Title: Quantum confined 2D metals as a novel material for optoelectronics and catalysis

 Abstract: 

Since the isolation of graphene by Novoselov in 2004, two-dimensional (2D) materials have ignited interest in the fields of material science, solid state physics, and electronics. Interest in 2D materials stems primarily from quantum confinement-induced changes in band structure compared with the bulk. 2D metals, a recent addition to the 2D materials family, are synthesized by confinement heteroepitaxy (CHet), with an epitaxial graphene (EG) capping layer enabling air stability. This dissertation focuses on CHet-grown 2D metals, covering electro-optic characterization using electric double layer (EDL) gating to 2D metal growth on microparticles for catalysis applications.

To modulate the electrical and optical properties of 2D Ga, a new EDL gating approach – alternating current (AC) EDL gating – is developed to generate large oscillating e-fields (0.22 V/nm) that drive significant changes in 2D Ga carrier density (∆n_RMS ∼ 20 %). This electronic modulation induces a measurable change in the microreflectivity of atomically thin (t ∼ 0.5 nm) 2D Ga (∆R/R ∼ 8 × 10⁻⁴). Through transfer matrix optical modeling and DFT calculations, the optical response was found to be dominated by a linear Stark shift (∼0.4 D), which is only possible in materials lacking inversion symmetry. Thus, these results show the first direct evidence of a permanent dipole moment in a 2D metal, in addition to the first demonstration of gating a 2D metal.

Extending 2D metal growth from wafers to SiC microparticles (d50 ∼ 300 μm) is the second major contribution of this work, inspired by interest in 2D metal catalysis. First, EG-covered SiC particles (EGCP) were synthesized via a high-temperature annealing process (1750 ^◦C). The results show that EG quality depends on facet type, as measured by Raman spectroscopy, with strained monolayer EG (average 2D peak FWHM of 38.5 ±7.5 cm⁻¹) on the Si-face (0001), highly crystalline EG on the C-face (0001) (crystallite size of (∼2.7 μm), and defective multilayer EG on the edge-faces. Polarized oriented (PO) Raman volume mapping reveals EG growth – not only at the surface – but also within the interior of the particles at crystallographic defects. This is a surprising discovery with potential usefulness in applications that require large surface area, including catalysis. Next, CHet was used to grow 2D Ga on the particles. Ultra low frequency (ULF) peaks detected in Raman spectra confirm the presence of 2D Ga on each facet type. Consistent with 2D Ga on wafers, Raman ULF peaks on the Si-face were detected at 26 cm⁻¹ and 54 cm⁻¹ although, on the C-face, a new Raman ULF peak emerged at ∼91 cm⁻¹. Raman ULF peaks were also detected on the edge-faces, but with increased variability due to heterogeneity in the SiC crystal structure. In preparation for directly comparing catalytic activity of 2D metals grown on wafers to particles, a custom-designed, alumina microreactor was 3D-printed and validated with Pt-covered SiC wafers. This microreactor enables gas-phase heterogeneous catalysis measurements on planar substrates with a total surface area of 4 cm². 

Collectively, this work advances the field of 2D metals, by reporting on their fundamental electrical and optical properties with EDL gating, and demonstrating scalable synthesis methods for high quality EG and air-stable 2D metals on particle substrates. Among other applications, this work could unlock the use of 2D metals in optoelectronics and heterogeneous catalysis.

Chair:

Dr. Susan Fullerton-Shirey, Department of Chemical and Petroleum Engineering, University of Pittsburgh

Committee Members:

Dr. Götz Veser, Department of Chemical and Petroleum Engineering,

University of Pittsburgh

Dr. James R. McKone, Department of Chemical and Petroleum Engineering,

University of Pittsburgh

Dr. Joshua A. Robinson, Department of Material Science,

Penn State University