Perpustakaan Digital ITB

Recent idea in incorporating charge and spin is implemeted in spintronic devices. For realizing the idea, spin degree of freedom can be generated by doping a small concentration of transition metal element in a non-magnetic semiconductor, forming diluted magnetic semiconductor (DMS). Room-temperature ferromagnetic DMS has been investigated to realize future application. Using ab inito modelling, electronic and magnetic properties of anatase TaxTi1–xO2 (x = 3.125%) are investigated. The properties are modified by additional M co-doping, forming TaxMyTi1–x–yO2 (M = Fe, Ni, Co) (x = y = 3.125 %). The main goals of this research are to model the properties of the systems by revealing band structures, total density of states (DOS), and projected DOS (PDOS). Method used is generalized gradient approximation (GGA) method, which are validated by calculating properties of pure anatase TiO2. Effective on-site Coulomb repulsion energy (Ueff) is included at d orbital to correct electronic properties. The pure anatase TiO2 system possesses semiconductor properties with bandgap (Eg) of 2.07 eV and 3.14 eV (0.80 Σ to Γ), for Ueff = 0 eV and 5.82 eV, respectively, as well as calculated valence band (VB) width values of 4.57 eV and 4.56 eV. Calculated PDOS has strong hybridization between Ti 3d and O 2p states. By Ta doping, TaxTi1–xO2 (x = 3.125%) system has half-metallic ferromagnetic properties with a total magnetic moment of 1.00 μB per Ta atom, mainly promoted from four Ti sites with a local magnetic moment of 0.31 μB per site. The spin-down band structure shows a value of Eg = 3.24 eV (0.25 Σ → Γ). By additional M co-doping treatments, TaxFeyTi1–x–yO2 (x = y = 3.125%) system possesses semiconductor ferromagnetic properties with a total magnetic of 5.00 μB per Fe atom, which mainly is from a delocalized local magnetic moment of 4.261 μB at Fe site. Spin-up and spin-down band structures show Eg of 2.94 eV (0.25 Σ → Γ). Besides, TaxNiyTi1–x–yO2 (x = y = 3.125%) system has half-metallic ferromagnetic properties with the total magnetic moment of 1.11 μB per Ni atom, which mainly is from a localized local magnetic moment of 1.577 μB at Ni site. Spin-down band structure shows Eg of 2.97 eV (Γ → Γ). TaxCoyTi1–x–yO2 (x = y = 3.125%) system posseses nonmagnetic semiconductor properties. Spin-up and spin-down band structure show Eg of 2.30 eV (Γ → M).For all the systems, the degeneracy level of Ti 3d, Fe 3d, Ni 3d, and Co 3d for each systems are decreased by Jahn-Teller distortion upon dzx and dzy states, compared with that of Ti 3d in TiO2. The calculated VB width of the systems are wider than that of TiO2 due to hybridizations of Ta 5d-O 2p in TaxTi1–xO2 and TaxFeyTi1–xO2, Ni 3d-O 2p in TaxNiyTi1–xO2, and Co 3d-O 2p in TaxCoyTi1–xO2. Beside showing detailed electronic and magnetic properties, this research suggests that TaxTi1–xO2 and TaxNiyTi1–xO2 can be oriented for transparent conducting oxide application, while TaxCoyTi1–xO2 and TaxFeyTi1–xO2 can be oriented for photocatalytic and DMS applications, respectively. Based on the succesful modelling of anatase TiO2-based systems, electronic and magnetic properties of rutile VxNiyTi1–x–yO2 (x = 0, 6.25%; y = 0, 6.25%) are investigated, The rutile VxTi1–xO2 (x = 6.25%) system possesses a n-type half-metallic ferromagnetic properties with a localized total magnetic moment of 1.00 μB per V atom, which mainly comes from V 3d states. By including Ueff, the properties are enhanced to be n-type semiconductor ferromagnetic. The rutile VxNiyTi1–x–yO2 (x = y = 6.25%) system possesses a p-type half-metallic ferromagnetic properties with a delocalized total magnetic moment of 1.36 μB per Ni atom, which mainly comes Ni 3d states, by including Ueff. Here, Ueff is important in predicting the newly-found properties of the rutile VxNiyTi1–x–yO2 systems. Overall, substitutional Ni doping enhances conductivity and ferromagnetism of rutile VxTi1–xO2. The calculation is extended to model electronic and optical properties of layered oxychalcogenides (LaO)CuCh (Ch = S, Se, Te), which show Eg (Γ → Γ) for Ch = S, Se, and Te of 1.67 eV, 1.44 eV, and 1.20 eV, respectively. Furthermore, calculated upper VB is divided into three states, i.e. antibonding and bonding states coming from hybridization of Cu 3d-t2g and Ch p, and nonbonding states coming from localized Cu 3d-eg states. Highest dielectric constants and optical dichroism are found in (LaO)CuTe system, while p-type conductivity is stronger in (LaO)CuSe system. Energy levels of plasmonic states can also be tuned by changing Ch element. These results are essential for novel functional device applications.