Xingkun Man

Tailoring nanostructures using copolymer nanoimprint lithography.

The generation of defect-free polymer nanostructures by nanoimprinting methods is described. Long-range nanorheology and shorter-range surface energy effects can be efficiently combined to provide alignment of copolymer lamellae over several micrometers. As an example, a perpendicular organization with respect to circular tracks is shown, demonstrating the possibility of writing ordered radial nanostructures over large distances.

Responsive behaviors of diblock polyampholyte brushes within self-consistent field theory.

Self-consistent field theory (SCFT) calculation has been performed to study the structure and stimuli-responsive behaviors of diblock polyampholyte (PA) brushes. Two kinds of brushes are considered: one formed by PA chains consisting of two strong polyelectrolyte blocks (system i) and the other formed by PA chains of a grafted strong acid block and an ungrafted weak base block (system ii). Density profiles and brush thickness are obtained. The chain trajectory (average position of each polymer segment) is also calculated to characterize the conformation of the grafted chains. For system i, the ungrafted blocks loop backward at low salt concentration and extend out at high salt concentration. For system ii, the charge fraction of the annealing block is independent of pH and becomes dependent on it at high salt concentration. As a result, pH has no effect on the brush structure at low salt concentration and takes effect at high salt concentration. That the salt concentration can switch on and off the responses of the PA brush to the pH stimuli may find application in building functional surfaces.

Organization of Block Copolymers using NanoImprint Lithography: Comparison of Theory and Experiments

We present NanoImprint lithography experiments and modeling of thin films of block copolymers (BCP). The NanoImprint technique is found to be an efficient tool not only to align lamellar phases perpendicularly to the substrate but also to get rid of in-plane defects over distances much larger than the natural lamellar periodicity. The modeling relies on self-consistent field calculations done in two- and three-dimensions and is found to be in good agreement with the experiments. It also offers some insight on the NanoImprint lithography setup and on the conditions required to perfectly ordered BCP lamellae.

Block copolymer films with free interfaces: ordering by nanopatterned substrates.

We study block copolymers (BCPs) on patterned substrates, where the top polymer film surface is not constrained but is free and can adapt its shape self-consistently. In particular, we investigate the combined effect of free interface undulations with wetting of the BCP film as induced by nanopatterned substrates. Under wetting conditions and for a finite volume of BCP material, we find equilibrium droplets composed of coexisting perpendicular and parallel lamellar domains. The self-assembly of BCPs on topographic patterned substrates is also investigated and it is found that the free interface induces mixed morphologies of parallel and perpendicular domains coupled with a nonflat free interface. Our study has some interesting consequences for experimental setups of graphoepitaxy and nanoimprint lithography.

Self-consistent field theory of directed self-assembly on chemically prepatterned surfaces

We use self-consistent field theory to investigate the self-assembly of AB diblock copolymers on chemically patterned substrates. The striped substrates consist of attractive domains alternating with neutral ones. Using our simulations, we observe the formation of different self-assembled morphologies in the polymer film and compute their formation free energy for various pattern widths and wetting conditions. We found that perpendicular lamellae are best formed when the A-attractive stripes have a width near half the natural lamellar period in bulk, but lamellar structures with wide bases are also formed in wide stripes. We also computed the formation energy of isolated dislocations and found that the energy cost is ≈ 50-110kT for ±15nm deviations from optimal commensurability conditions.

Defect-Free Perpendicular Diblock Copolymer Films: The Synergy Effect of Surface Topography and Chemistry

We propose a directed self-assembly mechanism toward obtaining defect-free perpendicular lamellar phases of diblock copolymer (BCP) thin films. In our numerical study, a thin BCP film having a flat top surface is cast on a unidirectional corrugated solid substrate. The substrate is treated chemically and has a weak preference toward one of the two BCP components. Employing self-consistent field theory (SCFT), we find that there is an enhanced synergy between two substrate characteristics: its topography (geometrical roughness) combined with a weak surface preference. This synergy produces the desired perpendicular lamellar phase with perfect in-plane ordering. Defect-free BCP lamellar phases are reproducible for several random initial states and are obtained for a range of substrate roughness and chemical characteristics, even for a unidirectional multimode substrate roughness. Our theoretical study suggests possible experiments that will explore the interplay between unidirectional substrate corrugation ...