Molecular dynamics simulations of laser-induced damage of nanostructures and solids

A theoretical approach to treat laser induced femtosecond structural changes in covalently bonded nanostructures and solids is described. Our approach consists in molecular dynamic simulations performed on the basis of time-dependent, many-body potential energy surfaces derived from tight-binding Hamiltonians. The shape and spectral composition of the laser pulse is explicitly taking into account in a non-perturbative way. We show a few examples of the application of this approach to describe the laser damage and healing of defects in carbon nanotubes with different chiralities and the ultrafast nonequilibrium melting of bulk germanium, initiated by the laser-induced softening and destabilization of transversal acoustic phonon modes.     

Nonequilibrium Thermodynamics of the First and Second Kind: Averages and Fluctuations

We elaborate and compare two approaches to nonequilibrium thermodynamics, the two-generator bracket formulation of time-evolution equations for averages and the macroscopic fluctuation theory, for a purely dissipative isothermal driven diffusive system under steady state conditions. The fluctuation dissipation relations of both approaches play an important role for a detailed comparison. The nonequilibrium Helmholtz free energies introduced in these two approaches differ as a result of boundary conditions. A Fokker-Planck equation derived by projection operator techniques properly reproduces long range fluctuations in nonequilibrium steady states and offers the most promising possibility to describe the physically relevant fluctuations around macroscopic averages for time-dependent nonequilibrium systems.     

Fluctuations of Quantum Currents and Unravelings of Master Equations

The very notion of a current fluctuation is problematic in the quantum context. We study that problem in the context of nonequilibrium statistical mechanics, both in a microscopic setup and in a Markovian model. Our answer is based on a rigorous result that relates the weak coupling limit of fluctuations of reservoir observables under a global unitary evolution with the statistics of the so-called quantum trajectories. These quantum trajectories are frequently considered in the context of quantum optics, but they remain useful for more general nonequilibrium systems. In contrast with the approaches found in the literature, we do not assume that the system is continuously monitored. Instead, our starting point is a relatively realistic unitary dynamics of the full system     

A Re-interpretation of the Concept of Mass and of the Relativistic Mass-Energy Relation

For over a century the definitions of mass and derivations of its relation with energy continue to be elaborated, demonstrating that the concept of mass is still not satisfactorily understood. The aim of this study is to show that, starting from the properties of Minkowski spacetime and from the principle of least action, energy expresses the property of inertia of a body. This implies that inertial mass can only be the object of a definition—the so called mass-energy relation—aimed at measuring energy in different units, more suitable to describe the huge amount of it enclosed in what we call the “rest-energy” of a body. Likewise, the concept of gravitational mass becomes unnecessary, being replaceable by energy, thus making the weak equivalence principle intrinsically verified. In dealing with mass, a new unit of measurement is foretold for it, which relies on the de Broglie frequency of atoms, the value of which can today be measured with an accuracy of a few parts in 10⁹.     

Dynamical and Geometric Phases of Bose-Einstein Condensates

By using of the Lewis-Riesenfeld invariant theory, dynamical and geometric phases of Bose-Einstein condensates are studied. The Aharonov-Anandan phase is also obtained under the cyclical evolution.     

Cosmic-coincidence problem and variable constants of physics

The standard model of cosmology is investigated using a time-dependent cosmological constant Λ and Newton gravitational constant G. The total energy content is described by the modified Chaplygin gas equation of state. It is found that the time-dependent constants coupled with the modified Chaplygin gas interpolate between the earlier matter to the later dark-energy dominated phase of the universe. We also achieve a convergence of the parameter ω→−1, almost at the present time. Thus our model fairly alleviates the cosmic-coincidence problem, which demands ω=−1 at the present time.     

Time Operators and Approximation of Continuous Functions

The main purpose of this paper is to study time operators associated with generalized shifts and determined by the Haar and Faber–Schauder bases on the space of continuous functions. It is given the characterization of the domains of the constructed time operators and their scalings. It is also shown how scalings of time operators affect the dynamics of associated semigroups of shift operators.     

Hamiltonian and Thermodynamic Modeling of Quantum Turbulence

The state variables in the novel model introduced in this paper are the fields playing this role in the classical Landau-Tisza model and additional fields of mass, entropy (or temperature), superfluid velocity, and gradient of the superfluid velocity, all depending on the position vector and another tree dimensional vector labeling the scale, describing the small-scale structure developed in ⁴He superfluid experiencing turbulent motion. The fluxes of mass, momentum, energy, and entropy in the position space as well as the fluxes of energy and entropy in scales, appear in the time evolution equations as explicit functions of the state variables and of their conjugates. The fundamental thermodynamic relation relating the fields to their conjugates is left in this paper undetermined. The GENERIC structure of the equations serves two purposes: (i) it guarantees that solutions to the governing equations, independently of the choice of the fundamental thermodynamic relation, agree with the observed compatibility with thermodynamics, and (ii) it is used as a guide in the construction of the novel model.     

Gravitational Perturbation of Garfinkle-Horowitz-Strominger Dilaton Black Hole and Quasinormal Modes

We research gravitational perturbation of Garfinkle-Horowitz-Strominger dilaton black hole and its quasinormal modes by using WKB approach proposed by Schutz, Will, Iyer and Konoplya. The quasinormal frequency with different angular momentum l is calculated in this paper. Our results show that, as the charge parameter b increase, both the real part and the absolute value of imaginary part of quasinormal frequency also increase, which means that the effect of charge in Garfinkle-Horowitz-Strominger dilaton background spacetime lead to higher frequency gravitational wave and the quasinormal modes damp at a rapider rate.     

硫酸根离子对草酸-硫酸混合酸中制备的多孔阳极氧化铝光致发光特性的影响

采用电化学阳极氧化法,分别在草酸、硫酸及两者不同浓度比的混合酸中制备了AAO薄膜样品,并分别观察了在250,296 nm光激发下的光致发光(PL)特性。结果表明:草酸和混合酸中制备的AAO薄膜,在250~550 nm范围内的光致发光与不同存在或分布形式的草酸杂质形成的发光中心相关。硫酸根离子对混合酸中制备的AAO薄膜的PL特性有很大影响,随硫酸根离子浓度的增加发光峰位逐渐蓝移。分析了出现上述实验现象的原因。     

On the origin of light emission from porous anodic alumina formed in sulfuric acid

We have investigated the photoluminescence (PL) property of porous anodic alumina membranes (PAAMs) formed on bulk Al foils in 0.3 M sulfuric acid. Different from that from the PAAMs formed in oxalic acid, the obtained PL spectra show two emission bands which have different origins. One centered at ∼465 nm (α-band) weakens its intensity in the PAAM annealed in O₂ and is thus attributed to optical transition in oxygen vacancies. The other in the blue (β-band) redshifts with increasing excitation wavelength. On the basis of spectral examinations and analyses, we ascribe the β-band to radiative recombination of carriers in the isolated hydroxyl groups at the surface of the pore wall, whereas the photogeneration of carriers takes place in oxygen vacancies in the pore wall. This work improves the understanding of the light-emitting property of the PAAMs formed in sulfuric and oxalic acid.     

Strong ultraviolet and violet photoluminescence from Si-based anodic porous alumina films

We have investigated photoluminescence (PL) from Si-based anodic porous alumina films formed by real-time controlled anodization of electron-beam evaporated Al films. As-anodized samples show three strong PL bands at 295, 340, and 395 nm. These bands blueshift and their intensities decrease after the samples are annealed. When the annealing temperature increases to 1000 °C, the blueshift becomes specially pronounced and meanwhile the structures of the films develop toward crystalline Al₂O₃. Based on discussions on the thermal annealing behaviors of the PL and PL excitation spectra, we suggest that optical transitions in oxygen-related defects, F⁺ (oxygen vacancy with one electron) centers, are responsible for the observed ultraviolet and violet PL. Received: 24 July 2000 / Accepted: 24 February 2001 / Published online: 3 May 2001     

Visible photoluminescence from Ge+-implanted SiO2 films thermally grown on crystalline Si

+ -implanted SiO₂ films is studied as a function of different fabricating conditions (implantation dose, annealing temperature and time). The SiO₂ films containing Ge nanocrystals exhibit two photoluminescence (PL) bands peaked at 600 nm and 780 nm. There are two excitation bands in the PL excitation (PLE) spectra. With variation in Ge nanocrystal size, the PL and PLE peak energies show no appreciable shift. The PL and PLE spectral analyses suggest that during the PL process, electron–hole pairs are generated by the E(l) and E(2) direct transitions inside Ge nanocrystals, which then radiatively recombine via luminescent centers in the matrix or at the interface between the nanocrystal/matrix. Received: 27 January 1998/Accepted: 18 March 1998     

1.54 μm emission mechanism of Er-doped zinc oxide thin films

Zinc oxide (ZnO) and Er-doped zinc oxide (ZnO:Er) thin films were formed by pulsed laser deposition, and characterized by photoluminescence (PL) and X-ray diffraction (XRD) in order to clarify the 1.54 μm emission mechanism in the ZnO:Er films. Er ions were excited indirectly by the 325 nm line of a He-Cd laser, and the comparison of the ultraviolet to infrared PL data of ZnO and ZnO:Er films showed that the 1.54 μm emission of Er³⁺ in ZnO:Er film appears at the expense of the band edge emission and the defect emission of ZnO. The crystallinity of the films was varied with the substrate temperature and post-annealing, and it was found that the intensity of the 1.54 μm emission is strongly related with the crystallinity of the films. There are three processes leading to the 1.54 μm emission; absorption of excitation energy by the ZnO host, energy transfer from ZnO to Er ions, and radiative relaxation inside Er ions, and it is suggested that the crystallinity plays an important role in the first two processes.     

Co/AAO纳米有序阵列复合结构的光致发光和光吸收特性

以草酸电解液制备的高度有序的纳米级多孔阳极氧化铝(AAO)为模板,采用交流电化学沉积工艺合成了Co/AAO纳米有序阵列复合结构。分别研究了AAO模板和Co/AAO结构的光致发光和光吸收特性。结果表明,AAO模板在350～550nm范围内存在较强的光致发光带,其峰位在395nm处;Co/AAO纳米有序阵列复合结构的光致发射峰位亦在395nm处,且发光峰强度随Co沉积量的增加而迅速减弱。实验发现,Co/AAO纳米有序阵列复合结构的光吸收边从近紫外至近红外的波段范围内发生大幅度红移,最大红移量甚至超过380nm。文章定性分析并解释了Co/AAO纳米有序阵列复合结构吸收边大幅度红移及其光发射峰位不变、而峰强减弱的主要原因。     

Effect of metal salts in the optical properties of polydiacetylenes

Films of polydiacetylene (PDA)-containing polyesters with metal salts of Zn, Eu and Gd were prepared. Ultraviolet-visible (UV-VIS) absorption and photoluminescent (PL) spectra indicated that the presence of the metal salts induced a small shift of their maxima to higher and lower energies, respectively. PDAs films with an ester group in allylic position to the conjugated system presented two emission bands. The higher energy emission band was assigned to carbonyl emission and the lower energy emission band to self-trapped excitons (STE). Indeed, the inclusion of metal salts incremented the intensity relation between the higher and lower emission energy bands. On the other hand, Raman spectroscopy measurements performed in PDAs films derived of pentyn-1-ol indicated that the metal salts induced a slight statistical shift in the triple and double bond signals to higher energies. In this way, the intensity variations and band shifts detected by Raman, UV-VIS and PL spectroscopies showed that the presence of metal salts could have a remarkable influence on the energy levels of these PDAs. Nonlinear optical (NLO) third harmonic generation (THG) measurements were performed in order to verify this affirmation as well as the cubic NLO performance of these materials.     

Porous ZnO nanobelts evolved from layered basic zinc acetate nanobelts

Novel porous ZnO nanobelts were successfully synthesized by heating layered basic zinc acetate (LBZA) nanobelts in the air. The precursor of LBZA nanobelts consisted of a lamellar structure with two interlayer distances of 1.325 and 0.99 nm were prepared using a low-temperature, solution-based method. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared spectroscopy are used to characterize the as-products. PL measurements show that the porous ZnO nanobelts have strong ultraviolet emission properties at 380 nm, while no defect-related visible emission is detected. The good performance for photoluminescence emission makes the porous ZnO nanobelts promising candidates for photonic and electronic device applications.     

Structural and photoluminescence characters of SnO2:Sb films deposited by RF magnetron sputtering

The Antimony-doped tin oxide (SnO₂:Sb) films have been prepared on glass substrates by RF magnetron sputtering method. The prepared samples are polycrystalline films with rutile structure of pure SnO₂ and have preferred orientation of (1 1 0) direction. XRD measurement did not detect the existence of Sb₂O₃ phase and Sb₂O₅ phase; Sb ions occupy the site of Sn ions and form the substitution doping. An intensive UV-violet luminescence peak near 392 nm is observed at room temperature. Photoluminescence (PL) properties influenced by sputtering power and annealing for the SnO₂:Sb films are investigated in detail and corresponding PL mechanism is discussed.     

Atomic layer deposition coating of ZnO shell for GaN-ZnO core-sheath heteronanowires

We have synthesized GaN-core/ZnO-shell nanowires and investigated effects of the ZnO coating. The X-ray diffraction pattern showed that as-synthesized samples are composed of GaN and ZnO. Transmission electron microscopy indicated that the deposited ZnO shell layer is poly-crystalline. The photoluminescence (PL) spectrum of GaN has been changed by the ZnO coating, where emission bands centered at roughly 1.9 eV, 2.5 eV, and 3.3 eV were newly added to the emissions from core GaN nanowires. We found that overall PL intensity has been significantly increased by coating the ZnO shell layers.