This document summarizes Paulo Sergio Fonseca's research project on growing and characterizing monolayer molybdenum disulfide (MoS2) using chemical vapor deposition (CVD). The goals were to grow monolayer MoS2 islands using CVD, characterize the material properties using techniques like photoluminescence spectroscopy and atomic force microscopy, fabricate devices on the MoS2 using electron beam lithography, and investigate the electrical properties through transport measurements. Key accomplishments included the successful growth and characterization of CVD monolayer MoS2, as well as fabricating a device to enable future transport studies of the material's electron mobility.

1. 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

2. 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

3. 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

4. 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

5. 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.cgi
http://imagebank.osa.org/getImage.xqy?img=dTcqLmxhcmdl
LG9lLTIwLTE5LTIxMjc4LWcwMDU
Fig 2: Step-by-step process of electron beam lithography on a substrate

6. Brief Introduction to AFM
Atomic 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 Å
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

7. Goals
Goal: 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

8. Accomplishments
1) CVD Grown Monolayer MoS2
1 𝜇m
10 𝜇m
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

9. Accomplishments
2) Material Characterization of Monolayer MoS2
Photoluminescence Spectroscopy:
1 𝜇m
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

10. 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

11. Accomplishments
3) Device fabrication on Monolayer MoS2
100x 50x 10x 5x
http://www.iop.org/news/14/apr/page_62966.html
http://nanotubes.rutgers.edu/img_res/1T-FET.png
http://nanotubes.rutgers.edu/img_cool/micro/RU_devices_M.png
5 𝜇m
10 𝜇m 100 𝜇m 200 𝜇m

12. 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

13. 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.

14. Acknowledgements
Special Thanks to my mentors:
Edwin Preciado
Ariana Nguyen
Velveth Klee
MacREU R’side

15. References
Plechinger, 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.