The melting mechanism in binary Pd0.25Ni0.75 nanoparticles: molecular dynamics simulations

The melting mechanism for Pd_0.25Ni_0.75 alloy nanoparticles (NPs) was investigated using molecular dynamics (MD) simulations with quantum Sutton-Chen many-body potentials. NPs of six different sizes ranging from 682 to 22,242 atoms were studied to observe the effect of size on the melting point. The melting temperatures of the NPs were estimated by following the changes in both the thermodynamic and structural quantities such as the total energy, heat capacity and Lindemann index. We also used a thermodynamics model to better estimate the melting point and to check the accuracy of MD simulations. We observed that the melting points of the NPs decreased as their sizes decreased. Although the MD simulations for the bulk system yielded higher melting temperatures because of the lack of a seed for the liquid phase, the melting temperatures determined for both the bulk material and the NPs are in good agreement with those predicted from the thermodynamics model. The melting mechanism proceeds in two steps: firstly, a liquid-like shell is formed in the outer regions of the NP with increasing temperature. The thickness of the liquid-like shell increases with increasing temperature until the shell reaches a critical thickness. Then, the entire Pd–Ni NP including core-related solid-like regions melts at once.     

Size effect in the melting and freezing behaviors of Al/Ti core-shell nanoparticles using molecular dynamics simulations

The thermal stability of Ti@Al core/shell nanoparticles with different sizes and components during continuous heating and cooling processes is examined by a molecular dynamics simulation with embedded atom method. The thermodynamic properties and structure evolution during continuous heating and cooling processes are investigated through the characterization of the potential energy, specific heat distribution, and radial distribution function(RDF). Our study shows that, for fixed Ti core size, the melting temperature decreases with Al shell thickness, while the crystallizing temperature and glass formation temperature increase with Al shell thickness. Diverse melting mechanisms have been discovered for different Ti core sized with fixed Al shell thickness nanoparticles. The melting temperature increases with the Ti core radius. The trend agrees well with the theoretical phase diagram of bimetallic nanoparticles. In addition, the glass phase formation of Al–Ti nanoparticles for the fast cooling rate of 12 K/ps, and the crystal phase formation for the low cooling rate of 0.15 K/ps. The icosahedron structure is formed in the frozen 4366 Al–Ti atoms for the low cooling rate.     

不同Ag壳厚度Au@Ag纳米粒子的调控制备表征及表面增强拉曼光谱的效应

采用化学还原法制备了以Au为核、包覆Ag的双金属核壳Au@Ag纳米粒子,并成功地用于表面增强拉曼光谱(SERS)分析测试。通过改变制备液中Ag/Au的量比来调控Ag壳包覆厚度。采用透射电子显微镜(TEM)和紫外-可见光谱仪(UV-Vis)对Au@Ag纳米粒子的构貌进行表征。TEM显示明显存在核壳结构,且Ag壳层随Ag/Au的量比的增加而逐渐变厚;UV-Vis表明随着Ag/Au的量比的增加,Au@Ag纳米粒子出现了Au核与Ag壳吸收峰的2个等离子体共振峰,同时伴随着Au峰的蓝移和Ag峰的红移。以双甲脒为分析物,考察了不同Ag/Au的量比时的Au@Ag纳米粒子的SERS活性。结果表明,SERS活性随Ag/Au的量比的增加先增大后减小,在6∶5时其SERS增强效应最佳,此时Ag壳厚度约为6 nm。以对巯基苯胺(4-ATP)、结晶紫(CV)和双甲脒为分析测试对象,对比了Au@Ag、Ag、Au 3种基底的SERS活性。结果表明,所制备的Au@Ag纳米粒子的SERS活性要明显优于单纯的Au、Ag纳米粒子。     

Emergent Upconversion Sustainable Micro‐Optical Trapping Device

Ultrathin‐thickness single‐junction Si‐based solar cells can be developed to enhance photoelectric conversion efficiency (PECE) approaching to Shockley–Queisser limit. However, loss of short circuit current is a crucial factor that dramatically affects PECE improvement. Even though many studies have focused on rare reflector architecture for facilitating near‐infrared radiation absorption, PECE is still constraint due to its fabrication cost. Herein, an upconversion sustainable micro‐optical trapping device is reported. Using a systematic procedure, a high upconversion performance core–shell‐nanoparticles (CSNPs) structure is synthesized. Accordingly, silica diatom microporous frustule is a good electromagnetic field localization chamber, upon which CSNPs are embedded through a microassemble synthesis. This emerging device can be support on ultrathin‐thickness single‐junction Si‐based solar cells as a rare absorber with its low preparation cost. In the experiment, CSNPs upconversion optical density by surface plasmon resonance of Au nanoparticle's enhancement can be increased five‐time greater than NaYF₄ without SiO₂ coating. A finite difference time domain simulation and real color luminescence images in this study are also demonstrated.     

Thermo-mechanical behavior of nano aluminum particles with oxide layers during melting

Molecular dynamics simulations were performed to study the thermo-mechanical behavior of nano aluminum particles coated with crystalline and amorphous oxide layers during melting. The analysis employs the Streitz–Mintmire potential, along with micro-canonical (NVE) and isobaric–isoenthalpic (NPH) ensembles. The effect of particle size in the range of 5–10 nm with oxide thickness in the range of 1–2.5 nm was investigated. The melting phenomenon was characterized using a combination of structural and thermodynamic parameters. Various fundamental processes, including structural changes, stress development, and phase transformations in both the aluminum core and the oxide shell, were examined and quantified systematically. The diffusion of aluminum cations through the oxide layer was also explored. In addition, a structural analysis was applied to determine the stress field in the oxide shell due to the volume dilatation in the aluminum core. In the particle-size range considered here, the oxide layer melts at ~1,100 K, substantially lower than the value for bulk alumina (2,327 K). The oxide thickness exerts a weak influence on the melting temperature of the shell. The aluminum core melts at a temperature considerably lower than its bulk value of 940 K, a situation comparable to that of a pure nano aluminum particle. This study is an important milestone in the development of a multi-scale theory for the ignition and combustion of nano-particulate aluminum.     

Localized Surface Plasmon Resonance of Cu@Cu2O core-shell nanoparticles: Absorption, Scattering and Luminescence

By co-deposition via RF-Sputtering and RF-PECVD methods and using Cu target and acetylene gas, we prepared Cu@Cu₂O core-shell nanoparticles on the a-C:H thin film at room temperature. Mie absorption of Cu cores, scattering from Cu₂O shell and luminescence that rises from carrier transfer in Cu@Cu₂O interface were employed to fit the whole range of visible extinction spectrum of these core-shells. From simulation it was found that scattering and luminescence have an important effect on the energy, width and shape of LSPR absorption peak. Shift of LSPR peak is more affected by the dielectric coefficient of shell than Cu core size particularly for Cu core diameter above 4 nm. Also, the LSPR absorption peak is damped by decreasing Cu core size and dielectric coefficient of shell. The energy of LSPR absorption peak is independent of shell thickness and host dielectric coefficient. The LSPR peak is damped by increasing shell thickness and host dielectric coefficient too. The scattering contribution in extinction spectra was affected more by shell size than dielectric coefficient. These points are important for detection techniques based on LSPR peak.     

金纳米球壳光学吸收的Mie理论分析

基于Mie散射理论研究了金壳厚度变化、内核尺寸变化及内核介质变化下金纳米球壳的吸收光谱.研究发现,随着金壳厚度的增加,颗粒光学吸收增加到最大值后逐渐降低;随着内核尺寸逐渐增加,金壳颗粒的光学吸收最大值逐渐减小.此外,还发现随着内核介电常数的增大,颗粒的光学吸收逐渐减弱,当内核为空心时,吸收最强.利用等离激元杂化理论及自由电子和振荡电子变化的竞争机制对上述现象进行了理论分析. 关键词： 金纳米球壳 等离激元共振 吸收光谱     

金纳米球壳表面等离激元共振波长调谐特性研究

理论研究了金纳米球壳的几何结构参数, 及物理参量对局 域表面等离激元共振波长调谐特性的影响. 结果表明, 随着壳层厚度的增大, 球壳消光共振峰先蓝移后红移, 高阶峰转向时对应的壳层厚度比低阶峰大, 且该厚度与球壳内径的比值随内径尺寸的增大而减小, 随内核材料或外界环境介电常数的增大而增大, 散射共振峰也有类似的移动规律. 利用电子杂化效应和相位延迟效应对该现象进行了理论解释.     

Study of the effect of varying core diameter,shell thickness and strain velocity on the tensile properties of single crystals of Cu–Ag core–shell nanowire using molecular dynamics simulations

Core–shell type nanostructures show exceptional properties due to their unique structure having a central solid core of one type and an outer thin shell of another type which draw immense attention among researchers. In this study, molecular dynamics simulations are carried out on single crystals of copper–silver core–shell nanowires having wire diameter ranging from 9 to 30 nm with varying core diameter, shell thickness, and strain velocity. The tensile properties like yield strength, ultimate tensile strength, and Young’s modulus are studied and correlated by varying one parameter at a time and keeping the other two parameters constant. The results obtained for a fixed wire size and different strain velocities were extrapolated to calculate the tensile properties like yield strength and Young’s modulus at standard strain rate of 1 mm/min. The results show ultra-high tensile properties of copper–silver core–shell nanowires, several times than that of bulk copper and silver. These copper–silver core–shell nanowires can be used as a reinforcing agent in bulk metal matrix for developing ultra-high strength nanocomposites.     

Light trapping and optical absorption enhancement in vertical semiconductor Si/SiO_2 nanowire arrays

The full potential of optical absorption property must be further cultivated before silicon(Si) semiconductor nanowire(NW) arrays become available for mainstream applications in optoelectronic devices. In this paper, we demonstrate both experimentally and theoretically that an SiO_2 coating can substantially improve the absorption of light in Si NW arrays.When the transparent SiO_2 shell is coated on the outer layer of Si NW, the incident light penetrates better into the absorbing NW core. We provide the detailed theoretical analysis by a combination of finite-difference time-domain(FDTD) analysis.It is demonstrated that increasing the thickness of the dielectric shell, we achieve 1.72 times stronger absorption in the NWs than in uncoated NWs.     

金银三层纳米管局域表面等离激元共振特性研究

相对于单一金属纳米材料,二金属复合纳米材料具有更大的潜在应用价值.基于时域有限差分方法,研究了SiO₂-Ag-Au和SiO₂-Au-Ag二金属三层纳米管的消光光谱,并对其局域表面等离激元共振(Localized Surface Plasmon Resonance,LSPR)特性进行了分析.研究发现,内核尺寸变大将导致上述两种金属纳米管LSPR峰红移;内层金属及外层金属壳层厚度增大均会导致其LSPR峰蓝移.银壳厚度变化对纳米管LSPR的调制作用大于金壳厚度变化造成的影响.上述现象可以利用等离激元杂化理论及自由电子和振荡电子变化的竞争机制进行分析.     

Non-monotonic size effects on the structure and thermodynamics of Coulomb clusters in three-dimensional harmonic traps

Finite-size effects on the static and thermodynamical properties of small three-dimensional clusters of identical charged particles confined by an harmonic trap are investigated using global optimization and numerical simulations. The relative stabilities of clusters containing up to 100 particles are estimated from the second energy derivatives, as well as from the energy gap between the two lowest-energy structures at a given size. We also provide a lower bound for the number of permutationally independent minima, as a function of size, up to n=75. Molecular dynamics and exchange Monte Carlo simulations are performed to get insight into the finite temperature behaviour of these clusters. By focusing on specific sizes, we illustrate the interplay between the stable structures, the possible competition between different isomers, and the melting point. In particular, we find that the orientational melting phenomenon known in two-dimensional clusters has an equivalent form in some three-dimensional clusters. The vibrational spectra, computed for all sizes up to 100, shows an increasing number of low-frequency modes, but comparing to hydrodynamical theory reveals strong correlation effects. Finally, we investigate the effects of the trap anisotropy on the general shape of Coulomb clusters, and on the melting point of a selected case.     

A Sensitive “Optical Nose” for Detection of Volatile Organic Molecules Based on Au@MOFs Nanoparticle Arrays through Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) is an effective technique for detecting toxic gas and volatile organic molecules (VOMs); however, recent SERS-based gas sensors have disadvantages and lack an effective approach to capture toxic gas and insufficient reproducibility of SERS substrates. Herein, a facile strategy is developed to integrate metal-organic frameworks with Au nanoparticle (NP) arrays to form Au@ZIF-8 NP arrays, which can be used as an “optical nose” based on SERS to detect toxic VOMs with good reproducibility and sensitivity. Toluene as a target molecule is recognized at ppm levels by the Au@ZIF-8 NP arrays in situ. And the analytical enhancement factor of Au@ZIF-8 NP arrays for toluene is about 1.2 × 10⁵. Importantly, this SERS substrate can also detect the 1-butanol molecule, which provides an idea for designing a universal VOM sensor. In addition, the coating method of the ZIF-8 shell can be extended to synthetize various NPs@ZIF-8 core–shell composites, such as Au nanospheres@ZIF-8, Au@Ag nanorods@ZIF-8, PS microspheres@ZIF-8, and Fe₂O₃ microellipsoids@ZIF-8 composites.     

Spatial dependence of the local field enhancement in dielectric shell coated silver nanospheres

The dependence of the local field factor around dielectric shell coated silver nanospheres was investigated by theoretical calculation as a function of the spatial distance. The local field factors in the dielectric shell are sensitive to the distance from particle center and shell thickness. When the shell dielectric constant is greater than that of surrounding medium, the maximum of local field factor at inner surface of the shell red shift and increases nonlinearly with increasing the shell thickness. On the contrary, when shell dielectric constant is less than that of surrounding medium, increasing the shell thickness leads the maximum of local field factor at inner surface blue shifts and decreases nonlinearly. However, with increasing the shell thickness, the maximum of local field factor at exterior surface of the shell always decrease nonlinearly. Furthermore, with increasing shell thickness, all these variations get gentle approach to a constant value when the shell thickness is two times of the core radius. When the core and shell diameter have fixed values, the local field factors in dielectric shell decrease with increasing the distance from particle center, but the peak position is not sensitive to the distance.     

Atomistic tight-binding computations of the structural and optical properties of CdTe/CdX (X=S and Se)/ZnS core/shell/shell nanocrystals

A study of CdTe/CdX (X=S and Se)/ZnS core/shell/shell nanocrystals is carried out using atomistic tight-binding theory and the configuration interaction method to provide information for applications in bioimaging, biolabeling, display devices and near-infrared electronic instruments. The calculations yield the dependences of the internal and external passivated shells on the natural behaviours of CdTe/CdX (X=S and Se)/ZnS core/shell/shell nanocrystals. The reduction of the optical band gaps is observed with increasing numbers of monolayers in the external ZnS shell due to quantum confinement. Interestingly, the optical band gaps of CdTe/CdS/ZnS core/shell/shell nanocrystals are greater than those of CdTe/CdSe/ZnS core/shell/shell nanocrystals. In the presence of an external ZnS-coated shell, electron–hole wave function overlaps, oscillation strengths, ground-state exchange energies and Stokes shift are improved, whereas ground-state coulomb energies and fine-structure splitting are reduced. The oscillation strengths, Stokes shift and fine-structure splitting are reduced with the increase in external ZnS shell thickness. The oscillation strengths, Stokes shift and fine-structure splitting of CdTe/CdS/ZnS core/shell/shell nanocrystals are larger than those of CdTe/CdSe/ZnS core/shell/shell nanocrystals. Reduction of the atomistic electron–hole interactions is observed with increasing external ZnS shell size. The strong electron–hole interactions are more probed in CdTe/CdS/ZnS core/shell/shell nanocrystals than in CdTe/CdSe/ZnS core/shell/shell nanocrystals.     

Thermally activated phase transitions in Fe-Ni core-shell nanoparticles

Fe-Ni core-shell nanoparticles are versatile functional materials, and their thermal stabilities are crucial for their performances in operating conditions. In this study, the thermodynamic behaviors of Fe-Ni core-shell nanoparticles are examined under continuous heating. The solid–solid phase transition from body centered cubic (bcc) to face centered cubic (fcc) in the Fe core is identified. The transition is accompanied with the generation of stacking faults around the core-shell interface, which notably lowers the melting points of the Fe-Ni core-shell nanoparticles and causes even worse thermal stability compared with Ni ones. Moreover, the temperature of the structural transformation is shown to be tuned by modifying the Ni shell thickness. Finally, the stress distributions of the core and the shell are also explored. The relevant results could be helpful for the design, preparation, and utilization of Fe-based nanomaterials.     

Pt-Au核-壳结构纳米粒子热稳定性的分子动力学研究

采用分子动力学方法结合嵌入原子势, 对Pt-Au核-壳纳米粒子的热稳定性进行了研究. 计算结果表明: Pt-Au纳米粒子的熔点明显高于Au纳米粒子而低于Pt纳米粒子. 通过计算Lindemann指数发现: 壳层中的Au首先熔化, 然后逐渐向内部扩展, 最终导致核中的Pt完全熔化; 熔化所经历的温度区间明显宽于单质纳米粒子, 而且该熔化过程呈现典型的两阶段熔化特征; 在两次熔化之间, 存在着固(核)液(壳)共存的结构. 关键词： 纳米粒子 熔化 分子动力学     

Investigation of the radiative lifetime in core–shell CdSe/ZnS and CdSe/ZnSe quantum dots

Using the one band effective mass approximation model we computed the optical properties of the spherical shaped CdSe/ZnS and Cdse/ZnSe core–shell quantum dot (CSQD). For each structure we calculated the charge carrier energies and corresponding wave functions. We investigated the dependence of the carrier energies on various parameters of the CSQD, including its size. Then we calculated the radiative recombination lifetime for the two types of CSQDs nanocrystals. We found that as the size of the dot is increased the optical gap of CSQD is reduced, resulting in a reduction in electron energies and an increase in hole energies. We have shown that the radiative recombination lifetime in the CdSe/ZnS and CdSe/ZnSe CSQDs decreased by increasing the shell thickness around the core of the QD. We also showed that the radiative lifetime in the CdSe/ZnS is less than that in the CdSe/ZnSe CSQDs and is sensitive to the size and nature of shell of the semiconductor's material.     

Research into the rationality and the application scopes of different melting models of nanoparticles

A rational melting model is indispensable to address the fundamental issue regarding the melting of nanoparticles. To ascertain the rationality and the application scopes of the three classical thermodynamic models, namely Pawlow, Rie, and Reiss melting models, corresponding accurate equations for size-dependent melting temperature of nanoparticles were derived. Comparison of the melting temperatures of Au, Al, and Sn nanoparticles calculated by the accurate equations with available experimental results demonstrates that both Reiss and Rie melting models are rational and capable of accurately describing the melting behaviors of nanoparticles at different melting stages. The former (surface pre-melting) is applicable to the stage from initial melting to critical thickness of liquid shell, while the latter (solid particles surrounded by a great deal of liquid) from the critical thickness to complete melting. The melting temperatures calculated by the accurate equation based on Reiss melting model are in good agreement with experimental results within the whole size range of calculation compared with those by other theoretical models. In addition, the critical thickness of liquid shell is found to decrease with particle size decreasing and presents a linear variation with particle size. The accurate thermodynamic equations based on Reiss and Rie melting models enable us to quantitatively and conveniently predict and explain the melting behaviors of nanoparticles at all size range in the whole melting process.

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Graphical abstract Both Reiss and Rie melting models are rational and capable of accurately describing the melting behaviors of nanoparticles at different melting stages. The former is applicable to the stage from initial melting to critical thickness of liquid shell, while the latter from the critical thickness to complete melting. The critical thickness of liquid shell decreases with decreasing particle size and a linear relationship between them is observed. This paper provides us an effective and convenient method to address the fundamental issue regarding the melting temperature of nanoparticles.

     

激光诱导充压柱壳破坏模式与参数阈值分析

通过数值计算模拟了激光诱导充压柱壳的热力破坏效应,研究了典型结构的动态爆裂过程,获得的破坏模式与实验结果基本一致。给出了三类典型破坏模式及其对应的参数范围,探讨了各类破坏模式的形成机理,并分析了不同光斑尺寸、壳体厚度条件下热软化效应对破坏内压阈值的影响,以及预内压与破坏时间的关系。研究结果表明:光斑半径越大、热软化程度越高,柱壳的破坏内压阈值越低,且破坏内压阈值随着壳体厚度的减小呈线性下降;给定激光参数和壳体参数下破坏时间随预充内压增大而减小并呈二次函数关系。给出了一种通过热软化程度预估激光诱导充压柱壳破坏时间的方法。