Tag: David Johnson

Presenter(s): Alexander Lygo − Physics, Chemistry

Faculty Mentor(s): David Johnson,

Poster 22

Funding: Vice President for Research and Innovation (VPRI) Undergraduate Fellowship, Presidential Undergraduate Research Scholars program

As potentially applicable in high-performance electronics and quantum computers, topological insulators and heterostructures containing them have recently garnered significant interest by materials scientists. Despite their imagined utility, these compounds have proven difficult to synthesize. In a recent study of a series of compounds, [BiSe1+δ]m[TiSe2] m with m = 1, 2, 3, it was observed that, for the m = 3 compound, the topological insulator Bi2Se3 formed upon deposition and was present at all annealing temperatures. To test if Bi2Se3 could be incorporated into a heterostructure, a series of (Bi- Se)3-TiSe2 precursors with varying Bi-Se ratios and layer thicknesses were prepared and annealed at various temperatures for 30 minutes. A combination of specular and in-plane diffraction indicated that select precursors formed a highly crystalline and crystallographically aligned compound containing BiSe, Bi2Se3, and TiSe2 and high-resolution electron microscopy revealed the stacking sequence of the constituents. X-ray fluorescence measurements reveal that the compound formed readily over a range of Bi-Se ratios. Electron transport measurements revealed metallic behavior and surprisingly high carrier mobility, compared to BiSe1+δ TiSe2. These results provide a synthetic route for preparing a high quality Bi2Se3 containing heterostructure with unexpected properties and with further research, a material with properties applicable to electronics or quantum computers may be discovered.

Presenter(s): Dylan Bardgett − Chemistry

Faculty Mentor(s): David Johnson

Poster 79

Research Area: Natural/Physical Science

From photovoltaics and semiconductors to optical and protective coatings, thin films have made their way into every facet of daily life. Driven by the significance of these materials, the scientific community has embarked on a global quest for cheap, nondestructive, and precise analytical techniques for determining the elemental composition and structure of thin films. One technique, known as X-Ray Fluorescence spectroscopy (XRF), uses the characteristic energies of photons fluoresced by an incident beam of X-rays to determine the relative amounts of different elements in a material. This study set out to take XRF a step further by measuring not only the relative quantity, but the exact quantity, directly proportional to the number of atoms, of each element in a film. Using a solid-state material synthesis method known as physical vapor deposition, thin films were deposited onto silicon substrates and heated to produce a wide range of crystalline compounds such as TiSe2, Bi2Se3, and MoSe2. Crystalline diffraction techniques were used to identify and confirm the structures of the films as well as their total thicknesses. These films were then scanned on an XRF spectrometer, and background signals were subtracted manually by measuring a fragment of the non-deposited silicon substrate set aside before the film deposition process. Data indicates that the intensity of fluoresced photons of a given energy characteristic of an atomic element scales linearly with the number of atoms of the corresponding element, so long as the total film thickness is less than roughly 100 nm. This study concludes that, when calibrated properly, XRF can be used to directly measure the quantities of specific elements in a film. These findings greatly bolster the viability of XRF spectroscopy as suitable analytical technique for the characterization of thin films.