RESEARCH

01  ||  Designing hierarchical material function from molecular to macroscopic scales.

A surprisingly broad range of problems in modern materials chemistry relates to structuring soft matter at scales well below 10nm. In this limit, an increasingly large fraction of functional groups lie at a boundary between two distinct chemical environments, which changes their behavior. We examine the behavior of functional groups in dimensionally confined environments, with a particular interest in defining structural motifs that use these environments to advantage for the design of new materials. In order for these precisely structured environments to be useful, we also develop routes to simultaneously control structure at sub-10-nm to macroscopic scales, and to maximize stability toward common device processing conditions.

02  ||  Chemistry in confinement.

Properties of matter change at the smallest scales. How can we take advantage of these changes in designing soft materials near the molecular limit? Inorganic nanostructures exhibit confinement effects that change their physical properties; often these are described in terms of a 'particle in a box' model that quantifies changes in electronic behavior based on material dimension. Similarly, the behavior of organic functional groups at interfaces differs from those in bulk solutions, which becomes increasingly important as larger fractions of atoms are located at an interface. Because Angstrom-scale changes in position of a functional group relative to an interface change function, we leverage nanoscale characterization techniques including scanning probe microscopy to examine sub-nm-scale structure. Combining this information with characterization of physical properties at larger scales (such as contact angle measurements) enables us to derive new material design principles. We are inspired by the chemistry of phospholipids, which create well-defined 2D polar/nonpolar boundaries at the cell membrane periphery that enable a broad range of chemistry central to normal biological function. The chemical diversity of lipids is vast -- many hundreds of structurally different amphiphiles use precisely arranged sets of functional groups, as well as a striking diversity of alkyl chains, to set the boundary structure. Transforming these molecules into striped phases, in which the alkyl chains lie parallel to the substrate, creates 1-nm-wide functional patterns of polar headgroups within a predominantly nonpolar layer structure. We find that these highly confined polar environments generate surprising new chemistry at the interface.

03  ||  Maximizing processability, scalability, and function per mass.

Designing materials with features at the 1-nm scale means building efficiently, with minimal waste. Simultaneously, to be useful in many applications, structures must be stable enough to survive reasonable device processing conditions, and scalable to areas of square centimeters or even square meters. Striped phases are similar in thickness to a graphene sheet (nearly an order of magnitude thinner than a conventional standing phase monolayer), but embed desired functional groups in nm-wide arrays. At the same time, the noncovalent nature of the assembly raises significant questions related to robustness and utility for practical applications. Our group develops routes for achieving very long range order (>100 μm2), improving processability (e.g. resistance toward desorption during solution processing) and for utilizing the functions embedded in the ultrathin molecular layer (e.g. in assembling nm-scale inorganic components on the template layer).

04  ||  Replicating structure and function in soft matter.

Controlling chemical information in materials across length scales from molecules to the macroscale provides new avenues to replicate function in complex matter, such as human tissue. Ongoing work in the group aims to apply scalable nanoscopic chemical and mechanical building blocks to design soft frameworks integrating critical functional elements to achieve control over the growth and differentiation of cells.

05  ||  Training the next generation of interdisciplinary scientists.

The questions we ask require us to design and synthesize new molecules with desired chemistry, assemble them, and characterize their function. Therefore, the research group is very interdisciplinary, drawing scientists with backgrounds ranging from materials characterization to organic and inorganic synthesis and theory. Students utilize facilities both within the laboratory and at the department's Analytical Instrumentation Center, and Purdue's Birck Nanotechnology Center and Life Sciences Microscopy Center. Techniques we use include scanning probe microscopies (including 2 AFMs and an STM as part of our lab), SEM, TEM, contact angle goniometry, XPS, fluorescence microscopy, CD, and PM-IRRAS. Students may also develop new instrumentation in conjunction with the Amy Instrumentation Facility, a unique resource in the Purdue Department of Chemistry.