分子−電極間軌域作用之於單分子電性： 乙炔−金電極與苯−碳奈米管電極之共價鍵結界面

Abstract

單分子電性研究的基本架構是「電極−分子−電極(electrode-molecule-electrode, EME)」，實驗方法多為反覆量測，經統計大量數據，報導EME機率最高的分子電性。每筆數據，代表著新的電極的生成與破壞，亦即量測值隨分子與電極的構型與作用而有所差別，而單分子導電性之分布，常寬於一個數量級，代表分子−電極界面的作用至關重要。本論文以模擬平面、單層台階、錐狀三種金電極構型為起始點，聚焦於與分子接觸的金原子之d-軌域，其5個軌域能量因軌域對稱性而隨著分子頭基為胺基(提供σ軌域)或乙炔基(提供π軌域)等特性變化。對分子兩端皆為乙炔基的EME，由於軌域形狀及能量的接近，乙炔基頭基的π和px軌域分別與電極原子dxz和dz2軌域混成。然而在零偏壓時僅能指認π−dxz混成軌域，在偏壓大於0.6 V，方能指認px−dz2混成軌域，表示EME的形成，使得接觸處的金原子之d軌域能量分裂。 就EME的電子傳輸效率而言，輔助傳輸的分子軌域越靠近費米能階(Fermi level)越有助於電子傳遞。越強的分子−電極作用，其混成分裂程度越大，反鍵結軌域能量應可更加靠近E_Fermi。我們以首尾不同的頭基設計分子整流元件，一端為氰基；而另一端是乙炔與丁二炔兩種頭基，後者在配位場理論中是較強的配位基，預期有較大的能階混成分裂程度。模擬與導電值量測的結果顯示丁二炔頭基分子的EME擁有較低的整流啟動電壓和較大的高偏壓電流值表現。 電極−分子間軌域對稱性的匹配程度影響其電子傳輸效率，而效率的高低則可作為開關元件的應用。我們模擬二苯乙炔(oligo(phenylene ethynylene), OPE2)稼接於奈米碳管(carbon nanotube, CNT)的EME，檢視其電性表現與OPE2的苯環平面相對於碳管夾角的相關性。當夾角由共平面的0o轉為90o時，CNT−OPE2的軌域由π−π軌域轉換成不利電子傳輸的π−σ軌域。對OPE2加入具氧化態差異的hydroquinone與quinone的取代基團，發現含有quinone的OPE2分子在夾角90o時，仍有效地輔助電子傳遞。我們以分子旋轉和氧化還原為輸入訊號，電子傳輸係數為輸出訊號，設計一個具有OR gate之邏輯閘。

The basic research structure for molecular electronics is “electrode−molecule−electrode (EME)”. Experimentally, EMEs are repeatedly formed and measured to obtain statistical value of the desired molecular electronic property. This means that with each EME formation, the geometry of the electrodes and EME itself is different. In retrospect, the range of single-molecular conductance value is often wider than one order of magnitude, and thus the interaction at the interface of molecule−electrode has profound impact on charge transport. In this thesis, EME simulations for three types of gold-electrode configurations (planar, stepped and pyramidal) are carried out to probe the change in d-orbital-energy of gold atoms when the headgroup in contact is amine (charge transport via σ orbital) or ethynyl (charge transport via π orbital). Due to similarity in orbital shape and energy, for the EME of the molecule with ethynyl at both terminals, the π and px orbital of ethynyl hybridize with dxz and dz2 orbital of the electrode atom, respectively. The px−dz2 hybridization can only be identified at bias above 0.6 V, implying that the d orbital energy of the gold atom in contact with the molecule split when bias is applied across EME. For charge transport efficiency in EME, the closer the molecular orbital is to E_Fermi, the more efficient the orbital can assist in charge transport. The stronger the molecular-electrode interaction, the higher the degree of energy hybridization and the closer the anti-bonding orbital would be to E_Fermi. Therefore, molecules with unsymmetrical headgroups, one ending with cyano group while the other ending with either ethynyl or butadienyl group, are designed such that the corresponding EMEs exhibit diode characteristics. Since butadienyl is a stronger ligand than ethynyl in ligand-field theory, the EME of the molecule with butadienyl terminal is expected to have higher degree of energy hybridization. Both simulations and experiments show that the EME of the molecule with butadienyl terminal has a lower onset potential for rectification as well as a larger current at high bias. The phenomenon that the degree of overlap between orbitals of electrode and molecule affects the charge transport efficiency of EME can be used to design a molecular switch. EME simulation of oligo(phenylene ethynylene) (OPE2) bridged between carbon nanotube (CNT) were performed to examine the correlation between the EME’s electrical performance and the twist angle of the benzene ring of OPE2 with respect to the CNT. When the twist angle is changed from 0o to 90o, the orbital of CNT−OPE2 transforms from the π−π orbital to the π−σ orbital, which is unfavorable for electron transport. Furthermore, with the modification of redox-active hydroquinone/ quinone on OPE2, it was found that with quinone modification the EME still effectively assist electron transport at a twist angle of 90o. Using rotation angle and redox state of the molecule bridged between CNT as input signals and transmission coefficient as output signals, a molecular OR gate is designed.