Polymer-free van der Waals method enables clean 2D material handling, boosting precision and reliability in nanoscale device fabrication.
(Nanowerk Spotlight) Atomically thin materials such as graphene, molybdenum disulfide (MoS₂), and hexagonal boron nitride (h-BN) hold promise for applications in nanoelectronics, photonics, and flexible electronic devices. Their ultrathin nature gives rise to remarkable electronic, optical, and mechanical properties, but it also exposes them to significant challenges during fabrication.
Chief among these is surface contamination introduced by standard handling techniques, which often rely on polymer-based supports to transfer or stack layers. Even after aggressive cleaning steps, residues from polymers like poly(methyl methacrylate) (PMMA) frequently remain. These traces can degrade performance by introducing unwanted doping, mechanical strain, or interfacial defects that disrupt charge transport and light-matter interactions.
Efforts to improve the cleanliness of fabrication processes have included the use of alternative transfer media such as polypropylene carbonate, metal films, and even ice. Others have employed mechanical cleaning strategies using tools like atomic force microscopy to physically sweep impurities off the material surface. While these methods help reduce contamination, they often trade off compatibility, scalability, or process control.
Importantly, many of these techniques still involve dissimilar materials at critical interfaces, such as using polymers or elastomers to pick up 2D flakes—materials that differ chemically and structurally from the flake itself. These mismatches affect adhesion, introduce strain, and complicate device integration.
More recent work has turned toward using van der Waals (vdW) interactions—weak electrostatic forces between adjacent atomic layers—as a non-invasive method for handling 2D materials. These forces can support stacking and manipulation without the need for adhesives or solvents.
However, most previous demonstrations used support layers made from materials different from the 2D layer being transferred. This results in non-uniform adhesion energy and limits precision in manipulating flakes. In addition, current vdW-based techniques often depend on complex equipment or require fine tuning of parameters such as metal coating thickness to achieve stable contact.
Researchers at the Gwangju Institute of Science and Technology have now reported a new fabrication method in the journal Advanced Materials (“Residue-Free Fabrication of 2D Materials Using van der Waals Interactions”) that addresses these limitations by using van der Waals interactions in a more direct and consistent way. Their approach replaces polymer-based handling with a residue-free stamp made of the same 2D material as the target flake. Using molybdenum disulfide (MoS₂) as both the stamp and the transferred layer, the technique eliminates chemical mismatch at the interface and enables clean, strain-free manipulation without requiring complex instrumentation or post-processing.
The fabrication procedure for residue-free MoS2 flakes. a) Schematic illustrations of the achievement of a residue-free region using the tape stamp. b) The SM and c) TM approaches for transferring the target flake onto a SiO2 substrate, along with AFM topography images of the transferred target flake (SM1 and TM1). These approaches enable the simultaneous achievement of both the residue-free stamp (RFS) and target flake. d) The results of accessible thickness and Rq for SM and TM. (Image: Reprinted from DOI:10.1002/adma.202418669, CC BY)
To prepare the stamp, the team exfoliated bulk MoS₂ using a tape-and-glass-slide system designed to expose clean, thin regions. The exfoliated flake was then brought into contact with a substrate using either a stamping or tearing method. Stamping involved separating clean layers from the rest of the flake based on interlayer adhesion differences, while tearing allowed thicker flakes to be removed by applying shear forces.
In both cases, the transfer process relied purely on vdW forces, avoiding exposure to solvents, heat, or other mechanical stress. Atomic force microscopy confirmed the transferred flakes had minimal roughness, with root-mean-square values as low as 0.06 nanometers—substantially better than those seen in standard methods.
The material quality was assessed using high-resolution transmission electron microscopy, electron diffraction, and Raman spectroscopy. These tools confirmed the transferred MoS₂ maintained a pristine hexagonal structure, showed no signs of lattice damage or oxidation, and exhibited uniform distribution of molybdenum and sulfur atoms. Notably, no traces of carbon or oxygen were detected, suggesting that no significant polymer or chemical residues were introduced during the process.
Field-effect transistors (FETs) fabricated from these clean flakes showed mobility values between 46 and 60 cm²/V·s and on/off current ratios up to 10⁸. These values exceed those achieved by many devices using conventional fabrication processes involving SiO₂ substrates and gold electrodes. The devices required no polymer resists during lithography, and their performance remained stable after vacuum annealing. Raman spectroscopy confirmed that the channel regions in the transistors were free from processing-induced strain, indicating that the mechanical steps used in the transfer had not disturbed the atomic lattice.
Beyond clean transfer, the method enabled a broad range of flake manipulation techniques. These included picking up and releasing flakes, stacking multiple layers with precise alignment, exfoliating partial layers, flipping flakes to reverse orientation, smoothing out wrinkles, and selectively removing defective or thin regions. Each of these operations relied on the well-matched adhesion between similar materials and the mechanical compliance of the residue-free stamp.
For instance, the pickup process was guided by the stronger adhesion between MoS₂ layers compared to the MoS₂–substrate interface, allowing the stamp to lift the flake cleanly. Release was achieved through lateral sliding, taking advantage of the low interlayer friction between 2D materials—a condition known as superlubricity. This interplay between adhesion, thickness, and bending stiffness determined which flakes could be reliably manipulated. Thinner flakes responded better to pickup and release, while thicker flakes resisted due to higher rigidity. These effects were supported by both experimental results and mechanical modeling.
Using this approach, researchers assembled multilayer heterostructures such as h-BN/MoS₂/graphene stacks. Each layer was deposited using the same material-based stamp system, achieving precise placement without introducing interfacial defects. Atomic force microscopy showed the resulting stacks were smooth and clean. Where blisters did appear at the interface, they were removed using a calibrated AFM-tip squeezing technique, which preserved the underlying layers while restoring flatness.
Raman analysis of the heterostructures showed minimal spectral shifts, indicating only mild reversible strain and no signs of lattice distortion or chemical modification. This confirmed that the entire manipulation sequence, including wrinkle smoothing and stacking, maintained the structural and electronic integrity of each layer. The process was also extended to other combinations of 2D materials, such as MoS₂/h-BN and graphene/h-BN, confirming the method’s flexibility and applicability beyond a single system.
The use of matching material interfaces, the absence of chemical treatment, and the capacity for precise manipulation all contribute to a process that is clean, controllable, and reproducible. By eliminating polymers, solvents, and high-temperature steps, the method avoids key sources of damage and variability that have hindered the practical use of 2D materials in devices.
While further work is needed to scale the process and expand its use in large-area manufacturing, this residue-free van der Waals method offers a reliable pathway for producing high-quality 2D interfaces. Its simplicity, adaptability, and cleanliness suggest broad potential for applications in nanoelectronics, sensors, and flexible systems where interfacial quality is critical.
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