Two-dimensional (2D) nanopores in materials like graphene, MoS2, WS2, hBN, and MXenes are emerging as versatile tools for single-molecule sensing, mimicking biological pores, and enabling applications such as water desalination, solute separation, and osmotic energy generation. MoS2 nanopores, in particular, are attractive due to their ultrathin nature (~0.65 nm), offering high spatial resolution for detecting DNA molecules down to single-nucleotide resolution and differentiating topological variations. Despite their potential, 2D nanopore devices face challenges related to stability and reliability, hindering their commercial application. Device yield, variability, and long-term stability are crucial performance metrics that have been poorly studied. Prior research has focused on improving the robustness of graphene nanopores through surface coatings, although these methods often compromise the membrane's thinness and introduce noise. This paper investigates the major reasons for the instability of monolayer MoS2 membranes and nanopores, aiming to address the issues of low yield and device failure. The authors hypothesize that delamination of the MoS2 monolayer from its substrate and chemical oxidation are primary contributors to instability. They propose solutions to mitigate these issues, enhancing the longevity and reliability of MoS2 nanopore devices.

The literature review highlights the significant advancements and challenges in the field of 2D nanopore devices. Studies on graphene nanopores, for instance, demonstrated the impact of surface modification on device noise and robustness. However, these modifications often increased the membrane thickness, hindering the high-resolution capabilities of these devices. The stability of 2D nanopore devices has been understudied, with limited research addressing the challenges in silicon-based systems. While progress has been made in the scalable growth and fabrication of high-quality MoS2, challenges remain, primarily concerning oxidation of the 2D material and delamination during ion-transport measurements. Existing studies highlight the detrimental effects of oxidation on 2D materials, causing morphological changes and impacting their stability in ambient conditions. The voltage-mediated delamination of 2D monolayers from their substrates has been observed, further emphasizing the need for improved stability.

The study employed a multifaceted approach to investigate and address the stability of MoS2 nanopores. A typical MoS2 nanopore device was fabricated by transferring a monolayer MoS2 onto a SiNx membrane with a pre-defined aperture. Nanopores were created using a TEM-based method or in situ electrochemical reaction (ECR). Ionic current measurements were performed to assess nanopore stability, observing instances of delamination and pore enlargement. To enhance stability, the researchers modified the SiNx substrate by applying a hexamethyldisilazane (HMDS) coating prior to MoS2 transfer. This hydrophobic coating aimed to improve the adhesion between the MoS2 and the substrate, reducing delamination. The effectiveness of the HMDS treatment was assessed by measuring the contact angle and surface free energy of the modified substrates. The effect of dissolved oxygen on MoS2 oxidation was examined using photoluminescence (PL) spectroscopy. Samples were exposed to aqueous solutions with varying oxygen concentrations, and the PL spectra were monitored over time to assess oxidation-induced changes. To study nanopore enlargement, MoS2 nanopores were incubated in aqueous solutions with different oxygen levels, and their size changes were characterized using TEM. Finally, long-term DNA sensing experiments were conducted to evaluate the stability of the improved nanopore devices under continuous operation. The translocation of 1 kbp double-stranded DNA through the MoS2 nanopores was monitored over several hours in a sealed flow cell with low oxygen concentration.

The study revealed that delamination of the MoS2 monolayer from the SiNx substrate and chemical oxidation were the primary causes of nanopore instability. Delamination resulted in a dramatic increase in ionic current, indicating a loss of the MoS2 membrane integrity. The researchers observed a voltage-dependent delamination process, with higher voltages accelerating the detachment. Surface modification of the SiNx substrate using HMDS significantly improved MoS2 adhesion, resulting in enhanced membrane stability. HMDS treatment increased the contact angle and decreased the surface free energy of the SiNx substrate, indicating the formation of a hydrophobic surface. Photoluminescence spectroscopy revealed that dissolved oxygen in the aqueous solution played a crucial role in inducing defects and oxidative degradation of MoS2. Lowering the oxygen concentration in the measurement buffer significantly reduced nanopore enlargement. Long-term DNA sensing experiments demonstrated the effectiveness of the stability enhancement strategies. Continuous DNA translocation measurements were performed for over 3 hours with minimal change in nanopore conductance, showcasing the improved long-term stability of the HMDS-modified MoS2 nanopores.

The findings of this study directly address the critical issue of stability in 2D nanofluidic devices, a major bottleneck for their widespread adoption. By identifying chemical oxidation and delamination as primary causes of instability, the researchers provided valuable insights into the underlying mechanisms of device failure. The successful demonstration of improved stability through surface modification and oxygen control offers practical strategies for developing more reliable and durable 2D nanopore sensors. The results highlight the importance of considering material-substrate interactions and environmental factors in the design and fabrication of these devices. The long-term DNA sensing experiments validated the enhanced stability, demonstrating the potential for reusable sensors and high-throughput biosensing applications.

This work significantly advances the field of 2D nanofluidic devices by providing critical insights into nanopore instability mechanisms and practical solutions for enhancing stability. The surface modification with HMDS and oxygen-controlled environment successfully increased nanopore lifespan and enabled long-term single-molecule DNA sensing. Future research could explore other surface modification strategies and investigate the influence of different 2D materials on nanopore stability. Further optimization of device fabrication and integration with advanced detection systems will pave the way for robust and reliable 2D nanopore sensors for various applications.

While the study successfully demonstrated improved nanopore stability, certain limitations exist. The long-term DNA sensing experiments were conducted under controlled conditions with low oxygen concentrations. The generalizability of these findings to other experimental conditions and different types of analytes requires further investigation. The study focused primarily on MoS2; extending the findings to other 2D materials would broaden the applicability of the proposed stabilization methods. The sample size of devices tested could be increased to enhance statistical significance.