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A model has been developed for the determination of the correct absolute size of nanoparticles using a disc centrifuge; the method does not require externally measured or literature derived particle densities. The principle of this method is the altered settling velocity of particles in fluids with different viscosities and/or densities, with the use of a linear regression analysis for the calculation of particle densities. This allows a fast particle density determination with at least two measurements using a disc centrifuge and a small subset of easily measurable parameters. Furthermore, correctness of the model is evaluated using viruses and nonbiological particles of known densities.
Assuntos
Centrifugação com Gradiente de Concentração/métodos , Nanopartículas/química , Centrifugação com Gradiente de Concentração/instrumentação , Tamanho da Partícula , Propriedades de Superfície , ViscosidadeRESUMO
(Ge(0.2)Se(0.8))(100-x)In(x) and (Ge(0.17)Se(0.83))(100-x)In(x) (x = 0, 5, 10, 15 at.%) chalcogenide glasses have been studied with high-energy x-ray diffraction, neutron diffraction and extended x-ray absorption spectroscopy at Ge, Se and In K-edges. The experimental data were modelled simultaneously with the reverse Monte Carlo simulation method. GeSe(4/2) tetrahedra are shown to be the main structural units in the binary and ternary glasses investigated. Indium bonds to the excess Se atoms in the ternary Ge-Se-In glasses. While the majority of In atoms have three Se neighbours, some In atoms may be tetrahedrally coordinated by Se.
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Germânio/química , Vidro/química , Índio/química , Método de Monte Carlo , Selênio/química , Simulação por Computador , Modelos Químicos , Espectroscopia por Absorção de Raios X , Difração de Raios XRESUMO
The structure of Zr(60)Cu(20)Fe(20) metallic glass has been studied with high-energy x-ray diffraction, neutron diffraction and extended x-ray absorption spectroscopy and modelled with the reverse Monte Carlo simulation technique. It is found that Cu and Fe atoms prefer Zr as a nearest neighbour. The mean interatomic distance between Cu/Fe and Zr atoms in the glass is remarkably shorter than the sum of the respective atomic radii. The coordination numbers for Cu/Fe-Cu/Fe pairs are very close to each other, suggesting a regular distribution of Cu and Fe atoms in the Zr(60)Cu(20)Fe(20) metallic glass.
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Cobre/química , Vidro/química , Compostos de Ferro/química , Modelos Moleculares , Método de Monte Carlo , Difração de Nêutrons , Difração de Raios X , Zircônio/química , Simulação por Computador , Espectroscopia por Absorção de Raios XRESUMO
We investigate the structure of liquid monotectic alloy Bi(30)Ga(70) above and below the critical point. The three-dimensional structure at 265 °C is modelled by means of the reverse Monte Carlo simulation technique using neutron and x-ray diffraction experimental data. It is shown that atomic segregation on the short-range scale exists in the liquid Bi(30)Ga(70) slightly above the critical temperature (T(C) = 262 °C). We present also the structure factors of Bi(30)Ga(70) liquid alloy under the critical point at 240 and 230 °C obtained with neutron diffraction to highlight the temperature effect in the atomic structure.
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The atomic structure of the liquid NiSi and NiSi(2) alloys is investigated by means of neutron diffraction experiments with isotopic substitution. From experimental data-sets obtained using four Ni isotopes, partial structure factors and pair correlation functions are obtained by applying a reverse Monte Carlo modelling approach. Both alloys were found to exhibit a strong tendency to hetero-coordination within the first coordination shell. In particular, covalent Si-Si bonds with somewhat greater distances seem to influence the structure of the liquid NiSi alloy.
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(As(0.4)S(0.6))(100-x)Ag(x) glasses (x = 0, 4, 8, 12 at.%) have been studied with high-energy x-ray diffraction, neutron diffraction and extended x-ray absorption spectroscopy at As and Ag K-edges. The experimental data were modelled simultaneously with the reverse Monte Carlo simulation method. Analysis of the partial pair correlation functions and coordination numbers extracted from the model atomic configurations revealed that silver preferentially bonds to sulfur in the As(2)S(3)-Ag ternary glasses, which results in the formation of homoatomic As-As bonds. Upon the addition of Ag, a small proportion of Ag-As bonds (N(AgAs)≈0.3) are formed in all three ternary compositions, while the direct Ag-Ag bonds (N(AgAg)≈ 0.4) appear only in the glass with the highest Ag content (12 at.%). Similar to the g- As(2)S(3) binary, the mean coordination number of arsenic is close to three, and that of sulfur is close to two, in the As(2)S(3)-Ag ternary glasses. The first sharp diffraction peak on the total structure factors of As(2)S(3) binary and (As(0.4)S(0.6))(100-x)Ag(x) ternary glasses is related to the As-As and As-S correlations.
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We present experiments on second-harmonic generation from arrays of magnetic split-ring resonators and arrays of complementary split-ring resonators. In both cases, the fundamental resonance is excited by the incident femtosecond laser pulses under normal incidence, leading to comparably strong second-harmonic signals. These findings are discussed in terms of Babinet's principle and in terms of a recently developed microscopic classical theory that leads to good agreement regarding the relative and the absolute nonlinear signal strengths. The hydrodynamic convective contribution is found to be the dominant source of second-harmonic generation--in contrast to a previous assignment [Science 313, 502 (2006)].
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Atomic structures of Ge(25)Sb(15)S(60) and Ge(35)Sb(5)S(60) glasses are investigated in the gamma-irradiated and annealed after gamma-irradiation states by means of high-energy synchrotron x-ray diffraction technique. The first sharp diffraction peak (FSDP) is detected at around 1.1 A(-1) in the structure factors of both alloys studied. The FSDP position is found to be stable for radiation/annealing treatment of the samples, while the FSDP intensity shows some changes between gamma-irradiated and annealed states. The peaks in the pair distribution functions observed between 2 and 4 A are related to the Ge-S, Ge-Sb, and Sb-Sb first neighbor correlations and Ge-Ge second neighbor correlations in the edge-shared GeS(42) tetrahedra, and S-S and/or Ge-Ge second neighbor correlations in the corner-shared GeS(42) tetrahedra. Three mechanisms of the radiation-/annealing-induced changes are discussed in the framework of coordination topological defect formation and bond-free solid angle concepts.
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Glassy As(25)Si(40)Te(35) has been studied by x-ray and neutron diffraction as well as x-ray absorption spectroscopy (EXAFS) at As and Te K-edges. Simultaneous modelling of the four independent measurements by means of the reverse Monte Carlo (RMC) simulation technique allowed the separation of partial pair distribution functions and estimation of the corresponding coordination numbers. It is shown that the atomic structure of As(25)Si(40)Te(35) glass can be presented as a three-dimensional network of twofold coordinated Te, threefold coordinated As and fourfold coordinated Si atoms.
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The method of angular photonic correlations of spontaneous emission is introduced as an experimental, purely optical scheme to characterize disorder in semiconductor nanostructures. The theoretical expression for the angular correlations is derived and numerically evaluated for a model system. The results demonstrate how the proposed experimental method yields direct information about the spatial distribution of the relevant states and thus on the disorder present in the system.
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Luminescence as a mechanism for terahertz emission from femtosecond laser-induced plasmas is studied. By using a fully microscopic theory, Coulomb scattering between electrons and ions is shown to lead to luminescence even for a spatially homogeneous plasma. The spectral features introduced by the rod geometry of laser-induced plasma channels in air are discussed on the basis of a generalized mode-function analysis.
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The lack of translational invariance perpendicular to the plane of a single quantum well causes equal probability for spontaneous emission to the left or right. Combining one emission path from the left and one from the right into a common detector leads to interference fringes for fundamentally indistinguishable paths corresponding to geometries where the same in-plane momentum is transferred to the quantum well. For all other paths, no interference is observed because of the entanglement between the photon and extended Bloch states of the many-body system. In multiple-quantum-well structures the interference can be controlled via the spacing between the wells.
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Time-resolved photoluminescence spectra after nonresonant excitation show a distinct 1s resonance, independent of the existence of bound excitons. A microscopic analysis identifies exciton and electron-hole plasma contributions. For low temperatures and low densities, the excitonic emission is extremely sensitive to details of the electron-hole-pair population making it possible to identify even minute fractions of optically active excitons.
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A fully microscopic theory is presented for interacting electrons, holes, photons, and phonons in semiconductor heterostructures. The formation dynamics and statistics of incoherent excitons are analyzed for different densities, lattice temperatures, and photonic environments. Luminescence experiments are shown to depend strongly on the photonic environment in contrast to suggested terahertz absorption measurements. Whereas luminescence in free space is dominated by plasma contributions, terahertz absorption should be able to directly measure excitonic populations.
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This study examined adult age differences in the accuracy, confidence ratings, and vividness ratings of veridical and suggested memories. After seeing either one or two exposures of a vignette depicting a theft, young adults (M = 19 years) and older adults (M = 73 years) were given misleading information that suggested the presence of particular objects in the episode. Memory accuracy was higher for younger adults than for older adults, and the frequency of falsely reporting the presence of suggested objects was greater for older adults than for young adults. Further, levels of confidence and vividness ratings of the perceptual attributes (colors, locations) of falsely recognized items were higher for older adults than for young adults. Both young adults and older adults used more perceptual references when describing veridical memories than when describing suggested memories. Age differences in the suggestibility of memory were attributed to nonspecific or nondissociated memory aging effects.
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Envelhecimento/fisiologia , Transtornos da Memória/diagnóstico , Adolescente , Adulto , Fatores Etários , Idoso , Idoso de 80 Anos ou mais , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Índice de Gravidade de Doença , Inquéritos e QuestionáriosRESUMO
The acquisition of cognitive skills often depends on 1 of (or a combination of) 2 processes, the execution of an algorithm, and the retrieval of problem instances. This study examined the effects of age and repetition of problem instances on the production and verification of solutions to 2 serially presented sets of alphabet arithmetic problems. Analyses of the parameters derived from power-function fits for individuals revealed age differences favoring young adults in improvement span, learning rate, and asymptote. For both age groups, the beneficial effects of repetitions on 1st-set response times were attributable to algorithmic speedup and to the retrieval of instances, whereas improvements in the speed of 2nd-set response times were attributable primarily to item retrieval.
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Cognição , Aprendizagem , Adolescente , Adulto , Idoso , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Tempo de ReaçãoRESUMO
This research examined age differences in the acquisition and reacquisition of instance-based automaticity. In 2 experiments, young and older adults were trained to enumerate targets presented in otherwise empty displays or in displays that contained distractors. Experiment 1 revealed that older adults required more practice to reach asymptote than young adults. For both age groups, modifications of the identities and locations of targets produced substantial disruptions in performance, whereas modifications of the identities or locations of distractors produced little interference. However, no age differences in the representations of instances in memory were obtained in participants who reached asymptote. Experiment 2 revealed age deficits in the long-term retention and rate of reacquisition of instance-based automaticity 18 months after initial training.
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Envelhecimento/fisiologia , Automatismo , Aprendizagem/fisiologia , Retenção Psicológica/fisiologia , Adolescente , Adulto , Fatores Etários , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Tempo de ReaçãoRESUMO
Memory for previously learned figural sequences and item-to-item covariations within figural sequences was examined under explicit and implicit instructional conditions in three age groups: young adults (17-23 years); middle-aged adults (35-45 years); and older adults (55-65 years). In Phase 1 of the experiment, the acquisition phase, half the subjects in each age group learned sequences of three to eight items in which the item-to-item changes conformed to an artificial grammar, and the other half of the subjects in each age group learned strings in which the item-to-item changes were nongrammatical. In Phase 2, the implicit/explicit test phase, subjects made forced-choice judgments about parts of the strings that they learned in Phase 1, under either explicit or implicit instructions. Analyses of Phase 2 data revealed that subjects in both instructional conditions used item-to-item covariations in making decisions about grammatical strings. However, use of previously learned covariations as well as the number of correct judgments about previously learned strings was greater in the explicit condition than in the implicit condition. An age-related deficit was found for explicit recognition of grammar-following sequences.
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Cognição , Julgamento , Aprendizagem , Reconhecimento Visual de Modelos , Adolescente , Adulto , Fatores Etários , Análise de Variância , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Fatores de TempoRESUMO
Adult age differences in the effects of different types of distractor interference on visual search were examined. Young adults (mean age = 18.5 years) and older adults (mean age = 69.5 years) performed a target-counting task that required a complete search of a visual display in each trial. Varying numbers of targets were presented alone in displays or were interspersed among eight distractor items that were either categorically related (letters) or conceptually related (numbers representing either the correct number or the incorrect number of targets in the display) to the target item (letter Q). An adult age difference in the speed of target enumeration was observed when targets were presented alone in the display. In addition, when targets appeared with distractors, both younger and older adults were penalized more by conceptually interfering distracters than by categorically related distractors. Results did not suggest an age-related decline in inhibitory processes.
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Envelhecimento/psicologia , Atenção , Percepção Visual , Adolescente , Adulto , Fatores Etários , Idoso , Cognição , Feminino , Percepção de Forma , Humanos , Masculino , Pessoa de Meia-Idade , Tempo de Reação , Análise e Desempenho de Tarefas , Fatores de TempoRESUMO
Visual asymmetry patterns related to skill were examined during a target-probe matching task in 24 skilled medical technologists and 24 matched controls. On each of 240 test trials, digitized replicas of specimens commonly encountered in medical laboratory diagnostics were shown centrally for 500 msec. Each target was immediately followed by a lateralized probe item for 120 msec that was either an exact copy (positive probe) or a distorted version (negative probe) of the target. Difficulty level of target-probe matching was manipulated on negative probe trials; half of the negative items consisted of difficult discriminations which were selected to assess the effects of domain-specific experience on detecting small differences in salient morphological features. Medical technologists exhibited a right visual field advantage, but were not different from the control subjects in speed or accuracy to positive probes or to easy negative probes. The observed left-hemisphere advantage in skilled visual processing is attributed to the beneficial effects of experience on the development of domain-specific visual analysis skills.