As several people mention, the answer depends on what you want to measure and how carefully you feel like doing it. Non-contact mode is, strictly speaking, the best mode to use. You never touch the sample, so you don't damage the tip or the sample. The tip 'feels' the van der waals potential. However, it is a very challenging technique, and one misstep in setting your feedback will crash the tip quite spectacularly. It is also usually best to do this in vacuum, adding to the complexity.

'tapping' mode is a good second option. Here the tip moves up and down so there is contact, but no shear on your sample. If you get the hang of setting your feedback loop you should be able to avoid much damage, and picking up much 'dirt' on the tip. This is often a good choice for soft materials. It works well in air without any special chambers and also works reasonably well in a fluid cell.

Contact mode is the simplest, but most aggressive technique. Here the tip is literally dragged across your sample at some preset normal force. If you have a soft sample this will often do the most damage to it, and will 'gunk' up the tip quite easily. If you have a hard sample, this will often do damage to the tip. Again if you get the feedback right you can usually minimize these issues. As an aside, because you are draging the tip, this is the only method that allows the measurement of lateral forces (friction) on these scales. Also, works well in an air or fluid environment.

It depends on your sample and the resolution you want to achieve. I usually manage with hard samples, like silicon substrate. I work in so called semi-contact mode in order to save tip and do not damage sample surface. Image resolution is still very good. I suggest semi-contact mode also with soft sample, eg. biological material, as contact mode can scratch sample surface and consequently contaminate tip apex.

Dear Erica Iacob ,

Thank you very much for your answer.It is very interesting to hear about semi-contact mode. Is semi-contact-mode used just for tip safety purpose than contact mode ?

In contact mode, What will be the minimum distance between sample and tip?

I think is a good option contact mode, if you are analyzing "hard" materials just take care with the contamination of the tip in some cases.

As several people mention, the answer depends on what you want to measure and how carefully you feel like doing it. Non-contact mode is, strictly speaking, the best mode to use. You never touch the sample, so you don't damage the tip or the sample. The tip 'feels' the van der waals potential. However, it is a very challenging technique, and one misstep in setting your feedback will crash the tip quite spectacularly. It is also usually best to do this in vacuum, adding to the complexity.

'tapping' mode is a good second option. Here the tip moves up and down so there is contact, but no shear on your sample. If you get the hang of setting your feedback loop you should be able to avoid much damage, and picking up much 'dirt' on the tip. This is often a good choice for soft materials. It works well in air without any special chambers and also works reasonably well in a fluid cell.

Contact mode is the simplest, but most aggressive technique. Here the tip is literally dragged across your sample at some preset normal force. If you have a soft sample this will often do the most damage to it, and will 'gunk' up the tip quite easily. If you have a hard sample, this will often do damage to the tip. Again if you get the feedback right you can usually minimize these issues. As an aside, because you are draging the tip, this is the only method that allows the measurement of lateral forces (friction) on these scales. Also, works well in an air or fluid environment.

it depends on the type of sample you are going to analyze. for soft samples-Non contact mode, for hard materials-Contact mode is used. if u have magnetic sample u can go with MFM.

Dear Sridar, by your replies I infere you are a beginner on this technique. Just for mention one, you dont have a minimum distance between sample and tip, you actually touch the sample in the contact mode..

The above answers from the colleagues tell you in which conditions the non contact, intermitent contact or contact modes are better or not, but I suggest you first read more about the basics of the technique.

Handbook of Microscopy for Nanotechnology edited by Nan Yao and Zhong Lin Wang is quite a nice textbook. Video tutorials on AFM are elsewhere in youtube.

Dear Sridar, by your replies I infere you are a beginner on this technique. Just for mention one, you dont have a minimum distance between sample and tip, you actually touch the sample in the contact mode..

The above answers from the colleagues tell you in which conditions the non contact, intermitent contact or contact modes are better or not, but I suggest you first read more about the basics of the technique.

Handbook of Microscopy for Nanotechnology edited by Nan Yao and Zhong Lin Wang is quite a nice textbook. Video tutorials on AFM are elsewhere in youtube

Dear F. Caballero-Briones,

Thank you very much for your answer. I am really new to AFM. in some ppt, I can see at least atomic distance between tip and sample in contact mode. I have attached that image( the plot didn't come to zero distance)... I am very happy if you clarify my doubt

Dear Sridhar, as Bartlomiej saids, what is contact indeed? in the so called contact mode you have to approach as much as the tip is able to even damage or being damaged by the sample, but at the microscopic level you could argue there is not contact at all because you are in the lenght of a chemical bond... In resume, contact mode means that, tip/sample touch therefore is the most damaging mode but the one that provides the highest resolution in AFM. Intermittent mode is gentler with sample and tip but as far as you are oscillating the tip you have some lost of resolution. But depending on the sample sometimes you must use one of another (soft samples for example, intermittent is better; conductive, friction modes need contact).

Dear Sridhar,

i think you should in any case use non contact to avoid tip contamination and for the highest resolution in air liquid and vacuum. The highest possible resolution in my view is atomic and inter-molecular resolution.

For true atomic resolution in air please see our recent paper http://pubs.acs.org/doi/abs/10.1021/nn501696q or soft samples HOPG and hard samples Graphene on SiC(0001). We can even resolve the single bonds between carbon atoms in air with hydrophilic bulk silicon tips which are common and hydrophobic very robust sapphire tips which are chemically inert, extremely wear resistant and usable on various samples even in thin solution films with atomic resolution http://journals.aps.org/prb/abstract/10.1103/PhysRevB.87.245415 .

Small, soft things like proteins will do better in tapping mode. Well anchored things, like nuclear pores in a synthetic lipid bilayer that are well attached to a mica surface can actually be imaged in contact mode, but with very small scan sizes. Large things like fixed cells can be imaged in tapping mode or contact mode. A lot of the success you have will depend on the probe you use and your experience with the machine. Setting the appropriate parameters for each condition is key. In the beginning, try several different probes and imaging modes before settling on the best one. It will take a long time before you find your "favorite" probe for the application. Patience is a virtue with AFM. Companies will often send sample packs out if you ask, they can also give you a good idea of how to image your particular sample.