Research Bits: Oct. 4

2D electrode for ultra-thin semiconductors

Researchers from the Korea Institute of Science and Technology (KIST), Japan’s National Institute for Materials Science, and Kunsan National University designed two-dimensional semiconductor-based electronic and logic devices, with electrical properties that can be selectively controlled through a new 2D electrode material, chlorine-doped tin diselenide (Cl-SnSe2).

The researchers noted that it is difficult to implement complementary logic circuits in conventional two-dimensional semiconductor devices because they only exhibit the characteristics of either N-type or P-type devices due to the Fermi-level pinning phenomenon.

By using Cl-SnSe2 as the electrode material, they found it was possible to freely control the characteristics of the N-type and P-type devices by minimizing defects with the semiconductor interface, enabling a single device to perform the functions of both N-type and P-type devices.

The team used the material to implement a high-performance, low-power, complementary logic circuit that can perform different logic operations such as NOR and NAND.

“This development will contribute to accelerating the commercialization of next-generation system technologies such as artificial intelligence systems, which have been difficult to use in practical applications due to technical limitations caused by the miniaturization and high integration of conventional silicon semiconductor devices,” said Do Kyung Hwang of the Center for Opto-Electronic Materials and Devices at KIST. “The developed two-dimensional electrode material is very thin; hence, they exhibit high light transmittance and flexibility. Therefore, they can be used for next-generation flexible and transparent semiconductor devices.”

Fast thin-film barium titanate

Researchers at Lawrence Berkeley National Laboratory and UC Berkeley synthesized a thin-film version of barium titanate (BaTiO3) where the polarization switches as quickly and efficiently as the bulk version.

Bulk BaTiO3 crystals respond quickly to a small electric field, flip-flopping the orientation of the charged atoms in a reversible but permanent manner even if the applied field is removed.

“We’ve known about BaTiO3 for the better part of a century and we’ve known how to make thin films of this material for over 40 years. But until now, nobody could make a film that could get close to the structure or performance that could be achieved in bulk,” said Lane Martin, a faculty scientist in the Materials Sciences Division at Berkeley Lab and professor of materials science and engineering at UC Berkeley.

A challenge to creating BaTiO3 films has been the high concentration of defects. To reduce these, the researchers used pulsed-laser deposition, which involves firing an ultraviolet laser light onto a ceramic target of BaTiO3 causes the material to transform into a plasma, which then transmits atoms from the target onto a surface to grow the film. “It’s a versatile tool where we can tweak a lot of knobs in the film’s growth and see which are most important for controlling the properties,” said Martin.

The method was able to achieve precise control over the deposited film’s structure, chemistry, thickness, and interfaces with metal electrodes.

By placing a film of BaTiO3 in between two metal layers, the team created tiny capacitors. Applying voltages of 100 mV or less and measuring the current that emerged showed that the film’s polarization switched within two nanoseconds and could potentially be faster. “This is a good early victory in our pursuit of low-power electronics that go beyond what is possible with silicon-based electronics today,” said Martin. “Unlike our new devices, the capacitors used in chips today don’t hold their data unless you keep applying a voltage.”

Electron microscope images show the precise atom-by-atom structure of a barium titanate (BaTiO3) thin film sandwiched between layers of strontium ruthenate (SrRuO3) metal to make a tiny capacitor. (Credit: Lane Martin/Berkeley Lab)

Next, the team plans to shrink the material down even thinner to make it compatible with real devices and study how it behaves. They will also work with collaborators at companies such as Intel to test the feasibility in first-generation electronic devices. “If you could make each logic operation in a computer a million times more efficient, think how much energy you save. That’s why we’re doing this,” said Martin.

Washable energy-harvesting fabric

Scientists at Nanyang Technological University Singapore and Tsinghua University developed a stretchable and waterproof fabric that turns energy generated from body movements into electrical energy.

The fabric uses a polymer that, when pressed or squeezed, converts mechanical stress into electrical energy. It is also made with stretchable spandex as a base layer and integrated with a rubber-like material to keep it strong, flexible, and waterproof. The fabric could be washed and folded without performance degradation and maintained a stable electrical output for up to five months.

“There have been many attempts to develop fabric or garments that can harvest energy from movement, but a big challenge has been to develop something that does not degrade in function after being washed, and at the same time retains excellent electrical output. In our study, we demonstrated that our prototype continues to function well after washing and crumpling. We think it could be woven into t-shirts or integrated into soles of shoes to collect energy from the body’s smallest movements, piping electricity to mobile devices,” said Pooi See Lee, materials scientist and NTU Associate Provost (Graduate Education) Professor.

The researchers said that the prototype fabric produces electricity in two ways: when it is pressed or squashed (piezoelectricity), and when it comes into contact or is in friction with other materials, such as skin or rubber gloves (triboelectric effect).

The material is comprised of a stretchable electrode made by screenprinting an ink comprising silver and styrene-ethylene-butylene-styrene (SEBS), a rubber-like material to make it more stretchable and waterproof. The stretchable electrode was then attached to a piece of nanofiber fabric made up of poly(vinylidene fluoride)-co-hexafluoropropylene (PVDFHPF), a polymer that produces an electrical charge when compressed, bent, or stretched, and lead-free perovskites.

“Embedding perovskites in PVDF-HPF increases the prototype’s electrical output. In our study, we opted for lead-free perovskites as a more environmentally friendly option. While perovskites are brittle by nature, integrating them into PVDF-HPF gives the perovskites exceptional mechanical durability and flexibility. The PVDF-HPF also acts an extra layer of protection to the perovskites, adding to its mechanical property and stability,” said Feng Jiang, an NTU PhD student.

The prototype fabric could generate 2.34 watts per square meter of electricity, enough to power small electronic devices, such as LEDs and commercial capacitors.

“Despite improved battery capacity and reduced power demand, power sources for wearable devices still require frequent battery replacements. Our results show that our energy harvesting prototype fabric can harness vibration energy from a human to potentially extend the lifetime of a battery or even to build self-powered systems. To our knowledge, this is the first hybrid perovskite-based energy device that is stable, stretchable, breathable, waterproof, and at the same time capable of delivering outstanding electrical output performance,” said Lee.

The team plans to explore whether the fabric could be adapted to harvest other forms of energy.