CVD Grow Thermal Conducting 2D-MoS₂ Nanostructures for Photodetection and SERS Applications

Abstract

Molybdenum disulfide (MoS2) is a layered transition metal dichalcogenide, which shows tunable bandgap and good thermal transport behaviour along with high absorption coefficient and mechanically flexible nature. These features of MoS2 make it suitable for use in next generation electronic and optoelectronic devices. This thesis entitled “CVD Grown Thermal Conducting 2D-MoS2 Nanostructures for Photodetection and SERS Applications” is focused on the synthesis of thermal conducting and semiconducting MoS2 nanostructures via chemical vapour deposition (CVD) technique and their photodetection and Surface-Enhanced Raman Scattering (SERS) applications. We have prepared three different morphologies of MoS2 nanostructures- horizontally grown interconnected network of few-layer MoS2 over Si substrate, horizontally grown triangular bi-layer MoS2 over SiO2/Si substrate and vertically oriented few-layer (VFL) MoS2 over Si substrate. We have characterized prepared MoS2 films by optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) techniques. To confirm the phase and the semiconducting nature of prepared MoS2, X-ray diffraction (XRD) and Raman spectroscopy techniques have been performed. The semiconducting nature of prepared MoS2 nanostructures have been confirmed via Photoluminescence (PL) spectroscopy. In the present work, thermal transport behaviour of triangular bi-layer MoS2 and VFL-MoS2 has been investigated using optothermal Raman technique. We have calculated the thermal conductivity (ks) of ~ 42 ± 8 W m-1 K-1 and interfacial thermal conductance (g) of ~ 1.264 ± 0.128 MW m-2 K -1 for triangular bi-layer MoS2 supported over SiO2/Si substrate. Higher thermal conductivity of ~ 100 ± 14 W m-1 K-1 has been found for VFL-MoS2 nanostructure, which can be associated with reduced phonon-defect scattering due to fewer defects and minimal strain in grown VFL-MoS2 nanostructure. Further, thermal sensitive quantum confinement phenomenon has been observed in above samples by performing temperature dependent PL study, which provides the information about the tunable nature of their bandgap suitable for optoelectronics application. Based on the thermal conducting and semiconducting nature of prepared MoS2 nanostructures, we have used these films for photodetection and SERS applications. The CVD grown MoS2 nanostructures are n-type in nature and hence they form p-n heterojunction with p-type Si substrate. We have successfully demonstrated the photodetection application of interconnected few-layer MoS2/Si heterojunction and VFL MoS2/Si heterojunction. We have observed the photoresponsivity of ~0.1413 A W-1 for few layer MoS2/Si heterojunction under white light illumination (0.15 mW cm-2 ) at -2 V bias. Higher photoresponsivity of ~7.37 A W-1 has been obtained for VFL-MoS2/Si heterojunction under green light illumination (0.15 mW cm-2) at -2 V bias, which can be attributed to the strong light absorption, intralayer carrier transport speed, and effective charge separation. Further, SERS application of prepared MoS2 nanostructures has been investigated to detect organic pollutants- Rhodamine 6G (R6G) and Methyl orange (MO). We have successfully detected R6G dye using all the prepared MoS2 nanostructures, while MO dye was detected using VFL-MoS2 nanostructure. The highest detection limit (10-10 M concentration for both the dyes) has been observed for VFL-MoS2 nanosheets over Si as SERS substrate. This high detection limit can be attributed to the enhanced light trapping and effective dye adsorption due to vertical morphology and vibronic-coupling-enabled charge transfer between MoS2 and organic dyes. The present thesis has been organized into seven chapters. The consecutive chapters are organized as follows Chapter 1 gives a brief introduction to 2D MoS2 nanostructure along with an overview of the current literature on thermal conductivity measurement, photodetection and SERS applications of MoS2 nanostructure. Chapter 2 describes the synthesis process of three different morphologies of MoS2 nanostructures. A concise overview of the characterization instruments like XRD, SEM, TEM, Raman, PL and AFM, is provided for structural and morphological studies of MoS2. This chapter also describes the current voltage measurement process and the preparation method of SERS substrate. Chapter 3 discusses the temperature and power-dependent Raman studies of horizontally grown triangular bi-layer MoS2 nanosheets over SiO2/Si substrate and vertically oriented few-layer (VFL) MoS2 nanosheets over Si substrate. Thermal conductivity calculation using Optothermal Raman technique has been described in detail. Chapter 4 discusses the thermal sensitive quantum confinement behaviour of triangular bi layer MoS2 and VFL-MoS2 nanostructures. Tunability of bandgap with temperature has been discussed using appropriate models. Chapter 5 describes the photodetection application of horizontally grown interconnected network of few-layer MoS2 and VFL-MoS2 over Si substrate using p-n junction mode under visible light illumination. Different optoelectronic parameters have been obtained and photodetection mechanism has been discussed in this chapter. Chapter 6 discusses the SERS detection of R6G and MO molecules using prepared different MoS2 nanostructures. The mechanisms like molecular resonance, excitonic resonance and vibronic coupling enable charge transfer have been discussed in detail. Chapter 7 thesis work has been summarized and the scope for the future work related to this field has been discussed.