Ansoon Kim

A novel TMSI–TOP precursor enables stable red CsPb(Br/I) ₃ PeNCs, where phosphonium iodide forms and TMSOH aids defect passivation, yielding LEDs with 21.69% EQE at 669 nm and verified reliability.

Chlorinated graphene (Cl-Gr) was synthesized by a non-toxic and facile electrochemical process. The Cl-Gr gas sensor demonstrates ultrafast NO ₂ detection and rapid recovery at room temperature, which is consistent with DFT analysis.

Platinum ditelluride (1T-PtTe₂) is a two-dimensional (2D) topological semimetal with a distinctive band structure and flexibility of van der Waals integration as a promising candidate for future electronics and spintronics. Although the synthesis of large-scale, uniform, and highly crystalline films of 2D semimetals system is a prerequisite for device application, the synthetic methods meeting these criteria are still lacking. Here, we introduce an approach to synthesize highly oriented 2D topological semimetal PtTe₂ using a thermally assisted conversion called tellurization, which is a cost-efficient method compared to the other epitaxial deposition methods. We demonstrate that achieving highly crystalline 1T-PtTe₂ using tellurization is not dependent on epitaxy but rather relies on two critical factors: (i) the crystallinity of the predeposited platinum (Pt) film and (ii) the surface coverage ratio of the Pt film considering lateral lattice expansion during transformation. By optimizing the surface coverage ratio of the epitaxial Pt film, we successfully obtained 2 in. wafer-scale uniformity without in-plane misalignment between antiparallelly oriented domains. The electronic band structure of 2D topological PtTe₂ is clearly resolved in momentum space, and we observed an interesting 6-fold gapped Dirac cone at the Fermi surface. Furthermore, ultrahigh electrical conductivity down to ∼3.8 nm, which is consistent with that of single crystal PtTe₂, was observed, proving its ultralow defect density.

We present the results of an interlaboratory study on work function (WF) measurements by ultraviolet photoelectron spectroscopy (UPS) conducted under the auspices of the Versailles Project on Advanced Materials and Standards. Two samples, gold (Au) film deposited on a flat Si(100) and highly oriented pyrolytic graphite (HOPG), were distributed to six different laboratories. Prior to UPS measurements, the samples underwent common sample pre-cleaning procedures: wet treatment and Ar+-sputtering in a vacuum for Au, and mechanical peeling in air for HOPG. Instrumental settings are adjusted for energy-scale calibration and sample bias optimization. The average reference WF value (WFref) and its expanded uncertainty (Uref) were calculated from each dataset that participants provided. The results of the Au sample demonstrated 5.40 ± 0.13 eV after Ar+ sputtering in vacuum, while the HOPG sample showed 4.62 ± 0.16 eV after mechanical peeling-off. Even though the participants used slightly different sample bias voltages and sample cleaning methods with various instruments, the results demonstrate remarkable consistency. Their consistency among the laboratories for both samples with high and low WF values provides a basis for establishing a new international standard for UPS in the surface chemical analysis community.

Photocorrosion of the anode participating in photo-electrochemical (PEC) water splitting is one of the obstacles for long-term stability. To prevent photocorrosion, an “electrically leaky” thick TiO₂ film was deposited onto an n-Si photoanode surface. However, the carrier transport mechanism through the thick dielectric layer and the interface between the dielectric layer and n-Si is still unclear. In order to explore the carrier transport mechanism, we only modulated the defect density of the protective TiO_{x(1.98≤x≤2.0)} film with no significant change in optical and physical properties, and chemical composition. The fact that the defect density of the TiO_x film is proportional to water-splitting activity allows us to explain the hole transport mechanism of the previously reported electrically leaky TiO₂ protection layer in the n-Si photoanode. For the defect-level optimization, controlled incorporation of defects into TiO_{x(1.94≤x≤2.0)} dramatically enhances the hole transport from the photoanode surface to the electrolyte solution. The influence of the protection layer defect density on the band structure and water-splitting activity of the photoanode system was explored. Mott–Schottky analysis of this system suggests that the defect level of the TiO_x films influences the band bending of n-Si, which governs the accessible density of defect states and the carrier recombination. Our photoanode consisting of the 50 nm-thick TiO_x protection layer with the optimal defect density retained about 85% of the initial current density after 100 h of PEC reaction.

Charge doping to Mott insulators is critical to realize high-temperature superconductivity, quantum spin liquid state, and Majorana fermion, which would contribute to quantum computation. Mott insulators also have a great potential for optoelectronic applications; however, they showed insufficient photoresponse in previous reports. To enhance the photoresponse of Mott insulators, charge doping is a promising strategy since it leads to effective modification of electronic structure near the Fermi level. Intercalation, which is the ion insertion into the van der Waals gap of layered materials, is an effective charge-doping method without defect generation. Herein, we showed significant enhancement of optoelectronic properties of a layered Mott insulator, α-RuCl₃, through electron doping by organic cation intercalation. The electron-doping results in substantial electronic structure change, leading to the bandgap shrinkage from 1.2 eV to 0.7 eV. Due to localized excessive electrons in RuCl₃, distinct density of states is generated in the valence band, leading to the optical absorption change rather than metallic transition even in substantial doping concentration. The stable near-infrared photodetector using electronic modulated RuCl₃ showed 50 times higher photoresponsivity and 3 times faster response time compared to those of pristine RuCl₃, which contributes to overcoming the disadvantage of a Mott insulator as a promising optoelectronic device and expanding the material libraries.

Synthetic nanofluidic diodes with highly nonlinear current-voltage characteristics are currently of particular interest because of their potential applications in biosensing, separation, energy harvesting, and nanofluidic electronics. We report the ionic current rectification (ICR) characteristics of porous anodic aluminum oxide membrane, whose one ends of nanochannels are closed by the barrier oxide layer. The membrane exhibits an intriguing pH-dependent ion transport characteristics, which cannot be explained by the conventional surface charge governed ionic transport mechanism. We reveal experimentally and theoretically that the space charge density gradient present across the 40-nm-thick barrier oxide is mainly responsible for the evolution of ICR. Based on our findings, we demonstrate for the first time the formation of a single 5 ~ 8 nm-sized pore in each hexagonal cell of the barrier oxide. The present work would provide viable information for the design and fabrication of future ultra-thin nanofluidic devices without being limited by the engineering of the nanochannel geometry or surface charge.

In the mutual calibration for determination of ultrathin oxide film thickness, the thickness measured by X-ray photoelectron spectroscopy (XPS) has been known to be representative offset traceable. The offset traceability of XPS has been proved in the Consultative Committee for Amount of Substance (CCQM) pilot study P-190. For the analysis of nm HfO₂ film thickness by XPS, a general equation is used involving the relative peak intensity (R_o) of pure HfO₂ film and the substrate. The difference in the surface polarity of HfO₂ film and substrate induces different amount of surface carbon contamination, which leads to the error in R_o value. In the plot of XPS and reference thickness of HfO₂ films, the correction for the carbon contamination results in the offset value of -0.014 nm, while a larger offset value of -0.219 nm was measured without the carbon correction. Furthermore, different from the thickness measurement of SiO₂ film, the reference geometry configuration is not important factor for the thickness measurement of ultrathin amorphous HfO₂ films.