Measurements of Single-Molecule Conductance by Conductive Atomic Force Microscopy with Tactile Feedback: the Effect of the Cantilever Force Constant

中文摘要 分子電子學的研究領域中，以電極–分子–電極作為測量平台，運用導電式原子力顯微術斷裂接合法(conductive atomic force microscopy break junction, c-AFM BJ)的技術，得以使分子跨接於探針與表面之間測量單分子導電性。本論文首次使用c-AFM監控分子拉伸過程，討論分子受力與導電值之關係。量測飽和烷雙硫醇(HS(CH2)nSH, n = 6, 8, 10)導電值，其結果與文獻相符，足以驗證本實驗方法之可行性。 本實驗選用的力學回饋系統(PicoAngler, NanoScopeIIIa)可將nanoNewton等級的作用力放大到操作者觸感可以感受的程度，以致於可以立即調整對分子的施力值於某個特定的區間。相較於這個領域常用的等待分子自發性吸脫附於兩電極間的方法，本實驗可獲得長時間(約450秒)的單一分子導電值。我們選用力常數為40、0.3及0.03 N/m的三種探針，將AFM施力大小與獲得的HS(CH2)8SH導電值做成二維統計圖進行比較。這個領域最常使用的探針之力常數為40 N/m，但因力常數大，壓電陶瓷的些微位移便足以斷裂金–金鍵結(1.5 nN)，即便在壓電陶瓷的最佳解析值，仍有5 nN的施力不準度，因此施力與分子導電值的相關性不明顯。力常數0.3及0.03 N/m的兩種探針，則可解析出導電值分佈隨施力增加而逐漸集中的趨勢。探針的力常數越小，越可減低壓電陶瓷及儀器擾動帶來的影響，並可以解析更多組導電值資訊。當探針拉力小(0.2 (± 0.1) nN)的狀態時，從I-t及I-V圖中皆發現有多組導電值的分佈，因為S–Au於bridge及hollow兩吸附能接近的吸附位作變動加上飽和烷構型轉換皆對導電值產生影響；當探針拉力大(0.8 (± 0.1) nN)的時候，S–Au僅吸附於atop位加上構型變動不易，導電值的分佈較為集中。Abstract In the field of molecular electronics, c-AFM BJ (conductive atomic force microscopy break junction) is one of the methods employed to create molecular junctions of metal–molecule–metal configurations through which single-molecule conductance can be measured. Reported in this thesis work is, for the first time, the acquisition of single-molecule conductance can be correlated with the applied stretching force at the molecular junction by taking advantage of a tensile sensor integrated in the c-AFM. Alkanedithiols (HS(CH2)nSH, n = 6, 8, and 10) are the model compounds and the consistence of their conductance values with literature reports validates this methodology. We construct two-dimensional (2D) histograms from the conductance and force traces using three kinds of force constant of tip (k = 40, 0.3, 0.03 N/m). We have found that (1) The conductance is independent of stretching force at k = 40 N/m. (2) The distribution of conductance become narrow with increasing stretching force at k = 0.3, 0.03 N/m. (3) Multi sets of conductance values are found at k = 0.3, reflecting the reduction of thermal fluctuation by using smaller force constant of tip. A force feedback system, PicoAngler, for fixing tip-substrate spacing to monitor the conductance of single molecular junction with a long lifetime (≈ 450 s) that enables detailed nature of the conductance of single molecules to be gained. When the stretching force is small (0.2 (± 0.1) nN), multi sets of conductance in I-t and I-V curves are produced due to change of conformation in an alkylene chain and binding sites (bridge or the hollow site) of thiolates on gold. On the contract, when the stretching force is large (0.8 (± 0.1) nN), conductance is centralized, resulted from only elongated conformers and the atop binding site