Recent progress in the assembly of nanodevices and van der Waals heterostructures by deterministic placement of 2D materials

Abstract: Designer heterostructures can now be assembled layer-by-layer with unmatched precision thanks to the recently developed deterministic placement methods to transfer two-dimensional (2D) materials. This possibility constitutes the birth of a very active research field on the so-called van der Waals heterostructures. Moreover, these deterministic placement methods also open the door to fabricate complex devices, which would be otherwise very difficult to achieve by conventional bottom-up nanofabrication approaches, and to fabricate fully-encapsulated devices with exquisite electronic properties. The integration of 2D materials with existing technologies such as photonic and superconducting waveguides and fiber optics is another exciting possibility. Here, we review the state-of-the-art of the deterministic placement methods, describing and comparing the different alternative methods available in the literature and we illustrate their potential to fabricate van der Waals heterostructures, to integrate 2D materials into complex devices and to fabricate artificial bilayer structures where the layers present a user-defined rotational twisting angle.

• The paper introduces deterministic placement methods that enable precise assembly of nanodevices and heterostructures.
• It compares techniques such as PDMS transfer and van der Waals pick-up to optimize speed, cleanliness, and twist control for twistronics.
• The study explores integrating 2D materials into complex devices while addressing challenges like interlayer contaminants and environmental degradation.

Recent Advances in Deterministic Placement of 2D Materials for Nanodevice Assembly

The paper by Frisenda et al. examines the advancements in the assembly of nanodevices and van der Waals heterostructures through the deterministic placement of two-dimensional (2D) materials. The deterministic placement of 2D materials has emerged as a significant area of focus, offering unprecedented precision and opening up new avenues in the fabrication of complex devices, which traditional methods cannot achieve efficiently. This paper provides a detailed overview and comparison of various deterministic placement techniques, evaluating them based on factors like cleanness, ease of implementation, and speed.

Deterministic Placement Methods

The paper categorizes and describes multiple deterministic placement techniques:

1. PMMA Carrying Layer Method: This technique, which primarily relies on PMMA as a carrier layer, enables high-mobility graphene devices on hexagonal boron nitride substrates. It involves a water-soluble release layer beneath the PMMA, facilitating the detachment and transfer of the flake.
2. Elvacite Sacrificial Layer: This method employs Elvacite as a sacrificial polymer layer, offering a simpler release mechanism through controlled melting. It allows the transfer of 2D materials onto substrates by leveraging low glass transition temperatures to detach materials from the adhesive backing.
3. Wedging Transfer Method: Utilizing water-mediated intercalation to separate 2D materials, this method is beneficial for handling uneven surfaces but is limited by the presence of surface imperfections, such as trapped water blisters and wrinkles.
4. PDMS Deterministic Transfer Method: Focused on all-dry processes without the need for polymer solvents, this method uses PDMS viscoelasticity for high-speed, capillary-free transfers, making it suitable for creating delicate suspended 2D materials without solvent residues.
5. Van der Waals Pick-up Transfer: This innovative technique leverages the natural adhesion forces between 2D materials to achieve cleaner transfers by avoiding direct polymer contact. However, it remains complex and is mainly applicable to heterostructure assemblies.

The comparison highlights that while PDMS-based methods provide advantages in speed and ease of use, van der Waals pick-up methods yield cleaner interfaces but require more steps and are limited to heterostructure applications.

Fabrication and Twistronics

The fabrication of van der Waals heterostructures is discussed in detail, showcasing the ability to use a broad array of 2D materials to create devices with unprecedented atomic-level precision. The deterministic placement methods enable the assembly of 2D layers with precise control over the twisting angle between them, a factor that has spurred interest in the emerging field known as "twistronics." The paper illustrates how controlling these twist angles produces novel superlattices with unique electronic and photonic properties that have been inaccessible through conventional growth methods like molecular beam epitaxy (MBE).

Integration into Complex Devices

The paper elaborates on integrating 2D materials into advanced devices, using deterministic placement to achieve complex architectures. Notable examples include the creation of high-performance photodetectors and optomechanical couplings with superconducting cavities. This facilitator of integration extends to placing 2D materials in non-traditional contexts, such as on lenses, optical fibers, and curved surfaces.

Challenges and Prospects

Despite significant progress, challenges such as interlayer contaminants and environmental degradation of sensitive 2D materials remain. The paper discusses approaches aiming to minimize these issues, such as careful lamination under inert conditions to prevent air degradation. Future research may further improve environmental stability and enhance the clean transfer of materials, making broader applications possible.

Conclusions

The deterministic placement of 2D materials has evolved to address practical and theoretical challenges in nanodevice fabrication effectively. This progress allows researchers to construct intricate heterostructures, explore novel electronic properties, and enhance device performance through methodical control over layer orientation and composition. The methodologies discussed in the paper pave the way for continuing advancements in materials science and nanotechnology, offering a robust framework for future explorations in 2D material applications. The ongoing development of these techniques is poised to unlock new potentials in nanoelectronics, optics, and beyond.