Force-deflection spectroscopy: a new method to determine the Young's modulus of nanofilaments

We demonstrate the determination of Young's modulus of nanowires or nanotubes via a new approach, that is, force-deflection spectroscopy (FDS). An atomic force microscope is used to measure force versus deflection (F-D) curves of nanofilaments that bridge a trench patterned in a Si substrate. The FD data are then fit to the Euler-Bernoulli equation to determine Young's modulus. Our approach provides a generic platform from which to study the mechanical and piezoelectric properties of a variety of materials at the nanoscale level. Young's modulus measurements on ZnS (wurtzite) nanowires are presented to demonstrate this technique. We find that the Young's modulus for rectangular cross section ZnS nanobelts is 52 +/- 7.0 GPa, about 30% smaller than that reported for the bulk.     

Mechanical properties of ultrahigh-strength gold nanowires

Nanowires have attracted considerable interest as nanoscale interconnects and as the active components of both electronic and electromechanical devices. Nanomechanical measurements are a challenge, but remain key to the development and processing of novel nanowire-based devices. Here, we report a general method to measure the spectrum of nanowire mechanical properties based on nanowire bending under the lateral load from an atomic force microscope tip. We find that for Au nanowires, Young's modulus is essentially independent of diameter, whereas the yield strength is largest for the smallest diameter wires, with strengths up to 100 times that of bulk materials, and substantially larger than that reported for bulk nanocrystalline metals (BNMs). In contrast to BNMs, nanowire plasticity is characterized by strain-hardening, demonstrating that dislocation motion and pile-up is still operative down to diameters of 40 nm. Possible origins for the different mechanical properties of nanowires and BNMs are discussed.     

Nanoindentation Measurements of the Mechanical Properties of Ni Thin Films: Effects of Film Microstructure and Substrate Modulus

This paper presents the results of nanoindentation measurements of the hardness and moduli of normally and obliquely deposited nanocrystalline Ni films on substrates of SiO₂, Si, and bulk Ni. Following an initial characterization of film microstructure and surface topography with atomic force microscopy (AFM), the paper examines the effects of film microstructure, film thickness, and substrate modulus on the measured film mechanical properties. Obliquely deposited films are shown to have lower hardness values than normally deposited films. The measured hardness values and material pile-up are also shown to depend significantly on the mismatch between the film modulus and substrate modulus. A framework is presented for quantifying the effects of substrate modulus mismatch on basic film mechanical properties.     

Elasticity and yield strength of pentagonal silver nanowires: In situ bending tests

This paper reports in situ mechanical characterization of silver nanowires (Ag NWs) inside a scanning electron microscope using a cantilevered beam bending technique. Measurements consisted in controlled bending of a cantilevered NW by the tip of an atomic force microscope glued to the force sensor. Relatively high degree of elasticity followed by either plastic deformation or fracture was observed in bending experiments. Experimental data were numerically fitted into the model based on the elastic beam theory and values of Young modulus and yield strength were extracted. Measurements were performed on twenty Ag NWs with diameters from 76 nm to 211 nm. Average Young modulus and yield strength were found to be 90 GPa and 4.8 GPa respectively. In addition, fatigue tests with several millions of cycles were performed and high fatigue resistance of Ag NWs was demonstrated.     

Mechanical elasticity of vapour-liquid-solid grown GaN nanowires

Mechanical elasticity of hexagonal wurtzite GaN nanowires with hexagonal cross sections grown through a vapour-liquid-solid (VLS) method was investigated using a three-point bending method with a digital-pulsed force mode (DPFM) atomic force microscope (AFM). In a diameter range of 57-135?nm, bending deflection and effective stiffness, or spring constant, profiles were recorded over the entire length of end-supported GaN nanowires and compared to the classic elastic beam models. Profiles reveal that the bending behaviour of the smallest nanowire (57.0?nm in diameter) is as a fixed beam, while larger nanowires (89.3-135.0?nm in diameter) all show simple-beam boundary conditions. Diameter dependence on the stiffness and elastic modulus are observed for these GaN nanowires. The GaN nanowire of 57.0?nm diameter displays the lowest stiffness (0.98?N?m(-1)) and the highest elastic modulus (400 ± 15?GPa). But with increasing diameter, elastic modulus decreases, while stiffness increases. Elastic moduli for most tested nanowires range from 218 to 317?GPa, which approaches or meets the literature values for bulk single crystal and GaN nanowires with triangular cross sections from other investigators. The present results together with further tests on plastic and fracture processes will provide fundamental information for the development of GaN nanowire devices.     

Nanomechanical characterization of one-step combustion-synthesized Al(4)B(2)O(9) and Al(18)B(4)O(33) nanowires

Two kinds of aluminum borate nanowires, Al(4)B(2)O(9) and Al(18)B(4)O(33), were successfully synthesized by a one-step combustion method through control of the Al:B atomic ratio and synthesis temperature. Both nanowires are single crystalline but have distinguishing growth habits. Nanoindentation tests were performed directly on individual nanowires to reveal their mechanical properties. A 70% reduction in elastic modulus was found in Al(18)B(4)O(33) nanowires compared with their bulk counterpart. Al(18)B(4)O(33) nanowires exhibited higher hardness and elastic modulus than Al(4)B(2)O(9) nanowires.     

Application of nanoindentation technique for structural characterisation of weld materials

Nanoindentation, a modern technique for investigation of mechanical properties of materials using a modified scanning force microscope (SEM), has been employed to examine two different types of boiler shell weld materials from nuclear power plants. These weld materials differ mainly in their manganese content and temperature treatment. The nanoindentation technique allows very small regions in grains to be investigated and different phase structures are distinguished using this technique. It is shown that a difference exists between the microhardness values, which integrate over several grains compared to the nanohardness measurements for weld materials made over smaller regions. The difference between the phase structures of these small regions is determined by a combination of both SFM imaging and the characteristic properties of force–depth curves which allow specific values of hardness and Young's modulus to be determined. Detailed measurements of the nanohardness and Young's modulus for various weld materials are given and these values are related to the material properties. It has been found that the hardness of the grains is always less in the centre and increases towards the grain boundary.     

Structural and electronic properties of AlX (X = P,As, Sb) nanowires: Ab initio study

Present paper discusses the structural stability and electronic properties of AlX (X = P, As and Sb) nanowires in its linear, zigzag, ladder, square and hexagonal type atomic configurations. The structural optimization has been performed in self consistence manner by using generalized gradient approximation with revised Perdew, Burke and Ernzerhof type parameterization. The study observes that in all the three nanowires, the square shaped atomic configuration is the most stable one. The calculated electronic band structures and density of states profile confirms the semiconducting behaviour of linear and zigzag shaped nanowires of AlP, whereas for AlAs and AlSb nanowires are metallic. The ground state properties have also been analysed in terms of bond length, bulk modulus and pressure derivative for all the nanowires along with their bulk counterpart. The lower bulk modulus of all the linear shaped geometries of AlX nanowires in comparison to its bulk counterpart indicates softening of the material at reduced dimension.     

Mechanical properties of microwave hydrothermally synthesized titanate nanowires

In this investigation titanate nanowires were synthesized by a microwave hydrothermal process and their nanomechanical characterization was carried out by a compression experiment via buckling instability using a nanomanipulator inside a scanning electron microscope. Nanowires of diameters 120-150?nm and length tens of microns can be synthesized by keeping a commercial nanoparticle inside a microwave oven at 350?W and 210?°C for 5?h. The nanowire was clamped between two cantilevered AFM tips attached to two opposing stages of the manipulator for nanomechanical characterization. The elasticity coefficients of the titanate nanowires were measured by applying a continuously increasing load and observing the buckling instability of the nanowires. The buckling behavior of a nanowire was analyzed from the series of SEM images of displacement of the cantilever attached to the nanowire due to application of load. The critical loads for different sized titanate nanowires were determined and their corresponding Young's modulus was computed with the Euler pinned-fixed end model. The Young's modulus of these microwave hydrothermal process synthesized titanate nanowires were determined to be approximately in the range 14-17?GPa. This investigation confirms the capability of the nanomanipulator via the buckling technique as a constructive device for measuring the mechanical properties of nanoscale materials.     

In situ observation of size-scale effects on the mechanical properties of ZnO nanowires

In this investigation, the size-scale in mechanical properties of individual [0001] ZnO nanowires and the correlation with atomic-scale arrangements were explored via in situ high-resolution transmission electron microscopy (TEM) equipped with atomic force microscopy (AFM) and nanoindentation (NI) systems. The Young's modulus was determined to be size-scale-dependent for nanowires with diameter, d, in the range of 40 nm ≤ d ≤ 110 nm, and reached the maximum of ～ 249 GPa for d = 40 nm. However, this phenomenon was not observed for nanowires in the range of 200 nm ≤ d ≤ 400 nm, where an average constant Young's modulus of ～ 147.3 GPa was detected, close to the modulus value of bulk ZnO. A size-scale dependence in the failure of nanowires was also observed. The thick ZnO nanowires (d ≥ 200 nm) were brittle, while the thin nanowires (d ≤ 110 nm) were highly flexible. The diameter effect and enhanced Young's modulus observed in thin ZnO nanowires are due to the combined effects of surface relaxation and long-range interactions present in ionic crystals, which leads to much stiffer surfaces than bulk wires. The brittle failure in thicker ZnO wires was initiated from the outermost layer, where the maximum tensile stress operates and propagates along the (0001) planes. After a number of loading and unloading cycles, the highly compressed region of the thinner nanowires was transformed from a crystalline to an amorphous phase, and the region near the neutral zone was converted into a mixture of disordered atomic planes and bent lattice fringes as revealed by high-resolution images.     

Molecular dynamics evaluation of strain rate and size effects on mechanical properties of FCC nickel nanowires

Based on molecular dynamics method, an atomistic simulation scheme for damage evolution and failure process of nickel nanowires is presented, in which the inter-atomic interactions are represented by employing the modified embedded atom potential. Extremely high strain rate effect on the mechanical properties of nickel nanowires with different cross-sectional sizes is investigated. The stress–strain curves of nickel nanowires at different strain rates subjected to uniaxial tension are simulated. The elastic modulus, yield strength and fracture strength of nanowires at different loading cases are obtained, and the effect of strain rate on these mechanical properties is analyzed. The numerical results show that the stress–strain curve of metallic nanowires under tensile loading has the trend identical to that of routine polycrystalline metals, and the yield strain of nanowires is independent of the strain rate and cross-sectional size. Based on the simulation results, a set of quantitative prediction formulas are obtained to describe the strain rate sensitivity of nickel nanowires on the mechanical properties, and the resulting formulas of the Young’s modulus, yield strength and fracture strength of nickel nanowires exhibit a linear relation with respect to the logarithm of strain rate. Furthermore, some comprehensive correlation equations revealing both the strain rate and size effects on mechanical properties of nickel nanowire are proposed through the numerical fitting and regression analysis, and the mechanical behaviors observed in this study are consistent with those from the experimental and available numerical results.     

Elasticity size effects in ZnO nanowires--a combined experimental-computational approach

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

In situ tensile testing of nanofibers by combining atomic force microscopy and scanning electron microscopy

A nanomechanical testing set-up is developed by integrating an atomic force microscope (AFM) for force measurements with a scanning electron microscope (SEM) to provide imaging capabilities. Electrospun nanofibers of polyvinyl alcohol (PVA), nylon-6 and biological mineralized collagen fibrils (MCFs) from antler bone were manipulated and tensile-tested using the AFM-SEM set-up. The complete stress-strain behavior to failure of individual nanofibers was recorded and a diversity of mechanical properties observed, highlighting how this technique is able to elucidate mechanical behavior due to structural composition at nanometer length scales.     

Elastic properties of thin poly(vinyl alcohol)-cellulose nanocrystal membranes

In spite of extensive studies on the preparation and characterization of nanocomposite materials, the correlation of their properties at the nanoscale with those in bulk is a relatively unexplored area. This is of great importance, especially for materials with potential biomedical applications, where surface properties are as important in determining their applicability as bulk characteristics. In this study, the nanomechanical characteristics of thin poly(vinyl alcohol) (PVOH)-poly(acrylic acid) (PAA)-cellulose nanocrystal (CNC) membranes were studied using the nanoindentation module in an atomic force microscope (AFM) and the properties were compared with the macro-scale properties obtained by tensile tests. In general, the elastic properties measured by nanoindentation followed the same trend as macro-scale tensile tests except for the PVOH 85-PAA 0-CNC 15 sample. In comparison to the macro-scale elastic properties, the measured elastic moduli with AFM were higher. Macro-scale tensile test results indicated that, in the presence of PAA, incorporation of CNCs up to 20?wt% improved the elastic modulus of PVOH, but when no PAA was added, increasing the CNC content above 10?wt% resulted in their agglomeration and degradation in mechanical properties of PVOH. The discrepancy between macro-scale tensile tests and nanoindentation in the PVOH 85-PAA 0-CNC 15 sample was correlated to the high degree of inhomogeneity of CNC dispersion in the matrix. It was found that the composites reinforced with cellulose nanocrystals had smaller indentation imprints and the pile-up effect increased with the increase of cellulose nanocrystal content.     

单晶硅纳米梁杨氏模量的弯曲测试

为研究纳米梁的力学特性,采用SOI晶圆制备了硅双端固支梁纳米梁,利用典型的原子力显微镜（AFM）弯曲测试方法测试了硅纳米梁〈100〉晶向的杨氏模量.AFM悬臂探针定位在纳米梁的中点处向下移动,纳米梁受到挤压发生弹性形变,形变过程存在一个最大形变点,在该点后,纳米梁被挤压在SOI硅片底层硅上.最大形变前的测试数据用于计算力-位移曲线斜率,最大形变后的测试数据用于计算探针的灵敏度,其实验值分别为0.792N/m和81.83nm/V.最终得到的杨氏模量为104GPa,该值小于体硅的杨氏模量,表面应力和缺陷可能是导致实验值偏小的原因.     

High-precision measurements of magnetostriction in nickel structures for magnetoresistive nanocontacts

Variations as small as 2 nm in the dimensions of a nickel microwire exposed to a magnetic field have been detected using an atomic force microscope specially made of nonmagnetic materials. Based on the results of these high-precision measurements, it is established that the ballistic magnetoresistance of nanocontacts formed by nickel electrodeposition between nickel nanowires is related to a considerable extent to the mutual displacement of nanowires caused by magnetostriction. Analogous measurements for nickel nanofilms did not reveal such displacements to within the experimental sensitivity. Therefore, film nanostructures are more suitable for the formation of nanocontacts with a minimum magnetostriction contribution.     

Effects of environmental degradation on near-fiber nanomechanical properties of carbon fiber epoxy composites

The fiber and matrix interphase is believed to control the fundamental load-transfer process and thereby bulk mechanical properties of composites. Carbon fiber reinforced composite properties have been qualitatively shown affected by moisture based degradation, however, quantitative characterization of the nanomechanical properties as a function of exposure time is still unknown. Here, quantitative measurement of the degraded epoxy matrix properties has been performed, taking advantage of the high scanning resolution of atomic force acoustic microscopy (AFAM). Composite samples were exposed to accelerated degradation and characterized using AFAM, showing a variation in elastic modulus of the matrix as a function of exposure.     

Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites

The local microstructure can be altered significantly by various fibre surface modifications, causing property differences between the interphase region and the bulk matrix. By using tapping mode phase imaging and nanoindentation tests based on the atomic force microscope (AFM), a comparative study of the sized fibre surface topography and modulus as well as the local mechanical property variation in the interphase of E-glass fibre reinforced epoxy resin and E-glass fibre reinforced modified polypropylene (PPm) matrix composites was conducted. The phase imaging AFM was found a highly useful tool for probing the interphase with much detailed information. Nanoindentation experiments indicated the chemical interaction during processing caused by a gradient profile in the modulus across the interphase region of γ-aminopropyltriethoxy silane (γ-APS) and polyurethane (PU)-sized glass fibre reinforced epoxy composite. The interphase with γ-APS/PU sizing is much softer than the PPm matrix, while the interphase with the γ-APS/PP sizing is apparently harder than the matrix, in which the modulus was constant and independent of distance away from the fibre surface. The interphase thickness varied between less than 100 and ≈300 nm depending on the type of sizing and matrix materials. Based on a careful analysis of ‘boundary effect’, nanoindentation with sufficient small indentation force was found to enable measuring of actual interphase properties within 100 nm region close to the fibre surface. Special emphasis is placed on the effects of interphase modulus on mechanical properties and fracture behaviour. The interphase with higher modulus and transcrystalline microstructure provided simultaneous increase in the tensile strength and the impact toughness of the composites.     

多孔氧化铝模板的纳米力学性能测试与分析

为了掌握多孔氧化铝模板的纳米力学性能,采用二次氧化法制备孔径在30～40nm之间且高度有序的纳米阵列氧化铝模板,并使用扫描电子显微镜(SEM)对其形貌进行表征;在原位纳米力学测试系统上进行微压痕实验,对样品表面力学性能(纳米硬度、模量)进行测试;利用原子力成像功能对实验区域扫描成像,在纳米尺度下观察和分析样品形貌.结果表明,AAO模板在同一深度处对应的硬度、模量值明显高于相应的基体材料铝,膜基体系的抗载能力明显提高;在压入深度为70～240nm时,AAO膜板的硬度和模量值分别为5.8GPa和106GPa,但从深度250nm时开始出现减小趋势;单晶铝与压针的接触为理想刚塑性接触,AAO模板与压针的接触为弹塑性接触.     

High molecular weight polyethylene nanospheres: synthesis physical and mechanical properties

The high molecular weight (MW) polyethylene (PE) particles of particle size varied from macro to micron to nanometer were synthesized by Grignard reagent. The microscopy analysis (scanning electron microscope, SEM; transmission electron microscope, TEM; and atomic force microscope, AFM) shows the spherical shape of PE particles. The effects of particle size, varies from macro to nanometer scale on crystal structure, crystallinity (chic), glass transition temperature (Tg), melting temperature (Tm), surface roughness and mechanical properties were studied. Differential scanning calorimetry (DSC) experiments show that the nanoparticles of PE are highly crystalline (chic approximately equal 72%). The crystal length of PE nanoparticles is found to be approximately 14 A. Although the Gibbs-Thomson equation is explained the depression of melting temperature (DeltaTm) by 5 degrees C, the impervious results of Tg are still not fully understood. The low roughness value (2 A) proves the presence of "atomic-scale-chain" folding at the surface of PE nanoparticles. A novel protocol is developed, and the elastic modulus of individual nanospherical PE particles is computed from 'force-distance' mapping curves of AFM. Hemispherical tungsten (W) tip was fabricated from focused ion beam and used as an indenter to measure the mechanical properties. It is found that the nano sized PE particles have higher elastic modulus (E = 1.2-1.4 GPa) compared to the bulk or macro sized PE (E = 0.6-0.7 GPa). The results corroborate the robustness of our experiments, since, the analogous results for macro sized particles match well with the literature.