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Bi-Weekly Seminar

Raman Spectroscopy of Ferroelectric Co3B7O13X (X=Cl,Br,I) Boracites (Phonons, X-Sublattice Instability, Raman Imaging of Twin Transformations)

Prof. Milko N. Iliev

by: Prof. Milko N. Iliev

Date: Friday January 23, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The boracites with general formula M3B7O13X (M=divalent metal, X=Cl,Br,I), shortly denoted as M-X, are the first known multiferroic materials. They exhibit a sequence of transitions from the high temperature paraelectric cubic phase to ferroelectric orthorhombic, monoclinic, trigonal phases, and finally to a monoclinic phase at low temperatures, where both ferroelectric and magnetic orders coexist. The lattice dynamics of boracites has been scarcely studied, the main problem with non-cubic phases being the coexistence of twin variants with different crystallographic and polarization orientation. We will present results of our detail temperature-dependent Raman study of Co-X and Ni-Br boracites. The spectra in the paraelectric cubic phase are analyzed in close comparison with results of ab initio (DFT) calculations of lattice dynamics. The analysis provides clear evidence for structural instability of the halogen sublattice, which triggers the ferroelectric cubic-to-orthorhombic transition. The spectra of the non-cubic ferroelectric phases of Co-Cl and Co-Br were obtained after Raman visualization of the twin variants. Using Raman microscopy imaging we were able to follow the twin-domain transformations through the crystallographic transitions, obtain Raman spectra from untwined domains in exact scattering configurations, determine the Raman mode symmetries, and assign Raman lines to definite atomic motions. The effect of elemental substitution at the X and M sites is also discussed.

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Bi-Weekly Seminar

Mechanisms and Detection of Biological Molecular Motors

Prof. John H. Miller

by: Prof. John H. Miller

Date: Friday October 31, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Rotary motors, including ATP synthase and the bacterial flagellar motor, play critical roles in living organisms. ATP synthase produces ATP, life's chemical currency of energy, in all three domains of life - bacteria, archaea, and eukarya. In humans, ATP synthase operates in the inner membranes of mitochondria. I will describe our recently developed electric field driven torque model of ion-driven rotary motors. The model predicts a scaling law that relates torque to the number of ion-carrying subunits in the rotor, the number of stators, and the ion motive force across the membrane. When the F0 complex of ATP synthase is coupled to F1, the model predicts a critical proton motive force below which ATP production drops to zero. In a human, such a drop in ATP would lead to unconsciousness and, eventually, death. We have also been measuring electromagnetic properties, such as impedance and harmonic responses, of live cells, mitochondria, and chloroplasts, in an effort to detect activity of active enzymes and changes in membrane potential. Dysfunction of mitochondrial enzymes has been implicated in type-2 diabetes, cancer, heart disease, Alzheimer's disease, and numerous specific mitochondrial disorders. Therefore, improved understanding of ATP synthase and other enzymes of mitochondrial respiratory chain is broadly significant to human health.

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Bi-Weekly Seminar

Electronic States and Dynamics at Semiconducting Polymer Heterojunction Interfaces

Prof. Eric  Bittner

by: Prof. Eric Bittner

Date: Friday September 05, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The optical-electronic properties of conjugated polymer-based electronic devices are acutely sensitive to the details of the intermolecular interactions and local environment. This is especially true at the interface between different semiconducting materials. In my talk I shall discuss our recent theoretical studies of OLEDS and solar cell materials based on polymer heterojunctions. Our theoretical approach combines modern quantum chemical methods based upon time-dependent density functional theory, projection operator techniques, and state of the art quantum dynamical methods for studying coupled electron/phonon systems. In my talk I shall discuss exciton breakup and recombination at interfaces as driven by phonons. I shall also talk about the possibility that interfacial triplet states may actually enhance the conversion efficiency of a heterojunction OLED device.

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Bi-Weekly Seminar

Quantum Measurement on Nano-Mechanical Resonators

Dr. Haibing H. Peng

by: Dr. Haibing H. Peng

Date: Friday July 11, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Harmonic oscillator has been well described in both classical mechanics and quantum mechanics. Recent advances in nano-fabrication technology make nano-mechanical resonator a model macroscopic system for investigating quantum behaviors in experiment, e.g., zero-point motion fluctuation. Studying the measurement (interaction) on quantum states of such macroscopic systems may lead to the achievement of ultimate sensitivity for many physical variables limited by quantum interactions. I will describe recent progress in pursuing the position detection limit governed by Heisenberg uncertainty principle and quantum back-action effects, in nano-mechanical resonators coupled to mesoscopic detectors such as single-electron transistors. I will also talk on the potentials of carbon-nanotube based devices in pushing the mechanics into quantum regime.

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Bi-Weekly Seminar

Bioconjugation onto Silicon Surfaces

 Chengzhi  Cai

by: Chengzhi Cai

Date: Friday June 27, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

We have recently prepared robust monolayers on silicon or silicon carbide surfaces by surface hydrosilylation. We have demonstrated that these monolayers are the most protein-resistant and stable monolayers reported to date. We have also developed a method for nanopatterning on the above monolayers with 10 nm resolution, and introduced handles on the monolayers for bioconjugation. Meanwhile, we are developing "click" reaction based methods for efficient bioconjugation onto surfaces and nanoparticles. We have used the method to functionalize silicon and silicon carbide surfaces with carbohydrates. The research is relevant to the development of efficient silicon-neuron interfaces.

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