Graphene, a two-dimensional carbon crystal, has been the focus of intense research since techniques were developed to extract it from graphite in the form of multi-layers1 and single layers.2 Graphene-based devices measured on substrates have revealed an impressive set of exotic electronic and optical properties with promising applications.3-7 Furthermore, suspended graphene has been shown to have exceptionally high electron mobilities8 and high strength.9,10 Due to its single-atom-thickness and the relatively low atomic number of carbon, suspended graphene is emerging as powerful platform for transmission electron microscopy (TEM). 10-12 In addition to serving as a nearly ideal substrate for TEM analysis, 13 it has been shown that electron-beam-induced-deposition (EBID) of carbon onto graphene can be achieved with high accuracy in a TEM.14 Graphene is also an exceptional substrate for nanopore biomolecule translocation, which is discussed in more detail on our graphene nanopore research page.

Electron beam nanosculpting of suspended graphene sheets

Our recent efforts include studies on suspended graphene sheets because of their superior mobilities compared to graphene placed on substrates. We have recently investigated the possibility to cut graphene sheets with electron beams and further sculpt them into arbitrary designs that may prove useful in graphene-based electronic and mechanical applications. For instance, fabricating narrow constrictions in graphene layers is of interest for electronic property engineering.15-23 Our work shows that graphene sheets can be controllably nanosculpted with few-nanometer precision by using focused electron-beam irradiation in a TEM at room temperature. To demonstrate this approach, we have realized nanopores, nanobridges and nanogaps, etched from graphene sheets. Such structures made by electron-beam irradiation are stable and do not evolve over time. Furthermore, we find that extensive removal of carbon does not introduce significant long-range distortions of the graphene sheet. Specifically, the sheets do not begin to fold, wrinkle, curl, or warp out of the focal plane during cutting. The ability to introduce features into suspended graphene sheets by electron-beam-induced cutting and reshaping with high spatial resolution expands their value as TEM compatible platforms and offers a route to fabricating graphitic structures for potential use in electrical, mechanical and molecular translocation studies.

Figure 1 TEM images of a suspended graphene sheet (a) before and (b) after a nanopore is made by electron beam ablation. (c) Higher magnification image of the nanopore. (d) Multiple nanopores made in close proximity to each other. (e) Folded edge of a graphene sheet showing lines corresponding to layer number. These lines are similar to those seen around the nanopores (Scale bars are 50, 50, 5, 10, 5 nm). (f) Average of intensity cross-sections taken along six different radial directions of the nanopore in (c), each starting at the edge and proceeding radially outward. (g) Average of six intensity cross-sections of the graphene sheet in (e), each taken perpendicular to and starting at the sheet edge.
Figure 2 (a) Two ~ 6 nm lines cut into a graphene sheet. (b) Electron irradiation is continued to create a ~ 5 nm wide bridge. (c) Higher resolution of the bridge shows clear atomic order. (d) Small gap opened in the nanobridge by additional electron irradiation. We note that the cut ends are closed. (Scale bars are 20, 10, 5, 5 nm).

Experimental Procedure

Graphene sheets were extracted from graphite by mechanical exfoliation2 on SiO2 substrates coated with ~ 400 nm of PMMA and then transferred to a suspended ~ 50 nm-thick suspended SiNx membrane substrate.24 Prior to transfer, arrays of ~ 1 μm square holes were patterned into the SiNx membranes by using electron beam lithography to create a resist mask and then exposing the surface to a SF6 reactive ion etch. Transfer was achieved by placing the SiNx membrane substrate onto the graphene sheets, adding a drop of isopropanol to bring the two substrates into close contact during its evaporation, and then dissolving the PMMA in acetone.14 Graphene sheets suspended over a hole in the SiNx membrane were identified in a TEM (JEOL 2010F operating at 200 kV). The number of graphene layers in a sheet could often be determined by imaging the edge of a folded region,11 in a manner similar to counting the number of tubes in a multi-walled nanotube. We have worked with samples ranging in thickness roughly from 1 – 20 graphene layers, though the majority of graphene sheets used in this work were composed of ~ 5 layers. Using a method described previously, arbitrary patterns were created in the graphene sheets by increasing the TEM magnification to ~ 800,000x, condensing the imaging electron beam to its minimum diameter, ~ 1 nm, and moving the beam position with the condenser deflectors.25 Nanosculpting was performed with the beam at cross-over in a diffusive mode. The beam current density at cross-over was ~ 0.3 A/cm2 and the exposure of the graphene sheets to the beam was ~ 1 s/nm2. All of the structures were made at room temperature.


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