Controlling the Geometries of Si Nanowires through Tunable Nanosphere Lithography

A tunable nanosphere lithography (NSL) technique is combined with metal-assisted etching of silicon (Si) to fabricate ordered, high-aspect-ratio Si nanowires. Non-close-packed structures are directly prepared via shear-induced ordering of the nanospheres. The spacing between the nanospheres is independent of their diameters and tuned by changing the loading of nanospheres. Nanowires with spacings between 110 and 850 nm are easily achieved with diameters between 100 and 550 nm. By eliminating plasma or heat treatment of the nanospheres, the diameter of the nanowires fabricated is nearly identical to the nanosphere diameter in the suspension. The elimination of this step helps avoid common drawbacks of traditional NSL approaches, leading to the high-fidelity, large-scale fabrication of highly crystalline, nonporous Si nanowires in ordered hexagonal patterns. The ability to simultaneously control the diameter and spacing makes the NSL technique more versatile and expands the range of geometries that can be fabricated by top-down approaches

Unusual Pressure Response of Vibrational Modes in Anisotropic TaS₃

We report on the unique vibrational properties of 2D anisotropic orthorhombic tantalum trisulfide (<i>o</i>-TaS₃) measured through angle-resolved Raman spectroscopy and high-pressure diamond anvil cell studies. Our broad-spectrum Raman measurements identify optical and low-frequency shear modes in pseudo-1D o-TaS₃ for the first time, and introduce their polarization resolved Raman responses to understand atomic vibrations for these modes. Results show that, unlike other anisotropic systems, only the S_∥ mode at 54 cm^–1 can be utilized to identify the crystalline orientation of TaS₃. More notably, high-pressure Raman measurements reveal previously unknown four distinct types of responses to applied pressure, including positive, negative, and nonmonotonic dω/d<i>P</i> behaviors which are found to be closely linked to atomic vibrations for involving these modes. Our results also reveal that the material approaches an isotropic limit under applied pressure, evidenced by a significant reduction in the degree of anisotropy. Overall, these findings significantly advance not only our understanding of their fundamental properties of pseudo-1D materials but also our interpretations of the vibrational characteristics that offer valuable insights about thermal, electrical, and optical properties of pseudo-1D material systems

In-Plane Optical Anisotropy and Linear Dichroism in Low-Symmetry Layered TlSe

In-plane anisotropy of layered materials adds another dimension to their applications, opening up avenues in diverse angle-resolved devices. However, to fulfill a strong inherent in-plane anisotropy in layered materials still poses a significant challenge, as it often requires a low-symmetry nature of layered materials. Here, we report the fabrication of a member of layered semiconducting A^IIIB^VI compounds, TlSe, that possesses a low-symmetry tetragonal structure and investigate its anisotropic light–matter interactions. We first identify the in-plane Raman intensity anisotropy of thin-layer TlSe, offering unambiguous evidence that the anisotropy is sensitive to crystalline orientation. Further <i>in-situ</i> azimuth-dependent reflectance difference microscopy enables the direct evaluation of in-plane optical anisotropy of layered TlSe, and we demonstrate that the TlSe shows a linear dichroism under polarized absorption spectra arising from an in-plane anisotropic optical property. As a direct result of the linear dichroism, we successfully fabricate TlSe devices for polarization-sensitive photodetection. The discovery of layered TlSe with a strong in-plane anisotropy not only facilitates its applications in linear dichroic photodetection but opens up more possibilities for other functional device applications