Charge density wave, a periodic modulation of electronic charge density often accompanied by a periodic lattice distortion, plays a vital role to induce exotic phenomena in condensed matter physics. In non-magnetic quantum materials, contrast inversion in scanning tunneling microscopy images, observed between opposite bias polarity, serves as a hallmark of the charge density wave. However, in itinerant ferromagnetic systems, charge density wave formation competes with magnetism: A charge density wave order typically reduces the density of states at the Fermi level, while the Stoner criterion for spontaneous spin polarization requires a high density of states at Fermi level. Therefore, direct real-space observation of such polarity-dependent contrast inversion in ferromagnetic materials remains elusive and experimentally challenging. Here, we demonstrate the observation of a charge density wave in itinerant ferromagnet Fe₅GeTe₂ associated with $\sqrt{3}$ × $\sqrt{3}$ superlattice, revealed through polarity-dependent scanning tunneling microscopy imaging. Importantly, we observe a gap-like dip at the Fermi level in tunneling spectra, serving additional evidence for the emergence of charge density wave in Fe₅GeTe₂. Interestingly, the strength of charge modulation can be systematically tuned by Fe1 vacancies and impurities, while the spectroscopic intensity shows a high sensitivity to surface degradation. Our finding provides an inspiring insight to charge density wave on the van der Waals ferromagnetic materials.

Monolayer (1L) transition metal dichalcogenides (TMDCs) have been attracting tremendous interest in recent years as promising candidate materials in atomic-scale optoelectronic devices due to their direct band gaps (1.5–2.2 eV) and strong light–matter interactions. Unfortunately, their practical applications are limited by low visible light absorption stemming from atomic thickness and negligible infrared response. Here, we report the triangular Sb₂O₃ microresonators in wide thickness and lateral size distributions grown on 1L TMDCs and their created significant broadband enhancement of light adsorption and photoresponse in 1L WSe₂ crystal via coexisting Fabry–Perot and whispering gallery type resonances. As an example of demonstration, 1L WSe₂ crystal coupled to Sb₂O₃ microresonators with widely distributed sizes exhibits the enhanced visible light absorption by up to 5 folds and the simultaneously extended near infrared (NIR) one of more than 50%. For application of 1L WSe₂ in photodetection, incorporation of Sb₂O₃ microresonators leads to significantly enhanced visible light responsivity by ~ 10⁴ order and expanded NIR one of more than 400 mA·W⁻¹. Similar results have been observed in the other 1L W(Mo) dichalcogenides coupled to Sb₂O₃ microresonators. This work provides a new route for development of the high-performance monolayer TMDCs-based optoelectronic devices.