Silicon-Immobilized Ferrocenyl Monolayers for All-Solid Molecular Charge Storage and Logic

The functionalization of technologically relevant conducting surfaces, such as oxide-free, hydrogen-terminated silicon (H-Si), with high-quality ferrocene (Fc) [1] and metal-complexed porphyrin [2] -terminated monolayers has been demonstrated to be a powerful bottom-up approach for the fabrication of electrically addressable charge-storage devices with low-power consumption.

Compared with metalloporphyrins, Fc is a much smaller molecule (average diameter of tetraphenylporphyrin and Fc are about 18 and 6.6 Å, respectively), and consequently is immobilized on silicon with a higher surface coverage, which gives rise to higher charge densities. Indeed, the surface coverages reached for high-quality ferrocenyl monolayers are in the range of (2.0–5.0) ×10^-10 mol cm^-2. This does not only allow for an extremely fast electron communication between the electroactive groups [3], but also yields charge densities in the range of 20–50 mC cm^-2. These values are thus much higher than those measured for Si/SiO₂ capacitors currently used in Dynamic Random Access Memories (5–10 mC cm^-2).

Very recently, we also demonstrated that tailor-made micrometer-sized patterns of such redox-active monolayers could behave as light-activated molecular memory cells operating at low voltages with unprecedented capacitance performances [4]. The characteristics of this stimuli-responsive device are of great scientific interest for data-processing applications, such as redox-based Boolean logic gates [5]. In our system, the switching of the capacitance upon light irradiation that is observed only at a certain electrical potential constitutes the principle of all-solid two-input AND logic gate without the need of chemical inputs in solution. Such performances can be ascribed to the judicious combination between a photoswitchable conducting/insulating silicon substrate and high-quality microstamped redox-active assemblies.

[1] Fabre, B. Acc. Chem. Res. 2010, 43, 1509–1518.

[2] Lindsey, J. S.; Bocian, D. F. Acc. Chem. Res. 2011, 44, 638–650.

[3] a) Hauquier, F.; Ghilane, J.; Fabre, B.; Hapiot, P. J. Am. Chem. Soc. 2008, 130, 2748–2749. b) Zigah, D.; Herrier, C.; Scheres, L.; Giesbers, M.; Fabre, B.; Hapiot, P.; Zuilhof, H. Angew. Chem. Int. Ed. 2010, 49, 3157–3160.

[4] Fabre, B.; Li, Y.; Scheres, L.; Pujari, S. P.; Zuilhof, H. Angew. Chem. Int. Ed. 2013, 52, 12024-12027.

[5] de Ruiter, G.; van der Boom, M. E. Acc. Chem. Res. 2011, 44, 563–573.