Molecular Adsorption to LiMo₃Se₃ Nanowire Film Chemiresistors

Abstract

Thin films of metallic nanowire bundles derived from the Chevrel compound LiMo3Se3 undergo reversible increases of their electrical resistance (up to 70%) upon exposure to vapors of organic solvents (Qi, X. B.; Osterloh, F. E. J. Am. Chem. Soc. 2005, 127 (21), 7666-7667). Using quartz crystal microbalance measurements with four analytes, we demonstrate here that the temporal and steady-state resistance changes of the films depend on the time following the adsorption and on the number of molecules that adsorb to the nanowire films at a given pressure. The adsorption ability of the films and the corresponding film resistance increase in the row: hexane < THF < ethanol < DMSO, closely following the polarities of the solvents. On average, approximately 10(5) analyte molecules per LiMo3Se3 unit are required to produce a measurable electrical response. Atomic force microscopy scans on nanowire films reveal that analytes deposit on top of the nanowire bundles and cause the films to swell by approximately 6% in volume. The temporal and steady-state resistance data of the LiMo3Se3 chemiresistors can be explained by assuming that coating of the nanowire bundles with analyte molecules reduces the interwire charge transport in the films.

Self-Assembly of LiMo3Se3 Nanowire Networks from Nanoscale Building-Blocks in Solution

“…The orange lines on the STEM image outline one bundle (on the right-hand side of the image) that splays out and joins up with other nanowires on the left-hand side of the image. The bright spots seen in the image are due to solvent residue . Figures b and c are STEM images of LiMo 3 Se 3 nanowire bundles with splayed ends, typical of the constituent materials of these networks.…”

“…It is easy to understand how a network of this type is formed by directed self-assembly, particularly in the context of the pronounced bonding anisotropy that exists within the solid crystal. A strong end-to-end interaction between the building-blocks guarantees strong ordering along the wire direction and ultimately the growth of long highly ordered wires that are responsible for the excellent conductivity of these films. ,,, On the other hand, the lateral interaction between building blocks is rather weak, leading to variable interwire spacing in a bundle and which in turn facilitates the interweaving of wires from one bundle to the next and ultimately the observed network morphology.…”

“…Golden et al observed thermally activated conductivity on a 1 μm thick LiMo 3 Se 3 foil, which showed a semiconducting-like resistance increase with decreasing temperature. Qi et al have suggested that analyte condensation on their LiMo 3 Se 3 film chemiresistors cause the interbundle separations to increase, resulting in break junctions which drastically affect the conductivity of the film. In both of these examples it seems likely that the continuous nature of the nanowire junctions described in our work play an important role in the conductivity behaviors they have observed.…”

“…Similarly, the Lieber group made use of scanning tunneling microscopy (STM) to image individual Mo 3 Se 3 wires and bundles but not networks or junctions of this material. More recently the group of Osterloh − has fabricated films of LiMo 3 Se 3 deposited from solution for sensor applications. Once again, large scale networks have been reported, but the mechanism of wire formation or the nature of the junctions between the wires was not studied.…”

First principles study of electronic and mechanical properties of molybdenum selenide type nanowires

“…Moreover, thin films of metallic Li 2 Mo 6 Se 6 nanowire ropes can be used as a chemical sensor. Xiubin et al 24,25 have showed that resistance of such a thin film changes upon adsorption of organic molecules, suggesting that the electrical conductance of the Mo 6 Se 6 nanowire may change with the type and number of adsorbed atoms or molecules.…”