Hexagonal boron nitride (h-BN), an insulating two-dimensional layered material, has recently attracted a great attention due to its fascinating optical, electrical, and thermal properties, and promising applications across the fields of photonics, quantum optics, and electronics. However, mechanically exfoliated bulk h-BN and h-BN films grown on catalytic metal substrates have been mainly used to study the fundamental properties, lacking in scalability for practical implementation of h-BN.

Here, we exploit the scalable approach to grow high-quality h-BN on Si-based nano-trenches and epitaxial gallium nitride (GaN) substrates by using metal-organic chemical vapor deposition (MOCVD). Firstly, the conformal growth of sp2hybridized few-layer h-BN over an array of Si-based nanotrenches with 45 nm pitch and the aspect ratio of ~ 7:1 was successfully accomplished by using pulsed-mode MOCVD. Surface-sensitive near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory calculations reveal that the B-O bonds formed on the non-catalytic SiO2 surface act as nucleation sites for the formation of mixed sp2-and sp3-hybridized BON2 and BN3 at the very initial stage of the pulsed-mode injection of MOCVD precursors, enables the conformal growth of few-layer sp2-hybridized h-BN with an excellent step coverage. We believe that these results can provide a broad avenue for the implementation of fascinating 2D materials for current state-of-the-art 3D Si-based architectures, overcoming the down-scaling limit. Secondly, it was found that under specific MOCVD growth conditions, a unique few-layer h-BN film can be grown on GaN substrates, in which the few-layer h-BN film is suspended on GaN nanoneedles. The combination of state-of-the-art microscopic and spectroscopic analyses, which includes fifth-order aberration-corrected scanning transmission electron microscopy, second harmonic generation, second-order resonant Raman scattering, and photoluminescence spectroscopy in the deep-ultraviolet range, revealed that the suspended h-BN films exhibit unprecedented atomic stacking. The mechanism underlying the formation of unique atomic stacking will be investigated through structural and electrical characterizations, as well as theoretical modeling. Our findings unveil new perspectives for the scalable synthesis of engineered h-BN polytypes.

Colloidal semiconductor nanocrystals allow for enriched color conversion essential to superior lighting and displays. These colloids span different types and heterostructures of semiconductors from colloidal quantum dots to wells. In this talk, we will present recent examples of semiconductor nanocrystal optoelectronic devices using atomically flat, tightly confined colloidal quantum wells (CQWs) [1-9]. By engineering quantum designs of the CQWs, we will show record high efficiency from their colloidal LEDs [10,11] and record high gain coefficients and low lasing thresholds from their colloidal lasers [12,13] in their classes.  We will also discuss the use of these nanocrystals embedded into carefully designed dielectric media to make colloidal metadevices [14]. We will finally present highly engineered quantum wells to make a new class of single-photon emitters with high quantum purity at room temperature [15]. Given their accelerating progress, these solution-processed nanocrystals offer great promise to challenge their epitaxial thin film counterparts in semiconductor optoelectronics.

Understanding the mechanisms underlying structural transformations in colloidal quantum dots is essential for optimizing their optical and electronic functionalities, which are inherently dictated by nanoscale structure and composition. In-situ transmission electron microscopy (TEM) has recently enabled direct visualization of dynamic processes in real time, offering unprecedented insights into how nanocrystals respond to external stimuli. In this talk, I will highlight three representative transformation pathways revealed through advanced in-situ TEM analyses. First, moisture-induced degradation of quantum-sized II–VI semiconductor nanocrystals is shown to proceed via well-defined amorphous intermediates, wherein adsorbed water molecules disrupt surface passivation, induce defect generation, and progressively destabilize the lattice.[1] Secondly, I will examine water-driven decomposition in metal halide perovskite nanocrystals, demonstrating how hydration initiates rapid lattice deformation, and facilitates their transition toward disordered and decomposed states.[2] Finally, I will present direct observations of off-stoichiometry–induced phase transitions in 2D CdSe quantum nanosheets, where subtle deviations from ideal composition lower the energy barrier for structural reorganization and drive the transformation from hexagonal wurtzite to cubic zinc blende phases.[3] Together, these findings establish a unified framework for environment- and composition-driven structural evolution in colloidal semiconductor nanocrystals and provide design principles for engineering more robust nanomaterials for next-generation optoelectronic applications.

In this talk, colloidal synthesis of bright europium halide perovskite nanocrystals and their derivatives is presented.

Europium halide perovskites are promising candidates for environmentally-benign blue light emitters with their narrow emission linewidth. However, the development of high-photoluminescence quantum yield (PLQY) colloidal europium halide perovskite nanocrystals (PNCs) is hindered by the limited synthetic methods and the elusive reaction mechanisms. Here, we provide an effective synthetic route for achieving high-PLQY deep-blue-emitting colloidal CsEuBr₃ PNCs. Using two Br-organic ligand precursors, oleylammonium bromide (OLAMHBr) and trioctylphosphine dibromide (TOPBr₂), we identified distinct phase evolution routes involving Eu²⁺:CsBr, Cs₄EuBr₆, and CsEuBr₃. The OLAMHBr precursor initially promotes the formation of Eu²⁺:CsBr phase, which reorganizes into the CsEuBr₃ perovskite phase via proton transfer. In contrast, the TOPBr₂ precursor induces the formation of core/shell Cs₄EuBr₆/CsBr PNCs, which subsequently transform into CsEuBr₃ through nucleophilic addition. The TOPBr₂ route exhibited enhanced CsEuBr₃ phase homogeneity, resulting in a significantly higher PLQY (40.5%; full-width-at-half-maximum (FWHM) = 24 nm at 430 nm), compared to the OLAMHBr route (16.5% at 418 nm). Notably, the phase-pure CsEuBr₃ PNCs demonstrated a world-record PLQY among the reported blue-emitting lead-free PNCs that exhibit a narrow emission linewidth (FWHM < 25 nm).[1] Our recent work on europium-doped metal halide nanocrystals will also be shortly presented, showing a high PLQY over 70% at pure-blue color range (458-476 nm). These works highlight the significant role of organic ligands (and halide precursors) in the colloidal synthesis of CsEuBr₃ PNCs and their potential as non-toxic, solution-processable blue-light emitters.

The search for high-performance circularly polarized luminescence (CPL) is an ongoing effort due to prospective applications spanning quantum communication, cryptography, and medical imaging. To realize these promises, potential emitters must be integrated into existing telecom and sensing architectures, with their emission tuned to the ubiquitous C-band around 1550 nm. Unfortunately, despite the growing library of CPL emitters, extracting C-band CPL remains elusive, and the few known candidates are lanthanide complexes limited by low quantum yields. Hybrid organic-inorganic nanocrystals are emerging as important candidates due to their outstanding optical properties and highly tunable structures. In this talk, I will discuss how chiral surface ligands affect the circular dichroism and CPL emission in halide perovskite nanoplatelets, and introduce our strategies to realize efficient CPL in the communication band through cation doping. By employing transmission electron microscopy and atomic scale electron energy loss mapping, we interrogate the mechanisms behind the CPL and reveal a structure-CPL relationship induced by chiral surface ligands. 

Colloidal semiconductor nanocrystals (NCs) combine outstanding optical properties, such as size-dependent emission and absorption, with robust photochemical stability. Recently, NCs have been employed as building blocks for mesoscale assemblies called superparticles (SPs), unlocking novel optoelectronic functionalities. Assembled SPs can behave as spherical microcavities by trapping light at their surface through optical whispering gallery modes. Under suitable excitation conditions, this can bring to population inversion and lasing. So far, most studies on optically active SPs revolve around cadmium-based NCs, limiting toxicity-sensitive applications. [1], [2] Here, we employ non-toxic copper indium sulfide/zinc sulfide (CIS/ZS) core/shell NCs, for superparticle assembly. We establish a synthetic approach based on the sequential growth of multiple zinc sulfide shells, resulting in photoluminescence quantum yield up to 75 %. We assemble these NCs into SPs, investigating the optical response of individual-SPs using steady-state and time-resolved spectroscopy under tunable laser excitation. These non-toxic artificial solids hold great promise for real-world applications in optoelectronics and photonics. 

Conventional blue-emitting materials often struggle to simultaneously achieve narrow emission linewidth, chemical stability, and compatibility with thin-film device fabrication. These limitations are especially pronounced in solution-processed systems, where uncontrolled nucleation frequently leads to rough and non-uniform films. Here, we present a vapor-phase strategy to synthesize uniform, covalently bonded organic-inorganic hybrid AgSePh thin films. By sequentially depositing metallic Ag and diphenyldiselenide (Ph2Se2) in the vapor phase and inducing amine-assisted crystallization, we obtain high-quality films that preserve the intrinsic deep-blue emission characteristics of AgSePh. The resulting films exhibit a peak emission at 466 nm with a narrow full width at half maximum (FWHM) of 16.6 nm, satisfying the high color-purity requirements for advanced display technologies. Using these films, we demonstrate the first AgSePh-based blue light-emitting diodes, confirming stable and intrinsic electroluminescence (EL) from this material. This work provides a practical route for integrating insoluble metal-organic chalcogenide (MOC) emitters into thin-film devices and highlights their potential as a robust platform for next-generation high color-purity blue displays.

Near-infrared photodetection and imaging devices play a crucial role in fields such as biological detection, information communication, and military meteorology. In traditional infrared imaging devices, the detector needs to be integrated with the readout circuit. However, the complexity of the integration process leads to higher device costs and larger pixel unit sizes, which limits the development of infrared imaging systems. By vertically integrating infrared detectors with visible light-emitting devices, the fabrication of infrared upconversion devices can significantly simplify the manufacturing process and facilitate the development of large-area, high-pixel-scale infrared imaging devices. In recent years, the performance of infrared detectors based on novel nanomaterial systems has rapidly improved, providing a new pathway for infrared detection and imaging technologies. We has developed an infrared upconversion imaging device structure based on quantum dots¹, expanding the detection range of upconversion imaging devices and enhancing their efficiency. In recent years, through the regulation of perovskite dimensions and nanostructures, highly efficient perovskite electroluminescent devices have been developed^2-4, including highly efficient semi-transparent light-emitting devices. Recently, leveraging perovskite luminescent thin films, we developed a simple, low-cost, and highly efficient infrared upconversion detection device, achieving an equivalent pixel scale of 7 million and a resolution line width of 11 micrometers⁵.

Extended shortwave infrared (eSWIR) photodetectors based on solution-processable semiconductors are attracting growing interest for applications including ranging, night vision, and gas sensing. Colloidal quantum dots (CQDs) are particularly appealing owing to their solution processability and facile bandgap tunability from the visible to the mid-infrared. However, most eSWIR-absorbing CQDs are based on toxic elements such as Hg and Pb, which are incompatible with RoHS directives. Moreover, the frequent need for cryogenic cooling and the intrinsically slow temporal response of many CQD devices significantly hinder their commercial deployment.

Here, we demonstrated an eSWIR photodetector employing silver telluride (Ag2Te) CQDs as a RoHS-compliant active material. Ag2Te CQDs feature a small effective mass, which is beneficial for achieving high carrier mobility and rapid response. To accommodate the complex monoclinic structure of Ag2Te, we rationally selected thiol ligands that enable efficient solid-state ligand exchange, thereby lowering trap density and enhancing carrier transport in the CQD films. These optimized CQD solids are integrated into a vertical p–n photodiode with a well-aligned energy-level landscape that promotes efficient charge extraction. The resulting device operates at room temperature and is free of regulated toxic elements, while exhibiting ultrafast temporal characteristics. Our best-performing photodetector achieved a fall time of 72 ns at 298 K, which, to our knowledge, represents the fastest response reported for CQD-based eSWIR photodetectors, including those based on toxic CQDs.

As next-generation electronic devices increasingly demand highly efficient and high-definition displays with flexible and deformable form factors, the development of nanocrystal (NC)-based light-emitting diodes (LEDs) has become a critical issue. Despite the intrinsic advantages of quantum dots (QDs) and perovskite nanocrystals (PeNCs)—such as high photoluminescence quantum yield, broad color gamut, and excellent color purity—progress in creating high-definition R, G, B subpixel patterns and efficient LEDs has been limited. In this presentation, we will discuss high definition and highly efficient nanocrystal based LEDs via transfer printing method. The transfer printing method ensures the close packing of nanocrystals, significantly reducing leakage current and boosting the external quantum efficiency. Furthermore, we explore the development of ultrathin and stretchable patterned LEDs using this innovative approach, demonstrating their potential for applications such as multicolor electronic tattoos. These deformable NC-based LEDs are poised to play a pivotal role in next-generation form factor displays, surpassing foldable and rollable technologies.

Colloidal quantum dot-based light-emitting diodes (QD-LEDs) are one of the future emissive displays and high-resolution patterning of quantum dot (QD) films is one of the preconditions for the practical use of QD-based emissive display. In this study, we introduce the ZnO interlayer by atomic layer deposition (ALD) to enhance the performance and lifetime of CdZnSeS/ZnS core/shell QD-LEDs. Employing direct optical lithography would be highly beneficial owing to its well-established process in the semiconductor industry. However, exposing the photoresist (PR) on top of the QD film deteriorates the QD film underneath. This is because the majority of the solvents for PR easily dissolve the pre-existing QD films. We present a conventional optical lithography process to obtain solvent resistance by reacting the QD film surface with diethylzinc precursors. It was confirmed that, by simple surface crosslinking of the QD surface and coating of the PR, a typical photolithography process can be performed to generate a red/green/blue pixel of 3000 PPI or more. QD electroluminescence devices were fabricated with all primary colors of QDs; moreover, compared to reference QD-LED devices, the patterned QD-LED devices exhibited enhanced brightness and efficiency. In addition, I will present results on strategies to suppress leakage current when operating QD displays in electroluminescent mode. Finally, I will discuss the results of validating QD-EL devices through a monolithic integration process directly implemented on the backplane.

High-resolution quantum-dot electroluminescent (QD-EL) devices are emerging as key components in next-generation optoelectronic systems, including emissive microdisplays and compact solid-state light sources [1]. This work introduces a solution-processable fabrication strategy that integrates printed electron-transport layers QD/organic nanohybrid emissive layers by inkjet or electrohydrodynamic (EHD) jet printing, enabling precise patterning and device fabrication under ambient air conditions.

The nanohybrid emissive layers—formed through controlled interactions between quantum dots and functionalized polymeric components—enhance charge balance, reduce trap-mediated recombination, and improve film uniformity and environmental robustness. Their solvent-orthogonal characteristics allow seamless integration into multilayer printed device stacks [2].

Particularly, EHD-jet printing enables ultra-fine patterning by generating droplets significantly smaller than the nozzle diameter. By optimizing jetting modes, nanohybrid features with pitches down to ~5 µm (≈5,080 pixels per inch, PPI) were produced, and fully functional EL pixels with lateral dimensions of ~21 µm (≈300 PPI) were fabricated entirely under ambient conditions. These results demonstrate that printing-based processes can reliably achieve high-resolution, display-grade QD-EL structures.

Importantly, the demonstrated printing strategy is not limited to a specific QD composition. While compatible with non III–V blue quantum dots, it can be extended across diverse QD families, enabling scalable pathways toward full-color RGB QD-EL devices and advanced optoelectronic applications.