Viktor Schneider

Light-Controlled Conductance Switching in Azobenzene-Containing MWCNT–Polymer Nanocomposites

We report on reversible light-controlled conductance switching in devices consisting of multiwalled carbon nanotube (MWCNT)-polymer nanocomposites blended with azobenzene molecules and photoisomerization of the latter. Both the azobenzene molecules and MWCNT, which are functionalized with carboxyl groups (MWCNT-COOH), are embedded independently in a poly(methyl methacrylate) matrix, and thin films are prepared by using a simple spin-coating technique. We demonstrate the feasibility of the present concept with a photocurrent switching amplitude of almost 10%.

Measurement of the force on microparticles in a beam of energetic ions and neutral atoms

The force on microparticles in an energetic ion beam is investigated experimentally. Hollow glass microspheres are injected into the vertically upward directed beam and their trajectories are recorded with a charge-coupled device camera. The net force on the particles is determined by means of the measured vertical acceleration. The resulting beam pressures are compared with Faraday cup measurements of the ion current density and calorimetric measurements of the beam power density. Due to the neutral gas background, the beam consists, besides the ions, of energetic neutral atoms produced by charge-exchange collisions. It is found that the measured composition of the drag force by an ion and a neutral atom component agrees with a beam model that takes charge-exchange collisions into account. Special attention is paid to the momentum contribution from sputtered atoms, which is shown to be negligible in this experiment, but should become measurable in case of materials with high sputtering yields.

An experiment for the investigation of forces on microparticles in ion beams

A novel experiment for the study of forces on microparticles in ion beams is presented. A broad beam ion source provides a vertically upward directed beam wherein 100 microm hollow glass spheres are injected. The particles are illuminated by a diode laser and recorded with a charge-coupled device camera. From the trajectories the acceleration and the net force on the particles are determined. Information on energetic neutral atoms is achieved, which is not accessible by electrostatic methods.

Measurement of the Force on Microparticles in an Energetic Ion Beam

Forces on microparticles in energetic ion beams are investigated experimentally. For this purpose, hollow glass microspheres are injected into a vertically upward directed beam. The particles are illuminated by a diode laser, and their scattered light is recorded with a charge-coupled device camera. From the trajectories, the acceleration and net force on the particles are determined. It is found that the force is significantly higher than the expected ion drag force. This additional part of the total force is explained with fast neutral atoms produced by charge-exchange collisions. Special attention is paid to the momentum contribution from sputtered atoms, which is shown to be negligible in this experiment.

Light-induced Conductance Switching in Photomechanically Active Carbon Nanotube-Polymer Composites

Novel, optically responsive devices with a host of potential applications have been demonstrated by coupling carbon nanomaterials with photochromic molecules. For light-induced conductance switching in particular, we have recently shown that carbon nanotube-polymer nanocomposites containing azobenzene are very attractive and provide stable and non-degradable changes in conductivity over time at standard laboratory conditions. In these composites, the photoswitching mechanisms are based on light-induced changes in electronic properties and related to the Pool-Frenkel conduction mechanism. However, no link between conductivity switching and the molecular motion of azobenzene chromophores could be found due to application of high elastic modulus polymer matrices. Here we report on single wall carbon nanotube-polymer nanocomposites with a soft polycaprolactone polymer host. Such a system clearly shows the transfer of light-induced, nano-sized molecular motion to macroscopic thickness changes of the composite matrix. We demonstrate that these photomechanical effects can indeed overshadow the electronic effects in conductivity switching behavior and lead to a reversion of the conductivity switching direction near the percolation threshold.

An optical trapping system for particle probes in plasma diagnostics

We present one of the first experiments for optically trapping of single microparticles as probes for low temperature plasma diagnostics. Based on the dual laser beam, counter-propagating technique, SiO2 microparticles are optically trapped at very large distances in low-temperature, low-pressure rf plasma. External forces on the particle are measured by means of the displacement of the probe particle in the trap. Measurements can be performed during plasma operation as well as without plasma. The paper focuses on the optical setup and the verification of the system and its principle. Three examples for the particle behavior in the trapping system are presented: First, we measured the neutral gas damping as a verification of the technique. Second, an experiment without a plasma studies the changing particle charge by UV light radiation, and third, by moving the probe particle in the vertical direction into the sheath or into the plasma bulk, respectively, the acting forces on the probe particle are measured.

Recent development for a plasma diagnostic with optically trapped microparticles

Summary form only given. The idea to use microparticles for plasma diagnostic purposes was implemented during the last years by several experiments as electrostatic or thermal probes1-5. In contrast to the commonly used diagnostic methods, microparticles rarely influence the surrounding plasma. However, the particle position and, thus, the measurement is mostly restricted to the plasma sheath region by the force balance. A change in the position, often possible only into one direction, is then associated with a considerable effort or just by changing the discharge and, thus, by changing the plasma parameters itself. Based on the principle of laser tweezing6, we present a noninvasive method for trapping and arbitrary manipulation of the microparticles position in the plasma7. We demonstrate how an externally applied force on the particle is determined by a position determination in the trap. Furthermore, we present the current stage of development as well as some possible plasma diagnostic applications.

On the use of microscopic test particles for non-conventional plasma sheath diagnostics

Summary form only given. The idea to use microscopic test particles as electrostatic and thermal probes in complex plasmas has been consequently developed during the last years1–3. Due to the force balance of the particles, however, it is very difficult to change their position in the plasma sheath without changing the external and internal plasma parameters. Recently, experiments have been performed where the confined particles are affected by additional centrifugal force4 or by laser radiation5.

Contact charging of microparticles for space propulsion and terrestrial applications

This paper reports on experiments aiming at an efficient technique for contact charging of conductive microparticles. Fine electrode structures generate high electric field strengths which charge the particles in contact with the high voltage electrode. Highly charged microparticles are a prerequisite for electrostatic microparticle acceleration in laboratory experiments and advanced concepts in space propulsion and terrestrial applications. Possible terrestrial applications are the treatment of surfaces and hypervelocity experiments, e.g. simulation of micrometeorites.