Surprising Molecular Length Dependence in Conduction through a Hybrid Organic-Inorganic System Sivan Kober, Gilad Gotesman, and Ron Naaman* Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel * S Supporting Information ABSTRACT: 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. SECTION: Physical Processes in Nanomaterials and Nanostructures C onduction properties of organic molecules have been the focus of many studies for years. The interest in this subject results from two origins: the first is the need for a basic understanding of the electronic properties of organic molecules, and the other stems from the idea that one may introduce interesting optoelectronic properties to electronic devices by using organic molecules. However, in practice, the subject was found to be quite complex due to the need to make a well- defined and reproducible contact between the organic and the inorganic, typically metal, electrodes and due to the molecular conformation, which is not always controllable. It is well-established that because those molecules are in general poor conductors, the conductivity decreases exponen- tially for short-range conduction and may decrease as a power law for longer ranges. This is indeed what has been observed by studying either conduction through single molecules or through self-assembled monolayers (SAMs). One approach suggests that hybrid devices may be used for introducing the properties of organic molecules to electronic devices, where organic molecules are mixed with metal nanoparticles (NPs). 1-3 There are two NP-based techniques: in the first, a dimer of NPs is connected by a bifunctional molecule (or an ensemble of molecules). Although in this method the NP-molecule structure is reproducible, 4 the contact between the NPs and the electrode is less controlled and is not well-defined. The second technique consists of two electrodes separated by a small gap, covered by a SAM, whereas metal NPs are deposited to bridge the gap. 2,5-7 Extension of the second method includes microscale devices that employ metal NPs mixed with organic molecules. 8-12 Here the main goal is to better understand the conduction mechanisms in the whole assembly and to probe how it is affected by the molecular properties. Indeed, in this case, one does not obtain the details observed when studying single-molecule-based devices. In the present work, we investigated the effect of the length of saturated organic molecules on the conduction properties of hybrid devices. The device is based on mixing gold NPs (GNPs) with alkanedithiols of different lengths, 1,3-propane- dithiol, 1,6-hexanedithiol, and 1,10-decanedithiol (C 3 DT, C 6 DT, and C 10 DT, respectively, Figure 1a), and depositing the hybrid mixture between two electrodes positioned 2 μm apart. The current versus voltage curves (I-V) were measured for different lengths of alkanedithiols. As a reference, devices with alkanemonothiols (C x MT) of lengths similar to the alkanedithiols were also measured. Because a large ensemble of molecules and GNPs is measured, the effect of the molecular structure is expressed by averaging the conduction properties of the entire ensemble. It will be shown that in such hybrid devices the molecule- length dependence of the conduction is not trivial and it deviates dramatically from the expectations based on the single- molecule properties. A microelectronic interdigitated device was built containing 33 pairs of electrodes (dimensions: 200 μm in length and 150 nm in height) with a total overlap length of 6600 μm and a 2 μm gap (Figure 1b). The devices were prepared by a conventional photolithography procedure on glass substrates and coated with a hybrid film containing alkanethiols and GNPs. The NPs were synthesized following the recipe described in the Supporting Information, and their size was verified by their plasmon resonance peak (Figure 2a). Received: May 5, 2013 Accepted: June 4, 2013 Published: June 4, 2013 Letter pubs.acs.org/JPCL © 2013 American Chemical Society 2041 dx.doi.org/10.1021/jz400943q | J. Phys. Chem. Lett. 2013, 4, 2041-2045