Stiffened ultrathin Pt films confirmed by acoustic-phonon resonances

Study of Elastic and Structural Properties of BaFe2As2 Ultrathin Film Using Picosecond Ultrasonics

We obtain the through-thickness elastic stiffness coefficient (C33) in nominal 9 nm and 60 nm BaFe2As2 (Ba-122) thin films by using picosecond ultrasonics. Particularly, we reveal the increase in elastic stiffness as film thickness decreases from bulk value down to 9 nm, which we attribute to the increase in intrinsic strain near the film-substrate interface. Our density functional theory (DFT) calculations reproduce the observed acoustic oscillation frequencies well. In addition, temperature dependence of longitudinal acoustic (LA) phonon mode frequency for 9 nm Ba-122 thin film is reported. The frequency change is attributed to the change in Ba-122 orthorhombicity (a-b)/(a+b). This conclusion can be corroborated by our previous ultrafast ellipticity measurements in 9 nm Ba-122 thin film, which exhibit strong temperature dependence and indicate the structural phase transition temperature Ts.

Mapping stress in polycrystals with sub-10 nm spatial resolution

From aircraft to electronic devices, and even in Formula One cars, stress is the main cause of degraded material performance and mechanical failure in applications incorporating thin films and coatings. Over the last two decades, the scientific community has searched for the mechanisms responsible for stress generation in films, with no consensus in sight. The main difficulty is that most current models of stress generation, while atomistic in nature, are based on macroscopic measurements. Here, we demonstrate a novel method for mapping the stress at the surface of polycrystals with sub-10 nm spatial resolution. This method consists of transforming elastic modulus maps measured by atomic force microscopy techniques into stress maps via the local stress-stiffening effect. The validity of this approach is supported by finite element modeling simulations. Our study reveals a strongly heterogeneous distribution of intrinsic stress in polycrystalline Au films, with gradients as high as 100 MPa nm^-1 near the grain boundaries. Consequently, our study discloses the limited capacity of macroscopic stress assessments and standard tests to discriminate among models, and the great potential of nanometer-scale stress mapping.

Nanomechanical probing of the layer/substrate interface of an exfoliated InSe sheet on sapphire

Van der Waals (vdW) layered crystals and heterostructures have attracted substantial interest for potential applications in a wide range of emerging technologies. An important, but often overlooked, consideration in the development of implementable devices is phonon transport through the structure interfaces. Here we report on the interface properties of exfoliated InSe on a sapphire substrate. We use a picosecond acoustic technique to probe the phonon resonances in the InSe vdW layered crystal. Analysis of the nanomechanics indicates that the InSe is mechanically decoupled from the substrate and thus presents an elastically imperfect interface. A high degree of phonon isolation at the interface points toward applications in thermoelectric devices, or the inclusion of an acoustic transition layer in device design. These findings demonstrate basic properties of layered structures and so illustrate the usefulness of nanomechanical probing in nanolayer/nanolayer or nanolayer/substrate interface tuning in vdW heterostructures.

Rhombohedral distortion analysis of ultra-thin Pt(111) films deposited under Ar–N2atmosphere

A rhombohedral analysis method for analysing the lattice distortion in a (111)-textured face-centred cubic film under rotationally symmetric stress is proposed. Because no material constants, such as diffraction elastic constants, are required, the expressions of the distortion, namely the angle and the lattice parameter, are universal and can be readily used to compare different films. Using this rhombohedral distortion analysis method, (111)-textured Pt films deposited under argon–nitrogen atmosphere are systematically investigated, and the thickness-dependent lattice deformation in as-deposited and annealed films is described by the two geometrical parameters of the rhombohedral cell.

Coherent control of GHz resonant modes by an integrated acoustic etalon

By carefully tuning the thickness of a compliant thin film placed within an acoustic cavity, we achieve coherent control of the cavity's acoustic resonances, analogous to the operation of an optical etalon. This technique is demonstrated using a supported membrane oscillator in which multiple high-frequency harmonic resonances are simultaneously optoexcited by an ultrafast laser. Theoretical and computational methods are used to analyze the selective strengthening or suppression of these resonances by constructive or destructive interference.

Simultaneous observation of acoustic resonance phenomena at both surfaces of a plate coated with thin layers

Acoustic resonance phenomena at the front and back surfaces of a plate coated with thin layers were successfully observed in the amplitude spectrum of the back surface echo. The amplitude ratio of spectra with and without layers takes its maximum and minimum values at the resonant frequencies of the front and back coatings and both frequencies can clearly be distinguished from each other. As an application, the thicknesses of the front and back coatings on a steel plate were measured simultaneously using their resonant frequencies, thus verifying the validity of the principle.

Precision and accuracy in film stiffness measurement by Brillouin spectroscopy

The interest in the measurement of the elastic properties of thin films is witnessed by a number of new techniques being proposed. However, the precision of results is seldom assessed in detail. Brillouin spectroscopy (BS) is an established optical, contactless, non-destructive technique, which provides a full elastic characterization of bulk materials and thin films. In the present work, the whole process of measurement of the elastic moduli by BS is critically analyzed: experimental setup, data recording, calibration, and calculation of the elastic moduli. It is shown that combining BS with ellipsometry a fully optical characterization can be obtained. The key factors affecting uncertainty of the results are identified and discussed. A procedure is proposed to discriminate factors affecting the precision from those affecting the accuracy. By the characterization of a model transparent material, silica in bulk and film form, it is demonstrated that both precision and accuracy of the elastic moduli measured by BS can reach 1% range, qualifying BS as a reference technique.

Transverse shear oscillator investigation of boundary lubrication in weakly adhered films

We investigate the boundary lubrication in weakly adhered molecularly thin films deposited between a sphere and a plane, below the sliding threshold. The shear contact stiffness and interfacial dissipation at the micrometer scale are determined with a high-frequency quartz oscillator. Two distinct behaviors are found as a function of the shear oscillation: a linear viscoelastic response at low amplitude and a nonlinear frictional microslip at high amplitude. A friction model is proposed to analyze the data, which allows evaluating the shear strength, the friction coefficient, and the interfacial viscosity at different solid interfaces under low load.

Ultrathin-film oscillator biosensors excited by ultrafast light pulses

Novel thin-film oscillator biosensors are developed using picosecond ultrasound method. 100-nm silicon-nitride thin films and 16-nm Pt thin films are used, and ultrashort light pulses are focused on their surfaces to excite the through-thickness resonance vibrations, which are detected by the delayed probe-light pulses using the optoelastic effect. Their fundamental resonance frequencies are 45 and 132 GHz, corresponding to theoretical mass sensitivities of 5.0×10(-5) and 2.2×10(-5) pg/cm(2)/Hz, respectively. These thin-film biosensors are used for detecting human immunoglobulin G (hIgG) with Staphylococcus aureus protein A nonspecifically immobilized on the film surfaces. Injection of a 5 nM analyte caused 2% decrease in the resonance frequency.

Linearized forward and inverse problems of the resonant ultrasound spectroscopy for the evaluation of thin surface layers

In this paper, linearized approximations of both the forward and the inverse problems of resonant ultrasound spectroscopy for the determination of mechanical properties of thin surface layers are presented. The linear relations between the frequency shifts induced by the deposition of the layer and the in-plane elastic coefficients of the layer are derived and inverted, the applicability range of the obtained linear model is discussed by a comparison with nonlinear models and finite element method (FEM), and an algorithm for the estimation of experimental errors in the inversely determined elastic coefficients is described. In the final part of the paper, the linearized inverse procedure is applied to evaluate elastic coefficients of a 310 nm thick diamond-like carbon layer deposited on a silicon substrate.

Unusual elastic behavior of nanocrystalline diamond thin films

This Letter studies the relationship between the elastic constants and the microstructure of nanocrystalline diamond thin films deposited by the chemical vapor deposition method doping various concentration of N2 gas. The elastic constants were measured by resonant ultrasound spectroscopy and picosecond laser ultrasounds. The increase of N2 gas decreases the diagonal elastic constants, but increases the off-diagonal elastic constants. The micromechanics calculation can explain this unusual elastic behavior, and it predicts thin graphitic phases at grain boundaries.

Mechanism of elastic softening behavior in a superlattice

We measured the out-of-plane elastic constants C(33) of a Co/Pt superlattice by picosecond ultrasound, and found that they were closely related to the thickness ratio of Co and Pt layers; C(33) was smaller than the prediction from the bulk values except for a specific thickness ratio. This behavior can be explained by the weak bonding at the interfaces that occurs to reduce the elastic strain energy, not by the interfacial strain. This view explained the relationship among C(33), the elastic strain energy, and perpendicular magnetic anisotropy.