UC Berkeley

The recent discoveries of ferroic properties, such as ferromagnetism and ferroelectricity, in two-dimensional van der Waals (2D vdW) materials have generated a surge of interest in the exploration of fundamental interaction in reduced dimensions and functional device applications.

On one hand, ferromagnetism in 2D vdW crystals enables better understanding of fundamental interactions in ultra-thin condensed matter systems. However, existing material candidates face limitations, such as the lack of room-temperature magnetic ordering. Here we report the room-temperature ferromagnetism in Co-doped graphene-like ZnO, a novel chemically stable 2D material, down to single-atom thickness. Through magneto-optic Kerr effect and superconducting quantum interference device measurements, we identify the spontaneous magnetization in such systems at room temperature and above. X-ray characterization shows that the Co atoms form Co2+ states in graphitic ZnO. By varying the Co doping level, we observe an exotic manipulation among paramagnetic, ferromagnetic, and disordered phases. Our development of Co-doped 2D ZnO opens an appealing path to 2D ferromagnetism persisting at room temperature with unique controllability.

On the other hand, the stabilization of 2D ferroelectricity overcomes the thickness limit. In contrast with conventional 3D ferroelectrics, 2D ferroelectrics show great advantages, including but not limited to, high mobility, perfect interface, allowing for non-destructive reading, being fatigueless. In this dissertation, we report the physical vapor deposition (PVD) of 2D vdW SnSe directly onto the silicon substrate. The synthesis process occurs at relatively low temperatures; the deposition temperature is 250 ℃. By varying the time of synthesis, we are able to manipulate the thickness of SnSe single crystals, which spans in a large range from 6 nm to the bulk. Out-of-plane ferroelectricity is detected in SnSe at room temperature (300 K) by means of piezoresponse force microscopy (PFM). More importantly, the ferroelectric switching voltage scales linearly with the sample thickness. A 6-nm-thick SnSe sheet displays sub-0.3 V switching, which is substantially smaller than those of previously reported 2D ferroelectrics and hafnium oxides. Moreover, the ferroelectric mechanisms are unraveled by transmission electron microscopy (TEM); AA stacking of vdW layers and interlayer sliding give rise to the vertical ferroelectric polarization. The ferroelectric dipole, as a new degree of freedom, can effectively manipulate the optical and electrical behaviors in SnSe. These results indicate that 2D SnSe is a promising candidate for future energy-efficient and non-volatile electronics or optoelectronics, which is highly compatible with the emergent complementary metal oxide semiconductor (CMOS) technologies.