Rough Gold?

[BYU_michael]

Hi I am new to the forum and I am trying to do dip-pen nanolithography with a nanoscope AFM at Brigham Young University. I am trying to reproduce the results in the paper by Piner et al entitled "Dip-Pen" Nanolithography.

It can be found here: http://www.sciencemag.org/cgi/content/full/283/5402/661

I am trying to reproduce the results in Figure 2a. When I do it I can see a faint outline of a square if I really use my imagination, but I have yet to find conclusive evidence. I deduced that my gold was too rough and that I needed to get atomically flat mica gold.

I am using 1-octadecanethiol and I am guessing that this thiol is approximately 2nm tall. Is this a correct assumption? Should I focus on getting flater gold or is there something else I can do?

Thanks in advance for all of your help!

Michael

[admin]

Hi Michael,

Writing MHA has a few parameters you have to watch out for. The roughness of the gold will make a difference in the line width of your features but if it is clean, it should not affect the writing.

The surface of the gold is critical. Gold will quickly and easily pick up contamination from the environment. Once it is covered in organic contamination you will not notice much deposition of small thiol molecules. You will loose contrast as the surrounding areas will be of a similar nature and the ink will have no binding to the surface. We typically evaporate 5-10 nm of Ti onto Si, then 25 nm of gold. Once the evaporation is complete we store the gold substrates under nitrogen to slow their exposure to the environment. Especially do not store any gold samples near an inked tip overnight or by used inkwells as the thiol molecules will coat the gold substrate in a matter of days.

The environmental conditions are also critical for depositing MHA on gold. In most situations you need a relative humidity of at least 30%. If the MHA is having dificulty writing feel free to improve that to 60% or even higher. The water miniscus between the tip and the surface drives the diffusion of the thiol molecules to the surface and must be present for quality writing.

While mica peeled gold gives great contrast to MHA since it is protected from the environment until the moment it is peeled, however it is not necessary. With mica gold you will be able to get a great topography scan of thiols on gold as the lack of surface roughness will cause the molecules to stand out from the background.

The 1-octadecane thiol should be about 2 nm. The best imagaing of SAM molecules on surfaces will be from either LFM scanning or Phase imaging which stand out very well on gold that has a grain size of even 100nm.

Good luck with the experiments in the lab,

let me know if you need anythingelse,

Al Smetana
Applications and Support Engineer