Synthesis of large, stable colloidal graphene quantum dots with tunable size

Article abstract of DOI:10.1021/ja1009376

We report a solution-chemistry-based approach to large, stable colloidal graphene quantum dots with uniform size and shape. The versatility of solution chemistry allows us to tune the structures of the graphenes and thus their properties.

Full text of DOI:10.1021/ja1009376

Published on Web 04/08/2010
Synthesis of Large, Stable Colloidal Graphene Quantum Dots with Tunable Size
Xin Yan, Xiao Cui, and Liang-shi Li*
Department of Chemistry, Indiana UniVersity, Bloomington, Indiana 47405
Received February 2, 2010; E-mail: li23@indiana.edu
Synthesis of colloidal semiconductor nanocrystals (or “quantum
dots”) with uniform sizes that approach bulk-material exciton Bohr
radius played an important role in our understanding of quantum
confinement.1 It has led to various practical applications in
bioimaging,2a-c lasing,2d-f photovoltaics,3a-c and light emitting
diodes.3d-f Graphene, consisting of a single atomic layer of
graphite, is a unique type of zero band gap semiconductor with an
infinite exciton Bohr radius due to a linear energy dispersion relation
of the charge carriers.4 As a result, quantum confinement could
take effect in graphenes of any finite size and is expected to result
in many interesting phenomena not obtainable in other semiconduc-
tor materials.5 However, in comparison with conventional semi-
conductors, the synthesis of graphene quantum dots is less
developed. The increasing face-to-face attraction between graphenes
with increasing size leads to rapidly decreasing solubility, which
has seriously limited the sizes achievable for stable colloidal
graphene nanostructures and prevented systematic studies of
quantum confinement in them.6
were purified with silica gel chromatography and confirmed with
standard characterization methods. To ensure good yields for the
final products, the connectivity among the phenyl groups in the
dendritic precursors was designed to avoid unwanted phenyl
rearrangement under the oxidative conditions.6a-c,8 The solubilizing
trialkyl phenyl groups were installed prior to the oxidation, thus
preventing aggregation of the final products when they were formed.
The oxidation reactions were stopped by quenching with methanol
after 2-3 h. Repetitive centrifugation, dissolution with toluene, and
precipitation with methanol led to quantum dot 2 as a deep red
and 1, 3 as black waxy solids. Here the synthetic routes are derived
from one we reported earlier that is applicable only for synthesizing
highly symmetric quantum dots.8 We have significantly improved
it so that we can tune the properties of the graphenes by varying
their symmetry, as well as increase the yield for 1.
Scheme 1. Synthesis of Graphene Quantum Dots 1-3a
Here we report the synthesis of large colloidal graphene quantum
dots with a uniform and tunable size, e.g., 1-3, through solution
chemistry. They consist of graphene moieties containing 168, 132,
and 170 conjugated carbon atoms, respectively, and are the largest
stable colloidal graphene quantum dots reported so far. Our work
is based on oxidative condensation reactions developed by Scholl,
Mu¨llen, and others,6a,7 with a new solubilization strategy recently
developed in our research group.8 The oxidation of polyphenylene
dendritic precursors that were synthesized through stepwise solution
chemistry led to fused graphene moieties. The stabilization of the
resultant graphenes is achieved by multiple 2′,4′,6′-triakyl phenyl
groups covalently attached to the edges of the graphene moieties.
The crowdedness on the edges of the graphene cores twists the
substituted phenyl groups from the plane of the core, leading to
alkyl chains closing the latter in all three dimensions. This results
in reduced face-to-face interaction between the graphenes, thus
effectively increasing their solubility and allowing us to make stable
graphene quantum dots larger than previously achieved.
a Conditions: (i) 3-(phenylethynyl)phenylboronic acid, Pd(PPh3)4, K2CO3,
toluene, EtOH, H2O, 80 °C; (ii) I2, tert-butyl nitrite, benzene, 5 °C; (iii)
4-(2′,4′,6′-trialkylphenyl)phenylborate, Pd(PPh3)4, K2CO3, toluene, EtOH,
H2O, 80 °C; (iv) tetraphenylcyclopentadienone, diphenyl ether, reflux; (v)
(a) n-BuLi, THF, -78 °C; (b) B(i-PrO)3, (c) HCl, H2O; (vi) 1,3,5-
triiodobenzene, Pd(PPh3)4, K2CO3, H2O, toluene, 80 °C; (vii) FeCl3, CH2Cl2,
CH3NO2; (viii) phenylboronic acid, Pd(PPh3)4, K2CO3, toluene, EtOH, H2O,
80 °C; (ix) pinacol; (x) 1,3-dibromo-5-iodobenzene, Pd(PPh3)4, K2CO3,
toluene, H2O, EtOH, 60 °C; (xi) 6, Pd(PPh3)4, K2CO3, toluene, H2O, 80
°C; (xii) 1,3-diphenylcyclopenta[e]pyren-2-one, diphenyl ether, reflux.
The synthesis of 1-3 is outlined in Scheme 1. We started from
small-molecule precursors, such as 3-iodo-4-bromoaniline (4) and
other substituted benzene derivatives, to synthesize two key
intermediates 5 and 6 (details in the Supporting Information).
Subsequent stepwise Suzuki coupling reactions led to polyphenylene
dendritic precursors for 1-3, which were then exposed to an excess
of FeCl3 in a dichloromethane/nitromethane mixture, yielding the
graphene quantum dots. All the intermediates to the quantum dots
Quantum dots 1-3 were identified with isotope-resolved mass
spectroscopy (MS), indicating their excellent uniformity in size.
They all are highly soluble in common organic solvents such as
chloroform, toluene, THF, etc. However, with conventional liquid
phase NMR spectroscopy no aromatic protons in 1-3 could be
detected, presumably because of dynamic aggregation of the
quantum dots in the NMR time scale. This is similar to soluble
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5944 J. AM. CHEM. SOC. 2010, 132, 5944–5945
10.1021/ja1009376  2010 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. MALDI-TOF MS spectra of quantum dots 1-3 (left to right), respectively. In the insets are the isotope-resolved patterns experimentally measured
(solid curves) with ones calculated (dotted curves) from the molecular structures of the quantum dots. In the spectra of 1 and 3, the small peaks marked by
asterisks are due to monochlorinated byproducts (M+34) in the last oxidation step.
nanographenes previously reported that contain fewer conjugated
carbon atoms.10a As a result, isotope-resolved MS with mild
ionization methods (e.g., MALDI-TOF) have been the only
applicable techniques for ensemble characterization of large
graphene nanostructures.10 Figure 1 shows the MALDI-TOF MS
results of 1-3, respectively. The comparison with spectra calculated
with the molecular formula of the quantum dots indicates the
elimination of the desired numbers of hydrogen atoms during the
oxidative condensation and the excellent size uniformity achieved
in the quantum dots. The structure of 1 was further confirmed with
infrared vibrational spectroscopy because of its high structural
symmetry and therefore the easy interpretation of the spectrum.8
As expected 1-3 have size- and shape-dependent optical
properties. Figure 2 shows their absorption spectra in dichlo-
romethane. Because of their larger sizes 1 and 3 have absorption
edges appearing at significantly longer wavelengths than 2. In
addition, all the spectra show R, p, and ꢀ bands in the order of
increasing energy from the absorption edges (e.g., marked by arrows
for 1 in Figure 2), a classic pattern observed in many polycyclic
aromatic hydrocarbons.11 The R and p transitions, which in 1 are
only weakly dipole-allowed due to the high molecular symmetry,
become significantly more pronounced in 2 as the symmetry reduces
(details in the Supporting Information), and the ꢀ transition that is
doubly degenerate in 1 appears as two separate bands in 2 or 3
since the reduced symmetry lifts the degeneracy.
both theoretical and experimental studies have shown that novel
electronic and magnetic properties may emerge as results of well-
defined edge structures in graphenes.5 Therefore the graphene
quantum dots may see a wide range of applications in optoelec-
tronics and molecular magnetism.
Acknowledgment. This work is supported by Indiana University
and the National Science Foundation (Grant 0747751).
Supporting Information Available: Detailed procedure for syn-
thesis and characterization of intermediates, IR spectra, and analysis
of UV-vis absorption bands of 1-3. This material is available free of
charge via the Internet at http://pubs.acs.org.
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Our work demonstrates the versatile synthesis of large, stable
colloidal graphene quantum dots with desired sizes and structures
enabled by a new solubilization strategy. Quantum dots 1 and 3
presented here, in particular, have large extinction coefficients
(Figure 2) in a wide spectral range from UV to near-infrared and,
thus, can serve as a new type of light harvesting media for
photovoltaics.8 We can further reduce the band gap of the graphenes
by increasing their sizes and tune their redox potentials by chemical
functionalization to tailor their properties for the devices. In addition,
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Figure 2. UV-vis absorption spectra of quantum dots 1-3 in solution.
Marked by arrows are the R, p, and ꢀ bands of 1, from right to left,
respectively.
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