Investigation On The Preparation,Phase And Structure Evolution Of Metallic (Alloy) Nanoparticles Based On In-Situ Electron Microscope

The fast developments of microelectronics have led to the continuous reduction of the critical dimension of micro-/nano-devices.As the dimension size decreases,the physical and chemical properties of nanometals and nanoalloys may be different from their bulk counterparts.Whether the theories established on bulk materials can still be valid on nanomaterials is unclear.Therefore,serving as the frame or interconnecting components,the nanometals are required to be thoroughly examined for their properties and behaviors so as to estimate whether they still fulfill the demand of next-generation devices.At present,researches in this field still face many challenges due to the requirements of precise manipulation of nanometals.On this issue,this thesis tries to use an electron beam in a TEM to prepare ultra-clean nanometals and nanoalloys,and to use them to investigate their structures,phases,properties,stability,etc.Both the extraordinary behaviors and mechanisms are explored and discussed.The main research contents and findings are summarized as follows:1.In-situ TEM-EAD fabrication of multifarious metallic nanoparticlesAn in-situ TEM-EAD fabrication method has been developed.Using this method,diverse nanoparticles have been fabricated.For Zn,Mg,Mn,Bi,and Pb precursors,metallic nanoparticles are obtained.For Sb,Ge,and Cr precursors,the yielded nanoparticles are the mixture of nanometals and nano-oxides.For Ag,Cu,Al and Ni precursors,only oxide nanoparticles are obtained.The main decisive factors determining the TEM-EAD process are the electron beam intensity and the saturated vapor pressure of the precursor material.The higher the saturated vapor pressure is,the easier the nanoparticles will be obtained.2.Structure and phase behaviors of Pb Bi bimetallic nanoparticles（1）Pb Bi alloy nanoparticles are obtained by the TEM-EAD method.It is found that the as-fabricated amorphous nanoalloys will precipitate Bi under electron beam irradiation,and the nanoparticles will be transformed into Janus heterostructures with featured morphologies.A model for the structure and phase evolution is proposed.The nano-phase evolution is compared with the bulk phase diagram.Similarities and differences between the segregations and their mechanisms lying behind are explored and discussed.Surface diffusion is supposed to be a key factor that causes the differences.（2）In addition to Janus type heterostructures（indicated as Type-Ⅰ）,other types of heterostructures have been found,which are named Type-Ⅱ,Type-Ⅲ,and Type-Ⅳ,respectively.Electron beam irradiation and in-situ heating experiments show that these different types of heterostructures have different thermal stability.Type-Ⅰ（Janus structure）is the most stable structure,while Type-Ⅱ,Type-Ⅲand Type-Ⅳare metastable.The TEM-EAD method provides an important chance to reveal the dynamic structure and phase evolution of nanoalloys which is hard to be observed by conventional methods due to the strict formation and preservation conditions.（3）The influences of surface conditions on the structure and phase evolution of Pb Bi nanoparticles are investigated.By comparing the evolutions of Pb Bi bimetallic nanoparticles with and without surface oxidation,surface chemistry-determined structure and phase behaviors are revealed.Under the irradiation of electron beam,the Pb Bi nanoparticles with oxide shells will be eroded,and Pb O_x hollow structures will form due to the Knotek-Feibelman effect.Generally,three types of hollow structures are obtained due to the different surface conditions,and a model is provided to illustrate their shape evolutions.3.Structure and phase of Mg Zn bimetallic nanoparticles（1）The TEM-EAD method is extended to fabricate Mg Zn bimetallic nanoparticles.The obtained Mg-Zn bimetallic nanoparticles have four different assembly morphologies,which are defined as Type-i,Type-ii,Type-iii,and Type-iv,respectively.The differences in phase between these different nanoparticle types are ascribed to the different distances of them from the precursor material.A thermal gradient is supposed to exist and leads to the different degrees of phase segregation.（2）The microstructures of the Mg Zn bimetallic nanoparticles are studied.It is found that the microstructures of Type-iv Mg Zn nanoparticles comprise Mg Zn₂ or Mg₄Zn₇ phases besides pure Mg.Type-iii is an amorphous solid solution alloy.Type-ii comprises Mg nanocrystals and nanoalloys with poor crystallinity.Type-i possesses complex structures,including Janus phases of Mg crystal+quasicrystal,Mg+Mg₂Zn₁₁,and Mg+Mg Zn₂.It is speculated that the transient aging environment during the formation of NPs is the main reason for the difference in microstructure.