Studies of the Dynamics of Alkane Nanoparticles

Our AFM and synchrotron x-ray scattering measurements on dotriacontane (C$_{32}$H$_{66}$ or C32) deposited on SiO$_{2}$-coated Si(100) substrates reveal mesa-shaped nanoparticles that have an orthorhombic structure in which the C32 molecules are aligned perpendicular to the SiO$_{2}$ surface.$^{2}$ To investigate their dynamical properties, we have used both the backscattering and disk chopper spectrometers at NIST covering a wide range of time scales (1 ps -- 4 ns). Elastic scans obtained on both spectrometers show step-like changes in intensity as a function of temperature indicating the existence of phase transitions below the bulk C32 melting point. One of these steps occurs at the crystalline-to-rotator phase transition of bulk C32, but there is a second step at still lower temperature of unknown origin. The similarity of the elastic scans on the two spectrometers suggests that the crystalline-to-rotator phase transition involves diffusive motion spanning a wide range of time scales. $^{2}$M. Bai\textit{ et al.}, Europhys. Lett. \textbf{79}, 26003 (2007).