Ž . Synthetic Metals 107 1999 1–6 www.elsevier.comrlocatersynmet Nano-scale protonic rectifier Jerzy J. Langer ) , Marcin Martynski ´ ´ A. Mickiewicz UniÕersity at Poznan, Faculty of Chemistry, Organic Semiconductor Laboratory, Marciniaka 2, PL-63100 Srem, Poland ´ Received 1 February 1998; received in revised form 1 February 1998; accepted 18 May 1999 Abstract The work describes a new approach to the molecular electronics: nano-scale protonic rectifier. The concept and experimental results are presented and discussed. We have developed a method to fabricate nano-scale protonic devices. Self-assembled monomolecular layer Ž . of 3-marcaptopropionic acid MPA has been spread onto a gold covered glass plate, then the second layer was formed with Ž . 3-aminopropyltriethoxysilane APTES and stabilised by condensation triethoxysilil ends. The structure and electrical properties of the Ž . complex layer were examined with scanning tunnelling microscope STM . We found a strong asymmetry of the I – V characteristic owing to rectifying behaviour of the layer. This is the first protonic rectifier of a nanometer thickness. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Molecular electronics; Molecular devices; Protonic rectifier 1. Introduction Molecular nanometer-scale devices have for decades been in the centre of interest for many physicists and w x chemists 1–8 . Recently, single electron tunnelling effects Ž and quantum size effects in isolated nanoparticles ‘‘quan- . tum dots’’, QD attract considerable attention, e.g., iso- wx lated C molecules have been used as ideal QDs 6 . Also 60 molecular rectifiers and transistors are still in the centre of wx interest 9 . In 1984 we have reported intramolecular recti- fying properties of a polarised N–O bond which is in fact one of the first subnanometer rectifiers. However, this is a totally intramolecular process observed only due to NMR wx experiments 3 . Our new and more practical approach to this problem combines molecular self-assembly and a rec- w x tifying behaviour based on the proton transport 10,11 . Self-assembly processes and supramolecular structures are the most promising for today’s materials science leading to nano-structured materials and nano-technology. A very interesting finding of great potential impact in this filed w x has recently been published by Stupp et al. 12 . Protons are well known to be highly mobile in coupled hydrogen bonding systems, e.g., water. Water behaves as an intrinsic protonic semiconductor which can be doped to form protonic analogues of p-type and n-type semiconduct- ) Corresponding author ing materials. Previously, a macroscopic rectifier diode based on protonic p–n junction has been constructed in our w x laboratory 10,11 . Now, we have developed a method to fabricate nano-scale protonic devices. Self-assembled Ž . monomolecular layer of 3-mercaptopropionic acid MPA has been spread onto a gold covered glass plate. Then, the second layer was formed with 3-aminopropyltriethoxysi- Ž . lane APTES owing to reaction between amino and car- boxylic groups and stabilised by condensation tri- Ž . ethoxysilil ends Fig. 1 . 2. Experimental The gold film freshly evaporated onto a glass substrate 3 mm = 3 mm cleaned carefully before use, or after an- nealing, was covered with MPA as a monomolecular layer formed during 15 h in a self-assembly process. MPA Ž 3 3 . solution in ethanol 0.2 cm MPA and 6 cm of ethanol was used at room temperature. The layer was washed with Ž . ethanol and dried in an inert atmosphere argon . Then, Ž MPA layer has been derivatised with APTES solution 0.2 cm 3 APTES and 10 cm 3 of ethanol, in 30–45 min at room . temperature giving a complex molecular system with a Ž . hydrophillic layer inside MPA-APS . The resulting layer was washed, dried and left in a clean box for at least 12 h to stabilise it due to condensation of triethoxysilil groups. 0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0379-6779 99 00108-3