Andrew Cupo: “Nanostructures and Phonon Anharmonicity in Atomically-Thin Black Phosphorus.” Alaa Moussawi: “Cascading Overload Failures in Power Grids: Analysis and Mitigation”

**Andrew Cupo**: **“Nanostructures and Phonon Anharmonicity in Atomically-Thin Black Phosphorus.”**

**Abstract**: Atomically-thin black phosphorus has been of interest recently [1] due to its high carrier mobility [2] and band gap which remains direct independent of the number of layers [3]. Using first-principles density functional theory (DFT) calculations we have investigated nanoribbons [4], nanopores [4], antidot lattices [5], and phonon anharmonicity in black phosphorus. We showed that the few-nm wide armchair and zigzag nanoribbons fabricated by collaborators have similar electronic properties as their single-layer counterparts. Furthermore, we rationalized the asymmetric opening of nanopores in black phosphorus under uniform irradiation by showing that the energy barrier for removing atoms from the edge is anisotropic in phosphorene. In addition, we explored the electronic properties of phosphorene antidot lattices. We demonstrated a tunable band gap due to quantum confinement with deviations from the general trend attributed to self-passivating edge morphologies. The spatial distribution of the band gap is bimodal with higher band gap atoms emanating from the zigzag nanoconstrictions, which reflects the material anisotropy. Lastly, we carried out ab initio molecular dynamics simulations in combination with the power spectrum method to show that phosphorene’s phonon frequencies decrease with increasing temperature. This accounts for the observed temperature dependence of the phonon frequencies from Raman spectroscopy [6].

[1] Quantum Confinement in Black Phosphorus-Based Nanostructures, A. Cupo and V. Meunier, Journal of Physics: Condensed Matter, 29 (28), 2017

[2] Achieving Ultrahigh Carrier Mobility in Two-Dimensional Hole Gas of Black Phosphorus, G. Long et al., Nano Letters, 16 (12), pp 7768-7773, 2016

[3] Direct Observation of the Layer-Dependent Electronic Structure in Phosphorene, L. Li et al., Nature Nanotechnology, 12, pp 21-25, 2017

[4] Controlled Sculpture of Black Phosphorus Nanoribbons, P. M. Das*, G. Danda*, A. Cupo* et al., ACS Nano, 10 (6), pp 5687-5695, 2016

[5] Periodic Arrays of Phosphorene Nanopores as Antidot Lattices with Tunable Properties, A. Cupo*, P. M. Das* et al., ACS Nano, 11 (7), pp 7494-7507, 2017

[6] Temperature Evolution of Phonon Properties in Few-Layer Black Phosphorus, A. Łapińska et al., The Journal of Physical Chemistry C, 120 (9), pp 5265-5270, 2016

**Alaa Moussawi**: **“Cascading Overload Failures in Power Grids: Analysis and Mitigation”**

**Abstract**: Cascading overload failures (blackouts) are a common and catastrophic vulnerability of spatially-embedded distributed flow networks that are poorly understood. The efficiencies that locally connected networks afford us come at an also high cost. With increasing energy demands taxing old infrastructures, power grids are currently operating at a critical phase where the capacity of these systems is approaching load demands. This highlights the importance of understanding the dynamics of power systems so that they can most effectively be utilized at this critical phase without major failure. Mitigation techniques will be presented, and their effectiveness under varying constraints will be investigated. A simple strategy for approximating the severity of multi-node failures will be presented. Finally, it will be shown that such networks exhibit a phase transition at a given capacity threshold. Moreover, we show that cascade size distributions measured in this region exhibit a power-law decay.