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  • 學位論文

脈衝電壓與溶劑性質對炔苯基苯盤狀分子於石墨表面排列結構的影響:掃描穿隧顯微術之研究

Structural Evolution of Molecular Assembly for Phenyl(ethynyl)benzenes at Liquid/Solid Interface: a Scanning Tunneling Microscopic Study

指導教授 : 陳俊顯

摘要


控制有機分子在表面的排列對於發展奈米級功能性材料為一重要議題。以光、熱、電等外界刺激(external stimuli)或改變實驗環境(溶劑、濃度)調控分子自組裝結構是為關鍵途徑。本論文第一部分利用掃描穿隧顯微術(scanning tunneling microscopy, STM)研究盤狀液晶分子六炔苯基苯(hexakis((3,4-bis(dodecyloxy)phenyl)ethynyl)benzene, HPB)於液固界面(苯基辛烷-石墨)之排列結構。我們發現利用STM探針施予脈衝電壓能改變分子的排列結構,施加3、4、5V的脈衝可分別誘導出六邊形排列(hexagonal)之單層、雙層及孔洞結構,推測原因為脈衝電壓誘導探針周圍的分子瞬間極化(polarized),而脈衝的偏壓施加方式讓基材表面帶相反電荷,因而使分子自溶液中沉降吸附於石墨基材。 論文的第二部分探究兩種液晶分子之排列結構與溶劑性質及分子濃度的影響。目標分子為修飾烷氧鏈的HPB以及修飾長烷鏈的1,2,4,5-四炔聯苯基苯(1,2,4,5-tetrakis(4-dodecyl-4'-ethynyl-biphenyl)benzene, TBPB)。HPB於表面形成六邊形單層結構,其排列不受溶劑種類影響;TBPB在不同溶劑中則呈現多種排列結構(polymorphs)。在弱吸附類溶劑中(苯基辛烷、1,2,4-三氯苯與正辛烷),TBPB皆呈現網狀結構且排列不受濃度影響。在強吸附類溶劑的十六烷溶液內,TBPB於高、中、低濃度條件分別出現直條狀、交錯及孔洞狀三種排列結構。溶劑十六烷共吸附(coadsorption)於TBPB分子烷鏈的空隙之間以凡得瓦力穩定結構,當TBPB濃度越稀,共吸附之溶劑分子數目越多,使分子由緻密排列的直條狀結構轉變成孔洞結構。

並列摘要


Research of external stimuli leading to controllable assembly of functional molecules has attracted much attention. Here we demonstrate that the reversible transformation of the molecular assembly can be triggered by short electrical pulses (3~5 V/10 microsecond), conveniently delivered by STM (scanning tunneling microscope). The model compound is an alkoxylated discotic nematogen (hexakis((3,4-bis(dodecyloxy)phenyl) ethynyl)benzene, HPB). Upon applying the pulses of 3, 4, and 5 V on the sample, the assembled pattern of HPB undergos, hexagonal, bilayer, and nanoporous, respectively, in a controllable fashion. This phenomenon is attributed to the deposition of tip-induced polarized molecules to the oppositely charged substrate whose local charge redistributes in associated with the electric stimuli. The second part of the thesis investigates how the assembly of alkylated discotic compounds is affected by the solvent properties. The examined compounds are hexakis((3,4-bis(dodecyloxy)phenyl)ethynyl) benzene (HPB) and 1,2,4,5-tetrakis(4-dodecyl-4'-ethynyl-biphenyl)benzene (TBPB). The solvents include 1-phenyloctane, 1,2,4-trichlorobenzene, and n-alkanes (n = 8, 12, 16). HPB monolayers adopt a hexagonal arrangement on graphite for all solvents used and HPB concentrations examined. For TBPB, weakly adsorbed solvents show no effect on the monolayer structure. In dodecane and hexadecane, TBPB exhibits polymorphs as a function of concentrations which involve coadsorption of solvent molecules due to van der Waal attraction from the interdigitation of their alkyl chains.

參考文獻


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