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Defects in Graphene: Generation, Healing, and Their Effects on the Properties of Graphene: A Review
Journal of Materials Science & Technology 31 (2015) 599e606
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Defects in Graphene: Generation, Healing, and Their Effects on the Properties of Graphene: A Review Lili Liu 1, Miaoqing Qing 1, Yibo Wang 1, Shimou Chen 2, * 1 2
Department of Chemistry, School of Science, Beijing Technology and Business University, Beijing 100048, China Key Laboratory of Green Process and Engineering, Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS), Beijing 100190, China
a r t i c l e i n f o Article history: Received 28 September 2014 Received in revised form 15 November 2014 Accepted 19 November 2014 Available online 20 January 2015 Key words: Graphene Vacancy Defect healing Band gap modulation Properties of defective graphene
Graphene has attracted immense investigation since its discovery. Lattice imperfections are introduced into graphene unavoidably during graphene growth or processing. These structural defects are known to significantly affect electronic and chemical properties of graphene. A comprehensive understanding of graphene defect is thus of critical importance. Here we review the major progresses made in defectrelated engineering of graphene. Firstly, we give a brief introduction on the types of defects in graphene. Secondly, the generation and healing of the graphene defects are summarized. Then, the effects of defects on the chemical, electronic, magnetic, and mechanical properties of graphene are discussed. Finally, we address the associated challenges and prospects on the future study of defects in graphene and other nanocarbon materials. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
1. Introduction Graphene has raised extensive interest in the worldwide for its extraordinary thermal, mechanical, electrical and other properties[1,2]. Among all properties, the unique electronic properties are assumed to be the most intriguing aspect of graphene, for example, outstanding ballistic transport properties and longest mean free path at room temperature[3], distinctive integral and half-integral quantum hall effect[4,5], the highest mobility to increase the speed of devices[6], and so on. The mobility of graphene is significantly higher than that of the widely-used Si, of approximately 1400 cm2 V1 s1. Consequently, graphene has been considered as a candidate material for applications in post-silicon electronics. Graphene has a honeycomb lattice structure and a unit cell that contains two carbon atoms. Just like in carbon nanotubes, there are two different kinds of graphene ribbon edges: armchair and zigzag, which can influence the electronic properties of graphene. However, most electronic applications are handicapped by the absence of a semiconducting gap in pristine graphene. For example, devices made from the zero-bandgap graphene are difficult to switch off, losing the advantage of the low static power consumption of the
* Corresponding author. Prof., Ph.D.; Tel.: þ86 10 82544800; Fax: þ86 10 82544875. E-mail address: [email protected] (S. Chen).
complementary metal oxide semiconductor (CMOS) technology. Quantitatively, the Ion/Ioff ratios for graphene-based field-effect transistors (GFETs) are less than 100[7], while any successor to the Si MOSFET should have excellent switching capabilities in the range of 104e107. Therefore, opening a sizeable and well-tuned band-gap in graphene is a significant challenge for graphene-based electrondevices, introducing defects have shown great potential on this important issue. The electronic and mechanical properties of graphene samples with high perfection of the atomic lattice are outstanding, but structural defects, which may appear during growth or processing, deteriorate the performance of graphene-based devices. However, deviations from perfection can be useful in some applications, as they make it possible to tailor the local properties of graphene and to achieve new functionalities. Like in any other real material, structural defects do exist in graphene and can dramatically alter its properties. Defects can also be deliberately introduced into graphene, for example, by irradiation or chemical treatments. Because sp2-hybridized carbon atoms can arrange themselves into a variety of different polygons to form different structures, the nonhexagonal rings may either introduce curvature in the sheet or leave it flat when the arrangement of polygons satisfies certain symmetry rules. This property is not included in other bulk crystals, for example, semiconductors such as silicon. Reconstructions in the atomic network permit a coherent defective lattice without undercoordinated atoms. Although they have no dangling bonds, these
http://dx.doi.org/10.1016/j.jmst.2014.11.019 1005-0302/Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
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reconstructed defects locally increase the reactivity of the structure and allow adsorption of other atoms on graphene[8]. In this review, we summarized the major progresses made in defect engineering of graphene, including the types of defects in graphene, the generation and healing of the graphene defects are summarized, as well as the effects of the defects on the chemical, electronic, magnetic, and mechanical properties of graphene. To keep the length manageable, our attention is mainly focused on the major developments of graphene defects in recent five years. 2. Different Type of Defects in Graphene There are two kinds of defects in graphene. One is point defects, typically vacancies or interstitial atoms are zero-dimensional. The other is on one dimensional line of defects. It is well-known that the defects are not always stationary and that their migration can have an important influence on the properties of a defective crystal. In graphene, each defect has certain mobility parallel to the graphene plane. The mobility might be immeasurably low, for example, for extended vacancy complexes, or very high, for example, for adatoms on an unperturbed graphene lattice. The migration is usually governed by an activation barrier which depends on the defect type and therefore increases exponentially with temperature.
formation of a five-membered and a nine-membered ring [V1(5-9) defect]. 2.3. Multiple vacancies Double vacancies (DV) can be created either by the coalescence of two SVs or by removing two neighboring atoms. Because no dangling bond is present in a fully reconstructed DV, two pentagons and one octagon [V2(5-8-5) defect] appear instead of four hexagons in perfect graphene. The atomic network remains coherent with minor perturbations in the bond lengths around the defect. Simulations indicate that the formation energy Ef of a DV is of the same order as that of an SV (about 8 eV)[11,12]. As two atoms are now missing, the energy permissing atom (4 eV per atom) is much lower than that for an SV. Hence, DVs are thermodynamically favored over SVs. Furthermore, the removal of more than two atoms may be expected to result in larger and more complex defect configurations. Generally, as an even number of missing atoms allows a full reconstruction (complete saturation of dangling bonds), such vacancies are energetically favored over structures with an odd number of missing atoms where an open bond remains[13]. 2.4. One dimensional defects
Since the graphene lattice can be reconstructed by forming nonhexagonal rings, the simplest example is the StoneeWales (SW) defect[9], which does not involve any removed or added atoms. As shown in Fig. 1, four hexagons are transformed into two pentagons and two heptagons [SW(55-77) defect] by rotating one of the CeC bonds by 90 . The SW(55-77) defect has a formation energy Ef ¼ 5 eV[10].
This kind of defect has been observed in many experimental studies of graphene[14e17]. In general, these line defects are tilt boundaries separating two domains of different lattice orientations with the tilt axis normal to the plane. Such defects can be thought of as a line of reconstructed point defects with or without dangling bonds[18e20], as shown in Fig. 3. One example is a domain boundary which has been observed to appear due to lattice mismatch in graphene grown on a Ni surface[17]. This defect makes up of an alternating line of pairs of pentagons separated by octagons (Fig. 3). Obviously, such a defect can be formed by aligning (5-8-5) divacancies along the zigzag lattice direction of graphene.
2.2. Single vacancies
2.5. Defects at the edges of graphene
The simplest defect in any material is the missing one lattice atom. Single vacancies (SV) in graphene have been experimentally observed by TEM[10]. As can be seen in Fig. 2, the SV undergoes a JahneTeller distortion which leads to the saturation of two of the three dangling bonds toward the missing atom. One dangling bond always remains owing to geometrical reasons. This leads to the
Each graphene layer is terminated by edges with the edge atom being either free or passivated with hydrogen atoms. The simplest edge structures are the armchair and the zigzag orientation. Defective edges can appear because of local changes in the reconstruction type or because of sustained removal of carbon atoms from the edges. This can be achieved by sputtering edge atoms[21,22].
2.1. StoneeWales defect
Fig. 1. StoneeWales defect SW(55-77), formed by rotating a carbonecarbon bond by 90 : (a) experimental TEM image of the defect; (b) its atomic structure as obtained from our Density functional theory (DFT) calculations (Copyright 2008 American Chemical Society)[10].
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Fig. 2. Single vacancy V1(5-9): (a) as seen in an experimental TEM image; (b) its atomic structure obtained from our DFT calculations (Copyright 2008 American Chemical Society)[10].
Under these conditions, armchair edges can be turned to zigzag edges[21]. An intermediate structure can be regarded as a defective edge. A simple example of an edge defect is the removal of one carbon atom from a zigzag edge. This leads to one pentagon in the middle of a row of hexagons at the edge. Other edge reconstructions result in different combinations of pentagons and heptagons at the edge as shown in Fig. 4[23]. Besides, hydrogen atoms and other chemical groups that can saturate dangling bonds at the edge under ambient conditions may be considered as disorder, dramatically increasing the number of possible edge defects. 3. Generation of Defects in Graphene The high formation energy of a single vacancy in graphene (7.5 eV) does not allow any detectable concentration of point
defects in thermal equilibrium at temperatures below the melting temperature. However, there are three mechanisms which can lead to nonequilibrium defects in graphene: (1) crystal growth; (2) irradiation with energetic particles, for example, electrons or ions; and (3) chemical treatment[8]. 3.1. Crystal growth Since the large-scale growth of a graphene layer does normally not occur slowly atom-by-atom from one nucleus but rather as a relaxation of a metal carbon system with many nuclei, for example, in chemical vapor deposition (CVD), it is natural to expect defects in the as-grown material. Generally, high temperature growth facilitates the relaxation toward thermal equilibrium, and defects can be annealed rapidly. However, defects are a well-known problem in
Fig. 3. (a) Grain boundary defect structure consisting of pentagon-pairs and octagons in graphene grown on a Ni substrate; (b) the DFT relaxed geometry of the defect structure; (c) the calculated adsorption energies for two domains are similar, but both are lower in energy than a third possible adsorption configuration with all carbon atoms on hollow sites (Copyright 2010 Nature Publishing Group)[17].
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Fig. 4. Different edge reconstructions in graphene: (a) reconstructed zigzag; (b) armchair edge; (c) (677) reconstructed armchair edge; (d) zigzag edge (Copyright 2008 American Physical Society)[23].
low temperature growth. Because of the high formation energy of vacancies and fast migration of adatoms in graphene, it is unlikely that there are any isolated vacancies in graphene after growth. This has been confirmed by the high carrier mobility in CVD-grown graphene, which would not be expected at a considerable density of vacancies. The temperature dependence of the mobility indicates that impurity scattering dominates at the interface even for the merged domains with the same orientation[24]. 3.2. Particle irradiation Irradiation of graphene with ions or electrons can produce point defects due to the ballistic ejection of carbon atoms[25e27]. The atom can be kicked out from graphene or adsorb on the sheet and migrate on its surface as an adatom. The effect of irradiation has been studied in detail by electron microscopy[28,29], where irradiation and imaging can be done with the same electron beam, and the formation of defects is observable in situ at atomic resolution. Uniform irradiation of larger areas results in a generation of randomly distributed vacancies. However, due to increased strain and/or under-coordinated atoms, the defective areas, for example, where a vacancy already exists, show an increased rate of defect formation. Defects can also be generated in preselected positions with a highly focused electron beam or by using masking techniques. Modern electron microscopes with aberration-corrected condensers allow focusing an electron beam onto a spot of approximately 1 Å in diameter thereby creating vacancies with almost atomic selectivity[30]. Another physical method which has been used for defect production in graphene is ion irradiation[31e35]. It can be used to selectively produce certain defects or to pattern and cut graphene with a precision down to 10 nm utilizing a focused ion beam[36,37]. However, for the bilayer graphene, contrary to theoretical estimates based on the conventional binary collision model, experimental results indicate that the number of defects in the lower layer of the bilayer graphene sample is smaller than that in the upper layer (as shown in Fig. 5)[35]. This observation is explained by in situ self-annealing of the defects.
3.3. Chemical methods The reactions of carbon atoms in a graphene layer with other species can lead to the loss of atoms and hence to defects. However, the high inertness of graphene (apart from edge positions that are highly reactive) only allows a very limited number of possible reactions at room temperature. Oxidation is the most common one, for example, in an oxidizing acid (HNO3 or H2SO4). In such a treatment it is possible to attach oxygen and hydroxyl (OH) or carboxyl (COOH) groups to graphene[38]. When graphene is covered more or less uniformly with hydroxyl or carboxyl groups, the material is called graphene oxide, which is essentially a highly defective graphene sheet functionalized with oxygen groups[39]. Plasma treatments and adsorption of atomic hydrogen on a graphene surface followed by its self-organization and hydrogen island formation can also be referred to in the context of graphene treatment by chemical methods[40]. 4. Healing of Defects in Graphene It has been directed that defects play a crucial role in tailoring the material properties of carbon-based structures such as graphite, carbon nanotubes, and graphene sheets [26,41,42]. For example, it was found that defects are responsible for the inherent ferromagnetic behavior of carbon-based materials, due to the presence around the defects of localized electron states with energies close to that of the Fermi level[43,44]. On the other hand, defects are well-known for their ability to scatter charge carriers and phonons, thus decreasing the ballistic transport path length and adversely affecting carrier mobility and thermal conductivity. The detrimental effects of defects are particularly pronounced in graphene films. For example, defects were held responsible for a remarkable reduction in charge carrier mobility in graphene films obtained by micromechanical cleavage[45]. The transport properties of graphene films produced by chemical methods, such as the exfoliation and chemical reduction of graphene oxide, have also been ascribed to defects[46]. In this respect, defects are undesirable, and the ability to “heal” them is
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Fig. 5. Schematic illustration of the experimental setups for creating defects on graphene. The samples were formed by subsequent transfer of 12C and 13C graphene sheets on Si/ SiO2 substrate and irradiated by Arþ ions with various doses followed by Raman probing. The right panel is a snapshot from molecular dynamics simulations showing a typical atomic configuration after an ion impact (Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA)[35].
important for generating carbon nanostructures with high electrical and thermal conductivities and, potentially, enhanced mechanical strength. 4.1. Thermal annealing One promising approach for removing crystalline lattice defects and restoring graphitic structures is high temperature processing in the presence of a hydrocarbon gas. Utilizing appropriate conditions, the hydrocarbon gas might decompose to supply carbon atoms that pez et al. demonstrated that can repair defective sites. Recently, Lo chemical vapor deposition (CVD) processing of chemically derived graphene films using ethylene carbon source improved film conductivity by two orders of magnitude, to room temperature (RT) values of 10e350 S cm1. The authors attributed the improvement to defect healing[47]. In addition, direct observation of the modification of defective sites on graphitic surfaces under gaseous hydrocarbon atmospheres was reported by Liu et al. [48]. They studied the reactivity of defects on highly oriented pyrolytic graphite (HOPG) surfaces exposed to acetylene at elevated temperatures by scanning tunneling microscopy (STM). 4.2. Self-healing By using first-principles calculations based on densityfunctional theory, Tsetseris et al. found that pairs of C adatoms
and clusters of four or more self-interstitials stay idle unless the system is heated to very high temperatures, while clustering of three C adatoms leads to removal of hillock-like features and creates mobile species, resulting in self-healing of defective structures[49]. Liu et al. found that in the single layer vacancy defects prefer to coalesce into larger vacancy holes, while in the multi-layer graphene the vacancies tend to be concentrated into a single hole in one layer through both intra- and inter-layer migrations (Fig. 6). The vacancy inter-layer migration is facilitated by the interaction of defects in neighboring layers[50]. 4.3. Healing by absorption Wang et al. proposed a strategy of controllable vacancy healing and N-doping of graphene[51], as shown in Fig. 7, vacancies can be healed by sequential exposure to CO and NO molecules in a twostep recipe. Firstly, the CO molecule can be easily absorbed at the site of graphene defects. Then, NO can remove the extra O atom in a chemical way by forming NO2 molecule that binds to graphene weakly. Encouraged by this observation, they further study the controllable N-doping in graphene with a similar procedure, which involved creating vacancy and subsequent exposure to NO molecules. That is to say, a combination of CO and NO molecules can potentially provide simultaneous healing and doping at a desirable ratio, which is very important for band-gap modulation of graphene.
Fig. 6. A diagram of the vacancy migration between graphene layers leads to vacancies holes formation in one layer and the self-healing of other layers (Copyright 2014 the Royal Society of Chemistry)[50].
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5. Properties of Defective Graphene 5.1. Chemical properties
Fig. 7. Schematic view of the vacancy healing and N-doping process of graphene with vacancies by CO and NO molecules (Copyright 2011 American Physical Society)[51].
4.4. Metal-assisted healing The healing of graphene grown from a metallic substrate is investigated by Karoui et al. using tight-binding Monte Carlo simulations[52]. At temperatures (ranging from 1000 to 2500 K), an isolated graphene sheet can anneal a large number of defects suggesting that their healings are thermally activated. Wherein, in the presence of a nickel substrate, a perfect graphene layer can be obtained. The probable mechanism is that the nickel carbon chemical bonds keep breaking and reforming around defected carbon zones, providing a direct interaction, necessary for the healing. As shown in Fig. 8, as in the case of the isolated graphene sheet, the sheet is far from being healed and a non-negligible concentration of defects remains at 1000 K (Fig. 8(b)). Once again, large rings are healed at 1000 and 1500 K, whereas pentagons and heptagons cancel out at higher temperatures. At 2500 K the graphene sheet is completely healed as seen in Fig. 8(c).
It is well-known that defects associated with dangling bonds should enhance the reactivity of graphene. Numerous simulations suggested that hydroxyl, carboxyl, or other groups could easily be attached to vacancy-type defects[53,54]. Simulations also showed that reconstructed defects without dangling bonds such as SW defects or reconstructed vacancies locally changed the density of p-electrons and may also increase the local reactivity[55,56]. For example, Pablo et al. found that when two phenyl groups are attached onto perfect graphene, the StoneeWales defect becomes more reactive than the 585 double vacancy and 555e777 reconstructed double vacancy[56]. The largest increase of reactivity is observed for the functional groups whose binding energy onto perfect graphene is small. Thus, the controlled creation of defects with a high spatial selectivity can be used for the local functionalization of graphene, and for the creation of graphene ribbons with the designed properties by various chemical methods. 5.2. Electronic properties Defects strongly affect the electronic properties of graphene. From a theoretical point of view, the Dirac equation has to be modified when defects are in the lattice. This will naturally have an impact on the electronic structure. The overlap of p-orbitals determines the electronic properties but is altered in the vicinity of structural defects. Firstly, bond lengths in the strain fields of defects are different from those in the perfect lattice. Secondly, defects lead to a local rehybridization of sigma and piorbitals which again change the electronic structure. A local curvature around defects
Fig. 8. Graphene sheet in the presence of the Ni lattice: (a) side and top views of the initial configurations; equilibrium configurations at (b) 1000 and (c) 2500 K (Copyright 2010 American Chemical Society)[52].
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Fig. 9. Schematic of strained graphene functionalization: (a) after being transferred to polydimethylsiloxane (PDMS), the polycrystalline graphene is strained by elongation; (b) after strain is applied, the aqueous solution of aryl diazonium salt is pipetted onto the graphene surface, the inset shows a photograph of several droplets on a graphene/PDMS substrate; (c) after functionalization, the solution is removed and the substrate rinsed and dried. Because of the increased reactivity of defect sites along boundaries, there is an increased concentration of functional molecules at these locations. (Copyright 2013 American Chemical Society)[67].
also has an influence on the rehybridization. Thirdly, all defects lead to scattering of the electron waves and change the electron trajectories[57,58]. 5.3. Magnetic properties Magnetism in pure carbon systems has recently been the subject of intense experimental and theoretical research, which is very important to understand a fundamental problem: the origin of magnetism in a system which traditionally has been thought to show diamagnetic behavior only. In addition to fullerenes, nanotubes, graphite, and nanodiamonds, magnetism was recently reported for graphene produced from graphene oxide [8,59e62]. Based on the calculations, the observed magnetic behavior in all these systems was explained in terms of defects in the graphitic network. Such defects have local magnetic moments and may give rise to flat bands and eventually to the development of magnetic ordering. Magnetism may also originate from impurity atoms which are nonmagnetic by themselves, but because the specific chemical environment give rise to local magnetic moments. 5.4. Mechanical properties The influence of defects on the mechanical properties of graphene has not yet been studied experimentally. However, based on a large body of experimental and theoretical data for carbon nanotubes[63e66], one can expect that point defects, in particular vacancies, will decrease the Young's modulus and tensile strength of graphene samples. Existence of defects may remarkably reduce the tensile strength, and mechanical properties should be decreased as the number of defects increase. Conversely, efficient reconstruction and healing of vacancy-type defects should minimize their detrimental effects. Line defects (dislocations) should be important for plastic deformation of graphene ribbons under tensile strain[8]. On the other hand, Bissett et al. found that mechanical strain can alter the structure of graphene, and dramatically increase the reactivity of the graphene. As shown in Fig. 9, the reaction rate of aryl diazonium functionalization can be increased up to 10 times by applying external strain to the graphene[67].
defects are frequently unavoidable in experiments; hence their effect on electronic properties of graphene, along with their healing to enrich the functionalities of graphene, should be investigated. Such studies can provide insight into the interplay of the defects in graphene and related nanocarbon materials and facilitate the rational design of novel carbon materials with new functionalities. Furthermore, the following key points are suggested for future studies of the graphene defects. (1) Most of the study on the influence of defects on the electronic properties of graphene is based on theoretical simulation, and graphene flakes with different sizes and borders are employed. This should have some difference with the real graphene materials. Experimental data directly reveal the defects effects on the electronic and other characteristics of graphene are urgently needed. (2) It is also very important to find a way to heal vacancy defects in graphene in a controlled way. It seems that a high temperature thermal annealing, adding foreign atoms or drawing support from some metal substrates can serve as efficient approaches to remove unwanted defects. However, it is still difficult to regulate the concentration of the defects in the real experiments, especially for the large area of graphene or batch processing. The nature of the healing mechanism should be explored. (3) There is a huge demand for developing the applications of graphene in electronics and optics, in which opening the band gap of graphene is the most important issue. Introducing defects have shown promising prospect in the bandgap modulation of graphene, even though the influences of the defects on pristine graphene are widely studied. More attention should be paid on the understanding of defects of graphene in the vicinity of surface, interface, reactant, and environment, etc. In-situ studies of the graphene and their prototype devices are also very important, which will provide new insights into the structures and properties of graphene, and boost further exploration of graphene applications.
6. Concluding Remarks
Acknowledgments
Graphene have a big role in nanoscience today because of their rich and promising electrical, mechanical and optical properties. However, achieving these properties requires understanding the underlying structure and its behavior. In addition to ideal systems,
This work was supported by the National Natural Science Foundation of China (Nos. 21276257 and 2110600) and the Research Foundation for Youth Scholars of Beijing Technology and Business University (No. QNJJ2014-14).
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Defects in Graphene: Generation, Healing, and Their Effects on the Properties of Graphene: A Review Lili Liu 1, Miaoqing Qing 1, Yibo Wang 1, Shimou Chen 2, * 1 2
Department of Chemistry, School of Science, Beijing Technology and Business University, Beijing 100048, China Key Laboratory of Green Process and Engineering, Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS), Beijing 100190, China
a r t i c l e i n f o Article history: Received 28 September 2014 Received in revised form 15 November 2014 Accepted 19 November 2014 Available online 20 January 2015 Key words: Graphene Vacancy Defect healing Band gap modulation Properties of defective graphene
Graphene has attracted immense investigation since its discovery. Lattice imperfections are introduced into graphene unavoidably during graphene growth or processing. These structural defects are known to significantly affect electronic and chemical properties of graphene. A comprehensive understanding of graphene defect is thus of critical importance. Here we review the major progresses made in defectrelated engineering of graphene. Firstly, we give a brief introduction on the types of defects in graphene. Secondly, the generation and healing of the graphene defects are summarized. Then, the effects of defects on the chemical, electronic, magnetic, and mechanical properties of graphene are discussed. Finally, we address the associated challenges and prospects on the future study of defects in graphene and other nanocarbon materials. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
1. Introduction Graphene has raised extensive interest in the worldwide for its extraordinary thermal, mechanical, electrical and other properties[1,2]. Among all properties, the unique electronic properties are assumed to be the most intriguing aspect of graphene, for example, outstanding ballistic transport properties and longest mean free path at room temperature[3], distinctive integral and half-integral quantum hall effect[4,5], the highest mobility to increase the speed of devices[6], and so on. The mobility of graphene is significantly higher than that of the widely-used Si, of approximately 1400 cm2 V1 s1. Consequently, graphene has been considered as a candidate material for applications in post-silicon electronics. Graphene has a honeycomb lattice structure and a unit cell that contains two carbon atoms. Just like in carbon nanotubes, there are two different kinds of graphene ribbon edges: armchair and zigzag, which can influence the electronic properties of graphene. However, most electronic applications are handicapped by the absence of a semiconducting gap in pristine graphene. For example, devices made from the zero-bandgap graphene are difficult to switch off, losing the advantage of the low static power consumption of the
* Corresponding author. Prof., Ph.D.; Tel.: þ86 10 82544800; Fax: þ86 10 82544875. E-mail address: [email protected] (S. Chen).
complementary metal oxide semiconductor (CMOS) technology. Quantitatively, the Ion/Ioff ratios for graphene-based field-effect transistors (GFETs) are less than 100[7], while any successor to the Si MOSFET should have excellent switching capabilities in the range of 104e107. Therefore, opening a sizeable and well-tuned band-gap in graphene is a significant challenge for graphene-based electrondevices, introducing defects have shown great potential on this important issue. The electronic and mechanical properties of graphene samples with high perfection of the atomic lattice are outstanding, but structural defects, which may appear during growth or processing, deteriorate the performance of graphene-based devices. However, deviations from perfection can be useful in some applications, as they make it possible to tailor the local properties of graphene and to achieve new functionalities. Like in any other real material, structural defects do exist in graphene and can dramatically alter its properties. Defects can also be deliberately introduced into graphene, for example, by irradiation or chemical treatments. Because sp2-hybridized carbon atoms can arrange themselves into a variety of different polygons to form different structures, the nonhexagonal rings may either introduce curvature in the sheet or leave it flat when the arrangement of polygons satisfies certain symmetry rules. This property is not included in other bulk crystals, for example, semiconductors such as silicon. Reconstructions in the atomic network permit a coherent defective lattice without undercoordinated atoms. Although they have no dangling bonds, these
http://dx.doi.org/10.1016/j.jmst.2014.11.019 1005-0302/Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
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reconstructed defects locally increase the reactivity of the structure and allow adsorption of other atoms on graphene[8]. In this review, we summarized the major progresses made in defect engineering of graphene, including the types of defects in graphene, the generation and healing of the graphene defects are summarized, as well as the effects of the defects on the chemical, electronic, magnetic, and mechanical properties of graphene. To keep the length manageable, our attention is mainly focused on the major developments of graphene defects in recent five years. 2. Different Type of Defects in Graphene There are two kinds of defects in graphene. One is point defects, typically vacancies or interstitial atoms are zero-dimensional. The other is on one dimensional line of defects. It is well-known that the defects are not always stationary and that their migration can have an important influence on the properties of a defective crystal. In graphene, each defect has certain mobility parallel to the graphene plane. The mobility might be immeasurably low, for example, for extended vacancy complexes, or very high, for example, for adatoms on an unperturbed graphene lattice. The migration is usually governed by an activation barrier which depends on the defect type and therefore increases exponentially with temperature.
formation of a five-membered and a nine-membered ring [V1(5-9) defect]. 2.3. Multiple vacancies Double vacancies (DV) can be created either by the coalescence of two SVs or by removing two neighboring atoms. Because no dangling bond is present in a fully reconstructed DV, two pentagons and one octagon [V2(5-8-5) defect] appear instead of four hexagons in perfect graphene. The atomic network remains coherent with minor perturbations in the bond lengths around the defect. Simulations indicate that the formation energy Ef of a DV is of the same order as that of an SV (about 8 eV)[11,12]. As two atoms are now missing, the energy permissing atom (4 eV per atom) is much lower than that for an SV. Hence, DVs are thermodynamically favored over SVs. Furthermore, the removal of more than two atoms may be expected to result in larger and more complex defect configurations. Generally, as an even number of missing atoms allows a full reconstruction (complete saturation of dangling bonds), such vacancies are energetically favored over structures with an odd number of missing atoms where an open bond remains[13]. 2.4. One dimensional defects
Since the graphene lattice can be reconstructed by forming nonhexagonal rings, the simplest example is the StoneeWales (SW) defect[9], which does not involve any removed or added atoms. As shown in Fig. 1, four hexagons are transformed into two pentagons and two heptagons [SW(55-77) defect] by rotating one of the CeC bonds by 90 . The SW(55-77) defect has a formation energy Ef ¼ 5 eV[10].
This kind of defect has been observed in many experimental studies of graphene[14e17]. In general, these line defects are tilt boundaries separating two domains of different lattice orientations with the tilt axis normal to the plane. Such defects can be thought of as a line of reconstructed point defects with or without dangling bonds[18e20], as shown in Fig. 3. One example is a domain boundary which has been observed to appear due to lattice mismatch in graphene grown on a Ni surface[17]. This defect makes up of an alternating line of pairs of pentagons separated by octagons (Fig. 3). Obviously, such a defect can be formed by aligning (5-8-5) divacancies along the zigzag lattice direction of graphene.
2.2. Single vacancies
2.5. Defects at the edges of graphene
The simplest defect in any material is the missing one lattice atom. Single vacancies (SV) in graphene have been experimentally observed by TEM[10]. As can be seen in Fig. 2, the SV undergoes a JahneTeller distortion which leads to the saturation of two of the three dangling bonds toward the missing atom. One dangling bond always remains owing to geometrical reasons. This leads to the
Each graphene layer is terminated by edges with the edge atom being either free or passivated with hydrogen atoms. The simplest edge structures are the armchair and the zigzag orientation. Defective edges can appear because of local changes in the reconstruction type or because of sustained removal of carbon atoms from the edges. This can be achieved by sputtering edge atoms[21,22].
2.1. StoneeWales defect
Fig. 1. StoneeWales defect SW(55-77), formed by rotating a carbonecarbon bond by 90 : (a) experimental TEM image of the defect; (b) its atomic structure as obtained from our Density functional theory (DFT) calculations (Copyright 2008 American Chemical Society)[10].
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Fig. 2. Single vacancy V1(5-9): (a) as seen in an experimental TEM image; (b) its atomic structure obtained from our DFT calculations (Copyright 2008 American Chemical Society)[10].
Under these conditions, armchair edges can be turned to zigzag edges[21]. An intermediate structure can be regarded as a defective edge. A simple example of an edge defect is the removal of one carbon atom from a zigzag edge. This leads to one pentagon in the middle of a row of hexagons at the edge. Other edge reconstructions result in different combinations of pentagons and heptagons at the edge as shown in Fig. 4[23]. Besides, hydrogen atoms and other chemical groups that can saturate dangling bonds at the edge under ambient conditions may be considered as disorder, dramatically increasing the number of possible edge defects. 3. Generation of Defects in Graphene The high formation energy of a single vacancy in graphene (7.5 eV) does not allow any detectable concentration of point
defects in thermal equilibrium at temperatures below the melting temperature. However, there are three mechanisms which can lead to nonequilibrium defects in graphene: (1) crystal growth; (2) irradiation with energetic particles, for example, electrons or ions; and (3) chemical treatment[8]. 3.1. Crystal growth Since the large-scale growth of a graphene layer does normally not occur slowly atom-by-atom from one nucleus but rather as a relaxation of a metal carbon system with many nuclei, for example, in chemical vapor deposition (CVD), it is natural to expect defects in the as-grown material. Generally, high temperature growth facilitates the relaxation toward thermal equilibrium, and defects can be annealed rapidly. However, defects are a well-known problem in
Fig. 3. (a) Grain boundary defect structure consisting of pentagon-pairs and octagons in graphene grown on a Ni substrate; (b) the DFT relaxed geometry of the defect structure; (c) the calculated adsorption energies for two domains are similar, but both are lower in energy than a third possible adsorption configuration with all carbon atoms on hollow sites (Copyright 2010 Nature Publishing Group)[17].
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Fig. 4. Different edge reconstructions in graphene: (a) reconstructed zigzag; (b) armchair edge; (c) (677) reconstructed armchair edge; (d) zigzag edge (Copyright 2008 American Physical Society)[23].
low temperature growth. Because of the high formation energy of vacancies and fast migration of adatoms in graphene, it is unlikely that there are any isolated vacancies in graphene after growth. This has been confirmed by the high carrier mobility in CVD-grown graphene, which would not be expected at a considerable density of vacancies. The temperature dependence of the mobility indicates that impurity scattering dominates at the interface even for the merged domains with the same orientation[24]. 3.2. Particle irradiation Irradiation of graphene with ions or electrons can produce point defects due to the ballistic ejection of carbon atoms[25e27]. The atom can be kicked out from graphene or adsorb on the sheet and migrate on its surface as an adatom. The effect of irradiation has been studied in detail by electron microscopy[28,29], where irradiation and imaging can be done with the same electron beam, and the formation of defects is observable in situ at atomic resolution. Uniform irradiation of larger areas results in a generation of randomly distributed vacancies. However, due to increased strain and/or under-coordinated atoms, the defective areas, for example, where a vacancy already exists, show an increased rate of defect formation. Defects can also be generated in preselected positions with a highly focused electron beam or by using masking techniques. Modern electron microscopes with aberration-corrected condensers allow focusing an electron beam onto a spot of approximately 1 Å in diameter thereby creating vacancies with almost atomic selectivity[30]. Another physical method which has been used for defect production in graphene is ion irradiation[31e35]. It can be used to selectively produce certain defects or to pattern and cut graphene with a precision down to 10 nm utilizing a focused ion beam[36,37]. However, for the bilayer graphene, contrary to theoretical estimates based on the conventional binary collision model, experimental results indicate that the number of defects in the lower layer of the bilayer graphene sample is smaller than that in the upper layer (as shown in Fig. 5)[35]. This observation is explained by in situ self-annealing of the defects.
3.3. Chemical methods The reactions of carbon atoms in a graphene layer with other species can lead to the loss of atoms and hence to defects. However, the high inertness of graphene (apart from edge positions that are highly reactive) only allows a very limited number of possible reactions at room temperature. Oxidation is the most common one, for example, in an oxidizing acid (HNO3 or H2SO4). In such a treatment it is possible to attach oxygen and hydroxyl (OH) or carboxyl (COOH) groups to graphene[38]. When graphene is covered more or less uniformly with hydroxyl or carboxyl groups, the material is called graphene oxide, which is essentially a highly defective graphene sheet functionalized with oxygen groups[39]. Plasma treatments and adsorption of atomic hydrogen on a graphene surface followed by its self-organization and hydrogen island formation can also be referred to in the context of graphene treatment by chemical methods[40]. 4. Healing of Defects in Graphene It has been directed that defects play a crucial role in tailoring the material properties of carbon-based structures such as graphite, carbon nanotubes, and graphene sheets [26,41,42]. For example, it was found that defects are responsible for the inherent ferromagnetic behavior of carbon-based materials, due to the presence around the defects of localized electron states with energies close to that of the Fermi level[43,44]. On the other hand, defects are well-known for their ability to scatter charge carriers and phonons, thus decreasing the ballistic transport path length and adversely affecting carrier mobility and thermal conductivity. The detrimental effects of defects are particularly pronounced in graphene films. For example, defects were held responsible for a remarkable reduction in charge carrier mobility in graphene films obtained by micromechanical cleavage[45]. The transport properties of graphene films produced by chemical methods, such as the exfoliation and chemical reduction of graphene oxide, have also been ascribed to defects[46]. In this respect, defects are undesirable, and the ability to “heal” them is
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Fig. 5. Schematic illustration of the experimental setups for creating defects on graphene. The samples were formed by subsequent transfer of 12C and 13C graphene sheets on Si/ SiO2 substrate and irradiated by Arþ ions with various doses followed by Raman probing. The right panel is a snapshot from molecular dynamics simulations showing a typical atomic configuration after an ion impact (Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA)[35].
important for generating carbon nanostructures with high electrical and thermal conductivities and, potentially, enhanced mechanical strength. 4.1. Thermal annealing One promising approach for removing crystalline lattice defects and restoring graphitic structures is high temperature processing in the presence of a hydrocarbon gas. Utilizing appropriate conditions, the hydrocarbon gas might decompose to supply carbon atoms that pez et al. demonstrated that can repair defective sites. Recently, Lo chemical vapor deposition (CVD) processing of chemically derived graphene films using ethylene carbon source improved film conductivity by two orders of magnitude, to room temperature (RT) values of 10e350 S cm1. The authors attributed the improvement to defect healing[47]. In addition, direct observation of the modification of defective sites on graphitic surfaces under gaseous hydrocarbon atmospheres was reported by Liu et al. [48]. They studied the reactivity of defects on highly oriented pyrolytic graphite (HOPG) surfaces exposed to acetylene at elevated temperatures by scanning tunneling microscopy (STM). 4.2. Self-healing By using first-principles calculations based on densityfunctional theory, Tsetseris et al. found that pairs of C adatoms
and clusters of four or more self-interstitials stay idle unless the system is heated to very high temperatures, while clustering of three C adatoms leads to removal of hillock-like features and creates mobile species, resulting in self-healing of defective structures[49]. Liu et al. found that in the single layer vacancy defects prefer to coalesce into larger vacancy holes, while in the multi-layer graphene the vacancies tend to be concentrated into a single hole in one layer through both intra- and inter-layer migrations (Fig. 6). The vacancy inter-layer migration is facilitated by the interaction of defects in neighboring layers[50]. 4.3. Healing by absorption Wang et al. proposed a strategy of controllable vacancy healing and N-doping of graphene[51], as shown in Fig. 7, vacancies can be healed by sequential exposure to CO and NO molecules in a twostep recipe. Firstly, the CO molecule can be easily absorbed at the site of graphene defects. Then, NO can remove the extra O atom in a chemical way by forming NO2 molecule that binds to graphene weakly. Encouraged by this observation, they further study the controllable N-doping in graphene with a similar procedure, which involved creating vacancy and subsequent exposure to NO molecules. That is to say, a combination of CO and NO molecules can potentially provide simultaneous healing and doping at a desirable ratio, which is very important for band-gap modulation of graphene.
Fig. 6. A diagram of the vacancy migration between graphene layers leads to vacancies holes formation in one layer and the self-healing of other layers (Copyright 2014 the Royal Society of Chemistry)[50].
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5. Properties of Defective Graphene 5.1. Chemical properties
Fig. 7. Schematic view of the vacancy healing and N-doping process of graphene with vacancies by CO and NO molecules (Copyright 2011 American Physical Society)[51].
4.4. Metal-assisted healing The healing of graphene grown from a metallic substrate is investigated by Karoui et al. using tight-binding Monte Carlo simulations[52]. At temperatures (ranging from 1000 to 2500 K), an isolated graphene sheet can anneal a large number of defects suggesting that their healings are thermally activated. Wherein, in the presence of a nickel substrate, a perfect graphene layer can be obtained. The probable mechanism is that the nickel carbon chemical bonds keep breaking and reforming around defected carbon zones, providing a direct interaction, necessary for the healing. As shown in Fig. 8, as in the case of the isolated graphene sheet, the sheet is far from being healed and a non-negligible concentration of defects remains at 1000 K (Fig. 8(b)). Once again, large rings are healed at 1000 and 1500 K, whereas pentagons and heptagons cancel out at higher temperatures. At 2500 K the graphene sheet is completely healed as seen in Fig. 8(c).
It is well-known that defects associated with dangling bonds should enhance the reactivity of graphene. Numerous simulations suggested that hydroxyl, carboxyl, or other groups could easily be attached to vacancy-type defects[53,54]. Simulations also showed that reconstructed defects without dangling bonds such as SW defects or reconstructed vacancies locally changed the density of p-electrons and may also increase the local reactivity[55,56]. For example, Pablo et al. found that when two phenyl groups are attached onto perfect graphene, the StoneeWales defect becomes more reactive than the 585 double vacancy and 555e777 reconstructed double vacancy[56]. The largest increase of reactivity is observed for the functional groups whose binding energy onto perfect graphene is small. Thus, the controlled creation of defects with a high spatial selectivity can be used for the local functionalization of graphene, and for the creation of graphene ribbons with the designed properties by various chemical methods. 5.2. Electronic properties Defects strongly affect the electronic properties of graphene. From a theoretical point of view, the Dirac equation has to be modified when defects are in the lattice. This will naturally have an impact on the electronic structure. The overlap of p-orbitals determines the electronic properties but is altered in the vicinity of structural defects. Firstly, bond lengths in the strain fields of defects are different from those in the perfect lattice. Secondly, defects lead to a local rehybridization of sigma and piorbitals which again change the electronic structure. A local curvature around defects
Fig. 8. Graphene sheet in the presence of the Ni lattice: (a) side and top views of the initial configurations; equilibrium configurations at (b) 1000 and (c) 2500 K (Copyright 2010 American Chemical Society)[52].
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Fig. 9. Schematic of strained graphene functionalization: (a) after being transferred to polydimethylsiloxane (PDMS), the polycrystalline graphene is strained by elongation; (b) after strain is applied, the aqueous solution of aryl diazonium salt is pipetted onto the graphene surface, the inset shows a photograph of several droplets on a graphene/PDMS substrate; (c) after functionalization, the solution is removed and the substrate rinsed and dried. Because of the increased reactivity of defect sites along boundaries, there is an increased concentration of functional molecules at these locations. (Copyright 2013 American Chemical Society)[67].
also has an influence on the rehybridization. Thirdly, all defects lead to scattering of the electron waves and change the electron trajectories[57,58]. 5.3. Magnetic properties Magnetism in pure carbon systems has recently been the subject of intense experimental and theoretical research, which is very important to understand a fundamental problem: the origin of magnetism in a system which traditionally has been thought to show diamagnetic behavior only. In addition to fullerenes, nanotubes, graphite, and nanodiamonds, magnetism was recently reported for graphene produced from graphene oxide [8,59e62]. Based on the calculations, the observed magnetic behavior in all these systems was explained in terms of defects in the graphitic network. Such defects have local magnetic moments and may give rise to flat bands and eventually to the development of magnetic ordering. Magnetism may also originate from impurity atoms which are nonmagnetic by themselves, but because the specific chemical environment give rise to local magnetic moments. 5.4. Mechanical properties The influence of defects on the mechanical properties of graphene has not yet been studied experimentally. However, based on a large body of experimental and theoretical data for carbon nanotubes[63e66], one can expect that point defects, in particular vacancies, will decrease the Young's modulus and tensile strength of graphene samples. Existence of defects may remarkably reduce the tensile strength, and mechanical properties should be decreased as the number of defects increase. Conversely, efficient reconstruction and healing of vacancy-type defects should minimize their detrimental effects. Line defects (dislocations) should be important for plastic deformation of graphene ribbons under tensile strain[8]. On the other hand, Bissett et al. found that mechanical strain can alter the structure of graphene, and dramatically increase the reactivity of the graphene. As shown in Fig. 9, the reaction rate of aryl diazonium functionalization can be increased up to 10 times by applying external strain to the graphene[67].
defects are frequently unavoidable in experiments; hence their effect on electronic properties of graphene, along with their healing to enrich the functionalities of graphene, should be investigated. Such studies can provide insight into the interplay of the defects in graphene and related nanocarbon materials and facilitate the rational design of novel carbon materials with new functionalities. Furthermore, the following key points are suggested for future studies of the graphene defects. (1) Most of the study on the influence of defects on the electronic properties of graphene is based on theoretical simulation, and graphene flakes with different sizes and borders are employed. This should have some difference with the real graphene materials. Experimental data directly reveal the defects effects on the electronic and other characteristics of graphene are urgently needed. (2) It is also very important to find a way to heal vacancy defects in graphene in a controlled way. It seems that a high temperature thermal annealing, adding foreign atoms or drawing support from some metal substrates can serve as efficient approaches to remove unwanted defects. However, it is still difficult to regulate the concentration of the defects in the real experiments, especially for the large area of graphene or batch processing. The nature of the healing mechanism should be explored. (3) There is a huge demand for developing the applications of graphene in electronics and optics, in which opening the band gap of graphene is the most important issue. Introducing defects have shown promising prospect in the bandgap modulation of graphene, even though the influences of the defects on pristine graphene are widely studied. More attention should be paid on the understanding of defects of graphene in the vicinity of surface, interface, reactant, and environment, etc. In-situ studies of the graphene and their prototype devices are also very important, which will provide new insights into the structures and properties of graphene, and boost further exploration of graphene applications.
6. Concluding Remarks
Acknowledgments
Graphene have a big role in nanoscience today because of their rich and promising electrical, mechanical and optical properties. However, achieving these properties requires understanding the underlying structure and its behavior. In addition to ideal systems,
This work was supported by the National Natural Science Foundation of China (Nos. 21276257 and 2110600) and the Research Foundation for Youth Scholars of Beijing Technology and Business University (No. QNJJ2014-14).
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