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Formation and structure of Ag, Ge and SiC nanoparticles encapsulated in boron nitride and carbon nanocapsules
Diamond and Related Materials 9 (2000) 911–915 www.elsevier.com/locate/diamond
Formation and structure of Ag, Ge and SiC nanoparticles encapsulated in boron nitride and carbon nanocapsules Takeo Oku a, *, Takafumi Kusunose a, Takamichi Hirata b, Rikizo Hatakeyama b, Noriyoshi Sato b, Koichi Niihara a, Katsuaki Suganuma a a Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan b Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
Abstract Boron nitride nanocapsules with silver nanoparticles and carbon nanocage structures with Ge and SiC nanoparticles were produced by the new chemical solution process and hybrid arc-discharge method. High-resolution electron microscopy, energy dispersive spectroscopy and X-ray diffraction showed the formation of silver and silver oxide nanoparticles encapsulated in boron nitride nanocages, which were synthesized from mixtures of boric acid, urea and silver nitrate by reduction at 300–700°C in hydrogen gas. Ge and SiC nanoparticles and nanowires encapsulated in carbon nanocapsules and nanotubes were also produced by direct current and radio frequency hybrid arc-discharge of C, Ge and Si elements. The present work indicates that the various boron nitride and carbon nanocage structures with electronic conductors, superhard materials, semiconductor nanoparticles and nanowires can be synthesized by the new chemical process and hybrid arc-discharge method, and the chemical synthesis of the boron nitride nanocapsules from the organic solution materials is a useful fabrication method for the mass production of lowdimensional nanocage structures at low temperatures compared to the ordinary arc-discharge method. © 2000 Elsevier Science S.A. All rights reserved. Keywords: BN phases; Carbon; Structure; TEM
1. Introduction Boron nitride and carbon (B–C–N ) fullerene materials such as fullerene clusters, nanotubes, nanocapsules, nanopolyhedra, cones, cubes and onions have great potential for studying materials in low dimensions with isolated environment [1–4]. Especially, cluster-included fullerene materials are intriguing for both scientific research and device applications such as cluster protection, cluster separation, nano-ball bearings, nano-optical–magnetic devices, catalysis and biotechnology. By controlling the size, layer numbers, included clusters, helicity and compositions, the cluster-included B–C–N nanocage structures with bandgap energy of 0–5 eV and non-magnetism are expected to show various electronic, optical and magnetic properties such as Coulomb blockade, photoluminescence and superparamagnetism. Recently, we have succeeded in the formation of * Corresponding author. Tel.: +81-6-6879-8521; fax: +81-6-6879-8522. E-mail address: [email protected] ( T. Oku)
carbon nanocapsules by thermal decomposition of polyvinyl alcohol with SiC clusters at 500°C in Ar gas atmosphere [5]. We have also produced Pd intercalated onions by electron beam irradiation [6 ]. These materials are expected to be used as solid state lubricants, nanoball bearings and magnetic devices. Although these metal nanoparticles were successfully enveloped inside spherical graphite sheets, few works have reported the encapsulation of semiconductors such as Ge, Si and SiC. Although these semiconductor nanoparticles are expected to show luminescence by the quantum size effect, they are easy to oxidize by exposure to air. Encapsulation of these semiconductor nanoparticles by graphite layers is expected to provide a stable surface structure and new properties. In addition to these conductive graphite sheets, insulating sheets such as boron nitride (BN ) are also needed for the control of electrons in future nanoscale devices, but few works have focused on the formation of BN nanocapsules [7,8]. In our previous work [9], BN nanocapsules with FeO and Au nanoparticles were produced x by an arc melting method, and the nanostructure was
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investigated by high-resolution electron microscopy. However, the amount of produced BN nanocapsules was very small (~1 mg), and it was difficult to evaluate, for example, optical, magnetic and electronic properties. In order to evaluate these properties, synthesis of a large amount of BN nanocapsules is mandatory. The purpose of the present work is two-fold. First, to develop a new synthetic process for a large amount (>1 g) of BN nanocapsules with nanoparticles which have electronic properties. Organic materials of urea [CO(NH ) ] and boric acid (H BO ) were selected in 22 3 3 the present work for the formation of BN layers. The mixture is expected to form BN layers by annealing in hydrogen gas [10]. Gold colloids have been used for the formation of single electron transistors [11,12] because of the easy control of cluster size [13]. In the present work, silver (Ag) nanoparticles with BN sheets were selected, which are expected to be used as quantum electronic devices. Silver nitrate was selected for the formation of silver nanoparticles in the present work. Furthermore, since these precursors are soluble in solvents, it is possible to mix them homogeneously in ionic state. Second, to synthesize carbon nanocapsules with semiconductor nanoclusters by direct current and radio frequency (DC–RF ) hybrid arc-discharge. In the present work, Ge and SiC with band structures of indirect transition were selected. Ge and SiC are extensively used and studied semiconductors in the electronic industrial field. It has been reported that the band structure of Si with indirect transition can be changed into that of direct transition by down-sizing of the nanoparticles [14]. To understand the formation mechanism of the nanocapsules, high-resolution electron microscopy (HREM ) [15,16 ] and energy dispersive spectroscopy (EDS ) were carried out for microstructure analysis. These studies will give us a guideline for designing and synthesis of the BN and carbon nanocage structures, which are expected as future nanoscale devices.
2. Experimental procedures For BN nanocapsule formation, the BN content was adjusted to 70 and 95 vol%. Boric acid, urea and silver nitrate were completely dissolved in deionized water, and this solution was dried in a rotating vacuum drier. The dried mixtures were reduced at 300 and 700°C in hydrogen gas for 7 h. Samples of carbon nanocapsules were prepared by pair anode arc-discharge in order to enhance the Ge and Si evaporation. A subanode consisting of a carbon cylinder with semiconductor powders is used, and a main anode of carbon rod was partially mixed with semiconductor powders. A control electrode or RF
antenna is installed above the arc point to generate an auxiliary plasma by DC–RF discharge around the arc plasma. The samples were investigated by X-ray diffraction ( XRD). Samples for HREM observations were prepared by dispersing materials on holey carbon grids using ethanol. HREM observations were performed with 1250 and 300 kV electron microscopes (ARM-1250 and JEM-3000F ) equipped with top and side entry goniometers having point-to-point resolutions of 0.12 and 0.17 nm, respectively. The EDS analysis was carried out by an EDAX system. To avoid sample damage by electron irradiation, the electron beam for HREM observations was minimized by using a smaller spot size.
3. Results and discussion A HREM image of Ag nanoparticles with BN matrix prepared at 700°C is shown in Fig. 1a. The particle size of Ag is in the range 10–20 nm, and all Ag nanoparticles are encapsulated in BN {002} sheets, which indicates the formation of BN nanocapsules. An XRD pattern of the BN nanocapsules prepared at 700°C is shown in Fig. 1b. Strong Ag reflections of 111, 200, 220 and 311 are observed. A weak peak of BN 002 is also observed, which indicates the formation of turbostratic BN. EDX spectra of the BN nanocapsules with Ag nanoparticles are shown in Fig. 1c. A nitrogen peak is as high as the boron peak, which indicates the formation of boron nitride. Ag peaks are from Ag nanoparticles, and a Cu peak is due to the HREM grid. Although oxygen atoms remain in the sample, no carbon is detected. Fig. 2a is a HREM image of a BN nanocapsule with Ag nanoparticle of ca. 10 nm, and lattice fringes of Ag {111} are observed in the clusters. The {002} planes of BN are observed around the Ag nanoparticles, and the number of BN sheets is four. A small amount of BN nanocapsules with AgO nanoparticles was observed, as shown in Fig. 2b. The incident beam is along the [111: ] direction of the AgO. The surface of the BN nanocapsule is enlarged in Fig. 2c. An epitaxial relationship of BN {002} and Ag {111} is observed as indicated by arrows, and the interface is very flat and clear. The formation mechanism of BN nanocapsules synthesized in the present work is believed to be described overall as follows: 2H BO +CO(NH ) +AgNO +H Ag+2BN 3 3 22 3 2 +6H O(+CO(+NO (. 2 x AgNO is soluble in water, and Ag+ and 2NO− would 3 3 be formed in the present ion exchanged water. After annealing at 300°C, silver clusters and amorphous BN with low nitrogen content were formed, and oxygen would be included in the BN matrix. After annealing at
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Fig. 1. (a) HREM image of Ag nanoparticles in BN nanocapsules prepared at 700°C. (b) XRD pattern of (a). (c) EDX spectra of (a).
700°C, t-BN layers were formed around the silver nanoparticles by the reduction and crystallization of BN matrix with H . 2 In the present work, we have succeeded in developing a new synthetic process for large amounts (>1 g) of BN nanocapsules with Ag nanoparticles, which is a sufficient quantity for various property measurements. We have also succeeded in producing BN nanocapsules at the ‘low’ temperature of 700°C. Previous BN nanocapsules had been produced by arc-discharge or arc-melting methods (2000–3000°C ), and it is difficult to control the formation of BN nanocapsules. In the present work, the control of BN nanocapsule formation is very easy and reliable because of the ordinary annealing technique. BN nanocapsules are expected to have various proper-
Fig. 2. HREM images of (a) BN nanocapsule with Ag nanoparticle, (b) BN nanocapsule with AgO nanoparticle, and (c) surface of BN nanocapsule.
ties. It is expected that silver nanoparticles encapsulated in BN nanocapsules will enable us to control a single electron through the insulator sheets, and they can be applied to single electron transistors [11,12]. Formation of the BN layer around nanoparticles is also useful for cluster protection. In the present work, AgO nanoparticles with a decomposition temperature of ~100°C were also encapsulated in BN nanocapsules, which indicates the cluster protection of AgO at elevated temperature of 700°C. In the present work, the formation process of nanoscale insulator sheets and cluster protection sheets around metal nanoparticles was developed.
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Fig. 3. HREM images of (a) carbon nanocapsules with SiC nanoparticles, (b) carbon nanocapsule and nanocage. (c) Enlarged image of SiC cluster encapsulated in carbon cage.
A HREM image of carbon nanocapsules with SiC nanoparticles is shown in Fig. 3a. SiC nanoparticles with sizes of 6–10 nm are observed. All SiC nanoparticles are encapsulated in graphite sheets, and the number of graphite sheets was in the range of three to 10 layers. A HREM image of the carbon nanocapsule and nanocage is shown in Fig. 3b. Five to six graphite layers are observed around the nanoparticle, and the size of the carbon nanocapsule is 4 nm. Lattice fringes with a distance of 0.25 nm, which corresponds to the distance of the {111} planes of b-SiC, are observed in the cluster. A carbon nanocage with three graphite sheets is observed. The tip of the cage is smeared. Fig. 3c is a HREM image of a carbon nanocapsule filled with an SiC nanocluster of size 2 nm. HREM images of Ge nanoparticles encapsulated in
Fig. 4. (a) HREM image of Ge nanoparticles encapsulated in carbon sheets. (b) Enlarged image of the nanocapsule. (c) Ge nanowire encapsulated in carbon nanotube.
graphite sheets are shown in Fig. 4a. In Fig. 4b, the Ge nanoparticle is surrounded by one to three graphite sheets. Fig. 4c is a HREM image of a Ge nanowire encapsulated in a carbon nanotube with one to two graphite sheets. The diameter is 10 nm, and the length is 70 nm. The Ge nanowire has microtwin structures, and the growth direction of the nanowire is 111 of Ge. Although many carbon nanocapsules with various elements and compounds have been prepared by an ordinary arc-discharge method, few nanocapsules filled with Ge and SiC have been reported. In the present work, a special DC–RF arc-discharge method was used, which results in their formation. The SiC nanoparticles would be formed by reaction of Si and C atoms in the arc plasma. The DC–RF arc-discharge plasma would
T. Oku et al. / Diamond and Related Materials 9 (2000) 911–915
be effective for the formation of semiconductor nanoparticles encapsulated in graphite sheets. In the present work, the sizes of the semiconductor Ge and SiC nanoparticles were reduced down to 10 nm, which indicates that the widening of the bandgap energy is expected by quantum size effects, and peculiar optical properties will be expected. Ge nanowires encapsulated in carbon nanotubes are also expected as one-dimensional devices. Si oxide and Ge oxide layers are not observed at the surface of the SiC and Ge nanoparticles, which indicates that the carbon nanocapsules are effective for cluster protection against oxidation in air.
4. Conclusions BN and carbon nanocapsules with Ag, Ge and SiC nanoparticles are synthesized by the new chemical solution process and hybrid arc-discharge method. The BN nanocages were synthesized from mixtures of boric acid, urea and silver nitrate by reduction at 300–700°C in hydrogen gas. Carbon nanocapsules and nanotubes were produced by DC–RF hybrid arc-discharge of C, Ge and Si elements. The present work indicates that the boron nitride and carbon nanocage structures with electronic conductors, superhard materials, semiconductor nanoparticles and nanowires can be synthesized by these new methods.
Acknowledgements The authors would like to acknowledge Professors K. Hiraga, E. Aoyagi, N. Motegi, T. Mieno, N.Y. Sato,
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H. Mase, M. Niwano, N. Miyamoto and T. Hirano for allowing them to use the electron microscopes and for experimental help. This work is partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan.
References [1] C.N.R. Rao, R. Seshadri, A. Govindaraj, R. Sen, Mater. Sci. Eng. R 15 (1995) 209. [2] Y. Saito, T. Yoshikawa, M. Okuda, N. Fujimoto, K. Sumiyama, K. Suzuki, A. Kasuya, Y. Nisina, J. Phys. Chem. Solids 54 (1993) 1849. [3] J. Sloan, J. Cook, M.L.H. Green, J.L. Hutchinson, R. Tenne, J. Mater. Chem. 7 (1997) 1089. [4] L. Rapoport, Y. Bilik, Y. Feldman, M. Homyonfer, S.R. Cohen, R. Tenne, Nature 387 (1997) 791. [5] T. Oku, K. Niihara, K. Suganuma, J. Mater. Chem. 8 (1998) 1323. [6 ] T. Oku, G. Schmid, K. Suganuma, J. Mater. Chem. 8 (1998) 2113. [7] N.G. Chopra, R.J. Luyken, K. Cherrey, V.H. Crespi, M.L. Cohen, S.G. Louie, A. Zettl, Science 269 (1995) 966. [8] M. Terauchi, M. Tanaka, H. Matsuda, M. Takeda, K. Kimura, J. Electron. Microsc. 46 (1997) 75. [9] T. Hirano, T. Oku, K. Suganuma, J. Mater. Chem. 9 (1999) 855. [10] T. Kusunose, Y.H. Choa, T. Sekino, K. Niihara, Ceram. Trans. 94 (1999) 443. [11] T. Sato, H. Ahmed, D. Brown, B.F.G. Johnson, J. Appl. Phys. 82 (1997) 696. [12] D.L. Klein, R. Roth, A.K.L. Lim, A.P. Alivisatos, P.L. McEuen, Nature 389 (1997) 699. [13] G. Schmid, Chem. Rev. 92 (1992) 1709. [14] W. Zhao, O. Schoenfeld, J. Kusano, Y. Aoyagi, T. Sugano, Jpn. J. Appl. Phys. 33 (1994) L899. [15] T. Oku, S. Nakajima, Surf. Sci. 407 (1998) L647. [16 ] T. Oku, J.-O. Bovin, Philos. Mag. A 79 (1999) 821.
Formation and structure of Ag, Ge and SiC nanoparticles encapsulated in boron nitride and carbon nanocapsules Takeo Oku a, *, Takafumi Kusunose a, Takamichi Hirata b, Rikizo Hatakeyama b, Noriyoshi Sato b, Koichi Niihara a, Katsuaki Suganuma a a Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan b Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
Abstract Boron nitride nanocapsules with silver nanoparticles and carbon nanocage structures with Ge and SiC nanoparticles were produced by the new chemical solution process and hybrid arc-discharge method. High-resolution electron microscopy, energy dispersive spectroscopy and X-ray diffraction showed the formation of silver and silver oxide nanoparticles encapsulated in boron nitride nanocages, which were synthesized from mixtures of boric acid, urea and silver nitrate by reduction at 300–700°C in hydrogen gas. Ge and SiC nanoparticles and nanowires encapsulated in carbon nanocapsules and nanotubes were also produced by direct current and radio frequency hybrid arc-discharge of C, Ge and Si elements. The present work indicates that the various boron nitride and carbon nanocage structures with electronic conductors, superhard materials, semiconductor nanoparticles and nanowires can be synthesized by the new chemical process and hybrid arc-discharge method, and the chemical synthesis of the boron nitride nanocapsules from the organic solution materials is a useful fabrication method for the mass production of lowdimensional nanocage structures at low temperatures compared to the ordinary arc-discharge method. © 2000 Elsevier Science S.A. All rights reserved. Keywords: BN phases; Carbon; Structure; TEM
1. Introduction Boron nitride and carbon (B–C–N ) fullerene materials such as fullerene clusters, nanotubes, nanocapsules, nanopolyhedra, cones, cubes and onions have great potential for studying materials in low dimensions with isolated environment [1–4]. Especially, cluster-included fullerene materials are intriguing for both scientific research and device applications such as cluster protection, cluster separation, nano-ball bearings, nano-optical–magnetic devices, catalysis and biotechnology. By controlling the size, layer numbers, included clusters, helicity and compositions, the cluster-included B–C–N nanocage structures with bandgap energy of 0–5 eV and non-magnetism are expected to show various electronic, optical and magnetic properties such as Coulomb blockade, photoluminescence and superparamagnetism. Recently, we have succeeded in the formation of * Corresponding author. Tel.: +81-6-6879-8521; fax: +81-6-6879-8522. E-mail address: [email protected] ( T. Oku)
carbon nanocapsules by thermal decomposition of polyvinyl alcohol with SiC clusters at 500°C in Ar gas atmosphere [5]. We have also produced Pd intercalated onions by electron beam irradiation [6 ]. These materials are expected to be used as solid state lubricants, nanoball bearings and magnetic devices. Although these metal nanoparticles were successfully enveloped inside spherical graphite sheets, few works have reported the encapsulation of semiconductors such as Ge, Si and SiC. Although these semiconductor nanoparticles are expected to show luminescence by the quantum size effect, they are easy to oxidize by exposure to air. Encapsulation of these semiconductor nanoparticles by graphite layers is expected to provide a stable surface structure and new properties. In addition to these conductive graphite sheets, insulating sheets such as boron nitride (BN ) are also needed for the control of electrons in future nanoscale devices, but few works have focused on the formation of BN nanocapsules [7,8]. In our previous work [9], BN nanocapsules with FeO and Au nanoparticles were produced x by an arc melting method, and the nanostructure was
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investigated by high-resolution electron microscopy. However, the amount of produced BN nanocapsules was very small (~1 mg), and it was difficult to evaluate, for example, optical, magnetic and electronic properties. In order to evaluate these properties, synthesis of a large amount of BN nanocapsules is mandatory. The purpose of the present work is two-fold. First, to develop a new synthetic process for a large amount (>1 g) of BN nanocapsules with nanoparticles which have electronic properties. Organic materials of urea [CO(NH ) ] and boric acid (H BO ) were selected in 22 3 3 the present work for the formation of BN layers. The mixture is expected to form BN layers by annealing in hydrogen gas [10]. Gold colloids have been used for the formation of single electron transistors [11,12] because of the easy control of cluster size [13]. In the present work, silver (Ag) nanoparticles with BN sheets were selected, which are expected to be used as quantum electronic devices. Silver nitrate was selected for the formation of silver nanoparticles in the present work. Furthermore, since these precursors are soluble in solvents, it is possible to mix them homogeneously in ionic state. Second, to synthesize carbon nanocapsules with semiconductor nanoclusters by direct current and radio frequency (DC–RF ) hybrid arc-discharge. In the present work, Ge and SiC with band structures of indirect transition were selected. Ge and SiC are extensively used and studied semiconductors in the electronic industrial field. It has been reported that the band structure of Si with indirect transition can be changed into that of direct transition by down-sizing of the nanoparticles [14]. To understand the formation mechanism of the nanocapsules, high-resolution electron microscopy (HREM ) [15,16 ] and energy dispersive spectroscopy (EDS ) were carried out for microstructure analysis. These studies will give us a guideline for designing and synthesis of the BN and carbon nanocage structures, which are expected as future nanoscale devices.
2. Experimental procedures For BN nanocapsule formation, the BN content was adjusted to 70 and 95 vol%. Boric acid, urea and silver nitrate were completely dissolved in deionized water, and this solution was dried in a rotating vacuum drier. The dried mixtures were reduced at 300 and 700°C in hydrogen gas for 7 h. Samples of carbon nanocapsules were prepared by pair anode arc-discharge in order to enhance the Ge and Si evaporation. A subanode consisting of a carbon cylinder with semiconductor powders is used, and a main anode of carbon rod was partially mixed with semiconductor powders. A control electrode or RF
antenna is installed above the arc point to generate an auxiliary plasma by DC–RF discharge around the arc plasma. The samples were investigated by X-ray diffraction ( XRD). Samples for HREM observations were prepared by dispersing materials on holey carbon grids using ethanol. HREM observations were performed with 1250 and 300 kV electron microscopes (ARM-1250 and JEM-3000F ) equipped with top and side entry goniometers having point-to-point resolutions of 0.12 and 0.17 nm, respectively. The EDS analysis was carried out by an EDAX system. To avoid sample damage by electron irradiation, the electron beam for HREM observations was minimized by using a smaller spot size.
3. Results and discussion A HREM image of Ag nanoparticles with BN matrix prepared at 700°C is shown in Fig. 1a. The particle size of Ag is in the range 10–20 nm, and all Ag nanoparticles are encapsulated in BN {002} sheets, which indicates the formation of BN nanocapsules. An XRD pattern of the BN nanocapsules prepared at 700°C is shown in Fig. 1b. Strong Ag reflections of 111, 200, 220 and 311 are observed. A weak peak of BN 002 is also observed, which indicates the formation of turbostratic BN. EDX spectra of the BN nanocapsules with Ag nanoparticles are shown in Fig. 1c. A nitrogen peak is as high as the boron peak, which indicates the formation of boron nitride. Ag peaks are from Ag nanoparticles, and a Cu peak is due to the HREM grid. Although oxygen atoms remain in the sample, no carbon is detected. Fig. 2a is a HREM image of a BN nanocapsule with Ag nanoparticle of ca. 10 nm, and lattice fringes of Ag {111} are observed in the clusters. The {002} planes of BN are observed around the Ag nanoparticles, and the number of BN sheets is four. A small amount of BN nanocapsules with AgO nanoparticles was observed, as shown in Fig. 2b. The incident beam is along the [111: ] direction of the AgO. The surface of the BN nanocapsule is enlarged in Fig. 2c. An epitaxial relationship of BN {002} and Ag {111} is observed as indicated by arrows, and the interface is very flat and clear. The formation mechanism of BN nanocapsules synthesized in the present work is believed to be described overall as follows: 2H BO +CO(NH ) +AgNO +H Ag+2BN 3 3 22 3 2 +6H O(+CO(+NO (. 2 x AgNO is soluble in water, and Ag+ and 2NO− would 3 3 be formed in the present ion exchanged water. After annealing at 300°C, silver clusters and amorphous BN with low nitrogen content were formed, and oxygen would be included in the BN matrix. After annealing at
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Fig. 1. (a) HREM image of Ag nanoparticles in BN nanocapsules prepared at 700°C. (b) XRD pattern of (a). (c) EDX spectra of (a).
700°C, t-BN layers were formed around the silver nanoparticles by the reduction and crystallization of BN matrix with H . 2 In the present work, we have succeeded in developing a new synthetic process for large amounts (>1 g) of BN nanocapsules with Ag nanoparticles, which is a sufficient quantity for various property measurements. We have also succeeded in producing BN nanocapsules at the ‘low’ temperature of 700°C. Previous BN nanocapsules had been produced by arc-discharge or arc-melting methods (2000–3000°C ), and it is difficult to control the formation of BN nanocapsules. In the present work, the control of BN nanocapsule formation is very easy and reliable because of the ordinary annealing technique. BN nanocapsules are expected to have various proper-
Fig. 2. HREM images of (a) BN nanocapsule with Ag nanoparticle, (b) BN nanocapsule with AgO nanoparticle, and (c) surface of BN nanocapsule.
ties. It is expected that silver nanoparticles encapsulated in BN nanocapsules will enable us to control a single electron through the insulator sheets, and they can be applied to single electron transistors [11,12]. Formation of the BN layer around nanoparticles is also useful for cluster protection. In the present work, AgO nanoparticles with a decomposition temperature of ~100°C were also encapsulated in BN nanocapsules, which indicates the cluster protection of AgO at elevated temperature of 700°C. In the present work, the formation process of nanoscale insulator sheets and cluster protection sheets around metal nanoparticles was developed.
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Fig. 3. HREM images of (a) carbon nanocapsules with SiC nanoparticles, (b) carbon nanocapsule and nanocage. (c) Enlarged image of SiC cluster encapsulated in carbon cage.
A HREM image of carbon nanocapsules with SiC nanoparticles is shown in Fig. 3a. SiC nanoparticles with sizes of 6–10 nm are observed. All SiC nanoparticles are encapsulated in graphite sheets, and the number of graphite sheets was in the range of three to 10 layers. A HREM image of the carbon nanocapsule and nanocage is shown in Fig. 3b. Five to six graphite layers are observed around the nanoparticle, and the size of the carbon nanocapsule is 4 nm. Lattice fringes with a distance of 0.25 nm, which corresponds to the distance of the {111} planes of b-SiC, are observed in the cluster. A carbon nanocage with three graphite sheets is observed. The tip of the cage is smeared. Fig. 3c is a HREM image of a carbon nanocapsule filled with an SiC nanocluster of size 2 nm. HREM images of Ge nanoparticles encapsulated in
Fig. 4. (a) HREM image of Ge nanoparticles encapsulated in carbon sheets. (b) Enlarged image of the nanocapsule. (c) Ge nanowire encapsulated in carbon nanotube.
graphite sheets are shown in Fig. 4a. In Fig. 4b, the Ge nanoparticle is surrounded by one to three graphite sheets. Fig. 4c is a HREM image of a Ge nanowire encapsulated in a carbon nanotube with one to two graphite sheets. The diameter is 10 nm, and the length is 70 nm. The Ge nanowire has microtwin structures, and the growth direction of the nanowire is 111 of Ge. Although many carbon nanocapsules with various elements and compounds have been prepared by an ordinary arc-discharge method, few nanocapsules filled with Ge and SiC have been reported. In the present work, a special DC–RF arc-discharge method was used, which results in their formation. The SiC nanoparticles would be formed by reaction of Si and C atoms in the arc plasma. The DC–RF arc-discharge plasma would
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be effective for the formation of semiconductor nanoparticles encapsulated in graphite sheets. In the present work, the sizes of the semiconductor Ge and SiC nanoparticles were reduced down to 10 nm, which indicates that the widening of the bandgap energy is expected by quantum size effects, and peculiar optical properties will be expected. Ge nanowires encapsulated in carbon nanotubes are also expected as one-dimensional devices. Si oxide and Ge oxide layers are not observed at the surface of the SiC and Ge nanoparticles, which indicates that the carbon nanocapsules are effective for cluster protection against oxidation in air.
4. Conclusions BN and carbon nanocapsules with Ag, Ge and SiC nanoparticles are synthesized by the new chemical solution process and hybrid arc-discharge method. The BN nanocages were synthesized from mixtures of boric acid, urea and silver nitrate by reduction at 300–700°C in hydrogen gas. Carbon nanocapsules and nanotubes were produced by DC–RF hybrid arc-discharge of C, Ge and Si elements. The present work indicates that the boron nitride and carbon nanocage structures with electronic conductors, superhard materials, semiconductor nanoparticles and nanowires can be synthesized by these new methods.
Acknowledgements The authors would like to acknowledge Professors K. Hiraga, E. Aoyagi, N. Motegi, T. Mieno, N.Y. Sato,
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H. Mase, M. Niwano, N. Miyamoto and T. Hirano for allowing them to use the electron microscopes and for experimental help. This work is partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan.
References [1] C.N.R. Rao, R. Seshadri, A. Govindaraj, R. Sen, Mater. Sci. Eng. R 15 (1995) 209. [2] Y. Saito, T. Yoshikawa, M. Okuda, N. Fujimoto, K. Sumiyama, K. Suzuki, A. Kasuya, Y. Nisina, J. Phys. Chem. Solids 54 (1993) 1849. [3] J. Sloan, J. Cook, M.L.H. Green, J.L. Hutchinson, R. Tenne, J. Mater. Chem. 7 (1997) 1089. [4] L. Rapoport, Y. Bilik, Y. Feldman, M. Homyonfer, S.R. Cohen, R. Tenne, Nature 387 (1997) 791. [5] T. Oku, K. Niihara, K. Suganuma, J. Mater. Chem. 8 (1998) 1323. [6 ] T. Oku, G. Schmid, K. Suganuma, J. Mater. Chem. 8 (1998) 2113. [7] N.G. Chopra, R.J. Luyken, K. Cherrey, V.H. Crespi, M.L. Cohen, S.G. Louie, A. Zettl, Science 269 (1995) 966. [8] M. Terauchi, M. Tanaka, H. Matsuda, M. Takeda, K. Kimura, J. Electron. Microsc. 46 (1997) 75. [9] T. Hirano, T. Oku, K. Suganuma, J. Mater. Chem. 9 (1999) 855. [10] T. Kusunose, Y.H. Choa, T. Sekino, K. Niihara, Ceram. Trans. 94 (1999) 443. [11] T. Sato, H. Ahmed, D. Brown, B.F.G. Johnson, J. Appl. Phys. 82 (1997) 696. [12] D.L. Klein, R. Roth, A.K.L. Lim, A.P. Alivisatos, P.L. McEuen, Nature 389 (1997) 699. [13] G. Schmid, Chem. Rev. 92 (1992) 1709. [14] W. Zhao, O. Schoenfeld, J. Kusano, Y. Aoyagi, T. Sugano, Jpn. J. Appl. Phys. 33 (1994) L899. [15] T. Oku, S. Nakajima, Surf. Sci. 407 (1998) L647. [16 ] T. Oku, J.-O. Bovin, Philos. Mag. A 79 (1999) 821.