I think this white particles contamination maybe sio2 particle.  XPS is necessary to identify the components of contamination. 

BTW: what kind of seal ring do you used in your CVD system? 

Thanks for your suggestion  Jia, I am using O ring of rubber with vacuum Grease.

If you make sure that the contamination is sio2 you should look out the composition of your ring and vacuum grease.

Thanks alot Jia.

SiO2 contamination is possible, however post-reaction XPS measurements will most likely indicate mainly Si-C bonds, as SiO2 is carbothermally reduced in the conditions for Gr growth (aprox 950 C, >1h, 1 Torr?)

Hi, Filip it may happen. But SEM image indicates white particle does not like graphene, most of white particles on periphery of graphene islands.

Indeed. I have yet to understand or read anything on that phenomenon. We limited the contamination by successively using several quartz tubes, that way their lifespan increases.

However, some papers talk about the reactivity of graphene edges which might be why you see those particles there (although I think in literature not thoroughly studied yet), look in the attached file, and in the following references:

Zhang, X. Y.; Wang, L.; Xin, J.; Yakobson, B. I.; Ding, F. J Am Chem Soc 2014, 136, (8), 3040-3047.

Wang, X. R.; Dai, H. J. Nat Chem 2010, 2, (8), 661-665.

but they do not experience this SiO2 contamination.

We suffered with our home built CVD by an increasing in time SiO2 contamination on graphene, we are almost sure it's caused by aging of the tube quartz mounted in our system, partially due to copper oxides penetration in quartz structure. We carefully avoided the use of vacuum grease inside the CVD chamber.

SiO2 contamination on our samples, appear, at SEM observation, as white sphere with a diameter about 10-100 nm. It seems deposited preferentially on copper boundary grain and in competition with graphene growth.

We partially resolver the problem changing frequently the tube with a new one, before it deteriorates. You can also do a post treatment on graphene with HF to remove the SiO2.

Hi Leoni, I am agree with your answer and  contamination on our samples was similar as you observed. I have already changed my quartz tube. Thank You.

Hi singh, have you resolved the contamination problem? I also encountered this problem recently. Thanks.

Hi Xiao, There was two type of contamination one from dirty quartz tube and another from silica vacuum grease. Size of white contaminated particle was 20 nm and 200 nm which came from quartz tube and silica grease respectively. If O ring for sealing the vacuum at a longer distance from furnace so there was no any contamination due to silica vacuum grease.  We have replaced my quartz tube with a longer quartz tube which solve my both problem. Thanks.

I actually published a paper on this contamination and easy solution to get rid of it.  Let me know if you have any further concerns. 

https://www.researchgate.net/publication/272156620_Silicon_Oxide_Contamination_of_Graphene_Sheets_Synthesized_on_Copper_Substrates_via_Chemical_Vapor_Deposition

Article Silicon Oxide Contamination of Graphene Sheets Synthesized o...

Hi Ruiz , I saw your paper and I think it will be very useful information for those people, who are using quartz tube for graphene growth. Thanks

Hi Ruiz , I saw your paper and I think it will be very useful information for those people, who are using quartz tube for graphene growth. Thanks

I believe our recent paper can also be helpful for people still looking for an explanation to similar problems. 

Article The Effects of Acid Pretreatment and Surface Stresses on the...