Modified Dry Transfer Technique

• Modified Dry Transfer Technique is a chemical-free method that uses thermal, mechanical, and polymer-assisted processes to relocate 2D materials onto various substrates.
• The approach minimizes contamination and strain by eliminating liquid chemicals, employing techniques like viscoelastic stamping, micro-stamper precision, and cryogenic cleaving.
• It enables scalable fabrication of van der Waals heterostructures and flexible devices by optimizing substrate pre-treatment, interface engineering, and adhesion control.

A modified dry transfer technique refers to a set of advancements in the transfer of two-dimensional (2D) materials—such as graphene, transition metal dichalcogenides (TMDCs), and hexagonal boron nitride (hBN)—from their growth or exfoliation substrates onto a variety of host platforms, without the use of liquid-phase chemicals during the critical transfer steps. Modifications over early protocols address key challenges such as area coverage, contamination, residue, strain, precision, substrate compatibility, and scalability. The breadth of modern dry transfer methodologies includes viscoelastic stamping, tape transfer, thermally or chemically triggered adhesion, micro-stamper (deterministic placement), and multi-material pickup. These techniques underpin critical progress in the reliable fabrication of van der Waals heterostructures, flexible electronics, precision optoelectronics, and quantum devices.

1. Key Principles and Methodological Variants

Early dry transfer methods utilized mechanical exfoliation directly onto substrates or simple viscoelastic PDMS stamps, often resulting in limited spatial control and flake size (Castellanos-Gomez et al., 2013). The ensuing suite of “modified” methods addresses these limitations via architectural, thermal, and chemical innovations:

• Thermal Release Tape Transfer: Epitaxial graphene is transferred from 4H-SiC onto arbitrary substrates (e.g., SiO₂, GaN, Al₂O₃) using a Nitto Denko Revalpha thermal release tape. Critical parameters include pre-cleaning and hydrophilization of the handle substrate, pressure-controlled wafer bonder steps (3–6 N/mm²), and mild heating above 120°C to trigger tape detachment (0910.2624).
• Deterministic Viscoelastic Stamping: Thin 2D flakes are mechanically exfoliated onto a PDMS-based viscoelastic stamp, enabling transparent optical alignment, sub-micron placement, and high-yield transfer upon slow, controlled peel thanks to the time-dependent elastic-to-viscous response (Castellanos-Gomez et al., 2013).
• Thermoplastic/Polymers with Triggered Adhesion: Poly(propylene carbonate) (PPC) films allow strong 2D flake adhesion at room temperature for exfoliation and identification; when heated (~70°C), adhesion decreases rapidly, enabling defect-free dry “release” onto hBN or similar targets (Kinoshita et al., 2019). Related strategies use polycaprolactone (PCL) or polycarbonate (PC) for tunable adhesion (Son et al., 2020, Rosser et al., 2019).
• Micro-Stamper Precision Methods: A sharp micro-stamper, pressing through the PDMS at the chosen flake location, achieves deterministic, contamination-free placement, as in “2D printer” protocols (Hemnani et al., 2018).
• Cryogenic/Temperature-Triggered Cleaving and Pickup: The PDMS stamp’s glass transition is exploited at cryogenic temperatures (–120°C), enabling mechanical cleavage and transfer of oxidatively and chemically sensitive materials (e.g., BSCCO, NbSe₂), and supporting deterministic stacking and twist-angle control (Patil et al., 30 May 2024).
• Polymer-Assisted and Oxide-Assisted Large-Area Peel: Polycaprolactone (PCL) or oxide layers are spin-coated or deposited for high-adhesion, large-area peel-off of films like MBE-grown TMDs, topological insulators, and 2D magnets, with minimized mechanical stress and preserved interfacial quality (Li et al., 22 Feb 2025, Xu et al., 24 Jan 2025).

A selection of these variations—including their key features and advantages—is summarized:

Method	Adhesion/Release Control	Notable Targets/Scenarios
Thermal Release Tape (TRT)	Heat-triggered release	Large-area epitaxial graphene (0910.2624)
Dry Viscoelastic Stamping (PDMS)	Peel rate & viscoelasticity	Multimaterial heterostructures (Castellanos-Gomez et al., 2013)
PPC/PC/PMMA Thermoplastics	Glass transition temperature	Clean vdW stacks, monolayer TMDs (Kinoshita et al., 2019, Rosser et al., 2019, Uwanno et al., 2015)
Micro-stamper Local Transfer	Local mechanical pressing	Zero-cross contamination on photonic chips (Hemnani et al., 2018)
Cryogenic PDMS Cleaving	Temperature-induced glassy PDMS	Chemically sensitive flakes, twist control (Patil et al., 30 May 2024)
PCL/High-k Oxide/Polymer Stack	Enhanced adhesion layer	Full-film transfer, wafer-scale MoS₂ (Li et al., 22 Feb 2025, Xu et al., 24 Jan 2025)

2. Substrate Preparation, Process Control, and Contamination Suppression

Successful dry transfer depends critically on substrate pre-treatment and interface management:

• Hydrophilization and Cleaning: O₂ plasma, SC1 cleaning, and megasonic rinsing are essential (e.g., 750 W O₂ plasma, 5 min; H₂O:NH₄OH:H₂O₂ 5:1:1, 40°C, 14 min) to guarantee adhesion and reduce voids (0910.2624).
• Thermal and Plasma-Assisted Surface Control: Post-transfer annealing (e.g., 250°C, 30 min) and solvent cleaning remove residues, while plasma cross-linking (Ar/BCl₃) prior to PMMA removal (the CASING approach) controls polymer swelling and undercut during acetone wash, preventing detachment or delamination of ultrathin 2D layers (Ghiami et al., 3 Dec 2024).
• Pre-Release Interface Engineering: In the PCL/oxide-assisted methods, the interface strength is finely tuned to balance full-film delamination from growth substrates (adapting to strong substrate-bonded MBE films) and conformation to flexible targets; detachment angle and mechanical support (e.g., using a roller) are carefully optimized to mitigate wrinkling and cracks (Li et al., 22 Feb 2025).
• Omission of Liquids: Purely “all-dry” methods—employing PVC/PDMS stamps and temperature-controlled adhesion—completely eliminate liquid solvents, with transfer yields above 80% for pre-patterned quantum emitter arrays (Mishuk et al., 28 Feb 2025). This avoidance of liquids abrogates capillary-driven contamination, bubble formation, and optical artifact introduction.

3. Transfer Quality, Metrology, and Performance Metrics

Metrological rigor is essential to quantify transfer quality, interface cleanliness, and device performance:

• Optical and Raman Spectroscopy: Quantification of flake coverage, uniformity, and quality (e.g., via the D/G and 2D/G Raman intensity ratios, the absence of D peak, and spatially resolved Raman/photoluminescence mapping) demonstrates the maintenance of high material integrity and minimal contamination (Fechine et al., 2014, Ghiami et al., 3 Dec 2024).
  • In thermal tape methods, Raman attenuation yields thickness via $I = I_{0} e^{-a d}$ with empirically measured a-values for extinction (0910.2624).
  • Absorption and cavity enhancement effects in optical stacks are tuned by precise control of suspension height and interface reflectivity (Rebollo et al., 2020).
• Morphological Analysis: AFM and SEM measurements confirm sub-nanometer increases in surface roughness post-transfer (ΔRMS ≈ 1 nm for PCL-based, versus ≈ 5 nm for PMMA methods), negligible wrinkling for well-controlled protocols, and clear optical contrast for monolayer identification in PPC/SiO₂ (Kinoshita et al., 2019, Li et al., 22 Feb 2025).
• Device Performance: Mobilities up to 1.9 × 10⁵ cm²/Vs for suspended CVD graphene (Liu et al., 2019), subthreshold swing as low as 68.8 mV/dec, and ON/OFF ratios ~10¹² for flexible MoS₂ FETs post transfer (Xu et al., 24 Jan 2025). Success rates in quantum emitter placement achieve 81.8% with no observable degradation in g^{(2)}(0) or emission spectra (Mishuk et al., 28 Feb 2025).
• Reproducibility and Statistical Analysis: The coefficient of variation (CV) for key device parameters (on-current, SS, V_th) shows marked improvement (e.g., CV_{on} = 0.338 vs 1.124 for dry vs wet transfer) indicative of enhanced uniformity (Ghiami et al., 3 Dec 2024).

4. Application Domains and Functional Demonstrators

Modified dry transfer techniques are foundational for a range of advanced applications and device platforms:

• Flexible and Transparent Electronics: TRT-based dry transfer and oxide peel-off enable integration of large-area, continuous monolayer TMDCs and graphene onto PET and other flexible substrates, underpinning the construction of soft logic, inverters, and tactile sensors with performance matching rigid-substrate implementations (0910.2624, Xu et al., 24 Jan 2025).
• Van der Waals Heterostructures: Dry pickup using hBN, MoS₂, ZnPS₃, and CrPS₄ enables vertical stacking with tunable band structure, clean interfaces, and encapsulation of air-sensitive or magnetic layers. Assembled heterostructures support Josephson tunneling, proximity-induced superconductivity, and engineered spin properties (Banszerus et al., 2017, Son et al., 2020).
• Ballistic and Quantum Devices: Suspended graphene FETs created via clean lift-off-resist (LOR) dry transfer exhibit ballistic transport and Fabry–Pérot interference with very low residual charge (n₀ ≈ 9 × 10⁸ cm⁻²) and high mobility (μ ≈ 1.9 × 10⁵ cm²/Vs) (Liu et al., 2019). Defect-tolerant quantum emitter arrays in hBN are integrated with high purity and spatial precision (Mishuk et al., 28 Feb 2025).
• Photonic and Nanophotonic Integration: PC-point transfer protocols enable contamination-free, micron-precision deposition of monolayer TMDCs onto ring resonators and waveguides for nonlinear and quantum optics, with careful management of heating/cooling and alignment (Rosser et al., 2019, Hemnani et al., 2018).
• Device Scalability and Industrialization: Wafer-scale transfer and full-film peel-off via modified dry methods support up to 4-inch contiguous area, with yields >90%, low defectivity, and high throughput—enabling circuit-level and robotic system integration (Xu et al., 24 Jan 2025, Li et al., 22 Feb 2025).

5. Critical Innovations, Challenges, and Future Prospects

The evolution of modified dry transfer techniques is marked by several critical advances:

• Strong, Tunable Adhesion Layers: The design of the interfacial adhesion—by optimization of polymer chemistry, glass transition profile, and mechanical support (roller, micro-stamper, oxide stack)—removes the tradeoff between full-film coverage and gentle release, crucial for MBE/vdW integration and for transferring air-sensitive or high-inertia materials (Li et al., 22 Feb 2025, Son et al., 2020).
• Precision Placement and Low Cross-Contamination: Micro-stamper and PC-point local pickup minimize cross-contamination by two to three orders of magnitude, ensuring that only pre-selected flakes are delivered, a requirement for high-density and high-value chiplet or photonic integration (Hemnani et al., 2018, Rosser et al., 2019).
• Minimization of Residue and Strain: All-dry, mechanical, and temperature-programmed peeling avoids the introduction of capillary forces, solvents, or swelling that would otherwise cause wrinkles, charge inhomogeneity, and impurity doping. Plasma-assisted surface chemistry further controls swelling and residue during sacrificial layer removal (Ghiami et al., 3 Dec 2024).

Continued challenges include balancing large-area scalability with ultralow mechanical and chemical strain, total elimination of oligomer residues (sometimes observed in PDMS-based protocols), and further reducing process complexity for wafer-level automation. A plausible implication is the future adoption of machine vision and robotic micro-assembly for rapid, operator-independent, and deterministic van der Waals heterostructure fabrication (Hemnani et al., 2018).

6. Comparative Perspective and Impact on 2D Material Technologies

Compared to conventional wet and exfoliation-based transfer methods, modern modified dry transfer:

• Achieves much larger, continuous-area and patterned film coverage;
• Provides unparalleled interface cleanliness, transparency, and uniformity;
• Demonstrates deterministic, scalable precision for heterogeneous integration;
• Minimizes property degradation across photonic, electronic, and quantum platforms.

Statistical improvements in device metrics and structural analyses confirm that these process modifications are enabling the reliable, scalable, and high-performance application of 2D materials in flexible electronics, optoelectronics, spintronics, and quantum technologies, from laboratory to industrial scale.