using a Veeco Dimension Icon atomic force microscope with a ScanAsyst-Air
AFM tip from Bruker Nano Inc. The sample grids were held on a slide
glass by copper tape/slide glass during scanning with a vacuum-assisted
lock on slide glass. The data were analyzed by using Nanoscope Analysis
2.0 software. The AFM tip was calibrated in thermal tuning mode with
a spring constant of 0.4 N/m.