Frederico B. Sousa

We develop a general framework for spin-resolved mem-emitters and illustrate it unequivocally using a monolayer MoSe2/clinochlore optoelectronic device. In mem-emitters, both the intensity and energy of light emission encode the history of external driving through slowly evolving internal state variables. When spin is included as an active degree of freedom, voltage-controlled memory dynamics give rise to spin-resolved hysteresis revealed through the degree of circular polarization under magnetic fields. Using polarization-resolved magneto-photoluminescence, we observe pronounced hysteresis in excitonic and trionic emission, whose loop topology depends on magnetic field and circular polarization. A minimal spin-resolved model shows that these features are generically governed by a polarity-dependent tuning of the effective Landé g-factor, with its voltage slope uniquely determining the orientation of the degree-of-circular-polarization loops. First-principles calculations suggest that interface-induced symmetry breaking provides a microscopic route to such bias-dependent spin responses, highlighting the broad relevance of spin-resolved memory effects in optoelectronic emitters.

Defect engineering is crucial for tuning 2D transition metal dichalcogenide properties for quantum and optoelectronic applications. While conventional photoluminescence (PL) and Raman spectroscopies are important characterization tools, their mapping in large area samples can be time‐consuming and lacks direct sensitivity for comprehensive defect characterization. Here, we introduce resonance‐enhanced four‐wave mixing (FWM) imaging for precise imaging and characterization of vanadium‐induced defect states in monolayers. Our multi‐modal investigation, integrating hyperspectral PL, Raman, and supported by density functional calculations, reveals nanoscale doping inhomogeneities, their influence on excitonic and vibrational properties. We observe resonance‐enhanced FWM signals correlating with vanadium‐induced defect regions, evidencing their unique nonlinear optical response. This work establishes FWM as an essential platform for high‐resolution, defect‐sensitive imaging, advancing defect‐engineered excitonic devices and enabling novel nonlinear quantum photonics.

Two-dimensional (2D) semiconductors such as monolayer WSe₂ have attracted significant interest for their quantum properties and potential as scalable single-photon emitters. However, conventional microphotoluminescence (μ-PL) techniques are fundamentally limited by optical diffraction, hindering access to critical nanoscale features such as strain gradients and localized quantum confinement. In this study, we utilize tip-enhanced photoluminescence (NanoPL) with a spatial resolution of ≈10 nm to directly image the emission landscape of monolayer WSe₂ on top of nanopillars at room temperature. Our results reveal two distinct localization regimes associated with leading theoretical models for single-photon activation and provide guidelines for deterministic nanoengineering of quantum light sources.

Moiré materials offer a versatile platform with engineered electronic structures, enabling the study of exotic quantum phenomena in strongly correlated and topological systems. From an optical perspective, the moiré pattern promotes the confinement of excitons giving rise to novel emissions depending on valleytronic, interlayer, and intralayer interactions. Here, we investigate the excitonic emissions of a twisted homobilayer WSe₂ that exhibits a moiré exciton with an intensity that depends on the application of a magnetic field perpendicular to the plane of the material. Our results suggest that the momentum mismatch ΔK, necessary to couple intralayer bright states from individual WSe₂ layers is promoted by magnetic field, giving rise to an interlayer excitonic emission with an intensity modulated by a magnetic field. This study opens new possibilities for manipulating the excitonic states of moiré materials, enabling the exploration of regimes characterized by smaller moiré pattern sizes and the dynamics involving interlayer and intralayer states.

Two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors exhibit interesting many-body effects even above room temperature due to their strong electron-hole interactions. For instance, low excitation densities lead to the well investigated exciton formation in these 2D TMDs and their heterostructures. The confinement of the moiré excitons and the delocalization of the interlayer excitons are among the novel excitonic phenomena presented by TMD-based heterostructures. However, the high excitation density responses of these 2D semiconductors and their heterostructures still lack solid understanding. In this work, we investigate the electron-hole plasma photoluminescence generated by high excitation densities in 2D MoSe₂-WSe₂ lateral heterostructures. Photoluminescence mapping and spectroscopy measurements at high pumping regimes reveal an enhanced light emission at the lateral heterojunctions. Ab initio calculations for a MoSe₂-WSe₂ lateral heterostructure with an alloyed interface of approximately the size of the heterojunctions of the experimental samples show good agreement with the experimental data. Additionally, the theoretical results provide an explanation for the observed enhancement of the photoluminescence at the heterojunctions and for the role of interfacial alloying in increasing the overlap of electron and hole wave functions at the interface. These observations reveal the localized character of the optical effects at heterojunctions of lateral TMD-based heterostructures under high excitation densities.

Transition metal dichalcogenides (TMDs) are promising sources of single-photon emitters (SPEs), which arise from defect-induced brightening of forbidden transitions. These SPEs, known as localized intervalley defect excitons, appear as doublets with an energy difference driven by electron–hole exchange interactions. Typically, spin forbidden transitions do not manifest in flat two-dimensional materials; however, the engineering of curved materials gives rise to novel phenomena. In this study, we engineered curved WSe₂ monolayers, enabling the direct identification of dark doublets associated with spin–flip transitions of intravalley defect excitons. We present a comprehensive first characterization of these dark doublets revealing an intriguing linear polarization with a 45° phase difference compared to the bright doublet emission. Additionally, a new fine structure splitting emerges from exchange interactions, coupling bright and dark intravalley transitions. This effect can be attributed to the nanopillar induced curvature, which tilts the magnetic moment away from the z-axis, leading to a mixing of in-plane and out-of-plane magnetic field effects. This mixing manifests in a spin–flip effect even in the out-of-plane configuration. Also, an unusually large in-plane g-factor of 4.5 suggests this mixed configuration. This discovery provides critical insights into the coupling mechanisms between dark and bright excitonic states, opening new avenues for exploiting exciton behavior in nanostructured materials.

In this review, we address the optical signatures of defects in two-dimensional transition metal dichalcogenides (2D TMDs), whether they occur unintentionally during growth or are deliberately introduced post-growth. We detail their primary responses as probed by photoluminescence (PL), magneto-PL, Raman, tip-enhanced PL and Raman, and nonlinear spectroscopies. Defects significantly impact the electronic, vibrational, magneto-optical, and nonlinear properties of TMDs, influencing outcomes based on application needs. This comprehensive overview highlights the distinctive optical fingerprints of various defects, providing guidance for their identification and characterization. Additionally, we discuss new optical phenomena induced by defects in TMD monolayers and future challenges in defect engineering for these materials. The insights from this review underscore the potential of TMDs for technological applications, with advancements in spectroscopy and defect engineering driving future innovations and enhancing our understanding of these materials.

Two-dimensional heterostructures have been crucial in advancing optoelectronic devices utilizing van der Waals materials. Semiconducting transition-metal dichalcogenide monolayers, known for their unique optical properties, offer extensive possibilities for light-emitting devices. Recently, a memory-driven optical device, termed a Mem-emitter, was proposed by using these monolayers atop dielectric substrates. The successful realization of such devices heavily depends on the selection of the optimal substrate. Here, we report a pronounced memory effect in a MoSe₂/clinochlore device, evidenced by an electric hysteresis in the intensity and energy of MoSe₂ monolayer emissions. This demonstrates both population- and transition-rate-driven Mem-emitter abilities. Our theoretical approach correlates these memory effects with internal state variables of the substrate, emphasizing that a clinochlore-layered structure is crucial for a robust and rich memory response. This work introduces a novel two-dimensional device with promising applications in memory functionalities, highlighting the importance of alternate insulators in the fabrication of van der Waals heterostructures.