College of Arts and Sciences

Photochromic molecules are characterized by a functional group whose configuration is modified by absorption of light, in a reversible manner. They could be at the basis of new electronic displays which would be activated by light irradiation. For the formation of ultra-thin electronic displays, researchers now try to adsorb them on metallic substrates. Two main questions are thus asked : Firstly, is it possible to self-assemble this type of molecule on a substrate. Second, when self-assembled in monolayer, do the molecules remain active under irradiation, and most importantly, can they be locally switched under STM tip Ordered self-assemblies have been successfully obtained with azo-benzene based photochromes,

Continue reading… Ordered self-assembly of molecules on gold substrates, for activated organic monolayers – Prof. Emmanuelle Lacaze

In this talk, I will first review the basic electronic properties of graphene. In particular, I will explain why the recently observed insulating phase of graphene at charge neutrality point in high magnetic field quantum Hall (QH) experiments is a big surprising. Then I will present a simple single-particle theory for this intriguing finding. We show that the magnetic field driven Peierls-type lattice distortion (due to the Landau level degeneracy) and random bond fluctuations compete with each other, resulting in a transition from a QH-metal state at relative low field to a QH-insulator state at high enough field at the charge neutrality point.

Continue reading… Topological transition of graphene from quantum Hall metal to quantum Hall insulator – Prof. XiangRong Wang

In the past few years a new class of solid state optical systems has been developed in which photons have an effective mass and a repulsive interaction between each other. These renormalized photons are known as “polaritons”. One way of looking at this type of system is as an optical medium with world-record nonlinearity, leading to new possibilities for modulating light. Another way of looking at this type of system is as an analogue of a gas of atoms, which can undergo Bose-Einstein condensation and can become superfluid, allowing us to study superfluidity in a new way. I will review the basic experimental methods and recent results of polariton superfluids.

Continue reading… Quantum Fluids of Light – David Snoke

Semiconductor nanowires are quasi-one-dimensional single-crystals that have emerged as promising materials for the development of photonic and electronic devices with enhanced performance. Nanowires offer solutions to some of the current challenges in science and engineering, but realization of their full potential will be ultimately dictated by the ability to control their structure, composition, and size with high accuracy. In this talk, I will discuss our recent results on the controlled growth, doping, and applications of III-V nanowires, as well as advanced electron microscopy techniques for direct correlation of structural and physical properties with high spatial resolution. We have developed a simple,

Continue reading… Semiconductor nanowires : from LEDs to solar cells – Silvija Gradecak

Spin-orbit coupling is a consequence of relativity but can be observed and used at the device scale to electrically initialize and manipulate electron spin polarization. Understanding how to exploit spin-orbit effects in non-magnetic semiconductors may enable the development of new devices with enhanced functionality and performance, such as spin-based devices that combine logic and storage and fast optical switches for information processing. In this talk, I will describe time- and spatially-resolved measurements of electron spin transport that enable sensitive measurements of the spin-orbit field and its dependence on applied electric fields and mechanical strain. These spin splittings also provide a mechanism for the electrical generation of spin polarization.

Continue reading… Mapping spin-orbit effects in semiconductors – Vanessa Sih

Small molecules constructed from familiar chemical components, but with an unconventional (reduced symmetry) molecular shape, hold promise for developing nematic liquid crystals with macroscopic biaxiality or even polarity. These properties, realized over practical temperature ranges using thermotropic compounds, could open new avenues in technologies including optical displays, mechanical sensors, and low-cost personal power generation. I will report on recent studies of short-range order – a guidance, if not a direct precursor, to rational development of biaxial/polar nematics – in three types of reduced symmetry thermotropic materials: bent-core (V-shaped) liquid crystal compounds and rod-like molecules containing either lateral branches (Y-shaped) or bridges (H-shaped).

Continue reading… Short-range order in nematic liquid crystals formed by reduced symmetry molecules – Sam Sprunt

Single spins associated with defects in diamond have emerged as a promising and versatile experimental system. They can be used as qubits in optically connected quantum networks, as sensors for magnetic imaging with sub-micron resolution, as readout heads for detecting and engineering quantum states of nano-mechanical oscillators, and even as probes in biological systems. I will discuss some of the key experimental progress and future prospects along these paths.

Continue reading… Hybrid Quantum Devices with Single Spins in Diamond – Gurudev Dutt

In the first part of the talk, I will tell you about the controversy about the position of the defect levels for the oxygen vacancy in ZnO and how we tried to resolve it. In the second part, I will discuss the case of nitrogen in ZnO. I will discuss why nitrogen on an oxygen site forms a deep rather than shallow acceptor level. However, it is known that there exists a shallow level related to nitrogen doping. The question is then what defect complex is responsible for this shallow level? I will try to convince you that a N2 molecule located on a Zn-site has all the expected behavior of a shallow acceptor.

Continue reading… Point defect studies in ZnO: oxygen vacancy and p-type doping – Walter Lambrecht

One route towards future electronics is to exploit interactions between coherent electron spin states and photons in semiconductor structures. This will require an understanding of the coherent evolution of spin states, the eventual decoherence of these states, and how these states interact with light, all in a scalable room-temperature system. In this seminar, I will present our work on spins in semiconductor nanocrystal quantum dots (NCQDs). This system provides a platform to study room-temperature coherent spin states and their interactions with light. In an NCQD, a spin optically initialized into a superposition of eigenstates remains coherent for approximately one nanosecond at room temperature.

Continue reading… Semiconductor nanocrystals for room-temperature coherent electronics: A flexible platform for manipulating spin coherence – Jesse Berezovsky

We live in a world whose technology is ruled by a small set of inorganic semiconductors, notably silicon. Research on organic semiconductors (OSCs), molecular materials based on organic compounds, seeks to supplement the reigning paradigm rather than to supplant it. In particular, OSCs may improve photovoltaics, LEDs, sensors, and flexible, cheap electronics. In this talk, I will describe recent work at Kent State on liquid crystalline (LC) semiconductors, a subclass of OSCs that offer distinct advantages and disadvantages relative to more common crystalline or polymeric organics. After a (very) short introduction to the physics of OSCs and LCs, I will discuss how the ability to align the molecules in LC OSCs over macroscopic distances can (profoundly) improve transport characteristics.

Continue reading… Shedding some light on liquid crystalline organic semiconductors – Brett Ellman

Nanomaterials, since their debut, have greatly advanced human knowledge from many aspects. For example, carbon-based nanomaterials, such as carbon nanotube and graphene, have been the subjects of intensive study over the last two decades and greatly improved our understanding of phenomena happening at the nanoscale. On the other hand, microelectromechanical systems, MEMS, research has thrived over the last few decades in the engineering field and brought along many new applications. In this talk, I will illustrate that, by combining nanomaterials with MEMS technology, even more new opportunities and new sciences can be unveiled. I will mostly focus two systems: 1.

Continue reading… Nanostructures in Motion: Probing Surface Science and Fracture Mechanics at Molecular Level – Zenghui Wang

Isolating the role of building block shape for self-assembly and packing provides insight into the ordering of molecules and the crystallization of colloids, nanoparticles, proteins, and viruses. We investigated a large group of polyhedra whose phase behavior arises solely from their anisotropic shape. At intermediate packing density, our results demonstrate a remarkably high propensity for thermodynamic self-assembly and structural diversity. We show that from simple measures of particle shape and local order in the fluid, the assembly of a given shape into a liquid crystal, plastic crystal, or crystal can be predicted. Towards higher density, packing considerations dominate. Good packings can often be distinct from what is observed to assemble from the disordered state.

Continue reading… Self-Assembly and Packing of Polyhedra into Complex Structures – Michael Engel

Nematic Liquid Crystals (NLCs) support strong nonlinear effects, most of them due to the high birefringence and non-local response. Light self-confinement via reorientational nonlinearity and nonlocality, yields to the creation of robust light filaments named ‘optical spatial solitons’, which can trap, switch and route optical signals. In the last ten years, the attention to NLC systems, due to their large and polarization dependent nonlinearity, allowed the observation of self-focusing and spatial solitons with a significant attention devoted to reduce thermal contributions to the nonlinear phenomena and lower the required optical power. In all the observed cases, self-confinement was observed over short distances (hundreds of micrometers) and with non-negligible thermo-optic effects.

Continue reading… Routing Light with Spatial Solitons: Light Localization and Steering in Liquid Crystals – Antonio DeLuca