Sub-10nm nanogap fabrication on suspended glassy carbon nanofibers

Glassy carbon nanofibers(GCNFs)are considered promising candidates for the fabrication of nanosensors for biosensing applications.Importantly,in part due to their great stability,carbon electrodes with sub-10 nm nanogaps represent an attractive platform for probing the electrical characteristics of molecules.The fabrication of sub-10 nm nanogap electrodes in these GCNFs,which is achieved by electrically stimulating the fibers until they break,was previously found to require fibers shorter than 2µm;however,this process is generally hampered by the limitations inherent to photolithographic methods.In this work,to obtain nanogaps on the order of 10 nm without the need for sub-2µm GCNFs,we employed a fabrication strategy in which the fibers were gradually thinned down by continuously monitoring the changes in the electrical resistance of the fiber and adjusting the applied voltage accordingly.To further reduce the nanogap size,we studied the mechanism behind the thinning and eventual breakdown of the suspended GCNFs by controlling the environmental conditions and pressure during the experiment.Following this approach,which includes performing the experiments in a high-vacuum chamber after a series of carbon dioxide(CO 2)purging cycles,nanogaps on the order of 10nm were produced in suspended GCNFs 52µm in length,much longer than the~2µm GCNFs needed to produce such small gaps without the procedure employed in this work.Furthermore,the electrodes showed no apparent change in their shape or nanogap width after being stored at room temperature for approximately 6 months.

Microplasma direct writing for site-selective surface functionalization of carbon microelectrodes

Carbon micro-and nanoelectrodes fabricated by carbon microelectromechanical systems(carbon MEMS)are increasingly used in various biosensors and supercapacitor applications.Surface modification of as-produced carbon electrodes with oxygen functional groups is sometimes necessary for biofunctionalization or to improve electrochemical properties.However,conventional surface treatment methods have a limited ability for selective targeting of parts of a surface area for surface modification without using complex photoresist masks.Here,we report microplasma direct writing as a simple,low-cost,and low-power technique for site-selective plasma patterning of carbon MEMS electrodes with oxygen functionalities.In microplasma direct writing,a high-voltage source generates a microplasma discharge between a microelectrode tip and a target surface held at atmospheric pressure.In our setup,water vapor acts as an ionic precursor for the carboxylation and hydroxylation of carbon surface atoms.Plasma direct writing increases the oxygen content of an SU-8-derived pyrolytic carbon surface from~3 to 27%while reducing the carbon-to-oxygen ratio from 35 to 2.75.Specifically,a microplasma treatment increases the number of carbonyl,carboxylic,and hydroxyl functional groups with the largest increase observed for carboxylic functionalities.Furthermore,water microplasma direct writing improves the hydrophilicity and the electrochemical performance of carbon electrodes with a contact-angle change from~90°to~20°,a reduction in the anodic peak to cathodic peak separation from 0.5 V to 0.17 V,and a 5-fold increase in specific capacitance from 8.82 mF∙cm−2 to 46.64 mF∙cm^(−2).The plasma direct-writing technology provides an efficient and easy-to-implement method for the selective surface functionalization of carbon MEMS electrodes for electrochemical and biosensor applications.