Electron Transport Through Single Molecular Junctions: Molecular Wires, Switches to Energy Storage Devices
The idea of building electronic devices using single molecule as active component was first proposed by Aviram and Ratner in the early seventies. Indeed, molecules are of great interest for application in electronic devices because of their small size, their recognition properties, their ability of self-organization and their possibility of chemical modification and customisation. Thus, the ability to measure and control charge transport across metal/molecule/metal junction is of considerable fundamental interest and represents a key step towards the development of molecular electronics.
During my PhD, I have employed STM break junctions (STM-BJ) and a complimentary mechanically controllable break junction (MCBJ) technique to scrutinise the electron transport properties of metal/molecule/metal junctions under well controlled experimental conditions (in liquid at room temperature)1,2. Using these two experimental techniques, I will try to demonstrate the devise independence and reproducibility of the charge transport characteristics of metal/molecule/metal junctions. Due to the fact that, large number of measurements possible for STM-BJ and MCBJ techniques provides robust statistical analysis of the conductance data and shows clear evidence for the formation of molecular junctions. Many configurations can be sampled and characterized quantitatively by analyzing statistically significant number of conductance-distance and /or current-voltage traces. In addition to the conductance features, the characteristic length analysis of the conductance-distance traces provides additional information about the stability and evolution of molecular junctions. Using these expertise, we systematically studied correlation between molecular structure and conductance properties of metal/molecule/metal junctions formed with a family of thiol terminated oligo (phenylene ethynylene) (OPE) molecular wires (role of molecular length, conjugation, HOMO-LUMO gap, solvent)2,3and roll of anchoring group on single molecular junction conductance4. Furthermore, we also systematically studied the conductance switching of the pyridyl terminated dimethyldyhydropyrene photo chrome molecules upon visible light irradiation4.
Increase of the molecular length and/or of the HOMO-LUMO gap leads to a decrease of the single junction conductance of the linearly conjugated OPE molecules. The experimental data simulations suggest a non-resonant tunneling mechanism involving hole transport through the molecular HOMO, with a decay constant Δ = (3.4 ± 0.1) nm-1 and a contact resistance Rc= 40kΩ per Au-S bond. Furthermore, decreasing the HOMO-LUMO gap by keeping the molecular length constant, we found that value of single molecular junction conductance was increasing. We also found effect of solvent on the single molecular junction conductances formed with mono thiol terminated OPE molecules. We also investigated the conductance properties of a photoswitchable dimethyldihydropyrene (DHP) derivative for the first time in single molecule junctions using the mechanically controllable break junction technique. We demonstrate that the reversible structure changes induced by photoisomerization of a single bis-pyridine-substituted DHP molecule are correlated with a large drop of the conductance value. We found a very high ON-OFF ratio (> 104) and an excellent reversibility of conductance switching5.
My present work is mainly focusing on modifying the Au, ITO, Pt electrode surfaces with functional molecules (electroactive or Photo active) and studying their properties at single molecule level under electrochemical conditions for molecular electronics and energy storage applications.
1. Hong, W.J et al., Beilstein J. Nanotechnol. 2011. 2, 699-713.
2. Kaliginedi et al., Journal of American chemical society. 2012, 134 (11), 5262–5275.
3. Kaliginedi et al., Journal of American chemical society. 2014, in preparation.
4. Moreno-Garcia et al., Journal of American chemical society. 2013, 135, 12228−12240.
5. Roldan, D., Kaliginedi et al., Journal of American chemical society. 2013, 135, 5974-5977.