Graphene is an extremely promising material for post-silicon electronics due to its outstanding physical and electronic properties. This article discusses doping, one of the most practical methods for tailoring graphene's properties for next-generation applications.
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What is the Need for Doping in Graphene Research?
Graphene, a novel material for the field of electronics devices, is particularly well-suited for investigating the unique electronic property in absolute two dimensions. However, the absence of a semiconducting gap in pristine graphene limits most electronic applications. Doping is considered one of the most viable practices for controlling the properties of graphene.
Benefits of Doping for Graphene Applications
Doping, which is the process of tuning the Fermi level (EF) by controlling the charge carrier concentration, has been extensively investigated.
Pristine graphene is a semimetal with zero band gap whose fermi level is at the Dirac point. However, doping graphene with foreign atoms such as nitrogen, boron, sulfur, phosphorus, and the first three halogens breaks the symmetry of graphene, shifts the fermi level, and widens the bandgap.
These changes, in turn, influence the properties of the doped graphene, such as thermal stability, magnetic moment, optical characteristics, electron mobility, reactivity, spin densities, and many more.
For example, after nitrogen (N) doping in the pristine graphene, the electrons are injected into graphene (electron donor), referred to as n-type doping. This causes the Fermi level to shift above the Dirac point, thereby opening the bandgap, making it a potential candidate for use in semiconductors. Additionally, because N-doped graphene exhibits enhanced magnetic properties, it is being investigated for use in spintronics devices.
Boron (B) doped graphene has been shown to induce p-type doping due to its hole acceptor properties. This results in the Fermi level shifting below the Dirac point. B/N is of particular interest in the case of co-doping. It is reported that graphene containing h-BN patches behaves identically to pristine graphene.
Graphene doped with sulfur exhibits a high catalytic activity. Fluorine and chlorine-doped graphene are direct bandgap semiconductors, while brominated graphene is an indirect bandgap semiconductor.
Approaches for Graphene Doping
Broadly, two approaches have been proposed: i) substituting carbon atoms in graphene lattice with heteroatoms and ii) chemical adsorption on the graphene surface.
The heteroatom doping can be used to open the bandgap and tune the Fermi level of graphene. The boron and nitrogen atoms are excellent candidates for doping by substituting dopants in graphene lattice because of their identical atomic sizes to those of carbon and their hole acceptor and electron donor characteristics for substitutional B- and N-doping, respectively. This approach, however, introduces defects in the graphene structure.
In comparison, chemical adsorption of electronically interacting dopants onto graphene surface causes a local perturbation in the graphene structure while preserving the graphene structure's integrity and thus the graphene's electrical properties.
State of Art: The Main Techniques of Graphene Doping
Scientists have reported using the CVD technique to synthesize nitrogen-doped graphene from methane and ammonia gaseous precursors using a 25 nm thickness of Cu foil. Nitrogen precursors react efficiently with intermediates formed from the carbon precursor to structure the graphene lattice, forming stable carbon-nitrogen bonds with pyridinic or pyrrolic nitrogen atoms.
In the case of boron, the Arc discharge technique is used to obtain B-doped graphene by passing a high-current between graphite electrodes in the atmosphere of diborane (B2H6) and hydrogen gas vapor. There are some reports on the synthesizing graphene containing h-BN domains using ammonia-borane (NH3-BH3) and methane via the CVD process.
Segregation growth and solvothermal synthesis are additional methods of synthesis. Additionally, post-treatment techniques such as thermal annealing and plasma treatment have been investigated, resulting in the injection of a controlled number of heteroatoms.
Recent Progress in Doped Graphene Applications
Numerous intrinsic properties of heterographene have prompted scientists to investigate its potential as a battery and supercapacitor material. A composite of N-doped graphene and MoS2 sheets demonstrated a high capacity of 1025 mAh/g at a current density of 100 mA/g for 100 cycles for reversible lithium storage without noticeable degradation.
Another study found that the boron monodoped system retained 93% of its maximum specific capacitance after 10,000 cycles, while the P/N codoped system retained 101% of its capacity after 1000 cycles.
Graphene is extremely inert and interacts with other substances very limitedly. Chemical doping, on the other hand, can introduce active sensing sites into graphene. For instance, S-doped graphene (SG) was used as a NO2 sensor and demonstrated a wide sensitivity range of 500 ppm to 100 ppm at room temperature.
The doping of graphene enables its application in optoelectronic devices. Boron-doped graphene that exhibits significant performance in organic light-emitting diodes (OLEDs), with an external quantum efficiency of 24.6%, has been discovered.
Another study demonstrated bromine-doped graphene for glucose detection in human blood serum.
Challenges and Future of Graphene Doping
Heteroatom doping has been shown to introduce defects and disorders into the graphene structure, resulting in a decrease in electronic mobility and consequent degradation of electrical device performance. The application prospects for heterographene will significantly improve if doped graphene can be grown inexpensively and reliably with a controlled defect count.
Nonetheless, doping is a promising method for opening a bandgap in graphene, making it a potential contender for applications in circuits beyond conventional CMOS technology. Its use is also likely to be seen in various other potential applications, including light-emitting diodes, sensors, catalytic applications, thin-film transistors, and in the health, water, and agricultural sectors.
Continue reading: Investigating Graphene Nanomaterials with Thermogravimetric Analysis.
References and Further Reading
Ullah, S., Shi, Q., Zhou, J., Yang, X., Ta, H., Hasan, M., Ahmad, N., Fu, L., Bachmatiuk, A. and Rümmeli, M., (2020) Advances and Trends in Chemically Doped Graphene. Advanced Materials Interfaces, 7(24), p.2000999. https://doi.org/10.1002/admi.202000999
Lee, H., Paeng, K. and Kim, I., (2018) A review of doping modulation in graphene. Synthetic Metals, 244, pp.36-47. https://doi.org/10.1016/j.synthmet.2018.07.001