Studies Of Transition Metal Dichalcogenides Materials On Preparation,Characterisation And Application On SERS

Transition metal dichalcogenides?TMDCs?are a class of materials with the formula MX₂,where M is a transition metal element from group IV?Ti,Zr,Hf and so on?,group V?for instance V,Nb or Ta?or group VI?Mo,W and so on?,and X is a chalcogen?S,Se or Te?.Due to the combination of different elements,there are up to 40 different types of transition metal sulfides.These bulk transition metal sulfides exhibit a variety of electrical properties,from metals to insulators.If the bulk TMDCs is stripped into two-dimensional material mode with single layer or a few layers,the edges and undersides of the prism structure of the TMDCs will be exposed,which will not only retain the original properties,but also lead to some additional properties or property changes due to the confine effect,for example,s-p_z orbital interaction between adjacent MX₂ layer disappears,widening of the band gap.The change of orbital hybridization will also change the indirect band gap of monolayer semiconductor type MX₂ materials such as MoS₂,WS₂ into direct band gap,which can further lead to the enhancement of photoluminescence.At the same time,the chemical properties of a single layer or a few layers of MX₂ are determined by the coordination bond type of the boundary.Due to these interesting properties,the TMDCs receive lots of attentions in many applications,such as using as the chemical catalyst in hydrogen production and hydrovulcanization,as electroactive materials in optoelectronic devices,as the energy storage materials in the lithium battery and super capacitor,as the saturable absorber material in laser lock mode,as Raman active substrate in the Surface Raman enhanced scattering?SERS?test and so on.In this paper,a two annealing process was proposed to achieve the direct growth of large area and high crystallization WS₂ thin films on the quartz substrate,and it is the first time to prove the WS₂ thin film possessing the SERS effect.Continuous and large-area MoS₂ thin films grow on metal nanoparticles directly using CVD method,and a highly sensitive and reproducibility SERS substrate was obtained,realizing the detection of malachite green,crystal violet and other chemical molecules.The thickness of MoS₂ thin film can be controlled by argon plasma and the composite structure of gold nanoparticles coated by single layer MoS₂ thin film was realized,and we further investigate the the effect of the thickness of MoS₂ on SERS performance.The monolayer MoS₂ film was used as the sub-nanometer spacer between metal nanoparticles to obtain ultra-sensitive SERS substrate.Finally,the multilayer metal nanoparticle structure is constructed by using MoS₂ or GO thin film as the nanospacer to regulate the intensity of hot spots between metal nanoparticles.The main works were as follows:?1?A facile fabrication of high-quality and large-area tungsten disulfide?WS₂?layers is demonstrated using a thermal decomposition of tetrathiotungstates??NH₄?₂WS₄?with two annealing process.During synthesis,the first annealing step is utilized to achieve lateral epitaxial growth of the WS₂ and create seamless and large-area WS₂ film.The second annealing step can offer an S-rich and high temperature condition,which is beneficial for the high quality of the WS₂ film.Scanning electron microscopy,Raman spectroscopy and atomic force microscopy confirm the presence of large-area and high-quality WS₂ film.The crucial role of the S,H₂ and the effect of the temperature during the experiment are also investigated.Furthermore,the potential application of the prepared WS₂ as a substrate for Raman enhancement is first discussed using R6G molecules as probe molecule.?2?Here we designed a new kind of SERS substrate referred to as a MoS₂-coated Cu nanopaticles?MoS₂@CuNPs?substrate for R6G detection.The Cu nanoparticles wrapped by a thin MoS₂ film were directly synthesized on flat quartz by two annealing process in a mixture of argon and hydrogen.Signals from a MoS₂@CuNPs SERS substrate were also demonstrated to have interesting advantages over pure CuNPs SERS,in terms of the excellent linear relationship between Raman intensity and analyte concentration,strongly background fluorescence quenching and being more stable against photo-induced damage and oxidation.The minimum detectable concentrations of R6G on MoS₂@CuNPs hybrids can be as low as10^-9M which is one order lower than that on the pure CuNPs substrates.This synthetic approach is facile,scalable and applicable to the MoS₂ film compound with other metals such as Ag,Au etc.?3?In this work,a facile and effective method for controlled-layer and large-area MoS₂films encapsulated Au nanoparticle hybrids is developed.With accurate Ar plasma treatment time control,the large-area MoS₂ layers can be obtained from monolayer to trilayer.The fabricated MoS₂@Au NPs with higher surface area exhibit excellent Raman enhanced effect for aromatic organic molecules?rhodamine 6G and crystal violet?and achieve the best when the monolayer MoS₂@AuNPs was obtained.The limit of detection is found to be as low as 1×10^-1010 M.The MoS₂@AuNPs was characterized by SEM,EDS,AFM,Raman spectroscopy,UV-Vis,XRD and HRTEM.?4?Here,3D hybrid plasmonic nanostructures using monolayer MoS₂ as sub-nanometer spacer with high density hot spots are fabricated.For the 3D hybrid assemblies,the hot spots exist not only on the 0.65 nm thick MoS₂ gap regions between large and small nanoparticles?NP?but also between small and small NP.The high-density hot spots induce prominently enhanced optical absorption,huge surface enhanced Raman scattering effect,excellent reproducibility,and ultralow limit of detection.Besides,the obvious electron transfer between MoS₂ and metal NP is also found in the 3D hybrid assemblies indicating the strong interactions between MoS₂ and Au or Ag.Combined with the long-term stability,the 3D hybrid assemblies should perform excellent in plasmon-enhanced sensor and photocatalysis applications.?5?We report a three-dimensional?3D?SERS substrate with different numbers of silver nanoparticle?Ag NP?layers using multilayer graphene oxide?GO?as a spacer.The SERS performance of the 3D nanostructure was investigated and it was found that the SERS effect increased as the number of Ag NP layers increased,and showed almost no change for more than four layers.We found that the SERS performance of the 3D nanostructures can be mainly attributed to the topmost hot spots which are closely related to the Ag NP layers in the 3D nanostructure.Furthermore,we explored 3D nanostructures with different Ag NP layers using the finite difference time domain method?FDTD?.The 3D SERS substrates also exhibit excellent detection capability.The limit of detection?LOD?was calculated down to 10^-15M for R6G and 10^-1212 M for CV.In addition,the reproducibility of the 3D SERS substrate was attributed obviously to the increasing number of Ag NP layers.Based on these promising results,the highly sensitive detection of molecules such as malachite green was demonstrated for food safety inspection.