High-resolution transmission electron microscopy study of electrically-driven reversible phase change in ge2sb2te5 nanowires

By combining high-resolution transmission electron microscopy (HRTEM) characterization and electrical measurements on a unique device platform, we study the reversible electrically-driven phase-change characteristics of self-assembled Ge(2)Sb(2)Te(5) nanowires. Detailed HRTEM analyses are used to correlate and understand the effect of full and intermediate structural transformations on the measured electrical properties of the nanowire devices. The study demonstrates that our unique approach has the potential to provide new information regarding the dynamic structural and electrical states of phase-change materials at the nanoscale, which will aid the design of future phase-change memory devices.     

Microscopic origin of the fast crystallization ability of Ge-Sb-Te phase-change memory materials

Ge-Sb-Te materials are used in optical DVDs and non-volatile electronic memories (phase-change random-access memory). In both, data storage is effected by fast, reversible phase changes between crystalline and amorphous states. Despite much experimental and theoretical effort to understand the phase-change mechanism, the detailed atomistic changes involved are still unknown. Here, we describe for the first time how the entire write/erase cycle for the Ge(2)Sb(2)Te(5) composition can be reproduced using ab initio molecular-dynamics simulations. Deep insight is gained into the phase-change process; very high densities of connected square rings, characteristic of the metastable rocksalt structure, form during melt cooling and are also quenched into the amorphous phase. Their presence strongly facilitates the homogeneous crystal nucleation of Ge(2)Sb(2)Te(5). As this simulation procedure is general, the microscopic insight provided on crystal nucleation should open up new ways to develop superior phase-change memory materials, for example, faster nucleation, different compositions, doping levels and so on.     

Highly scalable non-volatile and ultra-low-power phase-change nanowire memory

The search for a universal memory storage device that combines rapid read and write speeds, high storage density and non-volatility is driving the exploration of new materials in nanostructured form. Phase-change materials, which can be reversibly switched between amorphous and crystalline states, are promising in this respect, but top-down processing of these materials into nanostructures often damages their useful properties. Self-assembled nanowire-based phase-change material memory devices offer an attractive solution owing to their sub-lithographic sizes and unique geometry, coupled with the facile etch-free processes with which they can be fabricated. Here, we explore the effects of nanoscaling on the memory-storage capability of self-assembled Ge2Sb2Te5 nanowires, an important phase-change material. Our measurements of write-current amplitude, switching speed, endurance and data retention time in these devices show that such nanowires are promising building blocks for non-volatile scalable memory and may represent the ultimate size limit in exploring current-induced phase transition in nanoscale systems.     

Highly conductive coaxial SnO(2)-In(2)O(3) heterostructured nanowires for Li ion battery electrodes

Novel SnO(2)-In(2)O(3) heterostructured nanowires were produced via a thermal evaporation method, and their possible nucleation/growth mechanism is proposed. We found that the electronic conductivity of the individual SnO(2)-In(2)O(3) nanowires was 2 orders of magnitude better than that of the pure SnO(2) nanowires, due to the formation of Sn-doped In(2)O(3) caused by the incorporation of Sn into the In(2)O(3) lattice during the nucleation and growth of the In(2)O(3) shell nanostructures. This provides the SnO(2)-In(2)O(3) nanowires with an outstanding lithium storage capacity, making them suitable for promising Li ion battery electrodes.     

Synthesis and characterization of phase-change nanowires

Phase-change memory materials have stimulated a great deal of interest although the size-dependent behaviors have not been well studied due to the lack of method for producing their nanoscale structures. We report the synthesis and characterization of GeTe and Sb(2)Te(3) phase-change nanowires via a vapor-liquid-solid growth mechanism. The as-grown GeTe nanowires have three different types of morphologies: single-crystalline straight and helical rhombohedral GeTe nanowires and amorphous curly GeO(2) nanowires. All the Sb(2)Te(3) nanowires are single-crystalline.     

激光致溅射沉积Ge2Sb2Te5薄膜的结晶行为研究

利用ＸＲＤ研究了激光致溅射沉积Ｇｅ_２Ｓｂ_２Ｔｅ_５薄膜的结晶行为;研究发现,与热致相变不同的是,激光致相变只发生从非晶态到ＦＣＣ晶态结构的转变,从ＦＣＣ到ＨＣＰ的结构转变不再发生,这有利于提高相变光盘的信噪比．Ｇｅ_２Ｓｂ_２Ｔｅ_５薄膜的结晶程度受初始化功率和转速的影响．     

Synthesis of magnesium borate (Mg2B2O5) nanowires,growth mechanism and their lubricating properties

Large quantities of single crystalline magnesium borate nanowires of the form Mg₂B₂O₅ with typical diameter about 120–180 nm and length about 0.2 mm have been successfully synthesized by a new and simple method of heating the mixed tablet of Mg(BO₂)₂ and graphite directly in vacuum at 1200 °C for 1 h. The products have been characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectrometry and selected area electron diffraction. The process of the nucleation and the growth of nanowires have been analyzed by VS mechanism. The results of the lubricating properties show that the friction coefficient of the oil is significantly decreased by the addition of Mg₂B₂O₅ nanowires. Our results indicate that the new method we use is effective in synthesis of Mg₂B₂O₅ nanowires and the nanowires can be used as additive to antiwear nanodevices.     

In-situ TEM observation of repeating events of nucleation in epitaxial growth of nano CoSi2 in nanowires of Si

The formation of CoSi and CoSi2 in Si nanowires at 700 and 800 degrees C, respectively, by point contact reactions between nanodots of Co and nanowires of Si have been investigated in situ in a ultrahigh vacuum high-resolution transmission electron microscope. The CoSi2 has undergone an axial epitaxial growth in the Si nanowire and a stepwise growth mode was found. We observed that the stepwise growth occurs repeatedly in the form of an atomic step sweeping across the CoSi2/Si interface. It appears that the growth of a new step or a new silicide layer requires an independent event of nucleation. We are able to resolve the nucleation stage and the growth stage of each layer of the epitaxial growth in video images. In the nucleation stage, the incubation period is measured, which is much longer than the period needed to grow the layer across the silicide/Si interface. So the epitaxial growth consists of a repeating nucleation and a rapid stepwise growth across the epitaxial interface. This is a general behavior of epitaxial growth in nanowires. The axial heterostructure of CoSi2/Si/CoSi2 with sharp epitaxial interfaces has been obtained. A discussion of the kinetics of supply limited and source-limited reaction in nanowire case by point contact reaction is given. The heterostructures are promising as high performance transistors based on intrinsic Si nanowires.     

Orientations of nuclei during static recrystallization in a cold-rolled Mg-Zn-Gd alloy

The static recrystallization process of a cold-rolled Mg-Zn-Gd alloy was tracked by a quasi-in-situ electron backscatter diffraction method to investigate the orientations of nuclei.The results show that orientation distribution of nuclei is associated with nucleation mechanism.The continuous static recrystallization nuclei display similar orientations to the parent grains with TD orientation.Differently,discontinuous static recrystallization nuclei formed within the parent grains(TD-45~0 orientation) show random orientations and a variety of misorientation angles but preferred axes <5273> or <5270>.Interestingly,a special oriented nucleation is found.Discontinuous static recrystallization nuclei originated from boundaries of the parent grain(TD-70° orientation) show concentrated TD orientations in another side due to the preferred misorientation relationship 70°<1120>(∑18 b).It is speculated that these two special misorientation relationships are related to the dislocation type.     

激光致溅射沉积Ge2Sb2Te5薄膜的结晶行为研究

利用XRD研究了激光致溅射沉积Ge2Sb2Te5薄膜的结晶行为，研究发现，与热致相变不同的是，激光致相变只发生从非晶态到FCC晶态结构的转变，从FCC与HCP的结构转变不再发生，这有利于提高相变光盘的信噪比。Ge2Sb2Te5薄膜的结晶程度受初始化功率和转速的影响。     

Core-shell heterostructured phase change nanowire multistate memory

Phase-change memory, which switches reversibly between crystalline and amorphous phases, is promising for next generation data-storage devices. In this work, we present a novel, nonbinary data-storage device using core-shell nanowires to significantly enhance memory capacity by combining two phase-change materials with different electronic and thermal properties to engineer different onsets of amorphous-crystalline transitions. Electric-field induced sequential amorphous-crystalline transition in core-shell nanowires displays three distinct electronic states with high, low, and intermediate resistances, assigned as data "0", "1", and "2".     

Large-scale, hot-filament-assisted synthesis of tungsten oxide and related transition metal oxide nanowires

A scalable and versatile method for the large-scale synthesis of tungsten trioxide nanowires and their arrays on a variety of substrates, including amorphous quartz and fluorinated tin oxide, is reported. The synthesis involves the chemical-vapor transport of metal oxide vapor-phase species using air or oxygen flow over hot filaments onto substrates kept at a distance. The results show that the density of the nanowires can be varied from 10(6)-10(10) cm(-2) by varying the substrate temperature. The diameter of the nanowires ranges from 100-20 nm. The results also show that variations in oxygen flow and substrate temperature affect the nanowire morphology from straight to bundled to branched nanowires. A thermodynamic model is proposed to show that the condensation of WO(2) species primarily accounts for the nucleation and subsequent growth of the nanowires, which supports the hypothesis that the nucleation of nanowires occurs through condensation of suboxide WO(2) vapor-phase species. This is in contrast to the expected WO(3) vapor-phase species condensation into WO(3) solid phase for nanoparticle formation. The as-synthesized nanowires are shown to form stable dispersions compared to nanoparticles in various organic and inorganic solvents.     

Catalyst-free synthesis, structural, and mechanical characterization of twinned Mg2B2O5 nanowires

Mg2B2O5 nanowires with (010) twins were synthesized for the first time using a catalyst-free method. The microstructure of the Mg2B2O5 nanowires has been extensively studied by cross-sectional high-resolution transmission electron microscopy. Nanoindentation tests were performed directly on individual nanowires to probe their mechanical properties. It was found that the twinned Mg2B2O5 nanowires achieve comparable hardness but 19% decrease in elastic modulus compared to their bulk counterpart. The elastic softening mechanisms of the Mg2B2O5 nanowires are discussed with reference to their twin defects, size, and surface effects.     

Stability of single-wall silicon carbide nanotubes – molecular dynamics simulations

One-dimensional silicon carbide (SiC) nanotubes and nanowires are both realizable and may co-exist. The stability of SiC nanotubes relative to nanowires and against heating is still unknown. Using classical molecular dynamics simulations, the authors investigate the stabilities of SiC nanotubes; as a first step, the study focuses on single-wall nanotubes (SWNTs). The results show that SiC nanotubes are more stable than nanowires below a critical diameter of about 1.6 nm, while SiC nanowires are more stable than nanotubes beyond that. As temperature increases, melting takes place at about 1620 K in SiC nanotubes by heterogeneous nucleation from the non-hexagonal defects due to reconstruction at a free end, and at about 1820 K in nanotubes without free ends by homogeneous nucleation within the wall from thermally activated 5-7-7-5 defects. In both cases formation of Si–Si and C–C bonds proceeds melting.     

Minimum voltage for threshold switching in nanoscale phase-change memory

The size scaling of the threshold voltage required for the amorphous-to-crystalline transition in phase-change memory (PCM) is investigated using planar devices incorporating individual GeTe and Sb2Te3 nanowires. We show that the scaling law governing threshold switching changes from constant field to constant voltage scaling as the amorphous domain length falls below 10 nm. This crossover is a consequence of the energetic requirement for carrier multiplication through inelastic scattering processes and indicates that the size of PCM bits can be miniaturized to the true nanometer scale.     

Growth of ZnO nanowires on a patterned Au substrate

ZnO nanowires were site-selectively deposited on the catalytic gold-patterned substrates using a vapor transport process at low temperature. We observed that Au-Zn phase played an important role in initiating the ZnO nucleation, which was identified from the TEM observation of the interface between the substrate and ZnO nanowires. And then further growth of the nanowires on the ZnO surface was driven by the reaction of the Zn vapor and O₂ gas in vapor-solid growth mode. Finally, it was concluded that the site-specific deposition of ZnO nanowires was ascribed to the vapor-liquid-solid mechanism of Au, Zn vapor, and Au-Zn phases at the initial stage of ZnO nucleation.     

Growth of InN Nanowires with Uniform Diameter on Si(111) Substrates: Competition Between Migration and Desorption of In Atoms

The effects of the growth parameters on the uniformity and the aspect ratio of InN nanowires grown on Si(111) substrates have been studied systematically, and a modified quasi‐equilibrium model is proposed. The growth temperature is of great importance for both the nucleation of the nanowires and the migration of In and N atoms, thus affecting the uniformity of the InN nanowires. In order to improve the uniformity of the InN nanowires, both traditional substrate nitridation and pre‐In‐droplet deposition have been implemented. It is found that the substrate nitridation is favorable for the nucleation of InN nanowires. However, the initial In atoms adhered to the substrate are insufficient to sustain the uniform growth of the InN nanowires. We have found that the initial In droplet on the substrate is not only advantageous for the nucleation of the InN nanowire, but also favorable for the In atom equilibrium between the initial In droplets and the direct In flux. Therefore, InN nanowires with a uniform aspect ratio and optimal diameter can be achieved. The results reported herein provide meaningful insights to understanding the growth kinetics during the InN nanowires growth, and open up great possibilities of developing high‐performance group III‐nitride‐based devices.     

Identification of Particle Stimulated Nucleation during Recrystallization of AA 7050

Mechanical properties of polycrystalline metals are dependent upon the arrangement of microstructural features in the metal. Recrystallization is an important phenomenon that often occurs during thermo-mechanical processing of metals. This work focuses upon aluminum alloy 7050 and uses crystallographic texture and pair correlation functions of recrystallized grains to characterize the dominance of particle stimulated nucleation in the recrystallization process. The randomization of the recrystallization texture and similar pair correlation functions for the particle distribution and the recrystallization nuclei distribution indicate that particle stimulated nucleation controls the recrystallization behavior in this alloy.     

Compared growth mechanisms of Zn-polar ZnO nanowires on O-polar ZnO and on sapphire

Controlling the growth of zinc oxide nanowires is necessary to optimize the performance of nanowire-based devices such as photovoltaic solar cells, nano-generators, or light-emitting diodes. With this in mind, we investigate the nucleation and growth mechanisms of ZnO nanowires grown by metalorganic vapor phase epitaxy either on O-polar ZnO or on sapphire substrates. Whatever the substrate, ZnO nanowires are Zn-polar, as demonstrated by convergent beam electron diffraction. For growth on O-polar ZnO substrate, the nanowires are found to sit on O-polar pyramids. As growth proceeds, the inversion domain boundary moves up in order to remain at the top of the O-polar pyramids. For growth on sapphire substrates, the nanowires may also originate from the sapphire/ZnO interface. The presence of atomic steps and the non-polar character of sapphire could be the cause of the Zn-polar crystal nucleation on sapphire, whereas it is proposed that the segregation of aluminum impurities could account for the nucleation of inverted domains for growth on O-polar ZnO.     

Mechanisms of 1D crystal growth in reactive vapor transport: indium nitride nanowires

Indium nitride (InN) nanowire synthesis using indium (In) vapor transport in a dissociated ammonia environment (reactive vapor transport) is studied in detail to understand the nucleation and growth mechanisms involved with the so-called "self-catalysis" schemes. The results show that the nucleation of InN crystal occurs first on the substrate. Later, In droplets are formed on top of the InN crystals because of selective wetting of In onto InN crystals. Further growth via liquid-phase epitaxy through In droplets leads the growth in one dimension (1D), resulting in the formation of InN nanowires. The details about the nucleation and growth aspects within these self-catalysis schemes are rationalized further by demonstrating the growth of heteroepitaxially oriented nanowire arrays on single-crystal substrates and "tree-like" morphologies on a variety of substrates. However, the direct nitridation of In droplets using dissociated ammonia results in the spontaneous nucleation and basal growth of nanowires directly from the In melt surface, which is quite different from the above-mentioned nucleation mechanism with the reactive vapor transport case. The InN nanowires exhibit a band gap of 0.8 eV, whereas the mixed phase of InN and In(2)O(3) nanowires exhibit a peak at approximately 1.9 eV in addition to that at 0.8 eV.