Pillar Updates

It has been quite some time since my last post.  The fall was busy with classes and my General Exam which took the place of some of the this research as it allowed me time to evaluate my progress and the next steps to be taken in the completion of the project goals.
I also spent time working with colleagues on campus to create the nano-pillars, which I will updating today as we have made much progress in this area.

Originally the plan was use PolyStyrene NanoSpheres (PSNS) to create the pillars by first placing the spheres on the silicon surface and then performing Reactive Ion Etching (RIE) to etch the areas exposed between the beads and then remove the beads to leave the areas underneath the beads untouched and use them for the adsorption surfaces.  In the course of those experiments we found that the beads did not stand up to the power of the RIE process and changed shape during the etching leading the non-uniform shapes instead of pillars.  Those results were posted in January of 2013 if one wishes to take a look.
Even if that form of masking were successful, the smallest diameter we have been able to obtain a even somewhat decent monolayer with so far is 80-100 nm, which means the smallest pillars we would end up with would be 100 nm.  Our goal, however, is to reach pillars of 10 nm or so and therefore a different approach was needed.

Then it was thought that the beads could be used as a mask not for etching but for depositing a material into the interstitial spaces.  The beads could then be removed and the material would be used as the mask for etching.  Finally, removing the material would leave the untouched surface at the top of the pillar for adsorption studies.  The image above shows that the interstitial spaces of 100 nm beads would be approximately 15 nm.
It was later found that a group had already performed this process, with their citation given below

C.-W. Kuo, J.-Y. Shiu, P. Chen, G.A. Somorjai, Fabrication of Size-Tunable Large-Area Periodic Silicon Nanopillar Arrays with Sub-10-nm Resolution, The Journal of Physical Chemistry B, 107 (2003) 9950-9953.

That group used chromium sputtering in between the beads but we have decided to use thermal Nickel evaporation instead as the layer is more uniform and the source acts more like a point source so that the evaporation onto the surface is more anisotropic.

Above is an image of the surface of the silicon with evaporated Ni and beads removed.  There is a precleaning step which leads to a slight decrease in the diameter of the beads (and hence an increase in the bead separation and the interstitial spaces) and therefore the dots are roughly 25 to 30 nm instead of 15 nm.  However, if the cleaning step is avoided it is believed that smaller more uniform dots will be obtained.  There is Ni lace also scattered about the sample, arising from the difficulty in acquiring a widespread hexagonal monolayer of beads across the surface.  

Work will continue with these samples to perform the next step, RIE, in order to create the pillars and then remove the Ni with a transene etch. 

Work with the trenches is still ongoing, with more difficulty in reaching the silicon bottom of the smallest trench widths.  Trenches as small as 30 nm wide have been achieved but still not having a great deal of success in finding the silicon.   

Updates will be more frequent in coming from now on.

Finally, finally, finally, finally saw micelles on silica!

Sample E comparison Continued...

30 nm trenches and water damage

Micelles Finally!

Well it has been a while since I have posted but I have been working on the problem of obtaining images of micelles using the AFM.  Sprinkled in there as well were presentations and qualification exams which also took time but this received the bulk of experimental attention.  I finally got somewhere after emailing Dr. S. Manne who has quite a few publications which include micelle imaging and he suggested that I was walking before I learned to crawl (paraphrasing).  He suggested I try graphite first (which was the actual surface used in his paper) to get the basics down and to use contact mode to get a protocol which I could reproduce.  So I acquired the materials and started learning to use contact mode, as I had not used it before.  Dr. Paul Ashby gave a seminar here at the University of Oklahoma last year and gave the advice that I advance the stage and "wag" the tip to see if I had reached the surface yet.  I didn't seem to get anywhere with this (I understand why now) at first but continued with this mind set in contact mode.  The breakthrough came from understanding (not just using but understanding) the force curve capability of the scope.  The first two figures can help explain what I mean by this.  The tip is extended into the solution and if the surface is not close enough there is only interaction with the fluid surrounding the tip (air or water), which is just noise (not pictured) but when there is a surface which gives a repulsive or attractive force the curve looks something like the curve seen in the Figure 1 (air with clean silicon surface).  As the tip is advanced there is an increasing repulsive force and then an attraction, or snap-to point.  Then as the tip is pulled away there is a slight adhesion force which causes the force to not line up on top of the approaching line, and then there is a snap off point, the straight vertical line, as the tip suddenly leaves the surface.  Now if we now add water and micelles, we have added a layer to the surface which interacts with the tip.  Figure 2 shows a small blip for the approaching line, which is where the tip has broken through the micelle surface and is now at the actual surface.  As the tip is pulled away there is still an adhesion force and we see a snap-off.  The trick is set the imaging setpoint (the force applied to the tip which keeps its height constant) to be just before the snap-to point.  Any setpoint greater than this will push right through the micelle layer and you will only see the surface, which is smooth so you will essentially see nothing (many, many, many weeks/months/years of nothing).  The hardest part of most of this was translating the protocols of others using different scopes to the scope we have here at OU.  Anyway, the final products are seen in Figures 3 and 4, which are deflection images (working on translating to tapping mode currently).  The images show lines of micelles, which follow the grains of the surface.  For CTAB on graphite these are shown to be cylindrical micelles.  In order to verify this with our set-up a sharper tip will be used to check the height of the micelles.  A duller tip was used because I got tired of wasting all of my good, really sharp tips on seeing nothing so I used the slightly larger tip (but not so large that I would miss the micelles if they were there).  There is still a lot of work to do but now that I have finally been able to get this to work and have a protocol which I can reproduce we can move forward again instead of being stuck in once place!!  Next will work on imaging micelles using tapping mode (perhaps with a tip using magnetic backing to decrease the oscillation of the water) and then start working into the PMMA trenches.

Figure 1. Force Curve in air

Figure 2. Force curve in water with micelles on surface

Figure 3. CTAB micelles on surface following grains (deflection)

Figure 4.  CTAB micelles on surface (deflection)

A short update on cleaning and imaging procedures...

Let me be (probably not the first) one to say that if research didn't slog forward it probably wouldn't move at all.  I have been working with AFM in attempts to find the right combination of AFM probe and cleaning procedure (of both the substrate and the liquid cell) which will yield micelles on the surface.  I have been corresponding with contacts on campus which have experience with cleaning AFM probes (which I have mentioned before can become contaminated by the gel packing which the are stored and shipped in) but once again the difficulty of meshing schedules makes that endeavor a slow one.  I am meeting with another research specialist today in hopes of looking at the set up they use and replicating their procedure and apparatus somewhere in our building so that I, and other who are trained to use the AFM, will have access to it so that cleaning of AFM probes will not be such a hassle.  I was able to come up with a contamination free way of transporting probes across the university campus which will keep the probes safe, secure and clean following the cleaning, but if I am never able to get assistance in the cleaning then the container is not worth much (although I'm happy that the simple design we came up with works so well).  

On another AFM front I am also spending time trying to locate the 30 nm and smaller lines which is proving to be much more difficult than the 50 nm lines I have posted about before on here. Those lines, which were much more in number creating an array that later proved to be too big for the field size and may have contributed to the lines large widths, were also slightly sloped on the side walls which caused the trench bottom to be smaller than the top (as previously discussed) and made them much easier to find.  The new set of e-beam etches were not as sloped and were smaller (which is good!) are much harder to find even with smaller radii AFM tips.  So far I have worked with tips down to 1 nm but from literature I have found that without cleaning of the probes the contamination of the tips might be the reason that the lines are so difficult to find (The Journal of Physical Chemistry B, 1998. 102(22): p. 4288-4294. and The Journal of Physical Chemistry C, 2008. 112(38): p. 14902-14906. and The Journal of Physical Chemistry B, 1999. 103(40): p. 8558-8567. to name a few).  So once the cleaning procedure has been worked out I can tell if the lines and micelles are hard to image because of the contamination or because my procedure is still lacking.  Either way I will update regarding these matters soon!

AFM is a different animal...

Because the SEM technician who runs the microscopy lab at TU has been gone I have shifted focus primarily to imaging micelles on a plain silicon surface.  Although for as many papers of it I see it seems like this would be just a sit down at the AFM and get it done in a day or even a few days task, it has proven me wrong.  The main issue that I have run across is that working with micelles on the AFM requires a degree of art and a degree of science.  After speaking with someone this week who has worked with AFM for some time, although not on surfactants or micelles specifically, I came to realize that you really have to come to know each component that you're using personally.  I have tried three tips so far that did not show promise and have moved onto a new set of three which I am hoping will at least point me in the right direction.  I have also been in contact with Agilent and Nanosensors.com regarding these subjects and while they are helpful I have come to the conclusion that it will all come down to trial and error with tips and cleaning procedures.  It becomes pretty frustrating when I use the AFM for several hours in a day and come away with nothing other than "this wasn't a tip that will do the job" or "that cleaning procedure left too much on the surface".  However, there is always another procedure and another tip to try and with free samples from probe manufacturers I have plenty to work with.  Once I find "the tip", the plan is to purchase some and coat the backside of a couple with magnetic material which could (has been shown to) increase resolution by decreasing the motion of the solvent (in our case water) by the piezo.  A MAC mode nose cone will facilitate this by creating an oscillating magnetic field which will vibrate the tip alone without the piezo.  

Another problem which I am dealing with is that most AFM studies done on silicon use a cleaning procedure which leaves the surface hydrophilic, usually by washing in an RCA-1 cleaning solution.  For now I am just trying to achieve imaging micelles so I am following this procedure, but my work around in the end is to try a plasma cleaning step on the PMMA layered samples which should leave the exposed silicon in the trenches hydrophilic.  The time of the plasma will need to be short in order to prevent damage which could alter the roughness of the exposed surfaces.  This week I am trying the PPP-BSI-SPI, the PNP-TR-SPL and the SiNi AFM probes.  They are all soft cantilevers (less that .1 N/m and roughly 15 kHz).  The problem I have seen with these types of cantilevers so far is that the slightest environmental factor, whether it's air movement from closing the door to the room or sneezing too loudly, causes the cantilever to become erratic.  I have developed a few procedural additions which help to lessen their effect and will post on the efficacy of the new tips soon.