Microstructural Characterization Of Functional Materials And Studies Of Semiconductor Ultrafast Lattice Dynamics

The physical properties of functional materials are closely related to their microstructures.The further understanding of the microstructure is the basis for rational design and improvement of their physical properties.With the development of electron microscopy technology,more and more information on microstructure and dynamics can be discovered,which often play an important role in the physical properties of materials and their applications.This paper uses spherical-aberration corrected transmission electron microscope to study the structure of the misfit layered compounds Sr1-pCr2Se4-p,and finds that the incommensurate modulation structure along the c direction is the cause of the misfit layered structure(q1?4/5c*,q2?3/5c*);After that,the structure and physical properties of the layered material Na3Ni2BiO6 were studied,and Sr1.5Ni2BiO6 was obtained by the ion-replacement method.At the same time,this paper uses an ultrafast transmission electron microscope to study the ultrafast lattice dynamics of semiconductor Si,and observes the ultrafast expansion process of Si in the picosecond time scale,and finds that the thermal expansion behavior in the crystal plane direction,(111)and(220),has obvious anisotropy.This discovery can provide important physical information for the design and optimization of silicon-based high-speed optoelectronic devices.The work of this paper specifically includes the following aspects:(1)The ternary chalcogen compound A1-pCr2X4-p(A=Sr,Ba,Eu,Pb;X=S or Se)has a misfit layered structure and abundant physical properties.Studying of this complex crystal structure of this type is helpful to understand its physical properties.In this article,we analyzed the misfit layered structure and physical properties of Sr1-pCr2Se4-p.The structure of Sr1-pCr2Se4-p consists of a hexagonal framework with two channels filled with Sr6Cr2Se6 and Sr3Se atomic columns.These three substructures modulate each other in the c direction.Moreover,the diffuse scattering planes in Selected electron diffraction pattern(SAED)indicate that the filled atoms are in disorder.In addition,Sr1-pCr2Se4-p also owns semiconductor characteristics like other A1-pCr2X4-p.Due to the poor electron transmission capability of the ternary channel structure,its resistance is extremely large.At the same time,Sr1-pCr2Se4-p is ferrimagnet with Curie temperature Tc=150 K,remanence Br=0.069 emu,and the coercivity Hc=6.7 kOe.Magnetization reversal occurs at low temperatures with 10 Oe field applied.(2)03-type layered transition metal oxide Na3M(?)2M'(V)O6 is an important sodium ion electrode material,which can be used as the cathode of sodium ion battery for the high Na ion/extraction and insertion ability and high cycle stability.In this paper,Na3Ni2BiO6 was prepared by high-temperature solid-phase reaction in different atmospheres,and Sr1.5Ni2BiO6 was obtained by ion-replacement.The crystal structure was characterized by X-ray diffractometer(XRD),scanning electron microscope(SEM)and transmission electron microscope(TEM)techniques.And the magnetic properties were measured using a vibrating sample magnetometer(VSM)at low temperatures.This structure of Na3Ni2BiO6 is mainly determined by the honeycomb M2M'O6(M=Ni)layer,2/3 of which are the magnetic atom M and 1/3 the non-magnetic atom M'.This complex magnetic structure produces have received widespread attention for the possibility of magnetic frustration.Na3Ni2BiO6 shows long-range antiferromagnetic order at 11 K.The magnetic behavior of Sr1.5Ni2BiO6 is similar to Na3Ni2BiO6,and the Neel temperature is slightly higher.Furthermore,we synthesized Na3Ca2BiO6 and found that the introduction of Ca not only made it a completely paramagnet,but changed the entire honeycomb structure.(3)The ultrafast transmission electron microscope based on pump-probe technology is used to study the ultrafast lattice dynamics of the semiconductor Si.Under the irradiation of 520 um femtosecond laser(photon energy 2.38 eV),nanoparticles of Si(band gap 1.12 eV)were excited to produce lattice expansion.The evolution of lattice expansion over time was explored by ultrafast electron diffraction technology,and the crystal lattice was discovered the anisotropy of expansion along different crystal plane directions.Under the radiation of pulse laser with energy density of 22.5mJ/cm2,the expansion rate of the(111)crystal planes 0.25‰ smaller than that of(220).and its time constant of electron-phonon coupling is about 3 ps slower than(220).At the same time,we developed a data processing program for polycrystalline diffraction based on twodimensional Gaussian fitting,which made the data error around 0.02%,which ensured the credibility of the data results.