1. Introduction

As a kind of carbon nanomaterials, NDs have certain advantages over other nanomaterials, such as excellent chemical inertness, physical stability, biological compatibility and non-toxicity, as well as great bare surface which are conducive to surface modification and performance tailoring [1–3]. For above advantages, NDs as a potential carrier can be applied in drug delivery and genetic engineering, especially NDs with the size less than 5 nm, which can penetrate cell membranes with drug molecules or gene chains and enter cells for targeted therapy [4–6]. Even better, NDs can be metabolized and discharged out cells [7,8]. In addition, early studies have shown that after being added into the electrolytic cell and deposited on the electrode surface together with lithium ions, NDs can effectively inhibit the growth of lithium dendrites and greatly improve the performance of lithium batteries. Moreover, the smaller the size of NDs is, the more obvious the improvement will be [9].

Up to now, various methods for synthesizing NDs have been reported, which mainly fall into two categories including “bottom-up” methods and “top-down” methods. Both the chemical vapor deposition (CVD) and redox carbide are belong to “bottom-up”, the former process of which is complex and the equipment is expensive [10,11]; while products of redox carbide are complex, which make them difficult to be purified [12]. The mainstream methods belong to “top-down” category for synthesizing NDs are ultrasonic cavitation [13], ion beam radiation graphite [14], detonation [15], high-energy ball grinding of micro-diamond [16], and laser ablation in liquid [17–19]. Among them, laser ablation is attracting more and more attention because of the potential application for commercially industrial production as shown by numerous studies [20–22]. During this processing, high-temperature and high-pressure can be generated. This extreme physical condition can form local cavitation blasting which can result in a variety of nanoparticles including metals, ceramics, semiconductors, alloys and so on [21]. Streubel et al. found that the production efficiency of various metal nanoparticles is up to 4 g/h through picosecond pulsed laser ablation in liquid [23]. Even though the efficiency of femtosecond pulsed laser ablation in liquid (fs-PLAL) is not as good as that of picosecond laser [24], Donate-Buendia et al. proposed simultaneous spatial and temporal focusing (SSTF) to solve this problem leading to increasing the yield of gold nanoparticles by 2.4 times [25]. Qiu et al. obtained NDs with size distribution of 2-4 nm by femtosecond pulsed laser ablation of graphite in ethanol and glassy carbon in acetone successfully [26]. However, it is difficult to control the process of changes between carbon and diamond. Disappointedly, the yield of this method is very low, and products are unpurified, for which the carbon of non-diamond phase needs to be removed by acid boiling [27,28]. For the sake of stepping to industrial products, developing NDs with compact synthesis equipment, simple preparation technology, good dispersion, bare surface and high stability simultaneously is still full of challenge.

In this paper, NDs are obtained by using fs-PLAL to fragment diamond block. The NDs as confirmed by related characterization are all highly crystalline as well as high yield and pure. Moreover, through adjusting laser power, the UV-Vis absorption spectra and the ablation efficiency can be tuned. After modification of NDs’ surface by PEG_200N, they are able to emit relatively strong emission. This study provides a more environmental and economical way for the preparing of NDs which are more conductive to practical application.

2. Experimental section

NDs were synthesized by ablating bulk diamond in deionized (DI) water using a near-ultraviolet radiation of fs-laser (Light-Conversion Pharos) delivering laser pulses of ∼300 fs pulse width at 343 nm with a repetition rate of 90 KHz. The average pulse energy is 9.2 µJ·pulse⁻¹. Firstly, the bulk diamond target was cleaned with acetone, alcohol and deionized water successively in an ultrasonic cleaner. After that, the diamond target was fixed on the back inside of a quartz cuvette with a size of 12.5 mm × 12.5 mm × 45 mm, which was full of deionized water. The initial laser beam passed through a 3X beam expander and was focused on the surface of bulk diamond by a field lens with a 10 cm focal length. As shown in Fig. 1(a), the laser’s focus scanned on the substrate in concentric circles with separations of 1 µm for both the spots and lines controlled by F-theta system lasting for half an hour. The theoretical beam waist (2ω₀) estimated at the focal point in water was about 2 µm. In order to explore the influence of different ablation powers on ablation efficiency, six samples were obtained by changing the ablation powers (118 mW, 276 mW, 317 mW, 467 mW, 625 mW and 828 mW respectively). After ablation, all the samples were collected in clear glass bottles. It should be noted that in the process of ablation of diamond, graphite phase carbon will be emerge unexpectedly. And this part of products will exist in various forms, such as independent graphite particles, graphite layer coated on the surface of NDs, and graphite phase defects in NDs. Therefore, after ablation, 0.5 mL nitric acid was mixed with the colloidal solution following by ultrasound conducting for 60 min to remove the independent graphite particles and the graphite layer covering the surface of NDs completely. Biocompatible polyethylene glycol (PEG_200N, Sigma Aldrich, CAS: 25322-68-3) were used for the surface modification. 0.1 ml of PEG_200N was mixed with 2 ml acidified sample, and then the mixing solution was kept in a drying oven for 72 hours under 120°C. Then cool to room temperature, resting for 24 hours before characterization measurements. The samples were poured into six petri dishes, and the water was sublimated in a lyophilizer, leaving behind diamond powder and then being weighed.

 

Fig. 1. a) The synthetic scheme of NDs by fs-PLAL and process of surface passivation treatment. b) TEM image of NDs. c) HRTEM image of NDs. d) Size distribution of NDs. e) NDs colloid solution after surface passivation under natural light (left) and the emission photo irradiated by 343 nm laser (right).

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Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) were obtained on the JEOL JEM-2100F. TEM samples were obtained through dropping the NDs’ colloidal solution onto a 300 mesh copper grid supported by carbon membrane at room temperature and ambient pressure. The particles’ size and size distribution of NDs are analyzed through Nano Measurer software. The surface ligands of NDs were characterized by Fourier transform infrared spectroscopy (FTIR) conducted on Fourier transform infrared spectrometer (Nicolet 6700). FTIR samples were prepared by NDs colloid solution on the KBr infrared optical window. 2 mL colloidal solution was poured into a quartz cuvette (optical length = 10 mm) for the characterization of the UV-Vis absorption and emission spectra of NDs. The UV-Vis absorption spectra were recorded by UV3600 UV-Vis-NIR spectrophotometer (SHIMADZU, 200-1000 nm, 600 nm·min⁻¹). The emission spectra were carried out by F4600 fluorescence spectrophotometer (HITACHI, Japan, 280 nm to 420 nm). It is worth emphasizing that both UV-Vis absorption spectra and emission spectra have removed the background signal owing to the solvent and system. Raman spectra were conducted on JOBIN YVON T64000.

3. Results and discussion

The size and lattice structure of as-prepared NDs are characterized by TEM image (Fig. 1(b)) and HRTEM image (Fig. 1(c)) respectively. The size distribution of NDs is analyzed based on over 100 NDs showed by Fig. 1(d). As shown in Fig. 1(b), NDs present spherical morphology with rather uniform distribution and well dispersion. The HRTEM image in Fig. 1(c) exhibits the lattice structure of NDs. The crystal spacing is about 0.21 nm, which corresponds to the (111) crystal surface of cubic diamond phase. The size distribution histogram shows that the size distribution of NDs prepared by FLAL is around 2∼4 nm, with an average size of 3.0 nm. This size distribution meets current requirements of biomedicine, pharmacology and other applications in which the NDs size should be less than 5 nm [29].

In order to confirm the structure of NDs, the Raman spectra was conducted and compared to bulk diamond (Fig. 2(a)). The characteristic peak of bulk diamond is located at 1330 cm⁻¹, which is corresponding to sp³ hybrid orbital of carbon atom [30]. Obviously, Raman characteristic peak of the bulk diamond is sharp with an extremely narrow half-width. By contrast, the Raman peaks of NDs are located at 1328 cm⁻¹ and 1558 cm⁻¹, corresponding to peak D and peak G. The D peak is formed by the sp³ hybrid orbital of carbon atom proving diamond phase, and the G peak corresponds to the sp² hybrid orbital of carbon atom confirmed graphite phase. It is clear that graphite phase carbon exists in as-prepared NDs, which is consistent with previous reports [31]. To be noticed here, the strength of D peak of NDs is far stronger than the peak of G, and the intensity difference I_D/I_G is as high as 2.6, which is much higher than the previous report [26,32]. It should be emphasized that larger I_D/I_G value corresponds the higher purity of NDs, which means it is more favorable for practical application [33,34]. In addition, the strength of the characteristic peak of NDs is much lower than that of the bulk diamond. Moreover, peak broadening and downshift occur, which mainly caused by quantum confinement effect when the size of NDs is less than 6 nm [35].

 

Fig. 2. a) Raman spectra of NDs and bulk diamond; b) FTIR spectra of NDs with and without passivation.

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FTIR spectra of NDs with and without surface passivation are shown in Fig. 2(b). The FTIR spectral peaks of un-passivated NDs mainly appear in 3452 cm⁻¹ and 2914 cm⁻¹, which correspond to the stretching vibration of O-H bond and C-H bond respectively [36]. 1594.8 cm⁻¹ peak is corresponding to the in-plane bending vibration of C-H bond. These suggest that the surface of un-passivated NDs has not any ligand but a few hydroxyl group and hydrogen bonds, which are very conductive to the modification of its properties for special applications. Thus, PEG_200N is introduced to passivating NDs for its toxicity and biological compatibility [37]. As confirmed in Fig. 2(b), after surface passivation of NDs by PEG_200N, there appear a very distinct peak located at 1726cm⁻¹ caused by the stretching vibration of the C = O [38]. The peaks at 1241.4 cm⁻¹ and 1257 cm⁻¹ are caused by the symmetrical stretching vibrations from C-O-C [34,38]. C-O associative asymmetric stretching vibrations are found distributing in 1000-1200 cm⁻¹ [34]. Moreover, a new group –COOH is formed in the FTIR of passivated NDs [34]. It is found that the absorption peak of oxygen-containing groups gets stronger after being passivated by PEG_200N. That means the surface of the passivated NDs is oxidized to a greater extent than that of the un-passivated NDs. FTIR results manifest the surface of NDs is oxidized and a few number of carboxyl groups are introduced, which means the success of surface passivation.

To characterize the optical properties of NDs, UV-Vis absorption spectra are performed. As shown in Fig. 3(a), the absorption intensity presents an obvious dependence on laser ablation power. With the ablation power increasing, NDs’ absorption intensity gets stronger. According to Fig. 3(b), when the ablation power rises from 118 mW to 828 mW, the average ablation efficiency increases by 6.6 mg·h⁻¹. This indicates that the colloidal concentration of NDs gradually increases, which is why the absorption intensity of UV-Vis gradually increases with the rising of ablation power. The more important information revealed in Fig. 3(a) is that although different powers lead to different absorption intensity, the absorption peak is almost unchanged, as depicted in Fig. 3(b). The first absorption peak position is located at around 226 nm. Since the band gap of bulk diamond is 5.47 eV, namely ${E_g}$ is 5.47 eV. According to Equation:

 

Fig. 3. a) UV-Vis absorption spectra and b) ablation efficiency and absorption peak position of NDs fabricated by fs-PLAL method under different laser power.

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In order to better explore the optical properties of NDs with and without passivation, emission spectra were studied shown in Figs. 4(a)–4(d). The un-passivated NDs exhibit different emission properties under different excitation wavelength. Figure 4(a) displays that the dual emission phenomenon occurs under excitation of 260 nm and 280 nm which was also observed by Tan et al [26]. The reason can be due to two main emission mechanisms of NDs: the intrinsic direct transition of electrons (IDTE) and the defect energy trapping (DET) [40,41]. The emission peak located in 325 nm of NDs is caused by IDTE mechanism, which is the intrinsic emission of NDs. While the emission peak near 405 nm is due to the NDs’ DET mechanism. What’s more, the dual emission does not depend on the wavelength of the excitation light of 260 nm or 280 nm. However, Fig. 4(b) shows the dual emissions disappear only with 405 nm emission peaks remaining after NDs’ surface passivated by PEG_200N. Because after surface passivation, the surface oxidation degree of NDs is greatly increased which leads to DET radiation being dominant mechanism and then vanishing the intrinsic emission. Under the excitation from 300 nm to 420 nm, the emission peaks red-shift from 405 nm to 490 nm for un-passivated NDs and passivated NDs as shown in Figs. 4(c) and 4(d) respectively. By contrast, the emission intensities of passivated NDs are stronger than that of NDs without passivation. Especially excited by 340 nm, the emission intensity of passivated NDs is 4 times than that of un-passivated NDs. At the same time, the emission peaks are completely identical through comparing Figs. 4(c) and 4(d). The strongest emission of un-passivated NDs occurs with the main emission at 402 nm in blue region excited by 280 nm; while the maximum emission intensity after passivation occurs when the excitation laser at the wavelength of 340 nm and the fluorescence can be seen by naked eyes showed in Fig. 1(e) right set.

 

Fig. 4. The emission spectra of a) un-passivated and b) passivated NDs excited by 260 nm and 280 nm; The emission spectra of c) un-passivated NDs and d) passivated NDs excited by 320 nm ∼ 420 nm.

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4. Conclusion

In conclusion, NDs with an average particle size of 3.0 nm can be obtained by ablation diamond in deionized water with femtosecond laser, and the surface is clean without other ligands, which is very conductive to the performance modification of NDs and subsequent application research. The experiment results show that the ablation efficiency increases along with the laser ablation power. When the power is 828 mW, the ablation efficiency can reach to 8.2 mg/h, which meets the requirements of practical application. There are two absorption peaks in the UV-Vis spectrum of NDs, one is the absorption caused by the intrinsic transition around 226 nm, and the other is near 275 nm, which is attributed to the C = O bond caused by partially oxidation of NDs’ surface. Just because of this, the phenomenon of dual emission appears as excited by the 260 nm and 280 nm. The first emission peak which is due to the intrinsic transition of electrons located in 325 nm. As the excitation light wavelength changes from 260 nm to 420 nm, another emission peak gradually red-shifts from 405 nm to 490 nm, which is mainly caused by DET radiation. However, after the surface of NDs is passivated, the surface is greatly oxidized, DET radiation recombination increases, and then the fluorescence intensity of NDs increases, emitting visible blue fluorescence under the excitation of 343 nm laser. In fact, up to now, it is relatively difficult to synthesize NDs with size less than 5 nm, as well as good dispersion and bare surface condition, which further proves the advantages of fs-PLAL in synthesizing ultra-small nanomaterials.