- Email: [email protected]
Intensive blue-light emission from semiconductor GaN nanowires sheathed with BN layers
Chemical Physics Letters 383 (2004) 423–427 www.elsevier.com/locate/cplett
Intensive blue-light emission from semiconductor GaN nanowires sheathed with BN layers Jun Zhang *, Lide Zhang, Feihong Jiang, Zhenhong Dai Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, PR China and Department of Physics, Yantai University, Yantai 264005, PR China Received 15 July 2003; in final form 15 November 2003 Published online: 6 December 2003
Abstract Semiconductor GaN nanowires, sheathed with BN layers, were successfully synthesized by a simple chemical vapor deposition. The experimentally determined structure consists of a hexagonal wurtzite GaN core, and the outer shell of BN separated in the radial direction. The GaN-BN assembly-structure was about 40–50 nm in diameter and up to several hundreds of micrometers in length. The photoluminescence spectrum of the GaN-BN assembly-structure shows a very strong and broad blue-light emission, centered at 464 nm. The assembly-structures with semiconductor–insulator geometry, which take advantage of this self-organization mechanism for multi-element nanotube formation, have great prospects in fundamental physical science and applications in nanoscale optoelectronic devices (such as nanolasers). Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes (CNTs) in 1991 [1], much effort has been devoted to encapsulate metals and nonmetals inside the inner core of CNTs by various methods including chemical insertion, physical insertion, arc encapsulation and encapsulation via catalytic growth from the solid phase. Meanwhile, progress has also been made in fabricating solid nanowires by filling the hollow cavity of non-carbon nanotubes, such as BN nanotubes [2–5] and B–C–N nanotubes [6–9]. Zhang and co-workers [10] reported that they synthesized coaxial nanocable based on a SiC and SiO2 core sheathed with (BN)x Cy nanotubes by a laser-ablation method. Han et al. [11] showed that mass quantity of BN nanotubes and boron-doped carbon nanotubes were synthesized by a CNT-substitution reaction. Recently, Bando et al. [12] reported a method to put uniform BN coatings on SiC nanowires. Because, BN has unique chemical and physical properties, such as low density, *
Corresponding author. Fax: +865356901947. E-mail address: [email protected] (J. Zhang).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.11.057
high melting point, chemical inertness and high thermal conductivity in a wide rang of temperatures, it could be an ideal system for building semiconductor–insulator– semiconductor or semiconductor–insulator–metal heterojunction in the radial direction and has great potential applications in optoelectronic devices and electronic transportation. As a wide direct bandgap (3.4 eV, at room temperature) semiconductor [13], nanometer-scale GaN onedimensional structures (nanowires or nanorods) are known to have great prospects in fundamental physical science and novel nano-technological applications [14–16]. A number of research groups have developed various methods for synthesizing GaN one-dimensional nanostructures (nanowires or nanorods) [17–21]. In this communication, we first report the assembly system of two nitrides: uniform BN coatings on GaN nanowires to form GaN-BN assembly-structure. The GaN-BN assembly-structure is expected to be useful in nanoscale electronic devices and in the preparation of nanostructured material. To our knowledge, the synthesis of GaN-BN assembly-structure and their unique physical properties have not been reported to date.
424
J. Zhang et al. / Chemical Physics Letters 383 (2004) 423–427
2. Experimental In our experiment, GaN-BN assembly-structure was synthesized using a conventional thermal chemical vapor deposition (CVD) method. The reaction was carried out in a conventional furnace with a horizontal quartz tube. An Al2 O3 membrane was cleaned in a sonicating bath of acetone for about 30 min. A layer of Ni (about several tens of nanometers in thickness) was evaporated onto one side of the substrate under high vacuum (2 105 Torr). A 4:1:1 molar mixture of Ga, Ga2 O3 powder and B2 O3 nanoparticles was used as the starting material and was placed in an alumina crucible. The Ni coated substrate and the ceramic boat were transferred into a quartz tube. The substrate was typically placed at 2.0 cm from the center of the boat. This quartz tube was then placed inside a horizontal electronic resistance furnace with the center of the ceramic boat positioned at the center of the furnace and, the substrate placed downstream of the gas flow. The temperature of the furnace was rapidly increased to 1080 °C from room temperature about 8 min, and was kept at this temperature for 4 h and was held in a flowing ammonia atmosphere (400 standard cubic centimeters per minute). After the system was cooled down to room temperature, it was observed that light-yellow wool-like products appeared on the surface of the substrate and walls of the crucible.
3. Results and discussion
Fig. 1. (a) SEM image of the as-prepared products. (b) XRD pattern of the as-prepared products.
The morphology of the wool-like products was characterized by scanning electron microscopy (SEM) (JEOL JSM-5610LV). A typical SEM image in Fig. 1a shows that, the material resulting from the reaction of the mixture Ga2 O and B2 O3 with ammonia produced nanometer wire-like structures. The nanowires are straight and smooth and a maximum length of several hundreds of micrometers. The X-ray powder diffraction (XRD) pattern was recorded on a MXP18AHF (MAC Science Co. Ltd) X-ray diffractometer with Cu Ka ra). Fig. 1b shows a XRD pattern diation (k ¼ 1:54178 A of the products. XRD measurements shows that, the product is comprised of hexagonal GaN and BN. GaN has crystallized in the hexagonal structure, which is in good agreement with the reported values [13,17–21] and with the standard value for bulk hexagonal structure GaN (JCPDS 2-1078). Miller indices are indicated on the three strongest peaks of GaN (1 0 0), (0 0 2) and (1 0 1). Meanwhile, one can also see the XRD pattern of BN (JCPDS 18-0251, unindexed). The other strong diffraction peaks in the pattern correspond to the substrate of Al2 O3 . The structural and elemental analyses of an individual GaN-BN nanowire were performed using high-
resolution transmission electron microscopy (HRTEM) (JEOL JEM-2010, 200 kV), energy-dispersive X-ray fluorescence spectroscopy (EDS) (EDAX, DX-4) attached to the HRTEM. A low-magnified TEM image of the GaN-BN assembly-structure shown in Fig. 2a indicates that, an individual GaN nanowire is covered with a layer of BN. The nanowires have high aspect ratios with length of up to several hundreds of micrometers and diameter of several tens of nanometers. Fig. 2b is an enlarged image of the part of Fig. 2a indicated by the arrow. One can clearly see the structural properties of the GaN-BN assembly-structure. The image shows that, the nanowire has a crystalline core and a surrounding layer. The core is about 40–50 nm and the outer shell is about 10 nm. The outer shells extended all along the nanowire and adhered well to the surface. To further reveal the structure of GaN-BN and see lattice fringes of the GaN core and the BN sheath-layer, two HRTEM images are shown in Fig. 2c–d that come from the region (A–B) in Fig. 2b. The HRTEM image of the crystalline GaN core shown in Fig. 2c comes from the region A in Fig. 2b. In the image, the space of about 0.258 nm between arrowheads corresponds to the distance between
J. Zhang et al. / Chemical Physics Letters 383 (2004) 423–427
425
Fig. 3a at 190.95 eV could be assigned to BN. The binding energy spectrum of N(1s) shown in Fig. 3b shows a peak at about 398.40 eV, which is also attributed to BN. These data prove that the products are composed of GaN and BN, in agreement with the XRD results and HRTEM observations. The formation of the GaN-BN assembly-structures could be explained with a vapor–liquid–solid (VLS) growth mechanism, which has been widely used to synthesize one-dimensional nanowires. In our process, the reactions can be expressed as: 4GaðlÞ þ Ga2 O3 ðsÞ ! 3Ga2 OðgÞ
ð1Þ
Ga2 OðgÞ þ 2NH3 ðgÞ ! 2GaNðsÞ þ H2 OðgÞ þ 2H2 ðgÞ ð2Þ and B2 O3 ðsÞ þ 2NH3 ðgÞ ! 2BNðsÞ þ 3H2 OðgÞ
ð3Þ
The Ni grains deposited on the substrate act as the energetically favored site for absorption of gas-phase reactants. A 4:1 molar mixture of Ga–Ga2 O3 powder was used as the starting material for Ga2 O vapor from reaction (1) [22]. It is expected that, the GaN formation originate from the gas reaction (2) of the Ga2 O vapor Fig. 2. (a) Low-magnification TEM image of an individual GaN-BN assembly-structure. (b) An enlarged TEM image of 2(a) as indicated by the arrow. It shows that, there is an outer-shell layer covering the surface of the GaN nanowire. The image shows that the GaN-BN assembly-structure has uniform diameter. (c) A HRTEM lattice image of the wurtzite phase single-crystalline GaN core in region A of Fig. 2b. (d) A HRTEM lattice fringes of the outer shell layer in region B of Fig. 2b.
two (0 0 2) planes and the growth direction is indicated by an arrow. HRTEM image of the sheathed layer, which comes from the region B in Fig. 2b, is shown in Fig. 2d. From the HRTEM image, we can also see the lattice fringes of the sheathed layer. EDS spectrum to analyze the composition of the GaN-BN assemblystructure indicated that the individual nanowire consisted of Ga, N, C and Cu. Cu and C signals are generated from carbon-coated copper microgrid mesh that supported the nanowires. To determine whether BN is the outer shell or not, the composition of the as-synthesized product was further determined by X-ray photoelectron spectrometry (XPS) (Vgescalab-MkII). The base pressure of the analytical chamber was less than 109 mbar. Magnesium Ka radiation (hm ¼ 1253:6 eV) was used as the excitation source with an emission current of 20 mA and an anode voltage of 12 kV. A constant analyzer energy of 20 eV was adopted for the spectral data collection. The XPS spectrum of the as-synthesized products also demonstrates that, there are Ga, N, B and C elements in the product. The binding energy spectrum of B(1s) shown in
Fig. 3. XPS spectra from the as-prepared products. (a) The binding energy of B(1s). (b) The binding energy of N(1s).
426
J. Zhang et al. / Chemical Physics Letters 383 (2004) 423–427
with the flowing ammonia on the surface of the Ni grain. The solving of Ga and N atoms goes into the Ni grain to form Ni–Ga–N liquid-state droplet. The droplets continuously dissolve the gas phase generated by reaction (2) and reaction (3) to simultaneously form GaN and BN crystals when the droplets become saturated. Due to the formation from the same liquid droplet, according to the VLS growth mechanism of quasi-one-dimensional materials [23], the two crystalline phases would selfassemble to generate the GaN-BN composite structure. In our observation, a typical small-scale SEM image of a single nanowire in Fig. 4 may provide further details of VLS growth in GaN-BN assembly-structure. The image shows a long nanowire terminating in a nanoparticle at the tip. EDS analyses from the observed nanowirestem and the tip indicate that the Ni nanoparticles play a crucial role in the growth of GaN-BN assemblystructure based on the VLS mechanism, and the growth of the assembly system is governed by the presence of eutectic metal alloys at the end of nanowires. Therefore, the importance of the GaN-BN assembly-structure is not only the superior structure itself but also the control of the structure formation. It provides a unique process for making semiconductor nanowires sheathed with coating-layers. The outer shell can act as chemically inert protecting layers for GaN nanowires, and can show unique luminescence property as shown in Fig. 5. Photoluminescence (PL) spectrum of the GaN-BN assembly system was measured in a Hitachi 850 fluorescence spectrophotometer with a Xe lamp at room temperature. The excitation wavelength was 254 nm, and the filter wavelength was 310 nm. A very strong and broad blue-light emission feature at 464 nm was ob-
Fig. 5. Photoluminescence spectrum of GaN-BN assembly-structure.
tained from the PL spectrum of the GaN-BN assembly system shown in Fig. 5. The full-width at half maximum (FWHM) of the blue-light emission band is about 200 nm. In addition, a weak UV emission peak at 340 nm was also obtained. In order to compare the PL features of GaN-BN assembly-structure with the GaN nanowires, the PL spectrum of GaN nanowires was also measured shown in Fig. 5 (dash curve). One emission peak of the GaN nanowires at 364 nm was observed. The luminescence properties of the GaN-BN assemblystructure are different from that of the GaN nanowires (nanorods) [17–21] and GaN nanoparticles [24]. GaN is a direct bandgap semiconductor with wide band gap 3.4 eV (364.7 nm) at room temperature [13] and bulk hexagonal BN has a wide gap close to 5.8 eV (213.8 nm) gap [25]. The assembly-structure, which a shell of a high bandgap insulator is overgrown along the semiconductor nanowire, might provide further control over the optical properties of nanowire. The phenomena have also been found in experiment of semiconductor core/shell nanocrystal [26,27]. Because, of the enhanced confinement effects from the outer-layer BN, the energy band of GaN nanowire is compressed and shows obviously red-shift of energy structure. Therefore, the strong and broad peak, centered at 464 nm, might be ascribed to the GaN-BN assembly-structure. Of course, the existence of defects and surface states of the assembly system may cause a number of small structures. Further work is needed to clarify the underlying mechanism for the PL spectrum of the GaN-BN assembly system.
4. Conclusions
Fig. 4. A typical SEM image of a GaN-BN nanowire terminated with a nanoparticle at the tip.
Self-organization of assembly-structure with different components is also possible by changing chemicals in starting materials. The experimental results provide further support for the speculation of the GaN-based material assembly-structure. The optical and electronic properties of the structure can be revealed from
J. Zhang et al. / Chemical Physics Letters 383 (2004) 423–427
combining different nanowires and coating layers. Therefore the successful preparation of GaN-BN assembly-structure could be used to fabricate other assembly-structures, which offer opportunities for nanotechnological applications, such as solid-state injection nanolasers. Acknowledgements This work was supported by the Key Project of National Fundamental Research of China: Nanomaterials and Nanostructures (Grant No. 19994506) and the National Natural Science Foundation of China (Grant No. 60277023). References [1] S. Iijima, Nature 354 (1991) 56. [2] N.G. Chopra et al., Science 269 (1995) 966. [3] A. Loiseau, F. Willaime, N. Demoncy, G. Hug, H. Pascard, Phys. Rev. Lett. 76 (1996) 4737. [4] M. Terrones et al., Chem. Phys. Lett. 259 (1996) 568. [5] D. Golberg et al., Appl. Phys. Lett. 69 (1996) 2045. [6] O. Stephan et al., Science 266 (1994) 1683. [7] P. Redich et al., Chem. Phys. Lett. 260 (1996) 465. [8] Y. Zhang, H. Gu, K. Suenaga, S. Iijima, Chem. Phys. Lett. 279 (1997) 264.
427
[9] K. Suenaga et al., Science 278 (1997) 653. [10] Y. Zhang, K. Suenaga, C. Colliex, S. Iijima, Science 281 (1998) 973. [11] W. Han, P. Redlich, F. Ernst, M. Ruhle, Chem. Mater. 11 (1999) 3620; W. Han, P. Redlich, F. Ernst, M. Ruhle, Appl. Phys. Lett. 75 (1999) 1875. [12] C.C. Tang, Y. Bando, T. Sato, K. Kurashima, Adv. Mater. 14 (2002) 1046. [13] R. Dingle, K.L. Shaklee, Appl. Phys. Lett. 19 (1971) 5. [14] S.N. Mohammas, H. Morkoc, Prog. Quantum Electron. 20 (1996) 361. [15] F.A. Ponce, D.P. Bour, Nature (London) 386 (1997) 351. [16] S. Nakamura, Science 281 (1998) 956. [17] W. Han, S. Fan, Q. Li, Y. Hu, Science 277 (1997) 128. [18] X.F. Duan, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 188. [19] X.L. Chen, J. Li, Y. Cao, Y. Lan, H. Li, M. He, C. Wang, Z. Zhang, Z. Qiao, Adv. Mater. 12 (2000) 1432. [20] J. Zhang, L.D. Zhang, X.F. Wang, C.H. Liang, X.S. Peng, Y.W. Wang, J. Chem. Phys. 115 (2001) 5714. [21] C. Chen, C. Yeh, Adv. Mater. 12 (2000) 738. [22] C.M. Ballaks, R.E. Davis, J. Am. Ceram. Soc. 79 (1996) 2309. [23] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [24] Y. Xie, Y. Qian, W. Wang, S. Zhang, Y. Zhang, Science 272 (1996) 1926. [25] X. Blase, A. Rubio, S.G. Louie, M.L. Cohen, Europhys. Lett. 28 (1994) 335. [26] Oded Millo, D. Katz, Y.W. Cao, U. Banin, Phys. Rev. Lett. 86 (2001) 5751. [27] Y.W. Cao, U. Banin, J. Am. Chem. Soc. 122 (2000) 9692.
Intensive blue-light emission from semiconductor GaN nanowires sheathed with BN layers Jun Zhang *, Lide Zhang, Feihong Jiang, Zhenhong Dai Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, PR China and Department of Physics, Yantai University, Yantai 264005, PR China Received 15 July 2003; in final form 15 November 2003 Published online: 6 December 2003
Abstract Semiconductor GaN nanowires, sheathed with BN layers, were successfully synthesized by a simple chemical vapor deposition. The experimentally determined structure consists of a hexagonal wurtzite GaN core, and the outer shell of BN separated in the radial direction. The GaN-BN assembly-structure was about 40–50 nm in diameter and up to several hundreds of micrometers in length. The photoluminescence spectrum of the GaN-BN assembly-structure shows a very strong and broad blue-light emission, centered at 464 nm. The assembly-structures with semiconductor–insulator geometry, which take advantage of this self-organization mechanism for multi-element nanotube formation, have great prospects in fundamental physical science and applications in nanoscale optoelectronic devices (such as nanolasers). Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes (CNTs) in 1991 [1], much effort has been devoted to encapsulate metals and nonmetals inside the inner core of CNTs by various methods including chemical insertion, physical insertion, arc encapsulation and encapsulation via catalytic growth from the solid phase. Meanwhile, progress has also been made in fabricating solid nanowires by filling the hollow cavity of non-carbon nanotubes, such as BN nanotubes [2–5] and B–C–N nanotubes [6–9]. Zhang and co-workers [10] reported that they synthesized coaxial nanocable based on a SiC and SiO2 core sheathed with (BN)x Cy nanotubes by a laser-ablation method. Han et al. [11] showed that mass quantity of BN nanotubes and boron-doped carbon nanotubes were synthesized by a CNT-substitution reaction. Recently, Bando et al. [12] reported a method to put uniform BN coatings on SiC nanowires. Because, BN has unique chemical and physical properties, such as low density, *
Corresponding author. Fax: +865356901947. E-mail address: [email protected] (J. Zhang).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.11.057
high melting point, chemical inertness and high thermal conductivity in a wide rang of temperatures, it could be an ideal system for building semiconductor–insulator– semiconductor or semiconductor–insulator–metal heterojunction in the radial direction and has great potential applications in optoelectronic devices and electronic transportation. As a wide direct bandgap (3.4 eV, at room temperature) semiconductor [13], nanometer-scale GaN onedimensional structures (nanowires or nanorods) are known to have great prospects in fundamental physical science and novel nano-technological applications [14–16]. A number of research groups have developed various methods for synthesizing GaN one-dimensional nanostructures (nanowires or nanorods) [17–21]. In this communication, we first report the assembly system of two nitrides: uniform BN coatings on GaN nanowires to form GaN-BN assembly-structure. The GaN-BN assembly-structure is expected to be useful in nanoscale electronic devices and in the preparation of nanostructured material. To our knowledge, the synthesis of GaN-BN assembly-structure and their unique physical properties have not been reported to date.
424
J. Zhang et al. / Chemical Physics Letters 383 (2004) 423–427
2. Experimental In our experiment, GaN-BN assembly-structure was synthesized using a conventional thermal chemical vapor deposition (CVD) method. The reaction was carried out in a conventional furnace with a horizontal quartz tube. An Al2 O3 membrane was cleaned in a sonicating bath of acetone for about 30 min. A layer of Ni (about several tens of nanometers in thickness) was evaporated onto one side of the substrate under high vacuum (2 105 Torr). A 4:1:1 molar mixture of Ga, Ga2 O3 powder and B2 O3 nanoparticles was used as the starting material and was placed in an alumina crucible. The Ni coated substrate and the ceramic boat were transferred into a quartz tube. The substrate was typically placed at 2.0 cm from the center of the boat. This quartz tube was then placed inside a horizontal electronic resistance furnace with the center of the ceramic boat positioned at the center of the furnace and, the substrate placed downstream of the gas flow. The temperature of the furnace was rapidly increased to 1080 °C from room temperature about 8 min, and was kept at this temperature for 4 h and was held in a flowing ammonia atmosphere (400 standard cubic centimeters per minute). After the system was cooled down to room temperature, it was observed that light-yellow wool-like products appeared on the surface of the substrate and walls of the crucible.
3. Results and discussion
Fig. 1. (a) SEM image of the as-prepared products. (b) XRD pattern of the as-prepared products.
The morphology of the wool-like products was characterized by scanning electron microscopy (SEM) (JEOL JSM-5610LV). A typical SEM image in Fig. 1a shows that, the material resulting from the reaction of the mixture Ga2 O and B2 O3 with ammonia produced nanometer wire-like structures. The nanowires are straight and smooth and a maximum length of several hundreds of micrometers. The X-ray powder diffraction (XRD) pattern was recorded on a MXP18AHF (MAC Science Co. Ltd) X-ray diffractometer with Cu Ka ra). Fig. 1b shows a XRD pattern diation (k ¼ 1:54178 A of the products. XRD measurements shows that, the product is comprised of hexagonal GaN and BN. GaN has crystallized in the hexagonal structure, which is in good agreement with the reported values [13,17–21] and with the standard value for bulk hexagonal structure GaN (JCPDS 2-1078). Miller indices are indicated on the three strongest peaks of GaN (1 0 0), (0 0 2) and (1 0 1). Meanwhile, one can also see the XRD pattern of BN (JCPDS 18-0251, unindexed). The other strong diffraction peaks in the pattern correspond to the substrate of Al2 O3 . The structural and elemental analyses of an individual GaN-BN nanowire were performed using high-
resolution transmission electron microscopy (HRTEM) (JEOL JEM-2010, 200 kV), energy-dispersive X-ray fluorescence spectroscopy (EDS) (EDAX, DX-4) attached to the HRTEM. A low-magnified TEM image of the GaN-BN assembly-structure shown in Fig. 2a indicates that, an individual GaN nanowire is covered with a layer of BN. The nanowires have high aspect ratios with length of up to several hundreds of micrometers and diameter of several tens of nanometers. Fig. 2b is an enlarged image of the part of Fig. 2a indicated by the arrow. One can clearly see the structural properties of the GaN-BN assembly-structure. The image shows that, the nanowire has a crystalline core and a surrounding layer. The core is about 40–50 nm and the outer shell is about 10 nm. The outer shells extended all along the nanowire and adhered well to the surface. To further reveal the structure of GaN-BN and see lattice fringes of the GaN core and the BN sheath-layer, two HRTEM images are shown in Fig. 2c–d that come from the region (A–B) in Fig. 2b. The HRTEM image of the crystalline GaN core shown in Fig. 2c comes from the region A in Fig. 2b. In the image, the space of about 0.258 nm between arrowheads corresponds to the distance between
J. Zhang et al. / Chemical Physics Letters 383 (2004) 423–427
425
Fig. 3a at 190.95 eV could be assigned to BN. The binding energy spectrum of N(1s) shown in Fig. 3b shows a peak at about 398.40 eV, which is also attributed to BN. These data prove that the products are composed of GaN and BN, in agreement with the XRD results and HRTEM observations. The formation of the GaN-BN assembly-structures could be explained with a vapor–liquid–solid (VLS) growth mechanism, which has been widely used to synthesize one-dimensional nanowires. In our process, the reactions can be expressed as: 4GaðlÞ þ Ga2 O3 ðsÞ ! 3Ga2 OðgÞ
ð1Þ
Ga2 OðgÞ þ 2NH3 ðgÞ ! 2GaNðsÞ þ H2 OðgÞ þ 2H2 ðgÞ ð2Þ and B2 O3 ðsÞ þ 2NH3 ðgÞ ! 2BNðsÞ þ 3H2 OðgÞ
ð3Þ
The Ni grains deposited on the substrate act as the energetically favored site for absorption of gas-phase reactants. A 4:1 molar mixture of Ga–Ga2 O3 powder was used as the starting material for Ga2 O vapor from reaction (1) [22]. It is expected that, the GaN formation originate from the gas reaction (2) of the Ga2 O vapor Fig. 2. (a) Low-magnification TEM image of an individual GaN-BN assembly-structure. (b) An enlarged TEM image of 2(a) as indicated by the arrow. It shows that, there is an outer-shell layer covering the surface of the GaN nanowire. The image shows that the GaN-BN assembly-structure has uniform diameter. (c) A HRTEM lattice image of the wurtzite phase single-crystalline GaN core in region A of Fig. 2b. (d) A HRTEM lattice fringes of the outer shell layer in region B of Fig. 2b.
two (0 0 2) planes and the growth direction is indicated by an arrow. HRTEM image of the sheathed layer, which comes from the region B in Fig. 2b, is shown in Fig. 2d. From the HRTEM image, we can also see the lattice fringes of the sheathed layer. EDS spectrum to analyze the composition of the GaN-BN assemblystructure indicated that the individual nanowire consisted of Ga, N, C and Cu. Cu and C signals are generated from carbon-coated copper microgrid mesh that supported the nanowires. To determine whether BN is the outer shell or not, the composition of the as-synthesized product was further determined by X-ray photoelectron spectrometry (XPS) (Vgescalab-MkII). The base pressure of the analytical chamber was less than 109 mbar. Magnesium Ka radiation (hm ¼ 1253:6 eV) was used as the excitation source with an emission current of 20 mA and an anode voltage of 12 kV. A constant analyzer energy of 20 eV was adopted for the spectral data collection. The XPS spectrum of the as-synthesized products also demonstrates that, there are Ga, N, B and C elements in the product. The binding energy spectrum of B(1s) shown in
Fig. 3. XPS spectra from the as-prepared products. (a) The binding energy of B(1s). (b) The binding energy of N(1s).
426
J. Zhang et al. / Chemical Physics Letters 383 (2004) 423–427
with the flowing ammonia on the surface of the Ni grain. The solving of Ga and N atoms goes into the Ni grain to form Ni–Ga–N liquid-state droplet. The droplets continuously dissolve the gas phase generated by reaction (2) and reaction (3) to simultaneously form GaN and BN crystals when the droplets become saturated. Due to the formation from the same liquid droplet, according to the VLS growth mechanism of quasi-one-dimensional materials [23], the two crystalline phases would selfassemble to generate the GaN-BN composite structure. In our observation, a typical small-scale SEM image of a single nanowire in Fig. 4 may provide further details of VLS growth in GaN-BN assembly-structure. The image shows a long nanowire terminating in a nanoparticle at the tip. EDS analyses from the observed nanowirestem and the tip indicate that the Ni nanoparticles play a crucial role in the growth of GaN-BN assemblystructure based on the VLS mechanism, and the growth of the assembly system is governed by the presence of eutectic metal alloys at the end of nanowires. Therefore, the importance of the GaN-BN assembly-structure is not only the superior structure itself but also the control of the structure formation. It provides a unique process for making semiconductor nanowires sheathed with coating-layers. The outer shell can act as chemically inert protecting layers for GaN nanowires, and can show unique luminescence property as shown in Fig. 5. Photoluminescence (PL) spectrum of the GaN-BN assembly system was measured in a Hitachi 850 fluorescence spectrophotometer with a Xe lamp at room temperature. The excitation wavelength was 254 nm, and the filter wavelength was 310 nm. A very strong and broad blue-light emission feature at 464 nm was ob-
Fig. 5. Photoluminescence spectrum of GaN-BN assembly-structure.
tained from the PL spectrum of the GaN-BN assembly system shown in Fig. 5. The full-width at half maximum (FWHM) of the blue-light emission band is about 200 nm. In addition, a weak UV emission peak at 340 nm was also obtained. In order to compare the PL features of GaN-BN assembly-structure with the GaN nanowires, the PL spectrum of GaN nanowires was also measured shown in Fig. 5 (dash curve). One emission peak of the GaN nanowires at 364 nm was observed. The luminescence properties of the GaN-BN assemblystructure are different from that of the GaN nanowires (nanorods) [17–21] and GaN nanoparticles [24]. GaN is a direct bandgap semiconductor with wide band gap 3.4 eV (364.7 nm) at room temperature [13] and bulk hexagonal BN has a wide gap close to 5.8 eV (213.8 nm) gap [25]. The assembly-structure, which a shell of a high bandgap insulator is overgrown along the semiconductor nanowire, might provide further control over the optical properties of nanowire. The phenomena have also been found in experiment of semiconductor core/shell nanocrystal [26,27]. Because, of the enhanced confinement effects from the outer-layer BN, the energy band of GaN nanowire is compressed and shows obviously red-shift of energy structure. Therefore, the strong and broad peak, centered at 464 nm, might be ascribed to the GaN-BN assembly-structure. Of course, the existence of defects and surface states of the assembly system may cause a number of small structures. Further work is needed to clarify the underlying mechanism for the PL spectrum of the GaN-BN assembly system.
4. Conclusions
Fig. 4. A typical SEM image of a GaN-BN nanowire terminated with a nanoparticle at the tip.
Self-organization of assembly-structure with different components is also possible by changing chemicals in starting materials. The experimental results provide further support for the speculation of the GaN-based material assembly-structure. The optical and electronic properties of the structure can be revealed from
J. Zhang et al. / Chemical Physics Letters 383 (2004) 423–427
combining different nanowires and coating layers. Therefore the successful preparation of GaN-BN assembly-structure could be used to fabricate other assembly-structures, which offer opportunities for nanotechnological applications, such as solid-state injection nanolasers. Acknowledgements This work was supported by the Key Project of National Fundamental Research of China: Nanomaterials and Nanostructures (Grant No. 19994506) and the National Natural Science Foundation of China (Grant No. 60277023). References [1] S. Iijima, Nature 354 (1991) 56. [2] N.G. Chopra et al., Science 269 (1995) 966. [3] A. Loiseau, F. Willaime, N. Demoncy, G. Hug, H. Pascard, Phys. Rev. Lett. 76 (1996) 4737. [4] M. Terrones et al., Chem. Phys. Lett. 259 (1996) 568. [5] D. Golberg et al., Appl. Phys. Lett. 69 (1996) 2045. [6] O. Stephan et al., Science 266 (1994) 1683. [7] P. Redich et al., Chem. Phys. Lett. 260 (1996) 465. [8] Y. Zhang, H. Gu, K. Suenaga, S. Iijima, Chem. Phys. Lett. 279 (1997) 264.
427
[9] K. Suenaga et al., Science 278 (1997) 653. [10] Y. Zhang, K. Suenaga, C. Colliex, S. Iijima, Science 281 (1998) 973. [11] W. Han, P. Redlich, F. Ernst, M. Ruhle, Chem. Mater. 11 (1999) 3620; W. Han, P. Redlich, F. Ernst, M. Ruhle, Appl. Phys. Lett. 75 (1999) 1875. [12] C.C. Tang, Y. Bando, T. Sato, K. Kurashima, Adv. Mater. 14 (2002) 1046. [13] R. Dingle, K.L. Shaklee, Appl. Phys. Lett. 19 (1971) 5. [14] S.N. Mohammas, H. Morkoc, Prog. Quantum Electron. 20 (1996) 361. [15] F.A. Ponce, D.P. Bour, Nature (London) 386 (1997) 351. [16] S. Nakamura, Science 281 (1998) 956. [17] W. Han, S. Fan, Q. Li, Y. Hu, Science 277 (1997) 128. [18] X.F. Duan, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 188. [19] X.L. Chen, J. Li, Y. Cao, Y. Lan, H. Li, M. He, C. Wang, Z. Zhang, Z. Qiao, Adv. Mater. 12 (2000) 1432. [20] J. Zhang, L.D. Zhang, X.F. Wang, C.H. Liang, X.S. Peng, Y.W. Wang, J. Chem. Phys. 115 (2001) 5714. [21] C. Chen, C. Yeh, Adv. Mater. 12 (2000) 738. [22] C.M. Ballaks, R.E. Davis, J. Am. Ceram. Soc. 79 (1996) 2309. [23] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [24] Y. Xie, Y. Qian, W. Wang, S. Zhang, Y. Zhang, Science 272 (1996) 1926. [25] X. Blase, A. Rubio, S.G. Louie, M.L. Cohen, Europhys. Lett. 28 (1994) 335. [26] Oded Millo, D. Katz, Y.W. Cao, U. Banin, Phys. Rev. Lett. 86 (2001) 5751. [27] Y.W. Cao, U. Banin, J. Am. Chem. Soc. 122 (2000) 9692.