Cheng-Tai Kuo

Exfoliation and Raman Spectroscopic Fingerprint of Few-Layer NiPS3 Van der Waals Crystals

The range of mechanically cleavable Van der Waals crystals covers materials with diverse physical and chemical properties. However, very few of these materials exhibit magnetism or magnetic order, and thus the provision of cleavable magnetic compounds would supply invaluable building blocks for the design of heterostructures assembled from Van der Waals crystals. Here we report the first successful isolation of monolayer and few-layer samples of the compound nickel phosphorus trisulfide (NiPS3) by mechanical exfoliation. This material belongs to the class of transition metal phosphorus trisulfides (MPS3), several of which exhibit antiferromagnetic order at low temperature, and which have not been reported in the form of ultrathin sheets so far. We establish layer numbers by optical bright field microscopy and atomic force microscopy, and perform a detailed Raman spectroscopic characterization of bilayer and thicker NiPS3 flakes. Raman spectral features are strong functions of excitation wavelength and sample thickness, highlighting the important role of interlayer coupling. Furthermore, our observations provide a spectral fingerprint for distinct layer numbers, allowing us to establish a sensitive and convenient means for layer number determination.

Insulating-layer formation of metallic LaNiO3 on Nb-doped SrTiO3 substrate

We investigated the electronic structures of strongly correlated metallic LaNiO3 (LNO) and semiconducting Nb-doped SrTiO3 (Nb:STO) heterostructures by varying the LNO film thickness using in situ photoemission spectroscopy. We found that, contrary to other interfaces with SrTiO3 and LaAlO3, insulating LNO layers are formed between metallic LNO layers and Nb:STO. Such behavior seems to be related with an electron transfer from Nb:STO to LNO due to Schottky-barrier formation at the interface.