INNATE · Integrated Nanocrystal Tunnelling for Molecular Electronics

Integrated Nanocrystal Tunnelling for Molecular Electronics

The INNATE project has been concerned with the electrochemical and electronic behaviour of small monolayer-protected clusters (MPCs) of gold, to explore their potential for application in molecular electronics. The main achievements can be summarised as follows: - Highly monodisperse Au147C6S (C6S = n-hexanethiolate) MPCs have been synthesised, for which 16 single-electron charging states have been electrochemically resolved [1]. This number is the highest reported to date. - We have demonstrated for the first time that well-behaved quantised charging of gold MPCs is also possible in an air- and water-stable room temperature ionic liquid [1]. As ionic liquids have very attractive features including near-zero vapour pressure, considerable thermal stability and an electrochemical stability window that often exceeds 4 volt, this demonstration is particularly significant from the technological point of view. - The dynamics of the quantised cluster charging process in different ionic liquids has been studied in detail using voltammetric and impedance measurements [2] . We have discovered that the MPC charging rate can vary up to an order of magnitude depending on ionic liquid composition, which indicates that a judicious choice of electrolyte is crucial in the design of an electrochemical device. The differences in dynamics between the various ionic liquids have been rationalised in terms of varying chemical availability of free ions within the liquids, necessary to compensate the charges on the MPC cores. - We have demonstrated for the first time that dithiol-functionalised MPCs can disintegrate spontaneously on gold substrates, forming monoatomically high gold islands with low-coverage thiol/dithiol adlayers [3]. This is a strong indication that the ligand shell of MPC structures can be quite dynamic and its stability strongly linked to its environment. - We have demonstrated that for electrostatically stabilised nanocrystals, an electrostatic immobilisation approach can be a valuable alternative to covalent attachment [4]. This development is of particular significance for applications in electroanalysis and -catalysis [5]. - Using in situ scanning tunnelling microscopy, we have demonstrated that a redox-active adlayer (6-thiohexanoylferrocene) on a gold surface can generate nanoparticles following multiple oxidation/reduction cycles of the adlayer [6]. The clusters exhibit a narrow size distribution with an average diameter of (2.4 +/- 0.5) nm, corresponding to a single-particle capacitance of (1.5 +/- 0.2) aF. To our knowledge, this is the first demonstration of such spontaneous transformation. Both the redox resonance feature, which can be quantitatively understood based on existing models, and the multistate cluster resonance, for which proper models are still required, could be exploited in electrochemical/molecular electronic devices. In summary, the INNATE project has demonstrated the substantial application potential of nanocrystal-mediated tunnelling for molecular electronics, and in a broader context, the power of electrochemical methodologies in these areas. As this project was situated in the basic research field, its socio-economic impact must be seen in the longer term, after more applied studies have been carried out based on the principles demonstrated by us. [1] S.F.L. Mertens, K. Blech, A.S. Sologubenko, J. Mayer, U. Simon, T. Wandlowski, Electrochim. Acta 54, 2009, 5006-5010. [2] S.F.L. Mertens, G. Mészáros, T. Wandlowski, Phys. Chem. Chem. Phys., 12, 2010, 5417-5424. [3] S.F.L. Mertens, Z. Li, T. Wandlowski, Electrochem. Commun. 6/2010, in preparation. [4] S.F.L. Mertens, A. Bütikofer, L. Siffert, T. Wandlowski, Electroanalysis, submitted. 5/2010 [5] S.F.L. Mertens, A. Bütikofer, T. Wandlowski, D.J. Schiffrin, in preparation. [6] Z. Li, Y. Liu, S.F.L. Mertens, I.V. Pobelov, T. Wandlowski, J. Am. Chem. Soc., 132 , 2010, 8167 - 8193

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