College of Arts and Sciences

First principles calculations can be used to study many material properties from a fundamental point of view. This talk will cover the calculations of natural vibration frequencies (local vibrational modes) of impurities and defects in crystals. These vibration frequencies can be probed experimentally by infrared spectroscopy techniques. After a brief review of the computation techniques and details, two different cases of local vibrational modes will be explained. The first case covers the vibration frequencies of hydrogen atoms that form strong bonds with N or O in GaN and ZnO. In these cases, the vibration mode is very distinct from the crystal phonon modes.

Continue reading… Probing crystal defects by their vibrational modes – Sukit Limpijumnong

A carbon nanotube is a one-dimensional system in which confinement of charge carriers and an unusual band structure lead to a variety of interesting effects. Many electronic and optical properties of a nanotube depend strongly on its geometry — the way in which a two-dimensional lattice of carbon atoms is rolled up to form the nanotube. In contrast, recent Rayleigh scattering measurements by the Park group at Cornell reveal that the peak optical conductivity is approximately equal for all single-walled carbon nanotubes, independent of their geometric structure. In this talk, I will describe our efforts to understand the origin of this uniform peak conductivity.

Continue reading… Uniform Peak Conductivity in Single-Walled Carbon Nanotubes – Jesse Kinder

We investigate the nematic phase of a 4-cyanoresorcinol bisbenozate compound by varying its chain length from C4 to C9 using dielectric and electro-optic spectroscopy. The frequencies and dielectric strengths of the various modes are determined. The results of the sign reversal in the dielectric anisotropy are interpreted in terms of the relative dielectric strengths of the various modes, their relaxation frequencies the order parameter of the system. The results are found to be much more interesting and complex than known so far for the calamitic Mesogens-.

Continue reading… Sign reversal in dielectric anisotropy and dielectric relaxation in bent core liquid crystals – Jagdish Vij

Three-dimensional (3D) topological insulators (TIs) are a new state of quantum matter with a bulk gap generated by the spin orbit interaction and odd number of relativistic Dirac fermions on the surface. The robust surface states of TIs can be the host for many striking quantum phenomena, such as an image magnetic monopole induced by an electric charge and Majorana fermions induced by the proximity effect from a superconductor. Recently, several classes of materials were theoretically predicted to be the simplest 3D TIs whose surface states consist of a single Dirac cone. By investigating the surface state of these materials with angle-resolved photoemission spectroscopy (ARPES),

Continue reading… Experimental observation and manipulation of topological surface states – Yulin Chen

A single electron spin in an external magnetic field forms a two-level system that can be used to create a spin qubit. However, achieving fast single spin rotations, as would be required to control a spin qubit, is a major challenge. It is difficult to drive spin rotations on timescales that are faster than the spin dephasing time and to individually address a single spin on the nanometer scale. I will describe a new method for quantum control of single spins that does not involve conventional electron spin resonance (ESR). In analogy with an optical beam splitter, we use an anti-crossing in the energy level spectrum of our quantum dot “artificial atom”

Continue reading… Strong-arming electron spin dynamics – Jason Petta

The scaling of electronic devices such as field effect transistors to nanometer dimensions requires more precise control of individual dopants in semiconductor nanostructures, as statistical fluctuations can impact device performance and functionality. Toward this end, the scanning tunneling microscope (STM) is emerging as a useful tool for its capabilities of atomic manipulation, imaging and tunneling spectroscopy. I will discuss our STM studies of Mn acceptors within the surface layer of a p-doped GaAs crystal [1]. We start by sublimating Mn adatoms onto the GaAs (110) surface, prepared by cleavage in ultrahigh vacuum. A voltage pulse applied with the STM tip allows us to replace a Ga atom in the surface with the Mn atom,

Continue reading… Scanning tunneling microscopy studies of single magnetic ions in GaAs – Jay Gupta

Bilayer graphene, or two layers of graphene stacked together in Bernal stacking, is a unique two-dimensional electron system with hyperbolic bands and a band gap tunable by the application of an electric field through the two layers. In this talk, I will describe our experiments in understanding the mechanisms of carrier transport in a gapped bilayer graphene. I will also show accurate measurements of the effective mass of bilayer graphene, from which we determine its band structure. Our results show that electron-electron interaction renormalizes the bands of bilayer and suppresses the effective mass m*. The manifestation is qualitatively different from that in conventional 2D systems.

Continue reading… Electron-electron interaction and transport in bilayer graphene – Jun Zhu

The atomic-scale order of highly deformable yet chemically inert carbon frameworks animates a wide range of novel structural, optical, and electronic phenomena. For example, the division of surrounding space into two disconnected zones by an impenetrable suspended graphenic sheet enables adsorption of otherwise highly co-reactive species, such as alkali and halogen, in opposite subspaces, with an intense cross-sheet charge transfer that produces a new variant of ionic binding with a uncompensated electrostatic dipoles. New physics also results when the same species is adsorbed to both sides, with unusual “which-side” symmetry breaking at certain chemical potentials. New ways to induce gaps in graphene sheets can also be designed exploiting new stacking physics.

Continue reading… Nano is more than size: The role of geometry in the electronic structure of carbon nanostructures – Vince Crespi

Earth’s need for clean energy becomes more evident with each demonstration of the shortcomings of fossil and nuclear energy sources. All carbon-free and nuclear-free energy sources will play important roles in our energy future, but only solar energy can in principle provide all of our energy needs. I will describe current market and technology landscapes for photovoltaics, introduce the use of quantum dots (QDs) as electronic materials, and provide an overview of the developing field of colloidal QD-based thin film solar cells. Although many R&D efforts pursue the fabrication of thin film photovoltaic devices from solution-based particle and nanoparticle (QD) starting materials,

Continue reading… Colloidal Quantum Dot Solar Cells – Randy Ellingson

While electronic transport has been the focus of intensive research for nearly a century, thermal transport has proven difficult to quantify and model. However, a predictive model for thermal conductivity can improve our understanding of thermoelectric materials, thermal resistance barriers, nanoscale heat transport, and even geologic heat transfer. In this talk, I will discuss the development of a new first principles framework to model thermal transport in materials and nanostructures. Using density functional perturbation theory, we are able to calculate both harmonic and anharmonic interatomic force constants. Coupling these terms with a Boltzmann transport approach, we are able to demonstrate excellent agreement between the calculated and measured lattice thermal conductivities of technologically relevant semiconductors (silicon,

Continue reading… Ab-initio Heat Transfer: Predicting thermal transport in nanostructures and materials from the atoms up – Derek Stewart

Solution-processable thin-film solar cells can be competitive with silicon-based ones in terms of electricity output/cost ratio and therefore have great potential in solar energy utilization. Due to the requirement for efficient light harvesting, however, so far the most successful low-cost thin-film solar cells require materials containing either rare or toxic metals. In my talk I will discuss our efforts in making graphene, which is primarily made of carbon, an active component for solar energy conversion. Our work is based on the synthesis of solution-processable colloidal graphene quantum dots with tunable size and bandgap. Our spectroscopic studies have shown that the graphene quantum dots have some unique electro-optical properties,

Continue reading… Toward Graphene-Based Photovoltaics – Liang-shi Li

InN is an infrared bandgap semiconductor (although it hasn’t always been that way); ZnO is an ultraviolet bandgap material used in applications from gas sensors to breakfast cereals. Surprisingly, these ostensibly disparate materials are more closely related than we might think: for both, p-type doping is problematic, the surface exhibits significant electron accumulation, and undoped samples are characterized by a large background electron concentration. In this talk I will describe some of our work to date geared towards developing a better understanding of these and closely related materials, which have significant device potential for a large variety of applications.

Continue reading… InN and ZnO: Unexpected Commonalities – Steven Durbin

Many devices designed to guide and control waves rely on periodic structures on the wavelength scale: these include diffraction gratings (used to squeeze multiple signals onto a single optical fiber, and in our highest-powered lasers), photonic crystals (the most promising route to energy-efficient ultra-fast optical computation on a chip), meta-materials (allowing the control of waves in ways impossible in naturally-occurring media), and solar cells. I will present a class of efficient and accurate numerical methods based on boundary integral equations for the solution of wave propagation problems in piece-wise homogeneous periodic media. The main ideas behind these algorithms are (1) a novel representation of quasi-periodic Green’s functions that converges fast and (2) acceleration techniques based on equivalent sources and FFTs.

Continue reading… Accurate and efficient solutions of wave propagation problems in periodic media – Catalin Turc

Magnetic semiconductors having Curie temperature greater than 300 K are of interest for a wide variety of spintronic device applications. Short-range order has been reported to stabilize ferromagnetism in transition metal-doped III-V compound semiconductors. While both theory and experiment have centered on dilute magnetic semiconductors where the magnetic ions substitute randomly for cation sites, there is increasing evidence that correlated substitution needs to be considered. Evidence of correlated substitution comes from structural analysis (extended x-ray absorption fine structure (EXAFS)), magnetic property and recent magneto-optical property measurements as well as theory. Furthermore, there is growing theoretical and experimental support that ferromagnetic semiconductors with Curie temperatures well above room temperature can be formed through correlated substitution.

Continue reading… Ferromagnetic semiconductors and the role of disorder – Bruce Wessels

Nanosized magnetic materials continue to attract great interest due to their wide range of potential applications from data storage to medical diagnostics and therapy. Each application demands unique magnetic characteristics of the nanoparticles. Hence, the ability to tailor their properties is of considerable importance. A first step towards understanding the correlation between magnetic properties and nanostructure is the development of robust synthetic approaches to prepare magnetic materials with controllable nanoarchitectures. In this presentation the synthesis of metal and metal oxide magnetic nanoparticles using wet chemical approaches is presented. By carefully tuning the synthesis conditions, different sizes, shapes and compositions can be prepared.

Continue reading… Chemical Design of Magnetic Nanomaterials – Ana Cristina Samia

The dynamics response of individual cerebral vessels to sensory-stimuli is crucial to form a mechanistic understanding of functional imaging technologies, such as functional MRI (fMRI), as well as for understanding neurovascular dysfunction, as occurs in stroke and dementia. Using optical imaging technologies such as two-photon laser scanning microscopy and the rat primary sensory cortex as our animal model, we have characterized the stimulus-evoked cerebral hemodynamic response on the level of single arterioles and capillaries throughout a significant three-dimensional volume (~1-2mm3) in vivo. Further, we will relate this characterization to the underlying neuronal electrical activity and the angioarchitecture. In this talk,

Continue reading… Imaging 3D spatiotemporal hemodynamics of single cortical vessels in vivo using two-photon laser scanning microscopy – Peifang Tian

Graphene has rapidly risen in the past few years to become one of the most actively researched topics in condensed matter physics and nanoscience due to its numerous remarkable properties and potential applications. Perhaps the best known method to make graphene has been the “scotch tape” technique (to exfoliate graphite), used just a few years ago by graphene pioneers Geim and Novoselov (who were awarded the physics Nobel in 2010) to unlock the novel physics of graphene. This simple method, however, produces only very small graphene flakes (typically tens of microns) and therefore is not sufficient for large scale production of graphene to fully realize its potentials.

Continue reading… Stories of Large Scale Graphene – Yong Chen

Liquid crystal elastomers, sometimes called “artificial muscles,” combine the elastic properties of rubber with the molecular order properties of liquid crystals. These fascinating materials stretch, shrink, bend or flap in response to changes in temperature, illumination, or applied fields, due to strong coupling between orientational order and elastic strain. Their mechanical response under applied strain is also peculiar, showing in some geometries a pronounced plateau in the stress-strain curve, accompanied by formation of a striped microstructure with director rotation in alternating directions. We construct a mesoscale model of this system based on a simple free energy functional and implement it using finite element elastodynamics.

Continue reading… Modeling defects, microstructure, and shape evolution in orientationally ordered soft materials: nematic elastomers and lipid vesicles – Robin Selinger

Conjugated polymers are considered by many as leading candidates to produce the next generation of electronics. This belief is based upon several factors: (a) they can be solution-processed using established printing technologies to give flexible, lightweight functional thin films, allowing for low-cost and large-scale production. (b) Several already have desirable properties for practical applications. In the first part of this talk, I will discuss strategies to optimize transistors and show results for well-defined poly(3-alkylthiophene)s and block copolymers of poly(3-hexylthiophene). In the second part of the talk, I will talk about my research here at CWRU on polymer-based photovoltaics.

Continue reading… Polymeric materials for printable electronic applications: from synthesis to device characterization – Genevieve Sauve

The absorption of free linear chains in a polymer brush was studied with respect to chain size and compatibility with the brush by means of Monte Carlo simulations and Density Functional Theory / Self-Consistent Field Theory at both moderate and high grafting densities using a bead-spring model. Different concentrations of the free chains were examined. When free chains are incompatible with the brush, all oligomeric species are almost completely ejected by the polymer brush irrespective of their length. For compatible case, we find that in going from shorter to longer chains, the absorbed amount undergoes a sharp crossover from weak to strong absorption.

Continue reading… Absorption/Expulsion of Oligomers and Linear Macromolecules in a Polymer Brush – Sergei Egorov

A spectacular phenomenon that can occur in strongly correlated low dimensional systems is that of fractionalization. In such electronic systems, quasiparticles excitations can carry a fraction of the electron’s charge and can have anyonic quantum statistics which is neither fermionic nor bosonic. Here, mesoscopic ring geometries are introduced as a means of bringing out novel signatures of both charge fractionalization and non-Abelian anyonic statistics. It is shown that power maps of quasiparticle motion around a thin ring can act as measures of fractionalization complementary to recent cutting-edge studies in etched quantum wires. A proposal is presented for probing the non-Abelian statistics of recent tour de force realizations of fractional vortices in superconducting mesoscopic rings.

Continue reading… Fractionalization in Mesoscopic Rings – Smitha Vishveshwara

Coulomb and electromagnetic interactions between excitons and plasmons in nanocrystals cause several interesting effects: energy transfer between nanoparticles (NPs), plasmon enhancement, reduced exciton diffusion in nanowires (NWs), exciton energy shifts, Fano interference effect, and non-linear phenomena [1-3]. Using transport equations for excitons, we model exciton transfer in NWs and explain the origin of the blue shift of exciton emission observed during recent experiments with hybrid NW-NP assemblies [2]. We also look at optical responses of artificial light-harvesting complexes composed of chlorophylls, bacterial reaction centers, and NPs [3]. We show that, using superior optical properties of metal and semiconductor NPs, it is possible to strongly enhance the efficiency of light harvesting in such complexes [3].

Continue reading… Exciton-Plasmon Interactions and Fano Resonances in Nanostructures – Alexander Govorov

Nanopillar radial junctions achieved by embedding nanopillars in absorbing thin films have potential for improved performance in solar cells due to increased junction area and improved charge carrier collection. This type of structure is still in its initial stage of development by using expensive and complicated microfabrication processes. An economic approach to the fabrication of nanopillar p-n junction solar cells has been investigated recently using a simple two-step electrodeposition method. The nanopillar heterojunctions are composed of n-type ZnO nanopillar arrays embedded in p-type Cu2O thin films, showing improved performance as compared with the planar thin film structures. While much effort is needed to optimize the device overall performance,

Continue reading… Embedded nanopillars for solar cell applications – Jingbiao Cui

Disorder or variation of the periodic structure of 1D-photonic crystal can lead to defects in the reflection band, characterized by one or more spectrally narrow transmission peaks inside that band. At or near such defects, changes in the effective group velocity of the light result in interesting optical phenomena such as Faraday rotation enhancement and gain enhancement in a distributed feedback (DFB) photonic crystal laser. In this talk, we present experimental results on and a theoretical interpretation of magneto-optic rotation and DFB lasing within the reflection band of CLiPS polymer multilayers.

Continue reading… Studies of reflection-band defects in 1D polymeric photonic crystals – Guilin Mao

Have you ever received a shock when you touched a doorknob after shuffling across a carpeted floor? The culprit, known as triboelectric charging, is also responsible for phenomena as innocuous as a rubbed balloon that makes your hair stand on end, or as dramatic as a lightning strike. While it is familiar to every child, fundamental understanding of triboelectric charging is so poor that even the most basic questions are still being debated, such as whether the transferred charge species are electrons or ions. Scientific progress is difficult because triboelectric charging is a non-equilibrium process (separated surfaces are neutral at equilibrium) that involves changes in electron states and occurs at a level of one electron per 100,000 surface atoms (physical and/or chemical defects at this low level likely control the behavior).

Continue reading… Triboelectric Charging in Granular Systems – Daniel Lacks

Existing highly precise density functional method for electronic structure calculations are mostly restricted to the treatment of at maximum a few hundred inequivalent atoms. This limitation leaves many open questions in material science e.g. on complex defects and defect-defect interaction unresolved. KKRnano is a new massively parallel DFT-algorithm in the framework of the KKR Green function method which we developed and optimized for large-scaled applications of thousands of atoms. In order to deal with the enormous computational requirements of such calculations we have implemented four levels of parallelization, which allows for an efficient use of hundreds of thousands of processors on the latest generation of supercomputers as Blue Gene.

Continue reading… Massively parallel Density functional calculations for thousands of atoms: KKRnano – Alexander Thiess