Research And Development Of Ultrafast Transmission Electron Microscope And Structural Dynamics Of Nanomaterials

In recent years,the research on the ultra-fast motion process in the micro world has received more and more attention.Here we mainly introduces the construction and improvement of the first ultra-fast transmission electron microscope(UTEM)in China and the study of lattice dynamics process for multi-walled nanotube materials,including multi-walled carbon nanotubes and multi-walled boron nitride nanotubes.In addition,the in-situ heating technique was used to explore the anisotropic thermal expansion process of the multi-meter nanotubes.The thesis is unfolded as follows:1.Remarkable interlayer expansion and inner-layer inward contraction in the multi-walled boron nitride nanotubes(BNNTs)as the specimen temperature increases using in situ heating transmission electron microscopy.We measured the thermal expansion coefficients of multi-wall boron-nitrogen nanotubes in the radial and axial direction.Interestingly,a negative expansion of the thermal expansion coefficient arises with the linear temperature change in the axial direction.We believe that the observed inward contraction is due to the presence of strong constrain of the outer-layers on the radial expansion in the tubular structure upon in situ heating.The rise of specimen temperature upon heating can create pressure and stress toward the tubular center,which drive the lattice motion and yield inner diameter contraction for the multi-walled BNNTs.Using a simple model involving a wave-like pattern of layer-wise distortion,we discuss this peculiar structural alterations and the anisotropic thermal expansion properties for the tubular structures.2.The study of the electron-driven lattice dynamics of multi-walled carbon nanotubes(MWCNTs)with varying laser fluence and initial specimen temperature using the electron diffraction mode of ultra-fast electron microscopy.Our photoexcitation experiments demonstrate that nonthermal atomic motions in MWCNTs results in rapid intralayer expansions and interlayer contractions,whose strengths and rates depend on pump fluence.Our lab initio calculations support these findings and reveal that electrons excited from the?to the?*orbitals in carbon tube weaken the intralayer bonds while strengthen the?bonds along the radial direction.Moreover,by probing structural dynamics of MWCNTs at initial temperature of 300and 100 K,we uncover the concomitance of thermal and nonthermal dynamical processes and their mutual influence in MWCNTs.3.The study of ultrafast structural dynamics of multi-walled boron nitride nanotubes upon femtosecond optical excitation using ultrafast electron diffraction in a transmission electron microscope.Our experiments reveal the highly anisotropic lattice dynamics of BNNTS,especially in the non-thermal lattice changes directly driven by carriers,on a time scale with carbon nanotubes of the same tubular structure.The difference is that the axial non-thermal expansion and the radial non-thermal shrinkage reach a maximum value of 15 ps.In addition,we also found that the lattice heating process caused by carriers involves two time-scale lattice dynamic processes in the BNNTS,one can be understood as due to the electroacoustic coupling process,and the other is the Auger compound effect.Meanwhile,we used different laser energy densities to study the three-photon absorption process of boron-nitrogen nanotubes,and determined that the three-photon absorption coefficient and saturation power were 10~-1616 cm~3/W~2 and 50 GW/cm~2,respectively.