Functional Van der Waals Heterostructures of Low-Dimensional Materials for High-Performance Sensors

Abstract

Low-dimensional materials including zero-dimensional (0D), one-dimensional nanomaterials (1D) nanomaterials, and two-dimensional (2D) materials, have become promising candidates for functional devices. The electronic states of these materials are confined for at least one dimension, like quantum dots (QD) for 0D, carbon nanotubes (CNT) for 1D, graphene and transitional metal dichalcogenides (TMDCs) for 2D. Due to the quantum confinement of the charge carriers as a result of reduced dimensionality, low-dimensional materials exhibit unique properties including ultrahigh carrier mobility, excellent optical transparency, and thermal conductivity which can be distinctively different from their bulk materials.This thesis explores new applications of vdWs heterostructures of low dimensional materials for various types of sensors, particularly biosensors, photodetectors, and gas sensors. First, we applied the graphene/MoS2 heterostructure with plasmonic AuNPs on the top as substrates for surface-enhanced Raman spectroscopy (SERS). SERS enables single molecule through two enhancement mechanisms: electromagnetic (EM) enhancement and chemical enhancement (CM). Yet the microscopic mechanism of CM is still not well understood. Assembling the 2D heterostructures by stacking different 2D materials is an efficient approach for enhancing the Raman signals of adsorbed molecules. We demonstrated that the graphene/TMDCs heterostructure as an efficient substrate for SERS. Also, adding plasmonic AuNPs on top of the graphene/TMDCs heterostructure can further enhanced Raman signal due to EM enhancement. The interface dipole−dipole interaction in the heterostructure increases electron delocalization at the interface and increases the charge transfer probability between the Rhodamine 6G (R6G) molecule and the heterostructure which enhance the CM contribution to SERS signatures. R6G was used as an SERS probe molecule with a SERS sensitivity of 5×10−8 M using a nonresonance 633 nm laser, which is an order of magnitude higher than that reported on the AuNPs/graphene substrate using the same excitation. A higher SERS sensitivity of 5×10−10 M was obtained using resonance 532 nm laser excitation. Likewise, the 2D WS2/graphene vdW heterostructure is a promising platform for photodetection. On account of the limited light absorption in atomically thin layered materials, hybrid structures or heterostructures based on 2D materials can be employed to enhance the light absorption and the photoresponsivity. 2D materials are necessary to overcome the limitations of conventional bulk materials for optoelectronics applications. Due to the limited light absorption in atomically thin layered materials, hybrid structures or heterostructures based on 2D materials can be employed to enhance the light absorption and the photoresponsivity. Combining the high carrier mobility of graphene with a strong light absorbing material, such as semiconductor quantum dots (QDs) or other 2D materials like TMDCs, offer complementary to graphene that lacks the light absorption. In this type of two different combined materials, the incident light produces electron-hole pairs in light-absorbers such as semiconductor TMDCs, with one of the two types of carriers transferred to graphene and another one trapped in the TMDCs layer and gate the graphene channel. The type of carrier transferred to graphene can recirculate many times from source to drain before recombining with the trapped carrier and hence produce ultrahigh gain and photoresponsivity. However, light absorption in TMDC/graphene vdW heterostructures is still limited due to the small thickness of the TMDC as the photosensitizer. We demonstrated a photodetector based on a plasmonic WS2 nanodisks (NDs) with a lateral dimension of 200–400 nm on graphene vdW heterostructure. A strong localized surface plasmonic resonance (LSPR) can be gen