Structure control over soft matter on a molecular through nanoscopic lengthscale is a vital tool to optimizing properties for applications ranging from energy (solar and thermal) to biomaterials. For example, while molecular structure affects the electronic properties of semiconducting polymers, the crystal and grain structure greatly affect bulk conductivity, and nanometer lengthscale pattern of internal interfaces is vital to charge separation and recombination in photovoltaic and light emission effects. Similarly, biological materials gain functionality from structures ranging from monomeric sequence through chain shape through self-assembly. We work to both understand the effects of structure on properties and gain pattern control in these inherently multidimensional problems. We are particularly interested in materials for energy applications such as photovoltaics, fuel cells, and thermoelectrics.

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Why Control Structure?

Nanopatterning is the science of controlling the structure and behavior of matter at an intermediate scale between the molecule and the macroscopic world, that is the nanoscale (1-100 nm). Over the last several years, this field has garnered much attention as a primary step towards the fabrication of nanodevices. Many studies have demonstrated a sophisticated level of control over the self-assembling, coil-type polymer systems to produce nanometer scale patterns for lithography and secondary synthesis steps. Intricate block copolymer patterns may be formed and controlled by relying on the competition between unfavorable mixing and stretching of the dissimilar blocks.

Thermodynamics of Functional Polymer Self-Assembly

Nanoscale control and patterning of functional block copolymers presents a new challenge due to non-idealities in molecular conformation and mixing interactions that are present in these materials. In many cases, the functionality stretches the chain into a rod-like shape. Typical rod-like polymers include helical proteins and semiconducting polymers with rigid π-conjugated backbones. For example, rod-coil block copolymers with an amino acid-based rod blocks have been suggested as models for membrane structural proteins or DNA gels and for use as artificial membranes. There is also great interest in rod-coil block copolymers for use in organic electronics, where controlling the active layer morphology and interfacial structure in multicomponent devices on the 10 nm length scale of an exciton diffusion length is critical to optimizing photovoltaic device performance. As the stiffness of one of the blocks is significantly increased, however, a new class of intermolecular interactions is introduced including liquid crystallinity. A number of stunning and intriguing phases have been observed in rod-coil systems and the Segalman group has developed the first weakly segregated model system with which to probe the thermodynamics of self-assembly of these important materials.

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Example of Self-Assembling Functional Materials: Design Parameters of a Photovoltaic

The Segalman group is working on several projects in which we control the nanoscale structure of functional materials to enhance properties and performance. Specific applications range from energy (thermal, solar, and hydrogen fuel cells) to biology. Here, we'll show an example of how this works on a solar cell (phtovoltaic system).

Flexible, low cost plastic solar cells are an attractive alternative energy source however their viability is limited by poor efficiency. The cells are generally made of two materials: an electron donating (p-type) component in which light is absorbed to create an exciton (electron-hole bound pair) and an electron accepting (n-type) component which accepts the electron from the donor. The electron and hole are then transported back to the appropriate electrodes through the accepting and donating domains, respectively. This process dictates strict geometrical requirements on the device: the donor acceptor interfaces must be prevalent (occurring on order every 10nm) so that every created exciton may encounter an interface prior to recombination. Additionally, the donor and acceptor phases must form continuous pathways to the electrodes to allow for efficient charge transport and collection. This 10nm length scale patterning is difficult to achieve via conventional patterning techniques or by phase separation of polymer blends. Instead, we suggest the use of a bipolar block copolymer which self-assembles into the correct nanoscale pattern. While self-assembly is an alluringly inexpensive route to patterning on this length scale, the conjugated structure of a polymer backbone that leads to semiconducting properties, fundamentally alters thermodynamic behavior resulting in qualitatively different self-assembly. Our work centers on creating the controllable, predictable nanoscale patterns which are essential for efficient plastic electronics.

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Single Molecule Structure and Properties

About 90 percent of the world’s power (approximately 10 trillion Watts) is generated by heat engines that convert heat to mechanical motion, which can then be converted to electricity when necessary. However, nature requires us to pay a penalty for this process - all of the heat cannot be converted to power and about 15 trillion Watts of heat is released to the environment. If even a fraction of the low-grade lost heat can be converted to electricity in a cost-effective manner, the impact it would have on energy can be enormous, amounting to massive savings of fuel and atmospheric carbon dioxide. Thermoelectric energy converters can directly convert low-grade heat to electricity using semiconducting materials. These devices rely on a phenomenon called the Seebeck effect, in which a voltage is produced when a temperature differential is applied across a material. Dr. Segalman’s group, in collaboration with Dr. A. Majumdar, recently announced the discovery of the Seebeck effect in organic molecules when sandwiched between two metal particles. This is a significant step and major departure from traditional inorganic semiconductor materials. While the effect is quite small now, it is the first step in the direction of creating a new field of molecular thermoelectrics. Further research is being carried out to tune the relevant properties of the metal-molecule junctions using the chemistry of the molecules and its contact with the metal. The use of inexpensive organic molecules and metal nanoparticles offers the promise of low-temperature solution processing and low-cost plastic-like power generators and refrigerators. Furthermore, whenever molecules are connected to electrodes, a key feature is the alignment of the electronic energy levels which is critical for the operation and performance of molecular devices. In traditional microelectronics, this may be measured through the fabrication of a gated transistor, which is not currently possible on a single molecule lengthscale. This information, however, is also embedded in the measurement of thermopower. This is a fundamental step in the design and understanding of both molecular electronic devices for information processing and storage, as well as molecular solar cells for converting sunlight to electricity. This work is the subject of a recent article in Science.