- Email: [email protected]
One-dimensional growth mechanism of amorphous boron nanowires
20 June 2002
Chemical Physics Letters 359 (2002) 273–277 www.elsevier.com/locate/cplett
One-dimensional growth mechanism of amorphous boron nanowires Y.Q. Wang
a,*
, X.F. Duan a, L.M. Cao b, W.K. Wang
b
a
b
Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, People’s Republic of China Center for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received 23 January 2002; in final form 23 April 2002
Abstract High-density of arrays of self-oriented boron nanowires grown on silicon substrates were synthesized by radiofrequency magnetron sputtering with a target of highly pure boron and boron oxide mixture using argon as the sputtering atmosphere. TEM studies show that the conventional growth mechanisms such as Frank screw-dislocation mechanism and the vapor–liquid–solid mechanisms are not suitable for the one-dimensional growth of boron nanowires. The oxide-assisted cluster–solid mechanism for the Si and Ge crystalline nanowires is not completely suitable for our case. The vapor–cluster–solid mechanism is proposed for the well-aligned growth of the amorphous boron nanowires. Ó 2002 Elsevier Science B.V. All rights reserved.
One-dimensional nanostructures are of great interest because of their potential fundamental and practical implications in areas such as materials science, chemistry, physics and engineering [1,2]. Since the discovery of carbon nanotubes by Iijima [3], extensive researches on the nanotubes and nanowires [4–9] have been carried out. More recently, we reported a successful synthesis of wellaligned straight amorphous boron nanowires [10].
*
Corresponding author. Present address: H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, BS8 1TL, Bristol, UK. Fax: +86-10-6256-1422. E-mail addresses: [email protected], [email protected] (Y.Q. Wang).
The growth mechanism of one-dimensional nanowires is one of the controversial subjects. Two growth mechanisms for crystalline fibers or whiskers have been proposed: the screw-dislocation mechanism [11] and the vapor–liquid–solid (VLS) mechanism [12]. In VLS model, a liquid agent or catalyst is affixed to the tip of the nanowires to promote the one-dimensional growth, such as metal elements (Au, Fe, Ni, etc.) used in the syntheses of Si and Ge nanowires. Recently, Zhang et al. [13,14] proposed an oxide-assisted cluster–solid mechanism to explain the growth of Si and Ge nanowires on the basis of their experimental observations. In this Letter, a vapor–cluster–solid (VCS) mechanism is proposed to explain the wellaligned growth of the amorphous boron nanowires.
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 7 3 2 - 7
274
Y.Q. Wang et al. / Chemical Physics Letters 359 (2002) 273–277
A radio-frequency (rf) magnetron sputtering was employed in this work to prepare the aligned boron-nanowire arrays films. Our simple procedure for processing is described as follows. A mixture of pure boron powder (purity 99.9%) and high pure B2 O3 powder (purity 99.999%) of 40% in weight was compacted into a disk with a diameter of 60 cm to be used as the target. Cleaned Si(1 0 0) wafers were used as substrates. The substrates were placed on a temperature-controllable heater that is parallel to the target surface. Before sputtering, the vacuum chamber was first pumped to a base pressure of better than 4 105 Pa, and high pure argon gas (purity 99.999%) was then introduced into the chamber at a fixed flow rate of 30 sccm (standard cubic centimeters per minute). For sputtering deposition growth, the Si(1 0 0) substrate was first heated to 800 °C in the argon atmosphere, and thereafter growth was performed under the rf power of 800 W with the total pressure kept constant at 2 Pa. After 6 h of sputtering deposition, the silicon substrate was covered with pitchblack films of boron nanowires. We peeled off some black deposit of boron nanowires from the silicon substrates, ultrasonicated the deposit in ethanol for several min, and placed a drop onto a holey carbon-coated copper grid. An S-4200 scanning electron microscope (SEM) was used for morphological observation of the boron nanowires. A Philips CM200-FEG TEM equipped with a GIF (Gatan Imaging Filter, model 678) was used for high-resolution transmission electron microscopy (HRTEM), elemental mapping, and electron energy-loss spectroscopy (EELS) studies. A three-window method is used to study elemental mapping of boron and oxygen in order to clarify the existence of a boron oxide outer layer coating. The ionization edges selected for elemental mapping are listed as follows: B-K edge (188.5 eV), O-K edge (532 eV). The exposure time for the elemental mapping of B and O was 10 and 20 s, and the width of the energy windows DE was set to 10 and 20 eV, respectively. The EELS spectrum was acquired in the image mode with a half collection angle of 13 mrad. Fig. 1a shows a typical SEM image of the wellaligned boron nanowires. It can be seen from this image that the boron nanowires (with diameter
Fig. 1. (a) A typical SEM image of the well-aligned boron nanowires. (b) A typical HRTEM image of single boron nanowire.
around 40–50 nm) have a smooth surface and are straight along the axis. Fig. 1b shows a typical HRTEM image of single boron nanowire with a solid core. No crystalline fringes can be identified in the HRTEM image at the lattice-resolved scale. This indicates that the boron nanowires are amorphous. From HRTEM observation, it is very difficult to detect if there is an oxide-coating layer of BOx because the phase contrast of amorphous boron and amorphous BOx coating cannot be easily distinguished. In order to clarify the existence of oxygen in the boron nanowires, parallel EELS studies were carried out. The EELS spectrum of single boron nanowire is shown in Fig. 2, revealing the characteristic boron K-shell ionization edges (188
Y.Q. Wang et al. / Chemical Physics Letters 359 (2002) 273–277
Fig. 2. EELS spectrum acquired from single boron nanowire showing that there is a small peak at 532 eV corresponding to the oxygen K-edge.
275
eV). Careful examination of the EELS spectrum shows that there is a small peak at 532 eV, which corresponds to the K-shell excitation of oxygen. The magnified peak of the oxygen is shown in the inset of Fig. 2. This can demonstrate there is small amount of oxygen in the boron nanowire. In order to further investigate the distribution of boron and oxygen in the boron nanowires, elemental mappings of boron and oxygen were achieved for the straight boron nanowires (Fig. 3). It can be clearly seen that the boron is mainly distributed in the core (Fig. 3b), while oxygen is mainly located in the outer layer (Fig. 3c) of the boron nanowires. The importance of oxide or oxygen for the both nucleation and growth of the boron nanowires has
Fig. 3. (a) TEM image of single boron nanowire. (b) Elemental mapping of boron in the single boron nanowire. (c) Elemental mapping of oxygen in the single boron nanowire.
276
Y.Q. Wang et al. / Chemical Physics Letters 359 (2002) 273–277
been tested in an experiment (under the same conditions) using two targets (one is a mixture of B and B2 O3 , the other B only). The experiment showed the quantity of nanowire stopped increasing when pure B is used as the target. Moreover, the diameter of the boron nanowires (about 100 nm) using the pure B target is larger than that of the nanowires (40–50 nm) using the mixture target. Based on the above results, some points can be deduced: the screw-dislocation model loses its meaning for our case because the boron nanowires are amorphous. It seems that VLS model is not suited for the boron nanowires because no catalysts were added into the target and no metal or alloy particles can be found on the tips of the boron nanowires. According to the experimental results, we proposed that boron nanowires were grown by a VCS mechanism. The formation of the boron nanowires involves vapor phase generation of the substoichiometric boron oxides (BOx ) by sputtering of a mixture of boron and boron oxide, nanoclusters formation from the vapor phase separation, nucleation and growth of the boron nanowires on substrate, and nanowires self-assembly for orientation. A schematic model of the boron nanowire arrays formation is shown in Fig. 4. During the initial stage, Arþ ion sputtering of the target containing a mixture of boron and boron oxide generates a vapor of substoichiometric boron oxide (BOx ) (Fig. 4a). The vapor then condenses into BOx nanoclusters by the collisions of atoms and molecules during their motion towards the substrate (Fig. 4b). The nanoclusters reach and pile on the substrate surface (Fig. 4c). They undergo a phase separation of boron and boron oxide. The oxygen outdiffuses to the outer layer, and some of the oxygen escapes into vacuum (Fig. 4d). Ultimately, the densely packed boron nanowires will extend to the open space along the direction normal to the substrate surface to form regular arrays (Fig. 4e). Because the temperature was not high (800 °C) enough to allow recrystalliztion of the boron atoms, the boron nanowires adopted an amorphous structure instead of a crystalline one. In conclusion, high-density of arrays of selforiented amorphous boron nanowires grown on
Fig. 4. A schematic model for the growth of boron-nanowire arrays.
silicon substrates were synthesized by radio-frequency magnetron sputtering with a target of highly pure boron and boron oxide mixture using argon as the sputtering atmosphere. TEM studies show that the conventional growth mechanisms such as Frank screw-dislocation mechanism and the vapor–liquid–solid mechanisms are not suitable for the one-dimensional growth of amorphous boron nanowires. The oxide-assisted cluster–solid mechanism for the Si and Ge crystalline nanowires is not completely suitable for our case. The vapor–cluster–solid mechanism is proposed for the well-aligned growth of amorphous boron nanowires.
References [1] M.S. Dresselhaus, G. Dresselhaus, P. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, New York, 1996. [2] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [3] S. Iijima, Nature 354 (1991) 56.
Y.Q. Wang et al. / Chemical Physics Letters 359 (2002) 273–277 [4] D.P. Yu, C.S. Lee, I. Bello, X.S. Sun, Y.H. Tang, G.W. Zhou, Z.G. Bai, Z. Zhang, S.Q. Feng, Solid State Commun. 106 (1998) 403. [5] D.P. Yu, Z.G. Bai, Y. Ding, Q.L. Hang, H.Z. Zhang, J.J. Wang, Y.H. Zou, W. Qian, G.C. Xiong, H.T. Zhou, S.Q. Feng, Appl. Phys. Lett. 72 (1998) 3458. [6] H.Z. Zhang, D.P. Yu, Y. Ding, Z.G. Bai, Q.L. Hang, S.Q. Feng, Appl. Phys. Lett. 73 (1998) 3396. [7] Y.F. Zhang, Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835. [8] M. Morales, C.M. Lieber, Science 279 (1998) 208.
277
[9] J.R. Heath, F.K. Legoues, Chem. Phys. Lett. 208 (1993) 263. [10] Limin Cao, Ze Zhang, Liling Sun, Cunxiao Gao, Meng He, Yiqian Wang, Yancun Li, Xiangyi Zhang, Gong Li, Jun Zhang, Wenkui Wang, Adv. Mater. 13 (2001) 1701. [11] F.C. Frank, Disc. Faraday Soc. 5 (1949) 48. [12] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [13] Y.F. Zhang, Y.H. Tang, N. Wang, C.S. Lee, I. Bello, S.T. Lee, Phys. Rev. B 61 (2000) 4518. [14] Y.F. Zhang, Y.H. Tang, N. Wang, C.S. Lee, I. Bello, S.T. Lee, J. Cryst. Growth 197 (1999) 136.
Chemical Physics Letters 359 (2002) 273–277 www.elsevier.com/locate/cplett
One-dimensional growth mechanism of amorphous boron nanowires Y.Q. Wang
a,*
, X.F. Duan a, L.M. Cao b, W.K. Wang
b
a
b
Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, People’s Republic of China Center for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received 23 January 2002; in final form 23 April 2002
Abstract High-density of arrays of self-oriented boron nanowires grown on silicon substrates were synthesized by radiofrequency magnetron sputtering with a target of highly pure boron and boron oxide mixture using argon as the sputtering atmosphere. TEM studies show that the conventional growth mechanisms such as Frank screw-dislocation mechanism and the vapor–liquid–solid mechanisms are not suitable for the one-dimensional growth of boron nanowires. The oxide-assisted cluster–solid mechanism for the Si and Ge crystalline nanowires is not completely suitable for our case. The vapor–cluster–solid mechanism is proposed for the well-aligned growth of the amorphous boron nanowires. Ó 2002 Elsevier Science B.V. All rights reserved.
One-dimensional nanostructures are of great interest because of their potential fundamental and practical implications in areas such as materials science, chemistry, physics and engineering [1,2]. Since the discovery of carbon nanotubes by Iijima [3], extensive researches on the nanotubes and nanowires [4–9] have been carried out. More recently, we reported a successful synthesis of wellaligned straight amorphous boron nanowires [10].
*
Corresponding author. Present address: H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, BS8 1TL, Bristol, UK. Fax: +86-10-6256-1422. E-mail addresses: [email protected], [email protected] (Y.Q. Wang).
The growth mechanism of one-dimensional nanowires is one of the controversial subjects. Two growth mechanisms for crystalline fibers or whiskers have been proposed: the screw-dislocation mechanism [11] and the vapor–liquid–solid (VLS) mechanism [12]. In VLS model, a liquid agent or catalyst is affixed to the tip of the nanowires to promote the one-dimensional growth, such as metal elements (Au, Fe, Ni, etc.) used in the syntheses of Si and Ge nanowires. Recently, Zhang et al. [13,14] proposed an oxide-assisted cluster–solid mechanism to explain the growth of Si and Ge nanowires on the basis of their experimental observations. In this Letter, a vapor–cluster–solid (VCS) mechanism is proposed to explain the wellaligned growth of the amorphous boron nanowires.
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 7 3 2 - 7
274
Y.Q. Wang et al. / Chemical Physics Letters 359 (2002) 273–277
A radio-frequency (rf) magnetron sputtering was employed in this work to prepare the aligned boron-nanowire arrays films. Our simple procedure for processing is described as follows. A mixture of pure boron powder (purity 99.9%) and high pure B2 O3 powder (purity 99.999%) of 40% in weight was compacted into a disk with a diameter of 60 cm to be used as the target. Cleaned Si(1 0 0) wafers were used as substrates. The substrates were placed on a temperature-controllable heater that is parallel to the target surface. Before sputtering, the vacuum chamber was first pumped to a base pressure of better than 4 105 Pa, and high pure argon gas (purity 99.999%) was then introduced into the chamber at a fixed flow rate of 30 sccm (standard cubic centimeters per minute). For sputtering deposition growth, the Si(1 0 0) substrate was first heated to 800 °C in the argon atmosphere, and thereafter growth was performed under the rf power of 800 W with the total pressure kept constant at 2 Pa. After 6 h of sputtering deposition, the silicon substrate was covered with pitchblack films of boron nanowires. We peeled off some black deposit of boron nanowires from the silicon substrates, ultrasonicated the deposit in ethanol for several min, and placed a drop onto a holey carbon-coated copper grid. An S-4200 scanning electron microscope (SEM) was used for morphological observation of the boron nanowires. A Philips CM200-FEG TEM equipped with a GIF (Gatan Imaging Filter, model 678) was used for high-resolution transmission electron microscopy (HRTEM), elemental mapping, and electron energy-loss spectroscopy (EELS) studies. A three-window method is used to study elemental mapping of boron and oxygen in order to clarify the existence of a boron oxide outer layer coating. The ionization edges selected for elemental mapping are listed as follows: B-K edge (188.5 eV), O-K edge (532 eV). The exposure time for the elemental mapping of B and O was 10 and 20 s, and the width of the energy windows DE was set to 10 and 20 eV, respectively. The EELS spectrum was acquired in the image mode with a half collection angle of 13 mrad. Fig. 1a shows a typical SEM image of the wellaligned boron nanowires. It can be seen from this image that the boron nanowires (with diameter
Fig. 1. (a) A typical SEM image of the well-aligned boron nanowires. (b) A typical HRTEM image of single boron nanowire.
around 40–50 nm) have a smooth surface and are straight along the axis. Fig. 1b shows a typical HRTEM image of single boron nanowire with a solid core. No crystalline fringes can be identified in the HRTEM image at the lattice-resolved scale. This indicates that the boron nanowires are amorphous. From HRTEM observation, it is very difficult to detect if there is an oxide-coating layer of BOx because the phase contrast of amorphous boron and amorphous BOx coating cannot be easily distinguished. In order to clarify the existence of oxygen in the boron nanowires, parallel EELS studies were carried out. The EELS spectrum of single boron nanowire is shown in Fig. 2, revealing the characteristic boron K-shell ionization edges (188
Y.Q. Wang et al. / Chemical Physics Letters 359 (2002) 273–277
Fig. 2. EELS spectrum acquired from single boron nanowire showing that there is a small peak at 532 eV corresponding to the oxygen K-edge.
275
eV). Careful examination of the EELS spectrum shows that there is a small peak at 532 eV, which corresponds to the K-shell excitation of oxygen. The magnified peak of the oxygen is shown in the inset of Fig. 2. This can demonstrate there is small amount of oxygen in the boron nanowire. In order to further investigate the distribution of boron and oxygen in the boron nanowires, elemental mappings of boron and oxygen were achieved for the straight boron nanowires (Fig. 3). It can be clearly seen that the boron is mainly distributed in the core (Fig. 3b), while oxygen is mainly located in the outer layer (Fig. 3c) of the boron nanowires. The importance of oxide or oxygen for the both nucleation and growth of the boron nanowires has
Fig. 3. (a) TEM image of single boron nanowire. (b) Elemental mapping of boron in the single boron nanowire. (c) Elemental mapping of oxygen in the single boron nanowire.
276
Y.Q. Wang et al. / Chemical Physics Letters 359 (2002) 273–277
been tested in an experiment (under the same conditions) using two targets (one is a mixture of B and B2 O3 , the other B only). The experiment showed the quantity of nanowire stopped increasing when pure B is used as the target. Moreover, the diameter of the boron nanowires (about 100 nm) using the pure B target is larger than that of the nanowires (40–50 nm) using the mixture target. Based on the above results, some points can be deduced: the screw-dislocation model loses its meaning for our case because the boron nanowires are amorphous. It seems that VLS model is not suited for the boron nanowires because no catalysts were added into the target and no metal or alloy particles can be found on the tips of the boron nanowires. According to the experimental results, we proposed that boron nanowires were grown by a VCS mechanism. The formation of the boron nanowires involves vapor phase generation of the substoichiometric boron oxides (BOx ) by sputtering of a mixture of boron and boron oxide, nanoclusters formation from the vapor phase separation, nucleation and growth of the boron nanowires on substrate, and nanowires self-assembly for orientation. A schematic model of the boron nanowire arrays formation is shown in Fig. 4. During the initial stage, Arþ ion sputtering of the target containing a mixture of boron and boron oxide generates a vapor of substoichiometric boron oxide (BOx ) (Fig. 4a). The vapor then condenses into BOx nanoclusters by the collisions of atoms and molecules during their motion towards the substrate (Fig. 4b). The nanoclusters reach and pile on the substrate surface (Fig. 4c). They undergo a phase separation of boron and boron oxide. The oxygen outdiffuses to the outer layer, and some of the oxygen escapes into vacuum (Fig. 4d). Ultimately, the densely packed boron nanowires will extend to the open space along the direction normal to the substrate surface to form regular arrays (Fig. 4e). Because the temperature was not high (800 °C) enough to allow recrystalliztion of the boron atoms, the boron nanowires adopted an amorphous structure instead of a crystalline one. In conclusion, high-density of arrays of selforiented amorphous boron nanowires grown on
Fig. 4. A schematic model for the growth of boron-nanowire arrays.
silicon substrates were synthesized by radio-frequency magnetron sputtering with a target of highly pure boron and boron oxide mixture using argon as the sputtering atmosphere. TEM studies show that the conventional growth mechanisms such as Frank screw-dislocation mechanism and the vapor–liquid–solid mechanisms are not suitable for the one-dimensional growth of amorphous boron nanowires. The oxide-assisted cluster–solid mechanism for the Si and Ge crystalline nanowires is not completely suitable for our case. The vapor–cluster–solid mechanism is proposed for the well-aligned growth of amorphous boron nanowires.
References [1] M.S. Dresselhaus, G. Dresselhaus, P. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, New York, 1996. [2] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [3] S. Iijima, Nature 354 (1991) 56.
Y.Q. Wang et al. / Chemical Physics Letters 359 (2002) 273–277 [4] D.P. Yu, C.S. Lee, I. Bello, X.S. Sun, Y.H. Tang, G.W. Zhou, Z.G. Bai, Z. Zhang, S.Q. Feng, Solid State Commun. 106 (1998) 403. [5] D.P. Yu, Z.G. Bai, Y. Ding, Q.L. Hang, H.Z. Zhang, J.J. Wang, Y.H. Zou, W. Qian, G.C. Xiong, H.T. Zhou, S.Q. Feng, Appl. Phys. Lett. 72 (1998) 3458. [6] H.Z. Zhang, D.P. Yu, Y. Ding, Z.G. Bai, Q.L. Hang, S.Q. Feng, Appl. Phys. Lett. 73 (1998) 3396. [7] Y.F. Zhang, Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835. [8] M. Morales, C.M. Lieber, Science 279 (1998) 208.
277
[9] J.R. Heath, F.K. Legoues, Chem. Phys. Lett. 208 (1993) 263. [10] Limin Cao, Ze Zhang, Liling Sun, Cunxiao Gao, Meng He, Yiqian Wang, Yancun Li, Xiangyi Zhang, Gong Li, Jun Zhang, Wenkui Wang, Adv. Mater. 13 (2001) 1701. [11] F.C. Frank, Disc. Faraday Soc. 5 (1949) 48. [12] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [13] Y.F. Zhang, Y.H. Tang, N. Wang, C.S. Lee, I. Bello, S.T. Lee, Phys. Rev. B 61 (2000) 4518. [14] Y.F. Zhang, Y.H. Tang, N. Wang, C.S. Lee, I. Bello, S.T. Lee, J. Cryst. Growth 197 (1999) 136.