1 Introduction

Recent years have seen a rapid growth of interest by the scientific and engineering communities in the carbonic materials. In general, carbon-based materials play a major role in today’s science and technology and the discovery of graphene is the last of a long string of continuous advances in the science of carbon. Graphene sheets have unique electronic properties [1] and recently have been shown to be the strongest material known to man [2]. Indeed, amorphous and diamond-like carbons are currently used in many everyday life applications, for example, magnetic hard disk coatings, wear protective and anti-reflective coatings for tribological tools, engine parts, razor blades and sunglasses, biomedical coatings (such as hips or stents), and microelectromechanical systems [3, 4]. Graphitic carbon and, to an extent, carbon nanotubes are also utilized in batteries [5]. Applications in field emission displays, microwave amplifiers, transistors, supercapacitors, structural and conductive composites, photonic devices although all well beyond the proof-of-principle stage, have yet to make it to market.

As in the early days of nanotubes, graphene has proved difficult to produce. However, it is clear that large scale assembly is needed for the widespread application of this material. Several approaches have been developed to provide steady supply of graphene in large area and quantities amenable for mass applications. These include growth by chemical vapor deposition, segregation by heat treatment of carbon-containing substrates, and liquid-phase exfoliation. Other graphene preparation methods are based on unzipping of nanotubes and chemical or electrochemical reduction in exfoliated graphite oxide [6]. Chemical methods are combined with their own problems. High impurity level of final product and hazardous residue of production process beside the long time to pass through several chemical reactions to come to graphene is the problem of chemical methods.

Laser ablation of graphite target in liquid environment is another method for producing graphene [6, 7]. Laser wavelength, pulse width, spot size, intensity, as well as the ablation liquid environment and its temperature are strong tools to control the final productions of laser ablation process [8–13]. Mortazavi et al. [6] reported that in graphene-produced laser ablation, size of liquid environment molecules becomes important since they may penetrate between graphite target surfaces, leading to their ablation in the form of large sheets, i.e., graphene. In this experimental study, we have used liquid nitrogen as the ablation environment and the effect of laser pulse energy on the graphene-produced laser ablation method has been investigated experimentally.

Ablation liquid environment plays undeniable role on the characteristics of ablation products. Fabbro et al. [14] modeled the laser ablation process in three stages. In the first stage, during the laser heating, the generated pressure is typically 4–10 times greater than the corresponding one obtained in direct ablation. The second stage begins after the switch-off of the laser and is characterized by an adiabatic cooling of the plasma which maintains the applied pressure over a period which is about 2 times the laser pulse duration. Finally, the third stage concerns also the adiabatic cooling of the recombined plasma, but during this period, the exerted pressure is too small to realize a plastic deformation of the material. The pressure of the plasma plume on the surface of the target is directly proportional to the ablation liquid density. The density of the liquid nitrogen is smaller than water and other convenient ablation liquids. In the case of high pressure, usually atoms or molecules ablate from the surface of the target. In this case with increasing the pressure, the number of nanoparticles in comparison with nanosheets will increase. Decreasing the plasma pressure and the weak Van der Waals bonds between the graphite planes lead to increase the ablation products in the form of graphene planes. This is the main point of using liquid nitrogen as the ablation environment in the experiment.

This manuscript is organized as follow; following the introduction in Sect. 1, the experimental detail is described in Sect. 2. Section 3 is devoted to results and discussion, and conclusion is presented in Sect. 4.

2 Experimental

Carbon nanostructures were produced by pulsed laser ablation of a graphite plate (99.9 %) in liquid nitrogen environment. A graphite plate was placed on the bottom of an open glass cylindrical vessel filled with 20 mL of liquid nitrogen. Height of liquid nitrogen on the target was 0.5 cm. All equipment include the graphite plate was cleaned ultrasonically in alcohol, acetone, and deionized water before the experiments. Graphite target was ablated with the fundamental wavelength (1064 nm) of a pulsed Nd:YAG laser operated at 7 ns pulse width and 5 Hz repetition rate. To produce carbon nanostructure in liquid nitrogen, 5000 laser pulse was used. Output of laser was focused on the surface of graphite target using a 80 mm focal length convex lens. The spot size of laser pulse on the surface of target was calculated to be 30 μm. After the ablation process, liquid nitrogen was evaporated and the cylindrical vessel was filled with distilled water gradually. 0.5, 0.8, 1.1, 1.4, and 1.8 J/cm2 laser fluence was used to produce carbon nanostructures, which are labeled as samples 1–5, respectively. Pictures of carbon nanostructures suspensions are presented in Fig. 1.

Fig. 1
figure 1

Carbon nanostructures suspensions produced by laser ablation in liquid nitrogen

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A variety of analytical techniques were applied for the characterization of products. The optical properties of the nanoparticle solution were examined at room temperature by UV–Vis–NIR spectrophotometer from PG instruments Ltd. The crystalline structure of the carbon nanostructures was analyzed by X-ray diffraction (XRD) with Cu Kα radiation (λ = 1.54060 Å), using an STOE–XRD diffractometer. Their morphology was investigated by Hitachi S4160 field emission scanning electron microscope (FE-SEM). The suspensions were dried on Si substrates for FE-SEM imaging. Zeiss EM10C transmission electron microscope was employed to investigate the size and form of samples. Transmission electron microscopy (TEM) was done by depositing a drop of the concentrated suspension on a carbon-coated copper grid. Particle size and size distribution were measured by a dynamic light scattering (DLS) apparatus Malvern Zetasizer 3000HSA. Raman Thermo Nicolet disperse spectroscope from Almega was utilized to determine the structure, quality, and amount of graphene in dried drops of suspensions on glass. The samples were characterized by Fourier transform infrared (FTIR) spectroscopy (Nexus 870).

3 Results and discussion

Carbon nanostructure suspensions are expected to be gray or even dark gray. In the laser ablation of graphite sheet, usually several forms of carbon nanostructures such as carbon nanoparticles, graphite, graphene, and fluorine may be produced in the suspensions. Among carbon nanostructures, graphene nanosheet is transparent and colorless. The produced nanostructures suspensions, which are shown in Fig. 1, are colorless. It shows that they contain large amount of graphene sheets. From sample 1 to 5, the color of samples tends from colorless to bright gray. In this case with increasing the fluence of laser, the amount of graphene sheets in suspensions is decreased and other carbon nanostructures were produced.

The UV–Vis–NIR absorption spectra of samples in the 1-cm thickness quartz cuvette with the absorption of the distilled water as the baseline are shown in Fig. 2. The absorption peak due to n–π* transition of C=O band has been occurred around 306 nm [15, 16]. This absorption features are seen due to surface plasmon resonance (SPR) in the free electron cloud of carbonaceous material π electrons [17]. The corresponding wavelengths of absorption peaks for all produced samples are closed. In addition, it is noticeable that there was a promotion in absorption intensity of carbon nanostructure in sample 3 which was produced by 1.1 J/cm2 and its intensity depended on the amount of graphene and carbon nanoparticles in the suspension. The larger intensity of absorption peak for the carbon nanostructure produced by 1.1 J/cm2 confirms that the number of carbon nanoparticles produced in this fluence is greater than the other fluences.

Fig. 2
figure 2

UV-Vis–NIR absorption spectra of samples

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The FTIR spectra of samples are presented in Fig. 3. To record data, few drops of suspensions were dried on glass surface. Four distinct peaks were occurred in the FTIR spectra of samples at 700, 1650, 2100, and 3000–3700 cm−1. The peak at 700 cm−1 is due to vinyl or aromatic C–H bond. These bonds may be formed between carbon atoms and hydrogen atoms of water. The peak at 1650 cm−1 proves the presence of C=C bonds in the structure of produced samples. Both acyclic and conjugated C=C bonds oscillate at this wave number. The peak at 2100 nm shows the oscillation of C–N bonds. These bonds may be formed during the ablation process in liquid nitrogen. And the wide peak at 3000–3700 cm−1 shows the strong oscillation of O–H band of water environment. The intensity of an absorption band in FTIR spectra depends on the number of the specific bonds present. In this work, the intensity of absorption band in all samples is close to each other.

Fig. 3
figure 3

Typical FTIR spectra of samples

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The X-ray diffraction pattern of samples and graphite target are shown in Fig. 4. To record XRD data, few drops of suspensions were dried on silicon substrate. Graphite target peaks are at 2θ = 26.4°, 44.4°, and 55°. There is one peak at 2θ = 32.9° in the XRD spectrum of nanostructure produced by 0.5 J/cm2 (sample 1) due to fullerite structure of carbon lattice. There are two small peaks at 2θ = 44.4° and 55° in the XRD pattern of nanostructure produced by 0.8 J/cm2 (sample 2). These peaks show the similar lattice structure of nanostructure produced by 0.8 J/cm2 to graphite target, but they appeared very much weaker than the graphite target XRD peaks. The peak at 2θ = 44.4° was appeared in the XRD patterns of nanostructure produced with 1.1–1.8 J/cm2 laser fluence (samples 3–5). In the XRD spectrum of nanostructure produced with 1.1–1.8 J/cm2, three more peaks were occurred at 2θ = 17°, 25°, and 38°. The peak at 2θ = 17° is due to X-ray photons diffracted from fullerite structure of carbon lattice. Two peaks at 2θ = 25° and at 2θ = 38° show the lattice structure of carbon in the suspensions. In this experimental situation by increasing the laser fluence, the intensity of the peak belongs to X-ray photons diffracted from carbon material is increased. Results show that there is one peak in the nanostructure produced by 0.5 J/cm2 (sample 1). In the case of nanostructure produced by 0.8 J/cm2 (sample 2), lattice structure is very similar to the structure of graphite target and with increasing the laser fluence from 1.1 to 1.8 J/cm2 particles were ablated in atomic form from the target and in their recombination new structures such as carbon and fullerite were formed. Actually, we do not expect any XRD peaks from monolayer graphene. In this case, the nanostructures of produced with 0.5 and 0.8 J/cm2 laser fluence should be closed to graphene structure because of their weak peak. Results confirm that increasing the laser fluence leaded to ablating atomic carbons from graphite target, so new carbonic structures generate in suspensions.

Fig. 4
figure 4

X-ray diffraction of carbon nanostructures and graphite target

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Figure 5 shows the TEM micrographs of samples. In these micrographs, few-layer graphene and carbon nanoparticles can be seen. The graphene nanosheet exhibits transparent morphology. From TEM results, it was found that the laser fluence was a parameter that affected the type of carbon nanostructure produced by laser ablation. According to these data, we have larger amount of carbon nanoparticles in the samples produced at higher laser fluence. Shape of carbon nanoparticles which are produced by laser ablation of a graphite target is spherical. The generated carbon nanostructures at lower laser fluence (0.5 and 0.8 J/cm2) are graphene nanosheet with folded regions. Carbon nanostructures produced with 1.1–1.8 J/cm2 laser fluence are graphene nanosheet and spherical nanostructures. The mechanism of the graphene nanosheets formation is proposed based on the threshold fluence of laser pulse. This is the main mechanism which causes exfoliation of weakly bounded by Van der Waals forces graphene single-layer sheets at the laser fluence below the ablation threshold. On the opposite end, at the fluence above the ablation threshold, aggregation of carbon atoms into nanoclusters and nanoparticles in the ablated plume is a well-studied mechanism.

Fig. 5
figure 5

TEM micrographs of samples

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Figure 6 shows the FE-SEM micrographs of the samples. In these samples, sheet-like structures are formed; however, in 0.5 J/cm2 laser fluence (sample 1), produced sheets are adhered and these layers overlap with each other. Size of sheet-like structures produced with 0.8 J/cm2 laser fluence (sample 2) is larger than the other samples while the size of sheet-like structures produced with 1.1 J/cm2 laser fluence (sample 3) is smallest. It can be clearly seen that in the nanostructures produced with 1.4 J/cm2 (sample 4) the exfoliated-like structures was decorated with spherical structures. In nanostructures produced with 1.8 J/cm2 laser fluence (sample 5), the surface morphology was coarser than that of the other samples.

Fig. 6
figure 6

FE-SEM micrographs of samples

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Dynamic light scattering was used to determine the size distribution profile of the generated carbon nanostructures in suspension. The plots of size distribution of samples are presented in Fig. 7. Even though the all of the carbon nanostructures prepared in different laser fluence are not spherical, we have measured the size distribution for comparison. Significant variation in hydrodynamic sizes was observed in samples produced with different laser fluence. Result shows that the hydrodynamic sizes are broad and range from 1300 to 1600 nm for graphene nanosheets produced with 0.8 J/cm2 laser fluence (sample 2), but the range of hydrodynamic size distribution for graphene nanosheets produced with 0.5 J/cm2 laser fluence (sample 1) is 680–850 nm. The hydrodynamic size distributions for carbon nanostructures generated in higher laser fluence are between 450 and 750 nm for samples 3–5. The increased average size of carbon nanostructures in DLS study is because graphene nanosheets are formed in the sample. By increasing the amount of nanoparticles, the average size of carbon nanostructures in DLS is decreased. This result confirmed by TEM micrographs and UV–Vis–NIR absorption spectra of samples. According to TEM micrographs, more graphene nanosheets are formed in the sample 1–2 and the average size of these samples are larger than the other samples. Due to the higher amount of nanoparticles in the sample 3, the absorption peak of nanostructures in this sample is higher than the other samples and this result confirmed by DLS.

Fig. 7
figure 7

DLS plots of samples

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The Raman spectra of samples in the range of 1100–1700 cm−1 are presented in Fig. 8. Data have been recorded from the dried drops of suspension on a glass substrate with 0.1 cm−1 spectral resolution. The main characteristics in the Raman spectra of graphitic carbon-based materials are the G and D bands. The G band points out the optical E2g phonons at the Brillouin zone center caused by the bond stretching of sp 2 carbon. The D band represents the breathing mode of aromatic rings arising due to the defect in the sample [18].

Fig. 8
figure 8

Raman scattering spectra of samples in the range of 1100–1700 cm−1

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As can be seen in Fig. 8, the prominent G band at ~1580 cm−1 with intensity comparable to the D band at ~1350 cm−1 is indicative of minor structural disorder in graphene produced at 0.5–1.4 J/cm2 laser fluence [19–21]. The Raman spectrum of the produced graphene oxide at 1.8 J/cm2 laser fluence (sample 5) displays a broad D band at 1340 cm−1 and G band at 1590 cm−1 [20]. With decreasing the local defects in the graphene, the intensity of D band decreases. The small magnitude of the ratio of the integrated intensity of the disorder-induced D band to that of the G (I D/I G) indicates the increased fraction of sp 2 domains in these samples and suggests a satisfactory quality of graphene nanosheets produced in the liquid nitrogen. In order to determine the average domain size of the sp 2 in graphene, we employed the Tuinstra and Koenig relation, which relates the ratio of D and G bands intensities into the crystallite size as follows [18];

$$ \frac{{I_{\text{D}} }}{{I_{\text{G}} }} = \frac{{2 \times 10^{ - 10} (\lambda^{4} )}}{L} $$
(1)

where L is sp 2 domain size, λ is the laser wavelength which was used for Raman spectroscopy, and I D/I G is the intensity ratio of the D and G peaks. In this work, the calculated values of sp 2 domain size for samples 1–5 are 31.89, 34.19, 19.95, 32.90 and 16.29 nm, respectively. The calculated values of sp 2 domain size in samples 3 and 5 are smaller than the other samples. According to the other result of analyses such as UV–Vis–NIR absorption spectrum and DLS in the nanostructures produced with 1.1 J/cm2 (sample 3), the number of carbon nanoparticles produced in this fluence is greater than the other fluences. Small sp 2 domain size can be due to the higher amount of nanoparticles in this sample. In the nanostructures produced with 1.8 J/cm2 (sample 5), the small intensity of the G band in comparison with other samples indicates the decreased of sp 2 domain size in higher laser fluence.

Figure 9 displays the Raman shift due to the dried graphene suspension in the range of 1000–3000 cm−1. The 2D band is ascribe to double resonance transitions resulting in the production of two phonons with opposite momentum, and this band can be correlated with the number of graphene layers [22–24]. The 2D band is occurred at 2680 cm−1 in the spectra, due to production of graphene in liquid nitrogen [25]. Due to the added forces from the interactions between layers of AB-stacked graphene, as the number of graphene layers increases, the spectrum will become different from that of single-layer graphene, namely a splitting of the 2D peak into an increasing number of modes that can combine to give a wider, shorter, higher frequency peak [6]. As can be seen with increasing the laser fluence in samples 1–3, the intensity of 2D band in comparison with the intensity of D and G bands is increased. But in samples 4–5 when laser fluence increased, D and G bands are observed again, and the 2D band becomes weaker and broader. An enhanced 2D band is due to the reduction in graphene layers [26]. Such as sample 5, adding successive layers of graphene causes asymmetry in 2D band, and as a result, the 2D band splits into several overlapping modes.

Fig. 9
figure 9

Raman scattering spectra of samples in the range of 1000–3000 cm−1

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4 Conclusions

In summary, carbon nanostructures and graphene nanosheets synthesized using pulsed nanosecond Q-switched Nd:YAG laser ablation of the graphite in the liquid nitrogen. It is found that the number of graphene nanosheets and amount of carbon nanoparticles are proportional to the laser fluence. TEM micrographs and other analyses such as UV–Vis–NIR absorption spectrum and DLS verify that the generated carbon nanostructures are classified into two categories: few-layer graphene and carbon nanoparticles. The size distribution of samples decreased with increasing the laser fluence that can be attributed to increasing the amount of carbon nanoparticles. The larger intensity of absorption peak in sample produced at 1.1 J/cm2 laser fluence confirms that the number of carbon nanoparticles produced in this fluence is greater than the other fluences, and small sp2 domain size can be due to the higher amount of nanoparticles in this sample. The XRD patterns of samples show that the presence of graphene nanosheets decreased the degree of crystallinity. According to the result, more graphene nanosheets are formed in the samples produced at lower laser fluence and the average size of these samples are larger than the other samples. The peak position of 2D band for the generated samples confirms the formation of few-layer graphene. It is concluded that the characteristic of the carbon nanostructures produced by pulsed laser ablation in the liquid nitrogen can be controlled by laser fluence. The threshold fluence for the formation of graphene sheets in liquid nitrogen is 1.1 J/cm2. The best graphene sheets appeared in the production when the laser fluence was less than 1.1 J/cm2, and when the laser fluence exceeded 1.1 J/cm2, a noticeable amount of carbon nanostructures appeared in the productions.