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Articles 1 - 19 of 19
Full-Text Articles in Physics
Development And Investigation Of Intensity-Modulated Radiation Therapy Treatment Planning For Four-Dimensional Anatomy, Yelin Suh
Theses and Dissertations
Lung cancer is the leading cause of cancer-related deaths worldwide. Radiotherapy is one of the main treatment modalities of lung cancer. However, the achievable accuracy of radiotherapy treatment is limited for lung-based tumors due to respiratory motion. Four-dimensional radiotherapy explicitly accounts for anatomic motion by characterizing the motion, creating a treatment plan that accounts for this motion, and delivering this plan to the moving anatomy. This thesis focuses on the current problems and solutions throughout the course of four-dimensional radiotherapy. For characterization of respiratory-induced motion, patient tumor motion data were analyzed. It is shown that tumor motion can be significant …
Density Functional Investigations Of Pure And Ligated Clusters, Kristen Casalenuovo
Density Functional Investigations Of Pure And Ligated Clusters, Kristen Casalenuovo
Theses and Dissertations
Atomic clusters are attractive candidates for building motifs for new nano-assembled materials with desirable properties. At this nano-regime of matter, the size, shape, and composition of clusters changes their electronic structure and hence their properties. Computational modeling must work hand in hand with experiment to provide robust descriptions of the geometries and energetics of atomic clusters and how they might behave in a nano-assembled material. To this end, we have investigated three distinct species as model systems: antimony oxides SbxOy (x = 1, 2; y = 0 - 3), metal ion-solvent complexes Mm(NH3)n (M = Bi, Pb; m = 1 …
Band Bending In Gan, Michael Foussekis
Band Bending In Gan, Michael Foussekis
Theses and Dissertations
Steady-state and transient surface photovoltages in undoped GaN are studied in various environments (air, nitrogen, oxygen, vacuum) at room temperature and 400 K with a Kelvin probe attached to an optical cryostat. The results are explained within a phenomenological model accounting for the accumulation of photo-generated holes at the surface, capture of free electrons from the bulk over the near-surface potential barrier, and emission of electrons from surface states into the bulk. Mechanisms of surface photovoltage are discussed in detail. Photoadsorption and photodesorption of negatively charged species will either increase or decrease the surface potential and thus band bending. Oxygen …
Origin Of The Anatase To Rutile Conversion Of Metal-Doped Tio2, Sa Li, Puru Jena
Origin Of The Anatase To Rutile Conversion Of Metal-Doped Tio2, Sa Li, Puru Jena
Physics Publications
Extensive calculations using density functional theory enable us to explain the origin of the surprising room-temperature conversion of anatase to rutile phase of TiO2 when doped with Co and Ni, but not with Cu. Contrary to earlier suggestion, neither high spin nor strain of the transition metals is found to be responsible for this phase conversion. The driving mechanism, instead, is attributed to the increased interaction between Co and Ni atoms forming a linear chain in the rutile phase. We predict that Cr and Mn which have even larger spins than Co and Ni cannot induce this phase conversion.
Magnetic Properties Of Transition-Metal-Doped Zn1−Xtxo (T=Cr, Mn, Fe, Co, And Ni) Thin Films With And Without Intrinsic Defects: A Density Functional Study, Qian Wang, Qiang Sun, Puru Jena, Y. Kawazoe
Magnetic Properties Of Transition-Metal-Doped Zn1−Xtxo (T=Cr, Mn, Fe, Co, And Ni) Thin Films With And Without Intrinsic Defects: A Density Functional Study, Qian Wang, Qiang Sun, Puru Jena, Y. Kawazoe
Physics Publications
Theoretical calculations based on density-functional theory and generalized gradient approximation have been carried out in studying the electronic structure and magnetic properties of transition-metal-doped Zn1−xTxO (T=Cr, Mn, Fe, Co, and Ni) (112¯0) thin films systematically with and without intrinsic point defects (e.g., vacancies and interstitials), and as function of concentration and distribution of dopants and vacancies. Using large supercells and geometry optimization without symmetry constraint, we are able to determine the sites that metal atoms prefer to occupy, their tendency to cluster, the preferred magnetic coupling between magnetic moments at transition-metal sites, and the effect of intrinsic point defects on …
Electronic Behavior Of The Zn- And O-Polar Zno Surfaces Studied Using Conductive Atomic Force Microscopy, J. C. Moore, S. M. Kenny, C. S. Baird, H. Morkoç, A. A. Baski
Electronic Behavior Of The Zn- And O-Polar Zno Surfaces Studied Using Conductive Atomic Force Microscopy, J. C. Moore, S. M. Kenny, C. S. Baird, H. Morkoç, A. A. Baski
Physics Publications
We have used conducting atomic force microscopy (CAFM) to study the morphology and electronic behavior of as-received and air-annealed (0001) Zn- and (0001¯) O-polar surfaces of bulk ZnO. Both polar surfaces exhibit relatively flat morphologies prior to annealing, which rearrange to form well-defined steps upon annealing in air at 1050 °C for 1 h. Long-term exposure to air results in surface layer pitting and the destruction of steps for both the as-received and air-annealed (0001¯)surfaces, indicating its enhanced reactivity relative to the (0001) surface. CAFM I-V spectra for polar surfaces are similar and indicate Ohmic to rectifying behavior that depends …
Magnetic Response Of Core-Shell Cobalt Ferrite Nanoparticles At Low Temperature, K. Maaz, M. Usman, S. Karim, A. Mumtaz, S. K. Hasanain, M. F. Bertino
Magnetic Response Of Core-Shell Cobalt Ferrite Nanoparticles At Low Temperature, K. Maaz, M. Usman, S. Karim, A. Mumtaz, S. K. Hasanain, M. F. Bertino
Physics Publications
Cobaltferritenanoparticles (size: 26±4nm) have been synthesized by coprecipitation route. The coercivity of nanoparticles follows a simple model of thermal activation of particle moments over the anisotropy barrier in the temperature range of 30–300K in accordance with Kneller’s law; however, at low temperatures (
Photoelectron Imaging And Theoretical Investigation Of Bimetallic Bi1–2ga−0–2 And Pb−1–4 Cluster Anions, M. A. Sobhy, J. Ulises Reveles, Ujjwal Gupta, Shiv N. Khanna, A. W. Castleman Jr.
Photoelectron Imaging And Theoretical Investigation Of Bimetallic Bi1–2ga−0–2 And Pb−1–4 Cluster Anions, M. A. Sobhy, J. Ulises Reveles, Ujjwal Gupta, Shiv N. Khanna, A. W. Castleman Jr.
Physics Publications
We present the results of photoelectron velocity-map imaging experiments for the photodetachment of small negatively charged BimGan (m=1–2, n=0–2), and Pbn (n=1–4) clusters at 527 nm. The photoelectron images reveal new features along with their angular distributions in the photoelectron spectra of these clusters. We report the vertical detachment energies of the observed multiple electronic bands and their respective anisotropy parameters for the BimGan and Pbn clusters derived from the photoelectron images. Experiments on the BiGan clusters reveal that the electron affinity increases with the number of Ga atoms from n=0 to 2. The BiGa−2 cluster is found to be …
Doping Induced Anisotropic Growth In C60, Miao Miao Wu, Qiang Sun, Puru Jena, Yoshiyuki Kawazoe
Doping Induced Anisotropic Growth In C60, Miao Miao Wu, Qiang Sun, Puru Jena, Yoshiyuki Kawazoe
Physics Publications
Using density functional theory with generalized gradient approximation for exchange and correlation energy, we show that substitution of a Si atom at one of the C sites in C60 not only allows C59Si to have a hydrophobic head with a hydrophilic tail but also the Si atom acts as a seed for anisotropic growth of the heterofullerene. This is demonstrated by interacting C59Si with N7Sc and B8Si. The resulting complex structures exhibit enhanced electric dipole moments and anisotropy. Thus, doping induced anisotropic growth of nanostructures provides a novel route for the synthesis of bifunctional particles with atomic-level control on selectivity …
Stabilizing A 22 Karat Nanogolden Cage, Q. Wang, Q. Sun, P. Jena
Stabilizing A 22 Karat Nanogolden Cage, Q. Wang, Q. Sun, P. Jena
Physics Publications
Since the discovery of C60 fullerene, considerable efforts have been devoted to find other elements with similar hollow cage structures. However, search for hollow metallic cages with a diameter similar to that of C60 fullerene has been elusive. We describe a procedure for the rational design of metallic cages by suitably choosing their size, composition, and charge state. A 22 karat nanogolden cage with a diameter of about 8.5 Å and consisting of 12 Al and 20 Au atoms is found to be metastable, which can be stabilized by embedding a Mn4 cluster. In contrast to bulk Mn, which is …
Photoadsorption And Photodesorption For Gan, M. Foussekis, A. A. Baski, Michael A. Reshchikov
Photoadsorption And Photodesorption For Gan, M. Foussekis, A. A. Baski, Michael A. Reshchikov
Physics Publications
The effect of an ambient environment on the surface photovoltage and photoluminescence observed for GaN is studied. In air ambient the upward band bending gradually increases under UVillumination and is explained by the photoinduced chemisorption of surface adsorbates. Specifically, the increase in negative surface charge is consistent with the transfer of electrons from surface states or bulk to oxygen species physisorbed at the GaNsurface. In contrast, the upward band bending gradually decreases in vacuum under UVillumination and can be explained by the photoinduced desorption of these species. The photoadsorption and photodesorption of negatively charged species cause the surface depletion region …
Functionalized Heterofullerenes For Hydrogen Storage, Qiang Sun, Qian Wang, Puru Jena
Functionalized Heterofullerenes For Hydrogen Storage, Qiang Sun, Qian Wang, Puru Jena
Physics Publications
Using density functional theory, we show that Li decorated B doped heterofullerene (Li12C48B12) has the following desired properties of a hydrogen storagematerial. (1) The Li atoms remain isolated. (2) Through charge transfer to electron deficient C48B12 heterofullerene, the Li atoms become positively charged. (3) Each Li atom is able to bind up to three H2molecules, which remain in molecular form, and the binding energies of successive H2molecules are in the range of 0.135–0.172 eV/H2, suitable for ambient temperature storage. (4) The gravimetric density reaches the 9 wt % limit necessary for applications in the mobile industry.
Mg-Doped Gan Nanostructures: Energetics, Magnetism, And H2 Adsorption, Qian Wang, Qiang Sun, Puru Jena, Yoshiyuki Kawazoe
Mg-Doped Gan Nanostructures: Energetics, Magnetism, And H2 Adsorption, Qian Wang, Qiang Sun, Puru Jena, Yoshiyuki Kawazoe
Physics Publications
Using density functional theory and generalized gradient approximation for exchange and correlation potential we show that Mg-doped GaN nanocages and nanotubes can be magnetic with Mg-contributed spins distributed over the neighboring N sites. Mg atoms show no tendency for clustering due to the positive charge residing on them; they can trap hydrogen in molecular form via the charge polarization mechanism. The binding energies of hydrogen lie in the range of 0.1–0.2 eV/H2, which are ideal for storage applications under ambient thermodynamic conditions.
Highly Efficient (Cs8v) Superatom-Based Spin-Polarizer, Haiying He, Ravindra Pandey, J. Ulises Reveles, Shiv N. Khanna, Shashi P. Karna
Highly Efficient (Cs8v) Superatom-Based Spin-Polarizer, Haiying He, Ravindra Pandey, J. Ulises Reveles, Shiv N. Khanna, Shashi P. Karna
Physics Publications
Quantum transport through molecules and the possibility to manipulate spin has generated tremendous excitement. Here, we demonstrate unusual spin transport through a molecule of twoCs8V magnetic superatoms. Calculations based on density functional theory and nonequilibrium Green’s function methods find a much higher current for the spin-down charge carriers relative to the spin-up carriers in the model Au–(Cs8V)–(Cs8V)–Au device system with almost 100% spin polarization, indicating a highly efficient spin polarizer. The new behavior is rooted in strong coupling of the localized magnetic core on V and the itinerant electrons of the Cs shell atoms leading to nearly full spin polarization.
Magnetism In Assembled And Supported Silicon Endohedral Cages: First-Principles Electronic Structure Calculations, R. Robles, Shiv N. Khanna
Magnetism In Assembled And Supported Silicon Endohedral Cages: First-Principles Electronic Structure Calculations, R. Robles, Shiv N. Khanna
Physics Publications
First principles electronic structure calculations on a free CrSi12cluster, a (CrSi12)2 dimer, and CrSi12 clusters supported on Si(111) surfaces have been carried out within a gradient corrected density functional formalism using a supercell approach. The ground state of CrSi12 is a Cr centered hexagonal biprism of Si atoms in which the Cr spin moment is completely quenched. As two CrSi12 motifs are brought together, they form different composite units depending on initial direction of approach and, in most cases, the composite cluster is found to have a net spin moment. Cluster assemblies obtained by depositing CrSi12 motifs on a Si(111) …
Al13h−: Hydrogen Atom Site Selectivity And The Shell Model, A. Grubisic, X. Li, S. T. Stokes, K. Vetter, G. F. Ganteför, K. H. Bowen, P. Jena, B. Kiran, R. Burgert, H. Schnöckel
Al13h−: Hydrogen Atom Site Selectivity And The Shell Model, A. Grubisic, X. Li, S. T. Stokes, K. Vetter, G. F. Ganteför, K. H. Bowen, P. Jena, B. Kiran, R. Burgert, H. Schnöckel
Physics Publications
Using a combination of anion photoelectron spectroscopy and density functional theory calculations, we explored the influence of the shell model on H atom site selectivity in Al13H−. Photoelectron spectra revealed that Al13H− has two anionic isomers and for both of them provided vertical detachment energies (VDEs). Theoretical calculations found that the structures of these anionic isomers differ by the position of the hydrogen atom. In one, the hydrogen atom is radially bonded, while in the other, hydrogen caps a triangular face. VDEs for both anionic isomers as well as other energetic relationships were also calculated. Comparison of the measured versus …
Superhalogen Properties Of Cufn Clusters, Q. Wang, Q. Sun, P. Jena
Superhalogen Properties Of Cufn Clusters, Q. Wang, Q. Sun, P. Jena
Physics Publications
A first-principles calculation based on gradient corrected density functional theory reveals unusual properties of a Cu atom interacting with F. Up to six F atoms are bound to a single Cu atom with electron affinities steadily rising as successive F atoms are attached, reaching a peak value of 7.2 eV in CuF5. The large energy gaps between the highest occupied and lowest unoccupied molecular orbitals, both in neutral and anionic form, provide further evidence of their stability. These unusual properties brought about by involvement of inner shell 3d-electrons not only allow CuFn to belong to the class of superhalogens but …
Stable T2sin (T=Fe,Co,Ni,1≤N≤8) Cluster Motifs, R. Robles, S. N. Khanna
Stable T2sin (T=Fe,Co,Ni,1≤N≤8) Cluster Motifs, R. Robles, S. N. Khanna
Physics Publications
First principles studies on the geometry, electronic structure, and magnetic properties of neutral and anionic Fe2Sin, Co2Sin, and Ni2Sin (1≤n≤8) clusters have been carried out within a gradient corrected density functional framework. It is shown that these clusters display a variety of magnetic species with varying magnetic moment and different magnetic coupling between the two transition metal atoms. While Fe2Sin clusters are mostly ferromagnetic with large moments, Ni2Sin clusters are mostly nonmagnetic. Our studies of the variation of the binding energy upon addition of successive Si atoms and the gap between the highest occupied molecular orbital and the lowest unoccupied …
Tailoring Local Conductivity By The Formation Of Ag Nanoclusters In Sio2 Xerogel Films, Ricky Caperton
Tailoring Local Conductivity By The Formation Of Ag Nanoclusters In Sio2 Xerogel Films, Ricky Caperton
Theses and Dissertations
Porous silicon dioxide thin films were produced via dip-coating and doped with Ag+ by adding AgNO3 to the dipping solution. Nanoparticles were formed within the pores of these films by UV exposure. Nanoparticle formation was confirmed by UV-visible spectroscopy and Transmission Electron Microscopy (TEM). Conductive Atomic Force Microscopy (CAFM) showed that the conductivity of the films decreased upon exposure to UV. This decrease in the conductivity is most likely due to the clustering of charge carriers. Initially, Ag+ ions are attached to negatively charged pore walls in a dense packing network. Upon UV exposure (125 mW @ 266 nm), the …