P. Tzanetakis

Making silicon hydrophobic : wettability control by two-lengthscale simultaneous patterning with femtosecond laser irradiation

We report on the wettability properties of silicon surfaces, simultaneously structured on the micrometre-scale and the nanometre-scale by femtosecond (fs) laser irradiation to render silicon hydrophobic. By varying the laser fluence, it was possible to control the wetting properties of a silicon surface through a systematic and reproducible variation of the surface roughness. In particular, the silicon–water contact angle could be increased from 66° to more than 130°. Such behaviour is described by incomplete liquid penetration within the silicon features, still leaving partially trapped air inside. We also show how controllable design and tailoring of the surface microstructures by wettability gradients can drive the motion of the drops centre of mass towards a desired direction (even upwards).

Effective mobility and photocurrent in carbon nanotube–polymer composite photovoltaic cells

We examine the dark and the illuminated current?voltage (J?V) characteristics of poly(3-octylthiophene) (P3OT)/single-wall carbon nanotube (SWNT) composite photovoltaic cells as a function of SWNT concentration. Using an exponential band tail model, the influence of SWNT concentration on the J?V characteristics of the cells is analysed in terms of corresponding parameters such as effective hole mobility, short-circuit current, and open-circuit voltage. For the device with optimum 1% SWNT concentration, the increased photoresponse (~500 times) as compared to the pristine P3OT cell can be attributed partly to the increase (~50 times) in effective hole mobility, due to the reduction of localized states of the pristine P3OT matrix, and partly to the enhanced exciton extraction at the polymer/nanotube junctions.

Field Emission Properties of Low-Temperature, Hydrothermally Grown Tungsten Oxide

Tungsten oxide layers have been prepared on conductive glass substrates using aqueous chemical growth from a sodium tungstate precursor at low-temperature hydrothermal conditions. The deposits were then tested as cold electron emitters. Traceable layers could be deposited only within a narrow pH range of 1.5-2 at a time length not exceeding 4 h. Transmittance in the visible spectrum was found to decrease with deposition time. The presence of both monoclinic and hexagonal phases was always detected. At the longest deposition times and highest precursor concentrations, morphologies comprise randomly oriented spikes or rods. The overall emission performance is found to improve with growth time and precursor concentration. The role of morphology on the emission properties of the films is discussed.

Optimization of ultrafast laser generated low-energy ion beams from silicon targets

We demonstrate the possibility to manipulate the kinetic properties of ion beams generated by ultrafast laser ablation of silicon. The versatility in regulating the sub-keV ion flux is achieved by implementing adaptive control of the temporal shape of incident laser pulses. Tunable characteristics for the charged beams are obtained using excitation synchronized with the phase-transformation dynamics, exploiting transitions to volatile fluid states with minimal energetic expenses.

Photoinduced stress in hydrogenated amorphous silicon films

Photo-induced compressive stress ΔS in hydrogenated amorphous silicon (a-Si:H) has been studied using films deposited by plasma-enhanced or hot-wire chemical vapor deposition on crystalline silicon microcantilevers. The kinetics of ΔS(t) first rises with exposure time as t0.5 and follows a stretched exponential. The saturation values ΔSsat correspond to volume changes of about 10−3, which excludes the possibility that ΔS is a consequence of the light-induced creation of coordination defects. The highest-quality films have large initial stress, small values of the Young’s modulus, and a rapid approach of ΔS(t) towards saturation.

Imaging dielectric properties of Si nanowire oxide with conductive atomic force microscopy complemented with femtosecond laser illumination.

In most Si nanowire (NW) applications, Si oxide provides insulation or a medium of controlled electron tunneling. This work revealed both similarities and differences in the dielectric properties of NW oxide compared with that grown on wafers. The interface barrier to electron transit from the semiconductor to the dielectric and the threshold electric field for current flow are quite similar to those in the planar geometry. This is not true for the lowest currents measured which are not uniformly distributed, indicating variations of trap density in the gap of NW oxide.

Polymer-nanotube composite mats with improved field emission performance and stability.

The results of electron field emission from single walled carbon nanotubes (SWCNTs) mats deposited on different composite films of SWCNTs and poly(3-octylthiophene) (P3OT) semiconducting polymer are presented. Three different structures were tested: (a) dense and sparse SWCNT mats on n+ -Si; (b) SWCNT mats on composite films with different SWCNT-P3OT ratios; (c) composite films with different SWCNT-P3OT ratios on n+ -Si. The experiments show that there is a critical SWCNT-P3OT concentration in which the field emission stability of SWCNT mats is remarkably improved with a small reduction in the emission threshold compared to the optimum pristine SWCNT film. The contribution of the composite film morphology as well as the role of polymer-nanotube interaction on the emission performance are evaluated. The physical mechanism behind the stability of composite field emitters is also discussed.

Monitoring of the ion energy and current density at the surface of films grown by excimer laser ablation

A simple and easy to implement, ion time-of-flight (TOF), detection system has been developed and used to monitor the ions ejected during pulsed excimer laser ablation of solid and molten Si and Ge targets. The setup employs a Faraday cup (FC) detector with a high gain-bandwidth preamplifier and an adjustable voltage electrostatic barrier. The FC is capable of very long time, undisturbed, operation even with significant deposition of material on it. The analysis of the TOF ion signal and its modification by the barrier potential yields valuable quantitative information about the ion flux and kinetic energy at the surface of the growing film. The technique is capable of resolving atomic/cluster components of different charge to mass ratios. High ion fractions and ion energies are observed, in all cases studied, when the laser fluence is clearly above the ablation threshold.

Light induced stress in a-Si1-xGex:H alloys and its correlation with the Staebler-Wronski effect

Abstract Photoinduced compressional stress ΔS has been studied in hydrogenated amorphous silicon and silicon–germanium alloys, a-Si1−xGex:H with x=0, 0.4 and 0.67. The films were deposited by plasma enhanced chemical vapor deposition onto 4 μm thick crystalline silicon microcantilevers commonly used in scanning probe microscopy. The initial stress S0 of the films was obtained from the initial bending and the Youngs modulus from the cantilever resonance frequency. The kinetics of ΔS(t) follow a stretched exponential. ΔS(t) cannot be a consequence of photoinduced defect creation because ΔS continues to rise when defect creation has saturated and the largest ΔS corresponds to a relative volume change ΔV/V=1×10−3, too large for 4×1017 cm−3 defects. We observe a significant decrease in ΔS between x=0.4 and x=0.67 alloy composition just where defect creation is greatly diminished. We suggest that defect creation is associated with the time dependent large local strains in and around the volume elements of electron–hole recombination. ΔS is the time and spatial average of the local configuration changes.

Novel Aspects of Materials Processing by Ultrafast Lasers: From Electronic to Biological and Cultural Heritage Applications

Materials processing by ultrafast lasers offers several distinct possibilities for micro/nano scale applications. This is due to the unique characteristics of the laser-matter interactions involved, when sub-picosecond pulses are employed. Prospects arising will be discussed in the context of surface and in bulk laser induced modifications. In particular, examples of diverse applications including the development and functionalization of laser engineered surfaces, the laser transfer of biomolecules and the functionalization of 3D structures constructed by three-photon stereolithography will be presented. Furthermore, the removal of molecular substrates by ultrafast laser ablation will be discussed with emphasis placed on assessing the photochemical changes induced in the remaining bulk material. The results indicate that in femtosecond laser processing of organic materials, besides the well acknowledged morphological advantages, a second fundamental factor responsible for its success pertains to the selective chemical effects. This is crucial for the laser cleaning of sensitive painted artworks.