2, University of Georgia, Athens, Georgia, United States
A strong interest of the materials research community to quasi two-dimensional (2D) and quasi one-dimensional (1D) van der Waals materials has recently led to a renewed attention to layered materials that reveal the charge density wave (CDW) effects . Most of these materials belong to the group of transition metal dichalcogenides (TMDs) or trichalcogenides (TMTs) . Some of TMDs and TMTs have very high transitions temperatures, often above room temperature, to different CDW phases. The switching between CDW phases can be utilized for various device applications . This explains the motivations behind the search for new polymorphs of materials that can have CDW properties. In this presentation, we report results of Raman spectroscopy of recently discovered two new polymorph of Niobium Trisulfide, NbS3-IV and NbS3-V, which crystallize in the monoclinic space group P21/c and P21/m, respectively . Raman spectroscopy is a powerful tool for understanding CDW material structure and crystal lattice dynamics near the CDW transition points. The samples for this study have been prepared by the chemical vapor transport synthesis, and characterized using the X-ray diffraction and transmission electron microscopy. The Raman spectroscopy has been carried out in a wide temperature range from 90 K to 640 K using the low-wave-number filter. The peak assignment has been performed using ab initio calculations and comparison with experimental data for other polymorphs of this material. Temperature coefficients of higher frequency A1g mode for NbS3 – IV and NbS3 – V were determined to be approximately -0.01076±0.00032 cm-1/K and -0.01641±0.00241 cm-1/K, respectively. For NbS3 – V a significant deviation from a linear trend was observed at 180 K and 350 K suggesting possible phase transitions. The Raman spectroscopy results are in line with the preliminary electrical resistivity data.
This work was supported, in part, by the NSF EFRI 2-DARE project: Novel Switching Phenomena in Atomic MX2 Heterostructures for Multifunctional Applications and by UC-National Lab Collaborative Research and Training Program.
 R. Samnakay, D. Wickramaratne, T. R. Pope, R. K. Lake, T. T. Salguero, and A. A. Balandin, “Zone-folded phonons and the commensurate-incommensurate charge-density-wave transition in 1T -TaSe2 thin films,” Nano Letters, 15, 2965 (2015).
 G. Liu, S. Rumyantsev, M. A. Bloodgood, T. T. Salguero, M. Shur, and A. A. Balandin, “Low-frequency electronic noise in quasi-1D TaSe3 van der Waals nanowires,” Nano Letters, 17, 377 (2017).
 G. Liu, B. Debnath, T. R. Pope, T. T. Salguero, R. K. Lake, and A. A. Balandin, “A charge-density-wave oscillator based on an integrated tantalum disulfide–boron nitride–graphene device operating at room temperature,” Nature Nanotechnology, 11, 845 (2016).
 M. A. Bloodgood, P. Wei, E. Aytan, K. N. Bozhilov, A. A. Balandin, and T. T. Salguero, “Monoclinic structures of Niobium Trisulfide,” APL - Materials (accepted 2017).