Won Kook Choi

Metal organic chalcogenolates (MOCs) constitute a promising class of materials for optoelectronic applications owing to their unique 2D layered hybrid structure and inherent environmental stability. Among these materials, mithrene (silver phenylselenolate, AgSePh) is particularly compelling because of its sharp blue emission and notable anisotropic excitonic properties. However, conventional solvent-assisted mithrene synthesis methods are often associated with the introduction of chemical complexities as well as compromised film quality. Addressing these limitations, this study provides a robust solvent-free strategy for synthesizing high-quality mithrene thin films through precise pressure and temperature control in an inert gas environment, leading to optimized reaction kinetics. Comprehensive characterization through grazing incidence wide-angle X-ray scattering (GIWAXS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV-vis absorption, and photoluminescence (PL) spectroscopy revealed that the resulting films possess greatly improved crystallinity, enhanced excitonic absorption, and significantly greater PL emission than their solvent-processed counterparts. Notably, a previously unreported excitonic feature (X_α) was identified, possibly originating from the high structural coherence along the out-of-plane direction achieved through our method. This study not only provides an advanced solvent-free route for high-quality MOC thin film fabrication but also unlocks avenues for their broader integration into next-generation optoelectronic devices, semiconductors, and catalysts.

Fabricating freestanding films with pristine MXene aims at meeting all-around high standards of modern electronics through boosting high electrical conductivity, thermal conductivity, and robust mechanical strength. However, voids and defects arising during fabrication often limit the performance and versatility. An effective electrostatic repulsion-induced alignment strategy is suggested here to overcome the major obstacles in the industrial application of MXene films. This approach combines polydopamine (PDA)-modified MXene with two-dimensional (2D) tantalum disulfide (TaS₂) nanosheets to enhance the alignment and compactness of MXene layers. Incorporating a small amount of TaS₂ nanosheets, which carry negative charges, facilitates the alignment of PDA-modified MXene sheets during film assembly via electrostatic repulsion. This results in the fabrication of strong and well-ordered composite films with a synergistic effect between MXene and TaS₂. The resultant MXene/TaS₂ (MXT) films exhibit high mechanical strength (92 MPa), toughness (2.49 MJ·m^-3), excellent electrical conductivity (7932 S cm^-1), electromagnetic interference (EMI), shielding effectiveness (SE) (62 dB), and superior in-plane thermal conductivity (53.19 W·m^-1·K^-1). In addition, MXT films demonstrate remarkable Joule heating performance and environmental stability.

In this work, we report the first attempt to investigate the dependence of thioacetamide (TAA) on the size of ZnO nanoparticles (NPs) in forming ZnS nanostructures from ZnO. Size-controlled B(blue)_, G(green)_, and Y(yellow)_ZnO quantum dots (QDs) and NC (nanocrystalline)_ZnO NPs were synthesized using a sol–gel process and a hydrothermal method, respectively, and then reacted with an ethanolic TAA solution as a sulfur source. ZnO QDs/NPs began to decompose into ZnS QDs through a reaction with TAA for 5~10 min, so rather than forming a composite of ZnO/ZnS, ZnO QDs and ZnS QDs were separated and remained in a mixed state. At last, ZnO QDs/NPs were completely decomposed into ZnS QDs after a reaction with TAA for 1 h irrespective of the size of ZnO QDs up to ~50 nm. All results indicate that ZnS formation is due to direct crystal growth and/or the chemical conversion of ZnO to ZnS.

In this work, we report on the first attempt using size-controlled ZnO nanoparticles (NPs) to analyze role of thioacetamide (TAA) in forming ZnS nanostructures from ZnO. The size-controlled B(blue)_, G(green)_, Y(yellow)_ZnO QDs, and NC (nanocrystalline)_ZnO NPs were synthesized by sol-gel process as well as hydrothermal method, respectively and then reacted with an ethanolic TAA solution as sulfur source. After reacting with TAA, the formation of ZnS QDs was observed over time. The chemical reactions were identified using X-ray diffraction (XRD), Transmission Electron Microscope (TEM), UV-vis spectroscopy, and Photoluminescence (PL). All results indicate that ZnS formation is due to direct crystal growth and/or chemical conversion of ZnO to ZnS. Our results enable a broader understanding of the synthetic mechanisms involved in the use of TAA as a sulfur source in forming ZnS nanostructures from ZnO.

Non‐toxic heavy metal, low‐cost indium‐free blue emissive quantum dots (QDs) are critical for the realization of self‐emissive electroluminescent quantum dot (ELQD) light‐emitting diode (LED) displays. A facile and effective hybridization of ZnO QDs with functionalized polyaromatic hydrocarbons (f‐PAHs) to synthesize a very bright and stable blue‐light emissive quantum‐dot LED (QLED) using non‐toxic quasi‐type‐II ZnO@1‐Aminopyrene hybrid QDs is reported. The optimized QLEDs of ITO/PEDOT:PSS/TFB/ZnO@1‐Aminopyrene/TPBi/LiF/Al demonstrate very bright deep blue‐light electroluminescence of 3379 cd m⁻² centered at λ = 441 nm with a full‐width‐at‐half maximum of 41.7 nm and CIE 1931 color coordinates of 0.17 and 0.09, a luminous efficacy of 3.32 cd A⁻¹, power efficiency of 2.45 lm W⁻¹, external quantum efficiency of 2.35%, and a remarkably long lifetime T₅₀ of >17 830 h at 100 cd m⁻².

The widely used ZnO quantum dots (QDs) as an electron transport layer (ETL) in quantum dot light-emitting diodes (QLEDs) have one drawback. That the balancing of electrons and holes has not been effectively exploited due to the low hole blocking potential difference between the valence band (VB) (6.38 eV) of ZnO ETL and (6.3 eV) of CdSe/ZnS QDs. In this study, ZnO QDs chemically reacted with capping ligands of oleic acid (OA) to decrease the work function of 3.15 eV for ZnO QDs to 2.72~3.08 eV for the ZnO-OA QDs due to the charge transfer from ZnO to OA ligands and improve the efficiency for hole blocking as the VB was increased up to 7.22~7.23 eV. Compared to the QLEDs with a single ZnO QDs ETL, the ZnO-OA/ZnO QDs double ETLs optimize the energy level alignment between ZnO QDs and CdSe/ZnS QDs but also make the surface roughness of ZnO QDs smoother. The optimized glass/ITO/PEDOT:PSS/PVK//CdSe/ZnS//ZnO-OA/ZnO/Ag QLEDs enhances the maximum luminance by 5~9% and current efficiency by 16~35% over the QLEDs with a single ZnO QDs ETL, which can be explained in terms of trap-charge limited current (TCLC) and the Fowler-Nordheim (F-N) tunneling conduction mechanism.

Both PL and EPR are simultaneously adopted to systematically elucidate the defect centers of green luminescence (GL) as well as the EPR peak g = 1.96 of ZnO and the relationship between them. The PL of ZnO QDs reveals that GL of 2.21–2.31 eV disappears at excitation wavelengths > 400 nm. This is related to the electronic transition from conduction band (CB), or shallow donor defect centers, to deep defects of V_Zn‐H complexes at ≈0.9 eV above the valence band (VB). The EPR peak g = 1.96 emerged only when irradiated with light shorter than 400 nm, which is explicitly correlated with the electrons trapped in the CB or the shallow donors participating in the GL. A spin‐spin relaxation time (T₂) estimated from the peak‐to‐peak line width ΔH_pp in the EPR signal is ≈17.5–52.5 ns, which is two orders of magnitude longer than known values for bulk or thin‐film ZnO.