Journal Articles

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.

Acs Nano. 2022-09-26. DOI : 10.1021/acsnano.2c05201.

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.

Nanoscale Advances. 2022-09-02. DOI : 10.1039/d2na00409g.

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.

Physical Chemistry Chemical Physics. 2022-07-29. Vol. 24, num. 33, p. 19948-19955. DOI : 10.1039/d2cp02094g.

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.

Advanced Electronic Materials. 2022-04-29.  p. 2200123. DOI : 10.1002/aelm.202200123.

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.

2D Materials. 2022-04-01. Vol. 9, num. 2, p. 025030. DOI : 10.1088/2053-1583/ac5f6d.

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.

Acs Nano. 2022-03-22. Vol. 16, num. 3, p. 3695-3703. DOI : 10.1021/acsnano.1c07086.

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.

Acs Nano. 2022-03-15. Vol. 16, num. 4, p. 5719-5730. DOI : 10.1021/acsnano.1c10739.

Statistical distortion of supervised learning predictions in optical microscopy induced by image compression

E. Pomarico; C. Schmidt; F. Chays; D. Nguyen; A. Planchette et al. 

The growth of data throughput in optical microscopy has triggered the extensive use of supervised learning (SL) models on compressed datasets for automated analysis. Investigating the effects of image compression on SL predictions is therefore pivotal to assess their reliability, especially for clinical use. We quantify the statistical distortions induced by compression through the comparison of predictions on compressed data to the raw predictive uncertainty, numerically estimated from the raw noise statistics measured via sensor calibration. Predictions on cell segmentation parameters are altered by up to 15% and more than 10 standard deviations after 16-to-8 bits pixel depth reduction and 10:1 JPEG compression. JPEG formats with higher compression ratios show significantly larger distortions. Interestingly, a recent metrologically accurate algorithm, offering up to 10:1 compression ratio, provides a prediction spread equivalent to that stemming from raw noise. The method described here allows to set a lower bound to the predictive uncertainty of a SL task and can be generalized to determine the statistical distortions originated from a variety of processing pipelines in AI-assisted fields.

Scientific Reports. 2022-03-02. Vol. 12, num. 1, p. 3464. DOI : 10.1038/s41598-022-07445-4.

Zero-Bias Power-Detector Circuits based on MoS2 Field-Effect Transistors on Wafer-Scale Flexible Substrates

E. Reato; P. Palacios; B. Uzlu; M. Saeed; A. Grundmann et al. 

The design, fabrication, and characterization of wafer-scale, zero-bias power detectors based on 2D MoS2 field-effect transistors (FETs) are demonstrated. The MoS2 FETs are fabricated using a wafer-scale process on 8 mu m-thick polyimide film, which, in principle, serves as a flexible substrate. The performances of two chemical vapor deposition MoS2 sheets, grown with different processes and showing different thicknesses, are analyzed and compared from the single device fabrication and characterization steps to the circuit level. The power-detector prototypes exploit the nonlinearity of the transistors above the cut-off frequency of the devices. The proposed detectors are designed employing a transistor model based on measurement results. The fabricated circuits operate in the Ku-band between 12 and 18 GHz, with a demonstrated voltage responsivity of 45 V W-1 at 18 GHz in the case of monolayer MoS2 and 104 V W-1 at 16 GHz in the case of multilayer MoS2, both achieved without applied DC bias. They are the best-performing power detectors fabricated on flexible substrate reported to date. The measured dynamic range exceeds 30 dB, outperforming other semiconductor technologies like silicon complementary metal-oxide-semiconductor circuits and GaAs Schottky diodes.

Advanced Materials. 2022-02-17.  p. 2108469. DOI : 10.1002/adma.202108469.


2D MoS2 Nanopores: Wafer-scale Fabrication and Monolayer Stability for Long-term Single-Molecule Sensing

M. Thakur / A. Radenovic (Dir.)  

Biologically inspired solid-state nanopores are artificial openings or apertures in thin membranes similar to natural protein ion channels in a lipid bilayer of cell membranes. In solid-state nanopores, a thin insulating membrane with single or multiple pores separates two conductive salt solutions. When an electric field is applied across this membrane, electrically charged species such as ions pass through these nanopore(s), generating a nanopore ion current. In essence, nanopores are single-molecule sensors and valuable tools for studying biophysics. For instance, an intrinsically charged biomolecule such as DNA can be electrophoretically threaded through the nanopore, transiently blocking the ionic current characteristic of the molecule passing through the pore. With progress in two-dimensional (2D) materials, the marriage of nanopores with 2D materials – “2D nanopores” have emerged as a new class of ultra-thin membrane solid-state nanopores. Molybdenum disulfide (MoS2) is a 2D material with an atomic thickness (0.7 nm) that approaches the inter-base distance of two DNA bases and is a lucrative 2D material for the DNA sequencing application. However, there are inherent challenges and bottlenecks with using MoS2 due to sensitive fabrication and inherent challenges of the 2D materials leading to low device yield. In this thesis, I will demonstrate ways to improve high-throughput production and the development of more reliable and durable nanopore devices. In the second chapter, I will introduce MoS2 material as a 2D nanopore system and elaborate fabrication of nanopore substrates. I will discuss various problems and issues related to substrate fabrication, transfer, nanopore-creation, and nanopore measurements. Finally, I list a step-by-step protocol and troubleshooting guide for early-stage 2D nanopore researchers. In the third chapter, I specifically focus on “chip-scale” transfer strategies for MoS2 grown using chemical vapor deposition (CVD). I introduce two transfer approaches – direct-transfer and stamp-assisted- with just water as a medium. I will demonstrate these transfer approaches and discuss their advantages and limitations. Furthermore, I discuss hydrocarbon contamination with 2D materials and their implications in nanofluidics. In the fourth chapter, I demonstrate a scalable transfer from chip-scale to a larger “wafer-scale” for batch fabrication of nanopore substrates for single-molecule DNA sensing. With PDMS-based polymer, I will demonstrate 3-inch monolayer MoS2 transfer on nanopore substrates with 128 nanopore devices with high transfer efficiency (>70%). Moreover, the technique is etchant-free, and growth substrates are recyclable after transfer. In the fifth chapter, I study nanopore instability and address those issues. I address the delamination issue by chemically modifying Si-substrates with an organosilicon that increases the adherence of monolayer MoS2 layers. This surface pre-treatment helped reinforce 2D layer attachment to the substrate, increasing the nanofluidic devices’ durability. Further, we show that the nanopore enlargement due to dissolved oxygen in an aqueous solution can be considerably reduced in a low dissolved oxygen concentration in solution. These strategies improved MoS2 nanopore stability and enabled long-term DNA sensing. This will pave way toward more durable 2D nanopore sensors.

Lausanne, EPFL, 2022. 


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