| With the gradual advancement of intelligent society,miniaturized and high-performance gas sensors have been widely used in enterprise and household equipment.As one of the most widely used semiconductor materials,ZnO has unique advantages in gas detection.The sensing performance will be greatly improved as grain size gradually decreases to the level of quantum dot,so ZnO quantum dots,as one of high-performance gas-sensitive materials,have become the current research hotspot.In such context,the size effect 'of gas sensitivity in ZnO quantum dots was studied.The first principle calculation based on density function theory was' used to investigate the grain size effect of ZnO semiconductor,with a size ranging from 0.325 nm to 1.625 nm.The formation energy of oxygen vacancy increased from 3.855 eV to 6.256 eV,making it more difficult to produce defects.The densities of states of ZnO grains with complete and defective(110)surface were calculated,and it was concluded that carrier mobility and conductivity were positively correlated with grain size.And the total density of state was compared with the partial density of state by taking ZnO grain with a size of 0.975nm as an example.It was found that the lower valence band was mainly contributed by O 2s states,and the upper valence band was mainly composed of Zn 3d and O 2p states,accompanied by a small amount of Zn 4s states and Zn 4p states.In addition,the conduction band was mainly contributed by O 2p state,and also contained some Zn 4s and Zn 4p states.By comparing the density of states of ZnO grain with complete(110)surface and defective(110)surface with a size of 0.975 nm,it was found that the band gap width of ZnO grain with defective(110)surface was smaller than that of ZnO grain with complete(110)surface,making it easier for the electrons to jump from valence band to conduction band and improving their conductivity.The oxygen species of O-and O2-were considered as adsorbates on ZnO grains with defective(110)surface.When the grain size was lower than 1.3 nm,the adsorption energy of O-increased as grain size increased,while of that of O2-kept unchanged,indicating that the oxygen adsorption was dominated by O-.There was a competitive adsorption between both species when grain size was more than 1.3 nm.The adsorption of O-was inhibited by the incremental adsorption energy of O2-,which was the dominated species on the surface of ZnO grain.Based on the adsorption energy of oxygen ions(O-and O2-)on the ZnO grain with defective(110)surface with a size of 0.975 nm,the adsorption ratio of O-and O2-was 60%and 40%,respectively.The relationship between the response and concentration of gases was obtained by simulated calculation,which was consistent with the relationship between the response of ZnO gas sensor and concentration of gases in the experiment.A method for calculating the Debye length of ZnO grain was established,and its Debye length was 6.521 A as finally calculated.In summary,this paper used the first principle to study the size effect of ZnO gas-sensitive grains,and verified the results by theory and experiment,which proved the reliability of the results obtained by the first principle,promoted the preparation of ZnO gas sensors with high response and small size,and provided a new research method for improving the gas sensing characteristics of ZnO gas sensors. |
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