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Asymmetric diblock copolymer materials are known to self assemble into spherical microdomains, with spheres composed of the copolymer’s minority block formed within a matrix of the majority block.  In a thin film, these microdomains spontaneously arrange themselves into a locally well-ordered hexagonal lattice; local orientation extends over regions, or “grains,” with well defined boundaries.  Above the copolymer’s glass temperature, diffusion of molecules can lead to migration of microdomains and of grain boundaries.  One goal of this project is to measure grain boundary velocities as a function of their curvature to better understand the dynamics of these systems. 

Our experiment consists of preparing films with specifically tailored, long and narrow grains, and observing their subsequent evolution through atomic force microscopy (AFM).  Grains will be created by selectively aligning the spherical microdomains in a small area of a film using a shear stress applied by a flattened cantilever tip on an AFM.  The challenge in creating grains by localized shear stress has been to control the positioning of the AFM tip to remain close enough to the surface to generate sufficient lateral shear while avoiding damage to the molten polymer surface by direct contact with the tip.  We have tried to address this challenge in two ways: first by nullifying any preferential wetting between the tip and the sample by operating in a fluid (silicone oil) environment, and second by directly monitoring the deflection of the tip and adjusting its position accordingly as part of an active closed-loop feedback control system.   To date we have not succeeded in solving this vexing technical hurdle, but we continue to refine our techniques and remain reasonably optimistic that a solution is possible. 

Our work in two closely related areas also continues.  The first area involves the fundamental phase behavior of block copolymer materials on substrates in the presence of step edges.  It is well known that diblock copolymer films with thicknesses incommensurate with the microdomain lattice spacing can spontaneously assemble into regions of commensurate thicknesses.  It has also been observed that on substrates with long, straight, step-edge features, the microdomains in a block copolymer film can become preferentially aligned with the step edges, producing an array of periodic microdomains that is locally aligned in a fixed orientation.   To understand the interplay between these effects, we have assembled a collaboration of researchers at the National Institute of Standards and Technology (NIST), Virginia Commonwealth University, and Princeton University to systematically investigate the morphology and alignment of block copolymer thin films near step edges as a function of both the film thickness and the step edge height, using a multidimensional combinatorial study based on techniques recently developed at NIST.  Our findings indicate that the hexagonal lattice formed by a single layer of spheres on the low side of a step edge is aligned along the direction of the step edge only where the film on the high side is sufficiently thin to support only a wetting layer of copolymer material.  We see this work in part as a guide for tuning these two parameters in future studies of graphoepitaxy in block copolymer films; a manuscript based on this work is currently in preparation. 

A second related area has been developing Atomic Force Microscopy (AFM) imaging techniques for accurate metrology of thin films.  A major obstacle to accurate AFM imaging is “thermal drift,” which can cause small positioning errors that grow slowly over time, leading to significant distortion in AFM images.  We have recently developed a new technique for correcting this distortion using post-processing software; the result of our work in this area is a fully automated, software-based method for removing nonlinear (up to third order) drift in two dimensions from a single AFM image, using only a small rescan area as a reference.  A manuscript based on our work, recently reviewed by a leading microscopy journal, is currently under revision and will be resubmitted soon.  In addition, we have worked this summer to extend our technique to correct for AFM artifacts in the vertical axis, including thermal drift as well as electrostatic charging and spontaneous transitions between bistable cantilever oscillation states.  If successful, a description of this work would be published as an additional manuscript.

Finally, we have worked in collaboration with M.C. Leopold on monolayer protected nanoparticle film assemblies, studying the adsorption and thermodynamic properties of azurin protein on the films. This research involves using nanoparticles as a scaffold for azurin protein adsorption and electrochemistry by first assembling a thin film of alkanethiol-protected gold nanoparticles on an electrode.  A key component of this work is characterizing the nanoparticle assembly and its effect on the gold topography, a factor known to affect the electrochemical signal of the adsorbed protein.  Atomic force microscopy has been used as a tool to assess changes in topography before and after the assembly of MPCs with the working hypothesis being that the layer of nanoparticls may provide a more homogeneous, molecular-level adsorption environment for the azurin protein.  We have recently presented our findings at an ACS regional conference, and are currently preparing a manuscript based on this work.