Self-Catalyzed Growth of Vertically Aligned InN Nanorods by Metal-Organic Vapor Phase Epitaxy.Tessarek, C.; Fladischer, S.; Dieker, C.; Sarau, G.; Hoffmann, B.; Bashouti, M.; Göbelt, M.; Heilmann, M.; Latzel, M.; Butzen, E.; Figge, S.; Gust, A.; Höflich, K.; Feichtner, T.; Büchele, M.; Schwarzburg, K.; Spiecker, E.; Christiansen, S. in Nano Letters (2016). 16(6) 3415--3425.
Vertically aligned hexagonal InN nanorods were grown mask-free by conventional metal-organic vapor phase epitaxy without any foreign catalyst. The In droplets on top of the nanorods indicate a self-catalytic vapor-liquid-solid growth mode. A systematic study on important growth parameters has been carried out for the optimization of nanorod morphology. The nanorod N-polarity, induced by high temperature nitridation of the sapphire substrate, is necessary to achieve vertical growth. Hydrogen, usually inapplicable during InN growth due to formation of metallic indium, and silane are needed to enhance the aspect ratio and to reduce parasitic deposition beside the nanorods on the sapphire surface. The results reveal many similarities between InN and GaN nanorod growth showing that the process despite the large difference in growth temperature is similar. Transmission electron microscopy, spatially resolved energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy have been performed to analyze the structural properties. Spatially resolved cathodoluminescence investigations are carried out to verify the optical activity of the InN nanorods. The InN nanorods are expected to be the material of choice for high-efficiency hot carrier solar cells.
New insights into colloidal gold flakes: structural investigation, micro-ellipsometry and thinning procedure towards ultrathin monocrystalline layers.Hoffmann, B.; Bashouti, M. Y.; Feichtner, T.; Mavckovi'c, M.; Dieker, C.; Salaheldin, A. M.; Richter, P.; Gordan, O. D.; Zahn, D. R. T.; Spiecker, E.; Christiansen, S. in Nanoscale (2016). 8(8) 4529--4536.
High-quality fabrication of plasmonic devices often relies on wet-chemically grown ultraflat, presumably single-crystalline gold flakes due to their superior materials properties. However, important details about their intrinsic structure and their optical properties are not well understood yet. In this study, we present a synthesis routine for large flakes with diameters of up to 70 \($\mu$\)m and an in-depth investigation of their structural and optical properties. The flakes are precisely analyzed by transmission electron microscopy, electron backscatter diffraction and micro-ellipsometry. We found new evidence for the existence of twins extending parallel to the Au flake 111 surfaces which have been found to not interfere with the presented nanopatterning. Micro-Ellipsometry was carried out to determine the complex dielectric function and to compare it to previous measurements of bulk single crystalline gold. Finally, we used focused ion beam milling to prepare smooth crystalline layers and high-quality nanostructures with desired thickness down to 10 nm to demonstrate the outstanding properties of the flakes. Our findings support the plasmonics and nano optics community with a better understanding of this material which is ideally suited for superior plasmonic nanostructures.
Gold platelets for high-quality plasmonics.Hoffmann, Björn; Feichtner, Thorsten; Christiansen, Silke in Materials Today (2016). 19(4) 240--241.
Electromechanically Tunable Suspended Optical Nanoantenna.Chen, Kai; Razinskas, Gary; Feichtner, Thorsten; Grossmann, Swen; Christiansen, Silke; Hecht, Bert in Nano letters (2016). 16(4) 2680--5.
Coupling mechanical degrees of freedom with plasmonic resonances has potential applications in optomechanics, sensing, and active plasmonics. Here we demonstrate a suspended two-wire plasmonic nanoantenna acting like a nanoelectrometer. The antenna wires are supported and electrically connected via thin leads without disturbing the antenna resonance. As a voltage is applied, equal charges are induced on both antenna wires. The resulting equilibrium between the repulsive Coulomb force and the restoring elastic bending force enables us to precisely control the gap size. As a result the resonance wavelength and the field enhancement of the suspended optical nanoantenna can be reversibly tuned. Our experiments highlight the potential to realize large bandwidth optical nanoelectromechanical systems.
Evolutionary Optimization of Optical Antennas.Feichtner, Thorsten; Selig, Oleg; Kiunke, Markus; Hecht, Bert in Physical Review Letters (2012). 109(12) 127701.
The design of nanoantennas has so far been mainly inspired by radio-frequency technology. However, the material properties and experimental settings need to be reconsidered at optical frequencies, which would entail the need for alternative optimal antenna designs. Here we subject a checkerboard-type, initially random array of gold cubes to evolutionary optimization. To illustrate the power of the approach, we demonstrate that by optimizing the near-field intensity enhancement, the evolutionary algorithm finds a new antenna geometry, essentially a split-ring–two-wire antenna hybrid that surpasses by far the performance of a conventional gap antenna by shifting the n=1 split-ring resonance into the optical regime.
Atomically flat single-crystalline gold nanostructures for plasmonic nanocircuitry.Huang, Jer-Shing; Callegari, Victor; Geisler, Peter; Brüning, Christoph; Kern, Johannes; Prangsma, Jord C; Wu, Xiaofei; Feichtner, Thorsten; Ziegler, Johannes; Weinmann, Pia; Kamp, Martin; Forchel, Alfred; Biagioni, Paolo; Sennhauser, Urs; Hecht, Bert in Nature communications (2010). 1(9) 150----.
Deep subwavelength integration of high-definition plasmonic nanostructures is of key importance in the development of future optical nanocircuitry for high-speed communication, quantum computation and lab-on-a-chip applications. To date, the experimental realization of proposed extended plasmonic networks consisting of multiple functional elements remains challenging, mainly because of the multi-crystallinity of commonly used thermally evaporated gold layers. This can produce structural imperfections in individual circuit elements that drastically reduce the yield of functional integrated nanocircuits. In this paper we demonstrate the use of large (textgreater100 \($\mu$\)m(2)) but thin (\textless80 nm) chemically grown single-crystalline gold flakes that, after immobilization, serve as an ideal basis for focused ion beam milling and other top-down nanofabrication techniques on any desired substrate. Using this methodology we obtain high-definition ultrasmooth gold nanostructures with superior optical properties and reproducible nano-sized features over micrometre-length scales. Our approach provides a possible solution to overcome the current fabrication bottleneck and realize high-definition plasmonic nanocircuitry.
