Bottom-up fabrication of photoluminescent graphene quantum dots with uniform morphology

Article abstract of DOI:10.1021/ja204953k

Multicolor photoluminescent graphene quantum dots (GQDs) with a uniform size of ～60 nm diameter and 2-3 nm thickness were prepared by using unsubstituted hexa-peri-hexabenzocoronene as the carbon source. This result offers a new strategy to fabricate monodispersed GQDs with well-defined morphology.

Full text of DOI:10.1021/ja204953k

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pubs.acs.org/JACS
Bottom-Up Fabrication of Photoluminescent Graphene Quantum
Dots with Uniform Morphology
Ruili Liu,*,†,‡ Dongqing Wu,† Xinliang Feng,† and Klaus M€ullen*,†
†Max-Planck-Institut f€ur Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany
‡School of Environmental and Chemical Engineering, Shanghai University, Shangda Road 99, Shanghai 200444, P.R. China
S Supporting Information
b
Scheme 1. Processing Diagram for the Preparation of
Photoluminescent GQDs by Using HBC (1) as Carbon
Source
ABSTRACT: Multicolor photoluminescent graphene
quantum dots (GQDs) with a uniform size of ∼60 nm
diameter and 2À3 nm thickness were prepared by using
unsubstituted hexa-peri-hexabenzocoronene as the carbon
source. This result offers a new strategy to fabricate mono-
dispersed GQDs with well-defined morphology.
raphene, a two-dimensional (2D) honeycomb network
G
of sp2-hybridized carbon atoms, has received much atten-
tion because its unique properties, including high surface area,
electronic conductivity, and mechanical stability, make it very
attractive in physics, chemistry, and material science.1,2 It has
been both theoretically predicted and experimentally proved that
the morphology of graphene sheets, including their size, shape,
and thickness, can effectively determine their properties.3À5 For
instance, graphene sheets smaller than 100 nm, generally called
graphene quantum dots (GQDs),4 possess strong quantum
confinement and edge effects, which make them excellent
materials for the construction of nanoscaled optical and electro-
nic devices. To fabricate GQDs, Novoselov et al. used high-
resolution electron-beam lithography to carve graphene crystal-
lites to desired sizes;4 this method is limited by the requirement
for special equipment and an extremely low yield. On the other
hand, GQDs with sub-10 nm diameter were prepared from large
stack with each other via πÀπ interactions. Recently, our group
prepared a series of unprecedented carbon materials by the
pyrolysis of structure-defined aromatic molecules.13À15 The
composition, degree of graphitization, and physical and chemical
properties of these materials can be easily tuned by the selection
of pyrolysis conditions and precursors with different aromatic
frameworks. Inspired by these results, we envision that the
fabrication of GQDs by controlled pyrolysis of large PAHs could
be experimentally feasible.
Herein, we report for the first time the preparation of mono-
dispersed disk-like GQDs of ∼60 nm diameter and 2À3 nm
thickness with unsubstituted HBC (1, Scheme 1) as precursor by
a process of carbonization, oxidization, surface functionalization,
and reduction. To the best of our knowledge, these graphene
disks are the largest GQDs reported so far. Optical property
characterization indicates that the resultant GQDs possess a high
photoluminescence (PL) efficiency of 3.8%, which make these
graphene nanodisks promising candidates for biolabeled carriers
and nanodevices.
graphene oxide (GO) sheets via a hydrothermal route,6,7
a
reoxidization method,8 or an electrochemical avenue.9 However,
these approaches involve a nonselective “top-down” chemical
cutting process, which does not allow precise control over the
morphology and the size distribution of the products. Alterna-
tively, cyclodehydrogenation of polyphenylene precursors can
lead to GQDs with uniform shape through solution chemistry,10,11
but these GQDs are usually smaller than 4 nm, which is below the
processable scale of state-of-the-art lithography technology
(10 nm). Very recently, Loh et al. reported the fabrication of
geometrically well-defined GQDs by the cage-opening of fullerene
on ruthenium surfaces.12 However, a solution-processable and
scalable method toward monodispersed GQDs with precisely
tailored morphology and size ranging from 10 to 100 nm is still
lacking.
In this work, 1 was synthesized conveniently by the gram-scale
cyclodehydrogenation of commercially available hexaphenylben-
zene with high purity (Supporting Information). The wide-angle
X-ray scattering (WAXS) pattern for the as-made powder of
Hexa-peri-hexabenzocoronene (HBC) and other large poly-
cyclic aromatic hydrocarbons (PAHs) are generally regarded as
nanoscaled fragments of graphene.10 Similar to graphene, these
large PAHs have high thermal stability and a strong tendency to
Received: May 30, 2011
Published: September 06, 2011
r
2011 American Chemical Society
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Figure 1. WAXS patterns of as-made HBC, AG-600, -900, and -1200.
1 (Figure 1) depicts three well-resolved scattering peaks at 7.7,
13.5, and 15.5°, associated with 100, 110, and 200 reflections,
respectively, of 2D hexagonal symmetry with space group P6mm.
The unit cell parameter a0 is calculated to be 1.32 nm, consistent
with the length (1.35 nm) of 1.16 Additionally, a strong diffrac-
tion peak with 2θ = 25.3° is observed, corresponding to the πÀπ
stacking distance between two overlapped molecules of 1
(0.35 nm). By comparison with HBC derivatives with functional
groups, the condensed stacking of 1 can be attributed to the
absence of steric hindrance from the substituents, which favors
the formation of a graphitic framework with fewer defects during
the carbonization process.
Figure 2. (A,B) AFM topography images of GQD-1200 on mica
substrates with the width and height profiles along the line in the
images. (C) DLS and (D) TEM images of GQD-1200.
The overall procedure for the preparation of GQDs is
illustrated in Scheme 1. To prepare the artificial graphite, the
as-made powder of 1 was first pyrolyzed at different tempera-
tures, 600, 900, and 1200 °C, for 5 h with a heating rate of 1 °C/
min; the products are denoted as AG-600, AG-900, and AG-
1200, respectively. As indicated by their WAXS patterns, the
characteristic peak of graphite at 25.3° was observed in each case
(Figure 1), which clearly suggests that the artificial graphite was
generated by elimination of hydrogen and fusion to larger
graphitic structures. This scattering peak becomes narrower
and more intense as the pyrolysis temperature is increased from
600 to 1200 °C, indicating an increase in the degree of crystal-
linity. Wide-angle X-ray diffraction (XRD) and Raman analysis
further confirmed the high degree of crystallinity of AG-1200
(Figure S1). In the second step, the artificial graphite was
oxidized and exfoliated with a modified Hummers method.17
Subsequently, aqueous solutions of the resultant GOs were
heated to reflux for 48 h with oligomeric poly(ethylene glycol)
diamine (PEG1500N) and then reduced with hydrazine. The
FT-IR spectrum of the resultant graphene nanodisks provides
direct evidence of successful surface modification with PEG1500N
(Figure S2). According to the starting artificial graphite, the final
products are denoted as GQD-600, GQD-900, and GQD-1200,
respectively. Owing to their PEG substituents, the graphene
sheets prepared in this work exhibit very good dispersibility in
water. To investigate their morphologies, the suspensions of
graphene sheets were transferred to the surfaces of freshly
exfoliated mica and characterized by atom force microscopy
(AFM). It was found that GQD-600 consisted mainly of
disordered particles, while GQD-900 contained both particles
and disk-shape nanosheets (Figure S3). For GQD-1200,
Figure 3. UVÀvis absorption and PL emission spectra (recorded for
progressively longer excitation wavelengths from 320 to 480 nm in
20 nm increments) of GQD-1200 in water solution. Optical photograph
obtained under excitation at 365 nm.
homogeneous nanodisks of ∼60 nm diameter and 2À3 nm
thickness were found (Figure 2A,B). The thickness of these
nanodisks is 3À4 times higher than that of reduced GO, which
suggests that they contain more than one layer of graphene.6
Dynamic light scattering (DLS) and transmission electron
microscopy (TEM) studies of GQD-1200 further confirmed
the disk-like morphology. As shown in Figure 2C, the average
hydrodynamic diameter of GQD-1200 in water is 59.6 nm.
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Round graphene nanodisks of ∼60 nm diameter can also be
observed in the TEM image (Figure 2D).
1355. R.L. and D.W. gratefully acknowledge the National Natural
Science Foundation of China (20903066 and 21102091) and
Shanghai Pujiang Program (11PJ1403600 and 11PJ1405400) for
financial support.
According to the WAXS patterns of the artificial graphite
(Figure 1), the temperature of the thermal treatment is very
important for reorganizing 1 into more ordered architectures,
which subsequently decides the morphology of the graphene
nanodisks. With the elevated pyrolysis temperature, the contin-
uous graphitic domains in the artificial graphite grow gradually by
fusing 1 with neighboring molecules at different directions in-
plane. This might be the reason for the formation of round
nanodisks from AG-1200.
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The optical properties of GQDs hold the key for their future
applications in optoelectronic devices7,9 and biological sensors.
With this in mind, we recorded the UVÀvis absorption and PL
emission spectra of GQD-1200. The GQD-1200 suspension
shows a broad UVÀvis absorption with a weak shoulder at
280 nm (Figure 3), similar to chemically reduced graphene.18
Subsequent PL characterization indicated that the GQDs can
emit strong blue PL under excitation at 365 nm. As shown in
Figure 3, GQD-1200 exhibits an excitation-dependent PL beha-
vior, very similar to previously reported GQDs.6À9 When the
excitation wavelength changes from 320 to 480 nm, the PL peak
correspondingly shifts from 430 (violet) to 560 nm (yellow).
The most intense PL from the nanodisks appears under 400 nm
excitation and has a maximum at 510 nm. The bright and colorful
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The suspension of graphene nanodisks also shows extra-high
stability. Even after being kept for 1 year in air at room temper-
ature, it still exhibits a transparent appearance and strong PL,
which offers another advantage for its future applications.
In conclusion, we have developed an unprecedented strategy
to prepare multicolor PL GQDs with a uniform size of ∼60 nm
diameter and 2À3 nm thickness by using unsubstituted HBC as
the precursor. For the first time, our results demonstrate that
GQDs with ordered morphology can be obtained by pyrolysis
and exfoliation of large PAHs, and the morphology of the GQDs
can be influenced by the pyrolysis temperature. It is expected that
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can be further controlled by using different aromatic molecules,
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, XRD and
b
Raman spectra of AG-1200, FT-IR spectra of PL GQD-1200, and
AFM images of GQD-900. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
ruililiu@shu.edu.cn; muellen@mpip-mainz.mpg.de
’ ACKNOWLEDGMENT
This work was financially supported by the Max Plank Society
through the program EENERCHEM, the German Science Founda-
tion (Korean-German IR TG), and DFG Priority Program SPP
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dx.doi.org/10.1021/ja204953k |J. Am. Chem. Soc. 2011, 133, 15221–15223