This protocol uses origami-like self-folding methods to create 3D graphene-based polyhedrons. Two-dimensional graphene sheets possess extraordinary optical, electronic, and mechanical properties. By tailoring the shape of two-dimensional graphene, its physical, chemical, and optical properties can be tuned, which can introduce new material behaviors, allowing new application opportunities.
In this regard, integrating 2D graphene into the structures of functionalized, well-defined, three-dimensional polyhedrons has been of great interest recently in the graphene community. 3D graphene polyhedrons may be beneficial for many applications. Examples include gas and water-protected containers, molecular storage, delivery systems, encapsulation of liquid-based materials, for easy to observation in electron microscopy, functional optoelectronic devices, and matter materials.
A number of chemical routes for the production of 3D graphene-based structures have been introduced. However, these methods typically require strong chemical reactions, which affect the intrinsic properties of the graphene and present challenges in the construction of free-standing, hollow, 3D graphene polyhedrons. To overcome these limitations, this study shows a methodology for realizing multi-faced 3D microcubes with 2D graphene and graphene oxide by using origami-like self-folding.
To help give context to the overall fabrication process of the 3D graphene-based cubes, a simplified six-step overview will now be introduced prior to the detailed protocol. First, prepare the protection layers. Then, perform graphene-membrane transfer in patterning onto the protection layers.
Then, create metal surface patterning on the graphene membranes. Next, define polymer frames and hinges. Then, transform the 2D nets into 3D cubes via self-folding.
Lastly, remove the protection layers. Using an electron beam evaporator, deposit 10 nanometer-thick chromium and 300 nanometer-thick copper layers on the silicon substrate. Spin coat photoresist at 2500 RPM followed by baking at 115 degrees Celsius for 60 seconds.
Expose the designed 2D net areas to UV light on a contact mask aligner for 15 seconds and then develop for 60 seconds in developer. Rinse the sample with deionized water and blow-dry with an air gun. Deposit a 10 nanometer-thick chromium layer and lift off the remaining photoresist in acetone.
Rinse the sample with deionized water and blow-dry with an air gun. To pattern 2D nets with six-square aluminum oxide and chromium protection layers on the nets, spin coat photoresist at 2, 500 RPM followed by baking at 115 degrees Celsius for 60 seconds. Expose the designed six-square protection layers to UV light on a contact mask aligner for 15 seconds and develop for 60 seconds in developer.
Rinse the sample with deionized water and blow-dry with an airgun. Deposit a 100 nanometer-thick aluminum oxide layer in a 10 nanometer-thick chromium layer. Remove the remaining photoresist in acetone.
Rinse the sample with deionized water and blow-dry with an airgun. Starting with the 15 millimeter square piece of graphene adhered on copper foil, spin coat a thin PMMA layer at 3000 RPM on the surface of the graphene. Bake at 180 degrees Celsius for 10 minutes.
Place the PMMA graphene in copper foil-layered sheet, floating copper-side down in copper etchant for 24 hours to etch away the copper foil. After the copper foil is completely dissolved, leaving PMMA in graphene, transfer the floating PMMA-coated graphene onto the surface of a pool of deionized water using a microscope slide glass to remove any copper etchant residue. Repeat the transfer of the PMMA-coated graphene onto new deionized water pools several times to adequately rinse.
Transfer the floating PMMA-coated graphene onto another piece of graphene adhered on copper foil to obtain a bilayer graphene membrane. Thermally treat the double-layer graphene on copper foil on a hot plate at 100 degrees Celsius for 10 minutes. Remove the PMMA on top of the double-layer graphene on the copper foil in an acetone bath, leaving a graphene, graphene, and copper foil layer stack, followed by transferring to deionized water.
Repeat the graphene transfer one more time to get three stacked layers of graphene membranes. Place the PMMA, graphene, graphene, graphene, and copper foil layered sheet, floating copper side down in copper etchant for 24 hours to etch away the copper foil. Transfer the PMMA-coated three layers of graphene membranes onto the pre-fabricated aluminum oxide and chromium protection layers.
After transfer of the graphene, remove the PMMA with acetone. Then, dip the sample in deionized water and dry in air. Thermally treat the multi-layer graphene on the substrate on a hot plate at 100 degrees Celsius for one hour.
Spin coat photoresist at 2500 RPM and bake at 115 degrees Celsius for 60 seconds. UV expose the regions of photoresist directly above the square protection layer areas using a contact mask aligner for 15 seconds. Then develop for 60 seconds.
Remove the newly-uncovered unwanted graphene areas via an oxygen plasma treatment for 15 seconds. Remove the leftover photoresist in acetone. Rinse the sample with deionized water and dry in air.
Spin coat photoresist at 1700 RPM for 60 seconds on top of the previously fabricated aluminum oxide and chromium protection layers to obtain a 10 micrometer thick layer. Bake the photoresist at 115 degrees Celsius for 60 seconds and then wait for three hours. With the same mask used for patterning the aluminum oxide and chromium layers, UV expose the sample on a contact mask aligner for 80 seconds and develop for 90 seconds.
Rinse the sample with deionized water and blow-dry with an air gun. Perform a UV flood exposure of the entire sample without a mask for 80 seconds. Spin coat the prepared graphene oxide and water mixture on the sample at 1000 RPM for 60 seconds.
Perform the spin coating a total of three times. Dip the sample in developer to allow lift-off of unwanted graphene oxide. The lift-off process under a microscope is shown here with the footage accelerated.
Rinse the sample with deionized water and carefully blow-dry the sample with an air gun. Thermally treat the sample on a hot plate at 100 degrees Celsius for one hour. Create 20 nanometer-thick titanium patterns on top of the patterned graphene-based membranes.
Thermally treat the sample on a hot plate at 100 degrees Celsius for one hour. On top of the graphene-based membranes with titanium surface patterns, spin coat photoresist at 2500 RPM for 60 seconds to form a five micrometer-thick layer and bake at 90 degrees Celsius for two minutes. UV-expose the samples for 20 seconds, bake at 90 degrees Celsius for three minutes, and develop for 90 seconds.
Rinse the sample with deionized water and isopropyl alcohol, and carefully blow-dry the sample with an air gun. Post-bake the samples at 200 degrees Celsius for 15 minutes to enhance the mechanical stiffness of frames. To make the hinge pattern, spin coat photoresist at 1000 RPM for 60 seconds to form a 10 micrometer-thick film on top of the prefabricated substrate.
Bake at 115 degrees Celsius for 60 seconds and wait for three hours. UV-expose the sample on a contact mask aligner for 80 seconds and develop for 90 seconds. Rinse the sample with deionized water and carefully blow-dry the sample with an air gun.
To release the 2D structures, dissolve the copper sacrificial layer under the 2D nets in a copper etchant. Carefully transfer the released structures into a deionized water bath by using a pipette and rinse a few times to remove the residual copper etchant. Place the 2D structures in deionized water, heated above the melting point of the polymer hinges.
Monitor the self-folding in real time via optical microscopy and remove from the heat source upon successful assembly into closed cubes. After self-folding, remove the aluminum oxide and chromium protection layers with chromium etchant. Gently transfer the cubes into a deionized water bath and carefully rinse.
The optical images show the lithographic processes of the 2D graphene and graphene oxide net structures and the subsequent self-folding process. The self-folding process is monitored in real time via a high-resolution microscope. Both types of 3D graphene-based cubes are folded at about 80 degrees Celsius.
The figures lay out video-captured sequences showing the self-folding of 3D graphene-based cubes in a parallel manner. Under an optimized process, this approach shows a highest yield of approximately 90 percent. The figures show optical images of the 3D assembled graphene and graphene oxide-based cubes with and without surface patterns.
The overall size of the self-folded cubes is 200 micrometers wide by 200 micrometers long by 200 micrometers high. 20 nanometer-thick titanium-patterned features and UMN lettering are defined on each face of the 3D graphene-based cubes. The figures include Raman spectroscopy of 2D graphene-based nets and 3D graphene-based cubes.
The results show no noticeable changes in Raman peak position and intensity for both graphene and graphene oxide membranes after the self-folding. However, when protection layers are not used, noticeable changes in relative peak intensities were observed, indicating changes or damage to the properties of the graphene during the self-folding. This method allows the development of biomedical, electronic, and optical devices, including sensors and electric circuits using the numerous advantages of 3D configurations.
Furthermore, since the process uses an early image to only graphene-based materials, this method may be applied to other two-dimensional material, such as tren-ja-men-a ai chai-hus knives and black passports, thereby allowing this opportunity to be used in developing next-generation 3D configurations of 2D materials.
