Water-Mediated Assembly of Gold Nanoparticles into Aligned One-Dimensional Superstructures

This Article shows that water in ethanol colloids of gold nanoparticles enhances the formation of linear clusters and, more important for applications in electronics, determines their assembly on surfaces. We show by dynamic light scattering that ethanol colloids contain mainly monomers and dimers and that wormlike superstructures are mostly absent, despite UV–vis evidence of aggregation. Water added to the colloid as a cosolvent was found to enhance the number of clusters as well as their average size, confirming its role in linear self-assembly, on the scale of a few particles. Water adsorbed from the atmosphere during coating was also found to be a powerful lever to tune self-assembly on surfaces. By varying the relative humidity, a sharp transition from branched to linear superstructures was observed, showing the importance of water as a cosolvent in the formation of cluster superstructures. We show that one-dimensional superstructures may form due to long-range mobility of precursor clusters on wet surfaces, allowing their rearrangement. The understanding of the phenomenon allows us to statistically align both clusters and resulting superstructures on patterned substrates, opening the way to rapid screening in molecular electronics

Visualizing Local Morphology and Conductivity Switching in Interface-Assembled Nanoporous C₆₀ Thin Films

Carbon materials promise a revolution in optoelectronics, medical applications, and sensing provided that their morphology can be controlled down to the nanometer scale. Nanoporous materials are particularly appealing as they offer a drastically enlarged interfacial area compared to the corresponding planar materials. Entire fields such as organic solar cells, catalysis, or sensing may profit from an enlarged interface and facilitated molecular interaction between a carbon material and the environment. Nanoporous fullerene thin films obtained by the deposition of suspended nanoclusters of fullerene were already reported but suffered from the limitation of the size of these particles to over 100 nm. We study here a complementary method based on interfacial self-assembly forcing C₆₀ clusters to spontaneously form 2D percolating monolayers with most morphological features in the 5–20 nm range. Analysis of these films by means of electron microscopy and scanning probe microscopy proved their morphology to be a nanocomposite of crystalline beads embedded in an amorphous matrix of fullerenes. When contacted between two gold electrodes, these films show an intrinsic conductivity switching behavior. Their electrical conductivity could be reversibly switched on by applying a threshold electrical current and switched off by exposure to oxygen. Interestingly, the on-state exhibits an astonishing conductivity of over 10^–3 S/m. Kelvin probe force microscopy (KFM) was used to observe local changes in the distribution of electrical potential upon switching, on the relevant length scale of a few nanometers

100 GHz Plasmonic Photodetector

Photodetectors compatible with CMOS technology have shown great potential in implementing active silicon photonics circuits, yet current technologies are facing fundamental bandwidth limitations. Here, we propose and experimentally demonstrate for the first time a plasmonic photodetector achieving simultaneously record-high bandwidth beyond 100 GHz, an internal quantum efficiency of 36% and low footprint. High-speed data reception at 72 Gbit/s is demonstrated. Such superior performance is attributed to the subwavelength confinement of the optical energy in a photoconductive based plasmonic-germanium waveguide detector that enables shortest drift paths for photogenerated carriers and a very small resistance-capacitance product. In addition, the combination of plasmonic structures with absorbing semiconductors enables efficient and highest-speed photodetection. The proposed scheme may pave the way for a cost-efficient CMOS compatible and low temperature fabricated photodetector solution for photodetection beyond 100 Gbit/s, with versatile applications in fields such as communications, microwave photonics, and THz technologies

Data File 1: Harnessing nonlinearities near material absorption resonances for reducing losses in plasmonic modulators

Refractive Index of 75%HD-BB-OH and 25%YLD124 Originally published in Optical Materials Express on 01 July 2017 (ome-7-7-2168

Effect of Rigid Bridge-Protection Units, Quadrupolar Interactions, and Blending in Organic Electro-Optic Chromophores

A new organic electro-optic (EO) molecule was designed with two modifications aimed at increasing acentric order. The molecule is based on the well-known CLD donor-π bridge-acceptor template. The first structural modification introduces rigid aromatic fluorenyl and naphthyl site-isolation units (sterically bulky functional groups) to reduce aggregation. Site isolation units have been used in the past, but this is the first time that both the “front” and “back” of the CLD tetraene bridge have been modified with site-isolation units, and we had to introduce new synthetic methodology to do so. The second design element was the inclusion of cooperatively interacting aromatic dendron (HD) and fluoroaromatic dendron (FD) side groups to increase the acentric order. HD/FD units have previously been successfully used to increase EO performance, but we changed their location on the chromophore: they are attached to the donor and acceptor ends of the molecule to better match side chain ordering with the dipole moment of the molecule. Comparison chromophores were synthesized with alkyl (-MOM), hydroxyl (-OH), or HD units on the acceptor end of the molecule and either the traditional CLD bridge (T-bridge) or modified bridge (BB-bridge) for a family of eight chromophores. The HD/FD units increased glass transition temperature, <i>T</i>_g, by 4–21 °C, and the bulky bridge modification increased <i>T</i>_g by 27–44 °C, which is very beneficial as that results in extra thermal stability of the poling-induced acentric order. UV/vis absorbance spectroscopy shows that the site-isolation units reduce aggregation. Unfortunately, poor film formation of the neat materials precluded full chromophore evaluation in poling and <i>r</i>₃₃ experiments. The EO performance obtained for HD-BB-FD and HD-BB-OH was lower than expected, with <i>r</i>₃₃/<i>E</i>_p ≈ 1 nm² V^–2 at 1310 nm. We found that blending in 25 wt % YLD124 improved film-forming and poling efficiency. Due to the effect of blending and improved site isolation, <i>r</i>₃₃/<i>E</i>_p improved to 2.1–2.3 nm² V^–2 for 3:1 HD-BB-FD:YLD124, HD-BB-OH:YLD124, and HD-BB-MOM:YLD124, and <i>r</i>₃₃ as high as 351 pm V^–1 was obtained with 3:1 HD-BB-MOM:YLD124. Chromophore blends were also evaluated in plasmonic organic hybrid (POH) phase modulators with slot lengths of 5–20 μm. In POH devices, <i>r</i>₃₃ was as high as 325 pm V^–1 at 1260 nm and 220 pm V^–1 at 1520 nm. Overall, the increase in acentric order afforded by the HD/FD interactions was found to be small and resulted in no increase in <i>r</i>₃₃ due to the reduced number density. Ultimately, the increase in <i>r</i>₃₃/<i>E</i>_p afforded by the site isolation and blending resulted in a modest increase in <i>r</i>₃₃/<i>E</i>_p relative to YLD124, but combined with the increased <i>T</i>_g, the chromophore system is a significant improvement and points to an important design strategy