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Study of the growth of boron nanowires synthesized by laser ablation
Chemical Physics Letters 385 (2004) 177–183 www.elsevier.com/locate/cplett
Study of the growth of boron nanowires synthesized by laser ablation Yingjiu Zhang a,*, Hiroki Ago a, Motoo Yumura a,*, Satoshi Ohshima a, Kunio Uchida a, Toshiki Komatsu a, Sumio Iijima a,b,c a
Research Center for Advanced Carbon Materials, Central 5, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba 305-8565, Japan b NEC Corporation, Miyukigaoka, Tsukuba 305-8501, Japan c Faculty of Science and Technology, Meijo University, Terpaku-ku, Nagoya 468-8502, Japan Received 14 September 2003; in final form 1 December 2003 Published online: 14 January 2004
Abstract Boron nanowires with different morphologies and diameters have been fabricated by laser ablation. The effects of the synthesis temperatures, intensities of the laser beams, the types of the metal catalysts and the addition of H3 BO3 on the growth of the boron nanowires are studied. The systematic analyses show that the vapor–liquid–solid (VLS) model may play important roles in the growth of the boron nanowires. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Stimulated by the discovery of carbon nanotubes [1], nanomaterials, especially the one-dimensional nanomaterials have attracted much interest because of their potential fundamental characteristics and practical applications in areas such as materials science, physics, chemistry and industry [2,3]. As one of the most varied structural chemistries of all the elements in the periodic table, boron shows high melting temperature, low density, as well as hardness close to that of diamond and be proven as one of suitable elements to produce high temperature devices, light-weight protective armor and some products used in nuclear industry [4–6]. Although boron nanotubes have not been fabricated by now, theoretical calculation shows that the boron nanotubes may have some interesting properties [7,8], for e.g., may be more stable and have higher metallic conductivity compared with carbon nanotubes [7] while experiment shows that crystalline boron nanowires are semi-conductor and exhibit electrical properties consistent with those of elemental boron [9]. Additionally, boron
nanowires can be used as a template to synthesize MgB2 nanowires [10]. At beginning, only amorphous boron manowires are synthesized by vapor-transport method and magnetron sputtering [10,11]. Very recently, crystalline boron nanowires are synthesized by the chemical vapor deposition (CVD) method [9], the laser ablation [12,13] and the heating amorphous boron nanowires [14]. However, the synthesis parameters are not studied systemically in most of these works. In the present work, the single crystalline boron nanowires are fabricated on the surfaces of the targets by laser ablation using B/NiCo (or B/H3 BO3 /NiCo) target. Because the single-walled carbon nanotubes (SWNTs) and other kinds of nanowires are usually found at the downstream other than on the surface of the target [15–17], the growth of the products in the present work may not totally similar to that of other laser ablation works. The growth of the boron nanowires in different experimental parameters is studied systemically and its growth mechanism is discussed preliminary.
2. Experimental *
Corresponding authors. E-mail addresses: [email protected] (Y. Zhang), m.yumura@ aist.go.jp (M. Yumura). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.12.052
The growth system was similar to the one used in the work of synthesis of single-wall nanotubes [18]. A
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Y. Zhang et al. / Chemical Physics Letters 385 (2004) 177–183
pulsed, frequency-doubled Nd:Yttrium-aluminum-garnet (YAG) laser (wavelength, 532 nm; repetition, 10 Hz) was used to ablate the targets consisting of boron (and H3 BO3 ) and metals (listed in Table 1). Two quartz tubes, one with an outer diameter of about 55 mm and a length of 1200 mm was introduced as the chamber; another one was located at the center of the chamber with 25 mm in diameter and 800 mm in length. The diameter of the target was 16 mm and placed at the center of the inner tube. Argon gas flowed through the chamber at a rate of 80 sccm (standard cubic centimeter per minute) and was kept at a pressure of about 400 Torr. The laser beam power was about 3.5 W and the distance between the lens and the target was 860 mm. By changing the focal distance of the lens, the spot sizes of the beam irradiating the target changed and thus the intensities of the laser beam changed. Besides the intensity of laser beam, the effects of the other synthesis parameters, including synthesis temperature, compositions of the targets, are listed in Table 1. In order to control the diameters of the boron nanowires, the diametercontrolled catalytic particles were introduced in one experiment. The morphology and atomic structure of the product were characterized by using a scanning electron microscopy (SEM, S800) and a filed-emission gun transmission electron microscopy (TEM, JEM2000FX). The TEM was equipped with an energy dispersive spectrometer (EDS) and an electron energy loss spectrometer (EELS).
deposition is the amorphous ball-liked particles with most diameters less than 100 nm. In some of the particles, there are darker parts. There are Ni and Co elements in these darker parts while the amorphous parts are generally pure boron from EDS analysis. On the other hand, the one-dimensional products are present on the surfaces of the targets, which is different with most of the nanowires/nanotubes obtained by the laser ablation, because the later usually be found at the downstream end of the chamber. On the suitable synthesis parameters (Exp. 1), there are nanowires/fibers grown on the surface of target and generally, the sizes of these one-dimensional structures base on the distances from the pore hit by the laser beam. On the surface of the pore, no one-dimensional structure is found. At the periphery of the pore, needle-shaped rods as well as nanowires are found as shown in Fig. 1b. The diameter of the rods is about several hundreds nanometers and the aspect ratio is less than (or about) 10. At one end of these rods, there is a droplet whose diameter is lager than (similar to) that of the related rods. A few of nanowires are also observed in this area and the diameters of these nanowires are less than 100 nm. More nanowires are found at the farther position. In the area between the edge of the target and the pore made by the laser beam, a number of nanowires but no needleshaped rods are observed. These nanowires have diameters less than 100 nm and the aspect ratio is lager than 10 (Fig. 1c). 3.2. The structures and compositions of the products
3. Results and discussion 3.1. Morphologies of the products At the downstream, there is a great deal of gray deposition. SEM and TEM show (not shown here) that the
In Fig. 2a, the transmission electron microscopy (TEM) image of the nanowires is indicated. Some nanowires are found to have a droplet at one end, similar to needle-shaped rods. The typical EELS Spectra taken from the stem of the boron nanowire (Fig. 2b, c)
Table 1 The synthesis parameters and corresponding products No.
B (wt%)
Ni (wt%)
Co (wt%)
1
90
5
5
2 3 4 5 6 7 9 10 10 11 12 13a 14b
90 90 90 90 90 90 90 90 90 90 90 70 100
5 5 5 5 5 10 10 10 0 0 0 5 0
5 5 5 5 5 0 0 0 10 10 10 5 0
a b
W (W/mm2 )
T (°C)
Morphology
2
1250
5 0.7 0 2 2 2 2 2 2 2 2 2 2
1250 1250 1500 1200 1125 1250 1200 1125 1250 1200 1125 1250 1250
The diameters of the nanorods or nanowires base on distance from the pore formed by laser beam Mainly powders Mainly NWs Powders Low yield of NWs Few NWs Similar to Exp. 1 Similar to Exp. 5 Similar to Exp. 6 Fewer NWs than Exp. 1 Fewer NWs than Exp. 5 Very few NWs Straight NWs Boron nanowires with homogenous diameters
The left is H3 BO3 . The Co catalyst particles with controlled diameters are put near the target.
Y. Zhang et al. / Chemical Physics Letters 385 (2004) 177–183
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Fig. 1. SEM images of the products obtained at different positions on the surface of B/NiCo target (a) on the surface of the pore made by the laser beam, (b) at the periphery of the pore, (c) in the middle between the pore and the edge of the target.
show a clear peak corresponding to the K-shell ionization edge of the boron. No peaks are detected for possible impurities of Ni, Co and Carbon and thus, the stem of the boron nanowires is pure boron without impurities. However, EDS spectrum obtained from the droplet at one end of the nanowires shows clearly the peaks of B, Ni and Co (Fig. 2d). High-resolution electron microscopy (HREM) images provide more detail information about these boron nanowires. Fig. 3 is a HREM image of a typical boron nanowire with diameter less than 100 nm. HREM image shows that the nanowire is a single crystal but with some faults, such as strips of slightly disorder atoms. This imperfect properties of the single crystal is also revealed by the corresponding selected area electron diffraction (SAED) pattern inside Fig. 3, because some diffraction points in this SAED are divided into two (marked by arrow in SAED). However, the boron nanowire is not poly-crystalline and from SAED, its structure can be determined to be tetragonal one with lattice parameters a ¼ 0:875 nm, c ¼ 0:506 nm (X-ray diffraction card 74-0945) and its growth direction along [0 0 1]. After
Fig. 2. TEM analysis of the boron nanowires (a) TEM photo, (b, c) EELS spectra taken from the stem of one boron nanowire (d) EDS spectrum taken from the droplet at one end of the nanowire.
Fig. 3. HREM image and its corresponding SAED.
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analysis of a serials of SAED patterns and their related nanowires, it is found the main growth direction is along [0 0 1] one without many exceptions. 3.3. The effects of the catalysts and H3 BO3 Although the catalysts are not need in the works of synthesis of boron nanowires by magnetron sputtering [11] and other laser ablation method [13], no nanowires are found if no catalyst is used in the present work, even the synthesis temperature is as high as 1500 °C in Exp. 4. Several kinds of catalysts, including Ag, Co, Ni and NiCo, are tried to fabricate B nanowires. At any synthesis temperatures, no nanowires are found if using Ag catalyst. Both the yield and the morphology are not much different when Ni or NiCo catalysts are introduced while the yield is lower when Co is chosen as catalyst (Exp. 7, 10 and 11). It is different with the work of synthesis of SWNTs by laser ablation, because in the later condition, multiple catalysts usually result in a much high yield compared with only one kind of catalyst present [19]. Moreover, if Co catalyst particles with controlled diameters [20] are deposited on Al2 O3 particles which are dispersed on a substrate very near to the pure boron target, it is found that on the substrate, there are great number of the straight boron nanowires with about homogenous diameters appeared (Fig. 4, Exp. 14). This work is similar to the work of diameter-selected growth of GaP nanowires by means of diameter controlled catalytic particles [21]. Statistic results show that the distribution of the boron diameters is: smaller than 40 nm or larger than 70 nm, 10%, respectively; 40–50, 50– 60 and 60–70 nm, 35%, 28% and 17%, respectively. Compared to the results of the Exp. 1 (Fig. 1), this method can control the diameters of the boron nanowires in a certain degree. The morphology of the B nanowires is better when the B/H3 BO3 /NiCo target (in Exp. 13) is introduced.
Shown from Fig. 5a, many a brush of nanowires is covered on the surface of the target with higher yield. These brushes range from several microns to about 20 lm in size and looked like flower-shape in lower magnification while in the higher magnification, in fact these nanowires are disorderly distributed in the brushes, not grown form one growth center, which is different from the Ga2 O3 nanowires in Sharma et al. [22] work (Fig. 5b). Compared with Fig. 1c, these nanowires are much straight with somewhat homogenous diameters. Additionally, just at the upstream of the target, some nanowires with some big particles are also found while nearly no nanowires deposit on the corresponding position in other experiments. This means that the addition of H3 BO3 can change the morphology and increase the yield of boron nanowires. 3.4. The effects of the synthesis temperatures and the intensity of the laser beam At 1125 °C, only a few of nanowires are grown no matter what kinds of catalysts are introduced. At 1200 °C, nanowires can be easily found with lower yield compared with that at 1250 °C (Exp. 1, 5, 6). According to the VLS growth mechanism, the nanowires are grown form the droplets composing of catalysts and elements of nanowires (host elements). The formation of the solution of host elements and catalytic elements is necessary. The eutectic point of B/Ni and B/Co are 1110 °C, 1093 °C, respectively [23]. Thus, in the present work, the synthesis temperatures are usually needed higher than the eutectic points, which is contrary to some works to synthesize other kinds of nanowires/nanotubes [24]. It seems that the intensity of the laser beam directly affects the morphologies of the products. At the higher intensity (Exp. 2), few nanowires are found. At suitable intensity, such as in Exp. 1, the morphologies of the products are different according to the positions of the products as mentioned above. If the intensities are
Fig. 4. SEM images of the nanowires with homogenous diameters when diameter-selected Co particles are used (a) at lower magnification, (b) at higher magnification.
Y. Zhang et al. / Chemical Physics Letters 385 (2004) 177–183
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Fig. 5. SEM images of the boron nanowires when H3 BO3 is introduced (a) at lower magnification, many a brush is found on the surface of the target, (b) at higher magnification.
smaller in a certain range (Exp. 3), all the surfaces (except the surface of the pore made by the laser ablation) are nanowires. 3.5. The growth mechanisms of the nanowires First of all, it should be noted that the laser ablation is necessary, otherwise no nanowires will be found as Exp. 4 even the synthesis temperature is as high as 1500 °C. The laser beam provides the enough B and NiCo (or Ni, Co) atoms clusters, this process may be somewhat like the supply of gas source in common CVD method. Otherwise, the diffusion between B and metal catalyst at high temperature will not so intensive that can lead to the supersaturation of B from the B/NiCo system. VLS growth mechanism clearly plays an important role in the growth of boron nanowires in the present work. Firstly, the droplet composing of catalysts and host element (B) is present in one end of the nanowires and the size of the catalyst affects the diameter of the nanowires (Fig. 4), which is the property of the VLS growth mechanism. Second, without catalyst, no nanowires are found. Third, the kinds of the metal catalysts usually affect the growth of the nanowires. Here, Ni or NiCo have best catalytic function, Co, slightly worse while Ag has no catalytic effect. Although the reason why Ni and Co have slightly different function is unknown, the insolubility between Ag and B might give some explanations for this phenomenon. In fact, no phase diagram is available for the Ag–B system in the books such as Binary Alloy Phase Diagrams [23] because Ag and B are essentially mutually insoluble in both the solid and liquid states, even at as high as 1600 °C [25]. Without solubility, according to the VLS model, no nanowires will be grown. Although VLS model is usually introduced to explain the growth of nanowires (nanotubes) by laser ablation, this growth mechanism may direct the growth of boron
nanowires by a different way because we never obtain nanowires at the downstream even the weight of the deposition there is much higher compared with that of the deposition on the surface of target. Generally say, the growth of the SWNTs occurs from a liquid-like carbon–metal particles via supersaturation and segregation, and the main growth occurs in the process of the this kind particle be brought to the downstream by carrier gas [17]. No nanowires are obtained outside the target may indicate that the supply of both metal catalysts clusters and boron clusters can satisfy with the conditions for growth of nanowires through VLS only on the surface of the target. Although the reasons are unknown, if the boron target is thought as the growth substrate, even the common VLS mechanism can explain the phenomena in the present work. When the target is irradiated by the laser beam, some of the clusters of boron and metals will be ablated from the target and in the meantime, the temperature of the target will be increased due to acceptation of the energy from laser beam. The temperature must distribute according to the distance from the pore the laser beam hit. Generally, the longer distance, the lower temperature. In a certain range, the lower synthesis temperature will lead to a thinner diameter of the nanowire [24,26–28]. Therefore, at the periphery of the pore formed by laser ablation, the boron nanowires have larger diameters while the nanowires grown in the area between the edge of the target and the pore usually have thinner diameters. On the surface of the pore, due to the much high temperature and continued ablation, it is difficult to have suitable atmosphere for the growth of boron nanowires. Because both clusters (which provide the sources of growth) and big particles may be produced in the process of ablation, the density of the boron and metal clusters may change according to the intensity of the laser beam. At very high intensity, too many big particles are formed and the density of the atoms cluster
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may be drastically decreased as in Exp. 2, which may not provide enough boron source to grow nanowires and thus no nanowires are obtained. At suitable intensity, such as in Exp. 3, the density of element clusters may benefit the growth of the boron nanowires and the temperature of the whole target is not too high that benefit to grow thinner nanowires. The condition may similar to the growth of thinner nanowires in the middle between the pore and edge of the target in Exp. 1, thus all of the surface of target are covered by the nanowires. Besides the VLS model, oxygen-assisted model [29] or vapor–cluster–solid (VCS) growth mechanism [30] may also play some roles in the growth of boron nanowires when H3 BO3 is used as a reaction. At high temperature, 2H3 BO3 ðsÞ ! 2HBO2 ðsÞ þ 2H2 OðgÞ ! B2 O3 ðsÞ þ 3H2 OðgÞ In this decomposition process, some cavities will form in the target. Then B and B2 O3 will react with each other and form BOx . The new formed BOx vapor decomposes again and B nanowires may be grown in the new deposition process. BOx works as an oxygen source, may improve the growth of boron nanowires, which is similar to the growth of Si nanowires through oxygen-assisted mode [29] and amorphous boron nanowires by VCS model [30]. Additionally, the cavities formed due to the decomposition of H3 BO3 may increase the degree of the porosity in the target, which may also increase the yield of boron nanowires. It is consist with the work to increase the yield of SWNTs by mean of addition of easier decomposing reactants [31]. However, besides the reason why nearly no boron nanowires are found in the downstream as motioned above, there are still items related to the growth mechanism unknown. For e.g., (1) the reason why H3 BO3 can improve the morphology of boron nanowires; (2) when the composition percentages of Ni and Co in the target are very high, why boron plates (several 10 lm in width or length and about 100 nm in thickness, not shown here) other than boron nanowires are fabricated. These phenomena are interesting and should be further studied.
4. Conclusions Boron nanowires with diameter ranging from several tens nm to 1 lm have been synthesized by the laser ablation method on the surfaces of the targets. The VLS may play important roles in the growth of these boron nanowires while the synthesis temperatures, intensities of the laser beams, the types of the metal catalysts are the main synthesis effect factors on the growth. The phenomenon that the nanowires grow on the surface other than the downstream may help the understanding of the growth of the nanowires by laser ablation. The
controlling of the growth will benefit the practical application of the boron nanowires with different morphologies.
Acknowledgements This work is supported by Research Fund for Young Scientists from Ministry of Education, Culture, Sport, Science and Technology, Japan. We thank Dr. Du Xiwen in NIMS of Japan for his help on TEM analysis.
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Study of the growth of boron nanowires synthesized by laser ablation Yingjiu Zhang a,*, Hiroki Ago a, Motoo Yumura a,*, Satoshi Ohshima a, Kunio Uchida a, Toshiki Komatsu a, Sumio Iijima a,b,c a
Research Center for Advanced Carbon Materials, Central 5, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba 305-8565, Japan b NEC Corporation, Miyukigaoka, Tsukuba 305-8501, Japan c Faculty of Science and Technology, Meijo University, Terpaku-ku, Nagoya 468-8502, Japan Received 14 September 2003; in final form 1 December 2003 Published online: 14 January 2004
Abstract Boron nanowires with different morphologies and diameters have been fabricated by laser ablation. The effects of the synthesis temperatures, intensities of the laser beams, the types of the metal catalysts and the addition of H3 BO3 on the growth of the boron nanowires are studied. The systematic analyses show that the vapor–liquid–solid (VLS) model may play important roles in the growth of the boron nanowires. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Stimulated by the discovery of carbon nanotubes [1], nanomaterials, especially the one-dimensional nanomaterials have attracted much interest because of their potential fundamental characteristics and practical applications in areas such as materials science, physics, chemistry and industry [2,3]. As one of the most varied structural chemistries of all the elements in the periodic table, boron shows high melting temperature, low density, as well as hardness close to that of diamond and be proven as one of suitable elements to produce high temperature devices, light-weight protective armor and some products used in nuclear industry [4–6]. Although boron nanotubes have not been fabricated by now, theoretical calculation shows that the boron nanotubes may have some interesting properties [7,8], for e.g., may be more stable and have higher metallic conductivity compared with carbon nanotubes [7] while experiment shows that crystalline boron nanowires are semi-conductor and exhibit electrical properties consistent with those of elemental boron [9]. Additionally, boron
nanowires can be used as a template to synthesize MgB2 nanowires [10]. At beginning, only amorphous boron manowires are synthesized by vapor-transport method and magnetron sputtering [10,11]. Very recently, crystalline boron nanowires are synthesized by the chemical vapor deposition (CVD) method [9], the laser ablation [12,13] and the heating amorphous boron nanowires [14]. However, the synthesis parameters are not studied systemically in most of these works. In the present work, the single crystalline boron nanowires are fabricated on the surfaces of the targets by laser ablation using B/NiCo (or B/H3 BO3 /NiCo) target. Because the single-walled carbon nanotubes (SWNTs) and other kinds of nanowires are usually found at the downstream other than on the surface of the target [15–17], the growth of the products in the present work may not totally similar to that of other laser ablation works. The growth of the boron nanowires in different experimental parameters is studied systemically and its growth mechanism is discussed preliminary.
2. Experimental *
Corresponding authors. E-mail addresses: [email protected] (Y. Zhang), m.yumura@ aist.go.jp (M. Yumura). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.12.052
The growth system was similar to the one used in the work of synthesis of single-wall nanotubes [18]. A
178
Y. Zhang et al. / Chemical Physics Letters 385 (2004) 177–183
pulsed, frequency-doubled Nd:Yttrium-aluminum-garnet (YAG) laser (wavelength, 532 nm; repetition, 10 Hz) was used to ablate the targets consisting of boron (and H3 BO3 ) and metals (listed in Table 1). Two quartz tubes, one with an outer diameter of about 55 mm and a length of 1200 mm was introduced as the chamber; another one was located at the center of the chamber with 25 mm in diameter and 800 mm in length. The diameter of the target was 16 mm and placed at the center of the inner tube. Argon gas flowed through the chamber at a rate of 80 sccm (standard cubic centimeter per minute) and was kept at a pressure of about 400 Torr. The laser beam power was about 3.5 W and the distance between the lens and the target was 860 mm. By changing the focal distance of the lens, the spot sizes of the beam irradiating the target changed and thus the intensities of the laser beam changed. Besides the intensity of laser beam, the effects of the other synthesis parameters, including synthesis temperature, compositions of the targets, are listed in Table 1. In order to control the diameters of the boron nanowires, the diametercontrolled catalytic particles were introduced in one experiment. The morphology and atomic structure of the product were characterized by using a scanning electron microscopy (SEM, S800) and a filed-emission gun transmission electron microscopy (TEM, JEM2000FX). The TEM was equipped with an energy dispersive spectrometer (EDS) and an electron energy loss spectrometer (EELS).
deposition is the amorphous ball-liked particles with most diameters less than 100 nm. In some of the particles, there are darker parts. There are Ni and Co elements in these darker parts while the amorphous parts are generally pure boron from EDS analysis. On the other hand, the one-dimensional products are present on the surfaces of the targets, which is different with most of the nanowires/nanotubes obtained by the laser ablation, because the later usually be found at the downstream end of the chamber. On the suitable synthesis parameters (Exp. 1), there are nanowires/fibers grown on the surface of target and generally, the sizes of these one-dimensional structures base on the distances from the pore hit by the laser beam. On the surface of the pore, no one-dimensional structure is found. At the periphery of the pore, needle-shaped rods as well as nanowires are found as shown in Fig. 1b. The diameter of the rods is about several hundreds nanometers and the aspect ratio is less than (or about) 10. At one end of these rods, there is a droplet whose diameter is lager than (similar to) that of the related rods. A few of nanowires are also observed in this area and the diameters of these nanowires are less than 100 nm. More nanowires are found at the farther position. In the area between the edge of the target and the pore made by the laser beam, a number of nanowires but no needleshaped rods are observed. These nanowires have diameters less than 100 nm and the aspect ratio is lager than 10 (Fig. 1c). 3.2. The structures and compositions of the products
3. Results and discussion 3.1. Morphologies of the products At the downstream, there is a great deal of gray deposition. SEM and TEM show (not shown here) that the
In Fig. 2a, the transmission electron microscopy (TEM) image of the nanowires is indicated. Some nanowires are found to have a droplet at one end, similar to needle-shaped rods. The typical EELS Spectra taken from the stem of the boron nanowire (Fig. 2b, c)
Table 1 The synthesis parameters and corresponding products No.
B (wt%)
Ni (wt%)
Co (wt%)
1
90
5
5
2 3 4 5 6 7 9 10 10 11 12 13a 14b
90 90 90 90 90 90 90 90 90 90 90 70 100
5 5 5 5 5 10 10 10 0 0 0 5 0
5 5 5 5 5 0 0 0 10 10 10 5 0
a b
W (W/mm2 )
T (°C)
Morphology
2
1250
5 0.7 0 2 2 2 2 2 2 2 2 2 2
1250 1250 1500 1200 1125 1250 1200 1125 1250 1200 1125 1250 1250
The diameters of the nanorods or nanowires base on distance from the pore formed by laser beam Mainly powders Mainly NWs Powders Low yield of NWs Few NWs Similar to Exp. 1 Similar to Exp. 5 Similar to Exp. 6 Fewer NWs than Exp. 1 Fewer NWs than Exp. 5 Very few NWs Straight NWs Boron nanowires with homogenous diameters
The left is H3 BO3 . The Co catalyst particles with controlled diameters are put near the target.
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Fig. 1. SEM images of the products obtained at different positions on the surface of B/NiCo target (a) on the surface of the pore made by the laser beam, (b) at the periphery of the pore, (c) in the middle between the pore and the edge of the target.
show a clear peak corresponding to the K-shell ionization edge of the boron. No peaks are detected for possible impurities of Ni, Co and Carbon and thus, the stem of the boron nanowires is pure boron without impurities. However, EDS spectrum obtained from the droplet at one end of the nanowires shows clearly the peaks of B, Ni and Co (Fig. 2d). High-resolution electron microscopy (HREM) images provide more detail information about these boron nanowires. Fig. 3 is a HREM image of a typical boron nanowire with diameter less than 100 nm. HREM image shows that the nanowire is a single crystal but with some faults, such as strips of slightly disorder atoms. This imperfect properties of the single crystal is also revealed by the corresponding selected area electron diffraction (SAED) pattern inside Fig. 3, because some diffraction points in this SAED are divided into two (marked by arrow in SAED). However, the boron nanowire is not poly-crystalline and from SAED, its structure can be determined to be tetragonal one with lattice parameters a ¼ 0:875 nm, c ¼ 0:506 nm (X-ray diffraction card 74-0945) and its growth direction along [0 0 1]. After
Fig. 2. TEM analysis of the boron nanowires (a) TEM photo, (b, c) EELS spectra taken from the stem of one boron nanowire (d) EDS spectrum taken from the droplet at one end of the nanowire.
Fig. 3. HREM image and its corresponding SAED.
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analysis of a serials of SAED patterns and their related nanowires, it is found the main growth direction is along [0 0 1] one without many exceptions. 3.3. The effects of the catalysts and H3 BO3 Although the catalysts are not need in the works of synthesis of boron nanowires by magnetron sputtering [11] and other laser ablation method [13], no nanowires are found if no catalyst is used in the present work, even the synthesis temperature is as high as 1500 °C in Exp. 4. Several kinds of catalysts, including Ag, Co, Ni and NiCo, are tried to fabricate B nanowires. At any synthesis temperatures, no nanowires are found if using Ag catalyst. Both the yield and the morphology are not much different when Ni or NiCo catalysts are introduced while the yield is lower when Co is chosen as catalyst (Exp. 7, 10 and 11). It is different with the work of synthesis of SWNTs by laser ablation, because in the later condition, multiple catalysts usually result in a much high yield compared with only one kind of catalyst present [19]. Moreover, if Co catalyst particles with controlled diameters [20] are deposited on Al2 O3 particles which are dispersed on a substrate very near to the pure boron target, it is found that on the substrate, there are great number of the straight boron nanowires with about homogenous diameters appeared (Fig. 4, Exp. 14). This work is similar to the work of diameter-selected growth of GaP nanowires by means of diameter controlled catalytic particles [21]. Statistic results show that the distribution of the boron diameters is: smaller than 40 nm or larger than 70 nm, 10%, respectively; 40–50, 50– 60 and 60–70 nm, 35%, 28% and 17%, respectively. Compared to the results of the Exp. 1 (Fig. 1), this method can control the diameters of the boron nanowires in a certain degree. The morphology of the B nanowires is better when the B/H3 BO3 /NiCo target (in Exp. 13) is introduced.
Shown from Fig. 5a, many a brush of nanowires is covered on the surface of the target with higher yield. These brushes range from several microns to about 20 lm in size and looked like flower-shape in lower magnification while in the higher magnification, in fact these nanowires are disorderly distributed in the brushes, not grown form one growth center, which is different from the Ga2 O3 nanowires in Sharma et al. [22] work (Fig. 5b). Compared with Fig. 1c, these nanowires are much straight with somewhat homogenous diameters. Additionally, just at the upstream of the target, some nanowires with some big particles are also found while nearly no nanowires deposit on the corresponding position in other experiments. This means that the addition of H3 BO3 can change the morphology and increase the yield of boron nanowires. 3.4. The effects of the synthesis temperatures and the intensity of the laser beam At 1125 °C, only a few of nanowires are grown no matter what kinds of catalysts are introduced. At 1200 °C, nanowires can be easily found with lower yield compared with that at 1250 °C (Exp. 1, 5, 6). According to the VLS growth mechanism, the nanowires are grown form the droplets composing of catalysts and elements of nanowires (host elements). The formation of the solution of host elements and catalytic elements is necessary. The eutectic point of B/Ni and B/Co are 1110 °C, 1093 °C, respectively [23]. Thus, in the present work, the synthesis temperatures are usually needed higher than the eutectic points, which is contrary to some works to synthesize other kinds of nanowires/nanotubes [24]. It seems that the intensity of the laser beam directly affects the morphologies of the products. At the higher intensity (Exp. 2), few nanowires are found. At suitable intensity, such as in Exp. 1, the morphologies of the products are different according to the positions of the products as mentioned above. If the intensities are
Fig. 4. SEM images of the nanowires with homogenous diameters when diameter-selected Co particles are used (a) at lower magnification, (b) at higher magnification.
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Fig. 5. SEM images of the boron nanowires when H3 BO3 is introduced (a) at lower magnification, many a brush is found on the surface of the target, (b) at higher magnification.
smaller in a certain range (Exp. 3), all the surfaces (except the surface of the pore made by the laser ablation) are nanowires. 3.5. The growth mechanisms of the nanowires First of all, it should be noted that the laser ablation is necessary, otherwise no nanowires will be found as Exp. 4 even the synthesis temperature is as high as 1500 °C. The laser beam provides the enough B and NiCo (or Ni, Co) atoms clusters, this process may be somewhat like the supply of gas source in common CVD method. Otherwise, the diffusion between B and metal catalyst at high temperature will not so intensive that can lead to the supersaturation of B from the B/NiCo system. VLS growth mechanism clearly plays an important role in the growth of boron nanowires in the present work. Firstly, the droplet composing of catalysts and host element (B) is present in one end of the nanowires and the size of the catalyst affects the diameter of the nanowires (Fig. 4), which is the property of the VLS growth mechanism. Second, without catalyst, no nanowires are found. Third, the kinds of the metal catalysts usually affect the growth of the nanowires. Here, Ni or NiCo have best catalytic function, Co, slightly worse while Ag has no catalytic effect. Although the reason why Ni and Co have slightly different function is unknown, the insolubility between Ag and B might give some explanations for this phenomenon. In fact, no phase diagram is available for the Ag–B system in the books such as Binary Alloy Phase Diagrams [23] because Ag and B are essentially mutually insoluble in both the solid and liquid states, even at as high as 1600 °C [25]. Without solubility, according to the VLS model, no nanowires will be grown. Although VLS model is usually introduced to explain the growth of nanowires (nanotubes) by laser ablation, this growth mechanism may direct the growth of boron
nanowires by a different way because we never obtain nanowires at the downstream even the weight of the deposition there is much higher compared with that of the deposition on the surface of target. Generally say, the growth of the SWNTs occurs from a liquid-like carbon–metal particles via supersaturation and segregation, and the main growth occurs in the process of the this kind particle be brought to the downstream by carrier gas [17]. No nanowires are obtained outside the target may indicate that the supply of both metal catalysts clusters and boron clusters can satisfy with the conditions for growth of nanowires through VLS only on the surface of the target. Although the reasons are unknown, if the boron target is thought as the growth substrate, even the common VLS mechanism can explain the phenomena in the present work. When the target is irradiated by the laser beam, some of the clusters of boron and metals will be ablated from the target and in the meantime, the temperature of the target will be increased due to acceptation of the energy from laser beam. The temperature must distribute according to the distance from the pore the laser beam hit. Generally, the longer distance, the lower temperature. In a certain range, the lower synthesis temperature will lead to a thinner diameter of the nanowire [24,26–28]. Therefore, at the periphery of the pore formed by laser ablation, the boron nanowires have larger diameters while the nanowires grown in the area between the edge of the target and the pore usually have thinner diameters. On the surface of the pore, due to the much high temperature and continued ablation, it is difficult to have suitable atmosphere for the growth of boron nanowires. Because both clusters (which provide the sources of growth) and big particles may be produced in the process of ablation, the density of the boron and metal clusters may change according to the intensity of the laser beam. At very high intensity, too many big particles are formed and the density of the atoms cluster
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may be drastically decreased as in Exp. 2, which may not provide enough boron source to grow nanowires and thus no nanowires are obtained. At suitable intensity, such as in Exp. 3, the density of element clusters may benefit the growth of the boron nanowires and the temperature of the whole target is not too high that benefit to grow thinner nanowires. The condition may similar to the growth of thinner nanowires in the middle between the pore and edge of the target in Exp. 1, thus all of the surface of target are covered by the nanowires. Besides the VLS model, oxygen-assisted model [29] or vapor–cluster–solid (VCS) growth mechanism [30] may also play some roles in the growth of boron nanowires when H3 BO3 is used as a reaction. At high temperature, 2H3 BO3 ðsÞ ! 2HBO2 ðsÞ þ 2H2 OðgÞ ! B2 O3 ðsÞ þ 3H2 OðgÞ In this decomposition process, some cavities will form in the target. Then B and B2 O3 will react with each other and form BOx . The new formed BOx vapor decomposes again and B nanowires may be grown in the new deposition process. BOx works as an oxygen source, may improve the growth of boron nanowires, which is similar to the growth of Si nanowires through oxygen-assisted mode [29] and amorphous boron nanowires by VCS model [30]. Additionally, the cavities formed due to the decomposition of H3 BO3 may increase the degree of the porosity in the target, which may also increase the yield of boron nanowires. It is consist with the work to increase the yield of SWNTs by mean of addition of easier decomposing reactants [31]. However, besides the reason why nearly no boron nanowires are found in the downstream as motioned above, there are still items related to the growth mechanism unknown. For e.g., (1) the reason why H3 BO3 can improve the morphology of boron nanowires; (2) when the composition percentages of Ni and Co in the target are very high, why boron plates (several 10 lm in width or length and about 100 nm in thickness, not shown here) other than boron nanowires are fabricated. These phenomena are interesting and should be further studied.
4. Conclusions Boron nanowires with diameter ranging from several tens nm to 1 lm have been synthesized by the laser ablation method on the surfaces of the targets. The VLS may play important roles in the growth of these boron nanowires while the synthesis temperatures, intensities of the laser beams, the types of the metal catalysts are the main synthesis effect factors on the growth. The phenomenon that the nanowires grow on the surface other than the downstream may help the understanding of the growth of the nanowires by laser ablation. The
controlling of the growth will benefit the practical application of the boron nanowires with different morphologies.
Acknowledgements This work is supported by Research Fund for Young Scientists from Ministry of Education, Culture, Sport, Science and Technology, Japan. We thank Dr. Du Xiwen in NIMS of Japan for his help on TEM analysis.
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