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Transcript of Midterm Symposium Presentation
Fabrication and Characterization of Monolayer MoS2 Grown by CVD
Paulo Sergio Fonseca
Bartels Group
Program of Materials Science & Engineering
Department of Chemistry
MacREU R’side ‘15
Overview
Introduction to MoS2
Goals in MoS2 Research
Accomplishment in: Material Characterization of MoS2
Fabrication of Device
Summary
Acknowledgments
http://research.chem.ucr.edu/groups/bartels
Introduction
What are Transition Metal Dichalcogenides (TMDs)? TMDs are semiconductors with a chemical equation of MX2, where M represents a transition metal such as Mo or W and X is a chalcogen such as S ,Se or Te.
Material under current research is MoS2
WHY study MoS2 ?
- First and foremost: PRICE
- MoS2 is fairly inexpensive
- It is chemically and thermally stable
- MoS2 is desirable due to its ability to grow at the monolayer limit
- Tunable band gap availability
http://www.azonano.com/images/Article_Images/ImageForArticle_3605(1).jpg
Figure 1: Schematic diagram representing three atom layer thickness of monolayer MoS2
Figure 2: Energy band diagram for MoS2
Introduction (cont.) Synthesizing monolayer MoS2 :
- Two of the most popular ways to synthesize MoS2 is by Mechanical Exfoliation and Chemical Vapor Deposition (CVD)
CVD vs Exfoliation?
- CVD is the process by which a volatile precursor is introduced to a substrate which them react to form the desired deposit, the substrate
- Exfoliation is a “scotch tape” type of method in which continuous pealing of the substrate results in the desired product
Which method is preferable when growing monolayer MoS2 ?
In short, it has been proved that CVD grown monolayer MoS2 yields a higher Photoluminescence intensity than exfoliated MoS2
Figure 2: Schematic diagram depicting the process of Chemical Vapor Deposition
Figure 1: Schematic diagram depicting the process of Mechanical Exfoliation
http://m.iopscience.iop.org/1402-4896/2012/T146/014006/article
Introduction (cont.) What is Electron Beam Lithography ?
Electron beam lithography consists of scanning a focused beam of electrons on a surface in which is covered with electron sensitive films coined as “photoresist”. The ultimate goal of EBL is to design and draw custom shapes on your substrate.
Fig 1: Experimental setup of EBL machine
https://www.engr.ucr.edu/cgi-bin/nano/res/index.cgihttp://imagebank.osa.org/getImage.xqy?img=dTcqLmxhcmdlLG9lLTIwLTE5LTIxMjc4LWcwMDU
Fig 2: Step-by-step process of electron beam lithography on a substrate
Brief Introduction to AFMAtomic Force Microscopy is a type of scanning probe microscopy capable of high resolution that works by measuring the roughness of a desired sample
AFM is highly dependent on two types of forces:Van der Waals Forces ( if sample separation from probe is 1nm)
Electrostatic Forces (very small separation between probe and sample ~ 2.5
In AFM, there exists multiple ways to define the topography of a surface including:
Contact Mode
Tapping Mode
Non-Contact Mode (Works by oscillating the cantilever tip at a frequency such that the Van der Waals forces causes a change in frequency which defines the image topography
Figure 1: Schematic diagram of an AFM cantilever tip and sample
https://www.mtholyoke.edu/~menunez/ResearchPage/AFM_mechanism.gif
GoalsGoal: My overall goal is to better understand how monolayer MoS2 behaves as a 2D semi-conductor
Objective: To do so, I propose to:
1) Grow MoS2 monolayer islands via the method of CVD
2) Characterize such islands using Photoluminescence Spectroscopy and AFM
3) Fabricate a pattern on a monolayer island using Electron Beam Lithography
4) Deposit gold and titanium on the fabricated device using Electron Beam Deposition for transport measurements
5) Investigate the electrical properties of such fabricated device by conducting transport measurements
Accomplishments1) CVD Grown Monolayer MoS2
1 m 10 m
Figure 1: 100x (left) and 10x (right) images of monolayer MoS2 film grown by CVD
Figure 2: 100x (left) and 10x (right) images of monolayer MoS2 film grown by CVD
Accomplishments2) Material Characterization of Monolayer MoS2
Photoluminescence Spectroscopy:
1 m
Fig 1: 100x image of monolayer MoS2 film and its corresponding photoluminescence spectrum
Fig 2: 100x image of monolayer MoS2 film and its corresponding photoluminescence spectrum
Accomplishments
2) Material Characterization of Monolayer MoS2
Atomic Force Microscopy:
Using the software “Gwiddyon”, the step height of each triangle was averaged out to 0.7nm
Accomplishments3) Device fabrication on Monolayer MoS2
100x 50x 10x 5x
http://www.iop.org/news/14/apr/page_62966.htmlhttp://nanotubes.rutgers.edu/img_res/1T-FET.pnghttp://nanotubes.rutgers.edu/img_cool/micro/RU_devices_M.png
5 m
10 m 100 m 200 m
Summary
To wrap things up, my project consists of:
- Growing, patterning and characterizing monolayer MoS2
- Investigating the electrical properties of monolayer MoS2 by performing transport measurements on a fabricated device
What Has Been Done? - Grew monolayer MoS2 by CVD
- Characterized monolayer MoS2 islands via AFM and Raman
- Fabricated a device using EBL
Summary (cont.)
Next on the List:
- Investigate the electrical properties of a fabricated monolayer MoS2 device conducting transport measurements
Expectations:
- To gain a better understanding of the electron mobility values in devices based on CVD- grown monolayer MoS2. Through transport
measurements, I will confirm the proposed abilities of the material and improve on device specifications.
Acknowledgements
Special Thanks to my mentors:
Edwin Preciado
Ariana Nguyen
Velveth Klee
MacREU R’side ‘15
ReferencesPlechinger, G., J. Mann, E. Preciado, D. Barroso, A. Nguyen, J. Eroms, C. Schüller, L. Bartels, and T. Korn. "A Direct Comparison of CVD-grown and Exfoliated MoS 2 Using Optical Spectroscopy." Semiconductor Science and Technology Semicond. Sci. Technol. 29.6 (2014): 064008. Web.
Bao, Wenzhong, Xinghan Cai, Dohun Kim, Karthik Sridhara, and Michael S. Fuhrer. "High Mobility Ambipolar MoS2 Field-effect Transistors: Substrate and Dielectric Effects." Appl. Phys. Lett. Applied Physics Letters 102.4 (2013): 042104. Web.
MoS2." MoS2. Rahul Raghvendra, 24 Mar. 2015. Web. 28 July 2015.