Imaging of CNTs in a polymer matrix at low accelerating voltages using a SEM Original Research Article

As continued work of scanning electron microscope visualization of carbon nanotubes (CNTs) in epoxy matrix at high accelerating voltage (from 3 to 15 kV), this study focuses on the low accelerating voltage range (from 0.3 to 5.0 kV). In the CNT/epoxy composite, the CNT contrast is observed to invert two times as the accelerating voltage increases from 0.3 to 5.0 kV. At 0.7 kV the CNT contrast shifts from bright to dark, and after 1.5 kV bright CNTs are observed again. The contrast inversion and imaging mechanism of CNTs are interpreted in correlation with the secondary electron (SE) yield curve of epoxy. At 0.3–0.6 kV the epoxy is positively charged, which results in influx of electrons (from the ground) through the CNTs. These electrons lower the potential of the CNTs and enhance the local SE emission. The bright CNTs contrast at >1.5 kV is attributed to the high SE yield resulting from the trapping of injected primary electrons (PEs) in the CNTs. At 0.7 and 1.0 kV the dark CNT contrast may be attributed to the weak PEs–CNTs interaction or to the slight charging of epoxy. Further observation suggests that latter is more likely to occur.