Optimized Cathodoluminescence Microscopy of Buried Interfaces by Nanoscale Heterostructure Design

A simple approach to isolate a wide range of semiconductors from detrimental interactions with the environment is the encapsulation in hexagonal boron nitride (hBN) [1-3]. Although far-field optical microscopy is extensively used to characterize the optical properties of nanomaterials buried in such hBN-based heterostructures, it lacks the spatial resolution to map heterogeneity at the nanoscale. On the contrary, near-field optics can achieve nanoscale spatial resolution, but cannot easily access buried interfaces. Interestingly, cathodoluminescence (CL) can excite optically active nanomaterials buried in hBN using a nanoscale electron probe. We thus elect CL as an ideal technique to study a prototypical class of buried interfaces such as monolayers (MLs) of transition-metal dichalcogenides (TMDs) fully encapsulated in hBN. The encapsulation of TMDs in hBN has become a strict requirement to obtain high-quality devices, decoupled from unwanted perturbations due to the external dielectric environment [4]. TMD CL critically relies on the generation of electron-hole (e-h) pairs in hBN and their subsequent migration into the TMD ML, where they can radiatively recombine and emit light. Surprisingly, up to date there is no systematic study of the implications of e-h dynamics in hBN to the advantage of high-resolution CL mapping.<br/>Here, we address this issue and show tuning of the e-h dynamics through control of the hBN thickness. We rely on the atomically defined edges of TMD monolayers to show that the hBN thickness controls the CL spatial resolution and to demonstrate that the TMD CL resolution is limited by the e-h pair diffusion in hBN. For the first time, we are able to estimate a minimum threshold of the e-h diffusion length in bulk hBN, a crucial parameter for a deeper understanding of the optoelectronic properties of this material. We provide practical directions to optimize TMD CL by studying the most convenient hBN thickness that enables bright CL luminescence and sub-diffractive spatial resolution down to the nanoscale. We eventually corroborate our approach showing low-temperature hyperspectral maps of the heterogenous optical response of TMDs buried in hBN. We show that, even after encapsulation, the optical response of TMDs MLs is still highly sensitive to intrinsic heterogeneity due to structural defects [5] or complex strain fields [6], which critically emerge at the nanoscale. We thus combine the high spatial and spectral resolution of CL to disentangle different forms of perturbation of the TMD emission and demonstrate spatial resolution of very few tens of nanometers.<br/>In summary, using hBN-encapsulated TMD MLs as an exemplificative system, we show a strategy to access and map the properties of buried interfaces with nanoscale spatial resolution. Our method is applicable to a range of nanomaterials beyond TMDs, which paves the way for optimized, high-resolution CL experiments of buried, optically active interfaces. Moreover, we obtain this tuning by straightforward control of the encapsulation thickness. In the specific case of TMD MLs, our results show a promising strategy to disentangle different forms of heterogeneity in the TMD optical response and unveil finer details in their optoelectronic properties.<br/> <br/>[1] H. H. Fang et al., Adv. Funct. Mater. 28, 1800305 (2018).<br/>[2] M. Shanmugam et al., Nanoscale 5, 11275 (2013).<br/>[3] V. Raj et al., npj 2D Mater Appl. 5, 12 (2021).<br/>[4] A. Raja et al., Nat. Commun. 8, 15251, (2017).<br/>[5] B. Schuler et al., PRL 123, 076801 (2019).<br/>[6] T. Darlington et al., Nat. Nanotech. 15, 854 (2020).