If the capping agents are organic, they contain mainly carbon. As carbon is not very electron dense, you can't see it well on TEM as the electrons just pass through. That's why carbon is coated onto TEM grids to support your particles but not block the beam. If you want to image the coating you may need to use a negative stain, e.g. Uranyl acetate. This will pool around the outside of your capping agent, which will appear as a halo around your electron dense particles.

Hi Razium, actually TEM analysis is important part of characterization in nano-particle. Specially TEM is done for checking the morphology of particle whether it agglomerate or composite. But in case of nano you can also check the porosity and layer fomation. You get the structural information like octahedral or monoclinic etc. You can also check the d- spacing from TEM result at lower resolution like 1to 5 nm and it will support your XRD analysis.

Try to coat the grid of TEM with Gold and then again go for TEM.

i did it on copper grids, but in general sense in most of TEM images we don't see capping agents, why is that? does it have any thing to do with metallic nature of structures?

@ Tejas Bhosale Thank you for such nice answer but still i am confused, on the part that most of the particles we synthesize in solvents are capped to stop agglomeration, if so we cant capping agents be seen in TEM rather than just NPS itself

If the capping agents are organic, they contain mainly carbon. As carbon is not very electron dense, you can't see it well on TEM as the electrons just pass through. That's why carbon is coated onto TEM grids to support your particles but not block the beam. If you want to image the coating you may need to use a negative stain, e.g. Uranyl acetate. This will pool around the outside of your capping agent, which will appear as a halo around your electron dense particles.

Besides, you can do EDX over your Cu-nanoparticles to chek them.

Johanna Galloway @ thank you your answer was very helpful

Carlos Daza@ Please correct me if i am wrong EDX can not differentiate between surface and bulk material, it will not provide any information regarding surface of NPS,

i was wondering that why do we get heterogeneous distribution of particle size in a single sample that we make, even after optimization. If particle size is about 12 nm, why is so that no all particles are of that size, we get average size. can some one help me on the process of growth of nps

Razium, EDX give you the chemical identity of your NPs (i.e. which of your NPs are compossed of Cu), the penetration of EDX is only a few of micras, so the analysis includes some of the surface and bulk composition.

Carlos Daza@ ok that good i will try that..

For EDS in TEM there are no “bulk composition”, specimens are too thin. I suppose TEM grids with some support film were used; support film (pure carbon, or polymer) have a lot of carbon by itself. So, EDS is not a good method for this problem.

It is about contrast due to mass density. I did the same with metal nanoparticles trapped within dendrimer entanglements. Only metal nanoparticles are visible as darks nanosphères. Now, all depends the kind of capping agent you are dealing with. For instance, unlike metal, organic molecules are diffuse structures that do not block significant amounts of électrons. Then, it also dépends on the amount of capping agent and its compaction grade. Even inorganic coating agents with light atoms behave similarly. Quite dispersed nanoparticles (not stuck each other) is usually an accurate indicator of the presence of coating, capping or encapsulating agents between these nanospheres.

EDX is unsuitable because of two reasons: first, as Vladimir said before, the used grids may be made of a carbon film and will give ca strong carbon signal. Furthermore, these grids are mostly made out of a copper grid that holds the carbon film. This copper will also contribute to the EDX spectra because of stray radiation, so you will always see C and Cu in your spectrum.

There are grids with SiOx-films on other metals than copper (Au?) which should be used when you are interested in the detection of Cu/C in your particles.

if TEM image is not showing organic molecules means, is there any possibilities to check the capping agent around nanoparticles in SEM?

Dear Badmapriya,

now it is your turn again:

unfortunately I don't have the time to go into the matter of "capping agents" deeper.

But I guess, not only I myself would like to know WHAT NATURE (i.e. constituents of) your capping agent is made of.

So please specify in detail how your Cu-nanoparticles are "embedded", "bound to " or any other specific application technique you use to incorporate the copper nanoparticles. Thank you.

Otherwise you won't get more detailed information on your repeatedly asked question: "why can't we see the capping agent by TEM?" (cf. the most informative reply by Johanna (cf. Popular answer above: "If the capping agents are organic, they contain mainly carbon.., etc., etc.....").

Best regards,

Wolfgang

Depending on the density/thickness of your capping agent-- you might be able to get some resolution if you ask the TEM person to take images at lower magnification. At a lower magnification, the energy that is focused on your nanoparticles are not so strong that they "blast" away the capping agent. Again, depending on the density, you may get some type of information from SEM since you can easily control the energy of the electron beam when you are analyzing your sample.

To follow up on the polydispersity of your sample, there can be many reasons why it is not monodispersed. Some include: rate of stirring, temperature, sample prep cleanliness, quality of bulk chemicals, etc.

Thank again@ Thao Nguyen I did AFM analysis of the particles and got some very good results. The particles are very clear and homogeneous in size, however similar to TEM even in AFM there is no clue of capping agent, I did some washing and drying of the product before analysis, will this count? your answer for TEM and SEM is quiet clear I will definitely follow your opinion.

Maybe you can try it with EELS for the capping agents. Because the EELS mapping technique will help for element measurement, especially for the light element.

There is a non-conventional technique, freeze-fracture replicas, which you could see the capping arrangement and the non-organic nanotube arrangements. It is no easy to do but the results (images) are in majority of cases very convincent! As indirect method, you don't have the problem of the lack of contrast with the organic material. Replica is a thin layer made with Pt/C and C. Try it!