Manipulation and characterization of nanometer sized WS₂/MoS₂ heterostructures on graphene

In this work we demonstrate a method to transfer an MBE grown transition metal dichalcogenide (TMD) heterostructure (HS) to a SiO₂/Si wafer that is a more suitable substrate for investigation of optical properties and device fabrication. The original sample was prepared by heteroepitaxial growth of TMDs and graphene under UHV conditions. The resulting sample consists of lateral and vertical quasi-freestanding heterostructures WS₂/MoS₂/Graphene/Mo on Ir(111), with TMD islands of lateral size below 100nm. Scanning tunnelling spectroscopy (STS) reveals type-II band alignment for the lateral and a symmetric upward bend bending for the vertical HS. [1]



The large, continuous graphene layer beneath the TMD HSs allows for the transfer from Ir(111) to a SiO₂/Si wafer. AFM of the transferred HS reveals mostly uniform height across the sample area of interest. Micro-Raman spectroscopy confirms the presence of the TMD HS on SiO₂/Si wafer and fluorescence spectroscopy reveals the characteristic exciton resonances which blueshift approaching the low temperature (4.2 K) limit, confirming its semiconducting nature [2]. Comparison of optical bandgap with STS measurements of quasiparticle bandgap gives the value of binding energy for both WS₂ and MoS₂ exciton A.



Wide-field hyperspectral fluorescence microscopy yields spatially and spectrally resolved images of the sample revealing spectrally homogeneous areas over hundreds of microns.



The combination of MBE growth and electrochemical delamination transfer technique provides a controllable and scalable new method of preparing 2D materials on any desirable surface suitable for application in electronic and optoelectronic devices.

References

[1] B. Pielic et al 2021 “Electronic structure of quasi-freestanding WS2/MoS2 heterostructures” ACS Appl. Mater. Interfaces 2021, 13, 42, 50552–50563, 10.1021/acsami.1c15412

[2] K. P. O’Donnel, X. Chen, Appl. Phys. Lett. 58, 2924 (1991);