SEM instrumentation and sampling. SEM observations were performed on FEI Magellan 400L at 5 ´ 10 -5 Pa. SEM analysis was carried out at 3 ~ 6 kV beam acceleration, 13 ~ 25pA and a 2 mm working distance. Energy dispersive X-Ray spectroscopy (SEM-EDX) analysis was performed on AMETEK/EDAX Genesis APEX4 at working distance was set to 4.0 mm and at a beam acceleration voltage to 3 kV. EDX analysis was averaged over 20 measurements. Both SEM and SEM-EDX samples were prepared by dropping 20 µL of sample solution in toluene on a thin graphite layer.