Laboratory for Surface Modification (LSM)

Seminars Archives

October 2017 | November 2017 | December 2017

Thursday, November 02, 2017
Long-Range Correlated Dynamics in Glassy Materials and Its Role in Producing Stable Glasses
Zahra Fakhraai
University of Pennsylvania
Department of Chemistry
12:00 Noon CHEM 260

Nanometer-sized thin films of small organic molecules are widely used in applications ranging from organic photovoltaics and organic light emitting diodes, to protective coatings and high resolution nano-imprint lithography. Physical vapor deposition (PVD) is widely used in manufacturing ultra-thin layers of amorphous organic solids, with an underlying assumption that the properties of these layers are bulk-like. In this presentation, I demonstrate that films of organic glass-formers with thicknesses of 30 nm or less have dynamics significantly enhanced relative to the bulk dynamics at temperatures well below the glass transition temperature, Tg. I show that a sharp glass to liquid transition exists when the thickness of the layer is changed from 40 nm down to 20 nm. This significant change in the glass dynamics is due to the enhanced mobility at the air/glass interface and the length scale over which the effects of this perturbation can propagate due correlated dynamics in the bulk glass. As such, we show that these glassy systems have long-range correlated dynamics over length scales of about then times the size of the molecules, well exceeding their inter-molecular interaction range. We discuss the effect of variety of factors such as substrate interactions and molecular construct on the observed length scale. We show that this length scale is roughly the same in a wide range of organic systems, while it changes significantly when an inorganic glass such as amorphous selenium is studied. These results can also help elucidate fundamental mechanisms of glass transition phenomenon, a question that have attracted numerous theoretical and experimental studies in the past half century. Furthermore, we show that the enhanced surface dynamics along with long-range correlation in the dynamics can allow formation of stable glasses through the PVD process, enhancing our ability to engineer properties of glasses over a large range of properties.
Thursday, November 09, 2017
Molecularly Tunable Quantum Emitters
YuHuang Wang
University of Maryland
Department of Chemistry and Biochemistry
12:00 Noon CHEM 260

New properties arise as the size of a crystal reaches the Bohr radius of excitons. This phenomenon, known as quantum confinement, has enabled powerful synthetic strategies to control the optical and electronic properties of a material through size engineering. In this talk, we will discuss a fundamentally new approach that allows systematic tailoring of nanostructure excitons through covalently bonded surface functional groups that are themselves non-emitting. Specifically, we show that by varying the surface functional groups, a semiconducting carbon nanotube can be chemically converted to create a large series of distinct shortwave-infrared quantum emitters that are molecularly specific, systematically tunable, and significantly brighter than the parent semiconductor. In contrast with quantum confinement, where size matters, this new property-tailoring capability arises from the creation of fluorescent quantum defects that can be chemically controlled at the molecular level. I will discuss the exciting new opportunities that this new family of quantum emitters may open up for potential applications ranging from bioimaging and sensing to quantum information processing.
Thursday, November 16, 2017
Control of Light-Matter Interaction in Van Der Waals Materials
Vinod Menon
Department of Physics
The City College & Graduate Center of City University of New York (CUNY)
12:00 Noon CHEM 260

Two-dimensional (2D) Van der Waals materials have emerged as a very attractive class of optoelectronic material due to the unprecedented strength in its interaction with light. In this talk I will discuss approaches to enhance the strength of this interaction even further using microcavities, and metamaterials. I will first discuss the formation of strongly coupled exciton-photon quasiparticles (microcavity polaritons) at room temperature [1] and the valley polarization properties of these polaritons [2] in the 2D transition metal dichacogenide systems. Following this I will discuss the broadband enhancement of spontaneous emission from these 2D materials using photonic hypercrystals [3, 4]. Finally, I will also briefly discuss our recent work on room temperature single photon emission from hexagonal boron nitride [5] and the prospects of developing deterministic quantum emitters using them through strain engineering.
References
[1] X. Liu, et al., Nature Photonics 9, 30 (2015)
[2] Z. Sun et al., Nature Photonics 11, 491 (2017)
[3] T. Galfsky, et al., Nano Lett. 16, 4940 (2015)
[4] T. Galfsky, et al. Proc. Natl. Acad. Sci. 114, 5125 (2017).
[5] Z. Shotan, et al., ACS Photonics 3, 2490 (2016)
Thursday, November 30, 2017
THz Nonlinear Optics
Thomas E. Murphy
Department of Electrical and Computer Engineering
University of Maryland
12:00 noon CHEM 260

Nonlinear optical effects were first observed very soon after the firsts lasers were invented, and since those early days nonlinear optics has remained both an active area of research and practical ingredient in a number of important technologies and products. However, nonlinear optical effects are seldom observed in the far-infrared or terahertz spectral regime, in part because of the historical paucity of intense optical sources in this spectral range. In the past few years, there have been significant advances in our ability to produce bright, intense optical signals in the terahertz regime, which has enabled the study of THz nonlinear effects in materials and devices. We will present some of the new techniques for observing nonlinearities in uncharted spectral regime, and discuss the nature and origin of the nonlinear optical response. In particular, we will describe recent studies of THz nonlinearities in graphene and in silicon waveguides.

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