Effect of pH Conclusion

The pH of the 50 nm sphere solution was tested and found to be roughly 4.5.  From the literature, at 8.5 pH silica has a negative charge which hinders the adsorption of anionic surfactants (1).  Because our pH is much lower than this value we can say that the surface of the silica has a slightly positive charge which would increase the adsorption of the SDS (anionic) in the sphere solution.  The theory that this would increase the sphere adsorption as well would require further experimentation which is not the primary focus of this project. Therefore, we will conclude that the pH effect on the sphere adsorption is not a factor to be explored in enhancing the monolayer coverage for this project. 

However, the SDS adsorption to the silica as a result of the low pH may be a factor is creating a sharp relief.  It has been recommended that the samples be soaked in methanol to loosen the SDS on the surface of the silica.  To make sure that this soaking step does not affect the sphere monolayer, SEM will be performed prior to the soak and then again after the soak to determine the effect, if any, on the already deposited spheres.  

Post Memorial Day update

Hope everyone had a good Memorial Day Weekend! Today will be a short post.  Due to complications with the SEM I am not able to use it until later this week at the earliest.  Something important came up this weekend I was not able being the pH testing of the sphere diffusions as I had hoped but I should be able to do that tomorrow. On the personal side, following this weekend it is easier to understand the concept that the world continues to turn even when you want to take a moment to stand still.

Short term goals for the next 3 weeks

Next week I will be trying to remove some of the stuck on SDS from the surface of a deposited sample using a methanol soaking.  This step was recommended by the research scientist helping me with the etching process.  The idea would be to loosen and help the spreading out of the SDS in order that it does not create an etch mask on the surface of the silica.  I am not convinced that the spheres will remain in place during this process, but the worst that could happen is that the spheres became disordered and I know that it may make remove some of the SDS but at the cost of compromising the sphere locations.  I am hoping to etch again in the beginning of June using a different etch time and perhaps with smaller spheres.  We are getting close to the point where we will determine if this process will give us the surface we are looking for.  I anticipate that the answer to this question will be answered mid-June.  Memorial day weekend will be relaxing, only in the fact that I can spread out the work I hope to get done this weekend over three days instead of two, but still I am thankful for the holiday.  Everyone have a safe and enjoyable weekend!!

Adsorption variation due to pH effects

Depending on the pH of the solution in which the substrate(silica) is immersed the surface charge of the substrate tends to vary.  A higher pH will lead to a more negative charge on the silica surface.  The polystyrene sphere solution has SDS, an anionic surfactant, and surface sulfate groups from the synthesis of the of the spheres.  These sulfate groups lead to an overall anionic charge in the sphere solution prior to the addition of the SDS surfatant.  Yesterday I was able to discuss with a colleague  the possibility of the pH of the sphere solution causing a slight repulsion between the spheres and the substrate, due to the negative charge attached by the sulfate groups and the hydrophilic nature of the silica.  It seems a simple task to check the pH of a sample solution and test the effect of it's variation on the surface coverage of the substrate.  I will note that I feel as though speaking with a peer who is extraordinarily familiar with the effect of pH on adsorption, although the conditions are different for their project, was extremely helpful in making assumptions as to the nature of it's effect and whether or not considering the pH a factor was a major necessity.  

On a personal not, the frustration of not knowing if all of this work and produce the surface we need and whether or not what these small considerations are going to make any difference in the end is a little dis-hearting at times.  However, what worries me most is the time spent on this part of the project if we have to change methods.  I do not feel that it is time wasted because I am definitely learning a lot about how all of the little things affect adsorption.  That being said, I feel that research is exciting once you get results that tell you something new and I am excited and anxious for that part of the project to be here!  It's even more exciting because you know that you didn't cut corners and all that trial and error and work made a difference!  It's like a Christmas that comes early!

Surface Chemistry Alteration

I am looking into the possibility of altering the surface chemistry of the silica.  I am wondering that if by making the silica (hydrophilic) a hydrophobic surface if the monolayer coverage would not increase?  A few concerns I have are how the beads would react to a change in pH (my guess is they won't change very much) and whether or not the increased hydrophobicity would just create more areas of multilayers instead of increased monolayer coverage.  My reasoning behind this idea in the first place is the formation of layers of surfactants on hydrophobic graphite, which promotes the formation of surfactant morphologies which cover the surface to a greater extent than they would on a hydrophilic surface.  Therefore, because of the hydrophilicity of our silica, perhaps by altering the surface chemistry enough to create a hydrophobic layer we might be able to promote enhanced layering.  Another limitation is that any alteration of the surface must not interfere with the final etching process.  Another option that I will be investigating is how a change in pH of the sphere solution might increase (or decrease) sphere layering by increasing or decreasing the binding sites on the substrate with counter-ions.

Follow up of SEM of etched samples

The sample which was etched with the "diving board" structure masking part of the sample was viewed using SEM today.  First, using a top down view to look at the effect of the 5 min of etching on the spheres, it could be seen that the spheres had deformed and melted together in some areas.  It was not possible to tell about the depth of etching from this angle, so the sample was placed in an ultra-sonic methanol bath for 2 intervals of 5 minutes and cleaved down the middle. When viewing the sample on its edge we were able to view what had happened in different areas of the sample from the masking and etching areas.  In the ares that were not masked there was only a rough surface, nothing that would suggest spheres were ever attached to the surface at all.  There were areas that appeared noticeably higher than others though, suggesting that multi-layers may have masked those areas slightly.  As one viewed closer to the diving board masked area the surface because less rough and more uniformly etched until up near the top of the sample there were very small misshapen pillar formations.  These pillars are not flat enough for our purposes, but they do tell us that pillars were forming and that the polystyrene spheres did create pillar structures.  The length of etching will need to become shorter in order to tell if we will be able to create pillars with a flat enough surface that they can be used for our surfactant adsorption experiments.  Therefore the next step will be to try another round of etching with a shorter interval and view the results.  Right now we have been using one interval of etching, but I am wondering if having the a greater number of intervals that equal the same time might not serve our purposes better.  By increasing the interval number the heat that was generated and melted the spheres may be reduced.  So conclusions from today are that this method did create pillars, but that we need to see if shortening the etching time and/or increasing the etching intervals will provide a cleaner result.

SEM of etched layers

Performed SEM today on the etched samples.  The sample with 30 seconds of etching and the spheres removed showed a difference in height from the surrounding surface and where the spheres were.  With a longer etching time pillars might be clearer.  The SEM with the 50 nm spheres showed that there were monolayers but that their coverage was about even with that of multi-layers.  A conclusion that has been reached from this is that when using the Langmuir-Blodgett technique, at least with the parameters that we are using (solvent ratio, barrier speed and withdrawal speed) that the smaller the spheres the more difficult it is to control the monolayer coverage.  I'm a little nervous that after all this that the surface of the pillars might be insufficiently smooth for surfactant studies with AFM.  We will continue with this until we have pillars that show that separation in between the spheres is possible, which will come from etching for at least longer than 30 seconds before we think about trying another masking technique to create the nanostructures.

New Cleaning steps added

Have been using new pre-deposition cleaning procedure.  First the silica sample, which is already cut and has had an oxide layer grown on it thermally, are placed in a plastic beaker which contains methanol and is then put in an ultrasonic bath for 5 minutes.  This step is to loosen any debris which may be strongly adhered to the surface.  Next it is cleaned in a plasma cleaner for 10 min at 18 W.  Then it is placed in a solution of 1:1:5 hydrogen peroxide, ammonium hydroxide and DI water for 10 minutes at 75 degrees Celsius.  They are then removed and rinsed with DI water and dried with N2 and placed back in the plasma cleaner for 5 minutes at 10 W.  Then the sample is attached to the arm of the stepper motor and lowered into the sphere solution (2400 microliters sphere diffusion and 3600 microliters water which has been sonicated for 10 min) and the Langmuir-Blodgett barriers are advanced for 3 minutes while the sample is stationary.  The stationary step is to allow the spheres to build up a monolayer concentration near the surface of the substrate.  Then the substrate is withdrawn at 5 micrometers per second and while the barriers are advanced at 3 ticks above the slow mark.  A plastic ziplock bag is lowered around the set up to prevent any turbulent air currents from disrupting the deposition or bring contaminants into the solution.  Every 4 minutes the barriers are backed away from the substrate and restarted.  Once the process is complete, the substrate is placed in a sterile container horizontally.  The polystyrene sphere solution is pipetted back into its container and stored.  SEM was to be done today but due to scheduling conflicts was not able to secure a spot.  Looking forward to seeing how the etching from last week turned out.  Nice to finally see some results, promising or not, because at least it will point us in a direction for the next step.

First day of summer classes and Research

Today was the first day of summer classes and research.  Doing research with only one class feels like you have more time, but it can be deceiving because of the amount of time you will spend for that one class due to it being so condensed.  Implemented a few new cleaning procedures today including a methanol soaking of cleaned silica pieces to remove any dust that may have settled in between bead deposition trials.  Once the beads have dried sufficiently we will try and soak the sample in methanol to try and loosen the SDS that also adsorbed to the sample.  This will make the etching process easier as well as remove any contaminants that stuck to the surface during the deposition process.

Big Update

Although results from previous trials had shown that 1:1 EtOH/Water solvent gave promising results, more recent trials lead us to believe that a 2:1 ratio of ethanol to water gives more continuous coverage of nanospheres on the surface of the silica film, much of it being continuous monolayer with sporadic interruptions.  A concern which has been previously discussed is the evaporation of the alcohol in the solvent during the sonication and sphere deposition steps.  The greater amount of ethanol in the solvent makes for faster evaporation from the surface of the substrate as the spheres adsorb to the surface.  This leads to the conclusion that faster solvent evaporation leads to better monolayer formation, while at the same time making keeps the volume of the solvent constant.
Evaporation being primarily a mass transport matter has lead me to question the effects of air currents on the layer deposition phase.  To try and reduce the effects of these air currents, as well as any contamination in the air that may be settings on the sample or in the solution during the bead layering, a plastic cover has been fitted for the modified Langmuir-Blodgett trough apparatus.  Since using the cover the streakiness of the sample to the naked eye has been slightly reduced and monolayers have been forming consistently, which sporadic interruptions and multi-layers still, but the results are promising.  While the formation of monolayers is crucial, the formation of multi-layers may not cause problems for the final nano-pillar formation.
It is hypothesized that during the Reactive Ion Etching (RIE) step, only the areas covered in monolayer will allow ions to the surface on the substrate through the gaps between the spheres.  Areas where the beads are absent will be etched as well but will not have nanopillars and should be distinct from areas with nanopillar formation.  Where multi-layers have formed should have enough coverage with spheres that the areas between spheres will be masked by the above layers of spheres, leading to an overall masking of the area and prevention of etching.  We think that these areas will also be distinct from nanopillar regions under AFM and we will be able to test this hypothesis using SEM to see if the overall masking takes place.
Another method of sphere layering was tested for better results.  The method is referred to as the scooping technique (1).  The theory behind it is that by adding the sphere diffusion dropwise to a solution of water and ethanol that a monolayer of the spheres will form on the surface of the solution.  Then   a solution of SDS (Sodium Dodecyl Sulfate) is added to the solution with the floating monolayer to help the crystallization of the monolayer.  Next, the silica substrate that we are using would be dipped horizontally through the surface of the solution and allowed to soak.  Then the substrate is withdrawn at a constant speed and pick up the monolayer which has formed on the surface of the solution and it will adsorb as is to the surface of the substrate.
A few difference between the Langmuir-Blodgett technique and the scooping technique, to clear up any confusion, is that only a few drops of sphere diffusion is added to the solvent solution and the concentration of SDS is much lower because only a few drops of a dilute solution are added.  Then in the LB method the substrate is withdrawn vertically and the monolayer adsorbs at a constant rate and in the scooping method it is supposed to adsorb all at once. 
The technique did produce some samples that had intermittent layer formation but not in the quantity that the LB method produced and therefore the LB technique was used still.
The velocity of the barriers of the LB trough was increased to test their effect on the monolayer formation.  The increased speed of the barriers increased the coverage of spheres, in roughly the same ratio of monolayer to multi-layer, which is good because it meant that we had increased the amount of monolayers with which to create nanopillars without creating an excess of multilayers in the process. 
Four samples of 200 nm spheres on silica were produced, one for SEM prior to RIE and three for use with RIE, using these parameters.
Etching was performed with CF4 and O2. Prior to etching an unlayered sample was cleaned using ultrasonic methanol baths in 5 minute intervals.  This piece was used to determine the etch rate.  The clean sample was etched with an area of the sample covered to prevent etching in this area and the difference in height between the etched and unetched areas were determined using a profilometer and from this the etch rate was determined to be roughly 100 nm/min etching.  The rates of gas were 10 sccm CF4 and 5 sccm O2 at a pressure of 15 mTorr.  The RIE apparatus was a Trion Minilock II.  Power settings were 30 W for RIE and 350 W for inductively coupled plasma.    Images of the pre and post etching were taken on a Nikon Optical Microscope with above incident light.
The images showed that steps to create a cleaner surface prior to sphere deposition need to be taken in order to create a layered sample clear of debris.  A possible source of this debris is the oven in which oxide layer formation was done, as well as the scribing of the silicon wafer. Both will be investigated to determine if either can be modified to provide a cleaner sample.
 One of the layered samples was etched with the same parameters as above for 30 seconds.  The spheres were then removed in a ultrasonic methanol bath.  A second layered sample was etched under the same conditions, but a metal plate was attached that that it hung over half of the sample and it was etched for 5 minutes.  The metal plate blocked the path of direct etching ions but not ones coming from an angle.  This allowed for an etching gradient along the sample.  The idea was that the area of the sample fully exposed would be fully etched and the farther under the metal plate the spheres would be etched less and less, thus providing areas of different etching environments that could help lead us to the conditions that would be optimal for the etching  that we need.  When this sample was removed, it appeared that the spheres that were under the metal plate might have been removed and/or melted from naked eye inspection.  A concern brought up is that using a solution with too much surfactant may prevent even etching due to any etching prevention being caused by a surfactant film being present.  We will be able to do SEM next week on both samples in order to gain a better idea of what may have happened, as well as what conditions we will need and if anything about the process of sphere deposition may need to altered.

1. Y.J. Zhang, Journal of Alloys and Compounds, 450 (2008), 512-516