Palladium
diselenide (PdSe2), a peculiar noble metal
dichalcogenide, has emerged as a new two-dimensional material with
high predicted carrier mobility and a widely tunable band gap for
device applications. The inherent in-plane anisotropy endowed by the
pentagonal structure further renders PdSe2 promising for
novel electronic, photonic, and thermoelectric applications. However,
the direct synthesis of few-layer PdSe2 is still challenging
and rarely reported. Here, we demonstrate that few-layer, single-crystal
PdSe2 flakes can be synthesized at a relatively low growth
temperature (300 °C) on sapphire substrates using low-pressure
chemical vapor deposition (CVD). The well-defined rectangular domain
shape and precisely determined layer number of the CVD-grown PdSe2 enable us to investigate their layer-dependent and in-plane
anisotropic properties. The experimentally determined layer-dependent
band gap shrinkage combined with first-principle calculations suggest
that the interlayer interaction is weaker in few-layer PdSe2 in comparison with that in bulk crystals. Field-effect transistors
based on the CVD-grown PdSe2 also show performances comparable
to those based on exfoliated samples. The low-temperature synthesis
method reported here provides a feasible approach to fabricate high-quality
few-layer PdSe2 for device applications.
A new ultrahigh-resolution photoemission electron microscope called PEEM3 is being developed at the Advanced Light Source (ALS). An electron mirror combined with a sophisticated magnetic beam separator is used to provide simultaneous correction of spherical and chromatic aberrations. Installed on an elliptically polarized undulator beamline, PEEM3 will be operated with very high spatial resolution and high flux to study the composition, structure, electric and magnetic properties of complex materials.
To realize the origin of efficient spin injection at organic-ferromagnetic contact in organic spintronics, we have implemented the formation of quasi-molecular magnet via surface restructuring of a strong organic acceptor, tetrafluoro-tetracyano-quinodimethane (F4-TCNQ), in contact with ferromagnetic cobalt. Our results demonstrate a spin-polarized F4-TCNQ layer and a remarkably enhanced magnetic anisotropy of the Co film. The novel magnetic properties are contributed from strong magnetic coupling caused by the molecular restructuring that displays an angular anchoring conformation of CN and upwardly protruding fluorine atoms. We conclude that the π bonds of CN, instead of the lone-pair electrons of N atoms, contribute to the hybridization-induced magnetic coupling between CN and Co and generate a superior magnetic order on the surface.
Using x-ray photoemission electron microscopy and the magneto-optical Kerr effect, we have demonstrated a perpendicular magnetic anisotropy that could be due to exchange coupling between the ferromagnetic and antiferromagnetic layers. The results of magnetic imaging and hysteresis loops show that the magnetization of Fe and permalloy (Py) films orients from the in-plane to perpendicular direction, as an Mn underlayer is above a threshold value that depends on the Fe or Py layer thickness. Their thickness-dependent behaviors can be quantitatively described by a phenomenological model that takes into account the finite-size effect of the antiferromagnet on exchange coupling. The anisotropy energy extracted from the model and the thermal stability of perpendicular magnetization enhanced with the increase of the Mn underlayer further demonstrate the exchange coupling nature.
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