Leveraging Molecular Properties to Tailor Mixed-Dimensional Heterostructures beyond Energy Level Alignment

Abstract

The surface sensitivity and lack of dielectric screening in two-dimensional (2D) materials provide numerous intriguing opportunities to tailor their properties using adsorbed π-electron organic molecules. These organic–2D mixed-dimensional heterojunctions are often considered solely in terms of their energy level alignment, i.e., the relative energies of the frontier molecular orbitals versus the 2D material conduction and valence band edges. While this simple model is frequently adequate to describe doping and photoinduced charge transfer, the tools of molecular chemistry enable additional manipulation of properties in organic–2D heterojunctions that are not accessible in other solid-state systems. Fully exploiting these possibilities requires consideration of the details of the organic adlayer beyond its energy level alignment, including hybridization and electrostatics, molecular orientation and thin-film morphology, nonfrontier orbitals and defects, excitonic states, spin, and chirality. This Perspective explores how these relatively overlooked molecular properties offer unique opportunities for tuning optical and electronic characteristics, thereby guiding the rational design of organic–2D mixed-dimensional heterojunctions with emergent properties.

Angstrom-Scale Spectroscopic Visualization of Interfacial Interactions in an Organic/Borophene Vertical Heterostructure

“…In addition to enabling the introduction of long-term and reversible strains, molecular adsorption and STM tip manipulation offer a powerful means to achieve atomic control of strain and strain-coupled properties, such as phonon vibration, electronic structure, thermal conductivity, carrier mobility, and chemical reactivity. In particular, the band-gap structure and transport properties can be modified via strain engineering and further tailored by virtue of the tunable anisotropy of borophene polymorphs, which suggests tremendous promise for (opto-)­electronic applications . Moreover, the superconducting transition temperature T c can be dramatically tuned by subtly changing lattice parameters.…”

Donors, Acceptors, and a Bit of Aromatics: Electronic Interactions of Molecular Adsorbates on hBN and MoS$_2$ Monolayers

“…Hybrid materials formed by carbon-conjugated molecules adsorbed on low-dimensional semiconductors and insulators have been attracting attention due to the their structural versatility and electronic tunability. [1][2][3][4][5][6][7][8][9] Depending on their density on the substrate and on their physico-chemical characteristics, physisorbed moieties can introduce localized electronic states, [10][11][12] dispersive bands, 7 or a combination thereof. [13][14][15] The electronic structure of the interface results from the level alignment between the organic and inorganic components [16][17][18][19][20] and the hybridization between their electronic wave-functions.…”

“…The former is a known insulator, widely used as a substrate and/or as an encapsulating material in low-dimensional heterostructures, 17 which has been receiving increasing attention in surface and interface science, [31][32][33][34][35][36][37] for instance to sustain the growth of well-defined organic thin films. 9,38,39 MoS 2 belongs to the family of transition-metal dichalcogenides, the most promising emerging class of lowdimensional semiconductors. By performing geometry optimizations using the generalizedgradient approximation (GGA) and refining the analysis of the electronic structure using a range-separated hybrid functional, we rationalize how the nature of the constituents of the hybrid interface determines the level alignment and the projected density of states.…”

Mechanistic Investigation of Molybdenum Disulfide Defect Photoluminescence Quenching by Adsorbed Metallophthalocyanines

“…11 While MoS 2 has exceptional inherent optoelectronic properties, there are new opportunities in combining it with other classes of materials to form mixed-dimensional heterojunctions. 12,13 These interfaces leverage the unique properties of their constituent materials to realize new optoelectronic performance metrics, heterojunction device concepts, and emergent phenomena that are not possible in either isolated system. In this regard, the interface of molecular materials and monolayer MoS 2 is of particular interest due to the broad synthetic tunability, controllable assembly, and facile processing of the former as well as the exceptional optical and electrical properties of the latter.…”

“…Among the TMDs, MoS 2 is the most widely studied and represents an informative model system for exploring the fundamental physics and chemistry of TMDs . While MoS 2 has exceptional inherent optoelectronic properties, there are new opportunities in combining it with other classes of materials to form mixed-dimensional heterojunctions. , These interfaces leverage the unique properties of their constituent materials to realize new optoelectronic performance metrics, heterojunction device concepts, and emergent phenomena that are not possible in either isolated system. In this regard, the interface of molecular materials and monolayer MoS 2 is of particular interest due to the broad synthetic tunability, controllable assembly, and facile processing of the former as well as the exceptional optical and electrical properties of the latter. − Molecular heterojunctions have been used extensively to modulate the photoluminescence (PL) of monolayer MoS 2 , including quenching the PL by charge transfer, , augmenting the emission via energy transfer, enhancing Raman scattering, and increasing the PL quantum yield (PLQY) by defect passivation …”