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Synthesis of Strongly Fluorescent Graphene Quantum Dots by Cage-Opening Buckminsterfullerene

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† Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
‡ Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
§ Centre of Instrumental Techniques, Institute of Inorganic Chemistry of the AS CR, v.v.i., Husinec-Rez c.p. 1001, 250 68 Rez, Czech Republic
∥ Department of Biochemistry and Microbiology, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
*Address correspondence to [email protected]
Cite this: ACS Nano 2015, 9, 3, 2548–2555
Publication Date (Web):March 11, 2015
https://doi.org/10.1021/nn505639q

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Abstract

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Graphene quantum dots is a class of graphene nanomaterials with exceptional luminescence properties. Precise dimension control of graphene quantum dots produced by chemical synthesis methods is currently difficult to achieve and usually provides a range of sizes from 3 to 25 nm. In this work, fullerene C60 is used as starting material, due to its well-defined dimension, to produce very small graphene quantum dots (∼2–3 nm). Treatment of fullerene C60 with a mixture of strong acid and chemical oxidant induced the oxidation, cage-opening, and fragmentation processes of fullerene C60. The synthesized quantum dots were characterized and supported by LDI-TOF MS, TEM, XRD, XPS, AFM, STM, FTIR, DLS, Raman spectroscopy, and luminescence analyses. The quantum dots remained fully dispersed in aqueous suspension and exhibited strong luminescence properties, with the highest intensity at 460 nm under a 340 nm excitation wavelength. Further chemical treatments with hydrazine hydrate and hydroxylamine resulted in red- and blue-shift of the luminescence, respectively.

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The properties of graphene are closely governed by its geometry and chemical compositions. Such size-dependent properties can be observed in graphene nanoribbons and graphene nanoplatelets. (1, 2) Graphene quantum dots (graphene QDs) represent a class of zero-dimensional carbon nanoparticles with typical dimensions of ca. <20 nm. (3, 4) The low dimension of graphene QDs leads to unique quantum confinement (5) and edge effects resulting in exceptional fluorescence properties. (6, 7) Due to the high surface area, potential biocompatibility or low toxicity, and the availability of a π–π conjugated network with functionalizable surface groups, graphene QDs have been envisioned to fuel the development of research in electrochemical biosensors, drug delivery, bioimaging, and energy conversion. (6, 8)
Akin to the synthesis of graphene, graphene QDs can be prepared by either a top-down or bottom-up approach. The top-down approach involves cutting graphene sheets into graphene QDs by chemical ablation, (9-13) mechanical grinding, (14) or electrochemical synthesis. (15) On top of the requirement for expensive equipment, such treatments are usually difficult to control and often provide graphene QDs of various diameters (3–25 nm). Apart from that, the bottom-up approach includes chemical synthesis (16, 17) and ruthenium-catalyzed cage-opening of fullerene C60. (18) Although the synthesis of graphene QDs with specific sizes can be anticipated, the former involves tedious synthetic procedures, while the latter requires sophisticated equipment. With the current challenges and limitations, it is thus worthwhile to develop more efficient and affordable alternatives that are able to produce high-quality graphene QDs of specific sizes. Besides that, the fabrication of well-defined tiny graphene QDs (∼2–3 nm) will be advantageous for numerous applications. For example, the low toxicity of graphene QDs as compared to standard fluorescence probes using AIIBVI semiconductors like CdSe or rare earth fluorides rendered it valuable.
In fact, this has motivated us to investigate the possibility of cage-opening fullerene C60, which is a well-defined carbon nanomaterial with a diameter of ∼1 nm, via a top-down wet chemistry approach. As a spherical molecule, fullerene C60, or buckminsterfullerene, is made up of 60 carbon atoms packed into fused hexagons and pentagons. (19) As the main chemical reactivity principle of fullerene C60 is based on relieving the strain of the fullerene C60 cage, various derivatization reactions including cycloaddition, free radical addition, nucleophilic addition, and halogenation have been achieved. (20) In conjunction, the unzipping of carbon nanotubes to produce graphene nanoribbons under the presence of strong acid and oxidant has inspired us to investigate the possible cage-opening of fullerene C60 based on similar chemical treatment. As such, the treatment of fullerene C60 with concentrated sulfuric acid and potassium permanganate oxidant based on the modified Hummers method is anticipated to yield graphene quantum dots with well-defined diameters of 2–3 nm. The obtained graphene QDs were characterized by laser desorption ionization with time-of-flight mass spectrometry (LDI-TOF MS), transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), scanning tunneling microscopy (STM), Raman spectroscopy, Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), and luminescence analyses.
Further confirmation of the generation of graphene QDs, which should be highly oxidized due to the usage of concentrated acid and strong oxidant, was achieved by subsequent extensive chemical modifications. This includes esterification with trifluoroacetic anhydride and 4-nitrobenzoyl chloride, reduction with hydrazine hydrate, and reactions with hydroxylamine. In fact, the chemical modifications were accompanied by obvious changes on the luminescence properties of the modified graphene QDs.

Results and Discussion

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The cage-opening of fullerene C60 was attempted by subjecting it to a mixture of concentrated sulfuric acid, sodium nitrate, and potassium permanganate according to the modified Hummers method. (21) Similar chemical treatment has been performed to yield carbon nanoribbons from carbon nanotubes. (22) The reaction scheme is shown in Figure 1A. It was observed in the case of fullerene C60 that the quenched reaction mixture was clear yellow with negligible precipitation. In order to remove any soluble reaction byproducts, the clear yellow solution was dialyzed using a membrane with a cutoff below 1 kDa. The dialyzed material exhibited strong luminescence, as clearly visible in Figure 1B, where a blue laser pointer (405 nm) was used for luminescence excitation.

Figure 1

Figure 1. (A) Illustration of the oxidation and cage-opening of fullerene C60 with treatment of strong acid and chemical oxidant. (B) Luminescence of graphene QDs excited with a blue laser pointer (405 nm).

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The successful oxidation/cage-opening of fullerene C60 was first investigated by LDI-TOF MS. This is a very sensitive method to detect highly stable fullerene clusters. The LDI-TOF MS spectra of fullerene C60 before and after the oxidation/cage-opening procedure are shown in Figure SI1 (Supporting Information) for comparison. Although the disappearance of the fullerene C60 peak upon oxidation/cage-opening was anticipated, it was by no means a definite indicator of the exact structure of the graphene QDs. Indeed as shown in Figure SI1A (Supporting Information), the presence of an intense peak at m/z = 720 confirmed the presence of fullerene C60. Smaller carbon clusters were detected as well. The origin of small carbon clusters such as C6+, C7+, and C8+ has been reported not only for fullerenes (23) but also for graphite oxide that has undergone thermal reduction. (24) The disappearance of the peak at m/z = 720 in the LDI-TOF MS spectrum of graphene QDs in Figure SI1B (Supporting Information) confirmed the successful oxidation/cage-opening of fullerene C60 into graphene QDs. Moreover, the absence of obvious peaks beyond m/z = 720 ruled out the presence of fullerenols and suggested possible fragmentation processes of the cage-opened C60. In fact, only peaks of low m/z corresponding to small clusters that were composed of carbon, oxygen, and hydrogen were present.

Figure 2

Figure 2. (A) AFM image of graphene QDs and the corresponding height profile for the indicated particle. (B) STM image of graphene QDs and the corresponding height profile for the indicated particle.

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The morphology of graphene QDs was further investigated using AFM and STM. The aggregation of graphene QDs during solvent evaporation was observed by AFM analysis in Figure 2A. Such aggregation originated from the formation of noncovalent interactions or weak hydrogen bonding between oxygen functional groups present on the graphene QDs’ surface. The height of graphene QDs was about 0.6–1 nm and corresponded to typical values reported for graphene oxide and graphene nanoribbons. (1, 25) Subsequent analysis with STM performed under constant-current mode showed graphene QDs with lateral sizes within the range 7–10 nm and a height of approximately 0.7 nm, as shown in Figure 2B. This corresponded to the DLS measurement, whereby particles of sizes up to 15 nm were observed. In fact, the particle size measured by DLS in Figure SI2 (Supporting Information) showed a hydrodynamic radius in the range 1–15 nm with a maximum at 3 nm, which coincided with the estimated lateral size of graphene QDs upon successful oxidation/cage-opening of C60. The aggregation of graphene QDs in solution cannot be excluded since noncovalent interactions and hydrogen bonding between the oxygen functionalities are likely to occur. It should also be noted that the platelet characters of graphene QDs should be taken into account, and the relatively broad distribution indicated various orientations of platelet particles toward the laser and detector used for the measurements.
High-resolution TEM images were also collected to resolve the structure of the graphene QDs obtained from the oxidation/cage-opening of fullerene C60 (Figure SI3, Supporting Information). During the solvent evaporation process of the graphene QD suspension, aggregates were formed as also observed from AFM and STM analyses. Figure SI3A (Supporting Information) shows a low-magnification image of the graphene QD aggregates, while the electron diffraction pattern on the yellow-colored rectangular area in the inset confirmed the amorphous character of the graphene QD aggregates. A high-resolution image of the same aggregate shown in Figure SI3B (Supporting Information) supported the fully packed fullerene structure (fully packed graphene QDs were formed during the process of solvent evaporation). Subsequent high-resolution imaging in Figure SI3C (Supporting Information), which corresponded to the peripheral region of the aggregate (within the yellow oblong) in Figure SI3B (Supporting Information), showed the network geometry of graphene QDs in two adjacent blocks.

Figure 3

Figure 3. Raman spectra of graphene QDs measured with a (A) 532 nm laser at 5 W and (B) 325 nm laser at 2 mW. (C) FT-IR spectrum of graphene QDs obtained from the cage-opening of fullerene C60. (D) Survey, high-resolution (E) C 1s and (F) O 1s core-level XPS spectra of graphene QDs. High-resolution N 1s core-level spectrum is provided in Figure SI4C in the Supporting Information.

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Raman spectroscopy was subsequently performed to investigate the structure of graphene QDs. Two characteristic bands were observed on the graphene-based materials, mainly the D-band at 1350 cm–1 and G-band at 1580 cm–1. The D-band is associated with sp3-hybridized carbon atoms originating from defects and oxygen functional groups covalently bonded to the graphenic structure. The G-band arises from sp2-hybridized carbon atoms. The computed intensity of D-band to G-band (D/G ratio) reflects the crystalline quality of the graphene-based material. It should be noted that the majority of Raman signals for materials exhibiting strong luminescence properties are often masked by the intense luminescence background. The Raman spectrum of graphene QDs measured with a 532 nm laser is shown in Figure 3A (Raman spectrum of fullerene C60 is shown in Figure SI4A, Supporting Information). The strong luminescence background formed by this excitation wavelength revealed only a weak G-band signal originating from sp2-hybridized carbon atoms. To improve the signal-to-background ratio, a UV laser was subsequently used for the Raman measurement. The Raman spectrum of graphene QDs measured with a 325 nm laser is shown in Figure 3B. The strong G-band relative to the broad and low-intensity D-band indicated low disorder in the graphene QDs. The D/G ratio of graphene QDs was 0.41. This was drastically lower than typical values of D/G ratio observed on graphene oxide prepared from graphite. To note, contrasting Raman spectra obtained from lasers in the UV and vis regions have been reported by several authors on various carbon nanomaterials such as nanodiamonds and carbon nanotubes. (26-28) UV Raman spectroscopy typically exhibits a higher sensitivity toward carbon atoms with sp3 hybridization (D-band), whereas excitation in the visible region leads to enhancement of signals originating from sp2-hybridized carbon atoms (G-band). Due to this reason, the actual D/G ratio of graphene QDs can be even lower.
FT-IR spectroscopy enables the investigation of oxygen functional groups formed by the oxidation/cage-opening process of fullerene C60. The FT-IR spectrum in Figure 3C clearly shows O–H stretching located around 3400 cm–1. The peak at 1720 cm–1 due to C═O stretching proved the presence of a carboxylic acid group. Moreover, a C═O vibrational band was observed at 1370 cm–1, whereas a weak shoulder at 1390 cm–1 was attributed to the C–O vibration of a carboxylic acid group. The C═C vibration of sp2-hybridized carbon atoms was detected at 1600 cm–1, while a strong band with a maximum at 1050 cm–1 was designated as C–O stretching. It was surprising to note that C–O stretching of the hydroxyl group resulted only in a very weak vibrational band at 1230 cm–1. As a matter of fact, the presence of hydroxyl, ketone, and carboxylic acid functional groups indicated the successful cage-opening and a high degree of oxidation of fullerene C60.
XPS analysis was consequently carried out to determine the chemical composition of graphene QDs. The XPS survey spectrum of graphene QDs presented in Figure 3D clearly shows C 1s and O 1s peaks at ∼284.5 and ∼533 eV, respectively. Na 1s and N 1s peaks were also detected at ∼198 and ∼400.5 eV, respectively. Further quantitative analysis indicated a distribution of 63.96 at. % of C, 30.87 at. % of O, 3.19 at. % of N, and 1.98 at. % of Na. A trace amount of nitrogen observed in graphene QDs was introduced during the oxidation/cage-opening procedure from the usage of NaNO3 in conjunction with KMnO4 as an oxidation agent. (29) Moreover, residues of sodium also originated from NaNO3 as well as from NaOH, which was added to neutralize sulfuric acid prior to the dialysis purification process of graphene QDs. In fact, the relatively high content of oxygen confirmed the successful oxidation/cage-opening of fullerene C60. The survey spectrum of fullerene C60 (Figure SI4B, Supporting Information) contained only a C 1s peak at ∼285 eV without any traces of oxygen. Subsequently, a high-resolution C 1s core-level spectrum of graphene QDs was obtained. The C 1s peak in Figure 3E was fitted with Guassian–Lorentzian curves to quantitatively differentiate six different carbon moieties: C═C (284.4 eV), C–C/C–H (285.4 eV), C–O (286.3 eV), C═O (288.0 eV), O–C═O (289.0 eV), and a π–π* interaction (290.5 eV). The positions of the peaks were fitted according to typical fittings for graphite oxide. (30) The calculated abundances for the carbon functional groups were 0.8% C═C, 37.4% C–C/C–H, 48.9% C–O, 9.0% C═O, 3.8% O–C═O, and 0.1% π–π* interactions. In addition, a high-resolution O 1s core-level spectrum is shown in Figure 3F. The shape and binding energy peak position supported the C 1s core-level fitting results, whereby the majority of oxygen was bonded to carbon in the form of C–O. This was also in a good agreement with the data obtained from FT-IR spectroscopy that showed strong bands originating from the vibrations of the hydroxyl functional group.

Figure 4

Figure 4. (A) Luminescence of graphene QDs in dry form obtained by excitation with a 325 nm laser using various laser powers of 20 nW, 0.1 mW, and 0.2 mW. Measurement was performed using a microphotoluminescence spectrometer (see Experimental Section). (B) Dependence of graphene QD luminescence on the wavelength of excitation light. High-intensity line originated from the scattering of excitation light. The measurement was performed on dialyzed graphene QDs on a Cary Eclipse (see Experimental Section).

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Apart from that, the luminescence properties of graphene QDs were investigated using a tunable xenon light source and a microphotoluminescence system equipped with a He–Cd laser as excitation source. The almost linear dependence of luminescence intensity was observed for the excitation energy in the range from 20 nW to 0.2 mW. The obtained spectra are shown in Figure 4A. Broad luminescence was observed in the yellow-red region of the spectra with two maxima located at 630 and 700 nm. Especially for the application of quantum dots, it is very important to determine the luminescence intensity over the excitation wavelength. As such, graphene QDs were analyzed by a fluorimeter with a tunable excitation source using a Xe lamp on an aqueous suspension of the graphene QDs (Figure 4B). The highest intensity of luminescence was observed for 340 nm excitation wavelength with a maximum intensity at 460 nm. Obvious differences observed from the measurements on dry form and an aqueous suspension of graphene QDs indicated that the protonation of the carboxylic acid group as well as the interaction of other oxygen functional groups with water may critically affect the luminescence properties. Such drastic influences can originate from the very small size of the graphene QDs, which was formed only by several tenths of aromatic rings with a high concentration of oxygen functional groups, as determined by previous characterizations using various spectroscopic techniques. In fact, the synthesis of graphene QDs, which was performed several times to ascertain the reproducibility of the method, consistently gave graphene QDs with luminescence maxima in the range 450–465 nm (Figure SI5, Supporting Information).
Further chemical treatments on graphene QDs were performed to ascertain the presence and types of oxygen-containing groups. As summarized in Figure 5A, graphene QDs were subjected to treatment with trifluoroacetic anhydride, 4-nitrobenzoyl chloride, hydrazine hydrate, and hydroxylamine to target specific oxygen functionalities. The chemical modifications of graphene QDs were mainly determined by XPS analyses (Figures SI6–9, Supporting Information). The presence of hydroxyl groups on graphene QDs was verified by esterification reactions with trifluoroacetic anhydride (TFA) and 4-nitrobenzoyl chloride (NBC). The reactions were performed in dry phase whereby graphene QDs were suspended on a gold surface and treated with vapors of the reactants. XPS analyses on TFA-functionalized graphene QDs showed up to 2.9 at. % of fluorine (F/C ratio of 0.07). On the other hand, the reactivity of NBC with graphene QDs was lower than TFA given the moderate content of nitrogen (N/C ratio of 0.05). Nevertheless, the presence of a peak at 406 eV corresponding to the presence of a nitrophenyl group in the N 1s core-level spectrum of NBC-functionalized graphene QDs suggested the success of the functionalization. This characteristic peak was however not present on graphene QDs.

Figure 5

Figure 5. (A) Chemical transformation of functional groups on graphene QDs upon reactions with trifluoroacetic anhydride, 4-nitrobenzoyl chloride, hydrazine hydrate, and hydroxylamine. Dependence of graphene QD luminescence on the wavelength of excitation light for graphene QDs treated with (B) hydroxylamine and (C) hydrazine hydrate. High intensity of line originated from the scattering of excitation light.

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Graphene QDs were subsequently reacted with hydroxylamine, in which carboxylic acid and ketone functionalities would form hydroxamic acid and oximes, respectively. The significant broadening of a peak toward higher energy (up to 403 eV) in the N 1s core-level spectrum represented N–O bonds of oxime and hydroxamic acid and suggested the success of the functionalization. More importantly, the functionalization was accompanied by changes in its luminescence properties. The aqueous solution of NH2OH-functionalized graphene QDs showed a blue-shift phenomenon. An excitation wavelength of 310 nm resulted in an emission maximum of 425 nm, as shown in Figure 5B.
Subsequent treatment with hydrazine hydrate was performed to reduce the highly oxidized graphene QDs. Hydrazine-reduced graphene oxide has been previously reported to contain N–N moieties of pyrazole structure whereby one of the N atoms is pyridinic-like, while the other is quaternary-like. (31) In fact, the presence of pyrazole was obvious in the N 1s core-level spectrum, as the peak at 401.2 eV, which corresponded to the presence of quaternary-like nitrogen atoms, was prominent. This particular peak for quaternary nitrogen was absent in the N 1s core-level spectrum of graphene QDs. Moreover, a N/C ratio of 0.28 registered by the hydrazine-reduced graphene QDs compared to 0.05 of graphene QDs signified the unintentional covalent functionalization with nitrogen moieties, mainly pyrazole, upon hydrazine treatment. Interestingly, the hydrazine-reduced graphene on graphene QDs resulted in a red-shift of the emission maximum to 520 nm when an excitation wavelength of 420 nm was applied, as shown in Figure 5C.

Conclusion

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In conclusion, the present work has demonstrated the simultaneous oxidation and cage-opening of fullerene C60 to provide graphene quantum dots. The graphene QDs were synthesized by treating fullerene C60 with a mixture of concentrated sulfuric acid, sodium nitrate, and potassium permanganate. Detailed characterization including LDI-TOF MS, TEM, AFM, STM, XPS, DLS, FT-IR, and Raman spectroscopy analyses revealed the formation of aggregated small fragments consisting of carbon, oxygen, and hydrogen elements, which favored the production of graphene QDs. More importantly, the graphene QDs exhibited strong luminescence properties when excited at 340 nm. The highly oxygenated graphene QDs showcased their broad prospects for modifications through successful functionalization reactions. The luminescence properties varied according to the types of chemical treatments, whereby hydroxylamine-functionalized graphene QDs showed a blue-shift of the emission maximum, while hydrazine-reduced graphene QDs showed a red-shift of the emission maximum. All in all, the simplicity of this method in producing graphene QDs shows potential for further development for integration into practical devices or applications including optoelectronics and biological labeling.

Experimental Section

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Materials

The fullerene C60 (99.9%) was obtained from SES Research, TX, USA. Sulfuric acid (98%), potassium permanganate (99.5%), sodium nitrate (99.5%), hydrogen peroxide (30%), sodium hydroxide, hydrazine hydrate, and N,N-dimethylformamide (DMF) were obtained from Penta, Czech Republic. Hydroxylamine, 4-nitrobenzoyl chloride, and trifluoroacetic anhydride were obtained from Sigma-Aldrich, Czech Republic.

Procedures

Synthesis of Graphene QDs

The fullerene C60 was oxidized using Hummers method. (21) Fullerene C60 (2.5 g) and sodium nitrate (1.3 g) were stirred with sulfuric acid (98%, 57 mL). The mixture was then cooled to 0 °C. Potassium permanganate (7.5 g) was then added over a period of 2 h. During the next 4 h, the reaction mixture was allowed to reach room temperature before being heated to 35 °C for 30 min. The reaction mixture was then poured into a flask containing deionized water (125 mL) and heated to 70 °C for 15 min. The mixture was then poured into deionized water (0.3 L). The unreacted potassium permanganate and manganese dioxide were removed by the addition of 3% hydrogen peroxide. The reaction mixture was then neutralized using 1 M NaOH at pH = 8. The obtained graphene QD solution was purified by dialysis.

Reaction with Hydrazine Hydrate

Dialyzed graphene QDs (5 mL) were diluted with water (10 mL), and hydrazine hydrate (1 mL) was added. The reaction mixture was slightly alkalized with potassium hydroxide and heated under reflux for 24 h. Subsequently, the reaction mixture was concentrated under vacuum in order to remove unreacted hydrazine and to increase the concentration of graphene QDs. For the XPS analysis, 0.1 mL of the modified graphene QD solution was drop-casted onto a gold-coated silicon wafer and dried under vacuum for 5 h.

Reaction with Hydroxylamine

Dialyzed graphene QDs (5 mL) were diluted with water (10 mL), and hydroxylamine (1 mL) was added. The reaction mixture was heated under reflux for 24 h. Subsequently, the reaction mixture was concentrated under vacuum to remove unreacted hydroxylamine and to increase the concentration of graphene QDs. For the XPS analysis, 0.1 mL of modified graphene QD solution was drop-casted onto a gold-coated silicon wafer and dried under vacuum for 5 h.

Reaction with 4-Nitrobenzoyl Chloride

A dialyzed graphene QD solution (0.2 mL) was drop-casted onto a gold-coated silicon wafer and dried under vacuum. The wafer with graphene QDs was placed in 10 wt % of 4-nitrobenzoyl chloride in tetrahydrofuran (5 mL) for 24 h. Subsequently the wafer was removed from the solution, and unreacted 4-nitrobenzoyl chloride was washed off with dry THF and methanol and dried under vacuum at 50 °C for 24 h. The modified graphene QDs on the Au/Si wafer were directly used for XPS measurement.

Reaction with Trifluoroacetic Anhydride

A dialyzed graphene QD solution (0.2 mL) was drop-casted onto a gold-coated silicon wafer and dried under vacuum. The wafer with graphene QDs was placed on a holder over trifluoroacetic anhydride, and the flask was evacuated to 1 mbar. After 24 h, the wafer was removed from the trifluoroacetic anhydride atmosphere and dried under vacuum at 50 °C for 48 h. The modified graphene QDs on the Au/Si wafer were directly used for XPS measurement.

Equipment

An inVia Raman microscope (Renishaw, England) with a CCD detector was used for Raman and luminescence spectroscopy in backscattering geometry. A Nd:YAG laser (532 nm, 50 mW) with 50× magnification objective and He–Cd laser (325 nm, 22 mW) with 20× NUV objective were used for measurements. Instrument calibration was performed with a silicon reference which gave a peak centered at 520 cm–1 and a resolution of less than 1 cm–1. In order to avoid radiation damage, the laser power output used for this measurement was kept in a range of 20 nW to 5 mW. Prior to measurements, the dialyzed sample was ultrasonicated for 5 min, and then the suspension was deposited on a small piece of silicon wafer and dried.
Fluorescence measurement was performed on a Cary Eclipse fluorescence spectrometer (Varian, USA). The measurement was performed in quartz glass cuvettes using 1 mL of the graphene QD suspension.
For the measurement of atomic force microscopy images, the dialyzed sample was ultrasonicated for 5 min and the suspension of graphene QDs was drop-casted onto a freshly cleaved mica substrate. These measurements were carried out on an Ntegra Spectra from NT-MDT. The surface scans were performed in tapping (semicontact) mode. Cantilevers with a strain constant of 1.5 kN·m–1 equipped with a standard silicon tip with curvature radius less than 10 nm were used for all measurements. The measurement was performed under ambient conditions with a scan rate of 1 Hz and scan line of 512.
For the measurement of scanning tunneling microscopy images, the dialyzed sample was ultrasonicated for 5 min and the suspension of graphene QDs was drop-casted onto freshly cleaved highly oriented pyrolytic graphite. These measurements were carried out on an Ntegra Spectra from NT-MDT. The surface scans were performed in a constant current mode using a Pt–Ir tip.
High-resolution X-ray photoelectron spectroscopy was performed on an ESCAProbeP (Omicron Nanotechnology Ltd., Germany) spectrometer equipped with a monochromatic aluminum X-ray radiation source (1486.7 eV). Wide-scan surveys with subsequent high-resolution scans of C 1s and O 1s core levels were performed. The relative sensitivity factors were used in the evaluation of the carbon-to-oxygen (C/O) ratios from the survey spectra. The dialyzed sample was ultrasonicated for 5 min, and the suspension of graphene QDs was drop-casted (0.1 mL) onto a layer of gold (200 nm) freshly evaporated on a silicon wafer.
The morphology and structure of the cage-opened fullerenes were studied by using HR-TEM JEOL JEM-3010 operated at 300 kV (LaB6 cathode and point-to-point resolution 0.19 nm). For investigation of graphene, the suspension with water (1 mg/mL) was ultrasonicated for 15 min (75 W) before use. As specimen support for TEM investigation, a microscopic copper grid covered by a thin transparent carbon film was used. The samples were studied in both bright field and electron diffraction (SEAD) modes.
Samples for laser desorption/ionization with time-of-flight detector mass spectrometry were prepared similarly to the method described by Kuckova etal. (32) Dialyzed graphene QDs were ultrasonicated for 5 min, and 1 μL was directly dropped on the steel sample plate and air-dried. The mass spectra were acquired in positive reflector (for masses in the range 0–2000 Da) with a mass accuracy of 0.4 Da on the Bruker-Daltonics Biflex IV mass spectrometer fitted with a standard nitrogen laser (337 nm, 120 μJ). The spectra were analyzed using the XMASS (Bruker) and mMass software. (33) Each mass spectrum was the result of at least 100 laser pulses. The spectra were calibrated by Pepmix (Bruker Daltonics, Germany).
Fourier transform infrared spectroscopy measurements were performed on a Nicolet 6700 FTIR spectrometer (Thermo Scientific, USA). A Diamond ATR crystal and DTGS detector were used for all measurements, which were carried out in the range 4000–400 cm–1 at a resolution of 2 cm–1. A 0.1 mL amount of a dialyzed graphene QD suspension was drop-casted onto a silicon wafer and dried at room temperature. The measurement was performed directly on the material placed on the silicon wafer.
The dynamic light scattering was performed using a Zetasizer Nano ZS (Malvern, England). The measurement was performed at room temperature using disposable plastic cuvettes.

Supporting Information

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LDI-TOF MS analyses; particle size distribution analyses by DLS; HRTEM images; Raman and XPS spectra of fullerene C60; luminescence properties from another batch of graphene QDs; XPS survey and high-resolution spectra of TFA-, NBC-, and NH2OH-functionalized graphene QDs as well as hydrazine-reduced graphene QDs. This material is available free of charge via the Internet at http://pubs.acs.org.

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Author Information

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  • Corresponding Author
    • Martin Pumera - Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore;  Email: [email protected]
  • Authors
    • Chun Kiang Chua - Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
    • Zdeněk Sofer - Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
    • Petr Šimek - Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
    • Ondřej Jankovský - Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
    • Kateřina Klímová - Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
    • Snejana Bakardjieva - Centre of Instrumental Techniques, Institute of Inorganic Chemistry of the AS CR, v.v.i., Husinec-Rez c.p. 1001, 250 68 Rez, Czech Republic
    • Štěpánka Hrdličková Kučková - Department of Biochemistry and Microbiology, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
  • Notes
    The authors declare no competing financial interest.

Acknowledgment

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M.P. acknowledges a Tier 2 grant (MOE2013-T2-1-056; ARC 35/13) from the Ministry of Education, Singapore. Z.S, P.Š., O.J., and K.K. were supported by Czech Science Foundation (Project GACR No. 15-09001S) and by specific university research (MSMT No. 20/2015).

References

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    Kosynkin, Dmitry V.; Higginbotham, Amanda L.; Sinitskii, Alexander; Lomeda, Jay R.; Dimiev, Ayrat; Price, B. Katherine; Tour, James M.
    Nature (London, United Kingdom) (2009), 458 (7240), 872-876CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Graphene, or single-layered graphite, with its high crystallinity and interesting semimetal electronic properties, has emerged as an exciting 2-dimensional material showing great promise for the fabrication of nanoscale devices. Thin, elongated strips of graphene that possess straight edges, termed graphene ribbons, gradually transform from semiconductors to semimetals as their width increases, and represent a particularly versatile variety of graphene. Several lithog., chem. and synthetic procedures are known to produce microscopic samples of graphene nanoribbons, and one chem. vapor deposition process has successfully produced macroscopic quantities of nanoribbons at 950°C. Here, the authors describe a simple soln.-based oxidative process for producing a nearly 100% yield of nanoribbon structures by lengthwise cutting and unraveling of multiwalled carbon nanotube (MWCNT) side walls. Although oxidative shortening of MWCNTs has previously been achieved, lengthwise cutting is hitherto unreported. Ribbon structures with high water soly. are obtained. Subsequent chem. redn. of the nanoribbons from MWCNTs results in restoration of elec. cond. These early results affording nanoribbons could eventually lead to applications in fields of electronics and composite materials where bulk quantities of nanoribbons are required.
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  2. 2
    Luo, J.; Cote, L. J.; Tung, V. C.; Tan, A. T. L.; Goins, P. E.; Wu, J.; Huang, J. Graphene Oxide Nanocolloids J. Am. Chem. Soc. 2010, 132, 17667– 17669
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    Graphene oxide nanocolloids
    Luo, Jiayan; Cote, Laura J.; Tung, Vincent C.; Tan, Alvin T. L.; Goins, Philip E.; Wu, Jinsong; Huang, Jiaxing
    Journal of the American Chemical Society (2010), 132 (50), 17667-17669CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Graphene oxide (GO) nanocolloids-sheets with lateral dimension smaller than 100 nm-were synthesized by chem. exfoliation of graphite nanofibers, in which the graphene planes are coin-stacked along the length of the nanofibers. Since the upper size limit is predetd. by the diam. of the nanofiber precursor, the size distribution of the GO nanosheets is much more uniform than that of common GO synthesized from graphite powders. The size can be further tuned by the oxidn. time. Compared to the micrometer-sized, regular GO sheets, nano GO has very similar spectroscopic characteristics and chem. properties but very different soln. properties, such as surface activity and colloidal stability. Due to higher charge d. originating from their higher edge-to-area ratios, aq. GO nanocolloids are significantly more stable. Dispersions of GO nanocolloids can sustain high-speed centrifugation and remain stable even after chem. redn., which would result in aggregates for regular GO. Therefore, nano GO can act as a better dispersing agent for insol. materials (e.g., C nanotubes) in water, creating a more stable colloidal dispersion.
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  3. 3
    Li, L.-s.; Yan, X. Colloidal Graphene Quantum Dots J. Phys. Chem. Lett. 2010, 1, 2572– 2576
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    Colloidal Graphene Quantum Dots
    Li, Liang-shi; Yan, Xin
    Journal of Physical Chemistry Letters (2010), 1 (17), 2572-2576CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    A review. Graphene is a unique type of semiconductor with zero band gap and zero effective masses of charge carriers. Thus, in graphene quantum dots, one expects many interesting phenomena that are different from those in quantum dots of any other semiconductors. In addn., carbon is an element unique in chem. and in the society because of well-developed carbon chem. and the important roles that carbon materials have played in various technologies. In this perspective, the authors discuss recent developments 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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  4. 4
    Bacon, M.; Bradley, S. J.; Nann, T. Graphene Quantum Dots Part. Part. Syst. Charact. 2014, 31, 415– 428
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    4
    Graphene Quantum Dots
    Bacon, Mitchell; Bradley, Siobhan J.; Nann, Thomas
    Particle & Particle Systems Characterization (2014), 31 (4), 415-428CODEN: PPCHEZ; ISSN:1521-4117. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Graphene quantum dots (GQDs) are nanometer-sized fragments of graphene that show unique properties, which makes them interesting candidates for a whole range of new applications. This review article gives an overview of the synthesis, properties and applications of GQDs. Synthesis methods discussed include top-down and bottom-up approaches. Properties such as luminescence up- and down-conversion have been used in applications ranging from energy conversion to bio-analytics. This article provides an overview of the state-of-the-art and highlights promising findings as well as potential future directions of the research field.
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  5. 5
    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
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    Chaotic Dirac Billiard in Graphene Quantum Dots
    Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K.
    Science (Washington, DC, United States) (2008), 320 (5874), 356-358CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The exceptional electronic properties of graphene, with its charge carriers mimicking relativistic quantum particles and its formidable potential in various applications, have ensured a rapid growth of interest in this new material. We report on electron transport in quantum dot devices carved entirely from graphene. At large sizes (>100 nm), they behave as conventional single-electron transistors, exhibiting periodic Coulomb blockade peaks. For quantum dots smaller than 100 nm, the peaks become strongly nonperiodic, indicating a major contribution of quantum confinement. Random peak spacing and its statistics are well described by the theory of chaotic neutrino billiards. Short constrictions of only a few nanometers in width remain conductive and reveal a confinement gap of up to 0.5 eV, demonstrating the possibility of mol.-scale electronics based on graphene.
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  6. 6
    Shen, J. H.; Zhu, Y. H.; Yang, X. L.; Li, C. Z. Graphene Quantum Dots: Emergent Nanolights for Bioimaging, Sensors, Catalysis and Photovoltaic Devices Chem. Commun. 2012, 48, 3686– 3699
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    6
    Graphene quantum dots: emergent nanolights for bioimaging, sensors, catalysis and photovoltaic devices
    Shen, Jianhua; Zhu, Yihua; Yang, Xiaoling; Li, Chunzhong
    Chemical Communications (Cambridge, United Kingdom) (2012), 48 (31), 3686-3699CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A review. Similar to the popular older cousins, luminescent C dots (C-dots), graphene quantum dots or graphene quantum disks (GQDs) have generated enormous excitement because of their superiority in chem. inertness, biocompatibility and low toxicity. Besides, GQDs, consisting of a single at. layer of nano-sized graphite, have the excellent performances of graphene, such as high surface area, large diam. and better surface grafting using π-π conjugation and surface groups. Because of the structure of graphene, GQDs have some other special phys. properties. Therefore, studies on GQDs in aspects of chem., phys., materials, biol. and interdisciplinary science were in full flow in the past decade. In this Feature Article, recent developments in prepn. of GQDs are discussed, focusing on the main 2 approaches (top-down and bottom-down). Emphasis is given to their future and potential development in bioimaging, electrochem. biosensors and catalysis, and specifically in photovoltaic devices that can solve increasingly serious energy problems.
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  7. 7
    Zhu, S. J.; Tang, S. J.; Zhang, J. H.; Yang, B. Control the Size and Surface Chemistry of Graphene for the Rising Fluorescent Materials Chem. Commun. 2012, 48, 4527– 4539
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    7
    Control the size and surface chemistry of graphene for the rising fluorescent materials
    Zhu, Shoujun; Tang, Shijia; Zhang, Junhu; Yang, Bai
    Chemical Communications (Cambridge, United Kingdom) (2012), 48 (38), 4527-4539CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A review. Fluorescent graphene-based materials, labeled as a sort of fluorescent carbon-based nanomaterial, have drawn increasing attention in recent years. When the size and structure of graphene were controlled properly, photoluminescence was induced in graphene, resulting in the so-called fluorescent graphene (FG). FG has a size-, defect-, and wavelength-dependent luminescence emission, which is similar to traditional semiconductor-based quantum dots. Moreover, with excellent chem. stability, fine biocompatibility, low toxicity, up-conversion emission, pH-sensitivity, and resistance to photobleaching, FG promises to offer substantial applications in numerous areas: bioimaging, photovoltaics, sensors, etc. Currently, research works have allowed FG to be produced by many approaches ranging from simple oxidn. of graphene to cutting carbon sources and org. synthesis from small mols. The authors summarize the reported fluorescent graphenes, with emphasis on their category, properties, synthesis, and applications. A perspective on subsequent developments and a comparison of the features of FG with other fluorescent carbon-based materials are given.
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  8. 8
    Sun, H.; Wu, L.; Wei, W.; Qu, X. Recent Advances in Graphene Quantum Dots for Sensing Mater. Today 2013, 16, 433– 442
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    8
    Recent advances in graphene quantum dots for sensing
    Sun, Hanjun; Wu, Li; Wei, Weili; Qu, Xiaogang
    Materials Today (Oxford, United Kingdom) (2013), 16 (11), 433-442CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
    A review. Graphene quantum dots (GQDs) are a kind of 0D material with characteristics derived from both graphene and carbon dots (CDs). Combining the structure of graphene with the quantum confinement and edge effects of CDs, GQDs possess unique properties. In this review, we focus on the application of GQDs in electronic, photoluminescence, electrochem. and electrochemiluminescence sensor fabrication, and address the advantages of GQDs on phys. anal., chem. anal. and bioanal. We have summarized different techniques and given future perspectives for developing smart sensing based on GQDs.
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  9. 9
    Pan, D. Y.; Zhang, J. C.; Li, Z.; Wu, M. H. Hydrothermal Route for Cutting Graphene Sheets into Blue-Luminescent Graphene Quantum Dots Adv. Mater. 2010, 22, 734– 738
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    9
    Hydrothermal Route for Cutting Graphene Sheets into Blue-Luminescent Graphene Quantum Dots
    Pan, Dengyu; Zhang, Jingchun; Li, Zhen; Wu, Minghong
    Advanced Materials (Weinheim, Germany) (2010), 22 (6), 734-738CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The authors have developed a hydrothermal method for cutting preoxidized graphene sheets into ultrafine graphene quantum dots (GQDs) with strong blue emission. The cutting mechanism may involve the complete breakup of mixed epoxy chains composed of fewer epoxy groups and more carbonyl groups under hydrothermal conditions. Structural models for the GQDs in acidic and alkali media were established. The blue luminescence may originate from free zigzag sites with a carbene-like triplet ground state described as σ1π1. The study of the quantum confinement effect of the GQDs is currently under way. The discovery of the new luminescence from GQDs may expand the application of graphene-based materials to other fields such as optoelectronics and biol. labeling.
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  10. 10
    Zhu, S. J.; Zhang, J. H.; Liu, X.; Li, B.; Wang, X. F.; Tang, S. J.; Meng, Q. N.; Li, Y. F.; Shi, C.; Hu, R.etal. Graphene Quantum Dots with Controllable Surface Oxidation, Tunable Fluorescence and Up-Conversion Emission RSC Adv. 2012, 2, 2717– 2720
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    Graphene quantum dots with controllable surface oxidation, tunable fluorescence and up-conversion emission
    Zhu, Shoujun; Zhang, Junhu; Liu, Xue; Li, Bo; Wang, Xingfeng; Tang, Shijia; Meng, Qingnan; Li, Yunfeng; Shi, Ce; Hu, Rui; Yang, Bai
    RSC Advances (2012), 2 (7), 2717-2720CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    In this article, graphene quantum dots (GQDs) with tunable surface chem. (increasing oxidn. degree) were prepd. by an efficient two-step method. The GQDs have tunable fluorescence induced by the degree of surface oxidn., fine soly., high stability and applicable up-conversion photoluminescence (PL). The PL mechanism was investigated based on the surface structure and PL behaviors. More importantly, the GQDs have acid-base response property and can be applied as pH sensors.
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  11. 11
    Zhu, S. J.; Zhang, J. H.; Qiao, C. Y.; Tang, S. J.; Li, Y. F.; Yuan, W. J.; Li, B.; Tian, L.; Liu, F.; Hu, R.etal. Strongly Green-Photoluminescent Graphene Quantum Dots for Bioimaging Applications Chem. Commun. 2011, 47, 6858– 6860
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    11
    Strongly green-photoluminescent graphene quantum dots for bioimaging applications
    Zhu, Shoujun; Zhang, Junhu; Qiao, Chunyan; Tang, Shijia; Li, Yunfeng; Yuan, Wenjing; Li, Bo; Tian, Lu; Liu, Fang; Hu, Rui; Gao, Hainan; Wei, Haotong; Zhang, Hao; Sun, Hongchen; Yang, Bai
    Chemical Communications (Cambridge, United Kingdom) (2011), 47 (24), 6858-6860CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Strongly fluorescent graphene quantum dots (GQDs) have been prepd. by one-step solvothermal method with PL quantum yield as high as 11.4%. The GQDs have high stability and can be dissolved in most polar solvents. Because of fine biocompatibility and low toxicity, GQDs are demonstrated to be excellent bioimaging agents.
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  12. 12
    Ye, R.; Xiang, C.; Lin, J.; Peng, Z.; Huang, K.; Yan, Z.; Cook, N. P.; Samuel, E. L. G.; Hwang, C.-C.; Ruan, G., etal. Coal as an Abundant Source of Graphene Quantum Dots. Nat. Commun. 2013, 4, 2943.
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    12
    Coal as an abundant source of graphene quantum dots
    Ye Ruquan; Xiang Changsheng; Lin Jian; Peng Zhiwei; Huang Kewei; Yan Zheng; Cook Nathan P; Samuel Errol L G; Hwang Chih-Chau; Ruan Gedeng; Ceriotti Gabriel; Raji Abdul-Rahman O; Marti Angel A; Tour James M
    Nature communications (2013), 4 (), 2943 ISSN:.
    Coal is the most abundant and readily combustible energy resource being used worldwide. However, its structural characteristic creates a perception that coal is only useful for producing energy via burning. Here we report a facile approach to synthesize tunable graphene quantum dots from various types of coal, and establish that the unique coal structure has an advantage over pure sp2-carbon allotropes for producing quantum dots. The crystalline carbon within the coal structure is easier to oxidatively displace than when pure sp2-carbon structures are used, resulting in nanometre-sized graphene quantum dots with amorphous carbon addends on the edges. The synthesized graphene quantum dots, produced in up to 20% isolated yield from coal, are soluble and fluorescent in aqueous solution, providing promise for applications in areas such as bioimaging, biomedicine, photovoltaics and optoelectronics, in addition to being inexpensive additives for structural composites.
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  13. 13
    Dong, Y.; Lin, J.; Chen, Y.; Fu, F.; Chi, Y.; Chen, G. Graphene Quantum Dots, Graphene Oxide, Carbon Quantum Dots and Graphite Nanocrystals in Coals Nanoscale 2014, 6, 7410– 7415
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    Shang, N. G.; Papakonstantinou, P.; Sharma, S.; Lubarsky, G.; Li, M. X.; McNeill, D. W.; Quinn, A. J.; Zhou, W. Z.; Blackley, R. Controllable Selective Exfoliation of High-Quality Graphene Nanosheets and Nanodots by Ionic Liquid Assisted Grinding Chem. Commun. 2012, 48, 1877– 1879
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    14
    Controllable selective exfoliation of high-quality graphene nanosheets and nanodots by ionic liquid assisted grinding
    Shang, Nai Gui; Papakonstantinou, Pagona; Sharma, Surbhi; Lubarsky, Gennady; Li, Meixian; McNeill, David W.; Quinn, Aidan J.; Zhou, Wuzong; Blackley, Ross
    Chemical Communications (Cambridge, United Kingdom) (2012), 48 (13), 1877-1879CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Bulk quantities of graphene nanosheets and nanodots were selectively fabricated by mech. grinding exfoliation of natural graphite in a small quantity of ionic liqs. The resulting graphene sheets and dots are solvent free with low levels of naturally absorbed oxygen, inherited from the starting graphite. The sheets are only 2-5 layers thick. The graphene nanodots have diams. in the range of 9-29 nm and heights in the range of 1-16 nm, which can be controlled by changing the processing time.
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  15. 15
    Li, Y.; Hu, Y.; Zhao, Y.; Shi, G. Q.; Deng, L. E.; Hou, Y. B.; Qu, L. T. An Electrochemical Avenue to Green-Luminescent Graphene Quantum Dots as Potential Electron-Acceptors for Photovoltaics Adv. Mater. 2011, 23, 776– 780
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    An electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaics
    Li, Yan; Hu, Yue; Zhao, Yang; Shi, Gaoquan; Deng, Lier; Hou, Yanbing; Qu, Liangti
    Advanced Materials (Weinheim, Germany) (2011), 23 (6), 776-780CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The reported is an alternative electrochem. approach for direct prepn. of functional graphene quantum dots (GQDs) with an uniform size of 3-5 nm, which present a green luminescence and can be retained stably in water for several months without any changes. Polymer photovoltaic devices using GQDs as novel electron-acceptor materials are also demonstrated. Although without device optimization in this primary study, a power conversion efficiency of 1.28% was achieved.
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    Yan, X.; Cui, X.; Li, L. S. Synthesis of Large, Stable Colloidal Graphene Quantum Dots with Tunable Size J. Am. Chem. Soc. 2010, 132, 5944– 5945
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    16
    Synthesis of Large, Stable Colloidal Graphene Quantum Dots with Tunable Size
    Yan, Xin; Cui, Xiao; Li, Liang-shi
    Journal of the American Chemical Society (2010), 132 (17), 5944-5945CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We report a soln.-chem.-based approach to large, stable colloidal graphene quantum dots with uniform size and shape. The versatility of soln. chem. allows us to tune the structures of the graphenes and thus their properties.
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    Liu, R. L.; Wu, D. Q.; Feng, X. L.; Mullen, K. Bottom-Up Fabrication of Photoluminescent Graphene Quantum Dots with Uniform Morphology J. Am. Chem. Soc. 2011, 133, 15221– 15223
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    Bottom-Up Fabrication of Photoluminescent Graphene Quantum Dots with Uniform Morphology
    Liu, Ruili; Wu, Dongqing; Feng, Xinliang; Mullen, Klaus
    Journal of the American Chemical Society (2011), 133 (39), 15221-15223CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Multicolor photoluminescent graphene quantum dots (GQDs) with a uniform size of ∼60 nm diam. and 2-3 nm thickness were prepd. by using unsubstituted hexa-peri-hexabenzocoronene as the carbon source. This result offers a new strategy to fabricate monodispersed GQDs with well-defined morphol.
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    Lu, J.; Yeo, P. S. E.; Gan, C. K.; Wu, P.; Loh, K. P. Transforming C60 Molecules into Graphene Quantum Dots Nat. Nanotechnol. 2011, 6, 247– 252
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    18
    Transforming C60 molecules into graphene quantum dots
    Lu, Jiong; Yeo, Pei Shan Emmeline; Gan, Chee Kwan; Wu, Ping; Loh, Kian Ping
    Nature Nanotechnology (2011), 6 (4), 247-252CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    The fragmentation of fullerenes using ions, surface collisions or thermal effects is a complex process that typically leads to the formation of small carbon clusters of variable size. Here, we show that geometrically well-defined graphene quantum dots can be synthesized on a ruthenium surface using C60 mols. as a precursor. Scanning tunnelling microscopy imaging, supported by d. functional theory calcns., suggests that the structures are formed through the ruthenium-catalyzed cage-opening of C60. In this process, the strong C60-Ru interaction induces the formation of surface vacancies in the Ru single crystal and a subsequent embedding of C60 mols. in the surface. The fragmentation of the embedded mols. at elevated temps. then produces carbon clusters that undergo diffusion and aggregation to form graphene quantum dots. The equil. shape of the graphene can be tailored by optimizing the annealing temp. and the d. of the carbon clusters.
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    Kroto, H. W.; Heath, J. R.; Obrien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene Nature 1985, 318, 162– 163
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    C60: buckminsterfullerene
    Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smalley, R. E.
    Nature (London, United Kingdom) (1985), 318 (6042), 162-3CODEN: NATUAS; ISSN:0028-0836.
    Laser-induced vaporization of graphite produced a remarkably stable cluster, consisting of 60 C atoms. A truncated icosahedron is suggested, a polygon with 60 vertexes and 32 faces, 12 of which are pentagonal and 20 hexagonal. The C60 mol., which results when a C atom is placed at each vertex of this structure has all the valences satisfied by 2 single bonds and 1 double bond, has many resonance structures and appears to be arom.
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    Preparation of graphitic oxide
    Hummers, Wm. S., Jr.; Offeman, Richard E.
    Journal of the American Chemical Society (1958), 80 (), 1339CODEN: JACSAT; ISSN:0002-7863.
    See U.S. 2,798,878 (C.A. 51, 15080a).
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    Chua, C. K.; Sofer, Z.; Pumera, M. Graphene Sheet Orientation of Parent Material Exhibits Dramatic Influence on Graphene Properties Chem.—Asian J. 2012, 7, 2367– 2372
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    Graphene Sheet Orientation of Parent Material Exhibits Dramatic Influence on Graphene Properties
    Chua, Chun Kiang; Sofer, Zdenek; Pumera, Martin
    Chemistry - An Asian Journal (2012), 7 (10), 2367-2372CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The prodn. of graphene from various sources has garnered much attention in recent years with the development of methods that range from "bottom-up" to "top-down" approaches. The top-down approach often requires thermal treatment to obtain a few-layered and lowly oxygenated graphene sheets. Herein, we demonstrate the prodn. of graphene through oxidn. and thermal-redn./exfoliation of two sources of differently orientated graphene sheets: multiwalled carbon nanotubes (MWCNTs) and stacked graphene nanofibers (SGNFs). These two carbon-nanofiber-like materials have similar axial (length: 5-9 μm) and lateral dimensions (diam.: about 100 nm). We demonstrate that, whereas SGNFs exfoliate along the lateral plane between adjacent graphene sheets, carbon nanotubes exfoliate along its longitudinal axis and leads to opening of the carbon nanotubes owing to the built-in strain. Subsequent thermal exfoliation leads to graphene materials that have, despite the fact that their parent materials exhibited similar dimensions, dramatically different proportions and, consequently, materials properties. Graphene that was prepd. from MWCNTs exhibited dimensions of about 5000×300 nm, whereas graphene that was prepd. from SGNFs exhibited sheets with dimensions of about 50×50 nm. The d. of defects and oxygen-contg. groups on these materials are dramatically different, as are the electrochem. properties. We performed morphol., structural, and electrochem. characterization based on TEM, SEM, high-resoln. XPS, Raman spectroscopy, and cyclic voltammetry (CV) anal. on the stepwise conversion of the target source into the exfoliated graphene. Morphol. and structural characterization indicated the successful chem. and thermal treatment of the materials. Our findings have shown that the orientation of the graphene sheets in starting materials has a dramatic influence on their chem., material, and electrochem. properties.
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    A review with 115 refs. The synthesis of nanocryst. diamond films from C-contg. noble gas plasmas is described. The nanocrystallinity is the result of new growth and nucleation mechanisms, which involve the insertion of C2, C dimer, into C-C and C-H bonds, resulting in heterogeneous nucleation rates on the order 1010 cm-2 s-1. Extensive characterization studies indicated that phase-pure diamond is produced with a microstructure consisting of randomly oriented 3-15-nm crystallites. By adjusting the noble gas/H ratio in the gas mixt., a continuous transition from micro- to nanocrystallinity is achieved. Up to 10% of the total C in the nanocryst. films is located at 2 to 4 atom-wide grain boundaries. Because the grain boundary C is π-bonded, the mech., elec., and optical properties of nanocryst. diamond are profoundly altered. Nanocryst. diamond films are unique new materials with applications in fields as diverse as tribol., cold cathodes, corrosion resistance, electrochem. electrodes, and conformal coatings on MEMS devices.
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    Chua, Chun Kiang; Sofer, Zdenek; Pumera, Martin
    Chemistry - A European Journal (2012), 18 (42), 13453-13459CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Research on graphene materials has refocused on graphite oxides (GOs) in recent years. The fabrication of GO is commonly accomplished by using concd. sulfuric acid in conjunction with: a) fuming nitric acid and KClO3 oxidant (Staudenmaier); b) concd. nitric acid and KClO3 oxidant (Hofmann); c) sodium nitrate for in situ prodn. of nitric acid in the presence of KMnO4 (Hummers); or d) concd. phosphoric acid with KMnO4 (Tour). These methods have been used interchangeably in the graphene community, since the properties of GOs produced by these different methods were assumed as almost similar. In light of the wide applicability of GOs in nanotechnol. applications, in which presence of certain oxygen functional groups are specifically important, the qualities and functionalities of the GOs produced by using these four different methods, side-by-side, was investigated. The structural characterizations of the GOs would be probed by using high resoln. XPS, NMR, Fourier transform IR spectroscopy, and Raman spectroscopy. Further electrochem. applicability would be evaluated by using electrochem. impedance spectroscopy and cyclic voltammetry techniques. Our analyses highlighted that the oxidn. methods based on permanganate oxidant (Hummers and Tour methods) gave GOs with lower heterogeneous electron-transfer rates and a higher amt. of carbonyl and carboxyl functionalities compared with when using chlorate oxidant (Staudenmaier and Hofmann methods). These observations indicated large disparities between the GOs obtained from different oxidn. methods. Such insights would provide fundamental knowledge for fine tuning GO for future applications.
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    Chemistry - A European Journal (2013), 19 (6), 2005-2011CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Graphene has a wide range of potential applications, thus tremendous efforts have been put into ensuring that the most direct and effective methods for its large-scale prodn. are developed. The formation of graphene materials from graphene oxide through a chem. redn. method is still one of the most preferred routes. Numerous methods starting from various reducing agents have been developed to obtain near-pristine graphene sheets. However, most of the reducing agents are not mechanistically supported by classical org. chem. knowledge and of those that are supported, they are only theor. capable of, at most, reducing oxygen-contg. groups on graphene oxide to hydroxyl groups. The authors present a mechanistically proven method for the selective defunctionalization of hydroxyl groups from graphene oxide that is based on ethanethiol-aluminum chloride complexes and provides a graphene material with improved properties. The structural, morphol., and electrochem. properties of the graphene materials have been fully characterized based on high-resoln. XPS, Fourier transform IR spectroscopy, Raman spectroscopy, SEM, electrochem. impedance spectroscopy and cyclic voltammetry techniques. The obtained graphene materials exhibited high heterogeneous electron-transfer rates, low charge-transfer resistance, and high cond. as compared with the parent graphene oxide. Moreover, the selective defunctionalization of hydroxyl groups could potentially allow for the tailoring of graphene properties for various applications.
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    Chemically modified graphene platelets, produced via graphene oxide, show great promise in a variety of applications due to their electrical, thermal, barrier and mechanical properties. Understanding the chemical structures of chemically modified graphene platelets will aid in the understanding of their physical properties and facilitate development of chemically modified graphene platelet chemistry. Here we use (13)C and (15)N solid-state nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy to study the chemical structure of (15)N-labelled hydrazine-treated (13)C-labelled graphite oxide and unlabelled hydrazine-treated graphene oxide, respectively. These experiments suggest that hydrazine treatment of graphene oxide causes insertion of an aromatic N(2) moiety in a five-membered ring at the platelet edges and also restores graphitic networks on the basal planes. Furthermore, density-functional theory calculations support the formation of such N(2) structures at the edges and help to elucidate the influence of the aromatic N(2) moieties on the electronic structure of chemically modified graphene platelets.
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  • Abstract

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    Figure 1

    Figure 1. (A) Illustration of the oxidation and cage-opening of fullerene C60 with treatment of strong acid and chemical oxidant. (B) Luminescence of graphene QDs excited with a blue laser pointer (405 nm).

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    Figure 2

    Figure 2. (A) AFM image of graphene QDs and the corresponding height profile for the indicated particle. (B) STM image of graphene QDs and the corresponding height profile for the indicated particle.

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    Figure 3

    Figure 3. Raman spectra of graphene QDs measured with a (A) 532 nm laser at 5 W and (B) 325 nm laser at 2 mW. (C) FT-IR spectrum of graphene QDs obtained from the cage-opening of fullerene C60. (D) Survey, high-resolution (E) C 1s and (F) O 1s core-level XPS spectra of graphene QDs. High-resolution N 1s core-level spectrum is provided in Figure SI4C in the Supporting Information.

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    Figure 4

    Figure 4. (A) Luminescence of graphene QDs in dry form obtained by excitation with a 325 nm laser using various laser powers of 20 nW, 0.1 mW, and 0.2 mW. Measurement was performed using a microphotoluminescence spectrometer (see Experimental Section). (B) Dependence of graphene QD luminescence on the wavelength of excitation light. High-intensity line originated from the scattering of excitation light. The measurement was performed on dialyzed graphene QDs on a Cary Eclipse (see Experimental Section).

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    Figure 5

    Figure 5. (A) Chemical transformation of functional groups on graphene QDs upon reactions with trifluoroacetic anhydride, 4-nitrobenzoyl chloride, hydrazine hydrate, and hydroxylamine. Dependence of graphene QD luminescence on the wavelength of excitation light for graphene QDs treated with (B) hydroxylamine and (C) hydrazine hydrate. High intensity of line originated from the scattering of excitation light.

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    This article references 33 other publications.

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      A review. Graphene is a unique type of semiconductor with zero band gap and zero effective masses of charge carriers. Thus, in graphene quantum dots, one expects many interesting phenomena that are different from those in quantum dots of any other semiconductors. In addn., carbon is an element unique in chem. and in the society because of well-developed carbon chem. and the important roles that carbon materials have played in various technologies. In this perspective, the authors discuss recent developments 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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      A review. Graphene quantum dots (GQDs) are nanometer-sized fragments of graphene that show unique properties, which makes them interesting candidates for a whole range of new applications. This review article gives an overview of the synthesis, properties and applications of GQDs. Synthesis methods discussed include top-down and bottom-up approaches. Properties such as luminescence up- and down-conversion have been used in applications ranging from energy conversion to bio-analytics. This article provides an overview of the state-of-the-art and highlights promising findings as well as potential future directions of the research field.
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    5. 5
      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
      5
      Chaotic Dirac Billiard in Graphene Quantum Dots
      Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K.
      Science (Washington, DC, United States) (2008), 320 (5874), 356-358CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The exceptional electronic properties of graphene, with its charge carriers mimicking relativistic quantum particles and its formidable potential in various applications, have ensured a rapid growth of interest in this new material. We report on electron transport in quantum dot devices carved entirely from graphene. At large sizes (>100 nm), they behave as conventional single-electron transistors, exhibiting periodic Coulomb blockade peaks. For quantum dots smaller than 100 nm, the peaks become strongly nonperiodic, indicating a major contribution of quantum confinement. Random peak spacing and its statistics are well described by the theory of chaotic neutrino billiards. Short constrictions of only a few nanometers in width remain conductive and reveal a confinement gap of up to 0.5 eV, demonstrating the possibility of mol.-scale electronics based on graphene.
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    6. 6
      Shen, J. H.; Zhu, Y. H.; Yang, X. L.; Li, C. Z. Graphene Quantum Dots: Emergent Nanolights for Bioimaging, Sensors, Catalysis and Photovoltaic Devices Chem. Commun. 2012, 48, 3686– 3699
      6
      Graphene quantum dots: emergent nanolights for bioimaging, sensors, catalysis and photovoltaic devices
      Shen, Jianhua; Zhu, Yihua; Yang, Xiaoling; Li, Chunzhong
      Chemical Communications (Cambridge, United Kingdom) (2012), 48 (31), 3686-3699CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A review. Similar to the popular older cousins, luminescent C dots (C-dots), graphene quantum dots or graphene quantum disks (GQDs) have generated enormous excitement because of their superiority in chem. inertness, biocompatibility and low toxicity. Besides, GQDs, consisting of a single at. layer of nano-sized graphite, have the excellent performances of graphene, such as high surface area, large diam. and better surface grafting using π-π conjugation and surface groups. Because of the structure of graphene, GQDs have some other special phys. properties. Therefore, studies on GQDs in aspects of chem., phys., materials, biol. and interdisciplinary science were in full flow in the past decade. In this Feature Article, recent developments in prepn. of GQDs are discussed, focusing on the main 2 approaches (top-down and bottom-down). Emphasis is given to their future and potential development in bioimaging, electrochem. biosensors and catalysis, and specifically in photovoltaic devices that can solve increasingly serious energy problems.
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    7. 7
      Zhu, S. J.; Tang, S. J.; Zhang, J. H.; Yang, B. Control the Size and Surface Chemistry of Graphene for the Rising Fluorescent Materials Chem. Commun. 2012, 48, 4527– 4539
      7
      Control the size and surface chemistry of graphene for the rising fluorescent materials
      Zhu, Shoujun; Tang, Shijia; Zhang, Junhu; Yang, Bai
      Chemical Communications (Cambridge, United Kingdom) (2012), 48 (38), 4527-4539CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A review. Fluorescent graphene-based materials, labeled as a sort of fluorescent carbon-based nanomaterial, have drawn increasing attention in recent years. When the size and structure of graphene were controlled properly, photoluminescence was induced in graphene, resulting in the so-called fluorescent graphene (FG). FG has a size-, defect-, and wavelength-dependent luminescence emission, which is similar to traditional semiconductor-based quantum dots. Moreover, with excellent chem. stability, fine biocompatibility, low toxicity, up-conversion emission, pH-sensitivity, and resistance to photobleaching, FG promises to offer substantial applications in numerous areas: bioimaging, photovoltaics, sensors, etc. Currently, research works have allowed FG to be produced by many approaches ranging from simple oxidn. of graphene to cutting carbon sources and org. synthesis from small mols. The authors summarize the reported fluorescent graphenes, with emphasis on their category, properties, synthesis, and applications. A perspective on subsequent developments and a comparison of the features of FG with other fluorescent carbon-based materials are given.
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    8. 8
      Sun, H.; Wu, L.; Wei, W.; Qu, X. Recent Advances in Graphene Quantum Dots for Sensing Mater. Today 2013, 16, 433– 442
      8
      Recent advances in graphene quantum dots for sensing
      Sun, Hanjun; Wu, Li; Wei, Weili; Qu, Xiaogang
      Materials Today (Oxford, United Kingdom) (2013), 16 (11), 433-442CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
      A review. Graphene quantum dots (GQDs) are a kind of 0D material with characteristics derived from both graphene and carbon dots (CDs). Combining the structure of graphene with the quantum confinement and edge effects of CDs, GQDs possess unique properties. In this review, we focus on the application of GQDs in electronic, photoluminescence, electrochem. and electrochemiluminescence sensor fabrication, and address the advantages of GQDs on phys. anal., chem. anal. and bioanal. We have summarized different techniques and given future perspectives for developing smart sensing based on GQDs.
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    9. 9
      Pan, D. Y.; Zhang, J. C.; Li, Z.; Wu, M. H. Hydrothermal Route for Cutting Graphene Sheets into Blue-Luminescent Graphene Quantum Dots Adv. Mater. 2010, 22, 734– 738
      9
      Hydrothermal Route for Cutting Graphene Sheets into Blue-Luminescent Graphene Quantum Dots
      Pan, Dengyu; Zhang, Jingchun; Li, Zhen; Wu, Minghong
      Advanced Materials (Weinheim, Germany) (2010), 22 (6), 734-738CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors have developed a hydrothermal method for cutting preoxidized graphene sheets into ultrafine graphene quantum dots (GQDs) with strong blue emission. The cutting mechanism may involve the complete breakup of mixed epoxy chains composed of fewer epoxy groups and more carbonyl groups under hydrothermal conditions. Structural models for the GQDs in acidic and alkali media were established. The blue luminescence may originate from free zigzag sites with a carbene-like triplet ground state described as σ1π1. The study of the quantum confinement effect of the GQDs is currently under way. The discovery of the new luminescence from GQDs may expand the application of graphene-based materials to other fields such as optoelectronics and biol. labeling.
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    10. 10
      Zhu, S. J.; Zhang, J. H.; Liu, X.; Li, B.; Wang, X. F.; Tang, S. J.; Meng, Q. N.; Li, Y. F.; Shi, C.; Hu, R.etal. Graphene Quantum Dots with Controllable Surface Oxidation, Tunable Fluorescence and Up-Conversion Emission RSC Adv. 2012, 2, 2717– 2720
      10
      Graphene quantum dots with controllable surface oxidation, tunable fluorescence and up-conversion emission
      Zhu, Shoujun; Zhang, Junhu; Liu, Xue; Li, Bo; Wang, Xingfeng; Tang, Shijia; Meng, Qingnan; Li, Yunfeng; Shi, Ce; Hu, Rui; Yang, Bai
      RSC Advances (2012), 2 (7), 2717-2720CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      In this article, graphene quantum dots (GQDs) with tunable surface chem. (increasing oxidn. degree) were prepd. by an efficient two-step method. The GQDs have tunable fluorescence induced by the degree of surface oxidn., fine soly., high stability and applicable up-conversion photoluminescence (PL). The PL mechanism was investigated based on the surface structure and PL behaviors. More importantly, the GQDs have acid-base response property and can be applied as pH sensors.
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    11. 11
      Zhu, S. J.; Zhang, J. H.; Qiao, C. Y.; Tang, S. J.; Li, Y. F.; Yuan, W. J.; Li, B.; Tian, L.; Liu, F.; Hu, R.etal. Strongly Green-Photoluminescent Graphene Quantum Dots for Bioimaging Applications Chem. Commun. 2011, 47, 6858– 6860
      11
      Strongly green-photoluminescent graphene quantum dots for bioimaging applications
      Zhu, Shoujun; Zhang, Junhu; Qiao, Chunyan; Tang, Shijia; Li, Yunfeng; Yuan, Wenjing; Li, Bo; Tian, Lu; Liu, Fang; Hu, Rui; Gao, Hainan; Wei, Haotong; Zhang, Hao; Sun, Hongchen; Yang, Bai
      Chemical Communications (Cambridge, United Kingdom) (2011), 47 (24), 6858-6860CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Strongly fluorescent graphene quantum dots (GQDs) have been prepd. by one-step solvothermal method with PL quantum yield as high as 11.4%. The GQDs have high stability and can be dissolved in most polar solvents. Because of fine biocompatibility and low toxicity, GQDs are demonstrated to be excellent bioimaging agents.
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    12. 12
      Ye, R.; Xiang, C.; Lin, J.; Peng, Z.; Huang, K.; Yan, Z.; Cook, N. P.; Samuel, E. L. G.; Hwang, C.-C.; Ruan, G., etal. Coal as an Abundant Source of Graphene Quantum Dots. Nat. Commun. 2013, 4, 2943.
      12
      Coal as an abundant source of graphene quantum dots
      Ye Ruquan; Xiang Changsheng; Lin Jian; Peng Zhiwei; Huang Kewei; Yan Zheng; Cook Nathan P; Samuel Errol L G; Hwang Chih-Chau; Ruan Gedeng; Ceriotti Gabriel; Raji Abdul-Rahman O; Marti Angel A; Tour James M
      Nature communications (2013), 4 (), 2943 ISSN:.
      Coal is the most abundant and readily combustible energy resource being used worldwide. However, its structural characteristic creates a perception that coal is only useful for producing energy via burning. Here we report a facile approach to synthesize tunable graphene quantum dots from various types of coal, and establish that the unique coal structure has an advantage over pure sp2-carbon allotropes for producing quantum dots. The crystalline carbon within the coal structure is easier to oxidatively displace than when pure sp2-carbon structures are used, resulting in nanometre-sized graphene quantum dots with amorphous carbon addends on the edges. The synthesized graphene quantum dots, produced in up to 20% isolated yield from coal, are soluble and fluorescent in aqueous solution, providing promise for applications in areas such as bioimaging, biomedicine, photovoltaics and optoelectronics, in addition to being inexpensive additives for structural composites.
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    13. 13
      Dong, Y.; Lin, J.; Chen, Y.; Fu, F.; Chi, Y.; Chen, G. Graphene Quantum Dots, Graphene Oxide, Carbon Quantum Dots and Graphite Nanocrystals in Coals Nanoscale 2014, 6, 7410– 7415
      There is no corresponding record for this reference.
    14. 14
      Shang, N. G.; Papakonstantinou, P.; Sharma, S.; Lubarsky, G.; Li, M. X.; McNeill, D. W.; Quinn, A. J.; Zhou, W. Z.; Blackley, R. Controllable Selective Exfoliation of High-Quality Graphene Nanosheets and Nanodots by Ionic Liquid Assisted Grinding Chem. Commun. 2012, 48, 1877– 1879
      14
      Controllable selective exfoliation of high-quality graphene nanosheets and nanodots by ionic liquid assisted grinding
      Shang, Nai Gui; Papakonstantinou, Pagona; Sharma, Surbhi; Lubarsky, Gennady; Li, Meixian; McNeill, David W.; Quinn, Aidan J.; Zhou, Wuzong; Blackley, Ross
      Chemical Communications (Cambridge, United Kingdom) (2012), 48 (13), 1877-1879CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Bulk quantities of graphene nanosheets and nanodots were selectively fabricated by mech. grinding exfoliation of natural graphite in a small quantity of ionic liqs. The resulting graphene sheets and dots are solvent free with low levels of naturally absorbed oxygen, inherited from the starting graphite. The sheets are only 2-5 layers thick. The graphene nanodots have diams. in the range of 9-29 nm and heights in the range of 1-16 nm, which can be controlled by changing the processing time.
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    15. 15
      Li, Y.; Hu, Y.; Zhao, Y.; Shi, G. Q.; Deng, L. E.; Hou, Y. B.; Qu, L. T. An Electrochemical Avenue to Green-Luminescent Graphene Quantum Dots as Potential Electron-Acceptors for Photovoltaics Adv. Mater. 2011, 23, 776– 780
      15
      An electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaics
      Li, Yan; Hu, Yue; Zhao, Yang; Shi, Gaoquan; Deng, Lier; Hou, Yanbing; Qu, Liangti
      Advanced Materials (Weinheim, Germany) (2011), 23 (6), 776-780CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The reported is an alternative electrochem. approach for direct prepn. of functional graphene quantum dots (GQDs) with an uniform size of 3-5 nm, which present a green luminescence and can be retained stably in water for several months without any changes. Polymer photovoltaic devices using GQDs as novel electron-acceptor materials are also demonstrated. Although without device optimization in this primary study, a power conversion efficiency of 1.28% was achieved.
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    16. 16
      Yan, X.; Cui, X.; Li, L. S. Synthesis of Large, Stable Colloidal Graphene Quantum Dots with Tunable Size J. Am. Chem. Soc. 2010, 132, 5944– 5945
      16
      Synthesis of Large, Stable Colloidal Graphene Quantum Dots with Tunable Size
      Yan, Xin; Cui, Xiao; Li, Liang-shi
      Journal of the American Chemical Society (2010), 132 (17), 5944-5945CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We report a soln.-chem.-based approach to large, stable colloidal graphene quantum dots with uniform size and shape. The versatility of soln. chem. allows us to tune the structures of the graphenes and thus their properties.
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    17. 17
      Liu, R. L.; Wu, D. Q.; Feng, X. L.; Mullen, K. Bottom-Up Fabrication of Photoluminescent Graphene Quantum Dots with Uniform Morphology J. Am. Chem. Soc. 2011, 133, 15221– 15223
      17
      Bottom-Up Fabrication of Photoluminescent Graphene Quantum Dots with Uniform Morphology
      Liu, Ruili; Wu, Dongqing; Feng, Xinliang; Mullen, Klaus
      Journal of the American Chemical Society (2011), 133 (39), 15221-15223CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Multicolor photoluminescent graphene quantum dots (GQDs) with a uniform size of ∼60 nm diam. and 2-3 nm thickness were prepd. by using unsubstituted hexa-peri-hexabenzocoronene as the carbon source. This result offers a new strategy to fabricate monodispersed GQDs with well-defined morphol.
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    18. 18
      Lu, J.; Yeo, P. S. E.; Gan, C. K.; Wu, P.; Loh, K. P. Transforming C60 Molecules into Graphene Quantum Dots Nat. Nanotechnol. 2011, 6, 247– 252
      18
      Transforming C60 molecules into graphene quantum dots
      Lu, Jiong; Yeo, Pei Shan Emmeline; Gan, Chee Kwan; Wu, Ping; Loh, Kian Ping
      Nature Nanotechnology (2011), 6 (4), 247-252CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      The fragmentation of fullerenes using ions, surface collisions or thermal effects is a complex process that typically leads to the formation of small carbon clusters of variable size. Here, we show that geometrically well-defined graphene quantum dots can be synthesized on a ruthenium surface using C60 mols. as a precursor. Scanning tunnelling microscopy imaging, supported by d. functional theory calcns., suggests that the structures are formed through the ruthenium-catalyzed cage-opening of C60. In this process, the strong C60-Ru interaction induces the formation of surface vacancies in the Ru single crystal and a subsequent embedding of C60 mols. in the surface. The fragmentation of the embedded mols. at elevated temps. then produces carbon clusters that undergo diffusion and aggregation to form graphene quantum dots. The equil. shape of the graphene can be tailored by optimizing the annealing temp. and the d. of the carbon clusters.
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    19. 19
      Kroto, H. W.; Heath, J. R.; Obrien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene Nature 1985, 318, 162– 163
      19
      C60: buckminsterfullerene
      Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smalley, R. E.
      Nature (London, United Kingdom) (1985), 318 (6042), 162-3CODEN: NATUAS; ISSN:0028-0836.
      Laser-induced vaporization of graphite produced a remarkably stable cluster, consisting of 60 C atoms. A truncated icosahedron is suggested, a polygon with 60 vertexes and 32 faces, 12 of which are pentagonal and 20 hexagonal. The C60 mol., which results when a C atom is placed at each vertex of this structure has all the valences satisfied by 2 single bonds and 1 double bond, has many resonance structures and appears to be arom.
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    20. 20
      Hirsch, A.; Brettreich, M.; Wudl, F. Fullerenes: Chemistry and Reactions; Wiley, 2006.
      There is no corresponding record for this reference.
    21. 21
      Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide J. Am. Chem. Soc. 1958, 80, 1339– 1339
      21
      Preparation of graphitic oxide
      Hummers, Wm. S., Jr.; Offeman, Richard E.
      Journal of the American Chemical Society (1958), 80 (), 1339CODEN: JACSAT; ISSN:0002-7863.
      See U.S. 2,798,878 (C.A. 51, 15080a).
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    22. 22
      Chua, C. K.; Sofer, Z.; Pumera, M. Graphene Sheet Orientation of Parent Material Exhibits Dramatic Influence on Graphene Properties Chem.—Asian J. 2012, 7, 2367– 2372
      22
      Graphene Sheet Orientation of Parent Material Exhibits Dramatic Influence on Graphene Properties
      Chua, Chun Kiang; Sofer, Zdenek; Pumera, Martin
      Chemistry - An Asian Journal (2012), 7 (10), 2367-2372CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The prodn. of graphene from various sources has garnered much attention in recent years with the development of methods that range from "bottom-up" to "top-down" approaches. The top-down approach often requires thermal treatment to obtain a few-layered and lowly oxygenated graphene sheets. Herein, we demonstrate the prodn. of graphene through oxidn. and thermal-redn./exfoliation of two sources of differently orientated graphene sheets: multiwalled carbon nanotubes (MWCNTs) and stacked graphene nanofibers (SGNFs). These two carbon-nanofiber-like materials have similar axial (length: 5-9 μm) and lateral dimensions (diam.: about 100 nm). We demonstrate that, whereas SGNFs exfoliate along the lateral plane between adjacent graphene sheets, carbon nanotubes exfoliate along its longitudinal axis and leads to opening of the carbon nanotubes owing to the built-in strain. Subsequent thermal exfoliation leads to graphene materials that have, despite the fact that their parent materials exhibited similar dimensions, dramatically different proportions and, consequently, materials properties. Graphene that was prepd. from MWCNTs exhibited dimensions of about 5000×300 nm, whereas graphene that was prepd. from SGNFs exhibited sheets with dimensions of about 50×50 nm. The d. of defects and oxygen-contg. groups on these materials are dramatically different, as are the electrochem. properties. We performed morphol., structural, and electrochem. characterization based on TEM, SEM, high-resoln. XPS, Raman spectroscopy, and cyclic voltammetry (CV) anal. on the stepwise conversion of the target source into the exfoliated graphene. Morphol. and structural characterization indicated the successful chem. and thermal treatment of the materials. Our findings have shown that the orientation of the graphene sheets in starting materials has a dramatic influence on their chem., material, and electrochem. properties.
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    23. 23
      Mcelvany, S. W.; Ross, M. M.; Callahan, J. H. Characterization of Fullerenes by Mass-Spectrometry Acc. Chem. Res. 1992, 25, 162– 168
      There is no corresponding record for this reference.
    24. 24
      Jankovsky, O.; Hrdlickova Kuckova, S.; Pumera, M.; Simek, P.; Sedmidubsky, D.; Sofer, Z. Carbon Fragments are Ripped Off from Graphite Oxide Sheets during Their Thermal Reduction New J. Chem. 2014, 38, 5700– 5705
      There is no corresponding record for this reference.
    25. 25
      Schniepp, H. C.; Li, J.-L.; McAllister, M. J.; Sai, H.; Herrera-Alonso, M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. Functionalized Single Graphene Sheets Derived from Splitting Graphite Oxide J. Phys. Chem. B 2006, 110, 8535– 8539
      25
      Functionalized Single Graphene Sheets Derived from Splitting Graphite Oxide
      Schniepp, Hannes C.; Li, Je-Luen; McAllister, Michael J.; Sai, Hiroaki; Herrera-Alonso, Margarita; Adamson, Douglas H.; Prud'homme, Robert K.; Car, Roberto; Saville, Dudley A.; Aksay, Ilhan A.
      Journal of Physical Chemistry B (2006), 110 (17), 8535-8539CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      A process is described to produce single sheets of functionalized graphene through thermal exfoliation of graphite oxide. The process yields a wrinkled sheet structure resulting from reaction sites involved in oxidn. and redn. processes. The topol. features of single sheets, as measured by at. force microscopy, closely match predictions of first-principles atomistic modeling. Although graphite oxide is an insulator, functionalized graphene produced by this method is elec. conducting.
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    26. 26
      Ravindran, T. R.; Jackson, B. R.; Badding, J. V.; Raman, U. V. Spectroscopy of Single-Walled Carbon Nanotubes Chem. Mater. 2001, 13, 4187– 4191
      There is no corresponding record for this reference.
    27. 27
      Gruen, D. M. Nanocrystalline Diamond Films Annu. Rev. Mater. Sci. 1999, 29, 211– 259
      27
      Nanocrystalline diamond films
      Gruen, Dieter M.
      Annual Review of Materials Science (1999), 29 (), 211-259CODEN: ARMSCX; ISSN:0084-6600. (Annual Reviews Inc.)
      A review with 115 refs. The synthesis of nanocryst. diamond films from C-contg. noble gas plasmas is described. The nanocrystallinity is the result of new growth and nucleation mechanisms, which involve the insertion of C2, C dimer, into C-C and C-H bonds, resulting in heterogeneous nucleation rates on the order 1010 cm-2 s-1. Extensive characterization studies indicated that phase-pure diamond is produced with a microstructure consisting of randomly oriented 3-15-nm crystallites. By adjusting the noble gas/H ratio in the gas mixt., a continuous transition from micro- to nanocrystallinity is achieved. Up to 10% of the total C in the nanocryst. films is located at 2 to 4 atom-wide grain boundaries. Because the grain boundary C is π-bonded, the mech., elec., and optical properties of nanocryst. diamond are profoundly altered. Nanocryst. diamond films are unique new materials with applications in fields as diverse as tribol., cold cathodes, corrosion resistance, electrochem. electrodes, and conformal coatings on MEMS devices.
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    28. 28
      Ferrari, A. C. Raman Spectroscopy of Graphene and Graphite: Disorder, Electron-Phonon Coupling, Doping and Nonadiabatic Effects Solid State Commun. 2007, 143, 47– 57
      28
      Raman spectroscopy of graphene and graphite: Disorder, electron-phonon coupling, doping and nonadiabatic effects
      Ferrari, Andrea C.
      Solid State Communications (2007), 143 (1-2), 47-57CODEN: SSCOA4; ISSN:0038-1098. (Elsevier Ltd.)
      The authors review recent work on Raman spectroscopy of graphite and graphene. The authors focus on the origin of the D and G peaks and the 2nd order of the D peak. The G and 2 D Raman peaks change in shape, position and relative intensity with no. of graphene layers. This reflects the evolution of the electronic structure and electron-phonon interactions. The authors then consider the effects of doping on the Raman spectra of graphene. The Fermi energy is tuned by applying a gate-voltage. This induces a stiffening of the Raman G peak for both holes and electrons doping. Thus Raman spectroscopy can be efficiently used to monitor no. of layers, quality of layers, doping level and confinement.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXmt1yhtr0%253D&md5=b67986a7f5f92c4a5ab64950f3330d7a
    29. 29
      Chua, C. K.; Sofer, Z.; Pumera, M. Graphite Oxides: Effects of Permanganate and Chlorate Oxidants on the Oxygen Composition Chem.—Eur. J. 2012, 18, 13453– 13459
      29
      Graphite Oxides: Effects of Permanganate and Chlorate Oxidants on the Oxygen Composition
      Chua, Chun Kiang; Sofer, Zdenek; Pumera, Martin
      Chemistry - A European Journal (2012), 18 (42), 13453-13459CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Research on graphene materials has refocused on graphite oxides (GOs) in recent years. The fabrication of GO is commonly accomplished by using concd. sulfuric acid in conjunction with: a) fuming nitric acid and KClO3 oxidant (Staudenmaier); b) concd. nitric acid and KClO3 oxidant (Hofmann); c) sodium nitrate for in situ prodn. of nitric acid in the presence of KMnO4 (Hummers); or d) concd. phosphoric acid with KMnO4 (Tour). These methods have been used interchangeably in the graphene community, since the properties of GOs produced by these different methods were assumed as almost similar. In light of the wide applicability of GOs in nanotechnol. applications, in which presence of certain oxygen functional groups are specifically important, the qualities and functionalities of the GOs produced by using these four different methods, side-by-side, was investigated. The structural characterizations of the GOs would be probed by using high resoln. XPS, NMR, Fourier transform IR spectroscopy, and Raman spectroscopy. Further electrochem. applicability would be evaluated by using electrochem. impedance spectroscopy and cyclic voltammetry techniques. Our analyses highlighted that the oxidn. methods based on permanganate oxidant (Hummers and Tour methods) gave GOs with lower heterogeneous electron-transfer rates and a higher amt. of carbonyl and carboxyl functionalities compared with when using chlorate oxidant (Staudenmaier and Hofmann methods). These observations indicated large disparities between the GOs obtained from different oxidn. methods. Such insights would provide fundamental knowledge for fine tuning GO for future applications.
      >> More from SciFinder ®
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    30. 30
      Chua, C. K.; Pumera, M. Selective Removal of Hydroxyl Groups from Graphene Oxide Chem.—Eur. J. 2013, 19, 2005– 2011
      30
      Selective Removal of Hydroxyl Groups from Graphene Oxide
      Chua, Chun Kiang; Pumera, Martin
      Chemistry - A European Journal (2013), 19 (6), 2005-2011CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Graphene has a wide range of potential applications, thus tremendous efforts have been put into ensuring that the most direct and effective methods for its large-scale prodn. are developed. The formation of graphene materials from graphene oxide through a chem. redn. method is still one of the most preferred routes. Numerous methods starting from various reducing agents have been developed to obtain near-pristine graphene sheets. However, most of the reducing agents are not mechanistically supported by classical org. chem. knowledge and of those that are supported, they are only theor. capable of, at most, reducing oxygen-contg. groups on graphene oxide to hydroxyl groups. The authors present a mechanistically proven method for the selective defunctionalization of hydroxyl groups from graphene oxide that is based on ethanethiol-aluminum chloride complexes and provides a graphene material with improved properties. The structural, morphol., and electrochem. properties of the graphene materials have been fully characterized based on high-resoln. XPS, Fourier transform IR spectroscopy, Raman spectroscopy, SEM, electrochem. impedance spectroscopy and cyclic voltammetry techniques. The obtained graphene materials exhibited high heterogeneous electron-transfer rates, low charge-transfer resistance, and high cond. as compared with the parent graphene oxide. Moreover, the selective defunctionalization of hydroxyl groups could potentially allow for the tailoring of graphene properties for various applications.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtFKmsrs%253D&md5=3f62c8e9b18b6d15577c0aaa17666f40
    31. 31
      Park, S.; Hu, Y.; Hwang, J. O.; Lee, E.-S.; Casabianca, L. B.; Cai, W.; Potts, J. R.; Ha, H.-W.; Chen, S.; Oh, J.etal. Chemical Structures of Hydrazine-Treated Graphene Oxide and Generation of Aromatic Nitrogen Doping Nat. Commun. 2012, 3, 638
      31
      Chemical structures of hydrazine-treated graphene oxide and generation of aromatic nitrogen doping
      Park Sungjin; Hu Yichen; Hwang Jin Ok; Lee Eui-Sup; Casabianca Leah B; Cai Weiwei; Potts Jeffrey R; Ha Hyung-Wook; Chen Shanshan; Oh Junghoon; Kim Sang Ouk; Kim Yong-Hyun; Ishii Yoshitaka; Ruoff Rodney S
      Nature communications (2012), 3 (), 638 ISSN:.
      Chemically modified graphene platelets, produced via graphene oxide, show great promise in a variety of applications due to their electrical, thermal, barrier and mechanical properties. Understanding the chemical structures of chemically modified graphene platelets will aid in the understanding of their physical properties and facilitate development of chemically modified graphene platelet chemistry. Here we use (13)C and (15)N solid-state nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy to study the chemical structure of (15)N-labelled hydrazine-treated (13)C-labelled graphite oxide and unlabelled hydrazine-treated graphene oxide, respectively. These experiments suggest that hydrazine treatment of graphene oxide causes insertion of an aromatic N(2) moiety in a five-membered ring at the platelet edges and also restores graphitic networks on the basal planes. Furthermore, density-functional theory calculations support the formation of such N(2) structures at the edges and help to elucidate the influence of the aromatic N(2) moieties on the electronic structure of chemically modified graphene platelets.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC387otVemug%253D%253D&md5=0538cb797f9e8fcea2034952458aab4f
    32. 32
      Kuckova, S.; Hynek, R.; Nemec, I.; Kodicek, M.; Jehlicka, J. Critical Comparison of Spectrometric Analyses of Non-Mineral Blue Dyes and Pigments Used in Artworks Surf. Interface Anal. 2012, 44, 963– 967
      32
      Critical comparison of spectrometric analyses of non-mineral blue dyes and pigments used in artworks
      Kuckova, S.; Hynek, R.; Nemec, I.; Kodicek, M.; Jehlicka, J.
      Surface and Interface Analysis (2012), 44 (8), 963-967CODEN: SIANDQ; ISSN:0142-2421. (John Wiley & Sons Ltd.)
      The aim of this work was to find the lowest concns. of non-mineral blue dyes and pigments using IR spectroscopy with Fourier transformation, Raman spectroscopy and mass spectrometry based on laser desorption/ionisation-time of flight, in fresh and artificially aged model color layers, for which the detection of indigo, Prussian blue and copper phthalocyanine in differently prepd. samples could be reliably performed. This research was motivated by the fact that the presence of a particular blue dye allows at least approx. dating of artworks and therefore may be conducive to detg. their authenticity, as is shown on a Czech cubist painting in this paper. Copyright © 2012 John Wiley & Sons, Ltd.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVajtrk%253D&md5=00a3bc517c18df92bed2f411a639649c
    33. 33
      Strohalm, M.; Hassman, M.; Kosata, B.; Kodicek, M. Mmass Data Miner: An Open Source Alternative for Mass Spectrometric Data Analysis Rapid Commun. Mass Spectrom. 2008, 22, 905– 908
      33
      mMass data miner: an open source alternative for mass spectrometric data analysis
      Strohalm Martin; Hassman Martin; Kosata Bedrich; Kodicek Milan
      Rapid communications in mass spectrometry : RCM (2008), 22 (6), 905-8 ISSN:0951-4198.
      There is no expanded citation for this reference.
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    LDI-TOF MS analyses; particle size distribution analyses by DLS; HRTEM images; Raman and XPS spectra of fullerene C60; luminescence properties from another batch of graphene QDs; XPS survey and high-resolution spectra of TFA-, NBC-, and NH2OH-functionalized graphene QDs as well as hydrazine-reduced graphene QDs. This material is available free of charge via the Internet at http://pubs.acs.org.


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