First-Principles Study Of Molecular Energy Level Positions In 1,8-Octanedithiol Molecular Devices

As electronic devices continue to miniaturize,traditional silicon-based semiconductor electronic devices are approaching their size limit.Therefore,the preparation of single-molecule devices with a single molecule as the basic building block has been proposed as one of the solutions.Single-molecule devices have attracted wide attention from researchers due to their smaller volume,faster response speed,and lower energy consumption.In recent years,a series of highperformance molecular devices have been designed and fabricated,and researchers have systematically explored the factors that affect the electrical transport properties of molecular devices,including molecular structure,electrode materials,anchoring groups,molecular-electrode contact configuration,and the position of the frontier molecular orbitals relative to the Fermi level of electrodes to further improve the performance of single-molecule devices.Among them,the position of the frontier molecular orbitals relative to the Fermi level of electrodes is particularly important,and the mechanism is more complex to explore.Although a lot of researches have been carried out,there are still some problems that need to be further clarified,which to some extent hinders the further development of molecular devices.In this thesis,the first-principles calculations based on density functional theory are used to systematically study the effects of different molecule coverage,different electrode materials,and stretching and compressing the distance between two electrodes on the position of the frontier molecular orbitals relative to the Fermi level of electrodes.The specific research content and basic conclusions of this thesis are summarized as follows.1.The effects of molecule coverage and electrode materials on the position of the frontier molecular orbitals relative to the Fermi level of electrodes.It has been shown that the position of the frontier molecular orbital relative to the Fermi level of electrodes plays an important role in the conductivity and even the function of the device.Therefore,it is of great significance to study the factors affecting the position of the frontier molecular orbital relative to the Fermi level of electrodes and the influencing mechanism.By analyzing recent experimental and theoretical studies,we found that different molecule coverage and electrode materials have a significant effect on the position of the frontier molecular orbitals relative to the Fermi level of electrodes.However,the physical mechanism is not entirely clear.Therefore,we used first-principles calculations based on density functional theory to study the effects of different coverage(1/9,1/16,and 1/25)and different electrode materials(Au,Ag,and Pt)on the position of the HOMO(Highest Occupied Molecular Orbital)of 1,8-octanedithiol molecule devices relative to the Fermi level of electrodes.The calculation results show that the distance between the molecule’s HOMO and the Fermi level of electrodes is mainly affected by the work function of the electrodes and the contact dipole between molecule and electrode.It is noted that the work function of the electrodes and the contact dipole between molecule and electrode have a negative correlation and a positive correlation with the distance between the molecule’s HOMO and the Fermi level of electrodes,respectively.Specially,if the work function of the electrodes is larger,the distance between the molecule’s HOMO and the Fermi level of electrodes will become smaller.However,if the contact dipole between molecule and electrode is larger,the distance between the molecule’s HOMO and the Fermi level of electrodes seems also larger.When the molecule is adsorbed on same electrode surfaces with different coverage,the change trend of the distance between the molecule’s HOMO and the Fermi level of electrodes is determined by the contact dipole between molecule and electrode.When the molecule is adsorbed on Ag,Au,and Pt electrode surfaces with the same coverage,there is a linear relationship between the contact dipole and the coupling strength(i.e.,charge transfer),which all origin from molecule and electrode.Especially,if the contact dipole is larger,the coupling strength seem larger,which results from the larger amount of charge transfer between molecule and electrode.And the distance between the molecule’s HOMO and the Fermi level of electrodes is determined by the work function of the electrode,rather than the contact dipole(coupling strength)between molecule and electrode.2.The effects of stretching and compressing the distance between the electrodes on the position of the frontier molecular orbitals relative to the Fermi level of electrodes.Using MCBJ(Mechanically Controllable Break Junction)technology,the distance between the two electrodes can be precisely controlled,and thus the coupling strength between the molecule and the electrodes can be changed to affect the position of the frontier molecular orbitals relative to the Fermi level of electrodes.Amounts of theoretical and experimental reports have shown that the effects of stretching and compressing the distance between the electrodes and using different electrode materials to regulate the coupling strength between the molecule and the electrodes on the position of the frontier molecular orbitals relative to the Fermi level of electrodes are opposite.To explain this issue,we used first-principles calculations based on density functional theory to study the influence of stretching and compressing the distance between the electrodes on the position of the HOMO of 1,8-octanedithiol molecules device relative to the Fermi level of electrodes.If the distance between the electrodes via stretching and compressing,the change in the frontier molecular orbitals is mainly affected by the contact dipole between molecule and electrode.In other words,the charge transfer(i.e.,coupling strength)between molecule and electrode affects the change in energy level position.This thesis consists of the following four chapters: The first chapter is the introduction,which briefly introduces the development of molecular electronics,the factors that influence the electrical transport properties of molecular devices,and the factors that influence the position of the frontier molecular orbitals.The second and third chapters focus on the specific work carried out in this thesis.In the second chapter,we studied the influence of different molecular coverage(1/9,1/16,and 1/25)and different electrode materials(Au,Ag,and Pt)on the position of the HOMO of 1,8-octanedithiol molecular devices relative to the Fermi level of electrodes.In the third chapter,we studied the influence of stretching and compressing the distance between the electrodes on the position of the HOMO of 1,8-octanedithiol molecular devices relative to the Fermi level of electrodes.The last chapter provides a comprehensive summary of the work in this thesis and prospects for future work.