The electrode pair and nanogap fabrication: The pair of facing Au (99.99%) electrodes were fabricated by an optical-lithography technique on an n-type Si wafer of <111> orientation that was covered with 2-mm, thermally oxidized silicon layer. The initial separation was typically 2mm. The electrochemical fabrication of the nanogap electrodes was realized by using a CHI631A electrochemical workstation (CHI Co., USA). The deposition current in chronopotentiometry was 0.1mAcm 2. The electroplating solution was 15mm KAu(CN)2, with 0.3mm K3C6H5O7 and 0.3mm KH2PO4 as supporting electrolytes. These optimized experimental parameters, especially the current density, ensure that the Au deposits in a uniform and compact manner, which ensures a reasonably good stability of the fabricated nanogap in both solution and air. When the electroplating solution was diluted by a factor of 100, that is, the electroplating solution was 0.15mm KAu(CN)2 with 3mm K3C6H5O7 and 3mm KH2PO4, the largest fabricated gap width was about 10nm. All solutions were prepared with analytical grade reagents and Milli-Q water. The nanogap electrodes with a gap larger than 1nm were characterized with a LEO1530 scanning electron microscope (LEO Co., Germany). The nanogap electrodes with gaps smaller than 1nm were characterized with a Keithley 4200 semiconductor characterization system to measure the corresponding I-V curve of the gap.