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Nanoparticle size

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This post has 2 Replies | 4 Followers

Posts 25

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Don Chernoff Posted: Wed, Jan 23 2013 11:31 AM

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The following was posted by mistake in the "Off topic" forum. I'm interested in the same general question so I'm reposting it here so it may receive more attention.
-Don Chernoff
Don Chernoff, Ph.D., President, Advanced Surface Microscopy, Inc
www.asmicro.com
[business activities since 1990: analytical services in AFM, AFM probes,
consulting, training, calibration and test specimens,
calibration and measurement software, used NanoScope equipment.]

===Quote of Fabrizio's post===

Fabrizio posted on Fri, Apr 20 2012 1:02 PM

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. AFM topography images 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 analisys 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

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Posts 29

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Peter Eaton replied on Fri, Jan 25 2013 5:48 AM

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This is surprising. Usually the XRD will tend to underestimate the particles size, no overestimate it.

If you see features of 2 nm height 15nm across, they are probably spheres. You could do TEM to check (although they are pretty small, you need a reasonably good TEM).

I should think the most likely explanation is that the 2nm spheres are there, but are not representative of the samples, and you have lots of bigger stuff in the sample that was not imaged.

The other possibility is that the 2nm features are not actually silver, but some (non crystalline) contaminant.

Pete.

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Posts 35

Points 381

Dmitry Sokolov replied on Fri, Jan 25 2013 7:11 AM

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The shape factor can be addressed by embedding the particles in conductive epoxy followed by STM imaging of polished (= flat) sample. The particles should theoretically be scattered and oriented randomly. The randomness can also be addressed by the trimming and polishing of the same sample under the angle of your choice (45-90 degree should be sufficient for that).

The shape distortion artefacts could be caused by the probe-sample-substrate interaction. The sample was imaged in the "mechanical" AC AFM mode. Switching into physically different, for example "electrical" mode like STM / different substrate / probe or even different imaging conditions could be useful too. I would probably start with changing the imaging mode (STM).

Please let me know how it went or just share your screen in Skype when you see me online: http://confocal-manawatu.pbworks.com/w/page/23665663/Dmitry%20Sokolov

Cheers,
Dmitry

MIAWiki Knowledge Network /
Online Consultancy on Microscopy and Image Analysis

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Nanoparticle size

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