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Structural transformation in monolayer materials: A 2D to 1D transformation
Abstract
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.
... It can be pointed out that nanostructure materials with low-dimensional, for instance, 1D or 2D, are more competent to withstand volume change during the charge-discharge process (Wang et al., 2010a(Wang et al., , 2010b. Momeni et al. (2016aMomeni et al. ( , 2016b used the atomistic simulation method to reveal the intrinsic structural change of SnO 2 that brings down dimensions from 2D nanosheets to 1D nanorods of monolayer materials (Momeni et al., 2016a(Momeni et al., , 2016b. It is attractive for having a higher surface-area-to-volume ratio and supplies adequate absorption molecules aimed at various nanostructures (1D, 2D, and 3D) involved in a small space (Jung et al., 2008;Chang et al., 2020). ...
... It can be pointed out that nanostructure materials with low-dimensional, for instance, 1D or 2D, are more competent to withstand volume change during the charge-discharge process (Wang et al., 2010a(Wang et al., , 2010b. Momeni et al. (2016aMomeni et al. ( , 2016b used the atomistic simulation method to reveal the intrinsic structural change of SnO 2 that brings down dimensions from 2D nanosheets to 1D nanorods of monolayer materials (Momeni et al., 2016a(Momeni et al., , 2016b. It is attractive for having a higher surface-area-to-volume ratio and supplies adequate absorption molecules aimed at various nanostructures (1D, 2D, and 3D) involved in a small space (Jung et al., 2008;Chang et al., 2020). ...
... Besides, various creative findings have been reported in the literature on nanocomposites of SnO 2 that enhance remarkable visible-light-induced photodegradation of organic contaminants from wastewater (Mohanta et al., 2018;Mohanta and Ahmaruzzaman, 2021c, 2021b, 2021a. Likewise, heterojunction nanocomposites (Cu/ZnO, Pd/ZnO, etc.) demonstrated better results (⁓100% dye degraded) (Momeni et al., 2016a(Momeni et al., , 2016b than the rest reported in Table 4. The reason behind the higher degradation rate of dye molecules with reduced size of SnO 2 NPs has been reported that molecules with higher molar absorptivity are more stable and establish surface plasmon resonance (Heger et al., 2005). ...
Tin oxide (SnO2) with versatile properties is of substantial standing for practical application, and improved features of the material are demonstrated in the current issue through the integration of nanotechnology with bio-resources leading to what is termed as biosynthesis of SnO2 nanoparticles (NPs). This review reveals the recent advances in biosynthesis of SnO2 NPs by chemical precipitation method focused on distinct methodologies, characterization, and reaction mechanism along with a photocatalytic application for dye degradation. According to available literature reviews, numerous bio-based precursors selectively extracted from biological substrates have effectively been applied as capping or reducing agents to achieve the metal oxide NPs. The major precursor obtained from the aqueous extract of root barks of Catunaregam spinosa is found to be 7-hydroxy-6-methoxy-2H-chromen-2-one that has been proposed as a model compound for the reduction of metal ions into nanoparticles due to having highly active functional groups, being abundant in plants (67.475 wt%), easy to extract, and eco benign. In addition, the photocatalytic activity of SnO2 NPs for the degradation of organic dyes, pharmaceuticals, and agricultural contaminants has been discussed in the context of a promising bio-reduction mechanism of the synthesis. The final properties are supposed to depend exclusively upon a number of factors, e.g., particle size (< 50 nm), bandgap (< 3.6 eV), crystal defects, and catalysts dosage. With this contribution, it has been perceived not only to provide an overview of recent advances in the biosynthesis of SnO2 NPs but also to indicate the main issues in need aiming to show vision towards innovative outcomes.
... It should also be noticed that, although the strain effect on borophene has been investigated in detail [35,79], surface tension may also induce instability and even transform a monolayer to a nanotube without applying any external load [80]. In fact, surface tension can be viewed as the driving force for the phase transformation [81], and has two profound effects during the transformation: (i) it induces a large internal stress that triggers the transformation and (ii) maintains the integrity of the surface during the transformation [80]. ...
... It should also be noticed that, although the strain effect on borophene has been investigated in detail [35,79], surface tension may also induce instability and even transform a monolayer to a nanotube without applying any external load [80]. In fact, surface tension can be viewed as the driving force for the phase transformation [81], and has two profound effects during the transformation: (i) it induces a large internal stress that triggers the transformation and (ii) maintains the integrity of the surface during the transformation [80]. For covalently bonded materials with directional bonds, surface tension usually leads to the generation of dangling bonds and even surface reconfiguration [82]. ...
A new 2D material, borophene, has been grown successfully recently on single crystal Ag substrates. Three main structures have been proposed (, and striped borophene). However, the stability of three structures is still in debate. Using first principles calculations, we examine the dynamical, thermodynamical and mechanical stability of , and striped borophene. Free-standing and borophene is dynamically, thermodynamically and mechanically stable, while striped borophene is dynamically and thermodynamically unstable due to high stiffness along a direction. The origin of high stiffness and high instability in striped borophene along a direction can both be attributed to strong directional bonding. Our work shows a deep connection between stability and strength, and helps researchers to estimate accurately the mechanical performance of 2D materials.
... In the case of graphene structures with finite size of graphene platelets, the presence of edges may result in deviation from the concerted transformation kinetics by promoting initiation of transformation from the edges. [66,67] Atoms located on the edges may go under bond relaxation and reconstruction to reduce their excess energy, or they may bond with other edge carbon atoms, creating structures like half nanotubes. Such phase transitions may promote or suppress the formation of diamane nuclei at the edges. ...
... 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.
... However, they are generally focused on one aspect of the growth and do not capture the complete picture of the synthesis process. We may refer to classical Wulff construction 36 , atomistic simulations [37][38][39][40][41] , and diffuse interface methods [42][43][44][45][46][47][48][49] as examples of these efforts. However, so far, precise control of the uniformity and coverage of as-grown 2D materials is considered a formidable challenge due to intrinsic complexities associated with the growth involving multiple physics across a wide range of length and time scales. ...
Reproducible wafer-scale growth of two-dimensional (2D) materials using the Chemical Vapor Deposition (CVD) process with precise control over their properties is challenging due to a lack of understanding of the growth mechanisms spanning over several length scales and sensitivity of the synthesis to subtle changes in growth conditions. A multiscale computational framework coupling Computational Fluid Dynamics (CFD), Phase-Field (PF), and reactive Molecular Dynamics (MD) was developed – called the CPM model – and experimentally verified. Correlation between theoretical predictions and thorough experimental measurements for a Metal-Organic CVD (MOCVD)-grown WSe2 model material revealed the full power of this computational approach. Large-area uniform 2D materials are synthesized via MOCVD, guided by computational analyses. The developed computational framework provides the foundation for guiding the synthesis of wafer-scale 2D materials with precise control over the coverage, morphology, and properties, a critical capability for fabricating electronic, optoelectronic, and quantum computing devices.
... 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.
... [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.
... Multiple vacancies may exist in 2D materials such as double vacancies in graphene, resulting in (i) two pentagons and one octagon-V 2 (5-8-5) defect; or (ii) three pentagons and three heptagons-V 2 (555-777) defect; or (iii) four pentagons, a hexagon, and four heptagons-V 2 (5555-6-7777) defect 56 . The formation of defects with an even number of vacancies in graphene is energetically favorable due to the lack of any dangling bonds 57 , whereas a large number of vacancies may bend and wrap the 2D material 58 . Table 2 lists the typical values of formation and migration energies of these point defects from atomistic calculations. ...
The successful discovery and isolation of graphene in 2004, and the subsequent synthesis of layered semiconductors and heterostructures beyond graphene have led to the exploding field of two-dimensional (2D) materials that explore their growth, new atomic-scale physics, and potential device applications. This review aims to provide an overview of theoretical, computational, and machine learning methods and tools at multiple length and time scales, and discuss how they can be utilized to assist/guide the design and synthesis of 2D materials beyond graphene. We focus on three methods at different length and time scales as follows: (i) nanoscale atomistic simulations including density functional theory (DFT) calculations and molecular dynamics simulations employing empirical and reactive interatomic potentials; (ii) mesoscale methods such as phase-field method; and (iii) macroscale continuum approaches by coupling thermal and chemical transport equations. We discuss how machine learning can be combined with computation and experiments to understand the correlations between structures and properties of 2D materials, and to guide the discovery of new 2D materials. We will also provide an outlook for the applications of computational approaches to 2D materials synthesis and growth in general.
... This can be explained by the dominant surface tension effect in the 2L and 3L graphene systems. The surface stress, σ ij sr , in solid interfaces can be calculated using the Shuttleworth equation, 25 i.e., ...
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.
... The potential constants [26] are listed in Table 1. This interatomic potential is chosen because of its capability for reproducing the surface energy and surface stresses that are in agreement with experimental measurements [27][28][29]. The Wolf summation was used to take into account longrange electrostatic interactions [30], and a damping coefficient of 0.75 nm was selected to facilitate the convergence [31]. ...
... Kasra, et al. [14] used atomistic simulations and revealed an intrinsic structural transformation in monolayer materials that lowers their dimension from 2D nanosheets to 1D nanostructures Similarly, the chemical methods are used to synthesized NPs by electrodeposition, sol-gel process, chemical solution deposition, chemical vapour deposition, soft chemical method, Langmuir Blodgett method, catalytic route, hydrolysis, co-precipitation method and wet chemical method. Apart from that, the biological methods are using eco-friendly resources such as plant extracts, bacteria, and fungi, micro algae such as cyanobacteria, diatom, seaweed (macroalgae) and enzymes. ...
Biomolecules of live plants, plant extracts and microorganisms such as bacteria, fungi, seaweeds, actinomycetes, algae and microalgae can be used to reduce metal
ions to nanoparticles. Biosynthesized nanoparticle effectively controlled oxidative stress, genotoxicity and apoptosis related changes. Green biosynthesized NPs
is alternative methods, which is hydrophilic, biocompatible, non-toxic, and used for coating many metal NPs with interesting morphologies and varied sizes. The
reducing agents involved include various water-soluble plant metabolites (e.g. alkaloids, phenolic compounds, terpenoids, flavonoids, saponins, steroids, tannins and
other nutritional compounds) and co-enzymes. The polysaccharides, proteins and lipids present in the algal membranes act as capping agents and thus limit using
of non-biodegradable commercial surfactants. Metallic NPs viz. cobalt, copper, silver, gold, platinum, zirconium, palladium, iron, cadmium and metal oxides such as
titanium oxide, zinc oxide, magnetite, etc. have been the particular focus of biosynthesis. Bio-reduction mechanisms, characterization, commercial, pharmacological
and biomedical applications of biosynthesized nanoparticles are reviewed.
... Kasra, et al. [14] used atomistic simulations and revealed an intrinsic structural transformation in monolayer materials that lowers their dimension from 2D nanosheets to 1D nanostructures Similarly, the chemical methods are used to synthesized NPs by electrodeposition, sol-gel process, chemical solution deposition, chemical vapour deposition, soft chemical method, Langmuir Blodgett method, catalytic route, hydrolysis, co-precipitation method and wet chemical method. Apart from that, the biological methods are using eco-friendly resources such as plant extracts, bacteria, and fungi, micro algae such as cyanobacteria, diatom, seaweed (macroalgae) and enzymes. ...
Biomolecules of live plants, plant extracts and microorganisms such as bacteria, fungi, seaweeds, actinomycetes, algae and microalgae can be used to reduce metal
ions to nanoparticles. Biosynthesized nanoparticle effectively controlled oxidative stress, genotoxicity and apoptosis related changes. Green biosynthesized NPs
is alternative methods, which is hydrophilic, biocompatible, non-toxic, and used for coating many metal NPs with interesting morphologies and varied sizes. The
reducing agents involved include various water-soluble plant metabolites (e.g. alkaloids, phenolic compounds, terpenoids, flavonoids, saponins, steroids, tannins and
other nutritional compounds) and co-enzymes. The polysaccharides, proteins and lipids present in the algal membranes act as capping agents and thus limit using
of non-biodegradable commercial surfactants. Metallic NPs viz. cobalt, copper, silver, gold, platinum, zirconium, palladium, iron, cadmium and metal oxides such as
titanium oxide, zinc oxide, magnetite, etc. have been the particular focus of biosynthesis. Bio-reduction mechanisms, characterization, commercial, pharmacological
and biomedical applications of biosynthesized nanoparticles are reviewed.
... Their Young's modulus, Y, was obtained from the analysis of the stress versus strain graphs, which can be visualized in Figures 1b and 1c. We like to stress that unlike most materials, our phagraphene structures do not exhibit a linear regime at very low strain values, thus the Y values were obtained using the linear region [11] of the curves presented in Figure 1. For graphene at 300K (900 K) we obtained 1131 (1220) and 1317 (1250) GPa along x and y directions, respectively. ...
Recently, a new 2D carbon allotrope structure, named phagraphene (PG), was proposed. PG has a densely array of penta-hexa-hepta-graphene carbon rings. PG was shown to present low and anisotropic thermal conductivity and it is believed that this anisotropy should be also reflected in its mechanical properties. Although PG mechanical properties have been investigated, a detailed and comprehensive study is still lacking. In the present work we have carried out fully atomistic reactive molecular dynamics simulations using the ReaxFF force field, to investigate the mechanical properties and fracture patterns of PG membranes. The Young’s modulus values of the PG membranes were estimated from the stress-strain curves. Our results show that these curves present three distinct regimes: one regime where ripples dominate the structure and mechanical properties of the PG membranes; an elastic regime where the membranes exhibit fully planar configurations; and finally am inelastic regime where permanent deformations happened to the PG membrane up to the mechanical failure or fracture.
... Most of these applications are based on low-dimensional systems [27]. Nowadays it is well-known that, wurtzite (WZ) structures of group-III nitride by few number of (001)-layers transform into a new form of stable hexagonal graphene-like sheet structure [28,29]. Other types of low-dimensional systems are single-walled nanotubes and nanoribbons. ...
We have developed a tight-binding Hamiltonian model to study the band structure of group-III nitride nanostructures, AlN, GaN and InN in the form of monolayer sheets, nanotubes and nanoribbons . We have considered the semi-empirical tight-binding model taking into account the nearest neighbors. Based on our numerical results we obtained that d-orbitals are important for correct calculation of group-III nitride band structures. Also we have observed that the AlN, GaN and InN graphene-like sheets are semiconductors with indirect band gaps of 5.28 eV, 2.62 eV and 1.66 eV, respectively. We have seen that, our tight binding band gaps are larger than the corresponding band gaps obtained by DFT calculation. Also, we have investigated the effects of chirality of nanotubes and nanoribbons on the nitrides band structures and their gaps. Then, we have studied the influence of nanotube diameter and width of nanoribbon on the band structure. We observed that nanotubes and nanoribbons as the corresponding unzipped nanotubes have the same band structure. Moreover, we understand that the electronic properties are almost insensitive to diameter for nanotubes but width of nanoribbons is effective on the band structure.
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.
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.
The effect of elastic energy on nucleation and disappearance of a nanometer size intermediate melt (IM) region at a solid-solid (S1S2) phase interface at temperatures 120 K below the melting temperature is studied using a phase-field approach. Results are obtained for broad range of the ratios of S1S2 to solid-melt interface energies, kE, and widths, kδ. It is found that internal stresses only slightly promote barrierless IM nucleation but qualitatively alter the system behavior, allowing for the appearance of the IM when kE < 2 (thermodynamically impossible without mechanics) and elimination of what we termed the IM-free gap. Remarkably, when mechanics is included within this framework, there is a drastic (16 times for HMX energetic crystals) reduction in the activation energy of IM critical nucleus. After this inclusion, a kinetic nucleation criterion is met, and thermally activated melting occurs under conditions consistent with experiments for HMX, elucidating what had been to date mysterious behavior. Similar effects are expected to occur for other material systems where S1S2 phase transformations via IM take place, including electronic, geological, pharmaceutical, ferroelectric, colloidal, and superhard materials.
The microscopic kinetics of ubiquitous solid-solid phase transitions remain poorly understood. Here, by using single-particle-resolution video microscopy of colloidal films of diameter-tunable microspheres, we show that transitions between square and triangular lattices occur via a two-step diffusive nucleation pathway involving liquid nuclei. The nucleation pathway is favoured over the direct one-step nucleation because the energy of the solid/liquid interface is lower than that between solid phases. We also observed that nucleation precursors are particle-swapping loops rather than newly generated structural defects, and that coherent and incoherent facets of the evolving nuclei exhibit different energies and growth rates that can markedly alter the nucleation kinetics. Our findings suggest that an intermediate liquid should exist in the nucleation processes of solid-solid transitions of most metals and alloys, and provide guidance for better control of the kinetics of the transition and for future refinements of solid-solid transition theory.
Solid–solid (SS)(SS) phase transformations via nanometer-size intermediate melts (IMs)(IMs) within the SS interface, hundreds of degrees below melting temperature, were predicted thermodynamically and are consistent with experiments for various materials. A necessary condition for the appearance of IMs, using a sharp interface approach, was that the ratio of the energies of SS and solid–melt (SM)(SM) interfaces, kEkE, were >2. Here, an advanced phase-field approach coupled with mechanics is developed that reveals various new scale and interaction effects and phenomena. Various types of IM are found: (i) continuous and reversible premelting and melting; (ii) jump-like barrierless transformation to IMs, which can be kept at much lower temperature even for kE<2kE<2; (iii) unstable IMs, i.e. a critical nucleus between the SS interface and the IM. A surprising scale effect related to the ratio of widths of SS and SM interfaces is found: it suppresses barrierless IMs but allows IMs to be kept at much lower temperatures even for kE<2kE<2. Relaxation of elastic stresses strongly promotes IMs, which can appear even at kE<2kE<2 and be retained at kE=1kE=1. The theory developed here can be tailored for diffusive phase transformations, formation of intergranular and interfacial phases, and surface-induced phase transformations.
Understanding and measuring the size-dependent surface strain of nanowires are essential to their applications in various emerging devices. Here, we report on the diameter-dependent surface strain and Young's modulus of single-crystalline Co nanowires investigated by in situ X-ray diffraction measurements. Diameter-dependent initial longitudinal elongation of the nanowires is observed and ascribed to the anisotropic surface stress due to the Poisson effect, which serves as the basis for mechanical measurements. As the nanowire diameter decreases, a transition from the "smaller is softer" regime to the "smaller is tougher" regime is observed in the Young's modulus of the nanowires, which is attributed to the competition between the elongation softening and the surface stiffening effects. Our work demonstrates a new nondestructive method capable of measuring the initial surface strain and estimating the Young's modulus of single crystalline nanowires, and provides new insights on the size effect.
First principles calculations are carried to study the structural stability of SiC, GaN, InN, ZnO, and CdSe which are found to transform from a fourfold coordinated wurtzite (WZ) structure under ambient conditions to two different crystalline structures under loading of different triaxialities. Under hydrostatic compression, transformation into a sixfold coordinated rocksalt (RS) structure occurs, and under uniaxial compression along the [0001] direction and uniaxial tension along the [011̅ 0] crystalline direction (except SiC and GaN), transformation into a fivefold coordinated unbuckled wurtzite phase (HX) is observed. The lack of the WZ→HX transformation for SiC and GaN under uniaxial tension along the [011̅ 0] direction is because for these two materials the tensile stress required for the enthalpy of HX to become lower than the enthalpy of WZ is higher than their corresponding ultimate tensile strength. Critical stress levels for the transformations are found to depend on the formation energies of the WZ, HX, and RS structures which in turn are related to the ionicity of each material. The transformations are a manifestation of the tension-compression response asymmetry of these materials.
The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by
Bunn [Proc. Phys. Soc. London 47, 836 (1935)
], studies of its vibrational properties with Raman scattering in 1966 by
Damen et al. [Phys. Rev. 142, 570 (1966)
], detailed optical studies in 1954 by
Mollwo [Z. Angew. Phys. 6, 257 (1954)
], and its growth by chemical-vapor transport in 1970 by
Galli and Coker [Appl. Phys. Lett. 16, 439 (1970)
]. In terms of devices, Au Schottky barriers in 1965 by
Mead [Phys. Lett. 18, 218 (1965)
], demonstration of light-emitting diodes (1967) by
Drapak [Semiconductors 2, 624 (1968)
], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by
Minami et al. [Jpn. J. Appl. Phys. 13, 1475 (1974)
], ZnO/ZnSe n-p junctions (1975) by
Tsurkan et al. [Semiconductors 6, 1183 (1975)
], and Al/Au Ohmic contacts by
Brillson [J. Vac. Sci. Technol. 15, 1378 (1978)
] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by
Look and Claflin [Phys. Status Solidi B 241, 624 (2004)
]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures.
Arrays of regularly distributed CoO nano-octahedra are obtained by annealing Co nanowires at high temperatures. Both the size and the separation distance of the nano-octahedra can be controlled by tuning the annealing temperature. These self-assembled linear arrays of CoO nanocrystals result from the synergetic combination of the morphological transformation due to the intrinsic Rayleigh instability and the phase transformation due to the cobalt oxidation. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3518470]
The Silences of the Archives, the Reknown of the Story.
The Martin Guerre affair has been told many times since Jean de Coras and Guillaume Lesueur published their stories in 1561. It is in many ways a perfect intrigue with uncanny resemblance, persuasive deception and a surprizing end when the two Martin stood face to face, memory to memory, before captivated judges and a guilty feeling Bertrande de Rols. The historian wanted to go beyond the known story in order to discover the world of the heroes. This research led to disappointments and surprizes as documents were discovered concerning the environment of Artigat’s inhabitants and bearing directly on the main characters thanks to notarial contracts. Along the way, study of the works of Coras and Lesueur took a new direction. Coming back to the affair a quarter century later did not result in finding new documents (some are perhaps still buried in Spanish archives), but by going back over her tracks, the historian could only be struck by the silences of the archives that refuse to reveal their secrets and, at the same time, by the possible openings they suggest, by the intuition that almost invisible threads link here and there characters and events.
Many nanostructures experience structural transformation, either from one type of crystalline structure to another or from one orientation to another orientation within the same crystalline structure. Using a combination of ab initio calculations and analytical formulation, we show that such structural transformation is a direct result of surface reconstruction. In particular, our ab initio results show that infinite large ZnO nanoplates and infinite long ZnO nanowires transform from wurtzite to graphitic structure below a critical thickness or diameter. Our analytical formulation further shows that nanowires of finite length can go through the same structural transformation at larger critical diameters.
The arrays of piezoelectric, semiconducting ZnO nanowires (NW) grown on flexible plastic substrates can be used to convert mechanical energy into electrical energy using a conductive atomic force microscope. The estimated piezoelectric induced electrical power density of the NW array upon deflection of an AFM tip was on the order of milliwatts per centimeter squared, which is large enough to power a variety of MEMS, NEMS, and other nanoscale devices. The piezoelectric power generators that use ZnO NW arrays on flexible plastic substrates may be able to harvest energy from their environment for powering nanodevices and nanosystems. This flexible power source can find potential applications in implantable biosensors and biodetection, wireless self-powered sensors, and self-powering electronic devices.
Natural surfaces are often structured with nanometre-scale domains, yet a framework providing a quantitative understanding of how nanostructure affects interfacial energy, gamma(SL), is lacking. Conventional continuum thermodynamics treats gamma(SL) solely as a function of average composition, ignoring structure. Here we show that, when a surface has domains commensurate in size with solvent molecules, gamma(SL) is determined not only by its average composition but also by a structural component that causes gamma(SL) to deviate from the continuum prediction by a substantial amount, as much as 20% in our system. By contrasting surfaces coated with either molecular- (<2 nm) or larger-scale domains (>5 nm), we find that whereas the latter surfaces have the expected linear dependence of gamma(SL) on surface composition, the former show a markedly different non-monotonic trend. Molecular dynamics simulations show how the organization of the solvent molecules at the interface is controlled by the nanostructured surface, which in turn appreciably modifies gamma(SL).
We demonstrate the feasibility of using classical atomistic simulations, i.e. molecular dynamics and molecular statics, to study the piezoelectric properties of ZnO using core-shell interatomic potentials. We accomplish this by reporting the piezoelectric constants for ZnO as calculated using two different classical interatomic core-shell potentials: that originally proposed by Binks and Grimes (1994 Solid State Commun. 89 921-4), and that proposed by Nyberg et al (1996 J. Phys. Chem. 100 9054-63). We demonstrate that the classical core-shell potentials are able to qualitatively reproduce the piezoelectric constants as compared to benchmark ab initio calculations. We further demonstrate that while the presence of the shell is required to capture the electron polarization effects that control the clamped ion part of the piezoelectric constant, the major shortcoming of the classical potentials is a significant underprediction of the clamped ion term as compared to previous ab initio results. However, the present results suggest that overall, these classical core-shell potentials are sufficiently accurate to be utilized for large scale atomistic simulations of the piezoelectric response of ZnO nanostructures.
The harvesting of mechanical energy from ambient sources could power electrical devices without the need for batteries. However, although the efficiency and durability of harvesting materials such as piezoelectric nanowires have steadily improved, the voltage and power produced by a single nanowire are insufficient for real devices. The integration of large numbers of nanowire energy harvesters into a single power source is therefore necessary, requiring alignment of the nanowires as well as synchronization of their charging and discharging processes. Here, we demonstrate the vertical and lateral integration of ZnO nanowires into arrays that are capable of producing sufficient power to operate real devices. A lateral integration of 700 rows of ZnO nanowires produces a peak voltage of 1.26 V at a low strain of 0.19%, which is potentially sufficient to recharge an AA battery. In a separate device, a vertical integration of three layers of ZnO nanowire arrays produces a peak power density of 2.7 mW cm(-3). We use the vertically integrated nanogenerator to power a nanowire pH sensor and a nanowire UV sensor, thus demonstrating a self-powered system composed entirely of nanowires.
The structures of the polar surfaces of ZnO are studied using ab initio calculations and surface x-ray diffraction. The experimental and theoretical relaxations are in good agreement. The polar surfaces are shown to be very stable; the cleavage energy for the (0001)-Zn and (0001;)-O surfaces is 4.0 J/m(2) comparable to 2.32 J/m(2) for the most stable nonpolar (1010) surface. The surfaces are stabilized by an electronic mechanism involving the transfer of 0.17 electrons between them. This leads to 2D metallic surface states, which has implications for the use of the material in gas sensing and catalytic applications.
A novel nonpolar structure of 2 monolayer (ML) thick ZnO(0001) films grown on Ag(111) has been revealed, using surface x-ray diffraction and scanning tunneling microscopy. Zn and O atoms are arranged in planar sheets like in the hexagonal boron-nitride prototype structure. The observed depolarization is accompanied by a significant lateral 1.6% expansion of the lattice parameter and a 3% reduced Zn-O bond length within the sheets. The nonpolar structure stabilizes an atomically flat surface morphology unseen for ZnO surfaces thus far. The transition to the bulk wurtzite structure occurs in the 3-4 ML coverage range, connected to considerable roughening.
A phase-field approach for phase transformations (PTs) between three different phases at nonequilibrium temperatures is developed. It includes advanced mechanics, thermodynamically consistent interfacial stresses, and interface interactions. A thermodynamic Landau-Ginzburg potential developed in terms of polar order parameters satisfies the desired instability and equilibrium conditions for homogeneous phases. The interfacial stresses were introduced with some terms from large-strain formulation even though the small-strain assumption was utilized. The developed model is applied to study the PTs between two solid phases via highly disordered intermediate phase (IP) or intermediate melt (IM) hundreds of degrees below the melting temperature. In particular, the PTs in HMX energetic crystal via IM is analyzed. The effects of various parameters (temperature, ratios of widths and energies of solid-solid (SS) to solid-melt (SM) interfaces, elastic energy, and interfacial stresses) on the formation, stability, and structure of the IM within a propagating SS interface are studied. Interfacial and elastic stresses within SS interphase and their relaxation and redistribution with appearance of partial or complete IM are analyzed. Energy and structure of the critical nucleus (CN) of IM are studied as well. In particular, the interfacial stresses increase the aspect-ratio of the CN. Although including elastic energy can drastically reduce the energy of CN of IM, the activation energy of the CN of IM within SS interface increases when interfacial tension is taken into account. The developed thermodynamic potential can also be modified to model other multiphase physical phenomena, such as multi-variant martensitic PTs, grain boundary and surface-induced pre-melting and PTs, as well as developing phase diagrams for IPs.
The surface relaxation and the electronic structure of the two polar surfaces of ZnO have been investigated using ab initio supercell calculations. The relaxation of the slab, confined by the two different polar surfaces, compressed the Zn–O double layers, in particular for the (0 0 0-surface, where the Zn–O double-layer separation decreased to 48% of the bulk value. The calculated bandstructure revealed an occupied pz-surface state on the (0 0 0-surface, strongly localized to the surface O-atoms. The conduction band on the (0 0 0 1)-surface was split off from the bulk region to form a 2D metallic surface state. This enabled charge transfer from the (0 0 0- to the (0 0 0 1)-surface to quench the intrinsic dipole by a similar process that has been observed previously on the unreconstructed MgO(1 1 1)-surface.
An advanced three-phase phase-�eld approach (PFA) is suggested for a non-equilibrium phase interface
which contains an intermediate phase, in particular, a solid-solid interface with a nanometersized
intermediate melt (IM). Thermodynamic potential in the polar order parameters is developed,
which satis�es all thermodynamic equilibrium and stability conditions. Special form of the gradient
energy allowed us to include the interaction of two solid-melt interfaces via intermediate melt and
obtain a well-posed problem and mesh-independent solutions. It is proved that for stationary 1D
solutions to two Ginzburg-Landau equations for three phases, the local energy at each point is equal
to the gradient energy. Simulations are performed for � $ � phase transformations (PTs) via IM
in HMX energetic material. Obtained energy - IM width dependence is described by generalized
force-balance models for short- and long-range interaction forces between interfaces but not far
from the melting temperature. New force-balance model is developed, which describes phase �eld
results even 100K below the melting temperature. The e�ects of the ratios of width and energies
of solid-solid and solid-melt interfaces, temperature, and the parameter characterizing interaction
of two solid-melt interfaces, on the structure, width, energy of the IM and interface velocity are
determined by �nite element method. Depending on parameters, the IM may appear by continuous
or discontinuous barrierless disordering or via critical nucleus due to thermal
uctuations.
The IM may appear during heating and persist during cooling at temperatures well below than it
follows from sharp-interface approach. On the other hand, for some parameters when IM is expected,
it does not form, producing an IM-free gap. The developed PFA represents a quite general
three-phase model and can be extended to other physical phenomena, such as martensitic PTs,
surface-induced premelting and PTs, premelting/disordering at grain boundaries, and developing
corresponding interfacial phase diagrams.
A multiscale approach is pursued to develop a modified shear-lag model for capturing size-scale effects on electrostatic potential generated by a zinc oxide (ZnO) nanowire (NW) in a nanocomposite electrical generator (NCEG). The size-scale effect on elastic modulus of ZnO NWs is captured using a core-surface model. Closed form of governing equations are derived considering linear elasticity for axisymmetric problem and cylindrical coordinate system. Two different configurations based on parallel and series connecting of NCEGs for application in NEMS/MEMS devices are also studied. Parametric studies are performed for sample cases to demonstrate application of the developed model. It is shown that aspect ratio and diameter of NWs are crucial controlling parameters for determining the performance of nanocomposite electrical generators. Numerical results disclose that there is an optimum aspect ratio for each NW of specific diameter. It was also shown that despite the symmetry of loading with respect to mid-plane normal to the NW's axis, the electric potential is not symmetric.
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%.
A nanocomposite electrical generator composed of Zinc oxide nanowires (ZnO NWs) was modeled using continuum mechanics and Maxwell's equations. Axial loading was considered and the optimum aspect ratio of ZnO NWs for getting to maximum electric potential was calculated. The bonding between the ZnO NWs and the polymer matrix was considered to be perfect and the linear piezoelectric behavior was assumed. It was shown that the electric potential has maximum and minimum values of opposite signs at the extreme ends along the nanowire length. The maximum generated electric potential varies from 0.01717 for a NW with an aspect ratio of one to 0.61107 for a NW with an aspect ratio of thirty. The optimum aspect ratio of ZnO NW was defined as the difference between the maximum generated electric potential at which the difference becomes less than 1%, which results in an aspect ratio of 16. The results are a major step toward producing ZnO NWs for nanocomposite electrical generators with maximum performance.
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.
Semiconducting zinc oxide nanowires (NWs) and nanobelts (NBs) are a unique group of quasi-one-dimensional nanomaterial. This review mainly focuses on the rational synthesis, structure analysis, novel properties and unique applications of zinc oxide NWs and NBs in nanotechnology. First, we will discuss rational design of synthetic strategies and the synthesis of NWs via vapor phase and chemical growth approaches. Secondly, the vapor–solid process for synthesis of oxide based nanostructures will be described in details. We will illustrate the polar surface dominated growth phenomena, such as the formation of nanosprings, nanorings and nanohelices of single-crystal zinc oxide. Third, we will describe the unique and novel electrical, optoelectronic, field emission, and mechanical properties of individual NWs and NBs. Finally, we will illustrate some novel devices and applications made using NWs as ultra-sensitive chemical and biological nanosensors, solar cell, light emitting diodes, nanogenerators, and nano-piezotronic devices. ZnO is ideal for nanogenerators for converting nano-scale mechanical energy into electricity owing to its coupled piezoelectric and semiconductive properties. The devices designed based on this coupled characteristic are the family of piezotronics, which is a new and unique group of electronic components that are controlled by external forces/pressure.
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.
The size scale effect on the piezoelectric response of bulk ZnO and ZnO nanobelts has been studied using molecular dynamics simulation. Six molecular dynamics models of ZnO nanobelts are constructed and simulated with lengths of 150.97 Å and lateral dimensions ranging between 8.13 and 37.37 Å. A molecular dynamics model of bulk ZnO has also been constructed and simulated using periodic boundary conditions. The piezoelectric constants of the bulk ZnO and each of the ZnO nanobelts are predicted. The predicted piezoelectric coefficient of bulk ZnO is 1.4 C m−2, while the piezoelectric coefficient of ZnO nanobelts increases from 1.639 to 2.322 C m−2 when the lateral dimension of the ZnO NBs is reduced from 37.37 to 8.13 Å. The changes in the piezoelectric constants are explained in the context of surface charge redistribution. The results give a key insight into the field of nanopiezotronics and energy scavenging because the piezoelectric response and voltage output scale with the piezoelectric coefficient.
Based on the achievement of synthesis of ZnO nanowires in mass production, ZnO nanowires gas sensors were fabricated with microelectromechanical system technology and ethanol-sensing characteristics were investigated. The sensor exhibited high sensitivity and fast response to ethanol gas at a work temperature of 300 °C. Our results demonstrate the potential application of ZnO nanowires for fabricating highly sensitive gas sensors. © 2004 American Institute of Physics.
A distinction is made between the surface Helmholtz free energy F, and the surface tension ?. The surface energy is the work necessary to form unit area of surface by a process of division: the surface tension is the tangential stress (force per unit length) in the surface layer; this stress must be balanced either by external forces or by volume stresses in the body.
The surface tension of a crystal face is related to the surface free energy by the relation ?=F+A(dF/dA), where A is the area of the surface. For a one-component liquid, surface free energy and tension are equal. For crystals the surface tension is not equal to the surface energy. The standard thermodynamic formulae of surface physics are reviewed, and it is found that the surface free energy appears in the expression for the equilibrium contact angle, and in the Kelvin expression for the excess vapour pressure of small drops, but that the surface tension appears in the expression for the difference in pressure between the two sides of a curved surface.
The surface tensions of inert-gas and alkali-halide crystals are calculated from expressions for their surface energies and are found to be negative. The surface tensions of homopolar crystals are zero if it is possible to neglect the interaction between atoms that are not nearest neighbours.
Synthesis of aligned arrays of ultrathin ZnO nanotubes on a Si wafer coated with a thin ZnO film, is described. Systematic exploration of the effects were obtained during a systematic exploration of the effects of varying the hydrothermal growth conditions. The Si substrates were pre-coated, using PLD with a thin ZnO film shots. It was observed that the ratio of peak intensities of the UV and green yellow components in the PL spectra of the as-grown arrays is 0.32 and 3.4, implying that the nanotubes are of substantially higher crystal quality.
Ionic compounds pose extra challenges with the appropriate modeling of long-range coulombic interactions. Here, we study the mechanical properties of zinc oxide (ZnO) nanowires using molecular dynamic simulations with Buckingham potential and determine the suitability of the Ewald (Ann. Phys. 1921; 19) and Wolf (J. Chem. Phys. 1999; 110(17):8254–8282) summation methods to account for the long-range Coulombic forces. A comparative study shows that both the summation methods are suitable for modeling bulk structures with periodic boundary conditions imposed on all sides; however, significant differences are observed when nanowires with free surfaces are modeled. As opposed to Wolf's prediction of a linear stress–strain response in the elastic regime, Ewald's method predicts an erroneous behavior. This is attributed to the Ewald method's inability to account for surface effects properly. Additionally, Wolf's method offers highly improved computational performance as the model size is increased. This gain in computational time allows for modeling realistic nanowires, which can be directly compared with the existing experimental results. We conclude that the Wolf summation is a superior technique when modeling non-periodic structures in terms of both accuracy of the results and computational performance. Copyright © 2010 John Wiley & Sons, Ltd.
Molecular dynamics (MD) simulations are carried out to characterize the mechanical and thermal responses of
-oriented ZnO nanobelts with lateral dimensions of 21.22 Å×18.95 Å, 31.02 Å × 29.42 Å and 40.81Å × 39.89 Å over the temperature range of 300-1000 K. The Young's modulus and thermal conductivity of the nanobelts are evaluated. Significant surface effects on properties due to the high- surface-to-volume ratios of the nanobelts are observed. For the mechanical response, surface-stress-induced internal stress plays an important role. For the thermal response, surface scattering of phonons dominates. Calculations show that the Young's modulus is higher than the corresponding value for bulk ZnO and decreases by ~ 33% as the lateral dimensions increase from 21.22 Å × 18.95 Å to 40.81 Å × 39.89 Å. The thermal conductivity is one order of magnitude lower than the corresponding value for bulk ZnO single crystal and decreases with wire size. Specifically, the conductivity of the 21.22 Å × 18.95 Å belt is approximately (31-18)% lower than that of the 40.81 Å × 39.89 Å belt over the temperature range analyzed. A significant dependence of properties on temperature is also observed, with the Young's modulus decreasing on average by 12% and the conductivity decreasing by 50% as temperature increases from 300 K to 1000 K.
The thermal conductivity, thermal expansion coefficient (TEC), and the propagation velocity of longitudinal and transverse ultrasonic waves in ZnO-based ceramics are investigated in the temperature range from 300 to 1200 K with a porosity from 1.5 to 21%. The Young, shear, and bulk moduli and the Poisson ratio are calculated from the data on the propagation velocities of ultrasonic waves. Formulas are suggested to calculate the investigated parameters as a function of temperature and porosity.
A survey is given of surface‐structure determinations for the low index faces of compound semiconductors via analyses of elastic low‐energy electron diffraction intensities. The (100) face of (cubic‐NaCl structure) MgO is thought to exhibit a surface structure that differs little from that of a truncated bulk crystal. Small deviations from the truncated bulk geometry have been reported for the layered transition‐metal dichalcogenides MoS 2 and NbSe 2 . The cleavage faces of tetrahedrally coordinated compound semiconductors [i.e., zincblende (110), wurtzite (101¯0), and wurtzite (112¯0)] exhibit the same space‐group symmetry as the bulk crystal. The positions of the species in the uppermost atomic layer may be altered substantially from their bulk values, however, with the anion shifted outward, the cation inward, and the whole layer relaxed toward the bulk crystal. For the materials examined to date [i.e.,GaAs(110), ZnSe (110), ZnO(101¯0) and ZnO(112¯0)] the shifts in atomic position vary in magnitude but can exert profound effects on the electronic structure and chemical behavior of the associated surfaces.
The possibility is shown to fabricate a wide class of free-standing nano-objects based on few monolayers thick scrolled heterostructures. Using an ultra-thin film (1 ML GaAs:1 ML InAs), nanotubes with an inside diameter of ≈2 nm have been obtained, which constitutes the limiting size for this system. Molecular-beam-expitaxy overgrown structures with nanotubes embedded into GaAs have been obtained.
We tried to control preferred orientation of ZnOx films deposited by radio frequency (RF) magnetron sputtering, and to make the growth mechanisms clear. Zinc oxide has tetrahedral coordinates caused by sp3 hybridized orbits, and the (0001) plane has the lowest surface free energy. Therefore, the film grows with strong (0001) preferred orientation even on glass. Considering the formation of tetrahedral coordination in the vapor phase and the deposition rate, however, we succeeded to control the preferred orientation of ZnOx films on glass. The (112̄0) textured film was obtained under sputtering gas composition which deteriorates the formation of tetrahedral coordination in the vapor phase and the high deposition rate. Formation of each texture is strongly related to the formation of tetrahedral coordination in the vapor phase and on the substrate during sputtering. Therefore, (112̄0) textured film had higher carrier concentration than that of the (0001) textured film caused by existing excess Zn atoms. Moreover, the growth mechanism with considering the density of surface energy, and the applications of the control for the epitaxial growth, are discussed.
The structures of the reconstructed Ir(100), Pt(100) and Au(100) surfaces have been investigated. Low energy electron diffraction (LEED) patterns are analyzed and LEED intensity versus energy data are measured. A variety of structures is observed by LEED: Ir(100) exhibits a relatively simple (1 × 5) pattern; Pt(100) shows a series of closely related patterns, a typical representative of which has a structure; Au(100) usually exhibits a c(26 × 68) pattern, often inaccurately described in the literature as a (20 × 5) pattern. The reconstruction of Au(111) is also considered for comparison. Various plausible structural models are discussed, while laser simulation is used to lessen the number of these models. The analysis is completed in a companion paper where LEED intensity calculations are reported to determine the atomic locations.
A nanocomposite electrical generator composed of an array of zinc oxide nanowires is considered. The electric potential distribution along zinc oxide nanowires is modeled using continuum mechanics and Maxwell's equations for the case of axial loading. A perturbation technique is used for decoupling the constitutive equations. The governing differential equations are solved using a finite difference method. It is shown that a gradient of electric potential exists along the axis of the zinc oxide nanowires. Maximum and minimum values of electric potential exist at the extreme ends along the nanowire length and have opposite signs. The positive and negative voltages are separated by a zero-valued electric potential at the middle of the nanowire. It is also shown that the electric potential is a strong function of shear stress at the interface of matrix-nanowire. The proposed system and loading configuration can generate up to 160% more electric potential than the values reported for the nanowire in the bended configuration, which results in a more sustainable energy source.
When there is a dipole moment in the repeat unit perpendicular to the surface in an ionic crystal, lattice sums in the electrostatic energy diverge and the calculated surface energy is infinite. The cause of this divergence is demonstrated and the surfaces of any ionic or partly ionic material are classified into three types. Type 1 is neutral with equal numbers of anions and cations on each plane and type 2 is charged but there is no dipole moment perpendicular to the surface because of the symmetrical stacking sequence. Both these surfaces should have modest surface energies and may be stable with only limited relaxations of the ions in the surface region. The type 3 surface is charged and has a dipole moment in the repeat unit perpendicular to the surface. This surface can only be stabilised by substantial reconstruction. These conclusions are important for the analysis of the surface structure of ionic crystals.
Some generic design principles for a novel nano-fibrication approach, the nanomechanical architecture of strained bilayer films, in terms of geometric, physical, and processing parameters were discussed. It was shown that there exist fundamental geometric and physical conditions controlling the formation of nanotubes versus nanocoils. For an elastically isotropic film, the critical geometric condition for nanocoil formation was that the film width must be smaller than L0=2πR0, where R0 is the characteristic bending radius of the given bilayer film. For an anisotropic film, nanocoil formation was found to depend critically on the alignment of the most compliant direction of the film with respect to the film geometry.
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.
We present a detailed model describing the effects of surface stress on the equilibrium spacing and biaxial modulus of thin metal films. The model predicts that very thin films will equilibrate to a spacing in the plane substantially smaller than the bulk spacing for the material, and that this biaxial strain will vanish as the reciprocal of the film thickness. The model predicts enhancements in the biaxial modulus of thin metal films which also scale with the reciprocal of the film thickness. The magnitude of both the strain and the resulting change in biaxial modulus are proportional to the magnitude of the surface stress. We verified the predictions of the surface-stress model by performing molecular-dynamics computer simulations of thin metal films using an analytic form of the embedded-atom-method potential. The model was found to predict accurately the equilibrium properties of thin metal films.
The driving force for surface reconstruction is shown to be given by the (usually nonzero) difference between the surface energy and stress, leading to a ground state characterized by an isotropic stress tensor with vanishing shear components. The analytic results, illustrated by atomistic simulations of the missing row reconstruction of the Au(110) surface, permit likely reconstructions to be identified based on the stress and energy in the unreconstructed surface.
The rigorous size miniaturization of nanotechnology is continually generating new applications and new physical effects. We show here that nanotubes can be formed from thin solid films of almost any material at almost any position, once these films are released from their substrate. This exceptional design flexibility has useful implications, including for fluid transportation and capillarity on the nanometre scale, as well as offering the opportunity to extend fundamental investigations to a new diversity of materials, material systems and geometries.
Upon cooling, water freezes to ice. This familiar phase transition occurs widely in nature, yet unlike the freezing of simple liquids, it has never been successfully simulated on a computer. The difficulty lies with the fact that hydrogen bonding between individual water molecules yields a disordered three-dimensional hydrogen-bond network whose rugged and complex global potential energy surface permits a large number of possible network configurations. As a result, it is very challenging to reproduce the freezing of 'real' water into a solid with a unique crystalline structure. For systems with a limited number of possible disordered hydrogen-bond network structures, such as confined water, it is relatively easy to locate a pathway from a liquid state to a crystalline structure. For pure and spatially unconfined water, however, molecular dynamics simulations of freezing are severely hampered by the large number of possible network configurations that exist. Here we present a molecular dynamics trajectory that captures the molecular processes involved in the freezing of pure water. We find that ice nucleation occurs once a sufficient number of relatively long-lived hydrogen bonds develop spontaneously at the same location to form a fairly compact initial nucleus. The initial nucleus then slowly changes shape and size until it reaches a stage that allows rapid expansion, resulting in crystallization of the entire system.
Since the discovery of carbon nanotubes in 1991 (ref. 1), there have been significant research efforts to synthesize nanometre-scale tubular forms of various solids. The formation of tubular nanostructure generally requires a layered or anisotropic crystal structure. There are reports of nanotubes made from silica, alumina, silicon and metals that do not have a layered crystal structure; they are synthesized by using carbon nanotubes and porous membranes as templates, or by thin-film rolling. These nanotubes, however, are either amorphous, polycrystalline or exist only in ultrahigh vacuum. The growth of single-crystal semiconductor hollow nanotubes would be advantageous in potential nanoscale electronics, optoelectronics and biochemical-sensing applications. Here we report an 'epitaxial casting' approach for the synthesis of single-crystal GaN nanotubes with inner diameters of 30-200 nm and wall thicknesses of 5-50 nm. Hexagonal ZnO nanowires were used as templates for the epitaxial overgrowth of thin GaN layers in a chemical vapour deposition system. The ZnO nanowire templates were subsequently removed by thermal reduction and evaporation, resulting in ordered arrays of GaN nanotubes on the substrates. This templating process should be applicable to many other semiconductor systems.
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.
We demonstrate, by theoretical analysis and molecular dynamics simulation, a mechanism for fabricating nanotubes by self-bending of nanofilms under intrinsic surface-stress imbalance due to surface reconstruction. A freestanding Si nanofilm may spontaneously bend itself into a nanotube without external stress load, and a bilayer SiGe nanofilm may bend into a nanotube with Ge as the inner layer, opposite of the normal bending configuration defined by misfit strain. Such rolled-up nanotubes can accommodate a high level of strain, even beyond the magnitude of lattice mismatch, greatly modifying the tube electronic and optoelectronic properties.
This paper presents first-principles calculations for ultrasmall ZnO one-dimensional nanostructures. The calculations were done on ZnO nanowires and single-walled nanotubes with n atoms per periodic unit, where one periodic unit is made up of two ZnO layers. The calculations show that, for small n, a single-walled nanotube has lower energy than a nanowire. A crossover point near n = 38 is predicted. Vibrations and vibrational entropy of competing structures is discussed.
- I Levitas
- K Momeni
I. Levitas and K. Momeni, Acta Mater., 2014, 65, 125–132.
- C Huang
- M Boone
- D E Roberts
- M G Savage
- N Lagally
- H Shaji
- R Qin
- J A Blick
- F Nairn
- Liu
Huang, C. Boone, M. Roberts, D. E. Savage, M. G. Lagally,
N. Shaji, H. Qin, R. Blick, J. A. Nairn and F. Liu, Adv. Mater.,
2005, 17, 2860–2864.