The measurements of physical properties such as electrical characterization of single or few nano-objects like nanoparticles, nanoclusters and single molecules require the fabrication of nanoscale gaps between electrodes [1]. It is difficult to fabricate electrical contacts at this scale with standard lithography techniques but focused ion beam (FIB) can fabricate nanogaps by milling thin metallic wires on the insulation substrate. It is a not only a mask less technique but also enables to fabricate large numbers of nanogaps with reproducible properties at the room temperature [2]. The FIB milled nanogaps are employed to determine single or few nano-entities such as nanoparticles and molecules [3]. Due to limitation of ion beam diameter and profile, the size of FIB milled nanogap is more than 10 nm thus not applicable to measure single less than 10nm object. We have developed two strategies to create sub-5nm gaps 1) by a control and precise dosage of ions during milling 2) by electrodeposition of gold in 30nm FIB cut gaps

1) A series of nanogaps are milled with size range from sub-5 to 30nm in thin gold wires, through precise control of the applied ion dosages with a range between 4.6 and 9.2 x 10¹⁰ ions/cm  using focused gallium ion beam at 30KV. As shown in figure 1a, the nanogap is not fully milled with the ion dosage of 4.6×10¹⁰ ions/cm, whereas the large nanogaps are milled with high ion dosage of 9.2 x 10¹⁰ ions/cm having resistances in 100-1000TΩ range (figure 1b). With ion dosage of 5×10¹⁰ ions/cm the nanogap begin to show up in gold nanowire (figure 2), which is verified by the current-voltage (I-V) characteristic demonstrating the resistance up to 12 TΩ. The Fowler-Nordheim tunneling effect is also observed in the sub-5 nm. The application of Simmon’s model to the milled nanogaps and the electrical analysis indicates that the minimum effective nanogap size approaches to 2.3 nm

2) The 30nm FIB cut nanogaps (figure 1b) are connected to a potentiostat controlled by GPES software. A 10 µL drop of diluted gold chloride is added on top of a FIB cut gap. In one of the configuration when working electrode (WE) is connected to to one side of nanoelectrode setup, other side of setup is connected to reference electrode (RE) and counter electrode(CE) is placed inside the drop on silsicon dioxide as shown in figure 3, well defined and well-shaped reduction of gap size has occurred with the growth on both electrodes from deposition of gold atoms (figure 4). The calculated growth rate in the nanogap is in the order of 1 nm/s and growth rates depend directly on the deposition time. The current-voltage characteristics have demonstrated the open gap resistance of 5nm nanogap in the order of 300TΩ. The FIB templating also improves the shape of electrochemically grown nanogaps.

These two strategies allow us to create large number of nanogaps thus enabling to measure physical properties of a sub 5nm single nano-object.

References:

1.         Nicholas, P. and S. Kyung-Ah, Alligator clips to molecular dimensions. Journal of Physics: Condensed Matter, 2008. 20(37): p. 374116.

2.         Blom, T., et al., Fabrication and characterization of highly reproducible, high resistance nanogaps made by focused ion beam milling. Nanotechnology, 2007. 18(28): p. 285301.

3.         Jafri, S.H.M., et al., Nano-fabrication of molecular electronic junctions by targeted modification of metal-molecule bonds. Scientific Reports, 2015. 5: p. 14431.

Figures:

Figure 1: Scanning electron image of the focused Ga ion beam milling in gold thin wire with a) ion dosages of 4.6 x 10^10 ions/cm and b) 9.0 x 10^10 ions/cm

Figure 2: Scanning electron image of the focused Ga ion beam milling in gold thin wire with ion dosages of 5 x 10^10 ions/cm. Scale bar is 20nm

Figure 3: Schematic image of the experimental setup where it is optional to use one of the contact pads as WE and the other as RE, or both as WEs

Figure 4: Scanning electron image of nanogaps taken after electrodeposition by applying -1 µA with 0.2 mM gold chloride solution and deposition time of 30 seconds. Scale bar is 100 nm

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