Colloquium schedule. 

Refreshments at 3:30, Talk 4:00 - 5:00 p.m., in CII 3051, unless otherwise noted.

2017

Dec
6
2017
Low Center for Industrial Innovation (CII), Room 3051 4:00 pm

Nov
1
2017
Low Center for Industrial Innovation (CII), Room 3051 4:00 pm

Oct
25
2017
Low Center for Industrial Innovation (CII), Room 3051 4:00 pm

Oct
18
2017

Oct
4
2017
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.
 

Andrew Cupo, Alaa Moussawi, Physics, Applied Physics and Astronomy, Rensselaer
Low Center for Industrial Innovation (CII), Room 3051 4:00 pm

Apr
19
2017
"Classical vs. Quantum Decoherence and the Quantum- Classical Path Integral"

The path integral formulation of time-dependent quantum mechanics provides the ideal framework for rigorous quantum-classical or quantum-semiclassical treatments, as the spatially localized, trajectory-like nature of the quantum paths circumvents the need for mean-field-type assumptions. However, the number of system paths grows exponentially with the number of propagation steps. In addition, each path of the quantum system generally gives rise to a distinct classical solvent trajectory. This exponential proliferation of trajectories with propagation time is the quantum-classical manifestation of time nonlocality, familiar from influence functional approaches. A real-time quantum-classical path integral (QCPI) methodology has been developed. The starting point is the identification of two components in the effects induced on a quantum system by a polyatomic environment. The first, “classical decoherence mechanism” dominates completely at high temperature/low-frequency solvents and/or when the system-environment interaction is weak. Within the QCPI framework, the memory associated with classical decoherence is removable. A second, nonlocal in time, “quantum decoherence process” is also operative at low temperatures, although the contribution of the classical decoherence mechanism continues to play the most prominent role. The classical decoherence is analogous to the treatment of light absorption via an oscillating dipole, while quantum decoherence is primarily associated with spontaneous emission, whose description requires quantization of the radiation field. The QCPI methodology takes advantage of the memory-free nature of system-independent solvent trajectories to account for all classical decoherence effects on the dynamics of the quantum system in an inexpensive fashion. Inclusion of the residual quantum decoherence is accomplished via phase factors in the path integral expression, which is amenable to large time steps and iterative decompositions. The methodology can be used to perform an all-atom simulation of nonadiabatic processes in condensed phase environments with unprecedented accuracy. Applications to charge transfer reactions in solution will be discussed.

CII 3051 4:00 pm

Feb
22
2017
"Atomically thin and two-dimensional materials: Science and Applications"

In the past decade, there has been enormous progress in materials science and engineering at the very limits of quantum confinement. Ground-breaking discoveries and innovations have resulted from a range of atomically-thin systems such as carbon nanotubes, graphene, 2D transition metal dichalcogenides and other layered materials. These developments in ultrathin matter at the very quantum limit of stability have spearheaded an explosive growth in new science and technology.  This talk will attempt to outline the contributions of our research group in this exciting new field, including our attempts to develop and manipulate new types of atomically-thin materials, exploring the behaviour of charge, photons and phonons in them, and utilizing their unique properties to develop applications in the nanoelectronics, optoelectronics, sensing, detection, actuation, energy, and other areas. Through these discussions, I will try to motivate how quantum matter can potentially transform several important applications and enable them to operate at ultra-high and unprecedented performances.

Bio: Prof. Kar received his BSc degree in Physics (Honours) from Presidency College, Kolkata, in 1995, and obtained his MS (1998) and PhD (2004) degrees in Physics from the Indian Institute of Science, India. He worked as a postdoc at the Universitaet Karlsruhe, Germany, and Rensselaer Polytechnic Institute, USA (with Prof. PM Ajayan). He also held a research assistant professor position at RPI with Prof. Saroj Nayak, before joining Northeastern University, USA, as an assistant professor of physics, in 2010. Prof. Kar has published about 65 papers in peer-reviewed journals, including in Nature Nanotechnology, Nature Materials, Nature Photonics, Nature Communications, and Science Advances. Prof. Kar has presented over 45 invited talks worldwide, and has 6 patent applications. He currently serves as an Editorial Board member of Scientific Reports, and has served on several US and international grant application review panels. Prof. Kar enthusiastically promotes the science and technology of 2D materials and systems by regularly organizing conferences and symposia at top international physics and materials science congresses venues.

CII 3051 4:00 pm

Feb
8
2017
"Understanding light-matter interactions at the single-molecule level."

Metal nanoparticles sustain a collective oscillation of their free electrons, called a localized surface plasmon resonance (LSPR), when excited by an electromagnetic wave. When this incident wave is resonant with the LSPR frequency, the field intensity is strongly increased in the near field of the nanoantenna. Plasmonics thus provides a unique setting for the manipulation of light via the confinement of the electromagnetic field to regions well below the diffraction limit. This has opened up a wide range of applications based on extreme light concentration, including nanophotonic lasers and amplifiers optical metamaterials, biochemical sensing and antennas transmitting and receiving light signals at the nanoscale. However, many difficulties remain in experimentally measuring the shape, size, and enhanced field properties of the localized electromagnetic modes in the vicinity of the nano-particles due to the limitations of optical microscopy. In this seminar, I will discuss how we can unravel the coupling of light to a nano-antenna through single-molecule fluorescence imaging. This technique is a powerful tool to optically study structures beyond the diffraction limit by localizing isolated fluorophores and fitting the emission profile to the microscope point-spread function. By using the random motion of single dye molecules in solution to stochastically scan the surface, and by assessing emission intensity, wavelength, and density of emitters as a function of position, we gain new insight into the properties of these systems and pave the way for the development of better plasmonic devices.

CII 3051 4:00 pm
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Jan
25
2017
CII 3051 4:00 pm
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2016

Dec
6
2016
"Growth Of Organic Thin Films: Achieving the Morphology Needed For High Performance Devices

The performance of organic electronic devices such as OLEDs and organic photovoltaic (OPV) cells depends critically on the morphology of the grown layers of which they are comprised. For example, the emission zone of and OLED should be amorphous, yet alignment of the emitting species relative to the substrate plane can result in increased optical outcoupling. Further, OPV cells benefit from a nanocrystalline morphology to allow for rapid and efficient exciton diffusion and charge extraction. In our laboratory we have developed several growth technologies that have demonstrated the ability to precisely control the organic film morphology from the molecular to the macroscopic levels. In this talk I will focus on two growth processes that have consistently exhibited the most precise control of morphology, and in some cases device performance. These methods are the ultrahigh vacuum process of organic molecular beam deposition (OMBD) and organic vapor phase deposition (OVPD). OMBD can controllably deposit monolayer thick films in ultrapure environments. On the other hand, OVPD is based on volatilizing small molecule organics into a hot, inert carrier gas. The molecules are then transported to a cooled substrate where they are physisorbed. OVPD is now finding use in the manufacture of organic electronic devices due to its extraordinary ability to exquisitely control film morphology across large substrate areas while allowing for very rapid film deposition as required for high throughput device fabrication. Both methods of growth will be compared to the widely used and well-known process of high vacuum thermal evaporation.

The Department of Materials Science and Engineering and The Department of Physics, Applied Physics & Astronomy presents the Distinguished Lecture in Materials Science and Engineering
Sage 5510 4:00 pm
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Nov
2
2016
Darrin Communications Center (DCC) 337 4:00 pm
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Sep
21
2016
“Local representation of the electronic dielectric response function”

The microscopic electronic dielectric response function is a fundamental physical quantity that captures the many-electron correlation effect. Although it is non-local by definition, a local representation in real space can provide insightful understanding of its chemical nature and to improve the computational efficiency of first principles excited state methods. In a recent work [Phys. Rev. B 92, 241107(R), 2015], we have developed a local representation of the electronic dielectric response function, based on a spatial partition of the dielectric response into contributions from each occupied Wannier function using a generalized density functional perturbation theory. We show that the locality of the bare response function is determined by the locality of three quantities: Wannier functions of the occupied manifold, the density matrix, and the Hamiltonian matrix. In systems with a gap, the bare dielectric response is exponentially localized, which supports the physical picture of the dielectric response function as a collection of interacting local response that can be captured by a tight-binding model.

Several applications of the local response theory will be discussed: a) local descriptors in linear response, e.g., bond polarizability and bond screening constant, b) dielectric band structure interpolation using a local basis set, and c) direct formulation and evaluation of molecular polarizability in condensed phase beyond the dipole approximation.

This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

Dr. Deyu Lu, Brookhaven National Laboratory
Darrin Communications Center (DCC) 337 4:00 pm

Sep
14
2016
“Squishy Physics: Examples from polymers and active matter”

The assembly of objects into materials and the properties of those materials are dependent on their size and a comparison of their interactions to thermal excitations. Materials that are dominated by behaviors at atomic length scales are often either hard solids or soft liquids. However, there are many materials with structures in the nanometer to micron length scale with interaction energies comparable to thermal fluctuations. These materials are “squishy” with properties similar to both solids and liquids. In this talk, I will describe interesting and unique physics of squishy materials through examples from polymers and active matter.

Long polymers are able to change their configuration significantly through bond rotations with minimal changes in energy. The entropic effects that result often determine the response to external forcing (like fluid flows or electric fields). I will describe our work to efficiently incorporate these effects into computational models and to use those models to understand how to manipulate the dynamics of the polymers.

Active matter is a class of materials that is continually pushed out of equilibrium from every point within the material instead of from the outside. Because they are always out of equilibrium, their dynamics and assembly are not limited by equilibrium statistical mechanics. Other techniques must be used to understand them. I will describe our work to understand the roles of fluid flows in determining the structure and dynamics within active matter.

Darrin Communications Center (DCC) 337 4:00 pm