March 17, 2016

▲ The supercomputer ‘Titan’ and possibly a glimpse into the future of highly sensitive electronic information transmitting and processing devices based on spin-memory.

Heterostructures (referred to as Van der Waals {VdW}) are attracting a great deal of attention due to their diverse physical and chemical properties. A VdW heterostructure is assembled by stacking two or more different 2D semiconducting crystals on top of each other. The structure is grown by repeating the practice, the resulting stack represents an artificial material constructed in a certain sequence, akin to Lego blocks. There has been huge interest in these materials as they hold vast potential in helping science to find a new semiconductor that can replace silicon which decomposes and segregates in a natural environment. A VdW heterostructure can overcome the limitations of 2D crystals and offer an alternative for the construction of smaller and more powerful storage devices, supercomputers, administration of medicine and enhanced memory and graphics in hand held devices. Electrons within semiconductors roam freely and have internal states or ‘spins’. These spin currents exhibit magnetic order and can be tuned to prevent energy dissipation which occurs naturally when information is processed at vast speeds. However, not all VdW’s have this spin state; scientifically known as an antiferromagnetic state. PARK Je-Geun, a scientist from CCES, explains the unique qualities of their tested material NiPS₃: “The compound nickel phosphorus trisulfide (NiPS₃) is an intrinsically magnetic material and is an invaluable building block for the design for multi-layered VdW heterostructures.” The Center is the first to obtain monolayer and multilayer samples of magnetic VdW materials; the results lay the foundation for the development of future semiconductors that are high speed, low energy consuming and highly compact.

▲ Atomic structure and optical characterization of exfoliated NiPS₃.
(a) Schematic crystal structure. View perpendicular to layers, only top layer shown (upper schematic). The atoms contained in the section shaded grey are shown in view parallel to layers (lower schematic). Unit cell (dashed outlines) & covalent bonds within anions (grey lines). Ni: Nickel / P: Phosphorus / S: Sulfur

World’s first in Korea

The VdW material the IBS team experimented on belongs to a class of transition metals phosphorus trisulfides (MPS3) and, more importantly, exhibits antiferromagnetic order. With further developments, it can, theoretically, replace silicon as an ideal material for future magnetic semiconductors. The results compiled by the IBS team have never before been reported in the form of ultrathin sheets. The team’s scientific paper, published on February 15 in Scientific Reports, outlined the potential of their work as such: “Beyond their already fascinating properties, these VdW heterostructures and superlattices may exhibit even more exotic behavior. In particular, for the design of spintronic devices, VdW materials that exhibit magnetic order would be highly desirable building blocks.”

▲ Raman spectra of exfoliated NiPS₃.
(a) Spectra of a thick sheet (d 107 nm). These spectra are indistinguishable from those reported for bulk NiPS₃. (b) Spectra of thin NiPS₃ sheets (2–7 layers), together with the spectrum of a thick sheet shown in panel (a), and the substrate spectrum. All spectra were acquired in a single pass, under identical experimental conditions.

Methods of CCES & A Bright Future for 2D Materials

Using the well-established Scotch tape technique, the Korean team exfoliated flakes of NiPS₃ onto silicon capped by silicon oxide (SiO2). The resulting material was subjected to a heavy bombardment of high intensity lasers: a process called Raman spectroscopy, designed to provide specific information about molecular vibrations. Other forms of atomic scanning were performed to ascertain how different the atomic make-up of NiPS₃ is, in comparison, to its bulk form: MPS3. 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. According to the paper these results exhibit a “key significance of our results is that bulk MPS3 compounds exhibit magnetism, and antiferromagnetic ordering strongly influenced by interlayer coupling is known to take place at moderately low temperature.” Up until now, it is quite expensive to obtain magnetic monolayer oxides materials as it requires a high-end device and the material itself is less likely to be commercially available for technical use. This study proves that monolayer magnetic material can be obtained using magnetic atom like nickel (Ni) as well as many other magnetic atoms such as iron (Fe). The work of the IBS team is, tentatively, laying the foundations for the future study of spin-memory materials. Research is continuously driven by our collective scientific curiosity; we are within reach of precise control of electrons and atoms which would herald a new era of scientific exploration. The research team’s next mission is to obtain a magnetic monolayer material at moderate temperature. If they succeed, it will be very crucial step in commercializing magnetic semiconductors.

Neil Mannix

Notes for editors

- References
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 (2016) Exfoliation and Raman Spectroscopic Fingerprint of Few- Layer NiPS₃ Van der Waals Crystals, Scientific Reports, DOI: 10.1038/srep20904

- Media Contact
For further information or to request media assistance, please contact: Mr. Shi Bo Shim, Head of Department of Communications, Institute for Basic Science (+82-42-878-8189: sibo@ibs.re.kr) or Ms. Dahee Carol Kim, Department of Communications, Institute for Basic Science (+82-42-878-8133: clitie620@ibs.re.kr)

- About the Institute for Basic Science (IBS)
IBS was founded in 2011 by the government of the Republic of Korea with the sole purpose of driving forward the development of basic science in South Korea. It comprises a total of 50 research centers in all fields of basic science, including mathematics, physics, chemistry, life science, earth science and interdisciplinary science. IBS has launched 26 research centers as of March 2016. There are eight physics, one mathematics, six chemistry, eight life science, and three interdisciplinary research centers.