Chapter 1
Properties of Carbon: An Overview

Shengxi Huang, Johan Ek Weis, Sara Costa, Martin Kalbac and Mildred S. Dresselhaus

1.1 Overview of Properties

Carbon (element No. 6 in the periodic table) forms a variety of materials, including graphite, diamond, carbon fibers, charcoal, as well as newly discovered nanocarbon materials, such as fullerene, graphene, carbon nanotube, and graphene nanoribbon (GNR). Even though all are composed of the same atoms, different carbon materials can show very different physical and chemical properties, including electrical transport, optical and thermal properties, and chemical reactivity, depending on their structures.

Electrochemistry has been connected to carbon materials since the early days of electrochemistry research [1], and the discoveries of new carbon materials in the past decades have been accompanied by research advances concerning the doping of these materials using electrochemical techniques, with an emphasis on materials preparation, characterization, and applications.

Among the electrochemical techniques and characterization tools, vibrational and optical spectroscopies have been important. Electrochemical charge transfer, an important process in electrochemistry, influences not only the electronic structure of the materials but also their vibrational and optical properties, which are all dependent on the concentration of electrons and holes found in the solid. Therefore, valuable data can be obtained when electrochemistry and in situ Raman spectroscopy are applied simultaneously under the heading of spectroelectrochemistry. Such investigations have been carried out extensively on carbon nanomaterials in order to investigate the effects of electron and hole doping.

One advantage of electrochemistry over other experimental techniques is its ability to introduce higher levels of dopants and to make quantitative, reproducible measurements [2, 3]. The electrochemical setup including the choice of the particular electrolyte is an important factor that influences the doping efficiency in spectroelectrochemical experiments. Different electrolytes can thereby require larger electrode potentials than others to achieve the same doping levels, so that attention needs to be given to the choice of the electrolyte for studying a given material system. Failures to pay attention to such issues have, in the past, led to apparent inconsistencies between different sets of published data.

The basic carbon material introduced in this chapter is graphene, which is a single layer of crystalline graphite, because it is the basic building block behind sp2 carbon materials. Graphite, which represents nature's way to build up stacks of graphene layers into a bulk crystal, is then introduced briefly, together with the synthetic commercial product, highly oriented pyrolytic graphite (HOPG), which closely resembles graphite. Another commonly used nanocarbon material, carbon nanotube, is also discussed, which has deep scientific interest, and is also interesting along with its related carbon fiber analog for electrochemical commercial applications. Porous carbon is an sp2 carbon material useful for applications requiring a huge surface area, and is also discussed briefly. Diamond, which is commonly a symbol of societal wealth and prestige, is gaining more and more scientific attention recently due to its extraordinary properties in electrochemistry, quantum physics, and biology, and has promising applications in all of these fields. Finally, brief mention is made of other sp2 nanocarbon materials with significant current scientific interest, carbon nanoribbons and porous carbon, and these materials may someday find interest for electrochemical science and applications.

In this chapter, we will introduce some typical carbon materials that are widely studied in electrochemistry. Their properties, not restricted to their electrochemical properties, will be briefly described. Some characterization techniques, including spectroelectrochemistry, will be described when applied to selected carbon materials. A brief overview of the application of various carbon materials to electrochemistry will be included in this chapter, which will be concluded by an outlook to the future.

1.2 Different Forms of Carbon

1.2.1 Graphene

1.2.1.1 Optical Properties

It is widely established that graphene has numerous fascinating properties [4-9]. Though considered as a semimetal, graphene has unique electromagnetic/plasmonic effects compared to conventional noble metals [10, 11]. First, its plasma frequency in the long-wavelength limit is expressed as [12-15]

where is the Fermi energy of graphene, is the universal optical conductivity of graphene and is independent of any material parameters: [16, 17], is the unit charge, and is the permittivity. Note that the expression of plasma frequency for graphene is very different from that for metals which is [18]

where is the carrier density in graphene and is the carrier effective mass. We can see that and . Such a difference in the plasma frequencies between graphene and metals is due to the Dirac fermions in graphene, rather than to ordinary Schrödinger fermions in normal metals. The plasma frequency of graphene is in the terahertz range, which is 103 times lower than in metals, and which can be tuned through gating or doping [11, 19, 20], or by fabricating graphene ribbons with micron widths (see Figure 1.1), where is in the terahertz range. Here, differs with the ribbon widths and with the Fermi energy , as shown in Figure 1.1. Second, single-layer graphene has a linear dispersion relation and a uniform 2.293% light absorption across a wide frequency range [11, 21-23], resulting from its Dirac-cone band structure and linear energy-momentum relation E(k), as seen in Figure 1.2. Many works have studied the surface plasmonic properties of graphene or graphene ribbons, with different experimental techniques, including optical measurements, electron energy loss spectroscopy, angle-resolved photoemission spectroscopy, and surface tunneling spectroscopy [19, 20, 24-26], as further discussed in the cited references.

Figure 1.1 Control of the graphene plasmon resonance frequency by electrical gating and microribbon widths. (a) AFM (atomic force microscopy) images of graphene microribbons with widths of 1, 2, and 4 µm. Color bar of the height is shown on the right. (b) Fermi energy (EF) dependence of the graphene plasmon frequency (top axis gives related dependence on charge density |n|1/2) of ribbons with three different widths.

(Figure adapted from Ref. [20].)

Figure 1.2 Universal light absorbance and optical conductivity of graphene. (a) Schematic of Dirac-cone and interband optical transitions in graphene. (b) Optical absorbance (left axis) and optical sheet conductivity (right axis) of three graphene samples. The spectral range is from 0.5 to 1.2 eV. The black horizontal line shows the universal absorbance value of 2.293% per layer, with the variation within 10%. (c) The optical absorbance of graphene Sample 1 and Sample 2 over a smaller spectral range from 0.25 to 0.8 eV.

(Figure from Refs [22, 23].)

1.2.1.2 Electrical Properties and Tunability

One of the greatest advantages of studying graphene is that its transport and optical properties can be sensitively and controllably tuned by doping. The Fermi level can easily be shifted by introducing either electrons (n-doping) or holes (p-doping). Numerous ways of establishing a desired doping level have been investigated, for instance, by chemical doping [27, 28], electrochemical doping [29-33], electrostatically by top or back gating [34-36], and by the direct introduction of heteroatoms into the lattice [37].

One of the most studied and widely used techniques is to introduce the charge by top or back gating [34-36]. This technique is appealing due to its similarity to present use in gating field-effect transistors, which allows the knowledge and know-how learned from standard microelectronics to be used more widely in graphene electronics. However, one drawback of this approach is the extremely high gating potential (~100 V) that is required, because the present gate dielectrics have a relatively large thickness that restricts the gate capacitance value. For example, bias voltages as high as 80 V had to be used to achieve a carrier density of [36], and such a high bias voltage could cause charge trapping from the substrate, thereby altering the properties of both the substrate and the graphene.

Electrochemical doping, on the other hand, is significantly more efficient, insofar as voltages as small as 1.5 V are sufficient to reach charge carrier concentrations of . Higher doping levels can also be achieved by using a combination of a protecting layer and a liquid electrolyte [3] or using ferroelectric polymers [38]. Electrochemical doping is thus especially appealing when higher doping levels are desired, and these high doping levels are achieved by the electrical double layer (EDL) formed at the interface between the electrolyte solution and the graphene surface. The ions in the liquid are attracted by the graphene, which is charged by an opposite sign. These ions migrate to the surface, thus forming a very thin layer that performs as a capacitor with an extremely high capacitance value. Therefore, effective control of carrier densities in the graphene can be implemented...