Tuning The Properties Of Materials By Doping From First Principles

Doping is an important means to tune the properties of materials,such as the electrical,magnetic and structural properties.There are various ways to dope materials,for example,by doping phosphorus or gallium into semiconductor silicon,n-type or p-type semiconductor are obtained,which are used widely in electronic devices.Luminescent materials with different properties can be obtained by doping specific metal ions into some inorganic solid compounds.For example,the phosphors with red light can be obtained by doping europium ions into yttrium oxide.In this thesis,we studied the effect of ionic doping of cation on the structural stability of HfO2 which resulted into evolution of structure.Besides the ionic doping,we also studied the effects of carrier doping on magnetic properties in two-dimensional(2D)transition metal phospho-sulfur(selenium)compounds(MPX3,M=Mn,Fe,Ni,Co;X=S,Se).As an alternative material to Si,HfO2 has wide band gap and high dielectric constant,which is a candidate to overcome the size limit problem of SiO2/Si in MOS transistors,and has attracted extensive attention.In experiments,the ferroelectric properties of HfO2 films can be significantly improved by doping different elements(Si,Al,La,Y,Zr)and oxygen vacancy.In our study,we found that the doping ions can affect the stability of HfO2 crystal structure under certain conditions.By doping HfO2 with alkaline earth ions(A2+),the induced oxygen instability inevitably influences the structure stability of the HfO2 and an amorphous structure with ultra-high energy storage density and efficiency could be formed.Specifically,we notice that fluorite HfO2 and perovskite AHfO3 have similar face-centered metal sublattices,while the sites of metal atoms in fluorite HfO2 and perovskite AHfO3 are similar in lattice structure,the lattice sites and stoichiometric ratio of oxygen ions are different.In HfO2,the molar ratio of oxygen to metal is 2:1 and eight oxygen ions occupy the interstitial sites of Hf tetrahedrons.For AHfO3,the oxygen/metal molar ratio is reduced to 1.5:1 and oxygen ions are located at the connection-line of two same metal ions.The doped A2+ can affect the stability of O2+,and then adjust the stoichiometric ratio and distribution of O2+ and finally induce structural evolution between HfO2 and AHfO3.Interestingly.during this structure evolution,the A-Hf-O system collapses and forms an amorphous state due to the decrease of the stoichiometric ratio of O2+and the instability of O2+ in the proper doping concentration interval,while the molar ratio of oxygen/metal is not enough to form perovskite AHfO3 crystals.We performed first-principles calculations on the formation energy of oxygen vacancy(Ef(VO))to illustrate the dependence of oxygen stability on A2+concentration.The Ef(VO)in undoped cubic phase HfO2 crystal is positive,but shaply droped to negative which means that the O2+ is unstable and tends to form VO to maintain electric neutrality.Therefore,the stoichiometric ratio of the system is changed,which destroys the original system structure and causes the redistribution of O2+.In addition,we systematically summarized the effect of ionic doping of other elements on the structural stability of HfO2 thin films.Furthermore,we studied the modulations of magnetic properties of 2D antiferromagnet FePS3 and MnPSe3 by carrier doping.We demonstrated that the doped could induce magnetic transformation in 2D compounds.It is found that the doped FePS3 show an antiferromagnetic state to ferromagnetic state transition at a doping level of 0.3-0.9 electrons/cell and a ferromagnetic state to antiferromagnetic state transition at a doping level of more than 0.9 electrons/cell.We proposed that this phenomenon is due to competition between the interchain ferromagnetic order dominated by Stoner exchange and the antiferromagnetic order dominated by superexchange at different doping levels.For monolayer MnPSe3,AFM-FM transition occurs when electron or hole doping concentration is larger than 0.15e/unit cell.Our study on magnetic transformation of 2D MPX3 materials is helpful to design novel spintronic devices.The content of this thesis is divided into five chapters,the first chapter is the introduction of the background of our studies;the second chapter is the introduction of the first principle method of density functional theory;in chapter 3,the effect of ionic doping in HfO2 is discussed;in chapter 4,the effects of carrier doping on the magnetic properties of FePS3 and MnPSe3 are studied;chapter 5 summarizes.