1. If I have a sample of a nanosized material and modify its surface with some ligands [e.g. Ti(OCH2CH2CH3)4] and do EDS on the modified sample, is the elemental ratio correspond the ratio of atoms on the surface only or all atoms ii the sample including those in bulk?
2. For a nanoparticle of a know size how can the ratio of surface atoms to the total number of atoms be determined?
The ratios of elements in EDX is usually not very accurate, so it is probably not going to be able to tell you the information you seem to require. In the SEM, the beam penetration of the sample is dependant on the beam energy and the substrate you are imaging. Higher energy = more penetrating beam. The spot size of the beam that is able to emit xrays for a 20 keV beam interacting with silicon is about 5 microns, much larger than your nanoparticles. If you are interested in what is specifically on your surface, you need to try and use a surface sensitive technique, like xray photoemission spectroscopy (XPS).
I definitely agree with Johanna Galloway. I would use XPS or even Auger spectroscopy. Remind that XPS depth sensitivity is few nanometers (less than 4-5 nm for most samples). Even in that case, however, the method may not be that easy to be applied, if your nanoparticles are of a size comparable to the above depth sensitivity. A geometrical model should be adopted to properly interpret the XPS or Auger results.
as a matter of fact, what you probe with XPS will usually involve 2-4 nm depth within your target particles. Just consider this, and define a geometrical model for your NP/ligand interface.
1. I agree with above suggestions. However, one additional suggestion I would like to give is to use XPS or AES in grazing mode rather than normal mode. Grazing angle mode will let you have surface information upto 1-2 monolayers as well. EDX is not a good technique for eleemental cmposition of surface as the penetration depth of beam is high.
In a simple way, XPS may help us in terms of knowing the "bonding" nature of elements in the sample. You may understand the chemical and/or electronic state of the sample from the spectra, via the binding energy based peak detection. Stoichiometric level of any combined elements could be another interesting features to be investigated...
Actually problem is that I don't know how to distinguish different peaks based on the binding energy. Can you suggest me any reference from where I can study about basics of the analysis of XPS data?
Thank you.
During XPS analysis, you need to consider 2 main spectrum; The survey spectrum, and the narrow spectrum (for particular element to study). Try check my paper, M. A. Azam et al.,Appl. Surf. Sci. 258 (2011) 873 - 882. "Thermally oxidized aluminum as catalyst-support layer for vertically aligned
single-walled carbon nanotube growth using ethanol".
I think I used simple way of analyzing XPS data. I have cited several papers about the XPS.
Somehow, XPS is interesting! Good luck.
thank you for your contribution. but so far I haven't got any response regarding the second question
I am not so expert in atomic level analysis of nanoparticle. But what about a combination of HR-TEM analysis and computational/first-principle study?
Dear Lebohang,
providing a unique answer to your second question is not possible. It depends on too many possible conditions. Just the most evident features to take into account are:
- NP shape
- NP crystal structure (if any)
- surface chemical state (e.g., topmost oxidation)
Mohd Asyadi Azam suggested a good way, that can be applied in case of simple shapes (spherical, cubic,... NPs) and crystal structure of the NP.
You must take a look to the NP geometry/crystallinity via HR-TEM (sometimes normal TEM may be enaugh), and MOST IMPORTANTLY you must resort to model or 1st principle calculations to simulate the NP structure and get the surface-to-volume atomic ratio.
Do not forget that even in that case the result is just indicative, due to the usually strong influence of the environment on the same top surface chemistry and reconstruction.
1. You measure by EDX a volume of around 1cubic micro at 10keV primary electrom beam energy. (More energy means more penetration depth -> higher volume).
2. Try "a spherical approximation": number of atoms in a sphere with the radius r (from a known NP size) respect to a number of atoms on the sphere.
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