Unsupported Thin Films Research Articles

Quantification of Unsupported Thin-Film X-ray Spectra Using Bulk Standards.

A quantification model which uses standard X-ray spectra collected from bulk materials to determine the composition and mass thickness of single-layer and multilayer unsupported thin films is presented. The multivariate model can be iteratively solved for single layers in which each element produces at least one visible characteristic X-ray line. The model can be extended to multilayer thin films in which each element is associated with only one layer. The model may sometimes be solved when an element is present in multiple layers if additional information is added in the form of independent k-ratios or model assumptions. While the algorithm is suitable for any measured k-ratios, it is particularly well suited to energy-dispersive X-ray spectrometry where the bulk standard spectra can be used to deconvolve peak interferences in the thin-film spectra. The algorithm has been implemented and made available in the Open Source application National Institute of Standards and Technology DTSA-II. We present experimental data and Monte Carlo simulations supporting the quantification model.

Devitrification reduces beam-induced movement in cryo-EM.

As cryo-EM approaches the physical resolution limits imposed by electron optics and radiation damage, it becomes increasingly urgent to address the issues that impede high-resolution structure determination of biological specimens. One of the persistent problems has been beam-induced movement, which occurs when the specimen is irradiated with high-energy electrons. Beam-induced movement results in image blurring and loss of high-resolution information. It is particularly severe for biological samples in unsupported thin films of vitreous water. By controlled devitrification of conventionally plunge-frozen samples, the suspended film of vitrified water was converted into cubic ice, a polycrystalline, mechanically stable solid. It is shown that compared with vitrified samples, devitrification reduces beam-induced movement in the first 5 e Å-2 of an exposure by a factor of ∼4, substantially enhancing the contribution of the initial, minimally damaged frames to a structure. A 3D apoferritin map reconstructed from the first frames of 20 000 particle images of devitrified samples resolved undamaged side chains. Devitrification of frozen-hydrated specimens helps to overcome beam-induced specimen motion in single-particle cryo-EM, as a further step towards realizing the full potential of cryo-EM for high-resolution structure determination.

Carbon Supported Ni-Mo Catalysts for Reversible Alkaline Hydrogen Electrochemistry

Alkaline anion exchange membrane (AAEM) electrolyzers are attractive for cost-effective production of hydrogen using carbon-free electricity as the energy input. These devices facilitate the use of non-precious metal catalysts and ancillary components, which together offer significant reduction in capital cost. When combined with the low and still rapidly decreasing cost of renewable electricity, AAEM electrolyzers have the potential to meet the cost target of $2/kg for hydrogen production set by the US Department of Energy. Despite this promise, considerable work remains to develop efficient and stable catalysts and membranes for use under alkaline operating conditions. In this context, non-precious Ni-based catalysts are among the most promising for use as hydrogen-evolving cathodes. Our focus is on Ni-Mo composites, which have been studied and deployed for decades as alkaline hydrogen evolution catalysts, but the associated reaction mechanism remains relatively less explored.We recently showed that Ni-Mo catalysts comprise a Ni-rich metallic core surrounded by Mo-rich oxide shell, where the electrically resistive shell limits the overall catalytic activity in unsupported thin films. By contrast, incorporation of conductive carbon black supports significantly enhances catalyst mass activity toward the alkaline hydrogen evolution reaction (HER) without measurably altering the composition and structure of the active Ni-Mo composite. More recently, we have directly observed the evolution of the core-shell architecture in Ni-Mo catalysts using in situ environmental transmission electron microscopy (ETEM). These measurements suggest that thermal reduction of NiMoO4 precursors proceeds in at least two sequential events. When combined with HER catalytic activity measurements, the ETEM data show that the most competent Ni-Mo catalyst requires intimate contact between a metallic, Ni-rich face-centered cubic alloy phase and a Mo-rich suboxide phase. Carbon supported Ni-Mo catalyst composites were also found to be active for the alkaline hydrogen oxidation reaction (HOR), which lends to their potential use in unitized reversible fuel cell systems. Preliminary results suggest that the Ni-Mo mass loading relative to the carbon support has a stronger influence of its HOR activity than for the HER, suggesting that mass transfer plays a significant role in the practical HOR activity.

Long Lifetime and Coupling of Acoustic Vibrations of Gold Nanoplates on Unsupported Thin Films.

Radiation losses due to energy transfer to acoustic waves in the surroundings lead to short vibrational lifetimes of plasmonic nanoresonators and severely limits their applications in quantum technologies and mass sensing. Here we demonstrate a general method for improving the acoustic vibration lifetimes of Au nanoplates (flat structures with edge lengths of several microns and thicknesses of 10-40 nm) by using unsupported thin films instead of thick substrates, which blocks the outgoing acoustic waves. We experimentally confirm the vibrational quality factor improvement of Au nanoplates on Si3N4 and Al2O3/Si3N4 thin films with thicknesses less than 100 nm. Through this substrate phonon engineering, we successfully observe strong vibrational coupling between two Au nanoplates. Furthermore, the increased vibrational lifetimes allow us to study the vibrations of the nanoplates relative to the substrate and, for stacked nanoplates, relative to each other. The frequencies of these relative vibrational modes are well described by a simple coupled mass model, which provides the effective spring constants between interfaces separated by surface bonded molecules. We also demonstrate that the continuous thin films can be used for small plasmonic nanostructures (dimensions of several hundred nm), with similar increases in the vibrational lifetimes.

Theory of linear sweep voltammetry with diffuse charge: Unsupported electrolytes, thin films, and leaky membranes.

Linear sweep and cyclic voltammetry techniques are important tools for electrochemists and have a variety of applications in engineering. Voltammetry has classically been treated with the Randles-Sevcik equation, which assumes an electroneutral supported electrolyte. In this paper, we provide a comprehensive mathematical theory of voltammetry in electrochemical cells with unsupported electrolytes and for other situations where diffuse charge effects play a role, and present analytical and simulated solutions of the time-dependent Poisson-Nernst-Planck equations with generalized Frumkin-Butler-Volmer boundary conditions for a 1:1 electrolyte and a simple reaction. Using these solutions, we construct theoretical and simulated current-voltage curves for liquid and solid thin films, membranes with fixed background charge, and cells with blocking electrodes. The full range of dimensionless parameters is considered, including the dimensionless Debye screening length (scaled to the electrode separation), Damkohler number (ratio of characteristic diffusion and reaction times), and dimensionless sweep rate (scaled to the thermal voltage per diffusion time). The analysis focuses on the coupling of Faradaic reactions and diffuse charge dynamics, although capacitive charging of the electrical double layers is also studied, for early time transients at reactive electrodes and for nonreactive blocking electrodes. Our work highlights cases where diffuse charge effects are important in the context of voltammetry, and illustrates which regimes can be approximated using simple analytical expressions and which require more careful consideration.

A novel mesophase formed by top-shaped molecules in the bulk and unsupported thin films: A molecular dynamics study

We have used molecular dynamics simulations to investigate the ordering of top-shaped molecules in bulk phases and in unsupported thin films. Each rigid anisotropic molecule was composed of 11 Lennard-Jones interaction centers (beads). In an attempt to enhance the nematic stability in preference to smectic, the three central beads were assigned a larger Lennard-Jones diameter than the tail beads, giving the molecule a shape resembling a top. The molecular model was found to exhibit an unusual bulk mesophase with long-range orientational order and with molecular center-of-mass positions arranged in parallel interdigitated layers, with layer spacing smaller than half the length of the long axis of a molecule. However, despite the toplike molecular shape, no nematic phase was observed in the pressure range studied. Unsupported films of the isotropic liquid were cooled in order to locate a triple point between the novel mesophase, vapor, and isotropic liquid. At temperatures slightly above the triple point, enhanced surface ordering of molecules was found to occur in the unsupported film. At temperatures slightly below the triple point, the preferred molecular alignment in the unsupported film was parallel to the interface, in violation of arguments that have been proposed based on the relative enthalpies of various cleavage planes for close-packed structures.

Microstructure Development in Unsupported Thin Films

To better understand the role of the substrate in the microstructural evolution of thin films, unsupported nanocrystalline yttrium-stabilized zirconia (ZrO2:16%Y or YSZ) films were examined as a function of temperature and annealing time. Grain growth, texturing, and pinhole formation were measured using transmission electron microscopy (TEM) and electron diffraction. Films were produced and subsequently annealed on metallic grids using a previously developed technique that results in near full density films at low annealing temperatures. Microstructural evolution in these films was unique compared with constrained films. Grains were found to spheroidize much more readily, ultimately resulting in the formation of porosity and pinholes. Grain growth was found to stagnate at a size particular to each annealing temperature, presumably due to the effects of Zener pinning. It is proposed that the lack of substrate strain and confinement effects allows for the dominance of surface energetics with respect to microstructural evolution.

Development of pendant drop mechanical analysis as a technique for determining the stress–relaxation and water‐permeation properties of interfacially polymerized barrier layers

AbstractInterfacial polymerization (IP) involves the formation of solid polymeric films at the interface between aqueous and immiscible organic solutions via an interfacial polycondensation reaction between complementary monomeric reactants present in the two solutions. IP films are very thin (<0.25 μm) and form effective barrier layers in interfacially polymerized thin‐film composite membranes. Considerable difficulties are encountered in applying conventional characterization techniques to such unsupported IP thin films. This article describes the development of a novel technique, pendant drop mechanical analysis (PDMA), which can be used to study the mechanical and transport behavior of unsupported IP films. Experiments were conducted on films formed in a PDMA apparatus via the IP polymerization of m‐phenylene diamine and trimesoyl chloride (TMC). Stress–relaxation data obtained via PDMA were fitted with the Williams–Watts equation, and the results indicated a statistically significant dependence of the model parameters on the TMC concentration. Permeation experiments also demonstrated a statistically significant dependence of the membrane constant on the TMC concentration. The results provide unique insights regarding the relationship between structure and performance in unsupported IP films and suggest that network formation is enhanced in a concentration range of 0.1–0.3 wt % TMC. Although refinements are required, PDMA appears to be a promising technique for identifying optimum IP reaction conditions and assessing corresponding mechanical and transport characteristics. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2618–2628, 2003

Experimental determination of the surface ionisation in electron probe microanalysis

An experimental procedure is described for measuring the surface ionisation values ϕ(o) using thin-film measurements on a wide variety of substrates. The ϕ(o) values are determined by establishing the ratios of the film element X-ray intensities emitted from supported and unsupported thin films for a number of different film thicknesses and extrapolating towards a film thickness approaching zero. A number of 180 ϕ(o) data each for AlKα and PdLα X-radiations between 4 and 30kV, on substrates ranging from beryllium up to bismuth, have been collected in this way. The purpose of this work was to provide a systematic database on which a variety of existing expressions for ϕ(o) could be tested. In the final assessment procedure, in which we also included 108 data from literature, we compared the performance of the various expressions from literature for ϕ(o), as well as a newly developed one of our own. Our final conclusion is that there is little to be chosen between the three highest-ranking expressions.

Monte Carlo simulation of the interaction of electrons with supported and unsupported thin films

When an electron beam impinges on a solid, some particles of the beam come back and emerge from the target surface. These electrons are known as backscattered electrons. Due to their penetration below the surface and the resulting loss of small amounts of energy through ionisations, atomic electron excitations and plasmon emissions, the backscattered electrons are not reflected without dissipation of energy. A fraction of their energy is indeed lost in the solid before emerging. The electrons which are not backscattered are absorbed or transmitted. The sum of the absorbed, backscattered and transmitted electrons is equal to the total number of impinging electrons. The fractions of absorbed, backscattered and transmitted electrons depend on the film thickness, on the electron primary energy and on the material constituting the film. When the target is a surface film (i.e. a thin film deposited on a substrate of a different material) the number of backscattered electrons depends on the film thickness, on the electron primary energy and on the materials constituting the film and the substrate. In simulating the processes of penetration and backscattering of electrons impinging on supported thin films, with the Monte Carlo method, we have to take into account the interface between the film and the substrate along with the change in the scattering probabilities per unit length in passing from the film to the substrate and vice versa. This paper describes a Monte Carlo code written to simulate keV electrons interacting with solid targets, supported and unsupported thin films. Monte Carlo results concerning 5–20 keV electrons backscattered from various solid targets, unsupported thin films and films deposited on different substrates are then shown and compared with experimental and theoretical data.

Synthesis and Microstructural Characterization of Unsupported Nanocrystalline Zirconia Thin Films

A polymeric precursor spin‐coating technique is illustrated in which yttrium‐stabilized zirconia (YSZ) thin films are produced on Si, Al2O3, and NaCl at temperatures less than 350°C. High‐resolution transmission electron microscopy (HRTEM) examinations show that the YSZ films are nanocrystalline (grain size of less than 5 nm), fully dense, and have a stabilized cubic fluorite structure. Using the polymeric precursor spin coating method, unsupported nanocrystalline thin films of YSZ with thicknesses ranging from 30 to 1000 nm are prepared by transferring the films from a host substrate to metallic TEM grids with unsupported areas exceeding 1 mm2.

Structure and magnetism in thin films and multilayers of hexagonal ruthenium and iron

The results of ab initio calculations for the electronic structure and the magnetic moments of unsupported hexagonal thin Ru and Fe films, bulk Ru and Fe metals, and ${\mathrm{Ru}}_{5}{\mathrm{Fe}}_{5}$ multilayer are presented with the focus on the crystallographic phase stability of the Fe layers. The calculated equilibrium lattice parameters of hcp Ru and Fe solids reproduce accurately the values found experimentally. For Ru/Fe multilayers it is demonstrated that in an Fe part of a multilayer an unusual hexagonal stacking with a distorted bcc-like local environment is more stable than the standard hexagonal close-packed stacking. The same result was obtained in thin Fe films and it is shown to be associated with band structure effects of Fe at the Fermi level. For thin films the ground state lattice parameters and magnetic states of films up to four monolayers were determined by total energy minimization. The faults in hexagonal planes stacking and interdiffusion do not lead to nonmagnetic Fe layers at the interfaces which were observed experimentally.

Optical system for rapid materials characterization with the transient grating technique: Application to nondestructive evaluation of thin films used in microelectronics

This letter describes a noncontact, automated means for nondestructively measuring the physical (elastic, thermal, electronic, optical) properties of bulk samples, surfaces, supported or unsupported thin films, and multilayer assemblies. The method, which is based on the transient grating technique, uses specially constructed beam-shaping optics to manipulate excitation and probe laser beams for initiating and detecting motions in a sample. We illustrate the approach by determining the thicknesses and mechanical properties of metal films commonly used in microelectronics.

The effect of grain size on domain structure in unsupported thin films

The effect of grain size on domain structure in unsupported thin films with grains (70 - 250 nm) was studied by using TEM. It is found that there probably exists a critical grain size near 200 nm, at which an abrupt change in domain structure occurs. Above this size, domains, which are lamellar microtwins, appear in almost all of the coarse grains. The domain width decreases with grain size. Below this size, however, most of the grains are single-domain ones, while some irregular and curved domain walls appear only in a small portion of the fine grains. The number of the single-domain grains increases with decreasing grain size. Such a grain-size dependence of domain structure is correlated with the anomalous behaviours of the coercive field and dielectric constant in ferroelectric thin films. This interesting result has not been reported before.

45178 A new ultrasonic method for measuring elastic moduli in unsupported thin films: application to CuPd superlattices : Moreau, A.; Ketterson, J.B.; Davis, B. Journal of Applied Physics, vol. 68, no. 4, pp. 1622–1628 (15 Aug. 1990)

45178 A new ultrasonic method for measuring elastic moduli in unsupported thin films: application to CuPd superlattices : Moreau, A.; Ketterson, J.B.; Davis, B. Journal of Applied Physics, vol. 68, no. 4, pp. 1622–1628 (15 Aug. 1990)

Backscattering of electrons from multilayers.

The multiple reflection method of Schmidt [Ann. Phys. (Leipzig) 23, 671 (1907)] has been developed to study the general case of backscattering of electrons from multilayers. Exact expressions have been given for the absorption, backscattering, and transmission of electrons impinging on multilayers which reduce to previously deduced equations concerning supported and unsupported thin films.

Noninvasive Real-Time Evaluation of the Anisotropic Thermal Diffusivity in Thin Polymer Films for Electronics Packaging

AbstractWe describe an experimental method capable of evaluating the in- and out-of-plane components of the anisotropic thermal diffusivity in supported and unsupported thin polymer films. The technique is used to quantify the in-plane thermal diffusivity in films of PMDA/ODA (Dupont's P12545), BTDA/ODA/MPD (Dupont's P12555), BPDA/PPD (Dupont's P12611), BCB (Dow's Cyclotene 3022) and HFDA-APBP (Amoco's UD4212) with thicknesses in the 1–10 micron range.

Monte Carlo simulation of backscattered electrons and energy from thick targets and surface films

Electron-matter interaction is described by a Monte Carlo procedure in which the mean free path is calculated by using a screened Rutherford formula, while energy loss is computed by using the Kanaya and Okayama semiempirical expression. Monte Carlo simulation results of the backscattering coefficient have been compared with the available experimental data. The examined energy range was 5--30 keV, the atomic number range was 4--92, and the tilt angle range was 0\ifmmode^\circ\else\textdegree\fi{}--80\ifmmode^\circ\else\textdegree\fi{}. The agreement between simulated and experimental data is found to be excellent in the energy range 10--30 keV; also for 5 keV the agreement is very good when the atomic number is lower than 50. Then the mean backscattered energy was computed for bulk targets, unsupported thin films, and surface films.

On the elastic behavior of composition-modulated superlattices

Atomistic computer simulations are used to systematically investigate the role of interfacial disorder on the elastic behavior of composition-modulated superlattices of fcc metals, represented by simple Lennard–Jones potentials. The structures, energies, and average elastic properties of four types of superlattices with various degrees of interfacial disorder are computed as a function of the modulation wavelength along [001]. The four superlattice types studied include perfectly coherent, incoherent, and two types derived from these by introducing relative twists about [001] between alternating layers. A 20% lattice-parameter mismatch between the two modulating materials is assumed. Results are compared with our earlier work on unsupported thin films, grain-boundary superlattices, and incoherent superlattices with a 10% lattice-parameter mismatch. The degree of structural disorder at the interfaces is found to correlate well with the magnitude of the elastic anomalies, which cannot be accounted for by anisotropic lattice-parameter changes alone. The grain-boundary superlattices studied earlier are found to provide a good model limit for the elastic behavior of interfacially disordered dissimilar-material superlattices.

A new ultrasonic method for measuring elastic moduli in unsupported thin films: Application to Cu-Pd superlattices

We have developed a new ultrasonic method for measuring the phase velocity of longitudinal and shear waves in self-supporting thin films. The film is bonded at one end to an interdigital transducer (IDT) which emits a continuous wave signal. The film is supported at the other end and maintained flat under a weak tension. A receiving IDT detects the in-phase and quadrature components of the sound waves as they propagate. A liquid bond is used between the receiving IDT and the film to allow a continuous variation of the path length. From the longitudinal and shear wave velocities, we can deduce the flexural and shear moduli of the film. For films of uniaxial or higher symmetry, the Young’s and biaxial moduli can be expressed as combinations of the flexural and shear moduli. We applied this technique to Cu-Pd composition modulated thin films. Contrary to previous reports, we have not observed the supermodulus effect in either the flexural, shear, Young’s or biaxial moduli for modulation wavelengths between 13 and 36 Å.