Phase Separation in Ge-Rich GeSbTe at Different Length Scales: Melt-Quenched Bulk versus Annealed Thin Films.

Integration of the prototypical GeSbTe (GST) ternary alloys, especially on the GeTe-Sb2Te3 tie-line, into non-volatile memory and nanophotonic devices is a relatively mature field of study. Nevertheless, the search for the next best active material with outstanding properties is still ongoing. This search is relatively crucial for embedded memory applications where the crystallization temperature of the active material has to be higher to surpass the soldering threshold. Increasing the Ge content in the GST alloys seems promising due to the associated higher crystallization temperatures. However, homogeneous Ge-rich GST in the as-deposited condition is thermodynamically unstable, and phase separation upon annealing is unavoidable. This phase separation reduces endurance and is detrimental in fully integrating the alloys into active memory devices. This work investigated the phase separation of Ge-rich GST alloys, specifically Ge5Sb2Te3 or GST523, into multiple (meta)stable phases at different length scales in melt-quenched bulk and annealed thin film. Electron microscopy-based techniques were used in our work for chemical mapping and elemental composition analysis to show the formation of multiple phases. Our results show the formation of alloys such as GST213 and GST324 in all length scales. Furthermore, the alloy compositions and the observed phase separation pathways agree to a large extent with theoretical results from density functional theory calculations.

Wetting of surfaces decorated by gas-phase synthesized silver nanoparticles: Effects of Ag adatoms, nanoparticle aging, and surface mobility.

The wetting state of surfaces can be rendered to a highly hydrophobic state by the deposition of hydrophilic gas phase synthesized Ag nanoparticles (NPs). The aging of Ag NPs leads to an increase in their size, which is also associated with the presence of Ag adatoms on the surface between the NPs that have a strong effect on the wetting processes. Furthermore, surface airborne hydrocarbons were removed by UV-ozone treatment, providing deeper insight into the apparent mobility of the NPs on different surfaces and their subsequent ripening and aging. In addition, the UV-ozone treatment revealed the presence of adatoms during the magnetron sputtering process. This surface treatment lowers the initial contact angle of the substrates and facilitates the mobility of Ag NPs and adatoms on the surface of substrates. Adatoms co-deposited on clean high surface energy substrates will nucleate on Ag NPs that will remain closely spherical and preserve the pinning effect due to the water nanomeniscus. If the adatoms are co-deposited on a UV-ozone cleaned low surface energy substrate, their mobility is restricted, and they will nucleate in two-dimensional islands and/or nanoclusters on the surface instead of connecting to existing Ag NPs. This growth results in a rough surface without overhangs, where the wetting state is reversed from hydrophobic to hydrophilic. Finally, different material surfaces of transmission electron microscopy grids revealed strong differences in the sticking coefficient for the Ag NPs, suggesting another factor that can strongly affect their wetting properties.

Effect of Airborne Hydrocarbons on the Wettability of Phase Change Nanoparticle Decorated Surfaces.

We present here a detailed study of the wettability of surfaces nanostructured with amorphous and crystalline nanoparticles (NPs) derived from the phase-change material Ge2Sb2Te5 (GST). Particular attention was devoted to the effect of airborne surface hydrocarbons on surface wetting. Our analysis illustrates that a reversible hydrophilic-hydrophobic wettability switch is revealed by combined ultraviolet-ozone (UV-O3) treatments and exposure to hydrocarbon atmospheres. Indeed, the as-prepared surfaces exhibited a hydrophilic state after thermal annealing or UV-O3 treatment which can partially remove hydrocarbon contaminants, while a hydrophobic state was realized after exposure to hydrocarbon atmosphere. Using high-angle annular dark-field scanning transmission electron microscopy for the specially designed GST NP decorated graphene substrates, a network of hydrocarbon connecting GST NPs was observed. Our findings indicate that airborne hydrocarbons can significantly enhance the hydrophobicity of nanostructured surfaces. Finally, the experiments reveal that previously defined hydrophilic materials can be used for the design of hydrophobic surfaces even if the meniscus is highly adhered to a solid surface, which is in agreement with our qualitative model involving the contribution of the nanomeniscus formed between the substrate and a decorating NP.

Roughness-dependent wetting behavior of vapor-deposited metallic thin films.

We studied the wetting behavior of silver and copper thin films versus their kinetic roughening upon deposition at room temperature on glass substrates. Time-dependent height-height correlation functions were extracted from atomic force microscopy images, and they demonstrated a nonstationary growth front of the film roughness associated with a temporal evolution of the local surface slope. As a result, we tried to correlate the roughness statistical properties such as the root-mean-square (rms) roughness σ, the correlation length ξ, and the local surface slope (ρ≈σ/ξ) with the wetting behavior of the films' surfaces. The contact angle behavior was also studied by analyzing the variation of the energy of the system with water penetrating into the surface cavities, and the associated Laplace pressure induced by the local surface curvature. Hence, it was demonstrated that the wetting transition from a metastable Cassie-Baxter state to a Wenzel state as well as the penetration of a droplet into the surface crevices occur at the smaller local surface slopes for the higher surface energy material.

Dynamics of GeSbTe phase-change nanoparticles deposited on graphene.

Phase-change Ge2Sb2Te5 nanoparticles (NPs), that are promising for next-generation phase-change memory and other emerging optoelectronic applications, have been deposited on graphene support layers and analyzed using advanced transmission electron microscopy techniques allowing high quality atomic resolution imaging at accelerating voltages as low as 40 kV. The deposition results in about three times higher NP coverage on suspended graphene than on graphene containing an amorphous background support. We attribute this to the variation in surface energy of suspended and supported graphene, indicating that the former harvests NPs more effectively. Hydrocarbon contamination on the graphene profoundly enhances the mobility of the NP atoms and after prolonged (weeks) exposure to air resulted in more severe oxidation and spreading of NPs on the suspended graphene than on supported graphene because the network of hydrocarbons develops more extensively on the suspended rather than on the supported graphene. Due to this oxidation, GeO x shells are formed out of NPs having a uniform composition initially. The present work provides new insights into the structure and stability of phase-change NPs, graphene and their combinations.

Resolving Crystallization Kinetics of GeTe Phase-Change Nanoparticles by Ultrafast Calorimetry.

Chalcogenide-based phase change materials (PCMs) are promising candidates for the active element in novel electrical nonvolatile memories and have been applied successfully in rewritable optical disks. Nanostructured PCMs are considered as the next generation building blocks for their low power consumption, high storage density, and fast switching speed. Yet their crystallization kinetics at high temperature, the rate-limiting property upon switching, faces great challenges due to the short time and length scales involved. Here we present a facile method to synthesize highly controlled, ligand-free GeTe nanoparticles, an important PCM, with an average diameter under 10 nm. Subsequent crystallization by slow and ultrafast rates allows unravelling of the crystallization kinetics, demonstrating the breakdown of Arrhenius behavior for the crystallization rate and a fragile-to-strong transition in the viscosity as well as the overall crystal growth rate for the as-deposited GeTe nanoparticles. The obtained results pave the way for further development of phase-change memory based on GeTe with sub-lithographic sizes.

Shape and structural motifs control of MgTi bimetallic nanoparticles using hydrogen and methane as trace impurities.

In this work we report the influence of methane/hydrogen on the nucleation and formation of MgTi bimetallic nanoparticles (NPs) prepared by gas phase synthesis. We show that a diverse variety of structural motifs can be obtained from MgTi alloy, TiCx/Mg/MgO, TiCx/MgO and TiHx/MgO core/shell NPs via synthesis using CH4/H2 as a trace gas, and with good control of the final NP morphology and size distribution. Moreover, depending on the concentration of Ti and type of employed trace gas, the as prepared MgTi NPs can be tuned from truncated hexagonal pyramid to triangular and hexagonal platelet shapes. The shape of MgTi NPs is identified using detailed analysis from selected area electron diffraction (SAED) patterns and tomography (3D reconstruction based on a tilt series of Bright-Field transmission electron microscopy (TEM) micrographs). We observe the truncated hexagonal pyramid as a shape of MgTi alloy NPs in contrast to Mg NPs that show a hexagonal prismatic shape. Moreover, based on our experimental observations and generic geometrical model analysis, we also prove that the formation of the various structural motifs is based on a sequential growth mechanism instead of phase separation. One of the prime reasons for such mechanism is based on the inadequacy of Mg to nucleate without template in the synthesis condition. In addition, the shape of the TiCx/TiHx core, and the concentration of Mg have strong influence on the shape evolution of TiCx/MgO and TiHx/MgO NPs compared to TiCx/Mg/MgO NPs, where the thermodynamics and growth rates of the Mg crystal planes dominate the final shape. Finally, it is demonstrated that the core shape of TiCx and TiHx is affected by the Mg/Ti target ratio (affecting the composition in the plasma), and the type of the trace gas employed. In the case of CH4 the TiCx core forms a triangular platelet, while in the case of H2 the TiHx core transforms into a hexagonal platelet. We elucidate the reason for the TiCx/TiHx core shape based on the presence of (i) defects, and (ii) hydrogen and carbon adsorption on {111} planes that alter the growth rates and surface facet stabilization.

Rough Electrode Creates Excess Capacitance in Thin-Film Capacitors.

The parallel-plate capacitor equation is widely used in contemporary material research for nanoscale applications and nanoelectronics. To apply this equation, flat and smooth electrodes are assumed for a capacitor. This essential assumption is often violated for thin-film capacitors because the formation of nanoscale roughness at the electrode interface is very probable for thin films grown via common deposition methods. In this work, we experimentally and theoretically show that the electrical capacitance of thin-film capacitors with realistic interface roughness is significantly larger than the value predicted by the parallel-plate capacitor equation. The degree of the deviation depends on the strength of the roughness, which is described by three roughness parameters for a self-affine fractal surface. By applying an extended parallel-plate capacitor equation that includes the roughness parameters of the electrode, we are able to calculate the excess capacitance of the electrode with weak roughness. Moreover, we introduce the roughness parameter limits for which the simple parallel-plate capacitor equation is sufficiently accurate for capacitors with one rough electrode. Our results imply that the interface roughness beyond the proposed limits cannot be dismissed unless the independence of the capacitance from the interface roughness is experimentally demonstrated. The practical protocols suggested in our work for the reliable use of the parallel-plate capacitor equation can be applied as general guidelines in various fields of interest.

Strategies to initiate and control the nucleation behavior of bimetallic nanoparticles.

In this work we report strategies to nucleate bimetallic nanoparticles (NPs) made by gas phase synthesis of elements showing difficulty in homogeneous nucleation. It is shown that the nucleation assisted problem of bimetallic NP synthesis can be solved via the following pathways: (i) selecting an element which can itself nucleate and act as a nucleation center for the synthesis of bimetallic NPs; (ii) introducing H2 or CH4 as an impurity/trace gas to initiate nucleation during the synthesis of bimetallic NPs. The latter can solve the problem if none of the elements in a bimetallic NP can initiate nucleation. We illustrate the abovementioned strategies for the case of Mg based bimetallic NPs, which are interesting as hydrogen storage materials and exhibit both nucleation and oxidation issues even under ultra-high vacuum conditions. In particular, it is shown that adding H2 in small proportions favors the formation of a solid solution/alloy structure even in the case of immiscible Mg and Ti, where normally phase separation occurs during synthesis. In addition, we illustrate the possibility of improving the nucleation rate, and controlling the structure and size distribution of bimetallic NPs using H2/CH4 as a reactive/nucleating gas. This is shown to be associated with the dimer bond energies of the various formed species and the vapor pressures of the metals, which are key factors for NP nucleation.

Crystallization Kinetics of GeSbTe Phase-Change Nanoparticles Resolved by Ultrafast Calorimetry.

Although nanostructured phase-change materials (PCMs) are considered as the building blocks of next-generation phase-change memory and other emerging optoelectronic applications, the kinetics of the crystallization, the central property in switching, remains ambiguous in the high-temperature regime. Therefore, we present here an innovative exploration of the crystallization kinetics of Ge2Sb2Te5 (GST) nanoparticles (NPs) exploiting differential scanning calorimetry with ultrafast heating up to 40 000 K s-1. Our results demonstrate that the non-Arrhenius thermal dependence of viscosity at high temperature becomes an Arrhenius-like behavior when the glass transition is approached, indicating a fragile-to-strong (FS) crossover in the as-deposited amorphous GST NPs. The overall crystal growth rate of the GST NPs is unraveled as well. This unique feature of the FS crossover is favorable for memory applications as it is correlated to improved data retention. Furthermore, we show that methane incorporation during NP production enhances the stability of the amorphous NP phase (and thereby data retention), while a comparable maximum crystal growth rate is still observed. These results offer deep insight into the crystallization kinetics of nanostructured GST, paving the way for designing nonvolatile memories with PCM dimensions smaller than 20 nm.

Chaotic behavior in Casimir oscillators: A case study for phase-change materials.

Casimir forces between material surfaces at close proximity of less than 200 nm can lead to increased chaotic behavior of actuating devices depending on the strength of the Casimir interaction. We investigate these phenomena for phase-change materials in torsional oscillators, where the amorphous to crystalline phase transitions lead to transitions between high and low Casimir force and torque states, respectively, without material compositions. For a conservative system bifurcation curve and Poincare maps analysis show the absence of chaotic behavior but with the crystalline phase (high force-torque state) favoring more unstable behavior and stiction. However, for a nonconservative system chaotic behavior can take place introducing significant risk for stiction, which is again more pronounced for the crystalline phase. The latter illustrates the more general scenario that stronger Casimir forces and torques increase the possibility for chaotic behavior. The latter is making it impossible to predict whether stiction or stable actuation will occur on a long-term basis, and it is setting limitations in the design of micronano devices operating at short-range nanoscale separations.

Size-dependent and tunable crystallization of GeSbTe phase-change nanoparticles.

Chalcogenide-based nanostructured phase-change materials (PCMs) are considered promising building blocks for non-volatile memory due to their high write and read speeds, high data-storage density, and low power consumption. Top-down fabrication of PCM nanoparticles (NPs), however, often results in damage and deterioration of their useful properties. Gas-phase condensation based on magnetron sputtering offers an attractive and straightforward solution to continuously down-scale the PCMs into sub-lithographic sizes. Here we unprecedentedly present the size dependence of crystallization for Ge2Sb2Te5 (GST) NPs, whose production is currently highly challenging for chemical synthesis or top-down fabrication. Both amorphous and crystalline NPs have been produced with excellent size and composition control with average diameters varying between 8 and 17 nm. The size-dependent crystallization of these NPs was carefully analyzed through in-situ heating in a transmission electron microscope, where the crystallization temperatures (Tc) decrease when the NPs become smaller. Moreover, methane incorporation has been observed as an effective method to enhance the amorphous phase stability of the NPs. This work therefore elucidates that GST NPs synthesized by gas-phase condensation with tailored properties are promising alternatives in designing phase-change memories constrained by optical lithography limitations.

Synthesis and morphology of iron-iron oxide core-shell nanoparticles produced by high pressure gas condensation.

Core-shell structured Fe nanoparticles (NPs) produced by high pressure magnetron sputtering gas condensation were studied using transmission electron microscopy (TEM) techniques, electron diffraction, electron energy-loss spectroscopy (EELS), tomographic reconstruction, and Wulff shape construction analysis. The core-shell structure, which is composed of an Fe core surrounded by a maghemite (γ-Fe2O3) and/or magnetite (Fe3O4) shell, was confirmed by fast Fourier transform (FFT) analysis combined with EELS. It was found that the particle size and shape strongly depend on the gas environment. Moreover, extensive analysis showed that NPs with a size between 10-20 nm possess a truncated cubic morphology, which is confined by the 6 {100} planes that are truncated by the 12 {110} planes at different degrees. For NPs larger than 20 nm, the rhombic dodecahedron defined by the 12 {110} planes is the predominant crystal shape, while truncated rhombic dodecahedrons, as well as non-truncated and truncated cubic NPs, were also observed. The NPs without truncation showed a characteristic inward relaxation indicating that besides thermodynamics kinetics also plays a crucial role during particle growth.

Synthesis and exceptional thermal stability of Mg-based bimetallic nanoparticles during hydrogenation.

Here we report the extraordinary thermal stability of Mg rich bimetallic nanoparticles (NPs), which is important for hydrogen storage technology. The enhanced NP stability is accomplished because of two critical improvements: (i) no void development within NPs (nanoscale Kirkendall effect) during their formation and (ii) suppressed Mg evaporation and NP hollowing during Mg hydrogenation at elevated temperature. The mechanism leading to the improved thermal stability of Mg-based bimetallic NPs is shown to be due to MgH2 hydride formation before evaporation can take place. These findings were tested for various compositions of Mg with Ni, Cu, and Ti, which are interesting combinations of materials for hydrogen storage systems. To achieve this we first demonstrate the synthesis mechanism of Mg-Ni and Mg-Cu NPs, which is well controlled at the single particle level, in order to accomplish multi-shell, alloy and intermetallic structures of interest for hydrogen storage tests. Aberration corrected transmission electron microscopy was carried out to unravel the detailed atomic structure and composition of the bimetallic NPs after production, processing, and hydrogenation. Finally, a simple and effective methodology is proposed for tuning the composition of the Mg-based bimetallic NPs based on the temperature-dependent nucleation behavior of NPs in the gas-phase.

Tuning structural motifs and alloying of bulk immiscible Mo-Cu bimetallic nanoparticles by gas-phase synthesis.

Nowadays bimetallic nanoparticles (NPs) have emerged as key materials for important modern applications in nanoplasmonics, catalysis, biodiagnostics, and nanomagnetics. Consequently the control of bimetallic structural motifs with specific shapes provides increasing functionality and selectivity for related applications. However, producing bimetallic NPs with well controlled structural motifs still remains a formidable challenge. Hence, we present here a general methodology for gas phase synthesis of bimetallic NPs with distinctively different structural motifs ranging at a single particle level from a fully mixed alloy to core-shell, to onion (multi-shell), and finally to a Janus/dumbbell, with the same overall particle composition. These concepts are illustrated for Mo-Cu NPs, where the precise control of the bimetallic NPs with various degrees of chemical ordering, including different shapes from spherical to cube, is achieved by tailoring the energy and thermal environment that the NPs experience during their production. The initial state of NP growth, either in the liquid or in the solid state phase, has important implications for the different structural motifs and shapes of synthesized NPs. Finally we demonstrate that we are able to tune the alloying regime, for the otherwise bulk immiscible Mo-Cu, by achieving an increase of the critical size, below which alloying occurs, closely up to an order of magnitude. It is discovered that the critical size of the NP alloy is not only affected by controlled tuning of the alloying temperature but also by the particle shape.

Self-affine roughness influence on the friction coefficient for rubbers onto solid surfaces.

In this paper we investigate the influence of self-affine roughness on the friction coefficient mu(f) of a rubber body under incomplete contact onto a solid surface. The roughness is characterized by the rms amplitude w, the correlation length xi, and the roughness exponent H. It is shown that with increasing surface roughening at short and/or long length scales (decreasing H and/or increasing ratio w/xi, respectively), the maximum of the friction coefficient mu(f) shifts to lower sliding velocities. The latter occurs only for conditions of incomplete contact for small contact length scales lambda (>xi).