Design and Fabrication of Leaky MoS₂ Split-Nanoring Resonators for Biosensing Application

The emergence of novel atomically thin layered two-dimensional transition metal-dichalcogenides (TMDCs) such as MoS₂ is promising for biosensing [1,2] and optoelectronic applications [3]. In the existing literature, MoS₂has been explored in field-effect transistor platforms for sensing protein and DNA molecules [4,5]. Recently, stacked heterostructure of MoS₂ and graphene were explored for detecting Rhodamine 6G (R6G) molecules [6] and DNA hybridization [7].<br/>Here we propose a novel nanophotonic device platform using MoS₂ split-nanorings resonators array fabricated on SiO₂/Si substrate. We have explored MoS₂ because of its high refractive index (n &gt; 4 [8]) contrast with air and low absorption (k &lt; 1 [8]) both at monolayer and bulk. Also, the MoS₂ surface favors bonding with the carboxyl (-COOH) group, which makes it biocompatible. We have conducted finite difference time domain (FDTD) simulations for obtaining the best structural parameters by varying inner and outer diameter (D), periodicity (P), and height (H) of the split-nanorings array to study the resonance properties. We performed FDTD simulations by varying the number of polystyrene particles (100 nm diameter) randomly distributed on the device surface and calculated the resonance wavelength shift with the number of particles increased. Here, we have considered 100 nm-sized polystyrene particles as most of the bioanalytes have a similar refractive index to the polystyrene. One of the best-obtained design parameters is nanoring’s outer diameter- 200 nm, inner diameter- 100 nm, the height of MoS₂ nanoring- 150 nm, split gap- 80 nm with unit cell periodicity- 220 nm. The split gap in each nanoring has been introduced to leak out the resonant field available on the MoS₂ surface. Attachment of target bioanalytes on the MoS₂ surface disturbs the resonance field and induces a red-shift at resonance wavelength signifies the presence of bioanalytes and detection sensitivity is obtained by calculating the slope of the resonance shift curve with respect to the bioanalytes concentrations. The large area of uniform MoS₂ thin film will be grown by pulsed laser deposition (PLD) technique on SiO₂/Si substrates and the PLD growth parameters (substrate to target distance, chamber condition, post-growth annealing temperature and duration, number of pulses, etc.) are optimized in our lab. Split-nanoring structures will be patterned by electron beam lithography (EBL) technique on the PLD grown 150 nm MoS₂thin film and then reactive ion etching (RIE) will be performed to obtain MoS₂ nanoresonators array on SiO₂/Si substrates. Fabricated devices will be characterized by SEM, AFM, and optical reflection measurement will be carried out by our lab-made customized optical setup to get resonance conditions. We will present the design optimization and fabrication process of the MoS₂ nanoresonators array and demonstrate experimental measurements of varying concentrations of polystyrene beads of various dimensions to highlight the biosensing capability of the proposed platform.<br/><br/><b><u>References</u></b><b>:</b><br/>1. P. Singh, R. Gupta, M. Sinha, R. Kumar, and V. Bhalla, <i>Microchim. Acta</i>, 183, 1501−1506, 2016.<br/>2. S. Li, C. Hu, C. Chen, J. Zhang, Y. Bai, C. S. Tan, G. Ni, F. He, W. Li, and D. Ming, <i>ACS Appl. Bio Mater</i>., 4, 7, 5494–5502, 2021.<br/>3. K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, <i>Phys. Rev. Lett</i>., 105, 136805, 2010.<br/>4. D. Sarkar, W. Liu, X. Xie, A. C. Anselmo, S. Mitragotri, K. Banerjee, <i>ACS Nano,</i> 8, 3992−4003, 2014.<br/>5. C. Zhu, Z. Zeng, H. Li, F. Li, C. Fan, H. Zhang, <i>J. Am. Chem. Soc</i>., 135, 5998−6001, 2013.<br/>6. M. Alamri, R. Sakidja, R. Goul, S. Ghopry, and J. Z. Wu, <i>ACS Appl. Nano Mater.</i>, 2, 3, 1412–1420, 2019.<br/>7. P. T. Loan, W. Zhang, C. T. Lin, K. H. Wei, L. J. Li, C. H. Chen, <i>Adv. Mater.</i>, 26, 4838−4844, 2014.<br/>8. C. Hsu, R. Frisenda, R. Schmidt, A. Arora, S. M. de Vasconcellos, R. Bratschitsch, H. S. J. van der Zant, A. C. Gomez, <i>Advanced Optical Materials</i>, 7, 1900239,<br/>2019.