Synthesis, characterization and non-linear optical response of organophilic carbon dots

CARBON

x x x ( 2 0 1 3 ) x x x –x x x

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Letter to the Editor

Synthesis, characterization and non-linear optical response of organophilic carbon dots Athanasios B. Bourlinos a,f, Michael A. Karakassides b, Antonios Kouloumpis b, Dimitrios Gournis b, Aristides Bakandritsos c, Irene Papagiannouli d,e, Panagiotis Aloukos d,e, Stelios Couris d,e, Katerina Hola f, Radek Zboril f,*, Marta Krysmann g, Emmanuel P. Giannelis g,h,* a

Physics Department, University of Ioannina, Ioannina 45110, Greece Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece c Department of Materials Science, University of Patras, Rio 26504, Patra, Greece d Department of Physics, University of Patras, Rio 26504, Patra, Greece e Institute of Chemical Engineering Sciences (ICE-HT), Foundation for Research and Technology-Hellas (FORTH), Rio 26504, Patra, Greece f Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacky University, Olomouc 77146, Czech Republic g Materials Science and Engineering, Cornell University, Ithaca 14853, NY, USA h Center for Refining & Petrochemicals, KFUPM, Dhahran 31261, Saudi Arabia b

A R T I C L E I N F O

A B S T R A C T

Article history:

For the first time ever we report the nonlinear optical (NLO) properties of carbon dots (C-

Received 8 February 2013

dots). The C-dots for these experiments were synthesized by mild pyrolysis of lauryl gal-

Accepted 10 May 2013

late. The resulting C-dots bear lauryl chains and, hence, are highly dispersible in polar

Available online xxxx

organic solvents, like chloroform. Dispersions in CHCl3 show significant NLO response. Specifically, the C-dots show negative nonlinear absorption coefficient and negative nonlinear refraction. Using suspensions with different concentrations these parameters are quantified and compared to those of fullerene a well-known carbon molecule with proven NLO response.  2013 Elsevier Ltd. All rights reserved.

Carbon dots (C-dots) are a new and intriguing class of luminescent nanoparticles with properties complementary to the traditional metal-containing quantum dots [1]. The combination of multicolor and tunable emission, controlled surface chemistry, low toxicity, and solvent dispersability in one simple platform have made C-dots attractive for a range of optical, sensing and biomedical applications [1]. Research efforts have focused on ‘‘top down’’ or ‘‘bottom up’’ synthetic approaches. The latter involves mild pyrolysis of molecular

precursors [2,3]. As such it is straightforward, employs simple and inexpensive precursors, and usually proceeds in a single step providing surface-functionalized nanoparticles dispersible in aqueous or organic solvents depending on their surface functionality. While the photoluminescent and electroluminescent properties of C-dots have been thoroughly investigated no reports exist on their non linear optical (NLO) properties. This is somewhat puzzling since fullerenes, carbon nanotubes and carbon nanoparticles are all known to

* Corresponding authors. E-mail addresses: [email protected] (R. Zboril), [email protected] (E.P. Giannelis). 0008-6223/$ - see front matter  2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.05.017

Please cite this article in press as: Bourlinos AB et al. Synthesis, characterization and non-linear optical response of organophilic carbon dots. Carbon (2013), http://dx.doi.org/10.1016/j.carbon.2013.05.017

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be NLO active [4–8]. We report here for the first time the NLO behavior of C-dots synthesized from lauryl gallate. The gallate unit provides the carbon source while the covalently attached lauryl group the surface modifier of the C-dots. For the synthesis of these organophilic dots LG27 (Fig. 1), 1 g lauryl gallate powder (Aldrich) was loaded on a porcelain crucible and heated at 270 C for 2 h in air. The grey–black solid was extracted by 15 mL CHCl3 to produce a deep brown colloidal dispersion. The colloid was filtered off and then evaporated until completely dry. The solid residue was rinsed with 15 mL hexane and re-dried. Then it was re-dissolved in 15 mL CHCl3 and filtered off to give a clear dispersion. Other good solvents for LG27 include acetone, tetrahydrofuran and toluene. Elemental analysis for LG27 (% w/w): C, 63.50; H, 8.11; O, 28.39. LG27 shows G and D Raman bands typical of amorphous carbon (Fig. S1). The relative intensity of D/G ratio that represents the degree of disorder is ca. 0.5. Water-processable dots were obtained by dispersing LG27 in a water–acetone mixture (10:1 v/v) followed by filtration and subsequent stabilization with 2–3 drops of an aqueous solution of cetyltrimethylammonium chloride (25%). Stable suspensions can be prepared by first dissolving in acetone followed by dilution with the appropriate amount of water. The water–acetone dispersion is left to rest for 1 day prior to filtration and surfactant stabilization. This process results in a clear yet concentrated dispersion of C-dots in excess water that is remarkably stable for a long period of time (f = +44 mV). LG27 dots appear nearly spherical in Transmission Electron Microscopy (TEM) with sizes ranging between 3 and 15 nm (Fig. 2). A panoramic TEM view of a larger nanoparticle area is given in Fig. S2. That lauryl chains are present on the surface was confirmed by infrared spectroscopy (IR) and proton Nuclear Magnetic Resonance in CDCl3 (1H NMR). The IR spectrum of LG27 is clearly different from that of lauryl gallate precursor as a result of carbonization (Fig. S3). Nonetheless, the dots still exhibit characteristic absorptions below 3000 cm1 due to the –CH3 and –CH2– groups present in the lauryl modifier, as well as, the ester bond at 1680 cm1 (the ester links the lauryl chain to the carbon surface). In addition, the 1H NMR spectrum in CDCl3 shows sharp peaks at chemical shift <2 ppm also due to the lauryl modifier (Fig. S4). The optical spectra of LG27 in CHCl3 (Fig. 3) are typical of carbon dots, displaying no distinct absorption below 300 nm and continuous wavelength-depended emission in the visible range [1–3]. The corresponding excitation spectra are provided in Fig. S5. In general, emission is most intense in the blue and green regions (spectra not normalized). A visual demonstration of the luminescent properties of the dispersed dots in these regions is given as insets in Fig. 3. On the other hand,

Fig. 1 – Synthesis scheme of the LG27 organophilic dots.

Fig. 2 – TEM images of the LG27 carbon dots. Particle sizes range between 3 and 15 nm. Images depict some larger (top: 10–15 nm) and smaller (bottom: 3–5 nm) particles deposited from a chloroform or surfactant stabilized water–acetone dispersion, respectively.

the differences observed in the shape of the emission spectra at different excitation wavelengths might be caused by the different size of particles, different emissive traps on the surface or different fluorescent species. The nonlinear optical response of the LG27 C-dots was investigated using the Z-scan technique employing the second-harmonic, at 532 nm, from a 4 ns Q-switched Nd:YAG laser, operating at 10 Hz [9,10]. For the measurements several chloroform dispersions having different concentrations of LG27 were prepared. All samples exhibit significant nonlinear optical response for laser intensities 1–50 MW/cm2, consisting of both nonlinear absorption and refraction as shown by the representative ‘‘open-aperture’’ and ‘‘divided’’ Z-scans (Fig. 4). The ‘‘open-aperture’’ Z-scan shows a transmission peak, indicative of saturable absorption (SA) behavior, corresponding to negative nonlinear absorption coefficient b. The corresponding ‘‘divided’’ Z-scan shows a peak-valley transmission configuration, indicative of negative nonlinear refraction parameter c 0, corresponding to self-defocusing behavior. In the inset, the variation of the DTpv parameter as a function

Please cite this article in press as: Bourlinos AB et al. Synthesis, characterization and non-linear optical response of organophilic carbon dots. Carbon (2013), http://dx.doi.org/10.1016/j.carbon.2013.05.017

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3

4.0 3.5

Absorbance

3.0 2.5 2.0 1.5

(A)

1.0 0.5 0.0 200

250

300

350

400

450

500

550

600

Wavelength (nm) 1000 ex 350 ex 375 ex 400 ex 425 ex 450 ex 475 ex 500

PL (a.u.)

800

600

Fig. 4 – ‘‘Open-aperture’’ and ‘‘divided’’ Z-scans of a 2.8 mg/ mL LG27 dispersion in chloroform under 4 ns, 532 nm, 15 MW/cm2 laser excitation. The continuous lines are the fitting of the experimental points by the Z-scan theoretical expressions of the normalized transmittance. Inset: variation of the DTpv parameter as a function of the incident laser energy for two different concentration dispersions of the LG27 dots.

400

(B)

200

0 300

400

500

600

Wavelength (nm) Fig. 3 – Absorption (A) and emission (B) spectra of LG27 in CHCl3 (0.15 mg/mL). The emission spectra have been measured by exciting at different wavelengths (corresponding colored lines); the recorded spectra are not normalized. The excitation wavelengths (kex) are summarized in (B). Inset photo in (B): drop of the corresponding dispersion showing purple or green fluorescence under a fluorescence microscope for carbon dots (excitation wavelength: 360–370 nm for purple; 460– 495 nm for green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of the incident laser energy for two different concentration dispersions of the LG27 dots is presented. The slopes of the straight lines correspond to the nonlinear refraction parameter c 0 exhibited by each dispersion. Note that the solvent (i.e., the CHCl3) did not exhibit any nonlinear optical response under similar excitation conditions. Therefore, the Z-scans presented in Fig. 4 are solely due to the nonlinear optical response of the carbon dots. Note that under nanosecond laser excitation and for incident laser intensities as low as those used in the present study, there is no contribution from the solvent employed (e.g. independent from the solvent type). Using similar measurements performed on different concentration dispersions of LG27 the nonlinear absorption coefficient b and the nonlinear refractive parameter c 0 (i.e., n2), related to the imaginary and real parts of the third-order susceptibility v(3), respectively, have been determined, according

to the experimental methodology and data analysis described in detail elsewhere [6,9,10]. From these values the third-order nonlinear susceptibility v(3) was deduced. The values of the nonlinear optical parameters are summarized in Table 1. The values of another, benchmark carbon-based molecule, namely C60, are also included for comparison. Since the susceptibility v(3) depends on the number of molecules per volume, in order to make direct comparisons of the C-dots with fullerene, the values of the third-order susceptibility v(3) of the LG27 dispersions and of a C60-toluene solution have been normalized by the corresponding absorbance of each system at the laser excitation wavelength (532 nm) and have been also included in the Table 1. The corresponding quantity v(3)/a0 provides a figure of merit of the nonlinear optical response. As can be seen, the LG27 carbon dots possess significant nonlinear optical response comparable to that of C60, which is considered a benchmark carbon. Interestingly, aqueous dispersions of some onion-like carbon nanostructures, under very similar experimental conditions show nonlinear absorption only [6]. In fact, a nonlinear absorption parameter b of about 2 · 109 m/W has been reported for the onion-like carbons, which is close to the value of 0.8–1.4 · 109 m/W determined here for the C-dots. Concerning the origins of the observed optical nonlinearities of C-dots, they can be understood in terms of light-generated free-carriers (FCA) where electrons from the valence band can be excited to the intermediate states of the linear absorption tail through inter-band transitions, leading to the generation of a large number of free carriers, electrons and holes [11–13]. These free carriers can be further excited to higher levels into the conduction band by absorbing photons (intra-band transitions), giving rise to nonlinear absorption and nonlinear refraction. The interplay of parameters like

Please cite this article in press as: Bourlinos AB et al. Synthesis, characterization and non-linear optical response of organophilic carbon dots. Carbon (2013), http://dx.doi.org/10.1016/j.carbon.2013.05.017

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Table 1 – Third-order nonlinear optical parameters of LG27 carbon dots. Sample

a0 (cm1)

b (1011 m/W)

c 0 (1018 m2/W)

n2 (1011 esu)

v(3) (1013 esu)

v(3)/a0 (1013 esu cm)

LG27 2.8 mg/ml LG27 1.7 mg/ml C60 0.7 mg/ml

7.4 4.4 2.8

(140 ± 20) (80 ± 30) 80 ± 16

(100 ± 10) (52 ± 4) (86 ± 13)

(34 ± 3) (18 ± 1) (31 ± 5)

160 ± 10 90 ± 10 131 ± 20

22 ± 2 20 ± 2 65 ± 7

free carriers’ absorption cross-sections and de-excitation lifetimes, can explain the observed NLO response. In particular, the filling of the conduction band by excitation of electrons of the valence band can adequately explain the observed saturation of transmission (SA response) of the LG27 dots. In summary, we have synthesized new organophilic Cdots from mild pyrolysis of lauryl gallate. The C-dots are highly dispersible in polar organic solvents and show excitation dependent fluorescence similar to other C-dot systems. More importantly, in the first ever study the C-dots exhibit considerable third order non-linear optical response comparable to C60, thus, opening new possible applications for this new and intriguing class of carbon-based nanomaterials.

Acknowledgements We gratefully acknowledge the support by the Operational Program Research and Development for Innovations-European Regional Development Fund (project CZ.1.05/2.1.00/ 03.0058), Operational Program Education for Competitiveness (CZ.1.07/2.3.00/20.0017) and Award No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST). This work was additionally supported by the Grant Agency of the Czech Republic (P208/12/G016) and a student project of Palacky University (PrF_2013_028) as well as the European Union (European Social Fund-ESF) and Greek national funds through the Operational Program ‘‘Education and Lifelong Learning’’ of the National Strategic Reference Framework (NSRF)-Research Funding Program: THALIS and HERAKLEITOUS II. The authors would like to thank Dr. K. Safarova for her assistance in microscopy studies.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.carbon.2013.05.017.

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Please cite this article in press as: Bourlinos AB et al. Synthesis, characterization and non-linear optical response of organophilic carbon dots. Carbon (2013), http://dx.doi.org/10.1016/j.carbon.2013.05.017