3 research outputs found
Band alignment at interfaces of two-dimensional materials: internal photoemission analysis
The article overviews experimental results obtained by applying internal photoemission (IPE) spectroscopy methods to characterize electron states in single- or few-monolayer thick two-dimensional materials and at their interfaces. Several conducting (graphene) and semiconducting (transitional metal dichalcogenides MoS2, WS2, MoSe2, and WSe2) films on top of thermal SiO2 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.status: publishe
Excitation Intensity- and Size-Dependent Halide Photosegregation in CsPb(I<sub>0.5</sub>Br<sub>0.5</sub>)<sub>3</sub> Perovskite Nanocrystals
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(I0.5Br0.5)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
Evaluation of the effective work-function of monolayer graphene on silicon dioxide by internal photoemission spectroscopy
© 2019 Elsevier B.V. Internal photoemission of electrons from uncapped monolayer graphene to 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 oxide conduction band bottom was reproducibly found to be 4.1–4.2 eV. Moreover, this value was weakly sensitive to the contacting metal work function (Al, Cu, Au). This barrier height corresponds to an effective work function of graphene close to 5.0 eV, which is nearly 0.5 eV higher than the usually reported vacuum value.status: publishe