Hendrik Schlicke

Selective placement of faceted metal tips on semiconductor nanorods.

There is strong interest in the sequential synthesis of multicomponent nanostructures where there are segregated regions of different materials within one particle. To this end, there have been many reports on core–shell nanostructures as well as metal-semiconductor hybrids. Very recently, synthetic routes have even achieved the selective placement of one component onto another component of complex shape with a high degree of reproducibility. As an example, a metal (or semiconductor) can be selectively grown on the tips of semiconductor nanorods or onto semiconductor tetrapod arms. The potential utility of such structures was recently demonstrated in two different applications exhibiting the flexibility of such hybrid materials. In one study, the single particle conductivity was enhanced by many orders of magnitude when Au nanoparticles were selectively grown onto the ends of CdSe nanorods, compared with CdSe nanorods that did not have Au tips. In a second study, platinum tips were grown onto nanorods which had a CdSe seed embedded within a CdS nanorod. The catalytic activity of these particles with respect to hydrogen reduction in the overall water splitting reaction was then examined. It was demonstrated that the catalytic activity of the structure could be affected by either altering the length of the nanorod or by controlling the size of the CdSe seed. In prior work, the metal tips selectively deposited on the ends of semiconductor rods have been of undefined shape and faceting. Yet the catalytic activity and selectivity of the metal particles is strongly related to the crystal shape and specific surface facet on which the chemical reaction occurs. Herein, we demonstrate that it is possible to specifically control the faceting of metallic nanocrystals grown in colloidal solution on the tips of semiconductor nanorods by depositing cubeshaped Pt nanoparticles at the tips of both CdS and CdS/CdSe seeded nanorods. The cube-like particles were formed in two steps: an intitial deposition of Pt without shape control, followed by further growth in the presence of a molecule that favors defined facets. To initially grow platinum tips on the nanorods, a calculated quantity of nanorods was mixed with platinum acetylacetonate (Pt(acac)2), 1,2-hexadecanediol (HDD), oleic acid (OAC), and oleylamine (OAM) in a solution of diphenyl ether (see Supporting Information for experimental details). The procedure is similar to that reported by Habas et al., with slight modifications. This reaction yields small, Pt-metal particles of mostly undefined shapes on the rod tip. The metal particles were formed preferentially on either one or both tips ({0001} facets) of the nanorods (Figure 1, middle column), depending on the initial nanorod concentration. For our experiment we chose an initial rod concentration of 2.9 nmol (the amount of the Pt-precursor and a detailed description are given in the Supporting Information). This method achieved a high yield of nanorods having either one or two Pt-tips. HRTEM analysis of the as-synthesized samples did not show specific faceting of the small Pt tips (Supporting Information, Figure S4). To grow faceted, cube-like tips, we used carbon monoxide as both the metal-reducing and shape-directing agent. It has been well established that CO adsorbs preferentially to specific metal surfaces. In some cases, this selective binding can lead to directed growth of Pt atoms onto specific facets of the Pt nanoparticle. Carbon monoxide can also undergo oxidation to CO2 on selected Pt facets. [13] The behavior of CO molecules bound to Pt is also contingent on which facet the molecule is bound to. For example, CO molecules bound to the {100} surface of Pt are slightly more stretched and closer to the particle surface than COmolecules bound to the {111} facet of Pt. This is a result of the stronger binding of CO on the {100} Pt surface with respect to the {111} surface. In the case of platinum, these different binding [*] H. Schlicke, Dr. D. Ghosh, L.-K. Fong, Prof. A. P. Alivisatos Department of Chemistry, University of California Berkeley Berkeley, CA 94720 (USA) E-mail: alivis@berkeley.edu H. Schlicke, Dr. D. Ghosh, L.-K. Fong, Dr. H. L. Xin, Dr. H. Zheng, Prof. A. P. Alivisatos Materials Science Division, Lawrence Berkeley National Laboratory (LBNL) Berkeley, CA 94720 (USA) [] Present address: Department of Chemistry, Universit t Hamburg Martin-Luther-King-Platz 6, 20146 Hamburg (Germany)

Freestanding films of crosslinked gold nanoparticles prepared via layer-by-layer spin-coating

A new, extremely efficient method for the fabrication of films comprised of gold nanoparticles (GNPs) crosslinked by organic dithiols is presented in this paper. The method is based on layer-by-layer spin-coating of both components, GNPs and crosslinker, and enables the deposition of films several tens of nanometers in thickness within a few minutes. X-ray diffraction and conductance measurements reveal the proper adjustment concentration of the crosslinker solution of the critical is in order to prevent the destabilization and coalescence of particles. UV/vis spectroscopy, atomic force microscopy, and conductivity measurements indicate that films prepared via layer-by-layer spin-coating are of comparable quality to coatings prepared via laborious layer-by-layer self-assembly using immersion baths. Because spin-coated films are not bound chemically to the substrate, they can be lifted-off by alkaline underetching and transferred onto 3d-electrodes to produce electrically addressable, freely suspended films. Comparative measurements of the sheet resistances indicate that the transfer process does not compromise the film quality.

Controlling Molecular Conductance: Switching Off π Sites through Protonation

Conductance switching through chemical modification of a molecular bridge is a major goal in molecular electronics, with the potential to lead to molecule-based functional devices. In terms of switching speed, mechanisms that rely on only minor rearrangements of molecular structures are particularly promising. We demonstrate, based on density functional theory calculations combined with a coherent tunneling approach, how protonation and deprotonation of amine-substituted or amine-bridged model molecular wires can switch off and on π-sites and thus: a) remove or introduce interference features in the electron transmission, and b) decrease or increase coupling along a chain. This mechanism may also be relevant for interactions between molecular bridges and metal cations, for example, in sensor applications.

Fabrication of Strain Gauges via Contact Printing: A Simple Route to Healthcare Sensors Based on Cross-Linked Gold Nanoparticles

In this study, we developed a novel and efficient process for the fabrication of resistive strain gauges for healthcare-related applications. First, 1,9-nonanedithiol cross-linked gold nanoparticle (GNP) films were prepared via layer-by-layer (LbL) spin-coating and subsequently transferred onto flexible polyimide foil by contact printing. Four-point bending tests revealed linear response characteristics with gauge factors of ∼14 for 4 nm GNPs and ∼26 for 7 nm GNPs. This dependency of strain sensitivity is attributed to the perturbation of charge carrier tunneling between neighboring GNPs, which becomes more efficient with increasing particle size. Fatigue tests revealed that the strain-resistance performance remained nearly the same after 10.000 strain/relaxation cycles. We demonstrate that these sensors are well suited to monitor muscle movements. Furthermore, we fabricated all-printed strain sensors by directly transferring cross-linked GNP films onto soft PDMS sheets equipped with interdigitated electrodes. Due to the low elastic modulus of poly(dimethylsiloxane) (PDMS), these sensors are easily deformed and, therefore, they respond sensitively to faint forces. When taped onto the skin above the radial artery, they enable the well-resolved and robust recording of pulse waves with diagnostically relevant details.

Cross-Linked Gold-Nanoparticle Membrane Resonators as Microelectromechanical Vapor Sensors

We report a novel approach for the detection of volatile compounds employing electrostatically driven drumhead resonators as sensing elements. The resonators are based on freestanding membranes of alkanedithiol cross-linked gold nanoparticles (GNPs), which are able to sorb analytes from the gas phase. Under reduced pressure, the fundamental resonance frequency of a resonator is continuously monitored while the device is exposed to varying partial pressures of toluene, 4-methylpentan-2-one, 1-propanol, and water. The measurements reveal a strong, reversible frequency shift of up to ∼10 kHz, i.e., ∼5% of the fundamental resonance frequency, when exposing the sensor to toluene vapor with a partial pressure of ∼20 Pa. As this strong shift cannot be explained exclusively by the mass uptake in the membrane, our results suggest a significant impact of analyte sorption on the pre-stress of the freestanding GNP membrane. Thus, our findings point to the possibility of designing highly sensitive resonators, which utilize sorption induced changes in the membranes pre-stress as primary transduction mechanism.