These Developments Pave the Way for New Subsurface Imaging Techniques for Carbon Nanotubes

Lightweight composite materials consisting of polymer matrices with embedded carbon nanotubes (CNTs) offer a unique combination of stiffness and strength with very high thermal and electrical conductivities, but current knowledge on manipulating CNT properties and processes for different functions is preventing the widespread commercial use of these nanocomposite materials. New research will help establish Scanning Electron Microscopy (SEM) subsurface imaging as a simple method to provide quantitative 3-D information on CNT dispersions in polymer composites. Understanding the dispersion of carbon nanotubes embedded in a polymer matrix is crucial for manipulating CNT composite materials and improving their performance. High-resolution subsurface imaging methods are well-suited for this task, and while SEM is normally used for surface imaging, more reports are emerging of applying SEM to subsurface imaging. “There have been several open questions remaining for SEM subsurface imaging of CNTs,” says Dr. Minhua Zhao, a Visiting Researcher in the Center for Nanoscale Science and Technology at The National Institute of Standards and Technology (NIST), with joint appointment in Department of Materials Science and Engineering at University of Maryland. Zhao’s team conducted extensive experiments – including surface and cross sectional SEM imaging, beam-induced current measurement, focus ion beam milling, 3-D reconstruction from stereo SEM images – and Monte Carlo simulations that he says clarify issues and questions about SEM imaging, including:

1. Does the signal of subsurface CNTs originate from secondary electron (SE) yield emitted by the CNTs themselves, or from the near-surface polymer matrix?
2. Does the electron beam have to reach a given CNT for subsurface imaging?
3. What is the maximum SEM imaging depth for CNT polymer composites?
4. What is the role of SE recapture process for SEM subsurface imaging?

The SEM technique can be used to better understand the relationship between conductivity of CNT polymer composites and the dispersion of CNTs in a polymer matrix. Compared to Zhao’s team’s previous work on subsurface imaging based on electric force microscopy, the current technique has a much higher throughput and more quantitative depth information. SEM techniques are not limited to CNT-polymer composites; they are generally applicable to subsurface imaging of conducting nanostructures embedded in a dielectric matrix, such as graphene polymer composites or integrated circuit conductors covered by a dielectric layer. This may have significant implications in non-destructive imaging of silicon nanostructures embedded in silicon oxide for single electron transistors, high-resolution SEM overlay metrology or e-beam lithography, opening up a broad range of applications in nanotechnology.