The properties of pristine graphene are restricted due to its zero-gap band structure. However, the gapless symmetry of graphene can be altered through doping. Doped graphene could exhibit enhanced catalytic activity, higher charge mobility, superior photoresponses, and improved magnetic moments, chemical activity, which could revolutionize next-generation technologies.
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Graphene, the wonder material, has revolutionized the industry and driven the focus of researchers into two-dimensional materials. It has intriguing properties such as high mechanical strength, high flexibility, high transparency, high charge conductivity, a light weight, and stability.
In graphene, a single carbon atom is covalently bonded to three other carbon atoms forming a honeycomb structure. This honeycomb network is the basic building block for other important allotropes of carbon like carbon nanotubes and fullerenes. However, graphene has generated more interest due to its unique physical and chemical properties.
Graphene has shown potential applications in field-effect transistors (FETs), supercapacitors, and sensors. Since pristine graphene is a zero-gap band semimetal, its applications in various electronics and sensors are limited. Therefore, various methodologies have been proposed to tune the bandgap of graphene. Chemically doping the graphene is one of the proven methods for shifting the band edges.
How to Dope Graphene?
Shifting of the Fermi level of graphene, which is at the Dirac point for semimetal, to above or below the Dirac point determines p-type or n-type doped graphene, respectively. There are two categories of chemical doping: surface transfer doping and substitutional doping.
Surface transfer doping is achieved through the transfer of charges between graphene and surface adsorbed dopant. If the charge transfer is from dopant to graphene, the Dirac point shifts below the Fermi level resulting in n-type graphene. In contrast, if the charge transfer is from graphene to dopant, p-type graphene is the result.
Due to the high surface area of graphene, it is easy for dopants to adsorb to the surface. Adsorbents such as gas molecules, organic molecules, and metal atoms could act as dopants.
Water molecules adsorbed on the surface of graphene due to humidity have been reported to cause p-type doping. Studies have shown some other gas molecules like nitrogen dioxide induced p-type doping, while molecules with a strong donating group such as ethanol, ammonia, and carbon mono-oxide induce n-type doping in graphene.
When metal atoms are adsorbed to the graphene surface, charge transfer occurs to equilibrate the Fermi level positions. Aluminum, silver, and copper doped graphene form n-type while gold and platinum adsorbent atoms do p-type doping.
In substitutional doping, heteroatoms like nitrogen, boron, and sulfur replace the carbon atoms and break the graphene structure. N-type graphene is formed when dopants replace carbon with fewer valence electrons compared to carbon, such as boron. Dopants like nitrogen having more valence electrons compared to graphene creates p-type graphene.
There are other methods to alter the properties of graphene, such as functionalization and compositing. However, they are not as efficient as doping since a small amount of dopant can considerably change the properties of graphene. Moreover, doping can be accomplished through conventional methods.
Substitutional doping can be done through direct synthesis and post-treatment methods. A homogeneous doping is achieved when doped through direct synthesis methods such as chemical vapour deposition (CVD), solvothermal, and arc deposition.
In CVD, the doping efficiency is regulated by controlling the ratio between carbons and doping element sources. Solvothermal-based doping method is widely used because of its simple process, moderate synthesis conditions, and large quantity production. Arc discharge method has advantages of high purity and large-scale production and is widely used to prepare graphene and carbon nanotubes.
Post-treatment methods like thermal annealing and plasma treatment can result in only surface doping. Thermal annealing approaches utilize high temperatures to dope graphene, which involves annealing graphene or graphene oxide in a gaseous environment containing dopants. Plasma treatment-based doped graphene consists of a process containing the incorporation of foreign atoms and groups into scaffold surfaces.
Intriguing Properties of Doped Graphene
Through doping, the rigid zero-gap band structure of graphene can be changed to show new properties. Different dopants have different effects on the band structure and the carrier concentration of the doped graphene.
For example, in nitrogen-doped graphene, the lone pair of electrons from nitrogen is donated to graphene resulting in high electron mobility and forming n-type graphene. The concentration of nitrogen dopant in graphene strongly influences its electrochemical activity.
Studies show that more active sites were created with the increase in nitrogen dopant; however, the conductivity decreased. Another study based on boron and nitrogen-doped graphene showed influencing its semiconducting electronic properties.
Nitrogen-doped graphene has found applications in fuel cells due to increased active sites and spintronics due to its magnetic properties.
Another material phenomenon called photoluminescence (PL) is found significantly enhanced in nitrogen-doped graphene.
Applications of Doped Graphene
Doped graphene has many energy-related applications, such as in fuel cells and hydrogen evolution reactions. Through doping, the stability of graphene is altered, and the material starts interacting with the foreign species.
Doped graphene has been a strong candidate in batteries due to its increased storage capacities, light weight, and mechanical flexibility. For example, metal-doped graphene has advantages in lithium-ion batteries due to enhanced topographical defects, increased wettability, and enlarged interlayer distances. In another study nitrogen-sulfur codoped graphene demonstrated excellent long-term cyclic stability when used in supercapacitors.
They have shown many applications in sensors due to the increased active sites. A study reported the application of boron-doped graphene in the speedy response and immediate recovery of ammonia.
Light matter interactions in doped graphene have been utilized in light-emitting diodes, solar cells, photodiodes, and laser diodes. For example, Boron doped graphene had shown application in organic light-emitting diodes.
Various bio-related applications of doped graphene were explored. Doped graphene was used for probing DNAs and their structural dynamics. A study showed the application of bromine-doped graphene for detecting glucose in human blood serum.
Many studies on the properties of doped graphene with different dopants continue to be carried out by researchers. More properties are being explored, and more application into next-generation technologies is yet to come.
Continue reading: New Research Narrows the Gap for Graphene Nanoribbon Applications.
References and Further Reading
D Ghuge, A., R Shirode, A. and J Kadam, V. (2017) Graphene: a comprehensive review. Current drug targets, 18(6), pp.724-733. Available at: https://doi.org/10.2174/1389450117666160709023425.
Zhang, W., Wu, L., Li, Z. and Liu, Y. (2015) Doped graphene: synthesis, properties and bioanalysis. RSC Advances, 5(61), pp.49521-49533. Available at: https://doi.org/10.1039/C5RA05051K
Ullah, S., Shi, Q., Zhou, J., Yang, X., Ta, H.Q., Hasan, M., Ahmad, N.M., Fu, L., Bachmatiuk, A. and Rümmeli, M.H. (2020) Advances and trends in chemically doped graphene. Advanced Materials Interfaces, 7(24), p.2000999. Available at: https://doi.org/10.1002/admi.202000999.
Wang, H., Maiyalagan, T. and Wang, X. (2012) Review on recent progress in nitrogen-doped graphene: synthesis, characterization, and its potential applications. ACS catalysis, 2(5), pp.781-794. Available at: https://doi.org/10.1021/cs200652y
Choi, M.S., Nipane, A., Kim, B.S., Ziffer, M.E., Datta, I., Borah, A., Jung, Y., Kim, B., Rhodes, D., Jindal, A. and Lamport, Z.A. (2021) High carrier mobility in graphene doped using a monolayer of tungsten oxyselenide. Nature Electronics, 4(10), pp.731-739. Available at: https://doi.org/10.1038/s41928-021-00657-y.
Rao, C.N.R., Gopalakrishnan, K. and Govindaraj, A. (2014) Synthesis, properties and applications of graphene doped with boron, nitrogen and other elements. Nano today, 9(3), pp.324-343. Available at: https://doi.org/10.1016/j.nantod.2014.04.010.