Fabrication of Nano-porous Templates Using Molecular Self-Assembly of Block Copolymers for the Synthesis of NanostructuresLuke Soule and Jason TresbackCenter for Nanoscale Systems, Harvard University

Introduction

Traditional methods of fabricating nanowire and nanodot arrays are expensive and time consuming. A method for fabricating dense nanowires and dots on any substrate with any material deposited is highly favorable for solar cell and energy applications. Block copolymers have been used for fabricating nanostructures by several, mostly Russell et al [1] but few have used them for solar and energy applications. In the present work, PS-b-PMMA was spun cast on conductive, insulating, and semiconductor materials and used to make hexagonal “honeycomb” structures on the order of 30nms. Ni and Au were electrodeposited on the patterned substrate. Post lift-off, high density nanoparticle arrays were synthesized. MnO2 will be deposited, and dense nanowire arrays fabricated quickly and cheaply are highly desirable for several applications. Au nanoparticle arrays are favorable for plasmon enhanced solar cell applications. Nickel nanodots are useful for catalyst applications.

1 Gu, X., I. Gunkel, and T. P. Russell. "Pattern Transfer Using Block Copolymers."Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering

Sciences 371.2000 (2013): 20120306. Web..

Acknowledgements

S

Conclusions and Future Work

Experimental Methods

Block Copolymer Self Assembly

Results

Results

This work is funded by the NSF under the NNCI program. Program part of NSF summer REU program at Harvard University. Fabrication and analysis was performed at the Center for Nanoscale Systems at Harvard University. Electrodeposition was done at the Nocera lab at Harvard University.

This AFM phase image shows close hexagonal packing. Pores are around 40nms in diameter and are spaced around 35nms apart. This is 3 layers of PS-b-PMMA on a SiO2 on Si substrate.

Post development, an AFM height image was generated using a AC240BSA high aspect ratio tip. Line scan shows spacing and diameter. 1 layer ITO on glass

Nanoporous templates were developed on a variety of substrates. Templates were used to synthesize nanodot arrays of nickel and gold. Arrays can be used for plasmon enhanced solar cells, catalysts, and many other applications. Assembly was also seen on flexible substrates, which has a big potential for solar development. Nanoporous assembly also occurred between electrodes, allowing for electrical measurements of nanostructure arrays. MnO2 nanodot and wire arrays will be developed.

A Block copolymer is a long strand polymer that consists of groups of monomers “blocked” next to one another in a repeated fashion.

Where A is a polystyrene monomer and B is a methyl-methacrylate monomer.

Due to unfavorable segmental interactions, the two monomer blocks would like to repel each other. After annealing, the two strands self assemble into a variety of shapes and structures. The size of the structures is dictated by volume fraction and molecular weight.

AABBAABB

The 2:1 PS-b-PMMA, 82 kilodaltons block copolymer was used for synthesizing templates. Ag on Si, SiO2 on Si, and ITO on glass were the substrates used. A random brush copolymer was anchored to the surface to neutralize surface interactions.

The di-blocks were dissolved in toluene and spun cast on substrates at 4,000 RPM. Either 1 layer or 3 layers were spun based on desired thickness. Annealed for 48 hours.

Post anneal, templates were developed using UV, 200-400 mJ. This crosslinked PS and degraded PMMA. After a 5 minute acetic acid rinse, the templates were clean and ready for electrodeposition.

Nickel and gold were electrodeposited in microdomains. Using an acetone liftoff, very dense arrays of nanodots and nanowires were synthesized.

Si

Ag

PS/PMMA

Brush

Nickel and gold were deposited. AFM height image shows an average diameter of 50nm and a height of 20nm. Images show nickel (left) and gold (right) nanodots.

Deposits were also imaged and quantified using SEM. Image of nickel nanodots.