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In This Article

  • Summary
  • Abstract
  • Introduction
  • Protocol
  • Results
  • Discussion
  • Disclosures
  • Acknowledgements
  • Materials
  • References
  • Reprints and Permissions

Summary

Here, we present a protocol for fabrication of 3D graphene-based polyhedrons via origami-like self-folding.

Abstract

The assembly of two-dimensional (2D) graphene into three-dimensional (3D) polyhedral structures while preserving the graphene's excellent inherent properties has been of great interest for the development of novel device applications. Here, fabrication of 3D, microscale, hollow polyhedrons (cubes) consisting of a few layers of 2D graphene or graphene oxide sheets via an origami-like self-folding process is described. This method involves the use of polymer frames and hinges, and aluminum oxide/chromium protection layers that reduce tensile, spatial, and surface tension stresses on the graphene-based membranes when the 2D nets are transformed into 3D cubes. The process offers control of the size and shape of the structures as well as parallel production. In addition, this approach allows the creation of surface modifications by metal patterning on each face of the 3D cubes. Raman spectroscopy studies show the method allows the preservation of the intrinsic properties of the graphene-based membranes, demonstrating the robustness of our method.

Introduction

Two-dimensional (2D) graphene sheets possess extraordinary optical, electronic, and mechanical properties, making them model systems for the observation of novel quantum phenomena for next-generation electronic, optoelectronic, electrochemical, electromechanical, and biomedical applications1,2,3,4,5,6. Apart from the as-produced 2D layered structure of graphene, recently, various modification approaches have been investigated to observe new functionalities of graphene and seek new application opportunities. For example, modulating (or tuning) its physical properties (i.e., doping level and/or band gap) by tailoring the shapes or patterning of the 2D structure to a one-dimensional (1D) or zero-dimensional (0D) structure (e.g., graphene nanoribbon or graphene quantum dots) has been studied to obtain new physical phenomena including quantum confinement effects, localized plasmonic modes, localized electron distribution, and spin-polarized edge states7,8,9,10,11,12. In addition, varying the texture of 2D graphene by crumpling (often called kirigami), delamination, buckling, twisting, or stacking of multiple layers, or changing the graphene surface shape by transferring 2D graphene on top of a 3D feature (substrate) has been shown to change the graphene's wettability, mechanical characteristics, and optical properties13,14.

Beyond changing the surface morphology and layered structure of 2D graphene, assembly of 2D graphene into functionalized, well-defined, three-dimensional (3D) polyhedrons has been of great interest recently in the graphene community to obtain new physical and chemical phenomena15. In theory, the elastic, electrostatic, and van der Waals energies of 2D graphene-based structures can be leveraged to transform the 2D graphene into various 3D graphene-origami configurations16,17. Based on this concept, theoretical modeling studies have investigated 3D graphene structure designs, formed from nanoscale 2D graphene membranes, with possible uses in drug delivery and general molecular storage16,17. Yet, the experimental progress of this approach is still far from realizing these applications. On the other hand, a number of chemical synthetic methods have been developed to achieve 3D structures via template-assisted assembly, flow-directed assembly, leavening assembly, and conformal growth methods18,19,20,21,22. However, these methods are currently limited in that they cannot produce a 3D, hollow, enclosed structure without losing the intrinsic properties of the graphene sheets.

Here, a strategy for building 3D, hollow, graphene-based microcubes (overall dimension of ~200 µm) by using origami-like self-folding is outlined; overcoming the foremost challenges in the construction of free-standing, hollow, 3D, polyhedral, graphene-based materials. In origami-like, hands-free self-folding techniques, 2D lithographically patterned planar features (i.e., graphene-based membranes) are connected with hinges (i.e., thermal-sensitive polymer, photoresist) at various joints, thereby forming 2D nets which fold up when the hinges are heated to melting temperature23,24,25,26. The graphene-based cubes are realized with window membrane components composed of a few layers of chemical vapor deposition (CVD) grown graphene or graphene oxide (GO) membranes; both with the use of polymer frames and hinges. The fabrication of the 3D graphene-based cubes involves: (i) preparation of protection layers, (ii) graphene-membrane transfer and patterning, (iii) metal surface patterning on graphene-membranes, (iv) frame and hinges patterning and deposition, (v) self-folding, and (vi) removal of the protection layers (Figure 1). This article focuses mostly on the self-folding aspects of the 3D graphene-based cubes fabrication. Details on physical and optical properties of the 3D graphene-based cubes can be found in our other recent publications27,28.

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Protocol

CAUTION: Several of the chemicals used in these syntheses are toxic and may cause irritation and severe organ damage when touched or inhaled. Please use appropriate safety equipment and wear personal protective equipment when handling the chemicals.

1. Preparation of Aluminum Oxide and Chromium Protection Layers on a Copper Sacrificial Layer

  1. Using an electron-beam evaporator, deposit 10 nm thick chromium (Cr) and 300 nm thick copper (Cu) layers (sacrificial layer) on the silicon (Si) substrate (Figure 2a).
  2. Spin-coat a photoresist (PR)-1 at 2500 rpm followed by baking at 115 °C for 60 s.
  3. Expose the designed 2D net areas to ultraviolet (UV) light on a contact mask aligner for 15 s and develop for 60 s in developer-1 solution. Rinse the sample with deionized (DI) water and blow-dry with an air gun.
  4. Deposit 10 nm thick Cr layer and lift-off of the remaining PR-1in acetone. Rinse the sample with DI water and blow-dry with an air gun (Figure 2b).
  5. To pattern 2D nets with six square Al2O3/Cr protection layers on the 2D net, spin-coat a PR-1 at 2500 rpm followed by baking at 115 °C for 60 s.
  6. Expose the designed six square protection layers to UV light on a contact mask aligner for 15 s and develop for 60 s in developer-1 solution. Rinse the sample with DI water and blow-dry with an air gun.
  7. Deposit a 100 nm thick Al2O3 layer and 10 nm thick Cr layer. Remove remaining PR-1in acetone. Rinse the sample with DI water and blow-dry with an air gun (Figure 2c).

2. Preparation of Graphene and Graphene Oxide Membranes

Note: In this study, two types of graphene-based materials are used: (i) chemical vapor deposition (CVD) grown graphene and (ii) graphene oxide (GO).

  1. Preparation of multilayer CVD graphene membranes
    Note: To obtain multilayer graphene membranes, single-layer graphene is transferred three separate times using multiple polymethyl methacrylate (PMMA) coating/removal steps.
    1. Starting with a ~15 mm square piece of graphene adhered on Cu foil, spin-coat a thin PMMA layer at 3000 rpm on the surface of the graphene. Bake at 180 °C for 10 min.
    2. Place the PMMA/graphene/Cu foil-layered sheet floating Cu-side down in Cu etchant for 24 h to etch away the Cu foil.
    3. After the Cu foil is completely dissolved (leaving PMMA/graphene), transfer the floating PMMA-coated graphene onto the surface of a pool of DI water using a microscope slide glass to remove any Cu etchant residue. Repeat the transfer of the PMMA-coated graphene onto new DI water pools several times to adequately rinse.
    4. Transfer the floating PMMA-coated graphene onto another piece of graphene adhered on Cu foil (graphene/Cu) to obtain a bi-layer graphene membrane (forming a PMMA/graphene/graphene/Cu foil structure).
    5. Thermally treat the double-layer graphene on the Cu foil on a hot plate at 100 °C for 10 min.
    6. Remove the PMMA on top of the double-layer graphene on the Cu foil in an acetone bath (leaving a graphene/graphene/Cu foil layer stack), followed by transferring to DI water.
    7. Repeat the graphene transfer (2.1.1 - 2.1.5) one more time to get three stacked layers of graphene membranes. When step 2.1.4 is reached during the repeat process, instead of transferring the new PMMA-coated graphene sheet onto another piece of graphene/Cu, transfer the new PMMA-coated graphene onto the previously fabricated graphene double-layer from step 2.1.6 to form a PMMA/graphene/graphene/graphene/Cu foil layer combination. Then, repeat step 2.1.5 without modification.
    8. Place the PMMA/graphene/graphene/graphene/Cu foil-layered sheet floating Cu-side down in Cu etchant for 24 h to etch away the Cu foil.
    9. Transfer the PMMA-coated three-layers of graphene membranes (PMMA/graphene/graphene/graphene) onto the pre-fabricated Al2O3/Cr protection layers from section 1.
    10. After transfer of the graphene, remove the PMMA with acetone. Then, dip the sample in DI water and dry in air.
    11. Thermally treat the multi-layer graphene on the substrate on a hot plate at 100 °C for 1 h.
    12. Spin-coat PR-1 at 2500 rpm and bake at 115 ˚C for 60 s.
    13. UV expose the regions of PR-1 directly above the square protection layer areas using a contact mask aligner for 15 s and develop for 60 s in developer-1 solution.
    14. Remove the newly uncovered, unwanted graphene areas via an oxygen plasma treatment for 15 s.
    15. Remove the leftover PR-1 in acetone.
    16. Rinse the sample with DI water and dry in air (Figure 2d).
  2. Preparation of graphene oxide membranes
    Note: Traditional photolithography followed by a lift-off process via flood exposure is used to pattern the GO membranes.
    1. Spin-coat PR-2 at 1700 rpm for 60 s on top of the previously fabricated Al2O3/Cr protection layers to obtain a 10 µm thick layer. Bake the PR-2 at 115 °C for 60 s and then wait for 3 h.
    2. With the same mask used for patterning the Al2O3/Cr protection layer, UV expose the sample on a contact mask aligner for 80 s and develop for 90 s in developer-2 solution. Rinse the sample with DI water and blow-dry with an air gun.
    3. Perform a UV flood exposure of the entire sample without a mask for 80 s.
    4. Spin-coat the prepared GO and water mixture (15 mg of GO powder in 15 mL of DI water) on the sample at 1000 rpm for 60 s. Perform the spin-coating a total of 3 times.
    5. Dip the sample in developer-2 solution to allow lift-off of unwanted GO.
    6. Rinse the sample with DI water and carefully blow-dry the sample with an air gun.
    7. Thermally treat the sample on a hot plate at 100 °C for 1 h (Figure 2h).

3. Metal Surface Patterning on Graphene-Based Membranes

Note: A common photolithography process was conducted to achieve the surface patterning using a UV contact mask aligner and electron-beam evaporator (see 1.2 - 1.4).

  1. Create 20 nm thick titanium (Ti) patterns on top of the patterned graphene-based membranes.
  2. Thermally treat the sample on a hot plate at 100 °C for 1 h (Figure 2e for graphene and Figure 2i for GO).

4. Fabrication of Polymer Frames and Hinges

  1. On top of graphene-based membranes with Ti surface patterns, spin-coat PR-3 at 2500 rpm for 60 s to form a 5 µm thick layer and bake at 90 °C for 2 min.
  2. UV expose the samples for 20 s, bake at 90 °C for 3 min, and develop for 90 s in developer-3 solution.
  3. Rinse the sample with DI water and isopropyl alcohol (IPA) and carefully blow-dry the sample with an air gun.
  4. Post-bake the samples at 200 °C for 15 min to enhance the mechanical stiffness of the (PR-3) frames (Figure 2f for graphene and Figure 2j for GO).
  5. To make the hinge pattern, spin-coat PR-2 at 1000 rpm for 60 s to form a 10 µm thick film on top of the prefabricated substrate. Bake at 115 ˚C for 60 s and wait for 3 h.
  6. UV expose the sample on a contact mask aligner for 80 s and develop for 90 s in developer-2 solution.
  7. Rinse the sample with DI water and carefully blow-dry the sample with an air gun (Figure 2g for graphene and Figure 2k for GO).

5. Self-Folding in DI Water

Note: When the PR-2 hinges are melted (or reflow), a surface tension force is generated; hence, the 2D structures transform into 3D structures (a self-folding process).

  1. To release the 2D structure, dissolve the Cu sacrificial layer under the 2D nets in a Cu etchant (Figure 2l).
  2. Carefully transfer the released structure into a DI water bath by using a pipet and rinse a few times to remove the residual Cu etchant.
  3. Place the 2D structure in DI water heated above the melting point of the polymer (PR-2) hinges (Figure 2m).
  4. Monitor the self-folding in real-time via optical microscopy and remove from the heat source on successful assembly into closed cubes.

6. Removal of the Protection Layers

  1. After self-folding, remove the Al2O3/Cr protection layers with Cr etchant (Figure 2n).
  2. Gently transfer the cubes into a DI water bath and carefully rinse.

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Results

Figure 2 displays optical images of the lithographic processes of the 2D graphene and GO net structures and 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 ~80 °C. Figure 3 lays out video captured sequences showing the self-folding of 3D graphene-based cubes in a parallel manner. Under an op...

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Discussion

For the cubes fabricated with CVD graphene, because each face of a given cube is designed with an outer frame surrounding a ~160 × 160 µm2 area of free-standing graphene, a single sheet of monolayer graphene does not have the necessary strength to permit parallel processing of the cubes. For this reason, graphene membranes consisting of three layers of CVD graphene monolayer sheets are produced via three separate graphene transfers using multiple PMMA coating/removal steps. On the other hand...

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Disclosures

The authors have nothing to disclose.

Acknowledgements

This material is based upon work supported by a start-up fund at the University of Minnesota, Twin Cities and an NSF CAREER Award (CMMI-1454293). Parts of this work were carried out in the Characterization Facility at the University of Minnesota, a member of the NSF-funded Materials Research Facilities Network (via the MRSEC program. Portions of this work were conducted in the Minnesota Nano Center, which is supported by the National Science Foundation through the National Nano Coordinated Infrastructure Network (NNCI) under Award Number ECCS-1542202. C. D. acknowledges support from the 3M Science and Technology Fellowship.

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Materials

NameCompanyCatalog NumberComments
AcetoneFisher ChemicalA18P-4N/A
Aluminium oxideKurt J. Lesker CompanyEVMALO-1-2.599.99% Pure
APS Copper Etchant 100Transene Company, Inc.N/AN/A
Camera (for 3D image)NikonD51001080p Full HD, Effective pixels: 16.2 million, Sensorsize: 23.6 mm x 15.6 mm
CE-5 M Chromium Mask EtchantTransene Company, Inc.N/AN/A
Chemical deposition growth (CVD) systemCustomizedN/ALindberg/Blue Tube Furnace
ChromiumKurt J. Lesker CompanyEVMCR35J99.95% pure
Chromium Etchant 473Transene Company, Inc.N/AN/A
CopperKurt J. Lesker CompanyEVMCU40QXQJ99.99% pure
Developer-1 (MF319 developer)Microposit10018042N/A
Developer-2 (AZ developer)Merck performance Materials Corp.1005422496N/A
Developer-3 (SU-8 developer)MicroChemNC9901158N/A
Digital Hot PlateThermo ScientificHP131725Super-Nuvoa series, maximum temperature: 370 °C
E-Beam Evaporator SystemRocky Mountain Vacuum Tech.N/ARME-2000
Graphene oxideGoographeneN/APurity: ~ 99%; Single layer ratio: ~99%;  0.7-1.2 nm in thickness.
Isopropyl AlcoholFisher ChemicalA416-4N/A
Mask AlignerMidasMDA-400LJN/A
MicroscopeOmaxNJF-120AN/A
multiple polymethyl methacrylate (PMMA)MicroChem950 PMMA A9N/A
Oxygen plasma Technics Inc.SERIES 800Microscale reactive ion etching (RIE)
Photoresist-1 (S1813 Photoresist)Microposit10018348N/A
Photoresist-2 (SPR220 Photoresist)MicroChemSPR00220-7GN/A
Photoresist-3 (SU-8 Photoresist)MicroChemSU-8-2010N/A
ProfilometerTencor InstrumentsN/AAlpha-Step 200
RamanWITec Instruments Corp.Alpha300RConfocal Raman Microscope
Silicon WaferSiltronic AGN/A100mm diameter, N-type, one-side polish, resitivity: 560-840 Ω•cm
SpinnerBest ToolsS0114031123SMART COATER 100
TitaniumKurt J. Lesker CompanyEVMTI45QXQA99.99% Pure
Ultrasonic CleanerCrest UltrasonicsN/APowersonic series

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