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Domain wall free polar structure enhanced photodegradation activity in nanoscale ferroelectric BaxSr1-xTiO3Wang, Yaqiong; Zhang, Man; Jianguo, Liu; Zhang, Haibin; Li, Feng; Tseng, Chiao-Wei; Yang, Bin; Smith, Graham C.; Zhai, Jiwei; Zhang, Zhen; et al.Ferroelectric materials exhibit anomalous behavior due to the presence of domains and domain walls which are related to the spontaneous polarization inherent in the crystal structure. Control of ferroelectric domains and domain walls has been used to enhance device performances in ultrasound, pyroelectric detectors and photovoltaic systems with renewed interest in nanostructuring for energy applications. It is also known that the ferroelectric including domain walls can double photocatalytic rate and increase carrier lifetime from μs to ms However, there remains a lack of understanding on the different contributions of the domain and domain walls to photo-catalytic activities. Herein it is found, by comparing samples of nanostructured BaxSr1 xTiO3 with and without a polar domain, that the material with polar domains has a faster reaction rate (k=0.18 min-1) than the non polar one (k = 0.11 min-1). It is further revealed that the observed enhanced photoactivity of perovskite ferroelectric materials stems from the inherent polarization of the domain instead of domain walls. Here, the new understanding of the underlying physics of materials with a spontaneous dipole opens a door to enhance the performance of light induced energy harvesting systems.
Terahertz reading of ferroelectric domain wall dielectric switchingZhang, Man; Chen, Zhe; Yue, Yajun; Chen, Tao; Yan, Zhongna; Jiang, Qinghui; Yang, Bin; Eriksson, Mirva; Tang, Jianhua; Zhang, Dou; et al.Ferroelectric domain walls (DWs) are important nano scale interfaces between two domains. It is widely accepted that ferroelectric domain walls work idly at terahertz (THz) frequencies, consequently discouraging efforts to engineer the domain walls to create new applications that utilise THz radiation. However, the present work clearly demonstrates the activity of domain walls at THz frequencies in a lead free Aurivillius phase ferroelectric ceramic, Ca0.99Rb0.005Ce0.005Bi2Nb2O9, examined using THz time domain spectroscopy (THz-TDS). The dynamics of domain walls are different at kHz and THz frequencies. At low frequencies, domain walls work as a group to increase dielectric permittivity. At THz frequencies, the defective nature of domain walls serves to lower the overall dielectric permittivity. This is evidenced by higher dielectric permittivity in the THz band after poling, reflecting decreased domain wall density. An elastic vibrational model has also been used to verify that a single frustrated dipole in a domain wall represents a weaker contribution to the permittivity than its counterpart within a domain. The work represents a fundamental breakthrough in understanding dielectric contributions of domain walls at THz frequencies. It also demonstrates that THz probing can be used to read domain wall dielectric switching.
Ultrafast Electric Field-induced Phase Transition in Bulk Bi0.5Na0.5TiO3 under High Intensity Terahertz IrradiationYang, Bin; Zhang, Man; McKinnon, Ruth A.; Viola, Giuseppe; Zhang, Dou; Reece, Michael J.; Abrahams, Isaac; Yan, Haixue; University of Chester; Queen Mary University of London; Central South UniversityUltrafast polarization switching is being considered for the next generation of ferroelectric based devices. Recently, the dynamics of the field-induced transitions associated with this switching have been difficult to explore, due to technological limitations. The advent of terahertz (THz) technology has now allowed for the study of these dynamic processes on the picosecond (ps) scale. In this paper, intense terahertz (THz) pulses were used as a high-frequency electric field to investigate ultrafast switching in the relaxor ferroelectric, Bi0.5Na0.5TiO3. Transient atomic-scale responses, which were evident as changes in reflectivity, were captured by THz probing. The high energy THz pulses induce an increase in reflectivity, associated with an ultrafast field-induced phase transition from a weakly polar phase (Cc) to a strongly polar phase (R3c) within 20 ps at 200 K. This phase transition was confirmed using X-ray powder diffraction and by electrical measurements which showed a decrease in the frequency dispersion of relative permittivity at low frequencies.