Plasmonic hot carrier injection from single gold nanoparticles into topological insulator Bi2Se3 nanoribbons.

Plasmonic gold nanoparticles injecting hot carriers into the topological insulator (TI) interface of Bi2Se3 nanoribbons are studied by resonant Raman spectroscopy. We resolve the impact of individual gold particles with sizes ranging from 140 nm down to less than 40 nm on the topological surface states of the nanoribbons. In resonance at 1.96 eV (633 nm), we find distinct phonon renormalization in the Eg2- and A1g2-modes that can be associated with plasmonic hot carrier injection. The phonon modes are strongly enhanced by a factor of 350 when tuning the excitation wavelengths into interband transition and in resonance with the surface plasmon of gold nanoparticles. At 633 nm wavelength, a plasmonic enhancement factor of 18 is observed indicating a contribution of hot carriers injected from the gold nanoparticles into the TI interface. Raman studies as a function of gold nanoparticle size reveal the strongest hot carrier injection for particles with size of 108 nm in agreement with the resonance energy of its surface plasmon. Hot carrier injection opens the opportunity to locally control the electronic properties of the TI by metal nanoparticles attached to the surface of nanoribbons.

Conductance-strain behavior in silver-nanowire composites: network properties of a tunable strain sensor.

Highly flexible and conductive nano-composite materials are promising candidates for stretchable and flexible electronics. We report on the strain-resistance relation of a silver-nanowire photopolymer composite during repetitive stretching. Resistance measurements reveal a gradual change of the hysteretic resistance curves towards a linear and non-hysteretic behavior. Furthermore, a decrease in resistance and an increase in electrical sensitivity to strain over the first five stretching cycles can be observed. Sensitivity gauge factors between 10 and 500 at 23% strain were found depending on the nanowire concentration and stretching cycle. We model the electrical behavior of the investigated silver nanowire composites upon repetitive stretching considering the strain induced changes in the local force distribution within the polymer matrix and the tunnel resistance between the nanowires by using a Monte Carlo method.

Electrical and network properties of flexible silver-nanowire composite electrodes under mechanical strain.

Flexible and conductive silver-nanowire photopolymer composites are fabricated and studied under mechanical strain. The initial resistances of the unstretched flexible composites are between 0.27 Ω mm-1 and 1.2 Ω mm-1 for silver-nanowire concentrations between 120 µg cm-2 and 40 µg cm-2. Stretching of the samples leads to an increased resistance by a factor of between 72 for 120 µg cm-2 and 343 for 40 µg cm-2 at elongations of 23%. In order to correlate network morphology and electrical properties, micrographs are recorded during stretching. The Fiber Image Network Evaluation (FINE) algorithm determines morphological silver-nanowire network properties under stretching. For unstretched and stretched samples, an isotropic nanowire network is found with only small changes in fiber orientation. Monte-Carlo simulations on 2D percolation networks of 1D conductive wires and the corresponding network resistance due to tunneling of electrons at nanowire junctions confirm that the elastic polymer matrix under strain exhibits forces in agreement with Hooke's law. By variation of a critical force distribution the resistance curves are accurately reproduced. This results in a model that is dominated by quantum-mechanical tunneling at nanowire junctions explaining the electrical behavior and the sensitivity of nanowire-composites with different filler concentrations under mechanical strain.

Functional Printing of Conductive Silver-Nanowire Photopolymer Composites.

We investigated the fabrication and functional behaviour of conductive silver-nanowire-polymer composites for prospective use in printing applications. Silver-nanowires with an aspect ratio of up to 1000 were synthesized using the polyol route and embedded in a UV-curable and printable polymer matrix. Sheet resistances in the composites down to 13 Ω/sq at an optical transmission of about 90% were accomplished. The silver-nanowire composite morphology and network structure was investigated by electron microscopy, atomic force microscopy, profilometry, ellipsometry as well as surface sensitive X-ray scattering. By implementing different printing applications, we demonstrate that our silver nanowires can be used in different polymer composites. On the one hand, we used a tough composite for a 2D-printed film as top contact on a solar cell. On the other hand, a flexible composite was applied for a 3D-printed flexible capacitor.