Himchan Cho

Realizing high-resolution stretchable displays requires emissive materials that combine photopatternability with intrinsic stretchability. In colloidal nanocrystal films, however, the network formation needed for precise patterning often restricts molecular mobility, creating a fundamental trade-off with mechanical compliance. Here, we address this challenge using a benzophenone-based dual-crosslinker strategy that forms an elastomeric nanocrystal network. The tetrafunctional small-molecule crosslinker promotes efficient photopatterning, whereas the benzophenone-terminated polybutadiene crosslinker provides mechanical compliance. By tuning the crosslinker composition, the dual-crosslinking system enables direct optical patterning with line widths down to 2 μm and crack-free deformation up to 36% uniaxial strain. Both perovskite and InP-based nanocrystals largely retain their photoluminescence after processing, while the dual-crosslinking platform enables multicolor patterning and transfer to elastomeric substrates. QLEDs incorporating strained emissive layers maintain more than 90% of the maximum luminance of their unstretched counterparts. This dual-crosslinker design offers a general route to intrinsically stretchable, directly photopatternable colloidal nanocrystal emission layers.

As conventional digital computing becomes increasingly constrained by scalability and energy‐efficiency limits, computing based on intrinsic physical dynamics of devices has emerged as a promising alternative. In this context, three‐terminal devices have attracted attention because their additional terminal offers greater flexibility for implementing higher‐order dynamics than two‐terminal devices. Here, we present a dual‐state programmable oxide transistor (DUPOT) that exhibits a previously unreported form of high‐dimensional dynamical behavior with both the threshold voltage ( V _th ) and the saturation current ( I _sat ) independently tunable. Additionally, the programmed V _th state exhibits long‐term memory (LTM) characteristics, whereas the I _sat state shows short‐term memory (STM) behavior, enabling more complex computing functionalities. We elucidate its operating mechanisms and demonstrate robustness, and further showcase its use in time‐based cryptography, which fully exploits its rich dynamical behavior. Our array‐level demonstration supports diverse forms of time‐based cryptography, including time‐release encryption and time‐bound encryption, and can be extended to cloud cryptographic systems, marking a new milestone in the study of computing devices with higher‐order dynamics.

Metal halide–ligand complexes are widely employed to enhance the photoluminescence (PL) of colloidal quantum dots (QDs). In III–V materials such as indium phosphide (InP) QDs, these complexes are particularly effective at eliminating surface oxidative defects, yet the underlying mechanism and their interactions with metal carboxylates remain unclear. Here, we investigate how zinc oleate (Zn(OA)2), in combination with ZnX2–TOP complexes (ZXT; X = halides), promotes the removal of the surface oxide layer compared to ZXT-only treatment, with the extent of this promotion depending on the halide identity. The results suggest that ZXT can participate in TOP-mediated reactions with surface oxide species, but its effect on defect removal remains limited without Zn(OA)2. The presence of Zn(OA)2 appears to modify the original ZXT coordination state toward a less sterically hindered mixed-ligand Zn coordination environment. This modification facilitates more effective oxide removal and passivation of the oxide-removed surface, thereby suppressing reoxidation and trap formation, which, in turn, enhances the PL quantum yield (PLQY). Among the ZXT, ZnCl2–TOP enables the most efficient oxide removal at lower molar equivalents than ZnBr2–TOP and ZnI2–TOP. This synergistic pathway alleviates strain accumulation during metal chalcogenide shell growth, leading to more uniform shell formation and high-quality InP/ZnSe/ZnS core/shell QDs with a PLQY of 88.2%. This study provides insight into how ligand interactions influence oxidative defect removal and passivation, thereby offering a useful basis for the rational design of surface treatments for InP QDs.

Metal halide perovskites (MHPs) are promising materials for color‐conversion layers (CCLs) owing to their high absorption coefficients and narrow emission bandwidths. However, high‐resolution patterning of micrometer‐thick MHP CCLs remains challenging due to their susceptibility to optical and chemical damage during conventional patterning processes. Direct in situ photolithography has emerged as a potential solution by enabling high‐thickness pattern formation without directly exposing MHP emitters to harsh conditions, yet its achievable spatial resolution has remained limited. Here, we report a kinetically controlled direct in situ photolithography (KC‐ISP) approach for high‐resolution MHP CCLs. By controlling thiol–ene crosslinking kinetics and incorporating a precursor‐redistributing spin‐washing step, KC‐ISP enables high‐resolution patterning with feature sizes down to ∼3 µm while providing facile color tunability through controlled precursor composition. In addition, in situ crystallized MHPs exhibit a pronounced increase in photoluminescence quantum yield upon air exposure, attributed to phase redistribution and surface defect passivation under ambient conditions. As a result, our MHP CCLs with ∼4 µm thickness achieve blue uptake values of ∼80% and ∼98% for green and red emitters, respectively, with corresponding color‐conversion efficiencies of ∼45% and ∼30%, demonstrating the potential of KC‐ISP for high‐resolution micro‐display applications.

Deep‐blue perovskite light‐emitting diodes (PeLEDs) are essential for the full‐color commercialization of PeLED‐based displays. Although recent studies report external quantum efficiencies exceeding 10%, their application remains limited by low luminance (typically around 100 cd m ^{− 2} ) and spectral instability during operation. Here, a dual‐passivating additive‐based synergetic solid‐state halide exchange (DASH) process is introduced to address these challenges and realize highly bright and spectrally stable deep‐blue PeLEDs. The DASH process employs chlorodiphenylphosphine, a chlorine‐containing additive devoid of long alkyl chains that will otherwise hinder conductivity. This additive offers dual passivation via its lone‐pair electrons and chloride ions, while simultaneously enabling nondestructive chloride incorporation into pristine perovskite films, yielding improved morphology, enhanced photoluminescence quantum yield, and homogeneous halide composition. The emission spectrum is tunable from green (512 nm) to deep‐blue (434 nm), representing the largest color shift reported to date. The optimized PeLEDs exhibit peak emission at 459 nm with Commission Internationale de l'Éclairage coordinates of (0.141, 0.041), achieving a maximum luminance of 237.7 cd m ^{− 2} . This work establishes a passivation‐coupled halide‐exchange strategy that enables blue perovskite emitters to achieve both spectral stability and high luminance.

Intrinsically stretchable light‐emitting diodes (LEDs) are essential for next‐generation wearable and implantable optoelectronics. However, achieving high‐performance in intrinsically stretchable LEDs remains elusive due to the absence of a stretchable cathode that concurrently ensures efficient electron injection, mechanical compliance, and high optical reflectance. Here, we introduce a hybrid liquid metal—liquid metal particle (Hyb‐LM) cathode, engineered by selective rupture of surface liquid metal particles (LMPs), which facilitates their transformation into a continuous liquid metal (LM) layer. The resulting bilayer structure, comprising a surface LM layer and an underlying LMP layer, exhibits an exceptional combination of low work function (∼4.1 eV), high reflectance (∼90%), low sheet resistance (2.70 × 10 ^{− 2} Ω sq ⁻¹ ), and negligible resistance changes under 150% strain (R/R ₀ = 1.03 at 150% strain), overcoming fundamental limitations in state‐of‐the‐art stretchable cathodes. The Hyb‐LM cathode enables the realization of intrinsically stretchable organic LEDs with a low turn‐on voltage of 3.0 V, a maximum luminance of 17 670 cd m ⁻² , and a record‐high current efficiency of 10.35 cd A ⁻¹ , representing a critical advancement toward stretchable displays and implantable optoelectronics.

Magic-sized clusters (MSCs) serve as well-defined model systems for investigating surface-related photophysical properties due to their atomically precise structures and monodispersity. However, indium phosphide (InP) MSCs suffer from an extremely low photoluminescence quantum yield (PLQY < 1%) due to persistent surface trap states, which not only limit their utility but also hinder a fundamental understanding of their surface chemistry and photophysics. Herein, we introduce a surface-engineering strategy that overcomes this limitation by enabling the controlled in situ HF generation via Friedel-Crafts acylation chemistry, achieving a record-high PLQY of 18.1%. Comprehensive surface analyses reveal that the enhancement arises from phosphonate ligand exchange and surface oxide removal. These modifications suppress charge carrier trapping and inhibit exciton thermal quenching at room temperature. Moreover, the altered surface environment leads to red-shifted and broadened emission, not due to size heterogeneity but rather to surface-dependent electronic states in InP MSCs. Density functional theory simulations support this mechanism by demonstrating a reduced trap state density near the valence band edge and revealing that the presence of isomeric surface configurations with slightly different energy levels is responsible for the observed spectral broadening. These findings provide a molecular-level understanding of surface-dependent excitonic behavior and establish an effective strategy for overcoming the intrinsic limitations of luminescence efficiency in InP MSCs.

A dibenzocarbazole-based 4PADCB is introduced at the NiMgO _x /perovskite interface to enhance hole injection and suppress interfacial quenching. This interfacial strategy improves the peak EQE of green PeLEDs from 5.5% to 14.4%.

"My not-so-secret passion is establishing a strong safety culture in the lab… My favorite place on earth that science has taken me to is Sevilla, where I attended a conference this year…" Find out more about Himchan Cho in his Introducing…Profile.