Characteristics of Silver Nanomaterials and Their Plasmon-Induced Joining Mechanism
Author:Liang Tian Shou
Supervisor:zhou de jian
Using nanofabrication technology to manufacture the new generation electronic devices is a frontier topic to break through the physical limit described by Moore’s law.Fully understanding the characteristics of nanomaterials and accurately controlling the joining methods of nanomaterials are two important contents of nanofabrication technology.Silver nanomaterials have many excellent properties related to force,light,electricity,magnetism and heat,which are widely used in fabrication of microelectronic devices.In view of the fact that the research for the properties and joining methods of silver nanomaterials is in experimental stage,molecular dynamics method is used to further study the mechanical and thermodynamic properties of silver nanomaterials,and the gridless method for molecular dynamics is used to reveal the plasmon-induced joining process and mechanism of silver nanomaterials.(1)Molecular dynamics(MD)was used to simulate the tensile behavior of a series of five-fold twin silver nanowires with different lengths.The correlation between fracture morphology and fracture mode of silver nanowires is found,and the atomic behavior and mechanism during tensile failure process were also revealed.It is found that the brittle-fractured silver nanowires exist longer fracture size and conical fracture shape,and the ductile-failured silver nanowires produces smaller fracture size and pitted fracture shape.Further analysis of atomic behavior reveal that the tensile failure process of silver nanowires is controlled by various mechanisms: the failure mechanisms of short nanowires are stable dislocation nucleation in the first stage and stable dislocation movement assisted by pore expansion in the second stage;and the fracture mechanisms of long nanowires are from unstable dislocation nucleation in the first stage to fast necking in the second stage.In addition,we presents a mathematical model for predicting the fracture mode of nanowires,which is not only suitable for nanowires with necking failure,but also suitable for slip-band fracture.The calculated results are in good agreement with the simulated results and the measured values in the literature.(2)The size-dependent thermodynamic melting behavior and mechanism of silver nanomaterials were studied.The size effect of "melting drop" of silver nanomaterials is verified by using atomic structure characteristics and atomic bond fluctuation information,and the proposed normalized Lindman index.The size-dependent melting mode is also obtained.From the point of view of atomic physical state,the evolution process of atoms of silver nanoparticles during melting process was analyzed,and the atomic melting behavior and essence in different melting modes were revealed.The results show that the thermal instability of smaller silver nanoparticles directly results in the solid-solid transition,while various crystallographic facets that make up the surface of larger silver nanoparticles exist different melting characteristics.we also verified that the formation and movement of the solid-liquid interface in silver nanoparticles are caused by the "point defect" due to thermal activation.In addition,the melting process and Rayleigh instability of silver nanowires with different crystal structures were also discussed.(3)The joining behavior of silver nanoparticles due to photothermal effect of plasmon was studied.Firstly,the spectral distribution,photothermal effect,local electric field enhancement and hot spot distribution of silver single/double sphere nanoparticles exposed to photoelectric field were qualitatively analyzed by using finite element method.The laws of interaction between silver nanoparticles and photoelectric field were obtained.Secondly,by introducing an adaptive energy conversion factor,the molecular dynamics calculation method of the local two-temperature model was improved,and the iterative process of the energy/temperature conversion between electrons and lattices of silver nanoparticles was realized,which solves the problem of meshless multi-physical field coupling in a finite atomic system.Furthermore,the modified method was used to simulate the plasmon-induced joining of double-sphere silver nanoparticles,which show that the thermal conductivity of silver nanoparticles electronic subsystem has negligible effect on the temperature change of atomic lattice;the magnitude of input light flux has a significant effect on the joining behavior of silver nanoparticles;the joining process of silver nanoparticles has no significant correlation with the pulse width in the range of femtosecond to several picoseconds;when the pulse width is larger than several picoseconds,the joining process of nanoparticles lags behind with the increase of the pulse width,and the final joining results are basically the same.(4)The joining process of coaxial five-fold twinned and single crystal silver nanowires due to photothermal effect of plasmon were studied,and the joining behavior and mechanism of nanowires under different conditions were revealed.Firstly,the correlation between local electric field enhancement and geometrical shape of the nanowires was qualitatively analyzed by using finite element method.It is show that the larger the local curvature of the nanowires is,the greater the electric field enhancement is.Secondly,according to the self-limiting characteristics occurred during nanowires joining process due to plasmon,a mathematical model of energy action factor related to gap size was proposed.The simulation results show that the model is able to successfully simulate the self-limiting effect during joining process.Then,the effect of energy magnitude on the joining behavior of five-fold twinned silver nanowires was studied,which show that within a certain energy range the regular atomic lattice structure and twin grain boundary are able to be restored after the nanowires are joined.It is found that the nearly perfect lattice reconstruction is caused by the deposition of atoms layer by layer on the low energy dense-packed crystal surface.The effect of gap size on the joining behavior of nanowires was also studied,which show that an ablation of nanowires may occurs when the gap is larger than a certain value.Finally,the joining behavior of single crystal silver nanowires with the same size was studied under the same conditions,which shows that the joined single crystal nanowires exbit grain boundary in the target region.Through the research of this paper,not only the basic theory of mechanics and thermodynamics of silver nanomaterials are improved,but also the joining behavior and mechanism of silver nanomaterials under the photothermal effect of plasmon are revealed.The research results provide theoretical basis for the future development of nanojoining technology,and promote the development of new micro-electronic devices manufacturing technology.