Identification and Mapping of Mechanically Exfoliated

1H-MoS2 Flakes for Field-Effect Transistors

by Mariela Georgieva and Terrance O’Regan

ARL-TR-7021 August 2014

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ARL-TR-7021 August 2014

Identification and Mapping of Mechanically Exfoliated

1H-MoS2 Flakes for Field-Effect Transistors

Mariela Georgieva and Terrance O’Regan

Sensors and Electronic Devices Directorate, ARL

Approved for public release; distribution unlimited. 
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Identification and Mapping of Mechanically Exfoliated 1H-MoS2 Flakes for FieldEffect Transistors

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Mariela Georgieva and Terrance O’Regan

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14. ABSTRACT

Following the discovery of graphene, there has been increased interest in materials that allow for the construction of twodimensional (2D) devices. In this project we will focus on 1H-molybdenum disulfide (MoS2), which, like graphene, is a 

monolayer; unlike graphene, however, it has a band gap. 1H-MoS2 differs from silicon in that its band gap is direct and, like 

graphene, can be mechanically exfoliated (ME) to isolate it as a single molecular layer. We will lay the groundwork required 

for fabricating field-effect transistors and address the effects that the material microstructure has on transistor properties by 

comparing the quality of ME-MoS2 to MoS2 grown by chemical vapor deposition (CVD-MoS2). The layer count and material 

quality will be analyzed using Raman spectroscopy, photoluminescence spectroscopy, and atomic force microscopy (AFM) 

mapping, after which the transistors will be built using e-beam lithography.

15. SUBJECT TERMS

Exfoliating 1H-MoS2, identification, mapping

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Terrance O’Regan

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iii

Contents

List of Figures iv

Acknowledgments v

Student Bio vi

1. Introduction/Background 1

2. Experiment/Calculations 1

3. Results and Discussion 7

4. Conclusions 8

5. References 9

List of Symbols, Abbreviations, and Acronyms 11

Distribution List 12
iv

List of Figures

Fig. 1 A comparison of the functional modes of 1H-MoS2 and 2H-MoS2, adapted from 

Ataca et al.12...............................................................................................................................2

Fig. 2 Example of 1H- and 2H-MoS2. The thickness of 1H-MoS2 is usually 0.7 nm, and the 

thickness of 2H-MoS2 is usually 1.4 nm.14 Adapted from Radisavjevic et al..13......................3

Fig. 3. MoS2 flakes surrounded by a Si substrate. Under the green filter, the 1H-MoS2 flakes 

are the grey- green pieces. MoS2 becomes more yellow in color as the amount of layers 

increases. The dark green flakes are bilayer, the green are multilayer, and the yellow are 

bulk. ...........................................................................................................................................4

Fig. 4 a) Image of a PL plot for 1H-MoS2 and 2H-MoS2 flakes, showing the peak intensities 

of each. As layer count increases, the PL plot tends to flatten, with no discernible peak 

intensity for either the A or B exciton.3b) Image of a Raman plot for the same flakes, 

showing the silicon and functional mode peaks (zoomed in to show the difference 

between the functional mode peaks for the 1H and 2H), as well as a fourth peak where 

some of the laser leaks through the filter. For bulk MoS2, there is no silicon peak visible 

and no difference between in-plane and out-of-plane frequency because the silicon is 

covered by the MoS2 and molecular motion is equally restricted in all directions, as 

opposed to 1H-MoS2 where molecular motion is easier out of the plane because there is 

nothing restricting the atoms from moving.9.............................................................................5

Fig. 5 Various scans used to analyze Flake 66. a) Optical microscope image of the flake, 

where the 1H-MoS2 area of interest is the green/grey rectangle in the center. b) AFM 

image of the flake. The slightly lighter area is the 1H-MoS2 flake, showing the faint 

difference between the height of the molecular layer and that of the substrate. AFM 

images usually show a greater contrast between molecular layer and substrate, though in 

this case the bulk surrounding the molecular layer diminished the effect of the height 

difference between the molecular layer and the substrate. c) PL image of the flake, 

showing increased intensity with red and yellow coloring where the 1H-MoS2 flake is 

located. d) Raman image of the flake, where the 1H-MoS2 is shown as blue and bulk is 

red. .............................................................................................................................................6

Fig. 6 Examples of Raman and PL scans of devices built on MoS2. a, 1) The Raman 

intensity of the in-plane mode. a, 2) The PL intensity of the in-plane mode. b, 1) The 

Raman intensity of the A exciton. b, 2) The PL intensity of the A exciton.10..........................7
v

Acknowledgments

I wish to acknowledge the mentorship of Terrance O’Regan and Glen Birdwell, as well as the 

assistance of Frank J Crowne and Matin Amani.
vi

Student Bio

Mariela Georgieva is currently a junior at the University of Maryland, College Park, majoring in 

materials science and engineering in the Clark School of Engineering. Her past research 

experiences include gathering Earth sciences metrics and calibration data at NASA. Her future 

plans include going to graduate school to continue materials science studies.
1

1. Introduction/Background

Following the discovery of graphene, there has been increased interest in the use of materials that 

allow for the construction of two-dimensional (2D) devices. 2D materials both allow for more 

compact devices, as well as the potential to find a material to compete with, or at least 

compliment, silicon.

1 Graphene does not have a band gap, which means that there is no on/off 

switch, and, hence, does not lead one to consider graphene for logic applications. On the 

contrary, transition metal dichalcogenides (TMDs) do have band gaps, many of which are direct 

in nature. Consequently, this report will focus on 1H-molybdenum disulfide (MoS2), which has a 

direct band gap and can be exfoliated to a single molecular layer. If the quality of the material 

can be maintained during processing, MoS2 may prove to be a valuable resource for future 

technologies. This report aims to cover the process of finding useful molecular layer flakes on 

MoS2 that was mechanically exfoliated (ME-MoS2) by use of the Scotch Tape-method. The 

flakes will then be used to fabricate devices, and the material quality will be tracked throughout 

the device fabrication process. This tracking will allow for a comparison of material quality 

between naturally acquired ME-MoS2 and MoS2 grown in a lab by Chemical Vapor Deposition 

(CVD-MoS2). The comparison is important with regard to the fact that ME-MoS2 is of higher 

purity, but that CVD-MoS2 is easier to fabricate, and both are at risk for displaying degraded 

material quality following processing. As a result, it is important to study and compare the final 

material quality received at the end of the fabrication process to determine which would be a 

more realistic path to generate 1H-MoS2 to use for consistent device fabrication. In the future, 

the effects of each processing step will be qualified using non-invasive Raman spectroscopy, 

which is ideal for assessing material quality without actually making an impact on the material. 

Initially, when the (possibly multilayer) ME-MoS2 flakes were being analyzed, the layer count 

and material quality were analyzed using Raman spectroscopy, photoluminescence (PL) 

spectroscopy, and atomic force microscopy (AFM) mapping. The transistors are currently being 

built using electron beam lithography, and Raman will be used to test for damage. The spectra 

will be measured in the backscattering configuration using a 100× objective and either a 600 or 

1800 grooves/mm grating. The spot size of the laser will be ~342 nm, resulting in an incident 

laser power density ~140 μW/m2.

4

The comparison of the effects of the fabrication process on 

ME-MoS2 and CVD- MoS2 will allow us to correlate material degradation to device performance 

and innovate new ways to maintain material quality during fabrication. 

2. Experiment/Calculations

Molybdenum disulfide (MoS2) is of interest due to its electronic, optical, and catalytic properties, 

and also for its use as a dry lubricant. As the latter suggests, it is held together through van der 
2

Waals forces.

2

For this report, it is important to understand the difference between 1H-MoS2 and 

2H-MoS2. There is a change in functional modes between 1H-MoS2 and 2H-MoS2, as shown in 

Fig. 1. 1H-MoS2 is 2D and has a direct band gap, while 2H-MoS2 is three-dimensional (3D) and 

has an indirect band gap, which occurs as a result of stacking, shown in Fig. 2. 1H-MoS2 is also 

what is referred to as a single sheet, because it contains exactly 1 Mo and 2 S, while 2H-MoS2 is 

a monolayer made by stacking two layers of 1H-MoS2. Any greater number of layers is stacked 

2H-MoS2, which can be used to construct bulk MoS2.

12

Fig. 1 A comparison of the functional modes of 1H-MoS2

and 2H-MoS2, adapted from Ataca et al.12
3

Fig. 2 Example of 1H- and 2H-MoS2. The thickness of 1H-MoS2 is usually 

0.7 nm, and the thickness of 2H-MoS2 is usually 1.4 nm.

14 Adapted from 

Radisavjevic et al..

13

1H-MoS2 on SiO2 substrate is about 0.8–1.0 nm in height. The molecular layer, itself, is about 

0.5–0.7 nm, which is in accordance with the theoretical value of 6.15 Å. The increased thickness 

is due to absorbents on the surface of the substrate, which contribute to it.

2

It is possible that glue 

from the exfoliation process has become caught beneath the molecular layer, making it appear to 

be greater in height. 1H-MoS2 is also the first atomically thin material that is an effective emitter 

of light. It has a strong PL because of slower electronic relaxation arising from unique electronic 

structure.

2

Its strong PL makes it possible for use in photostable markers and sensors that can be 

adapted to probe nanoscale dimensions. The controllability of the band gap may be used to 

optimize the material’s use for photovoltaic applications.

3

It is also promising for use in logic 

circuits and optoelectronic devices, along with flexible and transparent substrates.

4

MoS2 molecular layers are visible on Si as a result of the effect of MoS2 on the reflected light. 

Light reflects differently off the surface of the sample based on whether MoS2 is present, or if 

there is only Si. This is the same reason that graphene is visible under an optical microscope, or

even to the naked eye.

6

The presence of even a small amount of graphene causes optical 

interference, so the reflected light is of a different intensity and there is enough contrast to see 

the graphene. It is important that the graphene be slightly opaque for this to occur, or there will 

not be enough interference to allow it to be visible; molecular layer graphene and 1H-MoS2 are 

both sufficiently opaque to be viewed by the human eye. The amount of contrast also differs 

based on the wavelength of light used to illuminate the sample and the thickness of the SiO2

layer. Using this, optimal contrast can be achieved by matching the thickness of the SiO2 layer 

with a color filter.

Contrast between the MoS2 flakes and the Si substrate was the initial technique for identifying 

1H-MoS2 flakes. An optical microscope was used to visually scan the chip using a 100× 

objective, with a grid created at MIT used to record the coordinates of the flakes to make them 

possible to relocate in the future for device fabrication. A 100× objective had to be used because 

most flakes were only a few microns in size, and the increased magnification also made it easier 
4

to distinguish between the color of molecular layer flakes and substrate. However, using this 

magnification on a 1 cm2chip meant that it took about 100 sweeps back and forth across the chip 

to cover the whole area. Because contrast increased with increased layer count, flakes with 

minimal contrast to the substrate were marked as potentially being molecular layers. Flakes

needed to be at least 3 × 3 μm to be suitable for device fabrication. Smaller flakes were also 

noted for the possibility that the shape allowed for a device to fit (most flakes are irregularly 

shaped, not a perfect 3 × 3 μm square) or for potential imaging or laser power tests to be done in 

the future. Imaging and the effects of high laser powers on molecular layers are also interesting 

topics to study, though they were not the subject of this report. An example of a flake with a 

combination of 1H-MoS2 and 2H-MoS2 is shown in Fig. 3, showing the extent of contrast 

between flakes used for the experiment and substrate, as well as an average size for most flakes 

studied.

Fig. 3. MoS2 flakes surrounded by a Si substrate. Under 

the green filter, the 1H-MoS2 flakes are the greygreen pieces. MoS2 becomes more yellow in color 

as the amount of layers increases. The dark green 

flakes are bilayer, the green are multilayer, and the 

yellow are bulk. 

Layer counts were confirmed with Raman spectroscopy, atomic force microscopy, and 

photoluminescence spectroscopy. AFM scans were initially conducted to check if what appeared 

to be 1H-MoS2 through the optical microscope was the appropriate thickness. Results from the 

AFM alone were not completely conclusive, due to the previously mentioned possibility of 

substrate effects, and it was necessary to perform additional scans to be certain that a flake was 

1H-MoS2. After AFM scans, PL was used to approximate layer counts. A higher intensity PL 

peak indicates a lower layer count. As the number of layers increased, the intensity of the 

photoluminescence decreased. This is due to the transition from indirect to direct band gap that 

occurs as the layers are stripped down to a single molecular layer.

11 Raman scans provided the 

last check to verify that the flakes were molecular layers. Figure 4 provides examples of the 
5

difference in PL and Raman values for 1H-MoS2 and 2H-MoS2, clearly showing the difference 

in the transition from direct to indirect band gap, as well as the change in functional modes.

Fig. 4 a) Image of a PL plot for 1H-MoS2 and 2H-MoS2 flakes, showing the peak intensities of each. As layer 

count increases, the PL plot tends to flatten, with no discernible peak intensity for either the A or B exciton.

3

b) Image of a Raman plot for the same flakes, showing the silicon and functional mode peaks (zoomed in to 

show the difference between the functional mode peaks for the 1H and 2H), as well as a fourth peak where 

some of the laser leaks through the filter. For bulk MoS2, there is no silicon peak visible and no difference 

between in-plane and out-of-plane frequency because the silicon is covered by the MoS2 and molecular 

motion is equally restricted in all directions, as opposed to 1H-MoS2 where molecular motion is easier out of 

the plane because there is nothing restricting the atoms from moving.

9

 

Checking the layer count with Raman involved determining the peak positions of the E' and A'1

functional modes. The frequency of in-plane E' mode decreases with thickness, while the out-ofplane A'1 mode increases with thickness.

7,8 Using these modes, the thickness can be determined 

on the atomic level using Raman. Raman spectroscopy is non-invasive and does not cause any 

damage to the material while taking measurements, unlike other techniques, such as AFM, which 

may scrape the surface of the material.

9

Figure 5 provides a reference of the images produced by 

the different scans, as well as what they mean in terms of distinction between 1H-MoS2 and 2HMoS2.
6

(a) (b)

(c) (d)

Fig. 5 Various scans used to analyze Flake 66. a) Optical microscope image of the flake, where the 1H-MoS2

area of interest is the green/grey rectangle in the center. b) AFM image of the flake. The slightly lighter 

area is the 1H-MoS2 flake, showing the faint difference between the height of the molecular layer and 

that of the substrate. AFM images usually show a greater contrast between molecular layer and 

substrate, though in this case the bulk surrounding the molecular layer diminished the effect of the 

height difference between the molecular layer and the substrate. c) PL image of the flake, showing 

increased intensity with red and yellow coloring where the 1H-MoS2 flake is located. d) Raman image 

of the flake, where the 1H-MoS2 is shown as blue and bulk is red. 

Electron-beam lithography (EBL) will be used to fabricate variable channel length field-effect 

transistors (FETs) and Hall bar structures directly onto 1H- and 2H-MoS2. A methyl 

methacrylate (MMA) and polymethyl methacrylate (PMMA) bilayer resist process will be used 

for all steps. The MoS2 layer will be patterned using a low-power inductively coupled plasma 

reactive ion etch (ICP-RIE) in a CH4/O2 plasma, and source and drain contacts will be deposited 

using e-beam evaporated Ti/Au (15/85 nm). In order to analyze any damage or variation in the 

MoS2 structural chemistry during processing, high-resolution Raman and PL imaging will be 

performed on the active device area after each processing step.

4

Figure 6 provides examples of 

devices built on MoS2 samples, and Raman and PL scans done regarding electrical performance.
7

Fig. 6 Examples of Raman and PL scans of devices built 

on MoS2. a, 1) The Raman intensity of the in-plane 

mode. a, 2) The PL intensity of the in-plane mode.

b, 1) The Raman intensity of the A exciton. b, 2) 

The PL intensity of the A exciton.

10

3. Results and Discussion

The chip sample studied was ~1 cm2in size and revealed 70 MoS2 flakes of interest, with six

potential 1H-MoS2 flakes of a reasonable size for processing devices. There were also high 

numbers of bilayer, multilayer, and bulk flakes that could be used to build devices, as well as a 

number of nanoscrolls that could prove promising to study. Nanoscrolls could be either 1H-MoS2

or 2H-MoS2, depending on how they formed. Though the main focus of this study was the 

effects of processing on 1H-MoS2, flakes with varying amounts of layers were also worth testing 

because of their potential to be useful and the fact that they are more abundant than the 1H-MoS2

flakes.

Fabrication is currently in process. Plans are to continue fabricating devices on all the eligible 

1H-MoS2 flakes, and then to test for basic transistor characteristics. Post-fabrication plans 

include measuring transistor performance, and obtaining the carrier mobility of ME-MoS2

molecular layers and comparing this to the value for CVD-MoS2. 
8

4. Conclusions

The experiment focused on analyzing the differences between the material characterization of 

CVD-MoS2 and ME-MoS2, using the same processes to build and analyze devices. At this point, 

potentially useful 1H-MoS2 flakes have been identified on ME-MoS2 using a variety of tools, 

including an optical microscope, Raman spectroscopy, photoluminescence spectroscopy, and 

atomic force microscopy mapping. Device fabrication is currently in processing, with plans for 

its conclusion before the end of the fiscal year.
9

5. References

1. Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, Grigorieva IV,

Firsov AA. Electric field effect in atomically thin carbon films. Science. 2004;306:666–669.

2. Li H, Zhang Q, Yap CCR, Tay BK, Edwin THT, Olivier A, Baillargeat D. From bulk to 

monolayer MoS2: evolution of raman scattering. Advanced Functional Materials.

2012;22:1385–1390.

3. Mak KF, Lee C, Hone J, Shan J, Heinz TF. Atomically thin MoS2: a new direct-gap 

semiconductor. Physical Review Letters. 2010;105:136805-1–136805-4.

4. Amani M. Chin ML, Birdwell AG, O’Regan TP, Najmaei S, Liu Z, Ajayan PM, Lou J, 

Dubey M. Electrical performance of monolayer MoS2 field-effect transistors prepared by 

chemical vapor deposition. Applied Physics Letters. 2013;102:193107-1–193107-4.

5. Butler SZ, Hollen SM, Cao L, Cui L, Gupta JA, Gutiérrez HR, Heinz TF, Hong SS, Huang J,

Ismach AF, Johnston-Halperin E, Kuno M, Plashnitsa VV, Robinson RD, Ruoff RS,

Salahuddin S, Shan J, Shi L, Spencer MG, Terrones M, Windl W, Goldberger JE. Progress, 

challenges, and opportunities in two-dimensional materials beyond graphene. ACS Nano.

2013;7:2898–2926.

6. Blake P, Hill EW, Neto AHC, Novoselov KS, Jiang D, Yang R, Booth TJ, Geim AK. 

Making graphene visible. Applied Physics Letters. 2007;91:063124-1–063124-3.

7. Lee C, Yan H, Brus LE, Heinz TF, Hone J, Ryu S. Anomalous lattice vibrations of singleand few-layer MoS2. ACS Nano. 2012;4:2695–2700.

8. Sahoo S, Gaur APS, Ahmadi M, Guinel MJ-F, Katiyar RS. Temperature dependent raman 

studies and thermal conductivity of few layer MoS2. ACS Nano. 2013;17:9042–9047

9. Dieing T, Hollricher O, Toporski J. Confocal raman microscopy. Springer: Berlin, 289; 

2010.

10. Najmaei S, Amani M, Chin ML, Liu Z, Birdwell AG, Ajayan PM, Dubey M, Lou J. 

Individual and collective role of grain boundaries in transport properties of molybdenum 

disulfide atomic layers. Submitted to Nano Letters, but has yet to be published.

11. Gutiérrez HR, Perea-López N, Elías AL, Berkdemir A, Wang B, Lv R, López-Urías F,

Crespi VH, Terrones H, Terrones M. Extraordinary room-temperature photoluminescence in 

triangular WS2 Monolayers. Nano Letters. 2012;DOI: 10.1021/nl3026357.

12. Ataca C, Topsakal M, Aktürk E, Ciraci S. A comparative study of lattice dynamics of threeand two-dimensional MoS2. The Journal of Physical Chemistry. 2011;115:16354–16351.
10

13. Radisavjevic B, Radenovic A, Brivio J, Giacometti V, Kis A. Single-layer MoS2 transistors. 

Nature Nanotechnology 2011, 6, 147–150.

14. Crowne FJ, Amani M, Birdwell AG, Chin ML, O’Regan TP, Najmaei S, Liu Z, Ajayan PM,

Lou J, Dubey M. Blue shifting of the a exciton peak in folded monolayer 1H-MoS2. 

arXiv:1307.1720
11

List of Symbols, Abbreviations, and Acronyms

2D two-dimensional

3D three-dimensional

AFM atomic force microscopy

CVD-MoS2 MoS2 grown by Chemical Vapor Deposition 

EBL Electron-beam lithography

FETs field-effect transistors

ICP-RIE inductively coupled plasma reactive ion etch

ME-MoS2 mechanically exfoliated

MMA methyl methacrylate

MoS2 1H-molybdenum disulfide 

PL photoluminescence 

PMMA polymethyl methacrylate

TMDs transition metal dichalcogenides 
12

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