Surprising Molecular Length Dependence in Conduction through a Hybrid Organic-Inorganic System.

A hybrid device made from gold nanoparticles connected by alkyldithiol molecules of different lengths was produced and its conduction properties were investigated for various lengths of the organic linker molecules. It was found that the conductivity increases with the length of the molecules. The surprising dependence of the conductivity on the molecules' length was explained by a model that takes into account the probability for forming continuous conductive paths for the different molecules.

Horizontal versus vertical charge and energy transfer in hybrid assemblies of semiconductor nanoparticles.

We studied the photoluminescence and time-resolved photoluminescence from self-assembled bilayers of donor and acceptor nanoparticles (NPs) adsorbed on a quartz substrate through organic linkers. Charge and energy transfer processes within the assemblies were investigated as a function of the length of the dithiolated linker (DT) between the donors and acceptors. We found an unusual linker-length-dependency in the emission of the donors. This dependency may be explained by charge and energy transfer processes in the vertical direction (from the donors to the acceptors) that depend strongly on charge transfer processes occurring in the horizontal plane (within the monolayer of the acceptor), namely, parallel to the substrate.

How isolated are the electronic states of the core in core/shell nanoparticles?

We investigated how isolated are the electronic states of the core in a core-shell (c/s) nanoparticles (NPs) from the surface, when the particles are self-assembled on Au substrates via a dithiol (DT) organic linker. Applying photoemission spectroscopy the electronic states of CdSe core only and CdSe/ZnS c/s NPs were compared. The results indicate that in the c/s NPs the HOMO interacts strongly with electronic states in the Au substrate and is pinned at the same energies, relative to the Fermi level, as the core only NPs. When the capping molecules of the NPs were replaced with thiolated molecules, an interaction between the thiol groups and the electronic states of the NPs was observed that depends on the properties of the NPs studied. Thiols binding to the NPs induce the formation of surface trap states. However, while for the core only CdSe NPs the LUMO states are strongly coupled to the surface traps, independent of their size, this coupling is size dependent in the case of the CdSe/ZnS c/s NPs. For a large core, the LUMO is decoupled from the surface trap states. When the core is small enough, the LUMO is delocalized and interacts with these states.

Self-assembly of nanoparticle arrays on semiconductor substrate for charge transfer cascade.

The current study focuses on the interaction between hierarchical structures of nanoparticles (NPs) and a semiconductor substrate on which they are assembled. Monolayer and bilayer assemblies of two different NPs were prepared on the surface of a GaAs substrate. The photoluminescence response of the bilayer assemblies depends on their hierarchy, namely on the ordering of differently sized nanoparticles with respect to the surface; however, the surface photovoltage does not. Based on these studies, it is possible to determine the importance of each of the possible quenching mechanisms for electron-hole pair excitation.

Selective surface patterning for the coadsorption of self-assembled gold and semiconductor nanoparticles.

The predetermined patterned adsorption of two types of nanoparticles on the same substrate may be of considerable importance in various applications, among others, to enhance the absorption of semiconductor nanoparticles by the plasmonic effect of metal NPs. We describe here a simple method for self-assembling 2D lateral patterns in which both gold and semiconductor nanoparticles are adsorbed, each in a predesigned area. Our method is based on a one-step lithographic process and the adsorption of two distinct self-assembled monolayers that can selectively bind only one type of nanoparticle.

Immobilizing a drop of water: fabricating highly hydrophobic surfaces that pin water droplets.

We describe the fabrication of a patterned, hydrophobic silicon substrate that can pin a water droplet despite its large contact angle. Arrays of nm tips in silicon were fabricated by reactive ion etching using polymer masks defined by photolithography. A droplet sitting on one class of these substrates did not fall even after the substrate was turned upside-down. The production allows the fabrication of large arrays of tips with a one-step simple etching process, along with silanization, to achieve a substrate with both very large contact and tilting angles.