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  • 1
    Electronic Resource
    Electronic Resource
    Structures and stabilities of doubly charged (MgO)nMg2+ (n=1–29) cluster ions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 4788-4796 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio perturbed ion plus polarization calculations are reported for doubly charged nonstoichiometric (MgO)nMg2+ (n=1–29) cluster ions. We consider a large number of isomers with full relaxations of the geometries, and add the correlation correction to the Hartree–Fock energies for all cluster sizes. The polarization contribution is included at a semiempirical level also for all cluster sizes. Comparison is made with theoretical results for neutral (MgO)n clusters and singly charged alkali–halide cluster ions. Our method is also compared to phenomenological pair potential models in order to assess their reliability for calculations on small ionic systems. The large coordination-dependent polarizabilities of oxide anions favor the formation of surface sites, and thus bulk-like structures begin to dominate only after n=24. The relative stabilities of the cluster ions against evaporation of an MgO molecule show variations that are in excellent agreement with the experimental abundance spectra. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478366
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  • 2
    Electronic Resource
    Electronic Resource
    Characterization of carbon films microstructure by atomic force microscopy and Raman spectroscopy (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 3443-3447 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Atomic force microscopy has been used to study the surface irregularities of hydrogenated and unhydrogenated carbon films grown by rf-powered glow discharge and dc-magnetron sputtering, respectively. In general films produced with the latter technique have rougher surfaces. In glow discharge produced samples, roughness can be reduced by increasing rf power during deposition. In sputtered films surface roughness is reduced by either lowering substrate temperature or reducing the power density during deposition. The relation between the microstructure and the crystalline state was done using Raman scattering.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357474
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  • 3
    Electronic Resource
    Electronic Resource
    Optical and structural characterization of ZnSe films grown by molecular beam epitaxy on GaAs substrates with and without GaAs buffer layers (1998)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 84 (1998), S. 1551-1557 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: ZnSe films with thickness between 800 and 7500 Å were grown on GaAs(100) by molecular beam epitaxy (MBE), and characterized by photoluminescence (PL), photoreflectance (PR), transmission electron microscopy (TEM), and high resolution x-ray diffraction. A first set of films was prepared with ZnSe directly grown on the GaAs substrate. Another set was prepared using an arsenic capped GaAs buffer layer grown on the GaAs substrate in a separated MBE system. PL studies at 18 K showed that the ZnSe films have more defects for samples grown directly on the GaAs substrate. The behavior of stacking faults and dislocations as a function of film thickness were investigated by TEM, and by the variation of the intensity of PL signals related to these defects. For both sets of samples the intensity of these signals decreased with increasing film thickness, but the decrease is steeper for films grown on GaAs buffer layers. A signal in PL spectra at ∼2.7 eV was observed only for the samples grown directly on GaAs substrates, it was associated with donor–acceptor transitions involving GaZn and VZn. The room temperature PR spectra showed, besides the GaAs and ZnSe band-gap signals, oscillations associated with the Franz–Keldysh effect due to internal electric fields. The strength of these fields was obtained by employing the asymptotic Franz–Keldysh model. A signal 22 meV below the GaAs band-gap energy was observed only in the PR spectra of the samples grown directly on GaAs substrates. This signal was associated with Zn interdiffused into the GaAs, and is correlated to the PL signal observed at 2.7 eV for the same set of samples. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.368222
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  • 4
    Electronic Resource
    Electronic Resource
    A study of Franz–Keldysh oscillations of GaAs/Si/GaAs and AlAs/Si/AlAs heterostructures (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 3616-3619 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have carried out a detailed study of Franz–Keldysh oscillations observed in the photoreflectance spectra of molecular beam epitaxy grown GaAs/Si/GaAs and AlAs/Si/AlAs heterostructures with a Si nominal thickness of two monolayers. The oscillations in the photoreflectance spectra were due to internal electric fields generated by graded p-n junctions created by Si diffusion. The data were analyzed employing the asymptotic Franz–Keldysh theory. It is concluded that different contributions from degenerate heavy and light hole bands, to transitions around the Γ point of the Brillouin zone, must be expected for different heterostructures depending upon the particular characteristics of the internal electric fields present in the sample.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357422
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  • 5
    Electronic Resource
    Electronic Resource
    Photoreflectance study of the substrate-film interface of GaAs homoepitaxial structures with different in situ substrate surface cleaning processes (1997)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 71 (1997), S. 338-340 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The substrate-film interface of GaAs homoepitaxial structures prepared by molecular beam epitaxy, employing three different in situ treatments for the substrate surface preparation: (1) cleaning by hydrogen radicals (H*), (2) exposure to trisdimethylaminoarsine (TDMAAs), and (3) the usual thermal cleaning under an arsenic flux have been studied. The concentrations of interfacial residual impurities of C and O were measured by secondary ion mass spectroscopy (SIMS). These impurities pin the Fermi level producing an interfacial electric field, whose strength was evaluated by photoreflectance spectroscopy. For semi-insulating substrates, the usual thermal cleaning process resulted in very high concentrations of C (2×1019 atoms/cm3) and O (1.3×1018 atoms/cm3) at the interface, producing a large electric field (1.3×106 V/m). The impurities were drastically diminished to below the SIMS detection limit by using the H* cleaning, and as a result the electric field was effectively reduced. On the other hand, we observed higher concentrations of impurities, and larger interfacial electric fields on Si-doped substrates. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.119564
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  • 6
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    Annealing temperature and barrier thickness effect on the structural and optical properties of silicon nanocrystals/SiO2 superlattices (2014)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2014-10-02
    Description: The effect of the annealing temperature and the SiO 2 barrier thickness of silicon nanocrystal (NC)/SiO 2 superlattices (SLs) on their structural and optical properties is investigated. Energy-filtered transmission electron microscopy (TEM) revealed that the SL structure is maintained for annealing temperatures up to 1150 °C, with no variation on the nanostructure morphology for different SiO 2 barrier thicknesses. Nevertheless, annealing temperatures as high as 1250 °C promote diffusion of Si atoms into the SiO 2 barrier layers, which produces larger Si NCs and the loss of the NC size control expected from the SL approach. Complementary Raman scattering measurements corroborated these results for all the SiO 2 and Si-rich oxynitride layer thicknesses. In addition, we observed an increasing crystalline fraction up to 1250 °C, which is related to a decreasing contribution of the suboxide transition layer between Si NCs and the SiO 2 matrix due to the formation of larger NCs. Finally, photoluminescence measurements revealed that the emission of the superlattices exhibits a Gaussian-like lineshape with a maximum intensity after annealing at 1150 °C, indicating a high crystalline degree in good agreement with Raman results. Samples submitted to higher annealing temperatures display a progressive emission broadening, together with an increase in the central emission wavelength. Both effects are related to a progressive broadening of the size distribution with a larger mean size, in agreement with TEM observations. On the other hand, whereas the morphology of the Si NCs is unaffected by the SiO 2 barrier thickness, the emission properties are slightly modified. These observed modifications in the emission lineshape allow monitoring the precipitation process of Si NCs in a direct non-destructive way. All these experimental results evidence that an annealing temperature of 1150 °C and 1-nm SiO 2 barrier can be reached whilst preserving the SL structure, being thus the optimal structural SL parameters for their use in optoelectronics.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 7
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    Structure, electronic properties, and aggregation behavior of hydroxylated carbon nanotubes (2014)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2014-11-04
    Description: We present a combined experimental and theoretical study to analyze the structure, electronic properties, and aggregation behavior of hydroxylated multiwalled carbon nanotubes (OH–MWCNT). Our MWCNTs have average diameters of ∼2 nm, lengths of approximately 100–300 nm, and a hydroxyl surface coverage θ ∼0.1. When deposited on the air/water interface the OH–MWCNTs are partially soluble and the floating units interact and link with each other forming extended foam-like carbon networks. Surface pressure-area isotherms of the nanotube films are performed using the Langmuir balance method at different equilibration times. The films are transferred into a mica substrate and atomic force microscopy images show that the foam like structure is preserved and reveals fine details of their microstructure. Density functional theory calculations performed on model hydroxylated carbon nanotubes show that low energy atomic configurations are found when the OH groups form molecular islands on the nanotube's surface. This patchy behavior for the OH species is expected to produce nanotubes having reduced wettabilities, in line with experimental observations. OH doping yields nanotubes having small HOMO–LUMO energy gaps and generates a nanotube → OH direction for the charge transfer leading to the existence of more hole carriers in the structures. Our synthesized OH–MWCNTs might have promising applications.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics (AIP)
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  • 8
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    Current-driven vortex domain wall motion in wire-tube nanostructures (2015)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2015-04-01
    Description: We have investigated the current-driven domain wall motion in nanostructures comprised of a pair of nanotube and nanowire segments. Under certain values of external magnetic fields, it is possible to pin a vortex domain wall in the transition zone between the wire and tube segments. We explored the behavior of this domain wall under the action of an electron flow applied in the opposite direction to the magnetic field. Thus, for a fixed magnetic field, it is possible to release a domain wall pinned simply by increasing the intensity of the current density, or conversely, for a fixed current density, it is possible to release the domain wall simply decreasing the magnetic external field. When the domain wall remains pinned due to the competition between the current density and the magnetic external field, it exhibits a oscillation frequency close to 8 GHz. The amplitude of the oscillations increases with the current density and decreases over time. On the other hand, when the domain wall is released and propagated through the tube segment, this shows the standard separation between a steady and a precessional regime. The ability to pin and release a domain wall by varying the geometric parameters, the current density, or the magnetic field transforms these wire-tube nanostructures in an interesting alternative as an on/off switch nano-transistor.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 9
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    Structural and optical properties of Al-Tb/SiO2 multilayers fabricated by electron beam evaporation (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-10-05
    Description: Light emitting Al-Tb/SiO 2 nanomultilayers (NMLs) for optoelectronic applications have been produced and characterized. The active layers were deposited by electron beam evaporation onto crystalline silicon substrates, by alternatively evaporating nanometric layers of Al, Tb, and SiO 2 . After deposition, all samples were submitted to an annealing treatment for 1 h in N 2 atmosphere at different temperatures, ranging from 700 to 1100 °C. Transmission electron microscopy confirmed the NML structure quality, and by complementing the measurements with electron energy-loss spectroscopy, the chemical composition of the multilayers was determined at the nanoscopic level. The average composition was also measured by X-ray photoelectron spectroscopy (XPS), revealing that samples containing Al are highly oxidized. Photoluminescence experiments exhibit narrow emission lines ascribed to Tb 3+ ions in all samples (both as-deposited and annealed ones), together with a broadband related to SiO 2 defects. The Tb-related emission intensity in the sample annealed at 1100 °C is more than one order of magnitude higher than identical samples without Al. These effects have been ascribed to the higher matrix quality, less SiO 2 defects emitting, and a better Tb 3+ configuration in the SiO 2 matrix thanks to the higher oxygen content favored by the incorporation of Al atoms, as revealed by XPS experiments.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 10
    Electronic Resource
    Electronic Resource
    Recursion relations for vibrational–rotational matrix elements involving two displaced Morse potentials (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 4327-4330 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a technique based on the hypervirial theorem along with a second quantization formalism, recursion relations for matrix elements and Franck–Condon factors of Morse potentials are obtained. These expressions can be used to calculate recursively matrix elements of the operators qk=(r−re)k, xs=exp[−as(r−re)], dl/dr l, or any combination of them, corresponding to vibrational–rotational states belonging to two distinctly separated Morse oscillators of arbitrary De. The proposed equations give accurate numerical results even when the traditional methods (numerical integration, asymptotic expansions, semiclassical, etc.) present serious inconveniences. The results are valid for any value of the power of the operator and of the quantum numbers v and J of both oscillators.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461756
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