tartaric acid etching protocol and validation.docxosujilab-wiki.eng.yale.edu/@api/deki/files/1426/=dimoulas... · Web viewprotocol for acid-etch ZnO nanorod forests using tartaric

protocol for acid-etch ZnO nanorod forests using tartaric acidauthors: Peter Dimoulas and Dr. Su Huang.last updated : August 1, 2014

introductionZnO nanorods efficiently transport electrons and have been used to increase the efficiency of polymer solar cells (PSCs). In PSC applications, it might be better to limit the diameter of nanorods, since they do not produce excitons themselves, to enable greater surface, or contact area to transfer electrons from the acceptor phase of PSCs to their substrate (e.g., ITO). The following is a protocol by which ZnO nanorod forests, grown on, and attached to a substrate can be etched with 0.1 mM tartaric acid.

materialswell-plate or small, 60mm diameter petri dishes (enough for 5 containers, or vessels per substrate)glassware: washed with alquinox, triple-rinsed with acetone, and then triple-rinsed with DI watertartaric acid, 0.1mMdistilled (DI) waterethanol (200 proof)droppers x 2timerhot plate (at 40°C)permanent marker

methods1) Obtain ZnO nanorod forests and label all well-plates or petri dishes (treatment, rinse 1-4,

and a final clean container).2) Dispense 0.1mM tartaric acid into appropriate container.3) Submerge ZnO nanorod forest (bound to substrate) into the acid.4) After at least 5 minutes, remove the substrate and place it in a clean vessel, then, using a

dropper, gently spray 2 mLs of DI water directly onto the substrate.1

5) Remove the substrate from the DI water, place it into a new, clean vessel and gently spray 2 mLs of ethanol directly onto the substrate.

6) Repeat step 5 two more times.7) Place the rinsed substrate with a final, clean vessel and allow the substrate to dry on a hot

plate for 20 minutes.

1 We tested 0.1mM for up to 10 minutes with reasonable results. We suggest that is possible to obtain additional etching without completely etching the nanorods; however, we did not characterize an upper limit to this protocol.

Validation study: tartaric acid etching of ZnO nanorod forests, day 5last updated: 28 July 2014

introduction, methods, and resultsZnO nanorod forests on silicon wafers with a thin film coating were obtained from Candice Veligra and Noga Kornblum (batch 8Jul’14, 84/17.5). Wafers were placed in small, 60mm diameter petri dishes with 0.1, 0.4, and 0.7 mM tartaric acid. Following acid exposure for 0, 2, 4, and 6 minutes all samples were rinsed with DI water, then triple-rinsed in ethanol by spraying using a dropper, and subsequently dried on a hot plate at 40°C. To measure nanorod diameter, using ImageJ: 1) a scale was set (provided within all SEM images), 2) a line was drawn across the width of every distinguishable nanorod (fig. 1), 3) a brightness/contrast was adjusted to leave only the lines drawn (fig. 2), which were used to measure their lengths. Similarly we also measured the height of resultant cones among etched samples (figs. 3 & 4). Please refer to the table below for sample-label configuration and results. For more information on the results obtained from SEM images visit:https://docs.google.com/spreadsheets/d/1CemryhdZzPcLVJQBJ6Ow5DTOfPzrhEJmYl5bJdFhDP8/edit#gid=0

sample ID

time mM notes from SEM images nanorod diameter:ave. (nm); stdev

nanorod cone height: ave. (nm); stdev

A 0 0.1 very dense nanorod forest present (fig. 1)

33.734; 7.127 -

B 2 0.1 very dense nanorod forest present, no evidence of etching

- 42.622; 10.111

C 4 0.1 etched, nanorod forest present (fig. 5)

30.40; 6.75 24.996; 8.631

D 6 0.1 etched, nanorod forest is present (fig. 6)

35.534; 6.795 37.188; 9.7234

E 0 0.4 not observed - -

F 2 0.4 etched, nanorod forest present (fig. 7)

32.407;18.226

G 4 0.4 no nanorods, only co-block polymer layer present

- -

H 6 0.4 not observed - -

I 0 0.7 not observed - -

J 2 0.7 no nanorods, only co-block polymer layer present

- -

K 4 0.7 no nanorods, only co-block polymer layer present

- -

L 6 0.7 not observed - -Discussion

We investigated methods of etching ZnO nanorods forests on block-co polymer, ITO substrates. After several trials, we obtained our most reliable results from tartaric acid of 0.1mM for a period of less than 10 minutes.

Because ZnO nanorods do not generate excitons themselves, but only transfer electrons, it would seem beneficial to limit their size accordingly. Etching of ZnO nanorods for use in PSCs has the potential to improve device performance. Dr. Su Huang fabricated and tested a P3HT-based device (P3HT was spin-coated over an etched ZnO nanorod forest over a block-co polymer and ITO substrate, over which MoO3 and Ag were thermally evaporated) and found that its performance was on par with that of a non-etched P3HT device. Although, no noticeable improvement was found among the few devices that we fabricated and tested it remains possible that such devices will realize improved performance with additional testing.

In terms of charactering etching of nanorod forests, we believe that there are three general modes: 1) rods get progressively shorter and thinner; 2) rods are etched to cones, which are progressively thinner and longer; 3) rods get shorter, with tips etched to cones with similar angles at their apex. Among our preliminary findings, we did not find differences in average nanorod diameter between etched and non-etched nanorods when we examined areas below which the diameter (of a given nanorod) was relatively constant. Additionally, among etched rods, we did not find that the heights of resultant cones were markedly different. Therefore, based on our preliminary findings, we believe that etching modes #2 and #3 are mostly likely.

Indeed, it seems that there may not necessarily be changes in nanorod diameter, with concurrent changes in nanorod height because the nanorods are being progressively etched from the top-down yielding a sharpened, spear–like appearance; however, this relationship may be more dependent on the spatial density of nanorod forests, rather than the degree of exposure to an acidic environment. Going forward we propose that more sophisticated digital analysis of SEM images may be able to distinguish between the two most likely modes by which nanorod forests are etched.

figure 1SEM image of sample A (day 5) showing ZnO nanorods not exposed to acid, with a line drawn across each nanorod that had distinguishable boundaries.

figure 2SEM image of sample A (day 5) showing only the lines drawn across each nanorod.

figure 3SEM image of sample D (day 5) showing etched ZnO nanorods not exposed to acid, with a line representing the height of a given cone for each nanorod that had distinguishable boundaries.

figure 4SEM image of sample D (day 5) showing the lines drawn representing the height of a given cone.

figure 5SEM image of sample C (day 5) showing etched ZnO nanorods.

figure 6SEM image of sample D (day 5) showing etched ZnO nanorods.

figure 7SEM image of sample F (day 5) showing etched ZnO nanorods.