A research team has achieved progress in studying van der Waals (vdW) contacts for two-dimensional (2D) electrical devices. The team developed an innovative all-stacking technique for fabricating 2D electrical devices, optimizing the interface contact between 2D materials and metal electrodes. The study was published in Nature Communications.

In conventional fabrication processes for 2D electrical devices, the deposition of metal electrodes is a critical step. High-energy metal atoms deposited on the surface of 2D materials can easily damage their lattices, ultimately degrading the device’s electrical performance. Therefore, achieving reliable electrical contact between 2D materials and metal electrodes is crucial for improving the performance of 2D electrical devices.

Recent research has shown that for 2D electronic devices, realizing the vdW contact holds promise for solving the above problems. The vdW contact refers to the interaction between 2D materials and metal electrodes through vdW forces. This method avoids introducing numerous defects and can achieve good electrical contact, offering better operability and potential for large-scale application.

To achieve reliable 2D vdW contacts, the team developed an all-stacking technique. This technique allowed for the direct stacking of metal electrodes onto 2D materials during the fabrication of 2D electrical devices, avoiding steps like metal deposition and thus protecting the 2D materials from damage while achieving excellent electrical performance.

Specifically, 2D electrical devices fabricated using this technique had sharp metal-semiconductor contact interfaces, with smooth and clear vdW gaps at the interface and no metal atom doping on the 2D material side. This indicated that high-quality vdW contact had been formed between the metal electrodes and the 2D semiconductor.

Due to the improved contact interface, 2D semiconductor transistors fabricated using this technique exhibited more than 95% reduction in off-state current and a 50% decrease in subthreshold swing compared to those fabricated using metal deposition processes. They also possessed a higher on-off ratio, making them more advantageous for low-power integrated circuits.

To demonstrate the potential of the all-stacking technique for wafer-scale manufacturing, the team fabricated an array of field-effect transistors based on monolayer molybdenum disulfide using this technique. The device yield was as high as 98.4%, exhibiting excellent consistency and stability.

The average on-off ratio of the transistor array was 6.8×10⁶, with 91.3% of the devices having an on-off ratio greater than 10⁶. The excellent performance and high consistency of the transistor array demonstrated the advantages of the all-stacking technique in achieving reliable contact for 2D electronic devices.

The all-stacking technique developed in this study optimizes the interfacial contact between 2D materials and metal electrodes, providing an efficient, high-quality, and universal approach for the preparation of 2D electronic devices. The study is expected to provide a new technical path for the industrial-level manufacturing of future 2D electronic devices.

The research team was led by Prof. Zeng Hualing, Prof. Qiao Zhenhua, and Prof. Shao Xiang from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS).

University of Missouri researchers have developed a way to create complex devices with multiple materials—including plastics, metals and semiconductors—all with a single machine.

The research, which was recently published in Nature Communications, outlines a novel 3D printing and laser process to manufacture multi-material, multi-layered sensors, circuit boards and even textiles with electronic components.

It’s called the Freeform Multi-material Assembly Process, and it promises to revolutionize the fabrication of new products.

By printing sensors embedded within a structure, the machine can make things that can sense environmental conditions, including temperature and pressure. For other researchers, that could mean having a natural-looking object such as a rock or seashell that could measure the movement of ocean water. For the public, applications could include wearable devices that monitor blood pressure and other vital signs.

“This is the first time this type of process has been used, and it’s unlocking new possibilities,” said Bujingda Zheng, a doctoral student in mechanical engineering at Mizzou and the lead author of the study. “I’m excited about the design. I’ve always wanted to do something that no one has ever done before, and I’m getting to do that here at Mizzou.”

One of the main benefits is that innovators can focus on designing new products without worrying about how to prototype them.

“This opens the possibility for entirely new markets,” said Jian “Javen” Lin, an associate professor of mechanical and aerospace engineering at Mizzou. “It will have broad impacts on wearable sensors, customizable robots, medical devices and more.”

Revolutionary techniques

Currently, manufacturing a multi-layered structure—such as a printed circuit board—can be a cumbersome process that involves multiple steps and materials. These processes are costly, time consuming, and can generate waste that harms the environment.

Not only is the new technique better for the planet, it’s inspired by systems found in nature.

“Everything in nature consists of structural and functional materials,” Zheng said. “For example, electrical eels have bones and muscles that enable them to move. They also have specialized cells that can discharge up to 500 volts to deter predators. These biological observations have inspired researchers to develop new methods for fabricating 3D structures with multi-functional applications, but other emerging methods have limitations.”

Specifically, other techniques fall short when it comes to how versatile the material can be and how precisely smaller components can be placed inside larger 3D structures.

The Mizzou team’s method uses special techniques to solve these problems. Team members built a machine that has three different nozzles: one adds ink-like material, another uses a laser to carve shapes and materials, and the third adds additional functional materials to enhance the product’s capabilities. It starts by making a basic structure with regular 3D printing filament, such as polycarbonate, a type of transparent thermoplastic. Then, it switches to laser to convert some parts into a special material called laser-induced graphene, putting it exactly where it’s needed. Finally, more materials are added to enhance the functional abilities of the final product.

“The I-Corps program is helping us identify market interests and needs,” Lin said. “Currently, we believe it would be of interest to other researchers, but we believe it will ultimately benefit businesses. It will shorten fabrication time for device prototyping by allowing companies to make prototypes in house. This technology, available only at Mizzou, shows great promise for transforming the way products are fabricated and manufactured.”