X-ray and neutron studies of nanoscale atomic diffusion in thin films and multilayers

In the present work we review some X-ray and neutron based techniques capable of measuring diffusion lengths in thin films with an accuracy of a fraction of a nanometer. The techniques have been used for studying both self-diffusion of the constituent species in a thin film, as well as interdiffusion at the interfaces in multilayers. The high accuracy of the techniques in depth profiling of an element or a specific isotope makes very low diffusivities ∼10⁻²³ m²/s, amenable to measurements, and allows one to study the subtle effects of factors like internal stresses or structural relaxation on self-diffusion in compositionally homogeneous films. Depth selectivity of X-ray standing wave technique in multilayers makes it possible to distinguish between diffusion at the two types of the interfaces, namely A-on-B and B-on-A, in a single multilayer structure.     

Magnetism of thin films and multilayers

The use of ultra-high-vacuum systems to deposit thin films and multilayers of magnetic materials has led to the observation of a wide range of new magnetic phenomena. These include: the perpendicular easy axis of many thin films, the enhancement of magnetic moments at surfaces and steps, oscillatory coupling across non-magnetic spacers, and the giant magnetoresistance. In this review I give an overview of these effects.     

Magnetization dynamics in thin films and multilayers

The behavior of spin waves is influenced by essentially all the parameters which characterize a magnetic material – exchange interactions, anisotropy, surface effects, dipolar interactions, phase transitions and imperfections. Thus measurements of spin wave frequencies can give important information on characterizing different magnetic materials and structures. In this paper we outline the major results and calculational methods for long-wavelength spin waves in thin films and multilayers. While the primary attention is on ferromagnet-based structures, long-wavelength spin waves in antiferromagnets are also discussed. We indicate how particular measurements of spin wave frequencies can be used to extract the fundamental parameters of the different structures.     

纳米ZnO和ZnO-SiO2复合薄膜的光学性质研究

通过溶胶凝胶(sol-gel)法分别在玻璃衬底上制备了ZnO纳米薄膜和ZnO-SiO2纳米复合薄膜,并利用紫外.可见光分光光度计对薄膜的光学性能进行了分析.可见光一紫外透射谱显示,随着Zn0溶胶浓度从0.7 mol/L降低到0.006 mol/L,制备的ZnO薄膜从只出现一个380 nm(对应的光学禁带宽度为3.27 eV)左右的吸收边到在380和320nm(对应的光学禁带宽度为3.76 ev)左右各出现一个吸收边,并且随着ZnO溶胶浓度的降低,在380-320 nm波段内的透过率明显提高.而ZnO-SiO2复合薄膜只在310 nm左右出现一个吸收边.SiOO2的包覆宽化了ZnO的禁带宽度,包覆后的禁带宽度可达到3.87 ev.     

Dynamical dielectric susceptibility of ferroelectric thin films and multilayers

The thickness dependence of the real and imaginary parts of the dynamical dielectric susceptibility is investigated phenomenologically for a multilayer structure consisting of alternating ferroelectric and paraelectric layers. It is shown that the frequency dependence of the linear dielectric response can be closely approximated by that of a damped harmonic oscillator, with the static susceptibility, relaxation time, and soft-mode frequency depending on the layer thickness and temperature. When the layer thickness and temperature are equal to their critical values corresponding to the onset of a size-driven ferroelectric phase transition, the static susceptibility and the relaxation time become anomalously large and then decrease with further increasing layer thickness. A spectrum of natural polarization oscillations is predicted to exist with thickness-dependent frequencies. This spectrum includes a soft-mode frequency which vanishes at the critical thickness and at the critical temperature. The frequency spectrum lies below the soft-mode frequency of a thick film (in which the gradient of polarization is negligible). The calculations are compared with experimentally measured dispersion of the dielectric response of a PbTiO₃-Pb_0.72La_0.28TiO₃ multilayer structure. The agreement between the theory and experiment is found to be good.     

纳米ZnO和ZnO-SiO2复合薄膜的光学性质研究

通过溶胶凝胶(sol-gel)法分别在玻璃衬底上制备了ZnO纳米薄膜和ZnO-SiO₂纳米复合薄膜,并利用紫外-可见光分光光度计对薄膜的光学性能进行了分析.可见光-紫外透射谱显示,随着ZnO溶胶浓度从0.7mol/L降低到0.006mol/L,制备的ZnO薄膜从只出现一个380nm(对应的光学禁带宽度为3.27eV)左右的吸收边到在380和320nm(对应的光学禁带宽度为3.76eV)左右各出现一个吸收边,并且随着ZnO溶胶浓度的降低,在380—320nm波段内的透过率明显提高.而Z 关键词： 纳米ZnO 2复合薄膜')" href="#">ZnO-SiO₂复合薄膜 溶胶凝胶法 透射率     

Excitonic diffusion in thin molecular films

The analyses of exciton diffusion in thin molecular films have shown that the diagonal elements of the diffusion tensor, usually called diffusion coefficients, depend on the layer index labeling layers in the direction of disturbed symmetry. The particular analysis was done for a thin film having four layers. In this structure only two layers are occupied by optical excitations. It means that in the four layer film two films occur in which optical excitations can travel. The subfilm contains a boundary layer that noticeably differs from the subfilm with internal layers. If the subfilm contains the boundary layer, the diffusion coefficient of this layer differs from the diffusion coefficient of any internal layer. If the subfilm contains two internal layers, the diffusion coefficient of these layers are equal, expectably from the viewpoint of physics. The exciton diffusion is very low due to the high exciton energies. This work was supported by the Serbian Ministry of Science and Technology: Grant No 1895.     

Inelastic nuclear resonant absorption of synchrotron radiation in thin films and multilayers

Inelastic nuclear resonant absorption of synchrotron radiation is an efficient and unique method for the direct measurement of vibrational density of states (VDOS) of thin films and interfaces that contain Mössbauer isotopes. This is demonstrated for the ⁵⁷Fe nuclear resonance in the case of amorphous and crystalline Tb–Fe alloy thin films and buried Fe/Cr interfaces in epitaxial α-Fe(0 0 1)/Cr(0 0 1) superlattices.

     

Contrasting friction and diffusion in molecularly thin confined films

We study, using fluorescence correlation spectroscopy, translational diffusion in molecularly thin liquids confined within a surface forces apparatus. The diffusion coefficient decreases exponentially from the edges towards the center of the Hertzian contact and further suggests the presence of a small number of distinct diffusion processes. This holds alike a crystallizable fluid (OMCTS) and a glass-former (1,2-propane diol), both of which displayed static friction. We conclude that friction, the average of an ensemble of molecules, masked massively heterogeneous molecular mobility.     

Anomalous electron diffusion in irradiated thin gold films

We report on topographical and electrical properties of an evaporated (as-prepared) thin gold film before and after irradiation with 200 keV Ar⁺ ions. TEM-investigations reveal for the as-prepared film voids and channels of small size, and a pronounced percolative structure with large scale inhomogeneities after irradiation. Magnetoresistance measurements carried out before and after irradiation yield the temperature dependence of the phase coherence lengthL , when analysing the experimental data within current theories for 2-dimensional (2d) weak localization. The results can be explained by normal electron diffusion if-for the as-prepared film-L is larger than the size of structural inhomogeneities, and-for the irradiated film-by anomalous electron diffusion ifL becomes smaller. The analysis ofL (T) for the irradiated film yields critical exponents for 2d-percolation, in good agreement with predictions of percolation theory.     

Hindered ion diffusion in polyelectrolyte/montmorillonite multilayers: Toward compartmentalized films

The European Physical Journal E - This report describes the construction of multicomposite ultrathin films using the layer-by-layer assembly technique. The films are built-up with two different...     

Modeling of exciton diffusion in amorphous organic thin films

Time-resolved photoluminescence spectroscopy of amorphous organic thin films of aluminum tris-(8-hydroxyquinoline) show emission spectra that redshift with time following excitation by ultrafast laser pulses. Based on reports of similar phenomena in other materials, we attribute this effect to the exciton diffusion between energetically dissimilar molecules by means of F?rster transfer. In analyzing results at 295, 180, 75, and 35 K, we show that existing theoretical treatments of exciton diffusion require two modifications to self-consistently fit our data: one must include spatial disorder in the model, and the energy dependence of F?rster transfer must be calculated using the donor-acceptor spectral overlap, instead of a Boltzman distribution. Monte Carlo simulations utilizing these changes yield results that are self-consistent with the observed spectral shifts.     

Anisotropic diffusion of single molecules in thin liquid films

The European Physical Journal E - Single molecule wide field imaging is applied to study the diffusion in ultrathin liquid films on solid surfaces. The results show a broad distribution of...     

Anisotropic diffusion of single molecules in thin liquid films

Single molecule wide field imaging is applied to study the diffusion in ultrathin liquid films on solid surfaces. The results show a broad distribution of diffusion coefficients. This is tentatively ascribed to an anisotropy of the diffusion coefficient perpendicular to the surface and a slow exchange of molecules between regions of different diffusion coefficients. We have evidence, that these changes as well as the slow motion perpendicular to the surface are related to the molecular layering of the liquid close to the surface.     

Kinetic pathways of diffusion and solid-state reactions in nanostructured thin films

Mass transport and solid-state reactions in nanocrystalline thin films are reviewed. It is illustrated that diffusion along different grain boundaries (GBs) can have important effects on the overall intermixing process between two pure films. These processes can be well characterized by a bimodal GB network, with different (fast and slow) diffusivities. First the atoms migrate along fast GBs and accumulate at the film surface. These accumulated atoms form a secondary diffusion source for back diffusion along slow boundaries. Thus the different GBs of the thin films can be gradually filled up with the diffusing atoms and composition depth profiles reflect the result of these processes. Similar processes can be observed in binary systems with intermetallic layers: instead of nucleation and growth of the reaction layer at the initial interface, the reaction takes place in the GBs and the amount of the product phase grows by the motion of its interfaces perpendicular to the GBs. Thus, the entire layer of the pure parent films can be consumed by this GB diffusion-induced solid-state reaction (GBDIREAC), and a fully homogeneous product layer can be obtained.     

Zinc oxide thin films prepared by means of Langmuir–Blodgett multilayers

Recieved: 7 April 1998/Accepted: 31 July 1998     

Extraordinary Hall effect on Fe-rich amorphous thin films and Fe-rich/Cu multilayers

In this study we investigated the magnetic and transport properties of thin Fe-rich amorphous films and Fe-rich/Cu multilayers. We compared the extraordinary Hall effect in these two types of samples and discussed it in terms of thickness and sample structure. The thicker films exhibited a strong in-plane magnetic anisotropy, and by decreasing film thickness both saturated Hall resistivity and Hall sensitivity increase. A Hall resistivity value of 20 μΩ cm is observed in 100 nm thick Fe-rich films at 12 K and a sensitivity of 1.3 Ω/T is obtained at room temperature. Electrical conductance increases and Hall resistivity decreases when the films are sandwiched with Cu.