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Single-Molecule studies of photo-conductance on photosynthetic molecular systems by SPM break-junction measurements

Periodic Report Summary 2 - PHOTOSYN-STM (Single-molecule studies of photo-conductance on photosynthetic molecular systems by SPM break-junction measurements)

Work carried toward the objectives

During the two-years outgoing phase, the researcher has been working on different methodologies to allow the identification of the formation of single metal-molecule-metal electric contacts and the characterisation of their charge transport behaviour. This stage involved the use of scanning tunnelling microscopy (STM) break-junction techniques as well as mechanical controllable break-junction (MCBJ) devices. New designs of the experimental array were pursued as well as the exploration of high frequency AC-modulated signals to more efficiently detect the formation of single-molecule contacts. Synthetic conjugated systems with new electrical behaviours have been intensively sought during the outgoing period, especially those with potential photoconductance properties.

During the return phase, the researcher has invested efforts primarily on the implementation of a new STM break-junction setup. The first single-azurin junctions have been the main target toward the study of charge transport in molecular system with biological relevance.

Main results achieved

During the course of this project, a robust methodology that allows the univocal detection of single-molecule junction formations has been developed. Using these methodologies, a number of new electrical behaviours in single-molecule contacts have been successfully achieved on the above mentioned conjugated molecules:

(i) Diode behaviour has been demonstrated in a single-molecule junction by using poly-phenyl blocks with symmetric binding groups at both sides as the attachment to the electrodes.
(ii) Low-band gap electronic structures in poly-aromatic fused rings systems such as perylene and coronene derivatives have been benefited to achieve a pronounce electrochemical gate effect with direct applications as single-molecule field effect transistors (FETs).
(iii) The coupling of the molecular pi-orbitals of a long pentaphenylene block with the metal electrodes has been quantified and used to design a single-molecule electromechanical device where the amount of current flow is modulated through a mechanical mean.
(iv) The transport mechanism through pentaphenylene polymer chains with varying length has also been studied in order to analyse the transition point from direct tunnelling to indirect hopping.
(v) Preliminary single-molecule photo-emission data using perylene derivatives and the previously studied molecular diode has been obtained.

During the last year period (return phase), the STM break-junction technique has been successfully setup at the host institution (IBEC). At the end of the return phase, a second fully operational STM break-junction setup has also been set up at the same lab. We decided to pick a robust redox protein such as blue Cu-azurin to study charge transport in single-protein junctions. The first single-azurin electrical contacts between two metallic electrodes were successfully built and characterised using the methodologies developed during the outgoing stage. The single-azurin junctions displayed a nice negative differential resistance (NDR) behaviour.

Impact of the results

The results achieved during this project have already had a significant impact in the field of molecular electronics as it is reflected by the amount and quality of the scientific publications derived from this work. The implication of these publications go beyond providing new scientific knowledge, and in fact, the new electrical capabilities demonstrated for these single-molecule devices constitute a prove of concept of their potential implementation in the near future technology.