High durability and stability of 2D nanofluidic devices for long-term single-molecule sensing

M. Thakur; N. Cai; M. Zhang; Y. Teng; A. Chernev et al. 

Nanopores in two-dimensional (2D) membranes hold immense potential in single-molecule sensing, osmotic power generation, and information storage. Recent advances in 2D nanopores, especially on single-layer MoS2, focus on the scalable growth and manufacturing of nanopore devices. However, there still remains a bottleneck in controlling the nanopore stability in atomically thin membranes. Here, we evaluate the major factors responsible for the instability of the monolayer MoS2 nanopores. We identify chemical oxidation and delamination of monolayers from their underlying substrates as the major reasons for the instability of MoS2 nanopores. Surface modification of the substrate and reducing the oxygen from the measurement solution improves nanopore stability and dramatically increases their shelf-life. Understanding nanopore growth and stability can provide insights into controlling the pore size, shape and can enable long-term measurements with a high signal-to-noise ratio and engineering durable nanopore devices.

Substitutional p‐type Doping in NbS2‐MoS2 Lateral Heterostructures Grown by MOCVD

Z. Wang; M. Tripathi; Z. Golsanamlou; P. Kumari; G. Lovarelli et al. 

Dataset for the paper High durability and stability of 2D nanofluidic devices for long-term single-molecule sensing

M. Thakur; N. Cai; M. Zhang; Y. Teng; A. Chernev et al. 

Information regarding the Dataset, corresponding to the paper: “Thakur, M., Cai, N., Zhang, M. et al. High durability and stability of 2D nanofluidic devices for long-term single-molecule sensing. npj 2D Mater Appl 7, 11 (2023). https://doi.org/10.1038/s41699-023-00373-5” This folder contains the raw data and complete package of codes used to analyze, view, save, and plot data for the publication titled “High durability and stability of 2D nanofluidic devices for long-term single-molecule sensing”. The code folder, “OpenNanopore-nanopore-tools”, can be used to plot raw data which corresponds to the figures in the paper and supplementary information.

High-Throughput Nanopore Fabrication and Classification Using Xe-Ion Irradiation and Automated Pore-Edge Analysis

M. Macha; S. Marion; M. Tripathi; M. Thakur; M. Lihter et al. 

Large-area nanopore drilling is a major bottleneck in state-of-the-art nanoporous 2D membrane fabrication protocols. In addition, high-quality structural and statistical descriptions of as-fabricated porous membranes are key to predicting the corresponding membrane-wide permeation properties. In this work, we investigate Xe-ion focused ion beam as a tool for scalable, large-area nanopore fabrication on atomically thin, free-standing molybdenum disulfide. The presented irradiation protocol enables designing ultrathin membranes with tunable porosity and pore dimensions, along with spatial uniformity across large-area substrates. Fabricated nanoporous membranes are then characterized using scanning transmission electron microscopy imaging, and the observed nanopore geometries are analyzed through a pore-edge detection and analysis script. We further demonstrate that the obtained structural and statistical data can be readily passed on to computational and analytical tools to predict the permeation properties at both individual pore and membrane-wide scales. As an example, membranes featuring angstrom-scale pores are investigated in terms of their emerging water and ion flow properties through extensive all-atom molecular dynamics simulations. We believe that the combination of experimental and analytical approaches presented here will yield accurate physics-based property estimates and thus potentially enable a true function-by-design approach to fabrication for applications such as osmotic power generation and desalination/filtration.

Wafer-scale MoS2 with water-vapor assisted showerhead MOCVD

M. Macha; H. G. Ji; M. Tripathi; Y. Zhao; M. Thakur et al. 

Among numerous thin film synthesis methods, metalorganic chemical vapor deposition performed in a showerhead reactor is the most promising one for broad use in scalable and commercially adaptable two-dimensional material synthesis processes. Adapting the most efficient monolayer growth methodologies from tube-furnace systems to vertical-showerhead geometries allows us to overcome the intrinsic process limitations and improve the overall monolayer yield quality. Here, we demonstrate large-area, monolayer molybdenum disulphide growth by combining gas-phase precursor supply with unique tube-furnace approaches of utilizing sodium molybdate pre-seeding solution spincoated on a substrate along with water vapor added during the growth step. The engineered process yields a high-quality, 4-inch scale monolayer film on sapphire wafers. The monolayer growth coverage, average crystal size and defect density were evaluated using Raman and photoluminescence spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and scanning transmission electron microscopy imaging. Our findings provide a direct step forward toward developing a reproducible and large-scale MoS2 synthesis with commercial showerhead reactors.

Stress induced delamination of suspended MoS2 in aqueous environments

M. Macha; M. Thakur; A. Radenovic; S. Marion 

Applying hydrostatic pressure with suspended 2D material thin membranes allows probing the effects of lateral strain on the ion and fluid transport through nanopores. We demonstrate how both permanent and temporary delamination of 2D materials can be induced by pressure and potential differences between the membrane, causing a strong mechanosensitive modulation of ion transport. Our methodology is based on in situ measurements of ionic current and streaming modulation as the supporting membrane is indented or bulged with pressure. We demonstrate how indirect measurements of fluid transport through delaminated MoS2 membranes is achieved through monitoring streaming current and potential. This is combined with TEM images of 2D material membranes before and after aqueous measurements, showing temporary delamination during mechanical or electrical stress. The obtained results allow a better understanding of measurements with supported 2D materials, i.e. avoiding misinterpreting measured data, and could be used to probe how the electrical field and fluid flow at the nanoscale influence the adhesion of supported 2D materials.

Stable Al2O3 Encapsulation of MoS2 ‐FETs Enabled by CVD Grown h‐BN

A. Piacentini; D. Marian; D. S. Schneider; E. González Marín; Z. Wang et al. 

Molybdenum disulfide (MoS2) has great potential as a two-dimensional semiconductor for electronic and optoelectronic application, but its high sensitivity to environmental adsorbents and charge transfer from neighboring dielectrics can lead to device variability and instability. Aluminum oxide (Al2O3) is widely used as an encapsulation layer in (opto)-electronics, but it leads to detrimental charge transfer n-doping to MoS2. Here, this work reports a scalable encapsulation approach for MoS2 field-effect transistors (FETs) where hexagonal boron nitride (h-BN) monolayers are employed as a barrier layer in-between each of the Al2O3 and MoS2 interfaces. These devices exhibit a significant reduction of charge transfer, when compared to structures without h-BN. This benefit of h-BN in the gate stack is confirmed by ab initio density functional theory calculations. In addition, the devices with h-BN layers show very low hysteresis even under ambient operating conditions.

Three-step, transfer-free growth of MoS2/WS2/graphene vertical van der Waals heterostructure

J. Sitek; I. Pasternak; K. Czerniak-Losiewicz; M. Swiniarski; P. P. Michalowski et al. 

Van der Waals heterostructures (vdWHSs) provide a unique playground to study fundamental physics and practical applications of two-dimensional (2D) materials. However, most 2D heterostructures are prepared by transfer, hindering their technological implementation. Here, we report the first chemical vapour deposition of monolayered MoS2/WS2/graphene vertical vdWHS without transfer step. By atomic force microscopy, photoluminescence, Raman spectroscopy, and secondary ion mass spectroscopy, we confirmed the vertical stacking of three different 2D materials. The use of WS2, graphene, and sapphire as growth substrates allowed us to describe the 2D materials growth process better. We determined that for the synthesis of 2D materials, only the chemical potential of the crystal formation and the substrate-layer adhesion energy are relevant factors. In addition, we used MoS2/WS2/graphene vdWHS to fabricate a photoresponsive memory device, showing the application potential of such heterostacks. Our results clarify the growth mechanisms of 2D materials and pave the way for the growth of more complex vdWHSs.

Engineering Optically Active Defects in Hexagonal Boron Nitride Using Focused Ion Beam and Water

E. Glushkov; M. Macha; E. Raeth; V. Navikas; N. Ronceray et al. 

Hexagonal boron nitride (hBN) has emerged as a promising material platform for nanophotonics and quantum sensing, hosting optically active defects with exceptional properties such as high brightness and large spectral tuning. However, precise control over deterministic spatial positioning of emitters in hBN remained elusive for a long time, limiting their proper correlative characterization and applications in hybrid devices. Recently, focused ion beam (FIB) systems proved to be useful to engineer several types of spatially defined emitters with various structural and photophysical properties. Here we systematically explore the physical processes leading to the creation of optically active defects in hBN using FIB and find that beam-substrate interaction plays a key role in the formation of defects. These findings are confirmed using transmission electron microscopy, which reveals local mechanical deterioration of the hBN layers and local amorphization of ion beam irradiated hBN. Additionally, we show that, upon exposure to water, amorphized hBN undergoes a structural and optical transition between two defect types with distinctive emission properties. Moreover, using super-resolution optical microscopy combined with atomic force microscopy, we pinpoint the exact location of emitters within the defect sites, confirming the role of defected edges as primary sources of fluorescent emission. This lays the foundation for FIB-assisted engineering of optically active defects in hBN with high spatial and spectral control for applications ranging from integrated photonics, to nanoscale sensing, and to nanofluidics.

High Performance Semiconducting Nanosheets via a Scalable Powder-Based Electrochemical Exfoliation Technique

R. A. Wells; M. Zhang; T-H. Chen; V. Boureau; M. Caretti et al. 

The liquid-phase exfoliation of semiconducting transition metal dichalcogenide (TMD) powders into 2D nanosheets represents a promising route toward the scalable production of ultrathin high-performance optoelectronic devices. However, the harsh conditions required negatively affect the semiconducting properties, leading to poor device performance. Herein we demonstrate a gentle exfoliation method employing standard bulk MoS2 powder (pressed into pellets) together with the electrochemical intercalation of a quaternary alkyl ammonium. The resulting nanosheets are produced in high yield (32%) and consist primarily of mono-, bi-, triatomic layers with large lateral dimensions (>1 mu m), while retaining the semiconducting polymorph. Exceptional optoelectronic performance of nanosheet thin-films is observed, such as enhanced photoluminescence, charge carrier mobility (up to 0.2 cm(2) V-1 s(-1) in a multisheet device), and photon-to-current efficiency while maintaining high transparency (>80%). Specifically, as a photoanode for iodide oxidation, an internal quantum efficiency up to 90% (at +0.3 V vs Pt) is achieved (compared to only 12% for MoS2 nanosheets produced via ultrasonication). Further using a combination of fluorescence microscopy and high-resolution scanning transmission electron microscopy (STEM), we show that our gently exfoliated nanosheets possess a defect density (2.33 x 10(13) cm(-2)) comparable to monolayer MoS2 prepared by vacuum-based techniques and at least three times less than ultrasonicated MoS2 nanoflakes. Finally, we expand this method toward other TMDs (WS2, WSe2) to demonstrate its versatility toward high-performance and fully scalable van der Waals heterojunction devices.