Scanning tunneling microscopy investigation of nanostructures produced by Ar+ and He+ bombardment of MoS2 surfaces

Abstract

Nanostructures were fabricated on natural MoS 2 crystals by bombardment with low doses of Ar + and He + with energies ranging from 100 to 5 keV. The bombarded surfaces were investigated with x-ray photoemission spectroscopy ͑XPS͒ and scanning tunneling microscopy ͑STM͒ in an ultrahigh vacuum environment. The ion exposures were low enough to ensure that the observed nanostructures can be associated with individual ion impacts. Argon ions ͑Ar + ͒ with energies of 100 eV or less remove very few, if any, sulfur atoms from the surface but STM and XPS studies reveal that the electronic structure of the MoS 2 surface is altered. Ar + with energies greater than 100 eV has a higher probability of sputtering sulfur atoms from the surface. The apparent size of the nanostructures in the STM images increased with Ar + energies up to about 1 keV and was dependent on the angle of incidence of the Ar + . Helium ion ͑He + ͒ sputtering of MoS 2 produced similar but smaller nanostructures when compared to Ar + at the same impinged ion energy. STM images showed bright ring-shaped features were created with He + energies greater than 500 eV. On the basis of XPS and current imaging tunneling spectroscopy investigations, the features are assigned to sulfur atom vacancies. A change in the surface doping type from n to p was observed upon light sputtering of the surface.

Influence of Stoichiometry on the Optical and Electrical Properties of Chemical Vapor Deposition Derived MoS2

“…The increase in electron doping can be explained by an increasing concentration of sulfur vacancies acting as electron donors, but it is perhaps surprising that the field-effect mobility also increases. 9 , 10 Below we examine whether the PL data are also consistent with the proposed variation in sulfur vacancy concentration, and then consider possible explanations for the trends in field effect mobility.…”

Oxide-mediated recovery of field-effect mobility in plasma-treated MoS 2

“…A recent study has shown that substoichiometric MoO 3− x can be volatile and even removed by Ar flow at low temperatures ( 36 ). In the O 2 /Ar plasma physical sputtering–dominated regime, the surface roughness is seen to increase by more than 1 nm at 28 s of exposure due to Ar + -related etching and redeposition of material on the surface ( 15 , 16 ). However, the unchanging surface roughness up until 8 s indicates initial direct conversion of MoS 2 into a planar oxide.…”

Native defects in bulk and monolayerMoS2from first principles

“…In addition to the native defects unintentionally introduced during growth, defects may also be produced deliberately at the postgrowth stage. This can be achieved by electron irradiation [38,45], ion bombardment [37,[46][47][48][49][50][51][52], chemical treatment such as selenization [53], plasma treatment [54], STM voltage pulsing [55,56], or vacuum annealing [32,37,[57][58][59]. Such processes are useful as they allow controlled modification of the material properties after the growth.…”