High Optical Abropsion from Thin Film MoS2 Nano-Flakes
Authors: Almansoori Muntaser , Rezk Ayman , Abdul Hadi Sabina , Askar Khalid , Nayfeh Ammar 
Published: August 2024

Due to their distinctive electronic and optical characteristics, transition metal dichalcogenides (TMDs) have become significant materials in optoelectronics. Molybdenum disulfide (MoS2) flakes have received significant interest due to their potential applications. The unique bandgap properties exhibited by MoS2 flakes render them highly appealing for applications in optoelectronic devices [1]. A direct bandgap in this material enables effective interactions between light and matter, which is significant for various applications, including transistors, photodetectors, and light-emitting diodes [2].
In this work, we synthesized our MoS2 flakes by exfoliating MoS2 powder following [3]’s recipe. We disperse 500 mg of MoS2 powder into 50 mL of N-Methyl-2-pyrrolidone (NMP)and then use a probe sonicator for the exfoliation process that runs continuously for 6 hours while keeping the sample cooled by a 0 °C ice bath. We later centrifuged at 1500 rpm for 60 min to remove any unexfoliated particles and then centrifuged again at 7500 rpm for 30 min to remove soluble impurities. After removing NMP, the acquired MoS2 flakes dispersed in 50 mL isopropyl alcohol. To study the optical properties of these MoS2 flakes, we prepared 3 x 3 cm2 fused silica substrate rinsed with acetone and DI water, respectively. Before deposition, we sonicate our MoS2 solution for 60 minutes in a bath sonicator. A precise pipet is used to drop-cast 20 uL of MoS2 on the fused silica substrate and wait 120 seconds to let it spread and settle. Afterward, we spin the sample at a low 150 rpm for 40 seconds, ensuring no spillovers. The sample was then carried to a UV-Vis Lambda 1050 spectrometer tool to measure the transmittance and reflectance of the sample over a range of wavelengths 250 to 1200 nm. Subsequently, we calculated absorbance values from this data. We repeat this on the same sample in steps of 20 uL of MoS2, acquiring spectrometer data for 20 to 200 uL of MoS2 on fused silica. The absorbance data showed a slope increase of absorption around 830 nm wavelength, reaching a peak around 660 nm, and It continues to absorb to 250 nm wavelength of the UV range.
Moreover, as we increased the MoS2 amount, the overall absorbance increased. At the same time, reflectance increased; however, the change slowed after 60 uL of MoS2. We suspect that the former is due to the increase in the overall thickness of the deposited material, which increases the chance of photons traversing the material to be absorbed. The latter effect could be attributed to texturing, as these deposited MoS2 flakes do not seem to fall flat and create a uniform film and additional reflection from the MoS2 surface. To study that, we prepared five other 3x3 cm2 MoS2 on silicon (Si) samples with 20-100 uL in steps of 20 uL using the same process described earlier. Looking at the morphology using an optical microscope and SEM images of MoS2 on Si, the images show random settlement of MoS2 flakes, leading to uneven coating and witnessing MoS2 flakes arranged and aggregated into islands similar to results reported by others [4]. Also, we found similar dips in reflectivity as reported by literature due to absorption of A and B excitons[5]. Additionally, we measured the electrical conductivity using a hall measurement system of 100 uL MoS2 sample and found that it exhibited n-type behavior with an average sheet resistance value of 2.91x102 Ω/sq. We aim to investigate further the properties and potential applications of these MoS2 flakes.
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[2] H. J. Jin, C. Park, H. H. Byun, S. H. Park, and S.-Y. Choi, “Electrically Modulated Single/Multicolor High Responsivity 2D MoTe2/MoS2 Photodetector for Broadband Detection,” ACS Photonics, vol. 10, no. 9, pp. 3027–3034, Sep. 2023, doi: 10.1021/acsphotonics.3c00143.
[3] X. Yu et al., “Hybrid Heterojunctions of Solution-Processed Semiconducting 2D Transition Metal Dichalcogenides,” ACS Energy Lett., vol. 2, no. 2, pp. 524–531, Feb. 2017, doi: 10.1021/acsenergylett.6b00707.
[4] N. Zebardastan et al., “2D MoS2 Heterostructures on Epitaxial and Self-Standing Graphene for Energy Storage: From Growth Mechanism to Application,” Advanced Materials Technologies, vol. 7, no. 4, p. 2100963, 2022, doi: 10.1002/admt.202100963.
[5] “Layer-number dependent reflection spectra of MoS2 flakes on SiO2/Si substrate.” Accessed: Nov. 23, 2023. [Online]. Available: https://opg-optica-org.libconnect.ku.ac.ae/ome/fulltext.cfm?uri=ome-8-10-3082&id=398132

Figure 1