Two-dimensional (2D) layered materials provide an ideal platform for engineering electronic and optical properties through strain control because of their extremely high mechanical elasticity and sensitive dependence of material properties on mechanical strain. In this paper, a combined experimental and theoretical effort is made to investigate the effects of mechanical strain on various spectral features of bilayer MoTe 2 photoluminescence (PL). We found that bilayer MoTe 2 can be converted from an indirect to a direct bandgap material through strain engineering, resulting in a photoluminescence enhancement by a factor of 2.24. Over 90% of the PL comes from photons emitted by the direct excitons at the maximum strain applied. Importantly, we show that strain effects lead to a reduction of the overall linewidth of PL by as much as 36.6%. We attribute the dramatic decrease of linewidth to a strain-induced complex interplay among various excitonic varieties such as direct bright excitons, trions, and indirect excitons. Our experimental results on direct and indirect exciton emission features are explained by theoretical exciton energies that are based on first-principles electronic band structure calculations. The consistent theory-experimental trend shows that the enhancement of PL and the reduction of linewidth are the consequences of the increasing direct exciton contribution with the increase of strain. Our results demonstrate that strain engineering can lead to a PL quality of the bilayer MoTe 2 comparable to that of the monolayer counterpart. The additional benefit of a longer emission wavelength makes the bilayer MoTe 2 more suitable for silicon-photonics integration due to the reduced silicon absorption.
Double doping, in which a single dopant molecule induces two charge carriers in an organic semiconductor (OSC), was recently experimentally observed and promises to enhance the efficiency of molecular doping. Here we present a theoretical investigation of p-type molecular double doping in a CN6-CP:bithiophene–thienothiophene OSC system. Our analysis is based on density functional theory (DFT) calculations for the electronic ground state. In a molecular complex with two OSC oligomers and one CN6-CP dopant molecule, we explicitly demonstrate double integer charge transfer and find the formation of two individual polarons on the OSC molecules and a dianion dopant molecule. We show that the vibrational modes and related infrared absorption spectrum of this complex can be traced back to those of the charged dopant and OSC molecules in their isolated forms. The near-infrared optical absorption spectrum calculated by time-dependent DFT shows features of both typical intramolecular polaron excitations and weak intermolecular charge transfer excitations associated with the doping-induced polaron states.
In article number 2100518, Johanna Heine and co‐workers report tunable white‐light emission from atomically thin sheets of the 1D hybrid perovskite [C7H10N]3[BiCl5]Cl. The unique structure enables self‐trapped‐exciton formation with white‐light emission, and reveals the thickness dependence of the exciton self‐trapping. This enables facile control of the emission color in next‐generation lighting and display technologies.
In the molecular doping of organic semiconductors (OSCs), achieving efficient charge generation and managing the energetic cost for charge release from local molecular charge-transfer complexes (CTCs) to the host matrix is of central importance. Experimentally, tremendous progress has been made in this direction. However, the relation between the OSC film structure on a nanoscopic level including different intermolecular geometrical arrangements and the macroscopic properties of doped OSC films is usually only established quite indirectly. Explicit microscopic insights into the underlying doping mechanisms and resulting electronic structure are still scarce and mostly limited to the study of the individual molecular constituents or isolated bimolecular dopant:host complexes. In the present study, we investigate n-type doping of the frequently investigated OSC materials ZnPC and F8ZnPc and their mixtures which are n-doped with 2-Cyc-DMBI in different dielectric environments. We report significant electronic differences for complexes with nominally the same material composition but different geometrical structures. One specific important finding in this context is that complexes containing two adjacent dopant molecules show much reduced ionization energy values which leads to substantially reduced energy cost for charge release. Furthermore, our results demonstrate that important trends toward macroscopic system behavior can already be obtained with increasing size and varying composition of the relatively small molecular dopant–host complexes considered, including systematic shifts in the Fermi level energies in the doped OSC.
Lewis-acid doping of organic semiconductors (OSCs) opens up new ways of p-type doping and has recently become of significant interest.
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