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The lubricating properties of water have been discussed extensively for millennia. Water films can exhibit high friction in the form of cold ice, or act as lubricants in skating and skiing when liquid. At the fundamental level, friction is the result of a balance between the rate of energy generation by phonon excitation during sliding, and drainage of the energy from the interface by coupling with bulk atoms. Using atomic force microscopy (AFM) we found that when H 2 O water intercalates between graphene and mica it increases the friction between the tip and the substrate, dependent on the thickness of water and graphene layers, while the magnitude of friction increase was reduced by D 2 O water intercalation. With the help of first-principles densityfunctional theory calculations we explain this unexpected behavior by the increased spectral range of vibration modes of graphene caused by water, and by the better overlap of the graphene vibration modes with the mica phonons, which favors more efficient dissipation of the energy. The larger increase in friction with H 2 O vs D 2 O shows that the high-frequency vibration modes of water molecules play a very important role in the transfer of the vibrational energy of the graphene to the phonon bath of the substrate.

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We report colloidal growth of PbSe nanosheets and finely size-tuned PbSe nanocrystals (NCs) via simple control of reaction parameters. The approach involves slow injection of precursors with excess amounts of oleic acid. Retarded growth, due to both the slow supply of precursors and the surfeit of oleic acid, causes attachment of PbSe NCs through the (110) planes, which are more reactive than the (100) facet, into a two-dimensional geometry. In contrast, such attachment processes can be prevented by impurities, e.g., Cd chalcogenide (CdSe or CdS) NCs dispersed in chloroform. For instance, the slow injection of Pb and Se precursors into a reaction solution containing Cd chalcogenide NCs results in the growth of spherical PbSe NCs, as the Cd chalcogenide NCs hinder the PbSe nuclei from merging via (110) planes. Compared to conventional rapid-injection methods, PbSe NCs grow slowly, which enables fine control of NC size. Ab initio calculations suggest that Cd precursors strongly bound on the surface of PbSe NCs may impede nanosheet formation and may slow PbSe NC growth.

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