Tailoring the growth of p-6P nanofibres using ultrathin Au layers: an organic-metal-dielectric model system

The influence of ultrathin Au cluster films on the growth of para-hexaphenyl (p-6P) fibres is investigated. Whereas p-6P at elevated temperatures forms long, mutually parallel fibres on plain mica, these fibres become shorter but taller on Au covered mica, up to a Au film thickness of approximately 8?nm. The degree to which fibres are mutually parallel decreases with increasing Au thickness. For thicker Au films the length of the fibres increases again, and their morphology changes from flat to faceted; for Au film thicknesses above 20?nm, fibre networks are formed. The spectroscopic properties of the fibres are not modified by the Au layer, enabling independent control of the fibre morphology by means of the intermediate metallic layer.     

Multicolor nanofiber based organic light-emitting transistors

Self-assembled, molecular crystalline nanofibers form microscale light-emitters for future nanophotonic applications. Such organic nanofibers exhibit many interesting optoelectronic properties, including polarized photo- and electroluminescence, waveguiding, and emission color tunability. Surface-grown nanofibers from two different molecules are implemented in an organic field-effect transistor platform by a double roll-printing scheme. Roll-printing multiple types of nanofibers onto the same device provides a fast and simple alternative to multilayer devices. The combination of nanofibers made from para-hexaphenylene and 5,5′-di-4-biphenyl-2,2′-bithiophene results in a nanofiber based organic light-emitting transistor (OLET) which emits both blue and green light. A comparison of measured electrical transport and electroluminescence (EL) properties results in a correlation between the threshold voltage for transport and the onset voltage for EL emission.     

In situ–Directed Growth of Organic Nanofibers and Nanoflakes: Electrical and Morphological Properties

Organic nanostructures made from organic molecules such as para-hexaphenylene (p-6P) could form nanoscale components in future electronic and optoelectronic devices. However, the integration of such fragile nanostructures with the necessary interface circuitry such as metal electrodes for electrical connection continues to be a significant hindrance toward their large-scale implementation. Here, we demonstrate in situ–directed growth of such organic nanostructures between pre-fabricated contacts, which are source–drain gold electrodes on a transistor platform (bottom-gate) on silicon dioxide patterned by a combination of optical lithography and electron beam lithography. The dimensions of the gold electrodes strongly influence the morphology of the resulting structures leading to notably different electrical properties. The ability to control such nanofiber or nanoflake growth opens the possibility for large-scale optoelectronic device fabrication.     

Finite Element Simulation of Photoacoustic Pressure in a Resonant Photoacoustic Cell Using Lossy Boundary Conditions

The finite-element method (FEM) is used to simulate the photoacoustic signal in a cylindrical resonant photoacoustic cell. Simulations include loss effects near the cell walls that appear in the boundary conditions for the inhomogeneous Helmholtz equation governing the acoustic pressure. Reasonably good agreement is obtained between theoretical results and experimental data. However, it was anticipated that loss mechanisms other than viscous and thermal boundary losses occur and should be included. Nevertheless, the feasibility to use FEM together with the derived boundary conditions to simulate the photoacoustic signal was demonstrated and good agreement with experiments for the actual resonance frequency and the quality factor of the cell was obtained despite its complicated geometry.     

Organic molecular nanotechnology

A new route to bottom-up organic nanotechnology is presented. Molecular building blocks with specific optoelectronic properties are designed and grown via directed self-assembly arrays of morphologically controlled light-emitting organic nanofibers on template surfaces. The fibers can be easily transferred from the growth substrate to device platforms either as single entities or as ordered arrays. Due to the extraordinary flexibility in the design of their optoelectronic properties they serve as key elements in next-generation nanophotonic devices.     

Organic nanofibers from thiophene oligomers

We report on the growth and morphology of α-quaterthiophene and α-sexithiophene clusters, needles (or “fibers”), and flat islands on muscovite mica (001). Samples are prepared by organic molecular beam deposition, and characterized ex situ by polarized fluorescence microscopy and by atomic force microscopy. The growth mechanism of such thiophene fibers is compared to that of para-phenylenes. Distinct needle orientations are present on the surface, their formation being explained by a combination of epitaxy and electric field assisted alignment.     

Charge transport in oligo phenylene and phenylene–thiophene nanofibers

Charge carrier mobilities have been measured in elongated crystalline aggregates with nanoscale cross-sectional dimensions. These molecular crystals are grown via dedicated epitaxial surface growth. They consist of well-ordered phenylene–thiophene (co-) oligomers that have been systematically tailored to study mobility of oligomers containing six rings and a variable number of thiophene rings. These organic nanoscale one-dimensional systems demonstrate charge mobilities as high as 1 cm² V⁻¹ s⁻¹. Density functional theory calculations have been performed in order to correlate the experimentally observed trends in mobility with molecular charge transport properties.     

Growth Control and Optics of Organic Nanoaggregates

Light‐emitting organic nanofibers made of phenyl molecules like para‐hexaphenyl (p‐6P) and grown on muscovite mica form a model system well‐suited for the study of optics in the sub‐wavelength regime. We demonstrate that p‐6P nanofibers can be grown with high control of the morphology of individual nanoaggregates and also of the mutual alignment of aggregates by the use of appropriate growth conditions and substrate surfaces. The nanofibers can be detached from the substrate, thus allowing one to study the optical response under a huge variety of fundamentally different conditions, from individual floating aggregates to dense bunches of interacting aggregates. We show examples of linear and nonlinear optical properties of the blue‐light‐emitting aggregates and mention possible applications in future submicrometer‐sized optoelectronics.     

Influence of geometry on hydrodynamic focusing and long-range fluid behavior in PDMS microfluidic chips

Details of hydrodynamic focusing in a 2D microfluidic channel-junction are investigated experimentally and theoretically, especially the effect on the focusing width of volumetric flow ratio r between main and side channels, as well as angle θ between channels. A non-linear relationship is observed where the focus width decreases rapidly with increasing r and levels off at higher values. For the dependence on θ, results from both experiments and modeling show that an increased focusing effect is obtained as θ approaches 90°. Long-range focusing is explored along a 1 cm long channel and it is observed that in the middle section of the channel, a smaller θ induces less divergence. This effect is of importance for microfluidic systems utilizing hydrodynamic focusing in long, straight channels.