Study On Size Effect Of Band Gap And Raman Frequency Of Semiconductor Nanoparticles

As the particle size of the material goes to the nanometer size range,it will exhibit different properties with the bulk material.Especially the semiconductor nanoparticles exhibit special optical,electrical,magnetic and thermodynamic properties.These properties will have extremely significant influence on the application of photoelectric conversion,storage equipment,sensor and display technique.Therefore,there have wide attention and a lot of researches on semiconductor nanoparticles.Most of the researches are carried on by experiments or simulations.At present,the more well-known theoretical models are quantum approximation theory and bond order-bond length-bond strength(BOLS)theory.However,these theoretical models have more variables,which make them complicated and variable.Therefore,it is a challenging task to establish a model which has few parameters and even can be applicable in the full size range,to analyze the size effect of semiconductor nanoparticles.In this paper,the bond number is introduced as the unique variable to describe the size dependence of the band gap and the Raman shift of semiconductor nanoparticles.As the size drops into nanoscale,the relative high surface energy makes the structure unstable.In order to obtain a more stable structure,the nanoparticles will take a special shape in order to reduce the surface energy,such as truc-octahedral(TO),icosahedral(IH),decahedral(DH)etc.As is known,to analyze bond number,the shape and size must be determined.Based on the previous researches,cubooctahedron(Cubo)has been widely used because of its stability in nanoscale.Moreover,the validity of Cubo in describing the shape of nanoparticles is confirmed by the fact of predicting the size-dependent cohesive energy and melting point of nanoparticles.Thus,we also use Cubo structure to describe the shape of the semiconductor nanoparticle.In this paper,the nanoparticles of IV semiconductor Si,II-VI semiconductor(Cd S,CdSe,ZnS,ZnSe,Cd Te),oxide semiconductor(SnO2,CeO2),III-V semiconductor(InP)are researched,and the change trends of their band gap and Raman frequency with size are analyzed by the thermodynamic method:1.The size-dependent band gap of semiconductor nanoparticles is established,where bond number is the only one parameter required to be known.The model shows that the band gap will increase gradually as the size decreases.Especially when D < 5nm,there is a significant increase trend for band gap.The model predictions are found to have good agreement with the corresponding experimental or simulation results.From this model,the band gap Eg(D)of nanomaterials ranges from Eg(?)to 2Eg(?),where Eg(?)represents the band gap of bulk material.The model shows that the band gap varies with the size,this is because the bond number decreases with size dropping,which can directly lead to the weaken of cohesive energy and then enhance band gap.In addition,the comparison between our model predictions and Yang's model is made.And one can find our predictions have a little smaller than that of Yang's,since the vacancy or the defects are not included in our model.However,the established model in this work is still simpler and valid in predicting the band gap of semiconductor nanoparticles.2.Through analyzing the change trends of both coordination number and thermal vibration amplitude with size,the size-dependent Raman frequency of semiconductor nanoparticles is constructed.In this model,bond number is still the only one parameter needed to be known.The model predictions are found to be consistent with a series of experimental or simulation results of single crystal,compounds and even semiconductor alloys.It is clear Raman shift exist in these semiconductor nanoparticles,if compared with bulk materials.One can find that Raman frequency gradually decreases as the size drops,and moreover the decrease trend become urgent at the lower end of size.The fact of Raman red shift for these semiconductor nanoparticles is also mainly due to the decrease in bond number with size dropping.This inevitably leads to the thermal instability of nanoparticles with the higher cohesive energy,and then the atoms become more active,resulting in the decrease in vibration frequency.The validity of the established model on Raman frequency means our model can be used to predict the Raman frequency of different semiconductor nanoparticles.Just because bond number is the only one parameter,the established model can be extended to other dimensional nanomaterials in principle,if bond number is calculated.