2D Excitonics with Atomically Thin Lateral Heterostructures

TL;DR

This review surveys atomically thin lateral heterostructures (LHs) built from transition metal dichalcogenides, focusing on how in-plane covalent interfaces create a lateral energy landscape that supports unidirectional exciton transport and charge-transfer (CT) excitons. It details bottom-up growth strategies—especially multi-step CVD, pulsed-laser chalcogen conversion, and one-pot CVD—and highlights how atomically sharp interfaces (roughly $2$–$3~\mathrm{nm}$) enable robust interfacial excitonic phenomena, observed via PL, DR, TEPL/TERS, and SHG. The article also covers CT exciton formation, propagation along and across interfaces, and lensing/funneling effects that concentrate excitons in engineered geometries, including triangular LHs and sub-wavelength islands, with diffusion coefficients up to $\mathcal{O}(10)$ $\mathrm{cm^{2}\,s^{-1}}$. Collectively, these findings position LHs as a versatile platform for tunable exciton transport, dipolar interfacial states, and potential room-temperature quantum optoelectronics, while outlining future directions such as time-resolved studies, tr-ARPES band-mapping, and multi-junction LH engineering. The review emphasizes dielectric and strain engineering as key levers to modulate CT excitons and interfacial transport, aiming toward scalable, planar excitonic devices.

Abstract

Semiconducting transition metal dichalcogenides (TMDs), such as MoSe$_2$ and WSe$_2$, exhibit unique optical and electronic properties. Vertical stacking of layers of one or more TMDs, to create heterostructures, has expanded the fields of moiré physics and twistronics. Bottom-up fabrication techniques, such as chemical vapor deposition, have advanced the creation of heterostructures beyond what was possible with mechanical exfoliation and stacking. These techniques now enable the fabrication of lateral heterostructures, where two or more monolayers are covalently bonded in the plane of their atoms. At their atomically sharp interfaces, lateral heterostructures exhibit additional phenomena, such as the formation of charge-transfer excitons, in which the electron and hole reside on opposite sides of the interface. Due to the energy landscape created by differences in the band structures of the constituent materials, unique effects such as unidirectional exciton transport and excitonic lensing can be observed in lateral heterostructures. This review outlines recent progress in exciton dynamics and spectroscopy of TMD-based lateral heterostructures and offers an outlook on future developments in excitonics in this promising system.

Figures (9)

• Figure 1: MoSe$_2$-WSe$_2$ lateral heterostructures a. Sketch of the exemplary MoSe$_2$-WSe$_2$ lateral heterostructure and b. the corresponding in-plane variation of the single-particle energies, showing a type-II alignment favoring the emergence of charge transfer (CT) excitons. a., b. are adapted from Rosati2023. c. Schematic of the CVD setup used for MoSe$_2$-WSe$_2$ growth on SiO$_2$/Si wafers with 300 nm of SiO$_2$. d. False-colored optical microscope images distinguishing MoSe$_2$ and WSe$_2$ domains. Reproduced with permission from Najafidehaghani2021, Copyright 2021, Wiley-VCH GmbH. e. Raman spectrum of MoSe$_2$ (lower panel) showing the $\text{A}^{\prime}_1$ peak at 241 cm$^{-1}$ and Raman spectrum of WSe$_2$ (upper panel) with the $\text{E}^{\prime}$/$\text{A}^{\prime}_1$ peak at 250 cm$^{-1}$. f. HAADF-STEM image of the MoSe$_2$-WSe$_2$ interface, with MoSe$_2$ (yellow) and WSe$_2$ (cyan) domains as well as the boundary region (red) are highlighted. Reproduced with permission from Beret2022, Copyright 2022, Nature Publishing Group.
• Figure 2: Tip-enhanced spectroscopy of the MoSe$_2$-WSe$_2$ interface at T = 300 K. a. Schematic of TERS and TEPL setup. b. Raman shift measured by TERS shows the $\text{A}^{\prime}_1(\Gamma)$ phonon line. The position of the interface is indicated with the black dashed line. c. PL emission energy obtained from Lorentzian fits of individual peaks. On the left of the interface the MoSe$_2$ neutral exciton (blue stars) contribution is visible while on the right of the interface the WSe$_2$ neutral (red stars) and dark (black stars) excitonic contributions are seen. d. Amplitude of the individual peaks obtained from TEPL shown in c. Figures reproduced from Beret2022, Copyright 2022, Nature Publishing Group.
• Figure 3: Photoluminescence from a MoSe$_2$-WSe$_2$ lateral heterostructure encapsulated in hBN at T = 4.7 K a. PL line scan across the interface of the LH. The neutral excitons ($\text{X}_0^{\text{MoSe}_2}$, $\text{X}_0^{\text{WSe}_2}$) and, the trions ($\text{T}^{\text{MoSe}_2}$, $\text{T}^{\text{WSe}_2}$) from the two materials of the LH are marked. The white dashed lines indicate the positions whose individual spectra are plotted in panel b. b. Spectra obtained in monolayer regions away from the interface showing MoSe$_2$ (orange) and WSe$_2$ (blue). MoSe$_2$ PL is dominated by emission from the neutral exciton $\text{X}_0$ at 1.638 eV and a weaker contribution from the trion (T) at 1.605 eV. The PL intensity from WSe$_2$ is characteristically lower than that of MoSe$_2$. In this case, the trion at 1.685 eV has a higher intensity compared to the neutral exciton at 1.717 eV. c. The cumulative (T + $\text{X}_0$ ) integrated PL from MoSe$_2$ (orange) and WSe$_2$ (blue) are plotted as a function of position in the line scan. Upon approaching the interface of the two materials, the intensity from MoSe$_2$ increases relative to that at positions away from the interface. These measurements are performed with a 633 nm He:Ne laser and a diffraction limited excitation spot.
• Figure 4: Differential white light reflectivity DR/R at lateral junction at T=4.1 K. a. Line scan of differential white light reflectivity DR/R measured across the interface of a $\text{MoSe}_2-\text{WSe}_2$ LH (same sample as Figure \ref{['Fig-PL']}). From position 0 µm to 2 µm, a resonance at 1.635 eV from MoSe$_2$ is observed, and from 2 µm onwards the resonance corresponding to WSe$_2$ appears, indicating that the strong oscillator strengths of these resonances are well maintained in the LH. b. DR resonance (black) and $X_0^{\text{MoSe}_2}$ (orange). Energies obtained from both of these measurements overlap well and appear fairly constant both in regions away from and close to the interface. c. DR resonance (black) and emission energy of $X_0^{\text{WSe}_2}$ (blue). Both of these show a position-dependent energy shift of about 8 meV.
• Figure 5: Charge transfer excitons in lateral heterostructures a. Band alignment in vertical and lateral heterostructures, showing a drastic reduction of the band offset in the latter. b.-c. Offset-driven localization of electrons and holes at the opposite sides of the interface thanks to the appearance of CT excitons. d. Theoretically predicted and e. experimentally measured cryogenic photoluminescence after an optical excitation at the interface. The appearance of a new CT exciton peak X$_{\text{CT}}$ located approximately 90 meV below the monolayer X$_{\text{Mo}}$ exciton is observed - in excellent theory-experiment agreement. Figures a., b.-c. and d.-e., are adapted respectively from Guo16, Lau18, and Rosati2023.