene vapors. The last step depicts sheets of graphene. The growing of linear polymer in Step 3 forces the graphite planes to separate. The subsequent heating of elastic polymer at temperatures greater than 4000C results in producing completely exfoliated graphic carbon. Similar work was performed by B.C. Brodie in 18595. The graphite was subjected to acids. The result- ing graphite oxide was called “graphone”. Graphone is a new material that adds magnetism to the graphene properties. The electrical and structural properties are retained, for the most part. Since magnetism and electronic spin are closely related, graphone has new qualities and paves the way to applications in spintronics. This is
Height, pm -350
1.0
0.5
c)
Frequency, a.u.
350
-700 0 700
FIGURE 6.1 (Continued) c) Graphene height distribution on SiO2 substrate (the outer curve) on h – BN (the inner curve).
112 Graphene for Defense and Security a low quality graphene according to Novoselov, et al (2011)6. The authors regard the above method as one not providing high quality graphene sheets since the graphite under consideration is intercalated with oxygen and hydroxyl groups.
As such, the groups imply an easy water penetration into the specimens.
Measurements confirm that mobility in chemically processed flakes is two to three orders of magnitude lower than in mechanically processed (exfoliated) graphene flakes. The chemical approach is, however, more practical than the mechanical one.
Thus, there is a future for the chemical techniques for e.g. solar cells, LEDs and some other applications where an enhanced electron mobility is important (such as transparent conductors)7.
Another alternative for producing single graphene sheets was suggested by Schniepp et al8. The suggested method belongs to the chemical graphite processing.
The authors described the received single graphene sheets as “functionalized”. The structural defects such as defects caused by oxidizing and reduction reactors and level structural defects are characteristic of the process. The final product includes also specific chemical defects, such as impurities (e.g. L – O – L (epoxy)). C – OH groups may exist between the graphene planes and at the edges – C – OH and - COOH8.
First, the graphene flakes were placed in an oxidizing solution of nitric, sulfuric acids and potassium chlorate for 96 hours, the time which is necessary to remove the interplannar space characteristic of graphite (0.34 nm wide) and to change the space to 0.654 – 0.75 nm which what the solid graphite has. Thermal exfoliation that uses preheated environment (T = 10500C) goes to a much higher temperature at a speed of more than 20000C per minute releasing single graphene sheets with the forced release of CO2 gas. A careful study of the graphene sheets, however, gives evidence
Graphite
k
k
k k k
k
k k
k
k
k k
k k
k k
K-GIC
FIGURE 6.2 Chemical process of graphene exfoliation4.
of defects both at the edges as well as at the base of the specimens. The defects are easily identified with Atomic Force Microscopy (AFM) and are obstacles for high quality of electronic devices but may be useful additives for composite materials.
6.1.1 BuLk exfoLiation
Bulk exfoliation of graphene has been one of the effective, low-cost methods for graphene nano-sheet (suspensions) that have superior electrical conductivity. Mixed colloidal suspensions exist for reduced graphene oxide and layered metal oxide nano- sheets. In order to reduce the number of defects in the graphene sheet, the exfoliation starts at the edges. The graphite is milled to produce edge-carbonated graphite (ECG) sheets, the size of which is in the range of 100 – 500 nm9. This process gives better quality graphene samples than by the means of chemical exfoliation since only outer surfaces are affected – the internal hexagonal structure remains intact. The carbox- ylated edges make the graphene samples dispersible in water which separates the graphene sheets in the sample. Subsequently, the edges are removed by heating the sample. The goal is to obtain transparent high conductivity films on substrates (e.g. on Si) for series production. The ECG process is depicted schematically in Fig. 6.3.
In Fig. 6.3 graphite oxide (GO) is depicted on the right-hand side. Direct annealing gives pure graphite. Exfoliation, in contrast, is applied to GO, producing graphene with plane defects. Exfoliated graphene has been produced on a large scale and its quality has been improved along the way10.
Another method of exfoliation is used for “expandable graphite”11. The exfoliation of graphite takes place at T = 10000C for 60 sec in the atmosphere of argon + 3% hydro- gen. The heating causes intercalants formation of volatile gaseous species. As a result, graphite expands and we receive a few layers of graphene sheets. The thermal exfolia- tion allows to control its parameters in order to reduce the number of graphene sheets to one or two. Subsequently, exfoliated graphene is submerged in a 1,2 dichloroethane (DCE) of poly (m-phenylenevinylene-co-2.5-dioctoxy-p-phenylevinylene). 30-minute
E
Ball Mill
Direct Milling
Graphite
H2SO4 HNO3 CO2
FIGURE 6.3 ECG (Edge – Corboxylated Graphite) process.
114 Graphene for Defense and Security sonication produces a homogeneous suspension. Sonication is used to weaken the interlayer stresses. A further centrifugation removes redundant large pieces leaving planes and ribbons of graphene. The PmPV polymer is used to make assist the soni- cation process to stabilize the process. Later, the PmPV is removed by calcination at 4000C. Obviously, the polymer binds the graphene by van der Waals forces preventing graphene from dispersing in the organic solvent. The sonication time may be opti- mized in order to control the process. In particular, to make nanoribbons the sonication time is reduced. AFM (Atomic Force Microscopy) may be employed for the product characterization that allows seeing ribbons and graphene sheets.
A possible application for nanoribbons is for field-effect transistors11. An oxi- dized p++Si wafer served as the back-gate of the FET. The ribbons with widths of 10 – 55 nm were placed on the wafer. The source and drain were formed by Pd (pal- ladium) contacts that were attached to the ribbons. The described design of the FET allows widening of the difference between “OFF” and “ON” states at the ribbon width of less than 10 nm. The data is given in Fig. 6.4.
Empirically, the band gap in the nanoribbon depends on the ribbon width, as Eg,≈0 8 eV. Nanotubes form a p – type semiconductor with the palladium contacts . that act like a Schottky barrier since the Pd work function is high. Taking this into account, the ON/OFF ratio becomes:
ION/ IOFF~exp E k T( g/ B ); (6.1)
10 20 30 40 50 60 70
0 100 101 102 103 104 105
W, nm 106
107
Ion/Ioff
FIGURE 6.4 The range of achievable rations of ON/OFF states with respect to the ribbon width.
It has been reported that the energy gaps are caused by electron confinement that is restricted by the width W and specific details of the edges of the nanoribbon’s configuration.
Experimentally, it has been found that graphene nanoribbons (GNR) always have a band gap. Nanotubes have fewer defects than nanoribbons which may suggest mak- ing nanotubes out of nanoribbons that, in its turn, may lead to better defined con- figurations of better quality12. Solution-produced nanoribbons give hole mobility on the order of 200 cm2/Vs. The fact that the nanoribbons always have bandgaps makes them well-suited for electronic devices.
An alternative to chemical exfoliation has been reported by Hernandez et al13. It is liquid phase exfoliation which implies a direct dispersion of graphite. Fig. 6.5 shows the usage of solvent N – methylpyrrolidone (NMP) for liquid-phase exfoliation. The counts of dispersion show the amount of dispersion vs. the number of layers. The exfoliation implies solving of powered graphite in the 0.01 mg/ml solvent, NMP.
The dispersed graphene was deposited on a carbon grid and then dried. The subse- quent TEM characterization showed that there were no oxidation defects. About 70% of the yield specimen contained 3 or less graphene layers (see Fig. 6.5).
2 4 6 8 10
0 10
20 30
N = 100 Counts
12 14
FIGURE 6.5 Counts of dispersion vs number of layers per sheet. Direct liquid phase exfo- liation with solvent NMP characterized by TEM13.
116 Graphene for Defense and Security
6.2 EPITAXIAL METHODS OF PRODUCING GRAPHENE LAYERS