Electric Behaviors of Metal String Complexes and Metal-String Modified Gold Clusters: Fabrication of Single-Molecule Transistors by Electromigration

本研究的目的在瞭解金屬串錯合物(extended metal-atom chains，EMACs)的介電常數及單分子導電度等電性。介電常數的部分，因粒徑1~4 nm的奈米金具有量子化充電效應(quantized double-layer charging，QDL)而產生類似電容的行為。這類奈米粒子的電容值除受粒徑影響，也決定於表面保護基之介電常數(dielectric constant)。本研究使用三苯基膦修飾之奈米金Au101(PPh3)21Cl5 (平均粒徑1.5 nm)與三核鎳之金屬串錯合物[Ni3(dpa)4(NCS)2]進行保護基置換反應(ligand place-exchange reactions)。藉差式脈波伏安法(differential pulse voltammetry，DPV)量測金屬串錯合物修飾之奈米金的電容值，推估金屬串錯合物的介電常數。但置換反應導致奈米金粒徑變化，難以獲得有再現性的數據。為降低奈米金粒徑變化程度，我們以Au101(PPh3)21Cl5為起始物，系統性地探討置換反應中，新保護基的濃度、長度與頭基對奈米金粒徑的影響，結果顯示奈米金的最終粒徑反比於保護基之頭基與金的吸附能力(如金屬串分子之−NCS−Au作用力)。 單分子電性部分所需的量測結構，乃利用電子束製程於Si基材上獲得寬約200 nm的金線，以電遷徙效應(electromigration)製作具分子級電極間距的類似於源、汲、閘之三電極系統。施加偏壓VSD量測兩電極間之待測分子電性，利用閘極電壓VG調整分子與電極能階達匹配(energy alignment)，調控分子導電性。目前製得單分子電晶體可量測[Ni3(dpa)4(NCS)2]之導電值，並藉由VG控制分子能階以改變分子導電行為。最終獲得導電值隨VSD與VG變化的分佈圖，以瞭解單分子電性與能階分佈。此技術可量測不同中心金屬核之金屬串錯合物(如Co、Cr和Ru等多核金屬串)，系統性地探討金屬串之分子結構、能階軌域與其導電行為的相關性。The goals of this research focus on the understanding of the dielectric constant and the single-molecule conductance of EMACs (extended metal-atom chains). Firstly, the dielectric constant of EMACs will be explored by the measurements of DPV (differential pulse voltammetry) for Au MPCs (monolayer-protected gold cluster) exhibiting quantized double-layer charging (QDL) which is dictated by both the core size and the dielectric constant of capping ligands. EMACs is place-exchanged with triphenylphosphine-modified Au MPCs, [Au101(PPh3)21Cl5], and DPV gives the capacitance of EMAC-capped MPCs and hence the dielectric constant of EMACs. However, the diameters of Au MPCs become polydisperse during exchange reactions, making the estimated dielectric constant unreliable. To keep unchanged the size of Au MPCs during exchange reactions, systematically examined factors are the concentration, chain length, and a range of headgroups of incoming ligands. The results suggest that the size evolution of Au MPCs is determined by the strength of headgroup–Au adsorption. For the exploration of single-molecule conductance, we fabricate single-molecule transistors by e-beam lithography and electromigration. The energy alignment between the electronic level of EMACs and Fermi level of electrodes can be achieved via the gate voltage. Preliminary results for [Ni3(dpa)4(NCS)2] show transistor-like behaviors. A color scale plot of conductance as a function of the bias voltage (VSD) and the gate voltage (VG) was obtained to help us to realize the molecular orbital levels. This technology can be utilized to investigate the electric properties for EMACs with metal centers such as Co, Cr, and Ru. This information will provide a comprehensive understanding concerning the correlation between the electronic structure of a molecule and its conductance