Facile laser-assisted synthesis of inorganic nanoparticles covered by a carbon shell with tunable luminescence

Abstract

We report a one-step strategy at ambient conditions for the production of hybrid inorganic core–carbon shell nanoparticles by means of pulsed laser ablation of inorganic targets (LiNbO₃, Au, and Si) in hydrocarbon liquids such as toluene and chloroform. The core of these spherical nanoparticles consists of the target material, whereas the shells are carbon structures (multilayer graphite-type carbon and amorphous carbon), which are formed due to the thermal decomposition of the organic liquid in contact with hot inorganic nanoparticles ejected from the bulk target. These carbon shells emit photoluminescence in the blue-green spectral region and the obtained luminescence, in which the luminescence band maximum position depends on the excitation wavelength, is analogous to the luminescence observed for the so-called “carbon dots”.

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1. Introduction

After the first publication that discovered the outstanding visible luminescence properties of the so called “carbon dots”¹ an avalanche growth of the number of publications on luminescent carbon nanomaterials has been observed.^2–4 These new carbonaceous materials are characterized by high optical absorptivity, high photoluminescence quantum yield (PL QY), chemical stability, biocompatibility, and low toxicity.⁵ In addition, practically all types of CDs, as well as some of the luminescent graphene (oxide) nanodots,^6–8 exhibit the dependence of the PL band position on the excitation wavelength, so that the PL can be tuned practically over the entire visible spectrum. It is also important to mention that the very high values of the PL QY of CDs (up to 0.80)⁴ are achieved only after special passivation and functionalization of their surface. Although the origin of CDs PL is still a matter of debate (see, for example, ref. 4), in recent years it has become commonly accepted that surface states are involved in the PL.^9–12

Carbon dots are usually believed to be helpful in biomedicine⁵ or in chemical catalysis.^13,14 Only very recently publications have appeared in which these new materials are considered as candidates for use in photonics and optoelectronics.¹⁵ Such a disproportion seems to be quite logical since carbon dots are normally produced as a suspension of nanometer-sized plain graphitic sheets, which consist of a few graphene monolayers. We believe that these carbonaceous nanostructures could find considerably wider applications in solid-state photonics and optoelectronics devices in cases where they are formed on underlying solid-state structures.

In this study, we propose a laser based approach, namely, pulsed laser ablation in liquid (PLAL) for the synthesis of such hybrid structures that contain luminescent graphitic layers deposited on solid-state materials. In the last decade, PLAL has emerged as an appealing alternative approach to generate a wide range of nanomaterials,^16–19 which is mainly due to the simplicity of the method. The main advantages of this technique include the green synthesis without chemical agents,¹⁶ the production of ligand-free nanoparticles,¹⁷ a possibility to work at ambient conditions, its versatility that allows in situ manipulations²⁰ and its high production yield.²¹ Recently, it has been reported that the synthesis of nanomaterials in organic solvents can influence the composition and structure of the products.^22–25

In the present work we report a new luminescent hybrid nanomaterial, which is produced by the pulsed laser ablation of an inorganic solid target in an organic (hydrocarbon) liquid. The material is fabricated in the form of spherical nanoparticles (NPs) with a core–shell structure, where the inorganic core is produced by the laser ablation of the target material, whereas the luminescent shell is a carbon structure (such as multilayer graphite or amorphous carbon), which is formed on the inorganic core surface due to the thermal decomposition of the hydrocarbon liquid that is in contact with the hot core nanoparticle. The carbon shell is shown to exhibit excitation-wavelength dependent emission in the visible spectral region, which is similar to “classical” CDs.

2. Experimental details

We used a 128°-y cut LiNbO₃ wafer (Castech), gold film of 99.99% purity (Goodfellow Cambridge Ltd.), and Si (100) p-type 20–30 ohm cm wafer (ITME) as laser ablation targets. Spectral grade toluene and chloroform (Aldrich) were used as liquids to perform pulsed PLAL process. A Q-switched pulsed Nd: YAG laser (Spectra-Physics Quanta-Ray) generating 355 nm pulses of 10 ns duration with a repetition rate of 30 Hz was used to ablate different targets in the liquids. The ablation pulse energy was fixed in all the experiments to about 20 mJ per pulse. The PLAL procedure consisted of exposing a solid target, which is immersed into a liquid in a vessel, to laser irradiation (usually for 5 minutes). The diameter of the laser spot on the sample was around 800 μm and the thickness of the liquid over the target was 5 mm. The vessel containing the liquid and the target was rotated during the PLAL process to achieve more uniform irradiation over the target surface. The PLAL process was performed with all controllable parameters constant (liquid layer thickness, pulse energy, vessel rotation velocity and laser beam configuration).

The PL spectra of the PLAL-produced suspensions were measured using a spectrofluorometer (FluoroMax-4, Horiba Jobin Yvon). The Raman characterization of dried PLAL products deposited on a Si wafer was done using a laser excitation wavelength of 632 nm. Fluorescence microscopy was performed, using a Nikon confocal microscope (A1sp, Nikon Instruments, Japan), on suspensions of NPs dried on glass. We acquired several confocal images that were excited both at 405 or 488 nm and collected PL in the spectral ranges 450–600 nm or 500–550 nm, respectively. We used a 60 × 1.4 NA objective lens and a pixel dwell time of 52 μs. TEM and HRTEM images were obtained with a Tecnai G² instrument operating at 100 kV. The preparation of the specimens was done by dropping a colloidal suspension over carbon-coated TEM copper grids. After complete evaporation of the liquid in air at room temperature the samples were transferred onto the electron microscope.

Dynamic light scattering (DLS) was performed using a Zetasizer Nano ZS90 (Malvern, USA) equipped with a 4.0 mW He–Ne laser operating at 633 nm and an Avalanche photodiode detector. Measurements were made at 25 °C for the nanoparticle solutions. The values were determined using the Smoluchowski approximation and were estimated as the average of three repeated measurements.

3. Results and discussion

By studying the suspensions of nanostructures formed by the nanosecond laser ablation of solid targets (LiNbO₃, Si, Au) in organic liquids, we have found that, independent of the target used, the suspensions produced in toluene or chloroform exhibit PL. Fig. 1 shows the confocal microscope images of the luminous materials produced by PLAL processing of a LiNbO₃ target in toluene (a) and chloroform (b) as well as by PLAL of a Au target in toluene (c) and CHCl₃ (d).

Fig. 1 Confocal microscopy images of NPs obtained by PLAL of a LiNbO₃ target in toluene (a) and CHCl₃ (b) shown in red false colour and of Au NPs obtained by PLAL of Au target in toluene (c) and CHCl₃ (d) shown in green false colour.

As one can see in Fig. 1, the emission is spotted. Given that LiNbO₃ and Au are not luminescent materials, clearly we should associate the origin of the PL with the carbon byproducts obtained during the PLAL process in organic liquids. It is interesting to note that the spatial distribution of the emission in Fig. 1a and c from one side and 1b, and 1d from another side are different: in the case of PLAL in toluene (1a and c) practically all the emitted light comes from isolated NPs, whereas in the case of PLAL in CHCl₃ the formation of a few μm agglomerates is clearly observed with emitted light coming not only from the NPs but also from the regions between them (“luminous clouds”). PLAL of the Si target in the same two solvents results in confocal luminescence images (not shown) that are similar to those presented in Fig. 1.

To understand what structures are responsible for the detected PL, the morphology of the nanostructures produced by PLAL in toluene and chloroform was studied using the TEM and HRTEM techniques. In Fig. 2 and 3 we show the results obtained with LiNbO₃ as the target. Qualitatively similar HRTEM data were obtained in the case of Si and Au targets (not shown).

Fig. 2 TEM images of the nanostructures obtained by the laser ablation of an LiNbO₃ target in toluene (a and c) and in chloroform (b and d). Scale bars are 100 nm (a and b) and 20 nm (c and d).

Fig. 3 HRTEM images of nanostructures obtained by the laser ablation of an LiNbO₃ target in toluene (A) and in chloroform (B).

In order to get insight of the morphology and structure of the luminescent product, conventional TEM and HRTEM analysis were performed. Analysis of TEM and HRTEM images of the LA products shows that, under the PLAL conditions used (time of ablation, excitation wavelength and energy), the laser ablation of the LiNbO₃ target immersed in toluene or chloroform yields perfect crystalline spherical LiNbO₃ NPs with diameters in a wide interval between 10 and 200 nm (Fig. 2). (Obtaining a narrow size distribution of the ablation-produced NPs was not the aim of the present work).

As one can see in Fig. 2, the nanostructures obtained by the ablation of the target in toluene from one side and in chloroform – from another side, are significantly different. Indeed, individual NPs coated by a shell are formed in the case of toluene (the shell is clearly visible in Fig. 2c), whereas PLAL in chloroform results in the formation of chain-like agglomerates, which consist of LiNbO₃ NPs as well as carbon-based NPs (the latter are visible as less dense objects in Fig. 2b). As Fig. 2d shows, the entire structure of the synthesized agglomerates is wrapped in an amorphous material, which can be identified as amorphous carbon (a-C). It should be noted that the structures described in Fig. 2 are similar to those observed earlier as a result of PLAL of a gold target in liquids. In particular, in ref. 26 the authors observed the formation of a multilayer carbon (graphitic) shell around gold NPs after PLAL in toluene, whereas in ref. 27 the authors found, as a result of laser ablation of an Au target in chloroform, the formation of chain-like aggregates which consisted mainly of amorphous carbon. It is worth noting, however, that in both publications no information was presented about the PL properties of the PLAL products.

The observed different structures of the PLAL products obtained in toluene (individual NPs) and chloroform (NPs aggregates) are unambiguously confirmed by the results of the dynamic light scattering experiments (Fig. S1†): ∼100 nm average diameter is obtained in the former case and ∼1 μm diameter in the latter.

Complementary to TEM, the HRTEM analysis reveals that the PLAL product has a highly crystalline structure. HRTEM images (Fig. 3) show that along with highly crystalline LiNbO₃ NPs, which are characterised by a 2.58 Å distance between diffraction fringes (which corresponds to the (020) orientation of the crystalline core, see Fig. 3A), multilayer shells are formed around the LiNbO₃ NP surface. The shells contain only amorphous phase (a-C) in the case of ablation in chloroform (Fig. 3B), whereas in the case of toluene one can identify a carbon multilayer (CML) structure deposited on the LiNbO₃ NP surface, which is covered by an amorphous shell. The interlayer distance for this CML is found to be 3.45 Å (Fig. 3A), which corresponds to a bulk graphite interplanar distance.

Similarly to the case of the LiNbO₃ target, the ablation of Au and Si targets in toluene resulted in the formation of multilayer graphitic shells around the target NPs, whereas in the case of ablation in CHCl₃ only a-C material is detected (not shown).

In order to obtain additional information on the origin of the products of the solid targets laser ablation in the two organic liquids, we also investigated their Raman spectra in the 1000–1800 cm⁻¹ region. The Raman spectra are presented in Fig. 4 (for the case of the LiNbO₃ target in toluene) and Fig. S2 and S3† for the other cases.

Fig. 4 Raman spectra of products of PLAL processing of a LiNbO₃ target in toluene (a) and chloroform (b). The noisy lines (in cyan colour) correspond to the experimental spectra, and the red solid lines represent the result of the experimental spectrum modelling by Lorentzian contours; the individual Lorentzian contours are shown by green dash lines. Positions of the contour maxima are also indicated.

In all cases the Raman spectra consisted of several very broad superimposed bands. Fitting by Lorentzian contours enabled us to select four main bands, which manifest themselves in each of the six measured spectra. There are three intensive bands with clearly visible maxima at ∼1340, ∼1450 and ∼1580 cm⁻¹ as well as one less intensive band with its maximum near 1150–1180 cm⁻¹, which are visible in the experimental Raman spectra as a shoulder.

Because all the target materials (LiNbO₃, Si and Au) do not exhibit any pronounced Raman lines in the investigated spectral region, one can suggest that the observed Raman bands belong to the PLAL products, which are produced due to the decomposition of organic liquids during the PLAL process. Indeed, it is well known that the Raman spectra of all carbons show several common features in the 800–2000 cm⁻¹ region, the so-called G and D peaks, which lie at around 1560 and 1360 cm⁻¹ for visible excitation, and the T peak, which is observed only for UV excitation at around 1060 cm⁻¹.²⁸ The G and D peaks are known to be due to sp² sites only.²⁸ The G peak is due to the bond stretching of all pairs of sp² atoms in both rings and chains.²⁹ The D peak is due to the breathing modes of sp² atoms in rings. Therefore, the observation of D and G bands as main lines in our spectra is unambiguous evidence in favor of the presence of sp² carbon structures. Without special investigations, including multiwavelength Raman studies,²⁸ it is impossible to evaluate the relative contributions of sp² and sp³ carbon in the laser ablation products and conclude about the relative contributions of different morphological carbon structures (disordered carbon, amorphous carbon, graphite-like carbon and others). We can only note that the large spectral width of the observed G and D bands is evidence in favour of the presence of highly disordered graphitic structures.²⁹

An interpretation of the intense band at 1450 cm⁻¹ requires more detailed analysis. Very often this Raman band at around 1450 cm⁻¹, which usually appears together with the band near 1150 cm⁻¹ in the case of nanocarbon materials, is assigned to the “phonon frequencies at the K and M points of the graphite equivalent Brillouin zone”.³⁰ However, the argumentation presented in ref. 31 convinced us that the ∼1150 and ∼1450 cm⁻¹ peaks should be assigned to the ν₁ and ν₃ modes of trans-polyacetylene (trans-PA) molecules.³² These modes are roughly the sum and difference combinations of C=C chain stretching and CH wagging modes. Because the Raman cross section of polyacetylene is very high as compared to the G and D bands of graphite,³¹ the relative amount of trans-PA in the PLAL products may be very small.

Finally, the very broad band, which exhibits its maximum at about 1690 cm⁻¹ in some of the Raman spectra of the obtained products, can be interpreted as a molecular C=C symmetric stretching vibration. Indeed, as indicated in the literature, this vibration is strong in Raman spectra and can be detected in the region of 1500–1900 cm⁻¹.³³ Concerning the formation of C=C bonds, even in the case of the use of CHCl₃ as an ablation medium, we expect that the very high temperature conditions of the laser ablation experiments can be responsible for this. Indeed, there are indications in the literature that, in the case of high-temperature chloroform pyrolysis (>850 °C), C₂H₄ is one of the major products.³⁴

All photoluminescence data obtained can be classified as follows. The PL spectra of the suspensions produced by PLAL of all used targets in toluene are similar. The main characteristic of these spectra is the dependence of the luminescence band on the excitation wavelength and more exactly a monotonous shift of the PL band maximum to the long wavelength side, from 350 to 570 nm, when increasing the excitation wavelength from 300 to 500 nm. A typical example of such behaviour is presented in Fig. 5 for the case of the suspension obtained by PLAL of the LiNbO₃ target in toluene.

Fig. 5 PL spectra of a suspension of NPs produced by PLAL of a LiNbO₃ target in toluene. The excitation of the PL was from 300 to 380 nm (a) and from 400 to 500 nm (b). Insets show normalized PL spectra. Absorption spectrum is also shown (solid grey line).

As one can observe in Fig. 5, excitation in the 300–340 nm interval results in two-band photoluminescence, the bands maxima are near 360 and 450 nm. Starting from 360 nm excitation, the further shift of the excitation wavelength from blue to the green side is accompanied by the single PL band shift towards long wavelengths without modification of the band spectral shape. The PL quantum yield of the suspension produced by PLAL of LiNbO₃ in toluene was about 0.01 under 360 nm excitation using quinine bisulfate in 0.10 N H₂SO₄ as a reference (PL quantum yield 0.53).³⁵ Similar PL quantum yield values are usually published in literature for as-synthesized CDs not subjected to additional passivation post-treatments. (Comprehensive photophysical investigation of the luminescent products is now in progress and the results obtained will be published elsewhere).

Contrary to the behaviour of the PL spectra of the suspensions produced in toluene, the suspensions in chloroform either do not demonstrate any (in the case of the LiNbO₃ and Au targets) or demonstrate weak (in case of the Si target) spectral position dependence of their PL band maxima on the excitation wavelength. A typical example of such behaviour is presented in Fig. S4.† As one can see from this figure, excitation at 300–400 nm results in the appearance of a single PL band with the maximum near 450 nm, and the following shift of the excitation to 500 nm is not accompanied by the appearance of a long wavelength-shifted PL band with a distinct band maximum. Instead, only the tail of the previous very broad 450 nm PL band is detected.

Finally, in case of the suspension produced by PLAL of the Si target in chloroform only a moderate shift of the PL band maximum is observed from about 500 to 550 nm when the excitation shifts from 300 to 500 nm (Fig. S5†).

When viewing Fig. S4 and S6† one can observe an interesting difference in absorption spectra of the suspensions produced by PLAL of a gold target in chloroform (Fig. S4†) and in toluene (Fig. S6†). Indeed, in case of chloroform, a distinct plasmonic band is observed with the maximum at 550 nm belonging to Au NPs, whereas the corresponding maximum is absent in the case of Au NPs produced in toluene. We suggest that two reasons may be responsible for such effect. First, in our experiments the average size of the Au NPs produced in toluene is less than 5 nm, and it has been shown earlier that for very small (2–3 nm diameter) Au NPs a plasmonic band is not observed.^36,37 On the other hand, Amendola et al. have shown²⁶ that the formation of a graphitic multilayer structure on the Au NP surface quenches the plasmonic absorption band, which can be ascribed to the formation of covalent bonds at the surface that exhaust/localize free electrons at the metal NP.³⁸

We would like to conclude the consideration of the PL properties of the produced hybrid nanoparticles with a discussion on the nature of the photoluminescence of CDs in general because it can shed light on the PL origin in our structures. In general, all proposed mechanisms of CDs photoluminescence can be divided onto two parts: (1) the mechanisms that consider PL to be a result of radiative deactivation (due to electron–hole recombination) of excitonic (collective) electronic states delocalized over a nanocrystal and (2) the mechanisms related to the radiative deactivation of surface states localized on specific chemical (functional) groups. Although in many earlier works the explanation of CDs PL was related with mechanism (1),^39–44 unambiguous data appeared in the last few years that weigh the scale on behalf of mechanism (2). In particular, very convincing results on the PL origin in CDs were obtained in ref. 45, where the photoluminescence of nanodiamonds was investigated. It is well known that “ideal” bulk diamond does not exhibit any visible PL and only the formation of crystallographic defects in the diamond crystal structure, where each C atom is sp³-hybridized, may result in the appearance of PL in the visible spectral region. In case of nanodiamonds, due to the extremely increased surface-to-volume ratio, the role of surface defect states in the formation of physical-chemical and photophysical properties of the nanostructures drastically increases. It was shown⁴⁵ that in spite of the fact that nanodiamonds are distinctly different from carbon quantum dots (with their essential contribution of sp²-hybridized carbon) from the point of view of their crystalline and electronic structure, nanodiamond colloids exhibit strong visible excitation-dependent fluorescence. This effect is unrelated to the size (i.e. electron confinement) effect and can be explained by the presence of several types of localized surface states, which are related with the functional groups residing on the nanodiamonds such as hydroxyl OH, carbonyl C=O, and carboxyl COOH groups.⁴⁵

Similar conclusions about the origin of visible PL in few-layer graphite-type structures were obtained in many other publications. Thus, in ref. 9, green luminescence in differently synthesized CDs, which possess different morphologies, and graphene oxide QDs were assigned to special edge states consisting of several carbon atoms on the edge of the carbon (graphite) backbone bound with carbonyl C=O or carboxyl COOH groups. In ref. 10, the PL of CDs was assigned to the presence of oxygen-containing C=O and –OH groups on the CD surface, which were chemically bound to a graphitic structure. At last, it is demonstrated in ref. 11, on the basis of careful optical investigations, that the so-called “carbon dots” are not “quantum dots” and are not even “dots”. Due to the presence of electronic anisotropy of the corresponding optical emission they cannot be considered as “dots”, but objects of zero dimension. The authors showed also that the CDs fluorescence response is not collective and represents a composition of individual emitters.¹¹ According to the presented results, the so-called “carbon dots” are considered in ref. 11 as nano-sized clusters assembling individual fluorophores, which are formed on the particle surface.

On the basis of the above mentioned literature data, the observed complex dependence of the PL spectra on excitation photon energy (Fig. 5), when two PL bands with maxima at about 350 and 450 nm are detected under 300–330 nm excitation and only the latter inhomogeneously broadened PL band is detected when excitation is realised in the 360–550 nm interval, which can be explained, in general terms, as follows.¹¹ Different functional groups (–OH, C=O, COOH) bound to the graphitic matrix form different luminescent centres, so that illumination by UV light (300–330 nm) preferably excites luminescent centres containing –OH groups, which possess PL with a maximum near 350 nm. The illumination by blue-green light excites mainly the centres containing C=O and COOH groups, which demonstrate inhomogeneously broadened PL in the green and even yellow region. We suggest that the absence of the blue PL band in case of LA in chloroform may be simply related with the fact that the content of hydrogen in CHCl₃ is considerably lower than that in toluene (C₆H₆), which prevents the formation of OH-related luminescent centres in a sufficient concentration.

4. Conclusion

In conclusion, we have reported a novel method for the fabrication of luminescent hybrid carbon–inorganic nanoparticles using a one-pot laser based strategy at ambient conditions. The material is in the form of nanoparticles with a core–shell structure, where the core is an inorganic spherical nanoparticle produced by laser ablation from the target material, whereas the luminescent shell is a carbon-based (graphite-type multilayer or amorphous carbon) structure. Our studies reveal an excitation-wavelength dependent carbon-based multicolour emission in the visible spectral region, which is similar to carbon dots. Our method is versatile and allows the generation of a large variety of core–shell structures by the proper selection of the target material used for the ablation. We also suggest that an analogous strategy can be used for the deposition of graphitic layers on preformed NPs immersed into an organic liquid such as toluene and irradiated by intensive laser pulses, which are well absorbed by the NPs to increase their temperature sufficiently for liquid decomposition. The proposed approach opens a route towards the fabrication of luminescent solid-state materials, which can find applications in photonic devices due to their high refractive index, high mechanical hardness, and the possibility of the use of plasmonic effects in the case of metallic cores.

Acknowledgements

This work was supported through the Spanish MCINN (project TEC2011-29120-C05-01), Generalitat Valenciana (Grant PROMETEOII/2014/059) and the EU Grant EU-FP7 NMP-246331 (NanoPV). We thank the Nikon Imaging Center at the Fondazione Istituto Italiano di Tecnologia for help with light microscopy.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07319g

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