Yuan Chen

Dilute magnetic semiconductors (DMSs) provide a platform for electrically controlling spin interactions; however, conventional systems face limited gate tunability and structural disorder. Here, we demonstrate gate-switchable magnetism in air-stable, dilute Fe-doped PtSe ₂ , a two-dimensional (2D) DMS that remains structurally homogeneous down to the atomically thin limit. Bulk crystals exhibit ferromagnetism with a Curie temperature of 320 kelvin, and this coupling persists in metallic devices down to seven layers. As thickness and carrier concentration further decrease, the system transitions to antiferromagnetic order, with a gate-tunable Néel temperature reaching 105 kelvin in five-layer semiconducting devices. Our first-principles calculations reveal a carrier-density–dependent crossover from Ruderman-Kittel-Kasuya-Yosida–mediated ferromagnetism to superexchange-driven antiferromagnetism. These findings demonstrate how induced magnetic order evolves from bulk to the 2D limit, providing a pathway to functional spintronic devices with electrically controlled magnetic states.

Hexagonal boron nitride (hBN) has been of great interest due to its ability to host several bright quantum emitters at room temperature. However, the identification of the observed emitters remains challenging due to spectral variability, as well as the lack of atomic defect structure information. In this work, we demonstrate the site-selective creation of blue emitters in exfoliated hBN flakes with high-energy ion irradiation. With the correlation analysis of cryogenic and temperature-dependent photoluminescence (PL) spectroscopy, we observe two zero phonon lines (ZPLs) at ∼432.8 and 454.3 nm. Photoluminescence excitation (PLE) measurements further confirm the emission origins of the two prominent lines. Scanning transmission electron microscopy (STEM) reveals that the dominant defect structures present in ion-irradiated samples are vacancy-type (V_x) and adatom(intercalant)-type (A_x). Together with first-principles GW-BSE (Bethe-Salpeter equation) calculations, we deduce that the observed blue emissions are likely related to boron intercalants (B_int). Our results not only discover a group of blue emissions in hBN but also provide insights into the physical origin of the emissions with local atomic structures in hBN.

Defect complexes can induce beneficial functionalities in two-dimensional (2D) semiconductors. However, understanding their formation mechanism with single-atom sensitivity has proven to be challenging for light elements using conventional transmission electron microscopy (TEM) techniques. Here, we demonstrate the atom-resolved formation of various one-dimensional (1D) defect complexes─consisting of rhenium dopants, sulfur interstitials, and sulfur vacancies─in monolayer MoS₂ by applying electron ptychography to our four-dimensional scanning transmission electron microscopy (4D-STEM) data sets. Our image resolution of 0.35 Å and a spatial precision of 2 pm allow us to achieve accurate matching between experimental structures and density functional theory (DFT) simulations at the atomic level. Additionally, we utilize out-of-focus ptychography to observe defect formation processes at dose rates comparable to those used in conventional TEM imaging, while maintaining a large field of view. This study demonstrates the systematic application of electron ptychography to extensive 4D-STEM data sets for quantitative defect imaging in 2D materials. We provide direct, atomically precise evidence that critical defect densities govern the formation of extended 1D defect complexes. For instance, we show that sulfur single-vacancy lines form when the vacancy density reaches 5 × 10¹³ cm^-2 and transform into double-vacancy lines beyond 8 × 10¹³ cm^-2. Rhenium-dopant lines emerge at a dopant concentration higher than 3 × 10¹³ cm^-2, where metastable sulfur interstitial-vacancy lines also form as the cumulative electron dose reaches 3 × 10⁵ e/Ų, initiating a local nucleation of the 1T' phase. Our results highlight the potential of electron ptychography for high-precision defect characterization and engineering in ultrathin 2D materials.

Point defects in crystalline solids behave as optically addressable individual quantum systems when present in sufficiently low concentrations. In two-dimensional (2D) semiconductors, such quantum defects hold potential as versatile single photon sources. Here, we report the synthesis and optical properties of Nb-doped monolayer WS₂ in the dilute limit where the average spacing between individual dopants exceeds the optical diffraction limit, allowing the emission spectrum to be studied at the single-dopant level. We show that these individual dopants exhibit common features of quantum emitters, including narrow emission lines (with linewidths <1 meV), strong spatial confinement, and photon antibunching. These emitters consistently occur within a narrow spectral range across multiple samples, distinct from common quantum emitters in van der Waals (vdW) materials that show large ensemble broadening. Analysis of the Zeeman splitting reveals that they can be attributed to bound exciton complexes comprising dark excitons and negatively charged Nb.

Precisely controlled impurity doping is of fundamental significance in modern semiconductor technologies. Desired physical properties are often achieved at impurity concentrations well below parts per million level. For emergent two-dimensional semiconductors, development of reliable doping strategies is hindered by the inherent difficulty in identifying and quantifying impurities in such a dilute limit where the absolute number of atoms to be detected is insufficient for common analytical techniques. Here we report rapid high-contrast imaging of dilute single atomic impurities by using conductive atomic force microscopy. We show that the local conductivity is enhanced by more than 100-fold by a single impurity atom due to resonance-assisted tunneling. Unlike the closely related scanning tunneling microscopy, the local conductivity sensitively depends on the impurity energy level, allowing minority defects to be selectively imaged. We further demonstrate subsurface impurity detection with single monolayer depth resolution in multilayer materials.

Two-dimensional (2D) semiconductors with point defects are predicted to host a variety of bound exciton complexes analogous to trions and biexcitons due to strong many-body effects. However, despite the common observation of defect-mediated subgap emission, the existence of such complexes remains elusive. Here, we report the observation of bound exciton (BX) complex manifolds in monolayer MoSe₂ with intentionally created monoselenium vacancies (V_Se) using proton beam irradiation. The emission intensity of different BX peaks is found to exhibit contrasting dependence on electrostatic doping near the onset of free electron injection. The observed trend is consistent with the model in which free excitons exist in equilibrium with excitons bound to neutral and charged V_Se defects, which act as deep acceptors. These complexes are more strongly bound than trions and biexcitons, surviving up to around 180 K, and exhibit moderate valley polarization memory, indicating partial free exciton character.