Hi. As a student who has been using the method developed by Moon et al to assemble PS spheres at an air/water interface for the past 2 years, I think I can provide some insight into the problem you are having.

When I first tried their method, I was not successful in achieving good packing either. I dispersed my amine functionalized 500 nm diameter PS spheres in 1-butanol and then drop cast them on water and found the spheres packed poorly. I tried using NaCl and IPA to improve the assemblies and that didn't work either. If any IPA or EtOH were present actually, the spheres would enter the sub-phase and the assembly would be a disaster. I have since improved the method to make good quality assemblies. I briefly describe my modifications in our recent work (Electrocatalysis 4(3)). Basically I heat the water subphase to ~ 55C before drop casting the 1 wt% solution of PS spheres in 1-butanol and then allow the assembly to cool before transferring to another substrate. When I do this assembly I don't use a defined volume or weight of PS spheres, I simply cast them until the surface is visibly "full" (the spheres no longer disperse but aggregate on the surface instead), so the size of the petri dish does not matter as long as you have enough material. The assemblies further improved when I modified the surfaces of the PS, but that work is still in publication, so I don't want to go into details about that. The more important thing that I want to note with that is the importance of your surface hydrophobicity/charge (I think my modification has increased the PS sphere hydrophobicity and decreased the surface charge to allow for better packing). Moreover, I want to mention that the method requires modification when I use different sized PS spheres (larger or smaller).

Finally, I would like to mention that I spent nearly 2 years prior to Moon et al. publishing their paper trying to using butanol dispersions of PS with an LB trough to make a close-packed layer and I was never able to get it to work. My spheres were simply not stable enough at the water surface to avoid being pushed into the sub-phase by the trough. Finding a way to keep the PS spheres on the water surface will likely be important if you go this route.

Also, there is some recent work published by Dr. Asher's group that might be useful:

http://onlinelibrary.wiley.com/doi/10.1002/anie.201105439/pdf

http://pubs.rsc.org/en/content/articlepdf/2013/TC/C2TC00794K

Dear George,

Your problem is a question of surface density of your particles. At low surface density, your system should form foam-like structures, a problem we studied in a series of papers (Physica A 236, 97-104 (1997); Molecular Physics 95, 371 (1998); Phys. Rev. E 58, 660 (1998); Physica A 276, 30-49 (2000); Journal of Colloid Interface Science 291, 1 (2005); J. Phys. Chem. B 110, 22230-22236 (2006)). In order to have crystalline-like order you need to increase the surface particle density, which if you have access to a Langmuir trough will be very easy to do by compressing the monolayer. Since you are using a Petri dish you can try to mimic a Langmuir trough by carefully sweeping the surface with a barrier to concentrate the particles on one side of your Petri dish. Another way is to increase the salt concentration of your subphase to about 2M. Since your particles are charged, this will prevent the majority of the particles to get solubilized into the subphase, remaining most of them on the surface. Good luck with your experiments.

Jaime

I agree. A Langmuir-Blodgett trough would help by allowing you to have a controlled compression of the nanosphere film to assist its organization. In addition, using the trough you should be able to transfer the film to a glass slide or other substrate for further study.

I agree also. In my case during synthesis TiC particles in liquid alloy Fe-C, at low surface volume of the system create foam-like structures. If size of the particles TiC decreased and volume density increased, ultra-fine dispersive composite is formed. Foam-like structures is not observed. This is not the same process, but part this is similar. This problem we studied in: E. Olejnik, M. Górny, T. Tokarski, B. Grabowska, A. Kmita, G. Sikora, Composite zones produced in iron castings by in situ synthesis of TiC carbides, Archives of Metallurgy and Materials, 58, 2 (2013) 465–471.

The above answers, especially those provided by Jaime and Walter seem quite relevant for your problem. Actually, regardless of the nature of the subphase (rigid support, liquid subphase) the formation of an open monolayer structure (foam-like network) is ubiquitous of the drying of nanoparticles suspension films when the concentration and thus the resulting (final) areal density is too low. Therefore, you might have used the right proportions given by the ACS Nano paper, but the wrong petri dish size (larger area), leading to a much lower final areal concentration of nanoparticles...So as suggested by the colleagues above, the first thing to test is increase the final surface concentration of nanoparticles by either increasing the nominal particles concentration, or by using a petri dishes with smaller cross-section areas. Now If you want to study your optical propeties on a substrate other than the aqueous subphase, a LB trough if accessible is by far more suited to both the fabrication of the close packed layer and its transfer to a rigid support. Hope these additional comments will be of some help...

Hi. As a student who has been using the method developed by Moon et al to assemble PS spheres at an air/water interface for the past 2 years, I think I can provide some insight into the problem you are having.

When I first tried their method, I was not successful in achieving good packing either. I dispersed my amine functionalized 500 nm diameter PS spheres in 1-butanol and then drop cast them on water and found the spheres packed poorly. I tried using NaCl and IPA to improve the assemblies and that didn't work either. If any IPA or EtOH were present actually, the spheres would enter the sub-phase and the assembly would be a disaster. I have since improved the method to make good quality assemblies. I briefly describe my modifications in our recent work (Electrocatalysis 4(3)). Basically I heat the water subphase to ~ 55C before drop casting the 1 wt% solution of PS spheres in 1-butanol and then allow the assembly to cool before transferring to another substrate. When I do this assembly I don't use a defined volume or weight of PS spheres, I simply cast them until the surface is visibly "full" (the spheres no longer disperse but aggregate on the surface instead), so the size of the petri dish does not matter as long as you have enough material. The assemblies further improved when I modified the surfaces of the PS, but that work is still in publication, so I don't want to go into details about that. The more important thing that I want to note with that is the importance of your surface hydrophobicity/charge (I think my modification has increased the PS sphere hydrophobicity and decreased the surface charge to allow for better packing). Moreover, I want to mention that the method requires modification when I use different sized PS spheres (larger or smaller).

Finally, I would like to mention that I spent nearly 2 years prior to Moon et al. publishing their paper trying to using butanol dispersions of PS with an LB trough to make a close-packed layer and I was never able to get it to work. My spheres were simply not stable enough at the water surface to avoid being pushed into the sub-phase by the trough. Finding a way to keep the PS spheres on the water surface will likely be important if you go this route.

Also, there is some recent work published by Dr. Asher's group that might be useful:

http://onlinelibrary.wiley.com/doi/10.1002/anie.201105439/pdf

http://pubs.rsc.org/en/content/articlepdf/2013/TC/C2TC00794K