Please elaborate your question. What is "both of coating and low vacumm"?

Both techniques have the disadvantage that the image resolutions reduces. VP has the advantages that you don't have to coat them, but you don't see the original surface anymore, especially if you use a simple splash coating. EDS will automatically get C or any other element in your spectr which might disturb if your samples contain this element and you don't know it.

Coating has the advantage that you can image practically any sample surface, charging or not. Using heavy elements the sample also impresses by strong image contrasts etc etc. As you hopefully see, there is not either or. We use both options and decide which is the better one, like the selection of different detectors. Any question like "Which detector in SEM is the best one?" is also useless.

I agree with Gert, however it depends on what do you want to see. If you need high magnification (for example 30k), I think it would be better to use coating technique. But if you do not need such high magnification (let say 3k is enough), I suggest to use VP mode. I worked with polymers, and I didn't need high magnification, so I used VP mode and resolution of captures were pretty good.

There is a counter argument that coating for very high magnifications is counter-productive as all you see is the coating material.

We use the W-low vacuum SEM for imaging uncoated only at relatively low magnifications, (up to x5,000) and the image 'quality' is perfectly acceptable for where gross morphology is the aim of the study. At higher magnifications the resolution will degrade with low vacuum operation. Using a metal coating will allow high vacuum operation and you can then push up the accelerating voltage and the resolution will improve, allowing higher magnifications, and observation of finer detail. Whilst the instrument is capable of x100,000, as proved by the gold on carbon check, it is not suitable for most real-life samples. There is a cut-off at about x50,000 where the grain of the coating becomes visible and you no longer see the surface of your sample (depending on your coating material - another topic!).

Using the FEG-SEM it is possible to work at low voltages (1kV) on uncoated material and get very good image 'quality' (I'm putting this is inverted commas as quality is a strange word - in certain circumstances you can achieve better 'quality' by reducing the resolution!). Anyway, using a fine grain metal coat in the FEG-SEM will tame unstable samples, but again when pushing the resolution in order to reach higher magnification, the grain becomes visible, so you see grain rather than sample up at ~x100,000. So the answer here is to go back to uncoated and work at very low voltages (100-500V), using beam deceleration or gentle beam as it is sometimes called. This will allow uncoated imaging in the upper magnification ranges x200,000 and above.

Since all samples are different and require different approaches, and SEMs have different capabilities, It would be more helpful if you could tell us what your material is, what you are trying to achieve, and with what instrumentation. 

well , thank you all for this informative answers ,

Ian J Slipper ..i'm working with different samples most of them are nano particles or non conductive polymers ,

and about the low voltage it is good but in this case we can not perform EDS .

Better go for bright or dark field STEM (scanning transmission electron microscopic technique) which is attached with TEM machine. It scan the surface and also it doesn't need any coating materials.

sakthi.

In my opinion the great interest of low-vacuum is that you can observe  samples that are impossible to observe on high vaccum like hydrated samples for example. I do not use LV when observation is possible in conventionnal mode because I can obtain better images with an FESEM with a coated sample up to a magnification of 200 000.

The spatial resolution of EDS is very poor in LV mode and it may be interesting to try low accelerating voltage to reduce this problem. Of course it does not work for all elements but the peaks of light elements can be more intense.