Wednesday, March 26, 2008

Simple Approach for High-Contrast Optical Imaging and Characterization of Graphene-Based Sheets

Simple Approach for High-Contrast Optical Imaging and Characterization of Graphene-Based Sheets

Inhwa Jung, Matthew Pelton, Richard Piner, Dmitriy A. Dikin, Sasha Stankovich, Supinda Watcharotone, Martina Hausner, and Rodney S. Ruoff

Nano Letters, 2007, 7(12), 3569-3575; Free (but not as pretty) at arXiv.

Being able to correlate film thickness with film color is a pretty big deal in the microelectronics field. Conventional lithographic/microfab techniques make use of the pretty colors that SiO2 gives at different thicknesses; although you can roughly tell the thickness of SiO2 films within 25nm using naked eye and a color chart, an ellipsometer is usually used to give a more precise measurement. Likewise, one can sometimes tell the thickness of other films on top of SiO2 through the same optical method.
Since the graphene field is very interested in the number of layers (or thickness) in a film, this approach has seen wide use since 2004. It's definitely a lot quicker and cheaper than AFM or SEM; unfortunately, for single layers of graphene, the color change for such a thin film is very small. This paper attempts to optimize the properties of the underlying SiO2 (and silicon nitride) to give the highest contrast for thin graphene oxide and graphene films.

The authors start by doing some math, which you can read the paper for; basically they try to optimize the amplitude of a reflected beam of light (at a certain thickness) by fooling with the wavelength of light used, it's incident angle, refractive indices of everything, and absorption coefficients. After they got these factors all dialed in to get a good angle and three good wavelengths to try, they put some graphene oxide layers on SiO2 (using methods described before) and took nice pictures (second column, below); then they applied heat to make the films more graphene-ish and took nice pictures of those (third column, below), using three different wavelengths (a, d, g):

CBC-style pretty pictures, although the colors are actually fake since these are just contrast maps. Don't let that distract you from the beauty of this work; they also did this on silicon nitride to give similar looking contrast maps, then tried the same approach with graphene (they don't mention which graphene they use; I'm guessing scotch-tape graphene).

What does all this mean? I think it means that they've tweaked the parameters for silicon oxide and silicon nitride layers to visualize overlying layers of graphene oxide and graphene, and they've also shown instances where the contrast will change after thermal treatment (implying that this technique can be used to quantify the reduction of graphene oxide to graphene).

Jung, I., Pelton, M., Piner, R., Dikin, D., Stankovich, S., Watcharotone, S., Hausner, M., Ruoff, R. (2007). Simple Approach for High-Contrast Optical Imaging and Characterization of Graphene-Based Sheets. Nano Letters, 7(12), 3569-3575. DOI: 10.1021/nl0714177


Anonymous said...

Dear Rob W,

I happened to stumble across your blog as I was looking for citations to my Nature Nanotech. paper from earlier this year.

Since I am heavily involved in chemical education as well as research, I put together a short video demonstrating how graphene synthesis can be done in aqueous solutions without the need for surfactants. Perhaps it will be of use to you in your graduate studies. Feel free to pass it along.

R.B. Kaner,
Department of Chemistry and Biochemistry

Rob W said...

Well done.

Anonymous said...

It's a trap!