Jan-Chi Yang

Domain Wall Geometry Controls Conduction in Ferroelectrics

A new paradigm of domain wall nanoelectronics has emerged recently, in which the domain wall in a ferroic is itself an active device element. The ability to spatially modulate the ferroic order parameter within a single domain wall allows the physical properties to be tailored at will and hence opens vastly unexplored device possibilities. Here, we demonstrate via ambient and ultrahigh-vacuum (UHV) scanning probe microscopy (SPM) measurements in bismuth ferrite that the conductivity of the domain walls can be modulated by up to 500% in the spatial dimension as a function of domain wall curvature. Landau-Ginzburg-Devonshire calculations reveal the conduction is a result of carriers or vacancies migrating to neutralize the charge at the formed interface. Phase-field modeling indicates that anisotropic potential distributions can occur even for initially uncharged walls, from polarization dynamics mediated by elastic effects. These results are the first proof of concept for modulation of charge as a function of domain wall geometry by a proximal probe, thereby expanding potential applications for oxide ferroics in future nanoscale electronics.

Ultrafast photoinduced mechanical strain in epitaxial BiFeO3 thin films

We studied ultrafast dynamics and photoinduced mechanical strain of BiFeO3 thin films by dual-color transient reflectivity measurements (ΔR/R). Anisotropic photostriction in BiFeO3 is found to be mainly driven by the optical rectification effect. Results of the photostriction at various thicknesses show that the estimated sound velocity along [110] direction of BiFeO3 is 4.76u2009km/s.

Role of measurement voltage on hysteresis loop shape in Piezoresponse Force Microscopy

The dependence of field-on and field-off hysteresis loop shape in Piezoresponse Force Microscopy (PFM) on driving voltage, Vac, is explored. A nontrivial dependence of hysteresis loop parameters on measurement conditions is observed. The strategies to distinguish between paraelectric and ferroelectric states with small coercive bias and separate reversible hysteretic and non-hysteretic behaviors are suggested. Generally, measurement of loop evolution with Vac is a necessary step to establish the veracity of PFM hysteresis measurements.

Distribution of electronic reconstruction at the n-type LaAlO3/SrTiO3 interface revealed by hard x-ray photoemission spectroscopy

We investigated the electronic reconstruction at the n-type LaAlO3/SrTiO3 interface with hard x-ray photoelectron spectroscopy (HAXPES) under grazing incidence. By exploiting the collapse of evanescent x-ray waves and the abrupt increase of x-ray absorption at the critical incidence angle, our HAXPES study reveals a 2% electronic reconstruction from Ti4+ to Ti3+ occurring near the interface. Such an electronic reconstruction also extends from the interface into SrTiO3 with a depth of about 48u2009A (∼12 unit cells) and an estimated total charge transfer of ∼0.24 electrons per two-dimensional unit cell.

Field enhancement of electronic conductance at ferroelectric domain walls

Ferroelectric domain walls have continued to attract widespread attention due to both the novelty of the phenomena observed and the ability to reliably pattern them in nanoscale dimensions. However, the conductivity mechanisms remain in debate, particularly around nominally uncharged walls. Here, we posit a conduction mechanism relying on field-modification effect from polarization re-orientation and the structure of the reverse-domain nucleus. Through conductive atomic force microscopy measurements on an ultra-thin (001) BiFeO3 thin film, in combination with phase-field simulations, we show that the field-induced twisted domain nucleus formed at domain walls results in local-field enhancement around the region of the atomic force microscope tip. In conjunction with slight barrier lowering, these two effects are sufficient to explain the observed emission current distribution. These results suggest that different electronic properties at domain walls are not necessary to observe localized enhancement in domain wall currents.Understanding the conductivity at the nominally uncharged domain walls in ferroelectrics is still far from complete. Here the authors report an enhanced conduction at domain walls in an ultra-thin (001) BiFeO3 film resulting from the formation of a field-induced meta-stable twisted domain nucleus.

Atomic-Scale Mechanisms of Defect-Induced Retention Failure in Ferroelectrics

The ability to switch the ferroelectric polarization using an electric field makes ferroelectrics attractive for application in nanodevices such as high-density memories. One of the major challenges impeding this application, however, has been known as retention failure, which is a spontaneous process of polarization back-switching that can lead to data loss. This process is generally thought to be caused by the domain instability arising from interface boundary conditions and countered by defects, which can pin the domain wall and impede the back-switching. Here, using in situ transmission electron microscopy and atomic-scale scanning transmission electron microscopy, we show that the polarization retention failure can be induced by commonly observed nanoscale impurity defects in BiFeO3 thin films. The interaction between polarization and the defects can also lead to the stabilization of novel functional nanodomains with mixed-phase structures and head-to-head polarization configurations. Thus, defect engineering provides a new route for tuning properties of ferroelectric nanosystems.

Multifunctionalities driven by ferroic domains

Considerable attention has been paid to ferroic systems in pursuit of advanced applications in past decades. Most recently, the emergence and development of multiferroics, which exhibit the coexistence of different ferroic natures, has offered a new route to create functionalities in the system. In this manuscript, we step from domain engineering to explore a roadmap for discovering intriguing phenomena and multifunctionalities driven by periodic domain patters. As-grown periodic domains, offering exotic order parameters, periodic local perturbations and the capability of tailoring local spin, charge, orbital and lattice degrees of freedom, are introduced as modeling templates for fundamental studies and novel applications. We discuss related significant findings on ferroic domain, nanoscopic domain walls, and conjunct heterostructures based on the well-organized domain patterns, and end with future prospects and challenges in the field.

A gate-free monolayer WSe 2 pn diode

Interest in bringing p- and n-type monolayer semiconducting transition metal dichalcogenides (TMD) into contact to form rectifying pn diode has thrived since it is crucial to control the electrical properties in two-dimensional (2D) electronic and optoelectronic devices. Usually this involves vertically stacking different TMDs with pn heterojunction or, laterally manipulating carrier density by gate biasing. Here, by utilizing a locally reversed ferroelectric polarization, we laterally manipulate the carrier density and created a WSe2 pn homojunction on the supporting ferroelectric BiFeO3 substrate. This non-volatile WSe2 pn homojunction is demonstrated with optical and scanningxa0probe methods and scanning photoelectron micro-spectroscopy. A homo-interface is a direct manifestation of our WSe2 pn diode, which can be quantitatively understood as a clear rectifying behavior. The non-volatile confinement of carriers and associated gate-free pn homojunction can be an addition to the 2D electron–photon toolbox and pave the way to develop laterally 2D electronics and photonics.Bringing together p- and n-type monolayers of semiconducting transition metal dichalcogenides results in the formation of atomically thin pn junctions. Here, the authors laterally manipulate carrier density to create a WSe2 pn homojunction on a supporting ferroelectric BiFeO3 substrate.