Colloidal Graphene Quantum Dots - The Journal of Physical

PERSPECTIVE pubs.acs.org/JPCL

Colloidal Graphene Quantum Dots Liang-shi Li* and Xin Yan Department of Chemistry, Indiana University, Bloomington, Indiana 47405

ABSTRACT Graphene is a unique type of semiconductor with zero band gap and zero effective masses of charge carriers. Thus, in graphene quantum dots, we expect many interesting phenomena that are different from those in quantum dots of any other semiconductors. In addition, carbon is an element unique in chemistry and in our society because of well-developed carbon chemistry and the important roles that carbon materials have played in various technologies. In this Perspective, we discuss the recent development and existing challenges regarding colloidal graphene quantum dots. This new quantum-confined system may lead to novel applications, and meanwhile, it can serve as a model system for understanding complex processes in carbon materials.

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hen the size of a semiconductor crystal is reduced to be comparable to the exciton Bohr radius of the bulk material, the boundary significantly modifies electron distribution, resulting in size-dependent properties such as band gap and energy relaxation dynamics.1-3 This phenomenon, known as quantum confinement, has been demonstrated in many semiconductor materials and led to practical applications in bioimaging, lasing, photovoltaics, and light-emitting diodes. In comparison with semiconductors in which quantum confinement has been extensively studied, graphene, consisting of a single atomic layer of graphite, is unique in many aspects. For example, bulk graphene has a zero band gap, and charge carriers have zero effective masses at band edges (Figure 1).4,5 As a result, the motion of the charge carriers can be described with relativistic quantum mechanics, leading to some exotic physical phenomena and remarkable properties not easily observable in other materials.4,5 This also results in an infinite exciton Bohr radius, and consequently, quantum confinement could take effect in graphene of any finite size. From a chemistry point of view, graphene is primarily made of carbon, of which the chemistry has been very well developed. Thus, it is possible to synthesize graphene through stepwise, well-defined solution chemistry to achieve structures with atomic precision that has not been possible for any other semiconductor materials.

Figure 1. Schematics of the energy dispersion relation in semiconductors with massive charge carriers (a) and in graphene (b) near their band edges. Here, Δk is the wavevector relative to band edges. In (a), the dispersion relation is parabolic, with E = p2Δk2/2m* for both the valence (lower half) and the conduction band (upper half); m* is the effective mass of the charge carriers, which may have different values for the two bands. Here, in the schematic, the valence band is shown to have larger effective mass. In (b), the dispersion relation appears linear near one of the six Dirac points (i.e., E = pνFΔk), indicating a zero effective mass for carriers in either the valence or the conduction band, where νF ≈ 106 m/s is the Fermi velocity.4,5 The bands are touching each other, yielding a zero band gap. Since the exciton Bohr radius is aB ∼ 1/μ*, where μ* is the reduced effective mass of an electron and a hole, when μ* f 0 at the Dirac points, the exciton Bohr radius diverges.

an essential role in determining their properties. Therefore, to make graphene nanostructures with well-defined properties, a solution chemistry approach is advantageous over exfoliation of graphite or chemical vapor deposition6,7 followed by etching since it offers us opportunities to control the graphenes on a molecular level. Furthermore, the solution chemistry approach enables us to synthesize solution-processable graphenes in large quantities, with identical molecular structures that allow for ensemble studies of their properties and interfacing them with other active materials for complex functions. In this Perspective, we discuss the latest progress on colloidal graphene quantum dots (QDs) synthesized through solution chemistry, highlighting emerging opportunities and challenges in exploring this new type of carbon materials.

This new quantum-confined system may lead to novel applications, and can serve as a model system for understanding complex processes in carbon materials.

Received Date: June 24, 2010 Accepted Date: August 6, 2010 Published on Web Date: August 12, 2010

The geometry and chemical nature of the edges of graphene nanostructures, in addition to their size and shape, play

r 2010 American Chemical Society

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DOI: 10.1021/jz100862f |J. Phys. Chem. Lett. 2010, 1, 2572–2576


Electronic and Optical Properties: What Can We Expect? While the electronic and optical properties of graphene QDs have seldom been investigated, we can anticipate many interesting phenomena by comparing them with previously studied quantum-confined systems such as semiconducting carbon nanotubes and other semiconducting inorganic QDs.8-10 Like carbon nanotubes, graphene consists of light atoms and thus has a small dielectric constant and weak spin-orbit coupling. These lead to strong carrier-carrier interactions and electronic states with well-defined spin multiplicity.9 Meanwhile, graphene QDs are zero-dimensional, and graphene has zero effective masses for charge carriers. Therefore, we expect some unique electric and optical properties in graphene QDs. Graphene QDs have much greater carrier-carrier Columbic interaction than other inorganic semiconductor QDs of similar sizes, which could have profound effects on both electronic levels in the QDs and energy relaxation dynamics. In semiconductor QDs, the electronic energy levels are dictated by quantum-size energy Eq due to the confinement, carrier-carrier Columbic interaction EC, and exchange interaction EX.1,2 In QDs of semiconductors with massive charge carriers confined in three dimensions, the scaling of the three terms versus the diameter of the QDs a is given by Eq ∼ h2/ 8m*a2 ∼ 1/a2, EC ∼ e2/4πεa ∼ 1/a, and EX ∼ 1/a3, where m* is the effective mass of the charge carriers, h the Planck's constant, e the charge of an electron, and ε the dielectric constant of the semiconductor. In contrast, because of the dimensionality of graphene and its massless charge carriers, the scaling for graphene QDs is instead given by Eq ∼ νFh/2a, EC ∼ 1/a, and EX ∼ 1/a2, where νF ≈ 106 m/s, is the Fermi velocity in graphene.11 By assuming a dielectric constant of 1 for graphene,10 we can further obtain Eq/EC ∼ 1. Thus, Eq and EC are comparable, and excitonic effects are important for graphene QDs of all sizes, whereas in very small particles of semiconductors with massive charge carriers, the Columbic interaction EC is only a small correction to Eq or EX. As an example, for a graphene QD with a diameter of 2.0 nm, the Columbic term EC ≈ 0.8 eV is much larger than those for CdSe or CdS,8 and thus, distinct excitonic features can be observed spectroscopically at room temperature. The size-dependent, discrete excitonic levels could significantly slow down the relaxation of high excited states (i.e., electron cooling) in graphene QDs due to a phonon bottleneck, which has been predicted in quantum-confined systems12 yet remains controversial.13 From a molecular point of view, radiationless relaxation between electronic states with energy spacing much larger than vibrational frequencies requires emission of multiple vibrational quanta, which reduces the FranckCondon factor in the coupling of the two states and thus reduces the relaxation rate. This is also referred to as the energy-gap law.14 In addition, strong carrier-carrier interactions could lead to generation of more than one exciton with one photon absorbed, a process particularly useful for improving the efficiency of photovoltaics,15 which is under debate in the semiconductor QD community16 and recently demonstrated in carbon nanotubes.17,18 Weak spin-orbit coupling means that the electronic states in graphene have well-defined spin multiplicity, and therefore,


excitons in graphene, like those in carbon nanotubes, have spin of either 0 or 1.9 However, the energy difference between the two spin states (i.e., singlet-triplet splitting), which is determined by exchange interaction, decreases with increasing size (EX ∼ 1/a2). As a result, according to the energy-gap law in radiationless energy relaxation, the reduced singlettriplet splitting could enhance spin-orbit coupling and thus coupling between the singlet and triplet manifolds.19 In small polycyclic aromatic hydrocarbons (a ≈ 0.5 nm), EX is on the order of 1 eV. For graphene QDs of nanometers size, it is in the range of several hundreds of meV, comparable to vibrational frequencies in the aromatic systems (e.g., stretching frequencies of CdC bonds ≈ 1600 cm-1 or 200 meV). Thus, when vibronic coupling in the singlet or triplet manifold is considered, the singlet and triplet manifolds overlap significantly in energy, leading to enhanced intersystem crossing. This was recently observed in photoexcited graphene QDs of ∼2.5 nm diameter.19 The singlet-triplet splitting was determined to be ∼175 meV, and intersystem crossing was so efficient that it competed with internal conversion among the states with the same multiplicity. As a result, the QDs emitted both fluorescence and phosphorescence at room temperature, with their relative intensity depending on the excitation energy. Since triplet states have a significantly longer lifetime, they could profoundly affect the chemical reactivity and other processes such as charge transfer or exciton migration in the graphenes. The triplet-state formation could also enable optical spin injection in the graphene QDs for spintronics, in which long spin decoherence time is advantageous for spin manipulation.20,21 Stabilization and Synthesis. Stabilization of graphene is an integral part of graphene synthesis because unstabilized graphene forms graphite. Structurally speaking, graphene belongs to polycyclic aromatic hydrocarbons, and there has been a long history of making large aromatic systems by fusing smaller aromatic compounds.22,23 Even though remarkable progress has been made in forming carboncarbon bonds and making extended conjugated systems,23 the rapidly decreasing solubility of graphenes with increasing size posed a tremendous challenge for the solution chemistry approach to large graphene nanostructures. A common strategy to solubilize conjugated systems has been lateral attachment of flexible side chains, which has been very successful in solubilizing small graphene molecules. The affinity between solvents and the flexible chains overcomes the intergraphene attraction, resulting in the graphenes being entropically pushed apart. However, since the maximum number of the flexible side chains and thus the chain-solvent interaction scales with the perimeter of the graphenes (∼a, with a being the diameter) while the intergraphene attraction scales with area (∼a2), for graphenes with increasing size, the intergraphene attraction rapidly overtakes the solubilization forces, making the current strategy less and less effective. We recently developed a new solubilization strategy for large graphene QDs by creating a three-dimensional “cage” around the graphene core.24 This was achieved by covalently attaching multiple 20 ,40 ,60 -trialkyl-substituted phenyl moieties (at the 10 -position) to the edges of the graphene (Figure 2). The crowding on the edges forces the peripheral phenyl groups to twist from the plane of the graphene, resulting in the alkyl

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ment. As aromatic hydrocarbons are well-known to show solvent-dependent optical properties, suspended graphene has some important differences from graphene on the substrate, both geometrically28 and electronically.29 This unique feature of graphene has been applied to fabricate sensors with single-molecule sensitivity.30 Moreover, the solution processability of the graphene QDs allows us to apply them with other active components for devices, such as photovoltaics,24 by taking advantage of their superior electro-optical and transport properties as well as the high natural abundance of its constituent elements. Since graphene has tunable band gap and large optical absorptivity, it can be used for light harvesting, as an alternative for more commonly used ones containing rare or toxic heavy metals. Their electronic levels of graphenes and their interfacing with other materials for charge-transfer processes can both be tuned with carbon chemistry. Graphene also has been shown to have very large charge mobilities, which could be useful for charge collection in solar cells. Challenges in the Chemistry of Graphene QDs. The versatile carbon chemistry potentially enables us to synthesize graphene QDs in a controlled fashion for various purposes. For example, their well-defined properties may make them useful model systems for understanding complex processes involving carbon materials.31-33 Of particular interest is that carbon as fractured nanotubes or small polyacenes catalyzes oxidative dehydrogenation of hydrocarbons, a class of reactions formerly restricted to transition metals and their oxides. Graphene QDs with zigzag edges were predicted to have interesting magnetic properties.34,35 Functionalization of the graphene cores with electron-donating or -withdrawing functional groups, as well as doping the graphene with heteroatoms, provides additional parameters to tune the electronic properties of the two-dimensional systems. However, to achieve the full potential of carbon chemistry in synthesizing graphene QDs with atomic precision, it is necessary to develop new purification and structural characterization techniques for such large conjugated systems.

Figure 2. Solubilization strategy for colloidal QDs. (a) A 20 ,40 ,60 trialkyl-substituted phenyl moiety is covalently attached to the edges of the graphene (at the 10 -position), so that the crowding on the edges forces the peripheral phenyl groups to twist from the plane of the graphene, resulting in the alkyl chains at the 20 ,60 -positions extending out of the plane and the one at the 40 -position extending laterally. (b) An energy-minimized geometry of the graphene QD 1 (in c), showing the “caging” of the graphene core (blue) by the alkyl chains (black) in three dimensions. (c) Structures of the three largest colloidal graphene QDs synthesized so far, containing 168, 132, and 170 conjugated carbon atoms, respectively. The structures of the QDs are controlled with stepwise solution chemistry so that they have excellent size uniformity (from refs 24 and 27).

chains at the 20 ,60 -positions extending out of the plane and the one at the 40 -position extending laterally.24 This leads to increased distance between graphenes in all three dimensions and thus greatly reduces the intergraphene attraction due to its short range.25 This approach is reminiscent of the well-known intercalation of graphite, in which inserting atoms or molecules between graphene layers significantly reduces the interlayer binding energy.26 With the new solubilization approach, we recently demonstrated the synthesis of stable graphene QDs that contain 132, 168, and 170 conjugated carbon atoms, respectively (1-3 in Figure 2c), the largest ones reported so far.24,27 They were synthesized from small aromatic compounds through stepwise organic chemical reactions and have excellent size uniformity. The intermediates to the QDs were purified with chromatography and confirmed with standard organic characterization techniques. In the last step, a polyphenylene dendrimer precursor was oxidized to fuse the phenyl rings, leading to the graphene QDs, which were then purified with repetitive precipitation and dissolution.24 To ensure the solubility of the final products, the dendrimeric precursors had the solubilizing trialkyl phenyl groups preinstalled. The resultant QDs 1-3 were highly soluble in common solvents such as chloroform, toluene, and so forth. Dynamic light scattering measurements show that the QDs are dispersed in forms of reversible oligomers, indicating the presence of residual intergraphene attraction in solution, despite the introduction of the solubilization groups.24 Colloidal graphene QDs not only allow us to investigate size- and shape-dependent properties of graphene that cannot be easily done on a single-sheet level but also enable us to vary their environment for property studies or novel functions. All electrons in graphene are on the surface. Therefore, electron distribution is subject to influences by the environ-


To achieve the full potential of carbon chemistry in synthesizing graphene QDs with atomic precision, it is necessary to develop new purification and structural characterization techniques. Modern purification techniques have played an essential role in achieving high purity in organic compounds, extension of which to graphene QDs however is not straightforward due to the large conjugation. No chemical reaction so far could achieve perfect yield in making polycyclic aromatic systems. Even with the most successful methods developed by M€ ullen et al.,23 rearrangement of phenyl groups occurs on some occasions,36,37 making the structures of the final products

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groups (such as 1),24 where the number of resonance peaks reveals the molecular symmetry. For graphene QDs with lower symmetry or more functional groups, however, theoretical calculation of vibrational frequencies is necessary to assist with the structural determination, which is challenging because of the size of the systems. Carbon materials have played an extremely important role in our society. They not only are the bases of many important technologies ranging from pencils, adsorbents, and metal strengtheners to batteries and many others, but they also constitute the largest supply of energy that we use today (i.e., coal). Since graphene can be considered as the basic architecture for all graphitic carbon materials, including fullerenes, carbon nanotubes, graphene, and graphite, synthesis and investigation of well-defined graphene nanostructures could greatly enhance our understanding of complex carbon materials. Graphene has many novel optical and electrical properties; thus, it could also lead to novel uses for carbon materials in its own right, such as sustainable use of carbon for energy applications.

sometimes unpredictable and thus limiting the geometry of the graphene QDs that can be obtained. Partially oxidized products and halogenated products also are present in most cases, which, even though in small amounts, may necessitate spectral selection techniques in optical studies, as has been done in studying other semiconductor QDs with size inhomogeneity. With normal phase liquid chromatography techniques, the large conjugated systems have high affinity to the polar stationary phases made of either silica or alumina through strong van der Waals interactions,38 so that they cannot be eluted. With reverse phase liquid chromatography or size exclusion chromatography, even though the QDs can be eluted due to reduced affinity, the small structural variation between graphene QDs and reaction byproducts is generally not sufficient for effective separation. Crystallization of the graphene QDs is also frustrated because the solubilization groups introduce packing disorder. Development of ensemble structural characterization of graphene QDs remains another challenge. Made through organic chemistry routes from smaller organic precursors, the graphene QDs have reached the size that some wellaccepted organic characterization techniques become ineffective. With the conventional NMR technique used for organic characterization, the most important structural information of the graphene QDs is contained in the aromatic proton resonance region that covers a narrow chemical shift range of ∼2 ppm. It is prone to suppression by peak broadening due to the slow tumbling of the QDs in solution and the fast spin-spin relaxation in the rigid structures. Oligomerization of the QDs in solution due to residual intergraphene attraction further aggravates the situation, limiting the usefulness of conventional solution NMR techniques for structurally characterizing the graphene QDs. Elemental analysis of the graphenes suffers from incomplete combustion and the difficulty in determining the hydrogen content with sufficient accuracy. The transmission electron microscopy (TEM), which is often used for screening other semiconductor QDs, is hindered by the low contrast from the carbon-containing graphene QDs, making it necessary to use large graphene sheets as the substrates for the imaging. In addition, since the properties of the graphene QDs are determined not only by their sizes and shapes but also by the edges, atomic resolution is necessary, and thus, TEM, as well as scanning tunneling microscopy (STM), becomes unpractical for analyzing large numbers of graphene QDs. High-resolution MALDI-mass spectroscopy (MALDI-MS) has been so far the best way to survey graphene QDs on an ensemble level. Isotope-resolved MALDI-MS spectra can be compared with simulated ones from proposed structures, yielding mass accuracy up to a fraction of an atomic mass unit. Since the species of concern are all large aromatics that differ little in structure, they are likely to have comparable ionization probabilities, and thus, MALDI-MS can yield reasonable, yet not quantitative, estimation of the composition. It can further be supplemented with IR spectroscopy, in which the aromatic C-H out-of-plane bending modes provide the fingerprint for identifying the substitution patterns of phenyl groups39 and thus the edges of the QDs. It is particularly useful for highly symmetric graphene QDs with few functional


AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: li23@ indiana.edu.

Biographies Liang-shi Li is currently an assistant professor at Indiana University. He got his B.S. and M.S. degrees at the University of Science and Technology of China and his Ph.D. at the University of California, Berkeley. He is interested in chemistry and physics of carbon-rich materials and their applications in renewable energy and neurosciences. Xin Yan received his B.S. and M.S. degrees in Polymer Chemistry and Physics at Jilin University, China. He has been a Ph.D. student at Indiana University since 2007. His research focuses on the synthesis and application of graphene quantum dots.

ACKNOWLEDGMENT We thank Dr. Allen Siedle at 3M and Professor John McGuire at Michigan State University for helpful discussions. We acknowledge funds from the National Science Foundation, ACS Petroleum Research Fund, and Indiana University for supporting this work.

REFERENCES (1) (2) (3)

(4) (5) (6) (7)

2575

Brus, L. E. Quantum Crystallites and Nonlinear Optics. Appl. Phys. A 1991, 53, 465–474. Efros, A. L.; Rosen, M. The Electronic Structure of Semiconductor Nanocrystals. Annu. Rev. Mater. Sci. 2000, 30, 475–521. El-Sayed, M. A. Small Is Different: Shape-, Size-, and Composition-Dependent Properties of Some Colloidal Semiconductor Nanocrystals. Acc. Chem. Res. 2004, 37, 326–333. Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183–191. Geim, A. K.; MacDonald, A. H. Graphene: Exploring Carbon Flatland. Phys. Today 2007, 35–41. Allen, M. J.; Tung, V. C.; Kaner, R. B. Honeycomb Carbon: A Review of Graphene. Chem. Rev. 2010, 110, 132–145. Park, S.; Ruoff, R. S. Chemical Methods for the Production of Graphenes. Nat. Nanotechnol. 2009, 4, 217–224.



(8) (9)

(10)

(11)

(12)

(13) (14) (15) (16)

(17)

(18)

(19)

(20) (21)

(22) (23) (24)

(25) (26) (27)

(28)

(29)

(30)

(31)

(32)

Scholes, G. D.; Rumbles, G. Exciton in Nanoscale Systems. Nat. Mater. 2006, 5, 683–696. Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Exciton Photophysics of Carbon Nanotubes. Annu. Rev. Phys. Chem. 2007, 58, 719–747. Brus, L. E. Commentary: Carbon Nanotubes, CdSe Nanocrystals, and Electron-Electron Interaction. Nano Lett. 2010, 10, 363–365. Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K. Chaotic Dirac Billiard in Graphene Quantum Dots. Science 2008, 320, 356–358. Bockelmann, U.; Bastard, G. Phonon Scattering and Energy Relaxation in Two-, One-, and Zero-Dimensional Electron Gases. Phys. Rev. B 1990, 42, 8947–8951. Klimov, V. In Semiconductor and Metal Nanocrystals; Klimov, V., Ed.; Marcel Dekker: New York, 2004; pp 159-214. Henry, B. R.; Siebrand, W. In Organic Molecular Photophysics; Birks, J. B., Ed.; John Wiley & Sons: London, 1973; Vol. 1. Nozik, A. J. Quantum Dot Solar Cells. Physica E 2002, 14, 115–120. McGuire, J. A.; Joo, J.; Pietryga, J. M.; Schaller, R. D.; Klimov, V. New Aspects of Carrier Multiplication in Semiconductor Nanocrystals. Acc. Chem. Res. 2008, 41, 1810–1819. Gabor, N. M.; Zhong, Z.; Bosnick, K.; Park, J.; McEuen, P. L. Extremely Efficient Multiple Electron-Hole Pair Generation in Carbon Nanotube Photodiodes. Science 2009, 325, 1367– 1371. Wang, S.; Khahzov, M.; Tu, X.; Zheng, M.; Krauss, T. D. Multiple Exciton Generation in Single-Walled Carbon Nanotubes. Nano Lett. 2010, 10, 2381–2386. Mueller, M. L.; Yan, X.; McGuire, J. A.; Li, L.-S. Triplet States and Electronic Relaxation in Photoexcited Graphene Quantum Dots. Nano Lett. 2010, 10, 2679–2682. Trauzettel, B.; Bulaev, D. V.; Loss, D.; Burkard, G. Spin Qubits in Graphene Quantum Dots. Nat. Phys. 2007, 3, 192–196. Wolf, S. A.; Awschalorn, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Spintronics: A Spin-Based Electronics Vision for the Future. Science 2001, 294, 1488–1495. Harvey, R. G. Polycyclic Aromatic Hydrocarbons; Wiley-VCH: New York, 1997. Wu, J.; Pisula, W.; Mullen, K. Graphenes as Potential Materials for Electronics. Chem. Rev. 2007, 107, 718–747. Yan, X.; Cui, X.; Li, B.; Li, L.-S. Large, Solution-Processable Graphene Quantum Dots As Light Absorbers for Photovoltaics. Nano Lett. 2010, 10, 1869–1873. Brennan, R. O. The Interlayer Binding in Graphite. J. Chem. Phys. 1952, 20, 40–48. Dresselhaus, M. S.; Dresselhaus, G. Intercalation Compounds of Graphite. Adv. Phys. 2002, 51, 1–186. Yan, X.; Cui, X.; Li, L.-S. Synthesis of Large, Stable Graphene Quantum Dots with Tunable Size. J. Am. Chem. Soc. 2010, 132, 5944–5945. Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. The Structure of Suspended Graphene Sheets. Nature 2007, 446, 60–63. Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Approaching Ballistic Transport in Suspended Graphene. Nat. Nanotechnol. 2008, 3, 491–495. Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6, 652–655. Burg, P.; Cagniant, D. In Chemistry and Physics of Carbon; Radovic, L. R., Ed.; CRC Press: Boca Raton, FL, 2008; Vol. 30.


(33)

(34) (35) (36)

(37)

(38) (39)

2576

Zhang, J.; Liu, X.; Blume, R.; Zhang, A. H.; Schlogl, R.; Su, D. S. Surface-Modified Carbon Nanotubes Catalyze Oxidative Dehydrogenation of n-Butane. Science 2008, 322, 73–77. Zhang, J.; Wang, X.; Su, Q.; Zhi, L. J.; Thomas, A.; Feng, X. L.; Su, D. S.; Schlogl, R.; Mullen, K. Metal-Free Phenanthrenequinone Cyclotrimer as an Effective Heterogeneous Catalyst. J. Am. Chem. Soc. 2009, 131, 11296–11297. Wang, W. L.; Meng, S.; Kaxiras, E. Graphene Nanoflakes with Large Spin. Nano Lett. 2008, 8, 241–245. Fernandez-Rossier, J.; Palacios, J. J. Magnetism in Graphene Nanoislands. Phys. Rev. Lett. 2007, 99, 177204. Muller, M.; Iyer, V. S.; Kubel, C.; Enkelmann, V.; Mullen, K. Polycyclic Aromatic Hydrocarbons by Cyclodehydrogenation and Skeletal Rearrangement of Oligophenylenes. Angew. Chem., Int. Ed. Engl. 1997, 36, 1607–1610. Ormsby, J. L.; Black, T. D.; Hilton, C. L.; Bharat; King, B. T. Rearrangements in the Scholl Oxidation: Implications for Molecular Architectures. Tetrahedron 2008, 64, 11370–11378. Fetzer, J. C. Large (C g 24) Polycyclic Aromatic Hydrocarbons; Wiley-Interscience: New York, 2000. Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Academic Press: New York, 1969.


Colloidal Graphene Quantum Dots - The Journal of Physical

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