Exciton and Excited-State Charge Transfer at 2D van der Waals Interfaces

Combining materials with diverse properties into two‐dimensional (2D) van der Waals heterostructures lies at the heart of electronic, optoelectronic, and photonic applications. Prerequisite is a significant degree of electronic or photonic coupling of the constituents across the heterointerface. Understanding and controlling these interactions is mandatory to achieve the desired functionality. This review focuses on the charge and energy transfer processes and their dynamics in a specific class of van der Waals heterostructures, namely such composed of semiconducting transition metal dichalcogenides and conjugated organic molecules. With the help of prototypical material combinations, the importance of a precise knowledge of the interfacial electronic structure is demonstrated as it governs the excited‐state dynamics. This review aims at providing basic design guidelines to achieve functional 2D organic/inorganic van der Waals heterostructures with final properties that can be designed by careful selection of the organic component.Peer Reviewe

Bicolour, large area, inkjet-printed metal halide perovskite light emitting diodes†

We demonstrate a bicoloured metal halide perovskite (MHP) light emitting diode (LED) fabricated in two sequential inkjet printing steps. By adjusting the printing parameters, we selectively and deliberately redissolve and recrystallize the first printed emissive layer to add a pattern emitting in a different color. The red light emitting features (on a green light emitting background) have a minimum size of 100 μm and originate from iodide-rich domains in a phase-segregated, mixed MHP. This phase forms between the first layer, a bromide-based MHP, which is partially dissolved by printing, and the second layer, an iodide-containing MHP. With an optimised printing process we can retain the active layer integrity and fabricate bicolour, large area MHP-based LEDs with up to 1600 mm2 active area. The two emission peaks at 535 nm and 710 nm are well separated and produce a strong visual contrast.Bundesministerium für Bildung und Forschung 10.13039/501100002347Helmholtz Energy Materials Foundry 10.13039/501100015608Peer Reviewe

Resonance Energy Transfer from Monolayer WS2 to Organic Dye Molecules: Conversion of Faint Visible-Red into Bright Near-Infrared Luminescence

The synergetic combination of transition metal dichalcogenides (TMDCs) with organic dye molecules in functional heterostructures is promising for various optoelectronic applications. Here resonance energy transfer (RET) from a red‐emitting WS2 monolayer (1L‐WS2) to a layer of near‐infrared (NIR) emitting organic dye molecules is demonstrated. It is found that the total photoluminescence (PL) yield of the heterostructures is up to a factor of eight higher as compared to the PL yield of pristine 1L‐WS2. This is attributed to the efficient conversion of the mostly non‐radiative excitons in 1L‐WS2 into radiative excitons in the dye layer. A type‐I energy level alignment of the 1L‐WS2/dye interface assures the emission of bright PL. From excitation density‐dependent PL experiments, it is concluded that RET prevails against defect‐assisted non‐radiative recombination as well as Auger‐type exciton‐exciton annihilation in 1L‐WS2. The work paves the way for employing organic dye molecules in heterostructures with TMDCs in nanoscale light‐emitting devices with improved efficiency and tunable color.Peer Reviewe

Strong coupling of monolayer WS2 excitons and surface plasmon polaritons in a planar Ag/WS2 hybrid structure

Monolayer (1L) transition metal dichalcogenides (TMDC) are of strong interest in nanophotonics due to their narrow-band intense excitonic transitions persisting up to room temperature. When brought into resonance with surface plasmon polariton (SPP) excitations of a conductive medium opportunities for studying and engineering strong light-matter coupling arise. Here, we consider a most simple geometry, namely a planar stack composed of a thin silver film, an Al2O3 spacer and a monolayer of WS2. We perform total internal reflection ellipsometry which combines spectroscopic ellipsometry with the Kretschmann-Raether-type surface plasmon resonance configuration. The combined amplitude and phase response of the reflected light at varied angle of incidence proves that despite the atomic thinness of 1L-WS2, the strong coupling (SC) regime between A excitons and SPPs propagating in the thin Ag film is reached. The phasor representation of rho corroborates SC as rho undergoes a topology change indicated by the occurrence of a double point at the cross over from the weak to the strong coupling regime. Our findings are validated by both analytical transfer matrix method calculations and numerical Maxwell simulations. The findings open up new perspectives for applications in plasmonic modulators and sensors benefitting from the tunability of the optical properties of 1L-TMDCs by electric fields, electrostatic doping, light and the chemical environment.Comment: 15 pages, 3 figure

Influence of the energy level alignment on charge transfer and recombination at the monolayer-MoS₂/organic hybrid interface

International audienceMonolayer (ML) transition-metal dichalcogenides (TMDCs) exhibit numerous unique optoelectronic features. This motivates recent efforts to combine TMDCs with organic semiconductors to form heterostructures with tailorable properties that feature the advantages of both materials. Here, we study the photoinduced charge transfer across hybrid interfaces of ML-MoS2 and a series of organic semiconductors─often used as hole transport materials─where we systematically tune the offsets of the frontier energy levels. Steady-state photoluminescence and ultrafast transient absorption spectroscopy reveal that a larger energy level offset causes a lower efficiency of photoinduced charge transfer but also a longer lifetime of the charge separated state. Both observations are explained in the framework of Marcus’ theory of electron transfer. In fact, our observations question direct electron–hole recombination across the hybrid interface as the main decay pathway for photogenerated carriers in the considered systems. Instead, back transfer of holes to ML-MoS2 is suggested as the key decay channel. Adding a 1 nm LiF interlayer causes a significant slowdown of interfacial carrier recombination while not suppressing free carrier formation. This strategy serves as a guideline for optimizing further hybrid systems toward high-performance ML-TMDC/organic-based optoelectronic devices

Modulating the luminance of organic light-emitting diodes: Via optical stimulation of a photochromic molecular monolayer at transparent oxide electrode

Self-assembled monolayers (SAMs) deposited on bottom electrodes are commonly used to tune charge carrier injection or blocking in optoelectronic devices. Beside the enhancement of device performance, the fabrication of multifunctional devices in which the output can be modulated by multiple external stimuli remains a challenging target. In this work, we report the functionalization of an indium tin oxide (ITO) electrode with a SAM of a diarylethene derivative designed for optically control the electronic properties. Following the demonstration of dense SAM formation and its photochromic activity, as a proof-of-principle, an organic light-emitting diode (OLED) embedding the light-responsive SAM-covered electrode was fabricated and characterized. Optically addressing the two-terminal device by irradiation with ultraviolet light doubles the electroluminescence. The original value can be restored reversibly by irradiation with visible light. This expanded functionality is based on the photoinduced modulation of the electronic structure of the diarylethene isomers, which impact the charge carriers' confinement within the emissive layer. This approach could be successfully exploited in the field of opto-communication technology, for example to fabricate opto-electronic logic circuits. © 2020 The Royal Society of Chemistry

Versatile and Scalable Strategy to Grow Sol-Gel Derived 2H-MoS2 Thin Films with Superior Electronic Properties: A Memristive Case

Transition metal dichalcogenides, such as molybdenum disulfide (MoS2), show peculiar chemical/physical properties that enable their use in applications ranging from micro- and nano-optoelectronics to surface catalysis, gas and light detection, and energy harvesting/production. One main limitation to fully harness the potential of MoS2 is given by the lack of scalable and low environmental impact synthesis of MoS2 films with high uniformity, hence setting a significant challenge for industrial applications. In this work, we develop a versatile and scalable sol-gel-derived MoS2 film fabrication by spin coating deposition of an aqueous sol on different technologically relevant, flexible substrates with annealing at low temperatures (300 °C) and without the need of sulfurization and/or supply of hydrogen as compared to cutting-edge techniques. The electronic and physical properties of the MoS2 thin films were extensively investigated by means of surface spectroscopy and structural characterization techniques. Spatially homogenous nanocrystalline 2H-MoS2 thin films were obtained exhibiting high chemical purity and excellent electronic properties such as an energy band gap of 1.35 eV in agreement with the 2H phase of the MoS2, and a density of states that corresponds to the n-type character expected for high-quality 2H-MoS2. The potential use of sol-gel-grown MoS2 as the candidate material for electronic applications was tested via electrical characterization and demonstrated via the reversible switching in resistivity typical for memristors with a measured ON-OFF ratio ≥102. The obtained results highlight that the novel low-cost fabrication method has a great potential to promote the use of high-quality MoS2 in technological and industrial-relevant scalable applications