Dear Albert

Yes, the concentration of compound is important parameter, where the concentration for the compound increase or decrease which effect on the structure of formed compound, also effect on the consistent phases. therefore the first test must be XRD to knew the shift of the structure (strain) that may be obtained, which will effect on the optical, electrical, morphological and topography properties for the formed compound. with best regards

Dear Albert

The concentration is very crucial to be tackle with, especially in the case of TEM, SEM and XPS techniques. More diluted the sample (solution/suspension) is, more accurate and clear results you will get. 

However description of the sample will help for giving a more explanatory answer.

Cheers

Definitely, slight variation in concentration would not effect much on the characterization like FTIR and XPS, however, microscopic image quality (not quantitative) would depend on how good the concentration is optimized.

Main effects of concentration in preparing graphene and graphene oxide relates to quality (number of layers, monodispersity,  defects in sheets) of final product. generally if you use low concentration, product with high quality will obtain. It is apparent that the quality of graphene product will impress strongly on final results of analysis. For instance if you use lower concentration, most probably you can see monolayer graphene by TEM analysis. Since FT-IR and XPS are qualitative analysis, so there is no significant difference in results for different GO concentrations.

If you want to use FTIR semi-quantitatively, I highly recommend controlling the concentration. Qualitatively, it should not make a big difference, but I would recommend reasonable high concentration to improve signal-to-noise ratio. It depends on how well you can control the sample preparation conditions though.

I agree with previous answers, that TEM and SEM work best if you have a highly diluted sample.

Dear Albert Sir

Concentration very essential part for the FTIR analysis so you should control the concentration according to Beer–Lambert law. The transmittance of material sample is related to its optical depth τ and to its absorbance A, and absorbance is directly related to the its concentration. You prepare the very dilute sample  on grid (Copper grid) for the TEM. Same sample can use for SEM analysis. Concentration also important for the XPS analysis to identify it  qualitative property and its binding energy. 

Thank you all guys, it's really useful information