Unfortunately this is a nearly universal problem and is one of the biggest issues with nanocomposite polymer films, which is a similar problem to what you are trying to do. The key is finding the right surface ligand which will give you good solvent interaction. If you can find something that will give a bit of a surface charge in solution, that can be an advantage for dispersion, depending on what polymer and solvent. I wish there were a simple recipe for this, but you have to look at the chemistry of the solvent and the polymer, along with the ligand interaction with the surfaces. If you also want good coupling to the surface, you may need a complimentary ligand on the surface to give a good interaction energy between the particles. Stating it generally (perhaps too much so) your interaction energy of the particles is greater than the interaction energy to either the solvent or the surface. You need to do something to your chemistry to make the particles "like" themselves less, and the solvent/surfaces more.

Unfortunately this is a nearly universal problem and is one of the biggest issues with nanocomposite polymer films, which is a similar problem to what you are trying to do. The key is finding the right surface ligand which will give you good solvent interaction. If you can find something that will give a bit of a surface charge in solution, that can be an advantage for dispersion, depending on what polymer and solvent. I wish there were a simple recipe for this, but you have to look at the chemistry of the solvent and the polymer, along with the ligand interaction with the surfaces. If you also want good coupling to the surface, you may need a complimentary ligand on the surface to give a good interaction energy between the particles. Stating it generally (perhaps too much so) your interaction energy of the particles is greater than the interaction energy to either the solvent or the surface. You need to do something to your chemistry to make the particles "like" themselves less, and the solvent/surfaces more.

You may try dip coating with high viscus solvent, not sure whether it works. Substrate surface modification with self-assembled monolayer or plasma treatment could be an alternative way to coat uniformly.

Cross-linking chemistry can be applied. If you can functionalize Ge NPs with -NH2 group and functionalize the target surface with -COOH or -NHS group, then covalent conjugation may work.

You may try spray coating. I used this method to prepare metal-organic framework thin films, it works well.

Try ultrasonic for dispersion and electrophoretic for deposition.

Which Jan Schröder described is similat to Langmuir-Blodgett film that I had wanted to suggest you. It is not so easy to produce monolayer with LB technique.

As Mark Davidson described well, solvent, and surface are also very important. with simple surface coating (BSA film or polymer) you can make surface that better than your original substrate.

I link an article about phase separation assisted monolayer may be it is also interesting for you

http://www.mech.utah.edu/~dmascaro/NOLED/Coe-Sullivan2005AdvFunctMat.pdf

I would propose 1) electrophoresis and 2) playing with mixtures of solvents or 3) adding some ions to the solvent (if it`s possible in your experiments) - e.g. during electrodeposition iodide ions make NPs well dispersed on the surface.

Some colleagues in our group who worked with nano-particles deposited quite homogeneously by dip coating with a dip robot. They deposited one to several monolayers this way.

How many layers have you coated?

I think your main problem is the lack of interaction between your particles and your solvents which hinders a good dispersion (see the answer made by Mark Davidson). On the basis of the conditions required by the use of the Van der Pauw technique, it seems difficult to use spray technique which would not guarantee a very homogeneous thickness of your coating, particularly with an unstable suspension of particles.

If you can work on the interaction between the solvent and your particles you could keep the spin-coating technique; it appears as the best option. If you can't, I also advise you to try to improve the dispersion of your particles by ultrasonication before casting et use perhaps a more viscuous mixture (if possible). Then, depending on the nature of your substrate and on the thickness you want to reach, you could try to cast thanks to a coast-master that gives rise to homogeneous composite films.

You shuld try spray deposition. Put an ultrasonic finger in the dispersion reservoir to avoid sedimantation.

As Mark Davidson and Audrey Potdevin indicate your principal problem is poor interaction between your NPs and your solvent matrix. The traditional method of overcoming this is to use a surfactant co-solvent. Without knowing your ligand and solvent chemistry, it is difficult to specify the best surfactant, but there is a substantial literature body to reference to determine the most appropriate one.

As noted by several others above, your deposition method is challenging. For thin films, 500nm), but will also require multiple deposition-anneal steps to get a uniform coating. The kinetic deposition methods suggested (spray, etc) will yield very thick films (>5 um), but will generally not yield very uniform thicknesses, which will be a challenge for doing conductivity testing.

Ultimately, you wish to perform Van der Pauw tests, for which you will need a very thin film that is electrically isolated from the substrate but well interfaced to your electrodes. Your interface layer between your thin film and your substrate (glass or Si are probably best) needs to be well tailored to minimize the NP-substrate interaction and to provide a uniform surface for thin film deposition. A high dielectric, such as Al2O3 or MgO, are traditional. ALD coating is probably best for these, but sputter coating will work.

R, Mehra, a colleague used dip coating of CdSe and CdS nanoparticles on different substrates with different ligand exchanges for her entire PhD work. That should a large number of samples.

Drop casting, spin coating and inking are the methods I'm familiar with for getting nanoparticles onto surfaces without doing ligand-exchange for thin film creation. Perhaps you can get better film creation if you treat the surface to make more/less hydrophillic, based on the capping agent you are using.

Thanks everyone for suggestions.

I read through all of the posts and would work with each of the suggestion.

David :

I can try adding a surfactant or polymer binder but i am really concerned as it can affect my Van der pauw measurements.

I haven't tried the Electrophoresis and it seems a possible way.

Audrey, Mark and William : Thanks for the detailed suggestion.

I came across this paper:

http://pubs.acs.org/doi/ipdf/10.1021/nl302555k , which says multi deposition by spin coating might help as you people suggested and might be useful.

You can try to apply the ultrasonic mixing (2h) of nanoparticles with the organic solvents and then the deposition of your suspension. If results will be correct inform me, please.

To get a homogeneous and films with one has to deposit several layers at least ten with proper heat treatment.

To get a homogeneous and films with good electrical connectivity one has to deposit several layers at least ten with proper heat treatment.

Hi Pratik. Up to what temperature the ligand capping is stable ? If you have access to a sprayer you can try to spray coat the same onto a substrate kept at a temperature of 90-100 oC. We successfully coated PVA as a protective layer to ZnO thin film using a spray coating.

To get a homogeneous and films with good electrical connectivity one has to deposit several layers at least ten with proper heat treatment. How many layers you have deposited? Please let me know.

R Mehra : I am still struggling with proper dispersion of my nanoparticles. But i still tried dispersing in chloroform and in benzonitrile and sonicating them for long as long as 2 hrs.Tried doing four layers by drop casting and it looked fine for initial measurements. Still want to find a better and effective way.

Thanks everyone.

For better uniformity you can apply electrophoresis

It some time happens in spin coating that homogeneous film cant be obtained. By two ways you can achieve homogeneous films 1. First nanoparticles should be homogeneous in the solution and have ability to adsorb on surface this can be achieved by using some polymer matrix like polyelectrolytes or other polymers with aromatic rings or unsaturated bonds to achieve homogeneous solution which have the property to adhere on the substrate. 2. Properly spread the solution on the substrate so that the surface covered uniformly then start spin coating first with slow speed and then followed by fast speed with proper heat treatment after each cycle and repeating these steps to achieve multiple layers to get the desired homogeneous surface. Hope it helps.

Did you try doctor-blading?

You should find the proper surface to immobilize nanoparticles.

We have developed an "adhesive" surface using self-assembling proteins on glass:

http://onlinelibrary.wiley.com/doi/10.1002/adfm.201502837/abstract

However, the results depends on the nanoparticle-nanoparticle interactions, nanoparticle-surface interactions, and nanoparticle-solvent interactions! You should find the best combination.

Jinxuan Liu can you please explain how you coated MOF because I am trying to coat a film of MOF and rGO compositie on a glass surface