1. Introduction

Graphene is a one-atomic thick sheet composed of sp2 carbon atoms arranged in a honeycomb lattice. Since first isolated in 2004, graphene has been received tremendous attention due to its intriguing properties such as high electron mobility, anomalous quantum Hall effect and long spin coherence length. Doping graphene with heteroatoms, replacing some of carbon atoms in a honeycomb lattice with different atoms, gives rise to some unusual phenomena that cannot be observed in non-doped graphene. It was reported that nitrogen (N)-doped graphene exhibits n-type semiconducting behavior, while pristine graphene shows semi-metallic behavior. It implies doping is a promising method of modifying the electronic structure of graphene, and plays an important role in band gap engineering. Defects and vacancies as well as impurities modify the electronic structure of graphene. Doping heteroatoms to graphene and introducing defects are essential to apply graphene to electronic devices. In this research I develop a method of synthesizing nitrogen-doped graphene and boron (B), N-doped graphene from heteroatom-containing organic molecules and elucidate the relationship between the molecular structures of source materials and the products. I also employ ultraviolet (UV)-irradiation to introduce defects into graphene.

2. Synthesis of Doped Graphene from Heteroatom-Containing Organic Compounds

2.1 Synthesis of N-Doped Graphene

N-doped graphene was synthesized on a Pt(111) surface from N-containing molecules via chemical vapor deposition process. Figure 1 shows the four N-containing molecules which were used as source materials of N-doped graphene, pyridine (C5H5N), acrylonitrile (C3H3N), julolidine (C(12)H(15)N) and melamine (C3H6N6). A Pt(111) surface was cleaned by cycles of Ar+ sputtering and annealing in an ultrahigh vacuum chamber. The substrate was heated at 500 °C and exposed to pyridine, acrylonitrile and julolidine ambient, while melamine was deposited from Knudsen-cell. X-ray photoelectron spectroscopy (XPS) measurements were conducted in situ for these samples.

Figure 2(a) shows the C 1s region of XPS spectra of the samples. The spectrum of the sample from ethylene is also shown in Fig. 2(a) as a reference since it is already known that graphene is formed from ethylene on a Pt(111) surface. C 1s peaks of the samples indicate graphene was presumably synthesized on a Pt surface from pyridine, acrylonitrile and julolidine because the peaks are positioned at around 284.0 eV, the same peak position as the sample from ethylene (Fig. 2(a)). In a region of N 1s (Fig. 2(b)), however, a peak was detected only for the sample from pyridine, while the samples from acrylonitrile and julolidine show no peaks. On the basis of these results, N-doped graphene was synthesized from pyridine, and non-doped graphene was obtained from acrylonitrile and julolidine. C 1s peak of the sample from melamine appears at higher binding energy, which cannot be assigned to graphene. The intensity ratio of C 1s and N 1s for the sample from melamine exhibits the sample contains more nitrogen than carbon. From this result, graphene cannot be formed from melamine by depositing it onto a heated Pt(111) surface.

It is obvious that the final product using this method strongly depends on the starting materials. This dependence can arise from the strength of chemical bonds in source molecules. The bond strength of skeletal bonds in a pyridine molecule is similar to each other. Therefore nitrogen atoms can be incorporated into graphene when carbon atoms construct sp2 network on a Pt surface kept at an appropriate heating temperature (Fig. 3(a)). In contrast, the single bonds in an acrylonitrile and a julolidine molecule are much more likely to be broken than the other skeletal bonds due to the weaker bond strength. Schematic illustration of growth model for acrylonitrile is shown in Fig. 3(b). Nitrogen atoms are separated off from carbon atoms and evaporate as volatile molecules such as HCN and C2N2.

2.2 Oxygen adsorption activity of N-doped graphene

Oxygen reduction reaction (ORR) activity is one of the most interesting topics of N-doped graphene. To elucidate the first step of the reaction, adsorption structure of oxygen on N-doped graphene was investigated by means of XPS. N-doped graphene synthesized from pyridine on a Pt(111) surface was exposed to 1.0×105 Pa oxygen for 10 min. Non-doped graphene from benzene was also exposed to oxygen.

No O 1s peak was detected for non-doped graphene. On the other hand, N-doped graphene exhibits O 1s peaks which can be assigned to oxygen functional groups. The relationship between N content in N-doped graphene and the amount of adsorbed oxygen is shown in Fig. 4. The higher the N content of N-doped graphene is, the more oxygen atoms are adsorbed. It is expected the nitrogen atoms incorporated in graphene modify the electronic structure of adjacent atoms, and those atoms can be active sites for oxygen adsorption. I also observed the changes in N 1s peak shape after oxygen adsorption. The peak can be fitted with two Gaussian functions of which the center peaks locate at graphitic N (N atoms bonded with three sp2 C neighbors) and pyridinic N (N atoms with two sp2 C neighbors, refer to Fig.5). The total amount of nitrogen does not change after the exposure to oxygen, while the intensity ratio of graphitic N to pyridinic N changes.

Based on the results, N atoms incorporated in graphene enhance the oxygen adsorption. Oxygen chemically adsorbs onto N-doped graphene and cause structural change around N atoms.

2.3 Synthesis of B, N-Doped Graphene

In addition to N-doped graphene, I synthesized B, N-doped graphene in a same way by using hexaphenylborazine (HPB, C(36)H(30)B3N3, shown in Fig.1(e)) as a source material. HPB was deposited onto a Pt(111) surface heated at various temperatures and in situ XPS measurements were taken. The C 1s spectra of the samples show a peak shifts to lower binding energy down to 284.0 eV with increasing substrate temperature (TS). N 1s peak becomes weaker as TS increases. N 1s peak can be observed up to 600 °C, and disappears at 800 °C. In the region of B 1s, B 1s peak can be detected as high as TS = 400 °C. Judged from these XPS data, HPB was decomposed on a heated Pt(111) surface and form graphene. Both boron and nitrogen doped graphene was synthesized at TS = 400 °C and N-doped graphene grew at TS = 600 °C, while non-doped graphene was obtained at TS = 800 °C.

3. Defect Formation on Graphene by Ultraviolet-Irradiation

The effect of UV-irradiation to graphene is investigated. Graphene was grown on several substrate, copper foil, Ru(0001) and Pt(111). Graphene on a Si wafer was also prepared by transferring graphene grown on Cu foil. I also used Highly Oriented Pyrolytic Graphite (HOPG) for comparison. Deuterium lamp was used as a UV-light source. Samples were UV-irradiated in various ambient at 10 Pa for 2 hours, followed by Raman spectroscopy.

Figure 6(a) shows the intensity ratios of the Raman D peak to the G peak (ID/IG). Graphene on a Si wafer (tGr) shows even higher ID/IG ratio, which indicates high density of defects, than graphene on other substrates. This result implies the suppression of defect formation by the interaction between graphene and substrates, because the Si surface hardly affects the electronic structure of graphene. The dotted line area in Fig. 6(a) is magnified in Fig. 6(b). The ID/IG ratio becomes higher for all irradiated samples except the one on a Pt(111) surface. The similar behavior in ID/IG ratio for graphene on a Ru(0001) surface and HOPG comes from the number of layers because multilayer graphene grows on a Ru(0001) surface while graphene on a Cu foil and a Pt(111) surface is a single layer. HOPG consists of a number of graphene layers, and hence exhibits a similar response to UV-irradiation. The difference in behavior between the graphenes on a Pt(111) surface and on Cu foil might be due to the Fermi level shift caused by metal contacts. The Fermi level of graphene shifts lower on a Pt surface, while contact with a Cu surface heightens the Fermi level. This opposite Fermi level shift affects the response of graphene to UV-light, and gives rise to different defect formation. It is clearly seen that samples irradiated in NH3 have higher ID/IG ratio. Photon energy of Deuterium lamp is enough high for photodissociation reaction of NH3 (NH3 → NH2 + H), which generates active radicals. Therefore such radicals attack graphene and cause high ID/IG ratio.

4. Summary

In my study, I developed methods of chemically doping graphene with heteroatoms and explore the defect formation of graphene by UV-irradiation. Several heteroatom-containing molecules were used in order to synthesize doped graphene on a Pt(111) surface by CVD process. I have clarified the relationship between the molecular structure and the potentiality of synthesizing doped graphene. Adsorption structure of oxygen on N-doped graphene was investigated with XPS. It was revealed that doped N atoms enhance the adsorption activity for oxygen, and structure of N-doped graphene changes by oxygen adsorption. Defect formation of graphene by UV-irradiation was also explored on several substrates and in several ambient. It became clear that defect formation is highly affected by substrates, and NH3 ambient causes high ID/IG ratio as NH3 produces active radicals through photodissociation reaction.

Fig. 1 Molecular structure of (a) pyridine, (b) acrylonitrile, (c) julolidine, (d) melamine, and (e) hexaphenylborazine

Fig. 2 XPS spectra of the samples in the region of (a) C 1s and (b) N 1s.

Fig. 3 Growth model of graphene on a heated Pt surface from (a) pyridine and (b) acrylonitrile.

Fig. 4 Amount of adsorbed oxygen as a function of nitrogen content in N-doped graphene.

Fig. 5 Schematic illustration of B,N-doped graphene. Gray, black and white balls represent carbon, nitrogen and boron atoms, respectively.

Fig. 6 (a) Intensity ratios of the Raman D peak to G peak. (b) The magnified figure of dotted line area in (a).

本論文は7章からなる.

第1章ではグラフェンに関しての基礎物理およびグラフェンの応用例について記述した後, ドープされたグラフェンおよび欠陥を含むグラフェンの性質および近年の研究例が述べられている.

第2章では本研究で用いられた実験手法の詳細が記載されている. まず初めに試料合成法として化学気相成長法(CVD法)の一般論およびCVD法の利点が述べられ,本研究における CVDグラフェンの作製手順と紫外線照射の手法・装置が記されている.次に試料の評価に必要な光電子分光法とその解析手法,ラマン分光法の原理およびグラフェンのラマンスペクトルの解釈について説明されている.

第3章では窒素ドープされたグラフェンについての研究内容が述べられている. 種々の窒素含有炭化水素分子を出発原料としたCVD合成を試した結果,ピリジンの縮重合で窒素ドープグラフェンが成長する条件を見出した.これは単一の原料から窒素ドープされたグラフェンを作製した最初の研究例である. また, 窒素ドープグラフェン成長の温度依存性, さらに他の原料分子からの生成物との比較からグラフェン成長のメカニズムについて言及しており, ドープされたグラフェンの成長に関する基礎的な理解を示した点で先駆的な研究である.

第4章ではhexaphenylborazine (HPB)を原料として用いることによりホウ素と窒素が共ドープされたグラフェンの作製・評価の研究について述べられている. 原料としてHPBを用い, 基板温度を変えることによりドープされる化学種およびドープ量の異なるグラフェンが得られることを示しており, 単一の分子から生成物の構造を制御した独自性の高い研究である. さらにHPBに関して量子化学計算により電子状態を調べ, これまで熱力学的なデータしか調べられていなかったHPBの構造やエネルギー状態について新たな知見を得た.

第5章では窒素ドープグラフェンへの酸素吸着に関する研究について述べられている.ドープされた窒素がグラフェンの酸素吸着活性を向上させていることを明らかにし, 吸着に伴う窒素ドープグラフェンの構造変化を明らかにした. 窒素ドープグラフェンは燃料電池への応用が期待されていることから, その電極反応の初期段階である酸素吸着の情報を得たことは応用の面からも有用であり, 炭素材料からなる燃料電池の開発において基礎的な知見をもたらすと考えられる.

第6章では紫外線照射によるグラフェンへの欠陥生成に関する研究について述べられている. 金属基板上のグラフェンに対し様々な雰囲気下で紫外線照射をすることにより, 欠陥生成の基板依存性および雰囲気依存性を明らかにしている. この研究から, グラフェンの層数が欠陥生成に影響をおよぼすことが示唆され, 単層グラフェンの場合には金属との相互作用が欠陥生成の反応性に関与していることが示された. これまでグラフェンの欠陥に関する研究例として紫外線を用いたものはほとんどなく, グラフェンと光の相互作用に着目したこの研究は独自性の高い研究であるといえる.

第7章では第3 - 6章の総括を行い,主要な結果を要約している.

以上のように, 本論文ではグラフェンへ異種原子をドープする方法の開発とその特性の測定(酸素吸着活性), およびグラフェンへの欠陥生成について実験データに基づく詳細な解析がなされている. 電子デバイスや触媒など, 産業への様々な応用が期待されているグラフェンについてこれらの基礎的な知見を得たことは大変重要であり, 本研究をベースとした更なる研究の展開が期待される.

なお, 本論文はCha Wen Chang氏(第4章), 難波江裕太氏(第4章), 柿本雅明氏(第4章), 宮田清藏氏(第4章), 齊木幸一朗氏(第3 – 6章)との共同研究であるが, 論文提出者が主体となって実験, 解析, 考察を行ったものであり, 論文提出者の寄与が十分であると判断する. したがって, 博士(理学)の学位を授けるのに十分な資格を有すると認める.