Although broad consensus exists that photoirradiation of mixed-halide lead perovskites leads to anion segregation, no model today fully rationalizes all aspects of this near ubiquitous phenomenon. Here, we quantitatively compare experimental, CsPb(I 0.5 Br 0.5 ) 3 nanocrystal (NC) terminal anion photosegregation stoichiometries and excitation intensity thresholds to a band gap-based, thermodynamic model of mixed-halide perovskite photosegregation. Mixed-halide NCs offer strict tests of theory given physical sizes, which dictate local photogenerated carrier densities. We observe that mixed-anion perovskite NCs exhibit significant robustness to photosegregation, with photosegregation propensity decreasing with decreasing NC size. Observed size-and excitation intensity-dependent photosegregation data agree with model predicted size-and excitation intensity-dependent terminal halide stoichiometries. Established correspondence between experiment and theory, in turn, suggests that mixed-halide perovskite photostabilities can be predicted a priori using local gradients of (empirical) Vegard's law expressions of composition-dependent band gaps.

Internal Photoemission (IPE) of electrons from uncapped monolayer graphene into insulating SiO 2 has been observed in samples prepared by water-intercalation based graphene transfer.The barrier height between the graphene Fermi level and the bottom of the oxide conduction band was reproducibly found to be 4.1-4.2 eV and weakly sensitive to the work function of the metal used to contact graphene monolayer (Al, Cu, Au). This barrier height corresponds to the effective work function of graphene close to 5.0 eV, which is nearly 0.5 eV higher than the usually reported vacuum value. Highlights• Internal photoemission of electrons has been detected from uncapped monolayer graphene • Effective work function of graphene has been determined to be 5.0 eV • EWF of graphene at the interface with SiO 2 is 0.5 eV higher than the vacuum value

The article overviews experimental results obtained by applying Internal PhotoEmission (IPE) spectroscopy methods to characterize electron states in single-or few-monolayer twodimensional (2D) materals and at their interfaces. Several conducting (graphene) and semiconducting (transitional metal dichalcogenides MoS2, WS2, MoSe2, and WSe2) films have been analyzed by IPE, which reveals significant sensitivity of interface band offsets and barriers to the details of the material and interface fabrication indicating violation of the Schottky-Mott rule. This variability is associated with charges and dipoles formed at the interfaces with van der Waals bonding as opposed to the chemically bonded interfaces of three-dimensional semiconductors and metals. Chemical modification of the underlying SiO2 surface is shown to be a significant factor, affecting interface barriers due to violation of the interface electroneutrality.

Key features of syntheses, involving the quaternary ammonium passivation of CsPbBr3 nanocrystals (NCs), include stable, reproducible, and large (often near-unity) emission quantum yields (QYs). The archetypical example involves didodecyl dimethyl ammonium (DDDMA+)-passivated CsPbBr3 NCs where robust QYs stem from interactions between DDDMA+ and NC surfaces. Despite widespread adoption of this synthesis, specific ligand–NC surface interactions responsible for large DDDMA+-passivated NC QYs have not been fully established. Multidimensional nuclear magnetic resonance experiments now reveal a new DDDMA+-NC surface interaction, beyond established “tightly bound” DDDMA+ interactions, which strongly affects observed emission QYs. Depending upon the existence of this new DDDMA+ coordination, NC QYs vary broadly between 60 and 85%. More importantly, these measurements reveal surface passivation through unexpected didodecyl ammonium (DDA+) that works in concert with DDDMA+ to produce near-unity (i.e., >90%) QYs.

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