S1-4
RF plasma Assisted Nitrogen Doping to Graphene Lattice
RFプラズマを用いたグラフェン格子への窒素ドープ
Koichiro Saiki
斉木 幸一朗Department of Complexity Science and Engineering, The University of Tokyo
Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8561, Japan
東京大学新領域 〒277-8561 柏市柏の葉5-1-5
Nitrogen doping is expected to provide several intriguing properties to graphene. Nitrogen plasma
treatment to highly oriented pyrolytic graphite (HOPG) and graphene oxide (GO) samples causes doping of
nitrogen atom into the graphene layer. The amount and the site of doped nitrogen atoms depend on theplasma treatment time and the initial state of material. The nitrogen doping changes the work function ofgraphene layer from 4.3 eV to 5.4 eV and the decreases the band gap of GO. The plasma treatment is foundto be a powerful tool to tailor the electronic states of graphene-based materials.

1. Introduction
Doping of heteroatoms is one of the mostpromising ways to tune the electronic properties ofgraphene for achieving various industrialapplications. Nitrogen (N)-doped graphene has
been investigated as the potential candidates ofcathode catalyst for fuel cells, anode materials oflithium-ion batteries, electrodes of bio-sensors, andultracapacitors, etc. In most of these studies
N-doped graphene has been synthesized by avariety of methods and the effect of N content anddoped site on the device performance has beenclarified. However, the origin of improvement has
not been well understood, especially on the basis ofelectronic structures. In the present work, we havesynthesized
nitrogen-doped
grapheneandinvestigated the effect of nitrogen doping on theelectronic structure of graphene.
2. Experimental
Synthesis of N-doped graphene has been carriedout mostly by direct growth or post treatment. In
the present work we exposed a highly oriented
pyrolytic graphite (HOPG) substrate and a graphene
oxide (GO) sheet to radio frequency (RF) nitrogenplasma. The plasma treatment has several
advantages as compared with conventional
chemical treatment: usability of safer gases,
decrease in substrate temperature, etc. In theexperiment the RF power was set at 0.1 W under
the working pressure of 0.1 Pa. The specimen was
not heated during plasma treatment.
3. Result and Discussion
3.1 Effect of nitrogen doping on the work functionof graphene [1]We have prepared two kinds of substrates: one

was pristine HOPG (p-HOPG) with large grains
and the other was damaged HOPG (d-HOPG)
which had been exposed to Ar plasma beforehand.
Nitrogen doping was carried out to both p-HOPG
and d-HOPG. There are several kinds of doped
sites as shown in Fig. 1. The nitrogen content

Fig. 1 Nitrogen doped sites in graphene.

with its chemical state was investigated by X-ray
photoelectron spectroscopy (XPS). Figure 2
shows the nitrogen content at each doped site as afunction of N 2 plasma treatment time. In the caseof p-HOPG, nitrogen atoms are doped mostly at
the graphitic site at the initial stage. As theplasma treatment proceeds, the relative content ofFig. 2 Nitrogen content at each doped-site asa function of N2 plasma treatment time for
p-HOPG. The figures denote total amount ofnitrogen content.

Fig. 3 Nitrogen content at each doped-site
as a function of N2 plasma treatment timefor d-HOPG. The figures denote the total
amount of nitrogen.

pyridinic site increases. After 10 min treatment
the nitrogen content is almost saturated and thepyridinic N has the largest content. In the case ofd-HOPG, however, the pyridinic N is the main
component from the initial stage as shown inFig.3. This difference is ascribed to the structure,
to which the nitrogen atom is incorporated. For
the nearly perfect lattice (p-HOPG) only way todope N is to substitute the constituent C atoms.
On the other hand, there exists a considerable
amount of edge of graphene lattice in d-HOPG. In
that case N atoms has a chance to replace theedge C atoms, resulting in doping at a pyridinicor pyrrolic site.

GO is a semiconductor or an insulator with aband gap larger than 2 eV [2]. RF plasma
treatment to GO causes not only nitrogen doping
but also reduction. The oxygen content decreases
with treatment time together with an increase ofnitrogen content. The feature of doping resembles
that of d-HOPG in a way that nitrogen atoms are
doped at a pyridinic or pyrrolic site from theinitial stage. Figure 5 shows the change ofvalence electronic states of GO with N 2 plasma
treatment. As-grown GO has a wide gap below
the Fermi level (0 eV). With increasing plasma
treatment time the valence band maximum
becomes shallower. Since the conduction
minimum is not known at the present stage, thedecrease of band gap is 1.1 eV at least. In the caseof chemical treatment by use of hydrazine, ittakes several tens of hours to reduce the band gap
by 1 eV [3]. The present plasma treatment isfound to be effective for reduction of GO.

30 min10 min5 min
1 min
GOFig. 5 Change of valence band electronicstates of GO with N2 plasma treatment.

4. Summary
Use of RF plasma is a promising way to tune theelectronic property of graphene-based materials.
We show the examples to control the work functionof HOPG and the band gap of GO.

Fig. 4 Change of work function with N2
plasma treatment time for p-HOPG andd-HOPG.

Figure 4 shows the change in work function
with increasing N 2 plasma treatment time. The
work function monotonously increases withnitrogen doping for d-HOPG, while it firstly
decreases and re-increases for p-HOPG. The
difference reflects the fact that graphitic or
pyridinic nitrogen decreases or increases the workfunction.
3.2 Effect of nitrogen doping on the band gap ofGO

Acknowledgments
This work was done and is being done bymembers in my Laboratory. The author would liketo thank especially Mr. Keishi Akada and Dr. Seiji
Obata for their collaboration.
References[1] K. Akada, T. Terasawa, G. Imamura, S. Obata, K.
Saiki, Appl. Phys. Lett. 104, 131601 (2014).
[2] S. Obata, H. Tanaka, and K. Saiki, Carbon 55, 126 133 (2013).
[3] A. Mathkar et al., J. Phys. Chem. Lett. 3, 986 (2012).