This talk will review collaborative efforts in characterizing the mechanical properties of graphene and related 2D materials. To fully characterize mechanical properties, we combine experimental tests consisting of nano-indentation of freely suspended membranes, with multiscale analysis that connects atomic-level density functional theory to nonlinear anisotropic continuum mechanics. These measurements have previously revealed that graphene is both ultrastiff and the strongest known material, with a strength that reflects the intrinsic breaking strength of the carbon lattice. We have more recently extended this technique to monolayer MoS2, polycrystalline CVD-grown graphene, and defective graphene. In addition, we are characterizing the strength of individual graphene grain boundaries of known tilt angle. I will also briefly discuss applications, such as flexible electronics and NEMS, that take advantage of this ultrahigh strength.

High special/energy resolution cathodoluminescence (CL) spectroscopy enables us to make precise investigation on the optical/electronic fine structures in nanostructures. The linear distribution of strain gradient from tensile to compression in bent ZnO nano/microwires provides ideal conditions to address the modification of the electronic structures by strain in semiconductor materials. Radial line scan of the CL spectroscopy along bent ZnO wires at liquid helium temperature shows very fine excitonic emission structures, which demonstrates systematic red shift towards the increase of tensile strain, and blue shift as well as excitonic peak splitting towards the increase of compressive strain. Strain-gradient is found to dominate the overall red-shift of the emission energy at a pure bending configuration.

The combination of deposition techniques in which self-assembly is observed and lithographic techniques has been widely used in recent electronic devices as well as in development research for future technologies. In the past years the deterministic releasing of thin films have attracted large interest due to the possibility of producing very thin membranes of a plethora of materials [1], as well as for the fabrication of micro- and nano-tubes that can be used as nano-reactors [2], resonant cavities [3], waveguides [4] and electronic devices [5]. In these structures the partial release of the layer strain also results in a change of the lattice configuration, modifying the material band structure and its optoelectronic properties [6, 7].
In general terms, rolled layers can be used to produce artificial radial super lattices of virtually any material combination, allowing for the integration of systems that are incompatible for high-quality conventional layer stacking upon deposition. In this work we have produced rolled-up structures in epitaxial layers of InGaAs / AlAs (HF etching sensitive) grown on a GaAs (001) substrate. On top of the InGaAs film, Cr layers with variable thickness are deposited by thermal evaporation, keeping a built-in strain on the metal/semiconductor interface. Lithographic trenches are defined for the selective etching of the sacrificial underlayer. The structures curl and produce tubes with well-defined radii. We use the resulting cylindrical geometry to probe elastic constants of the metal layers, as well as to submit them to fixed curvature status and induce controlled strain.
We are currently studying the elastic properties of additional very thin layers and nano-objects that may be transferred to the top of the metal films prior to rolling. Our intention is to retrieve reliable material constants and study properties that arise from such configuration.
Keywords: Rolled-up microtubes, strain release
[1] Y. F. Mei, G. S. Huang, et.al., Adv. Mater. 20, 4085 (2008).
[2] C. Deneke, N. Y. Jin-Phillipp, I. Loa, et. al., Appl. Phys. Lett. 84, 4475 (2004).
[3] K. Dietrich, Ch. Strelow, C. Schliehe, et.al., Nano Letters 10, 627 (2010).
[4] T. Kipp, H. Welsch, Ch. Strelow, Ch. Heyn, et.al., Phys.l Rev. Lett. 96, 077403 (2006).
[5] C. C. Bof Bufon, J. D. C. Gonzlez, D. J. Thurmer, et.al. Nano Lett. 10, 2506 (2010).
[6] A. Malachias, Ch. Deneke, B. Krause, C. Mocuta, et. al., Phys. Rev. B 79, 035301 (2009)
[7] Ch. Deneke, A. Malachias, S. Kiravittaya, et. al. Appl. Phys. Lett. 96, 143101 (2010)

While indium-tin oxide currently still tends to be considered as the most commonly used transparent conductive electrode, the anticipated need to replace indium by other earth-abundant metals has recently widened the spectrum of envisioned functional oxides for TCO applications, including thin film solar cells and flat panel displays. The main challenge in this search for the new "Holy Grail" is to arrive at a sufficiently large electrical conductivity while at the same time guaranteeing sufficient optical transparency in the visible and near-IR range. Tuning the mobility of charge carriers, rather than their concentration, has proven crucial in this respect [1]. In this work, we will report on scientific efforts aimed at quantifying the effect of elastic strain on both the electro-optical performance and the chemical reactivity of Al-doped zinc oxide (AZO).
The AZO thin films have been deposited by reactive magnetron sputtering upto a thickness ranging between 300 and 500 nm, both onto extra-clear glass substrates, as well as on oxidised (100) Si wafers. During deposition, the internal stress evolution in the growing film was monitored in-situ using high resolution (10 km^-1 every 0.1 sec) curvature measurements [2]. The resulting intrinsic growth-induced elastic strain, which was found to depend heavily on the oxygen partial pressure, could be further modulated by appropriately choosing the deposition temperature. The latter also induces an additional extrinsic thermal stress component, whose sign depends on the substrate used. As such, mean biaxial internal stresses could be achieved ranging from -600 MPa (compression) upto 800 MPa (tension).
The resulting charge carrier mobilities at room temperature, as obtained independently from Hall measurements, were found to range between 5 and 25 cm²/Vs. The maximum mobility was surprisingly obtained at the zero-stress condition, and together with a charge carrier concentration of about 10 10²⁰ cm^-3 gave rise to a record-low resistivity of 2.5 10^-4 Ohm.cm for our polycrystalline, (002) oriented films. From the stress-dependent optical transmission spectra in the range 200-1000 nm, the pressure coefficient of the optical bandgap was estimated from the corresponding Tauc plots to be 278 meV/GPa, indicative for a very high strain-sensitivity. Finally, on-line optical emission spectrometry was used to assess the reactivity of differently stressed films in 0.4% HCl media. In order to obtain a crater-like surface with high surface roughness, which is most beneficial for improved light trapping, mean compressive stresses turned out much more effective than tensile ones. At the same time, the sign of the internal stress was also found to induce significant differences in the etch rate evolution across the film thickness.
[1] K. Ellmer, J. Phys. D : Appl. Phys. 33 (2000) R17
[2] S. Michotte and J. Proost, Sol. Energ. Mat. & Sol. Cells 98 (2012) 253

The ideal strength of monolayer materials possessing semimetallic, semiconducting, and insulating ground states is computed using density functional theory. Previous work on graphene showed that a soft phonon mode at the K-point of the Brillouin zone limits the ideal strength under equibiaxial tension. Here we demonstrate that an analogous soft mode at the K-point exists in BN, graphane, and MoS₂ limiting the ideal strength of BN and MoS₂ under equibiaxial tension, while silicene does not exhibit such a mode. In BN and graphane the phonon instability corresponds to a Kekulé-like distortion similar to that of graphene, while MoS₂ has a distinct structural transformation in which 2/3 of the Mo sites distort towards trigonal pyramidal coordination. The phase transitions for BN, graphane, and MoS₂ are not accompanied by the opening of a band gap, which indicates that Fermi surface nesting is not a general explanation for phonon instabilities in strained monolayer materials. In the case of graphene the band gap opening decays with strain, illustrating the critical role of softening in components of the total energy other than band energy in driving the phonon instability.

Although the influence of geometry and defects on the electro-optical properties of nanostructures have been extensively studied, the effects of high mechanical strain levels have been largely overlooked up to now, as in situ tuning of such high mechanical strain levels has proven to be challenging. Using an electrostatically actuated nano-tensile testing device (EANAT), ultra-high strained nanostructures were realized, opening a new route for research on strained materials. In this paper we show the synthesis and monolithic integration of individual <111> oriented silicon nanowires in such EANAT devices via the vapor-liquid-solid (VLS) growth mechanism, enabling the application of pure tensile strain without any shear component while measuring the electrical and optical properties of such ultra-strained nanowires simultaneously. Strain levels, till to the fracture stress was achieved by varying the actuator voltage applied to the EANAT device. Confocal micro Raman spectroscopy was used to determine the strain level in the silicon nanowires. Therefore variations in the optical phonon frequency, in detail the downshifting of the first-order Stokes Raman band, are correlated to the strain in individual VLS grown silicon nanowires.
Even for strain amplitudes up to 3% no fatigue failures are observed for several hundred loading cycles.
For the maximum tensile strain the resistance of the Si nanowires decreased by an order of magnitude. The ability to fabricate single-crystal nanowires that are widely free of structural defects will it make possible to apply high strain without fracturing to other materials as well, therefore in any application where crystallinity and strain are important, the idea of making nanowires should be of a high value.

Single-crystalline ZnO NWs with uniform diameter, oriented along the c-axis of the wurtzite structure, were synthesized using chemical vapor deposition (CVD). The electrical conductance of an individual ZnO NW, which was subjected to constant voltage and consecutive uniaxial tensile tests, was measured using the four-probe method inside a transmission electron microscope (TEM). The electrical current was monitored with pico-ampere precision in real time. The in situ tensile straining was realized using a TEM nanoindenter coupled with a microfabricated push-to-pull (PTP) device, by which the force applied onto the NW was recorded (the tensile stress was then computed by dividing the load by the cross-sectional area). The strain was determined by measuring the distance between two carbon markers on the NW which were made using electron-beam assisted deposition. The stress-strain curve indicates that the ZnO NWs undergo pure elastic deformation before fracture. With the increase in the tensile strain and stress, the electrical conductance increased linearly. The piezoresistance coefficient of this ZnO NW was measured to be -1.86×10-10 Pa-1. Our results demonstrate that ZnO NWs have potential for measuring nano-Newton forces in force/pressure sensor applications.

This review will discuss history of strain Si and various methods of applying strain to Si including the use of SiGe virtual substrates, stress liners, ion implantation, embedded SiGe or Si:C etc. Implications of bi-axial vs uniaxial strain in current Si CMOS will be evaluated. The evolution of strain in Si CMOS since 90 nm node to current 22 nm node will be illustrated. It will become clear that straining Si is an integral part of performance scaling of CMOS. Even though initial application of strain Si was primarily limited to pFETs , adoption of strain in latest generation of nFETs will be examined in terms of its scaling. Strain scaling in pFETs via SiGe was somewhat straightforward, in that it could be achieved by simply increasing the Ge fraction in SiGe so long as critical thickness criteria was met but this is much harder to extend in nFETs which rely primarily on C substutionality because of thermodynamic constrains of C precipitation in Si. Since the device architecture in 14 nm and beyond CMOS will be dominated by FinFETs, there is a paradigm shift in achieving and scaling strain in such devices. The challenge of applying strain to 3D devices (FinFETs) is a topic of great interest currently and this will be critically examined. Finally, non uniform strain relaxation in patterned geometries in tight pitch, 2D and 3D devices will be addressed.

Semiconductor nanomembranes (NMs), extremely thin single-crystal sheets, show great promise as a new nanotechnology platform. Their properties (mechanical, electronic, charge transport, etc.) differ significantly from those of bulk counterparts. The primary drivers in these differences are 1) the extremely low flexural stiffness of NMs, relative to that of the bulk, 2) their large surface-to-volume ratio, and 3) their capacity for accepting high degrees of strain, which influences many functional properties of the material. Focussing here on Group IV NMs, these specialized features provide considerable potential for new or enhanced application: in fast flexible and quantum electronics; new nanophotonic, optoelectronic, and thermoelectric devices; chemical and biological sensors, microfluidics and biology, and opto- and electromechanical nanosystems.
We describe several unique outcomes resulting from the introduction of strain into Si, Ge, and SiGe NMs, outcomes that cannot or have not been achievable in other ways. 1) Ge can become a direct-bandgap material with sufficient biaxial strain. Thin sheets can accept much more strain that bulk material, as the strain energy increases with thickness. We mechanically strain Ge NMs to a high enough level to observe the change to direct-bandgap, something that has not been achieved in other ways. 2) We use strain to create single-crystal sheets of SiGe without disloca-tions, with Ge composition up to 40% so far, something not possible with conventional composi-tional-grading approaches. We use these as substrates to grow strained-Si two-dimensional elec-tron gases (2DEGs) and quantum cascade laser structures. 3) We create single-element electronic heterostructures via periodic strain modulation. 4) We create new strain symmetries in Si and SiGe with trilayer sandwich approaches. Additionally NMs as substrates influence the growth of quantum dot nanostructures. Measurements of resulting properties will be presented to demonstrate these unique outcomes.
Research supported by DOE, AFOSR, and NSF

Introduction
In the recent semiconductor devices, the carrier motilities are improved by the strained Si technology. However, it is difficult to measure both the quantity and direction of the stress applied to the device. We have reported that biaxial stress evaluation is possible using surface enhanced Raman scattering (SERS). It was achieved by the excitation of different optical phonon modes due to z-axis polarization component [1]. SERS also enhances Raman signal significantly, therefore useful to detect small signal from limited area. In this study, we performed SERS measurement to analyze the anisotropic biaxial stress in Si nano area induced by the patterned SiN stressor films and the channel region in the state-of-the-art CMOS devices.
Experiment
We prepared line-and-space patterned SiN films with narrow spacing as stressors deposited on Si (001) substrates. In order to obtain SERS effects, the silver particles were evaporated on the sample surface in 1.0×10-3 Pa with 1.0 Å/s average evaporation rate. The average particle size was approximately 30 nm. We used Nd:YAG laser (lambda; = 532 nm) as excitation light source and oil immersion lens (N.A. = 1.4) in the Raman measurements. Longitudinal optical (LO) and transverse optical (TO) phonon modes were selectively excited based on the Raman polarization selection rule [2]. TO active conditions were realized by the z polarization component in the surface Plasmon. We compared experimental results with finite element method (FEM) simulation. We also applied this method to evaluate the stress in the state-of-the-art CMOS whose channel region was exposed by gate removal.
Results and Discussion
Raman spectra under the LO and TO active conditions were obtained from locally strained Si with a width of 50 nm between patterned SiN. Strained Si peaks were observed under LO and TO active conditions and both shifted toward higher wavenumber from unstrained Si. Raman peak shifts of strained Si were evaluated by the multiple peak fitting. Stress values were calculated from the following equation: Δomega;LO = minus; 2.30 × σxx minus; 2.30 × σyy, Δomega;TO= minus; 2.88 × σxx minus; 0.54 × σyy [3], where σxx and σyy were defined as stresses along and across the SiN pattern, respectively. As results, the stresses were confirmed as σxx = minus; 0.90 GPa and σyy = minus; 0.45 GPa. This anisotropic biaxial stress field was also reproduced by the FEM simulations.
In this study, we succeeded to evaluate the biaxial stress in the nano area using SERS. The y direction stress has approximately half value of x direction stress, which was completely ignored in the case of uniaxial assumption in the conventional Raman evaluation. Therefore, SERS measurement is useful to evaluate anisotropy biaxial stress applied in nano area.

Strained-Si/relaxed-SiGe heterostructures are the foundation of Group IV quantum information processing. The fabrication of single-electron quantum devices requires a very thin layer of tensilely strained epitaxially grown Si confined between SiGe layers, to form a 2-dimensional electron gas (2DEG). Because they can be made as truly single crystals [1], SiGe nanomembranes (NMs) present a potentially significant advance for this field. The recently developed protocol for making single-crystal SiGe NMs rests upon our strain-engineering approach of growing Si/SiGe/Si trilayers, releasing them from the host, and then removing the Si layers to leave a fully elastically relaxed SiGe sheet, which serves as a substrate for further growth. We use MBE to grow structures that provide Si1-xGex (0.2 < x < 0.4) NMs. MBE is preferred to avoid pit formation common in CVD growth. We demonstrate, with high-resolution X-ray diffraction (XRD) and Raman spectroscopy, the single-crystal, defect-free nature of the SiGe NM and subsequent layers grown to create the strained-Si 2DEG.
Charge transport properties of these SiGe NMs and heterostructures grown on them are investigated with van der Pauw and Hall effect measurements over a range of temperatures and compared with results from compositionally graded SiGe substrates that contain strain inhomogeneities and mosaic [2]. The goal is to demonstrate that fabricating strained-Si/relaxed-SiGe 2DEGs with NMs will reduce the disorder potential that diminishes the quality of the 2DEG, and thus to advance application of quantum electronics and spintronics.
[1] D. M. Paskiewicz, B. Tanto, D. E. Savage, and M. G. Lagally, ACS Nano 2011, 5, 5814.
[2] P.G. Evans, D. E. Savage, J. R. Prance, C. B. Simmons, M. G. Lagally, S. N. Coppersmith, M. A. Eriksson, and T. U. Schülli, Adv Mater. 2012, 24, 5217.
Research supported by DOE.
Deborah M. Paskiewicz is currently affiliated to Argonne National Laboratory.

The natural evolution of functional materials architecture calls for their confinement in spatial and dimensional modes. Here, spatial confinement refers to inevitable attachment of materials to a substrate or an overlayer, for example. Dimensional constraint arises from confinement of material in 0- (i.e., nanocrystals), 1- (nanowires) and 2- (i.e., films/membranes) dimensions or their combination in 3-D. Further, by juxtaposing two or more functional materials in close proximity, there are exciting new opportunities for synergistic coupling of disparate phenomena in such hybrid confined materials systems. One of the common denominators in such systems is “spatial accommodation”, thus the elastic strain. In one version, nanopatterned oxides and their heterostructures experience and accommodate elastic strain through combination of symmetry, lattice and orientation and thermal mismatch. This can have dramatic effect on their localized properties as well as their coupling and synergistic behavior. On the other hand, nanoscale precipitates in bulk matrix often attain coherency below nominal threshold size where it may be favorable to elastically accommodate the volume strain than to introduce plastic deformation in the form of misfit dislocations. Here, elastic accommodation via coherency has profound impact on transport properties across such interfaces.

The presentation will cover the critical role of “strain” on two materials systems; nanopatterned oxides and nanostructured thermoelectrics. In nanopatterned oxides of ferroelectric ferrimagnetic and multiferoics, the size, shape and orientation of nanopattern on appropriate substrate have profound influence on the local polarization, magnetization and coupling behavior. These are mediated through elastic strain at the interface which alters the local crystallography, symmetry and electronic structure. Under appropriate 3-D epitaxy conditions which facilitate strain transfer, we show remarkable coupling between magnetization and polarization in nanoscale composite of ferroelectric BaTiO3 and ferrimagnetic Co2Fe2O4. In bulk thermoelectric of PbTe, coherent and semicoherent precipitates of other telluride systems (e.g., SrTe, BaTe, CaTe) provide for efficient electron transport across the interfaces while heat carrying phonon can be efficiently scattered to reduce lattice thermal conductivity. This synergistic effect results in high figure of merit ZT. In these materials systems, strain analysis is performed by a unique combination of synchrotron x-ray scattering and localized electron microscopy to enable quantitative and spatial mapping of strain. It will be argued that strain, particularly elastic strain, is an decisive parameter in many multifunctional materials which has the potential to tune both, local and macroscale properties and phenomena.

The performance of today&’s state of the art semiconductor electronic devices depends on charge transport within very small volumes of the active device regions. The structural, chemical and optical material properties in these small dimensions can critically affect the performance of these devices. Being able to quantitatively map strain with high spatial resolution and a large field of view is therefore crucial for developing modern microelectronic devices. The present contribution will cover the advances in nanometer scale characterization of the strain state in semiconductor heterostructures discussing various transmission electron microscopy (TEM) techniques. The focus will be on the image-based techniques such as high-resolution imaging (HRTEM and HRSTEM) and dark-field inline holography (DIH) as they are able to provide higher spatial resolution compared to diffraction-based methods. Although HRTEM has been shown to provide the required accuracy at the nanometer scale, optimization of imaging conditions is necessary for accurate strain measurements. Recent advances in aberration-corrected microscopy have made HRSTEM feasible allowing one to access structural, compositional and strain information simultaneously.
While high-resolution imaging is quite efficient for mapping relatively short-ranged strain fields induced by features limited in size, it becomes much more difficult if the strain fields extend beyond the field of view of a single high resolution image. To overcome this limitation, the DIH technique was developed. As an alternative to dark-field off-axis holography (DFEH), DIH relies on the reconstruction of the geometric phase from a focal series of filtered dark-field images using an exit-wave reconstruction algorithm. The present contribution will cover the guidelines for quantitative strain analysis including the TEM sample preparation requirements, the optimization of imaging conditions as well as reliable reconstruction of the geometric phase from focal series of dark field images. Strain mapping on technologically relevant structures including strain-engineered CMOS devices, InGaN-based LEDs and strain-compensated heterostructures will be demonstrated.

Local strain engineering of Si crystal lattice is important technology in the field of Si devices. For example, the lattice strain has been used to enhance mobility. However, it is difficult to determine strain distribution in Si nanostructures with both accuracy and high spatial resolution. In the case of general X-ray diffraction technique, which has the advantages in accuracy and non-destructive analysis, the sample volume is too small to evaluate the local strain and the obtained information is average value. On the other hand, transmission electron diffraction technique enables to measure the local strain with high spatial resolution. But strain might be relaxed because of the necessity of preparing cross-sectional samples. In the present paper, we estimate the strain depth distribution in periodic trenched silicon by measuring X-ray scattering patterns.
The periodic trenched silicon were fabricated on (001)-oriented Si wafers along the [110] direction with around 90nm pitch and about 250 nm height. The trenches were filled with SiO2. Some kinds of samples with different thermal conditions were prepared. The scattered X-rays from periodic trenched silicon interfere with each other and give rise to diffractions at additional reciprocal lattice points, which are elongated in the [00L] direction due to finite height of Si arrays [1]. The patterns around Si (111) diffraction spot were measured. The rod-shaped patterns are aligned in the [-110] direction based on the periodic arrays and separated from the diffraction spots of Si substrates. In order to measure the intensity of the rod-shaped scatterings, four-circle diffractometer and the synchrotron radiation at BL16XU of SPring-8 was employed.
The experiment data were compared with the X-ray kinematical simulations. Considering the average strain and local Gaussian-like-strain in the [001] direction, the rod-shaped patterns were explained well. In other words, this method can evaluate the strain depth distribution in periodic trenched silicon. As a result, it is suggested that the strain in silicon pattern is changed systematically corresponding to the thermal history of the trench-fill process.
[1] T. Takeuchi et al., J. Appl. Phys. 106, 073506 (2009).

Strain engineering is a key technology for modern semiconductor devices, as it enables an advantageous modification of the material properties e.g. by lifting the valley degeneracy. In silicon MOSFETs, strain leads to a performance improvement due to an enhancement of the carrier mobility. For germanium, high strain will eventually convert the material into a direct band gap semiconductor, which would thereby fulfill the prerequisite for efficient lasing. Traditionally, strain is induced either by the coherent growth of lattice mismatched layers, or locally by strain transfer using external stressor layers like silicon nitride.
Here, we present a complementary method to produce strain in a material layer of arbitrary thickness without any additional stressor films. The technique is based on the fabrication of suspended, constricted structures from a biaxially tensile pre-strained layer that is separable from its supporting substrate by selective etching. The method relies on elastic deformation and is applicable on the nano- and micrometer length scale. We obtained strain values of 4.5% in 13 nm thin silicon nanowires [1] and 3.1% in 1.5 µm thick Ge [2] which considerably exceed the limits known for strained layer growth.
We will present a discussion on the use of this new method for the fabrication of strained channels in CMOS and the realization of a Ge laser for integration in Si CMOS.
1. R. A. Minamisawa, M.J. Süess et al., Nature Comm. vol. 3, pp. 1096, Oct. 2012.
2. M. J. Süess, R. Geiger, et al. Nature Photonics, vol. 7, pp. 466-472, 2013.

How can the oxygen non-stoichiometry, oxygen diffusion and oxygen reduction kinetics be altered in functional oxides by lattice strain? This question has recently attracted wide interest for enabling a new route for attaining fast oxygen exchange and transport kinetics, and obtaining fundamental answers to this question is important for a range of technologies including high performance solid oxide fuel cells, oxide based sensors, separation membranes and memristors. By applying mechanical stresses, the inherent energy landscape of the reactions involved on the material can be altered. For example, in theory one can turn an endothermic reaction into an exothermic one, and reduce the energy barriers by applying lattice strain. This is the so-called mechano-chemical coupling, and when applied on electrochemical systems we can introduce the term mechano-electro-chemistry.
In this talk, we will discuss recent work that probes quantitatively the mechanisms by which elastic strain impacts surface reactivity and diffusion kinetics in fluorite and perovskite oxides. Computationally, using ab initio and atomistic simulations, we discovered that lattice strain can accelerate the ionic transport in 8%Y2O3-ZrO2 by reducing the ion migration energy barriers. Combining surface chemical characterization on model thin films with ab initio modeling, we demonstrated for the first time that tensile lattice strain favors oxygen exchange kinetics on both the oxygen-deficient (La,Sr)CoO3-d and (La,Sr)MnO3-d and the oxygen-excess Nd2NiO4+d by impacting the energies of oxygen adsorption and dissociation, oxygen vacancy formation, oxygen diffusion, cation segregation and the electron transfer process. The favorable impact of tensile lattice strain on these elementary reactions was validated at the collective level by quantifying the oxygen surface exchange and diffusion kinetics with isotope exchange measurements on (La,Sr)CoO3-d. These results together put forth elastic strain to be an important parameter that couples impressively to the kinetics of charge transport and reactivity in functional oxides.

Yttria-stabilized zirconia (YSZ) and gadolinia doped ceria (GDC) are traditional solid electrolyte materials for use in solid oxide fuel cells, oxygen sensors, and others due to their stability and moderately high ionic conductivity. Recently, lattice strain has been suggested to enable increased conductivity, but the results remain controversial and difficult to repeat. In this work, experimental evidence supporting a lattice strain effect on ionic conductivity in zirconia and ceria based thin films is presented and analyzed. Using RF reactive co-sputtering, cubic zirconia and ceria based thin films have been obtained with a thickness from ~100 nm to ~6 nm. Different substrates, substrate surface treatments, dopant concentrations, and thermal processing conditions were used to control the strain induced in the films. The ionic conductivity of the films was determined using impedance spectroscopy in the temperature range from 673 K to 1073 K and over a range of oxygen partial pressures. Strain was determined by x-ray diffraction and microscopy, including selected area electron diffraction.

Increasing the low temperature oxygen ion conductivity of solid electrolytes is crucial to wider commercialization of solid oxide fuel cells (SOFCs) in both portable and stationary power applications. Construction of nanostructured multilayers using two different oxygen ion conductors (e.g., doped zirconia and ceria) or one oxygen ion conductor and one insulating oxide (e.g., yttria stabilized zirconia (YSZ) and yttria) was recently reported to increase oxygen ion conductivity by orders of magnitude in comparison with that of either of their constituent layer materials. The presence of tensile lattice strains near the interfaces of the oxygen ion conductors is presumed to be the cause, but these experimental results have proven difficult to repeat. In this work, we detail our method and results of using nanostructured multilayers composed of YSZ and CexZr1-xO2 in order to systematically quantify the effect of interfacial lattice strains on oxygen ion conduction. The multilayers are deposited onto single crystal Al2O3 substrates via RF sputtering. Our unique fabrication method enables different lattice strains to be induced in the YSZ layers by controlling the Ce/Zr atomic ratio - and thereby the lattice parameter - in the alternating CexZr1-xO2 layers. Clear differences in interfacial structure are found as the lattice parameter differential between the layers is adjusted. Both the structural and electrical properties of these multilayers will be reported and discussed in this study.

Strain imposed on solid surfaces can drastically change the electronic structure and thereby chemisorption properties of these surfaces. Binding energies of simple adsorbates can be enhanced via expansive strain, and decreased via compressive strain. Similar effects of strain have been identified for the transition states of elementary reaction steps connecting reactants to products. As a result, the catalysis community has by now established strain as a major tool for manipulating catalytic properties of solid surfaces. In particular, the Sabatier principle suggests that for each reaction there is an optimal binding of key reaction intermediates; strain is a very effective way of moving towards that optimum. Combined with alloying, and through near surface alloys, and core-shell nanoparticles, strain has become a very effective and practical tool for manipulating surface reactivity and selectivity, a cornerstone for heterogeneous catalysis. Examples demonstrating the effect of strain on various heterogeneously catalyzed reactions will be discussed.

Strain has been recently proposed as a promising way of increasing the ionic conductivity of oxide-ion conductor materials, such as doped ZrO2 and CeO2. This interest was spurred by a study of Barriocanal and co-workers [1], who found an 8 orders of magnitude increase in the conductivity of 8% Y-doped ZrO2, when deposited on SrTiO3. A myriad of subsequent studies were then performed, but the results are contradictory, with some studies finding little or no change [2] while others found a significant enhancement [3] (though never 8 orders of magnitude).
In this presentation, I will report on some of the recent work we have performed on the effects of elastic strain on the conductivity of doped ceria. By using molecular dynamics simulations with accurate first-principles based interionic potentials [4], we can directly calculate the oxygen diffusion coefficient in both bulk Y-doped ceria and thin films, under various types of elastic lattice strain (isotropic and biaxial). Particular attention is paid to factors such as defect ordering and ion conductivity on the surface. We find, in general, that strain leads to a moderate enhancement of the ionic conductivity at the studied temperatures (800-1400 K). Recent, on-going work on interfaces between two different oxide-ion conductors, such as YSZ/CeO2, will also be presented.
[1] Barriocanal et al., Science 321, 676 (2008)
[2] Pergolesi et al., ACS Nano 6, 10524 (2012)
[3] Kosacki et al., Solid State Ionics 176, 1319 (2005)
[4] Burbano et al., Journal of Physics: Condensed Matter 23, 255402 (2011)

The strain-induced enhancement of ionic conductivity in oxide thin films and multi-layer structures is thought to be only due to the elastic strain at the hetero-phase boundaries by most experimental works. However, the non-negligible population of misfit and threading dislocations in the form of plastic strain at interfaces and in thin films is also expected to affect the defect chemistry and ion conduction. The role of dislocations on the local non-stoichiometry and diffusion kinetics has not yet been quantitatively assessed. Molecular Dynamics (MD), Autonomous Basin Climbing (ABC) method and Monte Carlo calculations are used to study the interaction of oxygen vacancies with an a/2<110>{100} edge dislocation in CeO₂, ceria. The interaction energy of the oxygen vacancy with dislocation is mapped in terms of the proximity of a vacancy with respect to the dislocation core. Both the oxygen vacancy and Ce³⁺, which forms upon the creation of oxygen vacancies in ceria, was found to prefer to stay in the core of the dislocation, and in the tensile strain field of the dislocation. This is because the larger radius of Ce³⁺ is elastically better accommodated in the tensile strain field. The impact of the dislocation on the diffusion of oxygen vacancies is being assessed by a combination of ABC calculations and kinetic simulations. The spatial extent of the dislocation's strain field in this model is on the order of 10 nm, comparable to the separation distance between misfit dislocations in thin ceria films on zirconia that is widely studied experimentally. This means that, in light of our findings, the distribution of defects and their mobility can be largely different in strained thin ceria films compared to bulk ceramics, and the effects of such dislocations must be taken into account in interpreting the experimental results.

Our recent work on MoS2 and other transition metal dichalcogenides (TMDC) has shown that these materials can behave as good catalysts for hydrogen evolution. Having a cheap alternative to Pt for this reaction makes these materials very attractive for such applications. The key feature seems to be the appropriate S-S bond which is just long enough to provide almost zero free energy change during the evolution reaction. This can be achieved if the bonds in the metallic 1T structure are stretched by 3%. While the metastable 1T phase shows a high catalytic activity, the more stable 2H phase does not. In order to investigate the thermodynamics and the strain state of this material at a given temperature, we construct a lattice gas model to describe the most stable 2H and 1T phases. The parameters of this model can be extracted for any TMDC system considered by fitting to total energies calculated from first-principles. More generally, however, Monte Carlo simulations allow us to deduce a phase diagram for this coarse grained model. This implies that the phase and domain structure, if any, can be calculated at any temperature and for any value of the model Hamiltonian parameters. From this phase diagram, we deduce the potential of any of the TMDC family of materials as useful and efficient catalysts for hydrogen evolution.

Lattice strain effects have received significant interest recently for altering the kinetics of oxygen ion diffusion and surface reactions in functional oxides used as electrodes and electrolytes of solid oxide fuel cells (SOFCs). For example, tensile strained perovskite oxide thin films, as exemplified by (La,Sr)CoO3, were shown to clearly exhibit enhanced charge transfer properties and oxygen reduction reactivity. Ruddlesden-Popper (RP) phases, with the chemical formula An+1MnO3n+1, have drawn wide interest as a competitive candidate for new cathode materials.The oxygen diffusion in these materials is highly anisotropic: the diffusion along the a-b plane is much faster than diffusion along the c-axis.While the oxygen exchange and diffusion in perovskite oxides is mediated by oxygen vacancies and is isotropic, RP phase oxides preferentially incorporate and transport oxygen interstitials along the a-b plane.Because of this structural anisotropy, the strain response of RP oxides in oxygen exchange and diffusion is expected also to be anisotropic, and likely stronger along the c-axis compared to the strain response of perovskite materials.
We systematically investigated the effect of lattice strain on surface chemistry, non-stoichiometry and oxygen surface exchange kinetics of Nd2NiO4 (NNO) epitaxial thin films as a model RP cathode. NNO thin films with both (100) and (001) orientations and tensile and compressive strain states were fabricated. High-resolution X-ray diffraction and in-situ X-ray photoelectron spectroscopy data are combined to assess the variation of oxygen non-stoichiometry and surface cation chemistry during annealing at high temperature. The strain along the c-axis, rather than the crystal orientation, was found to primarily determine the oxygen non-stoichiometry of the NNO oxide thin films. The NNO films with compressive strain along the c-axis are more reducible compared to the tensile strained films. Nd segregation and surface morphology evolution during annealing are more evident for the films on the NNO films with compressive strain along the c-axis. We attribute this to the elastic energy minimization along the rock-salt layers that is compressively stressed.Based on these observations, faster oxygen exchange kinetics is expected on the (100) oriented films with tensile strain along the c-axis. Investigation to determine the oxygen exchange kinetics directly and quantitatively with electrical conductivity relaxation method is ongoing.In this work, the strain response of RP phase represented by the model Nd2NiO4+δ composition was found to be anisotropic, and the strain along the c-axis was found to be a key parameter to alter the non-stoichiometry and surface chemistry. These results have important implications to surface oxygen exchange kinetics on SOFC cathodes, and enable a comparison of the strain response of different classes of oxides.

Using epitaxy and the misfit strain imposed by an underlying substrate, it is possible to strain oxide thin films to percent levels—far beyond where they would crack in bulk. Under such strains, the properties of oxides can be dramatically altered. For example, materials that are not ferroelectric or ferromagnetic in their unstrained state can be transmuted into ferroelectrics,^1,2 ferromagnets, or materials that are both at the same time.³ Strain can also be used to improve oxide photocatalytic materials,⁴ ferroelectrics,⁵ and tunable microwave dielectrics.⁶ Our results show that for oxide thin films, strain is a viable alternative to the traditional method of chemical substitutions for selecting between alternate ground states. This work was performed in collaboration with the coauthors listed in the references below.
1. J.H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y.L. Li, S. Choudhury, W. Tian, M.E. Hawley, B. Craigo, A.K. Tagantsev, X.Q. Pan, S.K. Streiffer, L.Q. Chen, S.W. Kirchoefer, J. Levy, and D.G. Schlom, “Room-Temperature Ferroelectricity in Strained SrTiO₃,” Nature430 (2004) 758-761.
2. M.P. Warusawithana, C. Cen, C.R. Sleasman, J.C. Woicik, Y.L. Li, L.F. Kourkoutis, J.A. Klug, H. Li, P. Ryan, L-P. Wang, M. Bedzyk, D.A. Muller, L.Q. Chen, J. Levy, and D.G. Schlom, “A Ferroelectric Oxide Made Directly on Silicon,” Science324 (2009) 367-370.
3. J.H. Lee, L. Fang, E. Vlahos, X. Ke, Y.W. Jung, L.F. Kourkoutis, J-W. Kim, P.J. Ryan, T. Heeg, M. Roeckerath, V. Goian, M. Bernhagen, R. Uecker, P.C. Hammel, K.M. Rabe, S. Kamba, J. Schubert, J.W. Freeland, D.A. Muller, C.J. Fennie, P. Schiffer, V. Gopalan, E. Johnston-Halperin, and D.G. Schlom, “A Strong Ferroelectric Ferromagnet Created by means of Spin-Lattice Coupling,” Nature466 (2010) 954-958.
4. C.M. Brooks, R.F. Berger, D. Chvostova, V. Trepakov, N.J. Podraza, L.F. Kourkoutis, T. Heeg, M. Bernhagen, R. Uecker, J. Schubert, D.A. Muller, A. Dejneka, C.J. Fennie, J.B. Neaton, and D.G. Schlom, “Manipulating the Band Structure of SrTiO₃ via Strain-Controlled Ferroelectric Phase Transitions,” (unpublished).
5. K.J. Choi, M.D. Biegalski, Y.L. Li, A. Sharan, J. Schubert, R. Uecker, P. Reiche, Y.B. Chen, X.Q. Pan, V. Gopalan, L.-Q. Chen, D.G. Schlom, and C.B. Eom, “Enhancement of Ferroelectricity in Strained BaTiO₃ Thin Films,” Science306 (2004) 1005-1009.
6. C.H. Lee, N.D. Orloff, T. Birol, Y. Zhu, V. Goian, E. Rocas, R. Haislmaier, E. Vlahos, J.A. Mundy, L.F. Kourkoutis, Y. Nie, M.D. Biegalski, J.-S. Zhang, M. Bernhagen, N.A. Benedek, Y. Kim, J.D. Brock, R. Uecker, X.X. Xi, V. Gopalan, D. Nuzhnyy, S. Kamba, D.A. Muller, I. Takeuchi, J.C. Booth, C.J. Fennie, and D.G. Schlom, “Exploiting Dimensionality and Defect Mitigation to Create Tunable Microwave Dielectrics,” (unpublished).

Recently Lummen et al. have reported direct imaging of low symmetry monoclinic phase in classic bulk ferroelectric crystals of BaTiO3[1]. The low symmetry phase, stabilized by internal elastic and electric interactions arising from a network of competing domains, demonstrated enhanced piezoelectric and nonlinear optical properties.
Since BE-PFM (Band Excitation-Piezoresponse Force Microscopy) technique offers unparalleled advantage in probing materials with low electric field, this technique has been utilized to image the monoclinic phase. The BE-PFM imaging shows a clear contrast between the monoclinic phase and the matrix tetragonal phase. In order to further understand how the BE-PFM data reflects the low symmetry materials properties, finite element modeling (FEM) were performed. Using the dielectric and piezoelectric properties calculated from phase-field simulations, we confirmed a negligible out-of-plane surface displacement, UZ, compared to the in-plane UX and UY for the PFM response of the in-plane domain system. Consequently, the origin of vertical PFM (VPFM) response observed in our BE-PFM imaging is only limited to the buckling of the cantilever arm, which is further induced by the in-plane displacements of the sample surface. Similar to our previous work[2], we performed additional FEM simulations, to analyze the relative con-tributions of the individual dij piezoelectric coefficients to UX and UY. The results indicate that the main contributions to UX are from d15, d24 and d33, and the main contributions to Uy are from d16 and d22.
Through a combination of experiments and modeling, we conclude that the suppressed UX piezoresponse from experimental BE-PFM data can be explained by the reduced d15 property for the monoclinic domains, and the induced UY piezoresponse can be explained by the induced d22 and d16 for the monoclinic domains. Most importantly, a large value of d22 and d23 were found for the monoclinic BaTiO3 of ~800 pC/N and ~500pC/N. This could be an explanation why BaTiO3 with engineered domain structures exhibits enhanced piezoelectric properties[3].
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1 Lummen et al., Thermotropic Phase Boundaries in Classic Ferroelectrics, to be submitted.
2 S Lei, et al. Origin of piezoelectric response under a biased scanning probe microscopy tip across a 180 degree ferroelectric domain wall. Phys. Rev. B 86, 134115 (2012).
3 S Wada, et. al. Enhanced piezoelectric properties of barium titanate single crystals with diffrent engineered-domain sizes. J. Appl. Phys. 98, 014109 (2005).

Intentional precipitation of arsenic in epitaxial GaAs allows one to obtain a material with a very high electrical resistivity and extremely short carrier lifetime, which is important for a variety of electronic and optical devices. The control over the precipitation is possible via delta-doping with isovalent impurities, such as Sb and P, which manage local strains and chemical potential [N.A.Bert et al. APL, 74, 1442 (1999)]. However, under certain conditions the local strains trigger the formation of dislocation loops [V.V.Chaldyshev et al. PRB, 79, 233304 (2009)].
In this paper the structure of low-temperature grown GaAs with equidistant delta-layers of Sb and P was studied by analysis of the X-ray curves, which was supported by optical absorption measurements and transmission electron microscopy. The simultaneous fitting of the X-ray reflectivity curve and diffraction ones for GaAs (004) and GaAs (115) crystallographic planes provided reliable information about the period of delta-layer superlattice, thickness of the Sb and P delta-layers, and amount of the excess As available for precipitation [M.V.Baidakova et al. Acta Cryst. B69, 30 (2013)]. Variation of these parameters was documented when the excess As precipitated into As nanoinclusions upon annealing.
We show that the Sb and P delta-layers impact differently on the As precipitation processes in the low-temperature grown GaAs. Strong intermixing and damage due to excess arsenic diffusion, precipitation and the formation of the dislocation loops is revealed for delta-Sb layers separated by 10 nm of the GaAs matrix. Delta-P layers separated by 10 nm of the GaAs matrix are much more stable against these factors. The combination of Sb and P delta-layers appears to be an effective tool for spatial patterning of the nanoinclusion array and prevention of the defect formation under annealing.

Engineering interfaces in perovskite/spinel heterostructures is challenging by structural differences. In particular, flat BaTiO3/CoFe2O4 epitaxial bilayers with the polar axis of BaTiO3 along the normal direction of the substrate have been elusive. The main difficulties are the strong tendency of (001)-oriented CoFe2O4 to three-dimensional growth, and the tensile epitaxial stress that CoFe2O4 imposes on BaTiO3. Here we show that these difficulties can be overcome. We have used kinetic growth limitations to fabricate flat c-oriented BaTiO3/CoFe2O4 epitaxial bilayers on La2/3Sr1/3MnO3/SrTiO3(001) by pulsed laser deposition assisted by high-pressure reflection high energy electron diffraction (RHEED). In-situ RHEED analysis of lattice strain during growth, has revealed that BaTiO3 grows relaxed on highly lattice-mismatched CoFe2O4, thus suppressing tensile epitaxial stress effects. As a result BaTiO3 is ferroelectric along the out-of-plane direction with bulk-like polarization. Also, the heterostructures present magnetoelectric coupling as revealed by the strong effects of an external magnetic field on the electrical permittivity at temperatures occurring structural transitions of BaTiO3. These results show that very high lattice mismatch in heteroepitaxy can be an opportunity rather than a limitation to integrate dissimilar materials with optimized functional properties.

The orientation, stress, and strain in single-crystalline samples of barium titanate have been determined and mapped using electron backscatter diffraction. Cross-correlation analysis of the electron backscatter diffraction patterns enabled the full (non-hydrostatic) strain tensor components to be mapped with spatial resolution of < 40 nm and strain resolution of < 0.0001. Measured structures ranged from micrometer-scale, parallel, lamellar domains to 100 nanometer-scale, isolated domains. Strains in the larger structures were dominated by strain in the minority a-domain bands with peak values of minus;0.006 determined in-plane, perpendicular to the intersection of the domain walls with the crystal surface, compared to +0.002 in the same direction for the majority c-domains. In distinction to the larger structures, strains in the smaller structures exhibited significant shear components. In both cases, the out-of-plane strains were negatively correlated with the in-plane strains and were about a factor of two smaller. The experimentally determined strains peaked at domain boundaries and suggest a contraction of the c-axis and an expansion of one of the a-axes as a domain wall is approached. The ratios of the in-plane and out-of-plane strains were consistent with the bulk elastic constants of barium titanate. Stress values determined from the strains and the elastic constants peaked at 750 MPa. The results point to the strong possibility of “flexoelectric” strain-gradient engineering in nano-scale ferroelectric materials.

Bardeen and Shockley proposed the notion of deformation potential in semiconductors over 60 years ago. Yet, its implications in materials properties have not fully appreciated, owing to the limited amount of strain that conventional materials can sustain. The recent emergence of ulrta-strength materials, which can sustain a significant fraction of their ideal strength, provides an unprecedented opportunity for exploring the effects of strain gradient on the quasiparticle dynamics. In this talk, I will discuss our results on elastic strain engineering of low-dimensional optoelectronic materials. I will discuss the possibility of creating a semiclassical charge carrier potential energy surface in low-dimensional semiconductors, by introducing an inhomogeneous elastic strain field. Numerical simulations and experimental measurements are combined to demonstrate the migration of charge carriers by strain gradient in low-dimensional systems, such monolayer molybdenum disulphide and ZnO micro/nanowires. The technological implications of these findings will be discussed.

Strain engineering can be utilized to tune the fundamental properties of semiconductor materials for applications in advanced electronic and photonic devices. Recently, the effects of large strain on the properties of nanostructures are being intensely investigated to further expand our insights into the physics and applications of such materials. Here we present results on controllable buckled cadmium-sulfide (CdS) optical nanowires (NWs), which show extremely large periodic energy bandgap tuning by >250 meV with strains within the elastic deformation limit. Polarization and spatially-resolved optical measurements reveal characteristics related to both compressive and tensile regimes, while micro-reflectance spectroscopy clearly demonstrate the effect of strain on the different types of excitons in CdS. Our works provides direct insights into the effect of large strains on the optical properties of NWs, which can be useful for designing mechanically responsive nanoelectronic or photonic systems.

The coupling between polarization and deformation in ferroelectrics give rise to piezoelectric, electrostrictive and flexoelectric effects. The flexoelectric effect which describes the linear dependence of polarization on strain gradient is very weak since the flexoelectric coefficient is on the magnitude of nC/m. However, when the system size scales down to nanometers, the flexoelectric effect becomes significant. With an extended phase-field model including flexoelectricity, we studied mechanical writing on ferroelectric thin films by atomic force microscope (AFM). The domain switching mechanism via flexoelectricity is analyzed. The competition between piezoelectric effect and flexoelectric effect is discussed. The effects of film thickness and geometry of the AFM tip are studied. Our simulation results show good agreement with experimental observations.

Strongly correlated electron materials offer many intriguing properties arising from competition of multiple electronic and structural phases. Different phases are often found to coexist in these materials across a wide range of length scales. Studying these materials at the single- or few-domain level allows probe of their intrinsic properties without the complication of phase inhomogeneity. Reduction in material size also allows strain engineering of the phase transition exceeding the bulk limit for novel device applications.
This presentation will focus on new insights gained and applications developed from strain engineering of vanadium dioxide nanomaterials as a representative strongly correlated electron material. By controlling the strain state and temperature independently, we map the phase diagram in previously unattainable phase space, probe the nonthermal lattice softening at highest precision, and discover abnormal effects in electronic thermal conductivity. By coupling the material with other structural materials, we develop new adaptive composites for actuation, sensing, and energy transduction with superior performance than existing technologies.

Structure and symmetry play an important role in determining electronic properties of periodic solids. The interplay between crystalline symmetry and electronic topology leads to new phases of matter dubbed topological crystalline insulators [1], which possess symmetry-protected surface states with unique and tunable electronic properties. In this talk, I will describe our theoretical prediction of topological crystalline insulators in the SnTe class of IV-VI semiconductors [2]. I will present k.p theory for topological surface states [3], and reveal their unprecedented properties with potential applications in tunable electronics and spintronics [4].
[1] L. Fu, Phys. Rev. Lett. 106, 106802 (2011)
[2] T. H. Hsieh, H. Lin, J. Liu, W. Duan, A. Bansil, and L. Fu, Nat. Commun., 3, 982 (2012).
[3] J. Liu, W. Duan, and L. Fu, arXiv:1304.0430
[4] Y. Okada, M. Serbyn et al, arXiv:1305.2823

Effective approach to control the magnetic state remains to be a critical problem in the application of novel spintronic devices. In this research, it has been found out that the magnetic moment of vacancy decorated h-BN can be controlled by biaxial elastic strain. Single vacancies and divacancies decorated h-BN under varied strains have been studied by spin polarized density functional calculations. The results indicate that there will be a switch of magnetic and electronic state under specific threshold of strain, accompanied by bond forming or breaking procedures. These findings provide a new way to control magnetism of spintronic devices made of low-dimensional nanomaterials.

Accelerating discovery and design of new electronic materials through computational screening involves filtering of a large number of candidates according to a set of desired properties. Topological insulators are becoming one of the most studied classes of novel materials because of their great potential for applications ranging from spintronics to quantum computers. In this presentation, we will explain how surface strain can be used as a metric to identify novel compounds and to play with electronic properties toward the ultimate goal of creating thermodynamically-driven quantum devices.

It is well known that epitaxial stress/strain conditions in thin films may result in stabilization of rather exotic and highly-distorted phases that would otherwise not exist in strain-free bulk materials. Biaxial planar stresses acting in plane parallel to heterogeneous layers can alter quite uniquely thermodynamic properties, structural parameters, elastic properties as well as magnetic and electronic states of thin films. It was also shown that there is a strong connection between the behavior of overlayers and bulk material at high shape deformations. Here we describe the epitaxy-induced changes generated by small and moderate stresses belonging to linear-elasticity regimes and introduce suitable quantum-mechanical approaches to highly-strained states. Focusing on elemental metals, transition-metal nitrides and half-metallic transition-metal oxides, selected structural, electronic and magnetic phase transformations will be studied and their impact on film properties will be shown.

In this report we investigate the electrical and mechanical properties of VO2 nanowires undergoing phase transitions induced by uniaxial tensile strain. Free-standing single-crystalline VO2 nanowires (NWs) with M1 and M2 structures were synthesized via the vapor transport method by controlling the growth conditions. Phase identification was carried out using Raman spectroscopy and synchrotron X-ray microdiffraction. By using an in situ electrical-contact-resistance push-to-pull (ePTP) MEMS device in TEM, uniaxial tensile strains were applied onto an individual VO2 NW under constant electrical voltage at both ambient and elevated temperatures. The electrical current was measured quantitatively in real time to monitor the strain-induced metal-insulator transition (MIT) and insulator-insulator transition (IIT); in the meantime, the applied force and selected area electron diffraction patterns were also recorded, which identify the accompanying structural phase transition (SPT). Our study uncovers quantitatively the evolution of the electrical property during the processes of IIT and MIT, as well as the mechanical property change during the M1-to-M2 and R-to-M2 SPTs.

We present an ab initio study of structural dependence of total energy of bulk As, Sb and Bi. The structures considered are the A7 ground state, bcc, fcc and simple cubic and they all have been newly described on equal footing with the help of a hexagonal unit cell. Best agreement with experimental atomic volume is found in case of local density approximation with spin orbit coupling.
We study several continuous deformation modes, including simultaneous trigonal distortion and change of the internal structural parameter as well as simultaneous volume change and trigonal distortion.
Total energies for these deformation modes calculated by pseudopotential VASP code are displayed in contour plots which are used to explain and predict structural parameters of As, Sb and Bi thin (0001) films on various substrates with the (111) cubic or (0001) hexagonal geometry.
An overall good agreement with available experimental data for thin films of Bi has been found. However, we also provide many theoretical predictions that may motivate experimentalists for a deeper study of the systems considered.

Piezoelectricity, a phenomenon known for centuries, is an effect that is about the production of electrical potential in a substance as the pressure on it changes. Wurtzite structures such as ZnO, GaN, InN and ZnS, due to the polarization of ions in a crystal that has non-central symmetry, a piezoelectric potential (piezopotential) is created in the crystal by applying a stress. The effect of piezopotential to the transport behavior of charge carriers is significant due to their multiple functionalities of piezoelectricity, semiconductor and photon excitation. By utilizing the advantages offered by these properties, a few new fields have been created. Electronics fabricated by using inner-crystal piezopotential as a “gate” voltage to tune/control the charge transport behavior is named piezotronics, with applications in strain/force/pressure triggered/controlled electronic devices, sensors and logic units. Piezo-phototronic effect is a result of three-way coupling among piezoelectricity, photonic excitation and semiconductor transport, which allows tuning and controlling of electro-optical processes by strain induced piezopotential. The objective of this talk is to introduce the fundamentals of piezotronics and piezo-phototronics and to give an updated progress about their applications in energy science (LED, solar) and sensors (photon detector and human-CMOS interfacing).
[1] W. Wu, X. Wen, Z.L. Wang “Pixel-addressable matrix of vertical-nanowire piezotronic transistors for active/adaptive tactile imaging”, Science, 340 (2013) 952-957.
[2] Z.L. Wang, Piezotronics and Piezo-phototronics, Springer, 2013.
[3] Z.L. Wang “Piezopotential Gated Nanowire Devices: Piezotronics and Piezo-phototronics”, Nano Today, 5 (2010) 540-552.
[3] Y.F. Hu, Y.L. Chang, P. Fei, R.L. Snyder and Z.L. Wang “Designing the electric transport characteristics of ZnO micro/nanowire devices by coupling piezoelectric and photoexcitation effects”, ACS Nano, 4 (2010) 1234-1240.
[4] Y.F. Hu, Y. Zhang, Y.L. Chang, R.L. Snyder and Z.L. Wang “Optimizing the Power Output of a ZnO Photocell by Piezopotential”, ACS Nano. 4 (2010) 4220-4224.; Corrections: 4 (2010) 4962 - 4962.
[5] Q. Yang, W.H. Wang, S. Xu and Z.L. Wang* “Enhancing light emission of ZnO microwire-based diodes by piezo-phototronic effect”, Nano Letters, 11 (2011) 4012-4017.
[6] Q. Yang, X. Guo, W.H. Wang, Y. Zhang, S. Xu, D.H. Lien, Z.L. Wang* “Enhancing sensitivity of a single ZnO micro/nanowire photodetector by piezo-phototronic effect”, ACS Nano, 4 (2010) 6285 - 6291.
[5] W.Z. Wu, Y.G. Wei and Zhong Lin Wang , Adv. Materials, “Strain-gated piezotronic logic nanodevices “, Adv. Materials, 22 (2010) 4711.
[7] Wenzhuo Wu and Zhong Lin Wang* “Piezotronic nanowire based resistive switches as programmable electromechanical memories”, Nano Letters, 11 (2011) 2779-2885.

Epitaxial strain has enabled dramatic control over the structure and properties of ferroelectrics. Modern ferroelectric films, including bilayer and superlattice heterostructures, provide access to exotic structures and properties. Here we will explore new routes to achieve large elastic strains and tune the ferroelectric properties. Our work in the use of compositionally graded heterostructures and free-standing thin-film multilayers pushes the edge of strain control of materials. Compositionally graded films possess a smooth variation in the composition throughout the thickness. We will discuss the evolution of crystal and ferroelectric domain structure as well as the dielectric, piezoelectric, pyroelectric, and ferroelectric properties of compositionally graded versions of PbZr_xTi_1-xO₃ (PZT) and other common ferroelectric systems. In PZT we will focus on compositions between PbZr_0.2Ti_0.8O₃ (tetragonal) and PbZr_0.8Ti_0.2O₃ (rhombohedral). Using pulsed-laser deposition, we have synthesized various thicknesses of films on a range of substrates and have explored single-layer, bilayer, and compositionally graded heterostructures. X-ray diffraction and piezoresponse force microscopy studies reveal, among the most surprising results, that certain versions of bilayer and compositionally graded heterostructures are found to be nearly coherently strained to the substrate (despite nearly 4% lattice mismatch) and that these films exhibit unexpected crystal and domain structures (e.g., tetragonal versions of PbZr_0.8Ti_0.2O₃). Such films exhibit dramatically reduced permittivity (<90 for certain compositionally graded films), large remnant polarizations, and horizontally shifted hysteresis loops (indicating built-in electric fields >200 kV/cm). These effects are the result the coupling between the polarization and strain gradients in the film (or flexoelectricity). This presentation will include aspects of experiment and hybrid modeling approaches that combine mechanical and order parameter evolution to understand how one can generate strains gradients >10⁵ m^-1, produce exotic properties and susceptibilities, and will explore potential of such approaches as we move towards epitaxial strain version 2.0.
[1] R. V. K. Mangalam, et al., Adv. Mater. 25, 1761 (2013).
[2] J. Karthik, et al., Phys. Rev. B 87, 024111 (2013).
[3] Z. Connell, et al. under review Phys. Rev. B (2013).

One-dimensional structures such as nanowires and nanotubes are potential materials for future nanoelectronics, optoelectronics, piezoelectric devices, sensors, and actuators. Due to length scale effects and higher surface-to-volume ratios, nanostructures can exhibit superior mechanical and electrical, as well as other length scale dependent properties. In this work, strain-rate controlled tensile experiments were conducted on silicon and zinc oxide nanowires using an electrical push-to-pull (EPTP) device in the SEM and TEM. The EPTP device is a MEMS-based uniaxial nanotensile test platform that has been designed for electromechanical characterization of one-dimensional as well as two-dimensional structures using a four point probe method. In the presentation, strain-induced electron mobility of Si and piezoelectric response of ZnO will be addressed. The electronic properties of such nanowires and how these properties can be tailored by applying strain will also be discussed.

We demonstrate, using phase-field simulations, a large and reversible average in-plane electromechanical strain (up to -0.3%) can be obtained in BaTiO3 nanoislands based on non-180 degree ferroelectric (FE) polarization switching. Such large ferroelastic strains can further be used to repeatedly switch the magnetic domains in a juxtaposed CoFeB thin-film layer between an in-plane and an out-of-plane direction. It thus allows a mechanical control of perpendicular magnetization which can be utilized to design novel high-density, low-power spin-electronic devices with purely voltage-driven means. Furthermore, influences of the substrate constraint and the lateral size of the heterostructured nanoisland on the electromechanical strain-driven magnetization switching are addressed, whereby an optimal lateral size and substrate strain condition are identified for illustration.

BiFeO3 (BFO) is one of the most intensely studied multiferroic materials due to the coexistence of a strong ferroelectric polarization and ferromagnetism at room temperature, and has more recently been recognized as a promising candidate for many photonic applications and devices. It has been shown that through biaxial epitaxial strain, BFO undergoes an iso-symmetric structural phase transition at room temperature from its rhombohedral parent-phase (R-BFO) to a super-tetragonal phase (T-BFO) under compressive strain (-4.5%)[1], and to an orthorhombic phase (O-BFO) under tensile strain (+1.0%)[2], where enhanced material properties have been reported across these transitions. Using optical second harmonic generation (SHG) analysis, we distinguish between the different BFO phases to confirm their structural point group symmetry, and we observe large nonlinear optical property enhancements in strained (T,O,R)-BFO thin film systems, which rival todays standard nonlinear optical materials.[2,3] The strain-tuned nonlinear optical property enhancements in BFO thin films make them potentially useful for optical applications and device integration.
This research has been supported by NSF MRSEC program DMR-0820404, and by ARO through agreement W911NF-08-2-0032.
[1] R. J. Zeches, M. D. Rossell, J. X. Zhang, A. J. Hatt, Q. He, C. H. Yang, A. Kumar, C. H. Wang, A. Melville, C. Adamo, G. Sheng, Y. H. Chu, J. F. Ihlefeld, R. Erni, C. Ederer, V. Gopalan, L. Q. Chen, D. G. Schlom, N. A. Spaldin, L. W. Martin, and R. Ramesh, Science 326, 977 (2009).
[2] J. C. Yang, Q. He, S. J. Suresha, C. Y. Kuo, C. Y. Peng, R. C. Haislmaier, M. A. Motyka, G. Sheng, C. Adamo, H. J. Lin, Z. Hu, L. Change, L. H. Tjeng, E. Arenholz, N. J. Podraza, M. Bernhagen, R. Uecker, D. G. Schlom, V. Gopalan, L. Q. Chen, C. T. Chen, R. Ramesh, and Y. H. Chu, Phys. Rev. Lett. 109, 247606 (2012).
[3] R. C. Haislmaier, N. J. Podraza, S. Denev, A. Melville, D. G. Schlom, V. Gopalan, Appl. Phys. Lett., accepted for publication (2013).

Graphene and boron nitride monolayers are two representative atomically thin two-dimensional materials [1]. However, the intrinsic zero-bandgap of graphene and large bandgap of boron nitride sheet limit their use in electronic transistors, etc. Capability to tailor their electronic and optical properties via chemical, physical, or mechanical approaches become extremely desirable. Here using first-principles density functional theory calculations we demonstrate that band gap of zigzag BN nanoribbons can be significantly tuned under uniaxial tensile strain due to reduced orbital hybridization upon elastic strain [2]. The change of band gap is also width dependent. In addition, we found sizable dipole moment and piezoelectric effect in these ribbons owing to structural asymmetry and hydrogen passivation. As two-dimensional atomically thin materials such as graphene have been demonstrated to be able to sustain large elastic strain [3,4], the predicted strain-engineered bandgap will offer new opportunities to optimize two-dimensional nanoribbons for applications such as electronic, piezoelectric, photovoltaic, and opto-electronic devices.
[1] The rise of graphene. A. K. Geim and K. S. Novoselov, Nature Materials 6, 183 (2007)
[2] Strain-engineering of band gaps in piezoelectric boron nitride nanoribbons. J. Qi, X. Qian, L. Qi, J. Feng, D. Shi, and J. Li, Nano Letters 12, 1224 (2012)
[3] Measurement of the elastic properties and intrinsic strength of monolayer graphene. C. Lee, X. D. Wei, J. W. Kysar, and J. Hone, Science 321, 385 (2008)
[4] Ab initio calculation of ideal strength and phonon instability of graphene under tension. F. Liu, P. M. Ming, and J. Li, Phys. Rev. B 76, 064120 (2007)

Perovskite ABO3 oxides display an amazing variety of phenomena that can be altered by subtle changes in the chemistry and internal structure. Most undergo non-polar structural distortions associated with a rotation of the BO6 octahedra about one or more of the crystal axes. These distortions are well known to control the charge/orbital, magnetic and electronic degrees of freedom. This strong coupling represents an opportunity to understand and create new functional materials that respond to an external perturbation in a useful way. In this talk I will discuss on ongoing work to rationally design from first principles novel multifunctional materials, such as multiferroics and systems displaying Mott transitions, by controlling structural degrees of freedom by means of epitaxial strain rather than relaying on chemical substitution alone.

Ferroelectric thin films often form twinned domain structures to relieve substrate mismatch strain or reduce the buildup of charge on film surfaces. These domain structures induce local stresses in the substrate, and their energy is influenced by the orientation and type of domain wall, the stiffness of the substrate and ferroelectric, and the domain sizes. Here, we employ an adaptive mesh phase-field model to study the effects of anisotropy of the substrate stiffness on the domain structures in (001)-oriented BiFeO3 thin films. We examine short-circuit films, where in-plane elastic properties tend to dominate the domain structure, as well as open-circuit films, where both in-plane and out-of-plane substrate properties can influence the domain structure. In addition, experimental BiFeO3 films on orthorhombic rare-earth scandates show a reduced number of ferroelastic domain variants, and we discuss the degree to which the reduced symmetry can be explained by stiffness anisotropy in the substrates.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department ofEnergy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

The phase change material Ge2Sb2Te5 (GST) is often used as the active material in prototype electrical memories. It can be switched between different structural states on sub-nanosecond time scales yet it is stable for years at room temperature. Electrically, the phase change results in three orders of magnitude change in the film&’s sheet resistance. Moreover, the structural phase transition is highly scalable thus electrical devices based on this material are a likely candidate to replace high density FLASH memories.
The amorphous to crystal phase transition can be thermally triggered. The temperature at which the phase transition occurs is dependent on stress within the material. Here the GST crystal growth rate was found to decrease with increased hydrostatic pressure. Furthermore, the stress tensor was found to influence the direction of Ge atomic movements during the phase transition; the Ge atoms tend to move along the direction of lowest stress. Interestingly, this effect has some similarities with a new class of material called Interfacial Phase Change Materials (iPCM) where Ge atomic movements are restricted to a single dimension at the interface between two different materials. Herein, we have used density functional theory molecular dynamics to study the effect of stress on the switching time of iPCMs. The results will aid the design of improved iPCM structures that take advantage of strain engineered interfaces.

The phase-field model of ferroelectric domains is extended to include the flexoelectric contributions by introducing strain gradient - polarization coupling. The classic ferroelectric BaTiO₃ is considered. It is shown that the Ising 180 degree ferroelectric domain wall, long-believed to be charge-neutral, contains both Bloch and Néel type polarization components, and is thus charged.This observation is also confirmed by DFT calculations. It is demonstrated that the additional polarization components are induced by the stress inhomogeneity around the domain wall through the flexoelectric effect. The magnitudes of the Bloch and Néel components are determined by the competing depolarization and flexoelectric fields with the polarization component parallel to the domain wall as high several mu;C/cm² and bound charge at the wall as high as 10⁶ C/m³. The results suggest the important role of flexoelectricity in determining the properties of a ferroelectric and the possibility to manipulate them through domain wall engineering.

Tightening or relaxing the tension on a drumhead will change the way the drum sounds. The same goes for drumheads made from atomically thin graphene, only instead of changing the sound, stretching graphene has a profound effect on the material&’s electrical properties. We recently reported the first study to show that a scanning tunneling microscope (STM) probe tip can continuously tune the deformation of a graphene drumhead, which in turn creates a localized strain field in the graphene and leads to the formation of a spatially confined quantum dot. The graphene drumheads were created by placing the graphene over etched holes in silicon dioxide. When the graphene is strained or stretched, the charge carriers get stirred in different directions instead of going in straight lines. Mathematically, the strain can be described as a fictitious magnetic field. Just as real magnetic fields make charges go in circles, the fictitious magnetic field also makes the carriers go in curved trajectories. Our atomistic simulations showed that the graphene drumhead in this study is strained symmetrically in the proximity of the STM probe tip, and further revealed that the resulting fictitious magnetic fields point up and down with a clover-leaf symmetry. Such fields force the carriers to go in a flower type pattern, thereby confining them around the apex area similar to confinement in a quantum dot. Creating semiconducting regions like quantum dots in graphene by simply straining, instead of cutting graphene might offer the best of both worlds: high speed and the band gap crucial to computing and other applications.

The deformation of nanocomposites containing graphene flakes with different numbers of layers has been investigated with the use of Raman spectroscopy. It has been found that there is a shift of the 2D band to lower wavenumber and that the rate of band shift per unit strain tends to decrease as the number of graphene layers increases. It has been demonstrated that band broadening takes place during tensile deformation for mono- and bi-layer graphene but that band narrowing occurs when the number of graphene layers is more than two. It is also found that the characteristic asymmetric shape of the 2D Raman band for the graphene with three or more layers changes to a symmetrical shape above about 0.4% strain and that it reverts to an asymmetric shape on unloading. This change in Raman band shape and width has been interpreted as being due to a reversible loss of Bernal stacking in the few-layer graphene during deformation.
It has been shown that the elastic strain energy released from the unloading of the inner graphene layers in the few-layer material (around 0.2 meV/atom) is similar to the accepted value of the stacking fault energies of graphite and few-layer graphene. It is further shown that this loss of Bernal stacking can be accommodated by the formation of arrays of partial dislocations and stacking faults on the basal plane.
This newly-discovered phenomenon has major implications for the use of few-layer graphene in a number of engineering applications. It will be demonstrated that this behavior leads to the reduction of the effective Young's modulus of few-layer graphene as the number of layers is increased. In addition, the effect of the reversible loss of Bernal stacking upon the electronic structure of few-layer graphene and possibility of using it to modify the electronic structure of few-layer graphene will be discussed.

Vertically aligned carbon nanotube (VACNT) structures exhibit nonlinear foam-like hysteretic behavior subjected to quasistatic compressive cyclic loadings. They also posses intrinsically graded functional properties that lead to strain localization and characteristic buckling. The presence of nonlinearity along with the ability to recover large strains (up to 80%) and high-energy absorption suggests their potential use in design and development of novel materials with unprecedented tunable properties.
We study the influence of the microstructure on the bulk mechanical properties of carbon nanotube structures subjected to impact loadings. We achieve this goal in two ways:
1. By tailoring the bulk density and stiffness through synthesis: we introduced hydrogen into the feedstock (carbon source: toluene; catalyst precursor: ferocene) in the thermal chemical vapor deposition (tCVD) process that enabled us to tune the bulk density and the effective stiffness of the VACNT structures.
2. By synthesizing micro-architectures of VACNT structures on two-dimensionally patterned silicon substrates using photolithography. We synthesized pillars, lines and concentric ring structures in the microscopic scale to understand the influence of the microstructural geometry on the bulk properties of VACNT structures.
These VACNT structures were tested subjected to impact loadings on a dynamic testing set up we developed in our laboratory. The dynamic displacement was measured by a non-contact optical method based on time-resolved, high-sensitivity geometric Moiré interferometry while a dynamic force sensor measured the dynamic force-time history. These measurements were then used to develop the dynamic constitutive response of the VACNT structures. A complementary high-speed camera with a microscopic lens was used to visually observe the deformation.
The VACNT structures exhibit highly nonlinear stress-strain behavior with hysteretic dissipation. They also show high resilience to impact: sequential periodic buckles formed as the striker compressed the specimen and more than 90% of the deformation fully recovered upon unloading. We were able to tune the global dynamic stress-strain behavior significantly by either engineering micro-structured geometries or by modifying the density. Different micro-architectures of VACNTs, with similar density, resulted in very different localized deformation mechanisms. The understanding of the influence of microstructure on the bulk mechanical behavior of the VACNTs and the ability to engineer different microstructures will enable the design of novel light weight materials and structures with unprecedented, tunable properties.

In the current presentation, we review some of our recent findings by in situ microscopy and related techniques. We demonstrated the ultra-large elasticity of face-centered cubic structural Cu nanowires [1], Cu-Zr [2, 3] and Zr-Nb [4, 5] metallic glass nanowires as well as the semi-conductor nanowires of ZnO [6], Si [7] and SiC [8] by up to 7% [1] elastic strain. With these exceptional properties of ultra-large elasticity and elastic strain, these nanowires possess ultra-large strength [3] and tunable bandgap [6]. These nanowires can be potentially used as structural component to enhance the strength of bulk materials and develop new functions and novel properties [9] and with tunable bandgap [6, 8], the electrical, optical properties and other functions of the nanowires can be tuned and designed. A new trend of elastic strain engineering with both of free-standing and compositional nanowires can be thus expected.
1 [email protected], 2 [email protected]
Reference:
[1] YH Yue et al., NANO LETTERS, 11, 3151 (2011)
[2] QS Deng et al., ACAT MATER., 59, 6511 (2011)
[3] L Tian et al., NATURE COMMUNICATIONS, 3, 609, DOI: 10.1038/ncomms1619, 2012
[4] QK Jiang et al., SCIENTIFIC REPORTS, 2, 852 (2012), DOI: 10.1038/srep00852
[5] QK Jiang et al., ACTA MATER. 61, 4689 (2013)
[6] B Wei et al., NANO LETTERS, 12, 4595-4599 (2012)
[7] K Zheng et al., NANO LETTERS, 9, 2471 (2009)
[8] RW Shao et al., APPL. PHYS. LETTERS, (2012)
[9] Hao et al., Science, 339, 1191 (2013)

Pluripotent, a term borrowed from cell biology, refers to a stem cell&’s ability to differentiate into different cell types. The greater the range of cell types, the greater the potency. In this presentation, we focus on the ability of any particular material to differentiate, via a simple processing procedure, into forms with a wide range of physical properties. In this discussion, we limit consideration to a material in the form of a sheet which is subject to a predetermined pattern of cuts. Design of the cut geometry/pattern is the same as the design of the materials properties. Surprisingly, a relatively simple set of cuts can enable flexible design. The basic idea is that cuts that nearly touch break the material into rotating units, subject to a particular connectivity. The cuts can be combined in a multi-level hierarchy and/or arranged with different motifs, to give a wide range of properties to the original material. By patterning the cut-motif and/or cut-level, spatially distributed properties can also be achieved. Design of final shape, final set of properties, their distribution, or combination of these by cuts can be a relatively straightforward inverse problem provided that the cut patterns remain simple. We demonstrate these concepts based upon a series of finite element calculations with a focus on stress-strain response. Then, we characterize the potency of hierarchical structures based upon geometric principles and provide an accurate, analytical estimate of properties for a set of cut geometries. Finally, we discuss how to do design the goemetry to achieve target properties.

The complex serpentine-like structures of single-wall carbon nanotubes grown by chemical vapor deposition on vicinal single-crystal quartz provide a remarkable system to study strain at the nanoscale [1]. Here [2] we develop in situ nanomanipulation and confocal Raman spectroscopy measurements to investigate the manipulation-induced local strain configurations exhibited by these systems. The measured strain profiles along the nanotubes, caused by nanomanipulation, are characterized by “zig-zag” behaviors, with characteristic slopes. We compare the experimental results with a mechanical model that considers an elastic line system in two dimensions, immersed in a viscous frictive media, to elucidate the richness in their local strain configurations. We demonstrate that the local strain profiles observed along the carbon nanotube serpentines are due to adhesion discontinuities, strain avalanches and strain memory effects. These results can be generally applied for one-dimensional systems sliding on a two-dimensional surface, although the main ideas can be extended to one-dimensional systems immersed in a three-dimensional medium, as well as to general friction problems, thus being a step forward in the understanding of nanotribology.
[1] P. T. Araujo, N. M. Barbosa Neto, H. Chacham, S. S. Carara, J. S. Soares, A. D. Souza, L. G. Canccedil;ado, A. B. de Oliveira, R. J. C. Batista, E. Joselevich, M. S. Dresselhaus, and A. Jorio, Nano Letters 12, 4110 (2012).
[2] L. A. M. Mussnich, H. Chacham, J. S. Soares, N. M. Barbosa Neto, N. Shadmi, E. Joselevich, L. G. Cancado, A. Jorio, to be published.

We present theoretical and experimental results on mechanical properties of monolayer and few-layer talc. We show, through AFM measurements, that talc can be mechanically exfoliated down to monolayer flakes. From first principles calculations, we obtain mechanical properties of monolayer talc. We find that the theoretical breaking strength of a talc monolayer, for uniaxial deformations, ranges from 29 to 33 N/m. These values are near graphene's record strength values. We also find that the flexural rigidity of monolayer talc is more than thirty times larger than that of graphene, but that it still can be bent to very small curvatures without fracturing. These predicted properties - that monolayer talc is strong, rigid, but not fragile-, together with a small calculated interlayer binding energy of 0.12 J/m2, are consistent with our experimental finding that few-layer talc depicts negative dynamic compressibility, but with magnitudes smaller than those of graphene, h-BN, and MoS2.

In situ transmission electron microscopy (TEM) provides dynamic observations of the physical behavior of materials in response to external stimuli such as temperature, environment, stress, and applied fields. One key advantage that in situ TEM probing techniques have is control over when the experiment begins so that clear before and after images or measurements can be obtained. For example, is possible to keep a material undeformed until contacted by a mechanical probe such as an indenter. Or, it is possible to stimulate a phase transformation with an electrical bias or even with the heat deposited by the electron beam from the TEM itself. This talk will present recent developments from a number of in situ TEM probing techniques, where deformation in metallic nanostructures or the actuation of phase transformations in functional oxides can be triggered either mechanically or electrically. In the field of dislocation plasticity, deformation is inherently a localized phenomenon so that combining digital image correlation with global stress and strain measurements makes it possible to measure the true local strain in a material. With functional materials, direct measurements of induced strain in the materials makes it is possible to probe the coupled electrical and mechanical properties of materials such as piezoelectric thin films and oxide nanowires. This talk will focus on recently developed testing methodologies for localized stress and strain measurements during in situ TEM mechanical and electrical probing experiments.

Geometry and confinement effects at the nanoscale can result in substantial modifications to a material&’s properties with significant consequences in terms of chemical reactivity, biocompatibility and toxicity. Although benefitting applications across a diverse array of environmental and technological settings, the long-term effects of these changes, for example in the reaction of metallic nanoparticles under atmospheric conditions, are not well understood. Here, we use the unprecedented resolution attainable with aberration-corrected scanning transmission electron microscopy to study the oxidation of cuboid Fe nanoparticles. Performing strain analysis at the atomic-level, we reveal that uncompensated strain at the Fe/Fe oxide interface leads to strain gradients within the growing oxide shell which enhance the transport of diffusing species ultimately driving oxide domain formation and the shape evolution of the particle. We conjecture that such a strain-gradient-enhanced mass transport mechanism may prove essential for understanding the reaction of nanoparticles with gases in general, and for providing deeper insights into ionic conductivity in strained nanostructures.

Microelectronics has been at the leading edge of nanofabrication, providing state-of-the-art methods and techniques. The basic building blocks are based on lithography and overlaying of thin films which often lead to configurations of stress that are difficult to determine or predict. Stress affects processing and defect formation, ultimately impacting reliability and performance. However, stressed thin films can also be used to introduce elastic strains in the electrically active region of a transistor. The structural change in the crystal lattice changes the electronic band configuration of the semiconductor material, improving the mobility of charge carriers. Silicon nitride (SiN) has been widely implemented as a low-cost solution for strained-silicon MOSFET devices. The compound, in addition of being compatible with silicon technology, can be tuned to develop compressive or tensile intrinsic stresses. The characteristics of the SiN film depend on the technique and parameters used for film growth. This impact on the local SixNy composition, the molecular packing of the material, and potentially on the mechanical properties. For example, the Young's modulus of silicon nitride is reported between 150 and 400 GPa. Deviations from the bulk values were as well reported for nanoscale film thicknesses. Therefore, determining with accuracy the elastic properties of materials used for nanoscopic elements is a major challenge for advanced MOSFET technology. A straightforward manner of extract the elastic parameters of an element, is to determine the imposed stresses and measure the corresponding strains. This is not such a simple task to be performed in nanoscale, and requires experimental techniques and procedures with appropriate spatial resolution. Recently, the invention of dark-field electron holography (DFEH) solved a fundamental difficulty associated to scale-of-measurement in the study of nanosystems. The technique allows strain fields to be mapped over micro-scale fields of view, keeping a nanometer spatial resolution. We use then DFEH to develop a method to study the local mechanical properties of small elements. The principle of our approach lies on the use of a crystalline matrix reference of identified mechanical properties as a “sensor”. The method is carried out under three fundamental steps: (i) determination of the stresses in the material of unknown elastic constants; (ii) measuring the strains imposed by this material in the “sensor” crystal of known elastic properties; (iii) determination of the elastic constants of the material under study by modeling the bahaviour of the coupled materials using elasticity theory. The demonstration in done using stressed SiN thin films of undetermined Young's modulus over a (001) Si substrate taken as the “sensor” material. For our film deposited by PECVD and having a biaxial stress of 1.6 GPa we determined that the Young&’s modulus is of 283+-30 GPa.

It has been well known that reducing the dimensions of materials down to the nanometer regime has significant effects on the mechanical properties of the materials. Extensive theoretical and experimental investigations have been carried out to explore various issues in this area. While theoretical investigations have been carried out for dimensions smaller than 20 nm because of calculation limitations, experimental investigations were usually conducted for dimensions larger than 20 nm due to the sensitivity limit of testing facilities or the inability to decouple thin films from their underlying bulk substrates.
Here we design an experiment to accurately extract the Young&’s modulus (E) of thin films with thickness less than 20 nm. Amorphous Al2O3 films with a thickness varying from 2 nm to 20 nm were coated on the surface of single-crystal GaAs nanowires with a constant diameter of 100 nm, forming a core/shell nanowire structure. The Young&’s modulus of Al2O3 film shell can be easily decoupled from that of the core/shell structure. The mechanical response of the nanowires was explored by in-situ compression transmission electron microscopy using a Hysitron PicoIndenter® with a flat punch. The Young moduli of the overall nanowire and the Al2O3 film were then extracted using finite element analysis by assuming E(total) = E(core)V(core) + E(shell)V(shell), in which V(core) and V(shell) represents the volume fraction of the core and the shell, respectively.

Silicon and copper are two materials used in semiconductor structures that do exhibit a large mismatch in the thermal expansion. Thermal cycling during the device&’s function or high temperature of around 400°C during production can induce stresses that potentially cause delamination in the device. Large stresses are built up in through-silicon-via (TSV) structures because of their dimensions. A TSV is typically a 5 to 25µm diameter hole in a Si wafer filled with copper that electrically connects front and back of the wafer. In-situ SPM imaging at elevated temperatures was used to examine plastic flow of copper caused by different thermal expansion coefficients of Si and Cu. A large expansion can cause device failure by delaminating thin film structures that are deposited over or close to the TSV.
Cu has an elastic anisotropy with a Young&’s modulus of 60GPa along <100> directions and 190GPa along <111> directions. Therefore, just by changing the crystal orientation, the thermally induced stress at the Si-Cu-interface can vary significantly and might increase above yield strength for a distinct orientation before others. Moreover, at temperatures of 400°C the plastic deformation by creep mechanisms happens at much smaller stress, and the processing time at these temperatures becomes another critical factor. The different aspects discussed above are being studied by nanoindentation. The anisotropy of elastic modulus can be determined in the microstructure. The Cu yield stress that varies by grain size and orientation does correlate with the hardness measurement. Moreover the creep behavior of copper at strain rates of 10^-4/s to 10^-6/s is studied by nanoscale dynamic indentation combined with temperature - a stress level that should not be exceeded can be determined. These experiments require a temperature control stage with fast heating so that it can reach up to 400°C in less than 60s controlled with a precisely calibrated PID-feedback loop. Creep tests can be performed over a long period using a dynamic referencing technique. A gas blend of argon (95%) and hydrogen (5%) is needed to prevent oxidation of the Cu surface, which can be observed starting from 200°C in ambient conditions.

It is well known that nanoindentation of diamond cubic (dc) semiconductors Si and Ge can induce transformations to a metallic (β-Sn) phase at around 10-12 GPa pressure on loading. During unloading, further phase transformations occur but the dc phase does not result following complete unloading. Rather, several metastable crystalline and phases can result at room temperature, depending on the unloading conditions. In this paper, we indicate that, in both Si and Ge, new and interesting metastable end phases can be stabilized at ambient pressure by built in stress.
Recently, we have shown that r8-Si is the dominant end phase following indentation of both dc and amorphous Si (a-Si) under appropriate slow unloading. This is surprising since the r8 phase of Si is expected from calculation and diamond anvil experiment to be unstable and transform to a bc8 phase at a pressure of around 2-3 GPa. Not only is this phase present at ambient pressure but it is stable enough that we have been able to confirm that it is a narrow band gap semiconductor and can be doped both p- and n-type. We show that this r8 phase is stabilized within the surrounding Si matrix by a compressive stress of around 2 GPa.
In Ge, we have found for the first time that r8-Ge is also the stable end phase following indentation loading and unloading of a-Ge. However, even though r8-Ge is expected from calculation to exist down to about 0.5 GPa (lower that the 2-3 GPa for Si), we find that it is unstable at room temperature and anneals to a hexagonal diamond (hd) phase after around 100 hours. This suggests that the compressive stress in the surrounding a-Ge matrix is considerable less than for Si and cannot effectively stabilise r8 Ge. In the case of r8 Ge we compare DFT calculations of the Raman line frequencies with experiment to distinguish r8 from other possible phases.

The use of strain to enhance the performance of devices is now a well-established technique in the microelectronics industry. Different techniques can be used to engineer strain but many of the processing steps used during device fabrication can introduce strain in a beneficial or adverse way. Measuring strain distributions accurately and locally is therefore of key importance to optimizing procedures and modeling performance.
Here, we present the state of the art in strain mapping at the nanoscale using dark-field electron holography (DFEH) [1]. Strain can be measured to high precision, with nanometre spatial resolution and for micron fields of view and is applicable to focused-ion beam (FIB) prepared samples. The method will be described, including calibrations and optimum conditions for specimen preparation. The problem of the relaxation of the thin samples will be addressed using a combination of finite-element modeling (FEM) and dynamical-scattering simulations.
The technique has been used for a wide range of applications, notably semiconductor devices and thin films. We will present measurements of strain in the active regions of strained-silicon MOSFET devices, strained multilayers and embedded quantum dots [2]. The current limitations of the technique will be discussed along with future areas of application. In particular, results will be shown for the recently installed I2TEM microscope (Hitachi), an instrument specifically designed for dark-field electron holography experiments.
[1] M.J. Hyuml;tch, F. Houdellier, F. Hüe, and E. Snoeck, Nature 453 (2008) 1086-1089. Nanoscale holographic interferometry for strain measurements in electronic devices.
[2] N. Cherkashin, S. Reboh, M.J. Hyuml;tch, A. Claverie, V.V. Preobrazhenskii, M.A. Putyato, B.R. Semyagin, and V.V. Chaldyshev, Appl. Phys. Lett. 102, 173115 (2013). Determination of stress, strain, and elemental distribution within In(Ga)As quantum dots embedded in GaAs using advanced transmission electron microscopy.

Numerous imaging methods have been developed over the last years in order to study materials on the nanoscale. In this context scanning diffraction microscopy is becoming a routine technique giving access to the structural properties under the effects of different environment (temperature, epitaxy, sample geometry, pressure, electric field ...) in particular strain, composition and orientation. A new technique and a new software package were developed at the ID01 ESRF beamline offering an easy to use scanning probe microscope with intriguing imaging qualities.
With a nanofocused xray beam (~180x300nm), the technique consists in a quick continuous mapping (K-Map) with sub micrometre resolution of a 2D real space map of a sample at a given Bragg condition.
These real space maps are recorded in continuous motion for every point across a rocking curve while recording 2D detector images using a suffiently fast pixel detector. In this way five dimensional data sets are produced, composed by millions of detector images. This requires an analysis in an automatic way by the user friendly software SOCS (Strain and Orientation Calculation Software) generating 2D maps of strain distribution and decoupling tilt from strain.
The K-Map technique and the SOCS software as well as several examples will be presented. e.g. Ge layers comprised in SiN/Ge/Si micro-structures, a candidate as active material for integrated light emitter applications, has been investigated and their elastic strain distribution has been extracted in few hours using the K-Map and SOCS. The mapping of Si strain distribution, in photovoltaic cells, induced by the Al back contact layer was also investigated and will be presented.

The recent advance in Electron Backscatter Diffraction known as High Resolution EBSD has permitted the strain tensor components and neighbour disorientation measurements to be mapped at resolutions better than 1 part in 10000. Currently, all measurements are with respect to a reference point in the sample which is chosen on the basis that it comes from an area in which the strain is zero or at least known. This latter limitation sometimes requires a separate sample to be present alongside that under investigation, a computer simulated pattern to be generated which can be used as reference or for insitu mechanical testing experiments, a before after pair of patterns has to be collected, the reference pattern being collected from the unstrained before test sample. Example data using all three methods has been obtained. All of these types of experiment however, require that the pattern centre coordinates PCX, PCY, and specimen to screen distance, Z, be accurately known in all circumstances to better than 5 parts in 10000. This has now been achieved using a circular aperture positioned in front of the phosphor screen on which the patterns are recorded. A calibration procedure is initially carried out that determines the relationship between the positions of the projection of the aperture centre onto the phosphor, the position of the shadow cast by the aperture, its radius and the PCX PCY and Z values. Once calibrated any future PCX PCY and Z values can be determined from the new position of the aperture shadow and its radius. This advance has been used in the mapping of strain in copper and nickel base polycrystalline samples. Localised strain values significantly in excess of the macroscopic yield point in metals have been observed and are thought to be associated with dislocation pile ups or local grain boundary incoherence.

Recently, stoichiometric FeRh has become of great interest for thermally assisted magnetic recording applications. Increasing temperatures above ~350K, FeRh undergoes a metamagnetic first order transition from an antiferromagnet to a ferromagnet. Upon this first order transition, the volume increases by ~1% and the resistivity is reduced by ~50%, suggesting the possibility to strain control this phase transition and exploiting the concomitant change of resistance as a magnetic memory element. Substrate induced epitaxial strain has previously been reported to alter the phase transition temperature of FeRh, however it is crucial to realize a controllable in-situ strain for magnetic memory device application. Here we report on the fabrication of epitaxial FeRh on a-/c-domain PbZr0.2Ti0.8O3/SrRuO3/NdScO3 heterostructures, in which the strain state of FeRh can be tuned by the ferroelectric domain architecture of PZT. Through the application of a 6V pulse between the FeRh and the bottom electrode SrRuO3, the ferroelectric domain architecture is altered and the electrical resistance of the FeRh can be reversibly changed by more than 20%. This approach allows the desired writing and reading of a magnetic memory device purely by electrical means.

We present recent experimental results of the grain size effect on thermomechanical response of superelastic NiTi. In situ tensile X-ray diffraction was carried out to study the pseudoelastic deformation mechanism of specimens with different grain size. Analysis of the diffraction profiles revealed that with decreasing grain size below 60 nm, there is a transition in pseudoelastic deformation mechanism. While above this grain size deformation is carried by transformation strain, below 60 nm the contribution of elastic strains as carrier of deformation increase . The smaller the grain size is the larger contribution of elastic strains. The above picture provides an in-depth knowledge of the grain size and elastic strain effects on the following thermomechanical properties:
1- Small hysteresis loop area.
2- Weak temperature dependence of transformation stress.
3- Extension of superelasticity window.
4- Small latent heat released during stress-induced phase transition.
5- Dependence of Young&’s modulus on grain size.
We further show that promoting the elastic strains via grain refinement has significant advantages for better performance of the material in non-isothermal applications such as monotonic and high frequency cyclic loading/unloading.
References:
1- Stress hysteresis and temperature dependence of phase transition stress in nanostructured NiTi------Effects of grain size Aslan Ahadi and QingPing Sun. Accepted in Applied Physics Letters.

The indentation of Ni was simulated using molecular dynamics to evaluate interface effects on plastic deformation during the first few nm of penetration. Ni samples with (111) and (100) surfaces were indented using EAM model potentials and two types of spherical indenter models. The first one is represented simply by the force it exerts on the indented surface: smooth, perfectly hard and including no interface effects. The second one simulates the crystalline surface of a diamond cubic structure with the [100] orientation perpendicular to the indented surface. Force reaction vs. penetration curves were generated for both cases and the plastic deformation was analyzed and compared to study the influence of interface effects. It was observed in both film surface orientations that the layers of Ni atoms nearest the contact surface were strongly affected by the diamond structure surface. The effect was less pronounced in the case of the (100) surface due to its better matching the (100) surface of the diamond structure. For the case of a (111) indented surface, interface effects resulted in plastic deformation being initiated at a greater penetration depth.

Managing the emerging internal mechanical stress in chips particularly if they are 3D-stacked is a key task to maintain performance and reliability of microelectronic products.
Microporous and mesoporous dielectrics used as insulating materials between on-chip interconnects are a crucial component in on-chip interconect stacks of leading-edge microelectronic products to reduce electrical signal delay and power loss. The main drawbacks of these porous dielectrics are their weak mechanical properties and their susceptibility to process-induced damage. New types of porous organosilicate glass (OSG) materials were developed based on sol-gel templating and spin-on deposition. Significantly improved mechanical properties like elastic modulus were achieved for these OSG materials by tuning the sol-gel chemistry to form (a) organic bridges between network silicon atoms, (b) polymer chains across interfaces, and (c) periodic pore arrangement. Based on previous studies, depending on silica, surfactant and solvent compositions, domains with different pore arrangements - cubic, hexagonal or disordered pores - were found in self-assembled porous organosilicate glass (SA-OSG) films.
In this talk, we will demonstrate the relationships between porosity, pore topology and elastic modulus based on finite element (FE) modeling and experimental studies at several OSG films. The experimental studies include positron annililation lifetime spectroscopy (PALS) and nanoindentation (NI) measurements.
The chemical-vapor-deposited CVD-OSG films with their broad pore size distribution follow the model of random overlapping spherical solids. SA-OSG films with p<0.3 follow the model of ordered non-overlapping spherical pores, while those with p>0.3 follow the model of random overlapping spherical pores. The plot on the pore topology map implies that the SA-OSG films studied have a threshold at p=0.3 at which pores start to be interconnected.

The stress produced during the growth of polycrystalline thin films can be very large and depends strongly on the growth conditions. For instance, stress in electrodeposited Ni films can be 500 MPa tensile if the film is grown rapidly and 500 MPa compressive if the film is grown slowly. We have developed a model for the residual stress that considers the stress that develops at the growing triple junction where the grain boundary forms between two adjacent grains. The model can explain the evolution of stress with film thickness as individual islands coalesce into a continuous film. The model also predicts that the steady-state stress depends on the parameter D/RL where D is the effective diffusivity, R is the growth rate and L is the grain size. Predictions of the model are compared with real-time measurements of stress evolution in thin films obtained using wafer curvature. We also discuss how growth effects can be used to spatially vary the stress in films grown on patterned substrates.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000

Introduction
The strained Silicon (Si) technique, which can enhance carrier mobility, is necessary to improve the performance of LSI. Silicon nitride (SiN) is a typical material for stressor. In the next generation devices, it is desired to establish the deposition technique with high thermal stability and stress controllability. The atomic layer deposition (ALD) has attracted much attention as a deposition technique to realize a good uniformity and high controllability of film thickness. On the other hand, the ALD SiN film, compared with conventional thermal chemical vapor deposition (CVD) or plasma CVD, has not achieved to introduce large strain [1]. In this study, we investigated a carbon or boron doped SiN film in order to increase the film stress and discuss the mechanism of stress control. In addition, the thermal stability was evaluated for practical applications.
Experiment
50-nm-thick ALD SiN films were deposited on Si (001) substrates using the batch-type ALD equipment (Tokyo Electron Tohoku, TELINDY-IRad) at 630 oC. Precursors, dichlorosilane (SiH2Cl2) and ammonia (NH3) gases, were alternately supplied in the deposition chamber. Carbon or boron was doped by simultaneous supply of ethylene (C2H4) or trichloroborane (BCl3), respectively. Carbon and boron concetrations in the film were approximately 6.5 % and 8.0 %, respectively. To investigate thermal stability, post-deposition annealing (PDA) was performed at 800 oC for 360 sec., or at 1000 oC for 0sec. (spike annealing). UV-Raman measurements using Ar ion laser (lambda; = 364 nm) were performed to evaluate the implied stress at the Si surface [2]. In addition, chemical bonding states of the impurity doped SiN films were evaluated by X-ray photoelectron spectroscopy (XPS).
Results and Discussion
The stress at the Si surface deposited with non-doped SiN film indicated 8.8 MPa. Meanwhile, we confirmed the stress enhancements of 10 % and 50 % by the carbon or boron doped SiN film, respectively. Difference of binding energy between Si2p and N1s photoelectron peaks were also increased by the intentional impurity doping. It is considered that impurity affected on the chemical bonding states in SiN films and the electro negativity of impurity is important factor for stress enhancement. Different stress behaviors were also observed after annealing process, depending on the doped impurity species. In the cases of carbon doping, PDA induced stress relaxation slightly larger than non-doped SiN. In the case of B doping, the stress drastically varied depending on the annealing conditions. The stress modification after PDA might be originated in the rearrangement of chemical bonding states. In conclusion, we have demonstrated that intentional impurity doping has a potential to enhance stress properties and the control of chemical bonding states contribute to the thermal stability.
References
[1] K. Nagata, et al., ECS. Trans., 53, 51 (2013).
[2] A. Ogura, et al., J. Appl. Phys., 45, 3007 (2006).

We investigate the interfaces and mechanical properties of sputtered, highly (100) and (111) textured Cu/Co multilayers when individual layer thickness, h, varying from 1 nm to 200 nm. In (100) Cu/Co multilayers, Co has primarily face-centered-cubic (FCC) structure and high density of inclined stacking faults (SFs), which provides extra hardening when h is large. In contrast in (111) textured Cu/Co, dependent on layer thickness, high density SFs and twins parallel to layer interface are observed. The mismatch strain between Cu/Co interfaces as the main reason to form these defects is discussed. When individual layer thickness is a few nanometers, both systems have fully coherent FCC interface. (111) Cu/Co has similar size dependent strengthening and peak hardness compared to (111) Cu/Ni multilayers. The peak strength of (100) Cu/Co may be dominated by the transmission of partial dislocations across interface, and hence it is lower than the peak strength of (100) Cu/Ni, which is dictated by transmission of full dislocations across interfaces.

Nowadays semiconductor heterostructures are essential for realizing advanced optoelectronic devices based on commonly available substrates. Owing to the wealth of electronic porperties of semiconductors, e.g. bandgap and carrier mobility, a broad range of device functions and emission wavelengths can be accessed [1]. However, defects associated with plastic relaxation of misfit strains often hamper full exploitation of the application spectrum of devices based on heterostructures. As a consequence, different strategies have been developed to overcome the deleterious effect of misfit related defects. One of these paths relies on reducing the size of the active regions into the nanoscale, where 3-dimensional elastic strain relaxation allows to eliminate or at least decrease the defect density [2]. In order to understand and to predict the critical dimensions for misfit dislocation formation in nanorods or -wires containing an axial heteroepitaxial interface, various theoretical approaches based on linear elasticity have been proposed in the last 15 years [3-5]. On closer examination, each of these approaches reveals strengths and weaknesses.
In view of the variety of heteroepitaxial material combinations of interest for optoelectronics and approximations made in the models, the present contribution addresses the issue of their applicability and accuracy/reliability. The focus is put on approaches describing the energetics of strained nanorods without as well as with one or one pair of existing misfit dislocations at equilibrium positions in the interface plane. In order to cover material assemblies of similar and dissimilar elastic parameters and different misfit, the approaches are exemplarily applied to GaSb/InP and InAs/GaAs semiconductor heterojunctions. We find that those concepts which calculate the total energy yield comparable critical dimensions, i.e. diameter and deposit height, with minor variations depending mainly on the considered misfit dislocation arrangement and conditions used for strain field determination. Most realistic assumptions are met by the finite-element method, which is however not expressible in analytic equations [5]. In contrast, the analytic approach by Zubia and Hersee leads to deviating results. One of its strengths is the prediction of strain partitioning between two materials at a heterojunction. As a major drawback, the stress fields do not fulfill the boundary conditions at the top surface of the deposit. Moreover, part of the deviations can be attributed to the employed energy areal density criterion, as it discards the strain energy in a significant volume of the nanoscale substrate and deposit regions.
References
[1] B.S. Meyerson: Proceedings of the IEEE 80 (1992) 1592
[2] D. Zubia, S.D. Hersee: J. Vac. Sci. Technol. B 18 (2000) 3514
[3] E. Ertekin et al.: J. Appl. Phys. 97 (2005) 114325
[4] F. Glas: Phys. Rev. B. 74 (2006) 121302(R)
[5] H. Ye et al.: Nanoletters 9 (2009) 1921

The mechanical properties of nanowires are very important for the structural reliability and even the functionality of the nanowires. A variety of experimental techniques, including uniaxial compression inside the electron microscope chamber, have been developed to measure the mechanical properties of the nanowires. Appropriate alignment between the applied force and the nanowire axial direction and uniform nanowire shape/diameter are required for accurate mechanical property measurement by uniaxial compression. However, it is extremely difficult to properly align the nanowire axial direction relative to the applied force direction and to grow nanowires without a tapering angle. These two factors have been largely ignored in previous nanowire mechanical characterization using uniaxial compression. In this presentation, we will demonstrate finite element analysis results on the effect of force misalignment and tapering angle of nanowire on the critical stress and critical strain of compression-induced buckling as well as the nanowire Young&’s modulus deduced from these critical stress values.

Recent studies show that nanotwins in certain face-centered-cubic (fcc) metals can lead to simultaneous increases in mechanical strength, while maintaining good ductility, electrical conductivity, and thermal stability. Nanoscale growth twins can be introduced during growth processes in metallic materials with low stacking fault energy (SFE). However, high SFE in many metals fundamentally inhibits growth twin formation, and typical approaches for increasing twin density yield little success. In this presentation, we summarize recent success in synthesizing Al films with high-density growth twins via template techniques in multilayer films with coherent interfaces. We will discuss mechanisms that enable the formation of high density growth twins in Al films. Mechanical properties of nanotwinned Al films will also be presented.

Shape memory alloy polycrystals have great potential in energy, actuation, and sensing technologies, but their applicability is limited by the propensity for intergranular brittle fracture, which results from mechanical interactions between grains during displacive martensitic transformations. The ductility of these alloys can be drastically improved by a strain engineering strategy, which is to introduce a ductile, non-transforming second phase into grain boundaries to reconcile transformation strains from neighboring grains. We study the Co-Ni-Al system, which can precipitate a second phase γ into the transforming austenite matrix β, primarily at grain boundaries. Such dual-phase microstructures are significantly more resistant to intergranular fracture, because γ can accommodate strain from neighboring β grains by undergoing plastic deformation (which is easily recoverable for high temperature applications). We study the crystallographic dependence of γ distribution and its ability to accommodate transformation strains utilizing electron backscatter diffraction (EBSD) and nanoindentation. We also model the strain distributions and accommodations during transformations, and further evaluate the effect of this grain boundary engineering technique quantitatively.