Regulation And Performance Prediction Of Low-dimensional Gallium Nitride Nanowires

Gallium nitride nanowires have been widely used in nano-lasers,sensors,high temperature and high power devices due to their excellent physical properties and high chemical stability.The key to the performance regulation of nanodevices can be considered as the regulation of energy band characteristics of nanomaterials.The results show that the band structure of nanowires can be controlled by doping,and the change of diameter,morphology,strain,surface and interface.For example,the H₂ sensor was designed at room temperature by depositing Pt particles on the surface of GaN nanowires.Based on density functional tight-binding theory and first-principles method,the effect of structure and metal adsorption on the band structure of GaN nanowires was studied,and the mechanism of the effect of different metal adsorption on the sensitivity of GaN nanowires was analyzed.The specific research contents and innovations are summarized as follows:1.The effect of strain and cross-section shape on the electronic structure of GaN nanowires was systematically simulated by density functional tight binding theory（DFTB）.Due to the quantum confinement effect,the band gap of GaN nanowires increases with the decrease of diameter.More importantly,the general linear expressions of band gap and surface area to volume ratio of GaN nanowires are obtained.The results can be used to predict the band gap of GaN nanowires because the cross-section shape and diameter of nanowires can be obtained in experiments.In addition,the linear relationship between the bandgap and the tensile strain in GaN nanowires is obtained.The results will provide a new alterrnative for the band gap control of GaN materials and guide the design of new nanodevices.2.The structure and band structure of zinc-blend/wurtzite（ZB/WZ）GaN superlattice nanowires were studied by first-principles method based on density functional theory.The results show that the Ga-N hexatomic rings near the interface of GaN superlattice nanowires are transformed into tera-atomic rings due to the different number of atomic hanging bonds at the interface.The homogeneous ZB/WZ interfacial structure weakens the tensile strain of nanowires in the contast of thet he perfect nanowires in a certain strain range（0-3%）.The blocking effect will disappear when the tensile strain is greater than this critical value（3%）.In addition,it is found that the band structure of one-dimensional periodic superlattice nanowires and three-dimensional periodic superlattice have different relationships with interfacial composition:（i）the band gap of three-dimensional periodic superlattices is a non-linear function of the proportion of interfacial composition,and shows obvious interfacial effect;（ii）the influence of interfacial composition on the band gap of superlattice nanowires has no specific law.This is due to the large surface area of nanowires,which results in less influence on the electronic structure of superlattice nanowires.By studying the micro-adsorption structure of metal atoms（e.g.Pt,Ag,Cu,Au）on the surface of GaN nanowires,the adsorption models of metal atoms on the surface of nanowires was proposed.Firstly,by analyzing the adsorption structure and energy of four metals on GaN nanowires,two common stable adsorption sites of four metals on GaN nanowires were obtained including the Center of Ga-N hexatomic ring（C site）and the site above the surface Ga-N chemical bond（B site）respectively.And the most stable adsorption structure was found at C site.Then,by calculating and comparing the density of state（DOS）of GaN nanowires before and after adsorption,the effects of different metal atom adsorption on the band structure and conductivity of GaN nanowires were obtained.After adsorbing metal atoms,the band gap of GaN nanowires decreases.Finally,bader charges of all atoms in the adsorbed nanowires are analyzed.Results show that Pt and Au atoms get electrons,the nanowires lose electrons,the carrier concentration in GaN nanowires decreases,and the conductivity of nanowires decreases when Pt and Au atoms are adsorbed on the surface of GaN nanowires.Pt and Au atoms acquire electrons and exhibit strong reducibility,thus reflecting the catalytic properties of metal atoms after adsorbing on the surface of GaN nanowires.While Ag and Cu atoms are adsorbed on the surface of GaN nanowires,the metal atoms lose electrons,the nanowires gain electrons,and the conductivity of the nanowires is enhanced.In addition,an obvious redistribution of surface charge of Pt,Au,Ag and Cu atoms is found after their adsorption and the surface charge reconstructions are different at different adsorption sites.4.Based on the stability model of metal adsorption on GaN nanowires obtained in previous studies,the first-principles method was used to study the gas sensing mechanism of metal adsorption on GaN nanowires for H₂ molecule.The adsorption energy of GaN nanowires adsorbed by H₂ molecule is the lowest on Pt（-1.969eV）.The band gap of GaN nanowires increased by 0.7eV after adsorption of H₂ molecule,and the surface charge was reconstructed significantly（the change of surface charge was0.17e）.This means that GaN nanowire gas sensor adsorbed by Pt metal has high sensitivity to H₂ molecule.The results will be helpful to understand the physical properties of metal functional nanowires and explain the mechanism of enhancing the sensitivity of gas sensors by metal adsorption.The potential application and enhancement mechanism of metal-adsorbed GaN nanowires in CO detection were further studied through the changes of adsorption structure and electronic structure before and after adsorption.The results show that surface metal adsorption not only increases the stable adsorption sites of CO on nanowire surface,but also decreases the adsorption energy of CO on nanowire surface,thus improving the sensitivity of GaN nanowire sensor to CO gas.It is also found that the adsorption structure and functional metals have significant effects on CO adsorption on the surface of GaN nanowires.In addition,the study shows that after CO adsorption,the DOS of GaN nanowires adsorbed by metals has changed significantly.This indicates that metal adsorption enhances the sensitivity of surface charge of GaN nanowires to CO molecules.At last we found that CO adsorption has the greatest influence on the electronic DOS of GaN nanowires adsorbed by Au and Cu metals,which means that GaN nanowire sensors adsorbed by Au and Cu metals are suitable in CO gas detecting.