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search.filters.applied.f.access: openAccess × End date: 2023 × Start date: 2020 × Type: Text × Subject: 2D materials × WGL Institute: PDI ×

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    The impact of ultraviolet laser excitation during Raman spectroscopy of hexagonal boron nitride thin films
    (Chichester [u.a.] : Wiley, 2020) Karim, Marwa; Lopes, Joao Marcelo J.; Ramsteiner, Manfred
    We utilized excitation in the ultraviolet (UV) spectral range for the study of hexagonal boron nitride (h-BN) thin films on different substrates by Raman spectroscopy. Whereas UV excitation offers fundamental advantages for the investigation of h-BN and heterostructures with graphene, the actual Raman spectra recorded under ambient conditions reveal a temporal decay of the signal intensity. The disappearance of the Raman signal is found to be induced by thermally activated chemical reactions with ambient molecules at the h-BN surface. The chemical reactions could be strongly suppressed under vacuum conditions which, however, favor the formation of a carbonaceous surface contamination layer. For the improvement of the signal-to-noise ratio under ambient conditions, we propose a line-scan method for the acquisition of UV Raman spectra in atomically thin h-BN, a material which is expected to play a key role in future technologies based on 2D van der Waals heterostructures. © 2020 The Authors. Journal of Raman Spectroscopy published by John Wiley & Sons Ltd
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    Evolution of Low-Frequency Vibrational Modes in Ultrathin GeSbTe Films
    (Weinheim : Wiley-VCH, 2021) Zallo, Eugenio; Dragoni, Daniele; Zaytseva, Yuliya; Cecchi, Stefano; Borgardt, Nikolai I.; Bernasconi, Marco; Calarco, Raffaella
    GeSbTe (GST) phase-change alloys feature layered crystalline structures made of lamellae separated by van der Waals (vdW) gaps. This work sheds light on the dependence of interlamellae interactions at the vdW gap on film thickness of GST alloys as probed by vibrational spectroscopy. Molecular beam epitaxy is used for designing GST layers down to a single lamella. By combining density-functional theory and Raman spectroscopy, a direct and simple method is demonstrated to identify the thickness of the GST film. The shift of the vibrational modes is studied as a function of the layer size, and the low-frequency range opens up a new route to probe the number of lamellae for different GST compositions. Comparison between experimental and theoretical Raman spectra highlights the precision growth control obtained by the epitaxial technique.