Dong Han Ha

Ultrathin PtSe₂, a member of the group-10 transition metal dichalcogenides, has emerged as a promising two-dimensional material due to its layer-dependent, tunable bandgap. Notably, a unique semiconductor-to-metal transition is predicted as the layer number of this material increases; however, pinpointing the exact critical thickness for this transition and reliably quantifying the energy gaps of the semiconducting layers remain formidable challenges. In this work, all-van der Waals assembled multiprobe schemes and planar tunnel junctions are employed to systematically investigate the thickness-sensitive charge transport properties and energy gaps of ultrathin PtSe₂ films. Temperature-dependent measurements reveal that PtSe₂ exhibits semiconducting behavior from monolayer to five layers, with a transition to a semimetallic state at six layers. Furthermore, using electron tunneling spectroscopy, we accurately quantify the energy gaps of monolayer, bilayer, and trilayer PtSe₂ and identifies that PtSe₂ in monolayer form behaves as an n-type semiconductor but intriguingly transitions to a p-type semiconductor in bilayer form. First-principles calculations highlight the importance of correctly evaluating interlayer distances to select the appropriate density functional theory functional, enabling reliable predictions of the critical thickness of ultrathin PtSe₂ for the semiconductor-to-metal transition and corresponding electronic structures.

Conductive hydrogel bioelectrodes that acquire human physiological information are vital in continuously monitoring various electrophysiological (EP) signals. However, poor adhesion during human physical/chemical activities limits their performance, adversely affecting the accuracy and continuity of biosignals. Herein, we report reduced graphene oxide (RGO) hydrogel-based epidermal bioelectrodes that can conformably adhere to the living skin and reliably record multiple EP signals during human activities. The RGO hydrogel bioelectrodes comprised polyvinyl alcohol (PVA) as the backbone material, RGO flakes as the conductive material, and polyacrylic acid (PAA) as the bioadhesive material. We have successfully developed RGO hydrogel-based EP bioelectrodes that exhibit several unique properties, including good adhesion strength (∼6.506 kPa), high electrical conductivity (∼0.11 S m⁻¹), and excellent in vitro biocompatibility (>90% cell viability). In addition, these adhesive bioelectrodes adhered firmly to different types of skin and could be reattached more than 10 times. The exceptional properties of our adhesive, biocompatible, and conductive graphene bioelectrodes were used to successfully monitor the electrooculogram, electromyogram, and electrocardiogram with human movements and sweat secretion. Our study provides a practical approach for monitoring high-fidelity EP biosignals from the skin, thus, overcoming the bottleneck of wearable bioelectronics.

Correction for ‘Raman spectroscopy study of K-birnessite single crystals’ by Dong Han Ha et al., J. Mater. Chem. A, 2025, 13, 617–626, https://doi.org/10.1039/D4TA06118G.

Raman studies of manganese dioxide (MnO₂), a crucial material for sustainable and innovative solutions in energy storage and environmental remediation, have predominantly been conducted on fine-grained aggregates, leaving the identification of Raman peaks open to debate. To address this, in this study the Raman spectra of potassium (K)-birnessite single crystals with varying crystal thickness, temperature, and polarization configuration are measured. An acoustic phonon mode of birnessite is identified, which is found to be sensitive to both crystal thickness and interlayer spacing, with its frequency increasing by approximately 35% when the c-axis lattice parameter is reduced from 0.70 to 0.65 nm by the removal of interlayer water. In contrast, the dependence of the optical phonon modes on crystal thickness and interlayer spacing is not particularly noticeable. It is demonstrated that the characteristic Raman peak of K-birnessite, observed at approximately 559 cm⁻¹, originates from a two-dimensional hexagonal configuration of cations and water molecules within the interlayer space, rather than from the MnO₆ octahedra. Additionally, the doubly degenerate vibrational mode of MnO₆ octahedra, corresponding to the motion of oxygen atoms in the basal plane, splits into two, confirming that the MnO₆ octahedra are distorted by the Jahn–Teller effect.

Electricity generation using the piezoelectric effect is an important energy harvesting method. In this study, a solar radiation pressure‐driven crater‐shaped light pressure electric generator (LPEG) device with a Pb(Zr_0.52, Ti_0.48)O₃ (PZT) piezoelectric layer and grained Ag layer is fabricated on GaAs(100) wafer. The electrical output of the device is improved by adsorbing Au nanoparticles (AuNPs) on the Ag layer with the surface of closely connected Ag nanoparticles (AgNPs). By controlling the size and concentration of the AuNPs, a maximum power density of 867.5 µW cm⁻² is obtained under a solar simulator (AM 1.5G), which represents a 151.7% improvement over the case without AuNP adsorption. The results of Raman spectra, finite‐difference time‐domain (FDTD) simulations, and COMSOL Multiphysics demonstrate that the newly formed hotspots between AgNPs─AuNPs and AuNPs─AuNPs enhance the electric field of the incident light significantly and extend the region where the electric field is maximally amplified to 600–800 nm, resulting in increased solar radiation pressure on the PZT piezoelectric layer.

A soft and implantable cracked reduced graphene oxide strain sensor with high strain sensitivity and durability was demonstrated, allowing for accurate measurement of in vivo biomechanical signals.

A light‐pressure electric generator (LPEG) device, which harvests piezoelectric energy using solar radiation enhanced by surface plasmons (SPs), is demonstrated. The design of the device is motivated by the need to drastically increase the power output of existing piezoelectric devices based on SP resonance. The solar radiation pressure can be used as an energy source by employing an indium tin oxide (ITO)/Ag double layer to excite the SPs in the near‐infrared (NIR) and visible light regions. The LPEG with the ITO layer generates an open‐circuit voltage of 295 mV, a short‐circuit current of 3.78 μA, and a power of 532.3 μW cm⁻² under a solar simulator. The power of the LPEG device incorporating the ITO layer increased by 38% compared to the device without the ITO layer. The effect of the ITO layer on the electrical output of the LPEG was analyzed in detail by measuring the electrical output when visible and NIR lights are incident on the device using optical bandpass filters. In addition, finite‐difference time‐domain simulation confirmed that the pressure of the incident light can be further amplified by the ITO/Ag double layer. Finally, the energy harvested from the LPEG was stored in capacitors to successfully illuminate red light‐emitting diodes.

Interfaces with surrounding materials, where charged impurities and surface roughness are present, have a significant impact on the electrical and optical properties of 2D materials. In the change of the phonon modes of MoS₂ accompanied by thickness variation, the portion caused by intrinsic factors and the portion caused by the interface effect are separated by examining the result of encapsulation with hexagonal boron nitride (hBN). For instance, the frequency of the A_1g peak of MoS₂ supported by SiO₂ decreases by ≈4 cm⁻¹ in air for a thickness reduction from ten layers to monolayer. Of this decrease, roughly 2 cm⁻¹ is attributable to the weakening of the van der Waals interlayer interaction, while the remaining 2 cm⁻¹ is due to the interface effect. The interface state, that is, the types and concentrations of impurities at the interface, between MoS₂ and SiO₂ is estimated to be similar to that between MoS₂ and air because the Raman properties when one surface of MoS₂ is in contact with SiO₂ and with air are identical within the measurement error. When entirely encapsulated with hBN, the width of the A_1g peak of few‐layer MoS₂ is significantly reduced, becoming comparable or equal to that of bulk MoS₂.