單分子電性量測之訊號擷取系統的改良及其紫質分子電性之研究

Improvement of the Acquisition System for STM-based Break Junction for Conductance Measurements of meso-Pyridylethynyl Porphyrins

許展翔(Chan-Hsiang Hsu)

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.2013.11012

摘要

分子電子學(molecular electronics)探討分子結構、能階與性質對元件電性表現的影響。瞭解單分子的導電值是發展分子電子學的入門步驟，最簡單的量測架構是「電極-分子-電極」(metal-molecule-metal junction, MMM junction)，大多藉由金屬材料的斷裂以製得量測之兩電極，在斷裂瞬間形成分子尺度的狹縫，若分子橋接於兩電極，即形成分子接點。藉由施加固定電壓或掃瞄電位，可分別獲得分子導電值或i-V曲線。形成分子接點的前後，量測系統的導電值將跨越多個數量級，此即儀器必須能夠涵括的量測範圍。
本論文工作改善本實驗室的量測硬體，將原本的三個數量級的導電值量測範圍提升至六個數量級，因此能在單一次的實驗量測中記錄到金原子接點(1 G0 = 77.5 uS, a conductance quantum)斷裂前到MMM junction斷裂後的導電值分布。作法是將量測數量級範圍大的動態訊號擷取卡外接於商業化的掃描穿隧顯微鏡(STM, scanning tunneling microscope)，並撰寫LabView程式控制STM-bj實驗(STM-based break junction method的簡稱)的數值擷取、運算與統計整理。
本實驗室曾研究一系列紫質分子之導電值，當時的STM-bj的測量範圍僅達三個數量級，須分兩次測量才能涵括較大範圍的導電值資訊。一種的參數設定是針對低導電值範圍的方法[STM I(s)法]，此設定未包含1 G0的數值，故不確定是否形成金原子接點；另一種針對高導電值範圍，則可確認金原子接點之形成[STM-bj法]。前者可獲得兩組導電值分布，其中較高的那組導電值則未見於後者的STM-bj法，且因儀器測量範圍之侷限，電流雜訊遠大於較低一組之導電值分布，故難以確定產生1 G0的實驗條件是否會伴隨形成低導電值的MMM junctions。本論文以改良過的軟硬體設備重新檢視此系列紫質分子，得自於STM-bj的導電值統計圖顯示1 G0的金原子接點及紫質低導電值接點，而無高導電值接點的導電峰值，可能是呈現高導電值的分子接點與表面需呈現特定夾角，而在STM-bj形成間隙的過程中，難以形成此位向。導電值-探針位移之二維統計圖的結果顯示分子接點斷裂時，電極間距與分子兩端氮原子之距離相當，與低導電值接點是頭基跨接於兩電極的推論吻合。STM I(s)法則觀察到三組導電值的分布，最高一組之數值大於先前STM I(s)法的導電值量測上限，推測此組接點由平躺於表面之紫質分子所形成。

關鍵字

分子電子學 ； 分子接點 ； 金屬紫質

並列摘要

Measuring the conductance of metal-molecule-metal (MMM) junctions is the most fundamental and direct way to investigate molecular electronics, which correlates the structure of molecules to their electrical behaviors under nanometer-scale. To create suitable widths of the electrode-gaps for the formation of MMM junctions, most experimental techniques start from the breaking events of metallic point contacts and monitor the current variation to detect the signal caused by the bridging of molecules. Devices with wide current windows are required to conduct the procedure mentioned above. A dynamic signal acquisition device with high resolving ability is connected to the commercial scanning tunneling microscopy (STM). The dynamic range of data acquired during STM break junction (STM-bj) is widened from 3 orders to 6 orders of magnitude, in the range of which it is made possible to record the conductance distributions of gold point contacts and MMM junctions in one experiment setup.
The conductance of a series of porphyrin derivatives were measured in the narrower dynamic range of the commercial STM before. In previous results, two sets of conductance distribution are shown in STM I(s) method, during which no gold point contacts occur; but the higher one is absence in STM-bj, which form gold point contacts in each cycle. However, the noise level of the commercial STM is much higher than the range of the lower one in the setup of STM-bj. It is unsure whether the gold point contacts hinder the more conductive junctions only. With the improvement of the data acquisition, the conductance and junction geometry of these porphyrin derivatives are probed by STM I(s) method and STM-bj again. In STM-bj, the conductance distributions of both gold point contacts and the less conductive junctions are recorded. It is clearly confirmed that only the more conductive junctions are hinders by gold point contacts and the possibility of possessing a tilted angle-confined geometries in these junctions is raised. The limits of the junction stretching distances are close to the distance between the two nitrogen atoms in the anchoring groups. This result further proves that the less conductive junctions bridge the electrodes by anchoring. In STM I(s) method, three set of conductance distribution are observed. The highest one is above the upper limit of the previous current windows and is attributed to the junction by lying porphyrins on the gold surfaces.

並列關鍵字

molecular electronics ； molecular junctions ； metalloporphyrins

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