Thank you for your response sir. So do I determine the size of the nano particle from the area of the circle or the diameter directly gives me the size of the nano particle?? Pls guide. Also let me know if this SEM image is clear enough or I need a better image. (pls note I have actually flattened the image using imagej).

Thanks a lot in advance.

Sandip, in any case, a single SEM image as the one you show gives a 2-dimensional representation of what you see near the surface. You cannot reliably tell about the sizes occurring along the electron beam. In any case, you have objects of very different sizes, already in 2D, many times some smaller objects seemingly lying on larger ones. Indeed, a size distribution analysis by some automated threshold method is next to impossible (my personal opinion).

To complement the information you have you can consider two further ways, still with the SEM.

a) take images at oblique incidence (e.g. around 50° off-normal), and experiment with smaller electron energies. this may give you a better notion of the objects in 3D.

b) create fractures of your film and inspect these ("from the side") to get a cross sectional view of your film.

All in all: take several (if not many) images to get an impression of the statistics involved. In microscopy the temptation is always there to be selective in choosing the objects we measure.

Thank fr ur answers. I have one more doubt on imagej. If I have nanocrytals then its not mandatory that I will have circular particles.. infact it might be a cube. But incase the x and y dimensions vary, how do I find the particle or rather nancrystal size??

You might have to write your own analysis code/plug-in, and to think of what would be the best definition given your problem. As you might know, the plug-ins are tools written by users such as you and me. Of course someone else might already have solved your problem -- you might have to search the web for the tool which suits you.

what are the other softwares equivalent to image j?