87 research outputs found
Surface-Induced Phase of Tyrian Purple (6,6′-Dibromoindigo): Thin Film Formation and Stability
No full textThe appearance of surface-induced phases of molecular crystals is a frequently observed phenomenon in organic electronics. However, despite their fundamental importance, the origin of such phases is not yet fully resolved. The organic molecule 6,6′-dibromoindigo (Tyrian purple) forms two polymorphs within thin films. At growth temperatures of 150 °C, the well-known bulk structure forms, while at a substrate temperature of 50 °C, a surface-induced phase is observed instead. In the present work, the crystal structure of the surface-induced polymorph is solved by a combined experimental and theoretical approach using grazing incidence X-ray diffraction and molecular dynamics simulations. A comparison of both phases reveals that π-π stacking and hydrogen bonds are common motifs for the intermolecular packing. In-situ temperature studies reveal a phase transition from the surface-induced phase to the bulk phase at a temperature of 210 °C; the irreversibility of the transition indicates that the surface-induced phase is metastable. The crystallization behavior is investigated ex-situ starting from the sub-monolayer regime up to a nominal thickness of 9 nm using two different silicon oxide surfaces; island formation is observed together with a slight variation of the crystal structure. This work shows that surface-induced phases not only appear for compounds with weak, isotropic van der Waals bonds, but also for molecules exhibiting strong and highly directional hydrogen bonds
Organic Nanofibers: Extending the Lasing Wavelength Coverage of Organic Semiconductor Nanofibers by Periodic Organic-Organic Heteroepitaxy (Advanced Optical Materials 2/2013)
No full textExtending the Lasing Wavelength Coverage of Periodically Grown Semiconductor Nanofibers by Organic-Organic Heteroepitaxy
No full textExtending the Lasing Wavelength Coverage of Organic Semiconductor Nanofibers by Periodic Organic-Organic Heteroepitaxy
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Lasing Performance of Organic Heteroepitaxial Nanofibers Realized by Periodic Deposition of Para-Sexiphenyl and Sexithiophene on Muscovite Mica
No full textMultiband laser action from organic-organic heteroepitaxial nanofibers
No full textWe report successful tuning of laser wavelength from ∼420 nm to ∼600 nm in epitaxially aligned nanofibers grown by periodic deposition of para-sexiphenyl (p6P) and sexithiophene (6T) on p-6P/muscovite mica templates. The nanofibers were photoexcited by subpicosecond pulses tuned to the lowest p6P absorption band, and the emission of 6T, whose coverage was kept in the submonolayer regime, was efficiently sensitized through resonance energy transfer (RET). The 6T lasing was achieved at room temperature with threshold fluences as low as 10 μJ/cm2 per pulse. Transient photoluminescence measurements, with picosecond resolution, showed that at these pump fluences the decay dynamics of 6T emission is independent of the excitation density, thereby demonstrating the attainment of room-temperature monomolecular lasing from epitaxially oriented 6T submonolayer aggregates. Main lasing properties remained unaltered upon direct photoexcitation of 6T below the p6P absorption edg
The photophysics of luminescence in multilayered organic nanofibers
No full textPeriodic deposition of para-hexaphenyl (p6P) and alfa-sexithiophene (6T) molecules on a muscovite mica substrate by hot wall epitaxy results in multilayered, crystalline nanofibers. By varying the relative thicknesses of the constituent materials, the fluorescence spectrum can be tuned within the red, green, and blue spectral range [1, 2]. Illumination of the multilayered nanofiber sample with UV light (3.8 eV photon energy) directly excites fluorescence from the p6P layers, while the 6T layers emit light due to sensitization from the excited p6P [2]. Illumination with blue light (2.5 eV photon energy), which is below the optical gap of p6P, excites photoluminescence (PL) only from the 6T layers [3]. Here, we probe the energy transfer and exciton recombination characteristics by temperature dependent PL spectroscopy using either 3.8 eV or 2.5 eV photons for stimulating the p6P or 6T layers, respectively. While the p6P quantum yield decreases by an order of magnitude between 6 K and 293 K, the 6T monolayer emission exhibits much weaker temperature dependence. We tentatively attribute this to the fact that crystalline p6P layers are H aggregates while the ultrathin 6T layers have a radically different aggregation state (most presumably J aggregates). The ratio between emission intensity of the sensitized and directly excited 6T monolayer increases by around 40% when increasing the temperature from 6 K to 293 K due to enhanced energy transfer from p6P to the 6T monolayer. To further elucidate the exciton dynamics, we have used temperature-dependent, time-resolved PL spectroscopy that can provide quantitative data on the exciton diffusion and energy transfer processes. This improved understanding of the photophysics in organic heterocrystals can aid in the development of new nanomaterials with desired optical properties.1 C. Simbrunner et al. ACS Nano 4, 6244 (2010)2 C. Simbrunner et al. ACS Nano 6, 4629 (2012)3 F. Quochi et al. Adv. Opt. Mat. 1, 117 (2013)<br/
The photophysics of luminescence in multilayered organic nanofibers
No full textPeriodic deposition of para-hexaphenyl (p6P) and alfa-sexithiophene (6T) molecules on a muscovite mica substrate by hot wall epitaxy results in multilayered, crystalline nanofibers. By varying the relative thicknesses of the constituent materials, the fluorescence spectrum can be tuned within the red, green, and blue spectral range [1, 2]. Illumination of the multilayered nanofiber sample with UV light (3.8 eV photon energy) directly excites fluorescence from the p6P layers, while the 6T layers emit light due to sensitization from the excited p6P [2]. Illumination with blue light (2.5 eV photon energy), which is below the optical gap of p6P, excites photoluminescence (PL) only from the 6T layers [3]. Here, we probe the energy transfer and exciton recombination characteristics by temperature dependent PL spectroscopy using either 3.8 eV or 2.5 eV photons for stimulating the p6P or 6T layers, respectively. While the p6P quantum yield decreases by an order of magnitude between 6 K and 293 K, the 6T monolayer emission exhibits much weaker temperature dependence. We tentatively attribute this to the fact that crystalline p6P layers are H aggregates while the ultrathin 6T layers have a radically different aggregation state (most presumably J aggregates). The ratio between emission intensity of the sensitized and directly excited 6T monolayer increases by around 40% when increasing the temperature from 6 K to 293 K due to enhanced energy transfer from p6P to the 6T monolayer. To further elucidate the exciton dynamics, we have used temperature-dependent, time-resolved PL spectroscopy that can provide quantitative data on the exciton diffusion and energy transfer processes. This improved understanding of the photophysics in organic heterocrystals can aid in the development of new nanomaterials with desired optical properties.1 C. Simbrunner et al. ACS Nano 4, 6244 (2010)2 C. Simbrunner et al. ACS Nano 6, 4629 (2012)3 F. Quochi et al. Adv. Opt. Mat. 1, 117 (2013)<br/
Organic Nanofibers: Extending the Lasing Wavelength Coverage of Organic Semiconductor Nanofibers by Periodic Organic–Organic Heteroepitaxy
No full textThin films of organic epitaxial nanofibers represent a viable strategy towards the realization of effective organic semiconductor lasers. Outlined on page 117, a novel approach based on organic-organic heteroepitaxy is taken by F. Quochi et al. to endow the nanofibers with a highly functional oligomer heterostructure, enabling random lasing with low thresholds in multiple bands across the visible spectrum
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