Session FB: Optoelectronics and Advanced Materials

Friday, October 22, 2010

Most density functional theory (DFT) calculations find band gaps that are 30-50 percent smaller than the experimental ones. Some explanations of this serious underestimation by theory include self-interaction and the derivative discontinuity of the exchange correlation energy. Several approaches have been developed in the search for a solution to this problem. Most of them entail some modification of DFT potentials. The Green function and screened Coulomb approximation (GWA) is a non-DFT formalism that has led to some improvements. Despite these efforts, the underestimation problem has mostly persisted in the literature. Using the Rayleigh theorem, we describe a basis set and variational effect inherently associated with calculations that employ a linear combination of atomic orbitals (LCAO) in a variational approach of the Rayleigh-Ritz type. This description concomitantly shows a source of large underestimation errors in calculated band gaps, i.e., an often dramatic lowering of some \textit{unoccupied energies} on account of the Rayleigh theorem as opposed to a physical interaction. We present the Bagayoko, Zhao, and Williams (BZW) method [Phys. Rev. B 60, 1563 (1999); PRB 74, 245214 (2006); and J. Appl. Phys. 103, 096101 (2008)] that systematically avoids this effect and leads (a) to DFT and LDA calculated band gaps of semiconductors in agreement with experiment and (b) theoretical predictions of band gaps that are confirmed by experiment. \textit{Unlike most calculations, BZW computations solve, self-consistently, a system of two coupled equations.} DFT-BZW calculated effective masses and optical properties (dielectric functions) also agree with measurements. We illustrate ten years of success of the BZW method with its results for GaN, C, Si, 3C-SIC, 4H-SiC, ZnO, AlAs, Ge, ZnSe, w-InN, c-InN, InAs, CdS, AlN and nanostructures. We conclude with potential applications of the BZW method in optoelectronic and advanced materials research.   

Friday, October 22, 2010

Surface plasmon nanophotonics is an emerging area which has manifested many potential applications for sensing, imaging, and communications. Surface plasmons are the free electron density oscillations on surfaces of metals. The free conduction electron density oscillations are always coupled with localized electromagnetic fields. An important property of surface plasmon-polaritons is the highly confined electromagnetic field near metal surfaces at the plasmon resonance. Although surface plasmons can confine electromagnetic energy in the nano-scale, a fundamental problem is the energy dissipation/loss in metal materials. In this talk, I will review recent progress in mitigating the loss of surface plasmon-polaritons and techniques for engineering surface plasmon-polaritons with hetero-dielectric nanolayers and nanostructures for various applications.   

Friday, October 22, 2010

Quantum electrodynamic coupling between excitons or phonons and plasmons has been of great interest for fundamental scientific research and photonic applications of lighting devices and bio-chemical sensing. Exciton-plasmon coupling of semiconductor quantum dots (SQDs) and metal nanoparticles (MNPs) provides high internal quantum efficiencies because of the localized surface plasmon resonance (LSPR) excitation and the faster coupling decay rates compare to the nonradiative decay rates. The enhancement and quenching of internal quantum efficiencies are determined by the coherent coupling condition and the balance between the faster resonant energy transfer from SQDs to MNPs than the nonradiative decay in SQDs and local field enhancement in the vicinity of MNPs. The resonant coupling of phonon-plasmon with analyte-linked MNPs also provides large enhancement of vibrational intensity in the analyte molecule because of strong LSPR and large polarizability of dimer-like MNP assemblies along the long-axis direction. Major physical origins of scattering enhancement could be the localized electromagnetic hot spots, the chemical energy transfer effects, and the spectral resonant excitation to the longitudinal plasmon modes. \textit{Acknowledgments:} This work at Hampton University was supported by the National Science Foundation (HRD-0734635 and HRD-0630372).   

Friday, October 22, 2010

Less weight, excellent mechanical properties, and high efficiency in absorbing electromagnetic (EM) wave make carbon nanotubes (CNTs) composites attractive for microwave technology applications. Multi-walled carbon nanotubes (MWNTs) have much higher performance-to-price ratio (PPR) than SWNTs do in the composite applications. In this work, we aim to study the effect of the outside diameter (OD) distributions of MWNTs on their microwave absorption properties. We have fabricated six groups of carbon nanotube/epoxy composite samples with various OD distributions. The weight percentages of MWNTs in the composites were controlled in the range from 1 to 10{\%}. We utilized a microwave resonant cavity technique to measure the microwave absorption properties of all the sixty samples around the central frequency of 9.968 GHz. Our results have shown that the maxima of EM wave absorptions for the six groups of samples were all around 7{\%} MWNTs weight percentage. We further studied the effective attenuations of the electric and magnetic fields in six groups of MWNT composite samples with the same (7 {\%}) MWNT blend in epoxy. The results show that, in general, the MWNTs with smaller diameters have higher microwave absorption at 9.968 GHz. However, sample group M5 (OD$<$8nm) shows unusual results, a lower microwave absorption than the other samples. We then used a scanning electron microscope (SEM) to study the morphologies of the MWNT samples. Based on the SEM analysis and microwave absorption measurements, we found that the efficiency of the microwave absorption of MWNT/Epoxy composites is strongly affected by the morphologies/structures of MWNTs in individual bundles.