Break Junction Measurements of Atomic and Molecular Junctions

Abstract

Single-molecule electronics, which uses individual molecules as active components in electronic devices, has garnered significant attention for its potential to revolutionize nanoscale technologies. To understand the fundamental electron transport in these single-molecule systems, break junction techniques, such as the scanning tunneling microscope break junction (STM-BJ) and mechanically controllable break junction (MCBJ), are crucial for investigating key properties of atomic and molecular junctions, including conductance, spin transport, and electrochemical gating effects.

In this thesis, using the STM-BJ technique, the rapid increase in the size of the gap formed immediately after breaking an atomic-scale gold contact (snapback distance) was measured in different environments. The snapback distance measured for a gold break junction in pure water was significantly reduced in an electrolyte containing halide anions. In the case of Br-, experiments under electrochemical control provided clear evidence that this reduction was caused by halide adsorption on the surface of the gold. This study represents the first direct investigation into the effect of anion adsorption on the snapback distance in gold (Au) nanostructures.

Lithographically fabricated MCBJ chips provide a more stable platform for gold atomic contacts and allow for precise measurements of single-molecule junctions with molecules. In this work, a lithographic MCBJ chip was designed and fabricated, and the fabrication process was optimized. The conductance of 1,5-pentanedithiol (PDT) and biphenyl-4,4'-dithiol (BPDT) was measured, validating the reliability of the fabricated MCBJ samples. Additionally, the conductance measurements of Au--PDT--Au and Au--BPDT--Au in anisole, which had not been previously reported, offer new insights and demonstrate the suitability of anisole as a promising solvent for single-molecule measurements. The application of a protective Al2O3 layer was confirmed through successful conductance measurements of 4,4’-bipyridine (BP) using Al2O3-coated gold atomic contacts, which had not been achieved with Al2O3-coated contacts in previous studies.

Cobalt atomic contacts, of interest for their potential in spintronics, posed fabrication challenges. To minimize the cobalt constriction and form reliable atomic contacts, notched-wire MCBJ, electrochemically-etched wire MCBJ, lithographic MCBJ, and electrochemically-coated MCBJ were fabricated. Conductance traces were measured in electrochemically-etched wire MCBJ and electrochemically-coated MCBJ, indicating the successful formation of cobalt atomic contacts. Although the results were preliminary, they demonstrated the feasibility of developing cobalt-based molecular junctions for future spintronic applications.

Date of Award	9 Jan 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Natasa Vasiljevic (Supervisor) & Walther Schwarzacher (Supervisor)