Î²-C3N4 nanocrystals: Carbon dots with extraordinary Morphologi- cal

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#-C3N4 nanocrystals: Carbon dots with extraordinary Morphological, Structural and Optical Homogeneity Alice Sciortino, Nicolò Mauro, Gianpiero Buscarino, Luisa Sciortino, Radian Popescu, Reinhard Schneider, Gaetano Giammona, Dagmar Gerthsen, Marco Cannas, and Fabrizio Messina Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05178 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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Chemistry of Materials

β-C3N4 nanocrystals: Carbon dots with extraordinary Morphological, Structural and Optical Homogeneity Alice Sciortino†,‡, Nicolò Mauro§,#, Gianpiero Buscarino†,◊, Luisa Sciortino†, Radian Popescu⊥, Reinhard Schneider⊥, Gaetano Giammona§, Dagmar Gerthsen⊥, Marco Cannas†, Fabrizio Messina†,◊,* † Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Via Archirafi 36, 90123 Palermo, Italy ‡ Dipartimento di Fisica e Astronomia, Università degli Studi di Catania, Via Santa Sofia 64, 95123 Catania, Italy § Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche (STEBICEF) , Università degli Studi di Palermo, Via Archirafi 32, 90123 Palermo, Italy # Fondazione Umberto Veronesi, Piazza Velasca 5, 20122 Milano, Italy. ◊ CHAB – ATeN Center, Università degli Studi di Palermo, Viale delle Scienze, Edificio 18, 90128 Palermo, Italy ⊥ Laboratory for Electron Microscopy, Karlsruhe Institute of Technology, Engesserstrasse 7, 76131 Karlsruhe, Germany ABSTRACT: Carbon nanodots are known for their appealing optical properties, especially their intense fluorescence tunable in the visible range. However, they are often affected by considerable issues of optical and structural heterogeneity, which limit their optical performance and limit the practical possibility of applying these nanoparticles in several fields. Here we developed a synthesis method capable of producing a unique variety of carbon nanodots displaying an extremely high visible absorption strength (ε>3·106 M(dot)-1cm-1) and a high fluorescence quantum yield (73%). The high homogeneity of these dots reflects in many domains: morphological (narrow size distribution), structural (quasi-perfect nanocrystals with large electronic bandgaps) and optical (non-tunable fluorescence from a single electronic transition). Moreover, we provide the proof of principle that an aqueous solution of these dots can be used as an active medium in a laser cavity, displaying a very efficient laser emission with dye-like characteristics, which reflects the benefits of such a highly homogeneous type of carbon-based nanodots.

INTRODUCTION The discovery of Carbon nanodots (CDs) as a new class of fluorescent nanomaterials1 revolutionized the idea of carbon as a “black” and not emitting material, initiating an entirely new field of studies on the extraordinary and attractive properties of this nanomaterial. CDs typically display strong linear and nonlinear2 absorption over the UV-VIS, and intense and tunable fluorescence bands.1 Their optical and photochemical properties lead to many different uses in biological,3 optoelectronic,4 photocatalytic5,6 and sensing applications,3 exploiting the ease, low cost and the variability of the synthesis procedures.7 Although CDs can be obtained by many different routes, some of which are very simple and starting from common, raw materials, there is a certain lack of reliable methods to control their size distributions, crystalline structures and surface functionalization, which are key elements determining their optical features.8 This absence led to study highly inhomogeneous samples, characterized by a broad size distribution9,10 and by an unstructured absorption spectrum without any definite features,1 increasing the complexity on understanding the emission processes, hindering further improvements of their optical properties and paralyzing the possibility to tailor them for specific applications. The very fluorescence

tunability of CDs,11 beneficial in some contexts, is indeed the sign of a dramatic dot-to-dot variability undesired for many applications, as recognized by very recent work.12 Engineering a new generation of homogeneous and well-controlled CDs, ideally endowed with a wide-bandgap crystalline structure and a single efficient emitting transition, would be an important step forward to outperform fluorescent semiconductor nanoparticles or molecular dyes. Only few works so far describe controlling procedures by which a subset of CDs is selected on the basis of a single feature (e.g. size or surface). 13-17 While these procedures are mostly meant to select a fraction with increased emission quantum yield (QY), the goal of identifying optically homogeneous types of CDs has mostly been neglected. Here, we develop a simple method to select and purify a type of CD with monodisperse size distribution and wellcontrolled structure, and we optimize the characteristics of the environment in order to get CDs displaying an extremely high absorption strength in the visible, combined with fluorescence quantum yield (QY) of 73% under excitation at 440 nm. This value is one of the highest in literature,16-20 where so large QYs are especially uncommon for visible excitations.12,21,22 We characterized our samples with structural and optical tech-

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niques and found out extremely homogeneous and beneficial characteristics in the morphological, structural and optical domains. Moreover, we demonstrate that an aqueous solution of these CDs can act as active laser medium inside a homemade optical cavity generating molecular dye-like laser emission. EXPERIMENTAL SECTION Atomic Force Microscopy (AFM). To perform AFM measurements, the solutions were diluted down to about 1 mg/L, deposited on a mica substrate and then dried in vacuum. AFM measurements were acquired in air by using a Bruker FASTSCAN microscope equipped with a closed-loop scanner (X, Y, Z maximum scan ranges: 35 µm, 35 µm, 3 µm, respectively). The scans were obtained in soft tapping mode by using FASTSCAN-A probes with apical radius of about 5 nm. Each AFM image was obtained with a pixel resolution comparable to the tip size. The diameter of the nanoparticles was estimated by evaluating their height from the AFM scans. High Resolution Transmission Electron Microscopy (HRTEM). The HRTEM measurements were performed on an aberration-corrected FEI Titan3 80-300 microscope at 300 keV electron energy. The sample were prepared at room temperature in air by the deposition of a drop of aqueous solution of CDs on a commercial 400 µm mesh Cu-grid (Plano 01824) covered by a holey amorphous carbon film, with a nominal thickness of 3 nm. Infrared Spectroscopy (IR). Infrared absorption spectrum reported in Figure 1-d were acquired on a N2-purged, Bruker VERTEX-70 spectrophotometer, in transmission geometry. The measurements were collected at room temperature under Nitrogen flux to avoid artifacts due to the residual water in the air. Samples were prepared by depositing drops of a concentrated CDs solution (1g⋅L-1) on a sapphire window and drying in vacuum. Steady State optical characterization. After the sample was purified, it was optically characterized by steady-state absorption and emission techniques. The spectra reported in Figure 2 were carried out on samples prepared by dissolving the CDs in water and in dimehylsulfoxide (DMSO). Steady-state absorption measurements were performed by a double beam spectrophotometer (JASCO V-560) in the 220-700 nm range in a 1 cm quartz cuvette. The emission spectra were recorded on diluted solutions (0.005 g/L) with a JASCO FP-6500 spectrofluorometer in a 1 cm cuvette with a 3 nm resolution bandwidth. To measure the quantum yield, we compared the fluorescence of CDs to that of fluorescein dissolved in water at pH=13 (QY=95%), used as a reference. All the measurements were performed at room temperature. Time resolved optical characterization. The data in Figure S3 were recorded on a sample prepared by dissolving the βCDs in milliQ water at a concentration of 0.005 g/L in a 1 cm quartz cuvette. The measurements were performed by the use of a tunable laser system (OPO – optical parametric oscillator) pumped by a Q-switched Nd:YAG laser, providing 5 ns pulses at 10 Hz repetition rate. The spectra were recorded on an intensified CCD camera, integrating the signal during temporal windows of 0.5 ns, after variable delays from the pump pulse. The accuracy of the time constant of the decay is about 0.2 ns. Pumping setup and laser cavity. The homemade laser cavity was pumped by the same tunable laser used for the time re-

solved optical characterization. A cylindrical lens (f=150 mm) was arranged in front of the gain medium (a solution of βCDs inserted in a 1 cm quartz cuvette). Two flat metallic mirrors were positioned near the cuvette in order to intercept and further amplify the generated laser beam.

RESULTS AND DISCUSSION Synthesis. The synthesis procedure of our sample consists in two steps pictured in Figure 1-a: the carbonization of the precursors and the separation/purification of the sample. We begin from an aqueous solution (10 mL of milliQ water) of citric acid monohydrate (3 g) and urea (3 g) (Sigma Aldrich). After the solutes are completely dissolved in water, the solution is exposed to microwave irradiation until a complete evaporation of water occurs.23 The result is a black powder made up of aggregates of CDs and, most likely, many other impurities.24 The collected quantity of the powder is about 30% in weight of the initial precursors. This powder is dissolved in water in order to carry out the separation/purification procedure. In order to get a sample with a suitable concentration for efficient column chromatography, this initial solution is concentrated to about 4 g⋅L-1 (as evaluated by freeze-drying part of the colloidal dispersions and weighing the residue) by using a rotating evaporator at 60 °C temperature, and 80-50 mbar pressure. Then, the use of size exclusion chromatography (SEC) technique allows to isolate the most luminescent fraction of the sample. The procedure was carried out using a glass column (100 cm length, 1.5 cm diameter) packed in turn with sephadex G25 (15 g), G15 (15 g) and G10 (30 g). Thus, the stationary phase consisted of three resins with increasing cut-off values. This specific combination of resins was selected on the basis of previous works in which a clear correlation between the hydrodynamic radius (Hr) and the average molecular weight (Mw) of random polymers was observed.25 Using that correlation, the limit of exclusion of commercially available resins was converted from Mw to Hr so as to select resins suitable to separate nanoparticles in the 1-6 nm range of diameters. During the SEC procedure, 5 ml of the CDs solutions were added on the stationary phase and, after the complete absorption, ultrapure water was added on the top as eluant (flow = 2.5 ml min-1). Then, we took 5 mL aliquots of the solution eluting from the column and analyzed them in detail. Therefore it was possible to identify four different fractions of CDs with distinct morphological and optical characteristics, observed to separate from the original sample for every SEC which was performed. The optical spectra of these fractions are reported in Figure S1. However, the aim of this work was identifying the sub-type of CDs with the highest optical performance and homogeneity. Therefore, we focused our attention on one specific fraction displaying particularly interesting characteristics, including a QY at least doubled with respect to the as-synthesized sample. This sample, referred to as βCD, is studied in detail in the rest of the paper. From a mass measurement obtained by lyophilization, we estimate that the quantity of βCD inside the initially synthesized nanopowder is about 13% of the total weight.


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Chemistry of Materials gram shown in Figure S4 which points out that the diameter of the sample in the X-Y directions is 6 േ2 nm. Comparing these data with those obtained by the AFM analysis, it is evident that the nanoparticles are practically spherical with a diameter of 6 nm. We performed infrared absorption measurements in transmission geometry. The spectrum is reported in Figure 1-d and it shows at least three clear peaks at 1710 cm-1, at 1600 cm-1, and 1380 cm-1, attributed to –COOH (carboxyl), CONH2 (amide), C-N vibrations, respectively, of the functional groups that are arranged on the surface of the dots. Optical characterization. The β-C3N4 structure of our CDs determines the optical characteristic of the sample and it is uncommon in the literature, even for highly N-doped CDs produced by similar methods,26-28 in striking contrast with the general view of CDs having either graphitic or amorphous core structures. Further studies will be necessary to find out to what extent the purification itself or some synthesis parameters (e.g. microwave power, synthesis time, precursor ratios…) contribute to isolate CDs with such a unique structure from other sub-types of CDs that can be formed from the same reagents.

Figure 1: (a) Scheme of the synthesis of βCDs: the microwaveinduced carbonization of an aqueous solution of citric acid and urea followed by a size exclusion chromatography which selects the highest luminescent fraction. (b) HRTEM image of a β–C3N4 particle having a diameter of 4.8 nm and its Fourier transform and the calculated diffraction pattern with Miller indices for bulk hexagonal β-C3N4. Characteristic lattice parameters are highlighted on the HRTEM image. (c) Distribution of diameters extracted from AFM measurements of the βCDs sample with a gaussian function for the sake of clarity. (d) IR absorption spectrum of βCDs in the spectral range 2000-1200 cm-1.

Structural and Morphological Characterization. The drop-casting of a diluted solution of βCDs on a mica surface produces a distribution of several thousands of dots per µm2 with negligible aggregation (Figure S2), as revealed by atomic force microscopy (AFM) measurements. Their AFM analysis makes evident that the average diameter (height from the substrate) is 6 nm and the size distribution is fairly narrow (FWHM≈2 nm, Figures 1-c and S2), if one compares it with most CDs described in the literature. This result reveals the good success of the purification process and it is one of the evidences of the high homogeneity which characterizes this sample. Through high resolution transmission electron microscopy measurements (HRTEM), we revealed the highly crystalline nature of the core as a β-C3N4 monocrystal of few nanometers (Figure 1-b). We investigated tens of different particles finding the same regular crystalline structure for all of them (some examples are shown in Figure S3), underlining again the strong homogeneity of the sample. The whole quantum dot in Fig. 1b represents a quasi-perfect β-C3N4 monocrystal with a hexagonal structure, as demonstrated by the good agreement between the sharp peaks of its Fourier transform and the calculated diffraction pattern of bulk hexagonal β-C3N4 (space group P63/m, space group number 176, a=b=6.38 Å and c=2.395 Å) in the [324]-zone axis (Fig. 1b). We also estimated the size of the dots in different TEM images with different magnifications and we obtained the histo-

Notably, β-C3N4 nanocrystals are characterized by a high band gap (∼4 eV).29 We speculate that this may be advantageous for the optical performance in order to avoid various possible types of losses. In fact, even if CD emissions are mostly due to surface states, the core structures may have an indirect but important role in determining the optical performance. For example, the low-bandgap core of common graphitic CDs may cause unwanted absorption reducing the QY, and may also introduce additional non-radiative dissipation channels affecting the fluorescence of surface chromophores. In contrast, βCDs should overcome both these drawbacks.

Figure 2: (a) Absorption spectrum (purple) and photoluminescence spectrum (red) of CDs in water. Inset: Comparison of photoluminescence spectra of a solution of βCDs in water (red) and in DMSO (green). The reported emission spectra were excited at 440



nm, but the characteristics of the emission band are excitationindependent, as inferred from panel b (b) Photoluminescence 2dimensional plot of βCDs in water. The dashed line indicates the emission spectrum plotted in panel (a). The oblique line across the entire plot derives from crossing of excitation and emission monochromators. (c) Photo of the emission of a solution of βCDs under the sunlight.

Their absorption spectrum is reported in Figure 2-a (purple curve) with the steady-state emission spectrum (red curve) collected from the aqueous solution of βCDs. The spectrum has a simple and well-defined structure, very different from most CDs. We see band-to-band core transitions below 300 nm and a single, sub-gap transition appearing as a relatively narrow Gaussian band peaking at 400 nm. Based on our previous solvatochromic studies of the unfractionated CDs, we model the band at 400 nm as a transition which involves the migration of an electron from the crystalline core of βCDs to the surface.31 In literature, CD absorption spectra are almost invariably reported in arbitrary units, rather than expressed as an absolute molar extinction coefficient ε. Here we used the size (6 nm – Figure 1c) and density (3.57 g cm-3 for β-C3N4, as from ref.30) of our dots to simply estimate the number of βCDs in a given mass, and so calculate ε from the measured absorption coefficient. In this way we obtain ε=3.4·106 M(dot)-1cm-1 at 400 nm, corresponding to an absorption cross section of σ=1.3⋅10-14 cm2. This is an extremely high value as compared with previous reports on carbon nanodots 12,31,32. In fact, the absorption strength of our βCDs, at the peak of their lowestenergy transition at 400 nm, is more than one order of magnitude higher than typical fluorescent molecular dyes,33 and even higher than CdSe or CdTe quantum dots of the same size34,35 (e.g. σ≈4⋅10-15 cm2 for 6.8 nm-sized CdSe dots on the peak of the lowest-energy transition).35 This high value confirms the idea that all βCDs homogeneously contribute to the same transition. The complete emission-excitation spectra of βCDs are displayed in the 2D-plot of Figure 2-b. In these plots, a horizontal cut isolates the emission spectrum at a fixed excitation wavelength and a vertical cut corresponds to the excitation spectrum at a certain emission wavelength. For instance, the dashed line yields the emission spectrum reported in Figure 2a. The 2D-plot shows a single, very homogeneous electronic transition with practically zero tunability of the emission, as from its symmetrically quasi-round shape. To confirm the great homogeneity of the sample, we also performed time resolved experiments and we found that the decay of the band is single-exponential (lifetime 5.0 ns) and that the lifetime of the band is completely independent of the emission wavelength (Figure S5). This result strikingly contrasts with usual reports of complex, multi-exponential decay for other CDs.36,37 From our previous studies, we know that the presence of H-bonding at the surface of carbon dots increases the non-radiative decay rate of the emission. 38 Considering these evidences, we dissolved these β-C3N4 nanocrystals in DMSO, and we found that their QY reaches indeed 73%, that is, an emission almost-free from non-radiative losses. Actually, the emission of βCDs appears extremely bright to the naked eye, as can be seen from the picture in Figure 2-c. Lasing emission. Interestingly, the large QY, homogeneous emission and intense absorption of our sample reflect in a very efficient lasing capability. After building a homemade reso-

nant cavity (Figure S6), we used the aqueous solution of βCDs as an active laser medium to generate lasing emission under pumping by nanosecond laser pulses at 440 nm. A photograph of the laser emission emitted in such conditions is shown in Figure 3-c. We found the lasing emission peaks at 568 nm, that is significantly redshifted from the emission band (Figure 3-a). In fact, the lasing wavelength should rather match the effective spectral maximum for stimulated emission, which was found at ≈560 nm (≈10 ps after photo-excitation) in our recent femtosecond pump-probe study.39 We found that the lasing emission FWHM is ∆λ∼10 nm (Figure 3-a), a rather large value likely due to the involvement of several longitudinal modes, because of the absence of wavelength-selective elements in the laser cavity.

Figure 3: (a) Comparison between the emission spectrum (purple) of the sample with its laser emission spectrum (red) excited at 430 nm. Inset: Output intensity of the laser emission of the sample plotted as a function of pump energy. The laser threshold is 1.5 mJ/pulse. (b) Normalized laser emission spectra excited at 410 nm (purple), 420 nm (blue), 430 nm (green) and 440 nm (red). (c) Photo of the laser emission of solution of βCDs. The lasing cuvette appears out of focus in the foreground of the photograph, while the green spot visible at the top center is the laser beam as it appears on a wall located at ≈1 m from the cuvette.

We explored a broad range of pump energies in order to find the lasing threshold, which is reached at 1.5 mJ/pulse with a cylindrical lens of 150 mm focal length, when the optical path of the medium is 1 cm and the solution (0,31 g⋅L-1) is inside a commercial quartz cell. In fact, lasing from aqueous βCDs is efficient enough that 4% reflection from cell surfaces is already sufficient to obtain a laser beam (as in Figure 3-c), although its intensity can be further enhanced by adding external mirrors. Interestingly, no degradation of the sample is detected, and lasing appears indefinitely stable under the intense pump light, differently with some previous works.40.The previous few works where CD-based lasing was achieved generally did not study the excitation-wavelength dependence of the laser emission, and typically achieved lasing under UV excitation at a fixed wavelength.12,41-44 Here, the lasing emission can be detected if the excitation wavelength explores the 410-440 nm (Figure 3-b) range and is practically independent of pump wavelength and intensity. Rather than displaying a tunable las-


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Chemistry of Materials ing, 45 our sample behaves more closely to a molecular laser dye, consistently with the homogeneous optical properties of these βCDs. In agreement with this picture, also the lasing bandwidth is independent of pump intensity, differently from previous findings of strongly inhomogeneous lasing, where the pump intensity determines the laser bandwidth through a change of the number of cavity modes participating to laser oscillations.45

CONCLUSIONS

In summary, we optimized a simple method to synthesize a type of CDs displaying peculiar and very promising optical properties. The tight control of the characteristics, as the size and the crystalline structure, seems very important to determine the very bright luminescence and strong absorption of these CDs. Furthermore, they display a unique degree of homogeneity of the optical response, unparalleled by other types of CD, and reflecting on their capability of generating dye-like lasing emission. These properties pave the way to the use of these nanomaterials in many different optical applications. For instance, these dots may be especially appealing to fabricate photovoltaic devices, exploiting their unusually high absorption strength to efficiently harvest solar photons. Our results can represent the first step to the engineering of the next generation of carbon-based nanodots with strictly controlled characteristics and well-designed crystalline structures, enhancing their potential to substitute other optical materials for a variety of ends.

ASSOCIATED CONTENT Supporting Information. Full UV/VIS characterization of the fractions obtained by SEC, additional AFM, HRTEM, and time-resolved fluorescence data on βCDs, scheme of the optical cavity used for lasing experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Email: [email protected]

ACKNOWLEDGMENT We acknowledge funding received from the Karlsruhe Nano Micro Facility (KNMF) – Proposal ID 2016-016-013649. We thank the LAMP group at the University of Palermo for support and stimulating discussions. Infrared measurements were carried out at the laboratories of ATeN center - CHAB.

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Î²-C3N4 nanocrystals: Carbon dots with extraordinary Morphologi- cal

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