Research ArticleENGINEERING

Soft nanocomposite electroadhesives for digital micro- and nanotransfer printing

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Science Advances  11 Oct 2019:
Vol. 5, no. 10, eaax4790
DOI: 10.1126/sciadv.aax4790
  • Fig. 1 Picking and placing micro objects using a ceramic-carbon SNE.

    (A) Schematic of a pick-and-place procedure wherein a micrometer-sized object in contact with an SNE surface can be picked up, while an external voltage is applied and placed back when the voltage is removed. (B) Schematics and scanning electron microscopy (SEM) images of a 200-μm SNE composed of Al2O3-coated carbon nanotubes (CNTs). (C) Snapshots of picking and placing an unpackaged LED chiplet (170 μm by 170 μm by 50 μm) using the Al2O3-CNT nanocomposite electroadhesive (200 μm by 200 μm by 50 μm). (D) Schematic of pull-off force measurement system using a colloidal AFM tip and load-displacement curves for a Al2O3-CNT SNE measured using a ~4-μm-diameter Pt-coated spherical tip with and without a bias voltage of 10 V. The inset SEM image shows the Pt-coated spherical tip. (E) Measured pull-off forces using the Pt-coated spherical tip against a reference surface (flat Al2O3-coated TiN; ~0.8 μN; black dashed line) and against the Al2O3-CNT SNE at different applied voltages (e.g., ~0.02 μN at 0 V and ~2.3 μN at 30 V; blue dashed line), measured by a conductive AFM. Error bars represent the SD in the set of force measurements with each applied voltage.

  • Fig. 2 Fibrous structure, roughness, and softness of SNE surface enabling switchable adhesion.

    (A) An SEM image of SNE showing the top entangled surface and vertically aligned fibers underneath. (B) Schematics showing structural behavior of the top surface when a microsphere is compressed and retracted (left) and showing the van der Waals force that scales with few contacting nanofibers and electrostatic attraction that scales with all the nanofibers on the SNE surface (right). (C) AFM images of the top SNE surface composed of entangled Al2O3-coated CNT fibers. The root mean square roughness within the 100-μm2 area is ~35 nm. (D) Load displacement of Al2O3-coated surfaces with and without SNE, measured by nanoindentation using a ~10-μm-diameter sapphire tip. (E) Pull-off forces of Al2O3-coated surfaces with and without SNE measured by AFM using the ~4-μm-diameter Pt-coated spherical tip. (F) Attractive pressure at distances from the Al2O3-CNT SNE surface measured using the 1.2 mm by 1.2 mm Au-coated flat tip.

  • Fig. 3 Switchable adhesion influenced by structural design and object conductivity.

    (A) Adhesion on/off ratio (ratio of pull-off force with an external voltage to that without voltage) of Al2O3-CNT SNE with different Al2O3 coating thicknesses (approximately 0.2, 1, and 10 nm) and different fiber densities (sparse and dense). (B) AFM images (top) images of CNTs coated with Al2O3 via ALD with 10 (left) and 50 (right) deposition cycles, respectively, and SEM images (bottom) of SNE top surfaces with different densities. (C) Pull-off forces against a conductive (Pt-coated; ~4 μm diameter) and a nonconductive (uncoated SiO2; ~10 μm diameter) spherical tip. (D) Schematics of polarization of conductive and dielectric objects when the SNE is in contact with the external voltage on.

  • Fig. 4 Manipulation of multiple micro/nano objects using patterned SNE.

    (A) A micro–pick-and-place map showing the capable object size range that can be picked (by the adhesion force estimated from Eq. 1; blue solid and dashed lines) and placed (estimated from the first term of Eq. 1; red solid line) against a general flat surface with the same coating material (estimated from eq. S2; black solid line) at different operating voltages (30 and 100 V). The dots on the top x axis are the demonstrated materials in this study. (B) An SEM image (60° tilted) of a micropatterned Al2O3-CNT SNE. (C) Optical microscope images of transfer printed polystyrene (PS) microspheres (500 nm in diameter) and (D) Ag nanowires (20 nm in diameter and <12 μm in length) on a SiO2 (thermally oxidized, 300 nm)/Si wafer. (E) An optical microscope image (30° tilted) of 5-μm SiO2 microspheres transfer printed on a polyethylene terephthalate (PET) film.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/10/eaax4790/DC1

    Supplementary Text

    Fig. S1. Schematics of the Al2O3-CNT SNE fabrication procedure.

    Fig. S2. Diagram and optical images of unpackaged LED chiplets (UT170-31, CREE) used in picking and placing demonstration.

    Fig. S3. Atomic force microscopy (AFM) system used for indentation tests with external voltages.

    Fig. S4. Adhesion on/off ratio (ratio of pull-off force with an external voltage to that without voltage) of Al2O3-coated surfaces with and without SNE on a TiN layer.

    Fig. S5. Optical microscope images of a paper towel showing its microscopically porous and rough surface.

    Fig. S6. Transmission electron microscopy images of Al2O3 coating on individual CNT fibers via ALD with different numbers of cycles.

    Fig. S7. Al2O3-CNT SNEs grown with different CVD procedures, exhibiting different CNT densities.

    Fig. S8. Pull-off forces of Al2O3-CNT SNEs with different heights according to applied voltages.

    Fig. S9. Pull-off forces of Al2O3-CNT SNEs measured by AFM using a 4 μm Pt-coated spherical tip with different applied external voltages and preloads.

    Fig. S10. Pull-off forces of Al2O3 coated TiN layer measured by AFM using a 4 μm Pt-coated spherical tip with different applied external voltages and preloads.

    Fig. S11. Pull-off forces of SNE measured by AFM indentation using a 10-μm SiO2 spherical tip with and without the shear motion.

    Fig. S12. Schematics of adhesion tests on an Al2O3-TiN surface using a microsphere.

    Fig. S13. Contact mechanics modeling of adhesion between an Al2O3-CNT nanocomposite surface and a microsphere.

    Fig. S14. Adhesive and gravitational forces along the object size.

    Fig. S15. Optical microscope images of PS microspheres picked from a spin-coated layer using a patterned SNE surface and placed on a silicon wafer surface, under on or off electrical voltage conditions.

    Fig. S16. Picking-and-placing of stainless steel (SS) microparticles, sized 15-45 μm, using a 200 μm by 200 μm Al2O3-CNT SNE, from a silicon wafer to a transparent substrate.

    Movie S1. Picking and placing of an unpackaged micro-LED chiplet using an Al2O3-CNT nanocomposite electroadhesive (side view).

    Movie S2. Picking and placing of a piece of paper towel using an Al2O3-CNT nanocomposite electroadhesive and polydimethylsiloxane block.

  • Supplementary Materials

    The PDF file includes:

    • Supplementary Text
    • Fig. S1. Schematics of the Al2O3-CNT SNE fabrication procedure.
    • Fig. S2. Diagram and optical images of unpackaged LED chiplets (UT170-31, CREE) used in picking and placing demonstration.
    • Fig. S3. Atomic force microscopy (AFM) system used for indentation tests with external voltages.
    • Fig. S4. Adhesion on/off ratio (ratio of pull-off force with an external voltage to that without voltage) of Al2O3-coated surfaces with and without SNE on a TiN layer.
    • Fig. S5. Optical microscope images of a paper towel showing its microscopically porous and rough surface.
    • Fig. S6. Transmission electron microscopy images of Al2O3 coating on individual CNT fibers via ALD with different numbers of cycles.
    • Fig. S7. Al2O3-CNT SNEs grown with different CVD procedures, exhibiting different CNT densities.
    • Fig. S8. Pull-off forces of Al2O3-CNT SNEs with different heights according to applied voltages.
    • Fig. S9. Pull-off forces of Al2O3-CNT SNEs measured by AFM using a 4 μm Pt-coated spherical tip with different applied external voltages and preloads.
    • Fig. S10. Pull-off forces of Al2O3 coated TiN layer measured by AFM using a 4 μm Pt-coated spherical tip with different applied external voltages and preloads.
    • Fig. S11. Pull-off forces of SNE measured by AFM indentation using a 10-μm SiO2 spherical tip with and without the shear motion.
    • Fig. S12. Schematics of adhesion tests on an Al2O3-TiN surface using a microsphere.
    • Fig. S13. Contact mechanics modeling of adhesion between an Al2O3-CNT nanocomposite surface and a microsphere.
    • Fig. S14. Adhesive and gravitational forces along the object size.
    • Fig. S15. Optical microscope images of PS microspheres picked from a spin-coated layer using a patterned SNE surface and placed on a silicon wafer surface, under on or off electrical voltage conditions.
    • Fig. S16. Picking-and-placing of stainless steel (SS) microparticles, sized 15-45 μm, using a 200 μm by 200 μm Al2O3-CNT SNE, from a silicon wafer to a transparent substrate.
    • Legends for movies S1 and S2

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Picking and placing of an unpackaged micro-LED chiplet using an Al2O3-CNT nanocomposite electroadhesive (side view).
    • Movie S2 (.mp4 format). Picking and placing of a piece of paper towel using an Al2O3-CNT nanocomposite electroadhesive and polydimethylsiloxane block.

    Files in this Data Supplement:

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