Superheating of small solid-argon bubbles in aluminum

Interfacial melting mechanism of nanocrystals determined by interfacial energy and interfacial stress

To clarify the interface melting mechanism, a unified analytical expression was developed to describe the depression and superheating of T_m(D) functions for metallic nanoparticles, nanostructures, and nanoparticles embedded in a coherent or incoherent interface. T_m(D) functions are determined by the sign of γ_ss, f_ss (or γ_sl and f_sl), and D₀ as caused by the change of interface environments. We found that there is TCIm(D) > TNSm(D) > TIIm(D) > TNPsm(D) for Ag nanocrystals within different interfaces. Moreover, for a given size, T_m(D)/T_m(∞) decreases with the reduction of γ_ss/f_ss for nanoparticles, nanostructures and nanoparticles embedded in incoherent interfaces, while an opposite trend occurs for the coherent interfaces. In addition, we also found that there is TNPsm(D)/T_m(∞) < TIIm(D)/T_m(∞) < TNSm(D)/T_m(∞), which is in agreement with the relation of γNPssl/fNPssl < γIIss/fIIss < γNSss/fNSss. By analyzing the γ_ss(D) (or γ_sl(D)), f_ss(D) (or f_sl(D)) and γ_ss(D)/f_ss(D) (or γ_sl(D)/f_sl(D)) functions of Ag nanocrystals and comparing with their T_m(D) functions, it is found that there is a high consistency between the variation of γ_ss(D)/f_ss(D) (γ_sl(D)/f_sl(D)) and T_m(D)/T_m, which reveals that the size dependence of T_m(D)/T_m is determined by γ_ss(D)/f_ss(D) (or γ_sl(D)/f_sl(D)). Our predictions show a good agreement with the available theoretical and experimental results.

Effect of density and Z-contrast on the visibility of noble gas precipitates and voids with insights from Monte-Carlo simulations

In this work, a detailed analysis of He, Ne, Ar, Kr and Xe precipitates in a complex borosilicate glass using transmission electron microscopy (TEM) with in-situ ion implantation is presented. With in-situ monitoring, the real-time dynamics of precipitate and void evolution under ion implantation was followed. Using appropriate equations of state and, Monte-Carlo simulations to supplement the TEM images, we then discuss in detail the possibility and ways of differentiating the precipitates of various noble gases from empty voids. It is shown that all the noble gases precipitate as inclusions of supercritical fluid. With the aid of the simulations, the crucial role played by the size and density of the precipitates and atomic number of the gas atoms in affecting the visibility of the precipitates is highlighted. The results show that the precipitates and voids can be unambiguously differentiated in the case of Xe and Kr whereas the precipitates of other lighter noble gases cannot be differentiated from the voids. However, the precipitate and void evolution under ion irradiation follow different dynamics, knowledge of which allows one to differentiate between the precipitates and voids even for lighter noble gases. Besides shedding light on the subject of noble gas precipitation and identification of the precipitates and voids, the study highlights the complexity in dissociating the behaviour of voids from the process of precipitate re-solution. This type of knowledge is pivotal in developing models describing the evolution of precipitates, voids and macroscopic porosity in a number of materials.

Interface-induced ordering of gas molecules confined in a small space

The thermodynamic properties of gases have been understood primarily through phase diagrams of bulk gases. However, observations of gases confined in a nanometer space have posed a challenge to the principles of classical thermodynamics. Here, we investigated interfacial structures comprising either O₂ or N₂ between water and a hydrophobic solid surface by using advanced atomic force microscopy techniques. Ordered epitaxial layers and cap-shaped nanostructures were observed. In addition, pancake-shaped disordered layers that had grown on top of the epitaxial base layers were observed in oxygen-supersaturated water. We propose that hydrophobic solid surfaces provide low-chemical-potential sites at which gas molecules dissolved in water can be adsorbed. The structures are further stabilized by interfacial water. Here we show that gas molecules can agglomerate into a condensed form when confined in a sufficiently small space under ambient conditions. The crystalline solid surface may even induce a solid-gas state when the gas-substrate interaction is significantly stronger than the gas-gas interaction. The ordering and thermodynamic properties of the confined gases are determined primarily according to interfacial interactions.

Phase change nanocomposites with tunable melting temperature and thermal energy storage density

Size-dependent melting decouples melting temperature from chemical composition and provides a new design variable for phase change material applications. To demonstrate this potential, we create nanocomposites that exhibit stable and tunable melting temperatures through numerous melt-freeze cycles. These composites consist of a monodisperse ensemble of Bi nanoparticles (NPs) embedded in a polyimide (PI) resin matrix. The Bi NPs operate as the phase change component whereas the PI resin matrix prevents nanoparticle coalescence during melt-freeze cycles. We tune melting temperature and enthalpy of fusion in these composites by varying the NP diameter. Adjusting the NP volume fraction also controls the composite's thermal energy storage density. Hence it is possible to leverage size effects to tune phase change temperature and energy density in phase change materials.

Modeling of superheating and undercooling of strained semiconductor nanocrystals in SiO2

Although superheating and undercooling of nanocrystals have been observed and simply modeled, physical aspects of the nanocrystals, like strain, have previously been ignored. In this article the size dependence of the melting and solidification point temperature of semiconductor nanocrystals embedded in an interface epitaxy free matrix is modeled, taking into account the strain state of the nanocrystals. A configurational entropy term is introduced to render the model applicable to any crystal size, including bulk. The model is validated on the system of InSb nanocrystals embedded in SiO(2) under a high compressive strain.

The kinetics of homogeneous melting beyond the limit of superheating

Molecular dynamics simulation is used to study the time-scales involved in the homogeneous melting of a superheated crystal. The interaction model used is an embedded-atom model for Fe developed in previous work, and the melting process is simulated in the microcanonical (N, V, E) ensemble. We study periodically repeated systems containing from 96 to 7776 atoms, and the initial system is always the perfect crystal without free surfaces or other defects. For each chosen total energy E and number of atoms N, we perform several hundred statistically independent simulations, with each simulation lasting for between 500 ps and 10 ns, in order to gather statistics for the waiting time τ(w) before melting occurs. We find that the probability distribution of τ(w) is roughly exponential, and that the mean value <τ(w)> depends strongly on the excess of the initial steady temperature of the crystal above the superheating limit identified by other researchers. The mean <τ(w)> also depends strongly on system size in a way that we have quantified. For very small systems of ~100 atoms, we observe a persistent alternation between the solid and liquid states, and we explain why this happens. Our results allow us to draw conclusions about the reliability of the recently proposed Z method for determining the melting properties of simulated materials and to suggest ways of correcting for the errors of the method.

Plasmon excitations by photoelectron emission from rare gas nanobubbles in aluminum

Aluminum bulk, surface, and multiple plasmons have been observed in the core-level spectra of rare gas (Ne, Ar, and Xe) nanobubbles in Al, whose intensities are even higher than those of Al metal. Both intrinsic and extrinsic bulk plasmons are detected, but they exhibit diametrically opposite intensity variation due to change in the size and implantation depth of the bubbles. Furthermore, the existence of bubble surface plasmon is demonstrated.

Large melting-point hysteresis of Ge nanocrystals embedded in SiO2

The melting behavior of Ge nanocrystals embedded within SiO2 is evaluated using in situ transmission electron microscopy. The observed melting-point hysteresis is large (+/-17%) and nearly symmetric about the bulk melting point. This hysteresis is modeled successfully using classical nucleation theory without the need to invoke epitaxy.

Effect of stress on melting and freezing in nanopores

A thermodynamic treatment of the freezing of fluids confined to nanosized closed pores is presented. The model includes the effects of pressure in the liquid, the volume change on solidification, and the strain energy in both the solidifying material and the wall material. When applied to the system of Pb droplets in Al, the model predicts an elevation of the melting point, in agreement with experiment.

Superheating and induced melting at semiconductor interfaces

We present ab initio density-functional simulations of the state of several semiconductor surfaces at temperatures near the bulk melting temperatures. We find that the solid-liquid phase-transition temperature at the surface can be altered via a microscopic (single-monolayer) coating with a different lattice-matched semiconducting material. Our results show that a single-monolayer GaAs coating on a Ge(110) surface above the Ge melting temperature can dramatically reduce the diffusion coefficient of the germanium atoms, going so far as to prevent melting of the bulk layers on the 10 ps time scale. In contrast, a single-monolayer coating of Ge on a GaAs(110) surface introduces defects into the bulk and induces melting of the top layer of GaAs atoms 300 K below the GaAs melting point. To our knowledge, these calculations represent the first ab initio investigation of the superheating and induced melting phenomena.

Argon nanobubbles in Al(111): a photoemission study

Two fundamental manifestations of Al conduction electron response to Ar atom core hole in the final state of photoemission have been studied in implanted Ar bubbles in Al(111). Ar 2p binding energy and the Doniach-Sunjić asymmetry of the core-level line shape vary systematically as functions of Ar+ implantation energy and number of ions bombarded (fluence). The observations are explained by relating the strength of Al conduction electron screening to the size of the Ar nanobubbles.

Extreme superheating and supercooling of encapsulated metals in fullerenelike shells

Nanometer-sized tin and lead crystals exhibit drastically altered melting and solidification behavior when encapsulated in fullerenelike graphitic shells. The melting transitions of encapsulated Sn and Pb nanocrystals are shown in an in situ electron microscopy study to occur at unexpectedly high temperatures, significantly higher than the melting point of the corresponding bulk materials. Atomistic simulations are used to show that the driving force for superheating is a pressure buildup of up to 3 GPa, that prevails inside graphitic shells under electron irradiation.

Gettering of metals by voids in silicon

The gettering efficiency of copper and platinum by cavities formed in silicon after high dose helium implantation and thermal processing has been investigated. The formation of helium bubbles and their evolution into cavities has been investigated by transmission electron microscopy; the measured values of void density, diameter and the width of the void layer can be interpreted by assuming a simple coalescence model. Metal impurities intentionally introduced in silicon by ion implantation are efficiently gettered inside these cavities, probably due to the large amount of unsatured bonds at the void internal surface. Processing at temperatures higher than 1000oC causes a release of the trapped metal atoms which can be gettered again by repeating the process. The method is demonstrated on real devices such as large area diodes (a particle detector) and bipolar transistors. The capability to localize in depth and across the wafer surface on the gettering sites allows the development of a new gettering engineering.