1. Environmental effects on the electrical properties of carbon nanotubes

Dan McCulley

The band gap of a bulk semiconductor is considered an intrinsic material property. In contrast, the band gap of a nanoscale semiconductor, such as a carbon nanotube (CNT), is sensitive to extrinsic factors. Our recent experiments test a long-standing prediction that CNT band gap will shrink almost 2-fold when the environment is changed from air to high-k dielectric. We have fabricated field-effect transistor devices from suspended semiconducting CNTs and measured electrical characteristics in a variety of dielectric environments, including air, vacuum, TiO2 coatings, and molecular liquids such as oil and isopropanol (k = 18). Some changes in electrical properties can be trivially explained by changes in electrostatic disorder, gate capacitance, mobility and band alignment. But, significantly, we find evidence for a nearly 2-fold band gap reduction when the environment changes from air to IPA. Two our knowledge this is a largest environmentally-induced change of band gap ever observed.

2. Frequency-dependent micromechanics of cellularized biopolymer networks

Chris Jones

Mechanical interactions between cells and the extracellular matrix (ECM) influence many cellular behaviors such as growth, differentiation, and migration. These are dynamic processes in which the cells actively remodel the ECM. Reconstituted collagen gel is a common model ECM for studying cell-ECM interactions \textit{in vitro} because collagen is the most abundant component of mammalian ECM and gives the ECM its material stiffness. We embed micron-sized particles in collagen and use holographic optical tweezers to apply forces to the particles in multiple directions and over a range of frequencies up to 10 Hz. We calculate the local compliance and show that it is dependent on both the direction and frequency of the applied force. Performing the same measurement on many particles allows us to characterize the spatial inhomogeneity of the mechanical properties and shows that the compliance decreases at higher frequencies. Performing these measurements on cell-populated collagen gels shows that cellular remodeling of the ECM changes the mechanical properties of the collagen and we investigate whether this change is dependent on the local strain and distance from nearby cells.