Correlating Growth Habit of Boron-Rich Low-Dimensional Materials

Article pubs.acs.org/crystal

Correlating Growth Habit of Boron-Rich Low-Dimensional Materials with Defect Structures by Electron Microscopy Zhiyang Yu,†,‡,§ Xin Fu,∥ Jun Yuan,⊥ Steffan Lea,⊥ Martin P. Harmer,§ and Jing Zhu*,†,‡ †

Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Beijing National Center for Electron Microscopy, Tsinghua University, Beijing 100084, China § Center for Advanced Materials and Nanotechnology, Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States ∥ General Research Institute for Nonferrous Metals, Beijing 100088, China ⊥ Department of Physics, The University of York, Heslington, York, YO10 5DD, U.K. ‡

ABSTRACT: Boron carbide and boron suboxide low-dimensional materials with α-rhombohedral symmetry, here simply referred to as boron-rich nanomaterials, exhibit a variety of growth habits, including rodlike fibers, slablike platelets, and some intermediary structures, as confirmed by scanning electron microscopy (SEM) observation. The defect structures of these variants have been thoroughly characterized by advanced transmission electron microscopy techniques, which reveal the prevalence of two basic contact twinning: parallel twinning and cyclic twinning. The growth habits and defect structures of boron-rich materials are found to be strongly correlated. This has been summarized by a growth habit map, which allows us to establish a quick strategy for the determination of defect structures by simple SEM shots in boron-rich materials, alleviating ceramists from the need and complexity in the identification of multitwinned structures.

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INTRODUCTION α-Rhombohedral boron-rich materials, mainly boron suboxide (nominal formula: B6O), boron carbide (nominal formula: B 4 C), and B-C-O compounds in this paper, possess extraordinary high strength and excellent oxidation resistance near room temperature.1 These properties make them attractive candidates for reinforcement phases in composites. Take nanoparticles as an example, aluminum−boron carbide composite brake pads have been realized, which exhibited an order of magnitude excellence in wear rates.2 By the addition of boron carbide particles into alumina, Liu et al. have successfully improved fracture toughness and flexural strength simultaneously.3 In addition, whiskers and platelets with the composition of boron carbide are widely utilized in ceramics composites, particularly in structural ceramic composites. By pressureless sintering of densified Al2O3 with B4C platelets/ whiskers as reinforcement phases, an increment of 6.3 MPa/ m1/2 has been achieved in the fracture toughness.4 In addition to their promising physical properties, boron-rich low-dimensional materials exhibit a variety of fascinating polymorphs, in the form of single crystal nanowires,5 parallel twinned nanowires,6 star-shaped nanowires with a 5-fold twinned cross section, 7−10 multiply twinned particles (MTPs),11−15 and parallel twinned platelets.13,14,16−22 In contrast to the well-studied single-crystal SiC nanobelts that are extensively used as reinforcement phases in the alumina matrix, the microstructures of boron-rich materials have not © 2013 American Chemical Society

been addressed in detail. In most cases, ceramist did not gain full insight into the defect structures of their products and mistakenly treated rodlike nanowires and slablike platelets as the same structure.4 The main reason why the understanding of their microstructure is impeded is the complexity and diversity in the growth habits. The twinning formation energy of αrhombohedral boron-rich materials is quite low; thus, the occurrence of stacking faults and microtwins is common in these system.23−25 Similar to the role of twinning in growth habit modification of noble metal nanostructures,26−28 the presence of two basic types of contact twining, for example, parallel twinning and cyclic twinning, is responsible for morphology diversity of boron-rich materials. This is because twinning triggers additional symmetry breaking in the rhombohedral lattice. Symmetry breaking is expected to occur either along the 5-fold axis for cyclic twinned structure or perpendicular to (001)r (The subscripts in the denotation of directions or planes refer to the rhombohedral representation for rhombohedral crystal structure.) twin planes for the parallel twinned structure. Twinning-induced symmetry breaking adds more diversity of the final morphology. Here, in this article, the authors conducted a comprehensive morphological categorization of α-rhombohedral boron-rich Received: November 12, 2012 Revised: April 5, 2013 Published: April 5, 2013 2269

dx.doi.org/10.1021/cg301657c | Cryst. Growth Des. 2013, 13, 2269−2276

Crystal Growth & Design

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Figure 1. (a) SEM morphology of commonly observed regular nanowires. The crystallinity of nanowires is improved by increasing the growth temperature. At 1300 °C, truncated side facets and re-entrant twin boundaries are well-resolved in high-resolution SEM, which indicates the pseudo5-fold symmetry in the cross section. (b) Diffraction patterns for regular nanowires fabricated at 1000 °C along several high symmetrical directions. The nanowire-beam orientation relationships have been marked in the schematics. (c, d) High-angle annular dark-field (HAADF) and highresolution electron microscopy (HREM) images for irregular nanowires. A strong size modulation in the diameter is evident. Microtwins have been indicated by the dotted lines.

section. The lower-sintering-temperature straight nanowires adopt a round surface. Thus, the structural determination must be done by TEM characterization. Our previous works demonstrate that most of these nanowires also possess distinct 5-fold-twinned cross sections.7,8,10 To reveal their “polycrystalline”-like structures, usually, the nanowires should be aligned to several high symmetric diffraction conditions, mostly at 0° and 18° orientations. Figure 1b shows typical diffraction patterns of nanowires with a smooth cross section at various titling angles, with the orientation between the electron beam and the diagonal of the T1 segment shown in the schematics. The diffraction patterns at 0° and 18° orientations can be understood as the superimposition of kinetic diffraction patterns from five twinned variants (T1−T5 segments), as confirmed by our previous works.7,8 By applying the serial tilting diffraction method, five twinned variants in the nanowire can be selectively revealed by careful examination of the diffraction patterns. Our diffraction analysis over more than 10 nanowires confirms that all of the straight nanowires, irrespective of their synthetic temperature, hold 5-fold twinned cross sections. Compared to regular nanowires, the nanowires in the second minor category are not straight in the longitudinal direction, as shown in Figure 1c. On the contrary, these nanowires show strong size modulation in the diameter and an irregular defined morphology. They are mostly detected at lower sintering temperature, below 1200 °C. High-resolution electron microscopy analysis of these nanowires reveals high densities of microtwins, as depicted by the lines in Figure 1d. Differing from the nanowires having a 5-fold twinned cross section, the second type of nanowires shows extensive parallel microtwins in the longitudinal direction. Platelets fall into three categories, which show a common thin-slab-like feature but differ in their plan-view morphology. The most commonly seen morphology is elongated [110]r platelets with acute tips forming an acute angle of ∼60° in both ends, as shown in Figure 2a. The other two morphologies are

materials in our study by SEM observation. Additionally, the defect structures, mainly in the form of twinning, are investigated by TEM techniques, including serial tilting diffraction analysis and HREM observations. The defect structures are found to have a strong correlation with growth habits. Therefore, a roadmap, presented with regard to structural evolution, is proposed to understand the role of twining defects in the determination of growth habits. In addition, the mass transport mechanism has been included to gain a full picture of the growth process. This roadmap forms a basis to understand the growth mechanism of α-rhombohedral boron-rich materials.

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EXPERIMENTAL SECTION

Boron-rich low-dimensional materials were fabricated by standard chemical vapor deposition. Details of the synthetic procedure can be found in our previous work.29 Pellets cold-pressed to 3 cm in diameter have been used as the precursor to produce boron-rich gaseous vapor. Later, the pellets that were rich in catalysts also served as the substrates for the high yield growth of boron materials at 1100−1500 °C. After the cooling of the furnace, large quantities of low-dimensional materials are recovered at the surface of the pellets. The morphology of boron-rich nanostructures harvested at the surface pellets was observed by a JEOL 6301F field emission scanning electron microscope (FESEM) and an Hitachi 5500 FESEM. In addition, their defect structures and chemical components were studied by a JEOL 2011 TEM, JEOL 2010F field emission TEM coupled with a Gatan Imaging Filter (GIF).

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RESULTS AND DISCUSSION Two kinds of nanowires that differed in morphology are found in the final product, as shown in Figure 1a,c. A majority of the nanowires are called “regular nanowires”, which are straight in the longitudinal direction, as indicated by the arrows. With the increments in sintering temperature, these nanowires tend to show well-defined acute edges in the sides faces (1300 °C nanowires), as well as re-entrant twinned boundaries, indicating that these nanowires consist of cyclic twinning in the cross 2270



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Figure 2. (a−c) SEM images of three categories of platelets differing in their morphology. The growth directions and the vertex angles at the ends have been labeled respectively. (d, e) Diffraction patterns for platelets aligned along the [11̅ ̅3]r zone axis. The fractional diffraction spots have been marked by the boxes. (f) Reconstructed reciprocal space for boron-rich platelets. The reciprocal lattice is elongated along [001]*r due to the presence of nanosized microtwins paralleling to the (001)r surface. Taking the reciprocal elongation as schematically shown by the green rod into consideration, the 1/3 fractional spots are well accounted for.

Figure 3. (a) Side-view bright-field image of an intermediary with the platelet tilted along the [11̅0]r orientation. For a better understanding of the geometry, the SEM image of a similar intermediary is given in Figure 5m. The insets show diffraction patterns recorded at regions A, B, and A + B. (b−d) High-resolution images acquired at regions A−C. In (c) and (d), the twinned variants imaged in each figure have been labeled in the schematics.

structural investigation of boron suboxide platelets, previously, we demonstrate that [110]r elongated platelets and rhombohedral platelets share identical defect structures,16 as they have

detected with a lower possibility. One has a well-defined rhombohedral shape (Figure 2b), and the other one (Figure 2c) shows an obtuse angle (∼120°) in both ends. In the 2271



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Figure 4. (a) Bright-field image of the second type of intermediary. The external morphology is shown in Figure 5n. Diffraction patterns are recorded at the platelet (b) and the nanowire (c).

Figure 5. Growth habits of low-dimensional α-rhombohedral boron-rich materials as a function of defect structures, sintering temperature, and chemical components.

the arrows in Figure 2a−c. These platelets are termed as [110]r elongated platelets, rhombohedral platelets, and [100] r elongated platelets in the following discussion, respectively. Besides the presence of nanowires and platelets, two intermediary structures, which hold both the structural characteristic of 5-fold twining and parallel twinning, are occasionally found after careful examination of final products. Figure 3 show one example of the interesting transient structure, with the crystal morphology shown in Figure 5m. A typical bright-field image is shown in Figure 3a, with the inset showing the corresponding electron diffraction patterns recorded at various regions. Some interesting features are revealed after close inspection. The pattern, recorded from the A region of the “edge-on” platelet, belongs to the [11̅0]r zone axis. HREM images recorded at regions A and C show exactly the same feature, which is an indication that the T1 segment structurally derives from one tip of region A. In the mean time, the pattern recorded at region B can be ascribed to the diffraction pattern from the [11̅ 3̅ ]r zone axis with forbidden diffraction spots indicated by the circles. These fractional spots suggest the existence of a large amount of microtwins in T3 or T4 segments (see Figure 3d for the geometry of twinned

identical diffraction patterns, as shown in Figure 2d. Figure 2e shows the diffraction pattern of [100]r elongated platelets along the same [1̅1̅3]r zone axis. All the diffraction patterns show faint forbidden diffraction spots, as indicated by the boxes. The reason for the fractional diffraction spots is the existence of high densities of (001)r microtwins running parallel to the large flat surface.16 As schematically shown in Figure 2f, the reciprocal lattice for platelets is elongated along the [001]*r direction due to the presence of nanosized microtwins.16 The Ewald sphere has been considered as a flat plane for simplification, as illustrated by the filled purple plane for the [1̅1̅3]r zone axis in Figure 2f. One can see that the introduction of reciprocal lattice elongation results in the forbidden diffraction spots, which sit exactly at the 1/3 fractional position of the original lattice. Likewise, the reconstructed reciprocal space can satisfactorily explain the formation of 1/2 fractional diffraction spots in our previous work.16 The similarity of diffraction patterns suggests that all the platelets we have examined possess a parallel twinned structure. It is believed that the formation of all of these platelets is promoted by the well-known twin plane reentrant edge (TPRE) mechanism.30,31 The major difference between these platelets is their growth direction, as depicted by 2272



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Figure 6. Schematic anatomies of boron-rich low-dimensional materials. The cross-sectional structure and the plane view (commonly observed external morphology in SEM) are shown in (a) and (b).

extensive TEM characterization of boron-rich low-dimensional materials, a growth habit roadmap has been proposed from the aspects of defect structure, growth temperature, and chemical component in Figure 5. In the view of defect structures, the structural evolution of boron-rich materials has been marked by the solid lines based on their structural correlations. Five individual twinned variants, labeled as T1−T5 segments, are more evident in the SEM at high growth temperatures, as shown in Figure 5a−d. The second category of nanowires, as shown in Figure 5e, actually contains a high amount of parallel (100)r microtwins. The parallel twinned defect structure is exactly the same as that of platelets. Therefore, the parallel twinned nanowires are believed to be an immature form of platelets, and when growth conditions permit, parallel twinned nanowires will be fully developed into rhombohedral platelets in Figure 5f. Further elongation of rhombohedral platelets will produce either [110]r or [100]r elongated platelets (Figure 5g− k), depending on the growth direction. In some cases, the growth direction of platelets occasionally changes, leading to a transient platelet morphology (Figure 5l). All the platelets share an identical parallel twinned structure. These two intermediary structures inherit both defect structures of 5-fold twinned nanowires and parallel twinned platelets, as schematically shown in Figure 5m,n. The 5-fold axis of cyclic twinned nanowires either inclines to or lies in the large flat (001)r surface of platelets. It is interesting to see that two types of basic contact twining, for example, parallel twinning and cyclic twinning, coexist in a single nanostructure. The structural evolution map directly points out the close correlation between defect structure and growth habits. The cyclic twining structure is associated with 5-fold twined nanowires, whereas parallel twining is strongly correlated to thin-slab-like platelets. The straightforward correlation presented here can be understood through the growth mechanism of boron-rich materials. Most of the boron-rich nanowires with cyclic twinned cross sections have been fabricated in our group, and we have conducted a detailed characterization of this distinct structure.7,8,10 The high occurrence of cyclic twinning in the

variants in the cross section). The 5-fold cross-sectional feature of the nanowire-like segment is identified, as the diffraction pattern recorded at regions A and B can be considered as the superimposed patterns along the [11̅0]r and [1̅1̅3]r zone axes, which matches well to the patterns at the 18° orientation discussed in Figure 1. The moiré fringe shown in Figure 3d can be understood as the overlapping of T1, T2, and T5 segments. This forms additional evidence for the existence of a 5-fold twinned cross section. The determination of such a complicated structure by an electron microscope confirms the coexistence of parallel twinning and cyclic twinning in one single boron-rich structure. Similar complexity in the crystal structure is found in the second category of transitional structures. A typical bright-field image is shown in Figure 4a. For a better understanding of its morphology, a typical SEM image is presented in Figure 5n. The electron diffraction pattern recorded in the platelet (region B) clearly shows fractional diffraction spots, as denoted by the dotted circles in Figure 4b. It is believed that there is a great amount of (001)r microtwins running parallel to the large flat (001)r surface. The diffraction pattern from region C exhibits a polycrystalline feature. Further examination of the pattern suggests that it is the superimposed pattern similar to the ones for 5-fold twinned nanowires at an 18° orientation (see Figure 1c). Comparison of the diffraction pattern from the platelet (Figure 4b) and nanowire (Figure 4c) indicates that either T3 or T4 segments for the 5-fold twinned nanowire are in the same orientation with the platelet. The crystallography analysis demonstrates that, for these two types of intermediary structures, several fragments of 5-fold twinned nanowires connected at one end of the platelets have exactly the same orientation with parallel twinned platelets. In our synthesis, the parallel twined platelets are mainly produced at high sintering temperatures (1400−1500 °C), whereas the cyclic twinned nanowires are the primary product at low sintering temperatures (1100−1200 °C). It is, therefore, reasonable to conclude that the nanowires sitting at the tip of the platelets might be described as the further epitaxial growth product when the growth of platelets slows down or stops. On the basis of an 2273



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reduced drastically compared with that of 2-D nucleation. The discussion on the growth habit in this paper is on a basis that the TPRE mechanism works to trigger further symmetry breaking. The only exception is for irregular nanowires where the growth process is controlled by the VLS mechanism. When the atomic structure of the surface is rough, irregular crystals, sometimes termed as euhedral crystals, are fabricated with the appearance of rough surfaces. Euhedral crystals are constantly found by Hubert14,23 and He15 as a byproduct of boron suboxide icosahedra particles. Some irregular platelets can be also understood as the result of the isotropy in surface energies.19,22 From the structural perspective, the growth of boron-rich materials is mainly controlled by the TPRE mechanism. As for the detailed mass transport mechanism, either the vapor− liquid−solid (VLS33) or the oxide-assisted growth (OAG34) mechanism can assist the TPRE mechanism in our study. When the temperature is below 1300 °C, iron particles sputtered on a silicon chip is the key factor in maintaining the high yield growth in a similar chemical vapor deposition experiment.35 Examination of the tips of low-dimensional products suggests the presence of iron-rich nanoparticles in most of the droplets we observed. This is a strong indication that the VLS mechanism dominates at lower temperatures and iron in the precursor plays a vital role as the growth catalyst. At higher temperatures above 1300 °C, there are always globule tips at the tip of platelets and nanowires. Further TEM analysis suggests that a large proportion of globules contain no crystalline iron, but an amorphous substance. EDS spectra obtained at the amorphous particles show strong Ba, B, and O peaks. Thus, it is speculated that the OAG growth mechanism is dominant at higher temperatures. Barium oxide is believed to be important in lowering the eutectic temperature of the precursor. To support this conclusion, we intentionally collected droplets on silicon chips without any iron nanoparticles on them. An aluminum boat containing BaO, B, and Fe3O4 precursors is put upstream as the boron source. The experimental design can be found in our previous work.35 The acquired EDS spectra on the droplets show no iron peak, but a strong Ba signal. This experimental design suggests that barium oxide forms a eutectic boron-rich melt that is easily vaporizing. Iron cannot squeeze into the eutectic melt and thus is absent in the droplet downstream. In addition, we find that the alumina crucible only turns to gray when a certain amount of barium oxide is put into the precursor. All of the evidence presented here supports our speculation. When the precursor is heated at a specific temperature, the iron-rich catalyst is formed by the chemical reaction between Fe3O4 and B. The byproduct of this reaction is the B2O2 molecule.36 BaO seldom reacts with B as the amount of BaB6 cannot be detected in XRD characterization. In turn, BaO will readily form a low-temperature eutectic with the B 2 O2 molecule. The melt is vaporized into barium- and boron-rich gas, which is further transferred to the catalyst. The condensation of boron-containing gas at low temperature might be favored on the surface of the iron boride catalyst, and thus, the VLS mechanism will direct the growth of boron-rich low-dimensional materials. Similar cases can be found when iron or Pt nanoparticles are used as the catalyst in Gao’s work.37,38As the sintering temperature is above the melting point of iron boride, for the high-temperature case, the VLS mechanism is suppressed. Now, the OAG mechanism is

boron-rich materials is due to the relative smaller strain energy when five twinned segments are trying to fit into a 360° solid angle along the 5-fold [001]r axis. The misfit angles for boron carbide cyclic twinned nanowires are between 4.55° and 5.25° excess.8 For the boron suboxide star-shaped nanowire, the 1° angular deficiency is negligible,7 as compared to the 7.35° angular deficiency in the face-centered cubic (fcc) structure. Consequently, the energy compensation to form cyclic twinned nuclei for boron-rich materials is low, enabling the easy formation of a 5-fold twinned structure. Once the nuclei are readily formed, the anisotropic growth will be favored along the [001]r 5-fold axis, due to the fact that the crystal growth at the twinned boundaries will be effectively accelerated by the TPRE mechanism.9 As the side faces of nanowires are bounded by low-energy surfaces, the straight profile of nanowires is maintained in the longitudinal directions, as shown in Figure 6. Our analysis of the growth habit suggests the strong correlation between cyclic twinned structures with the fiber-like nanowire morphology. As a consequence, for regular fibers, with or without catalyst, the cross-sectional structure is always with cyclic twining. The growth of irregular nanowires is believed to be an immature form of platelets. Under certain growth conditions, such as in a catalyst-guided VLS mechanism, the growth perpendicular to (001)r microtwins is favored because catalysts are well-suited at the tops of (001)r twin planes. Such a structure has been produced in low-pressure chemical vapor deposition via the VLS mechanism at a relatively lower growth temperature (∼1000 °C).6 As the diameter of the nanowire is largely controlled by the interfaces between the catalyst and the boron carbide phase, growth uncertainly will lead to modulation in diameter, as schematically depicted in Figure 6. The uneven dimension for the twinned segments accounts for strong diameter modulation for the irregular nanowires, whose growth habit is associated with the defect structure. The formation of a platelet-like morphology is directly correlated to their parallel twinned defect structures in the cross section, as shown Figure 6. As there are high densities of (001)r microtwins in the nuclei, the trough sitting in the twinning boundaries provides fast nucleation sites and promotes accelerated growth running parallel to the (001)r flat surface. In the meanwhile, the crystal growth perpendicular to the (001)r twin plane does not have such a geometry for easy nucleation. Therefore, crystal growth in the [001]r direction will not be speeded up. In other words, high densities of (001)r parallel twinning introduce further symmetry breaking in the rhombohedral lattice and results in a platelet morphology. All the platelets observed in the current study are confined by certain low-energy planes, such as the (001)r-type surface. As a result, both of the two ends are always enclosed by ∼60° acute angles or ∼120° obtuse angles, as shown in Figure 6. With regard to the side profiles, as the side facets of [110]r elongated platelets are terminated by (100)r and (010)r nanosized surfaces, instead of (11̅0)r planes (see Figure 6 for details), reasonably, they should exhibit a geometrical sawtooth profile. More details on the role of twinning in the growth habit modulation of platelets can be found elsewhere.16 It should be mentioned that an atomically smooth surface is the prerequisite for the correlation of growth habit and defect structure.32 When the low-energy surfaces are singular, boronrich materials tend to be enclosed by atomically flat surfaces. Therefore, the TPRE mechanism will dominate the growth mechanism as the nucleation barrier at the twinned boundary is 2274



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there is no attempt to correlate the growth habit of αrhombohedral boron-rich materials with twinning defects. Besides, the definition of whiskers is obscure in ceramics and materials science, as they should be properly referred to as single crystals in the form of filaments.41 Boron-rich materials show various diversity in morphologies, including single-crystal nanowires,5 parallel twinned nanowires,6 5-fold twinned nanowires,7−10 multiply twinned particles (MTPs),11−15 and parallel twinned platelets.13,14,16−22 From the microstructural view, if we exclude the form of MTPs, all the other varieties can be termed as whiskers. As a matter of fact, these variants differ in the defect structure. For example, in Lin’s work,4 if one takes a close look at the “double spear-like whiskers”, they are similar to the elongated platelets with parallel twinning discussed here, while their rodlike “whiskers” are actually nanowires with cyclic twinning in the cross section. A poor understanding of microstructure and growth habit might hinder structure− property investigation of boron-rich materials, as both numeral studies42 and experimental works43 have demonstrated that the cross section of reinforcement phases plays important roles in the mechanical properties of composites. The significant effect of microstructure in affecting the properties of ceramics reinforced composites should be taken into account for ceramists. We believe that our approach will be beneficial in building the connection between the microstructure and the property of boron-rich composites.

preferred as increasing the sintering temperature produces an abundant amount of boron-rich gas. Condensation of the gas forms nuclei, which keep absorbing boron-rich molecules. When the concentration of B-C-O in the droplet is saturated, boron-rich nanosized materials are precipitated while the barium element is preserved in the droplet as the growth continues. Therefore, when the boron-rich products are cooled down to room temperature, barium-rich amorphous droplets are detected at the tips. It should be noted that both of these two growth mechanisms work in our system and their contribution to the growth is mainly determined by the growth temperature. From the chemical composition aspect, we want to note that no correlation between chemical component and growth habit of boron-rich materials is detected in the growth habit roadmap. As confirmed by electron energy loss spectrum (EELS) quantification in our previous works,29 at higher growth temperatures (1400−1500 °C), the chemical component of platelets is close to boron suboxide. Lowering the growth temperature (1100−1200 °C) will shift chemical components approaching boron carbide. In between, platelets are composed of B-C-O solid solution. The chemical components of boron-rich nanostructures seem to be arbitrary, irrespective of their defect structures and growth habits. As all of these boron-rich phases in this paper share an identical crystal structure and similar defect structures, we believe the chemical deviation will not have strong effects on the growth habits. Our growth habit analysis paves a solid way for directly relating their growth morphology with defect structures. By simply looking at Figure 6 for the anatomies of boron-rich materials, one can easily determine the cross-sectional defect structures by taking one single SEM shot at reasonable magnification, without resorting to complicated diffraction patterns or HREM image interpretation. This will free ceramists from the complexity in structural identification of multitwinned materials and will be helpful in understanding the effect of defect structures on the property of composites when boron-rich low-dimensional materials are used as reinforcement additives. Boron-rich low-dimensional whiskers have been widely used as reinforcement phases in composites. Nevertheless, the structural insight into these whiskers has not been addressed in detail. For example, boron-rich platelets are often mistakenly recognized as single crystals.13,15,17−20,39 However, the αrhombohedral lattice is a high symmetric structure where all the [001]r directions are identical. Thus, if we do not include the effect of microtwins, it is difficult to explain the thin slab morphology since symmetry breaking always happens along the [001]r direction. The external morphology of elongated platelets as the most common products has been rarely questioned. There were some early works trying to reveal the defects of boron-rich platelets. The diffraction pattern of boron carbide 2D nanomaterials in Xu’s work shows evident fractional spots that are weaker in intensity, indicating the existence of high densities of microtwins.13 However, these spots have been misinterpreted as normal diffracting spots, and Xu treated these platelets as single crystals. Similar diffraction patterns can be found in boron and boron carbide whiskers.17 Even Carlsson17 and Jazirehpourf40 detected abundant amounts of parallel twinning from some “edge-on” platelets; they did not realize that actually parallel twinning is common in all of their elongated platelets. To the best of our knowledge, currently,

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CONCLUSION In summary, α-rhombohedral boron-rich nanomaterials have been thoroughly characterized by sophisticated TEM techniques, revealing an abundance of cyclic twinning and parallel twinning. These defect structures are found to be directly related to their growth habits. Therefore, growth habits of boron-rich materials have been summarized based on their structural correlation. This strategy not only can be utilized to understand the underlying growth mechanism at the atomic scale but also should be extended as an atlas for rapid determination of defect structures in boron-rich materials by a single SEM shot, without resorting to more complex TEM structural identifications.

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Phone: +86 10 62794026. Fax: +86 10 62772507. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work is financially supported by the National 973 Project of China and the Chinese National Nature Science Foundation. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work made use of the resources of the Beijing National Center for Electron Microscopy and that of The University of York. 2275



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