Pang Nganthavee, Week 3, More Spin-coating!

I began my third week at the lab by finishing up the analysis of the data that I collected during my second week here.

I forgot to include a photo of my drawer in the last blog post, but it's now all filled up with a bunch of samples from my first three weeks in the lab!

After I took the average of all the trials, graphed them, and organized them into a presentation for easier viewing and access, I had a short meeting with Jyo Lyn, my grad student, to look over my results together. Fortunately, unlike the first run of aging the silica solutions, everything turned out well this time. While the results from the first week showed no trend and was extremely inconsistent, the results from the second week followed what the primary article I was supposed to replicate reported. Whereas the refractive index of the films should decrease, the porosity and coating thickness of the silica solutions should increase as a function of time. Thus, now that I have successfully replicated the results of the primary article, I can now move on to the next step: learning how to make and spin-coat polystyrene, a type of polymer.

Although the spin-coating process of the two (silica nanoparticles and polystyrene) remained the same, the difference between them lies in the synthesis of each solution. Whereas 30 wt % silica are diluted to 5 wt % with deionized water, the polystyrene beads

Polystyrene beads

are diluted with toluene, which is a new substance that I have yet to work with in my lab experience so far.

To make low filler fraction nanocomposite films, I first had to know which concentration of polystyrene solution and spin-coated at which rpm (spin-coating speed) would give me what coating thickness on the silicon substrate. This is as ideally, when I put my bilayer films composed of both the polymer layer and the nanoparticle layer in the vacuum oven to allow it anneal together by capillary rise infiltration, I would want the polymer layer to be very similar in thickness to the thickest layer of the aged silica.

During the first two weeks, when I was replicating the experiment from a primary article, I found that after I allowed 5 wt % SM colloidal silica to age at 50°C for 70 hours, the thickness of the silica coating increased by around 20nm (from 97.42nm to 117.27nm). However, during this current aging session, I was planning to age it for a longer period (≈120 hours), which in turn would also result in a thicker silica layer. Thus, I would want the thickness of the polymer layer to be around 150-160nm instead of around 120nm in respect to the thicker silica layer.

To find out which exact weight percent and rpm of spin-coating, Jyo Lyn suggested that I should make two vials of PS solution at different concentrations: one at 3 wt % and the other at 5 wt %. After that, as the rpm of the spin-coater is also a factor that affects the thickness of the film (the coating thickness of the film decreases as a function of rpm), she also suggested that I should test out both concentrations at 6 different rpms (1000rpm to 6000rpm in 1000rpm intervals). After measuring the thickness of each film with the ellipsometer,

Ellipsometer

I would then be able to pick which concentration of polystyrene and at which rpm of spin-coating would I want to use as the parameters for synthesizing the nanocomposites in the next few days.

After completing the ellipsometry and looking over the recordeddata, I decided to use the 5 wt % polystyrene spin-coated at 4000rpm. At this specific concentration and rpm, I found that the resulting thickness of the polymer layer ranged between 150-160nm, which are exactly values that I hoped for. Also, it is important to note that these parameters are what I will keep constant throughout the rest of my trials in synthesizing the nanocomposite films, as the variable that I am aiming to change are only the aging time of the silica solutions along with the rpm at which I spin-coat the silica solution onto the polystyrene-coated silicon substrate.

Thus, using what I have learned so far with the preparation of the polystyrene and silica solutions, I am now able to begin attempting to make low filler fraction nanocomposite films and was what I did for the rest of the week.

After cleaning the silicon wafers, treating them with the plasma cleaner, I spin-coated the first layer of the bilayer film: the 5 wt % polystyrene. I then had to treat the polystyrene-coated wafer using the plasma cleaner once again to ensure that its surface is hydrophilic and clean. Finally, I proceeded to spin-coat the 5 wt % pH 7 silica onto the wafer and thus finish the process of making a bilayer film. After repeating this process 3 times at 4 different rpms (2000rpm, 3000rpm, 4000rpm and 5000rpm), one session of spin-coating is complete.

As of now, I still have one more silica spin-coating session of the last batch polystyrene-coated silicon substrates to do on Monday. Currently, I have only done 5 sessions for this particular run at times t = 0, 21, 26, 45 and 50. Therefore, looking ahead, after I am done with spin-coating this last batch of samples, I will probably use the ellipsometer to measure the thickness, refractive index, roughness and MSE of all of them and proceed to use the vacuum oven to heat them up and anneal the polystyrene and silica layers together. I will probably also have another meeting with Dr. Lee, my P.I., towards the end of next week to catch up and talk about the progress of my project.

A presentation compiling all the data I have collected so far. I will use this to help while discussing about my project during the meeting with my P.I.

In addition to learning how to prepare the polystyrene solutions, Jyo Lyn also taught me how to measure the porosity and refractive index of the nanoparticle framework of my silica-coated samples by using a liquid cell. After making sure that a 5 wt % silica-coated silicon wafer I wanted to measure was held securely with double-sided tape in the liquid cell, I placed it on the ellipsometer. After recording down the thickness and refractive index (via values of A, B, and C), I then had to repeat the same process when the sample is in water. To do so, I had to slowly inject deionized water into the liquid cell, while making sure there were no air bubbles present - air bubbles can affect the A, B and C values of refractive index due to potentially scattering the light that comes in from the ellipsometer. Finally, I would have to use an air gun to dry my wet sample and the liquid cell, and then repeat the process all over again for the rest of my samples.

After I have gathered and recorded down all my data in my lab notebook, I then will have to calculate the values of porosity and refractive index of the nanoparticle framework via the equations:

Equation for calculating porosity

Equation for calculating refractive index of the nanoparticle framework

Note*: nf,2 = refractive index in water, nf,1 = refractive index in air, p = porosity, nf,air = refractive index of air

Although I am still in the process of doing the calculations, the porosity should ideally increase while the refractive index of the nanoparticle framework should stay constant as a function of aging time.
Outside the lab this past weekend, my dad, Steph and I decided to hop on the subway to go to Temple University. As I will be taking TOEFL there in July, I thought that it would be better for me to go there at least once to check out where the building and testing room is, so that on the day, I would know exactly where to go. Nevertheless, this gave rise to an opportunity for us to explore Philadelphia beyond what I usually see. We had a great time!