The key factor in long-term use of batteries 9 is the formation of an electrically insulating solid layer that 10 allows lithium ion transport but stops further electrolyte 11 redox reactions on the electrode surface, hence solid 12 electrolyte interphase (SEI). We have studied a common 13 electrolyte, 1.0 M LiPF 6 /ethylene carbonate (EC)/diethyl 14 carbonate (DEC), reduction products on crystalline silicon 15 (Si) electrodes in a lithium (Li) half-cell system under 16 reaction conditions. We employed in situ sum frequency 17 generation vibrational spectroscopy (SFG-VS) with inter-18 face sensitivity in order to probe the molecular 19 composition of the SEI surface species under various 20 applied potentials where electrolyte reduction is expected. 21 We found that, with a Si(100)-hydrogen terminated wafer, 22 a Si-ethoxy (Si-OC 2 H 5) surface intermediate forms due to 23 DEC decomposition. Our results suggest that the SEI 24 surface composition varies depending on the termination 25

A phase transition within the ligand shell of core/shell quantum dots is studied in the prototypical system of colloidal CdSe/CdS quantum dots with a ligand shell composed of bound oleate (OA) and octadecylphosphonate (ODPA). The ligand shell composition is tuned using a ligand exchange procedure and quantified through proton NMR spectroscopy. Temperaturedependent photoluminescence spectroscopy reveals a signature of a phase transition within the organic ligand shell. Surprisingly, the ligand order to disorder phase transition triggers an abrupt increase in the photoluminescence quantum yield (PLQY) and full-width at half maximum (FWHM) with increasing temperature. The temperature and width of the phase transition shows a clear dependence on ligand shell composition, such that QDs with higher ODPA fractions have sharper phase transitions that occur at higher temperatures. In order to gain a molecular understanding of the changes in ligand ordering, fourier-transform infrared and vibrational sum frequency generation spectroscopies are performed. These measurements confirm that an order/disorder transition in the ligand shell tracks with the photoluminescence changes that accompany the order disorder ligand phase transition. The phase transition is simulated through a lattice model that suggests that the ligand shell is well-mixed, and does not 1 have completely segregated domains of OA and ODPA. Furthermore, we show that the unsaturated chains of OA seed disorder within the ligand shell.