Open Access Nano Express

Observation of ‘hidden’ planar defects in boron carbide nanowires and identification of their orientations

Zhe Guan1, Baobao Cao1, Yang Yang2, Youfei Jiang1, Deyu Li2 and Terry T Xu1*

  • * Corresponding author: Terry T Xu ttxu@uncc.edu

  • † Equal contributors

Author Affiliations

1 Department of Mechanical Engineering and Engineering Science, The University of North Carolina at Charlotte, Charlotte, NC 28223, USA

2 Department of Mechanical Engineering, Vanderbilt University, Nashville, TN 37235, USA

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Nanoscale Research Letters 2014, 9:30  doi:10.1186/1556-276X-9-30

Published: 15 January 2014

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 <a onClick="popup('http://www.nanoscalereslett.com/content/9/1/30/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.nanoscalereslett.com/content/9/1/30/mathml/M1">View MathML</a>) 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.

Keywords:
Boron carbide nanowires; Rhombohedral crystal system; Transmission electron microscopy; Planar defects