The Electronic Structure of Two Dimensional Materials
Thesis Defense
2:00 pm
Jorgensen Hall Room: 245
Iori Tanabe
Advisor: Prof. Peter A. Dowben
Abstract:
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: the trilayer graphene on top of the Co3O4(111)/Co(0001), the graphene on Co(0001), and multilayer hexagonal boron nitride, (h-BN)(0001), on Co(0001). The trilayer heterostructures of graphene/Co3O4(111)/Co(0001) exhibited an unusual MOKE hysteresis loops, with lack of remanence, for temperature up to 400 K. The magnetic force microscopy (MFM) measurements with the reference atomic force microscopy (AFM) revealed that the complex domain state was formed on graphene imprinting the Co magnetic domain structures. By way of comparison, graphene on the Co substrates without the Co3O4 interlayer yielded ordinary Co ferromagnetic hysteresis loops. The result suggests that graphene spin polarization was induced by the Co magnetization, mediated through the Co3O4 interlayer for the graphene/Co3O4(111)/Co(0001) system, while graphene merely acted as a spectator to cobalt for the graphene/Co(0001) system. Multilayer h-BN films were grown by atomic layer deposition on Co(0001) substrates. The MOKE measurements for the trilayer h-BN/CO samples again showed ordinary Co ferromagnetic hysteresis loops and no evidence of Co oxide formation after the prolonged air exposure. This last point suggests that trilayer h-BN films on Co inhibited air oxidation.
Secondly, the valence band structure and spin-orbit splittings of bulk 2H-WSe2, monolayer WSe2 and multilayer WS2 were investigated by means of angle-resolved photoemission spectroscopy (ARPES). The 2H-WSe2 revealed the significantly different band structures for states of even and odd reflection parities along the high symmetry points of the surface Brillouin zone. The experimental and theoretical band structures, for the most part, showed agreement. The spin-orbit splitting, ?SOC, 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), 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 the rigid energy shifts of the occupied and unoccupied band structures upon the 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 shifts for Co adsorped MoS2 and Na adsorped WSe2. The binding energy shifts exhibited the positive correlation with the difference of the work function between the metallic adlayer and the TMDs.
Advisor: Prof. Peter A. Dowben
Abstract:
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: the trilayer graphene on top of the Co3O4(111)/Co(0001), the graphene on Co(0001), and multilayer hexagonal boron nitride, (h-BN)(0001), on Co(0001). The trilayer heterostructures of graphene/Co3O4(111)/Co(0001) exhibited an unusual MOKE hysteresis loops, with lack of remanence, for temperature up to 400 K. The magnetic force microscopy (MFM) measurements with the reference atomic force microscopy (AFM) revealed that the complex domain state was formed on graphene imprinting the Co magnetic domain structures. By way of comparison, graphene on the Co substrates without the Co3O4 interlayer yielded ordinary Co ferromagnetic hysteresis loops. The result suggests that graphene spin polarization was induced by the Co magnetization, mediated through the Co3O4 interlayer for the graphene/Co3O4(111)/Co(0001) system, while graphene merely acted as a spectator to cobalt for the graphene/Co(0001) system. Multilayer h-BN films were grown by atomic layer deposition on Co(0001) substrates. The MOKE measurements for the trilayer h-BN/CO samples again showed ordinary Co ferromagnetic hysteresis loops and no evidence of Co oxide formation after the prolonged air exposure. This last point suggests that trilayer h-BN films on Co inhibited air oxidation.
Secondly, the valence band structure and spin-orbit splittings of bulk 2H-WSe2, monolayer WSe2 and multilayer WS2 were investigated by means of angle-resolved photoemission spectroscopy (ARPES). The 2H-WSe2 revealed the significantly different band structures for states of even and odd reflection parities along the high symmetry points of the surface Brillouin zone. The experimental and theoretical band structures, for the most part, showed agreement. The spin-orbit splitting, ?SOC, 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), 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 the rigid energy shifts of the occupied and unoccupied band structures upon the 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 shifts for Co adsorped MoS2 and Na adsorped WSe2. The binding energy shifts exhibited the positive correlation with the difference of the work function between the metallic adlayer and the TMDs.