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Heterogeneous Material Integration via Autogenous Transfer Printing Using a Graphene Oxide Release Layer
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Heterogeneous Material Integration via Autogenous Transfer Printing Using a Graphene Oxide Release Layer
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  • Il Ryu Jang
    Il Ryu Jang
    Digital Health Care R&D Department, Korea Institute of Industrial Technology (KITECH), Cheonan 31056, Korea
    Department of Robotics and Mechatronics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Korea
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  • Junwoo Yea
    Junwoo Yea
    Department of Robotics and Mechatronics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Korea
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  • Kyeong Jun Park
    Kyeong Jun Park
    Digital Health Care R&D Department, Korea Institute of Industrial Technology (KITECH), Cheonan 31056, Korea
    Department of Robotics and Mechatronics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Korea
  • Uhyeon Kim
    Uhyeon Kim
    Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea
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  • Kyung-In Jang
    Kyung-In Jang
    Department of Robotics and Mechatronics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Korea
  • Namjung Kim
    Namjung Kim
    Department of Mechanical Engineering, Gachon University, Sungnam 13120, South Korea
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  • Seok Kim
    Seok Kim
    Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea
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  • Hoe Joon Kim
    Hoe Joon Kim
    Department of Robotics and Mechatronics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Korea
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  • Hohyun Keum*
    Hohyun Keum
    Digital Health Care R&D Department, Korea Institute of Industrial Technology (KITECH), Cheonan 31056, Korea
    *Email: (H.K.) [email protected]
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ACS Applied Nano Materials

Cite this: ACS Appl. Nano Mater. 2024, 7, 1, 1019–1029
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https://doi.org/10.1021/acsanm.3c05028
Published December 29, 2023

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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The transfer printing method has drawn significant attention as a promising solution to overcome the limitation of substrate dependency in conventional microfabrication. However, several issues, such as pattern distortion, incompatibility of high-temperature processes, and low throughput, still pose challenges in achieving next-generation microfabrication. The present study utilizes graphene oxide (GO), with a thickness in the tens of nanometers, as the release layer to achieve stable, efficient, and highly scalable transfer printing. When an GO layer is exposed to the reducing agent, it undergoes the removal of existing functional groups, resulting in dimensional shrinkage and inducing microcrack formation. These microcracks serve as stress–concentration initiators between GO and the substrate, facilitating efficient exfoliation of the prepared layers above. The exceptional thermal stability of GO releasing layer allows the proposed method to be applied in transferring the high-temperature processed poly silicon and silicon dioxide patterns. Furthermore, the rapid processing time, confirmed through both experimental and numerical analysis, demonstrates a significant improvement in throughput compared to that of conventional transfer printing methods. Additionally, the proposed method involves a minimal aqueous process, effectively addressing pattern distortion issues in chemical sacrificial layer-releasing methods. The successful fabrication of a wearable resistance temperature detector embedded phototherapy device demonstrates the potential of the proposed method for advancing microfabrication techniques.

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Copyright © 2023 The Authors. Published by American Chemical Society

1. Introduction

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In recent years, thin film electronics have gained significant interest, largely driven by the rapid advancements in microfabrication techniques. Thus far, microfabrication of thin film electronics has focused on improvements in resist chemistry or with exposure methods to draw highly dense nanoscale patterns on a limited estate to generate more devices. (1−3) However, as the physical limitations are restraining further pattern miniaturization, researchers have pioneered to attain freeform electronics for unprecedented applications, which had been restricted by substrate dependency in conventional microfabrication. (4−6) In order to liberate from specific substrate reliance, the transfer printing method has emerged as a promising solution. (7−9) This method involves the initial fabrication of the device on a substrate termed a “donor substrate” via conventional microfabrication technique, followed by transferring onto a desired target substrate, through which substrate dependency associated with severe manufacturing conditions is circumvented while empowered by heterogeneous material integration.
The transfer printing method has successfully addressed the limitations of conventional fabrication methods and expanded the applications of electronics systems, such as organic thin film displays, (10) optoelectronics, (11−13) and wearable biosensors, (14) but there still remain several challenges that require further attention. Transfer printing typically involves a sacrificial layer between the epitaxy layer and the substrate to control the interfacial adhesion. Once the device is fabricated, the sample is immersed in an etchant solution to dissolve the sacrificial layer, thereby minimizing the interfacial adhesion and enabling a convenient transfer printing process. However, during such a sacrificial layer removal process, the convoluted fluidic motion of the etchant presumably distorts the device patterns, hindering the achievement of high transfer printing yields and the fabrication of sophisticated devices. (15−17) Additionally, sacrificial layer removal often relies on chemical diffusion at a thin interface, thereby prolonging the procedure and resulting in low throughput. Furthermore, commonly used polymer-based sacrificial layers are often susceptible to high temperatures, narrowing the list of compatible materials and device structures.
To tackle the issue of pattern distortion, researchers have extensively studied dry transfer printing as a potential solution. One approach involves utilizing lasers to control the interfacial adhesion and minimize pattern distortion. (18,19) However, this method is intrinsically limited to transparent substrates. Another proposed technique is to regulate the interfacial adhesion by utilizing the mismatch in coefficients of thermal expansion at heterogeneous interfaces. (20) This method allows high-throughput and large-area transfers. Nevertheless, the applicability of this approach is restricted by the material properties and its susceptibility to high-temperature processes. Therefore, further research is necessary to develop more versatile and reliable techniques to effectively overcome the limitations associated with pattern distortion, substrate dependency, and high-temperature processes.
The present work introduces a transfer printing method utilizing the functionalization of graphene oxide (GO). GO possesses distinct hydroxyl functional groups (e.g., hydroxyl group, carboxyl group, and epoxy group) on its graphene sheet layer, resulting in a larger interlayer spacing than graphene. (21,22) However, upon exposure to a reducing agent, the GO layer undergoes dimensional shrinkage due to the reduction of functional groups, leading to infinitesimal interfacial microcracks around the edge of the GO sheet. (23) These microcracks act as the crack initiators, facilitating the efficient delamination of the micropatterned multilayer prepared above the GO. It is noteworthy that exposing only the edge region of the device layer to the hydroiodic acid solution for a brief period of 45 s is sufficient to form interfacial nanoscale crack for exfoliation, reducing the distortion of the device pattern that may occur in other transfer printing techniques associated with complete removal of sacrificial layer with etchant. The dimensional shrinkage phenomenon observed in the GO film due to reduction bears similarities to the autogenous shrinkage observed in the cement hydration process, as indicated in previous studies. (24−26) Drawing from this analogy, the transfer printing technique that utilizes the reduction of the GO sheet is termed “autogenous shrinkage”. The principal merit of the technique resides in its compatibility with high temperatures with minimal pattern distortion while expediting the overall manufacturing procedure owing to GO’s exceptional thermal stability, partial liquid process nature, and rapid functionalization time. This paper first demonstrates the overall autogenous transfer printing procedure and analyzes the dimensional change phenomenon. The proposed approach is verified experimentally with an analytical analysis to fully understand the effect of microcracks on the transfer printing process via reduction. Lastly, the fabrication of a flexible thermistor-embedded phototherapy device is demonstrated to validate the effectiveness of the proposed method.

2. Results and Discussion

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Figure 1a shows a comprehensive overview of the autogenous transfer printing process. Initially, a Si wafer is used as the donor substrate due to its excellent compatibility with microfabrication. Oxygen plasma treatment creates a hydrophilic surface, enhancing the adhesion between the GO aqueous solution and the Si substrate. Then, the GO film as a release layer is spin-coated onto the substrate. For the deposition of the target layers, the process includes low-pressure chemical vapor deposition (LPCVD) at 700 °C for the polysilicon (poly-Si) layer and plasma-enhanced chemical vapor deposition (PECVD) at 300 °C for the silicon dioxide (SiO2) layer. Lastly, a polyimide (PI) layer is coated as a backbone layer to prevent mechanical failure (crack, buckling, tearing, etc.) during peeling.

Figure 1

Figure 1. (a) Overview of the fabrication process. (b) Schematic illustration depicting the atomic structure alteration of GO due to reduction.

Photolithography and reactive ion etching (RIE) selectively etch the PI and poly-Si/SiO2 layers, configuring the designed micropatterns and exposing the GO sheet. Subsequently, the GO film undergoes reduction using a reducing agent, resulting in autogenous shrinkage and structural alteration, as shown in Figure 1b. This process induces isotropic microcrack formation and nanoscale delamination at the exposed GO film interface boundary, facilitating subsequent transfer printing. The generated microcracks act as crack initiators, enabling crack propagation along the peeling direction, as demonstrated in Figure 2a. As a result, successful delamination of the poly-Si layer is achieved, as shown in Figure 2b,c. Energy-dispersive spectroscopy (EDS) is utilized to analyze the elemental composition of the transferred layer. The mapping images in Figure 2d,e confirm the successful and intact transfer of the poly-Si/SiO2 layer. The demonstrated transfer of the poly-Si/SiO2 layer, necessitating a high-temperature process of 700 and 300 °C, showcases the exceptional thermal stability of the proposed method.

Figure 2

Figure 2. Digital micrographs illustrating (a) the pickup of the Si layer using water-soluble tape and (b) the resulting transferred Si layer. (c) Optical micrograph of the transferred Si layer. EDS analysis of the transferred (d) Si and (e) SiO2 layer.

In addition to the exfoliation of representative semiconductor (poly-Si) and insulating material (SiO2), transfer printing of conductive gold (Au) and PI composite is experimentally demonstrated in Figure 3. Figure 3a,b presents the scanning electron microscopy (SEM) images of the patterned stack of Au–PI–GO layers on Si substrates after reduction. These images demonstrate that reduction induces diminutive delamination around the edge owing to the dimensional shrinkage of the GO interlayer, ensuring negligible distortion during the transfer process. Figure 3c,d depicts the surface morphology alteration at the edge of the pattern for the pristine sample and the sample with a reduction time of 45 s, which further approves our claim that structural integrity is disrupted only near the outermost boundaries. Figure 3e shows the EDS analysis, revealing clear differences in the Si, Au, O, and C elemental compositions between the exposed substrate and pattern. The EDS mapping data in Figure 3f indicate that the majority of carbon detection points are concentrated in the pattern area and only minimal carbon is detected outside the pattern. This result demonstrates that the GO sheet is also removed during the PI etching procedure, indicating that the substrate exercised for the autogenous transfer printing process is feasible to reuse, making the proposed technique economical and environmentally friendly.

Figure 3

Figure 3. (a, b) SEM images of the pattern stacked with Au–PI–GO layers on Si substrates. Surface morphology at the edge of the pattern for (c) the pristine sample and (d) the sample with a reduction of 45 s. (e) Elemental composition and (f) mapping of the surfaces with and without patterns for elemental analysis of Si, Au, O, and C.

Microcrack formation is optimized via surface morphology and atomic structure analysis to enhance the output yield and further scientifically validate the autogenous transfer printing technique. Figure 4 shows the surface morphology at the edge of the GO film. The pristine GO film exhibits a uniform thickness of 30 nm. However, during the reduction process, the height at the edge of the film significantly increases due to the formation of microcracks. Especially, the GO film reduced for 45 s shows a reduction in thickness of more than 30% compared to the pristine GO film, primarily due to volumetric change resulting from the elimination of functional groups. Furthermore, the height of the GO film at the edge reduced for 45 s demonstrates a substantial increase in thickness by 2.5 times. These findings strongly suggest the presence of microcracks in the GO film at the edge, which plays a crucial role in facilitating subsequent delamination processes.

Figure 4

Figure 4. Surface morphology of the edge of (a) pristine GO film, (b) GO film reduced for 20 s, and (c) GO film reduced for 45 s. Raman analyses of (d) pristine GO film, (e) GO film reduced for 20 s, and (f) GO film reduced for 45 s. XRD analyses of (g) pristine GO film, (h) GO film reduced for 20 s, and (i) GO film reduced for 45 s.

Figure 4 present the Raman spectra with the reduction time. As the reduction time increases, the G peak of the GO film is reduced, indicating the recovery of the carbon hexagonal network due to the elimination of functional groups. (27,28) Furthermore, the ID/IG value of the GO film remarkably increases with the reduction time, suggesting that the volumetric shrinkage induced by reduction leads to the generation of significant structural defects. Figure 4g–i displays the X-ray diffraction (XRD) patterns of the GO film with reduction time. The interlayer spacing of the GO film reduced for 20 s decreases by 0.54 Å due to shrinkage. The interlayer spacing is calculated from the peak position using Bragg’s law
2dsinθ=nλ
(1)
where d is interlayer spacing, θ is the angle of incidence, n is diffraction order, and λ is the wavelength of the X-ray beam (1.54 Å). Additionally, as the reduction time increases, the sharpness and height of the peak gradually decrease due to the degradation of crystallinity. In particular, the GO film reduced for 45 s shows no peak, indicating significant structural defects.
To further understand the delamination process using the GO film, the required peeling force for delaminating the device layer from the processed substrate is calculated from experimental data and theoretical equations. First, we measure the contact angle to calculate the work of adhesion between the oxygen plasma-treated Si substrate and the GO layer. Droplets of ethylene glycol (EG) and DI water with a volume of 5 μL were placed on the Si substrate and GO layer for contact angle measurement, as shown in Figure 5a. Using measured contact angle and known surface energies of liquids (γH2Od = 23.9 mJ/m2, γH2Op = 48.8 mJ/m2, γEGd = 29.2 mJ/m2, and γEGp = 18.3 mJ/m2), we calculate the dispersive and polar surface energies of Si substrate and GO layer using the following Fowkes equation (29,30)
2γldγsd+2γlpγsp=γl(1+cosθ)
(2)
where γld represents the dispersive surface energy of the liquids, γsd is the dispersive surface energy of the substrates, γlp is the polar surface energy of the liquids, and γsp is the polar surface energy of the substrates. γl is the total surface energy of liquids, which is the sum of the dispersive and polar surface energies.

Figure 5

Figure 5. (a) Contact angle measurement of DI water and EG droplets on Si and GO surfaces. The surface energy of Si and GO, and the work of adhesion in the table. (b) Required peeling force with reduction time.

Next, we calculate the work of adhesion between the Si substrate and GO (WSi/GO) by employing the dispersive and polar surface energies of Si (γSid and γSip) and GO (γGOp and γGOp) in the following equation:
WSi/GO=2γSidγGOd+2γSipγGOp
(3)
The table in Figure 5 presents the calculated surface energy and work of the adhesion values. These results indicate that the GO layer and Si substrate bond through weak van der Waals interactions, resulting in a moderate adhesion energy of 185 mJ/m2. This van der Waals heterostructure generally offers an optimal interfacial condition for the efficient delamination of the upper layer. (31−34) Second, the required peeling force can be predicted with the work of adhesion and angle of microcrack using the simplified Kendall model (35)
Fb=WSi/GO1cosα
(4)
where F represents the required peeling force for delamination of the layer, b is the width of the layer, and α is the inclination angle of the microcrack which is defined as the angle between the length and height of the microcrack. To quantify α, the surface profiler examined the morphology of microcracks over varying reduction times, as depicted in Figure S1. The findings suggest that the microcrack gradually develops as the reduction duration increases. From this observed morphology, the length and height of microcracks are determined, as shown in Figure S2a,b. Subsequently, the α is calculated using the formula arctan(microcrack height/microcrack length). The results show a progressive increase in α with the reduction time, as shown in Figure S2c. Therefore, because the required peeling force depends on α, as indicated by eq 4, the force decreases with reduction time, as shown in Figure 5b. This finding confirms that the reduction of GO film effectively controls the interfacial condition, ultimately enabling the efficient delamination process.
To comprehend and evaluate stress–concentration, finite element analysis (FEA) is performed to analyze the Von Mises stress and displacement of the delaminated layer when a small applied load of 1 μN in the thickness direction is exerted on the layer. The FEA model incorporates the silicon wafer and GO layer, as depicted in Figure S3. The geometries of microcracks in the GO film are sketched based on the measured surface morphology presented in Figure 4a–c. While no significant delamination is observed in the GO layer reduced for 0 and 20 s despite the applied peeling force, the GO layer reduced for 45 s exhibits considerable Von Mises stress concentrated at the microcrack, as shown in Figure 6a,b. Especially when the applied force is 15 mN, sufficient to trigger crack propagation, the MN/m2 scaled stress concentrates at the microcrack, as shown in Figure S4. To quantitatively assess the impact of the microcrack on the delamination performance, crack mouth opening displacement (CMOD) is introduced, indicating the extent of fracture toughness of materials with a crack. Figure 6c displays the significant CMOD values observed in the GO layer reduced for 45 s under the applied load. Such a rapid processing time enables high-throughput transfer printing compared to conventional transfer printing methods.

Figure 6

Figure 6. FEA results depicting the induced (a) stress and (b) displacement with reduction time under an applied peeling force of 1 μN. (c) CMOD as a function of applied peeling force.

To experimentally validate the theoretical prediction and FEA results, the delamination process of the patterned layer was demonstrated using water-soluble tape. As depicted in Figure 7a and the Supporting Information, the reduction of the GO film significantly improves the pickup yield of the patterned layer, with the sample reduced for 45 s, showing an exceptional pickup yield of 100%. It is expected that the longer reduction time presumably reduces the force required for exfoliation with larger CMOD, but it accompanies potential distortion of microscopic predefined device patterns, as shown in Figure S5. Hence, 45 s reduction time is optimal for triggering fracture propagation and facilitating transfer printing while maintaining the original configuration of micropatterns. Figure 7b shows the digital images illustrating the pickup of the patterned layer, the transferred layer on the water-soluble tape, and the Si substrate after the transfer. These images provide clear evidence that the transferred layer remains intact without any distortion of the patterns, indicating a successful delamination process.

Figure 7

Figure 7. (a) Pickup yield as a function of reduction time. Inset figures show the samples after delamination. (b) Digital images showing the pickup of the patterned layer, the transferred patterned layer on the water-soluble tape (point 1 indicates the transferred patterned layer), and the donor substrate after the transfer (point 2 that the location where the patterned layer had existed). Raman analyses of (c) point 1 and (d) point 2.

To validate our hypothesis regarding the fracture occurrence and propagation due to the edge delamination of the GO film, we performed Raman analysis on two points, as shown in Figure 7c,d: point 1, corresponding to the transferred patterned layer, and point 2, indicating the location where the patterned layer previously existed on the Si substrate. The Raman spectrum analysis confirms the presence of GO on the transferred layer. This observation strongly suggests that the crack propagates along the surface of the Si substrate, prompted by the edge delamination of the GO film, ultimately leading to the transfer of the GO film onto the device layer. These experimental results support our hypothesis regarding the role of edge delamination in the fracture and transfer process. Moreover, it is worth noting that the donor substrate shows an intact surface after the transfer process as both the GO layer and target layer are removed at the delamination and etching steps, as shown in Figures 3e,f and 7c,d. Consequently, the donor substrate can be reused, making the approach presented approach economical.
The proposed transfer printing technique is applied to fabricate a wearable phototherapy device. The device comprises several key components: a PI-based flexible layer, a Au electrode, LEDs, and a resistance temperature detector (RTD) sensor. Incorporating a PI layer with a moderate Young’s modulus ensures flexibility, enabling conformal attachment to human skin. Additionally, the Au electrodes serve as electrical lines to connect the LEDs, which function as light sources for the phototherapy process.
Upon operation of phototherapy, the LEDs emit light that interacts with biological tissue, potentially increasing body temperature due to inflammation. To monitor and optimize the operational temperature for phototherapy, a platinum (Pt)-based RTD sensor is integrated into the device, enabling the monitoring temperature. Figure 8a illustrates the composition of the device layer and overall structure, along with the fabrication process flow, which begins with a Si substrate known for its excellent microfabrication compatibility. To improve the adhesion between the substrate and the GO aqueous solution, we employ oxygen plasma to create a hydrophilic surface on the Si substrate, followed by spin-coating of the GO solution. Next, a flexible PI layer is applied as both the substrate and encapsulation layer. Photolithography and etch-back processes pattern the Au electrodes for the LED and RTD connections, followed by applying the PI layer for encapsulation. RIE then selectively etches the PI layer to create vias for the electrical connection between the Au electrode and RTD. The RTD electrodes are patterned using photolithography and lift-off processes, with Pt chosen as the material of RTD due to its superior thermal and chemical stability. (36) Further encapsulation is achieved by applying a PI layer. Subsequently, RIE is employed to selectively etch the PI layer, exposing the GO and electrode pads, which are essential for picking up the device layer and establishing an electrical connection. Only the device’s edge part is immersed in HI acid for 45 s to minimize damage to devices induced by the acid. After this process, the device is carefully picked up by using water-soluble tape. Subsequently, the device layer is transferred and bonded to a PDMS layer by using siloxane bonding. Finally, the water-soluble tape is dissolved in water to complete the process. Despite the aqueous solution treatment after transfer printing, the damage to the device layer is minimized due to the protection provided by the encapsulation layer. This demonstration validates the successful application of the proposed transfer printing method for flexible device fabrication.

Figure 8

Figure 8. (a) Composition of the device layer and overall structure along with the fabrication process flow. (b) Characterization of TCR for RTD. (c) Operation of the LED device with applied current. Phototherapy device on curved surfaces, including curved glass and human skin. (d) Resistance and temperature as a function of time with applied current. Inset figures show IR images with time under an applied current of 15 mA.

To measure the temperature coefficient of resistance (TCR) value, we measured the resistance of the RTD with varying device temperatures. Figure 8b illustrates a highly linear TCR value of 162 ppm/°C with an R2 value of 0.99. These results confirm that RTD holds the potential for temperature sensing while maintaining an intact condition with minimal damage, because of the encapsulation layer and partial liquid transfer process. The sensitivity of the RTD is can be easily tuned by adopting materials and designs for different applications. Digital images in Figure 8c demonstrate the LED’s brightness changes with different current values (1, 3, 5, and 10 mA) applied to the device. Additionally, we demonstrate the flexibility of the device by attaching it to curved surfaces, such as curved glass and human skin, because of the flexible PDMS and PI layers. Figure 8d shows the resistance and temperature changes with the applied current over time. The measured resistance changes of RTD due to the phototherapy device’s operation are converted to temperature using the characterized TCR value. The results reveal that the device exhibits temperature increases during operation, with a maximum temperature of 70 °C observed under an applied current of 15 mA. The infrared (IR) images further illustrate that the temperature elevation occurs not only on the LEDs due to their heat dissipation but also on the Au electrode due to joule heating. The proposed device demonstrates the potential of applying wearable phototherapy devices by diagnosing temperature using RTD to address overheating issues caused by LED operation and potential inflammation reactions in patients.

3. Conclusion

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This article introduces a novel transfer printing using the reduction of a 30 nm thick GO film. The proposed method successfully addresses the limitations of conventional methods including instability under high-temperature processes, low throughput, and pattern distortion. First, the proposed method demonstrates successful transfer of materials (poly-Si and SiO2) requiring high-temperature (700 and 300 °C, respectively) process, leveraging the exceptional thermal stability of GO film. Second, through a combination of experimental results and FEA, the optimal processing time of 45 s is confirmed, leading to a significant improvement in throughput. Lastly, the proposed method minimizes the need for a solution during the transfer process, effectively resolving the pattern distortion issues commonly encountered in conventional wet transfer printing methods. As a result, the method achieves an intact pattern transfer and an impressive pickup yield of 100%. Moreover, the proposed approach offers an economical advantage, as the donor substrate remains intact after the transfer process, allowing substrate recycling. To validate our claims, the article includes comprehensive experimental analysis, mechanical modeling, and in-depth discussions, revealing the underlying mechanisms of the proposed method. Additionally, the successful fabrication of wearable phototherapy devices further underscores the method’s potential for intricate and advanced heterogeneously integrated micro/nano freeform devices.

4. Methods

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4.1. Preparation of GO Solution and Coating on a Si Substrate

The GO was coated onto a Si substrate by using the following procedure. A 2 mg/mL aqueous solution of GO (763705, Sigma-Aldrich) was sonicated for an hour to achieve a homogeneous dispersion of GO. In parallel, the Si substrate underwent oxygen plasma treatment to create a hydrophilic surface. This treatment lasted for 15 s at an oxygen flow rate of 40 sccm and a power of 80 W, facilitating the uniform coating of the GO solution. Following oxygen plasma treatment, the GO solution was spin-coated onto the Si substrate at 3000 rpm for 30 s. Finally, the coated substrate was soft-baked on a hot plate at 150 °C for 10 min to remove the solvent and promote adhesion of the GO layer to the substrate.

4.2. Deposition and Patterning of Silicon Dioxide and Poly-Si

A 200 nm thick layer of silicon dioxide (SiO2) was deposited onto a Si substrate coated with graphene oxide (GO) at 300 °C by using a plasma-enhanced chemical vapor deposition (PECVD; KCT-800, KCT) process with silane (SiH4) and nitrous oxide (N2O) as processing gases. To prevent tearing or breakage during the transfer of the SiO2 layer, a PI layer was used as a backbone layer to enhance the mechanical properties of the thin SiO2 layer. The PI precursor (575798, Sigma-Aldrich) was spin-coated on top of SiO2 at 3000 rpm for 30 s and then underwent soft baking on a hot plate at 150 °C for 10 min. The sample was then thermally annealed in an oven at 250 °C for 4 h for imidization. A 6 μm thick AZ 4620 layer (AZ4620, Microchem) was photolithographically patterned on the sample to create the etching mask layer, and the PI and SiO2 layers were etched by using RIE (FabStar, TTL) with O2 and CHF3 gases, respectively.
For the poly-Si patterned sample, a 300 nm thick poly-Si layer was deposited on the GO-coated Si substrate through low-pressure chemical vapor deposition (LPCVD; BJM100) at 700 °C. The backbone and etching mask layer were then formed on the poly-Si layer by using the same procedure as the SiO2 sample. The PI and poly-Si layers were then etched using RIE with the O2 and SF6 gases, respectively.

4.3. Fabrication Process of a Phototherapy Device

The fabrication process started with a silicon wafer as a donor substrate. The Si substrate was treated with oxygen plasma to achieve a hydrophilic surface, which enhanced the adhesion between the GO solution and the Si substrate. A GO solution was then spin-coated onto the Si substrate at 3000 rpm for 30 s, followed by soft baking on a hot plate at 150 °C for 10 min to create the 30 nm thick GO layer. Subsequently, the PI precursor was then spin-coated at 3000 rpm for 30 s and then soft-baked on a hot plate at 150 °C for 10 min. This sample was then thermally annealed in an oven at 250 °C for 4 h for imidization.
The electrode for connecting the LED and RTD was formed by patterning the Au layer with conventional photolithography and wet etching methods. Initially, a 100 nm thick Au layer was sputtered on the sample, which was then photolithographically patterned with a 1.2 μm thick GXR 601 photoresist as an etching mask. The etchant selectively removed the Au layer, resulting in a patterned Au layer. An acetone solution eliminated the masking photoresist. For encapsulation, the PI layer was coated on the Au pattern by using the method mentioned above in the fabrication process.
To connect the RTD and Au layers, fabrication was performed. First, a 6 μm thick AZ 4620 photoresist as an etching mask was photolithographically patterned, and then the encapsulation layer on the junction of Au and RTD layers was selectively etched using RIE with O2 gas. The masking photoresist was removed by using an acetone solution. Next, a 1.2 μm thick AZ 5214 photoresist was photolithographically patterned, followed by sputtering of a 100 nm thick Pt layer. The Pt layer was then lifted off using acetone, resulting in RTD patterns. For encapsulation, the PI layer was coated on the RTD pattern using the same method as described earlier in the fabrication process.
To establish electrical connectivity and functionalize GO, selective etching of PI was performed to expose the electrode pads and the GO layer interface. A 10 μm thick AZ 4620 as an etching mask was photolithographically patterned, followed by RIE etching using O2 gas to remove the PI layer.

4.4. Delamination of the Device Layer and Integration with a PDMS Layer

Hydroiodic (HI) acid with a concentration of 57 wt % in water (210021, Sigma-Aldrich) was utilized as a reducing agent. The edge of the device layer was immersed in HI acid solution to induce the formation of the microcracks on the edge of the device layer. After the completion of reduction, water-soluble tape (Water-Soluble Wave Solder Tape 5414, 3M) picked up the device layer from the Si substrate. In parallel, a PDMS layer with a 1:10 ratio mixture (Sylgard 184, Dow-Corning) was spin-coated onto a glass substrate at 500 rpm for 30 s. The PDMS layer was then cured in a convection oven at 80 °C for 45 min. The fabricated PDMS and the device layers were integrated by using siloxane bonding. To functionalize the surface of the device layer, a 100 nm thick SiO2 was sputtered onto it. Subsequently, oxygen plasma treatment was performed for 30 s to activate the surfaces of both the PDMS and device layers. The siloxane bonding process was performed by bringing surfaces of PDMS and device layers into contact under pressure and heating conditions of 70 °C. Finally, the water-soluble tape was removed by using a water solution.

4.5. LED Integration with the Device Layer and Device Characterization

To integrate the LED (low-current 0603 SMD LED, Vishay Semiconductors) with the Au pad on the device, a conductive epoxy adhesive (silver conductive epoxy adhesive 8331, MG Chemicals) was applied to the Au pad. The LED was then precisely positioned on the Au pad by using a microscope and subsequently cured for 15 min at a temperature of 70 °C in a convection oven. A silver wire was connected to the electrode pad to establish an interface with a multimeter and a source meter. The multimeter (34401A, Agilent) was utilized to measure the resistance of RTD, which was employed for monitoring the device temperature. On the other hand, the source meter (2440 C, Keithley) was responsible for operating the LED. The IR camera (ETS320, FLIR) was employed to characterize the device’s temperature under the applied current.

4.6. Material Characterization

The field emission scanning electron microscope (FE-SEM, Hitachi/S-4800) was used for obtaining high-resolution images and elemental analysis of transferred patterns. Optical images of the patterns were also collected by using the Olympus MX60 upright microscope. The stylus profilometer (Bruker/Dektak XT) measured the profile of the delaminated edge of the device layer. The Raman imaging system (Nanobase/XperRAM) was employed to investigate the atomic structure of GO with a reduction time. The contact angle measurement system (Phoenix 150/S-EO) measured the contact angles of DI water and EG droplets on substrates to quantify the work of adhesion between the GO and Si substrates. Finally, an X-ray diffractometer (XRD, Panalytical/Empyrean) was used to analyze the crystallinity and interlayer spacing of GO.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsanm.3c05028.

  • Surface profile and morphology of microcracks with respect to reduction time (Figures S1 and S2); geometry/parameters of the FEA model (Figure S3); FEA results illustrating the induced stress distribution (Figure S4); digital images of the sample in a HI acid solution at various reduction times (Figure S5) (PDF)

  • Movie S1 (MP4)

  • Movie S2 (MP4)

  • Movie S3 (MP4)

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Author Information

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  • Corresponding Author
  • Authors
    • Il Ryu Jang - Digital Health Care R&D Department, Korea Institute of Industrial Technology (KITECH), Cheonan 31056, KoreaDepartment of Robotics and Mechatronics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Korea
    • Junwoo Yea - Department of Robotics and Mechatronics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Korea
    • Kyeong Jun Park - Digital Health Care R&D Department, Korea Institute of Industrial Technology (KITECH), Cheonan 31056, KoreaDepartment of Robotics and Mechatronics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Korea
    • Uhyeon Kim - Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, KoreaOrcidhttps://orcid.org/0009-0000-3540-0029
    • Kyung-In Jang - Department of Robotics and Mechatronics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, KoreaOrcidhttps://orcid.org/0000-0002-4664-5029
    • Namjung Kim - Department of Mechanical Engineering, Gachon University, Sungnam 13120, South KoreaOrcidhttps://orcid.org/0000-0002-2600-5921
    • Seok Kim - Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, KoreaOrcidhttps://orcid.org/0000-0003-3206-8061
    • Hoe Joon Kim - Department of Robotics and Mechatronics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, KoreaOrcidhttps://orcid.org/0000-0003-1180-7830
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This study was performed with the support of the R&D Program for Forest Science Technology (Project No. 2021396B10-2323-0107) provided by Korea Forest Service(Korea Forestry Promotion Institute), the basic science research program through the National Research Foundation of Korea (NRF) (2021R1C1C1011588), funded by the Ministry of Science and ICT of Korea, and the Korea Institute of Industrial Technology (KITECH EH-23-0014).

References

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This article references 36 other publications.

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    Linghu, C.; Zhang, S.; Wang, C.; Song, J. Transfer printing techniques for flexible and stretchable inorganic electronics. npj Flexible Electronics 2018, 2 (1), 26,  DOI: 10.1038/s41528-018-0037-x
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    Kim, K. S.; Lee, D.; Chang, C. S.; Seo, S.; Hu, Y.; Cha, S.; Kim, H.; Shin, J.; Lee, J.-H.; Lee, S. Non-epitaxial single-crystal 2D material growth by geometric confinement. Nature 2023, 614 (7946), 8894,  DOI: 10.1038/s41586-022-05524-0
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    Abhilash, T.; De Alba, R.; Zhelev, N.; Craighead, H. G.; Parpia, J. M. Transfer printing of CVD graphene FETs on patterned substrates. Nanoscale 2015, 7 (33), 1410914113,  DOI: 10.1039/C5NR03501E
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    Wang, C.; Linghu, C.; Nie, S.; Li, C.; Lei, Q.; Tao, X.; Zeng, Y.; Du, Y.; Zhang, S.; Yu, K. Programmable and scalable transfer printing with high reliability and efficiency for flexible inorganic electronics. Sci. Adv. 2020, 6 (25), eabb2393  DOI: 10.1126/sciadv.abb2393
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    Park, J.; Yoo, J.-H.; Grigoropoulos, C. P. Multi-scale graphene patterns on arbitrary substrates via laser-assisted transfer-printing process. Appl. Phys. Lett. 2012, 101 (4), 043110,  DOI: 10.1063/1.4738883
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    Luo, H.; Wang, C.; Linghu, C.; Yu, K.; Wang, C.; Song, J. Laser-driven programmable non-contact transfer printing of objects onto arbitrary receivers via an active elastomeric microstructured stamp. National science review 2020, 7 (2), 296304,  DOI: 10.1093/nsr/nwz109
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    Zheng, S.; Tu, Q.; Urban, J. J.; Li, S.; Mi, B. Swelling of graphene oxide membranes in aqueous solution: characterization of interlayer spacing and insight into water transport mechanisms. ACS Nano 2017, 11 (6), 64406450,  DOI: 10.1021/acsnano.7b02999
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    Qian, Y.; Zhang, X.; Liu, C.; Zhou, C.; Huang, A. Tuning interlayer spacing of graphene oxide membranes with enhanced desalination performance. Desalination 2019, 460, 5663,  DOI: 10.1016/j.desal.2019.03.009
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  • Abstract

    Figure 1

    Figure 1. (a) Overview of the fabrication process. (b) Schematic illustration depicting the atomic structure alteration of GO due to reduction.

    Figure 2

    Figure 2. Digital micrographs illustrating (a) the pickup of the Si layer using water-soluble tape and (b) the resulting transferred Si layer. (c) Optical micrograph of the transferred Si layer. EDS analysis of the transferred (d) Si and (e) SiO2 layer.

    Figure 3

    Figure 3. (a, b) SEM images of the pattern stacked with Au–PI–GO layers on Si substrates. Surface morphology at the edge of the pattern for (c) the pristine sample and (d) the sample with a reduction of 45 s. (e) Elemental composition and (f) mapping of the surfaces with and without patterns for elemental analysis of Si, Au, O, and C.

    Figure 4

    Figure 4. Surface morphology of the edge of (a) pristine GO film, (b) GO film reduced for 20 s, and (c) GO film reduced for 45 s. Raman analyses of (d) pristine GO film, (e) GO film reduced for 20 s, and (f) GO film reduced for 45 s. XRD analyses of (g) pristine GO film, (h) GO film reduced for 20 s, and (i) GO film reduced for 45 s.

    Figure 5

    Figure 5. (a) Contact angle measurement of DI water and EG droplets on Si and GO surfaces. The surface energy of Si and GO, and the work of adhesion in the table. (b) Required peeling force with reduction time.

    Figure 6

    Figure 6. FEA results depicting the induced (a) stress and (b) displacement with reduction time under an applied peeling force of 1 μN. (c) CMOD as a function of applied peeling force.

    Figure 7

    Figure 7. (a) Pickup yield as a function of reduction time. Inset figures show the samples after delamination. (b) Digital images showing the pickup of the patterned layer, the transferred patterned layer on the water-soluble tape (point 1 indicates the transferred patterned layer), and the donor substrate after the transfer (point 2 that the location where the patterned layer had existed). Raman analyses of (c) point 1 and (d) point 2.

    Figure 8

    Figure 8. (a) Composition of the device layer and overall structure along with the fabrication process flow. (b) Characterization of TCR for RTD. (c) Operation of the LED device with applied current. Phototherapy device on curved surfaces, including curved glass and human skin. (d) Resistance and temperature as a function of time with applied current. Inset figures show IR images with time under an applied current of 15 mA.

  • References


    This article references 36 other publications.

    1. 1
      Kim, S.; Wu, J.; Carlson, A.; Jin, S. H.; Kovalsky, A.; Glass, P.; Liu, Z.; Ahmed, N.; Elgan, S. L.; Chen, W. Microstructured elastomeric surfaces with reversible adhesion and examples of their use in deterministic assembly by transfer printing. Proc. Natl. Acad. Sci. U. S. A. 2010, 107 (40), 1709517100,  DOI: 10.1073/pnas.1005828107
    2. 2
      Linghu, C.; Zhang, S.; Wang, C.; Song, J. Transfer printing techniques for flexible and stretchable inorganic electronics. npj Flexible Electronics 2018, 2 (1), 26,  DOI: 10.1038/s41528-018-0037-x
    3. 3
      Zumeit, A.; Dahiya, A. S.; Christou, A.; Shakthivel, D.; Dahiya, R. Direct roll transfer printed silicon nanoribbon arrays based high-performance flexible electronics. npj Flexible Electronics 2021, 5 (1), 18,  DOI: 10.1038/s41528-021-00116-w
    4. 4
      Kim, K. S.; Lee, D.; Chang, C. S.; Seo, S.; Hu, Y.; Cha, S.; Kim, H.; Shin, J.; Lee, J.-H.; Lee, S. Non-epitaxial single-crystal 2D material growth by geometric confinement. Nature 2023, 614 (7946), 8894,  DOI: 10.1038/s41586-022-05524-0
    5. 5
      Park, J.; Lee, Y.; Lee, H.; Ko, H. Transfer printing of electronic functions on arbitrary complex surfaces. ACS Nano 2020, 14 (1), 1220,  DOI: 10.1021/acsnano.9b09846
    6. 6
      Kim, Y.; Suh, J. M.; Shin, J.; Liu, Y.; Yeon, H.; Qiao, K.; Kum, H. S.; Kim, C.; Lee, H. E.; Choi, C. Chip-less wireless electronic skins by remote epitaxial freestanding compound semiconductors. Science 2022, 377 (6608), 859864,  DOI: 10.1126/science.abn7325
    7. 7
      Carlson, A.; Bowen, A. M.; Huang, Y.; Nuzzo, R. G.; Rogers, J. A. Transfer printing techniques for materials assembly and micro/nanodevice fabrication. Adv. Mater. 2012, 24 (39), 52845318,  DOI: 10.1002/adma.201201386
    8. 8
      Kim, T.-H.; Cho, K.-S.; Lee, E. K.; Lee, S. J.; Chae, J.; Kim, J. W.; Kim, D. H.; Kwon, J.-Y.; Amaratunga, G.; Lee, S. Y. Full-colour quantum dot displays fabricated by transfer printing. Nat. Photonics 2011, 5 (3), 176182,  DOI: 10.1038/nphoton.2011.12
    9. 9
      Park, J. K.; Zhang, Y.; Xu, B.; Kim, S. Pattern transfer of large-scale thin membranes with controllable self-delamination interface for integrated functional systems. Nat. Commun. 2021, 12 (1), 6882,  DOI: 10.1038/s41467-021-27208-5
    10. 10
      Cok, R. S.; Hamer, J. W.; Bower, C. A.; Menard, E.; Bonafede, S. AMOLED displays with transfer-printed integrated circuits. J. Soc. Inf. Dispersion 2011, 19 (4), 335341,  DOI: 10.1889/JSID19.4.335
    11. 11
      Yoon, J.; Lee, S. M.; Kang, D.; Meitl, M. A.; Bower, C. A.; Rogers, J. A. Heterogeneously integrated optoelectronic devices enabled by micro-transfer printing. Adv. Opt. Mater. 2015, 3 (10), 13131335,  DOI: 10.1002/adom.201500365
    12. 12
      Li, H.; Xu, Y.; Li, X.; Chen, Y.; Jiang, Y.; Zhang, C.; Lu, B.; Wang, J.; Ma, Y.; Chen, Y. Epidermal inorganic optoelectronics for blood oxygen measurement. Adv. Healthcare Mater. 2017, 6 (9), 1601013,  DOI: 10.1002/adhm.201770044
    13. 13
      Song, Y. M.; Xie, Y.; Malyarchuk, V.; Xiao, J.; Jung, I.; Choi, K.-J.; Liu, Z.; Park, H.; Lu, C.; Kim, R.-H. Digital cameras with designs inspired by the arthropod eye. Nature 2013, 497 (7447), 9599,  DOI: 10.1038/nature12083
    14. 14
      Kim, C. H.; Lee, D. H.; Youn, J.; Lee, H.; Jeong, J. Simple and cost-effective microfabrication of flexible and stretchable electronics for wearable multi-functional electrophysiological monitoring. Sci. Rep. 2021, 11 (1), 14823,  DOI: 10.1038/s41598-021-94397-w
    15. 15
      Cha, S.; Cha, M.; Lee, S.; Kang, J. H.; Kim, C. Low-temperature, dry transfer-printing of a patterned graphene monolayer. Sci. Rep. 2015, 5 (1), 17877,  DOI: 10.1038/srep17877
    16. 16
      Abhilash, T.; De Alba, R.; Zhelev, N.; Craighead, H. G.; Parpia, J. M. Transfer printing of CVD graphene FETs on patterned substrates. Nanoscale 2015, 7 (33), 1410914113,  DOI: 10.1039/C5NR03501E
    17. 17
      Wang, C.; Linghu, C.; Nie, S.; Li, C.; Lei, Q.; Tao, X.; Zeng, Y.; Du, Y.; Zhang, S.; Yu, K. Programmable and scalable transfer printing with high reliability and efficiency for flexible inorganic electronics. Sci. Adv. 2020, 6 (25), eabb2393  DOI: 10.1126/sciadv.abb2393
    18. 18
      Park, J.; Yoo, J.-H.; Grigoropoulos, C. P. Multi-scale graphene patterns on arbitrary substrates via laser-assisted transfer-printing process. Appl. Phys. Lett. 2012, 101 (4), 043110,  DOI: 10.1063/1.4738883
    19. 19
      Luo, H.; Wang, C.; Linghu, C.; Yu, K.; Wang, C.; Song, J. Laser-driven programmable non-contact transfer printing of objects onto arbitrary receivers via an active elastomeric microstructured stamp. National science review 2020, 7 (2), 296304,  DOI: 10.1093/nsr/nwz109
    20. 20
      Heo, S.; Ha, J.; Son, S. J.; Choi, I. S.; Lee, H.; Oh, S.; Jekal, J.; Kang, M. H.; Lee, G. J.; Jung, H. H. Instant, multiscale dry transfer printing by atomic diffusion control at heterogeneous interfaces. Sci. Adv. 2021, 7 (28), eabh0040  DOI: 10.1126/sciadv.abh0040
    21. 21
      Zheng, S.; Tu, Q.; Urban, J. J.; Li, S.; Mi, B. Swelling of graphene oxide membranes in aqueous solution: characterization of interlayer spacing and insight into water transport mechanisms. ACS Nano 2017, 11 (6), 64406450,  DOI: 10.1021/acsnano.7b02999
    22. 22
      Qian, Y.; Zhang, X.; Liu, C.; Zhou, C.; Huang, A. Tuning interlayer spacing of graphene oxide membranes with enhanced desalination performance. Desalination 2019, 460, 5663,  DOI: 10.1016/j.desal.2019.03.009
    23. 23
      Pei, S.; Cheng, H.-M. The reduction of graphene oxide. Carbon 2012, 50 (9), 32103228,  DOI: 10.1016/j.carbon.2011.11.010
    24. 24
      Barcelo, L.; Moranville, M.; Clavaud, B. Autogenous shrinkage of concrete: a balance between autogenous swelling and self-desiccation. Cem. Concr. Res. 2005, 35 (1), 177183,  DOI: 10.1016/j.cemconres.2004.05.050
    25. 25
      Wu, L.; Farzadnia, N.; Shi, C.; Zhang, Z.; Wang, H. Autogenous shrinkage of high performance concrete: A review. Construction and Building Materials 2017, 149, 6275,  DOI: 10.1016/j.conbuildmat.2017.05.064
    26. 26
      Tang, S.; Huang, D.; He, Z. A review of autogenous shrinkage models of concrete. Journal of Building Engineering 2021, 44, 103412,  DOI: 10.1016/j.jobe.2021.103412
    27. 27
      Wu, J.-B.; Lin, M.-L.; Cong, X.; Liu, H.-N.; Tan, P.-H. Raman spectroscopy of graphene-based materials and its applications in related devices. Chem. Soc. Rev. 2018, 47 (5), 18221873,  DOI: 10.1039/C6CS00915H
    28. 28
      Moon, I. K.; Lee, J.; Ruoff, R. S.; Lee, H. Reduced graphene oxide by chemical graphitization. Nat. Commun. 2010, 1 (1), 16,  DOI: 10.1038/ncomms1067
    29. 29
      Kozbial, A.; Li, Z.; Conaway, C.; McGinley, R.; Dhingra, S.; Vahdat, V.; Zhou, F.; D’Urso, B.; Liu, H.; Li, L. Study on the surface energy of graphene by contact angle measurements. Langmuir 2014, 30 (28), 85988606,  DOI: 10.1021/la5018328
    30. 30
      Dalal, E. N. Calculation of solid surface tensions. Langmuir 1987, 3 (6), 10091015,  DOI: 10.1021/la00078a023
    31. 31
      Geim, A. K.; Grigorieva, I. V. Van der Waals heterostructures. Nature 2013, 499 (7459), 419425,  DOI: 10.1038/nature12385
    32. 32
      Polfus, J. M.; Muñiz, M. B.; Ali, A.; Barragan-Yani, D. A.; Vullum, P. E.; Sunding, M. F.; Taniguchi, T.; Watanabe, K.; Belle, B. D. Temperature-Dependent Adhesion in van der Waals Heterostructures. Advanced Materials Interfaces 2021, 8 (20), 2100838,  DOI: 10.1002/admi.202100838
    33. 33
      Rokni, H.; Lu, W. Direct measurements of interfacial adhesion in 2D materials and van der Waals heterostructures in ambient air. Nat. Commun. 2020, 11 (1), 5607,  DOI: 10.1038/s41467-020-19411-7
    34. 34
      Lee, C. H.; Kim, J.-H.; Zou, C.; Cho, I. S.; Weisse, J. M.; Nemeth, W.; Wang, Q.; Van Duin, A. C.; Kim, T.-S.; Zheng, X. Peel-and-stick: mechanism study for efficient fabrication of flexible/transparent thin-film electronics. Sci. Rep. 2013, 3 (1), 2917,  DOI: 10.1038/srep02917
    35. 35
      Kendall, K. Thin-film peeling-the elastic term. J. Phys. D: Appl. Phys. 1975, 8 (13), 1449,  DOI: 10.1088/0022-3727/8/13/005
    36. 36
      Bhattacharyya, P. Technological journey towards reliable microheater development for MEMS gas sensors: A review. IEEE Transactions on device and materials reliability 2014, 14 (2), 589599,  DOI: 10.1109/TDMR.2014.2311801
  • Supporting Information

    Supporting Information


    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsanm.3c05028.

    • Surface profile and morphology of microcracks with respect to reduction time (Figures S1 and S2); geometry/parameters of the FEA model (Figure S3); FEA results illustrating the induced stress distribution (Figure S4); digital images of the sample in a HI acid solution at various reduction times (Figure S5) (PDF)

    • Movie S1 (MP4)

    • Movie S2 (MP4)

    • Movie S3 (MP4)


    Terms & Conditions

    Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.