1 Introduction

Semiconductor manufacturing is an essential part of electronic devices and photovoltaic industry. In this process, a huge amount of silicon (Si) sludge is generated in powder form during the slicing and grinding of Si wafers and disposed of as industrial waste [1,2,3]. This practice constitutes a tremendous loss of material resources and energy and a potential threat to the environment. Thus, the recycling destinations of Si sludge are being considered to realize closed-loop sustainable manufacturing [4, 5].

Nanosized materials, such as the nanoparticles [6] and nanowires [7] of Si and silicon carbide (SiC) [8], have excellent cyclability and theoretical capacity and are expected to be applied as one of the most important anode alternatives for lithium-ion batteries. For other applications, 3C-SiC nanowires have excellent mechanical properties, field emission properties [9, 10], and biocompatibility [11, 12]. They have demonstrated remarkable dielectric and conductive properties [13] when coated with carbon (C). Coating SiC nanowires with amorphous carbon improve their field emission properties [14]. SiC nanowires and C-coated SiC nanowires, namely, SiC@C core–shell nanowires, have been proposed as anodes in lithium-ion batteries and as field emission displays [14] and nanoelectrodes [15, 16] that can be used inside living cells. They are expected to be applied in fields such as nanoelectronics with high durability and stability, even in harsh environments. On these backgrounds, tremendous industrial and social merits can be expected if Si sludge can be transformed into Si/SiC-based nanomaterials.

Previous studies have reported the generation of Si nanostructures such as Si nanoparticles [17, 18] and Si nanofibers [19] by irradiating Si slicing/grinding sludges with nanosecond and millisecond pulsed lasers. Si nanoparticles are also produced by femtosecond pulsed laser irradiation [20] and laser irradiation in liquid [21]. These methods generate nanostructures through the cooling and agglomeration of plumes generated during laser irradiation. By adding graphite powder to Si grinding sludge and irradiating the mixture with a millisecond pulsed laser, Minami et al. succeeded in producing graphene-coated 3C-SiC nanoparticles with a core–shell structure [22]. However, the possibility of producing Si/C composite nanowires with graphene coatings using Si sludge has never been reported.

In laser irradiation, the shape and velocity of the plume can be changed by varying the laser irradiation conditions, such as laser beam defocusing. These changes would affect the melting, evaporating, and cooling rates and the movement patterns of materials in the plume, which in turn lead to various mechanisms for nanomaterial growth and chemical reactions. The morphology of the resulting nanostructures may change from nanoparticles to nanowires, nanofibers, or other high aspect ratio nano-objects if the plume duration time is sufficiently long for their epitaxial growth.

This study aimed to produce Si/C composite nanowires with multilayer graphene coatings from Si grinding sludge. Si sludge generally contains impurities, such as SiC or diamond abrasive grains, making it difficult to be reused. Here, Si sludge was transformed once inside a plasma plume where a chemical reaction occurs. In the plasma plume, the abrasive grains also transform into Si and C species so that the existing SiC and diamond abrasive grains do not become impurities in the final product. The use of Si sludge as a precursor is of substantial importance from the perspective of waste reuse. The application of this composite is important because it is compatible with Si sludge and has the potential to transform waste into batteries and electronic materials. However, generating SiC nanowires from the mixed composite is technically more difficult compared with the SiC substrates previously reported [23]. Therefore, a series of experiments was conducted by laser defocusing, i.e., gradually changing the distance between the target and focal point, to explore the possible conditions in which nanowires can be produced by laser irradiation. The generated nanostructures were collected and characterized in detail. The effects of defocusing-induced changes in energy density and laser spot size on the morphology of the products were investigated. The plume volume change was also determined by measuring the target after irradiation and examining the amount of material scattered. The expectation is that Si/C composite nanowires with multilayer graphene coatings could be generated via laser defocusing. This work provides a potential method to synthesize anode materials for manufacturing advanced lithium-ion batteries and other electronic devices.

2 Experimental Methods

2.1 Target Preparation

The target generation process is shown in Fig. 1a. The Si sludge used in the experiment was obtained by grinding Si wafers, and its average grain size was 3.9 µm. The Si sludge was mixed with graphite powders with an average grain size of 50 µm (Kojundo Chemical Laboratory Co., Ltd., Japan) at a ratio of 1:2 (wt.%) and mill ground for 3 h using a pot mill. After pot milling, the Si particle size decreased to ~ 1 µm, and the C particle size was also slightly reduced, as shown in the scanning electron microscope (SEM) image in Fig. 1b. The mixed powder was placed in a tablet molding machine, and a 40 kN load was applied for 1 min at room temperature to produce a cylindrical Si/C mixed target with a diameter of 1.0 cm.

Fig. 1
figure 1

a Silicon (Si)/carbon (C) mixed target preparation process; b the scanning electron microscope (SEM) image of Si sludge and graphite powder after milling

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2.2 Laser Irradiation

Figure 2a shows a top view of the experimental setup for laser irradiation. In a chamber filled with argon gas at a pressure of 0.1 MPa, an ytterbium (Yb) single-mode CW laser (YLR-500-AC, IPG Photonics Co., USA) with a wavelength of 1070 nm, an output of 390 W, and Gaussian energy distribution was used to generate pulse waves with a pulse width of 10 ms. Each target was irradiated with a single shot in the experiment. The laser beam focused on the target surface using a condensing lens with a focal length of 200 mm. A glass substrate was placed 5 mm below the laser beam to collect the products generated during laser irradiation. As shown in Fig. 2b, the target was gradually relocated away from the focal point to four different defocused positions (15, 20, 24, and 28 mm) by horizontally moving the target holder. The changes in laser beam profile and spot size of the fiber laser system are shown in Fig. 3a. The light emitted from the fiber was collimated and then focused by a set of lenses.

Fig. 2
figure 2

Experimental setup for laser irradiation: a top view and b side view

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Fig. 3
figure 3

a Beam waist of a fiber laser; b change of beam radius with defocus distance; power density distributions under c on-focus condition and d defocused conditions

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The resulting beam waist \({\omega }_{0}\) can be calculated from the beam diameter D of the collimated light, the beam quality factor M2, the focal length f of the lens, and the wavelength λ of the laser as expressed by Eq. (1).

$$\omega_{0} = \frac{{2M^{2} \lambda f}}{{{\uppi }D}}$$
(1)

In this study, the beam quality factor was M2 = 1.05, and the measured laser beam diameter was D = 12.6 mm. Thus, the theoretical beam waist \(\omega_{0}\) was 11.4 µm. The expansion angle θ of the laser beam shown in Fig. 3b can be obtained using Eq. (2).

$$\theta = \frac{{M^{2} \lambda }}{{{\uppi }\omega_{0} }}$$
(2)

Using the value of θ, the spot radius \(\omega_{z}\) can be obtained by Eq. (3) when the defocus distance is L.

$$\omega_{z} { } \approx { }L\theta$$
(3)

When the laser power is P0, the power density can be obtained from Eq. (4):

$$P\left( r \right){ } = { }\frac{{2P_{0} }}{{{\uppi }\omega_{z}^{2} }}{\text{exp}}\left( { - \frac{{2r^{2} }}{{\omega_{z}^{2} }}} \right)$$
(4)

The calculated power density varies with defocus distance, as shown in Fig. 3c and d.

2.3 Material Characterization

The morphology of the products deposited on the glass substrate was observed using field emission scanning electron microscopy (FE-SEM, Inspect F50, FEI Company, USA) and transmission electron microscopy (TEM, Tecnai G2, FEI Company, USA). The deposits were collected in five single-shot irradiations at a defocus length from 15 to 28 mm to obtain adequate products for TEM observation. As shown in the TEM images, 25 arbitrary nanowires were extracted for the 15 mm defocused condition and 50 arbitrary nanowires for the 20, 24, and 28 mm defocused conditions. The diameter and length of the nanowires were measured to investigate the mechanism of nanowire growth. The composition of the nanoparticles was also evaluated from the selected area electron diffraction pattern by TEM.

The target was observed and measured using a laser microscope (VK-X1000, KEYENCE Co., Japan) to confirm the amount of material removed from the target after single-shot irradiation. The volume of the crater formed on the target surface by laser irradiation was estimated from the cross-sectional profile of the crater.

2.4 Plume Observation

The laser-induced plume was photographed using a high-speed camera FASTCAM Mini AX50 (Photron Ltd., Japan). Two types of shooting were carried out: general shooting mode and shooting with the Schlieren method. The general shooting mode is suitable for observation from the start of laser irradiation to the end of laser irradiation; however, the subsequent phenomenon is not visible when the light emitted by the laser irradiation ends. The Schlieren method allows observation of the plume after laser irradiation and is effective for understanding the plume [24]. A schematic of the light path in the Schlieren method plume observation is shown in Fig. S1 in the supplementary information. The plume to be observed was set in the collimated light produced by a point light source and two collimating lenses. The light was disrupted and refracted by the plume, allowing the density gradient of the plume to be visualized. In this study, a light source with a diameter of 4.3 mm was covered with a pinhole with a diameter of 1.5 mm. The illuminance at a distance of 10 cm from the light source was 3000 lx without the pinhole and 200 lx with the pinhole. A pinhole with a diameter of 1.5 mm was also placed in front of the high-speed camera as a knife edge. The plume can be observed after the irradiation has finished because a point light source is used. Changes in the shape and propagation rate of the plume can be visualized by observing the plumes produced by these two methods under focus and defocus conditions. In the general shooting mode, images were taken at a frame rate of 4000 fps and a shutter speed of 1/950,000 s. In shooting using the Schlieren method, images were taken at a frame rate of 10,000 fps and a shutter speed of 1/100,000 s. The distance propagated by the plume was measured from the Schlieren method shooting in 0.1 ms increments during the first 2 ms of irradiation and in 0.5 ms increments throughout the remaining irradiation, and the plume propagation speed was determined for each condition.

3 Results and Discussion

3.1 Defocus-Induced Nanoparticle/Nanowire Transition

Figure 4 shows the SEM images of the deposits under various conditions. Under the on-focus condition (Fig. 4a, b), almost all the products were nanoparticles. When the laser was defocused, nanowires were generated along with nanoparticles, and the number of nanowires increased with the defocus distance (Fig. 4c–f). The products included nanostructures that are too short to be called nanowires. However, in this study, we referred to this kind of nanostructures as nanowires to distinguish them from nanoparticles.

Fig. 4
figure 4

SEM images of nanostructures under defocused distances of a, b 0, c 15, d 20, e 24, and f 28 mm

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Figure 5 shows a TEM image of the nanowires at low magnification. Three types of nanowires were identified: nanowires with spherical or polygonal caps at the tip and nanowires without caps. Figure 6 shows a graph of nanowire diameters and lengths for each of the conditions under which nanowires were produced. The diameter of the nanowires (Fig. 6a) did not show a clear correlation with the defocus distance; but their length (Fig. 6b) showed a tendency to increase with the defocus distance. The length of the nanowires did not change much between the defocus distances of 20 and 24 mm. A possible reason is the nonuniformity of the target. The target used in this study is composed of powders of two different elements, each with a different grain size. After mixing, the material might have exhibited a nonuniform distribution on the target surface, resulting in small differences in the length of the nanowires produced. The frequencies of the nanowire aspect ratios (length/diameter) are shown in Fig. 7. The highest number of nanowires with an aspect ratio of < 5 were observed at a defocus distance of 15 mm, the highest number of nanowires with an aspect ratio between 5 and 10 were observed at defocus distances of 20 and 24 mm, and several nanowires with an aspect ratio exceeding 20 were observed at a defocus distance of 28 mm. The longer the defocus distance, the higher the aspect ratio of the nanowires produced.

Fig. 5
figure 5

The transmission electron microscopy (TEM) images of nanowires having a spherical caps, b polygonal caps, and c no caps under 28 mm defocused distance

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Fig. 6
figure 6

Change in nanowire: a diameters and b lengths with defocusing

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Fig. 7
figure 7

Frequencies figure of the nanowire aspect ratios under various defocused distances: a 15, b 20, c 24, and d 28 mm

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3.2 Material Composition/Structure Analysis

High-magnification TEM images of the nanoparticles are shown in Fig. 8. At the on-focus position, almost all nanoparticles were surrounded by graphene layers. This finding is consistent with the results of a previous study on the formation of core–shell particles [22]. The deposition of graphene layers under defocused conditions was also confirmed. Multilayer graphene deposition was also observed in the nanowires at high magnification, as shown in Fig. 9. On the basis of these results, a core–shell structure with graphene layers deposited on the nanoparticles and nanowires was generated under all conditions. The energy-dispersive X-ray spectroscopy (EDX) mapping images shown in Fig. 10 confirm that the nanowires were mainly composed of Si and C elements. Nanowires with caps (Fig. 10a, b) also contained Si and C elements in their wire and cap parts.

Fig. 8
figure 8

TEM images of graphene layers on the nanoparticles under defocused distance of a 0, b 15, c, d 20, e 24, and f 28 mm

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Fig. 9
figure 9

TEM images of graphene layers on the nanowires under defocused distance of a 15, b 20, c 24, and d 28 mm

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Fig. 10
figure 10

The energy-dispersive X-ray spectroscopy (EDX) mapping analysis of nanowires which have a, b with caps and c no caps generated under a defocused distance of 28 mm

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The electron diffraction patterns (Fig. 11) under all conditions were evaluated by TEM. Given that 3C-SiC and 6H-SiC have similar crystal structures, they have many common electron diffraction patterns [25,26,27]. However, the pattern for 6H-SiC {101} and {103} did not overlap with that of 3C-SiC [26, 27]. These patterns were used as an indicator to confirm the presence of 6H-SiC. 6H-SiC {101} was confirmed to be present under 20 (Fig. 11c), 24 (Fig. 11d, e), and 28 mm (Fig. 11f) defocused conditions, but the number of bright spots was small in all conditions. No pattern indicating {103} was observed in any of the conditions; the patterns common to 3C-SiC were all strongly bright, suggesting that 3C-SiC and 6H-SiC are both present, but the content of 3C-SiC may be higher than that of 6H-SiC.

Fig. 11
figure 11

The selected area electron diffraction patterns from TEM images under defocused distance of a 0, b 15, c 20, d, e 24, and f 28 mm

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In some TEM images, bright spots indicating C [28] (Fig. 11c, e) and Si [29] (Fig. 11e) were observed. The graphene layers being thinner than the nanoparticles and nanowires might have prevented the appearance of bright spots. In conclusion, the main product was multilayer graphene deposited 3C-SiC nanostructures with low contents of 6H-SiC and Si.

3.3 Material Volume Analysis

The targets after single-pulse irradiation were measured using a laser microscope. The cross-sectional profile of the holes created by these irradiations is shown in Fig. 12, and the relationship between the volume of the removed portion and defocusing is displayed in Fig. 13. The hole diameter increased with defocusing distance, but the hole depth became shallow. According to the 3D morphology measurement, the amount of removed material reached a maximum at a defocus of 15 mm and decreased with a further increase in defocused distance. In laser irradiation, the hole in the irradiated target becomes deeper and smaller in diameter as the energy level increases [30] because the amount of energy per unit area decreases with defocusing, as shown in Fig. 3c, d. This result showed that the generated plume volume increased due to the increase in energy per unit area. Photographs showing the holes caused by these irradiations are included in Fig. S2 in the supplementary information.

Fig. 12
figure 12

Cross-sectional profile of the holes created by laser irradiation

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Fig. 13
figure 13

Change in the volume of the removed portion with defocusing

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3.4 Plume Shape Comparison

Figure 14 shows images of the plume from the start of laser irradiation to 4.5 ms after the irradiation taken in the general shooting mode. The plume was generated immediately from the target irradiated at the focus position. When the target was moved in the defocus direction, additional time was required between irradiation and plume generation. This phenomenon occurred because the power density decreased due to defocusing, and the target seemed to require a few milliseconds to be sufficiently heated. Figure 15 shows images of the plume from the start of laser irradiation to the end of laser irradiation taken in the general shooting mode. In Fig. 15a–c, the plume extended beyond the screen. However, in Fig. 15d, e, the plume was contained within the area photographed, indicating that the length of the plume in the direction of propagation shortened as the laser defocused. Research using nanosecond pulsed lasers showed that the higher the laser energy, the larger the plume extends in the direction of the laser source [31]. The same phenomenon can also be observed with millisecond pulses.

Fig. 14
figure 14

High-speed camera images of plume propagation from the start of laser irradiation to 4.5 ms after the irradiation starts under defocused distance of a 0, b 15, c 20, d 24, and e 28 mm

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Fig. 15
figure 15

High-speed camera images of plume propagation from the start of laser irradiation to the end of the irradiation under defocused distance of a 0 mm, b 15 mm, c 20 mm, d 24 mm, and e 28 mm

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In addition to its length, the shape of the plume differs under focused and defocused conditions. An enlarged view of the plume in the focus condition and the plume in the 28 mm defocus distance is shown in Fig. 16. In the focused state, the plume moved in one direction without swirling. In the defocused state, the plume propagated in a swirling manner. For clarifying the behavior of the plume, the Schlieren image of the plume after the irradiation is shown in Fig. S3 in the supplementary information. At 24 and 28 mm defocus distances, the plume propagated slowly even after the irradiation was finished. The position of the plume progression was examined at 0.1 ms intervals by feeding the Schlieren image frame by frame, and the results are shown in Fig. 17. The approximate curves of the plot showed that the plume propagation speed for each condition was 12.85 mm/ms for the on-focus condition, 6.85 mm/ms for the 15 mm defocused condition, 3.14 mm/ms for the 20 mm defocused condition, 1.34 mm/ms for the 24 mm defocused condition, and 0.68 mm/ms in the 28 mm defocused condition. The growth rate of the plume was relatively slow in the defocused state.

Fig. 16
figure 16

Enlarged view of the plume under defocus distance of a 0 and b 28 mm

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Fig. 17
figure 17

Differences in plume diffusion distance per unit time due to laser defocusing

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3.5 Hypothesis of Nanowire Growth Mechanism

SiC@C core–shell nanoparticles and nanowires were obtained as final products, and their 3D microstructures are shown schematically in Fig. 18. SiC cores have excellent battery storage and electrolytic discharge properties, and graphene shells provide dielectric and conductive properties [8, 13]. In this section, the formation mechanism for these products was discussed. If the mechanism of composite material formation by laser can be clarified, then it could be applied to the formation of other composite materials. Composite materials, as typified by core–shell structures, are expected to be applied in a wide range of fields, such as power electronics [32, 33].

Fig. 18
figure 18

Schematic of Graphene-coated silicon carbide (SiC@C core–shell) nanoparticles and nanowires

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Laser irradiation of a solid target generates a plume [24]. The plume mainly consists of plasma and sublimated vapor [25], and nanodroplets are mixed in when millisecond pulses are irradiated [26, 27]. The plume is cooled down by collision with the ambient gas, resulting in nanoparticles or nanowires. The temperature and substance distributions inside the plume are complex, and the chemical reactions have a random nature. For example, while the inner part of the plume is under high temperature, the outer part of the plume could come into contact with the ambient gas and is prone to rapid cooling. Rapid cooling of the plume promotes nanoparticle production. Methods of nanoparticle production include femtosecond pulsed laser ablation [20] and nanosecond pulsed laser ablation [21] in water. Rapid cooling is an important factor in nanoparticle production; for example, the use of a short pulse width generates nanoparticles because of rapid cooling due to the low plume volume, and rapid cooling in water results in nanoparticles. Under the on-focus condition with millisecond pulsed lasers in this study, rapid cooling was facilitated by fast plume diffusion. The characteristic feature of a millisecond laser plume is that melting and evaporation occur intensely, so the plume volume is higher than that of nanosecond and femtosecond pulsed lasers, and the plume contains abundant droplets. Thus, the plume contains a mixture of droplets and gases, and the produced nanowires have different shapes depending on which state they originated from. Therefore, many types of products with different shapes and compositions are produced simultaneously, such as amorphous and crystalline nanoparticles and tipped and untipped nanowires. Some factors affect the morphology and the formation mechanism of the nanostructures produced by laser irradiation. Figure 19 shows a schematic of the possible nanowire formation mechanisms. The mechanisms of nanowire formation in this study can be classified into two categories: vapor–liquid–solid (VLS) growth (Fig. 19a) and vapor–solid (VS) growth (Fig. 19b).

Fig. 19
figure 19

Schematic of the hypothesized mechanism of SiC nanowire formation: a vapor–liquid–solid growth; b noncatalyst vapor–solid growth

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In VLS growth, a metal catalyst is usually used to form supersaturated droplets by supplying raw gas to the metal droplets from which nanowires are grown [34,35,36]. In this study, Si droplets took this position, although no metal catalyst was used. Si has droplet and gaseous plumes, and C has only a gaseous plume [22]. The formation of self-catalytic Si nanowires by Si droplets [37] and SiC nanoparticles through the reaction between Si droplets and carbon species has already been reported [38]. As shown in Fig. 19a, the diffusion of a large number of carbon species into the Si droplet produces Si–C particles, which have instability due to further continuous feeding and consequently form nanowires [39]. The nanowires produced by this mechanism are characterized by a cap at the tip, a remnant of the droplet, as shown in Fig. 5a, b. The spherical cap (Fig. 5a) is amorphous SiC, and the polygonal cap (Fig. 5b) is a shape indicative of crystalline SiC [39].

By contrast, nanowires synthesized by the VS method [10, 40, 41] are noncatalytically produced and characterized by the absence of a cap at the tip [42]. In this study, oxygen removal was not carried out for the starting materials, namely the Si grinding sludge and graphite powder. Hence, the surfaces are considered to be naturally oxidized, and the following reactions may happen [10]:

$${\text{2Si}}\left( {\text{g}} \right) + {\text{O}}_{{2}} \left( {\text{g}} \right) = {\text{2SiO}}\left( {\text{g}} \right)$$
(5)
$${\text{2C}}\left( {\text{g}} \right) + {\text{O}}_{{2}} \left( {\text{g}} \right) = {\text{2CO}}\left( {\text{g}} \right)$$
(6)
$${\text{2SiO}}\left( {\text{g}} \right) + {\text{2C}}\left( {\text{g}} \right) = {\text{2SiC}}\left( {\text{s}} \right) + {\text{O}}_{{2}} \left( {\text{g}} \right)$$
(7)
$${\text{SiO}}\left( {\text{g}} \right) + {\text{3CO}}\left( {\text{g}} \right) = {\text{SiC}}\left( {\text{s}} \right) + {\text{2CO}}_{{2}} \left( {\text{g}} \right)$$
(8)

When reactions (7) and (8) are repeated, nanowires are produced from the Si and C gases, as shown in Fig. 19b. In VS and self-catalytic VLS production methods, a continuous supply of raw material from the gas phase [41] and sufficient time for growth are essential for nanowire production.

Figure 20 shows a schematic of the defocusing-induced change in plume shapes, which is the cause of the transition of nanomaterial type from nanoparticles to nanowires. The hypothesized mechanism in Fig. 20 generally agreed with the phenomenon observed in Figs. 15 and 16. Under the on-focus condition, the plume moved in a single direction (Fig. 20a); thus, a few collisions occurred among the atom clusters and nanodroplets. By contrast, under the defocused conditions, the source material was supplied from various directions due to the swirling movement, resulting in intense collisions of atom clusters and nanodroplets in the plume (Fig. 20b).

Fig. 20
figure 20

Schematic of the plume structure under a on-focus condition and b defocused condition

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The velocity of the plume movement slowed down when the laser was defocused, suggesting that the hot plume travels a short distance through the ambient gas at room temperature, and cooling occurs slowly. Fast cooling and material feeding from only one direction could immediately transform nanodroplets into nanoparticles. By contrast, slow cooling and multidirectional material feeding could saturate the nanodroplets and allow sufficient time for the growth of nanowires or the formation of solid-phase nanowires from the gas phase. In addition to laser irradiation, another well-known method for producing SiC nanowires is to use a furnace kept at around 1400–1600 °C [43, 44]. SiC nanowires grow long when a high temperature is maintained for a long period of time. Under the defocusing condition, the nanowires were considered to have grown because the time during which this temperature range could be maintained was longer than that under the on-focusing condition. Previous studies have also shown that nanowires further grow when the pressure is increased from 13 to 101 kPa [19]. The formation of nanowires in this study indicated that the pressure inside the plume was expected to be increased to a similar or higher level compared with that during nanoparticle formation.

Regarding the mechanism of graphene-layer formation on the surfaces of nanoparticles and nanowires, the layers were coated after the SiC nanoparticles and nanowires were formed. The larger amount of graphite powder in the starting material than Si enabled carbon species to be present in the plume. As a result, a surplus of carbon species was available after the formation of SiC. C plumes have a longer residence time than Si plumes, and this time difference is suitable for C coating [22]. Graphene-layer formation on SiC substrates has also been investigated [45,46,47], and SiC is considered to have properties that facilitate the deposition of graphene layers on its surface.

4 Conclusions

Multilayer graphene-coated SiC nanowires were successfully produced through the laser irradiation of a mixed target of Si grinding sludge and graphite powder by using a suitable defocus distance. Conclusions are presented below.

  1. 1.

    The produced nanostructures are changed from nanoparticles to nanowires by shifting the target position and defocusing the laser beam.

  2. 2.

    The produced nanoparticles and nanowires have a core–shell structure covered by graphene layers, namely SiC@C core–shell nanoparticles and nanowires.

  3. 3.

    The nanowire growth involves two possible mechanisms: VLS and VS, resulting in nanowires with/without tip caps, respectively.

  4. 4.

    The plume generated under on-focus conditions propagates in a single direction at high speed, and that generated under defocus conditions swirls as a vortex moving at a low speed. Under such conditions, the slow cooling speed and multidirectional material supply promote nanowire growth.

  5. 5.

    The graphene layers are deposited by C gas diffusion after the SiC nanostructures are formed.

The produced nanoparticles and nanowires are expected to be used as anode materials for lithium-ion batteries. Nanowires are preferable because of their high strength and flexibility with respect to anode deformation. Nanowires are further expected to be used as materials for bioelectrodes and field emission displays. In this study, the nanowires were grown under noncatalytic conditions. Even for nanowires with chips, the chip part is already capable of self-catalysis without introducing another catalytic material, such as metal. Therefore, they can be used in a wide range of applications where metal contamination causes problems. The quantitative measurement of productivity is important when considering the industrial applicability of the proposed method. We will focus on this topic in our future work.

The results of this research broaden the reuse application of waste Si sludge and demonstrate the possibility of generating new Si/C composite nanomaterials with unique core–shell structures and compositing nanomaterials with other materials. The creation of these new materials from industrial waste is expected to contribute to sustainable manufacturing and industrial symbiosis for the realization of SDGs.