Evolution of optical phonons in CdS nanowires, nanobelts, and nanosheets

We report Raman scattering from single and ensemble CdS nanowires, nanobelts, and nanosheets. The Raman spectra of nanobelts and nanosheets are notably different from those of nanowires, exhibiting a strong enhancement of the multiphonon response. Moreover, the first-order longitudinal optical (LO) phonon energy systematically increases with increasing lateral size from nanowires to nanobelts, and to nanosheets. These results suggest that the optical phonons in the CdS nanostructures are influenced by strain, crystallinity, and exciton-LO phonon coupling.open342

Magnetic properties of pure and Gd doped EuO probed by NMR

An Eu NMR study in the ferromagnetic phase of pure and Gd doped EuO was performed. A complete description of the NMR lineshape of pure EuO allowed for the influence of doping EuO with Gd impurities to be highlighted. The presence of a temperature dependent static magnetic inhomogeneity in Gd doped EuO was demonstrated by studying the temperature dependence of the lineshapes. The results suggest that the inhomogeneity in 0.6% Gd doped EuO is linked to colossal magnetoresistance. The measurement of the spin-lattice relaxation times as a function of temperature led to the determination of the value of the exchange integral J as a function of Gd doping. It was found that J is temperature independent and spatially homogeneous for all the samples and that its value increases abruptly with increasing Gd doping.Comment: 14 pages, 10 figures, to be published in Physical Review

Crystal-field excitations and spin-phonon interactions in DyNi2B2C: Raman scattering study

We report polarized Raman results of magnetic superconductor DyNi2B2C (Tc=6.2 K, TN=10.3 K) to explore crystal-field (CF) excitations and spin-phonon interactions. In addition to the Ni-B1g phonon mode at 199 cm−1, we observed additional Raman modes at 124, 151, and 221 cm−1. By careful analysis of the temperature evolution of these modes, we attribute the 124 cm−1 excitation to a CF transition. The 151 and 221 cm−1 modes correspond to zone-folded phonons because they are completely quenched for T>TN. With increasing temperature across TN, the 124 cm−1 excitation diminishes rapidly in intensity and, interestingly, an additional mode appears at 119 cm−1. This excitation grows in intensity with increasing temperature toward 50 K and gradually decreases with increasing temperature further. We attribute the 119 cm−1 excitation to an excited CF transition from a low-lying CF level at 5 cm−1 to the higher CF level at 124 cm−1. Anomalous temperature-dependent behavior of the Ni-B1g phonon mode was observed in peak energy and in spectral width, i.e., both the phonon energy and the linewidth are enhanced in the vicinity of TN, suggesting the presence of strong spin-phonon interactions near the antiferromagnetic ordering temperature in DyNi2B2C.This article is from Physical Review B 82 (2010): 064423, doi:10.1103/PhysRevB.82.064423. Posted with permission.</p

Polarized Raman study of large built-in strain in monolayer WS2 grown on Au/W substrate

© 2022 Korean Physical Society. Strain plays a crucial role in the energy landscape of atomically thin two-dimensional materials. Here, we report polarized Raman results of WS2 monolayers having either rough or wrinkled surface morphologies. For rough WS2, the E′ phonon intensity exhibits polar behavior that deviates from the Raman polarization selection rules. Furthermore, with the formation of wrinkles on the WS2 surface, the E′ phonon splits into E′− and E′+ phonons. Polar plots of the E′− and E′+ phonon intensities as a function of polarization angle θ show that both phonons have strong polar dependence with orthogonal characteristics in their polarized intensity profiles: the E′− phonon intensity is maximum when the E′+ phonon intensity is diminished, and vice versa. Our result demonstrates that built-in strain in low-dimensional materials can be quantitatively identified by polarized Raman studies and further provides a useful strategy to achieve better device performance for which strain engineering is required.11Nsciescopuskc

Suppression of substrate-induced charge doping in hBN-encapsulated monolayer WS2

Strong coupling between a two-dimensional material and substrate is problematic in that interlayer charge transfer at the interface often complicates optoelectronic device applications. Such substrate-induced charge doping effects are particularly large for two-dimensional materials on SiO2/Si substrate. Here, we report a Raman study of monolayer WS2 encapsulated with hexagonal boron nitride (hBN) to unveil substrate-related charge doping phenomena under the influence of laser irradiation. Raman correlation analysis between the E′ and A1′ phonon frequencies of WS2 reveals that the encapsulation of WS2 with hBN leads to a screening of electron transfer from SiO2 to WS2. Further, photo-induced charge doping in WS2 is completely suppressed by hBN encapsulation. Our results demonstrate that understanding the photo-induced charge doping effects in two-dimensional heterostructures is useful in characterizing the role of hBN encapsulation and, thus, shed light on design strategies for efficient two-dimensional optoelectronic applications where a precise control of charge doping is required. © 2023 Korean Physical SocietyFALS

Raman imaging of strained bubbles and their effects on charge doping in monolayer WS2 encapsulated with hexagonal boron nitride

Interfacial defects significantly affect the optical and electronic properties of two-dimensional (2D) materials. Particularly, bubbles inevitably formed during the layer-by-layer fabrication of 2D heterostructures can cause spatially inhomogeneous distributions in charge density and strain, leading to modifications in emission efficiency and electronic structures that are markedly different depending on which interface the bubbles form. Here, we report spatially resolved Raman results of a hBN/WS2/hBN heterostructure over a large area in which microbubbles are present. Spatial variations in the optical phonon characteristics of both WS2 and hBN reveal that the bubbles are formed at the interface between the top hBN layer and the underlying WS2 monolayer. The presence of the hBN bubbles results in a relatively higher electron density of the underlying WS2 than that of the WS2 in the bubble-free surrounding flat region, possibly due to the flexoelectric effect of the bent hBN layer. In addition, the Grüneisen parameter of hBN is obtained using the relationship between E2g phonon frequency shifts and corresponding strain profiles of the hBN bubble layer. Our work inspires a more comprehensive understanding of charge and strain distributions under the effect of interfacial defects. © 2022 Elsevier B.V.FALS