Program Highlights

Nebraska MRSEC Partnership with the Hiroshima Synchrotron Radiation Center

Takashi Komesu, Keisuke Fukutani, and Peter A. Dowben
 Nebraska MRSEC

Kenya Shimada and Hideaki Iwasawa
Hiroshima Synchrotron Radiation Center

Angle-resolved photoemission spectroscopy (ARPES) is a powerful method for the exploration of the intricate details of the electronic structure of materials, details responsible for the material’s functional properties. The method is based on collecting and analyzing electrons of different wave vectors and energy emitted from a material as a result of exposure to a photon flux. High resolution light polarization dependent ARPES requires the use of synchrotrons – powerful sources of the photon radiation not available at a university setting.
The Nebraska MRSEC has established a long standing partnership with the Hiroshima Synchrotron Radiation Center to explore a wide variety of two-dimensional semiconducting materials including the transition metal chalcogenides, such as molybdenum disulfide, MoS₂. These materials are striking due to their potential to serve as efficient conducting channels in the field effects transistors – electronic devices lying in the heart of the semiconductor industry.  
ARPES makes possible to visualize changes in the position of electronic bands when alkali metal atoms are absorbed by MoS₂ (see Figure). The adsorption on sodium, Na, leads to a charge transfer to the MoS₂ surface causing an effect similar to electron doping of a semiconductor. The MoS₂ occupied valence band shifts rigidly to greater binding energy with a little change in the occupied state dispersion, but causing a narrowing of the MoS₂ bandgap and increased metallicity. This alkali metal adsorption on the surface, in fact, leads to band bending in the region of the surface, and thus dopes the surface region of MoS₂.
Six MRSEC papers have resulted from this international collaboration in the last 4 years.

These programs are supported by the National Science Foundation, Division of Materials Research, Materials Research Science and Engineering Program, Grant 0820521.

ARPES spectrum showing a shift of the occupied electronic bands when the alkali metal, sodium, is adsorbed on MoS₂. E_F denotes the Fermi energy.