## Nanosphere Lithography Frequently Asked Questions

Q: The smallest size of Ag nanoparticles. Nanosphere sizes that you employed range from 1.1 μm to 200 nm. The calculated sizes (Table 1) of the Ag nanoparticles (aSL) range from 255nm to 46 nm for triangle shape, and from 170 nm to 31 nm for hexagonal shape according to equations in your paper “Hulteen et al., JPC B, 1999, 103, 3854-3863” (Fig.1). While in another paper “Haynes and Van Duyne, JPCB 2001, 105 5599-5611”, the mentioned sizes of Ag nanoparticles ranged from 1000 nm to 20 nm. For the 20 nm Ag nanoparticles, is it the experimental value of Ag nanoparticles fabricated from 200 nm nano sphere?

A: You are correct in saying that we have only packed spheres in the size range of 200-1100 nm. In essence no one here has made 20 nm sized particles using the normal NSL technique. The smallest nanoparticles we have made using 200 nm diameter spheres have a perpendicular bisector of ~50 nm. We have not made nanoparticles with smaller spheres, but it is theoretically possible as you have shown in your calculation below. One way to make smaller nanoparticles is to use a variant on traditional NSL called Angle-resolved NSL. In this technique you deposit the metal at various angles greater then 0 degrees. This is stated in the review and in the following references:

“Angle-Resolved Nanosphere Lithography: Manipulation of Nanoparticle Size, Shape, and Interparticle Spacing”, C. L. Haynes, A. D. McFarland, M. T. Smith, J. C. Hulteen, R. P. Van Duyne, J. Phys. Chem. B, 106, 1898-1902 (2002).

“Dichroic Optical Properties of Extended Nanostructures Fabricated Using Angle-Resolved Nanosphere Lithography”, C. L. Haynes, R. P. Van Duyne, Nano Lett., 3, 939-943 (2003).

Q: 100 nm Nanosphere. I need to prepare some nanoparticles with sizes smaller than 20 nm. Based on the calculation on the relation between the sizes of Ag nanoparticles and the sizes of nanospheres (Table 1), I need nanospheres with sizes no more than 100 nm. I checked with the vendor about the properties of the nanospheres. The 100 nm white carboxyl-functionalized polystyrene latex nanospheres with the coefficient of variation less than 5% are not available. Could you please tell me other company from which I can obtain the 100nm nanosphere with CV% less than 5%?

A: Spheres with diameters below about 150 nm are difficult to make monodisperse. But here are a couple other companies that you could try: Duke Scientific and Bangs Laboratories. The smallest we have ever packed has been a sphere diameter of 160nm.

Q: May I use nanosphere with CV% of 15%? I am worried that the less uniform size of nanosphere will lead to the less uniform size of Ag nanoparticles.

A: With a CV greater the 5% you will not get uniform packing at all and the particles will not be very good. One of the critical criteria for getting good sphere packing is a CV of 5% or less.

Q: Other type nanospheres. Could I use white sulfate functionalized polystyrene latex nanospheres instead of white carboxyl-functionalized polystyrene latex nanospheres if I cannot find the later?

A: Yes, you can use spheres with other termination groups. Sulfate groups work just as well as carboxyl groups. You can also use Amine terminated groups, but the prep procedure for the glass is different.

Q: Acid Piranha solution. The main goal of the acid Piranha solution is to remove the organic compounds on the glass cover slip. Can we use other solutions to reach this goal, such as 3:1 = 65% HNO3 : 37% HCl (V/V)?

A: Using Aqua Regia is not necessary. Pirhana cleans the surface very well and it is less nasty then aqua regia. We also do not know how long you need to clean it so you would be completely on your own if you decide to change this step in the process.

Q: The wet cover slip. When we drop-coat the nanosphere on the surface of cover slip, we need to pick up one piece of this cover slip from water using tweezer. When we pick up the cover slip from water, there is still a layer of water on the surface of the cover slip, do we need to remove this layer of water?

A: You only want to dry one side of the coverslip. Keeping water on the other side of the coverslip is absolutely critical in being able to have the nanospheres flow over the surface evenly. The amount of water you have on the one side can be varied and should be to get the best packing. In different times of the year I have to use different amount of water on the surface, for example in the winter I use very dry substrates and in the summer I use very wet substrates.

Q: Drop-coating. Is it right that I still hold the glass cover slip using a tweezer during rotating my wrist to make the nanosphere solution spread evenly on the glass cover slip after I pippet the nanosphere solution on the cover slip and then lay down the pippet?

A: You have to hold the coverslip with the tweezers to be able to rotate it around. You want to make sure that you do not have the tweezers covering too much of the substrates though, only grab the very edge of the glass with the tweezers.

Q: Did you use the original concentration of Nanospheres as they purchased or did you dilute the solution? If you use the diluted solution, typically what are the dilute factors?

A: You always use the original concentrations. We have tried getting 10% solid solutions and hand packing with them by diluting them to 5%, but it does not work well. My suggestion is to stick with the 5% solids and use as is.

Q: Do we have to use the freshly purchased nanospheres? Typically, how stable the Nanosphere solutions are. I am wondering whether we can use one-year or two-year old Nanosphere solutions.

A: Sphere solutions typically only last for 2 years. Some of the very large and very small spheres last for 1-1.5 years. A good test to see if your sphere solution is still good is when you shake the bottle up if you see aggregates or clumps at all in the solution you should buy new spheres.

Q: Removal of Nanosphere. After deposition you remove the nanosphere from the substrate by ultrasonication. What is the sonicate power? As your paper described, the Nanosphere can be removed by ultrasonicating the substrate for 1-3 mins in ethanol solution. What are the good ways to make sure only nanospheres are removed, but not Ag nanoparticles? It appears that the Ag nanoparticles will also be released from the substrate by ultrasonicating the substrate in ethanol as described below?

A: We do not have a variable power ultrasonicator. We only have a standard Branson Sonicator that we use. You will sometimes run into problems where the particles will come off during the sonication process. The only real way to around this is to drop the power being used or make 4-6 samples at a time.

Q: Solvent for Ag nanoparticle release. When we release the Ag nanoparticles from the substrate, we still use absolute ethanol as releasing solvent. I am wonder if ethanol can work well. Are there other solutions working better than ethanol?

A: Ethanol is used in the releasing process because HDT is soluble in EtOH and that is why it is chosen. You could try other solvents, but we have not.

Q: Final Ag nanoparticle solution. After the Ag nanoparticles are capped by 1-HDT, they are released into the ethanol solution. Can we release them directly into aqueous buffer solution or nanopure water? I am planning to use MUA to replace HDT, and release the capped MUA-Ag nanoparticles into buffer solution or nanopure water solution. Hope to your suggestions.

A: My suggestion on this question is that as long as MUA is soluble in water then go ahead and give it a try and see if the particles come off in the same manner as when HDT is used as the capping agent. If it does it poses an interesting opportunity to modify the surface with different capping agents to have different functionalities

Q: AFM mode of operation. Would you please tell me which AFM operation modes (contact or tapping mode) and which tip radius (any special treatment of AFM tip or cantilever) you have use to characterize the nanospheres and Ag nanoparticles on substrates, respectively?

A: We always use tapping mode with our particles. Contact mode can rip the particles off the surface if too much force is applied. If you look at our papers all the info on the tips used is in there.

Q: What sphere size is the easiest to learn how to pack with?

A: You should start to practice sphere packing with larger spheres, like D=390 or 500 nm. These are easier to pack and you can get a feel for what good and bad substrates look like.

Q: Are there any difficulties packing on other substrates?

A: With NSL you can pack on essentially any substrate. Si, SiOx, and mica are the most common. Although, mica is a difficult substrate to work with if you are not an experienced sphere packer.

Q: What do the different areas on a substrate look like? What is a good area?

A: Here is a picture of three samples, D = 290, 390 and 500 nm. We will indicate the good and bad areas and you will be able to see hoe it gets more difficult to identify these as you pack smaller and smaller spheres.

Q: How are the Ag nanotriangles released from the glass substrate into solution after they have been fabricated by NSL?

A: The Ag nanotriangles are firstly made by NSL on glass slide, then incubated in 1 mM hexandecanethiol (other alkanethiol works as well) for 49-96 h. After incubation, rinse the samples thoroughly with neat ethonal. Lastly, sonicate the samples in ethanol for 3-5 min and you can observe the color on the glass slide is gone.

Q: Is it possible to perform this step without destroying the particles?

A: Yes, we have demonstrated that in our previous work. There are some fragments observed using TEM. However, a large amount of particles stay intact.

Q: How do you prevent the particles from aggregating in solution? Which solvent/capping agents can be used?

A: We found that alkanethiol (ethanol solution) can help to prevent the particles from aggregation.

Q: Do the spectral properties of the particles in solution and on the array resemble each other?

A: Yes. From ensemble measurement, the extinction maxium of the solution-phase particles are red-shifted from the surface-bound particles. Single nanoparticle measurement is under investigation.

Please refer to “Solution-Phase, Triangular Ag Nanotriangles Fabricated by Nanosphere Lithography,” A. J. Haes, J. Zhao, S. Zou,, C. S. Own, L. D. Marks, G. C. Schatz, and R. P. Van Duyne, J. Phys. Chem. B., 109, 11158-11162 (2005) for all the above questions.

Q: Have you ever tried redepositing the particles on a different surface and how does that influence the optical properties (I am assuming that the distribution of the redeposited particles might be different to the well controlled particle array)?

A: We never tried that.

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