Precise and Directed Assembly and Chemistry at the Atomic Scale

Open Access
Author:
Nanayakkara, Sanjni Ushika
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 23, 2006
Committee Members:
  • Paul S Weiss, Committee Chair
  • Christine Dolan Keating, Committee Member
  • Henry C Foley, Committee Member
  • Roy F Willis, Committee Member
Keywords:
  • scanning tunneling microscopy
  • single electron tunneling
  • substrate-mediated interactions
  • Coulomb blockade
  • scanning tunneling spectroscopy
  • catalysis
Abstract:
Atomic-scale studies of the assembly, chemistry and charge transport of individual adsorbates on well-defined surfaces and of nano-structures dispersed on self-assembled monolayer surfaces are presented. Ultrahigh vacuum variable temperature scanning tunneling microscopy (STM) was used to observe these systems directly with atomic resolution. The spectroscopic resolution enabled by low temperatures was utilized to identify adsorbates on atomically flat surfaces, as well as to probe charge transport through ex-situ assembled precise nano-scale structures. The role of the substrate in the reactivity and interactions between surface-bound species is necessary for a complete understanding of complex surface phenomena. Here, we have studied the atomic-scale reaction pathway of a surface-catalyzed reaction and utilized single-molecule spectroscopy to identify adsorbates. The Ullmann coupling reaction on Cu{111} refers to the coupling of two haloaromatic molecules to form a diaromatic molecule. The active site of the coupling step of this reaction has been identified. We have inferred the bonding geometry of surface-bound species through the vibrational modes observed for substituted haloaromatic molecules. Substrate-mediated indirect interactions between islands of bromine adatoms were investigated. The islands were formed at 600 K. The island-island interaction potential was measured by evaluating ~3000 inter-island distances from ~200 non-overlapping STM images that were acquired at 4 K. Preferred distances between the islands corresponded to half-multiples of the Fermi wavelength of Cu{111}. Conductance switching in 4,4´-di(ethynylphenyl)-1-benzenethiolate (OPE) and 4,4´-di(ethynylphenyl)-2´-nitro-1-benzenethiolate (NO2-OPE) molecules were investigated. These molecules have exhibited stochastic switching between discrete conductance states under ambient conditions and we have now addressed this phenomenon using variable temperature STM at 300, 77 and 4 K. Charge transport through nano-scale assemblies, ligand-stabilized precise undecagold clusters and gold nanoparticles has been investigated. These nanoparticles demonstrated Coulomb blockade as well as spectral hopping and diffusion. Apparent tunneling barrier height (ATBH) images acquired simultaneously with STM topographic images have enabled us to probe directly the alkanethiolate self-assembled monolayer (SAM)-substrate interface. Implications in the theories of the tunneling mechanism through an insulating thin film as well as the origin of features in STM images of SAMs are discussed.