Air tightness of hBN encapsulation and its impact on Raman spectroscopy of van der Waals materials

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Item type:	Article
Journal or Publication Title:	2D Materials
Publisher:	IOP
Volume:	7
Number of Issue or Book Chapter:	1
Page Range:	015012
Date:	8 November 2019
Institutions:	Physics > Institute of Experimental and Applied Physics > Chair Professor Weiss > Group Christoph Strunk
Projects:	SFB 1277: Emergente relativistische Effekte in der Kondensierten Materie: Von grundlegenden Aspekten zu elektronischer Funktionalität, KO 3612/3-1
Keywords:	Raman spectroscopy, sensitive 2D materials, photo-oxidation, hBN encapsulation, niobium diselenide
Dewey Decimal Classification:	500 Science > 530 Physics
Status:	Published
Refereed:	Yes, this version has been refereed
Created at the University of Regensburg:	Yes
Item ID:	41683

Abstract

Raman spectroscopy is a precious tool for the characterization of van der Waals materials, e.g. for the determination of the layer number in thin exfoliated flakes. For sensitive materials, however, this method can be dramatically invasive. In particular, the light intensity required to obtain a significant Raman signal is sufficient to immediately photo-oxidize few-layer thick metallic van der ...

Abstract

Raman spectroscopy is a precious tool for the characterization of van der Waals materials, e.g. for the determination of the layer number in thin exfoliated flakes. For sensitive materials, however, this method can be dramatically invasive. In particular, the light intensity required to obtain a significant Raman signal is sufficient to immediately photo-oxidize few-layer thick metallic van der Waals materials. In this work we investigated the impact of the environment on Raman characterization of thin NbSe2 crystals. We show that in ambient conditions the flake is locally oxidized even for very low illumination intensity. Based on this extreme sensitivity to the presence of light and oxygen, we could study the air-tightness of the hBN encapsulation method, the most common passivation method for a wide range of 2D material-based devices. We find that only fully encapsulated devices are reliably air-tight. On the contrary, a simple hBN cover from the top does not prevent a slow diffusion of oxygen between the SiO2 surface and the flake itself.

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