Korea Institute of Science and Technology

The demand for portable real-time monitoring technologies for air quality and hazardous gas detection has significantly increased due to the growing interest in healthy living. In particular, nitrogen dioxide (NO 2 ) is a major contributor to air pollution in urban areas, and its concentration can fluctuate rapidly in a short period of time due to traffic emissions and industrial activities. Therefore, it is necessary to develop an ultra-sensitive sensor with fast response and recovery at room temperature. In this study, we demonstrate an NO 2 gas sensor operated at room temperature using Ag-decorated Sr 2 Nb 3 O 10 (ASNO) nanosheets, fabricated through a simple solution-based exfoliation process. This sensor exhibits excellent structural stability and an enhanced response of 935 % to NO 2 gas due to catalytic effect of the Ag decorated surface. It demonstrates fast response and recovery characteristics, which makes it a good candidate for reliable real-time monitoring and repeated use. Room-temperature NO 2 sensing using Ag-decorated 2D nanosheets supports sensitive and selective urban air pollution monitoring.

While the interplay between phonon and boundary dictates thermal transport at the nanoscale, the spectral manipulation of phonon-boundary scattering is insufficiently demonstrated yet. Here, we choose a single-crystal diamond nanobeam, a material with one of the highest Debye temperature materials, to investigate the impact of modulated phonon-boundary scattering with a temperature knob. The thermal conductivity of nanobeams is measured from room temperature down to ∼140K, and we find that the value monotonically decreases scaling with T∼1.07. Compared to the model prediction based on the Boltzmann transport equation combined with ab initio calculation, we find that the experimental data increasingly deviate from the model prediction with diffuse phonon scattering as the temperature decreases. The deviation indicates the increasing portion of non-diffuse phonon-boundary scattering. Our analysis indicates that specular phonon-boundary scattering is more sensitive to temperature in single-crystal diamond compared to that of silicon. This work suggests that the diamond is a potential material platform to manipulate wave-like phonon conduction above 100K.

Cubic boron nitride (c-BN) and hexagonal boron nitride (h-BN) are known for their transparency and high 10B density, which provides a large thermal-neutron cross-section, yet their potential for space neutron shielding has not been explored. The fabrication of transparent c-BN films remains challenging, and the chemical vapor deposition growth of h-BN beyond 70nm, or with precise thickness control and high uniformity, has not been reported except by our group. Here, we present a space window design integrating an h-BN-based neutron shielding layer with advanced ceramic bulletproof layers and a γ-ray shielding layer. By incorporating C and O into h-BN, sp2-sp3 hybridized BN (HBN) reduces the refractive index mismatch with the SiO2 substrate, achieving 90.9% transmission at 550nm at 11.9µm thickness and enabling stable, transparent growth up to 79.2µm with minimized thermal expansion mismatch. The optically optimized HBN (B0.39N0.39C0.06O0.16) shows reduced boron content, but the enriched formation of 63.4% c-BN, with its higher boron density, compensates for this loss. The resultant density is 3.01g cm-3, evaluated from neutron-shielding probability, and HBN achieves the same neutron-shielding efficiency as h-BN at 3% reduced thickness.