Surface potential in KPFM

Hi, 

I've been running KPFM on MultiMode microscope Nanoscope IIIa for surface potential measurements of lipid microdomains in supported lipid monolayers. I was able to get good surface potential images while there are still some questions regarding potential artifacts and the sign of the surface potential I measured. Here are my questions:

1. I was able to get good images only if the lift distance of the probe is set to 0.5 to 2nm and with a high driving amplitude (9000mV) in lift mode while an optimal lift distance from 50-150nm are more commonly used in papers. What kind of the potential artifact I might have if my probe is too close to the surface? I don't see black or white stripes while scanning, however, the measured surface potential differences at the domain boundary is about 800 mV, which is much higher than I expected. 

2. The surface potential scale in the running software (NanoScope III v4.42r4) always appears to be positive. Assuming negatively charged domains appear to have a lower surface potential in a positive surface potential scale, then how do I determine the neutral point (V=0)? In another word, how do I assign absolute surface potential (including the sign) to different domains in the image assuming positively and negatively charged lipid domains both present in the membrane.

Thank you for your time. 

Eric