Article

# Atomic Defects Influenced Mechanics of II-VI Nanocrystals

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## Abstract

Mechanical properties of nanocrystals are influenced by atomic defects. Here, we demonstrate the effect of planar defects on the mechanics of ZnO nanorods using atomic force microscopy, high resolution transmission electron microscopy, and large scale atomistic simulation. We study two different conditionally grown single nanorods. One contains extended $I_{1}$ type stacking fault (SF) and another is defect free. The SF containing nanorods show buckling behaviors with reduced critical loading, whereas the other kinds show linear elastic behavior. We also studied the size dependence of elastic modulus and yield strength. The elastic modulus in both nanorods is inversely proportional to their size. Similar trend is observed for yield strength in the SF containing nanorods, however, the opposite is observed in the SF free nanorods. This first experimental and theoretical study will guide towards the development of reliable electromechanical devices.

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... In this paper, we combine molecular dynamics simulations with available experimental data to identify a repeatable energy absorbing concept through the reversible wurtzite-to-hexagonal phase transformation of ZnO nanopillars (NPs) subjected to cycles of uniaxial compressive loading and unloading at room temperature. The specific energy absorption obtained by molecular dynamics simulations is verified by experimental data from axial indention and tension of ZnO nanowires [20,23]. The time expected to revert back to the original wurtzite phase is validated by the hopping frequency between the wurtzite and tetragonal phases previously measured in transmission electron microscope [21]. ...
... Such relations have also been seen in the axial indentation force-displacement relations of ZnO nanowires in atomic force microscopy experiments [20]. Such relations allow the energy absorption to be assessed. ...
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... 28 Such defects not only form electrically active states in the band gap region but also increase threshold voltage and reduce the saturation current. 29,30 Although there is a wealth of information in the scientific literatures addressing the structural and optical properties 31,32,26 as well as different fabrication techniques and processing conditions for CdSe thin films, 33,34,30 only a few of them endeavored to bring the optoelectrical, microstructural, crystallographic and stoichiometric features altogether in a single platform to comprehend the effect of film thickness in attaining the desired physical properties in the present day single or multilayer functional devices. Therefore, in the present contribution, the variations in optical constants, dielectric and energy loss functions, optical transition parameters, optical dispersion parameters and their corresponding derivatives of TVD CdSe thin films on glass substrate have been studied as a function of film thickness under submicron level. ...
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... [9,10] Various theoretical and computational methods are utilized to study synthesis of 2D materials [11][12][13][14], specifically the diamondization process, e.g., Molecular Dynamics [7], DFT [4], and ab initio. [11,12] Among these methods, Molecular Dynamics has been widely used to study the phase transformation and surface effects in materials [17][18][19][20][21][22][23][24]. The solid-solid phase transformation, including the graphene→diamond transformation, is a complicated procedure. ...
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Diamond is the hardest superhard material with excellent optoelectronic, thermomechanical, and electronic properties. Here, we have investigated the possibility of a new synthesis technique for diamane and diamond thin films from multilayer graphene at pressures far below the graphite → diamond transformation pressure. We have used the Molecular Dynamics technique with reactive force fields. Our results demonstrate a significant reduction (by a factor of two) in the multilayer graphene → diamond transformation stress upon using a combined shear and axial compression. The shear deformation in the multilayer graphene lowers the phase transformation energy barrier and plays the role of thermal fluctuations, which itself promotes the formation of diamond. We revealed a relatively weak temperature dependence of the transformation strain and stresses. The transformation stress vs. strain curve for the bulk graphite drops exponentially for finite temperatures.
... This is a powerful technique which has been utilized to study phase transformation in various materials. 1,[24][25][26][27] We will calculate the transformation stress and strain for the formation of diamond thin films at different thermochemical conditions. We then analyzed the energy barrier for the diamond formation and the stability of the formed diamond structures. ...
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... Fig. 2a shows that the tensile critical stress of defective NBs is always smaller than its perfect counterpart, although the SF density has the opposite effect on compressive critical stress and leads to strengthening: A 63% increase in critical stress is experienced for highly defective NBs (925 SF/µm). It is worth noting that the smaller critical compressive stress for the low SF density is in agreement with an experimental report for ZnO NWs [35]. Fig. 2b depicts a monotonous enhancement in Young's Modulus by increasing the SF density under tension and compression: up to 26% and 14% under compressive and tensile loading, respectively, for a density of 925 SF/µm. ...
... It is worth mentioning that the strengthening methods in macroscale have their own setbacks, like a reduction in ductility [18] that can be overcome in nanoscale. However, on the other side of the spectrum, there is a long-standing notion that these defects are the root cause of structural weakening [19][20][21][22][23][24]; consequently, there is not a consensus on the exact effects of the defects on mechanical properties. Therefore, understanding the underlying physics opens a new avenue through which to predict, design, and fabricate a new class of one-dimensional materials with superior properties. ...
... Unfortunately, the stacking faults arise from the presence of one or more cubic bonds in the stacking sequence in the wurtzite structure, that are leading to the formation of electrically active states in the gap. Besides, the effects of a stacking fault on the device are the formation of a potential barrier on it, increase of the threshold voltage, and lowering of the saturation current of the device [28][29][30]. Many researchers studied the influence of a variety of changing growth conditions on the electronic properties of the nanostructured CdSe thin films [31,32]. ...
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This article surveys recent developments in the rational synthesis of single-crystalline zinc oxide nanowires and their unique optical properties. The growth of ZnO nanowires was carried out in a simple chemical vapor transport and condensation (CVTC) system. Based on our fundamental understanding of the vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls (including positional, orientational, diameter, and density control) have been achieved. Power-dependent emission has been examined and lasing action was observed in these ZnO nanowires when the excitation intensity exceeds a threshold (∼40 kW cm–2). These short-wavelength nanolasers operate at room temperature and the areal density of these nanolasers on substrate readily reaches 1 × 10¹⁰ cm–2. The observation of lasing action in these nanowire arrays without any fabricated mirrors indicates these single-crystalline, well-facetted nanowires can function as self-contained optical resonance cavities. This argument is further supported by our recent near-field scanning optical microscopy (NSOM) studies on single nanowires.
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A comparative morphological study of different ZnO nanostructures was carried out with different varying process parameters for energy harvesting. Molarity, temperature, growth duration and seed layer were such fundamental controlling parameters. The study brings out an outstanding piezoelectric coefficient (d 33) of 44.33 pm/V for vertically aligned ZnO nanorod structures, considered as the highest reported d 33 value for any kind of ZnO nanostructures. XRD analysis confirms wurtzite nature of this nanorod structure with [0001] as preferential growth direction. Semiconducting characteristic of nanorods was determined with temperature induced I/V characterization.
Article
The well-aligned and vertical ZnO nanowires were grown on PET substrates by the hydrothermal method. The novel piezoelectric nanogenerator was fabricated from ZnO nanowires and a Pt/ZnO nanowires electrode on a flexible PET substrate. A sample was compressed and bent bending generating internal stress in the PET substrate, which output a current of approximately 5x10-10 A without a source of vibration. The sample with 2% bending was also measured with vibration at a low frequency, yielding a maximum piezoelectric current of about 2.5x10-7 A, which is a four times the current of a non-bending sample. These results demonstrate that a little bending of a flexible substrate improves piezoelectric performance.
Article
One-dimensional (1D) zinc oxide nanostructures are the main components of nanogenerators and central to the emerging field of nanopiezotronics. Understanding the underlying physics and quantifying the electromechanical properties of these structures, the topic of this research study, play a major role in designing next-generation nanoelectromechanical devices. Here, atomistic simulations are utilized to study surface and size-scale effects on the electromechanical response of 1D ZnO nanostructures. It is shown that the mechanical and piezoelectric properties of these structures are controlled by their size, cross-sectional geometry, and loading configuration. The study reveals enhancement of the piezoelectric and elastic modulus of ZnO nanowires (NW) with diameter d > 1 nm, followed by a sudden drop for d < 1 nm due to transformation of NWs to nanotubes (NTs). Degradation of mechanical and piezoelectric properties of ZnO nanobelts (NBs) followed by an enhancement in piezoelectric properties occurs when their lower dimension is reduced to <1 nm. The latter enhancement can be explained in the context of surface reconfiguration and formation of hexagon-tetragon (HT) pairs at the intersection of (21[combining macron]1[combining macron]0) and (011[combining macron]0) planes in NBs. Transition from a surface-reconstructed dominant to a surface-relaxed dominant region is demonstrated for lateral dimensions <1 nm. New phase-transformation (PT) kinetics from piezoelectric wurtzite to nonpiezoelectric body-centered tetragonal (WZ → BCT) and graphite-like phase (WZ → HX) structures occurs in ZnO NWs loaded up to large strains of ∼10%.
Article
Harvesting waste energy through electromechanical coupling in practical devices requires combining device design with the development of synthetic strategies for large-area controlled fabrication of active piezoelectric materials. Here, we show a facile route to the large-area fabrication of ZnO nanostructured arrays using commodity galvanized steel as the Zn precursor as well as the substrate. The ZnO nanowires are further integrated within a device construct and the effective piezoelectric response is deduced based on a novel experimental approach involving induction of stress in the nanowires through pressure wave propagation along with phase-selective lock-in detection of the induced current. The robust methodology for measurement of the effective piezoelectric coefficient developed here allows for interrogation of piezoelectric functionality for the entire substrate under bending-type deformation of the ZnO nanowires.
Article
When the atomic-scale geometry of an interface or surface is known, its properties can be estimated and understood in terms of elementary theory of its electronic structure, or can be accurately calculated using full local-density theory. This is illustrated in terms of semiconductor heterojunctions and ideal semiconductor surfaces. The necessity for self-consistency is discussed in this connection. It is less certain that the atomic-scale geometries can be reliably predicted. This is discussed in terms of reconstruction of semiconductor surfaces, and the silicon surface in particular.
Article
The influence of surface elasticity on the piezoelectric potential of a bent ZnO nanowire is investigated using a modified core–shell (MC–S) model in which it is assumed that the elasticity in the shell (surface region) is an exponentially increasing function. Specifically, we analyse the effects of the nanowire radius and applied force on the piezoelectric potential with and without the surface elasticity effect, as well as the influence of surface elasticity on the deflection of the nanowire. The results demonstrate that both the nanowire radius and the applied force are key factors affecting the piezoelectric potential, and that the effect of surface elasticity is even more important, which should not be ignored, especially for dimensions below 100 nm.
Article
The mechanical properties of nickel nanowire at different temperatures are studied using molecular dynamics (MD) simulations. The inter-atomic interactions are represented by employing embedded-atom potential. In the case of uniaxial loading, the stress–strain curve at different strain rates is simulated. The effects of volume/surface ratio and temperature on mechanical properties of nickel nanowire are discussed. In particular, the loading–unloading process is modeled and the effect of unloading process on the stress–strain curve in the plastic region is investigated. Furthermore, the mechanical characterization in compression loading is carried out and the mechanism of deformation is elucidated based on the present model. The results of compression modeling show that the obtained yield stress is lower than the computed tensile yield stress.
Article
In this work, piezoelectricity of individual ZnO nanobelts grown along the [0 1 ī 0] direction is studied using piezoresponse force microscopy (PFM). It is found that the effective piezoelectric coefficient of these NBs, $d_{33}^{\mathrm{eff}}$ , is increasing from 2.7 pm/V at 30 kHz to 44 pm/V at 150 kHz. The results were explained by the Debye model, where structural inhomogeneity in our NBs was shown to be responsible for piezoelectric enhancement.
Article
By considering acoustic phonon mode displacements in nanowires, the piezoelectrically induced electric polarization vector and the associated potential are calculated. For the case of charge-free semiconductor nanowires, the piezo energies generated by strains applied in different directions are compared. For the directions considered, it is found that the maximum piezo energy in these nanowires is generated for strain applied in the vertical direction (i.e., along z-axis). Moreover, for these nanowires, energy generation in AlN and ZnO are found to be superior to GaN, just as expected based on past treatments of nanowires using phonons of bulk structures.
Article
A nanostructured surface may exhibit low adhesion or high adhesion depending upon fibrillar density, and it presents the possibility of realizing eco-friendly surface structures with desirable adhesion by mimicking the mechanics of fibrillar adhesive surfaces of biological systems. The current research uses a patterning technique to fabricate smart adhesion surfaces: one-, two- and three-level hierarchical synthetic adhesive structure surfaces with various fibrillar densities and diameters. The contact angles and contact angle hysteresis were measured to characterize the wettability. A conventional and a glass ball attached to an atomic force microscope (AFM) tip were used to obtain the adhesive forces via force-distance curves and to study the buckling behavior of a single fiber on the hierarchical structures.
Article
Nanocrystals of cadmium selenide exhibit a form of polytypism, with stable forms in both the wurtzite and zinc blende crystal structures. As a result, wurtzite nanorods of cadmium selenide tend to form stacking faults of zinc blende along the c-axis. These faults were found to preferentially form during the growth of the (001) face, which accounts for 40% of the rod's total length. Since II-VI semiconductor nanorods lack inversion symmetry along the c-axis of the particle, the two ends of the nanorod may be identified by this anisotropic distribution of faults.
Article
Piezoresponse force microscopy (PFM) is used to measure the effective piezoelectric coefficient (d33) of an individual (0001) surface dominated zinc oxide nanobelt lying on a conductive surface. Based on references of bulk (0001) ZnO and x-cut quartz, the effective piezoelectric coefficient d33 of ZnO nanobelt is found to be frequency dependent and varies from 14.3 pm/V to 26.7 pm/V, which is much larger than that of the bulk (0001) ZnO of 9.93 pm/V. The results support the application of ZnO nanobelts as nanosensors and nanoactuators.
Article
Developing wireless nanodevices and nanosystems are of critical importance for sensing, medical science, defense technology, and even personal electronics. It is highly desirable for wireless devices and even required for implanted biomedical devices that they be self-powered without use of a battery. It is essential to explore innovative nanotechnologies for converting mechanical energy (such as body movement, muscle stretching), vibrational energy (such as acoustic or ultrasonic waves), and hydraulic energy (such as body fluid flow) into electrical energy, which will be used to power nanodevices without a battery. This is a key step towards self-powered nanosystems. We have demonstrated an innovative approach for converting mechanical energy into electrical energy by piezoelectric zinc oxide nanowire (NW) arrays. The operation mechanism of the electric generator relies on the unique coupling of the piezoelectric and semiconducting properties of ZnO as well as the gating effect of the Schottky barrier formed between the metal tip and the NW. Based on this mechanism, we have recently developed a DC nanogenerator (NG) driven by the ultrasonic wave in a biofluid and a textile-fiber-based NG for harvesting low-frequency mechanical energy. Furthermore, a new field, ''nanopiezotronics'', has been developed, which uses coupled piezoelectric-semiconducting properties for fabricating novel and unique electronic devices and components. This Feature Article gives a systematic description of the fundamental mechanism of the NG, its rationally innovative design for high output power, and the new electronics that can be built based on a piezoelectric-driven semiconducting process. A perspective will be given about the future impact of the technologies.
Article
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].
Article
Mechanical instability and buckling characterization of vertically aligned single-crystal ZnO nanorods grown on different substrates including Si, SiC and sapphire (α-Al(2)O(3)) was done quantitatively by the nanoindentation technique. The nanorods were grown on these substrates by the vapor-liquid-solid (VLS) method. The critical load for the ZnO nanorods grown on the Si, SiC and Al(2)O(3) substrates was found to be 188, 205 and 130 µN, respectively. These observed critical loads were for nanorods with 280 nm diameters and 900 nm length using Si as a substrate, while the corresponding values were 330 nm, 3300 nm, and 780 nm, 3000 nm in the case of SiC and Al(2)O(3) substrates, respectively. The corresponding buckling energies calculated from the force displacement curves were 8.46 × 10(-12), 1.158 × 10(-11) and 1.092 × 10(-11) J, respectively. Based on the Euler model for long nanorods and the J B Johnson model (which is an extension of the Euler model) for intermediate nanorods, the modulus of elasticity of a single rod was calculated for each sample. Finally, the critical buckling stress and strain were also calculated for all samples. We found that the buckling characteristic is strongly dependent on the quality, lattice mismatch and adhesion of the nanorods with the substrate.
Article
ZnO is a wide band gap metal oxide with a very interesting combination of semiconducting, transparent optical and catalytic properties. Recently, an amplified interest in ZnO has appeared due to the impressive progress made in nanofabrication of tailored ZnO nanostructures and functional surfaces. However, the fundamental principles governing the structure of even the clean low-index ZnO surfaces have not been adequately explained. From an interplay of high-resolution scanning probe microscopy (SPM), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments, and density functional theory (DFT) calculations, we identify here a group of hitherto unresolved surface structures which stabilize the clean polar O-terminated ZnO(0001) surface. The found honeycomb structures are truly remarkable since their existence deviates from expectations using a conventional electrostatic model which applies to the opposite Zn-terminated (0001) surface. As a common principle, the differences for the clean polar ZnO surfaces are explained by a higher bonding flexibility of the exposed 3-fold coordinated surface Zn atoms as compared to O atoms.
Article
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.
Article
Molecular dynamics simulations of ZnO nanowires under tensile loading were performed and compared with simulations of TiO(2) wires to present size-dependent mechanical properties and super ductility of metal oxide wires. It is shown that while large surface-to-volume ratio is responsible for their size effects, ZnO and TiO(2) wires displayed opposite trends. Although the stiffness of both wires converged monotonically to their bulk stiffness values as diameter increases, bulk stiffness represented the upper bound for ZnO nanowires as opposed to the lower bound for TiO(2) wires. ZnO nanowires relaxed to either completely amorphous or completely crystalline states depending on wire thickness, whereas a thin amorphous shell is always present in TiO(2) nanowires. It was also found that when crystalline ZnO nanowires are stretched, necking initiated at localized amorphous regions to eventually form single-atom chains which can sustain strains above 100%. Such large elongations are not observed in TiO(2) nanowires. Using the analogy of a clothesline, an explanation is offered for the necessary conditions leading to super ductility.
Article
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.
Article
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.
Article
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.
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