Synthesis,Characterizations and Applications of WX2(X=S,Se) Single Crystals.pdf

1、Two dimensional (2D) layered materials have got extensive considerations for many specific applications owing to their interesting physical and chemical properties.In this dissertation,a comprehensive study on tungsten b

2、ased transition metal dichalcogenides single crystals is presented including synthesis,structural characterizations and various applications study by means of electrochemical supercapacitors,magnetic,optical and electric

3、al properties.
　　Firstly,high quality WX2 (X=S,Se) single crystals were successfully synthesized via an improved chemical vapor transport (CVT) technique.Morphological and structural characteristics of the as-grown la

4、yered single crystals were analyzed by using SEM,TEM,XPS,XRD,Raman and UV visible spectroscopy characterization techniques.
　　Secondly,to demonstrate its potential as promising electrode materials for electrochemical

5、supercapacitors,cyclic voltammetry (CV),electrochemical impedance spectroscopy (EIS) and charge-discharge analysis were performed on WS2 and WSe2 single crystals,revealing the comparison of energy storage performance.Not

6、ably,the obtained supercapacitor electrodes show excellent cycle stability and high capacitance retention of 80 and 99％ for WS2 and WSe2 respectively (even after 20,000 cycles),clearly suggesting their high potential in

7、electrochemical energy storage applications.
　　Thirdly,because of the interesting magnetic properties originating from layered lattices and strong spin-orbit coupling,layered transition metal dichalcogenides single cr

8、ystals are become important for new-generation spintronic applications.Herein,we present the CVT synthesis of a highly crystalline tungsten diselenide layered single crystal (WSe2),and subsequently doped with nickel (Ni)

9、 in order to tailor the intrinsic magnetic properties.The pristine WSe2 and Ni-WSe2 single crystals were comparatively characterized and analysed for magnetic properties from both experimental and computational aspects.I

10、t was found that the magnetic behaviour of pure WSe2 crystal would change from diamagnetic to ferromagnetic after Ni doping at tested temperature range of 10-300k.
　　After exploring the energy storage and magnetic pro

11、perties,WSe2 single crystal were studied for photovoltaic device application.Pristine WSe2 crystals were employed in CdTe solar cell.A layer of WSe2 single crystal was utilized as a buffer layer in between Cu back contac

12、t and CdTe absorbing layer to enhance the overall device efficiency.The inserted buffer layer avoid the penetration of back contact metal that diffuses through the layers and reduce the efficiency by creating the shunt p

13、aths,thus in this way WSe2 buffer layer increases the device efficiency.It also facilitates the flow of holes from CdTe to the metal electrode.For comparison purpose solar cell device with only Au metal was also prepared

14、.Device with only Au as metal electrode showed 4.11％ efficiency while fairly high values of Voc,Jsc and FF were observed for the device with WSe2 buffer layer with the overall device efficiency of 9.1％.
　　Finally,WSe2

15、 single crystal were exploited for electrical properties study perspective view.To explore the electrical properties of grown tungsten diselenide single crystals,field effect transistor was fabricated using successive st

16、eps of optical lithography and resist coating.Output characteristics of the fabricated FET device were drawn to analyse the conducting nature of our WSe2 single crystal.FET device was tested by applying different gate bi

17、as voltages (positive to negative values).It was observed that on increasing the gate voltages drain current shows increasing trend thereby showing n-type conducting behaviour of CVT synthesized WSe2 single crystals.WSe2