Large-Area, Two-Dimensional MoS₂ Exfoliated on Gold: Direct Experimental Access to the Metal–Semiconductor Interface

Abstract

Two-dimensional semiconductors such as MoS 2 are promising for future electrical devices. The interface to metals is a crucial and critical aspect for these devices because undesirably high resistances due to Fermi level pinning are present, resulting in unwanted energy losses. To date, experimental information on such junctions has been obtained mainly indirectly by evaluating transistor characteristics. The fact that the metal− semiconductor interface is typically embedded, further complicates the investigation of the underlying physical mechanisms at the interface. Here, we present a method to provide access to a realistic metal−semiconductor interface by large-area exfoliation of single-layer MoS 2 on clean polycrystalline gold surfaces. This approach allows us to measure the relative charge neutrality level at the MoS 2 −gold interface and its spatial variation almost directly using Kelvin probe force microscopy even under ambient conditions. By bringing together hitherto unconnected findings about the MoS 2 −gold interface, we can explain the anomalous Raman signature of MoS 2 in contact to metals [ACS Nano. 7, 2013, 11350] which has been the subject of intense recent discussions. In detail, we identify the unusual Raman mode as the A 1g mode with a reduced Raman shift (397 cm −1 ) due to the weakening of the Mo−S bond. Combined with our X-ray photoelectron spectroscopy data and the measured charge neutrality level, this is in good agreement with a previously predicted mechanism for Fermi level pinning at the MoS 2 −gold interface [Nano Lett. 14, 2014, 1714. As a consequence, the strength of the MoS 2 −gold contact can be determined from the intensity ratio between the reduced A 1g reduced mode and the unperturbed A 1g mode.

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

“…The A 1g mode red-shifts to 405 cm −1 and therefore the material here is p-doped with n h =0.1 × 10 13 cm −2 [29,34]. The observed change in doping is in striking contrast to values reported in literature so far, where MoS 2 usually shows n-type doping on Al 2 O 3 and SiO 2 [29,35,36], while p-type doping has been observed so far only on metallic substrates, as for example, Au [29,37]. P-type doping of MoS 2 can be achieved in different ways [34,38], e.g., also by adding appropriate reactants in the CVD process but the resulting carrier density determined in a field-effect transistor was an order of magnitude lower [39].…”

Signatures of the Dichalcogenide–Gold Interaction in the Vibrational Spectra of MoS2 and MoSe2 on Au(111)

“…16,17 Moreover, shifts or splittings of the Raman peaks can provide information about the interaction between the film and substrate. For MoS 2 films on gold, several experimental groups 7,8 have reported a downshift and a splitting of the E 2g (1) peak, whereas the A 1g peak develops a satellite at a lower frequency, slightly below 400 cm satellites had been observed for a WS 2 monolayer when covered with a thin gold layer. 18 For the MoS 2 monolayer, previous results for Au deposition had been indecisive: some researchers reported only a strain-induced splitting of the E 2g…”

“…In thin films, symmetry selection rules determine which of the composite modes in multilayered films are Raman-active; hence, this technique allows experimentalists to determine the number of layers in a film. , Moreover, shifts or splittings of the Raman peaks can provide information about the interaction between the film and substrate. For MoS 2 films on gold, several experimental groups , have reported a downshift and a splitting of the E 2g (1) peak, whereas the A 1g peak develops a satellite at a lower frequency, slightly below 400 cm –1 . Already before, similar satellites had been observed for a WS 2 monolayer when covered with a thin gold layer .…”

“…However, one of the best ways to obtain such samples is still the exfoliation of TMDCs from bulk with the help of metallic surfaces, in particular, gold surfaces. , In this paper, we report a computational study of the interaction of TMDCs, specifically MoS 2 and MoSe 2 , with a gold substrate. This substrate is of particular interest since it interacts with TMDCs sufficiently strongly ,, (compared to chemically inert and/or insulating substrates; see, e.g., refs , , ) while leaving the chemical structure of the TMDC intact. In addition to its role in exfoliation, the interface between TMDCs and metal surfaces is highly relevant for the formation of electronic contacts on TMDCs. − …”

“…By comparing the vibrational density of states (DOS) of adsorbed and free-standing monolayers, we disentangle the contributions of both mechanical strain effects and electronic interaction effects to vibrational frequency shifts. We aim at identifying fingerprints in the vibrational spectra characteristic of adsorption and attempt to rationalize the recent experimental finding of an additional Raman peak for MoS 2 adsorbed on gold. After reporting the computational details, we discuss the calculated vibrational spectra for both MoS 2 and MoSe 2 monolayers.…”