Scanning Tunneling Microscopy and Spectroscopy of Nanoscale Assemblies

Open Access
Author:
Blake, Meaghan Marie
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 28, 2010
Committee Members:
  • Paul S Weiss, Dissertation Advisor
  • Paul S Weiss, Committee Chair
  • John V Badding, Committee Member
  • Lasse Jensen, Committee Member
  • Seong H Kim, Committee Member
Keywords:
  • clusters
  • self-assembled monolayer
  • STM
  • STS
Abstract:
With the growing interest in building nanoscale structures with atomic-scale precision, it is important to understand the underlying chemical and physical factors that govern the assembly of these structures. To this end, we have used low-temperature scanning tunneling microscopy and spectroscopy to study individual molecular adsorbates related to heterogeneous catalysis and precise nanostructures tethered on self-assembled monolayers. High spectroscopic resolution enabled by low temperatures was employed to identify adsorbates on atomically flat surfaces, as well as to probe the electronic properties of precise cluster assemblies. The role of the substrate in the reactivity and interactions between surface-bound species is critical for a comprehensive understanding of complex surface phenomena. We have studied the atomic-scale reaction pathway of a surface-catalyzed reaction and utilized scanning tunneling spectroscopy to identify reactive intermediates and products in the Ullmann coupling reaction on Cu{111}. Chemical identification of fluorophenyl intermediates and biphenyl products was achieved by vibrational spectroscopy via inelastic tunneling spectroscopy. The strongest vibrational mode in the phenyl species disappears when the tilted intermediates couple to form biphenyl products. We infer that the surface normal component of the dipole moment is important in determining the transition strength in inelastic electron tunneling spectroscopy. The role of the substrate surface-state electrons in mediating long-range electronic interactions between islands of bromine adatoms on Cu{111} was investigated. We have quantified nearest neighbor island separations and found island-island distances correspond to half-multiples of the Fermi wavelength of Cu{111}, indicative of the role of the surface state in determining the lateral spacings between the islands at catalytically relevant temperatures. These substrate-mediated interactions are critical for developing new insight in the development of precisely nanostructured films and devices. The electronic properties of ligand-stabilized undecagold clusters and nanoparticles were investigated using scanning tunneling microscopy and spectroscopy. Significant spectral hopping and diffusion were observed across single and multiple clusters. Although precisely constructed and controlled, this spectral diffusion demonstrates the acute sensitivity of these clusters to their physical and chemical environment. This has enabled an understanding and ability to make unique measurements with unrivaled specificity and resolution, which can be applied to more complex systems, where understanding local electronic properties is key to device function. We have designed surfaces to capture superatom clusters, both in the solution and in the gas phase, that exhibit exceptional stability, in order to measure their electronic structure. Clusters of certain elements are known to exhibit “magic number” behavior that leads to their enhanced stability. Understanding how to tailor these superatoms with specific electronic, optical, and magnetic properties enables the fabrication of building blocks for cluster-assembled materials.