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.

Layered trihalides exhibit distinctive band structures and physical properties due to the sixfold coordinated 3d or 4d transition metal site and partially occupied d orbitals, holding great potential in condensed matter physics and advanced electronic applications. Prior research focused on trihalides with highly symmetric honeycomb-like structures, such as CrI₃ and α-RuCl₃, while the role of crystal anisotropy in trihalides remains elusive. In particular, the trihalide MoCl₃ manifests strong in-plane crystal anisotropy with the largest difference in Mo–Mo interatomic distances. Research on such material is imperative to address the lack of investigations on the effect of anisotropy on the properties of trihalides. Herein, we demonstrated the anisotropy of MoCl₃ through polarized Raman spectroscopy and further tuned the phonon frequency via strain engineering. We showed the Raman intensity exhibits twofold symmetry under parallel configuration and fourfold symmetry under perpendicular configuration with changing the polarization angle of incident light. Furthermore, we found that the phonon frequencies of MoCl₃ decrease gradually and linearly with applying uniaxial tensile strain along the axis of symmetry in the MoCl₃ crystal, while those frequencies increase with uniaxial tensile strain applied perpendicularly. Our results shed light on the manipulation of anisotropic light-matter interactions via strain engineering, and lay a foundation for further exploration of the anisotropy of trihalides and the modulation of their electronic, optical, and magnetic properties.

Two-dimensional (2D) materials with reversible phase transformation are appealing for their rich physics and potential applications in information storage. However, up to now, reversible phase transitions in 2D materials that can be driven by facile nondestructive methods, such as temperature, are still rare. Here, we introduce ultrathin Cu₉S₅ crystals grown by chemical vapor deposition (CVD) as an exemplary case. For the first time, their basic electrical properties were investigated based on Hall measurements, showing a record high hole carrier density of ~ 10²² cm⁻³ among 2D semiconductors. Besides, an unusual and repeatable conductivity switching behavior at ~ 250 K were readily observed in a wide thickness range of CVD-grown Cu₉S₅ (down to 2 unit-cells). Confirmed by in-situ selected area electron diffraction, this unusual behavior can be ascribed to the reversible structural phase transition between the room-temperature hexagonal β phase and low-temperature β’ phase with a superstructure. Our work provides new insights to understand the physical properties of ultrathin Cu₉S₅ crystals, and brings new blood to the 2D materials family with reversible phase transitions.

Light-matter interactions in low-dimensional quantum-confined structures can dominate the optical properties of the materials and lead to optoelectronic applications. In anisotropic layered silicon diphosphide (SiP₂) crystal, the embedded quasi-one-dimensional (1D) phosphorus–phosphorus (P–P) chains directly result in an unconventional quasi-1D excitonic state, and a special phonon mode vibrating along the P–P chains, establishing a unique 1D quantum-confined system. Alloying SiP₂ with the homologous element serves as an effective way to study the properties of these excitons and phonons associated with the quasi-1D P–P chains, as well as the strong interaction between these quasiparticles. However, the experimental observation and the related optical spectral understanding of SiP₂ with isoelectronic dopants remain elusive. Herein, with the photoluminescence and Raman spectroscopy measurements, we demonstrate the redshift of the confined excitonic peak and the stiffening of the phonon vibration mode $B_{1\mathrm{g}}^3$ of a series of Si(P_1−xAs_x)₂ alloys with increasing arsenic (As) compositions. This anomalous stiffening of $B_{1\mathrm{g}}^3$ is attributed to the selective substitution of As atoms for P atoms within the P–P chains, which is confirmed via our scanning transmission electron microscopy investigation. Such optical spectra evolutions with selective substitution pave a new way to understand the 1D quantum confinement in semiconductors, offering opportunities to explore quasi-1D characteristics in SiP₂ and the resulting photonic device application.

Strain engineering can serve as a powerful technique for modulating the exotic properties arising from the atomic structure of materials. Examples have been demonstrated that one-dimensional (1D) structure can serve as a great platform for modulating electronic band structure and phonon dispersion via strain control. Particularly, in a van der Waals material silicon diphosphide (SiP₂), quasi-1D zigzag phosphorus–phosphorus (P–P) chains are embedded inside the crystal structure, and can show unique phonon vibration modes and realize quasi-1D excitons. Manipulating those optical properties by the atom displacements via strain engineering is of great interest in understanding underlying mechanism of such P–P chains, however, which remains elusive. Herein, we demonstrate the strain engineering of Raman and photoluminescence (PL) spectra in quasi-1D P–P chains and resulting in anisotropic manipulation in SiP₂. We find that the phonon frequencies of SiP₂ in Raman spectra linearly evolve with a uniaxial strain along/perpendicular to the quasi-1D P–P chain directions. Interestingly, by applying tensile strain along the P–P chains, the band gap energy of strained SiP₂ can significantly decrease with a tunable value of ~ 55 meV. Based on arsenic (As) element doping into SiP₂, the strain-induced redshifts of phonon frequencies decrease, indicating the stiffening of the phonon vibration with the increased arsenic doping level. Such results provide an opportunity for strain engineering of the light–matter interactions in the quasi-1D P–P chains of SiP₂ crystal for potential optical applications.

Identifying air-stable two-dimensional (2D) ferromagnetism with high Curie temperature (T_c) is highly desirable for its potential applications in next-generation spintronics. However, most of the work reported so far mainly focuses on promoting one specific key factor of 2D ferromagnetism (T_c or air stability), rather than comprehensive promotion of both of them. Herein, ultrathin Cr_1–xTe crystals grown by chemical vapor deposition (CVD) show thickness-dependent T_c up to 285 K. The out-of-plane ferromagnetic order is well preserved down to atomically thin limit (2.0 nm), as evidenced by anomalous Hall effect observed in non-encapsulated samples. Besides, the CVD-grown Cr_1−xTe nanosheets present excellent ambient stability, with no apparent change in surface roughness or electrical transport properties after exposure to air for months. Our work provides an alternative platform for investigation of intrinsic 2D ferromagnetism and development of innovative spintronic devices.