Heterostructures: Raman revelations on van der Waals

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  • Published: Apr 1, 2016
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Heterostructures: Raman revelations on van der Waals

van der Waals sandwich

Atomic structure and optical characterization of exfoliated NiPS3. Credit: Image courtesy of Institute for Basic Science

Raman spectroscopy can be used to obtain the chemical fingerprint of a van der Waals material, according to researchers at the IBS Center for Correlated Electron Systems (CCES). Cheng-Tai Kuo, Michael Neumann, Karuppannan Balamurugan, Hyun Ju Park, Soonmin Kang, Hung Wei Shiu, Jin Hyoun Kang, Byung Hee Hong, Moonsup Han, Tae Won Noh, Je-Geun Park have reported the first successful atomically thin sheets of the magnetic Van der Waals material.

VdW materials are of increasing interest to materials scientists and technologists because of their diverse physical and chemical properties and their potential to be fashioned into a designer replacement for silicon semiconductors, for instance. In principle, a VdW heterostructure can be assembled by stacking two or more different two-dimensional semiconducting crystal layers on top of the other. Growth is possible by repeating the process several times to create a stack within which each layer is in a specified position in the stack to endow the composite with given characteristics. The hope is that VdW heterostructure would overcome limitations of 2D crystals and offer an alternative for the construction of smaller and more powerful microelectronic components.

Nickel phosphorus trisulfide

The team used the well-established sticky tape technique, to exfoliate flakes of nickel phosphorus trisulfide, NiPS3 on to silicon capped with silica. They then used Raman spectroscopy and other techniques to obtain specific information about their product.

With the current popularity of 2D VdW materials, the list of VdW materials is growing all the time. However, most of them are non-magnetic like graphene. This has put certain limitations on an otherwise most diverse exploration of material design. According to Je-Geun Park, the VdW material he and his colleagues tested contains a few layer nickel phosphorus trisulfide, NiPS3, "The compound is an intrinsically magnetic material and is an invaluable building block for the design for multi-layered VdW heterostructures." It is very rare that monolayer and multilayer, ultrathin samples of magnetic VdW materials have been obtained. The results could lay the foundations for the development of future semiconductors that are high speed, low energy consuming and very compact. VdW heterostructures and superlattices also exhibit the kind of exotic behaviour that might be useful in future spintronic devices.

Stark differences

The team recorded stark differences in the Raman spectra of thin NiPS3 from the bulk material and the Raman spectra varied clearly between sheets of different layer numbers. They point out in their paper that bulk metal phosphorus trisulfide compounds are known to exhibit magnetism and antiferromagnetic ordering strongly influenced by interlayer coupling at moderately low temperatures. However, it has been quite expensive to obtain magnetic monolayer oxide materials making them commercially inaccessible. The new study demonstrates that monolayer magnetic materials can be obtained using a magnetic metal, in this case nickel as well as other metals including, iron. The work thus takes the first steps towards spin-memory materials. The next step will be to demonstrate a magnetic monolayer material at relatively low temperature, which could ultimately lead to commercial magnetic semiconductors.

"My group is pushing on two fronts," Park told us. "One is obviously on how to utilize the newly found atomically thin sample for applications. The other is on how to use it to test some of fundamental questions." He adds that, "My ultimate goal is to use it together with other VdW materials or something else to produce heterostructures."

Related Links

Sci Rep, 2016, 6, 20904: "Exfoliation and Raman Spectroscopic Fingerprint of Few-Layer NiPS3 Van der Waals Crystals"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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