Researchers use ultrasonic waves to assemble copper nanosheets into uniform, conductive coatings on curved and flexible surfaces for electronics and sensors.
(Nanowerk Spotlight) The ability to deposit conductive films onto surfaces with irregular shapes is becoming increasingly important for electronics manufacturing. Applications such as wearable sensors, biomedical implants, and flexible energy devices require materials that can conform to three-dimensional or flexible substrates.
Traditional methods like vacuum deposition, sputtering, and chemical vapor deposition have been widely used to create high-quality conductive films. However, these methods are expensive, require cleanroom environments, and are best suited for flat, rigid surfaces. This makes them poorly suited to producing films on curved or textured geometries, and limits their relevance for soft or wearable electronics.
To overcome these constraints, researchers have explored solution-based techniques such as spray-coating, inkjet printing, and blade-coating. These offer better compatibility with flexible materials and allow for scalable processing. Yet they struggle with precision and uniformity when applied to complex surfaces.
Moreover, the conductive fillers used in these methods—such as metal nanoparticles or nanowires—come with their own limitations. Their small size or elongated shapes often result in poor film connectivity, uneven adhesion, or uncontrolled aggregation. These problems restrict the electrical and mechanical performance of the resulting films.
One possible solution involves the use of two-dimensional (2D) metal nanosheets—ultrathin plate-like structures that offer a large contact area with the substrate and fewer electrical junctions. Among available metals, copper stands out for its high electrical conductivity, low cost, and resistance to oxidation when prepared with specific crystal orientations.
In particular, copper nanosheets with a (111) orientation display good chemical stability and surface smoothness, which help them adhere more effectively to substrates. However, existing techniques have been unable to reliably assemble these nanosheets into uniform films on non-flat surfaces. The challenges lie not just in maintaining control over their arrangement, but also in ensuring that they bond adequately with the substrate without overlapping or detaching.
In a study published in Advanced Materials (“Omni‐Directional Assembly of 2D Single‐Crystalline Metal Nanosheets”), researchers from Jeonbuk National University and collaborating institutions describe a new approach to this problem using ultrasonic waves to assemble copper nanosheets in an omni-directional manner. Their method uses cavitation—the rapid collapse of microbubbles induced by high-frequency sound waves in a liquid medium—to drive the nanosheets toward hydrophilic surfaces, where they adhere and form monolayer or near-monolayer films.
The process works on surfaces with a range of geometries and materials, including flat wafers, capillary tubes, and flexible polymers. It does not require vacuum systems or elevated temperatures and relies only on commercially available solvents and equipment.
Ultrasonic-driven assembly of copper nanosheets. A) A schematic of the ultrasonic-driven assembly process. Ultrasonic waves generate cavitation bubbles in the solvent, which collapse and produce microjets that propel copper nanosheets toward the substrate, facilitating uniform deposition. B) Comparison of copper nanostructures after ultrasonication. SEM images, photographs, and schematic diagrams of copper nanoparticles (0D), copper nanowires (1D), and copper nanosheets (2D) on substrates post-ultrasonication; inset shows a scale bar of 5 mm. The 2D copper nanosheets exhibit superior substrate coverage and uniformity compared to their 0D and 1D counterparts. C) Cavitation formation and influence of acoustic forces. Schematic representation of cavitation bubble formation during ultrasonication and the resulting acoustic forces acting on the 2D metal nanosheets. D) Possible assembly scenarios under ultrasonication. Schematic diagrams illustrating two different outcomes: (①) overlapping cooper nanosheets leading to non-uniform assembly and lift-off, and (②) well-assembled, non-overlapping copper nanosheets resulting in uniform monolayer films. (Image: reprinted from DOI:10.1002/adma.202501632, CC BY) (click on image to enlarge)
The researchers began by observing an unexpected effect during routine ultrasonication of copper nanosheet dispersions. After sonication, the interior walls of the glass vial showed a shiny, metallic coating—evidence that the nanosheets were assembling onto the substrate in response to cavitation forces. Further investigation revealed that this behavior could be reproduced and controlled. When ultrasound waves propagate through a liquid, they create cavitation bubbles that collapse near solid surfaces.
The resulting microjets propel suspended particles toward the surface. In this case, the flat geometry of the nanosheets ensures a high contact area, promoting van der Waals interactions—weak but collectively significant forces that help secure the nanosheets to the substrate.
To test how nanosheet morphology affects assembly, the team compared the behavior of zero-dimensional copper nanoparticles, one-dimensional copper nanowires, and two-dimensional copper nanosheets under identical conditions. While the particles and wires failed to assemble uniformly, the nanosheets adhered consistently and covered the substrate with minimal overlap. This confirmed the role of geometry in determining whether cavitation forces would lead to stable film formation.
Surface chemistry also played a critical role. The team found that the process only worked reliably when the nanosheets were dispersed in non-polar solvents such as chloroform. In these solvents, the hydrophobic copper nanosheets avoided remaining in solution and instead adsorbed onto the hydrophilic substrate to reduce overall surface energy.
By contrast, polar solvents tended to coat the substrate with solvent molecules, preventing the nanosheets from making contact and resulting in poor film formation. Oxidized copper nanosheets, which have rougher surfaces and altered chemical characteristics, also performed poorly regardless of solvent. The increased roughness disrupted adhesion, and cavitation bubbles trapped in the gaps caused the sheets to dislodge during sonication.
The researchers identified a clear relationship between film quality, processing time, and nanosheet concentration. Longer sonication times and higher concentrations improved surface coverage up to a point, but excessive exposure led to diminishing returns. After 25 minutes of sonication, cavitation forces began to damage the nanosheets or dislodge them from the substrate. The best results were achieved with smaller nanosheets (around 5 micrometers in lateral dimension), which were better able to fill gaps and form continuous films. Under optimal conditions, the team achieved surface coverage as high as 88 percent.
To better understand the limits of this method, the researchers extended the process to silver nanosheets, which share a similar (111) crystal orientation but differ in surface energy and roughness. Silver nanosheets adhered less effectively and reached a maximum surface coverage of just over 38 percent. This confirmed that both surface roughness and material-specific surface energy affect how well the nanosheets assemble under ultrasonic conditions.
One of the key advantages of this approach is its ability to pattern films without using masks or lithography. The researchers chemically modified the substrate to include both hydrophilic and hydrophobic regions. When immersed in the nanosheet solution and exposed to ultrasound, only the hydrophilic areas became coated with copper. This selective assembly allowed for precise localization of conductive films, opening the door to patterned electronic circuits or sensors.
To demonstrate real-world applicability, the team fabricated a functional heater by assembling copper nanosheets onto the exterior of a capillary tube. The film, which had a resistance of approximately 128 ohms, heated efficiently under low voltages. Infrared imaging showed that the tube reached 57 degrees Celsius within 25 minutes when powered at just 2.5 volts. The film remained stable under conditions of high humidity, elevated temperature, and long-term storage, confirming its durability and suitability for practical use.
Ultrasonic-driven omni-directional assembly of copper nanosheets (Cu NS) on complex geometries and flexible substrates. A) Schematic of Cu NS deposition on complex 3D structures. The teapot exemplifies the potential for uniformly coating complex 3D surfaces using ultrasonication. B) Photographs of the assembly of Cu NS on various substrates and objects. From left to right: a transparent glass teapot before Cu NS coating, illustrating the original substrate; a vial containing the Cu NS dispersion with the glass teapot submerged during the ultrasonication process; the same teapot after coating, with a uniform layer of Cu NS on its surface. The inset on the right provides a close-up view of the nanosheet coverage. C) Examples of Cu NS–assembled objects demonstrating the versatility of the coating technique. From left to right: A small pumpkin-shaped object, a dolphin-shaped figurine, a glass die with assembled Cu NS, and a flexible copper-coated substrate. (Image: reprinted from DOI:10.1002/adma.202501632, CC BY) (click on image to enlarge)
The researchers also showed that their method could be extended to a wide range of objects and surfaces. They successfully coated decorative glass objects with complex shapes, flexible polymer sheets, and even the inner curves of a teapot spout. These results highlight the adaptability of ultrasonic-driven assembly to non-planar substrates—a major limitation of conventional deposition methods.
This study not only introduces a new technique for fabricating conductive films but also provides insight into the physical mechanisms that govern particle-substrate interactions under ultrasonic fields. The combination of cavitation physics, surface chemistry, and nanosheet morphology offers a tunable platform for building conductive coatings on a wide variety of materials. By avoiding the need for vacuum chambers, precise nozzles, or patterning steps, this approach simplifies the fabrication of next-generation electronic components that require flexibility, durability, and compatibility with complex geometries.
Get our Nanotechnology Spotlight updates to your inbox!
Thank you!
You have successfully joined our subscriber list.
Become a Spotlight guest author! Join our large and growing group of guest contributors. Have you just published a scientific paper or have other exciting developments to share with the nanotechnology community? Here is how to publish on nanowerk.com.
