1. Jonathan Jordan
Dr. Jie Shan
PHYS 390
December 18, 2013
Final Report
Abstract:
The purpose of this course was to experience working in a physics laboratory at a
top tier research institution. The focus of this course was the study of the optical
properties of two-dimensional materials facilitated by professor Jie Shan’s group. After
learning the proper procedures, the lab work consisted of working nine hours a week
cleaning substrates, material exfoliation, searching for monolayers of samples, and
performing Photoluminescence tests. The main material worked with was p-type doped
Molybdenum Disulfide. Due to time constraints the optical properties of the second
material Gallium Selenide were not tested.
Background:
Photoluminescence occurs when electrons in a semiconductor fall from their
excited states releasing the band gap energy as a photon. For this to happen, electrons
first need to be excited, in this case using photon excitation. The light created from the
release of energy of the electrons is called photoluminescence. Experimentally,
photoluminescence of a material can be tested using a laser that emits photons larger than
the band gap energy of the material, which excites electrons. The photon emission can
then be recorded using a CCD camera. In particular, two-dimensional materials similar to

2. Molybdenum diselenide are studied using PL tests because they have a much greater PL
intensity than multiple layers of the same material. Molybdenum Disulfide is also a
particularly interesting material because its PL can be altered through chemical doping.
P-type and n-type doping are when impurities are added to a material, which alters the
number of holes and/or electrons in a material. P-type doping is when there are more
holes than electrons while n-type doping is when there are more electrons than holes.
Studying p-type doped Molybdenum Disulfide allows us to study the changes in PL when
this material is doped. The PL intensity of Molybdenum Disulfide changes with p-type
doping and n-type doping. Photoluminescence spectroscopy is used to study electronic
states of materials, electron hole interactions, surface state density, surface related
electronic transition, electronic band bending, and near surface bulk properties.
Procedures:
With the direction, instruction, and supervision of graduate students Keliang He
and Zefang Wang, procedures were learned and put to use in the laboratory, including
cleaning substrates, material exfoliation, searching for monolayers of samples, and
performing optical tests. Silicon substrates with an oxide layer were cleaned chemically
as well as with sonication to remove any dust particles. The cleaning process is important
because materials in monolayers are likely to bond with other particles. After cleaning the
substrates, the materials in crystal form were exfoliated mechanically using scotch tape to
separate layers of each material. After exfoliating the materials they were transferred to
the Silicon substrates. The substrates are then placed on a microscope slide and are ready
to be searched. The samples are searched using 100x magnification to find atomically

3. thin layers of the materials. After finding monolayers of each material, their location was
recorded using photographs taken by the microscope software. Pictures were taken at
2.5x, 10x, 20x, 50x, and 100x magnification so that the monolayers could be located
easily. After finding the samples, they were optically tested using a 532 nm laser. To
perform the PL test the laser was aligned and focused and the CCD camera was cooled to
a very low temperature. The materials were then tested using the laser, the CCD, and
WinSpec software. The integration time of the PL test ranged from ten seconds to fifty
seconds. The PL test was used to confirm that samples found were indeed monolayers
and used to show the optical properties of the monolayers of p-type doped Molybdenum
Disulfide.
Experimentation:
The following are pictures of monolayers of p-type doped Molybdenum
Disulfide at 100x magnification.
The smaller sample in the image on the left is approximately 5 micrometers by 3
micrometers. The larger sample is approximately 6 micrometers by 3.5 micrometers. The
sample in the image on the right is approximately 8 micrometers by 3 micrometers.

4. The following is the result of a typical photoluminescence test on the monolayer of the p-
type doped Molybdenum Disulfide using 100x magnification, with a light filter of OD 2
and integration times of 20 seconds.
The majority of the samples had the same shape with the peak in the same location,
which means they were likely monolayers according to the results in Tunable
Photoluminescence of Monolayer MoS2 via Chemical Doping2. The samples with
differently shaped graphs were likely not monolayers of the material. The PL intensity,
which is plotted on the y-axis, did vary from sample to sample.
The following are images of possible monolayers for Gallium Selenide at 100x
magnification.

5. The image on the left shows a sample that is approximately 7 micrometers by 5.5
micrometers. The image in the center shows a sample that is approximately 10
micrometers by 5 micrometers. The image on the right is approximately 7 micrometers
by 7 micrometers.
Discussion:
Physics 390 offered a valuable hands-on experience in the world of experimental
physics research. I plan to continue this educational experience in the spring term. The
results of the photoluminescence for MoS2 suggested that the samples found were indeed
monolayers. The variation in the intensity from sample to sample could be a result of
different doping levels within the material. The variation could have also been a result of
the experimental setup including the laser alignment. Better experimental setup yields
better results and the equipment may not have been used to its full potential. In the future
laser alignment and calibration should be improved upon. Future experimentation should
include the testing of Gallium Selenide and the further testing of doping in Molybdenum
Disulfide. A device could be created where electrodes could be attached to a large sample
of Molybdenum Disulfide to adjust the doping level, which would allow for the
measurement of the change in the PL based directly on the change in the chemical doping
of the material.

6. References
[1] Mak, K. F., Lee, C., Hone, J., Shan, J., & Heinz, T. F. (2010). Atomically Thin MoS2: A New
Direct-Gap Semiconductor. Physical Review Letters, 105(13), 1-4.
[2] Mouri, S., Miyauchi, Y., & Matsuda, K. (n.d.). Tunable Photoluminescence of Monolayer
MoS2 via Chemical Doping.
[3] Singha, A., Dhar, P., & Roy, A. (2005). A Nondestructive Tool for Nanomaterials: Raman
and Photoluminescence Spectroscopy. American Journal of Physics, 73(3), 224-233.