Metallic Nanofilms Research Articles

Interplay between quantum size effect and strain effect on growth of nanoscale metal thin films

We develop a theoretical framework to investigate the interplay between quantum size effect (QSE) and strain effect on the stability of metal nanofilms. The QSE and strain effect are shown to be coupled through the concept of "quantum electronic stress. First-principles calculations reveal large quantum oscillations in the surface stress of metal nanofilms as a function of film thickness. This adds extrinsically additional strain-coupled quantum oscillations to surface energy of strained metal nanofilms. Our theory enables a quantitative estimation of the amount of strain in experimental samples, and suggests strain be an important factor contributing to the discrepancies between the existing theories and experiments.

Atom-atom interactions in continuous metallic nanofilms

Physical reasons of the existence of the nanosystem-size dependence of the potential energy of the neighboring atoms are considered, and a thermodynamic validation of this dependence is given. Solid nanofilms of monoatomic metallic systems having an fcc structure are simulated by the molecular-statics method with the Morse and Sutton-Chen potentials.

Non-Fourier heat conduction study for steady states in metallic nanofilms

As a fundamental theory of heat transfer, Fourier’s law is valid for most traditional conditions. Research interest in non-Fourier heat conditions is mainly focused on heat wave phenomena in non-steady states. Recently, the thermomass theory posited that, for steady states, non-Fourier heat conduction behavior could also be observed under ultra-high heat flux conditions at low ambient temperatures. Significantly, this is due to thermomass inertia. We report on heat conduction in metallic nanofilms from large currents at low temperatures; heat fluxes of more than 1×1010 W m−2 were used. The measured average temperature of the nanofilm is larger than that based on Fourier’s law, with temperature differences increasing as heat flux increased and ambient temperature decreased. Experimental results for different film samples at different ambient temperatures reveal that non-Fourier behavior exists in metallic nanofilms in agreement with predictions from thermomass theory.

Piezoquartz resonator as an in situ method for studying the phase transitions in thin metal and alloy films

A new method based on a quartz resonator is proposed to study the melting-solidification phase transformations in metal and alloy nanofilms. The melting-solidification temperature hysteresis is studied, and the supercoolings upon the solidification of bismuth and gold-germanium films on a carbon substrate are determined. The results obtained by the new method are shown to agree well with the reported data.

Optical constants of Ni nanofilms

A previously proposed ellipsometric technique for determining the refractive indices, extinction indexes, and thicknesses of metal nanofilms at optical measurements in air has been implemented for nickel films. The optical constants found for nickel, n2 = 2.04 ± 0.05 and k2 = 3.75 ± 0.05 (almost = 632.8 nm), are in agreement with the results of measurements in high vacuum. The technique proposed makes it possible to measure film thicknesses in the range of 10–30 nm with an error as small as 1 nm. However, for nickel films thinner than 10 nm, the experimental data differ significantly and deviate more strongly from the calculated values, in contrast to titanium films. Possible reasons for the discrepancy between the calculated and experimental data are discussed.

Nonlinear Plasmon-Photon Interaction Resolved byk-Space Spectroscopy

Metallic nanostructures support extreme localization and enhancement of optical fields via surface-plasmon (SP) resonances. Although SP are associated with giant enhancements of nonlinear phenomena such as second-harmonic generation (SHG), the role of SP in the process, whether as a field-enhancing catalyst or as a quasiparticle converted in the interaction, has remained experimentally elusive. We demonstrate how k-space spectroscopy can distinguish between the plasmonic and photonic SHG processes that occur in a metal nanofilm when it is optically driven via the Kretschmann geometry. The results revealed a nonlinear interaction where two SP annihilate to create a second-harmonic photon. This knowledge has implications for realizing the inverse process, plasmonic parametric down-conversion, which could act as a coherent source of entangled SP pairs.

Control synthesis and numerical analysis of gold nanofilms after electric-field-assisted, one-step deposition

Gold nanofilms consisting of networked, cross-linked nanowires were synthesised in large quantities by the integration of electrochemical deposition and dielectrophoresis. The resultant products were characterised by scanning electron microscopy and atomic force microscopy. On the basis of finite-element modelling of the electrode system in terms of the theory of dielectrophoresis and electromagnetism, the authors found that the growth of gold nanowires responded highly to the inhomogeneous electric fields. Also, the morphologies of the gold nanofilm evolve with the deposition time and frequency. Numerical simulations and theoretical analyses offer useful insights into the mechanism. The above study provides strong evidence that this electrochemical and template-free route is a powerful and universal technique to the synthesis for improving nanofilms performance. This strategy can be transplanted into the synthesis of other metallic nanofilms, such as silver and copper.

Thermodynamic modeling of particle formation and reshaping in metallic catalyst nanofilms for carbon nanotube growth

In this study a model based on thermodynamics was derived to calculate the critical and stable sizes of various elemental catalyst nanoparticles (Mo, Fe, Co, Ni and Au) formed from their nanofilms for the growth of carbon nanotubes. In the first step, the modeling of heterogeneous transformation of films into nanoparticles was considered based on the size dependence of Gibbs free energy changes. At the second step, the reshaping of nanoparticles to nanorods was obtained based on the Laplace–Young equation. The proposed model was compared with the existing experimental data. The results of this study were in good agreement with the available experimental data.

Digital image correlation of metal nanofilms on SU-8 for flexible electronics and MEMS

This paper presents strain measurements using digital image correlation of common microelectronic metal nanofilms deposited onto a polymer substrate (SU-8), which has applications in flexible electronics and nano/microsystems reliability analysis. The novel experimental method is based on digital image correlation coupled with microtensile test apparatus for the in situ investigation of the deformation behaviour of the deposited thin films under uniaxial tensile loading. One of the key features of the method is the real-time two-dimensional strain field measurements on a bare thin film surface, during the deformation process, without any initial speckle or grid deposition. The outstanding performances of the method, having a spatial resolution of 0.7 µm, allow one to envisage further studies related to the understanding of the mechanical behaviour of such thin films and, in particular, the damage localization process.

Quantum theory of the electromagnetic response of metal nanofilms

We develop a quantum theory of electron confinement in metal nanofilms. The theory is used to compute the nonlinear response of the film to a static or low-frequency external electric field and to investigate the role of boundary conditions imposed on the metal surface. We find that the sign and magnitude of the nonlinear polarizability depends dramatically on the type of boundary condition used.

Investigation of thermomechanical responses in ultrafast laser heating of metal nanofilms

The present work uses two-temperature model and Cattaneo's constitutive model to study the thermomechanical responses and the size effect on energy transport during ultrafast laser heating of Au nanofilms. It is shown that the grain size effects on thermophysical properties, heat transport and thermal stresses are rather evident when the average grain diameter is less than the electron mean free path. The study on heat transport shows that there are two heat waves propagating in the lattice during nonequilibrium heating of films. It is found that the classical thermoelastic theory can not reveal the generation of the ultrafast thermal stresses during nonequilibrium heating; however, it can approximately describe the stress evolution in the stage of thermal equilibrium.

Measurements of electron—phonon coupling factor and interfacial thermal resistance of metallic nano-films using a transient thermoreflectance technique

Using a transient thermoreflectance (TTR) technique, several Au films with different thicknesses on glass and SiC substrates are measured for thermal characterization of metallic nano-films, including the electron-phonon coupling factor G, interfacial thermal resistance R, and thermal conductivity Ks of the substrate. The rear heating-front detecting (RF) method is used to ensure the femtosecond temporal resolution. An intense laser beam is focused on the rear surface to heat the film, and another weak laser beam is focused on the very spot of the front surface to detect the change in the electron temperature. By varying the optical path delay between the two beams, a complete electron temperature profile can be scanned. Different from the normally used single-layer model, the double-layer model involving interfacial thermal resistance is studied here. The electron temperature cooling profile can be affected by the electron energy transfer into the substrate or the electron-phonon interactions in the metallic films. For multiple-target optimization, the genetic algorithm (GA) is used to obtain both G and R. The experimental result gives a deep understanding of the mechanism of ultra-fast heat transfer in metals.

Metallic glass nanofilms

In this paper, we report for the first time the spontaneous formation of Zr-based metallic glass nanofilms by developed dynamic forced-shear-rupture technique of hat-shaped specimens. The obtained nanofilms have about 100 nm thickness and other two geometrical dimensions can reach micrometer scales. Their glassy nature and structural stability were solidly identified. It was found that electrons with the wavelength of less than 0.165 Å could make the metallic glass nanofilms transparent. Furthermore, it is clearly shown that shear-banding instability still afflicts such 100-nm-thick metallic glass nanofilms.

Theoretical and experimental study on the heat transport in metallic nanofilms heated by ultra-short pulsed laser

In this paper the mechanism of heat transport in metallic nanofilms under ultra-short pulsed laser heating is examined theoretically and experimentally. In order to easily understand the non-equilibrium heat transport in metallic nanofilms the study of heat transport behavior is first carried out in dielectrics. The analyses indicate that there may be two kinds of wave phenomena in dielectrics subjected to a periodic surface temperature. One is the thermal wave governed by the C–V model based hyperbolic equation and the other is the diffusive wave governed by the Fourier model based parabolic equation. According to the hyperbolic two step model for non-equilibrium heat transport, such two kinds of wave phenomena can also occur simultaneously in the metallic nanofilms under pulsed laser heating, where the diffusive wave is induced by the electron temperature oscillation at the surface due to the non-equilibrium between electrons and lattices. Unlike the propagation speed of the thermal wave, the propagation speed of the diffusive wave depends not only on the medium properties but also the period of the temperature oscillation at the boundary. Hence, the propagation speed of the diffusive wave in the electron gas may be of as high as 10 6 m s −1, when the laser pulse duration is less than 1 ps. A transient thermoreflectance (TTR) system has been built to measure the transient electron temperature responses caused by the femtosecond laser heating and a pump–probe technique is used to ensure the femtosecond temporal resolution in the experiments. Different from the commonly used front heating-front detecting (FF) method for measuring the material properties, a rear heating-front detecting (RF) method is applied, so that measuring the propagation speed of heat becomes available. The non-equilibrium heat diffusion model is used to fit the measured normalized electron temperature profiles of 27.2 nm, 39.9 nm and 55.5 nm Au films. The best-fitted coupling factor G basically agrees with the theoretical value 2.3 × 10 16 W m −3 K −1. The propagation speed of the diffusive wave in the electron gas can be obtained by comparing the measured delay time of peak electron temperatures of Au films with different thicknesses. The average propagation speed of the temperature oscillation or diffusive wave in Au films for the range of thickness from 27.2 nm to 55.5 nm is equal to 8.1 × 10 5 m s −1, which is close to the value predicted by the non-equilibrium heat diffusion model.

Shell structure and phase relations in electronic properties of metal nanowires from an electron-gas model

The electronic and dynamic properties of metal nanowires are analyzed by using a minimal electron-gas model (EGM), in which the nanowire is treated as a close system with variable Fermi energy as a function of nanowire radius. We show that the planar surface energy and the curvature energy from the EGM are reasonably consistent with those from previous stabilized-jellium-model calculations, especially for metals with low electron densities. The EGM shell structure due to the fillings of quantum-well subbands is similar to that from the stabilized jellium model. The crossings between subbands and Fermi energy level for the metal nanowire correspond to cusps on the chemical-potential curve versus nanowire radius, but inflection points on the surface-free-energy curve versus the radius, as in the case of metal nanofilms. We also find an oscillatory variation in electron density versus radius at the nanowire center with a global oscillation period which approximately equals half Fermi wavelength. Wire string tension, average binding energy, and thermodynamic stability from the EGM are in good agreement with the data from previous first-principles density-functional theory calculations. We also compare our model with those from previous reported free-electron models, in which the nanowire is treated as an open system with a constant Fermi energy. We demonstrate that the fundamental thermodynamic properties depend sensitively on the way that the potential wall is constructed in the models.

Donor–acceptor nonradiative energy transfer mediated by surface plasmons on ultrathin metallic films

Selected properties of donor–acceptor energy transfer in the presence of surface plasmon coupled emission (SPCE) on metallic nanofilms are demonstrated. These properties of surface plasmon mediated energy transfer (SPMET) are for the first time compared to those of traditional energy transfer (ET) based on the same donor–acceptor system. The presence of plasmons significantly accelerates energy transfer as revealed by the results of fluorescence intensity decay. In particular, the rise time of acceptor fluorescence intensity upon donor excitation is 10 times shorter in the presence of SPCE. It is also observed that contrary to ET the sensibilized acceptor emission in SPMET is totally linearly polarized.

Erratum: Quantum size effects in metal nanofilms: Comparison of an electron-gas model and density functional theory calculations [Phys. Rev. B80, 155404 (2009)

Erratum: Quantum size effects in metal nanofilms: Comparison of an electron-gas model and density functional theory calculations [Phys. Rev. B<b>80</b>, 155404 (2009)

Hollow nanocylinder: Multisubband superconductivity induced by quantum confinement

Quantization of the transverse electron motion in high-quality superconducting metallic nanowires and nanofilms results in the formation of well-distinguished single-electron subbands. They shift in energy with changing thickness, which is known to cause quantum-size superconducting oscillations. The formation of multiple subbands results in a multigap structure induced by the interplay between quantum confinement and Andreev mechanism. We investigate multisubband superconductivity in a hollow nanocylinder by numerically solving the Bogoliubov-de Gennes equations. When changing the inner radius and thickness of the hollow nanocylinder, we find a crossover from an irregular pattern of quantum-size superconducting oscillations, typical of nanowires, to an almost regular regime, specific for superconducting nanofilms. At this crossover the multigap structure becomes degenerate. The ratio of the critical temperature to the energy gap increases and approaches its bulk value while being reduced by $20--30\text{ }\mathrm{%}$ due to Andreev-type states driven by quantum confinement in the irregular regime.

Measurement of the electroconductivity of nonmagnetic metallic nanofilms at ultra-high frequencies

It is shown that the electroconductivity of nonmagnetic metallic films may be measured by means of coaxial and waveguide impedance measurement instruments.

Vavilov-Cherenkov like effect in metal nanofilms

The so-called surface plasmon polaritons, i.e., natural waves with a low phase velocity (much lower than the speed of light in a vacuum), exist in silver, gold, and copper nanofilms and nanowires. Electrons that are relatively slow in comparison with those that emit Cherenkov light in a homogeneous medium produce plasmons. The dispersion relations for the corresponding plasmons and the emission angles of plasmons with corresponding frequencies are calculated. It is shown that devices based on detecting Cherenkov light in nanofilms and nanowires can be used to detect low-energy electrons.