Seid Jebril

Crystal growth behaviour in Au-ZnO nanocomposite under different annealing environments and photoswitchability

The growth of gold nanoparticles and ZnO nanorods in atom beam co-sputtered Au-ZnO nanocomposite (NC) system by annealing at two different ambient conditions is demonstrated in this work. Annealing in a furnace at 600 °C (air environment) confirmed the formation of ZnO nanorods surrounded with Au nanoparticles. In-situ annealing inside a transmission electron microscope (TEM) led to the formation of gold nanocrystals with different polygonal shapes. TEM micrographs were obtained in real time at intermediate temperatures of 300 °C, 420 °C, and 600 °C under vacuum. The growth mechanisms of Au nanocrystals and ZnO nanorods are discussed in the framework of Au-Zn eutectic and Zn-melting temperatures in vacuum and air, respectively. Current-voltage responses of Au-ZnO NC nanorods in dark as well as under light illumination have been investigated and photoswitching in Au-ZnO NC system is reported. The photoswitching has been discussed in terms of Au-ZnO band-diagram.

Plasmonic properties of vapour-deposited polymer composites containing Ag nanoparticles and their changes upon annealing

The effect of temperature on microstructure and optical properties of nanocomposite films containing Ag nanoparticles embedded in a polymer matrix of Teflon AF was investigated in detail. Temperature effects were studied in two modes: the effect of temperature during preparation of the nanocomposites and post-deposition heat treatment. Substrate heating during deposition leads to a decrease in the condensation coefficient up to the glass transition temperature of the polymer and increases beyond this temperature. During heat treatment after deposition metal diffuses into the polymer and leads to a change in the microstructure of the nanocomposites resulting in larger metal cluster size, an increase in the interparticle distance and more spheroidal shaped clusters. An increase in the substrate temperature during deposition can lead to both blue shifts and red shifts of the plasmon peak position. Changes in the microstructure upon heat treatment above the glass transition temperature are caused by diffusion of metal clusters into the polymers. In this regime, the peak wavelength of the plasmon band shifts towards shorter wavelength with increasing temperature.

Procedures and Properties for a Direct Nano-Micro Integration of Metal and Semiconductor Nanowires on Si Chips

1-dimensional metal and semiconductor nanostructures exhibit interesting physical properties, but their integration into modern electronic devices is often a very challenging task. Finding the appropriate supports for nanostructures and nanoscale contacts are highly desired aspects in this regard. In present work we demonstrate the fabrication of 1D nano- and mesostructures between microstructured contacts formed directly on a silicon chip either by a thin film fracture (TFF) approach or by a modified vapor-liquid-solid (MVLS) approach. In principle, both approaches offer the possibilities to integrate these nano-meso structures in wafer-level fabrications. Electrical properties of these nano-micro structures integrated on Si chips and their preliminary applications in the direction of sensors and field effect transistors are also presented.

Simple Ways to Complex Nanowires and Their Application

Many syntheses exist for nanowire fabrication. This development was driven by the promise of novel applications. In order to realize these applications, simple and rapid ways have to be found for organized nanowire fabrication, because applications often require large area coverage with nanostructures or low production costs for mass fabrication. Furthermore, contact formation to nanowires is often difficult to achieve. One solution is the fabrication of nanowires within thin film cracks. Two examples for this method will be shown, one for flexible electronics, the other for the integration in microchips. The first is discussing about the change in properties of polymer foils by covering them with a large amount of parallel and crossed nanowires, leading to a variation of the optical and electrical conductivity behaviour. The other is showing the fabrication of ZnO nanowires and their integration into microchips, it is demonstrated that they can be used as field effect transistors for sensor applications.

Employing Thin Film Failure Mechanisms to Form Templates for Nano-electronics

Recently, we showed that thin film stresses can be used to form well aligned and complex nanowire structures [1]. Within this approach we used stress to introduce cracks in a thin film. Subsequent vacuum deposition of metal leads to the formation of a metal layer on the thin film and of metal nanowires in the cracks of the film. Removal of the thin film together with the excess metal cover finishes the nanowire fabrication on the substrate. As stress can be intentionally introduced by choosing an appropriate thin film geometry that leads to a stress concentration, the cracks and consequently the nanowires can be well aligned. Meanwhile, we have demonstrated how to form thousands of parallel aligned nanowires, x-and y-junctions or nanowires with macroscopic contacts for sensor applications, simply by applying fracture mechanics in thin films. Christiansen and Gosele called this approach “constructive destruction” in a comment in Nature Materials [2]. This gives a hint how to overcome some problems of the approach, arising from the limits of thin film fracture. A generalization of the fracture approach by being “more destructive” can overcome this limitations. For example, it is difficult to form pairs of parallel wires with a nanometer distance of the pair, but a micrometer separation between the individual pairs. Structures like this are useful for many contact applications including sensor arrays or field effect transistors. As well as thin film fracture, thin film delamination can be well controlled by fracture mechanics. Our latest experiments show that the combination of both, fracture and delamination, forms an ideal shadow mask for vacuum deposition. Cracks with delaminated sides were used as templates for the deposition of pairs of parallel wires consisting out of different materials with only a few 10 nm separation. First, a metal was sputter deposited under an angle of approx. 45° through the delaminated crack, which was used as a shadow mask. Afterwards, a second deposition metal is deposited under the opposite 45° angle with respect to the sample normal, having the crack located in the middle between both deposition sources. The angle, the delamination height and the crack width determine the separation of the nanowire contacts. We present several examples which show how these mechanisms of mechanical failure of thin films can be turned into useful templates for various nanostructures. We will focus here on two thin film systems, that can be easily deposited in every lab. These are wet chemically deposited photo-resist and flash evaporated amorphous carbon. These examples are compared with finite element simulations of the thin film stress with the ANSYS program. Moreover, we show how the delamination cracks can be also used as masks for the removal of material. Channals with a width down to 20 nm produced by ion beam sputtering are shown.

Using Thin Film Stress for Nanoscaled Sensors

Thin film stress is often seen as an unwanted effect in micro- and nanostructures. Since recent years, we could employ thin film stress as a useful tool to create nanowires. By creating stress at predetermined breaking points, e.g., in microstructured photo resist thin films, cracks occur on the nanoscale in a well defined and reproducible manner [ ]. By using those as a simple mask for thin film deposition, nanowires can be created. More recently this fabrication scheme could be improved by utilizing delamination of the thin film, in order to obtain suitable shadow masks for thin film deposition in vacuum [ ]. Now, these stress based nanowires can be integrated in microelectronic devices and used as field effect transistors or as hydrogen sensors [ ]. For the functional part of the sensor, it was proposed that thin film stress created by hydrogen adsorption in the nanowire is the driving force. In terms of function, thin films can be also applied on free standing nanoscale whiskers or wires to modify their mechanical features or adding additional functionality. As a second example for the utilization of thin film stress, recent experiments on a piezoelectric and magnetostrictive material combination will be presented. These piezoelectric-magnetostrictive nano-composites are potential candidates for novel magnetic field sensors [ ]. In these composites the magnetostriction will be transferred to the piezoelectric component, resulting in a polarization of the piezoelectric material, that can be used as the sensor signal. The results of two different composite layouts will be presented and discussed with a special focus on the comparison between classical macroscopic composites and the novel nanocomposites.

Au-ZnO: A tunable plasmonic nanocomposite for SERS and switching

In present work we focus on synthesis and properties of Au-ZnO nanocomposite and demonstrate its applications. Au-ZnO nanocomposites were synthesized by atom beam co-sputtering and sequential annealing. Optical absorption studies revealed the tunable surface plasmon resonance of gold nanoparticles. Annealing beyond eutectic temperature resulted in formation of Au-nanoparticles supported by ZnO nanorods as confirmed by high resolution transmission electron microscopy. Au-ZnO nanocomposite was used to study the surface enhanced Raman spectroscopy in fullerene molecule (C70). The Au-ZnO exhibits excellent switching behaviour enabling its potential use in day-today life.