Point-defects Diffusion And Phonon Trapping In Nanowires

Nanowires have become an important element in micro and nano electromechanical systems(MEMS/NEMS),energy collection/conversion/storage and nanoscale sensors.Accordingly,these nanowires' physical properties,such as mechanical,electrical and thermal properties,play dominant role on the performance of devices.In recent years,lots of research about the preparation and properties of nanowires have been conducted.But most of them are based on the prefect single crystal structure.In experiments,these nanowire structures are usually exposed in complicated multi-field coupling environments and prepared as imperfect material including various crystal defects.However,the research on these complex problems is relatively lagged although they are very important and can not be avoided.Here by using molecular dynamics,finite element method and theoretical derivation,we studied the special mechanical response induced by point defects' diffusion and the thermoelectric property of nanowires enhanced by phonon trapping.The main points could be summarized as follows.1)The ultra-strong anelastic property is discovered in the Zn O nanowire with point defects.With theoretical derivation,a new theoretical continuum model is proposed for point defects diffusion in nanowire.By this model,the ultra-strong anelastic behavior could be explained perfectly,which is caused by long-range diffusion of point defects in the Zn O nanowire.The applied stress gradient within the nanowire induces the chemical potential gradient of point defects.It drives the point defects to diffuse in long distance,and also makes the mechanical properties of nanowires to be time-dependent.The results of the theoretical model are verified by the experimental results.Based on this model,we also extend the typical Gorsky relaxation to non-linear anelasticity for the diffusion of multiple point defects.2)The coupling effect between relaxation behavior and mechanical vibration of nanowire is revealed.By coupling diffusion equation and vibration equation of nanowire,the mechanical response behavior of nanowires with point defects is studied here.And the dependence on all relevant nanowire parameters,such as size,Young's modulus,density,diffusion coefficient of point defects,etc.are considered here.It is also explained that why some bended nanowires need to vibrate violently after the release while other nanowires are slowly returning to its original position without vibration.Meanwhile,quantitative research about damping effect generated by the coupling vibration is also conducted in this part.The results from molecular dynamics validate the accuracy of the theoretical model in further.3)The phonon trapping effect of pearl-necklace-shaped nanowires is illustrated for enhancing the thermoelectric property.Inspired by the successful fabrication of pearl-necklace-shaped nanowires in experiments,we calculate the thermal conductivity of this special nanowire with non-equilibrium molecular dynamics method,and find that the pearl-necklace-shaped nanowire has a strong phonon localization effect,which makes it possess a lower thermal conductivity compared with the smooth nanowire.At the same time,a simplified phonon scattering theoretical model is also presented to explain this phenomena,which provides a practical method to further control the thermal conductivity of the nanowire.Our results have presented a new idea and route for controlling the mechanical and thermal performance of nanowire.At the same time,the research also provides a good reference for experiments and designing of nano devices.