Physics Commons^™

Fine Structure Of The Red Luminescence Band In Undoped Gan, Michael A. Reshchikov, A. Usikov, H. Helava, Yu. Makarov

Physics Publications

Many point defects in GaN responsible for broad photoluminescence (PL) bands remain unidentified. Their presence in thick GaN layers grown by hydride vapor phase epitaxy (HVPE) detrimentally affects the material quality and may hinder the use of GaN in high-power electronic devices. One of the main PL bands in HVPE-grown GaN is the red luminescence (RL) band with a maximum at 1.8 eV. We observed the fine structure of this band with a zero-phonon line (ZPL) at 2.36 eV, which may help to identify the related defect. The shift of the ZPL with excitation intensity and the temperature-related transformation of …

Green Luminescence In Mg-Doped Gan, Michael A. Reshchikov, Denis Demchenko, J. D. Mcnamara, S. Fernández-Garrido, R. Calarco

Physics Publications

A majority of the point defects in GaN that are responsible for broad photoluminescence (PL) bands remain unidentified. One of them is the green luminescence band (GL2) having a maximum at 2.35 eV which was observed previously in undoped GaN grown by molecular-beam epitaxy in Ga-rich conditions. The same PL band was observed in Mg-doped GaN, also grown in very Ga-rich conditions. The unique properties of the GL2 band allowed us to reliably identify it in different samples. The best candidate for the defect which causes the GL2 band is a nitrogen vacancy (VN). We propose that transitions of electrons …

Electronic Structure And Magnetic Properties Of Mn And Fe Impurities Near The Gaas (110) Surface, M. R. Mahani, M. Fhokrul Islam, A. Pertsova, C. M. Canali

Physics Publications

Combining density functional theory calculations and microscopic tight-binding models, we investigate theoretically the electronic and magnetic properties of individual substitutional transition-metal impurities (Mn and Fe) positioned in the vicinity of the (110) surface of GaAs. For the case of the [Mn2+]0 plus acceptor-hole (h) complex, the results of a tight-binding model including explicitly the impurity delectrons are in good agreement with approaches that treat the spin of the impurity as an effective classical vector. For the case of Fe, where both the neutral isoelectronic [Fe3+]0 and the ionized [Fe2+]− states are relevant to address scanning tunneling microscopy (STM) …

18-Electron Rule Inspired Zintl-Like Ions Composed Of All Transition Metals, Jian Zhou, Santanab Giri, Purusottam Jena

Physics Publications

Zintl phase compounds constitute a unique class of compounds composed of metal cations and covalently bonded multiply charged cluster anions. Potential applications of these materials in solution chemistry and thermoelectric materials have given rise to renewed interest in the search for new Zintl ions. Up to now these ions have been mostly composed of group 13, 14, and 15 post-transition metal elements and no Zintl ions composed of all transition metal elements are known. Using gradient corrected density functional theory we show that the 18-electron rule can be applied to design a new class of Zintl-like ions composed of all …

First-Principles Studies On Graphene-Supported Transition Metal Clusters, Sanjubala Sahoo, Markus E. Gruner, Shiv N. Khanna, Peter Entel

Physics Publications

Theoretical studies on the structure, stability, and magnetic properties of icosahedral TM13 (TM = Fe, Co, Ni) clusters, deposited on pristine (defect free) and defective graphene sheet as well as graphene flakes, have been carried out within a gradient corrected density functional framework. The defects considered in our study include a carbon vacancy for the graphene sheet and a five-membered and a seven-membered ring structures for graphene flakes (finite graphene chunks). It is observed that the presence of defect in the substrate has a profound influence on the electronic structure and magnetic properties of graphene-transition metal complexes, thereby increasing the …

The Viability Of Aluminum Zintl Anion Moieties Within Magnesium-Aluminum Clusters, Haopeng Wang, Yeon Jae Ko, Xinxing Zhang, Gerd Gantefoer, Hansgeorg Schnoeckel, Bryan W. Eichhorn, Puru Jena, Boggavarapu Kiran, Anil K. Kandalam, Kit H. Bowen Jr.

Physics Publications

Through a synergetic combination of anion photoelectron spectroscopy and density functional theory based calculations, we have investigated the extent to which the aluminum moieties within selected magnesium-aluminum clusters are Zintl anions. Magnesium-aluminum cluster anions were generated in a pulsed arc discharge source. After mass selection, photoelectron spectra of Mg m Al n ⁻ (m, n = 1,6; 2,5; 2,12; and 3,11) were measured by a magnetic bottle, electron energy analyzer. Calculations on these four stoichiometries provided geometric structures and full charge analyses for the cluster anions and their neutral cluster counterparts, as well as photodetachment transition energies (stick spectra). Calculations …

Aluminum Zintl Anion Moieties Within Sodium Aluminum Clusters, Haopeng Wang, Xinxing Zhang, Yeon Jae Ko, Andrej Grubisic, Xiang Li, Gerd Gantefoer, Hansgeorg Schnoeckel, Bryan W. Eichhorn, Mal-Soon Lee, Puru Jena, Anil K. Kandalam, Boggavarapu Kiran, Kit H. Bowen Jr.

Physics Publications

Through a synergetic combination of anion photoelectron spectroscopy and density functional theory based calculations, we have established that aluminum moieties within selected sodium-aluminum clusters are Zintl anions. Sodium–aluminum cluster anions, Na m Al n ⁻, were generated in a pulsed arc discharge source. After mass selection, their photoelectron spectrawere measured by a magnetic bottle, electron energy analyzer. Calculations on a select sub-set of stoichiometries provided geometric structures and full charge analyses for both cluster anions and their neutral cluster counterparts, as well as photodetachment transition energies (stickspectra), and fragment molecular orbital based correlation diagrams.

Temperature Dependence Of Defect-Related Photoluminescence In Iii-V And Ii-Vi Semiconductors, Michael A. Reshchikov

Physics Publications

Mechanisms of thermal quenching of photoluminescence (PL) related to defects in semiconductors are analyzed. We conclude that the Schön-Klasens (multi-center) mechanism of the thermal quenching of PL is much more common for defects in III–V and II–VI semiconductors as compared to the Seitz-Mott (one-center) mechanism. The temperature dependencies of PL are simulated with a phenomenological model. In its simplest version, three types of defects are included: a shallow donor, an acceptor responsible for the PL, and a nonradiative center that has the highest recombination efficiency. The case of abrupt and tunable thermal quenching of PL is considered in more detail. …

Vector Acoustic Intensity Around A Tuning Fork, Daniel A. Russell, Justin Junell, Daniel O. Ludwigsen

Physics Publications

The acoustic intensity vector field around a tuning fork is investigated. Theory for a longitudinal quadrupole source predicts a well-defined transition between near-field and far-field, with significant circulation of sound energy in the near-field. Vector components of the time-averaged intensity were measured using a two-microphone intensity probe and found to agree well with predictions from theory. The vector intensity map is interpreted, and shown to provide useful information about the near-field of an acoustic source.

Hydroxyl-Decorated Graphene Systems As Candidates For Organic Metal-Free Ferroelectrics, Multiferroics, And High-Performance Proton Battery Cathode Materials, Menghao Wu, J. D. Burton, Evgeny Y. Tsymbal, Xiao Cheng Zeng, Puru Jena

Physics Publications

Using a first-principles method we show that graphene based materials, functionalized with hydroxyl groups, constitute a class of multifunctional, lightweight, and nontoxic organic materials with functional properties such as ferroelectricity, multiferroicity, and can be used as proton battery cathode materials. For example, the polarizations of semihydroxylized graphane and graphone, as well as fully hydroxylized graphane, are much higher than any organic ferroelectric materials known to date. Further, hydroxylized graphene nanoribbons with proton vacancies at the end can have much larger dipole moments. They may also be applied as high-capacity cathode materials with a specific capacity that is six times larger …

Communication: In Search Of Four-Atom Chiral Metal Clusters, Xinxing Zhang, Bradley Visser, Martin Tschurl, Evan Collins, Yi Wang, Qian Wang, Yawei Li, Qiang Sun, Puru Jena, Gerd Gantefoer, Ulrich Boesl, Ulrich Heiz, Kit H. Bowen Jr.

Physics Publications

A combined study utilizing anion photoelectron spectroscopy and density functional theory was conducted to search for four-atom, chiral, metal, and mostly metal clusters. The clusters considered were AuCoMnBi−/0, AlAuMnO−/0, AgMnOAl−/0, and AuAlPtAg−/0, where the superscripts, −/0, refer to anionic and neutral cluster species, respectively. Based on the agreement of experimentally and theoretically determined values of both electron affinities and vertical detachment energies, the calculated cluster geometries were validated and examined for chirality. Among both anionic and neutral clusters, five structures were identified as beingchiral.

Magnetic Properties Of Co2-Xtmxc And Co3-Xtmxc Nanoparticles, Meichun Qian, Shiv N. Khanna

Physics Publications

Using synthetic chemical approaches, it is now possible to synthesize transition metal carbides nanoparticles with morphology, where the transition metal layers are embedded with intervening layers of carbon atoms. A composite material consisting of Co2C and Co3C nanoparticles has been found to exhibit unusually large coercivity and energy product. Here, we demonstrate that the magnetic moments and the anisotropy can be further enhanced by using a combination of Co and other transition metals (TM). Our studies are based on mixednanoparticles Co2−x TM xC and Co3−x TM xC, in which selected Co sites are replaced with 3d transition elements Cr, Mn, …

Structure And Properties Of Mn4cl9: An Antiferromagnetic Binary Hyperhalogen, Yawei Li, Shunhong Zhang, Qian Wang, Puru Jena

Physics Publications

Calculations based on density functional theory show that the structure of Mn₄Cl₉ anion is that of a Mn atom at the core surrounded by three MnCl₃ moieties. Since Mn is predominantly divalent and MnCl₃ is known to be a superhalogen with a vertical detachment energy (VDE) of 5.27 eV, Mn₄Cl₉ can be viewed as a hyperhalogen with the formula unit Mn(MnCl₃)₃. Indeed, the calculated VDE of Mn₄Cl₉ anion, namely 6.76 eV, is larger than that of MnCl₃ anion. More importantly, unlike previously discovered hyperhalogens, …

Superlinear Increase Of Photoluminescence With Excitation Intensity In Zn-Doped Gan, Michael A. Reshchikov, Anita J. Olsen, Marilyn F. Bishop, Tom Mcmullen

Physics Publications

We have observed a superlinear increase of photoluminescence (PL) intensity in a narrow range of excitation intensities for Zn-doped GaN. The characteristic intensity at which the abrupt increase occurs increases with increasing temperature. This is unlike the usual observations for defects in semiconductors in which the PL intensity increases linearly with excitation intensity, saturating at high intensity because defects become saturated with photogenerated charge carriers. The observed phenomenon is attributed to a redirection of electron and hole flow from nonradiative centers at low excitation intensity to a recombination path via the Zn-Ga acceptor at high excitation intensity. This is the …

Unusually Large Spin Polarization And Magnetoresistance In A Femg8-Femg8 Superatomic Dimer, Lin Zhu, Meichun Qian, Shiv N. Khanna

Physics Publications

Electronic transport across a FeMg8 magnetic superatom and its dimer has been investigated using a density functional theory combined with Keldysh nonequilibrium Green's-function formalism. For a single cluster, our studies for the cluster supported in various orientations on a Au(100) surface show that the transport is sensitive to the contact geometry. Investigations covering the cases where the axes of Mg square antiprism are 45°, perpendicular, and parallel to the transport direction, show that the equilibrium conductance, transferred charge, and currentpolarizations can all change significantly with orientation. Our studies on the transport across a magnetic superatom dimer FeMg8–FeMg8 focus on the …

Enhanced Biocidal Activity Of Au Nanoparticles Synthesized In One Pot Using 2, 4-Dihydroxybenzene Carbodithioic Acid As A Reducing And Stabilizing Agent, Syeda Sohaila Naz, Nazar Ul Islam, Muhammad Raza Shah, Syed Sartaj Alam, Zafar Iqbal, Massimo Bertino, Louis Franzel, Afifa Ahmed

Physics Publications

Background

The conjugation of gold nanoparticles with biocides such as natural products, oligosaccharides, DNA, proteins has attracted great attention of scientists recently. Gold NPs covered with biologically important molecules showed significant enhancement in biological activity in comparison with the activity of the free biocides. However, these reports are not very systematic and do not allow to draw definitive conclusions. We therefore embarked in a systematic study related to the synthesis and characterization of biocidal activities of Au nanoparticles conjugated to a wide variety of synthetic and natural biomolecules. In this specific report, we investigated the activity of a synthetic biocide, …

Quantification Of Biomass And Cell Motion In Human Pluripotent Stem Cell Colonies, Thomas A. Zangle, Jennifer Chun, Jin Zhang, Jason C. Reed, Michael A. Teitell

Physics Publications

Abstract

Somatic cell reprogramming to pluripotency requires an immediate increase in cell proliferation and reduction in cell size. It is unknown whether proliferation and biomass controls are similarly coordinated with early events during the differentiation of pluripotent stem cells (PSCs). This impasse exists because PSCs grow in tight clusters or colonies, precluding most quantifying approaches. Here, we investigate live cell interferometry as an approach to quantify the biomass and growth of HSF1 human PSC colonies before and during retinoic acid-induced differentiation. We also provide an approach for measuring the rate and coordination of intracolony mass redistribution in HSF1 clusters using …

Blue Luminescence And Zn Acceptor In Gan, Denis Demchenko, Michael A. Reshchikov

Physics Publications

In this paper, we present a comparison of exchange-tuned hybrid density functional calculations with experimental data obtained for the Zn acceptor in GaN. Since this acceptor is one of the few reliably identified defects in GaN, we use Zn-doped GaN as a test case for the widely used HSE06 hybrid functional method of calculations of defect properties in semiconductors. Here, we present the experimental results of luminescence measurements in Zn-doped GaN from which we obtain Zn acceptor defect levels. They are compared with theoretically calculated defect thermodynamic and optical transition levels as well as the zero-phonon line associated with this …

Tunable Thermal Quenching Of Photoluminescence In Mg-Doped P-Type Gan, Michael A. Reshchikov, J. D. Mcnamara, S. Fernández-Garrido, R. Calarco

Physics Publications

We have studied the thermal quenching of the ultraviolet luminescence band with a maximum at about 3.25 eV in p-type Mg-doped GaN. The characteristic temperature of the thermal quenching of photoluminescence (PL) gradually shifted to higher temperatures with increasing excitation intensity. This effect is explained by a population inversion of charge carriers at low temperatures, which suddenly converts into a quasiequilibrium population as the temperature increases above the characteristic value. Tunable quenching of PL has been observed only in some of the GaN:Mg samples. The absence of the tunable quenching of PL in another group of GaN:Mg samples is preliminarily …

The Effect Of Sulfur Covalent Bonding On The Electronic Shells Of Silver Clusters, Anthony F. Pedicini, Arthur C. Reber, Shiv N. Khanna

Physics Publications

The nature of the bonding in Ag n S m ^0/− clusters, n = 1–7; m = 1–4, has been analyzed to understand its effect on the electronic shell structure of silver clusters. First-principle investigations reveal that the sulfur atoms prefer 2 or 3-coordinate sites around a silver core, and that the addition of sulfur makes the planar structures compact. Molecular orbital analysisfinds that the 3p orbitals of sulfur form a bonding orbital and two weakly bonding lone pairs withsilver. We examine the electronic shell structures of Ag 6Sm, which are two electrons deficient of a spherical closed electronic shell …

An All-Electron Density Functional Theory Study Of The Structure And Properties Of The Neutral And Singly Charged M-12 And M-13 Clusters: M = Sc-Zn, G. L. Gutsev, C. W. Weatherford, K. G. Belay, B. R. Ramachandran, P. Jena

Physics Publications

The electronic and geometrical structures of the M ₁₂ and M ₁₃ clusters where M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn along with their singly negatively and positively charged ions are studied using all-electron density functional theory within the generalized gradient approximation. The geometries corresponding to the lowest total energy states of singly and negatively charged ions of V₁₃, Mn₁₂, Co₁₂, Ni ₁₃, Cu ₁₃, Zn ₁₂, and Zn ₁₃ are found to be different from the geometries of the corresponding neutral parents. The computed …

Magnetic Hollow Cages With Colossal Moments, Menghao Wu, Puru Jena

Physics Publications

A comprehensive study of the interaction of transition metal clusters with B, C, N, O, and Si reveal novel structure and properties: Co12C6, Mn12C6, and Mn24C18 clusters form stable ferromagnetic hollow cages with total magnetic moments of 14 μB, 38 μB, and 70 μB, respectively. Replacement of C with B, N, O, or Si has significant impact on their structure and magnetic properties. For example, Mn20Si12 cluster forms a ferrimagnetic dodecahedral hollow cage with a total magnetic moment of 36 μB while Mn12N6, X12C6 (X = Ni, Cu, Pd, Pt), and Cu12O6 possess no magnetic moment, although they retain hollow …

High-Frequency Rapid B-Mode Ultrasound Imaging For Real-Time Monitoring Of Lesion Formation And Gas Body Activity During High-Intensity Focused Ultrasound Ablation, Madhu Sudhan Reddy Gudur, Ronald Kumon, Yun Zhou, Cheri X. Deng

Physics Publications

Abstract:

The goal of this study was to examine the ability of high-frame-rate, high-resolution imaging to monitor tissue necrosis and gas-body activities formed during high-intensity focused ultrasound (HIFU) application. Ex vivo porcine cardiac tissue specimens (n = 24) were treated with HIFU exposure (4.33 MHz, 77 to 130 Hz pulse repetition frequency (PRF), 25 to 50% duty cycle, 0.2 to 1 s, 2600 W/cm2). RF data from Bmode ultrasound imaging were obtained before, during, and after HIFU exposure at a frame rate ranging from 77 to 130 Hz using an ultrasound imaging system with a center frequency of 55 MHz. …

High-Frequency Ultrasound M-Mode Imaging For Identifying Lesion And Bubble Activity During High-Intensity Focused Ultrasound Ablation, Ronald Kumon, Madhu Sudhan Reddy Gudur, Yun Zhou

Physics Publications

Effective real-time monitoring of high-intensity focused ultrasound (HIFU) ablation is important for application of HIFU technology in interventional electrophysiology. This study investigated rapid, high-frequency M-mode ultrasound imaging for monitoring spatiotemporal changes during HIFU application. HIFU (4.33 MHz, 1 kHz PRF, 50% duty cycle, 1 s, 2600 – 6100 W/cm2 ) was applied to ex-vivo porcine cardiac tissue specimens with a confocally and perpendicularly aligned high-frequency imaging system (Visualsonics Vevo 770, 55 MHz center frequency). Radiofrequency (RF) data from M-mode imaging (1 kHz PRF, 2 s × 7 mm) was acquired before, during, and after HIFU treatment (n = 12). Among …

Acoustic Testing And Modeling: An Advanced Undergraduate Laboratory, Daniel A. Russell, Daniel O. Ludwigsen

Physics Publications

This paper describes an advanced laboratory course in acoustics, specifically targeted for students with an interest in engineering applications at a school with a strongly integrated industrial co-op program. The laboratory course is developed around a three-pronged approach to problem solving that combines and integrates theoretical models, computational models, and experimental data. The course is structured around modules that begin with fundamental concepts and build laboratory skills and expand the knowledge base toward a final project. Students keep a detailed laboratory notebook, write research papers in teams, and must pass laboratory certification exams. This paper describes the course layout and …

Two-Step Thermal Quenching Of Photoluminescence In Zn-Doped Gan, Michael A. Reshchikov

Physics Publications

We observed tunable two-step thermal quenching of photoluminescence in high-resistivity Zn-doped GaN. The characteristic temperatures of the first and second steps increase with increasing excitation intensity. The effect is explained within a phenomenological model involving shallow donors, nonradiative deep donors, and two types of acceptors.

Tuning Magnetic Properties Of Graphene Nanoribbons With Topological Line Defects: From Antiferromagnetic To Ferromagnetic, Min Kan, Jian Zhou, Qiang Sun, Qiang Wang, Yoshiyuki Kawazoe, Puru Jena

Physics Publications

Zigzag-edged graphene nanoribbons are antiferromagnetic in cross-edge coupling and unsuitable for spintronics applications. Two new strategies of tuning antiferromagnetism (AFM) to ferromagnetism (FM) in graphene nanoribbons are introduced through topological line defects composed of pentagonal and octagonal rings, and their ability to induce magnetic transition is probed by using density functional theory. The resulting exchange energy is found to be large enough for ferromagnetism to be observed at room temperature. Both strategies are experimentally feasible, and the results suggest that defect engineering may provide a novel path to manipulate the magnetic properties of graphene nanoribbons.

Determination Of The Absolute Internal Quantum Efficiency Of Photoluminescence In Gan Co-Doped With Si And Zn, M. A. Reshchikov, M. Foussekis, J. D. Mcnamara, A. Behrends, A. Bakin, A. Waag

Physics Publications

The optical properties of high-quality GaN co-doped with silicon and zinc are investigated by using temperature-dependent continuous-wave and time-resolved photoluminescence measurements. The blue luminescence band is related to the Zn_Ga acceptor in GaN:Si,Zn, which exhibits an exceptionally high absolute internal quantum efficiency (IQE). An IQE above 90% was calculated for several samples having different concentrations of Zn. Accurate and reliable values of the IQE were obtained by using several approaches based on rate equations. The concentrations of the Zn_Ga acceptors and free electrons were also estimated from the photoluminescence measurements.

Electrical Transition Of (3,3) Carbon Nanotube On Patterned Hydrogen Terminated Si(001)-2 X 1 Driven By Electric Field, Bikash C. Gupta, Shyamal Konar, Puru Jena

Physics Publications

Structure, energetics, and electrical properties of (3,3) carbon nanotube(CNT) supported on patterned hydrogen terminated Si(001): 2 × 1 surface are studied using density functional theory. Our investigation reveals that an otherwise metallic (3,3) CNT when supported becomes semiconducting with a band gap of ≈0.5 eV due to its strong interaction with the surface. During adsorption, Si-C bonds form at the interface and charges transfer from Si surface to CNT. The Si-C bonds at the interface are partially covalent and partially ionic in nature. Under the application of an external electric field, the bandgap of the supported CNT reduces to zero, …

Ferromagnetism In Li Doped Zno Nanoparticles: The Role Of Interstitial Li, Saif Ullah Awan, S. K. Hasanain, Massimo F. Bertino, G. Hassnain Jaffari

Physics Publications

ZnO nanoparticles doped with Li (Zn_1−yLi_yO, y ≤ 0.1) have been investigated with emphasis on the correlation between their magnetic, electronic, and structural properties. In particular, defects such as interstitial Li and Zn atoms, substitutional Li atoms, and oxygen vacancies have been identified by X-ray photoelectron spectroscopy(XPS) and their respective roles in stabilization of the magnetic moment are discussed. X-ray diffraction(XRD) and XPS give clear evidence of Li presence at both substitutional and interstitial sites. XPS studies further show that the amount of substitutional Li defects (Li_zn) and interstitial Li defects (Li_i) …