Batch-Fabricated PDMS Templates for the Robotic Transfer of 2D Materials

Abstract

Robotic stacking of van der Waals heterostructures has been at the verge thanks to the convergence between artificial intelligence (AI) and two-dimensional (2D) materials research. Key ingredients to fulfill this pursuit often include algorithms to identify layer compounds on chips, hard-wares to realize sophisticated operations of motion and/or rotation in a microscale, and, as importantly, highly-standardized and uniform transfer stamps that are often used in picking up layered materials under a microscope. Here, we report a hot-casted-droplet batch fabrication method for polydimethylsiloxane (PDMS) templates tailored for dry transfer of 2D materials. Controlled precursor formulation, degassing, and motorized-syringe dispensing produce dome-shaped PDMS templates with ultra-smooth surfaces (root-mean-square roughness about 0.3 nm at relatively low curing temperatures). By tuning the curing temperature, the reproducible and controllable apex curvature allows precisely defined contact area between the organic adhesive film and substrate, via thermal expansion. Our results further reveals thermalmechanical behaviors with different casting parameters of such PDMS domes. This scalable and parameterized fabrication protocol gives rise to uniform transfer-stamps with ultra-smooth surface, which may be beneficial for future AI-driven robotic assembly of 2D material heterostructures.

Figures (5)

• Figure 1: Schematic of the PDMS-based stamp for dry transfer of 2D materials. The stamp consists of a glass slide, a hemispherical PDMS template, and a polycarbonate (PC) film. Vertical motion controlled by a precision mechanical stage allows the PC film to pick up 2D flakes via controlled adhesion. The flexible PDMS template enables gradual, programmable contact with the target substrate, supporting iterative assembly of multilayer van der Waals heterostructures and quantitative AI-assisted robotic transfer.
• Figure 2: Preparation of the PDMS precursor and crosslinking reaction mechanism. (a) Mixing the base prepolymer and crosslinking agent at a 10:1 weight ratio. (b) Mechanical stirring to ensure homogeneous blending. (c) Vacuum degassing to eliminate entrapped air bubbles. (d) Schematic illustration of the Platinum-catalyzed crosslinking reaction during thermal curing. (e) Schematic of the arrayed microinjector system for precise and reproducible batch dispensing of PDMS templates, the system integrates a motorized stage, an injector array, and a temperature-controlled hotplate, enabling uniform deposition of PDMS droplets onto preheated glass slides for subsequent 2D material transfer.
• Figure 3: Surface roughness characterization of PDMS templates. (a) Three-dimensional AFM topography of a typical PDMS template cured at 120 ℃. (b) High-resolution AFM height map obtained from a $1~\mu\mathrm{m}^2$ area at the template apex. (c) Height profile extracted along the dashed line in panel (b). (d) Root-mean-square roughness ($R_q$) as a function of curing temperature, with error bars indicating the standard deviation. Insets show the corresponding roughness distribution histograms. (e) Comparison of PDMS surface roughness prepared using metal, resin, and photopolymer molds versus the proposed batch-fabrication method. Dashed lines and color gradients are included as visual guides to distinguish the different fabrication routes.
• Figure 4: Temperature-dependent geometry and mass of batch-fabricated PDMS templates. (a) Apex curvature radius ($R$) as a function of curing temperature, with error bars representing the standard deviation. The top-right inset shows a representative hemi-ellipsoidal PDMS template and its geometric parameters: $r_a$ (major semi-axis) and $r_b$ (minor semi-axis). Histograms display the distribution of $R$ obtained from 20 replicate templates at each curing temperature. (b) Evolution of $R$, $r_a$, and $r_b$ with curing temperature, illustrating the coordinated dimensional changes induced by accelerated cross-linking dynamics at elevated temperatures. (c) Template mass as a function of curing temperature, demonstrating excellent batch-to-batch reproducibility across all curing conditions.
• Figure 5: Thermomechanical behavior of PDMS templates at different curing temperatures. (a) Schematic of the dry-transfer setup, including the transfer stamp, motorized precision stage, and hotplate. (b) Optical images showing the formation of initial Newton's rings and the gradual expansion of the circular contact area toward the target 2D material edge. (c) Programmed and measured substrate temperature profiles during the transfer process. (d) Contact radius versus substrate temperature ($T_s$) during thermal cycling for PDMS templates fabricated at different curing temperatures. Forward and backward arrows indicate heating and cooling processes, respectively. (e) Corresponding thermal expansion rate as a function of substrate temperature for the same template sets, with arrows denoting heating/cooling directions.