Investigation Of Charge Transport Characteristics Of Single-molecule Devices Responding To High-frequency Excitations

The development of molecular electronics is to solve the theoretical limit size problem with the miniaturization of semiconductor devices.At present,more and more single-molecule devices with specific functions have been manufactured from theory to reality.However,some typical problems that have long restricted the industrialization of single-molecule devices still need to be solved.On the one hand,single-molecule devices need to have better electromechanical stability to resist mechanical oscillation.However,the characterizations of the electromechanical stability of molecular machines still lack clear quantitative indicators.What’s more,the single-molecule devices need higher operating frequency to surpass the performance of traditional silicon-based semiconductor devices,but most of the current molecular electronics research on single-molecule devices is still restricted to the low-frequency or even DC range.This thesis focus on the theme of "how to construct single-molecule devices with high mechanical robustness and high-frequency electrical response".The corresponding scientific research was carried out by introducing high-frequency external excitation into a single molecular device and then analyzing its electrical response,based on independently building and modifying scientific instruments and independently coding data analysis programs.The main contents and results of this thesis are below:1.It was found that tolane with dithiol terminals(OPE-2)exhibits different degrees of radiation-enhanced charge transport to varying wavelengths in its absorption region,and the degree of enhancement is consistent with the intensity of ultraviolet/visible absorption.But in contrast,tolane with bi-pyridyl terminals(OAE-2)and 1,8-octanedithiol(ODT)shows no response upon illumination.An interplay between theory and experiment confirmed that the energy level alignment between the charged state and the triplet state directly determines the enhancement of single-molecule photoconductance2.The Alpha value method was introduced to study the high-frequency mechanical oscillation response of the molecular devices.The STM-BJ set-up was built and modified,and the Alpha value analysis technique was introduced by applying high-frequency,small-amplitude sinusoidal mechanical oscillations to the hovering molecular junction.It was found that S-Tn:Tn-S supramolecular junctions with even-numbered thiophene rings have higher electromechanical stability.By simplifying the single-molecule and supramolecular junction as a spring model,we found that the S-Tn:Tn-S supramolecular junctions with evennumbered thiophene rings have stronger intermolecular interactions and smaller strain distributions.Theoretical calculations found that the strength of the intermolecular interaction in the supramolecular junction is originate from the torsional freedom of the thiophene rings.3.Single-molecule device impedance and impedance spectroscopy measuring method was proposed and the corresponding scientific instruments were developed.We set up a single-molecule impedance measuring set-up based on a logarithmic amplifier by applying high-frequency electrical excitation to the hovering molecular junction and then collecting its electrical response.We set up a single-molecule impedance spectroscopy measurement set-up based on a lockin amplifier,which can realize the measurement of the impedance spectroscopy of molecular devices by changing the frequency of the bias excitation.