Uncovering the (un-)occupied electronic structure of a buried hybrid interface

The energy level alignment at organic/inorganic (o/i) semiconductor interfaces is crucial for any light-emitting or -harvesting functionality. Essential is the access to both occupied and unoccupied electronic states directly at the interface, which is often deeply buried underneath thick organic films and challenging to characterize. We use several complementary experimental techniques to determine the electronic structure of p-quinquephenyl pyridine (5P-Py) adsorbed on ZnO(10-10). The parent anchoring group, pyridine, significantly lowers the work function by up to 2.9 eV and causes an occupied in-gap state (IGS) directly below the Fermi level $E_\text{F}$. Adsorption of upright-standing 5P-Py also leads to a strong work function reduction of up to 2.1 eV and to a similar IGS. The latter is then used as an initial state for the transient population of three normally unoccupied molecular levels through optical excitation and, due to its localization right at the o/i interface, provides interfacial sensitivity, even for thick 5P-Py films. We observe two final states above the vacuum level and one bound state at around 2 eV above $E_\text{F}$, which we attribute to the 5P-Py LUMO. By the separate study of anchoring group and organic dye combined with the exploitation of the occupied IGS for selective interfacial photoexcitation this work provides a new pathway for characterizing the electronic structure at buried o/i interfaces

Electrostatic Field Driven Alignment of Organic Oligomers on ZnO Surfaces

We study the physisorption of organic oligomers on the ZnO($10\bar{1}0$) surface using first-principles density-functional theory and non-empirical embedding methods. We find that both in-plane location and orientation of the molecules are completely determined by the coupling of their quadrupole moments to the periodic dipolar electric field present at the semiconductor surface. The adsorption is associated with the formation of a molecular dipole moment perpendicular to the surface, which bears an unexpected linear relation to the molecule-substrate interaction energy. Long oligomers such as sexiphenyl become well-aligned with stabilization energies of several 100 meV along rows of positive electric field, in full agreement with recent experiments. These findings define a new route towards the realization of highly-ordered self-assembled arrays of oligomers/polymers on ZnO($10\bar{1}0$) and similar surfaces.Comment: 4 pages, 4 figure

Cross-sectional TEM preparation of hybrid inorganic/organic materials systems by ultramicrotomy

Preparation of hybrid inorganic-organic systems (HIOS) for transmission electron microscopy (TEM) in cross sectional view is the key for understanding the interfacial structure. Strikingly different materials properties like hardness, cleavability and heat sensitivity limit the number of applicable preparation strategies. Successful preparation of a HIOS system combining ZnO and para-sexiphenyl (6P) is realized by ultramicrotomy. It is shown that the alignment of the cutting plane with respect to the (0001) cleavage plane of ZnO plays a decisive role for successful preparation of extended TEM lamellae and the preservation of the HIOS structure. In particular, for (0001) oriented ZnO substrates the optimum cut direction is parallel to the HIOS interface. In cross-sectional high-resolution TEM images (100) lattice planes of 6P are observed proving the appropriate preparation strategy.Peer Reviewe

Direct observation of state filling at hybrid tin oxide organic interfaces

Electroluminescence (EL) spectra of hybrid charge transfer states at metal oxide/organic type-II heterojunctions exhibit bias-induced spectral shifts. The reasons for this phenomenon have been discussed controversially and arguments for either electric field-induced effects or the filling of trap states at the oxide surface have been put forward. Here, we combine the results of EL and photovoltaic measurements to eliminate the unavoidable effect of the series resistance of inorganic and organic components on the total voltage drop across the hybrid device. For SnOx combined with the conjugated polymer [ladder type poly-(para-phenylene)], we find a one-to-one correspondence between the blue-shift of the EL peak and the increase of the quasi-Fermi level splitting at the hybrid heterojunction, which we unambiguously assign to state filling. Our data are resembled best by a model considering the combination of an exponential density of states with a doped semiconductor. Published under license by AIP Publishing

Polarized white light from hybrid organic/III-nitrides grating structures.

Highly polarised white light emission from a hybrid organic/inorganic device has been achieved. The hybrid devices are fabricated by means of combining blue InGaN-based multiple quantum wells (MQWs) with a one-dimensional (1D) grating structure and down-conversion F8BT yellow light emitting polymer. The 1D grating structure converts the blue emission from unpolarised to highly polarised; Highly polarised yellow emission has been achieved from the F8BT polymer filled and aligned along the periodic nano-channels of the grating structure as a result of enhanced nano-confinement. Optical polarization measurements show that our device demonstrates a polarization degree of up to 43% for the smallest nano-channel width. Furthermore, the hybrid device with such a grating structure allows us to achieve an optimum relative orientation between the dipoles in the donor (i.e., InGaN/GaN MQWs) and the diploes in the acceptor (i.e., the F8BT), maximizing the efficiency of non-radiative energy transfer (NRET) between the donor and the acceptor. Time-resolved micro photoluminescence measurements show a 2.5 times enhancement in the NRET efficiency, giving a maximal NRET efficiency of 90%. It is worth highlighting that the approach developed paves the way for the fabrication of highly polarized white light emitters

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 colo

Excited State Charge Transfer Enabling MoS2 Phthalocyanine Photodetectors with Extended Spectral Sensitivity

Monolayer ML transition metal dichalcogenides TMDCs are an attracting new class of two dimensional direct band gap semiconducting materials for optoelectronic device applications. The combination of TMDCs with organic semiconductors holds the promise to further improve device properties with added functionality. Here, we demonstrate that excited state charge transfer from a thin organic absorber layer, i.e., metal free phthalocyanine H2Pc , enhances the photoresponse of ML MoS2 dramatically at the same time also significantly extending it to spectral regions where the TMDC is transparent. The fundamental processes enabling this boost in photodetector performance are unraveled by a combination of photoemission PES , photoluminescence PL , and photocurrent action spectroscopy. Direct and inverse PES reveal a type II energy level alignment at the MoS2 H2Pc interface with a large energy offset of 1 eV, which is sufficient to drive the excited state charge transfer. Time resolved PL measurements evidence highly efficient dissociation of excitons generated in H2Pc when they are in contact with MoS2. Exciton dissociation results in the formation of a charge separated state at the hybrid interface with an energy gap of ca. 1.2 eV, in accordance with PES. This state then dissociates into free carriers and markedly contributes to the current in the photodetector, as demonstrated by photocurrent action spectroscopy. This reveals that the photoconductivity within the MoS2 ML is generated by light directly absorbed in the TMDC and, notably, with comparable efficiency by the absorption by H2Pc. The present demonstration of a highly efficient carrier generation in TMDC organic hybrid structures paves the way for future nanoscale photodetectors with very wide spectral sensitivit

Efficient light emission from inorganic and organic semiconductor hybrid structures by energy level tuning

The fundamental limits of inorganic semiconductors for light emitting applications, such as holographic displays, biomedical imaging and ultrafast data processing and communication, might be overcome by hybridization with their organic counterparts, which feature enhanced frequency response and colour range. Innovative hybrid inorganic/organic structures exploit efficient electrical injection and high excitation density of inorganic semiconductors and subsequent energy transfer to the organic semiconductor, provided that the radiative emission yield is high. An inherent obstacle to that end is the unfavourable energy level offset at hybrid inorganic/organic structures, which rather facilitates charge transfer that quenches light emission. Here, we introduce a technologically relevant method to optimize the hybrid structure's energy levels, here comprising ZnO and a tailored ladder-type oligophenylene. The ZnO work function is substantially lowered with an organometallic donor monolayer, aligning the frontier levels of the inorganic and organic semiconductors. This increases the hybrid structure's radiative emission yield sevenfold, validating the relevance of our approach

The Schottky Mott Rule Expanded for Two Dimensional Semiconductors Influence of Substrate Dielectric Screening

A comprehensive understanding of the energy level alignment mechanisms between two dimensional 2D semiconductors and electrodes is currently lacking, but it is a prerequisite for tailoring the interface electronic properties to the requirements of device applications. Here, we use angle resolved direct and inverse photoelectron spectroscopy to unravel the key factors that determine the level alignment at interfaces between a monolayer of the prototypical 2D semiconductor MoS2 and conductor, semiconductor, and insulator substrates. For substrate work function amp; 934;sub values below 4.5 eV we find that Fermi level pinning occurs, involving electron transfer to native MoS2 gap states below the conduction band. For amp; 934;sub above 4.5 eV, vacuum level alignment prevails but the charge injection barriers do not strictly follow the changes of amp; 934;sub as expected from the Schottky Mott rule. Notably, even the trends of the injection barriers for holes and electrons are different. This is caused by the band gap renormalization of monolayer MoS2 by dielectric screening, which depends on the dielectric constant amp; 949;r of the substrate. Based on these observations, we introduce an expanded Schottky Mott rule that accounts for band gap renormalization by amp; 949;r dependent screening and show that it can accurately predict charge injection barriers for monolayer MoS2. It is proposed that the formalism of the expanded Schottky Mott rule should be universally applicable for 2D semiconductors, provided that material specific experimental benchmark data are availabl