Texture-dependent twin formation in nanocrystalline thin Pd films

Nanocrystalline Pd films were produced by electron-beam evaporation and sputter deposition. The electron-beam-evaporated films reveal randomly oriented nanograins with a relatively high density of growth twins, unexpected in view of the high stacking fault energy of Pd. In contrast, sputter-deposited films show a clear 〈1 1 1〉 crystallographic textured nanostructure without twins. These results provide insightful information to guide the generation of microstructures with enhanced strength/ductility balance in high stacking fault energy nanocrystalline metallic thin films.     

Hydrogen behavior in nanocrystalline titanium thin films

Nanocrystalline titanium films of different thicknesses, sputtered on sapphire substrates, were charged electrochemically with hydrogen. Hydrogen absorption and the thermodynamics of the nanocrystalline Ti–H thin film system were studied using electromotive force (EMF) measurements. The phase boundaries obtained from the EMF–pressure–concentration curves were confirmed by X-ray diffraction, complemented by in situ stress measurements during hydrogen charging. The change in the stress increase with hydrogen concentration was found to be in good agreement with the obtained phase boundaries. In comparison to bulk Ti–H system, considerable changes, such as shifted phase boundaries, and narrowed and sloped miscibility gaps, were observed in Ti–H thin films. These changes vary among the films of different crystalline orientation and are attributed to both microstructural effects and stress contributions. The influence of the initial crystallographic growth orientation of Ti films on the measured thermodynamic isotherms, phase transitions and stress development is discussed in detail.     

Effect of Pd doping on the microstructure and gas-sensing performance of nanoporous SnOx thin films

Pristine and Pd-doped nanoporous SnO_x thin films were fabricated via a sol–gel route. The Pd-doped film exhibited enhanced H₂ gas-sensing performance, in terms of higher sensitivity and shorter response time. Structural characterization was performed to investigate the effect of Pd doping on the microstructure evolution of the films. The grain and pore size of Pd-doped film, as measured using transmission electron microscopy and grazing-incidence small-angle X-ray scattering (GISAXS), are both smaller than those of undoped film. In particular, the pore size evolution of the films during annealing was quantitatively monitored in situ using synchrotron-based GISAXS. Knudsen gas diffusion and depletion layer models were employed to evaluate the microstructure influence on the gas sensitivity semi-quantitatively. The results suggest that the microstructure of the Pd-doped film is critical for improving the gas sensitivity but cannot account for the total sensitivity enhancement, implying other mechanisms could play a more important role.     

A corrosion study of nanocrystalline copper thin films

Thin nanocrystalline, compact films, based on the copper–nitrogen system, up to 2.5 μm thickness and 3.5% nitrogen, were deposited by magnetron sputtering at different partial pressure ratios of N₂ and Ar, without formation of Cu_xN compounds, the nitrogen concentration influencing grain size (down to 30 nm) and film homogeneity. Electrochemical corrosion properties were investigated using polarization curves and electrochemical impedance spectroscopy in 0.5 M NaCl aqueous solution, and compared with pure bulk copper; morphology was examined by scanning electron microscopy. Significant variations in corrosion currents between samples were attributed to grain size and structural defects on the grain boundaries.     

Modeling the effects of impurities on microstructure formation in nanocrystalline nickel thin films

The Atomic Kinetic Lattice Monte Carlo method was used to model the effects of a clustering impurity species on the deposition of nanocrystalline nickel films onto (001) copper. The deposition model was partly parameterized by physical inputs from atomistic calculations of copper-nickel alloys. The grain size and shape is shown to depend on the concentration of impurities in the deposited film. Low concentrations yield elongated columnar grains, while higher concentrations produce much smaller and more equiaxed grains. For more information, contact Corbett C. Battaile, Sandia National Laboratories, P.O. Box 5800, MS 1411, Albuquerque, NM 87111, USA; (505) 844-7039; fax (505) 844-9781; e-mail ccbatta@sandia.gov.     

Studies of photovoltaic properties of nanocrystalline thin films of CdS-CdTe

Nanocrystals of CdS and CdTe were synthesized by aqueous chemical route. From the optical absorption spectra the particle sizes (diameter) were estimated to be around 7 and 4 nm for CdS and CdTe, respectively. The photovoltaic device was fabricated using these nanocrystalline materials on an indium tin oxide (ITO) coated glass substrate using a spin coating method. From the room temperature photoluminescence study a drastic quenching of photoluminescence the CdS-CdTe thin film was observed. Light intensity dependent current-voltage measurements of CdS-CdTe thin film shows photovoltaic effect; with increase in light intensity the current density increases, however, the open circuit voltage does not show any change. The low efficiency of the device has been explained on the basis of the defects and diffusion of Te ions into CdS.     

Microstructure and grain growth kinetics of nanocrystalline SnO2 thin films prepared by pulsed laser deposition

The isothermal grain growth of SnO₂ thin films prepared by pulsed laser deposition techniques was investigated at Si (100) substrate temperatures between 300 and 450 °C with 50 °C intervals for different annealing times. X-ray diffraction patterns proved that the average grain sizes are in the range of 2.4–27.8 nm. The grain growth data were analyzed using two different models. The first model, assuming normal grain growth as that in conventional polycrystalline materials, yields large grain growth exponent (n) and extremely low activation energy (Q). Although it can describe the evolution of grain sizes, it fails to give satisfactory physical interpretation of n and Q, both beyond the theoretical predictions. The second model is based on the structural relaxation of the interface component in nanocrystalline materials. In this case, the ordering of distorted interfaces by structural relaxation proceeds with grain growth. This structure relaxation model not only describes the evolutions of grain growth well, but also makes reasonable attribution of the low activation energy to the short-range rearrangement of atoms in the interface region as well.     

Origins of microstructure and stress gradients in nanocrystalline thin films: The role of growth parameters and self-organization

The development of depth gradients of texture, morphology and stresses in thin nanocrystalline films was experimentally demonstrated for a nanocrystalline CrN film by means of position-resolved synchrotron X-ray nanodiffraction and explained by atomistic processes at the growing film surface and the effect of interfaces, both controlled by the deposition conditions. Controllable changes in the energy of incident particles adjusted by bias voltages ranging from ?40 to ?120 V affect the competitive growth of grains with different orientations, induce disruption of grain growth and thus give rise to structural variations across the film thickness. Subsequent changes in the volume fraction of grain boundaries and film texture were found to be responsible for changes in the residual stress state as defect generation proceeds to different extents in the interior of differently oriented grains and in the interfacial area. While the defect density predominantly affects the development of intrinsic stress, the variation in the number of weakly bonded atoms of grain boundaries determines the thermal stress component. The structural dependence of both stress components thus contributes to the characteristic development of stress gradients in thin nanocrystalline films.     

On plastic strain recovery in freestanding nanocrystalline metal thin films

In a recent article [J. Rajagopalan, J.H. Han, M.T.A. Saif, Science 315 (2007) 1831–1834], we have reported substantial (50–100%) plastic strain recovery in freestanding nanocrystalline metal films (grain size 50–65 nm) after unloading. The strain recovery was time dependent and thermally activated. Here we model the time evolution of this strain recovery in terms of a thermally activated dislocation propagation mechanism. The model predicts an activation volume of ≈42b³ for the strain recovery process in aluminum.     

Amorphous and nanocrystalline sputtered Mg–Cu thin films

Binary Mg–Cu amorphous alloys were first fabricated in 1980s via liquid quenching. In this study, the Mg_1−xCu_x (x varying from 38 at.% to 82 at.%) partially amorphous thin films are prepared via co-sputtering. Upon thermal annealing, the Mg₂Cu or MgCu₂ nanocrystalline phases are induced in the Mg-rich or Cu-rich thin films, respectively. Due to the presence of fine nanocrystalline Mg₂Cu or MgCu₂ particles in the Mg–Cu amorphous matrix, the as-sputtered thin films show satisfactory Young's modulus 100 GPa and hardness 4 GPa.     

甲烷体积分数对纳米金刚石薄膜形貌的影响

目的研究不同甲烷体积分数对纳米金刚石(NCD)薄膜生长的影响,实现较小晶粒尺寸、高平整度的NCD薄膜的制备。方法采用微波等离子体增强化学气相沉积的方法制备NCD薄膜,以CH4/H2为气源,在生长阶段控制其他条件不变的前提下,探讨不同甲烷体积分数对NCD晶粒尺寸、表面形貌以及表面粗糙度的影响。采用SEM、XRD等观测NCD薄膜的表面形貌和晶粒尺寸大小,并利用Raman对NCD薄膜的不同散射峰进行分析。结果随着甲烷体积分数的增加,薄膜晶粒尺寸有减小的趋势。甲烷体积分数较低时,晶形比较完整,但致密度较小;甲烷体积分数较高时,晶形杂乱无章,但致密度较好。当甲烷体积分数为9%时NCD薄膜平均粒径达到最小,为21.3 nm,表面粗糙度较好,但非晶金刚石成分开始大量生成,NCD薄膜质量开始变差;当甲烷体积分数为8%时其形貌最好,且此时最小表面粗糙度小于20 nm。通过Raman分析可知NCD薄膜中出现了硅峰和石墨烯特征峰。结论甲烷体积分数对NCD薄膜形貌有较大影响,甲烷体积分数为8%时是表面平整度由较差变好再逐渐变差的分界点,且平均晶粒尺寸为23.6 nm,薄膜表面具有较好的平整度。     

Microstructures and electrical conductance of silver nanocrystalline thin films on flexible polymer substrates

Using sputtering technique, silver nanocrystalline thin films were deposited on two different polymer substrates, namely pretreated nylon and polyester fabrics, under identical conditions. It is found that the difference of the electrical conductance between the two samples is quite large. The highest conductivity obtained from the coated nylon fabric is 2.4 × 10⁷ Sm^− 1, indicating the feasibility to fabricate wearable electrodes using silver coated fabrics. However, the electrical conductance of the coated polyester fibre is found to be lower by six orders of magnitude. Transmission electron microscopy reveals that the silver grains coated on the polyester fibre are spatially separated while the silver grains coated on the nylon fibre are connective. The pre-deposition surface conditions of the fibres are suggested to be responsible to the remarkable differences in the electrical conductance and microstructures of the silver nanocrystalline films.     

Plasma polymerization of amine-containing thin films and the studies on the deposition kinetics

Plasma-polymerized thin films prepared from amine-containing saturated (propylamine) and unsaturated (allylamine and propargylamine) precursors were investigated in terms of the deposition rate, polymerization kinetics and thin film morphology as function of the applied power. Moreover, a mathematical model was established to correlate the physicochemical properties to the deposition kinetics under plasma polymerizations. It was found that the thin film deposition rate was primarily dictated by the amine bond dissociation energy (NH₂ BDE) of precursors. On the other hand, surface chemistry (N/C ratio, surface imine and amine contents) guided the growth of L-929 fibroblast cells on the prepared amine-functionalized surfaces. A close relationship among the deposition kinetics, the physicochemical and biological properties of the deposited amine-containing plasma polymers was presented.     

A comparison of the strength of multilayers,thin films and nanocrystalline compacts

A survey of the yield strength and hardness of copper-based materials shows a progressive lowering of the strength with respect to the extrapolation of the Hall–Petch relation as the length scale of the microstructure decreases. The overall trend can be modeled by scaling the elastic screening length for the dislocation line tension with the microstructural length scale, as proposed by Scattergood and Koch.     

Buckling and post-buckling kinetics of compressed thin films on viscous substrates

Compressively stressed films and islands can buckle provided their substrates can flow or creep. A linear-stability theory, based upon plate theory, is developed which determines the onset, rate of growth and wavelength of the buckling instability for compressively-stressed elastic films on finite-thickness viscous substrates. Although the condition for the onset of the buckling instability of the film on a glass layer is the same as that for a compressively-stressed free-standing film, the instability of the film on the viscous substrate grows slowly, with a typically long characteristic time. The linear stability analysis is extended to include the effects of interfacial shear stresses and shear deformation of the film within a rigorous linear elasticity calculation. The corrections to the original critical unstable wavelength and maximally unstable mode are found to be minimal, especially for small to moderate values of the compressive stress. The role of substrate elasticity is shown to have a higher order effect in modifying the growth rate. An approximate non-linear theory is developed that predicts the saturation of the buckling instability at intermediate times, followed by a long time coarsening of the buckling wavelength and a decrease of the stress within the film. The present analysis provides the tools necessary for designing stress relaxation strategies.     

Growth kinetics of plasma deposited microcrystalline silicon thin films

A continuum model for the nucleation and growth of microcrystalline silicon thin films from SiH₄/H₂ discharges is presented. The simulation considers mass balances and surface coverage with adspecies and silicon clusters up to the size where they can be considered as thermodynamically stable. The model is combined to a plasma gas phase simulator and a simulator of thin film morphology and is used for studying the growth differences in two different regions, the center and the edge of a 30 × 30 cm² substrate. The predictions for the film growth rate, film crystallinity and surface roughness in both regions are presented and discussed together with the main processes governing nucleation and growth and the slow step for stable nanocrystal formation.