Development of a Digital Measurement System for Inelastic Tunneling Spectroscopy of Single Molecular Junctions
Year: 2015
Type: dissertation
Abstract: The miniaturization of the silicon-based transistor is drawing ever closer to the physical limits set by quantum tunneling. In molecular electronics, quantum tunneling is the working principle of single molecular devices. Molecular electronics is thus one of the possible solutions for moving beyond the limits of the current technology. In order to realize single molecular electronics in devices, a complete understanding of the characteristics of a molecular junction is necessary. A challenge in the eld of molecular electronics is to determine if a target single molecule is measured as expected. To con rm the existence of a molecule in a junction, inelastic electron tunneling spectroscopy (IETS) is useful since it can resolve molecular vibrations. The activated vibrational modes by electron-phonon coupling open new conduction channels in the junction and thus change the charge transport characteristics. IETS can thus provide fruitful insight in the characteristics of single molecular junctions. This research has focused on developing and applying a fully digital low-noise method for measuring IETS in a mechanically controllable break junction. The measurement system was developed using LabVIEW to realize a digital lock-in ampli er for IETS measurement. Running the signal through solid-state circuits adds pink noise (1=f), impacting the signal-to-noise ratio. Hence, the constructed system has an inherent advantage to conventional methods using commercially available lock-in ampli ers since there is no need to connect an external adder circuit. Taking advantage of the freedom of signal design, a carrier wave (CW) sweep type was developed. Using the CW sweep type allows for an order of magnitude higher IV curve resolution during the IETS measurement. A fully automated IETS measurement setup was developed with sophisticated automatic ampli er gain and automatic control of the break junction process. The automated system allows for probing IETS in di erent single molecular conformations and also for investigating the e ect of stretching a molecule. Several molecules were investigated, with focus on an oligothiophene molecule (5T). The developed measurement system can e ectively measure IETS in single molecular junctions at a low noise level. During stretching of the 5T molecule, a Au-S peak blue shift was observed indicating stretching of the bond. New vibrational modes were also observed near molecular detachment due to stretching, indicating charge transfer through the entire molecule.
Author: Jimmy Franchi
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W1938351545