Electrical Tunability of Domain Wall Conductivity in LiNbO3 Thin Films

Domain wall nanoelectronics is a rapidly evolving field, which explores the diverse electronic properties of the ferroelectric domain walls for application in low‐dimensional electronic systems. One of the most prominent features of the ferroelectric domain walls is their electrical conductivity. Here, using a combination of scanning probe and scanning transmission electron microscopy, the mechanism of the tunable conducting behavior of the domain walls in the sub‐micrometer thick films of the technologically important ferroelectric LiNbO₃ is explored. It is found that the electric bias generates stable domains with strongly inclined domain boundaries with the inclination angle reaching 20° with respect to the polar axis. The head‐to‐head domain boundaries exhibit high conductance, which can be modulated by application of the sub‐coercive voltage. Electron microscopy visualization of the electrically written domains and piezoresponse force microscopy imaging of the very same domains reveals that the gradual and reversible transition between the conducting and insulating states of the domain walls results from the electrically induced wall bending near the sample surface. The observed modulation of the wall conductance is corroborated by the phase‐field modeling. The results open a possibility for exploiting the conducting domain walls as the electrically controllable functional elements in the multilevel logic nanoelectronics devices.     

Intrinsic Conductance of Domain Walls in BiFeO3

Ferroelectric domain walls exhibit a number of new functionalities that are not present in their host material. One of these functional characteristics is electrical conductivity that may lead to future device applications. Although progress has been made, the intrinsic conductivity of BiFeO₃ domain walls is still elusive. Here, the intrinsic conductivity of 71° and 109° domain walls is reported by probing the local conductance over a cross section of the BiFeO₃/TbScO₃ (001) heterostructure. Through a combination of conductive atomic force microscopy, high‐resolution electron energy loss spectroscopy, and phase‐field simulations, it is found that the 71° domain wall has an inherently charged nature, while the 109° domain wall is close to neutral. Hence, the intrinsic conductivity of the 71° domain walls is an order of magnitude larger than that of the 109° domain walls associated with bound‐charge‐induced bandgap lowering. Furthermore, the interaction of adjacent 71° domain walls and domain wall curvature leads to a variation of the charge distribution inside the walls, and causes a discontinuity of potential in the [110]_p direction, which results in an alternative conductivity of the neighboring 71° domain walls, and a low conductivity of the 71° domain walls when measurement is taken from the film top surface.     

Atomic Level 1D Structural Modulations at the Negatively Charged Domain Walls in BiFeO3 Films

Charged domain walls (CDWs) show great potentials to mediate the properties of ferroelectrics. Direct mapping of these domain walls at an atomic scale is of critical importance for understanding the domain wall dominated properties. Here, based on aberration‐corrected scanning transmission electron microscopy, tail‐to‐tail CDWs at 71°, 109°, and 180° domains in BiFeO₃ thin films have been identified. 2D mappings demonstrate 1D structural modulations with alternate lattice expansions and clockwise/counterclockwise lattice rotations at these CDWs. Such behaviors of CDWs reveal a remarkable contrast to the uncharged domain walls and imply delicate interactions between bound charges and structural compensations of domain wall. These results are expected to provide new information on domain wall structures and shed some light on the understanding of domain wall properties in ferroelectrics.     

Ferroelectric BiFeO3 as an Oxide Dye in Highly Tunable Mesoporous All‐Oxide Photovoltaic Heterojunctions

As potential photovoltaic materials, transition‐metal oxides such as BiFeO₃ (BFO) are capable of absorbing a substantial portion of solar light and incorporating ferroic orders into solar cells with enhanced performance. But the photovoltaic application of BFO has been hindered by low energy‐conversion efficiency due to poor carrier transport and collection. In this work, a new approach of utilizing BFO as a light‐absorbing sensitizer is developed to interface with charge‐transporting TiO₂ nanoparticles. This mesoporous all‐oxide architecture, similar to that of dye‐sensitized solar cells, can effectively facilitate the extraction of photocarriers. Under the standard AM1.5 (100 mW cm^?2) irradiation, the optimized cell shows an open‐circuit voltage of 0.67 V, which can be enhanced to 1.0 V by tailoring the bias history. A fill factor of 55% is achieved, which is much higher than those in previous reports on BFO‐based photovoltaic devices. The results provide here a new viable approach toward developing highly tunable and stable photovoltaic devices based on ferroelectric transition‐metal oxides.     

Preservation of Surface Conductivity and Dielectric Loss Tangent in Large‐Scale,Encapsulated Epitaxial Graphene Measured by Noncontact Microwave Cavity Perturbations

Regarding the improvement of current quantized Hall resistance (QHR) standards, one promising avenue is the growth of homogeneous monolayer epitaxial graphene (EG). A clean and simple process is used to produce large, precise areas of EG. Properties like the surface conductivity and dielectric loss tangent remain unstable when EG is exposed to air due to doping from molecular adsorption. Experimental results are reported on the extraction of the surface conductivity and dielectric loss tangent from data taken with a noncontact resonance microwave cavity, assembled with an air‐filled, standard R100 rectangular waveguide configuration. By using amorphous boron nitride (a‐BN) as an encapsulation layer, stability of EG's electrical properties under ambient laboratory conditions is greatly improved. Moreover, samples are exposed to a variety of environmental and chemical conditions. Both thicknesses of a‐BN encapsulation are sufficient to preserve surface conductivity and dielectric loss tangent to within 10% of its previously measured value, a result which has essential importance in the mass production of millimeter‐scale graphene devices demonstrating electrical stability.