Space Charge Transfer in Hybrid Inorganic/Organic Systems

We discuss density functional theory calculations of hybrid inorganic/organic systems (HIOS) 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 (F4TCNQ) on the ZnO(000$\bar{1}$) 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 HIOS and important for device design

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

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

Effect of electric field on the photoluminescence of polymer-inorganic nanoparticles composites

We report on the effect of electric field on the photoluminescence, PL, from a composite consisting of a conjugated polymer mixed with zinc oxide nanoparticles. We have found that in the absence of electric field PL emission from the composite film has two maxima in the blue and green-yellow regions. Application of a voltage bias to planar gold electrodes suppresses the green-yellow emission and shifts the only PL emission maximum towards the blue region. Current-voltage characteristics of the polymer-nanoparticles composite exhibit the non-linear behavior typical of non-homogeneous polymer-inorganic structures. Generation of excited states in the composite structure implies the presence of several radiative recombination mechanisms including formation of polymer-nanoparticle complexes including exciplex states and charge transfer between the polymer and nanoparticle that can be controlled by an electric field.Comment: 5 pages, 5 figures. accepted for publication in Solid State Communication

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

2-(4-Chloro­phen­yl)-5-{3,4-dibut­oxy-5-[5-(4-chloro­phen­yl)-1,3,4-oxadiazol-2-yl]thio­phen-2-yl}-1,3,4-oxadiazole

In the title compound, C28H26Cl2N4O4S, the dihedral angles between the two chloro­phenyl rings and the two oxadiazol rings are 10.51 (4)° and 13.55 (3)°, respectively. The thio­phene ring is oriented at dihedral angles of 5.59 (4)°, 8.33 (4)° and 4.41 (4)°, 11.05 (3)°, respectively, with respect to the two oxadiazol and the two chloro­phenyl rings. The intra­molecular C—H⋯O hydrogen bond results in the formation of a five-membered ring. In the crystal structure, π–π contacts between the oxadiazol rings, the chloro­phenyl rings and the chloro­phenyl and oxadiazol rings [centroid–centroid distances = 3.428 (3) Å, 3.750 (3) Å and 3.768 (3) Å, respectively] are present