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Park, Jun Hong,Vishwanath, Suresh,Wolf, Steven,Zhang, Kehao,Kwak, Iljo,Edmonds, Mary,Breeden, Michael,Liu, Xinyu,Dobrowolska, Margaret,Furdyna, Jacek,Robinson, Joshua A.,Xing, Huili Grace,Kummel, Andr American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.34
<P>To fabricate practical devices based on semiconducting two-dimensional (2D) materials, the source, channel, and drain materials are exposed to ambient air. However, the response of layered 2D materials to air has not been fully elucidated at the molecular level. In the present report, the effects of air exposure on transition metal dichalcogenides (TMD) and metal dichalcogenides (MD) are studied using ultrahigh-vacuum scanning tunneling microscopy (STM). The effects of a 1-day ambient air exposure on MBE-grown WSe2, chemical vapor deposition (CVD)-grown MoS2, and MBE SnSe2 are compared. Both MBE grown WSe2 and CVD-grown MoS2 display a selective air exposure response at the step edges, consistent with oxidation on WSe2 and adsorption of hydrocarbon on MoS2, while the terraces and domain/grain boundaries of both TMDs are nearly inert to ambient air. Conversely, MBE-grown SnSe2, an MD, is not stable in ambient air. After exposure in ambient air for 1 day, the entire surface of SnSe2 is decomposed to SnOx and SeOx as seen with X-ray photoelectron spectroscopy. Since the oxidation enthalpy of all three materials is similar, the data is consistent with greater oxidation of SnSe2 being driven by the weak bonding of SnSe2.</P>
Anisotropic AC Magnetic Susceptibility in (Ga,Mn)As Films
Xiang Li,Sining Dong,Taehee Yoo,Xinyu Liu,Sanghoon Lee,Furdyna, Jacek K.,Dobrowolska, Margaret IEEE 2015 IEEE transactions on magnetics Vol.51 No.11
<P>We report a systematic investigation of ac magnetic susceptibility in (Ga,Mn)As films as a function of temperature and magnetic field, carried out in parallel with dc magnetization measurements. The temperature dependence of ac susceptibility χ<SUB>ac</SUB> shows anisotropic behavior: 1) a single peak in χ<SUB>ac</SUB> is observed close to T for the [11̅0] orientation of the driving ac field; 2) a single peak is also seen close to 22 K for the field along [110]; and 3) peaks at both these temperatures are observed for the field applied along [100]. A detailed analysis of the ac and dc data unambiguously indicates that the peak near T is related to the paramagnetic-to-ferromagnetic phase transition, with the ferromagnetic domains nucleating with their easy axes aligned with the [11̅0] direction, providing a clear picture of uniaxial domain behavior near T . The peak near 22 K, on the other hand, is related to the onset of biaxial domain structure in (Ga,Mn)As induced by the competition between uniaxial and cubic anisotropy. More specifically, the ac susceptibility peak near T<SUB>C</SUB> involves 180° magnetization flips along the [11̅0] easy axis of the domains, while the peak near 22 K originates from magnetization wobbling between two biaxial easy axes separated by a small angle.</P>