Graduate Thesis Defense - Current

Speaker: Kazi J. Tasnim (PhD Defense)

Abstract: Due to the unique structural and electronic properties along with the practical applications in current and near future, investigations about the group-IV graphene-like two-dimensional (2D) sheets have been accelerated. Among them, 2D SiC and GeC sheets are polar materials with in-plane charge transfer from Si (Ge) to C atoms. An interesting question is how the electrostatic force, triggered by such in-plane charge transfer, plays a role in stabilizing the vertical heterostructures formed by 2D SiC and GeC sheets beyond vdW interaction. To answer this question, we have systematically investigated, the effects of the electrostatic interaction between layers of 2D SiC/GeC bilayer heterostructure with different stacking and out-of-plane species ordering within the framework of density functional theory. The results illustrated that, in addition to the weak vdW interaction, the electrostatic interlayer interaction can also induce the pi - pi orbital hybridization between adjacent layers, with the strong hybridization in the cases of Si-C and C-Ge species orderings but the weak hybridization between layers for C-C ordering. In particular, in the cases of either Si-C or C-Ge species orderings, the attractive electrostatic interlayer interaction stabilizes the inter-layer distance and the vdW interaction makes the system attain a lower cohesive energy. On the contrary, in the case of the C-C species ordering, the vdW interaction mostly controls the inter-layer distance and the repulsive electrostatic inter-layer force has less effect in here. The ways how the layers are stacked and how the species are ordered in the structure can greatly affect the band structure of the hybrid 2D SiC/GeC bilayer heterostructure. When the hybrid system is stacked in a specific way (e.g., AA stacking with Si-C/C-Ge ordering or AB stacking with C-C ordering), it shows an indirect band gap nature. On the other hand, a direct band gap behavior can be achieved under different stacking configurations (e.g., AB stacking with Si-C or C-Ge or Si-Ge ordering, or AA stacking with C-C/Si-Ge ordering). This means that the band gap of the 2D polar hybrid heterostructure can be adjusted depending on how the layers are arranged and how the species are ordered. Additionally, the SiC/GeC hybride structure has a weak type-II band alignment characteristic. These findings indicate that the hybrid structure holds promise for applications in light-emitting diodes and laser emitters.

The structural anisotropy in the principal axes (i.e., along the armchair and the zigzag directions) in black phosphorus (bP) leads to a wide range of applications in nanoelectronics, energy application, etc. However, its high reactivity in the air limits its practical applications. One approach to overcome this problem is to make alloys of bP with other materials with similar structural properties. Black arsenic (b-As) is a good candidate and black As_xP_1-x alloys have attracted experimentalists to synthesize. Since the most stable phase for As is gray As with the β phase,  and the most stable phase for P is the bP with the α phase, it is crucial to understand the whole aspects of the AsxP1-x alloys including (i) their structural and electronic properties in the β and α phases, (ii) the energetics between two phases, and (iii) the effect of Li intercalation and high pressure on their structural properties. In the second part of my research project, we have conducted a comprehensive theoretical study to deeply understand the aspects of AsxP1-x alloys. We have found that (1) structurally, the black As_xP_1-x alloys are stabilized in anisotropic puckered structures with the AB stacking, keeping the α phase as in bP. The gray AsxP1-x alloys, on the other hand, are stabilized in buckled structures with ABC stacking, keeping the  β phase as in the gray As.  The distribution of As and P atoms in the black AsxP1-x alloys prefers to form armchair As-As and P-P bonds, aligning along zigzag direction. But the distribution of As and P atoms in gray As_xP_1-x alloys prefers to form in-plane As-P bonds, instead of As-As or P-P bonds; (2) electronically, alloying widens the band gap compared to pure bP and b-As in black As_xP_1-x alloys, making the materials semiconducting with tunable direct-indirect nature, while the semi-metallic feature in the gray As and blue P is still kept in gray AsxP1-x alloys; (3) there is a critical As concentration, below it black As_xP_1-x alloys are energetically more favorable, but above it gray As_xP_1-x alloys are more favorable; (4) a local structural transformation or phase segregation was found in black As_xP_1-x alloys under either Li intercalation or the high pressure, providing pathways for structural phase transitions and leading to the new type of materials.

Speaker:Dinushika Vithanage (PhD Defense)

Abstract: Black arsenic phosphorus (b-As_yP_1-y) alloys have emerged as intriguing materials within the realm of two-dimensional (2D) materials, following the discovery of black phosphorus (BP). These alloys possess capability to overcome major limitations of BP and exhibit potential for tunability and enhancement of properties making them promising materials for a wide range of applications. Inspired by the intriguing findings obtained for BP, this research focuses on understanding the structural modifications that can be achieved in b-As_yP_1-y alloys through the application of stimuli such as intercalation and high pressure.
In the initial phase, b-As_yP_1-y alloys were synthesized using the chemical vapor transport (CVT) method and thoroughly characterized. The subsequent investigation focused on studying the structural evolution of b-As_yP_1-y alloys during lithium (Li) intercalation, with varying As concentration (y). In-situ Raman spectroscopy, facilitated by a dedicated in-situ electrochemical cell, was employed to analyze the real-time vibrational modes of the alloys during Li intercalation. The vibrational modes of b-As_yP_1-y alloys encompass eight distinct modes, representing P-P bonds (A¹_g, A²_g, 𝐵_2𝑔), As-As bonds (A¹_g, A²_g, 𝐵_2𝑔), and As-P bonds (two modes). During the initial stages of the intercalation process, a monotonic redshift was observed in all vibrational modes of b-AsyP1-y samples due to the softening of each mode caused by the intercalation-driven donor-type charge-transfer from Li to b-As_yP_1-y. Above a specific intercalation threshold, the emergence of a new peak, identified as the E_g mode of gray As, indicated the presence of an intercalation-driven structural phase segregation process. Furthermore, the A_1g mode of gray As emerged after this intercalation threshold and closely overlapped with the A²_g Raman mode of b-As_yP_1-y. Beyond the intercalation threshold, all peaks exhibited an upshift due to the co-existence of gray As with b-As_yP_1-y alloys, causing strain and phonon mode hardening. In the sample with the highest As concentration, phase segregation occurred during the synthesis process. Computational modeling revealed the co-existence of gray As in b-As_yP_1-y alloys with high As concentrations and the occurrence of local structural segregation during the intercalation process at a critical Li concentration.
In the final stage, the structural evolution of b-As_yP_1-y alloys under hydrostatic pressure was investigated using in-situ Raman spectroscopy with a Diamond Anvil Cell (DAC). The experiments revealed pressure-induced changes in vibrational modes, leading to the observation of two distinct pressure regimes. In Region I, all vibrational modes showed a monotonic upshift, indicating phonon hardening due to hydrostatic pressure. In Region II, As_0.4P_0.6 and As0.6P0.4 alloys displayed a linear blueshift at a reduced rate, suggesting local structural reorganization with less bond compression. Notably, As_0.8P_0.2 exhibited anomalous behavior in Region II. Interestingly, the pressure range also revealed the emergence of new peaks corresponding to the E_g mode and A_1g mode of the gray-As phase, indicating compressive strain-induced structural changes. The anomalous changes in Region II confirmed the formation of a new local structure, characterized by elongation of the P-P, As-As, and As-P bonds along the zigzag direction within the b-As_yP_1-y phase, possibly near the grain boundary. Additionally, the gray-As phase underwent compressive structural changes. This study highlights the significance of intercalation and pressure in inducing structural transformations and exploring novel phases in two-dimensional (2D) materials, including b-As_yP_1-y alloys.

Speaker: Safia Abdullah R Alharbi (PhD Defense)

Abstract: Recently, two-dimensional (2D) heterostructures have attracted extreme attention in nanomaterials science. They have been successfully fabricated and applied to nanotechnology in many fields, such as nanoelectronics, solar cells, sensors, energy stores, quantum information, etc. The most common heterostructures are 2D-lateral heterostructure (LH) and 2D-vertical heterostructure (VH) where each of them exhibits unique features depending on the direction of assembly, i.e., along in-plane or out-of-plane direction.
Beyond the van der Waals-VH which possess of van der Waals (vdW) interaction, there are other types of heterostructures made of 2D polar materials that possess different types of chemical bonding nature, e.g., chemical bonds with less (e.g., 𝑆𝑖𝐶 monolayer) or more (e.g., 𝐺𝑒𝐶 and 𝑆𝑖𝐺𝑒 monolayers) charge transfer between atoms, forming covalent bonds with a certain ionicity. The goal of this work focused on shedding light on the physical aspects of 2D LH and VH, constructed by such polar materials (e.g., 𝑆𝑖𝐶, 𝐺𝑒𝐶, and 𝑆𝑖𝐺𝑒 monolayers). This work is a theoretical study by employing Density Functional Theory to unravel the unique physical properties of such heterostructures.
Because an artificial strain will be induced by the lattice mismatch in building heterostructures, the effect of strain on the electronic properties of 𝑆𝑖𝐶, 𝐺𝑒𝐶, and 𝑆𝑖𝐺𝑒 monolayers was first investigated. It was found that these monolayers can tolerate strain up to 8%, and such strain can induce modifications on the physical properties. Interestingly, it was found that 𝑆𝑖𝐶 and 𝐺𝑒𝐶 monolayers undergo a direct-indirect band gap transition; while 𝑆𝑖𝐺𝑒 monolayer undergoes a metal-semimetal transition, which made them attractive candidates for building heterostructures.
Second, a systematically study on the aspect of 2D polar-LH of 𝑆𝑖𝐶/𝐺𝑒𝐶 and 𝑆𝑖𝐺𝑒/𝐺𝑒𝐶 has been conducted. It was found that the synergistic effect of the lattice mismatch induced strain, the chemical bonding nature at the interface, and quantum confinement can lead to several interesting phenomena. For instance, their electronic properties can be modulated by tuning the domain size, the chemical bonding nature, and the designing of interface. Accordingly, a lateral spontaneous p-n junction triggered by the in-plane charge transfer was detected, which implies promising applications such as visible light photocatalyst.
Third, the roles of the stacking species arrangement and the interlayer interactions (including vdW and electrostatic forces) on stabilizing the structure and modulating electronic properties of 2D polar-VH of 𝑆𝑖𝐺𝑒/𝐺𝑒𝐶 were deeply studied. It was found that, in addition to the redistribution of the in-plane net-charge transfer, a net charge redistribution also occurs between layers and leads to a polarization in the interfacial region that induces a built-in electric field and helps to reduce the recombination of photogenerated electron-hole pairs.

Speaker: Milinda Bharatha Kalutara Koralalage (PhD Defense)

Abstract: Lithium-Sulfur (Li-S) batteries have become a promising candidate to meet the current energy storage demand, with its natural abundance of materials, high theoretical capacity of 1672 mAhg^-1, high energy density of 2600 Whkg^-1, low cost and lower environmental impact. Sulfide based solid state electrolytes (SSEs) have received greater attention due to their higher ionic conductivity, compatible interface with sulfur-based cathodes, and lower grain boundary resistance. However, the interface between SSEs and cathodes has become a challenge in all solid-state Li-S batteries due to the rigidity of the participating surfaces. A hybrid electrolyte containing SSE coupled with a small amount of ionic liquid, was essential to improve the interface contact of the SSE with the electrodes.

Coating-based cathodes were successfully fabricated using water-based carboxymethyl cellulose (CMC) solution and Styrene butadiene rubber (SBR) as the binder with low sulfur loading (0.70 mgcm^-2) as well as high sulfur loading (4.0 mgcm^-2). Solid-state composite powder-based cathodes pressed onto SSE (loading 4.0 mgcm^-2) with enhanced electronic and ionic conductivity were fabricated with SP:S and SSE.

Ionic Liquids (IL) prepared using Lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI) as salt,  with premixed pyrrolidinium bis(trifluoromethyl sulfonyl)imide (PYR) as solvent and 1,3-dioxolane (DOL) as diluent were used to wet both SSE-electrode interfaces. The effect of IL dilution, co-solvent amount, LiTFSI concentration, C rate at which the batteries are tested and the effect of SSE inside the cathode, were systematically studied and optimized to develop a QSSLSB with higher capacity retention and cyclability. LiTFSI (2M) dissolved in PYR:DOL(1:1) found to be optimum IL combination for low sulfur loading QSSLSBs reaching 500 mAh/g after 100 cycles while LiTFSI (3M) in PYR:DOL(1:3) was the optimum IL concentration for higher loading QSSLSBs reaching 400 mAh/g after 100 cycles.

This work reports promising results of quasi-solid-state electrolyte Li-S batteries (QSSLSB) based on novel Li6PS5F0.5Cl0.5 Li-argyrodite solid-state electrolyte (SSE) with minute amount of IL, Super P-Sulfur (SP:S) cathode, and Li-anode. It also offers a new insight into the intimate interfacial contacts between the SSE and carbon-sulfur cathodes, which will be critical for improved electrochemical performance of quasi-solid-state lithium-sulfur batteries with high sulfur loading in the future.