Cellulose nanofiber (CNF) applications like resin reinforcement are developed due to its light and strong mechanical properties. Visualization of the fiber structure using the SEM is indispensable from research and development to production and quality control stages. However, complicated pretreatments such as dehydration and drying are required for the SEM evaluation because CNFs are typically dispersed in liquid. If the purpose is fiber-length measurement, a simple processing method of drying the suspension on a flat substrate may be adoptable. Since CNF fibers are very thin, SEM observation conditions for them must be set in consideration of the signal from the substrate generated by the incident electron beam transmission. Figure 1 shows the observation result of CNFs at landing voltage of 200 V. The CNFs dispersed in water were dropped onto a silicon substrate with hydrophilicity treatment, spin-coated, and air-dried. CNFs could be confirmed with high contrast, and the fiber length was also measurable. Figure 2 shows SEM images of CNF obtained with the landing voltages varied from 20 V to 500 V. The numerical value below each. image indicates the contrast obtained from the image intensity on the red line in the image by dividing the intensity at the substrate by that at the CNF. The contrast was the highest at 200 V. According to Monte Carlo simulations, the primary electron penetration depth was approximately 1.7 nm at 200 V and approximately 3 nm at 300 V. As the diameter of the CNF is approximately 3 nm, setting the landing voltage to 200 V or lower is thought to contribute the high contrast observation as shown in Figure 1 by reducing signal entry from the substrate. A simple pretreatment method of CNFs was shown effective for some evaluations of them by FE-SEM ultra-low voltage observation.
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