Monolayer formation update

Work has continued in attempting to create monolayers.  SEM was performed and showed that monolayers have indeed been produced, but they are slightly patchy.  The Langmuir-Blodgett trough is still being used but have slightly altered the method for withdrawal and hoping this will form full monolayers.  Another option to be tried in this coming week is to make new solutions of spheres using a 1:1 mixture of H20 and MeOH to try and affect the patchiness of the layers.

Working with silica update

Silicon wafers were placed in an oven at 900 degrees C for 3 hours in order to apply a thermal oxide layer for SiO2.  They were then cut using a diamond tipped etching tool and have been running tests using the modified Langmuir-trough setup to observe the film on the surface.  So far have used 100 and 200 nm sphere solutions with films that are almost fully complete after examination with the naked eye.  However, there are still a few spots which should not be bare and therefore we will be working on changing the speed of the trough "floaters", which maintain the surfactant concentration near the surface of the substrate as it's being withdrawn.  My hope is that by increasing the speed and making sure that the substrate does not come into contact with the floaters, the increased surfactant concentration near the substrate surface will allow for a more even coating of the spheres.  If this does not increase the uniformity of the film then another option would be to increase the SDS concentration in an attempt to increase the concentration of individual spheres (non-aggregated) near the liquid-air interface.

Two week update

So I have made progress in creating monolayers of polystyrene microspheres on glass.  Because I have been using glass slides as practice I am a little nervous about how changing to the silica substrate will affect the monolayer formation.  Using previous literature I have performed 7 trials with various sphere sizes in either water or a water/methanol mixture to improve formation and film continuity. Using an optical microscope I can see the formation of these layers with 2 micron sized spheres but am unable to say with certainty about the 500 and 200 nm sized spheres.  The glass coverage is greater with decreasing sphere size and when using methanol and 2 micron spheres the coverage was greater but from what I can tell using a microscope it appears that a multilayer may have formed over most of the surface with small patches of monolayers scattered throughout.  I have repeated the trial and the sample is being dried overnight and observations will be made again tomorrow.  I will begin using silica next week and hopefully will be able to begin taking SEM images of these in order to determine with certainty the formation of this monolayer.  The literature that I'm using was performing these trials on highly ordered pyrolytic graphite and saw the best film coverage on this so I may have to see what I with silica and consider other options.

Multiple Day Update

Have been working in the lab but forgot to update the blog on my progress.
Last week I mixed a 6 mL solution of 2 micron latex spheres (1% wt, 2.4 mL latex solution, 3.6 mL nanopure water) and added SDS as a spreading agent (31.093 mM).  The solution was then sonicated fo 10 min and a piece of glass (microscope slide cover) was lowered about 4 mm into the solution and withdrawn at roughly 1 micron/second for 1 hour and 12.5 minutes.  The glass was allowed to dry for 140 min and then viewed under an optical microscope.  The film appeared to be a monolayer of latex spheres but was was patchy.  To help fix this I will be performing this again with a modified version of a Langmuir-Blodgett trough to keep an effective concentration of spheres near the surface of the glass as it's withdrawn.  Will add an update of this shortly.
I have also cleaned a silicon wafer using a plasma cleaner and ammonium hydroxide/hydrogen peroxide solution and cleaved a small area for AFM.  The RMS roughness of the silicon piece was determined to be 0.268 nm.

Switching to E-Beam lithography

So after some trial and error we have found that ion beam milling will not be a suitable substitute for e-beam lithography in fabricating our controlled roughness sample.  The ion beam redeposits gallium as it mills and that could create a terrace but would have added roughness from the gallium.  Also this technique creates a V shaped channel which I am concerned will create a surface on which additional surfactant adsorption could occur and effect the adsorption on the terrace surface.  This would defeat the purpose of having a terrace separating adsorbed layer from the rest of the surface.  We will need to wait for the e-beam to be ready to use and while this is a small setback, it provides time to learn to use the Quartz crystal micro balance and practice using the liquid cell with AFM.  Happy Holidays!

Impact and implications of AFM imaging studies

Just read Surfactant adsorbed layer structure at solid/solution interfaces: impact and implications of AFM imaging studies (Surfactant Adsorbed Layer Structure; Warr, G.G., Current Opin. in Coll. & Interface Sci., 2000, 5, 88-94) and I feel that it is one of the most helpful in understanding AFM studies that I've read so far.  I believe this is because it was published when AFM was a newer technology and was just beginning to be used for adsorption studies, so less information is skipped over because it has become more common knowledge.  I found the topic of transforming cylindrical and spherical structures into mono- or bilayer sheets using ion exchange very helpful.  Structures that originally begin as cylinders or spheres transition into flat sheets over time (18 hrs in the article) and "has been attributed to slow exchange of CTA+ for the potassium ions in the mica lattice." (Warr, 90) Then by increasing the ion concentration (they used caesium as an example but mention potassium, lithium and hydrogen as well) they are able to transfer back into cylinders and spheres.  Useful information for research to be done using adsorption.

CTAB adsorbed to various substrates

Have been going over Surface-Induced Phase Behavior of Alkyltrimethylammonium Bromide Sufactants Adsorbed to Mica, Silica, and Graphite(Surface-Induced Phase Behavior; Liu,J.F.; Ducker, W.A., J. Phys. Chem., 1999, 103, 8558-8567) for the last few days before finals and had to reread some of it following finals to catch back up.  I found it very interesting and helpful in coming up with methods for preparing samples and interpreting AFM images.  As someone still fairly new to adsorption research I especially found the section on variation of chemical potential by changing one or several different parameters including temperature, concentration, concentration of a salt which acting as a co- or counter-ion, the substrate and the length of the alkyl chain.
              I feel that the best surfactant to begin the trials with would be CnTAB due to the amount of literature on its use as well as the ease of changing the length of a surfactant molecule without changing the head group by switching between n-lengths.  The article makes use of a temperature controlling apparatus on their AFM which may be of some use to us as well but if we are not able to use something to keep the temperature constant while taking images of the AFM we may be able to see the changes in morphology and phase behavior as the temperature changes with time in the process of taking the image.
         I have been reading literature to do with many substrates but have been making more notes on the methods of preparation of silica and silicon substrates because of the substrate we will be using first will be silicon which has been milled using reactive ion etching.  We will then place the samples in an oven to promote the formation of an oxide layer to the surface of the sample. An interesting test that was used was the "steam test" for silica substrates, which seems fairly simple and checks for the uniformity of the hydrophilic character of the substrate.  The test is to run steam over the sample and check for a thin layer of water over the whole sample.  If it is a patchy layer of water then the hydrophilic nature is not uniform and must be addressed before using the sample.