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

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 MoS 2 exfoliated on ultraflat Au films (0.16−0.21 nm roughness) and finally transferred to an insulating Al 2 O 3 substrate. Raman mapping and correlative analysis of the E′ and A 1 ′ peak positions revealed a moderate tensile strain (ε ≈ 0.2%) and p-type doping (n ≈ −0.25 × 10 13 cm −2 ) of 1L MoS 2 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 MoS 2 on Au, with a broad distribution of tunneling barrier values (Φ B from 0.7 to 1.7 eV) consistent with p-type doping of MoS 2 . After the final transfer of 1L MoS 2 on Al 2 O 3 /Si, the strain was converted to compressive strain (ε ≈ −0.25%). Furthermore, an n-type doping (n ≈ 0.5 × 10 13 cm −2 ) was deduced by Raman mapping and confirmed by electrical measurements of an Al 2 O 3 /Si back-gated 1L MoS 2 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.

Growth of p-doped 2D-MoS$_2$ on metal oxides from spatial atomic layer deposition

“…3c, that points towards a significant density of excess charges. From the Raman mode positions we again infer that the MoS 2 here is single layer [31], n-doped with n e =0.2 × 10 13 cm −2 , and experiences a tensile strain of 0.3 % [28][29][30]. The strain is comparable to that observed on the SiO 2 substrate (see Fig.…”

“…3c), exhibiting an even smaller difference of 20 cm −1 . The E 1 2g mode shifts to 385 cm −1 which shows that the strain of the material is now negligible with <0.02 % and thus comparable to the reference value of freestanding MoS 2 [29,33]. This relaxation is probably connected to the reduced size of the flakes.…”

“…From the position of the Raman modes we deduced the strain and doping of the grown flakes following the approaches presented in Refs. [28][29][30]. We find that MoS 2 on SiO 2 exhibits a tensile strain of 0.45 % and is highly n-doped with n e =1.0 × 10 13 cm −2 , and on sapphire it shows a compressive strain of −0.3 % and is similarly n-doped with n e =1.2 × 10 13 cm −2 .…”

Multiscale Investigation of the Structural, Electrical and Photoluminescence Properties of MoS2 Obtained by MoO3 Sulfurization

“…Top-down synthesis approaches used to separate MoS 2 from bulk crystals, such as mechanical exfoliation [18,19], gold-assisted exfoliation [20][21][22][23][24], and liquid exfoliation [25], are not suitable to ensure the reproducibility and thickness control on a wafer scale required for high-end electronic applications. For this reason, bottom-up approaches as Chemical Vapour Deposition (CVD) [26,27], Pulsed Laser Deposition (PLD) [28], Molecular Beam Epitaxy (MBE) [29], and Atomic Layer Deposition (ALD) [30] represent the most promising methods to obtain a reproducible thin film of TMDs on a large area.…”

Substrate-Driven Atomic Layer Deposition of High-κ Dielectrics on 2D Materials

“…More recently, the direct thermal ALD of an ultra-thin and pinhole-free Al 2 O 3 layer has been demonstrated on monolayer (1 L) MoS 2 residing on a gold substrate [22]. During the last few years, several research groups have investigated mechanical exfoliation on gold as an effective way to separate large-area (cm 2 ) 1 L MoS 2 membranes from bulk molybdenite crystals by exploiting the strong S-Au interactions [71][72][73]. Furthermore, the exfoliated membranes could be transferred from gold to insulating substrates, demonstrating electrical and optical properties comparable to the best-quality MoS 2 crystals [71,74].…”