Surface potential measurement using Kelvin probe force Microscopy

KPFM on Bruker’s Dimension ICON AFM is used to record images of surface topography and contact potential difference (V_CPD) between the conducting tip and sample surface. Kelvin probe force microscopy (KPFM) measurements are performed using the lift-mode technique. In the lift-mode, the surface topography is acquired in tapping mode along a single line profile. Following this, the mechanical excitation of the cantilever is turned off and a second scan is executed along the same line following the topographic profile at a user specific lift-height (LH) from the sample surface, recording local variations in CPD. During the second scan, the tip-sample distance is constant and is equal to d_AFM + LH, where d_AFM represents the tip-sample distance during the topographic scan. During our measurements LH was kept at 50 nm while an AC bias (V_ac) of 500 mV is applied between the tip and sample. No DC bias is applied. Silicon tips coated with conductive Pt-Ir and a radius of 20 nm are used to record KPFM measurements. The spring constant and resonant frequency of these tips was 2.8 N/m and 75 kHz, respectively. Due to their relatively lower spring constant, these tips are known for providing large mechanical deflection compared with various other tapping mode probes. In order to obtain a value for the absolute V_CPD of a single NR, square portions are selected from a single NR which is then used to obtain multiple points across the surface of a single NR.