Dry Transfer of van der Waals Crystals to Noble Metal Surfaces To Enable Characterization of Buried Interfaces

Abstract

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have been explored for many optoelectronic applications. Most of these applications require them to be on insulating substrates. However, for many fundamental property characterizations, such as mapping surface potential or conductance, insulating substrates are nonideal as they lead to charging and doping effects or impose the inhomogeneity of their charge environment on the atomically thin 2D layers. Here, we report a simple method of residue-free dry transfer of 2D TMDC crystal layers. This method is enabled via noble-metal (gold, silver) thin films and allows comprehensive nanoscale characterization of transferred TMDC crystals with multiple scanning probe microscopy techniques. In particular, intimate contact with underlying metal allows efficient tip-enhanced Raman scattering characterization, providing high spatial resolution (<20 nm) for Raman spectroscopy. Further, scanning Kelvin probe force microscopy allows high-resolution mapping of surface potential on transferred crystals, revealing their spatially varying structural and electronic properties. The layer-dependent contact potential difference is clearly observed and explained by charge transfer from contacts with Au and Ag. The demonstrated sample preparation technique can be generalized to probe many different 2D material surfaces and has broad implications in understanding of the metal contacts and buried interfaces in 2D material-based devices.

Direct Optoelectronic Imaging of 2D Semiconductor–3D Metal Buried Interfaces

“…The electron beam evaporation technique was used to deposit Au (see Methods and Figure S9). 22 Thermal evaporation and sputtering were also investigated (see Supporting Information S8 for details). Due to the high kinetic energy of incoming Au atoms during evaporation, Au atoms are likely to strongly couple with S atoms on the MoS 2 surface, leading to an intimate electronic contact with negligible tunneling or Schottky barriers.…”

“…The sample fabrications process (Supplementary Figure S9) is identical to the process described in a previous paper . Briefly, mono- and bilayer MoS 2 were mechanically exfoliated on a Si/SiO 2 substrate.…”

“…The sample fabrications process (Supplementary Figure S9) is identical to the process described in a previous paper. 22 Briefly, mono-and bilayer MoS 2 were mechanically exfoliated on a Si/SiO 2 substrate. A Au layer of 100 nm was deposited asexfoliated MoS 2 by either e-beam evaporator (Kurt J. Lesker PVD 75 PRO-Line e-beam evaporator) or a thermal evaporator (Kurt J. Lesker Nano 36 thermal evaporator).…”

The Intricate Love Affairs between MoS2 and Metallic Substrates

“…[ 2b ] The polymer‐free nature of this transfer, which leaves surfaces free from residual contamination, is of significant advantage also. [ 6 ] Despite these research efforts, it is currently unknown whether this method can also be applied to other metals, predicted to exhibit even stronger binding with MoS 2 than Au. [ 7 ]…”

Uncovering topographically hidden features in 2D MoSe2 with correlated potential and optical nanoprobes

“…In this study, topographically-hidden features within large area MoSe 2 flakes were evaluated using KPFM and correlated to local TEPL mapping, enabling a nondestructive quantitative evaluation of grain boundaries, edges, and oxidized regions within the structure. Both of these techniques were performed without the need to transfer the film from the growth substrate and are entirely non-destructive, unlike the case for tip-enhanced Raman spectroscopy, which requires transparent substrates 33 or transfer to metallic films 34,35 . By evaluating the monolayer 2D materials on the native growth substrates, intrinsic nanoscale features of these exciting materials on silicon substrates can be evaluated independent of the influence of film transfer.…”