Formation of Highly Ordered Self-Assembled Monolayers on Two-Dimensional Materials via Noncovalent Functionalization

Abstract

Functionalized two-dimensional materials (2DMs) are attracting much attention due to their promising applications in nanoscale devices. Producing continuous and homogeneous surface assemblies with a high degree of order has been challenging. In this work, we demonstrate that by noncovalently self-assembling molecular platforms on 2DMs, high-quality and highly ordered monolayers can be generated. The high degree of order and uniformity of the self-assembled monolayer layers were confirmed by a variety of analytic techniques including time-of-flight secondary ion mass spectrometry, scanning tunnelling microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. Furthermore, by selectively enhancing the molecular vibrations of the molecular platform, via a combination of graphene-enhanced Raman spectroscopy (GERS) and surface-enhanced Raman spectroscopy (SERS), we were able to determine the orientation of self-assembled molecular platforms with respect to the surface normal. The selective enhancement of the vibrational modes occurs by taking advantage of the distance dependence of the Raman enhancement either by the graphene surface (GERS) or the silver nanoparticules (SERS) that are located on top of the self-assembled monolayer.

Enhancement of 1T‐MoS2 Superambient Temperature Stability and Hydrogen Evolution Performance by Intercalating a Phenanthroline Monolayer

“…Notably, the Phen molecule lies almost entirely over the ridges of the nanorelief (Figure 5b and Figure S6), differing in this respect with the previously described alkylamine guests tending to occupy the valleys [31] . Such a disposition of the Phen molecules provides the shortest C−S and N−S distances thus favoring involvement of the aromatic π‐system in the interaction with sulfur (Figure 5c) similarly to the recently reported π‐stacked MoS 2 assembly with another aromatic heterocycle, triazatriangulene [52] . Water molecule stabilizes the PhenH + cation implicating its additional H atom in NH…O hydrogen bonding, simultaneously forming the OH…N and OH…S bonds with second nitrogen and sulfur atoms as H‐donor (Figure 5d).…”

Modulating the Optoelectronic Properties of MoS2 by Highly Oriented Dipole-Generating Monolayers

“…To resolve this problem, we have recently utilized azobenzene-decorated triazatriangulenes (AzoTATA) as promising candidates for the highly oriented noncovalent surface functionalization of 2DMs . Originally developed by Herges and coworkers for functionalizing planar gold crystals, − these functional molecules feature a photoresponsive azobenzene head group (Figure A, orange A) bound via ethynyl spacer to a triazatriangulene platform group (Figure A, red T).…”

“…Originally developed by Herges and coworkers for functionalizing planar gold crystals, − these functional molecules feature a photoresponsive azobenzene head group (Figure A, orange A) bound via ethynyl spacer to a triazatriangulene platform group (Figure A, red T). This broad aromatic platform moiety adsorbs to planar surfaces in a well-ordered fashion across the face of the material with a packing density of 0.75 molecules per nm –2 , with the azobenzene head group directed perpendicular to the surface normal. This highly oriented architecture not only ensures facile separation of the azobenzene functionality from the 2DM surface but likewise strictly directs the functional group perpendicular to the surface normal.…”

Light-Programmable Logic-in-Memory in 2D Semiconductors Enabled by Supramolecular Functionalization: Photoresponsive Collective Effect of Aligned Molecular Dipoles

“…28,31,32 Recently, this class of molecule has been proven to enhance Raman signals and modulate the work functions of 2D materials. 33,34 However, production of the TATA platform requires a multistep cumbersome synthesis, whereas a thermodynamically driven approach based on molecular self-assembly would be more convenient. Among others, metal porphyrins and phthalocyanines are wellestablished molecular scaffolds that are usually flatly physisorbed on 2D materials and could also provide such functionality for growing axially bonded azobenzene ligands in a more versatile and modular manner.…”

“…The latter would enable maximization of the influence on the electronic properties of 2D materials underneath. Several previous works showed that triazatriangulenium (TATA) ions on the surface could be an appropriate platform to bind azobenzene with ethinyl or phenyl spacers where the photoisomerization of freestanding azobenzene is observed by scanning tunneling microscopy (STM). ,, Recently, this class of molecule has been proven to enhance Raman signals and modulate the work functions of 2D materials. , However, production of the TATA platform requires a multistep cumbersome synthesis, whereas a thermodynamically driven approach based on molecular self-assembly would be more convenient. Among others, metal porphyrins and phthalocyanines are well-established molecular scaffolds that are usually flatly physisorbed on 2D materials and could also provide such functionality for growing axially bonded azobenzene ligands in a more versatile and modular manner. − As we have previously demonstrated, the axial coordination of metal and functional pyridines is a viable route to generate a vertical electrical field to monolayer MoS 2 thanks to different alignments and magnitudes of molecular dipoles (taking advantage of the confinement of crystal field of octahedral transition metal d-orbital of zinc and cobalt phthalocyanine). ,− The method has also been proven recently for stable growth of larger π-conjugated pyridinic ligands .…”