Graphene, a single atomic layer of carbon with a hexagonal crystal structure, has been heavily investigated in the past decade for its many unique material properties. With its high electrical and thermal conductivity, near optical transparency, chemical inertness, and high mechanical strength, graphene is considered a very promising two-dimensional material for a broad range of applications, including energy storage, optical displays and sensors [1,2].
One challenge of working with graphene is the ability to process the material without compromising its structural integrity. With maximum applied powers of 10’s watts, our plasma cleaners are designed to affect surfaces only on an atomic level and are well-suited for treating graphene. In a series of application notes, we explore the multiple ways researchers have applied plasma treatment to tune the various material properties of graphene.
NITROGEN DOPING OF GRAPHENE
By using nitrogen (N2) gas, plasma can be applied to incorporate N atoms into the graphene structure and, in turn, modify its electronic properties. Nitrogen is considered an ideal element for doping carbon materials because of its similar atomic size and favorable electron configuration to form strong covalent bonds with carbon . The following papers describe applying N2 plasma to create N-doped graphene with semiconducting characteristics.
Wang et al. studied the modification of the intrinsic properties of graphene (as opposed to creating composite or hybrid graphene materials) to enhance its performance in electrocatalysis and biosensing applications. By controlling the N2 plasma treatment time, they produced N-doped graphene with dopant concentrations of 0.11-1.35 at. %. Based on TEM imaging, a schematic was proposed where N-doped graphene maintained a planar, two-dimensional structure, even after plasma treatment. The authors suggested that N atoms substitute into the graphene lattice with the presence of periodic defect sites. This induced a change in the electronic band structure and opened the energy bandgap to create semiconducting graphene.
Further assessment of the chemically-modified graphene was performed through electrochemical measurements and fabrication of biosensor devices with the graphene as the electrode material. Wang et al. found that N-doped graphene displayed good electrocatalytic activity for reduction of H2O2 and high selectivity and sensitivity for glucose biosensing. However, extended plasma exposures (>40 min) led to diminished electocatalytic activity, indicating destruction of the graphene layer.
Yanilmaz et al. investigated optimal N2 plasma conditions to produce N-doped graphene for potential electronic and optical applications. They also found that N dopant concentration increased with plasma treatment time. Optimal process conditions were required to bind N to plasma-generated defects (step edges) without formation of excessive defects that can dismantle the planar, 2D structure of graphene. Prolonged plasma treatment time (>30 min) and higher power destroyed the lattice structure and etched graphene without doping. Effective doping was attained with 15 min plasma treatment at Medium RF power, where the resulting N-doped graphene was homogeneous across a 20 μm x 20 μm area. Experimental results and theoretical calculations suggested that periodic adsorption of N atoms occurs on top of the C atoms rather than through N substitution within the graphene lattice.
Relevant Articles from Harrick Plasma Users
- Wang Y, Shao Y, Matson D, Li J and Lin Y. “Nitrogen-Doped Graphene and Its Application in Electrochemical Biosensing”. ACS Nano 2010 4(4): 1790-1798.
- Yanilmaz A, Tomak A, Akbali B, Bacaksiz C, Ozceri E, Ari O, Senger RT, Selamet Y and Zareie HM. “Nitrogen doping for facile and effective modification of graphene surfaces”. RSC Adv. 2017 7: 28383-28392.
Supplemental References (Do Not Report Using Harrick Plasma Equipment)
 Allen MJ, Tung VC and Kaner RB. “Honeycomb Carbon: A Review of Graphene”. Chem. Rev. 2010 110(1): 132-145.
 Choi W, Lahiri I, Seelaboyina R and Kang YS. “Synthesis of Graphene and Its Applications: A Review”. Crit. Rev. Solid State 2010 35(1): 52-71.
 Wang H, Maiyalagan T and Wang X. “Review on Recent Progress in Nitrogen-Doped Graphene: Synthesis, Characterization, and Its Potential Applications”. ACS Catal. 2012 2: 781-794.