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Synthesis and optical properties of aluminum nitride nanowires prepared by arc discharge method
Journal of Alloys and Compounds 465 (2008) 562–566
Synthesis and optical properties of aluminum nitride nanowires prepared by arc discharge method Longhai Shen a,∗ , Taimin Cheng b , Lijun Wu a , Xuefei Li c , Qiliang Cui c a
b
School of Science, Shenyang Ligong University, Shenyang 110168, PR China Department of Mathematics and Physics, Shenyang Institute of Chemical Technology, Shenyang 110142, PR China c National Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China Received 12 September 2007; received in revised form 1 November 2007; accepted 1 November 2007 Available online 20 February 2008
Abstract AlN nanowires with hexagonal structure were successfully synthesized by direct reaction of aluminum with nitrogen gas using arc discharge method. The wurtzite AlN nanowires have an average diameter of 40 nm and a length of several tens micrometer. The growth direction of most single-crystalline AlN nanowires is perpendicular to the [0 0 1] direction, while it is also found that the AlN nanowires grow along [0 0 1] direction. The vapor–solid growth mechanism can explain the formation of the AlN nanowires. Raman spectroscopy studies of the AlN nanowires reveal that the stress is rather low and the crystallinity is close to bulk AlN. The UV spectrum of the AlN nanowires shows that the absorption edge at 6.23 eV is comparable with that of the bulk AlN. The photoluminescence of the AlN nanowires suggests that the emission band at 506 nm may be ascribed to the deep level defect due to nitrogen vacancy. © 2007 Elsevier B.V. All rights reserved. Keywords: Aluminum nitrides; Nanowires; Crystal morphology; Arc discharge; Optical properties
1. Introduction One-dimensional (1D) nanostructured materials have attracted much attention because of their unique properties derived their high surface area and low dimensionality [1,2], which are particularly important not only for understanding fundamental concepts underlying the observed optical, electronic, and mechanical properties of materials, but also for being potentially applicable to nanoscale devices [3–6]. Therefore, the synthesis and properties of various 1D nanostructured materials are enthusiastically stimulated since the discovery of carbon nanotubes [7]. As an important III–V semiconductor, aluminum nitride (AlN) has a wide band gap (6.2 eV), a large exciton binding energy, and a very small electron affinity. These attractive physical properties make it potential application in surface acoustic wave, ultraviolet sensor, and field-emission devices [8–10]. Consequently, 1D AlN nanostructures are of fresh interest at present
∗
Corresponding author. Tel.: +86 24 24682191. E-mail address: [email protected] (L. Shen).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.11.007
due to its promising application in optoelectronic and fieldemission nanodevice [11,12]. Very recently, several routes have been developed to prepare 1D AlN nanowires, such as carbon nanotubes confined reaction [13], confined method of anodic porous aluminum template [14], extended vapor–liquid–solid growth technique [15] and direct reaction of Al or Al alloy in a mixture of nitrogen and ammonia gas (N2 + NH3 ) [16,17]. All these method, the flowing NH3 was employed as necessary nitrogen source and a crucial condition for synthesis of AlN nanowires. Additionally, the NH3 flux and ratio of NH3 to N2 also was accurately controlled and the reaction time is usually not less than two hours. In this paper, we successfully synthesized abundant AlN nanowires in five minutes by direct reaction of Al with N2 using direct current arc discharge method without catalyst and template. The optical properties of the assynthesized AlN nanowires are also investigated. 2. Experimental The direct current arc discharge apparatus used in this experiment has been described elsewhere [18]. In brief, the pure metal Mo (purity 99.99%) rod (10 mm in diameter and 160 mm in length) was used as the cathode. The Al (purity 99.99%) column (30 mm in diameter, 30 mm in height) as raw material
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Fig. 1. (a)–(d) The SEM images of the as-synthesized product.
was tightly inserted into a water-cooled anode copper crucible. The distance is about 20 mm between the tip of the Mo cathode and the Al column. Before arc discharging, the reaction chamber was evacuated to below 10−2 Torr and flushed with pure Ar gas (purity 99.99%) to remove the residual oxygen, and then 400 Torr pure N2 (purity 99.99%) was filled into the chamber as reaction gas. When the arc was ignited, the discharge voltage and current was kept in the ranges of 35–40 V and 100 A, respectively. The arc discharge process was maintained for 5 min. Finally, a layer of white product was suspended around Mo cathode. The morphology was determined by field-emission scanning electron microscopy (SEM, XL 30 ESEM). The crystal structure of white as-synthesized products was characterized by X-ray diffractometer (XRD, D8 DISCOVERGADDS) with Cu K␣ radiation (λ = 0.15418 nm). The microstructures of the nanowires were studied by high-resolution transmission electron microscopy (HRTEM, JEOL-2010 FEF, 200 kV). The Raman scattering measurements were performed by Raman spectroscopy (Renishaw 1000) with a confocal microscopy at room temperature, using excitation wavelengths of 488 nm (Argon ion laser). The optical absorption was measured using an ultraviolet–visible spectroscopy (UV-3150, Shimadzu). Photoluminescence spectrum (PL) at room temperature was measured using an Argon ion laser as the excitation source (325 nm).
˚ and c = 4.985 A, ˚ which match well with those of of a = 3.101 A bulk AlN crystal (JCPDS card No. 76-0702). No other impurity phases were found in the XRD pattern. When the AlN nanowires are vertically aligned on the substrate, the strong intensity of the (0 0 2) peak indicates that the AlN crystals grew preferentially along the [0 0 1] direction [19]. Since the AlN nanowires are randomly planished on the no-peaks silicon substrate in our XRD measure, the strong intensity of the (0 0 2) peak in our XRD curve indicates that most AlN nanowires grew preferentially along the direction perpendicular to [0 0 1] direction [20]. Fig. 3(a) shows the TEM image of a single AlN nanowire with uniform diameter about 20 nm. The corresponding HRTEM and fast Fourier transform images for the AlN nanowire are shown
3. Results and discussion Fig. 1(a)–(d) shows the SEM images of the as-synthesized product without affecting their original nature. Fig. 1(a) shows clearly a high density of AlN nanowires, which reveals that the nanowires have the length about tens of micrometers. The high-magnification SEM image of these nanowires is shown in Fig. 1(b), which shows the nanowires have a uniform diameter about 40 nm. Additionally, the quasi-aligned AlN nanowires are also observed, as shown in Fig. 1(c) and (d). These nanowires grow across like fence, or bind together, forming many bundles. Fig. 2 shows a typical XRD pattern of the collected white product. All the peaks in the XRD pattern could be indexed to the hexagonal wurtzite phase of AlN with the lattice constants
Fig. 2. The XRD pattern of the as-synthesized white product.
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Fig. 3. (a) TEM image of a representative AlN nanowire, (b) HRTEM image of an single AlN nanowire. Inset: The corresponding fast Fourier transform images of the HRTEM image. (c) HRTEM image of an AlN nanowire, the growth direction is perpendicular to the c-axis direction. (d) HRTEM image of an AlN nanowire, the growth direction is along the c-axis direction.
in Fig. 3(b). The HRTEM and fast Fourier transform images reveal that the synthesized AlN nanowire is single crystalline with hexagonal wurtzite structure. The adjacent spacing of the lattice fringes in the HRTEM image is 0.271 nm, which corresponds to the (1 0 0) plane of wurtzite AlN. It indicates that the AlN nanowire grows along [1 2 0] direction, being vertical to [0 0 1] direction. Fig. 3(c) shows the HRTEM image of another AlN nanowire. The interlayer spacing of 0.248 nm corresponds to the (0 0 2) plane of wurtzite AlN. This indicates that the growth direction of this AlN nanowire was also vertical to [0 0 1] direction. In addition, it is also found that the AlN nanowire grew along the [0 0 1] direction, as shown in Fig. 3(d). This indicates that not all AlN nanowires should grow along the direction perpendicular to the [0 0 1] direction under arc-discharge system. Non-uniform growth direction of AlN nanowires is most likely caused by kinetic crystal-growth
process under non-equilibrium provided by arc-discharge process. Two well-established growth mechanisms for 1D AlN nanostructures are vapor–solid (VS) and metal-catalyst-assisted vapor–liquid–solid (VLS) model [15–17,19]. In the VS process, a vapor species generated from evaporation or chemical reaction is transported and condensed onto a substrate. In the VLS process, the metal-catalytic-assisted growth is governed by a liquid–solid interface. A liquid droplet, supersaturated by vapor species, is definitely located at the growth front of the nanowire and acts as the catalytic active site. In our experiment, no catalyst was employed for the AlN nanowires growth, meanwhile no solidified spherical droplet was also observed at the tip of the nanowire from the result of TEM. This indicates the growth of the AlN nanowires is dominated by VS growth mechanism. The thermal convection produced in arc discharge process can auto-
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matically provide a vapor transport and condensation process, which is responsible for the formation of AlN nanowires. During the arc discharge process, the Al vapors are generated from the Al anode under accelerated electron bombardment, and then react with the nitrogen radicals from the N2 atmosphere parallel to the high temperature plasma processes. The Al–N species are formed in plasma, and then Al–N species were transported by the thermal convection to a certain temperature zone to form into AlN nuclei. The AlN nuclei absorb continually the Al and N atom, and then the AlN crystal grow up along certain direction and form AlN nanowires. Raman scattering is an excellent tool for studying the crystalline quality and presence of lattice defects. It is well known that the space group of hexagonal wurtzite AlN crystal is C46v (P63 mc) with all atoms occupying the C3v sites. Six first-order Raman active modes, 2E2 , 1A1 (TO), 1A1 (LO), 1E1 (TO), and 1E1 (LO), may be present [21]. The Raman spectrum of the assynthesized AlN nanowires was shown in Fig. 4. Six signals at 248.3 cm−1 , 612.2 cm−1 , 656.3 cm−1 , 668.8 cm−1 899.6 cm−1 and 907.9 cm−1 are indexed to the E2 (low), A1 (TO), E2 (high), E1 (TO), A1 (LO), and E1 (LO) phonon modes, respectively, which are signatures for wurtzite AlN as reported previously for bulk and film structure [22,23]. Two characteristic peaks assigned to the lattice vibration modes A1 (TO) and E2 (high) are usually employed to analyze the stress state information and nanosize effects of AlN nanocrystal [16,24]. The frequency of symmetric and strong E2 (high) phonon mode at 656.3 cm−1 is comparable with that observed in the bulk AlN crystal (656 cm−1 ) [23], while the A1 (TO) phonon mode at 612.2 cm−1 appears a little asymmetric. The Gauss fitted Raman spectrum, as shown in inset of Fig. 4, gives the full width at half maximum (FWHM) of A1 (TO) and E2 (high), which are 8.5 cm−1 and 6.14 cm−1 , respectively. These FWHM values are comparable with 7.0 cm−1 and 6.4 cm−1 for A1 (TO) and E2 (high) modes reported for AlN crystallite, respectively [25]. The nanomaterials, due to the size confinement effect and internal stress, usually make the Raman peaks appear broadened and asymmetric [26]. From the analysis of the linewidth and frequency of two char-
acteristic peaks, the as-synthesized AlN nanowires have low internal stress and are not obviously affected by size confinement effect. The band gap information of the as-synthesized AlN nanowires was characterized by UV spectroscopy. The Fig. 5 shows the absorption spectrum of the AlN nanowires at room temperature recorded from 190 nm to 400 nm. The absorption edge is at 199 nm, corresponding to the band gap of 6.23 eV, which is compared with the band gap of bulk AlN. The room-temperature photoluminescence spectra of the AlN nanowires were shown in Fig. 6. It exhibits a very broad emission bands from 380 nm (3.26 eV) to 650 nm (1.91 eV) with a center peak at about 506 nm (2.45 eV). Obviously, the emission bands cannot be attributed to band edge emission of AlN, but may originate from defect luminescence due to several separated defect state inside the AlN band gap. The very broad emission bands may result from the surface defect of AlN nanowires. The emission peak centered at about 380 nm (3.26 eV) of AlN nanomaterials has generally been ascribed to the transition from the shallow donor level to the excited states of the deep acceptor
Fig. 4. Raman spectra of the as-synthesized sample. Inset: Gauss fitted Raman spectrum of two characteristic peaks (the solid line is the original experimental data and the dashed lines are the Gauss fitted data).
Fig. 6. Photoluminescence spectrum of the as-synthesized AlN nanowires.
Fig. 5. The absorption spectrum of as-synthesized AlN nanowires.
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level. The shallow and deep levels are related to the separated ON (oxygen impurity) ion and ON –VAl (Al vacancy) defect complex, respectively [27,28]. The emission peaks at about 481 nm in AlN nanocones, and 519 nm in Aligned AlN Nanorods, are attributed to the nitrogen deficiency [19,29]. This indicate that the emission peak at 506 nm may be related to the transition from deep donor level of nitrogen vacancy at about 2.5 eV above the valence band to the ground sate. 4. Conclusions High density of wurtzite AlN nanowires with length about several tens micrometer and average diameter 40 nm were successfully synthesized through the direct reaction of Al and N2 without catalyst and template. The arc discharge system provides automatically a chemical-vapor transport and condensation process. The formation of AlN nanowires can be understanded by vapor–solid growth mechanism. The characterization of Raman spectrum results indicates that the crystallization of AlN nanowires is close to single crystal bulk AlN. The UV studies reveal that the absorption edge of AlN nanowires is at 6.23 eV. The PL spectrum displays a yellow emission band centered at 506 nm, which may have potential application in light-emitting nanodevices. Acknowledgments This work was financially supported by the Natural Science Foundation of China (No. 10647138), Scientific Research Foundation of the Educational Bureau of Liaoning Province (No. 20060667), and National Basic Research Program of China (No. 2005CB724400). We thank Dongshan Zhao at center of electron Microscopy in Wuhan University for his technical support. References [1] T. Ruecks, K. Kim, E. Joselevich, G.Y. Tseng, C. Cheung, C.M. Lieber, Sience 289 (2000) 94. [2] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [3] Q. Wu, Z. Hu, X.Z. Wang, Y. Chen, J. Phys. Chem. B 107 (2003) 9726. [4] M.S. Gudiksen, L.J. Lauhon, J. Wang, D.C. Smith, C.M. Lieber, Nature 415 (2002) 617.
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Synthesis and optical properties of aluminum nitride nanowires prepared by arc discharge method Longhai Shen a,∗ , Taimin Cheng b , Lijun Wu a , Xuefei Li c , Qiliang Cui c a
b
School of Science, Shenyang Ligong University, Shenyang 110168, PR China Department of Mathematics and Physics, Shenyang Institute of Chemical Technology, Shenyang 110142, PR China c National Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China Received 12 September 2007; received in revised form 1 November 2007; accepted 1 November 2007 Available online 20 February 2008
Abstract AlN nanowires with hexagonal structure were successfully synthesized by direct reaction of aluminum with nitrogen gas using arc discharge method. The wurtzite AlN nanowires have an average diameter of 40 nm and a length of several tens micrometer. The growth direction of most single-crystalline AlN nanowires is perpendicular to the [0 0 1] direction, while it is also found that the AlN nanowires grow along [0 0 1] direction. The vapor–solid growth mechanism can explain the formation of the AlN nanowires. Raman spectroscopy studies of the AlN nanowires reveal that the stress is rather low and the crystallinity is close to bulk AlN. The UV spectrum of the AlN nanowires shows that the absorption edge at 6.23 eV is comparable with that of the bulk AlN. The photoluminescence of the AlN nanowires suggests that the emission band at 506 nm may be ascribed to the deep level defect due to nitrogen vacancy. © 2007 Elsevier B.V. All rights reserved. Keywords: Aluminum nitrides; Nanowires; Crystal morphology; Arc discharge; Optical properties
1. Introduction One-dimensional (1D) nanostructured materials have attracted much attention because of their unique properties derived their high surface area and low dimensionality [1,2], which are particularly important not only for understanding fundamental concepts underlying the observed optical, electronic, and mechanical properties of materials, but also for being potentially applicable to nanoscale devices [3–6]. Therefore, the synthesis and properties of various 1D nanostructured materials are enthusiastically stimulated since the discovery of carbon nanotubes [7]. As an important III–V semiconductor, aluminum nitride (AlN) has a wide band gap (6.2 eV), a large exciton binding energy, and a very small electron affinity. These attractive physical properties make it potential application in surface acoustic wave, ultraviolet sensor, and field-emission devices [8–10]. Consequently, 1D AlN nanostructures are of fresh interest at present
∗
Corresponding author. Tel.: +86 24 24682191. E-mail address: [email protected] (L. Shen).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.11.007
due to its promising application in optoelectronic and fieldemission nanodevice [11,12]. Very recently, several routes have been developed to prepare 1D AlN nanowires, such as carbon nanotubes confined reaction [13], confined method of anodic porous aluminum template [14], extended vapor–liquid–solid growth technique [15] and direct reaction of Al or Al alloy in a mixture of nitrogen and ammonia gas (N2 + NH3 ) [16,17]. All these method, the flowing NH3 was employed as necessary nitrogen source and a crucial condition for synthesis of AlN nanowires. Additionally, the NH3 flux and ratio of NH3 to N2 also was accurately controlled and the reaction time is usually not less than two hours. In this paper, we successfully synthesized abundant AlN nanowires in five minutes by direct reaction of Al with N2 using direct current arc discharge method without catalyst and template. The optical properties of the assynthesized AlN nanowires are also investigated. 2. Experimental The direct current arc discharge apparatus used in this experiment has been described elsewhere [18]. In brief, the pure metal Mo (purity 99.99%) rod (10 mm in diameter and 160 mm in length) was used as the cathode. The Al (purity 99.99%) column (30 mm in diameter, 30 mm in height) as raw material
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Fig. 1. (a)–(d) The SEM images of the as-synthesized product.
was tightly inserted into a water-cooled anode copper crucible. The distance is about 20 mm between the tip of the Mo cathode and the Al column. Before arc discharging, the reaction chamber was evacuated to below 10−2 Torr and flushed with pure Ar gas (purity 99.99%) to remove the residual oxygen, and then 400 Torr pure N2 (purity 99.99%) was filled into the chamber as reaction gas. When the arc was ignited, the discharge voltage and current was kept in the ranges of 35–40 V and 100 A, respectively. The arc discharge process was maintained for 5 min. Finally, a layer of white product was suspended around Mo cathode. The morphology was determined by field-emission scanning electron microscopy (SEM, XL 30 ESEM). The crystal structure of white as-synthesized products was characterized by X-ray diffractometer (XRD, D8 DISCOVERGADDS) with Cu K␣ radiation (λ = 0.15418 nm). The microstructures of the nanowires were studied by high-resolution transmission electron microscopy (HRTEM, JEOL-2010 FEF, 200 kV). The Raman scattering measurements were performed by Raman spectroscopy (Renishaw 1000) with a confocal microscopy at room temperature, using excitation wavelengths of 488 nm (Argon ion laser). The optical absorption was measured using an ultraviolet–visible spectroscopy (UV-3150, Shimadzu). Photoluminescence spectrum (PL) at room temperature was measured using an Argon ion laser as the excitation source (325 nm).
˚ and c = 4.985 A, ˚ which match well with those of of a = 3.101 A bulk AlN crystal (JCPDS card No. 76-0702). No other impurity phases were found in the XRD pattern. When the AlN nanowires are vertically aligned on the substrate, the strong intensity of the (0 0 2) peak indicates that the AlN crystals grew preferentially along the [0 0 1] direction [19]. Since the AlN nanowires are randomly planished on the no-peaks silicon substrate in our XRD measure, the strong intensity of the (0 0 2) peak in our XRD curve indicates that most AlN nanowires grew preferentially along the direction perpendicular to [0 0 1] direction [20]. Fig. 3(a) shows the TEM image of a single AlN nanowire with uniform diameter about 20 nm. The corresponding HRTEM and fast Fourier transform images for the AlN nanowire are shown
3. Results and discussion Fig. 1(a)–(d) shows the SEM images of the as-synthesized product without affecting their original nature. Fig. 1(a) shows clearly a high density of AlN nanowires, which reveals that the nanowires have the length about tens of micrometers. The high-magnification SEM image of these nanowires is shown in Fig. 1(b), which shows the nanowires have a uniform diameter about 40 nm. Additionally, the quasi-aligned AlN nanowires are also observed, as shown in Fig. 1(c) and (d). These nanowires grow across like fence, or bind together, forming many bundles. Fig. 2 shows a typical XRD pattern of the collected white product. All the peaks in the XRD pattern could be indexed to the hexagonal wurtzite phase of AlN with the lattice constants
Fig. 2. The XRD pattern of the as-synthesized white product.
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Fig. 3. (a) TEM image of a representative AlN nanowire, (b) HRTEM image of an single AlN nanowire. Inset: The corresponding fast Fourier transform images of the HRTEM image. (c) HRTEM image of an AlN nanowire, the growth direction is perpendicular to the c-axis direction. (d) HRTEM image of an AlN nanowire, the growth direction is along the c-axis direction.
in Fig. 3(b). The HRTEM and fast Fourier transform images reveal that the synthesized AlN nanowire is single crystalline with hexagonal wurtzite structure. The adjacent spacing of the lattice fringes in the HRTEM image is 0.271 nm, which corresponds to the (1 0 0) plane of wurtzite AlN. It indicates that the AlN nanowire grows along [1 2 0] direction, being vertical to [0 0 1] direction. Fig. 3(c) shows the HRTEM image of another AlN nanowire. The interlayer spacing of 0.248 nm corresponds to the (0 0 2) plane of wurtzite AlN. This indicates that the growth direction of this AlN nanowire was also vertical to [0 0 1] direction. In addition, it is also found that the AlN nanowire grew along the [0 0 1] direction, as shown in Fig. 3(d). This indicates that not all AlN nanowires should grow along the direction perpendicular to the [0 0 1] direction under arc-discharge system. Non-uniform growth direction of AlN nanowires is most likely caused by kinetic crystal-growth
process under non-equilibrium provided by arc-discharge process. Two well-established growth mechanisms for 1D AlN nanostructures are vapor–solid (VS) and metal-catalyst-assisted vapor–liquid–solid (VLS) model [15–17,19]. In the VS process, a vapor species generated from evaporation or chemical reaction is transported and condensed onto a substrate. In the VLS process, the metal-catalytic-assisted growth is governed by a liquid–solid interface. A liquid droplet, supersaturated by vapor species, is definitely located at the growth front of the nanowire and acts as the catalytic active site. In our experiment, no catalyst was employed for the AlN nanowires growth, meanwhile no solidified spherical droplet was also observed at the tip of the nanowire from the result of TEM. This indicates the growth of the AlN nanowires is dominated by VS growth mechanism. The thermal convection produced in arc discharge process can auto-
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matically provide a vapor transport and condensation process, which is responsible for the formation of AlN nanowires. During the arc discharge process, the Al vapors are generated from the Al anode under accelerated electron bombardment, and then react with the nitrogen radicals from the N2 atmosphere parallel to the high temperature plasma processes. The Al–N species are formed in plasma, and then Al–N species were transported by the thermal convection to a certain temperature zone to form into AlN nuclei. The AlN nuclei absorb continually the Al and N atom, and then the AlN crystal grow up along certain direction and form AlN nanowires. Raman scattering is an excellent tool for studying the crystalline quality and presence of lattice defects. It is well known that the space group of hexagonal wurtzite AlN crystal is C46v (P63 mc) with all atoms occupying the C3v sites. Six first-order Raman active modes, 2E2 , 1A1 (TO), 1A1 (LO), 1E1 (TO), and 1E1 (LO), may be present [21]. The Raman spectrum of the assynthesized AlN nanowires was shown in Fig. 4. Six signals at 248.3 cm−1 , 612.2 cm−1 , 656.3 cm−1 , 668.8 cm−1 899.6 cm−1 and 907.9 cm−1 are indexed to the E2 (low), A1 (TO), E2 (high), E1 (TO), A1 (LO), and E1 (LO) phonon modes, respectively, which are signatures for wurtzite AlN as reported previously for bulk and film structure [22,23]. Two characteristic peaks assigned to the lattice vibration modes A1 (TO) and E2 (high) are usually employed to analyze the stress state information and nanosize effects of AlN nanocrystal [16,24]. The frequency of symmetric and strong E2 (high) phonon mode at 656.3 cm−1 is comparable with that observed in the bulk AlN crystal (656 cm−1 ) [23], while the A1 (TO) phonon mode at 612.2 cm−1 appears a little asymmetric. The Gauss fitted Raman spectrum, as shown in inset of Fig. 4, gives the full width at half maximum (FWHM) of A1 (TO) and E2 (high), which are 8.5 cm−1 and 6.14 cm−1 , respectively. These FWHM values are comparable with 7.0 cm−1 and 6.4 cm−1 for A1 (TO) and E2 (high) modes reported for AlN crystallite, respectively [25]. The nanomaterials, due to the size confinement effect and internal stress, usually make the Raman peaks appear broadened and asymmetric [26]. From the analysis of the linewidth and frequency of two char-
acteristic peaks, the as-synthesized AlN nanowires have low internal stress and are not obviously affected by size confinement effect. The band gap information of the as-synthesized AlN nanowires was characterized by UV spectroscopy. The Fig. 5 shows the absorption spectrum of the AlN nanowires at room temperature recorded from 190 nm to 400 nm. The absorption edge is at 199 nm, corresponding to the band gap of 6.23 eV, which is compared with the band gap of bulk AlN. The room-temperature photoluminescence spectra of the AlN nanowires were shown in Fig. 6. It exhibits a very broad emission bands from 380 nm (3.26 eV) to 650 nm (1.91 eV) with a center peak at about 506 nm (2.45 eV). Obviously, the emission bands cannot be attributed to band edge emission of AlN, but may originate from defect luminescence due to several separated defect state inside the AlN band gap. The very broad emission bands may result from the surface defect of AlN nanowires. The emission peak centered at about 380 nm (3.26 eV) of AlN nanomaterials has generally been ascribed to the transition from the shallow donor level to the excited states of the deep acceptor
Fig. 4. Raman spectra of the as-synthesized sample. Inset: Gauss fitted Raman spectrum of two characteristic peaks (the solid line is the original experimental data and the dashed lines are the Gauss fitted data).
Fig. 6. Photoluminescence spectrum of the as-synthesized AlN nanowires.
Fig. 5. The absorption spectrum of as-synthesized AlN nanowires.
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level. The shallow and deep levels are related to the separated ON (oxygen impurity) ion and ON –VAl (Al vacancy) defect complex, respectively [27,28]. The emission peaks at about 481 nm in AlN nanocones, and 519 nm in Aligned AlN Nanorods, are attributed to the nitrogen deficiency [19,29]. This indicate that the emission peak at 506 nm may be related to the transition from deep donor level of nitrogen vacancy at about 2.5 eV above the valence band to the ground sate. 4. Conclusions High density of wurtzite AlN nanowires with length about several tens micrometer and average diameter 40 nm were successfully synthesized through the direct reaction of Al and N2 without catalyst and template. The arc discharge system provides automatically a chemical-vapor transport and condensation process. The formation of AlN nanowires can be understanded by vapor–solid growth mechanism. The characterization of Raman spectrum results indicates that the crystallization of AlN nanowires is close to single crystal bulk AlN. The UV studies reveal that the absorption edge of AlN nanowires is at 6.23 eV. The PL spectrum displays a yellow emission band centered at 506 nm, which may have potential application in light-emitting nanodevices. Acknowledgments This work was financially supported by the Natural Science Foundation of China (No. 10647138), Scientific Research Foundation of the Educational Bureau of Liaoning Province (No. 20060667), and National Basic Research Program of China (No. 2005CB724400). We thank Dongshan Zhao at center of electron Microscopy in Wuhan University for his technical support. References [1] T. Ruecks, K. Kim, E. Joselevich, G.Y. Tseng, C. Cheung, C.M. Lieber, Sience 289 (2000) 94. [2] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [3] Q. Wu, Z. Hu, X.Z. Wang, Y. Chen, J. Phys. Chem. B 107 (2003) 9726. [4] M.S. Gudiksen, L.J. Lauhon, J. Wang, D.C. Smith, C.M. Lieber, Nature 415 (2002) 617.
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