Interactions among electronic and lattice degrees-of-freedom are foundational to various phases in condensed-matter physics, yet the dynamic interplay between excitonic and phononic quasiparticles represents an equivalent, underexplored frontier. Moiré superlattices provide an ideal platform for realizing these interactions by offering localized intralayer excitons (IALX) and ultralow-energy collective lattice modes, such as phasons. Here, by optically suppressing ultrafast charge-transfer (CT) to interlayer excitons in WSe2/WS2 heterostructures, we uncover dynamics of moiré IALX revealing long lifetimes (τ > 1000 ps) arising from localized Wannier and in-plane CT nature. We then observe moiré intralayer intervalley biexcitons with binding energy 16 meV, with long lifetimes due to moiré confinement. Furthermore, we find time-domain signatures of strong coupling between moiré-IALX and 10 micro-eV phasons, evidenced as twist-angle-dependent GHz oscillations in IALX dynamics. Our findings establish moiré superlattices as interacting hybrid quantum systems and for engineering non-equilibrium phenomena, as well as for GHz-scale optoelectronics.

Exciton-phonon interactions govern the optical response of semiconductors, yet disentangling multiple coupling channels in lead halide perovskites remains challenging. We investigate CsPbBr3 microcrystals using photoluminescence, Raman, and reflectance spectroscopy at low temperature, revealing the simultaneous presence of high-energy and Rashba excitons, each accompanied by distinct phonon replica series. High-energy exciton replicas are uniquely spaced by approximately 9 meV, whereas Rashba exciton replicas exhibit a characteristic approximately 6 meV spacing, indicating the specificity of the exciton-phonon coupling. k-means clustering applied to a large low-temperature photoluminescence data set reveals these replica features are prevalent. With increasing temperature, replica features broaden and merge, evolving into a dominant longitudinal optical phonon coupling regime at room temperature. This work establishes direct spectroscopic evidence for concurrent, exciton-specific phonon coupling within a single material, offering pathways to engineer light-matter interactions for optoelectronic and phonon-photon-based quantum device applications.

Atomic reconstruction in twisted transition metal dichalcogenide heterostructures leads to mesoscopic domains with a uniform atomic registry, profoundly altering the local potential landscape. While interlayer excitons in these domains exhibit strong many-body interactions, the extent and impact of quantum confinement on their dynamics remains unclear. We reveal that quantum confinement persists in these flat, reconstructed regions. Time-resolved photoluminescence spectroscopy uncovers multiple, finely spaced (∼1 meV) interlayer exciton states and correlated emission with enormous lifetime variation from subnanosecond to over 100 ns across a 10 meV energy-window. Cascade-like transitions confirm that these states originate from reconstructed domains acting as potential wells, further supported by calculations. At high excitation rates, we observe anomalous nonlinear dynamics, specifically transient photoluminescence suppression followed by gradual recovery, a phenomenon we term "quantum siphoning". Our results demonstrate that quantum confinement and nonlinear dynamics persist beyond ideal moiré paradigm, potentially enabling applications in sensing and modifying dynamics via strain engineering.

We demonstrate wide-field magnetic imaging of current flow in hydrogen-terminated diamond field-effect transistors (FETs) through in-substrate nitrogen-vacancy (NV) centers. Hydrogen termination of the diamond surface induces a two-dimensional hole gas (2DHG), while an ensemble of near-surface NV centers located similar to 1 mu m below the surface enables operando magnetic imaging of current flow with micrometer-scale spatial resolution. The FETs were electrically characterized over a range of drain-source biases, V ds = 0 to -15 V, and gate voltages, V gs = +3 to -9 V, followed by in situ wide-field NV magnetometry during device operation. Magnetic field maps and reconstructed current-density distributions directly visualize current injection at the source-drain contacts and transport beneath the hBN-gated channel. Magnetic field maps reveal current-density variations in the channel region owing to non-uniformities or defects in the gate dielectric. In addition, we observe a pronounced enhancement of the drain current (similar to 600 to 900 mu A) and a shift in the apparent threshold voltage during laser illumination, reflecting photo-induced changes in channel electrostatics. By correlating gate-dependent magnetic images with simultaneous electrical measurements, we directly link spatial current distributions to FET transfer characteristics, providing new insight into buried interface transport and non-uniform gating effects in the transistor channel. As the methodology is compatible with top-gated FETs, it can be used to map channel current distributions with micrometer resolution in emerging channel materials, such as 2D materials and wide-bandgap channels, and establish wide-field NV magnetometry as a powerful platform for probing charge transport in transistors and van der Waals dielectric heterostructures.

Three-dimensional all-inorganic perovskites with low optical bandgaps, with the possibility of hosting spontaneous polarization, as realized in CsGeX 3 (X = Cl-, Br-, and I-), hold significant promise in the field of ferroelectric photovoltaics, with the possibility of charting a different path from their more celebrated Pb-based analogues. Depending on the octahedral tilting within the [MI6]4- framework, CsPbI3 exhibits temperature-driven phase transitions involving cubic, tetragonal, and orthorhombic phases, which influence its optoelectronic properties, such as optical bandgaps, photoluminescence, and charge carrier dynamics. The temperature-dependent studies of CsGeI3 below room temperature are limited, and the role of the 4s2 lone pair of Ge in stabilizing various polymorphs has not been explored. In this work, we investigate CsGeI3, establishing a distinct structural evolution arising from the interplay of the stereochemical activity of 4s2 lone-pair electrons on the Ge2+ cation and octahedral tilting. Temperature-dependent powder and single-crystal X-ray diffraction measurements reveal a series of polar-polar structural phase transitions arising from subtle changes in the Ge2+ off-centering and lattice distortions. We also present nearly identical thermal evolutions of photoluminescence and absorption spectra of CsGeI3 over a wide temperature range, suggesting the existence of a low exciton binding energy. In addition, temperature-dependent second-harmonic generation measurements confirm the polar nature of all phases of CsGeI3 at room temperature and below.

Molybdenum disulfide (MoS2) is a promising 2D transition metal dichalcogenide (TMD) for optoelectronics and quantum technologies, but scalable synthesis and defect engineering remain challenging. Oxygen-assisted chemical vapor deposition (O-CVD), which introduces in situ oxygen during growth, shows excellent potential in resolving both issues at once; however, oxygen's underlying mechanistic role remains unclear. Here, we combine oxygen dosing experiments, density functional theory (DFT), computational fluid dynamics (CFD), and ab initio molecular dynamics (AIMD) to uncover the dual role of oxygen in O-CVD. First, AIMD reveals that oxygen increases MoO3 sublimation and enhances Mo3O9 supply. Concomitantly, DFT reveals that sulfur oxides (bulkier than pure S2) limit the formation of reactive MoS6 intermediates. Subsequently, by experimentally varying the oxygen flow parameters and correlating them with CFD, we decouple oxygen's roles in source-poisoning prevention (MoO3 evaporation) and growth regulation. We find that a low sulfur-to-oxygen (S:O2) ratio at the MoO3 boat and substrate during nucleation, and a high S:O2 ratio at the substrate during growth, is the key to obtaining large-area high-quality monolayer MoS2, confirmed by our optical measurements. Based on our understanding, we present a kinetic phase diagram that establishes controlled oxygen dosing as a tuning parameter for scalable, defect-controlled monolayer MoS2 synthesis.

Heterostructure-based photodetectors often face performance limitations due to strong interlayer coupling and restricted band modulation. An alternative approach is engineering doped 2D TMDs, particularly through hole doping, to achieve tunable electronic and optoelectronic properties. In this work, we report oxygen passivation of selenium vacancies in MoSe2 (OP-MoSe2) and compare it with vacancy-rich MoSe2 (VSe-MoSe2) in terms of structural, chemical, and optoelectronic characteristics, demonstrating its application in weak-light photodetection. Raman spectroscopy, PL spectroscopy, and X-ray photoelectron spectroscopy confirm that oxygen passivation improves structural quality and optical performance. Low-temperature PL reveals reduced inhomogeneous broadening and biexcitonic features, indicating modulation of excitonic interactions. First-principles calculations show oxygen preferentially occupies vacancy sites, effectively passivating them and suppressing defect-related states. Consequently, OP-MoSe2 photodetectors achieve outstanding performance under 530 nm illumination, with responsivity of 0.74 × 105 A/W, detectivity of ~1014 Jones, at 89 nW/cm2, and a low noise-equivalent power of 0.087 fW/Hz1/2. Furthermore, we demonstrate weak-light tracking of moving objects at varying speeds, mimicking security surveillance conditions. These results establish oxygen-passivated MoSe2 as a promising platform for high-performance, low-light photodetection. This work underscores oxygen passivation as an effective defect engineering strategy for high-performance, ultra-sensitive 2D photodetectors.

Single-photon emitters (SPEs) hosted by two-dimensional (2D) semiconducting materials are envisioned for next-generation quantum applications. However, SPE creation in 2D semiconductors on rigid substrates like SiO2/Si via nanoindentation is a technological gap, critical for interfacing SPEs with photonic circuits and cavities. Here, we report a protocol for deterministically creating SPEs in monolayer WSe2 on SiO2/Si substrates using a sharp diamond AFM (atomic force microscope) tip. A displacement-controlled indentation process is developed, allowing indent depths > 150 nm necessary for creating SPEs. Sharp defect peaks ( 200 μeV) are observed in cryogenic (4K) photoluminescence (PL) spectrum at nanoindented sites and are stable upto 120K. 76

Two-dimensional magnetic materials (2D-MM) are an exciting playground for fundamental research, and for spintronics and quantum sensing. However, their large-grain large-area synthesis using scalable vapour deposition methods is still an unsolved challenge. Here, we develop a tailored approach for centimetre-scale growth of semiconducting 2D-MM CrCl3 films on mica substrate, via physical vapour transport deposition. A controlled synthesis protocol, enabled via innovations concerning light management, very-high carrier-gas flow, precursor flux, and oxygen/moisture removal, is critical for wafer-scale growth. Optical, stoichiometric, structural, and magnetic characterization identify crystalline, phase-pure 2D-MM CrCl3. Substrate temperature tunes thickness of films from few-layers to tens of nanometres. Further, selective-area growth and large-area transfer are demonstrated. Substrate-dependent growth features are explained by density functional theory and state-of-the-art machine learning interatomic potential-based atomic-scale simulations. This scalable vapour deposition approach can be applied for growth of several 2D-MM, and low growth temperature ( 500 C) will enable creation of hybrid heterostructures.

Exciton-phonon interactions govern the optical response of semiconductors, yet disentangling multiple coupling channels in lead halide perovskites remains challenging. We investigate CsPbBr3 microcrystals using photoluminescence, Raman and reflectance spectroscopy at low temperature, revealing the simultaneous presence of high-energy and Rashba excitons, each accompanied by distinct phonon replica series. High-energy exciton replicas are uniquely spaced by approximately 9 meV, whereas Rashba exciton replicas exhibit a characteristic approximately 6 meV spacing, indicating the specificity of the exciton-phonon coupling. Unsupervised machine learning applied to a large low-temperature photoluminescence dataset reveals these replica features are prevalent. With increasing temperature, replica features broaden and merge, evolving into a dominant longitudinal optical phonon coupling regime at room temperature. This work establishes direct spectroscopic evidence for concurrent, exciton-specific phonon coupling within a single material, offering new pathways to engineer light-matter interactions for optoelectronic and phonon-photon-based quantum device applications.

Transition metal dichalcogenide (TMD) nanoscrolls (NSs), specifically MoS 2 NSs, present unique structural and optical properties, exhibiting prominent photoluminescence (PL) signals despite their multilayer nature. This study investigates the structural and spectroscopic characteristics of MoS 2 NSs, correlating them to show the effects of reduced interlayer interactions on excitons of MoS 2 NSs. The reduction in interlayer interaction arises from two main factors: (1) symmetry-broken mixed stacking due to misalignment between layers and (2) a highly inhomogeneous strain profile generated by the Archimedean spiral geometry with positive eccentricity. Transmission electron microscopy, field emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and second harmonic generation measurements confirm these findings. Low-temperature PL spectroscopy explores the impact of reduced interlayer interactions on exciton properties such as exciton–phonon coupling and oscillator strength. This study provides crucial insights into the structure, stacking and unique optical properties of TMD NSs, advancing the understanding of interlayer interactions and their impacts in complex quasi-one-dimensional nanostructures.

Single photon emitters (SPEs) are building blocks of quantum technologies. Defect engineering of two-dimensional (2D) materials is ideal to fabricate SPEs, wherein spatially deterministic and quality-preserving fabrication methods are critical for integration into quantum devices and cavities. Existing methods use combination of strain and electron irradiation, or ion irradiation, which make fabrication complex, and limited by surrounding lattice damage. Here, only ultra-low energy electron beam (e-beam) irradiation (5 keV) is utilized to create dilute defect density in hBN-encapsulated monolayer MoS2, with ultra-high spatial resolution (<50 nm, extendable to 10 nm). Cryogenic photoluminescence spectra exhibit sharp defect peaks, following power-law for finite density of single defects, and characteristic Zeeman splitting for MoS2 defect complexes. The sharp peaks have low spectral jitter (<200 mu eV), and are tunable with gate-voltage and e-beam energy. Use of low-momentum electron irradiation, ease of processing, and high spatial resolution, will disrupt deterministic creation of high-quality SPEs.

Traditional magnetization dynamics measurements in van der Waals antiferromagnets rely on millimeter‐sized crystals to achieve detectable signal levels. However, conventional approaches become inadequate when investigating micrometer‐scale few‐layer samples, which host unique layer‐dependent phenomena including tunable magnon–magnon interactions and thickness‐controlled mode hybridization. Here, we develop a method enabling ferromagnetic resonance (FMR) measurements on micrometer‐sized antiferromagnetic samples (CrCl 3 ) using a conventional vector network analyzer (VNA)‐based FMR setup. Our approach combines a protective polycarbonate (PC) coating to prevent delamination, mechanical damage, and aging‐induced degradation, with a precision alignment technique that guarantees stable signal acquisition through optimal positioning relative to the excitation source. FMR measurements reveal robust spin dynamics in PC‐protected CrCl 3 , with clearly resolved acoustic and Kittel modes, showing a reduced interlayer exchange field compared to bulk crystals. Remarkably, the magnetic properties remained unchanged after 3 months under ambient conditions, confirming the effectiveness of the PC coating. Thus, our method enables reliable FMR studies of micrometer‐scale van der Waals magnets, bridging the gap between bulk and few‐layer investigations and paving the way for exploring layer‐dependent spin phenomena.

Atomic reconstruction in twisted transition metal dichalcogenide heterostructures leads to mesoscopic domains with uniform atomic registry, profoundly altering the local potential landscape. While interlayer excitons in these domains exhibit strong many-body interactions, extent and impact of quantum confinement on their dynamics remains unclear. Here, we reveal that quantum confinement persists in these flat, reconstructed regions. Time-resolved photoluminescence spectroscopy uncovers multiple, finely-spaced interlayer exciton states ( 1 meV separation), and correlated emission lifetimes spanning sub-nanosecond to over 100 nanoseconds across a 10 meV energy window. Cascade-like transitions confirm that these states originate from a single potential well, further supported by calculations. Remarkably, at high excitation rates, we observe transient suppression of emission followed by gradual recovery, a process we term "quantum siphoning". Our results demonstrate that quantum confinement and competing nonlinear dynamics persist beyond the ideal moire paradigm, potentially enabling applications in quantum sensing and modifying exciton dynamics via strain engineering.

The strong coupling of a molecular electronic transition with a quantized radiation field can result in modified photophysics compared to its uncoupled counterparts. Often, such changes are attributed to kinetic factors, overlooking the possible modifications to intermolecular interactions. The spin-cast films of chlorin e6 trimethyl ester (Ce6T) show an excitonic coupling band in absorption resulting from their ground-state intermolecular interactions and subsequent excimer-like emission upon photoexcitation. Interestingly, the electronic strong coupling (ESC) of the Ce6T Soret and Q-band suppresses the intermolecular excitonic interactions that otherwise exist in the Ce6T thin films and brings back the monomer-like emission characteristics. Our experiment provides a unique tool to tune the molecular assembly without involving chemical modifications. Our results suggest that ESC can induce modification to the intermolecular interaction forces that hold together the molecular assemblies in the ground state, which is a significant step toward understanding the fundamentals of polaritonic chemistry in detail.

Chemical vapor deposition (CVD) is the most widespread approach for two-dimensional (2D) material synthesis, yet control of nucleation density remains a major hurdle towards large-area growth. We find that precursor flux, a function of gas velocity and precursor concentration, is the critical parameter controlling nucleation. We observe that for a vertically aligned substrate, the presence of a cavity/slot in the substrate-supporting plate creates an enhanced growth zone for 2D-MoS2. The effect of this confined space on nucleation density is experimentally verified by electron microscopy. To understand this intriguing observation, we developed a hyper-realistic multiphysics computational fluid dynamics model, i.e., a digital twin of our CVD reactor, which reveals that space confinement achieves nearly-zero gas velocities. Digital twin-informed calculations indicate a significantly lower metal precursor flux at the confined space during the initial stages of growth, while precursor concentration is uniform across the substrate. The digital twin also makes an important prediction regarding a large time-lag between the set temperature, reactor environmental temperature, and substrate temperature, with implications for nucleation and growth. We offer a framework for designing confined spaces to control nucleation via regulating precursor flux, and for simulating reactor parameters for rapid optimization via the digital-twin model.

The properties of two-dimensional (2D) materials are heavily influenced by their dielectric environment. Nanoscale variations in thickness and trench depth of the substrate are often used to vary the optical properties, which involve testing multiple substrates. In this study, we develop grayscale electron-beam lithography (g-EBL) to vary substrate thicknesses (staircase structure) or trenches of different depths (holey structure) on the same SiO2/Si substrate. Monolayer MoS2 transferred onto these structures shows variations in Raman and photoluminescence (PL) intensities, consistent with the multireflection model (MRM). In holey structures, we observe a PL (Raman) enhancement of over 150 (90) times compared with the MoS2 placed directly on the substrate. Raman peak shifts are systematically analyzed using the MRM model, which considers trench-depth-dependent power absorption with implications for thermal conductivity measurements and thermal engineering applications. Our fabrication technique is also useful for creating photonic/phononic band gap crystals with varying depths, designing diffractive optical elements, and developing micro/nanoelectromechanical systems (M/NEMS).

Two-dimensional transition metal dichalcogenides are leading materials for next-generation optoelectronics, but fundamental problems stand enroute to commercialization. These problems include, firstly, the widely debated defect- and strain-induced origins of intense low-energy broad luminescence peaks (L-peaks) observed at low temperatures. Secondly, the role of oxygen in tuning the properties via chemisorption and physisorption is intriguing but challenging to understand. Thirdly, our physical understanding of the benefits of hexagonal boron nitride (hBN) encapsulation is inadequate. Using a series of samples, we decouple the contributions of oxygen, defects, adsorbates, and strain on the optical properties of monolayer MoS2. The defect origin of the L-peak is confirmed by temperature- and power-dependent photoluminescence (PL) measurements, with a dramatic redshift of similar to 130 meV for oxygen-assisted chemical vapour deposition (O-CVD) samples compared with exfoliated samples. Anomalously, the O-CVD samples show high A-exciton PL at room temperature (cf exfoliated), but reduced PL at low temperatures, attributed to the strain-induced direct-to-indirect bandgap crossover in low-defect O-CVD MoS2. These observations are consistent with our density functional theory calculations and are supported by Raman spectroscopy. In the exfoliated samples, the charged O adatoms are identified as thermodynamically favourable defects, and create in-gap states. The beneficial effect of encapsulation originates from the reduction of charged O adatoms and adsorbates. This experimental-theoretical study uncovers the type of defects in each sample, enables an understanding of the combined effect of defects, strain, and oxygen on the band structure, and enriches our understanding of the effects of encapsulation. This work proposes O-CVD as a method for creating high-quality materials for optoelectronics.

Trapped materials at the interfaces of two-dimensional heterostructures (HS) lead to reduced coupling between the layers, resulting in degraded optoelectronic performance and device variability. Further, nanobubbles can form at the interface during transfer or after annealing. The question of what is inside a nanobubble, i.e., the trapped material, remains unanswered, limiting the studies and applications of these nanobubble systems. In this work, we report two key advances. First, we quantify the interface quality using RAW format optical imaging (unprocessed image data) and distinguish between ideal and non-ideal interfaces. The HS/substrate ratio value is calculated using a transfer matrix model and is able to detect the presence of trapped layers. The second key advance is the identification of water as the trapped material inside a nanobubble. To the best of our knowledge, this is the first study to show that optical imaging alone can quantify interface quality and find the type of trapped material inside spontaneously formed nanobubbles. We also define a quality index parameter to quantify the interface quality of HS. Quantitative measurement of the interface will help answer the question whether annealing is necessary during HS preparation and will enable creation of complex HS with small twist angles. Identification of the trapped materials will pave the way toward using nanobubbles for optical and engineering applications.