Nano-electrodes are one of the key elements in the creation of novel nano-devices as well as in the measurement of electronic properties of single molecules. In order to create such devices, gap spacings ranging from several nanometers to several ten nanometers are necessary. Much of the effort in nano-electrode structuring has been dedicated to the creation of small gap spacings. But in many nano-devices based on nano-electrodes, parameters such as gap resistance as well as reproducibility and quantity of nanogaps are essential.
In this work, we present the fabrication and detailed structural and electrical characterisation of highly reproducible and high resistance 20nm wide Au nano-electrodes. The electrodes were fabricated by a combination of photolithography (PLi), electron beam lithography (EBL) and focused ion beam milling (FIB). Each structuring technique was used in an optimal way in order to minimize both the total processing time and the amount of Ga+ ions that are implanted during the FIB milling process. EBL was used to fabricate thin wires, contact pads were deposited by PLi and the nanogap was milled by scanning a focussed ion beam across the wire.
From transmission and scanning electron microscopy images, the atomic scale structure of the nanogaps is visible. Though the FIB process might induce surface amorphisation, in the Au electrodes, atomic lattice planes range up to the electrode surface. The nano-gaps produced in this work can be structured with a reproducibility of ±2.4 nm. The gap resistance, between 300 and 1300 TÙ, is, to our knowledge, the highest hitherto reported for such nanogaps. The functionality of the electrodes was demonstrated by ac dielectrophoretic trapping of colloid gold nanoparticles (GNPs). The electrodes were shown to be stable enough to both endure currents high enough to induce fusing of the trapped GNPs as well as empty gap voltages as high as 5 V. The presented electrode design opens thus the possibility for fabricating large numbers of high quality nanoelectrodes. The high resisistivity enables these electrodes to be used in tunnel devices and for current measurements of highly resistive molecules and nanoparticles.
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