Observation of ‘hidden’ planar defects in boron carbide nanowires and identification of their orientations
© Guan et al.; licensee Springer. 2014
Received: 25 November 2013
Accepted: 5 January 2014
Published: 15 January 2014
The physical properties of nanostructures strongly depend on their structures, and planar defects in particular could significantly affect the behavior of the nanowires. In this work, planar defects (twins or stacking faults) in boron carbide nanowires are extensively studied by transmission electron microscopy (TEM). Results show that these defects can easily be invisible, i.e., no presence of characteristic defect features like modulated contrast in high-resolution TEM images and streaks in diffraction patterns. The simplified reason of this invisibility is that the viewing direction during TEM examination is not parallel to the (001)-type planar defects. Due to the unique rhombohedral structure of boron carbide, planar defects are only distinctive when the viewing direction is along the axial or short diagonal directions (, , or ) within the (001) plane (in-zone condition). However, in most cases, these three characteristic directions are not parallel to the viewing direction when boron carbide nanowires are randomly dispersed on TEM grids. To identify fault orientations (transverse faults or axial faults) of those nanowires whose planar defects are not revealed by TEM, a new approach is developed based on the geometrical analysis between the projected preferred growth direction of a nanowire and specific diffraction spots from diffraction patterns recorded along the axial or short diagonal directions out of the (001) plane (off-zone condition). The approach greatly alleviates tedious TEM examination of the nanowire and helps to establish the reliable structure–property relations. Our study calls attention to researchers to be extremely careful when studying nanowires with potential planar defects by TEM. Understanding the true nature of planar defects is essential in tuning the properties of these nanostructures through manipulating their structures.
KeywordsBoron carbide nanowires Rhombohedral crystal system Transmission electron microscopy Planar defects
Planar defects, such as stacking faults and twins, naturally exist in some as-synthesized one-dimensional (1D) nanostructures . In addition to assisting the growth of nanostructures , these defects can affect the mechanical , electrical , thermal , and optical  properties of 1D nanostructures. Thus, it is crucial to know their nature such as existence, distribution, and orientation within each 1D nanostructure while establishing the structure–property relations. So far, transmission electron microscopy (TEM) has been one major technique commonly used to characterize the structure of individual 1D nanostructures and reveal the nature of planar defects . However, due to the sophistication of the TEM technique, sometimes, experimental artifacts could be erroneously interpreted or lead to controversy [6–10]. To date, most planar defect-related studies have been focused on 1D nanostructures made of silicon, silicon carbide, III-V (e.g., GaAs, InP), or II-IV compounds (e.g., ZnO, CdSe) whose crystal structures are either cubic or hexagonal [8–15].
Boron carbide 1D nanostructures have attracted increasing attention in the last few years because of their potential applications in nanocomposites and thermoelectric energy conversion [16–25]. Most reported boron carbide 1D nanostructures were synthesized by carbothermal reduction or chemical vapor deposition at approximately 1,100°C [16–23]. Field emission [18, 23], photoluminescence , mechanical [21, 23], and thermal conductivity  properties of these 1D nanostructures were reported. However, due to the complicated rhombohedral crystal structure, detailed structural characterization especially on planar defects that could greatly affect the properties of boron carbide 1D nanostructures has not yet gained enough attention, and the structure–property relations have not been established. In our previous study , about one hundred as-synthesized boron carbide nanowires were subjected to TEM study, during which each nanowire was examined throughout the full tilting range allowed by the configuration of our microscope. Approximately 75% examined nanowires were found to have planar defects, while the remaining 25% were planar defect-free-like. The defected nanowires were further categorized into two groups: transverse faults (TF) nanowires with planar defects perpendicular to the preferred growth direction of nanowires and axial faults (AF) nanowires with planar defects parallel to the preferred growth direction of nanowires. The determination of defects’ existence and fault orientations (TF or AF) within each nanowire was based on the characteristic features presented in TEM results, including modulated contrast in high-resolution TEM (HRTEM) images and streaks in diffraction patterns.
In this work, more extensive TEM examination and model simulation were performed to gain a deeper understanding of the nature of planar defects in the aforementioned boron carbide nanowires to answer two questions. (1) Do planar defect-free boron carbide nanowires really exist? Literature review shows that due to its relatively low stacking fault energy (75 mJ/m2) , planar defects have been frequently observed in bulk boron carbides independent of the synthesis methods [27–30]. It has also been reported that the density of planar defects decreases as the synthesis temperature increases . However, the planar defects were still detectable by TEM from bulk samples synthesized at 2,100°C . Considering the common existence of planar defects in bulk boron carbides and the relatively low temperatures researchers used to synthesize boron carbide 1D nanostructures, one may naturally ask ‘Can boron carbide nanowires synthesized at approximately 1,100°C be planar defect-free? Or defects always exist but sometimes are not found by TEM?’ (2) If planar defects exist in all of our as-synthesized boron carbide nanowires, can their orientations be determined from TEM results showing no characteristic features (i.e., results from the off-zone directions as discussed later)? It is expected that different orientations of planar defects could have distinctive effects on the properties of these nanowires, similar to that physical properties of superlattices could be very different along their in-plane and cross-plane directions [31, 32]. Therefore, it is important to know the fault orientation of each boron carbide nanowire when establishing the structure–property relations.
In this paper, a thorough discussion on observing planar defects in boron carbide nanowires by TEM is presented. Results show that planar defects can be easily invisible in boron carbide nanowires even after a full range of tilting examination. Extra attention must be paid and reliable conclusion can only be made based on the results from different viewing directions (i.e., zone axes). Furthermore, a new approach is developed to determine the fault orientations of those boron carbide nanowires whose planar defects are invisible in TEM results. The approach can be extended to other 1D nanostructures whose crystal structure is not rhombohedral.
Boron carbide nanowires were synthesized by co-pyrolysis of diborane and methane over nickel-coated semiconductor substrates at relatively low temperatures in a home-built low-pressure chemical vapor deposition system . The as-synthesized nanowires were first transferred from substrates to a small block of elastomeric polydimethylsiloxane (PDMS) by a gentle stamping process. Individual boron carbide nanowires were selected and picked up by a sharp probe mounted on an in-house assembled micromanipulator and then transferred to a TEM grid layered with lacy carbon support film. This operation was done under an optical microscope equipped with long working distance objective lenses. In each mesh of the TEM grid, only one nanowire was placed. During TEM study, each nanowire was subjected to a full range of tilting examination. The tilting range was set by the configuration of our microscope, as described later. For the nanowire that appeared to be planar defect-free in the initial round of TEM examination, it would be picked up by the sharp probe and repositioned onto another region of the lacy carbon support film for reexamination. This challenging and tedious reposition-reexamination process was repeated several times for some nanowires to reveal the true nature of planar defects inside them.
A JEOL JEM-2100 LaB6 transmission electron microscope, Akishima-shi, Japan, was used to characterize boron carbide nanowires. The microscope is equipped with an analytical high-resolution pole piece, which can realize a point resolution of 0.23 nm, a lattice resolution of 0.14 nm, and a specimen tilting range of ±30° in both X and Y directions. A JEOL double-tilt holder was used to realize the wide angle of tilting. It is worth pointing out that the 60° in total tilting range is comparable to or even wider than that of the most microscopes researchers used to study 1D nanostructures. The operation acceleration voltage used for this study was 200 kV.
Software packages CrystalMaker® and SingleCrystal™, Oxfordshire, UK, were used to construct, display, and manipulate three-dimensional models of boron carbide unit cell and nanowires, as well as to simulate corresponding electron diffraction patterns. All crystallographic indexes used in this paper are expressed in the rhombohedral notation for convenience of discussion (see Additional file 1 for conversion between the rhombohedral notation and the hexagonal notation).
Results and discussion
The existence of ‘hidden’ defects
As briefly pointed out in our previous report , wide angle of tilting during TEM examination is needed to reveal the existence of planar defects in as-synthesized boron carbide nanowires. Figure 1c shows the TEM results of a nanowire that seems to be planar defect-free due to the lack of modulated contrast in the image and streaks in the electron diffraction pattern. However, after tilting the nanowire to a different zone axis, all ‘hidden’ planar defects emerged as clearly shown in Figure 1d, revealing a TF nanowire. This example undoubtedly demonstrates that one cannot conclude that a nanowire is planar defect-free based on TEM results obtained from one single viewing direction. A full range of tilting examination from multiple zone axes is necessary to obtain a reliable conclusion.
As mentioned in the ‘Background’ section, although in our previous study, approximately 25% of boron carbide nanowires appear to be planar defect-free based on the full range of tilting examination, we are wondering whether these nanowires are really without any planar defects. Recently, using the reposition-reexamination process described in the ‘Methods’ section, we clarified this issue. Figure 1e is a low magnification TEM image of a boron carbide nanowire. An initial full range of tilting examination suggests that the nanowire is planar defect-free, as shown in Figure 1f. However, after repositioning the nanowire (Figure 1g) and reexamination, the ‘hidden’ planar defects are revealed in Figure 1h and the nanowire is identified as an AF nanowire. This example further demonstrates that the existence of planar defects cannot be fully revealed by observation from one single zone axis. Moreover, in specific occasions, even after a full range of tilting examination limited by the configuration of a microscope, there is still a possibility of neglecting the existence of planar defects. In our current study, twenty five planar defect-free-like nanowires were subjected to multiple rounds of reposition and reexamination, and planar defects were seen from all of them eventually. This new finding strongly suggests that planar defects exist in all of our as-synthesized boron carbide nanowires. However, these defects are not always visible from routine characterization.
The origin of ‘hidden’ defects
It is now clear that during TEM examination, planar defects can be easily invisible in boron carbide nanowires. Analysis indicates that the simplified reason for this invisibility is that the viewing direction is not along some specific directions parallel to planar defects.
A roadmap consisting of simulated diffraction patterns of major low index zone axes within the (001) plane is shown in Figure 2c. During TEM examination, this roadmap can help us judge if it is possible to tilt to the next zone axis according to the calculated angle between different zone axes. For example, it is nearly impossible to obtain results from both and  zone axes on the same nanowire because the calculated inter-axial angle (57.1°) is close to the tilting limit of our TEM specimen holder (60°). In the roadmap, there are four independent patterns such as those from , , , and  directions, as grouped in four colors. During TEM examination, planar defects can be seen along directions of , , and  whose diffraction patterns are asymmetric and with streaks in them. While viewing along the  direction, the layered faults feature is hidden because of the mirror symmetry. In addition, planar defects are more distinctive when viewing along directions of and  than that of (see Additional file 1 for comparison between experimental results obtained from the aforementioned four different zone axes). Therefore, in our real TEM practice, only results from the two independent directions: and  are recorded and analyzed.
There are a total of six equivalent -type and -type directions in the rhombohedral system, as drawn in orange and blue lines in Figure 2b. Characteristic features of planar defects can be observed by TEM when the viewing direction is along the rhombohedral axes or the short diagonal within the (001) plane, i.e., the directions of , , and . These three directions (outlined in orange) are denoted as in-zone directions. Meanwhile, the other three directions: , , and , located out of the (001) plane (marked in blue), are denoted as off-zone directions, due to the fact that planar defects are invisible from them.
Now the difficulty to visualize planar defects in boron carbide nanowires becomes obvious. If the viewing direction is not parallel to planar defects, the defects will be invisible. In addition, even if the viewing direction is parallel to planar defects, depending on the initial orientation of the viewing direction, planar defects may also not be observed. For example, if the initial viewing direction (i.e., without any tilting of the specimen holder) is along the  direction within the (001) plane, it is then impossible to see any characteristic features of planar defects even after a full range of tilting examination. This is due to that approximately ±33° is needed to tilt from the  direction to the in-zone directions:  or , according to the roadmap shown in Figure 2c. This required titling angle exceeds the tilting limit of ±30° for our specimen holder.
In short, planar defects in boron carbide nanowires are likely hidden during TEM examination. There are only three specified in-zone directions, along which planar defects can be easily seen. The discussed difficulty of identifying ‘hidden’ planar defects in boron carbide nanowires calls attention to researchers to pay great cautions when analyzing microstructures of 1D nanomaterials with a complicated rhombohedral crystal structure. Although planar defects in boron carbide 1D nanostructures were neglected or misinterpreted in some previous publications [16, 17, 19, 23], some research groups have realized this issue just like us. For instance, the two recent papers on α-rhombohedral boron-based nanostructures  and fivefold boron carbide nanowires  set good examples, in which abnormal weak diffraction spots were specifically studied and a serial tilting electron diffraction method was conducted to reveal cyclic and parallel twinning inside individual nanostructures. Different from these two works, our work focuses on planar defect-free-like nanowires whose experimental results are more deceptive (i.e., showing no clue of defects from either TEM images or electron diffraction patterns) and presents out correct approaches to investigate these nanowires.
Identification of fault orientations from the off-zone results
Based on the aforementioned results, we believe that planar defects exist in all of our as-synthesized boron carbide nanowires. During TEM examination, planar defects are invisible in some nanowires even after a full range of tilting examination. Additional manipulation to reposition these nanowires on TEM grids can help to meet the in-zone condition and eventually reveal the planar defects and their fault orientations (i.e., AF or TF). However, this process is challenging and tedious, especially if multiple times of nanowire manipulation is needed. So without the reposition-reexamination process, is it possible to identify the fault orientation from results obtained from the off-zone directions? With the help of CrystalMaker® and SingleCrystal™, a new approach has been developed to achieve this goal.
Simulated cases along the three off-zone directions
Simulated results for determination of fault orientation within a nanowire whose TEM results are from the off-zone directions
Alignment of the projected preferred growth direction in the diffraction pattern
TF case 1
Through and 110 spots
TF case 2
Through and 101 spots
Through and 011 spots
AF case 1
Perpendicular to the tie line between and 010 spots
AF case 2
Perpendicular to the tie line between 010 and spots
AF case 3
Perpendicular to the tie line between 011 and spots
Experimental validation of the simulated cases
To verify that the above simulation results indeed can be used to predict the fault orientations of boron carbide nanowires, experimental TEM data from both in-zone and off-zone conditions have to be found on the same nanowire, which turns out to be extremely challenging. It is simply impossible to achieve this goal without multiple rounds of the reposition-reexamination operation on a single nanowire, during which the nanowire could be lost or broken.
In brief, an approach to identify the fault orientation of a nanowire based on TEM results from the off-zone condition was developed. The key of this approach is to analyze the geometrical relation between the projected preferred growth direction of a nanowire and certain diffraction spots from its diffraction patterns recorded along the off-zone directions. Comparison with experimental data shows that this approach correctly identifies the fault orientation in a boron carbide nanowire without going through the tedious reposition-reexamination process. Knowing the fault orientation of each nanowire could help us to establish reliable structure–property relations of boron carbide nanowires.
In summary, a thorough discussion on the observation of planar defects in boron carbide nanowires is presented. There are two major findings. (1) Planar defects can easily become invisible during TEM examination, in which case, observation along different zone axes is a must when studying the nature of planar defects. A roadmap based on simulated diffraction patterns along several low index zone axes parallel to planar defects is constructed to facilitate the practical TEM examination. (2) An approach has been developed to determine the fault orientation (i.e., transverse faults or axial faults) within a nanowire even if the planar defects are not revealed by TEM, which could facilitate further examination of the nanowire and help to establish the structure–property relations. Although our discussion is focused on boron carbide nanowires, the above two major findings are useful when studying other 1D nanostructures. This study prompts us to use cautions when drawing the conclusion of ‘planar defect-free’ 1D nanostructures, especially for those made of materials with relatively low stacking fault energy. Last but not the least, it is worth pointing out that the current study is on long straight portions of boron carbide nanowires only. For boron carbide nanowires with kinks, new phenomena might be observed in the kinked portions, which is currently under investigation.
We appreciate the financial support from the National Science Foundation (DMR 1308509 and 1308550, CMMI 0748090 and CBET 1067213). We are grateful to the multiuser facilities at UNC Charlotte including the TEM facility established by the NSF-MRI award 0800366 and the SEM lab within the Department of Mechanical Engineering and Engineering Science. We thank Dr. Timothy Gutu on his initial work on this project.
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