d.w.rowlands [at] gmail.com
Master's Thesis (2015)
DW Rowlands, "Xenon Difluoride Etching and Molecular Oxygen Oxidation of Silicon by Reactive Scattering", Massachusetts Institute of Technology M.S. Thesis, (2015).
While both molecular fluorine (F2) and xenon difluoride (XeF2) fluorinate Si(100)2x1 surfaces at coverages up to one monolayer, fluorine is unable to cleave Si-Si bonds that ultimately leads to etching silicon at coverages above one monolayer (reaction probability 9×10–4) while XeF2 does so readily (reaction probability 0.6). Previous studies have demonstrated that both fluorine and xenon difluoride react with silicon via atom-abstraction mechanisms at low coverages, and that the XeF radicals produced by atom abstraction from xenon difluoride dissociate in the gas phase, producing a source of F radicals that may contribute to Si-Si bond cleavage by formation of multiple F-Si bonds. In addition, it has been shown that van der Waals clusters of F2 with a Xe atom and with a Kr atom have reaction probabilities with silicon at one monolayer of fluorine coverage of roughly 0.9 and 0.04, respectively, suggesting that the effect of mass of the incident gas molecules is important in activating the cleavage of silicon-silicon bonds through a multiple collision process.
A model based on a classical harmonic oscillator linked via a variable damping term to the thermal energy of the lattice is proposed to test the importance of the mass of the gas molecules to energy transfer and the duration of the vibrational excitation of the silicon-silicon lattice bonds. Computer simulations using this model suggest that the mass of the gas molecule does not affect the overall energy transferred to the silicon. However, the heavy mass and the resulting multiple collisions do extend the duration of the silicon excitation and the duration during which the gas molecule is in contact with the vibrationally excited silicon lattice.
The techniques that have been used to study the reactivity of fluorine compounds with silicon are potentially of use to the study of another problem: why triplet molecular oxygen (O2) is able to dissociatively chemisorb across singlet silicon-silicon dimer bonds to form the singlet Si-O product. Several mechanisms, including atom abstraction and non-adiabatic ladder climbing of (O2)2 van der Waals dimers are suggested to explain this reaction and a series of experiments to test the hypothesized mechanisms are proposed.
In addition, a detailed account of the ultra-high-vacuum molecular beam surface scattering apparatus used for these studies is provided, including a detailed description of its operation and maintenance procedures.
Gordon Research Conference Poster (2013)DW Rowlands, MR Blair, J-G Lee, R Hefty, and ST Ceyer, "Making F2 Heavy: Activated Etching of Si by van der Waals Molecules”, Poster at Gordon Research Conference on Dynamics at Surfaces, (2013).
The reaction probability of XeF2, a chemically stable covalent molecule, with a fluorinated Si(100) 2x1 surface to form the volatile etch product SiF4 is 103---104 times higher than that of F2, even though the reaction of Si with F2 is 20 kcal/mol more exothermic. This study shows that the reactivity difference arises from greater Si lattice vibrational excitation caused by the impact of the more massive XeF2. This effect is probed by increasing the mass of F2 by attaching it to a Kr or Xe atom via formation of a van der Waals complex, Kr(F2) and Xe(F2). These clusters are produced by high-pressure adiabatic expansion of a mixture of F2 and an inert gas in a triply differentially pumped, low-energy (plasmaless) molecular beam. Both complexes have the chemical characteristics of molecular fluorine, but the mass of covalently bound KrF2 or XeF2. The probability (P) of fluorine reacting with the fluorinated Si surface is observed to increase dramatically with increased mass, with P = 5±1x10-4 for F2, 0.08±0.02 for Kr(F2) and 0.9±0.1 for Xe(F2). The larger mass serves to increase energy transfer to the surface upon collision, as verified by time-of-flight measurements of the Kr and Xe velocities before and after interaction with the surface. The increased energy transfer results in greater lattice vibrational excitation. In turn, the vibrationally excited Si lattice has a higher probability of reaction with the fluorine carried on the inert gas atom, leading to Si etching. This study shows for the first time that the energy transferred to a surface as a result of a molecule's initial collision with it plays a critical role in the reaction probability in a molecule-surface interaction.
Chemistry Student Seminar Talk (2013)
DW Rowlands, "Silicon Surface Chemistry with Supersonic Molecular Beams", Talk at Massachusetts Institute of Technology Chemistry Student Seminar, (2013).
Undergraduate Senior Thesis (2009)
DW Rowlands, "Producing Safe Spin-Polarized Metabolites for Magnetic Resonance Imaging", California Institute of Technology Undergraduate Chemistry Senior Thesis, (2009).
There is significant interest in improved sensitivity and decreased measurement times for nuclear magnetic resonance and magnetic resonance imaging of small molecules, especially those with 13C and 15N spin labels. A source of spin polarizations oforder unity is spin-exchange optical pumping, which produces 129Xe with fractional polarization of order unity, about 105 higher than is achieved at equilibrium in high field. The research presented is the development and testing of components of an apparatus intended to bring target molecules, such as acetic acid, into the close (about 1 nm) contact with hyperpolarized 129Xe atoms as needed to effect rapid equalization of their spin temperatures through dipolar couplings. Specifically, an apparatus has been designed and is being built with the intention of rapidly depositing a gaseous mixture of xenon and the target molecule as a homogeneous solid under a strong magnetic field, then dropping the field strength to allow spin equalization through dipolar couplings followed by rapid production of a liquid sample of the target molecule at room temperature.
High School Senior Thesis (2005)
DW Rowlands, “Analysis of Light Pollution Spectra”, Eleanor Roosevelt High School (Greenbelt, Maryland) Senior Thesis, (2005).
Light pollution is artificially-produced light which serves no useful purpose. It may be light directed toward places that do not need illumination or light that is directed directly into the sky. There are many different sources of light pollution. One way in which light pollution sources differ is their spectra—different light sources have different spectra and thus alter the night time sky glow in different ways. One can determine the proportions of light pollution produced by different light sources.
Spectra were taken in three directions: Baltimore, MD, Washington, DC, and Annapolis, MD. It should be noted that Laurel and Columbia, MD are in the same general direction as Baltimore and that Bowie, MD is in the same general direction as Annapolis, MD. Spectra of Washington were also taken both in the early evening and around midnight. Lines produced by sodium, mercury, scandium, thorium, lithium, oxygen, neon, and high-pressure sodium were detected. Sodium D lines and scandium lines were stronger in Baltimore than in any other city, suggesting that low pressure sodium and metal halide lamps are more prevalent there. Mercury and sodium lines increased over the course of the night, suggesting that the proportion of the sky glow produced by mercury vapor and high pressure sodium lamps increases over the course of the night. This study showed that the breakdown of light pollution spectra does change over the course of a night and that different cities do produce different sky glow spectra.