Spectroscopic Analysis of Polymer and Transition Metal Dichalcogenide Interface for Photodetection Applications

Two-dimensional transition-metal dichalcogenides (2D TMDs) have been extensively studied for photodetection applications due to their strong optoelectronic behavior in the visible range and tunable band gap. [1] However, the optoelectronic properties of 2D TMDs can deteriorate due to surface defects and exposure to air and water. [2] Researchers have used a variety of materials to combat this degradation and passivate the surface, but polymers are ideal for passivating 2D TMDs for photodetection applications as many are transparent in the visible range. [3] Researchers have used polymers to perform significant modulation of the optoelectronic properties of hybrid devices via doping, passivation, dielectric screening, and capacitive effects. [4] These schemes illustrate that polymers can contribute multiple mechanisms to the polymer/2D TMD interface. In this work, the 2D TMD and polymer interface is assessed spectroscopically for three common polymers to evaluate the efficacy of each polymer in passivating the MoS₂ surface for photodetection applications. Evaluating this interface with spectroscopic methods, especially with ultrafast and dynamic pump probe measurements, mimics the physics occurring during photodetection operation and thus provides a clearer understanding of the fundamental physics at play.<br/><br/>For this study, a representative group of three commonly available polymers were chosen to understand the relative passivation effects: parylene N (Pa-N), polymethyl methacrylate (PMMA), and polyvinylidene difluoride-trifluoroethylene (PVDF-TrFE). Pa-N is an extremely inert polymer used frequently in organic electronics as a dielectric and biomedical devices as a chemical barrier. [5] PMMA is an electron beam photoresist frequently used to encapsulate 2D TMD devices for passivation. [6] PVDF-TrFE is an electroactive polymer that has been shown to be piezoelectric, pyroelectric, and ferroelectric, and as a result, is widely applied for flexible sensors. [7] PMMA and PVDF-TrFE have functional groups containing oxygen and fluorine that would be expected to better fix sulfur vacancies or provide doping than Pa-N.<br/><br/>Spectroscopic measurements including Raman spectroscopy, photoluminescence spectroscopy, and time-resolved terahertz spectroscopy (TRTS) were performed on these samples to observe the effect of polymer passivation on a MoS₂ monolayer. The photoluminescence measurements were done with a hyperspectral microscope to facilitate greater averaging of the photoluminescence response across the sample surface. The results of the Raman and photoluminescence spectroscopy show an n-doping effect on the MoS₂ spectra after coating with Pa-N and PMMA through a downshift in the Raman peaks and a redshift in the photoluminescence peak. In the TRTS measurements, the peak of the response was higher for all polymer samples, which suggests more charge is generated in polymer passivated samples upon excitation either from doping or successful passivation. For PVDF-TrFE, the lifetime of the carriers was longer than any other sample. These results indicate that unpoled PVDF-TrFE best passivates the surface of the MoS₂ of the polymers tested. Further work is being done to understand the exact mechanism by which it passivates the surface, but it is suspected that the fluorine present in the PVDF-TrFE contributes electrons that help to fill the sulfur vacancies. This effect could be used successfully in photodetection to help improve the detectivity.<br/><br/>[1] H. S. Nalwa, <i>RSC Adv.</i>, vol. 10, no. 51, pp. 30529–30602, 2020. [2] W. Park et al., <i>Nanotechnology,</i> vol. 24, no. 9, pp. 095202, 2013. [3] J. Ma et al. <i>Appl. Phys. Lett.</i>, vol. 113, no. 1, 2018. [4] K. Cho et al., <i>ACS Nano</i>, vol. 13, no. 9, pp. 9713-9734, 2019. [5] S. Gholizadeh et al., <i>Scientific Reports</i>, vol. 13, no. 1, pp. 4262, 2023. [6] U. Ali et al., <i>Polymer Reviews</i>, vol. <i>55</i>, no. 4, pp. 678-705, 2015. [7] B. Stadlober et al., <i>Chem. Soc. Rev.</i>, vol. 48, no. 6, pp. 1787–1825, 2019.