**4.1 Historical development of graphene doping techniques**

Recently a significant amount of research has been dedicated toward the manipulation of the physicochemical and electrical properties of graphene to specifically tailor it for various applications. One means to achieve this is through chemically functionalizing the graphene, involving modification of its carbon *sp*<sup>2</sup> honeycomb structure [18]. This chemical functionalization in turn necessitates chemically doping the atomic lattice of graphene with atoms from other compatible elements of the periodic table, essentially modifying graphene lattice originally undoped into a heteroatomically doped one.

The technique of doping through inducing charge carriers comprising either holes or electrons may be divided into two broad categories, which are as follows:


is a process whereby carbon atoms in the graphene lattice are substituted with other atoms, leading to either *p*-type or *n*-type conductivity [20]. Likewise, *surface transfer* describes a nondestructive technique for inducing charge carriers, in this case within the graphene lattice involving charge transfer between surface adsorbates and the graphene [21].

Doping by surface transfer may occur as the result of two different mechanisms—*electronical doping* and *electrochemical doping* [22]. Electronic doping is due to the direct transfer of charge between the graphene and adsorbate. In the presence of a differing electronic chemical potential, the doping type is controlled by the position of the graphene Fermi level relative to the highest occupied (HOMO) and lowest occupied (LUMO) level molecular orbitals of the adsorbate. While graphene is usually *n*-doped when the adsorbate HOMO lies above graphene Fermi level, *p*-type doping occurs when the LUMO of the adsorbate is found below the graphene Fermi levels [7]. The representation of molecular orbitals levels to the graphene Fermi levels for (a) *p*-type and (b) *n*-type doped graphene is shown in **Figure 8**. In contrast to electrical doping, electrochemical doping is a time-dependent process influenced by various factors that include the reaction rate and diffusion rate of molecular species [23].

The focus here is on inducing *p*-type doping in graphene through chemical doping. Chemical heteroatom doping of graphene is generally performed using either a one-step or two-step synthesis method. The one-step method involves employing CVD to introduce both carbon and boron sources into the chamber while heating the copper foil at high temperatures [24].

The alternative two-step synthesis process for boron doping includes thermal annealing [25] and rapid Wurtz-type reactive coupling [26] techniques, among others. Nevertheless, such two-step methods generally involve more complex experimental setups, tend to result in defects present in the doped graphene films, and require the use of toxic chemicals as the source/precursor as well as relatively high temperatures. These factors clearly limit the types of substrates that may be practically used [27].

However, a recently developed technique known as the spin-on dopant (SOD) process has made it possible to avoid these shortcomings in large part [28]. This

#### **Figure 8.**

*Relative position of highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of an adsorbate to the Fermi level of graphene for (a) p-type and (b) n-type dopants [6].*

method, which we have adopted for producing *p*-type doping in graphene and shall subsequently be described in more comprehensive detail, requires only a relatively basic experimental setup without the need for toxic precursor gases.
