Exfoliation of large-area monolayers is important for fundamental research and technological implementation of transition-metal dichalcogenides. Various techniques have been explored to increase the exfoliation yield, but little is known about the underlying mechanism at the atomic level. Here, we demonstrate gold-assisted mechanical exfoliation of monolayer molybdenum disulfide, up to a centimeter scale. Detailed spectroscopic, microscopic, and first-principles density functional theory analyses reveal that strong van der Waals (vdW) interaction between Au and the topmost MoS2 layer facilitates the exfoliation of monolayers. However, the large-area exfoliation promoted by such strong vdW interaction is only achievable on freshly prepared clean and smooth Au surfaces, while rough surfaces and surfaces exposed to air for more than 15 min result in negligible exfoliation yields. This technique is successfully extended to MoSe2, WS2, WSe2, MoTe2, WTe2, and GaSe. In addition, electrochemical characterization reveals intriguing interactions between monolayer MoS2 and Au. A subnanometer-thick MoS2 monolayer strongly passivates the chemical properties of the underlying Au, and the Au significantly modulates the electronic band structure of the MoS2, turning it from semiconducting to metallic. This could find applications in many areas, including electrochemistry, photovoltaics, and photocatalysis.

Non-volatile resistive switching, also known as memristor 1 effect in two terminal devices, has emerged as one of the most important components in the ongoing development of high-density information storage, brain-inspired computing, and reconfigurable systems 2-9 . Recently, the unexpected discovery of memristor effect in atomic monolayers of transitional metal dichalcogenide sandwich structures has added a new dimension of interest owing to the prospects of size scaling and the associated benefits 10 . However, the origin of the switching mechanism in atomic sheets remains uncertain. Here, using monolayer MoS2 as a model system, atomistic imaging and spectroscopy reveal that metal substitution into sulfur vacancy results in a non-volatile change in resistance. The experimental observations are corroborated by computational studies of defect structures and electronic states. These remarkable findings provide an atomistic understanding on the non-volatile switching mechanism and open a new direction in precision defect engineering, down to a single defect, for achieving optimum performance metrics including memory density, switching energy, speed, and reliability using atomic nanomaterials.

Gold-mediated exfoliation of MoS 2 has recently attracted considerable interest. The strong interaction between MoS 2 and Au facilitates preferential production of centimeter-sized monolayer MoS 2 with near-unity yield and provides a heterostructure system noteworthy from a fundamental standpoint. However, little is known about the detailed nature of the MoS 2 –Au interaction and its evolution with the MoS 2 thickness. Here, we identify the specific vibrational and binding energy fingerprints of this interaction using Raman and X-ray photoelectron spectroscopy, which indicate substantial strain and charge doping in monolayer MoS 2 . Tip-enhanced Raman spectroscopy reveals heterogeneity of the MoS 2 –Au interaction at the nanoscale, reflecting the spatial nonconformity between the two materials. Micro-Raman spectroscopy shows that this interaction is strongly affected by the roughness and cleanliness of the underlying Au. Our results elucidate the nature of the MoS 2 –Au interaction and guide strain and charge doping engineering of MoS 2 .

Mechanical exfoliation of 2D materials yields high-quality crystals popular with researchers in fundamental scientific disciplines but its scalability is severely limited. This method generates 2D monolayers tens or hundreds of microns in lateral sizes on most substrates, often after an elaborate surface treatment. [1] Gold-mediated exfoliation of chalcogenides, chlorides, thiophosphates, black phosphorus, and black arsenic, with a robust control of the near-unity monolayer yield at a millimeter-/centimeterscale, has recently emerged as a viable solution to the scalability issues, [2] and has been adopted in various branches of applied research and engineering. [3] In the case of transition metal dichalcogenides (TMDCs), the root of the preferential mono layer exfoliation has been attributed to the strong interactions between gold and chalcogenides, which have been explored in different facets of science for decades. [4] However, it has recently been shown that the interaction between TMDCs and Au is non-covalent and van der Waals (vdW) in its nature, inferred from the sizeable S-Au equilibrium distance (3.5 Å) and binding energies in the Au-MoS 2 heterostructure. [2c,5] The vdW interaction therefore facilitates the transfer of the TMDC monolayers onto non-metallic substrates, which restore their semiconducting characteristics exploitable in optoelectronics, photovoltaics, and related themes. [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] Here, we study the ability of different metallic substrates to exfoliate large-area monolayer MoS 2. We find that gold is by far the best substrate, outperforming all other metals by at least two orders of magnitude in terms of the lateral size of the MoS 2 , thanks to the unique ability of Au to resist oxidation and the sizeable interfacial strain in the Au-MoS 2 heterostructure. A moderate exfoliation yield is achieved for other precious metals, including Pt, Pd, and Ag, while hardly any exfoliated material is found on base metals, including Cu, Ni, Co, Cr, and Ti, which suffer from significant oxidation of their surface upon exposure to air. A correlation between the maximum lateral Mechanical exfoliation yields high-quality 2D materials but is challenging to scale up due to the small lateral size and low yield of the exfoliated crystals. Gold-mediated exfoliation of macroscale monolayer MoS 2 and related crystals addresses this problem. However, it remains unclear whether this method can be extended to other metals. Herein, mechanical exfoliation of MoS 2 on a range of metallic substrates is studied. It is found that Au outperforms all the other metals in their ability to exfoliate macroscale monolayer MoS 2. This is rationalized by gold's ability to resist oxidation, which is compromised on other metals a...

Metals have been increasingly used as substrates in devices based on two-dimensional (2D) materials. However, the high reflectivity of bulk metals results in low optical contrast (<3%) and therefore poor visibility of transparent mono- and few-layer 2D materials on these surfaces. Here we demonstrate that by engineering the complex reflectivity of a purpose-designed multilayer heterostructure composed of thin Au films (2-8 nm) on SiO/Si substrate, the optical contrast of graphene and graphene oxide (GO) can be significantly enhanced in comparison to bulk Au, up to about 3 and 5 times, respectively. In particular, we achieved ∼17% optical contrast for monolayer GO, which is even 2 times higher than that on bare SiO/Si substrate. The experimental results are in good agreement with theoretical simulations. This concept is demonstrated for Au, but the methodology is applicable to other metals and can be adopted to design a variety of high-contrast metallic substrates. This will facilitate research and applications of 2D materials in areas such as plasmonics, photonics, catalysis and sensors.

Extraordinarily high optical contrast is instrumental to research and applications of two-dimensional materials, such as, for rapid identification of thickness, characterisation of optical properties, and quality assessment. With optimal designs of substrate structures and light illumination conditions, unprecedented optical contrast of MoS2 on Au surfaces exceeding 430% for monolayer and over 2600% for bilayer is achieved. This is realised on custom-designed substrates of near-zero reflectance near the normal incidence. In particular, by using an aperture stop to restrict the angle of incidence, high-magnification objectives can be made to achieve extraordinarily high optical contrast in a similar way as the low-magnification objectives, but still retaining the high spatial resolution capability. The technique will allow small flakes of micrometre size to be located easily and identified with great accuracy, which will have significant implications in many applications.

A multilayered biosensor was constructed and found to detect trinitrotoluene (TNT) in ppb concentrations in air both prior to and after detonation of TNT without use of a liquid phosphate buffered saline (PBS) superstrate. The biosensor surface was fabricated from a monoclonal antibody for TNT covalently bound to an 11,11'-dithio-bis(succinimidoylundecanoate) (DSU) self-assembled monolayer immobilized on a thin gold film bonded to a BK7 glass slide. The binding between the immobilized antibody and TNT antigen was detected using surface plasmon resonance spectroscopy (SPRS). Biosensor specificity for TNT was demonstrated with chemical homologues as well as against an unrelated explosive, RDX.

Laser-controlled reduction of individual graphene oxide films provide unprecedented work function tuning with millivolt precision.