PFM imaging of ferroelectric domains in single-crystal tungsten ditelluride. Image: FLEET.
PFM imaging of ferroelectric domains in single-crystal tungsten ditelluride. Image: FLEET.

In a paper published in Science Advances, researchers at the University of New South Wales (UNSW) in Australia describe the first observation of a native ferroelectric metal. This study, which was funded through the ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), has uncovered the first example of a native metal with bistable and electrically switchable spontaneous polarization states – the hallmark of ferroelectricity.

Ferroelectricity can be considered an analogy to ferromagnetism. A ferromagnetic material displays permanent magnetism; in layperson's terms, it is simply, a 'magnet' with north and south poles. A ferroelectric material, likewise, displays an analogous electrical property called a permanent electric polarization, which originates from electric dipoles consisting of equal, but oppositely charged, ends or poles. In ferroelectric materials, these electric dipoles exist at the unit cell level and give rise to a non-vanishing permanent electric dipole moment.

This spontaneous electric dipole moment can be repeatedly transitioned between two or more equivalent states or directions upon application of an external electric field. This property is utilized in numerous ferroelectric technologies, such as nano-electronic computer memory, RFID cards, medical ultrasound transducers, infrared cameras, submarine sonar, vibration and pressure sensors, and precision actuators. Conventionally, ferroelectricity has been observed in materials that are insulating or semiconducting rather than metallic, because conduction electrons in metals screen-out the static internal fields arising from the dipole moment.

"We found coexistence of native metallicity and ferroelectricity in bulk crystalline tungsten ditelluride (WTe2) at room temperature," explains study author Pankaj Sharma at UNSW. "We demonstrated that the ferroelectric state is switchable under an external electrical bias and explain the mechanism for 'metallic ferroelectricity' in WTe2 through a systematic study of the crystal structure, electronic transport measurements and theoretical considerations."

WTe2 belongs to a class of materials known as transition metal dichalcogenides (TMDCs). To confirm its metallic behavior, the researchers conducted spectroscopic electrical transport measurements and studied it with conductive-atomic force microscopy (c-AFM). They conducted piezo-response force microscopy (PFM) to map the polarization, detecting lattice deformation due to an applied electric field, and directly visualized the ferroelectric domains – ie, the regions with oppositely oriented direction of polarization – in freshly-cleaved WTe2 single crystals.

Meanwhile, researchers at the University of Nebraska conducted first-principles density functional theory (DFT) calculations that confirmed the experimental findings of the electronic and structural origins of the ferroelectric instability of WTe2.

"A van der Waals material that is both metallic and ferroelectric in its bulk crystalline form at room temperature has potential for new nano-electronics applications," says co-author Feixiang Xiang at UNSW.

This story is adapted from material from FLEET, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.