Strain, Doping, and Electronic Transport of Large Area Monolayer MoS2 Exfoliated on Gold and Transferred to an Insulating Substrate.

Salvatore Ethan Panasci,Marco Cannas,Simonpietro Agnello,Filippo Giannazzo,Franco M Gelardi,Emanuela Schilirò,Giuseppe Greco,Fabrizio Roccaforte

doi:10.1021/acsami.1c05185

Abstract

Gold-assisted mechanical exfoliation currently represents a promising method to separate ultralarge (centimeter scale) transition metal dichalcogenide (TMD) monolayers (1L) with excellent electronic and optical properties from the parent van der Waals (vdW) crystals. The strong interaction between Au and chalcogen atoms is key to achieving this nearly perfect 1L exfoliation yield. On the other hand, it may significantly affect the doping and strain of 1L TMDs in contact with Au. In this paper, we systematically investigated the morphology, strain, doping, and electrical properties of large area 1L MoS2 exfoliated on ultraflat Au films (0.16–0.21 nm roughness) and finally transferred to an insulating Al2O3 substrate. Raman mapping and correlative analysis of the E′ and A1′ peak positions revealed a moderate tensile strain (ε ≈ 0.2%) and p-type doping (n ≈ −0.25 × 1013 cm–2) of 1L MoS2 in contact with Au. Nanoscale resolution current mapping and current–voltage (I–V) measurements by conductive atomic force microscopy (C-AFM) showed direct tunneling across the 1L MoS2 on Au, with a broad distribution of tunneling barrier values (ΦB from 0.7 to 1.7 eV) consistent with p-type doping of MoS2. After the final transfer of 1L MoS2 on Al2O3/Si, the strain was converted to compressive strain (ε ≈ −0.25%). Furthermore, an n-type doping (n ≈ 0.5 × 1013 cm–2) was deduced by Raman mapping and confirmed by electrical measurements of an Al2O3/Si back-gated 1L MoS2 transistor. These results provide a deeper understanding of the Au-assisted exfoliation mechanism and can contribute to its widespread application for the realization of novel devices and artificial vdW heterostructures.

Highlights

Semiconducting transition metal dichalcogenides (TMDs) are a class of two-dimensional (2D) layered materials with the general chemical formula MX2, where M is a transition metal (Mo, W, ...) and X is a chalcogen (S, Se, ...), which are characterized by strong in-plane bonds and weak van der Waals interactions between the layers.[1]
We investigated the morphology, strain, doping, and electrical properties of 1L MoS2 exfoliated on ultraflat Au films and transferred to an Al2O3/Si substrate
Roughness of 0.16 nm, as deduced from the tapping mode atomic force microscopy (AFM) image reported in Figure S2 of the Supporting Information

Summary

Introduction

Semiconducting transition metal dichalcogenides (TMDs) are a class of two-dimensional (2D) layered materials with the general chemical formula MX2, where M is a transition metal (Mo, W, ...) and X is a chalcogen (S, Se, ...), which are characterized by strong (covalent) in-plane bonds and weak van der Waals (vdW) interactions between the layers.[1]. In spite of the reported progress in the scalable exfoliation (lithiation/sonication,[18] electrochemical exfoliation,[19] etc.) and large area deposition of TMDs (chemical vapor deposition,[20,21] molecular beam epitaxy,[22] pulsed laser deposition,[23] etc.), mechanical exfoliation still remains a method of choice for investigating basic physical phenomena and demonstrating new device concepts, due to the superior quality of the material produced by this approach.[24,25]. The so-called “gold-assisted” mechanical exfoliation approach showed the possibility of separating large area (cm2) monolayer MoS2 (1L MoS2) from a bulk crystal stamp by exploiting the strong affinity between a gold film and the topmost sulfur atoms of MoS2.26−29 The exfoliation process from the bulk stamp can be repeated many times, producing flakes with reproducible geometry at each exfoliation step, with a size limited only by the dimensions of currently available bulk samples.

Methods

Results

Conclusion