Custom-Designed Glassy Carbon Tips for Atomic Force Microscopy

Abstract

Glassy carbon is a graphenic form of elemental carbon obtained from pyrolysis of carbon-rich precursor polymers that can be patterned using various lithographic techniques. It is electrically and thermally conductive, mechanically strong, light, corrosion resistant and easy to functionalize. These properties render it very suitable for Carbon-microelectromechanical systems (Carbon-MEMS) and nanoelectromechanical systems (Carbon-NEMS) applications. Here we report on the fabrication and characterization of fully operational, microfabricated glassy carbon nano-tips for Atomic Force Microscopy (AFM). These tips are 3D-printed on to micro-machined silicon cantilevers by Two-Photon Polymerization (2PP) of acrylate-based photopolymers (commercially known as IP-series resists), followed by their carbonization employing controlled pyrolysis, which shrinks the patterned structure by ≥98% in volume. Tip performance and robustness during contact and dynamic AFM modes are validated by morphology and wear tests. The design and pyrolysis process optimization performed for this work indicate which parameters require special attention when IP-series polymers are used for the fabrication of Carbon-MEMS and NEMS. Microstructural characterization of the resulting material confirms that it features a frozen percolated network of graphene sheets accompanied by disordered carbon and voids, similar to typical glassy carbons. The presented facile fabrication method can be employed for obtaining a variety of 3D glassy carbon nanostructures starting from the stereolithographic designs provided by the user.

Keywords: AFM tip; Carbon-NEMS; glassy carbon; pyrolysis; two-photon polymerization.

Figure 1

(a–d) SEM images of IP-Dip tips; and (e,f) IP-L tip before and after carbonization (indicated by the left-to-right arrows). Examples of (g) high aspect ratio; and (h) ultra-small carbon tips fabricated on silicon wafers; (i) Typical glassy carbon tips fabricated on a chip with three (previously) tip-less cantilevers. The aspect ratio and height h of each tip is given at the lower left of the SEM images. Scale bars: (a–g) 10 µm; (h) 1 µm; (i) 100 µm.

Figure 2

Atomic Force Microscopy (AFM) images recorded with glassy carbon tips. (a) Calibration grating CS-20NG; (b) calibration grating TGT1, with cross-sections indicated by solid lines in (c,d), respectively. AFM images of (e) a carbon nanofiber mat and (f) electroplated copper.

Figure 3

Wear tests for glassy carbon tips. (a) Pre- and post-wear tip-apex profiles and (b) corresponding post-wear SEM micrograph at 10 µm/s scan velocity. Magnified image of the apex of the tip in (b) (highlighted by a circle) is shown in (c); (d) pre- and post-wear tip-apex profiles and (e) corresponding post-wear SEM micrograph at 100 µm/s scan velocity; (f) Measured change in tip radii at 10 and 100 µm/s scan velocities at various sliding distances.

Figure 4

Microstructural characterization of IP-Dip-derived glassy carbon. (a) Raman spectrum; (b) Transmission Electron Microscopy (TEM) image showing the microstructure observed in the majority of the material (inset: Focused Ion Beam (FIB) milled sample used for TEM analysis); (c) TEM image featuring a graphitizing region; (d) Pictorial representation of possible 3D arrangement of graphene sheets in the material (locations highlighted by circles); (e) TEM image featuring strongly folded graphene sheets. Scale bars: (b) 5 nm; (c,e) 2 nm.