Of course a remote diagnostic is difficult, but here are some thoughts (no guarantee for the right one being in the mix):

-adhesion forces in vacuum are always higher than in air (explainable by the

Lifshitz Theory of Van der Waals Force, see Wikipedia for a start). However, the difference you have there is too high to be explained just by that

-your assumption with hydrocarbon removal may indeed be a factor, it may also be a different substance. It may also be that the particles are somewhat humid (what solvent did you use for production?) and you are observing a drying effect which makes the surface more adhesive. The drying would also explain why it stays strongly adhesive after venting again.

As I said, these are just some thoughts, no guarantee for completion.

Hi Mohammad, I would suspect the difference between the ambient and vacuum measurements would mainly be due to the removal of water adsorbed onto the surfaces of the probe and wafer. Once "cleaned" in the vacuum in such a way it will take some time for fresh water vapour to accumulate once back at atmospheric pressure. I suspect if you repeat the measurement over time (after exposing to atmospheric pressure,) you will find the force needed gradually drops again (depending on humidity etc.).

My speciality is STM, but I remember from many talks in conferences that AFM at ambient conditions has always water. I remember this was plaguing someone's experiments who was attempting to do non-contact AFM in ambient conditions. If you really need to do the experiments in 'ambient' pressures I think the best strategy is to try to do it in some sort of dry atmosphere (argon?).

Dear All

Thank you kindly for your answers.

We have also come to this conclusion that there is always a 1-2nm hydrocarbon contamination film on the silicon wafer which is (highly probably) removed in vacuum. It explains why in vacuum we measure higher forces. We also separately observed that once this contamination film is gone, it does not quickly come back to the surface, at least within a few hours.

Another reason is the wearing of the colloidal probe surface due to a large number of experiments we have done. I measured the roughness of the ball before and recently after the tests and found that it has become kind of flat. Therefore, the larger contact area increases the adhesive area as well and then, the total adhesion force.

There is a point that the adsorbed water on a silicon wafer (also perhaps on other hydrophilic surfaces) does not disappear even in vacuum which means that there is always some monolayers of water molecules which contribute to the total van der Waals forces.

Hi Mohammad, you could look at using UVB light in vaccum to remove additional water vapour also, just to see if there is a noticeable difference.

Hi Mohammad.

I recently did some work in a dedicated environmental high vacuum AFM using 50nm spherical tip and a HOPG cleaved surface.

At vacuum (~10^-5Pa) - my adhesion force was 2-3nN

At 80% relative humidity - my adhesion was 6-7nN

I think those force you report are extremely large - probably in part due to size of probe - maybe also a spring constant issue (if spring constant too large, you might not get the required force sensitivity)

But would also suggest that accurate, and regular spring constant calibration along with laser sensitivity measurements before each experiment is good practice.