Theory and spatially resolved vibrational spectroscopy of confined water in carbon nanotubes

Water confined in carbon nanotubes (CNTs) has been a topic of significant experimental and theoretical interest for several decades. The nature of the confined water is still unclear. Atomic vibrations are the property directly correlated with molecular configurations and bonding. Here we report initial results of a comprehensive theory-experiments project on sample fabrication and characterization of water-filled CNTs, spatially-resolved scanning transmission electron microscopy (STEM) and monochromated (high energy resolution) electron energy loss spectroscopy (EELS) used to probe individual CNTS, and theoretical vibrational spectra based on density-functional-theory molecular-dynamics simulations. The EEL spectra show a peak at 460 meV with no corresponding peak at 420 meV (the O-H stretch mode of water at room temperature). Simulations have been performed for both single- and double-wall nanotubes, allowing full vibrational freedom for all carbon atoms. Comparison with results obtained with fixed-position carbon atoms enable detailed analysis of the origins of observed peaks and frequency shifts in terms of the bonding rearrangements caused by the CNT vibrations. The role of the effective confined-water density is elucidated via a novel way to characterize this density.