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Atomic Defects Influenced Mechanics of II-VI Nanocrystals
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 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.
... Among them, ZnO nanostructures has attracted continued interests due to their easy synthesis [15] and application in the field of energy generation [16,17]. In particular, reversible wurtzite-to-hexagonal and wurtzite-to-tetragonal transformations have been predicted by molecular dynamics simulations [18,19] and further confirmed by experiments [20,21] in ZnO nanostructures, in examples that show computations can be applied to guide expensive and time-consuming trial-and-error methods in laboratories [22]. ...
... 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. ...
We show that repeatable energy absorption can be obtained via the reversible wurtzite-to-hexagonal phase transformation of ZnO nanopillars at room temperature. The effect is demonstrated using molecular dynamics simulations and available experimental data. With uniaxial compressive strains up to 22.1% along the [0001] orientation, a ZnO nanopillar with a lateral dimension of 5.5 nm can produce average specific energy absorption on the order of 26.7 J g⁻¹ under quasistatic cyclic loading and 11.1 J g⁻¹ under rapid loading. The theoretical maximum of the specific energy absorption is 41.0 J g⁻¹ which can be approached at nanopillars with lateral sizes above 55 nm. These values are comparable to that of widely used aluminum foams. The effects of inversion domain boundaries and sample size on the repeatable energy absorbing capacity are discussed. The findings open an avenue for ZnO nanostructures in energy absorption and dissipation applications.
... 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. ...
... 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. ...
Nano-crystalline CdSe thin films of different thicknesses under sub-micron range were deposited on glass substrate via thermal evaporation route. A gradual deterioration in film crystallinity confirmed by XRD line profile analysis has been accompanied by a reduction in Cd to Se molar ratio as the film thickness decreases. A coordinated microstructural and crystallographic orientation distribution analysis explicitly demonstrated that CdSe tends to grow in nano-sized columns with hexagonal c-axis parallel to its growth direction on glass substrate. A thickness dependence of structural evolution was discussed in terms of aspect ratio of the columnar structure and dispersion in orientation of hexagonal (002) basal plane. The variation in the spectra of optical constants [n(λ), k(λ)] obtained from Swanepoel envelop method was interpreted in terms of crystallographic defects arising from stoichiometric disorder which was also accounted for the observed thickness dependent shifts in band gap and valence band split energy. The bathochromic shifts in dielectric and energy loss functions, optical conductivity, skin depth and cut-off energy were discussed in detail along with the variations in their spectral shapes in connection with the dispersion in the real and imaginary parts of complex refractive index, which might shed a new light upon holistic comprehension of thickness dependent optical properties of other chalcogenide semiconducting thin films.
... Efficient computational tools have been developed to perform simulations within timespans not accessible to experimental studies [22,23]. We may refer to large-scale continuum simulations , phase-field simulations for capturing the microstructure [48][49][50][51][52][53][54][55], MD simulations to capture the atomistic mechanisms [2,47,[56][57][58][59][60][61][62][63][64][65][66][67], and multiscale simulations to capture the broad spectrum of materials and processes response [24,25,46,[68][69][70][71][72][73]. Out of several computational methods, molecular dynamics simulation allows the capturing of materials evolution with atomistic accuracy, including the radiation damage mechanism. ...
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.
... It can also be combined with material databases such as Thermo-Calc ( Ref 19) to predict various phases that form during the printing process ( . The interface and defect energies can be determined using atomistic simulations (Ref [22][23][24][25][26][27][28][29][30] to determine the free parameters of the phase-field model (Ref [31][32][33][34]. ...
Aluminum alloys are among the top candidate materials for in-space manufacturing (ISM) due to their lightweight and relatively low melting temperature. A fundamental problem in printing metallic parts using available ISM methods, based on the fused deposition modeling (FDM) technique, is that the integrity of the final printed components is determined mainly by the adhesion between the initial particles. Engineering the surface melt can pave the way to improve the adhesion between the particles and manufacture components with higher mechanical integrity. Here, we developed a phase-field model of surface melting, where the surface energy can directly be implemented from the experimental measurements. The proposed model is adjusted to Al 7075-T6 alloy feedstocks, where the surface energy of these alloys is measured using the sessile drop method. Effect of mechanics has been included using transformation and thermal strains. The effect of elastic energy is compared here with the corresponding cases without mechanics. Two different geometric samples (cylindrical and spherical) are studied, and it is found that cylindrical particles form a more disordered structure upon size reduction compared to the spherical samples.
... Life on earth shows significant evidence of chaotic patterns [14][15][16][17][18][19][20] . One of the unresolved questions is if these complexities arise from the fundamental self-assembly of matters and the role of defects in them [21][22][23] . Here, an obvious question remains, are complex patterns in life-essential or just an accidental phenomenon? ...
Complex pattern formation is an essential characteristic of plants and their ageing, growth, and evolution. Perception towards these patterns is an intrinsic nature of plant-dependent animals for coexistence. Areca nut is considered to be addictive for humans and has increased adverse health effects. A large number of areca nuts has been studied since 2017 to develop a low-cost tool for the LMICs to categorise areca nuts. We present the first finding to identify similarities among complex networks of differently aged areca nuts. We investigated the internal patterns of differently aged areca nuts randomly chosen from the same age group. We developed a smartphone camera-based high-resolution measurement with comprehensive biophysical mathematics and a quantum mechanical concept called density of states (DOS). We found that the DOS can provide a unique coefficient to represent age and ageing together using a single number. The average of these single numbers for less aged nuts and highly aged nuts are 4.9 and 3.8, respectively. If a fruit looks aged from its external morphology as well as internal morphology, our method identifies the intrinsic similarities between two phenotypic expressions without implementing any computationally expensive search algorithm. We have also conducted further analyses of local DOS, Fourier decomposition, correlation study, spectral decomposition, and structural similarity index.
... [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. ...
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. ...
Mono- and few-layer graphene exhibit unique mechanical, thermal, and electrical properties. However, their hardness and in-plane stiffness are still not comparable to the other allotrope of carbon, i.e. diamond. This makes layered graphene structures to be less suitable for application in harsh environments. Thus, there is an unmet need for the synthesis of atomically thin diamond films for such applications that are also stable under ambient conditions. Here, we demonstrate the possibility for the synthesis of such diamond films from multilayer graphene using the molecular dynamics approach with reactive force fields. We study the kinetics and thermodynamics of the phase transformation as well as the stability of the formed diamond thin films as a function of the layer thickness at different pressures and temperatures for pristine and hydrogenated multilayer graphene. The results indicate that the transformation conditions depend on the number of graphene layers and surface chemistry. We revealed a reduction in the transformation strain by up to 50% while transformation stress has reduced by as much as five times upon passivation with hydrogen atoms. While the multilayer pristine graphene to diamond transformation is shown to be reversible, hydrogenated multilayer graphene structures had formed a metastable diamond film. Our simulations have further revealed temperature-independence of transformation strain, while transformation stresses are strong functions of temperature.
... 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]. ...
In this work, nano-crystalline CdSe thin films were deposited by thermal evaporation technique from CdSe nano-powders on glass substrates. The deposited films were characterized by using optical transmission measurements, and X-ray diffraction (XRD). X-ray line profile analysis revealed that the CdSe films are nano-crystalline and having a wurtzite (hexagonal) structure. These nano-crystalline CdSe films were preferentially oriented along the (002) plane with a c-axis perpendicular to the substrate surface. The crystallite size of these films varied from 16 to 36 nm as measured from X-ray line broadening. The variation of film thickness had a great influence on the microstructural parameters, such as micro-strain, stacking-fault probabilities, and dislocation densities. The obtained results demonstrate that the micro-strain, dislocation density, and stacking fault probabilities decrease by increasing the film thickness. The estimated refractive indices of the films fitted well with Sellmeier dispersion formula over a spectral range, 200 nm and 2500 nm. The effective resonance absorption wavelengths were between 610 and 723 nm. Optical parameters of nano-crystalline CdSe films were investigated extensively, and interpreted in the frame of Wemple-DiDomenico single oscillator model. These parameters provided valuable physical insights, such as the relationship between the ionicity of the CdSe films and microstructural parameters. The studied samples obviously had two distinct regions in Tauc plot, except for that sample CS04, due to the strong influence of strain and dislocations.
Oxide ceramics serve as crucial functional structural materials with vast applications across multiple domains of human production and everyday life. However, their inherent brittleness and heightened susceptibility to defects render...
Oxide crystal ceramics are commonly hard and brittle, when they are bent they typically fracture. Such mechanical response limits the use of these materials in emerging fields like wearable electronics. Here, a polymer‐induced assembly strategy is reported to construct orderly assembled TiO2 crystals into continuous nanofibers that are stretchable, bendable, and even knottable. Ball‐milling the spinning sol and curved‐drafting the electrospun nanofibers significantly improve the molecular structural order and reduce pore defects in the precursor nanofibers. Using this method, continuous TiO2 nanofibers, in which orderly assembled TiO2 nanocrystals (brick) are connected by twin grain boundaries or an amorphous region (mortar), are formed after sintering. Mechanical measurements and finite element analysis simulation indicate that the dislocation slip of “bricks” and the elastic deformation of “mortar” render the nanofibers with a small bending rigidity of ≈22 mN and a small elastic modulus of ≈20.8 Gpa, thus displaying properties associated with both soft and hard matter. More importantly, the reported approach can be easily extended to synthesize a wide range of soft, yet tough ceramic membranes, such as ZrO2 and SiO2. Flexible ceramic nanofibers are fabricated using a ball‐milling‐ and curved‐drafting‐assisted electrospinning technique. With this strategy, continuous TiO2 nanofibers, in which orderly assembled TiO2 nanocrystals are connected by twin or amorphous grain boundaries, are formed after sintering. The single nanofiber is knottable, while the nanofiber membrane can be folded without fracture, displaying properties associated with both soft and hard matter.
Until now, we could not engineer Nature's ability to dynamically handle diffusing single molecules at liquid-phase as it takes place in pore-forming proteins and tunnelling-nanotubes. Consistent handling of individual single molecules in a liquid will be of paramount importance to fundamental molecular studies and technological benefits, like single-molecule level separation and sorting for early biomedical diagnostics, microscopic studies of molecular interactions and electron/optical microscopy of molecules and nanomaterials. We can consistently resolve the dynamics of diffusing single molecules if they are confined within a uniform dielectric environment at nanometre length-scales. Uniform dielectric environment is the key characteristic since intrinsic electronic properties of molecules gets modified while interacting any surfaces, and the effect is not same from one dielectric surface to another. We present a dynamic nanofluidic detection of optically active single molecules in a liquid. An all-silica nanofluidic environment was used to electrokinetically handle individual single-molecule where molecular shot noise got resolved. We recorded the single-molecule motion of small fragments of DNA, carbon-nanodots, and organic fluorophores in water. The electrokinetic 1D molecular mass transport under two-focus fluorescence correlation spectroscopy (2fFCS) showed confinement-induced modified molecular interactions (due to various inter-molecular repulsive and attractive forces), which have been theoretically interpreted as molecular shot noise. Our demonstration of high-throughput nanochannel fabrication, 2fFCS-based 1D confined detection of fast-moving single molecules and fundamental understanding of molecular shot noise may open an avenue for single-molecule experiments where physical manipulation of dynamics is necessary.
One-dimensional semiconductor nanowires often contain polytypic structures, owing to the co-existence of different crystal phases. Therefore, understanding the properties of polytypic structures is of paramount importance for many promising applications in high-performance nanodevices. Herein, we synthesized nanowires of typical III-V semiconductors, namely, gallium phosphide and gallium arsenide by using the chemical vapor transport method. The growth directions ([111] and [211]) could be switched by changing the experimental conditions, such as H 2 gas flow; thus, various polytypic structures were produced simultaneously in a controlled manner. The nanobeam electron diffraction technique was employed to obtain strain mapping of the nanowires by visualizing the polytypic structures along the [111] direction. Micro-Raman spectra for individual nanowires were collected, confirming the presence of wurtzite phase in the polytypic nanowires. Further, we fabricated the photodetectors using the single nanowires, and the polytypic structures are shown to decrease the photosensitivity. Our systematic analysis provides important insight into the polytypic structures of nanowires.
Cesium lead halide (CsPbX3) nanocrystals have emerged as a new family of materials that can outperform the existing semiconductor nanocrystals due to their superb optical and charge transport properties. However, the lack of a robust method to produce quantum dots with controlled size and high ensemble uniformity has been one of the major obstacles in exploring the useful properties of excitons in 0-dimensional nanostructure of CsPbX3. Here, we report a new synthesis approach enabling the precise control of the size based on the equilibrium rather than kinetics, producing CsPbX3 quantum dots nearly free of heterogeneous broadening in their exciton luminescence. The high level of size control and ensemble uniformity achieved here will open the door to harnessing the benefits of excitons in CsPbX3 quantum dots for photonic and energy-harvesting applications.
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.
Understanding the effects of pressure-induced deformations on the optoelectronic properties of nanomaterials is important not only from the fundamental point of view but also for potential applications such as stress sensors and electromechanical devices. Here, we describe the novel insights into these piezochromic effects gained from using a linear-scaling density functional theory framework and an electronic enthalpy scheme, which allow us to accurately characterize the electronic structure of CdS nanocrystals with a zincblende-like core of experimentally relevant size. In particular, we focus on unravelling the complex interplay of size and surface (phenyl) ligands with pressure. We show that pressure-induced deformations are not simple isotropic scaling of the original structures and that the change in HOMO-LUMO gap with pressure results from two competing factors: (i) a bulk-like linear increase due to compression, which is offset by (ii) distortions and disorder and, to a lesser extent, orbital hybridization induced by ligands affecting the frontier orbitals. Moreover, we observe that the main peak in the optical absorption spectra is systematically red-shifted or blue-shifted, as pressure is increased up to 5 GPa, depending on the presence or absence of phenyl ligands. These heavily hybridize the frontier orbitals, causing a reduction in overlap and oscillator strength, so that at zero pressure, the lowest energy transition involves deeper hole orbitals than in the case of hydrogen-capped nanocrystals; the application of pressure induces greater delocalization over the whole nanocrystals bringing the frontier hole orbitals into play and resulting in an unexpected red shift for the phenyl-capped nanocrystals, in part caused by distortions. In response to a growing interest in relatively small nanocrystals that can be difficult to accurately characterize with experimental techniques, this work exemplifies the detailed understanding of structure-property relationships under pressure that can be obtained for realistic nanocrystals with state-of-the-art first-principles methods and used for the characterization and design of devices based on these and similar nanomaterials.
Quantifying and characterising atomic defects in nanocrystals is difficult and low-throughput using the existing methods such as high resolution transmission electron microscopy (HRTEM). In this article, using a defocused wide-field optical imaging technique, we demonstrate that a single ultrahigh-piezoelectric ZnO nanorod contains a single defect site. We model the observed dipole-emission patterns from optical imaging with a multi-dimensional dipole and find that the experimentally observed dipole pattern and model-calculated patterns are in excellent agreement. This agreement suggests the presence of vertically oriented degenerate-transition-dipoles in vertically aligned ZnO nanorods. The HRTEM of the ZnO nanorod shows the presence of a stacking fault, which generates a localised quantum well induced degenerate-transition-dipole. Finally, we elucidate that defocused wide-field imaging can be widely used to characterise defects in nanomaterials to answer many difficult questions concerning the performance of low-dimensional devices, such as in energy harvesting, advanced metal-oxide-semiconductor storage, and nanoelectromechanical and nanophotonic devices.
Ferroelectricity in ZnO is an unlikely physical phenomenon. Here, we show ferroelectricity in undoped [001] ZnO nanorods due to zinc vacancies. Generation of ferroelectricity in a ZnO nanorod effectively increases its piezoelectricity and turns the ZnO nanorod into an ultrahigh-piezoelectric material. Here using piezoelectric force microscopy (PFM), it is observed that increasing the frequency of the AC excitation electric field decreases the effective d33. Subsequently, the existence of a reversible permanent electric dipole is also found from the P–E hysteresis loop of the ZnO nanorods. Under a high resolution transmission electron microscope (HRTEM), we observe a zinc blende stacking in the wurtzite stacking of a single nanorod along the growth axis. The zinc blende nature of this defect is also supported by the X-ray diffraction (XRD) and Raman spectra. The presence of zinc vacancies in this basal stacking fault modulates p–d hybridization of the ZnO nanorod and produces a magnetic moment through the adjacent oxygen ions. This in turn induces a reversible electric dipole in the non-centrosymmetric nanostructure and is responsible for the ultrahigh-piezoelectric response in these undoped ZnO nanorods. We reveal that this defect engineered ZnO can be considered to be in the competitive class of ultrahigh-piezoelectric nanomaterials for energy harvesting and electromechanical device fabrication.
Tetragonal barium titanate nanoparticles (BTNPs) have been exploited as nanotransducers owing to their piezoelectric properties, in order to provide indirect electrical stimulation to SH-SY5Y neuron-like cells. Following application of ultrasounds to cells treated with BTNPs, fluorescence imaging of ion dynamics revealed that the synergic stimulation is able to elicit a significant cellular response in terms of calcium and sodium fluxes; moreover, tests with appropriate blockers demonstrated that voltage-gated membrane channels are activated. The hypothesis of piezoelectric stimulation of neuron-like cells was supported by lack of cellular response in the presence of cubic non-piezoelectric BTNPs, and further corroborated by a simple electro-elastic model of a BTNP subjected to ultrasounds, according to which the generated voltage is compatible with the values required for the activation of voltage-sensitive channels.
Basal-plane stacking faults are an important class of optically active
structural defects in wurtzite semiconductors. The local deviation from the 2H
stacking of the wurtzite matrix to a 3C zinc-blende stacking induces a bound
state in the gap of the host crystal, resulting in the localization of
excitons. Due to the two-dimensional nature of these planar defects, stacking
faults act as quantum wells, giving rise to radiative transitions of excitons
with characteristic energies. Luminescence spectroscopy is thus capable of
detecting even a single stacking fault in an otherwise perfect wurtzite
crystal. This review draws a comprehensive picture of the luminescence
properties related to stacking faults in GaN. The emission energies associated
with different types of stacking faults as well as factors that can shift these
energies are discussed. In this context, the importance of the quantum-confined
Stark effect in these zinc-blende/wurtzite heterostructures, which results from
the spontaneous polarization of wurtzite GaN, is underlined. This discussion is
extended to zinc-blende segments in a wurtzite matrix. Furthermore, other
factors affecting the emission energy and linewidth of stacking fault-related
peaks as well as results obtained at room temperature are addressed. The
considerations presented in this article should be transferable also to other
wurtzite semiconductors.
This review focuses on the growth, properties and novel applications of aligned arrays of ZnO nanowires (NWs) and nanobelts (NBs) for nanogenerators and nano-piezotronics. Owing to the semiconducting and piezoelectric dual properties of ZnO crystals, novel applications are introduced using aligned ZnO NWs, such as nanogenerators. These unique properties and applications will have profound impacts in many areas, such as self-powered nanodevices and nanosystems for in situ, real-time and implantable biosensing and biodetection, self-powering for defence and commercial applications, and remote sensing for space technology. The article provides detailed illustrations about the synthesis method, mechanical, electrical and optical properties, as well as the underlying mechanism for piezoelectronic devices and systems.
We demonstrate the feasibility of ab initio studies of piezoelectricity within an all-electron scheme. The focus of our analysis is on wurtzite ZnO; for comparison, some results are also presented for the related materials BeO and ZnS. The comparative study is performed in order to understand the microscopic origin of the peculiar behavior of ZnO, whose piezoelectric response is the strongest among the tetrahedrally bonded semiconductors. In all such materials, the piezeoelectric effect results from two different terms of opposite sign: these are usually referred to as the ‘‘clamped-ion’’ and the ‘‘internal-strain’’ contributions. Cancellation among them is least effective in ZnO, where the dominant effect is due to a rigid-ion-like mechanism. Furthermore, we compute the spontaneous polarization of ZnO and we discuss the puzzling agreement between our calculated value and a very indirect experimental estimate of the same quantity.
In this paper, we present experimental results describing enhanced readout of the vibratory response of a doubly clamped zinc oxide (ZnO) nanowire employing a purely electrical actuation and detection scheme. The measured response suggests that the piezoelectric and semiconducting properties of ZnO effectively enhance the motional current for electromechanical transduction. For a doubly clamped ZnO nanowire resonator with radius ~10 nm and length ~1.91 µm, a resonant frequency around 21.4 MHz is observed with a quality factor (Q) of ~358 in vacuum. A comparison with the Q obtained in air (~242) shows that these nano-scale devices may be operated in fluid as viscous damping is less significant at these length scales. Additionally, the suspended nanowire bridges show field effect transistor (FET) characteristics when the underlying silicon substrate is used as a gate electrode or using a lithographically patterned in-plane gate electrode. Moreover, the Young's modulus of ZnO nanowires is extracted from a static bending test performed on a nanowire cantilever using an AFM and the value is compared to that obtained from resonant frequency measurements of electrically addressed clamped–clamped beam nanowire resonators.
ZnO nanostructures, due to their novel properties, are promising components in a wide range of nanoscale devices for future applications. This article provides a comprehensive review of the current research activities that focus on the synthesis, characterization and applications of ZnO based nanostructures. We briefly describe the most commonly applied methodologies for the synthesis of ZnO nanostructures. A range of remarkable characteristics is then presented, organized into sections describing the optical, electrical and mechanical properties. Finally, we include a brief analysis of the possible future trends for the application of this interesting semiconductor oxide for hydrogen storage and biosensors. These studies constitute the basis for developing versatile applications of ZnO nanostructures.
The size dependence of the piezoelectricity in ZnO nanofilms is investigated using ab initio density-functional theory calculations. The effective piezoelectric constant of ZnO nanofilms increases monotonically with increasing film thickness in the nanoscale simulated in the present work, and surprisingly, exceeds that of the bulk ZnO when the thickness is greater than 2.4 nm. The enhancement over the bulk value reaches 11% when the film thickness is 2.9 nm. (c) 2007 American Institute of Physics.
We measure the elastic modulus of a single horizontal ZnO nanorod [NR] grown by a low-temperature hydrothermal chemical process on silicon substrates by performing room-temperature, direct load-controlled nanoindentation measurements. The configuration of the experiment for the single ZnO NR was achieved using a focused ion beam/scanning electron microscope dual-beam instrument. The single ZnO NR was positioned horizontally over a hole on a silicon wafer using a nanomanipulator, and both ends were bonded with platinum, defining a three-point bending configuration. The elastic modulus of the ZnO NR, extracted from the unloading curve using the well-known Oliver-Pharr method, resulted in a value of approximately 800 GPa. Also, we discuss the NR creep mechanism observed under indentation. The mechanical behavior reported in this paper will be a useful reference for the design and applications of future nanodevices.
Semiconductor nanowires are unique as functional building blocks in nanoscale electrical and electromechanical devices. Here, we report on the mechanical properties of ZnO nanowires that range in diameter from 18 to 304 nm. We demonstrate that in contrast to recent reports, Young's modulus is essentially independent of diameter and close to the bulk value, whereas the ultimate strength increases for small diameter wires, and exhibits values up to 40 times that of bulk. The mechanical behavior of ZnO nanowires is well described by a mechanical model of bending and tensile stretching.
Extended and oriented nanostructures are desirable for many applications, but direct fabrication of complex nanostructures with controlled crystalline morphology, orientation and surface architectures remains a significant challenge. Here we report a low-temperature, environmentally benign, solution-based approach for the preparation of complex and oriented ZnO nanostructures, and the systematic modification of their crystal morphology. Using controlled seeded growth and citrate anions that selectively adsorb on ZnO basal planes as the structure-directing agent, we prepared large arrays of oriented ZnO nanorods with controlled aspect ratios, complex film morphologies made of oriented nanocolumns and nanoplates (remarkably similar to biomineral structures in red abalone shells) and complex bilayers showing in situ column-to-rod morphological transitions. The advantages of some of these ZnO structures for photocatalytic decompositions of volatile organic compounds were demonstrated. The novel ZnO nanostructures are expected to have great potential for sensing, catalysis, optical emission, piezoelectric transduction, and actuations.
We have converted nanoscale mechanical energy into electrical energy by means of piezoelectric zinc oxide nanowire (NW) arrays. The aligned NWs are deflected with a conductive atomic force microscope tip in contact mode. The coupling of piezoelectric and semiconducting properties in zinc oxide creates a strain field and charge separation across the NW as a result of its bending. The rectifying characteristic of the Schottky barrier formed between the metal tip and the NW leads to electrical current generation. The efficiency of the NW-based piezoelectric power generator is estimated to be 17 to 30%. This approach has the potential of converting mechanical, vibrational, and/or hydraulic energy into electricity for powering nanodevices.
Reducing the dimensions of materials to atomic scales results in a large portion of atoms being at or near the surface, with lower bond order and thus higher energy. At such scales, reduction of the surface energy and surface stresses can be the driving force for the formation of new low-dimensional nanostructures, and may be exhibited through surface relaxation and/or surface reconstruction, which can be utilized for tailoring the properties and phase transformation of nanomaterials without applying any external load. Here we used atomistic simulations and revealed an intrinsic structural transformation in monolayer materials that lowers their dimension from 2D nanosheets to 1D nanostructures to reduce their surface and elastic energies. Experimental evidence of such transformation has also been revealed for one of the predicted nanostructures. Such transformation plays an important role in bi-/multi-layer 2D materials.
Flexible nanogenerators that efficiently convert mechanical energy into electrical energy have been extensively studied because of their great potential for driving low-power personal electronics and self-powered sensors. Integration of flexibility and stretchability to nanogenerator has important research significance that enables applications in flexible/stretchable electronics, organic optoelectronics, and wearable electronics. Progress in nanogenerators for mechanical energy harvesting is reviewed, mainly including two key technologies: flexible piezoelectric nanogenerators (PENGs) and flexible triboelectric nanogenerators (TENGs). By means of material classification, various approaches of PENGs based on ZnO nanowires, lead zirconate titanate (PZT), poly(vinylidene fluoride) (PVDF), 2D materials, and composite materials are introduced. For flexible TENG, its structural designs and factors determining its output performance are discussed, as well as its integration, fabrication and applications. The latest representative achievements regarding the hybrid nanogenerator are also summarized. Finally, some perspectives and challenges in this field are discussed.
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.
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.
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.
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%.
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.
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.
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.
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.
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,
, 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.
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.
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.
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.
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.
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.
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].
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.
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.
In this investigation, the size-scale in mechanical properties of individual [0001] ZnO nanowires and the correlation with atomic-scale arrangements were explored via in situ high-resolution transmission electron microscopy (TEM) equipped with atomic force microscopy (AFM) and nanoindentation (NI) systems. The Young's modulus was determined to be size-scale-dependent for nanowires with diameter, d, in the range of 40 nm ≤ d ≤ 110 nm, and reached the maximum of ∼ 249 GPa for d = 40 nm. However, this phenomenon was not observed for nanowires in the range of 200 nm ≤ d ≤ 400 nm, where an average constant Young's modulus of ∼ 147.3 GPa was detected, close to the modulus value of bulk ZnO. A size-scale dependence in the failure of nanowires was also observed. The thick ZnO nanowires (d ≥ 200 nm) were brittle, while the thin nanowires (d ≤ 110 nm) were highly flexible. The diameter effect and enhanced Young's modulus observed in thin ZnO nanowires are due to the combined effects of surface relaxation and long-range interactions present in ionic crystals, which leads to much stiffer surfaces than bulk wires. The brittle failure in thicker ZnO wires was initiated from the outermost layer, where the maximum tensile stress operates and propagates along the (0001) planes. After a number of loading and unloading cycles, the highly compressed region of the thinner nanowires was transformed from a crystalline to an amorphous phase, and the region near the neutral zone was converted into a mixture of disordered atomic planes and bent lattice fringes as revealed by high-resolution images.
Molecular dynamics 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.
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.
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 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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