## 6 Outlook and Future Directions  
In recent years, the advancement of 2D material fabrication has opened new possibilities for creating high-performance electronic devices. One significant challenge in the deterministic assembly of van der Waals heterostructures through mechanical stacking is the reduction of residues trapped between the stacked layers. These residues often form bubbles and ripples, which limit the effective clean surface area available for final device fabrication [110, 111]. As highlighted by [PERSON] et al. [9], eliminating residues is particularly important for thinner devices, like single and bilayer field-effect transistors (FETs). Such thin devices experience drastic deterioration in electronic properties, including carrier mobility and homogeneity, due to environmental factors, like residual water and oxygen in the transfer chamber. Standard glovebox chambers, which only achieve partial oxygen and water pressures down to \(10^{-4}\) mbar, fail to provide a sufficiently clean environment, leading to surface damage and inefficient self-cleaning processes in thin crystals. This results in rough interface morphology and complex local strain, contributing to poor device performance. Furthermore, the use of argon or nitrogen environments, even with high-quality gas purification systems, does not completely eliminate hydrocarbon contamination or bubble formation, further degrading the quality of van der Waals interfaces. As highlighted in Section 5.1 of this review, despite experimental demonstrations showing the potential for fabricating heterostructures under high vacuum, these methods are not yet widely adopted due to their complexity, cost, and scalability issues. Additionally, the transfer method might still involve exposing the 2D materials to air at certain stages of the assembly process [5, 88]. Developing cleaner transfer methods or implementing these techniques in controlled ultra-high vacuum environments will be crucial research directions in the near future. A recent review by [PERSON] et al. [78] provides an overview of recent efforts to develop clean 2D materials and devices. The second challenge is the inherent sample-to-sample variability in manual transfer processes, which are highly dependent on operator skills. This variability complicates not only the comparison of samples produced in different laboratories but also those made in the same laboratory by different researchers or at different times. To address this, the development of robotic approaches [85] using computer vision and machine learning algorithms to autonomously identify, pick up, and transfer materials is an urgent future avenue to be pursued. This is particularly important for research focused on complex heterostructure architectures. With such advancements, van der Waals heterostructures could offer extensive flexibility in the damage-free integration of distinct materials, surpassing the limitations of traditional chemical integration. Lastly, it is important to develop new experimental setups and techniques for using 2D materials to fabricate more complex systems with enhanced reliability and functionality. Recently, unique and modified techniques have been developed for the deterministic transfer of 2D materials, such as the pick-and-place approach [85], the polyvinyl alcohol (PVA) method [92], viscoelastic stamping [96], Evalcite method [121], the wedging method [125], patterned pick-and-place stamping method [126], patterned monolayer transfer [127], and metal-assisted transfer [128]. Each of these methods helps transfer 2D materials efficiently but is limited by its shortcomings and scope of implementation, such as obtaining a high yield of quality flakes, avoiding cross-contamination of the entire substrate from undesired flake transfer, demonstrating repeatability in device performance, and avoiding undesired residues on flakes [89].