Su Ying Quek's research while affiliated with National University of Singapore and other places

We show that excitonic resonances and interexciton transitions can enhance the probability of spontaneous parametric down-conversion, a second-order optical response that generates entangled photon pairs. We benchmark our ab initio many-body calculations using experimental polar plots of second harmonic generation in NbOI2, clearly demonstrating the relevance of excitons in the nonlinear response. A strong double-exciton resonance in 2D NbOCl2 leads to giant enhancement in the second order susceptibility. Our work paves the way for the realization of efficient ultrathin quantum light sources.

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 superfluidity1,2 and topological orders with long-range quantum entanglement3,4. While such collective quantum states have been reported in sophisticated multi-layered heterostructures1,2,4–8, 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 materials5,8,10–12.

We show that excitonic effects in 2D crystals enhance the probability of spontaneous parametric down-conversion (SPDC), a second-order optical response that leads to the generation of entangled photon pairs. We formulate the second-order response within many-body theory and perform first principles calculations on NbOX$_2$ (X = I, Cl), 2D crystals of interest in non-linear optics. Excitonic resonances together with interexciton couplings lead to order-of-magnitude enhancements in the second-order response. Our work paves the way for the realization of efficient ultrathin quantum light sources.

Hexagonal boron nitride (hBN) has been a centre of 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 report two new blue quantum emitters with zero phonon line (ZPL) centred around 460 nm and 490 nm in hBN powders. We further demonstrate that the blue emissions can be created by high temperature annealing or high energy ion irradiation in exfoliated hBN flakes. Scanning transmission electron microscopy (STEM) reveals that the dominant defect structures present in ion irradiated sample are vacancy-type (V­ x ­) and adatom(intercalant)-type (A ­x ­). Together with first principle GW-BSE (Bethe-Salpeter equation) calculation, the observed blue emissions at 490 nm may be due to boron intercalants (B int ). Our results not only discover a new group of blue quantum emissions in hBN, but also provide an insight on the physical origin of the emissions by correlating the emission wavelength with local atomic structures in hBN.

Atomic defects in two-dimensional (2D) semiconductors are promising single exciton traps for achieving quantum emission. However, excessively high density of defects often makes it impractical to optically address the quantum nature of such quasi-particles, preventing strategic exciton engineering. Here, we report a versatile strategy for dilute acceptor impurity doping of monolayer WS 2 , which enables the study of isolated bound excitons. We observe two distinct types of acceptor-bound excitons: one of them is characterized by a large binding energy, allowing its observation at room temperature and the other by an extremely narrow linewidth (< 1 meV), showing strong resemblance to quantum emitters in WSe 2 . Polarization-resolved spectroscopy and magnetic field dependence reveal their strong contribution of free exciton character. In particular, the latter type exhibits characteristics of acceptor-bound dark excitons. Our work demonstrates dilute impurity doping as a promising strategy towards optically addressable quantum states in monolayer semiconductors.

Using state-of-the-art ab initio GW many-body perturbation theory calculations, we show that monolayer MoS2 on Au is a p-type contact, in contrast to the vast majority of theoretical predictions using density functional theory. The predominantly n-type behaviour observed experimentally for MoS2/Au junctions can be attributed to the presence of sulfur vacancies, which pin the Fermi level. GW calculations on WSe2/Au junctions likewise predict p-type contacts for pristine WSe2 and n-type contacts for junctions with selenium vacancies. Experimentally, WSe2/metal junctions are predominantly p-type or ambipolar, with p-type junctions being observed for selenium-deficient WSe2, suggesting that selenium vacancies are not effective in pinning the Fermi level for WSe2/metal junctions. We rationalize these apparently contradictory results by noting that selenium vacancies in WSe2 are readily passivated by oxygen atoms. Taken together, our state-of-the-art calculations clearly elucidate the relation between contact polarity and atomic structure. We show that non-local exchange and correlation effects are critical for determining the energy level alignment and even the contact polarity (in the case of MoS2 on Au). We further reconcile a large body of experimental literature on TMDC/metal contact polarities by consideration of the defect chemistry.

Since the isolation of graphene in 2004, two-dimensional (2D) materials research has rapidly evolved into an entire subdiscipline in the physical sciences with a wide range of emergent applications. The unique 2D structure offers an open canvas to tailor and functionalize 2D materials through layer number, defects, morphology, moiré pattern, strain, and other control knobs. Through this review, we aim to highlight the most recent discoveries in the following topics: theory-guided synthesis for enhanced control of 2D morphologies, quality, yield, as well as insights toward novel 2D materials; defect engineering to control and understand the role of various defects, including in situ and ex situ methods; and properties and applications that are related to moiré engineering, strain engineering, and artificial intelligence. Finally, we also provide our perspective on the challenges and opportunities in this fascinating field.

Defect engineering of atomically thin semiconducting crystals is an attractive route to developing single‐photon sources and valleytronic devices. For these applications, defects with well‐defined optical characteristics need to be generated in a precisely controlled manner. However, defect‐induced optical features are often complicated by the presence of multiple defect species, hindering the identification of their structural origin. Here, we report systematic generation of optically active atomic defects in monolayer MoS2, WS2, MoSe2, and WSe2 via proton‐beam irradiation. Defect‐induced emissions are found to occur ≈100 to 200 meV below the neutral exciton peak, showing typical characteristics of localized excitons such as saturation at high‐excitation rates and long lifetime. Using scanning transmission electron microscopy, it is shown that freshly created chalcogen vacancies are responsible for the localized exciton emission. Density functional theory and ab initio GW plus Bethe‐Salpeter‐equation calculations reveal that the observed emission can be attributed to transitions involving defect levels of chalcogen vacancy and the valence band edge state.