مواد نو، ويژگي هاي نو و كاربردهاي نو: تحقيقي بر پايه نظريه تابعي چگالي

چكيده انگليسي :

Zahra Nourbakhsh z nourbakhsh@ph iut ac ir Department of Physics Isfahan University of Te nology Isfahan 84156 83111 IranDegree Ph D Supervisors Prof Hadi Akbarzadeh and Dr S Javad Hashemifar AbstractDensity Functional eory DFT is one of the most successful and common theories in materialscience whi can be used to study new or complicated compounds and structures On the basis ofdensity functional pseudopotential calculations in this thesis we study two di erent systems In the rst part of this thesis apters two three and four we employ DFT for more under standing of the ex ange interaction in the novel p ferromagnetic CaC and CaN compounds in theRS ZB WZ and NiAs structures We show that the generalized gradient functional give rises to ahalf metallic or nearly half metallic electronic structure for these systems Comparing the struc tural properties in the ferromagnetic and nonmagnetic states clari es that the bonding propertiesare almost independent of the ex ange interaction in these systems It is observed that the inter atomic ex ange interaction in these systems is quite strong and controls the spli ing of the spinresolved bond points In the third apter our focus is on CaC compound and we study structural magnetic dynamical and me anical properties of this compound in the nonmagnetic B2 anti NiAs NA and CrB B33 structures as well as the four magnetic RS ZB WZ NA structures It isargued that the ZB WZ NA and RS structures are more ionic while the NA B2 and B33 structuresare more covalent systems As a result of that the nonmagnetic B33 CaC is the energetically pre ferred system while the more ionic structures prefer a ferromagnetic ground state with high Fermilevel spin polarization e observed ferromagnetism in the more ionic systems is a ributed to thesharp partially lled p states of carbon atom in the system In the framework of density functionalperturbation theory the phonon spectra of these systems are computed and the observed dynami cal instabilities of the NA and B2 structures are explained in terms of the covalent bonds betweencarbon atoms e calculated Helmholtz and Enthalpy free energies indicate the highest stabilityof the B33 structure in a wide range of temperatures and pressures Several me anical propertiesof the dynamically stable structures of CaC are determined from their phonon spectra In the second part we demonstrate the occurrence of quantum con nement QC in the quan tum slabs of crystallite silicon embedded in hydrogenated a Si e QC whi can signi cantlyincrease the e ciency of the solar cells a racts many a entions in photovoltaic industry e op tical bandgap Eg of quantum con ned nc Si H is higher than bulk Si and it is tunable by angingthe thi ness of the slabs We show and clarify the occurrence of a new type of con nement inwhi only valence band states are spatially localized within a nanograin was derived and there isno barrier for con ning electrons within the c Si seeds QC in quantum slabs of crystallite silicon whi are con ned only in one dimensional proves QC in quantum wires and quantum dots ofnc Si H whi are con ned in two and three dimensional respectively Keywords Density functional theory ab initio calculations magnetization atomic orbitals s p d f com pounds of group II IV and II V crystal structures half metal phonon frequencies semicon ductors photovoltaic solar cells localization quantum con nement silicon nano structures amor phous silicon and HOMO and LUMO states

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مواد نو، ويژگي هاي نو و كاربردهاي نو: تحقيقي بر پايه نظريه تابعي چگالي

محاسبات ابتدا به ساكن , مغناطش , اوربيتال اتمي , جايگزيدگي , حصر كوانتومي , نانو ساختار سيليكون , ساختار بي نظم سيليكون