Scientists Use Force to ‘Write’ Atomically Thin Superconductors

For decades, heat has been the workhorse of materials manufacturing, providing the energy needed to drive the chemical reactions that create everything from semiconductors to advanced electronic materials. But what if, instead of heating an entire device, scientists could use mechanical force to trigger those same reactions only where they were needed?

A close collaboration between Penn, led by Robert Carpick, John Henry Towne Professor in Mechanical Engineering and Applied Mechanics, and Princeton University, led by Sanfeng Wu, Associate Professor of Physics, has demonstrated exactly that. Together, the team has developed a technique that uses the tip of an atomic force microscope (AFM) to “write” an atomically thin quantum material with nanometer precision. Published in Physical Review X, the study introduces a new manufacturing approach that could enable more energy-efficient fabrication of next-generation electronic and quantum devices.

“Our work shows that mechanical force can complement and sometimes even replace heat as a catalyst for creating new materials through mechanochemistry,” says Carpick. “Using the AFM, we’ve developed an energy-efficient tool for making entirely new materials exactly where we want them.”

The atomic force microscope (AFM) used for these experiments (left) and an electron microscopy image of the type of AFM tip used to apply force to the sample in the experiments, shown making a nanoscale contact with a flat substrate (right).

From Discovery to Manufacturing

The project builds on recent work led by Wu, who discovered that heating two materials together — a thin layer of molybdenum ditelluride placed over patterned palladium — causes them to react and form an entirely new atomically thin material with the chemical formula Pd₇MoTe₂.

The material behaves as a superconductor at low temperatures, meaning it conducts electricity with no resistance. Moreover, at room temperature it is extremely flat and atomically thin, which could be useful for future technologies. 

But, by using heat as the main manufacturing approach to create Pd₇MoTe₂, there was a practical limitation: heating caused the new material to spread outward wherever the reaction could occur, offering little control over its final shape. 

For researchers hoping to build electronic devices, that limited precision paired with the energy cost of heating the entire material posed a significant challenge.

An illustration of the setup for thermally triggered growth of the 2D metal palladium on a single layer transition metal dichalcogenide (MoTe₂) detailed in the study conducted by the Princeton team in 2024 shows the surprising spreading of extremely uniform palladium (precisely 7-atom-layer) and the new material Pd₇MoTe₂ (in purple) produced through the heat process alone (source: Figure 1 in Jia et al., 2024).

Using Mechanical Force to Manufacture

The collaboration between Penn and Princeton began when Shuai Zhang, a former postdoctoral researcher in the Carpick group, connected the dots. 

“My postdoctoral research focused on mechanochemistry, and Rob and I had been looking for a two-dimensional material system to explore it,” says Zhang. “When I came across the Princeton team’s work using heat to drive chemical bonding, we immediately saw an opportunity to ask whether mechanical force could do the same.”

Working with Wu and his team at Princeton as well as Andrew Rappe, Blanchard Professor of Chemistry in Penn Arts & Sciences, whose group provided theoretical modeling to explain the underlying mechanisms, the researchers combined expertise in materials science, mechanics and computational chemistry to turn that question into a new approach to materials synthesis.

They began by gently warming the material stack to initiate the reaction, after which they lowered the temperature to prohibit further thermal growth. They then used the nanoscale tip of an atomic force microscope to press and slide across carefully chosen regions, applying mechanical force precisely where the new material was beginning to form. Remarkably, the forces triggered the growth of the same material locally at much reduced temperatures.

The result was a dramatic reduction in the energy needed to produce the new metal due to the force applied, and the team is able to do so  at desired locations and with custom patterns and nanoscale precision. Rather than relying on heat to drive the reaction across the material, the researchers could use mechanical force to locally trigger its formation. The resulting Pd₇MoTe₂, just ten atomic layers thick, formed only where the microscope tip was applied. This allowed the researchers to effectively “draw” conducting pathways approximately 50 nanometers wide — about 1/1,000th the width of a human hair — directly into the two-dimensional material stack.

“With the thermal growth technique, it is very difficult to control the exact material pattern and device geometry,” says Wu. “By applying force with a nanoscale tip, we can reduce the reaction temperature from around 200°C to near room temperature and, because the tip acts like a nano-writer, precisely write arbitrary patterns of these new 2D superconductors according to design. This is a breakthrough in constructing 2D material devices.”

“Heat and force really become a one-two punch,” adds Carpick. “The force allows us to work at much lower temperatures, meaning this approach is more energy efficient while giving us precise control over where the material grows.”

And the ability to control where the material forms also gave the researchers an unprecedented way to investigate a fundamental question: Why does this particular reaction produce a material exactly seven layers thick?

“This project really grabbed our attention because of the team’s ability to synthesize a specific seven-atom-layer metal sheet on a three-atom-layer semiconductor,” says Rappe. “From there, we set out to figure out why seven layers specifically, not one or 100, was part of why this material had such unique properties. The balance of mechanical and chemical forces in this manufacturing technique led to amazing control over the material.”

A Breakfast Analogy to Visualize the Making of this New Metal

Creating metals only a few atoms thick has long challenged materials scientists, and visualizing the process can be even harder, especially for non-experts.

Carpick likens the process to laying a thin crepe over sticks of butter resting on a warm pan.

“Think of the palladium as the sticks of butter and the molybdenum ditelluride as a crepe laying on top of the butter. As the butter softens, it spreads beneath the crepe,” he explains. “Only here, instead of simply spreading, the materials, the butter and the crepe or the palladium and the molybdenum ditelluride, react together to create something entirely new.”

The analogy helps illustrate why conventional heating alone offers little control. Once the reaction begins, it naturally and uncontrollably expands outward.

The Penn Engineer’s mechanochemical approach changes that dynamic, allowing researchers to guide the reaction with remarkable precision rather than simply letting it spread like melting butter.

In the new mechanochemical manufacturing process, the AFM tip presses onto the newly formed metal, shown in purple, and can drag out the line of its formation into specific patterns as thin as 50 nanometers wide. This is an advancement from the previous manufacturing process using heat alone, which could only produce the superconducting metal in an unpredictable and uncontrollable shape, similar to the way butter melts in a hot pan.

Collaborating Toward Future Quantum Devices

Because the material can be written directly into custom patterns, the approach opens possibilities far beyond creating a new material.

The researchers have already fabricated nanoscale wires and prototype device structures directly through the writing process, without relying on conventional lithographic patterning.

“This work simply wouldn’t have happened without collaboration,” says Carpick. “The NSF-funded Center for the Mechanical Control of Chemistry, of which Andrew Rappe and I are both members, brings together researchers who think about chemistry, mechanics, materials and theory in different ways. That environment gave us the opportunity to connect ideas that otherwise might never have come together, including the ability to collaborate with the Princeton group, making this advance possible.”

Looking ahead, the team believes mechanochemical manufacturing could provide a new way to build future nanoelectronics, superconducting circuits and quantum devices while reducing energy consumption and avoiding the high temperatures that can damage other components during fabrication.

Learn more about the work being done in the Carpick lab here.

This research was supported by the U.S. Air Force Office of Scientific Research (FA9550-23-1-0140 and FA9550-25-1-0354); the National Science Foundation through the Center for the Mechanical Control of Chemistry (CHE-2303044), the Materials Research Science and Engineering Center (MRSEC) program (DMR-2011750) and the National Nanotechnology Coordinated Infrastructure Program (NNCI-2025608); Princeton University’s Imaging and Analysis Center and Princeton Catalysis Initiative; the Gordon and Betty Moore Foundation (GBMF11946); the Fundamental Research Funds for the Central Universities (226-2025-00104); and the Japan Society for the Promotion of Science (JSPS KAKENHI Grant Nos. 21H05233 and 23H02052) and the World Premier International Research Center Initiative (WPI), MEXT, Japan. The atomic force microscopy work was carried out at the Singh Center for Nanotechnology.