Structure of <i>p</i>‑Sexiphenyl Nanocrystallites in ZnO Revealed by High-Resolution Transmission Electron Microscopy

The structure of <i>para</i>-sexiphenyl (6P) nanocrystallites embedded in ZnO single crystals is resolved by cross-sectional high-resolution transmission electron microscopy (HRTEM) combined with image contrast simulations and X-ray diffraction measurements. The hybrid structures are prepared by subsequent physical vapor deposition of 6P on ZnO­(1010) templates followed by overgrowth with ZnO. Application of ultramicrotomy for HRTEM specimen preparation and imaging under different focus conditions provides direct access to the atomic and molecular structure of the hybrid interface and the organic inclusion. The hybrid stacks reveal a high structural perfection. The 6P nanocrystallites maintain a structure as in the bulk crystal. Individual 6P lattice planes can be traced up to the lateral and top interfaces with ZnO, indicating that all interfaces are defined on an atomic/molecular level. Further evaluation of the HRTEM images reveals peculiarities of 6P growth on ZnO­(1010). The common 6P β-phase coexists here with the rarely reported γ-phase. The ZnO surface structure induces two mirror-symmetric in-plane preferential orientations of the 6P nanocrystallites. The ZnO surface topography, on the other hand, is critical for the structural perfection of 6P. Although conformal growth is observed, ZnO step edges induce characteristic stacking faults in 6P nanocrystallites

Light-Controlled “Molecular Zippers” Based on Azobenzene Main Chain Polymers

Single strands of azobenzene main chain polymers exhibiting alkyl side chains can be largely and reversibly contracted and extended with light. We show that upon self-assembly in a thin layered film they act as “molecular zippers” that can be opened and closed with UV- and blue light, respectively. Simultaneously <i>in situ</i> recorded time-resolved X-ray diffraction and optical spectroscopy measurements, together with scanning force microscopy show that upon the light-induced <i>E → Z</i> isomerization of the main chain azobenzenes the layered film morphology remains, while the initially highly ordered alkyl side chains become disordered. Already the <i>E → Z</i> isomerization of about 20% of all azobenzene chromophores triggers a complete disorder of the alkyl chains. The kinetics of this partial amorphization of the film is about 18 times slower than the ensemble kinetics of the initial azobenzene photoisomerization. This is the first demonstration of a rigid main chain polymer film with reversibly photoswitchable side chain crystallinity

Lattice Matching as the Determining Factor for Molecular Tilt and Multilayer Growth Mode of the Nanographene Hexa-<i>peri</i>-hexabenzocoronene

The microstructure, morphology, and growth dynamics of hexa-<i>peri</i>-hexabenzocoronene (HBC, C₄₂H₁₈) thin films deposited on inert substrates of similar surface energies are studied with particular emphasis on the influence of substrate symmetry and substrate–molecule lattice matching on the resulting films of this material. By combining atomic force microscopy (AFM) with X-ray diffraction (XRD), X-ray absorption spectroscopy (NEXAFS), and in situ X-ray reflectivity (XRR) measurements, it is shown that HBC forms polycrystalline films on SiO₂, where molecules are oriented in an upright fashion and adopt the known bulk structure. Remarkably, HBC films deposited on highly oriented pyrolytic graphite (HOPG) exhibit a new, substrate-induced polymorph, where all molecules adopt a recumbent orientation with planar π-stacking. Formation of this new phase, however, depends critically on the coherence of the underlying graphite lattice since HBC grown on defective HOPG reveals the same orientation and phase as on SiO₂. These results therefore demonstrate that the resulting film structure and morphology are not solely governed by the adsorption energy but also by the presence or absence of symmetry- and lattice-matching between the substrate and admolecules. Moreover, it highlights that weakly interacting substrates of high quality and coherence can be useful to induce new polymorphs with distinctly different molecular arrangements than the bulk structure

Formation of Carboxy- and Amide-Terminated Alkyl Monolayers on Silicon(111) Investigated by ATR-FTIR, XPS, and X‑ray Scattering: Construction of Photoswitchable Surfaces

We have prepared high-quality, densely packed, self-assembled monolayers (SAMs) of carboxy-terminated alkyl chains on Si(111). The samples were made by thermal grafting of methyl undec-10-enoate under an inert atmosphere and subsequent cleavage of the ester functionality to disclose the carboxylic acid end-group. X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (GIXD) indicate a surface coverage of about 50% of the initially H-terminated sites. In agreement, GIXD implies a rectangular unit mesh of 6.65 and 7.68 Å side lengths, containing two molecules in a regular zigzag-like substitution pattern for the ester- and carboxy-terminated monolayer. Hydrolysis of the remaining H–Si(111) bonds at the surface furnished HO–Si(111) groups according to XPS and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) studies. The amide-terminated alkyl SAM on Si(111) assembled in a 2-(6-chloro-1<i>H</i>-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HCTU)-mediated one-pot coupling reaction under an inert atmosphere, whereby the active ester forms in situ prior to the reaction with an amino-functionalized photoswitchable fulgimide. ATR-FTIR and XPS studies of the fulgimide samples revealed closely covered amide-terminated SAMs. Reversible photoswitching of the headgroup was read out by applying XPS, ATR-FTIR, and difference absorption spectra in the mid-IR. In XPS, we observed a reversible breathing of the amide/imide C1s and N1s signals of the fulgimide. The results demonstrate the general suitability of HCTU as a reagent for amide couplings to carboxy-terminated alkyl SAMs and the on-chip functionalization toward photoswitchable Si(111) surfaces

Carbazole–Phenylbenzotriazole Copolymers as Absorber Material in Organic Solar Cells

An alternating copolymer comprising a 2,7-functionalized carbazole donor and a 2-phenyl-2<i>H-</i>benzotriazole acceptor with an octyldodecyloxy substituent was synthesized. The polymer was blended with [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) and incorporated as absorber layer into solution processed organic solar cells. By adding the processing additive 1,8-diiodooctane (DIO) to the host solvent 1,2-dichlorobenzene (DCB), the solar cell fill factor increased to remarkable 70% and the power conversion efficiency approached 4.6%. Low-energy scanning transmission electron microscopy (low-keV STEM) investigations indicated a finer bulk morphology of the active layer upon deposition from DCB:DIO. Further, the low-energy shoulder of the absorption spectrum was enhanced, indicating stronger polymer aggregation. According to external quantum efficiency measurements, the enhanced absorption also promoted better photon harvesting. Grazing incidence X-ray diffraction experiments revealed face-on polymer aggregates being beneficial for the vertical hole transport

Thin-Film Texture and Optical Properties of Donor/Acceptor Complexes. Diindenoperylene/F6TCNNQ vs Alpha-Sexithiophene/F6TCNNQ

In this work, two novel donor/acceptor (D/A) complexes, namely, diindenoperylene (DIP)/1,3,4,5,7,8-hexafluoro-tetracyanonaphthoquinodimethane (F6TCNNQ) and alpha-sexithiophene (6T)/F6TCNNQ, are studied. The D/A complexes segregate in form of π–π stacked D/A cocrystals and can be observed by X-ray scattering. The different conformational degrees of freedom of the donor molecules, respectively, seem to affect the thin-film crystalline texture and composition of the D/A mixtures significantly. In equimolar mixtures, for DIP/F6TCNNQ, the crystallites are mostly uniaxially oriented and homogeneous, whereas for 6T/F6TCNNQ, a mostly 3D (isotropic) orientation of the crystallites and coexistence of domains of pristine compounds and D/A complex, respectively, are observed. Using optical absorption spectroscopy, we observe for each of the two mixed systems a set of new, strong transitions located in the near-IR range below the gap of the pristine compounds: such transitions are related to charge-transfer (CT) interactions between donor and acceptor. The optical anisotropy of domains of the D/A complexes with associated new electronic states is studied by ellipsometry. We infer that the CT-related transition dipole moment is perpendicular to the respective π-conjugated planes in the D/A complex

Molecular Reorganization in Organic Field-Effect Transistors and Its Effect on Two-Dimensional Charge Transport Pathways

Charge transport in organic thin film transistors takes place in the first few molecular layers in contact with the gate dielectric. Here we demonstrate that the charge transport pathways in these devices are extremely sensitive to the orientational defects of the first monolayers, which arise from specific growth conditions. Although these defects partially heal during the growth, they cause depletion of charge carriers in the first monolayer, and drive the current to flow in the monolayers above the first one. Moreover, the residual defects induce lower crystalline order and charge mobility. These results, which are not intuitively explained by electrostatics arguments, have been obtained by combining <i>in situ</i> real time structural and electrical characterization together with <i>ex situ</i> AFM measurements, on thin films of a relevant n-type organic semiconductor, <i>N</i>,<i>N</i>′-bis(<i>n</i>-octyl)-dicyanoperylene-3,4:9,10-bis dicarboximide grown by sublimation in a quasi-layer-by-layer mode at different substrate temperatures