Research Projects

Tuning valley properties in 2D semiconductors by magnetic proximity effects

The information in the digital world is encoded by two distinct states of a physical system, such as the “on” and “off” states of a transistor. An emerging field called “valleytronics” proposes to use the valley degree of freedom for information processing and storage, promising electronic devices with high speed and low power consumption. An “energy valley” is the local extremum of the electronic bands in a crystalline solid, not much different from the peaks and valleys in mountains. A single layer of a transition metal dichalcogenide (TMD) that contains only 3 atomic layers is just such a system with two distinct valley states that may be used for valleytronics.

The two valleys of the TMDs possess the same energy but opposite momentum. Light with opposite circular polarizations can be used to access and control different valleys. However, from the point of view of device integration, it is desirable to control valleys using means other than light helicity. This often requires lifting the valley degeneracy. Recent experiments demonstrated that an external magnetic field can be used to pull the energy levels apart, and split the energies of the valley states. However, the splitting obtained is rather modest for any practical use. In our recent work, we demonstrated that a ferromagnetic material can used as an “amplifier” to enlarge the effective magnetic field by more than an order of magnitude, achieving a greatly enhanced valley splitting.

Large, tunable and non-volatile valley splitting allows convenient control of valley polarization, e.g. by an electric field, thus offering new paradigm for information processing.

  1. Zhao, C. et al. “Enhanced valley splitting in monolayer WSe2 due to magnetic exchange field”, Nature Nanotech., 12, 757 (2017).
  2. Taheri, P. et al. “Growth mechanism of largescale MoS2 monolayer by sulfurization of MoO3 film”, Mater Res. Express 3, 075009 (2016).
  3. J. Wang, et al. “Robust ferromagnetism in Mn-doped MoS2 nanostructures”, Appl. Phys. Lett. 109, 092401 (2016).

Chalcogenide Perovskites – An emerging class of unconventional semiconductors

Perovskites refer to a class of crystalline compounds adopting the generic chemical formula ABX3, where cation “B” has six nearest-neighbor anions “X” and cation “A” sits in a cavity formed by eight corner-shared BX6 octahedra. They demonstrate a rich spectrum of physical phenomena such as characteristics of a 2D electron gas, ferroelectricity/piezoelectricity, ferromagnetism, colossal magnetoresistance, multiferroicity, ionic conductivity, and superconductivity. Most widely studied perovskites are metal-oxide or organic-inorganic halides. The power conversion efficiency of solar cells made of halide perovskites has witnessed an unprecedented rate of increase, from an initial PCE of 3.8% in 200918 to above 22% in 2016. Their toxicity and instability, however, have remained major roadblocks for their practical applications.

As opposed to their oxide and halide siblings, chalcogenide perovskites have received little attention, despite being synthesized more than a half century ago. Theoretical studies predict that some of these materials are direct bandgap materials with strong light absorption and good carrier mobility and are therefore particularly attractive for photovoltaic and optoelectronic applications. Our subsequent experimental effort has confirmed that a prototypical chalcogenide perovskite BaZrS3 possesses a bandgap value close to 1.7-1.8 eV, in good agreement with theoretical predictions.

In this project we aim to investigate this novel class of unconventional semiconductors. Their ionic nature of bonding could make them highly defect tolerant, which is beneficial to carrier transport for photovoltaic and electronic applications in demanding environment. The truly remarkable attributes of chalcogenide perovskites lie in their intrinsic multifunctionality, owing to their proximity to oxide perovskites with diverse physical properties and their strong coupling to visible and infrared light. They may only possess ferroelectricity and/or ferromagnetism together with semiconducting properties in a single-phase material, but these functionalities may be programmable in real time by external knobs such as an electric field, magnetic field and light. Such capabilities, absent in conventional materials, should provide extra degrees of freedom that could transform the device design paradigm.

  1. Gross, N et al. “Stability and band gap tuning of the chalcogenide perovskite BaZrS3 in Raman and optical investigations at high pressure”, Phys. Rev. Appl., 8, 044014 (2017).
  2. Perera, S et al. “Chalcogenide perovskites–an emerging class of ionic semiconductors”, Nano Energy 22, 129-135 (2016).