My Ph.D. work 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.
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.
I'm currently working as a materials scientist at Brookhaven National Lab. My research in the nanocenter focuses on how processing effects (thermal fields, optical fields, nanoparticle additives, etc.) can drastically alter self-assembly behavior. These 'stimulated self-assembly' effects can in fact be used to control morphology formation, orient nanostructures, and alter assembly kinetics and pathways.
In addition, I'm responsible for the CFN involvement in the X9 x-ray scattering beamline at NSLS. The X9 beamline is available to external users through a peer-reviewed proposal system. Potential users can contact me if they wish to learn more about the instrument's capabilities, or how to become a user.