Elisa Riedo Among various novel materials, two-dimensional molybdenum disulfide (MoS2) is of particular interest due to its large band gap, low dielectric constant, and heavy carrier effective mass. Currently, a key issue in creating high performing field-effect transistors (FETs) based on MoS2 and other transition metal dichalcogenides (TMDC) films is the poor quality of the metal contacts fabricated on these atomic layers, and the difficulty to pattern dopants. Here, we report a strategy to fabricate metal contacts on 2D materials with high reproducibility [1]. Our approach is based on a double polymer stack chemical etching/lift off process combined with thermal scanning probe lithography (t-SPL) [1, 2]. Using t-SPL, top-gated and back-gated monolayer MoS2 FETs are fabricated with different metals as direct contacts on the MoS2. The approach does not require vacuum, allows for in situ simultaneous patterning and imaging of a monolayer of MoS2, can achieve sub-10 nm resolution, gives rise to no resist contamination, and completely eliminates damage from either electrons or photons. As a result, the t-SPL fabricated FETs exhibit on/off ratios up to 1010, Schottky barrier heights (SBH) close to 0 mV, and sub threshold swings as low as 64 mV/dec without using negative capacitors or hetero-stacks, outperforming EBL results in literature. Furthermore, we demonstrate a new integration of t-SPL with a flow-through reactive gas cell to achieve a nanoscale control of the local thermal activation of defects in monolayer MoS2. The t-SPL activated nanopatterns can present either p- or n-type doping on demand, depending on the used gasses, allowing the realization of field effect transistors, and p-n junctions with precise sub-nm spatial control and a rectification ratio over 104 [3].
The ability to replicate the microenvironment of biological tissues creates unique biomedical possibilities for stem cell applications. Current fabrication methods are limited by either the control on feature size and shape, or by the throughput and size of the replicas. Here, a novel platform is reported that combines tSPL with innovative methodologies for the low-cost and high-throughput nanofabrication of large area quasi-3D bone tissue replicas with high fidelity, sub-15 nm lateral precision, and sub-2 nm vertical resolution [4]. This bio-tSPL platform features a biocompatible polymer resist that withstands multiple cell culture cycles, allowing the reuse of the replicas, further decreasing costs and fabrication times. The as-fabricated replicas support the culture and proliferation of human induced mesenchymal stem cells, which display broad therapeutic and biomedical potential. Furthermore, it is demonstrated that bio-tSPL can be used to nanopattern the bone tissue replicas with amine groups, for subsequent tissue-mimetic biofunctionalization. The achieved level of time and cost-effectiveness, as well as the cell compatibility of the replicas, make bio-tSPL a promising platform for the production of tissue-mimetic replicas to study stem cell-tissue microenvironment interactions, test drugs, and ultimately harness the regenerative capacity of stem cells and tissues for biomedical applications.
[1] “Patterning metal contacts on monolayer MoS2 with vanishing Schottky barriers using thermal nanolithography”, Nature Electronics, 2, 17–25 (2019) https://doi.org/10.1038/s41928-018-0191-0
[2] “Advanced Scanning Probe Lithography”, Nature Nanotechnology, 9, 577 (2014) DOI: 10.1038/NNANO.2014.157
[3] “Spatial defects nanoengineering for bipolar conductivity in MoS2”, Nature Communication, (2020) https://doi.org/10.1038/s41467-020-17241-1
[4] “Cost and Time Effective Lithography of Reusable Millimeter Size Bone Tissue Replicas With Sub-15 nm Feature Size on A Biocompatible Polymer” Advanced Functional Materials, (2021) https://doi.org/10.1002/adfm.202008662
Professor Elisa Riedo received her Ph.D. in Physics in a joint program between the University of Milano and the European Synchrotron Radiation Facility in Grenoble, France. She then worked as Post Doc at the École Polytechnique Fédérale de Lausanne (EPFL). In 2003 she was hired as Assistant Professor at the Georgia Institute of Technology in the School of Physics, where she was promoted to Associate Professor with tenure in 2009 and to full Professor in 2015. From 2016 to summer 2018, she worked as Nanoscience Professor at the CUNY Advanced Science Research Center (ASRC), as well as she was a Physics Professor at the City College of New York. Since 2018, she is a Professor at the NYU Tandon School of Engineering in the department of Chemical and Biomolecular Engineering, where she is director of the picoForce Lab. Her research is focused on new scanning probe microscopy-based methods to study and fabricate materials and solid/liquid interfaces at the nanoscale. Highlights from her research are the invention of thermochemical scanning probe lithography, the discovery of the exotic viscoelasticity of water at the interface with a solid surface, and the development of new methods to study materials’ elasticity and friction with sub-nm resolution. Applications of her work range from fundamental understanding of nanoscale matter to fabrication of the building blocks for next generation of electronics, biomedical, sensing, and photonics devices. In 2013, Dr. Riedo was elected APS Fellow, in the Division of Condensed Matter Physics, for her atomic force microscopy studies of nanoscale friction, liquid structure and nanotube elasticity, and the invention of thermochemical nanolithography.
Learn more at www.picoForceLab.org.