ALTERING THE SURFACE AND INTERLAYER COMPOSITION OF 2D MATERIALS

The use of two-dimensional nanomaterials in technology, such as plasmonic sensors and optoelectronic devices, requires precise control of composition and dimensions. The large surface area of 2D materials naturally creates opportunities to investigate surface science and intrinsic quasi-surface science in ultra thin samples. As a species is brought in contact with these materials, either at the surface or an interlayer space, charge transfer may occur. The extent of this charge transfer can alter the carrier concentration of the material, effectively doping it. However, if this charge transfer is large in magnitude, it may also lead to a chemical reaction and a physical change of the surface composition. Here, I investigate 2D materials after these charge transfer events (e.g. surface and interlayer composition, and carrier concentration). Specifically, I focus on altering the carrier concentration of graphene though various methods and explore the role of the physical dimensions on the emerging properties. Additionally, I characterize the composition of a 2D material as it is oxidizes due to charge transfer with oxidant species and then spatially-resolve the resulting surface oxide composition. Graphene is a semi-metal with a carrier concentration of 1012 cm2; however by altering the carrier concentration of this 2D material, it can become metallic. When graphene is doped, it behaves as an exceptional plasmonic material, demonstrating tunable plasmons in the THz and mid-infrared regimes. Despite investigation of graphene plasmons in various arrays, there is limited research in anti-dot (e.g. hole or nanomesh) lattices, especially at very high carrier concentrations. Here, we explore the plasmons that emerge in graphene anti-dot lattices, with low to high levels of doping (0.1–1.35 eV) corresponding to in-plane carrier concentrations of 1 1012—5 1014 cm2. Furthermore, we investigate the effect of geometric variations (i.e. radius, periodicity, number of layers) on the the plasmon resonance. We find that the plasmon resonance of these few-layer, anti-dot lattices can be blue-shifted well into the near infrared regions of the electromagnetic spectrum by varying geometric parameters and levels of doping, opening new opportunities for graphene in NIR plasmonics. Finally, we propose methods to further increase tunability, potentially enabling this material to have a plasmon in the visible spectrum, as well as schemes to realize these structures experimentally. Phosphorene is emerging as an important two-dimensional semiconductor, but controlling the surface chemistry of phosphorene remains a significant challenge. This material readily undergoes surface degradation in ambient environments, due to significant charge transfer events with oxidants. Here we show that controlled oxidation of phosphorene determines the composition and spatial distribution of the resulting oxide. We used X-ray photoemission spectroscopy to measure the binding energy shifts that accompany oxidation. We interpreted these spectra by calculating the binding energy shift for 24 likely bonding configurations, including phosphorus oxides and hydroxides located on the basal surface or edges of flakes. After brief exposure to high-purity oxygen or high-purity water vapor at room temperature, we observed phosphorus in the +1 and +2 oxidation states; longer exposures led to a large population of phosphorus in the +3 oxidation state. To provide insight into the spatial distribution of the oxide, transmission electron microscopy was performed at several stages during the oxidation. We found crucial differences between oxygen and water oxidants: while pure oxygen produced an oxide layer on the van der Waals surface, water oxidized the material at pre-existing defects, such as edges or steps. We propose a mechanism based on the thermodynamics of electron transfer to interpret these observations. This work opens a route to functionalize the basal surface or edges of 2D black phosphorus through site-selective chemical reactions and presents the opportunity to explore the synthesis of 2D phosphorene oxide by oxidation.Doctor of Philosoph

Interpreting core-level spectra of oxidizing phosphorene: Theory and experiment

We combine ab initio density functional theory calculations with the equivalent cores approximation to determine core-level binding-energy shifts at phosphorus sites caused by oxidation of phosphorene. We find that presence of oxygen increases the core-level binding energies of P atoms and expect binding-energy shifts of up to 6 eV in highly defective geometries. We have identified likely binding geometries of oxygen that help to interpret the observed core-level photoemission spectra in samples at different stages of oxidation and allow us to determine the fractions of specific local geometries

Experimental Demonstration of an Electride as a 2D Material

Because of their loosely bound electrons, electrides offer physical properties useful in chemical synthesis and electronics. For these applications and others, nano-sized electrides offer advantages, but to-date no electride has been synthesized as a nanomaterial. We demonstrate experimentally that Ca$_2$N, a layered electride in which layers of atoms are separated by layers of a 2D electron gas (2DEG), can be exfoliated into two-dimensional (2D) nanosheets using liquid exfoliation. The 2D flakes are stable in a nitrogen atmosphere or in select organic solvents for at least one month. Electron microscopy and elemental analysis reveal that the 2D flakes retain the crystal structure and stoichiometry of the parent 3D Ca$_2$N. In addition, the 2D flakes exhibit metallic character and an optical response that agrees with DFT calculations. Together these findings suggest that the 2DEG is preserved in the 2D material. With this work, we bring electrides into the nano-regime and experimentally demonstrate a 2D electride, Ca$_2$N

Control of Surface and Edge Oxidation on Phosphorene

Phosphorene is emerging as an important two-dimensional semiconductor, but controlling the surface chemistry of phosphorene remains a significant challenge. Here, we show that controlled oxidation of phosphorene determines the composition and spatial distribution of the resulting oxide. We used X-ray photoemission spectroscopy to measure the binding energy shifts that accompany oxidation. We interpreted these spectra by calculating the binding energy shift for 24 likely bonding configurations, including phosphorus oxides and hydroxides located on the basal surface or edges of flakes. After brief exposure to high-purity oxygen or high-purity water vapor at room temperature, we observed phosphorus in the +1 and +2 oxidation states; longer exposures led to a large population of phosphorus in the +3 oxidation state. To provide insight into the spatial distribution of the oxide, transmission electron microscopy was performed at several stages during the oxidation. We found crucial differences between oxygen and water oxidants: while pure oxygen produced an oxide layer on the van der Waals surface, water oxidized the material at pre-existing defects such as edges or steps. We propose a mechanism based on the thermodynamics of electron transfer to interpret these observations. This work opens a route to functionalize the basal surface or edges of two-dimensional (2D) black phosphorus through site-selective chemical reactions and presents the opportunity to explore the synthesis of 2D phosphorene oxide by oxidation