Elsevier

Carbon

Volume 159, 15 April 2020, Pages 579-585
Carbon

High-quality nitrogen-doped graphene films synthesized from pyridine via two-step chemical vapor deposition

https://doi.org/10.1016/j.carbon.2019.12.095Get rights and content

Abstract

Modulation of the electrical properties of graphene is of significant importance in advancing graphene electronics: it can be achieved by a Fermi level shift induced by electron acceptor/donor doping. Suitable doping methods involving low-temperature processes and offering long-term stability are imperative to practical applications for such materials. Here, we demonstrate a two-step chemical vapor deposition (CVD) technique for direct synthesis of N-doped graphene film from a pyridine feed-stock at 300 °C under ambient pressure. We extended the synthesis—classified into nucleation and lateral growth steps—by controlling the carbon partial pressure in the processing gases. This led to large-area, continuous N-doped graphene films of excellent quality with full surface coverage: for example, a film size of 2 in2, optical transmittance of 97.6%, and electron mobility of 1400 cm2 V−1 s−1. Our modified CVD method is expected to facilitate the direct synthesis of N-doped graphene in device manufacturing processes toward practical applications while keeping the underlying devices intact.

Introduction

Graphene has attracted intensive interest because of its outstanding physical and electrical properties, such as high carrier mobility, high thermal conductivity, flexibility and transparency, owing to the two-dimensional honeycomb structure of carbon atoms [[1], [2], [3], [4]]. The simple modulation of the electrical properties of graphene makes it a promising candidate for various nanoelectronic applications. Such modulation can be achieved by introducing a Fermi level shift employing electron acceptor/donor doping [[5], [6], [7], [8], [9], [10], [11], [12], [13]]. Several approaches such as surface treatment of using polymer electrolytes [5,6], decoration using metal [7] and metal oxide nanoparticles [8,9], and substitution of carbon atoms with other atoms, e.g., nitrogen and boron [[10], [11], [12], [13], [14], [15], [16]], have been taken to achieve such modifications. Using surface treatment and dopant decoration techniques, charge transfer between graphene and dopants can be facilitated to easily modulate the electrical properties of graphene [6,9]. However, the doping effects decline with time because the dopants lose the electrons to oxygen or water molecules in air [5,7]. Substitutional doping is, however, a suitable method in terms of doping stability arising from strong chemical bonds between carbon and dopant atoms [11]. To achieve substitution doping of carbon atoms with other atoms, e.g. nitrogen, graphene has been grown using methane as a carbon source and an ammonia dopant source in a chemical vapor deposition (CVD) process, wherein pristine graphene obtained from methane was post-annealed with NH3 at a high temperature above 850 °C for breaking C–C bonds and forming C–N bonds [10,11]. In addition, nitrogen-doped graphene films were synthesized via the CVD process from a mixture of CH4 and NH3 gases at 800 °C [12] or pyridine as N-containing aromatic hydrocarbon molecules above 930 °C [13,14]. However, the high-temperature process requires expensive and precise equipment and makes direct deposition of graphene in electronic device manufacturing processes infeasible due to the severe physical damage posed to substrates (e.g., metals, semiconductors, and the junctions between them) underneath the graphene. Therefore, synthesizing large-area N-doped graphene films at low temperatures is imperative for reducing the fabrication cost.

Several groups have investigated N-doped graphene synthesis at relatively lower temperatures employing laser-induced synthesis, plasma-enhanced CVD (PECVD), and thermal CVD using N-containing aromatic hydrocarbon molecules [[17], [18], [19], [20], [21], [22]]. Recently, low-temperature synthesis of N-doped graphene at 600 °C was demonstrated by laser-induced synthesis using a N-doped SiC substrate, but the atomic concentration of nitrogen in the graphene was very low at around 0.6 at.% [17]. In another study, N-doped graphene was achieved at 435 °C using PECVD, but defective graphene was formed due to the damage inflicted on the graphene surface by energetic plasma ions [18]. Furthermore, synthesis of N-doped graphene films at an even lower temperature (230–500 °C) has been demonstrated using nitrogen-containing organic compounds such as pyridine [19,20], acrylonitrile [20], pentachloropyridine [21], and methyl-and ketone-form molecules [22] as the carbon source. These works have enabled the synthesis of N-doped graphene at lower temperatures than those required for fabricating graphene from methane using low-pressure CVD (LPCVD), but have not led to large-area and high-quality graphene films. Therefore, significant challenges for the low-temperature synthesis of graphene film suitable for practical application still remain.

Among organic molecules, pyridine is the most favorable carbon source for the direct synthesis of high-quality N-doped graphene at temperatures as low as 300 °C without additional dopant or post-annealing processes. However, methods for achieving large-area, continuous N-doped graphene films at low temperatures has remained elusive [19]. In our previous work, the oxygen-free ambient pressure chemical vapor deposition (APCVD) technique was found to be favorable in the production of continuous graphene films using benzene at 300 °C [23]. In contrast to benzene-derived graphene, synthesis of pyridine-derived graphene is limited to the planar direction because the asymmetric molecule restricts graphene flakes to anisotropic and discontinuous growth. In this study, we developed a two-step synthesis route to improve the surface coverage of continuous N-doped graphene synthesized at temperatures as low as 300 °C, wherein a 2nd step with a higher carbon concentration than that in the 1st step could facilitate lateral growth. This led to full surface coverage and high-quality N-doped graphene films prepared under significantly reduced synthesis temperatures (below 400 °C).

Section snippets

Synthesis of N-doped graphene by one-step APCVD

Cu foil (0.127 mm, Alfa Aesar) was used as a catalytic substrate to synthesize graphene. The Cu foil was electrochemically polished by using phosphoric acid to remove native oxide and form smooth surface. The foil was then loaded into a 1-inch diameter CVD fused quartz tube [24]. Prior to graphene synthesis, the quartz tube was pumped down to ∼10−4 Torr and then purged with argon several times to remove the air contained in CVD system. In order to increase the grain size and improve the

Results and discussion

Using the “one-step synthesis process,” N-doped graphene was synthesized from pyridine (the nitrogen-containing carbon source) on pre-annealed Cu at 300 °C in 5–180 min under constant carrier gas flow (red line of schematic in Fig. S1). In order to rule out the effect of the annealing process for formation of graphene, we measured Raman spectra of electrochemically polished Cu, annealed Cu, and graphene-grown Cu (Fig. S2a). Raman spectra of the electrochemically polished Cu and annealed Cu

Conclusion

We demonstrated synthesis of large-area, continuous N-doped graphene films using a pyridine carbon source at the low temperature of 300 °C via a two-step APCVD process. This two-step synthesis process consists of nucleation and lateral growth steps. Controlling the carbon pressure maintained the growth rate during the entire process and led to the formation of large-area, continuous, N-doped monolayer graphene films. N-doped graphene grown at 300 °C showed a film size of 2 in2, optical

Author contributions

Son M and Ham MH conceived and designed the research. Son M synthesized the N-doped graphene and performed the characterizations of the samples. Chee SS, Kim SY, Lee W, Kim YH, Oh BY, Hwang JY and Lee BH assisted in doing experiments and analyses. Son M wrote the manuscript with input from Ham MH. All authors discussed the results and commented on the manuscript, and have given approval to the final version of the manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1A6A3A11049947), the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (20182020700110), Industrial Technology Innovation Program funded by the Ministry of trade, Industry & Energy (MOTIE) of the Republic of Korea (10052853), and

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