S. A. Syed Asif

Nanoindentation and contact stiffness measurement using force modulation with a capacitive load-displacement transducer.

We have implemented a force modulation technique for nanoindentation using a three-plate capacitive load-displacement transducer. The stiffness sensitivity of the instrument is ∼0.1 N/m. We show that the sensitivity of this instrument is sufficient to detect long-range surface forces and to locate the surface of a specimen. The low spring mass (236 mg), spring constant (116 N/m), and damping coefficient (0.008 Ns/m) of the transducer allows measurement of the damping losses for nanoscale contacts. We present the experimental technique, important specimen mounting information, and system calibration for nanomechanical property measurement.

Quantitative imaging of nanoscale mechanical properties using hybrid nanoindentation and force modulation

In this article, we present a quantitative stiffness imaging technique and demonstrate its use to directly map the dynamic mechanical properties of materials with nanometer-scale lateral resolution. For the experiments, we use a “hybrid” nanoindenter, coupling depth-sensing nanoindentation with scanning probe imaging capabilities. Force modulation electronics have been added, enhancing instrument sensitivity and enabling measurements of time dependent materials properties (e.g., loss modulus and damping coefficient) not readily obtained with quasi-static indentation techniques. Tip–sample interaction stiffness images are acquired by superimposing a sinusoidal force (∼1 μN) onto the quasi-static imaging force (1.5–2 μN), and recording the displacement amplitude and phase as the surface is scanned. Combining a dynamic model of the indenter (having known mass, damping coefficient, spring stiffness, resonance frequency, and modulation frequency) with the response of the tip–surface interaction, creates maps o...

Ultrahigh stress and strain in hierarchically structured hollow nanoparticles

Nanocrystalline materials offer very high strength but are typically limited in their strain to failure, and efforts to improve deformability in these materials are usually found to be at the expense of strength. Using a combination of quantitative in situ compression in a transmission electron microscope and finite-element analysis, we show that the mechanical properties of nanoparticles can be directly measured and interpreted on an individual basis. We find that nanocrystalline CdS synthesized into a spherical shell geometry is capable of withstanding extreme stresses (approaching the ideal shear strength of CdS). This unusual strength enables the spherical shells to exhibit considerable deformation to failure (up to 20% of the spheres diameter). By taking into account the structural hierarchy intrinsic to novel nanocrystalline materials such as this, we show it is possible to achieve and characterize the ultrahigh stresses and strains that exist within a single nanoparticle during deformation.

Mechanics and Dynamics of the Strain-Induced M1–M2 Structural Phase Transition in Individual VO2 Nanowires

The elastic properties and structural phase transitions of individual VO(2) nanowires were studied using an in situ push-to-pull microelectromechanical device to realize quantitative tensile analysis in a transmission electron microscope and a synchrotron X-ray microdiffraction beamline. A plateau was detected in the stress-strain curve, signifying superelasticity of the nanowire arising from the M1-M2 structural phase transition. The transition was induced and controlled by uniaxial tension. The transition dynamics were characterized by a one-dimensionally aligned domain structure with pinning and depinning of the domain walls along the nanowire. From the stress-strain dependence the Youngs moduli of the VO(2) M1 and M2 phases were estimated to be 128 ± 10 and 156 ± 10 GPa, respectively. Single pinning and depinning events of M1-M2 domain wall were observed in the superelastic regime, allowing for evaluation of the domain wall pinning potential energy. This study demonstrates a new way to investigate nanoscale mechanics and dynamics of structural phase transitions in general.

In situ nanomechanical testing in focused ion beam and scanning electron microscopes.

The recent interest in size-dependent deformation of micro- and nanoscale materials has paralleled both technological miniaturization and advancements in imaging and small-scale mechanical testing methods. Here we describe a quantitative in situ nanomechanical testing approach adapted to a dual-beam focused ion beam and scanning electron microscope. A transducer based on a three-plate capacitor system is used for high-fidelity force and displacement measurements. Specimen manipulation, transfer, and alignment are performed using a manipulator, independently controlled positioners, and the focused ion beam. Gripping of specimens is achieved using electron-beam assisted Pt-organic deposition. Local strain measurements are obtained using digital image correlation of electron images taken during testing. Examples showing results for tensile testing of single-crystalline metallic nanowires and compression of nanoporous Au pillars will be presented in the context of size effects on mechanical behavior and highlight some of the challenges of conducting nanomechanical testing in vacuum environments.

Incipient plasticity in metallic thin films

The authors have compared the incipient plastic behaviors of Al and Al–Mg thin films during indentation under load control and displacement control. In Al–Mg, solute pinning limits the ability of dislocations to propagate into the crystal and thus substantially affects the appearance of plastic instabilities as compared to pure Al. Displacement control allows for a more sensitive detection of such instabilities, as it does not require collective dislocation motion to the extent required by load-controlled indentation in order to resolve a yield event. This perception is supported by in situ transmission electron microscopy observations.

3D Porous Graphene by Low-Temperature Plasma Welding for Bone Implants

3D scaffolds of graphene, possessing ultra-low density, macroporous microstructure, and high yield strength and stiffness can be developed by a novel plasma welding process. The bonding between adjacent graphene sheets is investigated by molecular dynamics simulations. The high degree of biocompatibility along with high porosity and good mechanical properties makes graphene an ideal material for use as body implants.

Nanomechanical actuation from phase transitions in individual VO2 micro-beams

The mechanical response due to structural phase transitions in individual VO2 nanowires was quantitatively investigated in situ in a transmission electron microscope. The electron beam was employed to locally heat the nanowire, and stress-strain was measured in situ using a microfabricated push-to-pull device. Tensile loading was found to increase the metal-insulator transition temperature and decrease the insulator-insulator transition temperature, consistent with the phase diagram of VO2. These phase transitions resulted in an axial mechanical response of the VO2 nanowires, an effect that can potentially be used to actuate nanostructures or gauge the local temperature change induced by electron beam irradiation.

Atomically thin gallium layers from solid-melt exfoliation

A unique way to synthesize innovative 2D gallenene. Among the large number of promising two-dimensional (2D) atomic layer crystals, true metallic layers are rare. Using combined theoretical and experimental approaches, we report on the stability and successful exfoliation of atomically thin “gallenene” sheets on a silicon substrate, which has two distinct atomic arrangements along crystallographic twin directions of the parent α-gallium. With a weak interface between solid and molten phases of gallium, a solid-melt interface exfoliation technique is developed to extract these layers. Phonon dispersion calculations show that gallenene can be stabilized with bulk gallium lattice parameters. The electronic band structure of gallenene shows a combination of partially filled Dirac cone and the nonlinear dispersive band near the Fermi level, suggesting that gallenene should behave as a metallic layer. Furthermore, it is observed that the strong interaction of gallenene with other 2D semiconductors induces semiconducting to metallic phase transitions in the latter, paving the way for using gallenene as promising metallic contacts in 2D devices.

High Plastic Strain of Silica Microparticles under Electron Beam Irradiation

The studies of irradiation damage in silica are of significant interest because of its application in nuclear reactors, nuclear waste containers, optical fibers, and semiconductor devices [1,2]. Although there are a number of publication showing the effect of electrons, ions, protons, alpha-particles irradiation on microstructural changes of silica, understanding deformation behavior under applied stress of irradiated sample is still lacking [2,3]. In this work, we investigate plastic flow and failure behavior of amorphous silica particles under compressive stress inside a scanning electron microscopy (SEM).