Thanks Toli, may I ask you what method you are using to get single sheets of graphene?

centrifuge is a pretty easy method. Get graphene from different centrifuge speed then look at them under AFM.

With high centrifuge speed, big graphene piece will precipiate so you can get small graphene in the solution.

But the centrifuge makes them aggregated (bigger particles), so I have to sonicate them again, then the data will not be reliable, because now, the size is changed.

Actually how long we should sonicate to get good dispersion for Graphene Oxide, usually i am doing sonication for 3 hrs, in 200w power sonicator (bath not probe) but if i centrifuge it at 2000rpm these particles will precipitated, actually which rpm is preferable and please recommend the sonicaton time for the stable GO dispersion.

How about dissolve the graphene particle in a solution and keep this for few hours without agitation or stirring. Then collect the solution through several fraction ( recommended 4-5 fractions ) such as top-upper fraction, then top-upper fraction then middle fraction and finally the bottom fraction. The lower/bottom fraction will contains the bigger graphene compare to that of upper fraction. All the best

thank u nurunnabi one more doubt, is it possible to find out the concentration of GO and how long we can keep the dispersion ( stability).

Dear Gnanaprakasam

GO was prepared from natural graphite by the Hummers method. GO (100 mg) was loaded in a 250 ml round bottom flask and water (100 ml) was then added, which yielded an inhomogeneous yellow-brown dispersion. This dispersion was sonicated using a bath cleaner(150 W) until it became clear with no visible particulate matter. Hydrazine hydrate (1.00 ml) was then added and the solution was heated in an oil bath at 100 °C under a water-cooled condenser for 24 h over which the reduced GO gradually precipitated out as a black solid. This product was isolated by filtration over a medium fritted glass funnel, washed copiously with water (500 ml)and methanol (500 ml), and dried on the funnel under a continuous air flow through the solid product cake. The product was characterized with SEM, Raman, XPS, NMR spectroscopy, TGA,elemental analysis and electrical conductivity

Ultrasonication for 24 h

Best of luck

Dear Toli,

How to prepare monolayer graphene? and what method you prefer for preparing single layer graphene?

http://www.sciencedirect.com/science/article/pii/S0008622313001152

see this paper where authors use simple filter paper and physical filtration to separate different sized RGO