Beyond Molecular Assemblies and Scanning Tunneling Microscopy

Open Access
Author:
Kim, Moonhee
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 25, 2011
Committee Members:
  • Paul S Weiss, Dissertation Advisor
  • Scott T Phillips, Committee Chair
  • Paul S Weiss, Committee Member
  • Christine Dolan Keating, Committee Member
  • Anne M Andrews, Committee Member
Keywords:
  • scanning tunneling microscopy
  • self-assembled monolayers
  • SAMs
  • photon scanning tunneling microscopy
  • evanescent wave coupling
  • anthracene
  • photodimerization
  • regioselective photoreaction
  • adamantanethiol
  • 2-adamantanethiol
  • 2AD
  • displacement
  • Hydrogen bonding interaction
  • amide containing alkanethiol
  • 3-mercapto-N-nonylpropionamide
  • 1ATC9
Abstract:
We introduce the self-assembled monolayers (SAMs) of n-alkanethiols on Au{111} as a standard molecular assembly to understand the physical chemistry of organic molecules on solid surfaces. We apply these molecular assemblies to create new interfaces and to exert control over single molecules. Scanning tunneling microscopy (STM) is used to characterize the molecular orientations, structures, phases, and intermolecular interactions within molecular assemblies, in real space, at the sub-nanometer scale. We take advantage of the high spatial resolution of STM to obtain fundamental insight into molecular phenomena. Other surface characterization techniques, including infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry complement and support our STM measurements. Photon STM is used to investigate optical phenomena of single organic molecules with Ångström-scale precision. Evanescent wave coupling by total internal reflection adds chemical specificity to STM function. High quality, thin Au{111} films deposited on c-cut sapphire prisms efficiently couple light to the tunneling junction. Annealing significantly improves substrate quality of both the sapphire and Au{111} surfaces, providing more stable tunneling junctions. Lock-in detection with light modulation improves the photoresponse registry in the photon STM. One promising candidate for efficient organic solar cells, C60-tethered 2,5-dithienylpyrrole triads, was isolated within a well-ordered n dodecanethiolate monolayer to measure the photocurrent on single triad molecules, which will determine the intrinsic photovoltaic efficiency at the molecular level. Increased photoconductivity of isolated photovoltaic or photoconductive molecules demonstrates the function and potential of photon STM. Photoreactions of conjugated organic molecules may be allowed by selection rules but not observed in solution reactions because of unfavorable reaction geometries. We have used defect sites in self-assembled alkanethiolate monolayers on gold surfaces to direct geometrically unfavorable photochemical reactions between individual organic molecules. High conductivity and stochastic switching of anthracene terminated phenylethynylthiolates within alkanethiolate monolayers, as well as in-situ photochemical transformations, have been observed and distinguished with the STM. Ultraviolet light absorbed during imaging increases the apparent heights of excited molecules in STM images, evidence that we are probing electronically excited states. The formation of 2-adamantanethiolate SAMs on Au{111} and their displacement by n dodecanethiol are investigated using STM, X ray photoelectron spectroscopy, and infrared reflection absorption spectroscopy. Well-ordered 2 adamantanethiolate monolayers undergo rapid and significant molecular exchange upon exposure to n dodecanethiol solutions, but their structures and intermolecular interactions template the growth of n dodecanethiolate domains. Annealing 2 adamantanethiolate monolayers at 78 ºC decreases the density of vacancy islands, while increasing the overall order and the average domain sizes in the films. This results in slower displacement by n dodecanethiol molecules, as compared to unannealed monolayers. The secondary sulfur position on the adamantyl cage alters the lattice structure and the exchange of 2 adamantanethiolate monolayers by alkanethiols relative to 1 adamantanethiol monolayers. We report the electrochemically driven phase transformation of a linear nanostructure in amide-containing alkanethiol, 3 mercapto N nonylpropionamide (1ATC9) SAMs. Hydrogen-bonding interactions between buried amide groups cause a multi-step electrochemical desorption, which involves an unusual phase change. This reaction can be exploited to produce a less dense, textured structure. Single-component 1ATC9 SAMs prepared in solution at room temperature for 24 hours consist of two phases with different apparent heights in STM images; these phases are readily manipulated by controlling solution temperature and deposition time. Intermolecular hydrogen-bonding interactions produce films with high thermal stability. The presence of two independent cathodic peaks in 1ATC9 monolayer voltammograms indicates two-step reductive desorption. A monolayer phase transition occurs after the first cathodic peak, transforming a close-packed monolayer into a striped phase that is energetically favored at low surface-thiolate density. Scanning tunneling microscopy, cyclic voltammetry, infrared reflection absorption spectroscopy, and X ray photoelectron spectroscopy reveal electrochemical nanostructuring, driven by partial reductive desorption and strong interchain hydrogen bonding. The resultant striped, low-coverage phase is inaccessible by other synthetic preparations, except by controlled dosing in ultrahigh vacuum.