銠鎳異核金屬串於二氯甲烷及三氯苯混合溶劑之導電值研究

Single-Molecule Conductance of Rh and Ni Heterometallic String Complexes in Mixed Solvents of Dichloromethane and 1,2,4-Trichlorobenzene

蔡豐任(Feng-Jen Tsai)

指導教授 ： 陳俊顯

國立臺灣大學/理學院/化學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.11004

摘要

本研究以掃描式穿隧顯微破裂接合點法(STM-bj)測量異核金屬串分子(heteronuclear metal string complexes, HMSCs) [RhRhRhNiNi(tpda)4(NCS)2] (tpda2‒：tripyridylamido dianion)的單分子導電值，此分子的幾何結構是以四個吡啶胺配基呈螺旋狀結構與中心金屬配位形成一維金屬串分子，且軸向配基為異硫氰基(NCS‒)可吸附於金電極。STM-bj為控制STM探針撞擊及遠離基材，重複地進行此過程產生電極‒分子‒電極的接合點(MMM junctions)並紀錄探針遠離時的電流值(稱為I(s) trace)。由I(s) trace的圖形可以發現當探針撞擊基材兩端金原子會產生融合(fuse)並可形成金原子串接合點。此時在1 G0(金原子串的量子導電值，G0 = 77.5 microsiemens)或其倍數位置會呈現階梯平台狀，斷裂後電流急遽下降(sharp drop)並產生原子尺寸狹縫及電極結構，分子即有機率架接至此狹縫並呈現階梯平台狀。隨著探針遠離基材MMM junction會斷裂並再次呈現sharp drop。將所記錄的I(s) traces加以統計可得到分子導電值並計算金原子串及分子斷裂的探針拉升距離即可得到分子在接合點中所拉升的長度(step length)。在實驗過程中分別以甲苯(toluene)液體及1,2,4-三氯苯(TCB)做為溶劑時，皆無法得到明顯的導電峯值。若改以二氯甲烷(DCM)及TCB混合溶劑進行量測，則可得到明顯的導電峯值。DCM與TCB體積比值為50/50、20/80及11/89 (v/v)時，量測的導電峯值分別為2.2× 10^(‒5)、1.3× 10^(‒4)及4.8× 10^(‒4) G0。欲探討金屬原子間作用力對導電值的影響，須先釐清與溶劑相關的現象，於是檢視了銠鎳異核及鎳同核金屬串分子包含三核的Ni3、RhRhNi、NiRhNi及五核的Ni5及RhRhNiNiNi。實驗結果顯示(1)中心金屬的排列若是對稱，異核及同核金屬串分子在不同溶劑比例下量測到的導電峯值類似，step length也類似。(2)中心金屬的排列若是不對稱，在不同溶劑比例下量測到的導電值皆不相同且step length也有所不同。由以上的實驗結果搭配目前既有的文獻中環境因素對導電值影響的研究，推測最有可能的因素為頭基在不同溶劑下存在的型態不同，導致電極與頭基之間的作用程度在不同溶劑比例下有所差異進而影響導電值。

關鍵字

分子導電值 ； 銠鎳異核金屬串 ； 掃描式穿隧顯微破裂接合點法

並列摘要

Metal string complexes (MSCs), in which the metal centers are co-linear and stably coordinated by four oligopyridylamido ligands, have been demonstrated a unique category of conductive molecular wires. Bond orders which describe the strength of metal-metal interactions are qualitatively well correlated with the single-molecule conductance of the MSCs. Previous examples are limited to homometallic string complexes in which the metal centers are composed of the same element. Explored in this thesis work are the conductive properties of heterometallic string complexes (HMSCs). Specifically, the sequences of the metal cores are RhRhNi, NiRhNi, RhRhNiNiNi, and RhRhRhNiNi. Also examined are Ni3 and Ni5 MSCs. Methods of compound purification and crystallization are improved. The experimental protocols and data analysis are also modified. The conductance measurements are carried out through electrode-molecule-electrode junctions created by an STM (scanning tunneling microscope) tip which repeatedly impinges into and retracts from the substrate. In this study, the tip-substrate contact is ensured by an external device, while it was not monitored in our earlier work. The conductance histograms are prepared by pooling all acquired conductance traces while, in the past, only those with step-like features were selectively used. The results show that the conductance histograms are featureless and that it is very difficult to determine the conductance values for pentametallic string complexes, even for Ni5 MSC. X-ray crystallographic data reveal that solvent molecules, present in the unit cell by previous purification methods, are absent for the new samples. Accordingly, mixed solvents exhibiting a range of polarity are utilized to improve the solubility of MSCs. The conductance peaks are slightly more pronounced in a more polar environment. The conductance peaks of Ni3, Ni5, and NiRhNi are solvent-independent, while the peak positions of asymmetric RhRhNi, RhRhNiNiNi, and RhRhRhNiNi shift associated with the solvent utilized. According to published researches in the effect of environment on molecular junction, we proposed that solvent molecules will affect the head group and alter gold-sulfur contact.

並列關鍵字

molecular conductance ； Rh an Ni heteometallic metal string complexes ； STM-bj

參考文獻

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