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Title:Tip-based nanolithography and application to molybdenum disulfide devices
Author(s):Chen, Sihan
Director of Research:King, William P
Doctoral Committee Chair(s):King, William P
Doctoral Committee Member(s):Bashir, Rashid; Nam, SungWoo; van der Zande, Arend M
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):scanning probe lithography
single molecule sensing
Abstract:Advances in nanotechnology rely on the capability to fabricate nanometer scale structures and devices. Tip-based nanolithography (TBN) is a low-cost and versatile unconventional nanolithography technique that can be used to prototype novel devices. This dissertation reports the study of tip-based polymer nanostructure fabrication and the application of TBN to fabricate MoS2 devices. The main contributions of this work are (i) understanding the effects of polymer properties and polymer melt flow on the size and shape of polymer nanostructures deposited from a heated tip, (ii) fabricating monolayer MoS2 nanoribbon transistors as narrow as 30 nm using TBN, (iii) improving the mobility of hBN encapsulated monolayer MoS2 transistors by cleaning and smoothing the interfaces with contact mode atomic force microscopy, (iv) demonstrating single-molecule sensing with monolayer MoS2 nanoribbon-nanopore devices. For TBN of polymer nanostructures, this work found nanometer scale capillary-driven flow governs polymer nanostructure deposition from a heated tip. Heated atomic force microscope (AFM) tips deposited 50 – 350 nm wide poly(methyl methacrylate) nanoribbons over a wide range of polymer molecular weight and deposition conditions. Both ribbon width and height increase with tip temperature and decrease with tip speed and polymer molecular weight. The width of the deposited polymer nanoribbons depends on capillary number (Ca) and is independent of tip temperature, tip speed and polymer molecular weight. Uniform and continuous deposition occurs only when Ca << 1, indicating that capillary-driven flow governs polymer transport. For application of TBN to MoS2 devices, this study described the fabrication of the first working transistor from monolayer MoS2 with a channel width smaller than 100 nm to our knowledge. Seven devices were fabricated with channel width ranging from 30 nm to 370 nm. The electrical properties (mobility, on/off ratio and subthreshold swing) of MoS2 field-effect transistors (FETs) degrade after nanoribbon formation, while atomic layer deposition passivation improves mobility and on/off ratio but degrades subthreshold swing. One 30-nm-wide monolayer FET achieved a mobility of 8.53 cm2/Vs and an on/off ratio of 2×105, on par with multilayer MoS2 nanoribbon FETs reported to date. This study also found cleaning and smoothing the interfaces with contact mode AFM could improve the mobility of hBN encapsulated monolayer MoS2 FETs. The AFM cleaning process flattened hBN encapsulated monolayer MoS2 and reduced the photoluminescence peak width of monolayer MoS2, both of which indicate a reduction in interface traps. The mobility of four hBN encapsulated monolayer MoS2 FETs increased by an average of 73% from 21.95±2.27 cm2/Vs to 38.03±6.12 cm2/Vs after AFM cleaning. Finally, this work demonstrated DNA sensing with solid state nanopores integrated with monolayer MoS2 nanoribbon transistors. Nanopores of 4-10 nm were drilled in ~ 100 nm wide hBN encapsulated monolayer MoS2 nanoribbons. Translocation of 3kbp dsDNA through a monolayer MoS2 nanoribbon-nanopore device was detected in ionic channel. Example events of simultaneous detection of DNA translocation in ionic current and MoS2 current were obtained. Improved understanding of TBN of polymer nanostructures enhances the reproducibility and scalability of tip-based polymer nanofabrication. Monolayer MoS2 nanoribbon field-effect transistors can be used as the building block for next generation low-power electronics. MoS2 FET- nanopore devices could potentially enable fast and cheap DNA sequencing.
Issue Date:2019-08-08
Rights Information:Copyright 2019 Sihan Chen
Date Available in IDEALS:2020-03-02
Date Deposited:2019-12

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