Tungsten Nitride (W5N6): An Ultraresilient 2D Semimetal.

Two-dimensional transition metal nitrides offer intriguing possibilities for achieving novel electronic and mechanical functionality owing to their distinctive and tunable bonding characteristics compared to other 2D materials. We demonstrate here the enabling effects of strong bonding on the morphology and functionality of 2D tungsten nitrides. The employed bottom-up synthesis experienced a unique substrate stabilization effect beyond van-der-Waals epitaxy that favored W5N6 over lower metal nitrides. Comprehensive structural and electronic characterization reveals that monolayer W5N6 can be synthesized at large scale and shows semimetallic behavior with an intriguing indirect band structure. Moreover, the material exhibits exceptional resilience against mechanical damage and chemical reactions. Leveraging these electronic properties and robustness, we demonstrate the application of W5N6 as atomic-scale dry etch stops that allow the integration of high-performance 2D materials contacts. These findings highlight the potential of 2D transition metal nitrides for realizing advanced electronic devices and functional interfaces.

Deep-Ultraviolet and Helicity-Dependent Raman Spectroscopy for Carbon Nanotubes and 2D Materials.

Recent progress of Raman spectroscopy on carbon nanotubes and 2D materials is reviewed as a topical review. The Raman tensor with complex values is related to the chiral 1D/2D materials without mirror symmetry for the mirror in the propagating direction of light, such as chiral carbon nanotube and black phosphorus. The phenomenon of complex Raman tensor is observed by the asymmetric polar plot of helicity-dependent Raman spectroscopy using incident circularly-polarized lights. First-principles calculations of resonant Raman spectra directly give the complex Raman tensor that explains helicity-dependent Raman spectra and laser-energy-dependent relative intensities of Raman spectra. In deep-ultraviolet (DUV) Raman spectroscopy with 266 nm laser, since the energy of the photon is large compared with the energy gap, the first-order and double resonant Raman processes occur in general k points in the Brillouin zone. First-principles calculation is necessary to understand the DUV Raman spectra and the origin of double-resonance Raman spectra. Asymmetric line shapes appear for the G band of graphene for 266 nm laser and in-plane Raman mode of WS2 for 532 nm laser, while these spectra show symmetric line shapes for other laser excitation. The interference effect on the asymmetric line shape is discussed by fitting the spectra to the Breit-Wigner-Fano line shapes.

Interacting Phonons between Layers in Raman Spectra of Carbon Nanotubes inside Boron Nitride Nanotubes.

We present the resonant Raman spectra of a single-wall carbon nanotube inside a multiwall boron nitride nanotube (SWNT@BNNT). At EL = 1.58 eV, SWNT@BNNT exhibited resonant Raman spectra at 807 (ωBN) and 804 cm-1 (ωGr). Their intensities almost disappeared at EL = 2.33 eV. We assigned ωBN to the out-of-plane BN phonon mode that coupled with ωGr. At EL = 4.66 eV, the G+ and G- bands of the SWNT@BNNT red-shifted 3.8 cm-1 compared with the SWNT, suggesting the interwall interactions between the in-plane modes of SWNT and BNNT. Moreover, the E2g mode of the BNNT in SWNT@BNNT appeared at 1370.3 ± 0.1 cm-1, which is undistinguishable for EL < 3 eV because of the overlap with the D band frequency. The assignment of the present Raman spectra was confirmed through the first-principles calculations.

Characterizing temperature-dependent optical properties of (MA0.13FA0.87) PbI3 single crystals using spectroscopic ellipsometry.

In this paper, we present spectroscopic ellipsometry measurements of (MA0.13FA0.87)PbI3 single crystals assessed at photon energies of 0.73-6.42 eV and at temperatures between 4.4 and 400 K. At room temperature, the refractive index was dispersed as a function of frequency, which is typical of a semiconductor. The absorption spectrum exhibited several electronic transitions. We estimated a room temperature direct band gap of 1.66 ± 0.02 eV and exciton binding energy of 40 meV. With decreasing temperature, the refractive index increased. The room-temperature thermo-optic coefficients were -1.7 × 10-4 and -2.5 × 10-4 K-1 at wavelength of 600 and 1200 nm. The exciton peak position and bandgap energy exhibited a redshift, which was attributed to a reverse ordering of the band structures. Additionally, an anomaly in exciton peak position and bandgap occurred at approximately 100-200 K due to the structural phase transition. This phenomenon was associated with the coexistence of MA/FA-disordered and MA/FA-ordered domains. Our results provide a foundation for the technological development of lead halide perovskites-based photonic devices at various temperatures.