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In situ observation of size-scale effects on the mechanical properties of ZnO nanowires
Nanotechnology
Abstract
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.
... Elastic properties of ZnO NWs have been measured by all common techniques employed for mechanical characterization of one-dimensional nanostructures including tensile test [36][37][38][39][40][41], resonance method [42][43][44][45][46][47], cantilevered beam bending [48][49][50][51], three-point bending [52,53], nanoindentation [54,55], buckling [40,56] and contact resonance atomic force microscopy (AFM) [57]. ...
... Note that Manoharan et al. [48] provided only the average value for the range of diameters; therefore, it is presented as a dashed line on the graph. Please also note that variation of properties with diameters (in the given case increase of the measured Young's modulus with a decrease of the diameters) is a well-known phenomenon for nanoscale systems [36,38,42,56,[78][79][80] and will be discussed further in the article. When we speak about the discrepancy in the results in the present review, our concerns are related to the discrepancy in the reported elastic modulus of ZnO NWs when we compare similar diameters. ...
... The measured modulus values in quasistatic nanomechanical characterization are therefore influenced by the electromechanical coupling resulting in Young's modulus of ZnO NW being different from the bulk. We, however, would not be too critical to quasi-static tests, even for piezoelectric materials with low dielectric constant like ZnO, since there are a number of results reported (for instance [38,40,46,52,56] and others) that are close or even exceed bulk values, obtained in the dynamic modes. Moreover, He et al. [38] compared the results of uniaxial tensile test measurements of ZnO NWs to previously published electric-filedinduced resonant method measurements on the same material. ...
In this critical review, we call attention to a widespread problem related to the vast disagreement in elastic moduli values reported by different authors for nanostructures made of the same material. As a particular example, we focus on ZnO nanowires (NWs), which are among the most intensively studied nanomaterials due to their remarkable physical properties and promising applications. Since ZnO NWs possess piezoelectric effects, many applications involve mechanical deformations. Therefore, there are plenty of works dedicated to the mechanical characterization of ZnO NWs using various experimental and computational techniques. Although the most of works consider exactly the same growth direction and wurtzite crystal structure, reported values of Young’s modulus vary drastically from author to author ranging from 20 to 800 GPa. Moreover, both – diameter dependent and independent – Young’s modulus values have been reported. In this work, we give a critical overview and perform a thorough analysis of the available experimental and theoretical works on the mechanical characterization of ZnO NWs in order to find out the most significant sources of errors and to bring out the most trustable results.
... In addition, the Young's modulus of ZnO nanowires increases with decreasing wire diameter as shown in Fig. 5.1f; when the wire diameter is ! 200 nm, the elastic modulus reaches that of bulk ZnO [618]. These unique properties are ascribed to the high surface-to-volume ratios of nanopores, nanowires and nanoparticles. ...
... (e) The yield strength of NP Au increased with decreasing ligament diameter [617]. (f) The elastic modulus as a function of wire diameter for ZnO nanowires [618]. Reprinted with permission from Refs. ...
... Reprinted with permission from Refs. [450,617,618,629]. discussion, we briefly summarize the major conclusions derived from the cellular model. ...
There is a significant demand for the discovery of advanced materials that can survive high temperature and high doses of irradiations for the next generation nuclear reactors. Materials subjected to high dose irradiation by energetic particles often experience severe damage in the form of drastic increase of defect density, and significant degradation of their mechanical and physical properties. Extensive studies on radiation effects in materials in the past few decades show that, although nearly no materials are immune to radiation damage, the approaches of deliberate introduction of certain types of defects in materials before radiation are effective in mitigating radiation damage. Nanostructured materials with abundant internal defects have been extensively investigated for various applications. However, their impact on the alleviation of radiation damage remains less well understood. In this review article, we summarize and analyze the current understandings on the influence of various types of internal defect sinks on reduction of radiation damage in primarily nanostructured metallic materials, and partially on some nanoceramic materials (nitrides and oxides). We also point out open questions and future directions that may significantly improve our fundamental understanding on radiation damage in nanomaterials. The field of radiation damage in nanostructured materials is an exciting and rapidly evolving new arena, enriched with challenges and opportunities. The integration of extensive research effort, resources and expertise in the field materials science, nuclear science and technology, advanced microscopy, physics, mechanics, chemistry, and modeling and simulations may eventually lead to the design of advanced nanomaterials with unprecedented radiation tolerance.
... Semiconducting ZnO (with its direct band gap E g ¼ 3.37 eV [1]) is one of the very few dominant materials for nanoscale technologies, by virtue of its low cost, negligible environmental impact, and the wide variety of possible nanostructures that can be conveniently fabricated from it [2][3][4][5][6]. Such zero-and one-dimensional (1D) geometries as nanobelts [2,4,7], nanorods and nanowires [4,8,9], and nanorings and nanohelixes [2,4] have been made from ZnO and integrated with other microscopic and nanoscopic building blocks [10,11], including core-shell arrangements [12]. Prospective applications of these nanostructures as device components range from field-effect transistors [13,14] and power generators for wearable and biomedical electronics [15], including nanorobots [4,16], to photodetectors and photovoltaics [16][17][18][19][20][21][22][23], as well as mechanical [24] and chemical sensors [18,25]. ...
... Variations of elastic properties of w-ZnO nanowires as their size, d, diminishes have been thoroughly investigated [8,35,[63][64][65][66][67][68]. There is a general agreement that nanowires start to stiffen when d becomes smaller than d c ∼ 300 nm; however, one group of studies reported that such changes are not significant until the diameter is less than 10-70 nm [64,65,67], while the other group finds substantial stiffening effects at diameters right below d c [8,35,63,66]. ...
... Variations of elastic properties of w-ZnO nanowires as their size, d, diminishes have been thoroughly investigated [8,35,[63][64][65][66][67][68]. There is a general agreement that nanowires start to stiffen when d becomes smaller than d c ∼ 300 nm; however, one group of studies reported that such changes are not significant until the diameter is less than 10-70 nm [64,65,67], while the other group finds substantial stiffening effects at diameters right below d c [8,35,63,66]. ...
Attractive mechanical, optical, and electronic properties of semiconducting ZnO nanowires make them prime candidates for a variety on energy-harvesting technologies, including photovoltaics and piezoelectric nanogenerators. In order to enhance the efficiency and versatility of such devices, it is paramount to elucidate the connections between the different property realms, i.e., to establish how mechanical distortions can affect the electronic and optical response of the nanowires, depending on their size, shape, and morphology. For example, it was recently demonstrated that band-gap downshifts of up to −0.1 eV can be induced in monolithic ZnO nanowires by an application of tensile strain [see Wei et al., Nano Lett. 12, 4595 (2012)]. Here, we conduct mesoscale-level, finite-element-method-based modeling of the coupled elastic and electronic properties of both already-synthesized monolithic ZnO nanowires and yet-to-be-fabricated Zn-ZnO core-shell structures with diameters ranging from 100 to 800 nm. Our investigation suggests that, after an optimization of the size, shape, and mutual crystallographic orientations of the core and shell regions, core-shell nanowires can exhibit downward band-gap shifts of up to −0.3 eV (i.e., approximately 10% of the stress-free ZnO band-gap value) under tensile distortions, which can greatly expand the utility of such nanostructures for optoelectronic applications.
... Although scattering of measured Young's modulus of NWs exists, most studies supported the conclusion that the Young's modulus of NWs is a function of their diameters. 45,69,73,132,133 For example, the Young's modulus of ZnO NWs increases from 140 to 210 GPa when the diameter decreases from 550 to 17 nm [ Fig. 12(a)], 69 while the same trend on the size effect on Young's modulus has also been reported in GaAs NWs [ Fig. 12 54 It is well accepted that the size-dependency of the Young's modulus of NWs stems from the reconstruction of their surface structure, including surface atoms rearrangement to remove the tangling bonds 51,69,73,132,133 and formation of surface native oxide layer, 54,72 and the large surface-to-volume ratios of NWs result in significant increase in the contribution of the surface reconstruction to the overall mechanical properties of the NWs. The difference between the Young's moduli on the surface and in the bulk plays increasingly crucial roles in the Young's modulus of NWs by decreasing their diameters. ...
... Although scattering of measured Young's modulus of NWs exists, most studies supported the conclusion that the Young's modulus of NWs is a function of their diameters. 45,69,73,132,133 For example, the Young's modulus of ZnO NWs increases from 140 to 210 GPa when the diameter decreases from 550 to 17 nm [ Fig. 12(a)], 69 while the same trend on the size effect on Young's modulus has also been reported in GaAs NWs [ Fig. 12 54 It is well accepted that the size-dependency of the Young's modulus of NWs stems from the reconstruction of their surface structure, including surface atoms rearrangement to remove the tangling bonds 51,69,73,132,133 and formation of surface native oxide layer, 54,72 and the large surface-to-volume ratios of NWs result in significant increase in the contribution of the surface reconstruction to the overall mechanical properties of the NWs. The difference between the Young's moduli on the surface and in the bulk plays increasingly crucial roles in the Young's modulus of NWs by decreasing their diameters. ...
... Similarly, ZnO NWs also experienced a crystal-to-amorphous transition in highly compressed regions after a number of loading and unloading cycles. 133 A summary of the fracture modes and failure strain, including the values of plastic strain in some cases, is listed in Table I. ...
The mechanical behaviors of nanowires (NWs) are significantly different from those of their bulk materials because of their small dimensions. Determining the mechanical performance of NWs and understanding their deformation behavior are crucial for designing and manufacturing NW-based devices with predictable and reproducible operation. Owing to the difficulties to manipulate these nanoscale materials, nanomechanical testing of NWs is always challenging, and errors can be readily introduced in the measured mechanical data. Here, we survey the techniques that have been developed to quantify the mechanical properties and to understand the deformation mechanisms of NWs. We also provide a general review of the mechanical properties and deformation behaviors of NWs and discuss possible sources responsible for the discrepancy of measured mechanical properties. The effects of planar defects on the mechanical behavior of NWs are also reviewed.
... The electromechanical properties of ZnO NWs are significantly influenced by defects [4,6], NW diameter [7], cross-sectional geometry [8][9][10], polarization orientation [11], and growth direction [11,12]. This dependence was attributed to the surface effect and structural anisotropy at the nanoscale level [13] and to the associated reconfiguration of surface atoms that led to a stiffer crystal, resulting in a stronger NW than its bulk form [14,15]. For example, Agrawal et al [16] performed a uniaxial tension test of ZnO NWs and found that their respective fracture stress and failure strain can be as high as 9.53 GPa and 6.2%, respectively, which are 500% larger than those reported for bulk ZnO. ...
... However, density functional theory investigations were mainly limited to NWs with very small diameters (D NW 5 nm) and short lengths (a single unit cell) due to the computational burden required to model larger systems [11]. This order of magnitude difference in the dimensions of NWs from existing experimental works [1,15,16,22] limited our ability to understand the deformation mechanism of NWs used in engineering applications. On the other hand, MD can simulate much larger NWs, which allows us to tailor their properties and design novel nanoscale systems that utilize their multifunctionality [23]. ...
Currently, numerous articles are devoted to examining the influence of geometry and charge distribution on the mechanical properties and structural stability of piezoelectric nanowires (NWs). The varied modeling techniques adopted in earlier works dictated the outcome of the different efforts. In this article, comprehensive molecular dynamic studies are conducted to determine the influence of varied interatomic potentials (partially charged rigid ion model [PCRIM], ReaxFF, charged optimized many-body [COMB], and Buckingham), geometrical parameters (cross-section geometry, wire diameter, and length), and charge distribution (uniform full charges versus partially charged surface atoms) on the resulting mechanical properties and structural stability of zinc oxide (ZnO) NWs. Our optimized parameters for the Buckingham interatomic potential are in good agreement with the existing experimental results. Furthermore, we found that the incorrect selection of interatomic potentials could lead to excessive overestimate (61%) of the elastic modulus of the NW. While NW length was found to dictate the strain distribution along the wire, impacting its predicted properties, the cross-section shape did not play a major role. Assigning uniform charges for both the core and surface atoms of ZnO NWs leads to a drastic decrease in fracture properties.
... If the effect of surface elasticity is neglected, surface elasticity 0 s E = , namely, the dimensionless elastic modulus ratio 0 γ = , the analytical solution can be reduced to the equation (15), which is the results without surface elasticity effect, but only with the effect of flexoelectricity. ...
... However, our theoretical result should be more reasonable, for the reason that, firstly, actually, in the process of experimental operation, the primary captured voltage comes from the top side or its vicinity of the ZnO nanowire when the AFM tip scanning over the nanowire, that is to say, the influence of flexoelectricity is almost zero or very weak. Secondly, most researches have proved that surface effects become the important effect for the nanoscale elements [14][15][16]; especially the effect of surface elasticity cannot be ignored. Thus, it is possible that surface elasticity becomes one of key factors influencing the piezoelectric potential. ...
In this work, a rapid model is established to study the effects of flexoelectricity and surface elasticity on the piezoelectric potential of a bent ZnO nanowire. Based on the piezoelectric theory and core-surface model, the distribution of piezoelectric potential of the ZnO nanowire is investigated. The analytical solution shows that the flexoelectricity and surface elasticity both significantly influence the piezoelectric potential. However, the effect of flexoelectricity is longitudinal dependent, which vanishes on the top side of nanowire, but only left surface elasticity effect on the potential. Simulation results show that the maximum value of potential on the top side of nanowire is about ± 220.5mV, of which result is lower compared to other theoretical models, but it should be more reasonable.
... When the dimensions of materials and structures gradually evolve from macroscopic to micro/nano scales, the size effect significantly affects the mechanical behaviour of components and materials. Asthana et al [8] conducted an in situ observation of size effects on the mechanical properties of ZnO nanowires. When the diameter of the nanowire decreased from 110 to 40 nm the Young's modulus increased from 189.4 (±2.5) to 249.3 (±1.8) ...
... When these nanowires are used in the nano-electromechanical system (NEMS) as active components, such as nanorelays, nanoresonators, or switches (ON-OFF devices), the mechanical properties play an essential role in the reliability and manufacturability of these nano-devices under processing and working conditions. Therefore, it has become increasingly important to understand the mechanical properties of these nanowires, and many investigations have been reported on mechanical behaviors, including the elasticity [9][10][11], anelasticity [12], plasticity [13,14] and strength [15] of these components. ...
Molecular dynamics simulations using a three-body potential show that the melting and mechanical behaviors of silicon nanowires are strongly dependent on their cross-section area. For the wire with a small cross-section area, rearrangements of surface atoms greatly affect thermal stability in a relatively low temperature regime. For these wires with a relatively large area, while some surface atoms adjust their positions, most of the interior atoms hold their tetrahedra packing patterns. At a high temperature, the accumulation of structural disorder can quickly extend into the entire wire, which resembles the melting of the bulk phase. By applying the uniaxial tensile, these silicon nanowires present the typical mechanical behavior of plastic materials. The atomic local stress in the necking region is apparently larger than that outside of the necking region. As the cross-section area becomes large, both the yield strength and tensile strength increase. With the increasing temperature, the elasticity decreases significantly.
... It was established that this characteristic is determined mainly by the shear nature of deformation in an inhomogeneous system, increases with an increase in the relative stiffness of whiskers, and decreases significantly with an increase in the volumetric content of the matrix. In the general case, shear deformations in the interphase layer are associated with a longitudinal shear in the direction of the whiskers Source: (Gusev and Lurie, 2009;Kumar and Talreja, 2003;Asthana et al., 2011). ...
It is known that the mechanical properties of fiber-reinforced composites are controlled by the conditions of contact between the fiber and the matrix. In this regard, great efforts of mechanics are directed to developing various techniques to improve the quality of the interface. The most common are: modification of the fiber surface, improvement of chemical interactions, or the addition of a third phase (interfacial layer) between the fiber and the matrix. The most common are: modification of the fiber surface, improvement of chemical interactions, or a third phase (interfacial layer) between the fiber and the matrix. In this study, the authors aim to examine the effective dynamic properties of a whiskered layer of fibers in modified composites, taking into account the structural characteristics of the interfacial layer – its thickness – length of whiskers, volumetric content of whiskers, and their mechanical properties. The dynamic performance of the whiskered layer surrounding the base fiber in modified composites was estimated. The whiskered layer is considered a fibrous composite formed by nanoscale whiskers grown on the surface and a matrix. An epoxy binder or a viscoelastic polymer is considered as a matrix. An approximate model was used. The effective characteristics of the whiskered layer were modeled and determined as the properties of a transversally isotropic fibrous system with the isotropy axis coinciding with nanowhiskers in the whiskered layer. A feature of the whiskered layer is that the density of whiskers varies with distance from the fiber surface. Therefore, it depends on the length of the nanowhiskers (the thickness of the interfacial layer). In this case, it turns out that the bulk for the matrix in the whiskered layer, even at the maximum density of nanowhiskers grown on the fiber surface and for sufficiently thin interfacial layers, is very significant. Fuzzy fiber composite, nanofibers, epoxy binder, damping properties.
... When the dimensions of materials and structures gradually evolve from macroscopic to micro/nano scales, the size effect significantly affects the mechanical behaviour of components and materials. Asthana et al [8] conducted an in situ observation of size effects on the mechanical properties of ZnO nanowires. When the diameter of the nanowire decreased from 110 to 40 nm the Young's modulus increased from 189.4 (±2.5) to 249.3 (±1.8) ...
The adhesive contact problem between a rigid cylindrical punch and gradient nanostructured (GNS) coating is investigated by considering the size effect. The laminated plate model is applied to characterize the material properties of GNS coating in plane strain couple stress elasticity. By using the Fourier integral transform and transfer matrix method, the governing singular integral equation(s) for two dimensional adhesive contact problem are obtained. Numerically calculated results are presented to analyse the effect of characteristic material length, the adhesion parameter and nonhomogeneous parameter on the mechanical response of GNS coating for the adhesive contact problem. The paper explored the nano scale contact of GNS coating with shear modulus varying as a function of depth according to an exponential function or the power law function. The present results provide a way to improve the contact deformation and damage of Nano-Electromechanical System (NEMS) by adjusting the gradient index of GNS coating.
... Existing studies of ZnO NW mechanical responses have been focused on the measurement of the effective axial Young's modulus and its variation with the characteristic size [11][12][13][14][15]. This characterisation approach stems from their ability to sustain large and recoverable deformations prior to failure, which is advantageous for ZnO NW based piezoelectric and electromechanical devices. ...
Surface modification via adsorbates is significant for property prediction in nanostructures where surface effect is dominant. This is especially vital for zinc oxide (ZnO) nanowires (NWs) which has no native passivation layer. As water is an ubiquitous environmental factor and its aggregation on ZnO surface is favoured, molecular statics (MS) simulations are used to study the deformation of ZnO with surface water adsorption in the finite strain regime (up to 0.1). Three types of water covered surface structures are considered to examine their effects on the size-dependence of linear (Y0) and nonlinear (Y1) elastic moduli. The pathway of adsorption to impact NWs is identified by revealing the radial distribution of Y0, Y1 and residual stress for the NWs. The physical origins of the water adsorption effects are further discussed in terms of the layer-wise equilibrium structure and potential energy variation.
... A. Young's Modulus and critical stress were calculated and plotted in Fig.22. These aspect ratios are much smaller than the critical buckling aspect ratio of ZnO NWs, (L/D)crit=40, that obtained from experimental [51] [52], and assures of no buckling instability in our simulations. The width was kept constant to eliminate the size effect on results and solely focus on the effect of aspect ratio. ...
The advent of nanomaterials has opened a new avenue for designing and fabricating
materials with unique properties, e.g., superior mechanical properties. Based on a common
notion, the perfect structures are assumed to exhibit better mechanical properties, such as
higher yield strength and Young’s modulus. Therefore, researchers have devoted an
extensive amount of time to decrease defect concentration by fabricating materials with the
micro/nanoscale, e.g., nanowires (NWs) and nanobelts (NBs), to enhance the mechanical
characteristics of the system. However, defects are a part of the fabrication process and
precise control over synthesizing procedure is needed to eliminate them from the material.
In this work, we showed, with the help of the classical molecular dynamics method, that
these inherited defects can be employed as a microstructural feature to improve the
mechanical properties of low dimensional nanomaterial, i.e., defect engineering. Our
results indicate that the NWs with a high density of I1 stacking faults (I1-SFs) show higher
compressive/tensile critical stress (14% increase), as well as Young’s Modulus (37%
increase), in comparison to the perfect structure over a wide range of temperature: ranged from 0 K to 500 K. Such an improvement is in agreement with the in-situ experimental
measurements of highly defective GaAs NWs, and can be justified by interplay between
surface stresses and the intrinsic stress field of locked SFs. The SF-induced stresses are
partially relaxed by raising the temperature for this non-trivial strengthening.
Moreover, a specific stress relaxation mechanism, twin boundary formation, was found to
take place in highly defected NWs, which further postponed the phase transition from
hexagonal (HX) to cubic and subsequently boosted the toughness of NWs; this
phenomenon appears as a stress plateau in highly defected NWs. Numerous parametric
studies on the system variables, such as cross-section geometry, aspect ratio, width, and SF
distribution, were performed to find the optimum design. Our results demonstrated the
promise and applicability of this strengthening method over a wide temperature range and
geometrical features. This novel method, defects engineering, adds a new parameter to the
design-space of materials and also paves the way to the fabrication of a new class of
materials with superior mechanical properties, including higher stiffness, strength, and
ductility.
... 19 −24 In nanowires, in particular, the field effect becomes more relevant in electrical conduction as the diameter decreases, and the mechanical flexibility is enhanced. 19,23 Additionally, the absorption spectrum widely varies with the diameter. 21,22 These structure-induced variations in nanowire properties lead to a batch-to-batch variability in the performance of the devices. ...
We demonstrate the facile, rational synthesis of monodispersedly sized zinc oxide (ZnO) nanowires from randomly sized seeds by hydrothermal growth. Uniformly shaped nanowire tips constructed in ammonia-dominated alkaline conditions serve as a foundation for the subsequent formation of the monodisperse nanowires. By precisely controlling the sharp tip formation and the nucleation, our method substantially narrows the distribution of ZnO nanowire diameters from σ = 13.5 nm down to σ = 1.3 nm and controls their diameter by a completely bottom-up method, even initiating from randomly sized seeds. The proposed concept of sharp tip based monodisperse nanowires growth can be applied to the growth of diverse metal oxide nanowires and thus paves the way for bottom-up grown metal oxide nanowires-integrated nanodevices with a reliable performance.
... Therefore, different NBs with different lengths (10, 20, 30, and 40 nm) and the same width (9 nm) were simulated, and their corresponding Young's Modulus and critical stress are plotted in Fig. 10. The modeled aspect ratios are much smaller than the critical buckling aspect ratio of ZnO NWs, (L/D) crit = 40, obtained from experimental studies [44,45], and subsequently eliminate the concern regarding buckling instability in our simulations. The width was kept constant to eliminate the effect of size on the results and allow sole focus on the aspect ratio as a design parameter. ...
... Recently, self-powered electrical devices have experienced tremendous developments for information processing, biomedical systems and energy storage relying on nanogenerators (NGs), which harvest the mechanical energy from ambient sources and convert them into electrical energy via electromechanical coupling effects. Among other nanostructures, ZnO nanowire (NW) is an outstanding candidate for NGs [1] because of its excellent mechanical properties withstanding high strain as well as its high piezoelectric coefficients [2,3,4,5]. Moreover, upon combining piezoelectricity and semiconductivity, ZnO has paved a way to many other applications of the so-called piezotronics [6]. ...
Nanowire based semiconductors are promising for nanogenerators. However, there exist limited numerical tools to analyze these type of structures taking into account effects which are of particular importance at nanoscale. Therefore, we present a finite deformation NURBS based formulation to model a multifunctional material that couples strain, strain gradient, polarization and free charge carriers simultaneously. Specifically, the weak form and consistent linearization of the piezoelectric semiconductor including flexoelectricity and non-local elasticity are introduced. The nonlinear equations are then discretized and solved by utilizing isogeometric analysis (IGA) which fulfills the [Formula presented] continuity requirement. Several numerical examples are performed to investigate the influence of flexoelectricity and non-local elasticity in ZnO piezoelectric semiconductor nanowires under large deformation. The formulation developed in this work can contribute to the development of novel nanoelectromechanical coupling devices such as flexoelectric nanogenerators.
... In Equation (2), c λµ is the Young's modulus, e iµ the stress piezoelectric coefficient, and ij is the permittivity constant, where i, j = 1, 2, 3 and λ, µ = 1, 2, 3, 4, 5, 6. To describe an applied force in a random direction, we use e iµ , which for ZnO [38] is given by: ...
With the consistent shrinking of devices, micro-systems are, nowadays, widely used in areas such as biomedics, electronics, automobiles, and measurement devices. As devices shrunk, so too did their energy consumptions, opening the way for the use of nanogenerators (NGs) as power sources. In particular, to harvest energy from an object’s motion (mechanical vibrations, torsional forces, or pressure), present NGs are mainly composed of piezoelectric materials in which, upon an applied compressive or strain force, an electrical field is produced that can be used to power a device. The focus of this work is to simulate the piezoelectric effect in different ZnO nanostructures to optimize the output potential generated by a nanodevice. In these simulations, cylindrical nanowires, nanomushrooms, and nanotrees were created, and the influence of the nanostructures’ shape on the output potential was studied as a function of applied parallel and perpendicular forces. The obtained results demonstrated that the output potential is linearly proportional to the applied force and that perpendicular forces are more efficient in all structures. However, nanotrees were found to have an increased sensitivity to parallel applied forces, which resulted in a large enhancement of the output efficiency. These results could then open a new path to increase the efficiency of piezoelectric nanogenerators.
... Since the effects of size have important roles to play in determining mechanical behaviors; thus, they have received wide attention via experimental and numerical studies. [26][27][28][29][30] Besides, MEMS test-bed device was designed to study mechanical properties of ZnO nanowires. 31 It is also worth to mention that extensive efforts have been made to investigate optical properties of nanowires via FDTD simulation techniques, such as bending losses of optical silica, polystyrene and zinc oxide nanowires, 32 the effects of length, diameter and filling ratio on optical absorption of gallium arsenide (GaAs) nanowire arrays, 33 investigation of waveguiding and optical emission properties of ZnO or GaN NWs 34 or optical properties of nanowire/Si structure. ...
The main objective of this study is to investigate the influences of mechanical strain on optical properties of ZnO nanowire (NW) before and after embedding ZnS nanowire into the ZnO nanowire, respectively. For this work, commercial finite element modeling (FEM) software package ABAQUS and three-dimensional (3D) finite-difference time-domain (FDTD) methods were utilized to analyze the nonlinear mechanical behavior and optical properties of the sample, respectively. Likewise, in this structure a single focused Gaussian beam with wavelength of 633 nm was used as source. The dimensions of ZnO nanowire were defined to be 12280 nm in length and 103.2 nm in diameter with hexagonal cross-section. In order to investigate mechanical properties, three-point bending technique was adopted so that both ends of the model were clamped with mid-span under loading condition and then the physical deformation model was imported into FDTD solutions to study optical properties of ZnO nanowire under mechanical strain. Moreover, it was found that increase in the strain due to the external load induced changes in reflectance, transmittance and absorptance, respectively.
... As one-dimensional nanomaterials with large surface to volume ratio, NWs also have a mechanical behavior that greatly depends on their size [7]. There have been substantial efforts in research to characterize NW mechanical properties [8,9]. The Curtin-Murdoch model was generated to describe the surface energy of bending curvature-dependent NWs by introducing a surface flexural stiffness [10][11][12]. ...
Mechanical properties of size-dependent nanowires are important in nano-electro-mechanical systems (NEMS), and have attracted much research interest. It is desperate to characterize the size effect of nanowires at atmosphere directly to broaden its practical application instead of in high vacuum situation as reported previously. In this study, we systematically studied the Young's modulus of vertical ZnO nanowires at atmosphere. The diameters range from 48nm to 287 nm, with resonance method by non-contact atomic force microscopy (AFM). The values of Young's modulus at atmosphere present extremely strong increase tendency with the diameter decrease of nanowire by stronger surface atomic bonds, comparing with that in vacuum. A core-shell model for nanowires is proposed to explore the Young's modulus enhancement at atmosphere, which is correlated with atoms of oxygen occurred near nanowire surface. The new model is more accurate to analyze the mechanical behavior of nanowires at atmosphere comparing with the model in vacuum. Furthermore, it is possible to use this characterization method to measure the size-related elastic properties of similar wire-sharp nanomaterials at atmosphere and estimate the corresponding mechanical behavior. The study of size-dependent Young's modulus in ZnO nanowires at atmosphere will improve the understanding of mechanical properties of nanomaterials as well as providing guidance for applications in NEMS, nanogenerator, biosensor and other related areas.
... Zhu et al. (2014) gave a buckling analysis of an elastic plate with attached thin films with intrinsic stresses. On the other hand, the effect of surface elasticity on the elastic properties of nanowires was confirmed through various approaches (Asthana et al. 2011, Yao et al. 2012, Zheng et al. 2010. The above-mentioned researches mainly focus on vibration of nanobeams under the condition of conservative forces if any. ...
This paper studies on dynamic and stability behavior of a clamped-elastically restrained nanobeam under the action of a nonconservative force with an emphasis on the influence of surface properties on divergence and flutter instability. Using the Euler-Bernoulli beam theory incorporating surface effects, a governing equation for a clamped-elastically restrained nanobeam is derived according to Hamilton‟s principle. The characteristic equation is obtained explicitly and the force-frequency interaction curves are displayed to show the influence of the surface effects, spring stiffness of the elastic restraint end on critical loads including divergence and flutter loads. Divergence and flutter instability transition is analyzed. Euler buckling and stability of Beck‟s column are some special cases of the present at macroscale.
... With the development of commercial microelectromechanical systems (MEMS), such as the Nanofactory Instruments AB holder [21], or Hysitron systems [22], in situ TEM techniques have become invaluable tools in materials science (see for example a recent review in [23]). With such tools, the mechanical properties of a large variety of nanotubes, nanoparticles, nanowires, micro-pillars and even surfaces and coatings have been studied [24][25][26][27][28][29][30][31]. ...
Incomplete fuel burning inside an internal combustion engine results in creation of soot in the form of nanoparticles. Some of these soot nanoparticles (SNP) get adsorbed into the lubricating oil film present on the cylinder walls, which adversely affects the tribological performance of the lubricant. In order to better understand the mechanisms underlying the wear caused by the SNPs, it is important to understand the SNP behavior and to characterize potential changes in their mechanical properties (e.g. hardness) caused by (or during) mechanical stress.
In this study, the behavior of individual SNPs originating from diesel engines was studied under compression. The experiments were performed in a transmission electron microscope using a nanoindentation device. The nanoparticles exhibited elasto-plastic behavior in response to consecutive compression cycles. From the experimental data, the Young's modulus and hardness of the SNPs were calculated. The Young's modulus and hardness of the nanoparticles increased with compression cycles. Using an electron energy loss spectroscopy technique, it was shown that the sp2/sp3 ratio within the compressed nanoparticle decreases, which is suggested to be the cause of the increase in elasticity and hardness.
In order to corroborate the experimental findings, molecular dynamics simulations of a model SNP were performed. The SNP model was constructed using carbon and hydrogen atoms with morphology and composition comparable to those observed in the experiment. The model SNP was subjected to repeated compressions between two virtual rigid walls. During the simulation, the nanoparticle exhibited elasto-plastic behavior like that in the experiments. The results of the simulations confirm that the increase of the elastic modulus and hardness is associated with a decrease in sp2/sp3 ratio.
... At the same time, if a spread of Y NW (d NW ) curve distributions evaluated theoretically (figure 5) is noticeable (due to different methods of calculations and their parameters), then a similar spread of experimental measurements is markedly larger (see figures 5 and 6). Obviously, this difference is caused not only by principally different methods of NW synthesis and arrangement but also by a variety of techniques used for Young's modulus measurement (uniaxial tensile test, cantilevered beam bending, 3-point bending, resonance method, nanoindentation, buckling test, etc) [9,[12][13][14][15][16][47][48][49][50]. The reported experimental results for the Young's modulus of the wurtzite ZnO bulk crystal oriented along [0 0 0 1] are also distributed in a range of values (110-140 GPa) [2,4]. ...
Morphologically reproducible wurtzite-structured zinc oxide nanowires (ZnO NWs) can be synthesized by different methods. Since ZnO NWs have been found to possess piezoelectricity, a comprehensive study of their mechanical properties, e.g. deformations caused by external compression or stretching, is one of the actual tasks of this paper. We have calculated wurtzite-structured [0001]-oriented ZnO NWs whose diameters have been varied within 1–5 nm and 1–20 nm ranges when using either ab initio (hybrid DFT-LCAO) or force-field (molecular mechanical) methods, respectively (the minimum diameter d_NW of experimentally synthesized NWs has been estimated on average to be ~20 nm). When using both chosen calculation approaches, the
values of Young’s moduli determined for the mentioned ranges of NW diameters have been found to be qualitatively compatible (168–169 GPa for 5 nm NW thickness), whereas results of molecular-mechanical simulations on Y_NW for 20 nm-thick NWs (160–162 GPa) have been qualitatively comparable with those experimentally measured along the [0001] direction of NW loading. In all the cases, a gradual increase of the NW diameter has resulted in an asymptotic decrease of Young’s modulus consequently approaching that (Y_b) of wurtzite-structured ZnO bulk along its [0001] axis. The novelty of this study is that we combine the computation methods of quantum chemistry and molecular mechanics, while the majority of previous studies with the same aim have focused on the application of different classical molecular dynamical methods.
... The overall size dependence of Young's Modulus at a nanoscale was frequently reported, using both experimental 2,35 and theoretical approaches, 4,33,42,43 and was associated with the surface stress contribution. One well-known explanation is based on the core−shell model, 44 where a shell (outer layers) is under compressive stress due to surface stresses and the core (inner layers) is under tension. ...
Moving to nanoscale is a path to get perfect materials with superior properties. Yet defects, such as stacking faults (SFs), are still forming during the synthesis of nanomaterials and, according to common notion, degrade the properties. Here, we demonstrate the possibility of engineering defects to, surprisingly, achieve mechanical properties beyond those of the corresponding perfect structures. We show that introducing SFs with high density increases the Young’s Modulus and the critical stress under compressive loading of the nanowires above those of a perfect structure. The physics can be explained by the increase in intrinsic strain due to the presence of SFs and overlapping of the corresponding strain fields. We have used the molecular dynamics technique and considered ZnO as our model material due to its technological importance for a wide range of electromechanical applications. The results are consistent with recent experiments and propose a novel approach for the fabrication of stronger materials.
... Therefore, it is important to characterize the size-dependent properties of PNs. From the experimental perspective, micro-electro-mechanical-systems (MEMS) in situ transmission electron microscopy (TEM) [32,33] or in situ scanning electron microscopy (SEM) [34] has been successfully adopted to characterize the mechanical properties of PNs while the piezoelectric properties of some materials have been studied by using the piezoelectric force microscopy (PFM) [35,36]. There is a general agreement concluded from the experiments that the properties of PNs are size-dependent, e.g., as the nanowire diameter decreases from 80 nm to 20 nm, the Young's modulus of ZnO nanowire increases from ∼140 to 160 GPa [32]; the fracture strain and strength of ZnO nanowires also increase as the nanowire diameter decreases [34]; the piezoelectric constants of GaN nanowires were reported up to six times that of the bulk values [37]. ...
Piezoelectric nanomaterials (PNs) are attractive for applications including sensing, actuating, energy harvesting, among others in nano-electro-mechanical-systems (NEMS) because of their excellent electromechanical coupling, mechanical and physical properties. However, the properties of PNs do not coincide with their bulk counterparts and depend on the particular size. A large amount of efforts have been devoted to studying the size-dependent properties of PNs by using experimental characterization, atomistic simulation and continuum mechanics modeling with the consideration of the scale features of the nanomaterials. This paper reviews the recent progresses and achievements in the research on the continuum mechanics modeling of the size-dependent mechanical and physical properties of PNs. We start from the fundamentals of the modified continuum mechanics models for PNs, including the theories of surface piezoelectricity, flexoelectricity and non-local piezoelectricity, with the introduction of the modified piezoelectric beam and plate models particularly for nanostructured piezoelectric materials with certain configurations. Then, we give a review on the investigation of the size-dependent properties of PNs by using the modified continuum mechanics models, such as the electromechanical coupling, bending, vibration, buckling, wave propagation and dynamic characteristics. Finally, analytical modeling and analysis of nanoscale actuators and energy harvesters based on piezoelectric nanostructures are presented.
In this paper, we present the results of mechanical measurement of single nanowires (NWs) in a repeatable manner. Substrates with specifically designed mechanical features were used for NW placement and localization for measurements of properties such as Young's modulus or tensile strength of NW with an atomic force microscopy (AFM) system. Dense arrays of zinc oxide (ZnO) nanowires were obtained by one-step anodic oxidation of metallic Zn foil in a sodium bicarbonate electrolyte and thermal post-treatment. ZnO NWs with a hexagonal wurtzite structure were fixed to the substrates using focused electron beam-induced deposition (FEBID) and were annealed at different temperatures in situ. We show a 10-fold change in the properties of annealed materials as well as a difference in the properties of the NW materials from their bulk values with pre-annealed Young modulus at the level of 20 GPa and annealed reaching 200 GPa. We found the newly developed method to be much more versatile, allowing for in situ operations of NWs, including measurements with different methods of scanning probe microscopy.
Ferritic-martensitic steels, such as T91, are candidate materials for high-temperature applications, including superheaters, heat exchangers, and advanced nuclear reactors. Considering these alloys’ wide applications, an atomistic understanding of the underlying mechanisms responsible for their excellent mechano-chemical properties is crucial. Here, we developed a modified embedded-atom method (MEAM) potential for the Fe-Cr-Si-Mo quaternary alloy system—i.e., four major elements of T91—using a multi-objective optimization approach to fit thermomechanical properties reported using density functional theory (DFT) calculations and experimental measurements. Elastic constants calculated using the proposed potential for binary interactions agreed well with ab initio calculations. Furthermore, the computed thermal expansion and self-diffusion coefficients employing this potential are in good agreement with other studies. This potential will offer insightful atomistic knowledge to design alloys for use in harsh environments.
Nanomaterials usually demonstrate remarkable physical, chemical and mechanical properties and are thus ideal to serve as building blocks of devices pertinent to various research fields. With continuous miniaturization of devices and their components, a fundamental understanding of the size impact on the deformation mechanisms and mechanical behaviors becomes more important than before. Deformation mechanisms and consequently mechanical properties such as strength, hardness and toughness may change significantly with decreasing materials size, especially in the nanometer-scale regime. However, investigating the deformation mechanism necessitates capturing the microstructural evolution during the deformation process under atomic-scale resolution and at real-time, which is technically demanding for most characterization methods. Facilitated by the development in transmission electron microscopy (TEM) imaging techniques and nanofabrication technologies, in situ TEM based mechanical testing methods are now capable of simultaneous real-time atomic-scale imaging and quantitative mechanical testing, and thus proved to be the most effective tool for pursuing a mechanistic understanding of the atomic-scale deformation mechanisms. This chapter begins with a brief introduction of in situ nanomechanical TEM history, followed by an overview of principal in situ TEM based nanomechanical testing methods including sample preparation, mechanical loading, and force and deformation measurements. At last, important recent achievements via in situ nanomechanical TEM will also be highlighted and summarized.
The difference in the mechanical properties of bulk materials and nanoscale structures has been studying for more than two decades. Understanding mechanical properties of wide bandgap nanowires plays an important role in design of nanodevices and nanosystems working under application of stress/strain. The mechanical properties of nanowires are characterized by various methods, including bending/buckling and nano‐indentation and resonance testing. Various parameters, which describe basic mechanical properties, have been reported for nanowires, including mechanical strength, toughness, plasticity, and modulus of elasticity. In general, the mechanical failure of nanowires is associated defects. Nanowires typically exhibit better mechanical properties compared with bulk materials due to the lower number of defects per unit length. This chapter discusses the impact of defects and nanowire dimensions on the mechanical properties of nanowires.
Zinc oxide (ZnO) is a popular material and is used in various fields such as optical devices, field emission devices, biological/biomedical, security printing, and sensors because of its catalytic, electrical, optoelectronic, and photochemical properties. There are many methods that have been developed in the literature for the fabrication of ZnO nanostructures. The properties of ZnO can be further tuned by doping for better technological applications. This chapter provides a brief introduction of ZnO and its practical applications.
Based on the established bond moduli of surface atoms ys and interior ones yi in a nanostructure, surface modulus Ys(D) and interior modulus Yi(D) are modeled with D being size. And then the Young's modulus Y(D) is determined through considering the volume fraction of Ys(D) and Yi(D). It is found that the variations of Ys(D), Yi(D) and Y(D) are evident, where cubooctahedron is taken as the shape of nanoparticles. In Cu and diamond nanoparticles, although their Ys(D) and Yi(D) present the decrease trend with D dropping, the increase in Y(D) of Cu nanoparticles but the decrease of Y(D) for diamond nanoparticles are existing. As expected, the contribution of Ys(D) is important, since it is Ys(D) > Y0 mainly leading to Y(D) > Y0 for Cu nanoparticles, and the similar case happens for diamond nanoparticles when D < 2.8 nm, where Ys(D) < Y0 and Y(D) < Y0 are seen with Y0 being bulk Young's modulus. However, it is also pointed out that the contribution of Yi(D) can not be ignored and it is no longer a constant with Yi(D) ≤ Y0. This can be reflected by, one hand the different change trend of Ys(D) and Y(D) in Cu nanoparticles, and the other hand the case where Ys(D) > Y0 but Y(D) < Y0 for diamond nanoparticles as D > 2.8 nm. All this implies that the reason why Y(D) greatly rely on size and have different change trend for different materials can be clarified by considering Ys(D) and Yi(D).
First-principles DFT methods complement the experimental study of the binary oxides (ZnO, TiO, ZrO, HfO)-based nanostructures. We begin each section of this chapter with a short discussion of the results of the corresponding bulk crystal and nanosheet properties calculations. This information is important for understanding the structure and properties of binary oxide-based nanotubes and nanowires.
Mechanical properties of ferroelectric materials at the nanoscale have received growing interest over the past years due to new developments in scientific instrumentation and novel materials that allow for the study of so far scarcely investigated and /or hidden nanoscale phenomena. The use of atomic force microscopy (AFM) as the main investigation tool has shed new light onto various areas of interest, from non-destructive probing of Young’s modulus to mechanically triggered phase transitions at nanoscale regions in ferroelectrics. Together with various theoretical approaches, these scanning probe microscopy based concepts provide a powerful platform for nanoscale mechanical property studies. Here we provide an overview of recent developments and investigations, and give an outlook to future opportunities in this research area.
A high-performance multilayer coating with the external, flexible and silicon-doped hydroxyapatite (Si-HA) nanorods was designed using bionics. Plasma electrolytic oxidation (PEO) and microwave hydrothermal (MH) method were used to rapidly deposit this multilayer coating on a titanium (Ti) substrate, applied for 5 and 10 min, respectively. The bioactive multilayer coating was composed of four layers and the outermost layer was an external growth layer that consisted of many Si-HA nanorods with a single-crystal structure. The Si-HA nanorods exhibited a good flexibility, likely because of their complete single-crystal structures, smooth surfaces, and suitable diameters and lengths. This multilayer coating with a high surface energy was super-hydrophilic and exhibited good in vitro bioactivities, such as good apatite formation ability, good cell spreading and high osteogenic gene expression levels. After implantation in tibia of rabbits for 16 weeks, almost no soft tissues were formed at the MH treated PEO implant-bone interface. A direct bone contact interface was formed by a bridging effect of the flexible Si-HA nanorods, which further produced a high implant-bone interface bonding strength. The current results demonstrated that the bioactive multilayer layers with the flexible Si-HA nanorods displayed a very good osseointegration ability, showing promising applications in biomedical fields.
The primary challenge to exploit the nanowire as a truly one‐dimensional building block in nanoscale devices is the clear incorporation of scale effects into the operational performance. Size‐dependent behavior in physical properties of nanowires is the subject of intense experimental and computational studies for more than two decades. In this review, the measurement techniques and computational approaches to study scale effects on mechanical properties of nanowires are reviewed for fcc metallic, silicon, and zinc oxide structures. Advantages and disadvantages of each measurement tool are summarized with data reported in the literature. A similar comparison is carried out for computational techniques. Contradictions in the literature are highlighted with an assessment of research needs and opportunities, among which the plastic behavior of gold nanowires and elastic properties of silicon nanowires can be primarily mentioned. Furthermore, challenges associated with the coupling of measurement methods and modeling approaches are summarized. Finally, points of agreement between experimental measurements and computational studies are discussed paving the way for the utilization of nanowires in future nanoscale devices.
Lorsque leurs dimensions deviennent nanométriques, les matériaux présentent généralement des propriétés bien différentes de celles mesurées aux échelles supérieures. Ainsi, concernant les propriétés mécaniques, il est, par exemple, souvent fait état d’une résistance accrue à la déformation plastique. Toutefois, une majorité des travaux dans ce domaine concerne des systèmes à une dimension, tels que les nanofils et les nanopiliers. Nos connaissances des propriétés mécaniques d’un autre type de système ’nano’, à savoir les nanoparticules, restent actuellement limitées, ce qui est surprenant en regard de leur immense champ d’applications.Les travaux ici présentés portent sur les propriétés mécaniques de nanoparticules sphériques de matériaux métalliques de structure cubique à faces centrées (aluminium, nickel, cuivre). Ils ont été conduits à l’aide de simulations de dynamique moléculaire de compression uniaxiale.Ces dernières permettent d’analyser finement les mécanismes de plasticité à l’échelle atomique.Deux axes principaux d’étude ont été retenus : l’influence de la taille des nanoparticules et géométrie de la surface de contact dans la gamme de taille étudiée (4-80 nm) lors des premiers stades de déformation plastique. Nous montrons ainsi que cette dernière influe sur la limite d’élasticité, ainsi que sur le mode de déformation plastique, tel que le maclage.
ZnO nanowires are currently used in many application fields. Thermal stability is often a concern in terms of the mechanical response and, in particular, for the elasticity of the nanowires. Literature works focused, to a certain degree, on the nanowires heating response. Anyhow, no experimental data are nowadays available in literature on the low- and very low-temperature exposures. In the present study, deep-cryogenic treatment was performed on vertically aligned ZnO nanowires produced by metal organic chemical vapor deposition. The critical buckling stress and strain of individual nanowires was not significantly influenced by the cryogenic exposure, while the bulk ZnO halved.
Post mortem characterization techniques are essential for the understanding of the lubrication mechanisms of complex tribological systems. The information obtained using these techniques can be used to propose hypothesis of mechanisms that have then to be definitively validated by probing the interfacial material in real time during the friction test. To go further in the understanding of the action modes of some tribological systems, it is important to set up an analytical methodology using in situ experimental techniques in order to (i) probe directly the behavior of the interfacial material in the contact zone and (ii) dissociate, for a better understanding, the different components of the tribological stress (pressure and shear). There are several techniques that combine mechanical stress and in situ analysis (Raman tribology, Infra-Red tribology, EXAFS under pressure …). However, the most interesting techniques are certainly those that permit a visualization of the contact area in real time area during the friction test, down to a nanometer scale. The objective of this chapter is to present through some examples the potential of in situ friction tests done inside a transmission electron microscope.
In situ tensile tests show atypical defect motions in the brittle NTO nanowire (NW) within the elastic deformation range. After brittle fracture, elastic recovery of the NTO NW is followed by reversible motion of the defects in a time-dependent manner. In situ cyclic loading-unloading test shows that these mobile defects shift back and forth along the NW in accordance with the loading-unloading cycles and eventually restore their initial positions after the load is completely removed. The Exsistence of the defects within the NTO NWs and their motions do not lead to plastic deformation of the NW. The atypical defect motion is speculated to be the result of glidibility of the TiO6 layers where weakly bonded cation layers are inbetween. Exploration of the above novel observation can establish new understandings of the deformation behavior of superlattice nanostructures.
A liquid-phase method to grow strictly ordered arrays of ZnO nanorods is introduced. Silicon substrates with a sol-gel pre-deposited seed layer of ZnO were used to grow nanostructures of zinc oxide. The types, the content of precursors and the technological parameters of sol-gel synthesis have been determined and their effects on the morphology and crystallinity of the investigated materials were evaluated. We demonstrated that an appropriate choice of parameters of the hydrothermal synthesis is necessary to ensure a directional change in the properties and morphology of the resulting materials. This is a pre-requisite for the fabrication of MEMS.
The elastic modulus of ZnO nanowires was measured using a resonance method based on laser Doppler effect and their fracture strains were determined via two-point bending with the aid of optical
nanomanipulation. The elastic moduli of ZnO nanowires with diameters of 78 to 310 nm vary from 123 to 154 GPa, which are close to the bulk value of 140 GPa and independent of the diameters and
surface defects. However, the fracture strains of the ZnO nanowires depend significantly on their diameters, increasing from 2.1% to 6.0% with the decrease in diameter from 316 to 114 nm. Postmortem
TEM analysis of the ends of the fractured nanowires revealed that fracture initiated at surface defects. The Weibull statistical analysis demonstrated that a greater defect depth led to a smaller
fracture strain. The surface-defect dominated fracture should be an important consideration for the design and application of nanowire-based nanoelectromechanical systems.
To get and provide better insights into the structure of bionanocomposites and the nature of bioinorganic interfaces, a wide set of characterization techniques may be efficiently used, including chemical analysis and spectroscopic investigations of particle–biomolecule interactions. In addition, numerous methods are available to investigate optical, mechanical, magnetic, and biological properties of nanocomposites. This last section of the book presents a selection of the most commonly used techniques with emphasis on their specific application to bionanocomposites.
Hard tungsten carbide (WC) with brittle behavior is frequently applied for mechanical purposes. Here, ultralarge elastic bending deformation is reported in defect-rare WC [0001] nanowires; the tested bending strain reaches a maximum of 20% ± 3.33%, which challenges the traditional understanding of this material. The lattice analysis indicates that the dislocations are confined to the inner part of the WC nanowires. First, the high Peierls-Nabarro barrier hinders the movement of the locally formed dislocations, which causes rapid dislocation aggregation and hinders long-range glide, resulting in a dense distribution of the dislocation network. In this case, the loading is dispersed along multiple points, which is then balanced by the complex internal mechanical field. In the compressive part, the possible dislocations predominantly emerge in the (0001) plane and mainly slip along the axial direction. The disordered shell first forms at the tensile side and prevents the generation of nanocracks at the surface. The novel lattice kinetics make WC nanowires capable of substantial bending strain resistance. Analytical results of the force-displacement (F-d) curves based on the double-clamped beam model exhibit an obvious nonlinear elastic characteristic, which originates fundamentally from the lattice anharmonicity under moderate stress.
Accurate grasp of the mechanical properties of one dimensional nanomaterials, especially young's modulus, plays a crucial role in the design and safely service of flexible electronic devices and implanted biomedical sensors. Nanobridge tests, in which an atomic force microscope (AFM) functioned a test machine exerts a force to bend a nanowire suspended across a trench or a hole with two ends fixed on a template or substrate, are widely used in the characterization of mechanical properties of nanowires. Adhesion contact deformation occurs inevitably during nanobridge testing between the AFM tip and the tested sample, thereby underestimating the Young's modulus of a tested nanowire and causing pseudo-size effect in determined Young's modulus. The present work systematically investigates the adhesion contact deformation in nanobridge tests and provides an analytical approach to evaluate the contact deformation and to determine Young's modulus. To illustrate the developed methodology, AFM nanobridge tests
Transmission electron microscopy (TEM) and related techniques allow for imaging of nanomaterials to determine the material size, shape, composition, and crystal structure. In situ TEM measurements allow for observation of dynamic processes, such as nanowire growth. This chapter describes the application of ex situ and in situ TEM techniques to the analysis of nanowires/nanorods as a subset of nanomaterials. Nanowires refer to anisotropic metal, semiconductor, metal oxide, and/or alloyed structures that may be cylindrical solids, core-shell structures, or hollow tube-like structures. Herein, “bottom-up” nanowires – those synthesized by monomer addition or particle aggregation – are considered for how their analyses may be aided by TEM-based techniques. Lithographically defined wire-like structures, carbon nanotubes, and graphene-based scroll structures are not considered in this chapter.
Surface effects have been observed to contribute significantly to the mechanical response of nanoscale structures. The newly proposed energy-based coarse-grained atomistic method Multiresolution Molecular Mechanics (MMM) [Q. Yang, A.C. To, Comput. Methods in Appl. Mech. Eng. 283 (2015) 384-418] is applied to capture surface effect for nanosized structures by designing a surface summation rule SRS within the framework of MMM. Combined with previously proposed bulk summation rule SRB, the MMM summation rule SRMMM is completed. SRS and SRB are consistently formed within SRMMM for general finite element shape functions. Analogous to quadrature rules in finite element method (FEM), the key idea to the good performance of SRMMM lies in that the order or distribution of energy for coarse-grained atomistic model is mathematically derived such that the number, position and weight of quadrature-type (sampling) atoms can be determined. Mathematically, the derived energy distribution of surface area is different from that of bulk region. Physically, the difference is due to the fact that surface atoms lack neighboring bonding. As such, SRS and SRB are employed for surface and bulk domains, respectively. Two- and three-dimensional numerical examples using the respective 4-node bilinear quadrilateral, 8-node quadratic quadrilateral and 8-node hexahedral meshes are employed to verify and validate the proposed approach. It is shown that MMM with SRMMM accurately captures corner, edge and surface effects with less 0.3% degrees of freedom of the original atomistic system, compared against full atomistic simulation. The effectiveness of SRMMM with respect to high order element is also demonstrated by employing the 8-node quadratic quadrilateral to solve a beam bending problem considering surface effect. In addition, the introduced sampling error with SRMMM that is analogous to numerical integration error with quadrature rule in FEM is very small.
Applications of nanowires into future generation nanodevices require a complete understanding of the mechanical properties of the nanowires. A great research effort has been made in the past two decades to understand the deformation physics and mechanical behaviors of nanowires, and to interpret the discrepancies between experimental measurements and theoretical predictions. This review focused on the characterization and understanding of the mechanical properties of nanowires, including elasticity, plasticity, anelasticity and strength. As the results from the previous literature in this area appear inconsistent, a critical evaluation of the characterization techniques and methodologies were presented. In particular, the size effects of nanowires on the mechanical properties and their deformation mechanisms were discussed.
Single-crystalline Si3N4 nanobelts (NBs) were successfully synthesized in a size-controlled manner using graphite, nanosilicon and nanosilica powders in N2 gas. The average width of Si3N4 NBs increased with an increase in the flow rate of N2 gas, with 264 nm at 50 ml min⁻¹, 295 nm at 100 ml min⁻¹ and 341 nm at 200 ml min⁻¹. The room-temperature photoluminescence (PL) spectra showed that the synthesized α-Si3N4 nanobelts with three different sizes all had two strong emission peaks located in the yellow and red spectral range, which could be mainly attributed to the incorporation of a small amount of Al in the NBs, and the larger size could also lead to a slight red shift. The Young's modulus of Si3N4 NBs was measured with a hybrid scanning electron microscope/scanning probe microscope (SEM/SPM) system by means of the modulated nanoindentation method. The Young's modulus of Si3N4 NBs strongly depended on their size, decreasing from about 548.6 to 455.1 GPa, which can be explained by surface effects and defect-related effects arising due to their nanometer size. These findings provide a method to tailor the optical and mechanical properties of the NBs in a controlled manner over a wide range of Young's modulus values.
Molecular-dynamics simulations were employed to study deformations on nickel nanowires subjected to uniaxial strain at 300 K using a recently reported embedded-atom (many body) model potential. This embedded-atom model can reproduce exactly the experimental second-order and third-order elastic moduli as well as the phase stability, equation of state and phonon frequency spectra are also in good agreement with experiments. Strong influence was observed in the Young modulus and force constant due to surface effects when considering nanowires with different cross sections. Applying strain rates, from 0.05 to 15%ps-1, we found elastic behavior up to 11.5% strain with corresponding stress of 9.4 GPa. At low strain rates (<0.05%ps-1) the system passes through plastic deformations although keeping the crystalline structure. This ductile process is showed by several snapshots. At this low strain rate regime we observed that the nanowires shows superplasticity. For high strain rates (>~7%ps-1) the system changes continuously from crystalline to amorphous phase. Although this amorphization occurs with no use of liquid quenching or introduction of chemical or physical disorder, so being a different and interesting process, the amorphous resulted is unstable. We studied this instability monitoring the recrystallization process.
We report on the performance of a microelectromechanical system (MEMS) designed for the in situ electron and x-ray microscopy tensile testing of nanostructures, e.g., carbon nanotubes and nanowires. The device consists of an actuator and a load sensor with a gap in between, across which nanostructures can be placed, nanowelded, and mechanically tested. The load sensor is based on differential capacitance measurements, from which its displacement history is recorded. By determining the sensor stiffness, the load history during the testing is obtained. We calibrated the device and examined its resolution in the context of various applications of interest. The device is the first true MEMS in which the load is electronically measured. It is designed to be placed in scanning and transmission electron microscopes and on x-ray synchrotron stages.
A NiTi intermetallic compound was cold rolled at room temperature by 30% and 60% thickness reductions, and microstructures were studied by means of transmission electron microscopy (TEM). In the cold-rolled samples we observed both a phase of nanometer-sized crystals and an amorphous phase. A substantially high dislocation density, 1013 to 1014/cm2, was evident in the transition region between crystalline and amorphous phases. A simple estimate of the elastic energy arising from this dislocation density is of the same order as the crystallization energy, suggesting that dislocation accumulation is a major driving force for amorphization in cold-rolled NiTi.
This paper addresses the design and optimization of thermal actuators employed in a novel MEMS-based material testing system. The testing system is designed to measure the mechanical properties of a variety of materials/structures from thin films to one-dimensional structures, e.g. carbon nanotubes (CNTs) and nanowires (NWs). It includes a thermal actuator and a capacitive load sensor with a specimen in-between. The thermal actuator consists of a number of V-shaped beams anchored at both ends. It is capable of generating tens of milli-Newton force and a few micrometers displacement depending on the beams' angle and their number. Analytical expressions of the actuator thermomechanical response are derived and discussed. From these expressions, a number of design criteria are drawn and used to optimize the device response. The analytical predictions are compared with both finite element multiphysics analysis (FEA) and experiments. To demonstrate the actuator performance, polysilicon freestanding specimens cofabricated with the testing system are tested.
Oxide and nitride nanotubes have gained attention for their large surface areas, wide energy band gaps, and hydrophilic natures for various innovative applications. These nanotubes were either grown by templates or multistep processes with uncontrollable crystallinity. Here the authors show that single crystal ZnO nanotubes can be directly grown on planar substrates without using catalysts and templates. These results are guided by the theory of nucleation and the vapor-solid crystal growth mechanism, which is applicable for transforming other nanowires or nanorods into nanotubular structures. (c) 2007 American Institute of Physics.
A mathematical framework is developed to study the mechanical behavior of material surfaces. The tensorial nature of surface stress is established using the force and moment balance laws. Bodies whose boundaries are material surfaces are discussed and the relation between surface and body stress examined. Elastic surfaces are defined and a linear theory with non-vanishing residual stress derived. The free-surface problem is posed within the linear theory and uniqueness of solution demonstrated. Predictions of the linear theory are noted and compared with the corresponding classical results. A note on frame-indifference and symmetry for material surfaces is appended.
A systematic experimental and theoretical investigation of the elastic and failure properties of ZnO nanowires (NWs) under
different loading modes has been carried out. In situ scanning electron microscopy (SEM) tension and buckling tests on single ZnO NWs along the polar direction [0001] were conducted.
Both tensile modulus (from tension) and bending modulus (from buckling) were found to increase as the NW diameter decreased
from 80 to 20 nm. The bending modulus increased more rapidly than the tensile modulus, which demonstrates that the elasticity
size effects in ZnO NWs are mainly due to surface stiffening. Two models based on continuum mechanics were able to fit the
experimental data very well. The tension experiments showed that fracture strain and strength of ZnO NWs increased as the
NW diameter decreased. The excellent resilience of ZnO NWs is advantageous for their applications in nanoscale actuation,
sensing, and energy conversion.
KeywordsZnO nanowire-mechanical property-size effect-Young’s modulus-fracture
The mechanical properties of the single ZnO rod were studied using nanoindentation. The hardness and Young’s modulus of the polar (0001) and nonpolar (01 1 0) faces were tested and the results demonstrate a face dependence variation in both properties. The mechanical behavior of the ZnO nano/microrod is discussed in conjunction with its morphology, structure, and defect effects.
Surface and interface stresses in solids are defined and their role in the thermodynamics of solids is presented. A discussion concerning the physical meaning of these quantities is given, along with a review of selected theoretical calculations and experimental measurements. It is shown that for a solid phase with one or more of its dimensions smaller than about 10 nm, the surface and interface stresses can be principal factors in determining the equilibrium structure and behavior of the solid. In particular, the effects of surface and interface stresses on thin films are reviered along with the related topic of surface reconstructions in metals.
Using molecular dynamics simulations with a many-body force field, we studied the deformation of single crystal Ni and NiCu random alloy nanowires subjected to uniform strain rates but kept at 300 K. For all strain rates, the Ni nanowire is elastic up to 7.5% strain with a yield stress of 5.5 GPa, far above that of bulk Ni. At high strain rates, we find that for both systems the crystalline phase transforms continuously to an amorphous phase, exhibiting a dramatic change in atomic short-range order and a near vanishing of the tetragonal shear elastic constant perpendicular to the tensile direction. This amorphization which occurs directly from the homogeneous, elastically deformed system with no chemical or structural inhomogeneities exhibits a new mode of amorphization.
In situ transmission electron microscopy is an established experimental technique that permits direct observation of the dynamics and mechanisms of dislocation motion and deformation behavior. In this article, we detail the development of a novel specimen goniometer that allows real-time observations of the mechanical response of materials to indentation loads. The technology of the scanning tunneling microscope is adopted to allow nanometer-scale positioning of a sharp, conductive diamond tip onto the edge of an electron-transparent sample. This allows application of loads to nanometer-scale material volumes coupled with simultaneous imaging of the material's response. The emphasis in this report is qualitative and technique oriented, with particular attention given to sample geometry and other technical requirements. Examples of the deformation of aluminum and titanium carbide as well as the fracture of silicon will be presented.
We report the development of a material testing system for in situ electron microscopy (EM) mechanical testing of nanostructures. The testing system consists of an actuator and a load sensor fabricated by means of surface micromachining. This previously undescribed nanoscale material testing system makes possible continuous observation of the specimen deformation and failure with subnanometer resolution, while simultaneously measuring the applied load electronically with nanonewton resolution. This achievement was made possible by the integration of electromechanical and thermomechanical components based on microelectromechanical system technology. The system capabilities are demonstrated by the in situ EM testing of free-standing polysilicon films, metallic nanowires, and carbon nanotubes. In particular, a previously undescribed real-time instrumented in situ transmission EM observation of carbon nanotubes failure under tensile load is presented here.
• carbon nanotube
• microelectromechanical system
• nanomechanics
• nanowires
ZnO is a wide bandgap semiconductor with a direct bandgap of 3.32eV at room temperature. It is a candidate material for ultraviolet LED and laser. ZnO has an exciton binding energy of 60 meV, much higher than that of GaN. It is found to be significantly more radiation hard than Si, GaAs, and GaN, which is critical against wearing out during field emission. Furthermore, ZnO can also be made as transparent and highly conductive, or piezoelectric. ZnO nanotips can be grown at relatively low temperatures, giving ZnO a unique advantage over the other nanostructures of wide bandgap semiconductors, such as GaN and SiC. In the present work, we report the selective growth of ZnO nanotips on various substrates using metalorganic chemical vapor deposition. ZnO nanotips grown on various substrates are single crystalline, n-type conductive and show good optical properties. The average size of the base of the nanotips is 40 nm. The room temperature photoluminescence peak is very intense and sharp with a full-width-half-maximum of 120 meV. These nanotips have potential applications in field emission devices, near-field microscopy, and UV photonics.
The mechanical resonance of a single ZnO nanobelt, induced by an alternative electric field, was studied by in situ transmission electron microscopy. Due to the rectangular cross section of the nanobelt, two fundamental resonance modes have been observed corresponding to two orthogonal transverse vibration directions, showing the versatile applications of nanobelts as nanocantilevers and nanoresonators. The bending modulus of the ZnO nanobelts was measured to be ∼52 GPa and the damping time constant of the resonance in a vacuum of 5×10-8 Torr was ∼1.2 ms and quality factor Q=500. © 2003 American Institute of Physics.
Semiconducting and piezoelectric zinc oxide has two important structure characteristics: the multiple and switchable growth directions: 〈010〉, 〈20〉, and 〈0001〉; and the {0001} polar surfaces. The fast growth directions create nanobelts of different crystallographic facets, and the polar surfaces result in bending of the nanobelt for minimizing the spontaneous polarization energy. A combination of these distinct growth characteristics results in a group of unique nanostructures, including several types of nanorings, nanobows, platelet circular structures, Y-shape split ribbons, and crossed ribbons. We present here the as-grown nanoarchitectures naturally created by combining some of the fundamental structure configurations of ZnO, which could be unique for many applications in nanotechnology.
Process of nanostructure formation and crystalline-to-amorphous transformation (CTAT) by high pressure torsion have been studied
for various intermetallic compounds, such as, TiNi, ZrCu and Ni3Al. These compounds have been chosen to elucidate the effect of crystal structure on CTAT. Crystal structures of the samples
were B2 or B19’ martensite for TiNi, Cm martensite for ZrCu and L12 for Ni3Al.
Surface and interface stresses, which are intrinsic thermodynamic quantities associated with all types of solid surfaces and interfaces, are reviewed. These stresses can significantly modify the thermodynamics of small scale solids as well as result in significant variations in the structure and physical behavior of materials when the characteristic microstructural length scale is reduced to the nanometer range. Examples of these surface and interface stress effects are presented. (C) 1997 Elsevier Science S.A.
The solid-state amorphization and subsequent crystallization of the quenched ‘white tin’ high-pressure phase of GaSb alloys containing 20–80 at.% Sb were studied at ambient pressure by differential scanning calorimetry and dilatometry. Within the experimental uncertainty, the heat of both amorphization and crystallization did not change over the homogeneity region for the ‘white tin’ phase (47.5–52.5 at.% Sb); the heats were exothermic and equal to 3.5 ± 0.5 and 8.3 ± 1.0 kJ/mol, respectively. The amorphization was accompanied by a sample volume expansion of about 25%. The processes of amorphization and crystallization involved a multistage process.
Light and heavy neutron-irradiation damage of highly oriented pyrolytic graphite ( HOPG) crystals was examined by means of X-ray diffraction and high-resolution high-voltage transmission electron microscopy (TEM). From the X-ray data analysis, it was found that there is an average increase of about 3% in the c-axis lattice parameter of the unit cell of graphite for lightly neutron-irradiated HOPG. However, the c-axis lattice parameter could not be estimated from the HOPG sample having the highest dose of neutron irradiation under the present investigation, because the X-ray profile was highly asymmetrical. This increase in the c-axis lattice parameter is attributed to lattice expansion due to the static displacement of atoms after neutron irradiation. Local structure analysis by TEM shows that the 0002 lattice spacing for the above-mentioned HOPG samples has been increased by up to 10% as a result of the neutron irradiation. This increase in c-axis lattice spacing can be ascribed to the fragmentation of the crystal lattice into nanocrystallites, breaking and bending of the 0002 straight lattice fringes, appearance of dislocation loops, and extra interstitial planes within the fragmented nanocrystallites. All these changes are a result of the static displacement of atoms after neutron irradiation.
Atomistic simulations with modified embedded atom method (MEAM), embedded atom method (EAM) and surface embedded atom method (SEAM) potentials reveal that, at certain sizes, a face centered cubic (fcc) gold ⟨100⟩ nanowire reorients into an fcc ⟨110⟩ nanowire. In MEAM simulations, the reorientation consists of two successive processes. First, surface stress and thermal vibrations cause the fcc ⟨100⟩ nanowire to transform into a body centered tetragonal (bct) nanowire. Second, the bct nanowire becomes unstable with respect to shear and transforms into an fcc ⟨110⟩ nanowire. In EAM and SEAM simulations a different reorientation mechanism exists. The surface stress in the fcc ⟨100⟩ nanowire induces slip on a {111}⟨112⟩ system. Progressive slip on adjacent {111} planes changes the stacking sequence of these {111} planes from ABCABC to ACBACB, and the nanowire reorients into an fcc ⟨110⟩ nanowire. The difference in reorientation mechanisms is rooted in the differences in the unstable stacking fault energy and orientation dependence of electron density in the potentials. In spite of these differences, the final structures of the reoriented nanowires are the same, which helps to explain why uniform fcc ⟨110⟩ nanowires are observed much more often in experiments than nanowires of other orientations.
The size dependence of elastic modulus of ZnO nanofilms is investigated by using atomistic simulations. The strain energy and elastic stiffness of the surface and interior atomic layers, as well as interlayer interactions, are decoupled. The surface stiffness is found to be much lower than that of the interior layers and bulk counterpart, and with the decrease of film thickness, the residual tension-stiffened interior atomic layers are the main contributions of the increased elastic modulus of nanofilms.
Data on the elastic properties of single crystals have been collected from the literature published through mid-1964. The elastic properties of isotropic aggregates (Young's modulus, Poisson's ratio, shear modulus, bulk modulus, compressibility, velocity of shear waves, and the velocity of compressional waves) are calculated according to the schemes of Voigt and Reuss. The tables include about 1100 determinations. (Author)
Uniaxial tensile measurements have been performed on [0001]-oriented zinc oxide nanowires (NWs) with diameters ranging from 18 to 204 nm using a homemade in situ mechanical testing system. Diameter dependence of tensile modulus (TM) is further compared with that of bending modulus (BM, shown earlier). With diameters of NWs decreasing in an intermediate range (about 30–120 nm), TM increases slower than BM, while it gets close to the latter with diameters decreasing below 30 nm; for rather large diameters, they both tend to the bulk modulus. A core-shell model is developed based on diameter-dependent and radial-distributed elastic stiffening in NWs and well explains our experimental results.
CARBON nanotubes are predicted to have interesting mechanical properties—in particular, high stiffness and axial strength—as a result of their seamless cylindrical graphitic structure1–5. Their mechanical properties have so far eluded direct measurement, however, because of the very small dimensions of nanotubes. Here we estimate the Young's modulus of isolated nanotubes by measuring, in the transmission electron microscope, the amplitude of their intrinsic thermal vibrations. We find that carbon nanotubes have exceptionally high Young's moduli, in the terapascal (TPa) range. Their high stiffness, coupled with their low density, implies that nanotubes might be useful as nanoscale fibres in strong, lightweight composite materials.
We show how contact resonance atomic force microscopy (CR-AFM) can be used to accurately determine the radial elastic moduli of [0001] ZnO nanowires with diameters smaller than 150 nm. Using a realistic tip−nanowire contact geometry, we find the radial indentation modulus from CR-AFM data and the tangential shear modulus from friction-type measurements. Both moduli show a pronounced increase when the wire diameter is reduced below 80 nm. The size dependence of the elastic properties can be explained by a core−shell model that accounts for a bulk-like core and an elastically stiffer surface layer.
ZnO can appear as nanowires, nanobelts, and nanocombs, which are attractive for various applications. However, this has prevented the growth of desired nanostructures without other trace morphologies. Here we demonstrated a mechanism for selective growth of pure and long ZnO nanowires. This was obtained by placing a gold film at a high-temperature zone so that ZnO nanowires with controllable densities can be grown on adjacent bare substrates at lower temperature zones. The concentration gradients of gold and ZnO vapors are responsible for this selective growth, which could be applicable for selective growth of ZnO nanobelts and nanocombs in the future.
Molecular dynamics simulations are performed to characterize the response of zinc oxide (ZnO) nanobelts to tensile loading. The ultimate tensile strength (UTS) and Young's modulus are obtained as functions of size and growth orientation. Nanobelts in three growth orientations are generated by assembling the unit wurtzite cell along the [0001], , and crystalline axes. Following the geometric construction, dynamic relaxation is carried out to yield free-standing nanobelts at 300 K. Two distinct configurations are observed in the [0001] and orientations. When the lateral dimensions are above 10 Å, nanobelts with rectangular cross-sections are seen. Below this critical size, tubular structures involving two concentric shells similar to double-walled carbon nanotubes are obtained. Quasi-static deformations of belts with and orientations consist of three stages, including initial elastic stretching, wurtzite-ZnO to graphitic-ZnO structural transformation, and cleavage fracture. On the other hand, [0001] belts do not undergo any structural transformation and fail through cleavage along (0001) planes. Calculations show that the UTS and Young's modulus of the belts are size dependent and are higher than the corresponding values for bulk ZnO. Specifically, as the lateral dimensions increase from 10 to 40 Å, decreases between 38–76% and 24–63% are observed for the UTS and Young's modulus, respectively. This effect is attributed to the size-dependent compressive stress induced by tensile surface stress in the nanobelts. and nanobelts with multi-walled tubular structures are seen to have higher values of elastic moduli (~340 GPa) and UTS (~36 GPa) compared to their wurtzite counterparts, echoing a similar trend in multi-walled carbon nanotubes.
Effective stiffness properties (D) of nanosized structural elements such as plates and beams differ from those predicted by standard continuum mechanics (Dc). These differences (D-Dc)/Dc depend on the size of the structural element. A simple model is constructed to predict this size dependence of the effective properties. The important length scale in the problem is identified to be the ratio of the surface elastic modulus to the elastic modulus of the bulk. In general, the non-dimensional difference in the elastic properties from continuum predictions (D-Dc)/Dc is found to scale as αS/Eh, where α is a constant which depends on the geometry of the structural element considered, S is a surface elastic constant, E is a bulk elastic modulus and h a length defining the size of the structural element. Thus, the quantity S/E is identified as a material length scale for elasticity of nanosized structures. The model is compared with direct atomistic simulations of nanoscale structures using the embedded atom method for FCC Al and the Stillinger-Weber model of Si. Excellent agreement between the simulations and the model is found.
The mechanical properties of individual zinc oxide (ZnO) nanowires, grown by a solid–vapour phase thermal sublimation process, were studied in situ by transmission electron microscopy (TEM) using a home-made TEM specimen holder. The mechanical resonance is electrically induced by applying an oscillating voltage, and in situ imaging has been achieved simultaneously. The results indicate that the elastic bending modulus of individual ZnO nanowires were measured to be ~58 GPa and the damping time constant of the resonance in a vacuum of 10−8 Torr was ~14 ms. A nanobalance was built and the mass of the nanoparticle attached at the tip of a nanowire was measured. The ZnO nanowires are promising in potential applications as nanocantilevers and nanoresonators.
We have examined the nanomechanics of a carbon nanotube by a manipulation technique using a scanning electron micro-scope. Young's modulus of the nanotube, estimated from the buckling under force acting on the axial direction of the nanotube, agrees well with the value estimated from the bending under force acting on the side face. This indicates that the nanotube can be treated as an isotropic material in conventional mechanics. The adhesion force between the side face of the nanotube and a pit wall in a 4.7 GB digital versatile disk is estimated to be ∼10 nN using atomic force microscopy. This value is 160 times less than the value estimated using force curve measurement. This discrepancy is due to the finding that the value estimated from the force curve includes not only the adhesion but also the friction force.
An energetic electron beam has been used to stimulate crystallization of spatially isolated amorphous regions in Si, Ge, GaP, and GaAs at 30 and 300 K. In the four materials it was found that crystallization was induced even when the energy of the electron beam was less than that required to create point defects in the crystalline structure. The rate of crystallization depended on the material and on the electron energy. In all materials, the rate decreases as the electron energy increases from 50 keV (the lowest electron energy used), reaching a minimum value at an electron energy slightly below the displacement threshold voltage. Above the displacement threshold, the regrowth rate again increases with increasing electron energy. The possible role of electron‐beam heating was studied both theoretically and experimentally. Calculations suggested heating effects were negligible and this was confirmed by in situ ion implantations and electron irradiations performed at 30 K, where subthreshold electrons stimulated crystallization. The subthreshold and low‐temperature results are consistent with the model that the crystallization process is dependent on the creation of defects (dangling bonds and kinks) at the crystalline‐amorphous (c‐a) interface. The crystallization stimulated by the subthreshold electron beams suggests that electronic excitation of the bonds along the c‐a interface can induce the amorphous to crystalline transition. © 1995 American Institute of Physics.
We report the development of a method for quantitative, in situ nanoindentation in an electron microscope and its application to study the onset of deformation during the nanoindentation of aluminum films. The force–displacement curve developed shows the characteristic “staircase” instability at the onset of plastic deformation. This instability corresponds to the first appearance of dislocations in a previously defect-free grain. Plastic deformation proceeds through the formation and propagation of prismatic loops punched into the material, and half loops that emanate from the sample surface. These results represent the first real time observations of the discrete microstructural events that occur during nanoindentation. © 2001 American Institute of Physics.
The elastic constants of ZnO nanostructures control their elastic energy and thereby are important to their function in converting strain energy to electricity. This letter presents the size dependence of Young’s moduli of ZnO nanoplates, according to density-functional-theory-based ab initio calculations. Our results show that Young’s moduli of (0001)/(000 1 ) , (1 1 00) , and (11 2 0) nanoplates increase as size decreases. For (0001)/(000 1 ) nanoplate, Young’s moduli vary discontinuously with size, due to a phase transformation from wurtzite to graphitic structure. Further, our analyses show that the increase of moduli is due to surface stiffening and bulk nonlinear elasticity.
Wurtzitic ZnO is a wide-bandgap (3.437 eV at 2 K) semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films. Most of these applications require only polycrystalline material; however, recent successes in producing large-area single crystals have opened up the possibility of producing blue and UV light emitters, and high-temperature, high-power transistors. The main advantages of ZnO as a light emitter are its large exciton binding energy (60 meV), and the existence of well-developed bulk and epitaxial growth processes; for electronic applications, its attractiveness lies in having high breakdown strength and high saturation velocity. Optical UV lasing, at both low and high temperatures, has already been demonstrated, although efficient electrical lasing must await the further development of good, p-type material. ZnO is also much more resistant to radiation damage than are other common semiconductor materials, such as Si, GaAs, CdS, and even GaN; thus, it should be useful for space applications.
We report here, an investigation on electrical and structural-microstructural properties of an individual ZnO nanobelt via in situ transmission electron microscopy using an atomic force microscopy (AFM) system. The I-V characteristics of the ZnO nanobelt, just in contact with the AFM tip indicates the insulating behavior, however, it behaves like a semiconductor under applied stress. Analysis of the high resolution lattice images and the corresponding electron diffraction patterns shows that each ZnO nanobelt is a single crystalline, having wurtzite hexagonal structure (a = 0.324 nm, c = 0.520 66 nm) with a general growth direction of [100].
An experimental and computational approach is pursued to investigate the fracture mechanism of [0001] oriented zinc oxide nanowires under uniaxial tensile loading. A MEMS-based nanoscale material testing stage is used in situ a transmission electron microscope to perform tensile tests. Experiments revealed brittle fracture along (0001) cleavage plane at strains as high as 5%. The measured fracture strengths ranged from 3.33 to 9.53 GPa for 25 different nanowires with diameters varying from 20 to 512 nm. Molecular dynamic simulations, using the Buckingham potential, were used to examine failure mechanisms in nanowires with diameters up to 20 nm. Simulations revealed a stress-induced phase transformation from wurtzite phase to a body-centered tetragonal phase at approximately 6% strain, also reported earlier by Wang et al. (1) The transformation is partial in larger nanowires and the transformed nanowires fail in a brittle manner at strains as high as 17.5%. The differences between experiments and computations are discussed in the context of (i) surface defects observed in the ZnO nanowires, and (ii) instability in the loading mechanism at the initiation of transformation.
The bending Young's modulus of ZnO nanobelts was measured by performing three-point bending tests directly on individual nanobelts with an atomic force microscope (AFM). The surface-to-volume ratio has no effect on the bending Young's modulus of the ZnO nanobelts for surface-to-volume ratios ranging from 0.017 to 0.035 nm(2) nm(-3), with a belt size of 50-140 nm in thickness and 270-700 nm in width. The bending Young's modulus was measured to be 38.2 +/- 1.8 GPa, which is about 20% higher than the nanoindentation Young's modulus of 31.1 +/- 1.3 GPa. The ZnO nanobelts exhibit brittle fracture failure in bending but some plastic deformation in indentation.
The mechanisms governing the tensile behavior of TiO(2) nanowires were studied by molecular dynamics simulations. Nanowires below a threshold diameter of about 10 A transformed into a completely disordered structure after thermodynamic equilibration, whereas thicker nanowires retained their crystalline core. Initial elastic tensile deformation was effected by the reconfiguration of surface atoms while larger elongations resulted in continuous cycles of Ti-O bond straightening, bond breakage, inner atomic distortion, and necking until rupture. Nanowires have much better mechanical properties than bulk TiO(2). Nanowires below the threshold diameter exhibit extraordinarily high stiffness and toughness and are more sensitive to strain rate.
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.
We describe in detail, the design, construction, and testing of a specimen holder that allows for the nanoindentation of surfaces while viewing in cross-section in a high voltage transmission electron microscope (TEM). This nanoindentation specimen holder, having three-axis position control of a diamond indenter in combination with micromachined specimens, allows for the first time the dynamic observation of subsurface microstructure evolution under an indenter tip. Additionally, the sample design techniques that have been developed for these procedures may eliminate the need for TEM specimen preparation for additional ex situ nanoindentation experiments. Initial experimental results from in situ indentation of Si samples in the high voltage electron microscope are reported here to demonstrate the capability of this new specimen holder.
Materials performance in structural applications is often restricted by a transition from ductile response to brittle fracture
with decreasing temperature. This transition is currently viewed as being controlled either by dislocation mobility or by
the nucleation of dislocations. Fracture experiments on tungsten single crystals reported here provide evidence for the importance
of dislocation nucleation for the fracture toughness in the semibrittle regime. However, it is shown that the transition itself,
in general, is controlled by dislocation mobility rather than by nucleation.
Several researchers have demonstrated, through experiments and analysis, that the structure and properties of nanometre-scale materials can be quite different to those of bulk materials due to the effect of surfaces. Here we use atomistic simulations to study a surface-stress-induced phase transformation in gold nanowires. The emergence of the transformation is controlled by wire size, initial orientation, boundary conditions, temperature and initial cross-sectional shape. For a <100> initial crystal orientation and wire cross-sectional area below 4 nm(2), surface stresses alone cause gold nanowires to transform from a face-centred-cubic structure to a body-centred-tetragonal structure. The transformation occurs roughly when the compressive stress caused by tensile surface-stress components in the length direction exceeds the compressive stress required to transform bulk gold to its higher energy metastable crystal structure.
An atomic force microscopy (AFM) based technique is demonstrated for measuring the elastic modulus of individual nanowires/nanotubes aligned on a solid substrate without destructing or manipulating the sample. By simultaneously acquiring the topography and lateral force image of the aligned nanowires in the AFM contacting mode, the elastic modulus of the individual nanowires in the image has been derived. The measurement is based on quantifying the lateral force required to induce the maximal deflection of the nanowire where the AFM tip was scanning over the surface in contact mode. For the [0001] ZnO nanowires/nanorods grown on a sapphire surface with an average diameter of 45 nm, the elastic modulus is measured to be 29 +/- 8 GPa.
We report a size dependence of Young's modulus in [0001] oriented ZnO nanowires (NWs) with diameters ranging from 17 to 550 nm for the first time. The measured modulus for NWs with diameters smaller than about 120 nm is increasing dramatically with the decreasing diameters, and is significantly higher than that of the larger ones whose modulus tends to that of bulk ZnO. A core-shell composite NW model in terms of the surface stiffening effect correlated with significant bond length contractions occurred near the {1010} free surfaces (which extend several layers deep into the bulk and fade off slowly) is proposed to explore the origin of the size dependence, and present experimental result is well explained. Furthermore, it is possible to estimate the size-related elastic properties of GaN nanotubes and relative nanostructures by using this model.
Large strain plasticity is phenomenologically defined as the ability of a material to exhibit an exceptionally large deformation rate during mechanical deformation. It is a property that is well established for metals and alloys but is rarely observed for ceramic materials especially at low temperature ( approximately 300 K). With the reduction in dimensionality, however, unusual mechanical properties are shown by ceramic nanomaterials. In this Letter, we demonstrated unusually large strain plasticity of ceramic SiC nanowires (NWs) at temperatures close to room temperature that was directly observed in situ by a novel high-resolution transmission electron microscopy technique. The continuous plasticity of the SiC NWs is accompanied by a process of increased dislocation density at an early stage, followed by an obvious lattice distortion, and finally reaches an entire structure amorphization at the most strained region of the NW. These unusual phenomena for the SiC NWs are fundamentally important for understanding the nanoscale fracture and strain-induced band structure variation for high-temperature semiconductors. Our result may also provide useful information for further studying of nanoscale elastic-plastic and brittle-ductile transitions of ceramic materials with superplasticity.
- S L Mensah
- V K Kayastha
- I N Ivanov
- D B Geohegan
S. L. Mensah, V. K. Kayastha, I. N. Ivanov, D. B. Geohegan and Y. K. Yap. Appl.
PhysLetts. 90, 113108 (2007).