First-principles Studies On The Rectification Mechanisms And Performance Modulations Of ?-? Single-molecule Diodes

With the rapid development of conventional semiconductor technology,the size of transistors has been shrinking continuously to the nanoscale where the quantum effect of electrons is important and nonnegligible.However,it is a nontrivial feat to further improve the performance of electronic devices by reducing their characteristic size due to the limitations of fabrication technologies.As a burgeoning field,molecular electronics aims to reproduce the varieties of functions of traditional electronic devices and even to obtain wholly new functions by constructing molecular devices using molecules as building blocks,which is believed one of the most promising ways to overcome the scaling limits of conventional semiconductor technology.In recent years,with the advances of techniques and methods for molecular devices,a series of molecular devices with excellent performance have been designed and synthesized.However,the requirements for preparing stable and working molecular devices are very harsh,and complete understandings for the transport mechanisms of many molecular devices and factors influencing their performance are still lacking,which hiders the further applications of molecular devices.In particular,there are some prominent problems for single-molecule diodes,such as poor repeatability,low rectification ratio and unstable rectification performance,which needs further experimental and theoretical devotions.Recently,based on the technique of self-assembled monolayers?SAMs?,a series of single-molecule diodes constructed by?-?type molecules have been synthesized experimentally,which manifest high rectification ratios.However,the understandings of their rectification mechanisms and researches on the modulations of the rectification performance are still highly lacking,especially from the theoretical point of view.Therefore,here by using the state-of-the-art nonequilibrium Green's function?NEGF?method combined with density functional theory?DFT?,electron transport properties of?-?type single-molecule diodes have been systematically investigated,and mechanisms of rectification and modulations on the rectification performance have been correspondingly discussed.The more detailed contents and results of this thesis are summarized as follows.1.Effect of the site of?-conjugated group Fc in the tridecanethiolate backbone on the rectification characteristics in ferrocenyl-embedded tridecanethiolate single-molecule rectifiers.The geometrically structural asymmetry in single-molecule diodes is the key factor for rectification.Therefore,changes in the molecular structure can affect the rectification of the diodes.Here,by sequentially changing the position of Fc group within the tridecanethiolate backbone in ferrocenyl-embedded tridecanethiolate(SC_nFc_13-n,n=0-13)diodes sandwiched between two symmetric Ag?111?electrodes,the reversal of the rectification direction observed in the experiment has been reproduced,where asymmetric Ag and Ga₂O₃/EGaIn electrodes were adopted.The first-principles results show that frontier molecular orbitals?FMOs?of the diodes are spatially localized on the Fc group.Asymmetrically positioning the Fc group within the tridecanethiolate backbone causes asymmetric evolutions of the FMOs under forward and backward bias voltages.Varying the site of Fc group within the tridecanethiolate backbone modifies the spatial distributions of the conducting FMOs and hence changes the rectification direction of the molecular diode.The asymmetric evolutions of the spatially localized FMOs can be well fitted by our proposed model.2.Effect of coupling strength of van der Waals interaction between?-conjugated terminal group Fc and electrode on the rectification performance of ferrocenyl-undecanethiolate single-molecule diodes.The coupling strength between the?-conjugated terminal group and electrode is one of the important factors that influence the rectification performance of?-?type single-molecule diodes.By comprehensively analyzing the experimental data for?-?type single-molecule diodes that chemically adsorb on the substrate electrode with metal-S bond and physically contact on the top electrode with the ending?-conjugated terminal group via van der Waals interactions,it is found that different fabrication schemes will lead to a large difference in the rectification ratios for single-molecule diodes with similar structures or even the same one.Taking SC₁₁Fc single-molecule diode as an example,here the coupling strength between the?-conjugated terminal Fc group and electrode is modulated by changing the tilt angle?of Fc group with respect to the electrode surface.By analyzing the influence of coupling strength in the rectification for this kind of molecular devices,the mechanism of the large difference in rectification ratio for similar molecular devices in the experimental results is explained.It is revealed that the alignments between the conducting FMOs and Fermi energy of electrodes are prominently varied by changing the coupling strength of van der Waals interaction between SC_n Fc molecule and electrode.And this further significantly influences their rectification performance.Weaker coupling strength is in favor of rectification improvement,and the rectification ratio can be improved by 2 orders of magnitude.This work provides a feasible way to modulate the rectification performance of single-molecule diodes based on SC_nFc or structurally similar systems.3.Mechanisms of the odd-even effect in rectifying performance of ferrocenyl-n-alkanethillate single-molecule diodes.There is an odd-even effect in the rectification performance of SC_nFc single-molecule diodes as the methylene unit number n is varied from 6 to 15.The diodes with n_even have larger rectification ratios than those with n_odd when Au substrate electrode is used.Our theoretical investigation reveals that the monotonic evolutions of the FMOs under positive and negative bias voltages are responsible for the rectification.Further analysis reveals that the odd-even effect in the rectification ratios is attributed to different alignments between FMOs and Fermi energy of electrodes for SC_n Fc with n_odd and n_even,which originates from the odd-even dependent orientation of the Fc terminal group with respect to the adjacent electrode surface.And the odd-even effect in the rectification ratios is completely reversed when the Au electrode is replaced by Ag electrode,that is,the diodes with n_even have smaller rectification ratios than those with n_odd.The reversal of the odd-even effect in rectification ratios is attributed to the different Au-S-C and Ag-S-C bond angles for alkanethiolates,which leads to the reversal for the odd-even dependent orientation of the Fc terminal group with respect to electrode surface.4.Odd-even effect of rectification in 4'-methyl-2,2'-bipyridyl-terminated n-alkanethiolate single-molecule diodes.The electron transport properties of single-molecule diodes composed of a 4'-methyl-2,2'-bipyridyl-terminated n-alkanethiolate?SC_nBIPY,n=6-14?have been theoretically investigated.Similarly to SC_n Fc single-molecule diodes,there is also an odd-even effect in the rectification ratios for SC_nBIPY when the methylene unit number n is varied.Differently,it has been found that the odd-even effect can be reversed by tuning the magnitude of the applied bias voltage and the length of the alkyl chain.Detailed analysis suggests that the FMOs in the diodes spatially localize on the BIPY terminal group and evolve monotonically under external bias voltage,which results in the rectification of the diodes.The alignments of the FMOs with the Fermi energy is essentially affected by the orientation-dependent interactions between the terminal group and the electrode,which closely relies on the odevity of n.This further contributes to the odd-even effect in the rectification of the diodes at low bias voltages,where the diodes are in the non-resonant charge transport regime.However,when the bias voltage increases and the diodes enter the resonant transport regime.As the increase of the bias voltage,the conducting FMOs approach to the Fermi energy and meanwhile the spatial distributions of the wavefunctions become more localized and less conductive,which will lead to the inversion of the odd-even effect in rectification at large bias voltages.While,for a moderate bias voltage,a longer alkyl chain tends to drive the diodes to enter the resonant transport regime and facilitate the inversion of the odd-even effect in rectification.This thesis consists of seven parts.The first chapter introduces the background of molecular electronics and summarizes the functional molecular devices which have been studied in recent years,where the progress of molecular diodes and corresponding rectification mechanisms are especially focused.In the second chapter,the DFT and NEGF method as well as their combination to self-consistently calculate the charge transport of two-probe systems are summarized.The studied results included in this thesis using the above-mentioned method are presented in chapter three to chapter six.In the third chapter,taking the SC_nFcC_13-n single-molecule as an example,the mechanism of the rectification in?-?type molecular diodes is theoretically understood.Based on this,effect of the site of the?-conjugated group in the alkanethiolate backbone on the rectification characteristics has been investigated.In the next chapter,taking SC₁₁Fc for an example,effect of coupling strength of van der Waals interaction between?-conjugated group and electrode on the rectification performance of?-?type molecular diodes has been investigated,where the tilt angle of?-conjugated group with respected to electrode is varied.This work is helpful for understanding some previous experimental observations.In chapter five,the rectification characteristics of a series of SC_nFc single-molecule diodes have been focused when the length of alkyl chain varies.In addition,effect of electrode material on the rectification characteristics is also investigated.Chapter six devotes to the SC_nBIPY single-molecule diode,for which effects of length of alkyl chain and external bias voltage on the rectification characteristics have been explored.Compared to the SC_n Fc single-molecule diode,the theoretical study reveals different mechanisms for the odd-even effect of rectification and a new factor to trigger the reversal of the odd-even behavior of rectification ratios.This study is helpful for deeper understandings of the rectification characteristics and ways to modulate the rectification performance of?-?type molecular diodes.Finally,in chapter seven,a summary of the studies in this thesis is presented and then the development of molecular rectifiers in the future is expected.