Ravi Agrawal

Elasticity Size Effects in ZnO Nanowires−A Combined Experimental-Computational Approach

Understanding the mechanical properties of nanowires made of semiconducting materials is central to their application in nano devices. This work presents an experimental and computational approach to unambiguously quantify size effects on the Youngs modulus, E, of ZnO nanowires and interpret the origin of the scaling. A micromechanical system (MEMS) based nanoscale material testing system is used in situ a transmission electron microscope to measure the Youngs modulus of [0001] oriented ZnO nanowires as a function of wire diameter. It is found that E increases from approximately 140 to 160 GPa as the nanowire diameter decreases from 80 to 20 nm. For larger wires, a Youngs modulus of approximately 140 GPa, consistent with the modulus of bulk ZnO, is observed. Molecular dynamics simulations are carried out to model ZnO nanowires of diameters up to 20 nm. The computational results demonstrate similar size dependence, complementing the experimental findings, and reveal that the observed size effect is an outcome of surface reconstruction together with long-range ionic interactions.

Giant Piezoelectric Size Effects in Zinc Oxide and Gallium Nitride Nanowires. A First Principles Investigation

Nanowires made of materials with noncentrosymmetric crystal structure are under investigation for their piezoelectric properties and suitability as building blocks for next-generation self-powered nanodevices. In this work, we investigate the size dependence of piezoelectric coefficients in nanowires of two such materials - zinc oxide and gallium nitride. Nanowires, oriented along their polar axis, ranging from 0.6 to 2.4 nm in diameter were modeled quantum mechanically. A giant piezoelectric size effect is identified for both GaN and ZnO nanowires. However, GaN exhibits a larger and more extended size dependence than ZnO. The observed size effect is discussed in the context of charge redistribution near the free surfaces leading to changes in local polarization. The study reveals that local changes in polarization and reduction of unit cell volume with respect to bulk values lead to the observed size effect. These results have strong implication in the field of energy harvesting, as piezoelectric voltage output scales with the piezoelectric coefficient.

Experimental-Computational Investigation of ZnO nanowires Strength and Fracture

An experimental and computational approach is pursued to investigate the fracture mechanism of [0001] oriented zinc oxide nanowires under uniaxial tensile loading. A MEMS-based nanoscale material testing stage is used in situ a transmission electron microscope to perform tensile tests. Experiments revealed brittle fracture along (0001) cleavage plane at strains as high as 5%. The measured fracture strengths ranged from 3.33 to 9.53 GPa for 25 different nanowires with diameters varying from 20 to 512 nm. Molecular dynamic simulations, using the Buckingham potential, were used to examine failure mechanisms in nanowires with diameters up to 20 nm. Simulations revealed a stress-induced phase transformation from wurtzite phase to a body-centered tetragonal phase at approximately 6% strain, also reported earlier by Wang et al. (1) The transformation is partial in larger nanowires and the transformed nanowires fail in a brittle manner at strains as high as 17.5%. The differences between experiments and computations are discussed in the context of (i) surface defects observed in the ZnO nanowires, and (ii) instability in the loading mechanism at the initiation of transformation.

Effect of Growth Orientation and Diameter on the Elasticity of GaN Nanowires. A Combined in Situ TEM and Atomistic Modeling Investigation

We characterized the elastic properties of GaN nanowires grown along different crystallographic orientations. In situ transmission electron microscopy tensile tests were conducted using a MEMS-based nanoscale testing system. Complementary atomistic simulations were performed using density functional theory and molecular dynamics. Our work establishes that elasticity size dependence is limited to nanowires with diameters smaller than 20 nm. For larger diameters, the elastic modulus converges to the bulk values of 300 GPa for c-axis and 267 GPa for a- and m-axis.

Multiscale Experiments: State of the Art and Remaining Challenges

In this article we review recent advances in experimental techniques for the mechanical characterization of materials and structures at various length scales with an emphasis in the submicron- and nanoregime. Advantages and disadvantages of various approaches are discussed to highlight the need for carefully designed experiments and rigorous analysis of experimentally obtained data to yield unambiguous findings. By examining in depth a few case studies we demonstrate that the development of robust and innovative experimentation is crucial for the advancement of theoretical frameworks, assessment of model predictive capabilities, and discovery of new physical phenomena.

Large-scale density functional theory investigation of failure modes in ZnO nanowires.

Electromechanical and photonic properties of semiconducting nanowires depend on their strain states and are limited by their extent of deformation. A fundamental understanding of the mechanical response of individual nanowires is therefore essential to assess system reliability and to define the design space of future nanowire-based devices. Here we perform a large-scale density functional theory (DFT) investigation of failure modes in zinc oxide (ZnO) nanowires. Nanowires as large as 3.6 nm in diameter with 864 atoms were investigated. The study reveals that pristine nanowires can be elastically deformed to strains as high as 20%, prior to a phase transition leading to fracture. The current study suggests that the phase transition predicted at approximately 10% strain in pristine nanowires by the Buckingham pairwise potential (BP) is an artifact of approximations inherent in the BP. Instead, DFT-based energy barrier calculations suggest that defects may trigger heterogeneous phase transition leading to failure. Thus, the difference previously reported between in situ electron microscopy tensile experiments (brittle fracture) and atomistic simulations (phase transition and secondary loading) (Agrawal, R.; Peng, B.; Espinosa, H. D. Nano Lett. 2009, 9 (12), 4177-2183) is elucidated.

An energy-based model to predict wear in nanocrystalline diamond atomic force microscopy tips

Atomic force microscopy (AFM) is one of the most powerful techniques to probe surfaces and material properties at the nanoscale, and pattern organic and inorganic molecules. In all cases, knowledge of the tip geometry and its evolution with continued use is essential. In this work, a broadly applicable energy model for the evolution of scanning probe tip radii during use is presented based on quantitative wear experiments. Experiments were conducted using AFM probes made of both undoped and nitrogen-doped diamond. Undoped diamond probes were found to be nearly ten times more wear resistant than commercially available silicon nitride probes. For a constant applied force, a linear relationship between wear volume and total dissipation energy is identified. The change in tip radius was also found to be proportional to the square root of scan distance, x0.5.Atomic force microscopy (AFM) is one of the most powerful techniques to probe surfaces and material properties at the nanoscale, and pattern organic and inorganic molecules. In all cases, knowledge of the tip geometry and its evolution with continued use is essential. In this work, a broadly applicable energy model for the evolution of scanning probe tip radii during use is presented based on quantitative wear experiments. Experiments were conducted using AFM probes made of both undoped and nitrogen-doped diamond. Undoped diamond probes were found to be nearly ten times more wear resistant than commercially available silicon nitride probes. For a constant applied force, a linear relationship between wear volume and total dissipation energy is identified. The change in tip radius was also found to be proportional to the square root of scan distance, x0.5.