Availableonlineatwww.sciencedirect.com SCIENC DIRECT E olid communications ELSEVIER Solid State Communications 132(2004)263-268 Growth of platelike and branched single-crystalline Si3N4 whiskers Weiyou Yang, Zhipeng Xie,, Jingjing Li, Hezhuo Miao, Zhang. Linan an State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing, 100084, People's Republic of China Laboratory of Excited State Process, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, People's Republic of China Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816. USA Received 11 June 2004: received in revised form 13 July 2004: accepted 14 July 2004 by C. Tejedor Available online 4 August 2004 Abstract In this communication, we report for the first time the growth of platelike and branched Si N4 whiskers via catalyst-assisted yrolysis of polymeric precursors. The as-prepared whiskers are single crystalline with a uniform thickness and wi thickness and width of the Si3N4 whiskers range from 200 to 300 nm and 800 to 1200 nm, respectively. The platelike whiskers grew along [010] direction, while the branches grew along [001 direction. a growth mechanism based liquid-gas-solid reaction/crystallization is proposed. The formation of platelike whiskers instead of cylindrical whiskers is attributed t pic growth at an early nucleation/growth stage. C 2004 Elsevier Ltd. All rights reserved. PACS:81.10,81.05Z Keywords: A SiN4 whiskers; B. Crystal growth; C. Crystal structure and symmetry 1. Introduction have been devoted to the synthesis of one-dimension Si3N4 structures. Si3N4 nanorods [5-7] have been synthesized by Due to its excellent thermal and mechanical properties, mild benzene-thermal route [51, carbothermal reduction [6] Si3N4 is an important engineering material for high and template method [7 Si3 N4 nanowires [8-17] have been temperature structural applications [1, 2]. In addition, synthesized by carbothermal reduction and nitriding reac- Si3 Na is also a wide band gap (5.3 ev) semiconductor in tion at high temperatures [8-13], combustion under a high which midgap levels can be introduced to tailor its N2 pressure[14], hot-filament CVD and microwave plasma electronic/optic properties by properly doping [3, 4]. Similar heating method [15-17. Most recently, Si,N4 nanobelts to the group Ill-N compounds(such as Gan and alN), have been synthesized via a vapor-solid thermal reaction Si3N4 could be an excellent host materials in terms of between ammonia and silicon monoxide [18] mechanical strength, thermal/chemical stability and high In this communication, we report the synthesis of dopant concentration, thus promises for microelectronic/ platelike and branched Si3, whiskers via a new method. optic devices that can operate at high temperatures an namely catalyst-assisted pyrolysis of polymer precursors. radiation environments. In the recent years, extensive efforts Si3N4 nanorods [21] have been synthesized by using the similar Corresponding author. Fax: +86-10-627-94603 technique. While branched structures have been synthesized E-mailaddresses:ywyol(@mails.tsinghua.edu.cn(Z.Xie), n various materials [22-35, to the best of our knowledge, this is the first time that branched Si3 Na have been reported. 038-1098 front matter 2004 Elsevier Ltd. All rights reserved doi:10.1016jsc200407.032
Growth of platelike and branched single-crystalline Si3N4 whiskers Weiyou Yanga , Zhipeng Xiea,*, Jingjing Lia , Hezhuo Miaoa , Ligong Zhangb , Linan Anb,c a State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing, 100084, People’s Republic of China b Laboratory of Excited State Process, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun, People’s Republic of China c Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816, USA Received 11 June 2004; received in revised form 13 July 2004; accepted 14 July 2004 by C. Tejedor Available online 4 August 2004 Abstract In this communication, we report for the first time the growth of platelike and branched Si3N4 whiskers via catalyst-assisted pyrolysis of polymeric precursors. The as-prepared whiskers are single crystalline with a uniform thickness and width. The thickness and width of the Si3N4 whiskers range from 200 to 300 nm and 800 to 1200 nm, respectively. The platelike a-Si3N4 whiskers grew along [010] direction, while the branches grew along [001] direction. A growth mechanism based on solid– liquid–gas–solid reaction/crystallization is proposed. The formation of platelike whiskers instead of cylindrical whiskers is attributed to an anisotropic growth at an early nucleation/growth stage. q 2004 Elsevier Ltd. All rights reserved. PACS: 81.10; 81.05.Z Keywords: A. Si3N4 whiskers; B. Crystal growth; C. Crystal structure and symmetry 1. Introduction Due to its excellent thermal and mechanical properties, Si3N4 is an important engineering material for high temperature structural applications [1,2]. In addition, Si3N4 is also a wide band gap (5.3 eV) semiconductor in which midgap levels can be introduced to tailor its electronic/optic properties by properly doping [3,4]. Similar to the Group III-N compounds (such as GaN and AlN), Si3N4 could be an excellent host materials in terms of mechanical strength, thermal/chemical stability and high dopant concentration, thus promises for microelectronic/ optic devices that can operate at high temperatures and radiation environments. In the recent years, extensive efforts have been devoted to the synthesis of one-dimension Si3N4 structures. Si3N4 nanorods [5–7] have been synthesized by mild benzene-thermal route [5], carbothermal reduction [6] and template method [7]. Si3N4 nanowires [8–17] have been synthesized by carbothermal reduction and nitriding reaction at high temperatures [8–13], combustion under a high N2 pressure [14], hot-filament CVD and microwave plasma heating method [15–17]. Most recently, Si3N4 nanobelts have been synthesized via a vapor–solid thermal reaction between ammonia and silicon monoxide [18]. In this communication, we report the synthesis of platelike and branched Si3N4 whiskers via a new method, namely catalyst-assisted pyrolysis of polymer precursors. Si3N4 nanowires [19], Si3N4 nanobelts [20] and SiC nanorods [21] have been synthesized by using the similar technique. While branched structures have been synthesized in various materials [22–35], to the best of our knowledge, this is the first time that branched Si3N4 have been reported. 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.07.032 Solid State Communications 132 (2004) 263–268 www.elsevier.com/locate/ssc * Corresponding author. Fax: C86-10-627-94603. E-mail addresses: ywy01@mails.tsinghua.edu.cn (Z. Xie), xzp@mail.tsinghua.edu.cn (Z. Xie)
W. Yang et al. Solid State Communications 132(2004)263-268 Furthermore, the trunks and branches of the branched branches were also observed(Fig. 2). It can be seen that structures reported previously are nanowires [22-28], the branches grow outward from the main stem, exhibiting nanotubes [29 nanoneedles [30,31, or nanoprisms [351, T-shaped(Fig. 2(a)), cross-shaped(Fig. 2(b), comb-like both trunks and branches of the branched Si3 N4 reported (Fig. 2(c))and feather-like(Fig. 2(d)) morphology. The here possess belt-like morphologies. It is anticipated that the typical stem of the structures is platelike, and the thickness novel structures could be useful in fabricating three and width of the stem are about 200 and 1000 nm dimensional composites and micro-/nano-devices. respectively. In the comb-like and feather-like structures. the branches possess belt-like cross-section with a uniform thickness and width along the growth direction which is are 2. Experiment fundamentally different from those previously observed in other material systems, where the branches possess A polyureasilazane(Ceraset, Kion Corporation, US) morphology of either wires or tubes or needles [22-301 was used as the precursor in this study. The as receive The branches are typically 50 nm in thickness and Ceraset, which is liquid at temperature, was first c1000 nm in width, and the length can be up to more solidified by heat-treatment at 260C in N2. The obtained than ten micrometers. The branches are separated with a solid was then crushed into fine powders by high-energy ball uniform distance on the stem and parallel to each other nilling for 24 h with an addition of 3 wt %o FeCl2 powders without sub-branches. The angle between the stem and (Beijing Bei Hua Fine Chemicals Company Lt. Beijin branch is about90° China). The ball-milled powder mixture was then placed XRD pattern (Fig. 3)of the synthesized whiskers high purity alumina crucible and pyrolysized in suggests that pyrolyzed products contain both a-Si3N4 and conventional furnace under flowing ultra-high purity B-Si3N4 phases. The broad hump at lower angle regions nitrogen. The powder mixture was heated to 1450C at suggests that the unconverted powders remains amorphous 10C/min and held there for 4 h followed by furnace-cool. The structures of platelike and branched Si3N4 whiskers The experiments were also performed on the samples ere further characterized with TEM. Fig. 4(a) shows the without FeCl2 additives for comparison typical TEM image of the platelike Si3 N4 whiskers. EDS The morphology, structure and composition of the analysis shows that the platelike whiskers consist of Si and pyrolysis products were characterized using field emission N elements only. The inset picture in Fig. 4(a) is the canning electron microscopy (SEM, JSM-6301F, JEOL, corresponding select area electron diffraction (SAED) Japan), X-ray diffraction (XRD, Automated D/Max-RB, pattern that is identical over the entire whisker, indicating Rigaku, Japan) with Cu Ko radiation(=1.54178 A), and that the whisker is a single crystal and shows the hexagonal transmissio on electron microscope (TEM, JEOL-2010F, structure of a-Si3N4, where a=0.77541 nm and c Japan) equipped with energy dispersive X-ray spectrum 0.56217 nm (JCPDS Card No. 41-0360). Examination on (EDS) more than 10 platelike whiskers suggests that [010] is the only growth direction for the platelike whiskers. Fig. 4(b) shows a Si3Na stem at the stage to nucleate branches; 3. Result and discussion numerous triangle droplets can be observed on the side of the stem. The inset picture in Fig. 4(b) is the typical EDS Fig. 1(a)is a typical SEM image of the as-pyrolyzed spectrum for the triangle droplets, which reveals that the products, showing that, relatively high-density whiskers droplets contain Fe element with a little amount of Cr and Ni have grown homogeneously on the top of the powder Cu elements come from the copper grid, and Cr and Ni may matrix. Closer examination under high magnification(Fig. have come from the impurities of the FeCl2 catalyst 1(b)and(c)) reveals that the cross-sections of the Sin This result indicates the catalyst growth of the branches whiskers are rectangular with the thickness ranges from 200 Fig 4(c)is a typical TEM image of T-shaped whiskers. The to 300 nm and the width from 800 to 1200 nm. Within each single crystalline nature of the branched structures is individual whisker. the thickness and width are unifon confirmed by SAED SAED patterns recorded from different along its entire length, which can be up to several areas(indicated by the white circles)in a single dendrite are millimeters. The SEM observation shows that the surfaces shown in Fig. 4(d). It is found that the SaEd patterns of the whiskers are smooth and clean. besides regular recorded from different areas of the dendrite are almost whiskers, intercrossed whiskers were also observed (Fig. identical, suggesting that the whole branched structure I(c). Fig. 1(d) is a typical SEM image of the tip of th stem+ branches)is a single crystal. The SAEd patterns can whisker, showing the triangle morphology. There are no be indexed to the hexagonal structures of a-Si3 N4. The liquid droplets at the tip, which was typically observed in growth directions of the stem and branch were [010] and vapor-liquid-solid (VSL) growth at the presence of [001], respectively catalysts [36], indicating the reported whiskers grew by No whiskers were formed in the samples without FeCl2 fundamentally different mechanism. additives, suggesting the catalytic growth of the Si3N Beside the platelike whiskers, the whiskers with
Furthermore, the trunks and branches of the branched structures reported previously are nanowires [22–28], nanotubes [29], nanoneedles [30,31], or nanoprisms [35], both trunks and branches of the branched Si3N4 reported here possess belt-like morphologies. It is anticipated that the novel structures could be useful in fabricating threedimensional composites and micro-/nano-devices. 2. Experiment A polyureasilazane (Cerasete, Kion Corporation, US) was used as the precursor in this study. The as-received Ceraset, which is liquid at room temperature, was first solidified by heat-treatment at 260 8C in N2. The obtained solid was then crushed into fine powders by high-energy ball milling for 24 h with an addition of 3 wt% FeCl2 powders (Beijing Bei Hua Fine Chemicals Company Lt. Beijing, China). The ball-milled powder mixture was then placed in a high purity alumina crucible and pyrolysized in a conventional furnace under flowing ultra-high purity nitrogen. The powder mixture was heated to 1450 8C at 10 8C/min and held there for 4 h followed by furnace-cool. The experiments were also performed on the samples without FeCl2 additives for comparison. The morphology, structure and composition of the pyrolysis products were characterized using field emission scanning electron microscopy (SEM, JSM-6301F, JEOL, Japan), X-ray diffraction (XRD, Automated D/Max-RB, Rigaku, Japan) with Cu Ka radiation (lZ1.54178 A˚ ), and transmission electron microscope (TEM, JEOL-2010F, Japan) equipped with energy dispersive X-ray spectrum (EDS). 3. Result and discussion Fig. 1(a) is a typical SEM image of the as-pyrolyzed products, showing that, relatively high-density whiskers have grown homogeneously on the top of the powder matrix. Closer examination under high magnification (Fig. 1(b) and (c)) reveals that the cross-sections of the Si3N4 whiskers are rectangular with the thickness ranges from 200 to 300 nm and the width from 800 to 1200 nm. Within each individual whisker, the thickness and width are uniform along its entire length, which can be up to several millimeters. The SEM observation shows that the surfaces of the whiskers are smooth and clean. Besides regular whiskers, intercrossed whiskers were also observed (Fig. 1(c)). Fig. 1(d) is a typical SEM image of the tip of the whisker, showing the triangle morphology. There are no liquid droplets at the tip, which was typically observed in vapor–liquid–solid (VSL) growth at the presence of catalysts [36], indicating the reported whiskers grew by fundamentally different mechanism. Beside the platelike whiskers, the whiskers with branches were also observed (Fig. 2). It can be seen that the branches grow outward from the main stem, exhibiting T-shaped (Fig. 2(a)), cross-shaped (Fig. 2(b)), comb-like (Fig. 2(c)) and feather-like (Fig. 2(d)) morphology. The typical stem of the structures is platelike, and the thickness and width of the stem are about 200 and 1000 nm, respectively. In the comb-like and feather-like structures, the branches possess belt-like cross-section with a uniform thickness and width along the growth direction, which is are fundamentally different from those previously observed in other material systems, where the branches possess morphology of either wires or tubes or needles [22–30]. The branches are typically 50 nm in thickness and w1000 nm in width, and the length can be up to more than ten micrometers. The branches are separated with a uniform distance on the stem and parallel to each other without sub-branches. The angle between the stem and branch is about 908. XRD pattern (Fig. 3) of the synthesized whiskers suggests that pyrolyzed products contain both a-Si3N4 and b-Si3N4 phases. The broad hump at lower angle regions suggests that the unconverted powders remains amorphous. The structures of platelike and branched Si3N4 whiskers were further characterized with TEM. Fig. 4(a) shows the typical TEM image of the platelike Si3N4 whiskers. EDS analysis shows that the platelike whiskers consist of Si and N elements only. The inset picture in Fig. 4(a) is the corresponding select area electron diffraction (SAED) pattern that is identical over the entire whisker, indicating that the whisker is a single crystal and shows the hexagonal structure of a-Si3N4, where aZ0.77541 nm and cZ 0.56217 nm (JCPDS Card No. 41-0360). Examination on more than 10 platelike whiskers suggests that [010] is the only growth direction for the platelike whiskers. Fig. 4(b) shows a Si3N4 stem at the stage to nucleate branches; numerous triangle droplets can be observed on the side of the stem. The inset picture in Fig. 4(b) is the typical EDS spectrum for the triangle droplets, which reveals that the droplets contain Fe element with a little amount of Cr and Ni (Cu elements come from the copper grid, and Cr and Ni may have come from the impurities of the FeCl2 catalyst). This result indicates the catalyst growth of the branches. Fig. 4(c) is a typical TEM image of T-shaped whiskers. The single crystalline nature of the branched structures is confirmed by SAED. SAED patterns recorded from different areas (indicated by the white circles) in a single dendrite are shown in Fig. 4(d). It is found that the SAED patterns recorded from different areas of the dendrite are almost identical, suggesting that the whole branched structure (stemCbranches) is a single crystal. The SAED patterns can be indexed to the hexagonal structures of a-Si3N4. The growth directions of the stem and branch were [010] and [001], respectively. No whiskers were formed in the samples without FeCl2 additives, suggesting the catalytic growth of the Si3N4 whiskers. 264 W. Yang et al. / Solid State Communications 132 (2004) 263–268
w. Yang et al. Solid State Communications 132(2004)263-268 265 I um 1 SEM micrographs of platelike Si,N4 whiskers: (a) the morphology of the pyrolyzed product under low magnification;(b) shows ectangular cross section of the whiskers with clean surface; (c)shows the intercrossed platelike whiskers; (d)shows the triangle-shaped tip of the whisker without no liquid drop Typically catalyst-assisted growth of one-dimensional converted to an amorphous ceramic with an apparent structures is through VLS mechanism [361, which is composition of SiCo. No84 at -1000C at 0.1 MPa N characterized by the presence of a catalyst droplet at the The material was stable up to -1450C, where it crystal- tips of the structures and requires the continuous supplement lized to Si3 N4 and free carbon [38]. It can be seen that at of gaseous species. Previous study [37 on the pyrolysis of present heat-treatment conditions(1450C, 0.1 MPa N2). Ceraset without catalyst revealed that the polysilazane was there is no Si-containing gaseous phase. That, together with 3 ur Fig. 2. SEM micrographs of branched Si3N4 whiskers: (a)T-shaped branched whiskers; (b) cross-shaped branched whiskers;(c) comb-like branched whiskers;(d) feather-like branched whiskers
Typically catalyst-assisted growth of one-dimensional structures is through VLS mechanism [36], which is characterized by the presence of a catalyst droplet at the tips of the structures and requires the continuous supplement of gaseous species. Previous study [37] on the pyrolysis of Ceraset without catalyst revealed that the polysilazane was converted to an amorphous ceramic with an apparent composition of SiC0.99N0.84 at w1000 8C at 0.1 MPa N2. The material was stable up to w1450 8C, where it crystallized to Si3N4 and free carbon [38]. It can be seen that at present heat-treatment conditions (1450 8C, 0.1 MPa N2), there is no Si-containing gaseous phase. That, together with Fig. 1. SEM micrographs of platelike Si3N4 whiskers: (a) the morphology of the pyrolyzed product under low magnification; (b) shows rectangular cross section of the whiskers with clean surface; (c) shows the intercrossed platelike whiskers; (d) shows the triangle-shaped tip of the whisker without no liquid drop. Fig. 2. SEM micrographs of branched Si3N4 whiskers: (a) T-shaped branched whiskers; (b) cross-shaped branched whiskers; (c) comb-like branched whiskers; (d) feather-like branched whiskers. W. Yang et al. / Solid State Communications 132 (2004) 263–268 265
W. Yang et al. Solid State Communications 132(2004)263-268 released N, gas. Further reaction of the solid SiCn and the ed in a liquid cr:C-Si aL-Si3N. This supersaturated liquid phase then reacted with N2 gas to precipitate the Si3N4 whiskers(shown in Fig. 5(a)). The formation of silicon nitride. instead of silicon or silicon carbide. is due to that the silicon nitride is the most stable phase at the processing conditions. However, the mechan- ism that governed the formation of platelike instead of cylindrical whiskers is rather difficult to understand. The observation of the triangle-shaped tips(Fig. 1(d)suggests a strongly anisotropy growth of the nuclei at earlier stage. It is believed [19, 20 that at the beginning of the nucleation/ growth of the Si3 N4 nuclei, the growth of the nuclei occurs along all directions simultaneously. However, the growth rate along thickness directions is much slower than that along width direction due to the anisotropy nature of Si3N4 Fig. 3. XRD pattern of as-pyrolyzed products, indicating the crystal structure. Consequently, the growth along axial and coexistence of a- and B-Si3N width direction resulted in the formation of triangle-shaped tips. The growth along width direction stops after it reaches a certain value limited by the confining effect of the liquid the fact of the absence of catalyst droplet at the tips of the phase droplets and further growth only occurs along axial whiskers(Fig. I(d), suggests that the growth of the direction to form platelike whiskers whiskers reported here is not via VLS mechanism. In imilarly, a two-stage SlGs growth mechanism is previous studies [ 19-21 we proposed a solid-liquid-gas proposed for the growth of the branched structures. Firstly, solid (SLGS)reaction/crystallization growth mechanism fo silicon nitride platelike stems were formed via SLGS the one-dimensional structures via catalyst-assisted pyrol- mechanism. Secondly, the impurity elements such as Fe, ysis of polymeric precursors. At the beginning of the Ni and Cr were then deposited onto the surfaces of the stem process, the amorphous SiCN reacted with Fe to form a uniformly. Another possible source for the impurity liquid Si-Fe-C alloy at a temperature higher than the elements is the amorphous layer that generally formed on eutectic temperature of Si-Fe-C ternary system, meanwhile the surface of liquid-solid grown structures [39]. Such I um I um um Fig 4. TEM images and SAEl elike and branched Si3 N4 whiskers. (a) A typical TEM he platelike whiskers with the corresponding SAED patter rystalline structure with a preferred grow shows the nucleation stage of the branched whisker; (c)a M image of the T-shaped branched whisker; and(d)s m and the branch show the branched whisker is a single c ith the preferred growth direction of [010] and [001] for the stem
the fact of the absence of catalyst droplet at the tips of the whiskers (Fig. 1(d)), suggests that the growth of the whiskers reported here is not via VLS mechanism. In previous studies [19–21], we proposed a solid–liquid–gas– solid (SLGS) reaction/crystallization growth mechanism for the one-dimensional structures via catalyst-assisted pyrolysis of polymeric precursors. At the beginning of the process, the amorphous SiCN reacted with Fe to form a liquid Si–Fe–C alloy at a temperature higher than the eutectic temperature of Si–Fe–C ternary system, meanwhile released N2 gas. Further reaction of the solid SiCN and the liquid alloy resulted in a liquid supersaturated with silicon. This supersaturated liquid phase then reacted with N2 gas to precipitate the Si3N4 whiskers (shown in Fig. 5(a)). The formation of silicon nitride, instead of silicon or silicon carbide, is due to that the silicon nitride is the most stable phase at the processing conditions. However, the mechanism that governed the formation of platelike instead of cylindrical whiskers is rather difficult to understand. The observation of the triangle-shaped tips (Fig. 1(d)) suggests a strongly anisotropy growth of the nuclei at earlier stage. It is believed [19,20] that at the beginning of the nucleation/ growth of the Si3N4 nuclei, the growth of the nuclei occurs along all directions simultaneously. However, the growth rate along thickness directions is much slower than that along width direction due to the anisotropy nature of Si3N4 crystal structure. Consequently, the growth along axial and width direction resulted in the formation of triangle-shaped tips. The growth along width direction stops after it reaches a certain value limited by the confining effect of the liquid phase droplets and further growth only occurs along axial direction to form platelike whiskers. Similarly, a two-stage SLGS growth mechanism is proposed for the growth of the branched structures. Firstly, silicon nitride platelike stems were formed via SLGS mechanism. Secondly, the impurity elements such as Fe, Ni and Cr were then deposited onto the surfaces of the stem uniformly. Another possible source for the impurity elements is the amorphous layer that generally formed on the surface of liquid–solid grown structures [39]. Such Fig. 3. XRD pattern of as-pyrolyzed products, indicating the coexistence of a- and b-Si3N4. Fig. 4. TEM images and SAED patterns of the platelike and branched Si3N4 whiskers. (a) A typical TEM image of the platelike whiskers with the corresponding SAED pattern shows the single-crystalline structure with a preferred growth direction of [010]; (b) shows the nucleation stage of the branched whisker; (c) a typical TEM image of the T-shaped branched whisker; and (d) SAED patterns of the stem and the branch show the branched whisker is a single crystal with the preferred growth direction of [010] and [001] for the stem and branch, respectively. 266 W. Yang et al. / Solid State Communications 132 (2004) 263–268
w. Yang et al. Solid State Communications 132(2004)263-268 SiNC particles Si-Fe-C [010 (b) Fig. 5.(a) a schematic diagram showing the growth mechanism of the platelike whiskers; (b)a two-staged ( based on a) growth mechanism of the purities were then reacted with Si3N4 [2]R K. Govila, J Mater. Sci. 20(1985)4345. uid spots consisting of Si, Fe, Ni and Furth reaction [3] F. Munakata, K. Matsuo, K. Furuya, YJ of the spots and the stem resulted in Si-supersaturated liquid. L Ishikawa, Appl. Phys. Lett. 74(1999)3498 The belt-like branches then nucleated and grew via reaction [4] A.R. Zanatta, L.A. O Nunes, Appl. Phys. Lett. 72(1998)3127 of N2 gas and the supersaturated liquid at the surfaces of the [5] F. Xu, x. Zhang. w. Xi, J. Hong. Y. Xie, Chem. Lett. 32 liquid droplets as shown in Fig. 5(b) (2003)600 16 Y H. Gao, Y. Bando, K. Kurashima, T. Sato. Microsc. Microanal. 8(2002 )5. [7 w. Han, S Fan, Q 4. Concl Lett.71(1997)2271 [8X.C. Wu, W.H. Song, B. Zhao, W.D. Huang, M.H. Pu In summary, platelike and branched whiskers were Y P. Sun, J.J. Du, Solid State Commun. 115(2000)683. synthesized via catalyst-assisted pyrolysis of polymeric [9] L.D. Zhang, G W Meng, F Phillipp. Mater. Sci Eng. A 286 precursors. The platelike whiskers are single crystals with a (2000)34 uniform thickness and width, ranging from 200 to 300 nm [10] Y. Zhang, N. Wang, S Gao, R. He, S Miao, J. Liu, J. Zhu, and 800 to 1200 nm, respectively. The preferred growth X Zhang, Chem. Mater. 14(2002)3564. directions of the platelike whiskers are [010]. For branched [11 Y H Gao, Y. Bando, K. Kurashima, T Sato, J Appl. Phys. 91 (2002)1515 structures, the whole structure is a single crystalline with the stem that grew along [010] direction and the branch that [12]G. Gundiah. G.V. Madhav, A. Govindaraj, Md M. Seikh, C N R. Rao, J Mater. Chem. 12(2002)1606. grew along [001] direction. The branches also possess belt- [13]CC. Tang, X.x. Ding, X.T. Huang, ZW.Gan,w.Liu like morphology. A solid-liquid-gas-solid reaction/crystal- S.R. Qi,YXLi, J.P. Qu, L Hu, Jpn J Appl. Phys. 41(2002) ization growth mechanism is proposed for both platelike and branched whiskers. The formation of platelike whiskers [14] H Chen, Y. Cao, X Xiang, J. Li, C. Ge, J. Alloy Compd. 325 instead of cylindrical whiskers is attributed to the anisotropy (2001)L1 growth at earlier stage. [15] Y Chen, L Guo, D.T. Shaw, J Cryst. Growth 210 (2000)527 [16]H Cui, B.R. Stoner, J Mater Res. 16(2001)3111 [17 M K Sunkara, S. Sharma, H. Chandrasekaran, M. Talbott, K. Krogman, G. Bhimarasetti, J Mater. Chem. 14(2004)590. Acknowledgements [18]L. Yin, Y. Bando, Y. Zhu, Y. Li, Appl. Phys. Lett. 83(2003) The authors acknowledge the finance support from the [19]w. Yang, Z. Xie, J. Li, H. Miao, L. Zhang, L.An,J.Am. National Natural Science Foundation of China(Grant No Ceram Soc. 2004: in review 50372031)and 'Hundred Person program of Chinese [20] w. Yang, Z. Xie, H Miao, H Ji, L. Zhang, L.An,JAm. Academy of Science Ceram Soc. 2004: in [21] W. Yang, H Miao, Z Xie, L. An, Chem. Phys. Lett. 383(2004)441 [22] L Cao, K. Hahn, Y. Wang, C. Zhang. C. Gao.Y Li References X Zhang, L Sun, W. Wang, M. Adv Mater. 14(2002) [1] G. Ziegler, J. Heinrich, C. wott later. Sci. 22(1987 [23] J Jian, X Chen, w. Wang, L Dai, Y. Xu, Appl. Phys. A 76 (2003)291
impurities were then reacted with Si3N4 stem to nucleate liquid spots consisting of Si, Fe, Ni and Cr. Further reaction of the spots and the stem resulted in Si-supersaturated liquid. The belt-like branches then nucleated and grew via reaction of N2 gas and the supersaturated liquid at the surfaces of the liquid droplets as shown in Fig. 5(b). 4. Conclusion In summary, platelike and branched whiskers were synthesized via catalyst-assisted pyrolysis of polymeric precursors. The platelike whiskers are single crystals with a uniform thickness and width, ranging from 200 to 300 nm and 800 to 1200 nm, respectively. The preferred growth directions of the platelike whiskers are [010]. For branched structures, the whole structure is a single crystalline with the stem that grew along [010] direction and the branch that grew along [001] direction. The branches also possess beltlike morphology. A solid–liquid–gas–solid reaction/crystallization growth mechanism is proposed for both platelike and branched whiskers. The formation of platelike whiskers instead of cylindrical whiskers is attributed to the anisotropy growth at earlier stage. Acknowledgements The authors acknowledge the finance support from the National Natural Science Foundation of China (Grant No. 50372031) and ‘Hundred Person’ program of Chinese Academy of Science. References [1] G. Ziegler, J. Heinrich, C. Wo¨tting, J. Mater. Sci. 22 (1987) 3041. [2] R.K. Govila, J. Mater. Sci. 20 (1985) 4345. [3] F. Munakata, K. Matsuo, K. Furuya, Y.J. Akimune, I. Ishikawa, Appl. Phys. Lett. 74 (1999) 3498. [4] A.R. Zanatta, L.A.O. Nunes, Appl. Phys. Lett. 72 (1998) 3127. [5] F. Xu, X. Zhang, W. Xi, J. Hong, Y. Xie, Chem. Lett. 32 (2003) 600. [6] Y.H. Gao, Y. Bando, K. Kurashima, T. Sato, Microsc. Microanal. 8 (2002) 5. [7] W. Han, S. Fan, Q. Li, B. Gu, X. Zhang, D. Yu, Appl. Phys. Lett. 71 (1997) 2271. [8] X.C. Wu, W.H. Song, B. Zhao, W.D. Huang, M.H. Pu, Y.P. Sun, J.J. Du, Solid State Commun. 115 (2000) 683. [9] L.D. Zhang, G.W. Meng, F. Phillipp, Mater. Sci. Eng. A 286 (2000) 34. [10] Y. Zhang, N. Wang, S. Gao, R. He, S. Miao, J. Liu, J. Zhu, X. Zhang, Chem. Mater. 14 (2002) 3564. [11] Y.H. Gao, Y. Bando, K. Kurashima, T. Sato, J. Appl. Phys. 91 (2002) 1515. [12] G. Gundiah, G.V. Madhav, A. Govindaraj, Md.M. Seikh, C.N.R. Rao, J. Mater. Chem. 12 (2002) 1606. [13] C.C. Tang, X.X. Ding, X.T. Huang, Z.W. Gan, W. Liu, S.R. Qi, Y.X. Li, J.P. Qu, L. Hu, Jpn. J. Appl. Phys. 41 (2002) L589. [14] H. Chen, Y. Cao, X. Xiang, J. Li, C. Ge, J. Alloy Compd. 325 (2001) L1. [15] Y. Chen, L. Guo, D.T. Shaw, J. Cryst. Growth 210 (2000) 527. [16] H. Cui, B.R. Stoner, J. Mater. Res. 16 (2001) 3111. [17] M.K. Sunkara, S. Sharma, H. Chandrasekaran, M. Talbott, K. Krogman, G. Bhimarasetti, J. Mater. Chem. 14 (2004) 590. [18] L. Yin, Y. Bando, Y. Zhu, Y. Li, Appl. Phys. Lett. 83 (2003) 3584. [19] W. Yang, Z. Xie, J. Li, H. Miao, L. Zhang, L. An, J. Am. Ceram. Soc. 2004; in review. [20] W. Yang, Z. Xie, H. Miao, H. Ji, L. Zhang, L. An, J. Am. Ceram. Soc. 2004; in press. [21] W. Yang, H. Miao, Z. Xie, L. Zhang, L. An, Chem. Phys. Lett. 383 (2004) 441. [22] L. Cao, K. Hahn, Y. Wang, C. Scheu, Z. Zhang, C. Gao, Y. Li, X. Zhang, L. Sun, W. Wang, M. Ru¨hle, Adv. Mater. 14 (2002) 1294. [23] J. Jian, X. Chen, W. Wang, L. Dai, Y. Xu, Appl. Phys. A 76 (2003) 291. Fig. 5. (a) a schematic diagram showing the growth mechanism of the platelike whiskers; (b) a two-staged (based on a) growth mechanism of the branched whiskers. W. Yang et al. / Solid State Communications 132 (2004) 263–268 267
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