Structural and electronic properties of MoS2 and MoSe2 monolayers grown by chemical vapor deposition on Au(111).

The exceptional electronic and photonic properties of the monolayers of transition metal dichalcogenides including the spin-orbit splitting of the valence and conduction bands at the K points of the Brillouin zone make them promising for novel applications in electronics, photonics and optoelectronics. Scalable growth of these materials and understanding of their interaction with the substrate is crucial for these applications. Here we report the growth of MoS2 and MoSe2 monolayers on Au(111) by chemical vapor deposition at ambient pressure as well as the analysis of their structural and electronic properties down to the atomic scale. To this aim, we apply ultrahigh vacuum surface sensitive techniques including scanning tunneling microscopy and spectroscopy, low-energy electron diffraction, X-ray and angle-resolved ultraviolet photoelectron spectroscopy in combination with Raman spectroscopy at ambient conditions. We demonstrate the growth of high-quality epitaxial single crystalline MoS2 and MoSe2 monolayers on Au(111) and show the impact of annealing on the monolayer/substrate interaction. Thus, as-grown and moderately annealed (<100 °C) MoSe2 monolayers are decoupled from the substrate by excess Se atoms, whereas annealing at higher temperatures (>250 °C) results in their strong coupling with the substrate caused by desorption of the excess Se. The MoS2 monolayers are strongly coupled to the substrate and the interaction remains almost unchanged even after annealing up to 450 °C.

Patterned Growth of Transition Metal Dichalcogenide Monolayers and Multilayers for Electronic and Optoelectronic Device Applications.

A simple, large area, and cost-effective soft lithographic method is presented for the patterned growth of high-quality 2D transition metal dichalcogenides (TMDs). Initially, a liquid precursor (Na2 MoO4 in an aqueous solution) is patterned on the growth substrate using the micromolding in capillaries technique. Subsequently, a chemical vapor deposition step is employed to convert the precursor patterns to monolayer, few layers, or bulk TMDs, depending on the precursor concentration. The grown patterns are characterized using optical microscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and photoluminescence spectroscopy to reveal their morphological, chemical, and optical characteristics. Additionally, electronic and optoelectronic devices are realized using the patterned TMDs and tested for their applicability in field effect transistors and photodetectors. The photodetectors made of MoS2 line patterns show a very high responsivity of 7674 A W-1 and external quantum efficiency of 1.49 × 106 %. Furthermore, the multiple grain boundaries present in patterned TMDs enable the fabrication of memtransistor devices. The patterning technique presented here may be applied to many other TMDs and related heterostructures, potentially advancing the fabrication of TMDs-based device arrays.

Chemical Vapor Deposition of High-Optical-Quality Large-Area Monolayer Janus Transition Metal Dichalcogenides.

One-pot chemical vapor deposition (CVD) growth of large-area Janus SeMoS monolayers is reported, with the asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. The formation of these 2D semiconductor monolayers takes place upon the thermodynamic-equilibrium-driven exchange of the bottom Se atoms of the initially grown MoSe2 single crystals on gold foils with S atoms. The growth process is characterized by complementary experimental techniques including Raman and X-ray photoelectron spectroscopy, transmission electron microscopy, and the growth mechanisms are rationalized by first principle calculations. The remarkably high optical quality of the synthesized Janus monolayers is demonstrated by optical and magneto-optical measurements which reveal the strong exciton-phonon coupling and enable an exciton g-factor of -3.3.

Photoactive ultrathin molecular nanosheets with reversible lanthanide binding terpyridine centers.

In recent years, functional molecular nanosheets have attracted much attention in the fields of sensors and energy storage. Here, we present an approach for the synthesis of photoactive metal-organic nanosheets with ultimate molecular thickness. To this end, we apply low-energy electron irradiation induced cross-linking of 4'-(2,2':6',2''-terpyridine-4'-yl)-1,1'-biphenyl-4-thiol self-assembled monolayers on gold to convert them into functional ∼1 nm thick carbon nanomembranes possessing the ability to reversibly complex lanthanide ions (Ln-CNMs). The obtained Ln-CNMs can be prepared on a large-scale (>10 cm2) and inherit the photoactivity of the pristine terpyridine lanthanide complex (Ln(III)-tpy). Moreover, they possess mechanical stability as free-standing sheets over micrometer sized openings. The presented methodology paves a simple and robust way for the preparation of ultrathin nanosheets with tailored photoactive properties for application in photocatalytic and energy conversion devices.