Study On Quantum Size Effect Of ?-? Semiconductor Nanomaterials

The tunable band gap due to quantum size effect enables a tunable optical absorption shift wavelength for semiconductor nanomaterials.Since the appearance of semiconductor nanomaterials,quantum size effect has become a research hotspot in semiconductor field.Predicting accurately the size-dependent band gaps of semiconductor nanomaterials is not only one of the basic problems in nanoscience,but also a goal pursued by researchers.Up to now,however,it is still difficult to give accurate and reliable calculation results of energy gap shifts in the whole nanometer region,using existing theoretical models such as the Effective Mass Approximation,Empirical Tight-Binding Model,Empirical Pseudopotential Method,K.P perturbation method,etc.A new chemical bond theoretical method for the quantum size effect shift parts is proposed in this paper,which is suggested that the energy gap shift of semiconductor nanomaterials arises from the sum of surface effects shift and quantum effect shift parts.When the size of nanomaterials is small,the effect of surface effect on energy gap shift cannot be ignored.This method is suitable for the energy gap shift calculation of binary nanocrystalline semiconductors and multiplexed nano semiconductor alloys with different sizes.It is not necessary to introduce any tunable parameters or data parameters which are difficult to obtain.First principle model based on density functional theory is used to calculate the band and density of states of ?-? semiconductor crystals with sphalerite and wurtzite structure,respectively.The structural optimization of the ?-? semiconductor crystals is performed to obtain the structural parameters of the semiconductor nanocrystals,which can be used as the initial calculation parameters of the internal chemical bond characteristics of the nanocrystals.A semiconductor nanocage structure containing only one layer of atoms is established and its structure is optimized.The obtained structural parameters can be used as initial calculation parameters for the surface chemical bond parameters.Using the hot-injection method,CdTe nanocrystals are prepared and then characterized by UV-Vis absorption spectroscopy,X-ray diffraction,transmission electron microscopy and Raman spectroscopy.The partical sizes of prepared samples are 1.5,2.5,4.0 and 6.0nm,respectively,with zinc blende structure,and corresponding band gaps are 2.59,2.01,1.93 and 1.89 eV,respectively.Based on the chemical bond theoretical method,the chemical bond characteristics and band gaps of prepared samples are calculated.A good agreement between the experimental and calculated band gaps confirms the correction of our theoretical method.The chemical bond characteristics and band gaps of ?-? binary semiconducting nanocrystals with spheroidite/wurtzite structures and different sizes are calculated using the proposed chemical bond theoritical method.Calculation results show that the changes of coordination numbers for surface atoms,the proportion of surface atoms and surface chemical bonds play important effects on chemical bond characteristics.The band gaps increase with the decreased partical sizes induced by quantum confinemet effect and surface effect which cannot be neglected when the partical size is smaller than 10 nm.Using the improved chemical bond theory method,we calculate the chemical bond characteristics and energy gap values of ternary ?-? semiconductor nanoalloys with different sizes and structures.The results show that composition of semiconductor nanoalloys also play an important role on the energy gap shift beside the partical size.Therefore,the energy gap can be regulated by controlling the size and the alloy composition to obtain nanomaterials with the specific energy gap values.The calculation results also show that when the size and componet content are certain,the crystal structure play a smaller effect on energy gap.Therefore,we can calculate the energy gap with the typical crystal structure to simplify the calculation.The energy gap of type-? semiconductor nanoalloy have been calculated,which show a non-monotonic change with the component content.The main reason is that the electronic energy levels of the components in the alloy are staggered.When the doping concentration changes,the energy gap will show red shift,i.e.energy gap has a minimum value.The trend of energy gap chaning with the componet content for type-? quaternary semiconductor nanoalloys are predicted,which also show the red shift.