Hybrid Frenkel-Wannier excitons facilitate ultrafast energy transfer at a 2D-organic interface

TL;DR

This work investigates hybrid Frenkel-Wannier excitons at 2D/TMD-OSC interfaces and their role in ultrafast energy transfer. It combines femtosecond momentum microscopy with $G_{0}W_{0}$+$BSE$ calculations for the WSe$_2$/PTCDA heterostructure to identify a hybrid exciton, $hX$, whose wavefunction mixes intralayer Frenkel-like $HOMO\rightarrow LUMO$ with interlayer $VBM\rightarrow LUMO$ contributions. Experimentally and theoretically, the $hX$ forms via a Förster-type energy transfer ($FRET$) from optically excited K-excitons, with onset around $66$ fs, reaching a steady-state population by ~ $150$ fs and decaying on ~ $1.9$ ps, while K- and Σ-excitons remain predominantly intralayer. The results show no strong orbital hybridization of single-particle states, indicating energy transfer is mediated by dipole-dipole interactions rather than band hybridization, and demonstrate how mixed intra-/interlayer character can enhance interfacial energy conversion. This provides a framework for engineering interfacial excitons to optimize energy and charge transfer in 2D-organic heterostructures.

Abstract

Two-dimensional transition metal dichalcogenides (TMDs) and organic semiconductors (OSCs) have emerged as promising material platforms for next-generation optoelectronic devices. The combination of both is predicted to yield emergent properties while retaining the advantages of their individual components. In OSCs the optoelectronic response is typically dominated by localized Frenkel-type excitons, whereas TMDs host delocalized Wannier-type excitons. However, much less is known about the spatial and electronic characteristics of excitons at hybrid TMD/OSC interfaces, which ultimately determine the possible energy and charge transfer mechanisms across the 2D-organic interface. Here, we use ultrafast momentum microscopy and many-body perturbation theory to elucidate a hybrid exciton at an TMD/OSC interface that forms via the ultrafast resonant Förster energy transfer process. We show that this hybrid exciton has both Frenkel- and Wannier-type contributions: Concomitant intra- and interlayer electron-hole transitions within the OSC layer and across the TMD/OSC interface, respectively, give rise to an exciton wavefunction with mixed Frenkel-Wannier character. By combining theory and experiment, our work provides previously inaccessible insights into the nature of hybrid excitons at TMD/OSC interfaces. It thus paves the way to a fundamental understanding of charge and energy transfer processes across 2D-organic heterostructures.

Figures (12)

• Figure 1: Sample layout and electronic structure of the hybrid WSe$_2$/PTCDA heterostructure.a Sketch of the layered sample structure and real-space photoemission image. The real-space region-of-interest addressed in the momentum-resolved photoemission measurement is marked by the white dashed circle, while the hBN flake and the WSe$_2$ monolayer are indicated by colored lines. b Experimentally determined superstructure of PTCDA adsorbed on WSe$_2$ monolayer (cf. extended Fig. \ref{['extFig_SamplePrep']}). c Energy-momentum cut of the static photoemission spectrum along the $\Gamma$-K direction of the WSe$_2$/PTCDA heterostructure measured at 50 K. d Energy distribution curves taken at the momenta indicated in c. The dispersive spin-split WSe$_2$ bands (VB1, VB2) and the non-dispersive HOMO level are marked by arrows. e Overview of the type-I energy level alignment of the TMD/OSC heterostructure, as determined from static photoemission spectroscopy (c,d), the $G_{0}W_{0}$ calculation (f), and scanning tunneling spectroscopy experiments reported in refs. Zheng16acsnanoGuo22nanoresearch. f Unfolded single-particle energy landscape of the WSe$_2$/PTCDA heterostructure as retrieved from the scissor-shifted $G_{0}W_{0}$ calculation in a 4$\times$4$\times$1 supercell (cf. extended Fig. \ref{['extFig_calcBandstructure']}c and Methods).
• Figure 1: Analysis of the real-space structure of the WSe$_2$/PTCDA heterostructure.a Sketch of the layered sample structure. b, c Optical microscope and photoemission real-space image of the sample before PTCDA evaporation. The different flakes are marked with the same colors as used in a. Note that the marked WSe$_2$ area corresponds to an intact monolayer (without cracks) whereas the complete monolayer as seen by the contrast in b and c was larger. d LEED pattern of monolayer PTCDA on multilayer WSe$_2$ recorded with a beam energy of 24 eV. The red circles correspond the superstructure defined by the matrix $M = 1.586.784.391.32$. e The presence of a well-ordered PTCDA monolayer on the WSe$_2$ monolayer is confirmed by the appearance of umklapp-scattering replicas of the WSe$_2$ band structure, here shown for the valence band maximum at the K point (red circles). The replicas can be directly compared to the LEED pattern in d.
• Figure 2: Energy- and momentum-resolved identification of the excitonic photoemission signatures.a,b Energy-filtered momentum maps and c momentum-filtered energy-distribution-curves of excitonic photoemission signatures. The data is obtained by integrating over the pump-probe delays from 100 to 500 fs and the application of the non-negative matrix factorization formalism (cf. Methods and extended Fig. \ref{['extFig_NMF']}). a The K and $\Sigma$ valleys of the WSe$_2$ Brillouin zone are indicated by the corners of an orange and a grey hexagon, respectively. The blue circle with a radius of $\sqrt{k_x^2+k_y^2}\approx 1.7$ Å-1 corresponds the the expected mean radius of the simulated momentum distribution of the LUMO of PTCDA (cf. extended Fig. \ref{['extFig_POT']}). c The EDCs are filtered in momentum for the K (orange), $\Sigma$ (grey), and molecular (blue) photoemission signatures (see extended Fig. \ref{['extFig_EDCs']} for chosen region of interests) and fitted with a single or two Gaussian peaks (cf. Methods). The resulting peak energies are marked with a horizontal bar in the plot and the corresponding exciton energies E$_{\rm exc}^i$ are summarized in Table \ref{['table:fit_edcs']}.
• Figure 2: Direct comparison of ARPES data collected on pristine monolayer WSe$_2$ and the WSe$_2$/PTCDA heterostructure.a,b,e,f Momentum-maps collected at energies of the valence bands (bottom row) and at energies of excitonic photoemission signatures (top row). Photoemission signatures of the HOMO and the hX are labeled by arrows in f and b, respectively. c,g Simulated momentum maps from DFT calculations of the LUMO and HOMO of PTCDA using the plane wave model of photoemission Puschnig09sci and accounting for the herringbone structure and the different mirror and rotational domains shown in d. The DFT data are extracted from reference puschnig2020organic.
• Figure 3: Reciprocal-space representation of the Bloch states and molecular orbitals contributing to the K-exciton and the hX wavefunction.a Absorption spectrum of WSe$_2$/PTCDA retrieved by G$_0$W$_0$+BSE calculations. The oscillator strengths of the contributing excitons are indicated as solid lines where all values below one (dark-excitons) are set to one for visibility. Excitons with and without contributions from PTCDA orbitals are distinguished in blue and yellow, respectively. b Backfolded Brillouin zone according to the theoretical superstructure (see extended Fig. \ref{['extFig_calcBandstructure']}a). c,d The two lowest-lying excitons marked by arrows in a are analyzed in detail in reciprocal space using the backfolded Brillouin zone (b). The electron and hole contributions are marked in red and cyan, respectively. c While the K-exciton wavefunction is purely composed of TMD valence and conduction band states (WSe$_2$ VBM and CBM), d the hX wavefunction has contributions from the TMD valence bands (WSe$_2$ VBM) and from the PTCDA HOMO and LUMO orbitals. e Visualization of the electron-hole transitions that contribute to the wavefunction of K-exciton, $\Sigma$-exciton, and hybrid exciton (hX). The hX wavefunction is of partial intra- and interlayer composition and built up by HOMO$\rightarrow$LUMO and VBM$\rightarrow$LUMO transitions, respectively. f Illustration of the hX in the exciton picture. The intralayer and interlayer electron-hole transitions are expected to be nearly degenerate in energy because of the stronger electron-hole interaction of the pure HOMO-LUMO exciton compared to the VBM-LUMO exciton. Mixing of the transitions leads to the formation of a new bound hybrid excitonic state at lower exciton energies, i.e., the hX.