Research Experience

Non-equilibrium Self-Assembly

Self-assembly is canonically thought of as an equilibrium process, where the materials order according to energy-minimization. The important role of non-equilibrium effects is becoming increasingly clear: self-assembly is influenced by kinetic trapping, metastable states, coexisting phases, and process-history dependent order. While this is often viewed as a nuisance (kinetics limiting the quality of order that one can easily achieve), it is also possible to exploit these non-equilibrium effects to control order-formation in self-assembly.

I am working on a suite of methods to control the ordering of block-copolymer thin films, exploiting non-equilibrium effects. For instance, I demonstrated how extreme thermal gradients, generated using photothermal methods, can enhance grain coarsening kinetics, and allow the morphology to be aligned. Moreover, these methods allow one to probe the complex pathway-dependent ordering histories of block-copolymers. More generally, I am interested in demonstrating how contrived arrangements of block-copolymer materials (e.g. layering) can be used to induce the formation of morphologies that do not appear in the native (bulk equilibrium) phase diagram.

Templated Self-Assembly (Postdoctoral Research)

I worked as a postdoctoral guest researcher at NIST, on problems in templated and directed self-assembly. In particular, I studied how model self-assembling systems (especially block-copolymers) can be influenced by a variety of directing forces, such as topographical templates, rough interfaces, and thermal fields. For instance, I showed that robust biasing of the self-assembly process can be achieved using simple nanoparticle-treated substrates, since these inherently rough surfaces drastically alter the self-assembly energy landscape.

Integral to my work at NIST was the development of measurement techniques and methodologies for characterizing nanostructures. In particular we identified through industrial partners that knowledge of the orientational distribution of structures is a crucial parameter for serious use of self-assembly. For instance, alignment of block-copolymer cylinders can be used as a mask for subsequent etching, enabling production of extremely small-scale devices. The angular distribution of the cylinder axes plays a critical role in determining the quality of any pattern transfer, yet many techniques (e.g. AFM) can only probe the surface order. I thus been developed scattering techniques (rotational SANS, GISAXS, off-specular reflectivity, etc.) in order to quantitatively determine the orientational order in these systems.

This work generated considerable interest in the materials science and scattering communities. My research was highlighted in a NIST press release. After presenting at the American Physical Society in New Orleans, our work was highlighted in APS News (May 2008), and in an issue of Nanomaterials News (Vol 4, issue 4, 22 April 2008). We presented the details of our scattering work at the American Conference on Neutron Scattering in Santa Fe, where Robert Briber (U. Maryland) highlighting our work on rotational SANS in the plenary lecture. My talk was also selected for a writeup for the meeting summary. This work was recognized by the NIST Materials Laboratory with the prestigious Associate Award, given to guest researchers who have made a substantial individual contribution to NIST's scientific mission. Our related work on how stress can control block-copolymer morphology was highlighted in an issue of C&E News.

Photoresponsive Polymers (Ph.D. Research)

My Ph.D. work at McGill University focused on the peculiar all-optical patterning known to occur in the azobenzene-polymer system. In 1995, it was discovered that the free surface of azo-polymer thin films would spontaneously deform in response to light gradients. In essence, the material is forming a surface hologram that reproduces any impinging light pattern. This single-step, all-optical patterning/lithography is fast, efficient, and indefinitely stable at room temperature. Multiple holograms can be superimposed, and in fact the features can be thermally erased. Thus the process is stable yet reversible when required. The fundamental mechanism of this process was not understood, and my Ph.D. was largely geared towards solving this question.

I first wrote a cellular automata computer model to analyze the temperature distribution during laser inscription. This work allowed me to exclude thermal models as possible explanations for the mass-transport phenomenon. I also measured the thermal erasure of photo-induced surface features, using a variety of techniques from combinatorial material science. These experiments showed conclusively that the mass-transport requires coordinated polymer motion over long length-scales (~150 nm) and not merely molecular diffusion. Finally, neutron reflectivity studies were used to identify photo-mechanical effects in this system. It was found that light can be used to photo-expand and photo-contract the azo material. These photo-mechanical effects appear to be the origin for the unique mass transport.