We study the properties of water molecules adjacent to a hydrophobic molecular layer with vibrational sum-frequency generation spectroscopy. We find that the water molecules at D2O/hexane, D2O/heptane, and D2O/polydimethylsiloxane interfaces show an enhanced ordering and stronger hydrogen-bond interactions than the water molecules at a D2O/air interface. With increasing temperature (up to 80 °C) the water structure becomes significantly less ordered and the hydrogen bonds become weaker.

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The bandgap tunability of mixed-halide perovskites makes them promising candidates for light-emitting diodes and tandem solar cells. However, illuminating mixed-halide perovskites results in the formation of segregated phases enriched in a single halide. This segregation occurs through ion migration, which is also observed in single-halide compositions, and whose control is thus essential to enhance the lifetime and stability. Using pressure-dependent transient absorption spectroscopy, we find that the formation rates of both iodide- and bromide-rich phases in MAPb(Br x I 1– x ) 3 reduce by 2 orders of magnitude on increasing the pressure to 0.3 GPa. We explain this reduction from a compression-induced increase of the activation energy for halide migration, which is supported by first-principle calculations. A similar mechanism occurs when the unit cell volume is reduced by incorporating a smaller cation. These findings reveal that stability with respect to halide segregation can be achieved either physically through compressive stress or chemically through compositional engineering.

As a surface-specific technique, vibrational sum-frequency generation (SFG) is used in a wide range of applications where soft matter or solid interfaces are to be probed on a molecular level through their vibrational modes. In recent years, phase-specific sum-frequency generation (PS-SFG, also known as heterodyne-detected SFG) spectroscopy has been increasingly replacing its predecessor (direct SFG, also known as homodyne SFG) as the experimental technique of choice for characterizing interfacial structure. The technique enables phase sensitive measurements, allowing for the determination of the real and imaginary parts of the interfacial vibrational response function and thereby the unambiguous identification of molecular orientation. This phase-sensitivity requires, however, a complete understanding of the complex optical properties of the sample and of their effect on the signal. These optical properties significantly influence the raw spectral data from which the real and imaginary parts of the second-order susceptibility are retrieved. We show that it is essential to correct the data appropriately to infer the true molecular response. The current study presents a detailed description of the physical contributions to the phase-resolved spectrum, allowing a direct comparison between the phase-resolved spectrum and that obtained using the well-understood direct detection method in a step-by-step data analysis process. In addition to phase sensitivity, PS-SFG has been shown to increase the sensitivity compared to traditional (direct) SFG spectroscopy. We present a quantitative comparison between theoretical limits of the signal-to-noise ratio of both techniques, which shows that for many systems the signal-to-noise ratio is very similar for direct- and phase-specific SFG signals.

We study the structure and orientation of water molecules at water/alkane and water/polydimethylsiloxane interfaces with surface specific intensity and heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. We observe that the hydrogen-bond structure of the water molecules is enhanced at these interfaces compared to the water/air interface. We also find that the water molecules at the interface show a net orientation of their O-H groups pointing towards to the hydrophobic layer.