Engineering 2D material exciton lineshape with graphene/h-BN encapsulation

TL;DR

The study addresses shaping optical excitations in atomically thin TMDs by engineering near-field coupling in van der Waals heterostructures with graphene/graphite and h-BN. It reports Fano-like asymmetric exciton lineshapes at the lowest excitons $X_A$ and $X_B$ for WS$_2$, MoSe$_2$, and WSe$_2$ when interfaced with graphene/graphite, and symmetric Lorentzian lineshapes for h-BN encapsulation; in WSe$_2$/graphene, trion emission is suppressed with a neutral exciton redshift of $44\, ext{meV}$ and a binding-energy reduction of $30\, ext{meV}$. A retardation-aware 2D optical-conductivity model explains the lineshapes as electromagnetic coupling to the surrounding conductive environment, predicting that the asymmetry grows with the substrate’s imaginary dielectric contribution $\text{Im}\{\epsilon_{\text{sub}}\}t_{\text{sub}}$ and can be captured by modified Fano parameters $(q,a)$. The results demonstrate a route to tailor narrow optical modes and exciton energies in vdWHs, enabling new nanophotonic device functionalities through precise stacking and thickness control.

Abstract

Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Remarkable advances have been achieved through alloying, chemical and electrical doping, and applied strain. However, the integration of TMDs with other 2D materials in van der Waals heterostructures (vdWHs) to tailor novel functionalities remains largely unexplored. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton lineshape and charge state. Fano-like asymmetric spectral features are produced in WS$_{2}$, MoSe$_{2}$ and WSe$_{2}$ vdWHs combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe$_{2}$/graphene with a neutral exciton redshift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron-beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.

Figures (12)

• Figure 1: Spectra of TMD monolayer with graphite and/or h-BN encapsulation: EELS spectra of (a) WS$_2$, (b) MoSe$_2$, and (c) WSe$_2$ monolayers in different configurations of freestanding, supported or encapsulated with h-BN and/or thin graphite measured at $T = 110$ K. The configuration of each spectrum is color-coded by the arrows and roman numerals on the sketches in the upper part of the panels. All spectra are normalized with respect to the total intensity of the elastic (zero-loss) peak (ZLP) and vertically offset for clarity. Insets in (a) and (c) show the optical absorption spectrum (orange curves) on similarly produced Gr/WS$_2$/Gr and h-BN/WSe$_2$/h-BN heterostructures at 5 K and 150 K, respectively, compared to their respective EELS spectrum. The inset in (b) shows the comparison of experimentally measured (lines) and modelled (dots) EELS spectra for a MoSe$_2$ monolayer encapsulated in thin graphite (asymmetric lineshape) and h-BN (Lorentzian lineshape).
• Figure 2: Modelled spectra versus parameters of the encapsulating layers: Evolution of the 2D-modelled EELS spectral shape as a function of (a) graphite and (b) h-BN encapsulation thickness. Solid lines represent TMD monolayer + encapsulation, while dashed lines are calculated for encapsulation material only. (c) Fitted parameters q and a of the modified Fano-like profile [Eq. \ref{['Eq:Fano_model']}] as a function of the encapsulation properties, specifically represented by the real versus imaginary part of the dielectric function multiplied by the thickness of the corresponding encapsulating layer, $\mathrm{Re}\{\epsilon_\textrm{sub}\}t_\textrm{sub}$ and $\mathrm{Im}\{\epsilon_\textrm{sub}\}t_\textrm{sub}$, respectively.
• Figure 3: Near-field coupling of h-BN-encapsulated TMD monolayers with graphene: (a) Optical micrograph of a h-BN/graphene+WSe$_2$/h-BN heterostructure with the different constituents outlined. (b) EELS spectra of h-BN encapsulated WSe$_2$ and graphene/WSe$_2$ heterostructures, highlighting the clear redshift in the presence of graphene (gr). (c) CL and EELS spectra from identical nanometric regions allows for a clear assignment of the emission peaks, including a prominent low-energy trion emission in the absence of graphene, as well as a clear quenching of the neutral exciton ($\sim$7$\times$ lower) but no obvious trion emission.
• Figure SI1: Fabrication of the mixed encapsulation sample of MoSe$_2$ monolayer. (a) Optical micrograph of a h-BN/ graphite + MoSe$_2$ + graphite heterostructure imaged on the polymer stamp with the different layer constituents outlined. (b) Optical micrograph of the same heterostructure after dropping onto a holey carbon support TEM grid and washing off any polymer residues. Micrographs in (a,b) are each spliced from two separate images due to differences in focus in the top graphite and top h-BN areas. (c) Bright-field STEM image of the heterostructure with the Gr/MoSe$_2$/Gr measurement regions circled. (d) Bright-field STEM image of the h-BN/MoSe$_2$/Gr region of the vdWH stack, with the h-BN/MoSe$_2$/Gr and h-BN/MoSe$_2$ measurement areas circled in solid and dotted lines, respectively. Holes in the TEM grid carbon support are 1.2 $\mu$m in size. Folds, trapped dirt and bubbles of few hundreds of nm size can be observed in the images of (c,d) as dark contrast.
• Figure SI2: Nano-beam electron diffraction patterns of WS$_2$ monolayer in (a,b) Gr/WS$_2$/Gr and (c,d) freestanding WS$_2$ configurations at sample tilt-angles of 0 and 448 mrad. The diffraction spots boxed in purple are of the same index order used to compare the roughness of the WS$_2$ monolayer. Diffraction spots highlighted by grey hexagons and white circles are from WS$_2$ and graphite, respectively.