Date of Degree

9-2022

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Gustavo E. Lopez

Advisor

Carlos A. Meriles

Committee Members

Vinod M. Menon

Cyrus E. Dreyer

Maria C. Tamargo

Subject Categories

Condensed Matter Physics | Materials Chemistry

Keywords

Point-defects, color centers, semiconductors, rare earths, spectroscopy, first-principles

Abstract

For many years, atomic point-defects have been readily used to tune the bulk properties of solid-state crystalline materials, for instance, through the inclusion of elemental impurities (doping) during growth, or post-processing treatments such as ion bombardment or high-energy irradiation. Such atomic point-defects introduce local ‘incompatible’ chemical interactions with the periodic atomic arrangement that makes up the crystal, resulting for example in localized electronic states due to dangling bonds or excess of electrons. When present in sufficient concentrations, the defects interact collectively to alter the overall bulk properties of the host material. In the low concentration limit, however, point-defects can serve as interesting nanoscale or quantum objects, giving rise to localized magnetic moments (unpaired electrons) or addressable optical and electronic transitions otherwise absent in the bulk material. These effects have been particularly promising in semiconductors and insulators, where isolated point-defects can behave as optically addressable ‘artificial atoms’ with narrow-band and tunable single-photon emission, and remarkable sensitivities to local fluctuations in temperature, magnetic or electric fields, strain, and/or surface effects, even at room temperature. Such atomic point-defects are naturally present in any semiconductor, but not all of these are optically active within the electromagnetic regions of interest (known as defect-related color centers), while typically only a fraction of them feature addressable unpaired electrons (an important element for nanoscale sensing and/or quantum applications). Further, identifying their underlying atomic structures is challenging and required to generate them on demand.

The work presented in this thesis aims at contributing to the collective efforts of studying and identifying native defect-related color centers in relevant wide bandgap semiconductors, while providing an alternative approach towards engineering defect-related color centers with interesting optoelectronic properties for quantum and nanoscale applications. The approach towards defect engineering presented in this work builds on the well-known, robust, and high-quality atomic-like properties of the rare-earth ions, such as cerium and erbium, and the material advantages that 2-dimensional wide bandgap semiconductors, such as hexagonal boron nitride and tungsten disulfide, offer for device integration. First, confocal Raman and fluorescence spectroscopy techniques are combined to isolate single native color centers in cubic boron nitride nanocrystals that feature room-temperature narrow-band single-photon emission within the visible. Secondly, fluorescence and X-ray photoelectron spectroscopy are combined with first-principles calculations as an alternative approach towards the identification of point-defects and impurities present in highly fluorescent hexagonal boron nitride thin flakes. Lastly, cerium-doped hexagonal boron nitride and erbium-doped tungsten disulfide are studied via spectroscopic and first-principles techniques and proposed as alternative material platforms for nanoscale and quantum applications based on engineered rare-earth related color centers. Further considerations towards material integration, photonic interactions with small rare-earth based color center ensembles, and improved techniques for quantitative computational descriptions of rare-earth based color centers are also discussed.

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