Solid-state synthesis of silver nanowires by green chemistry and mechanistic aspects influencing nanowire growth and plasmonic study in Au thin film/MoS2 heterostructures for surface enhanced Raman spectroscopy

UNCG Author/Contributor (non-UNCG co-authors, if there are any, appear on document)
Harish Chevva (Creator)
The University of North Carolina at Greensboro (UNCG )
Web Site:
Jianjun Wei

Abstract: This thesis has a research focus on novel nanomaterials synthesis, fabrication and the fundamental understanding of the process and properties regarding surface enhanced Raman spectroscopy application. The two projects are: 1) a novel solid-state growth of silver nanowires (AgNWs), and 2) fabrication of 2D layered MoS2 on gold film for surface enhanced Raman spectroscopy (SERS). Solid-state and bottom-up growth of nanowires-based research has been specific to semi-conductor nanowires, with rare research reports on the growth of metal nanowires in the similar manner. Synthesis of AgNWs has been around in the research filed for more than a decade. Most of the synthesis processes reported till now were all solution-based processes involving complex synthesis set-up employing harsh conditions for good yield. This research presents a solid-state green synthesis of AgNWs with detailed growth analysis, reporting key factors influencing the self-growth of the nanowires. Our synthesis process is simple, performed at room temperatures and uses bio-compatible chitosan polymer as base for silver nanoparticles (AgNPs) synthesis and AgNWs growth. A new way of blocking oxygen by thin film coating, to avoid oxidative etching of Ag atoms which hinders growth of AgNWs, is investigated. Role of Cl- ions and influence of its concentration to initiate the growth AgNWs is examined thoroughly with the help of UV-Vis, XRD and EDS analysis. Finally, role of time in the continuous growth of AgNWs is studied for seven weeks long under SEM. AgNWs/Chitosan polymer platform is directly employed in SERS application to detect Raman active probe Rhodamine 6G (R6G) at a detection limit of 10-7M. This report, for the first time, presents an inexpensive synthetic and growth mechanism for solid-state and bottom-up growth of metal nanowires. This research helps in paving a new way for research involved with Ag nanowires. 2D materials other than graphene which made their presence strong in research field are transition metal dichalcogenides (TMDCs). Among a range of TMD materials, Molybdenum disulfide (MoS2) has shown promising impact in various fields like optoelectronics, SERS based biosensors, photonics and electrochemistry. SERS enhancement observed in semiconductor (1.8ev) monolayer MoS2 is due to various factors like surface plasmons, excitons and charge transfer. But, monolayer MoS2 exhibit a modest Raman enhancement (<100) when compared to other 2D materials like graphene. Multilayer MoS2 which have lower bandgap (1.4ev to 1.6ev) exhibits no SERS enhancement due to increase in the thickness of the material. Lower SERS enhancement property of monolayer MoS2 is addressed by incorporation of nanoparticles on the surface of MoS2 and with the combination of MoS2 and metal nanostructures. But, there are very less reports on dealing with few layers MoS2 SERS enhancement. Here, in this report we observed raise in intensity of primary Raman signals E2g1 and A1g of few layer MoS2 with respect to change in thickness of Au film substrates on which MoS2 is deposited. Thorough optical and Raman analysis proved that along with monolayer MoS2, few layers MoS2 which are deposited on gold film also exhibit promising results in showing SERS enhancement for the detection of Raman active probe R6G. Further, thickness dependence of underneath Au film influencing the SERS property of particularly few layers of MoS2 is validated using Raman analysis supported by FDTD simulation data. The results proved thickness of 150nm Au showing highest enhancement in both increasing the intensity of MoS2 Raman peaks and in detection of R6G molecule at the concentration of 10-6M. These findings and understanding of the behavior of few layer MoS2/gold film expand the use of MoS2 on wide range of SERS applications.

Additional Information

Language: English
Date: 2019
2D material, Biosensors, MoS2, Silver nanowires, Solid-state silver nanowires
Raman spectroscopy
Molybdenum disulfide

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