(461b) Understanding Ion-Mobility Of Aerosol Nanowires And Its Application To The Measuement Of Length

In a previous experimental study, we demonstrated that gas phase electrophoretic mobility separation enables one to classify diameter-selected carbon nanotubes (CNTs) by length [Kim, S. H. and Zachariah, M. R. Nanotechnology 16 (2005) 2149-2152] and have subsequently used this capability to track the growth rate of CNT's in free-flight [Kim, S. H. and Zachariah, M. R., J. Phys. Chem. B 110 (2006) 4555-4562]. In this talk, we also develop a theoretical model to describe the behavior of nanotubes (or nanowires) undergoing Brownian rotation in an electric field, and provide a more rigorous interpretation of the experimental results. The Bolzmann expression for the orientation probability includes both the free charge energy as well as the polarizability energy. In the theoretical model, we computed the orientation-averaged electrical mobility and the precipitating time of a rotating nanowire in DC electric field in the free-molecular limit. This analysis was used to obtain the precipitation time of a nanowire in a differential mobility analyzer (DMA). Based on the theoretical model and its comparison with experimental measurement, we found that (i) a stronger electric field was required to select longer nanowires for a given diameter and (ii) shorter nanowires with the aspect ratio of <~30 (df=15 nm) freely rotate for applied electric field up to ~1kV/cm, while longer nanowires (>~30) were aligned by the electric field. The experimentally determined nanowire length was in fair agreement with that computed by our theoretical model, in which the effect of the nanowire's alignment on mobility classification in various electric fields was accounted for. The over prediction of the model for longer nanowires is shown to result from the bent structure of the longer nanowires. The methodology should be generic to other nanostructures with a high aspect ratio.