Excitation Energy Transfer in Nanohybrid System of Organic Molecule and Inorganic Transition Metal Dichalcogenides Nanoflake

TL;DR

This work develops a quantitative theory of excitation energy transfer from a single 6P molecule to a finite MoS$_2$ nanoflake using an $11$-band tight-binding model with hydrogen passivation to remove edge states and a configuration-interaction-based description of MoS$_2$ excitations, simplified to uncorrelated electron-hole pairs for efficiency. EET rates are computed via Fermi's golden rule, incorporating Coulomb coupling between molecular transition charges and semiconductor transition charges, along with spectral overlap encoded in donor/acceptor DOS. Key findings show that molecule-to-nanoflake energy transfer dominates the process, with rates strongly dependent on vertical distance, lateral position, and nanoflake size, while the coupling magnitudes remain modest (sub-10 meV) and governed by spectral resonance conditions. The framework offers a practical, parameter-free route to understand and optimize non-contact EET in TMDC-organic hybrids and can be extended to few-layer TMDC heterostructures for device-relevant applications.

Abstract

Excitation energy transfer (EET) in an organic/inorganic nanohybrid system, composed of a single \textit{para}-sexiphenyl (6P) molecule physisorbed on a finite-sized MoS$_2$ nanoflake, is investigated theoretically. % The electronic structure of the MoS$_2$ nanoflake is described by using an 11-band tight-binding model, in which edge states are passivated with H atoms to restore a well-defined bandgap. % Within a configuration-interaction scheme, excitonic states are constructed and, for computational efficiency, approximated by uncorrelated electron-hole pairs in the relevant high-energy window. % The EET rates are evaluated via Fermi's golden rule, incorporating Coulomb coupling, thermal broadening, and spectral overlap between the molecular excitation and the MoS$_2$ nanoflake's electron-hole pairs. % Our results reveal that energy transfer from the molecule to the nanoflake is the dominant process, and its efficiency depends strongly on the size of the MoS$_2$ nanoflake, as well as the molecule's vertical distance and lateral position relative to the nanoflake.

Figures (6)

• Figure 1: Schematic illustration of nanohybrid system formed by the H-passivated MoS$_2$ nanoflake and the 6P molecule. The upper part shows the top and the side views, while the lower part displays an enlarged top view (yellow sphere: S, gray sphere: Mo, cyan sphere: H, green sphere: C). $x$, $y$, $z$ denote Cartesian coordinates.
• Figure 2: Density of single-electron states $|\varphi_a ({\bf r})|^2$ referring to unpassivated (a) the conduction-band like state and (b) valence-band like state, as well as (c,d) the H-passivated cases. (e) Total DOS for unpassivated (black solid line) and passivated (red dashed line) cases.
• Figure 3: Top view on exciton transition densities $\rho$ for the MoS$_2$ nanoflake and the 6P molecule. Shown are isosurfaces embedded in the atomic lattice. The isosurfaces are obtained as the real part of $\rho_{\alpha 0}$ referring to the (a) 1st, (b) 3rd and (c) 10th exciton level of the MoS$_2$ nanoflake as well as (d) the single excitation of the 6P molecule. Blue and red colored parts indicate positively and negatively charged areas, respectively.
• Figure 4: Absolute values of the EET coupling $V_{e0, a\bar{a}\,g}$ between MoS$_2$ electron-hole pairs and the 6P molecule (red sticks) drawn versus energies of the electron-hole pairs. The molecule is placed at the center of the MoS$_2$ nanoflake with a minimal distance of 2.0 Å. The black (blue) solid line is the combined DOS $\mathcal{D}^{(\text{D})}_{e\to g}$ ($\mathcal{D}^{(\text{A})}_{g\to e}$) from Eq. \ref{['D2']} (Eq. \ref{['D1']}).
• Figure 5: EET rates $k_{\rm mol \to sem}$ (a) and EET couplings $|V_{e0, a\bar{a}\,g}|$ (b) versus the distance along $z$-axis between the MoS$_2$ surface and the 6P molecule. The side lengths of MoS$_2$ nanoflakes are set to 60, 70 and 74 Å.