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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
What are the chemical aspects of graphene as a novel 2D material and how do they relate to the molecular structure? This book addresses these important questions from a theoretical and computational standpoint.
Graphene Chemistry: Theoretical Perspectives presents recent exciting developments to correlate graphene’s properties and functions to its structure through state-of-the-art computational studies. This book focuses on the chemistry aspect of the structure-property relationship for many fascinating derivatives of graphene; various properties such as electronic structure, magnetism, and chemical reactivity, as well as potential applications in energy storage, catalysis, and nanoelectronics are covered. The book also includes two chapters with significant experimental portions, demonstrating how deep insights can be obtained by joint experimental and theoretical efforts.
Topics covered include:
- Graphene ribbons: Edges, magnetism, preparation from unzipping, and electronic transport
- Nanographenes: Properties, reactivity, and synthesis
- Clar sextet rule in nanographene and graphene nanoribbons
- Porous graphene, nanomeshes, and graphene-based architecture and assemblies
- Doped graphene: Theory, synthesis, characterization and applications
- Mechanisms of graphene growth in chemical vapor deposition
- Surface adsorption and functionalization of graphene
- Conversion between graphene and graphene oxide
- Applications in gas separation, hydrogen storage, and catalysis
Graphene Chemistry: Theoretical Perspectives provides a useful overview for computational and theoretical chemists who are active in this field and those who have not studied graphene before. It is also a valuable resource for experimentalist scientists working on graphene and related materials, who will benefit from many concepts and properties discussed here.
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Seitenzahl: 856
Veröffentlichungsjahr: 2013
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Contents
Cover
Title Page
Copyright
List of Contributors
Preface
Acknowledgements
Chapter 1: Introduction
References
Chapter 2: Intrinsic Magnetism in Edge-Reconstructed Zigzag Graphene Nanoribbons
2.1 Methodology
2.2 Polyacene
2.3 Polyazulene
2.4 Edge-Reconstructed Graphene
2.5 Conclusion
Acknowledgments
References
Chapter 3: Understanding Aromaticity of Graphene and Graphene Nanoribbons by the Clar Sextet Rule
3.1 Introduction
3.2 Armchair Graphene Nanoribbons
3.3 Zigzag Graphene Nanoribbons
3.4 Aromaticity of Graphene
3.5 Perspectives
Acknowledgements
References
Chapter 4: Physical Properties of Graphene Nanoribbons: Insights from First-Principles Studies
4.1 Introduction
4.2 Electronic Properties of Graphene Nanoribbons
4.3 Mechanical and Electromechanical Properties of GNRs
4.4 Summary
Acknowledgements
References
Chapter 5: Cutting Graphitic Materials: A Promising Way to Prepare Graphene Nanoribbons
5.1 Introduction
5.2 Oxidative Cutting of Graphene Sheets
5.3 Unzipping Carbon Nanotubes
5.4 Beyond Oxidative Cutting
5.5 Summary
References
Chapter 6: Properties of Nanographenes
6.1 Introduction
6.2 Synthesis
6.3 Computation
6.4 Geometry of Zigzag-Edged Hexangulenes
6.5 Geometry of Armchair-Edged Hexangulenes
6.6 Geometry of Zigzag-Edged Triangulenes
6.7 Magnetism of Zigzag-Edged Hexangulenes
6.8 Magnetism of Zigzag-Edged Triangulenes
6.9 Chimeric Magnetism
6.10 Magnetism of Oligocenes, Bisanthene-Homologs, Squares and Rectangles
6.11 Concluding Remarks
Acknowledgment
References
Chapter 7: Porous Graphene and Nanomeshes
7.1 Introduction
7.2 Transition State Theory
7.3 Gas and Isotope Separation
7.4 Conclusion and Perspectives
Acknowledgement
References
Chapter 8: Graphene-Based Architecture and Assemblies
8.1 Introduction
8.2 Fullerene Polymers
8.3 Carbon Nanotube Superarchitecture
8.4 Graphene Superarchitectures
8.5 C60/Carbon Nanotube/Graphene Hybrid Superarchitectures
8.6 Boron-Nitride Nanotubes and Monolayer Superarchitectures
8.7 Conclusion
Acknowledgments
References
Chapter 9: Doped Graphene: Theory, Synthesis, Characterization, and Applications
9.1 Introduction
9.2 Substitutional Doping of Graphene Sheets
9.3 Substitutional Doping of Graphene Nanoribbons
9.4 Synthesis and Characterization Techniques of Doped Graphene
9.5 Applications of Doped Graphene Sheets and Nanoribbons
9.6 Future Work
Acknowledgments
References
Chapter 10: Adsorption of Molecules on Graphene
10.1 Introduction
10.2 Physisorption versus Chemisorption
10.3 General Aspects of Adsorption of Molecules on Graphene
10.4 Various Ways of Doping Graphene with Molecules
10.5 Enhancing the Graphene-Molecule Interaction
10.6 Conclusion
References
Chapter 11: Surface Functionalization of Graphene
11.1 Introduction
11.2 Functionalized Graphene: Properties and Challenges
11.3 Theoretical Approach
11.4 Interaction of Graphene with Specific Atoms and Functional Groups
11.5 Surface Functionalization of Graphene Nanoribbons
11.6 Conclusions
References
Chapter 12: Mechanisms of Graphene Chemical Vapor Deposition (CVD) Growth
12.1 Background
12.2 The Initial Nucleation Stage of Graphene CVD Growth
12.3 Continuous Growth of Graphene
12.4 Graphene Orientation Determination in CVD Growth
12.5 Summary and Perspectives
References
Chapter 13: From Graphene to Graphene Oxide and Back
13.1 Introduction
13.2 From Graphene to Graphene Oxide
13.3 Modeling Using PBC Models
13.4 From Graphene Oxide back to Graphene
13.5 Concluding Remarks
Acknowledgement
References
Chapter 14: Electronic Transport in Graphitic Carbon Nanoribbons
14.1 Introduction
14.2 Theoretical Background
14.3 From Graphene to Ribbons
14.4 Graphene Nanoribbon Synthesis and Processing
14.5 Tailoring GNR’s Electronic Properties
14.6 Thermoelectric Properties of Graphene-Based Materials
14.7 Conclusions
Acknowledgements
References
Chapter 15: Graphene-Based Materials as Nanocatalysts
15.1 Introduction
15.2 Electrocatalysts
15.3 Photocatalysts
15.4 CO Oxidation
15.5 Others
15.6 Conclusion
Acknowledgements
References
Chapter 16: Hydrogen Storage in Graphene
16.1 Introduction
16.2 Hydrogen Storage in Molecule Form
16.3 Hydrogen Storage in Atomic Form
16.4 Conclusion
Acknowledgements
References
Chapter 17: Linking Theory to Reactivity and Properties of Nanographenes
17.1 Introduction
17.2 Nanographenes with Only Armchair Edges
17.3 Nanographenes with Both Armchair and Zigzag Edges
17.4 Nanographene with Only Zigzag Edges
17.5 Quinoidal Nanographenes
17.6 Conclusion
References
Chapter 18: Graphene Moiré Supported Metal Clusters for Model Catalytic Studies
18.1 Introduction
18.2 Graphene Moiré on Ru(0001)
18.3 Metal Cluster Formation on g/Ru(0001)
18.4 Two-dimensional Au Islands on g/Ru(0001) and its Catalytic Activity
18.5 Summary
Acknowledgments
References
Index
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Library of Congress Cataloging-in-Publication Data
Graphene chemistry : theoretical perspectives / edited by De-en Jiang, Zhongfang Chen. pages cm Includes bibliographical references and index. ISBN 978-1-119-94212-2 (hardback) 1. Graphene. I. Jiang, De-en, 1975– II. Chen, Zhongfang, 1971– QD341.H9G693 2014 546′.68142–dc23 2013015047
A catalogue record for this book is available from the British Library.
ISBN: 978-1-119-94212-2 (Hardback)
List of Contributors
Preface
The book before you is about the chemistry of graphene from theoretical perspectives. The sp2 carbon and its various manifestations are dear to all chemists’ hearts. Graphene is no exception. No doubt, graphene is a new wonder material. Since its isolation in 2004, graphene has rapidly risen to be one of the hottest stars for various disciplines of basic and applied sciences due to its many exciting unusual characteristics. Over 8000 papers related to graphene were published in 2012, and graphene keeps occupying the headlines of scientific news media.
As stated by John Dewey, “Every great advance in science has issued from a new audacity of the imagination.” The discovery of graphene was a strong proof. In 1947, Canadian theorist Philip Russell Wallace predicted the relativistic behavior of electrons of graphene using the tight-binding approximation, though no one believed then that such a monolayer could exist. The groundbreaking experiments on realizing graphene by mechanical cleavage in 2004 totally changed our minds, and led to the Nobel Prize for Physics in 2010 being given to Andre Geim and Konstantin Novoselov. This path immediately reminds us of the history of fullerenes, especially C60, which was theoretically predicted by Eiji Osawa 1970, experimentally realized in 1985, and won Richard Smalley, Robert Curl and Harold Kroto the Nobel Prize for Chemistry in 1996.
Chemistry will play an increasingly important role in realizing graphene’s potentials, as synthesizing targeted graphene nanosystems, scaling up the synthesis, controlling defects, introducing functional groups, making composites, and so on, all involve chemistry. This book intends to provide a comprehensive state-of-the-art understanding of graphene chemistry at the atomic level from theoretical perspectives, especially on the structure-property-function relationship. The diverse group of contributors are leading experts and their dedication has made this book possible. Computational and theoretical chemists who have not studied graphene before will find this book useful in giving them an overview of the field. Graduate students in computational nanosciences will find this book particularly helpful in learning how one addresses important problems in an interesting and hot topic. Experimentalists in graphene will benefit from the many concepts and interesting properties predicted here for many novel graphene systems; what’s more, this book has two chapters with a significant experimental portion, which demonstrates how deeper insights can be obtained by joint experimental and theoretical efforts. Even experts in the theoretical studies of graphene will find this book interesting thanks to the diverse topics covered in this book.
Future discoveries, innovation, and development in graphene chemistry will continue to be accelerated by modern theoretical and computational studies. We hope that the readers in both theoretical and experimental communities will enjoy this book, and discover more of the magic power of the sp2 carbon.
Acknowledgements
We thank generous financial support from the funding agencies: the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy (D.J.); US Department of Defense and National Science Foundation (Z.C.). We are grateful to Sarah Hall of John Wiley & Sons, Ltd for initiating the idea of this book and working through the book proposal with us; and to Sarah Tilley of John Wiley & Sons, Ltd for helping us deliver this book. We thank all the authors who contributed to this book and the reviewers for their time; and Ms. Adi Shinar for creating the cover image.
1
Introduction
De-en Jianga and Zhongfang Chenb
aChemical Sciences Division, Oak Ridge National Laboratory, USA
bDepartment of Chemistry, Institute for Functional Nanomaterials, University of Puerto Rico, USA
From the breakthrough discovery in 2004 [1], to the award of the Nobel Prize for Physics to Geim and Novosolov in 2010, it took only six years for graphene to reach the pinnacle of scientific research. However, this is not the first time that sp2 carbon has won a Nobel Prize. Remember that Smalley, Kroto, and Curl won the Nobel Prize for Chemistry in 1996 for their discovery of fullerenes in 1985 [2]; only that took 11 years. What's more interesting is what the late Smalley said in his Nobel lecture: “Carbon has this genius of making a chemically stable two-dimensional, one-atom-thick membrane in a three-dimensional world. And that, I believe, is going to be very important in the future of chemistry and technology in general.” We have to admire his foresight of the explosion of interest in graphene 10 years later.
The fact that graphene won the Nobel Prize for Physics also reflects the community's focus on the physics aspect of graphene research. However, any large-scale application of graphene would undoubtedly rely on the chemistry of graphene. The expanding interest in graphene oxide through the Hummers method [3] or chemical-vapor deposition [4] approach to graphene synthesis are just two typical examples. The versatility of chemistry presents endless opportunities in graphene research.
Why do we focus on the theoretical perspectives of graphene chemistry? On one hand, there are books already discussing the experimental aspects of graphene chemistry; on the other hand, graphene provides an ideal proving ground for testing theoretical methods and computational imagination. In addition, sp2 or graphitic carbons are the basis of many important materials, such as carbon fibers for building cars and planes; activated carbons for supercapacitors; and graphite and hard carbons for lithium-ion batteries. Understanding graphene chemistry would also lay a foundation of understanding of complex carbonaceous materials. Therefore, the aim of this book is to deliver a comprehensive view of graphene chemistry from various theoretical and computational perspectives.
As a truly two-dimensional system, the honeycomb lattice of graphene has given rise to many interesting physical properties. However, no size is infinite in the real world and we eventually come to the edge of the graphene sheet just like the vast ocean greeting the shore line. Thus, we can expect that edge geometry will have a profound effect on the π-electronic structure.
A chemist relates to graphene by thinking about benzene: graphene is nothing but fused benzene rings. The geometric character is the same: each carbon atom has sp2 hybridization and contributes one 2pz electron for π bonding. However, when contrasting the electronic structure of benzene with that of graphene, we can immediately see the difference: the benzene molecule has a large HOMO-LUMO gap (Figure 1.1), while graphene is a zero-gap semiconductor where the conduction band and the valence band touch at one point at the Fermi level (Figure 1.2). How does the electronic structure evolve from a large-gap six-membered ring to a zero-gap lattice of infinite number of fused rings? This is a rather intriguing question. It turns out that the chemical details matter. Let's take acenes, or linearly fused benzene rings, as an example. Quite a few theoretical papers have been devoted to examining the evolution of the electronic structure with the number of benzene rings in these interesting systems, starting with the earlier studies by Whangbo et al. [5] and Kivelson et al. [6]. In the experimental community, researchers have been trying to synthesize ever longer acenes. The record so far seems to be a nonacene derivative which has nine fused rings as its core [7]. Chapter 2 examines acenes or polyacenes by applying the effective valence bond model and the density-matrix-renormalization-group method, compared to density functional theory (DFT). More often than not, the linearly fused six-membered rings may not be as happy as alternating five-membered and seven-membered rings. This leads to a class of molecules called fused-azulenes or polyazulenes which also forms on the graphene edge due to reconstruction. Chapter 2 addresses this system too.
Figure 1.1 The π-orbitals of the benzene molecule
Figure 1.2 The graphene lattice (left) and its electronic density of states (right) computed using density functional theory
Extending acenes infinitely results in a ribbon with two zigzag edges. The width of the ribbon can also be further increased while preserving the two zigzag edges. What is unique about the zigzag edge is the edge state, as predicted by Fujita et al. [8–9] from the tight-binding method. This pioneering study has inspired many follow-up theoretical investigations and experiments. DFT calculations in particular, revealed that the edge state also possesses a radical-like chemical reactivity [10]. Chapter 4 examines the electronic, mechanical, and electromechanical properties of both zigzag-edged and armchair-edged graphene ribbons.
The uniqueness of the zigzag edge not only manifests itself in ribbons but also in nanographenes. This is fundamentally related to the inherent instability of the zigzag edge. Furthermore, this may be due to the difficulty that π-electrons have in forming localized double bonds since the consecutive zigzag edges do not support bond-length variation well [11]. In contrast, this is not a problem for armchair edges. The geometry-related π-electron distribution is captured by the Clar's sextet rule which seems to be particularly useful in predicting stable nanographenes [12] and ribbons. Chapter 3 seeks to understand aromaticity of graphene and graphene nanoribbons from the perspective of Clar's sextet rule, while Chapter 4 also briefly discusses the electronic structure of nanographenes or finite graphene flakes as the authors call them. Chapter 6 focuses on nanographene exclusively where the authors interrogates C–C bond length variation, orbital energies, and magnetization with the nanographene's size.
A most natural way to relate carbon nanotubes and graphene ribbons is probably to think of unzipping a carbon nanotube to a graphene ribbon. Amazingly, this idea, put forward for fun in 2007 [10] (Figure 1.3), was realized by two experiments two years later through plasma etching [13] and chemical oxidation [14]. Chapter 5 examines the mechanisms of various ways to cut open not only carbon nanotubes, but also graphene itself.
Figure 1.3 Cutting open a carbon nanotube leads to a graphene nanoribbon. Reprinted with permission from [10] © 2007 American Institute of Physics
A straightforward way to utilize the one-atom thickness of graphene is to use it as a membrane, since a membrane's permeance is inversely proportional to its thickness. But a perfect graphene is impermeable to molecules as small as helium [15]. Hence, it is necessary to create holes to empower the graphene sheet for membrane separations; for example, the pore as shown in Figure 1.4 was computationally shown to be highly selective towards H2 [16] and CO2 [17]. More interestingly, this idea of using porous graphene to sieve gas molecules was recently experimentally confirmed through a clever setup [18]. Chapter 7 gives a detailed discussion of how to apply transition state theory to understand porous graphene and graphene nanomeshes for gas and isotope separations.
Figure 1.4 H2 molecule inside a subnanometer pore in the graphene sheet. Electron density contours of both H2 and the porous graphene are shown. Reprinted with permission from [16] © 2009 American Chemical Society
Using Lego bricks, a kid will let the imagination fly and build all sorts of things. Similarly, when given sp2-carbon building blocks, namely, fullerene, carbon nanotubes, and graphene, scientists can assemble them into sp2-hybridized carbon-based superarchitectures, such as fullerene polymers, nanotube clathrates, nanopeapods, nanobuds, nanofunnels, and nanofoams. Chapter 8 reviews the recent efforts in designing the prototypes of such materials and discusses their potential applications in electronics and energy storage.
Doping is an important topic in terms of graphene chemistry, as it is the major approach to opening and tuning the electronic band gap of graphene for device applications, and for modifying the graphene surface to improve reactivity and its interface with other materials for desired applications. Chapter 9 reviews different ways to dope graphene and their resulting changes in graphene's properties, as well as the characterization of doped graphenes and their applications.
Interaction between graphene and a foreign atom, functional group, or molecule is also interesting. From a chemical perspective, this is directly related to functionalization of graphene, a hot topic in itself [19]. From a physical perspective, one immediately thinks about sensing. The preference of π-electron delocalization in graphene dictates that converting a sp2 carbon to sp3 won't be easy. However, this could happen in a pair-wise fashion [20]. Chapter 10 examines in detail the physical adsorption of molecules on graphene, while Chapter 11 focuses on the chemical interaction of functional groups such as –OH with the graphene sheet.
Chemical vapor deposition is the method of choice for large-scale production of high-quality graphene. Various substrates especially copper have been used for CVD growth of graphene. Chapter 12 discusses the mechanistic insights into graphene CVD growth obtained by DFT computations for the nucleation stage and the kinetic Wulff plots for the growth stage.
Graphene oxide is the preferred medium by chemists and materials scientists for manipulating graphene due to its easy preparation and processability [21]. Due to the difficulties in experimentally characterizing the oxygen-containing groups, computational methods are of great help. Chapter 13 discusses the mechanistic insights into oxidation of graphene and reduction of graphene oxide from DFT computation.
Applications of graphene in electronic devices rely on our understanding how the electrons in graphene nanostructures respond to applied potentials. In other words, how do the factors governing the nanographene electronic structure (such as zigzag edges versus armchair edges, doping, boundary shape, etc.) manifest in electronic transport? Chapter 14 reviews recent progress in elucidating the electronic transport of graphene nanoribbons and their assemblies.
Graphene has many applications in energy-related fields, such as batteries [22–23], pseudo-capacitors [24–25] and electrochemical double-layer capacitors [26]. More excitingly, nitrogen doped graphene and carbon nanotubes have been found to be excellent fuel cell catalysts [27]. Chapter 15 covers a variety of topics related to catalysis by graphene, including oxygen reduction reaction, photocatalysis, and CO oxidation. Chapter 16 focuses on hydrogen storage by graphene-based materials.
Chapter 17 and Chapter 18 are unique in this book in that they have a significant experimental portion. This is to show the ever-increasing need to integrate computational insights into experimental discoveries. Chapter 17 is written by experts in synthesis of nanographenes who examine the challenge, strategy, and reward in making novel nanographene molecules of unique electronic and magnetic properties, often predicted by theory. Chapter 18 examines the growth of metal clusters on the moiré structure of graphene on the Ru(0001) surface: both surface science experiments and large-scale DFT calculations are used to understand catalysis in such a model system.
The chapters in this book aim to provide the reader a comprehensive snapshot of our present understanding of graphene chemistry from theoretical perspectives. We hope that both experimental and computational researchers will find reading through the chapters a rewarding experience.
References
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2. Kroto, H. W.; Heath, J. R.; Obrien, S. C.; Curl, R. F.; and Smalley, R. E., “C-60 – Buckminsterfullerene,” Nature, 1985, 318, 162–163.
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5. Whangbo, M. H.; Hoffmann, R. and Woodward, R. B., “Conjugated one and two-dimensional polymers,” Proc. R. Soc. London A, 1979, 366, 23–46.
6. Kivelson, S. and Chapman, O. L., “Polyacene and a new class of quasi-one-dimensional conductors,” Phys. Rev. B, 1983, 28, 7236–7243.
7. Purushothaman, B.; Bruzek, M.; Parkin, S. R.; Miller, A. F. and Anthony, J. E., “Synthesis and structural characterization of crystalline nonacenes,” Angew. Chem.-Int. Edit., 2011, 50, 7013–7017.
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9. Nakada, K.; Fujita, M.; Dresselhaus, G. and Dresselhaus, M. S., “Edge state in graphene ribbons: Nanometer size effect and edge shape dependence,” Phys. Rev. B, 1996, 54, 17954–17961.
10. Jiang, D. E.; Sumpter, B. G. and Dai, S., “Unique chemical reactivity of a graphene nanoribbon's zigzag edge,” J. Chem. Phys., 2007, 126, 134701.
11. Jiang, D. E. and Dai, S., “Electronic ground state of higher acenes,” J. Phys. Chem. A, 2008, 112, 332–335.
12. Jiang, D. E. and Dai, S., “Circumacenes versus periacenes: HOMO-LUMO gap and transition from nonmagnetic to magnetic ground state with size,” Chem. Phys. Lett., 2008, 466, 72–75.
13. Jiao, L. Y.; Zhang, L.; Wang, X. R.; Diankov, G. and Dai, H. J., “Narrow graphene nanoribbons from carbon nanotubes,” Nature, 2009, 458, 877–880.
14. Kosynkin, D. V.; Higginbotham, A. L.; Sinitskii, A.; Lomeda, J. R.; Dimiev, A.; Price, B. K. and Tour, J. M., “Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons,” Nature, 2009, 458, 872-U5.
15. Bunch, J. S.; Verbridge, S. S.; Alden, J. S.; van der Zande, A. M.; Parpia, J. M.; Craighead, H. G. and McEuen, P. L., “Impermeable atomic membranes from graphene sheets,” Nano Lett., 2008, 8, 2458–2462.
16. Jiang, D. E.; Cooper, V. R. and Dai, S., “Porous graphene as the ultimate membrane for gas separation,” Nano Lett., 2009, 9, 4019–4024.
17. Liu, H.; Cooper, V. R.; Dai, S. and Jiang, D. E., “Windowed carbon nanotubes for efficient CO2 removal from natural gas,” J. Phys. Chem. L., 2012, 3, 3343–3347.
18. Koenig, S. P.; Wang, L. D.; Pellegrino, J. and Bunch, J. S., “Selective molecular sieving through porous graphene,” Nat. Nanotechnol., 2012, 7, 728–732.
19. Sarkar, S.; Bekyarova, E. and Haddon, R. C., “Chemistry at the Dirac point: Diels-Alder reactivity of graphene,” Accounts Chem. Res., 2012, 45, 673–682.
20. Jiang, D. E.; Sumpter, B. G. and Dai, S., “How do aryl groups attach to a graphene sheet?,” J. Phys. Chem. B, 2006, 110, 23628–23632.
21. Kim, J.; Cote, L. J. and Huang, J. X., “Two dimensional soft material: new faces of graphene oxide,” Accounts Chem. Res., 2012, 45, 1356–1364.
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2
Intrinsic Magnetism in Edge-Reconstructed Zigzag Graphene Nanoribbons
Zexing Qu and Chungen Liu
Institute of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, China
Graphene, a two-dimensional carbon-based monolayer honeycomb lattice, is considered to be a prospective material for future electronics [1–3]. One-dimensional nano-structure strips of graphene a few nanometers in width, known as graphene nanoribbons (GNR) have been chemically synthesized and studied in laboratory, [4,5] which quickly attracted attention from theorists [4,5].
Zigzag-terminated graphene nanoribbons (ZGNRs) are of particular interest, due to observed room-temperature ferromagnetism as well as indirect evidence of a magnetic edge state, suggesting potential applications for spintronics [2,10–14]. However, numerous theoretical studies on pure and hydrogen-passivated ZGNRs using density functional theory (DFT) methods have predicted an anti-ferromagnetic (AFM) ground state with a minor band gap of approximately several meV between the lowest ferromagnetic (FM) and AFM states [15–18]. It seems that, instead of being an inherent feature of GNRs, room-temperature magnetism could come from easily generated edge or in-body structure defects.
Edge defects, which could shift the edge states from their Fermi energy level (EF), play a critical role in tuning the electronic properties of graphene nanoribbons [17,19–21]. Among them, a novel mechanism is the planar edge reconstructions on zigzag edges, resulted in the appearance of alternating pentagon-heptagon carbon rings on those edges. It is believed that the energy barrier involved in edge reconstruction is less than 1 eV [21]. Such reconstructed structures are usually denoted Rc-ZGNRs. As revealed by density functional theory (DFT) computations, a Rc-ZGNR edge is energetically more favorable by 0.35 eVÅ than a normal zigzag edge [19,22]. The existence of different lengths of alternating pentagon-heptagon structure on ZGNR edges has already been confirmed by aberration-corrected transmission electron microscopy (TEM) observations [23,24].
Furthermore, spin-polarized DFT calculations on one-edge reconstructed ZGNRs indicated an FM ground state, with a finite magnetic moment arising from the opposite (unreconstructed) zigzag edge [19], which supplies a yet to be confirmed theoretical foundation of the ferromagnetism of GNRs, since that the calculated energy gap between the lowest FM and AFM states is too small to maintain a stable magnetic ground state at room temperature [17]. At this stage, the origin of room-temperature magnetism of ZGNRs remains an open problem.
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