AFM vs. XRD to measure nanoparticle size

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fabrizioBobba's picture
AFM vs. XRD to measure nanoparticle size

 Dear friends, 

Im working with a multimodeV AFM in AC mode to characterize the size and shape of Ag nanoparticles. Samples are prepared by dropping the nanoparticles on mica sheet. Nanoparticles are clearly visible and well dispersed without forming large aggregates.   Results are showing an average heigth of about 2 nm with a  lateral size of about 15 nm. I supposed that this was due to the tip convolution effect. I used TESP with a nominal apex radius guaranteed <12 nm.

I also performed XRD analysis and I  used the Debye-Scherrer formula to recover the nanoparticle size :  D=0.9*Lambda/(FHWM*cos(Theta)) where lambda is the wave length (0.1541 nm) FWHM is the full width at half maximum of the selected diffraction peak  and Theta is the correspective diffraction angle.   The calculated average value of the particle size by means of XRD seems to be in agreement with the lateral diameter values extrapolated by grain analysis of AFM images but not with the heigths. 
This was surprising me as I supposed AFM height values were more meaningful. Of course I checked with the z-calibration gratings TGZ01 from Mikromash . This calibration test showed the expected step heigths of 20 nm.

I looked in literature and I found several scientific papers claiming a perfect agreement of XRD  results with AFM lateral size measuraments. But neither an esplicit value of nanoparticles heigths nor the z-scale bar in AFM images was reported.  

Anyone can help me to clarify my doubts?

Thanks in advance Fabrizio

mohan1986physics's picture
 Dear Friend,

 Dear Friend,
     First of all we should be very clear about the theoritical determination of grain size analysis by X-ray diffraction analysis.  The method which you have used to determine the average grain size is Debye scherrer method.  But there are some more X-ray line profile analysis to determine the grain size than this Debye scherrer method say W-H plot, modified W-H plots like UDM, USDM, UDEDM plot and size-strain plot.  Compare to Debye Scherrer's method these line profile analysis will give u better results (means more approximate grain size).  Whats special about these methodologies are, the strain, energy densities between the particles will be taken into account while determining the grain size through these W-H series of analysis.  It should be noted that they are all only approximate values of grain size and it may have certain error percentages too.  
     In conclusion, Debye scherrer formula is least approximation of grain size compared with W-H and modified W-H plots.  Also, In Debye scherre formula, the constant 0.9 can be used only when u have spherical morphology of particles, if the morphology of your grains vary from spherical shape, then no longer debye scherrer's equation will help u determine the grain size.  
     Literatures are available about the x-ray line profile analysis to determine particle size.  Try to get those papers.  

with regards