Structural Evolution of Molecular Assembly for Phenyl(ethynyl)benzenes at Liquid/Solid Interface: a Scanning Tunneling Microscopic Study
Date Issued
2012
Date
2012
Author(s)
Hsu, Yu-Ju
Abstract
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.
Subjects
scanning tunneling microscopy
liquid-solid interface
electrical pulses
solvent-coadsorption
Type
thesis
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