We recently demonstrated a controllable and reproducible method to obtain suspended monolayer graphene nanoribbons with atomically defined edge shape [1]. Our method exploits the electron-beam of a Scanning Transmission Electron Microscope (accelerated at 300 kV) to create vacancies in the lattice by knock-on damage and pattern graphene in any designed shape. The small beam spot size (0.1 nm) enables close-to-atomic cutting precision, while heating graphene at 900 K during the patterning process avoids formation of beam-induced Carbon deposition and allows self-repair of the graphene lattice.  Self-repair mechanism is essential to obtain well-defined (zig-zag or armchair) edge shape and, if the electron beam dose is lowered, to perform non-destructive imaging of the graphene nanoribbons.

Drawing the electron-beam path with a software script, we were able to obtain reproducible graphene nanoribbons with sub 10 nm width. Using an in-house built microscopy holder equipped with electrical feedthroughs, we performed 2 and 4 wire measurements on several graphene nanoribbons, with different number of layers. Wide nanoribbons (width> 50 nm) exhibited ohmic behaviour, with conductivity linearly proportional to the width. Narrower ribbons (10 nm < width < 50 nm) displayed non-linear current-voltage (IV) relationship at low temperature (4 K, ex-situ), possibly indicating the opening of a band gap. The narrowest ribbon was 1.5 nm wide, with strong non-linear IV also at room temperature.

Electrical measurements were also performed in the high temperature range (300 K – 900 K), from which we concluded that thermal generated carriers give the main contribution to  electrical conductivity above room temperature.

Ackwnoledgements: This work was supported by ERC funding,  project 267922 – NEMinTEM 

References

[1]        Q.Xu, M. Wu, G. F. Schneider, L. Houben, S.K. Malladi, C. Dekker, E. Yucelen, R.E. Dunin-Borkowski, and H.W. Zandbergen, ACS Nano  7 (2), 1566-1572, 2013

Figures:

Figure. 1. STEM images of a four layer graphene sample, showing the sculpting procedure realized in-situ with the microscope electron beam. Graphene is fully suspended and it is displayed in grey/white color, the empty background appears in black.

Figure 2. STEM image of a graphene nanoribbon, with a constriction of 1.5 nm width. The ribbon is stable at room temperature, also when removed from the microscope environment.

Figure 3. Current-voltage plot of the same nanoribbon showed in Figure 2, measured at room temperature, and high vacuum (10-6 mbar). A transport gap of approximately 500 meV can be extrapolated from the plot.

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