Sylke Blumstengel

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 structures 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 structures radiative emission yield sevenfold, validating the relevance of our approach.

Space-charge transfer in hybrid inorganic-organic systems.

We discuss density functional theory calculations of hybrid inorganic-organic systems that explicitly include the global effects of doping (i.e., position of the Fermi level) and the formation of a space-charge layer. For the example of tetrafluoro-tetracyanoquinodimethane on the ZnO(0001[over ¯]) surface we show that the adsorption energy and electron transfer depend strongly on the ZnO doping. The associated work function changes are large, for which the formation of space-charge layers is the main driving force. The prominent doping effects are expected to be quite general for charge-transfer interfaces in hybrid inorganic-organic systems and important for device design.

Electronic coupling of optical excitations in organic/inorganic semiconductor hybrid structures

The epitaxial growth of small conjugated molecules on ZnO-based surfaces is studied. A weak substrate interaction allows for the preparation of organic layers with well-defined morphology and electronically intact interfaces without the need for extra passivation. Nonradiative energy transfer from inorganic quantum wells to various molecules is identified by optical spectroscopy. The strength of the dipole-dipole mediated coupling between Wannier and Frenkel excitons is as large as 2meV. In hybrid structures with type-II energy level alignment, charge separation occurs at the organic/inorganic interface as well. These findings render organic/ZnO hybrid structures interesting for light-emitting as well as photovoltaic applications benefiting from favorable properties of both material classes.

Vacuum-processable ladder-type oligophenylenes for organic–inorganic hybrid structures: synthesis, optical and electrochemical properties upon increasing planarization as well as thin film growth

A novel synthetic route to even-numbered ladder-type oligo(p-phenylene)s (LOPPs) carrying no solubilizing groups to facilitate vacuum-processing is presented. The influence of increasing bridging adjacent phenylene units on the optical and electrochemical properties is discussed in the series of p-sexiphenyl 6P, terfluorene 3F, and ladder-type sexiphenyl L6P. The influence of the extension of the π-system is taken into consideration as well. Furthermore it is shown that highly ordered thin films of L6P on alumina surfaces can be prepared by organic molecular beam deposition (OMBD).

Electrostatic-field-driven alignment of organic oligomers on ZnO surfaces.

We study the physisorption of organic oligomers on the strongly ionic ZnO(1010) surface by using first-principles density-functional theory and nonempirical embedding methods. It turns out that the in-plane variation of the molecule-substrate interaction energy and the bonding dipole in the vertical direction are linked up by a linear relationship originating from the electrostatic coupling of the molecule with the periodic dipolar electric field generated by the Zn-O surface dimers. Long oligomers with a highly axial π-electron system such as sexiphenyl become well oriented with alignment energies of several 100 meV along rows of a 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 or polymers on ZnO(1010) and similar surfaces.

An Inorganic/Organic Semiconductor “Sandwich” Structure Grown by Molecular Beam Epitaxy

2009 WILEY-VCH Verlag Gmb Inorganic/organic semiconductor hybrid structures offer new functionalities that can not be achieved by the individual components alone. In order to benefit from the complementary properties of the two material systems, electronic coupling across the inorganic/organic interface is required. Nonradiative energy transfer as well as charge-carrier separation could be observed on properly designed specimens. These findings are not only interesting from a fundamental point of view, but are also of direct practical relevance. Combination of the high carrier mobility in inorganic semiconductors with the very strong and easily wavelength-tuneable absorption-emission features of organic molecules opens up new vistas for light-emitting or photovoltaic devices. Two approaches regarding the growth of semiconductor hybrid structures are pursued currently. One approach employs wet-chemically produced semiconductor nanocrystals, the other relies on epitaxial semiconductor quantum well (QW) structures. For the latter, ZnO/ZnMgO, InGaN/GaN, and GaAs/ AlGaAs have been employed successfully. High purity standards and excellent structural control are merits of epitaxial growth. However, the epitaxial hybrid structures fabricated so far have in common that a sole organic layer is merely deposited on top of the inorganic QW. Subsequent overgrowth by the inorganic material appeared to be impossible because typical temperatures applied in semiconductor epitaxy range between about 500 and 1000 8C and are thus not compatible with organic molecules. As found out recently, ZnO and some of its ternaries are remarkable exceptions as these compounds can be grown with high quality by molecular beam epitaxy (MBE) at temperatures as low as room temperature. Exploiting this unique potential, all-epitaxial periodic organic/inorganic hybrid structures come into reach. There are several crucial points, however, which have to be solved in order to prepare such hybrid structures. First, the molecules must survive the overgrowth with their electronic and optical properties remaining unchanged. ZnO is grown by radical-source MBE with the oxygen being provided by an rf-plasma source that generates a flux of highly reactive atomic species on the sample surface. It must be assured that the molecules survive such a harsh environment. Second, electronic coupling between the subcomponents must persist and, third, the inorganic overlayer should preferably grow in a coherent epitaxial mode. In this work, we will concentrate on the first two points. Moreover, we will demonstrate that ZnO/organic/ZnO sandwich structures form planar waveguides that support stimulated emission of the enclosed organic layer. The inorganic/organic sandwich (IOS) structures are grown under ultrahigh vacuum conditions in aMBE apparatus equipped with interconnected growth chambers for the two material systems. This ensures well-defined organic/inorganic interfaces free of extrinsic defects. For the inorganic part below the organic layer, the standard epitaxy regime is employed. At first, a 30-nm-thick nucleation layer, either ZnO or ZnMgO, is deposited at TS1⁄4 280 8C on an a-plane sapphire substrate and subsequently annealed at 560 8C. Then, a ZnO epilayer or ZnO/ZnMgO QW structure is grown at TS1⁄4 350 8C. Additional annealing steps (TS1⁄4 560 8C) at each side of the QW are introduced to assure atomically abrupt interfaces. The part atop the organic layer is a ZnO film with a typical thickness of 50–200 nm. For its growth, the substrate temperature is lowered to TS 100 8C. In the absence of the organic layer, a two-dimensional layer-by-layer mode is indeed established at such low temperatures. As depicted in Figure 1a, clear RHEED (Reflection high-energy electron diffraction) oscillations in the specular beam intensity are observed each of which corresponding to the deposition of one monolayer. Furthermore, the streaky RHEED diffraction pattern in the inset of Figure 1a is indicative of high crystalline perfection and an atomically flat surface. As organic molecule we chose 2,7-bis(biphenyl-4-yl)20,70-di-tert-butyl-9,90-spirobifluorene (SP6) with the chemical structure shown in Figure 1b. SP6 is amorphous in the condensed phase and considered as a promising candidate for organic solid-state lasers. The choice of SP6 is also triggered by the previous observation of strong excitonic coupling with ZnO/ ZnMgO QW structures. The molecules are deposited on the inorganic underlay at TS1⁄4 20 8C at a growth rate of 0.1 nmmin 1 resulting in closed and homogeneous films, both on ZnO and ZnMgO. The amorphous character of SP6 makes it unlikely that single-crystalline overgrowth of ZnO can be accomplished. Indeed, the RHEED pattern features Debye rings indicating that the ZnO atop of SP6 is polycrystalline. On the other hand, AFM images taken on completed ZnO/SP6/ZnO structures reveal that ZnO films with a very smooth surface morphology are formed. In the example depicted in Figure 1c, the root mean square roughness is only 0.7 nm. Therefore, though polycrystalline, the growth process is well-defined endorsing the choice of SP6 also from this point of view. The specific design used for assessing the viability of the ZnO/ SP6/ZnO IOS consists of a ZnO/ZnMgO QW structure followed by 5-nm-thick SP6 and 150-nm-thick ZnO (TS1⁄4 100 8C). The ZnMgO barrier below the QW is 600 nm thick, while the upper

Calculating Optical Absorption Spectra of Thin Polycrystalline Organic Films: Structural Disorder and Site-Dependent van der Waals Interaction.

We propose a new approach for calculating the change of the absorption spectrum of a molecule when moved from the gas phase to a crystalline morphology. The so-called gas-to-crystal shift Δm is mainly caused by dispersion effects and depends sensitively on the molecule’s specific position in the nanoscopic setting. Using an extended dipole approximation, we are able to divide Δm= −QWm in two factors, where Q depends only on the molecular species and accounts for all nonresonant electronic transitions contributing to the dispersion while Wm is a geometry factor expressing the site dependence of the shift in a given molecular structure. The ability of our approach to predict absorption spectra is demonstrated using the example of polycrystalline films of 3,4,9,10-perylenetetracarboxylic diimide (PTCDI).

Cascade energy transfer versus charge separation in ladder-type oligo(p-phenylene)/ZnO hybrid structures for light-emitting applications

Usability of inorganic/organic semiconductor hybrid structures for light-emitting applications can be intrinsically limited by an unfavorable interfacial energy level alignment causing charge separation and nonradiative deactivation. Introducing cascaded energy transfer funneling away the excitation energy from the interface by transfer to a secondary acceptor molecule enables us to overcome this issue. We demonstrate a substantial recovery of the light output along with high inorganic-to-organic exciton conversion rates up to room temperature.

Uniform and Efficient UV-emitting ZnO/ZnMgO Multiple Quantum Wells Grown by Radical-Source Molecular Beam Epitaxy

Five-fold stacked ZnO/ZnMgO quantum wells are fabricated by radical-source molecular beam epitaxy on a-plane sapphire, employing low-temperature growth for the ternary component and appropriate annealing steps performed at each interface. Transmission electron microscopy images reveal that the ZnO/ZnMgO interfaces are abrupt and smooth on an atomic scale. Threading dislocations originating from the interfacial region between substrate and the ZnMgO nucleation layer are largely annihilated during growth of a subsequent 600 nm thick ZnMgO buffer. The residual dislocation density in the well region is sufficiently low to allow for efficient exciton emission up to room temperature.

Nanoscale transport of excitons at the interface between ZnO and a molecular monolayer

Time-resolved near-field optical microsopy maps exciton transport in a hybrid system. Within the 100 ps photoluminescence lifetime, an equilibrium distribution of surface and bound excitons displays lateral diffusion on a 50 nm length scale.