Abstract
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 ([100], [010], 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.
Highlights
Planar defects, such as stacking faults and twins, naturally exist in some as-synthesized one-dimensional (1D) nanostructures [1]
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 transmission electron microscopy (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? 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]
(1) Planar defects can become invisible during TEM examination, in which case, observation along different zone axes is a must when studying the nature of planar defects
Summary
Planar defects, such as stacking faults and twins, naturally exist in some as-synthesized one-dimensional (1D) nanostructures [1]. Literature review shows that due to its relatively low stacking fault energy (75 mJ/m2) [26], planar defects have been frequently observed in bulk boron carbides independent of the synthesis methods [27,28,29,30]. 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? 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
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