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Twin-plane reentrant edge growth of rhombohedra boron suboxide platelets
ARTICLE IN PRESS Journal of Crystal Growth 312 (2010) 1789–1792
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Twin-plane reentrant edge growth of rhombohedra boron suboxide platelets Zhiyang Yu a,b, Jun Jiang a,b, Jun Yuan c, Jing Zhu a,b,n a b c
Beijing National Center for Electron Microscopy, Tsinghua University, Beijing 100084, China Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Department of Physics, The University of York, York YO10 5DD, UK
a r t i c l e in fo
abstract
Article history: Received 27 November 2009 Received in revised form 19 January 2010 Accepted 25 February 2010 Communicated by M. Schieber Available online 6 March 2010
Large quantities of rhombohedra and elongated rhombohedra boron suboxide platelets with flat (0 0 1) surface have been synthesized through conventional solid state reaction. Detailed structural investigations by selected area electron diffraction (SAED) and high-resolution electron microscopy (HRTEM) of these platelets are presented. We present the direct experimental observation of extensive lateral (0 0 1) microtwins in rhombohedra platelets and they give rise to the fractional diffractions spots. It is believed that the growth of these rhombohedra platelets is prompted by the twin-plane reentrant edge (TPRE) mechanism. The transition from rhombohedra platelets to elongated rhombohedra platelets in morphology is probably the result of catalytic growth at the apexes of the platelets. This proposed growth model can be representative of various platelets with low defects formation energy, especially in twinned crystals having a rhombohedra structure. Besides, the presence of extensive microtwins will yield interesting physical properties and probably results in the broadening of photoluminescence (PL) spectra from the rhombohedra and elongated rhombohedra platelets. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Twinning A1. Characterization A1. Growth models A1. Nanomaterials A1. Crystal morphology B2. Semiconducting materials
1. Introduction Platelets form a distinct class of nanostructures which have potential applications in field effect transistor [1], gas- and bio-sensors [2,3], and nanoactuators [4]. Platelets can be produced by a variety of methods including thermal evaporation [5], chemical vapor deposition [6], solution method [7] and so on. However, the underlying growth mechanism has not received so much attention as those focused on nanotubes, nanowires and nanoparticles. The most systematic growth mechanism studies of platelets have been carried out on face centered cubic (FCC) based materials, such as silicon [8–11] and germanium dendrites [12,13], nanocrystalline diamonds [14], silver platelets [15] and silver halide tabular platelets [16–19]. Anisotropic growth of the platelets can be unambiguously attributed to the preferential nucleation at the reentrant grooves of lateral twin planes, that is, the twin-plane reentrant edge (TPRE) mechanism, or the W–H–S growth mechanism [13,20]. In-situ observation during the growth of Si faceted dendrites [10] and silver bromide tabular crystal [21] has lend direct support that the observed platelet morphology is consistent with that as predicted by TPRE. Rhombohedra lattice (space group R3¯m) is a high symmetric structure where all the /0 0 1S directions are identical. A schematic
model of rhombohedra platelet is given in Fig. 1a. Crystals with rhombohedra structure will often exhibit rhombohedra and elongated rhombohedra ‘thin slab’ morphology in previous research [22–32]. However, the high occurrence of platelet-like morphology in materials with rhombohedra symmetry is not allowed as crystal growth along the [0 0 1] direction must be retarded. Besides, systematic structural and growth mechanism investigations are lacking in the distinct rhombohedra platelets. In this paper, extensive /0 0 1S microtwins have been revealed in rhombohedra and elongated rhombohedra platelets based on diffraction pattern analysis and high-resolution transmission electron microscopy (HRTEM) observations. We not only propose that the vapor assisted TPRE mechanism can successfully account for the high occurrence of rhombohedra platelets, but also discuss the catalyst aided TPRE mechanism for the elongated rhombohedra platelets. It is suggested that the TPRE mechanism can also operate in rhombohedra based materials, not just confined in platelets with FCC crystal structure. In addition, platelets with physical properties enhanced by microtwins will have promising future in applications, such as thermoelectric power generation [33] and mechanical components [34].
2. Experimental section n
Corresponding author at: Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. Tel.:+ 86 10 62794026; fax: + 86 10 62772507. E-mail address: [email protected] (J. Zhu). 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.02.039
Boron suboxide platelets were prepared in a tube furnace by conventional solid state reaction as described previously [26]. In short, fine powders of BaO, B and Fe3O4 in a mole ratio of
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Fig. 1. (a) A schematic model of rhombohedra platelet with rhombohedra symmetry. (b) SEM image of platelets grown on the substrate. (c and d) Magnified images of rhombohedra platelet and elongated rhombohedra platelet, respectively. (e) The overall length along [ 1 1 0] (L[1 1 0]) as a function of the side length along [1 0 0] (L[1 0 0]). P is the abbreviation for platelet.
1.84:28.52:1.77 were cold pressed into pellets (4 MPa) and sintered at 1400 1C for 1 h under a flowing argon atmosphere (200 sccm). Initial characterization revealed that the process could be tuned to produce either predominately cyclic-twinned nanowires [35] or platelets [26] of boron suboxide. Platelets were the dominate product at high temperature ranging from 1400 to 1500 1C. The obtained platelets were characterized by field emission scanning electron microscope (FESEM, JEOL-6301 F) and transmission electron microscope (TEM, JEOL-2011). Photoluminescence (PL) spectra of the platelets were collected by a Confocal Microscopic Raman Spectrometer (RM2000 made in Renishaw UK) using a 633 nm laser source.
3. Results and discussions Plenty of platelets with rhombohedra symmetry (Fig. 1a) were found on the substrate as shown in Fig. 1b. The platelets fall into two categories: rhombohedra platelets (Fig. 1c) and elongated rhombohedra platelets (Fig. 1d). They share a common flat (0 0 1) surface and two distinct acute tips. To reveal the internal relationship between them, a statistical analysis (Fig. 1e) of their characteristic dimensions (the overall length defined as L[110] and the side length defined as L[1 0 0] in Fig. 1c, d), was conducted. The data attributed to rhombohedra platelets form a straight line fit as expected of their symmetry. More importantly, this line is the
lower bound for the overall length of the elongated rhombohedra platelets (L[1 1 0]) at any given width. It clearly suggests that these two kinds of platelets share a common growth path and the elongated ones are the elongation of rhombohedra platelets along the [1 1 0] direction. Detailed structural investigations are crucial in understanding the underlying growth mechanism. A bright field image of a rhombohedra platelet and its corresponding diffraction patterns taken along the [1¯ 0 1], [1¯ 0 2] and [1¯ 1¯ 3] zone axes are presented in Fig. 2a–d, respectively. Fig. 2e–h show the corresponding figures for an elongated rhombohedra platelet. The strong diffraction spots can be ascribed to the rhombohedra lattice of boron suboxide, as highlighted by the shaded parallelograms and triangles in Fig. 2b–d and f–h. In addition, 1/2(2 2 1) and 1/3(3 3 2) fractional diffraction spots are detected and marked out by squares in Fig. 2c, d, g and h. Their presence in boron-rich platelets has been reported in previous research without detailed discussion [26] or the diffraction patterns are mis-interpreted [23,25,27–29,31,32]. Fractional diffraction spots along specific zone axes are the result of structural incompleteness in crystals [36]. However, it is difficult to detect the intrinsic defects as they are perpendicular to the electron beam direction. Consequently these rhombohedra platelets have often been treated as single crystal by mistake. Platelets which accidentally stand ‘edge-on’ on the carbon holy film were surveyed in order to give direct experimental evidence for the structural defects. The HRTEM images, taken from a typical elongated rhombohedra platelet in side view (Fig. 3a), clearly show extensive lateral (0 0 1) twins in Fig. 3b, c. With fine twin spacing shown above, the reciprocal lattice evolves into reciprocal rods along the [0 0 1]n direction and results in the fractional spots in Fig. 2c, d, g and h. It is assumed that fine random (0 0 1) microtwins are also present in rhombohedra platelets, as it is consistent with the low twin formation energy of boron-rich nanostructure [37]. The twin-plane reentrant edge (TPRE) growth mechanism can successfully account for the morphology of rhombohedra platelets. It is suggested that the growth of the rhombohedra platelets can be prompted by a vapor assisted TPRE mechanism involving B2O2 like vapor [38,39]. The nucleus is immersed in a homogenous vapor atmosphere (Fig. 4a), which is quite similar to the melt-based environment for silicon [8–11], germanium [12,13] and silver halide dendrites [16–19]. The presence of extensive (0 0 1) microtwins can induce anisotropic growth of platelets as the preferential nucleation and growth at the twin-plane reentrant grooves (Fig. 4b). Indeed, all the platelets are coated with a uniform oxide layer (1–2 nm, Fig. 3b, c) which is efficient in catching vapor molecules. It suggests the vapor assisted TPRE mode takes effects during the growth of the rhombohedra platelets. It should be noted that the rhombohedra platelets are not the dominant product in this experiment owing to the addition of large amount of catalyst in the precursor. Our results suggest that TPRE mechanism can also be aided by the catalytic growth. Catalytic particles are not always observed at the apexes of platelets in this experiment. If the argon flow was decreased from 200 to 20 sccm, catalytic particle can be mostly preserved at one apex of platelets. There is high possibility that catalytic particles be blown off or consumed at the end of growth process. Consequently, catalytic particles locating at the apex will facilitate the vapor incorporation and accelerate the growth along the side facets of platelets (Fig. 4c). The limited diffusion length of atoms along the side ledge will trigger the elongation of platelets along the [1 1 0] direction. The side length of boron suboxide platelet is around 1 micron as estimated from Fig. 1e, consistent with the diffusion length in literatures [40,41]. Moreover, it is reasonable that the advent of globular particles at the apex will induce further elongation along the [1 1 0] direction if we
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Fig. 2. (a) Bright field image of rhombohedra platelet. The systematic diffraction patterns are presented along the [1¯ 0 1] (b) [1¯ 0 2] (c) and [1¯ 1¯ 3] (d) zone axes, respectively. Some fractional diffraction spots are highlighted by the square in (c and d). (e–h) Corresponding structural investigations for an elongate d rhombohedra platelet. The ring patterns in (f–h) come from the gold film sputtered on the platelet for calibration only.
Fig. 3. (a) Side view of an ‘edge on’ elongated rhombohedra platelet. (b and c) HRTEM images taken from two tips reveal extensive (0 0 1) microtwins as highlighted by the solid lines.
reconsider the close relationship of both two platelets in Fig. 1e. The growth mode is presented here to give a clear physical insight into the high occurrence of rhombohedra and elongated rhombohedra platelet-like morphology both in this experiment and previous research. Actual growth process can be quite complicate to involve two or more mechanisms. For instance, we suspect that both vapor assisted (Fig. 4a and b) and catalyst aided (Fig. 4c) TPRE mechanisms take effect during the growth of the elongated rhombohedra platelets. This proposed principle can be applicable to related systems having a rhombohedra crystal structure, such as boron carbide rhombohedra platelets [22] and their elongated varieties [23,25], boron suboxide fibers [26], boron nanocones [28] and nanoribbons [27], Bi2Te3 [29,30] and Sb2Te3 platelets [31,32] where similar growth characteristics are present. The underlying growth mechanism is previously attributed to simple VS [25] or conventional VLS mechanism [23,26] without taking the role of
structural defects into account, even though in most cases the fractional spots are observed. The presence of extensive (0 0 1) microtwins in these platelets may yield interesting physical effects. Fig. 5 shows room temperature PL spectra recorded from a rhombohedra platelet and three elongated rhombohedra platelets. The similarity of these spectra also suggests that both the rhombohedra and elongated rhombohedra platelets share the same chemical component and crystal structure. A broad PL emission can be detected at Red wavelengths, peaking around 760–775 nm. The full-width-at-half-maximum (FWHM) of the spectra can be determined to be 280 710 meV for all the observed platelets. The broad PL emission is probably the result of random extensive (0 0 1) microtwins with small twin spacing ranging from 1–5 nm, as quantum confinement will widen the PL spectrum [42]. Besides, Enin [33] concluded that 20-fold increase in Seebeck coefficient could be induced by a large density of microtwins at 1000 1C in boron carbide. This condition may be satisfied in the heavily twinned boron-rich nanostructures. Understanding the growth mechanism of twinned platelets will be helpful in rationally designing nanostructures with enhanced properties.
4. Conclusion In conclusion, large densities of (0 0 1) microtwins in distinct rhombohedra and elongated rhombohedra platelets have been systemically revealed by SAED and HRTEM observations. It is suggested that boron suboxide rhombohedra platelets are prompted by the vapor assisted TPRE mechanism. The elongated rhombohedra platelets can be related to rhombohedra ones by subsequent catalytic growth at the apex. This proposed growth mechanism presents a unified description of the various rhombohedra platelets in related research. Moreover, our results point out that the TPRE mechanism is also applicable in rhombohedra based materials. Understanding the growth mechanism involved will be beneficial in rationally designing a new group of nanostructures and understanding related physical properties.
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Fig. 4. (a) Vapor assisted TPRE growth model for rhombohedra platelets. Extensive (0 0 1) microtwins lead to alternating reentrant grooves which serve as preferential nucleation sites indicated by the arrows. (b) The as grown platelet will exhibit distinct rhombohedra morphology. (c) Catalyst aided TPRE growth of elongated rhombohedra platelets. The boron-rich molecules are first absorbed by the catalyst and further transferred to the twin-plane reentrant edge to prompt the growth of platelets. A diffusion limit will be applied by the catalyst. Consequently the platelets will be elongated along the [1 1 0] direction.
Fig. 5. Room temperature PL spectra from a rhombohedra platelet and three elongated rhombohedra platelets with a laser excitation of 633 nm.
Acknowledgements This research is supported by the National Basic Research Program (grant no. 2002CB613500) from Chinese Ministry of Science and Technology. Zhiyang Yu appreciates the exchange visit offered by the University of York which results the final version of this paper. References [1] M.S. Arnold, P. Avouris, Z.W. Pan, Z.L. Wang, J. Phys. Chem. B. 107 (2003) 659–663. [2] E. Comini, G. Faglia, G. Sberveglieri, Z.W. Pan, Z.L. Wang, Appl. Phys. Lett. 81 (2002) 1869. [3] C. Yu, Q. Hao, S. Saha, L. Shi, X.Y. Kong, Z.L. Wang, Appl. Phys. Lett. 86 (2005). [4] M.H. Zhao, Z.L. Wang, S.X. Mao, Nano Lett. 4 (2004) 587–590. [5] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947–1949. [6] S.Y. Bae, H.W. Seo, J. Park, H. Yang, J.C. Park, S.Y. Lee, Appl. Phys. Lett. 81 (2002) 126–128.
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Twin-plane reentrant edge growth of rhombohedra boron suboxide platelets Zhiyang Yu a,b, Jun Jiang a,b, Jun Yuan c, Jing Zhu a,b,n a b c
Beijing National Center for Electron Microscopy, Tsinghua University, Beijing 100084, China Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Department of Physics, The University of York, York YO10 5DD, UK
a r t i c l e in fo
abstract
Article history: Received 27 November 2009 Received in revised form 19 January 2010 Accepted 25 February 2010 Communicated by M. Schieber Available online 6 March 2010
Large quantities of rhombohedra and elongated rhombohedra boron suboxide platelets with flat (0 0 1) surface have been synthesized through conventional solid state reaction. Detailed structural investigations by selected area electron diffraction (SAED) and high-resolution electron microscopy (HRTEM) of these platelets are presented. We present the direct experimental observation of extensive lateral (0 0 1) microtwins in rhombohedra platelets and they give rise to the fractional diffractions spots. It is believed that the growth of these rhombohedra platelets is prompted by the twin-plane reentrant edge (TPRE) mechanism. The transition from rhombohedra platelets to elongated rhombohedra platelets in morphology is probably the result of catalytic growth at the apexes of the platelets. This proposed growth model can be representative of various platelets with low defects formation energy, especially in twinned crystals having a rhombohedra structure. Besides, the presence of extensive microtwins will yield interesting physical properties and probably results in the broadening of photoluminescence (PL) spectra from the rhombohedra and elongated rhombohedra platelets. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Twinning A1. Characterization A1. Growth models A1. Nanomaterials A1. Crystal morphology B2. Semiconducting materials
1. Introduction Platelets form a distinct class of nanostructures which have potential applications in field effect transistor [1], gas- and bio-sensors [2,3], and nanoactuators [4]. Platelets can be produced by a variety of methods including thermal evaporation [5], chemical vapor deposition [6], solution method [7] and so on. However, the underlying growth mechanism has not received so much attention as those focused on nanotubes, nanowires and nanoparticles. The most systematic growth mechanism studies of platelets have been carried out on face centered cubic (FCC) based materials, such as silicon [8–11] and germanium dendrites [12,13], nanocrystalline diamonds [14], silver platelets [15] and silver halide tabular platelets [16–19]. Anisotropic growth of the platelets can be unambiguously attributed to the preferential nucleation at the reentrant grooves of lateral twin planes, that is, the twin-plane reentrant edge (TPRE) mechanism, or the W–H–S growth mechanism [13,20]. In-situ observation during the growth of Si faceted dendrites [10] and silver bromide tabular crystal [21] has lend direct support that the observed platelet morphology is consistent with that as predicted by TPRE. Rhombohedra lattice (space group R3¯m) is a high symmetric structure where all the /0 0 1S directions are identical. A schematic
model of rhombohedra platelet is given in Fig. 1a. Crystals with rhombohedra structure will often exhibit rhombohedra and elongated rhombohedra ‘thin slab’ morphology in previous research [22–32]. However, the high occurrence of platelet-like morphology in materials with rhombohedra symmetry is not allowed as crystal growth along the [0 0 1] direction must be retarded. Besides, systematic structural and growth mechanism investigations are lacking in the distinct rhombohedra platelets. In this paper, extensive /0 0 1S microtwins have been revealed in rhombohedra and elongated rhombohedra platelets based on diffraction pattern analysis and high-resolution transmission electron microscopy (HRTEM) observations. We not only propose that the vapor assisted TPRE mechanism can successfully account for the high occurrence of rhombohedra platelets, but also discuss the catalyst aided TPRE mechanism for the elongated rhombohedra platelets. It is suggested that the TPRE mechanism can also operate in rhombohedra based materials, not just confined in platelets with FCC crystal structure. In addition, platelets with physical properties enhanced by microtwins will have promising future in applications, such as thermoelectric power generation [33] and mechanical components [34].
2. Experimental section n
Corresponding author at: Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. Tel.:+ 86 10 62794026; fax: + 86 10 62772507. E-mail address: [email protected] (J. Zhu). 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.02.039
Boron suboxide platelets were prepared in a tube furnace by conventional solid state reaction as described previously [26]. In short, fine powders of BaO, B and Fe3O4 in a mole ratio of
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Fig. 1. (a) A schematic model of rhombohedra platelet with rhombohedra symmetry. (b) SEM image of platelets grown on the substrate. (c and d) Magnified images of rhombohedra platelet and elongated rhombohedra platelet, respectively. (e) The overall length along [ 1 1 0] (L[1 1 0]) as a function of the side length along [1 0 0] (L[1 0 0]). P is the abbreviation for platelet.
1.84:28.52:1.77 were cold pressed into pellets (4 MPa) and sintered at 1400 1C for 1 h under a flowing argon atmosphere (200 sccm). Initial characterization revealed that the process could be tuned to produce either predominately cyclic-twinned nanowires [35] or platelets [26] of boron suboxide. Platelets were the dominate product at high temperature ranging from 1400 to 1500 1C. The obtained platelets were characterized by field emission scanning electron microscope (FESEM, JEOL-6301 F) and transmission electron microscope (TEM, JEOL-2011). Photoluminescence (PL) spectra of the platelets were collected by a Confocal Microscopic Raman Spectrometer (RM2000 made in Renishaw UK) using a 633 nm laser source.
3. Results and discussions Plenty of platelets with rhombohedra symmetry (Fig. 1a) were found on the substrate as shown in Fig. 1b. The platelets fall into two categories: rhombohedra platelets (Fig. 1c) and elongated rhombohedra platelets (Fig. 1d). They share a common flat (0 0 1) surface and two distinct acute tips. To reveal the internal relationship between them, a statistical analysis (Fig. 1e) of their characteristic dimensions (the overall length defined as L[110] and the side length defined as L[1 0 0] in Fig. 1c, d), was conducted. The data attributed to rhombohedra platelets form a straight line fit as expected of their symmetry. More importantly, this line is the
lower bound for the overall length of the elongated rhombohedra platelets (L[1 1 0]) at any given width. It clearly suggests that these two kinds of platelets share a common growth path and the elongated ones are the elongation of rhombohedra platelets along the [1 1 0] direction. Detailed structural investigations are crucial in understanding the underlying growth mechanism. A bright field image of a rhombohedra platelet and its corresponding diffraction patterns taken along the [1¯ 0 1], [1¯ 0 2] and [1¯ 1¯ 3] zone axes are presented in Fig. 2a–d, respectively. Fig. 2e–h show the corresponding figures for an elongated rhombohedra platelet. The strong diffraction spots can be ascribed to the rhombohedra lattice of boron suboxide, as highlighted by the shaded parallelograms and triangles in Fig. 2b–d and f–h. In addition, 1/2(2 2 1) and 1/3(3 3 2) fractional diffraction spots are detected and marked out by squares in Fig. 2c, d, g and h. Their presence in boron-rich platelets has been reported in previous research without detailed discussion [26] or the diffraction patterns are mis-interpreted [23,25,27–29,31,32]. Fractional diffraction spots along specific zone axes are the result of structural incompleteness in crystals [36]. However, it is difficult to detect the intrinsic defects as they are perpendicular to the electron beam direction. Consequently these rhombohedra platelets have often been treated as single crystal by mistake. Platelets which accidentally stand ‘edge-on’ on the carbon holy film were surveyed in order to give direct experimental evidence for the structural defects. The HRTEM images, taken from a typical elongated rhombohedra platelet in side view (Fig. 3a), clearly show extensive lateral (0 0 1) twins in Fig. 3b, c. With fine twin spacing shown above, the reciprocal lattice evolves into reciprocal rods along the [0 0 1]n direction and results in the fractional spots in Fig. 2c, d, g and h. It is assumed that fine random (0 0 1) microtwins are also present in rhombohedra platelets, as it is consistent with the low twin formation energy of boron-rich nanostructure [37]. The twin-plane reentrant edge (TPRE) growth mechanism can successfully account for the morphology of rhombohedra platelets. It is suggested that the growth of the rhombohedra platelets can be prompted by a vapor assisted TPRE mechanism involving B2O2 like vapor [38,39]. The nucleus is immersed in a homogenous vapor atmosphere (Fig. 4a), which is quite similar to the melt-based environment for silicon [8–11], germanium [12,13] and silver halide dendrites [16–19]. The presence of extensive (0 0 1) microtwins can induce anisotropic growth of platelets as the preferential nucleation and growth at the twin-plane reentrant grooves (Fig. 4b). Indeed, all the platelets are coated with a uniform oxide layer (1–2 nm, Fig. 3b, c) which is efficient in catching vapor molecules. It suggests the vapor assisted TPRE mode takes effects during the growth of the rhombohedra platelets. It should be noted that the rhombohedra platelets are not the dominant product in this experiment owing to the addition of large amount of catalyst in the precursor. Our results suggest that TPRE mechanism can also be aided by the catalytic growth. Catalytic particles are not always observed at the apexes of platelets in this experiment. If the argon flow was decreased from 200 to 20 sccm, catalytic particle can be mostly preserved at one apex of platelets. There is high possibility that catalytic particles be blown off or consumed at the end of growth process. Consequently, catalytic particles locating at the apex will facilitate the vapor incorporation and accelerate the growth along the side facets of platelets (Fig. 4c). The limited diffusion length of atoms along the side ledge will trigger the elongation of platelets along the [1 1 0] direction. The side length of boron suboxide platelet is around 1 micron as estimated from Fig. 1e, consistent with the diffusion length in literatures [40,41]. Moreover, it is reasonable that the advent of globular particles at the apex will induce further elongation along the [1 1 0] direction if we
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Fig. 2. (a) Bright field image of rhombohedra platelet. The systematic diffraction patterns are presented along the [1¯ 0 1] (b) [1¯ 0 2] (c) and [1¯ 1¯ 3] (d) zone axes, respectively. Some fractional diffraction spots are highlighted by the square in (c and d). (e–h) Corresponding structural investigations for an elongate d rhombohedra platelet. The ring patterns in (f–h) come from the gold film sputtered on the platelet for calibration only.
Fig. 3. (a) Side view of an ‘edge on’ elongated rhombohedra platelet. (b and c) HRTEM images taken from two tips reveal extensive (0 0 1) microtwins as highlighted by the solid lines.
reconsider the close relationship of both two platelets in Fig. 1e. The growth mode is presented here to give a clear physical insight into the high occurrence of rhombohedra and elongated rhombohedra platelet-like morphology both in this experiment and previous research. Actual growth process can be quite complicate to involve two or more mechanisms. For instance, we suspect that both vapor assisted (Fig. 4a and b) and catalyst aided (Fig. 4c) TPRE mechanisms take effect during the growth of the elongated rhombohedra platelets. This proposed principle can be applicable to related systems having a rhombohedra crystal structure, such as boron carbide rhombohedra platelets [22] and their elongated varieties [23,25], boron suboxide fibers [26], boron nanocones [28] and nanoribbons [27], Bi2Te3 [29,30] and Sb2Te3 platelets [31,32] where similar growth characteristics are present. The underlying growth mechanism is previously attributed to simple VS [25] or conventional VLS mechanism [23,26] without taking the role of
structural defects into account, even though in most cases the fractional spots are observed. The presence of extensive (0 0 1) microtwins in these platelets may yield interesting physical effects. Fig. 5 shows room temperature PL spectra recorded from a rhombohedra platelet and three elongated rhombohedra platelets. The similarity of these spectra also suggests that both the rhombohedra and elongated rhombohedra platelets share the same chemical component and crystal structure. A broad PL emission can be detected at Red wavelengths, peaking around 760–775 nm. The full-width-at-half-maximum (FWHM) of the spectra can be determined to be 280 710 meV for all the observed platelets. The broad PL emission is probably the result of random extensive (0 0 1) microtwins with small twin spacing ranging from 1–5 nm, as quantum confinement will widen the PL spectrum [42]. Besides, Enin [33] concluded that 20-fold increase in Seebeck coefficient could be induced by a large density of microtwins at 1000 1C in boron carbide. This condition may be satisfied in the heavily twinned boron-rich nanostructures. Understanding the growth mechanism of twinned platelets will be helpful in rationally designing nanostructures with enhanced properties.
4. Conclusion In conclusion, large densities of (0 0 1) microtwins in distinct rhombohedra and elongated rhombohedra platelets have been systemically revealed by SAED and HRTEM observations. It is suggested that boron suboxide rhombohedra platelets are prompted by the vapor assisted TPRE mechanism. The elongated rhombohedra platelets can be related to rhombohedra ones by subsequent catalytic growth at the apex. This proposed growth mechanism presents a unified description of the various rhombohedra platelets in related research. Moreover, our results point out that the TPRE mechanism is also applicable in rhombohedra based materials. Understanding the growth mechanism involved will be beneficial in rationally designing a new group of nanostructures and understanding related physical properties.
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Fig. 4. (a) Vapor assisted TPRE growth model for rhombohedra platelets. Extensive (0 0 1) microtwins lead to alternating reentrant grooves which serve as preferential nucleation sites indicated by the arrows. (b) The as grown platelet will exhibit distinct rhombohedra morphology. (c) Catalyst aided TPRE growth of elongated rhombohedra platelets. The boron-rich molecules are first absorbed by the catalyst and further transferred to the twin-plane reentrant edge to prompt the growth of platelets. A diffusion limit will be applied by the catalyst. Consequently the platelets will be elongated along the [1 1 0] direction.
Fig. 5. Room temperature PL spectra from a rhombohedra platelet and three elongated rhombohedra platelets with a laser excitation of 633 nm.
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