五核異金屬串的單分子電性： 分子-電極間電子耦合與負微分電阻性質之探討

Abstract

在分子電子學的領域中，分子電性是在金屬-分子-金屬(metal-molecule-metal, MMM)的架構下進行量測。理想分子-電極接觸界面的建立與功能性分子導線的設計，對於實現分子電子元件而言是項重要的議題。良好的分子-電極接觸界面會提升電子在接合點的傳遞效率，可以提高分子導電性。為了要探究影響金屬-分子-金屬接合點的原因，以HOMO-LUMO能隙大的飽和烷為分子主體，兩端官能基為‒SH、‒NCS及‒CN，並以金、鈀和鉑三種金屬為電極材料。運用STM-bj (scanning tunneling microscopy break junction)量測單分子導電值，可得結果如下，使用鈀電極的單分子導電值較金電極提升2-3.5倍，且各種頭基與金屬電極的組合皆呈現兩組導電值。對於接觸導電值而言，以鈀作為電極較金電極增加2-5倍，顯示鉑電極的選用是有助於電子在接觸界面的傳遞。根據鍵結角度的計算與Mayer鍵序的分析，頭基與鈀和鉑的界面會形成pi特性，而與金的界面則是以sigma特性為主。在接觸界面處的pi特性形成額外通道提供電子的傳遞，因此對於量測到的單分子電性與接觸導電值而言，分子-電極接觸界面的電子耦合扮演重要的影響因素。此外，藉由Landauer方程式的拆解可獲得分子主體電阻值，對於相同分子主體而言，所推衍出的電阻值會與頭基和電極材料的種類無關。利用tight bonding 模型可說明，拆解出的電阻值只和分子主體的能階有關，並顯示出分子主體結構的導電能力。 為了調控金屬串的導電性，將具有強金屬-金屬作用力的雙核釕金屬和鎳串摻混，合成出Ni–Ru–Ru–Ni–Ni為骨架的異金屬串錯合物。由測量到的導電性顯示，[NiRu2Ni2(tpda)4(NCS)2]的導電值較[Ni5(tpda)4(NCS)2]增加4倍。藉由DFT/B3LYP的分析發現，Ru2的摻混可增加金屬串的平均鍵序並使HOMO-LUMO能隙變窄，故較五核鎳金屬串來得導電。此外，[NiRu2Ni2(tpda)4(NCS)2]在金、鈀和鉑電極上的I-V曲線皆呈現負微分電阻的特性，根據能階軌域的計算與NDR (negative differential resistance)波峰隨金、鈀和鉑電極功函數的位移，可推衍出產生負微分電阻的機制。由於Ru2所貢獻的能階會造成HOMO附近出現不連續能階分佈，電極費米能階與不連續能階間的能量匹配，會導致電子的傳遞由與分子能階匹配的共振穿隧至直接形式的穿隧，因而造成導電值的下降。因此，[NiRu2Ni2(tpda)4(NCS)2]產生負微分電阻的原因，是在HOMO附近的不連續能階分佈所導致。

The development of ideal molecule−electrode contacts and the design of functional molecular wires are critical for the realization of molecular electronics. The good molecule−electrode contacts exhibit efficient charge transportation and thus confer large single-molecule conductance. To derive the intrinsic properties of the MMM contact, the conductance of a series of alkanes terminated with–SH, –NCS, and –CN on Au, Pd and Pt were carried out by using the method of STM-bj. The results show the single-molecule conductance via Pt contact is 2~5-fold superior to those via Au contact. Among the three headgroups, –SH bears the largest contact conductance and –CN is smallest. Such disparity in their conductance can be ascribed to the degree of the headgroup–electrode coupling. Simulated bond angles and Mayer bond order at the contact suggest that π characters are significantly involved at Pt and Pd contacts, while σ characters is preferably adopted at Au contacts. These findings demonstrate that the electronic coupling at the contact plays an important factor on contact conductance and on the measured single-molecule conductance. Moreover, we purpose that the resistance of molecular backbone can be extracted via Landauer formula. The resistance values for the same framework are found to be independent of teminal headgroup and electrode material, manifesting that this approach can evaluate quantitatively the resistance of functional moieties from the measured value. To tune electric conductance of extended metal atom chains, the first pentametal EMAC (extended metal-atom chain) of heteronuclear backbone was synthesized by mixing a weakly coupled nickel-atom chain with an Ru2 unit, which has strong metal-metal interactions. The resulted Ni‒Ru‒Ru‒Ni‒Ni is 4-fold more conductive than that of its pentanickel analogue. DFT/UB3LYP analysis shows that the incorporation of the Ru2 unit enhances metal-metal interaction and thus results in the conductance superior to that of pentanickel EMAC. Single-molecule I-V characteristic of NiRu2Ni2(tpda)4(NCS)2 exhibits NDR (negative differential resistance) behavior, unobserved for pentanickel or pentaruthenium complexes. A plausible explanation is derived based on the simulation of energy level and the correlation of the NDR peak positions with the EFermi of Au, Pd, and Pt. The energy levels contributed by the Ru2 moiety make the frontier orbitals discrete such that the molecular conductance decreases upon ramping the electrode EFermi from where aligned with the MOs to nonresonant regimes. Thus, the discrete levels near HOMO are accounted for NDR phenomena.