Effect of organometallic clamp properties on the apparent diversity of tensile response of nanowires

Mechanical Characterization of Two-Segment Free-Standing ZnO Nanowires Using Lateral Force Microscopy

Mechanical characterization of quasi one-dimensional nanostructures is essential for the design of novel nanoelectromechanical systems. However, the results obtained on basic mechanical quantities, such as Young's modulus and fracture strength, show significant standard deviation in the literature. This is partly because of diversity in the quality of the nanowire, and partly because of inappropriately performed mechanical tests and simplified mechanical models. Here we present orientation-controlled bending and fracture studies on wet chemically grown vertical ZnO nanowires, using lateral force microscopy. The lateral force signal of the atomic force microscope was calibrated by a diamagnetic levitation spring system. By acquiring the bending curves of 14 nanowires, and applying a two-segment mechanical model, an average bending modulus of 108 ± 17 GPa was obtained, which was 23% lower than the Young's modulus of bulk ZnO in the [0001] direction. It was also found that the average fracture strain and stress inside the nanowire was above 3.1 ± 0.3 % and 3.3 ± 0.3 GPa, respectively. However, the fracture of the nanowires was governed by the quality of the nanowire/substrate interface. The demonstrated technique is a relatively simple and productive way for the accurate mechanical characterization of vertical nanowire arrays.

Extracting information from noisy data: Strain mapping during dynamic in-situ SEM experiments

Micromechanical testing techniques can reveal a variety of characteristics in materials that are otherwise impossible to address. However, unlike to macroscopic testing, these miniaturized experiments are more challenging to realize and analyze, as loading and boundary conditions can often not be controlled to the same extent as in standardized macroscopic tests. Hence, exploiting all possible information from such an experiment seems utmost desirable. In the present work, we utilize dynamic in-situ microtensile testing of a nanocrystalline equiatomic CoCrFeMnNi high entropy alloy in conjunction with initial feature tracking to obtain a continuous two-dimensional strain field. This enables an evaluation of true stress-strain data as well as of the Poisson's ratio and allows to study localization of plastic deformation for the specimen. We demonstrate that the presented image correlation method allows for an additional gain of information in these sophisticated experiments over commercial tools and can serve as a starting point to study deformation states exhibiting more complex strain fields.

Multiplicity of dislocation pathways in a refractory multiprincipal element alloy

Refractory multiprincipal element alloys (MPEAs) are promising materials to meet the demands of aggressive structural applications, yet require fundamentally different avenues for accommodating plastic deformation in the body-centered cubic (bcc) variants of these alloys. We show a desirable combination of homogeneous plastic deformability and strength in the bcc MPEA MoNbTi, enabled by the rugged atomic environment through which dislocations must navigate. Our observations of dislocation motion and atomistic calculations unveil the unexpected dominance of nonscrew character dislocations and numerous slip planes for dislocation glide. This behavior lends credence to theories that explain the exceptional high temperature strength of similar alloys. Our results advance a defect-aware perspective to alloy design strategies for materials capable of performance across the temperature spectrum.

Advances in mechanical characterization of 1D and 2D nanomaterials: progress and prospects

Last several decades have sparked a tremendous interest in mechanical properties of low dimensional systems specifically 1D and 2D nanomaterials, in large, due to their remarkable behavior and potential to possess unique and customizable physical properties, which have encouraged the fabrication of new structures to be tuned and utilized for targeted applications. In this critical review we discuss examples that represent evolution of the mechanical characterization techniques developed for 1D and 2D nanomaterials, with special emphasis on specimen fabrication and manipulation, and the different strategies, tools and metrologies, employed for precise positioning and accurate measurements of materials’ strength, elastic modulus, fracture toughness as well as analysis of failure modes. We focus separately on techniques for the mechanical characterization of 1D and 2D nanomaterials and categorize those methods into top-down and bottom-up approaches. Finally, we discuss advantages and some drawbacks in most common methodologies used for 1D and 2D specimen testing and outline future possibilities and potential paths that could boost the development of more universal approaches for technologically viable solutions which would allow for more streamlined and standardized mechanical testing protocols to be developed and implemented.

Atypical Defect Motions in Brittle Layered Sodium Titanate Nanowires

In situ tensile tests show atypical defect motions in the brittle Na₂Ti₃O₇ (NTO) nanowire (NW) within the elastic deformation range. After brittle fracture, elastic recovery of the NTO NW is followed by reversible motion of the defects in a time-dependent manner. An in situ cyclic loading-unloading test shows that these mobile defects shift back and forth along the NW in accordance with the loading-unloading cycles and eventually restore their initial positions after the load is completely removed. The existence of the defects within the NTO NWs and their motions does not lead to plastic deformation of the NW. The atypical defect motion is speculated to be the result of the glidibility of the TiO₆ layers, where weakly bonded cation layers are in between. Exploration of the above novel observation can establish new understandings of the deformation behavior of superlattice nanostructures.

In situ Bragg coherent X-ray diffraction during tensile testing of an individual Au nanowire

Systematic tensile tests were performed on single defect-free 〈110〉 Au nanowires grown by physical vapor deposition while simultaneously recording three-dimensional Bragg peaks using coherent X-rays. The trajectory of three-dimensional Bragg peaks in reciprocal space during tensile testing allowed for measurements of the evolution of strains and rotations of the nanowire, thus sensitively uncovering the full deformation geometry of the nanowire. The transition from elastic to plastic deformation is accompanied by rotations of the nanowire as quantified by analysis of the motion of Bragg peaks, showing the importance of boundary conditions in interpreting nanoscale mechanical deformations.

Growth and nanomechanical characterization of nanoscale 3D architectures grown via focused electron beam induced deposition

Nanomechanical measurements of platinum-carbon 3D nanoscale architectures grown via focused electron beam induced deposition (FEBID) were performed using a nanoindentation system in a scanning electron microscope (SEM) for simultaneous in situ imaging. Compression tests were used to estimate the modulus of the platinum-carbon deposits to be in the range of 8.6-10.5 GPa. Cantilever arm bend tests resulted in a modulus estimation of 15.6 GPa. Atomic layer deposition was used to conformally coat FEBID structures with a thin film of Al₂O₃, which strengthened the structures and increased the measured modulus. Cycled load-displacement testing at various load rates of nano-truss structures was also performed, demonstrating a viscoelastic response in the FEBID material. Finally, load-displacement tests of a variety of 3-dimensional nanoarchitectures with and without Al₂O₃ coatings were measured.

In situ thermomechanical testing methods for micro/nano-scale materials

The advance of micro/nanotechnology in energy-harvesting, micropower, electronic devices, and transducers for automobile and aerospace applications has led to the need for accurate thermomechanical characterization of micro/nano-scale materials to ensure their reliability and performance. This persistent need has driven various efforts to develop innovative experimental techniques that overcome the critical challenges associated with precise mechanical and thermal control of micro/nano-scale specimens during material characterization. Here we review recent progress in the development of thermomechanical testing methods from miniaturized versions of conventional macroscopic test systems to the current state of the art of in situ uniaxial testing capabilities in electron microscopes utilizing either indentation-based microcompression or integrated microsystems. We discuss the major advantages/disadvantages of these methods with respect to specimen size, range of temperature control, ease of experimentation and resolution of the measurements. We also identify key challenges in each method. Finally, we summarize some of the important discoveries that have been made using in situ thermomechanical testing and the exciting research opportunities still to come in micro/nano-scale materials.

Measuring surface dislocation nucleation in defect-scarce nanostructures

Linear defects in crystalline materials, known as dislocations, are central to the understanding of plastic deformation and mechanical strength, as well as control of performance in a variety of electronic and photonic materials. Despite nearly a century of research on dislocation structure and interactions, measurements of the energetics and kinetics of dislocation nucleation have not been possible, as synthesizing and testing pristine crystals absent of defects has been prohibitively challenging. Here, we report experiments that directly measure the surface dislocation nucleation strengths in high-quality 〈110〉 Pd nanowhiskers subjected to uniaxial tension. We find that, whereas nucleation strengths are weakly size- and strain-rate-dependent, a strong temperature dependence is uncovered, corroborating predictions that nucleation is assisted by thermal fluctuations. We measure atomic-scale activation volumes, which explain both the ultrahigh athermal strength as well as the temperature-dependent scatter, evident in our experiments and well captured by a thermal activation model.

Strain- and defect-mediated thermal conductivity in silicon nanowires

The unique thermal transport of insulating nanostructures is attributed to the convergence of material length scales with the mean free paths of quantized lattice vibrations known as phonons, enabling promising next-generation thermal transistors, thermal barriers, and thermoelectrics. Apart from size, strain and defects are also known to drastically affect heat transport when introduced in an otherwise undisturbed crystalline lattice. Here we report the first experimental measurements of the effect of both spatially uniform strain and point defects on thermal conductivity of an individual suspended nanowire using in situ Raman piezothermography. Our results show that whereas phononic transport in undoped Si nanowires with diameters in the range of 170-180 nm is largely unaffected by uniform elastic tensile strain, another means of disturbing a pristine lattice, namely, point defects introduced via ion bombardment, can reduce the thermal conductivity by over 70%. In addition to discerning surface- and core-governed pathways for controlling thermal transport in phonon-dominated insulators and semiconductors, we expect our novel approach to have broad applicability to a wide class of functional one- and two-dimensional nanomaterials.

Temperature controlled tensile testing of individual nanowires

We present a novel experimental method for quantitatively characterizing the temperature-dependent mechanical behavior of individual nanostructures during uniaxial straining. By combining a microelectromechanical tensile testing device with a low thermal mass and digital image correlation providing nm-level displacement resolution, we show successful incorporation of a testing platform in a vacuum cryostat system with an integrated heater and temperature control. Characterization of the local sample temperature and time-dependent response at both low and high temperature demonstrates a testing range of ∼90-475 K and steady-state drift rates less than 0.04 K/min. In situ operation of the tensile testing device employing resistively heated thermal actuators while imaging with an optical microscope enables high-resolution displacement measurements, from which stress-strain behavior of the nanoscale specimens is deduced. We demonstrate the efficacy of our approach in measuring the temperature dependence of tensile strength in nominally defect-free ⟨110⟩ Pd nanowhiskers. We uncover a pronounced sensitivity of the plastic response to testing temperature over a range of ∼300 K, with an ultimate strength in excess of 6 GPa at low temperature. The results are discussed in the context of thermally activated deformation mechanisms and defect nucleation in defect-free metallic nanostructures.

Quantitative in situ TEM tensile fatigue testing on nanocrystalline metallic ultrathin films

A unique technique to perform quantitative in situ transmission electron microscopy (TEM) fatigue testing on ultrathin films and nanomaterials is demonstrated. The technique relies on a microelectromechanical system (MEMS) device to actuate a nanospecimen and measure its mechanical response. Compared to previously demonstrated MEMS-based in situ TEM techniques, the technique takes advantage of two identical capacitive sensors on each side of the specimen to measure electronically elongation (with nm resolution) and applied force (with μN resolution). Monotonic and fatigue tests were performed on nanocrystalline gold ultrathin film specimens that were manipulated and fixed onto the MEMS device without the use of a focused ion-beam microscope (and therefore, importantly, without any associated surface damage). The major advantage of the technique is its capability to use TEM imaging solely for high magnification microstructural observations while the MEMS device provides continuous tracking of the material's response, thereby expanding the capabilities of MEMS-based techniques towards more complex in situ TEM nanomechanical tests, such as fatigue tests.