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.
Researchers around the world have observed the formation of molecularly ordered structures of unknown origin on the surface of titanium dioxide (TiO) photocatalysts exposed to air and solution. Using a combination of atomic-scale microscopy and spectroscopy, we show that TiO selectively adsorbs atmospheric carboxylic acids that are typically present in parts-per-billion concentrations while effectively repelling other adsorbates, such as alcohols, that are present in much higher concentrations. The high affinity of the surface for carboxylic acids is attributed to their bidentate binding. These self-assembled monolayers have the unusual property of being both hydrophobic and highly water-soluble, which may contribute to the self-cleaning properties of TiO This finding is relevant to TiO photocatalysis, because the self-assembled carboxylate monolayers block the undercoordinated surface cation sites typically implicated in photocatalysis.
Anisotropic etchants selectively reveal a specific crystallographic plane. Although prized industrially, these etchants are poorly understood because they target specific defect sites on a surface. New methods, which rely on a combination of scanning tunneling microscopy, kinetic Monte Carlo simulations, and infrared spectroscopy, have been developed to quantify these reactions. By correlating the measured reaction rates with the structure of the defects, information about reaction mechanisms can be obtained. These techniques have also been extended to allow for the quantification of impurity reactions such as the reaction of dissolved O2, and of nonetching additives, such as alcohols. A complementary macroscopic technique, which utilizes microfabricated arrays of miscut surfaces to measure orientation-dependent kinetics, is also described.
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