Su Ying Quek

Achieving deterministic control over crystal phase at the atomic limit remains a fundamental challenge for atomically thin metals, where subtle differences in atomic registry can produce large changes in electronic and optical functionality. Here, we establish interfacial defect templating as a general materials‐design strategy for phase engineering in confined two‐dimensional metals, using monolayer silver as a model system. By tailoring the defect chemistry of a graphene overlayer, we selectively stabilize two competing crystalline phases of two‐dimensional Ag at the graphene/SiC interface: a near‐commensurate phase promoted by vacancy and line defects in epitaxial graphene, and a denser phase favored beneath intrinsically sp ³ ‐rich zero‐layer graphene. Multimodal characterization reveals distinct lattice registries, electronic structures, and charge transfer to the graphene overlayer for each phase. First‐principles calculations show that phase selectivity arises from a competition between kinetically favored nucleation pathways and thermodynamically preferred packing configurations, explaining both controlled phase formation and long‐term evolution. The defect‐programmed Ag phases exhibit strongly contrasting linear and nonlinear optical responses, enabling phase‐tunable optical functionality at atomic thickness. More broadly, this work reframes defects as deliberate design elements for programming structure–property relationships in confined two‐dimensional metals.

In excitonic systems, where the relative motion of electrons and holes is constrained by Coulomb interaction, how exciton effects influence the magnitude of shift current─a phase-coherent shift of photoexcited Bloch electrons and holes found in noncentrosymmetric materials─has been a subject of ongoing theoretical interest. Here, we report observation of exciton-enhanced shift current in uniaxially strained monolayer MoS₂. We obtain energy-dependent shift current conductivity tensor elements by measuring polarization-resolved photocurrent spectrum in multiterminal configurations, revealing a series of features that are absent in the linear optical spectra. We show that these features can be attributed to higher-order excitons and that their large shift vector plays an important role in their observation. We further demonstrate that electrostatic gating can modulate the shift current by two orders of magnitude in a nonmonotonous manner, revealing the interplay between free carrier conductivity and exciton oscillator strength.

The electronic relaxation dynamics of two-dimensional silver polar metal heterostructures (2D-PMets), isolated with two different Ag lattice structures, were studied with femtosecond transient absorption (fs-TA) spectroscopy. The two 2D Ag phases, called Ag₍₁₎ and Ag₍₂₎, differ in atomic packing density, which leads to phase-specific ultralow frequency (ULF) phonon modes and visible electronic absorption transitions. Time-resolved kinetic traces for both phases were fit to a biexponential decay function, with the first decay component pertaining to ultrafast electronic relaxation and the second corresponding to carrier-phonon scattering. The first decay time constant τ₁ is 2 ∼ 2 ps for Ag₍₁₎ and ∼ 1 ps for Ag₍₂₎. The shorter τ₂ in Ag₍₂₎ is attributed to increased carrier-phonon scattering probability in more close-packed lateral structures. The results indicate that atomic-level structure controls energy flow in spatially confined 2D materials.

Substitutional transition metal (TM) point defects have recently been controllably introduced in two-dimensional (2D) transition metal dichalcogenides. We identify quantum defect candidates through a first-principles materials discovery approach with 25 TM elements substituting Mo and W in 2D MoS₂ and WSe₂, respectively. We elucidate trends in the charge transition levels for these 50 systems and report the existence of defects with spin-triplet ground states and a zero-field splitting (ZFS) in the terahertz (THz) regime, in contrast to typical gigahertz values. These defects can couple to resonant near-infrared radiation, providing a route to applications as high-fidelity qubits controlled by spin-dependent optical transitions. The THz ZFS implies that these high-fidelity operations can take place at higher temperatures compared to the case for GHz qubits. Our results also point toward the possibility of realizing a single-photon THz emitter. This work broadens the scope of quantum defects, highlighting the opportunities for next-generation THz quantum technologies─an area of growing interest given the rapid advancement in the development of THz sources and detectors.

Hexagonal boron nitride (hBN) is a wide-band-gap semiconductor that is promising as a host material for solid-state quantum technologies through defect engineering. It has been shown that boron atoms can be removed from the lattice upon irradiation by electrons or light ions, creating boron vacancies and boron interstitials. While the optical properties of boron-vacancy-derived defects have been studied extensively, little is known about the optical properties of boron-interstitial-derived defects. In this work, we use state-of-the-art first-principles calculations to predict the electronic and optical properties of boron interstitials (B_int) and defect complexes comprising B_int and substitutional carbon impurities at boron and nitrogen sites (C_B and C_N). These carbon impurities can be present in as-grown hBN and can also be introduced intentionally. We demonstrate that these complexes are expected to be stable at room temperature. Our GW-Bethe-Salpeter equation (BSE) calculations show that B_int-C_B and B_int-C_N have low-energy optical transitions that are isolated in energy, making them suitable as single-photon emitters. Together with constrained density functional theory calculations to capture the red shift due to emission, we predict that B_int-C_B and B_int-C_N have zero phonon lines at ∼2.0 eV and ∼2.6 eV, respectively. Defects involving B_int are likely to be the source of blue emitters recently observed in regions several microns away from ion-irradiated parts of hBN. Our work sheds light on these recent experiments and introduces a fresh perspective to the field of quantum emitters in hBN─we show that defects related to B_int are potential single-photon emitters that can be intentionally created in hBN.

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.

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.

The coexistence of correlated electron and hole crystals enables the realization of quantum excitonic states, capable of hosting counterflow superfluidity and topological orders with long-range quantum entanglement. Here we report evidence for imbalanced electron–hole crystals in a doped Mott insulator, namely, α-RuCl₃, through gate-tunable non-invasive van der Waals doping from graphene. Real-space imaging via scanning tunnelling microscopy reveals two distinct charge orderings at the lower and upper Hubbard band energies, whose origin is attributed to the correlation-driven honeycomb hole crystal composed of hole-rich Ru sites and rotational-symmetry-breaking paired electron crystal composed of electron-rich Ru–Ru bonds, respectively. Moreover, a gate-induced transition of electron–hole crystals is directly visualized, further corroborating their nature as correlation-driven charge crystals. The realization and atom-resolved visualization of imbalanced electron–hole crystals in a doped Mott insulator opens new doors in the search for correlated bosonic states within strongly correlated materials.

Strongly correlated electrons enable the realization of a plethora of quantum states of matter, such as Wigner crystallization, fractional quantum Hall effect, and high-temperature superconductivity. When correlated electrons and holes are allowed to coexist, they become intertwined and fuel the pursuit of quantum excitonic states harbouring counterflow superfluidity^1,2 and topological orders with long-range quantum entanglement^3,4. While such collective quantum states have been reported in sophisticated multi-layered heterostructures^1,2,4–8, realizing and controlling such quantum states in a single natural strongly correlated material has remained challenging due to the fast particle recombination. Here, we report the creation of imbalanced electron-hole crystals in a doped multi-orbital honeycomb Mott insulator, α-RuCl₃, through gate-tunable non-invasive van der Waals (vdW) doping from graphene. The absence of layer separation allows the immediate visualization of electron-hole crystals via scanning tunneling microscopy (STM). Real-space imaging reveals two completely different charge orderings at the lower Hubbard band (LHB) and the upper Hubbard band (UHB) energies, whose origin can be attributed to the correlation-driven honeycomb hole crystal composed of hole-rich Ru sites and rotational symmetry breaking paired electron crystal composed of electron-rich Ru-Ru bonds, respectively. Moreover, a gate-induced transition of electron-hole crystals can be directly visualized, further corroborating their nature as correlation-driven charge crystals⁹. The realization and atom-resolved visualization of imbalanced electron-hole crystals in a doped multi-orbital honeycomb Mott insulator, combined with a gate-tunable electron reservoir, opens new doors in the search for exotic correlated bosonic states within strongly correlated materials^5,8,10–12.