Noise and Sensitivity Characteristics of Solid-State Nanopores with 2-D Membranes
Authors: Park Kyeong-Beom , Kim Hyung-Jun , Kim Hyun-Mi , Kim Ki-Bum 
Published: February 2020

Since the idea was proposed that it might be possible to detect each nucleotides of DNA by using nano-scale pores in 1996, the nanopore detection method has widely researched as a new solution of DNA sensing method as well as a single molecule sensing. In particular, nanopores with 2-D membranes have been highlighted to be able to perform DNA sequencing at single base-pair resolution, which means that the thickness of a single layer of (0.3 – 0.7 nm) is well-matched with the spacing of each nucleotide (0.32 - 0.52 nm). And also, since the thinner membrane promises to enhance the higher blockade signal, 2-D membrane is expected to improve the signal to noise ratio (SNR). As the 2-D membrane, graphene is first introduced in solid-state nanopores for the detection of DNA translocation and thereafter Boron Nitride (BN) and Molybdenum disulfide (MoS2) are also reported to be an alternative 2-D layer. Although these 2-D membranes promise high signal and vertical resolution due to its thin thickness, high flicker noise issues (or 1/f noise) have been reported to weaken sensitivity. Initially, because graphene membrane has naturally hydrophobic property and has native pinhole defects, the hydrophobicity and leakage currents of 2-D membrane were suspected as the origin of high 1/f noise. To improve the noise property, the hydrophilic TiO2 or Al2O3deposition on top of graphene by atomic layer deposition (ALD) were conducted and showed the reduced noise level. However, recent works about the noise properties of graphene suggested that the mechanical instability of 2-D membrane is the underlying cause of the high flicker noise.
In this manuscript, we developed 2-D membrane nanopore platform with low-noise level to achieve the high signal to noise ratio. For the improved noise properties, we first performed 2-D film transfer on a low noise platform based on pyrex substrate for the reduction of dielectric noise. The dielectic noise is also the one of main issues of solid-state nanopores and pyrex substrate platform can dramatically improve the dielectric noise due to the low dielectric constant and dieletric loss. In terms of the reduction of 1/f noise, we used multi layered 2-D membrane on SiNx supporting membrane with a small aperture size of less than 100 nm to enhance the mechanical stability. The resulting multi-layerd nanopores significantly show the reduced dielectric and 1/f noise, when it compared to devices based on single layered and Si substrate. This results support the tentative conclusion that the mechanical fluctuation of 2-D membrane may be the main cause of high flicker noise. Also, the improved noise properties allow high measurement bandwidths to 100 kHz, which is important to obtain the precise time domain for single DNA molecule transport. Finally, we observed a double-stranded DNA translocation on few layered nanopore with high SNR around 50 at 10 and 100 kHz bandwidths. The translocation signal is significantly different depending on the filter frequency employed, which shows the importance of improving both the spatial and temporal resolution of the nanopores.