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Author: Thamarasee Jeewandara, Phys.org

Stimulus response, self-folding, two-dimensional (2-D) layered materials have interesting functions in flexible electronics, wearable devices, biosensors, and photonics applications. However, scalability limitations and lack of design tools hinder high integration and reliable functionality. In a new report now published on Advanced Intelligent Systems, Qi Huang and a team of scientists in chemical and biomolecular engineering and electrical and computer engineering at Johns Hopkins University in the United States proposed a mass production strategy to create Reversible self-folding structure of single-layer graphene. The material can be used in microfluidics and micromechanical systems. As a proof of concept, they implemented complex functional devices in the form of rings, polyhedrons, flowers and origami birds. Then they integrated gold electrodes into the construct to increase its detection sensitivity. The experiment proposed a comprehensive framework to rationally design and manufacture scalable and complex 3-D, self-folding optical and electronic devices by folding 2-D single-layer graphene.

Develop 3-D microstructures from 2-D precursors

The development of 3-D integrated microstructures from wafer-level 2-D precursors can be used in many fields, including optics, electronics, robotics, and biomedical engineering. However, it is still difficult to realize on-wafer or independent and reversible two-dimensional layered material hybrid devices. In this work, Huang et al. The folding mechanics of differentially cross-linked SU8 was studied, that is, epoxy-based ultraviolet (UV) cross-linkable negative photoresist based on commercial resin, and the interaction between light and flexible graphene-gold (Au)-SU8 3-D. micro structure. The team introduced several new ideas through experiments and simulations, and demonstrated the self-folding SU8 graphene microstructure. They changed the degree of crosslinking of SU8 by adjusting the UV dose to develop a physics-based coarse-grained model that includes the effects of UV on material mechanics and volume changes. They then used the method to provide example 3-D shapes, including origami birds. The method also includes multi-layer very large-scale integration (VLSI) calculation methods. This method allows simple connections to electrodes and other electronic, optical or microfluidic modules. These studies showcase 3-D graphene hybrid functional devices suitable for robotics, wearable devices, and photonics.

Reasonably design 3D self-folding SU8 structure

Huang et al. Two methods that allow the reversible folding of differentially cross-linked SU8 films were tested, including double-layer and gradient methods. For these two versions, they first deposited a 50 nm thick thermally evaporated copper sacrificial layer on the wafer or glass slide. In the two-layer method, they used photolithography to pattern the SU8 two-layer film with a fully cross-linked bottom layer and a partially cross-linked top layer to promote bending away from the wafer. Then, they spin-coated the SU8 layer onto the material and adjusted the double-layer pattern by dipping them in acetone to produce a self-folding precursor. When the solvent is transferred from acetone to water, the adjusted structure can be reversibly folded and unfolded. By changing the thickness of the pattern, they assembled curved beams with different radii and various 3D shapes. The team also changed the dose ratio of ultraviolet radiation to increase the degree of pattern folding. They noticed how to achieve different folding angles by changing the thickness and degree of crosslinking. This work provides the design criteria required to achieve the controlled bending and geometry of the SU8 microstructure. The simulation is an accurate reproduction of the experimental folding shape.

Convert graphene into 3-D shape based on self-folding SU8 structure

The self-folding structure can importantly support the transformation of flat single-layer graphene to a 3-D shape. This integration process includes several key steps. First, the team used the polymethylmethacrylate (PMMA) method to transfer a single-layer graphene grown using chemical vapor deposition from a copper-plated wafer to a sacrificial copper-plated silicon substrate. Then use Raman spectroscopy, Huang et al. As expected, note the peak corresponding to the single-layer graphene deposited on SU8. After that, they patterned graphene through photolithography and plasma etching, and realized the self-curling of the graphene-SU8 structure, which was reversibly rolled/unrolled in water and acetone. This self-rolling graphene-SU8 integration process occurs at the wafer level and helps to include other elements, including gold wires or patterns, to form functional electronic or optical devices.

Develop ultra-thin deformable smart materials.

Materials scientists usually study the electronic and optical applications of graphene based on the unique physical properties, high mechanical strength and stability of materials. Due to its optoelectronic properties, the high charge carrier mobility of graphene at ambient temperature reveals its potential applications in high-frequency and high-speed devices. However, the light absorption and light-matter interaction of graphene is very low for atomically thin planar graphene-based devices. Huang et al. Therefore, the optical transparency of SU8 is used to develop optical devices based on 3-D self-folding graphene to form flexible optical devices and wearable devices. They created a multi-volume 3-D graphene structure to overcome the limitation of poor absorption of single-layer graphene. Then, the scientists used a planar graphene-gold-SU8 photodetector and tested the substrate by irradiating each gold electrode with a 488 nm laser. Compared with the SU8 side, when the laser irradiation is directly incident on the graphene side, the photovoltage is higher. The decrease in light is due to the absorption of light by the SU8 film. The photovoltage generated in the work mainly comes from the overlapping area of ​​gold and graphene.

Form a chip-integrated 3-D graphene-SU8 structure and photodetector

As a proof of concept, Huang et al. Developed complex origami design and chip integrated structure. To assemble them, they patterned the copper sacrificial layer and graphene, and controlled the UV irradiation in specific areas to selectively fold the SU8 microstructure, while the other parts were kept flat. This complex structure is very important for soft robots with graphene-gold interfaces for remote light energy harvesting applications. On-chip assembly design is also very important in optoelectronics, Huang et al. An angle-resolved photodetector with a self-folding SU8 graphene photodetector array is used for illustration. Using light, they show different light responses according to the angle of the laser and the structure of the material. The team also used simulation to determine the angular resolution response.

In this way, Qi Huang and colleagues developed a highly parallel process to assemble 3-D flexible graphene microstructures. This method has three main advantages,

Optically transparent photoresists can be spin-coated and remain relatively flexible. These structures are stable in the air and can form a better lightweight alternative to silicon-based modules for integration in flying and swimming robots. The main basis of the self-folding mechanism relies on differential swelling driven by chemical solvents to promote folding/unfolding motion. The team hopes to use this method to create a series of 3D microstructures for wearable devices, mobile robots, biosensors, and energy harvesting devices. Further explore the use of 3-D curved graphene to stay ahead of the curve. More information: Huang Q. et al. Solvent-responsive self-folding of 3D photosensitive graphene architecture, advanced intelligent system, doi: doi.org/10.1002/aisy.202000195

Freitag M. et al. The photoconductivity of biased graphene, Nature Photonics, doi: doi.org/10.1038/nphoton.2012.314

Koppens FHL etc. Photodetectors based on graphene, other two-dimensional materials and hybrid systems, Nature Nanotechnology, doi.org/10.1038/nnano.2014.215 Journal information: Nature Photonics, Nature Nanotechnology

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