Thermomechanical nanomolding of nanowires spurs unpredictable phases


April 20, 2023

(Nanowerk News) Sometimes to make a big break, you have to start very small.

One way that scientists can get the most out of certain quantum materials is by creating nanoscale structures that generate new properties on the surface and edges of the material. The Cornell researchers used a relatively straightforward thermomechanical nanomolding process to fabricate single-crystalline nanowires that can activate metastable phases that are difficult to achieve with conventional methods.

Scanning electron microscopy image offers a top-down view of single-crystalline Mo nanowires4P3 made by thermomechanical nanomolding.

“We are very interested in this nanomolding synthesis method because it allows us to make various types of materials to nanoscale quickly and easily, but with some controls that other nanomaterials synthesis methods do not have, especially control over morphology and size. ,” says Judy Cha, Ph. D. ’09, professor of materials science and engineering at Cornell Engineering, who led the project.

The team paper, “Nanomolding of Metastable Mo4P3,” published in Affairs (“Nanomolding of metastable Mo4P3). The lead author of this paper is postdoctoral researcher Mehrdad Kiani. Scanning electron microscopy image offers a top-down view of the single-crystalline Mo4P3 nanowires fabricated by thermomechanical nanomolding. Scanning electron microscopy image offers a top-down view of single-crystalline Mo nanowires4P3 made by thermomechanical nanomolding. (Image: Cornell University)

In thermomechanical nanomolding, a material is consolidated into a bulk raw material, loaded into a porous mold and pressed at high temperature for several hours. The resulting structure is then separated from the feedstock – in this case, by ultrasonic vibrations, a process known as sonication – and deposited on a silicon wafer or other surface.

The benefit of this process is that nanoscale solid quantities of material can be printed at temperatures well below their melting point, representing easy processing conditions. This allows a wide range of materials to be harnessed for untapped exotic properties, similar to the way graphene revolutionized conduction in electronics.

Cha’s team has been experimenting with molybdenum monophosphide (MoP), which is a topological compound.

“Topological metals are predicted to have decreasing resistance as you move down to smaller sizes, and MoP is not only topological but also has a very high carrier density (electrons per volume), which will further help lower resistance,” said Kiani.

Cha and his team have previously shown that nanomolding of topological nanowires can accelerate the discovery of new electrical properties for applications such as quantum computing, microelectronics, and clean energy catalysts. These nanowires would be especially suitable for the interconnection between the billions of transistors in integrated circuits.

Earlier this year, the group demonstrated that MoP nanowires have such a low resistivity that they outperform copper interconnects.

“It was a surprising find,” said Cha. “But the challenge is, we have to keep making the MoP smaller and smaller, and the methods we’re using are not getting us there. So then came the nanomolding method, and we looked at it as a way to make smaller MoP nanowires to continue to check if their resistivity was still much lower than that of copper.”

Instead, they found the process of printing the nanowires changed the crystal structure of MoP to a different composition: Mo4P3.

“It’s not something we expected. And what is even more surprising is this Mo phase4P3 not the steady phase you usually get,” said Cha. “Now we realize that this printing method has the potential to provide us with a metastable phase.”

Metastable Mo4P3resistivity is about 75% higher than MoP, so MoP is still the most promising candidate for interconnection.

“This really expands our exploration space for new materials. And who knows what the odds are? said Cha. “When graphene was first discovered it was not at all clear whether we could use it in golf balls, for example, to think of mundane applications. For now, we want to find the next example of Mo4P3another metastable phase that we can capture and then fabricate into nanowires.”


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