ARTICLE IN PRESS Journal of Crystal Growth i (ulllll-ll Contents lists available at sciverse science direct Journal of Crystal Growth ELSEVIER ournalhomepagewww.elsevier.com/locate/jcrysgro Vapor-liquid-solid growth route to aIN nanowires on Au-coated Si substrate by direct nitridation of Al powder Leshu Yu, Yingying Lv, Xiaolan Zhang, Yiyue Zhang, Ruyi Zou, Fan Zhang eyChemistry and Chemical Engineering. Shangrao Normal University. Jiangxi 334000,China Jiangsu Marine Resources Development Research Institute of Huaihai Institute of Technology, Jiangsu 222001, China ARTICLE INFO ABSTRACT he past several decades vapor-liquid-solid (VLS)growth mechanism has been used for constructing Received 15 March 2011 one dimensional (lD)AIN nanostructures though the clear observation of metallic catalyst particles on top of individual ID nanostructure is rare. Using Au thin film on Si substrate as metallic catalyst, fine ccepted 17 August 2011 AIN nanowires were grown through the nitridation of A powder in this study. The systematic Communicated by j.M. Redwing haracterizations including scanning electron microscopy(SEM), transmission electron microscopy TEM) and energy dispersive X-ray spectroscopy(EDX) have confirmed the existence of metallic catalyst particles on the top of each AIN nanowire. Therefore the AlN nanowires growth is indeed Keywords: reaction temperature were also explored for the growth of AIN nanowires. Incidentally some A2. Vapor-lid nanostructures such as faceted cross-sectional nanorods nanobelt and nanocomb were also obtained B1. One-dimensional nanomaterials via vapor-solid growth mechanism on the Si substrate B2 Semiconducting lll-V materials c 2011 Elsevier B V. All rights reserved controlling the geometry of final products. To date people usually attribute the vs growth of 1d nanostructures to the help of Among many methods to fabricate one-dimensional (1D) defects, which play the role of effective collecting sites for nanomaterials, vapor-phase synthesis is the most extensively incoming atoms and result in the anisotropic crystal used one due to the low cost, easy operation, large scale and AIN is an important member of group-lll nitrides w high crystallization of the product [1. Various mechanisms applications in optoelectronic and electronic devices responsible for 1D growth in the vapor phase have been studied 1D nanostructures are of great interest both for fundamental or several decades. Vapor-liquid-solid (VLS) process and vapor- research and emerging applications. Recently great progress solid(vs)process are most successful for constructing 1D nanos- was made in preparing some 1D tube, wire, belt- and cone-as tructure though the details are not unambiguously verified up to well as hierarchy-like AN nanostructures [10-14]. Among these date. The vls growth was originally proposed by Wagner and Ellis AIN morphologies, AIN nanowires can be used as main blocks to explain micrometer-sized whiskers in 1960s [2, and recently building for electromechanical nanoscale devices. However, to adopted by many scientific research groups to generate numerous our knowledge, few aln nanowires have been synthesized via vls organic 1D nanostuctures3-5] a typical VLS process probably growth with the clear observations of metallic catalyst particles occurs after meeting the two prerequisites: (1)nano-sized metal- on the top of nanowires [15]. Although many research group lic liquid droplets and (2)sufficiently low vapor pressure of the speculated that as-synthesized ain nanowires were grown via used precursor [1. 4. The former limits the lateral growth of an VLS mechanism, no observations of metallic catalyst particles on individual and promotes anisotropic crystal growth; while the the top of nanowires made their claims inconvincible [16-18]. latter suppresses secondary nucleation growth[1, 4. Nowadays, Very recently, Hu et al. demonstrated the detailed Vls growth the vls growth process has become a widely used approach to process of the sparse AIN nanowires from the nitridation of Al-Ni construct 1D nanostructures with uniform diameter along length alloys at the high reaction temperature up to 1100C using [6-8]. In contrast to the vLs growth, the studies on the vs growth the bimetallic phase equilibrium diagram [19 In this report, mechanism were relatively lagged due to the difficulty in the fine Aln nanowires with uniform diameter were obtained on Au-c substrate through the nitridation of Al powde The direct observation of metallic catalyst particles on the top Corresponding author. Tel. +86 7938150637: fax: +867938150621 of each AIN nanowire, herein further supports that the E-mailaddress:yuleshu2008@126.com(LYu). AIN nanowires growth mechanism is indeed VLs process. 0022-0248S-see front matter e 2011 Elsevier B V. All rights reserved. doi:10.1016/ .crysgro.2011.08025 Please cite this article as: L. Yu, et aL, ]. Crystal Growth(2011). doi: 10.1016 j-jcrysgro 2011.08.025
Vapor–liquid–solid growth route to AlN nanowires on Au-coated Si substrate by direct nitridation of Al powder Leshu Yu a,n , Yingying Lv a , Xiaolan Zhang a , Yiyue Zhang a , Ruyi Zou a , Fan Zhang b a School of Chemistry and Chemical Engineering, Shangrao Normal University, Jiangxi 334000, China b Jiangsu Marine Resources Development Research Institute of Huaihai Institute of Technology, Jiangsu 222001, China article info Article history: Received 15 March 2011 Received in revised form 22 July 2011 Accepted 17 August 2011 Communicated by J.M. Redwing Keywords: A1. X-ray diffraction A2. Growth from vapor A2. Vapor–liquid–solid B1. One-dimensional nanomaterials B2. Semiconducting III–V materials abstract In the past several decades vapor–liquid–solid (VLS) growth mechanism has been used for constructing one dimensional (1D) AlN nanostructures though the clear observation of metallic catalyst particles on top of individual 1D nanostructure is rare. Using Au thin film on Si substrate as metallic catalyst, fine AlN nanowires were grown through the nitridation of Al powder in this study. The systematic characterizations including scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) have confirmed the existence of metallic catalyst particles on the top of each AlN nanowire. Therefore the AlN nanowires growth is indeed accomplished via VLS process. The VLS-generated conditions including thickness of Au film and reaction temperature were also explored for the growth of AlN nanowires. Incidentally some other AlN nanostructures such as faceted cross-sectional nanorods, nanobelt and nanocomb were also obtained via vapor–solid growth mechanism on the Si substrate. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Among many methods to fabricate one-dimensional (1D) nanomaterials, vapor-phase synthesis is the most extensively used one due to the low cost, easy operation, large scale and high crystallization of the product [1]. Various mechanisms responsible for 1D growth in the vapor phase have been studied for several decades. Vapor–liquid–solid (VLS) process and vapor– solid (VS) process are most successful for constructing 1D nanostructure though the details are not unambiguously verified up to date. The VLS growth was originally proposed by Wagner and Ellis to explain micrometer-sized whiskers in 1960s [2], and recently adopted by many scientific research groups to generate numerous inorganic 1D nanostuctures [3–5]. A typical VLS process probably occurs after meeting the two prerequisites: (1) nano-sized metallic liquid droplets and (2) sufficiently low vapor pressure of the used precursor [1,4]. The former limits the lateral growth of an individual and promotes anisotropic crystal growth; while the latter suppresses secondary nucleation growth [1,4]. Nowadays, the VLS growth process has become a widely used approach to construct 1D nanostructures with uniform diameter along length [6–8]. In contrast to the VLS growth, the studies on the VS growth mechanism were relatively lagged due to the difficulty in controlling the geometry of final products. To date people usually attribute the VS growth of 1D nanostructures to the help of defects, which play the role of effective collecting sites for incoming atoms and result in the anisotropic crystal growth [1]. AlN is an important member of group-III nitrides with various applications in optoelectronic and electronic devices [9], and its 1D nanostructures are of great interest both for fundamental research and emerging applications. Recently great progress was made in preparing some 1D tube-, wire-, belt- and cone- as well as hierarchy-like AlN nanostructures [10–14]. Among these AlN morphologies, AlN nanowires can be used as main blocks building for electromechanical nanoscale devices. However, to our knowledge, few AlN nanowires have been synthesized via VLS growth with the clear observations of metallic catalyst particles on the top of nanowires [15]. Although many research groups speculated that as-synthesized AlN nanowires were grown via VLS mechanism, no observations of metallic catalyst particles on the top of nanowires made their claims inconvincible [16–18]. Very recently, Hu et al. demonstrated the detailed VLS growth process of the sparse AlN nanowires from the nitridation of Al–Ni alloys at the high reaction temperature up to 1100 1C using the bimetallic phase equilibrium diagram [19]. In this report, the fine AlN nanowires with uniform diameter were obtained on Au-coated Si substrate through the nitridation of Al powder. The direct observation of metallic catalyst particles on the top of each AlN nanowire, herein further supports that the AlN nanowires growth mechanism is indeed VLS process. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.08.025 n Corresponding author. Tel.: þ86 793 8150637; fax: þ86 793 8150621. E-mail address: yuleshu2008@126.com (L. Yu). Please cite this article as: L. Yu, et al., J. Crystal Growth (2011), doi:10.1016/j.jcrysgro.2011.08.025 Journal of Crystal Growth ] (]]]]) ]]]–]]]
ARTICLE IN PRESS L Yu et al /Journal of Crystal Growth I(un)Ill-lll Furthermore, the vls growth of AIN nanowires was also disc AIN nanostructures such as comb-like faceted cross-sectional and in terms of the existence and the size of Au catalyst belt-like were also prepared via VS growth mechanism on the Si substrate and the reaction temperature. Incidentally some 300nm 100nm Fig. 1. Structural and morphological characterizations of the aIn nanowires obtained at 900C:(a)XRD pattern, revealing the product hexagonal phase(b)SL ttached a round ball, which was circled in red. (d)TEM image of a typical AIN nanowire clearly demonstrating the existence of the catalyst, left inset is another Tl ntaining two AIN nanowires with catalyst attached on top of them, and right insets are the edx curves of the top and the body of the displayed AIN respectively. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article as: L Yu, et al. ]. Crystal Growth(2011). doi: 10.1016 j-jcrysgro 2011.08.025
Furthermore, the VLS growth of AlN nanowires was also discussed in terms of the existence and the size of Au catalyst on Si substrate and the reaction temperature. Incidentally some other AlN nanostructures such as comb-like, faceted cross-sectional and belt-like were also prepared via VS growth mechanism on the Si substrate. Fig. 1. Structural and morphological characterizations of the AlN nanowires obtained at 900 1C: (a) XRD pattern, revealing the product hexagonal phase. (b) SEM image, showing as-prepared AlN wire-like morphology with several decades micrometer in length. (c) Close-up of a local in a, indicating on the top of each AlN nanowire is attached a round ball, which was circled in red. (d) TEM image of a typical AlN nanowire clearly demonstrating the existence of the catalyst, left inset is another TEM image containing two AlN nanowires with catalyst attached on top of them, and right insets are the EDX curves of the top and the body of the displayed AlN nanowire, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 2 L. Yu et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]] Please cite this article as: L. Yu, et al., J. Crystal Growth (2011), doi:10.1016/j.jcrysgro.2011.08.025
ARTICLE IN PRESS L Yu et al/ Journal of Crystal Growth I(un)I 2. Experimental section from TEM image that a typical AIN nanowire with uniform diameter of ca. 100 nm contained two distinct parts: (1) the blacl Si(100)substrates, unless otherwise mentioned, were depo end and (2)the gray rod-like body. EDX curve indicated the black ited by 2-5 nm thick Au film via a magnetic sputtering process. end is composed of Au, Al, N and the inevitable O elements; while An alumina boat, carrying the mixture of pre-milled aluminum the gray rod composed of Al, n and the inevitable o elements. The powder and calcium fluoride powder with the weight ratio of 1: 1. left inset in d also revealed the existence of the round metallic was placed inside a quartz tube. The distance between Si ball at the top of each nanowire. Therefore, TEM image is substrate (with Au-coated face facing the mixture)and the consistent with SEM. Based on the observation of Au catalyst at mixture was about 0.5 cm. The quartz tube was housed in a the top of each AIn nanowire, it is reasonable to conclude that the flushed with Ar flow several times to remove the oxygen and process whose detailed growing steps are shown in Fig.2 Vls conventional tubular furnace. This system was evacuated and growth mechanism for AIN nanowires in this work is vi moisture. Then the furnace was heated to 900-1000oC at the rate of 10C/min under 60 sccm of ammonia/nitrogen atmosphere 3. 2. VLS-generated conditions for the growth of AIN nanowires (NH3 4 vol%). After allowing the reaction to proceed for 45 min, the furnace was cooled to ambient temperature under the ar For From the characterizations shown in Fig. 1, it is clear that fine comparison,the experiments were also performed using bare Si AIN nanowires with uniform diameter over the entire length substrates without metallic catalyst and/or at high reaction could be readily obtained via VLS growing mechanism. As men- temperature up to 1100C. The products obtained were charac tioned above, a typical vlS process probably occurs after meeting terized by X-ray diffraction(XRD; Philips X'pert Pro diffract- the two prerequisites:(1)nano-sized metallic droplets and(2) ometer),scanning electron microscopy (SEM; JEOL JSM-6300) sufficiently low vapor pressure of the used precursor. Herein the and transmission electron microscopy (TEM; JEOL-JEM-1005) growth process is also related to the two main factors equipped with an energy-dispersion X-ray spectrometer(EDX). When the thickness of au film on si substrate is much more Note: do not use strong ultrasonic very long time when than 5 nm, for instance 12 nm in our previous report, the aIn preparing the samples for TEM observation in order to keep the nanoflowers, nanocones and nanowires were obtained on metallic catalyst from falling off on the top of nanowires Au-coated substrates in the range of 900-1000C via VS growth mechanism [11]. This could be explained by the diagram of Au film melting point vs. its thickness(Fig 3a), calculated from the 3. Results and discussions ormula Tm/Tb=1-1. 1281/3D, where D and Tm are the thickness and the corresponding melting point of the Au film on Si 3.1. Structural and morphological analysis substrate, and Tb is the melting point of the bulk Au 20. If Au film on Si substrates is 8 nm in thickness, its melting point is as gh as 1000'C, indicating the Au film on Si substrate is in solid at 33. 22, 36.05, 37.94, 49.84 and 59.37 can be indexed as the state. Hence VLs growth mechanism is prohibited for preparing wurtzite aln phase with lattice parameters of a=3. 111 A and AlN nanowires. The thickness of Au film thereafter the size of At C=4.978A(PDF# 760566): other peaks were resulted from nanoparticles on Si substrate played a key role of determining the exposed Si substrate, suggesting the deposited film of aln growth model of 1D nanowires [21. On the contrary, if no nanowires on si substrate ry thin. The strongest reflection metallic catalyst was used in the reaction, rhombic cross-sec of the(002)plane in all the diffraction peaks indicates that the tional aIn nanorods [22](Fig. 4a-c)or some AIN nanowires and as-prepared ain crystals grew perpendicularly to the basal plane nanobelts(Fig. 4d, e)were obtained via VS growth process at 900 r 1000C, respectively. The non-selective fabrication of SEM images(Fig. 2b, c) shows the morphologies of the as- nanostructures reveals vs growth is inferior to VLS growth synthesized products on Au-coated Si substrate obtained at On the other hand a low metallic Al vapor pressure is also very 900C. A number of randomly distributed aln nanowires covered important to generate VLs growth model. Fig. 3b is a plot of the Si substrate(Fig 2b), but most them kept growing upward. It Al vapor pressure vS. heating temperature, calculated by the is worth noting that the top of each AIN nanowire( Fig. 2c)is formula of logP=A/T+B+ClogT, where A, B and C are constants attached by a round ball as circled with red round. We believe [23. Al vapor pressure is increasing sharply when the heating these are the metallic catalyst particle, which is crucial in the temperature is up to 1100C. High vapor pressure of used preparations of inorganic 1D nanomaterials via Vls growth precursor is in favor of rowth model due to the fast mechanism. Furthermore each AIN nanowire is smooth and has nucleation for the 1D anisotropic crystal growths [1 In this work uniform diameter over its entire length. The nanowires have the addition of inert salt into al powder helps to increase the al lengths of a few tens of micrometer and diameters ranging from gaseous pressure resulting in the higher metallic Al vapor 50 to 100 nm( Fig. 2c). The detailed structure of AIN nanowires pressure than the calculated value from Fig 3b. Hence when the were further characterized by tEM and eDX (Fig. 1d). It is seen reaction temperature was much higher than 1000C, vs growth AIN, Al and N species 5 nm in thickness nanoparticles AIN, Al and N species Fig. 2. Schematic diagram of the preparation of AIN nanowires via VlS growth process in this preparation. Please cite this article as: L. Yu, et aL, ]. Crystal Growth(2011). doi: 10.1016 j-jcrysgro 2011.08.025
2. Experimental section Si (1 0 0) substrates, unless otherwise mentioned, were deposited by 2–5 nm thick Au film via a magnetic sputtering process. An alumina boat, carrying the mixture of pre-milled aluminum powder and calcium fluoride powder with the weight ratio of 1:1, was placed inside a quartz tube. The distance between Si substrate (with Au-coated face facing the mixture) and the mixture was about 0.5 cm. The quartz tube was housed in a conventional tubular furnace. This system was evacuated and flushed with Ar flow several times to remove the oxygen and moisture. Then the furnace was heated to 900–1000 1C at the rate of 10 1C/min under 60 sccm of ammonia/nitrogen atmosphere (NH3 4 vol%). After allowing the reaction to proceed for 45 min, the furnace was cooled to ambient temperature under the Ar. For comparison, the experiments were also performed using bare Si substrates without metallic catalyst and/or at high reaction temperature up to 1100 1C. The products obtained were characterized by X-ray diffraction (XRD; Philips X’pert Pro diffractometer), scanning electron microscopy (SEM; JEOL JSM-6300) and transmission electron microscopy (TEM; JEOL-JEM-1005) equipped with an energy-dispersion X-ray spectrometer (EDX). Note: do not use strong ultrasonic very long time when preparing the samples for TEM observation in order to keep the metallic catalyst from falling off on the top of nanowires. 3. Results and discussions 3.1. Structural and morphological analysis XRD pattern (Fig. 1a) shows that, the diffraction peaks located at 33.221, 36.051, 37.941, 49.841 and 59.371 can be indexed as the wurtzite AlN phase with lattice parameters of a¼3.111 A and ˚ c¼4.978 A (PDF# 760566); other peaks were resulted from ˚ exposed Si substrate, suggesting the deposited film of AlN nanowires on Si substrate was very thin. The strongest reflection of the (0 0 2) plane in all the diffraction peaks indicates that the as-prepared AlN crystals grew perpendicularly to the basal plane along [0 0 1]. SEM images (Fig. 2b, c) shows the morphologies of the assynthesized products on Au-coated Si substrate obtained at 900 1C. A number of randomly distributed AlN nanowires covered the Si substrate (Fig. 2b), but most them kept growing upward. It is worth noting that the top of each AlN nanowire (Fig. 2c) is attached by a round ball as circled with red round. We believe these are the metallic catalyst particle, which is crucital in the preparations of inorganic 1D nanomaterials via VLS growth mechanism. Furthermore each AlN nanowire is smooth and has uniform diameter over its entire length. The nanowires have lengths of a few tens of micrometer and diameters ranging from 50 to 100 nm (Fig. 2c). The detailed structure of AlN nanowires were further characterized by TEM and EDX (Fig. 1d). It is seen from TEM image that a typical AlN nanowire with uniform diameter of ca. 100 nm contained two distinct parts: (1) the black end and (2) the gray rod-like body. EDX curve indicated the black end is composed of Au, Al, N and the inevitable O elements; while the gray rod composed of Al, N and the inevitable O elements. The left inset in d also revealed the existence of the round metallic ball at the top of each nanowire. Therefore, TEM image is consistent with SEM. Based on the observation of Au catalyst at the top of each AlN nanowire, it is reasonable to conclude that the growth mechanism for AlN nanowires in this work is via VLS process whose detailed growing steps are shown in Fig. 2. 3.2. VLS-generated conditions for the growth of AlN nanowires From the characterizations shown in Fig. 1, it is clear that fine AlN nanowires with uniform diameter over the entire length could be readily obtained via VLS growing mechanism. As mentioned above, a typical VLS process probably occurs after meeting the two prerequisites: (1) nano-sized metallic droplets and (2) sufficiently low vapor pressure of the used precursor. Herein the growth process is also related to the two main factors. When the thickness of Au film on Si substrate is much more than 5 nm, for instance 12 nm in our previous report, the AlN nanoflowers, nanocones and nanowires were obtained on Au-coated substrates in the range of 900–1000 1C via VS growth mechanism [11]. This could be explained by the diagram of Au film melting point vs. its thickness (Fig. 3a), calculated from the formula Tm/Tb¼1–1.1281/3D, where D and Tm are the thickness and the corresponding melting point of the Au film on Si substrate, and Tb is the melting point of the bulk Au [20]. If Au film on Si substrates is 8 nm in thickness, its melting point is as high as 1000 1C, indicating the Au film on Si substrate is in solid state. Hence VLS growth mechanism is prohibited for preparing AlN nanowires. The thickness of Au film thereafter the size of Au nanoparticles on Si substrate played a key role of determining the growth model of 1D nanowires [21]. On the contrary, if no metallic catalyst was used in the reaction, rhombic cross-sectional AlN nanorods [22] (Fig. 4a–c) or some AlN nanowires and nanobelts (Fig. 4d, e) were obtained via VS growth process at 900 or 1000 1C, respectively. The non-selective fabrication of AlN nanostructures reveals VS growth is inferior to VLS growth. On the other hand, a low metallic Al vapor pressure is also very important to generate VLS growth model. Fig. 3b is a plot of Al vapor pressure vs. heating temperature, calculated by the formula of logP¼A/TþBþClogT, where A, B and C are constants [23]. Al vapor pressure is increasing sharply when the heating temperature is up to 1100 1C. High vapor pressure of used precursor is in favor of VS growth model due to the fast nucleation for the 1D anisotropic crystal growths [1]. In this work the addition of inert salt into Al powder helps to increase the Al gaseous pressure, resulting in the higher metallic Al vapor pressure than the calculated value from Fig. 3b. Hence when the reaction temperature was much higher than 1000 1C, VS growth Si Substrate Au film less than 5 nm in thickness Aggregating into nanoparticles via heating Saturation nucleation Nanowires growth AlN, Al and N species AlN, Al and N species Fig. 2. Schematic diagram of the preparation of AlN nanowires via VLS growth process in this preparation. L. Yu et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]] 3 Please cite this article as: L. Yu, et al., J. Crystal Growth (2011), doi:10.1016/j.jcrysgro.2011.08.025
ARTICLE IN PRESS L Yu et aL Journal of Crystal Growth I(nlIll-lll 00110012001300 Thickness(nm) Fig. 3. Diagrams of Au film melting point vs. its thickness(a) and al vapor pressure vs heating temperature(b b 100nm le 200nm g nm 200mm Fig 4. SEM and TEM images of some various AIN nanostructures obtained on Si substrates by the nitridation of Al powder: at 900C(a, b, c): 1000C(d, e)and 1100C(f g). dominates for the preparation of ID ures regardless growths in of the controllable construction of AN of the existence of metallic catalyst or dy Fig 4f and g tures. Our experiment results are also embodied in previc shows the ain nanocombs as well from many research groups [11, 14, 22, 24). This nanobelts and hierarchical structure, which were grown via Vs could be well understood from the primary conditions of vs and VLs mechanism at 1100C because secondary nucleation happened in growth, because Vs begins to happen from the defects, which is the presence of evident gaseous Al pressure. It is seen from the uncontrollable in the preparation process, while VLS starts under the obtained results above that, VS growths may be inferior to VLs crystal direction of catalyst which is certain in reactions [21, 25 Please cite this article as: L Yu, et al. ]. Crystal Growth(2011). doi: 10.1016 j-jcrysgro 2011.08.025
dominates for the preparation of 1D AlN nanostructures regardless of the existence of metallic catalyst or not in this study. Fig. 4f and g shows the AlN nanocombs as well as a minority of nanowires, nanobelts and hierarchical structure, which were grown via VS mechanism at 1100 1C because secondary nucleation happened in the presence of evident gaseous Al pressure. It is seen from the obtained results above that, VS growths may be inferior to VLS growths in aspect of the controllable construction of AlN nanostructures. Our experiment results are also embodied in previous reports from many other research groups [11,14,22,24]. This comparison could be well understood from the primary conditions of VS and VLS growth, because VS begins to happen from the defects, which is uncontrollable in the preparation process, while VLS starts under the crystal direction of catalyst which is certain in reactions [21,25]. 1µm Fig. 4. SEM and TEM images of some various AlN nanostructures obtained on Si substrates by the nitridation of Al powder: at 900 1C (a, b, c); 1000 1C (d, e) and 1100 1C (f, g), respectively. Fig. 3. Diagrams of Au film melting point vs. its thickness (a) and Al vapor pressure vs. heating temperature (b). 4 L. Yu et al. / Journal of Crystal Growth ] (]]]]) ]]]–]]] Please cite this article as: L. Yu, et al., J. Crystal Growth (2011), doi:10.1016/j.jcrysgro.2011.08.025
ARTICLE IN PRESS L Yu et al/ Journal of Crystal Growth I(un)I Overall, in order to produce fine AIN nanowires with good [71 LS. Yu, Y W. Ma, Z Hu, CV. D Low-temperature, synthesis route to GaN selectivity, a thin Au film acting as liquid metallic catalyst and a on silicon substrate, Joumal of Crystal Growth 310(2008) low vapor pressure of metallic Al are required for VLS growth model. [81 M.O. He, P.Z. Zhou, S.N. Mohammad, GL Harris, J.B. Halpern, R Jacobs, 4. Conclusions [91 J.H. Edgar, S. Strite. L. Akasaki, H. Amano, C.W. Etzel, Properties, Processing and Applications of Gallium Nitride and Related Semiconductors, INSPEC, The fine ain nanowires with uniform diameter over their entire ength have been synthesized on thin Au film-coated Si substrate. The [10] Q. Wu, Z Hu, x.Z Wang. Y.N. Lu, X Chen, H.Xu, YChen, Synthesis and convincing observations from SEM, TEM and EDX confirm the VLS f American Chemical Society 125(2003)10176-101 growth mechanism of AIN nanowires. Furthermore, the VLs growth 111 LS. Yu, ZHu, Y W. Ma, K.F. Huo, Y Chen, H Sang. W.W. Lin, Y.N. Lu, Evolution conditions for AIN nanowires were also explored. when the thicknes of Au film on Si substrate and the reaction temperature are less than ubstrate by direct nitridation of aluminum precursor, Diamond and Relate 5 nm and 1000C, respectively, the liquid state for Au nanoparticle [12] Q Wu, Z Hu, XZ Wang Y. Chen, Y.N. Lu, Synthesis and optical characteriza- and low gaseous pressure for metallic Al are thus sustained. In tion of aluminum nitride nanobelts, Journal of Physical Chemistry B 107 other Al ructures such as faceted cross- 2003)9726-9729 sectional nanorods, nanobelts and nanocombs have been synthesized [13] C Liu, Z Hu,. Wu, X.Z Wang. YChen H Sang. J.M. Zhu, S.Z. Deng NSXu, on Si substrate without metallic catalyst and or high reaction temperature up to1100℃ 114)LW. Yin. Y. Band. Y C. Zhu. M.S. L, Y.B. L. D 18-1322 Golberg. Growth and field of hierarchical single-crystalline wurtzite AN nanoarchitectures. Advanced Materials 17(2005)110-114 Acknowledgments [15 H.U. Jooa, B -K Minb, w.-s. Jung, Characteristics of aluminum nitride anowhiskers grown via the vapor-liquid-solid mechanism, Physica E 40 c This work was supported by the Natural Science Foundation of [16] Q. Wu, Z Hu, X.Z. Wang Y.N. Lu, K.F. Huo, S.Z. Deng. N.S. Xu, B Shen, China (21161016) the Fourth Characteristic Specialty Fou properties of aluminium nitride nanowires, Journal of Materials Chemistry of University from Ministry of Education(TS11524), the [17 V. Cimalla, Ch Foerster, D. Cengher, K. Tonisch, O Ambacher, Growth of AIN (G111607, GJ10613) and Key Laboratory of Jiangxi founde tal organic chemical vapour deposition, Pysica Status Solidi 243(2006)1476-1480 [19] C.Y. He, x.Z Wang. Q. Wu, Z. Hu Y.w. Ma, J-J. Fu, Y. Chen, Phas References plid growth mechanism, Journal of American Society132(2010)4843-4847 [20] KK Nanda, S.N. Sahu, S N. Behera, Liquid-drop model for the size-d Physical Review A 66(2002)(013 al nanostructures: synthesis, characterization, and applications. [21 K.H. ChoL kK. cho Advanced Materials 15(2003)353-385 B-Ga203 nanowires on the basis of catalyst size, Journal of Cryst [2] RS. Wagner, W.C. Ellis, Vapor-liquid-solid mechanism of single crystal 311(2009)1195-120 [22] LS. Yu, Y.Y. Lv, P.Y. Liu, X.Q. Yu, Synthesis of rhombic and triangular cross- rystal GaN AIN nanorods on si substrate via thermal cvd. materials letters 65 nical Society 122(2000)188-185 solid nanowire growth, [23] J.C. Bailar. H ] Emeleus, S.R. Nyholm, A.F. Trotman-dickenson, Comprehensive of American Chemical Society 123(2001)3165-3166 5 H L Liu, J-J. Wu, K.H. Chen, L.C. Chen, [24 S.C. Shi, S. Chattopadhyay, C.F. Chen, K.H. Chen, LC. Chen, Structural J.Y. Peng, Catalytic growth and characterization of gallium nitride nanowires. ers4l8(2006)152-157 [61 Y.J. Chen, J.B. Li. YS. Han, X. Z. Yang. J.H. Dai, The effect of Mg vapor source [25 C.H. Ye, Recent progress in understanding the growth mechanism of one- e formation of Mgo whiskers and sheets, Journal of Crystal growth 245 dimensional nanostructures by vapor pha rocesses, Science of Advanced Materials2(2010)365-377 Please cite this article as: L. Yu, et aL, ]. Crystal Growth(2011). doi: 10.1016 j-jcrysgro 2011.08.025
Overall, in order to produce fine AlN nanowires with good selectivity, a thin Au film acting as liquid metallic catalyst and a low vapor pressure of metallic Al are required for VLS growth model. 4. Conclusions The fine AlN nanowires with uniform diameter over their entire length have been synthesized on thin Au film-coated Si substrate. The convincing observations from SEM, TEM and EDX confirm the VLS growth mechanism of AlN nanowires. Furthermore, the VLS growth conditions for AlN nanowires were also explored. When the thickness of Au film on Si substrate and the reaction temperature are less than 5 nm and 1000 1C, respectively, the liquid state for Au nanoparticle and low gaseous pressure for metallic Al are thus sustained. In addition, some other AlN nanostructures such as faceted crosssectional nanorods, nanobelts and nanocombs have been synthesized on Si substrate without metallic catalyst and/or high reaction temperature up to 1100 1C. 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