The Electronic Structure of Two Dimensional Materials
This thesis details new insights into the electronic structure of various two-dimensional materials. Firstly, by means of magneto optical Kerr effect (MOKE) we compared the magnetic properties of two-dimensional samples on ferromagnetic substrates: trilayer graphene on top of Co3O4(111)/Co(0001), graphene on Co(0001), and multilayer hexagonal boron nitride, (h-BN)(0001), on Co(0001). While graphene on cobalt without Co3O4 interlayer and trilayer h-BN on cobalt showed ordinary Co ferromagnetic hysteresis loops, the trilayer heterostructures of graphene/Co3O4(111)/Co(0001) exhibited an unusual MOKE hysteresis loop, with lack of remanence, for temperature up to 400 K. Magnetic force microscopy measurements with the reference atomic force microscopy revealed that the complex domain state was formed on graphene imprinting the Co magnetic domain structures.
Secondly, the valence band structure and spin-orbit splitting of bulk 2H-WSe2, monolayer WSe2 and multilayer WS2 were investigated by means of angle-resolved photoemission spectroscopy (ARPES). The bulk 2H-WSe2 revealed the significantly different band structures for states of even and odd reflection parities along both the Γ– K and Γ–!M lines at the surface Brillouin zone edge. The experimental and theoretical band structures, for the most part, showed agreement. The spin-orbit splitting, ΔSOC, at the top of the valence band at K was measured to be 513 ± 10 meV for monolayer WSe2, 490 ± 10 meV for bulk 2H-WSe2 and 420 ± 20 meV for multilayer WS2. The multilayer WS2(0001), grown by chemical vapor deposition (CVD), showed n-type character as characterized by both ARPES and transistor measurements.
Lastly, the influence of the metal adsorption on bulk 2H-MoS2(0001) and bulk 2H-WSe2(0001) were studied by various kinds of photoemission spectroscopy methods including ARPES and inverse photoemission spectroscopy (IPES). We observed rigid
energy shifts of the occupied and unoccupied band structures upon Co or Na adsorption on the MoS2 and WSe2. Na adsorption enhanced the n-type character of MoS2 while Co adsorption enhanced the p-type character of WSe2. Interestingly, we detected only a negligible energy shift for Co adsorped MoS2 and Na adsorped WSe2. The binding energy shifts exhibited a positive correlation with the difference of the work function between the metallic adlayer and the TMDs.