Femtosecond laser synthesis of nitrogen-doped luminescent carbon dots from acetonitrile
Introduction
Carbon dots is an emerging class of nanomaterials which hold promise in the fields of bioimaging, photocatalysis, sensing, as theranostic agents, components of light emitting diodes etc. [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]] Fluorescent carbon nanoparticles are currently regarded as competitors to more common semiconductor quantum dots due to their enhanced photostability, facile postsynthetic modification, low toxicity, environmental friendliness and low fabrication cost [2,11]. Importantly, optical properties of CDs are highly tunable and can be adjusted by changing their size, morphology, dopants and surface modification [[12], [13], [14], [15], [16]]. As a consequence, a lot of effort was invested in their synthesis and chemical modification which resulted in a plethora of synthetic methods which, in turn, could be formally split into two general approaches: top-down and bottom-up synthesis [3,5]. While the former is based on breaking down larger carbon materials by laser irradiation, arc discharge, ultrasonication or electrolysis, the latter one involves solution-based thermolysis of organic precursors [17] and often turns to be a method of choice for the synthesis of CDs. However, fine control of particle size, crystallinity, morphology and chemical composition of CDs still remains a challenge [15], motivating the research towards the development of conceptually new approaches to the synthesis of carbon nanomaterials. Moreover, fabrication of CD-based optical devices like LEDs or photonic crystals requires ability to precisely arrange small quantities of CDs into microscale patterns. Thus far, only a few exotic methods for microscale space-selective synthesis have been demonstrated [18]. Direct femtosecond laser writing is a potentially powerful technique for manufacturing of 2D or even 3D micropatterns of CDs with high accuracy and spatial resolution similar to laser writing of semiconductor quantum dots [19]. Our technique can take advantage of effective nonlinear absorption in strong optical fields of tightly focused laser pulses to carry out synthesis of CDs in a microscale reaction volume as small as hundreds of nanometers.
Another feature of femtosecond pulses is an ability to produce strong photoionization even in optically transparent materials and, thus, allow reaction pathways which are unavailable for thermally activated reactions and produce CDs with novel properties from novel precursors. Despite potential advantages of direct laser writing, only a very few reports of pulsed laser bottom-up synthesis of luminescent CDs from organic molecules have been published [20,21], and in all cases only arenes were employed as precursors. Potential of laser pulses to generate CDs from non-aromatic molecules remains completely unexplored.
Here we describe an efficient synthesis of CDs under intensive femtosecond laser irradiation of acetonitrile which is a cheap and readily available starting material for the production of N-doped CDs. Nitrogen doping is a common way for enhancing optical or catalytic properties of CDs [15,22]. Finally, acetonitrile is a stable material, which graphitizes only under extreme conditions [23], and to the best of our knowledge, production of CDs from acetonitrile by other methods has never been reported. The results below demonstrate that femtosecond laser-induced approach has untapped potential in bottom-up synthesis of (heteroatom-doped) carbon nanomaterials.
Section snippets
Materials and methods
Synthesis of nanoparticles. Acetonitrile (2 mL) was placed in a scintillation vial and irradiated with femtosecond laser pulses at 800 nm wavelength amplified by regenerative amplifier (Spitfire, Spectra-Physics) and focused by a spherical lens with numerical aperture of NA = 0.3. The pulse repetition rate, duration and energy were 1 kHz, 50 fs and 1.4 mJ, respectively. Based on the estimate of the laser beam at the waist the fluence is 1.7 × 104 J/cm2 and the power density is 1.7·1017 W/cm2.
Results and discussion
Irradiation of neat acetonitrile with 50 fs laser pulses at 800 nm with a pulse repetition rate of 1 kHz resulted in a gradual colour change to yellow already after 10 min and then to dark-brown after 6 h of permanent irradiation (Fig. 1a). The observed darkening of the reaction mixture was accompanied by the formation of a significant amount of pale-yellow precipitate, which was isolated by filtration and dried under vacuum.
Characterization of the solid by the atomic force microscopy (AFM) and
Conclusion
In conclusion, we have developed a novel simple method for the production of N-doped CDs from acetonitrile as a source of the N atoms, which is a promising approach to the synthesis of (heteroatom doped) CDs. Importantly, the method is well-reproducible and new CDs exhibit unique physical characteristics distinctive from those of obtained via traditional approaches. Principal characteristics of acetonitrile-derived CDs, such as positions of emission peaks, luminescence quantum yield and mean
CRediT authorship contribution statement
Artyom A. Astafiev: Writing - original draftWriting – original draft, Conceptualization, Methodology, Investigation, Writing. Aleksander M. Shakhov: Investigation, Validation, Methodology. Andreii S. Kritchenkov: Investigation, Methodology. Victor N. Khrustalev: Investigation, Methodology. Denis V. Shepel: Investigation. Victor A. Nadtochenko: Conceptualization, Supervision. Alexander G. Tskhovrebov: Writing - original draftWriting – original draft, Conceptualization, Supervision, Methodology,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the RFBR, Grants No. 18-03-01121 and 19-53-52007. This work was supported by the Ministry of Education and Science of the Russian Federation (award no. 075-03-2020-223 (FSSF-2020-0017)) and the RUDN University Strategic Academic Leadership Program. Experiments were performed using equipment of Semenov FRCCP CCE (no.506694).
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