Surface Electronic and Structural Characterization of Nanoparticles Based BiFeO3 Thin Films

Abstract

Bismuth iron oxide (BFO) possesses ABO3 type distorted Perovskite structure with simultaneous exhibition of ferroelectric and ferromagnetic properties. BiFeO3 has long range cycloidal spin structure with high TC = 1100K and TN = 640K respectively. However, phase stability and high leakage current are some problems interrelated with BiFeO3 thin films. These problems can be overcome by substitution of suitable elements at A or B site or both or by using suitable solvents. In this research work the surface and structural characterizations of bismuth iron oxide nanoparticles based thin films have been discussed. Sol gel method was adapted to synthesize BiFeO3 nanoparticles. Acetic acid and Ethylene Glycol were used as solvents for synthesis purpose by varying calcination temperatures as 100˚C, 200˚C and 300˚C. Anomalous behavior of dielectric constant was observed for BiFeO3 NPs even at room temperature. Increased value of dielectric constant (52.82 to 85.2 (log f =4)) was observed for EG based NPs after calcination at 300˚C. Increase in dielectric constant, along with anomalous behavior, with increased activation energy was observed with increase in characterization temperature (30oC– 210oC). Increase in conductivity with increase in temperature was termed as correlated barrier hopping mechanism. Polarization vs Electric field (P-E) curves indicated ferroelectric nature of nanoparticles even under as-synthesized conditions. Maximum polarization (Pmax ~12.22 µC/cm2) was observed for EG based nanoparticles calcined at 300oC. Bismuth iron oxide nanoparticles were also synthesized using sol-gel method with changing Bi/Fe ratio as 0.9, 0.95, 1.0, 1.05, 1.10, 1.15 and 1.20. XRD and Raman spectroscopy results confirmed the formation of phase pure crystalline rhombohedrally distorted Perovskite of BiFeO3 without the use of any post thermal treatment. Bi/Fe ratios 0.9-1.05, 1.15 and 1.20 resulted in amorphous nanoparticles. HRTEM image showed nanoparticles with size ~6-8nm. Increase in dielectric constant from 718 to 2815 (log f =3.0) was observed with increase in Bi/Fe ratio to 1.10. Increase in dielectric constant has been associated with increase in crystallinity of nanoparticles. Grain and grain boundary resistances increased as Bi/Fe ratio was increased to 1.10. Impedance analysis showed negative temperature coefficient behavior for BiFeO3 nanoparticles. Increase in saturation magnetization to 9.121emu/g with increase in Bi/Fe ratio is attributed to suppression of helical spin structure and reduction in magneto crystalline anisotropy. Bi1-xCaxFeO3 nanoparticles (x = 0.0-0.5) were synthesized with sol-gel method. These nanoparticles were characterized without any post thermal treatment. XRD results showed formation of BiFeO3 phase at dopant concentrations 0.0-0.3. Higher dopant concentration resulted in inclusion of calcium oxide phase in BiFeO3. BiFeO3 phase formation and incorporation of dopant in the lattice was further confirmed using Raman analysis and EDS. Nanoparticles of size ~7.2nm were observed using HRTEM. Saturation magnetization of bismuth iron oxide nanoparticles increased from 9.202emu/g to 21.097emu/g with reduction in magneto crystalline anisotropy. Magnetic ordering with blockingPage v temperature of ~88K (applied field 500Oe) was obtained using FC/ZFC curves. Bi1-xLaxFeO3 (x=0.0-0.5) thin films were prepared with sol-gel method. Formation of Perovskite BiFeO3 with rhombohedral distortion in unit cell was confirmed using XRD. Dielectric constant increased from 36.8 to 287 (log f =5.0) with increase in x from 0.0 to 0.3. Bi1-xBaxFeO3 thin films were prepared using sol-gel method and annealed at 300˚C. Phase pure BiFO3 was observed in XRD patterns without any trace of non-Perovskite bismuth phases till dopant concentration 0.2. BaO phase was observed at high dopant concentration x = 0.25 and 0.3. Films with dopant concentration x = 0.2 showed dielectric constant and tangent loss of 153.61 and 0.0058 (log f = 4.0) respectively. Activation energy of 1.8eV for Bi0.8Ba0.2FeO3 thin films indicated that oxygen vacancies were immobile thus leading to enhanced insulating behavior of BiFeO3. Bi1-xBaxFeO3 films showed ferromagnetic behavior with high saturation magnetization of 139.35emu/cm3 at dopant concentration 0.2. Nickel doped bismuth iron oxide nanoparticles were prepared by sol-gel route. A series of nanoparticles was prepared by varying their dopant concentration as 0 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt% & 5 wt%. XRD results confirmed formation of phase pure BiFeO3 till 4 wt% dopant concentration. Whereas peak corresponding to nickel oxide was observed at higher dopant concentration of 5 wt % due to increase of dopant concentration beyond certain limit. MH loops showed soft ferromagnetic behavior of nanoparticles. An increase in the value of saturation magnetization was observed from 0 wt% to 4 wt% (i.e., from 9.12 emu/g to 32.15 emu/g). This increase in saturation magnetization was attributed to suppression of helical spin structure. Further increase of dopant concentration to 5 wt% resulted in decrease of magnetization values due to appearance of NiO peak in XRD pattern. Nanoparticles synthesized by using 4 wt% carrier concentration resulted in high dielectric constant of 440.144(log f=5.0). Reduction in oxygen vacancies was observed by doping with Ni2+. Surface Photovoltage Spectroscopy (SPV) method has been used to characterize the surface of low dimensional structures important for applications in the electronics and optical fields. The detection of electronic transitions in BiFeO3 thin films helped in obtaining evidence of crystal structure induced phase changes in BiFeO3. The response of SPV was related to the band gap which was found to vary with the variation of type of solvent, ratio or substitutional variations. The work function of bismuth iron oxide (i.e., 4.79eV to 5.22 eV) matched well with the previously reported values under various conditions studied and optimized in the present study. Thus, the material prepared and optimized during this research work has the potential application in various electronic and spintronic devices because of its phase purity along with strengthened ferromagnetic nature at low synthesis / annealing temperatures.