Atomic scale investigations of the thermal and electron induced chemistry of small molecules on platinum(111) as revealed by scanning tunneling microscopy

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Título:
Atomic scale investigations of the thermal and electron induced chemistry of small molecules on platinum(111) as revealed by scanning tunneling microscopy
Autor: Schwendemann, Todd Charles
Assuntos: 37 Inorganic, Organic, Physical, And Analytical Chemistry Scanning Tunneling Microscopy, Surface Chemistry ; Scanning Tunneling Microscopy, Surface Chemistry
Descrição: The work presented here can be divided into two parts: 1) an experimental and analysis section dealing with the investigation of small molecules such as methyl bromide, carbon dioxide, diatomic nitrogen, methane and methane?s photochemical derivative methyl radical adsorbed onto the Pt(111) surface, and 2) A detailed explanation of the current STM and chamber, with included designs and detailed instructions for operation and maintenance of both the STM and chamber. The investigations of the methyl bromide molecule show interesting dipole-dipole interactions on the Pt(111) surface. With a (6 x 3) lattice being described as the full monolayer that was created by overdosing and annealing to 104 K. The (6 x 3) lattice is shown to occupy top sites and three fold hollow sites on the Pt(111) surface giving rise to a very sharp and symmetrically split ν2 RAIRS mode, and the absence of the ν5 mode in RAIRS is indicative that the molecules are all aligned with their C-Br bond parallel to the surface normal. Additional sub-monolayer structures were observed that had components that were not aligned with the surface normal. The submonolayer lattices ranging from a structured 0.12 ML to a random coverages estimated at 0.20 ML, to a shift in the (6 x 3) lattice resulting in a high local line coverage of 0.33 ML. Analysis of the CO2 molecules adsorbed onto the Pt(111) surface shows that there is a preferred high temperature dosing that results in a thermodynamically stable system of a (3 x 3) lattice consisting of both horizontal and vertical molecules. The coverage of the (3 x 3) lattice of vertical molecules is 0.11 ML which can be assigned to the RAIRS peak of 2287 cm-1. The vertical molecules are seen to occupy the hollow sites within the horizontal (3 x 3) lattice. The low temperature dosage of multilayers and annealing, to 78 K, show that the (3 x 3) lattice is compressed into a lattice of (5 x 3) with some of the molecules in the unit cell that are incommensurate with the Pt(111) lattice. However, isolated unit cells of the horizontal (3 x 3) lattices remain after the compression which allows a single vertical CO2 molecule to occupy the hollow site resulting in the characteristic 2277 cm-1 peak in RAIRS. The resulting local coverage of the (5 x 3) lattice is calculated to be 0.40 ML. Methane was found to adsorb onto the Pt(111) lattice in a (√3 x √3) configuration yielding a coverage of 0.33 ML in perfect agreement with previous coverage assignments. With a full coverage of methane adsorbed onto the crystal surface an ArF excimer laser was used to photodissociate the molecules to create methyl radicals that could be imaged by STM. After photochemical deposition of methyl radicals and annealing the surface to 175 K, the STM was used to image the surface. The methyl radical were estimated to arrange in a (√3 by √3) lattice same as the methane, and imaged as roughly 0.4 ? high protrusions from the surface with a diameter of 5.5 ?. The last molecule that was studied, was the adsorption of diatomic nitrogen on the Pt(111) surface at a temperature of 25 K. Due to the very low desorption temperature of N2 (i.e. 46 K) and the relatively high temperature of the crystal, only chemisorbed molecules were able to be resolved. The results confirm that diatomic nitrogen binds to the top side of the Pt(111) step edge in agreement with Yates RAIRS studies, and calculations by Norskov. However, there was observed a stable cluster of molecules bound to the lower side of the step edge in a (2 x 2) lattice configuration which has previously unknown before these images and is the most likely source of the photoactivity of nitrogen molecules on the crystal surface. It is the hopes of this author that the experiments described within the dissertation lead to new and better understanding of the role that the microscopic scale structures of adsorbates on the surface play in catalysis. Also that the general information of STM design, construction and tip fabrication will be useful to all students who follow me in working on the STM UHV chamber as well as be a reference for anyone interested in scanning tunneling microscopy work.
Editor: University of Virginia, Charlottesville, VA
Data de criação/publicação: 2006
Idioma: Inglês

Atomic scale investigations of the thermal and electron induced chemistry of small molecules on platinum(111) as revealed by scanning tunneling microscopy

Schwendemann, Todd Charles

University of Virginia, Charlottesville, VA 2006

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