Roana Melina de Oliveira Hansen

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.

AC-driven light emission from in situ grown organic nanofibers

In-situ grown organic nanofibers have been prepared on metal electrodes patterned by electron beam lithography. A systematic investigation shows that the light emission from these nanofibers driven by an AC gate voltage depends nonlinearly on the amplitude of the AC gate voltage and linearly on the frequency of the gate voltage, which indicates that a model involving thermally assisted charge-carrier tunneling can be applied. The photoluminescence spectra of parahexaphenylene (p6P) and α-sexithiophene (6T) nanofibers illustrate that the emission color of the in-situ grown nanofibers can be tuned by depositing two types of discontinuous organic layers on the same platform. Electroluminescence from two nanofiber thin films suggests that the relative light emission contribution from the two organic molecules can be varied by changing, e.g., the nominal thickness of the two materials.

Light-emission from in-situ grown organic nanostructures

Organic crystalline nanofibers made from phenylene-based molecules exhibit a wide range of extraordinary optical properties such as intense, anisotropic and polarized luminescence that can be stimulated either optically or electrically, waveguiding and random lasing. For lighting and display purposes, the high quantum yield and the easy tunability of the color by changing the molecular building blocks are especially important. The application of such nanostructures as electrically driven light-emitters requires integration with suitable metal electrodes for efficient carrier injection. Here, we demonstrate the implementation of a method for achieving such nanostructure integration. The method relies on growing the nanostructures directly between metal electrodes on a substrate that has been specially designed to guide the nanostructures growth. We present results in terms of morphological characterization and demonstrate how appropriate biasing with an AC gate voltage enables electroluminescence from these in-situ grown organic nanostructures.

Electrical properties of in-situ grown and transferred organic nanofibers

Para-hexaphenylene (p6P) molecules have the ability to self-assemble into organic nanofibers, which exhibit a range of interesting optical and optoelectronic properties such as intense, polarized luminescence, waveguiding and lasing. The nanofibers are typically grown on specific single-crystalline templates, such as muscovite mica, on which mutually parallel nanofibers are self-assembled upon vapor deposition of the organic material under high vacuum conditions. Besides such single-crystalline templates, the nanofibers can also be grown on non-crystalline gold surfaces, on which the orientation of the nanofibers can be manipulated by structuring the gold surface prior to parahexaphenylene (p6P) deposition. In this work it is demonstrated, how such organic nanofiber growth can be controlled by modifying the design of the underlying gold structures prior to growth. Here, the investigated designs include pinning lines and gratings. We demonstrate how gold gratings fabricated on an insulating substrate can enable electrical contact to in-situ grown p6P nanofibers. Furthermore, the electrical characteristics of in-situ grown fibers are compared to that of transferred p6P nanofibers. The transferred nanofibers are initially grown on muscovite mica, and subsequently transferred onto a target substrate by drop casting, and electrodes are applied on top by a special shadow mask technique.

Controlled growth of organic nanofibers on nano- and micro-structured gold surfaces

Nanofibers made from para-hexaphenylene (p6P) molecules hold unique optoelectronic properties, which make them interesting candidates as elements in electronic and optoelectronic devices. Typically these nanofibers are grown on specific single-crystalline substrates, on which long, mutually parallel nanofibers are formed. However, the lack of ability to further process these substrates restrains their use in devices. In this work, a novel method for in-situ growth of p6P nanofibers on nano- and micro-structured gold surfaces is presented. The substrates are prepared by conventional microfabrication techniques such as lithography, etching and metal deposition, which increase their potential as device platforms. The results presented here demonstrate, that both the growth direction and the nanofiber length can be controlled by placement of nano- and micro-structured lines on the substrate. It is shown that the preferred growth direction of the nanofibers is perpendicular to these structures whereas their length scales are limited by the size and placement of the structures. This work therefore demonstrates a new technique, which can be useful within future organic nanofiber based applications.

Functionalizing micro-cantilevers for meat degradation measurements

This paper presents an investigation of the functionalization of micro-cantilevers in order to bind to specific biogenic amines related to meat degradation, as for example cadaverine. The micro-cantilevers were functionalized with the compost 1,4,8,11 - tetraazacyclotetradecane (cyclam), which is binding to cadaverine molecules on gas phase. Different functionalization conditions were investigated, by immersing gold coated AFM cantilevers in cyclam solutions at different concentrations, for different functionalization times, and for different post-annealing treatments. The results show different morphologies for different conditions, and specific binding properties for different morphologies. The samples were exposed to gas phase cadaverine at different concentrations and to meat sample. Changes in resonance frequency of the cantilevers (due to mass increase by cadaverine binding) were measured using an atomic force microscope (AFM) and were in the range of few kHz, corresponding to a mass change of about 600 femtogram (for 4 seconds exposure to cadaverine).

Flexible PCPDTBT:PCBM solar cells with integrated grating structures

We report on development of flexible PCPDTBT:PCBM solar cells with integrated diffraction gratings on the bottom electrodes. The presented results address PCPDTBT:PCBM solar cells in an inverted geometry, which contains implemented grating structures whose pitch is tuned to match the absorption spectra of the active layer. This optimized solar cell structure leads to an enhanced absorption in the active layer and thus improved short-circuit currents and power conversion efficiencies in the fabricated devices. Fabrication of the solar cells on thin polyimide substrates which are compatible with the lithographically processed grating structures are done in order to obtain the efficiency enhancement in thin, flexible devices.