The prototypical organic-oxide interface: intra-molecular resolution of sexiphenyl on In2_2O3_3(111)

The performance of an organic-semiconductor device is critically determined by the geometric alignment, orientation, and ordering of the organic molecules. While an organic multilayer eventually adopts the crystal structure of the organic material, the alignment and configuration at the interface with the substrate/electrode material is essential for charge injection into the organic layer. This work focuses on the prototypical organic semiconductor para-sexiphenyl (6P) adsorbed on In$_2$O$_3$(111), the thermodynamically most stable surface of the material that the most common transparent conducting oxide, indium tin oxide (ITO) is based on. The onset of nucleation and formation of the first monolayer are followed with atomically-resolved scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Annealing to 200$^\circ$C provides sufficient thermal energy for the molecules to orient themselves along the high-symmetry directions of the surface, leading to a single adsorption site. The AFM data suggests a twisted adsorption geometry. With increasing coverage, the 6P molecules first form a loose network with poor long-range order. Eventually the molecules re-orient and form an ordered monolayer. This first monolayer has a densely packed, well-ordered (2$\times$1) structure with one 6P per In$_2$O$_3$(111) substrate unit cell, i.e., a molecular density of 5.64$\times$10$^{13}$ cm$^{-2}$

Pulsed-laser growth of In2O3 thin films on YSZ(111) substrates

Indium(III) oxide (In2O3) is a wide bandgap semiconductor and belongs to the class of transparent conductive oxides, which combine high electrical conductivity and optical transparency in the visible region. Achieving a better understanding of the atomicscale surface characteristics by investigating well-defined single-crystal model systems is of paramount importance to optimize the functionality of In2O3-based technology. Undoped In2O3 single crystals are not commercially available, and synthetically grown ones are usually very small, limiting the investigation by area-averaging techniques such as temperature programmed desorption and X-ray photoelectron spectroscopy (XPS). To compensate for the lack of large In2O3 single crystals, we have prepared well-ordered and atomically-flat In2O3(111) thin films, with a thickness of few hundreds of nanometres. The films were grown on Y-stabilized zirconia (111) substrates by pulsed laser deposition (PLD). Their structure, chemical composition, and morphology were characterized by electron (RHEED, LEED) and x-ray diffraction (XRD), XPS, atomic-force microscopy (AFM), and scanning tunneling microscopy (STM). By optimizing the growth parameters (temperature and oxygen background pressure) and investigating their effect on the film morphology and structure, we could obtain In2O3(111) films exhibiting properties comparable to the best single crystalline samples available. According to AFM measurements after growth, such films exhibit atomically-flat terraces with an average terrace width of ∼ 150 nm, which increases the typical terrace width of an In2O3 single crystal by a factor of 3−4. XRD reveals that the In2O3 film adopts the cubic bixbyite structure. The out-of-plane lattice parameter, as well as the typical peak widths, are comparable to those of single-crystalline samples, indicating high crystalline quality. STM investigations show a very good agreement with the STM results of In2O3 single crystals, both on the small and on the large scale. The similarity in high resolution STM measurements strongly promotes the use of the grown films as an equivalent replacement of In2O3(111) single crystals for different experimental setups.10

Prototypical Organic–Oxide Interface: Intramolecular Resolution of Sexiphenyl on In₂O₃(111)

The performance of an organic semiconductor device is critically determined by the geometric alignment, orientation, and ordering of the organic molecules. Although an organic multilayer eventually adopts the crystal structure of the organic material, the alignment and configuration at the interface with the substrate/electrode material are essential for charge injection into the organic layer. This work focuses on the prototypical organic semiconductor para-sexiphenyl (6P) adsorbed on In₂O₃(111), the thermodynamically most stable surface of the material that the most common transparent conducting oxide, indium tin oxide, is based on. The onset of nucleation and formation of the first monolayer are followed with atomically resolved scanning tunneling microscopy and noncontact atomic force microscopy (nc-AFM). Annealing to 200 °C provides sufficient thermal energy for the molecules to orient themselves along the high-symmetry directions of the surface, leading to a single adsorption site. The AFM data suggests an essentially planar adsorption geometry. With increasing coverage, the 6P molecules first form a loose network with a poor long-range order. Eventually, the molecules reorient into an ordered monolayer. This first monolayer has a densely packed, well-ordered (2 × 1) structure with one 6P per In₂O₃(111) substrate unit cell, that is, a molecular density of 5.64 × 10¹³ cm^–2

In Vitro Cytotoxicity of D18 and Y6 as Potential Organic Photovoltaic Materials for Retinal Prostheses

Millions of people worldwide are diagnosed with retinal dystrophies such as retinitis pigmentosa and age-related macular degeneration. A retinal prosthesis using organic photovoltaic (OPV) semiconductors is a promising therapeutic device to restore vision to patients at the late onset of the disease. However, an appropriate cytotoxicity approach has to be employed on the OPV materials before using them as retinal implants. In this study, we followed ISO standards to assess the cytotoxicity of D18, Y6, PFN-Br and PDIN individually, and as mixtures of D18/Y6, D18/Y6/PFN-Br and D18/Y6/PDIN. These materials were proven for their high performance as organic solar cells. Human RPE cells were put in direct and indirect contact with these materials to analyze their cytotoxicity by the MTT assay, apoptosis by flow cytometry, and measurements of cell morphology and proliferation by immunofluorescence. We also assessed electrophysiological recordings on mouse retinal explants via microelectrode arrays (MEAs) coated with D18/Y6. In contrast to PFN-Br and PDIN, all in vitro experiments show no cytotoxicity of D18 and Y6 alone or as a D18/Y6 mixture. We conclude that D18/Y6 is safe to be subsequently investigated as a retinal prosthesis