Recently scientists all over the world have been examining components of ever smaller, virtually molecular dimensions. An international team from the Institute of Physics of the Czech Academy of Sciences and the Tokyo Institute of Technology has developed a new method which will contribute to the miniaturization of electric circuits in electronics. They have published their discovery in the prestigious scientific journal Chemical Science.
When examining the properties of molecules potentially useable in miniature circuits, scientists encounter a number of problems. One of them is understanding the configuration of molecule contacts with the metal surfaces of electrodes that influences important properties of junctions, e.g. their conductance. The international team established in collaboration between the Institute of Physics of the Czech Academy of Sciences and the Tokyo Institute of Technology has managed to significantly contribute to addressing this obstacle.
“The new method will enable to check the geometry of the interface between metal electrodes and a molecule. We have thus taken a step towards overcoming one of the main challenges in the realization of stable and reproducible single molecule circuits,” says the leader of the Czech team from the Department of Thin Films and Nanostructures of the Institute of Physics Héctor Vázquez. “The success has been achieved in collaboration with our Japanese colleagues whose measurements we have identified with specific types of a bond using numerical simulations. It is the combination of different techniques that forms the basis of the successful new method.”
Fig. 1. Experimental setup where there are two golden electrodes linked by a single “conduction” molecule (a single molecule junction).
The linking of the molecule to source and drain electrodes is done via chemical bonds established between linking functional groups on a molecule (linkers) and atoms of golden electrodes. The properties of the junction (including the important conductance) are strongly affected by the details of the bonding geometry. This is particularly relevant for the most commonly used linkers containing sulphur.
This geometry, however, changes quickly in the conditions under which experiments are conducted most frequently – in solution or in ambient conditions, and at room temperature – and cannot be detected easily. The changes in geometry then lead to significant variations (up to 2 orders of magnitude) in conductance of the junction and thus significantly impede the investigation of molecule suitability for the use in microelectronics.
Through the combination of different techniques, the scientists managed to distinguish between three binding configurations of a molecule (see Fig. 2) – bridge, hollow or atop conformations.
The group of Manabu Kiguchi at the Tokyo Institute of Technology performed simultaneous surface enhanced Raman scattering and current-voltage measurements. The group of Héctor Vázquez at the Institute of Physics carried out density functional theory (DFT) based simulations. Variations in conductance and in Raman frequencies characteristic of the molecule measured experimentally were thus matched to specific configurations. By applying a small voltage, the scientists also managed to induce transitions between the different binding sites.
Based on the article “Identifying the molecular adsorption site of a single molecule junction through combined Raman and conductance studies”, published in Chemical Science, Issue 25, 2019. Authors of the study:
Satoshi Kaneko1, Enrique Montes2, Sho Suzuki1, Shintaro Fujii1, Tomoaki Nishino1, Kazuhito Tsukagoshi3, Katsuyoshi Ikeda4, Hideaki Kano5, Hisao Nakamura6, Héctor Vázquez2 and Manabu Kiguchi1
Chem. Sci. 10, 6261-6269 (2019), DOI: 10.1039/C9SC00701F
1Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1 W4-10 Ookayama, Meguro-ku, Tokyo 152-8511, Japan.
2Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, Prague CZ-162 00, Czech Republic.
3International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan.
4Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan.
5Institute of Applied Physics, University of Tsukuba Tennodai 1-1-1, Tsukuba 305-8573, Japan.
6CD-FMat, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan.