Projects
With the advent of the 2D era (est. 2004) and bloom of twistronics (est. 2018) in the field of condensed matter physics, many new phases of matter as well as unique manifestations of known phenomena have been uncovered in 2D moiré structures. 2D ferroelectricity, being one of them, was pioneered in our group. My main focus have been two distinct forms of ferroelectricity based on graphene and hBN moirés, as well as new functionalities arising from these structures. I employed a variety of characterization, fabrication, and measurement techniques, ranging from electrical transport to capacitance to optics, in my studies.
I’ve also included “my child setup” in the lab – a VTI refrigerator that I led and oversaw from market research, purchasing, installation, to our own hands-on design and wiring of the probe and sample holder.
A type of ferroelectricity never seen before, where the electrical polarization is continuously tunable. We fabricate graphene-hBN moiré devices and measure the ferroelectric behavior with electrical transport. A novel “electronic ratchet effect” is uncovered, with its name coined in analogy to the working of a mechanical ratchet. Such phenomenon presents a unique platform for achieving advanced synaptic behaviors for next generation neuromorphic computing devices.
2D sliding ferroelectrics, a new member in the class of ferroelectric material, enables two polarization states from atomic translation between two low energy stacking configurations. I combined sliding ferroelectricity with the unconventional ferroelectricity in a new moiré structure, which allows for analog-digital dual mode functionality in the same device, reconfigurable without any circuitry switching. Such devices offer a powerful combination of accuracy and low energy consumption, the balance of which drives versatile and practical neural network performance.
(Publication in preparation)
Both of the two types of ferroelectricity discovered in stacking engineered 2D structures, sliding and unconventional ferroelectricity, differ fundamentally from conventional 3D ferroelectricity in terms of microscopic mechanism of polarization switching. This leaves the switching dynamics of these emergent ferroelectric phases an open question. We study the switching speed and endurance of both types of 2D ferroelectrics with ultra fast pulsing down to nanoseconds with combined capacitance and transport measurements.
(Publication in preparation)
Conventional ferroelectrics have the problem of fundamental size limit due to the increasing influence of depolarization field with decreasing size. 2D sliding ferroelectrics overcome the thickness limit by realizing stable polarization in two atomic layers, with a thickness of ~7Å. I am probing the limit of its lateral size limit, investigating its switching dynamics and robustness against domain wall pinning in miniature devices down to 50nm with nanosecond pulsing measurements.
(Publication in preparation)
An array of 50nm ferroelectric field effect transistors (FeFETs) I fabricated. ▶
Flatbands characteristic of moiré structures can enable unusual I-V responses such as negative differential conductance. When accelerated under a high in-plane electric field, carriers in bilayer graphene-hBN moiré reach the van Hove singularity in the bandstructure, thereby decelerating or even producing electron-hole plasma. Within this regime, the photocurrent is sensitive to single photon absorption and switches between bistable states, which allows for a new mechanism of single photon detection. We also found that such new mechanism allows for single photon detection at higher temperature (25K) and at longer wavelengths (11.3um in mid-IR) than existing ones.
Capacitance is a clean technique for detecting thermodynamic gaps with high sensitivity, and is also capable of probing layer-specific properties by adjusting the measurement geometry. We employed a cryogenic on-chip capacitance bridge amplifier based on high electron mobility transistor to probe the hysteretic charging sequence in various ferroelectric moiré through Landau level gap measurements.
Out of plane ferroelectric polarization can cause a deflection of a sharp tip (~tens of nanometer) close to the ferroelectric, which allows for high resolution polarization imaging. One can also apply an electric field between the tip and a backgate below the ferroelectric to switch its polarization. I applied such techniques on parallel-stacked hBN, a type of sliding ferroelectrics, to visualize its switching process.
◀ Ferroelectric domains “breathing” under opposite electric bias, imaged with electrical force microscopy.
When bringing graphene into contact with a metallic tip under humid environment, a water meniscus is formed around the contact area. The water, together with the SiO2 substrate, in essence forms an RC circuit in series upon which one can apply an AC voltage. The dissipative component, at the graphene region under water, produces significant heat to initiate a chemical reaction that oxidizes the graphene. This new nanolithography is versatile and applicable also to van der Waals heterostructures of graphene.
Drawing boxes with an AFM tip to isolate regions of interest on a ▶
twisted hBN-graphene heterostructure
A moiré potential has demonstrated to be a powerful tool to modify electronic bandstructure and induce exotic topological and strong correlation effects. However, in many cases, direct moiré formation is subject to practical constraints such as moiré period limit or the constituent material is unsuitable for certain experimental techniques. I now use twisted bilayer graphene and other moiré structures pioneered in our group as a substrate to imprint tunable moiré potential on a target material, where in-situ doping of the moiré layer tunes the target material across phase transitions, providing insights to the mechanism of the formation of strongly correlated phases.
(Study in progress, details confidential)
Image adapted from Nature Materials 23, 165–166 (2024).
I designed the wiring and sample mount of our latest VTI measurement probe, incorporating copper tape filters to minimize thermal noise transmission to sample. I led 4 other group members to design and fabricate the PCB board for the chip carrier, and completed the wiring and assembly. The system is now working under good condition.
